The Chemistry of Allelopathy - American Chemical Society

11 Effects of Allelochemicals on Mineral Uptake and Associated Physiological Processes Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 14, 2016 | http://pubs.acs.org Publication Date: December 17, 1985 | doi: 10.1021/bk-1985-0268.ch011

N E L S O N Ε. B A L K E Department of Agronomy, University of Wisconsin, Madison, WI 53706

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, i t s concentration, and environmental conditions (especially pH). Interference i n the growth of one plant by another plant can result from either competition f o r nutrients, water, and l i g h t or from chemicals released from one plant (donor) that a f f e c t the second plant ( r e c e i v e r ) . The l a t t e r phenomenon is known as allelopathy Cl) and may be as important as competition f o r influencing plant growth i n both natural and a g r i c u l t u r a l ecosystems. Many n a t u r a l l y occurring compounds (primarily secondary metabolites) produced by plants have been found to a l t e r the growth of plants (1). Although micro-organisms also produce compounds that can 0097-6156/ 85/0268-0161 $06.00/ 0 © 1985 American Chemical Society

Thompson; The Chemistry of Allelopathy ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 14, 2016 | http://pubs.acs.org Publication Date: December 17, 1985 | doi: 10.1021/bk-1985-0268.ch011

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THE CHEMISTRY OF ALLELOPATHY

affect plant growth, t h i s review w i l l be concerned with only allelochemicals known to be synthesized by higher plants. Although many physiological and biochemical processes i n plants are affected by various allelochemicals, i n most instances the d e t a i l s of the mechanism of action of a p a r t i c u l a r allelochemical have not been elucidated. Because soil mediates the transfer of most allelochemicals (except perhaps v o l a t i l e compounds) from a donor to a receiver, plant roots are often the f i r s t tissues to contact an allelochemical. Thus, i t is not surprising that root growth and development are inhibited i n many instances of allelopathy (17*3) One of the primary physiological functions of plant roots is the absorption of mineral nutrients. Therefore, i t is l o g i c a l that the influence of a l l e l o p a t h i c interactions on mineral absorption by plant roots has been investigated. Although the d e f i n i t i o n of allelopathy includes stimulation as well as i n h i b i t i o n of growth by allelochemicals (1, 4), allelochemi c a l s that d e f i n i t i v e l y a f f e c t mineral absorption by plant roots have been found to primarily i n h i b i t , rather than stimulate, the process. The f i r s t part of t h i s review presents evidence that a l t e r a t i o n of mineral absorption is a physiological mechanism of allelopathy. Possible physiological and biochemical bases f o r the i n h i b i t i o n of mineral absorption by allelochemicals are then discussed. Influence of F e r t i l i t y Level on A l l e l o p a t h i c Interactions The f i r s t finding to suggest that mineral absorption might be i n h i b i t e d by allelochemicals was a report i n 1912 by Schreiner and Skinner (5) showing that the i n h i b i t i o n of growth by phenolic compounds could be modified by adding nutrients to the water culture. Glass (6) also found that addition of mineral nutrients decreased the i n h i b i t o r y a c t i v i t y of a mixture of phenolics found i n soil under bracken f e r n (Pteridium equllinum L. Kuhn). More s p e c i f i c a l l y , nitrogen f e r t i l i z e r helped a l l e v i a t e the i n h i b i t i o n of growth i n birdsfoot t r e f o i l (Lotus cornlculatus L.) caused by extracts of t a l l fescue (Festuca arundinacea Schreb.) (7), and soluble phosphorus f e r t i l i z e r largely overcame the deleterious e f f e c t s of goldenrod (Solidago canadensis L.) extracts on n u t r i t i o n and growth of maple (Acer saccharum Marsh.) seedlings (8). Increased l e v e l s of nitrogen and phosphorous reduced the i n h i b i t i o n of barley (Hordeum vulgare L.) growth caused by jr-coumaric and v a n i l l i c acids (8a). Although other explanations are possible, these e f f e c t s of f e r t i l i z e r s suggest that i n h i b i t i o n of mineral absorption was responsible f o r the observed i n h i b i t i o n of growth. A l t e r a t i o n of Mineral Concentration i n Intact Plants One of the primary bases f o r the hypothesis that allelochemicals a f f e c t mineral uptake rests with the fact that the concentration of minerals i n receiver plants can be altered by donor plants by a mechanism other than competition for minerals. In general, studies i n support of t h i s hypothesis have involved measuring the amount of mineral present per weight of a p a r t i c u l a r plant part following absorption of minerals by the roots of the intact receiver. Although f o r many of these studies one can question whether


