The Chemistry of Allelopathy - American Chemical Society

3 Assessment of the Allelopathic Effects of Weeds on Field Crops in the Humid Midsouth C. D. ELMORE

Downloaded by NATL TAIWAN UNIV on July 1, 2015 | http://pubs.acs.org Publication Date: December 17, 1985 | doi: 10.1021/bk-1985-0268.ch003

Southern Weed Science Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Stoneville, MS 38776

Weed interference, which consists of the combined effects of competition and allelopathy, i s responsible for m i l l i o n s of dollars i n crop loss i n the humid Mid-South. The a l l e l o p a t h i c component of interference has not been c l e a r l y determined under field conditions. A l i t e r a t u r e review indicated allelopathy (as determined by greenhouse and laboratory assay) to be a potential factor i n the loss caused by many weeds of the Mid-South; e.g. johnsongrass [Sorghum halepense (L.) Pers.], purple nutsedge (Cyperus rotundus L.), yellow nutsedge (Cyperus esculentus L.), prostrate spurge (Euphorbia maculata L.) and velvetleaf (Abutilon theophrasti Medic.). In addition, wheat (Triticum aestivum L.) straw, p r i c k l y sida (Sida spinosa L.), and pigweeds (Amaranthus sp.) have been p r e l i m i n a r i l y implicated as having a l l e l o p a t h i c effects on f i e l d crops i n the humid Mid-South. Although allelopathy by weeds i s a potential factor i n crop losses, no published reports have unequivocally shown an a l l e l o p a t h i c effect on the weed-crop associations i n the f i e l d i n the Mid-South. Therefore, a needed first approach to the demonstration of allelopathy i n the f i e l d i s to determine if weed residues have an a l l e l o p a t h i c effect on Mid-South crops. Then, either a s t a t i s t i c a l or b i o l o g i c a l approach could be devised to p a r t i t i o n weed interference into i t s components, competition and allelopathy.

Weeds are an enormous problem a f f e c t i n g f i e l d crops i n the Mid-South. They reduce y i e l d s , increase the cost of production, reduce the quality of the harvested produce, and decrease the value of the marketed product. These effects of weeds are present wherever crops are grown; however, they seem to be more devastating This chapter not subject to U.S. copyright. Published 1985, American Chemical Society

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


22

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

and pernicious i n warm, humid temperate, and tropic regions than i n cooler and d r i e r climates. How weeds produce their effect i s the subject of continuing research. As rather recently defined, the negative effect of weeds on crop plants includes both competition and allelopathy (!) and has been termed interference. Previous weed science research considered the competition component foremost and, usually, solely (e.g. 2). Allelopathy, however, has received some attention recently, with a number of symposia and reviews devoted to a l l e l o p a t h i c e f f e c t s . Some of these cover the e f f e c t s of weeds on crops (3, 4, 5). In this chapter, I review the possible role of allelopathy i n weed interference i n crop production i n the Mid-South. A second objective of t h i s review i s to indicate where research i s needed and to suggest p o t e n t i a l l i n e s of future research, especially with respect to the developing role of conservation t i l l a g e practices i n t h i s region. A l l e l o p a t h i c Weeds The p r i n c i p a l summer annual row crops i n the Mid-South include cotton (Gossypium hirsutum L . ) , corn (Zea mays L.) soybeans [(Glycine max (L.) Merr.], grain sorghum (Sorghum b i c o l o r (L.) Moench.), peanuts (Arachis hypogea L . ) , and tobacco (Nicotiana tabacum L . ) . Sugarcane [(Saccharum officinarum L.) (not an annual crop)], the small grains wheat (Triticum aestivum L.) and r i c e (Oryza sativa L.) and small-acreage specialty crops such as sunflower (Helianthus annuus L.) and syrup crops are not considered i n t h i s review. The common and troublesome weeds present i n these s i x crops i n the 13 Southern states (VA, NC, SC, GA, FL, AL, MS, TN, KY, AR, LA, TX and OK) are regularly surveyed by State Extension Weed S p e c i a l i s t s . The most recent survey includes 59 weeds which are common or troublesome i n these six f i e l d crops (6). Twenty weeds on that l i s t have been implicated as being a l l e l o p a t h i c (Table 1); eleven are among the 10 most common i n at least one of the s i x major f i e l d crops i n the Southern states. Ten of these 11 weeds are considered to be among the 10 most troublesome, usually for the same crop i n which i t was found to be common. There i s one weed [bermudagrass (Cynodon dactylon (L.) Pers] that i s on the most troublesome l i s t for cotton and tobacco, but not on any most common list. S i m i l a r l y , giant f o x t a i l (Setaria faberi Herrm.) i s a common weed i n tobacco, but apparently not a troublesome one. An interesting feature of t h i s l i s t i s that some of our most troublesome and d i f f i c u l t to control weeds are not suspected a l l e l o p a t h i c s . Many weeds, such as morningglory (Ipomoea spp.), cocklebur (Xanthium strumarium L . ) , and sicklepod (Cassia o b t u s i f o l i a L . ) , which make almost a l l l i s t s of serious weed species, are strong competitors with l i t t l e or no evidence of a l l e l o p a t h i c a c t i v i t y . In f a c t , cocklebur i s probably the epitome of what a competitor can be, being even more devastating than



