A laboratory bioassay showed that inhibitors of gibberellin synthesis (flurprimidol, paclobutrazol, and uniconazole) reduced plant height but did not affect physiological parameters such as photosynthesis, respiration, and chlorophyll content in two weedy submersed aquatic plants, hydrilla and Eurasian watermilfoil. Eurasian watermilfoil was sensitive to these compounds at concentrations as low as 0.75 μg L−1. Hydrilla sensitivity was in the range of 75 to 750 μg L−1. The three compounds reduced main and lateral stem lengths in hydrilla; however, at 75 μg L−1 the number of lateral stems and roots was greatly increased over untreated controls resulting in a stoloniferous growth habit. Dominant growth form of treated Eurasian watermilfoil was a single shortened stem with numerous compacted buds. Photosynthesis, respiration, and chlorophyll content were not affected in either plant at nontoxic dosages in which stem reduction was obtained. Both plants required only a 24-h exposure to maintain main stem length reduction for 6 wk after transfer to untreated medium. Our results indicate that these gibberellin synthesis inhibitors would be effective in reducing plant height in aquatic systems.

Control of monoecious and dioecious hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] biotypes required a minimum 2 days of exposure to diquat (6,7-dihydrodipyrido [1,2-α:2′,1′-c] pyrazinediium ion) at a concentration of 0.25 mg/L under laboratory conditions. When treatment concentration was increased to 2.0 mg/L diquat, the minimum required contact time was reduced to 6 or 12 h depending on plant growth stage. Early growth emerging from sprouting tubers appeared to be more susceptible to diquat treatments. Herbicide uptake was linear during a 4-day exposure to 14C-diquat up to 1.0 mg/L, and the amounts of radioactivity in plant tissue varied proportionally with ambient levels of 14C-diquat in water. The lethal concentration of diquat in hydrilla tissue was estimated to be 81 μg/g plant dry weight when hydrilla was exposed to diquat at 0.25 mg/L for 2 days.

Growth and morphology of two U.S. hydrilla biotypes grown under 10- and 16-h photoperiods in outdoor aquaria were investigated. The dioecious hydrilla biotype elongated extensively under both photoperiods and reached the water surface within 2 to 3 weeks after planting. In contrast, the monoecious biotype grew more densely near the sediment, primarily by producing a high number of horizontal stems, root crowns, and higher shoot densities. Monoecious hydrilla grown from tubers produced new tubers after 4 weeks of exposure to the 10-h photoperiod, and the number of tubers produced was five- to sevenfold higher in the monoecious than in the dioecious biotype. Furthermore, monoecious hydrilla produced tubers under both 10- and 16-h photoperiods, with tuber production much higher under the shorter photoperiod. Dioecious hydrilla grown from tubers under similar conditions produced new tubers after 8 weeks only under the 10-h photoperiod. No tubers were produced by the dioecious biotype throughout a 10-week exposure to the 16-h photoperiod.

Selected biological and physiological parameters of hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] biotypes were compared in growth chambers, glasshouse, or outside aquaria. Salinity tolerance was similar for both biotypes with an injury threshold of approximately 13 parts per thousand (ppt). The registered aquatic herbicides diquat (6,7-dihydrodipyrido [1,2-α:2′,1′-c] pyrazinediium ion) and dipotassium and monoamine salts of endothall (7-oxabicyclo [2.2.1] heptane-2,3-dicarboxylic acid)4 produced similar injury to both biotypes. Biomass of the monoecious plants, growing in controlled environments, declined under short days. Tuber production in monoecious plants was greatest under short days and was significantly higher than in dioecious plants under the same conditions. Growth response to temperature was similar among all plants and was retarded at 15 C. Tuber germination occurred at lower temperatures in the monoecious plants, which was indicative of a lower temperature tolerance. Vegetative propagules were the only perennating structures observed in the monoecious biotype, and regrowth was entirely from tubers and turions. The annual growth habit, in conjunction with rapid and abundant propagule production, adapts the monoecious biotype to northern areas with short growing seasons.

