Measurement of the Respiratory Quotient of Plant Tissues in a Constant Gaseous Environment

Abstract

In the course of plant physiological investigations it is often necessary to determine the respiratory activity of living tissues. The rate of carbon dioxide evolution has been used frequently as an index of respiration since it is relatively easy to measure. Under some conditions, however, the carbon dioxide released does not reflect certain biochemical changes associated with respiration which may be more closely correlated with oxygen absorption. If both gases are measured simultaneously, and the respiratory quotient (CO2/O2) calculated, a better interpretation of attendant phenomena may be given than if an analysis is made of only one gaseous component of the system. For example, erroneous conclusions may be drawn concerning the influence of low oxygen tensions, and the effects of various absorbed salts on the formation of organic acids in excised barley roots when C02 measurements alone are used as a basis for measuring the respiratory processes. Many methods for determining the respiratory quotients of living tissues have been proposed according to which samples of gas were withdrawn from an enclosed chamber and then subsequently analyzed for carbon dioxide and oxygen. Objections to these methods were based on changes in composition of the gaseous environment during the experimental period and in the necessity of great accuracy in the analytical estimations. Major improvements were developed by Fernandez (2), who provided a closed system with a Bunsen valve pump for circulating the gases through alkali towers to absorb the carbon dioxide, and an oxygen generator controlled by an operator to replace electrolytically an oxygen deficit indicated by a manometer. His apparatus was adopted in a slightly modified manner by Wetzel (7) and more recently in revised form by Bennet-Clark (1). The latter investigator introduced a non-pulsating pump and the automatic addition of