Effects of free‐air CO2 enrichment and drought on root growth of field grown maize and sorghum

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 × 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 f ree‐a ir c arbon dioxide e nrichment 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.