Elevated CO2 Enhances Leaf Water Use Efficiency in Tobacco Plants (Nicotiana tabacum 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.