The Methods for Determining the Mutation Frequency after Seed Irradiation in Rice

In mutation breeding, mutation frequencies are expressed as the proportion of X1 ears which segregate mutant progeny. Since mutations occur as sectors in X1 ear and the size of mutated sectors varies with increasing dose, the procedure used every ear for calculation of the mutation frequency might result in underestimates of mutation frequency. Using the method of the segregation frequency of mutants in mutated X1-ear-branch progenies, it could be shown that the primary branch was traced back to a single cell of rice seed embryo. Mutation frequencies were measured on the X1 ear, X2 plant or X1 ear-branch basis. The deviation from the linear relationship between dose and the frequency of mutations seemed to be less on the X1 ear-branch basis. Consequently, it was proposed to measure the mutation frequency per original treated cell as the proportion of primary branches of X1 ear which segregated mutant progeny. Furthermore, the mutation frequencies and the segregation ratios were less in the lower primary branches of X1 ear. It was supposed that even in haplontic selection the course of the elimination process was dependent on the environment in which the X2 seeds were produced.