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

Elevated atmospheric CO₂ concentrations ([eCO₂]) 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 CO₂ 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[CO₂] under water deficit. Moreover, we investigated the contribution of plant net photosynthesis rate (P ) and transpiration rates (T _r) toward WUE in water deficit conditions and e[CO₂] using graphical vector analysis (GVA). In summary, e[CO₂] 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[CO₂] 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 CO₂ concentration (a[CO₂]) and e[CO₂], respectively. The e[CO₂]-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[CO₂] 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.