Allelopathic Effects Of Rye (Secale Cereale L.) And Their Implications For Weed Management - A Review

Abstract

Allelopathy offers potential for selective biological weed management through 
the production and release of allelochemicals from leaves, flowers, seeds, 
stems, and roots of living or decomposing plant materials (Weston, 1996).  Under 
appropriate conditions, allelochemicals may be released in quantities 
suppressive to developing weed seedlings.  Allelopathy is strongly coupled with 
inherent stresses of the crop environment, including insects and disease, 
temperature extremes, nutrient and moisture variables, radiation, and herbicides 
(Einhellig, 1996).  These stress conditions often enhance allelochemical 
production, thus increasing the potential for allelopathic interference.  Rye is 
an example of a plant which provides excellent weed suppression through both 
allelopathic and competitive mechanisms.  Rye residues maintained on the soil 
surface release 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) and a breakdown 
product 2(3H)-benzoxazalinone (BOA) (Barnes and Putnam, 1987) both of which are 
strongly inhibitory to germination and seedling growth of several dicot- and 
monocotyledenous plant species.  Further, microbially produced transformation 
products of BOA demonstrate several fold increases in phytotoxic levels (Chase 
et al., 1991a; Gagliardo and Chilton, 1992).  Hence a variety of natural 
products contribute to the herbicidal activity of rye residues.  Several studies 
have demonstrated the allelopathic characteristics of rye residues and root 
exudates containing DIBOA and BOA.  For example, experiments have shown marked 
reductions in germination and growth of several problem agronomic weeds 
including barnyardgrass (Echinochloa crusgalli L.), common lambsquarters 
(Chenopodium album L.), common ragweed (Ambrosia artemisiifolia L.), green 
foxtail [Setaria viridis (L.) Beauv.], and redroot pigweed (Amaranthus 
retroflexus L.) (Putnam and DeFrank, 1983; Shilling et al., 1985).  Researchers 
are optimistic the results of these and future studies will provide the 
necessary information to develop alternative weed management strategies and 
cropping systems to enhance the sustainability of agriculture.


Introduction
Economic and environmental constraints of crop production systems have 
stimulated interest in alternative weed management strategies.  Allelopathy 
offers potential for selective biological weed management through the production 
and release of allelochemicals from leaves, flowers, seeds, stems, and roots of 
living or decomposing plant materials (Weston, 1996).  The term allelopathy 
refers to biochemical interactions among plants, including those mediated by 
microorganisms (Molisch, 1937).  This broad definition of allelopathy is 
appropriate because considerable research has indicated the involvement of 
microorganisms and lower plants in production of phytotoxins (Gagliardo and 
Chilton, 1992; Putnam, 1986).  A variety of allelochemicals have been 
identified, including the phenolic acids, coumarins, terpenoids, flavonoids, 
alkaloids, glycosides, and glucosinolates (Barnes, Putnam, and Burke, 1986; 
Blum, 1995).  Allelopathic inhibition typically results from the combined action 
of a group of allelochemicals which, collectively, interfere with severe 
physiological processes (Blum, 1996).  Allelopathy is strongly coupled with 
inherent stresses of the crop environment, including insects and disease, 
temperature extremes, nutrient and moisture variables, radiation, and herbicides 
(Einhellig, 1996).  These stress conditions often enhance allelochemical 
production, thus increasing the potential for allelopathic interference.
Rye is an example of a plant which provides excellent weed suppression through 
both competitive and allelopathic mechanisms.  Rye and its residues reduce weed 
and crop growth by modifying the microenvironment and releasing allelochemicals 
(Putnam et al., 1983; Rice, 1984).  Rye produces a dense canopy, which is more 
competitive than weeds for light, moisture, and nutrients.  In addition, rye 
residues on the soil surface reduce weed germination and growth by shading, 
lowering soil temperatures, moderating diurnal temperature fluctuation, and 
acting as a physical barrier (Barnes and Putnam, 1986; Putnam et al., 1983; 
Rice, 1984).  Rye residues maintained on the soil surface release phytotoxic 
acids which strongly inhibit germination and seedling growth of several dicot- 
and monocotyledenous plant species (Barnes et al., 1987).
