Effects of salinity on photosynthesis in maize probed by prompt fluorescence, delayed fluorescence and P700 signals

Salinity affects the photochemical activity of photosystems (PSs) in plants. However, it is not clear which components in the photosynthetic processes are sensitive to salt stress. In this study, simultaneous measurements of prompt chlorophyll a fluorescence (PF), delayed chlorophyll a fluorescence (DF) and modulated 820 nm reflection (MR) were employed to investigate the effect of salt stress on the entire photosynthetic electron chain in maize leaf tissues, including the PSII donor side, electron transfer between PSII and PSI, and the PSI acceptor side. For the PF transients, salt stress induced a pronounced K-band; a positive L-band; a significant reduction in PIABS, RC/CSO, TRO/ABS, and ETO/TRO; and a significant increase in ABS/RC and REO/ETO. Analysis of the normalized MR kinetics showed that the re-reduction kinetics of P700(+) and PC+ became slower and occurred at later times under salt treatment. For the DF signals, a decrease in the amplitude of the DF induction curve and a change in the shape of both the induction curve and the decay curve were observed under salt stress. These results suggest that salt stress decreased the number of active PSII reaction centers, impaired the connectivity between independent PSII units, destroyed the oxygen-evolving complex, and limited electron transport beyond the primary quinone acceptor QA(-). In contrast, the photochemical activity of PSI was largely unscathed. The results obtained from measuring three simultaneous signals were in good agreement.