Nitrogen nodules

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 ... 

FIGURE 15.2 The bacteria that inhabit these nodules on the roots of a pea plant are responsible for fixing atmospheric nitrogen and making it available to the plant. 

A. L. Virtanen, S. Von Hausen, and T. Laine, Investigations on the root nodule bacteria of leguminous plants XIX. Influence of various factors on the excretion of nitrogenous compounds from nodules. Journal of Agricultural Science 27 332 (1937). 

J. A. Downie, A. Economou, A. K. Scheu, A. W. B. John.son, J. L. Firmin, K. E. Wilson, M. T. Cubo, A. Mavridou, C. Marie, A. Davies and B. P. Surin, The Rhizohium legumino.sarum bv. viciae NodO protein compensates for the exported signal made by the host-specific nodulation genes. Nitrogen Fixation Achievements and... 

M. P. Bec-Fcrte, H. B. Krishnan, D. Prome. A. Savagnac, S. G. Pueppke. and J.-C. Prome, Stractures of nodulation factors from the nitrogen lixing soybean symbiont Rhizobium fredii USDA257. Biochemistry. (.( l 1782 (1994). 

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). 

L. L. Barton, G. V. John.son, K. Schitoskcy, and M. Wertz. Sidcrophore-mediated iron metabolism in growth and nitrogen fixation by alfalfa nodulated with Rhizobium meliloii. J. PhuU Nmr. 79 1201 (1996). 

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. 

Minimization of agricultural losses from soil toxins Toxins from soils appear to be responsible for inhibition of nitrogen fixation, metabolism and nodulation in legumes. Removal of toxins could be achieved by proper adsorption techniques and also by growing companion plants that contribute organic matter to microoranisms which help to destroy or degrade toxic chemicals. 

Two weeks after planting in the pipes, the plants were thinned to 35 pipe per pipe each and the cups to one plant each, and the treatments begun. Each first, third and fifth day of the week for twelve weeks the pipes were flushed with three liters of tap water poured in the elbow end. The water flowed past the plant root systems and drained out the screened end of the pipes into a flask. One hundred milliliter aliquots of this water ( root exudate ) were used to water the soybean plants in the cups three times weekly. After each flushing, two liters of a low nitrogen (50 ppm N) complete nutrient solution (Peter s Hydro-sol ) were added to each pipe. The soybean plants in cups were watered as needed at other times with tap water. On alternate weeks the soybean plants were fertilized with the complete nutrient solution. At 4, 8 and 12 weeks after the root exudate treatments started eighty soybean plants (10 treatments x 2 soybean varieties x 4 blocks) were randomly chosen for analysis. The soil was washed free of the plant roots and each soybean plant was divided into roots, nodules, stems, leaves and fruits. The plant parts were dried at 105°C for four days and weighed. 

Nodule-forming bacteria (legume bacteria) live in symbiosis with the root system of legumes (e.g. beans). They can reduce nitrogen to ammonia with the aid of a molybdenum-sulfur complex. 

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