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Carbon dioxide and water are crucial resources for plant growth. With anthropogenic fossil fuel emissions, CO2 availability is and has been increasing since the last glacial maximum. Simultaneously water availability is expected to decrease and the frequency and severity of drought episodes to increase in large parts of the world. How plants respond to these two changes will help in understanding plants' responses to climate of the future. Here we sought to understand how drought affects plant traits responses to CO2 and whether there are trade-offs in responsiveness to low and elevated CO2 and drought. We grew seedlings of seven C-3 annuals at past low (160 mu l l(-1)), ambient (450 mu l l(-1)) and elevated (750 mu l l(-1)) CO2. At each concentration plants were subjected to well-watered conditions (100% soil water availability, SWA), 40% SWA or 20% SWA. We measured biomass allocation, relative growth rate, tissue N concentration, and gas exchange. Compared to well-watered conditions plant size was an important element in the absolute response to SWA decrease, i.e. the smaller, slow growing species were unaffected by drought at low CO2. Plants allocated less mass to root tissue at low CO2 contrasting with increased root mass fraction at lower SWA at ambient CO2. Across all traits measured, we found mostly additive effects of CO2 and water. As due to climate change regions become more drought prone these results suggest CO2 fertilization will not counteract the effects of reduced water availability.