Phosphate Compounds in Vertebrate Red Blood Cells

This review brings together and discusses the significance ofexisting information about water-soluble (small molecule) organicphosphate constituents of red blood cells in different vertebratespecies, with emphasis on 2,3 diphosphoglycerate (DPG), inositolpentaphosphate (IP₅) ATP and guanosine triphosphate (GTP), compoundswhich may play an important role in respiratory physiology bymodifying the affinity of hemoglobin for oxygen. Results onthe distribution and concentration of these compounds in redcells of vertebrate animals can be summarized as follows 1)DPG High in mammals (except cats and ruminants) Absent in crocodilianssquamata and fishes. High briefly in the bird embryo absentin adult. High briefly in turtle embryo low in juvenile lowto absent in adult 2 IP₅. High in birds. Absent in mammals,crocodilians squamata and fishes (with the exception of Arapaimagigas). Low in turtles 3 ATP Intermediate in mammals. High inbirds and turtles. Very high in squamata Intermediate to veryhigh in fishes. Low in crocodilians 4) GTP Very low in mammalsbirds, reptiles and amphibians (except for small pool in Ranatadpole). Low to very high in fishes.     

Decreased affinity for oxygen of cytochrome-c oxidase in Leigh syndrome caused by SURF1 mutations

Mutations in the gene SURF1 prevent synthesis of cytochrome-c oxidase (COX)-specific assembly protein and result in a fatal neurological disorder, Leigh syndrome. Because this severe COX deficiency presents with barely detectable changes of cellular respiratory rates under normoxic conditions, we analyzed the respiratory response to low oxygen in cultured fibroblasts harboring SURF1 mutations with high-resolution respirometry. The oxygen kinetics was quantified by the partial pressure of oxygen (PO₂) at half-maximal respiration rate (P₅₀) in intact coupled cells and in digitonin-permeabilized uncoupled cells. In both cases, the P₅₀ in patients was elevated 2.1- and 3.3-fold, respectively, indicating decreased affinity of COX for oxygen. These results suggest that at physiologically low intracellular PO₂, the depressed oxygen affinity may lead in vivo to limitations of respiration, resulting in impaired energy provision in Leigh syndrome patients. oxygen kinetics; mitochondrial disease     

Role of blood-oxygen transport in thermal tolerance of the cuttlefish, Sepia officinalis

Mechanisms that affect thermal tolerance of ectothermic organismshave recently received much interest, mainly due to global warmingand climate-change debates in both the public and in the scientificcommunity. In physiological terms, thermal tolerance of severalmarine ectothermic taxa can be linked to oxygen availability,with capacity limitations in ventilatory and circulatory systemscontributing to oxygen limitation at extreme temperatures. Thepresent review briefly summarizes the processes that definethermal tolerance in a model cephalopod organism, the cuttlefishSepia officinalis, with a focus on the contribution of the cephalopodoxygen-carrying blood pigment, hemocyanin. When acutely exposedto either extremely high or low temperatures, cuttlefish displaya gradual transition to an anaerobic mode of energy productionin key muscle tissues once critical temperatures (T_crit) arereached. At high temperatures, stagnating metabolic rates anda developing hypoxemia can be correlated with a progressivefailure of the circulatory system, well before T_crit is reached.However, at low temperatures, declining metabolic rates cannotbe related to ventilatory or circulatory failure. Rather, wepropose a role for hemocyanin functional characteristics asa major limiting factor preventing proper tissue oxygenation.Using information on the oxygen binding characteristics of cephalopodhemocyanins, we argue that high oxygen affinities (= low P₅₀values), as found at low temperatures, allow efficient oxygenshuttling only at very low venous oxygen partial pressures.Low venous PO₂s limit rates of oxygen diffusion into cells,thus eventually causing the observed transition to anaerobicmetabolism. On the basis of existing blood physiological, molecular,and crystallographical data, the potential to resolve the roleof hemocyanin isoforms in thermal adaptation by an integratedmolecular physiological approach is discussed.     

