2. NITROGEN fiXATION ENABLES THE
NITROGEN IN THE AIR TO BE USED FOR
PLANT GROWTH
In a closed ecological system, the nitrate required for plant growth is derived from the
degradation of the biomass.
In contrast to other plant nutrients (e.g., phosphate or sulfate), nitrate cannot be
delivered by the weathering of rocks.
Smaller amounts of nitrate are generated by lightning and carried into the soil by rain
water (in temperate areas about 5 kg N/ha per year).
3. Due to the effects of civilization (e.g., car traffic, mass animal production, etc.), the
amount of nitrate, other nitrous oxides and ammonia carried into the soil by rain can be
in the range of 15 to 70 kg N/ha per year.
Fertilizers are essential for agricultural production to compensate for the nitrogen that is
lost by the withdrawal of harvest products. For the cultivation of maize, for instance, per
year about 200 kg N/ha have to be added as fertilizers in the form of nitrate or ammonia.
Ammonia, the primary product for the synthesis of nitrate fertilizer, is produced from
nitrogen and hydrogen by the Haber-Bosch process:
4. HABER-BOSCH PROCESS:
• If it were not for the production of nitrogen fertilizer by Haber-Bosch synthesis, large
parts of the world’s population could no longer be fed.
5. N2 FIXATION
• The majority of cyanobacteria and some bacteria are able to synthesize ammonia from
nitrogen in air. A number of plants live in symbiosis with N2-fixing bacteria, which supply
the plant with organic nitrogen. In return, the plants provide these bacteria with
metabolites for their nutrition.
• The symbiosis of legumes with nodule-inducing bacteria (rhizobia) is widespread and
important for agriculture. Legumes, which include soybean, lentil, pea, clover, and lupines,
form a large family (Leguminosae) with about 20,000 species.
6. • A very large part of the legumes have been shown to form a symbiosis with rhizobia. In
temperate climates, the cultivation of legumes can lead to an N2 fixation of 100 to 400
kg N2/ha per year.
• Therefore legumes are important as green manure; in crop rotation they are an
inexpensive alternative to artificial fertilizers.The symbiosis of the water fern Azolla with
the cyanobacterium Nostoc supplies rice fields with nitrogen.
• N2-fixing actinomycetes of the genus Frankia form a symbiosis with woody plants such
as the alder or the Australian casuarina. The latter is a pioneer plant on nitrogen-
deficient soils
7.
8. LEGUMES FORM A SYMBIOSIS WITH
NODULE-INDUCING BACTERIA
• The nodule-inducing bacteria include, among other genera, the genera Rhizobium,
Bradyrhizobium, and Azorhizobium and are collectively called rhizobia.
• The rhizobia are strictly aerobic gram-negative rods, which live in the soil and grow
heterotrophically in the presence of organic compounds.
• Some species (Bradyrhizobium) are also able to grow autotrophically in the presence
of H2, although at a low growth rate.
9.
10. • The uptake of rhizobia into the host plant is a controlled infection. The molecular basis
of specificity and recognition is still only partially known. The rhizobia form species-
specific nodulation factors (Nod factors). These are lipochito-oligosaccharides that
acquire a high structural specificity (e.g., by acylation, acetylation, and sulfatation).
• They are like a security key with many notches and open the house of the specific host
with which the rhizobia associate.The Nod factors bind to specific receptor kinases of
the host, which are part of signal transduction chain.
11.
12. • In this way the “key” induces the root hair of the host to curl and the root cortex cells
to divide, forming the nodule primordium. After the root hair has been invaded by the
rhizobia, an infection thread forms. which extends into the cortex of the roots, forms
branches there, and infects the cells of the nodule primordium.
• A nodule thus develops from the infection thread. The morphogenesis of the nodule is
of similarly high complexity to that of any other plant organ such as the root or shoot.
• The nodules are connected with the root via vascular tissues, which supply them with
substrates formed by photosynthesis.
13. • The bacteria, which have been incorporated into the plant cell, are enclosed by a
peribacteroid membrane (also called a symbiosome membrane), which is formed by
the plant. The incorporated bacteria are thus separated from the cytoplasm of the host
cell in a so-called symbiosome
• In the symbiosome, the rhizobia differentiate to bacteroids. The volume of these
bacteroids can be 10 times the volume of individual bacteria. Several of these
bacteroids are surrounded by a peribacteroid membrane
