The significance of the cadmium species in uptake and metabolism of cadmium in crop plants

The uptake of cadmium‐115 by intact seedlings of maize vas investigated with respect to time, concentration and interactions with various cations. An appreciable fraction of the cadmium absorbed by the roots was displaced by desorption and consequently constitutes the exchangeable fraction. Only a small proportion of the cadmium was absorbed in the presence of calcium indicating that this fraction is taken up predominantly by metabolically mediated processes. The transport of cadmium to the shoots was markedly reduced by the addition of zinc to the culture solution. The effects of supplying various cadmium compounds to barley seedlings were investigated hydroponically at a concentration of 0.5 ppm cadmium. Dry weights, mean longest lengths and cadmium, zinc and iron concentrations of root and shoot material were established per treatment. No visible growth effects were noted in any of the treatments. Significant concentrations of cadmium were found in all plants ranging from high concentrations associated with cadmium supplied as the chloride to low concentrations in plants grown in the presence of cadmium sulphide. Addition of cadmium to the culture solutions enhanced the concentration of zinc in the shoots but reduced the iron concentration. The chemical species of cadmium present in various crop plants was investigated by applying high voltage paper electrophoresis, polyacrylamide gel electrophoresis and Sephadex column filtration techniques. It was concluded that the predominant water soluble form of cadmium present in the 8 crop species test was ionic cadmium2+ (>90%). An association between protein and cadmium‐115 was established from extracts of labelled plant tissue. The use of column filtration techniques indicated definite protein/cadmium correlations, although this could be duplicated by spiking inactive plant material with tracer cadmium prior to extraction. Experiments using sulphur‐35 supplied simultaneously with cadmium‐115 to plants grown hydroponically indicated the affinity of cadmium for sulphur containing proteins. Whether this process has toxic implications or represents a possible natural detoxification mechanism within plants is discussed.