CaCl2 promotes the cross adaptation of Reaumuria trigyna to salt and drought by regulating Na+, ROS accumulation and programmed cell death

Reaumuria trigyna, a salt-secreting xerophytic shrub endemic to arid desert regions of northwest China, is extremely adaptable to salt and aridity. In this study, we used PEG to simulates drought stress and investigated the effect of NaCl and CaCl2 on R. trigyna seedlings exposed to drought stress. Exogenous application moderate NaCl and CaCl2 were found to stimulate the growth and alleviate drought stress in R. trigyna seedlings. Moderate NaCl and CaCl2 combined treatment increased fresh weight and decreased electrolyte leakage, and malondialdehyde (MDA) content in R. trigyna seedlings under drought stress. Simultaneously, leaf senescence and root damage induced by drought stress were alleviated, with programmed cell death (PCD) related genes expression down-regulated. Among them, the application of CaCl2 under drought and salt treatment is the most effective way to increase osmotic regulators content, antioxidant enzymes activities, and related genes expressions of plants under drought stress, which scavenged excess reactive oxygen species (ROS) and alleviated oxidative damage caused by drought stress. Meanwhile, CaCl2 can reduce the content of Na+and the ratio of Na+/K+ by promoting the outflow of Na+ and inflow of Ca2+, as well as the expression of ion transporter gene, and reduce the ionic toxicity caused by drought and salt cross adaptation. The principal component analysis (PCA) showed that the relevant beneficial indicators were positively correlated with the combined treatment. These results indicated that moderate NaCl can positively regulates defense response to drought stress in R. trigyna, while CaCl2 can significantly promote this process.