ALBERT

Notes: Hydraulic conductivity (K) in the soil and xylem declines as water potential (Ψ) declines. This results in a maximum rate of steady-state transpiration (Ecrit) and corresponding minimum leaf Ψ (Ψcrit) at which K has approached zero somewhere in the soil–leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area (AR:AL), the loss of rhizosphere K limited Ecrit and Ψcrit. Above the threshold, loss of xylem K from cavitation was limiting. The AR:AL threshold ranged from 〉 40 for coarse soils and/or cavitation-resistant xylem to 〈 0·20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species’ hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their AR:AL threshold. Corollary predictions are (1) within a soil type the AR:AL should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the AR:AL should increase and maximum cavitation resistance should decrease.

Notes: Rhizosheaths (sheaths of sand grains that form around the roots of some grasses) are common in perennial grasses that colonise sandy substrates. It has been hypothesised that rhizosheaths increase water availability by increasing the efficiency of water absorption. Others have suggested that rhizosheaths act as storage reservoirs for water. In either case rhizosheaths undoubtedly play an important role in the water relations of these grasses.In an attempt to evaluate the main function of rhizosheaths, we developed a finite element cylindrical water flow model which enabled us to simulate water uptake by Oryzopsis hymenoides (Roem, and Shult.) Ricker. This model allowed us to estimate total water uptake by root systems with and without rhizosheaths and to compare these values to the extra water stored within the rhizosheath. The results of this study suggest that the presence of rhizosheaths is more important in reducing the total resistance to water flow within the rhizosphere than in enhancing water storage.

Notes: Abstract The traditional reference evaporation with empirical crop factor approach to irrigation scheduling can now be improved upon (due to the accessibility of personal computers) by using a more dynamic description of the factors affecting crop water uptake. The soil–water balance (SWB) model, which quantifies water uptake as a water-supply- or evaporative-demand-limited process, was successfully adapted to estimate the water-use of pea (Pisum sativum L. cv. Puget) under both well-watered and water-stressed conditions. A growth analysis experiment in Pretoria, South Africa, provided the necessary crop input parameters to the model. Simulations of soil water deficit and canopy growth compared well with independent data sets in a water-stress field trial. The model, developed in a user-friendly format, can be used as a generic crop irrigation scheduling tool, for full or deficit irrigation conditions, provided that specific crop growth parameters are known.

Notes: Summary Soil water potential is measured either by measuring some property of the soil which changes with the water potential, or by equilibrating the liquid or gas phase of the water in some reference medium with the liquid phase of the soil and measuring some property of the water in the reference medium. Devices commonly used to measure soil water potential are tensiometers, thermocouple psychrometers, electrical resistance sensors, thermal conductivity sensors, and correlations with water contents of the soil or of filter paper which has been equilibrated with the soil. The principles governing the operation of each of these devices will be discussed.

Notes: Abstract The F3C Cold Plasma Analyzer (CPA) instrument on theFreja spacecraft is designed to measure the energy per unit charge (E/Q) of ions oe electrons in the range 0〈E/Q〈200 V and complements the observations made by the F3H Hot Plasma Experiment. The CPA sensor, which is deployed on a boom, is an electrostatic analyzer which produces angle/energy images of particles incident on the sensor in a plane perpendicular to the boom axis. Charged particles incident normal to the CPA sensor housing axis of symmetry, which coincides with the boom axis, pass through collimators and enter a semi-spherical electrostatic analyzer which disperses particles in energy and azimuthal angle of arrival onto an imaging MCP detector thus producing images of the particle distributions in a plane perpendicular to the boom axis. Measurements are transmitted either as discrete 16×16 (angle/energy) images or as parameters related to the incident particle distribution function. Pixels in the discrete images are separated approximately equally in azimuthal angle while the 16 energy bins are separated approximately geometrically in energy. The ratio of the maximum to minimum energy imaged is programmable up to a maximum of more than a factor of ten, and the energy range itself is also under the control of the processor and can be varied by more than an order of magnitude. The density dynamic range of the sensor is increased by the introduction of an electrostatic gating system between the entrance aperture and the analyzer which can be used to duty-cycle low-energy electrons into the sensor thus keeping the count rate within appropriate levels. To reduce the effects of spacecraft induced perturbations on the lower-energy particle distributions, the sensor portion of the instrument is deployed on a 2 m long boom, perpendicular to the spacecraft spin axis. Spacecraft rotation is used to recover complete (4π) angle/energy distributions every half spin period. In addition, the sensor skin may be biased with respect to the spacecraft ground to offset effects due to spacecraft charging. Current to the skin is monitored, making the exterior of the sensor equivalent to a large cylindrical Langmuir probe. Two separate processing paths for signals from the MCP anode may be chosen; ‘slow’ and ‘rast’. The ‘slow’ pulse processing path provides discrete angle/energy images at a nominal rate of 10 images per second and a peak ‘burst mode’ rate of 100 images per second. The ‘fast’ analog or current mode path provides crude parameterized estimates of densities, temperatures and drift velocities at nominal rates of up to 1000 parameters per second with a burst rate near 6000 parameters per second. Observations of cold ions and electrons in an unperturbed ionospheric plasma are presented which demonstrate the functionality of the instrument. Suprathermal ion observations in a transverse ion energization or acceleration region are also shown which demonstrate many of the small-scale features of these events.

