The effect of cadmium on nodulation and N₂(C₂H₂)-fixation by dry beans (Phaseolus vulgaris L.)

Experiments were conducted to determine the toxicity of Cd on the nodulation, N₂(C₂H₂)-fixation, and growth of dry beans. Plants were grown in the greenhouse with hydroponic solutions to which 0-500 μm Cd/liter had been added. The total N content of shoots, nodule weight, nodule number, and N₂(C₂H₂)-fixation were significantly decreased at 10 μm Cd/liter. Nodulation was completely inhibited at 500 μm Cd/liter. Nodulation and the N₂(C₂H₂)-fixation rate were negatively correlated with solution Cd concentration (r = -0.97). Cadmium uptake and translocation were slow; therefore, Cd levels were highest in the roots, much lower in the nodules, and lowest in the shoot tissue. For example, at 20 μm Cd/liter, respective tissue Cd levels averaged 431, 80, and 35 μg Cd/g, on a dry weight basis. Cadmium levels in roots or shoots from comparable Cd treatments were essentially the same whether the plants obtained their N by fixation or from nutrient solution NO₃. Root Cd concentrations were exponentially correlated with nodule number, nodule weight, and acetylene reduction, with correlation coefficients of -0.99, -0.99, and -0.97, respectively. Shoot Cd concentrations were logarithmically correlated with nodule number, nodule weight, and acetylene reduction, with correlation coefficients of -0.89, -0.91, and -0.84, respectively.