Cadmium-induced structural and ultrastructural changes in the vascular system of bush bean stems

The effects of a phytotoxic cadmium concentration (4.45 . 10−5 M) on the structure and ultrastructure of bean plant stems were analysed by light (LM), transmission electron (TEM) and scanning electron microscopy (SEM). Cadmium significantly reduced both the number and the size of tracheary elements. Cadmium-induced electron dense depositions, which seemed to obstruct partially some xylem vessels, were found only in the later maturing tracheary elements (helical, scalariform or reticulate structure), but not in the annular structured early differentiated ones. Plants exposed to Cd showed less fiber development than the control plants. In the plants treated with Cd abnormally high amounts of calcium oxalate crystals were found in the paratracheary parenchyma cells of the bottom of the stems. The levels of soluble Ca2+ in the expressed stem sap of Cd-treated plants was significantly decreased, while substantial amounts of soluble Cd were detected. The probable mechanisms of the structural alterations observed are discussed.