Lemna Species Research Articles

Combination effects of herbicides on plants and algae: do species and test systems matter?

Risk assessment of herbicides towards non-target plants in Europe is currently based solely on tests on algae and floating aquatic plants of Lemna sp. Effects on terrestrial non-target species is not systematically addressed. The purpose of the present study was to compare combination effects of herbicide mixtures across aquatic and terrestrial test systems, and to test whether results obtained in the traditional aquatic test systems can be extrapolated to the terrestrial environment. This was done by evaluating ten binary mixtures of nine herbicides representing the seven most commonly used molecular target sites for controlling broadleaved weeds. Data were evaluated statistically in relation to the concentration addition model, and for selected concentrations to the independent action model. The mixtures were tested on the terrestrial species Tripleurospermum inodorum (L.) Schultz-Bip. (Scentless Mayweed) and Stellaria media (L.) Vill. (Common Chickweed), and on the aquatic species Lemna minor L. (Lesser duckweed) and the alga Pseudokirchneriella subcapitata (Korschikov) Hindak. For the two mixtures of herbicides with the same molecular site of action, the joint effect was additive. For the eight mixtures of herbicides with different sites of action, two of the mixtures were consistently antagonistic across species, while for the remaining six mixtures the joint effect depended on the species tested. This dependence was, however, not systematic, in the sense that none of the species or test systems (terrestrial versus aquatic) had a significantly higher probability of showing synergistic or antagonistic joint effects than others. Synergistic interactions were not observed, but approximately 70% of the mixtures of herbicides with different sites of action showed significant antagonism. Hence, the concentration addition model can be used to estimate worst-case effects of mixtures of herbicides on both terrestrial and aquatic species. Comparing the sensitivity of the species to a 10% spray drift event showed that the terrestrial species were more vulnerable to all herbicides compared with the aquatic species, emphasising the importance of including terrestrial non-target plants in herbicide risk assessment.

TREATMENT OF HIGH-CONCENTRATION SWINE WASTEWATER BY ANAEROBIC DIGESTION AND AN AQUATIC PLANT SYSTEM

The treatment of high-strength swine wastewater by anaerobic digestion combined with an aquatic plant system was investigated. Anaerobic digestion of swine wastewater gave volatile solids (VS) removal efficiencies of 43.3%, 52.1% and 54.5% for hydraulic retention times (HRTs) of 20, 30, 40 days, respectively. The removal efficiencies of VS, total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD) decreased with increasing VS volumetric loading rate (VLR). Higher organic removal efficiency was observed at longer HRTs for the same VS volumetric loading rate. As VS volumetric loading rate increased, biogas production increased and the methane content of the biogas decreased. Experiments using duckweed (Lemna species) as an aquatic macrophyte gave the following results. In the case of nitrogen, removal efficiency was above 60% and effluent concentration was below 10.0 mg/L when the influent ammonia-N loading was about . In the case of phosphorus, removal efficiency was above 55% and effluent concentration was below 2.0 mg/L when the influent -P loading was about . In addition, crude protein and phosphorus content of duckweed biomass increased from 15.6% to 41.6% and from 0.8% to 1.6%, respectively, as the influent nutrient concentration increased. The treatment of high-strength swine wastewater by anaerobic digestion combined with an aquatic plant system offers good performance in terms of organics and nutrient removal for relatively low operation and maintenance costs. The results indicate that under appropriate operational conditions, the effluent quality is within the limits set by Korean discharge criteria.

Conserved Expression Profiles of Circadian Clock-related Genes in Two Lemna Species Showing Long-day and Short-day Photoperiodic Flowering Responses

The Lemna genus is a group of monocotyledonous plants with tiny, floating bodies. Lemna gibba G3 and L. paucicostata 6746 were once intensively analyzed for physiological timing systems of photoperiodic flowering and circadian rhythms since they showed obligatory and sensitive photoperiodic responses of a long-day and a short-day plant, respectively. We attempted to approach the divergence of biological timing systems at the molecular level using these plants. We first employed molecular techniques to study their circadian clock systems. We developed a convenient bioluminescent reporter system to monitor the circadian rhythms of Lemna plants. As in Arabidopsis, the Arabidopsis CCA1 promoter produced circadian expression in Lemna plants, though the phases and the sustainability of bioluminescence rhythms were somewhat diverged between them. Lemna homologs of the Arabidopsis clock-related genes LHY/CCA1, GI, ELF3 and PRRs were then isolated as candidates for clock-related genes in these plants. These genes showed rhythmic expression profiles that were basically similar to those of Arabidopsis under light-dark conditions. Results from co-transfection assays using the bioluminescence reporter and overexpression effectors suggested that the LHY and GI homologs of Lemna can function in the circadian clock system like the counterparts of Arabidopsis. All these results suggested that the frame of the circadian clock appeared to be conserved not only between the two Lemna plants but also between monocotyledons and dicotyledons. However, divergence of gene numbers and expression profiles for LHY/CCA1 homologs were found between Lemna, rice and Arabidopsis, suggesting that some modification of clock-related components occurred through their evolution.

