In Iran, monochoria is a noxious weed in fields of transplanted rice. Two field experiments were conducted to assess the efficacy of soil-applied and foliar-applied herbicides to control monochoria in transplanted rice. Prepackaged herbicides (triafamone plus ethoxysulfuron applied at 40 g ai ha−1, pyrazosulfuron-ethyl plus pretilachlor applied at 382.5 g ai ha−1, and pendimethalin plus clomazone applied at 1,200 g ai ha−1) reduced monochoria biomass by 100%, 100%, and 14%, respectively; and a single application of flucetosulfuron at 30 g ai ha−1, pendimethalin at 990 g ai ha−1, thiobencarb at 2,750 g ai ha−1, and pretilachlor at 1,000 g ai ha−1 reduced monochoria biomass by 100%, 99%, 75%, and 56%, respectively, compared with a nontreated control. Tank-mixed bensulfuron-methyl at 45 g ai ha−1 applied with pretilachlor, thiobencarb, or pendimethalin provided 100% control of monochoria. Rice height, and straw and grain yield were greater after herbicide treatments than those of the nontreated and hand-weeded controls, indicating the advantages of chemical control of monochoria over manual weeding. Full-season monochoria interference reduced rice grain yield by 32%. In the second study, the herbicides triafamone plus ethoxysulfuron, flucetosulfuron, 2,4-D at 1,080 g ai ha−1, dicamba plus 2,4-D at 928 g ai ha−1, bispyribac-sodium at 31.25 g ai ha−1, bentazon plus MCPA at 1,150 g ai ha−1, pyribenzoxim at 30 g ai ha−1, and propanil at 5,400 g ai ha−1 applied to foliage at 4- to 5-leaf seedlings of monochoria provided ≥97% control and prevented 100% of its regrowth, with the exception of propanil. This study shows that monochoria control can be achieved by using a variety of residual and foliar-applied herbicides with different mechanisms of action.

Outdoor experiments were conducted at two locations in Minnesota to characterize the growth, development, and vegetative reproduction potential of giant burreed grown with and without wild rice. Giant burreed growth was influenced by environment more than the presence of wild rice. Shoots emerged from corms 2 to 3 wk after planting (WAP), and continued throughout the growing season. Giant burreed attained a maximum height of 135 cm with an average 36 shoots/plant. Rhizome production started 4 to 6 WAP, and continued throughout the growing season. Corm formation at the base of shoots started 8 WAP and continued throughout the growing season. Rhizomes grew to a maximum cumulative length of 2500 cm/plant spreading in all directions and produced up to 250 buds/plant. Corm and rhizome bud viability was 15 and 10%, respectively. These data suggest that giant burreed grown in wild rice should be controlled by 2 to 3 wk after emergence to prevent vegetative reproduction.

Waterhyacinths grown outdoors were treated with combinations of 0 or 22 kg ai ha-1 of the plant growth retardant paclobutrazol and 0, 25, or 250 waterhyacinth weevils per culture tank under conditions of low (25% coverage) or high (100% coverage) plant densities. The low plant density treatments simulated a regrowth or colonization situation. Weevil feeding activity, canopy height, plant volume, leaf production, and leaf mortality were monitored biweekly for 22 weeks or until all plants had died. In high plant densities, weevil infestations increased leaf mortality rates while decreasing production rates, causing eventual plant death. At low plant densities, waterhyacinth plants partially compensated for weevil feeding by increased leaf production rates, thus enabling them to survive until the end of the study. Paclobutrazol reduced canopy height for about 3 months but had little effect on weevil feeding intensities. Combinations of weevils and paclobutrazol gave complete control of waterhyacinths regardless of plant densities. The combined effects were synergistic, with accelerated leaf mortality rates exceeding production rates leading to early plant death. This study demonstrated the potential for increasing effectiveness of waterhyacinth weevils using integrated strategies.

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.

Akinetes (spores) of the green, filamentous alga Pithophora oedogonia (Montagne) Wittrock were more copper resistant than filamentous cells, tolerating copper concentrations as high as 4 μg·ml-1 (16 μg·ml-1 copper sulfate pentahydrate). The localization of copper in viable and nonviable cells was conducted using cell fractionation and cytochemical/ultrastructural methods. In copper-exposed viable akinetes, copper was bound primarily to the outer layers of the cell wall. In nonviable cells, copper was found randomly distributed throughout the cytoplasmic and vacuolar regions. No evidence for compartmentation of copper as vacuolar or intranuclear deposits was demonstrated. Copper uptake into the cell wall was associated with release of equimolar amounts of calcium, magnesium, and zinc. When viable copper-treated akinetes were allowed to recover in copper-free medium, the copper was gradually released from the cell wall, presumably to the culture medium. A portion of the copper also appeared to be redistributed to new cell wall material during the akinete germination process. The differential tolerance of P. oedogonia akinetes to copper in comparison to filamentous cells appears to be due to a combination of factors that include cell surface/volume ratios, metabolic activity, and cell wall copper-binding components.

