Mechanistic Insights into Selenium-Induced Tolerance of Cucumber (Cucumis sativus L.) Seedlings to Alkaline Stress.

Saline-alkali stress severely restricts cucumber (Cucumis sativus L.) growth by disrupting ion balance, water status, photosynthesis, and redox homeostasis. Here, we examined the effects of exogenous selenium (Se) on cucumber seedlings exposed to NaHCO3 stress. Se supplementation improved plant growth and root activity and partly restored photosynthetic performance by maintaining chlorophyll content, gas exchange, and chlorophyll fluorescence. Se reduced oxidative injury through lower ROS and MDA levels and by enhancing antioxidant enzyme activities together with the AsA-GSH cycle. In parallel, Se moderated ion toxicity by limiting Na+ accumulation, increasing K+, Ca2+, and Mg2+ uptake, and stimulating H+-ATPase and H+-PPase activities. Enhanced TCA cycle activity and organic acid accumulation suggested improved energy metabolism and ionic regulation. Se also promoted osmotic adjustment via soluble sugars and proline, and upregulated aquaporin genes (PIP1;2 and PIP2;4) to sustain water transport. Moreover, Se increased salicylic acid levels by upregulating CsPAL and CsICS, pointing to a role of SA signaling in Se-induced tolerance.