Effects of Oxygen on Metabolism by the Glycollate Pathway in Leaves

Segments of wheat leaves were supplied in the light with ¹⁴C-labelledserine or glucose in atmospheres containing different concentrationsof O₂ and zero or 350 parts/10⁶ CO₂. Some O₂ was necessary forsucrose synthesis from either serine or glucose but sucrosesynthesis from glucose depended on reactions with a high affinityfor O₂ whereas sucrose synthesis from serine depended both onreactions with high and low affinities for O₂. In the presenceof CO₂ sucrose synthesis from serine was decreased when theO₂ concentration was increased from 20 to 80% by volume andCO₂ was liberated; sucrose synthesis from glucose was almostunaffected by the same change in conditions. Also, in an atmospherecontaining 80% O₂ and 350 parts/10⁶ CO₂, radioactivity from[¹⁴C]serine, was incorporated into glycine. This was not truefor glucose feeding. Hence glucose provides a substrate forsucrose synthesis but not for photorespiration whereas serineis used for both processes in the presence of CO₂; in the absenceof CO₂ glucose provides substrate for both sucrose synthesisand photorespiration and serine metabolism to sucrose is restricted.     

Methods for detecting local intestinal ischemic anaerobic metabolic acidosis by PCO2

Rozenfeld, Ranna A., Michael K. Dishart, Tor IngeTønnessen, and Robert Schlichtig. Methods for detecting localintestinal ischemic anaerobic metabolic acidosis byPCO₂. J. Appl. Physiol. 81(4): 1834-1842, 1996.Gut ischemia isoften assessed by computing an imaginary tissue interstitial pH fromarterial plasma HCO₃ and thePCO₂ in a saline-filled balloontonometer after equilibration with tissuePCO₂ (Pti_CO2).Pti_CO2 mayalternatively be assumed equal to venous PCO₂(Pv_CO2) in that region of gut. The ideais that as blood flow decreases, gutPti_CO2 andPv_CO2 will increase to the maximumaerobic value, i.e., maximum respiratoryPv_CO2(Pv_CO2 rmax). Above a "critical" anaerobic threshold, lactate(La) generation, bytitration of tissue HCO₃, should raisePti_CO2abovePv_CO2 rmax.During progressive selective whole intestinal flow reduction insix pentobarbital-anesthetized pigs, we usedPCO₂ electrodes to test thehypotheses that criticalPti_CO2is achieved earlier in mucosa than in serosa and thatPv_CO2 rmax,computed using an in vitro model, predicts criticalPti_CO2. Wedefined criticalPti_CO2 as theinflection ofPti_CO2-Pv_CO2vs. O₂ delivery(O₂)plots. CriticalO₂for O₂ uptake was 12.55 ± 2 ml ^· kg^{1 ·} min¹.Critical Pti_CO2 for mucosaand serosa was achieved at similar whole intestineO₂(13.90 ± 5 and 13.36 ± 5 ml ^· kg^{1 ·} min¹,P = NS). CriticalPti_CO2 (129 ± 24 and 96 ± 21 Torr) exceeded Pv_CO2 rmax(62 ± 3 Torr). During ischemia,La excretion into portalvenous blood was matched by K⁺excretion, causing Pv_CO2 to increaseonly slightly, despitePti_CO2 risingto 380 ± 46 (mucosa) and 280 ± 38 (serosa) Torr. These resultssuggest that mucosa and serosa become dysoxic simultaneously, thatischemic dysoxic gut is essentially unperfused, and that in vitropredictedPv_CO2 rmaxunderestimates criticalPti_CO2.

     

Combined heart rate and activity improve estimates of oxygen consumption and carbon dioxide production rates

