Temperature stress can alter the photosynthetic efficiency and secondary metabolite concentrations in St. John's wort

Temperature stress is known to cause many physiological, biochemical and molecular changes in plant metabolism and possibly alter the secondary metabolite production in plants. The hypothesis of the current study was that temperature stress can increase the secondary metabolite concentrations in St. John's wort. Plants were grown under controlled environments with artificial light using cool white fluorescent lamps and CO 2 enrichment and 70-day-old plants were subjected for 15 days to different temperature treatments of 15, 20, 25, 30 and 35 °C before harvested. Major aim of the study was to increase the major secondary metabolites in St. John's wort by applying temperature stress and to evaluate the physiological status of the plant especially the photosynthetic efficiency and peroxidase activity of the leaf tissues exposed to different temperatures under precisely controlled environmental factors. Results revealed that relatively high (35 °C) or low (15 °C) temperatures reduced the photosynthetic efficiency of the leaves of St. John's wort plants and resulted in low CO 2 assimilation. Net photosynthetic rates and the maximal quantum efficiency of PSII photochemistry of the dark adopted leaves (ϕ p max) decreased significantly in the leaves of plants grown under 35 or 15 °C temperature treatments. High temperature (35 °C) treatment increased the leaf total peroxidase activity and also increased the hypericin, pseudohypericin and hyperforin concentrations in the shoot tissues. These results provide the first indication that temperature is an important environmental factor to optimize the secondary metabolite production in St. John's wort and controlled environment technology can allow the precise application of such specific stresses.