Application of salicylic acid and silicon can enhance drought stress tolerance in Scrophularia striata L.

Water deficit is a major environmental stress significantly affecting plant growth and productivity. This research examined the molecular and biochemical responses of Scrophularia striata, a traditional Iranian medicinal plant, to drought stress. It also evaluated whether the separate and combined application of salicylic acid (SA) and silicon (Si) could alleviate these negative effects. The present study was conducted in a greenhouse with a four-factor factorial completely randomized design (FCRD) experimental design with three replications. The treatments included combinations of drought stress level 50% of field capacity (FC50%), SA at two levels (0 and 100 mg L^{- 1}), as well as Si at two levels (0 and 1 g L^{- 1}). The evaluated biochemical parameters included β-carotene, α-tocopherol, and beta-amyrin (βA) content, as well as changes in the expression of genes involved in the terpenoid pathway. The mean comparison revealed that drought stress lowered the levels of β-carotene, α-tocopherol, and βA. This suggests that the adaptive metabolites changed in response to adverse environmental conditions. It reduced the expression of biosynthesis genes such as geranyl diphosphate synthas (GPPS), isopentenyl diphosphate isomerase (IPPI), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), farnesyl pyrophosphate synthase (FPS), and beta-amyrin synthase (βAS). The use of SA heightened the expression of the isopentyl diphosphate isomerase gene. When salicylic acid (SA) and Si were employed together, they elevated the levels of β-carotene, α-tocopherol, and βA under FC50% drought stress conditions. The findings provide important insights into the transcriptional and metabolic reprograming of S.striata under drought stress. These results have significant implications for the development of drought-resilient of medicinal plants increasingly confronting climate changes stresses.