Inhibition of mineralization

C. S. SikesandA. Wierzbicki. Stereospecific and nonspeciflc inhibition of mineral scale and ice formation. In Proceedings Volume. 51st Annu NACE Int Corrosion Conf (Corrosion 96) (Denver, CO, 3/24-3/29), 1996. 

Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

Although the definition of allelopathy Includes stimulation as well as Inhibition of growth by allelochemicals (1., 4), allelochemicals that definitively affect mineral absorption by plant roots have been found to primarily Inhibit, rather than stimulate, the process. The first part of this review presents evidence that alteration of mineral absorption Is a physiological mechanism of allelopathy. Possible physiological and biochemical bases for the Inhibition of mineral absorption by allelochemicals are then discussed. 

Increased levels of nitrogen and phosphorous reduced the Inhibition of barley (Hordeum vulgare L.) growth caused by -coumaric and vanillic acids (8a). Although other explanations are possible, these effects of fertilizers suggest that Inhibition of mineral absorption was responsible for the observed Inhibition of growth. 

Leachates of Donor Residue. Use of leachates of donor plant residue results In much less total material being put Into the growth medium of the receiver. Thus, this Is a more refined manner In which to test for allelopathic Inhibition of mineral absorption. 

Inhibition of Mineral Absorption in Excised Roots. More conclusive evidence that allelochemicals can inhibit mineral absorption has been obtained using purified allelochemicals and excised plant roots as the experimental system (Table 1). Use of excised roots eliminates the possibility that exists with intact plants that inhibition of translocation rather than absorption is responsible for decreased mineral content. Use of purified allelochemicals rather than plant debris or leachates allows more definitive conclusions to be reached regarding the capacity of allelochemicals to inhibit mineral absorption. 

Several general characteristics of the results compiled in Table I are worthy of mention. Compared to the variety of chemicals postulated to be involved in allelopathy (1), few specific compounds have been tested for inhibition of mineral absorption. The most extensively studied compounds are the phenolic acids, probably because of their being ubiquitously found in nature (1). Also, several flavonoids are inhibitory to mineral absorption (Table I). Both of these groups of compounds are often cited as being responsible for allelopathic interactions between plants. 

Two additional characteristics of the inhibition of mineral absorption by phenolic acids were observed. The inhibition of both P0 absorption (27) and K+ absorption (31, 32) was reversed when the phenolic acid was removed from the absorption solution. Harper Balke (32) found this reversibility to be dependent upon pH the lower the pH, the less the reversal. Also, kinetic plots of the inhibition of mineral absorption showed that the phenolic acids did not competitively inhibit either P0 (26, 28) or K+ (31) absorption. Rather, ferulic acid inhibited PO -absorption in a noncompetitive (26) or uncompetitive (28) manner and jr-hydroxybenzoic acid inhibited K+ absorption in an uncompetitive manner (31). 

By no means have all the potential allelochemicals produced by plants been isolated and identified. There are certainly unknown compounds that are active at very low concentrations. When these compounds are purified, physiological bases for their action will need to be determined. To say that inhibition of mineral absorption is the sole, primary mechanism of action of allelochemicals is unjustified. However, undoubtedly some allelochemicals can inhibit mineral transport and alter other membrane phenomena. 

Another possible role for inhibition of mineral growth by pyrophosphate and possibly nucleotides has been discussed in detail elsewhere114. ... 

Inhibition of mineral transformation by biological molecules is thus potentially a very selective process. Often only a trace amount of the inhibitor will be required to prevent... 

Uses. Propargyl alcohol is a component of oil-well acidizing compositions, inhibiting the attack of mineral acids on steel (see Corrosion and CORROSION control). It is also employed in the pickling and plating of metals. 

Contaminated soil has been inoculated with bacteria of established capacity for degradation of chlorobenzoates (Hickey et al. 1993). In the presence of added biphenyl, mineralization of PCBs was shown, although it was pointed out that there may exist incompatibility between the prodnction of chlorocatechols from chlorobenzoates and their inhibition of dihydroxybiphenyl-2,3-dioxygenase that catalyzes the ring fission of many PCBs. 

Hydrogen cyanide is highly endothermic. It polymerises violently when there is no inhibitor present. If the heat liberation brings the medium to 284°C, it causes the compound to detonate. The presence of a base catalyses the hydrogen cyanide polymerisation. Nevertheless, this reaction is inhibited by mineral acids. The most common polymerisation inhibitor is phosphoric acid. 

