Incorporation of thymidine into onion root meristematic cell nuclei in presence of hydroxyurea and its role in recovery of mitotic activity

Hydroxyurea treatment of onion roots induced mitotic block which was released by transfer of bulbs to water, and also to some extent by addition of cold or ³H-thymidine to hydroxyurea solutions. In presence of hydroxyurea there was noted very intense incorporation of ³H-thymidine into cell nuclei, giving labelling index of 40-70%. However, all the mitotic figures appearing in presence of hydroxyurea and ³H-thymidine were unlabelled. On the other hand, labelled mitotic figures were obtained when roots incubated with ³H-thymidine in presence of hydroxyurea had been transferred to water. Incorporation of ³H-uridine was unaffected by hydroxyurea. The results show that hydroxyurea arrests onion root meristematic cells, either in the S phase and the G₂ phase. Enhanced incorporation of ³H-thymidine in the presence of hydroxyurea, and release by added thymidine of the mitotic block indicate that hydroxyurea induces in onion root meristematic cells a particular shortage of thymidylate.