Steady-state Canopy Gas Exchange: System Design and Operation

Commercial growth chambers can be altered to measure wholeplant and canopy CO 2 fluxes, and the modifications greatly improve traditional growth studies.Measurement of CO 2 fluxes can provide a nondestructive measurement of daily growth, as well as photosynthetic and respiratory efficiency.Recent advances in instrumentation have significantly improved photosynthesis measurements, and the techniques have been refined in small-scale (single-leaf), gas-exchange systems.Although whole-plant applications in larger growth chambers have developed slowly, our knowledge has increased dramatically in the last decade.For example, two major books on controlled environments in the late 1970s did not refer to the use of growth chambers for photosynthesis measurement (Langhans, 1978;Tibbitts and Kozlowski, 1979).Methods used to measure photosynthesis in canopies are similar to those used in single-leaf, gas-exchange systems, and it is useful to understand these principles before scaling-up to canopies.Singleleaf, gas-exchange techniques, devices, construction details, calibrations, and calculations have been reviewed in several recent articles (Field and Mooney, 1990a, 1990b;Field et al., 1989;Parkinson and Day, 1990).McCree (1986) described a system for whole-plant gas exchange in the laboratory; canopy systems for the field were reviewed by Garcia et al. (1990) and Reicosky (1990).Apparent biological responses in gas-exchange systems are often caused by changes in instrument calibration and electronic noise (which will be discussed later).Thus, it is necessary to fully understand both instrument and organism responses.Gas-exchange components must be calculated from several simultaneous measurements, and it is imperative to understand the instrumentation from which these measures and components are derived.Several books provide a solid background in measurement fundamentals related to gas exchange (Coombs et al., 1985;Goel and Norman, 1990;Hashimoto et al., 1990;Marshall and Woodward, 1985;Pearcy et al., 1989).This paper describes the use of a commercial growth chamber for canopy photosynthesis, respiration, and transpiration measurements.The system was designed to measure transpiration via water vapor fluxes, and the importance of this measurement is discussed.Procedures for continuous measurement of root-zone respiration are described, and new data is presented to dispel myths about sources of water vapor interference in photosynthesis and in the measurement of CO 2 by infrared gas analysis.Mitchell (1992) has described the fundamentals of various approaches to measuring photosynthesis.Because our system evolved from experience with other types of single-leaf and canopy gas-exchange systems, it is useful to review advantages and disadvantages of different systems as they apply to various research objectives. Closed systemsClosed systems require the least instrumentation but cannot be used to measure steady-state photosynthesis.The CO 2 concentration