Effects of substrate and shading on the growth of two submerged macrophytes

Excessive nutrient loading may cause a shift from submerged macrophyte dominance to free-floating macrophyte dominance. Tolerance and persistence of submerged plants in response to shade may be key characteristics in determining when/if such a shift occurs in shallow eutrophic lakes. This study examines how the cover of floating macrophyte (Lemna minor) and shade of dark mesh affect the growth and photosynthetic efficiency of two submerged plants (Vallisneria natans and Myriophyllum spicatum) on different nutrient substrates. We found that low- and mid-cover intensities generally enhanced the leaf/shoot growth of both submerged plants under all cover and substrate types. The relative growth rates (RGR) were slightly enhanced under the treatment of Lemna with low- and mid-intensity cover on both nutrient-rich substrates. The leaf/shoot growth and RGR of both submerged macrophytes generally increased more under Lemna cover than mesh cover. The photosynthetic efficiency (F _v/F _m value) typically increased with the duration of treatment and the cover densities. In addition, these two macrophytes with contrasting growth forms have markedly different growth and survival strategies in response to covers. These results strengthen the hypothesis that submerged plants can successfully develop under a low-intensity cover of floating vegetation on nutrient-rich substrate.     

Effects of aquatic vegetation type on denitrification

In a microcosm ¹⁵N enrichment experiment we tested the effect of floating vegetation (Lemna sp.) and submerged vegetation (Elodea nuttallii) on denitrification rates, and compared it to systems without macrophytes. Oxygen concentration, and thus photosynthesis, plays an important role in regulating denitrification rates and therefore the experiments were performed under dark as well as under light conditions. Denitrification rates differed widely between treatments, ranging from 2.8 to 20.9 ??mol N m^?2 h^?1, and were strongly affected by the type of macrophytes present. These differences may be explained by the effects of macrophytes on oxygen conditions. Highest denitrification rates were observed under a closed mat of floating macrophytes where oxygen concentrations were low. In the light, denitrification was inhibited by oxygen from photosynthesis by submerged macrophytes, and by benthic algae in the systems without macrophytes. However, in microcosms with floating vegetation there was no effect of light, as the closed mat of floating plants caused permanently dark conditions in the water column. Nitrate removal was dominated by plant uptake rather than denitrification, and did not differ between systems with submerged or floating plants.     

Long‐term allelopathic control of phytoplankton by the submerged macrophyte Elodea nuttallii

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Growth limitation of submerged aquatic macrophytes by inorganic carbon

1. This study determined the effects of CO₂ and HCO₃- enrichment on in situ growth of two submerged macrophytes, Elodea canadensis and Callitriche cophocarpa, in two Danish lakes: Lake Hampen and Lake Væng. Lake Hampen is an oligotrophic low-alkaline lake (0.4 meq ^?1) and Lake Væng is mesotrophic with an alkalinity of 1.1 meq 1-^?1. In Lake Hampen experiments were carried out throughout the growth season, whereas experiments in Lake Væng were restricted to late summer. The CO₂ and HCO₃-enrichment procedures used increased the concentration of free-CO₂ by 500–1000 μM and the concentration of HCO₃- by about 80 μM. 2. The concentration of free-CO₂ in Lake Hampen was about five times atmospheric equilibrium concentration (55 μM) in early summer declining to virtually zero at the end of summer. 3. Under ambient conditions Callitriche, which is restricted to CO₂ use, was unable to grow and survive in both lakes. In contrast, Elodea, which has the potential to use HCO₃- in photosynthesis, grew at rates varying from 0.046 to 0.080 day^?1 over the season. 4. Under CO₂ enrichment the growth rate of Callitriche varied from 0.089 to 0.124 day^?1 and for Elodea from 0.076 to 0.117 day^?1 over the season. Enrichment with HCO₃-affected Elodea only and only to a limited extent. This may be a result of insufficient increase in [HCO₃-] upon enrichment or to a limited capacity of the plants to take up HCO₃-. 5. The substantial stimulation of in situ growth of Elodea and Callitriche by enhanced concentrations of free-CO₂ shows that inorganic carbon is an important determinant of growth of submerged macrophytes and that inorganic carbon limitation of in situ growth may be a common phenomenon in nature, even in lakes with an alkalinity as high a 1 meq 1-^?1. Inorganic carbon, however, is only one of many parameters important for growth, and the growth rates of Elodea at both ambient and high free-CO₂ were closely coupled to day length and photon irradiance, indicating that light had an ultimate control on growth.     

