Allelopathy: An Overview - ACS Symposium Series (ACS Publications)

Chapter 2

Allelopathy: An Overview Elroy L. Rice

Downloaded by NORTH CAROLINA STATE UNIV on November 7, 2013 | http://pubs.acs.org Publication Date: January 8, 1987 | doi: 10.1021/bk-1987-0330.ch002

Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019

Our increasing knowledge of allelopathy is aiding greatly in our understanding of many ecological phenomena. Our increasing awareness of conditions under which certain crop residues cause allelopathic effects on subsequent crops should enable us soon to guard against detrimental effects and to manage rotation to take advantage of stimulatory effects. Available evidence indicates that it will be possible, through breeding/or biotechnology, to develop crop cultivars that will inhibit growth of the chief weeds in a given area through allelopathic action and thus decrease the need for synthetic weed killers. We are already able to use allelopathic companion crops or residues of allelopathic crop plants and weeds to control weed growth in some crops and orchards. Our understanding of allelopathic interactions between various plant species has been used advantageously in reforestation, and future developments are encouraging. Considerable information is available concerning types of chemicals involved in allelopathy, and some information is available concerning movement of the chemicals from plants and factors determining their effectiveness after egression from plants. Nevertheless, these areas of allelopathy are probably the ones that merit the strongest research emphasis in the near future. T h e o p h r a s t u s (1), a b o u t 3 0 0 B . C . , s t a t e d t h a t c h i c k p e a ( C i c e r a r i e t i n u m ) does not r e i n v i g o r a t e t h e ground as other r e l a t e d p l a n t s (legumes) do b u t " e x h a u s t s " i t instead. H e pointed out also that c h i c k p e a destroys weeds. P l i n y (2) r e p o r t e d i n t h e 1st c e n t u r y A . D . t h a t c h i c k p e a , b a r l e y ( H o r d e u m vulgare), fenugreek (Trigonella foenum-graecum), a n d bitter vetch (Vicia ervilia) all "scorch up" cornland. In s p i t e o f t h e e a r l y s u g g e s t i o n s c o n c e r n i n g a p p a r e n t a l l e l o p a t h i c e f f e c t s , no s o l i d s c i e n t i f i c e v i d e n c e w a s o b t a i n e d t o s u p p o r t t h e s u g g e s t i o n s until t h e present c e n t u r y . T h e t e r m a l l e l o p a t h y w a s c o i n e d by M o l i s c h i n 1937 t o r e f e r t o b i o c h e m i c a l i n t e r a c t i o n s b e t w e e n a l l t y p e s o f p l a n t s , i n c l u d i n g m i c r o o r g a n i s m s t r a d i t i o n a l l y p l a c e d i n t h e p l a n t k i n g d o m (3). H i s discussion i n d i c a t e d that he meant the t e r m to cover both inhibitory a n d stimulatory biochemical interactions. 0097-6156/87/0330-0008$06.00/0 © 1987 A m e r i c a n C h e m i c a l Society

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

2.

RICE

Allelopathy:

An

Overview


A v e r y i m p o r t a n t p o i n t c o n c e r n i n g a l l e l o p a t h y is t h a t i t s e f f e c t d e p e n d s o n a c h e m i c a l c o m p o u n d b e i n g a d d e d t o t h e e n v i r o n m e n t . It is t h u s s e p a r a t e d f r o m c o m p e t i t i o n w h i c h i n v o l v e s the r e m o v a l or r e d u c t i o n of s o m e f a c t o r f r o m t h e e n v i r o n m e n t t h a t is r e q u i r e d by s o m e o t h e r p l a n t o r m i c r o o r g a n i s m sharing the habitat. M u l l e r (4) s u g g e s t e d the term interference to refer to the overall influence of one plant (or m i c r o o r g a n i s m ) on a n o t h e r . I n t e r f e r e n c e would thus encompass both allelopathy and c o m p e t i t i o n . E v i d e n c e i n d i c a t e s t h a t a l l e l o p a t h i c c o m p o u n d s get out of plants by v o l a t i l i z a t i o n , e x u d a t i o n f r o m r o o t s , l e a c h i n g f r o m plants or residues by r a i n , or d e c o m p o s i t i o n o f r e s i d u e s (5). The goals of this paper are to discuss some of the major g e n e r a l i z a t i o n s t h a t c a n be m a d e a b o u t 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 s o m e e x a m p l e s of suggested r o l e s , a n d focus on d e s i r a b l e f u t u r e r e s e a r c h applications. O n l y a few of the p e r t i n e n t i n v e s t i g a t i o n s are c i t e d in i l l u s t r a t i n g major p r i n c i p l e s . Allelopathy in Plant Pathology Spores of most p a r a s i t i c fungi r e m a i n ungerminated while l o c a t e d in their s i t e o f p r o d u c t i o n (6). T h i s c a n be due t o s e v e r a l f a c t o r s , o n e o f w h i c h i s p r o d u c t i o n by t h e s p o r e s o f f u n g i s t a t i c a g e n t s t h a t a r e e x c r e t e d i n t o t h e w a t e r around the spores. These s e l f - i n h i b i t o r s generally assure dispersal of viable ungerminated spores. Endogenous germination stimulators that c o u n t e r a c t i n h i b i t i o n by s e l f - i n h i b i t o r s o c c u r i n m a n y s p o r e s . N o n a n a l a n d 6 - m e t h y l - 5 - h e p t e n - 2 - o n e w e r e i s o l a t e d f r o m uredospores of U r o m y c e s and P u c c i n i a (7). T h e s e c o m p o u n d s s t i m u l a t e g e r m i n a t i o n of s t e m rust spores that contain m e t h y l f e r u l a t e as t h e i r i n h i b i t o r , b u t t h e y do n o t s t i m u l a t e g e r m i n a t i o n o f s p o r e s t h a t c o n t a i n a d i m e t h o x y c i n n a m a t e as i n h i b i t o r . M o s t p a r a s i t e s have to s u r v i v e prolonged periods of t i m e a p a r t f r o m the host p l a n t . C o n s e q u e n t l y , the f o r m a t i o n of r e s t i n g p r o p a g u l e s , such as s c l e r o t i a , c o n s t i t u t e s a c r i t i c a l p a r t of the p a r a s i t e ' s l i f e c y c l e . Several o b s e r v a t i o n s i n d i c a t e t h a t f o r m a t i o n o f s c l e r o t i a m a y be s t i m u l a t e d b y a l l e l o c h e m i c a l s (8,9). B r a n d t a n d R e e s e (10) c o n c l u d e d t h a t V e r t i c i l l i u m dahliae produces a diffusible morphogenetic factor that stimulates p r o d u c t i o n of m i c r o s c l e r o t i a . When low c o n c e n t r a t i o n s of the diffusible f a c t o r w e r e added to c u l t u r e s of the p a t h o g e n , the hyphae s w e l l e d and became c o n s t r i c t e d , septation was increased, and c e l l walls became thickened. K e r r (11) g r e w s t e r i l e s e e d l i n g s i n s i d e c e l l o p h a n e b a g s b u r i e d i n s o i l i n o c u l a t e d w i t h P e l l i c u l a r i a f i l a m e n t o s a and found an intense d e v e l o p m e n t o f t h e p a t h o g e n on t h e c e l l o p h a n e o p p o s i t e t h e r o o t s o f t h e t w o s u s c e p t i b l e h o s t s , l e t t u c e a n d r a d i s h , b u t no s t i m u l a t i o n o p p o s i t e t o m a t o r o o t s , w h i c h a r e n o t s u s c e p t i b l e . B u x t o n (12) a l s o d e m o n s t r a t e d a d e f i n i t e s p e c i f i c i t y i n r e l a t i o n t o t h e g e r m i n a t i o n o f s p o r e s o f F u s a r i u m o x y s p o r u m f. p i s i i n t h e e x u d a t e s of t h r e e pea v a r i e t i e s d i f f e r i n g in s u s c e p t i b i l i t y to this p a t h o g e n . E x u d a t e f r o m a w i l t - r e s i s t a n t v a r i e t y i n h i b i t e d spore g e r m i n a t i o n , whereas exudate from a susceptible plant stimulated such germination. In s o i l t h a t h a s n o t h a d r e c e n t a d d i t i o n s o f p l a n t r e s i d u e o r o t h e r organic m a t e r i a l , m i c r o b i a l respiration proceeds at a low rate (13). M o r e o v e r , f u n g i a p p a r e n t l y e x i s t m o s t l y as s p o r e s i n a s t a t e o f f u n g i s t a s i s . T h i s m i c r o f l o r a u s u a l l y r e s p o n d s t o t h e a d d i t i o n o f p l a n t r e s i d u e by s p o r e germination, increased respiration, and growth. These responses w e r e i n d u c e d by v o l a t i l e c o m p o n e n t s f r o m a l f a l f a t o p s , c o r n l e a v e s , w h e a t s t r a w , bluegrass clippings, tea leaves, and tobacco leaves, even when the residue


