Manganese toxicity in bush bean as affected by concentration of manganese and iron in the nutrient solution

An experiment was conducted to clarify the relationship between Mn toxicity and Fe deficiency in bush snap bean (Phaseolus vulgaris L. cv. ‘Wonder Crop No. 2'). Seedlings were grown in full strength Hoagland No. 2 solution at pH 6.0 for ten days. Six concentrations of Mn as MnCl₂.4H₂O were used in combination with three concentrations of Fe as FeEDTA.

Toxicity symptoms, induced by low levels of Mn (0.1 ppm and above), included: small brown necrotic spots and veinal necrosis on primary leaves; necrosis on primary leaf petioles; interveinal chlorosis, with or without brown necrotic spots, on trifoliate leaves; and brown necrotic spots on stipules. Manganese toxicity symptoms were alleviated or prevented by increasing Fe concentration in the nutrient solution.

Manganese concentration in the leaves increased with increasing Mn and decreased with increasing Fe concentration in the nutrient solution, Iron concentration in the roots increased with increasing Fe concentration in the nutrient solution; however, Fe concentration in the leaves was not significantly affected by increasing Mn concentration in the solution culture. Manganese toxicity symptoms developed when Mn concentration in the leaves reached about 120 ppm.

A decrease in the Fe/Mn ratio in the nutrient solution resulted in a proportionate decrease in that of the leaves. Manganese toxicity symptoms occurred when the Fe/Mn ratio in the solution was 10.0 and below, or when the ratio in the leaves was less than 1.5. The ratio of Fe/Mn in the solution required for optimum growth of ‘Wonder Crop No. 2’ bean, without Mn toxicity symptoms, was in the range of 20.0 to 25.0.

Results indicate that the chlorosis on bush bean leaves induced by excessive Mn in the nutrient solution was due to excessive accumulation of Mn and not to Fe deficiency.