Phenolic acid mixtures

The notion of action by specific allelochemical compounds is also unjustified. Detailed study of some allelochemicals in active species has shown the presence of phenolic acid mixtures and other phenolic derivatives or terpenes. I think that we can never talk about the action of a single substance everywhere many compounds having different biological activity act simultaneously, perhaps mutually increasing their activity. As a rule, such allelochemicals are the intermediate products of soil humus, synthesis, or the ground detritus in aquatic ecosystems (11). High concentrations of these substances are lethal, moderate ones inhibit growth processes, and low concentrations stimulate them. 

Figure 27.13 Chromatovoltammogram of a phenolic acid mixture using volt-ammetric-amperometric detection. Conditions scan rate, 2.0 V/s W2 potential, -0.2 V vs. Ag/AgCl flow rate, 1.0 mL/min. Peak identities (in order of elution) gentisic acid, 80 pmol vanillic acid, 130 pmol caffeic acid, 220 pmol p-coumaric acid, 520 pmol ferulic acid, 410 pmol sinapic acid, 580 pmol. [Reprinted with permission from Ref. 25.]... 

Cecil -A 7 phenolic acid mixture 0.5 pmol 2542 Blum et al.9... 

Gently stir 5 u.b.w.—2.75 p.b.v. of sulfuric acid into the phenol/acid mixture... 

Very slowly (one drop every 10 seconds) stir the water/nitric acid/sulfuric acid mixture to the phenol/sulfuric acid mixture. If the temperature of the phenol/acid mixture starts to approach 40°C (104°F), then stop adding the acid mixture (but continue stirring) and allow the phenol/acid mixture to cool to below 30°C (86°F) before continuing. 

After all the water/nitric acid/sulfuric acid mixture has been added to the phenol/acid mixture, continue stirring for 10 minutes. 

In summary the rate of depletion (i.e., root uptake and microbial utilization) varies with type phenolic acid present, concentration, pH, time of day, time of day of treatment, number of treatments, composition of phenolic acid mixtures, whether uptake is apoplastic or symplastic, phenolic acid-utilizing microbial populations present on roots and in the nutrient solution, and aeration. Phenolic acid treatments of seedlings in nutrient culture modify microbial populations on root surfaces (rhi-zoplane) and in the nutrient solutions. Once taken up by roots, phenolic acids were distributed throughout seedlings. Highest concentrations, however, were retained in the roots. 

Fig. 2.20 The effects of multiple treatments of 7- (a) and 4- (b) equal-molar phenolic acid mixtures on cucumber seedling rhizosphere bacterial populations that can utilize phenolic acids as sole carbon sources, where CFU equals colony-forming units. Seedlings were grown in Cecil A sou. The 7-phenoUc acid mixture was composed of caffeic.p-coumaric, ferulic.p-hydroxybenzoic, sinapic, syringic, and vanUlic acids. The 4-phenohc add mixture was composed ofp-coumaric, fer-ulic,p-hydroxybenzoic, and vaniUic acids. Figure based on data from Blum et al. (2000). Plenum Publishing Corporation, data used with permission of Springer Science and Business Media...

Detectable population changes in laboratory systems of bulk-soil and rhizosphere phenolic-acid utilizing microorganisms to phenolic acid enrichment are a function of a variety of soil physicochemical and biotic factors including type of phenolic acid, phenolic acid enrichment concentrations, presence of other available organic compounds, nuttition, soil type, and initial microbial populations. Responses of microbes to phenolic acids or phenolic acid mixtures also varied with the type of microbe (e.g bacteria, actinomycetes, or fungi). 

In summary, observed effects of individual phenolic acids or phenolic acid mixtures were similar to what had been observed in nutrient culture but the response times and the magnitude of effects (see Section 2.4.8 for direct comparison) were slower and lower, respectively. Relative potencies of phenolic acids were lower when compared to nutrient culture. Increasing the number of phenolic acids in a mixture of phenolic acids reduced the concentrations of the individual phenolic acids required for a given percent inhibition. The presence of other readily available organic compounds (inhibitory or non-inhibitory) also reduced the concentration of phenolic acids required for a given percent inhibition. The addition of nitrate or nutrient solution reduced the inhibitory activity of phenolic acids. The inhibition of methionine, an amino acid, on the other hand was enhanced by the addition of nitrate. Finally phenolic acid effects were greater under acidic than under neutral conditions. 

Fig. 2.24 Relationships (a) between percent stimulation of rhizosphere bacteria that can utilize phenolic acids as sole carbon sources and percent inhibition of absolute rates of leaf expansion of cucumber seedlings growing in CecU A soU treated with a 0.6 p.mol/g soil 4-equal-molar phenolic acid mixture (a = 0.50), where CPU equals colony-forming units and the 4-phenoUc...

Total pmol/g soil of phenolic acid mixture... 

Indeijit, Streibig JC, Olofsdotter M (2002) Joint action of phenolic acids mixtures and its significance in aUelopathy research. Physiol Plant 114 422 28 Indeijit, Weston LA (2000) Are laboratory bioassays for allelopathy suitable for prediction of field responses J Chem Ecol 26 2111-2118... 

Effects of wheat shoot (a) and sunflower leaf (b) tissues, a phenolic acid mixture composed of equal-molar concentrations of p-coumaric acid, ferulic acid, p-hydroxybenzoic acid, and vanillic acid (a, b r =... 

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