Measurement of vulnerability to water stress‐induced cavitation in grapevine: a comparison of four techniques applied to a long‐vesseled species

Among woody plants, grapevines are often described as highly vulnerable to water‐stress induced cavitation with emboli forming at slight tensions. However, we found native embolism never exceeded 30% despite low xylem water potentials (Ψ_x) for stems of field grown vines. The discrepancy between native embolism measurements and those of previous reports led us to assess vulnerability curve generation using four separate methods and alterations (i.e. segment length and with/without flushing to remove embolism prior to measurement) of each. Centrifuge, dehydration and air‐injection methods, which rely on measurement of percentage loss of hydraulic conductivity (PLC) in detached stems, were compared against non‐invasive monitoring of xylem cavitation with nuclear magnetic resonance (NMR) imaging. Short segment air‐injection and flushed centrifuge stems reached >90 PLC at Ψ_x of‐0.5 and ?1.5 MPa, respectively, whereas dehydration and long‐segment air‐injection measurements indicated no significant embolism at Ψ_x > ?2.0 MPa. Observations from NMR agreed with the dehydration and long segment air‐injection methods, showing the majority of vessels were still water‐filled at Ψ_x > ?1.5 MPa. Our findings show V. vinifera stems are far less vulnerable to water stress‐induced cavitation than previously reported, and dehydration and long segment air‐injection techniques are more appropriate for long‐vesseled species and organs.     

Comparative biology of pollination systems in the African-Malagasy genus Brownleea (Brownleeinae: Orchidaceae)

The role of pollinators in the evolution of the African-Malagasy orchid genus Brownleea (Brownleeinae) was investigated. Taxa show specialization for pollination by bees ( B. parviflora , B. recurvata ), short-proboscid flies ( B. galpinii ), and long-proboscid flies ( B. macroceras , B. coerulea ). All species in the genus produce nectar, but some ( B. coerulea , B. galpinii ) appear to mimic flowers of abundant sympatric species as an additional strategy to attract pollinators. Species investigated in terms of their breeding systems ( B. coerulea , B. parviflora , B. macroceras , B. galpinii ) are reliant on pollinator visits for seed production. Self-pollination results in strong inbreeding depression during embryo formation. A phylogeny of the genus, constructed using data from the ITS1-5.8S-ITS2 region and morphology, indicates that fly pollination is likely to be basal in the genus, and that there has been a single shift to bee pollination.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 65–78.     

Body mass of prefledging Emperor Geese Chen canagica: large-scale effects of interspecific densities and food availability

We studied body mass of prefledging Emperor Geese Chen canagica at three locations across the Yukon–Kuskokwim Delta, Alaska, during 1990–2004 to investigate whether large-scale variation in body mass was related to interspecific competition for food. From 1990 to 2004, densities of Cackling Geese Branta hutchinsii minima more than doubled and were c . 2–5× greater than densities of Emperor Geese, which were relatively constant over time. Body mass of prefledging Emperor Geese was strongly related (negatively) to interspecific densities of geese (combined density of Cackling and Emperor Geese) and positively related to measures of food availability (grazing lawn extent and net above-ground primary productivity (NAPP)). Grazing by geese resulted in consumption of ≥ 90% of the NAPP that occurred in grazing lawns during the brood-rearing period, suggesting that density-dependent interspecific competition was from exploitation of common food resources. Efforts to increase the population size of Emperor Geese would benefit from considering competitive interactions among goose species and with forage plants.     

Grapevine species from varied native habitats exhibit differences in embolism formation/repair associated with leaf gas exchange and root pressure

Drought induces xylem embolism formation, but grapevines can refill non‐functional vessels to restore transport capacity. It is unknown whether vulnerability to embolism formation and ability to repair differ among grapevine species. We analysed in vivo embolism formation and repair using x‐ray computed microtomography in three wild grapevine species from varied native habitats (Vitis riparia, V. arizonica, V. champinii) and related responses to measurements of leaf gas exchange and root pressure. Vulnerability to embolism formation was greatest in V. riparia, intermediate in V. arizonica and lowest in V. champinii. After re‐watering, embolism repair was rapid and pronounced in V. riparia and V. arizonica, but limited or negligible in V. champinii even after numerous days. Similarly, root pressure measured after re‐watering was positively correlated with drought stress severity for V. riparia and V. arizonica (species exhibiting embolism repair) but not for V. champinii. Drought‐induced reductions in transpiration were greatest for V. riparia and least in V. champinii. Recovery of transpiration after re‐watering was delayed for all species, but was greatest for V. champinii and most rapid in V. arizonica. These species exhibit varied responses to drought stress that involve maintenance/recovery of xylem transport capacity coordinated with root pressure and gas exchange responses.     

Oxygen Transport in the Avian Egg at High Altitude

Oxygen transport to avian embryo tissues occurs by three steps,two of which are driven by diffusion. This results in a seriesof stepwise decrements in PO₂ between atmosphere and tissue.The PO₂ decrements for embryos of the domestic fowl incubatedat different altitudes are used here to examine potential adaptationsto hypobanc hypoxia. With exposure to moderate hypoxia embryosof the domestic fowl appear to maintain adequate tissue oxygenation.Adaptive adjustments in the shell, shell membranes and chorioallantoiscomplex were not observed. However, hemoglobin O₂ affinity wasincreased and preliminary evidence suggests a redistributionof blood flow to maintain adequate oxygenation in higher priorityareas of embryonic tissue. At severe hypoxia, embryos of thedomestic fowl show decreased O₂ consumption, embryo mass andlengthened incubition period. Thus at severe hypoxin the embryoof the domestic fowl does not appear to provide a realisticmodel. Evidence from avian embryos of species native to highaltitude suggest that they are able to maintain adequate tissueoxygenation even at severe hypoxia. Preliminary evidence suggeststhat some of the blood vascular system and tissue level adaptationspresent in the chicken embryo are also present in species nativeto high altitude. One of these, an increase in embryonic hemoglobin-O₂affinity which is physiologically mediated in the chicken embryois genetically-based in the embryo of the native high-altitudespecies.