ALLELOPATHY

Definition and Concept species is often not accomplished. According to Ferguson and Rathinasabapathi, According to Colquhoun, effective “allelopathy refers to the beneficial or harmful demonstration of allelopathy on plant growth effects of one plant on another plant, both crop and development and its reliable application in and weed species, by the release of chemicals agricultural pest management have been from plant parts by leaching, root exudation, minimal. The use of allelopathic cover crops volatilization, residue decomposition and other such as rye has resulted in the greatest processes in both natural and agricultural application of allelopathy in agriculture. systems.” Very little is known about the mode of action of First, some definitions that relate to allelopathy. allelochemicals or how plants of a particular species avoid reputed allelochemical effects in ! Allelochemical–for the purposes of this a natural setting. Even though there is a article, a toxic chemical produced by a plant growing body of literature that implicates ! Phytochemical–a chemical compound that allelopathy in plant-to-plant relationships occurs naturally in a plant andinteractions in a natural agricultural setting, ! Toxicity–the negative effect of a substance on the translation of this knowledge into its use or a plant place in managing weeds in agricultural ! Synthetic herbicide–herbicides formed production systems has been slow. through a chemical process or chemical synthesis The study of allelopathy and discerning its importance for agricultural production With the continuing, increasing occurrence of systems will only be advanced if current and herbicide-resistant (HR) weeds in agricultural forthcoming research results can be translated systems, there is renewed interest in into new technologies that can be used for determining how/if allelopathy might be used weed management and/or reduced as a possible option in weed control systems. dependence on synthetic herbicides.

Allelopathic effects can be and have been History and Future measured in controlled environment settings such as a laboratory or greenhouse, but Dr. Stephen Duke, USDA-ARS Research Leader replicating the effects in the field has been and at Oxford, MS, published an excellent summary continues to be difficult. on the current status of research on allelopathy and the possible implications for agriculture. Research has left little doubt that the presence His commentary article was published in 2010 of a phytochemical that is toxic to plants of in Vol. 25 of the “Allelopathy Journal”. Below another species will have an adverse effect on are summary points taken from this article. those plants. However, equating the toxicity of the phytochemical to its availability and effect ! Results of past research with allelopathy most in the natural environment of the target plant often have demonstrated correlative rather WWW.MSSOY.ORG. Aug. 2015 1

than cause and effect relationships. produced source of an allelochemical that is ! Effects often attributed to allelopathy may toxic to a receiving plant is more important actually be a result of competition for than the static concentration of the toxic resources between/among plants. allelochemical in the soil at any measurement ! There is evidence that chemical detection of a period. competing plant species can trigger the ! Determining/understanding the mechanisms of production of an allelochemical. allelopathic activity, followed by utilizing that ! Past research has often shown that many information to manage weeds in agricultural plants produce compounds that, in high systems, is the challenge that is faced by enough concentrations, are toxic to other scientists. plants in the absence of soil. Such ! Transgenically imparting or enhancing experiments suggest the potential for but do allelopathy in crop species will be a positive not prove allelopathy in a natural setting. step toward utilizing allelopathy in ! Extrapolation of results from experiments agriculture. conducted in a lab setting in the absence of ! Research is needed to ascertain soil microbial soil is virtually impossible. involvement in allelopathy; e.g. do soil ! Allelochemicals from living plants and dead microbes transform allelochemicals to more or plant matter are constantly entering the soil less toxic forms, does exposure of soil environment and this is almost impossible to microbes to a particular allelochemical over a measure. Also, these allelochemicals are period of time increase their activity on the either absorbed in the water portion of the soil allelochemical, do allelochemicals change the or are adsorbed to soil particles, so the amount soil microbial makeup. of the allelochemical that is available at any ! Increased interest in allelopathy is fueled by one time is difficult to measure or predict. the need to reduce synthetic herbicide use in ! Reputed allelochemicals are often inactivated agriculture, and the desire to find or identify in a soil environment due to their instability, natural products or allelopathic processes to degradation by soil microbes, or other control HR weeds. interactions with soil. Also, those that are ! Using transgenics to produce crop cultivars water-soluble are subject to leaching from the that have enhanced allelopathic properties is root zone of potential target species. being undertaken, but this area of research is ! Methods that are being developed can be used insignificant compared to that of using to determine if the loss of allelochemicals transgenics to develop herbicide resistance in from the soil due to the above processes is crop species. offset by the producing plant’s exuding or ! In the near future, there is little reason to secreting more of the allelochemical. expect the development of allelochemicals for ! Allelochemicals that are steadily produced by use as natural herbicides. a donor plant and released into the soil may not seem too significant, but may become so if Wheat Allelopathy this steady release and subsequent accumulation of the allelochemical by the Soybean–wheat doublecropping is a widely- receiving plant is in an amount to cause an used cropping system in the Midsouth. An adverse reaction. In other words, a steadily often-stated allelopathic effect is that of wheat WWW.MSSOY.ORG. Aug. 2015 2

