Systemic antioxidant effects of low-dose ultraviolet treatment on tobacco leaves are mediated by hydrogen peroxide

Exposing a single leaf of tobacco plants to ultraviolet (UV) radiation, we detected eustress-like pigment and antioxidant responses not only in the directly treated leaf itself but also in the leaf above it (the systemic leaf), which was not directly exposed to UV radiation. To the best of our knowledge, this study is the first to investigate the systemic UV responses in plants. The experiments used low-dose, supplemental UV irradiation indoors (two hours daily, for two days), which had no significant effect on photosynthesis in the directly exposed leaves but influenced their antioxidant status. Systemic leaves showed increased H₂O₂ levels, indicating the involvement of reactive oxygen species in the underlying complex signalling cascade. Metabolic changes in systemic leaves are driven by increased carbon dioxide assimilation, supported by more open stomata. Compared with the leaves of untreated plants of the same age, systemic leaves exhibited higher adaxial flavonoid levels and more efficient H₂O₂ housekeeping (higher peroxidase and catalase activities and lower superoxide dismutase activities). The role of H₂O₂ in the systemic UV effect was supported by another experiment, showing that direct H₂O₂ treatment increased the H₂O₂ levels in a systemic manner in the leaves above the treated ones. Notably, unlike its direct effects, the systemic effects of UV radiation did not enhance hydroxyl radical scavenging activity, indicating the unique nature of direct UV-driven responses. The possibility presented here to induce eustress-like antioxidant responses in distal leaves without direct UV exposure could be relevant to indoor plant growth systems as a potential biofortification tool.