Murray Unkovich1 and David Layzell2
School Agriculture, Food and Wine, The University of Adelaide, PMB 1, Glen Osmond SA 5064. Australia. murray.unkovich@adelaide.edu.au
Department of Biological Sciences, University of Calgary, 2500 University Drive N.W., Calgary AB Canada T2N 1N4
Abstract
Despite the fact that N2 gas comprises 78% of the earth’s atmosphere, there is some evidence that its concentration in legume nodules may be less than that needed to saturate the activity of nitrogenase, the bacterial enzyme responsible for fixing N2 into NH4+ needed for protein and DNA synthesis in biological systems. This paper reviews that evidence. If this hypothesis were true, to meet their demand for fixed N, legumes would need to produce more nodules, bigger nodules or more nitrogenase enzyme activity per nodule. Given the high energy cost of legume nodules, N2 limited nitrogenase could reduce legume productivity.
We provide an overview of the published literature on 15N isotope enrichment associated with nitrogenase activity and evidence to suggest that in legumes nitrogenase may not be N2 saturated. We then consider the biophysical properties of legume nodules, and the biochemical properties of the nitrogenase enzyme to explore a possible explanation for the isotopic data and the stated hypothesis.