Effects of Allelopathic Chemicals on Crop Productivity - ACS

8 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 8, 2015 | http://pubs.acs.org Publication Date: April 26, 1985 | doi: 10.1021/bk-1985-0276.ch008

Effects of Allelopathic Chemicals on Crop Productivity

F. A. EINHELLIG Department of Biology, University of South Dakota, Vermillion, SD 57069

Biochemical interactions among plants (allelopathy) result from the a c t i v i t y of a diverse group of compounds synthesized by higher plants and microorganisms. Commonly accepted representatives include scopoletin, f e r u l i c acid, p-hydroxybenzoic a c i d , catechin, amygdalin, p a t u l i n , and juglone. A l l e l o p a t h i c regulation of plant growth and development depends on the concentration, combination of substances, edaphic and climatic factors, interaction with other stresses, and species s e n s i t i v i t y . The source of allelochemicals i n a g r i c u l t u r a l f i e l d s may be the weeds, crops, or microorganisms. Yields may be affected by (a) the i n h i b i t o r y or stimulatory effect of a crop on the subsequent crop, (b) the capacity of crop plants to i n h i b i t weeds, and (c) production losses due to a l l e l o pathic weeds. Allelochemical interference with germination or growth of a crop can occur from direct effects on metabolism, or i n d i r e c t l y through effects on nitrogen f i x a t i o n and other microorganism a c t i v i t y . Many physiol o g i c a l processes are altered by a l l e l o p a t h i c chemicals, but it has been d i f f i c u l t to determine the primary mechanism involved for a s p e c i f i c compound. Both avoidance and application strategies may be employed to utilize allelochemicals for improving crop production. These include management of crop sequences, u t i l i z a t i o n of a l l e l o p a t h i c crop residues, breeding crops for weed control, and development of allelochemicals as herbicides. Investigations over the l a s t three decades have provided abundant evidence that plants and animals often produce products that affect the growth, development, d i s t r i b u t i o n , and behavior of other organisms (1,2) . C o l l e c t i v e l y , these natural substances are termed allelochemicals, or allelochemics. They often impart plant r e s i s t ance to insects, nematodes, and pathogens. Likewise, following their release into the environment through v o l a t i l i z a t i o n , leaching, root exudation, or tissue decomposition, some allelochemicals regulate the

0097-6156/85/0276-0109$06.50/0 © 1985 American Chemical Society

In Bioregulators for Pest Control; Hedin, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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d i s t r i b u t i o n o f p l a n t s and t h e v i g o r o f p l a n t growth. These i n t e r a c t i o n s a r e t h e phenomenon o f a l l e l o p a t h y , which i n c l u d e s a l l b i o c h e m i c a l i n t e r r e l a t i o n s h i p s among p l a n t s , b o t h h i g h e r p l a n t s and microorganisms. A l l e l o p a t h i c i n t e r f e r e n c e occurs i n agroecosystems, and i t i s one o f t h e many f a c t o r s t h a t i n f l u e n c e c r o p p r o d u c t i v i t y (3). The e f f e c t s o f s u c h i n t e r a c t i o n s may be e i t h e r s t i m u l a t o r y o r i n h i b i t o r y , b u t t h e major documentation o f a l l e l o c h e m i c a l e f f e c t s i n agronomy has been t h a t o f growth i n h i b i t i o n . The purpose o f t h i s paper i s t o d i s c u s s t h e major g e n e r a l i z a t i o n s t h a t c a n be made about a l l e l o p a t h i c i n t e r a c t i o n s , p r o v i d e examples o f t h e r o l e i n a g r i c u l t u r e , and f o c u s on some c o n s i d e r a t i o n s f o r t h e f u t u r e . Only a few o f the s a l i e n t i n v e s t i g a t i o n s w i l l be c i t e d i n i l l u s t r a t i n g major principles. I m p l i c i t i n t h e concept o f a l l e l o p a t h y i s t h e r e c o g n i t i o n t h a t t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s i n (a) t h e c a p a c i t y o f s p e c i e s and v a r i e t i e s t o produce a l l e l o c h e m i c a l s , (b) t h e s e n s i t i v i t i e s o f v a r i o u s p l a n t s t o a l l e l o p a t h i c compounds, and (c) p l a n t r e s p o n s e s during the various stages of the l i f e c y c l e . The p r o d u c i n g and r e c e i v i n g p l a n t s may be t h e same o r d i f f e r e n t s p e c i e s . The r e c e i v i n g p l a n t s may be growing c o n c u r r e n t l y w i t h t h e p r o d u c e r s , o r they may be found sequent t o t h e p r o d u c e r s . Crop p l a n t s most o f t e n c o n t a c t a l l e l o c h e m i c a l s by t h e p r e s e n c e o f t h e s e s u b s t a n c e s i n t h e s o i l com partment. The s c h e m a t i c o f F i g u r e 1 i l l u s t r a t e s some o f t h e p o t e n t i a l f i e l d i n t e r r e l a t i o n s h i p s and c o m p l e x i t i e s . I n a d d i t i o n to these a s p e c t s , as a l l e l o c h e m i c a l s move t h r o u g h t h e p h y s i c a l environment t h e i r q u a n t i t y , r e s i d e n c e t i m e , and b i o l o g i c a l a c t i v i t y f l u c t u a t e widely.

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Allelopathy Effect on Crop Productivity

