Estuarine gradients and the growth and development of Agapanthia villosoviridescens, (Coleoptera), a stem-borer of the salt marsh halophyte Aster tripolium

Summary Phytophagous insects of estuarine salt marshes which live inside their host plants are not directly exposed to estuarine gradients. Host plant quality, however, may change along the estuary as a result of the direct effects of these gradients; as a consequence growth and development of endophagous insects may be influenced. The results of a study of the life cycle of Agapanthia villosoviridescens (Coleoptera, Cerambycidae), a stem-borer of the halophyte Aster tripolium, on three salt marshes along the Westerschelde estuary (the Netherlands) are in line with this hypothesis. It was shown that in upstream direction (1) mean larval weights were consistently higher during the entire period of larval development; (2) the percentage of late instars on a given sampling date generally was higher; (3) the percentage of larvae which underwent successful metamorphosis increased. Furthermore, (4) dry weight of the imagos was highest on the least saline marsh. The effects of estuarine gradients on the Aster host plants was indicated by differences in growth and chloride content between the populations of the three marshes. The non-overlapping geographic distribution of Agapanthia villosoviridescens and its host plant Aster tripolium on the Westerschelde salt marshes may be related to the effects of estuarine gradients on the suitability of the host plant.     

Estuarine gradients determining the quality of tidal marsh halophytes as host plants for endophagous insect larvae: Experimental evidence

Phytophagous insects of estuarine tidal marshes which live inside their host plants, are, in contrast to the plants, not directly exposed to the estuarine salinity gradient. Previous field studies, however, have shown that patterns of growth and development ofAgapanthia villosoviridescens larvae, stem-borers of the halophyteAster tripolium, gradually change on tidal marshes along the Westerschelde estuary (HEMMINGA and VAN SOELEN, 1988). In the present study we carried out a laboratory experiment in which we followed growth ofA. villosoviridescens larvae from two different Westerschelde tidal marshes; the larvae either were kept inA. tripolium stems from their own marsh, or they were kept in stems from the other marsh. It was found that larvae from both tidal marshes showed larger weight increases inA. tripolium stems from the least saline marsh. Apparently, differences in host plant quality between stems of the two marshes exist. The results lend support to the hypothesis that growth and development, and distribution and abundance of phytophagous insects in estuarine tidal marshes may be indirectly influenced by estuarine gradients,via the host plant quality which changes along the estuary.     

Growth and protein profile responses in the halophyte sea aster (Aster tripolium L.) suspension-cultured cells to salinity

We established salt tolerant cell lines, which survived and grew under high salinity conditions with 150 mM (S-150) and 300 mM (S-300) NaCl, to study the effects of salt stress on the proliferation and protein profile of these cells in the halophyte sea aster,Aster tripolium L. These salt-adapted cell lines were produced from leaves and selected by repeated suspension subculture in media containing NaCl every 25 days for five cycles. S-150 cells displayed no inhibition in their growth compared to control cells maintained under non-stressed conditions. S-150 cells exhibited approximately a 15-fold increase in both fresh and dry weight during the 25 days under saline conditions. S-300 cells showed positive growth under severe salt stress, but their dry matter gain was significantly less than that of the S-150 cells, with only a 2.5-fold increase in dry weight. We also detected changes in the protein profile of salt-adapted cells with two specifically induced polypeptides (basic 58.4 and acidic 24.8 kDa) and one enhanced polypeptide (basic 15.1 kDa) in the soluble fraction, and one specifically induced polypeptide (42.0 kDa) in the insoluble fraction.     

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.     