11.

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163

Allelopathic Effects on Mineral Absorption


competition was t r u l y eliminated and only a l l e l o p a t h i c e f f e c t s were being observed, some studies have c l e a r l y eliminated competition by physically separating the donor from the receiver. These l a t t e r , more convincing, experiments have used four basic approaches: (1) leachates from roots of donors growing separately from receivers, (2) donor residue incorporated into the medium i n which receivers are grown, (3) extracts of donor residue applied to receivers, and (4) s p e c i f i c allelochemicals added to the growth medium of receivers. Donor and Receiver Grown Together. Chambers and Holm (9) reported that one common bean (Phaseolus vulgaris L.) plant absorbed l e s s Ρθ£~ ( P ) when grown i n association with other bean, redroot pigweed (Amaranthus retroflexus L.) or green f o x t a i l (Setaria v l r i d i s L.) plants than when grown alone. Although t h i s r e s u l t could be interpreted as r e s u l t i n g from competition f o r P , one donor bean plant reduced ^2p uptake as much as two, three, or four bean plants. Also, pigweed absorbed seven times as much 3^P bean but had less e f f e c t on **2p uptake bv the receiver than did donor bean plants, which absorbed less ^P than pigweed. Thus, although competition was not eliminated i n t h i s study, the r e s u l t s suggested that allelopathy was responsible f o r the decreased 32 uptake. Two reindeer-moss species (Cladonia rangiferina L. and Cladonia a l p e s t r i s L.) decreased P0^~ concentration i n jack pine (Plnus banksiana Lamb.) and white spruce (Picea glauca Moench) (10). In addition, £. rangiferina decreased Ν concentration of the jack pine. However, K , C a , and M g concentrations i n both receivers were not altered by either donor. Although mycorrhizae were associated with receiver plants i n a l l the treatments, the authors concluded that impairment of e i t h e r mycorrhizal function i n absorbing P0$~ or P0^~ translocation i n the receiver was responsible for decreased P0$~ uptake. They suggested that allelopathy might be responsible. Two studies have suggested that quackgrass (Agropyron repens L.) can reduce uptake by maize (Zea mays L.) plants. Bandeen and Buchholtz (11) found that K , but not Ν or P0^~, content of corn was decreased by quackgrass growing with maize. Because high l e v e l s of f e r t i l i z a t i o n did not overcome the e f f e c t of quackgrass, the authors concluded that competition was not responsible (11, 12). Furthermore, when part of the receiver's root system was placed i n nutrient solution ( s p l i t - r o o t technique), the receiver grew much better. The authors concluded that quackgrass caused an a l l e l o p a t h i c i n h i b i t i o n of K and Ν uptake by maize (12). A l l the above studies suffer from the p o s s i b i l i t y that competition influenced the r e s u l t s . Any time the donor and receiver are grown together, the p o s s i b i l i t y of competition cannot be dismissed t o t a l l y . Thus, the donor and receiver must be grown separately to convincingly eliminate competition. 32

32

a s

3

P

+

2 +

2+

+

+

Donor and Receiver Grown Separately. When donor and receiver plants are grown separately, a method must be devised to transfer the allelochemical(s) from the donor to the receiver. This can be accomplished by applying leachates from the donor to the receiver. One method of doing t h i s is to arrange pots of the donor and pots of the receiver i n a " s t a i r s t e p " arrangement (13) so that nutrient


