Cell-cycle dependency of radiosensitivity and mutagenesis in fertilized egg cells of rice, Oryza sativa L. : 2. X-ray sensitivity and mutation rate during a cell cycle.

In order to examine changes in survival and mutation rates during a cell cycle in higher plant, fertilized egg cells of rice were irradiated with X-rays at 2 h intervals for the first 36 h after pollination, i.e., at different phases of the first and second cell cycles. The most sensitive phase in lethality was late G1 to early S, followed by late G2 to M, which were more sensitive than the other phases. In both M1 and M2 generations, sterile plants appeared most frequently when fertilized egg cells were irradiated at G2 and M phases. Different kinds of mutated characters gave rise to the respective maximum mutation rates at different phases of a cell cycle: namely, albino and viridis were efficiently induced at early G1, xantha at early S, short-culm mutant at mid G2, heading-date mutant at M to early G1. The present study suggests the possibility that the differential mutation spectrums concerning agronomic traits are obtained by selecting the time of irradiation after pollination.