Respiratory Control in the Transition from Water to Air Breathing in Vertebrates

SYNOPSIS. Studies on extant bimodally breathing vertebratesoffer us a chance to gain insight into the changes in respiratorycontrol during the evolutionary transition from water to airbreathing. In primitive Actinopterygian air-breathingfishes(Lepisosteus and Amid), gill ventilation is driven by an endogenouslyactive central rhythm generator that is powerfully modulatedby afferent input from internally and externally oriented branchialchemoreceptors, as it is in water-breathing Actinopterygians.The effects of internal or external chemoreceptor stimulationon water and air breathing vary substantially in these aquaticair breathers, suggesting that their roles are evolutionarilymalleable. Air breathing in these bimodal breathers usuallyoccurs as single breaths taken at irregular intervals and isan on-demand phenomenon activated primarily by afferent inputfrom the branchial chemoreceptors. There is no evidence forcentral CO₂/pH sensitive chemoreceptors and air-breathing organmechanoreceptors have little influence over branchial- or air-breathingpatterns in Actinopterygian air breathers. In the Sarcopterygianlungfish Lepidosiren and Protopterus, ventilation of the highlyreduced gills is relatively unresponsive to chemoreceptor ormechanoreceptor input. The branchial chemoreceptors of the anteriorarches appear to monitor arterialized blood, while chemoreceptorsin the posterior arches may monitor venous blood. Lungfish respondvigorously to hypercapnia, but it is not known whether theseresponses are mediated by central or peripheral chemoreceptors.A major difference between the Sarcopterygian and Actinopterygianbimodal breathers is that lungfish can inflate their lungs usingrhythmic bouts of air breathing, and lung mechanoreceptors influencethe onset and termination of these lung inflation cycles. Thecontrol of breathing in amphibians appears similar to that oflungfish. Branchial ventilation may persist as rhythmic buccaloscillations in most adults, and stimulation of peripheral chemoreceptorsin the aortic arch or carotid labyrinths initiates short boutsof breathing. Ventilation is much more responsive to hypercapniain adult amphibians than in Actinopterygian fishes because ofcentral CO₂/pH sensitive chemoreceptors that act to convertperiodic to more continuous breathing patterns when stimulated.     

The Effect of External pH on the Apparent CO2 Affinity of Chlorella saccharophila

The carbon dioxide compensation point of the unicellular greenalga, Chloretla saccharophila, was determined in aqueous mediumby a gas chromatographic method. Compensation points decreasedmarkedly from 63 cm³ m^–3 at an external pH of 4.0 to 3.2cm³ m^–3 at pH 8.0 and were not affected by the O₂ concentrationof the medium. The calculated CO² concentration required tosupport the half-maximum photosynthetic rate of the algal cellsranged from 6.0 mmol m_–3 at an external pH of 60 to 1.5mmol m^–3 at pH 8.0 and these values were not affectedby O₂ concentration. The K_m(CO₂) of nbulose-l,5-bisphosphatecarboxylase isolated from cells grown either at pH 4.0 or pH8.0 was determined to be 64 mmol m^–3. These results indicatethat loss of CO₂ by photorespiration does not occur in C. saccharophilacells at acid pH and the disparity between the apparent affinityfor CO₂ of the intact cells and that of the carboxylase indicatesthe operation of a ‘CO₂ concentrating mechanism’in this alga at acid pH. Key words: Acidophilic alga, bicarbonate transport, Chlorella saccharophila, compensation point, CO₂ affinity, PH, RuBP carboxylase     

Central ventilatory control in the South American lungfish, <Emphasis Type="Italic">Lepidosiren paradoxa:</Emphasis> contributions of pH and CO<Subscript>2</Subscript>

