Photoinhibition of Suaeda salsa to chilling stress is related to energy dissipation and water-water cycle

To investigate the photoprotection of energy dissipation and water-water cycle, a C-3 euhalophytic herb, Suaeda salsa L., was exposed either to chilling temperature (4A degrees C) accompanied by moderate irradiance (600 mu mol m(-2) s(-1)) (CM) and/or to chilling temperature (4A degrees C) accompanied by low irradiance (100 mu mol m(-2) s(-1)) (CL). During chilling stress, both the maximal photochemical efficiency of PSII (F-v/F-m) and the oxidizable P700 decreased in S. salsa leaves either under CM or CL, which indicated the severe photoinhibition. Relative to F-v/F-m, the oxidizable P700 decreased markedly under CL, which indicated that PSI was more sensitive to CL treatment than PSII. Initial fluorescence, number of closed PSII centers, and nonphotochemical quenching increased under CM, but more markedly under CL in S. salsa leaves. Activity of superoxide dismutase and ascorbate peroxidase was higher under CM than that under CL. The production of reactive oxygen species (ROS) decreased first and then increased under both treatments, but the content of O-2 center dot(-) and H2O2 was higher under CL than that under CM after 12 h of chilling stress. These results suggested that photoinhibition in S. salsa might be related to the accumulation of reactive oxygen species (ROS) induced by excess energy. The water-water cycle could not dissipate energy efficiently under CL, which caused the great accumulation of ROS.