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Some i n v e s t i g a t o r s have p o s t u l a t e d t h a t a l l e l o p a t h i c s u b s t a n c e s i n higher p l a n t s are immediately d e t o x i f i e d a f t e r r e l e a s e . While such t r a n s f o r m a t i o n s o c c u r , a t times the r e s u l t a n t p r o d u c t s can be of higher t o x i c i t y . F o r example, h y d r o j u g l o n e i s o x i d i z e d to j u g l o n e , a v e r y p o t e n t quinone t h a t i s i n h i b i t o r y t o some s p e c i e s a t 10"° M l e v e l s ( 4 ) . A c y a n o g e n i c g l u c o s i d e of peach r o o t s , a m y g d a l i n , y i e l d s hydrogen c y a n i d e , b e n z a l d e h y d e , and s u b s e q u e n t l y o t h e r i n h i b i t o r s a s s o c i a t e d w i t h t h e peach r e p l a n t problem ( 1 ) . M i c r o b i a l a c t i v i t y i n t r a n s f o r m a t i o n s , p l u s the m e t a b o l i c p r o d u c t i o n of d i v e r s e a l l e l o c h e m i c a l s by many m i c r o o r g a n i s m s , add t o the c o m p l i c a t i o n s i n d e t e r m i n i n g the r o l e of a l l e l o p a t h i c i n t e r f e r e n c e i n crop p r o d u c t i o n . D u r i n g r e s i d e n c e i n the s o i l compartment, impacts of such compounds on c r o p p l a n t s may a l s o be m o d i f i e d by m o i s t u r e , t e m p e r a t u r e , and o t h e r s o i l f a c t o r s (5-10). The number and d i v e r s i t y of compounds i m p l i c a t e d i n a l l e l o p a t h y a r e r a p i d l y growing. A c e t i c a c i d and a few o t h e r major i n t e r m e d i a t e s o f m e t a b o l i s m have been r e p o r t e d as a l l e l o p a t h i c a g e n t s , but most of t h o s e t h a t have been i d e n t i f i e d a r e secondary compounds t h a t a r i s e from t h e s h i k i m i c a c i d and a c e t a t e pathways, or r e s u l t as h y b r i d s from t h e s e s y n t h e s i s pathways. R i c e (1) c l a s s i f i e d the compounds i n t o f o u r t e e n c h e m i c a l c a t e g o r i e s , p l u s a m i s c e l l a n e o u s group. S e v e r a l major groups o f h i s scheme a r e the t e r p e n o i d s and s t e r o i d s , a l k a l o i d s and c y a n o h y d r i n s , l o n g - c h a i n f a t t y a c i d s and p o l y a c e t y l enes, u n s a t u r a t e d l a c t o n e s , t a n n i n s , c i n n a m i c a c i d d e r i v a t i v e s , b e n z o i c a c i d d e r i v a t i v e s and o t h e r s i m p l e p h e n o l s , coumarins, and flavonoids. Each c a t e g o r y i s not e q u a l l y i m p o r t a n t , and c e r t a i n l y o n l y a f r a c t i o n o f the n a t u r a l l y o c c u r r i n g compounds t h a t c o u l d be named under each c a t e g o r y a r e a l l e l o p a t h i c i n n a t u r e . Some, such as many o f the t e r p e n o i d s and p o l y a c e t y l e n e s , may f u n c t i o n i n a v o l a t i l e s t a t e , but most of the c u r r e n t documentation i n a g r o e c o systems i n v o l v e s w a t e r - s o l u b l e compounds. Numerous p h e n o l i c coinpounds have been i m p l i c a t e d i n a l l e l o p a t h y , and d e r i v a t i v e s o f c i n n a m i c a c i d , b e n z o i c a c i d , and coumarin have been t h o s e most o f t e n i d e n t i f i e d from h i g h e r p l a n t s (11). Common ones i n c l u d e s c o p o l e t i n , e s c u l e t i n , and the p h e n o l i c a c i d s ; f e r u l i c , p-coumaric, c a f f e i c , v a n i l l i c , p - h y d r o x y b e n z o i c , and c h l o r o g e n i c . The known l i s t of c h e m i c a l s i n v o l v e d i n a l l e l o p a t h y c o n t i n u e s to expand and examples g i v e n s h o u l d not be a u t o m a t i c a l l y assumed to be the most i m p o r t a n t a g e n t s . B e t t e r i s o l a t i o n techniques are e x p e d i t i n g the i d e n t i f i c a t i o n of a d d i t i o n a l s u b s t a n c e s , some w i t h higher b i o l o g i c a l a c t i v i t y than those noted. Over 10,000 secondary p l a n t compounds a r e known (12) , most have not been t e s t e d as a l l e l o p a t h i c a g e n t s , and many thousands more a r e p r o b a b l y p r e s e n t i n p l a n t s . The i n f a n c y of t h i s a r e a of r e s e a r c h i s i l l u s t r a t e d by the f a c t t h a t p r e s e n t l y more t h a n 2,000 d i f f e r e n t a l k a l o i d s have been i s o l a t e d from o v e r 3,000 s p e c i e s of p l a n t s ( 1 3 ) , y e t o n l y a few have been e v a l u a t e d f o r t h e i r a c t i v i t y i n growth r e g u l a t i o n (1). Similarly, f l a v o n o i d s are p r o b a b l y the l a r g e s t c l a s s of p h e n o l i c compounds i n f l o w e r i n g p l a n t s , y e t o n l y a few, such as m y r i c i t i n , q u e r c e t i n , and k a e m p f e r o l , have been t e s t e d f o r p o t e n t i a l a l l e l o p a t h i c e f f e c t s . Assessment of a l l e l o c h e m i c a l s f o r s p e c i f i c i n v o l v e m e n t i n growth i n h i b i t i o n or s t i m u l a t i o n and a n a l y s e s of the mechanisms of such a c t i o n s a r e u r g e n t l y needed to d e t e r m i n e t h e i r r o l e s i n crop production.


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and I n t e r a c t i o n s o f A l l e l o c h e m i c a l s

Interactions Involving Allelochemicals. P r o d u c t i o n o f many s e c o n d ary s u b s t a n c e s by h i g h e r p l a n t s i s m o d i f i e d by a number o f e n v i r o n m e n t a l f a c t o r s . M i n e r a l d e f i c i e n c i e s , c o l d t r e a t m e n t , UV l i g h t , h e r b i c i d e s , and o t h e r s t r e s s c o n d i t i o n s t y p i c a l l y cause an i n c r e a s e i n the q u a n t i t y o f some o f the common a l l e l o p a t h i c c h e m i c a l s i n p l a n t s (14-21). L i m i t e d work i n the U.S.S.R. even s u g g e s t s t h a t a l l e l o c h e m i c a l s w h i c h a r e r e c e i v e d by a p l a n t may r e g u l a t e gene a c t i v i t y t h a t c o n t r o l s the q u a n t i t y o f c h e m i c a l p r o d u c t i o n by t h a t p l a n t (22). These i n t e r a c t i o n s between the p l a n t and i t s e n v i r o n ment do not p r e c l u d e the f a c t t h a t the b a s i c g e n o t y p i c c a p a c i t y f o r s y n t h e s i s o f a l l e l o c h e m i c a l s v a r i e s e x t e n s i v e l y even among c u l t i v a r s of a c r o p (23 2Λ). 9

