Cell-cycle dependency of radiosensitivity and mutagenesis in fertilized egg cells of rice, Oryza sativa L. : 1. Autoradiographic determination of the first DNA synthetic phase.

To determine the time and duration of the first and second DNA synthetic phases in fertilized egg cells and central cells of rice, a total of 753 ovules were sampled at 2 h intervals during the first 30 h after pollination and exposed to (3)H-thymidine for 2 h at 25 °C. Autoradiographic observation of labeled nuclei was made for fertilized egg cells, as well as for central and antipodal cells. The first and second DNA synthetic phases in fertilized egg cells were found 8-12 h and 21-25 h after pollination, respectively. The durations of each cell-cycle phase in the egg cell were estimated to be 4-6 h for G1, 4 h vor S and for G2, and 2 h for M. In the central cell, the first DNA synthesis took place at 3-4 h after pollination, i.e., immediately after fertilization, followed by the formation of the primary endosperm nucleus. Antipodal cells also showed labeled nuclei in the early stages after fertilization. The first divisions of fertilized egg cell and primary endosperm nucleus were observed at 16-18h and at 4-6 h after pollination, respectively. The present observations suggest that sperm and egg nuclei participate in fertilization with haploid amount (1C) of DNA and fertilized egg cell originates thus in 2C state.