| Effects of fast neutron radiation on Einkorn, Emmer
and bread wheats M. S. SWAMINATHAN and A. T. NATARAJAN Indian Agricultural Research Institute, New Delhi, India Seeds and seedlings of various crop plants are being treated with different chemical and physical mutagens in the course of the Mutation Research Programme in progress at the Indian Agricultural Research Institute. As a part of this work, dry seeds of Triticum monococcum (var. Japanese Early), T. dicoccum (var. Khapli) and T. aestivum (var. C. 591) were irradiated for 3 hours with fast neutrons of source strength 109NF/Cm2/Sec. over a 2pi solid angle at the cascade generator of the Tata Institute of Fundamental Research, Bombay, using the reaction d(Be9B10)Alpha. The seeds were germinated in petridishes; some seedlings were transplanted into pots and some were used for the study of somatic cell division, which was done in Feulgen root tip squashes prepared from material fixed in acetic-alcohol (1:3) 24, 48 and 96 hours after germination. Among the seedlings grown in pots, plant vigour and survival decreased in the following order: T. dicoccum. T. aestivum and T. monococcum. In material irradiated with thermal neutrons, Konzak and Singleton (1954) have reported a similar superiority in resistance of the tetraploid wheat. We have observed and many other authors have also reported, that for a given dose of X-ray of B radiation, thete is an increase in resistance with increasing polyploidy. The results with thermal and fast neutron radiation where the hexaploid shows a higher degree of sensitivity than the tetraploid are therefore different from those observed with the other radiations. The study of the effects on somatic cell division showed the occurrence of chromosome breaks at metaphase and dicentric bridges and fragments at anaphase in all the three species. The mean number of chromosome breaks per cell in the different species was as below:
A striking feature in the hexaploid wheat was the presence of a large number of cells (nearly 40%) with each one of the 42 chromosomes fragmented. Such extensive breakage was rare in the diploid and tetraploid wheats. The frequency of occurrence of breaks in different cells was found to follow a Poisson distribution in the case of the diploid and teraploid wheats, thus indicating that the formation of the chromosome break is unaffected by the presence or absence of other breaks in the cell. In the case of the hexaploid, however, the distribution of breaks did not fit into the Poisson pattern. It appears from the data that in the hexaploid wheat, the advantage of ploidy is offset by the occurrence of extensive chromosome breakage in a large number of cells. This may probably account for it being more sensitive to neutron radiation than the tetraploid. It appears likely that threshold of breakage may exist beyond which breakage at any one region may facilitate further breakage among the chromosomes. |
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