Evaluation of a tiller inhibition (tin) gene in wheat

Reduced tillering cereals have been proposed as being advantageous under terminal drought conditions through their presumed reduction in leaf area and increased partitioning of assimilate towards fertile stems. The reduced leaf area should reduce preanthesis transpiration and conserve soil water for grain filling while the partitioning of a greater proportion of biomass into fertile stems should subsequently result in more efficient partitioning of assimilate towards grain. The tiller inhibition (tin) gene reduces the number of tillers produced by spring wheat plants and this study was undertaken to assess the agronomic potential of this gene in addition to investigating the physiological changes associated with it on a plant and crop basis. The inhibition of tiller bud growth by the tin gene and the phenomenon known as "stunting" are linked and the latter appears to be an extreme manifestation of the former whereby the development of the mainstem apex is retarded. High light intensity, long photoperiod and low minimum temperatures are required to induce stunting, while high C02 induces associated traits such as reduced leaf length. The accumulation of biomass did not appear to be significantly affected by the presence of the tin gene. The reduction in tiller number was compensated for by more assimilate being partitioned into those tillers that were produced. The presence of the tin gene resulted in higher harvest index values, indicating more efficient partitioning of biomass into grain, as well as larger spikes with more kernels spike-1 at low densities. However, as spike densities of the tin lines approached 450 spike m-2, the differences in the characteristics associated with the tin gene disappeared. Leaf area index was not reduced by the presence of the tin gene as plants were able to produce longer and wider leaves. Using epidermal cells as an example, the increase in leaf length was due primarily to an increase in the number, rather than the length, of the leaf cells. The maximum rate of cellular division was also increased by the presence of the tin gene. Yield was not significantly altered by the presence of the tin gene under most conditions where it was tested. There were however significant changes in the composition of yield with kernel weight increased by 5 to 6% for the spring lines containing the tin gene relative to the near-isogenic pairs, and although the number of kernels spike-1 was often greater for the tin lines, kernels m-2 was reduced due to lower spike densities. The application of nitrogen increased spike densities in the tin lines, but not to similar densities as those produced by the freely tillering near-isogenic pairs. Water extraction was the same for the lines with and without the tin gene under field conditions, except at one site at maturity where the tin lines extracted more than their near-isogenic pairs. Root length density appeared to be unaffected by the tin gene. The intrinsic benefits of the tin gene appeared to be higher harvest index values, shorter stems and higher kernel weights as these traits were exhibited in the tin lines when plants were grown to achieve the same spike densities as their near-isogenic pairs. Most other traits such as increased number of spikelets spike-1, increased number of kernels spike-1 and increased stem length density appear to be contingent on spike densities being lower for the tin lines. Stem water soluble carbohydrates were also as high or higher for the tin lines under field conditions when spike densities were low, although when grown to achieve the same spike density the levels were the same or lower in the tin lines than their near-isogenic pairs