Lungfish represent a probable sister group to the land vertebrates. Lungfish and tetrapods share features of respiratory control, including central, peripheral and intrapulmonary CO₂ receptors. We investigated whether or not central chemoreceptors in the lungfish, L. paradoxa, are stimulated by CO₂ and/or pH. Ventilation was measured by pneumotachography for diving animals. The fourth cerebral ventricle was equipped with two catheters for superfusion. Initially, two control groups were compared: (1) catheterized animals with no superfusion and (2) animals superfused with mock CSF solutions at pH = 7.45; PCO₂ = 21 mmHg. The two groups had virtually the same ventilation of about 40 ml BTPS kg⁻¹ h⁻¹ (P > 0.05). Next, PCO₂ was increased from 21 to 42 mmHg, while pH_CSF was kept at 7.45, which increased ventilation from 40 to 75 ml BTPS kg⁻¹ h⁻¹. Conversely, a decrease of pH_CSF from 7.45 to 7.20 (PCO₂ = 21 mmHg) increased ventilation to 111 ml BTPS kg⁻¹ h⁻¹. Further decreases of pH_CSF had little effect on ventilation, and the combination of pH_CSF = 7.10 and PCO₂ = 42 mmHg reduced ventilation to 63 ml BTPS kg⁻¹ h⁻¹.     

Refixation of respiratory CO2 in the ears of C3 cereals

The spatial arrangement of tissues within the ears of cereals,and gas exchange measurements on intact ears of barley and durumwheat suggest that respiratory CO₂ associated with grain-fillingprocesses, may be refixed close to its site of evolution. Apparentrefixation of respiratory CO₂ in intact ears was compared withthat in flag leaves, by feeding both organs with ¹⁴C-labelledsucrose and trapping ¹⁴CO₂ released by respiration. Apparentrefixation in the ears was twice that measured in flag leafblades of durum wheat genotype Durelle. In ears, the capacityof refixation of respiratory CO₂ at 210 mmol mol^–1 O₂ranged from 55% in barley genotype Roxana to 75% in barley genotypeHatif, and 60% in duwm wheat genotype Bidi 17. A low O₂ concentrationincreased refixation of respiratory CO₂ by up to 90%, 92% and82%, respectively. The occurrence of CO₂ refixation in the field,in a set of 12 barley genotypes grown under irrigated and rainfedMediterranean field conditions, was consistent with observedcarbon isotope ratios (     

Alkalization of the Medium by Unicellular Green Algae during Uptake Dissolved Inorganic Carbon

When Chlorella vulgaris 11h, Chlorella vulgaris C-l, Chlamydomonasreinhardtii, Chlamydomonas moewusii, Scenedesmus obliquus, orDunaliella tertiolecta were illuminated in with 0.5 mM NaHCO₃,the pH of the medium increased in a few minutes from 6 to about9 or 10. The alkalization, which was accompanied by O₂ evolution,was dependent on light, external dissolved inorganic carbon(DIC) as HCO^-₃, and algae grown or adapted to a low, air-levelCO₂ in order to develop a DIC concentrating mechanism. Therewas little pH increase by algae without a DIC concentratingprocess from growth on 3% CO₂ in air. Photosynthetic O₂ evolutionwithout alkalization occurred using either internal DIC or externalCO₂ at acidic pH. The PH increase stopped between pH 9 to 10,but the alkalization would restart upon re-acidification betweenpH 6 and 8. Alkalization was suppressed by the carbonic anhydraseinhibitors, acetazolamide, ethoxyzolamide or carbon oxysulfide.The pH increase appeared to be the consequence of the externalconversion of HCO^–₃ into CO₂ plus OH^– during photosynthesisby cells with a high affinity for CO₂ uptake. Cells grown onhigh CO₂ to suppress the DIC pump, when given low levels ofHCO^–₃ in the light, acidified the medium from pH 10 to7. Air adapted Scenedesmus cells with a HCO^–₃ pump, aswell as a CO₂ pump, alkalized the medium very rapidly in thelight to a pH of over 10, as well as slower in the dark or inthe light with DCMU or without external DIC and O₂ evolution.Alkalization of the medium during photosynthetic DIC uptakeby algae has been considered to be part of the global carboncycle for converting H₂CO₃ to HCO^–₃ and for the formationof carbonate salts by calcareous algae from the alkaline conversionof bicarbonate to carbonate. These processes seem to be a consequenceof the algal CO₂ concentrating process. ¹Present address: Department of Biology, Faculty of Science,Niigata University, Niigata, 950-21 Japan.     

