Biomass Accumulation and C N Partitioning in Soybean Plants in Response to Drought Stress and Elevated Atmospheric CO2 Concentration

Elevated carbon dioxide (e[CO₂]) promotes plant photosynthetic activity and growth, which mitigates the adverse effects of drought. The ability of soybean to fix nitrogen (N) from the air may sustain plant N nutrition under e[CO₂], and thus may influence carbohydrate metabolism. This study systematically analysed the interaction mechanism between elevated CO₂ concentration and drought stress on the regulation of carbon and nitrogen metabolism in soybean. In this study, the effects of e[CO₂] and aridity on the activity of carbohydrate-metabolising enzymes, carbon (C) and N partitioning in soybean plants were investigated. The findings indicated that e[CO₂] enhanced leaf biomass by 66% (Drought stress/DS) and 31.6% (Well-watered/WW) in comparison to plants raised under ambient CO₂, but decreased stem biomass by 28.6% (DS) and 35.5% (WW), with no effect on root biomass. e[CO₂] stimulated dry matter (18%DS, 16% WW) and C (17% DS, 16% WW) partitioning into leaf, whereas drought decreased it. Phosphofructokinase and sucrose synthase activity in leaves were increased in response to e[CO₂], especially for the drought-stressed plants. e[CO₂] depressed the leaf cytoplasmic invertase and cell wall invertase activities, while drought stress reversed such effects. In addition, e[CO₂] reduced leaf and stem N concentration, especially in well-watered plants, but elevated root N concentration under drought. Drought had little effect on N partitioning, while e[CO₂] increased the N partitioning to leaf (17% DS, 15% WW) and root (3% DS, 2% WW). The PCA plot further indicated there is a link between some of the C-catalysing enzyme activity and dry matter partitioning in soybean plants subjected to e[CO₂] and aridity treatment. These findings suggest that the alleviation of drought in soybean plants under e[CO₂] is mainly achieved by promoting root nitrogen nutrition distribution and leaf carbohydrate synthesis pathways. These observations provide a greater understanding of the adaptation to future elevated CO₂ and drought environments in soybean plants.