An Automated CO2Injection Circuit for Measurements of Rates of Photosynthesis

Abstract

There have been many descriptions of circuits incorporating infra-red gas analysers that can be used for measurements of photosynthetic rates (Sestak, Catsky, and Jarvis, 1971). These can be classified into three groups simply described as closed, open, and semi-closed circuits (Heath, 1969). Although closed circuits are the simplest to construct, they have been criticized on the grounds that they do not allow steady-state measurements of photosynthesis (Heath, 1969). Of the other two, the open circuit has been most widely used. In this system the CO2 concentration is measured before it enters and after it leaves the assimilation chamber. Photo synthetic rate is determined from measurement of depletion of CO2 in the chamber and its accuracy is dependent upon the precise control and recording of flow rate as well as careful measurement of CO2 concentration. In addition, if rates of photosynthesis at varying CO 2 concentrations are to be measured then either pre-mixed gases of known concentration must be available, or an expensive system of gas mixing pumps used. In the semi-closed circuit, CO2 is added to maintain a constant concentration in the assimilation chamber with the gas analyser acting as a null-point sensor regulating the supply of CO 2. The advantage in using this system is that accurate measurements of flow rates in the circuit are not required and that the CO2 concen tration can be maintained at any level within the range of the infra-red gas analyser. However, great precision is needed in the injection of CO2 into the circuit, especially when small volumes of pure CO2 are used. Koller and Samish (1964) and Redshaw and Meidner (1970) have described systems for injecting CO2 into a closed circuit but both depend on manual adjustment of gas flow and the measurement of flow of CO2 or a C02/air mixture into the circuit. We describe here a system which does not require measurement of flow rate and is fully automated with respect to injection of very small doses of pure CO 2.