Differential response of salt stress on Brassica juncea: photosynthetic performance, pigment, proline, D1 and antioxidant enzymes.

To investigate the ability of sensitive and tolerant variety of Brassica juncea to adapt to a saline environment in a field, we examined the activities of antioxidant enzymes in relation to photosystem II, chlorophyll a fluorescence, photosynthetic pigment concentration, protein (D1) and proline in plants exposed to salt stress. We observed a greater decline in the photosynthetic rate (∆F/Fm') and electron transport rate (ETRsat) and saturating photosynthetically active photo flux density (PPFDsat) under salt stress in var. Urvashi (sensitive variety) than in var. Bio902 (tolerant variety). Var. Urvashi was found to be more sensitive to high salinity. In var. Bio902, chlorophyll a, chlorophyll b and total chlorophyll concentrations were higher than in the sensitive variety. Proline and protein contents were also higher in var. Bio902 as compared to their lower accumulation in var. Urvashi. The improved performance of the var. Bio902 under high salinity was accompanied by an increase in ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (CAT, EC 1.11.1.6), though no salt-dependent increase in the activity of superoxide dismutase (SOD, EC 1.15.1.1) was observed. However, no such increase in APX and CAT was observed in var. Urvashi, though there was significant increase in SOD activity. These results suggest that var. Bio902 is potentially more tolerant to salt damage and is associated with better adaptive responses found in var. Bio902 than var. Urvashi. Increased photoinhibition in var. Urvashi as observed by its reduced thylakoid membrane protein, D1 probably results from the greater photosynthetic damage caused by salt stress than var. Bio902.