Pontederia Cordata Research Articles

Field efficacy and predicted host range of the pickerelweed borer, Bellura densa , a potential biological control agent of water hyacinth

The North American noctuidmoth Bellura densa offers promise as abiological control agent for use in Africa andother countries invaded by water hyacinth. Anaugmentative release at a pond in Florida, USA,eliminated water hyacinth within a few months. Laboratory studies, though, indicated thatoviposition was indiscriminate and thatdevelopment was completed on taro (Colocasia esculenta [Araceae]) as well as onseveral Pontederiaceae. Acceptability of taroas a larval food plant was confirmed in thefield when larvae were found in isolated standsof taro in Florida. Evidence of use of Peltandra virginica (Linnaeus) (Araceae) wasnoted at another site. The distribution oflarval damage was compared at a site containinga mixture of 97% taro and 3% pickerelweed(Pontederia cordata). Larvae damaged87% of the pickerelweed compared to only about5% of the taro, suggesting spillover ontotaro. In another study, 416 larvae wereliberated into a concrete tank containing waterhyacinth (818 plants) surrounded by taro (96plants). Three months later, taro accountedfor only 4% of the damaged plants, less thanthe 11% expected if host selection had beenrandom. In a similar study, larvae wereliberated onto water hyacinth in a large tankdivided into thirds, with pickerelweed or taroat either end and water hyacinth in the middle. The distributions of F1 egg masses andincidence of damage 3 months later indicatedthat pickerelweed was preferred over taro, but26% of the taro plants were damaged. Weconclude that while B. densa prefersplants in the Pontederiaceae, it is notrestricted to this plant family. Plants in theAraceae would be at risk if this insect werereleased outside of North America, particularlyin cropping situations near water hyacinthinfestations. Bellura densa could beuseful for water hyacinth management in theU.S. if effective augmentation strategies weredeveloped.

Host Plants of the Grasshopper Cornops aquaticum (Bruner) (Orthoptera: Acrididae) in the Wetland of Poconé, MT, Brazil

The grasshopper Cornops aquaticum (Bruner) lives permanently on aquatic macrophytes in floodplains of tropical South America. Its host plants and feeding preference were determined by field observations, crop analysis and feeding tests in the laboratory. Macrophytes were sampled and their petioles were examined for potential egg deposition in order to reveal host plant specificity. Sub-samples were dissected in the laboratory and eggs were counted directly whereas the rest of the sample was kept in net-covered water tanks aiming to verify hatching of nymphs. C. aquaticum fed and oviposited on Eichhornia azurea and Pontederia cordata (Pontederiaceae) in the field. Out of 140 fore-guts analyzed from free-living grasshoppers, 75% contained Pontederiaceae tissues. In the laboratory, however, it accepted plant species from other families. Tests of acceptance revealed that from 19 plant species offered, one by one, 16 were accepted. The seven plant species, which were accepted with highest frequency, were selected for tests of feeding rates. These rates were equal for all seven species. When four or five food plant species were offered simultaneously, the host species were not always preferred. The fact that in the laboratory more host-plant species were accepted than in the field indicates that the host range in the field is determined by other ecological factors than the plant chemistry. Host plant selection by C. aquaticum can be related to the relative abundance of the macrophyte species in the field and to the protection, which they offer against predation. Considering the emerging parts of the macrophytes, E. azurea represents the most abundant resource in the floodplains. Moreover, the cryptic coloration of the grasshopper on E. azurea lessens predation risk. That is why, apart from being potentially polyphagous, C. aquaticum presents a considerable oligophagy in the field, feeding on few species of Pontederiaceae.

Equatorial Aperture Pattern in Monocots: Same Definition Rules as in Eudicots? The Example of Two Species of Pontederiaceae

Pontederia cordata L. and Eichhornia paniculata (Spreng.) Solms (Pontederiaceae, monocots) produce disulculate pollen grains. The formation of intersporal callose walls was followed in these two species by staining callose during male meiosis. A new type of cytokinesis that ends with the formation of conspicuous callose knobs at future aperture sites was observed. This observation indicates that the formation of the disulculate pattern is linked to cytokinesis. Thus, equatorial aperture patterns of monocots and aperture patterns of eudicots appear to be defined by similar mechanisms.