Moon, Jon K., and Nancy F. Butte. Combined heart rateand activity improve estimates of oxygen consumption and carbon dioxideproduction rates. J. Appl. Physiol.81(4): 1754-1761, 1996.Oxygen consumption(O₂) andcarbon dioxide production (CO₂) rates were measuredby electronically recording heart rate (HR) and physical activity (PA).Mean daily O₂ andCO₂ measurements by HR andPA were validated in adults (n = 10 women and 10 men) with room calorimeters. Thirteen linear and nonlinear functions of HR alone and HR combined with PA were tested as models of24-h O₂ andCO₂. Mean sleepO₂ andCO₂ were similar to basalmetabolic rates and were accurately estimated from HR alone[respective mean errors were 0.2 ± 0.8 (SD) and0.4 ± 0.6%]. The range of prediction errorsfor 24-h O₂ andCO₂ was smallestfor a model that used PA to assign HR for each minute to separateactive and inactive curves(O₂, 3.3 ± 3.5%; CO₂, 4.6 ± 3%). There were no significant correlations betweenO₂ orCO₂ errors and subject age,weight, fat mass, ratio of daily to basal energy expenditure rate, orfitness. O₂,CO₂, and energy expenditurerecorded for 3 free-living days were 5.6 ± 0.9 ml ^· min^{1 ·} kg¹,4.7 ± 0.8 ml ^· min^{1 ·} kg¹,and 7.8 ± 1.6 kJ/min, respectively. Combined HR and PA measured 24-h O₂ andCO₂ with a precisionsimilar to alternative methods.

     

Estimating exercise stroke volume from asymptotic oxygen pulse in humans

Whipp, Brian J., Michael B. Higgenbotham, and Frederick C. Cobb. Estimating exercise stroke volume from asymptotic oxygenpulse in humans. J. Appl. Physiol.81(6): 2674-2679, 1996.Noninvasive techniques have been devisedto estimate cardiac output () during exercise toobviate vascular cannulation. However, although these techniques arenoninvasive, they are commonly not nonintrusive to subjects'spontaneous ventilation and gas-exchange responses. We hypothesizedthat the exercise stroke volume (SV) and, hence, might be accurately estimated simply from the response pattern of twostandardly determined variables:O₂ uptake(O₂) and heart rate (HR).Central to the theory is the demonstration that the product of and mixed venousO₂ content is virtually constant (k) during steady-state exercise. Thus from the Fickequation, O₂ =  ^· Ca_CO2  k, whereCa_CO2 is the arterialCO₂ content, theO₂ pulse(O₂-P) equalsSV ^· Ca_CO2  (k/HR). Because the arterial O₂ content(Ca_O2) is usually relatively constant innormal subjects during exercise,O₂-P should change hyperbolicallywith HR, asymptoting atSV ^· Ca_O2. Inaddition, because the asymptoticO₂-P equals the slope (S) of thelinear O₂-HR relationship,exercise SV may be predicted as S/Ca_O2.We tested this prediction in 23 normal subjects who underwent a 3-minincremental cycle-ergometer test with direct determination ofCa_O2 and mixed venous O₂content from indwelling catheters. The predicted SV closely reflected the measured value (r = 0.80). Wetherefore conclude that, in normal subjects, exercise SV may beestimated simply as five times S of the linearO₂-HRrelationship (where 5 is approximately 1/Ca_O2).

     

Influence of modified oxygen and carbon dioxide atmospheres on mint and thyme plant growth, morphogenesis and secondary metabolism in vitro

. Growth (fresh weight) and morphogenesis (production of leaves, roots and shoots) of mint (Mentha sp. L.) and thyme (Thymus vulgaris L.) shoots were determined under atmospheres of 5%, 10%, 21%, 32%, or 43% O₂ with either 350 or 10,000 µmol mol^-1 CO₂. Plants were grown in vitro on Murashige and Skoog salts, 3% sucrose and 0.8% agar under a 16/8-h (day/night) photoperiod with a light intensity of 180 µmol s^-1 m^-2. Growth and morphogenesis responses varied considerably for the two plant species tested depending on the level of O₂ administered. Growth was considerably enhanced for both species under all O₂ levels tested when 10,000 µmol mol^-1 CO₂ was added as compared to growth responses obtained at the same O₂ levels tested with 350 µmol mol^-1 CO₂. Mint shoots exhibited high growth and morphogenesis responses for all O₂ levels tested with 10,000 µmol mol^-1 CO₂. In contrast, thyme shoots exhibited enhanced growth and morphogenesis when cultured in ₁% O₂ with 10,000 µmol mol^-1 CO₂ included compared to shoots cultured under lower O₂ levels. Essential oil compositions (i.e. monoterpene, piperitenone oxide from mint and aromatic phenol, thymol from thyme) were analyzed from CH₂Cl₂ extracts via gas chromatography from the shoot portion of plants grown at all O₂ levels. The highest levels of thymol were produced from thyme shoots cultured under 10% and 21% O₂ with 10,000 µmol mol^-1 CO_2,and levels were considerably lower in shoots grown under either lower or higher O₂ levels. Higher levels of piperitenone oxide were obtained from mint cultures grown under ₁% O₂ with 10,000 µmol mol^-1 CO₂ compared to that obtained with lower O₂ levels.     

