Physiological, Biochemical, and Transcriptional Responses under Drought Stress in Capparis spinosa L.

Drought stress profoundly alters plant physiological and biochemical processes and can modulate secondary metabolite accumulation. We evaluated growth, water status, antioxidant-related metabolites, and expression of key genes of the rutin pathway in Capparis spinosa L. grown at 100, 75, 50, and 25% field capacity (FC). Severe drought (25% FC) markedly reduced shoot length, leaf number/area, and leaf biomass, but increased total phenolics and total flavonoids (mg/g DW). Because biomass declined under severe stress, the total flavonoid yield per plant decreased. Rutin concentration increased with drought, whereas quercetin decreased. qRT‑PCR showed that 50% FC maximized expression of 4-coumaroyl CoA ligase (4CL), flavonoid 3 'hydroxylase (F3 'H), flavonol synthase (FLS), and flavonol-3-O-glucoside l-rhamnosyltransferase (RT) (3.76‑, 4.24‑, 13.05‑, and 8.61‑fold vs. control, respectively). Although 25% FC enhanced per‑gram metabolite concentrations, it strongly impaired growth. In contrast, 50% FC provided a pragmatic balance between biomass and metabolite output, suggesting that moderate, controlled water deficit can be used to improve the production of bioactive flavonoids in C. spinosa cultivated in arid regions.