| All the five untreated seeds germinated normally but manifested severe
necrosis three weeks after their emergence; only six out of ten treated
seeds germinated. These plants grew normally and did not show necrosis till
the flag-leaf stage, when very sligh yellowing of leaves was observed. The
hybrid nature of the plants was established by the seed color of M1
plants which was red like the male parent. This indicates that either both
the severe alleles present in these varieties were inactivated or changed
from severe to very mild form. The latter hypothesis seems to be more plausible
as the necrosis was observed when the M2 generation material
(five plants) was screened. The degree of necrosis varied from mild to moderate
type. Observations recorded on the number of plants showing necrosis (Table 1) indicated that a ratio fitting 5 Necrotic: 11 Normal in case of families from three plants while the other two gave abberant ratios. NARULA, SRIVASTAVA and SRIVASTAVA (1970) have shown that if the phenotype of the F1 hybrid does not express necrosis, the segregation pattern in the F2 generation conforms to 5 necrotic: 11 normal, because more than two dozes as against two of the F1 are necessary for the manifestation of hybrid necrosis in certain weakly necrotic crosses. It, therefore, gives an indication that the severe alleles for necrosis present in these test varieties have been transformed through irradiation into weak types. SHARMA (1969) who followed the same approach of precluding hybrid necrosis and obtained chimeras at tiller and at leaf level, in addition to normal plants, which set adequate seeds for growing M2 generation, also observed the same phenomenon of higher mutational rate. If these loci be really so highly mutable, it may be interesting to ascertain whether there is any spontaneous mutation in the natural population, which may account to some extent for varietal impurity mentioned by HERMSEN (1963). This approach to preclude hybrid necrosis makes it feasible for the plant breeders to manipulate cross-combinations hitherto not possible without resorting to time-consuming and devious means. Authors are grateful to Drs. H. K. JAIN, M. V. RAO and D. P. MISRA for their keen interest in these studies. Literature Cited ANAND, S. C., B. S. GILL and R. P. JAIN 1969. Necrosis in intervarietal crosses of wheat. Indian Jour. Genet. 29 : 131-134. HERMSEN, J. G. Th. 1963. The genetic basis of hybrid necrosis in wheat. Genetica 33 : 245-287. TSUNEWAKI, K. and H. KIHARA 1962. Comparative gene analysis of common wheat and its ancestoral species. I. Necrosis. Jap. J. Genet. 37 : 474-484. NARULA, P. N., O. P. SRIVASTAVA and P. S. L. SRIVASTAVA 1970. Genetics of hybrid necrosis in bread wheat. Indian Jour. Genet. SHARMA, D. 1969. Use of radiations for breaking hybrid necrosis in wheat. Euphytica 18 : 66-70. ZEVEN, A. C. 1966. Geographical distribution of genes causing hybrid necrosis in wheat. Euphytica 15: 281-284. (Received March 13, 1970) |
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