Maximum mutation frequency in gamma-ray and thermal neutron exposures in barley

The wide variation of 29-fold at maximum in sensitivity was detected when the seeds of 528 barley varieties were exposed to gamma ray. A major gene controlling radiosensitivity was detected. The correlation between radiosensitivity and chlorophyll mutation frequency was not observed, therefore, the varieties with less resistance showed lower maximum mutation yield. Much higher exposure than that ordinarily applied gave proportionally higher mutation yield. The observation of the PMC of M1 plants revealed that reciprocal translocation was induced at high frequency reaching 38 % at maximum in 40 kR or higher exposure. The selection of fertile M1 spikes eliminated effectively transmissible sterility except the highest exposure of 50 kR. The use of very high dose followed by the selection of fertile spikes in M1 may be useful for obtaining rare mutants. The relation of mutation frequency to seed sterility in M1 was similar in the exposures to gamma ray and thermal neutrons. The highest frequency of mutation in neutron treatment was higher than that in 60 kR of gamma ray. The B-10 enrichment of seeds did not heighten the maximum mutation yield in thermal neutron exposure, but conspicuously elevated mutation frequency per unit flux. (Kako, I.)