Dose-response relationships between herbicides with different modes of action and growth of Lemna paucicostata: an improved ecotoxicological method.

Lemna species are widely used in ecotoxicological research to assess the phytotoxicity of environmental compounds. The purpose of the work described in this paper was to further refine the Lemna spp. bioassay, making it more robust and useful in comparing results between laboratories. A nondestructive image analysis method was used to measure growth as affected by herbicides during a time course. We tested 26 herbicides, with as many as 19 different modes of action, on leaf area growth of Lemna paucicostata to establish complete dose-response relationships. By using 3,5-dichlorophenol as a reference compound, different EC50 (concentration that inhibited growth by 50%) values of the herbicides can be compared. The EC50 values ranged from 0.003 microM for sulcotrione to 407 microM for asulam. Complete dose-response parameters were obtained that will better allow comparison of these results to those of other laboratories. These results should give useful information to anyone who works with Lemna species to address questions on herbicide residues in environmental compartments, screening of new herbicidal compounds, or the assessment of phytotoxic side effects of any compound.

Callus induction and regeneration in Spirodela and Lemna.

The development of tissue culture systems in duckweeds has, to date, been limited to species of the genus Lemna. We report here the establishment of an efficient tissue culture cycle (callus induction, callus growth and plant regeneration) for Spirodela oligorrhiza Hegelm SP, Spirodela punctata 8717 and Lemna gibba var. Hurfeish. Significant differences were found among the three duckweed species pertaining to carbohydrate and phytohormone requirements for callus induction, callus growth and frond regeneration. In vitro incubation with poorly assimilated carbohydrates such as galactose ( S. oligorrhiza SP and L. gibba var. Hurfeish) and sorbitol ( S. punctata 8717) as sole carbon source yielded high levels of callus induction on phytohormone-supplemented medium. Sorbitol is required for optimal callus growth of S. oligorrhiza SP and S. punctata 8717, while sucrose is required for callus growth of L. gibba var. Hurfeish. Sucrose either alone ( S. oligorrhiza SP, L. gibba var. Hurfeish) or in addition to sorbitol ( S. punctata 8717) is required for frond regeneration.

Mechanisms of EDDHA effects on the promotion of floral induction in the long-day plant Lemna minor (L.)

EDDHA added in an optimal concentration (20.5 mumol.L-1) to a modified Pirson-Seidel nutrient solution induces flowering in some clones of the species Lemna minor, Lemna gibba and Spirodela polyrrhiza, which in the absence of EDDHA in the same nutrient solution do not flower. By adding EDDHA (20.5 mumol.L-1), floral induction under LD conditions is optimally promoted in the long-day (LD) species Lemna minor. After adding EDDHA to the nutrient solution, before floral induction and during flowering, Zn, Mn and Cu content is significantly increased in plants. Zn-EDDHA (0.86 mumol.L-1), Mn-EDDHA (1.51 mumol.L-1) and Cu-EDDHA (0.12 mumol.L-1), when used individually, greatly promote flowering under LD conditions as compared to flowering in the same nutrient solution with an equivalent quantity of Zn, Mn or Cu in the nonchelate form. If, on the other hand, Zn-EDDHA and Mn-EDDHA are added to the nutrient solution together (instead of Zn and Mn in nonchelate form), their effect on the promotion of flowering is less than in the case of their individual use. This shows that there is antagonism between Zn-EDDHA and Mn-EDDHA that is eliminated by adding EDDHA to the nutrient solution. We obtained the highest percentage of flowering plants (i.e. 74%) if we added EDDHA (20.5 mumol.L-1) to the nutrient solution containing Mn, Zn and Cu in chelate form. 74% of flowering plants actually means that flowering was achieved in all physiologically mature plants. Our results show that EDDHA promotes floral induction in Lemna minor under LD conditions, especially through chelating Zn, Mn and Cu, and, in addition, through eliminating the antagonism between Mn and Zn chelates EDDHA. Zn-EDDHA (0.86 mumol.L-1) also promote floral differentiation, especially cell division of microspore mother cells into dyads and those into microspore tetrads, which can be seen in microphotographs. When investigating possible pathways through which Mn-EDDHA, Zn-EDDHA and Cu-EDDHA promote flowering, we studied the effects of various concentrations of IAA and sucrose added to the nutrient solution as well. The results support the hypothesis that one of the possible pathways in which Mn-EDDHA promotes floral induction is through auxin oxidase, whereas Zn-EDDHA and Cu-EDDHA probably promote it through the enhancement of the photosynthesis and synthesis of sucrose.

