Aggregative responses of brent geese on salt marsh and their impact on plant community dynamics

The aggregative responses and habitat preferences of a generalist herbivore, the dark-bellied brent goose Branta bernicla bernicla, feeding on salt marshes are examined in relation to vegetation community characteristics and the abundances of individual plant species. In the autumn, feeding was strongly concentrated on the low marsh, which had the highest biomass of the preferred food plant, Salicornia europaea. There was a strong aggregative response of the geese to the abundance of S. europaea. A decline in the availability of S. europaea led to an increase in the pattern of aggregation in relation to the two other major food plants on the low marsh, Aster tripolium and Puccinellia maritima. The availability of these food plants, however, reached critically low levels in mid-winter and the geese abandoned the low marsh for the high marsh. Within the high marsh, the plant communities selected tended to be dominated by the inedible species Limonium vulgare. The food plants selected were P. maritima in the winter and P. maritima and Triglochinmaritimum in the spring. On the high marsh, aggregative responses were shown to both P. maritima and T. maritimum, but in both cases, aggregation increased up to a critical level of biomass, and then declined. The prevention of grazing with exclosures for 3 years led to an increase in the abundance of P. maritima on both high and low marshes. This change was slight on high marsh but pronounced on low marsh, where S. europaea showed a decrease in abundance in the exclosures over this time. The implications of the aggregative responses for the population dynamics of P. maritima and S. europaea are discussed. Received: 11 September 1997 / Accepted: 12 December 1997     

Effects of environmental context on the susceptibility of Atriplex patula to attack by herbivorous beetles

The susceptibility of plants to attack by insect herbivores often depends on local environmental conditions. This study documents variation in herbivore damage by the chrysomelid beetle Erynephala maritima to the annual forb Atriplex patula in two microhabitats within New England salt marshes: bare patches and dense matrix vegetation. Environmental conditions within bare patches differ from those within matrix vegetation in a number of ways. Bare patches are characterized by the absence of perennial grasses and rushes (matrix vegetation) and greater levels of physical stress, and are rapidly colonized by the fugitive annual, Salicornia europaea, a second host plant of these beetles. Surveys of herbivore damage across three marshes revealed that A. patula in bare patches had a greater proportion of leaves damaged by beetles than those within matrix vegetation. Presence or absence of matrix vegetation and presence or absence of S. europaea were experimentally manipulated to determine the proximate cause of this pattern. The presence of S. europaea significantly increased the susceptibility of A. patula to herbivory in experimental plots. Both the extent of herbivore damage to plants and the proportion of plants damaged through time were greater in treatments with S. europaea than in controls, regardless of the presence or absence of matrix vegetation. Plants in S. europaea addition treatments were also less likely to survive to reproduction. Decreased survival appears to result from increased herbivory, suggesting that the negative effect of S. europaea on A. patula is mediated indirectly through shared insect herbivores. These results support the hypothesis that indirect interactions between alternative host plants, mediated by insect herbivores, can be important in natural communities. Received: 9 January 1999 / Accepted: 29 April 1999     

Impact of different sheep grazing intensities on salt marsh vegetation in northern Germany

Abstract. Mainland salt marshes in Schleswig-Holstein (northern Germany) have been grazed intensively by sheep for several decades. In 1988 experimental sites were established in the lower and middle salt marsh of Sönke-Nissen-Koog and subjected to different grazing intensities. From 1989 to 1993 the impact of sheep grazing on the composition and structure of the salt marsh vegetation was studied through the yearly analysis of permanent plots, vegetation mapping and measurements of the vegetation height. The intensively grazed site (10 sheep/ha) is covered by a short monotonous Puccinellia maritima sward with Salicornia europaea and Suaeda maritima. Halimione portulacoides and Aster tripolium, especially flowering plants, are rare. On the sites with 1.5 and 3 sheep/ha Puccinellia maritima remained dominant. The population density of Salicornia europaea decreases after reduction of the grazing intensity, whereas Suaeda maritima finds optimal growing conditions. Stands of Halimione portulacoides and flowering Aster tripolium plants are rare near the sea dike but their cover and size increases further away from the dike. In the plot with 1.5 sheep/ha the height of the vegetation increases along the gradient from the dike towards the tidal flats, due to local differences in actual grazing intensity. When grazing is stopped, Puccinellia maritima is successively replaced by Festuca rubra, Halimione portulacoides and Aster tripolium. High variability of vegetation height indicates structural diversity. Patches of higher and lower vegetation correspond with the distribution pattern of different plant species. In terms of nature conservation cessation of grazing is recommended.     

