| Summary: | Leaf-level water use efficiency (<i>WUE<sub>i</sub></i>) is often used to predict whole plant water use efficiency (<i>WUE<sub>wp</sub></i>), however these measures rarely correlate. A better understanding of the underlying physiological relationship between <i>WUE<sub>i</sub></i> and <i>WUE<sub>wp</sub></i> would enable efficient phenotyping of this important plant trait to inform future crop breeding efforts. Although <i>WUE<sub>i</sub></i> varies across leaf age and position, less is understood about the regulatory mechanisms. <i>WUE<sub>i</sub></i> and <i>WUE<sub>wp</sub></i> were determined in Australian (cv. Krichauff) and UK (cv. Gatsby) wheat cultivars. Leaf gas exchange was measured as leaves aged and evaluated in relation to foliar abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylic acid (ACC) concentration, chlorophyll content and Rubisco activity. Carbon dioxide (CO<sub>2</sub>) assimilation (<i>A</i>) declined more rapidly as leaves aged in the lower <i>WUE<sub>wp</sub></i> genotype Gatsby. Both ACC concentration and Rubisco activity declined as leaves aged, but neither explained the variation in <i>A</i>. Further, stomatal conductance (<i>g<sub>s</sub></i>) and stomatal sensitivity to ABA were unchanged as leaves aged, therefore <i>WUE<sub>i</sub></i> was lowest in Gatsby. Maintenance of <i>A</i> as the leaves aged in the Australian cultivar Krichauff enabled greater biomass production even as water loss continued similarly in both genotypes, resulting in higher <i>WUE<sub>wp</sub></i>.
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