I t s h o u l d b e emphasized t h a t a complex o f s u b s t a n c e s i s g e n e r a l l y i n v o l v e d when a l l e l o p a t h i c i n t e r f e r e n c e s o c c u r , o f t e n w i t h each below a t h r e s h o l d l e v e l f o r impact. T h i s i s i l l u s t r a t e d by the c o m b i n a t i o n s o f p h e n o l i c a c i d s found i n decomposing c r o p r e s i d u e s (25-27) and from s o i l s (28-34). I n a l l e l o p a t h i c s i t u a t i o n s which i m p l i c a t e p h e n o l i c a c i d s , s o i l q u a n t i t i e s o f f e r u l i c , p-coumaric, and c a f f e i c a c i d s have ranged from below 10 t o above 1,000 ppm f o r each compound (11,35). The lower end o f t h i s spectrum i s below a c o n c e n t r a t i o n r e q u i r e d f o r an e f f e c t i n c u r r e n t b i o a s s a y s . However, a d d i t i v e and s y n e r g i s t i c e f f e c t s have been documented f o r combina t i o n s o f c i n n a m i c a c i d s (35), b e n z o i c a c i d s (36), b e n z o i c and c i n n a m i c a c i d s (37) , and p - h y d r o x y b e n z a l d e h y d e w i t h coumarin ( 3 8 ) . Each o f the a l l e l o c h e m i c a l s i n t h e s e t e s t s was not e q u a l l y t o x i c , but they c o n t r i b u t e d i n c r e m e n t a l l y t o i n h i b i t i o n o f g e r m i n a t i o n and growth. Whereas c o m b i n a t i o n s o f many a l l e l o c h e m i c a l s have not been d e t e r m i n e d , i t appears t h a t b o t h a d d i t i v e and s y n e r g i s t i c i n t e r a c t i o n s a r e e x t r e m e l y i m p o r t a n t under f i e l d c o n d i t i o n s . A l l e l o p a t h i c c h e m i c a l s may a l s o a c t i n c o n c e r t w i t h r e s i d u a l quantities of herbicides. Our t e s t s demonstrated t h a t a c o m b i n a t i o n of t r i f l u r i n and f e r u l i c a c i d i n h i b i t e d sorghum [So/igkum bldoLoh. (L.) Moench] g e r m i n a t i o n and s e e d l i n g growth more t h a n e i t h e r a l o n e (39). Likewise, a t r a z i n e s t r e s s acted cooperatively with f e r u l i c a c i d i n s t u n t i n g oat (Avtna Acutiva L.) s e e d l i n g s . Obviously, a l l e l o p a t h y i s o n l y one o f the s e v e r a l s t r e s s f a c t o r s o f the c r o p environment. S t r e s s c o n d i t i o n s from h e r b i c i d e s , a l l e l o c h e m i c a l s , t e m p e r a t u r e extremes, and m o i s t u r e d e f i c i t s may work i n c o n j u n c t i o n as t h e y impact on c r o p p r o d u c t i o n (10,39,40). I n d i r e c t Modes o f A c t i o n . A l l e l o c h e m i c a l s may e i t h e r a f f e c t c r o p p l a n t s d i r e c t l y b y i n t e r f e r e n c e w i t h m e t a b o l i c f u n c t i o n s , o r the e f f e c t s may b e i n d i r e c t t h r o u g h a c t i o n s on a s s o c i a t e d o r g a n i s m s . Examples o f the l a t t e r r e s u l t from e f f e c t s on organisms o f the n i t r o g e n c y c l e , on m y c o r r h i z a l f u n g i , and on d i s e a s e s u s c e p t i b i l i t y and r e s i s t a n c e . P l a n t s s u b j e c t e d t o a l l e l o p a t h i c s t r e s s have l e s s v i g o r o u s growth and a r e o f t e n more s u s c e p t i b l e t o d i s e a s e ( 6 ) . I t a l s o has l o n g been a p p a r e n t t h a t p l a n t s i n n a t u r a l systems have c o n s i d e r a b l e d e f e n s e a g a i n s t d i s e a s e , i n s e c t damage, and h e r b i v o r e g r a z i n g , w i t h a good p a r t o f t h i s b e i n g due t o the q u a n t i t y and q u a l i t y o f s e c o n d a r y compounds they c o n t a i n . Crop b r e e d i n g programs have r e s u l t e d i n more t h a n 75% o f the a g r i c u l t u r a l l a n d i n the U.S.



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b e i n g p l a n t e d t o v a r i e t i e s r e s i s t a n t t o some b a c t e r i a , f u n g i , o r v i r u s (41,42). I n f o r m a t i o n on t h e s e r e l a t i o n s h i p s has been c o m p i l e d by H e d i n (43), and t h e r o l e o f secondary compounds and mechanisms i n p r o t e c t i v e f u n c t i o n s have been r e c e n t l y r e v i e w e d (12,44). Thus, i n t h i s summary I w i l l o n l y a d d r e s s a l l e l o c h e m i c a l e f f e c t s o t h e r t h a n those i n v o l v i n g disease r e l a t i o n s h i p s . R i c e and co-workers c l e a r l y demonstrated t h a t when cropped-out f i e l d s i n Oklahoma were abandoned, t h e s u c c e s s i o n o f p l a n t c o l o n i z a t i o n was p a r t i a l l y c o n t r o l l e d by a l l e l o p a t h i c s u b s t a n c e s produced by e a r l y i n v a d i n g weeds w h i c h reduced t h e a c t i v i t y o f n i t r o g e n - f i x i n g organisms, i n c l u d i n g f r e e - l i v i n g f i x e r s , Rkizobium spp., and b l u e green a l g a e ( 1 ) . These c h e m i c a l s were a l s o r e s p o n s i b l e f o r r e d u c i n g nitrification. E v i d e n c e from s t u d i e s o f s e v e r a l n a t u r a l ecosystems s u g g e s t s t h a t n i t r i f i c a t i o n may be i n c r e a s i n g l y i n h i b i t e d d u r i n g s u c c e s s i o n , so t h a t c l i m a x communities r e t a i n more n i t r o g e n i n t h e r e d u c e d form ( 1 ) . S i m i l a r f a c t o r s can a l t e r n i t r o g e n l e v e l s i n agricultural fields. When r i c e (Osiyza Acubiva L.) r e s i d u e i s l e f t i n t h e f i e l d i n Taiwan and n o t burned, y i e l d o f t h e subsequent c r o p o f soybeans [Glycine, max (L.) Merr.] i s d e p r e s s e d . T h i s i s due t o t h e r e l e a s e o f p h e n o l i c a c i d s , and perhaps o t h e r s u b s t a n c e s which i n h i b i t n o d u l a t i o n and heme p r o d u c t i o n , s u p p r e s s i n g a c t i v i t y o f t h e n i t r o g e n - f i x i n g b a c t e r i a (45). A l t h o u g h t h e e x a c t cause o f a u t o t o x i c i t y o f a l f a l f a (M&dicago Aotivci L.) and s e v e r a l o t h e r f i e l d legumes has n o t been d e t e r m i n e d (46,47) , t o some degree i t may r e l a t e t o a l l e l o p a t h i c i n h i b i t i o n o f t h e i r symbiont a s s o c i a t e s . S e v e r a l i n 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 have been r e p o r t e d i n forestry. Walnut (Juglan nigfia L . ) p l a n t a t i o n s i n t h e C e n t r a l U.S. o f t e n have European b l a c k a l d e r [A£nuo gùjubino&a (L.) G a e r t n . ] , a h o s t f o r n i t r o g e n f i x a t i o n , i n t e r p l a n t e d w i t h walnut as a n u r s e c r o p . However, i n 8-13 y e a r s t h e a l d e r s d i e o u t on p o o r l y d r a i n e d s i t e s due to j u g l o n e t o x i c i t y (4,48). Some o f t h i s e f f e c t may be on t h e n i t r o g e n - f i x i n g organisms. A l s o , t h e r e i s growing e v i d e n c e t h a t an i m p o r t a n t a l l e l o p a t h i c impact may o c c u r on m y c o r r h i z a l f u n g i . Brown and M i k o l a (49) r e p o r t e d t h a t r e i n d e e r l i c h e n , p a r t i c u l a r l y CZadoviÀCL odipQÀtHÂÀ, i n h i b i t e d m y c o r r h i z a l symbiont s, r e s u l t i n g i n l e s s phosphorus u p t a k e and s u p p r e s s i o n o f p i n e and s p r u c e s e e d l i n g growth i n F i n l a n d f o r e s t s . As p o i n t e d o u t by F i s h e r (50) and Rose e t a l . (51), a l l e l o p a t h i c s u p p r e s s i o n o f f u n g a l growth and r o o t c o l o n i z a t i o n may e x p l a i n f a i l u r e s o f r e f o r e s t a t i o n by c o n i f e r s p e c i e s i n d i s t u r b e d sites. U n f o r t u n a t e l y , s i m i l a r i n f l u e n c e s on m i n e r a l n u t r i t i o n o f agronomic c r o p s have n o t been a s s e s s e d . D i r e c t Modes o f A c t i o n . Evaluation o f the e f f e c t s of a l l e l o c h e m i c a l s on c r o p p l a n t s has g e n e r a l l y been i n terms o f a l t e r a t i o n s i n germina t i o n o r some a s p e c t o f s e e d l i n g growth. O f t e n s e e d l i n g growth i s d i m i n i s h e d by lower l e v e l s o f an i n h i b i t o r t h a n g e r m i n a t i o n , and l o n g - t e r m growth may r e f l e c t e f f e c t s n o t e v i d e n t i n s h o r t - t e r m b i o a s s a y s (_35,4_). N e i t h e r g e r m i n a t i o n n o r s e e d l i n g growth b i o a s s a y s p r o v i d e s e v i d e n c e f o r t h e mechanisms o f growth r e g u l a t i o n , and t h e s e mechanisms a r e c u r r e n t l y n o t w e l l u n d e r s t o o d (11,52). I t appears e v i d e n t t h a t w i t h t h e many d i f f e r e n t c a t e g o r i e s o f compounds t h a t have been i d e n t i f i e d , a v a r i e t y o f mechanisms o f a c t i o n must e x i s t . A n o t h e r d i f f i c u l t y i n d e f i n i n g mechanisms o f a c t i o n i s t h a t a s p e c i f i c compound may a f f e c t s e v e r a l m e t a b o l i c f u n c t i o n s , and as



