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

摘要

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 degrees C and 42 degrees C for tobacco leaves. We found that (1) the thylakoid proton conductance was significantly enhanced at 42 degrees C, leading to a decline in trans-thylakoid proton gradient (Delta pH); (2) this low Delta pH at 42 degrees 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 degrees 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.