A Search for the Allelopathic Agents in Diffuse Knapweed - American

Chapter 22

A Search for the Allelopathic Agents in Diffuse Knapweed A. D. Muir , W. Majak , F. Balza , and G. H. N. Towers 1

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Agriculture Canada Research Station, 3015 Ord Road, Kamloops, British Columbia, Canada, V2B 8A9 Botany Department, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1W5

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: January 8, 1987 | doi: 10.1021/bk-1987-0330.ch022

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The a l l e l o p a t h i c potential of knapweed (Centaurea diffusa) was investigated in field and pot t r i a l s . Field studies failed to show any phytotoxic effects from diffuse knapweed l i t t e r . In pot t r i a l s , knapweed seedlings did not inhibit seed germination or seedling development of three rangeland grass species. Under rangeland conditions, knapweed did not appear to influence soil microbial activity or affect levels of soil phenolics. Phenolic acids or sesquiterpene lactones were not detected in root exudates of knapweed. These results suggest that allelopathy is not a significant factor in the spread of knapweed. Diffuse (Centaurea diffusa) and spotted knapweed (C. maculosa) are two introduced weeds that have expanded to occupy large areas of rangeland in the dry Interior of the Pacific Northwest. The apparent ability of knapweed to invade established, productive rangeland has been attributed to the action of allelochemicals produced by knapweed 0,.2). The potential allelopathic effect of plants such as knapweed is an area of considerable interest, with researchers looking to exploit this phenomenon in agricultural situations, or simply to explain patterns of plant distribution (_3). In 1 963 , Fletcher and Renney (_2) detected a plant growth inhibitor in knapweed and they suggested an allelopathic role for the substance(s) that could promote the spread of knapweed. Although these results are widely quoted, the inhibitor has never been identified nor has its existence been confirmed. Twenty years l a t e r , knapweed allelopathy was reexamined in our laboratory. A series of diffuse knapweed isolates were found to be inhibitory to ryegrass germination in petri dish assays 0097-6156/87/0330-0238$06.00/0 © 1987 American Chemical Society

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


22.

MUIR ET AL.

Allelopathic

Agents in Diffuse

Knapweed

239

(4^. These s t u d i e s i n d i c a t e d t h a t the s h o o t s of diffuse knapweed c o n t a i n e d s i g n i f i c a n t q u a n t i t i e s of s e s q u i t e r p e n e l a c t o n e s , w i t h the germacranο 1ide cnicin t h e m o s t a b u n d a n t . T h e s e s q u i t e r p e n e l a c t o n e f r a c t i o n was inhibitory i n b o t h seed g e r m i n a t i o n and s e e d l i n g growth a s s a y s b u t t h e p u r i f i e d c n i c i n was l a r g e l y i n a c t i v e at t h e c o n c e n t r a t i o n s t e s t e d ( 4 0 0 p p m ) . A p o l a r f r a c t i o n was also found to inhibit ryegrass seed germination. The inhibitory effect o f t h e p o l a r f r a c t i o n was n o t due t o osmotic effects. It soon became a p p a r e n t t h a t b e f o r e a c h e m i c a l agent c o u l d be i m p l i c a t e d u n d e r f i e l d c o n d i t i o n s , t h e subtle nature of the allelopathic effect would have to be c a r e f u l l y e l u c i d a t e d . It should be possible to demonstrate the r e l e a s e of the c h e m i c a l agent i n t o the soil environment and its a c c u m u l a t i o n to phytotoxic concentrations. An e x a m p l e of this phenomena is the a c c u m u l a t i o n of the t h i o p h e n e , α-terthienyl, i n the root zone of common m a r i g o l d s ( T^ a_£e_ t e s_ e_ r_ e_ c_ t_ a_ ) (_5)« If knapweed i s a l l e l o p a t h i c , i s the e f f e c t p r e s e n t at a l l stages of p l a n t development or i s the e f f e c t most e v i d e n t at the v u l n e r a b l e s e e d l i n g stage? The m a i n o b j e c t i v e s i n the p r e s e n t study were to: (a) d e t e r m i n e the p h y t o t o x i c i t y of knapweed l i t t e r , (b) t e s t t h e i n h i b i t o r y e f f e c t of k n a p w e e d s e e d l i n g s on t h e g r o w t h of g r a s s s e e d l i n g s , and (c) d e t e r m i n e t h e effect o f k n a p w e e d on s o i l m i c r o b i a l r e s p i r a t i o n and s o i l phenol concentrations . Method s Knapweed l i t t e r , c o l l e c t e d from a s e v e r e l y i n f e s t e d site, was a i r - d r i e d and g r o u n d to pass a 20-mesh screen. To determine the p h y t o t o x i c i t y of the l i t t e r , the ground material was a p p l i e d t o m^ p l o t s o n a p a s t u r e site in 1981 a n d o n t w o r a n g e l a n d s i t e s i n 1 9 8 2 . S a n d ( 6 5 0 g) was c o m b i n e d w i t h e a c h k n a p w e e d t r e a t m e n t to p r o d u c e an e v e n d i s p e r s a l o f l i t t e r . The l i t t e r was a p p l i e d i n N o v e m b e r and g r a s s y i e l d s , e s t i m a t e d on a d r y m a t t e r b a s i s , were d e t e r m i n e d i n the f o l l o w i n g June. A series of pot t r i a l s using rangeland soil was conducted to determine if knapweed seedlings could i n h i b i t the g r o w t h of g r a s s s e e d l i n g s . Knapweed seeds and seeds of the f o l l o w i n g grasses were grown i n 6 cm diameter pots: crested wheatgrass (Agropyron c r i s tatum), Whitmar wheatgrass (A^ s p i c a t u m v a r . i n e r m e ) and S h e r m a n big bluegrass (Poa am ρ 1 a ) . Whitmar wheatgrass and the bluegrass are commercial selections derived from native s t o c k and c r e s t e d w h e a t g r a s s i s an i n t r o d u c e d species, w e l l a d a p t e d t o I n t e r i o r r a n g e l a n d s . E a c h g r a s s was grown a l o n e (10 s e e d l i n g s per pot) o r as a c o m p a n i o n species w i t h knapweed, i n w h i c h case t h e r e were 5 g r a s s seedlings and 5 k n a p w e e d seedlings per pot to keep p l a n t densities uniform. E m e r g e n c e r a t e s were c u m u l a t e d on a d a i l y basis (6^) a n d t h e s e e d l i n g s were h a r v e s t e d and w e i g h e d after