9


10

ALLELOCHEMICALS: ROLE IN AGRICULTURE A N D FORESTRY

was s e p a r a t e d f r o m the soil by a 5 - c m a i r gap. T h e r e was a r a p i d o u t g r o w t h of hyphae f r o m the soil surface t o w a r d the residue before any g r o w t h of f u n g i c o u l d be s e e n i n t h e p l a n t m a t e r i a l . V a p o r s f r o m d i s t i l l a t e s o f w a t e r e x t r a c t s of the various plant residues m e n t i o n e d had s i m i l a r e f f e c t s on g r o w t h of fungi and m a r k e d l y i n c r e a s e d n u m b e r s of b a c t e r i a and the respiratory rate of m i c r o o r g a n i s m s in soil samples. W i t c h w e e d ( S t r i g a a s i a t i c a ) is a n e c o n o m i c a l l y i m p o r t a n t r o o t p a r a s i t e affecting many warm-season grasses, including such important crop plants as c o r n , g r a i n s o r g h u m , a n d s u g a r c a n e . V i a b l e w i t c h w e e d s e e d s m a y r e m a i n d o r m a n t i n t h e s o i l f o r m a n y y e a r s (l*f). The seeds w i l l usually not g e r m i n a t e unless p r e t r e a t e d in a w a r m , m o i s t e n v i r o n m e n t for several days before exposure to a c h e m i c a l c o m p o u n d exuded f r o m the roots of a host p l a n t or s o m e non-host p l a n t s . O n e such c o m p o u n d , s t r i g o l , was i s o l a t e d f r o m t h e r o o t e x u d a t e o f c o t t o n a n d h a s p r o v e d t o be a p o w e r f u l s t i m u l a n t o f w i t c h w e e d s e e d g e r m i n a t i o n . J o h n s o n , R o s e b e r y a n d P a r k e r (15) r e p o r t e d the synthesis and t e s t i n g of s e v e r a l analogs of s t r i g o l , and some w e r e powerful seed g e r m i n a t i o n s t i m u l a n t s for species of b o t h S t r i g a and O r o b a n c h e . C o t t o n is n o t a h o s t p l a n t f o r S t r i g a o r O r o b a n c h e , a n d i t i s n o t e w o r t h y t h a t t h e s t r u c t u r e s of t h e g e r m i n a t i o n s t i m u l a n t s e x u d e d f r o m t h e r o o t s o f t h e h o s t p l a n t s r e m a i n u n k n o w n (16). T h e h a u s t o r i a o f t h e p a r a s i t e s do n o t f o r m w h e n t h e p l a n t s a r e g r o w n a x e n i c a l l y , but are rapidly i n d u c e d in the presence of the host roots or host r o o t e x u d a t e s (17). S e v e r a l h a u s t o r i a l - i n d u c i n g compounds have now been c h a r a c t e r i z e d . Xenognosin A and Β were identified in gum t r a g a c a n t h , an e x u d a t e of A s t r a g a l u s g u m m i f e r , a n d s o y a s a p o g e n o l Β w a s i d e n t i f i e d i n r o o t s of L e s p e d e z a sericea. A l l e l o p a t h y in N a t u r a l Ecosystems P a t t e r n i n g of v e g e t a t i o n . C u r t i s a n d C o t t a m (18) o b s e r v e d t h e f a i r y - r i n g p a t t e r n o f t h e p r a i r i e s u n f l o w e r H e l i a n t h u s r i g i d u s , w h i c h is due t o a pronounced r e d u c t i o n in plant numbers, s i z e , and i n f l o r e s c e n c e s in the c e n t e r of the c l o n e . T h e y subsequently d e m o n s t r a t e d t h a t the p a t t e r n was due to a u t o t o x i n s p r o d u c e d by d e c a y of dead parts of the s u n f l o w e r . Prostrate knotweed, Polygonum aviculare, rapidly encroaches into b e r m u d a g r a s s lawns and the grass dies in p a t c h e s of p r o s t r a t e k n o t w e e d w h i l e b e r m u d a g r a s s at the edges of the k n o t w e e d patches turns y e l l o w . Soil minus l i t t e r was c o l l e c t e d under a P . a v i c u l a r e stand and under a b e r m u d a g r a s s stand and used to g r o w b e r m u d a g r a s s (19,20). S o i l c o l l e c t e d i n M a r c h under k n o t w e e d m a r k e d l y i n h i b i t e d seed g e r m i n a t i o n and seedling g r o w t h of bermudagrass c o m p a r e d w i t h soil f r o m under b e r m u d a g r a s s . D e c a y i n g roots and shoots of p r o s t r a t e k n o t w e e d reduced seed g e r m i n a t i o n and seedling g r o w t h of bermudagrass. A d d i t i o n a l l y , root exudates of k n o t w e e d r e d u c e d s e e d l i n g g r o w t h of b e r m u d a g r a s s . E l e v e n a l l e l o c h e m i c a l s i n h i b i t o r y to g r o w t h o f b e r m u d a g r a s s w e r e i s o l a t e d f r o m s o i l u n d e r p r o s t r a t e k n o t w e e d , w h e r e a s none of these o c c u r r e d in soil under b e r m u d a g r a s s (20,21). F o u r w e r e p h e n o l i c s and seven w e r e l o n g - c h a i n f a t t y a c i d s . V e g e t a t i o n under the trees in Japanese red pine, Pinus d e n s i f l o r a , f o r e s t s is s p a r s e d e s p i t e t h e f a c t t h a t t h e i n t e r i o r o f t h e s e f o r e s t s is o n e o f the b r i g h t e s t a m o n g f o r e s t s (22,23). Many other forests have dense undergrowths of herbs in spite of m u c h l o w e r l i g h t i n t e n s i t i e s . V a r i o u s p a r t s of red pine and the soil under i t c o n t a i n e d c h e m i c a l s t o x i c to m a n y p o t e n t i a l understory plants. Thus, it was concluded that allelopathy probably plays an i m p o r t a n t role in r e t a r d i n g understory g r o w t h .


2.

RICE

Allelopathy:

An

Overview

11


L y c o r i s r a d i a t a is a d o m i n a n t s p e c i e s a l o n g r o a d s i d e s a n d s l o p e s i n J a p a n , and i t appears to p r e v e n t some other plant species f r o m e m e r g i n g a n d g r o w i n g n e a r i t (24). U e k i and T a k a h a s h i found t h a t the bulbs of L . r a d i a t a exuded two a l l e l o c h e m i c a l s that m a r k e d l y reduced root growth of several weedy species usually o c c u r r i n g in the same general areas w i t h L y c o r i s . M o r e o v e r , the same compounds w e r e found in the soil adjacent to Lycoris bulbs. P l a n t s u c c e s s i o n . In t h e t a l l g r a s s p r a i r i e r e g i o n o f O k l a h o m a a n d K a n s a s , there are four m a i n successional stages when fields that are i n f e r t i l e are abandoned f r o m c u l t i v a t i o n : a pioneer w e e d stage t h a t persists for only 2-3 y e a r s , a n a n n u a l - g r a s s s t a g e t h a t l a s t s f o r 9 t o 13 y e a r s , a p e r e n n i a l b u n c h g r a s s s t a g e t h a t r e m a i n s f o r 30 y e a r s or l o n g e r a f t e r a b a n d o n m e n t , a n d t h e c l i m a x p r a i r i e (25). T h e e v i d e n c e is s t r o n g t h a t t h e p i o n e e r w e e d s t a g e disappears rapidly because the species are e l i m i n a t e d through strong a l l e l o p a t h i c i n t e r a c t i o n s (5). A r i s t i d a o l i g a n t h a , p r a i r i e t h r e e a w n , the d o m i n a n t o f t h e s e c o n d s t a g e , i n v a d e s n e x t a p p a r e n t l y b e c a u s e i t is n o t i n h i b i t e d b y t h e a l l e l o c h e m i c a l s p r o d u c e d a n d is a b l e t o g r o w w e l l i n s o i l t h a t is s t i l l t o o l o w i n n i t r o g e n t o s u p p o r t s p e c i e s t h a t i n v a d e l a t e r i n s u c c e s s i o n (26). A . oligantha and several pioneer species produce allelochemicals that inhibit g r o w t h of R h i z o b i u m and f r e e - l i v i n g n i t r o g e n - f i x i n g organisms, and n o d u l a t i o n a n d h e m o g l o b i n f o r m a t i o n i n l e g u m e s (5). T h i s i n d i r e c t e v i d e n c e suggested that b i o l o g i c a l nitrogen f i x a t i o n was slowed in the first two stages o f s u c c e s s i o n . K a p u s t k a a n d R i c e (27) m e a s u r e d n i t r o g e n f i x a t i o n r a t e s i n soils of the pioneer w e e d stage, the a n n u a l grass stage, and the c l i m a x p r a i r i e using the a c e t y l e n e r e d u c t i o n t e c h n i q u e . The rate was about four t i m e s as h i g h i n t h e c l i m a x s o i l as i n t h e p i o n e e r w e e d s t a g e a n d a b o u t f i v e t i m e s as h i g h i n t h e c l i m a x as i n t h e a n n u a l g r a s s s t a g e , t h u s s u b s t a n t i a t i n g the i n d i r e c t evidence. T h e s l o w i n g of n i t r o g e n f i x a t i o n i n the f i r s t t w o successional stages probably gives A r i s t i d a oligantha a selective advantage in c o m p e t i t i o n w i t h species h a v i n g higher n i t r o g e n r e q u i r e m e n t s and causes it to r e m a i n for a l e n g t h y p e r i o d . T h e r e is a s t r o n g e v i d e n c e t h a t n i t r i f i c a t i o n is s l o w e d i n t h e l a t e r s t a g e s o f s u c c e s s i o n c a u s i n g a v a i l a b l e n i t r o g e n t o be p r e s e n t c h i e f l y a s a m m o n i u m n i t r o g e n (28,29,30). T h i s should help to conserve nitrogen b e c a u s e t h e a m m o n i u m i o n is a d s o r b e d by t h e n e g a t i v e l y c h a r g e d m i c e l l e s i n t h e s o i l a n d is n o t r e a d i l y l e a c h e d b e l o w t h e d e p t h o f r o o t i n g o r w a s h e d away into streams. T h e r e is e v i d e n c e that tannins, phenolic acids, f l a v o n o i d s , a n d c o u m a r i n s m a y be i m p o r t a n t i n h i b i t o r s o f n i t r i f i c a t i o n (30, 31). The nitrogen c o n c e n t r a t i o n gradually increases to the point where some l a t e r species c a n i n v a d e . T h i s a p p a r e n t l y results in less i n h i b i t i o n of n i t r o g e n f i x a t i o n and m o r e i n h i b i t i o n of n i t r i f i c a t i o n . T h u s , the rate of a d d i t i o n o f n i t r o g e n is i n c r e a s e d a n d t h e r a t e of l o s s o f n i t r o g e n is decreased. E v e n t u a l l y t h e c o n c e n t r a t i o n o f n i t r o g e n is i n c r e a s e d t o t h e point where c l i m a x species can invade. U r b a n i z a t i o n a r o u n d l a r g e c i t i e s in J a p a n has g r e a t l y changed the p l a n t c o m m u n i t i e s because of the c r e a t i o n of bare areas or serious d i s t u r b a n c e of n a t u r a l e c o s y s t e m s (32-34). W e e d s u c c e s s i o n on urban w a s t e l a n d is s i m i l a r t o t h a t i n o l d - f i e l d s i n s o m e p a r t s o f t h e U . S . A . w i t h A m b r o s i a a r t e m i s i i f o l i a b e i n g t h e f i r s t - y e a r d o m i n a n t f o l l o w e d by S o l i d a g o a l t i s s i m a a n d E r i g e r o n s p p . f o r a f e w y e a r s , a n d n e x t by M i s c a n t h u s s i n e n s i s . N u m a t a a n d his c o l l e a g u e s f o u n d t h a t S . a l t i s s i m a a n d E r i g e r o n a n n u u s b o t h p r o d u c e p o l y a c e t y l e n i c m e t h y l e s t e r s (one i n S o l i d a g o a n d t h r e e i n E .



