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

Abstract

1. 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.2. We performed a meta-analysis on the interactive effects of eCO 2 and drought on plant water relations, photosynthesis, biomass production and allocation.3. 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 wellwatered 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. 4. 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. K E Y W O R D S biomass, drought, elevated CO 2 , gas exchange, global change ecology, water potential, water use efficiency | 2837

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