Allozyme studies inLemnaceae: variation and relationships inLemnasectionsAlataeandBiformes

SummaryCrawford, D. J., Landolt, E., Les, D. H. & Kimball, R. T.: Allozyme studies inLemnaceae: variation and relationships inLemnasectionsAlataeandBiformes.– Taxon 50: 987–999. 2001.–ISSN 0040–0262.Allozyme variation was examined within and among three species ofLemna(Lemnaceae) that have been viewed as closely related based on morphological and DNA sequence data. These includeL. aequinoctialis, L. perpusilla, andL. tenera, with the former two species the only members of sect.Alataeand the latter the sole species of sect.Biformes. Lemna aequinoctialisis widely distributed in tropical and subtropical regions whereas the other two species have much narrower distributions. Sampling from one population ofL. aequinoctialisrevealed several multilocus genotypes but a deficiency of heterozygotes, which agrees with observations that the species can self‐fertilise and produce seed in addition to reproducing clonally. Two series of localized populations revealed several multilocus genotypes in each group, but no heterozygotes were detected at any of the variable loci. Twenty single clones from much of the geographic range ofL. aequinoctialiscontain the same alleles detected in the local populations; no pattern of large‐scale geographic variation was detected. Allozyme data support the recognition ofL. perpusillaas distinct fromL. aequinoctialis, but the two species share much higher identities at allozyme loci than either does with any other species ofLemna, thus agreeing with morphological and plastid DNA sequence data indicating that the two are closely related yet distinct species.Lemna tenerashares very low allozyme identities withL. aequinoctialisandL. perpusilla.Neither allozymes nor plastid sequences support the hypothesis thatL. perpusillais a hybrid betweenL. aequinoctialisandL. turionifera.Molecular data indicate that the three species form a strongly supported monophyletic group, with the common ancestor ofLemna teneraandL. aequinoctialis‐L. perpusillahaving diverged first, much later divergence occurring between the common ancestor ofL. aequinoctialisandL. perpusilla.

Removal of municipal solid waste COD and NH 4–N by phyto-reduction: A laboratory–scale comparison of terrestrial and aquatic species at different organic loads

Laboratory scale tests on phytodepuration of raw and pre-treated leachate from municipal sanitary waste were carried out with four vegetable aquatic and terrestrial species at different organic loads. We used the terrestrial species Stenotaphrum secundatum and the free-floating aquatic species Lemna minor, Eichhornia crassipes and Myriophyllum verticellatum to purify leachate from municipal solid waste. The organic load characterized by COD varied from 2–30 g m −2 day −1. Blanks using tap water served as controls. Duration of the experiments varied from 9–90 days. Maximum concentrations in the experiments were 1600 mg l −1 COD and 300 mg l −1 NH 4–N for S. secundatum. Best results in terms of COD, BOD, and ammonia removal were obtained for raw leachate with COD=2 g m −2 day −1 in free water surface (FWS) wetlands, and with 2 and 5 g m −2 day −1 in subsurface flow (SSF) wetlands. Results show that for pretreated leachate (labeled c) low in BOD and NH 4–N, the aquatic species showed low removal and stress even at the lowest load of COD=2 g m −2 day −1. We cannot say if this is due to the pretreatment itself or the chemical or microbial composition of this leachate. The Stenotaphrum system operated well with this load of leachate c. For untreated leachate (type a and b) the removal and plant growing conditions seemed good at COD=2 g m −2 day −1. For S. secundatum a load of COD=5 g m −2 day −1 operated well. All loads above COD=5 g m −2 day −1 caused low removal and stress, and the green parts of the plants disappeared. Oxygen was, however, consumed throughout the experimental period. For pretreated leachate (type c), the removal of COD were low (−24 to 17%) but good for NH 4–N (52–91%). This leachate also experienced high ammonia removal from the beginning of the experiments, probably due to existing consortia of nitrifying bacteria in it. Statistical analysis shows that the S. secundatum and L. minor systems maintained higher oxygen levels than the M. verticellatum and E. crassipes systems, when operated with tap water. For Lemna minor, this may be due to a better capacity for transporting oxygen into the water. With leachate all S. secundatum systems have higher oxygen levels than the aquatic systems, basically because the water content of the soil has been kept well below saturation. S. secundatum shows a significantly lower removal of COD than did the aquatic systems at a loading of COD=2 g m −2 day −1 of raw leachate. There is no significant difference between the systems in the removal of NH 4–N at a loading of COD=2 g m −2 day −1 of both types of leachate. E. crassipes has a lower removal of NH 4–N than M. verticellatum and S. secundatum at a loading of 5 g m −2 day −1 of COD of both types of leachate. In our experiments, it appears that the amount of free ammonia explains the toxicity of the leachate to the plants. This, however, does not exclude other possible toxic factors.

