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

Abstract The elevated CO 2 concentration (eCO 2 ) is expected to improve plant water relations and carbon (C) uptakes, with a potential to mitigate drought stress. However, the interactive effects of eCO 2 and drought on plant physiology and growth are not clear. We performed a meta‐analysis on the interactive effects of eCO 2 and drought on plant water relations, photosynthesis, biomass production and allocation. We found that eCO 2 did not lead to the conservation of soil water, but improved leaf water status under drought conditions as evidenced by a higher leaf relative water content (LRWC) and a less negative midday leaf water potential, resulting from reduced stomatal conductance ( g s ) and increased root to shoot ratio. Elevated CO 2 retarded the response of g s to drought, which may be mediated by the decrease in leaf abscisic acid concentration under eCO 2 and drought. Drought imposed stomatal limitations on photosynthesis ( A ), which was alleviated by eCO 2 via increasing intercellular CO 2 concentration ( C i ). This led to a stronger A response to eCO 2 under drought, supporting the ‘low C i effect’. However, no interaction of eCO 2 and drought was detected on plant biomass production. Intrinsic water use efficiency (iWUE) increased proportionally with eCO 2 , while plant‐scale WUE was less responsive to eCO 2 . C 3 plants had advantages over C 4 plants in terms of A and biomass production under eCO 2 and well‐watered conditions rather than under eCO 2 and drought conditions. Drought caused a greater reduction in biomass for woody plants than for herbs. Plants growing in pots showed greater decreases in the physiology and biomass under drought than those growing in field. Synthesis . These findings suggest that eCO 2 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.