T H E C H E M I S T R Y OF A L L E L O P A T H Y

164

solution flows out the bottom of a donor pot into the top of a receiver pot. The pots can be separated s u f f i c i e n t l y to eliminate competition f o r l i g h t between the donor and the receiver. A simpler method is to f l u s h donor pots with nutrient solution and apply the leachate to receiver pots. Both of these techniques have been used to study a l l e l o p a t h i c a l t e r a t i o n of mineral absorption. Leachates from sand i n which hoop pine (Araucaria cunninghamii Ait.) slash pine (Pinus e l l i o t t i i Engl.), or crows ash ( F l i n d e r s i a a u s t r a l i s R. Br.) was growing decreased the P0$~ concentration of hoop pine seedlings growing i n sand or soil (14). Ν and K status was not affected s i g n i f i c a n t l y . S i m i l a r l y , when leachates from roots of Anthoxanthum odoratum, Lolium perenne, Plantago lanceolata, or Trifolium repens were applied to pots containing each of the same four species i n d i v i d u a l l y , some of the exudates decreased P0^~ uptake some receivers whereas other exudates stimulated P0^~ uptake (15). Using the stairstep design, Walters and Gilmore (16) found that leachates from the rhizosphere of fescue altered the mineral content of sweetgum (Liquidambar s t y r a c i f l u a L.) seedlings. K and M g concentrations i n the leaves increased and P0$~ concentration i n the roots decreased whereas Ν and C a were not a f f e c t e d .


+

+

2+

2 +

Donor Residue Incorporated into Growth Medium of Receiver. Because plant l i t t e r , i n 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 D o l l (18, 19). In a f i e l d study, ten weed and crop residues were Incorporated i n d i v i d u a l l y into plots into which maize or soybean (Glycine max L. Merr.) were planted subsequently. Most of the residues had no effect on N, P0$", or K"*" concentration i n the two crops. However, sunflower (Helianthus annus L.) residue increased P0$~ content i n soybean; barnyardgrass (Echlnochloa c r u s - g a l l i L. Beauv.), yellow f o x t a i l (Setaria lutescens Wiegel), and soybean residue decreased Ν content i n maize; common lambsquarter (Chenopodium album L.) and redroot pigweed increased K content i n both maize and soybean; sunflower residue increased ΡΟζ content i n soybean; and barnyardgrass increased K content i n soybean (18). A similar study conducted i n a controlled-environment f a c i l i t y with f i v e weed residues against maize and soybean again showed l i t t l e e f f e c t on Ν and IT uptake (19). Ragweed (Ambrosia a r t e m i s i f o l i a L.) decreased Ν content i n soybean: ragweed and velvetleaf (AbutiIon theophrasti Medic.) increased Ρθ|~ content i n soybean. However, a l l f i v e residues increased ΚΛ content i n both crops. Young and Bartholomew (20) incorporated Namarthria a l t i s s i m a (Poir.) Stepf. and Hubb. root residue into soil. The tops of Desmodium intorturm ( M i l l . ) Urb. that grew i n that soil i n the greenhouse contained less ΡΟς , although N, K , and Mg^ l e v e l s were not affected. The authors concluded that a l l e l o p a t h i c substances were responsible f o r the decrease i n Ρθ£~ content. In another greenhouse study, sunflower debris increased the P0$"" concentration of redroot pigweed but did not a l t e r the concentrations of Ν or K (21). When nutrient solutions were added, the P0$~ and Ν responses were the same, but K was s l i g h t l y increased by the debris. +

+

1-

T

+

+


11.

