IRON-DEFICIENCY AND ZINC TOXICITY IN SOYBEAN GROWN IN NUTRIENT SOLUTION WITH DIFFERENT LEVELS OF SULFUR

A typical symptom of iron (Fe) deficiency in plants is yellowing or ch lorosis of leaves. Heavy metal toxicity, including that of zinc (Zn), is often also expressed by chlorosis and may be called Fe chlorosis. I ron deficiency and Zn toxicity were evaluated in soybean (Glycine max [L.] Merr.) at two levels each of Zn (0.8 and 40 mu M), Fe (0 and 20 m u M), and sulfur (S) (0.02 and 20 mM). Reduction in dry matter yield a nd leaf chlorosis were observed in plants grown under the high level o f Zn (toxic level), as well as in the absence of Fe. Zinc toxicity, la ck of Fe, and the combination of these conditions reduced dry matter y ield to the same extent when compared to the yield of the control plan ts. The symptoms of Zn toxicity were chlorosis in the trifoliate leave s and a lack of change in the orientation of unifoliate leaves when ex posed to light. The main symptoms of Fe deficiency were chlorosis in t he whole shoot and brown spots and flaccid areas in the leaves. The la tter symptom did not appear in plants grown with Fe but under Zn toxic ity. It seems that Fe deficiency is a major factor impairing the growt h of plants exposed to high levels of Zn. Under Zn toxicity, Fe and Zn translocation from roots to shoots increased as the S supply to the p lants was increased.