Radiation-induced somatic mutations and the loss of reproductive integrity in Tradescantia stamen hairs

Abstract

Inflorescences of Tradescantia (clone 02), a diploid hybrid heterozygous for flower color, were given a 16-hr exposure (25–500 R) of 60Co γ-rays from a 3200 Ci source. Morphological abnormalities induced in stamen hairs were noted for a period of 20 days and records kept of the frequency and position of mutant cells and mutant events and of the killing effect (loss of reproductive integrity) in the hairs. The results show that these effects are highly dependent upon the developmental stage of the hair at the time of exposure. Giant cells and abnormally shaped cells were induced both apically and interstitially at the higher levels of radiation exposures. Branching of the hairs was also observed. Such effects occur very rarely in unirradiated flowers. An increased number of mutant pink cells per stamen appeared about six days postirradiation. They increased with increasing exposure and time, reached a peak about 14 days after treatment, then decreased in number up to 20 days. The frequency of mutant events (single cell or multicellular mutations) per stamen reached the highest value for each exposure at about 10 or 11 days after treatment, leveled off, and declined after about 13 days. This indicates that at the time of irradiation these buds contained the maximum number of meristematic stamen hair cells. Mutation rate varied with exposure and time, but at 11 days after irradiation with 100 R it was estimated to be 5·29 × 10 −4 per R per meristematic cell. The percentage of stunted hairs (those that had lost their reproductive integrity) per stamen showed an increase with increasing exposure. But at exposures up to 300 R some sort of recovery phenomenon was noticed. A typical exponential survival curve was found with a D 37 of 270 R and and extrapolation number of 1·81. Exposures of 200–300 R are optimal for producing mutations and are not sufficiently high to induce lethality to an extent which would mask the mutation sensitivity of stamen hair cells. The results thus indicate that there exists, over a limited time and within a certain exposure range, an approximately linear relationship between radiation exposures and the frequencies of mutant cells and of mutant events.