Photosynthetic Linear Electron Flow Drives CO(2) Assimilation in Maize Leaves

Photosynthetic organisms commonly develop the strategy to keep the reaction center chlorophyll of photosystem I, P700, oxidized for preventing the generation of reactive oxygen species in excess light conditions. In photosynthesis of C(4) plants, CO(2) concentration is kept at higher levels around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) by the cooperation of the mesophyll and bundle sheath cells, which enables them to assimilate CO(2) at higher rates to survive under drought stress. However, the regulatory mechanism of photosynthetic electron transport for P700 oxidation is still poorly understood in C(4) plants. Here, we assessed gas exchange, chlorophyll fluorescence, electrochromic shift, and near infrared absorbance in intact leaves of maize (a NADP-malic enzyme C(4) subtype species) in comparison with mustard, a C(3) plant. Instead of the alternative electron sink due to photorespiration in the C(3) plant, photosynthetic linear electron flow was strongly suppressed between photosystems I and II, dependent on the difference of proton concentration across the thylakoid membrane (ΔpH) in response to the suppression of CO(2) assimilation in maize. Linear relationships among CO(2) assimilation rate, linear electron flow, P700 oxidation, ΔpH, and the oxidation rate of ferredoxin suggested that the increase of ΔpH for P700 oxidation was caused by the regulation of proton conductance of chloroplast ATP synthase but not by promoting cyclic electron flow. At the scale of intact leaves, the ratio of PSI to PSII was estimated almost 1:1 in both C(3) and C(4) plants. Overall, the photosynthetic electron transport was regulated for P700 oxidation in maize through the same strategies as in C(3) plants only except for the capacity of photorespiration despite the structural and metabolic differences in photosynthesis between C(3) and C(4) plants.