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SUMMARYEight dwarf and semi-dwarf varieties, covering a range of genetic sources of dwarfism, and four tall varieties of wheat were tested for their response to a range of GA concentrations by release of the enzyme -amylase. The two dwarfs, Minister Dwarf and Tom Thumb, were found to be insensitive compared with the remaining 10 varieties; among the latter, differences in minimum levels of sensitivity were found that were unrelated to height phenotype.Transplant experiments, in which embryos of one genotype were grown on the endosperm of another, showed that the lack of response was a function only of the endosperm tissue and that GA release from the embryos of insensitive varieties during germination was similar to that of sensitive genotypes.

SUMMARYThe major genes for dwarfism in Norm 10 wheat, Rht 1 and Rht 2 are located on chromosomes 4A and 4D. The gene for extreme dwarfness from Tom Thumb, Rht 3, is located close to, or is allelic with Rht 1 on chromosome 4A. An F2 telocentric mapping technique has been employed to locate these genes by using their gibberellin insensitive phenotype. Gai/Rht 1 and Gai/Rht 3 were estimated to be 13 map units from the centromere on the arm of chromosome 4A and Gai/Rht 2 15 map units from the centromere on the long arm of chromosome 4D. These results suggest that the genes are part of a homoeoallelic series.

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