Life histories of Colpomenia sinuosa and Hydroclathrus clathratus (Scytosiphonaceae, Phaeophyceae) in culture

Life-history studies in culture were carried out on Colpomenia sinuosa (Mertens ex Roth) Derbès et Solier and Hydroclathrus clathratus (C. Agardh) Howe (Scytosiphonales, Phaeophyceae) from Japan. These species showed a heteromorphic life-history pattern with an alternation between erect thalli bearing plurilocular zoidangia and prostrate thalli bearing ectocarpoid plurilocular and unilocular zoidangia. Plurizoids released from erect and prostrate thalli developed into prostrate thalli. Unizoids, however, developed into erect thalli. Prostrate thalli produced plurilocular zoidangia in long-day conditions and unilocular zoidangia in short-day conditions at 10-20°C. Prostrate thalli of C. sinuosa formed ascocysts. Germlings of both species did not grow at 5°C.     

Differential responses of three grasses to defoliation,water and light availability

Grazing not only modifies the structure and functioning of grasslands, it also changes micro-environmental conditions that alter the availability of resources. The aim of this study was to analyze the response of grasses with different photosynthetic pathways (C₃/C₄), growth forms (prostrate/erect), and grazing responses (increaser/decreaser) to defoliation and resource availability. In a greenhouse, we performed a factorial experiment with three factors: defoliation, light, and water and three species: Axonopus affinis (C₄ prostrate, increaser), Coelorachis selloana (C₄ erect, decreaser), and Bromus auleticus (C₃ erect, decreaser). We measured the relative growth rate (RGR), biomass assignment, and specific leaf area. The RGR of both C₄ species was affected by light availability, while the decreaser C₃ did not respond to any factor. Biomass allocation to leaves and stolons changed with the interaction between light and water in the C₄ prostrate species (increaser). In the C₄ erect grass (decreaser), biomass allocation was more affected by defoliation under low levels of light and water. Low light availability and defoliation reduced the assignment to leaves, while the allocation to rhizomes increased. Species-specific responses to resources availability that are modified by grazing were related to photosynthetic pathway, growth form, and grazing responses. Biomass allocation was related to strategies to avoid and/or tolerate grazing. The investment to leaves was limited by light and water availability in prostrate species, while in erect grasses it was controlled by defoliation and water availability. Our results highlight the importance of species responses to changes in resource availability associated to grazing regimes.     

Polysiphonia perforans sp. nova (Ceramiales,Rhodophyta) from the Mediterranean Sea

ABSTRACT

A new species of the genus Polysiphonia from the Mediterranean Sea is described. It is an ecorticate species, with 4 pericentral cells, showing prostrate axes from which erect axes arise. Rhizoids are formed by pericentral cells in a median position remaining in open connection with them. Erect axes are straight, simple throughout or pseudodichotomous at the base then simple or with one (rarely two-three) orders of branching; trichoblasts and scar cells not observed. Only tetrasporangial plants were found. They show ellipsoid tetrasporangia borne in short straight series, often interrupted by sterile segments. The new species is also characterized by a peculiar habit consisting of prostrate axes, adhering to lower faces of Peyssonnelia spp. by means of rhizoids growing upward, from which erect upright axes perforating thalli of the supporting species arise. A comparison with the related species of Polysiphonia was also carried out.     

Severely degraded dunes of the southern Kalahari: local extinction,persistence and natural re‐establishment of plants

This study aimed to quantify and understand the impact of severe land degradation on plant attributes and diversity on dunes of the southern Kalahari. Heavy grazing pressure in particular resulted in a significant decline of canopy cover and species number in annual and perennial life forms; forb and graminoid growth forms; erect and prostrate habits; and leafy stem, tussock and stoloniferous architectures. However, no significant change was found in shrub and tree forms which persisted without apparent new recruitment. Under these conditions, species diversity dropped sharply and a number of species, mainly graminoids, became apparently locally extinct. The perennial shrub, Crotalaria cf. spartioides, showed the converse with a high frequency of establishing seedlings on the degraded dunes. Changes in relative dominance show that the extreme treatment favours perennial over annual, woody over graminoid and forb, erect over prostrate and leafy stem over stoloniferous and tussock. Some of these results and certain species and soil responses differ from those reported from grazing studies elsewhere, and are possibly no longer directly related to the impact of the primary grazing pressure but to the secondary effect of subsequent instability of the dunes.     