The Challenges of Living in Hypoxic and Hypercapnic Aquatic Environments

Organisms living in coastal waters, and especially estuaries,have long been known to have behavioral or physiological mechanismsthat enable them toexist in water containing low amounts ofoxygen. However, the respiratory consumption of oxygen thatgenerates hypoxia is also responsible for producing significantamounts of carbon dioxide. An elevation of carbon dioxide pressurein water will cause a significant acidosis in most aquatic organisms.Thus, the combination of low oxygen and elevated carbon dioxidethat occurs in estuaries represents a significant environmentalchallenge to organisms living in this habitat. Organisms maymaintain oxygen uptake in declining oxygen conditions by usinga respiratory pigment and/or by making adjustments in the convectiveflow of water and blood past respiratory surfaces (i.e., increasecardiac output and ventilation rate). Severe hypoxia may resultin an organism switching partially or completely to anaerobicbiochemical pathways to sustain metabolic rate. There is alsoevidence to suggest that organisms lower their metabolism duringhypoxic stress. Elevated water CO₂ (hypercapnia) produces anacidosis in the tissues of organisms that breathe it. This acidosismay be wholly or partially compensated (i.e., mechanisms returnpH to pre-exposure levels), or may be uncompensated. Some studieshave examined the effects on organisms of exposure simultaneouslyto hypoxia and hypercapnia. This article reviews some of thespecific adaptations and responses of organisms to low oxygen,to high carbon dioxide, and to the cooccurrence of low oxygenand high carbon dioxide     

On the Oxygen Affinity of Bird Blood

In oxygen affinity characteristics bird blood appears to haveseveral features that distinguish it from mammalian blood. Fordomesticated species at least the range of oxygen half saturationvalues is extremely wide. A difference in the shape of the oxygendissociation curve has been recorded by several authors withan increase in sigmoidocity with increasing oxygen saturation.There is evidence that the oxygen affinity determining organicphosphate of bird red blood cells inositol pentaphosphate (IP5)is relatively metabolically inert. This suggests that modulationof blood oxygen affinity is primarily achieved by altering theIP5 hemoglobin interaction rather than varying IP5 levels perse. In contrast to mammals carbon dioxide has no direct effecton whole blood oxygen affinity for some bird species (hen chickgoose) or it may cause the oxygen affinity to increase (pigeonflamingo). Carbon dioxide is a blood oxygen affinity modulatorof some flexibility its effect in both direction and magnitudebeing dependent on the hemoglobin type red cell pH and organicphosphate levels. The physiological significance of these distinguishingfeatures is discussed.     

Respiratory Function of the Hemocyanins

Recent studies clearly demonstrate the respiratory importanceof the hemocyanins in each of the three animal phyla in whichthey occur. Despite their generally low oxygen affinity, hemocyaninscan be highly oxygenated at the site of gas exchange with themedium as well as deoxygenated at the tissues. The functionalrange of a hemocyanin oxygen transport system is severely limitedhowever by environmental change. These systems function underincipient hypoxia due largely to responses of blood pH whichare not fully understood a normal Bohr shift is accompaniedby a rise in blood pH and a reverse Bohr shift by a decreasein blood pH. In both instances blood oxygen affinity increasesand its oxygenation state at the gill remains high in spiteof its lower Po₂. Dilution of the blood at low salinity generallyalters its oxygenation properties both oxygen affinity and cooperativity.These properties may or may not be restored by concomitant changesin blood pH, which depend on the various mechanisms of osmoticadaptation. Within a homogeneous taxon the oxygenation properties of a hemocyaninappear to be highly conservative showing little interspecificadaptation except to extreme changes in the mode of gas exchange.Unlike that in vertebrates air-breathing in crustaceans is accompaniedby an increase in blood oxygen affinity. Similar oxygen affinitiesin latitudinally separated species result in optimal functioningof the system at the same temperature, corresponding to differentseasons. In eurythermal species a temperature acclimation ofoxygen affinity extends the operating range of the crustaceanhemocyanins but they cannot deoxygenate at very low temperatures. Unsolved problems of hemocyanin function include specific effectsof pH and CO₂ the basis of which is not entirely clear, andthe postulated occurrence in native blood of both dialyzableand non-dialyzable substances that modify oxygen affinity theidentity of which is unknown. With the exception of the crustacean oxygen carrier the hemocyaninsconfer a respiratory advantage over their predecessors. Butthe oxygen carrying capacity of crustacean blood never reachesthe levels found in the annelids and molluscs due to the colloidosmotic pressure of the relatively low molecular weight hemocyaninand to the drop in blood hydrostatic pressure accompanying theloss of a fluid skeleton. The selection of a blood oxygen carrierwith an apparently limiting combination of respiratory and osmoticproperties is obscured by the uncertain phylogenetic positionof the phylum.     

