Theory of Evolution by Natural Selection

CHAPTER 6

Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.

6.1 Photosynthesis Is the Conversion of Carbon Dioxide into Simple Sugars

Photosynthesis is the process by which energy from the Sun, in the form of shortwave radiation, is harnessed to drive a series of chemical reactions that result in the fixation of CO2 into carbohydrates (simple sugars) and the release of oxygen (O2) as a by-product. The portion of the electromagnetic spectrum that photosynthetic organisms use is between 400 and 700 nanometers (nm; roughly corresponding to the visible portion of the spectrum) and is referred to as photosynthetically active radiation (PAR).

The process of photosynthesis can be expressed in the simplified form shown here:

6CO2+12H2O→C6H12O+6O2+6H2O6CO2+12H2O→C6H12O + 6O2 +6H2O

The net effect of this chemical reaction is the use of six molecules of water (H2O) and the production of six molecules of oxygen (O2) for every six molecules of CO2 that are transformed into one molecule of sugar C6H12O6. The synthesis of various other carbon-based compounds—such as complex carbohydrates, proteins, fatty acids, and enzymes—from these initial products occurs in the leaves as well as other parts of the plant.

Photosynthesis, a complex sequence of metabolic reactions, can be separated into two processes, often referred to as the light-dependent and light-independent reactions. The light-dependent reactions begin with the initial photochemical reaction in which chlorophyll (light-absorbing pigment) molecules within the chloroplasts absorb light energy. The absorption of a photon of light raises the energy level of the chlorophyll molecule. The excited molecule is not stable, and the electrons rapidly return to their ground state, thus releasing the absorbed photon energy. This energy is transferred to another acceptor molecule, resulting in a process called photosynthetic electron transport. This process results in the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and of NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate [NADP]) from NADP+. The high-energy substance ATP and the strong reductant NADPH produced in the light-dependent reactions are essential for the second step in photosynthesis—the light-independent reactions.

In the light-independent reactions, CO2 is biochemically incorporated into simple sugars. The light-independent reactions derive their name from the fact that they do not directly require the presence of sunlight. They are, however, dependent on the products of the light-dependent reactions and therefore ultimately depend on the essential resource of sunlight.

The process of incorporating CO2 into simple sugars begins in most plants when the five-carbon molecule ribulose biphosphate (RuBP) combines with CO2 to form two molecules of a three-carbon compound called phosphoglycerate (3-PGA).

CO2+RuBP→23−PGACO2+   RuBP  →  2  3-PGA

1-carbon5-carbon3-carbonmoleculemoleculemolecule1-carbon5-carbon3-carbonmoleculemoleculemolecule

This reaction, called carboxylation, is catalyzed by the enzyme rubisco (ribulose biphosphate carboxylase-oxygenase). The plant quickly converts the 3-PGA formed in this process into the energy-rich sugar molecule glyceraldehyde 3-phosphate (G3P). The synthesis of G3P from 3-PGA requires both ATP and NADPH—the high-energy molecule and reductant that are formed in the light-dependent reactions. Some of this G3P is used to produce simple sugars (C6H12O6), starches, and other carbohydrates required for plant growth and maintenance; the remainder is used to synthesize new RuBP to continue the process. The synthesis of new RuBP from G3P requires additional ATP.

In this way, the availability of light energy (solar radiation) can limit the light-independent reactions of photosynthesis through its control on the production of ATP and NADPH required for the synthesis of G3P and the regeneration of RuBP. This photosynthetic pathway involving the initial fixation of CO2 into the three-carbon PGAs is called the Calvin–Benson cycle, or C3 cycle, and plants employing it are known as C3 plants (Figure 6.1).

The C3 pathway has one major drawback. The enzyme rubisco that drives the process of carboxylation also acts as an oxygenase; rubisco can catalyze the reaction between O2 and RuBP. The oxygenation of RuBP results in the eventual release of CO2 and is referred to as photorespiration (not to be confused with the process of cellular respiration discussed herein). This competitive reaction to the carboxylation process reduces the efficiency of C3 photosynthesis by as much as 25 percent.

Some of the carbohydrates produced in photosynthesis are used in the process of cellular respiration—the harvesting of energy from the chemical breakdown of simple sugars and other carbohydrates. The process of cellular respiration (also referred to as aerobic respiration) occurs in the mitochondria of all living cells and involves the oxidation of carbohydrates to generate energy in the form of ATP.

C6H12O6+6O2→6CO2+6H2O+ATPC6H12O6+6O2→6CO2+6H2O +ATP

Because leaves both use CO2 during photosynthesis and produce CO2 during respiration, the difference in the rates of these two processes is the net gain of carbon, referred to as net photosynthesis .

Net photosynthesis=Photosynthesis −  RespirationNet photosynthesis  =  Photosynthesis −  Respiration

The rates of photosynthesis and respiration, and therefore net photosynthesis, are typically measured in moles CO2 per unit leaf area (or mass) per unit time (μmol/m2/s).

6.2 The Light a Plant Receives Affects Its Photosynthetic Activity

Solar radiation provides the energy required to convert CO2 into simple sugars. Thus, the availability of light (PAR) to the leaf directly influences the rate of photosynthesis (Figure  6.2). At night, in the absence of PAR, only respiration occurs and the net uptake of CO2 is negative. The rate of CO2 loss when the value of PAR is zero provides an estimate of the rate of respiration. As the Sun rises and the value of PAR increases, the rate of photosynthesis likewise increases, eventually reaching a level at which the rate of CO2 uptake in photosynthesis is equal to the rate of CO2 loss in respiration. At that point, the rate of net photosynthesis is zero. The light level (value of PAR) at which this occurs is called the light compensation point (LCP). As light levels exceed the LCP, the rate of net photosynthesis increases with PAR. Eventually, photosynthesis becomes light saturated. The value of PAR, above which no further increase in photosynthesis occurs, is referred to as the light saturation point. In some plants adapted to extremely shaded environments, photosynthetic rates decline as light levels exceed saturation. This negative effect of high light levels, called photoinhibition, can be the result of “overloading” the processes involved in the light-dependent reactions.

6.3 Photosynthesis Involves Exchanges between the Plant and Atmosphere

The process of photosynthesis occurs in specialized cells within the leaf called mesophyll cells (see Figure 6.1). For photosynthesis to take place within the mesophyll cells, CO2 must move from the outside atmosphere into the leaf. In terrestrial (land) plants, CO2 enters the leaf through openings on its surface called stomata (Figure 6.3) through the process of diffusion. Diffusion is the movement of a substance from areas of higher to lower concentration. CO2 diffuses from areas of higher concentration (the air) to areas of lower concentration (the interior of the leaf). When the concentrations are equal, an equilibrium is achieved and there is no further net exchange.

diffusion.

Two factors control the diffusion of CO2 into the leaf: the diffusion gradient and stomatal conductance. The diffusion gradient is defined as the difference between the concentration of CO2 in air adjacent to the leaf and the concentration of CO2 in the leaf interior. Concentrations of CO2 are often described in units of parts per million (ppm) of air. A CO2 concentration of 400 ppm would be 400 molecules of CO2 for every 1 million molecules of air. Stomatal conductance is the flow rate of CO2 through the stomata (generally measured in units of μmol/m2/s) and has two components: the number of stoma per unit leaf surface area (stomatal density) and aperture (the size of the stomatal openings). Stomatal aperture is under plant control, and stomata open and close in response to a variety of environmental and biochemical factors.

As long as the concentration of CO2 in the air outside the leaf is greater than that inside the leaf and the stomata are open, CO2 will continue to diffuse through the stomata into the leaf. So as CO2 diffuses into the leaf through the stomata, why do the concentrations of CO2 inside and outside the leaf not come into equilibrium? The concentration inside the leaf declines as CO2 is transformed into sugar during photosynthesis. As long as photosynthesis occurs, the gradient remains. If photosynthesis stopped and the stomata remained open, CO2 would diffuse into the leaf until the internal CO2 equaled the outside concentration.

When photosynthesis and the demand for CO2 are reduced for any reason (such as a reduction in light), the stomata tend to close, thus reducing flow into the leaf. The stomata close because they play a dual role. As CO2 diffuses into the leaf through the stomata, water vapor inside the leaf diffuses out through the same openings. This water loss through the stomata is called transpiration.

As with the diffusion of CO2 into the leaf, the rate of water diffusion out of the leaf will depend on the diffusion gradient of water vapor from inside to outside the leaf and the stomatal conductance (flow rate of water). Like CO2, water vapor diffuses from areas of high concentration to areas of low concentration—from wet to dry. The relative humidity (see Section  2.5, Figure 2.15) inside a leaf is typically greater than 99 percent, therefore there is usually a large difference in water vapor concentration between the inside and outside of the leaf, resulting in the diffusion of water out of the leaf. The lower the relative humidity of the air, the larger the diffusion gradient and the more rapidly the water inside the leaf will diffuse through the stomata into the surrounding air. The leaf must replace the water lost to the atmosphere, otherwise it will wilt and die.

6.4 Water Moves from the Soil, through the Plant, to the Atmosphere

The force exerted outward on a cell wall by the water contained in the cell is called turgor pressure. The growth rate of plant cells and the efficiency of their physiological processes are highest when the cells are at maximum turgor—that is, when they are fully hydrated. When the water content of the cell declines, turgor pressure drops and water stress occurs, ranging from wilting to dehydration. For leaves to maintain maximum turgor, the water lost to the atmosphere in transpiration must be replaced by water taken up from the soil through the root system of the plant and transported to the leaves.

