Minerals absorption

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 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. 

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. 

Donor Residue Incorporated Into Growth Medium of Receiver. Because plant litter, In addition to root exudates, Is a potential source of allelochemicals (17), several studies have Investigated the Influence of residues of plants on mineral absorption by receivers. Two extensive studies have been done by Bhowmik and Doll (18, 19). 

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. 

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. 

These two experiments provide the strongest data showing that allelochemicals can inhibit mineral absorption by intact plants. 

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. 

Table I. Effects of naturally-occurring phenolic compounds on mineral absorption in excised roots.

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. 

The short time periods (10 min to 4 hr) over which absorption was measured (Table 1) helps support the hypothesis that certain allelochemicals inhibit mineral absorption directly. Under acidic conditions (pH 4.0) salicylic acid inhibited K+ absorption within 1 min (32). The degree of inhibition remained constant over time when salicylic acid inhibited 1C " absorption (32) and when vanillic acid inhibited P0 absorption (28). Thus, at least phenolic acids appear to inhibit absorption rapidly and consistently. 

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). 

Two studies have used single cells to study the effect of phenolic acids on mineral absorption. In sterile cell cultures of Paul s Scarlet rose, 100 pM ferulic acid inhibited Rb+ absorption in about 10 min when the cells were 4-5 days old (37). Uptake from 0.2 mM RbCl was inhibited about 25% and absorption from 5.0 mM RbCl was inhibited 45%. Absorption by 10-day-old cells was affected little. Salicylic acid at 10 pM inhibited PO - absorption by Scenedesmus, a unicellular green alga (38). These studies show that allelochemicals inhibit mineral absorption in cellular systems as well as tissue systems (Table I). 

Based on this model of active mineral absorption, one can hypothesize several ways that allelochemicals could Inhibit mineral absorption (1) alter the PD, (2) Inhibit ATPases, (3) decrease cellular ATP content, and (4) alter membrane permeability to Ions. 

Benzoic acid derivatives also altered the electrical potential across the cell membrane in neurons of the marine mollusk Navanax lnermls (46). Salicylic acid (1-30 mM) caused a depolarization very rapidly (1-2 min) and decreased the ionic resistance across the membrane. As pH was decreased, more salicylic acid was required to reverse the effect of pH on the membrane potential (47). This result is contradictory to the influence of pH on the amount of salicylic acid required to affect mineral absorption in roots (32). The ability of a series of salicylic and benzoic acid derivatives to increase PD correlated with their octanol/water partition coefficients and pKa values (48). The authors proposed that the organic acid anions bound directly to membranes to produce the observed results. 

Effects of Allelochemlcals on ATPases. Several flavonoid compounds inhibit ATPase activity that is associated with mineral absorption. Phloretin and quercetin (100 pM) inhibited the plasma membrane ATPase Isolated from oat roots (33). The naphthoquinone juglone was inhibitory also. However, neither ferulic acid nor salicylic acid inhibited the ATPase. Additional research has shown that even at 10 mM salicylic acid inhibits ATPase activity only 10-15% (49). This lack of activity by salicylic acid was substantiated with the plasma membrane ATPase Isolated from Neurospora crassa (50) however, the flavonols fisetln, morin, myricetin, quercetin, and rutin were inhibitory to the Neurospora ATPase. Flavonoids inhibited the transport ATPases of several animal systems also (51-53). Thus, it appears that flavonoids but not phenolic acids might affect mineral transport by inhibiting ATPase enzymes. 

Effects of Allelochemlcals on ATP Supply. Allelochemlcals might decrease the ATP content of tissue by either increasing ATP utilization or decreasing ATP production. Some allelochemlcals that inhibit mineral absorption decrease ATP content of plant tissues. Salicylic acid decreased the ATP content of oat roots in a pH dependent manner (Figure 3). This result suggested that mitochondrial production of ATP was decreased in the tissue. On the other hand, Tillberg (38) found that salicylic acid and cinnamic acid increased the ATP content of Scenedesmus. Various flavonoids inhibited ATP production by mitochondria Isolated from cucumber (Cucumls satlvus L.) hypocotyls (54). Flavones such as kaempferol were more inhibitory than the corresponding flavanones. Substituted cinnamic acids such as caffeic acid were not inhibitory. 

Alteration of Membrane Permeability. The ability of allelochemicals to alter membrane permeability and thus inhibit mineral absorption has been investigated in detail with only phenolic acids. Salicylic acid induced the efflux of PO5 (28) and 1C" (42) from barley roots, but -hydroxybenzoic acid did not cause the efflux of K+... 

It is quite possible that phenolic acids may produce more than one effect on the cellular processes responsible for mineral absorption. The potential sites of action discussed above all involve cellular membranes in some way. Which mechanism of action is predominant in a given situation may depend upon the concentration of allelochemicals present and the conditions (e.g. pH) of the plant/chemical interaction. 

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. 

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. 

Phytate (myo-inositol hexaphosphate Fig. 15.3, structure 33) is found in many food species and can be considered as a phytochemical. Its role in the plant is primarily as a phosphate store in seeds, but it is found in other tissues as well, for example, tubers (Harland et al., 2004). Phytate and its hydrolysis products are anti-nutrients that chelate metal ions and thus reduce their bioavailability (Persson et al., 1998 House, 1999). This is particularly a problem with cereal grains, but pre-processing can improve mineral absorption from these foods (Agte and Joshi, 1997). There is some concern that high phytate foods could also contain higher levels of toxic heavy metals caused by natural accumulation. Plants also contain phytate-degrading enzymes that can also influence metal ion bioavailability (Viveros et al., 2000). 

We have developed an in vitro digestion procedure, not as a substitute for in vivo studies, but as a useful adjunct. Our initial objective was to develop an in vitro procedure for measuring exchangeability, the fraction of the food mineral which exchanges with an extrinsic isotope tracer added to the food. This was expected to facilitate the measurement of food mineral absorption in humans by the extrinsic tag method. Secondary objectives were to determine if in vitro mineral solubility could be used to estimate potential... 

Fractionation Methods. Ultrafiltration and gel filtration are nondestructive methods which, based on limited experience, can be used for fractionation of mineral complexes from digests. In earlier studies mineral absorption on the gel material was a problem. Lonnerdal (30) introduced a method of treating dextran gels with sodium borohydride in order to eliminate the mineral-binding sites on the gel. In preliminary studies we have recovered more than 90 of Ca, Mg, Fe, Zn and P in samples applied to a borohydride-treated gel column (Sephadex G-50, Pharmacia Fine Chemicals, Piscataway, NJ). Recovery of Ca (Table IV) and Mg, Fe and Zn from ultrafiltration was also good. 

It is possible that plants could bring about the concentration of buried actinides over many years by the Goldschmidt or Verhadskii principle (68). This is a cyclic process of mineral absorption from soil depths, death and mineral release into the upper layers of the soil where the cations become complexed with the humates. In some recently published work Cleveland and Rees (69) report that they found little evidence... 

Asvarujanon, P., Ishizuka, S., and Hara, H. (2004). Inhibitory effects of psyllium on rat mineral absorption were abolished by reduction of viscosity with partial hydrolysis. Biosci. Biotechnol. Biochem. 68,1737-1742. 

Casein Mineral absorption promoters Improved mineral absorption in intestine... 

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