Brassinosteroid Enhances Cucumber Stress Tolerance to NaHCO3 by Modulating Nitrogen Metabolism, Ionic Balance and Phytohormonal Response

Under NaHCO₃ stress, exogenous 24-epibrassinolide (EBR) markedly alleviated Na⁺ accumulation in cucumber plants, thereby decreasing the Na⁺/K⁺, Na⁺/Mg²⁺, and Na⁺/Ca²⁺ ratios. This mitigation was accompanied by elevated concentrations of K⁺, Ca²⁺, and Mg²⁺, as well as enhanced expression of the <i>NHX</i> and <i>SOS1</i> genes. In addition, the activities of plasma membrane H⁺-ATPase, vesicular membrane H⁺-ATPase, and vesicular membrane H⁺-PPase were significantly increased, contributing to the maintenance of ionic balance in cucumber plants. NaHCO₃ stress disrupted nitrogen metabolism, as evidenced by reductions in the activities of NR, GS, GOGAT, GOT, and GPT, along with altered GDH activity. These disruptions led to an accumulation of NH₄⁺ and substantial decreases in NO₃^--N and total nitrogen content. Exogenous EBR alleviated these effects by enhancing the activities of NR, GS, GOGAT, GOT, and GPT, countering the prolonged suppression of GDH activity, and restoring NO₃^--N and total nitrogen levels. Consequently, EBR application reduced NH₄⁺ toxicity induced by alkali stress. Additionally, NaHCO₃ stress increased ABA accumulation while decreasing IAA and GA₃ content in cucumber seedlings. In contrast, exogenous EBR application elevated IAA and GA₃ levels and increased the IAA/ABA and GA₃/ABA ratios, thus maintaining hormonal equilibrium under alkali stress. Collectively, these findings highlight that exogenous EBR enhances the alkaline tolerance of cucumber plants by regulating nitrogen metabolism, ion homeostasis, and phytohormonal responses.