Denitrification in the periphyton associated with plant shoots and in the sediment of a wetland system supplied with sewage treatment plant effluent

Seasonal variation in denitrification and major factors controlling this process were determined in sediment, microbial communities attached to plant shoots (periphyton) and in the water of a Phragmites and an Elodea-dominated stand of a constructed wetland system between May 1997 and February 1998. The wetland was supplied with effluent from a sewage treatment plant. The denitrification rate in periphyton on plants shoots (expressed per shoot area) was always considerably higher than in the sediment and varied with the chlorophyll-a content of the periphyton in the course of the year. The algae in the periphyton provided attachment surfaces and probably also organic compounds to the denitrifying bacteria. Decreases in periphyton biomass and denitrification rate in the Phragmites and Elodea-dominated stands during the growing season were associated with enhanced shading by Phragmites shoots or a floating layer of macro-algae and Lemna spp., respectively. Light availability and the denitrification rate of periphyton increased again after the Phragmites shoots were cut in October. Nitrate appeared to limit the denitrification rate in the sediment. Periphyton denitrification rates were mostly lower on Elodea shoots than on Phragmites shoots, in spite of the higher living algal biomass on Elodea shoots. This difference was associated with lower nitrate concentrations in the Elodea-dominated stand. In the two stands, the daily denitrification rates in periphyton on shoots of Phragmites australis (44.4–121 mg N m^–2 stand area d^–1) and Elodea nuttallii (14.8–33.1 mg N m^–2 d^–1) were clearly more important than rates in the sediment (0.5–25.5 mg N m^–2 d^–1) or the water (0.4–3.9 mg N m^–2 d^–1). The presence of few bacteria attachment sites or low organic carbon availability possibly resulted in low denitrification rates in the water. Denitrification appeared to be a major process in nitrate removal from the through-flowing water in this wetland system.     

Development of a zooplankton community including <Emphasis Type="Italic">Heterocope saliens</Emphasis> Lillijeborg in an experiment with hydrophytes

The influence of the submerged plants Ceratophyllum demersum L. and Elodea canadensis Michx. and the floating plant Hydrocharis morsus-ranae L. on the species composition and quantitative parameters of a zooplankton community was studied experimentally. Among submerged vegetation, the development of the predaceous calanid Heterocope saliens Lilljeborg was suppressed. An increase in the number of zooplankter species was observed in all experimental ecosystems with hydrophytes. The species similarity of zooplankton was higher between communities with plants of the same ecological group than with plants of different groups. The highest average zooplankton biomass, as determined by the abundance of Daphnia longispina O.F. Müller and Simocephalus vetulus (O.F. Müller), was observed in experiments with Elodea. The highest average abundance over the experimental period was recorded among Ceratophyllum, where the abundance of Rotifera, chydorids, and copepods common in hydrophyte beds was higher than in other versions of the experiment.     

Morphological and Morphometrical Variations of Selected Rotifer Species in Response to Predation: a Seasonal Study of Selected Brachionid Species from Lake Xochimilco (Mexico)

Allelopathy of filamentous green algae (FGA) has been less studied than that of macrophytes. Little Budworth Pool, Cheshire, UK is a small, shallow, clear-water lake with high TP concentrations, very high NO_3-N concentrations, only moderate phytoplankton density, high FGA growth (mainly Spirogyra sp.) and no submerged plants. Experiments were carried out to test the possible allelopathic effects of Spirogyra on the phytoplankton of this lake and on a submerged plant Elodea nuttallii. Changes in phytoplankton growth, phytoplankton species dynamics and species composition were apparently not influenced by allelopathy of live or decaying Spirogyra. A shift from diatom (Cyclotella sp) – cryptomonad (Chroomonas acuta and Cryptomonas erosa) dominance to Chlorococcales (Micractinium pusillum, Monoraphidium contortum and Scenedesmus opoliensis) – Volvocales (Chlorogonium elongatum and Pandorina morum) dominance was recorded in both control and FGA treatments, suggesting an effect of nutrient enrichment. Nutrient concentrations and differences in competitiveness among phytoplankton species can also explain differences in their growth rates in Spirogyra filtrate. Spirogyra also did not influence apex number per plant, shoot length or growth rate of E. nuttallii. This FGA species probably cannot control phytoplankton or E. nuttallii growth in nutrient rich conditions through allelopathy.     

