Studies on the Radiation Breeding in the genus Mentha : (XIII)Effects of Irradiation on Pollen

The effects of pollen grain irradiation on pollen-tube mitosis were investigated. The data were obtained from artificial culture. For irradiation gamma-rays were applied with 60Co at a dose rate 65 KR/hr. The doses ranged from 1 to 80 KR. 1) Inhibition of mitosis increased with increasing dose; namely the higher the dose, the more reduced was the frequency of nuclear division and the more delayed was the time of its occurrence. 2) In M, rotundifolia the generative nucleus remained mostly undiveded in showing chromosome elements even at 5KR, while at 20KR a few nuclei consisted of chromatin masses or chromosomes. The pollen-tubes of M. rotundifolia did not grow smoothly on the media. In natural condition, therefore, their pollen-tube mitosis may be more radio-resistant. 3) At 80 KR irradiation of M. spicata pollen the mitotic division of the generative nuoleus was almost completely inhibited. 4) In M. arvensis the generative nucleus did not divide into 2 sperm nuclei above 25 KR, while relatively many generative nuclei with chromosomes were observed at 50 KR. 5) The pollen grains of M, arvensis L. var. piperascens had the highest radioresistance. At 50-80KR the generative nucleus underwent complete mitotic division. 6) Radio-sensitivity of pollen grains varied widely according to the species. In general, their pollen-tube mitosis was much more radio-resistant than the meiosis in PMC's and mitosis in root tips.