Exogenous GSH protects tomatoes against salt stress by modulating photosystem II efficiency, absorbed light allocation and H2O2-scavenging system in chloroplasts

The effects of exogenous GSH (reduced glutathione) on photosynthetic characteristics, photosystem II efficiency, absorbed light energy allocation and the H2O2-scavenging system in chloroplasts of salt-stressed tomato (Solanum lycopersicum L.) seedlings were studied using hydroponic experiments in a greenhouse. Application of exogenous GSH ameliorated saline-induced growth inhibition, the disturbed balance of Na+ and Cl- ions and Na+/K+ ratios, and the reduction of the net photosynthetic rate (P-n). GSH also increased the maximal photochemical efficiency of PSII (F-v/F-m), the electron transport rate (ETR), the photochemical quenching coefficient (q(P)), and the non-photochemical quenching coefficient (NPQ). In addition, GSH application increased the photochemical quantum yield (Y(II)) and relative deviation from full balance between the photosystems (beta/alpha-1) and decreased the PSII excitation pressure (1-q(P)) and quantum yield of non-regulated energy dissipation (Y(NO)) in leaves of salt-stressed tomatoes without BSO (L-buthionine-sulfoximine, an inhibitor of key GSH synthesis enzyme gamma-glutamylcysteine synthetase) or with BSO. Further, the addition of GSH depressed the accumulation of H2O2 and malondialdehyde (MDA), induced the redistribution of absorbed light energy in PSII reaction centers, and improved the endogenous GSH content, GSH/GSSH ratio and activities of H2O2-scavenging enzymes (including superoxidase dismutase (SOD), catalase (CAT), peroxidase (POD) and key enzymes in the AsA-GSH cycle and Grx system) in the chloroplasts of salt-stressed plants with or without BSO. Therefore, GSH application alleviates inhibition of salt-induced growth and photosynthesis mainly by overcoming stomatal limitations, improving the PSII efficiency, and balancing the uneven distribution of light energy to reduce the risk of ROS generation and to mediate chloroplast redox homeostasis and the antioxidant defense system to protect the chloroplasts from oxidative damage. Thus, GSH may be used as a potential tool for alleviating salt stress in tomato plants.