The effects of 60Co gamma radiation on seeds, seedlings and callus tissue cultures of Phaseolus vulgaris L. var. Manitou were compared. Tissues were subjected to various doses (0·5-40 kilorad) of gamma radiation at 200 rad/min and subsequently grown for 4-8 weeks. Seeds irradiated with 10-12 krad produced stunted seedlings which later exhibited delayed flowering, and poor seed set; although the seeds germinated, no normal seedlings were formed at 15 krad. However, when 8-day-old seedlings were irradiated, severe inhibition of growth occurred at 2 krad, followed by degeneration of the apical meristem and death at 5 krad. Callus grown in suspension cultures in a modified White's medium supplemented with 2, 4-D kinetin and yeast extract showed significant stimulation of growth (P< 0.01) at 0.5 krad followed by a gradual decrease from 1 to 10 krad. Cells in such cultures showed mitotic inhibition, degeneration of cytoplasm, increased polyploidy and reduced plating efficiency. Callus growth at 20-30 krad was drastically reduced and ceased completely at 40 krad. The color of the callus continued to darken with increasing doses. There is a striking difference in the radiosensitivity of seeds, seedlings and callus tissue; the young seedlings are most radiosensitive, followed by seeds, and callus tissue is most tolerant. © 1970 Pergamon Press.
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