The effect of rht genotype and temperature on coleoptile growth and dry matter partitioning in young wheat seedlings

Coleoptile length in wheat (Triticum aestivum L.) can be affected by several factors, including genotype, height-reducing genes and environmental factors, including temperature. There is little information on how these factors influence rate and duration of coleoptile growth to determine final coleoptile length in wheat. Coleoptile growth was determined for eight genotypes representing four different height-reducing genes: gibberellic acid (GA)-sensitive, standard height (rht), GA-sensitive semidwarfs (Rht8); and GA-insensitive, semidwarfs (Rht2). These were grown in the dark at three temperatures (12, 16 and 20 degreesC) and coleoptile lengths measured every 12 degrees Cd. Logistic growth curves were fitted to coleoptile growth data for each genotype with thermal time as the explanatory variable. Differences in final coleoptile length were largely attributable to differences in rate of coleoptile elongation although there were small differences in duration of growth between genotypes. The longer coleoptile of the rht wheats was achieved through the fastest rate of coleoptile elongation. Coleoptiles of Rht8 wheats were equivalent in final length to rht wheats at 107 mm, but achieved this through a slower growth rate (2.10 mm degrees Cd-1) combined with an increased duration of growth (57 degrees Cd). In contrast, the shorter coleoptiles of Rht2 wheats resulted from 25% slower rates of elongation than either Rht8 or rht. There were no interactions between the components of coleoptile growth and temperature, although a longer duration and a fastest rate of growth combined to increase coleoptile length at 12 degreesC compared with either 16 or 20 degreesC. In a second experiment, dry matter partitioning and length of coleoptile, subcrown internode (SCI), shoot and roots were determined after 200 degrees Cd. In Rht2, the SCI and shoot were short while roots were longer than either Rht8 or rht. Reduced dry matter (DM) partitioning to the coleoptile and SCI and DM retention in the seed reduced the endosperm-use efficiency (EUE) of Rht compared with rht. EUE was poor also in Rht8, apparently through increased respiratory losses. Reduced partitioning of dry matter to coleoptiles and the SCI in Rht2 increased the root : shoot ratio compared with rht or Rht8. We conclude that either increased rate or duration of coleoptile growth could be targeted in a breeding program that aims to increase coleoptile length in wheat.