165

Allelopathic Effects on Mineral Absorption

BALKE


Although these studies u t i l i z i n g incorporated debris are valuable because they show the potential f o r allelochemicals to be released from plant l i t t e r , they suffer from a disadvantage. The amount of debris added and i t s carbon to nitrogen r a t i o might lead to alterations i n nutrient contents i n the soil as the result of p r o l i f e r a t i o n or s h i f t s i n populations of micro-organisms. Thus, a control i n which a material of s i m i l a r C/N r a t i o but lacking allelochemicals needs to be included f o r such studies to be conclusive. The above studies did not include such controls and thus are not d e f i n i t i v e . Leachates of Donor Residue. Use of leachates of donor plant residue results i n much less t o t a l material being put into the growth medium of the receiver. Thus, t h i s is a more refined manner i n which to test for a l l e l o p a t h i c i n h i b i t i o n of mineral absorption. Walters and Gilmore (16) incorporated either roots or leaves from fescue into the sand i n the donor pots of a stairstep design. The subsequent c i r c u l a t i o n of nutrient solution leached chemicals from the donor debris and carried them into the receiver pots. K , Mg , and C a contents i n the leaves of sweetgum were increased, P0$~ content was decreased, and Ν content did not change when leachates from fescue roots or leaves were applied. Fescue roots and leaves produced the same results on Ν and POj content i n sweetgum roots as leaves, but Κ , Mg** , and Ca^ content i n sweetgum roots was variable. Mulches of aster (Aster novae-angliae L.) or goldenrod placed on the top of pots i n which sugar maple were grown resulted i n lower PO^", C a , and M g content and higher K content i n the maple (8). The authors concluded that allelochemicals were leached from the mulches and caused the altered mineral contents i n the receiver. P0$~ content i n needles of red pine (Pinus resinosa A i t . ) was reduced when red pine trees were watered with aqueous extracts of Lonicera t a t a r i c a or Solidago gigentea f o l i a g e (22). +

2+

2 +

2 +

2+

+

Specific Chemicals. Very few studies have tested d i r e c t l y the influence of s p e c i f i c allelochemicals on mineral uptake by plants. In a l l the previous papers c i t e d i n t h i s review, no attempt was made to i s o l a t e and i d e n t i f y the chemicals responsible f o r the a l t e r a t i o n of mineral content i n the receiver. Obviously, i f we are to prove that some allelochemicals act by i n h i b i t i n g mineral absorption, i n d i v i d u a l chemicals must be tested (23). Olmsted and Rice (24) reported that chlorogenic and gallotannic acids decreased the amount of K and C a removed from nutrient solutions by seedlings of redroot pigweed during 12 days of growth. Because chlorogenic acid is a phenolic compound found i n sunflower, Hall et a l . (21) investigated the influence of chlorogenic acid on mineral uptake by redroot pigweed. The compound increased N, decreased P0^~, and did not affect K concentration during 7 weeks of pigweed growth. Another phenolic compound, c a f f e i c acid, altered the mineral content of Argyrodendron t r i f o l i o l a t u m a f t e r 6 months (25). Z n , Mn , and ΡΟζ contents increased at 10 ppm c a f f e i c acid, but 50 ppm decreased M n and P0$"~ compared to t h e i r concentrations at 10 ppm. C o l l e c t i v e l y , these experiments show that mineral concentrations +

