Protective roles of exogenously applied gallic acid in Oryza sativa subjected to salt and osmotic stresses: effects on the total antioxidant capacity

The aim of our study was to examine whether exogenously applied with gallic acid (GA) enhances the tolerance of rice cultivars to polyethylene glycol (PEG)induced osmotic stress or salt (NaCl) stress. After two Oryza sativa L. cultivars, tolerant cultivar Pokkali and sensitive cultivar IR-28, were hydroponically-grown for 3 weeks, seedlings were treated with GA (0.75 and 1.5 mM), 120 mM NaCl and 20 % PEG-6000 producing the same osmotic potential (-0.5 MPa) and GA and stress combination for 72 h. PEG had greater reduction in growth rate (RGR) and water content (RWC) than that of NaCl in both cultivars. Salt and PEG decreased the maximum photochemical efficiency (F-v/F-m), the photochemical quenching coefficient (qP) and the actual quantum yield (Phi(PSII)), even more pronounced at NaCl in IR-28. Also, a notable increase in lipid peroxidation (TBARS) and hydrogen peroxide (H2O2) content was observed with PEG as compared to NaCl treatment. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase in Pokkali and, catalase (CAT) and peroxidase (POX) in IR-28 were induced to a greater extent by PEG. The sensitivity of plants to stress was higher in IR-28 than in Pokkali. Also, comparing the injury between the NaCl and PEG stresses, it was greater under PEG-mediated osmotic stress. However, compared with the stress-treated plants alone, GA added to NaCl-stressed Pokkali significantly decreased H2O2 and TBARS contents, and enhanced the activities of SOD, CAT, POX and APX as well as increase of RGR, osmotic potential (Psi(Pi)), F-v/F-m and proline. In PEG-treated IR-28, GA strongly suppressed H2O2 and TBARS contents, up-regulated SOD and APX activities and increased RGR, RWC and Psi(Pi). It could be concluded that the both GA concentrations alleviated NaCl and PEG toxicity by increased the level of antioxidant activity and photosynthetic efficiency in rice.