The objective of this paper is to review current literature that characterizes 
the ability of rye to interfere with other crops and weeds through competition 
and allelopathy.

Rye allelochemicals identified
Understanding allelopathy may hold the key to new weed management strategies.  
However, the difficulty of distinguishing chemical interference from competition 
has hindered studies of allelopathy in natural and cultivated plant communities 
(Weidenhamer, 1996).  To positively identify the effects of chemical 
interference in future studies, Fuerst and Putnam (1983) proposed a series of 
criteria, modeled after Koch's postulates in microbiology, for proving a 
hypothesis of allelopathy.  These are: (i) identify and quantify specific 
symptoms of interference; (ii) isolate, identify, and synthesize the toxin, 
characterizing its biological activity through bioassays; (iii) simulate the 
interference through applications mimicking natural rate; and (iv) quantify the 
amount of toxin released to the environment and taken up by the target plant.
Despite the acknowledgment of these essential criteria researchers remain 
skeptical about the feasibility of designing experiments that conclusively test 
the toxin hypothesis of plant interaction (Harper, 1977; Williamson, 1990).  
Challenges posed by allelochemicals include: (i) continual application in the 
exact dosage released from its source plant; (ii) altered toxicity of 
allelochemicals through degradation; and (iii) interaction of allelochemicals in 
their effects thereby requiring knowledge of the chemical complex, the 
concentration of each component, and the mechanism of release of each component.
Many studies of allelopathic residues have been conducted with rye because of 
its substantial biomass production and apparent phytotoxicity.  Putnam and 
DeFrank (1983) reported that rye residue reduced the emergence of common ragweed 
by 43%, green foxtail by 80%, redroot pigweed by 95%, and common purslane 
(Portulaca olearacea L.) by 100%.  Shilling et al. (1985) reported that a 
surface mulch of desiccated rye in a no-till system reduced aboveground biomass 
of common lambsquarters by 99%, redroot pigweed by 96%, and common ragweed by 
92% compared to an unmulched tilled control.  Creamer et al. (1996) demonstrated 
that allelochemicals could be leached from rye shoot residue and used as a 
control to separate the physical effects of weed suppression of surface rye 
mulch from other types of interference.  Leached rye inhibited emergence of 
eastern black nightshade (Solanum ptycanthum Dun.) by 98%.
Barnes and Putnam (1983) evaluated and confirmed that residues and aqueous 
extracts of rye shoots were toxic to several plant species.  Guided by criteria 
suggested by Fuerst and Putnam (1983) they proposed to identify and characterize 
the most toxic compounds through separation fractions and bioassays of their 
relative activity on the germination and seedling growth of cress (Lepidium 
sativum L. 'Curly') (Barnes et al., 1987).  Sequential partitioning of aqueous 
extracts against a series of solvents of increasing polarity separated the most 
active compounds in the Et2O fraction based on a cress root growth assay.  
Bioassays after thin layer chromatography (TLC) indicated two major zones of 
toxicity.  Further separation of the Et2O extract revealed two new phytotoxic 
benzoxazinones in rye.  The compounds were identified as 2,4-dihydroxy-1,4(2H)-
benzoxazin-3-one (DIBOA) and its decomposition product, 2(3H)-benzoxazolinone 
(BOA).  The pure DIBOA compound was subsequently assayed for activity on cress 
and showed reduced root and shoot length, but showed little effect on seed 
germination at these concentrations.
