Screening for increased coleoptile length in semidwarf wheat

The short coleoptiles of Rht-B1b and Rht-D1b semidwarf wheats can contribute to poor seedling establishment when sowing seed at depth or into stubble. Alternative dwarfing genes are available to reduce plant height but which allow selection for increased coleoptile length. Response to selection for genes conferring greater coleoptile length requires that phenotypic variation for coleoptile length is largely attributable to genotypic differences and not to sampling variation that would otherwise reduce heritability. The influence of sampling on coleoptile length was investigated for a range of ear-position and environmental factors commonly encountered when screening breeding populations for coleoptile length. Changes in floret position within the spikelet produced significant (p <0.01) changes in coleoptile length but these were attributable to differences in kernel size, not floret position per se. Seed source effects were small except for weather-damaged seed where reductions in falling numbers were associated with small yet significant (p <0.05) reductions in coleoptile length. Differences in coleoptile length among varieties were substantially larger than for effects associated with either grain size or seed production source. Standard height (rht) and gibberellic acid (GA)-sensitive Rht8/9 semidwarf wheat varieties produced coleoptiles that were between 25 to 40% longer than GA-insensitive Rht-B1b and Rht-D1b semidwarf varieties. Furthermore, increasing mean soil temperature from 11 to 19 °C reduced coleoptile length by 20% in rht and Rht8/9 wheats and by as much as 50% in Rht-B1b and Rht-D1b semidwaffs. Our results indicate that selection for increased coleoptile length can be made readily in a wheat-breeding program which includes GA-sensitive parents.