You may recall from basic physics that work—the displacement of matter, such as transporting water from the soil into the plant roots and to the leaves—requires the transfer of energy. The measure of energy available to do work is called Gibbs energy (G), named for the U.S. physicist Willard Gibbs, who first developed the concept in the 1870s. In the process of active transport, such as transporting water from the ground to an elevated storage tank using an electric pump, the input of energy to the system is in the form of electricity to the pump. The movement of water through the soil–plant–atmosphere continuum, however, is an example of passive transport, a spontaneous reaction that does not require an input of energy to the system. The movement of water is driven by internal differences in the Gibbs energy of water at any point along the continuum between the soil, plant, and atmosphere. The Second Law of Thermodynamics states that the transfer of energy (through either heat or work) always proceeds in the direction from higher to lower energy content (e.g., from hot to cold). Therefore, a gradient of decreasing energy content of the water between any two points along the continuum must exist to enable the passive movement of water between the soil, plant, and atmosphere.

The measure used to describe the Gibbs energy of water at any point along the soil–plant–atmosphere continuum is called water potential (ψ). Water potential is the difference in Gibbs energy per mole (the energy available to do work) between the water of interest and pure water (at a standard temperature and pressure). Plant physiologists have chosen to express water potential in terms of pressure, which has the dimensions of energy per volume, and is expressed in terms of Pascals (Pa = 1 Newton/m2). By convention, pure water at atmospheric pressure has a water potential of zero and the addition of any solutes or the creation of suction (negative hydrostatic pressure) will function to lower the water potential (more negative values).

We can now examine the movement of water through the soil–plant–atmosphere continuum as a function of the gradient in water potential. As previously stated, the transfer of energy will always proceed from a region of higher energy content to a region of lower energy content, or in the case of water potential, from areas of higher water potential to areas of lower water potential. We can start with the exchange of water between the leaf and the atmosphere in the process of transpiration. When relative humidity of the atmosphere is 100 percent, the atmospheric water potential (ψatm) is zero. As values drop below 100 percent, the value of Gibbs energy declines, and ψatm becomes negative (Figure 6.4). Under most physiological conditions, the air within the leaf is at or near saturation (relative humidity ∼ 99 percent). As long as the relative humidity of the air is below 99 percent, a steep gradient of water potential between the leaf (ψleaf) and the atmosphere (ψatm) will drive the process of diffusion. Water vapor will move from the region of higher water potential (interior of the leaf) to the region of lower water potential (atmosphere)—that is, from a state of high to low Gibbs energy.

As water is lost to the atmosphere through the stomata, the water content of the cells decreases (turgor pressure drops) and in turn increases the concentration of solutes in the cell. This decrease in the cell’s water content (and corresponding increase in solute concentration) decreases the water potential of the cells. Unlike the water potential of the atmosphere, which is determined only by relative humidity, several factors determine water potential within the plant. Turgor pressure (positive pressure) in the cell increases the plant’s water potential. Therefore, a decrease in turgor pressure associated with water loss functions to decrease water potential. The component of plant water potential as a result of turgor pressure represents hydrostatic pressure and is represented as ψp.

Increasing concentrations of solutes in the cells are associated with water loss and will lower the water potential. This component of plant water potential is termed osmotic potential (ψπ) because the difference in solute content inside and outside the cell results in the movement of water through the process of osmosis.

The surfaces of larger molecules, such as those in the cell walls, exert an attractive force on water. This tendency for water to adhere to surfaces reduces the Gibbs energy of the water molecules, reducing water potential. This component of water potential is called matric potential (ψm). The total water potential ψ at any point in the plant, from the leaf to the root, is the sum of these individual components:

ψ = ψp + ψp + ψmψ = ψp + ψp + ψm

The osmotic and matric potentials will always have a negative value, whereas the turgor pressure (hydrostatic pressure) component can be either positive or negative. Thus, the total potential can be either positive or negative, depending on the relative values of the individual components. Values of total water potential at any point along the continuum (soil, root, leaf, and atmosphere), however, are typically negative and the movement of water proceeds from areas of higher (zero or less negative) to lower (more negative) potential (from the region of higher energy to the region of lower energy). Therefore, the movement of water from the soil to the root, from the root to the leaf, and from the leaf to the atmosphere depends on maintaining a gradient of increasingly negative water potential at each point along the continuum (Figure 6.4).

ψatm < ψleaf < ψroot < ψsoilψatm < ψleaf < ψroot < ψsoil

Drawn by the low water potential of the atmosphere, water from the surface of and between the mesophyll cells within the leaf evaporates and escapes through the stomata. This gradient of water potential is transmitted into the mesophyll cells and on to the water-filled xylem (hollow conducting tubes throughout the plant) in the leaf veins. The gradient of increasingly negative water potential extends down to the fine rootlets in contact with soil particles and pores. As water moves from the root and up through the stem to the leaf, the root water potential declines so that more water moves from the soil into the root.

Water loss through transpiration continues as long as (1) the amount of energy striking the leaf is enough to supply the necessary latent heat of evaporation (see Section 2.5), (2) moisture is available for roots in the soil, and (3) the roots are capable of maintaining a more negative water potential than that of the soil. At field capacity, water is freely available, and soil water potential (ψsoil) is at or near zero (see Section 4.8). As water is drawn from the soil, the water content of the soil declines, and the soil water potential becomes more negative. As the water content of the soil declines, the remaining water adheres more tightly to the surfaces of the soil particles, and the matric potential becomes more negative. For a given water content, the matric potential of soil is influenced strongly by its texture (see Figure 4.10). Soils composed of fine particles, such as clays, have a higher surface area (per soil volume) for water to adhere to than sandy soils do. Clay soils, therefore, are characterized by more negative matric potentials for the same water content.

As soil water potential becomes more negative, the root and leaf water potentials must decline (become more negative) if the potential gradient is to be maintained. If precipitation does not recharge soil water, and soil water potentials continue to decline, eventually the gradient between the soil, root, and leaf cannot be maintained, and at that point, the stomata close to stop further water loss through transpiration. However, this closure also results in stopping further uptake of CO2. The value of leaf water potential at which stomata close and net photosynthesis ceases varies among plant species ( Figure 6.5 ) and reflects basic differences in their biochemistry, physiology, and morphology.

The rate of water loss varies with daily environmental conditions, such as humidity and temperature, and with the characteristics of plants. Opening and closing the stomata is probably the plant’s most important means of regulating water loss. The trade-off between CO2 uptake and water loss through the stomata results in a direct link between water availability in the soil and the plant’s ability to carry out photosynthesis. To carry out photosynthesis, the plant must open its stomata; but when it does, it loses water, which it must replace to live. If water is scarce, the plant must balance the opening and closing of the stomata, taking up enough CO2 while minimizing the loss of water. The ratio of carbon fixed (photosynthesis) per unit of water lost (transpiration) is called the water-use efficiency.

We can now appreciate the trade-off faced by terrestrial plants. To carry out photosynthesis, the plant must open the stomata to take up CO2. But at the same time, the plant loses water through the stomata to the outside air—water that must be replaced through the plant’s roots. If its access to water is limited, the plant must balance the opening and closing of stomata to allow for the uptake of CO2 while minimizing water loss through transpiration. This balance between photosynthesis and transpiration is an extremely important constraint that has governed the evolution of terrestrial plants and directly influences the productivity of ecosystems under differing environmental conditions (see Chapter 20).

6.5 The Process of Carbon Uptake Differs for Aquatic and Terrestrial Autotrophs

A major difference in CO2 uptake and assimilation by aquatic autotrophs (submerged plants, algae, and phytoplankton) versus terrestrial plants is the lack of stomata in aquatic autotrophs. CO2 diffuses from the atmosphere into the surface waters and is then mixed into the water column. Once dissolved, CO2 reacts with the water to form bicarbonate (HCO3 −). This reaction is reversible, and the concentrations of CO2 and bicarbonate tend toward a dynamic equilibrium (see Section 3.7). In aquatic autotrophs, CO2 diffuses directly from the waters across the cell membrane. Once the CO2 is inside the cell, photosynthesis proceeds in much the same way as outlined previously for terrestrial plants.

One difference between terrestrial and aquatic autotrophs is that some aquatic species can also use bicarbonate as a carbon source. However, the organism must first convert it to CO2 using the enzyme carbonic anhydrase. This conversion can occur in two ways: (1) active transport of bicarbonate into the cell followed by conversion to CO2 or (2) excretion of the enzyme into adjacent waters and subsequent uptake of converted CO2 across the membrane. As CO2 is taken up, its concentration in the waters adjacent to the organism decline. Because the diffusion of CO2 in water is 104 times slower than in the air, it can easily become depleted (low concentrations) in the waters adjacent to the organism, reducing rates of uptake and photosynthesis. This constraint can be particularly important in still waters such as dense seagrass beds or rocky intertidal pools.

6.6 Plant Temperatures Reflect Their Energy Balance with the Surrounding Environment

Both photosynthesis and respiration respond directly to variations in temperature ( Figure 6.6 ). As temperatures rise above freezing, both photosynthesis and respiration rates increase. Initially, photosynthesis increases faster than respiration. As temperatures continue to rise, the photosynthetic rate reaches a maximum related to the temperature response of the enzyme rubisco. As temperatures continue to rise, photosynthetic rate declines and respiration rate continues to increase. As temperatures rise further, even respiration declines as temperatures reach critical levels. The temperature response of net photosynthesis is the difference between the rate of carbon uptake in photosynthesis and the rate of carbon loss in respiration (see Figure 6.6). Three values describe the temperature response curve: Tmin, Topt, and Tmax. The values Tmin and Tmax are, respectively, the minimum and maximum temperatures at which net photosynthesis approaches zero (meaning no net carbon uptake). Topt is the temperature, or range of temperatures, over which net carbon uptake is at its maximum.

