Water deficit is one of the most detrimental environmental challenges that seriously impacts plant growth and agricultural productivity. In the current investigation, alterations induced by drought stress and exogenous application of sodium nitroprusside (SNP), as a NO donor, at the physiological, biochemical and molecular levels were studied in lemon balm (Melisa officinalis L.) plant. Partial fragments of chlorophyllide a oxygenase (CAO) and Rubisco activase (RCA) genes were also isolated and sequenced in this study. Obtained results showed that limited water markedly induced oxidative damage and decreased the chlorophyll content and the level of relative water content (RWC) of the plant and resulted in significant inhibition of growth parameters. However, exogenous NO ameliorated the adverse impacts of limited water and accompanied with the better plant growth. Under this condition, supplementation of NO elevated both enzymatic and non-enzymatic antioxidant systems as indicated by increasing in activities of superoxide dismutase and catalase and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals and ferric reducing antioxidant power (FRAP) capacities and greater accumulation of phenolic compounds. In this regard, stressed plants treated with NO exhibited lowered levels of lipid peroxidation, hydrogen peroxide and electrolyte leakage, increased transcript abundance of RCA and chlorophyll synthase (CHLG) genes and higher free proline and RWC levels. Results of the present study may be helpful in understanding mechanisms involved in NO-mediated drought adaptability in lemon balm.
