Exogenously applied glycine betaine regulates some chemical characteristics and antioxidative defence systems in lettuce under salt stress

We investigated the effects of exogenous glycine betaine (GB) applications on antioxidant enzyme activity, dry matter, and the contents of organic acids, amino acids, total antioxidants, and total phenolics in lettuce, Lactuca sativa, under salt stress. The treatments included four concentrations of GB (0, 5, 10, and 25 mM) and two levels of salinity (0 and 100 mM of NaCl). The 25 mM GB treatment increased dry matter and the content of total phenolics in lettuce plants compared to the non-GB-treated plants under salt stress. Salinity (100 mM NaCl without GB) significantly reduced dry matter, total phenolic content, and total antioxidant content in the plants. However, the lettuce plants grown under salt stress generally had higher amino acid and organic acid contents than those grown under non-salinity conditions. GB treatments had different effect on amino acid and organic acid contents under salinity conditions. Superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) activities were elevated under the 100 mM NaCl and 0 mM GB treatments, whereas higher concentrations of GB decreased them under salt stress. The 25 mM GB treatment mitigated the negative effect of salt stress and increased the dry matter by 44% compared to the plants that were treated with 100 mM NaCl and 0 mM GB. The results suggested that exogenous GB treatments could ameliorate the tolerance of lettuce to salt stress by increasing the total antioxidants and total phenolics, and regulating antioxidant enzyme activity, and altering the contents of organic acids and amino acids.