Comparative effects of copper and zinc toxicity and tolerance ofHyptis suaveolens(L.) Poit. andHelianthus annuusL.

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Soil application of copper and zinc to the plants Hyptis suaveolens (L.) Poit. and Helianthus annuus L. caused phytotoxic symptoms and decreased growth in terms of dry weight and chlorophyll content. Copper was found to be more toxic to sunflower than to Hyptis. In Hyptis, much of the absorbed copper was retained in the roots. Imbalances, in the levels of Fe, Mn, Cu and Zn were greater for sunflower which exhibited severe phytotoxicity. Zinc treatment caused a greater reduction in the biological yield of Hyptis. At the highest level of 1200 ppm Zn, dry weight yield of Hyptis was only 57.3% as against 66.4% in sunflower. The content of K and Mg in leaves and Ca levels in roots of Zn‐treated sunflower markedly increased. In general, the phytotoxic effects of Zn were found to be less severe than those of Cu, at the concentrations employed in this experiment.