| The RBE values of different characters were calculated roughly from Table
1. For seedling height the RBE value of fission neutrons to that of
gamma-rays was found to be 10-15. It was calculated for seed fertility as
about 10 for diploids and as about 50 for tetra- and hexaploids and also
for chlorophyll mutation as 25 or more in diand tetraploid. In general,
the RBE value was lower for the characters observed in earlier stages than
at maturity, especially in polyploids. 3. gamma-rays and 14 MeV neutrons on einkorn wheat Dry seeds of Triticum monococcum were exposed in August 1963 to monoenergetic 14 MeV neutrons from T (d, n) reaction neutron generator in the Hiroshima University by courtesy of Dr. H. YOSHINAGA. The neutron intensity was 9 x109 neutrons/sec on average for 94 - 281 min, excluding gamma-ray contamination of less than 10 per cent. For comparison gamma-ray exposure was carried out simultaneously with 3.1 kr/hr for 102-510 min with the 6,000 curie 137Cs source in our institute. Germination, survival in the field, seed fertility and chlorophyll mutation were investigated. The results of the present experiment are roughly in good accord with those of our earlier works (MATSUMURA 1961, 1964) with 14 MeV neutrons obtained from 3H (d, n) 4He reaction produced in the 250 kV Cockcroft-Walton accelerator in the Biology Division, Oak Ridge National Laboratory. The data summed up from both experiments are given in Table 2. The RBE of 14 MeV neutrons to that of gamma and X-rays was calculated from the table. It was found to be abont 10 for germination rate showing a 10 % depression as compared with the control and about 13 for seedling height showing a 25 % depression. Also the RBE value was calculated as about 15 for seed fertility showing a 30 % depression and for chlorophyll mutation rate amounting to 5 %. It was generally lower for characters observed in earlier than in later stages, at maturity. According to the above mentioned experiments with fission and fast neutrons from Po-Be, it is assumed that they are more effective than 14 MeV neutrons, and also that fast neutrons (Po-Be) are clearly the most effective. In general, the RBE of fast neutrons is clearly larger for higher plants than for animals and microorganisms, and it increases with increasing LET (linear energy transfer). The extraordinary large RBE values for mutations in higher plants strongly indicate, as also other evidences do (STADLER 1932, 1954, MATSUMURA et al. 1963. KONDO 1964), that the majority of ionizing - radiation induced mutations are due to chromosome aberrations or gross deletions. |
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