Radiation genetics in wheat—IX: Differences in effects of gamma-rays and 14 MeV, fission and fast neutrons from Po-Be

Abstract

Dry seeds of Triticum monococcum were exposed to Cs 137 γ-rays or to monoenergetic 14 MeV neutrons obtained from the T ( d, n) reaction in a neutron generator. For both radiations the higher the dose, the more delayed the seedling growth and the more reduced both survival rate and seed fertility. However the frequency of chlorophyll mutations increased with increase in dose. These results are in good accord with those obtained in our earlier work with 14 MeV neutrons provided by the Oak Ridge National Laboratory, U.S.A. (ORNL). (11,12) From the data from both experiments the RBE of 14 MeV neutrons vs. γ- and X-rays was found to be about 10 for germination rate, 13 ~ 15 for seedling growth and about 15 for seed fertility and chlorophyll mutation rate. The fission neutron (average 1–5 MeV) irradiations of dry seeds at three ploidy levels were conducted in the ORNL reactor and parallel γ-irradiations from Co 60 were also carried out at ORNL. In general, T. monococcum ( n=7) was the most sensitive to fission neutrons and γ-rays. There was, as expected, no significant difference between T. durum ( n=14) and T. vulgare ( n=21). The higher the dose of fission neutrons and γ-rays, the more delayed were germination and seedling growth, and the more reduced was seed fertility. As expected, the higher the ploidy, the lower the frequency of chlorophyll mutations. For seedling height the RBE value of fission neutrons to that of γ-rays was found to be 10 ~ 15 in the three ploidy series. For seed fertility the RBE was about 10 for diploids and about 50 for tetra- and hexaploids and for chlorophyll mutations an RBE of 25 or more was determined for di- and tetraploids. In general, the RBE value was higher for the characters observed at maturity than in earlier stages, especially in polyploids. Fast neutrons of mean energy 4·7 MeV (16) were obtained from a Po-Be source. Doses of 75 ~ 125 rad were not markedly effective in inhibiting seed germination and seedling growth or decreasing survival and seed fertility in T. monococcum, but were effective in inducing chlorophyll mutations. It is concluded that these neutrons are more effective than 14 MeV and fission neutrons, and that the RBE value for chlorophyll mutation frequency is about 40 against γ-rays.