METHODS FOR THE CONTINUOUS MEASUREMENT OF O-2 CONSUMPTION AND H2 PRODUCTION BY NODULATED LEGUME ROOT SYSTEMS

The details of two systems for measuring O-2 uptake and H-2 production in flow-through gas systems used to study nodule physiology and bioch emistry are presented here. Both are constructed from commercially ava ilable fuel cells. Oxygen uptake measurements are based upon the diffe rential signal from paired detectors exposed to sample and blank gas s treams. Because of the very small signal:background ratio needed to de tect O-2 uptake against atmospheric O-2 concentration, these detectors were mounted in thermally controlled aluminium blocks designed to avo id changes in back pressure. Also to avoid apparent concentration diff erences 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 t o that used for O-2, although the requirements for temperature and bac k pressure control are much less stringent for this application. The l imit for differential detection of O-2 uptake in air was about 25 ppm (1 mu mol l(-1)) and for H-2 production 1 ppm (0.04 mu mol l(-1)). Lin earity checks for the two detectors over the range 4-90% O-2 and 15-75 0 ppm H-2 gave regression coefficients of >0.999 and neither detector was significantly affected by changing the background mixing gas from N-2 to Ar or He provided that flow rates and back pressures remained c onstant. Water vapour had no effect on the H-2 detectors, but caused s mall baseline shifts during O-2 uptake measurements which were obviate d with silica gel drying filters. Changes in gas stream pO(2) produced small baseline shifts with the H-2 detectors, but did not effect the magnitude of the H-2 signal. Two examples are provided of the use of t hese detectors together with the soyabean/USDA16 symbiosis in a flow-t hrough system furnished with an infrared gas analyser (IRGA) to measur e CO2 production.