Circumnutation behavior of an exotic honeysuckle vine and its native congener: influence on clonal mobility

Virtually all plant parts rotate slightly about a central axis, a movement called circumnutation, but vines show exaggerated circumnutation. This study contrasts circumnutation in two congeneric twining vines, specifically focusing on differences in erect and prostrate shoots, and examines the impact of circumnutation on exploitation of available climbing supports and exploration for more distant ones. Elongating shoots of Lonicera japonica and L. sempervirens growing in a common garden were classed as (1) erect but not climbing, (2) climbing on a trellis, or (3) prostrate, and their circumnutation quantified by tracking the compass direction of shoot tips. To quantify the impact of different circumnutation behaviors, the climbing success of erect shoots and the maximum dispersion and rooting success of prostrate shoots were measured. Erect shoots of both species circumnutated at similar rates (averaging 31°/h), and did not differ in their success rate of exploiting trellises (averaging 76.8%). Prostrate shoots differed, with those produced by L. japonica having reduced circumnutation. In contrast, prostrate shoots of L. sempervirens continued to circumnutate as much as erect shoots. The specialized circumnutation behavior of the prostrate shoots of L. japonica results in increased rooting success and maximum dispersion compared to the unspecialized shoots of L. sempervirens.     

Karyotyping of female and male Hediste japonica (Polychaeta, Annelida) in comparison with those of two closely related species, H. diadroma and H. atoka

Karyotypes of females and males of the brackish-water polychaete Hediste japonica sensu stricto, collected from the Ariake Sea, Japan, were examined by using regenerating tails. We used the Giemsa staining method and a computer-assisted image-analyzing system for the identification of each chromosome pair. The somatic chromosome number was 2n=28. The presence of an XX-XY (male heterogametic) sex chromosome system was determined from well-spread metaphase plates of somatic cells. The type of sex chromosomes related with phenotype exactly. The metacentric Y chromosome was much larger than the submetacentric X chromosome. All autosomes were metacentric. The karyotype of this species was compared with those of the other two closely related species (H. diadroma and H. atoka). The karyotypes of all the three species were similar to one another.     

Spread of introduced Caulerpa species in macroalgal habitats

A short-term field experiment was designed to identify layers of Mediterranean macroalgal assemblage conducive to successful spread of two introduced Caulerpa species (Bryopsidales, Chlorophyta). By manipulation of species presence, three experimental assemblages were obtained: (1) encrusting algae, having removed the turf and erect species; (2) encrusting and turfing algae, having removed erect species; (3) encrusting, turfing and erect algae, that is, unmanipulated assemblages, which served as a control. Fragments of the two introduced species Caulerpa taxifolia (Vahl) C. Agardh and Caulerpa racemosa (Forsskål) J. Agardh were transplanted in each of the three assemblages. Width of the colony, blade density and percentage of the substratum covered by the two species were measured.

The susceptibility of the indigenous community to the spread of Caulerpa species was related to type of assemblage. Blade density and amount of substratum covered by the two Caulerpa species were different between species and generally greater for C. taxifolia than for C. racemosa. Overall, the spread of these species was strongly dependent on the type but not directly on the complexity of the assemblage. Turf was more favourable than encrusting species alone, while the least advantageous habitat was where the macroalgal assemblage is composed of encrusting, turf and erect species. In other words, increased number of species in the assemblage reduces invasion of the Caulerpa species but the type of algae in the assemblage is likely to be more important than number of species. The presence of turf promotes the spread of Caulerpa species.     

The influence of light quality on the development of the brown algae Petalonia and Scytosiphon

Petalonia fascia (O. F. Müll.) Kuntze, P. zosterifolia (Reinke) Kuntze and Scytosiphon lomentaria (Lyngb.) J. Ag. have been cultured in white, blue and red light of equal quantum irradiance at 15°C. Hairs, knot-filaments and Ralfsia-like crusts were formed only in blue light, whereas the prostrate system of red-grown plants consisted entirely of sparingly-branched and mostly uniseriate filaments. The production of erect thalli from the prostrate system was controlled or stimulated by red light, but these erect thalli became fertile only in the presence of blue light. All three species exhibited all of these types of response, although specific differences in the degree of certain types of response were observed.     

