Frequency and spectrum of mutations induced by radiations in some varieties of bread wheat

1. The frequency and types of mutations induced by different dosages of X-rays, fast and thermal neutrons and beta particles from P32 and S35 were studied in two fully awned (C. 591 and H. 167) and two awnless strains (N.P. 809 and H. 389) of bread wheat. 2. Phenotypic changes affecting only some of the tillers of a plant occurred in the M1 generation. Speltoid-normal chimeras were the most frequent. From the fact that the induced change affected the whole ear and never part of an ear, it is suggested that each ear arises either from a single initial cell or a few cells which have been affected alike. 3. Albina mutations occurred in the M2 progenies of C. 591 but not in the others. From the varietal variation in the incidence of chlorophyll mutations, it seems likely that there are differences in the number of genes controlling chlorophyll development in different varieties. 4. Similar types of viable mutations occurred in all varieties, the speltoid mutation being predominant. Though a large number of viable mutations was recovered, they belonged to only a few types. 5. No awnless types were found in the M2 and subsequent progenies of C. 591 and H. 167. Fully awned mutations occurred readily in the awnletted variety N.P. 809. In crosses with the control, the awned mutants gave monogenic segregation. It is suggested that the awned mutants of N.P. 809 arise from either the deletion or inactivation of the awn suppressor gene B1. The completely awnless variety H 389 has two epistatic genes, B1 and B2a. While half-bearded mutants caused by the deletion of B1 occur frequently in the M2 and subsequent generations a fully bearded mutant was found in the M3 progeny of a family which segregated for half bearded mutations in M2. Awning can thus be readily induced in awnless varieties of bread wheat. 6. The breeding behaviour of all the mutations isolated in M2 was followed. Speltoids either bred true or segregated into normal and speltoid plants. Compactoids mostly segregated into compactoid, sub-compactoid and normal plants but true-breeding compactoids were also found. Erectoids, plants with adventitious spikelets, multiple carpel mutants and other variants seldom bred true but gave rise to a few plants of the parental phenotype in their progenies