HETEROSIS, OVERDOMINANCE FOR GRAIN-YIELD, AND ALPHA-AMYLASE ACTIVITY IN F1 HYBRIDS BETWEEN NEAR-ISOGENIC RHT DWARF AND TALL WHEATS

The Rht-B1b, Rht-D1b and Rht-B1c alleles for reduced height in wheat ( the Norin 10 and Tom Thumb dwarfing genes previously known as Rht1, Rh t2 and Rht3) were exploited in combinations to generate a near-continu ous range of plant heights, from 53 cm to 123 cm, amongst near-isogeni c homozygotes and F-1 hybrids. Pleiotropic yield effects of Rht genes were measured in both homozygous (intravarietal) and heterozygous (int ervarietal) genetic backgrounds. Heterosis due to overdominance of Rht genes was detected among intravarietal hybrids. The effects of hetero zygosity at other genetic loci (mean dominance) were determined, indep endently of Rht effects, from comparisons between intravarietal and in tervarietal F-1 hybrids. Genotypes of intermediate plant heights gave maximum yields, in agreement with other trials of the homozygous lines , so that heterosis (hybrid exceeding best parent) for Rht yield effec ts was observed in crosses between tall and dwarf isogenic pairs. This heterosis combined additively with increased mean weight per grain in intervarietal crosses, generating the highest overall grain yields in hybrids with semi-dwarf stature in heterozygous genetic backgrounds. The Rht-B1c allele showed single-gene overdominance for grain yield, a lso the production of alpha-amylase in ripening grains of Maris Huntsm an was effectively inhibited in the Rht-B1a/c intravarietal hybrid. Th e Rht-B1c allele thus offers advantages for both grain yield and grain quality in the heterozygous condition and should be considered as an alternative to the conventional semi-dwarfing genes Rht-B1b and Rht-D1 b for F-1 varieties in environments conductive to preharvest sprouting .