Effects of X-irradiations on Cosmos bipinnatus with particular reference to induction of somatic mutations (Preliminary report)

Abstract

For study of somatic mutations two clones of Cosmos bipinnatus were used. One was heterozygous both fcr flower color and flower-type, whiie the other was heterozygous for flower color only. Induction of somatic mutations was easily detected by the appearance of streaks with sharp outlines of different colors on a background of normal color of the ray florets. Plants of both the clones were irradiated at different stages of flower development, which was controlled by giving short day treatments for various periods of time following continuous long day treatment. The average height of plants decreased as a result of irradiation. The decrease in plant height increased with the increase in dose of irradiation. Narrow streaks to wide sectors of red, Iight pink, and' white colors were observed on a background of normal color of Clone I and mutant sectors of light pink and white colors were observed on a background of normal red color of Clone II. Mutation rate was calculated on the basis of percentage of flowers with mutations. It increased with the dose up to an optimum dose, after which a decrease in mutation rate was observed with the increase in dose. Highest mutation rate was observed after 30 days, in Clone I and after, 10 days short day treatment in Clone II at 2 Kr. Some difference in the two clones was noted as regards the stage at which niaximum :mutations were induced. The number of flowers having mutant sectors bigger than half the width of the ray floret was generally larger at the vegetative and younger stages of flower development than at the advanced stages. The number of mutations increased up to a few weeks after initiation of flowering, but later on it gradually decreased. Greater decrease in number of flowers with mutations was observed as more time elapsed between irradiation and flowering. But the proportion of flowers with bigger mutant sectors increased after greater lapse of time between irradiation and flowering. Flowers with physiological injuries and morphological abnormalities were observed on irradiated plants of both the clones.