Copper content and growth of excised hydrilla [Dioecious Hydrilla verticillata (L.f.) Royle # HYLLI] apical shoot segments were determined following exposure to copper sulfate (CuSO4), copper-triethanolamine (Cu-TEA), and copper-ethylenediamine (Cu-EDA). For all copper formulations, inhibition of growth was related to the amount of copper associated with the excised shoots. At equal copper exposure, the Cu-EDA formulation produced the greatest inhibition of growth and generally the highest copper levels in the plants. The Cu-EDA formulation inhibited dry weight gain by more than 80% 3 weeks after a 2-h exposure to 2.0 or 4.0 ppmw copper. Under similar conditions, CuSO4 or Cu-TEA produced 60% inhibition. The presence or absence of light during a 2-h exposure had no effect on the efficacy of uptake of copper from any of the formulations. Formulation-dependent differences in the mechanism of copper uptake is suggested because rinsing of exposed shoots with dilute acid (0.01N HNO3) removed copper from shoots treated with CuSO4 or Cu-TEA but not from those treated with Cu-EDA.

Hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] was grown for 4 to 16 weeks in pans filled with either an organic muck-sand soil, sand, or sand mixed with Osmocote, Esmigran, and dolomite under outdoor conditions in plastic-lined pools with flowing pond water. Dry weight for plants cultured in sand plus the fertilizers was dependent on the concentration of fertilizer and was from 6 to 14 times that of plants cultured in sand alone. Dry weight was also higher for three treatments of sand amended with fertilizer than for plants cultured in the organic muck-sand soil. Water temperature for different growth periods influenced dry weight of hydrilla cultured with all three root media. Tuber production was independent of three levels of fertilizer for 16 weeks of plant growth, but plant weight was dependent on the concentration of nutrients in the root zone. Of nine plant tissue nutrients measured, only phosphorus in both the shoots and roots was dependent on the level of fertilizer in the root zone. This suggests that growth of hydrilla is controlled by nutrients in the root zone. The use of sand amended with various levels of fertilizers may be a way to simulate fertility levels of sediments as a method to study aquatic sites for their potential to support growth of hydrilla.

Thirty-eight percent of monoecious hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] grown from tubers produced new tubers after 28 days exposure to a 10-h photoperiod. One hundred percent of the plants grown at a 10-h photoperiod produced tubers by 56 days while only thirty-eight percent of those grown at a 12-h photoperiod did so. Plants grown at 14- or 16-h photoperiods did not produce tubers. Tubers appeared to be produced at the expense of new root and shoot tissue. Dioecious hydrilla (female) grown under similar conditions did not produce tubers by 56 days at any photoperiod examined. Relative growth rates (total dry weight) for both types did not differ with photoperiod and ranged between 81 ± 63 and 284 ± 52 mg·g-1·wk-1 (regression coefficient ± standard error; n = 30). In general, total chlorophyll (a+b) was greater for dioecious than for monoecious plants. The ratios of chlorophyll “a” to chlorophyll “b” and carotenoids to chlorophyll a changed with increasing photoperiod and plant age in a similar manner for both monoecious and dioecious plants.

A fiber system for controlled delivery of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} was evaluated for its potential use in the management of the submersed aquatic weed hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] in slow-moving water. The release of fluridone from the fibers was first order and could be extended over several days simply by using fibers of larger diameters. Fibers of 0.8- and 1.2-mm diam released fluridone over a period of 40 and 50 days, respectively. Both effectively controlled hydrilla at a rate of 2.2 kg ai/ha in a 4-month study in replicated pools with water flow adjusted to provide one volume change every 24 h. Under the same conditions, the conventional liquid formulation was ineffective and a pellet formulation provided only marginal control.

Subterranean turions (tubers) of hydrilla lose viability when desiccated. Experimental data showed that freshly collected tubers had a moisture content between 50 and 60% and more than 90% viability. When desiccated, there was an approximate 2% increase in tuber mortality with each percent decline in moisture content. However under field conditions, the tuber bank within the exposed sediments of a northern Texas reservoir showed no decline in number or tuber viability throughout a 12-mo continuous drawdown. Apparently, the buried tubers were never subject to sufficient dessication to damage them. Finally, an experimental pond with an extensive hydrilla tuber bank was manipulated through six flood/drawdown cycles to determine the effects of short-term drawdowns on tuber survival and quiescence. Initially, the pond had a tuber bank of about 676 and 305 tubers m−2 in the shallow and deep zones, respectively. Although the tuber number was reduced to fewer than 15 to 30 tubers m−2 by these repetitive drawdowns, hydrilla tubers were not eradicated from the pond.