The relationship between developmental stages of flower-buds and somatic mutations induced by acute X- and chronic gamma-irradiation in Cosmos bipinnatus

Abstract

Experiments were conducted on the effects of acute X-, and chronic γ-irradiation of different stages of flower development in Cosmos bipinnatus with particular reference to the induction of somatic mutations. Clone I was heterozygous both for flower colour and flower type, while Clone II was heterozygous for flower colour only. Stage of flower development was controlled by giving short-day treatment prior to irradiation. Plants were irradiated with 2 kR of X-rays at the vegetative stage and after 10, 20 and 27 days of short-day treatment. For chronic irradiation two groups of growing plants of Clone I were transplanted to the gamma field, and received a total exposure of about 3 and 2 kR of γ-rays. Somatic mutations appeared as narrow streaks to wide sectors of red, light pink and white colours on a background of normal colour of the ray florets. The maximum number of flowers with mutations was observed in Clone I after 20 days of short-day treatment. The relative frequency of flowers with colour mutations varied with the clone and stage of flower development at the time of irradiation. Narrower sectors of mutated colour with a higher frequency were observed at advanced stages of flower development; wider sectors of mutated colour with a lower frequency were induced at vegetative and younger stages of flower development. The number of mutations decreased as greater time elapsed between irradiation and flowering but the frequency of mutant sectors of larger size increased. Various types of physiological injuries were observed. The most commonly observed morphological abnormality was the dissection of ray florets. Mutation rate was very low after chronic γ-irradiation as compared with the acute X-irradiation.