Morpho-Physio-Biochemical Responses of Sweet Basil Plants to Integrated Application of Silicon and Salicylic Acid under Water Supply Restrictions

Abstract

Drought stress can markedly reduce plant growth and development, leading to considerable yield losses in sweet basil (Ocimum basilicum L.). Individual application of silicon (Si) and salicylic acid (SA) has the potential to mitigate the detrimental effects of drought stress; however, their combined effect is largely unknown. The aim of this study was to evaluate the efficacy of Si and SA, both independently and in concert, in mitigating the deleterious impacts of drought stress on sweet basil plants. A factorial experiment was implemented using a completely randomized design, incorporating soil application of three Si levels (0, 30, and 60 kg ha^–1), foliar application of three SA levels (0, 100, and 200 mg L^–1), and three soil moisture levels (50, 75, and 100% field capacity ‘FC’). Leaf area, shoot dry matter, leaf yield, irrigation water productivity, net photosynthetic rate, and stomatal conductance were declined by 54–78%, 55–66%, 77–84%, 55–68%, 42–70%, and 73–92%, respectively, at 50% FC in contrast to conditions at 100% FC, while electrolyte leakage, free proline concentration, total phenol concentration, and total flavonoid concentration were increased by 77–130%, 173–330%, 87–148%, and 101–169%, respectively, across Si and SA doses. The treatment of 60 kg Si ha^–1 in conjunction with 100 mg SA L^–1 emerged as the most efficacious treatment. This combination resulted in a 174% augmentation in leaf area, a 91% enhancement in shoot dry matter, a 98% increase in leaf yield, a 63% increase in irrigation water productivity, a 28% rise in leaf relative water content, and a 112% increase in total phenol concentration at 50% FC, when compared to plants grown under the same soil moisture level without Si and SA supplementation. Additionally, this treatment combination reduced electrolyte leakage by 26% compared to the plants not receiving Si and SA at 50% FC. The performance of plants under this combination at 75% FC was superior to that of the control plants even under optimal conditions at 100% FC for some parameters, underscoring the drought-mitigating potential of Si and SA in sweet basil. The combination of Si (60 kg ha^–1) as a soil amendment and SA (100 mg L^–1) applied as a foliar spray could be an effective strategy for improving the drought tolerance ability of sweet basil and enhancing its performance under both water-stressed and well-watered conditions.