Notes: Abstract Penetration of a layer of fibre by wind reduces its effectiveness as a barrier to heat flow. In the literature, the dependence of coat or clothing insulation I(u) on windspeed u is usually described by a relation of the form I(u) = I(0) − au 1/2, where a is a constant. Re-analysis reveals that it is more appropriate to treat coat conductance (proportional to 1/I) as a linear function of windspeed. Vapour conductance can also be treated as a linear function of windspeed.

Notes: Summary Thermal resistance and heat gain from simulated solar radiation were measured over a range of wind velocities in black and white pigeon plumages. Plumage thermal resistance averaged 39% (feathers depressed) or 16% (feathers erected) of that of an equivalent depth of still air. Feather erection increased plumage depth four-fold and increased plumage thermal resistance about 56%. At low wind speeds, black plumages acquired much greater radiative heat loads than did white plumages. However, associated with the greater penetration of radiation into light than dark plumages, the radiative heating of white plumages is affected less by convective cooling than is that of black plumages. Thus, the heat loads of black and white plumages converge as wind speed is increased. This effect is most prominent in erected plumages, where at wind speeds greater than 3 ms−1 black plumages acquire lower radiative heat loads than do white plumages. These results suggest that animals with dark-colored coats may acquire lower heat loads under ecologically realistic conditions than those forms with light-colored coats. Thus, the dark coat colors of a number of desert species and the white coat color of polar forms may be thermally advantageous. These results are used to test a new general model that accounts for effects of radiation penetration into a fur or feather coat upon an animal's heat budget. Even using simplifying assumptions, this model's predictions closely match measured values for plumages with feathers depressed (the typical state). Predictions using simplifying assumptions are less accurate for erected plumages. However, the model closely predicts empirical data for erected white plumages if one assumption is obviated by additional measurements. Data are not sufficient to judge whether this is also the case for erected black plumages.

Notes: Summary Equations are derived that quantify the component thermal resistances to heat transfer in small birds and account explicity for the effects of variation in these resistances. Heat transfer theory is used to quantify external resistances, and an experiment was conducted to estimate body resistance (r b, plumage plus tissues) as a function of external temperature and wind speed. The value ofr b decreased with wind speed, and decreased as air temperature approached 0°C. Heat transfer from small birds is shown to be relatively independent of external resistances and mainly dependent onr b. The predictions of the new theoretical equations are shown to agree well with existing empirical data.

Notes: Summary Spectral measurements were made of solar ultraviolet-B (UVB) irradiance at Sutton Bonington (52° 50′N, 1° 15′W) under cloudless skies using a Licor LI 1800 scanning spectroradiometer. The finite bandpass of the instrument and the steep shape of the UVB spectra caused overestimation of irradiance at short wavelengths. Spectra were corrected mathematically for these effects. The corrected spectra were compared to estimates of global UVB irradiance as a function of zenith angle and amount of ozone. Comparisons were made at 300 nm, 310 nm and 320 nm. Estimates were significantly greater (p = 0.05) than the measurements except at 320 nm where differences were not significant. The differences may have been the result of overestimation of UVB at short wavelengths, since some of the assumptions on which the estimates were based may not be valid for Sutton Bonington conditions.