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.

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.

The effect of 1.0 ppmw fluridone {1-methyl-3-phenyl-5-[3-trifluoromethyl)phenyl]-4(1H)-pyridinone} on growth of American pondweed (Potamogeton nodosus Poir.) and sago pondweed (Potamogeton pectinatus L.) was examined in plants kept in darkness or under various numbers of 12-h photoperiods. When plants were exposed to 1.0 ppmw fluridone for 1 to 10 12-h days, no stunting of American pondweed occurred until 21 days after treatment. Thirtythree days after treatment, plants that had received 4 to 10 days' exposure were most stunted. Sago pondweed was slightly more susceptible than American pondweed (87% and 50% reduction in length, respectively) 37 days after a 10-day exposure. Chlorophylls a and b were significantly lower in both species 14 days after all fluridone treatments. American pondweed plants exposed to fluridone continuously (1.0 ppmw) for 15 days, but given 0, 3, 6, 9, 12, or 15 12-h days were stunted only after having received at least six photoperiods. Stunting in sago pondweed was only slightly light-dependent. Neither species absorbed significantly more 14C-labeled fluridone after 2, 4, or 14 photoperiods compared to plants kept in darkness for the same time.

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.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the United States Environment Protection Agency (USEPA)-approved herbicide that provides relatively long-term systemic control. Mature (6-wk-old) and young (freshly planted from 10-cm apical shoot apices) hydrilla were grown in 540 L fiberglass vaults under short- (natural 8 to 10 hr light/14 to 16 hr dark photoperiod) or long- (artificially extended 16 h light/8 h dark photoperiod) day greenhouse conditions. Fluridone treatments of 0, 1, 5, and 10 µg L−1 were applied after 2 wk and maintained within each population and photoperiodic regime throughout the study. Short days promoted subterranean turion formation, but this effect was reduced by long days and 5 and 10 µg L−1 fluridone. Fluridone caused a reduction in chlorophyll and carotenoid levels but the effect on anthocyanin content was variable. Short days caused elevated anthocyanin, and this effect was diminished by fluridone. Fluridone reduced the abscisic acid content of mature apical stems and was higher under short days in younger plants. These studies provide further evidence that fluridone can be used as a fall herbicide application to reduce turion production.

Lanceleaved waterplantain is an exotic weed of rice that was introduced into Australia in the 1930s. Since its introduction, it has spread throughout much of the rice-growing region in southeastern Australia. The variability of lanceleaved waterplantain in these regions was studied using polymerase chain reaction-based, inter simple sequence repeat (ISSR) analysis. Deoxyribonucleic acid fingerprints from samples of the weed from southeastern Australia were compared between locations and with two samples of common waterplantain, a closely related species. The analysis indicated that there were two distinct groups of lanceleaved waterplantain that correlated with location. From the results of multidimensional scaling analysis, it is hypothesized that the Griffith group did not arise from hybridization between lanceleaved waterplantain and common waterplantain, and that it is more likely that the group arose from a separate introduction into the area. It is also suggested that there is seed movement between areas in the Murray Valley and Colleambally Irrigation areas. The implications of this variation for biological control of the weed are discussed.

Florida hydrilla populations have shown an alarming increase in resistance to fluridone, an herbicide used extensively for controlling invasive US hydrilla populations. A rapid PCR and sequencing method was developed to identify and screen hydrilla genomic DNA for three previously identified phytoene desaturase (pds) gene mutations that confer resistance to fluridone. Ninety hydrilla accessions were screened for fluridone resistant genotypes including 46 accessions from the US and 44 accessions from 15 other countries. In Florida, hydrilla from five of nine sites tested was heterozygous for wild-type and herbicide-resistant alleles. Additionally, a new resistant population was identified from Lake Seminole in Georgia, the first genetically confirmed strain of resistant hydrilla outside of Florida. All resistance-conferring mutations were located on the same homologous haplotype of US dioecious hydrilla. All other hydrilla samples tested possessed only wild type alleles, including monoecious strains that had been exposed to fluridone. Management implications are discussed.

