Allelochemicals - American Chemical Society

C h a p t e r 32

Interactions Among Allelochemicals and Other Stress Factors of the Plant Environment F. A. Einhellig

Downloaded by CORNELL UNIV on July 24, 2016 | http://pubs.acs.org Publication Date: January 8, 1987 | doi: 10.1021/bk-1987-0330.ch032

Department of Biology, University of South Dakota, Vermillion, SD 57069

Current evidence indicates allelopathic inhibition most often results from the combined action of several different chemicals. A specific allelochemical may be present at a concentration below i t s growth inhibition threshold and s t i l l affect growth. Several combinations of allelochemicals have been shown to have either additive or synergistic action. Other work demonstrates that the action of phenolic acids is interrelated with nutrient conditions and temperature, moisture, and herbicide stress. Grain sorghum and soybean seedlings grown under relatively hot conditions exhibited a ferulic acid inhibition threshold at only one-half the concentration required under moderate temperatures, indicating stress interactions. Additive inhibition occurred when phenolic acids were tested in conjunction with moisture stress. Recent work showed that the growth of seedlings subjected to ferulic acid and atrazine together was suppressed more than with either alone. Allelochemicals may also promote damage from disease organisms. Hence, associated physical and chemical stress conditions may either enhance the inhibitory action of allelochemicals or result in an additive incremental detriment to plant growth.

It has been difficult to make an absolute connection between a suspected allelochemical inhibitor and the reduction in plant germination, growth, or function that characterizes a particular allelopathic situation. One reason this cause-effect relationship has been hard to establish is that the quantity of a biologically active compound recovered from the environment typically has been below the level required for inhibition in bioassays. Thus, a constant concern and argument against allelopathy has been that the level of an allelochemical in a natural setting is inadequate to be effective in growth regulation. However, the literature on allelopathy is replete with situations where several different 0097-6156/87/0330-0343$06.00/0 © 1987 American Chemical Society

Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


344

ALLELOCHEMICALS: ROLE IN AGRICULTURE AND

FORESTRY

chemicals have been i d e n t i f i e d and i n f e r r e d as substances which could cause interference. In f a c t , i n most of the cases where the putative chemicals have been sought a number of compounds with b i o l o g i c a l a c t i v i t y have been found. This suggests the p o s s i b i l i t y that a l l e l o p a t h i c interference may be the result of the simultaneous action of several compounds. Receiving plants often contact allelochemicals through the s o i l medium, yet the information on a v a i l a b i l i t y from the s o i l i s minimal (1). Much of what i s known concerning allelochemicals i n the s o i l references phenolic acids and closely related phenolic structures. Because these compounds have received more scrutiny than others, they w i l l be a central focus i n the subsequent discussion. Reported concentrations of p-coumaric, f e r u l i c , p-hydroxybenzoic, v a n i l l i c , and other phenolic acids i n the s o i l have varied according to what a l l e l o p a t h i c species colonized the area, abundance and duration of plant residue, s o i l type, environmental factors, time of year, and method of extraction. Although i n d i v i d u a l phenolic acids i n the s o i l may exceed 1,000 jag/g of s o i l (2-4) , i t i s probable that only a f r a c t i o n of t h i s contributes to allelopathy, because a large percentage may not be b i o l o g i c a l l y a c t i v e . For example, Whitehead et a l . (5) found that water extracts of p-coumaric, p-hydroxybenzoic, and v a n i l l i c acids from s o i l under quackgrass [Agropyron repens (L.) Beauv.] were equivalent to one micromolar or less for each compound in the s o i l solution. An extraction that might simulate limed conditions, 5% Ca(0H)2» gave values i n the 10 to 100 jiM range. These values are t y p i c a l , yet they are below concentrations that have been used i n tests for b i o l o g i c a l a c t i v i t y . A c h a r a c t e r i s t i c feature of allelopathy i s that the i n h i b i t o r y effects of a l l e l o p a t h i c compounds are concentration dependent. Dose-response curves with known compounds show an i n h i b i t i o n threshold. Below t h i s l e v e l either no measurable e f f e c t occurs, or stimulation may r e s u l t . Although the concentration of a compound required to exceed the i n h i b i t i o n threshold varies extensively according to d i f f e r e n t s e n s i t i v i t i e s among species and also among phases of the growth cycle for higher plants, the concept of an i n h i b i t i o n threshold seems consistent. Thus, i t i s reasonable to evaluate how, and i f , a subthreshold concentration of an allelochemical may contribute to a l l e l o p a t h i c interference. Also i n need of evaluation i s how environmental conditions may influence the deleterious action of an allelochemical and the concentration required for an e f f e c t . Such interactions are e s p e c i a l l y pertinent for those environmental situations that place some degree of stress on plant functions. This paper w i l l review the l i t e r a t u r e on the cooperative action of known allelochemicals. It w i l l also focus on the increasing evidence that the b i o l o g i c a l importance of these substances, especially i n low concentrations, depends on associated environmental conditions. Inhibition by Combinations of Allelochemicals The a l l e l o p a t h i c p o t e n t i a l of plants has often been evaluated from tests of the b i o l o g i c a l a c t i v i t y of v o l a t i l e s , leachates, and root exudates, or from aqueous extracts of the tissue. A l t e r n a t i v e l y , assessment of i n h i b i t o r s i n the s o i l associated with a suspected