The temperature of the leaf, not the air, controls the rate of photosynthesis and respiration; and leaf temperature depends on the exchange of thermal energy between the leaf and its surrounding environment. Plants absorb both shortwave (solar) and longwave (thermal) radiation (see Section 2.1). Plants reflect some of this solar radiation and emit longwave radiation back to the atmosphere. The difference between the radiation a plant receives and the radiation it reflects and emits back to the surrounding environment is the net radiation balance of the plant (Rn). The net radiation balance of a plant is analogous to the concept of the radiation balance of Earth (see Chapter 2, Figure 2.3). Of the net radiation absorbed by the plant, some is used in metabolic processes and stored in chemical bonds—namely, in the processes of photosynthesis and respiration. This quantity is quite small, typically less than 5 percent of Rn. The remaining energy raises the temperature of the leaves and the surrounding air. On a clear, sunny day, the amount of energy plants absorb can raise internal leaf temperatures well above ambient (air or water temperature). Internal leaf temperatures may go beyond the optimum for photosynthesis and possibly reach critical levels (see Figure 6.6).

To maintain internal temperatures within the range of tolerance (positive net photosynthesis), plants must exchange thermal energy (heat) with the surrounding environment. The transfer of heat between the plant and the surrounding air (or water) is governed by the Second Law of Thermodynamics—thermal energy flows in only one direction, from areas of higher temperature to areas of lower temperature.

The primary means by which terrestrial plants dissipate heat are evaporation and convection; aquatic plants do so primarily by convection. Evaporation occurs in the process of transpiration. Recall from Chapter 3 that the phase change of water from a liquid to a gas (evaporation) requires an input of thermal energy (540 calories or 2260 joules per gram [g] of water). As waters transpires from the leaves of plants to the surrounding atmosphere through the stomata, the leaves lose thermal energy and their temperature declines through evaporative cooling (see Section 3.2). The ability of terrestrial plants to dissipate heat by evaporation is dependent on the rate of transpiration. The transpiration rate is in turn influenced by the relative humidity of the air and by the availability of water to the plant (see Section 6.3).

Convection is the transfer of heat energy through the circulation of fluids (Chapter 2), whereas conduction is the transfer of thermal energy through direct contact (between two objects). For convection to occur, the surface of the leaf must first transfer thermal energy between the adjacent molecules of air or water through conduction. The direction of this conductive exchange depends on the difference between the temperature of the leaf and the surrounding air. If the leaf temperature is higher than that of the surrounding air, there is a net transfer of heat from the leaf to the surrounding air. Thermal energy is then transported from the air adjacent to the surface of the leaf to the surrounding air through the process of convection, the circulation of fluids.

The transfer of heat from the plant to the surrounding environment is influenced by the existence of the boundary layer , which is a layer of still air (or water) adjacent to the surface of each leaf. The environment of the boundary layer differs from that of the surrounding environment (air or water) because it is modified by the diffusion of heat, water, and CO2 from the plant surface. As water is transpired from the stomata, the humidity of the air within the boundary layer increases, reducing further transpiration. Likewise, as thermal energy (heat) is transferred from the leaf surface to the boundary layer, the air temperature of the boundary layer increases, reducing further heat transfer from the leaf surface. Under still conditions (no air or water flow), the boundary layer increases in thickness reducing the transfer of heat and materials (water and CO2) between the leaf and the atmosphere (or water). Wind or water flow functions to reduce the size of the boundary layer, allowing for mixing between the boundary layer and the surrounding air (or water) and reestablishing the diffusion or temperature gradient between the leaf surface and the bulk air.

Leaf size and shape also influence the thickness and dynamics of the boundary layer, and therefore, the ability of plants to exchange heat through convection. Air tends to move more smoothly (laminar flow) over a larger surface than a smaller one, and as a result, the boundary layer tends to be thicker and more intact in larger leaves. Deeply lobed leaves, and small, compound leaves (Figure 6.7) tend to disrupt the flow of air, causing turbulence that functions to reduce the boundary layer and increase the exchange of heat and water.

The relative importance of evaporation and convection to the maintenance of leaf temperatures (dissipation of heat) is dependent on the physical environment. In locations where water is available, such as regions of high mean annual precipitation, most of the dissipation of heat can occur through transpiration (evaporation) as plants open stomata to support the uptake of CO2. As conditions become drier, however, transpiration becomes limited and the average leaf size of species decreases, enhancing heat loss through convection (see Figure 6.18b

6.7 Constraints Imposed by the Physical Environment Have Resulted in a Wide Array of Plant Adaptations

We have explored variation in the physical environment over Earth’s surface: the salinity, depth, and flow of water; spatial and temporal patterns in climate (precipitation and temperature); variations in geology and soils (Part One). In all but the most extreme of these environments, autotrophs harness the energy of the Sun to fuel the conversion of CO2 into glucose in the process of photosynthesis. To survive, grow, and reproduce, plants must maintain a positive carbon balance, converting enough CO2 into glucose to offset the expenses of respiration (photosynthesis > respiration). To accomplish this, a plant must acquire the essential resources of light, CO2, water, and mineral nutrients as well as tolerate other features of the environment that directly affect basic plant processes such as temperature, salinity, and pH. Although often discussed—and even studied as though they are independent of each other—the adaptations exhibited by plants to these features of the environment are not independent, for reasons relating to the physical environment and to the plants themselves.

Many features of the physical environment that directly influence plant processes are interdependent. For example, the light, temperature, and moisture environments are all linked through a variety of physical processes (Chapters 2– 4). The amount of solar radiation not only influences the availability of light (PAR) required for photosynthesis but also directly influences the temperature of the leaf and its surroundings. In addition, air temperature directly affects the relative humidity, a key feature influencing the rate of transpiration and evaporation of water from the soil (see Section 2.5, Figure 2.15). For this reason, we see a correlation in the adaptations of plants to variations in these environmental factors. Plants adapted to dry, sunny environments must be able to deal with the higher demand for water associated with higher temperatures and lower relative humidity, and they tend to have characteristics such as smaller leaves and increased production of roots.

In other cases, there are trade-offs in the ability of plants to adapt to limitations imposed by multiple environmental factors, particularly resources. One of the most important of these trade-offs involves the acquisition of above- and belowground resources. Allocating carbon to the production of leaves and stems increases the plant’s access to the resources of light and CO2, but it is at the expense of allocating carbon to the production of roots. Likewise, allocating carbon to the production of roots increases access to water and soil nutrients but limits carbon allocation to the production of leaves. The set of characteristics (adaptations) that allow a plant to successfully survive, grow, and reproduce under one set of environmental conditions inevitably limits its ability to do equally well under different environmental conditions. We explore the consequences of this simple premise in the following sections.

6.8 Species of Plants Are Adapted to Different Light Environments

The amount of solar radiation reaching Earth’s surface varies diurnally, seasonally, and geographically (Chapter 2, Section  2.3). However, a major factor influencing the amount of light (PAR) a plant receives is the presence of other plants through shading (see Section 4.2 and Chapter 4, Quantifying Ecology 4.1). Although the amount of light that reaches an individual plant varies continuously as a function of the area of leaves above it, plants live in one of two qualitatively different light environments—sun or shade—depending on whether they are overtopped by other plants. Plants have evolved to possess a range of physiological and morphological adaptations that allow individuals to survive, grow, and reproduce in these two different light environments (see this chapter, Field Studies: Kaoru Kitajima).

Plant species adapted to high-light environments are called shade-intolerant species, or sun-adapted species. Plant species adapted to low-light environments are called shade-tolerant species, or shade-adapted species. Shade-tolerant and shade-intolerant species differ across a wide variety of phenotypic characteristics that represent adaptations to sun and shade environments. One of the most fundamental differences between shade-intolerant and shade-tolerant plant species lies in their patterns of photosynthesis in response to varying levels of light availability. Shade-tolerant species tend to have a lower light saturation point and a lower maximum rate of photosynthesis than shade-intolerant species (Figure  6.8). These differences relate in part to lower concentrations of the photosynthetic enzyme rubisco (see Section 6.1) found in shade-tolerant plants. Plants must expend a large amount of energy and nutrients to produce rubisco and other components of the photosynthetic apparatus. In shaded environments, low light, not the availability of rubisco to catalyze the fixation of CO2, limits the rate of photosynthesis. Shade-tolerant (shade-adapted) species produce less rubisco as a result. By contrast, production of chlorophyll, the light-harvesting pigment in the leaves, often increases. The reduced energy cost of producing rubisco and other compounds involved in photosynthesis lowers the rate of leaf respiration. Because the LCP is the value of PAR necessary to maintain photosynthesis at a rate that exactly offsets the loss of CO2 in respiration (net photosynthesis = 0), the lower rate of respiration can be offset by a lower rate of photosynthesis, requiring less light. The result is a lower LCP in shade-tolerant species. However, the same reduction in enzyme concentrations that is associated with lower rates of respiration limits the maximum rate at which photosynthesis can occur when light is abundant (high PAR; see Figure 6.8), lowering both the light saturation point and the maximum rate of photosynthesis. The lower maximum rates of photosynthesis inevitably result in a lower rate of net carbon gain and growth rate by shade-tolerant species as compared to shade-intolerant species when growing under high light levels (see Figure 6.8).

Field Studies Kaoru Kitajima

Department of Botany, University of Florida, Gainesville, Florida

A major factor influencing the availability of light to a plant is its neighbors. By intercepting light, taller plants shade individuals below, influencing rates of photosynthesis, growth, and survival. Nowhere is this effect more pronounced than on the forest floor of a tropical rain forest, where light levels are often less than 1 percent of those recorded at the top of the canopy (see Section 4.2). With the death of a large tree, however, a gap is created in the canopy, giving rise to an “island” of light on the forest floor. With time, these gaps in the canopy eventually close because individuals grow up to the canopy from below or neighboring trees expand their canopies, which once again shades the forest floor.