Primary determinants of macrophyte community structure in 62 marshes across the Great Lakes basin: latitude, land use, and water quality effects

We collected water quality, land use, and aquatic macrophyte information from 62 coastal and inland wetlands in the Great Lakes basin and found that species richness and community structure of macrophytes were a function of geographic location and water quality. For inland wetlands, the primary source of water quality degradation was inputs of nutrients and sediment associated with altered land use, whereas for coastal wetlands, water quality was also influenced by exposure and mixing with the respective Great Lakes. Wetlands within the subbasins of the less developed, more exposed upper Great Lakes had unique physical and ecological characteristics compared with the more developed, less sheltered wetlands of the lower Great Lakes and those located inland. Turbid, nutrient-rich wetlands were characterized by a fringe of emergent vegetation, with a few sparsely distributed submergent plant species. High-quality wetlands had clearer water and lower nutrient levels and contained a mix of emergent and floating-leaf taxa with a diverse and dense submergent plant community. Certain macrophyte taxa were identified as intolerant of turbid, nutrient-rich conditions (e.g., Pontederia cordata, Najas flaxilis), while others were tolerant of a wide range of conditions (e.g., Typha spp., Potamogeton pectinatus) occurring in both degraded and pristine wetlands.

Metalaxyl toxicity, uptake, and distribution in several ornamental plant species.

Phytoremediation depends on the ability of plants to tolerate and assimilate contaminants. This research characterized the interaction between several ornamental plant species and the fungicidal active ingredient, metalaxyl [N-(2,6-dimethylphenyl)-N-(methoxyacetyl)alanine methyl ester]. Species evaluated included sweetflag (Acorus gramineus Sol. ex Aiton), canna (Canna hybrida L. 'Yellow King Humbert'), parrotfeather [Myriophyllum aquaticum (Vell.) Verdc.], and pickerelweed (Pontederia cordata L.). Metalaxyl tolerance levels for each species were determined by exposing plants for 7 d to solutions containing 0, 5, 10, 25, 50, 75, or 100 mg metalaxyl L-1 aqueous nutrient media. Response endpoints included fresh mass production after 7 d exposure and 7 d post-exposure and quantum efficiency using dark-adapted (Fv/Fm) and light-adapted (fluorescence yields) plants. Metalaxyl uptake and distribution within the plant was determined by growing plants in aqueous nutrient media containing 1.18 x 10(6) Bq L-1 [14C]metalaxyl (0.909 mg L-1) for 1, 3, 5, or 7 d. Plant tissues were combusted and analyzed by liquid scintillation counting. Metalaxyl had no effects on the endpoints measured, except for fresh mass production of sweetflag at the 75 and 100 mg L-1 treatment levels. However, leaf necrosis was apparent in most species after 5 d exposure to concentrations greater than 25 mg L-1. Metalaxyl removal from the spiked nutrient media ranged from 15 to 60% during the 7-d exposure period. The majority of metalaxyl removed from the solution was detected within individual plants. In nearly all cases, activity from the radiolabeled pesticide accumulated in the leaves. Uptake of metalaxyl was correlated with water uptake throughout the 7 d. These results suggest that all species examined may be good candidates for incorporation into a phytoremediation scheme for metalaxyl.

Phytotoxicity, uptake, and distribution of14C-simazine inAcorus grameniusandPontederia cordata

