Agriculture Reference
In-Depth Information
The most extensive sets of papers are from Denison and colleagues in the United
States and United Kingdom and Provorov and colleagues in Russia. Kiers et al. (2008)
have summarised a number of different hypotheses regarding cooperation between
symbionts. These are shared interest, kin selection in the rhizosphere, amino-acid
cycling and bacteroid environment, and post-infection sanctions. All are shown to have
limitations as a universal answer. Approaching the problem from a slightly different
angle, Provorov et al. (2008) suggested that there are two types of gene systemoperating
in the bacteria, partly confounded by horizontal transfer between bacterial genomes.
The first system is directed by Darwinian, frequency-dependent selection and operates
at the level of host recognition and penetration, i.e, features related to pathogenesis.
The second, directed by kin selection, operates at the level of nitrogen fixation and traits
beneficial to the host. Another approach has been to consider the relations between
the two symbionts as being analogous to collective bargaining between a firm and a
worker's union (Ak¸ay & Roughgarden, 2007). Details of these models are beyond the
scope of this topic. More relevant is the range of levels of effectiveness of nodules in
many areas of the globe, for example in different savanna regions (see Section 2.2 in
Chapter 2).
5.11 Constraints on nitrogen fixation in agriculture
and the environment
Many papers, reviews and even topics have been written on factors affecting nitrogen
fixation. Most of these have been on crop species, which are usually mesophytes.
Thus their growth and nitrogen fixation would be expected to be reduced by all the
usual agents: drought, flooding, high and low temperature, salinity and pH extremes.
Hartwig (1998) has discussed these in a more ecological context and notes that data
for some 'stresses' are ambiguous. Sprent (1999) took up the challenge and reviewed
the growth of non-crop species in diverse environments. This section will extend that
review and also include some recent work on crop species. There is considerable
evidence for the latter, that plants dependent on nitrogen fixation are more sensitive to
stress than those grown on combined nitrogen, but this may not be true for legumes
that live naturally in an 'extreme' environment.
5.11.1 Waterlogging, drought and salinity
Most crop species such as peas and beans cannot tolerate long periods in waterlogged
soil. This is a function of the whole plant, not just nodules. However, as indicated
earlier, some legumes occur naturally in wet conditions and some, such as
Discolobium
pulchellum
(Lourerio et al., 1994), will only nodulate when submerged. Although it is
possible to growsome legumes hydroponically and obtain root hair infection, in natural
waterlogged conditions, when oxygen is usually limiting, infection is via wounds
associated with lateral root emergence. As discussed above, this may be the primitive,
default position.