240

ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY

s i x weeks. The two w h e a t g r a s s e s were a l s o g e r m i n a t e d i n soil c o l l e c t e d from the r o o t zone of knapweed. Soil from the root zone of n a t i v e g r a s s e s was u s e d as a c o n t r o l . This experiment was r e p e a t e d using soils c o l l e c t e d at a second site. The m e t h o d s for determining soil r e s p i r a t i o n and phenols were s i m i l a r to those d e s c r i b e d p r e v i o u s l y (]_)· Air-dried soil samples ( 40 g ) f r o m t h e r o o t z o n e w e r e saturated with water, incubated i n sealed containers at 25°C and r e s p i r a t i o n r a t e s w e r e d e t e r m i n e d by t r a p p i n g evolved carbon dioxide in a l k a l i . Soil phenols were d e t e r m i n e d by t r e a t i n g s o i l s a m p l e s w i t h 2 Ν NaOH f o r 1 hr at room temperature and m e a s u r i n g the extracted phenolics c ο 1 ο r i me t r i c a 1 1 y (jO. Organic matter was d e t e r m i n e d b y t h e W a l k l e y - B l a c k m e t h o d (9_)« Results

and

Discussion

The rapid expansion of knapweed i n f e s t a t i o n s , the development of knapweed monocultures, and the f l u s h of grass after a p p l i c a t i o n of the h e r b i c i d e picloram (4amino-3,5,6- t r i ch1οr ορi c ο 1i ηi c acid) are f i e l d observations often given as e v i d e n c e for knapweed a l l e l o p a t h y . However, c a r e f u l e x a m i n a t i o n of a number of s i t e s where d i f f u s e knapweed has r e c e n t l y appeared failed to r e v e a l any of the c l a s s i c symptoms of allelopathy. There w a s no e v i d e n c e of s y s t e m a t i c plant distribution, z o n e s o f i n h i b i t i o n , o r o b v i o u s s i g n s o f t o x i c e f f e c t s on o t h e r p l a n t s . A d e t r i m e n t a l i m p a c t of knapweed on f o r a g e p r o d u c t i o n has been suggested (l_(^,JjO b u t t h e s e studies were conducted on r a n g e l a n d already depleted with a h i s t o r y of severe overgrazing. Furthermore, the d u r a t i o n of t h e s e s t u d i e s was t o o s h o r t t o e s t a b l i s h a d e f i n i t e c o r r e l a t i o n between knapweed i n v a s i o n and a d e c l i n e i n forage yield. When k n a p w e e d l i t t e r was a p p l i e d t o r a n g e l a n d sites (Table I), there was no s i g n i f i c a n t decrease in grass yield i n the f o l l o w i n g s p r i n g . The d a t a s u g g e s t e d the opposite, possibly due to a m u l c h e f f e c t . In a dense stand of knapweed, i t s above-ground biomass is approximately 2 0 0 g / m^ i n t h e f a l l . At r a t e s three times this l e v e l no i n h i b i t i o n was observed, and therefore i t is u n l i k e l y that accumulated l i t t e r transfers an i n h i b i t o r y agent to the s o i l . I n p o t t r i a l s t h