Submitted to: Photosynthesis Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2001
Publication Date: 1/1/2002
Citation: Bunce, J.A. 2002. Sensitivity of infrared water vapor analyzers to oxygen concentrations and errors in stomatal conductance. Photosynthesis Research. 71:273-276. Interpretive Summary: Infrared analyzers are used to measure stomatal conductance, a characteristic of plant leaves which affects their rate of water loss and their rate of photosynthesis. Measurements of stomatal conductance are important in analyzing crop adaptations to the environment. The potential for large errors in estimates of stomatal conductance due to effects of oxygen concentration on the sensitivity of infrared water vapor analyzers has not been recognized. In this work we tested three types of infrared water vapor analyzers for changes in sensitivity depending on the oxygen concentration, and found that large errors in estimates of stomatal conductance can occur unless the change in analyzer sensitivity with oxygen concentration is taken into account. This information will improve the accuracy of measurements of stomatal conductance by crop physiologists in analyzing plant responses to the environment.
Technical Abstract: Use of infrared analyzers to measure water vapor concentrations in photosynthesis systems is becoming common. It is known that sensitivity of infrared carbon dioxide and water vapor analyzers is affected by the oxygen concentration in the background gas, particularly for absolute analyzers, but the potential for large errors in estimates of stomatal conductance due to effects of oxygen concentration on the sensitivity of infrared water vapor analyzers is not widely recognized. It was found that changing from either zero or 2% to 21% oxygen in nitrogen decreased the sensitivity to water vapor for all three types of infrared water vapor analyzers by about 4%. The resulting error in calculated stomatal conductance would depend strongly on the leaf to air vapor pressure difference and leaf temperature, and also on whether leaf temperature was directly measured or calculated from energy balance. Examples of measurements of gas exchange on soybean leaves under glasshouse conditions indicated that changing from 21% to 2% oxygen produced an artifactual apparent increase in stomatal conductance which averaged about 30%. Such errors could affect inferences about the carbon dioxide dependence of the sensitivity of photosynthesis to oxygen.