Elevated CO ₂ Enhances Leaf Water Use Efficiency in Tobacco Plants ( <scp> <i>Nicotiana tabacum</i> </scp> L.) Under Water Deficit by Modulating Stomatal Behaviour

ABSTRACT The frequency and intensity of drought are projected to increase globally, threatening plant growth. However, the role of elevated CO 2 (e[CO 2 ]) in plant drought recovery, particularly with respect to photosynthesis and water use efficiency (WUE), remains unclear. The study investigated the impact of e[CO 2 ] (800 μmol mol −1 ) on the gas exchange parameters, stomatal morphology and distribution, and leaf tissue structure of tobacco plants under various water levels (full irrigation [FI], mild water deficit [MI], moderate water deficit [MO] and severe water deficit [SE]) within growth chambers. The results revealed that stomatal limitations significantly reduced CO 2 assimilation under MI to MO, whereas non‐stomatal limitations became more prominent under more severe deficit. Moreover, the instantaneous water use efficiency ( WUE n ) and intrinsic water use efficiency ( WUE i ) of tobacco plants grown under e[CO 2 ] were approximately twofold greater than those of plants grown under ambient [CO 2 ] (a[CO 2 ]). Moreover, e[CO 2 ] led to distinct adjustment strategies in response to varying water availability, thereby alleviating the adverse effects of water deficit on plants. These findings indicate that e[CO 2 ] enhances leaf water use efficiency in tobacco plants by modulating stomatal behaviour and increase our understanding of the potential mechanism through which the CO 2 fertilization effect improves plant drought resistance.