An Allozyme Study of Two Sibling Species of Lemna (Lemnaceae) with Comments on Their Morphology, Ecology and Distribution

CRAWFORD, D. J. (Department of Plant Biology, The Ohio State University, Columbus, OH 43210), E. LANDOLT (Geobotanisches Institut ETH, Zurichbergstrasse 38, CH-8044, Zurich, Switzerland) AND D. H. LES (Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269). An allozyme study of two sibling species of Lemna (Lemnaceae) with comments on their morphology, ecology and distribution. Bull. Torrey Bot. Club 123: 1-6. 1996.-Enzyme electrophoresis was employed to examine divergence between Lemna mninuta H.B.&K. and L. valdiviana Phil., two nearly indistinguishable species comprising sect. Uninerves of Lemna. Fifteen presumptive loci were resolved for 25 clones of Lemna minuta and 26 of L. valdiviana. Genetic identity between the two species is 0.70, which is near the mean value for congeneric species of flowering plants, but is much higher than values reported for other species of Lemnaceae. With the exception of two clones, the taxonomic designations of the strains correlated with monomorphism for alternative alleles at two MDH loci. The two species are highly differentiated at two other loci as well. Although the taxa are morphologically similar, they differ in geographic distribution, flavonoid chemistry, and certain physiological and ecological attributes. The relatively high identity at allozyme loci compared to other Lemnaceae suggests that L. minuta and L. valdiviana are closely related and recently diverged sister species.

Internal eutrophication of aquatic ecosystems: mechanisms and possible remedies

Summary Eutrophication is an important problem in aquatic environments in Europe. In the Netherlands, many aquatic macrophytes have strongly declined while, at present their former habitats are characterized by non-rooting species such as Lemna species, Spirodela polyrhiza (L.) Schleiden and Azollla filiculoides Lamark. The experiments reveal that processes that cause eutrophication without the external input of nutrients (i.e. internal eutrophication) play an important role in the eutrophication of semi-aquatic habitats. The alkaline nature and high sulphate content of the river Rhine water which is allowed into large parts of the peaty lowlands of the Netherlands is suggested to be responsible for this internal eutrophication. Internal eutrophication appears to be caused by increased mineralisation of organic matter in semi aquatic and aquatic habitats due to the alkalinisation of sediments. Alkalinisation is caused by the production of bicarbonate owing to the reduction of sulphate and the alkaline nature of the inlet water. Next, free iron levels in the sediment have decreased due to increased iron sulphide precipitation and decreased iron inputs by seepage. Finally, sulphide and phosphorus levels have increased after the exhaustion of mobile iron in the sediment. The results also show that the mobilisation of phosphorus from the sediment is prevented by the addition of iron to iron depleted-sediments. Some possible remedies that can prevent or diminish internal eutrophication are discussed.