Feeding and growth of Asellus aquaticus (Isopoda) on food items from the littoral of Windermere, including green leaves of Elodea canadensis

SUMMARY. In the laboratory, Asellus aquaticus devoured intact green leaves from growing shoots of the aquatic macrophyte Elodea canadensis. In four collections of A. aquaticus on Elodea in a lake (Windermere), c. 20% of the specimens contained in their guts fragments of green Elodea leaves; this material and pieces of oak (Quercus) were identified from characteristic leaf hairs. Some specimens had also eaten the filamentous alga Oedogonium. Fluorescence microscopy is a useful aid for screening invertebrates that may have eaten living plant tissues. Immature A. aquaticus, with an initial mean body length of c. 3 mm, wet weight c. 1 mg, were grown through sexual maturity over a 49-day period at 15°C in a series of twenty-two experiments (six to twelve isolated specimens in each experiment) comparing growth rates on different foods, including instances where no food was given. Animals were fed on a variety of items collected from the littoral of Windermere, plus some laboratory cultures of algae and bacteria. The highest mean specific growth rate (5.8% day^?1) was obtained on young Elodea leaves mechanically shaken to remove epiphytes. Other diets yielding fast growth rates (3.7–5.3% day^?1) were young growing leaves of Elodea with few epiphytes and older green and brown living leaves covered with a thick growth of epiphytic algae, epiphytic algae removed from Elodea, plastic imitation Elodea immersed in the lake until covered with attached algae, epilithic algae on stones, Oedogonium, and decaying oak leaves. Slower growth (1.3–2.2% day^?1) and poorer survival was obtained on the following: a pure culture of the bacterium Sphaerotilus natans; cultured bacteria from lakewater; the filamentous algae Cladophora and Stigeoclonium both with and without epiphytes; faecal matter from Asellus; freshly killed Asellus; lake sediment. Some growth (mean = 0.7% day^?1) and 50% survival for 21 days occurred in ‘starved’ animals kept in filtered, sterilized lakewater. Better survival and slightly faster growth (1.0–1.5% day^?1) occurred in ‘starved’ animals kept in filtered and unfiltered lakewater. Growth of A. aquaticus was also experimentally determined from birth in animals fed on young green Elodea leaves and on decaying oak leaves. On both diets, growth was curvilinear and approximately exponential from birth to sexual maturity reached at c. 2mg wet weight in 46–60 days at 15°C. In older specimens the relative growth rate gradually fell over a period of 50 days, representing a more linear phase of growth during sexual maturity.     

Combined effects of water column nitrate enrichment, sediment type and irradiance on growth and foliar nutrient concentrations of Potamogeton alpinus

1. High water column NO₃^? concentrations, low light availability and anoxic, muddy sediments are hypothesised to be key factors hampering growth of rooted submerged plants in shallow, eutrophic fresh water systems. In this study, the relative roles and interacting effects of these potential stressors on survival, growth, allocation of biomass and foliar nutrient concentrations of Potamogeton alpinus were determined in a mesocosm experiment using contrasting values of each factor (500 versus 0 μmol L^?1 NO₃^?; low irradiance, corresponding to the eutrophic environment, versus ambient irradiance; and muddy versus sandy sediment). 2. Low irradiance, high NO₃^? and sandy sediment led to reduced growth. In a muddy sediment, plants had lower root : shoot ratios than in a sandy sediment. 3. Growth at high NO₃^? and on the sandy sediment resulted in lower foliar N and C concentrations than in the contrasting treatments. The C : N ratio was higher at high NO₃^? and on the sandy sediment. Foliar P was higher on the muddy than on the sandy sediment but was not affected by irradiance or NO₃^?. The N : P ratio was lowest at high NO₃^? on the sandy sediment. 4. Total foliar free amino acid concentration was lowest on sand, low irradiance and high NO₃^?. Total free amino acid concentration and growth were not correlated. 5. Turbidity and ortho‐PO₄^3? concentration of the water layer were lower at high water column NO₃^? indicating that the growth reduction was not associated with increased algal growth but that physiological mechanisms were involved. 6. We conclude that high water column NO₃^? concentrations can significantly reduce the growth of ammonium preferring rooted submerged species such as P. alpinus, particularly on sediments with a relatively low nutrient availability. Further experiments are needed to assess potential negative effects on other species and to further elucidate the underlying physiological mechanisms.     