Transport of Oxygen by the Blood of the Land Crab, Gecarcinus lateralis

Parameters relating to transport of oxygen were measured inthe pericardial blood and venous outflow from the first walkingleg of Gecarcinus lateralis. O₂-equilibrium curves of the hemocyaninof G. lateralis were found to be sigmoid and, at 27°C andpH 7.45, to have a half-saturation pressure of about 17 mm Hgoxygen. Average partial pressures of oxygen as measured by O₂-electrodewere 32 mm Hg in pericardial blood and 9 mm Hg in the venoussamples. Analysis of the O₂-content in corresponding samplesby the Van Slyke technique revealed an average of 2.17 volumes% O₂-capacity for whole blood, 1.45 volumes % for pericardialblood, and 0.61 volumes % for venous blood. Estimates basedon the Van Slyke analyses indicated an average pO₂ of 29 and14 mm Hg in pericardial and venous samples, respectively. Thesefigures agree fairly well with those obtained by means of O₂-electrodes.Of the oxygen carried to the tissues, about 94% is carried asoxyhemocyanin and about 6% is carried in physical solution.As the blood passes through the gills, the hemocyanin, on anaverage, becomes 80–85% saturated with oxygen and returnedfrom the tissues 18–45% saturated with oxygen. These resultsindicate that the blood of G. lateralis has a higher O₂-capacitythan the blood of most other decapod crustaceans for which similarinformation is available. In addition, the blood of G. lateralistransports more oxygen to the tissues per unit volume than doother crustacean bloods.     

Nitric oxide regulation of mitochondrial oxygen consumption II: Molecular mechanism and tissue physiology

Nitric oxide (NO) is an intercellular signaling molecule; among its many and varied roles are the control of blood flow and blood pressure via activation of the heme enzyme, soluble guanylate cyclase. A growing body of evidence suggests that an additional target for NO is the mitochondrial oxygen-consuming heme/copper enzyme, cytochrome c oxidase. This review describes the molecular mechanism of this interaction and the consequences for its likely physiological role. The oxygen reactive site in cytochrome oxidase contains both heme iron (a₃) and copper (Cu_B) centers. NO inhibits cytochrome oxidase in both an oxygen-competitive (at heme a₃) and oxygen-independent (at Cu_B) manner. Before inhibition of oxygen consumption, changes can be observed in enzyme and substrate (cytochrome c) redox state. Physiological consequences can be mediated either by direct "metabolic" effects on oxygen consumption or via indirect "signaling" effects via mitochondrial redox state changes and free radical production. The detailed kinetics suggest, but do not prove, that cytochrome oxidase can be a target for NO even under circumstances when guanylate cyclase, its primary high affinity target, is not fully activated. In vivo organ and whole body measures of NO synthase inhibition suggest a possible role for NO inhibition of cytochrome oxidase. However, a detailed mapping of NO and oxygen levels, combined with direct measures of cytochrome oxidase/NO binding, in physiology is still awaited. mitochondria; cytochrome oxidase     

