C4 photosynthesis in a single C3 cell is theoretically inefficient but may ameliorate internal CO2 diffusion limitations of C3 leaves

Attempts are being made to introduce C-4 photosynthetic characteristics into C-3 crop plants by genetic manipulation. This research has focused on engineering single-celled C-4-type CO2 concentrating mechanisms into C-3 plants such as rice. Herein the pros and cons of such approaches are discussed with a focus on CO2 diffusion, utilizing a mathematical model of single-cell C-4 photosynthesis. It is shown that a high bundle sheath resistance to CO2 diffusion is an essential feature of energy-efficient C-4 photosynthesis. The large chloroplast surface area appressed to the intercellular airspace in C-3 leaves generates low internal resistance to CO2 diffusion, thereby limiting the energy efficiency of a single-cell C-4 concentrating mechanism, which relies on concentrating CO2 within chloroplasts of C-3 leaves. Nevertheless the model demonstrates that the drop in CO2 partial pressure, pCO(2), that exists between intercellular airspace and chloroplasts in C-3 leaves at high photosynthetic rates, can be reversed under high irradiance when energy is not limiting. The model shows that this is particularly effective at lower intercellular pCO(2). Such a system may therefore be of benefit in water-limited conditions when stomata are closed and low intercellular pCO(2) increases photorespiration.