The structure of the phytophagous insect fauna on the introduced weed Solidago altissima in Switzerland

Solidago altissima L. was introduced into Europe as an ornamental plant from North America more than 100 years ago and the phytophagous insect fauna of it was recently examined in Switzerland where it has become an important weed in disturbed habitats. Rhizomes and aerial parts were examined and all insects collected in summer were tested in a no-choice feeding test. 18 out of 55 phytophagous insects were found feeding on S. altissima in Switzerland and for the remaining 37 the relation with the plant was not determined. The insects that have expanded their host range to feed on S. altissima since its introduction to Switzerland are almost solely opportunistic, unspecialized ectophages not closely attuned to the growth cycle of S. altissima. Only 4% of the insects were specialists and 9% endophagous. In contrast in North America S. altissima supports 25% specialists and 17% endophages. The native Solidago virgaurea L. in Switzerland supports many more specialists (28%) and endophages (23%) than S. altissima here. Possible reasons why almost none of these have switched to S. altissima are discussed. A comparison is made between the number of insects recruited by S. altissima in Switzerland with the number recruited by the exotic plants Heracleum mantegazzianum Som. & Lev. (Apiaceae) in Switzerland and Xanthium occidentale Bertol. (Asteraceae) in Australia. Reasons for different levels of insect recruitment are discussed. The high number of phytophagous insect species found on S. altissima in North America shows that there should be no shortage of possible control agents and any insects imported into Switzerland for the biological control of S. altissima would find largely unexploited food sources awaiting them.     

Salt tolerance in Aster tripolium L. I. The effect of salinity on growth

Abstract A study of the growth of the maritime halophyte Aster tripolium L. has been carried out over a range of salinity treatments. The regression approach to growth analysis using frequent small harvests has been used to allow ‘continuous’ measurement of growth over a period of 36 d. Salinity was applied with the major ions present in ratios typical of those found in seawater. Growth was inhibited in terms of both dry weight production and leaf expansion at salinity levels equivalent to 0.625 strength sea water (full culture solution 300) and above, with the greatest effect being seen in terms of leaf area. Aster tripolium did not show increased succulence at high salinity, leaf fresh weight to dry weight ratio in fact declined, whilst leaf fresh weight per unit area remained constant. It should be noted that the plants exhibit low growth rates due to the low light intensity used.     

Host plant use of Phyllotreta nemorum: do coadapted gene complexes play a role?

The view of (insect) populations as assemblages of local subpopulations connected by gene flow is gaining ground. In such structured populations, local adaptation may occur. In phytophagous insects, one way in which local adaptation has been demonstrated is by performing reciprocal transplant experiments where performance of insects on native and novel host plants are compared. Trade-offs are assumed to be responsible for a negative correlation in performance on alternative host plants. Due to mixed results of these experiments, the importance of trade-offs in host plant use of phytophagous insects has been under discussion. Here we propose that another genetic mechanism, the evolution of coadapted gene complexes, might also be associated with local adaptation. In this case, however, transplant experiments might not reveal any local adaptation until hybridization takes place. We review the results we have obtained in our work on the host plant use of the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae), and propose a hypothesis involving coadapted genes to explain the distribution of genes that render P. nemorum resistant to defences of one of its host plants, Barbarea vulgaris R. Br. (Cruciferae).     

Arbuscular mycorrhizal fungi enhance root cadmium and copper accumulation in the roots of the salt marsh plant Aster tripolium L.

It is known that vegetation plays an important role in the retention of heavy metals in salt marshes by taking up and accumulating the metals. In this study, we investigated whether arbuscular mycorrhizal fungi (AMF) increase Cd and Cu uptake and accumulation in the root system of the salt marsh species Aster tripolium L., and whether indigenous AMF isolated from polluted salt marshes have higher capacity to resist and alleviate metal stress in A. tripolium than isolates of the same species originated from non-polluted sites. Plants inoculated with Glomus geosporum, either isolated from a polluted salt marsh site (PL isolate) or from a non-polluted site (NP isolate), and non-mycorrhizal (NM) plants were compared in a pot experiment at four different Cd and Cu concentrations. Cd had no effect in root colonization, whereas high concentrations of Cu decreased colonization level in plants inoculated with the NP isolate. AM colonization did not increase plant dry weight or P concentration but influenced root Cd and Cu concentrations. Inoculation with PL and NP isolates enhanced root Cd and Cu concentrations, especially at highest metal addition levels, as compared to NM plants, without increasing shoot Cd and Cu concentrations. There was no evidence of intraspecific variation in the effects between AMF isolated from polluted and non-polluted sites, since there were no differences between plants inoculated with PL or NP isolate in any of the tested plant variables. The results of this study showed that AMF enhance metal accumulation in the root system of A. tripolium, suggesting a contribution of AMF to the sink of metals within vegetation in the salt marshes.     

