Hydrilla (Hydrilla verticillata Royle) is an exotic aquatic angiosperm which has become a problematic weed throughout the southern United States. Infestation by aquatic weeds like hydrilla increase loss of valuable water through evapotranspiration, choke waterways used for transportation, depress real estate values, and may present health hazards. Control strategies including herbicide treatment and mechanical harvesting have proven costly and of short term benefit. One reason for this is the vegetative dormant buds which allow the plant to overwinter and reestablish rapidly after attempts at control. This research focuses on elucidation of the bud formation process in hydrilla to provide the basis for development of a control strategy which combines use of herbicides with disruption of bud formation and hence the capacity for regrowth.
The environmental and hormonal control of vegetative dormant bud formation was studied in the laboratory using algal-free cultures of hydrilla. Bud formation is stimulated by a photoperiod of less than 12 hours. This environmental stimulus appears to be sensed by the phytochrome system and hormonally induced by abscisic acid. Field sampling at Lake Conroe, Texas, confirmed, where possible, laboratory results and indicated that artificially prolonged photoperiods prevent bud formation.
Ethylene, both in laboratory and greenhouse cultures, significantly reduced bud formation at a dose rate of 0.1 mg/l every two days. Coumarin significantly reduced bud formation when applied at 10('-6) M but information in the literature indicates that the mode of action of coumarin may be induced ethylene formation in the plant (Morgan and Cowell, 1970).
Growth characteristics of hydrilla make it suitable for use in a bioassay for the assessment of the relative phytotoxicity of aquatic pollutants. Algae-free cultures of hydrilla were grown in the laboratory in 10% Hoagland's medium. Evaluation of chemical toxicity was obtained starting from two different life stages of the plant: dormant buds and apical fragments. Dormant bud production was also monitored in order to evaluate the potential for life cycle and chronic studies.
Hydrilla appeared more sensitive to ammonium carbamate than were duckweeds and more sensitive to phenol than was a unicellular green alga. Growth rate was not quantitatively related to toxicant concentration, but yield, dormant bud sprouting, and dormant bud production were related. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI