Climate models predict an increase in atmospheric CO2 concentration and prolonged droughts in some parts of the Amazon, but the effect of elevated CO2 is still unknown. Two experiments (ambient CO2 -400 ppm and elevated CO2 -700 ppm) were conducted to assess the effect of drought (soil at 50% field capacity) on physiological parameters of Carapa. At ambient CO2 concentration, light-saturated net photosynthetic rate (PNsat) was reduced by 33.5% and stomatal conductance (gs) by 46.4% under

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... , but the effect of drought on PNsat and gs was nullified at elevated CO2. Total plant biomass and leaf area production were also reduced (42-47%) by drought. By changing leaf traits, Carapa is able to endure drought, as the consumptive use of water was reduced under drought (32-40%). The improvement of PNsat under elevated CO2 and water stress and the leaf plasticity of Carapa broaden our understanding of the physiology of Amazonian trees. Additional key words: chlorophyll fluorescence; leaf water potential; nonphotochemical quenching; water-use efficiency. use of water; ETRelectron transport rate; FC -field capacity; Fm -maximal fluorescence yield of the dark-adapted state; Fm' -maximal fluorescence yield of the light-adapted state; Fs -steady-state fluorescence yield; Fv/Fm -maximal quantum yield of PSII photochemistry; Fv -variable fluorescence; gs -stomatal conductance; Jmax -maximum electron transport rate; F0 -minimal fluorescence yield of the dark-adapted state; Jmax25 -Jmax at 25°C; LMA -leaf mass per area ratio; NPQ -nonphotochemical quenching; T -temperature in Kelvin; PN -net photosynthetic rate; PNsat -light-saturated PN; PNmax -light-and CO2-saturated PN; RH -relative humidity; TNC -total non-structural carbohydrates; Vcmax -maximum carboxylation rate of Rubisco; Vcmax25 -Vcmax at 25°C; VPDL -leaf-to-air vapor pressure difference; WT -total dry matter (biomass); WUEi -intrinsic water-use efficiency; Γ -CO2-compensation point; Γ* -Γ in absence of mitochondrial respiration; ΨL -leaf water potential; ФPSII -effective quantum yield of PSII photochemistry. Acknowledgments: To the Ministry of Science, Technology, Innovations and Communications (MCTIC/ INPA PRJ 15.120), National Council for Scientific and Technological Development -CNPq (Grant 302041/2015-0 and scholarship for MFO) and Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM) for supporting this research. We thank the Editor and the anonymous reviewers and for their constructive and useful comments.