Distribution and concentration of cadmium in root tissue of Populus alba determined by scanning electron microscopy and energy-dispersive x-ray microanalysis

In a polluted environment with potentially toxic elements, plants may play a relevant role on contaminant absorption or stabilization. The aim of this study was to discriminate two poplar clones in their tolerance to high Cd concentrations (50 μM) in the growth solution and to show the potential of poplar in the cleaning-up of Cd-contaminated substrate. Chemical analyses to determine the content of heavy metals in biological samples involve their destruction by digestion with concentrated acids, preventing the localization of potentially toxic elements in situ. In contrast, scanning electron microscopy equipped with energy-dispersive x-ray microanalysis may provide information on element localization and chemical composition of biological samples. Scanning electron microscopy allows for the observation of samples in a dry or wet state, at high magnifications and good field depth with a minimum preparation, and the possibility to combine structural and analytical information by energy-dispersive x-ray microanalysis and digital images. Although, energy-dispersive x-ray microanalysis has relatively low detection limits, it is useful in establishing distribution maps of potentially toxic elements inside cells and tissues. Tissue concentration and localization of Cd (and other elements) in root tips of Populus alba clones (6K3 and 14P11) were investigated, using two different types of scanning electron microscope (ambient temperature and low temperature) both coupled with energy-dispersive x-ray microanalysis. These techniques were useful to investigate structural modifications and to identify in situ concentration and distribution of Cd in poplar roots, establishing indirect correlations between accumulation and localization of the metal. Overall, observations suggested differential patterns between 6K3 and 14P11 clones in accumulating Cd within the root profile, though overall concentration and content of Cd in the root system, determined by atomic absorption spectrophotometry, did not differ between clones. The possibility that these accumulation patterns arise from differences in uptake processes and structural properties is discussed and related to tolerance mechanisms.