Elevated CO reduces the adverse effects of drought stress on a high-yielding soybean (Glycine max (L.) Merr.) cultivar by increasing water use efficiency.

Soybean (Glycine max (L.) Merr.) is the world's most important grain legume. The impacts of climate change such as elevated CO and drought on soybean physiological and morphological responses are not well understood. This study evaluated the effects of elevated CO (ambient concentration + 200 mmol mol) and drought stress (35-45% of relative water content) on soybean leaf photosynthesis, chlorophyll fluorescence, stress physiological indexes, morphological parameters, biomass and yield over 2 years at the open-top chamber (OTC) experimental facility in North China. We found that drought decreased intrinsic efficiency of PSII (F'/F'), effective quantum yield of PSII photochemistry (Φ), photochemical quenching coefficient (qP), and yield of soybean, increased nonphotochemical quenching (NPQ), peroxidase (POD), and malondialdehyde (MDA), but had no effect on superoxide dismutase (SOD) or soluble sugar content. Elevated [CO] increased net photosynthetic rate (P), water-use efficiency (WUE), Φ, qP, SOD, soluble sugar content and yield of soybean. Elevated [CO] enhanced the positive effects of drought on WUE, but reduced the negative effects of drought on Φ and qP. Elevated [CO] enhanced the resistance to drought by improving the capacity of photosynthesis and WUE in soybean leaves.