Electron Transport and Oxidative Phosphorylation in Arum Spadix Mitochondria

Arum spadix mitochondria exhibited a rapid cyanide-resistantoxygen uptake when oxidizing malate, NADH₂ or succinate, anda slower, cyanide-sensitive oxygen uptake when oxidizing ascorbate+tetramethylphenylenediamine(TMPD). Cytochrome oxidase does not therefore appear to functionas the terminal oxidase in the presence of cyanide, and therather low cytochrome c oxidase activity obtained using ascorbate+TMPDmay exclude it from possessing a major role even in the absenceof cyanide. ATP synthesis has been shown to accompany substrateoxidation. In the presence of antimycin A the P: O ratio accompanyingmalate oxidation was reduced by half, while phosphorylationaccompanying NADH₂ or succinate oxidation was almost completelyabolished. It is proposed that electrons from exogenous NADH₂enter the electron transport chain at a site after that whereendogenous NADH2 donates electrons and that electrons from exogenousNADH₂ are not coupled to ATP synthesis at site 1. The cyanide-resistant,non-phosphorylating electron-transport pathway may functionin the absence of cyanide and account for the low efficiencyof energy conservation observed in this tissue.     

Active uptake of inorganic carbon by Chlorella saccharophila is not repressed by growth in high CO2

Regulation of transport of dissolved inorganic carbon (DIC)in response to CO₂ concentration in the external medium hasbeen compared in two closely-related green algae, Chlorellaellipsoidea and Chlorella saccharophila. C. ellipsoidea, whengrown in high CO₂, had reduced activities of both CO₂ and transport and DIC transport activitieswere increased after the cells had acclimated to air. However,high CO₂-grown C. saccharophila had a comparable level of photosyntheticaffinity for DIC to that of air-grown C. ellipsoidea and thiswas accompanied by a capacity to accumulate high internal concentrationsof DIC. The high photosynthetic affinity and the high intracellularDIC accumulation did not change in cells grown in air exceptthat the occurrence of external carbonic anhydrase (CA) in air-grownC. saccharophila stimulated the intracellular DIC accumulationin the absence of added CA. These data indicate that activeDIC transport is constitutively expressed in C. saccharophila,presumably because this alga is insensitive to the repressiveeffect of high CO₂ on DIC transport. This strongly supportsthe existence of a direct sensing mechanism for external CO₂in Chlorella species, but also indicates that external CA isregulated independently of DIC transport in Chlorella species. Key words: Carbonic anhydrase, Chlorella, CO₂-insensitive, DIC transport, wild type     

Hypoxic Reptiles: Blood Gases, Body Temperature and Control of Breathing

Our contribution to this symposium is a review of recent modelsand experimental cdata on oxygen homeostasis in vertebrateswith normal intracardiac shunts; i.e., amphibians and reptiles.We focus on the interactions among hemoglobin function, bodytemperature regulation, and cardiovascular shunts under normalconditions (i.e., breathing fresh air at or near sea level)and during external hypoxia (e.g., altitude, burrows) and internalhypoxia (e.g., anemia, hemorrhage). Mathematical models andexperimental data suggest that animals with venous admixturefrom cardiovascular shunts will show biphasic arterial and mixedvenous Po₂ responses to warming; i.e., first increasing andthen, as the dissociation curve shifts too far to the right,decreasing. This has implications for many physiological functionsincluding oxygen consumption by tissues, control of breathing,as well as preferred body temperature and its regulation. Wepresent some of the recent experiments that have explored theseimplications.     

Low incidence of atrial septal defects in nonmammalian vertebrates

The atrial septum enables efficient oxygen transport by separating the systemic and pulmonary venous blood returning to the heart. Only in placental mammals will the atrial septum form by the coming‐together of the septum primum and the septum secundum. In up to one of four placental mammals, this complex morphogenesis is incomplete and yields patent foramen ovale. The incidence of incomplete atrial septum is unknown for groups with the septum primum only, such as birds and reptiles. We found a low incidence of incomplete atrial septum in 11 species of bird (0% of specimens) and 13 species of reptiles (3% of specimens). In reptiles, there was a trabecular interface between the atrial septum and the atrial epicardium which was without a clear boundary between left and right atrial cavities. In developing reptiles (four squamates and one crocodylian), the septum primum initiated as a sheet that acquired perforations and the trabecular interface developed late. We conclude that atrial septation from the septum primum only results in a low incidence of incompleteness. In reptiles, the atrial septum and atrial wall develop a trabecular interface, but previous studies on atrial hemodynamics suggest this interface has a very limited capacity for shunting.     

