EFFECTS OF SULFUR SUPPLY ON SOYBEAN PLANTS EXPOSED TO ZINC TOXICITY

Application of most waste or by-product material increases the zinc (Zn) concentration in soils markedly. This investigation was conducted to determine if enhanced sulfur (S) supplied as sulfate (SO4) would modify the toxic effects of excess Zn. Soybean (Glycine max [L.] Men. cv. Ransom) was grown for two weeks in nutrient solutions containing ranges in Zn (0.8 to 80 mu M) and S (0.02 to 20 mM). Root and shoot conditions were observed, dry weights measured, and Zn concentration determined. Zinc-toxicity symptoms started about one week after transplanting young plants to nutrient solutions. Symptoms including chlorosis, especially in the trifoliate leaves, and change in orientation of unifoliate leaves were mild in 20 mu M-, intermediate in 40 mu M-, and severe in 80 mu M Zn-containing solutions. Dry weight was reduced in plants exposed to 20, 40, and 80 mu M in. Plants grown in 40 mu M Zn and 20 mM S survived longer than those grown in lower S concentrations and showed alleviation of the chlorosis in trifoliate leaves. The change in the orientation of the unifoliate leaves due to in toxicity, however, was not affected by S. Zinc contents in shoots grown at toxic Zn levels were higher in 20 mM- than in lower S-containing nutrient solutions. High S supply (20 mM) increased Zn translocation from roots to shoots. Besides increasing the Zn translocation from roots to shoots, it seems that S nutrition may also be a factor helping the plants to cope with high levels of-Zn in their tissues.