Agronomic assessment of the durum Rht18 dwarfing gene in bread wheat

The wheat Green Revolution Rht-B1b and Rht-D1b dwarfing alleles are associated with increased grain yields but also with reduced early growth and seedling emergence, especially if sowing conditions are unfavourable. The gibberellic acid-responsive, mutagen-derived Rht18 dwarfing gene was backcrossed from durum wheat (Triticum turgidum subsp. durum L.) cv. Icaro into tall bread wheat (Triticum aestivum L.) cv. Halberd using phenotypic selection for reduced plant height. The Rht18 allele was confirmed among homozygous BC1F2-derived, F-5:7 recombinant inbred lines by using a chromosome 6AS-linked, microsatellite molecular marker (Xwms4608), and then assessed for agronomic performance across multiple field sites ranging in yield from 3.6 to 6.4 t/ha. The Rht18-containing lines were significantly (P < 0.05) shorter in height (-24%) and reduced in plant lodging (-51%) compared with tall sister lines. Reductions in plant height were associated with significant increases in grain yield (+16%), reflecting increases in grain number (+21%), number of spikes (+7%) and number of grains per spike (+12%). Coleoptile length, early shoot biomass and ground cover percentage were unaffected by the presence of the Rht18 dwarfing gene. Comparisons of effects of gibberellic acid-insensitive Rht-B1b and Rht18 on early growth and agronomic performance were assessed separately for a set of 30 BC5F6-derived Halberd near-isogenic lines in the field in 2015. Ground cover and coleoptile length were significantly greater for Rht18 lines, whereas plant height, lodging, harvest index, grain number and yield were similar for Rht-B1b and Rht18 sister lines. Reduced lodging and increased grain number and yield, together with greater coleoptile length, indicate a potentially useful role for Rht18 in improving wheat performance.