Carbon isotope discrimination and stomatal function in C4 grasses
Water scarcity is a perennial dilemma which is projected to intensify over the next fifty years due to an increasing population, frequent droughts associated with climate change, and water mismanagement. Globally, 80-90% of water is used in agriculture  and in Australia, 75% of agricultural water usage is allocated towards irrigation. However, water resources such as river basins and groundwater are nearing/exceeding their extractable limits  leading to major impacts on crop yield, resulting in a world food crisis. To alleviate this crisis, we need to improve crop resiliency to drought and this can be achieved by improving water-use efficiency (WUE).
Grasses account for the majority of food production throughout the world and also dominate the semi-arid and canopy understory of Australia. Almost 47% of the grass species assimilate CO2 from the atmosphere using C4 photosynthesis , a variant of the common C3 photosynthesis. C4 plants use a carbon concentrating mechanism (CCM) which enables them to achieve higher CO2 fixation per unit of water loss translating to better WUE than their C3 counterparts, mainly because it enables them to suppress a wasteful process called photorespiration . Amongst grasses, rapid stomatal response to water limitation is an additional factor that has contributed to their success and productivity . Therefore, C4 photosynthesis and advanced stomatal control within grasses is an imperative combination to pursue for the optimisation of leaf WUE.
The major substrate of photosynthesis (CO2) exists in two stable isotopes (13CO2 and 12CO2) each with slight difference in atomic mass and abundance, with the former being heavier and scarcer. Breeding crops with improved WUE has been successful by examining the integrated ratio (13C/12C) of the two naturally occurring stable isotopes of carbon incorporated during photosynthesis . This technique has provided high throughput selection of wheat (a C3 grass) varieties with improved WUE, namely “Drysdale” and “Rees”, and are now used commercially in Australia . While this method is straightforward among C3 grasses, CCM within C4 grasses makes it difficult to implement. Thus, examining the isotope signatures (13C/12C) of major leaf metabolites could give insights into the fluxes of 13C within the C4 CCM, and ultimately aid the understanding of WUE mechanisms among C4 grasses.
The goals of my PhD project are to (1) better understand the link between stomatal responses and leaf WUE by exploring the physiological mechanisms underpinning highly responsive C4 grass stomata, particularly species native to Australia; and (2) investigate the role of 13CO2 and its redistribution in C4 leaf biochemical processes during and after photosynthesis.
The outcomes of my PhD will provide critical knowledge elucidating stomatal regulation of grasses and biochemical controls of carbon fluxes within the C4 leaves, allowing for the development of screening tools for breeding crops with improved WUE.
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Assoc. Professor Oula Ghannoum (HIE-WSU), Assoc. Professor Zhong-Hua Chen (SSH-WSU), Dr. Hilary Suart-Williams (RSB-ANU)