Allelopathic Chemicals: Their Potential Uses for Weed Control in
Agroecosystems
Zewdu Kebede
Department of Plant Pathology and Weed Science
Colorado State University
Fort Collins, Colorado 80523
Abstract
The importance of allelopathy in nature and in agroecosystem
has attracted researcher's attention with the main goal of using
the phenomenon in biological control of weeds. currently, active
involvement of scientists from different disciplines made
allelopathy a multidisciplinary subject, and transformed the
research from basic to applied, enabling use of allelopathy in
agriculture and forestry. Screening accessions of allelopathic
crops and natural vegetation for their ability to reduce weeds is
the basic approach for utilizing the phenomenon. The active
principle isolated from coffee (Coffea arabica) by chromatography
and identified as 1,3,7-trimethyxanthnine showed complete
germination inhibition of Amaranthus spinosus and seven other
noxious weed species through amylase activity inhibition. In
other long term study weeds were reduced by 40% when winter rape
was used as an intermediate crop, and 48% when grown in spring.
It was also found post harvest cropping of cruciferous plants in
corn fields reduced weeds by upto 96%. Sweet potato reduced
yellow nutsedge dry weight to less than 10%. The most polar
serially extracted sweet potato periderm tissue was highly
inhibitory to yellow nutsedge root growth. Its hexane extract
having non polar components of the periderm tissue, inhibited
velvetleaf, prosso millet, black nightshade, and red root pigweed
germination. Its ethyl acetate fraction was inhibitory (in
addition to the abov weeds) to goosegrass, tall morningglory,and
coffee senna. Medicarpin (3-hydroxy-9-metoxypterocarpan) is a
compound produced by matured alfalfa that is an autotoxic to its
seedlings and other nearby plants, thus gaining a competitive
advantage. From a holistic poit of view, research potential and
use of allelopathyin an agroecosystem is wide. The richness of
agricultural techniques, crop rotation, cover cropping, and
related practices allow researchers to evaluate and make use of
allelopathic chemicals for weed management in agricultural
systems.
Introduction
Allelopathy, the chemical mechanism of plant interference,
is characterized by a reduction in plant emergence or growth,
reducing their performance in the association. The term
allelopathy was originated by Molisch (1937) to refer to
reciprocal suffering of two organisms. The term has since
appeared commonly in literature to plant/plant biochemical
interactions that causes detrimental effects, and it has now been
recognized as an important ecological factor in plant
interactions(Chou, 1990). According to Putnam (1988), Chemicals
with allelopathic potential are present (commonly in conjugated
form) in almost all plants and in many tissues, like leaves,
stems, flowers, fruits, seeds and roots. Under specific
conditions, these chemicals are released in to the environment
(atmosphere or rhizosphere) by means of volatilization, leaching,
decomposition of residues, and root exudation (Chou, 1990), in
ample quantities and long persistence to affect a neighboring or
successional plant. These processes are also affected by
environmental complex, and are not easy to single them out
(Einhellig, 1987; Rice, 1987). The science of allelopathy has
progressed its descriptive and foundations to provide a base to
aid crop production.
Nowadays, allelopathy is concerned in research involving
sustainable agriculture, also referred as organic, low input,
biodynamic or resource conserving. Allelopathy has been used in
agricultural practices, such as weed control, intercroppings,
nutrient recycling, and low-external input farming practices
(Chou,1986; Fisher, 1987; Rizvi and Rizvi, 1987). The
allelopathic plant products are known to offer a vast array of
secondary compounds which have the potential to be used directly
as herbicide substitutes or as structural leads for new synthetic
herbicides. Many of the early investigations into allelopathy
were a result of crop phytotoxicity problems observed in
agriculture (Putnam etal., 1990).
The number of options to be considered in discovery and
development of a natural product as a herbicide is known to be
larger than for a synthetic herbicide. Furthermore, molecular
complexity, limited environmental stability and low herbicidal
activity of many natural products are not encouraging (Duke and
Lydon, 1993).
