EFFECTS OF SULFUR SUPPLY ON SOYBEAN PLANTS EXPOSED TO ZINC TOXICITY

Application of most waste or by-product material increases the zinc (Z n) concentration in soils markedly. This investigation was conducted t o determine if enhanced sulfur (S) supplied as sulfate (SO4) would mod ify the toxic effects of excess Zn. Soybean (Glycine max [L.] Men. cv. Ransom) was grown for two weeks in nutrient solutions containing rang es in Zn (0.8 to 80 mu M) and S (0.02 to 20 mM). Root and shoot condit ions were observed, dry weights measured, and Zn concentration determi ned. Zinc-toxicity symptoms started about one week after transplanting young plants to nutrient solutions. Symptoms including chlorosis, esp ecially in the trifoliate leaves, and change in orientation of unifoli ate 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 m M S survived longer than those grown in lower S concentrations and sho wed alleviation of the chlorosis in trifoliate leaves. The change in t he orientation of the unifoliate leaves due to in toxicity, however, w as 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 seem s that S nutrition may also be a factor helping the plants to cope wit h high levels of-Zn in their tissues.