Drought-induced changes in photosynthetic electron transport in maize probed by prompt fluorescence, delayed fluorescence, P700 and cyclic electron flow signals

The effect of drought on the prompt chlorophyll a fluorescence (PF) transient (OJIP), delayed chlorophyll alpha fluorescence (DF), modulated 820-nm reflection (MR), energy conversion efficiencies in photosystems (PS) I and II, and cyclic electron flow (CEF) activity in two maize hybrids with contrasting drought tolerance was investigated. Our aim was to identify the target site of drought stress on the photosynthetic electron transport chain and investigate the relevance of the CEF pathway to the drought tolerance of maize plants. The OJIP analysis showed that drought stress, depending on its duration, decreased F-P, increased F-J, and induced a pronounced K-band and a positive L-band. Moreover, OJIP parameters, including PIABS, RC/CSO, TFO/ABS, and ETO/TRO, were significantly reduced. The DF analysis showed that the values of I-1 and I-2 in the induction curve and L-1 and L-2 derived from the decay curve decreased progressively with the duration of drought stress. The MR analysis showed that drought stress inactivated both the fast decrease and slow increase phases of the MR transient, resulting in a gradual decrease in both V-PSI and VPSII-PSI. The energy conversion analysis showed that drought stress decreased the PSI photochemical quantum yield Y(I) and PSII photochemical quantum yield Y(II). Compared to the tolerant hybrid, the drought-induced changes in the sensitive hybrid were stronger and appeared at an earlier treatment stage. The CEF activity analysis showed that the CEF pathway under drought stress operated for a longer time in the tolerant hybrid than that in the sensitive hybrid. The above results indicate that drought stress damaged the donor and acceptor sides of PSII, the PSII reaction center and the acceptor side of PSI and decreased the efficiency of both PSI and PSII and the capacity of electron transfer. The CEF pathway might play an important role in the tolerance of the maize photosynthetic electron transport chain to drought stress.