straw on soybeans that follow wheat in a ! Wheat accessions/genotypes/varieties differ in doublecrop system, or that of wheat straw on their allelopathic effect toward other plant weed seed germination and emergence. species. ! Research on wheat allelopathy towards weeds A summary of the results from an extensive has progressed to the identification of wheat literature search follow. allelochemicals, and further to the identification of genetic markers associated ! Both wheat residue and wheat seedling with this trait. Presently, there is no allelopathy are currently being researched for significant agronomic achievement resulting their potential utilization in weed from this knowledge. management. ! Wheat varieties have not been developed that ! Results reported from greenhouse experiments have enhanced biosynthesis of allelochemicals where wheat straw or wheat straw leachate for weed control, although this field of was used have shown a perceived adverse research and development is of interest to a allelopathic effect of wheat straw on early number of researchers. growth of soybeans.

! Results from field studies using the same In summary, there is no evidence that the variables as those used in greenhouse studies phytotoxicity of wheat straw on soybeans is a are not correlated with those of the greenhouse field problem. Thus, perceived allelopathy studies; i.e., the same results have not been effects that were measured in the greenhouse obtained from field studies. ! In field studies where leaving wheat straw on but not in field environments are likely the soil surface or incorporating the straw into tempered by the soil environment. Also, N the soil resulted in stunted early growth of immobilization by the wheat straw probably soybeans, adding supplemental N at soybean results in lack of sufficient available N needed planting overrode the effect. In treatments for early-season soybean growth before where wheat straw was removed or burned, nodulation and subsequent N fixation occurs. the effect did not occur. Thus, N Even though past research indicates that wheat immobilization rather than allelopathy was varieties release allelochemicals that have an likely responsible for the measured effects. adverse effect on some weed species, this has ! Wheat plants produce and release toxic not led to a weed management system that substances that inhibit several weed species, incorporates this concept. and many of these compounds have been identified. This has led to the perception that A 1982 “Weed Science” article states that wheat allelopathy has the potential for the further screening of different weed species management of weeds. against wheat allelopathic toxins in field ! It is believed that the constant exposure of situations is needed before wheat allelopathy weed plants to continuously released toxic can be a factor in weed control. This is still allelochemicals would create a stress true. environment for the weeds, thereby resulting in a reduced impact of the target weed in a A statement from Molecular approaches in cropping system. improving wheat allelopathy published in 2005 WWW.MSSOY.ORG. Aug. 2015 3

