Physiological and biochemical characteristics of FN1, a ‘fixation impaired’ mutant of pea (Pisum sativum L.)

In contrast to the classification of FN1 as a fix minus mutant (Postma et al., 1990), in these experiments, nodules of the FN1 mutant (formed by Rhizobium leguminosarum RCR 1045) showed acetylene reduction activity, but at a much lower level than in the Rondo parent line. The presence of nitrogenase was confirmed by immunodetection. Microelectrode measurements of O 2 gradients across the nodule cortex indicated that the oxygen diffusion barrier in mutant nodules appeared to be functioning normally. Leghaemoglobin (Lb) haem concentration in FN1 nodules was only 40% and 25% of that in the Rondo parent line at 3 and 4 weeks, respectively, whereas immunodetection of the apoprotein suggested reductions to 57% and 46%, respectively. FN1 nodules also contained substantially lower amounts of ononitol, aspartate and asparagine in comparison with Rondo nodules. Sucrose concentration was similar, but starch content was 2-fold higher in the mutant nodules. Of the 15 enzymes of C and N metabolism investigated, only sucrose synthase, phosphoenolpyruvate carboxylase, alanine pyruvate aminotransferase, glutamine synthetase, and glutamate synthase were significantly lower in the nodules of the FN1 mutant. In contrast to other fix minus or fix impaired mutants, alcoholic fermentation was not induced in FN1 nodules.