32.

EINHELLIG

Stress Factors of the Plant Environment

345

a l l e l o p a t h i c plant has been routine. Such evaluations almost always deal with a complex matrix of biochemicals. When the bioassays have demonstrated effects on germination or growth, subsequent work on i d e n t i f i c a t i o n of the responsible allelochemicals has often followed. Although these searches have seldom been exhaustive, they have consistently revealed more than one compound with b i o l o g i c a l a c t i v i t y . I t has not been uncommon to i s o l a t e ten or more compounds which may include several different chemical classes (6-9). However, i t has been d i f f i c u l t to determine the quantity of each that might be functional i n the environment, and generally the r e l a t i v e contribution of each allelochemical to growth i n h i b i t i o n has not been evaluated. ι Some investigators have tested equimolar mixtures of chemicals they have isolated as the agents i n a l l e l o p a t h i c situations (10-12) , and a few have attempted to simulate combinations from f i e l d situations. Glass (13) grew plants hydroponically i n a solution which reproduced the phenolic acid conditions found i n the s o i l associated with Pteridium aquilinum (L.) Kuhn. This mixture of 39 uM p-hydroxybenzoic acid, 49 jiM v a n i l l i c acid, 42 μΜ p-hydroxycinnamic acid, and 4 μΜ f e r u l i c acid altered the root growth of barley (Hordeum vulgare L.) and several other species. In a study of hackberry (Celtis laevigata L.) allelopathy, Lodhi (3) found that the combined e f f e c t of p-coumaric, f e r u l i c , and c a f f e i c acids at the concentrations found i n s o i l underneath these trees was much more i n h i b i t o r y to seed germination than the e f f e c t of each chemical (at i t s s o i l concentration) tested separately. Weaver and K l a r i c h (14) reported an increase i n r e s p i r a t i o n rate i n wheat plants that were exposed i n the f i e l d to v o l a t i l e substances, presumably monoterpenes, from Artemisia tridentata Nutt., but the r e l a t i v e amount of d i f f e r e n t terpenes was not ascertained. A recent study of Lupinus albus L. showed the a l l e l o p a t h i c e f f e c t s of a mixture of q u i n o l i z i d i n e alkaloids which approximated that excreted from germinating seeds and seedlings (15). More d e f i n i t i v e e f f o r t s have been made to ascertain the concerted action of allelochemicals by quantitatively comparing the action of a mixture of substances with the a c t i v i t y of each component part (Table I ) . Although most of these studies have been with derivatives of benzoic acid, cinnamic acid, and coumarin, some evaluations of other compounds have occurred. Asplund (16) reported that the phytotoxic monoterpenes, camphor, pulegone, and borneol, exhibited marked synergistic action on root growth. Up to 100-fold enhancement was found using two compounds simultaneously, demonstrating that b i o l o g i c a l a c t i v i t y could occur with concentrations two orders of magnitude below the threshold f o r a single compound. Wallace and whitehead (17) showed the synergistic action of v o l a t i l e fatty acids, and their work demonstrated the value of recognizing that s i m i l a r compounds may have d i f f e r e n t a c t i v i t i e s . I t took ten times as much acetic acid to i n h i b i t wheat (Tritiown aestivum L.) as butyric or propionic acid. We have investigated the concurrent action of some of the more commonly reported phenolic allelochemicals by testing these compounds at, or below, their i n h i b i t i o n threshold (18-21). Our f i r s t experiments showed that a combination of 5 mM each of p-coumaric and f e r u l i c acids reduced grain sorghum [Sorghum bicotor (L.) Moench.] germination appreciably more than separate treatments


ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY

346

Table I .

Quantitative Assessment of E f f e c t s of Combinations of Allelochemicals a


Chemical C l a s s C o n e , of S i n g l e B i o a s s a y E f f e c t Cpd. i n Mixture

5

Ref.

Monoterpenes

0.017-0.68 μΚ/L

G

Syn

16

Fatty Acids

0.27 - 3.2 mM

RE

Syn

17

2.5 - 5.0 mM 0.125-0.25 mM

G SG

Syn Syn

18

2.5 - 5.0 mM 0.5 mM

G SG

Syn Syn

19

Phenolic Acids, etc. FA,pCA

VA,pHB

FA,VA,pCA

G,SG

3.3 mM

£CnA,pCA,FA,CA

CA,FA,pCA CA,FA,VA CA,FA,pCA,pHB, PRO,SIN,SYR,VA

G = germination;

Syn Syn Syn

21

1.0 mM

G

Several Add

22

1.0 - 3.0 mM

G

Add,Ant

23

0.5 mM

RE

Ant

24

0.125- 0.5 mM

SG

Syn,Add,Ant

25

RE = root elongation;

^Syn = s y n e r g i s t i c ; Add = additive; CA = c a f f e i c ;

20

G RE SE

1.0 - 2.5 mM 0.25 - 1.0 mM 0.04 - 0.1 mM

Coumarin,CGA,FA, HCnA,p CA,pHBAL,PYR

Syn,Ant

CGA = chlorogenic;

SG = seedling growth

Ant = antagonistic

FA = f e r u l i c ;

HCnA = hydrocinnamic; pCA = p-coumaric; pHB = p-hydroxybenzoic; pHBAL = p-hydroxybenzaldehyde; PRO = protocatechuic; PYR = pyrocatechol; SIN = sinapic; SYR = s y r i n g i c ; VA = v a n i l l i c ; £CnA = t-cinnamic



3 2.