2 +

+

2 +

2+

2+




166

i n plants can be altered when plants grow i n situations where allelopathy may e x i s t . The concentration of many d i f f e r e n t minerals can be a l t e r e d , but for any p a r t i c u l a r mineral, a p a r t i c u l a r donor/receiver combination may produce an increase, a decrease, or no change i n the mineral concentration i n the receiver. This is not surprising because both competition and concentrations of natural products w i l l d i f f e r from donor to donor and even within a single donor depending upon growth conditions. Thus, i t is d i f f i c u l t to reach any general conclusions from experiments with l i v i n g tissue, debris, or leachates from donors because the i d e n t i t y and quantity of the p o t e n t i a l allelochemicals present are not known. Furthermore, the long time periods of these experiments (12 days to 10 months) make i t impossible to conclude that allelochemicals d i r e c t l y a l t e r mineral absorption. An equally f e a s i b l e explanation is that the chemicals i n h i b i t growth and t h i s i n d i r e c t l y a l t e r s the concentration of minerals i n the t i s s u e s . One way to address the direct nature of the a l l e l o p a t h i c a l t e r a t i o n of mineral absorption is to measure absorption over short periods of time. Anthoxanthum adoraturn, Lollurn perenne, Plantago lanceolata, or T r i f o l i u m répens plants grown i n leachates from l i v i n g Plantago lanceolata roots f o r 3 days subsequently absorbed less P0^~ during a 4-hour exposure to 32p than did untreated roots (15). Intact roots of soybean absorbed less P during a 90-minute exposure to f e r u l i c acid than did unexposed roots (26). F e r u l i c acid did not a f f e c t the percentage of absorbed ^2p that was translocated to the shoot. Thus, both an a l l e l o p a t h i c leachate (15) and a pure allelochemical (26) affected P absorption during short enough time periods that growth would have been minimal. These two experiments provide the strongest data showing that allelochemicals can i n h i b i t mineral absorption by intact plants. I n h i b i t i o n of Mineral Absorption i n Excised Roots. More conclusive evidence that allelochemicals can i n h i b i t mineral absorption has been obtained using p u r i f i e d allelochemicals and excised plant roots as the experimental system (Table 1). Use of excised roots eliminates the p o s s i b i l i t y that e x i s t s with intact plants that i n h i b i t i o n of translocation rather than absorption is responsible for decreased mineral content. Use of p u r i f i e d allelochemicals rather than plant debris or leachates allows more d e f i n i t i v e conclusions to be reached regarding the capacity of allelochemicals to i n h i b i t mineral absorption. Several general c h a r a c t e r i s t i c s of the results compiled i n Table I are worthy of mention. Compared to the variety of chemicals postulated to be involved i n allelopathy (1), few s p e c i f i c compounds have been tested f o r i n h i b i t i o n of mineral absorption. The most extensively studied compounds are the phenolic acids, probably because of t h e i r being ubiquitously found i n nature 01). Also, several flavonoids are i n h i b i t o r y to mineral absorption (Table I ) . Both of these groups of compounds are often c i t e d as being responsible for a l l e l o p a t h i c interactions between plants. Absorption of both cations and anions can be i n h i b i t i e d by various allelochemicals. Most studies have been conducted with R b (a tracer f o r K ) or P0^~ but CI" and N O 3 absorption (34) and even phenolic glycoside absorption (27) was shown to be inhibited by several flavonoids . The lack of s p e c i f i c i t y of these 3 2

3 2

8 6

+

+


11.

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Allelopathic Effects on Mineral

167

Absorption

Table I . E f f e c t s of naturally-occurring phenolic compounds on mineral absorption i n excised roots.

CHEMICAL CLASS Chemical

Plant Species^

Mineral ion

Chemical Treatment Cone. Time Ref.


SIMPLE PHENOLS Hydroquinone

Hv

+

K

5

mM

3 hr

(27)

PHENOLIC ACIDS ^ BENZOIC S a l i c y l i c acid

Hv

POJj-

500

uM

3 hr

(28-30)

Hv

K

+

250

μΜ

3 hr

(31)

As

K

+

500

μΜ

1 hr

(32)

Hv

pojj"

500

μΜ

3 hr

(28,

250

μΜ

3 hr

(31)

500

μΜ

1 hr

(26)

500

μΜ

1 hr

(32)

100

μΜ

10 min

(33)

10

μΜ

ι» hr

(3i)

10

μΜ

hr

(3i)

CINNAMIC F e r u l i c acid

+

Hv

K

Gm

Pojj"

As

K

+

As

K

+

Ta

CI", NO^

30)

NAPHTHOQUINONES Juglone FLAVANONES Naringenin

POjj" ISOFLAVONES Genistein

N03

Ta

C1",

As

K

+

100

μΜ

10 min

(33)

As

K

+

100

μΜ

10 min

(33)

FLAVONOLS Kaempferol DIHYDROCHALCONES Phloretin

—^Abbreviations

of plant species:

As, Avena sativa; Gm, Glycine

max; Hv, Hordeum vulgare; Ta, Triticum aestivum. —^Several additional benzoic and cinnamic acid derivatives were ^Several additior tested ( 2 8 - 3 1 ) .


