Nitrogen root development

The texture and structure of a soil also affect plant response to nitrogen additions through effects on root development and depth of penetration. A restricted root system limits the ability of the plant to assimilate nutrients and also limits its uptake of water. Since the movement of water through soils by capillarity is slow and limited, the plant roots must literally go after it. A deep root system, therefore, assures that a much greater proportion of the supply in the soil is available to the plant. In addition, soil compaction limits the absorption of the rainfall. Such unfavorable conditions do of course lower the efficiency of applied nitrogen. 

The possibility that limited root development is attributable to insufficient available major and minor nutrients has received considerable attention. Sometimes the addition of available nitrogen and phosphorus has increased the depth and extension of root systems, but in most cases the increase has not been marked, and there was no certainty as to how much of the increases obtained was due to the nutrients per se, and how much to the mechanical operations involved in the addition of the nutrients. Where the A-horizon is present, most of the nutrients needed for growth in the subsoil are readily obtained from the topsoil, but in exposed subsoils added available nitrogen and phosphorus may be very essential for extensive root proliferation. If the physical conditions are satisfactory, plants produce a surprisingly large amount of roots on a minimum of nutrients they appear to send out roots in search of nutrients, particularly nitrogen. 

As world supplies of petroleum are depleted, and as the Earth s population steadily increases, society will be forced to develop more efficient ways to make fertilizer. Genetic engineering offers a promising solution. There is a remarkable bacterium that lives in the roots of leguminous plants such as soybeans, peas, and peanuts. This organism can convert molecular nitrogen into ammonia. The plant and the bacterium have a... 

Many companies spiecialize in the production of chemicals grouped in chemical trees characterized by the same chemical roots (compounds) or the same/similar method of manufacturing. Examples are the Lonza trees based upon (I) hydrogen cyanide, (2) ketene (H2C=C=0) and diketene (4-methyleneoxetan-2-one), and (3) nitrogen heterocycles. A different t3q)e of tree is that of DSM Chemie Linz, which branches out from ozonolysis as the core technology (Stinson, 1996). Wacker Chemie has developed its chemical tree leading to acetoacetates, other acylacetates, and 2-ketones (Stinson, 1997). Table 1.1 shows examples of fine chemicals. 

Com, leafy greens and root crops also do well in manured soil. Irish potatoes and sweet potatoes are exceptions. Irish potatoes tend to develop scab and sweet potatoes to crack when fertilized with manure. Peas and beans require httle if ary supplemental nitrogen and do not respond well to manure apphcations. 

Soil-borne bacteria of the family Rhizobiaceae and leguminous plants form a symbiotic relationship during which a new organ, the root nodule, is developed. Within these root nodules the bacteria fix atmospheric dinitrogen and the product of nitrogen fixation, ammonia, is exported to the plant [69,70]. Root nodules develop from primordia which are established at specific sites in the root cortex shortly after Rhizobium infection. The peptide enod40 is believed to play a critical role in inducing the de-differentiation and the mitotic division of root cortical cells, i.e. the initial steps in nodule development. This however, is not entirely undisputed [3,4,69-72]. 

Advances are also being made in the more complex area of the molecular biology of symbiotic nitrogen fixation.1469 Genes in Rhizobia involved in recognition, infection and nodulation have been identified, while some progress has been made with the plant genes and proteins involved in the development of root nodules. 

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