How these extreme variations in availability of light at the forest floor have influenced the evolution of rain forest plant species has been the central research focus of plant ecologist Kaoru Kitajima of the University of Florida. Kitajima’s work in the rain forests of Barro Colorado Island in Panama presents a story of plant adaptations to variations in the light environment that includes all life stages of the individual, from seed to adult.

Within the rain forests of Barro Colorado Island, the seedlings of some tree species survive and grow only in the high-light environments created by the formation of canopy gaps, whereas the seedlings of other species can survive for years in the shaded conditions of the forest floor.

In a series of experiments designed to determine shade tolerance based on patterns of seedling survival in sun and shade environments (see Figure 6.10), Kitajima noted that seed mass (weight) is negatively correlated with seedling mortality rates. Interestingly, large-seeded species not only had higher rates of survival in the shade but also exhibited slower rates of growth after germination. Intuitively, one might think that larger reserves of energy and nutrients within the seed (larger mass) would allow for a faster rate of initial development, but this was not the case. What role does seed size play in the survival and growth of species in different light environments? An understanding of these relationships requires close examination of how seed reserves are used.

The storage structure or structures within a seed are called the cotyledon. Upon germination, cotyledons transfer reserve materials (lipids, carbohydrates, and mineral nutrients) into developing shoots and roots. The cotyledons of some species serve strictly as organs to store and transfer seed reserves throughout their life span and are typically positioned at or below the ground level (Figure  1a). The cotyledons of other species develop a second function—photosynthetic carbon assimilation. In these species, the cotyledons function as “seed leaves” and are raised above the ground (Figure 1b). As Kitajima’s research has revealed, the physiological function of cotyledons is crucial in determining the growth response of seedlings to the light environment.

The smaller seeds of the shade-intolerant species had photosynthetic cotyledons and developed leaves earlier than did shade-tolerant species with their larger storage cotyledons. These differences reflect two different “strategies” in the use of initial seed reserves. Shade-intolerant species invested reserves in the production of leaves to bring about a rapid return (carbon uptake in photosynthesis), whereas shade-tolerant species kept seed reserves as storage for longer periods at the expense of growth rate.

Having used their limited seed reserves for the production of leaves, the shade-intolerant species responded to light availability earlier than did the shade-tolerant species. And without sufficient light, mortality was generally the outcome.

So the experiments revealed that larger seed storage in shade-tolerant species does not result in a faster initial growth under shaded conditions (Figure 2). Rather, these species (shade-tolerant) exhibit a conservative strategy of slow use of reserves over a prolonged period. In a later study, Kitajima established that the lower relative growth rate of shade tolerant seedlings is associated with an increased storage of nonstructural carbohydrates (sugars and starches) in the stem and roots (Figure 3a), enabling them to cope with periods of biotic (herbivory and disease) and abiotic (shade) stress. Results of the study show a significant positive relationship between seedling survival during the first year under shaded conditions and the storage of nonstructural carbohydrates (Figure 3b).

Whether shade tolerant or intolerant, once seedlings use up seed reserves, they must maintain a positive net carbon gain as a prerequisite for survival (see Section 6.7). What suites of seedling traits allow some species to survive better than others in the shade? To answer this question, Kitajima grew seedlings in the experimental sun and shade environments for an extended period beyond the reserve phase. Under both sun and shade conditions, shade-tolerant species had a greater proportional allocation to roots (relative to leaves), thicker leaves (lower SLA), and as a result, a lower photosynthetic surface area than did shade-intolerant species. As a result, the relative growth rates of shade-intolerant species were consistently greater than those of the shade-tolerant species, both in sun and shaded conditions (see Figure 2).

Whereas the characteristics exhibited by the shade-intolerant species reflect strong natural selection for fast growth within light gaps, shade-tolerant species appear adapted to survive for many years in the understory, where their ability to survive attacks by pathogens and herbivores is enhanced by their well-developed reserves within the root system.

Bibliography

1. Kitajima, K. 1994. “Relative importance of photosynthetic and allocation traits as correlates of seedling shade tolerance of 15 tropical tree species.” Oecologia 98: 419–428.

2. Kitajima, K. 1996. Ecophysiology of tropical tree seedlings. In Tropical forest plant ecophysiology (S. Mulkey, R. L. Chazdon, and A. P. Smith, eds.), 559–596. New York: Chapman & Hall.

3. Kitajima, K. 2002. “Do shade-tolerant tropical tree seedlings depend longer on seed reserves?” Functional Ecology 16:433–444.

4. Myers, J. A., and Kitajima, K. 2007. “Carbohydrate storage enhances seedling shade and stress tolerance in a neotropical forest.” Journal of Ecology 95:383–395.

1. What processes might create gaps in the forest canopy?

2. How might seed size influence the method of seed dispersal from the parent plant?

The variations in photosynthesis, respiration, and growth rate that characterize plant species adapted to different light environments are illustrated in the work of plant ecologist Peter Reich and colleagues at the University of Minnesota. They examined the characteristics of nine tree species that inhabit the cool temperate forests of northeastern North America (boreal forest). The species differ widely in shade tolerance from very tolerant of shaded conditions to very intolerant. Seedlings of the nine species were grown in the greenhouse, and measurements of maximum net photosynthetic rate at light saturation, leaf respiration rate, and relative growth rate (growth rate per unit plant biomass; see Quantifying Ecology 6.1)) were made over the course of the experiment (Figure 6.9). Species adapted to lower light environments (shade-tolerant) are characterized by lower maximum rates of net photosynthesis, leaf respiration, and relative growth rate than are species adapted to higher light environments (shade-intolerant).

The difference in the photosynthetic characteristics between shade-tolerant and shade-intolerant species influences not only rates of net carbon gain and growth but also ultimately the ability of individuals to survive in low-light environments. This relationship is illustrated in the work of Caroline Augspurger of the University of Illinois. She conducted a series of experiments designed to examine the influence of light availability on seedling survival and growth for a variety of tree species, both shade-tolerant and shade-intolerant, that inhabit the tropical rain forests of Panama. Augspurger grew tree seedlings of each species under two levels of light availability. These two treatments mimic the conditions found either under the shaded environment of a continuous forest canopy (shade treatment) or in the higher light environment in openings or gaps in the canopy caused by the death of large trees (sun treatment). She continued the experiment for a year, monitoring the survival and growth of seedlings on a weekly basis. Figure  6.10 presents the results for two contrasting species, shade-tolerant and shade-intolerant.

The shade-tolerant species (Myroxylon balsamum) showed little difference in survival and growth rates under sunlight and shade conditions. In contrast, the survival and growth rates of the shade-intolerant species (Ceiba pentandra) were dramatically reduced under shade conditions. These observed differences are a direct result of the difference in the adaptations relating to photosynthesis and carbon allocation discussed previously. The higher rate of light-saturated photosynthesis resulted in a high growth rate for the shade-intolerant species in the high-light environment. The associated high rate of leaf respiration and LCP, however, reduced rates of survival in the shaded environment. By week 20 of the experiment, all individuals had died. In contrast, the shade-tolerant species was able to survive in the low-light environment. The low rates of leaf respiration and light-saturated photosynthesis that allow for the low LCP, however, limit rates of growth even in high-light environments.

Interpreting Ecological Data

1. Q1. In general, how do net photosynthesis and leaf respiration vary with increasing shade tolerance for the nine boreal tree species? What does this imply about the corresponding pattern of gross photosynthesis with increasing shade tolerance for these species?

2. Q2. Based on the data presented in graphs (a) and (b), how would you expect the light compensation point to differ between Populus tremuloides and Picea glauca?

3. In addition to the differences in photosynthesis and growth rate, shade-tolerant and shade-intolerant species also exhibit differences in patterns of leaf morphology. The ratio of surface area (measured in centimeters squared [cm2]) to weight (g) for a leaf is called the specific leaf area (SLA; cm2/g). The value of SLA represents the surface area of leaf produced per gram of biomass (or carbon) allocated to the production of leaves. Shade-tolerant species typically produce leaves with a greater specific leaf area. This difference in leaf structure effectively increases the surface area for the capture of light (the limiting resource) per unit of biomass allocated to the production of leaves. Marc Abrams and Mark Kubiske of Pennsylvania State University examined leaf morphology of 31 tree species inhabiting the forests of central Wisconsin. The researchers measured both SLA and leaf thickness for individuals of each species growing in full sunlight and in the shaded conditions of the understory. Shade-tolerant species show a consistent pattern of higher SLA and lower leaf thickness than shade-intolerant species (Figure 6.11). Their data also illustrate a second important point regarding plant adaptations: plant species exhibit phenotypic plasticity in response to the light environment. Individuals of both shade-tolerant and shade-intolerant species exhibit an increase in SLA and a reduction in leaf thickness when growing under shaded conditions compared to open, sunny conditions (see Figure 6.11). The increased surface area of leaves in the shade functions to increase the photosynthetic surface area, partially offsetting the reduced rates of photosynthesis.

4. The dichotomy in adaptations between shade-tolerant and shade-intolerant species reflects a fundamental trade-off between characteristics that enable a species to maintain high rates of net photosynthesis and growth under high-light conditions and the ability to continue survival and growth under low-light conditions. The differences in biochemistry, physiology, and leaf morphology exhibited by shade-tolerant species reduce the amount of light required to survive and grow. However, these same characteristics limit their ability to maintain high rates of net photosynthesis and growth when light levels are high. In contrast, plants adapted to high-light environments (shade-intolerant species) can maintain high rates of net photosynthesis and growth under high-light conditions but at the expense of continuing photosynthesis, growth, and survival under shaded conditions.

