[³h]thymidine Utilization in the Primary Root of Vicia Faba L.

SUMMARY The size of the endogenous pool of TdR in roots of Vicia faba was found to influence the rate of [ 3 H]TdR incorporation into DNA. Thus, increasing the size of this pool, as a result of treatment with TdR, resulted in a decrease in the amount of [ 3 H]TdR incorporated into DNA, while decreasing the size of the endogenous TdR pool, resulted in increased incorporation of [ 3 H]TdR into DNA. The size of the TdR pool could be decreased by excising the roots or by removing the cotyledons. TdR was found to be transported to the root apex from the cotyledons in young V. faba plants. The pattern of distribution of 3 H‐compounds in the primary root of V. faba was found to change with increase in duration of the period of exposure to [ 3 H]TdR. These changes were a consequence of the movement of 3 H‐compounds from the base of the root to the apex and of the fact that DNA synthesis was greater in the root apex than in more basal segments.