Enzymatic Antioxidant Defense System of Scots Pine Seedlings Under Conditions of Progressive Manganese Deficiency

Manganese (Mn) is a crucial micronutrient for plants. The impaired function of the oxygen-evolving complex in Photosystem II (PSII) due to Mn deficiency is believed to result in the overproduction of reactive oxygen species and the induction of an enzymatic antioxidant system. In our study, we investigated the effects of progressive Mn deficiency (the difference in Mn content between the needles of control and Mn-deficient plants increased from 17-fold at the beginning of the experiment to 59-fold at the end) on the activities of superoxide dismutase (SOD), catalase, ascorbate peroxidase, and guaiacol peroxidase in the roots and needles of Scots pine seedlings. We found that the soluble protein content in plant organs under Mn deficiency was maintained at a level comparable to that of the control. Regardless of the severity of Mn deficiency, the needles of the Mn-deficient plants presented twofold lower SOD activity than the needles of the control plants. These differences were observed even when Mn deficiency did not negatively affect plant growth. Additionally, the total SOD activity in the needles of both plant groups was determined solely by the activity of the Cu/Zn-containing SOD isozymes. Compared with the control plants, Mn deficiency did not result in an increase in any of the studied H₂O₂-degrading enzymes in the needles of the seedlings. In contrast, the needles of the Mn-deficient plants presented a lower level of guaiacol peroxidase activity. Despite the inhibition of root growth, Mn deficiency led to changes in the balance of the enzymatic antioxidant system in plant roots. The data obtained suggest that the lack of activation of SOD and other antioxidant enzymes in Scots pine seedlings against the background of progressive Mn deficiency is due to the reduced ability of PSII to generate ROS under these conditions.