The performance of the leaf mining microlepidopteran Bucculatrix maritima (Stt.) on the salt marsh halophyte,Aster tripolium (L.), exposed to different salinity conditions

Summary The performance of phytophagous insects is influenced by the nutritional quality of the food plant, which may vary with environmental conditions. Hardly any information exists on food-plant mediated effects of variable soil salinity on the performance of phytophagous insects. Conspicuous differences in salinity levels, however, are found in soils of intertidal wetlands such as salt marshes and mangroves. The growth of larvae of Bucculatrix maritima, a leaf miner of the salt marsh halophyte Aster tripolium, was studied on the host plant along the salinity gradient of the Westerschelde estuary (S.W. Netherlands). In addition, its performance on A. tripolium grown on low or high salinity culture medium was investigated experimentally. Although salinity conditions significantly influenced the chemistry of the host plants, insect performance seemed almost unaffected, although near the mouth of the estuary high environmental salinities may have caused some inhibition of larval growth. The results contrast with our previous studies on the stem-borer Agapanthia villosoviridescens, which showed that growth and development was conspicuously influenced by the changing characteristics of Aster tripolium along the estuarine salinity gradient. The location-dependent qualities of halophytes in an estuary thus appear to have species-specific effects on insect performance. We hypothesize that this phenomenon contributes to the existence of non-identical distribution patterns of phytophagous insects associated with the same halophyte in an estuary.     

Salt stress effects on growth,pigments, proteins and lipid peroxidation in <Emphasis Type="Italic">Salicornia persica</Emphasis> and <Emphasis Type="Italic">S. europaea</Emphasis>

The effects of NaCl stress on growth, water status, contents of protein, proline, malondialdehyde (MDA), various sugars and photosynthetic pigments were investigated in seedlings of Salicornia persica and S. europaea grown in vitro. Seeds were germinated under NaCl (0, 100, 200, 300, 400, 500 and 600 mM) on Murashige and Skoog medium for 45 d. The shoot growth of both species increased under low NaCl concentration (100 mM) and then decreased with increasing NaCl concentrations. In contrast to S. persica, root length in S. europaea reduced steadily with an increase in salinity. Proline content in S. persica was higher than in S. europaea at most NaCl concentrations. Proline, reducing saccharide, oligosaccharide and soluble saccharide contents increased under salinity in both species. In contrast, contents of proteins and polysaccharides reduced in both species under salt stress. MDA content remained close to control at moderate NaCl concentrations (100 and 200 mM) and increased at higher salinities. MDA content in S. europaea was significantly higher than S. persica at higher salinities. Salt treatments decreased K⁺ and P contents in seedlings of both species. Significant reduction in contents of chlorophylls and carotenoids due to NaCl stress was also observed in seedlings of both species. Some differences appeared between S. persica and S. europaea concerning proteins profile. On the basis of the data obtained, S. persica is more salt-tolerant than S. europaea.     

