Enhancement of photosystem II photoreactions and high pH stability in thylakoids from cotyledonary leaves of Brassica juncea raised under sodium chloride stress

Abstract

Enhancement in the rates of photosystem (PS) II‐catalysed O2 evolution was observed with thylakoids isolated from cotyledonary leaves of Brassica juncea seedlings raised in medium containing 200 mM NaCl compared to controls. The nature of this enhancement in PS II activity by NaCl treatment has been investigated. The pH profile showed an optimal PS 11(H2O→FeCN) activity at pH 7.5 in thylakoids of both controls and NaCl‐raised seedlings. However, the thylakoids from NaCl‐raised plants showed significantly higher PS II activity at all the pHs (from 6.0 to 9.0) with maximal difference occurring at alkaline pHs. Besides the enhanced electron transport rates, light‐induced proton uptake was about 30% higher in the thylakoids from NaCl‐raised plants than in the control thylakoids. The 3‐(3,4‐dichlorophenyl)‐1, 1‐dimethylurea (DCMU)‐induced enhancement in the room temperature fluorescence was significantly higher in thylakoids from NaCl‐grown plants compared to controls, suggesting more efficient PS II photochemical reactions in thylakoids from NaCl‐raised seedlings. The analysis of the changes in the fluorescence yield of Chl also showed an enhancement in Fv level in NaCl‐raised plants, suggesting that NaCl treatment stimulates the photochemistry of PS II. Analysis of light saturation kinetics of PS II [H2O→2,6‐dichloro‐p‐benzoquinone (DCBQ) or phenylenediamine (PD)] activity showed that the extent of stimulation by NaCl under rate‐limiting light intensities was greater than at rate‐saturating intensities. This result indicates that NaCl treatment not only alters the electron transport rates but also the relative quantum yield. Incubation of thylakoids at pH 9.0 inhibited FeCN‐supported PS II activity more drastically in control than in NaCl‐treated thylakoids. Recovery from alkaline‐pH‐induced inhibition occurred when the thylakoids were resuspended at pH 7.5. Reversal in PS II activity was significantly higher in the thylakoids of NaCl‐raised plants than in the control thylakoids. These results suggest that the seedlings raised at moderately elevated levels of NaCl have thylakoids with enhanced stability and PS II photochemical functions.