The sesquiterpene lactone parthenin, one of the major toxins in an obn
oxious weed, parthenium (Parthenium hysterophorus L.), was toxic at 50
ppm to the floating aquatic weeds pistia (Pistia stratiotes L.) and l
emna (Lemna pausicostata Hegelm.) and at 100 ppm to water hyacinth (Ei
chhornia crassipes Mart Solmns.), salvinia (Salvinia molesta Mitchell)
, azolla (Azolla nilotica Decne.), and spirodella (Spirodella polyrhiz
a L. Schleid). The lethal dose for the submerged weeds najas (Najas gr
aminea Del.), ceratophyllun (Ceratophyllum demersun L.), and hydrilla
(Hydrilla verticillata L. f. Royle) was 25 ppm. The submerged aquatic
weeds were more sensitive to parthenin. Water hyacinth was used as a r
epresentative for studying the phytotoxicity of pathenin on aquatic we
eds. Inhibition of water hyacinth by parthenin was associated with dec
line in water use, root dysfunction, excessive leakage of solutes from
roots indicative of massive damage to cellular membranes, loss of deh
ydrogenase activity in the roots, and loss of chlorophyll in the leave
s. Plant death occurred in a period of one to two weeks. Parthenin phy
totoxicity is gradually lost in an aquatic environment as a lethal dos
e became nonlethal in about 30 days under outdoor conditions. Possible
buildup of a toxin concentration may affect population dynamics and a
shift in the aquatic weed flora in the immediate area of parthenium s
tands. Accumulation of the toxin in an aquatic environment, however, a
t a level sufficient to produce such changes in a natural ecosystem as
a consequence of rain washing parthenium plants and leaching of toxin
from their residue appears to be unlikely.