12

a n n u u s ) t h a t i n h i b i t s e e d g e r m i n a t i o n o f A . a r t e m i s i i f o l i a , M. s i n e n s i s a n d a species of T a g e t e s , and g r o w t h of r i c e seedlings. T h e p h y t o t o x i n found i n Solidago was also e x t r a c t e d f r o m soil in a stand of the species, and the concentration present was sufficient to regulate germination and growth of associated species. A v e r y d i l u t e s o l u t i o n (5 p p m ) o f t h r e e o f t h e phytotoxins inhibited growth of A . a r t e m i s i i f o l i a in soil. K o b a y a s h i e t a l . (35) f o u n d t h a t t h e r o o t s o f S o l i d a g o a l t i s s i m a c o n t a i n 250-400 p p m o f t h e C . - p o l y a c e t y l e n e , c i s - d e h y d r o m a t r i c a r i a e s t e r ( c i s D M E ) . T h e y found t h a r soil under a stand of the Solidago c o n t a i n e d 6 ppm of c i s - D M E plus t r a n s - D M E . B o t h compounds were found to i n h i b i t g r o w t h of rice seedlings. Three C . «-polyacetylenes were i d e n t i f i e d f r o m m e t h a n o l extracts of Erigeron annuus, E . canadensis, E . floribundus, and E . philadelphicus. These were the c i s - and t r a n s - m a t r i c a r i a ester and the c i s l a c h n o p h y l l u m ester. A l l w e r e i n h i b i t o r y to seed g e r m i n a t i o n of A m b r o s i a a r t e m i s i i f o l i a a n d s e e d l i n g g r o w t h of r i c e a t a c o n c e n t r a t i o n of 5 p p m or above. Kobayashi et a l . concluded that the dominance of Solidago altissima a n d E r i g e r o n s p p . i n t h e s e c o n d s t a g e o f s e c o n d a r y s u c c e s s i o n is p r o b a b l y due t o t h e i r p r o d u c t i o n o f a c e t y l e n i c c o m p o u n d s w h i c h s t r o n g l y i n h i b i t g r o w t h of many other plant species. T h e y suggested also that the r e l a t i v e l y s h o r t p e r i o d o f o c c u p a t i o n by S. a l t i s s i m a a n d E r i g e r o n s p p . m a y be a consequence of the a c c u m u l a t i o n of such polyacetylenes in the soil to the point where they are t o x i c to these species also.


n

Allelopathy in Manipulated Ecosystems A l l e l o p a t h y i n f o r e s t r y . W a l t e r s a n d G i l m o r e (36) n o t e d t h a t h e i g h t g r o w t h of s w e e t g u m , L i q u i d a m b a r s t y r a c i f l u a , w a s less i n plots c o n t a i n i n g f e s c u e , F e s t u c a arundinacea, than in adjacent plots without fescue. C h e m i c a l and physical soil factors did not appear to explain the differences. G r o w t h of sweetgum was c o r r e l a t e d w i t h residual phosphorus and magnesium, but this correlation was achieved across a l l experimental plots without respect to the presence or absence of f e s c u e . S e e d i n g of fescue into pots c o n t a i n i n g sweetgum seedlings resulted in a reduction in dry weight increment of s w e e t g u m u p t o 9 5 % . E l i m i n a t i o n o f c o m p e t i t i o n t h r o u g h use o f a s t a i r s t e p apparatus suggested that an allelopathic mechanism was involved. L e a c h a t e s f r o m the rhizosphere of live fescue, dead fescue roots, and dead f e s c u e l e a v e s c a u s e d r e d u c t i o n s i n d r y w e i g h t i n c r e m e n t s o f s w e e t g u m up t o 6 0 % . C h e m i c a l analysis of sweetgum seedlings from the stairstep experiment suggested that fescue leachates decreased absorption of phosphorus and nitrogen. T u b b s (37) f o u n d t h a t s u g a r m a p l e s e e d l i n g s i n h i b i t e d g r o w t h o f seedlings of yellow birch despite the apparent absence of c o m p e t i t i o n i n nursery e x p e r i m e n t s . R o o t e l o n g a t i o n of b i r c h w a s r e t a r d e d by exudates of a c t i v e l y g r o w i n g root tips of sugar m a p l e . When seedlings of these species were grown together in aerated nutrient solution, the number of actively growing root tips of b i r c h f o r m e d each day w a s inversely c o r r e l a t e d w i t h the a c t i v i t y o f t h e a l l e l o c h e m i c a l p r o d u c e d b y m a p l e , as i n d i c a t e d b y t h e retardation of elongation of yellow birch roots. A l d e r species are often i m p o r t a n t in forests because of the f i x a t i o n of n i t r o g e n by F r a n k i a i n nodules on t h e i r roots. J o b i d o n a n d T h i b a u l t (38) observed growth depression of alders near balsam poplar, Populus b a l s a m i f e r a , stands. Water e x t r a c t s of leaf l i t t e r and buds, and fresh leaf l e a c h a t e s of b a l s a m poplar i n h i b i t e d seed g e r m i n a t i o n and r a d i c l e a n d hypocotyl growth of green alder, Alnus crispa v a r . mollis, seedlings. There was m a r k e d i n h i b i t i o n of root hair d e v e l o p m e n t and necrosis of the r a d i c l e



2.

RICE

Allelopathy:

An

Overview

13

m e r i s t e m s . T h e a v e r a g e n u m b e r of nodules on a l d e r p l a n t s t r e a t e d w i t h any one of the three b a l s a m e x t r a c t s d e s c r i b e d a b o v e was o n l y 5 1 % of t h a t of c o n t r o l p l a n t s (39). A c e t y l e n e reduction (nitrogen fixation) was decreased 6 2 % by green a l d e r p l a n t s t r e a t e d w i t h the m o s t c o n c e n t r a t e d bud a n d l e a f litter extracts. C e r t a i n t r e e s p e c i e s s u c h as B e t u l a p e n d u l a a n d P i c e a a b i e s f a i l t o d e v e l o p in a s s o c i a t i o n w i t h h e a t h e r , C a l l u n a v u l g a r i s (40,41). This a p p a r e n t l y r e s u l t s f r o m t h e p r o d u c t i o n by h e a t h e r o f a n a l l e l o c h e m i c a l t o x i c to g r o w t h o f m y c o r r h i z a e o f B e t u l a a n d P i c e a . F r u t i c o s e s o i l l i c h e n s a r e o f t e n a l l e l o p a t h i c to the g r o w t h of m y c o r r h i z a e and f o r e s t t r e e seedlings a l s o (42). R e m o v a l o f r e i n d e e r m o s s (a l i c h e n ) i n f i e l d t e s t s r e s u l t e d i n a c c e l e r a t e d g r o w t h of pine and s p r u c e . A l l e l o p a t h y in a g r i c u l t u r e . S c h r e i n e r and his associates published s e v e r a l p a p e r s s h o r t l y a f t e r 1900 w h i c h i n d i c a t e d t h a t c e r t a i n c r o p p l a n t s p r o d u c e c o m p o u n d s i n h i b i t o r y t o g r o w t h o f t h e s a m e a n d o t h e r c r o p p l a n t s (5). M c C a l l a a n d D u l e y (43,44) r e p o r t e d t h e a l l e l o p a t h i c e f f e c t s o f d e c a y i n g wheat residues in 1948-1949, and many papers on a l l e l o p a t h i c e f f e c t s of crop plants have been published in the past three decades. The unharvested parts of r i c e plants are generally m i x e d w i t h the soil b e c a u s e t h i s h a s b e e n t h o u g h t t o be b e n e f i c i a l . It h a s b e e n observed h o w e v e r , t h a t p r o d u c t i v i t y of the second c r o p of r i c e in a paddy is less than t h a t o f t h e f i r s t c r o p . C h o u a n d L i n (45) f o u n d t h a t a q u e o u s e x t r a c t s o f d e c o m p o s i n g r i c e residues in soil r e t a r d e d r a d i c l e g r o w t h of r i c e seedlings and g r o w t h of rice plants. M a x i m u m t o x i c i t y o c c u r r e d in the first m o n t h of decomposition and declined thereafter. Some t o x i c i t y persisted for four months in the paddies. F i v e inhibitory phenolic acids were identified from decaying rice residues and several unidentified allelochemicals were isolated. In t h e s o u t h e r n p a r t o f T a i w a n , a c r o p o f r i c e is o f t e n followed i m m e d i a t e l y by a l e g u m e c r o p . Y i e l d s o f s o y b e a n s h a v e b e e n i n c r e a s e d b y s e v e r a l h u n d r e d k i l o g r a m s p e r h e c t a r e by b u r n i n g t h e r i c e s t r a w p r i o r t o planting the soybeans. R i c e e t a l . (46) h y p o t h e s i z e d t h a t t h e d e c r e a s e d y i e l d s in unburned f i e l d s m a y r e s u l t f r o m an i n h i b i t i o n of n i t r o g e n f i x a t i o n by R h i z o b i u m i n t h e n o d u l e s o f t h e s o y b e a n p l a n t s . T h e f i v e p h e n o l i c a c i d s i d e n t i f i e d by C h o u a n d L i n a n d s t e r i l e e x t r a c t s o f d e c a y i n g r i c e s t r a w i n s o i l m a r k e d l y i n h i b i t e d g r o w t h of R h i z o b i u m . T h e phenolics also reduced nodule n u m b e r s and h e m o g l o b i n c o n t e n t of the nodules of t w o bean v a r i e t i e s . M o r e o v e r , e x t r a c t s of decomposing rice straw in soil reduced n i t r o g e n f i x a t i o n (acetylene reduction) in Bush B l a c k Seeded beans. It h a s b e e n o b s e r v e d f o r s o m e t i m e i n S e n e g a l i n w e s t A f r i c a t h a t g r o w t h o f s o r g h u m is d e c r e a s e d m a r k e d l y f o l l o w i n g s o r g h u m i n s a n d y s o i l s b u t n o t i n s o i l s h i g h i n m o n t m o r i l l o n i t e (47). S i m i l a r r e s u l t s o c c u r r e d i n t h e g r o w t h of s o r g h u m seedlings w h e n roots or tops of s o r g h u m w e r e added to sandy soils in l a b o r a t o r y e x p e r i m e n t s . N o i n h i b i t i o n r e s u l t e d , h o w e v e r , when the residues w e r e added to soil h i g h in m o n t m o r i l l o n i t e . W a t e r e x t r a c t s of roots or tops r e t a r d e d g r o w t h of sorghum seedlings in sandy soils s i m i l a r l y . I n o c u l a t i o n w i t h T r i c h o d e r m a v i r i d e or an u n k n o w n species of A s p e r g i l l u s e l i m i n a t e d the i n h i b i t o r y e f f e c t s of aqueous e x t r a c t s of sorghum roots in a short t i m e . Several weeks were required, however, to detoxify nonsterile f i e l d s o i l a f t e r a d d i t i o n o f r o o t r e s i d u e s o f s o r g h u m . It w a s c o n c l u d e d t h a t the m i c r o f l o r a in the sandy soils of S e n e g a l w e r e not able to d e t o x i f y the soil fast enough to p r e v e n t i n h i b i t i o n of subsequent crops of s o r g h u m . Some c r o p residues and weeds appear to s t i m u l a t e g r o w t h of other plants. C h o p p e d a l f a l f a added to soil s t i m u l a t e d the g r o w t h of t o m a t o ,


