Survival of Cornus sericea L. stem cortical cells following immersion in liquid helium

Abstract Cold-acclimated stems of red-osier dogwood (Cornus sericea L.) were sampled in midwinter and early spring and subjected to the following low temperature treatments: (a)0 →?40 → 0°C; (b) 0 →?40 →? 196 → 0°C; (c) 0 →?40 →?196 →?269 →?196 → 0°C; (d) 0 →?40 →?269 →?196 → 0°C; (e) 0 →?196 → 0°C; (f) 0 →?269 →?196 →0°C. The cortical parenchyma cells of the outer stem layers survived exposure to ?269°C when pre-frozen to ?40°C and either transferred directly to ?269°C or to ?196°C and then to ?269°C (treatments c and d). Acclimated stems transferred to a greenhouse (22°C) 2 weeks prior to the low temperature treatments deacclimated and were not able to survive freezing to ?10°C. Cortical cells of stem samples taken in March, near the time when dogwood naturally deacclimates, survived ?196°C (treatment b), but not ?269°C (treatment cord). Thus, the freezing tolerance of dogwood varies seasonally from near ?10°C to below ?269°C.     

Etude Ultrastructurale du Genre Enteromonas da Fonseca (Zoomastigophorea) et Révision de l'Ordre des Diplomonadida Wenyon

RESUME. Deux espèces d'Enteromonas sont observées, provenant, l'une de l'intestin de Triton, l'autre des crottes du Lapin domestique. La cellule piriforme porte un noyau antérieur et 4 flagelles insérts près du pôle ventral du noyau. Le flagelle récurrent (R) est logé dans une dépression ventrale ou cytostome. Les cinétosomes, disposés en une paire antérieure (#1, #2) et une paire postérieure (#3, R), sont liés entre eux par des microfibrilles. Une fibre microtubulaire située au-dessus du noyau est reliée au cinétosome #1. Une autre fibre microtubulaire sous-nucléaire est homologue de la fibre microtubulaire croisée qui existe chez les cellules de Diplozoaires. Le cytostome est bordé par 2 lèvres: la gauche proéminente et armée par plusieurs rangées de microtubules, la droite contenant seulement une mince fibre microtubulaire associée à des microfibrilles. Le cytostome occupe les 2/3 de la face ventrale. Le flagelle récurrent pénètre dans le cytostome puis dépasse l'extrémite de la cellule. Les Bactéries sont phagocytées au fond du cytostome, entre les 2 lèvres distendues. Elles sont digérées dans les nombreuses vacuoles et les corps résiduels sont évacués par rupture de la membrane cellulaire. L'ergastoplasme est concentré près de la périphérie de la cellule. Il n'y a pas de mitochondrie ni d'appareil de Golgi. Dans les kystes observés la cellule plurinucléée est enfermée dans une enveloppe kystique microfibrillaire, les axonèmes sont libres dans le cytoplasme. Les formes diplomonades sont nombreuses et ressemblent aux cellules d'Hexamita, excepté par le cytostome qui est différent. Dans ces formes, les 2 monades sont souvent disposées selon une symétrie axiale binaire mais quelquefois elles sont associées de façon plus anarchique. La cinétide d'Enteromonas est organisée comme celle d'un zoïde de Diplozoaire. Il est possible que le genre Enteromonas soit à l'origine des Diplomonadida et que l'état diplomonadien transitoire chez Enteromonas se soit stabilisé ensuite chez les Diplomonadida. Enteromonas apparaît plus primitif que les autres genres de Diplomonadida aussi nous proposons de créer 2 sous-ordres: celui des Enteromonadina avec le genre Enteromonas et celui des Diplomonadina avec les genres Trepomonas, Trigonomonas, Hexamita, Spironucleus, Octomitus, Giardia. La disposition des cinétosomes et l'existence du cytostome sont les principaux caractères communs entre Enteromonas et les Retortamonadida, cependant les fibres annexes ne sont pas homologues. Une étude plus complète de la division nucléaire et cellulaire de ces 2 ordres de Zooflagellés est nécessaire pour donner un meilleur schéma évolutif. SYNOPSIS. Fine structure of 2 species of Enteromonas, one from the intestine of the salamander, Triturus vulgaris, and another from the feces of domestic rabbit, Oryctolagus cuniculi, is described. The pyriform cell has an anteriorly located nucleus. The 4 flagella originate from an area near the anterior end of the nucleus. The recurrent flagellum (R) is lodged in a ventral depression or cytostome. The kinetosomes, arranged into 2 pairs, anterior (#1, #2) and posterior (#3, R), are interconnected by microfibrils. One microtubular fiber, connected to kinetosome #1, is situated near the anterior surface of the nucleus. Another, subnuclear, microtubular fiber is homologous to the “crossed'’fiber found in Diplozoa. The cytostome is bordered by 2 lips: the preeminent left lip is equipped with several rows of microtubules, while the right lip contains only a thin microtubular fiber associated with microfibrils. The cytostome occupies 2/3 of the ventral surface. The recurrent flagellum passes over the anterior surface of the cell and then comes to lie in the cytostome. The bacteria are phagocytosed in the bottom part of the cytostome between the 2 distended lips. They are digested in numerous vacuoles. The undigested residual bodies are evacuated by a rupture of the cell membrane. The ergastoplasm is concentrated near the cell periphery. Mitochondria and the Golgi apparatus are absent. In the cyst stage, the multinucleate cell is enclosed in a microfibrillar membrane; the axonemes lie free in the cytoplasm. Diplomonad forms of Enteromonas resembling Hexamita are numerous, except that the cytostome is different in these 2 genera. In such forms, the arrangement of the 2 individuals often has binary axial symmetry, but on occasion they are associated in a more anarchic fashion. The mastigont of Enteromonas is organized like that of a single zooid of a diplozoon. It is possible that the genus Enteromonas is ancestral to Diplomonadida and that the diplomonad state, transitory in Enteromonas, became permanently established in Diplomonadida. Enteromonas appears to be more primitive than the other genera of Diplomonadida. Thus we propose 2 suborders: Enteromonadina, subord. nov. with the genus Enteromonas, and Diplomonadina Wenyon, emend., with the genera Trepomonas, Trigonomonas, Hexamita, Spironucleus, Octomitus, Giardia. The arrangement of the kinetosomes and the existence of a cytostome are the principal characters common to Enteromonas and Retortamonadida, while their “accessory'’fibers are not homologous. A more complete study of division of the 2 zooflagellate orders is necessary for the presentation of a more detailed evolutionary scheme of these groups.     

