Viability analysis of a threatened amphibian population: modelling the past,present and future

Conservation actions that have maintained populations in the past may not necessarily do so in the future. Population viability analysis provides one tool for exploring the impact of management actions on large temporal scales. However, there are relatively few long‐term data sets that provide the demographic and environmental data demanded by such models. Using a 37‐yr data set, we used RAMAS Metapop to model the persistence of natterjack toads Epidalea (Bufo) calamita on a heathland in southern Britain. A retrospective analysis showed that the best fit between the predicted population trajectories and the real population was when the management carried out was modelled as an increase in K of 150 toads yr^?1. However, even if ongoing management continues to improve K by a further 40–60 toads yr^?1 over the next 50 yr, the population still has an extinction risk of at least 60% if other factors remain unchanged. Sensitivity analyses and simulated management scenarios indicated that the population was most sensitive to changes in the survival of juvenile (i.e. 1–2 yr old) toads. In addition, if the frequency and severity of pond desiccation increases, the risk of extinction was predicted to increase as a result of reduced recruitment. Low levels of extinction risk occurred irrespective of K when juvenile survival was enhanced in combination with low frequency and severity of pond desiccation. The models suggest that populations that are responding to management against a background of natural fluctuations may remain vulnerable to extinction for several decades. These extinction risks may increase if habitat management fails to offset reductions in recruitment and juvenile survival caused by environmental change.     

Caesium uptake and toxicity in the cyanobacterium <Emphasis Type="Italic">Nostoc muscorum</Emphasis>

A NH₄⁺ transport-defective mutant and a K⁺ transport-defective mutant of the cyanobacterium Nostoc muscorum were analysed with regard to percentage survival as a function of CsCl toxicity and Cs⁺ uptake activity. Neither survival nor Cs⁺ uptake was affected in either of the two mutants when compared with the wild type. The results indicate that the toxicity of Cs⁺ is determined at more than one cellular site in this organism.     

Population regulation in Bromus rubens and B. mollis: Life cycle components and competition

Summary A series of Bromus rubens and B. mollis populations were sampled in the coastal range and northern part of the Central Valley of California in order to study their population ecology in demographic terms. Quantitative estimates were obtained on plants collected directly in nature, and their progenies in controlled environments with randomized block design in the greenhouse.Two parameters of population growth — the intrinsic rate of increase, r, and the carrying capacity, K-were estimated by using the logistic model (r=ln R and K=equilibrium population size). It was found that B. mollis is a relatively K-type species, while B. rubens is a relatively r-type species.The effects of density on competition between individuals in pure and mixed populations of B. mollis and B. rubens were studied. In both species, increasing density induced greater mortality and a striking plastic reduction in the size and reproductive potential of the individuals. Further, B. rubens showed a relatively greater mortality and less plastic response to densities than B. mollis in both pure and mixed stands. Two different types of plasticity were considered: one in response to changing density (d-plasticity); and the other in response to changing enironmental conditions (e-plasticity). High plasticity in one of them need not imply that the other one is high too. B. rubens showed higher e-plasticity, but lower d-plasticity than B. mollis.The relationships between r, K and competitive ability were discussed. Two types of K-strategy were distinguished: one involving greater nonreproductive effort with longer life span, or lowered mortality (Type-I) and the other with density-induced adjustments in body size along with survival in higher numbers (Type-II). Different populations of these two Bromus species showed different values of r and K (Type-II) and different competitive abilities. It was found that higher r was usually accompanied by lower K (Type-II), while higher K (Type-II) was accompanied by lower competitive ability, which in turn is correlated with higher d-plasticity. In general, coexistence was predicted on the basis of estimates derived from the interspecific competition experiments.     

