Agriculture Reference
In-Depth Information
species being nodulated by different genera of endophyte, as in the case of
Lotononis
,
where some species are nodulated by
Methylobacterium nodulans
and others by an as-
yet-unnamed species from the
-Proteobacteria (Yates et al., 2007). In
Lebeckia
, some
species are nodulated by
- and others by
-Proteobacteria (Phallane et al., 2008).
4.5 Competition
If a host is grown in a soil that contains several strains of bacteria that can poten-
tially nodulate it, who gets in? Do the numbers of bacteria in the rhizosphere play a
part? Graham (2008) discusses the use of the term 'competitiveness' and concludes
that it covers a variety of aspects of the relationship between the legume host and
potential endophytes and suggests that it really means 'success in nodule occupancy'.
Clearly this must include the more traditional features of competitiveness, such as
ability to persist in the soil in the absence of a host, to grow actively in the rhizo-
sphere and attach to host roots when the host is reintroduced. This further requires
that the bacteria are in the correct position with respect to the host root. Thus in
the early stages of growth, they need to be near the crown of the root system, but
later they may need to be deeper in the soil. Further, the root architecture of the host
and its phenology (vegetative or reproductive phases of growth) can influence rhizo-
bial population structure, as shown for
Rhizobium leguminosarum
biovar
vicieae
around
Pisum sativum
plants (Depret & Laguerre, 2008). It is possible that different rhizobial
genotypes may be better at producing effective nodules at different stages of host
development. Graham (2008) stressed that it is necessary that a 'competitive' strain
completes the infection events in a 'timely' manner. Ampomah et al. (2008) found
that strains of
Sinorhizobium
that are unable to catabolise the sugar trehalose could
produce more nodules on some host genotypes, even when they were poor colonis-
ers, the accumulation of trehalose in the infecton threads apparently giving them an
advantage.
Competitiveness is also affected by edaphic factors. Of these, pH, temperature, de-
siccation and nutrient status have beenmost widely studied. For example, acid-tolerant
strains of
R. tropici
are more likely to be recovered from nodules grown in acid than
neutral soils and fast-growing rhizobia from soybean nodules at alkaline pH (reviewed
by Graham, 2008). Soil nutrients, both macro and micro, can affect rhizobia directly,
as they have to live as saprophytes in the absence of host plants. Wielbo et al. (2007)
found that within
R. leguminosarum
, strains that could use the widest range of nu-
trients, especially organic and amino acids, were the most competitive. In addition,
soil factors may affect competition via effects on the host plant (Sadowski, 2005). Al-
though combined nitrogen, usually in the form of nitrate or ammonium nitrate, has
been widely found to inhibited nodulation, its mechanism of action on competition
is not well understood. Because of the bias of research towards agricultural systems
where nitrate is the predominant form of soil nitrogen, it is often not realised that
many legumes can nodulate and fix nitrogen in the presence of ammonium, which
is the predominant form of nitrogen in many natural soils and in most forests. In
the mimosoid tree
Mimosa caesalpinifolia
and the papilionoid tree
Gliricida sepium
,re-
spectively, nitrate inhibited nodulation, but ammonium at the same concentration