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.     

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.     

Salt tolerance of the annual halophyte <Emphasis Type="Italic">Cakile maritima</Emphasis> as affected by the provenance and the developmental stage

Though halophytes are naturally adapted to salinity, their salt-tolerance limits are greatly influenced by their provenance and developmental stage. In the present study, physio-biochemical responses of two Tunisian ecotypes of the oilseed coastal halophyte Cakile maritima (Brassicaceae) to salinity (0–400 mM NaCl) were monitored during germination and vegetative growth stages. Tabarka and Jerba seeds were collected from humid or arid climatic areas, respectively. Plant response to salinity appeared to depend on the ecotype and salinity levels. Increasing salinity inhibited germination process. Jerba seeds were found to be more salt tolerant than the Tabarka ones. At the autotrophic stage of growth and under salt-free conditions, Jerba was less productive than Tabarka (in terms of dry matter accumulation), but plant biomass production and leaf expansion (area and number) of the former ecotype were progressively improved by 100 mM NaCl, as compared to the control. In contrast, at the same salt concentration, these parameters decreased under increasing salinity in Tabarka (salt sensitive). Leaf chlorophyll content was reduced at severe salinity, but this effect was more conspicuous in the sensitive Tabarka plants. Na⁺ contents in the Jerba and Tabarka leaves collected from the 400 mM NaCl-treated plants were 17- and 12-fold higher than in the respective controls. This effect was accompanied by a significant reduction in the leaf K⁺, Mg²⁺ and Ca²⁺ contents, especially in the salt-treated Tabarka. A significant accumulation of proline and soluble carbohydrates in leaves was found during the period of intensive leaf growth. These organic compounds likely play a role in leaf osmotic adjustment and in protection of membrane stability at severe salinity.     

Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids

Ecological specialisation on different host plants occurs frequently among phytophagous insects and is normally assumed to have a genetic basis. However, insects often carry microbial symbionts, which may play a role in the evolution of specialisation. The bacterium Regiella insecticola is a facultative symbiont of pea aphids (Acyrthosiphon pisum) where it is found most frequently in aphid clones feeding on Trifolium giving rise to the hypothesis that it may improve aphid performance on this plant. A study in which R. insecticola was eliminated from a single naturally infected aphid clone supported the hypothesis, but a second involving two aphid clones did not find the same effect. We created a series of new pea aphid–R. insecticola associations by injecting different strains of bacteria into five aphid clones uninfected by symbionts. For all aphid clones, the bacteria decreased the rate at which aphids accepted Vicia faba as a food plant and reduced performance on this plant. Their effect on aphids given Trifolium pratense was more complex: R. insecticola negatively affected acceptance by all aphid clones, had no effect on the performance of four aphid clones, but increased performance of a fifth, thus demonstrating genetic variation in the effect of R. insecticola on pea aphid host use. We discuss how these results may explain the distribution and frequency of this symbiont across different aphid populations. Julia Ferrari and Claire L. Scarborough contributed equally to the work.     

The effect of lead contamination of soils and air on its accumulation in pollen

Summary Concentrations of heavy metals in the honey, derived fromAster tripolium was the starting-point for this investigation. Lead content of pollen inAster tripolium andTaraxacum officinale is mainly due to airborne lead. Accumulation of other heavy metals such as zinc and copper in pollen occur mainly via a translocation process from roots to flowers.     

Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L.