3.

ELMORE

Allelopathic Effects of Weeds

23

johnsongrass to soybeans (17). Perhaps the right tests have not been reported for these weed species ( E i n h e l l i g , at t h i s symposium, reports that cocklebur i s i n fact highly a l l e l o p a t h i c to soybeans). More could be said, but s u f f i c e i t to say that weed competition i s a strong force; i n some cases, i t i s the only force needed by weeds to exert t h e i r detrimental effect on crops. One of the features of allelopathy, as pointed out by Rice (18), i s that i t i s s p e c i f i c . Certain species, but not others, are affected by the allelochemics produced by a plant. This suggests that allelopathy by a weed must be p o s i t i v e l y demonstrated for each crop. References to s p e c i f i c reports of demonstrated allelopathy by a weed on a crop are given i n Table 1. In some cases [crabgrass ( D i g i t a r i a sanguinalis (L.) Scop.), spurge (Euphorbia spp.), e t c . ] , no crop i s l i s t e d i n the allelopathy column. Table I provides general information on the i d e n t i f i e d a l l e l o p a t h i c weeds and the crops they a f f e c t . More detailed information on two of these weeds, johnsongrass and purple nutsedge, w i l l be presented to show the tenuous nature of the evidence for allelopathy. These two species are undoubtedly a l l e l o p a t h i c , at least under c e r t a i n conditions. Rigorous proof that allelopathy i s the agent responsible for even a s p e c i f i c portion of the interference exerted by them i s not e a s i l y attained, however, even though these are the weeds with the best research data available of those weeds that occur i n the Mid-South. Johnsongrass. Johnsongrass was one of the f i r s t weeds to be implicated as having a l l e l o p a t h i c p o t e n t i a l (19). Friedman and Horowitz (20) demonstrated that dried and fragmented pieces of johnsongrass rhizomes added to s o i l (50 g/kg) and l a t e r (2 and 4 months) extracted with d i s t i l l e d water inhibited seedling development of barley (Hordeum vulgare L . ) , mustard [Brassica nigra (L.) Koch.] and wheat. In a subsequent test, barley sown i n the s o i l to which the rhizomes had been placed, but removed after 2 or 4 months, also seemed to be inhibited i n seedling development (21). Ethanolic extracts of the s o i l from which the rhizome fragments had been removed also showed a l l e l o p a t h i c a c t i v i t y . This same technique worked with cotton as the test species (10). In a similar experiment, Lolas and Coble (2L) showed that johnsongrass was a l l e l o p a t h i c to soybeans. They collected s o i l from a johnsongrass-infested f i e l d and removed the rhizomes before assay. A regression analysis was used to test the johnsongrass rhizome biomass for a l l e l o p a t h i c a c t i v i t y . These tests demonstrate that rhizomes of johnsongrass have b i o l o g i c a l a c t i v i t y and that the a c t i v i t y i s residual. The allelochemic usually associated with Sorghum species i n general, and with johnsongrass i n p a r t i c u l a r , i s the cyanogenic glucoside dhurrin Q ) or i t s decomposition product (p-hydroxybenzaldehyde). Whether these compounds act as allelochemics i n f i e l d situations i s unknown and somewhat suspect, since they would surely be immobilized or altered i n most s o i l situations, as was shown for


In The Chemistry of Allelopathy; Thompson, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985. corn, cotton peanuts, sorghum soybeans, tobacco corn, cotton peanuts, sorghum soybeans cotton, peanuts soybeans, tobacco

cotton, peanuts soybeans, tobacco cotton

tobacco

Pigweeds Amaranthus spp.