14



cucumber, lettuce, and several other p l a n t s (48). The stimulatory a l l e l o c h e m i c a l w a s i d e n t i f i e d as 1 - t r i a c o n t a n o l . S u b s e q u e n t t e s t s w i t h t h i s c o m p o u n d h a v e given v a r i a b l e r e s u l t s , but a d d i t i o n of c a l c i u m or l a n t h a n u m salts to the t r i a c o n t a n o l s o l u t i o n appears to m a k e the s t i m u l a t o r y a c t i v i t y c o n s i s t e n t (49). A s t e r o i d , b r a s s i n o l i d e , has b e e n i s o l a t e d f r o m rape ( B r a s s i c a n a p u s ) a n d a l d e r ( A l n u s ) p o l l e n (49). O n e nanogram applied to a bean plant causes significant growth increases. Weeds versus crop plants. V e l v e t l e a f , A b u t i l o n t h e o p h r a s t i , is a s e r i o u s w e e d of s e v e r a l c r o p s in the U n i t e d S t a t e s and C a n a d a . A v e r a g e y i e l d r e d u c t i o n s of soybeans under a v a r i e t y of v e l v e t l e a f densities, p l a c e m e n t s , a n d d u r a t i o n o f i n t e r f e r e n c e r a n g e d f r o m 14 t o 4 1 % (50-52). R e d u c t i o n s i n c o t t o n y i e l d s r a n g e d f r o m 44 t o 1 0 0 % ( 5 3 , 5 4 ) . A l l t h e c i t e d r e s e a r c h e r s a t t r i b u t e d the r e d u c t i o n s in c r o p yields to c o m p e t i t i o n a l t h o u g h none p e r f o r m e d e x p e r i m e n t s t o d e t e r m i n e w h e t h e r a l l e l o p a t h y m i g h t be i n v o l v e d . Numerous other researchers have found velvetleaf to have marked a l l e l o p a t h i c p o t e n t i a l (55-58). W a t e r e x t r a c t s of v e l v e t l e a f residues w e r e s l i g h t l y a l l e l o p a t h i c ( 5 - 2 4 % i n h i b i t i o n ) t o r a d i c l e a n d c o l e o p t i l e g r o w t h of c o r n a n d to h y p o c o t y l g r o w t h o f s o y b e a n s ( 5 7 , 5 8 ) . D e c a y i n g r e s i d u e s w e r e highly a l l e l o p a t h i c (50% or m o r e i n h i b i t i o n ) to height g r o w t h and fresh w e i g h t i n c r e a s e of shoots of b o t h c o r n and soybeans in double pot experiments. P u r p l e n u t s e d g e , C y p e r u s r o t u n d u s , w a s l i s t e d by H o l m (59) as o n e o f t h e t e n w o r s t w e e d s i n t h e w o r l d , a n d i n t e r f e r e n c e by t h i s w e e d c a u s e d r e d u c t i o n s i n y i e l d s of v a r i o u s c r o p s r a n g i n g f r o m 2 3 t o 8 9 % (5). It is n o t e w o r t h y t h e r e f o r e t h a t n u m e r o u s w o r k e r s have found purple nutsedge to b e s t r o n g l y a l l e l o p a t h i c . S o i l p r e v i o u s l y i n f e s t e d w i t h t h i s w e e d f o r 9 t o 12 w e e k s s i g n i f i c a n t l y reduced g e r m i n a t i o n of m u s t a r d , b a r l e y , and c o t t o n seeds; and soil infested for only 6 weeks s i g n i f i c a n t l y reduced g e r m i n a t i o n o f m u s t a r d a n d c o t t o n s e e d s (60). E t h a n o l e x t r a c t s of the p r e v i o u s l y i n f e s t e d s o i l i n h i b i t e d r a d i c l e g r o w t h of b a r l e y a l s o . D e c o m p o s i n g t u b e r s of p u r p l e n u t s e d g e r e d u c e d r o o t a n d t o p g r o w t h o f b a r l e y (61), s o r g h u m (62), a n d s o y b e a n s (62). S o m e p o l y p h e n o l s (63) a n d s e s q u i t e r p e n e s (64,65) w e r e i s o l a t e d f r o m tubers and other parts of purple nutsedge. Seven sesquiterpenoids were i d e n t i f i e d in the s t e a m d i s t i l l a t e of soil in w h i c h purple nutsedge was g r o w i n g (65) a n d t h e s a m e c o m p o u n d s w e r e i s o l a t e d f r o m e s s e n t i a l o i l i n p u r p l e n u t s e d g e (66). S e v e r a l of the compounds i d e n t i f i e d were previously shown to i n h i b i t e l o n g a t i o n of w h e a t c o l e o p t i l e segments in the presence of i n d o l e a c e t i c a c i d a n d s e c o n d l e a f s h e a t h g r o w t h of r i c e s e e d l i n g s i n t h e p r e s e n c e o f g i b b e r e l l i n A - (64). D e c a y i n g g r o u n d - i v y ( G l e c h o m a h e d e r a c e a ) l e a v e s (2 g p e r k g o f s o i l ) m a r k e d l y s t i m u l a t e d both root and shoot growth of downy brome (Bromus t e c t o r u m ) a n d r a d i s h ( R a p h a n u s s a t i v u s ) (67). R a d i s h root g r o w t h was s t i m u l a t e d 1354% in one e x p e r i m e n t . M o r e o v e r , root e x u d a t e s of g r o u n d - i v y s i g n i f i c a n t l y s t i m u l a t e d b o t h r o o t a n d s h o o t g r o w t h of r a d i s h . In f a c t , t h e r o o t s a t t a i n e d t a b l e s i z e i n o n l y 14 d a y s . C r o p plants versus w e e d s . B o t h t h i n and dense f i e l d stands of K e n t u c k y - 3 1 f e s c u e w e r e o b s e r v e d by P e t e r s (68) t o be r e l a t i v e l y f r e e o f w e e d s . E x t r a c t s of fescue, sand c u l t u r e s , and s p l i t - r o o t - s y s t e m e x p e r i m e n t s d e m o n s t r a t e d that fescue produced t o x i c c h e m i c a l s w h i c h exuded f r o m the roots and i n h i b i t e d g r o w t h of w i l d m u s t a r d a n d b i r d s f o o t t r e f o i l . T h r e e t h o u s a n d a c c e s s i o n s of the U S D A c o l l e c t i o n of o a t , A v e n a , g e r m p l a s m were screened for their a b i l i t y to exude s c o p o l e t i n , a c o m p o u n d



2.

