Carbon dioxide fertilization effect on plant growth under soil water stress associates with changes in stomatal traits, leaf photosynthesis, and foliar nitrogen of bell pepper (Capsicum annuum L.)

•Elevated CO2 concentration increases the plant growth and leaf photosynthesis, even under water stress conditions.•This stimulated leaf photosynthesis at higher CO2 concentration is associated with stomatal diffusion processes under water stresses.•Elevated CO2 concentration enhances water use efficiency under water stresses.•This CO2 fertilization effect under water stress associates with stomatal traits, leaf photosynthesis, and foliar carbohydrates. Understanding the processes and mechanisms of crops in response to elevated CO2 concentration and water stress is critical for accurately projecting the potential risk of climate change on global agriculture productivity. We examined effects of elevated CO2 concentration on plant growth, stomatal traits, leaf gas exchange, and biochemistry of bell peppers under water stresses with environmental growth chambers controlling CO2 concentration at ambient (400 μmol mol−1) or elevated (800 μmol mol−1) CO2 level along a soil water gradient including full irrigation (75–85 % FC), mild water stress (65–75 % FC), moderate water stress (55–65 % FC), and severe water stress (45–55 % FC). We found that rising atmospheric CO2 concentration dramatically enhanced the plant biomass of bell peppers through the strong CO2 fertilization effect even under the mild and moderate water stresses. The CO2 fertilization effect on plant growth under water stress was directly confirmed by the boosted leaf photosynthesis, which can be attributed to the increased stomatal number and the larger stomatal openness as well as the more regular spatial distribution patterns of bell pepper plants grown at high CO2 concentration and water stress. As a result, the leaf-level water use efficiency was enhanced due to the stimulated leaf photosynthesis and the reduced leaf transpiration under high CO2 concentration, even for these bell pepper plants are suffering water stresses. Nevertheless, this CO2 fertilization effect on plant growth and leaf photosynthesis were generally decreased along the soil water gradients, indicating that the CO2 fertilization effect may partially be mitigated or even offset by water availability as evidenced by the little impact of high CO2 concentration on the total biomass of pepper plants under severe water stress. Our results suggest that water stress may lower the CO2 fertilization effect on plant growth of bell pepper plants, thus current ecological processes models based on the strong CO2 fertilization effect may