Salicylic acid, hydrogen peroxide and calcium-induced saline tolerance associated with endogenous hydrogen peroxide homeostasis in naked oat seedlings

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This study investigates the role of salicylic acid (SA), hydrogen peroxide (H2O2) and calcium chloride (CaCl2) singly or in combination, in inducing naked oat plant tolerance to sodium chloride (NaCl). Two-week-old naked oat plants were pretreated with both single and double of 0.5 mM SA, 0.5 mM H2O2 and 5 mM CaCl2 by adding them to the culture solution for 24 h. At the end of the pretreatment, the plants were subjected to 200 mM NaCl exposure for 7 days. Data were collected on plant biomass, H2O2 level, antioxidant enzyme activity, non-enzymatic antioxidant content and malondialdehyde (MDA) content. Results showed that exposure to salt significantly inhibited plant growth, and the shoot and root dry weights were reduced 47.5% and 63.4%, and the H2O2 levels elevated 5.8 and 2.4 times in comparison with those in the control, respectively. Under the saline stress, the activities of superoxide dismutase (SOD) and catalase (CAT) were induced, but the contents of ascorbic acid (AA) and glutathione (GSH) decreased, and MDA largely accumulated. The various pretreatments efficiently counteracted the salt-caused growth inhibition, especially with H2O2 + CaCl2 the shoot and root dry weights reduced only 9.4% and 24.4% of the non salt-stressed plants. The determination of endogenous H2O2 level demonstrated that the pretreatments induced H2O2 accumulation, with H2O2 + CaCl2 being most efficient, but the effect was transient. After 7 days of saline stress, the H2O2 contents in the pretreated shoots and roots accounted for 23.7–41.8% and 31.7–57.3% of the non-pretreated plants, varying according to the different pretreatments. Under saline stress, SOD and CAT further increased, AA and GSH maintained higher levels and MDA decreased in the pretreated plants compared to the untreated plants. With application of diphenylene iodonium (DPI) during the pretreatment, which inhibited the accumulation of H2O2, the ameliorative effect of the pretreatment on salt-caused plant growth inhibition was reduced. However, applied DPI at the immediate end of the pretreatment did not alter its favorable role, indicating a H2O2 peak formed at the early time of saline stress might play an important role in regulating plant tolerance to saline stress.