Elevated CO2 reduces O3 flux and O3-induced yield losses in soybeans: possible implications for elevated CO2 studies

Soybeans were grown for three seasons in open-top field chambersto determine (1) whether elevated CO₂ (360 versus 700 µmolmol^–1) alleviates some of the yield loss due to pollutantO₃, (2) whether the partial stomatal closure resulting fromchronic O₃ exposure (charcoal-filtered air versus 1.5 ambientconcentrations) is a cause or result of decreased photosynthesis,and (3) possible implications of CO₂/O₃ interactions to climatechange studies using elevated CO₂. Leaf conductance was reducedby elevated CO₂, regardless of O₃ level, or by exposure to O₃alone. As.a result of these effects on conductance, high CO₂reduced estimated midday O₃ flux into the leaf by an averageof 50% in charcoal-filtered air and 35% in the high O₃ treatment.However, while exposure to O₃ reduced seed yields by 41% atambient CO₂ levels, the yield reduction was completely amelioratedby elevated CO₂. The threshold midday O₃ flux for yield lossappears to be 20–30 nmol m^–2 s^–1 in this study.Although elevated CO₂ increased total biomass production, itdid not increase seed yields. A/C_i curves show a large reductionin the stomatal limitation to photosynthesis due to elevatedCO₂ but no effect of O₃. These data demonstrate that (1) reducedconductance due to O₃ is the result, and not the cause, of reducedphotosynthesis, (2) 700 µmol mol^–1 CO₂ can completelyameliorate yield losses due to O₃ within the limits of theseexperiments, and (3) some reports of increased yields underelevated CO₂ treatments may, at least in part, reflect the ameliorationof unrecognized suppression of yield by O₃ or other stresses. Key words: Stomatal limitation, elevated CO₂, O₃ flux, Glycine max, yield suppression     

Identification of CO2 Chemoreceptors in Helix pomatia

SYNOPSIS. Gas exchange in pulmonate snails of the family Helicidaeoccurs through a highly vascularized diffusion lung known asthe mantle. The extent of ventilation of the mantle dependsupon the duration and size of opening of an occlusible poreknown as the pneumostome. In Helix aspersa and Helix pomatia,pneumostomal size and frequency of opening are exquisitely sensitiveto CO₂. Respiratory CO₂ chemosensitivity resides in a discreteregion of the subesophageal ganglia. The discharge pattern ofmany neurons in the chemoreceptor area changes during stimulationwith CO₂. However, the electrophysiological response to CO,stimulation alone does not discriminate between CO₂ chemoreceptorcells and CO₂-insensitive neurons active in the pneumostomalresponse to CO₂. We identified a subset of CO₂-sensitive neuronsfrom the larger population of neurons active during CO₂ stimulation.The action potential discharge frequency of CO₂ chemosensoryneurons increased in response to CO₂ stimulation. An increaseddischarge frequency of CO₂-sensitive neurons was associatedwith increased pneumostomal opening, and both the size and thefrequency of pneumostomal opening increased during CO₂ stimulation.Injecting depolarizing current into individual chemosensoryneurons elicited opening of the pneumostome in the absence ofCO₂. Action potential generation in response to CO₂ was independentof synaptic transmission. Removal of individual CO₂-sensitivecells or inhibition of action potential generation in CO₂-sensitivecells reduced or eliminated pneumostomal responses to CO₂. CO₂sensitivity in chemoreceptor cells required extracellular calcium,but not sodium. Substituting barium for calcium supported chemoreceptoractivity. In summary, we have identified respiratory related,chemosensory neurons that are CO₂ sensitive in the absence ofsynaptic input.     

