PsbS confers limited adaptive benefit to C ₄ photosynthesis under fluctuating light

Adaptation of plant photosynthesis to dynamic light conditions experienced in natural environments is achieved through specific protective mechanisms. Energy-dependent non-photochemical quenching (qE), regulated by Photosystem II Subunit S (PsbS), is a key process facilitating acclimation to fluctuating light in C ₃ plants, which operate conventional photosynthesis. C ₄ plants, which include some of the world’s most productive and agriculturally important crops, have evolved a distinct high-efficiency photosynthetic pathway. Little is known about the role of specific processes, like qE, in acclimation of C ₄ plants to dynamic light environments. We generated gene-edited lines of the model C ₄ grass Setaria viridis lacking PsbS, which were found to be deficient in qE. This deficiency resulted in a modest increase in PSII photoinhibition and a CO ₂ assimilation penalty under light stress in short-term experiments, but photosynthesis and growth under fluctuating light were unaffected. Instead, keeping Photosystem I oxidised through photosynthetic control, negative feedback regulation of the Cytochrome b ₆ f complex, was critical. Therefore, unlike in C ₃ plants, qE does not provide a significant adaptive advantage to C ₄ plants under dynamic light conditions. These findings provide important insights into the biology of C ₄ plants and help prioritise future strategies for improving the productivity and resilience of C ₄ crops.