Carbon isotope discrimination and stomatal function in C4 grasses

Abstract

Water scarcity is projected to intensify over the next fifty years due to the increasing population and changing climate. Hence, efforts are channelled towards breeding crops with improved water-use efficiency (WUE) to mitigate the negative impact of the future insecurity of food, feed, and biofuel. Leaf-level crop improvement is one key target, but to reach maximal improvements, we will need to exploit the physiological, anatomical, and biochemical diversity of grasses, the family which includes the all-important cereal crops. The success of grass productivity can be partly attributed to a finely-regulated balance between CO2 assimilation (Anet) and transpiration through highly responsive stomata, as well as the evolution of a CO2 concentrating mechanism (CCM) - the C4 cycle. The CCM of C4 photosynthesis endows C4 grasses with higher leaf-level instantaneous (WUEi) and intrinsic water use efficiency (iWUE) than C3 counterparts. In C3 plants, iWUE is directly correlated with the carbon isotope composition (d13C) of the leaf dry matter through their separate relationships with a common parameter, the ratio of intercellular to ambient CO2 (Ci/Ca). However, leaf d13C and iWUE in C4 plants are not consistently correlated because of the CCM and factors including post-photosynthetic carbon isotope fractionation. The two main objectives of this thesis were to (1) investigate how stomatal responses regulate leaf iWUE by exploring the physiological mechanisms underpinning the highly responsive C4 grass stomata, and (2) elucidate the impact of post-photosynthetic fractionation, particularly carbohydrate metabolism, on leaf d13C. The two objectives serve the overall goal of better understanding the physiological factors that control leaf d13C, iWUE and their relationship in C4 plants. I utilised C3 and C4 grasses with different photosynthetic types and C4 biochemical subtypes (NADP-ME, NAD-ME, PCK) grown under different environmental conditions. The outcomes from this study will provide fundamental knowledge and understanding of stomatal and photosynthetic regulation of iWUE in C4 grasses, allowing for the development of screening tools for breeding crops with improved iWUE.

Date of Award	2019
Original language	English