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.     

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     

Effect of oxygen on photosynthesis and biosynthesis of glycolate in photoheterotrophically grown cells of Rhodospirillum rubrum

Photosynthetic CO₂ fixation was studied using cells of Rhodospirillumrubrum grown heterotrophically on malate or butyrate. Ratesof CO₂ fixation were higher in the malategrown cells than inthe butyrate-grown bacteria but ribulosebisphosphate (RUP₂)carboxylase/oxygenase activities were higher in the extractsprepared from the butyrate-grown bacteria. The photosyntheticCO₂ fixation in the butyrate-grown R. rubrum cells was inhibitedby KCN, and the inhibitory effect of O₂ on CO₂ fixation wasreversed when cells were returned to an N₂ atmosphere. In themalate-grown cells, photosynthetic CO₂ fixation was insensitiveto KCN and the inhibitory effect exerted by O₂ was practicallyirreversible. ¹⁴CO₂ was not incorporated into glycolate by either malate-or butyrate-grown cells in an N₂ atmosphere, but small amountsof labeled glycolate were found in malate- and butyrate-growncells in air or 100% O₂. Glycolate excreted by these cells in100% O₂ was measured colorimetrically and its identity establishedby mass spectrometry. When the O₂ atmosphere was labeled with¹⁸O₂, only one of the carboxyl oxygens of the excreted glycolatewas labeled, and the enrichment of ¹⁸O in this carboxyl oxygenrelative to the ¹⁸O₂ provided was greater than 80%. These studiesshow that significant glycolate production by R. rubrum onlyoccurs in the presence of O₂ and that in both malateand butyrate-growncells, the glycolate so formed is presumably produced via RuP₂oxygenase. ¹ Paper No. 46 in the series "Structure and Function of ChloroplastProteins", and research supported in part by research grantsfrom the Japanese Ministry of Education (No. 211113), the TorayScience Foundation (Tokyo), and the Nissan Science Foundation(Tokyo). (Received August 19, 1978; )     

A dual-respiration chamber system with automated calibration

Schoffelen, Paul F. M., Klaas R. Westerterp, Wim H. M. Saris, and Foppe Ten Hoor. A dual-respiration chambersystem with automated calibration. J. Appl.Physiol. 83(6): 2064-2072, 1997.This studycharacterizes respiration chambers with fully automated calibration.The system consists of two 14-m³pull-type chambers. Care was taken to provide a friendly environment for the subjects, with the possibility of social contact during theexperiment. Gas analysis was automated to correct for analyzer driftand barometric pressure variations and to provide ease of use. Methodsused for checking the system's performance are described. Thegas-analysis repeatability was within 0.002%. Results of alcohol combustion (50-350 ml/minCO₂) show an accuracy of 0.5 ± 2.0 (SD) % for O₂consumption and 0.3 ± 1.6% forCO₂ production for 2- to 24-hexperiments. It is concluded that response time is not the main factorwith respect to the smallest practical measurement interval (duration);volume, mixing, gas-analysis accuracy, and levels ofO₂ consumption andCO₂ production are at leastequally important. The smallest practical interval was 15-25 min,as also found with most chamber systems described in the literature. We chose to standardize 0.5 h as the minimum measurementinterval.

     

Chronic intermittent hypoxia increases sympathetic responsiveness to hypoxia and hypercapnia

We sought to determine whether chronic exposure tointermittent hypoxia (CIH) increases sympathetic responsiveness tosubsequent chemoreflex stimulation. Sprague-Dawley rats were exposed to30 days of CIH: exposure chamber%O₂ [fractionalconcentration of chamber O₂(Fc_O2)]nadir 6.5-7% with return to 21% each minute for 8 h/day duringthe diurnal sleep period (Exp group). Sham controls (SC group) weresimilarly handled but kept at 21%Fc_O2 andcompared with unhandled controls (UC group). Rats were then anesthetized with urethan, and preganglionic cervical sympathetic activity (CSA), diaphragm electromyogram, arterial pressure, and electrocardiogram were recorded while the rats were spontaneously breathing 100% O₂, room air, 10%O₂, 12%CO₂, and 10%O₂-12%CO₂. CSA and heart rate were alsorecorded during phenylephrine infusion to assess baroreceptor function.Mean arterial pressure was significantly greater in Exp than in SC andUC rats during all conditions (P < 0.05). A vasopressor response to 10%O₂-12%CO₂ was observed only in Exp rats.CSA was greater in Exp than in SC and UC rats during 10%O₂, 12%CO₂, and 10%O₂-12%CO₂ but not during room-air exposure. A significant increase in CSA compared with room air wasnoted during 10% O₂, 12%CO₂, and 10%O₂-12%CO₂ in Exp but not in SC or UCrats. No differences in baroreceptor function were observed amonggroups. We conclude that CIH leads to increased sympatheticresponsiveness to chemoreflex stimulation.