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a r e s u l t , i t has been seldom p o s s i b l e t o s o r t p r i m a r y from s e c o n d a r y effects. R e p o r t s on p h y s i o l o g i c a l e f f e c t s a r e most numerous f o r p h e n o l i c compounds t h a t a r e d e r i v a t i v e s of c i n n a m i c and b e n z o i c a c i d s , o r c l o s e l y r e l a t e d compounds w i t h the coumarin s k e l e t o n . Two p r i m a r y mechanisms of a c t i o n t h a t have been s u g g e s t e d f o r p h e n o l i c a c i d s a r e t h e i r i n t e r a c t i o n s w i t h phytohormones (53,54) and t h e i r e f f e c t s on membranes (55). E a r l y work i n d i c a t e d t h a t d i p h e n o l i c compounds s y n e r g i z e IAA-induced growth by c o u n t e r a c t i n g IAA ( i n d o l e - 3 - a c e t i c a c i d ) d e s t r u c t i o n , and monophenols s t i m u l a t e d e c a r b o x y l a t i o n , w i t h b o t h a c t i o n s r e d u c i n g growth (56). Numerous subsequent i n v e s t i g a t i o n s have demonstrated t h a t p h e n o l s may i n t e r a c t w i t h the o x i d a s e enzyme system and c o n t r o l the l e v e l of IAA (57). A r e c e n t study r e p o r t e d t h a t o x i d a t i o n of IAA was c o u p l e d t o the c o o x i d a t i o n of p h e n o l s through the f o r m a t i o n of H2O2 as an i n t e r m e d i a t e (58). P o l y p h e n o l s may b l o c k the a c t i o n of g i b b e r e l l i c a c i d , a n t a g o n i z i n g growth (59). I n d i r e c t hormonal e f f e c t s on c r o p s , such as e l e v a t e d e t h y l e n e l e v e l s i n the s o i l as the r e s u l t o f m i c r o b i a l m e t a b o l i s m of p l a n t r e s i d u e , have a l s o been i m p l i c a t e d i n a l l e l o p a t h y (60). However, the e v i d e n c e t h a t p h e n o l i c a c i d s and p o l y p h e n o l s cause t h e i r e f f e c t t h r o u g h a c t i o n on phytohormones i s not the complete p i c t u r e . P h e n o l i c a c i d s a r e known to a l t e r p h o t o s y n t h e t i c and r e s p i r a t i o n r a t e s , cause s t o m a t a l c l o s u r e , r e d u c e c h l o r o p h y l l c o n t e n t , modify the f l o w of c a r b o n i n t o v a r i o u s m e t a b o l i c p o o l s , and a l t e r n u t r i e n t up take i n a f f e c t e d t i s s u e (61-73). A common denominator f o r t h e s e m u l t i p l e e f f e c t s appears to be the a c t i o n of p h e n o l i c compounds on membranes. They a r e s o l u b l e i n membranes, and cause a r e d u c t i o n i n i o n a c c u m u l a t i o n i n c e l l s (71-73). S e v e r a l p h e n o l i c a c i d s cause membrane d e p o l a r i z a t i o n , e s p e c i a l l y a t low pH, i n c r e a s i n g membrane p e r m e a b i l i t y t o i o n s (72,73). T h i s a c t i o n u n d o u b t e d l y i m p a i r s the p r o t o n g r a d i e n t and A T P - d r i v e n i o n t r a n s p o r t . L o g i c a l l y , the e f f e c t s p h e n o l i c a c i d s have on membranes c o u l d d i s t u r b the w a t e r b a l a n c e and m i n e r a l n u t r i t i o n of s e e d l i n g s , and r e s e a r c h i n my l a b o r a t o r y has e s t a b l i s h e d such a r e l a t i o n s h i p . F e r u l i c and p-coumaric a c i d s a l t e r e d the water b a l a n c e of g r a i n sorghum and soybean s e e d l i n g s (55,74). Even a t t r e a t m e n t l e v e l s t h a t were below the t h r e s h o l d f o r s h o r t - t e r m growth r e d u c t i o n , water s t r e s s was e v i d e n t by t h e d e p r e s s i o n of s e e d l i n g water p o t e n t i a l . Sorghum t r e a t e d w i t h 0.25 mM f e r u l i c o r p-coumaric a c i d s had midday l e a f water p o t e n t i a l s of a p p r o x i m a t e l y -10 b a r s , compared to -5 b a r s f o r the c o n t r o l s . Treatments w i t h p h e n o l i c compounds t h a t reduced growth a l s o were c o r r e l a t e d w i t h p a r t i a l s t o m a t a l c l o s u r e (55,63,64). V i s i b l e symptoms o f a l l e l o p a t h i c e f f e c t s a r e o f t e n q u i t e s i m i l a r to t h o s e found w i t h n u t r i e n t d e f i c i e n c i e s , and i n a few c a s e s i n t e r f e r e n c e w i t h n u t r i e n t uptake has been documented. I n l a b o r a t o r y experiments d e s i g n e d to t e s t the e f f e c t s o f f e r u l i c a c i d on the m i n e r a l c o n t e n t of sorghum, we found lower t i s s u e l e v e l s of phos p h o r u s , p o t a s s i u m , and magnesium accompanying reduced growth o f s e e d l i n g s (75). Whether such growth e f f e c t s can be m o d i f i e d by i n c r e a s e d f e r t i l i t y i s not c l e a r . Our p r e l i m i n a r y work i n d i c a t e d t h a t i n h i b i t i o n by f e r u l i c a c i d was not overcome by i n c r e a s e d n u t r i e n t l e v e l s i n the growth medium, but Stowe and Osborn (76) r e p o r t e d n u t r i e n t augmentation c o u l d a l l e v i a t e growth r e d u c t i o n s from p h e n o l i c a c i d s . The e v i d e n c e c i t e d shows t h a t p h e n o l i c a c i d s



8.

