Phenolic acid depletion

Although all of these approaches to estimate microbial activity (i.e., enzyme activity, utilization of substrates, formation of products, respiration, or changes in microbial populations) could be determined, only changes in microbial populations that can utilize phenolic acids as a sole carbon source have been related to phenolic acid depletion from soil solutions.3... 

Given the bacterial populations that utilized p-coumaric acid as a sole carbon source and the physicochemical (e.g., constant temperature, adequate nutrition and moisture) and biotic conditions of these two laboratory systems, utilization of p-coumaric acid ranged from 0.6 to 5.0 pg/g soil/h for the open systems and 8.6 pg/g soil/h for the closed system. The pg values for the open system represent steady-state rates as modified by nutrition, while the pg values for the closed system represent maximum rates. Whether such rates ever occur in field soils is not known, since the physicochemical and biotic environments of field soils are so different from those of laboratory systems. Laboratory soil systems provide potential rates of utilizations, but until field rates are determined the importance of microbial activity in phenolic acid depletion from soil solutions will not be known. 

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). 

The higher concentrations of phenolic acids required for a given percent inhibition between the two systems stem from the fact that nutrient cultures have a much more consistent environment than soil culture systems in that water, nutrients, and phenolic acids are evenly distributed in the treatment container and thus are readily available to interact with root surfaces. Soil systems, on the other hand, are much more complex heterogeneous environments in which roots must compete with a variety of soil sinks (e.g., clays, organic matter, and microbes) for water, nutrients, and phenolic acids. There is also mechanical resistance to the movement of water, nutrients, and phenolic acids and the growth of roots in soils. The slower development of inhibition after treatment and the slower recovery after phenolic acid depletion in soil systems is very likely related to the slower growth of seedlings in soil culture. 

In summary inhibition of phenolic acid treatments took longer to develop and took higher concentrations in soil culture than in nutrient culture. Recovery after phenolic acid depletion in the root zone was also slower in soil culture than in nutrient culture. 

Singh, U.P., Sarma, B.K., Singh, D.P., Bahadur, A. Studies on exudates-deplete sclerotial development in Sclerotium rolfsii and the effect of oxalic acid, sclerotial exudates, and culture filtrate on phenolic acid induction in chickpea (Cicer arietinum). Can J Microbiol 2002 48 443-448. 

The phenolic acids of interest here [caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), p-coumaric acid (p-hydroxycinnamic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), sinapic acid (3,5-dimethoxy-4-hydroxyxinnamic acid), p-hydroxybenzoic acid, syringic acid (4-hydroxy-3,5-methoxybenzoic acid), and vanillic acid (4-hydroxy-3-methoxybenzoic acid)] (Fig. 3.1) all have been identified as potential allelopathic agents.8,32,34 The primary allelopathic effects of these phenolic acids on plant processes are phytotoxic (i.e., inhibitory) they reduce hydraulic conductivity and net nutrient uptake by roots.1 Reduced rates of photosynthesis and carbon allocation to roots, increased abscisic acid levels, and reduced rates of transpiration and leaf expansion appear to be secondary effects. Most of these effects, however, are readily reversible once phenolic acids have been depleted from the rhizosphere and rhizoplane.4,6 Finally, soil solution concentrations of... 

Mycophenolate mofetil (Cellcept) is an immunosuppressant approved for prophylaxis of organ rejection in patients with renal, cardiac, and hepatic transplants. Myco-phenolic acid, the active derivative of mycophenolate mofetil, inhibits the enzyme inosine monophosphatase dehydrogenase (IMPDH), thereby depleting guanosine nucleotides essential for DNA and RNA synthesis. Moreover, mycophenolic acid is a fivefold more potent inhibitor of the type 11 isoform of IMPDH found in activated B- and T-lymphocytes and thus functions as a specific inhibitor of T- and B-lymphocyte activation and proliferation. The drug also may enhance apoptosis. 

Cucumber seeds and seedlings have associated with them substantial microbial populations that are difficult to eliminate because microbes are not only found on and in the cutinized surface of the seed coat but can also be found internally within the seed (Leben 1961 Mundt and Hinkle 1976). Depletion of phenolic acids from nutrient solutions thus represent uptake by roots and microbial utilization. By replacing the nutrient solution (control) and nutrient-phenolic acid solutions (treatments) every other day, microbial populations were kept in check and phenolic acid concentrations were brought back to the original treatment concentrations. However, since phenolic acid treatments changed microbial populations on the rhizoplane (Fig. 2.9)... 

In addition, depletion rates also varied with the type of phenolic acid present, the number of phenolic acids tfeatments received by seedlings, the number and concentrations of phenolic acids present in a solution, and the level of aeration of the nutrient solution (Blum and Dalton 1985 Blum et al. 1985a Shann and Blum 1987a Lyu et al. 1990 Lyu and Blum 1990 Lehman and Blum 1999b Blum and Gerig 2005). For example ... 

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. 

Con 2 Phenolic acids were rapidly lost from surface wheat cover crop residues after desiccation and thus release of phenolic acids to soils was limited in time, roughly 3 or 4 weeks, for the cover crop residues tested. The rate of and time for depletion of phenolic acids from surface residues was determined largely by the extent and frequency of rainfall/irrigation events. For most of the growing season phenolic acids in soil extracts were, in fact, largely derived from older decomposed... 

Net depletion of phenolic acid by 12 day-old cucumber seedlings grown in a growth chamber (a r — 0.78) and by 14-18 day-old cucumber seedlings grown in a light bank (b r > 0.79), where FER equals femlic acid and POH equals p-hydroxybenzoic acid. Nutrient solutions were aerated. Initial pH values for nutrient solutions of (a) were 5.5. Initial pH values for (b) varied as indicated. 

The net depletion of phenolic acids in the absence or presence of a second phenolic acid at equal-molar concentrations from nutrient solution by 15-day old cucumber seedlings growing in a light bank. Where FER equals ferulic acid, PCO equals p-coumaric acid, and VAN equals vanillic acid and data in (a) are depletion of ferulic acid, b depletion for p-coumaric acid, and c depletion for vanillic acid. Nutrient solutions were not aerated and had an initial pH of 5.5. The absence of standard error bars indicates that the error bars are smaller than the symbols representing the mean. Figures based on data from Lyu et al. (1990). Plenum Publishing Corporation, data used with permission... 

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