Need and Rationale for Development of Allelopathic Chemicals
Total crop losses from weed interference are estimated at
$9-10 billion annually in USA (Chandler, 1985; Putnam and Weston,
1986). Weed researchers have commonly interpreted this loss to be
from weed competition. However, few researchers have found
allelopathic potentials in plant performances after residue
amendments and/or bioassay using extracts, leachates, or exudates
(Einhelling and Leather, 1988).
As far as agricultural business is required to continue, the
need for new herbicides is inevitable. Currently, about two-
third, by volume, of the pesticides used in agricultural
production are herbicides (Duke and Lydon, 1993). The potential
for undesirable environmental contamination from herbicides is
relatively high, and there is a need for environmentally safe
herbicides that are equally or more effective and selective than
currently available synthetic herbicides. There is a strong
feeling that allelopathic research can be applied to so many
current weed problems (Putnam, et al., 1983) and nutrient
recycling (Alsaadawi, 1987). The present emphasis on reduced- or
no-tillage agriculture will depend on herbicides for weed
control. On the other hand, the increasing incidence of herbicide
resistance is creating a demand for new herbicides with
unexploited mechanism of action. Thus, the need for new
herbicides becomes obvious to solve the dilemma of the continued
demand for herbicides while older herbicides are removed from the
production fields for environmental, toxicological or economical
purposes. Natural plant compounds released by crops, weeds, or
their residues may offer solutions to some of these needs
(Putnam, 1988).
Studies on the allelopathic effects of weed species on crop
plants needs to be designed to understand the causative factors
responsible for the strong interference of some weeds with
various crops in agricultural lands and with weeds under natural
settings, which may be applied for weed management.
The Role of Allelopathy in Weed Control
The recognized importance of allelopathy in agricultural
practices has been increased with the main objectives of using
this phenomenon in biological control of weeds (Rice, 1984). One
approach to utilize this phenomenon is suggested to screen
accessions of allelopathic crops for their ability to reduce
weeds, and a few crops have been evaluated in this aspect
(Leather, 1982). Sorghum (Sorghum bicolor) is known to provide a
good weed killing capacity (Putnam, et al., 1983) and others
showed that it is also autotoxic and should be rotated with other
crops for maximum yield (Leon, 1976). Alsaadawi et al., (1985)
conducted a screening experiment to examine the activity of
sorghum root exudates of 100 cultivars to inhibit germination and
seedling development of Pigweed (Amaranthus retroflexus) in a
sand culture medium. A high variability was observed in the
ability of the test cultivars to alter seed germination and/or
seedling growth of the weed. They found 82% of the control
reduction in seed germination in 25 cultivars. They also found 10
cultivars inhibiting A. retroflexus growth by more than 79% of
control. Upon collection and characterization of root exudates
from one of the most phytotoxic cultivars, the researchers found
that neutral fraction of the exudates was more inhibitory than
the acidic and basic fractions under field pH condition. In
addition, cultivars which were found non-toxic through root
exudation, showed considerable toxicity through aqueous extracts
and decaying materials. Thus, it can be suggested that some
sorghum cultivars have a potential inhibitory activity against
weeds and requirement for commercial herbicides could
significantly be reduced.
Einhelling and Leather (1988) used sorghum and soybean
Glycin max seedling bioassay to demonstrate inhibition from
Kochia Kochia scopria L., Jerusalem artichoke Helianthus
tuberosus L., Cocklebur Xanthium strumarium L. velvetleaf, giant
ragweed Ambrosia trifida L., and curly dock Rumex crispus L.
The inhibitory compounds occurring in sorghum plants are
mostly phenols (Guenzi and McCalla, 1966), and the mechanism of
action of some of the phenolic compounds such as syrigic,
caffeic, and protocatechuic acids are tested on cowpea (Vigna
sinensis) seedlings using sand culture medium. Seedling growth,
chlorophyll a and b , total chlorophyll, chlorophyll a/b ratio,
and the uptake of N, P, K, Fe and Mo were significantly reduced
(Alsaadawi, 1992). He also found that reduction in dry weight,
chlorophyll content, and ion uptake in cowpea were highly
correlated.