LIFE HISTORY OF COLPOMENIA SINUOSA (SYCTOSIPHONACEAE,PHAEOPHYCEAE) IN THE AZORES1

Colpomenia sinuosa (Mertens ex Roth) Derbès and Solier (Scytosiphonaceae, Phaeophyceae) is a common species on the rocky intertidal shores of the Azores, where reproductive gametophytes occur throughout the year. Life‐history studies of this species were carried out in culture, and both sexual and asexual reproduction were observed. Anisogamous gametes fused to form zygotes. The zygotes gave rise to a filamentous prostrate sporophyte generation bearing unilocular sporangia, under both short‐day and long‐day conditions at 15 and 22°C, and to both unilocular and plurilocular sporangia, under the lower temperature condition. Unispores developed into gametophytes, and plurispores gave rise to filamentous sporophytes. Asexual reproduction was carried out by unfused female gametes and asexual plurispores produced from the same gametophyte. Unfused gametes developed into filamentous prostrate sporophytes producing unilocular sporangia in both culture conditions, and unispores released from the sporangia gave rise to gametophytes. Asexual plurispores from field gametophytes, under both culture conditions, developed directly into new gametophytes. The species exhibited three types of life history: a heteromorphic, diplohaplontic; a heteromorphic, monophasic (both with alternation between the erect and filamentous prostrate thalli); and a monomorphic, monophasic.     

The last two pathway-specific enzyme activities of hexosamine biosynthesis are present in Blastocladiella emersonii zoospores prior to germination

Forward direction assays have been developed for the last two pathway-specific enzymes of hexosamine biosynthesis using crude extracts from Blastocladiella emersonii zoospores. The specific enzyme activities measured are substantially higher than those reported with enzyme preparations from other organisms. During the development of one of the assays, another enzyme activity was observed which converts one of the intermediates of the pathway, N-acetylglucosamine-6-phosphate, to N-acetylglucosamine. The finding of these three enzyme activities in zoospore extracts completes the demonstration that all the enzyme activities necessary to synthesize some 2% by weight as chitin early during zoospore germination (de novo cell wall formation) pre-exist in the zoospore. This demonstration is consistent with the conclusion that the hexosamine pathway is regulated at the post-translational level during zoospore germination.     

PHYLOGENETIC IMPLICATIONS OF DIVERSE SEED TYPES,CHROMOSOME NUMBERS,AND POLLEN MORPHOLOGY IN HOUSTONIA (RUBIACEAE)

Seed and pollen morphology were studied by light microscopy and scanning electron microscopy in 39 North and Central American species of Houstonia (including Hedyotis, but excluding Oldenlandia). Chromosome counts were obtained for eight taxa, of which five lacked previous chromosome data. A chromosome number of n = 17 for Houstonia gracilis is a new base number for the genus. Seed external morphology in the genus is very diverse, including variation in compression, margins, testa surfaces, and elaboration of ventral cavities or depressions and hilar ridges or their absence. Three types of pollen apertures are recognized: colporate with type A os, colpororate, and colporate with type B os, the last the most advanced type, occurring in H. caerulea and related species. The 39 species are arranged in twelve groups, based on correlation of seed, pollen, and chromosome data. Geographic distribution provides supplementary evidence for the distinctness and integrity of the six principal groups each composed of 2–9 species. Five of the six minor groups each with one species need chromosome data to facilitate future taxonomic decisions. Chromosome numbers of x = 6, 7, 8, 9, 10, 11, 13, and 17 are now known in this genus, and phylogenetic implications of the combined data are discussed.     

Microcladia exserta sp. nov. (Ceramiaceae,Rhodophyta) from the East coast of South Africa

Microcladia exserta, a new species of the red algal genus Microcladia Greville (Ceramiaceae, Ceramiales), is described from the Natal coast of South Africa. This small, creeping alga, an epiphyte on the coralline alga Amphiroa anceps (Lamarck) Decaisne, is distinguished from other species in the genus by the following combination of characteristics: the prostrate habit, the exerted position of the tetrasporangia, and the presence in the cortex of numerous and conspicuous vesicle cells. Evidence is presented to demonstrate that the criterion of an erect habit in Microcladia vs. a prostrate habit in Herpochondria used to separate these genera is not sound.     

Comparative vessel anatomy of arctic deciduous and evergreen dicots

Arctic tundra plant species exhibit striking variation in leaf character and growth form. Both are likely related to differences in vessel anatomy, and all may affect responses to climate changes in the Arctic. To investigate the relationships among leaf character, growth form, vessel anatomy, and susceptibility to freeze-thaw-induced xylem cavitation, xylem vessel characteristics were compared among six deciduous and six evergreen arctic dicot species of erect and prostrate growth forms. We hypothesized that deciduous and erect species would have larger and longer vessels than evergreen and cushion/mat-forming species. Vessel lengths, diameters, and densities were measured for each species. Theoretical vessel flow rates were calculated using Poiseuille's law for ideal capillaries. Flow rates were used to determine the susceptibility of vessels to cavitation induced by freeze-thaw events that may become more frequent with global warming. Vessel diameters were larger in deciduous species compared to evergreens, and in shrubs/trees vs. cushion/mat-forming plants. Vessel length distributions, however, did not differ for growth form or leaf character. Vessel density was greater in cushion/mat-forming species than in shrub/tree species. Deciduous plants showed a greater contribution to total conductivity by relatively larger vessels than evergreens. One of the deciduous species, Vaccinium uliginosum, is predicted to be susceptible to freeze-thaw-induced cavitation. These results have important implications for future arctic species composition and plant community structure.     