Long‐term variation in the littoral food web of an acidified mountain lake

Evidence of chemical recovery from acidification in European freshwaters has emerged in recent years, with many previously damaged systems responding to decades of reduced acidifying emissions. Biological recovery, however, has often lagged behind, and this has been ascribed to several possible mechanisms, including inertia in the food web. We examined two decades of change in hindcasted food webs for Lochnagar, a Scottish mountain lake, to make inferences about the potential dynamical stability of the system and to assess the prospects for future biological recovery. Although community composition tracked temporal changes in acidity, this was neither sustained nor directional, and mainly manifested as shifts in relative abundances rather than the establishment of more acid‐sensitive species. The food web was highly interconnected and reticulate, especially in years when species richness was low, and subsidized by external inputs of detritus. Among the primary consumers, generalist herbivore–detritivores maintained feeding links with the scant algal resources, which appeared insufficient to support viable populations of specialist grazers. Together, these characteristics, which are shared with many other acidified freshwaters, are likely to make the community dynamically stable and resistant to invasions of potential new colonists, thereby slowing the pace of future biological recovery.     

Soil carbon balance in a clonal Eucalyptus plantation in Congo: effects of logging on carbon inputs and soil CO2 efflux

Soil CO₂ efflux was measured in clear‐cut and intact plots in order to quantify the impact of harvest on soil respiration in an intensively managed Eucalyptus plantation, and to evaluate the increase in heterotrophic component of soil respiration because of the decomposition of harvest residues. Soil CO₂ effluxes showed a pronounced seasonal trend, which was well related to the pattern of precipitation and soil water content and were always significantly lower in the clear‐cut plots than in the intact plots. On an annual basis, soil respiration represented 1.57 and 0.91 kg_C m^?2 yr^?1 in intact and clear‐cut plots, respectively. During the first year following harvest, residues have lost 0.79 kg_C m^?2 yr^?1. Our estimate of heterotrophic respiration was calculated assuming that it was similar to soil respiration in the clear‐cut area except that the decomposition of residues did not occur, and it was further corrected for differences in soil water content between intact and clear‐cut plots and for the cessation of leaf and fine root turnover in clear cut. Heterotrophic respiration in clear‐cut plots was estimated at 1.18 kg_C m^?2 yr^?1 whereas it was only 0.65 kg_C m^?2 yr^?1 in intact plots (41% of soil respiration). Assumptions and uncertainties with these calculations are discussed.     

Vector specificity of barley yellow dwarf virus serotypes and variants in southwestern Idaho