Time‐dependent effects of fertilization on plant biomass in floating fens

Abstract. A cross‐over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m^?2 N, 5 g.m^?2 P, both elements and unfertilized control). The above‐ground biomass production of vascular plants was co‐limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N‐fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P‐fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N‐fertilization vascular plant growth was still co‐limited by N and P, but one year after P‐fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N‐fertilizer had mostly ‘disappeared’ from N‐fertilized plots, whereas the availability of P remained markedly enhanced in P‐fertilized plots. In addition, P‐fertilization enhanced the uptake of N by plants the following year. The time‐dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P‐fertilization on the availability of N; (4) contrasting effects of N‐ and P‐fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above‐ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non‐limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer‐term experiments are needed, so that indirect effects have time to develop and be detected.     

FACTORS AFFECTING SURFACE ALKALINE PHOSPHATASE ACTIVITY IN THE BROWN ALGA FUCUS SPIRALIS AT A NORTH SEA INTERTIDAL SITE (TYNE SANDS,SCOTLAND)1

Surface alkaline phosphatase activity (APA) was measured in the tips of Fucus spiralis L. thalli over an approximately 1-year period, using small plants from upper littoral rock pools at a site on the east coast of Scotland. Maximum APA (4.4 μmol p-NP·g dry wt^?1·h^?1) occurred in April, coincident with a sharp increase in tissue N:P ratio of the tips and the probable period of most rapid growth. Enzymatic activity was not correlated with any of the physical and chemical factors (temperature, pH, external phosphate, salinity) measured in the pools or nutrient concentrations (C, N, P) in the apical part of the thallus. No obvious correlation was found between APA and the position of a plant on the shore. However, APA was significantly higher in plants emersed at low tide than in plants submerged during the whole tidal cycle. There was a significant positive correlation between APA and tissue P within a single thallus, with the highest value at the tip (which includes the meristematic zone). Changes in APA of Fucus spiralis appear to result from a complex interaction of biotic and environmental factors, including stress by emersion, growth period, rapid changes in tissue nutrient content and N:P ratio, and regzon of thallus.     

Transpiration does not control growth and nutrient supply in the amphibious plant Mentha aquatica

Mentha aquatica L. was grown at different nutrient availabilities in water and in air at 60% RH. The plants were kept at 600 mmol m^?3 free CO₂ dissolved in water (40 times air equilibrium) and at 30 mmol m^?3 CO₂ in air to ensure CO₂ saturation of growth in both environments. We quantified the transpiration-independent water transport from root to shoot in submerged plants relative to the transpiration stream in emergent plants and tested the importance of transpiration in sustaining nutrient flux and shoot growth. The acropetal water flow was substantial in submerged Mentha aquatica, reaching 14% of the transpiration stream in emergent plants. The transpiration-independent mass flow of water from the roots, measured by means of tritiated water, was diverted to leaves and adventitious shoots in active growth. The plants grew well and at the same rates in water and air, but nutrient fluxes to the shoot were greater in plants grown in air than in those that were submerged when they were rooted in fertile sediments. Restricted O₂ supply to the roots of submerged plants may account for the smaller nutrient concentrations, though these exceeded the levels required to saturate growth. In hydroponics, the root medium was aerated and circulated between submerged and emergent plants to minimize differences in medium chemistry, and here the two growth forms behaved similarly and could fully exploit nutrient enrichment. It is concluded that the lack of transpiration from leaf surfaces in a vapour-saturated atmosphere, or under water, is not likely to constrain the transfer of nutrients from root to shoot in herbaceous plants. Nutrient deficiency under these environmental conditions is more likely to derive from restricted development and function of the roots in waterlogged anoxic soils or from low porewater concentrations of nutrients.     

