The phenomenon of mutant heterosis was studied in barley using F-1 hybrids developed from crosses involving two mutants derived from the same parent variety (067AR x 032AR), or from crosses of mutants with their parent varieties (Diva x 228DV; 437MG x Mg 4170). The agronomic performance (plant height, tillers per plant, kernel number and weight per plant) of F-1 hybrids was evaluated in relation to the corresponding parent variety used for mutant development. Wide-space sowing (0.06 m(2)/plant) was applied to allow the full expression of hybrid vigour. The hybrids significantly exceeded their parent varieties in various yield parameters in each year of the experiment. Depending on the cross, the level of heterosis ranged from 39.45% to 82.1% for kernel number and from 44.45% to 91.2% for kernel weight, as an average of observations over three years. The increased yield of hybrids was accompanied by an increase in spikes/plant, without significant changes, in most cases, in yield components such as kernels/spike and 1000-kernel weight. The height of F-1 plants remained similar to the corresponding parent variety. Doubled haploids (DH) were developed through anther culture from three F-1 hybrids to determine whether the heterotic effect could be fixed, even partly, in homozygous lines. Twenty-eight, 42, and 68 DH lines per cross were produced, and evaluated in the field, space-planted at 0.06 m(2)/plant, together with hybrids and parent varieties. The best-performing DH lines were evaluated again the following season in a conventional field trial. We were able to select 1.5-14.3% of 'F-1-performing' DH lines per heterotic cross combination, which, in two years of studies, reached the yield of heterotic hybrids and significantly out-yielded the parent variety in plot. The yield potential of DH lines was better expressed under a wide sowing density, but even under normal growing conditions, the yield of the best selected DH lines surpassed their corresponding parent varieties by 17.7-30.1%.
