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Tree growth enhancement under elevated [CO2] is much smaller than originally anticipated; yet carbon overabundance can lead to increased wood carbon storage and to stomata] downregulation and hence reduced water use. Notably, all three outcomes increase tree drought resistance. Here we studied growth, water relations, and nonstructural carbohydrates of 60 lemon saplings growing in CO2-controlled rooms at the same greenhouse, under 400, 650, and 850 ppm (CO2]. At each [CO2] level, 10 saplings were exposed to 1-month dry-down after 2 months of standard irrigation, followed by re-watering for another month. The other 10 saplings served as controls. Under drought, tree growth was maintained at elevated, but not ambient, CO2, linked with mild vs. severe tree water stress (leaf water potential of 3.5 at elevated and 5.5 MPa at ambient [CO2]). Stomatal downregulation with increasing [CO2] meant that leaf transpiration and diurnal plant water-use were 13-46% lower at elevated vs. ambient [CO2] but photosynthesis was still 15-25% higher. CO2-induced increases in root and shoot starch were transient but significant. Our results suggest that when predicting tree growth in a warmer and drier future, concomitant atmospheric CO2 concentration must be considered. In young lemon trees, elevated CO2 partially compensated for drought effects on tree growth and water status, and might delay some of the effects of the anthropogenic climate change.
