Chloroplast ATP synthase restricts photosynthesis under fluctuating light in tomato but not in maize

Photosynthesis in fluctuating light requires coordinated adjustments of diffusion conductance and biochemical capacity, but the role of chloroplast ATP synthase activity (g_H⁺) in dynamic photosynthesis is not well understood. In this study, we measured gas exchange, chlorophyll fluorescence and electrochromic shift signals in fluctuating light for leaves of tomato (Solanum lycopersicum) and maize (Zea mays). During the transition from sun to shade, simultaneous increases in g_H⁺, effective quantum yield of PSII, and net CO₂ assimilation rate (A_N) occurred in tomato but uncoupled in maize, indicating that g_H ⁺ limited A_N during the sun-to-shade transition in tomato but not in maize. During the shade-to-sun transition, g_H ⁺ increased simultaneously with stomatal conductance, mesophyll conductance and Rubisco carboxylation capacity in tomato, suggesting that g_H⁺ is an overlooked factor affecting light induction of A_N in tomato. By comparison, g_H ⁺ maintained at high levels in maize and its A_N was mainly restricted by stomatal conductance. Our results reveal that the kinetics of g_H⁺ in fluctuating light differs between species, and chloroplast ATP synthase may be a potential target for improving dynamic photosynthesis in crops such as tomato.