Phytoremediation depends on the ability of plants to tolerate and assimilate contaminants. We have been interested in the use of common ornamental plants to ameliorate the effects of pesticide waste on golf courses and ornamental plant nurseries. This research characterized the interaction between two ornamentals, Acorus gramenius and Pontederia cordata, and the herbicide simazine. Simazine tolerance levels for the ornamentals were determined by exposing plants to 0, 0.01, 0.03, 0.1, 0.3, 1.0, or 3.0 mg L−1 in aqueous nutrient media for 7 d. Response endpoints included fresh weight gains for both species after 7 d exposure and 7 d postexposure. Quantum efficiency of P. cordata was also measured using dark-adapted (Fv/Fm) and light-adapted (fluorescence yields) plants. Simazine uptake and distribution within each plant was determined by exposing plants to [14C]-simazine in nutrient media (0.24 mg L−1) for 1, 3, 5, or 7 d. Plant tissues were combusted and analyzed by liquid scintillation counting. Pontederia cordata fresh weight gains were reduced 76 and 70% by 1.0 and 3.0 mg L−1, respectively, after 7 d exposure. Acorus gramenius fresh weight gains were reduced 103, 124, and 144% at 0.3, 1.0, and 3.0 mg L−1 following 7 d exposure, respectively. Photosynthetic efficiency measured on dark-adapted P. cordata (Fv/Fm) was reduced 21, 47, and 71% by 0.3, 1.0, and 3.0 mg L−1, respectively, whereas photosynthetic efficiency measured on light-adapted plants (fluorescence yields) were reduced 9, 25, 59, 87, and 96%, respectively, by 0.03, 0.1, 0.3, 1, and 3 mg L−1. Simazine activity in solution was reduced 45 and 34% over 7 d with A. gramenius and P. cordata, respectively. By day 7, activity was distributed throughout the plants, but predominantly in the leaves. Uptake of simazine was correlated with water uptake throughout the 7 d. These results suggest that A. gramenius and P. cordata may be good candidates for incorporation in a phytoremediation scheme for simazine.

Modified AFLP technique for rapid genetic characterization in plants.

The standard amplified fragment-length polymorphism (AFLP) technique was modified to develop a convenient and reliable technique for rapid genetic characterization of plants. Modifications included (i) using one restriction enzyme, one adapter molecule and primer, (ii) incorporating formamide to generate more intense and uniform bands and (iii) using agarose gel electrophoresis. Sea oats (Uniola paniculata L.), pickerel-weed (Pontederia cordata L.), Bermudagrass (Cynodon dactylon L.) and Penstemon heterophyllus Lindl. were used to determine the ability to generate adequate resolution power with both self- and cross-pollinated plant species including cultivars, ecotypes and individuals within populations. Reproducibility of bands was higher in all the AFLP experiments compared to random amplified polymorphic DNA (RAPD). Formamide with or without bovine serum albumin improved band intensities compared to dimethyl sulfoxide and the standard reaction mixture with no organic solvents. Comparison between RAPD and modified AFLP using sea-oats population samples proved that modified AFLP exhibits (i) a low number of faint bands with increased specificity of amplified bands, (ii) a significantly higher number of polymorphic loci per primer, (iii) less primer screening time, (iv) easy scoring associated with fewer faint bands and (v) greatly enhanced reproducibility. The technique described here can be applied with a high degree of accuracy for plant genetic characterization.

Root Cell Ultrastructure in Developing Aerenchyma Tissue of Three Wetland Species

Patterns of root cortex cell development and ultrastructure were analysed in Sagittaria lancifolia L., Thalia geniculata L. and Pontederia cordata L. using scanning and transmission electron microscopy (SEM, TEM). In all three species, cortex cells were arranged in radial columns extending from the endodermis to the hypodermis/epidermis. During gas space formation, the cortex cells elongated parallel to the root radius and shrank in the plane perpendicular to the radius leaving long and thin rows of cortex cells extending from the endodermis to the epidermis. Although the cortex cells appeared collapsed in tissue with well-developed gas spaces, TEM revealed that the cortical cells as well as the epidermal cells maintained intact membranes and many normal organelles. Formation of root cortex tissue with well-developed gas spaces does not require cell death in these species. Living cortex cells in root tissue with mature gas spaces could provide a symplastic pathway for transport between the root stele and the living epidermal cells.

Response of Exotics to Restored Hydroperiod at Dupuis Reserve, Florida

AbstractIn 1990 and 1991, 40 ditch plugs were constructed to restore a more natural hydroperiod (i.e., duration and depth of flooding) on portions of the 8,900‐ha Dupuis Reserve in south Florida. Vegetation transects and digital water level recorders were installed at three sites to monitor changes in vegetation relative to improved hydrologic conditions. Increased hydroperiod resulted in the elimination of Paspalum notatum (bahia grass), an exotic species introduced for cattle forage. Panicum repens (torpedo grass), another introduced species, formed dense monotypic stands in response to increased hydroperiod, but was unable to penetrate areas where Panicum hemitomon (maidencane) already existed. Frequency of occurrence and density of two desirable species, Pontederia cordata (pickerelweed) and maidencane, increased relative to higher inundation frequencies, although maidencane preferred more moderate water depths. Pickerelweed density increased significantly after the second year and was well established on two sites by the fourth year of monitoring.