EINHELLIG


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can r e d u c e t h e uptake o f major n u t r i e n t s , y e t i n c e r t a i n s i t u a t i o n s t h e s e same s u b s t a n c e s may be exuded t o b e n e f i t p l a n t n u t r i t i o n . In times o f s t r e s s , some p l a n t s r e l e a s e o r t h o d i h y d r o x y p h e n o l s , such as c a f f e i c a c i d , i n t o the r o o t medium and thus mediate t h e movement o f i r o n i n t o t h e r o o t by c h e l a t i o n (72,78). Thus, i t i s d i f f i c u l t t o make g e n e r a l i z a t i o n s about t h e e f f e c t s o f p h e n o l i c a l l e l o c h e m i c a l s on m i n e r a l n u t r i t i o n , even though we know more about t h e mode o f a c t i o n of t h e s e compounds than any o t h e r a l l e l o p a t h i c a g e n t s . In subsequent d i s c u s s i o n s , o t h e r examples w i l l be g i v e n w h i c h s u p p o r t t h e view t h a t a l l e l o p a t h i c i n t e r f e r e n c e w i t h c r o p s may o p e r a t e through e f f e c t s on water b a l a n c e and m i n e r a l n u t r i t i o n . A number o f a l l e l o p a t h i c c h e m i c a l s o t h e r than p h e n o l i c a c i d s may a l s o have t h e i r i n i t i a l e f f e c t s on c e l l u l a r and o r g a n e l l e membranes. A d i s r u p t i o n o f membranes n o t o n l y a f f e c t s n u t r i e n t t r a n s p o r t , b u t r e s p i r a t o r y c o u p l i n g , p h o t o s y n t h e s i s r e a c t i o n s , and o t h e r membrane associated processes. Allelochemical Interference

i n Agricultural Fields

A l l e l o p a t h i c Weeds. Weed i n f e s t a t i o n s a r e a major f a c t o r r e d u c i n g crop y i e l d s , and i t i s n o t s u r p r i s i n g t h a t a r e c e n t e s t i m a t e o f c o s t s o f weed c o n t r o l and w e e d - i n c u r r e d l o s s e s was $14 b i l l i o n a n n u a l l y i n t h e U.S. ( 7 9 ) . I n t e r f e r e n c e from weedy s p e c i e s i s o f t e n due b o t h t o c o m p e t i t i o n and a l l e l o p a t h y , and f i e l d s t u d i e s have n o t g e n e r a l l y s e p a r a t e d t h e s e f a c t o r s . However, a l l e l o p a t h i c p o t e n t i a l has been r e p o r t e d f o r more t h a n 70 weedy s p e c i e s , and t h e l i s t o f weed-crop i n t e r a c t i o n s i s growing. These i n c l u d e some o f t h e tough e s t weeds t o c o n t r o l , and some c a u s i n g t h e g r e a t e s t economic damage i n major c r o p s . Examples o f t h e a l l e l o p a t h i c e f f e c t s o f a few weeds w i l l i l l u s t r a t e t h e b r e a d t h o f t h e i r impact. One o f t h e most complete s t u d i e s o f a l l e l o p a t h i c i n t e r f e r e n c e has been o f VaKthoyiivm kyateAopkosuiA L. , a t r o p i c a l American weed which has t a k e n over many c u l t i v a t e d a r e a s o f I n d i a (80-83). A l l e l o c h e m i c a l s i d e n t i f i e d from VaJVtktwium i n c l u d e c a f f e i c , v a n i l l i c , f e r u l i c , c h l o r o g e n i c , and a n i s i c a c i d s , p l u s t h e s e s q u i t e r p e n e lactone, parthenia. Both a e r i a l p l a n t p a r t s and the r o o t s c o n t r i b u t e to t h e problem. F i e l d s t u d i e s were conducted t o a s c e r t a i n a l l e l o p a t h i c e f f e c t s on y i e l d . D r i e d PoAtkzvU.Uïïi l e a v e s mixed i n t o t h e s o i l of f i e l d p l o t s reduced t h e y i e l d o f cowpea (Vigna &yU> L . ) , tomato (LycopQAAicon QAOJuZzvitum L . ) , r a g i (EXeoo/tne coHRcana G a e r t n . ) , and beans (PhaAoZouA VuZgaAsU L . ) , w i t h n o d u l a t i o n a l s o reduced i n the legumes. However, growth o f b a j r a (VznviiAZtum typko-id&um R i c h . ) was s t i m u l a t e d , i n d i c a t i n g t h e s u b t l e d i f f e r e n c e s among s p e c i e s . Corn (Zea may& L.) p r o d u c t i o n systems a r e an example where a number o f a l l e l o p a t h i c weeds have been i d e n t i f i e d t h a t c a n p o t e n t i a l l y reduce y i e l d . Corn growth i s i n h i b i t e d by r o o t r e s i d u e s and whole p l a n t l e a c h a t e s o f g i a n t f o x t a i l {SztcUiia faabQJbLL Herrm.); root r e s i d u e s o f g i a n t f o x t a i l , y e l l o w f o x t a i l [ S . gZauca (L.) Beauv. ] and c r a b g r a s s [V^igÂXcUvioi &anguinaLU> ( L . ) Scop. ] ; rhizomes and r e s i d u e s o f q u a c k g r a s s \_kg>wpyKOYl KZpQM (L.) Beauv] $ and r e s i d u e s o f v e l v e t l e a f (Abuutilon tkzophAOAti Medic.) and y e l l o w nutsedge (CypQAuA QACuZzntuA L.) (84-88). V e l v e t l e a f r e s i d u e caused t h e g r e a t e s t s u p p r e s s i o n o f c o r n h e i g h t and w e i g h t i n sandy and l i g h t t e x t u r e d s o i l , w i t h r e d u c t i o n s up t o 50%. Y e l l o w nutsedge r e s u l t e d



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i n g r e a t e r i n h i b i t i o n as t h e p e r c e n t a g e o f sand i n t h e s o i l m i x t u r e i n c r e a s e d , and when t h e c o r n seeds were i n c l o s e p r o x i m i t y t o t h e residue. These f i n d i n g s c o n f i r m t h e importance o f s o i l c o n d i t i o n s and r e i n f o r c e an e a r l i e r c o n c l u s i o n t h a t t h e e x t e n t o f c r o p damage may b e r e l a t e d t o t h e chance e n c o u n t e r o f r o o t s w i t h l o c a l i z e d a r e a s of h i g h t o x i c i t y (6,89). I n t e r e s t i n g l y , a r e c e n t o p i n i o n survey ranked f o x t a i l s , v e l v e t l e a f , and quackgrass among t h e f i v e weeds c a u s i n g the most economic damage i n c o r n (90). J o h n s o n g r a s s [SokQhum kalp(LVU>& (L.) P e r s . ] was a n o t h e r one o f t h i s group, and i t s a l l e l o p a t h i c e f f e c t s a g a i n s t s e v e r a l p l a n t s a r e w e l l known ( 1 ) . My own i n v e s t i g a t i o n s have f o c u s e d on a g g r e s s i v e weeds common t o the N o r t h C e n t r a l P l a i n s . The weeds i n v e s t i g a t e d a r e common i n g r a i n sorghum and soybean f i e l d s , thus s e e d l i n g s o f t h e s e c r o p s have been u t i l i z e d i n bioassays. A l l e l o p a t h i c e f f e c t s have been documented from c u r l y dock (Rumex οΛΛΛραΔ L . ) , g i a n t ragweed (hnbhObAJL ttvifaida L . ) , v e l v e t l e a f , c o c k l e b u r (Kantkium bthxmoJiLvm L . ) , J e r u s a l e m a r t i