Shilling et al. (1985) had previously identified and implicated (-phenyl-lactic 
acid (PLA) and (-hydroxbutyric acid (HBA) in rye residue toxicity.  The relative 
activity of these compounds was compared with DIBOA and BOA (Barnes and Putnam, 
1987).  Overall, DIBOA and BOA were consistently more inhibitory (2 to 30 times) 
than PLA and HBA to germination and seedling growth of all weeds and crops 
tested.  Of the four chemicals, DIOBA was most active against the monocot 
species and BOA was most inhibitory to germination of dicot species.  On 
average, the dicots were 30% more sensitive than the monocots to all rates of 
all chemicals tested.  Chlorosis was a symptom of injury by rye residues on 
several indicators and may be related to the effects of DIBOA and BOA on 
photophosphorylation and electron transport.
Once in the soil system, the benzoxazinones produced by rye would be susceptible 
to microbial transformation by various soil microbes.  For DIBOA and BOA to be 
involved in long-term allelopathic activity, they must be sufficiently resistant 
to such microbial transformations.  Alternatively, if the parent compounds are 
metabolized, it is conceivable that biologically active metabolites may be 
involved in the overall allelopathic process.
In a following experiment, Nair et al. (1990) thoroughly mixed BOA with sterile 
and non-sterile soils and allowed them to incubate at 26 C for 10 days.  The 
methanol extract of the control soil (sterile) was pale yellow, while the 
experimental soil (non-sterile) extract was intensely orange.  Using TLC a dark 
red compound appeared in only one spot.  This compound was removed and 
recrystallized from hexane-acetone to yield orange-red needles which was further 
characterized as 2,2'-oxo-1,1'-azobenzene (AZOB), an azoperoxide, produced from 
rye.  A similar experiment was conducted starting with DIBOA and the resulting 
extractions and purifications yielded BOA, AZOB, and unreacted DIBOA.  The 
sterile soils did not produce any AZOBs, suggesting that these compounds are 
produced by soil microbes.  A similar field study using commercial BOA (which 
demonstrated identical characteristics) also produced AZOB, suggesting that the 
microbes are present in the environment.
Barnes and Putnam (1987) found dicotyledonous species to be approximately 30% 
more sensitive to BOA and DIBOA than were monocotyledonous species.  Initial 
assays, conducted with cress and barnyardgrass as indicators, indicated a high 
degree of toxicity of AZOB to radicle elongation of both species.  It appeared 
AZOB was much more toxic than either BOA or DIBOA, and therefore could 
contribute to the overall toxicity of rye residues.
In a subsequent study by Chase et al. (1991b), Acinetobacter calcoaceticus, a 
gram-negative bacterium isolated from field soils in Michigan, was found to be 
responsible for the biotransformation of BOA to AZOB.  In these transformation 
studies, soil inoculated with A. calcoaceticus indicated that the production of 
AZOB increased linearly with the concentration of BOA in sterile soil and showed 
a quadratic trend in non-sterile soils.
Gagliardo and Chilton (1992) were skeptical of the conclusions reached by Chase 
et al. (1991b) which identified A. calcoaceticus as the microbial agent 
responsible for the transformation of BOA to AZOB.  Soil microorganisms are 
known to transform substituted anilines into azo compounds (Bartha and Pramer, 
1972).  However, microbial formation of an oxygen-oxygen bond between phenols is 
uncommon.  The published structure of AZOB indicated an element of symmetry in 
the pigment leading to six carbon and four hydrogen signals of C12H8N2O2.  All 
isomeric structures with the required symmetry can be rejected based on 
published melting point and spectral data except one, o-benzoquinone azine 
reported to be the product obtained by silver(II) oxide oxidation of o-
aminophenol (Ortiz et al., 1972).  The ultraviolet-visible spectrum and high 
melting point reported for this red oxidation product is the same as that 
reported for AZOB; therefore it appeared that the red soil transformation 
product and the product of silver(II) oxide oxidation of o-aminophenol might be 
identical.
After further investigation, Gagliardo and Chilton (1992) found that the product 
of silver(II) oxidation of o-aminophenol is in fact not o-benzoquinone azine, 
but rather 2-amino-3H-phenoxazin-3-one, from which it is indistinguishable by 
TLC, mass spectral fragmentation, and UV spectra.  They also identified the red 
pigment produced from BOA by non-sterile soil as 2-amino-3H-phenoxazin-3-one.  
