Zinc‐dependent changes in ESR signals, NADPH oxidase and plasma membrane permeability in cotton roots

The effect of Zn2+on the plasma membrane permeability and superoxide radical (O2‐) formation in roots was studied with cotton (Gossypium hirsutumL. cv. Delta‐pine 15/21) plants grown in nutrient solution with different Zn2+supply. Compared to Zn‐sufficient plants, the plasma membrane permeability of Zn‐deficient plants was increased as indicated by a 3‐, 5‐ and 2.5‐fold increase in root cell leakage of K+, NO3‐and organic carbon compounds, respectively. Resupply of Zn2+to Zn‐deficient plants for 12 h substantially decreased this leakage. The effects of Zn2+on membrane permeability were closely correlated with the levels of O2‐measured by electron spin resonance (ESR) spectroscopy in the microsomal membrane fraction and in the cytosol fraction of root cells. The amplitudes of the O2‐‐derived Tiron ESR signal also coincided with a O2‐‐generating oxidase activity which was strongly dependent on the presence of NADPH and FAD. The results suggest that Zn2+directly affects the integrity of the plasma membrane, at least in part, by interfering with O2‐generation by a membrane‐bound NADPH oxidase.