Episodic Breathing in Frogs: Converging Hypotheses on Neural Control of Respiration in Air Breathing Vertebrates

SYNOPSIS. The episodic, or intermittent, breathing of frogsand many ectothermic vertebrates results in important fluctuationsof arterial blood gases. This pattern of breathing differs fromthe rhythmic and continuous alternation of inspiration observedin most homeotherms, which maintain O₂ and CO₂ levels withinnarrow ranges. These differences in pattern of breathing indicatethat the respiratory control systems of ectotherms and homeothermsdiffer substantially. The results of recent studies using invitro brainstemspinal cord preparations of adult frogs and premetamorphictadpoles (Rana catesbeiana and Rana pipiens) demonstrate, however,that the mechanisms for rhythm generation and pattern formationdescribed previously for mammals are also key features of therespiratory control system of frogs. These findings thereforesupport the hypothesis that the respiratory control system ishighly conserved amongst air breathing vertebrates, whetherthey breathe continuously or episodically.     

The Optimal Oxygen Equilibrium Curve: A Comparison Between Environmental Hypoxia and Anemia

SYNOPSIS. Internal hypoxia in vertebrates occurs during anemia,when blood oxygen (0₂) carrying capacity is reduced, or duringexposure to environmental hypoxia. In non-altitude adapted vertebrates,exposure to environmental hypoxia results in a change in bloodO₂ affinity which, in some cases is beneficial to tissue O₂delivery. In contrast, the elevation in blood O₂ carrying capacityobserved in almost all vertebrates is always beneficial to tissueO₂ delivery (assuming no large changes in blood viscosity) andmay be more important than changes in blood O₂ affinity in maintainingtissue O₂ delivery. Experimental anemia in vertebrates results in a decrease inblood O₂ affinity which is always beneficial to tissue O₂ delivery.The reduction in affinity is brought about by an increase inthe organic phosphate to hemoglobin ratio (NTP:Hb) within thered cell. In fish NTP:Hb decreases during environmental hypoxiabut increases during anemia indicating that NTP regulation isquite different between these treatments despite the similarityof these treatments at the tissue level.     

Hypoxia, temperature, and pH/CO2 effects on respiratory discharge from a turtle brain stem preparation

Johnson, Stephen M., Rebecca A. Johnson, and Gordon S. Mitchell. Hypoxia, temperature, andpH/CO₂ effects on respiratory discharge from a turtle brain stem preparation. J. Appl. Physiol. 84(2): 649-660, 1998.An in vitrobrain stem preparation from adult turtles (Chrysemyspicta) was used to examine the effects of anoxia andincreased temperature and pH/CO₂on respiration-related motor output. At pH ~7.45, hypoglossal (XII)nerve roots produced patterns of rhythmic bursts (peaks) of discharge(0.74 ± 0.07 peaks/min, 10.0 ± 0.6 s duration) that werequantitatively similar to literature reports of respiratory activity inconscious, vagotomized turtles. Respiratory discharge was stable for 6 h at 22°C; at 32°C, peak amplitude and frequency progressivelyand reversibly decreased with time. Two hours of hypoxia had no effecton respiratory discharge. Acutely increasing bath temperature from 22 to 32°C decreased episode and peak duration and increased peakfrequency. Changes in pH/CO₂increased peak frequency from zero at pH 8.00-8.10 to maxima of0.81 ± 0.01 and 1.44 ± 0.02 peaks/min at 22°C (pH 7.32) and32°C (pH 7.46), respectively;pH/CO₂ sensitivity was similar atboth temperatures. We conclude that1) insensitivity to hypoxiaindicates that rhythmic discharge does not reflect gasping behavior,2) increased temperature altersrespiratory discharge, and 3)central pH/CO₂ sensitivity isunaffected by temperature in this preparation (i.e.,Q₁₀ ~1.0).