Stimulation of nodulation in field peas (Pisum sativum) by low concentrations of ammonium in hydroponic culture

Although the inhibitory effects of high concentrations of mineral N (> 1.0 mM) on nodule development and function have often been studied, the effects of low, static concentrations of NH₄⁺ (< 1.0 mM) on nodulation are unknown. In the present experiments we examine the effects of static concentrations of NH₄⁺ at 0, 0.1 and 0.5 mM in flowing, hydroponic culture on nodule establishment and nitrogenase activity in field peas [Pisum sativum L. cv. Express (Svalöf AB)] for the initial 28 days after planting (DAP). Peas grown in the presence of low concentrations of NH₄⁺ had significantly greater nodule numbers (up to 4-fold) than plants grown without NH₄⁺. Nodule dry weight per plant was significantly higher at 14, 21 and 28 DAP in plants grown in the presence of NH₄⁺, but individual nodule mass was lower than in plants grown without NH₄⁺. The nodulation pattern of the plants supplied with NH₄⁺ was similar to that often reported for supernodulating mutants, however the plants did not express other growth habits associated with supernodulation. Estimates of N₂ fixation indicate that the plus-NH₄⁺ peas fixed as much or more N₂ than the plants supplied with minus-NH₄⁺ nutrient solution. There were no significant differences in nodule numbers, nodule mass or NH₄⁺ uptake between the plants grown at the two concentrations of NH₄⁺. Nodulation appeared to autoregulate by 14 DAP in the minus-NH₄⁺ treatment. Plant growth and N accumulation in the minus-NH₄⁺ plants lagged behind those of the plus-NH₄⁺ treatments prior to N₂ fixation becoming well established in the final week of the experiment. The plus-NH₄⁺ treatments appeared not to elicit autoregulation and plants continued to initiate nodules throughout the experiment.     

Growth retardation induced by nutritional deficiency or abscisic acid in Lemna gibba: The relationship between growth rate and endogenous cytokinin content

The relationship between endogenous cytokinin content and relative growth rate (RGR) was studied in cultures of Lemna gibba L. G3 supplied with daily doses of mineral nutrients that were increased exponentially over time. At the optimal level of nutrient supply the RGR was 30–35% day^-1. The RGR was regulated by adjusting the rate of nitrogen supply, or it was restricted by addition of 0.5 M abscisic acid (ABA). Another approach used to investigate the specific roles of nitrogen (N) and phosphorus (P), was to transfer optimally growing plants to media without N or P but otherwise complete. The plants were harvested at regular intervals for determination of the RGR and levels of cytokinins of the isopentenyladenosine (iPA) and zeatinriboside (ZR) types with an enzyme-linked immunosorbent assay (ELISA). Levels of both iPA- and ZR-type cytokinins decreased when nitrogen was applied to cultures in growth limiting amounts. The cytokinin levels decreased more rapidly than the RGR when either N or P was lacking in the medium, suggesting an early influence of nutrient availability on cytokinin levels which in turn may induce adaptive response by the plant. RGR retardation induced by ABA did not affect cytokinin levels during the first 4 days of the treatment, and the later effects were small. The experiments gave no indication that ABA is involved in the adaptation response of Lemna plants to nutritional stress.Abbreviations ABA - abscisic acid - BAP - benzylaminopurine - ELISA - enzyme-linked-immunosorbent-assay - iP - isopentenyladenine - iPA - isopentenyladenosine - PBS - phosphate-buffered saline - PVP - polyvinylpyrrolidone - RGR - relative growth rate - R_N - relative nitrogen addition rate - Z - trans-zeatin - ZR - trans-zeatin riboside     

Impacts of a submerged plant (Elodea canadensis) on interactions between roach (Rutilus rutilus) and its invertebrate prey communities in a lake littoral zone

1. Using 5‐m² field enclosures, we examined the effects of Elodea canadensis on zooplankton communities and on the trophic cascade caused by 4–5 year old (approximately 16 cm) roach. We also tested the hypothesis that roach in Elodea beds use variable food resources as their diet, mainly benthic and epiphytic macroinvertebrates, and feed less efficiently on zooplankton. Switching of the prey preference stabilises the zooplankton community and, in turn, also the fluctuation of algal biomass. The factorial design of the experiment included three levels of Elodea (no‐, sparse‐ and dense‐Elodea) and two levels of fish (present and absent). 2. During the 4‐week experiment, the total biomass of euplanktonic zooplankton, especially that of the dominant cladoceran Daphnia longispina, decreased with increase in Elodea density. The Daphnia biomass was also reduced by roach in all the Elodea treatments. Thus, Elodea provided neither a favourable habitat nor a good refuge for Daphnia against predation by roach. 3. The electivity of roach for cladocerans was high in all the Elodea treatments. Roach were able to prey on cladocerans in Elodea beds, even when the abundance and size of these prey animals were low. In addition to cladocerans, the diet of roach consisted of macroinvertebrates and detrital/plant material. Although the biomass of macroinvertebrates increased during the experiment in all Elodea treatments, they were relatively unimportant in roach diets regardless of the density of Elodea beds. 4. Euplanktonic zooplankton species other than Daphnia were not affected by Elodea or fish and the treatments had no effects on the total clearance rate of euplanktonic zooplankton. However, the chlorophyll a concentration increased with fish in all the Elodea treatments, suggesting that fish enhanced algal growth through regeneration of nutrients. Thus, our results did not unequivocally show that Elodea hampered the trophic cascade of fish via lowered predation on grazing zooplankton. 5. In treatments with dense Elodea beds (750 g FW m^?2), chlorophyll a concentration was always low suggesting that phytoplankton production was controlled by Elodea. Apparently, the top‐down control of phytoplankton biomass by zooplankton was facilitated by the macrophytes and operated simultaneously with control of phytoplankton production by Elodea.     