Application of Lemna species in ecotoxicological studies of heavy metals and organic biocides

Owing to their small size and easy manipulation under aseptic conditions, duckweeds (Lemnaceae) provide good material for studying the effects of toxicants and organic biocides. Applying static acute ecotoxicological tests the responses of three duckweed species to different chemicals were compared. In each species the IC 50 values of copper and Bonion biocide reached 10 mg/1 and 15 mg/1, respectively. Comparing the two Lemna species L. minor proved to be more sensitive than L. gibba. Simultaneously with the measurement of the productivity the changes in the CO 2 and O 2 exchange were also determined during the static acute tests of biocide (methylene bisthiocyanate) by means of a gas analyser (Quadrupole MS) in special growth chambers. There were no correlations between the respiration of duckweds and the concentration of biocide. Differences were found in the intensity of the photosynthesis as a function of the biocide treatment.

Duckweed, A Useful Strategy for Feeding Chickens: Performance of Layers Fed with Sewage-Grown Lemnacea Species

Layer performance and egg quality were assessed in hens fed sewage-grown Lemna species (duckweed) in order to examine the safety and efficacy of this plant as a feedstuff for poultry. Dried Lemna gibba was included in the diets of two commercial strains of laying hens at 0, 15, 25, and 40% inclusion. Egg production and egg weights were compared with those of hens fed a standard isocaloric and isonitrogenous control diet At all levels of Lemna, hens maintained egg production and had mean egg weights similar to layers fed a control diet. Eggs from Leghorn hens fed 15 and 25% Lemna had higher protein content than control eggs. Also, the addition of Lemna to the diets significantly increased yolk pigmentation, an important commercial value for this plant. Lemna species may be a useful substitute for soybean and some fish meal in layer hen diets, especially in countries where some of these commodities are imported.

Flowering and Endogenous Levels of Plant Hormones in Lemna Species

Flowering and Endogenous Levels of Plant Hormones in Lemna Species

Phytochrome Control of Root Growth Rate in Lemna Species

Phytochrome Control of Root Growth Rate in Lemna Species

Flowering and Endogenous Levels of Benzoic Acid in Lemna Species

Flowering and Endogenous Levels of Benzoic Acid in Lemna Species

PHYTOCHROME VARIATION AND REVERSION IN LEMNA MINOR, L. GIBBA G3 AND L. PAUCICOSTATA 6746

Abstract— Dual wavelength phot00ometry showed an increase of phytochrome in darkness in partially etiolated plants of three heterotrophically cultured Lemna species. In all three species, Pfr formed by a brief illumination decreased in subsequent darkness with half‐lives of 2–6 h. In L. minor and L. gibba, dark reversion was observed during the first 8h of darkness; in L. paucicostata, however, only when the plants were in a slightly anaerobic condition. Under continuous illumination, phytochrome decay was not influenced by light intensity. Actinic doses of far red resulted in non‐photoreversible changes of absorbance. In material exposed to red for longer than 12 h, these changes increased after preceding actinic doses of red light. This effect may result in erroneous phytochrome estimations.

Gegensätzliche Geschlechtsausprägung von Blütenstandsanlagen der Langtagpflanze Lemna gibba und der Kurztagpflanze Lemna paucicostata in vitro

Inflorescence primordia of two species of the genus Lemna develop on synthetic agar media in vitro. On medium consisting of agar, salts of the PIRSON-SEIDEL-solution, sucrose, kinetin, gibberellic acid, indoleacetic acid, some vitamins, nucleotides and casein hydrolysate the explants of the long-day plant Lemna gibba and of the short-day plant Lemna paucicostata develop in a similar manner. Young, fully meristematic primordia (stage 2) show only little or no growth response, whereas primordia of both species with a pistil length of ≥ 0.08 mm (stage 3) always undergo some growth. With stage 4 full development of all flower organs occurs, if medium composition is adequate. With stage 5 not only mature flowers develop, but also fructification and development of ovules to unripe seeds occur. On a more simple medium containing 2 % sucrose and 1 mg/1 kinetin as the sole organic additions, better development occurs especially in the younger stages. On this medium, however, the relative development of stamens and pistils is altered. In Lemna gibba pistil development is enhanced, which leads to feminisation. In Lemna paucicostata growth of stamens is enhanced, which leads to masculinisation of the explants. This different developmental behaviour of the two photoperiodic reaction types presumably is due to a different endogenous hormone production in the meristems of both species.

