Effect of Cadmium on the Nucleoli of Meristematic Cells of Onion Allium Cepa L: an Ultrastructural Study

This study of the effect of cadmium on nucleolar ultrastructure was carried out with meristematic cell populations of Allium cepa L. Meristems, grown at 25 degrees C, were treated with 10 ppm cadmium chloride. Conventional and silver staining techniques were carried out, and the ultrastructure was analyzed using electron microscopy. Observation showed alterations in the nucleoli of the cells that had been treated with cadmium and this effect varied according to the time of exposure to the metal. After 4h of treatment, nucleolar segregation was observed in interphase, probably because of the effect of cadmium on the synthesis of ribosomal RNA precursors. A decrease in the fibrillar to granular component ratio also occurred in the cells exposed to Cd2+ for 8 h. Some changes were observed in the G1 cells; their chromatin still remained very condensed, and prenucleolus bodies remained scattered within the nucleus. At the same time, there was a large amount of interchromatin granules. These changes produced by cadmium resembled those produced during inhibition of RNA synthesis. The fibrillar bodies, another morphologic feature, resulting from a blocked transcription, were also evidenced. All these observations suggest that one of the ways that cadmium exercises its toxicity is by altering the biosynthesis of the preribosomal RNA precursor.