Endothelin-1 mediates prostaglandin F(2alpha)-induced luteal regression in the ewe

A diversified series of experiments was conducted to determine the potential role of endothelin-1 (ET-1) in ovine luteal function. Endothelin-1 inhibited basal and LH-stimulated progesterone production by dispersed ovine luteal cells during a 2-h incubation. This inhibition was removed when cells were preincubated with cyclo-D-Asp-Pro-D-Val-Leu-D-Trp (BQ123), a highly specific endothelin ET(A) receptor antagonist. Administration of a luteolytic dose of prostaglandin F(2alpha) (PGF(2alpha)) rapidly stimulated gene expression for ET-1 in ovine corpora lutea (CL) collected at midcycle. Intraluteal administration of a single dose of BQ123 to ewes on Day 8 or 9 of the estrous cycle mitigated the luteolytic effect of PGF(2alpha). Intramuscular administration of 100 microg ET-1 to ewes at midcycle reduced plasma progesterone concentrations for the remainder of the estrous cycle. Following pretreatment with a subluteolytic dose of PGF(2alpha), i.m. administration of 100 microg ET-1 caused a rapid decline in plasma progesterone and shortened the length of the estrous cycle. These data complement and extend previously published reports in the bovine CL and are the strongest evidence presented to date in support of a role for ET-1 in PGF(2alpha)-mediated luteal function in domestic ruminants.     

PHOTIC ORIENTATION BY TWO POINT-SOURCES OF LIGHT

General formulæ are derived for the orientation of phototropic organisms in a field illuminated by two point-sources of light in the same horizontal plane. It is shown that formulæ previously found may all be derived as special cases of these general equations.     

Spectral and Structural Measures of Northwest Forest Vegetation at Leaf to Landscape Scales

We report a multiscale study in the Wind River Valley in southwestern Washington, where we quantified leaf to stand scale variation in spectral reflectance for dominant species. Four remotely sensed structural measures, the normalized difference vegetation index (NDVI), cover fractions from spectral mixture analysis (SMA), equivalent water thickness (EWT), and albedo were investigated using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. Discrimination of plant species varied with wavelength and scale, with deciduous species showing greater separability than conifers. Contrary to expectations, plant species were most distinct at the branch scale and least distinct at the stand scale. At the stand scale, broadleaf and conifer species were spectrally distinct, as were most conifer age classes. Intermediate separability occurred at the leaf scale. Reflectance decreased from leaf to stand scales except in the broadleaf species, which peaked in near-infrared reflectance at the branch scale. Important biochemical signatures became more pronounced spectrally progressing from leaf to stand scales. Recent regenerated clear-cuts (less than 10 years old) had the highest albedo and nonphotosynthetic vegetation (NPV). After 50 years, the stands showed significant decreases in albedo, NPV, and EWT and increases in shade. Albedo was lowest in old-growth forests. Peak EWT, a proxy measure for leaf area index (LAI), was observed in 11- to 30-year-old stands. When compared to LAI, EWT and NDVI showed exponentially decreasing, but distinctly different, relationships with increasing LAI. This difference is biologically important: at 95% of the maximum predicted NDVI and EWT, LAI was 5.17 and 9.08, respectively. Although these results confirm the stand structural variation expected with forest succession, remote-sensing images also provide a spatial context and establish a basis to evaluate variance within and between age classes. Landscape heterogeneity can thus be characterized over large areas—a critical and important step in scaling fluxes from stand-based towers to larger scales.     

Non-linear dynamics of a pelagic ecosystem model with multiple predator and prey types

Using numerical techniques, we explored the dynamics of a one-dimensional,six-component nutrient–phytoplankton–zooplankton(NPZ) model in which zooplankton grazed on a mixed prey field.Five alternative functional forms were implemented to describezooplankton grazing, and the form for predation on mesozooplanktonwas prescribed by a product of a specific predation rate (h)and the mesozooplankton concentration raised to a power (q),which we varied between one and two. With all five grazing functions,Hopf bifurcations, where the form of the solution transitionedbetween steady equilibrium and periodic limit cycles, persistedacross the q–h parameter space. Regardless of the valuesof h and q, with some forms of the grazing function, we wereunable to find steady equilibrium solutions that simultaneouslycomprised non-zero concentrations for all six model components.Extensions of Michaelis–Menten-based single resource grazingformulations to multiple resources resulted in periodic solutionsfor a large portion of the q–h space. Conversely, extensionsof the sigmoidal grazing formulation to multiple resources resultedin steady solutions for a large portion of q–h parameterspace. Our results demonstrate the consequences of the functionalform of biological processes on the form of the model solutions.Both the steady or oscillatory nature of state variable concentrationsand the likelihood of their elimination are important considerationsfor ecosystem-modelling studies, particularly when attemptingto model an ecosystem in which multiple phytoplankton and zooplanktoncomponents are thought to persist simultaneously for at leasta portion of the seasonal cycle.     

Components and Controls of Water Flux in an Old-growth Douglas-fir–Western Hemlock Ecosystem

We report measurements of rates of sap flow in dominant trees, changes in soil moisture, and evaporation from coarse woody debris in an old-growth Douglas-fir–western hemlock ecosystem at Wind River, Washington, USA, during dry periods in summer. The measurements are compared with eddy-covariance measurements of water-vapor fluxes above the forest (E_e) and at the forest floor (E_u) to examine the components of ecosystem water loss and the factors controlling them. Daily values of E_u were about 10% of E_e. Evaporation from coarse woody debris was only about 2% of E_e. Transpiration (E_t), estimated by scaling sap-flow measurements accounted for about 70% of (E_e– E_u); transpiration from subdominant trees may account for the remainder. The daily total change in soil moisture (E_s) in the top 30 cm was larger than the net change, probably because of hydraulic redistribution of soil water by roots. Observed differences between E_s and E_e were probably because roots also extract water from greater depth, and/or because the measuring systems sample at different spatial scales. The ratio of E_t to E_s decreased with decreasing soil water content, suggesting that partitioning in water use between understory and overstory changed during the season. The rate of soil drying exceeded E_e early in the day, probably because water vapor was being stored in canopy air space and condensed or adsorbed on tree stems, lichens, and mosses. The daily variation of E_e with vapor-pressure deficit showed strong hysteresis, most likely associated with transpiration of water stored in tree stems and branches.