Roots, plant

For most crops, other than rice, urea in the soil must first undergo hydrolysis to ammonia and then nitrification to nitrate before it can be absorbed by plant roots. One problem is that in relatively cool climates these processes are slow thus plants may be slow to respond to urea fertilization. Another problem, more likely in warmer climates, is that ammonia formed in the soil hydrolysis step may be lost as vapor. This problem is particularly likely when surface appHcation is used, but can be avoided by incorporation of the urea under the soil surface. Another problem that has been encountered with urea is phytotoxicity, the poisoning of seed by contact with the ammonia released during urea hydrolysis in the soil. Placement of urea away from the seed is a solution to this problem. In view of the growing popularity of urea, it appears that its favorable characteristics outweigh the extra care requited in its use. 

Pseudomonas. These gram-aegative bacteria are a diverse group of microbes that iahabit plants, water, and sod. Pseudomonads are metabohcaHy versatile, capable of carrying out chemical transformations, mineralization of organic compounds, and colonization on plant roots (16). The use of Pseudomonads strains ia the clean up of chemical wastes and od spills has drawn considerable attention. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

The weathering process which eventually reduces the rock of the parent material to the inorganic constituents of soil comprises both physical and chemical changes. Size reduction from rocks to the colloidal state depends not only upon the mechanical action of natural forces but also on chemical solubilisation of certain minerals, action of plant roots, and the effects of organic substances formed by biological activity. 

Soil reaction (pH) The relationship between the environment and development of acid or alkaline conditions in soil has been discussed with respect to formation of soils from the parent rock materials. Soil acidity comes in part by the formation of carbonic acid from carbon dioxide of biological origin and water. Other acidic development may come from acid residues of weathering, shifts in mineral types, loss of alkaline or basic earth elements by leaching, formation of organic or inorganic acids by microbial activity, plant root secretions, and man-made pollution of the soil, especially by industrial wastes. 

The SAH water potential determines many aspects of their behavior in the soil. The processes of water redistribution in the soil, its transport to the plant roots, and assimilation follow the osmotic laws and are regulated by the thermodynamic potential. 

Amino Acids. Early observations on the liberation of amino acids by plant roots were reviewed by Loehwing (94), Rademacher (121), and Borner (12). Free amino acids have been isolated from soil fractions (119), and the excretion of a variety of ninhydrin-positive compounds by plant roots has been demonstrated under controlled conditions by Katznelson et al. (18), Rovira (121), and Pearson and Parkinson (115). 

Information concerning the metabolic function of unusual naturally occurring plant constituents might be used to develop new growth regulators. An example of a development that arose in this way began with the observation by Tolbert (143) that as much as 30% of the total soluble phosphorus in plant roots, and sometimes in the leaves,... 

Pinacolone, o-(diphenylphosphino)benzoyl-coordination chemistry, 2, 401 Ping-pong reactions copper(II) complexes, 5, 717 Piperidine, /V-hydroxy-metal complexes, 2, 797 P a values azole ligands, 2, 77 Plant roots amino acids, 2, 962 carboxylic adds, 2,962 Plants... 

Transfer of toxic chemicals among ecosystem compartments often occurs. For example, trace metals may be absorbed by plant roots or deposited onto the... 

Sensing of soil drying by the plant root system and the resulting regulation of shoot physiology... 

King, G.J. Hussey, C.E., Jr Turner, V.A. (1986). A protein induced by NaCl in suspension cultures of Nicotiana tabacum accumulates in whole plant roots. Plant Molecular Biology, 7, 441-9. 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

In some cases pectinolytic enzymes have been associated with virulence and it is generally accepted that pectinolysis by these bacteria facilitates their entry and spread in plant tissue. In Rhizohium, these enzymes may play a role in the root infection process that precedes nodule formation (Hubbell et al 1978). A. irakense has never been reported to be pathogenic on plants. It can therefore be speculated that moderate and strictly regulated pectinolysis of A. irakense facilitates entry in the outer cortex of plants roots, since A. irakense has been isolated from surface-sterilized roots. It is likely that breakdown of plant polysaccharides by root colonizing bacteria can provide them with extra carbon source. 

Simon HM, CE Jahn, LT Bergerud, MK Sliwinski, PJ Weimer, DK Willis, RM Goodman (2005) Cultivation of mesophilic soil crenarchaeotes in enrichment cultures from plant roots. Appl Environ Microbiol 71 4751-4760. 

Bhadra R, DG Wayment, JB Hughes, V Shanks (1999) Confirmation of conjugation processes during TNT metabolism by axenic plant roots. Environ Sci Technol 33 446-452. 

The interaction of plants and bacteria in the root system plays an important role in the remediation of contaminants, and may involve the use of plant exudates to stimulate the growth of degradative bacteria as well as the penetration of the soil by plant roots (Kuiper et al. 2004) ... 

Fletcher JS, RS Hegde (1995) Release of phenols by perennial plant roots and their potential importance in bioremediation. Chemosphere 31 3009-3016. 

Patten CL, BR Glick (2002) Role of Pseudomonas putida indoleacetic acid in development of the host plant root system. Appl Environ Microbiol 68 3795-3801. 

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