choke (HelsiantkuA άώοΛολαλ L . ) , and Kockia [Kockia &copa/iia(L.) Schrad.] (55,91-94). Aqueous e x t r a c t s from each o f the weeds i n h i b i t e d t h e growth o f sorghum s e e d l i n g s i n n u t r i e n t s o l u t i o n when the e x t r a c t from 1 g f r e s h l e a f weight was i n c o r p o r a t e d i n 60 ml o f n u t r i e n t medium. P r o g r e s s i v e l y g r e a t e r growth r e d u c t i o n s and o t h e r v i s i b l e t o x i c i t y symptoms o c c u r r e d w i t h h i g h e r l e v e l s o f e x t r a c t s . S p e c i e s d i f f e r e n c e s were a p p a r e n t , w i t h some weeds r e d u c i n g sorghum growth a t l e s s than 1 g i n 120 m l o f medium. When d r i e d - s h o o t resi due from Kockia, J e r u s a l e m a r t i c h o k e , o r c o c k l e b u r were added t o s o i l , g e r m i n a t i o n was n o t m o d i f i e d . However, a f t e r two weeks, b o t h shoot and r o o t d r y w e i g h t s o f sorghum were l e s s i n s o i l c o n t a i n i n g 0.63% (w/w) weed r e s i d u e , o r h i g h e r . Based on f i e l d samplings from weed-infested areas, these residue l e v e l s could e a s i l y occur i n the surface decimeter o f the s o i l . I n s e v e r a l i n s t a n c e s , soybeans had a d i f f e r e n t s e n s i t i v i t y t o t h e s e a l l e l o p a t h i c weeds t h a n sorghum. F o r example, soybean growth was s i g n i f i c a n t l y reduced i n e x t r a c t t r e a t m e n t s c o n t a i n i n g 1 g f r e s h weight o f Kockia i n 240 ml o f the n u t r i e n t medium, i n d i c a t i n g t h a t soybeans were more s e n s i t i v e t h a n sorghum t o a l l e l o c h e m i c a l s i n t h e Kockia e x t r a c t . I n c o n t r a s t , i n h i b i t i o n o f soybeans grown i n s o i l c o n t a i n i n g Kockia r e q u i r e d a h i g h e r r e s i d u e l e v e l (2.5%) than t h e i n h i b i t i o n t h r e s h o l d f o r sorghum. The l o w e s t Koc/l/ta-residue amend ment t o s o i l (0.63%) s t i m u l a t e d t h e growth o f soybeans. Subsequent t e s t s w i t h v e l v e t l e a f , Kockia, J e r u s a l e m a r t i c h o k e , and c o c k l e b u r showed t h a t t h e i r a l l e l o p a t h i c a c t i o n a l t e r e d water b a l a n c e (55,94,95). Growth r e d u c t i o n s i n sorghum and soybean s e e d l i n g s i n n u t r i e n t s o l u t i o n amended w i t h e x t r a c t s from t h e s e weeds c o r r e l a t e d w i t h h i g h d i f f u s i v e r e s i s t a n c e s and low l e a f water p o t e n tials. S t o m a t a l c l o s u r e o c c u r r e d i n p l a n t s t r e a t e d w i t h the more concentrated extracts. D e p r e s s i o n s i n water p o t e n t i a l were due t o a r e d u c t i o n i n b o t h t u r g o r p r e s s u r e and o s m o t i c p o t e n t i a l . A lower r e l a t i v e water c o n t e n t was a l s o found i n v e l v e t l e a f - t r e a t e d p l a n t s . These impacts on water b a l a n c e were n o t from o s m o t i c f a c t o r s . A l l e l o c h e m i c a l s from t h e s e weeds have n o t been t h o r o u g h l y a s c e r t a i n e d but t h e p r e s e n t e v i d e n c e shows t h a t some c o n t a i n p h e n o l i c i n h i b i t o r s . L o d h i (96) r e p o r t e d t h a t Kockia c o n t a i n s f e r u l i c a c i d , c h l o r o g e n i c a c i d , c a f f e i c a c i d , m y r i c e t i n , and q u e r c e t i n . As n o t e d e a r l i e r , a n e f f e c t on p l a n t - w a t e r r e l a t i o n s h i p s i s one mechanism a s s o c i a t e d w i t h the a c t i o n o f f e r u l i c a c i d .


