Metal toxicity and sulfur metabolism in Chinese cabbage

Copper and zinc are essential nutrients for the plants; however, they become rapidly phytotoxic at elevated concentrations in the root environment. Exposure of Chinese cabbage to elevated Cu2+ (≥ 2 μM) or Zn2 + concentrations (≥ 5 μM) resulted in leaf chlorosis and subsequently in a loss of photosynthetic activity and a strongly reduced biomass production of both root and shoot. Cu is a redox-active metal, which has the potential to accelerate the formation of reactive oxygen species in plant tissue. However in Chinese cabbage, the Cu-induced chlorosis was most likely not due to the formation of reactive oxygen species but the consequence of a negatively affected chloroplast development. UV-A+B radiation and elevated Cu2+ concentra- tions had negative synergistic effects on biomass production, pigment content on the quantum yield of photosystem II. The uptake and metabolism of sulfur and nitrogen were differentially affected at elevated Cu2+ or Zn2+ concentrations. Both Cu2+ and Zn2+ exposure resulted in an increased sulfate uptake by the roots and in enhanced total sulfur content of the shoot, which could be ascribed partially to an accumulation of sulfate. Moreover, exposure resulted in a strongly enhanced level of water-soluble non-protein thiols in the root, which only partially could be ascribed to a metal-induced synthesis of phy- tochelatins. The nitrate uptake by the root was decreased upon Cu2+ or Zn2+ exposure, demonstrating the absence of a mutual regulation of the uptake of sulfate and nitrate. There was no direct relation between the sulfur metabolite levels viz. total sulfur, sulfate and water-soluble non-protein thiols and the ex- pression and activity of the sulfate transporters and the expres- sion of APS reductase at elevated Cu and Zn concentrations. Apparently, the presumed signal transduction pathway involved in their regulation appeared to be overruled or bypassed at high tissue Cu and Zn levels. It is doubtful that the Cu2+ or Zn2+-in- duced effects on the uptake and metabolism of sulfate have any adaptive significance in the detoxification of these metals in Chinese cabbage. 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. The uptake of nitrate appeared to be closely linked to the growth rate of the plant, even at toxic Zn levels. The total N content was strongly decreased in the shoot at toxic Zn concentrations. The decrease in total N in the shoot could only partly be ascribed to a decrease in nitrate content. In the root, how- ever, the total N content remained unaffected at elevated Zn2+ concentrations.