Plants nitrogen fixation

The cell envelopes prevent rapid diffusion of 02 into the cells but do permit rapid enough entry of N2 to maintain the observed rate of fixation of N2.72 In actinomycetes of the genus Frankia, which forms root nodules with woody plants, nitrogen fixation occurs in vesicles that are sheathed by multiple layers of hopanoid lipids (see Chapter 22).73... 

Soil Nutrient. Molybdenum has been widely used to increase crop productivity in many soils woddwide (see Fertilizers). It is the heaviest element needed for plant productivity and stimulates both nitrogen fixation and nitrate reduction (51,52). The effects are particularly significant in leguminous crops, where symbiotic bacteria responsible for nitrogen fixation provide the principal nitrogen input to the plant. Molybdenum deficiency is usually more prominent in acidic soils, where Mo(VI) is less soluble and more easily reduced to insoluble, and hence unavailable, forms. Above pH 7, the soluble anionic, and hence available, molybdate ion is the principal species. 

W. B. Silvester, iu P. S. Nutman, ed.. Symbiotic Nitrogen Fixation in Plants, Cambridge University Press, London, 1976, p. 521. 

Nitrogen fixation Conversion of atmospheric nitrogen into organic nitrogen compounds available to green plants a process that can be carried out only by certain strains of soil bacteria. 

Nitrogen fixation takes place in a wide variety of bacteria, the best known of which is rhizobium which is found in nodules on the roots of leguminous plants such as peas, beans, soya and clover. The essential constituents of this and all other nitrogen-fixing bacteria are ... 

Moreover, the lipo-chitooligosaccharides, also known as nod factors, permit nitrogen fixation by which plants and symbiotic Rhizobia bacteria can reduce atmospheric nitrogen to the ammonia that is utihzed by the plant, thus making available nitrogen compounds to other living organisms. 

Fig. 10-13. The links between the cycling of C, N, and O2 are indicated. Total primary production is composed of two parts. The production driven by new nutrient input to the euphotic zone is called new production (Dugdale and Goering, 1967). New production is mainly in the form of the upward flux of nitrate from below but river and atmospheric input and nitrogen fixation (Karl et al, 1997) are other possible sources. Other forms of nitrogen such as nitrite, ammonia, and urea may also be important under certain situations. The "new" nitrate is used to produce plankton protoplasm and oxygen according to the RKR equation. Some of the plant material produced is respired in the euphotic zone due to the combined efforts...

Besides nitrogen fixation, the only other major source of reduced nitrogen is the decomposition of soil or aquatic organic matter. This process is called ammonification. Heterotrophic bacteria are principally responsible for this. These organisms utilize organic compounds from dead plant or animal matter as a carbon source, and leave behind NH3 and NHJ, which can then be recycled by the biosphere. In some instances heterotrophic bacteria may incorporate a complete organic molecule into their own biomass. The majority of the NH3 produced in this way stays within the biosphere however, a small portion of it will be volatilized. In addition to this source, the breakdown of animal excreta also contributes to atmospheric... 

Biological nitrogen fixation as a result of agricultural practices (e.g., planting clover so as to replenish nitrogen on farmlands). 

Bums, R. C. and Hardy, R. W. F. (1975). "Nitrogen Fixation in Bacteria and Higher Plants." Springer-Verlag, New York. 

Plants take up inorganic nitrogen, either as atmospheric Nj, through symbiotic nitrogen fixation in legumes such as pulses, some shrubs and trees. 

Model studies in nitrogen fixation and cobalamin chemistry Nitrogen-fixing plants... 

Rhizobium have been reported, but to date there is little evidence that siderophores produced by pseudomonads are beneficial for promoting nodulation and nitrogen fixation. In experiments examining the role of siderophore production on nodulated clover plants, siderophore-defective mutants were shown to stimulate growth of nodulated clover plants similarly to the siderophore-producing parent strain (119). 

R. M. Boddey, S. Urquiaga, V. Reis, and J. Doberreiner, Biological nitrogen fixation a.ssociated with sugar cane. Plant Soil 137 111 (1991). 

Nevertheless, cereal plants can interact with endosymbionts, capable of nitrogen fixation in other species, and be stimulated in their productivity. The odds of soil life are balanced for some bacteria by their interactivity at rhizosphere level, and a realm of exchanged signals dictates entry into hormonally reprogrammed root sites. Specificity for partner plant species is part of a fine speciation process that actively involves the bacterial nodulation genes, and continues to drive their variation dynamics. 

J. I. Sprent, F. R. Minchin, and R. Parson, Evolution since Knoxville were rhizobia wise to inhabit land plants New horizon. in nitrogen fixation (R. Palacios, J. Mora and W. E. Newton, eds.), Kluwer Academic Publishers, Dordrecht, 1993. 

J. F. Witty, P. J. Keay, P. J. Frogatt, and P. J. Dart, Algal nitrogen fixation on temperate arable fields the Broadbalk experiment. Plant Soil 52 151-164 (1979). 

---