5. Quantifying Ecology 6.1 Relative Growth Rate

6. When we think of growth rate, what typically comes to mind is a measure of change in size during some period of time, such as change in weight during the period of a week (grams weight gain/week). However, this conventional measure of growth is often misleading when comparing individuals of different sizes or tracking the growth of an individual through time. Although larger individuals may have a greater absolute weight gain when compared with smaller individuals, this may not be the case when weight gain is expressed as a proportion of body weight (proportional growth). A more appropriate measure of growth is the mass-specific or relative growth rate. Relative growth rate (RGR) expresses growth during an observed period of time as a function of the size of the individual. This calculation is performed by dividing the increment of growth during some observed time period (grams [g] weight gain) by the size of the individual at the beginning of that time period (g weight gain/total g weight at the beginning of observation period) and then dividing by the period of time to express the change in weight as a rate (g/g/time).

Using RGR to evaluate the growth of plants has an additional value; RGR can be partitioned into components reflecting the influences of assimilation (photosynthesis) and allocation on growth—the assimilation of new tissues per unit leaf area (g/centimeters squared [cm2]/time) called the net assimilation rate (NAR) , and the leaf area per unit of plant weight (cm2/g), called the leaf area ratio (LAR).

The NAR is a function of the total gross photosynthesis of the plant minus the total plant respiration. It is the net assimilation gain expressed on a per-unit leaf area basis. The LAR is a function of the amount of that assimilation that is allocated to the production of leaves—more specifically, leaf area—expressed on a per-unit plant weight basis.

The LAR can be further partitioned into two components that describe the allocation of net assimilation to leaves, the leaf weight ratio (LWR) , and a measure of leaf density or thickness, the specific leaf area (SLA) . The LWR is the total weight of leaves expressed as a proportion of total plant weight (g leaves/g total plant weight), whereas the SLA is the area of leaf per gram of leaf weight. For the same tissue density, a thinner leaf would have a greater value of SLA.

The real value of partitioning the estimate of RGR is to allow for comparison, either among individuals of different species or among individuals of the same species grown under different environmental conditions. For example, the data presented in Table 1 are the results of a greenhouse experiment in which seedlings of Acacia tortilis (a tree that grows on the savannas of southern Africa) were grown under two different light environments: full sun and shaded (50 percent full sun). Individuals were harvested at two times (at four and six weeks), and the total plant weight, total leaf weight, and total leaf area were measured. The mean values of these measures are shown in the table. From these values, estimates of RGR, LAR, LWR, and SLA were calculated. The values of RGR are calculated using the total plant weights at four and six weeks. NAR was then calculated by dividing the RGR by LAR. Because LAR varies through time (between weeks four and six), the average of LAR at four and six weeks was used to characterize LAR in estimating RGR. Note that the average size (weight and leaf area) of seedlings grown in the high-light environment is approximately twice that of seedlings grown in the shade. Despite this difference in size, and the lower light levels to support photosynthesis, the difference in RGR between sun- and shade-grown seedlings is only about 20 percent. By examining the components of RGR, we can see how the shaded individuals are able to accomplish this task. Low-light conditions reduced rates of photosynthesis, subsequently reducing NAR for the individuals grown in the shade. The plants compensated, however, by increasing the allocation of carbon (assimilates) to the production of leaves (higher LWR) and producing thinner leaves (higher SLA) than did the individuals grown in full sun. The result is that individuals grown in the shade have a greater LAR (photosynthetic surface area relative to plant weight), offsetting the lower NAR and maintaining comparable RGR.

These results illustrate the value of using the RGR approach for examining plant response to varying environmental conditions, either among individuals of the same species or among individuals of different species adapted to different environmental conditions. By partitioning the components of plant growth into measures directly related to morphology, carbon allocation, and photosynthesis, we can begin to understand how plants both acclimate and adapt to differing environmental conditions.

1. 1When plants are grown under dry conditions (low water availability), there is an increase in the allocation of carbon to the production of roots at the expense of leaves. How would this shift in allocation influence the plant’s LAR?

2. 2understory. Shade-tolerant species show a consistentNitrogen availability can directly influence the rate of net photosynthesis. Assuming no change in the allocation of carbon or leaf morphology, how would an increase in the rate of net photosynthesis resulting from an increase in nitrogen availability influence RGR? Which component of RGR would be influenced by the increase in net photosynthesis?

6.9 The Link between Water Demand and Temperature Influences Plant Adaptations

As with the light environment, a range of adaptations has evolved in terrestrial plants in response to variations in precipitation and soil moisture. As we saw in the previous discussion of transpiration (see Section 6.3), however, the demand for water is linked to temperature. As air temperature rises, the saturation vapor pressure will likewise rise, increasing the gradient of water vapor between the inside of the leaf and the outside air and subsequently the rate of transpiration (see Section 2.5). As a result, the amount of water required by the plant to offset losses from transpiration will likewise increase with temperature.

Plants exhibit both acclimation and developmental plasticity (both forms of phenotypic plasticity; see Section 5.4 ) in response to changes in water availability and demand on a variety of timescales. When the atmosphere or soil is dry, plants respond by partially closing the stomata and opening them for shorter periods of time. In the early period of water stress, a plant closes its stomata during the hottest part of the day when relative humidity is the lowest (Figure 6.12). It resumes normal activity in the afternoon. As water becomes scarcer, the plant opens its stomata only in the cooler, more humid conditions of morning. Closing the stomata reduces the loss of water through transpiration, but it also reduces CO2 diffusion into the leaf and the dissipation of heat through evaporative cooling. As a result, the photosynthesis rate declines and leaf temperatures may rise. Some plant species, such as evergreen rhododendrons, respond to moisture stress by an inward curling of the leaves. Others show it in a wilted appearance caused by a lack of turgor in the leaves. Leaf curling and wilting allow leaves to reduce water loss and heat gain by reducing the surface area exposed to solar radiation.

Interpreting Ecological Data

1. Q1. How does specific leaf area (cm2 of leaf area per gram of leaf weight) change for leaves grown in the shade as compared to the open (full sun)? Does the relationship differ for shade-tolerant and shade-intolerant species (same direction of change)?

2. Q2. How does the observed change in leaf thickness for leaves growing in the shade relate to the changes in specific leaf area (think about what a higher specific leaf area implies about leaf thickness)?

3. Q3. Are the observed changes in leaf morphology under shaded conditions an example of phenotypic plasticity?

Interpreting Ecological Data

Q1. How does relative humidity change with temperature? Why (see Section 2.5, Figure 2.15)?

Q2. How does stomatal conductance (the opening and closing of stomata) respond to changes in relative humidity?

Q3. What is the advantage to the plant of partially closing the stomata during the middle of the day? How would the decline in stomatal conductance influence net photosynthesis?

Prolonged moisture stress inhibits the production of chlorophyll, causing the leaves to turn yellow or, later in the summer, to exhibit premature autumn coloration. As conditions worsen, deciduous trees may prematurely shed their leaves—the oldest ones dying first. Such premature shedding can result in dieback of twigs and branches.

Plants also exhibit developmental plasticity in response variations in the availability of water to meet the demands of transpiration. During development, plants respond to low soil water availability by increasing the allocation of carbon to the production of roots while decreasing the production of leaves (Figure 6.13a). By increasing its production of roots, the plant can explore a larger volume and depth of soil for extracting water. The reduction in leaf area decreases the amount of solar radiation the plant intercepts as well as the surface area that is losing water through transpiration. The combined effect is to increase the uptake of water per unit leaf area while reducing the total amount of water that is lost to the atmosphere through transpiration.

The decline in leaf area with decreasing water availability is actually a combined effect of reduced allocation of carbon to the production of leaves (Figure 6.13b) and changes in leaf morphology (size and shape). The leaves of plants grown under reduced water conditions tend to be smaller and thicker (lower specific leaf area; see Section 6.8) than those of individuals growing in more mesic (wet) environments (Figure 6.13c).

On an evolutionary timescale, a wide array of adaptations has evolved in plant species in response to variations in the availability of water relative to demand. In some species of plants, referred to as C4 plants and CAM plants, a modified form of photosynthesis has evolved that increases water-use efficiency in warmer and drier environments. The modification involves an additional step in the conversion (fixation) of CO2 into sugars.

In C3 plants, the capture of light energy and the transformation of CO2 into sugars occur in the mesophyll cells (see Section 6.1). The products of photosynthesis move into the vascular bundles, part of the plant’s transport system, where they can be transported to other parts of the plant. In contrast, plants possessing the C4 photosynthetic pathway have a leaf anatomy different from that of C3 plants (see Figure 6.3). C4 plants have two distinct types of photosynthetic cells: the mesophyll cells and the bundle sheath cells. The bundle sheath cells surround the veins or vascular bundles (Figure 6.14). C4 plants divide photosynthesis between the mesophyll and the bundle sheath cells.

In C4 plants, CO2 reacts with phosphoenolpyruvate (PEP), a three-carbon compound, within the mesophyll cells. This is in contrast to the initial reaction with RuBP in C3 plants. This reaction is catalyzed by the enzyme PEP carboxylase , producing oxaloacetate (OAA) as the initial product. The OAA is then rapidly transformed into the four-carbon molecules of malic and aspartic acids, from which the name C4 photosynthesis is derived. These organic acids are then transported to the bundle sheath cells (see Figure 6.14). There, enzymes break down the organic acids to form CO2, reversing the process that is carried out in the mesophyll cells. In the bundle sheath cells, the CO2 is transformed into sugars using the C3 pathway involving RuBP and rubisco.