RICE

Allelopathy:

An

Overview

15

known to have root-growth-inhibiting properties (69). Twenty-five accessions exuded more blue-fluorescing material (characteristic of scopoletin) f r o m t h e i r roots than a standard oat c u l t i v a r (Garry). Four a c c e s s i o n s e x u d e d u p t o t h r e e t i m e s as m u c h s c o p o l e t i n a s G a r r y o a t s . W h e n o n e o f t h e s e w a s g r o w n i n s a n d c u l t u r e f o r 16 d a y s w i t h a w i l d m u s t a r d , g r o w t h of t h e m u s t a r d w a s s i g n i f i c a n t l y less t h a n t h a t o b t a i n e d when the weed was grown with Garry oats. Moreover, plants grown in close association with the toxic accession were chlorotic, stunted, and twisted i n d i c a t i v e o f c h e m i c a l e f f e c t s r a t h e r t h a n c o m p e t i t i o n . It a p p e a r s p o s s i b l e t h e r e f o r e to breed a l l e l o p a t h i c genes into standard c u l t i v a r s to a i d i n w e e d control. A l l e l o p a t h i c crop plants have already been used e x p e r i m e n t a l l y in weed control. L e a t h e r (70) f o u n d o n e o f t h i r t e e n g e n o t y p e s o f t h e c u l t i v a t e d s u n f l o w e r t e s t e d t o b e v e r y a l l e l o p a t h i c t o s e v e r a l w e e d s . In a 5 year field study w i t h oats and sunflower grown in r o t a t i o n , the weed density was s i g n i f i c a n t l y less than i n c o n t r o l plots w i t h oats only. P u t n a m a n d D e F r a n k (71) t e s t e d r e s i d u e s o f s e v e r a l f a l l - a n d s p r i n g planted crops for weed control i n M i c h i g a n . T h e plants were desiccated by the h e r b i c i d e s glyphosate or paraquat, or by f r e e z i n g . T e c u m s e h wheat a n d B a l b o a r y e r e s i d u e s r e d u c e d w e e d g r o w t h b y up t o 8 8 % . M u l c h e s o f s o r g h u m or s u d a n g r a s s a p p l i e d t o a p p l e o r c h a r d s i n e a r l y s p r i n g r e d u c e d w e e d b i o r n a s s by 9 0 % a n d 8 5 % , r e s p e c t i v e l y . In a 3 - y e a r s e r i e s o f f i e l d t r i a l s , s o r g h u m residues reduced populations of c o m m o n purslane by 7 0 % and of s m o o t h c r a b g r a s s b y 9 8 % (72). C h e m i c a l N a t u r e of A l l e l o p a t h i c C o m p o u n d s A l l e l o p a t h i c compounds consist of a wide variety of c h e m i c a l types w h i c h a r i s e t h r o u g h e i t h e r t h e a c e t a t e o r t h e s h i k i m i c a c i d p a t h w a y (5). T h e s e c o m p o u n d s range f r o m v e r y s i m p l e gases a n d a l i p h a t i c compounds to complex multi-ringed aromatic compounds. Only a few examples are mentioned below. A c e t i c and butyric acids were among the toxins produced during d e c o m p o s i t i o n o f r y e r e s i d u e s (73), a n d s a l t s o f a c e t i c , p r o p i o n i c , a n d b u t y r i c acids were the chief p h y t o t o x i n s produced in d e c a y i n g wheat straw (74). A s i m p l e l a c t o n e , parasorbic a c i d , f r o m the fruit of m o u n t a i n a s h , i n h i b i t s s e e d g e r m i n a t i o n a n d a l s o h a s a n t i b a c t e r i a l a c t i o n (75). Another s u c h c o m p o u n d , p a t u l i n , is p r o d u c e d b y s e v e r a l f u n g i , i n c l u d i n g P é n i c i l l i u m u r t i c a e , w h i c h produced large a m o u n t s of the substance when growing on w h e a t s t r a w (76). L o n g - c h a i n f a t t y a c i d s have l o n g been r e p o r t e d to be i m p o r t a n t a l l e l o c h e m i c a l s p r o d u c e d by a l g a e (77). These compounds were recently r e p o r t e d t o be p o t e n t t o x i n s i n d e c a y i n g r e s i d u e s o f a h i g h e r p l a n t , P o l y g o n u m a v i c u l a r e (21). P o l y a c e t y l e n e s a r e a p p a r e n t l y d e r i v e d f r o m l o n g c h a i n f a t t y a c i d s (78), a n d e v i d e n c e is i n c r e a s i n g t h a t t h e y a r e i m p o r t a n t a l l e l o p a t h i c c o m p o u n d s (35,79). α-Terthienyl p r o d u c e d by roots of m a r i g o l d , Tagetes e r e c t a , caused 5 0 % m o r t a l i t y in seedlings of four test species in c o n c e n t r a t i o n s f r o m 0 . 1 5 t o 1.93 p p m (79). J u g l o n e is t h e o n l y q u i n o n e i d e n t i f i e d as a n a l l e l o p a t h i c c o m p o u n d f r o m h i g h e r p l a n t s (5). It i s p r o d u c e d b y w a l n u t t r e e s a n d is a p o t e n t i n h i b i t o r . N u m e r o u s a n t i b i o t i c s p r o d u c e d by m i c r o o r g a n i s m s a r e quinones, i n c l u d i n g t h e t e t r a c y c l i n e a n t i b i o t i c s s u c h a s a u r e o m y c i n (80). Simple phenols, phenolic acids derived from benzoic a c i d , and phenolic acids derived from c i n n a m i c a c i d have been the most c o m m o n l y identified



16


a l l e l o p a t h i c c o m p o u n d s p r o d u c e d by h i g h e r p l a n t s . The most common a l l e l o p a t h i c compounds identified in soil under a l l e l o p a t h i c plants are £hydroxybenzoic, v a n i l l i c , £-coumaric, and f e r u l i c acids. C o u m a r i n s are l a c t o n e s of o - h y d r o x y c i n n a m i c acids in w h i c h side c h a i n s o f t e n a r e i s o p r e n o i d (78). C o u m a r i n , e s c u l i n , a n d p s o r a l e n (a f u r a n o c o u m a r i n ) a l l strongly i n h i b i t seed g e r m i n a t i o n . Such i n h i b i t o r s are p r o d u c e d by a v a r i e t y of l e g u m e s and c e r e a l g r a i n s . F l a v o n o i d s are widespread in higher plants and a few 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 . P h l o r i z i n i n a p p l e r o o t s is t o x i c t o y o u n g a p p l e trees and often causes d i f f i c u l t y in replanting old apple orchards. N u m e r o u s f l a v o n o i d s a n d t h e i r g l y c o s i d e s a r e p r o d u c e d by s p e c i e s f r o m t h e t a l l g r a s s p r a i r i e and post oak/blackjack oak forest and are i n h i b i t o r y to n i t r i f y i n g b a c t e r i a a n d t o s e e d g e r m i n a t i o n (31). Several hydrolyzable and condensed tannins have been i m p l i c a t e d in a l l e l o p a t h y (5). T h e y h a v e b e e n i d e n t i f i e d as g r o w t h a n d g e r m i n a t i o n i n h i b i t o r s i n d r y f r u i t s (81), a s g r o w t h r e t a r d e r s of n i t r o g e n - f i x i n g a n d n i t r i f y i n g b a c t e r i a i n s e v e r a l p l a n t s , a n d as r e d u c e r s o f s e e d l i n g g r o w t h i n s e v e r a l p l a n t s (5). H i g h e r p l a n t s p r o d u c e a g r e a t v a r i e t y o f t e r p e n o i d s (78) b u t o n l y a f e w of these have been i m p l i c a t e d in a l l e l o p a t h y . The monoterpenoids are the major c o m p o n e n t s of e s s e n t i a l oils of plants and they are the p r e d o m i n a n t terpenoid inhibitors that have been identified from higher plants. Many f u n g i (82) a n d a l g a e (83) p r o d u c e t e r p e n o i d a l l e l o c h e m i c a l s a l s o . There are only a few instances in w h i c h amino acids have been i m p l i c a t e d in a l l e l o p a t h y and in most cases the s p e c i f i c a m i n o acids have not been i d e n t i f i e d . R h i z o b i t o x i n e is p r o d u c e d b y c e r t a i n s t r a i n s o f R h i z o b i u m j a p o n i c u m a n d is a n o n p r o t e i n a m i n o a c i d (84). S e v e r a l of the p h y t o t o x i n s p r o d u c e d by p a t h o g e n i c m i c r o o r g a n i s m s a r e p o l y p e p t i d e s a n d r e l a t e d g l y c o p e p t i d e s (82). Many alkaloids have been i m p l i c a t e d in p l a n t - a n i m a l c h e m i c a l i n t e r a c t i o n s b u t f e w h a v e b e e n a s s o c i a t e d w i t h a l l e l o p a t h y (85). Several a l k a l o i d s w e r e d e m o n s t r a t e d by E v e n a r i (75) t o be s t r o n g i n h i b i t o r s o f s e e d germination. L i t t l e r e c e n t w o r k has been done on a l k a l o i d s e x c e p t for c a f f e i n e (78). α - P i c o l i n i c a c i d is a m i c r o b i a l a l k a l o i d w i t h t o x i c a c t i o n o n p l a n t s (82). O n e o f t h e m o r e a c t i v e s y n t h e t i c h e r b i c i d e s on t h e m a r k e t , p i c l o r a m ( D o w ' s T o r d o n ) , is a c h l o r i n a t e d p i c o l i n i c a c i d d e r i v a t i v e . C y a n o h y d r i n s have been i m p l i c a t e d in a l l e l o p a t h y in several instances. D h u r r i n occurs in grain sorghum seedlings and the seedlings c o n t a i n e n z y m e s t h a t h y d r o l y z e d h u r r i n to g l u c o s e , H C N (hydrogen c y a n i d e ) , a n d j>h y d r o x y b e n z a l d e h y d e (86). T h e s i t u a t i o n is s i m i l a r i n J o h n s o n g r a s s , S o r g h u m h a l e p e n s e , a v e r y a l l e l o p a t h i c w e e d (87). B o t h the H C N and £hydroxybenzaldehyde are potent a l l e l o c h e m i c a l s . H C N and benzaldehyde a r e p r o d u c e d by t h e h y d r o l y s i s o f a m y g d a l i n p r e s e n t i n p e a c h r o o t r e s i d u e s (88) . H C N a n d b e n z a l d e h y d e a r e i n h i b i t o r y t o g r o w t h o f p e a c h s e e d l i n g s a n d apparently cause the peach replant problem in old peach orchards. M u s t a r d o i l s , s u c h as a l l y l i s o t h i o c y a n a t e , a r e p r o d u c t s o f t h e h y d r o l y s i s o f m u s t a r d o i l g l y c o s i d e s (78). M u s t a r d o i l s a r e p r o d u c e d by a l l o r g a n s o f p l a n t s b e l o n g i n g t o t h e C r u c i f e r a e ( m u s t a r d f a m i l y ) (75), a n d a r e s t r o n g i n h i b i t o r s of seed g e r m i n a t i o n and m i c r o b i a l g r o w t h . M a n y a n t i b i o t i c s p r o d u c e d by v a r i o u s m i c r o o r g a n i s m s a r e n u c l e o s i d e s (5). A m o n g these are nebularine, c o r d y c e p i n , and n u c l e o c i d i n . The only k n o w n p u r i n e s i n h i g h e r p l a n t s s h o w n t o be i n v o l v e d i n a l l e l o p a t h y a r e c a f f e i n e , t h e o p h y l l i n e , p a r a x a n t h i n e , and theobromine f r o m the c o f f e e t r e e (89) .


