2. What
is allelopathy?
When?
Who ?
Which are the main allelochemicals?
Site of production?
Mechanism of action?
Constraints in using allelopathy as a weed
management tool?
Practical applicability of allelopathy in weed
management?
3. Allelopathy
refers to all biochemical
interactions,stimulatory or
inhibitory,among plants including
m.org(Molisch,1937)
Detrimental
effect of chem.or exudates
produced by one living plant sp.on
germn,growth&dev.of other plant or
m.org sharing same habitat.
(Evenari,1949)
4.
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
plant.(Rice,1974)
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
5. from the Latin words allelon ‘of
each other’ and pathos ‘to suffer’, refers to the
chemical inhibition of one species by another.
Allelopathy,
The
source of allelochemicals in agricultural
fields may be the weeds, crops or
microorganisms.
7. Many
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:
Lucerne
Rice
Sorghu
m
Buckwh
eat
Red
clover
sweet
clover
Barley
Wheat
• Trifoli
8. Varieties
There
can be a great deal of difference in
the strength of allelopathic effects between
different crop varieties.
Specificity
A crop which is strongly allelopathic
against one weed may show little or no
effect against another.
9. Autotoxicity
Allelopathic
chemicals may suppress same
sp.
Lucerne is particularly known for this kind
of autotoxicity.
The toxic effect of wheat straw on following
wheat crops is also well known.
11. Environmental
factors
Several factors impact on the strength of the
allelopathic effect, especially soil fertility.
Low fertility increases the production of
allelochemicals.
After incorporation the alleopathic effect
declines fastest in warm wet conditions and
slowest in cold wet conditions.
12. There are two types:
1) True type - the release into the environment of
compounds that are toxic in the form in which they
are produced.
2) Functional type - the release into the environment of
a substance that is toxic as the result of
transformation by micro-organisms
13. • Wheat,
• alfalfa,
• cowpea
Auto
Allo
allelopathy
• Lentil
residues
on wheat
allelopathy
Residual
• Maize – on
chenopodium
album,
• Sorghum on
abutylon
• theophrasti
Concurrent
/direct
allelopathy
• Instantane
ous direct
effect
• sorghum
21. Volatalization
Arid
and semi arid environment
Mostly terpinoid group.
Released from special glands on stems or
leaves
Susceptible plants absorbs through cuticle
directly from air or adsorbed on dry soil and
taken up.
22. Leaching
Through
aqueous solutions (rain,dew)
Mature leaves are more susceptible
Exudation
Metabolites
exuded from roots to surrounding
atm.
Pottential source of allelopathic effect
25. I
N
H
I
B
I
T
Celldivision
and elongation
Gibberllin or IAA(growth hormones)
Mineral uptake
Nitrification (nitrosomonas-furilic acid)
Respiration
Stomatal opening
Protein synthesis and org.acid metabolism
Specific enzymatic activities.
Retardation of photosynthesis
27. Rye, some varieties of barely, oats as well as buckwheat are
effective inhibiting the growth habit of a large number of weed
species.
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
density
28. •
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.
29. •
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.
30. Dry
leaf leachates of female plants of
cannabis sativa caused maximum reduction in
bio.activities of Parth.hysterophores.
Aqueous
leachate
concn(%)
Germination
Shoot fresh wt(g)
Fresh leaf
leachate
Dry leaf
leachate
Fresh leaf
leachate
Dry leaf
leachate
Control
87(0.0)
89(0.0)
2.50(0.0)
2.32(0.0)
50
80(8.1)
68(23.6)
1.96(21.6)
1.65(28.9)
100
76(12.6)
45(49.4)
1.89(24.4)
1.10(52.66)
(Sing and thapar,2003)
31. 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
germination.
32. 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 .
.
33. 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
not available.
35. Where
do we stand now - Research?
Research is ongoing to identify allelopathic
effects and to identify genes responsible for
allelopathy.
This should lead, in time, to
recommendations for using allelopathy in
weed management and to breeding of new
varieties.
36. As
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
herbicide”
37. Weed
science,basics and applications
(T K das)
LA Weston - Agronomy Journal, 1996 crops.org
www.organicweeds.org.in