     

Measurement of the Carbon Dioxide Compensation Point and the Rate of Loss of 14CO2 in the Light and Dark in Some Bryophytes

The CO₂ compensation point at 25 °C and 250 µEinsteinsm^–2 s^–1 wasmeasured for 27 bryo-phyte species, andwas found to be in the range of 45–160 µl CO₂ I^–1air. Under the same conditions Zea mays gave a value of 11 µlI^–1 and Horde um vulgare 76 µI^–1. The rate of loss of photosyntheticallyfixed ¹⁴CO₂ in the light and dark in six bryophytes (three mosses,two leafy liverworts, one thalloid liverwort) was determinedin CO₂-free air and 100% O₂. The rate of ¹⁴CO₂ evolution inthe light was less than that in the dark in CL₂-free air, butin 100% O₂ the rate in the light increased, so that in all butthe leafy liverworts it was greater than that in the dark. Raisingthe temperature tended to increase the rate of 14CO₂ evolutioninto CO₂-free air both in the light and dark, so that the light/dark(L/D) ratio did not greatly vary. The lower rate of loss of¹⁴CO₂ in the light compared tothe dark could be due to partialinhibition of ‘dark respiration’ reactions in thelight, a low rate of glycolate synthesis and oxidation, or partialreassimilation of the ¹⁴CO₂ produced, or a combination of someor all of these factors.     

Furosemide reduces accumulated oxygen deficit in horses during brief intense exertion

Hinchcliff, K. W., K. H. McKeever, W. W. Muir, and R. A. Sams. Furosemide reduces accumulated oxygen deficit inhorses during brief intense exertion. J. Appl.Physiol. 81(4): 1550-1554, 1996.We theorizedthat furosemide-induced weight reduction would reduce the contributionof anaerobic metabolism to energy expenditure of horses during intenseexertion. The effects of furosemide on accumulatedO₂ deficit and plasma lactateconcentration of horses during high-intensity exercise were examined ina three-way balance randomized crossover study. Nine horses completedeach of three trials: 1) a control(C) trial, 2) a furosemide-unloaded(FU) trial in which the horse received furosemide 4 h before running, and 3) a furosemide weight-loaded(FL) trial during which the horse received furosemide and carriedweight equal to the weight lost after furosemide administration. Horsesran for 2 min at ~120% maximalO₂ consumption. Furosemide (FU)increased O₂ consumption (ml ^· 2 min^{1 ·} kg¹)compared with C (268 ± 9 and 257 ± 9, P < 0.05), whereas FL was notdifferent from C (252 ± 8). AccumulatedO₂ deficit (ml O₂ equivalents/kg) wassignificantly (P < 0.05) lowerduring FU (81.2 ± 12.5), but not during FL (96.9 ± 12.4), thanduring C (91.4 ± 11.5). Rate of increase in blood lactateconcentration (mmol ^· 2 min^{1 ·} kg¹)after FU (0.058 ± 0.001), but not after FL (0.061 ± 0.001), was significantly (P < 0.05) lower than after C (0.061 ± 0.001). Furosemide decreased theaccumulated O₂ deficit and rate ofincrease in blood lactate concentration of horses during briefhigh-intensity exertion. The reduction in accumulatedO₂ deficit in FU-treated horseswas attributable to an increase in the mass-specific rate ofO₂ consumption during thehigh-intensity exercise test.