is “Wheat allelopathy has great potential in Corn Allelopathy integrated weed management systems. Concerted research efforts have been made Corn allelopathy arguably has received less toward the development of wheat varieties attention than allelopathy of other crops such with high allelopathic activity”. However, as rice, wheat, and grain sorghum. wheat varieties that may have been released Since corn is often rotated with soybeans in the with this trait are not advertised in the US. Midwest, and is increasingly so in the Midsouth, Articles used to develop the above summary the effect of possible corn allelopathy on about wheat allelopathy are: following soybeans is of interest to Midsouth producers. However, there is negligible Techniques for identifying tolerance of soybean information to indicate any such effect. to phytotoxic substances in wheat straw, by Herrin, Collins, and Caviness (“Crop Science”, So, the points below are statements about Vol. 26, 1986). possible corn allelopathy in general. Soybean-Wheat Doublecropping: Implications ! As with extracts from the residues of other from straw management and supplemental crops, extract from corn residue inhibits nitrogen by Hairston, Sanford, Pope, and germination and growth and development of Horneck (“Agronomy Journal”, Vol. 79, 1987). corn seedlings in laboratory experiments (Martin, McCoy, and Dick, “Agron. Journal”, Molecular approaches in improving wheat Vol. 82, 1990; Elmore and Abendroth). allelopathy, by Wu. However, there is no indication in the literature that this has been manifested in a Allelopathy in wheat, by Wu, Pratley, Lemerle, field environment. and Haig (“Ann. Appl. Biol.”, Vol. 139, 2001). ! Using corn allelopathy is a hoped-for and perceived possible alternative to weed Allelochemicals in wheat: cultivar difference in management with synthetic herbicides or the exudation of phenolic acids, by Wu, Haig, herbicide-resistant hybrids (Pratley 2006). Pratley, Lemerle, and An (“J. Agric. Food This is a worthwhile alternative weed Chem.”, Vol. 49, 2001). management approach in corn, but, it has not been successfully accomplished in production Evaluation of seedling allelopathy in 453 wheat environments. accessions against annual ryegrss by the equal- ! In field experiments, Elmore and Abendroth compartment-agar method, by Wu, Pratley, state “...yields are reduced when corn follows Lemerle, and Haig (“Aust. J. Agric. Res.”, Vol. 51, corn....”, or to use common jargon, there is a 2000) “yield drag” for corn following corn. Allelopathic potential of wheat straw on selected Allelopathy is often implicated but rarely if weed species, by Steinsiek, Oliver, and Collins ever proven in field environments because of (“Weed Science”, Vol. 30, 1982). weather vagaries, amount and makeup of the residues, and unknown factors such as soil microbial activity. More than likely, allelopathy is used as a catch-all term for the WWW.MSSOY.ORG. Aug. 2015 4

myriad unidentified causes of this proven management tool in the near future. occurrence. Sorghum Allelopathy According to Heggenstaller (Pioneer, 2012), Grain sorghum is not a major crop acreage- “corn residues are a major factor contributing wise in Mississippi, with only 105,000 acres to lower yields for corn following corn harvested in 2014 and an estimated 95,000 compared to corn rotated with soybean, acres to be harvested in 2015. However, it is particularly in no-till management.” Major recognized as being superior to corn in both points from this study are: drought and heat tolerance (Crop Management, ! With corn following corn, residue Nov. 2010). Therefore, it has the potential to management seems to be the key to avoiding become a significant rotation partner with the yield-reducing effects of corn stover. This soybeans in a dryland production system. involves offsetting stover’s potential for producing negative effects by reducing the The recent incursion of the sugar cane aphid interference from corn residue through into Midsouth sorghum fields may further limit removal (baling) of a portion (approximately sorghum production in the region, either one-half) of the stover. because of is severe effect on sorghum yield or ! Results suggest that corn after corn with the increased cost of production associated stover removal may produce yields that are with its control. similar to those from corn rotated with Previously, I stated that corn allelopathy soybeans. arguably has received less attention than has ! No-till continuous corn production is allelopathy of other crops. Conversely, amenable to stover removal due to the high sorghum allelopathy has received considerable amounts of residue that are produced and attention. Sorghum’s allelopathic properties remain on the soil surface in the absence of are more pronounced than those of most other tillage. plants that have been studied. ! It is proffered from previous research that as much as half of the corn residue can be Research findings are presented here to removed without negatively affecting the support sorghum’s allelopathic effect and the beneficial effects of that residue. potential usefulness of that property in cropping systems. Additional discussion of possible corn allelopathy is presented by Lykins in the April A set of publications from the USDA-ARS 2012 issue of “Corn and Soybean Digest”. Natural Products Utilization Research Unit in Oxford, MS provides results from a continuing In summary, and to be concise, corn investigation of sorghum’s allelopathy. allelopathy has not been specifically identified Scientists responsible for these publications as a factor in US corn production systems, are F. Dayan, I. Alsaadawi, A. Rimando, Z. Pan, S. especially as it relates to soybeans rotated with Baerson, A. Bimsing, S. Duke, L. Pratt, D. Cook, corn. There is no evidence that corn and I. Kagan. allelopathy will be utilized as a weed WWW.MSSOY.ORG. Aug. 2015 5

