Elevated CO2 alleviates adverse effects of drought on plant water relations and photosynthesis: a global meta-analysis

1. The elevated CO2 concentration (eCO2) is expected to improve plant water relations and carbon (C) uptakes, with a potential to mitigate drought stress. However, the interactive effects of eCO2 and drought on plant physiology and growth are not clear.

2. We performed a meta-analysis on the interactive effects of eCO2 and drought on plant water relations, photosynthesis, biomass production and allocation.

3. We found that eCO2 did not lead to conservation of soil water, but improved leaf water status under drought conditions as evidenced by a higher leaf relative water content and a less negative midday leaf water potential, resulting from reduced stomatal conductance (gs) and increased root to shoot ratio. Elevated CO2 retarded gs response to drought, which may be mediated by decreases in leaf abscisic acid concentration under eCO2 and drought. Drought imposed stomatal limitations on photosynthesis (A), which was alleviated by eCO2 via increasing intercellular CO2 concentration (Ci). This led to a stronger A response to eCO2 under drought, supporting the "low Ci effect". However, no interaction of eCO2 and drought was detected on plant biomass production. Intrinsic water use efficiency (iWUE) increased proportionally with eCO2, while plant-scale WUE was less responsive to eCO2 regardless of water availability. The advantages of eCO2 on C3 plants over C4 plants under well-watered conditions diminished under drought conditions. Within C3 plants, drought caused a greater reduction in biomass for woody plants than for herbs. Biomass declined progressively as drought prolonged for plants growing in both ambient CO2 and eCO2. The physiology and biomass of plants growing in pots showed more negative responses to drought than those growing in field. Biomass increase in free-air carbon dioxide enrichment experiments was significantly less than those in growth chamber and open top chamber experiments.

4. Synthesis. These findings suggest that eCO2 can alleviate the adverse impacts of drought on plant water relations and C sequestration, and are of significance in the prediction of plant growth and ecosystem productivity under global changes.