Exogenous spermidine improves salt tolerance of pecan-grafted seedlings via activating antioxidant system and inhibiting the enhancement of Na+/K+ ratio

Salt stress is abiotic stress that negatively impacts plant growth. Exogenous spermidine (SPD) has been proposed as a priming agent that effectively alleviates various abiotic stresses in plants. To evaluate the alleviation role of exogenous SPD application against salt stress, we studied the physiological and biochemical changes, and the expression of gene encoding antioxidant enzymes and phytohormones in the SPD-treated pecan-grafted seedling. The results revealed that for the seedlings growing under salt stress environment, a drop in the total chlorophyll content and photosynthetic capability was observed. These effects subsided when the seedlings were treated with SPD. Moreover, the malondialdehyde level and Na+/K+ ratio, as well as the damage of the chloroplast ultrastructure in the seedlings, were markedly suppressed after SPD treatment. Compared with the non-SPD-treated counterparts, the exogenous SPD application promoted higher antioxidant enzyme activities and their corresponding gene expression. At the same time, the induction of abscisic acid and ethylene was suppressed. The results indicate that the exogenous SPD plays a vital role in alleviating salt-incurred phytotoxicity and oxidative damage in pecan-grafted seedlings. The remissive role of exogenous SPD is implicated by its interplay with the expression of the antioxidant enzymes and phytohormones in the pecan-grafted seedlings.