     

Uptake and Accumulation of Inorganic Carbon by a Freshwater Diatom

Colman, B. and Rotatore, C. 1988. Uptake and accumulation ofinorganic carbon by a freshwater diatom.—J. exp Bot 39:1025–1032. The mechanism of uptake of inorganic carbon and its accumulationhas been studied in the freshwater diatom Navicula pelliculosa.No external carbonic anhydrase could be detected, although itwas detected in cell extracts. The rate of photosynthetic O₂evolution, in media in the range pH 7.5–8.5, exceededthe calculated rate of CO₂ supply 2- to 5-fold, indicating thatHCO^–₃ was taken up by the cells. At an external pH of7.5, the internal pH, measured by ¹⁴C-dimethyloxazolidine-2,4-dione distribution between the cells and the medium, was pH7.6 in the light and pH 7.4 in the dark. Accumulation of inorganiccarbon was determined by the silicone oil centrifugation methodand inorganic carbon pools of 23.5 mol m^–3 were found,a concentration 21.6-fold that in the external medium. The resultsindicate an active accumulation of inorganic carbon againstpH and concentration gradients in this diatom, probably by activeHCO^–₃ uptake. Key words: Bicarbonate transport, carbon dioxide, carbonic anhydrase, CO₂ affinity, CO₂ concentrating mechanism, internal pH, Navicula pelliculosa     

Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chora corallina

Carbonic anhydrase (CA, EC. 4.2.1.1 [EC] ) activity in air-grown Characorallina was detected mainly in the intracellular fraction,most of which composed of chloroplasts and cytoplasmic gel,and not on the cell surface. Only minor levels of CA activity,on the basis of equivalent volumes, were detected in the cellsap and the cytoplasmic sol. The maximum rate of photosynthetic O₂ evolution by air-grownChara corallina at pH 6.0 was twice that at pH 7.6, while theapparent K_m for external inorganic carbon (C_i) at pH 7.6 wasabout three times that at pH 6.0. However, the apparent K_m(CO₂)was about three times larger at pH 6.0 than at pH 7.6. The K_m(C_i)-valueat pH 7.6 increased severalfold in the presence of acetazolamide(AZA), an inhibitor of CA, but no inhibition was observed atpH 6.0. The pH-dependence may be due to differences in the permeabilityof AZA at the given pH values. Fixation of ¹⁴CO₂ at 20 µMand of H¹⁴CO₃^– at 200 µM over the course of 5 swas very similar at pH 7.4. Addition of CA significantly suppressedthe photosynthetic ¹⁴CO₂-fixation but it stimulated the H¹⁴CO₃^–-fixation.This result indicates that free CO₂ is an active species ofC_i that is incorporated into the cell during photosynthesis. These results together suggest the following: (1) Free CO₂ isutilized for photosynthesis, (2) CA is mainly located insidethe cell and functions to increase the affinity for CO₂ in photosynthesisby facilitating the supply of CO₂ from the plasmalemma to thesite of CO₂-fixation. ³Present address: Biological Laboratory, The University of theAir, Wakaba 2-11, Chiba, 260 Japan. (Received December 9, 1988; Accepted March 22, 1989)     

CO2‐metabolism in early tetrapods revisited: inferences from osteological correlates of gills,skin and lung ventilation in the fossil record

