Genetic response ofAntirrhinum majus to acute and chronic plant irradiation

Abstract

Clonal lines ofAntirrhinum majus heterozygous for flower color have been used to study somatic mutations induced by irradiating plants with chronic gamma rays from cobalt-60 and acute X rays. Pink or white flecks and sectors of various sizes on the purple flowers are interpreted as resulting from mutational events at different times in flower development. Acute X-ray treatment of pre-flowering stages caused no visible change in flowers opening prior to the 15th day thereafter. On the 16th to 19th days there was dense mottling, and by the 20th or 21st day numerous eight-cell mutant spots could be counted. The number of spots induced by 1,000 r of X rays was used to obtain a preliminary estimate of an average mutation rate for the three genes studied of 6.33×10−6 per cell per roentgen. Chronic irradiation in a gamma radiation field and greenhouse showed a linear response of mutation rate to daily dose rate, the actual rate being subject to seasonal variation. There were occasional indications of non-linearity but further tests have failed to show any significantly higher mutagenic efficiency of dose rates below 6 r/day. In plants removed from chronic gamma irradiation the frequency of mutation spots returns to the spontaneous level after about six weeks. The appearance, however, of mutant sectors on plants several months after exposure to both acute and chronic radiation indicates the induction of persistent genetic changes. Progeny tests of seeds and cuttings are planned to test the nature of these genetic events.