Response of wheat growth, grain yield and water use to elevated <scp>CO</scp>₂ under a Free‐Air <scp>CO</scp>₂ Enrichment (<scp>FACE</scp>) experiment and modelling in a semi‐arid environment

<jats:title>Abstract</jats:title><jats:p>The response of wheat crops to elevated <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> (<jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub>) was measured and modelled with the Australian Grains Free‐Air <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> Enrichment experiment, located at Horsham, Australia. Treatments included <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> by water, N and temperature. The location represents a semi‐arid environment with a seasonal <jats:styled-content style="fixed-case">VPD</jats:styled-content> of around 0.5 <jats:styled-content style="fixed-case">kP</jats:styled-content>a. Over 3 years, the observed mean biomass at anthesis and grain yield ranged from 4200 to 10 200 kg ha<jats:sup>−1</jats:sup> and 1600 to 3900 kg ha<jats:sup>−1</jats:sup>, respectively, over various sowing times and irrigation regimes. The mean observed response to daytime <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub> (from 365 to 550 μmol mol<jats:sup>−1</jats:sup> <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub>) was relatively consistent for biomass at stem elongation and at anthesis and <jats:styled-content style="fixed-case">LAI</jats:styled-content> at anthesis and grain yield with 21%, 23%, 21% and 26%, respectively. Seasonal water use was decreased from 320 to 301 mm (<jats:italic>P</jats:italic> = 0.10) by <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub>, increasing water use efficiency for biomass and yield, 36% and 31%, respectively. The performance of six models (APSIM‐Wheat, APSIM‐Nwheat, CAT‐Wheat, <jats:styled-content style="fixed-case">CROPSYST</jats:styled-content>,<jats:styled-content style="fixed-case"> OLEARY</jats:styled-content>‐<jats:styled-content style="fixed-case">CONNOR</jats:styled-content> and <jats:styled-content style="fixed-case">SALUS</jats:styled-content>) in simulating crop responses to <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub> was similar and within or close to the experimental error for accumulated biomass, yield and water use response, despite some variations in early growth and <jats:styled-content style="fixed-case">LAI</jats:styled-content>. The primary mechanism of biomass accumulation via radiation use efficiency (<jats:styled-content style="fixed-case">RUE</jats:styled-content>) or transpiration efficiency (<jats:styled-content style="fixed-case">TE</jats:styled-content>) was not critical to define the overall response to <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub>. However, under irrigation, the effect of late sowing on response to <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub> to biomass accumulation at <jats:styled-content style="fixed-case">DC</jats:styled-content>65 was substantial in the observed data (~40%), but the simulated response was smaller, ranging from 17% to 28%. Simulated response from all six models under no water or nitrogen stress showed similar response to <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub> under irrigation, but the differences compared to the dryland treatment were small. Further experimental work on the interactive effects of <jats:styled-content style="fixed-case">eCO</jats:styled-content><jats:sub>2</jats:sub>, water and temperature is required to resolve these model discrepancies.</jats:p>