     

Ventilatory and metabolic responses to ambient hypoxia or hypercapnia in rats exposed to CO hypoxia

Gautier, Henry, Cristina Murariu, and Monique Bonora.Ventilatory and metabolic responses to ambient hypoxia orhypercapnia in rats exposed to CO hypoxia. J. Appl. Physiol.83(1): 253-261, 1997.We have investigated at ambienttemperatures (T_am) of 25 and5°C the effects of ambient hypoxia(Hx_am; fractional inspired O₂ = 0.14) and hypercapnia(fractional inspiredCO₂ = 0.04) on ventilation (),O₂ uptake(O₂), andcolonic temperature (T_c) in 12 conscious rats before and after carotid body denervation (CBD). Therats were concomitantly exposed to CO hypoxia (Hx_CO; fractional inspired CO = 0.03-0.05%), which decreases arterial O₂ saturation by ~25-40%.The results demonstrate the following. 1) AtT_am of 5°C, in both intact andCBD rats,/O₂ islarger when Hx_am orCO₂ is associated withHx_CO than with normoxia. At T_am of 25°C, this is also thecase except for CO₂ in CBD rats. 2) AtT_am of 5°C, the changes inO₂ andT_c seem to result from additiveeffects of the separate changes induced byHx_am,CO₂, andHx_CO. It is concluded that, inconscious rats, central hypoxia does not depress respiratory activity.On the contrary, particularly whenO₂ is augmented during acold stress, both/O₂during Hx_CO and the ventilatoryresponses to Hx_am andCO₂ are increased. The mechanismsinvolved in this relative hyperventilation are likely to involvediencephalic integrative structures.

     

Pulmonary diffusing capacities for oxygen-labeled CO2 and nitric oxide in rabbits

Heller, Hartmut, Gabi Fuchs, and Klaus-DieterSchuster. Pulmonary diffusing capacities foroxygen-labeled CO₂ and nitric oxide in rabbits.J. Appl. Physiol. 84(2): 606-611, 1998.We determined the pulmonary diffusing capacity(DL) for¹⁸O-labeledCO₂(C¹⁸O₂)and nitric oxide (NO) to estimate the membrane component of therespective gas conductances. Six anesthetized paralyzed rabbits wereventilated by a computerized ventilatory servo system. Single-breath maneuvers were automatically performed by inflating the lungs with gasmixtures containing 0.9%C¹⁸O₂or 0.05% NO in nitrogen, with breath-holding periods ranging from 0 to1 s forC¹⁸O₂and from 2 to 8 s for NO. The alveolar partial pressures of C¹⁸O₂and NO were determined by using respiratory mass spectrometry. DL was calculated from gasexchange during inflation, breath hold, and deflation. We obtainedvalues of 14.0 ± 1.1 and 2.2 ± 0.1 (mean value ± SD)ml · mmHg¹ · min¹forDL_C¹⁸O2and DL_NO,respectively. The measured DL_C¹⁸O2/DL_NOratio was one-half that of the theoretically predicted value accordingto Graham's law (6.3 ± 0.5 vs. 12, respectively).Analyses of the several mechanisms influencing the determination ofDL_C¹⁸O2and DL_NOand their ratio are discussed. An underestimation of the membranediffusing component for CO₂ isconsidered the likely reason for the lowDL_C¹⁸O2/DL_NOratio obtained.

     

Physiological and Genomic Consequences of Intermittent Hypoxia: Selected Contribution: Chemoreflex responses to CO2 before and after an 8-h exposure to hypoxia in humans

The ventilatorysensitivity to CO₂, in hyperoxia, is increased after an 8-hexposure to hypoxia. The purpose of the present study was to determinewhether this increase arises through an increase in peripheral orcentral chemosensitivity. Ten healthy volunteers each underwent 8-hexposures to 1) isocapnic hypoxia, with end-tidalPO₂ (PET_O2) = 55 Torr and end-tidal PCO₂(PET_CO2) = eucapnia; 2)poikilocapnic hypoxia, with PET_O2 = 55 Torr and PET_CO2 = uncontrolled;and 3) air-breathing control. The ventilatory response toCO₂ was measured before and after each exposure with theuse of a multifrequency binary sequence with two levels of PET_CO2: 1.5 and 10 Torr above the normalresting value. PET_O2 was held at 250 Torr.The peripheral (Gp) and the central (Gc) sensitivities were calculatedby fitting the ventilatory data to a two-compartment model. There wereincreases in combined Gp + Gc (26%, P < 0.05),Gp (33%, P < 0.01), and Gc (23%, P = not significant) after exposure to hypoxia. There were no significant differences between isocapnic and poikilocapnic hypoxia. We conclude that sustained hypoxia induces a significant increase inchemosensitivity to CO₂ within the peripheral chemoreflex.

