Characterization of PSI recovery after chilling-induced photoinhibition in cucumber (Cucumis sativus L.) leaves

By simultaneously analyzing the chlorophyll a fluorescence transient and light absorbance at 820 nm as well as chlorophyll fluorescence quenching, we investigated the effects of different photon flux densities (0, 15, 200 μmol m−2 s−1) with or without 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on the repair process of cucumber (Cucumis sativus L.) leaves after treatment with low temperature (6°C) combined with moderate photon flux density (200 μmol m−2 s−1) for 6 h. Both the maximal photochemical efficiency of Photosystem II (PSII) (Fv/Fm) and the content of active P700 (ΔI/Io) significantly decreased after chilling treatment under 200 μmol m−2 s−1 light. After the leaves were transferred to 25°C, Fv/Fm recovered quickly under both 200 and 15 μmol m−2 s−1 light. ΔI/Io recovered quickly under 15 μmol m−2 s−1 light, but the recovery rate of ΔI/Io was slower than that of Fv/Fm. The cyclic electron transport was inhibited by chilling-light treatment obviously. The recovery of ΔI/Io was severely suppressed by 200 μmol m−2 s−1 light, whereas a pretreatment with DCMU effectively relieved this suppression. The cyclic electron transport around PSI recovered in a similar way as the active P700 content did, and the recovery of them was both accelerated by pretreatment with DCMU. The results indicate that limiting electron transport from PSII to PSI protected PSI from further photoinhibition, accelerating the recovery of PSI. Under a given photon flux density, faster recovery of PSII compared to PSI was detrimental to the recovery of PSI or even to the whole photosystem.