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

Abstract

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.

Keywords: Air pollution; Antiozonant; EDU; Physiological stress; Plant protection.