Agronomy Journal • Volume 106, Issue 3 • 2014 789 Notes & Unique Phenomena Carbon Dioxide Control in an Open System that Measures Canopy Gas Exchanges Jeffrey T. Baker,* Dennis C. Gitz III, Paxton Payton, Katrina J. Broughton, Michael P. Bange, and Robert J. Lascano Published in Agron. J. 106:789–792 (2014) doi:10.2134/agronj13.0450 Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. ABSTRACT 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. J.T. Baker, USDA-ARS, Plant Stress and Water Conservation Laboratory, 302 West I-20, Big Spring, TX, 79720; D.C. Gitz, P. Payton, and R.J. Lascano, USDA-ARS, Plant Stress and Water Conservation Laboratory, 3810 4th Street, Lubbock, TX 79415; K.J. Broughton and M.P. Bange, CSIRO Plant Industry, Locked Bag 59, Narrabri, 2390 NSW, Australia. Received 24 Sept. 2013. *Corresponding author (Jef.Baker@ars.usda.gov). Abbreviations: A, net assimilation; CETA, Canopy EvapoTranspiration and Assimilation chamber; [CO 2 ], atmospheric CO 2 concentration; ET, evapotranspiration; T, transpiration. In general, C 3 plants usually respond to elevated [CO 2 ] with an increase in A and a reduction in transpiration (T). However, there is evidence to suggest that plants evolved and adapted to the low [CO 2 ] of the earth’s past and this may con- strain plant responses to current and future projected increases in [CO 2 ] (Sage and Coleman, 2001). Indeed, interspecifc variability among rice ( Oryza sativa L.) cultivars in response to [CO 2 ], (Ziska and Teramura, 1992; Ziska et al., 1996; Baker, 2004; Baker et al., 2005) point to the potential of selecting or breeding elite cultivars that may be highly responsive to anticipated future levels of atmospheric [CO 2 ]. Ideally, current breeding programs that select for resistance to stresses like drought in water-limited areas of the world would beneft from facilities for controlling [CO 2 ]. Because of a lack of correlation between single-leaf A and crop yield, extensive eforts to breed crop cultivars with high leaf-level A failed to result in the release of higher-yielding crop cultivars (Nelson, 1988) and in fact even led to some cultivars with lower yield potential (Evans, 1990). In contrast with leaf-level A, whole canopy A does correlate with crop biomass production and it is generally agreed that it is this parameter that needs to increase to achieve future yield enhancements (Peng et al., 2000). While reports on leaf-level gas exchanges are relatively common, experimental systems that can measure whole canopy A and water use in situ are less common. Fewer still are experimental systems that measure whole canopy A and T or evapotranspiration (ET) while simultaneously controlling [CO 2 ]. Canopy scale gas exchanges have been measured using several micrometeorological approaches including Bowen ratio energy balance, as well as weighing lysimeters and a rather wide array of chamber systems of varying sophistication. Field chambers can be broadly classifed as either closed or open systems. Closed system chambers are typically transiently sealed or placed over the canopy for brief periods and canopy A and ET are determined from the loss of chamber atmospheric CO 2 and the increase in chamber air H 2 O, respectively (cf. Steduto et al., 2002; Perez-Priego et al., 2010). Open or fow-through chambers measure canopy gas exchange from the di ferential between incoming and outgoing gas concentrations and by measuring air fow rate through the system (cf. Burkart et al., 2007; Müller et al., 2009). Examples of experimental systems capable of measuring whole canopy gas exchanges while simultaneously controlling [CO 2 ] include open-top feld chambers (Leadley and Drake, 1993; Ham et al., 1995), horizontal fow-through feld chambers such as the one described in this paper, whole-tree chambers (WTC; Barton et al., 2010) and naturally sunlit Soil, Plant, Atmosphere Research (SPAR) facilities (Baker et al., 1990; Reddy et al., 2001). Whole canopy gas exchange measurements using open system feld chambers have also been Published February 28, 2014