The Host Range ofEccritotarsus catarinensis(Heteroptera: Miridae), a Potential Agent for the Biological Control of Waterhyacinth (Eichhornia crassipes)

Abstract The host range of Eccritotarsus catarinensis was determined using 33 plant species to assess the risk of using this insect, a native of South America, for the classical biological control of waterhyacinth ( Eichhornia crassipes ) in Australia and Papua New Guinea. The results, in conjunction with the results of Hill et al. (1999) who tested 67 species (mostly South African), strongly suggest that E. catarinensis is restricted to the Pontederiaceae, a family of aquatic plants. All five species of Pontederiaceae in the Australian testing, E. crassipes, Pontederia cordata, Monochoria vaginalis, M. cyanea, and M. australasica, were suitable for insect development. Colonies persisted for at least four generations on E. crassipes, P. cordata, and M. vaginalis. Two-way choice and multiple-choice preference trials were conducted and discussed. E. catarinensis did not exhibit a clear preference for waterhyacinth over other Pontederiaceae in these trials. Most oviposition occurred into the upper surface of the lamina during laboratory testing despite observations that the underside was preferred in the field. Although not considered suitable for release in Australia, this insect may be useful in other countries where more serious waterhyacinth problems occur and where M. vaginalis is a serious weed, such as in Southeast Asia.

Competitive reduction of noxious Lyngbya wollei mats by rooted aquatic plants

Establishment of native emergent or floating-leaved plants appears to offer natural and effective means to minimize the nuisance impacts of the mat-forming cyanobacterium Lyngbya wollei. This research has identified two species ( Pontederia cordata and Potamogeton nodosus) which offer the potential for long-term competitive control of nuisance L. wollei mats in shallow waters. Both species significantly impacted the distribution and total mass of L. wollei mats within planted experimental plots compared to adjacent unplanted control plots. The established plants forced a redistribution of the L. wollei biomass within the water column, preventing the formation of the characteristic summertime floating mats, the most objectionable of the many nuisance aspects of this species. In addition, the L. wollei biomass was reduced by more than 50% in planted plots. This reduction in mass may have been related to reduced light and nutrient availability within the vegetated plots. In addition, oxygenation and acidification of the benthic region may have promoted the decomposition and remineralization of the nuisance mat.

Regulation of root-associated methanotrophy by oxygen availability in the rhizosphere of two aquatic macrophytes

The relative importance of oxygen for root-associated methanotrophy was examined by using sediment-free, intact freshwater marsh plants (Pontederia cordata and Sparganium eurycarpum) incubated in split chambers. The root medium contained approximately 100 (mu)M methane. Methane oxidation was calculated from the difference between methane loss from chambers in the presence and absence of 1 mM 1-allyl-2-thiourea, a methanotrophic inhibitor. When the root medium was oxic, methane oxidation accounted for 88 and 63% of the total methane depletion for S. eurycarpum and P. cordata, respectively; the remainder represented diffusional loss to the atmosphere via roots, stems, and leaves. Under suboxic conditions, methane oxidation was not detectable for S. eurycarpum but accounted for 68% of total methane depletion for P. cordata. The introduction of a biological oxygen sink, Pseudomonas aeruginosa, resulted in complete loss of methane oxidation in S. eurycarpum chambers under oxic conditions, while methane consumption continued (51.6% of total methane depletion) in P. cordata chambers. The differences between plant species were consistent with their relative ability to oxygenate their rhizospheres: during a suboxic incubation, dissolved oxygen decreased by 19% in S. eurycarpum chambers but increased by 232% for P. cordata. An in situ comparison also revealed greater methanotrophic activity for P. cordata than S. eurycarpum.

