| Summary: | Global climate change will modify plants in terms of growth and physiology. To better understand the consequences of this effect, the responses of the leaf water relations and nitrogen (N) use efficiency of barley and tomato plants to elevated CO<sub>2</sub> (<i>e</i>[CO<sub>2</sub>], 800 ppm) combined with progressive drought stress at two levels of N supply (N1, 0.5 g N pot<sup>−1</sup> and N2, 1.0 g N pot<sup>−1</sup>) were studied. The plants were grown in two separate phytotrons at ambient CO<sub>2</sub> (<i>a</i>[CO<sub>2</sub>], 400 ppm) and <i>e</i>[CO<sub>2</sub>], respectively. The leaf physiological parameters as well as carbon (C) and N concentrations were determined; plant growth, water and N use efficiencies were evaluated. The results showed that <i>e</i>[CO<sub>2</sub>] increased photosynthesis and water use efficiency (WUE) while decreased specific leaf area (SLA) in both species, whereas N supply level differentially influenced WUE in barley and tomato plants. The abscisic acid (ABA)-induced stomatal closure during progressive soil drying varied between the two species where the stomatal conductance (g<sub>s</sub>) of barley plants was more sensitive to leaf ABA than tomato plants, though CO<sub>2</sub> environment did not affect the response in both species. Compared to <i>a</i>[CO<sub>2</sub>], <i>e</i>[CO<sub>2</sub>] reduced plant transpiration rate (T<sub>plant</sub>) in barley but not in tomato. <i>e</i>[CO<sub>2</sub>] increased the leaf C:N ratio ([C:N]<sub>leaf</sub>) in plants by enhancing leaf C concentration ([C]<sub>leaf</sub>) in barley and by dilution of leaf N concentration ([N]<sub>leaf</sub>) in tomato, respectively, but N2 substantially decreased [C:N]<sub>leaf</sub>, and thus, N treatment was the dominant factor controlling [C:N]<sub>leaf</sub>. Collectively, appropriate N supply may modulate the acclimation of plants to <i>e</i>[CO<sub>2</sub>] and soil water deficits. This study provides some novel insights into N management of different plant species for adapting to future drier and CO<sub>2</sub>-enriched environment.
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