The application of carbon isotope discrimination to wheat improvement

Liam Dixon

Among the many factors that reduce wheat (Triticum aestivum L.) productivity, drought stress is considered the single greatest threat worldwide. Rising temperatures coupled with unpredictable precipitation patterns expected under future climate scenarios may exasperate the issue. For these reasons, there is a critical need to develop drought tolerant varieties. In conjunction with selection for yield under dry conditions, breeding programs advance drought tolerance by selecting for physiological traits beneficial to yield in target breeding environments. As a proxy for stomatal conductance and photosynthetic capacity, carbon isotope discrimination (∆) has been identified as a potentially valuable drought tolerance trait in certain environments. The utility of grain ∆ as a selection criterion was evaluated for target breeding environments of Washington State. Across the five field location-years, ∆ demonstrated an overall mild relationship to yield with variation in this relationship among high and low precipitation environments. This, coupled with low heritability and limited prospects for improving selection efficiency, contributed to the conclusion that direct selection for grain ∆ would be ineffective for Washington State breeding programs. However, an analysis of high yielding low and high ∆ genotypes in each location-year led to the proposal that grain ∆ may be useful as a relatively high-throughput screening tool for the identification of more specific drought-tolerance traits, including water use behavior (conservative and rapid water use), deep roots, reliance on stem reserve carbohydrates, and photosynthetic capacity. These traits are challenging to evaluate directly, making an indirect, albeit less accurate, measurement valuable. As a novel use for grain ∆, this application requires verification. Genotypes suspected of particular traits based on ∆ analysis may be tested for those traits. While methodologies exist for determining root architecture, reliance on stem reserves, and photosynthetic capacity, methods for studying water use behavior are less established. To address this knowledge gap, a gravimetric method capable of monitoring plant transpiration under various water conditions was developed. In total, this thesis makes a modest contribution to the development of drought tolerant varieties in Washington State, and will hopefully be beneficial to a broader audience of researchers working on similar issues.

The application of carbon isotope discrimination to wheat improvement

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