2.

RICE

Allelopathy:

An

Overview

17


F a c t o r s D e t e r m i n i n g E f f e c t i v e n e s s of A l l e l o c h e m i c a l s S o m e a l l e l o c h e m i c a l s have been shown to be bound by the h u m i c m a t e r i a l i n t h e s o i l a n d p r e s u m a b l y i n a c t i v a t e d (90). When k n o w n a m o u n t s of t a n n i c a c i d w e r e a d d e d t o a p r a i r i e s o i l t h a t c o n t a i n e d no t a n n i c a c i d , a m i n i m u m o f 4 0 0 p p m h a d t o be a d d e d b e f o r e a n y c o u l d be r e c o v e r e d i m m e d i a t e l y . It is n o t e w o r t h y , t h e r e f o r e , t h a t as s m a l l a c o n c e n t r a t i o n as 30 p p m a d d e d t o the same soil r e d u c e d the nodule number of h e a v i l y i n o c u l a t e d legumes growing in the soil. O b v i o u s l y , some of the bound t a n n i c a c i d r e m a i n e d b i o l o g i c a l l y a c t i v e (91). S o m e plants e x e r t g r e a t e r a l l e l o p a t h i c e f f e c t s in f i n e - t e x t u r e d than in c o a r s e - t e x t u r e d soils and evidence indicates that the g r e a t e r r e t e n t i o n c a p a c i t y o f t h e f i n e t e x t u r e d s o i l s f o r a t l e a s t s o m e a l l e l o c h e m i c a l s m a y be i m p o r t a n t in the a c c u m u l a t i o n of physiologically a c t i v e c o n c e n t r a t i o n s of these c h e m i c a l s (92-94). M a n y a l l e l o c h e m i c a l s a r e d e c o m p o s e d i n s o i l , e i t h e r a b i o t i c a l l y (37) o r by microorganisms (95-100). Obviously, the attainment of active c o n c e n t r a t i o n s o f a l l e l o c h e m i c a l s i n s o i l d e p e n d s o n t h e r e l a t i v e r a t e s of a d d i t i o n a n d i n a c t i v a t i o n . It is i m p o r t a n t t o u n d e r s t a n d a l s o t h a t m i c r o b i a l d e c o m p o s i t i o n of a l l e l o c h e m i c a l s does not n e c e s s a r i l y r e s u l t in a decrease in 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 , t h e r e v e r s e m a y be t r u e . H y d r o j u g l o n e is o x i d i z e d i n s o i l t o j u g l o n e , a q u i n o n e t h a t is i n h i b i t o r y t o s o m e s p e c i e s a t a 10" M c o n c e n t r a t i o n (101). Isoflavonoids p r o d u c e d by red c l o v e r a r e d e c o m p o s e d t o e v e n m o r e t o x i c p h e n o l i c c o m p o u n d s (95); a n d t o r e p e a t , amygdalin from peach roots is c h a n g e d to hydrogen cyanide and b e n z a l d e h y d e w h i c h c a u s e t h e p e a c h r e p l a n t p r o b l e m (88), a n d p h l o r i z i n f r o m a p p l e r o o t s is d e c o m p o s e d t o s e v e r a l p h e n o l i c c o m p o u n d s t h a t a p p e a r t o be r e s p o n s i b l e f o r t h e a p p l e r e p l a n t p r o b l e m ( 1 0 0 ) . It is a l s o i m p o r t a n t t o u n d e r s t a n d t h a t m o s t a l l e l o p a t h i c e f f e c t s apparently result f r o m the c o m b i n e d actions of several a l l e l o c h e m i c a l s , o f t e n w i t h e a c h b e l o w a t h r e s h o l d c o n c e n t r a t i o n f o r i m p a c t . In a l l e l o p a t h i c situations w h i c h i m p l i c a t e phenolic acids, soil concentrations have ranged f r o m b e l o w 10 t o a b o v e 1000 p p m f o r e a c h c o m p o u n d . T h e l o w e r e n d o f t h e s p e c t r u m is b e l o w 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 a n e f f e c t i n c u r r e n t bioassays. A d d i t i v e and synergistic e f f e c t s have been demonstrated, h o w e v e r , f o r c o m b i n a t i o n s o f c i n n a m i c a c i d s (102), b e n z o i c a c i d s (103), b e n z o i c a n d c i n n a m i c a c i d s (104), a n d £ - h y d r o x y b e n z a l d e h y d e w i t h c o u m a r i n ( 1 0 5 ) . It a p p e a r s t h a t s u c h c o m b i n e d i n t e r a c t i o n s m a y b e v e r y i m p o r t a n t under field conditions. It is r e v e a l i n g t o c o n s i d e r m i c r o b i a l d e c o m p o s i t i o n o f a l l e l o p a t h i c c o m p o u n d s i n r e l a t i o n to s y n e r g i s m . As discussed above, partial d e c o m p o s i t i o n of one c o m p o u n d may result in the presence of s e v e r a l a c t i v e compounds, which may exert synergistic allelopathic effects. Thus, partial decomposition could increase allelopathic a c t i v i t y , rather than decrease it. The D i r e c t i o n of F u t u r e R e s e a r c h in A l l e l o p a t h y M o s t of our present knowledge c o n c e r n i n g a l l e l o p a t h y has been o b t a i n e d in the past three decades. T h u s , i t is a v e r y y o u n g f i e l d o f s c i e n c e a n d r e s e a r c h s h o u l d be c o n t i n u e d i n a l l a r e a s o f a l l e l o p a t h y i n v e s t i g a t e d i n t h e past. T h e point has been r e a c h e d , h o w e v e r , w h e r e c e r t a i n areas need special emphasis. E v e n though many types of c h e m i c a l compounds have been i m p l i c a t e d in allelopathy, there are probably many highly important ones t h a t have been o v e r l o o k e d . T e c h n i q u e s are now a v a i l a b l e to i d e n t i f y a l l e l o c h e m i c a l s m u c h more rapidly and a c c u r a t e l y than in the past, and



18


many more chemists are doing research in allelopathy. Therefore, there s h o u l d be s p e c i a l e m p h a s i s i n t h i s a r e a . There have been many d e m o n s t r a t e d i n s t a n c e s of a l l e l o p a t h i c a c t i o n on the p a r t of m a n y p l a n t s p e c i e s w h e r e no a l l e l o c h e m i c a l s w e r e i d e n t i f i e d . M o s t species for w h i c h g o o d e v i d e n c e e x i s t s o f a l l e l o p a t h i c p o t e n t i a l , s h o u l d p r o b a b l y be e x a m i n e d again also w i t h new techniques and e x p e r t i s e . It is i m p o r t a n t t o i d e n t i f y t h e a l l e l o p a t h i c c o m p o u n d s i n t h e s u b s t r a t e (soil or w a t e r ) of the a l l e l o p a t h i c p l a n t and to d e t e r m i n e w h e t h e r these c o m p o u n d s h a v e c o m e f r o m t h e p l a n t , a r e p r o d u c e d by p a r t i a l d e c o m p o s i t i o n o f o t h e r c o m p o u n d s , o r a r e s y n t h e s i z e d by m i c r o o r g a n i s m s u s i n g c a r b o n s o u r c e s f r o m t h e p l a n t . It is i m p o r t a n t t o k e e p i n m i n d t h a t t h e a l l e l o p a t h i c c o m p o u n d s p r o d u c e d b y b a c t e r i a , f u n g i , a n d a l g a e a r e j u s t as m u c h a p a r t o f t h e s c i e n c e o f a l l e l o p a t h y as a r e t h o s e p r o d u c e d d i r e c t l y by p l a n t s . T h e r e is a l a r g e b o d y o f i n d i r e c t e v i d e n c e , b u t o n l y a r e l a t i v e l y s m a l l body of direct evidence, c o n c e r n i n g the movement of allelopathic compounds f r o m plants that produce them and the uptake and t r a n s l o c a t i o n o f t h e s e c o m p o u n d s b y n e i g h b o r i n g p l a n t s . T h i s is no d o u b t t h e w e a k e s t l i n k in our c h a i n of i n f o r m a t i o n c o n c e r n i n g the phenomenon of a l l e l o p a t h y . T h e r e is a n u r g e n t n e e d , t h e r e f o r e , f o r c a r e f u l r e s e a r c h i n t h i s a r e a . P o t e n t i a l a l l e l o p a t h i c c o m p o u n d s n e e d t o be t a g g e d ( w i t h r a d i o i s o t o p e s ) i n s u s p e c t e d a l l e l o p a t h i c p l a n t s , a n d p a t h s of t h e c o m p o u n d s s h o u l d be t r a c e d out of the donor plant and i n t o and t h r o u g h a f f e c t e d a c c e p t o r p l a n t s . S u c h investigations should i n c l u d e studies of the possible movement of a l l e l o p a t h i c c o m p o u n d s f r o m donor to a c c e p t o r t h r o u g h n a t u r a l root or s t e m g r a f t s , m y c o r r h i z a l fungi, and h a u s t o r i a l c o n n e c t i o n s of p a r a s i t i c plants (106-109). After allelochemicals have been identified in the substrate, c o n c e n t r a t i o n s s h o u l d be c a l c u l a t e d , a n d t h r e s h o l d c o n c e n t r a t i o n s for a c t i v i t y s h o u l d be d e t e r m i n e d a g a i n s t t e s t p l a n t s u s i n g c o m b i n a t i o n s o f compounds present in the substrate, in addition to individual ones. U n d o u b t e d l y , many i m p o r t a n t a l l e l o p a t h i c e f f e c t s have been overlooked b e c a u s e o f t h e use o f s i n g l e a l l e l o c h e m i c a l s i n d e t e r m i n i n g t h r e s h o l d c o n c e n t r a t i o n s for a c t i v i t y . A m o d e r a t e a m o u n t o f i n f o r m a t i o n is a v a i l a b l e c o n c e r n i n g t h e f a c t o r s a f f e c t i n g c o n c e n t r a t i o n s of phenolics in plants, and a l i t t l e research has been c o m p l e t e d c o n c e r n i n g f a c t o r s a f f e c t i n g c o n c e n t r a t i o n s of a l k a l o i d s and terpenoids. L i t t l e i n f o r m a t i o n is a v a i l a b l e c o n c e r n i n g f a c t o r s affecting c o n c e n t r a t i o n s of o t h e r types of a l l e l o p a t h i c c o m p o u n d s ; thus, r e s e a r c h is urgently needed in this a r e a . There is a c r i t i c a l n e e d f o r more study of f a c t o r s affecting i n a c t i v a t i o n and e f f e c t i v e n e s s of a l l e l o c h e m i c a l s a f t e r they m o v e out of d o n o r p l a n t s . V e r y l i t t l e is k n o w n c o n c e r n i n g t h e b i n d i n g o f t h e s e c h e m i c a l s i n s o i l a n d t h e e f f e c t s o f t h e b i n d i n g o n t h e i r a c t i v i t y . V i r t u a l l y n o t h i n g is k n o w n c o n c e r n i n g the role of t e x t u r e in the a c c u m u l a t i o n of a l l e l o c h e m i c a l s to p h y s i o l o g i c a l l y a c t i v e c o n c e n t r a t i o n s . T e m p e r a t u r e stress markedly a c c e n t u a t e s the a l l e l o p a t h i c e f f e c t s of f e r u l i c a c i d on g r o w t h of s o r g h u m a n d s o y b e a n s (110). T h e r e a r e o b v i o u s l y m a n y s t r e s s f a c t o r s w h i c h c o u l d a f f e c t response of a p l a n t or m i c r o o r g a n i s m to a g i v e n a l l e l o c h e m i c a l or combination of allelochemicals, and such interactions should be investigated. T h e s u r f a c e h a s j u s t b e e n s c r a t c h e d i n d e t e r m i n i n g t h e m e c h a n i s m s by w h i c h the d i f f e r e n t kinds of a l l e l o p a t h i c compounds e x e r t t h e i r a c t i o n s . T h e r e f o r e , i t is i m p o r t a n t t h a t m u c h m o r e r e s e a r c h be d o n e i n t h i s a r e a o f allelopathy.