EINHELLIG


341

of these chemicals (18). In tests with sorghum seedlings grown i n nutrient solution amended with p-coumaric and f e r u l i c acids, the threshold for growth reduction was less than l/20th the l e v e l required to reduce germination. Cooperative i n h i b i t o r y action of these phenolics was obvious since seedlings grown with 0.125 mM p-coumaric or f e r u l i c acids were s i g n i f i c a n t l y stimulated, whereas growth of plants i n a combination of the two was i n h i b i t e d . Similar studies showed the three-way i n t e r a c t i o n of p-coumaric, f e r u l i c , and v a n i l l i c acids on seed germination was s y n e r g i s t i c , while v a n i l l i c acid seemed to antagonize some of the i n h i b i t i o n of the other two on shoot elongation (20). Colby's (26) analysis was used as an index for judging potential interactions among four cinnamic acids (21). Based on this c r i t e r i o n , concentrations of 0.04 mM t-cinnamic acid and 0.1 mM f e r u l i c , p-coumaric, and c a f f e i c acids had synergistic effects on sorghum growth when applied i n combinations of two, three, and a l l four. Similar cooperative effects may also occur with mixtures of allelochemicals of d i f f e r e n t chemical categories. Work now i n progress (unpublished data) has shown s y n e r g i s t i c i n h i b i t i o n of sorghum germination and growth by a three-way combination of a flavonoid ( r u t i n ) , a coumarin (umbelliferone), and a benzoic acid ( s a l i c y l i c acid) . Antagonism among these three occurred i n Lernna minor L . bioassays, demonstrating that species vary i n their response. Investigations using above-threshold concentrations also indicate that several phenolic compounds i n a mixture can have at least a cumulative e f f e c t . Duke et a l . (23) concluded from probit analysis of data on lettuce seed germination that p-coumaric and f e r u l i c acids produced additive i n h i b i t i o n . Blum et a l . (25) tested eight phenolics i n various combinations of two or three on cucumber (Cucumis sativa L . ) leaf expansion, and applied regression analysis. The e f f e c t s of mixtures tested ranged from s y n e r g i s t i c to antagonistic, and the authors concluded that the nature of the response depended on the magnitude of i n h i b i t i o n associated with each compound, the compounds i n the mixture, and the factor measured. Methods for assessing the j o i n t action of i n h i b i t o r s have many d i f f i c u l t i e s , especially when used for studying the r e l a t i v e l y low concentrations that are most probable i n a f i e l d s i t u a t i o n . As pointed out by Morse (27) and Nash (28), nearly a l l methods have t h e i r shortcomings and even an unambiguous d e f i n i t i o n of terms such as synergism has remained elusive. However, these shortcomings and problems i n communication should not be allowed to detract from the b i o l o g i c a l importance of the j o i n t action of chemicals functioning in allelopathy. The preponderance of evidence indicates: (a) an allelochemical seldom acts alone, (b) concentrations considerably below i n h i b i t i o n thresholds i n bioassays may be b i o l o g i c a l l y active, and (c) the j o i n t action of several compounds can influence plant growth and functions. Viewed i n this manner, an a l l e l o p a t h i c substance that i s released into the environment can j u s t l y be considered one of several stresses that may influence plant d i s t r i b u t i o n and vigor of plant growth. Interaction Between Allelopathy and Mineral N u t r i t i o n An early observation that a l l e l o p a t h i c e f f e c t s might be subject to other environmental conAmg^t^^Ctl6ilfôâJiS06^q^f luence noted

Library 1155 16th St., N.W. Washington, D.C. 20038 Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


348


FORESTRY

from a d d i n g n u t r i e n t s (29). S e v e r a l i n v e s t i g a t i o n s i n the l a s t decade have demonstrated t h a t p h e n o l i c a l l e l o p a t h y may be more s e v e r e under low f e r t i l i t y , and r a i s i n g the n u t r i e n t l e v e l can s u p p r e s s some o f t h e a l l e l o c h e m i c a l e f f e c t (13,30). Stowe and Osborn (30) r e p o r t e d t h a t t o x i c i t y of v a n i l l i c (25 and 50 ppm) and p - c o u m a r i c (5 and 10 ppm) a c i d s t o b a r l e y p l a n t s depended i n t i m a t e l y on n u t r i e n t c o n c e n t r a t i o n s . Two-way a n a l y s i s of v a r i a n c e showed a d e f i n i t e i n t e r a c t i o n between p h e n o l i c t r e a t m e n t s and n i t r o g e n and phosphorus l e v e l s . V a n i l l i c acid inhibited barley growth i n a manner dependent upon phosphorus s u p p l y , and p - c o u m a r i c a c i d e f f e c t s were dependent upon n i t r o g e n l e v e l s . At low n u t r i e n t l e v e l s b o t h p h e n o l i c s were s i g n i f i c a n t l y i n h i b i t o r y , s u g g e s t i n g soil f e r t i l i t y might be v e r y i m p o r t a n t i n p h e n o l i c a l l e l o p a t h y . H a l l e t a l . (31) found pigweed (Amaranthus retroflexus L.) grown i n s o i l amended w i t h c h l o r o g e n i c a c i d was s t u n t e d and the p l a n t s had a r e d u c e d phosphorus c o n t e n t , but t h e s e e f f e c t s were overcome by a d d i n g a n i t r o g e n - p h o s p h o r u s - p o t a s s i u m supplement. Indeed, case s t u d i e s i n d i c a t e i n p u t s o f n i t r o g e n and phosphorus have a l l e v i a t e d a l l e l o p a t h i c i n h i b i t i o n from g o l d e n r o d (Solidago canadensis L.), t a l l f e s c u e (Festuca arundinacea S c h r e b . ) , and s u n f l o w e r (Helianthus