The extra step in the fixation of CO2 gives C4 plants certain advantages. First, PEP does not react with oxygen, as does RuBP. This eliminates the inefficiency that occurs in the mesophyll cells of C3 plants when rubisco catalyzes the reaction between O2 and RuBP (photorespiration), leading to the production of CO2 and a decreased rate of net photosynthesis (see Section 6.1). Second, the conversion of malic and aspartic acids into CO2 within the bundle sheath cells acts to concentrate CO2. The CO2 within the bundle sheath cells can reach much higher concentrations than in either the mesophyll cells or the surrounding atmosphere. The higher concentrations of CO2 in the bundle sheath cells increase the efficiency of the reaction between CO2 and RuBP catalyzed by rubisco. The net result is generally a higher maximum rate of photosynthesis in C4 plants than in C3 plants.

To understand the adaptive advantage of the C4 pathway, we must go back to the trade-off in terrestrial plants between the uptake of CO2 and the loss of water through the stomata. Resulting from the higher photosynthetic rate, C4 plants exhibit greater water-use efficiency (CO2 uptake/H2O loss; see Section 6.4). That is, for a given degree of stomatal opening and associated water loss in transpiration, C4 plants typically fix more carbon in photosynthesis. This increased water-use efficiency can be a great advantage in hot, dry climates where water is a major factor limiting plant growth. However, it comes at a price. The C4 pathway has a higher energy expenditure because of the need to produce PEP and the associated enzyme, PEP carboxylase.

The C4 photosynthetic pathway is not found in algae, bryophytes, ferns, gymnosperms (includes conifers, cycads, and ginkgos), or the more primitive flowering plants (angiosperms). C4 plants are mostly grasses native to tropical and subtropical regions and some shrubs characteristic of arid and saline environments, such as Larrea (creosote bush) and Atriplex (saltbush) that dominate regions of the desert southwest in North America. The distribution of C4 grass species in North America reflects the advantage of the C4 photosynthetic pathway under warmer and drier conditions (Figure 6.15). The proportion of grass species that are C4 increases from north to south, reaching a maximum in the southwest.

In the hot deserts of the world, environmental conditions are even more severe. Solar radiation is high, and water is scarce. To counteract these conditions, a small group of desert plants, mostly succulents in the families Cactaceae (cacti), Euphorbiaceae, and Crassulaceae, use a third type of photosynthetic pathway—crassulacean acid metabolism (CAM). The CAM pathway is similar to the C4 pathway in that CO2 initially reacts with PEP and is transformed into four-carbon compounds using the enzyme PEP carboxylase. The four-carbon compounds are later converted back into CO2, which is transformed into glucose using the C3 cycle. Unlike C4 plants, however, in which these two steps are physically separate (in mesophyll and bundle sheath cells), both steps occur in the mesophyll cells but at separate times (Figure 6.16).

CAM plants open their stomata at night, taking up CO2 and converting it to malic acid using PEP, which accumulates in large quantities in the mesophyll cells. During the day, the plant closes its stomata and reconverts the malic acid into CO2, which it then fixes using the C3 cycle. Relative to both C3 and C4 plants, the CAM pathway is slow and inefficient in the fixation of CO2. But by opening their stomata at night when temperatures are lowest and relative humidity is highest, CAM plants dramatically reduce water loss through transpiration and increase water-use efficiency.

In addition to adaptations relating to modifications of the photosynthetic pathway, plants adapted to different soil moisture environments exhibit a variety of physiological and morphological characteristics that function to allow them to either tolerate or avoid drought conditions. Plant species adapted to xeric conditions typically have a lower stomatal conductance (lower number and size of stomata) than species adapted to more mesic conditions. This results in a lower rate of transpiration but also functions to decrease rates of photosynthesis. Because of the higher diffusion gradient of water relative to CO2, the reduction in stomatal conductance functions to increase water-use efficiency.

(Data from Mokany et al. 2006.)

Plant species adapted to drier conditions tend to have a greater allocation of carbon to the production of roots relative to aboveground tissues (greater ratio of roots to shoots), particularly leaves (Figure 6.17). This pattern of carbon allocation allows the plant to explore a larger volume and depth of soil for extracting water. The decline in leaf area in more xeric environments is actually a combined effect of reduced allocation of carbon to the production of leaves and changes in leaf morphology (size and shape). The leaves of plant species adapted to xeric conditions tend to be smaller and thicker (lower specific leaf area; see Section 6.9) than those of species adapted to more mesic environments (Figure 6.18). In some plants, the leaves are small, the cell walls are thickened, the stomata are tiny, and the vascular system for transporting water is dense. Some species have leaves covered with hairs that scatter incoming solar radiation, whereas others have leaves coated with waxes and resins that reflect light and reduce its absorption. All these structural features function to reduce the amount of energy striking the leaf, enhance the dissipation of heat through convection (see Section 6.6, Figure 6.7), and thus, reduce the loss of water through transpiration. In tropical regions with distinct wet and dry seasons, some species of trees and shrubs have evolved the characteristic of dropping their leaves at the onset of the dry season (see Section 2.6). These plants are termed drought deciduous. In these species, leaf senescence occurs as the dry season begins, and new leaves are grown just before the rainy season begins.

Although the decrease in leaf area and corresponding increase in biomass allocated to roots observed for plant species adapted to reduced water availability functions to reduce transpiration and increase the plant’s ability to acquire water from the soil, this shift in patterns of allocation has consequences for plant growth. The reduced leaf area decreases carbon gain from photosynthesis resulting in a reduction in plant growth rate.

6.10 Plants Exhibit Both Acclimation and Adaptation in Response to Variations in Environmental Temperatures

As sessile organisms, terrestrial plants are subject to wide variations in temperature on a number of spatial scales and timescales. As we discussed in Chapter 2, at a continental to global scale, temperatures vary with latitude (see Section 2.2). At a local to regional scale, temperatures vary with elevation, slope, and aspect. Seasonal changes in temperature are influenced by both latitude and position relative to the coast (large bodies of water; see Section 2.7 , whereas diurnal (daily) changes in temperature occur everywhere. These patterns of temperature variation are consistent and predictable, and evolution has resulted in a variety of adaptations that enable plants to cope with these variations.

When examined across a range of plant species inhabiting different thermal environments, Tmin, Topt, and Tmax (see Figure  6.6) tend to match the prevailing environmental temperatures. Species adapted to cooler environments typically have a lower Tmin, Topt, and Tmax than species that inhabit warmer climates (Figure 6.19). These differences in the temperature response of net photosynthesis are directly related to a variety of biochemical and physiological adaptations that act to shift the temperature responses of photosynthesis and respiration toward the prevailing temperatures in the environment. These differences are most pronounced between plants using the C3 and C4 photosynthetic pathways (see Section 6.9). C4 plants inhabit warmer, drier environments and exhibit higher optimal temperatures for photosynthesis (generally between 30°C and 40°C) than do C3 plants (Figure 6.20). This is in large part because of the higher Topt for PEP carboxylase as compared to rubisco (see Section 6.9).

Although species from different thermal habitats exhibit different temperature responses for photosynthesis and respiration, these responses are not fixed. When individuals of the same species are grown under different thermal conditions in the laboratory or greenhouse, divergence in the temperature response of net photosynthesis is often observed (Figure 6.21). In general, the range of temperatures over which net photosynthesis is at its maximum shifts in the direction of the thermal conditions under which the plant is grown. That is to say, individuals grown under cooler temperatures exhibit a lowering of Topt, whereas those individuals grown under warmer conditions exhibit an increase in Topt. This same shift in the temperature response can be observed in individual plants in response to seasonal shifts in temperature (Figure 6.22). These modifications in the temperature response of net photosynthesis are a result of the process of acclimation—reversible phenotypic changes in response to changing environmental conditions (see Section 5.4).

In addition to the influence of temperature on plant carbon balance, periods of extreme heat or cold can directly damage plant cells and tissues. Plants that inhabit seasonally cold environments, where temperatures drop below freezing for periods of time, have evolved several adaptations for survival. The ability to tolerate extreme cold, referred to as frost hardening, is a genetically controlled characteristic that varies among species as well as among local populations of the same species. In seasonally changing environments, plants develop frost hardening through the fall and achieve maximum hardening in winter. Plants acquire frost hardiness—the turning of cold-sensitive cells into hardy ones—through the formation or addition of protective compounds in the cells. Plants synthesize and distribute substances such as sugars, amino acids, and other compounds that function as antifreeze, lowering the temperature at which freezing occurs. Once growth starts in spring, plants lose this tolerance quickly and are susceptible to frost damage in late spring.

Producing the protective compounds that allow leaves to survive freezing temperatures requires a significant expenditure of energy and nutrients. Some species avoid these costs by shedding their leaves before the cold season starts. These plants are termed winter deciduous, and their leaves senesce during the fall. The leaves are replaced during the spring, when conditions are once again favorable for photosynthesis. In contrast, needle-leaf evergreen species—such as pine (Pinus spp.) and spruce (Picea spp.) trees—contain a high concentration of these protective compounds, allowing the needles to survive the freezing temperatures of winter.

Although evolution has resulted in an array of physiological and morphological mechanisms that enable plant species to adjust to the prevailing environmental temperatures, these adaptations have a cost. Most mechanisms (particularly biochemical) for both acclimation and adaptation to temperature involve trade-offs between performance at higher temperatures and performance at lower temperatures. For example, shifts of enzymes and membranes (both acclimation and adaptation) to low temperatures generally result in poor performance (or maladaptation) to high temperatures, that is, shifts in Tmin are associated with a corresponding shift in Tmax . In addition, reductions in Topt are typically associated with a decline in maximum rates of net photosynthesis and growth.