The development of fluridone resistance by hydrilla has significantly impacted hydrilla management, and research is ongoing to develop alternate herbicides for effective hydrilla control. We determined the potential cross-resistance in fluridone-resistant hydrilla to other bleaching herbicides norflurazon, mesotrione, and topramezone-methyl. Phytoene, β-carotene, and chlorophyll contents as a function of hydrilla biotype and herbicide treatment were evaluated. Hydrilla shoot tips were collected from fluridone-susceptible (S) and -resistant (R) biotypes and exposed to 5, 25, 50, 75, and 100 µg L−1 of herbicide. The susceptible biotype showed an increase in phytoene and a decrease in β-carotene and chlorophyll contents when treated with 5 µg L−1 fluridone, whereas higher doses of fluridone were required to affect these pigments in the resistant biotype. There was no difference in response by S and R biotypes to mesotrione and topramezone-methyl, with both biotypes showing significant affects on pigment contents at 5 µg L−1. Higher doses of norflurazon were required to affect these pigments in the R compared to the S biotype. The S biotype had EC50 values of 11.7, 12.2, and 4.7 µg L−1, whereas the R biotype had EC50 values of 56.6, 41.1, and 41.7 µg L−1 fluridone for phytoene, β-carotene, and chlorophyll contents, respectively. There was no difference in EC50 for phytoene, β-carotene, and chlorophyll values between the hydrilla biotypes for mesotrione and topramezone-methyl herbicides. In fluridone-susceptible and -resistant hydrilla biotypes, EC50 values for phytoene, β-carotene, and chlorophyll were 12.4 to 11.8, 10.2 to 13.2, and 3.1 to 4.6 µg L−1 mesotrione and 12.6 to 13.5, 13.3 to 11.9, and 4.6 to 5.7 µg L−1 topramezone-methyl, respectively. For norflurazon, S and R biotypes had EC50 values of 33.1, 45.4, and 40.6 µg L−1 and 84.6, 81.0, and 92.7 µg L−1 for phytoene, β-carotene, and chlorophyll, respectively. These studies confirmed negative cross-resistance of fluridone-resistant hydrilla to mesotrione and topramezone-methyl and a positive cross-resistance to norflurazon.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only U.S. Environment Protection Agency (USEPA)–approved herbicide that provides relatively long-term systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. Fluridone-susceptible (S) and several fluridone-resistant (R1 to R5) hydrilla biotypes varying in resistance levels were maintained in 950-L tanks under ambient sunlight and day-length conditions. To correlate varying levels of fluridone resistance to ploidy in hydrilla, flow cytometric analysis was performed. Differential ploidy levels (diploid, 2n = 2x = 16; triploid, 2n = 3x = 24; and tetraploid, 2n = 4x = 32) were reported among different hydrilla biotypes, plants within each biotype, and within shoot tissues of the same plant. Triploid plants were predominant in all biotypes. Diploid plants were observed in all hydrilla biotypes except the susceptible hydrilla (S). Plants with tetraploidy were rare within biotypes. The diploid, triploid, and tetraploid plants had nuclear DNA contents of 2.43 to 2.73 pg, 3.44 to 3.71 pg, and 4.64 to 4.90 pg, respectively, and no differences were observed among plants with same ploidy for nuclear DNA content in different hydrilla biotypes. Endoreduplication patterns were observed in diploid plants of R1 and R3 biotypes. However, no plant with higher ploidy levels (triploid or tetraploid) in any hydrilla biotypes showed endoreduplication.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only herbicide approved by the U.S. Environment Protection Agency that provides systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. Hydrilla biotypes of varying fluridone resistance levels were maintained in 900-L tanks under natural atmospheric conditions from September 2004 to September 2005 in the absence of fluridone. Hydrilla shoot tips were collected from each biotype during September 2004 (at planting), December 2004 (3 mo after planting [MAP]), March 2005 (6 MAP), June 2005 (9 MAP), and September 2005 (12 MAP) and exposed to 5, 10, 15, 20, 30, and 50 µg L−1 fluridone to assess changes in susceptibility to this herbicide over time. Nonlinear regression analysis was used to calculate EC50 values for phytoene and β-carotene (effective fluridone concentration to increase/decrease the phytoene/β-carotene content in hydrilla plant tissue by 50% over the untreated control) at each time interval. EC50 values did not change in the susceptible hydrilla biotype over time. The EC50 values for phytoene and β-carotene for the susceptible biotype were 7.5 and 8.9 µg L−1, respectively, at planting and 7.6 and 9.4 µg L−1, respectively, at 12 MAP. Resistant hydrilla biotypes (R1–R5) also showed no change in EC50 phytoene values over time. Although, EC50 β-carotene values in resistant biotypes R1, R3, R4, and R5 did not change over time, R2 recorded a reduction in EC50 β-carotene at 12 MAP. Also, a 0.5-point decrease in resistance factor was observed for all resistant biotypes. Future long-term studies are needed to evaluate stability of resistant hydrilla biotypes in the absence of fluridone selection pressure.