Morphological differentiation and geographical distribution of the Lemna gibba-lemna minor group

A 100 strains of Lemna gibba L. s.l. and 350 strains of L. minor L. s.l. have been morphologically investigated from a collection of living strains. The following characters have been considered (Table I): shape and size of root-cap, shape and size of fronds, structure of the upper surface, gibbosity, innervation, pigmentation and turion formation. A new feature, useful in differentiation of L. gibba s.l. and L. minor s.l. is given in the number and disposition of the nerves (Fig. 2). The systematic position of L. disperma Hegelmaier and L. parodiana Giadelli could not be clarified because there were not enough flowering plants. Both taxa seem to occur within the ecological and geographical limits of L. gibba s. str. L. minor s.l. can be subdivided into three taxa according to morphological, geographical and ecological features: L. minor L. s. str., L. obscura (Austin) Daubs and L. turionifera spec. nov. L. turionifera differs from L. minor mainly by the formation of turions and the pigmentation of the lower surface of fronds. L. obscura is able to produce gibbous fronds; further, the prominent papilla at the apex and the pigmentation of the lower surface are typical. Measurements performed in aquatic habitats of various Lemna species in North America show that L. gibba colonizes waters that show the highest concentration of calcium, magnesium, sodium, nitrogen and phosphorus. The lowest concentrations have been found in waters with L. minor s. str. (Table II). The geographical distribution of L. gibba s.l. is distinctly limited by climatic boundaries towards too cold, too warm and too wet regions. The boundary against too cold areas is fairly coincident with the January isotherm of about −1°C. The limit against warmer areas is given by the 18°C isotherm of the three coolest months. Further limits are given by precipitations of at the utmost 90 mm/year or by the absence of a dry period during the growing season. The regions situated outside the 10° C isotherm for the three warmest months are too cold for L. minor s.l.; on the other hand, areas where the isotherm for the three coolest months is higher than 15°C are apparently too warm. L. turionifera occupies more continental regions of North America and eastern Asia, characterized by low amount of precipitations and low minimal temperatures. L. minor s. str. grows in rather a moist climate, being rare or absent in more arid regions. L. obscura is confined to the south-eastern United States that have mild winters and hot humid summers. The distributional limits have to be explained partly by the competition between various species of Lemnaceae.

Cytological variation within Lemna L.

Three levels of cytological variation were found within the genus Lemna: (1) intra-individual variation, i.e. aneusomaty and mixoploidy; (2) variation within populations; and (3) “racial” variation resulting in populations and clones that were cytologically uniform yet represented various cytotypes. The rather wide range of chromosome numbers (2 n= 20, 30, 36, 40, 42, 50, 60, 70, 80) suggests that various mechanisms operate in somatic tissues of the Lemna species. However, the material is highly unsuitable for a more detailed analysis. 2 n = 20 and 2 n = 30 might represent an occasional reversion to lower levels of polyploidy. Aneuploid chromosome numbers occurring in Lemna might result from mitotic distrubances or arise through structural rearrangements of chromosomes. However, pronounced aberrations were virtually absent in the material studied. The most frequently observed differences in chromosome number within mixoploid plants corresponded to a single genome. It might be supposed that a selective endoduplication operated only in some sectors of the given nuclei, resulting in this particular trend of polyploidization. It is assumed that mechanisms giving rise to aneusomaty and mixoploidy may be tools in the differentiation of taxa that manifest a pronounced asexual reproduction.

Título en español.

Se determinó el efecto de los herbicidas Fluometuron, Prometryne, Ametryne y Diuron sobre el crecimiento autotrófico de las especies acuáticas Lemna major L. y Lemna perpusilla Torr. El Fluometuron resultó menos tóxico a ambas especies de Lemna. El Diuron fué el herbicida más tóxico a L. major, mientras Ametryne fué el más tóxico a L. perpusilla. Las concentraciones del herbicida requeridas para causar 50 por ciento de inhibición del crecimiento de L. major fueron las siguientes: a) Fluometuron, 2.6 X 10-6 M; b) Prometryne, 3.5 X 10-7 M; c) Ametryne, 2.30 X 10-7 M. Las concentraciones necesarias para causar 50 por ciento de inhibición del crecimiento de L. perpusilla fueron las siguientes: a) Fluometuron, 2.05 X 10-6 M; b) Diuron, 6.4 X 10-8 M; c) Prometryne, 5.4 X 10-8 M y d) Ametryne, 4.6 X 10-8 M. Se concluyó que la aplicación de estos herbicidas a concentraciones micromolares constituye un riesgo para las dos especies de Lemna sometidas a tratamiento.