A genetic component of extinction risk in mammals

Genetic factors may play an important role in species extinction but their actual effect remains poorly understood, particularly because of a strong and potentially masking effect expected from ecological traits. We investigated the role of genetics in mammal extinction taking both ecological and genetic factors into account. As a proxy for the role of genetics we used the ratio of the rates of nonsynonymous (amino acid changing) to synonymous (leaving the amino acid unchanged) nucleotide substitutions, K_a / K_s. Because most nonsynonymous substitutions are likely to be slightly deleterious and thus selected against, this ratio is a measure of the inefficiency of selection: if large (but less than 1), it implies a low efficiency of selection against nonsynonymous mutations. As a result, nonsynonymous mutations may accumulate and thus contribute to extinction. As a proxy for the role of ecology we used body mass W, with which most extinction‐related ecological traits strongly correlate. As a measure of extinction risk we used species’ affiliation with the five levels of extinction threat according to the IUCN Red List of Threatened Species. We calculated K_a / K_s for mitochondrial protein‐coding genes of 211 mammalian species, each of which was characterized by body mass and the level of threat. Using logistic regression analysis, we then constructed a set of logistic regression models of extinction risk on ln(K_a / K_s) and lnW. We found that K_a / K_s and body mass are responsible for a 38% and a 62% increase in extinction risk, respectively. Given that the standard error of these values is 13%, the contribution of genetic factors to extinction risk in mammals is estimated to be one‐quarter to one‐half of the total of ecological and genetic effects. We conclude that the effect of genetics on extinction is significant, though it is almost certainly smaller than the effect of ecological traits. Synthesis Mutation provides the material for evolution. However, most mutations that play a role in evolution are slightly deleterious and thus may contribute to extinction. We assess the role of mitochondrial DNA mutations in mammalian extinction risk and find it to be one‐quarter to one‐half of the total of mutation and body mass effects, where body mass represents an integral measure of extinction‐related ecological traits. Genetic factors may be all the more important, because ecological traits associated with large body mass would both promote and protect from extinction, while mutation accumulation caused by low effective population size seems to have no counterbalance.     

Current-voltage relationships of potassium channels in the plasmalemma ofAcetabularia

Summary The outer membrane of mechanically prepared protoplasmic droplets fromAcetabularia mediterranea has been investigated by patch-clamp techniques. These membranes are shown to consist of physiologically intact plasmalemma. With the Cl^– pump inhibited, microscopic currents through K⁺-selective channels were studied. These currents compare well with macroscopic K⁺ currents as previously determined by standard microelectrode techniques and tracer flux measurements. There is about one K⁺ channel per m² in the plasmalemma. The current-voltage relationship (I–V curve) of the main open channel (channel A) is sigmoid over a voltage range between about –100 and +100 mV with saturation currents of about ±10 pA. A second species (or different state of channel A) of K⁺-selective channels (channel B) differs from channel A by smaller saturation currents (about ±7 pA) and a much smaller open probability. The open probability of channel A increases from almost zero at large negative voltages to about 1/2 at large positive voltages. Taking the closed times into account, the mean steady-stateI–V curve of channel A displays outward rectification about the equilibrium voltage for K⁺ and negative slope conductance at larger negative voltages. The open channelI–V curve of the open channels A and B, the changes of theI–V curve of the open channel A upon variation of the external K⁺ concentration, as well as the mean steady-stateI–V curves of channel A are described by simple reaction kinetic models, the parameters of which are determined to fit the experimental data. The results are discussed with respect to data from other K⁺ channels in plants and with respect to regulation of the cytoplasmic K⁺ concentration inAcetabularia.     

Effect of sudden change in potassium concentration on <Emphasis Type="Italic">Penaeus latisulcatus</Emphasis> Kishinouye survival,osmolality and health in inland saline water cultures