2.

RICE

Allelopathy:

An Overview

19

I have e m p h a s i z e d to this point the need for research i n a l l the basic areas of allelopathy. Such work c o u l d open up new horizons for applied research in the field of allelopathy; in fact, the results form the foundation of t h e e n t i r e f i e l d . T h i s emphasis o n basic r e s e a r c h should i n no w a y d e t r a c t f r o m the value and need for more progress in the various applied areas of a l l e l o p a t h y . In r e a l i t y , o n l y a r e l a t i v e l y s m a l l a m o u n t o f r e s e a r c h h a s b e e n c a r r i e d out concerning the roles of allelopathy i n natural or any of the m a n made or m a n - a l t e r e d ecosystems. O n l y a f e w of t h e more obvious areas i n need of a t t e n t i o n will be mentioned here. M u c h research is needed on t h e q u a n t i t a t i v e e f f e c t s on crop yields of i n t e r f e r e n c e by most of our serious weeds, and on the relative c o n t r i b u t i o n s of a l l e l o p a t h y a n d c o m p e t i t i o n to the t o t a l i n t e r f e r e n c e by e a c h w e e d species. C r o p - c r o p relationships need t o be i n v e s t i g a t e d m u c h m o r e thoroughly to d e t e r m i n e w h i c h crops c a n follow others w i t h the least i n h i b i t o r y o r most s t i m u l a t o r y e f f e c t s . M o r e emphasis' should be p l a c e d on investigations of s t i m u l a t o r y a l l e l o p a t h i c e f f e c t s , because these e f f e c t s have been largely ignored i n the past. Possible a u t o t o x i c i t y should be i n v e s t i g a t e d also t o d e t e r m i n e if i t is unwise t o c u l t i v a t e t h e same c r o p continuously without rotation. R e s e a r c h i n t h e use of a l l e l o p a t h y i n b i o l o g i c a l w e e d c o n t r o l should be vigorously p u r s u e d . T h i s should i n c l u d e t h e use o f m u l c h e s o f a l l e l o p a t h i c p l a n t s ; r o t a t i o n o f c r o p s i n w h i c h o n e o r m o r e o f t h e c r o p p l a n t s is a l l e l o p a t h i c to major weeds; use of a l l e l o p a t h i c cover crops; underplanting of a l l e l o p a t h i c c o m p a n i o n crops in orchards, vineyards, e t c . ; a n d the development (through breeding or genetic engineering) of crop c u l t i v a r s w h i c h c a n c o n t r o l major weeds in a given a r e a through a l l e l o p a t h i c a c t i v i t y . M o r e r e s e a r c h is n e e d e d o n t h e p o s s i b l e u s e o f c e r t a i n a l l e l o c h e m i c a l s a s herbicides or as s t r u c t u r a l models for h e r b i c i d e d e v e l o p m e n t . The very broad area of allelopathic interactions between microorganisms and plants has been largely ignored by researchers. There has been some study of e f f e c t s o f s e l e c t e d w e e d y species on f r e e - l i v i n g a n d symbiotic nitrogen fixers and on nitrifiers in natural ecosystems but v i r t u a l l y n o t h i n g has been done on these r e l a t i o n s h i p s i n other e c o s y s t e m s . M u c h more r e s e a r c h needs to be done also on t h e a n t a g o n i s t i c e f f e c t s o f plants on soil-borne plant pathogens, and on the e f f e c t s of a l l e l o c h e m i c a l s i n the p r e d i s p o s i t i o n of plants to i n f e c t i o n by pathogens (111-113). T h e r e is a pressing need also for better understanding of the production by m i c r o o r g a n i s m s of a l l e l o c h e m i c a l s in soil or w a t e r that a f f e c t g r o w t h of plants. This extends also to the partial decomposition of allelochemicals from plants, which produces more active compounds or simply more compounds which can increase allelopathic effects through additive or synergistic action. Obviously these suggestions for future research i n allelopathy a r e only a few of t h e large numbers that could be given. H o p e f u l l y , however, they may give some impetus to progress i n some v i t a l areas of a l l e l o p a t h y .

Literature Cited 1. Theophrastus. (ca 300 B.C.) "Enquiry into plants and Minor Odours and Weather Signs". 2 Vols.; transi, to English by Hort, Α.; W. Heinemann: London, 1916. 2. Plinius Secundus, C. (First Century A.D.) "Natural History". 10 Vols., transi, to English by Rackam, H.; Jones, W.H.S.; Eichholz, D. E. Howard University Press: Cambridge, Mass., 1938-1963.



20

ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY

3. Molisch, H. "Der Einfluss einer Pflanze auf die andere—Allelopathie"; Gustav Fischer: Jena, 1937. 4. Muller, C. H, Vegetatio 1969, 18, 348-57. 5. Rice, E. L. "Allelopathy"; 2d ed.; Academic Press: Orlando, Florida, 1984. 6. Bell, A. A. In "Report of the Research Planning Conference on the Role of Secondary Compounds in Plant Interactions (Allelopathy)"; McWhorter, C. G.; Thompson, A. C.; Hauser, E. W. Eds.; USDA, Agricultural Research Service: Tifton, Georgia, 1977; pp. 64-69. 7. French, R. C.; Graham, C. L.; Gales, A. W.; Long, R. K. J. Agric. Food Chem. 1977, 25, 84-88. 8. Chet, J.; Henis, Y. Ann. Rev. Phytopathol. 1975, 13; 169-92. 9. Willetts, H. J. Biol. Rev. 1972, 47, 515-36. 10. Brandt, W. H.; Reese, J. E. Am. J. Bot. 1964, 51, 922-27. 11. Kerr, A. Aust. J. Biol. Sci. 1956, 9, 45-52. 12. Buxton, E. W. Trans. Brit. Mycol. Soc. 1957, 40, 145-54. 13. Menzies, J. D.; Gilbert, R. G. Soil Sci. Soc. Am. Proc. 1967, 31, 49596. 14. Pepperman, A. B., Jr.; Blanchard; Ε. J. In "The Chemistry of Allelopathy"; Thompson, A. C. Ed.; American Chemical Society: Washington, D.C., 1985, pp. 415-25, 15. Johnson, A. W.; Rosebery, G.; Parker, C. Weed Res. 1976, 16, 223. 16. Dailey, O. D., Jr.; Vail, S. L. In "The Chemistry of Allelopathy"; Thompson, A.C., Ed.; American Chemical Society: Washington, D.C.; 1985; pp. 427-35. 17. Lynn, D. G. In "The Chemistry of Allelopathy"; Thompson, A. C., Ed.; American Chemical Society: Washington, D.C., 1985; pp. 55-81. 18. Curtis, J. T.; Cottam, G. Bull. Torrey Bot. Club 1950, 77, 187-91. 19. AlSaadawi, I. S.; Rice, E. L. J. Chem. Ecol. 1982, 8, 993-1009. 20. AlSaadawi, I. S.; Rice, E. L. J. Chem. Ecol. 1982, 8, 1011-23. 21. AlSaadawi, I. S.; Rice, E. L.; Karns, T. Κ. B. J. Chem. Ecol. 1983, 9, 761-74. 22. Lee, I. K.; Monsi, M. Bot. Mag. (Tokyo) 1963, 76, 400-13. 23. Kil, B.S. Ph.D. Dissertation, Chung-Ang University, Iri, Korea, 1981. 24. Ueki, K.; Takahashi, M. Intern. Chem. Congr. Pacific Basin Soc. Honolulu Hawaii, 1984, Abstract 02F11. 25. Booth, W. E. Am. J. Bot. 1941, 28, 415-22. 26. Rice, E. L.; Penfound. W. T.; Rohrbaugh, L. M. Ecology 1960, 41, 224-28. 27. Kapustka, L. Α.; Rice, E. L. Soil Biol. Biochem. 1976, 8, 497-503. 28. Rice, E. L. Ecology 1964, 45, 824-37. 29. Rice, E. L.; Pancholy, S. K. Am. J. Bot. 1972, 59, 1033-40. 30. Rice, E. L.; Pancholy, S. K. Am. J. Bot. 1973, 60, 691-702. 31. Rice, E. L.; Pancholy, S. K. Am. J. Bot. 1974, 61, 1095-1103. 32. Numata, M.; Kobayashi, Α.; Ν. Ohga. In "Fundamental Studies in the Characteristics of Urban Ecosystems"; Numata, M. Ed.; 1973; pp. 59-64. 33. Numata, M.; Kobayashi, Α.; Ohga, N . In "Studies in Urban Ecosystems"; Numata, M., Ed.; 1974; pp. 22-25. 34. Numata, M.; Kobayashi, Α.; Ohga, N . In "Studies in Urban Ecosystems"; Numata, M., Ed.; 1975; pp. 38-41. 35. Kobayashi, Α.; Morirnoto, S.; Shibata, Y.; Yamashita, K., Numata, M. J. Chem. Ecol. 1980, 6, 119-31. 36. Walters, D. T.; Gilmore, A. R. J. Chem. Ecol. 1976, 2, 469-79. 37. Tubbs, C. H.Forest Sci. 1973, 19, 139-45. 38. Jobidon, R.; Thibault, J. R. Bull. Torrey Bot. Club 1981, 108, 413-18. 39. Jobidon, R.; Thibault, J. R. Am. J. Bot. 1982, 69, 1213-23.