The [1H]NMR of aminophenoxazinone contains all of the signals reported for AZOB 
(Nair et al., 1990), with the same chemical shifts and multiplicities, but, in 
addition, contains two singlets and a broad singlet due to an NH2 not reported 
for the Michigan soil transformation product.
Additional similarities between AZOB and 2-amino-3H-phenoxazin-3-one offered by 
Gagliardo and Chilton (1992) were that non-sterile soil converts the 
allelochemical BOA into the phytotoxic pigment aminophenoxazinone, sterile soil 
does not.  The probable route of transformation of BOA is its hydrolysis to o-
aminophenol (likely requiring microorganisms such as A. calcoaceticus), followed 
by oxidation to aminophenoxazinone.  Sterile soil in contact with air is capable 
of accomplishing the subsequent oxidation of o-aminophenol into 
aminophenoxazinone.  Additional studies conducted with aminophenoxazinone 
indicated that it is an order of magnitude more phytotoxic than BOA and 
therefore has the potential for increasing the allelopathic effect of rye mulch.
A response to Gagliardo and Chilton's paper (1992) was not found in the 
literature.  In any event, it appears to be commonly accepted that the two 
cyclic hydroxamic acids (DIBOA and BOA) are responsible for the base-line 
phytotoxic activity of rye mulch and microbially transformed products of BOA can 
often dramatically enhance phytotoxic levels.

Implications of rye allelochemicals for weed management
Chase et al. (1991a) conducted a study to determine the allelopathic effects of 
rye compounds (DIBOA, BOA, and AZOB) on several plant species including garden 
cress, barnyardgrass, cucumber (Cucumis sativus L.), and snap bean (Phaseolus 
vulgaris L.).  They found that larger-seeded and deeper-seeds species were less 
sensitive to the allelochemicals.  This was likely due to the highest 
concentrations of allelochemicals being near the soil surface where small seeded 
species typically germinate.  It appears that selectivity can be achieved based 
on seed size and seed placement; the same principle that has allowed the 
selective use of synthetic herbicides.  However, the regulation and placement of 
allelochemicals in the field will be much more difficult.
Perez and Ormeno-Nunez (1991) studied the effects of rye root exudates on wild 
oats (Avena fatua L.).  They stated that while hydroxamic acids (e.g., DIBOA and 
BOA) have demonstrated allelopathic effects, the ability of a plant to exude 
them as a defensive response has not been shown.  GC and HPLC analysis of roots 
and root exudates of rye cultivars with high hydroxamic acid levels in their 
leaves, demonstrated the presence of these compounds in their roots and root 
exudates.  Bioassays employing these root exudates inhibited root growth of wild 
oats, suggesting allelopathic interference.  It was determined by Friebe et al. 
(1997) that DIBOA and BOA inhibit the plasma membrane H+-ATPase of chloroplasts 
and mitochondria.  The location of the enzyme in the plasma membrane implies 
early interactions with absorbed allelochemicals.
Perez and Ormeno-Nunez (1991) added that simply identifying roots with high 
contents of hydroxamic acids is not adequate for the selection of varieties with 
allelopathic potential.  Root exudate analysis will also be required.  It was 
reported that stress and other factors such as plant age, plant nutrition, 
light, and moisture can greatly increase root exudation (Nye and Tinker, 1977).  
The combination of these factors with high hydroxamic acid content will be 
required to select and develop allelopathically superior rye cultivars.  In a 
subsequent study (1993) Perez and Ormeno-Nunez identified the ability of rye 
(cultivar 'Forrajero-Baer') to reduce wild oat biomass by 84% and 86% compared 
to wheat and forage oats, respectively.  The main hydroxamic acid found in rye 
was DIBOA.  This compound exists in the plant as the glucoside DIBOA-glc the 
attached glucose molecule provides stability to DIBOA and prevents autotoxicity 
within the plant.  DIBOA-glc is readily hydrolyzed to DIBOA glucosidase when the 
tissue is wounded, breaking cellular membranes that separate the two compounds 
(Niemeyer, 1988).