Nutrient release and resuspension generated by ruffe (Gymnocephalus cernuus) and chironomids

1. Benthivorous fish may play an important role in internal nutrient loading. Ruffe are highly specialised, feeding exclusively on bottom animals; thus all nutrients released via their feeding are derived from the bottom and are new to the water column. The fish can also release nutrients from the sediment through resuspension while searching for food. 2. The aim of this study was to estimate experimentally in the laboratory the effect on water quality of resuspension and nutrient release by ruffe and bottom animals (chironomids). 3. Ruffe released nutrients during 8 h experiments as follows: total P 1.4, dissolved PO₄ 0.6, total N 24.0 and NH₄‐N 15.9 μg g^?1 WW h^?1. A decreasing trend in mass‐specific release was observed over time, probably because of starvation. The mass‐specific release of total N and NH₄‐N decreased as the mean weight of fish increased. The mean ratio of excreted N : P was 32. 4. In 26 h experiments with sediment and both ruffe and chironomids, ruffe increased nutrient concentrations and turbidity values significantly but chironomids had an effect only on turbidity. Neither ruffe nor chironomids affected the ratio of inorganic N : P concentrations. An interaction between ruffe and chironomids was found for turbidity. 5. According to these results, benthivorous fish may increase nutrient concentrations in the water column and need to be taken into account when estimating internal loading.     

Receptor modifiers indicate that 4-aminobutyric acid (GABA) is a potential modulator of ion transport in plants

GABA (4-aminobutyric acid) is a ubiquitousnon-protein amino acid that accumulates rapidly inplants in response to stress. GABA was firstidentified in plants (potato tubers) and animals(brain tissue) 50 years ago. Although GABA is nowrecognized as the most important inhibitoryneurotransmitter in the mammalian central nervoussystem (CNS), the role of GABA in plants remainsunclear. Studies were performed using Lemna toinvestigate the possibility that GABA elicits aresponse in plants that may be related to that of asignaling molecule as described for GABA effects onthe CNS. Lemna growth was increased 2 to 3-foldby 5 mM GABA, but growth was strongly inhibited by 0.5mM of the isomers 3-aminobutyric acid and2-aminobutyric acid. Growth promotion by GABA wasrapidly terminated by addition of 2-aminobutyric acidto the culture medium, but inhibitory effects of2-aminobutyric acid were not reversed by GABAregardless of amounts added. Promotion of Lemnagrowth by GABA was associated with an increase inmineral content of treated plants in a dose dependentmanner. Results support the hypothesis that GABAactivity in plants involves an effect on ion transportand an interaction with a receptor. Evidence for GABAreceptors in Lemna was obtained from experimentswith pharmacological agents that have been used toidentify GABA receptors in animals. GABA mediatedpromotion of Lemna growth was inhibited bybicuculline and picrotoxin, which are respectivelycompetitive and non-competitive antagonists of GABAreceptors in the CNS. Growth inhibition bybicuculline was not relieved by increasing the amountsof GABA in the medium, indicating that the alkaloid isnot acting, as in the CNS, by competitive antagonismof GABA at GABA receptor sites. Baclofen, a GABAagonist that promotes GABA activity in animalssignificantly increased GABA mediated promotion ofLemna growth. These findings and the knownaction of GABA in regulating ion channels in animalssuggests a way that GABA could amplify the stressresponse in plants.     

Interaction between the macrophyte <Emphasis Type="Italic">Stratiotes aloides</Emphasis> and filamentous algae: does it indicate allelopathy?