The aim of the project was to determine the effects of sudden change in potassium concentration in inland saline water on the survival, osmolality and condition indices of western Australian King Prawn—Penaeus latisulcatus Kishinouye. Australia has large volumes of inland saline water that could be used for aquaculture but is often deficient in K⁺. Western king prawn is a candidate species for culture in inland saline water. Such waters require K⁺ fortification for prawn survival and growth. Trials were conducted in tanks to determine the effect of sudden change in K⁺ concentration in inland saline water samples on the survival, osmolality and condition indices of western king prawns. Prawns in tanks were acclimated to inland saline water procured from Wannamal—an inland location in Western Australia. After 3 days of acclimation, prawns were subjected to sudden increase in medium K⁺ concentration over 1 h, ranging from 80% to 100% of the marine water K⁺ concentration by adding potassium chloride. Identical inland saline water was added to the control tanks over the same time period. Survival, ingestion rate and osmoregulatory capacity (OC) were then recorded over 19 days. At the conclusion of the trial, survival ranged from 71% to 78% in the potassium-enriched tanks whereas 100% mortality was observed in the control tanks by day 11. Ingestion rate of prawns was significantly higher in the experimental tanks than in the control. Osmoregulatory capacity of potassium-enriched prawns was significantly lower post- than pre-ionic change and significantly higher at the conclusion of the trial than both pre- and post-ionic change. There was no significant difference in OC between water types at any time period. These results indicate that prawns can tolerate sudden increase in K⁺ content in inland saline water and the higher K⁺ concentration increases survival and OC, but other measures indicate the prawns were experiencing stressful conditions. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected Papers from the 9th Conference of the International Society for Salt Lake Research     

Dynamic light-scattering studies of internal motions in DNA. I. Applicability of the rouse-zimm model

The intermediate scattering function G(K,t) for any polymer model obeying a linear separable Langevin equation can be expressed in terms of the eigenvalues and eigenvectors of its normal coordinate transformation. An algorithm for the extract numerical evaluation of G(K,t) for linear Rouse-Zimm chains in the presence of hydrodynamic interaction has been developed. The computed G(K,t)² were fit to C(t) = A exp(?t/τA) + B, and apparent diffusion coefficients calculated according to D_app ≡ 1/(2τ_AK²). G(K,t)² was surprisingly well-fit by single-exponential decays, especially at both small and large values of Kb, where K is the scattering vector and b the root-mean-squared subunit extension. Plots of D_app vs K² in-variably showed a sigmoidal rise from D₀ at K² = O up to a constant plateau value at large K²b². Analytical expression for G(K,t), exact in the limit of short times, were obtained for circular Rouse-Zimm chains with and without hydrodynamic interaction, and also for free-draining linear chains, and in addition for the independent-segment-mean-force (ISMF) model. The predicted behaviors for G(K,t) at large Kb (or KR_G) was found in all cases to be single-exponential with 1/τ ∝ K² at large Kb, in agreement with the computational results. A simple procedure for estamating all parameter of the Rouse-Zimm model from a plot of D_app vs K² is proposed. Experimental data for both native and pH-denatured calf-thymus DNA in 1.0M Nacl with and without EDTA clearly plateau behavior of D_app at large values of K, in harmony with the present Rouse-Zimm and ISMF theories, and in sharp contrast to previous predictions based on the Rouse-Zimm model.     

Properties of Shaker-type Potassium Channels in Higher Plants

Potassium (K⁺), the most abundant cation in biological organisms, plays a crucial role in the survival and development of plant cells, modulation of basic mechanisms such as enzyme activity, electrical membrane potentials, plant turgor and cellular homeostasis. Due to the absence of a Na⁺/K⁺ exchanger, which widely exists in animal cells, K⁺ channels and some type of K⁺ transporters function as K⁺ uptake systems in plants. Plant voltage-dependent K⁺ channels, which display striking topological and functional similarities with the voltage-dependent six-transmembrane segment animal Shaker-type K⁺ channels, have been found to play an important role in the plasma membrane of a variety of tissues and organs in higher plants. Outward-rectifying, inward-rectifying and weakly-rectifying K⁺ channels have been identified and play a crucial role in K⁺ homeostasis in plant cells. To adapt to the environmental conditions, plants must take advantage of the large variety of Shaker-type K⁺ channels naturally present in the plant kingdom. This review summarizes the extensive data on the structure, function, membrane topogenesis, heteromerization, expression, localization, physiological roles and modulation of Shaker-type K⁺ channels from various plant species. The accumulated results also help in understanding the similarities and differences in the properties of Shaker-type K⁺ channels in plants in comparison to those of Shaker channels in animals and bacteria.     