In Taiwan, Chou (1990) found that rice (Oryza sativa)
planted twice a year in a monoculturte system reduced the second
crop yield by about 25% in areas of poor water drainage. During
the fallow period, rice stubble is left in the field after
harvesting, and incorporated into the soil for decomposition.
During decomposition, a great quantity of phytotoxins were
produced resulting in the suppression of rice growth and
subsequent grain yields. Chou and Chiou (1979) found aqueous of
paddy soil collected and bioassayed to be phytotoxic. In pot
experiments, Chou (1993) also found rice seedlings growing poorly
in the decomposed straw and soil mixture. He found dark brown
roots with abnormal and enlarged cells in the retarded plants.
Furthermore, he observed an increase in phytotoxicity with
increased straw level, and the toxicity of aqueous extracts of
decomposing rice residue was still persistent after 16 weeks of
decomposition. He also extracted the rice straw and soil mixture
with ethanol, purified, and identified the phytotoxic compounds
to be p-hydroxybenzoic, syringic, vanillic, ferulic, acetic,
o-hydroxyphenylacetic, propionic, and butryric acids. In his
previous work, Chou (1983) found that most of these compounds to
be autotoxic to rice. This idea offers a clue to make bioassay in
weed species to test range of phytotoxicity and their potential
use as herbicides.
Putnam et al.(1990) have drilled rye into field plots during
early October, over-wintered and killed with glyphosate (N-
phosphonomethyl glycine) when it attained about 5metric tons/ha,
and evaluated its weed control capacity in vegetable and fruit
cropping systems over a 10-year period. Their experiments
indicated that living rye has a strong interference ability
against weeds (Table 1), providing excellent weed control prior
to annual crop establishment or provides soil cover during the
dormant period in perennial crops.
Table 1. Effect of spring -planted living ryecover crop on
early season biomass/sq m of large crabgrass, common ragweed
and common lambsquarters in spinks loamy sand
They also found rye residue providing an excellent weed
control for a 30-60 day period (Table 2). This duration of weed
control has been found adequate to provide many annual crops a
chance to occupy the space and to form a canopy to shade-out
weeds for the remainder of the season. According to Einhelling
and Leather, (1988), the high biomass production and toxicity of
rye make it a particularly effective weed suppressant cereal.
After cover crop kill, Worsham (1991) emphasized use of selective
postemergence herbicides as needed, especially for grasses and
perennial weeds.
Table 2. Weed suppression with surface residues
of rye compared with populus wood shavings
The most active allelochemicals from rye residues were found
to be hydroxamic acids which occur as glycosides in the living
plants (Barnes, et al. 1987). The glucosyl conjugate of 2,4-
dihydroxy-1,4(2H)-benzoxazin-3-one (GDIBOA) forms the aglycone
(DIBOA) upon injury or death of the rye plant (Fig. 1). DIBOA
degrades to another active compound, 2(3H)-benzoxazalinone (BOA)
(Figure 1). Both compounds are Known to have strong inhibitory
action on germinating dicotyledons weed seedlings (Barnes and
Putnam, 1983; Barnes and Putnam, 1986), and Some monocot weeds
(Barnes, et al., 1983) (Figure 2).
Fig. 1. Hydrolysis of GDIBOA Fig. 2. Response of
to form the aglycon (DIBOA) barnyardgrass to various
and rearanged BOA.(Barnes,"83) conc.of DIBOA and BOA.
BOA could be transferred by soil microbes to a more active
compound, 2,2'-oxo-1,'-azobenzene (AZOB) through rearrangement
and combining of two BOA molecules. Putnam et al. (1990) found
AZOB to be 7 to 10 times more active than DIBOA on most weed
species tested. Their research result leads to the idea that both
plant and microbial compounds contribute to the allelopathic
response.
