Intensity of Oxidative Processes and Enzymes Activity of Antioxidant System in the Pinus sylvestris and Larix sibirica Needles under Different Levels of Technogenic Pollution

Industrial emissions from aluminum production, containing a wide range of inorganic and organic pollutants, have a negative impact on trees. The expression and effectiveness of trees defense responses to technogenic stressors are largely determined by the functioning of their antioxidant system. Accordingly, the aim of this study was to investigate changes in the activity of main antioxidant enzymes in the needles of Scots pine (Pinus sylvestris L.) and Siberian larch (Larix sibirica Ledeb.)—dominant species in the boreal forests of the Irkutsk region—along a pollution gradient caused by aluminum smelter emissions, and to evaluate how these changes affect pollutant induced oxidative stress. The concentrations of fluorine, sulfur, heavy and light metals, as well as 16 priority polycyclic aromatic hydrocarbons (PAHs), were determined in the needles of both species. Based on cluster analysis, five pollution levels were identified: background, low, moderate, high, and critical. It was established that the accumulation capacity of L. sibirica for pollutants is higher than of P. sylvestris. The accumulation of pollutants in needles leads to oxidative stress, which is manifested by increased concentrations of H2O2 and malondialdehyde by 1.3–4.6 times. The most significant changes were observed in pine needles. Analysis of the enzymatic component of the antioxidant system revealed that its functioning has species-specific features. In P. sylvestris, peroxidase and catalase activity are higher than in L. sibirica, and pine peroxidase maintains activity over a wide pH range (3.0–11.0). Its activity remained high even under critical pollution, whereas catalase activity under the same load decreased by 1.5–2.0 times compared to background levels. Pine needles also showed greater diversity of peroxidase isoforms, which may indicate broader adaptive capabilities of its enzymatic system. Correlation analysis revealed a negative relationship between catalase activity and the content of fluorine, heavy metals and PAHs, and a positive relationship for peroxidase, which indicates different functions of these enzymes in the plant defense system activated by technogenic emissions. The obtained results confirm our hypothesis of significant differences in the defense systems of the two conifer species, manifested in the development of oxidative stress and the functioning of the antioxidant system. These differences should be taken into account when assessing the condition of forest ecosystems in industrial zones.