Potassium deficiency triggers the development of dormant cells (akinetes) in Aphanizomenon ovalisporum (Nostocales,Cyanoprokaryota)1

Akinetes are spore‐like nonmotile cells that differentiate from vegetative cells of filamentous cyanobacteria from the order Nostocales. They play a key role in the survival and distribution of these species and contribute to their perennial blooms. Various environmental factors were reported to trigger the differentiation of akinetes including light intensity and quality, temperature, and nutrient deficiency. Here, we report that deprivation of potassium ion (K⁺) triggers akinete development in the cyanobacterium Aphanizomenon ovalisporum. Akinetes formation is initiated 3 d–7 d after an induction by K⁺ depletion, followed by 2–3 weeks of a maturation process. Akinete formation occurs within a restricted matrix of environmental conditions such as temperature, light intensity or photon flux. Phosphate is essential for akinete maturation and P‐limitation restricts the number of mature akinetes. DNA replication is essential for akinete maturation and akinete development is limited in the presence of Nalidixic acid. While our results unequivocally demonstrated the effect of K⁺ deficiency on akinete formation in laboratory cultures of A. ovalisporum, this trigger did not cause Cylindrospermopsis raciborskii to produce akinetes. Anabaena crassa however, produced akinetes upon potassium deficiency, but the highest akinete concentration was achieved at conditions that supported vegetative growth. It is speculated that an unknown internal signal is associated with the cellular response to K⁺ deficiency to induce the differentiation of a certain vegetative cell in a trichome into an akinete. A universal stress protein that functions as mediator in K⁺ deficiency signal transduction cascade, may communicate between the lack of K⁺ and akinete induction.     

Predicting protein pKa by environment similarity

A statistical method to predict protein pK_a has been developed by using the 3D structure of a protein and a database of 434 experimental protein pK_a values. Each pK_a in the database is associated with a fingerprint that describes the chemical environment around an ionizable residue. A computational tool, MoKaBio, has been developed to identify automatically ionizable residues in a protein, generate fingerprints that describe the chemical environment around such residues, and predict pK_a from the experimental pK_a values in the database by using a similarity metric. The method, which retrieved the pK_a of 429 of the 434 ionizable sites in the database correctly, was crossvalidated by leave‐one‐out and yielded root mean square error (RMSE) = 0.95, a result that is superior to that obtained by using the Null Model (RMSE 1.07) and other well‐established protein pK_a prediction tools. This novel approach is suitable to rationalize protein pK_a by comparing the region around the ionizable site with similar regions whose ionizable site pK_a is known. The pK_a of residues that have a unique environment not represented in the training set cannot be predicted accurately, however, the method offers the advantage of being trainable to increase its predictive power. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Hydraulic adjustment of maple saplings to canopy gap formation

The leaf-specific hydraulic conductivity (K _L) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (A _L/A _S) and the specific hydraulic conductivity of xylem tissue (K _S). In environments with high atmospheric vapor pressure deficit (VPD), K _L may increase to support higher transpiration rates. We predicted that saplings of Acerrubrum and A.pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher K _L and lower A _L/A _S than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and K _S increased with sapling size for both species. In A. rubrum, K _S did not differ between the two environments but lower A _L/A _S (P=0.05, ANCOVA) led to higher K _L for gap-grown saplings (P < 0.05, ANCOVA). In A. pensylvanicum, neither K _S, A _L/A _S, nor K_L differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and carbon assimilation of A.pensylvanicum. Maximum stomatal conductance for A.pensylvanicum increased with K _L (r ²=0.75, P < 0.05). A hypothetical large A. pensylvanicum sapling (2 m tall) had 2.4 times higher K _L and 22 times greater daily carbon assimilation than a small (1 m tall) sapling. Size-related hydraulic limitations in A.pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A.pensylvanicum sapling size (r ²=0.69, P < 0.05). Calculations indicated that small A.pensylvanicum saplings (low K _L) could not transpire at the rate of large saplings (high K _L) without reaching theoretical thresholds for xylem embolism induction. The coordination between K _L and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also limits transpiration. The K _S of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing K _L. However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result of the increase in K _L and K _S with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation for small saplings in the gap environment. Received: 18 February 1997 / Accepted: 23 June 1997     

