ANALYTICAL BIOCHEMISTRY 254, 272–282 (1997) ARTICLE NO. AB972416 The Simultaneous Measurement of Low Rates of CO 2 and O 2 Exchange in Biological Systems J. R. Willms,* A. N. Dowling,* Z. M. Dong,* S. Hunt,* B. J. Shelp,† and D. B. Layzell* *Department of Biology, Queen’s University, Kingston, Ontario K7L 3N6, Canada; and †Department of Horticultural Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada Received June 16, 1997 ing tissues. When electrons from water are used in An instrument for measuring low rates of biological photosynthetic CO 2 fixation resulting in O 2 production, O 2 exchange using an open-flow gas analysis system is or electrons from carbohydrate metabolism reduce O 2 described. A novel differential O 2 sensor that is capa- to H 2 O in respiratory O 2 consumption, the exchange of ble of measuring as little as 0.4 Pa O 2 against a back- O 2 and CO 2 in each case is both equal and opposite. ground of ambient air (20,900 Pa O 2 ), yet has a dynamic However, if there is reductant flow to electron sinks range of {2000 Pa O 2 (i.e., {ca. 2% O 2 ) is described. other than CO 2 fixation or O 2 uptake, an imbalance in Baseline drift was typically less than 0.025 Pa min 01 . the rate of CO 2 and O 2 exchange occurs. These alterna- The differential O 2 sensor was incorporated into a res- tive electron sinks may include oil or protein synthesis, piratory quotient/photosynthetic quotient analyzer the reduction of nitrate in roots or leaves of plants, or that contained other environmental sensors for atmo- the fixation of N 2 gas in aerobic bacteria or legume spheric pressure, absolute O 2 and CO 2 concentration, nodules. Sources of reducing power for respiration temperature of the differential O 2 sensor block, and other than carbohydrate (oil, protein, organic acids) differential pressure between reference and sample may also result in an imbalance in the exchange of O 2 streams. Protocols for how these sensors can be used and CO 2 . to calibrate the differential O 2 sensor and to improve The term, respiratory quotient (RQ) is widely used as its stability with time are described. Together, the dif- a metabolic index for animals or nonphotosynthesizing ferential O 2 sensor, the environmental sensors, and the plant tissues and is calculated as simple calibration techniques allow for simultaneous, noninvasive, and accurate measurements of O 2 and RQ Å0CER/OER, [1] CO 2 exchange in tissues with metabolic rates as low as about 0.1 mmol O 2 or CO 2 h 01 . Example data are pro- vided in which O 2 differentials of 3 to 41 Pa O 2 were where CO 2 is produced (i.e., CER is positive) and O 2 is measured in an open-flow system.  1997 Academic Press consumed (i.e., OER is negative). An RQ of less than 1.0 indicates either catabolism of substrates that are more highly reduced than carbohydrates (e.g., fat, pro- tein) or anabolism of products that are less reduced than carbohydrates. RQ values greater than 1.0 indi- The rate of O 2 exchange (OER) 1 and CO 2 exchange cate organic acid metabolism or the synthesis from car- (CER) from biological systems can provide valuable in- bohydrates of products that are highly reduced, and RQ formation concerning the flow of reducing power in liv- values around unity reflect carbohydrate metabolism. The term photosynthetic quotient (PQ) is used in 1 Abbreviations used: CA, infrared analyzer measuring absolute studies of photosynthetic organisms in the light, and CO 2 (Pa); CER, CO 2 exchange rate (mmol h 01 ); DW, dry weight; FW, is calculated as fresh weight; MV, mixing volumes; OA, sensor for absolute O 2 con- centration (kPa); OD, sensor for differential O 2 concentration (Pa) between the reference and analytical gas streams; OER, O 2 exchange PQ Å0OER/CER, [2] rate (mmol h 01 ); PA, sensor for atmospheric pressure (kPa); PD, sen- sor for differential pressure (Pa) between the reference and analytical where O 2 is evolved (i.e., OER is positive) and CO 2 is gas streams; PQ, photosynthetic quotient; RQ, respiratory quotient; TO, sensor for temperature (°C) of the OD sensor block. fixed (i.e., CER is negative). Photosynthetic production 272 0003-2697/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.