Section 5 — Mutagenesis 



5.112. Effect of Gamma-radiation on Cytology of 

 Microsporogenesis in Pinus rigida. Francois 

 Mergen (New Haven, U.S.A.). 



A cesium-137 (9500 curies) source was placed 

 in a natural pitch pine-oak forest at the Brook- 

 haven National Laboratory. Beginning on No- 

 vember 22, 1961, the forest was exposed to gam- 

 ma-rays 20 hr each day. Pitch pine trees were 

 exposed to radiation levels ranging from a high 

 of 960 r/day to a level of normal background 

 radiation. Microsporangiate strobili were col- 

 lected at regular intervals to follow the effects of 

 gamma radiation on the cytology of micro- 

 sporogenesis. 



Trees receiving 82 r/day and higher were un- 

 able to resume cell division and differentiation 

 during the spring of 1962, while microspore 

 mother cells were formed on trees receiving 75 

 r/day. At 56 r/day meiosis with subsequent for- 

 mation of tetrads took place, and the highest 

 level at which mature microspores were collected 

 was 14 r/day. A slight reduction in the length of 

 the strobili was observed at 0.4 r/day, and at 

 3.7 r/day they were significantly shorter than the 

 controls. The percentage of cells with visible 

 chromosome aberrations increased with an in- 

 crease in exposure, e.g. 0.36 r/day = 1 per cent, 

 7 r/day = 2 per cent, 11 r/day == 12 per cent, 

 56 r/day == 76 per cent. Illustrations of these 

 chromosome aberrations are given. The data will 

 be published in full in Radiation Botany. 



5.113. Dependence of the Effect of Dose Rate on the 

 Physiological State of Irradiated Objects. N. I. 



Nuzhdin, R. L. Dozortseva and N. S. Sa- 



MOKHVALOVA (MOSCOW, U.S.S.R.). 



of forced and organic dormancy. The frequency 

 of cells with chromosome aberrations (bridges 

 and fragments) in the first mitoses of rootlet tips 

 was under study. The rootlets of non-irradiated 

 seeds of the same generation served as a control. 



The cytological study of the rootlets of seeds 

 irradiated in the state of forced dormancy showed 

 a definite difference in the frequency of cells 

 with chromosome aberrations. The irradiation 

 with high intensity led to the appearance of more 

 chromosome aberrations than the irradiation 

 with low intensity (4 1.46 ± 2.38 per cent and 30.79 

 ± 1.43 per cent for one strain and 20.16 d: 1.20 

 percent and 10.38 ± 0.82 per cent for the other, 

 respectively). 



The results of the irradiation of seeds in the 

 state of organic dormancy were not the same. 

 In the cells of the rootlets of seeds germinated 

 immediately after they were brought out from 

 the state of organic dormancy the chromosome 

 aberrations percentage was in fact the same for 

 the irradiation both with high and low intensity 

 (9.45 d: 1.56 per cent and 6.21 ± 1.5 per cent for 

 one strain and 11.97 ± 1.12 per cent and 10.51 : 

 0.97 per cent for the other, respectively). In the 

 seeds which were brought out from the state of 

 organic dormancy at the same time, but grown 

 after 10 days' storage in air-dry state the influence 

 of irradiation intensity is distinctly seen. The high 

 dose rate produces a higher yield of cells with 

 chromosome aberrations than the same dose but 

 of lower intensity (23.39 ± 1.36 per cent and 

 14.39 = 1.09 per cent, respectively). 



The data obtained indicate that irradiation- 

 induced chromosome damage cannot be con- 

 sidered as a mechanical effect or a direct result of 

 separate ionization events. They mainly depend 

 upon the physiological processes in the irradiated 

 cells. 



Sax (1939) made an attempt to explain the 

 difference in the frequency of exchange-type chro- 

 mosome aberrations when using the same dose 

 of different intensity by his hypothesis that this 

 difference is determined by not-identic possibility 

 of the broken chromosome ends recombination. 

 The results obtained by Lane (1951) and Koller 

 (1956) led to physiological explanation of nature 

 of the different effect of irradiation intensity. 



The hypothesis of Sax can be applied only in 

 the case of irradiation of dividing cells which 

 have active metabolism. In view of this peculiar- 

 ity we irradiated air-dry seeds of the Odessky-17 

 and Wintering Moskovsky strains of barley by 

 y-rays Co- &0 with doses of 12 and 10 kr. In the 

 1st case the dose rate was 423 and 55 r/min, in 

 the 2nd one 301 and 43 r/min. The irradiated 

 seeds were in two physiologically different states 



5.114. Genetic Advance in the Height of Plants In- 

 duced by Irradiation in Wheat. Katarina Bo- 

 rojevic (Novi Sad, Yugoslavia). 



The height of plant as a quantitative character 

 is controlled by polygenic system and it may be 

 changed by irradiation. For this reason the dry 

 seed of wheat was treated with the different 

 dosages of X rays and thermal neutrons. 



In the Ri both irradiations reduced the height 

 of plants from 1 to 32 per cent depending on the 

 dosage. The variability of this character was in- 

 creased only in the group treated with high 

 dosages. 



In the R 2 and R 3 the height of plants remain 

 reduced in all treated groups. Compared with 



96 



