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N 2 + 8H + + 8e -
2NH 3 + H 2
2H +
Ureides, phaseoloid nodules
Amide
exporting
nodules
Asparagine
export
2NH + + 2 glutamate
glutamine
Figure 5.2 Outline of the process of fixation of nitrogen gas into organic nitrogen compounds. For further
details see Sprent (2001) or most major textbooks. The initial reduction of nitrogen occurs in bacteroids,
with all other reactions taking place in host plant cells. Ureides are allantoin and allantoic acid.
group of tribes, including Psoraleae (Kanu et al., 2008) are unusual in exporting the
ureides, allantoin and allantoic acid (Sprent, 2000, 2001). Lotus exports amides, which
is another reason for its not being a perfect model legume representing determinate
nodules. Ureide, but not amide exporters tested also produce
-phenethylamine, es-
pecially at flowering stage, but the significance of this compound to the legumes
concerned is not clear (Fujihara et al., 2002). The general reactions of nitrogen fixation
are summarised in Fig. 5.2.
5.8.1 The hydrogen enigma
It has been known for many years that the production of hydrogen is an inevitable
consequence of the nitrogenase reaction (Fig. 5.2). Some bacteroids have the ability
to recycle this hydrogen, thus saving some of the energy wasted in its production.
This was regarded as a good thing, except that experiments indicated no advantage to
nodules having an uptake hydrogenase. A further complication is that the expression
of uptake hydrogenase is under control of the host plant (Brito et al., 2008). It has
been established that, at least in some circumstances, hydrogen production by legume
nodules alters the balance of the soil microflora and may contribute to the benefits of
legumes in crop rotation (Dong et al., 2003). This does not always seem to be the case,
as Peoples et al. (2008) could find little evidence that hydrogen production by soybeans
affected either soybean growth or growth of a subsequent maize crop. In any case, it is
difficult to see how hydrogen production is of direct benefit to the legumes producing
it, so the jury is still out on this question.
5.9 Nodule effectiveness
Chapter 4 considered some aspects of specificity and competition. Both are relevant to
the formation of effective nodules. All are affected by temporal and spatial heterogene-
ity in the environment. This context-dependent approach to co-evolution is discussed
by Heath & Tiffin (2007). Generally, effectiveness is judged by the amount of nitrogen
in a symbiotic plant, compared with the same host genotype grown on combined nitro-
gen. In the best scenario, symbiotic plants may contain as much or even more nitrogen
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