Agriculture Reference
In-Depth Information
Box 4.2
Biochemistry of N
2
Fixation in Free-living Organisms and Symbiotic Associations
N
2
reduction requires much energy. Approximately 16 moles of ATP are
required per mole of N
2
reduced (ATP stands for the energy-carrying molecule
adenosine triphosphate). The
nitrogenase
enzyme catalyzes the splitting of the
dinitrogen molecule (N
N) and the reduction of each part to NH
3
. NH
3
very
rapidly combines with C compounds to form amino acids, the building blocks of
proteins.
The amount of N fixed in symbiotic associations, especially those involving
legumes, is much greater than that fixed by free-living organisms. The host plant
supplies a large amount of energy through the oxidation of carbohydrates. The
bacteria in the nodule (now called bacteroids) are supplied with O
2
by the pigment
leghemoglobin
, which has a very high affinity for O
2
. By this means, the bacteroids
can continue to respire, but the O
2
partial pressure at the surface of the bacteroids,
where N
2
reduction occurs, is kept very low. When N
2
fixation is active in a
nodule, the tissue has a characteristic pinkish-red color because of the functional
leghemoglobin; this can be assessed by cutting open nodules detached from
legumes in the field.
Successful N
2
fixation depends on factors that operate at any one of several
points in the complex sequence of root hair infection, nodule initiation, and N
2
reduction. The main factors and how they operate are summarized in figure
B4.2.1.
Figure B4.2.1
Factors affecting legume nodulation and the effectiveness of N
2
fixation.
Inadequate
Low pH and
low Ca content
carbohydrate
supply and moisture
stress reduce a
nodulated
inhibit root
hair infection
and nodule
initiation.
root's fixation
capacity.
Because of the large
Nodulated legumes
energy demand for
need more P, Mo, and
fixation, nodulation
is suppressed when
NH
4
+
, is readily
Cu than non-nodulated
legumes,
and have a unique
requirement for Co.
mineral N, especially
available.
Effective and
noneffective bacterial
strains compete.
A noneffective strain
produces many nodules
, yet
Nodulation and N
2
fixation are stimulated by
simultaneous infection
of the roots by
vesicular-arbuscular
mycorrhizae, due
that do not fix N
2
their presence inhibits
to the enhanced
supply of P to the plant.
nodulation by an
effective strain.
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