Enhanced Superoxide Radical Production in Roots of Zinc-Deficient Plants

The production of superoxide radical (⁠O2–⁠) was studied in roots of cotton (Gossypium hirsutum L. cv. Deltapine 15/21), bean (Phaseolus vulgaris L. var. Prélude) and tomato (Lycopersicon esculentum L. cv. Super marmande) plants grown in nutrient solution with different Zn concentrations. Using Tiron as a spin-probe, electron spin resonance (ESR) was employed for the measurements of O2– levels. In the 48 000 g and 140 000 g supernatants of cotton root extracts the amplitude of the Tiron ESR signals reflecting O2– production steeply increased with the appearance of visual Zn deficiency symptoms in the shoots. The changes in the amplitude of the Tiron ESR signals were closely correlated with an NADPH-dependent O2– generating oxidase activity with a high pH optimum. Increases in NADPH-dependent O2– generation were also found in root extracts of Zn-deficient bean and tomato plants. In all experiments re-supply of Zn to deficient plants for 12 h or 24 h markedly decreased O2– generation. Further, with advancing Zn deficiency rates of NADPH oxidation increased and the activities of superoxide dismutase (SOD) and catalase decreased. The results suggest that cotton, bean and tomato roots possess an NADPH-dependent O2– generating activity which is affected by the Zn nutritional status of the plants. Under Zn deficiency, enhanced O2– generation and impaired detoxification of Of and H2O2 could lead to elevated levels of O2– and O2– -derived oxidizing O2 species and thus to increased peroxidation of membrane lipids.