| Boron effects upon gamma-ray and thermal neutron irradiations
in Einkorn wheat; RBE of heavy particles from 10B(n, Alpha)7Li reaction
S. MATSUMURA, S. KONDO and T. MABUCHI National Institute of Genetics, Misima, Japan In order to study RBE for chromosome aberrations and gene mutations of (Alpha + 7Li) from the thermal neutron capture reaction 10B(n, Alpha)7Li, compared with gamma-rays, seeds of Triticum monococcum flavescens soaked in water and 0.1, 0.5 and 1.0% aqueous solutions of borax for 2 days were exposed to 60Co gamma - rays at the doses of 0.5, 1, 2 and 3 kr, and to thermal neutrons in JRR - 1, of which the integrated flux ranged from 1.3 to 10.4x 1011 nth/cm2 for 25 - 200 seconds (cf. WIS No. 13 : p 4). Chromosome aberrations in PMC's of the X1 plants and chlorophyll mutations in the X2 seedlings were investigated. The frequency of chromosome aberrations increased markedly with the increase of the dose of gamma - rays and thermal neutrons, and especially with increasing borax concentrations for the neutron treatments. Assuming that the chromosome aberration frequency versus gamma - ray dose curve is linear and independent of dose rate as well as of borax concentrations, the overall effective doses of thermal neutrons of 1.3 x 1011 and 5.2 x 1011 nth/cm2 can be converted to equivalent gamma - ray doses for the various borax concentrations. The frequency of chlorophyll mutations increased roughly in a linear relation to the dose of gamma - rays and thermal neutrons and also markedly with increasing borax concentrations only for thermal neutron treatments, as could have been expected. There was no significant boron effect on the mutation frequency induced by gamma - irradiation. Therefore, the data for gamma - treatments were pooled. They show a linear relationship between the mutation frequency and the dose of gamma - rays. The chlorophyll mutation rate per kr of gamma - rays is 1.5%. Assuming a similar linear relationship, frequency-dose curves were drawn for neutron treatments. Table 1 lists in line 4 the RBE doses for chromosome aberrations and chlorophyll mutations under various irradiation conditions calculated from dose-response curves mentioned above. Line 5 gives the increases in RBE dose due to the added boron, which were calculated by subtracting the corresponding RBE doses for the treatment at zero borax concentration. Line 6 gives the absorbed doses (rad per 1011 nth / cm2) due to neutron capture by added boron, which have been calculated by the first collision dose. The RBE values of the heavy particles listed in line 7 have been obtained by dividing the RBE dose values given in line 5 with absorbed dose values given in line 6. The RBE values obtained for chromosome aberrations in PMC's and for chlorophyll mutations were 23 + or - 10 and 29 + or - 10, respectively. These figures are of the same order as those for fast neutrons previously reported for wheat and barley. Most of the present results are explicable on the assumption that chromosome breakage in wheat requires many ionizations to occur within a chromosome and that the majority of radiation-induced chlorophyll mutations result not from point mutations but from chromosome breakage events. |