168


chemicals towards i n d i v i d u a l minerals needs to be studied further. Another l i m i t a t i o n to the studies i n Table I is the small number of plant species tested. Primarily monocotyledonous plants have been studied, although McClure et a l . (26) found f e r u l i c acid i n h i b i t o r y i n soybean. The r e s t r i c t i o n of studies to monocots is probably because the mechanism of mineral absorption has been more f u l l y elucidated with monocots. Harper and Balke (32) reported some minor differences i n the i n h i b i t i o n of K absorption by s a l i c y l i c acid among oats (Avena sativa L.), wheat (Triticum aestivum L.), barley, and maize roots. Chemical concentrations of O.1 to O.5 mM were used routinely f o r these studies (Table I ) , but concentrations as low as 10 yM (naringenin and genistein) (34) and as high as 5 mM (hydroquinone) (27) were i n h i b i t o r y . Phenolic compounds i n the 10 to 100 yM concentration range have been extracted from soil (35, 36). Thus, i t would appear that most of these studies have been performed at the upper l i m i t of phenolic concentrations that can be expected i n s o i l s . However, because several phenolics are usually found together i n s o i l s and many phenolics i n h i b i t mineral absorption (28, 31), additive or even s y n e r g i s t i c i n h i b i t i o n from mixtures of phenolics can be expected. Also, pH has a dramatic e f f e c t on the degree of i n h i b i t i o n produced by a given concentration of a phenolic acid. As pH was decreased, K absorption was i n h i b i t e d more by s a l i c y l i c acid (Figure 1). Thus, at a c i d i c pH's much lower concentrations of phenolic acids are required to i n h i b i t mineral absorption. The short time periods (10 min to 4 hr) over which absorption was measured (Table 1) helps support the hypothesis that c e r t a i n allelochemicals i n h i b i t mineral absorption d i r e c t l y . Under a c i d i c conditions (pH 4.0) s a l i c y l i c acid i n h i b i t e d K absorption within 1 min (32). The degree of i n h i b i t i o n remained constant over time when s a l i c y l i c acid inhibited absorption (32) and when v a n i l l i c acid inhibited Ρ0^~ absorption (28). Thus, at least phenolic acids appear to i n h i b i t absorption rapidly and c o n s i s t e n t l y . Two additional c h a r a c t e r i s t i c s of the i n h i b i t i o n of mineral absorption by phenolic acids were observed. The i n h i b i t i o n 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 t h i s r e v e r s i b i l i t y to be dependent upon pH; the lower the pH, the less the r e v e r s a l . Also, k i n e t i c plots of the i n h i b i t i o n of mineral absorption showed that the phenolic acids did not competitively i n h i b i t either Ρθ£~ (26, 28) or K (31) absorption. Rather, f e r u l i c acid i n h i b i t e d P0^~absorption i n a noncompetitive (26) or uncompetitive (28) manner and £-hydroxybenzoic acid i n h i b i t e d K absorption i n an uncompetitive manner (31). Two studies have used single c e l l s to study the e f f e c t of phenolic acids on mineral absorption. In s t e r i l e c e l l cultures of Paul's Scarlet rose, 100 yM f e r u l i c acid i n h i b i t e d Rb absorption i n about 10 min when the c e l l s were 4-5 days old (37). Uptake from O.2 mM RbCl was i n h i b i t e d about 25% and absorption from 5.0 mM RbCl was inhibited 45%. Absorption by 10-day-old c e l l s was affected l i t t l e . S a l i c y l i c acid at 10 yM inhibited P0^~ absorption by Scenedesmus, a u n i c e l l u l a r green alga (38). These studies show that allelochemicals i n h i b i t mineral absorption i n c e l l u l a r systems as well as tissue systems (Table I ) .