DENSITY-DEPENDENT SELECTION ON CONTINUOUS CHARACTERS: A QUANTITATIVE GENETIC MODEL

A quantitative genetic model of density-dependent selection is presented and analysed with parameter values obtained from laboratory selection experiments conducted by Mueller and his coworkers. The ecological concept of r- and K-selection is formulated in terms of selection gradients on underlying phenotypic characters that influence the density-dependent measure of fitness. Hence the selection gradients on traits are decomposed into two components, one that changes in the direction to increase r, and one that changes in the direction to increase K. The relative importance of the two components is determined by temporal fluctuations in population density. The evolutionary rate of r and K (per-generation changes in r and K due to the genetic responses of the underlying traits) is also formulated. Numerical simulation has shown that with moderate genetic variances of the underlying characters, r and K can evolve rapidly and the evolutionary rate is influenced by synergistic interaction between characters that contribute to r and K. But strong r-selection can occur only with severe and continuous disturbances of populations so that the population density is kept low enough to prevent K-selection.     

Lactoperoxidase-catalyzed iodination of sodium and potassium ion-activated adenosine triphosphatase in the Madin-Darby canine kidney epithelial cell line and canine renal membranes

Experiments are described in which the large chain of (Na⁺ + K⁺)-ATPase is labeled by lactoperoxidase-catalyzed iodination either at its extracytoplasmic surface exclusively or at both its extracytoplasmic and its cytoplasmic surfaces simultaneously. The former was accomplished by labeling intact cells of the Madin-Darby canine kidney line, and the latter by labeling open membrane vesicles, also from canine kidney. A comparison of the specific radioactivities for the large chain from the open membranes and the large chain from the Madin-Darby canine kidney cells reveals that the former was labeled approximately 5-fold more extensively. This indicates that the large chain of (Na⁺ + K⁺)-ATPase is situated in the membrane such that more of its mass protrudes into the cytoplasm than into the extracytoplasmic environment.     

Klebsiella pneumoniae survives within macrophages by avoiding delivery to lysosomes

Klebsiella pneumoniae is an important cause of community‐acquired and nosocomial pneumonia. Evidence indicates that Klebsiella might be able to persist intracellularly within a vacuolar compartment. This study was designed to investigate the interaction between Klebsiella and macrophages. Engulfment of K. pneumoniae was dependent on host cytoskeleton, cell plasma membrane lipid rafts and the activation of phosphoinositide 3‐kinase (PI3K). Microscopy studies revealed that K. pneumoniae resides within a vacuolar compartment, the Klebsiella‐containing vacuole (KCV), which traffics within vacuoles associated with the endocytic pathway. In contrast to UV‐killed bacteria, the majority of live bacteria did not co‐localize with markers of the lysosomal compartment. Our data suggest that K. pneumoniae triggers a programmed cell death in macrophages displaying features of apoptosis. Our efforts to identify the mechanism(s) whereby K. pneumoniae prevents the fusion of the lysosomes to the KCV uncovered the central role of the PI3K–Akt–Rab14 axis to control the phagosome maturation. Our data revealed that the capsule is dispensable for Klebsiella intracellular survival if bacteria were not opsonized. Furthermore, the environment found by Klebsiella within the KCV triggered the down‐regulation of the expression of cps. Altogether, this study proves evidence that K. pneumoniae survives killing by macrophages by manipulating phagosome maturation that may contribute to Klebsiella pathogenesis.     

