We tested the independent and interactive effects of nitrogen (N; 0 and 80 kg ha-1), phosphorus (P; 0, 40 and 80 kg ha-1), and ozone (O3) application/exposure [ambient concentration (AA), 1.5 × AA and 2.0 × AA] for five consecutive months on biochemical traits of the O3-sensitive Oxford poplar clone. Plants exposed to O3 showed visible injury and an alteration of membrane integrity, as confirmed by the malondialdehyde by-product accumulation (+3 and +17% under 1.5 × AA and 2.0 × AA conditions, in comparison to AA). This was probably due to O3-induced oxidative damage, as documented by the production of superoxide anion radical (O2-, +27 and +63%, respectively). Ozone per se, independently from the concentrations, induced multiple signals (e.g., alteration of cellular redox state, increase of abscisic acid/indole-3-acetic acid ratio and reduction of proline content) that might be part of premature leaf senescence processes. By contrast, nutrient fertilization (both N and P) reduced reactive oxygen species accumulation (as confirmed by the decreased O2- and hydrogen peroxide content), resulting in enhanced membrane stability. This was probably due to the simultaneous involvement of antioxidant compounds (e.g., carotenoids, ascorbate and glutathione) and osmoprotectants (e.g., proline) that regulate the detoxification processes of coping with oxidative stress by reducing the O3 sensitivity of Oxford clone. These mitigation effects were effective only under AA and 1.5 × AA conditions. Nitrogen and P supply activated a free radical scavenging system that was not able to delay leaf senescence and mitigate the adverse effects of a general peroxidation due to the highest O3 concentrations.
Keywords: Antioxidant compounds; Detoxification; Nitrogen; Oxidative damage; Phosphorus; Premature senescence.
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