Carbon Dioxide Control in an Open System that Measures Canopy Gas Exchanges

Atmospheric carbon dioxide concentration ([CO 2 ]) affects both C 3 carbon net assimilation (A) as well as crop water use. Methods for measuring whole canopy gas exchange responses under [CO 2 ] enrichment are needed for breeding programs aiming to develop crop cultivars resistant to stresses like drought in a future higher CO 2 world. Previously we developed and tested a portable, open transparent chamber system for measuring canopy gas exchanges. Here we describe further development of this system by adding the capability of controlling [CO 2 ]. Pure CO 2 injection into the system was accomplished with a data logger operated mass flow controller attached to a high pressure CO 2 gas cylinder. Across the full range of chamber air flow rates, [CO 2 ] enrichment controls were within ± 12 µmol mol −1 of the desired set point. Following an abrupt user‐selected change in chamber air flow rate, [CO 2 ] enrichment controls were re‐established within 3 to 5 min.