annuus L.) (31-33). S e v e r a l p h e n o l i c a c i d s and many n o n s p e c i f i c a l l e l o p a t h i c c o n d i t i o n s have been shown to a l t e r the m i n e r a l c o n t e n t of p l a n t s , and c e r t a i n l y p h e n o l i c a l l e l o c h e m i c a l s may p e r t u r b c e l l u l a r f u n c t i o n s i n a number of ways t h a t a r e o f importance t o p l a n t n u t r i t i o n (34 ,_35) . However, r a i s i n g f e r t i l i t y does not always s u p p r e s s a l l e l o p a t h i c i n h i b i t i o n , and t h e i n t e r r e l a t i o n s h i p s between t h e s e two f a c t o r s a r e s t i l l not c l e a r . Bhowmik and D o l l (36) showed t h a t a l l e l o p a t h i c i n h i b i t i o n o f c o r n and soybeans by r e s i d u e s o f f i v e a n n u a l weeds was n o t a l l e v i a t e d by s u p p l e m e n t a l n i t r o g e n o r phosphorus. S i m i l a r l y , an i n c r e a s e i n f e r t i l i z e r d i d not overcome i n h i b i t i o n o f c o r n by q u a c k g r a s s o r c i r c u m v e n t the a u t o t o x i c i t y of berseem c l o v e r (Trifolium alexandrium L.) (37,38). Even when r a i s i n g n u t r i e n t l e v e l s r e l e a s e s i n h i b i t i o n , i t does not mean t h a t a l l e l o p a t h y was i n o p e r a t i v e under the o r i g i n a l c o n d i t i o n s . These i n s t a n c e s s i m p l y i l l u s t r a t e the i m p o r t a n c e o f t h e i n t e r a c t i o n o f the two s t r e s s c o n d i t i o n s . Enhancement of A l l e l o c h e m i c a l E f f e c t s by

Temperature

Stress

I o f t e n o b s e r v e d t h a t under greenhouse c o n d i t i o n s t h e r e was c o n s i d e r a b l e v a r i a t i o n i n the e f f e c t a p a r t i c u l a r c o n c e n t r a t i o n o f a p h e n o l i c i n h i b i t o r had on the growth of s e e d l i n g s . Since p r o c e d u r e s used i n t h e s e b i o a s s a y s were q u i t e u n i f o r m , i t was l o g i c a l t h a t e n v i r o n m e n t a l f a c t o r s were i n f l u e n c i n g the r e s u l t s . T h i s was v e r i f i e d when we t e s t e d the h y p o t h e s i s t h a t t e m p e r a t u r e o f the growth environment m o d i f i e d a l l e l o c h e m i c a l a c t i o n (39). Grain sorghum and soybean [Glycine max (L.) M e r r . ] s e e d l i n g s were t r e a t e d w i t h s e v e r a l l e v e l s of f e r u l i c a c i d , t h e n each t r e a t m e n t group was s u b d i v i d e d and t h e two s u b s e t s h e l d a t d i f f e r e n t t e m p e r a t u r e s w i t h l i g h t i n t e n s i t y the same f o r t h e two e n v i r o n m e n t s . The two t e m p e r a t u r e regimes i n each experiment were w i t h i n the normal range the s e e d l i n g s might e x p e r i e n c e under f i e l d c o n d i t i o n s . However, the h i g h e r t e m p e r a t u r e s would g e n e r a l l y be c o n s i d e r e d more stressful.