Tolerance of Selected Ornamentals for Phytoremediation of Atrazine, Simazine, and Metalaxyl Residues in Water

Atrazine, simazine, and metalaxyl residues are often present in sprayer rinsates and in runoff water following application of the formulated products. As an initial step in the development of a constructed wetland for the phytoremediation of these pesticides in water, several plant species were evaluated for their tolerances to each. Plant species were chosen based on their aesthetics, tolerance to wetland conditions, and their potential to produce much vegetative growth. Species included: Acorus gramenius, Canna hybrida `King Humbert', Myriophyllum aquaticum, and Pontederia cordata. Plants were exposed to various concentrations of each pesticide dissolved in 10% Hoagland's nutrient media for 7 days. Tests were conducted under metal halide lamps with a light intensity of 400 μmol/m2 per s and a photoperiod of 16 h light: 8 h dark. Test endpoints measured included 7-day fresh mass production and chlorophyll fluorescence. A completely randomized statistical design with four replications of each concentration was utilized for each plant species. These tests indicate that all plant species were susceptible to atrazine and simazine in the 0.1 to 1 μg/ml range. Effected plants displayed concentration-dependent degrees of chlorosis and necrosis. Plants were more tolerant to metalaxyl concentrations in water. However, leaf chlorosis and necrosis did occur at concentrations greater than 25 μg/ml. Future research will quantify the uptake and mineralization potential for these plants and pesticides.

Germination of Macrophytes from a Delaware River Tidal Freshwater Wetland

LECK, M. A. (Biology Department, Rider University, Lawrenceville, NJ 08648). Germination of macrophytes from a Delaware River tidal freshwater wetland. Bull. Torrey Bot. Club 123: 48-67. 1996. The effects of burial and soil moisture (seeds were on the surface, or buried at 1 and 5 cm with drained, saturated, or flooded water levels), inundation (Petri dishes, and open and closed jars), and storage (-0 and -100% RH), as well as of light and various temperature regimes, were examined. Focus was on ten species (Amaranthus cannabinus (L.) Sauer, Ambrosia trifida L., Bidens laevis (L.) BSP, Impatiens capensis Meerb., Peltandra virginica (L.) Schott. & Endl., Phalaris arundinacea L., Pilea pumila (L.) A. Gray, Polvgonum arifblium L., Polygonum punctatum Elliott, and Sagittaria latifolia Willd.); five others were also considered to a limited extent (Alisma subcordatum Raf., Carex lurida Wahlenb., Cicuta maculata L., Cuscuta gronovii Willd., and Pontederia cordata L.). Species varied in their germination responses. Those with the same seed bank strategy (transient or persistent) had different dormancy mechanisms although seeds with persistent seed banks required temperatures >50C and germination for most was enhanced by light. Responises to burial and moisture level showed that larger seeded species were able to germinate from greater depth, but germination was greater in drained samples. There were three types of responses to inundation: (1) requirement for at least modest amounts of oxygen, (2) requirement for hypoxic conditions, and (3) those having good germination regardless of inundation regime. Seed bank losses and relatively short seed bank longevity could be accounted for by germination following afterripening, adverse effects of prolonged inundation, failed gerrnination (e.g., germination at depths that preclude the seedling from reaching the surface), and predation. Differences between the seed characteristics of species of this and those of other wetlands relate to the transient nature of the seed bank and to wetland hydrology. Variable responses to drying have implications for wetland restoration projects.

Use of aquatic and terrestrial plants for removing phosphorus from dairy wastewaters

