Physiological and Biochemical Characteristics of FN1, a “Fix Minus Mutant” of Pea (Pisum sativum L)

In spite of information available from the literature (Postma et al., 1990), in our experiment nodules on the FN1 mutant (formed from R. leguminosarum RCR 1045) were able to fix some nitrogen. The FN1 acetylene reduction activity, measured on detached nodules in a flow system, was between 5 and 16% of the Rondo parent line. This activity was apparently reduced between 21 and 28 days, suggesting that the plant was unable to sustain effective nodules for more than a few weeks. The presence of nitrogenase was also confirmed by immunodetection. Microelectrode measurements of O2 gradients across of the nodule cortex showed that in parent and mutant nodules O2 concentration decreased rapidly and was close to zero within 350 μm of the nodule surface. Lb concentration in FN1 nodules (colorimetric assay) was only 25% of the Rondo parent line. FN1 nodules contained 2 to 5 fold lower amounts of the ononitol, aspartate and asparagine in comparison with Rondo nodules. Sucrose concentration was similar, but starch content 2 fold in the mutant nodules which suggests there was no block in carbon supply to mutant nodules from the shoot. Of the 15 enzymes of C and N metabolism investigated, only sucrose synthase, phosphoenolpyruvate carboxylase and alanine pyruvate aminotransferase were significantly lower in the nodules of the FN1 mutant. The data from immunodetection of host nodule proteins confirmed the reduced level of sucrose synthase in mutant nodules.