Photosynthetic Linear Electron Flow Drives CO₂ Assimilation in Maize Leaves

• Main Authors: Ginga Shimakawa, Chikahiro Miyake
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: International Journal of Molecular Sciences
• Subjects: photosynthesis; linear electron flow; C₄ plants; P700 oxidation; ferredoxin
• Online Access: https://www.mdpi.com/1422-0067/22/9/4894

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 C₄ plants, CO₂ 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 CO₂ at higher rates to survive under drought stress. However, the regulatory mechanism of photosynthetic electron transport for P700 oxidation is still poorly understood in C₄ plants. Here, we assessed gas exchange, chlorophyll fluorescence, electrochromic shift, and near infrared absorbance in intact leaves of maize (a NADP-malic enzyme C₄ subtype species) in comparison with mustard, a C₃ plant. Instead of the alternative electron sink due to photorespiration in the C₃ 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 CO₂ assimilation in maize. Linear relationships among CO₂ 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 C₃ and C₄ plants. Overall, the photosynthetic electron transport was regulated for P700 oxidation in maize through the same strategies as in C₃ plants only except for the capacity of photorespiration despite the structural and metabolic differences in photosynthesis between C₃ and C₄ plants.