Leaf photosynthesis and its genetic improvement from the perspective of energy flow and CO₂ diffusion

Single-leaf photosynthesis is a fundamental process in plant biomass production, and is a major research topic in crop physiology. This paper reviews the recent achievements of research on the physiological determinants of the photosynthetic capacity from the perspective of energy flow and CO₂ diffusion. Measurement of chlorophyll fluorescence is a popular method to diagnose the function of photosystem II, and is useful to assess the susceptibility to photoinhibition and allocation of energy, which are keys to improving both stress resistance and photosynthetic productivity. Mesophyll conductance (g_m) is the conductance to CO₂ diffusion from intercellular airspaces to the chloroplast, and was long thought to be determined by leaf anatomical properties. However, recent studies showed that environmental conditions affect g_m. It is possible that g_m is affected by the gating of the CO2-permeable aquaporins (cooporins). Stomatal morphology is revealed to be an important factor affecting gas exchange both in crop plants and in Arabidopsis thaliana. The knowledge of the stomatal differentiation in Arabidopsis will be applicable to various crops. g_m, stomatal conductance (g_s) and leaf nitrogen content are the main factors to cause difference in leaf photosynthesis among rice lines, and recent activities are conducted to find genes to manipulate these factors. Although the association of leaf photosynthesis with crop productivity still has a large 'missing link', these achievements strongly suggest that the leaf photosynthetic capacity can be genetically improved in crop species.