Delayed incorporation of H3-thymidine by primordial cells

Roots of Vicia faba were treated with H³-thymidine (1 c/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.This research has been supported by the U.S.A.E.C. [Grant AT (11-1) 1625-12].