Zinc exposure has differential effects on uptake and metabolism of sulfur and nitrogen in Chinese cabbage

Zinc (Zn) is a plant nutrient; however, at elevated levels it rapidly becomes phytotoxic. In order to obtain insight into the physiological background of its toxicity, the impact of elevated Zn2+ concentrations (1 to 10 mu M) in the root environment on physiological functioning of Chinese cabbage was studied. Exposure of Chinese cabbage (Brassica pekinensis) to elevated Zn2+ concentrations (>= 5 mu M) in the root environment resulted in leaf chlorosis and decreased biomass production. The Zn concentrations of the root and shoot increased with the Zn2+ concentration up to 68-fold and 14-fold, respectively, at 10 mu M compared to the control. The concentrations of the other mineral nutrients of the shoot were hardly affected by elevated Zn2+ exposure, although in the root both the Cu and Fe concentrations were increased at >= 5 mu M, whereas the Mn concentration was decreased and the Ca concentration strongly decreased at 10 mu M Zn2+. The uptake and metabolism of sulfur and nitrogen were differentially affected at >= 5 mu M Zn2+. Zn2+ exposure resulted in an increase of sulfate uptake and the activity of the sulfate transporters in the root, and in enhanced total sulfur concentration of the shoot, which could be ascribed partially to an accumulation of sulfate. Moreover, Zn2+ exposure resulted in an up to 6.5-fold increase in water-soluble non-protein thiol (and cysteine) concentration of the root. However, nitrate uptake by the root and the nitrate and total nitrogen concentrations of the shoot were decreased upon Zn2+ exposure, demonstrating the absence of a mutual regulation of the uptake and metabolism of sulfur and nitrogen at toxic Zn levels. Evidently, elevated Zn2+ concentrations in the root environment did not only disturb the uptake, distribution and assimilation of sulfate, it also affected the uptake and metabolism of nitrate in Chinese cabbage.