Phytochemical and physiological changes in the leaves of St. John's wort plants under a water stress condition

Water stress is known to increase the secondary metabolites concentration in plant tissues and severe water stress conditions may cause oxidative stress due to the formation of reactive oxygen species and photoinhibitory damage. Current study was undertaken to evaluate the changes in the physiological status especially the photosynthetic efficiency and the biochemical profile of the leaf tissues of St. John's wort (Hypericum perforatum  ) plants exposed to water stress. The net photosynthetic rates of the leaves of plants grown under a water stress condition were significantly low compared with those of the control. The maximal quantum efficiency of PSII photochemistry (ϕpmax) of the dark adopted leaves was similar for both wilted and recovered plants although these values were significantly low compared with those of the control. The leaf tissues of the plants under a water stress condition had a significantly higher capacity to detoxify oxygen radicals with an about 2.5-fold increase over the antioxidant potential of the leaves of non-treated (control) or recovered plants. In the leaf tissues of plants grown under a water stress condition, both hypericin and pseudohypericin concentrations reduced with time and on Day 12 of the treatment, the concentration was significantly lower than that of the control; in contrast, the hyperforin concentration increased significantly and the value was nearly double after 12 days of the treatment. Under a water stress condition, the hyperforin concentration was about 70-fold higher than the total hypericins concentration in the leaf tissues; in case of control it was only 10–12 times higher. The results also indicate that the major secondary metabolite, hyperforin concentration of 62-day-old plants was three- to four-fold higher than the previously reported values for 1–2-year-old field-grown plants.