A MICROFOUNDATION OF PREDATOR‐PREY DYNAMICS

ABSTRACT. Predator‐prey relationships account for an important part of all interactions betweenspecies. In this paper we provide a microfoundation for such predator‐prey relations in afood chain. Basic entities of our analysis are representative organisms of species modeled similar to economic households. With prices as indicators of scarcity, organisms are assumed to behave as if they maximize their net biomass subject to constraints which express the organisms' risk of being preyed upon during predation. Like consumers, organisms face a ‘budget constraint’ requiring their expenditure on prey biomass not to exceed their revenue from supplying own biomass. Short‐run ecosystem equilibria are defined and derived. The net biomass acquired by the representative organism in the short term determines the positive or negative population growth. Moving short‐run equilibria constitute the dynamics of the predator‐prey relations that are characterized in numerical analysis. The population dynamics derived here turn out to differ significantly from those assumed in the standard Lotka‐Volterra model.     

A DIURNAL RHYTHM IN THE RATE OF LIGHT-INDUCED ELECTRON FLOW IN MAIZE BUNDLE SHEATH STRIPS

Abstract— A technique for the rapid mechanical isolation of bundle sheath strips from Zea mays was used to study the rate of light-induced electron flow as a function of time of day. Seedlings were raised for 10 days in light/dark cycles and then exposed to various intensities of light. The rate of light-induced electron flow was measured five times during the 24-h cycle using a whole cell, H₂O to methyl viologen assay in the presence of gramicidin. The uncoupled rate of electron flow reached a maximum at midday and then decreased for the remainder of the cycle.
If at the end of the 14-h dark period plants were either maintained in darkness, or exposed to very low irradiance white light. then no rhythmicity was observed. If continuous light at higher irradiance was used, then only one additional peak of activity was observed. The minimal light irradiance necessary to allow the continued expression of the rhythm in light/dark cycles is in the range between 11 to 54 W m_-2. A 15-min bright-light (54 W m_-2) pulse administered at dawn is not sufficient to stimulate the expression of the rhythm indicating that the light during the light/dark cycle is doing more than just acting as a "light-on" synchronizing signal. An imposed dark period is also necessary if the rhythm is to be expressed for more than one cycle.     

REACTIVITY OF PHOTOCHEMICALLY-GENERATED LIPID HYDROPEROXIDES IN CELL MEMBRANES WITH GLUTATHIONE PEROXIDASE

The ability of glutathione peroxidase (Gpx) to catalyze the reductive inactivation of photochemically-generated lipid hydroperoxides (LOOHs) was investigated, using hematoporphyrin derivative (HPD) as a photosensitizing agent and erythrocyte ghosts as membrane targets. Glutathione peroxidase was reactive toward photoperoxidized membranes only after their exposure to phospholipase A2 (PLA2). Iodometrically-determined LOOH values were typically 30-40% greater than values measured by enzymatic assay using Gpx and glutathione reductase. A consistent result was obtained when photooxidized membranes were treated with PLA2 and GSH/Gpx followed by iodometric assay, viz. persistence of approximately 40% of the starting LOOH. Whereas photooxidized egg phosphatidylcholine liposomes underwent total LOOH loss when incubated with PLA2 and GSH/Gpx, no net loss was observed with photooxidized cholesterol/dimyristoyl-phosphatidylcholine liposomes. The results suggest that cholesterol hydroperoxides in ghost membranes account for the Gpx-resistant fraction of LOOHs.     

Perturbation Analysis for Wetting Fronts in Richard's Equation

Perturbation methods are used to study the interaction of wetting fronts with impervious boundaries in layered soils. Solutions of Richards' equation for horizontal and vertical infiltration problems are considered. Asymptotically accurate solutions are constructed from outer solutions and boundary-layer corrections. Results are compared with numerical simulations to demonstrate a high level of accuracy.     

TEMPORAL AGGREGATION AND TESTING FOR TIMBER PRICE BEHAVIOR

ABSTRACT. Different harvest timing models make different assumptions about timber price behavior. Those seeking to optimize harvest timing are thus first faced with a decision regarding which assumption of price behavior is appropriate for their market, particularly regarding the presence of a unit root in the timber price time series. Unfortunately for landowners and investors, the literature provides conflicting guidance on this subject. One source for the ambiguous results of unit root tests of timber prices may involve data problems. We used Monte Carlo simulations to show that aggregating observations below their observed rate resulted in similar power reductions and empirical size distortions across three classes of unit root tests. Moving‐average error structures can also affect power and sizes of tests on period‐averaged data. Such error structures can also be created by the kind of temporal averaging common in reported timber prices. If we take timber prices at their face value and therefore ignore these sampling error and temporal aggregation complications, we find that unit root tests on southern timber prices support a unit root in 158 out of 208 product‐deflation combinations tested, random walks in 38 of the series found to be nonsta‐tionary, and stationarity in none. However, if we recognize temporal aggregation errors, unit root tests more commonly favor stationarity, especially for pulpwood stumpage. Because price trends for sawtimber and pulpwood products may behave differently even in the same region, stochastic harvest timing models must be developed that allow their multiple products to follow different price paths.     

CHARACTERIZATION OF THE FAST LIGHT-EVOKED ELECTRICAL RESPONSES FROM LEAVES

Abstract— A fast electrical response with a millisecond time course and no detectable latency has been recorded from the front surface of a bean leaf uniformly illuminated by an intense flash. The response is biphasic with the tip of the leaf always going negative with respect to the base of the leaf during the first msec regardless of the electrode positions and direction of light. Careful checks were made to eliminate the possibility that the response is due to some type of electrode artifact or to a photovoltaic, photogalvanic or photoconductive process in the leaf. Several properties of this response, including temperature dependence, flash energy dependence, polarity of the response with respect to leaf orientation and the effect of external polarizing voltages were investigated in order to compare it with fast photoresponses recently detected from other biological materials such as the vertebrate eye, pigment epithelium, and skin.     

INTRACELLULAR FLUORESCENCE OF PHOTOSENSITIZING PORPHYRINS AT DIFFERENT CONCENTRATIONS OF MITOCHONDRIA

Abstract— Photofrin II, which contains the most efficient components of hematoporphyrin derivative with regard to tumor phototherapy, was measured fluorometrically in single cells of three squamous cell lines with different mitochondrial concentrations. Using the method of time-resolved fluorescence microscopy, two components—probably monomeric and dimeric species—were differentiated according to the lifetimes of their excited states. The intensity ratio between these components was found to depend on the concentration of mitochondria. This may be due to a monomerization of those porphyrin molecules which are retained in mitochondrial sites.