Which are the main allelochemicals?
Site of production?
Mechanism of action?
Constraints in using allelopathy as a weed
Practical applicability of allelopathy in weed
refers to all biochemical
inhibitory,among plants including
effect of chem.or exudates
produced by one living plant sp.on
germn,growth&dev.of other plant or
m.org sharing same habitat.
The term allelopathy refers to any direct or
indirect inhibitory effect by the production,
of chemicals by a plant which can influence
the growth and development of another
For weed management we are interested in
the inhibition of one plant (the weed or
weeds) by another (usually the crop or weed)
through the production of allelochemicals
from the Latin words allelon ‘of
each other’ and pathos ‘to suffer’, refers to the
chemical inhibition of one species by another.
source of allelochemicals in agricultural
fields may be the weeds, crops or
Elroy L. Rice
crops have been reported as showing allelopathic
properties at one time or another …
some crops such as oats seem to clean fields of weeds
better than others. The list also includes:
can be a great deal of difference in
the strength of allelopathic effects between
different crop varieties.
A crop which is strongly allelopathic
against one weed may show little or no
effect against another.
chemicals may suppress same
Lucerne is particularly known for this kind
The toxic effect of wheat straw on following
wheat crops is also well known.
Several factors impact on the strength of the
allelopathic effect, especially soil fertility.
Low fertility increases the production of
After incorporation the alleopathic effect
declines fastest in warm wet conditions and
slowest in cold wet conditions.
There are two types:
1) True type - the release into the environment of
compounds that are toxic in the form in which they
2) Functional type - the release into the environment of
a substance that is toxic as the result of
transformation by micro-organisms
• Maize – on
• Sorghum on
Forms of allelopathic
against against against against
Source of inhibitors
Shoots and foliages
Shoots and foliages
Effect of several waste land weeds on parthenium suppression
Extent of suppresssion (%)
and semi arid environment
Mostly terpinoid group.
Released from special glands on stems or
Susceptible plants absorbs through cuticle
directly from air or adsorbed on dry soil and
aqueous solutions (rain,dew)
Mature leaves are more susceptible
exuded from roots to surrounding
Pottential source of allelopathic effect
By weathering and micro organisms.
Gibberllin or IAA(growth hormones)
Nitrification (nitrosomonas-furilic acid)
Protein synthesis and org.acid metabolism
Specific enzymatic activities.
Retardation of photosynthesis
How allelochemicals introduced into plant??
Rye, some varieties of barely, oats as well as buckwheat are
effective inhibiting the growth habit of a large number of weed
Root excudates of wheat and oats contained phenolic acid which
has more detrimental effects on wild mustard.
Sunflower crop inhibit the growth of certain weeds.
Cucumber which strongly inhibit the growth of wild mustard
Root inhibition of rye grass weed is influenced by wheat seed
The leaf litter and root exudates of some Eucalyptus species are
allelopathic for certain soil microbes and plant species.
The tree of heaven , Ailanthus altissima, produces allelochemicals
in its roots that inhibit the growth of many plants.
Rice allelopathy depends on variety and origin: Japonica rice is
more allelopathic than Indica and Japonica-Indica hybrid. More
recently, critical review on rice allelopathy and the possibility for
weed management reported that allelopathic characteristics in rice
are quantitatively inherited and several allelopathy-involved traits
have been identified.
Continuous cropping of legume crop alfaalfa has been created to
auto toxicity in low land weed.
Corn gluten meal (CGM) is a natural preemergence
weed control used in turfgrass, which reduces
germination of many broadleaf and grass weeds.
Garlic mustard is an invasive plant species.Its success
may be partly due to its excretion of an unidentified
allelochemical that interferes with mutualisms between
native tree roots and their mycorrhizal fungi.
The black walnut (Juglans nigra) produces the
allelochemical juglone, which affects some species.
leaf leachates of female plants of
cannabis sativa caused maximum reduction in
bio.activities of Parth.hysterophores.
Shoot fresh wt(g)
(Sing and thapar,2003)
Reduction in fresh wt of shoots
• Reduced biosynthesis of photosynthates
Reduced seed germination
• High amount of all.chem present in dried leaf
extract which respiration and metabolic process
responsible for biosynth. Of PROTEASE,
amylase,IAA and there by inhibit seed
1.Difficulty in exudate collection
2.Poor knowledge of conditions under which
exudates are released
3.The exact concn in which allelochemicals
are released is unknown.
4.Difficulty to identify which weed is
responsible for the observed allelopathy.
5.Presence of autoallelopathy.
6.Lab to field gap .
6. Information about which crops are effective
against which weeds is limited
7. Information about which are the most
allelopathic varieties of a particular crop is
Sources of plant and microbial phytotoxins with promising herbicidal
P -hydrobenzoic acid
do we stand now - Research?
Research is ongoing to identify allelopathic
effects and to identify genes responsible for
This should lead, in time, to
recommendations for using allelopathy in
weed management and to breeding of new
outlined in the previous discussions there
are many potential problems with
attempting to use allelopathy as a practical
tool still if we overcome them to some
extend allalopathy is the best “Natural
science,basics and applications
(T K das)
LA Weston - Agronomy Journal, 1996 crops.org