Agriculture Reference
In-Depth Information
rhizosphere. In phosphorus deficient soils, nitrogen fixation by legumes is indirectly
enhanced by the formation of arbuscular mycorrhizae largely through an improved
uptake of phosphorus and other deficient elements (Marschner and Dell, 1994).
Uptake of other elements. A large number of macro- and micronutrient elements (and
some non-nutrient elements) may be taken up by external hyphae and transferred to the
plant: nitrogen, calcium, potassium, zinc, sulphate, copper, amongst others (Marschner
and Dell, 1994). As indicated above, mycorrhizae may increase the uptake of heavy
and other potentially toxic metals or exclude them from their tissues.
3.2.1.3
N-fixation by prokaryotes
Symbiotic nitrogen fixation is a unique process whereby plants acquire atmospheric
nitrogen to meet their nutritional requirements through associations with prokaryotes
that possess the nif (nitrogen fixing) genes (see Section IV.3.1.2.2). It is therefore a
highly challenging area for research and technical development since, in favourable
circumstances, certain prokaryote-plant associations can routinely fix up to
per cropping cycle and sometimes more (see e.g., Rinaudo et al., 1983; Toomsan et al .,
1995). On a world-wide basis, an overall estimate for biological N-fixation is Mt per
year (Sprent, 1984).
Almost all of the higher plants forming nodulating N-fixing associations with
Rhizobial bacteria (Eubacteria) (Table IV. 10) occur in the family Leguminosae or in
the taxonomically-diverse range of trees and shrubs associated with N-fixing
Actinobacteria of the genus Frankia. Alnus (Betulaceae) and Casuarina (Casuarinales)
are perhaps the best-known genera associated with Frankia.
While most legumes fix N in root-associated nodules, species in a number of genera,
including Aeschynomene (20 species), Sesbania (4 species) and several others may have
active N-fixing nodules on their stems (Duhoux et al., 1993). The number of nodules
may thus be 5 to 10 times higher than in other plants and populations of this shrub
are able to fix up to over a cropping period (Rinaudo et al., 1983). Similarly,
two species of Casuarina also form active actinorhizae on their stems which make
a notable contribution to their N nutrition. In both these groups, the structures are
associated with the formation of adventitious roots and are most important in humid
environments (Duhoux et al., 1993).
Establishment of symbiotic associations
The mutualistic association between legumes and rhizobial bacteria starts as an infection
of the root tissues which later becomes a mutualistic relationship. Not all legumes can
be infected by Rhizobia, and both tropical and temperate plants form associations with
different species of Rhizobium and related genera (Sprent, 1984). Bacteria enter the root
through the tips of absorbing hairs, and multiply to form infection cords which grow
through the hair down to the root cortex (Figure IV.41). These cortex cells then actively
multiply to form meristem cells which are colonised by the Rhizobia. At this stage, the
cells have changed into so-called bacteroids which are aggregated into colonies of up to
20 cells covered by an envelope called the peribacteroid membrane. Bacteroids have
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