Concerning cover crop and residue management, many
researchers reported that cover crops of wheat, barley, sorghum
and sudangrass have been used effectively to suppress range of
weed species (Barnes and Putnam, 1983; Putnam and DeFrank, 1983;
Putnam et al., 1983; Shilling et al., 1985). Barnes et al. (1986)
reported that weed biomass in a cover crop of living rye was
reduced 90% over unplanted controls and even a mulch of 40-day-
old spring-planted rye gave 69% reduction. Soyabeans and
sunflowers planted without tillage into desiccated rye mulch gave
over 90% reduction in the biomass of common lambsquarter
Chenopodium album L., redroot pigweed Amaranthus retroflexus L.
and common ragweed Ambrosia artemisiifolia L. compared to tillage
and no rye.
Pegeon pea Cajanus cajan and velvet bean Mucuna deeringiana
reduced weeds to the level where farmers do not weed at all
(Hepperly et al. 1992). These legumes have even controlled purple
nutsedge Cyperus rotundus, the most damaging weed world wide.
Hegde and Miller (1990) verified long term
allelopathy/autotoxicity of alfalfa by comparing germination and
growth of alfalfa and sorghum. Both effects were implicated in
the growth inhibition of the two crops.
Alsaadawi et al. (1986) found that sorghum plants from seeds
exposed to gama rays have more inhibitory activity in their root
exudates, aqueous extracts, and decaying residues against growth
of A. retroflexus. Regarding genome enhancement, the possibility
for exploiting allelopathy include genetic manipulation of crops
to increase their capability for weed control. In this respect,
Fay and Duke (1977) found accessions of oats exuding three times
as much scopoletin as a common cultivar. Putnam and Duke (1974
also found differences in toxicity among 500 cucumber accessions,
and one allelopathic accession reduced total weed fresh weight to
about 1/3 of the weed biomass found with pioneer, commonly grown
cultivar. This approach may increase the allelopathic potential
of some crops withought causing genetic damages. Einhelling and
Leather (1988) suggested that an alternative to genome changes is
to develop chemicals to manipulate crop metabolism to achieve an
increase in allelophatic chemical production.
Natural Plant-Derived Compounds with Herbicidal Potential
Most of the natural products that cause allelopathy are a
subset of the array of secondary compounds synthesized by plants
and microorganisms, and most of the currently identified
compounds are products of the shikimic acid and acetate pathway
(Rice, 1984). The common ones include phenolics like cinnamic and
benzoic acids, coumarines, tannins, and flavonoids; terpenoids;
and a few alkaloids, steroids, and quinones (Einhelling and
Leather, 1988).
Although most of the simple phenolic acids and flavonoids
are known to be allelochemicals, they seem to be weekly
phytotoxic in soil and have little selectivity. Many of the
phenolic compounds ( e.g. salicylic acid and p-hydroxybenzoic
acid), at very high rates (56 to 112 kg/ha), are effective
against weeds and are relatively non-selective (Duke and Lydon,
1993). However, the authors suggest that synthetic modification
of these compounds increase their efficacy and selectivity. In
this respect, the halogenated benzoic acid herbicides like
dicamba, chloramben, and picloram are derived from benzoic acid,
a phenolic plant product. Phenolic quinones such as juglone and
lawsone are among the more phytotoxic phenolic compounds (Spencer
et al., 1986). Phenolic derivatives, such as the dihydroquinone
sorgoleone, produced by Sorghum bicolor, are extremely phytotoxic
in hydroponic culture(Einhellig and Souza, 1992). Synthetic
derivatives of coumarines have been known to be good herbicides.
Other aromatic compounds, such as DIBOA, are as active in
reducing plant growth as many herbicides (Barnes et al., 1986).
Most phenolic compounds are unlikely to significantly affect
weed growth singly under natural conditions. Because of this,
some investigators have proven that synergism between these weak
phytotoxins produces a significantly phytotoxic combination by
additive effects (Duke and Lydon, 1993).
Phytotoxity in crop-weed competition also involves
terpenoids. For example, the monoterpene 1,8-cineole is strongly
phytotoxic, its derivative cinmethylin has been seriously
considered for herbicide development (Grayson et al. 1987).
Alkaloids like colchicine and vinblastine, and terpenoids like
taxol that stop plant growth by interference with mitosis have
modes of action identical to certain synthetic herbicides
(Vaughan and Vaughn, 1988).
