Differences in structure of adventitious roots in Salix clones with contrasting characteristics of cadmium accumulation and sensitivity

Various clones of Salix spp. have contrasting characteristics of accumulation, translocation to shoots and of sensitivity to cadmium (Cd). The aim was to investigate the structure of adventitious roots and find out if differences between groups of clones in root anatomy accounted for differences in relation to Cd. Stem cuttings of eight clones of Salix spp. with different combinations of high or low Cd accumulation, translocation of Cd to shoots and sensitivity to Cd, were cultivated for 3 weeks in hydroponics containing 100 muM Ca(NO3)(2). No Cd was added in this experiment. Equal-sized roots were selected for hand-sectioning and fluorescence staining to detect the beginning of Casparian band formation and suberin lamellae deposition in endodermis. In addition, root apices were fixed, embedded, sectioned longitudinally and transversally, and stained. The image analysis system LUCIA was used for quantitative evaluation of tissue differences. The structure of adventitious root apices showed an intermediate-open type of root apical meristem and the clones differed in organization of root apices. Clones with low accumulation of Cd and high Cd tolerance had smaller meristematic zones and more extensive vacuolation of cells in the root apices than clones characterized by high accumulation of, and high sensitivity to, Cd. The apoplastic barriers, exo- and endodermis, were developed relatively close to the apex. In both layers the first ontogenetic stage, the Casparian band development, was followed by the second stage, the suberin lamellae deposition. This process started in the endodermis, preferentially against phloem poles, which is a common phenomenon also in other plant species. However, preferential development of exodermis in the sectors against phloem poles was observed in this study for the first time in plants. Development of endodermal Casparian bands in clones characterized by high accumulation of Cd occurred more distant from the root tip than in clones with low accumulation. Furthermore, the suberin lamellae were more distant from the root tip in clones with high translocation of Cd compared with those with low translocation. This indicates that apoplastic movement of Cd into the stele and the upward translocation may vary due to the endodermal anatomy. The proportion of root apoplastic barriers, exodermis and endodermis as well as epidermis to other root tissues was significantly increased in clones with higher tolerance to Cd ions, indicating the importance of these tissues in protection of the root against toxic effects of Cd.