6.11 Plants Exhibit Adaptations to Variations in Nutrient Availability

Plants require a variety of chemical elements to carry out their metabolic processes and to synthesize new tissues (Table 6.1). Thus, the availability of nutrients has many direct effects on plant survival, growth, and reproduction. Some of these elements, known as macronutrients, are needed in large amounts. Other elements are needed in lesser, often minute quantities. These elements are called micronutrients, or trace elements. The prefixes micro– and macro– refer only to the quantity of nutrients needed, not to their importance to the organism. If micronutrients are lacking, plants fail as completely as if they lacked nitrogen, calcium, or any other macronutrient.

Table 6.1

Essential Elements in Plants

Alternate View

Of the macronutrients, carbon (C), hydrogen (H), and oxygen (O) form the majority of plant tissues. These elements are derived from CO2 and H2O and are made available to the plant as glucose through photosynthesis. The remaining six macronutrients—nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)—exist in varying states in the soil and water, and their availability to plants is affected by different processes depending on their location in the physical environment (see Chapters 3 and 4). In terrestrial environments, plants take up nutrients from the soil. Autotrophs in aquatic environments take up nutrients from the substrate or directly from the water.

The rate of nutrient absorption (uptake per unit root) depends on concentrations in the external solution (soil or water; Figure 6.23a). As the availability (concentration) of nutrients at the root surface declines, the rate of absorption declines, which eventually results in a decline in tissue nutrient concentrations (Figure 6.23b). In the case of nitrogen, the decrease in leaf concentrations has a direct effect on maximum rates of photosynthesis (Figure 6.23c) through a reduction in the production of rubisco and chlorophyll (see Section 6.1). In fact, more than 50 percent of the nitrogen content of a leaf is in some way involved directly with the process of photosynthesis, with much of it tied up in these two compounds.

In response to reduced nutrient availability, carbon is allocated to root growth at the expense of shoot growth, resulting in an increase in the ratio of roots to shoots ( Figure 6.24 ). The increased production of roots is an example of phenotypic plasticity and allows the plant to compensate for the decrease in nutrient absorption per-unit root by increasing the root area and soil volume from which nutrients are absorbed. Despite the shift in patterns of carbon allocation to compensate for reduced nutrient availability, decreased rates of photosynthesis and reduced allocation to leaves resulting from increased allocation to roots inevitably lead to a reduction in growth rate.

We have seen that geology, climate, and biological activity alter the availability of nutrients in the soil (see Chapter  4). Consequently, some environments are relatively rich in nutrients, and others are poor. How do plants from low-nutrient environments succeed?

Species that inhabit low nutrient environments exhibit a wide array of phenotypic characteristics that enable them to survive, grow, and reproduce under reduced nutrient levels in the soil or water. Compared with species from more fertile soils, species characteristic of infertile soils usually exhibit a low absorption rate. Consequently, in comparison with species from high-nutrient environments, species from infertile soils absorb considerably less nutrient under high-nutrient conditions but similar quantities, and in some cases, even more nutrients at extremely low availability. In addition, plants adapted to low-nutrient environments generally have a greater allocation of carbon to the production of roots and subsequently have a higher ratio of roots to shoots.

Because plants require nutrients for synthesizing new tissue, in physiological terms, it is growth that creates demand for nutrients. Conversely, as we have seen, the plant’s uptake rate of the nutrients directly influences its growth rate. This relationship may seem circular, but the important point is that not all plants have the same inherent (maximum potential) rate of growth. In Section 6.8 (see Figure 6.8), we saw how shade-tolerant plants have an inherently lower rate of photosynthesis and growth than shade-intolerant plants do, even under high-light conditions. This lower rate of photosynthesis and growth translates to a lower demand for resources, including nutrients. The same pattern of reduced photosynthesis occurs among plants that are characteristic of low-nutrient environments. Figure 6.25 shows the growth responses of two grass species when soil is enriched with nitrogen. The species that naturally grows in a high-nitrogen environment keeps increasing its rate of growth with increasing availability of soil nitrogen. The species native to a low-nitrogen environment reaches its maximum rate of growth at low to medium nitrogen availability. It does not respond to further additions of nitrogen.

Some plant ecologists suggest that a low maximum growth rate is an adaptation to a low-nutrient environment. One advantage of slower growth is that the plant can avoid stress under low-nutrient conditions. A slow-growing plant can still maintain optimal rates of photosynthesis and other metabolic processes crucial for growth under low-nutrient availability. In contrast, a plant with an inherently high rate of growth will show signs of stress.

Another hypothesized adaptation to low-nutrient environments is leaf longevity (Figure 6.26). Leaf production has a cost to the plant. This cost can be defined in terms of the carbon and other nutrients required to grow the leaf. At a low rate of photosynthesis, a leaf needs a longer time to “pay back” the cost of its production. As a result, plants inhabiting low-nutrient environments tend to have longer-lived leaves. A good example is the dominance of pine species on nutrient poor, sandy soils in the coastal region of the southeastern United States. In contrast to deciduous tree species, which shed their leaves every year, these pines have needles that live for up to three years.

6.12 Plant Adaptations to the Environment Reflect a Trade-off between Growth Rate and Tolerance

As we have seen in the preceding discussion, plant adaptations to the abiotic (physical and chemical) environment represent a fundamental trade-off between phenotypic characteristics that enable high rates of photosynthesis and plant growth in high resource/energy environments and the ability to tolerate (survive, grow, and reproduce) low resource/energy conditions (Figure 6.27). The basic physiological processes, particularly photosynthesis, function optimally under warm temperatures and adequate supplies of light, water, and mineral nutrients. As environmental temperatures get colder and supplies of essential resources decline, plants respond through a variety of mechanisms that function to both increase access to the limiting resource or enhance the ability of the plant to function under the reduced resource/energy conditions. For example, species adapted to low-light environments exhibit lower rates of respiration that enable the maintenance of positive rates of photosynthesis under low-light levels (reduced LCP), but at the same time these characteristics reduce maximum rates of photosynthesis and plant growth under high-light levels. Likewise, characteristics that enable plant species to successfully grow in arid and drought-prone environments, such as increased production of root, reduced leaf area, and smaller leaf size, enhance its ability to access water and reduce rates of water loss through transpiration; however, these same characteristics limit growth rates under mesic conditions.

What has emerged in our discussion is a general pattern of evolutionary constraints and trade-offs (costs and benefits) such that the set of phenotypic characteristics that enhance an organism’s relative fitness under one set of environmental conditions inevitably limit its relative fitness under different environmental conditions. The set of phenotypic characteristics that enhance a species’ carbon gain (photosynthesis and plant growth) under high resource/energy environments limit its tolerance (survival and growth) of low resource/energy conditions. Conversely, the phenotypic characteristics that enable a species to survive, grow, and reproduce under low resource/energy conditions limit its ability to maximize growth rate in high resource/energy environments. This basic concept will provide a foundation for later discussions of the interactions of plant species under different environmental conditions (e.g., competition) and how patterns of plant species distribution and abundance change across the landscape.

Ecological Issues & Applications Plants Respond to Increasing Atmospheric CO2

In Chapter 2 we discussed that atmospheric concentrations of CO2 have been rising exponentially since the mid-19th century from preindustrial levels of approximately 280 ppm to current levels of 400 ppm (as of June 2013; Chapter 2, Ecological Issues & Applications). In addition to influencing the planet’s energy balance (Section 2.1) and the pH of the oceans (Chapter  3, Ecological Issues & Applications), rising atmospheric concentrations of CO2 have a direct influence on terrestrial plants.

Recall that CO2 diffuses from the air into the leaf through the stomatal openings (see Section 6.3). The rate of diffusion is a function of two factors: the diffusion gradient (the difference in CO2 concentration between the air and the leaf interior) and stomatal conductance. Therefore, for a given stomatal conductance, an increase in the CO2 concentration of the air will increase the diffusion gradient, subsequently increasing the movement of CO2 into the leaf interior. In turn, the increased concentration of CO2 within the leaf (mesophyll cells) will result in a greater rate of photosynthesis. The higher rates of diffusion and photosynthesis under elevated atmospheric concentrations of CO2 have been termed the CO2 fertilization effect .

The increased rate of photosynthesis under elevated CO2 is in large part a result of reduced photorespiration (Section 6.1). The higher internal concentrations of CO2 increase the affinity of rubisco to catalyze the reaction of RuBP with CO2 (photosynthesis) rather than with O2 (photorespiration). Because photorespiration can reduce photosynthetic rates by as much as 25 percent, the reduction, or even elimination of photorespiration under elevated CO2 greatly enhances potential rates of net photosynthesis. Because C4 plants avoid photorespiration (see discussion of C4 pathway in Section 6.9) they do not exhibit the same increase in photosynthesis under elevated CO2 (Figure 6.28).

A second observed response of plants to elevated CO2 is a reduction in stomatal conductance. Recall that stomatal conductance has two components: the number of stoma per unit area (stomatal density) and aperture (the size of the stomatal openings). In the short term, the observed decrease in stomatal conductance under elevated CO2 is caused by a reduction in the aperture (partial closure of the stomata); in the long term, developmental plasticity has been shown to result in a decline in the stomatal density. As with the partial closure of stomata in response to decreased relative humidity (see Section 6.3 and Figure 6.12), this decrease in stomatal conductance functions to reduce rates of transpiration to a greater degree than CO2 uptake and photosynthesis, and therefore results in an increase in water-use efficiency (ratio of photosynthesis to transpiration; see Section 6.3).

