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Effect of Cadmium on Symbiotic Soybean Plants¹

ABSTRACT

The potential for environmental contamination by cadmium (Cd) has increased significantly in recent years. Since Cd may be hazardous to living systems, a study was conducted to investigate: (i) the existence of Rhizobium japonicum strains tolerant to Cd, (ii) the Cd effect on the symbiosis between host soybean plants (Glycine max (L.) Metr.) and R. japonicum strains with different Cd tolerances, and (iii) interactions and distribution of Cd and other elements in the host when biomass is decreased by Cd.

The existence of R. japonicum strains tolerant to Cd was assessed by comparing growth curves of 10 different strains growing in a basal medium with gluconate, yeast extract, L-arabinose, and Cd (0, 3.5, 7, 14, and 28 µg/ml). Strain 3Ilb110 (110) exhibited a tolerance to Cd after an initial lag in growth, while strain 3Ilb123 (123) was susceptible.

Strains 110 and 123 were used to inoculate plants growing in nutrient solution with Perlite as a support medium to evaluate the plant-rhizobial interactions in the presence of 0, 2.2, 6.7, 10.1, and 20.2 µg Cd/ml, respectively. Plants were harvested and tops, roots, and nodules were analyzed 23 days after starting Cd application.

Cadmium decreased dry matter production of tops, roots, and nodules. Plants inoculated with strain 123 accumulated significantly more dry matter in nodules than those with strain 110. However, in the presence of 0 and 2.2 µg Cd/ml, plants inoculated with strain 110 accumulated significantly more N than plants inoculated with strain 123.

Nutrient imbalances were observed in the presence of Cd. It is suggested that the interactions of Cd with some nutrients may contribute to Cd toxicity in soybean plants. In this study the most pronounced Cd effect was on Fe and Mn nutrition, rather than Zn as had been previously reported.

Key Words: Glycine max • Rhizobium japonicum • heavy metals • nitrogen fixation • nutrients • serology

¹ Paper no. 6532 of the Journal Series of the North Carolina Agric. res. Serv., Raleigh, NC 27650.

² Former Graduate Student and Professor of Soil Science, North Carolina State Univ., Raleigh, respectively. Dr. Borges is currently Associate Professor, Depto de Microbiologia, Univ. Fed. de Vicosa, Minas Gerais, Brazil.

Received for publication August 29, 1980.