Johnsongrass Sorghum halepense (L.) Pers

Purple nutsedge Cyperus rotundus L.

Yellow nutsedge Cyperus esculentus L.

Spurge Euphorbia spp.

Giant f o x t a i l Setaria f a b e r i Herrm.

Sunflower Helianthus annuus L.

Common corn, peanuts cotton, sorghum soybeans, tobacco

Weed Large crabgrass D i g i t a r i a c i l a r i s (Retz.) Koel

cotton

corn, cotton peanuts, sorghum

corn, cotton peanuts, sorghum soybeans, tobacco

corn, cotton peanuts, sorghum soybeans

corn, sorghum soybeans, tobacco

Crops Affectgd Troublesome corn, sorghum

Table I. Weeds With Reported A l l e l o p a t h i c A c t i v i t y Found i n Row Crops i n the Southern United States


sorghum, soybeans (13)

corn (12)

corn, soybeans (13)

cotton (10)

corn (8) soybeans (9)

soybeans, corn (JJ

Allelopathic

X

X

Ο •η > r r m r Ο

H

m

χ

m π

H

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

corn, tobacco

Lambsquarters Chenopodium album L.

Common purslane Portulaca oleracea L.

Sandbur Cenchrus spp.

corn,

corn, sorghum soybeans

Broadleaf signalgrass Brachiaria spp.

Table

page

soybeans (15)

soybeans (14)

cotton (10)

I . C o n t i n u e d on next

sorghum

cotton, peanuts

soybeans

cotton, soybeans

sorghum, soybeans tobacco

cotton, tobacco

Prickly sida Sida spinosa L.

Kochia Kochia scoparia (L.) Schrad

Velvetleaf Abutilon theophrastl Medic. Ragweed Ambrosia spp.

Cynodon dactylon (L.) Pers.

Bermudagrass


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

Jimsonweed Datura stramonium L.

h

~

"

-

Crops Affectgd Troublesome

^

(From 1984 SWSS Research

~

sorghum, corn (16)

Allelopathic

i f the weed i s one of the top 10 common weeds i n that crop i n the Mid-South. i f the weed i s one of the top 10 troublesome weeds i n that crop i n the Mid-South. i f a report exists of s p e c i f i c a l l e l o p a t h i c a c t i v i t y of that weed on the crop.

i f i t occurs as a weed i n any of the row crops of the Mid-South.

-

F i e l d bindweed Convolulus arvensis L.

A weed i s l i s t e d .Report) (6). A crop i s l i s t e d ^A crop i s l i s t e d A crop i s l i s t e d

-

Common

Shattercane Sorghum b i c o l o r (L.) Moench

1

Weed

Table I. (Continued)



3.

ELMORE


27

f e r u l i c acid (22). Recent evidence would implicate other chemical classes of compounds not previously associated with allelopathy (22). Perhaps i t i s not these compounds, but other undetected highly toxic compounds or even species with short h a l f - l i f e periods, that are responsible. These experiments which have been used to document allelopathy of johnsongrass to cotton and soybeans are either i n v i t r o tests using Johnsongrass l i t t e r (dead decaying fragments of johnsongrass rhizomes), or greenhouse studies with l i t t e r and l i v e plant material. In the f i e l d with an i n f e s t a t i o n of johnsongrass, how i s allelopathy distinguished and partitioned from competition? Table II i s an i l l u s t r a t i o n that may be useful. Within any paired treatment i n that experiment where the soybean herbicide injury was the same, and the only differences were between presence or absence of johnsongrass, the y i e l d was greater when johnsongrass was absent, even though the control of the weed was nearly complete i n the johnsongrass p l o t s . This, of course, i s a confounded experiment, because competition for 3 weeks may have also been responsible. Other research would suggest, however, that three weeks of competition by johnsongrass i s not s u f f i c i e n t to reduce y i e l d (2). The implication here i s that the herbicide treated johnsongrass plants released some factor(s) into the environment of the developing soybean plant and reduced the y i e l d by 135 kg/ha. This 135 kg/ha then i s probably the a l l e l o p a t h i c component of the 430 kg/ha reduction caused by interference from uncontrolled rhizome johnsongrass. This would mean that allelopathy accounts for approximately 30% of the johnsongrass interference. Such an analysis i s obviously too s i m p l i s t i c , the a l l e l o p a t h i c e f f e c t may be much more i n larger, healthier plants, or i f injured, dead or metabolically altered plants produce or leach more allelochemics, even l e s s . Thus, the a l l e l o p a t h i c factor i s very d i f f i c u l t to p a r t i t i o n from competition, as was suggested by Fuerst and Putnam (Z§), Dekker et a l . (2£) and Elmore et a l . (27)

.