The aquatic macrophyte Stratiotes aloides Linnaeus, which has recently received attention in studies on allelopathy, has been shown to suppress phytoplankton growth. In the Netherlands, S. aloides often co-occurs with floating filamentous algae. However, filamentous algae are generally absent in close proximity to S. aloides, resulting in gaps in filamentous algae mats. We analyzed whether those gaps may be caused by allelopathic substances excreted by S. aloides or by nutrient depletion. We studied in a field survey the colonization of natural S. aloides by filamentous algae and determined in situ nutrient concentrations in natural S. aloides stands. To analyze the relative importance of allelopathy and nutrient competition in the interaction between S. aloides and filamentous algae, we carried out field experiments. Introduction of artificial (non-allelopathic) plants in natural S. aloides stands enabled us to compare the colonization by filamentous algae of both Stratiotes sp. and artificial plants. The filamentous algae were absent in close vicinity to S. aloides. Significantly lower concentrations of ortho-phosphate and potassium were observed close to S. aloides as compared with the filamentous algae. In the field experiments the artificial plants were rapidly colonized by filamentous algae, mainly Cladophera Kützing and Spirogyra Link, while all natural plants remained free of such algae. Additionally, most nutrient concentrations did not significantly differ in the proximity of artificial or natural stands of S. aloides. The concentrations of the major growth-limiting nutrients, phosphate and nitrate, were significantly higher and nonlimiting in natural Stratiotes stands. Our main conclusion is that, although allelopathic interactions between S. aloides and filamentous algae do occur under natural conditions, nutrient competition between the two can also be an important factor.     

Effects of Zn<Superscript>2+</Superscript> on nodulation and growth of a South American actinorhizal plant, <Emphasis Type="Italic">Discaria americana</Emphasis> (Rhamnaceae)

Discaria americana is a xerophytic shrub which lives in symbiosis with an actinomycete of the genus Frankia. The objective of this paper was to investigate the effects of high soil Zn²⁺ concentrations on growth and nodulation on the association Discaria americana–Frankia with the aim of determining if this association is suitable for improving contaminated soils. Two experiments were performed in 1 dm³ pots containing soil and different Zn additions, from 0 to 2,000 mg Zn²⁺ kg⁻¹ dry soil, with or without N fertilization. Zn additions strongly delayed shoot and root growth, but once growth was initiated, the biomass production of the plants supplied with moderate Zn amounts did not differ from the control plants. Zn reduced the final nodule number, but not the total nodule biomass. At the end of the experiment only the highest Zn treatments showed a lower nodule weight than the control plants, while N addition completely inhibited nodulation. It is concluded than Zn reduces the number of Frankia infections, but once the actinomycete is inside the roots, nodules can continue growing according to plant demand for N, compensating the reduced nodule number with more biomass. On the other hand, there is a toxic effect of Zn itself on plants when present in very high concentrations.     

Growth, photosynthesis and acclimation by two submerged macrophytes in relation to temperature

In this study we examine the influence of temperature on growth, photosynthetic performance and acclimation of two submerged macrophyte species, Elodea canadensis L.C. Rich and Ranunculus aquatilis (L.) Wimmer. The plants were grown at 5, 10 and 15°C and a photon flux density of 300 μmol m^?2 s^?1 (PAR) in a medium with an alkalinity of 0.85 meq l^?1 bubbled with atmospheric air containing 400?ppm CO₂. In general, growth rates of both species increased with temperature with a Q ¹⁰ varying from 2.3 to 3.5. An exception was Elodea at 5°C, where growth was nearly arrested. Temperature effects on ambient rates of net photosynthesis and photosynthetic capacity followed the pattern observed for growth. Dark respiration was not suppressed for Elodea at 5°C and both species had a Q ¹⁰ of 2.3. The light-use efficiency (α^I) for photosynthesis declined with increasing growth temperature for Ranunculus. For Elodea no difference in α^I was observed between 10 and 15°C; at 5°C, however, α^I was reduced by about 30%. Both species acclimated to temperature as shown in a series of experiments in which the plants were exposed to a change in temperature. Acclimation was faster for shoots transferred from low to high temperature, where growth rates stabilised after a few days; for shoots transferred to low temperature growth rates still changed after 22 days. Although acclimation was evident, the changes in the metabolic apparatus were insufficient to balance effects of temperature. It is suggested that temperature may affect local distribution of the two species and their ability to grow in turbid or deep water.