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

Abstract

Attempts are being made to introduce C4 photosynthetic characteristics into C3 crop plants by genetic manipulation. This research has focused on engineering single-celled C4-type CO 2 concentrating mechanisms into C3 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 C3 leaves generates low internal resistance to CO2 diffusion, thereby limiting the energy efficiency of a single-cell C4 concentrating mechanism, which relies on concentrating CO2 within chloroplasts of C3 leaves. Nevertheless the model demonstrates that the drop in CO2 partial pressure, pCO2, that exists between intercellular airspace and chloroplasts in C3 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 pCO2. Such a system may therefore be of benefit in water-limited conditions when stomata are closed and low intercellular pCO2 increases photorespiration.