Foliar application of yeast extract mitigates water deficit stress and elicits hypericin and phenolic production in Hypericum perforatum L.

Yeast extract has emerged as a bio-elicitor capable of modulating secondary metabolism and stress tolerance in plants, but its impact on St John's Wort (Hypericum perforatum L.) remains unexplored. Therefore, the interactive effects of yeast extract (0, 3, and 6 g L) and irrigation intervals (7, 10, and 13 days) on hypericin and phenolic production in this medicinal herb were investigated in a field experiment. The prolonged irrigation intervals decreased biomass during both seasons. Hypericin content peaked under the 10-day irrigation interval but declined in the 13-day irrigation interval. Foliar spraying of yeast extract improved biomass, chlorophyll a, b, carotenoids, relative water content, and hypericin concentration across all water regimes. Yeast extract application reduced hydrogen peroxide and malondialdehyde contents in water deficit-subjected plants due to increased activity of superoxide dismutase and catalase, and elevated levels of total phenol and flavonoid contents in the leaves. The highest contents of hypericin and phenolics were recorded with applying 6 g L yeast extract under the 10-day irrigation interval, corresponding with the strongest 2,2-diphenylpicrylhydrazyl scavenging activity and ferric-reducing power in the leaves. These findings suggest that yeast extract spraying might be a promising approach for enhancing the productivity and quality of medicinal plants under water deficit.