+

+

+

+

+

+

+


11.

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Allelopathic Effects on Mineral

Absorption

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Mechanism(s) of Action It would be valuable to determine how allelochemicals i n h i b i t mineral absorption because such knowledge might indicate how to control the i n h i b i t i o n . Several recent reviews outline the fundamental biochemical and biophysical aspects of mineral absorption by plant roots (39-41), and the reader is referred to them for detailed discussion of the process of mineral absorption. Mineral ions move into c e l l s i n response to an e l e c t r i c a l p o t e n t i a l difference (PD) that is maintained across the plasma membrane and tonoplast. Part ( d i f f u s i o n potential) of t h i s PD a r i s e s from d i f f e r e n t permeabilities of cations and anions through the membranes, but the major portion (electrogenic potential) of the potential is produced by an electrogenic pump. The electrogenic pump moves e l e c t r i c a l charge (ions) across the membrane. These pumps require metabolic energy and are believed to be ATPase (ATP phosphohydrolase) enzymes located i n the plasma membrane and tonoplast. In root c e l l s these ATPases u t i l i z e ATP produced by mitochondria and are believed to pump H out of the cytoplasm. Based on this model of active mineral absorption, one can hypothesize several ways that allelochemicals could i n h i b i t mineral absorption: (1) a l t e r the PD, (2) i n h i b i t ATPases, (3) decrease c e l l u l a r ATP content, and (4) a l t e r membrane permeability to ions. +

A l t e r a t i o n of E l e c t r i c a l Potential (PD). Study of the influence of allelochemicals on the e l e c t r i c a l potentials across plant c e l l membranes has been r e s t r i c t e d to phenolic acids. Glass and Dunlop (42) reported that at pH 7.2, 500 μΜ s a l i c y l i c acid depolarized the e l e c t r i c a l potential i n epidermal c e l l s of barley roots. The e l e c t r i c a l potential changed from -150 mV to -10 mV within 12 min. Recovery of the PD was very slow over about 100 min when the s a l i c y l i c acid was removed. As the concentration of the allelochemical was increased, the extent of depolarization increased, but the time required f o r depolarization and recovery were constant. S a l i c y l i c acid depolarized PD i n epidermal c e l l s of oat roots also. At pH 4.5, 500 μΜ s a l i c y l i c acid caused a transient hyperpolarization followed by a dramatic depolarization to about -45 mV (Figure 2). Removal of s a l i c y l i c acid produced a transient, p a r t i a l r e p o l a r i z a t i o n . At pH 6.5, s a l i c y l i c acid d i d not a f f e c t PD. These results with d i f f e r e n t pH's are consistent with the influence of s a l i c y l i c acid on K absorption i n oat roots (32). Other l i p o p h i l i c weak acids have been shown to a l t e r PD i n plant c e l l s . Benzoic and butyric acids (1 μΜ) rapidly depolarized the PD i n oat c o l e o p t i l e c e l l s at pH 6.0 to about -100 mV (43). Higher concentrations (10 mM) of butyrate produced hyperpolarization. Butyrate also hyperpolarized a p i c a l c o r t i c a l c e l l s of maize roots (44) . The hyperpolarization was accentuated at pH 5.5 compared to pH 6.4. In c e l l s of the fungus Neurospora crassa, 5 mM butyrate (pH 5.8) produced a transient hyperpolarization followed by depolarization (45) . The chemical also increased the pH of the cytoplasm of the c e l l s . Whether phenolic acids also increase cytoplasmic pH is not +


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The Chemistry of Allelopathy - American Chemical Society

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