Heavy metal toxicities in vegetable crops

Fourteen species of vegetable crops were grown in solution culture with addition of zinc, in order to investigate the symptom of zinc toxicity and relative tolerance of vegetable species to zinc. HOAGLAND's No. 1 solution was used as the basic nutrient solution. The concentrations of zinc supplied were 0.05 (control), 0.3, 1, 3, 10, and 30ppm. All of the solutions were adjusted to pH 6. The plants were grown under differential treatment for four weeks.1. With the increasing concentration of zinc in the nutrient solution, the plant growth was generally inhibited. In the 30ppm zinc treatment, most vegetable crops showed especially poor growth accompanied with chlorosis resembling iron deficiency.2. The relative tolerance of vegetable crops to zinc was evaluated according to the concentration of zinc in the nutrient solution corresponding to a 50 per cent reduction in dry weight of tops. Welsh onion did not show the 50 per cent yield reduction even in the highest zinc treatment. In Japanese hornwort, carrot, and celery the 50 per cent yield reduction was observed between the 10 and 30ppm zinc treatments, and in several crops including fruit vegetables, Cruciferous vegetables, and lettuce between the 3 and 10ppm zinc treatments. Spinach was estimated to be most susceptible, which showed the 50 per cent yield reduction between the 1 and 3 ppm zinc treatments.3. In most crops, the concentration of zinc in leaves of control plants was less than 100ppm in dry matter and increased with the increasing supply of zinc in the nutrient solution. There was no evident relation between the zinc accumulating capacity in leaves and the relative tolerance of vegetable crops to zinc. The accumulation of zinc in roots was generally higher than that in leaves. The concentration of zinc in leaves associated with the 50 per cent reduction of dry weight yield was generally higher in the tolerant crops than in the susceptible crops.4. The concentration of iron in leaves was not so markedly influenced by the zinc treatments. The concentration of manganese in leaves increased with the increasing supply of zinc up to the 3 or 10 ppm zinc treatment, and then markedly decreased.5. The content of nitrogen, phosphorus, calcium or magnesium in leaves was not related to the growth inhibition by the excess zinc supply.