Inter- and intra-specific variation in accumulation of cadmium by peanut, soybean, and navybean

Abstract

Production of summer grain legumes like peanut, soybean, and navybean is expanding into irrigated or high rainfall areas on more acid, lighter textured soils in coastal areas of north-eastern Australia. A history of intensive use of phosphatic fertilisers, combined with soil properties which generally enhance phytoavailability of cadmium (Cd), have produced concerns about the likely quality of grain legumes produced in these areas. This paper reports field and pot experiments which examine the effect of grain legume species and variety on Cd accumulation when grown across a range of soil types. Results clearly show that both peanut and soybean accumulate Cd in seeds at levels greater than the maximum permitted concentration (MPC, 0·05 mg Cd/kg) even on soils with relatively low total or available Cd concentrations (<0·5 mg/kg). The relative risk of MPC exceedance in marketable seeds or kernels was peanut > soybean > navybean, with the differences between peanut and navybean apparently correlated with differences in total plant Cd uptake. Cadmium concentrations in plant tops always exceeded that in seeds or kernel, and the testa in peanut kernel was shown to contain Cd concentrations that were 50 times greater than that in the embryonic axis and cotyledons.

Significant (P < 0·05) variation in Cd content (at least 2-fold) was recorded among peanut varieties, with lesser variation evident among a limited sample of commercial navybean varieties. Comparison of results for 11 peanut varieties grown at each of 2 locations suggested strong genotype environment interactions determining kernel Cd concentration.

Highly significant (P < 0·01) linear relationships were established between soil Cd in the cultivated layer (0-20 cm; 0·1 M CaCl₂ extraction) and seed Cd content in field-grown soybean. However, despite observations of an apparent relationship between soil Cd (CaCl₂ extraction) and peanut kernel Cd in pot studies, relationships between soil Cd in the cultivated layer and kernel Cd could not be reproduced in field trials. Kernel Cd concentrations from field-grown peanut plants were generally higher than those from pot trials, despite using soil collected from the cultivated layer (0-20 cm) of the field site for the potting medium. The presence of significant levels of Cd to approximately 60 cm in the soil profile and a general decline in pH_w with depth suggest the lack of correlation between soil test Cd in the top 20 cm and kernel Cd in field-grown plants may be at least partly due to Cd uptake from deeper soil layers.