Physiological, photochemical and ionic responses of sunflower seedlings to exogenous selenium supply under salt stress

Since the specific ionic and photochemical mechanisms of Se-mediated adaptation to salt stress is not yet clear, the present study was performed to address this issue. Sunflower (Helianthus annuus L.) plants were grown under controlled conditions, and soils of Se treatment were fertilized with selenium (5 mg kg(-1) as Na2SeO4) before filling the pots. Four weeks after sowing, salinity treatment using NaCl salt was started as EC = 15 dS m(-1). Salt stress significantly decreased seedling growth, whereas application of Se alleviated the detrimental effect of NaCl stress. Salt treatment decreased maximum quantum yield of photochemistry (about 12%), inferred oxygen evolving complex activity (F-v/F-o) (about 39%), and calculated Performance Index (PIABS) (about 75%) comparing with control plants; however, Se ameliorated these effects. Consequently, the photosynthetic apparatus of the Se-supplied seedlings exhibited enhanced resistance to salt stress, compared to those under non-Se condition. Salt stress increased Na content and reduced K concentration, whereas shoot and root K were both significantly increased in seedlings pretreated with Se. Under saline conditions, treatment with Se enhanced the glutathione peroxidase activity, and also resulted in a significant reduction of malondialdehyde contents. These results suggest that Se decreases salt-induced damages through stimulating the antioxidant activities, which is associated with the improvement of the K/Na ratio needed for normal photochemical functioning, resulting in better plant growth under salt stress.