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Increasing CO2 concentration ([CO2]) is thought to induce climate change and thereby increase air temperatures and the risk of drought stress, the latter impairing crop growth. The objective of this study was to investigate the effects of elevated [CO2] and drought stress on root growth of one maize genotype (Zea mays cv. Simao) and two sorghum genotypes (Sorghum bicolor cv. Bulldozer and Sorghum bicolor x Sorghum sudanense cv. Inka) under the cool moderate climate of Central Europe. It was hypothesized that root growth stimulation due to elevated [CO2] compensates for a reduced root growth under drought stress. Therefore, we established an experiment within a free-air carbon dioxide enrichment system (FACE) in 2010 and 2011. Sorghum and maize genotypes were grown under ambient [CO2] (385 ppm CO2) and elevated [CO2] (600 ppm CO2) and in combination with restricted and sufficient water supply. Elevated [CO2] decreased root length density (RLD) in the upper soil layers for all genotypes, but increased it in deeper layers. Higher [CO2] enhanced specific root length (SRL) of "Simao" and "Bulldozer," however, did not affect that of "Inka." "Simao" achieved a higher SRL than the sorghum genotypes, indicating an efficient investment in root dry matter. Although elevated [CO2] affected the root growth, no interaction with the water treatment and, consequently, no compensatory effect of elevated [CO2] could be identified.