This study investigated the interaction of NaCl-salinity andelevated atmospheric CO₂ concentration on gas exchange, leafpigment composition, and leaf ultrastructure of the potentialcash crop halophyte Aster tripolium. The plants were irrigatedwith five different salinity levels (0, 25, 50, 75, 100% seawatersalinity) under ambient and elevated (520 ppm) CO₂. Under salineconditions (ambient CO₂) stomatal and mesophyll resistance increased,leading to a significant decrease in photosynthesis and wateruse efficiency (WUE) and to an increase in oxidative stress.The latter was indicated by dilations of the thylakoid membranesand an increase in superoxide dismutase (SOD) activity. Oxidativestress could be counteracted by thicker epidermal cell wallsof the leaves, a thicker cuticle, a reduced chlorophyll content,an increase in the chlorophyll a/b ratio and a transient declineof the photosynthetic efficiency. Elevated CO₂ led to a significantincrease in photosynthesis and WUE. The improved water and energysupply was used to increase the investment in mechanisms reducingwater loss and oxidative stress (thicker cell walls and cuticles,a higher chlorophyll and carotenoid content, higher SOD activity),resulting in more intact thylakoids. As these mechanisms canimprove survival under salinity, A. tripolium seems to be apromising cash crop halophyte which can help in desalinizingand reclaiming degraded land. Key words: Aster tripolium, cash crop halophyte, elevated CO₂, gas exchange, oxidative stress, photosynthesis, salt tolerance, ultrastructure, water use efficiency Received 29 July 2008; Revised 8 October 2008 Accepted 9 October 2008     

Phytophagous insect-woody sprout interactions in tropical eucalypt forest. II. Insect community structure

Abstract Sucking insects constituted 79% of all phytophagous insects collected from woody sprouts in the ground layer of a tropical eucalypt forest. Mobile insect groups such as non-psyllid Hemiptera and Orthoptera were relatively frequent in this environment compared to temperate, Eucalyptus-dominated vegetation. The high fire frequency of the tropical eucalypt forest may favour mobile insect groups. The capture of sucking insects and caterpillars peaked in dry season samples. Other patterns of abundance of phytophagous insect groups showed little consistency in their seasonal trends between host species or between vegetation types within host species. Disparities between chewing insect abundance in daytime samples and the damage chewing insects cause, may result from disproportionate consumption by large, mainly nocturnal insects, such as members of the Orthoptera. In this study, 21% of insect species were specialists on single plant species. This study suggested that insect abundance reflected the growth patterns of woody sprouts after regular burning, rather than that plant growth and development were tuned to the pressures of insect herbivory.     

The influence of proline on diet selection: sex-specific feeding preferences by the grasshoppers Ageneotettix deorum and Phoetaliotes nebrascensis (Orthoptera: Acrididae)

Diet selection based on the level of proline in an insect's host plant has been observed for a number of phytophagous insects, but few studies have examined potential differences in feeding preferences between males and females. The level of proline among an insect's host plants, particularly in drought-stressed plants, can be highly variable and often is positively correlated with soluble nitrogen levels. Additionally, proline is known to participate in a number of physiological functions in insects. We tested the effect of proline as a feeding stimulant in reproductively active grasshoppers using the graminivorous Ageneotettix deorum and the generalist, but mostly graminivorous, Phoetaliotes nebrascensis. Feeding preference tests using diets with representative free amino acid and sucrose levels but varying proline levels (zero, normal and 3 x normal) were examined. The feeding preference exhibited by both species was sex-specific, although the sex-specific response was more pronounced in P. nebrascensis than in A. deorum. Females of both species displayed preferences for diets high in proline. Males of neither species exhibited a preference for proline when responses were averaged over all treatment levels. However, within specific treatment combinations, male A. deorum preferred diets with high proline over diets with zero proline. These results suggest that diet selection for specific nutrients may vary between males and females because of differences in their physiological status and, possibly, differences in the nutritional requirements associated with reproduction. These results also suggest that subtle shifts in the concentration of individual nutrients within an insect's host plant may greatly influence insect feeding patterns.     

Salt resistance and the environment of the cell wall of some halophytes

It was shown for Cochlearia anglica roots in comparison with Phaseolus vulgaris, roots that: (a) the cell walls of the halophyte are heavy; (b) they have a high Ca²⁺ content; (c) they may be a convenient means of transfer for Na⁺.In Aster tripolium as well as in Atriplex littoralis NaCl alters the composition of the various pectic fractions and maintains a low pectinemethylesterase activity and a great cellulase activity. This effect may be responsible for the great plasticity of the cell walls which allows an increased cell growth and leads to the formation of succulent organs, therefore to the dilution of salt within the cells.     