Characterization of the rat Na+/nucleoside cotransporter 2 and transport of nucleoside-derived drugs using electrophysiological methods

The Na⁺-dependent nucleoside transporter 2 (CNT2) mediates active transport of purine nucleosides and uridine as well as therapeutic nucleoside analogs. We used the two-electrode voltage-clamp technique to investigate rat CNT2 (rCNT2) transport mechanism and study the interaction of nucleoside-derived drugs with the transporter expressed in Xenopus laevis oocytes. The kinetic parameters for sodium, natural nucleosides, and nucleoside derivatives were obtained as a function of membrane potential. For natural substrates, apparent affinity (K_0.5) was in the low micromolar range (12–34) and was voltage independent for hyperpolarizing membrane potentials, whereas maximal current (I_max) was voltage dependent. Uridine and 2'-deoxyuridine analogs modified at the 5-position were substrates of rCNT2. Lack of the 2'-hydroxyl group decreased affinity but increased I_max. Increase in the size and decrease in the electronegativity of the residue at the 5-position affected the interaction with the transporter by decreasing both affinity and I_max. Fludarabine and formycin B were also transported with higher I_max than uridine and moderate affinity (102 ± 10 and 66 ± 6 µM, respectively). Analysis of the pre-steady-state currents revealed a half-maximal activation voltage of about –39 mV and a valence of about –0.8. K_0.5 for Na⁺ was 2.3 mM at –50 mV and decreased at hyperpolarizing membrane potentials. The Hill coefficient was 1 at all voltages. Direct measurements of radiolabeled nucleoside fluxes with the charge associated showed a ratio of two positive inward charges per nucleoside, suggesting a stoichiometry of two Na⁺ per nucleoside. This discrepancy in the number of Na⁺ molecules that bind rCNT2 may indicate a low degree of cooperativity between the Na⁺ binding sites. two-electrode voltage clamp; concentrative nucleoside transport; presteady-state currents     

The physiology of lipid storage and use in reptiles

Lipid metabolism is central to understanding whole‐animal energetics. Reptiles store most excess energy in lipid form, mobilise those lipids when needed to meet energetic demands, and invest lipids in eggs to provide the primary source of energy to developing embryos. Here, I review the mechanisms by which non‐avian reptiles store, transport, and use lipids. Many aspects of lipid absorption, transport, and storage appear to be similar to birds, including the hepatic synthesis of lipids from glucose substrates, the transport of triglycerides in lipoproteins, and the storage of lipids in adipose tissue, although adipose tissue in non‐avian reptiles is usually concentrated in abdominal fat bodies or the tail. Seasonal changes in fat stores suggest that lipid storage is primarily for reproduction in most species, rather than for maintenance during aphagic periods. The effects of fasting on plasma lipid metabolites can differ from mammals and birds due to the ability of non‐avian reptiles to reduce their metabolism drastically during extended fasts. The effect of fasting on levels of plasma ketones is species specific: β‐hydroxybutyrate concentration may rise or fall during fasting. I also describe the process by which the bulk of lipids are deposited into oocytes during vitellogenesis. Although this process is sometimes ascribed to vitellogenin‐based transport in reptiles, the majority of lipid deposition occurs via triglycerides packaged in very‐low‐density lipoproteins (VLDLs), based on physiological, histological, biochemical, comparative, and genomic evidence. I also discuss the evidence for non‐avian reptiles using ‘yolk‐targeted’ VLDLs during vitellogenesis. The major physiological states – feeding, fasting, and vitellogenesis – have different effects on plasma lipid metabolites, and I discuss the possibilities and potential problems of using plasma metabolites to diagnose feeding condition in non‐avian reptiles.     

