Abstract
Drought stress poses a serious threat to global agriculture, affecting plant growth, physiology, and biochemical processes, thereby impacting food security. Supplementation of phytohormones regulates plant physiological processes and improves tolerance to abiotic stress. In our study, we applied glycine betaine (GB), a non-toxic, highly soluble signaling molecule that plays an important role in protecting plants from environmental stress. To assess the role of with and without exogenous GB against fourteen days of prolonged drought stress (60% and 30% field capacity) on two high-yielding barley varieties, BARI barley-6 (sensitive) and BARI barley-9 (tolerant), with control plants were maintained at 90% field capacity. Results showed that both varieties exhibited a significant reduction in biomass, leaf relative water content, and photosynthetic activity under drought stress, while increasing the accumulation of proline and ROS, which indicates oxidative damage. In contrast, foliar application of GB improved growth, photosynthetic pigments, and net photosynthetic rate. It also helped to detoxify ROS by boosting the activities of antioxidant enzymes such as CAT, APX, POD, and GST while upregulating secondary metabolites like phenolic and flavonoid contents, maintaining membrane integrity, and regulating osmotic balance under water-deficient conditions. Overall, GB enhanced the drought tolerance of both barley varieties by modulating various physiological and biochemical processes. Our findings provide insights into GB-induced adaptation mechanisms in plants that combat water scarcity and may help to develop drought-resilient crops.