Halophyte and glycophyte salt tolerance at germination and the establishment of halophyte shrubs in saline environments

Saline sites suffer variations in surface salinity, available soil water, temperature, soil crust strength and other factors which can influence germination and establishment. For establishment to occur the germinating seed must capitalise on a window of opportunity. This window can be widened by placing seeds in a low-salt niche, covering the seeds with a mulch (such as vermiculite), spraying the seed and mulch placement with a coating which may stabilise the favourable situation and raise soil temperature. In this paper it is shown that using seeds collected from plants of Atriplex amnicola which produce many volunteer seedlings in their vicinity can assist establishment from direct seeding. These seeds had the ability to germinate under saltier and cooler conditions than seeds from A. amnicola bushes which did not produce volunteers. Seeds of a halophyte (Atriplex lentiformis) and a non-halophyte (Medicago sativa) are able to imbibe water from a saline substrate in a similar manner. The water enables the seeds of both species to mobilise stored growth materials and produce and elongate radicles. When the seedlings try to erect a hypocotyl and spread their cotyledons, the non-halophyte, in a saline medium, becomes flaccid, distorted and dies. The halophyte seedling shows evidence of high salt tolerance in the form of succulence of cotyledons and trichomes on true leaves even before they are visible and goes on to successfully develop a functioning plant. Nevertheless, germination of halophyte seeds is inhibited or severely reduced at salinity levels above 250 mM NaCl and slowed and reduced progressively up to those levels.     

Responses of the stomata of Aster tripolium to calcium and sodium ions in relation to salinity tolerance

In previous work, the stomata of the maritime halophyte Astertripolium L. were shown to close when NaCl concentrations risein the vicinity of the guard cells. Further studies have nowrevealed important effects of calcium on the ionic responsesof the stomata. When the guard cells were presented with KCl,Ca²⁺ suppressed opening in a manner similar to that which hasbecome familiar in other species such as Commelina communisL. However, in the presence of NaCl, Ca²⁺ had the opposite effect,reducing the closing response to NaCl. This pattern of behaviouris discussed in relation to known salt effects on membranes,but the underlying physiological basis remains obscure. A previous study led to the hypothesis that the closing responseof the stomata to Na⁺ ions may make an important contributionto the salinity tolerance of this species. Here we report thatincreasing supplies of Ca²⁺ ions reduce the effect of salinityon stomatal conductance in the whole plant as well as in theisolated epidermis. This finding is consistent with the wellestablished role of calcium in increasing resistance to salinity:in the presence of high calcium the plant can tolerate a greatersalt intake, and hence there is a reduced need for transpirationto be restricted by partial stomatal closure. Key words: Sodium, calcium, Aster tripolium, stomata, salinity tolerance     

Endophytic insect communities of two prairie perennials (Asteraceae: Silphium spp.)

Little is known of the biology of most insects that are endemic to prairie ecosystems of North America, with the exception of large and conspicuous species. In particular, species that are sequestered within plant tissues are commonly overlooked. In this paper, we assess the biodiversity of endophytic insects that inhabit stems of Silphium laciniatum L. and S. terebinthinaceum Jacquin (Asteraceae), endemic plants of tallgrass prairies. Endophytic herbivores, gall wasps Antistrophus rufus Gillette and A. minor Gillette (Hymenoptera: Cynipidae) and stem-boring larvae of the beetle Mordellistena aethiops Smith (Coleoptera: Mordellidae) were attacked by 10 species of natural enemies. We report new host plant associations for herbivores, and new host insect associations for parasitoids. The two plant species differed significantly in their densities of gall wasps and the vertical dispersion of galls within stems. Interactions within and between trophic levels attest to the biodiversity of endophytic insect communities, and the specialized nature of these insects suggests they are highly vulnerable to habitat conservation practices that involve destruction of dead vegetation.