! Sorgoleone with its lipid resorcinol analogue in weed inhibition, and that this has potential is a potent phytotoxin produced by grain applications in production agriculture. sorghum root hairs, and likely accounts for much of the allelopathy attributed to sorghum. Wortman, Schmidt, and Lindquist (Crop Sorgoleone is one of the most studied Management Vol. 13, Dec. 2014) conducted allelochemicals. experiments with sudangrass (Sorghum bicolor ! Increased production of sorgoleone by (L.) Moench var. sudanense) in a range of soil sorghum roots was measured when a crude mixtures. A summary of their results follows. extract from velvetleaf roots was added to the growth medium.  Sudangrass root exudates and shoot ! Sorgoleone can be absorbed through the residue both reduced emergence of green hypocotyl and cotyledonary tissues of foxtail grass from soil. germinating seedlings, thus leading to the  Sudangrass root exudates and decomposing possibility that its effects are the result of shoot residue acted synergistically to delay photosynthesis inhibition in young seedlings. emergence of green foxtail from soil. ! Research results suggest that sorghum roots  These results indicate that sudangrass has have the capacity to continuously exude the potential to provide measurable sorgoleone into the soil. As this phytotoxic reductions in grass weed seed emergence exudate is released directly into the soil, its from soil. The resulting recommendation is action is similar to that of a soil-applied that producers use sudangrass as a cover herbicide. crop and incorporate both shoot and root ! The potential for continuous exudation of residues into the soil to realize maximum sorgoleone into the soil may sustain its weed suppression effect. presence and subsequent activity in soil over a much longer period than that of an applied Roth, Shroyer, and Paulsen (Agronomy Journal, herbicide. Vol. 92, 2000) reported that wheat yields ! Sorgoleone is persistent in soil, but it is following grain sorghum were reduced by 15 mineralized by microorganisms over time. and 30% compared to fallow when the Scientists at the above USDA-ARS Unit are ! sorghum residue had been tilled or left on the developing information needed to genetically soil surface with no tillage, respectively. Their increase production of sorgoleone in sorghum. results suggest that tillage of the sorghum

stover abated but did not completely offset the Einhellig and Rasmussen reported in the effect of the allelopathic compounds in the “Journal of Chemical Ecology” (Vol. 15, 1989) sorghum stover. They found no differences in that a grain sorghum crop reduces weediness tolerance to sorghum residue among wheat in the following crop year compared to corn hybrids. and soybeans. This effect was primarily on broadleaf weeds, and involved both delayed In summary, there is strong research evidence emergence and growth inhibition of weeds that sorghum has an allelopathic effect on during the growing season. They concluded plants of a following crop, and on weed plants that allelopathic conditions from growing grain that may appear in a growing sorghum crop. sorghum must be considered as a major factor WWW.MSSOY.ORG. Aug. 2015 6

It is anticipated that sorghum’s allelopathic Vol. 93, 2001), isothiocyanates released by a effect will be the first to be exploited for use in turnip-rape mulch were determined to production agriculture. However, the suppress weed germination after incorporation application of this trait has yet to be realized. of the mulch into the soil. They concluded that Certainly, this property of sorghum needs the potential of cover crops to release these further attention so that its potential for use in compounds should be further investigated as a weed control and curtailing the use of tool for weed control/suppression in synthetic herbicides can be determined and integrated cropping systems. hopefully become a reality. In a recent report in “Agronomy Journal” (Vol. Cover Crops and Allelopathy 104, 2012), Lawley, Teasdale, and Weil determined that early and competitive fall Cover crops have long been recognized for growth of a forage radish cover crop is the their potential to provide soil cover that will dominant mechanism for early-spring weed curtail erosion between crop growing seasons, suppression resulting from use of this species and to provide residue that is available to as a cover crop. They did not measure any increase soil organic matter. allelopathic activity that limited seed germination or seedling development. Thus, With the increasing occurrence of HR weeds, cover crops are now being evaluated for their they concluded that cover crop management allelopathic potential to control weeds. strategies should be directed towards practices that ensure maximum cover crop development Current thought is that cover crops and their in the fall to ensure maximum physical weed- residues may provide weed suppression suppression activity the following spring. through their physical presence on the soil surface and/or by the release of Cereal cover crops will produce more biomass than will legume cover crops (Reddy, “Weed allelochemicals that may inhibit weed seed Technology”, Vol. 15, 2001). This increased germination and/or early seedling development (Weston, “Agronomy Journal”, physical barrier, coupled with the slower Vol. 88, 1996; Weston, 2005). Weston also degradation of residues from cereals compared suggests that allelopathic crops offer potential to that of legumes, should result in more and for the development of herbicides, as well as longer-lasting weed control/suppression from for providing germplasm from which to select using cereal cover crops. for allelopathic products and chemistry. There are five important points regarding the Thus, allelopathic potential of cover crops for use of cover crops for weed control either by weed suppression is touted, and research has physical suppression or by allelopathy. shown that cover crop extracts can inhibit  Differentiating between allelopathy and the early plant development (Kelton, Price, and mulching effect of cover crops is difficult. As Mosjidis, Intech Press, 2012; Wortman, Schmidt, stated above, it is accepted that increased and Lindquist). cover crop biomass on the soil surface can suppress weeds, but to what extent and with In a study by Petersen et al (“Agronomy Journal”, WWW.MSSOY.ORG. Aug. 2015 7

