FACTORS AFFECTING THE REDUCTION OF ACETYLENE BY ROOT NODULES OF LUPINUS SPECIES

A number of parameters affecting acetylene reduction by root nodules of Lupinus angustifolius, L. cosentinii and L. luteus have been studied. Acetylene reduction could be satisfactorily measured in air provided a sufficiently high pC₂H₂ was used. However, evacuation of air from containers was not harmful to the nodule system. The maximal rate of acetylene reduction occurred at 23°C but the rate was hardly less at 33°C. Plant age had a marked effect upon the rate of acetylene reduction in its nodules. Rates of acetylene reduction in nodules were maintained at high levels after the onset of flowering but rapid reduction in nodule activity occurred after pods were filled. Decapitation of plants had little or no effect on acetylene reduction by the nodules, but removal of the nodules reduced their activity by 70–85%. Brief rinsing of the nodules and rapid drying with blotting paper had little effect on their acetylene reducing activity, but prolonged wetting markedly decreased it. Nodulated roots and whole plants responded similarly to pO₂. Acetylene reduction was zero in the absence of oxygen, increased to a maximum rate by 0.2–0.25 atm O₂, remained constant up to 0.4–0.5 atm O₂, and then declined sharply to nearly zero at 0.9 atm. We detected no diurnal rhythm, attributable to effects of light on supply of photosynthate, with any of the three species.