The Effect of Chronic Low-Level Gamma Radiation on Polytrichum commune. I. Growth and Development of Gametophores from Irradiated Sporocytes and Spores

Abstract

Spores and gametophores of Polytrichum commune exhibited radio- resistance and what appeared to be a stimulatory response during the first 10-15 weeks following irradiation at 1,000, 1,600, 2,200, and 2,800 roentgens of chronic gamma radiation, especially those receiving the smallest dose. However, a de- layed inhibitory response was noted a few months after the plants were trans- ferred to soil. Sporocytes exposed to the same levels of radiation were markedly radiosensitive and produced atypical protonemal filaments and shoots with stunted gametophores at all four levels of radiation. Bryophytes and other nonvascular terrestrial plants are believed to be less sensitive to ionizing radiation than the vascular plants (Brodo, 1964; Pullum & Erbisch, 1972; Woodwell, 1967). However, little is known of the possible changes these nonvascular plants will undergo as the result of a gradual increase in the level of radiation. Exposing bryophytes to greater doses of radiation than they receive in their natural habitats can give some clue as to the kinds of changes, if any, that could occur in these plants in the future. Since changes in size or shape of organs might eventually result in the evolution of new taxa or in the extinction of a species, it would be interesting to know if, and to what extent, these nonvascular plants will undergo morphological and anatomical alteration if the level of radiation continues to rise in their environments. Polytrichum commune was selected for investigation because of its local abundance, apparent hardiness, large size (which facilitated handling), and because little information is available on the effects of ionizing radiation on this species. Snyder (1961) investi- gated the effects of ten specific doses of X-irradiation on the germination pattern of spores of P. commune. The dosage ranged from 100 to 5,000 r (roentgen), and the number of spores that germinated was inversely proportional to the dose. There was no inhibition of spore germination under 300 r, and the greatest decrease in germination and stunting of protonemal growth occurred at 1,000 r and higher. Five thousand r was lethal for all spores. Our investigation consisted of four parts. The first three dealt with the effects of irradiation on sporocytes, mature spores, and mature gametophores. Since P. commune plants have been observed to survive extreme drought (McCormick & Platt, 1962;