Deciphering the role of low molecular weight antioxidants in the sensitivity of Melissa officinalis L. to realistic ozone concentrations

To decipher the role of low molecular weight antioxidants in the sensitivity of Melissa officinalis L. (lemon balm, an aromatic plant, widely cultivated for pharmaceutical, food, beverage and cosmetic purposes) to realistic ozone (O-3) concentrations, plants of this species were exposed to the gaseous pollutant (80 ppb for 5 h), and investigated for their antioxidative systems and stress tolerance. Ozone treated leaves tried to cope with the increased oxidative pressure by improving the synthesis of most of the tested antioxidant compounds (e.g. superoxide dismutase, ascorbic acid, phenols and total carotenoids), whereas catalase and proline were not increased. The biosynthesis of rosmarinic acid (i.e. the dominant active phenolic compound of the investigated species) was also triggered by O-3. These cellular processes were orchestrated during and after the exposure, inducing a partial and transient control of the reactive oxygen species (i.e. hydrogen peroxide and superoxide anion) and an increase of the antioxidant capacity throughout the recovery phase (12-48 h from the beginning of the exposure). However, they were not able to protect the cell structure, as demonstrated by the macroscopic damages observed on both leaf surfaces of fumigated plants.