A microcosm study was conducted in Okeechobee, County, Florida, to evaluate the P removal potential of several aquatic macrophytes and “unconventional” terrestrial crops cultured on dairy lagoon wastewaters. Existing conventional spray field crops in this region that are irrigated with dairy wastewaters are estimated to remove 23 mgP/m 2-day. Effects of season and hydraulic retention time (HRT) on P uptake by the floating macrophytes water hyacinth ( Eichhomia crassipes) and duckweed ( Lemna obscura) were evaluated using half-strength primary lagoon effluent as a growth medium. Maximum P uptake rates by water hyacinths in February and July were 59 and 200 mgP/m 2-day, respectively, whereas P uptake by duckweed was identical (20 mgP/m 2-day) during both periods. During July, wastewater total P concentrations were reduced from 7.3 mgP/l to 0.2 mgP/l by water hyacinth and to 2.4 mgP/l by duckweed at a 7-day HRT. Wastewater P and N removal in excess of that accounted for by plant uptake was observed in both water hyacinth and duckweed microcosms. Ten emergent macrophytes were screened in secondary lagoon effluent for their P uptake potential, including cattail ( Typha domingensis); pickerelweed ( Pontederia cordata); canna lily ( Canna flaccida); bulltongue ( Sagittaria lancifolia); arrowhead ( Sagittaria latifolia); lizard's tail ( Saururus cernuus); green arum ( Peltandra virginica); giant reed ( Phragmites australis); soft rush ( Juncus effusus); and bulrush ( Scirpus validuus) Most species exhibited increased tissue N and P contents and greater biomass yields when supplemental N and P fertilization was provided. Canna lily and pickerelweed were the two species that provided highest rates of foliage P uptake from the enriched wastewater (uptake rates of 173 and 66 mgP/m 2-day, respectively). Shoot:root biomass ratios for canna lily and pickerelweed were 1.2 and 0.7, respectively, so “whole-plant” P uptake rates were likely twice the values shown above. Several studies were conducted with the terrestrial crops alemangrass ( Echinochloa polystachia). paragrass ( Brachiaria mutica), floralta limprograss ( Hemarthria altissima) and bermudagrass ( Cynodon dactylon). Effects of primary lagoon effluent application rates (4.5 vs. 8.9 cm/week) and water depth on biomass production and P uptake were evaluated. From February to May, P uptake by bermudagrass, paragrass, alemangrass and floralta limpograss cultured at a 4.3 cm/wk application rate (with drainage of the leachate 24 h after effluent application) was 34, 29, 28 and 16 mgP/m 2-day, respectively. Phosphorus uptake by alemangrass (22 mgP/m 2-day) and paragrass (18 mgP/m 2-day) was reduced by cultivation in 9 cm of standing water. Phosphorus uptake (based on foliage harvests) by floralta limpograss (41 mgP/m 2-day) and alemangrass (49 mgP/m 2-day) was enhanced by cultivation at a 8.9 cm/week application rate, due both to increased biomass production and increased tissue P concentrations.

044 VARIETAL DIFFERENCES IN EARLY EX VITRO GROWTH AND DEVELOPMENT OF MICROPROPAGATED PONTEDERIA CORDATA L.

Micropropagation is a commercially viable and ecologically sound method for producing native herbaceous wetland plants used for wetland revegetation projects. The ability to rapidly screen, select and store germplasm of wetland species genotypes with desirable characteristics of growth rate and habit, nutrient uptake capacity, and/or substrate preference would significantly impact how micropropagated wetland plants are marketed. Early screening of plantlet growth ex vitro may provide an efficient method to select for specific characteristics of growth rate and habit. Five micropropagated lines of Pontederia cordata of differing phenotype were established in vitro from Florida populations. Rooted microcuttings were established ex vitro in a shallow outdoor tank. Growth and development were monitored over a 9 week period. Significant differences in shoot growth and number, leaf area and number, flowering and dry weights were observed between the different Pontederia cordata varieties.

Locus of methane release and mass‐dependent fractionation from two wetland macrophytes

Factors responsible for the alteration of CH4 isotopic composition during gas transport by emergent macrophytes were investigated in Pontederia cordata (Pickerelweed) and Sagittaria lancifolia (Bull tongue). Measured rates of CH4 emission from petioles and leaves indicated that the locus of gas release from these plants is the petiole and not the leaf. Methane concentration profiles of gases within petioles further indicated that most CH4 is emitted from the lower portion of the petiole near the waterline. Ethane and propane tracer experiments confirmed mass‐dependent fractionation during gas transport through these plants. After injection with a gas mixture, both plant types emitted ethane 12% faster than propane. Twenty‐five minutes or more after injection, ethane was found to be depleted in gas within petioles relative to the injected gas. The degree of fractionation observed for the 13CH4/12CH4 and ethane/propane pairs was similar to values predicted by kinetic gas theory.