Fluridone has been widely used for control of the submersed aquatic weed hydrilla in Florida for over 25 years. Recently, some hydrilla biotypes were suspected of having developed resistance to fluridone. Laboratory studies were conducted to monitor changes in phytoene and β-carotene contents as a function of suspected susceptible and resistant hydrilla biotypes to fluridone treatment. Hydrilla shoot tips from each biotype were exposed to 5, 10, 15, 20, 30, and 50 μg L−1 fluridone. Higher β-carotene and lower phytoene content was observed in all resistant hydrilla biotypes compared with the susceptible. The susceptible biotype showed an increase in phytoene or a decrease in β-carotene content when treated with as little as 5 μg L−1 of fluridone, whereas much higher doses were needed for the resistant biotypes. EC50 β-carotene values of 9 and 63 μg L−1 fluridone were found in the susceptible and the most resistant biotype, respectively. Consistent levels of hydrilla injury occurred at phytoene/β carotene index values of 5.5–7 and occurred at exposure to 5–10 μg L−1 fluridone in susceptible and 50 μg L−1 in the most resistant biotype. A resistance factor (R/S) was calculated for each hydrilla biotype which ranged from as low as 2X (R1 and R2) to 7X (R5). Aggressive spread of fluridone resistant dioecious hydrilla in aquatic ecosystems can severely impact hydrilla management, and consequently cause substantial and long-lasting ecological and economic problems throughout the southern United States.

Alligatorweed is well known for its potassium (K+)-accumulating capabilities and its strong resistance to undesired growth conditions. The results of this study revealed properties of K+ accumulation and its contribution to drought stress in alligatorweed. In addition, we attempted to characterize the molecular mechanisms of K+ accumulation in this plant. Alligatorweed plants showed a consistent increase in biomass in response to external K+ concentrations, ranging from micromolar levels up to 50 mmol L−1; K+ was also accumulated accordingly in the plants. The stem was the most K+-accumulating organ, accumulating up to 13% of the K+. Moreover, this K+ superaccumulation caused improved resistance to drought stress. The apparent K+ uptake by the roots showed a typical high-affinity property, and the Michaelis constant increased at higher rates of plant K+ in the starting materials. Furthermore, three putative, K+-uptake transporter complementary DNAs (cDNAs) were isolated from alligatorweed (ApKUP1, ApKUP2, and ApKUP3, respectively) using degenerated primers and rapid amplification of cDNA end techniques. The expression of ApKUP1 and ApKUP3 was predominately localized to the leaves, whereas ApKUP2 was expressed throughout the entire plant. The expression of ApKUP1 and ApKUP3 was stimulated in the stems and roots when K+ was depleted from the external medium. Moreover, ApKUP3 expression was enhanced in the stem in response to abscisic acid treatment and drought stress. In conclusion, our findings provide further insight into the mechanisms of K+ accumulation linked to K+ uptake in alligatorweed.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only U.S. Environment Protection Agency (USEPA)–approved herbicide that provides relatively long-term systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. Several biotypes of hydrilla varying in resistance levels were maintained in 950-L tanks under ambient sunlight and day-length conditions from September 2004 to September 2005 in absence of fluridone. Phenotypic measurements were performed during this 1-yr period to monitor differences in growth and reproductive physiology. All fluridone-resistant biotypes (except R3) were growing at the same rate or greater than the susceptible hydrilla. These data suggested that there are no deleterious effects on growth and reproductive physiology because of development of fluridone resistance. Aggressive spread of fluridone-resistant dioecious hydrilla in aquatic ecosystems can severely affect hydrilla management and, consequently, cause substantial and long-lasting ecological and economic problems throughout the southern United States.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only U.S. Environment Protection Agency (USEPA)–approved herbicide that provides relatively long-term systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. One susceptible and five fluridone-resistant biotypes of hydrilla varying in resistance levels were maintained in 950-L tanks under ambient sunlight and day-length conditions from September 2004 to September 2005 in absence of fluridone. Because fluridone is an inhibitor of the enzyme phytoene desaturase (PDS), the gene for PDS (pds) was cloned from fluridone-susceptible and -resistant hydrilla biotypes. Somatic mutations in amino acid 304 of hydrilla PDS are known to confer herbicide resistance. We determined pds sequence from these hydrilla biotypes at planting and 12-mo after planting. Two independent mutations at the arginine 304 codon of pds were found in the resistant hydrilla plants. The codon usage for arginine 304 is CGT, and a single point mutation yielding either serine (AGT) or histidine (CAT) was identified in different resistant hydrilla biotypes. There were no differences at codon 304 in the PDS protein of any hydrilla biotype 12-mo after planting. Several other mutations were also found in resistant pds alleles, though their possible role in herbicide resistance is unclear.