Relationships between net photosynthetic rate and secondary metabolite contents in St. John's wort

Hypericum perforatum L. (St. John's wort), a traditional herb with antidepressive and wound healing properties, was grown under 100, 300 and 600 μmol m−2 s−1 photosynthetic photon flux (PPF) each with 500, 1000 and 1500 μmol mol−1 CO2, with the objective of maximizing the biomass and secondary metabolite production. Plants grown in the field (1770 μmol m−2 s−1 PPF and 380 μmol mol−1 CO2) were used as a control. On day 45, total fresh and dry mass, and the number of stem nodes were greatest in plants grown under 600 μmol m−2 s−1 PPF with 1500 μmol mol−1 CO2 concentration (HH-treatment) and were 29, 30 and 4 times greater, respectively, than those of the control. Leaf net photosynthetic rate (Pn) increased with increasing PPF and/or CO2 concentration, with the highest value occurring in the plants grown under the HH-treatment. Secondary metabolite contents (mg/plant) in the leaf tissues were generally higher in the plants grown under the controlled environments than those in the field (control). Hypericin and pseudohypericin contents were highest in the HH-treatment, being 30 and 41 times greater, respectively, than those of the control. Hypericin and pseudohypericin contents in the leaf tissues increased with increasing Pn. The second order polynomial correlations between Pn and hypericin, and between Pn and pseudohypericin contents with R2 of 0.82 and 0.79, respectively, were obtained. Moreover, total hypericin (hypericin + pseudohypericin) concentration (mg g−1 leaf DM) in leaf tissues was related to both PPF and CO2 concentration as expressed by second order polynomial correlations (R2 = 1). Therefore, growing St. John's wort plants under a controlled environment can enhance biomass and secondary metabolite production by increasing net photosynthetic rate.