Ozone primes changes in phytochemical parameters in the medicinal herb Hypericum perforatum (St. John's wort)

Plants produce secondary metabolites promoting adaptation to changes in the environment and challenges by pathogenic microorganisms. Ozone (O-3), a crucial gaseous air pollutant, has been reported to alter the chemical composition of plants and this also involves changes in bioactive secondary compounds. The objective of this work was to assess under controlled conditions the possible role of O-3 (110 ppb, 5 h) as an elicitor of antioxidant compounds in Hypericum perforatum L. (St. John's Wort, SJW) plants at two stages, vegetative (VS, three-months old) and reproductive (RS, six-months-old) stage. At the end of the treatment, we found an early and transient burst of hydrogen peroxide (H2O2, + 35% in comparison to controls), a less efficient metabolism of the Halliwell-Asada cycle (as confirmed by the unchanged levels of ascorbate) and the up-regulation of the reduced/oxidized glutathione ratio (about 2-fold higher than control), which represent a typical initial stress response in VS plants. After 24 h of exposure, a significant reduction of H2O2 levels was observed that could be dependent on the activation of peroxidase activity (+ 26%), accumulation of total phenols, flavonoids and anthocyanins (+45, + 58 and + 2%, respectively) and increased antioxidant potential. A modification of the pool of soluble phenolics (at the quantitative and qualitative levels) was observed in VS and RS plants, so confirming its involvement in detoxification mechanisms. Our results show the potential of using O-3 to enhance the concentration of several antioxidant phytochemicals and consequently to increase the beneficial properties of SJW plants at different phenological stages.