Molecular Cloning and Evolutionary Analysis of Hemoglobin α-Chain Genes in Bats

Bats are the only mammals with the capacity for powered flight. When flying, they need abundant energy and oxygen. According to previous works, the hemoglobin (Hb) oxygen loading function of bats is insensitive to variations in body temperature, although different bat species have different heat sensitivity. We cloned Hb α-chain sequences from eight bat species to investigate whether they have different characteristics. We found that Hb in the bat lineages is under purifying selection, which accords with the importance of its function in bats. Three turn regions in bat Hb, however, have distinct evolutionary rates compared with those of other mammals, and the codons in these regions have an accelerated rate of evolution. These codons are under divergent selection in bats. These changes in Hb may have occurred in response to the physiological requirements of the species concerned, as adaptations to different lifestyles.     

Divergent selection for aerobic capacity in rats as a model for complex disease

Based upon ideas about evolution, we put forth the argumentthat the capacity to transfer energy via aerobic metabolismis such a central feature of mammalian biology, that it mustalso be the primary determinant of complex disease. From this,we hypothesized that artificial selection on low and high capacityfor aerobic exercise would create lines that can be used todefine the divide between health and disease. In 1996 we beganlarge-scale divergent selection for aerobic treadmill runningcapacity in a widely heterogeneous stock of rats (N:NIH). Byten generations we developed lines of low capacity runners (LCR)and high capacity runners (HCR) that on average differed by317%. As a correlated trait, body mass increased at each generationin the LCR while the body mass decreased in the HCR. The linesalso separated for key factors of systemic oxygen transportcapacity such as maximal oxygen consumption (VO₂max), tissueperfusion, capillary density, and oxidative enzyme activity(citrate synthase and B-HAD). We also tested our hypothesisthat differences in aerobic energy transfer would produce ratsthat contrast for risk factors associated with complex disease.Indeed, the lines separated for cardiovascular risk factorsincluding differences in blood pressure, cardiac contractility,visceral adiposity, plasma free fatty acids, and triglycerides.The decrease in aerobic capacity was also associated with lowamounts of several proteins required for mitochondrial function.     

Mechanism of Electron Flow through the QB Site in Photosystem II. 1. Kinetics of the Reduction of Electron Acceptors at the QB and Plastoquinone Sites in Photosystem II Particles from the Cyanobacterium Synechococcus vulcanus

Various derivatives of benzoquinone (BQ) were found to be reducedat two sites [the Q_B and plastoquinone (PQ) sites] in photosystemII particles from Synechococcus vulcanus, and the relationshipbetween the structures of BQs and the kinetics of such reductionat each site were studied. Affinities of BQs for both the Q_B and the PQ site and the maximumturnover rates at the two sites were estimated by computer simulationof the dependence on the concentration of the BQ of the rateof oxygen evolution. Affinities of BQs for the Q_B sites determinedby this method agreed well with those determined from competitionbetween BQs and DCMU for the Q_B site. All methyl-substituted BQs had low affinities for the Q_B site,and tetramethyl-p-benzoquinone had almost no affinity. An increasein the number of chlorine atoms in the quinone ring increasedthe affinity, and the position of such substitutions had a greateffect on the affinity when two positions on the ring were occupiedby two chlorine atoms or methyl groups. The affinities of BQs for the PQ site were almost the same forall BQs tested in this experiment but the maximum turnover ratesat this site varied greatly from one derivative to another. The results are consistent with the hypothesis that the bindingof the PQ molecule to the Q_B site is attributable not to itsquinone ring but to its isoprenoid chain and, moreover, thatthe electron transfer through the Q_B site occurs not by replacementof the PQ molecule but by donation of electrons to another PQmolecule. (Received September 20, 1994; Accepted February 27, 1995)     