One of the most important physiological changes during the conquest of land by vertebrates was the increasing reliance on lung breathing, with the concomitant decrease in importance of gill breathing. The main problem involved here was to cope with the excessive accumulation of CO₂ in the body and to avoid respiratory acidosis. In the past, several often mutually contradicting hypotheses of CO₂‐elimination via skin, lungs and gills in early tetrapods have been proposed, based on theoretical physiological considerations and comparison with extant air‐breathing fishes and amphibians. This study proposes a revised scenario of CO₂‐elimination in early tetrapods based on fossil evidence, that is recently identified osteological correlates of gills, skin structure and mode of lung ventilation. In stem tetrapods of the Devonian and Carboniferous, O₂‐uptake via the lungs by buccal pumping was decoupled from CO₂‐release via internal gills, and the rather gas‐impermeable skin played a minor role in gaseous exchange. The two main lineages of crown‐group tetrapods, the amphibian and amniote lineage, used different strategies of CO₂‐elimination. As in stem tetrapods, O₂‐uptake and CO₂‐release remained always largely decoupled in temnospondyls, which ventilated their lungs via buccal pumping and relied mainly on their internal gills for CO₂‐release. Temnospondyls were not able to reduce their internal gills before their skin became more gas permeable and their body size was reduced, to shift from internal gills to the skin as the major site of CO₂‐elimination, a pattern that is retained in most lissamphibians. In contrast, internal gills were lost very early in stem amniote evolution. This was associated with the evolution of the more effective aspiration pump that allowed the elimination of the bulk of CO₂ via the lungs, leading to a coupled O₂‐uptake and CO₂‐loss in stem amniotes and later in amniotes.     

Assessing the genetic relatedness of higher ozone sensitivity of modern wheat to its wild and cultivated progenitors/relatives

Modern wheat (Triticum aestivum L.) is one of the most ozone(O₃)-sensitive crops. However, little is known about its geneticbackground of O₃ sensitivity, which is fundamental for breedingO₃-resistant cultivars. Wild and cultivated species of winterwheat including donors of the A, B and D genomes of T. aestivumwere exposed to 100 ppb O₃ or charcoal-filtered air in opentop chambers for 21 d. Responses to O₃ were assessed by visibleO₃ injury, gas exchange, chlorophyll fluorescence, relativegrowth rate, and biomass accumulation. Ozone significantly decreasedlight-saturated net photosynthetic rate (–37%) and instantaneoustranspiration efficiency (–42%), but increased stomatalconductance (+11%) and intercellular CO₂ concentration (+11%).Elevated O₃ depressed ground fluorescence (–8%), maximumfluorescence (–26%), variable fluorescence (–31%),and maximum photochemical efficiency (–7%). Ozone alsodecreased relative growth rate and the allometric coefficient,which finally reduced total biomass accumulation (–54%),but to a greater extent in roots (–77%) than in the shoot(–44%). Winter wheat exhibited significant interspeciesvariation in the impacts of elevated O₃ on photosynthesis andgrowth. Primitive cultivated wheat demonstrated the highestrelative O₃ tolerance followed by modern wheat and wild wheatshowed the lowest. Among the genome donors of modern wheat,Aegilops tauschii (DD) behaved as the most O₃-sensitive followedby T. monococcum (AA) and Triticum turgidum ssp. durum (AABB)appeared to be the most O₃-tolerant. It was concluded that thehigher O₃ sensitivity of modern wheat was attributed to theincreased O₃ sensitivity of Aegilops tauschii (DD), but notto Triticum turgidum ssp. durum (AABB) during speciation. Key words: Biomass, Chl a fluorescence, genome, ozone sensitivity, relative growth rate, stomatal conductance, winter wheat Received 20 September 2007; Revised 30 November 2007 Accepted 16 January 2008     

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO₂/H⁺ is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K⁺ channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO₂/H⁺ levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca²⁺, gap junctions, oxidative stress, glial cells, bicarbonate, CO₂, and neurotransmitters. The normal target for these signals is generally believed to be a K⁺ channel, although it is likely that many K⁺ channels as well as Ca²⁺ channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO₂- and/or H⁺-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO₂/H⁺. hypercapnia; brain stem; ventilation; peripheral chemoreceptor; glia; gap junction; glomus; channel; calcium; potassium; carbonic anhydrase; taste receptor; nociception     