Rhizospheric methane oxidation determined via the methyl fluoride inhibition technique

Methane oxidation rates in the rhizosphere of aquatic macrophytes were quantified by development of a technique employing a recently described inhibitor of methane oxidation, methyl fluoride. Unlike other inhibitors, methyl fluoride appears to be nontoxic to the plants, allowing them to act as natural conduits, transporting the inhibitor from the headspace to the rhizosphere. Increases in methane emissions were recorded after closed chamber methyl fluoride incubations, primarily in greenhouse (Pontederia cordata and Sagittaria landfolia) experiments with some preliminary outdoor and field (Oryza sativa and Typha latifolia) data. Comparison of emissions before and after incubation indicated oxidation of 23 to 90% of the methane produced (defined as CH4 emission in the absence of oxidation) in greenhouse studies and 10 to 47% in field and outdoor studies. A comparison of 1.5 and 3.0% methyl fluoride chamber headspace incubations as well as initial dose response data indicated that the lower concentration was sufficient to obtain inhibition of methane oxidation in the greenhouse studies without significantly affecting methanogenesis. Inhibition was possible with one 16‐ to 18‐hour incubation period. Methyl fluoride within the rhizosphere disappeared after approximately 1 week due to plant ventilation and possible bacterial uptake.

Pollen Removal From Tristylous Pontederia Cordata: Effects of Anther Position and Pollinator Specialization

Stamens produce male gametophytes and expose them to the pollination process, so that stament characteristics should reflect selection on male function. For animal—pollinated plants, restricting removal in favor of using all available pollinators should generally promote pollen dispersal, so that staminal characteristics should limit pollen removal by individual pollinators. Here we describe the influences of anther position on pollen removal from Pontederia cordata, a tristylous plant with anthers at three distinct positions: exserted from the flower (long—level anthers), at the mouth of the flaring tubular perianth (mid—level anthers), and near the perianth based (short—level anthers). To measure pollen removal, we presented unvisited flowers to freely foraging bees and counted the number of pollen grains left in flowers after 1—4 visits. The pollinators included two generalist bumble bees (Bombus impatiens and B. vagans: Apidae) and Melissodes apicata (Anthophoridae), a specialist with morphological and behavioral adaptations for collecting pollen from P. cordata. Analysis of pollen removal incorporated the effects of bee species, anther position, pollen size, flower length, and the number and cumulative duration of bee visits. Anther position and bee species significantly influenced pollen removal, although the differences between bee species were not related to pollinator specialization. Long—level anthers dehisced early, lost an average of 39% of their pollen during their first visit, and did not consistently contact small pollinators, such as M. apicata. Short—level anthers restricted pollen removal more (28% lost) than long—level anthers, but dehisced more slowly and were subject to unpredictable pollen removal. Mid—level anthers combine the advantages of more exserted and inserted positions without suffering the corresponding disadvantages as they restricted removal (25% lost), dehisced relatively early, and seemed to consistently contact all pollinators. The overall benefits of positioning anthers within the perianth mouth, the position occupied by mid—level anthers in P. cordata, may explain the prevalence of this morphology among bee—pollinated plants with flaring, tubular, monomorphic flowers.

Concentrations of zinc and chromium in aquatic macrophytes from the sudbury and muskoka regions of Ontario, Canada

Root and shoot samples of Eriocaulon septangulare, Nuphar variegatum, Nymphaea odorata and Pontederia cordata were collected from 15 lakes in central Ontario during the summer of 1988 to investigate possible relationships between zinc and chromium levels in aquatic macrophytes and water and sediment variables. Although concentrations of zinc and chromium differed greatly among the four species, both metals were consistently higher in Eriocaulon. Generally, root and rhizome tissue contained higher zinc and chromium than shoot tissues of the same species and site. Zinc concentrations (dry weight) ranged from 6·3 μg g −1 in Nuphar shoots to 87·7 μg g −1 in whole Eriocaulon. Chromium ranged from 0·23 μg g −1 in Pontederia shoots to 23·9 μg g −1 in whole Eriocaulon. No significant trends were detected throughout the growing season in macrophyte or sediment concentrations of either metal. Results of multiple linear regression analyses of several water quality and environmental variables on Eriocaulon indicated that sediment zinc was the best predictor of plant zinc, and sediment chromium and calcium were the best predictors of plant chromium.