32.

EINHELLIG


349

Seedling response over a 10-day treatment period showed a s i g n i f i c a n t interaction e f f e c t (two-way analysis of variance) between temperature and f e r u l i c acid. Although several morphological features were d i f f e r e n t , the dry weights of control plants i n the two temperature regimes were equivalent at the end of each experiment. Effects of equimolar concentrations of f e r u l i c acid were more severe at the higher temperatures. The threshold concentration f o r i n h i b i t i o n of sorghum growth was 0.2 mM f e r u l i c acid at 37° C average day temperature, while 0.4 mM f e r u l i c acid was required f o r i n h i b i t i o n with an average day temperature of 29° C. Both shoot and root weight reductions evidenced t h i s difference i n an i n h i b i t i o n threshold. Soybeans were more sensitive than sorghum to both temperature and f e r u l i c acid, but a s i m i l a r interaction between temperature and f e r u l i c acid occurred i n these experiments. Soybeans grown with a day temperature of 34° C and 0.1 mM f e r u l i c acid were s i g n i f i c a n t l y i n h i b i t e d , weighing 63% as much as control plants, while at 23° C even 0.25 mM f e r u l i c acid-treated plants were not stunted as severely. These experiments c l e a r l y established that r e l a t i v e l y hot environmental conditions enhanced allelochemical inhibition. Several other evidences that temperature v a r i a t i o n can a l t e r the extent of a l l e l o p a t h i c i n h i b i t i o n have been reported. Glass (13) subjected barley seedlings to a mixture of phenolic acids with subgroups at 5, 10, 15, 20, 25, and 30° C. The phenolic acid mixture suppressed root growth (fresh weight) i n each environment over a 14-day growth period, but the extent of i n h i b i t i o n was more severe at the extremes of low and high temperature. Steinsiek et a l . (40) reported leachates from wheat straw caused a more marked i n h i b i t i o n of germination and growth of sensitive weeds, such as ivy leaf morningglory \_Ipomoea hederaoea (L.) Jacq.], when incubated at 35° C than at 30 or 25° C. Both temperature and photosynthetic photon f l u x density altered the a l l e l o p a t h i c e f f e c t s of residues of redroot pigweed (Amaranthus retro flexus L.) and yellow f o x t a i l \Setaria glauoa (L.) Beauv.] on corn (41). The i n h i b i t o r y e f f e c t s of these weed residues were less when corn was grown with moderate l i g h t and at 30/20° C light/dark conditions, as compared to a lower l i g h t l e v e l and temperature. However, a l l e l o p a t h i c effects of the weed residues on soybean were not overcome at the more moderate conditions. Concurrent Action of Allelochemicals and Moisture Stress Phenolic acids interfere with many major physiological processes of higher plants (35). These disruptions of function include an a l t e r a t i o n of plant water balance. We found depression of leaf water potential to be an early indicator of allelochemical stress from f e r u l i c and p-coumaric acids (42). Likewise one mechanism of a l l e l o p a t h i c action by cultivated sunflower, velvetleaf (Abutilon theophrasti M e d i c ) , Kochia [Kochia scoparia (L.) Schrad.], and several other weeds was water stress (43-45). Since some allelochemicals i n t e r f e r e with plant-water relationships, i t seemed l o g i c a l that t h e i r action might be most c r i t i c a l at times when plants are under water stress from other causes. We i n i t i a t e d several experiments to determine the impact of allelochemicals acting simultaneously with moisture stress (46).



