Synergistic Effect of Soil Amendment with Silicon and Foliar Application of Gibberellic Acid on Growth, Herbage Yield, and Physio-Biochemical Responses of Drought-Affected Ocimum basilicum L.

Sweet basil (Ocimum basilicum L.) is an herbaceous plant widely renowned for its medicinal and aromatic properties. It is susceptible to drought stress, which markedly reduces its vegetative growth, essential oil yield, along with alterations in secondary metabolites. Although the individual soil application of silicon (Si) and foliar application of gibberellic acid (GA₃) have been shown to enhance growth, herbage yield, and physiological responses of sweet basil under drought stress, it is hypothesized that their combined or synergistic effect, which remains largely unexplored, would be even more effective in mitigating drought stress. The objective of this experiment was to discern the potential role of soil application of Si and foliar application of GA₃, both individually and in combination, in alleviating the adverse consequences of water scarcity on sweet basil. The current polyhouse study was conducted under a completely randomized design with three factors, such as soil application of three Si doses (0 [Si₀], 30 [Si₃₀], and 60 [Si₆₀] kg ha^–1) applied in the form of monosilicic acid with a 20% Si content, foliar application of three GA₃ doses (0 [GA₃-0], 50 [GA₃-50], and 100 [GA₃-100] mg L^–1), and three soil water regimes (field capacity [FC] 50%: FC₅₀, 75%: FC₇₅, and 100%: FC₁₀₀). Shoot length, dry weight of shoot, leaf number, herbage yield, water productivity, solute potential (Ψ_s), and net photosynthetic rate (P_n) were declined by 27–33%, 53–58%, 41–45%, 67–73%, 52–68%, –0.26 to –0.49 MPa, and 39–58%, respectively, at FC₅₀ in comparison to FC₁₀₀ across Si and GA₃ doses. The combined soil supply of Si₃₀ and foliar application of GA₃-50 was the most effective treatment, leading to a 31% increase in leaf area, a 91% rise in herbage yield, an 86% enhancement in water productivity, a 149% increase in P_n, a 96% rise in total phenol concentration, and a 186% boost in total flavonoid concentration at FC₅₀ as compared to those plants raised under similar soil moisture level without Si and GA₃ supplementation. Moreover, the performance of some parameters of the plants treated with the same combination at FC₇₅ was better than the control plants grown at optimal conditions of FC₁₀₀, underlining the drought-alleviating potential of Si and GA₃ in the cultivation of sweet basil. Exogenous application of Si₃₀ as a soil supplement and GA₃-50 as a foliar spray appears to be a promising technique for enhancing drought resilience of sweet basil plants and optimizing its growth potential in both well-irrigated and water-stressed conditions.