Plants with symptoms of barley yellow dwarf virus (BYDV) obtained in infection feeding assays of aphids collected in the field in Idaho between 1986 and 1988 were tested for virus transmissibility by possible aphid vectors. Isolates obtained during 1987–1988 were also tested with a range of polyclonal antisera which distinguished PAV, MAV, SGV, RPV and RMV serotypes. In 1989 some Idaho (ID) BYDV isolates, maintained as standards for comparison, were serotyped and tested for aphid transmissibility, using 11 species of aphids. There was not always the expected correspondence between serotype and vector specificity for ID isolates. For isolates obtained from field-collected Rhopalosiphum padi, vector transmissibility and serotype corresponded with previous reports; however, 44% of isolates which were serotyped as RMV were also transmissible by species other than Rhopalosiphum maidis. Similarly, the transmissibility of the ID laboratory standards did not always conform to the reported vector specificity of serotypes. The laboratory ID-MAV culture was transmitted by Metopolophium dirhodum and Myzus persicae as well as by Sitobion avenae. The laboratory ID-SGV culture was transmitted by R. padi and 5. avenae as well as by Schizaphis graminum. The ID-RPV culture was transmitted by S. graminum and Rhopalosiphum insertum as well as R. padi. Both of two laboratory ID-RMV cultures were transmissible by R. insertum and R. padi transmitted one of them. The results indicate that, for isolates collected in Idaho, vector specificity cannot be assumed from their serotypes.     

Theoretical habitat templets, species traits, and species richness: floodplain vegetation in the Upper Rhône River

• 1 The floodplain vegetation at approximately 100 sites located in nine different habitat types of the Upper Rhône River, France, was surveyed three times over the past 27 years. Information on species traits of the higher plants comprising the Rhône floodplain vegetation was based on studies conducted between Geneva, Switzerland, and Lyon, France.
• 2 These data were structured using a ‘fuzzy coding’ technique and then examined using ordination analyses to investigate: (i) relationships among species traits; (ii) habitat utilization; (iii) the relationship between species traits and habitat utilization; and (iv) trends of species traits and species richness in the framework of spatial–temporal habitat variability to test predictions of the river habitat templet and the patch dynamics concept.
• 3 Size, number of descendants per reproductive cycle, number of reproductive cycles per individual, and the regeneration potential of an individual were positively related with each other, whereas the degree of attachment to the soil decreased, and the reproductive period shifted from autumn/late summer towards early summer/spring, as size increased.
• 4 The habitat utilization by the higher plants of the floodplain revealed a double lateral gradient: the first was from the banks of the temporary waters to terrestrial flats; the second from aggrading pebble to aggrading silt habitats. These gradients were related to gradients in water saturation, oxygen conditions, nutrient loading, and nutrient retention of the soils.
• 5 A significant relationship between species traits and habitat utilization was observed for the floodplain vegetation, i.e. plant communities used particular habitat types with a particular set of species trait modalities (= categories).
• 6 Patterns of species trait modalities were significantly related to temporal and spatial habitat variability but only modalities of the trait ‘parental care’ conformed to trends predicted from theory.
• 7 No trends were observed when species richness of different habitat types was considered in the framework of spatial–temporal habitat variability.
• 8 Although the habitats of the Upper Rhône clearly act as a templet for the species traits of the floodplain vegetation, the lack of agreement between observations and predictions on trends in species traits and richness in terms of habitat variability suggest that important elements of theory should be rejected. However, human-induced changes in these habitats are too recent when compared with the longer time periods required for floodplain vegetation to respond to such changes.

     

Resource quality and stoichiometric constraints on stream ecosystem functioning

1. Resource quality and stoichiometric imbalances in carbon : nutrient ratios between consumers and resources can influence key ecosystem processes. In many streams, this has important implications for food webs that are based largely upon the utilization of terrestrial leaf‐litter, which varies widely among litter types in its value as a food source for detritivores and as a substrate for microbial decomposers. 2. We measured breakdown rates and macroinvertebrate colonization of leaf‐litter from a range of native and exotic plants of differing resource quality and palatability to consumers [e.g. carbon : nitrogen : phosphorus (C : N : P) ratios, lignin and cellulose content], in a field experiment. We also measured C : N : P ratios of the principal leaf‐shredding invertebrates, which revealed strong stoichiometric imbalances across trophic levels: C : N and C : P ratios typically differed by at least one order of magnitude between consumers and resources, whereas N : P imbalances were less marked. Application of the threshold elemental ratio approach, which integrates animal bioenergetics and body elemental composition in examining nutrient deficiency between consumers and resources, revealed less marked C : P imbalances than those based on the simpler arithmetic differences described above. 3. Litter breakdown rates declined as nutrient imbalances widened and resource quality fell, but they were independent of whether resources were exotic or native. The principal drivers of total, microbial and invertebrate‐mediated breakdown rates were lignin : N, lignin : P and fungal biomass, respectively. However, multiple regression using orthogonal predictors yielded even more efficient models of litter breakdown, as consumers responded to more than one aspect of resource quality. For example, fungal biomass and litter C : N both influenced invertebrate‐mediated breakdown. 4. Large stoichiometric imbalances and changes in resource quality are likely to have serious consequences for stream ecosystem functioning, especially when riparian zones have been invaded by exotic plant species whose chemical composition differs markedly from that of the native flora. Consequently, the magnitude and direction of change in breakdown rates and, thus, resource depletion, will be driven to a large extent by the biochemical traits (rather than taxonomic identity per se) of the resident and invading flora.