Water Deficit Modulates the CO(2) 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.