Developmental patterns of growth have been studied in nine spring and 10 winter wheat cultivars and in a number of crosses involving both groups. Among the spring cultivars five responded to vernalization while four did not. There was a very wide range in responsiveness to vernalization among the winter cultivars, ranging from the responsive Winter Minflor to Jones Fife which appeared not to respond at all to the '30-day vernalization test'. The spring habit of growth was governed by three dominant genes, any one of which was able to inhibit the expression of the winter habit. Progress has been made in establishing relationships among several spring cultivars. Those carrying the gene Sk, either alone or in combination with others, appeared to be non-responsive to vernalization. In the absence of Sk all spring cultivars so far tested exhibited a positive response. Winter selections made from spring-winter crosses always resembled the winter parent with respect to the intensity of the winter characteristic. While all winter wheats carry recessive alleles at all three loci, the differences in expression which exist between them appear to have been due to the presence of multiple recessive alleles at these loci. Further evidence of the association of leaf and spikelet numbers with days to ear emergence is presented.
Earlier studies of the genetics of winter-spring habit in wheat, in conjunction with the present studies, have led to the determination of the genotypes of eleven spring cultivars. Four spring cultivars each possessed two inhibitors which are the subject of further study. One notable cultivar -WW15, known to have a very high yield potential -possesses the single inhibitor Vm,, making it only one step removed from the winter wheats.Evidence is presented for the existence of at least one additional inhibitor, the identity of which is currently under investigation.
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