Purple nutsedge. Purple nutsedge, possibly the world's worst weed (28) , has been s i m i l a r l y tested for allelopathy. Friedman and Horowitz (20, ^29), and Horowitz (10), i n experiments s i m i l a r to those discussed above for johnsongrass, have shown that fragments of purple nutsedge tubers and rhizomes i n h i b i t growth of wheat, mustard, barley and cotton. In a separate study (2J.) they further i d e n t i f i e d the allelochemics as phenolic acids. Other work with purple nutsedge has shown i t to be a very strong competitor, especially i n the humid tropics (30). Season-long interference from purple nutsedge reduced y i e l d s of g a r l i c (Allium sativum L.) by 89% and other vegetable crops s i g n i f i c a n t l y . However, corn y i e l d was not reduced by a large population of purple nutsedge i n the humid tropics when enough nitrogen f e r t i l i z e r was added, suggesting that the p r i n c i p a l mode of interference by purple nutsedge i s competition for nutrients (2).


In The Chemistry of Allelopathy; Thompson, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985. 91a

Ob

No Yes

Not Not

0 0

...

89a

Od

...

95a

88b

...

93a

...

100a

...

... Od

99a

... Od

...

99a

99a

94b

93b

...

68c

72c

(°'\ \'°)

Od Od

15c 15c 35a 35a

13b 13b 25a 25a 0c 0c

Od Od 20b 20b

0c 0c

7ab 7ab 10a 10a

24b 24b 34a 34a Od Od

0c 0c 5b 5b

Od Od 5c 5c

Soybean injury Late Early Early July Sept. July

13b 13b 25a 25a

Late June

l,440ab 1,010g

l,270cd l,130ef l.HOf 980g

a

Soybean yield (kg/ha) 1,450a l,310cd l,350bc l,220de

.according to Duncan's multiple range test. ^ E l l i p s i s (...) indicates that johnsongrass control was not evaluated because the weed was not present, reproduced with permission from Weed Science (24). C o p y r i g h t 1980, Weed S c i e n c e S o c i e t y of America.

treated treated

6 6 6 6

No Yes No Yes

and and and and

3 3 3 3

1.1 1.1 2.2 2.2

91a

...

89a

77c

ab Johnsongrass control' Early Late Late Early Sept. July June July

No Yes No Yes

Presence of johnsongrass

Indications of Possible A l l e l o p a t h i c Effects of Johnsongrass on Soybeans

after emergence (weeks) 3 3 3 3

Rate of metriflufen (kg/ha) 1.1 1.1 2.2 2.2

Time of

Table I I .


S

r r m r Ο

η >

Ο

H

g

π χ m

H X m

3. E L M O R E

29



In a replacement-series study, Elmore et^ a l . (27) have shown that purple nutsedge i s a stronger competitor than four other species, including cotton. The data are i n s u f f i c i e n t to document allelopathy, but nutsedge tuber residue has been shown to affect cotton seedling development (31)· In a study at Stoneville on radish (Raphanus sativus L.) (Table I I I ) , purple nutsedge residue did not reduce y i e l d i n the f i e l d . However, plots with the weed present, even where weed control was attempted with herbicide and a plant feeding insect (Bactra) (32) so that weed growth was reduced, resulted i n severe y i e l d reductions. Here we see that residues were not toxic but l i v e , growing nutsedge was. A complicating factor was a heavy r a i n soon after planting. The results of t h i s study would suggest that f i e l d Table I I I .

