Allelochemicals Biologica... - Name

108
RAMANATHAN KATHIRESAN, CLIFFORD H. KOGER , KRISHNA N. REDDY
seedling development of other species. Inderjit and Dakshini (1991) reported several
phenolic compounds extracted from leachates of cogongrass foliage and roots/rhizomes
reduced germination and shoot and root length of mustard [Brassica juncea (L.)
Czern and Coss.] and tomato (Lycopersicon esculentum Mill.). Inderjit and Dakshini
(1991) also found phenolic compounds in leachates of soil collected near the
rhizosphere of cogongrass as well as up to 3 m away that were not present in control
soils. These reports suggest that allelopathic effect of cogongrass is species-specific.
Thus, allelochemicals from cogongrass may serve as potential leads in discovery of
new selective herbicides.
2.4. Plant Products for Allelopathic Control of Weeds
Presence of abundant moisture in wetlands allow faster transport of allelochemicals
from the applied plant products to the target weeds. Aquatic systems have the advantage
of no soil-interface that restricts the activity of allelochemicals on susceptible flora.
As a result, use of plant products for allelopathic control of weeds has more potential
in wetlands and aquatic systems than soil containing terrestrial systems. Carrot grass
(Parthenium hysterophorus L.), an invasive obnoxious weed originated from Mexico
has shown to be allelopathic on another introduced invasive aquatic weed water
hyacinth [Eichhornia crassipes (Mart.) Solms. Laubach] in India. Dried residues of
leaves and flower of carrot grass applied in to the water at 0.5% (w/v) killed water
hyacinth within one month. Residues of leaves of carrot grass showed the highest
biological activity followed by flowers, stem and roots. The flowers and leaves of
carrot grass also had higher total phenolic acid levels in the medium which was
responsible for the inhibition (Pandey et al., 1993). Dry powder of an epiphytic plant
Cassytha sp dissolved in water infested with water hyacinth at 1-2% (w/v) resulted in
complete desiccation of leaves of water hyacinth with in 15 days and drastic reduction
in biomass (Kauraw and Bhan, 1994). Parthenin, a sesquiterpene lactone that is one
of the major toxins in the weed, proved lethal to water lettuce (Pistia stratiotes Linn.)
and duck weed (Lemna perpusilla Torr.) at 50 ppm concentration and to water hyacinth
at 100 ppm concentration. The mechanism of allelopathic inhibition by parthenin
was identified to be associated with decline in water use, root dysfunction, excessive
leakage of solutes resulting in massive damage to cellular membranes, loss of
dehydrogenase activity in the roots and destruction of chlorophyll in the leaves (Pandey,
1996a).
Among several allelochemicals (p-hydroxybenzoic acid, anisic acid, salicylic acid,
coumaric acid, fumaric acid, tannic acid, gallic acid, chlorogenic acid, vanillic acid,
caffeic acid and ferulic acid), p-hydroxybenzoic acid had highest phytotoxicity on
aquatic weeds and was lethal at 50 ppm to all weeds tested, including floating aquatic
weeds like salvinia (Salvinia molesta Mitchell), azolla (Azolla nilotica Decne. ex
Mett.), spirodella (Spirodella polyrhiza (L.) Schieiden) and lemna (Lemna
paucicostaha Hegelm.), and submerged weeds like hydrilla (H. verticilata),
ceratophyllum (Ceratophyllum demersum L.) and najas (Najas graminea Del.).
However, p-hydroxybenzoic acid was lethal for water hyacinth and water lettuce only