Heavy Metal Toxicities in Vegetable Crops

III. The Effect of Manganese Concentration in the Nutrient Solution on the Growth of Vegetable Crops

Abstract

Fourteen species of vegetable crops were grown in solution culture in order to evaluate the relative tolerance of the vegetable crops to manganese. Hoagland′s No. 1 solution was used as the basic nutrient solution. Iron, in the form of Fe-EDTA, was supplied at the rate of 3ppm. Manganese was added at levels of 0.5 (control), 3, 10, 30, and 100ppm. The pH of the solution was adjusted to 6.0. The plants were grown under differential treatment for four weeks.
1. The relative tolerance of the vegetable crops to manganese was evaluated in terms of the concentration of manganese in the nutrient solution which corresponded to a 50 per cent reduction in top dry weight. This point was not reached even at the highest manganese level, 100ppm, in radish, Welsh onion, pepper, and Japanese honewort (Cryptotaenia japonica HASSK.); it was reached between 30 and 100ppm in lettuce, spinach, carrot, eggplant, and cabbage, between 10 and 30ppm in celery and tomato, and between 3 and 10ppm in turnip, kidney bean, and cucumber.
2. Symptoms of manganese toxicity in various species showed considerable variety. Brown necrotic spots and chlorosis were observed in many species; in general both of these symptoms were developed at lower manganese levels in less tolerant species than in more tolerant species. Brown necrotic spots occurred in the marginal area of older leaves or along the lower portion of stems. At least part of the chlorosis resembled iron deficiency symptom, but most chlorosis occurred as marginal chlorosis which was not identical with iron deficiency. Excess of manganese induced no serious abnormality in roots except a brownish discoloration in some species.
3. In most crops the concentration of manganese in leaves was less than 200ppm on a dry weight basis in the control and markedly increased with the increase of manganese in the nutrient solution. The more the species was tolerant to manganese, the more the accumulation of the element in leaves corresponding to the 50 per cent yield reduction increased. The threshold value of manganese concentration in leaves associated with the occurrence of brown necrotic spots was generally lower in less tolerant species than in more tolerant species.
4. The effect of excessive manganese on the iron concentration in leaves varied with the species. No clear relationship was found between the occurrence of chlorosis and the iron concentration in leaves.
5. The concentration of copper, zinc, nitrogen, phosphorus, and potassium in leaves had no relationship to the manganese toxicity, while the concentration of calcium and magnesium in leaves in some species was considerably decreased by the excess of manganese.