Increased Kernel Number in Norin 10-Derived Dwarf Wheat: Evaluation of the Cause

This study of the effect of major Norin 10 dwarfing genes (Rht1, Rht2) on kernel number in spring wheat emphasised three near-isogenic pairs of spring wheat, differing in the presence or absence of both genes. Plants were grown under controlled environment conditions with equal light intensities at their tops and were restricted to main stems by repeated tiller trimming. The dwarf wheats had a higher proportion of shoot dry weight in the spike at anthesis, an effect which field studies suggest is closely associated with their production of more kernels per unit land area. Stems of dwarf genotypes were somewhat shorter as early as 50 days before anthesis (i.e. before floral initiation) but relative differences became especially great commencing at 35 days before anthesis. Apex and spike lengths were never very different. A greater proportion of dry matter was distributed to leaf lamina from 50 days before anthesis until the end of lamina growth in dwarf wheats, while partitioning to stems was lower from 50 days before anthesis onwards. Partitioning to spikes was only higher in the 15 days preceding anthesis. Stem water-soluble carbohydrate (WSC) contents at anthesis were greater in dwarf wheats but maximum spike WSC contents reached at about 15 days before anthesis were lower. Spike morphogenesis including floret production and grain setting did not differ generally, except for a tendency with dwarf wheats for a longer floral initiation to anthesis interval and for more kernels per unit of spike weight at anthesis. The major unique feature of dwarf genotypes, the higher proportion of dry matter partitioned to the spike, appeared to be due to reduced competition from growing stems in dwarf wheats rather than intrinsic differences in the growth potential of their spikes.