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Applications of 5 × 10-4 M Zn(II) and 3 × 10-5M Cd(II) depressed but did not block root growth in Allium cepa L. Both heavy metals were indirect clastogens at these moderate concentrations, for they induced the appearance of chromosomal bridges, but no breaks were detected in the first ana-telophases. On the other hand, Zn and Cd respectively potentiated six and eleven times the direct clastogenic effect of the ß-irradiation produced by [3H]thymidine incorporation into DNA. The chromosomal breaks at mitosis were more abundant when [3H] was incorporated into early replicating DNA. On the other hand, when the incorporation took place in the last half of interphase, the subsequent G2 as well as the prophase were extended (17-fold for Zn and 27-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). [ABSTRACT FROM AUTHOR]