A potassium transport mutant of Saccharomyces cerevisiae

A mutant in Saccharomyces cerevisiae required one hundred times more K⁺ than wild type for the same half maximal growth rate. Mutant cells and wild type cells grown at millimolar K⁺ did not show significant differences in Rb⁺ transport. In the mutant, a rapid K⁺ loss induced by azide or incubation (4 h) in K⁺-free medium decreased the Rb⁺ transport K _m by one half; in the wild type, those treatments decreased the Rb⁺ K _m twenty and one hundred times, respectively. Mutant and wild type did not show significant differences in Na⁺ transport and in the Na⁺ inhibition of Rb⁺ transport, either in normal-K⁺ cells or in K⁺-starved cells. The results suggest that either two systems or one system with two interacting sites mediate K⁺ transport in S. cerevisiae.Abbreviations YPD yeast-peptone-dextrose medium     

Detecting multimodality in saccadic reaction time distributions in gap and overlap tasks

In many cases the distribution of saccadic reaction times (SRT) deviates considerably from a unimodal distribution and may often exhibit several peaks. We present a statistical approach to determining the number and form of the individual peaks. The overall density of the reaction times f _i (t), i=1…M obtained in M different experiments with the same subject is described as the sum of K basis functions x _k (t),k=1…K with different weights and an error term. A change in the experimental conditions is assumed to cause a change in the weights, not in the basis functions. We minimize the square of the difference (measured data minus approximation), divided by the error of the data. Incrementing K step by step we determine the necessary number of basis functions. This method is applied to data of six subjects tested in different saccade tasks. We detect five different modes: two in the range 80–140 ms (express modes), two in the range 145–190 ms (fast-regular mode) and one at about 230 ms (slow-regular mode). These modes are located at about the same positions for different subjects. The method presented here not only proves statistically the existence of several modes in SRT distributions but also allows the distributions to be described by a few characteristic numbers that go beyond the mean values and standard deviations. Received: 24 March 1997 / Accepted: 5 December 1997     

Kinetic studies of a (Na++ K++ Mg2+)ATPase in sugar beet roots. III. A proposed model for the (Na++ K+) activation and its significance for field properties

A microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase preparation from sugar beet roots was used. The activation by simultaneous addition of Na⁺ and K⁺ at different levels was examined in terms of steady state kinetics. The observed data can be summarized in the following way: 1. The apparent affinity between the enzyme and the substrate MgATP depends on the ratio between Na⁺ and K⁺. At low Na⁺ concentration (below 5 mM), the apparent K_m decreases with increasing concentrations of K⁺ (1–20 mM). At 5 mM Na⁺, the K⁺ level does not change the apparent K_m, while at Na⁺ levels above 10 mM, the apparent K_m between enzyme and substrate increases with increasing concentration of K⁺. 2. When the MgATP concentration is kept constant, homotropic cooperativity (concerning one type of ligand) and heterotropic cooperativity (concerning different types of ligands) exist in the activation by Na⁺ and K⁺. The Na⁺ binding is cooperative with different K_m values and Hill coefficients (n) in the presence of low and high concentration of K⁺. At low Na⁺ level (< 5 mM). a negative cooperativity exists for Na⁺ (n_Na < 1) which is more pronounced in the presence of high [K⁺]. When the concentration of Na⁺ is raised the negative cooperativity disappears and turns into a positive one (n_Na > 1). Only K⁺ binding in the presence of low [Na⁺] shows cooperativity with a Hill coefficient that reflects changes from negative to positive homotropic cooperativity with increasing concentrations of K⁺ (n_K < 1 → n_K > 1). In the presence of [Na⁺] > 10 mM, the changes in n_k are insignificant. 3. A model is proposed in which one or two different K sites and one or two Na sites control the catalytic activity, with multiple interactions between Na⁺, K⁺ and MgATP. 4. In the presence of Na⁺ (< 10 mM), K⁺ is probably bound to two K sites, one of which translocates K⁺ through the membrane by an antiport Na⁺/K⁺ mechanism. This could be connected with an elevated K⁺ uptake in the presence of Na⁺ and could therefore explain some field properties of sugar beets.     