Y i e l d of Radish (Raphanus sativus L.) Growing i n Purple Nutsedge

Treatment

3

Yield -gfwg 504 a ^^® 78 128 Residue augmented with greenhouse

Weed and residue free , 1 Weedfree /with residue^ With weeds/controlled y i t h weeds/uncontrolled Planted nutsedge stand k i l l e d . grown ovendried residue. Planted nutsedge stand control was attempted with postdirected glyphosate (rope wick) and Bactra release. Growth of nutsedge was ^educed but not k i l l e d . Means followed by the same l e t t e r are not d i f f e r e n t according to Duncan's Multiple Range Test. c

research may produce s i g n i f i c a n t l y d i f f e r e n t results from greenhouse pot studies. This may be because plants have a much larger s o i l volume to exploit i n the f i e l d than i n small pots i n the greenhouse, and r a i n f a l l could leach allelochemics out of the s o i l p r o f i l e . Therefore, studies with both purple nutsedge and johnsongrass suffer the same d e f i c i e n c i e s : lack of rigorous proof of a l l e l o p a t h i c e f f e c t s i n the f i e l d , although the case for johnsongrass may be better substantiated than for purple nutsedge. Future research. Ample research has been conducted to demonstrate the a l l e l o p a t h i c p o t e n t i a l of numerous weedy species, and of t h e i r associated crops. Future research should be directed to discerning how much of t h i s potential i s realized i n the f i e l d under humid, warm conditions such as that found i n the Southeast United States. Newman (33), i n a r e f l e c t i v e a r t i c l e , concludes that — i t cannot be said with confidence that allelopathy has any significance for agriculture." The evidence so f a r c e r t a i n l y substantiates h i s conclusion. To get a better f e e l f o r allelopathy i n the f i e l d , f i e l d studies should minimally include the weed and weed residues. 1 1


30

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

The experimental design should include a weed-free control, a weedy control, a weed free but with weed residue treatment, and at least one treatment with weeds and residue present. Various levels of weeds and weed residue could be incorporated into the study. Residue management. In an agriculture based on a f u l l t i l l a g e one-crop-per-year system, crop and weed residues present no management d i f f i c u l t i e s . Residues are simply incorporated into the s o i l s u f f i c i e n t l y ahead of planting to allow decomposition. Green manure cover crops, which were at one time i n vogue, required a management scheme that may have been forgotten today. The cover crop had to be turned under s u f f i c i e n t l y ahead of planting to avoid interference from fermenting residues on the germinating crop seed. The stubble mulch studies of the 1940 s (34) showed convincingly that crop residues on the s o i l surface can i n t e r f e r e with production. The current best practice for double cropping wheat and soybeans i n M i s s i s s i p p i i s to burn the wheat stubble, and then plant soybeans n o - t i l l i n the residue-free surface (35). Wheat straw may be a l l e l o p a t h i c to soybeans (36), but i t also interferes with f i e l d operations. Management of plant residues may be as simple as determining what residue-crop combinations are compatible or, perhaps as Putnam and de Frank (37) have suggested, the a l l e l o p a t h i c potential of a plant residue may be altered by chemical treatment. In many respects, crop production has advanced l i t t l e i n the l a s t two decades. More tools (herbicides and insecticides) are available for use by producers, but i n many cases we have not f u l l y u t i l i z e d lessons learned e a r l i e r . Residue management seems to be one of those cases. In n o - t i l l agriculture, residue management techniques w i l l have to be developed—by engineering, chemical, or b i o l o g i c a l means—for the system to function.


1

Conclusion Allelopathy should be studied and the weed-crop combinations that develop with new management constraints should be evaluated. However, easy solutions to pest problems w i l l not be forthcoming by manipulating allelopathy. For allelopathy to operate, the weed or weed residue must be present, and then i t does not matter much to the producer whether the detrimental effect i s due to competition or to allelopathy. Research on allelopathy may have some serendipitous r e s u l t s , however, because, as Newman (33) points out new herbicides may be discovered that are analogous to phytotoxins produced by higher plants, or allelochemic production may be blocked i n weeds, or toxin production may be altered i n decaying plant residues by chemical manipulation. Although no a g r i c u l t u r a l l y useful information has surfaced from allelopathy studies thus f a r , the future i s open for innovative solutions.


3.

