Agriculture Reference
In-Depth Information
Rhizobia populations have been shown to decrease during periods of drought in the absence
of host plants (Furseth et al., 2011). Even though certain strains may react slightly differently, the
correlation between a decreasing population and decreasing soil moisture is strong (Pena-Cabriales
and Alexander, 1979). Rhizobium populations decline rapidly at first, then slowly after a period of
time. Interestingly, when the soil is moistened, the population will stabilize, but not increase, then
continue to decrease on further drying (Pena-Cabriales and Alexander, 1979). These results suggest
that periods of drought may be more hazardous to rhizobia populations than periodic flooding, par-
ticularly in situations with no host plants (Furseth et al., 2011). Furseth et al. (2011) reported that the
use of seed-applied rhizobia inoculants following severe flooding is not likely to produce a positive
seed yield or quality under soil and environmental conditions.
7.6.7 C
arBon
d
IoxIde
Increasing CO
2
levels in the legume plant ambient improve the growth of legumes and, conse-
quently, nodulation and N fixation (Gibson and Jordan, 1983). Improvements in N fixation with
an increasing CO
2
concentration in the atmosphere by red clover (
Trifolium pretense
L.) (Wilson
et al., 1933), white clover (
Trifolium repens
L.) (Masterson and Sherwood, 1978), soybean (
Glycine
ma
x L. Merr.) (Gibson and Jordan, 1983), and pea (
Pisum sativum
) (Phillips et al., 1976) have been
reported. The positive response can be attributed to higher levels of photosynthesis in the presence
of increased ambient CO
2
and presumably to increased photosynthate supply to the roots (Gibson
and Jordan, 1983).
7.6.8 I
nfluenCe
of
o
ther
m
ICroorGanIsm
on
r
hIzoBIa
The production of substances toxic to rhizobia by various soil organisms has been attributed to the
poor colonization of
Rhizobium trifolii
in some soils (Gibson and Jordan, 1983). Actinomycetes may
affect colonization and nodulation by
Rhizobium japonicum
, but not all actinomycetes are antago-
nistic to rhizobia (Antoun et al., 1978).
7.6.9 p
athoGens
and
p
redators
Various pathogens and predators such as fungi, viruses, nematodes, and insects can adversely
affect the legume-
Rhizobium
symbiosis. Soil fungi may have a direct effect on nodulation through
their inhibition of rhizobia (Gibson, 1974). Tu (1979) reported that nodule formation in soybean by
Rhizobium japonicum
was reduced by the pathogen
Phytophthora megasperma
. Tu et al. (1970)
also reported that soybean plants infected with mosaic and bean pod mottle viruses showed reduced
nodulation, and lower levels of N
2
fixation, with some evidence of host variety differences in the
magnitude of the effect. Similarly, reductions in N
2
fixation by white clover (
Trifolium repens
L.)
and dry bean (
Phaseolus vulgaris
L.) infected with a bean yellow mosaic virus have been reported
by Smith and Gibson (1960) and Orellana and Fan (1978). Gibson and Jordan (1983) reported that
although virus particles have been found in nodules, it is likely that the effect of viral infection on
nodulation and N fixation is a consequence of the reduced physiological vigor of the host plants.
Barker et al. (1972) reported that roots infected with nematodes can reduce the nodulation of vari-
ous legumes because nematodes prefer nodule tissue rather than root tissue of host plants. Soybean
N
2
fixation has been reduced when roots were infected by nematodes (Barker et al., 1972). Gibson
and Jordan (1983) reviewed the literature on the effect of insects on nodulation of many legumes,
including the faba bean (
Vicia faba
L.), pea (
Pisum sativum
L.), and cowpea (
Vigna unguiculata
L.), and reported that nodule formation and, consequently, N
2
fixation was adversely affected. He
further reported that insects prefer nodule tissue over root tissue.
