Flavonoids are key secondary metabolites that are biologically active and perform diverse functions in plants such as stress defense against abiotic and biotic stress. In addition to its importance, no comprehensive information has been available about the secondary metabolic response of Populus tree, especially the genes that encode key enzymes involved in flavonoid biosynthesis under drought stress. In this study, the quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the expression of flavonoid biosynthesis genes (PtPAL, Pt4-CL, PtCHS, PtFLS-1, PtF3H, PtDFR, and PtANS) gradually increased in the leaves of hybrid poplar (P. tremula × P. alba), corresponding to the drought stress duration. In addition, the activity and capacity of antioxidants have also increased, which is positively correlated with the increment of phenolic, flavonoid, anthocyanin, and carotenoid compounds under drought stress. As the drought stress prolonged, the level of reactive oxygen species such as hydrogen peroxide (H₂O₂) and singlet oxygen (O₂⁻) too increased. The concentration of phytohormone salicylic acid (SA) also increased significantly in the stressed poplar leaves. Our research concluded that drought stress significantly induced the expression of flavonoid biosynthesis genes in hybrid poplar plants and enhanced the accumulation of phenolic and flavonoid compounds with resilient antioxidant activity.

Growth performance and enzymatic-antioxidant activities of Moringa oleifera were investigated under simultaneous effect of water availability and plant growth regulators. Plant growth regulators i.e., cytokinin (50 mg L−1), thiourea (5 mM), benzyl amino purine (BAP @ 50 mg L−1), salicylic acid (50 mg L−1), hydrogen peroxide (120 μM) and ascorbic acid (50 mg L−1) were exogenously applied to moringa plants at three water holding capacity levels, 100, 70, and 40% in a completely randomized design with three replications. A decrease in shoot length, leaf score and number of branches was recorded at 100 and 40% water holding capacity as the plants respond well at 70% water holding capacity. BAP, cytokinin and salicylic acid were found as effective plant growth regulators in improving drought tolerance in moringa plants even at 40% water holding capacity. On the basis of these findings, moringa can be suggested to cultivate as a fodder crop under water deficit conditions and its tolerance can be further improved by foliar application of benzyl amino purine and ascorbic acid.