Methods for the continuous measurement of O2 consumption and H2 production by nodulated legume root systems

The details of two systems for measuring O2 uptake and H2 production in flow-through gas systems used to study nodule physiology and biochemistry are presented here. Both are constructed from commercially available fuel cells. Oxygen uptake measurements are based upon the differential signal from paired detectors exposed to sample and blank gas streams. Because of the very small signal: background ratio needed to detect O2 uptake against atmospheric O2 concentration, these detectors were mounted in thermally controlled aluminium blocks designed to avoid changes in back pressure. Also to avoid apparent concentration differences arising because of pressure differences at the detectors it is important that sample, control and calibration gas flow rates are the same. Hydrogen detectors were mounted in an aluminium block similar to that used for O2 although the requirements for temperature and back pressure control are much less stringent for this application. The limit for differential detection of O2 uptake in air was about 25 ppm (1 μmol l−1) and for H2 production 1 ppm (0.04 μmol l−1). Linearity checks for the two detectors over the range 4–90% O2 and 15–750 ppm H2 gave regression coefficients of > 0.999 and neither detector was significantly affected by changing the background mixing gas from N2 to Ar or He provided that flow rates and back pressures remained constant. Water vapour had no effect on the H2 detectors, but caused small baseline shifts during O2 uptake measurements which were obviated with silica gel drying filters. Changes in gas stream pO2 produced small baseline shifts with the H2 detectors, but did not effect the magnitude of the H2 signal. Two examples are provided of the use of these detectors together with the soyabean/USDA16 symbiosis in a flow-through system furnished with an infrared gas analyser (IRGA) to measure CO2 production.