Carbon Dioxide Effects on Fruit Respiration. I. Measurement of Oxygen Uptake in Continuous Gas Flow

Fruit physiologists have been concerned with carbon dioxide as a product of the respiratory process and as a factor in the environment surrounding the fruit. Measurement of CO, evolution was favored over determinations of oxygen uptake because of the simplicity of the determination, but a number of investigators realized that CO2 may be produced by anaerobic as well as aerobic pathways of metabolism. Therefore, carbon dioxide is not a sufficient index for establishing the nature of the biological oxidation in the material under study. This realization is of particular significance when fruits are subjected to atmospheric conditions that differ materially from those in air. The interest in modified atmospheres was prompted by the desirability of prolonging storage life, and the chief emphasis was on the observation of fruit quality in increased carbon dioxide and reduced oxygen in the storage environment as compared to ordinary air. Few investigators have concentrated on a systematic change in one component while maintaining constancy in the other. Still fewer have determined respiration as a function of CO, tension, since as long as CO2 measurements were employed by conventional techniques it was not feasible to determine the small amount of carbon dioxide added to the atmosphere containing a relatively high concentration of carbon dioxide. Attempts to develop infrared analysis to achieve such determinations proved excessively complicated. Lack of suitable analytical methods had limited the use of oxygen consumption as a measure of respiration in intact tissues, until Pauling, Wood, and Sturdivant (3) took advantage of the fact that oxygen has an unusually high paramagnetic susceptibility while most other gases are only slightly diamagnetic. They devised a simple and sensitive means of measuring this property of gases and built an instrument which would detect very small changes in the partial pressure of oxygen. The paramagnetic principle was used by the A. 0. Beckman Co. to develop a commercial oxygen analyzer. Since their null type instrument was capable of determining changes in oxygen concentration as small as 0.02 %, it seemed suited to the measurement of respiratory activity in atmospheres high in carbon dioxide. This instrument is characterized by excellent sensitivity, freedom from interference, and linearity over the required range. It also has the unique advantages of requiring no specially calibrated gas mixtures for standardization and of being easily adaptable for automatic sampling and recording. Since the gas is not altered by the determination it may be trapped for other analyses. On the other hand, it has the disadvantage, also inherent in most other methods, that the flow rate must be determined precisely. Several years ago the A. 0. Beckman Co. (Process Instrument Div., Beckman Instrument Co., Fullerton, Cal.) agreed to build an instrument designed to automatically sample and record respiratory activity of fruit subjected to gas mixtures high in CO, and containing from one to 25 % 02 Principle of the Oxygen Analyzer. A schematic illustration of an oxygen analyzer is shown in figure 1. This diagram was supplied by the Beckman Go. and depicts a simplified instrument which illustrates the principle of the analyzer. The paramagnetic detector is shown in the left section of the diagram and consists of a dumbbell-shaped test body suspended on a quartz fiber in the non-uniform field of a permanent magnet. The test body is free to rotate on the fiber in response to magnetic and electrostatic forces. The test body itself is paramagnetic and in the absence of a paramagnetic gas tends to orient in the position of maximum magnetic flux. As oxygen is admitted to the unit, the test body tends to rotate out of the position of maximum magnetic flux in response to the gas becoming more paramagnetic. The degree of rotation is proportional to the difference between the volume magnetic susceptibilities of the test body and the gas that it displaces. A mirror attached to the quartz fiber just above the dumbbell-shaped test body reflects a beam of light to the apex of a front silvered prism, which divides and directs the reflected light beam to two photocells in proportion to the angle of deflection of the test body. Thus any deflection from the null position of the test body causes an unbalance of the output of the two photocells which results in movement of the recorder pen. A precision potentiometer. 1 Received revised manuscript Dec. 20, 1961.