Limitations to soybean photosynthesis at elevated carbon dioxide in free-air enrichment and open top chamber systems

• Limitations to soybean photosynthesis at elevated CO2 were assessed in free-air and open top chamber systems by in situ leaf gas exchange measurements in one location.
• Photosynthesis at high light was limited by VCmax in both exposure systems on all days and temperatures sampled.
• Photosynthesis measured at low light was down-regulated at elevated CO2, with no net stimulation by elevated CO2.

It has been suggested that the stimulation of soybean photosynthesis by elevated CO2 was less in free-air carbon dioxide enrichment (FACE) systems than in open top chambers (OTC), which might explain smaller yield increases at elevated CO2 in FACE systems. However, this has not been tested using the same cultivars grown in the same location. I tested whether soybean photosynthesis at high light and elevated CO2 (ambient + 180 μmol mol−1) was limited by electron transport (J) in FACE systems but by ribulose-bisphosphate carboxylation capacity (VCmax) in OTC. FACE systems with daytime and continuous CO2 enrichment were also compared. The results indicated that in both cultivars examined, midday photosynthesis at high light was always limited by VCmax, both in the FACE and in the OTC systems. Daytime only CO2 enrichment did not affect photosynthetic parameters or limitations, but did result in significantly smaller yields in both cultivars than continuous elevation. Photosynthesis measured at low photosynthetic photon flux density (PPFD) was not higher at elevated than at ambient CO2, because of an acclimation to elevated CO2 which was only evident at low measurement PPFDs.