Distribution of 15 N within Pea, Lupin, and Soybean Nodules

Abstract

The 15N abundance of some, but not all, legume root nodules is significantly elevated compared to that of the whole plant. It seems probable that differences in ',N enrichment reflect differences in the assimilatory pathway of fixed N. In that context, we have determined the distribution of naturally occurring 15N in structural fractions of nodules from soybean (Glycine max L. Merr.), yellow lupin (Lupinus luteus), and pea (Pisum sativum) nodules and in chemical components from soybean nodules and to a lesser extent, pea and lupin nodules. None of the fractions of pea nodules (cortex, bacteriod, or host plant cytoplasm) was enriched in 1'N. The differences among bacteriods, cortex, and plant cytoplasm were smaller in lupin than in soybean nodules, but in both, bacteriods had the highest 15N enrichment. In soybean nodules, the 15N abundance of bacteriods and cortex was higher than plant cytoplasm, but all three fractions were more enriched in 15N than the entire plant. Plant cytoplasm from soybean nodules was fractionated into protein-rich material, nonprotein alcohol precipitable material (NA), and a low molecular weight fraction. The N of the latter was further separated into N of ureides, nucleotides and free amino acids. Most of these components were either similar to or lower in 15N abundance than the plant cytoplasm as a whole, but the NA fraction showed unusual 15N enrichment. However, the percentage of nodule N in this fraction was small. NA fractions from yellow lupin and pea nodules and from soybean leaves were not enriched in 15N. Nor was the NA fraction in ruptured bacteriods and cortical tissue of soybean nodules. Variation among soybean nodule fractions in the preponderance in protein of different amino acids was not large enough to explain the differences in 15N abundances among them. A hypothesis, consistent with all known data, conceming the mechanism leading to the observed excess 15N of lupin and soybean bacteriods is offered. Nodules of certain, but not all, N2-fixing plants have a substantially higher "N abundance than that of nonnodular tissues (7, 15, 18, 19), while the "N abundances ofnonnodular tissues are usually similar to each other in "N abundance. When atmospheric N2 is the sole source of N to the N2-fixing plant, the 6"N value2 of the entire plant is usually within 2%o of that of atmospheric N2 (16).