Seiji Matsumura

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 Cs137 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 Co60 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.

RADIATION GENETICS IN WHEAT. VII. COMPARISON OF RADIATION EFFECTS OF BETA- AND GAMMA-RAYS ON DIPLOID WHEAT

Abstract Seeds of Triticum monococcum flavescens were soaked in 32 P and 131 I solutions for 2 days before sowing, to compare the radiation effects of beta-rays with those of gamma-rays. Radioactive solutions of pH 6–7 contained 0·05–0·8 mc/gr 32 P and 0·2–0·8 mc/g 131 I. For comparison, seeds soaked in water for 2 days were exposed to gamma-radiation with 60 Co at the dosages 2·5, 5, 10 and 20 kr. The growth of seedlings, height of mature plants, single-spike fertility, and chromosome aberrations of treated plants in X 1 and chlorophyll mutations in X 2 were compared for beta- and gamma-irradiation. The results of the first experiments in 1957/58 were generally confirmed by those obtained in 1958/59. The higher the dosage of beta- and gamma-rays, the more delayed were emergence and growth of seedlings and the lower were survival rate, height of mature plants, and fertility. The relation between the inhibition of seedling growth and dosage of beta- and gamma-radiations coincides roughly with that between the decrease of survival rate or fertility and dosage. There was no emergence of seedlings at 20 kr gamma-radiation and 0·8 mc/g 32 P beta-radiation. The effects of beta-radiation from 0·15–0·2 mc/g 32 P and 0·8 mc/g 131 I solutions correspond roughly to those of 2·5 kr gamma-radiation. As to chromosome aberrations and chlorophyll mutations, the effects of 2·5 kr gamma-radiation coincide roughly with those of 0·1 mc/g 32 P and 0·6–0·8 mc/g 131 I solution. If we assume that the effects of beta-radiation are confined only to the embryo, we find from calculation that the 0·2 mc/g 32 P solution equals about 2·4 krad. This will account for the present data.