Kinetics of high-affinity K+ uptake in plants, derived from K+-induced changes in current-voltage relationships

To investigate coupled, charge-translocating transport, it is imperative that the specific transporter current-voltage (IV ) relationship of the transporter is separated from the overall membrane IV relationship. We report here a case study in which the currents mediated by the K⁺-H⁺ symporter, responsible for high-affinity K⁺ uptake in Arabidopsis thaliana (L.) Heynh. cv. Columbia roots, are analyzed with an enzyme kinetic reaction scheme. The model explicitly incorporates changes in membrane voltage and external substrate, and enables the derivation of the underlying symport IV relationships from the experimentally obtained difference IV data. Data obtained for high-affinity K⁺ transport in A. thaliana root protoplasts were best described by a 1:1 coupled K⁺-H⁺ symport-mediated current with a parallel, outward non-linear K⁺ pathway. Furthermore, the large predictive value of the model was used to describe symport behaviour as a function of the external K⁺ concentration and the cytoplasmic K⁺ concentration. Symport activity is a complex function of the external K⁺ concentration, with first-order saturating kinetics in the micromolar range and a strong activity reduction when external K⁺ is in the millimolar range and the membrane depolarises. High cytoplasmic K⁺ levels inhibit symport activity. These responses are suggested to be part of the feedback mechanisms to maintain cellular K⁺ homeostasis. The general suitability of the model for analysis of carrier-mediated transport is discussed. Received: 23 November 1996 / Accepted: 22 April 1997     

Reversible phosphorylation regulation of NADPH-linked polyol dehydrogenase in the freeze-avoiding gall moth, Epiblema scudderiana: role in glycerol metabolism

Larvae of the goldenrod gall moth, Epiblema scudderiana, use a freeze avoidance strategy of cold hardiness to survive the winter. A key metabolic adaption that supports subzero survival is the accumulation of large amounts of glycerol as a colligative antifreeze. Production of glycerol relies on polyol dehydrogenase (PDH) which catalyzes the NADPH‐dependent conversion of glyceraldehyde into glycerol. Kinetic analysis of PDH from E. scudderiana revealed significant changes in properties as a result of subzero temperature acclimation; the K_m for glyceraldehyde in 5°C‐acclimated larvae was 7.0 mM and doubled in ? 15°C‐exposed larvae. This change suggested that PDH is regulated by a state‐dependent covalent modification. Indeed, high and low K_m forms could be interconverted by incubating larval extracts in vitro under conditions that stimulated either endogenous protein kinases or protein phosphatases. Protein kinase incubations doubled the K_m glyceraldehyde of the 5°C enzyme, whereas protein phosphatase incubations decreased the K_m of the ? 15°C enzyme by about 50%. PDH was purified by ion exchange and affinity chromatography steps and then subjected to electrophoresis. Staining with ProQ Diamond phosphoprotein stain showed a much higher phosphate content of PDH from ? 15°C‐acclimated larvae, a result that was further confirmed by immunoblotting that showed a much greater phosphoserine content on the ? 15°C enzyme. These experiments established that PDH is regulated by state‐dependent reversible phosphorylation in E. scudderiana and suggest that this regulatory mechanism makes a significant contribution to controlling the synthesis, maintenance, and degradation of glycerol pools over the winter months. © 2011 Wiley Periodicals, Inc.     

Root hydraulic conductance of six forest trees: possible adaptive significance of seasonal changes

ABSTRACT

Root hydraulic conductance (K_R was measured in terms of the ratio of volume flow through roots (F) and to the pressure (P) driving the flow in six forest trees growing in habitats characterized by different water availabilities Le. Acer campestre L., Castanea sativa Miller, Fraxinus ornus L., Fraxinus oxycarpa Bieb., Ceratonia siliqua L. and Olea oleaster Hoffmg. et Link. Measurements were made in May, August and November 1996. K_R as normalized for unit leaf surface area (K_RL), was higher in species growing in humid environments (A. campestre and C. sativa) than in others (C. siliqua and O. oleaster) typical of aria zones. A. campestre and C. sativa showed declining K_RL values from spring to autumn while the latter had highest K_RL values in summer. This is in agreement with the typical drought avoidance strategy of C. siliqua which is based on large water losses balanced by equal water uptake from the soil. Plots of F and of A (leaf surface area) to K_RL as well as annual percent changes in F, A_L and K_RL suggest that changes in K_RL are mainly due to analogous changes in F, except for O. oleaster where opposite balanced changes in F and A_L contributed in maintaining the K_RL constant from August to November.