Mixtures of aldehydes with surfactants are active in preventing corrosion, in particular in the presence of mineral or organic acids [646]. The aldehyde may be trans-cinnamaldehyde. The surfactant may be N-dodecylpyridinium bromide or the reaction product of trimethyl-1-heptanol with ethylene oxide [645]. Such aldehyde and surfactant mixtures provide greater and more reliable corrosion inhibition than the respective compositions containing aldehydes alone. 

Although many physiological and biochemical processes In plants are affected by various allelochemicals, In most Instances the details of the mechanism of action of a particular allelochemical have not been elucidated. Because soil mediates the transfer of most allelochemicals (except perhaps volatile compounds) from a donor to a receiver, plant roots are often the first tissues to contact an allelochemical. Thus, It Is not surprising that root growth and development are Inhibited In many Instances of allelopathy (1.-3) One of the primary physiological functions of plant roots Is the absorption of mineral nutrients. Therefore, It Is logical that the Influence of allelopathic Interactions on mineral absorption by plant roots has been Investigated. 

In all the previous papers cited In this review, no attempt was made to isolate and identify the chemicals responsible for the alteration of mineral content in the receiver. Obviously, if we are to prove that some allelochemlcals act by inhibiting mineral absorption, individual chemicals must be tested (23). 

Furthermore, the long time periods of these experiments (12 days to 10 months) make it impossible to conclude that allelochemicals directly alter mineral absorption. An equally feasible explanation is that the chemicals inhibit growth and this indirectly alters the concentration of minerals in the tissues. 

Another limitation to the studies in Table 1 is the small number of plant species tested. Primarily monocotyledonous plants have been studied, although McClure et al. (26) found ferulic acid inhibitory in soybean. The restriction of studies to monocots is probably because the mechanism of mineral absorption has been more fully elucidated with monocots. Harper and Balke (32) reported some minor differences in the inhibition of K+ absorption by salicylic acid among oats (Avena sativa L.), wheat (Triticum aestlvum L.), barley, and maize roots. 

Two hypotheses have been proposed to explain how phenolic acids directly increase membrane permeability. The first is that the compounds solubilize into cellular membranes, and thus cause a "loosening" of the membrane structure so that minerals can leak across the membrane (28-30, 42). Support for this hypothesis comes from the fact that the extent of inhibition of electrical potentials correlates with the log P (partition coefficient of a compound between octanol and water) for various benzoic and cinnamic acid derivatives (Figure 5). 

Although several allelochemicals (primarily phenolic acids and flavonoids) have been shown to inhibit mineral absorption, only the phenolic acids have been studied at the physiological and biochemical levels to attempt to determine if mineral transport across cellular membranes can be affected directly rather than indirectly. Similar and even more definitive experiments need to be conducted with other allelochemicals that are suspected of inhibiting mineral absorption. Membrane vesicles isolated from plant cells are now being used to elucidate the mechanism of mineral transport across the plasma membrane and tonoplast (67, 68). Such vesicle systems actively transport mineral ions and thus can serve as simplified systems to directly test the ability of allelochemicals to inhibit mineral absorption by plant cells. 

Hydraulic fluids themselves cannot be measured in blood, urine, or feces, but certain chemicals in them can be measured. Aliphatic hydrocarbons, which are major components of mineral oil hydraulic fluids and polyalphaolefin hydraulic fluids, can be detected in the feces. Certain components of organophosphate ester hydraulic fluids leave the body in urine. Some of these fluids inhibit the enzyme cholinesterase. Cholinesterase activity in blood can be measured. Because many other chemicals also inhibit cholinesterase activity in blood, this test is not specific for organophosphate ester hydraulic fluids. This test is not available at most doctor s offices, but can be arranged at any hospital laboratory. See Chapters 2 and 6 for more information. 

A combined addition of a chain-breaking inhibitor and a hydroperoxide-breaking substance is widely used to induce a more efficient inhibition of oxidative processes in polyalkenes, rubbers, lubricants, and other materials [3 8]. Kennerly and Patterson [12] were the first to study the combined action of a mixture, phenol (aromatic amine) + zinc dithiophosphate, on the oxidation of mineral oil. Various phenols and aromatic amines can well serve as peroxyl radical scavengers (see Chapter 15), while arylphosphites, thiopropionic ethers, dialkylthio-propionates, zinc and nickel thiophosphates, and other compounds are used to break down hydroperoxide (see Chapter 17). Efficient inhibitory blends are usually prepared empirically, by choosing such blend compositions that induce maximal inhibitory periods [13],... 

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