Recovery of Pisum root meristems after mitotic-inhibitory treatments with 3H-thymidine. Inhibition of cell-cycle progression by unincorporated 3H-thymidine.

Primary root meristems of Pisum sativum recover form a 3H-thymidine-induced reduction in mitotic activity once the roots are no longer exposed to exogenous 3H-thymidine. Cells arrested in G2 during 3H-thymidine treatment apparently do not divide for at least 16 hours after treatment, whereas cells remaining in G1 and S do divide and thereby account for recovery. Recovery occurs only when meristems are no longer exposed to exogenous (i.e. unincorporated) 3H-thymidine, suggesting that cytoplasmic irradiation from unincorporated 3H-thymidine prevents cellular recovery from 3H-thymidine-induced inhibition of cell progression through the mitotic cycle. Concentrations of 14C-thymidine which result in cytoplasmic irradiation nearly equivalent to that achieved with 3H-thymidine, but much lower levels of nuclear irradiation, also prevent recovery from 3H-thymidine-induced inhibition of mitotic activity, but do not alone produced such inhibition. These results support the contention that cytoplasmic irradiation prevents recovery from the effects of nuclear irradiation. Unincorporated 3H-thymidine also prevents recovery from sucrose deprivation in stationary phase G2 cells which have not incorporated 3H-thymidine into nuclear DNA.