ELMORE


31


Acknowledgments. The cooperation of Κ. E. Frick in supplying Bactra for purple nutsedge control is appreciated. The helpful reviews of Drs. S. 0. Duke, L. G. Heatherly, G. W. Cathey, and G. H. Egley for this manuscript were greatly appreciated. Literature Cited 1. Harper, J. L. In "Mechanisms in Biological Competition". Milthrope, F. L. Ed., Univ. Press, Cambridge. 1961. pp. 1-40. 2. Zimdahl, R. L. "Weed-crop competition. A review." Int. Plant Prot. Center, Oregon State Univ., Corvallis, OR. 1980. 3. Patterson, D. T. In "Research Methods in Weed Science", So. Weed Sci. Soc., Champaign, IL. 1984. 4. Putnam, A. R.; W. B. Duke. Ann. Rev. Phytopathol. 1978, 16, 431-451. 5. Putnam, A. R.; Defrank, J.;Barnes, J. P. J. Chem. Ecol. 1983, 9, 1001-1010. 6. Elmore, C. D. So. Weed Sci. Soc. Research Report. 1984, 37, 192-198. 7. Bhowmik, P. C.; Doll, J. D. Agron. J. 1982, 74, 601-606. 8. Beltiano, J.; Montaldi, E. R. Rev. Facultad de Agronomic, 1982, 3, 265-269. 9. Lolas, P. C.; Coble, H. D. Weed Sci. 1982, 30, 589-593. 10. Horowitz, M. Expl. Agric. 1973, 9, 263-273. 11. Dorst, C. D.; Doll, J. D. Weed Sci. 1980, 28, 229-233. 12. Bell, D. T.; Koeppe, D. E. Agron. J. 1972, 64, 321-325. 13. Irons, S. M.; Burnside, O. C. Weed Sci. 1982, 30, 372-377. 14. Dekker, J.; Meggitt; W. F. Weed Res. 1983, 23, 91-101. 15. Pope, Dan, personal communication. 16. Guenzi, W. D.; McCalla, T. M. Soil Sci. Soc. Am. Proc. 1962, 26, 456-458. 17. McWhorter, C. G.; Hartwig, Ε. E. Weed Sci. 1972, 20, 56-59. 18. Rice, E. L. "Allelopathy"; Academic Press. N.Y. 1974. 19. Abdul-Wahab, A. S.; Rice, E. L. Bull. Torrey Bot. Club 1967, 94, 486-497. 20. Friedman, T.; Horowitz, M. Weed Res. 1970, 10, 382-385. 21. Horowitz, M.; Friedman, T. Weed Res. 1971, 11, 88-93. 22. Dalton, B. R.; Blum, U.; Weed, S. B. J. Chem. Ecol. 1983, 9, 1185-1201. 23. Lehle, F. C.; Putnam, A. R. Plant. Physiol. 1982, 69, 1212-1216. 24. Azlin, W. R.; McWhorter, C. G. Weed Sci. 1980, 29, 138-143. 25. Fuerst, E. P.; Putnam, A. R. J. Chem. Ecol. 1983, 9, 937-944. 26. Dekker, J. H.; Meggitt, W. F.; Putnam, A. R. J. Chem. Ecol. 1983, 9, 945-981. 27. Elmore, C. D.; Brown, Μ. Α.; Flint, E. P. Weed Sci. 1983, 31, 200-207. 28. Holm, L. G.; Plucknett, D. L.; Pancho, J. V.; Herberger, J. P. "The world's worst weeds". Univ. Press of Hawaii, Honolulu, HI. 29. Friedman, T.; M. Horowitz. Weed Sci. 1971, 19, 398-401. 30. William, R. D.; Warren, G. F. Weed Sci. 1975, 23, 317-323. 31. Elmore, C. D.; Clarke, L. E. Proc. 1981 Meeting, Weed Sci. Soc. Am. 1981, p. 87. 32. Frick, Κ. E. Environ. Entom. 1982, 11, 938-945. 33. Newman, Ε. I. Pestic. Sci. 1982, 13, 575-582.


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THE CHEMISTRY OF ALLELOPATHY 34. McCalla, T. M.; Duley, F. L. S o i l S c i . Soc. Am. Proc. 1949, 14, 196-199. 35. Sanford, J . O. Agron. J . 1982, 74, 1032-1035. 36. Steinsiek, J . W.; O l i v e r , L. R.; C o l l i n s , F. C. Weed S c i . 1982, 30, 495-497. 37. Putnam, A. R.; DeFrank, J . Crop Prot. 1983, 2, 173-181.


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The Chemistry of Allelopathy - American Chemical Society

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