Protective role of zinc against oxidative damage in plants.

It was envisaged to evaluate Zn deficiency response on components of the antioxidative system in Zea mays L. plants differing in tolerance to zinc deficiency stress. Consequently, after screening of inbred maize lines, studies were made on non-enzymatic and enzymatic components of the antioxidative system in maize lines CM 115 (moderately susceptible) CM 120 (highly susceptible) subjected to (i) Zn deficiency (0.1µ M Zn) and recovery therefrom and (ii) the interactive effect on Zn stress and light intensity in the antioxidative system of these two maize lines. As compared to the Zn sufficient plants (1µM Zn), the severity of deficiency symptoms, such as condensation of internodes, development of white coloration and bronze necrotic lesions on leaves and decrease in dry matter production in Zn deficient plants of the maize line CM 120 was more than in CM 115.Under Zn deficiency, the decrease in chlorophyll and carotenoids and increase in lipid peroxidation as observed by the accumulation of MDA, was more in line CM 120 than in CM 115 indicating a higher oxidative damage in the Zn susceptible line CM 120 than in the moderately susceptible line CM115. In the Zn susceptible maize line CM 120, decrease in ascorbate concentration under Zn stress was associated with increase in the activity of ascorbic peroxidase and decrease in the activity of glutathione reductase. Activity staining of the gels revealed 4 superoxide dismutase (SOD) isoforms, 2 each of Mn SOD and Cu/Zn SOD. The activity of SOD and the expression of each isoform especially Cu/Zn SOD was saverely reduced in Zn deficient CM 115 and CM 120, but more so in the later. In maize line CM 115 that had moderate susceptibility to Zn deficiency, ascorbate, dehydroascorbate, APX, SOD and GR and their isoforms increased in response to Zn deficiency, providing better protection against oxidative damage. Alleviation of Zn deficiency by supply of sufficient Zn (1µM Zn) to Zn deficient plants partially increased the content of chlorophyll, carotenoid and ascorbate and the activities of Sod, APX and GR and decreased the MDA content.Under high light intensity, plants response to Zn deficiency was similar to that under normal intensity of light but the oxidative damage was more pronounced than under normal intensity of light. Judged from the loss (degradation) of chlorophyll and carotenoids and enhanced MDA content, high light intensity caused larger photo-oxidative damage in response to Zn deficiency in highly susceptible maize line CM 120 than in maize line CM115. Relatively higher activity of APX and GR and high ascorbate concentration in Zn deficient plants of maize line CM 115 than in maize line CM 120 could contribute better tolerance to oxidative damage under conditions of high light intensity.