Agriculture Reference
In-Depth Information
2002). Furthermore, rhizobia are frequent rhizosphere colonizers of a wide range of plants and
may also inhabit nonleguminous plants endophytically. In these rhizospheric and endophytic
habitats, they may exhibit several plant growth-promoting effects, such as hormone production,
phosphate solubilization, and the suppression of pathogens (Sessitsch et al., 2002).
Biological N fixation by legumes offers more flexible management than fertilizer N because
the pool of organic N becomes slowly available to nonlegume species (Peoples et al., 1995a,b).
Concomitant with N
2
fixation, the use of legumes in rotations offers control of crop diseases and
pests (Graham and Vance, 2000; Sessitsch et al., 2002). Microorganisms capable of N
2
fixation
may be divided into two groups, that is, those living free and those living symbiotically with higher
plants. Microorganisms that form a symbiotic association have the greatest agricultural significance
for N
2
fixation.
Rhizobium
and
Bradyrhizobium
, living symbiotically with legumes, are the most
important N
2
fixers. According to Pohlhil (1981), there are about 19,700 known legume species
of which about 80% grow symbiotically with
Rhizobium
or
Bradyrhizobium
. About 200 of these
legume species are used as crop plants (Mengel et al., 2001).
Costs associated with commercial fertilizers continue to increase in response to energy prices
and the large demand by domestic and international agricultural enterprises (Silveira et al., 2013).
Alternative fertilizer options that maintain optimum forage production with minimum environmen-
tal impacts are the use of legume-grass mixtures in pastures for cattle raising. Dinitrogen-fixing
legumes provide levels of N for pasture growth (Evers, 1985; Ocumpaugh, 1990), increase forage
quality (Franzluebbers et al., 2004), and also enhance animal performance compared with grass
alone (Hoveland et al., 1978).
Biological N fixation has a special importance in the twenty-first century due to the high cost
of N fertilizers and low N use efficiency in the cropping systems, which may cause environmental
pollution. In addition, biological N fixation is an important and integral component of sustainable
agricultural systems (Sessitsch et al., 2002). Hence, the objective of this chapter is to review the
latest information on biological N fixation by legumes. This information can be used by research
scientists in planning their experiments and discussing experimental results related to N
2
fixation
by legume crops. In addition, agronomists can also use this information in planning their cropping
systems to improve N use efficiency.
7.2 MECHANISM OF NODULE FORMATION
One of the unique features of legumes is the formation of a symbiosis between the plant and
soil bacteria of the genus
Rhizobium
, which results in the fixation of atmospheric N (Frame and
Newbould, 1986). Gibson and Jordan (1983) reported that although nodule formation is regarded
as a general characteristic of legumes, not all legumes have been reported to nodulate. The sub-
family Papilionoideae contains a high proportion of nodulating genera (95%) (Allen and Allen,
1981). When nodulating legume plant growth is normal, the bacteria strain infects the roots and
stimulates nodulation. There are several strains that are responsible for nodulation in legumes.
The bacteria are free living in the soil, but fix N only in symbiosis (a mutually beneficial relation-
ship) with the host legume. The most common type of symbiosis is between members of the plant
family Leguminosae (also called Fabaceae) and soil bacteria, collectively called rhizobia, which
include representatives of the genera
Rhizobium
,
Bradyrhizobium
,
Azorhizobium
,
Sinorhizobium
,
and
Photorhizobium
(Epstein and Bloom, 2005). The attraction or recognition of a suitable host
plant by rhizobia is through host plant lectins (carbohydrate-binding proteins) which interact selec-
tively with microbial cell surface carbohydrates and serve as determinants of recognition or host
specificity (Bauer, 1981).
Once the N
2
fixation begins, the nodule and its bacterial inhabitants maintain their depen-
dency upon the host plant for both C and N. The reduction potential and ATP generated through
photosynthesis are used to drive the carbon reduction cycle. Photoassimilates produced are
