Rice (oryza Sativa L.) Exhibits Differential Responses to Salt Stress and Salt Shock in Electron Transfer Efficiency

The study of crop response to soil salinity has traditionally focused on the effects of varying salt concentrations, with less attention given to the methodologies of salt application. This research aimed to elucidate the differential impacts of two distinct salinity exposure methods on the photosynthetic characteristics of rice. Plants subjected to a gradual increase in salinity experience salt stress, whereas those immediately exposed to high salinity levels encounter a salt shock condition. Our findings revealed that rice plants accumulated comparable levels of shoot Na+ under both salt stress and salt shock at 4 and 10 days after the onset of stress. No significant differences were observed in the steady-state photosynthetic rate and electron transport rate (ETR). In contrast, salt stress markedly reduced the relative variable fluorescence at the ‘J’ step (Vj), thereby enhancing the probability of an exciton transferring an electron beyond the primary quinone electron acceptor (QA−) (ΨEo). This led to a notable increase in the activity of leaf photosystem II (PSII), as evidenced by the Performance Index on absorption basis (PIabs), which was twice as high as that of the control group. This mechanism is believed to facilitate the activation of alternative metabolic pathways. Additionally, kinetic measurements of light potential indicated that salt stress improved the initial induction of ETR after a brief exposure to high photosynthetically active radiation (PAR) compared to the control. This effect was not observed in the salt shock treatment group. Our results suggest that plants exhibit distinct physiological responses to salt stress versus salt shock, and the potential positive influence on leaf photosynthetic activity during salt stress may have been previously underestimated.