what resulting value in a soybean production another species will have an adverse effect on system is not known. those plants. However, equating the toxicity of  The variability in allelopathic effects from the phytochemical to its availability and effect plant residues presently negates their in the natural environment of the target plant consideration as a stand-alone weed control species is often not accomplished. option in large-scale crop production systems.  A likely system will be using cover crops that Research is needed to ascertain soil microbial are proven to physically or allelopathically involvement in allelopathy; e.g. do soil suppress weeds to offset some herbicide use. microbes transform allelochemicals to more or  The additional cost associated with using less toxic forms, does exposure of soil microbes cover crops in a crop production system must to a particular allelochemical over a period of be considered (Reddy, “Weed Technology”, time increase their activity on the Vol. 15, 2001). In other words, the additional allelochemical, do allelochemicals change the cost of using cover crops for potential weed soil microbial makeup. control must be compensated for by increased soybean yield and/or reduced herbicide Concerted research efforts have been made usage/cost. Otherwise, producers will be toward the development of wheat varieties reluctant to insert cover crops into soybean with high allelopathic activity. However, wheat production systems for any reason. varieties that may have been released with this  To maximize the weed-suppression capacity trait are not advertised in the US. of an allelopathic cover crop such as Corn allelopathy has not been specifically sudangrass, shoot residues should not be identified as a factor in US corn production removed by grazing or haying (Wortman, systems, especially as it relates to soybeans Schmidt, and Lindquist). rotated with corn. There is no evidence that corn allelopathy will be utilized as a weed Summary management tool in the near future.

Increased interest in allelopathy is fueled by There is strong research evidence that the desire to reduce synthetic herbicide use in sorghum has an allelopathic effect on plants of agriculture and to find or identify natural a following crop, and on weed plants that may products or allelopathic processes to control appear in a growing sorghum crop. It is HR weeds. anticipated that sorghum’s allelopathic effect will be the first to be exploited for use in Allelopathic effects can be and have been production agriculture. However, the measured in controlled environment settings application of this trait has yet to be realized. such as a laboratory or greenhouse, but replicating the effects in the field has been and The variability in allelopathic effects from plant continues to be difficult. residues presently negates their consideration as a stand-alone weed control option in large- Research has left little doubt that the presence scale crop production systems. of a phytochemical that is toxic to plants of Cover crops that are proven to physically or WWW.MSSOY.ORG. Aug. 2015 8

allelopathically suppress weeds to offset some herbicide use may be a consideration in a soybean production system. However, the costs associated with this practice and the uncertain magnitude of the effect from using this practice will affect its adoption.

The likelihood of weeds becoming resistant to allelochemicals is unknown, so if they become an important component in weed management, this will be a factor to consider.

Allelochemicals that may be forthcoming will have to be selective in their effect; i.e., they must only adversely affect targeted plants such as weeds. Using transgenics to produce crop cultivars that have enhanced allelopathic properties is being undertaken, but this area of research is insignificant compared to that of using transgenics to develop herbicide resistance in crop species.

In the near future, there is little reason to expect the development of allelochemicals for use as natural herbicides. The principle of allelopathy may have the greatest application in low-input agricultural systems. The study of allelopathy and discerning its importance for agricultural production systems will only be advanced if current and forthcoming research results can be translated into new technologies that can be used for weed management and/or reduced dependence on synthetic herbicides. Composed by Larry G. Heatherly, Revised Aug. 2015, [email protected]

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