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ZINC PHYTOTOXICITY: PHYSIOLOGICAL RESPONSES AND DIAGNOSTIC CRITERIA FOR TISSUES AND SOLUTIONS

We studied zinc phytotoxicity with lettuce seedlings in solution culture. Zinc absorption in lettuce (Lactuca sativa L. cv. Climax) consisted of two distinct uptake phases; a slow uptake phase below 5 μM and a rapid uptake phase above 5 μM of solution Zn. In both phases the absorption rate, as measured by Zn concentration in the tissue, increased with solution concentration, but the rate of increase was much faster in the second phase. The rate of change of Zn absorption during the first phase was not materially affected by the Ca concentration of the nutrient solution. The second phase of Zn absorption (the potentially phytotoxic range) was substantially decreased as solution Ca concentration was increased. Zinc tissue concentrations showed relatively little variation between normal tissue and those clearly showing initial symptoms of Zn phytotoxicity. Zinc phytotoxicity as expressed by reduced growth appeared to be a direct result of the accumulation of Zn in tissues to toxic levels with the average tissue concentration being 500 mg L−1 Zn at threshold toxicity. This accumulation of Zn was associated with the second uptake phase. The time required to accumulate toxic levels of tissue Zn is determined by the uptake rate, which was a function of a number of parameters, including both Zn and Ca concentrations. If the threshold of latent toxicity is defined as the lowest solution concentration that has the potential to cause toxicity with extended exposure, then for lettuce this threshold is 5 μM Zn in the external solution, the concentration associated with the initiation of the second uptake phase.