Drought-induced Changes In Photosynthesis Of Maize At Seedling Stage Probed By Prompt Fluorescence,delayed Fluorescence,P700 And Cyclic Electron Flow Signals

Maize(Zea mays L.)is an important food and forage crop in China,which plays an important role in ensuring the safety of Chinese food production.Drought is one of the common non-biologica stresses in agricultural production.Drought and water shortage affect the photosynthetic carbon assimilation process and kernel yield of maize.During the photosynthetic light reaction,electrons released from water in PSII are ultimately transferred to NADP via PSI,forming a forward electron flow.Meanwhile,the accumulation of electrons in the transport chain between PSI and PSII caused by the forward electron flow can be transferred back to the reaction center(RC)of PSII,forming a backward electron flow.Additionally,a cyclic electron flow(CEF)around PSI also occurs,driving ATP synthesis without the concomitant generation of NADPH.Many studies have observed a drought-induced decrease in photosynthesis,However,comprehensive and detailed information on the drought-induced changes in the light reaction of photosynthesis is still unavailable.The effect of drought on the prompt chlorophyll a fluorescence(PF)transient(OJIP),delayed chlorophyll a 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 main results of the experiment are as follows:(1)The OJIP analysis showed that drought stress,depending on its duration,decreased Fp,increased FJ,and induced a pronounced K-band and a positive L-band,which indicated that drought stress increased non-radiative dissipation of PSII antenna chlorophylls,decreased number of photosynthetic apparatus with fully closed PSII RCs,impairment at the PSI acceptor side and/or denaturation and degradation of chlorophyll proteins,limited the electron transfer beyond QA,destroyed the OEC and impaired the electron transfer capacity on the donor side of PSII,derecased the energetic connectivity of the PSII units.Compared to the tolerant hybrid,the drought-induced changes in the sensitive hybrid were stronger and appeared at an earlier treatment stage,indicated that the damage of PSII activity of sensitive hybrids was greater than that of drought-tolerant materials.(2)The MR analysis showed that the lowest point of the fast decrease phase of the MR/MRO transient appeared at a later time in the duration of the drought treatment for both hybrids,which indicated that the PSI oxidation activity was impaired by drought stress.At the end of the experiment,the slow phase of the MR/MRO curve of sensitive hybrids disappeared completely,while that of drought-tolerant hybrids still existed,indicated that the drought stress on the PSI reducing activity of sensitive hybrids was greater than that of drought tolerance hybrids.(3)The DF analysis showed that the values of I1 and I2 in the induction curve and L1 and L2 derived from the decay curve decreased progressively with the duration of drought stress.These results suggest that drought stress impaired the backward electron transfer capacity and resulted in a weak charge recombination and subsequent excited PSII antenna chlorophyll repopulation.The increased I2/I1 value of Nongda95 suggests the drought stress decreased PSII donor-side electron transfer capacity of sensitive hybrids(4)The energy conversion analysis showed that drought stress decreased Y(Ⅰ),Y(Ⅱ),Y(ND),Y(NPQ)and increased Y(NO),Y(NA).These results suggest that drought stress decreased the photochemical efficiency of PSII and PSI,impaired the xanthophyll-cycle-related active energy dissipation,pumped fewer electrons to the intersystem electron transport chain and induced passive energy dissipation.Compared to the tolerant hybrid,the drought-induced changes in the sensitive hybrid were stronger and more obviously,indicated that the damage on the energy conversion process of PSII and PSI in sensitive hybrids was greater than that of drought tolerance hybrids.(5)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 CEF pathway might play an important role in the tolerance of the maize photosynthetic electron transport chain to drought stress.