Coordination of cyclic electron flow and alternative electron sinks adjusts photosynthetic regulation in maize under fluctuating light during combined heat and drought stress

Field-grown maize is frequently exposed to fluctuating light (FL) combined with heat and drought during summer, yet the underlying photosynthetic regulatory mechanisms remain poorly understood. We examined chlorophyll fluorescence, the P700 redox state, and electrochromic shift signals in maize leaves subjected to FL under individual and compound stresses of moderate heat and drought. Following short-term FL treatment, photosystem I (PSI) photoinhibition was not exacerbated by any stress treatment, whereas photosystem II (PSII) photoinhibition was significantly aggravated by heat and compound stresses. Individual moderate heat stress minimally affected photosynthetic electron flow and the trans-thylakoid proton gradient (ΔpH). Under drought and compound stress, linear electron flow was substantially suppressed while cyclic electron flow (CEF) was stimulated persistently, enhancing the ΔpH formation during high-light phases. Additionally, alternative electron sinks were strongly activated during the initial one minute of low-to-high light transition under compound stress. Coordinated CEF and alternative electron sinks alleviated the PSI acceptor side limitation, thereby protecting PSI against photoinhibition in maize under FL during drought and compound stress. These findings provide insights into photosynthetic regulation in C4 crops under complex environmental conditions.