350


FORESTRY

Seeds were germinated i n vermiculite for f i v e days, then transplanted to 80-ml opaque v i a l s containing a complete nutrient medium. The following day, seedlings were treated by transferring them to nutrient solutions amended with f e r u l i c acid (0.1 or 0.25 mM), an osmoticum (e.g., -0.2 MPa), or both. After ten days growth in the greenhouse, plant dry weights were compared by analysis of variance and Colby's (26) analysis was applied for evaluating potential interactions. A l l osmotic agents may create some a n c i l l a r y e f f e c t s , but at the l e v e l s used i n these experiments equivalent results were obtained with either polyethylene g l y c o l 4000, KC1, or NaCl. The results of these experiments c l e a r l y established that the simultaneous actions of water stress and an allelochemical are more deleterious than either alone. This combination e f f e c t was especially evident with r e l a t i v e l y minor stresses from each source, as shown with data from one experiment (Table I I ) . Treatment of 0.1 mM f e r u l i c acid had no e f f e c t on growth, while nutrient media with an osmotic adjustment of -0.2 MPa caused some suppression of sorghum seedlings. These two together s i g n i f i c a n t l y reduced sorghum below the e f f e c t caused by the NaCl alone. Apparently moisturestressed plants were more sensitive to f e r u l i c acid. Dry weights of plants grown i n the combination of 0.25 mM f e r u l i c acid and -0.2 MPa were s i g n i f i c a n t l y below those observed i n the separate treatments, and Colby s analysis suggested the combined action at these levels was more than additive. Replicate experiments gave similar r e s u l t s . We found comparable cooperative effects i n studies of seed germination, except that both a lower water potential and higher phenolic acid concentration were required to achieve a germination i n h i b i t i o n threshold. Experiments using a matrix of four levels of f e r u l i c acid and four levels of moisture stress demonstrated that the combined action was additive under more s t r e s s f u l levels of the i n d i v i d u a l factors than i n the previous t e s t s . Duke et a l . (23) tested the germination of lettuce seeds treated with phenolic acids (1 mM) at water potentials (D-mannitol) of 0, -0.2, -0.4, and -0.6 MPa. The combined action of low water p o t e n t i a l and exposure to phenolic acids resulted i n an additive detriment to germination, and the authors concluded from probit analysis that the mechanism of action from these sources was s i m i l a r . Whatever their mechanisms, moisture stress and phenolic acids appear to work together i n l i m i t i n g growth of plants. Although i n d i r e c t and probably quite rare, another route has been reported for allelochemical interference with plant-water relationships. Lovett and D u f f i e l d (47) i d e n t i f i e d benzylamine as an allelochemical i n the leaf washings from the cruciferous weed Cconelina sativa (L.) Crantz. Subsequent work showed benzylamine induced hydrophobic conditions i n the s o i l , and these conditions could reduce water a v a i l a b i l i t y for plant growth (48). Thus, i n d i r e c t action through changes in s o i l structure could be p a r t i a l l y responsible for adverse e f f e c t s on linseed (Linseed usitatissirnvon L.) and could enhance more d i r e c t a l l e l o p a t h i c e f f e c t s . 1

Joint Action of Herbicides

and

Allelochemicals

Opportunity exists i n agroecosystems for two sources of chemical interference, natural and synthetic. The o r i g i n of allelochemicals


Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987. +8..7 65.4

63..6 56..7

300.6+25.9bc 267.9+20.3c

171. .1+12.9cd 155, ,6+11.9d

129, .5+13.5de

112, .3+ 8.9e

1

!

Positive difference from predicted = synergism.

product of % of control f o r single treatments

Means (N = 15) i n a column not followed by the same l e t t e r are s i g n i f i c a n t l y d i f f e r e n t , P

Allelochemicals - American Chemical Society

Recommend Documents