Allelochemicals Biologica... - Name

ALLELOPATHY FOR WEED CONTROL 115
and to detect the sensitivity of the target weed to minute quantities of allelopathic
substances, incised plant parts of aquatic weeds would serve better than whole plants.
For screening of different allelopathic substances for inhibition of water hyacinth, a
specific bio-assay method was designed with cut leaves of water hyacinth (Kathiresan,
2000; Kannan, 2002). This bioassay involved exposure of cut leaves of water hyacinth
to graded doses of plant materials to be tested (dissolved in water). The leaves of
water hyacinth plants (with healthy leaves submerged in water) were detached by
cutting the petiole with a razor blade with care to retain the incision point below
water level. The detached portions of leaf with a part of petiole intact was kept
submerged in water for 90 seconds to ensure that no air was trapped internally. Then
these leaves were transferred to scintillation vials with water where in different plant
products were dissolved and individually compared with an untreated control. The
percent fresh weight reduction of the cut leaves was calculated using the formula
Initial weight of the cut leaves – weight after 24 hr of treatment
X 100
Initial weight of cut leaves
5.2. Dose Response Studies
Even if a plant product proves appreciably allelopathic on aquatic weeds in bio-assay,
tracing the pattern of allelopathic inhibition with graded doses of the plant product
becomes vital as aquatic systems requires enormous quantities of plant product due to
the quantum of water body (dilution), which in many cases is not practically feasible.
Furthermore, this dose response has to be plotted for differing morpho-physiological
states of the weed that occurs in common, as the quantity of allelochemical required
for a knock down effect is less with a small statured weed compared to that of larger
sized weed. For this purpose, before screening of allelopathic plant products on water
hyacinth, the latter was classified in to different morphological stages (large, medium
and small) prevalent in the state of Tamilnadu, India using discriminant analysis
(Kannan and Kathiresan, 1998). Both whole plant and cut leaf bio-assays were done
on each of the three stages of water hyacinth. Higher (10 to 30 g L -1 ) doses were used
in whole plants to impart a near lethal effect on the whole plant and lower doses were
used in cut leaf bio-assays to cause some allelopathic injury, if not lethal. The doses
used for water hyacinth cut leaf bio-assay were 30, 25, 20, 15 10, 5 2.5, 1, 0.5, 0.25,
and 0.1 g L -1 . The dose response data revealed that cut leaf-bioassay was superior to
whole plant assay. For example, C. amboinicus at as low as 0.1 g L -1 dose caused 24%
fresh weight reduction in water hyacinth within a week of exposure (Kathiresan,
2000). Therefore, cut leaf bio-assay could be useful to detect allelopathic potential of
plant products which otherwise may have been missed if whole plant assay was used.
In contrast, the dose response study data revealed that lethal doses for large, medium,
and small plants of water hyacinth were relatively closer. C. amboinicus at low (10 g
L -1 ) dose caused death of the water hyacinth after 20 days whereas at high (25 g L -1 )
dose caused death within one week.