There is a coupled decrease in respiration and nitrogenase activity of nodules of many legume symbioses induced by exposure to acetylene in the presence of 21% O2. The respiratory costs of nitrogenase activity can be determined directly and distinguished from respiratory costs for growth and maintenance of roots and nodules, using the linear regression of respiration on nitrogenase activity. The regression gradient represents the carbon costs for the transfer of one pair of electrons by nitrogenase in terms of moles CO2 released per mole of ethylene produced. The intercept of the regression is the growth and maintenance respiration of nodules or nodulated roots. Exposure to acetylene at decreased or increased oxygen concentrations in the range from 10% to 70% resulted in a wider range of values for CO2 production and nitrogenase activity that fell on the same regression line as values obtained during the acetylene-induced decline at 21% oxygen. Oxygen concentrations below 10% increased significantly the proportion of anaerobic respiration and produced changes in nitrogenase activity not correlated with CO2 production. Provided that these limits are not exceeded, oxygen-induced changes in nodule activity in the presence of acetylene can be used to measure the efficiency of those symbioses which do not exhibit an acetylene-induced decline at a fixed oxygen concentration.

Respiratory cost (moles CO2/mole ethylene) remained relatively constant with plant age for detached pea nodules (2.8), attached nodulated roots of lucerne (2.5) and detached nodulated roots of field bean (4.2). However, for lucerne and field beans the proportion of total root respiration coupled to nitrogenase declined with time.

A survey of 13 legume species gave values from 2 to 5 moles CO2/mole C2H4Rhizobium strain and host-dependent variations in efficiency were found.

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