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 micro M Ca(NO(3))(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.

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Summary• The structure and development of the cortical layers, especially the endodermis and exodermis, and changes in the cortex caused by the secondary growth of vascular tissues are described in the adventitious roots of gentian ( Gentiana asclepiadea ).• Sections along the whole axis of the soil-grown roots were observed using light microscopy; fluorescence microscopy was used to determine developmental stages of the endodermis and exodermis.• Both endodermis and exodermis develop in three stages: Casparian band formation, suberin lamellae deposition and secondary thickening of walls. After the onset of cambial activity (20 mm from apex) cortical cells expand tangentially and subdivision of individual cells starts between 20 mm and 60 mm from apex. Highly differentiated endodermal cells are divided by 0 -19 new anticlinal walls, exodermal cells by 0 -3 and parenchymatous mid-cortex by 0 -1.• The additional anticlinal cell walls of the endodermis and exodermis possess neither Casparian bands nor suberin lamellae. Suberin lamellae remain continuous on the surface of extended tangential walls of both layers. There is a correlation between increasing diameter of the secondary vascular tissues and the number of endodermal cells created by subdivision of the original cells.

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