Allelopathic

K. H. Park, K. Moody, S. C. Kim, and K. U. Kim, Allelopathic activity and determination of allelochemicals from sunflower (Helianilws annuus L.) root exudate.s. 11. Elucidation of allelochemicals from sunflower exudate.s. Korean J. Weed Sci. 12 173 (1992). 

Flavonoids and other phenolic compounds released by plant roots have important functions in plant-pathogenic interactions, feeding deterrence, nematode resistance, and allelopathic interactions they also serve as signal molecules for the establishment of symbiotic associations (72,149). However, a detailed analysis of signaling pathways involved in these interactions is currently available in only a limited number of cases (see also Chap. 7). 

C. S. Tang and C. C. Young, Collection and identification of allelopathic compounds from the undisturbed root. system of Bigalte Lompograss iHemarthia alti.s-sima). Plant Physiol. 69 155 (1982). 

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. 

Chemicals with allelopathic potential are present in virtually all plant tissues, including leaves, stems, roots, rhizomes, flowers, fruits, and seeds. Whether these compounds are released from the plant to the environment in quantities sufficient to elicit a response, remains the critical question in field studies of allelopathy. Allelochemics may be released from plant tissues in a variety of ways, including volatilization, root exudation, leaching, and decomposition of the plant residues. 

Although allelopathic interactions have been observed for centuries, the science of allelopathy is in its infancy. Much needs to be accomplished, and it will require joint efforts of scientists from several disciplines. Although by no means a complete list, the following areas need intensive study. 

Muller reported that black mustard (Brassica nigra) can form almost pure stands after invading annual grasslands of coastal southern California (25). This was attributed to inhibitors released from the dead stalks and leaves which do not permit germination and growth of other plants. These observations provide agronomists hope that similar results could be exploited with crops, specifically to achieve almost pure stands of crops (over weeds) by use of an allelopathic mechanism. 

Another approach is the utilization of allelopathic rotational crops or companion plants in annual or perennial cropping systems (19). Living rye (Secale cereale L.) and its residues have been shown to provide nearly complete suppression of a variety of agroecosystem weeds (33). Similarly, residues of sorghums, barley, wheat and oats can provide exceptional suppression of certain weed species Q ). Although some progress has been made on identifying the allelochmicals from these plants, much remains to be accomplished. 

Assessment of the Allelopathic Effects of Weeds on Field Crops in the Humid Midsouth... 

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. 

The common and troublesome weeds present in these six crops in the 13 Southern states (VA, NC, SC, GA, FL, AL, MS, TN, KY, AR, LA, TX and OK) are regularly surveyed by State Extension Weed Specialists. The most recent survey includes 59 weeds which are common or troublesome in these six field crops (6). Twenty weeds on that list have been implicated as being allelopathic (Table 1) eleven are among the 10 most common in at least one of the six major field crops in the Southern states. Ten of these 11 weeds are considered to be among the 10 most troublesome, usually for the same crop in which it was found to be common. There is one weed [bermudagrass (Cynodon dactylon (L.) Pers] that is on the most troublesome list for cotton and tobacco, but not on any most common list. Similarly, giant foxtail (Setaria faberi Herrm.) is a common weed in tobacco, but apparently not a troublesome one. 

An interesting feature of this list is that some of our most troublesome and difficult to control weeds are not suspected allelopathics. Many weeds, such as morningglory (Ipomoea spp.), cocklebur (Xanthium strumarium L.), and sicklepod (Cassia obtusifolia L.), which make almost all lists of serious weed species, are strong competitors with little or no evidence of allelopathic activity. In fact, cocklebur is probably the epitome of what a competitor can be, being even more devastating than... 

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.

A regression analysis was used to test the johnsongrass rhizome biomass for allelopathic activity. 

Table I. Weeds With Reported Allelopathic Activity Found in Row Crops in the Southern United States...

A crop is listed if a report exists of specific allelopathic activity of that weed on the crop. 

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. 

Grimmer ( 4) recatmended that special terms be used for the allelopathic chemicals based on where they are produced and which plants are affected by them. These special terms include ... 

---