Biogeochemical interactions between iron and sulphate in freshwater wetlands and their implications for interspecific competition between aquatic macrophytes

1. Wetlands are threatened by desiccation, eutrophication and changing water quality, generally leading to greatly altered biogeochemical processes. Sulphate pollution can lead to severe eutrophication and sulphide toxicity, but may also interact with the availability of iron and other metals. 2. In the present study, we examined the biogeochemical interactions between sulphate and iron availability, and their effects on aquatic macrophytes, in a field experiment with enclosures. The natural iron supply by groundwater was mimicked by adding iron to the sediment, and the effect of increased sulphate concentrations in the surface water was also studied. The enclosure experiment was performed in a mesotrophic, anaerobic ditch in a peat meadow reserve in the Netherlands. In all enclosures, three Stratiotes aloides plants were introduced to serve as indicator species. 3. Addition of sulphate led to the mobilisation of phosphate, whereas addition of iron or both iron and sulphate did not affect P mobilisation. Growth of S. aloides was decreased by both iron addition and sulphate addition (sulphide toxicity). Addition of iron under sulphidic conditions, however, led to mutual detoxification of both toxicants (iron and sulphide) and did not decrease S. aloides growth. The uptake of metals was highest in the treatment involving sulphate addition, probably as a result of increased mineralisation of the peat soil. 4. Growth of Elodea nuttallii, which grew naturally in the enclosures, was stimulated by iron or iron plus sulphate addition. It did not, however, grow in the enclosures with sulphate addition, as a result of sulphide toxicity or sulphide‐induced iron deficiency. Under iron‐rich conditions, E. nuttallii appeared to be a better competitor than S. aloides and depressed the growth of the latter species. 5. We propose that the growth of S. aloides is directly regulated by interactions between sulphide and iron and indirectly by the effects of both compounds on the competitive strength of E. nuttallii. In general, we conclude that biogeochemical interactions between sulphate and iron can have a strong influence on plant species composition in freshwater wetlands, because of direct effects or changes in the competitive strength of plant species related to differential sensitivity to either iron or sulphide.     

Competition between Salicornia europaea and Atriplex prostrata (Chenopodiaceae) along an experimental salinity gradient

Salicornia europaea L. is a halophyte that often occupies the lowestand most saline (>3.5% total salt) areas of salt marshes. Atriplexprostrata Boucher is less salt tolerant than S. europaea and oftengrows in a less saline (<2.0% total salts) zone adjacent to S. europaea. The purpose of this experiment was to determine thecompetitive outcome when these two species are grown at differentsalinities to ascertain the extent salinity and competition affect plantzonation. Plants were grown in a de Wit replacement series at 85, 170,and 340 mM NaCl in half-strength Hoagland's no. 2 nutrient solution fortwo months. There was a significant effect of salt concentration,competition, and their interaction on biomass production of S. europaea plants. However, only salt concentration significantly affectedbiomass production of A. prostrata plants. Results of thisexperiment confirmed the results of other studies that demonstrated thatthe more salt tolerant species were less competitive at lower salinities. Atriplex prostrata was the better competitor at 85 mM NaCl, whereasS. europaea was the better competitor at 340 mM NaClbecause growth of A. prostrata was inhibited. At 170 mMNaCl, A. prostrata biomass production decreased more than S. europaea biomass in mixed culture.     

Growth strategies of four salt marsh plants on Mankyung River estuary in Korea

The emergence of seedlings, the length of roots and shoots, and the biomass of four dominant plant species and shore height were measured to investigate the growth strategy of these plants on the salt marsh of Mankyung River estuary. Four salt marsh plants showed a distinctive zonation, for example, Suaeda japonica was predominantly spread around the low salt marsh, Atriplex gmelini and Aster tripolium were in the middle, and S. asparagoides was in the upper part of the marsh. In terms of emergence of seedlings, S. japonica appeared first followed by A. gmelini, S. asparagoides, and A. tripolium. The growth strategies of halophytes were as follows: S. japonica germinated earlier than the other halophytes so that its root grew rapidly and extensively at the beginning of growth. This species adopted a continuous germination strategy, allowing growth whenever favorable conditions were provided. A. gmelini germinated later than S. japonica, as a quasi-simultaneous germination type, it showed the highest germination rate within the shortest time. Aster tripolium germinated later than any other halophyte. Since this species exhibited characteristics between the continuous germination type and the quasi-simultaneous germination type, it did not show a very high germination rate. Instead, it showed continuous germination and consistent growth of both above-ground and underground parts. Suaeda asparagoides showed an especially high emergence rate at the beginning of its growth. However, the high density retarded its growth until the middle stage. Its roots extended longer than the other halophytes, allowing it to grow well in the dry conditions of the upper marsh.     