Most of our fundamental understanding about the response of plants to elevated CO2 has come from experiments in controlled environments, greenhouses, and open-top chambers. However, because these techniques can alter the environment surrounding the plant, the use of free-air CO2 enrichment (FACE) experiments—in which plants are grown at elevated CO2 in the field under fully open-air conditions—provide scientists with the best estimates of how plants will respond to increasing atmospheric concentrations of CO2 in natural ecosystems (Figure 6.29). Elizabeth Ainsworth and Alistair Rogers of the University of Illinois conducted a meta-analysis of the results of FACE experiments and summarized the current understanding of the response of plant species to elevated CO2 concentrations. Averaged across all plant species grown at elevated CO2 (567 ppm) in FACE experiments, stomatal conductance was reduced by 22 percent (Figure 6.30a). There was significant variation among different plant groups. On average, trees, shrubs, and forbs showed a lower percentage decrease in stomatal conductance as compared to C3 and C4 grasses and herbaceous crop species. Although studies have shown a reduction in stomatal density in a wide variety of species when grown under elevated CO2, the observed decrease in stomatal conductance in the FACE experiments was not significantly influenced by a change in stomatal density.

Elevated CO2 stimulated light-saturated photosynthetic rates (see Figure 6.2) in C3 plants grown in FACE experiments by an average of 31 percent (Figure 6.30b). The magnitude of increase in photosynthetic rates, however, varied with plant type and environment. Trees showed the largest response to elevated CO2, whereas shrub species showed the smallest response. There was a surprising increase in photosynthetic rates of C4 crop species, however; this stimulation of photosynthesis at elevated CO2 was an indirect effect of reduced stomatal conductance. The reduction in stomatal conductance is associated with improved soil water status (Figure  6.31) as a result of reduced transpiration. Increased rates of photosynthesis in the C4 crops sorghum and maize (corn) were associated with improved water status or were limited to periods of low rainfall.

The effects of long-term exposure to elevated CO2 on plant growth and development, however, may be more complicated. Plant ecologists Hendrik Poorter and Marta Pérez-Soba of Utrecht University in the Netherlands reviewed the results from more than 600 experimental studies examining the growth of plants at elevated CO2 levels. These studies examined a wide variety of plant species representing all three photosynthetic pathways: C3, C4, and CAM (Section 6.9). Their results revealed that C3 species respond most strongly to elevated CO2, with an average increase in biomass of 47 percent (Figure  6.32). Data on the response of CAM species were limited, but the mean response for the six species reported was 21 percent. The C4 species examined also responded positively to elevated CO2, with an average increase of 11 percent.

Interpreting Ecological Data

1. Q1. How does soil moisture at both soil depths (0.0–0.15 m [circles] and 0.15–0.30 m [squares]) differ between ambient and elevated CO2 chambers during the month of June?

2. Q2. How is the increase in soil moisture under elevated CO2 a result of reduction in stomatal conductance and lower rates of transpiration?

3. Q3. How might the increased soil moisture under elevated CO2 during the summer months affect net photosynthesis of plants (in addition to the direct enhancement of net photosynthesis by elevated CO2)?

On average within C3 species, crop species show the highest biomass enhancement (59 percent) and wild herbaceous plants the lowest (41 percent). Most of the experiments with woody species were conducted with seedlings, therefore covering only a small part of their life cycle. The growth stimulation of woody plants was on average 49 percent.

In some longer-term studies, the enhanced effects of elevated CO2 levels on plant growth have been short-lived (Figure  6.33). Some plants produce less of the photosynthetic enzyme rubisco at elevated CO2, reducing photosynthesis to rates comparable to those measured at lower CO2 concentrations; this phenomenon is known as downregulation. Other studies reveal that plants grown at increased CO2 levels allocate less carbon to producing leaves and more to producing roots.

One factor that has been shown to influence the magnitude of the response of photosynthesis to elevated CO2 is the availability of nitrogen. Under elevated CO2 the ability of the plant to acquire adequate nitrogen and other essential resources to support an enhanced growth potential has been shown to lead to reductions in the production of rubisco and thereby functions to reduce rates of photosynthesis (downregulation) and plant growth.

Summary

Photosynthesis and Respiration 6.1

Photosynthesis harnesses light energy from the Sun to convert CO2 and H2O into glucose. A nitrogen-based enzyme, rubisco, catalyzes the transformation of CO2 into sugar. Because the first product of the reaction is a three-carbon compound, this photosynthetic pathway is called C3 photosynthesis. Cellular respiration releases energy from carbohydrates to yield energy, H2O, and CO2. The energy released in this process is stored as the high-energy compound ATP. Respiration occurs in the living cells of all organisms.

Photosynthesis and Light 6.2

The amount of light reaching a plant influences its photosynthetic rate. The light level at which the rate of CO2 uptake in photosynthesis equals the rate of CO2 loss as a result of respiration is called the light compensation point. The light level at which a further increase in light no longer produces an increase in the rate of photosynthesis is the light saturation point.

CO2 Uptake and Water Loss 6.3

Photosynthesis involves two key physical processes: diffusion and transpiration. CO2 diffuses from the atmosphere to the leaf through leaf pores, or stomata. As photosynthesis slows down during the day and demand for CO2 lessens, stomata close to reduce loss of water to the atmosphere. Water loss through the leaf is called transpiration. The amount of water lost depends on the humidity. Water lost through transpiration must be replaced by water taken up from the soil.

Water Movement 6.4

Water moves from the soil into the roots, up through the stem and leaves, and out to the atmosphere. Differences in water potential along a water gradient move water along this route. Plants draw water from the soil, where the water potential is the highest, and release it to the atmosphere, where it is the lowest. Water moves out of the leaves through the stomata in transpiration, and this reduces water potential in the roots so that more water moves from the soil through the plant. This process continues as long as water is available in the soil. This loss of water by transpiration creates moisture conservation problems for plants. Plants need to open their stomata to take in CO2, but they can conserve water only by closing the stomata.

Aquatic Plants 6.5

A major difference between aquatic and terrestrial plants in CO2 uptake and assimilation is the lack of stomata in submerged aquatic plants. In aquatic plants, there is a direct diffusion of CO2 from the waters adjacent to the leaf across the cell membrane.

Plant Energy Balance 6.6

Leaf temperatures affect both photosynthesis and respiration. Plants have optimal temperatures for photosynthesis beyond which photosynthesis declines. Respiration increases with temperature. The internal temperature of all plant parts is influenced by heat gained from and lost to the environment. Plants absorb longwave and shortwave radiation. They reflect some of it back to the environment. The difference is the plant’s net radiation balance. The plant uses some of the absorbed radiation in photosynthesis. The remainder must be either stored as heat in the plant and surrounding air or dissipated through the processes of evaporation (transpiration) and convection.

Interdependence of Plant Adaptations 6.7

A wide range of adaptations has evolved in plants in response to variations in environmental conditions. The adaptations exhibited by plants to these features of the environment are not independent for reasons relating to the physical environment and to the plants themselves.

Plant Adaptations to High and Low Light 6.8

Plants exhibit a variety of adaptations and phenotypic responses (phenotypic plasticity) in response to different light environments. Shade-adapted (shade-tolerant) plants have low photosynthetic, respiratory, metabolic, and growth rates. Sun plants (shade-intolerant) generally have higher photosynthetic, respiratory, and growth rates but lower survival rates under shaded conditions. Leaves in sun plants tend to be small, lobed, and thick. Shade-plant leaves tend to be large and thin.

Alternative Pathways of Photosynthesis 6.9

The C4 pathway of photosynthesis involves two steps and is made possible by leaf anatomy that differs from C3 plants. C4 plants have vascular bundles surrounded by chlorophyll-rich bundle sheath cells. C4 plants fix CO2 into malate and aspartate in the mesophyll cells. They transfer these acids to the bundle sheath cells, where they are converted into CO2. Photosynthesis then follows the C3 pathway. C4 plants are characterized by high water-use efficiency (the amount of carbon fixed per unit of water transpired). Succulent desert plants, such as cacti, have a third type of photosynthetic pathway, called CAM. CAM plants open their stomata to take in CO2 at night, when the humidity is high. They convert CO2 to malate, a four-carbon compound. During the day, CAM plants close their stomata, convert malate back to CO2, and follow the C3 photosynthetic pathway.

Adaptations to Temperature 6.10

Plants exhibit a variety of adaptations to extremely cold as well as hot environments. Cold tolerance is mostly genetic and varies among species. Plants acquire frost hardiness through the formation or addition of protective compounds in the cell, where these compounds function as antifreeze. The ability to tolerate high air temperatures is related to plant moisture balance.

Plant Adaptations to Nutrient Availability 6.11

Terrestrial plants take up nutrients from soil through the roots. As roots deplete nearby nutrients, diffusion of water and nutrients through the soil replaces them. Availability of nutrients directly affects a plant’s survival, growth, and reproduction. Nitrogen is important because rubisco and chlorophyll are nitrogen-based compounds essential to photosynthesis. Uptake of nitrogen and other nutrients depends on availability and demand. Plants with high nutrient demands grow poorly in low-nutrient environments. Plants with lower demands survive and grow, slowly, in low-nutrient environments. Plants adapted to low-nutrient environments exhibit lower rates of growth and increased longevity of leaves.

Trade-off between Growth and Tolerance 6.12

Plant adaptations to the abiotic environment represent a fundamental trade-off between phenotypic characteristics that enable high rates of photosynthesis and plant growth in high resource/energy environments and the ability to tolerate (survive, grow, and reproduce) under low resource/energy conditions.

Plant Response to Elevated CO2 Ecological Issues & Applications

Plants exhibit two primary responses to CO2: an increase in photosynthesis and a reduction in stomatal conductance. The increase in photosynthesis occurs primarily in C3 plant species and is a response to reduced photorespiration. The decrease in stomatal conductance functions to increase water-use efficiency. Increased rates of photosynthesis result in an increase in growth rates.

(Smith 120-121)

Smith, Thomas M. Smith and Robert L. Elements of Ecology, 9th Edition. Pearson Learning Solutions, 06/2016. VitalBook file.

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