Agriculture Reference
In-Depth Information
fixed N for growth and development. If a given legume is grown on a specific area, N-fixing bacteria
may be present. Under this situation, inoculation with N-fixing bacteria is not required. Otherwise,
seeds of a legume should be inoculated with an appropriate strain of bacteria. Nowadays, commer-
cial inoculants are available for legume species.
Both
Rhizobium
and
Bradyrhizobium
belong to the Rhizobiaceae family but are distinguished
by their genetic properties and host specificity. In this respect,
Bradyrhizobium
species are not so
selective as
Rhizobium
species (Werner, 1987). Some crop species can be infected by both types
of N-fixing bacteria. For example, pegenon pea (
Cajanus cajan
) may be infected by
Rhizobium
as well as by
Bradyrhizobium
. Mengel et al. (2001) reported that
Bradyrhizobium
yielded a much
higher N
2
fixation rate than
Rhizobium
, although
Rhizobium
grew faster than
Bradyrhizobium
.
This difference is explained by a higher activity of the tricarboxylic acid cycle enzymes in
Bradyrhizobium
as compared with
Rhizobium
(Mengel et al., 2001).
Bradyrhizobium
is able to
store appreciable amounts of a heteropolysaccharide in the nodules (Streeter and Salminen, 1993).
There are several legumes that fix biological N. However, most of the biological association stud-
ied between rhizobia and legumes is between soybean (
Glycine max
L. Merr.), pea (
Pisum sativum
L.), and clover (
Trifolium repens
L.) and the bacteria of the genus
Rhizobium
or
Bradyrhizobium
.
Appunu et al. (2009) reported that soybean, an important N
2
-ixing legume, forms N-fixing root
nodules with diverse bacteria belonging to different genera and species. The slow-growing rhizobia
that effectively nodule soybean are distributed among five different
Bradyrhizobium
species. These
species are
Bradyrhizobium japonica
,
Bradyrhizobium elkanii
,
Bradyrhizobium liaoningense
,
Bradyrhizobium canariense
, and
Bradyrhizobium yuanmingense
.
Rhizobia infect cells within the roots of the legume, causing the formation of root outgrowths,
called nodules, in which the bacteria proliferate and fix N
2
for export to the plant (Layzell and
Moloney, 1994). The dinitrogen fixation requires or consumes a lot of energy. Hence, the yield of
legumes is lower than that of cereals and N
2
is one of the reasons to divert production energy to
nodule energy or N fixation energy.
Sprent (1989) has grouped legume nodules into two groups. In the determinate nodules group
fall the nodules of soybean and dry bean (
Phaseolus vulgaris
L.). Nodules of these two species are
usually spherical and all the infected cells within each individual nodule are of approximately the
same age. In contrast, indeterminate nodules like pea, clover, and alfalfa are generally elongated
due to the presence of a distinct meristem that continues to divide and produce new infected tissue
throughout the life of the nodule. In both nodule types, the bacteria-infected cells are located in the
central region of the nodule and are surrounded by layers of uninfected cortical cells (Layzell and
Moloney, 1994).
Bauer (1981) reported that a broad range of evidence indicates that nodulation is self-regulated
and optimized by the host. Ineffective nodules (i.e., nodules in which dinitrogen fixation does not
take place) are usually formed in greater numbers than effective nodules. Hence, it can be concluded
that nodule development is subject to negative feedback regulation by substances produced in effec-
tive nodules. Similarly, nodules are often found to occur in clusters, especially after a delayed
inoculation, indicating that the frequency of subsequent nodulation below the cluster is diminished
by self-regulation (Bauer, 1981).
Perennial legumes fix N during any time of active growth. In annual legumes, N fixation peaks
at flowering. With seed formation, it ceases and the nodules fall off the roots. Rhizobia return to the
soil environment to await their next encounter with legume roots. These bacteria remain viable in
the soil for 3-5 years, but often at too low a level to provide significant optimal N-fixation capacity
when legumes are replanted. Several articles or books are available on nodule formation in legumes.
Readers may consult these articles, that is, nodule development (Robertson and Farden, 1980), char-
acteristics of free-living and bacteroid rhizobia (Tsien et al., 1977), soil ecology and genetics (Abe
and Higashi, 1979),
Rhizobium
-legume symbiosis (Dart, 1977), and infection of legumes by rhizo-
bia (Bauer, 1981).
