A Fine-Control Carbon Dioxide Injection Circuit for the Measurement of Rates of Net Photosynthesis by Infra-Red Gas Analysis

Abstract

The measurement of rates of net photosynthesis by infra-red gas analysis has in the past been carried out in open, closed, and semi-closed circuits, all of which are unsuitable for factorial experiments involving, as the dependent variate, predeter mined rates of photosynthesis at measured steady-state carbon dioxide concentra tions (Heath, 1969). In addition, the closed circuit does not allow for steady-state measurements and the open circuit must be operated with an expensive flow-meter of great accuracy at variable high volume flows. Other processes such as trans piration and leaf temperature will, of course, be afFected by the variable high volume flows even if there is a good mixing fan in the chamber. As an alternative to varying the main flow, air-carbon dioxide mixtures must be supplied by expensive mixing pumps. In the semi-closed circuit a special air-carbon dioxide mixture must be provided under constant pressure and the total through-flow will remain variable with possible effects on processes other than photosynthesis. The carbon dioxide injec tion circuit described below removes most of the shortcomings mentioned and has the advantage of using ordinary laboratory equipment. Carbon dioxide injection in such quantities as are assimilated by the plant material in the chamber (i.e. as are necessary to maintain a constant carbon dioxide concentration in a closed system), has hitherto been from a carbon dioxide cylinder with a pressure-reducing valve in combination with a fine needle valve and capillary soap-bubble meter (Meidner, 1967, 1968). Results obtained were satisfactory, but the unavoidable play in even the best spring-loaded needle valve made fine adjustments during an experiment time-consuming and difficult. The