In a Leaucaena leucosephla plantation, Chou (1990) found an
almost total lack of understory after 3 to 4 years of growth,
except its own seedlings. He stated that the absence of weeds is
because of a heavy accumulation of Leaucaena plant residues, such
as leaves, and branches. Aqueous extracts of fresh leaves, seeds,
litter, and soil showed significant phytotoxic effects on many
test species, but Leucaena. Decomposing leaves also suppressed
growth of plants in pots. He also identified ten phytotoxins
using paper and thin-layer chromatography, uv-visible
spectrophotometry, and high performance liquid chromatography.
The chemicals included mimosine, quercetin, and gallic
protocatechuic, p-hydroxybenzoic, p-hydroxyphenylacetic,
vanillic, caffeic, and p-coumaric acids.
Chou (1990) concluded that the exclusion of understory
plants is because of the allelopathic effect of Leucaena, where
he observed the pattern most clearly in the area with a heavy
accumulation of its leaves during a year with drought and heavy
winds.
Abdul-Rahman and Habib, (1986) investigated phytotoxicity of
extracts of bermudagrass Cynodon dactylon, Johnson grass Sorghum
halepense and thumble pigweed Amaranthus albus against doddor
Cuscuta campestris on alfalfa. Herbicides like glyphosate,
dimethyltetrachloroterephthlate (DCPA), and metribuzine were also
included for comparison. In their experiment, they found that the
test concentrations of all weeds controlled dodder plant
significantly, and the percent of control increased with increase
in extract concentration (Table 3), while the increase in percent
of control was parallel with the increase of alfalfa growth. The
potential of weed extracts in controlling dodder was the same
with DCPA and glyphosate. In most tests, the weed extracts
exhibited less alfalfa injury than the other test herbicides. It
was also found that the weed extracts contained several phenolic
compounds, and apparently bermudagrass contained the highest of
all total phenols.
Habib and Abdul-Rahman (1988) tested phytotoxic potential of
the above weeds plus wall goosefoot weed Chenopodium murale in
controlling the dodder parasite under lathhouse and field
conditions. Their extracts of all weeds significantly increased
control of dodder parasite, while bermudagrass and wall goosefoot
weed showed the highest ability to control the dodder plant.
Analysis of total phenols indicated that the higher toxicity of
bermudagrass was correlated with a higher amount of phenols it
produces. The researchers have isolated phenolic compounds which
have been recognized as major phytotoxins in allelopathic plant
species.
Table 3. Weedicidal spectrum 1,3,7-trimethylxanthine
isolated from seeds of coffee (Cofea arabica)
(Rizvi et al. 1981).
In another experiment, Abdul-Rahman and Al-Naib(1986) found
that aqueous extracts and root exudates of bermudagrass
significantly inhibited germination and growth of cotton and the
weeds prosopis Lagongchium farctum, Johnson grass and cocklebur
Xanthium strumarium which also in the cotton field. In the
chemical analysis, they have found numerous plant germination and
growth inhibitory phenolic compounds.
Coffee Coffee arabica L. and tea Camellia sinensis
(L.)O.Kuntze are known for toxic alkaloid production and their
possible allelopathic or autotoxic influences (Rizvi et al.,1981;
Rizvi et al., 1987; Suzuki and Waller, 1987; Waller et al.,
1986). High concentration of 1, 3, 7-trimethylxanthine (1,3,7-T)
or caffeine have been isolated from soils around coffee trees.
They also found related purine alkaloids and fatty acids which
demonstrated allelopathic activity. Rizvi et al. (1981) proposed
that since caffeine exerted differential action on several plant
species, it might be a useful selective herbicide. Rizvi and
Rizvi (1992) also found reduction in amylase activity in
germinating seeds of Amaranthus spinosa after treating with
1,3,7-T (an alkaloid). They also found that the alkaloid
inhibited seven other noxious weeds (Table 4) at various
concentrations.
Table 4. Dodder control and alfalfa injury and
dry weightas affected by different herbicidal
and plant extract treatments (Rizvi and Rizvi, 1992).
