Recovery of Pisum Root Meristems after Mitotic-inhibitory Treatments with ³H-thymidine

Summary

Primary root meristems of Pisum sativum recover from 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 Gl 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 produce 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.