Differential effect of shade,water and soil type on emergence and early survival of three dominant species of the Atacama Desert

Understanding the regeneration niche of species may allow us to gain insight into how communities are structured. In deserts, the regeneration niche is usually related to spaces beneath shrubs where shade cast by shrubs creates microenvironments that benefit seedlings and where even small amounts of rain may favour germination and establishment. Shade and water may also interact with different types of soils. However, species may have different requirements for germination and seedling survival. We could expect that shrub species with different drought tolerances exhibit different responses to the combination of these factors. We ask if responses of dominant species of the Atacama Desert to abiotic factors (shade, water and soil type) are related to their drought tolerance, a topic not exhaustively explored in shrubs growing in true deserts. We conducted two factorial experiments. The first one was designed to evaluate how shade (microhabitat) in combination with water may affect germination (emergence) and early survival. In the second experiment, we assessed the influence of shade in relation to soil type. Each species responded distinctively to the three variables under study, but in general, their emergence responses were more influenced by water (more water, greater emergence) than by microhabitat or soil type. Survival was influenced both by microhabitat and by water and was higher under shade and abundant water. Soil type affected only one of our species in terms of emergence. Species responses in general depended on their tolerance to stress. In one species, there was indication of a seed–seedling conflict. Our results show similar species responses to environmental constraints but also more or less unique responses that are related to their tolerance to drought and which may ultimately permit species coexistence. We found that shade may not be important for germination but may be crucial for survival in dry years.     

The growth form composition of plant communities in the ecuadorian páramos

A growth form classification for the plants of the Ecuadorian páramos is proposed, consisting of ten forms: stem rosettes, basal rosettes, tussocks, acaulescent rosettes, cushions and mats, upright shrubs, prostrate shrubs, erect herbs, prostrate herbs, and trailing herbs.The growth form composition of 192 samples of páramo vegetation from twelve different regions in Ecuador was analysed using multivariate techniques to determine physiognomic types. The distribution of growth forms was significantly related to altitude, rock cover, bare ground and to measures of disturbance and exposure.The dominant growth forms in all the samples belonged to one of Hedberg's (1964) five types, but other forms were also significant components of the vegetation. The majority of Ecuadorian páramo vegetation showed a relatively consistent growth form composition, dominated by tussocks. The accompanying growth forms were mostly acaulescent rosettes, cushions, upright shrubs, prostrate shrubs, erect herbs and prostrate herbs, sometimes with stem rosettes, basal rosettes or trailing herbs.At higher altitudes, the dominance of tussocks was reduced. At first, acaulescent rosettes became dominant, but higher still their dominance was shared with cushions. At the highest altitudes of all, where plant cover was thin, no single growth form was dominant. In other locations where plant cover was sparse, once again no single growth form was dominant. In humid páramos, stem rosettes were co-dominant with tussocks or erect herbs. Basal rosettes, erect herbs and prostrate herbs were locally co-dominant at higher altitudes.The growth form composition of the Ecuadorian páramos showed similarities with other tropical alpine regions, though no comparable quantitative data for these regions are available yet. The quantitative determination of growth form composition may also lead to a better understanding of community structure and the mechanisms which govern it.     

The Novel Oomycide Oxathiapiprolin Inhibits All Stages in the Asexual Life Cycle of Pseudoperonospora cubensis - Causal Agent of Cucurbit Downy Mildew

Oxathiapiprolin is a new oomycide (piperidinyl thiazole isoxazoline class) discovered by DuPont which controls diseases caused by oomycete plant pathogens. It binds in the oxysterol-binding protein domain of Oomycetes. Growth chambers studies with detached leaves and potted plants showed remarkable activity of oxathiapiprolin against Pseudoperonospora cubensis in cucurbits. The compound affected all stages in the asexual life cycle of the pathogen. It inhibited zoospore release, cystospore germination, lesion formation, lesion expansion, sporangiophore development and sporangial production. When applied to the foliage as a preventive spray no lesions developed due to inhibition of zoospore release and cystospore germination, and when applied curatively, at one or two days after inoculation, small restricted lesions developed but no sporulation occurred. When applied later to mature lesions, sporulation was strongly inhibited. Oxathiapiprolin suppressed sporulation of P. cubensis in naturally-infected leaves. It exhibited trans-laminar activity, translocated acropetaly from older to younger leaves, and moved from the root system to the foliage. Seed coating was highly effective in protecting the developed cucumber plants against downy mildew. UV microscopy observations made with cucumber leaves infected with P. cubensis revealed that inhibition of mycelium growth and sporulation induced by oxathiapiprolin was associated with callose encasement of the haustoria.     

