Allelopathy weeds

There is extensive evidence that allelopathy may contribute to patterning of vegetation in natural ecosystems (11). Distinct zones of inhibition are present under and adjacent to a variety of woody species, and often toxins from their litter are implicated (J2). One might speculate that aggressive perennial weed species quickly gain dominance by exploiting allelopathic mechanisms. 

How weeds produce their effect is the subject of continuing research. As rather recently defined, the negative effect of weeds on crop plants includes both competition and allelopathy (1.) and has been termed interference. Previous weed science research considered the competition component foremost and, usually, solely (e.g. 2). Allelopathy, however, has received some attention recently, with a number of symposia and reviews devoted to allelopathic effects. Some of these cover the effects of weeds on crops (3, 4, 5). In this chapter, I review the possible role of allelopathy in weed interference in crop production in the Mid-South. A second objective of this review is to indicate where research is needed and to suggest potential lines of future research, especially with respect to the developing role of conservation tillage practices in this region. 

One of the features of allelopathy, as pointed out by Rice (18). is that it is specific. Certain species, but not others, are affected by the allelochemics produced by a plant. This suggests that allelopathy by a weed must be positively demonstrated for each crop. References to specific reports of demonstrated allelopathy by a weed on a crop are given in Table 1. In some cases [crabgrass (Digitaria sanguinalis (L.) Scop.), spurge (Euphorbia spp.), etc.], no crop is listed in the allelopathy column. 

Table I provides general information on the identified allelopathic weeds and the crops they affect. More detailed information on two of these weeds, johnsongrass and purple nutsedge, will be presented to show the tenuous nature of the evidence for allelopathy. These two species are undoubtedly allelopathic, at least under certain conditions. Rigorous proof that allelopathy is the agent responsible for even a specific portion of the interference exerted by them is not easily attained, however, even though these are the weeds with the best research data available of those weeds that occur in the Mid-South.

Future research. Ample research has been conducted to demonstrate the allelopathic potential of numerous weedy species, and of their associated crops. Future research should be directed to discerning how much of this potential is realized in the field under humid, warm conditions such as that found in the Southeast United States. Newman (33), in a reflective article, concludes that "—it cannot be said with confidence that allelopathy has any significance for agriculture." The evidence so far certainly substantiates his conclusion. To get a better feel for allelopathy in the field, field studies should minimally include the weed and weed residues. 

The term allelopathy, when first proposed by Molisch (1 ), referred to either the beneficial or detrimental interaction between all types of plants and microorganisms. As presently used, this definition is generally accepted. Since 1970 a concerted effort has been made to understand the phenomenon of allelopathic interaction. The many interpretations resulting from these studies are well documented in the literature (2-4). An area currently receiving considerable attention is the allelopathic effect resulting from weed-crop and weed-weed interactions (2, 5-7). One study conducted by Wilson and Rice (7) showed that the common sunflower, Helianthus annuus L., possessed allelopathic properties. Realizing the inherent potential... 

Relating the effects caused by specific allelochemicals to those caused by an allelopathic plant is complicated because the inhibitory substances released from a plant are often unknown, and generally several different compounds are involved. However, the actions of the weeds studied in our investigations have certain parallels to those found from pCA and FA. The allelopathic nature of Kochi a, Jerusalem artichoke, and cocklebur was established, since both aque-ous extracts and weed residues reduced sorghum growth. The data show a concentration dependency characteristic of allelopathy, and some difference in toxicity among the three weeds was observed with cockle-bur the most toxic. 

Ninety weed and crop species common to the southeastern U. S. were tested for phytotoxin content to discover candidates for an allelopathy testing program. Plants were collected in the spring, summer and fall in a three county area of northeastern Mississippi. Plants were maintained in sealed plastic... 

The presence of crop residues has been reported to both increase (5, 6) and decrease crop yields ( 7) and not tilling to increase certain difficult to control weeds (8). However, other reports indicate that the presence of certain mulches can reduce the biomass of certain weeds (9-15) and allow for higher crop yields ( 5, 6). Thus, under certain conditions, mulches can suppress certain weed species, but determining the reason(s) presents many logistical problems, especially under field conditions. To determine the cause(s), the physical and chemical (i.e., allelopathy) effects of the mulch and the role of soil disturbance (or the lack of, as would be the case in a no-till system) must be separated. 

The purposes of the studies reported here were to (1) characterize the effects of rye and wheat mulch and soil disturbance on certain weed species, (2) determine if allelopathy was involved,... 

Murphy, S.D. (1999). Is there a role for pollen allelopathy in biological control of weeds. In Allelopathy Update, (Ed., S.S. Narwal) Vol. 2 321-332. Science Publishers, Enfield, Plymouth, USA. 

Baranenko, V.V. (2001). Pea chloroplasts under clino-rotation lipid peroxidation and superoxide dismutase activity. Advances in Space Research 27 973-976. Bhowmik, P.C. and Inderjit (2003). Challenges and opportunities in implementing allelopathy for natural weed management. Crop Protection 22 661-671. 

Murphy, S. D. (2001). The role of pollen allelopathy in weed ecology. Weed Technology 15 867-872. 

Keywords Allelopathy Cover crops Crop residues Intercropping Mulching Organic farming Weeds... 

Weed management in organic agriculture use preventive methods such as an appropriate crop rotation, precise soiled preparation before sowing crops, narrow seed spacing, etc. (Labrada 2003). Many of them include ways in which allelopathy (often together with competition) could play an important role (Fig. 14.1). 

Fig. 14.1 Utilization of allelopathy in organic weed management. Weed management in organic agriculture includes four groups of different methods for weed reduction. Allelopathy plays an important role in some cultural and biological methods...

Hence, allelopathy alone can not be a sufficient tool for weed control. Combinations of methods that prevent weed germination and control weeds have to be used (Rasmussen 2004). 

Providing weed suppression through the use of allelopathic cover crops is an important method of weed control in organic farming and it is one of the best possibilities of allelopathy application (Sullivan 2003a). Besides, growing of cover... 

Intercropping may facilitate weed control if intercrops are more weed competitive than sole crops or are able to suppress weed growth through allelopathy (the weed species is more susceptible to such phytotoxin than crops). If intercrops do not suppress weeds more than sole crops, they should provide yield advantages due to better utilization of resources or by converting resources than in case of sole crops (Liebman and Dyck 1993). 

Allelopathy found hopeful utilization in plant protection against parasite weeds. The main world parasitic species are the witchweeds (Striga spp.), broomrapes (Orobanche spp.), and eventually dodder (Cuscuta spp.). Witchweeds and broom-rapes attack many economically important crops especially throughout the semi-arid regions. Early detection of parasitic weed infestation and protection is difficult because of the growth habit of a root parasite and huge production of dust-like seeds viable up to 20 years (Kebreab and Murdoch 1999). 

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