Abstract
The role of chilling temperatures on photoinhibition of photosystems I and II (PSI and PSII) under weak light has been examined in cucumber, a chilling-sensitive plant. The extent of PSII photoinhibition, determined by pulse-modulated fluorescence in vivo, is closely related to the redox state of the PSII electron acceptor Q A, measured as a fluorescence parameter, 1 − q p. On the other hand, the extent of PSI photoinhibition, which is only observed in chilling-sensitive plants at chilling temperatures, cannot be related to the redox state of Q A, suggesting that the underlying mechanism is different from that of PSII photoinhibition. Chilling treatment at low photon flux densities is found to enhance cyclic electron flow around PSI. Both PSI photoinhibition and enhanced cyclic electron flow show similar temperature dependence, with the threshold temperature at 10°C.
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