Mwaja et al., (1995) found that rye toxicity is influenced by fertility regime 
and production environment.  The concentrations of BOA and DIBOA were highest in 
shoot tissues when rye was grown under low or moderate fertility rather than 
high fertility.  Ether extracts of dried rye shoots were also less inhibitory 
when grown under high fertility regimes.  Based on their field studies, rye 
residues and their allelochemicals can effectively control redroot pigweed for 
four to eight weeks, depending on weather conditions.
Duration of cover crop residue on the soil surface often determines the extent 
of an effective weed control period.  Yenish et al. (1995) found that 50% of the 
initial content of rye residue disappeared by 105 days after clipping.  However, 
the combined active compound concentrations of DIBOA-glc, DIBOA, and BOA 
disappeared 168 days after clipping.  Therefore, the reported duration of weed 
suppression by the rye cover crop more closely followed disappearance of 
allelochemical from rye residue than disappearance of the residue itself.
In another study by Yenish et al (1996) cover crops including rye, crimson 
clover, subterranean clover, and hairy vetch were evaluated in no-till corn to 
determine their ability to replace herbicides.  Rye consistently produced the 
largest amount of biomass among the cover crops and resulted in the highest corn 
yields.  However, weed control by cover crops alone was inconsistent or 
inadequate.  Pre-emergence herbicides were needed for adequate season-long weed 
control and overall greatest corn yield.
Rye cover crop residue has shown to be effective at reducing light transmittance 
(quality and quantity) and soil temperature which in turn can reduce or delay 
germination and emergence of certain weed species (Teasdale and Mohler, 1993).  
However, Teasdale and Mohler acknowledged that higher soil moisture under cover 
crop residue could have variable effects on weed seed germination.  During 
periods of drought, residue could maintain soil moisture at levels more 
favorable for germination than bare soil.
In recent years several weed species have developed resistance to specific 
herbicide families (Gressel et al., 1982).  In controlled studies, 
Przepiorkowski and Gorski (1994) evaluated the effects of rye residues on 
germination and growth of three triazine-resistant weed species, barnyardgrass, 
willowherb (Epilobium ciliatum Rafin), and horseweed (Conyza canadensis L.).  
Barnyardgrass seed germination was generally not influenced by rye roots and 
associated soil, which supported previous studies (Barnes et al., 1986; Shilling 
et al., 1985).  Both willowherb and horseweed seed germination were sensitive to 
rye-root soil mixtures.  However, this effect did not increase in severity with 
increasing seeding rates of rye.  Thus, it appears that after an initial 
threshold effect was obtained, additional reductions in seed germination did not 
occur with increased rye seeding rates.  The growth of barnyardgrass was 
severely reduced by rye residues but the growth of the two dicot species were 
only slightly reduced.

Conclusions
While much progress has been made to isolate and characterize the 
allelochemicals of rye and their interactions with microorganisms, the 
indisputable proof of allelopathy in these interference studies has not been 
presented (Weidenhamer, 1996).  While appropriate experimental designs and 
techniques have not yet been developed to satisfy the criteria proposed by 
Fuerst and Putnam (1983), researchers tend to agree that the primary phytotoxic 
compounds in rye are the cyclic hydroxamic acids 2,4-dihydroxy-1,4(2H)-
benzoxazin-3-one (DIBOA) and a breakdown product 2(3H)-benzoxazalinone (BOA).  
Thus, researchers have continued to study the effects of DIBOA and BOA on 
several plant species.  The mounting evidence of allelopathic interference 
exerted by rye is certainly encouraging and future manipulations of its 
phytotoxic tendencies will be greatly welcomed by individuals interested in 
alternative weed management strategies and cropping systems.


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