Ethylene is released by many plant tissues and ripening
fruits, and used agriculturally to stimulate witchweed Striga
asiatica seed germination. This gas at 1.5 kg/ha is being used
effectively as a soil injection to trigger "suicidal" germination
of the parasitic weed to deplete the seed reserve in the soil
(Epley, 1975). According to Putnam (1988), a germination
stimulant from a natural host (sorghum) for Striga was recently
identified as p-benzoquinone compound in both an active and
inactive forms.
Chen et al. (1991) compared Artemisinin, an allelochemical
isolated from Artemisia annua, with commonly used herbicides,
2,4-D and glyphosate in mung bean Phaseolus aureus seedlings.
They found that artemisinin had the same level of growth
inhibition as glyphosate. Duke et al., (1987) also suggested that
the selectivity of artemisinin and its potency ( Chen and
Leather, 1990) would be additional advantages to consider the
chemical as a potential natural herbicide.
In general, allelopathic chemical groups useful or
potentially useful in agriculture include flavonoids (tricin,
kaempferol, and quercetin), polyacetylenes, quinones (juglonee),
and terpenes (mono- and sesquiterpenes) (Putnam, 1988). Many of
these compounds and their derivatives are believed to serve as
models for new herbicide or growth hormone development.
complementarity of allelochemicals and herbicides can also
be exploited as means of weed control. An herbicide applied along
with allelopathic conditions could have supportive action,
affecting the same or different weed species. A reduced level of
herbicide may be feasible to provide weed control when it
operates simultaneous with allelopathic conditions. Einheling and
Leather, (1988) suggested that manipulation of timing,
formulation and application rate of a herbicide used to kill or
supplement a cover crop can enhance phytotoxicity, improving weed
control.
Commercialization of allelophatic chemicals
High demands for environmentally safer herbicides and growth
regulators lead to development of natural products for these
purposes. Moreover, many crop production systems are nowadays
facing an increase in herbicide-resistant weed biotypes as well
as changes in the composition of weed species more closely
related to the crops they infest. These factors plus the
increasing cost and difficulty of developing synthetic new
herbicides indicate that new and safe weed control strategies
need to emerge (Duke 1986). With this regard, some plant
compounds have served as structural templates for herbicides.
According to Eihelling and Leather (1988) many phenoxy herbicides
are auxin analogs, while benzoic acid compounds are frequently
known in allelopathy, and the herbicidal utility of their
halogenated derivatives (TBA, TIBA, dicamba, etc) is practical.
Several weeds were effectively suppressed when sprayed with the
common allelochemicals salicilic acid, P-hydroxybenzoic acid,
hydroquinone, and umbelliferone (Shettle and Balke, 1983).
Cinmethylin (by Shell Co.) is a cineole alcohol with the
addition of a substituted benzil group, which represents a new
class of herbicide in the market. It controls many annual grasses
and some broadleaf weeds, at comparable rate with alachlor
herbicide (Einhellig and Leather, 1988) .
Conclusion
Ample information is available and agricultural production
needs are causes for exploiting allelopathy to benefit production
systems. Current farming is in a transition of reduction of
tillage and less chemical use. Reducing tillage restricts weed
seeds to poor germination sites and by utilizing natural
phytotoxins leaching from plant residues, the germination of
seeds and growth of many weeds can be inhibited. There are many
crop species known for their allelopathic potential. Herbicides
can be used to supplement cover crops in reduced tillage
practices.
Crop plant breeding for genetic manipulation and
allelopathic potential against weeds is believed to solve, in
part, weed problems. Use of proper type and amount of crop and
weed mulch should be considered for weed management. Residue
management, crop rotation, timing of operations and proper
agronomic practices needs to be identified for specific areas of
production to make use of allelopathic conditions.
Natural plant products may provide clues to new and safe
herbicide chemistry. Thus, modifying these natural products could
give more active, selective and persistent herbicides. Promising
results have been shown by selecting for allelopathic crops and
including them in the rotation.
New target sites of action can be exploited for natural
phytotoxins. Thus, many of the allelochemicals have potential as
herbicides or as templates for new herbicide classes.
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