Zinc-induced oxidative damage, antioxidant enzyme response and proline metabolism in roots and leaves of wheat plants

The purpose of this study was to evaluate antioxidative responses and proline metabolism in roots and leaves of wheat seedlings after treatment with different zinc (Zn) concentrations (0, 0.5, 1 and 3 mM) for 6 days. A notable reduction in Zn content was observed in 0.5 mM Zn-treated leaves, but a significant elevation in response to 1 and 3 mM Zn treatment. Significant increases in Zn levels were observed in roots exposed to all applied Zn concentration. The highest Zn concentration resulted in significant reduction in the amount of total chlorophyll (chl) and chl a, while chl b content decreased under all applied Zn concentrations. In wheat leaves, Zn excess caused an insignificant enhancement of hydrogen peroxide (H2O2) content as well as unaltered malondialdehyde (MDA) level. Unchanged superoxide dismutase (SOD) activity, increased peroxidase (POD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX) activities were also observed in the leaves of Zn-treated seedlings. By contrast, higher H2O2 and MDA contents in Zn-treated roots were correlated with the stimulation of SOD and the inhibition of POD and GR. There were significant enhancements of soluble sugar and proline in both leaves and roots of wheat seedlings under Zn stress, but the increased rate of proline was higher in the roots than in the leaves. Differently, soluble protein content due to Zn treatment was lower in the leaves but higher in the roots, as compared with untreated seedlings. Additionally, ornithine δ-aminotransferase in both leaves and roots was stimulated by Zn stress, but different Zn concentrations exhibited inhibitory effect on glutamate kinase activity in wheat seedlings. In contrast, all applied Zn concentration resulted in an elevation of proline dehydrogenase activity in the leaves while the highest Zn concentration inhibited this parameter in the roots.

► Antioxidative responses and proline metabolism in response to zinc treatment.
► Chl b was more sensitive to Zn stress in comparison with chl a.
► An important role of APX and GR in scavenging ROS was suggested in the leaves.
► Glutathione-ascorbate cycles may not be due to H2O2 scavenging in the roots.
► Differences in the mechanisms of proline metabolism in the leaves and roots.