I'm working on a bio spinach lab right now, our purpose is to find to factors that cause the different photosynthesis rate between spinach, chard,...

I'm working on a bio spinach lab right now, our purpose is to find to factors that cause the different photosynthesis rate between spinach, chard, and collard. I found lots of practical things about how to growing them but cant find any document that discuss their specific difference in the structures, I need help! What is the structural difference between spinach, chard, and collard that cause their different photosynthesis rate