The Changing State of Surfactant Lipids: New Insights from Ancient Animals

SYNOPSIS. Pulmonary surfactant is a mixture of phospholipids(including disaturated phospholipids), cholesterol and proteinslining the air-liquid interface within the lung. Surfactantacts to reduce surface tension, thereby increasing lungcomplianceand also preventing edema. The saccular lungs, or other gas-holdingstructures, of nonmammals have 7–70% more surfactant/cm²of surface than lungs of mammals. Nonmammalian surfactant actsas an antiglue that decreases the inflation pressures of collapsedlungs by reducing the adherence of apposing epithelial surfaces.The autonomic nervous system appears to be the primary systemcontrolling release of surfactant in nonmammals. The lipid compositionis highly conserved within the vertebrates, except that surfactantof teleost fish is dominated by cholesterol whereas tetrapodsurfactant consists primarily of disaturated phospholipids (DSP).The dipnoan Neoceratodus forsteri demonstrates a "fish-type"surfactant profile while the other derived dipnoans demonstratea surfactant profile similar to that of tetrapods. Homologyof the surfactant protein SP-A within the vertebrates pointsto a single evolutionaryorigin for the system and indicatesthat fish surfactant is a "protosurfactant". Amongst the tetrapods,the relative proportions of DSP and cholesterol vary in responseto lung structure, habitat, and body temperature (T_b) but notin relation to phytogeny. The cholesterol content of surfactantis elevated in species with simple saccular lungs, in aquaticspecies, and in species with low T_b. The DSP content is highestin complex lungs, particularly ofaquatic species or specieswith high T_b. The cholesterol content of surfactant also increasesin response to acute decreases in Tb in lizards and torpid marsupials,presumably to maintain fluidity of the lipid mixture.     

Photosynthesis and photorespiration in soybean [Glycine max (L.) Merr.] chronically exposed to elevated carbon dioxide and ozone

The effects of elevated carbon dioxide (CO₂ and ozone (O₃) onsoybean (Glycine max (L.) Merr.] photosynthesis and photorespiration-relatedparameters were determined periodically during the growing seasonby measurements of gas exchange, photorespiratory enzyme activitiesand amino acid levels. Plants were treated in open-top fieldchambers from emergence to harvest maturity with seasonal meanconcentrations of either 364 or 726 µmol mol^–1 CO₂in combination with either 19 or 73 nmol mol^–1 O₃ (12h daily averages). On average at growth CO₂ concentrations,net photosynthesis (A) increased 56% and photorespiration decreased36% in terminal mainstem leaves with CO₂ enrichment. Net photosynthesisand photorespiration were suppressed 30% and 41%, respectively,by elevated O₃ during late reproductive growth in the ambientCO₂ treatment, but not in the elevated CO₂ treatment. The ratioof photorespiration to A at growth CO₂ was decreased 61% byelevated CO₂ There was no statistically significant effect ofelevated O₃ on the ratio of photorespiration to A. Activitiesof glycolate oxidase, hydroxypyruvate reductase and catalasewere decreased 10–25% by elevated CO₂ and by 46–66%by elevated O₃ at late reproductive growth. The treatments hadno significant effect on total amino acid or glycine levels,although serine concentration was lower in the elevated CO₂and O₃ treatments at several sampling dates. The inhibitoryeffects of elevated O₃ on photorespiration-related parameterswere generally commensurate with the O₃-induced decline in A.The results suggest that elevated CO₂ could promote productivityboth through increased photoassimilation and suppressed photorespiration. Key words: Photorespiration, CO₂-enrichment, ozone, climate change, air pollution     

