Salt pretreatment alleviated salt-induced photoinhibition in sweet sorghum

Sweet sorghum is an important energy crop. This study aimed to investigate the effects of salt pretreatment on the interaction between photosystem II (PSII) and photosystem I (PSI) upon salt stress. In this study, sweet sorghum was pretreated with 150 mM NaCl for 10 days, and subsequently, the pretreated plants were subjected to severe salt stress at 300 mM NaCl. PSII and PSI photoinhibition occurred in non-pretreated plants after 4 days of salt stress, as the maximum quantum yield of PSII (Fv/Fm) and the maximal photochemical capacity of PSI (△MR/MR0) significantly decreased, and their normal coordination was destroyed. The significant positive correlation between Fv/Fm and △MR/MR0 under salt stress indicated that PSII photoinhibition was in relation to PSI photoinhibition, and PSI photoinhibition might lead to PSII photoinhibition through inhibiting electron transport at the acceptor side of PSII. Salt stress did not induce PSII photoinhibition in salt-pretreated plants, and thus, salt pretreatment protected PSI against photoinhibition not by aggravating PSII photoinhibition. Salt pretreatment mitigated the decrease in CO2 assimilation, reduced the feedback inhibition on photosynthetic electron transport and then contributed to suppressing PSI and PSII photoinhibition in sweet sorghum under salt stress. Therefore, the normal coordination between PSII and PSI was maintained in salt-pretreated plants. In conclusion, salt pretreatment ensured normal PSII and PSI coordination by preventing photoinhibition in sweet sorghum under salt stress.