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F₃ Yield Response to F₂ Selection for Gibberellic Acid Insensitivity in Eight Wheat Crosses¹

The hypothesis that the Gai/Rht alleles have a positive pleiotropic effect on grain yield was investigated by examining the F₃ yield responses to F₂ seedling selection for GA₃ insensitivity within eight wheat(Triticum aestivum L.) crosses. Seedlings were liberally sprayed with a 100 ppm solution of GA₃ (Berelex) at the three-leaf stage. Seven days after spraying they were visually sorted into two groups, those that were insensitive and those responsive to applied GA₃ as expressed by elongated internodes and a general etiolated appearance. Significant F₃ yield increases were obtained in response to F₂ seedling selection for GA₃ insensitivity in two out of four crosses with the Gai/Rht 1 genotype and three out of four crosses with the Gai/Rht 2 genotype. In seven out of the eight crosses, the highest yielding F₃ lines came from the GA₃ insensitive populations, but the large Least Significant Differences that resulted from the use of two replicate randomized complete block yield experiments meant the differences among lines were generally not significant. Selection for GA₃ insensitivity was simple and rapid, however, the visual classification into only two groups in this study meant that the so-called GA₃ sensitive group contained plants heterozygous for GA₃ insensitivity. Actual measurement of the height to the second leaf node was the most reliable means of selection for GA₃ response among seedlings. Selection for reduced plant height at the F₂ seedling stage resulted in subsequent F₃ yield increase because of the positive pleiotropic effect of the Gai/Rht genotype on yield. The technique can be integrated with any breeding methodology.

Key Words: Triticum aestivum L. • Pleiotropism • Dwarfing genes

² Geneticist, Victorian Crops Research Institute, Private Bag 260, Horsham, Victoria 3400, Australia, and senior associate, Faculty of Agriculture and Forestry, Univ. of Melbourne, Parkville, Victoria 3052, Australia.

Received for publication April 11, 1980.