Evidence for the Occurrence of Feedback Inhibition of Photosynthesis in Rice

The response of photosynthesis in the flag leaf of rice (Oryzasativa) to elevated CO₂ or reduced O₂ was investigated relativeto other environmental factors using steady-state gas exchangetechniques. We found under moderate conditions of temperatureand photosynthetic flux density (PFD) (26°C and 700µmolquanta m^–2s^–1 similar to growth conditions) photosynthesisin the flag leaf of rice during heading and grain filling saturatedat near ambient levels of CO₂, with a concomitant loss of O₂sensitivity, when a high stomatal conductance was maintainedby high humidity (low vapor pressure deficit). Under 18°Cthere was near complete loss of O₂ sensitivity of photosynthesisat normal ambient levels of CO₂. This is in contrast to thelarge enhancement of photosynthesis by supra-atmospheric levelsof CO₂ and sub-atmospheric levels of O₂ by suppression of photorespirationwhen there is no limitation on utilizing the initial productof CO₂ assimilation (triose-P) as predicted from Ribulose-l,5-bisphosphatecarboxylase/oxygenase (Rubisco) kinetic properties. Thus, lossof sensitivity to CO₂ and O₂ has been previously explained asa limitation on utilization of triose-P to synthesize carbohydrates.Under high PFD at 25°C, the rate of photosynthesis in ricedeclined over a period of hours around midday, while the intercellularlevels of CO₂ remained constant suggesting a limitation on utilizationof photosynthate. Short-term fluctuations in climatic factorsincluding temperature, light and humidity could result in afeedback limitation on photosynthesis in rice which may be exacerbatedby rising CO₂. (Received March 12, 1998; Accepted May 14, 1998)     

Intraclass diversification of immunoglobulin heavy chain genes in the African lungfish

Lungfish (Dipnoi) are the closest living relatives to tetrapods, and they represent the transition from water to land during vertebrate evolution. Lungfish are armed with immunoglobulins (Igs), one of the hallmarks of the adaptive immune system of jawed vertebrates, but only three Ig forms have been characterized in Dipnoi to date. We report here a new diversity of Ig molecules in two African lungfish species (Protopterus dolloi and Protopterus annectens). The African lungfish Igs consist of three IgMs, two IgWs, three IgNs, and an IgQ, where both IgN and IgQ originated evidently from the IgW lineage. Our data also suggest that the IgH genes in the lungfish are organized in a transiting form from clusters (IgH loci in cartilaginous fish) to a translocon configuration (IgH locus in tetrapods). We propose that the intraclass diversification of the two primordial gnathostome Ig classes (IgM and IgW) as well as acquisition of new isotypes (IgN and IgQ) has allowed lungfish to acquire a complex and functionally diverse Ig repertoire to fight a variety of microorganisms. Furthermore, our results support the idea that “tetrapod-specific” Ig classes did not evolve until the vertebrate adaptation to land was completed ~360 million years ago.     

Oxygen Exchange in Relation to Carbon Assimilation in Water-stressed Leaves During Photosynthesis

In a study on metabolic consumption of photosynthetic electronsand dissipation of excess light energy under water stress, O₂and CO₂ gas exchange was measured by mass spectrometry in tomatoplants using ¹⁸O₂ and ¹³CO₂. Under water stress, gross O₂ evolution(E_O), gross O₂ uptake (U_O), net CO₂ uptake (P_N), gross CO₂ uptake(TPS), and gross CO₂ evolution (E_C) declined. The ratio P_N/E_Ofell during stress, while the ratios U_O/E_O and E_C/TPS rose.Mitochondrial respiration in the light, which can be measureddirectly by ¹²CO₂ evolution during ¹³CO₂ uptake at 3000 µll^{–1 13}CO₂, is small in relation to gross CO₂ evolutionand CO₂ release from the glycolate pathway. It is concludedthat PSII, the Calvin cycle and mitochondrial respiration aredown-regulated under water stress. The percentages of photosyntheticelectrons dissipated by CO₂ assimilation, photorespiration andthe Mehler reaction were calculated: in control leaves morethan 50 % of the electrons were consumed in CO₂ assimilation,23 % in photorespiration and 13 % in the Mehler reaction. Undersevere stress the percentages of electrons dissipated by CO₂assimilation and the Mehler reaction declined while the percentageof electrons used in photorespiration doubled. The consumptionof electrons in photorespiration may reduce the likelihood ofdamage during water deficit.