Daily Changes of Amino Acids in Soybean Leaflets Are Modified by CO₂ Enrichment

The effects of CO₂ enrichment on plant growth and on nitrogen partitioning were examined using soybean [Glycine max (L.) Merr. cv. Clark] leaflets. Plants were grown from single seeds in matching controlled environment chambers. Continuous ambient CO₂ partial pressures were from 38 to 40 Pa and elevated CO₂ treatments were 68 to 70 Pa. Total above ground biomass, total leaf area and specific leaf weight of soybean were increased 78%, 58% and 33%, respectively, in response to CO₂ enrichment when measured 25 days after sowing. Total chlorophyll (a+b) was 25% greater in third trifoliolate soybean leaflets in response to CO₂ enrichment but total soluble protein did not differ between treatments. These and other measurements indicated that soybean plants were nitrogen sufficient in this study. Variations of total soluble amino acids were observed in soybean leaflets and these were enhanced by CO₂ enrichment when measurements were performed mid-day. However, concentrations of total amino acids were similar in both CO₂ treatments by end of the photoperiod. Glycine was lower in the elevated compared to the ambient CO₂ treatment suggesting that rates of photorespiration were diminished by elevated CO₂. Alanine increased 20% in response to CO₂ enrichment. Overall, changes of soluble amino acids in response to CO₂ enrichment were smaller than in other crop species and a temporal shift occurred in the daily accumulation of amino acids in soybean leaflets.