The Importance of Water-Soluble Carbohydrates to the Maintenance of Grain Yield and Yield Quality in Durum Wheat (Triticum durum Desf.)

Abstract

<p>Species of wheat (<i>Triticum</i> spp.) have the ability to accumulate and remobilise large amounts of stem water-soluble carbohydrates (WSC) and direct them into grain formation. This constitutes a desirable trait to incorporate into germplasm when grown in regions with frequent terminal drought.While the accumulation and subsequent remobilisation of WSC is known to vary with variety, environment and cultural practices in bread wheat, little is known about the response of durum wheat (<i>Triticum durum</i> Desf).</p> <p>This research thesis investigated the importance of post-anthesis WSC in grain formation and final yield and quality in durum wheat in Australia. Four major experiments comprising three field and one greenhouse, investigated the historical, agronomic and genetic influences on anthesisWSCaccumulation and subsequent remobilisation for grain yield and quality.</p> <p>In the first field experiment, the historical changes in the anthesis WSC accumulation and post-anthesis remobilisation, based on the date varieties were released, was examined for 24Australiandurum wheat varieties. Anthesis WSC accumulation, post-anthesis remobilisation and mobilisation efficiencies have increased over the time since the first variety was released in 1956, with modern varieties accumulating more assimilates at anthesis, utilising more of the accumulated assimilates for grain formation and hence achieving higher mobilisation efficiencies than the older varieties.However, the gains in WSC accumulation utilisation did not contribute significantly to grain yield and quality which was in contradiction to the results published by other authors in bread wheat. Thelow or no contribution of WSC to grain yield and quality was attributed to favourable growing conditions and hence the absence of post-anthesis water stress.</p> <p>The second experiment examined the effect of irrigation on anthesis WSC accumulation and remobilisation in eight durum wheat varieties in the field. Irrigation significantly affected WSCdynamics with a reduction in irrigation water application increasing WSC accumulation anthesis,WSC mobilisation and remobilisation efficiency. However, correlation analysis between WSC and yield parameters found a non-significant or very weak relationship. The non-significant or low correlation suggests that irrigation and possibly high soil water availability post-anthesis reduced WSC mobilisation and hence WSCs made a lower contribution to grain yield and quality.</p> <p>In the third experiment, the genetic effect of reduced height (rht) genes on WSC dynamics in124durum wheat genotypes was examined in a field trial. Neither the widely used rht-1 nor the newer rht-18 gene, had any effect on WSC dynamics in durum wheat genotypes. Dwarfing genes were introduced into wheat during the Green Revolution, however, plant height was not affected by the gene type in this study. This contradicts the findings from other authors who reported significant differences in plant height as result of the different dwarfing genes. Correlations between WSC and yield components were either non-significant or very weak. The non-significant correlation between WSC parameters and yield parameters indicated that, yield gains were made irrespective of the contribution of WSC. NDVI and LAI readings revealed total ground cover was attained at a very early stage which is an indication of high germination rate and seedling vigour. As a result, the superior seedling establishment derived from the long coleoptiles present in the rht-18 lines did not play any role in final yield determination. It is concluded that, under favourable growing conditions,WSC dynamics has no effect on grain yield and quality and the rht-18 allele provided no superior yield benefits when compared to the rht-1 allele.</p> <p>The final experiment, conducted in the greenhouse, examined the effect of nitrogen and post-anthesis water stress on WSC remobilisation and grain yield and quality for selected durum wheat genotypes. There was a significant N × post-anthesis water stress interaction for WSCmobilisation efficiency, grain weight and thousand grain weight both in main tillers and the whole plant. Therewas a significantly lower WSC mobilisation efficiency in the high N × well-watered treatments as compared to the high N × drought stressed, low N × drought stress and lowN×well-watered treatments for both main tillers and the whole plant. This further supports the concept that under favorable growing conditions, the contribution of WSC to grain filling is reduced. The higher WSC mobilisation efficiency, which is an indication of plant stress, was associated with low, drought stress or a combination of both.</p> <p>The higher WSC mobilisation efficiency in the stressed treatments however did not translate to higher grain weights which is consistent with results from the three previous field-based experiments. Both grain weight and thousand grain weight were significantly higher in inhighN×well-watered treatments for both main tillers and the whole plant. The higher WSCmobilisation efficiency with corresponding lower grain weight and thousand grain weight may be attributed to the shortfall in concurrent assimilation for grain formation in stressed treatments. Even though the shortfall was catered for by higher remobilisation of stored WSC, the overall effect was far lower than the concurrent assimilation from photosynthesis in the unstressed treatments.</p> <p>In all experiments, there was high degree of variation in anthesis WSC accumulation, WSCutilisation and WSC remobilisation efficiencies due to treatment effects, with the exception being the right study in Chapter 4. However, these variations did not directly affect grain yield and quality.This study concludes that WSC may not play any active or direct role in yield formation and grain yield and quality in durum wheat under conditions of adequate resource supply. However, as reported in other studies this can be an important factor in drier environments and will become an important factor with global heating and the consequent effect in marginal growing regions.</p>