INFLUENCE OF HORMONAL TREATMENTS ON THE GROWTH OF TWO HALOPHYTIC SPECIES OF SUAEDA

Experiments were made to determine the interaction between the growth regulating substances, gibberellic acid and kinetin, and salinity on the growth and development of two species of Suaeda. We found that the growth of Suaeda maritima var. macrocarpa, an obligate halophyte, was greatly stimulated with treatments by both of these hormones in controls not treated with sodium chloride. These results suggest that halophytes grow poorly under nonsaline conditions because of some type of hormonal imbalance. GA₃ was found to stimulate growth at all salinities for both S. maritima var. macrocarpa and S. depressa, while kinetin proved to be inhibitory to growth and elongation of plants at higher salinities. Chlorophyll content decreased with salt treatments and was not significantly influenced by hormonal treatments. GA₃ had little influence on water content in roots and shoots, whereas plants treated with kinetin generally had reduced water contents.     

Performance of two laboratory‐scale horizontal wetlands under varying influent loads treating artificial sewage

The performance of two laboratory‐scale horizontal subsurface‐flow constructed wetlands(CWs) treating artificial sewage in response to varying influent components and loads was investigated. Acidification with a pH of 3.0 was detected under an organic carbon load of 100 mg/day, which further inhibited the activity of the denitrification process. With an increase in the carbon load to 240 mg/day, the pH was significantly elevated to 6.0. However, a negative effect of sulphide as a product of sulphate reduction was observed on the removal of ammonium, the plants (Juncus effusus), and the organic carbon degradation. With a produced sulphide concentration of about 3.5 mg/L, the ammonium removal decreased from 100% to 30% under an inflow load of 100 mg/day, and the number of healthy stalks of J. effuses was reduced from about 14 000/m² to less than 6000/m². The removal of organic carbon decreased from 94% to 68% under an influent load of 240 mg/day, when the sulphide concentrations reached up to about 8–10 mg/L. The production of sulphide was not immediately controlled by stopping the inflow sulphate load to remove the negative effect of the sulphide toxicity, thus indicating an immobilization of the deposited reduced sulphur compounds. Moreover, the effect of a nitrate dosage on the sulphide control was also examined, but was shown to be only evident under the conditions of a low organic carbon input.     

Germination and early establishment of lower salt-marsh species in grazed and mown salt marsh

Abstract. Lower salt-marsh species occur more in the grazed than in the mown sites of a salt marsh in Schiermonnikoog in the Netherlands. This was demonstrated by a sowing experiment which determined which characteristic of the stand structure, height of the canopy or percentage of bare soil, is responsible for this difference. The total number of seedlings which emerged was negatively related to the height of the canopy and positively to the percentage of bare soil. The survival of seedlings of Suaeda maritima and Plantago maritima could be explained by the height of the canopy and for the latter species also by the percentage of bare soil. The survival of Aster tripolium and Plantago maritima was higher in grazed than in mown sites. Since the amount of bare soil was higher than seemed necessary for germination and survival, it was concluded that the taller canopy was responsible for the absence of lower salt-marsh species in the mown sites.     

