ULTRASTRUCTURAL EFFECTS AND LOCALIZATION OF LOW CADMIUM CONCENTRATIONS IN BEAN ROOTS

The influence of a low concentration (0.5-mu-M) on the growth and ultrastructure of bean (Phaseolus vulgaris L.) roots was analysed at 24 h intervals for 144 h following the start of Cd supply. After 144 h exposure to Cd the cellular and subcellular location of Cd was determined both histochemically by the silver sulphide method and by energy dispersive X-ray microanalysis. A significant decrease in root elongation was observed after 48 h exposure, due to the inhibition of root cell division by Cd. Formation of cell wall ingrowths similar to those of transfer cells were observed in the hypodermis, suggesting a Cd-induced alteration of ion transport. Our results on the location of Cd by the silver sulphide method, at the light microscope level, suggest that Cd principally accumulates in the outer cortex cells, with very little in the endodermis. At the subcellular level, silver deposits indicating the presence of Cd were mainly found in vacuoles and nuclei and, to lesser extent, in the cytoplasm and plastids, but not in cell walls. X-ray microanalysis confirmed the presence of Cd in vacuoles and nuclei, but failed to detect Cd in any other cell compartment. The advantages and disadvantages of the histochemical and X-ray microanalytical methods for the localization of small amounts of Cd in plants are discussed.