Phenolic acid breakdown

Blum, U. Effects of microbial utilization of phenolic acids and their phenolic acid breakdown products on allelopathic interactions. J Chem Ecol 1998 24 685-708. 

Fig. 2.12 The decline of 0.5 mM p-coumaric acid (a) and the accumulation and decline of initial phenolic acid breakdown products (b) in nutrient solutions (pH 5.0) surrounding roots of 12 day-old cucumber seedlings. Breakdown products are in p-coumaiic acid equivalence. Nutrient solutions were aerated or not aerated. Figures reproduced from Blum and Geiig (2005). Figures used with permission of Springer Science and Business Media...

However, if phenolic acid from tissues are primarily utilized ( used up ) to stimulate phenolic-acid utilizing microbes within the bulk soil and/or the rhi-zosphere/rhizoplane, then the inhibition of cucumber seedlings could be due to a promoter/modifier/inhibitor complex dominated by other inhibitors and any remaining phenolic acids and phenolic acid breakdown products, and... 

Soil microorganisms produce many compounds that are potentially toxic to higher plants. Examples include members of the following antibiotics (1-6), fatty and phenolic acids (7-12), amino compounds (13-15), and trichothecenes (16, 17). "Soil sickness" and "replant problems" have been reported where certain crops or their residues interfere with establishment of a subsequent crop (18, 19). Toxins resulting from microbial activity sometimes are involved, but it is often unclear whether these are synthesized de novo in microbial metabolism or are breakdown products of the litter itself (20). 

Regardless of the source, phenolic acids are ultimately broken down to gaseous products such as CO2 and methane. This breakdown occurs by three general methods (i) aerobic respiration, using molecular oxygen as an electron acceptor, the end product being CO2, (ii) anaerobic respiration with electron acceptors such as nitrate and (iii) anaerobic fermentation with phosphorylation reactions involving no external electron acceptor (50). 

The decrease in the anthocyanin concentration results from both breakdown reactions and stabilization reactions. In breakdown reactions (Section 6.3.3), free anthocyanins are broken down by heat into phenolic acids (mainly malvidin) and by violent oxidation, mainly delphinidin, petuni-din and cyanidin. They are highly sensitive to quinones and the action of oxidases, either directly or in combination with caftaric acid. This acid may even react in the (nucleophilic) quinone form and bond to anthocyanin s (electrophilic) node 8 as a carbinol base. 

Decomposition of the substituent effect into the effect on the neutral molecule and the appropriate charged species, that is, either the conjugate acid or base, is revealing. For phenol acidity, the breakdown of the substituent effect into the two components indicates quite clearly that the interaction in the substituted phenoxide ion is the dominant one (10). In other words, p-nitrophenol is a strong acid primarily because the p-nitrophenoxide ion is relatively stable. The same is true for aniline acidity. 

Complete solution changes for multiple phenolic acid treatments were used because phenolic acids supplied to seedlings in the nutrient culture system disappeared from the nutrient solution within 24-48 h (Blum and Dalton 1985 Blum and Gerig 2005). This was due to microbial metabolism, physical breakdown, and/or root uptake. Since we did not want to confound nutrient and phenolic acid effects, complete solution changes were made. An additional benefit of this approach was to reset phenolic acid concentrations to the initial treatment levels for each solution change. This was important since recovery of seedling processes occurred rapidly after phenolic acid depletion (Blum and Dalton 1985 Blum and Rebbeck 1989 Blum and Gerig 2005). 

Multiple additions of phenolic acids were used because phenolic acid concentrations in soil decline rapidly after each addition of phenolic acids (Blum et al. 1987 Blum and Gerig 2006). This was due to microbial metabolism, physical breakdown, root uptake, and/or soil particle sorption. Recovery of seedling processes, although considerably slower than in nutrient culture, also occurred in seedling-soil systems (Blum et al. 1987 Blum and Gerig 2006). To maintain inhibition for extended time periods multiple additions of phenolic acids were required. 

Utilization of phenolic acids by microbes can be substantial as long as conditions are appropriate, e.g., adequate nutrition, moisture, pH, and temperature. However, a variety of other factors can also influence microbial utilization of phenolic acids. For example, phenolic acid utilization by microbes was reduced when more readily available carbon sources were present in the soil such as glucose and phenylalanine (Fig. 2.24 Blum et al. 1993 Pue et al. 1995) or other sources of available carbon, i.e., roots (Blum et al. 1999a). The presence of methionine, however, did not influence the rate of phenolic acid utilization by soil microbes (Pue et al. 1995). Differential utilization of carbon sources by soil microbes is fairly common (Martin and Haider 1979 Harder and Dijkhuizen 1982 Papanastasiou 1982 Sugi and Schimel 1993). Thus the relationship between phenolic acid-utilizing microbes and their carbon environment is complex. There is yet another aspect that adds to this complexity. Initial microbial breakdown products of phenolic acids are frequently other phenolic acids. For example, ferulic acid is converted to caffeic acid or vanillic acid and these are converted to protocatechuic acid. Next the ring structure of the... 

Con 3 Microbes in the bulk soil, rhizosphere, and rhizoplane rapidly utilized phenolic acids as a carbon source. In most instances, the resulting breakdown products were short lived and much less inhibitory or neutral in their effects, thus soil solution concentrations are generally low. Accumulation of breakdown products requires an anaerobic environment. 

Bertin C, Harmon R, Akaogi M, Weidenhamer ID, Weston LA (2009) Assessment of the phytotoxic potential of m-tyrosine in laboratory soil bioassays. J Chem Ecol 35 1288-1294 Blum U (1996) Allelopathic interactions involving phenolic acids. J Nematol 28 259-267 Blum U (1998) Effects of microbial utilization of phenolic acids and their phenolic add breakdown products on allelopathic interactions. J Chem Ecol 24 685-708... 

When the leaving group is better, breakdown can occur directly from A. This is the case when R"0 is a phenolate anion. The mechanism also depends upon the pH and the presence of general acids and bases because the position of the equilibria among the tetrahedral intermediates and their rates of breakdown are determined by these factors. 

Atmospheres polluted by oxidising agents, e.g. ozone, chlorine, peroxide, etc. whose great destructive power is in direct proportion to the temperature, are also encountered. Sulphuric acid, formed by sulphur dioxide pollution, will accelerate the breakdown of paint, particularly oil-based films. Paint media resistant both to acids, depending on concentration and temperature, and oxidation include those containing bitumen, acrylic resins, chlorinated or cyclised rubber, epoxy and polyurethane/coal tar combinations, phenolic resins and p.v.c. 

The breakdown of organic contaminants present in the MU water to produce ammonia (from nitrogenous contaminants), phenols and carboxylic acids (from humic and fulvic acids), and tri-halomethanes (from the by-products of chlorination)... 

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