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排序方式: 共有174条查询结果,搜索用时 15 毫秒
1.
YONG-JIANG ZHANG FREDERICK C. MEINZER GUANG-YOU HAO FABIAN G. SCHOLZ SANDRA J. BUCCI FREDERICO S. C. TAKAHASHI RANDOL VILLALOBOS-VEGA JUAN P. GIRALDO KUN-FANG CAO WILLIAM A. HOFFMANN & GUILLERMO GOLDSTEIN 《Plant, cell & environment》2009,32(10):1456-1466
Size-related changes in hydraulic architecture, carbon allocation and gas exchange of Sclerolobium paniculatum (Leguminosae), a dominant tree species in Neotropical savannas of central Brazil (Cerrado), were investigated to assess their potential role in the dieback of tall individuals. Trees greater than ∼6-m-tall exhibited more branch damage, larger numbers of dead individuals, higher wood density, greater leaf mass per area, lower leaf area to sapwood area ratio (LA/SA), lower stomatal conductance and lower net CO2 assimilation than small trees. Stem-specific hydraulic conductivity decreased, while leaf-specific hydraulic conductivity remained nearly constant, with increasing tree size because of lower LA/SA in larger trees. Leaves were substantially more vulnerable to embolism than stems. Large trees had lower maximum leaf hydraulic conductance ( K leaf ) than small trees and all tree sizes exhibited lower K leaf at midday than at dawn. These size-related adjustments in hydraulic architecture and carbon allocation apparently incurred a large physiological cost: large trees received a lower return in carbon gain from their investment in stem and leaf biomass compared with small trees. Additionally, large trees may experience more severe water deficits in dry years due to lower capacity for buffering the effects of hydraulic path-length and soil water deficits. 相似文献
2.
SANDRA KNAPP F.L.S. 《Botanical journal of the Linnean Society. Linnean Society of London》1991,106(1):73-89
The Solanum sessile species group is one of eight putatively monophyletic subgroups of the large and diverse Solanum section Geminata. The species group comprises ten species of mostly northern South American trees and shrubs. Characters and character states peculiar to Solanum are discussed in detail. A cladistic analysis of the sessile group yielded four equally parsimonious trees which represent two basic tree topologies. The consensus cladogram is composed of three main clades. Biogeographic analysis using the consensus tree revealed no components in common to the three clades. Possible geographical scenarios are discussed with reference to the geological history of northern South America. 相似文献
3.
The interaction between the Toxoplasma parasitophorous vacuole and vimentin-type intermediate filaments in Vero cells was investigated via immunofluorescence microscopy. A significant rearrangement of host cell vimentin around the Toxoplasma parasitophorous vacuoles occurs throughout the course of infection. Host cell vimentin associates with the parasitophorous vacuoles within an hour after invasion. This vimentin overcoating of the vacuole is initiated at the host cell nuclear surface. During parasite multiplication, vimentin retains a closely defined association with the cytosolic surface of the parasitophorous vacuole. In addition, the vimentin intermediate filaments originating from the host cell nuclear surface are progressively rearranged around the enlarging parasitophorous compartment. During infections, the order of vimentin cytoskeleton is normal throughout the cell and appears redefined only at the vicinity of the parasitophorous vacuole. Depolymerization of the intermediate filaments was achieved with the phosphatase inhibitors okadaic acid and calyculin A. Disruption of the intermediate filament networks resulted in displacement of the parasitophorous vacuoles from the host cell nuclear surface. The data indicate that host cell vimentin binds to the Toxoplasma parasitophorous vacuoles and that the host intermediate filament network serves to dock the parasite compartment to the host cell nuclear surface. 相似文献
4.
J. R. COOK SANDRA SMITH HAGGARD PATRICIA HARRIS 《The Journal of eukaryotic microbiology》1976,23(3):368-373
SYNOPSIS. In populations of Euglena gracilis strain Z synchronized by cultivation on a repetitive light-dark cycle, chloroplasts undergo cyclic changes in structure. During most of the light period chloroplasts are relatively compact with closely appressed lamellae; during the dark (division) period the chloroplasts become quite distended. This change persists for at least one cycle even when the cells are left in continuous light, suggesting that the periodicity may be related more to the age of the cell than to a direct effect of light. In addition, the pyrenoid in synchronized cells has a transient existence, being present only in the first half of the light period. 相似文献
5.
Leaf structure and defence control litter decomposition rate across species and life forms in regional floras on two continents 总被引:17,自引:2,他引:17
6.
The importance of pre-thicket conifer plantations for nesting Hen Harriers Circus cyaneus in Ireland
MARK W. WILSON SANDRA IRWIN DAVID W. NORRISS STEPHEN F. NEWTON KEVIN COLLINS THOMAS C. KELLY & JOHN O'HALLORAN 《Ibis》2009,151(2):332-343
Hen Harriers Circus cyaneus are threatened across much of their range and their conservation requires appropriate habitat management. The locations of 148 Hen Harrier nests found in the Republic of Ireland during national breeding surveys in 2000 and 2005 were used to assess nest-site selection. The distribution of these nests was compared to distributions of randomly located points to investigate selection at the scale of the nest-site and landscape. The main nesting habitats selected were pre-thicket stage of first and, particularly, second rotation plantations, mostly of exotic conifers. There was no evidence that the area of post-closure plantations negatively affected Hen Harrier nest distribution. There was a positive correlation across study areas between changes in numbers of Hen Harrier nests between 2000 and 2005 and changes in the area of pre-thicket second rotation plantations over the same period. The overall effect of plantation forests on breeding Hen Harriers in Ireland therefore appears to be positive. However, this study did not consider the effects of plantation habitats on breeding success. Improved grassland was strongly avoided as a nesting habitat. Furthermore, after controlling for the influence of nesting habitat on nest location, landscapes with a high percentage cover of improved grassland were also avoided. Further agricultural intensification of grassland in areas where Hen Harriers breed is likely to have a negative impact on this species. These results are required for the development of management strategies for the conservation of this species. 相似文献
7.
