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.