Natural ¹⁵N Abundance of NH₄⁺, Amide N, and Total N in Various Fractions of Nodules of Peas, Soybeans and Lupins

Abstract

Nodules of certain N₂-fixing root nodules are substantially enriched in ¹⁵N compared with non-nodular tissues. This enrichment usually resides largely within bacteroids. Isotope discrimination associated with export of ammonia(um) from the bacteroid would result in ¹⁵N enrichment of NH₄⁺ within bac- teroids. Bacteroid protein synthesis from this pool of ¹⁵N enriched NH₄⁺ would then account for enrichment of the bacteroids. Measurements of ¹⁵N abundances of total N and free NH₄⁺ in nodule fractions from lupins (Lupinus luteus), soybeans (Glycine max) and peas (Pisum sativum) showed this was not the case. With the inocula used in experiments reported here, lupin and soybean nodules were enriched in ¹⁵N, while pea nodules were not. There was no correlation between ¹⁵N abundances of NH₄⁺ and total N in the nodule fractions (r= 0.445, P> 0.2). We conclude that isotope discrimination associated with ammonia(um) transport does not explain the ¹⁵N elevation of lupin and soybean nodules. We also conclude, on the basis of the large isotope effect for the equilibrium between NH₄⁺ and NH₃, that most of the ammonia(um) is exported from bacteroids as NH₄⁺ rather than NH₃.

We also measured the ¹⁵N abundance of free amide N. There was a strong correlation between ¹⁵N abundances of free amide N and total N in nodule fractions (r=0.924, P<0,001), suggesting that amide N is a significant source of N to the amino acid pools from which proteins are synthesised.