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R e s i d u e amendments t o s o i l changed t h e water s t a t u s o f s e e d l i n g s i n some t r e a t m e n t s . However, t h e g r o w t h - i n h i b i t i o n t h r e s h o l d f o r a p a r t i c u l a r weed r e s i d u e i n s o i l was t y p i c a l l y lower t h a n t h e l e v e l r e q u i r e d t o r e d u c e water p o t e n t i a l and i n c r e a s e d i f f u s i v e r e s i s t a n c e . T h i s p r o b a b l y r e s u l t e d from t h e combined a c t i o n o f a number o f a l l e l o p a t h i c c h e m i c a l s , w i t h c e r t a i n ones d i s r u p t i n g o t h e r a s p e c t s of m e t a b o l i s m . I t i s t o o e a r l y t o s u g g e s t how o f t e n a l l e l o p a t h i c weed-crop i n t e r a c t i o n s a f f e c t water u p t a k e , b u t i t i s i n t e r e s t i n g t o n o t e t h a t a r e c e n t r e p o r t i n d i c a t e d t h a t p r o d u c t i o n l o s s e s from q u a c k g r a s s c o u l d be overcome by i r r i g a t i o n ( 9 7 ) . S e v e r a l a l l e l o p a t h i c weeds a l t e r m i n e r a l n u t r i t i o n (98,99), and such an a c t i o n on a c r o p would l i k e l y r e s u l t i n lower y i e l d s . Chambers and Holm (100) r e p o r t e d beans t o o k up l e s s phosphorus when they were grown i n a s s o c i a t i o n w i t h pigweed (Ama/ianthuA HQÂA0falzxuA L.). Corn s t u n t e d by q u a c k g r a s s was low i n n i t r o g e n and p o t a s s i u m , and y e t heavy f e r t i l i z a t i o n d i d n o t improve y i e l d (86,101). S i m i l a r l y , Bhowmik and D o l l (88) r e p o r t e d above-ground r e s i d u e s o f common l a m b s q u a r t e r (Ckzno podium aZbum L , ) , pigweed, v e l v e t l e a f , and y e l l o w f o x t a i l reduced c o r n and soybean growth i n d e p e n d e n t l y o f n i t r o g e n and phosphorus augmentation. Their data d i d not i n d i c a t e t h a t growth i n h i b i t i o n was r e l a t e d t o n u t r i e n t u p t a k e . Undoubtedly, a b e t t e r u n d e r s t a n d i n g o f mechanism o f weed i n t e r f e r e n c e w i t h m e t a b o l i s m c o u l d l e a d t o a v o i d a n c e management s t r a t e g i e s . Crop-crop I n t e r a c t i o n s . Crop r e l a t i o n s h i p s i n v o l v e b o t h a u t o t o x i c i t y ( s e l f - i n h i b i t i o n ) and t h e e f f e c t s t h a t one c r o p may have on a d i f f e r ent c r o p . The l a t t e r can be s t i m u l a t o r y as w e l l as i n h i b i t o r y . W h i l e the knowledge base f o r most o f t h e s e i n t e r a c t i o n s i s embryonic, a few examples i l l u s t r a t e t h e i r p o t e n t i a l . Farm o p e r a t o r s have r e c o g n i z e d f o r q u i t e some time t h a t a drop i n p r o d u c t i o n c a n o c c u r under c o n t i n u o u s c r o p p i n g i n some f i e l d s . T h i s has been r e f e r r e d t o as s o i l s i c k n e s s , o r a sod-bound c o n d i t i o n for grasses. O f t e n , t h e cause i s unknown, b u t t h e problem f o r s e v e r a l legumes may be due t o a l l e l o p a t h y . In s o i l sickness of red c l o v e r (JtbifaoLLum pficutzyiAZ L . ) , i s o f l a v o n o i d and p h e n o l i c a c i d t o x i n s have been i m p l i c a t e d (102), K a t z n e l s o n (46) t e n u o u s l y con c l u d e d t h a t t h e problem i n Berseem c l o v e r (T atzxandxiviin L.) was from d i s t u r b a n c e s i n phosphorus u p t a k e , b u t nematodes were t h e major cause o f P e r s i a n c l o v e r (T. h.2Au.pÎYicutum L.) s o i l s i c k n e s s . A u t o t o x i c i t y i n a l f a l f a appears t o o c c u r ( 4 7 ) , a l t h o u g h t h e s p e c i f i c a l l e l o c h e m i c a l s have n o t been d e t e r m i n e d and t h e e x t e n t o f t h e problem v a r i e s w i t h s o i l and c l i m a t i c c o n d i t i o n s . A n o t h e r example o f a u t o t o x i c i t y has been o b s e r v e d f o r t h e growth o f p i g e o n pea [CajanuA cajan ( L . ) M i l l s p . J , a t r o p i c a l bush legume c u l t i v a t e d i n P u e r t o Rico. H e p p e r l y and D i a z (103) r e p o r t e d i n s t a n c e s where y i e l d dropped d r a s t i c a l l y under c o n t i n u o u s p l a n t i n g and t h e r e was l i t t l e response to supplemental f e r t i l i t y . They s u g g e s t e d p h y t o t o x i c i t y from t e r p e n o i d s , p o l y p h e n o l s , and o t h e r a l l e l o c h e m i c a l s was r e s p o n s i b l e f o r the d e c l i n e i n y i e l d of pigeon pea. f

Major c e r e a l crops may s u f f e r s i m i l a r p r o b l e m s . P l a n t i n g o f wheat (T/UZLcum aOAtivum L.) each y e a r i n t h e p r o d u c t i o n b e l t s o f the U.S., A u s t r a l i a , and t h e U.S.S.R. sometimes r e s u l t s i n d e c l i n i n g y i e l d s , e s p e c i a l l y when t h e s t r a w i s l e f t and a c o o l , wet s e a s o n occurs. R e s e a r c h i n Nebraska i n d i c a t e d t h a t p a t u l i n , an u n s a t u r a t e d



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l a c t o n e produced by s e v e r a l f u n g i , was i n v o l v e d (8,104,105). A major s o u r c e o f p a t u l i n was V&YiLcJJUUjum UAticaz B a i n i e r w h i c h f l o u r i s h e s d u r i n g d e c o m p o s i t i o n of wheat straw. About 40% o f a l l s o i l m i c r o organisms which they i s o l a t e d produced s u b s t a n c e s t h a t reduced p l a n t growth. Thus, o t h e r m i c r o b i a l t o x i n s may a l s o be i n v o l v e d . Kimber (106) found t h a t b o t h i m m o b i l i z a t i o n o f n i t r o g e n and p h y t o t o x i c e f f e c t s were i m p o r t a n t i n s u p p r e s s i o n o f wheat g e r m i n a t i o n by wheat straw. S t a t i s t i c s on c o r n f o l l o w i n g c o r n the p r e v i o u s y e a r show t h a t y i e l d i s about 10 b u / a c r e l e s s t h a n c o r n f o l l o w i n g soybeans, and t h i s i s not due t o f e r t i l i t y (107). The d i f f e r e n c e i n y i e l d may be due t o i n h i b i t i o n from c o r n r e s i d u e (108), s t i m u l a t i o n from soybean r e s i d u e , or a c o m b i n a t i o n of b o t h . C e r t a i n l y the y i e l d d i f f e r e n t i a l i n d i c a t e s t h a t the h i s t o r i c a l p r a c t i c e o f c r o p r o t a t i o n was b e n e f i c i a l . Crop impact on a subsequent p l a n t i n g o f a d i f f e r e n t s p e c i e s was a l l u d e d t o p r e v i o u s l y i n t h e d i s c u s s i o n of y i e l d r e d u c t i o n s i n soy beans f o l l o w i n g r i c e . E a r l y work of P a t r i c k and c o l l e a g u e s demon s t r a t e d e x t e n s i v e p h y t o t o x i c i t y from a v a r i e t y o f decomposing c r o p r e s i d u e s (6,89). G u e n z i and M c C a l l a (25) r e p o r t e d t h a t o a t s , wheat, sorghum, and c o r n r e s i d u e s c o n t a i n e d w a t e r - s o l u b l e a l l e l o c h e m i c a l s , i n c l u d i n g f e r u l i c , p-coumaric, s y r i n g i c , v a n i l l i c , and p-hydroxyb e n z o i c a c i d s , which a f f e c t e d t h e g e r m i n a t i o n and growth of wheat, c o r n , and sorghum. They a l s o found much h i g h e r l e v e l s o f t h e s e p h e n o l i c a c i d s under farm o p e r a t i o n s t h a t l e f t s i g n i f i c a n t r e s i d u e s on t h e s u r f a c e (29,109), O t h e r examples demonstrate t h a t c r o p a l l e l o p a t h i c problems encompass a range o f s i t u a t i o n s , from v e g e t ables to f o r e s t r y . Asparagus (A6pa/iaguA 0^IdÎYiaJUA L.) i s a u t o t o x i c as w e l l as a l l e l o p a t h i c to o t h e r v e g e t a b l e s , and p a r t o f the t o x i c e f f e c t s may r e s u l t from i n t e r a c t i o n s w i t h p a t h o g e n i c TuACUbium spp. (110). W a l t e r s and G i l m o r e (111) found t h a t f e s c u e (ïtetuca CUuiyidinacza Shreb.) i n t e r f e r e d w i t h e s t a b l i s h m e n t and growth o f sweetgum (LiquidambaA Atysiaciuùia L , ) . S t u d i e s where c o m p a r a t i v e e f f e c t s were e l i m i n a t e d through use o f a s t a i r s t e p a p p a r a t u s i n d i c a t e d an a l l e l o p a t h i c mechanism caused t h e growth r e d u c t i o n s . C h e m i c a l a n a l y s i s o f sweetgum s e e d l i n g s t r e a t e d w i t h f e s c u e l e a c h a t e s showed t h a t growth i n h i b i t i o n was a s s o c i a t e d w i t h an i m p a i r e d a b s o r p t i o n o f phosphorus an n i t r o g e n . O b v i o u s l y , c o n d i t i o n s of decomposi t i o n , a l l e l o c h e m i c a l enhancement o f d i s e a s e , t h e n a t u r e o f the secondary p r o d u c t s from m i c r o b i a l a c t i v i t y , and i n t e r a c t i o n s among a l l e l o c h e m i c a l s are a l l s i g n i f i c a n t v a r i a b l e s i n i n t e r c r o p a l l e l o pathy. The uniqueness o f t h e c h e m i c a l environment f o r each c r o p sequence and s i t u a t i o n w i l l c o n t i n u e t o confound p r e c i s e a n a l y s e s of e f f e c t s on y i e l d . R e i n f o r c e m e n t needs t o be g i v e n to the f a c t t h a t c e r t a i n c r o p r e s i d u e s may be s t i m u l a t o r y . I n 1975, i t was r e p o r t e d t h a t s m a l l amendments of chopped a l f a l f a t o the s o i l s t i m u l a t e d the growth o f tomato, cucumber (Cucuma> Acutivum , l e t t u c e (LacXaca Acutiva L . ) , and s e v e r a l o t h e r s . T r i a c o n t a n o l , a l o n g - c h a i n a l c o h o l which i s a component o f t h e waxy c o a t o f some l e a v e s , was i s o l a t e d as the e f f e c t i v e compound (112). U n f o r t u n a t e l y , numerous f i e l d t e s t s w i t h t r i a c o n t a n o l have not c o n s i s t e n t l y g i v e n growth s t i m u l a t i o n , Agrostemmin was i s o l a t e d from c o r n c o c k l e (AgfLOAtowma gÂjtkougo L , ) , and s t r o n g c l a i m s have been made about i t s c a p a b i l i t y f o r enhancing wheat y i e l d (113). B r a s s i n o l i d e , a s t e r o i d t h a t has growth s t i m u l a t o r y c a p a b i l i t i e s i n e x c e e d i n g l y s m a l l amounts, was i s o l a t e d from