SEBASTIEN RIOUX PAQUETTE GARY D. SHORE SANDRA M. BEHNCKE FRANOIS‐JOSEPH LAPOINTE EDWARD E. LOUIS 《Molecular ecology resources》2005,5(3):527-530
The radiated tortoise (Geochelone radiata) is an endangered endemic species from Madagascar that inhabits the semiarid spiny forest of the southern part of the island. Habitat destruction and illegal harvesting greatly threaten this species, as attested by the significant reduction of its distribution area in the past 30 years. In order to undertake appropriate conservation actions, it is essential to acquire a better knowledge of its genetic structure. For this study, 145 blood samples were collected from three populations of radiated tortoises in southwestern Madagascar. Eight microsatellite loci were found to be polymorphic, and allelic diversity and observed heterozygosity were high for all markers. 相似文献
8.
SANDRA GILLESPIE 《Ecological Entomology》2010,35(6):737-747
1. Bumblebees are important pollinators in North America and are attacked by a range of parasites that impact their fitness; however, few studies have investigated the extent or causes of parasitism in North America. 2. This study used a 2‐year multi‐site survey of bumblebee parasitism to ask: (i) how common are parasitoid conopid flies and the internal parasites Crithidia bombi and Nosema bombi in Massachusetts; and (ii) what factors are correlated with parasitism? 3. Infection rates by all three parasites were higher in this study than previously documented in North America. Overall, conopids infected 0–73% of bees in each sample, C. bombi infected 0–82% of bees, and N. bombi infected 0–32%. 4. Conopid flies infected female bees more than males and intermediate‐sized bees more than large or small bees. Crithidia bombi infection rates were higher in certain bee species and sites, and exhibited a unimodal pattern of prevalence over time. Nosema bombi parasitism was higher in male than female bees. 5. Infection by N. bombi in two rare bumblebee species was higher than expected based on parasitism rates of common bee species but C. bombi infection was lower. If high prevalence of N. bombi in these bumblebee species is common, parasitism may be a potential cause of their decline. 6. Given the documented effects of these parasites, the high levels of infection may affect bee populations in Massachusetts and threaten the stability of their valuable ecosystem services. 相似文献
9.
SANDRA PANCHALINGAM ASHIWEL S. UNDIE 《Journal of receptor and signal transduction research》2013,33(2):125-146
Dopamine agonist-stimulated [35S]GTPγS binding to membrane G proteins was studied in select brain regions under experimental conditions that permit the activation of receptor coupling to the G proteins Gi, Gs, or Gq. Agents studied were agonists known to be effective at various dopamine receptor/effector systems and included quinelorane (D2-like/Gi), SKF38393 (D1-like/Gq, D1-like/Gs), SKF85174 (D1-like/Gs), and SKF83959 (D1-like/Gq). Dopamine and SKF38393 significantly stimulated [35S]GTPγS binding to normal striatal membranes by 161% and 67% above controls. Deoxycholate, which enhances agonist-induced phospholipase C (PLC) stimulation, markedly enhanced the agonistic effects of dopamine and SKF38393 to 530% and 637% above controls, respectively. The enhancing effects of deoxycholate were reversed if it was washed off the membranes before agonist addition. The thiol-reducing agent, dithiothreitol, completely abolished the effects of SKF38393 and SKF83959, whereas SKF85174 effects were augmented. Agonist responses were concentration-related, and highest efficacies were obtained in the hippocampus, thus paralleling both the brain regional distribution and agonist efficacies previously observed in phosphoinositide hydrolysis assays. These findings suggest that D1-like receptor conformations that mediate agonist stimulation of Gs/adenylylcyclase may be structurally different from those that mediate Gq/PLC activation. Although the exact mechanism of deoxycholate's effect awaits elucidation, the results are consistent with the emerging concept of functional selectivity whereby deoxycholate could create a membrane environment that facilitates the transformation of the receptor from a conformation that activates Gs/adenylylcyclase to one that favors Gq/PLC signaling. 相似文献
10.
MARTIN CARR BARRY S. C. LEADBEATER SANDRA L. BALDAUF 《The Journal of eukaryotic microbiology》2010,57(1):56-62
ABSTRACT. The choanoflagellates are a widespread group of heterotrophic aquatic nanoflagellates, which have recently been confirmed as the sister-group to Metazoa. Asexual reproduction is the only mode of cell division that has been observed within the group; at present the range of reproductive modes, as well as the ploidy level, within choanoflagellates are unknown. The recent discovery of long terminal repeat retrotransposons within the genome of Monosiga brevicollis suggests that this species also has sexual stages in its life cycle because asexual organisms cannot tolerate retrotransposons due to the rapid accumulation of deleterious mutations caused by their transposition. We screened the M. brevicollis genome for known eukaryotic meiotic genes, using a recently established "meiosis detection toolkit" of 19 genes. Eighteen of these genes were identified, none of which appears to be a pseudogene. Four of the genes were also identified in expressed sequence tag data from the distantly related Monosiga ovata . The presence of these meiosis-specific genes provides evidence for meiosis, and by implication sex, within this important group of protists. 相似文献