Knapweed

Inhibition of tomato and barley plants growing in soils infested with Centaurea repens (knapweed) was reported by Fletcher and Renney (38). A toxic component was isolated in highest concentration from the foliage of knapweed. The inhibitor was considered to be an indole alkaloid or auxin precursor because of its ultraviolet absorption spectrum and the positive reactions obtained with Salkowski and Ehrlich reagents. The presence of the inhibitor was considered to explain partially the rapid establishment of Centaura spp. in almost pure stands. 

Composites -have been reported (8) to contain a wide variety of sesquiterpene lactones and lactones of this type have been previously found in both Russian knapweed and yellow starthistle (9,10,11,12). The wide range of biological activity associated with sesquiterpene lactones, e.g., cytotoxicity, phytotoxicity, antineoplasticity, etc. (8) suggested the possibility that these compounds may play a role in the allelopathy of both Russian knapweed and yellow starthistle. 

Plant Material. Russian knapweed and yellow starthistle were collec-ted in June along Highway 4, Contra Costa County, CA, between Brentwood and Stockton. The aerial parts of the plants were air dried, ground in a hammerraill with a 1/8" screen and extracted sequentially with Skellysolve-F and ether. 

The biological evaluation of the compounds isolated from Russian knapweed and yellow starthistle has led to some interesting features. 

Alford ER, Perry LG, Qin B, Vivanco 1M, Paschke MW (2007) A putative allelopathic agent of Russian knapweed occurs in invaded soils. Soil Biol Biochem 39 1812-1815... 

Cyclo(Pro-Tyr), also known as maculosin, is a phytotoxin produced by the fungus Altemaria alternata, and has been found to cause black necrotic lesions on the leaves of spotted knapweed. The mechanism underlying the phytotoxic action of cyclo(Pro-Tyr) may lie in its ability to inhibit ribulose-l,5-diphosphate carboxylase. ... 

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 (i., ). 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 ( ). 

In 1963, 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 later, 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... 

It soon became apparent that before a chemical agent could be implicated under field conditions, the subtle nature of the allelopathic effect would have to be carefully elucidated. It should be possible to demonstrate the release of the chemical agent into the soil environment and its accumulation to phytotoxic concentrations. An example of this phenomena is the accumulation of the thiophene, a-terthienyl, in the root zone of common marigolds (e r e c t a ) ( ) If knapweed is allelopathic, is the effect present at all stages of plant development or is the effect most evident at the vulnerable seedling stage ... 

The main objectives in the present study were to (a) determine the phytotox1c1ty of knapweed litter, (b) test the inhibitory effect of knapweed seedlings on the growth of grass seedlings, and (c) determine the effect of knapweed on soil microbial respiration and soil phenol concentrations. 

Knapweed litter, collected from a severely infested site, was air-dried and ground to pass a 20-mesh screen. To determine the phytotoxicity of the litter, the ground material was applied to m plots on a pasture site in 1981 and on two rangeland sites in 1982. Sand (650 g) was combined with each knapweed treatment to produce an even dispersal of litter. The litter was applied in November and grass yields, estimated on a dry matter basis, were determined in the following June. 

TABLE I. The Effect of Fall Application of Alr-drled, Ground Knapweed on the Yield of Grasses In Spring... 

TABLE IV. Respiration Rates, Phenolic Acid Levels and Organic Matter in Knapweed and Non-knapweed Soils. 

TABLE V. The Effect of Four Levels of p-Coumaric acid on the Emergence Rate and Yield of Knapweed and Two Grasses Grown in Pots... 

In summary, a number of hypotheses on knapweed allelopathy were tested and the conlusions from the various experiments were as follows ... 

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