The Morphology of Protonema and Bud Formation in the Bryales

The development of protonema from spore in aseptic culture wasfollowed in species representing 14 genera and 7 orders of theBryales. In no case could a distinction be made between sharplydefined ‘chloronema’ and ‘caulonema’stages. In all species, with the exception of the anomalous Cinclidotusfontinaloides, the protonema developed a markedly heterotrichoushabit, with prostrate filaments of relatively unlimited growthand erect filaments of more restricted development. The twotypes of filament were usually distinct in cellular characteristicsas well as in the direction of growth. In some cultures a thirdtype of filament grew downwards into the medium. Shoot buds were found to occupy diverse positions in differentspecies, but the commonest position was near the base of a primaryfilament of the erect branch system. Shoot buds were formedas readily in artificial light of sufficient intensity as innatural daylight.     

DEUCALION GEN. NOV. AND ANISOSCHIZUS GEN. NOV. (CERAMIACEAE,CERAMIALES), TWO NEW PROPAGULE-FORMING RED ALGAE FROM SOUTHERN AUSTRALIA1

Two new propagule-farming red algae from southern Australia, Deucalion levringii (Lindauer) gen. et comb. nov. and Anisoschizus propaguli gen. et sp. nov., are described and defined largely on their development in laboratory culture. Deucalion is included in the tribe Compsothamnieae on the basis of its subapical procarp and alternate distichous branching. It differs from the other genera included in that tribe in that it produces 3-celled propagules, polysporangia, a subapical cell of the fertile axis which bears 3 pericentral cells, and an apparently post-fertilization involucre which develops from the hypogenous and sub-hypogenous cells of the fertile axis. Its gametophyte morphology has been elucidated in culture, as only sporophytes are known from the field. Gametophytes do not appear to produce propagules. Anisoschizus is provisionally included in the tribe Spermothamnieae on the basis of its subdichotomous branching, possession of a prostrate system and the production of polysporangia. It differs from the other genera of the tribe in the production of 2-celled propagules. Observations on the germination of the “monosporangia” of Mazoyerella arachnoidea and Monosporus spp. indicate that they are analagous to the propagules of Deucalion and Anisoschizus. The nature of these propagules and their role in recycling the parent plant are discussed and contrasted with true monosporangia. It is recommended that Monosporus be maintained as a form genus containing representatives from more than one tribe, as exemplified by plants from Lord Howe I. provisionally identified as M. indicus Boergesen which have both prostrate and erect, as opposed to only erect, axes.     

EFFECTS OF HERBIVORE SIZE AND HUNGER LEVEL ON PERIPHYTON COMMUNITIES1

The effects of herbivore size and hunger level were tested on lotic periphyton community structure and ash-free dry mass (AFDM). My hypotheses were 1) that small herbivores would remove more periphyton per unit biomass than large herbivores of the same species because of energetic demands and 2) that within the same size class, starved herbivores would remove more periphyton than nonstarved herbivores. The herbivore used was the prosobranch snail Elimia clavaeformis Lea. Seven treatments were employed: 1) no snails (control); 2) small “starved” snails; 3) large “starved” snails; 4) small and large “starved” snails; 5) small fed snails; 6) large fed snails; and 7) small and large fed snails. Although snails removed significant amounts of periphyton AFDM relative to controls, neither snail size nor degree of starvation had a significant effect on loss of total AFDM. Small snails removed significantly more erect forms of Stigeoclonium tenue (C. A. Ag.) Kütz. than large snails, but snail size had no other significant effect. Starved snails removed significantly more Cocconeis placentula Ehr. than fed snails, suggesting that after the more susceptible growth forms (e.g. erect Stigeoclonium) were removed, Cocconeis cells became more vulnerable to grazing by hungry snails. When small and large snails were combined in chambers, large snails gained weight over time, whereas small snails lost weight. These results suggest that intraspecific competition may occur within populations of Elimia.