e r e w a s n o s i g n i f i c a n t i n h i b i t i o n (P > 0.05) of g r a s s seedling emergence rates when t h e t h r e e grasses were g e r m i n a t e d i n the presence of knapweed seed (Table II). Knapweed had a s i g n i f i c a n t l y higher g e r m i n a t i o n r a t e and i t s s e e d l i n g s emerged f a s t e r than the grass species. This should give knapweed a s i g n i f i c a n t advantage in the f i e l d . There was a significant d e c l i n e (P < 0 . 0 5 ) i n t h e y i e l d o f Sherman big bluegrass but not for the wheatgrasses (Table III). The p o t t r i a l s w e r e o b s e r v e d f o r s i x weeks and t h e r e w e r e no o b v i o u s s i g n s o f p h y t o t o x i c i t y . When t h e r o o t zones


22.

MUIR ET AL.

TABLE

Agents in Diffuse

Knapweed

241

I. The E f f e c t of F a l l A p p l i c a t i o n of G r o u n d K n a p w e e d on t h e Y i e l d o f Grasses i n Spring

Knapweed application rate (g/m ) 2


Allebpathic

Grass Yield (g/m )

Air-dried,

a

2

Crested wheatgrass (rangeland)

Crested wheatgras s (pasture)

Bluebunch wheatgrass (rangeland)

100 200 400 600

158 129 160 148

124 94 93 119

165 99 134 127

Mean

149

108

131

Control

142

71

110

E a c h v a l u e i s t h e mean o f t h r e e d e t e r m i n a t i o n s . T r e a t e d p l o t s were not s i g n i f i c a n t l y d i f f e r e n t ( P > 0.05) f r o m t h e c o n t r o l s u s i n g D u n c a n s M u l t i p l e Range test.

TABLE

I I . Emergence Rates of Three Grasses Grown i n the Presence and A b s e n c e o f Knapweed

Emergence

Rate

0

a

in

Pots

(seedlings/day)

0

Knapweed

Crested wheatgrass

Whitmar wheatgrass

Knapweed

5

3

4

Crested wheatgras s Whi t m a r wheatgrass Sherman b i g bluegrass

6

1

0

Sherman big bluegrass

Companion species

5 6

2

3 3

B a s e d on f i v e s e e d s o f e a c h s p e c i e s p e r p o t . E a c h v a l u e i s t h e mean o f f i v e d e t e r m i n a t i o n s , T-tests were u s e d t o compare means. S i g n i f i c a n t d i f f e r e n c e s (P > 0.05) w e r e n o t d e t e c t e d .


242

ALLELOCHEMICALS: ROLE IN AGRICULTURE AND FORESTRY

TABLE

I I I . Y i e l d s of T h r e e G r a s s e s Grown i n P o t s P r e s e n c e and Absence of Knapweed

Yield


Knapweed

( mg/plant

3

i n the

)

3

Crested Whitmar wheatgrass wheatgrass

3

Sherman big bluegrass

Companion species Knapweed

83

53

Crested wheatgras s Whi tmar wheatgras s Sherman b i g bluegrass

68

72

88 107

45

14

37 22

Each v a l u e i s the mean of f i v e d e t e r m i n a t i o n s . T - t e s t s w e r e u s e d t o c o m p a r e means. S i g n i f i c a n t differences were o n l y d e t e c t e d f o r Sherman b i g b l u e g r a s s yields (P

A Search for the Allelopathic Agents in Diffuse Knapweed - American

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