THE GENOTOXICITY OF ZN(II) AND CD(II) IN ALLIUM-CEPA ROOT MERISTEMATIC CELLS

Applications of 5x10(-4) M Zn(II) and 3x10(-5) M Cd(II) depressed but did not block root growth in Allium cepn L. Both heavy metals were ind irect clastogens at these moderate concentrations, for they induced th e appearance of chromosomal bridges, but no breaks were detected in th e first ana-telophases. On the other hand, Zn and Cd respectively pote ntiated six and eleven times the direct clastogenic effect of the beta -irradiation produced by [H-3]thymidine incorporation into DNA. The ch romosomal breaks at mitosis were more abundant when [H-3] was incorpor ated into early replicating DNA. On the other hand, when the incorpora tion took place in the last half of interphase, the subsequent G(2) as well as the prophase were extended (1.7-fold for Zn and 2.7-fold for Cd), and the Sexcess lesions were repaired before anaphase. The whole cycle experienced 1.5 and 2.0-fold increases in the presence of Zn and Cd in relation to that in control meristems (15 h).