Exogenous application of melatonin protects bean and tobacco plants against ozone damage by improving antioxidant enzyme activities, enhancing photosynthetic performance, and preventing membrane damage.

Ozone (O3) pollution is harmful to plants and ecosystems. Several chemicals have been evaluated to protect plants against O3 deleterious effects. However, they are not adequately efficient and/or the environmental safety of their application is questioned. Hence, new chemicals that provide sufficient protection while being safer for environmental application are needed. This study investigates the response of two O3-sensitive plant species (Phaseolus vulgaris L. cv. Pinto and Nicotiana tabacum L. cv. Bel-W3) leaf-sprayed with deionized water (W, control), ethylenediurea (EDU, 1 mM) or melatonin at lower (1 mM) or higher (3 mM) concentrations (Mel_L and Mel_H, respectively), and then exposed to a square wave of 200 ppb O3, lasting 1 day (5 h day-1) for bean and 2 days (8 h day-1) for tobacco. In both species, the photosynthetic activity of O3-exposed plants was about halved. O3-induced membrane damage was also confirmed by increased malondialdehyde (MDA) byproducts compared to control (W). In EDU- and Mel-treated bean plants, the photosynthetic performance was not influenced by O3, leading to reduction of the incidence and severity of O3 visible injury. In bean plants, Mel_L mitigated the detrimental effect of O3 by boosting antioxidant enzyme activities or osmoprotectants (e.g. abscisic acid, proline, and glutathione transferase). In Mel_L-sprayed tobacco plants, O3 negatively influenced the photosynthetic activity. Conversely, Mel_H ameliorated the O3-induced oxidative stress by preserving the photosynthetic performance, preventing membrane damage, and reducing the visible injuries extent. Although EDU performed better, melatonin protected plants against O3 phytotoxicity, suggesting its potential application as a bio-safer and eco-friendlier phytoprotectant against O3. It is worth noting that the content of melatonin in EDU-treated plants remained unchanged, indicating that the protectant mode of action of EDU is not Mel-related.