Zinc Tolerance

Zinc is the heavy metal most often in the highest concentrations in wastes arising in industrialized communities (21). Zinc exclusion from uptake, or binding in the cell walls, does not seem to contribute to zinc tolerance (24,25). Zinc exclusion might exist in Scots pine (Pinus sylvestris L.), where certain ectomycorrhizal fungi retain most of the zinc in their mycelia, resulting in the ability of the plant to tolerate zinc (26). Infections with ectomycorrhizal fungi are beneficial for the growth and development of pecan (27). These fungi are highly specialized parasites that do not cause root disease. They are symbiotic, thus gaining substance from the root and contributing to the health of the root.

Tolerance is achieved through sequestering zinc in the vacuoles, and zinc remains low in the cytoplasm of tolerant plants, whereas zinc is stored in the cytoplasm of non-tolerant clones (28). Positive correlation between organic acids such as citrate and malate with zinc in tolerant plants indicates a mechanism of zinc tolerance (29,30). Zinc tolerance in tufted hair grass (Deschampsia caespitosa Beauvois) was increased in plants supplied with ammonium as compared to nitrate nutrition. This effect apparently is caused by greater accumulation of asparagine in the cytoplasm of ammonium-fed plants, which form stable complexes with asparagines and zinc (31).

Foliar application of chelates is inefficient because of poor absorption of the large organic molecules through cuticles (32,33). Foliar ZnSO₄ treatments are toxic to peach leaves (34) and to many other species, probably because sulfur accumulates on leaves and results in salt burn. A zinc nitrateammonium nitrate-urea fertilizer (NZNTM; 15% N, 5% Zn; Tessenderlo Kerley Group, Phoenix, AZ, U.S.A.) did not burn peach leaves. Apparently, NZN-treated peach leaves do not suffer from salt burn because the nitrate in NZN is readily absorbed in response to the need of leaves for nitrogen in protein synthesis thus not accumulating on the surface to cause leaf burn (34).