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rape p o l l e n . Commercial a n a l o g u e s , c a l l e d b r a s s i n o s t e r o i d s , have been s y n t h e s i z e d and some suggest t h e s e a r e a p r o t o t y p e of a new group o f p l a n t growth r e g u l a t o r s (114). Whatever the f u t u r e h o l d s , c e r t a i n crop sequences may improve y i e l d s even when the d e t a i l s o f a l l e l o c h e m i c a l involvement remain a mystery. Crop I n h i b i t i o n o f Weeds. The a l l e l o p a t h i c c a p a c i t y o f c r o p s t o s u p p r e s s weed growth has immediate u t i l i t y f o r management s t r a t e g i e s and, i f h e r i t a b l e f a c t o r s f o r a l l e l o c h e m i c a l s can be i d e n t i f i e d , t h e s e c o u l d be i n c o r p o r a t e d i n t o commercial c u l t i v a r s . The s e v e r a l t h r u s t s o f r e s e a r c h on weed c o n t r o l have been t o (a) i d e n t i f y c u l t i v a r s and a c c e s s i o n s w i t h h i g h a l l e l o p a t h i c p o t e n t i a l , (b) i s o l a t e p r i m a r y a l l e l o c h e m i c s , and (c) d e v e l o p f i e l d p r o t o c o l f o r c a p i t a l i z i n g on a l l e l o c h e m i c a l s from c r o p p l a n t s . Putnam and Duke (115) s c r e e n e d f o r p h y t o t o x i c i t y i n seed s o u r c e s from 526 a c c e s s i o n s o f cucumber and found 3% o f the a c c e s s i o n s i n h i b i t e d the i n d i c a t o r s p e c i e s , p r o s o m i l l e t (Panicum miLiac2.um L.) and w h i t e mustard (B/LO64/CC0. kOtfa Moench) , by more t h a n 75%. In the f i e l d , some a c c e s s i o n s reduced weed p o p u l a t i o n s more t h a n 50%, b u t l e s s weed s u p p r e s s i o n o c c u r r e d under p e r i o d s o f i n c r e a s e d r a i n f a l l (116) . Fay and Duke (23) approached the q u e s t i o n of a l l e l o p a t h i c e x p r e s s i o n from o a t s i n a d i f f e r e n t way. They s c r e e n e d 3,000 a c c e s s i o n s o f Avtna spp. germ plasm f o r o u t p u t o f s c o p o l e t i n , a known i n h i b i t o r from o a t s . Some exuded up to t h r e e times as much s c o p o l e t i n as a s t a n d a r d c u l t i v a r , and t h e s e were the most a c t i v e i n s u p p r e s s i n g w i l d mustard [BHOAAica kabOA (D.C.) L.C. Wheeler] i n sand c u l t u r e . However, i n loamy-sand s o i l t h i s a c t i v i t y was l o s t . The c a p a c i t y f o r weed s u p p r e s s i o n a l s o has been shown f o r soybeans and s u n f l o w e r s (HoJiiantkuA annuuA L . ) . Greenhouse e v a l u a t i o n of 141 a c c e s s i o n s o f soybeans on two weeds, [HfLÙYiWVtiriUL (LchioldQA (L.) Gaertner] and Alop&CUSuM myobuJioidte Huds. , i l l u s t r a t e d t h a t some l i n e s s t i m u l a t e d t h e s e weeds, whereas o t h e r s were i n h i b i t o r y , and the s e n s i t i v i t y o f the two t e s t s p e c i e s was q u i t e d i f f e r e n t (117) , L e a t h e r (24,118) r e p o r t e d a l l e l o p a t h i c - m e d i a t e d weed s u p p r e s s i o n i n a f i v e year f i e l d study u s i n g a sunflower-oat-sunflower rotation. Weed d e n s i t y i n c r e a s e d i n a l l p l o t s d u r i n g the s t u d y , but i n the r o t a t i o n w i t h s u n f l o w e r s the i n c r e a s e i n weed d e n s i t y was s i g n i f i c a n t l y l e s s than i n p l o t s w i t h o u t s u n f l o w e r s . Work i s now i n p r o g r e s s on i s o l a t i o n and c h a r a c t e r i z a t i o n o f the c h e m i c a l s i n v o l v e d . U n f o r t u n a t e l y , s u n f l o w e r s may a l s o be d e l e t e r i o u s t o o t h e r c r o p s . Lower y i e l d s than e x p e c t e d have been r e p o r t e d f o r c r o p s f o l l o w i n g s u n f l o w e r s i n South Dakota, and our l a b o r a t o r y t e s t s showed t h a t sorghum grown i n s o i l amended w i t h s u n f l o w e r r e s i d u e was s t u n t e d and

e x h i b i t e d water s t r e s s (119). The p o t e n t i a l f o r u s i n g a l l e l o p a t h i c c r o p r e s i d u e s i n weed c o n t r o l has been e v a l u a t e d i n M i c h i g a n over the l a s t decade. Putnam and DeFrank (120) r e p o r t e d t h a t r e s i d u e s o f b a r l e y (Hotiddum vuZgaAQ. L . ) , o a t s , wheat, r y e (Szcatd C2A

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