Sodium plays a more important role than potassium and chloride in growth of <Emphasis Type="Italic">Salicornia europaea</Emphasis>

Salicornia europaea is a succulent euhalophyte that belongs to the Chenopodiaceae family. It is found that moderate concentration of NaCl can dramatically stimulate the growth of S. europaea plants. To elucidate the mechanism underlying the phenomenon, morphological and physiological changes of S. europaea in response to different ions, including cations (Na⁺, K⁺, Li⁺, Cs⁺) and anions (Cl⁻, NO₃ ⁻, CH₃COO⁻) were investigated, and the effects of Na⁺, Cl⁻ and K⁺ on the growth of S. europaea were also studied. Na⁺ was more effective than K⁺ and Cl⁻ in stimulating shoot succulence, cell expansion, and stomatal opening. Plants treated with Na⁺ (including NaCl, Na⁺, NaNO₃) showed better plant growth, increased photosynthesis and less cell membrane damage than those untreated and treated with 200 mM of Cl⁻ and K⁺ (including KCl and KNO₃). Both SEM-X-Ray microanalysis and flame emission results revealed that well developed S. europaea plants had a higher content of sodium but lower potassium and chlorine. It is concluded that sodium plays a more important role in the growth and development of S. europaea than potassium and chloride.     

Inhibitory effect of Salicornia europaea on the marine alga Skeletonema costatum

Exploiting the negative biochemical interference between plants and algal species has been suggested as a method to control harmful algal blooms. In this work, we investigated the inhibitory effect of the salt marsh halophyte Salicornia europaea against the marine alga Skeletonema costatum. S. europaea suppressed the growth of S. costatum in a nutrient-sufficient co-culture system, indicating that the inhibition of algal growth was because of the phytotoxic effect of S. europaea, rather than nutrient competition. We tested aqueous and organic extracts from S. europaea roots against S. costatum. The organic extracts inhibited growth and affected the cell size and chlorophyll a content of S. costatum in a dose-dependent manner. Among the three tested organic extracts, the methanol extract had the greatest effects on S. costatum, followed by butanol extract, and then the chloroform extract. Two flavonoids, rutin and quercetin-3-??-D-glucoside, were identified in the methanol extract by high performance liquid chromatography. The concentration of rutin was much higher than that of quercetin-3-??-D-glucoside. In an algal bioassay, rutin inhibited the growth of S. costatum and the inhibitory effect increased with increasing rutin concentration and with decreasing initial algal density. Therefore, we concluded that S. europaea negatively affects the growth of S. costatum, and that rutin, a metabolite of S. europaea, may play a role in this inhibitory effect.     

Sulphide release from anoxic sediments in relation to iron availability and organic matter recalcitrance and its effects on inorganic phosphorus recycling

This research aims to analyse the sediment capacity to buffer free sulphide release in three coastal lagoons which differ in terms of eutrophication level, tide influence and primary producer communities. A preliminary estimate of soluble reactive phosphorus (SRP) regeneration coupled with sulphide fluxes is also made. Sediment profiles of ferrous and ferric iron and reduced sulphur pools were determined in three stations in the Bassin d'Arcachon (South West France), in one site in the Etang du Prévost lagoon (Southern France), and in three stations in the Sacca di Goro lagoon (Northern Italy). Laboratory experiments were also conducted by incubating sediment slurries. Slurries from the French lagoons were also enriched with about 2% d.w. of organic detritus obtained from the dominant macrophytes of each site, namely Zostera noltii and Ruppia cirrhosa (Bassin d'Arcachon), and Ulva rigida (Etang du Prévost). In the Sacca di Goro, slurry experiments were conducted at two sites with different salinity range, sediment composition and hydrodynamics.Field data showed that concentrations of available iron (Fe(II)+Fe(III)) ranged from a minimum of 28.5 µmol cm^–3 (Etang du Prévost) to a maximum of 275.7 µmol cm^–3 (Sacca di Goro). Moreover, in the French lagoons, acid volatile sulphide (AVS) accumulation in the superficial sediment was related to ferrous iron concentrations. Laboratory experiments showed that in spite of strong reducing conditions, sulphide and SRP release was weaker in iron-rich sediments and in those enriched with the most refractory organic matter. The highest fluxes were detected in sediment slurries from the Etang du Prévost, which had the lowest iron content, supplied by 2% of the labile detritus from Ulva rigida. In this case, SRP release was directly related to sulphide production.Two factors seem significant to evaluate the buffer capacity against free sulphide and SRP release from anoxic sediment: organic matter biodegradability, which forces sediment toward reducing conditions, and iron availability, which can affect sulphide mobility as well as the iron hydroxide-phosphate-sulphide system.     