2. RICE Allelopathy: An Overview

40. 41. 42. 43. 44. 45. 46. 47.


48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83.

21

Handley, W. R. C. Bull. Forest Comm., London, 1963, No. 36. Robinson, R. K. J. Ecol. 1972, 60, 219-24. Brown, R. T.; Mikola, P. Acta Forest. Fenn. 1974, 141, 1-22. McCalla, T. M.; Duley, F. L. Science 1948, 108, 163. McCalla, T. M.; Duley, F. L. Soil Sci. Soc. Amer. Proc. 1949, 14, 196-99. Chou, C. H.; Lin, H. J. J. Chem. Ecol. 1976, 2, 353-67. Rice, E. L.; Lin, C. Y.; Huang, C. Y. J. Chem. Ecol. 1981, 7, 333-44. Burgos-Leon, W.; Ganry, F.; Nicou, R.; Chopart, J. L.; Dommergues, Y. Agron. Trop. 1980, 35, 319-34. Ries, S. Κ.; Went, V.; Sweeley, C. C.; Leavitt, R. A. Science 1977, 195, 1339-41. Maugh, T. H., II. Science 1981, 212, 33-34. Oliver, L. R.; Weed Sci. 1979, 27, 183-8. Staniforth, D. W. Weeds 1965, 13, 191-3. Hagood, E. S., Jr.; Bauman, T. T.; Williams, J. L., Jr.; Schreiber, M. M. Weed Sci. 1980, 28, 729-34. Chandler, J. M. Weed Sci. 1977, 25, 151-58. Robinson, E. L. Weed Sci. 1976; 24, 353-55. Elmore, C. D. Weed Sci. 1980, 28, 658-60. Colton, C. E.; Einhellig, F. A. Am. J. Bot. 1980, 67, 1407-13. Bhowmik, P. C.; Doll, J. D. Proc. North Central Weed Cont. Conf. 1979, 34, 43-45. Bhowmik, P. C.; Doll, J. D. Agron. J. 1982, 74, 601-6. Holm, L. Weed Sci. 1969, 17, 113-18. Friedman, T.; Horowitz, M. Weed Sci. 1971, 19, 398-401. Horowitz, M.; Friedman, T. Weed Res. 1971, 11, 88-93. Lucena, J. M.; Doll, J. Revista Comalfi 1976, 3, 241-56. Komai, K.; Ueki, K. Weed Res. (Japan) 1975, 20, 66-71. Komai, K.; Iwamura, J.; Ueki, K. Weed Res. (Japan) 1977, 22, 14-18. Komai, K.; Ueki, K. Weed Res. (Japan) 1980, 25, 42-47. Komai, K.; Sato, S.; Ueki, K. Mem. Fac. Agr. Kinki Univ. 1982, 15, 3341. Rice, E. L. In "Advances in Allelopathy"; Putnam, A. R.; Tang, C. S., Eds.; John Wiley: New York (In press). Peters, Ε. J. Crop Sci. 1968, 8, 650-53. Fay, P. K.; Duke, W. B. Weed Sci. 1977, 25, 224-28. Leather, G. R. Weed Sci. 1983, 31, 37-42. Putnam, A. R.; DeFrank, J. Proc. IX Int. Cong. Plant Protection, 1979, pp. 580-82. Putnam, A. R.; DeFrank, J. Crop Prot. 1983, 2, 173-81. Patrick, Z. A. Soil Sci. 1971, 111, 13-18. Tang, C. S.; Waiss, A. C., Jr. J. Chem. Ecol. 1978, 4, 225-32. Evenari, M. Bot. Rev. 1949, 15, 153-94. Norstadt, F. Α.; McCalla, T. M. Science 1963, 140, 410-11. Spoehr, Η. Α.; Smith, J. H. C.; Strain, H. H.; Milner, H. W.; Hardin, G. J. "Fatty Acid Antibacterials from Plants," Carnegie Institution of Washington, 1949, Pub. 586. Robinson, T. "The Organic Constituents of Higher Plants"; 5th ed.; Cordus Press: North Amherst, Mass., 1983. Campbell, G.; Lambert, J. D. H.; Arnason, T.; Towers, G. H. N . J. Chem. Ecol. 1982, 8, 961-72. Whittaker, R. H.; Feeny, P. P. Science 1971, 171, 757-70. Varga, M.; Koves, E. Nature 1959, 183,401. Owens, L. D. Science 1969, 165, 18-25. Fenical, W. J. Phycol. 1975, 11, 245-59.



22

ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY

84. Owens, L. D.; Thompson, J. F.; Fennessey, P. V. J. Chem. Soc., Chem. Commu. 1972, 1972, 715. 85. Rice, E. L. "Pest Control with Nature's Chemicals: Allelochemicals and Pheromones in Gardening and Agriculture"; University of Oklahoma Press: Norman, 1983. 86. Conn, E. E.; Akazawa, T. Fed. Proc. 1958, 17,205. 87. Abdul-Wahab, A. S.; Rice, E. L. Bull. Torrey Bot. Club 1967, 94, 486-97. 88. Patrick, Z. A. Can. J. Bot. 1955, 33, 461-86. 89. Chou, C. H.; Waller, G. R. J. Chem. Ecol. 1980, 6, 643-54. 90. Wang, T. S. C.; Yeh, K. L.; Cheng, S. Y.; Yang, T. K. In "Biochemical Interactions among Plants"; U.S. Nat. Comm. for IBP, Ed.; National Academy Sciences: Wash. D.C., 1971; pp. 113-20. 91. Blum, U.; Rice, E. L. Bull. Torrey Bot. Club 1969, 96, 531-44. 92. Ahshapanek, D. C. Ph.D. Dissertation, University of Oklahoma, Norman, 1962. 93. Muller, C. H.; del Moral, R. Bull. Torrey Bot. Club 1966, 93, 130-37. 94. del Moral, R.; Muller, C. H. Am. Midi. Natur. 1970, 83, 254-82. 95. Chang, C. F.; Suzuki, Α.; Kumai, S.; Tamura, S. Agri. Biol. Chem. 1969, 33, 398-408. 96. Bonner, J. Bot. Gaz. 1946, 107, 343-51. 97. Henderson, Μ. Ε. Κ.; Farmer, V. C. J. Gen. Microbiol. 1955, 12, 37-46. 98. Kunc, F. Folia Microbiol. 1971, 16, 41-50. 99. Turner, J. Α.; Rice, E. L. J. Chem. Ecol. 1975, 1, 41-58. 100. Borner, H. Contrb. Boyce Thompson Inst. 1959, 20, 39-56. 101. Rietveld, W. J. J. Chem. Ecol. 1983, 9, 295-308. 102. Einhellig, F. Α.; Schon, M. K.; Rasmussen, J. A. J. Plant Growth Regul. 1982, 1, 251-58. 103. Einhellig, F. Α.; Rasmussen, J. A. J. Chem. Ecol. 1978, 4, 425-36. 104. Rasmussen, J. Α.; Einhellig, F. A. Plant Sci. Letters 1979, 14, 69-74. 105. Williams, R. D.; Hoagland, R. E. Weed Sci. 1982, 30, 206-12. 106. Bjorkman, E. Physiol. Plant. 1960, 13, 308-27. 107. Graham, B. F., Jr.; Bormann, F. H. Bot. Rev. 1966, 32, 255-92. 108. Woods, F. W.; Brock, K. Ecology 1964, 45, 886-89. 109. Atsatt, P. R. In "Biochemical Coevolution"; Chambers, K. L . Ed.; Biol. Colloquium #29, Oregon State University Press: Corvallis; pp. 53-68. 110. Einhellig, F. Α.; Eckrich, P. C. J. Chem. Ecol. 1984, 10, 161-70. 111. Patrick, Ζ. Α.; Toussoun, Τ. Α.; Snyder, W. C. Phytopathology 1963, 53, 152-61. 112. Toussoun, Τ. Α.; Patrick, Z. A. Phytopathology 1963, 53, 265-70. 113. Patrick, Ζ. Α.; Koch, L. W. Can. J. Bot. 1963, 41, 747-58. RECEIVED

June 9, 1986


Allelopathy: An Overview - ACS Symposium Series (ACS Publications)

Recommend Documents