Foliar application of glutamic acid enhances drought tolerance in poplar by coordinating physiological, molecular, and phyllosphere microbial responses

Drought induced water scarcity is a major abiotic stress that adversely disrupts the physiological and biochemical functioning of the plants. To address this challenge, exogenous application of various compound have emerged as promising strategy. The present study investigates the effects of varying concentrations of Coumarin (20, 40, 60 μm), Glutamic Acid (10, 20, 30 mM), and Glycine Betaine (50, 100, 150 mM) on improving drought tolerance in Populus deltoides. Analysis revealed that foliar treatments substantially enhanced morphological and biochemical traits. Coumarin was most effective in promoting plant height (37%) while Glutamic Acid improved leaf area, leaf count, chlorophyll content (15%) and photosynthetic efficiency across all tested concentrations. Both Glutamic Acid (GA) and Glycine Betaine (GB) positively modulate osmolyte concentrations including proline, soluble sugars and proteins, contributing to enhanced drought resilience. Antioxidant enzymatic activities varied across treatments, underscoring their role in fortifying the plant’s defense mechanisms under stress. A showed a strong correlation with the E (r = 0.91), and moderate correlation with both Gs (r = 0.75) and total chlorophyll content (r = 0.79). Results indicate that carbon gain is closely linked to water loss through stomatal control, particularly under drought stress CAT and SOD exhibited a strong positive correlation. Moreover, relative expression of stress-responsive genes P5CS and DREB was markedly up-regulated by GA3, followed by GB. Additionally, Glutamic Acid treatment also promoted the growth of drought-tolerant bacterial strains such as Bacillus cereus, Bacillus subtilis, and Bacillus safensis. Overall, Glutamic acid emerged as the most effective treatment, followed by Glycine Betaine, in mitigating the detrimental impacts of drought stress, enhancing stress resilience and supporting the specific microbial activity which provide one more approach for plant tolerance to drought.