Drought, but not salinity, determines the apparent effectiveness of halophytes colonized by arbuscular mycorrhizal fungi

The halophytes Plantago maritima, Aster tripolium, Artemisia santonicum, Puccinellia limosa, Festuca pseudovina and Lepidium crassifolium from two different saline soils of the Hungarian steppe were examined for colonization by arbuscular mycorrhizal fungi (AMF). The salt aster (A. tripolium) and the sea plantain (P. maritima) were examined more thoroughly by recording root colonization parameters, the salt content in the soil and monthly precipitations in 2001 and 2002. Mycorrhizal colonization was maximal in late spring to early summer and had a second peak later in the autumn. Arbuscule formation and overall mycorrhizal colonization appeared to be inversely correlated with the intensity of rainfall at the investigated sites. The results suggest that, in addition to seasonality, drought may play an important role in governing mycorrhizal activity in saline habitats. In greenhouse experiments, conditions in which AMF could overcome the inhibitory effects of sodium chloride on establishing plant–mycorrhizal symbiosis were not met.     

Stomatal responses to sodium ions in Aster tripolium: a new hypothesis to explain salinity regulation in above-ground tissues

A study has been made of the ionic relations of stomata of Aster tripolium L., a maritime halophyte which colonizes coastal saltmarshes. The results obtained allow us to add this species to the growing list for which an involvement of K⁺ transport in stomatal movements has been demonstrated. However, an additional and ecologically important characteristic was found: there was a suppression of stomatal opening by increasing NaCl concentrations. A new hypothesis is offered of the mechanism for controlling salt and water relations in A. tripolium, a species which does not possess glands or other means of excreting salt. It is suggested that when the capacity of the tissues to accumulate salt in cell vacuoles is exceeded, the concentration of Na⁺ ions in the apoplast around the guard cells begins to rise. This causes partial stomatal closure, reduces transpiration and increases water-use-efficiency. Therefore, the flow of salt into the leaves is reduced but growth (and the manufacture of the new photosynthates required to support it) can continue. Aster tripolium can be added to the small list of known species which readily yield isolated epidermis suitable for detailed stomatal studies. Throughout this study, we have compared its stomatal physiology with C. communis, which has been thoroughly investigated in the past.     

The effect of cattle and sheep grazing on salt-marsh vegetation at Skallingen,Denmark

The aggregated effect of cattle and sheep grazing on Puccinellion maritimae and other salt-marsh vegetation has been studied together with changes in species composition, the percentage cover of each species, total cover and the percentage of bare ground, six years after grazing had been prevented by construction of experimental exclosures. The results of these experiments are discussed in relation to the natural development of the vegetation that occurred in a permanent plot in the ungrazed part of the salt marsh. Six years without grazing caused a marked increase in total cover and a dramatic reduction in the amount of bare ground. The species composition of the Puccinellia maritima community did not change during these six years, but all species increased in cover. Whereas Artemisia maritima migrated into the Festuca rubra community and only two species Festuca and Halimione portulacoides, increased in cover, all other species showed reduced cover and Salicornia europaea disappeared from the plot after six years without grazing. During the same period of time, three species, Salicornia europaea, Suaeda maritima, and Glaux maritima, disappeared from the plot in the ungrazed marsh as a result of natural development. During thirty-five years the vegetation originally dominated by Puccinellia maritima and Salicornia europaea has changed into a community dominated by Halimione portulacoides, whereas the grazed salt marsh is still dominated by Puccinellia maritima and Salicornia europaea. These results are discussed in relation to the literature on the effect of cattle and sheep grazing on salt-marsh vegetation.     

