Water Deficit Modulates the CO2 Fertilization Effect on Plant Gas Exchange and Leaf-Level Water Use Efficiency: A Meta-Analysis

Elevated atmospheric CO 2 concentrations ([eCO 2 ]) and soil water deficits significantly influence gas exchange in plant leaves, affecting the carbon-water cycle in terrestrial ecosystems. However, it remains unclear how the soil water deficit modulates the plant CO 2 fertilization effect, especially for gas exchange and leaf-level water use efficiency (WUE). Here, we synthesized a comprehensive dataset including 554 observations from 54 individual studies and quantified the responses for leaf gas exchange induced by e[CO 2 ] under water deficit. Moreover, we investigated the contribution of plant net photosynthesis rate ( P n ) and transpiration rates ( T r ) toward WUE in water deficit conditions and e[CO 2 ] using graphical vector analysis (GVA). In summary, e[CO 2 ] significantly increased P n and WUE by 11.9 and 29.3% under well-watered conditions, respectively, whereas the interaction of water deficit and e[CO 2 ] slightly decreased P n by 8.3%. Plants grown under light in an open environment were stimulated to a greater degree compared with plants grown under a lamp in a closed environment. Meanwhile, water deficit reduced P n by 40.5 and 37.8%, while increasing WUE by 24.5 and 21.5% under ambient CO 2 concentration (a[CO 2 ]) and e[CO 2 ], respectively. The e[CO 2 ]-induced stimulation of WUE was attributed to the common effect of P n and T r , whereas a water deficit induced increase in WUE was linked to the decrease in T r . These results suggested that water deficit lowered the stimulation of e[CO 2 ] induced in plants. Therefore, fumigation conditions that closely mimic field conditions and multi-factorial experiments such as water availability are needed to predict the response of plants to future climate change.