DELAYED INCORPORATION OF H3-THYMIDINE BY PRIMORDIAL CELLS

Roots of Vicia faba were treated with H3-thymidine (1 μc/ml) for 1 h and incorporation of H3-TdR into nuclei of primordia was studied. Large primordia (>1,500 cells) did not incorporate H3-TdR. Cells of small primordia did not incorporate the labeled precursor immediately but did so after a delay of several hours. The frequency of labeled nuclei became similar to that of lateral meristems only after a delay of 12-14 h. The gradual increase in labeling index also occurs in colchicine treated cells, which do not divide; this shows that the rise in labeling index is not due solely to the division of labeled cells. It has been estimated from H3-TdR and colchicine labeled cells that small primordia have a population of cells in which intermitotic time is about 12 h. The delay in incorporation appears to indicate that pools of H3-TdR can be maintained in small primordia for several hrs and that the precursor is not used until the cells synthesize thymidine kinase. © 1968 Springer-Verlag.