Salt regulation in halophytes

Summary Ion concentration and saturation water content were measured in various aged leaves of halophytes growing in saline soils east of lake Neusiedlersee (Austria).All species investigated showed a substantial sodium accumulation within the maturing organs accompanied by a considerable potassium decline. In most species chloride concentration rises distinctly with increasing leaf age, too, whereas concentration shifts of alkaline earth ions and of sulfate (except in Plantago maritima, Lepidium crassifolium and Crypsis aculeata) are of comparably less importance.Saturation water increases markedly in succulent species (Suaeda maritima, Chenopodium glaucum, Spergularia media, Lepidium crassifolium) and to a less degree in xerophytic monocotyledons (Puccinellia distans, Crypsis aculeata, Bolboschoenus maritimus). However, this surplus of water in older leaves is not sufficient to dilute the salt to such an extent that a rise in concentration can be prevented (except chloride in Suaeda maritima and Chenopodium glaucum).Rosette plants (Triglochin maritimum, Plantago maritima, Scorzonera parviflora, Aster tripolium) with the ability to renew their leaves continuously throughout the growth period are characterized by only insignificant changes of saturation water content with increasing leaf age. In these plants, shedding of old salt-saturated leaves is thought to be the main strategy for salt regulation.A modification of Steiner's classical concept of different salt regulation types is proposed, based on original findings about salt regulation in Austrian halophytes and on new bibliographical data upon additionally revealed regulatory principles in halophytes and saltaffected nonhalophytes.Respectfully and thankfully dedicated to Prof. Dr. H. Kinzel on the occasion of his 50th birthday.     

How do Mycorrhizas Affect C and N Relationships in Flooded Aster tripolium Plants?

The mycorrhizal associations established between plants and fungi have multiple effects on plant growth, directly affecting stress tolerance. This work aimed to explore arbuscular mycorrhizal (AM) effects on carbon and nitrogen relationships of Aster tripolium L. and consequently on its flooding tolerance. Mycorrhizal and non-mycorrhizal juvenile plants were submitted to non-flooding and tidal flooding conditions for 56 d. Tidal flooding reduced biomass, but the presence of mycorrhiza had an ameliorating effect. The AM symbioses seem to have, like flooding, a stressful effect on A. tripolium at an early stage of plant development. However, once the plant was established, an improvement of growth performance of plants with mycorrhiza under flooding conditions was observed. The better tolerance of AM plants to flooding was mediated through an improvement of the osmotic adjustment of the plant tissues (higher concentrations of soluble sugars and proline) and through the increment of nitrogen acquisition in tidal-flooded plants.     

Effects of thiosulphate on haemocyanin oxygen affinity in the isopod Saduria entomon (L.) and the brown shrimp Crangon crangon (L.)

The sulphide-tolerant brackish water isopod Saduria entomon has haemocyanin with a high affinity for oxygen. Sulphide is oxidized in the hepatopancreas to thiosulphate. Oxygen needed for this oxidation is transported bound to haemocyanin. Thiosulphate was found to improve haemocyanin oxygen affinity in S. entomon. The modulating effect was time and concentration dependent. The effect was greater at higher pH, i.e. the Bohr effect was greater (−1.82) compared to that seen without thiosulphate (−1.36). S. entomon exposed to simultaneous severe hypoxia and sulphide had a low haemolymph pH and the partial pressure of oxygen required to reach 50% saturation was even lower than that measured in vitro. This is ascribed to the combined effects of thiosulphate and lactate as well as unknown co-factors. Thiosulphate had no effect at all in the sulphide and hypoxia non-tolerant natantian Crangon crangon. It is suggested that thiosulphate can be important as a haemocyanin modulator for crustaceans digging in sulphide-rich sediments or constantly exposed to sulphide. Accepted: 11 August 1999