Responses of photosystem I compared with photosystem II to combination of heat stress and fluctuating light in tobacco leaves

•Photosynthetic responses to heat and fluctuating light were investigated in tobacco.•Heat induces the stimulation of CEF in high light phases under fluctuating light.•The stimulation of CEF facilitates the electron sink via photorespiration.•Photoinhibition under fluctuating light was unaffected by moderate heat stress.•Coordination of CEF and photorespiration favors photoprotection under heat stress. Moderate heat stress is usually accompanied with fluctuating light in summer. Although either heat stress or fluctuating light can cause photoinhibition of photosystems I and II (PSI and PSII), it is unclear whether moderate heat stress accelerate photoinhibition under fluctuating light. Here, we measured chlorophyll fluorescence, P700 redox state and the electrochromic shift signal under fluctuating light at 25 °C and 42 °C for tobacco leaves. We found that (1) the thylakoid proton conductance was significantly enhanced at 42 °C, leading to a decline in trans-thylakoid proton gradient (ΔpH); (2) this low ΔpH at 42 °C did not decrease donor-side limitation of PSI and thermal energy dissipation in PSII; (3) the activation of cyclic electron flow (CEF) around PSI was elevated at 42 °C; and (4) the moderate heat stress did not accelerate photoinhibition of PSI and PSII under fluctuating light. These results strongly indicate that under moderate heat stress the stimulation of CEF protects PSI under fluctuating light in tobacco leaves.