Exogenous spermidine improved the salinity-alkalinity stress tolerance of grapevine (Vitis vinifera) by regulating antioxidant system, Na⁺/K⁺ homeostasis and endogenous polyamine contents

Salinity-alkalinity stress severely limits the growth and yield of grapes. Spermidine (Spd), a type of polyamine, has been found to play a critical role in improving plant tolerance against abiotic stresses. However, the effect of exogenous Spd on the tolerance to salinity and alkalinity of grape seedlings and its mechanism are poorly understood. In the present study, we investigated the function of exogenous Spd in the salinity-alkalinity stress response of grape seedlings (Vitis vinifera L.). The results showed that salinity-alkalinity stress led to yellowing and wilting of grape leaves, and a decrease in the photosynthetic rate, while exogenous Spd significantly alleviated the damage of salinity-alkalinity stress and maintained the photosynthetic capacity. Exogenous Spd can improve the activity of antioxidant enzyme, and thereby scavenge the reactive oxygen species (ROS) induced by salinity-alkalinity stress. Moreover, exogenous Spd reduced Na+ accumulation and improved K+ content in grape leaves under salinity-alkalinity stress by upregulated the expression levels of VvNHXP (Na+/H+ antiporter) and VvHKT2 (potassium transporter) genes. In addition, exogenous Spd treatment promoted the accumulation of endogenous free Spd and spermine (Spm), and decreased the free putrescine (Put) content in leaves of grape seedlings under salinity-alkalinity stress. Taken together, these results suggest that exogenous Spd can effectively alleviate the salinity-alkalinity stress suffered by grape seedlings, which may contribute to the development of the grape industry in saline alkali areas.