The Transport and Metabolism of Urea in Chara australis: III. TWO SPECIFIC TRANSPORT SYSTEMS

Reciprocal competitive inhibition studies were used to showthat N-methyl-urea (NMU), acetamide and urea all compete forbinding to a common transport system, designated system I andthat this system is one of two specific mechanisms transportingurea in Chara. System I binds urea with a Km of about 0–3mmol m-3 and is strongly influenced by metabolic controls. SystemI is active and electrogenic and may be energized by the couplingof urea uptake to an influx of protons. This is the first reportof an electrogenic urea transport system in an alga. The secondspecific mechanism for urea transport, designated system II,binds urea with a relatively low affinity (K_m c. 7–0 mmolm-3) and does not transport NMU to a significant extent. SystemII is less subject to metabolic control than system I and, thoughit may be active, is not electrogenic. Key words: Urea, methylurea, proton cotransport, metabolic control     

The Physiological Ecology of the Zebra Mussel, Dreissena polymorpha, in North America and Europe

SYNOPSIS. The zebra mussel, Dreissena polymorpha (Pallas), wasintroduced into North America in 1986. Initial North American(N.A.) studies suggested that physiological responses variedbetween N.A. and European populations. However, literature reviewindicates agreement on most aspects of physiological adaptationincluding: respiratory responses; hypoxia/anoxia tolerance;salinity limits; emersion tolerance; freezing resistance; environmentalpH limits; calcium limits; starvation responses; and bioenergeticpartitioning. The main differences among N.A. and European musselsappear to be elevated upper thermal limits and temperaturesfor optimum growth among N.A. populations. N.A. zebra musselsprobably originated from the northern shore of the Black Seain the warmest portion of the mussel's European range. However,most European physiological data come from northern Europe wherepopulations may be adapted to colder temperatures. Alternatively,N.A. research suggests that mussels may have a capacity forseasonal temperature acclimatization such that responses recordedin warmer N.A. waters may be different from those recorded innorthern Europe even after short-term laboratory acclimation.Studies of genetic variation and physiological response amongEuropean and N.A. D. polymorpha populations are required toelucidate the basis for physiological differentiation. Recentlyevolved D. polymorpha has poor resistance adaptations comparedto unionacean and sphaeriid bivalves with longer freshwaterfossil histories. Poor resistance adaptations make it less suitedfor stable habitats, instead, its high fecundities, early maturity,and rapid growth are adaptations to unstable habitats whereextensive resistance adaptations are of little value.     

Nutrient-Limited Growth Rates: Roles of Nutrient-Use Efficiency and of Adaptations to Increase Uptake Rate

When stressed by low nutrient availability, young sunflowerplants (Helianthus annuus) showed responses seen in many otherspecies: increases in root uptake capacity (V^max, l/K_m), root:shoot ratio, and putative nutrient-use efficiency, nUE=l/(tissuenutrient content). A straightforward mechanistic model is derivedfor relative growth rate (RGR) in solution culture in termsof these factors. A linear regression based on the model indicatesa negative role for nUE, which violates a premise of the model.A revised model proposes that primary adaptations are only inuptake rate and growth or nutrient allocations, and these actthrough the photosynthetic utility of nutrient. The tissue nutrientcontent and associated nUE become dependent quantities. Thepredictions for RGR, as tested by linear regression, are improved.The model predicts that nUE can increase as external solutionconcentration decreases, but decreases with increased uptakeadaptations in one given environment. The decrease in nUE compromisespotential gains in RGR from uptake adaptations, and makes increasesin root: shoot ratio a nearly insignificant contributor to earlyRGR. The model and associated regression analyses are generalizedfor additional adaptations such as increased root fineness andfor different quantitative ways that a nutrient may limit photosynthesis.The model and analyses are also generalized to plant growthin soil and growth without functional balance between root andshoot. Key words: Relative growth rate, Helianthus annuus, nutrient stress, nutrient use efficiency, functional balance