Genotypic increases in coleoptile length improves stand establishment, vigour and grain yield of deep-sown wheat

Timely sowing is critical for achieving high grain yields in winter cereals. However, inadequate seed-zone moisture for germination commonly delays sowing to reduce biomass and subsequent yield in semi-arid environments. Sowing deep to reach soil moisture is often avoided by growers of Rht-B1b and Rht-D1b semi-dwarf wheat as these wheat show poor emergence when sown deep. Their reduced cell elongation associated with insensitivity to endogenous gibberellins, results in shorter coleoptiles and smaller early leaf area. Alternative dwarfing genes responsive to endogenous gibberellins (e.g. Rht8) are available for use in wheat breeding. These reduce plant height without affecting coleoptile length and offer potential to select longer coleoptile wheat for deep sowing. Nine semidwarf (Rht8, Rht-B1b, and Rht-D1b) and seven tall (rht) wheat genotypes were sown at depths of 50, 80 and 110 mm at three locations in 2 or 3 years. Coleoptile lengths measured in a growth cabinet at four temperatures (11, 15, 19 and 23 °C) were strongly correlated with coleoptile length (r p = 0.77-0.79 **) and plant number (r p = 0.49 *-0.79 **) in deep-sown plots in the field. Furthermore, differences in coleoptile length were genetically correlated with greater numbers of emerged seedlings (r g = 0.97 **), shallower crown depth (-0.58 **), greater seedling leaf area (0.59 **) and seedling biomass (0.44 *). Wheat containing the Rht-B1b or Rht-D1b dwarfing genes produced significantly (P < 0.01) shorter coleoptiles (97 mm) than both Rht8 (118 mm) and tall (117 mm) wheat. In turn, compared with emergence from 50 mm depth, the Rht-B1b and Rht-D1b wheat produced significantly fewer seedlings at 110 mm sowing depth (-62%) than either Rht8 (-41%) or tall (-37%) wheat. Effects of deep sowing early in the season were maintained with reductions in spike number and biomass at both anthesis and maturity. Kernel number was also reduced with deep sowing leading to reductions in grain yield. Over all entries, genotypic increases in plant number were associated with increases in fertile spike (r g = 0.61 **) and kernel number (0.21 *), total biomass (0.26 *) and grain yield (0.28 *). Reduction in spike number and grain yield with deep sowing was smallest for the Rht8 (-18 and -10%) and rht (-15 and -7%) wheat, and largest for the Rht-B1b/D1b (-39 and -16%) wheat. Plant height and coleoptile length were independent among Rht8 and tall wheat genotypes. This study demonstrates the importance of good seedling emergence in achieving high wheat yields, and the potential use of alternative dwarfing genes such as Rht8 in development of long coleoptile, reduced height wheat suitable for deep sowing. © 2006 Elsevier B.V. All rights reserved.