     

Response of Wheat Seedlings to Low O2 Concentrations in Nutrient Solution: I. GROWTH, O2 UPTAKE AND SYNTHESIS OF FERMENTATIVE END-PRODUCTS BY ROOT SEGMENTS

Root growth of 7-d-old wheat (Triticum aestivum cv. Gamenya)seedlings was impaired at dissolved O₂ concentrations of 0.01and 0.055 mol m^–3 O₂, while growth at 0.115 mol m^–3O₂ was the same as that in continuously aerated controls (0.26mol m^–3 O2). Oxygen uptake by apical (0–2 mm), expanding (2–4mm) and expanded (10–12 mm) tissues of the roots decreasedbelow 0.16, 0.09 and 0.05 mol m^–3 O₂, respectively. Thishierarchy is consistent with the metabolic rates of these tissues.There was a small (c. 9%) inhibition of O₂ uptake and some netsynthesis of ethanol and alanine in root apices at 0.115 molm^–3 O₂. Significant amounts of anaerobic end-productsaccumulated at 0.055 mol m^–3 O₂ and even more so at 0.01mol m^–3 O₂, indicating that oxidative phosphorylationwas strongly inhibited. Net alanine synthesis increased in fully expanded (10–16mm) tissues exposed to <0.003–0.01 mol m^–3 O₂,and this increase was accompanied either by a proportionallysmaller increase in the concentration of other free amino acidsor by a net decrease in free amino acid levels excluding alanine.This suggests that alanine was synthesized as an end-productof anaerobic catabolism and did not accumulate simply becauseof decreased net protein synthesis. Comparing the carbon flow to CO₂, ethanol, lactate and alaninein roots at 0.01 mol m^–3 O₂ with carbon loss as CO₂ inaerated roots suggests that carbon flow to products of metabolismwas not greatly enhanced due to O₂ deficiency. This infers,but does not prove that, in wheat, generation of energy duringperiods of O₂ deficiency is not enhanced due to a Pasteur effect. Key words: Anaerobic, fermentation, oxygen, wheat     

Effect of Elevated CO2 on Stomatal Density and Distribution in a C4 Grass and a C3 Forb under Field Conditions

Two common tallgrass prairie species, Andropogon gerardii, thedominant C₄ grass in this North American grassland, and Salviapitcheri, a C₃ forb, were exposed to ambient and elevated (twiceambient) CO₂ within open-top chambers throughout the 1993 growingseason. After full canopy development, stomatal density on abaxialand adaxial surfaces, guard cell length and specific leaf mass(SLM; mg cm^-2) were determined for plants in the chambers aswell as in adjacent unchambered plots. Record high rainfallamounts during the 1993 growing season minimized water stressin these plants (leaf xylem pressure potential was usually >-1·5 MPa in A. gerardii) and also minimized differencesin water status among treatments. In A. gerardii, stomatal densitywas significantly higher (190 ± 7 mm^-2; mean ±s.e.) in plants grown outside of the chambers compared to plantsthat developed inside the ambient CO₂ chambers (161 ±5 mm^-2). Thus, there was a significant 'chamber effect' on stomataldensity. At elevated levels of CO₂, stomatal density was evenlower (P < 0·05; 121 ± 5 mm^-2). Most stomatawere on abaxial leaf surfaces in this grass, but the ratio ofadaxial to abaxial stomatal density was greater at elevatedlevels of CO₂. In S. pitcheri, stomatal density was also significantlylower when plants were grown in the open-top chambers (235 ±10 mm^-2 outside vs. 140 ± 6 mm^-2 in the ambient CO₂ chamber).However, stomatal density was greater at elevated CO₂ (218 ±12 mm^-2) compared to plants from the ambient CO₂ chamber. Theratio of stomata on adaxial vs. abaxial surfaces did not varysignificantly in this herb. Guard cell lengths were not significantlyaffected by growth in the chambers or by elevated CO₂ for eitherspecies. Growth within the chambers resulted in lower SLM inS. pitcheri, but CO₂ concentration had no effect. In A. gerardii,SLM was lower at elevated CO₂. These results indicate that stomataland leaf responses to elevated CO₂ are species specific, andreinforce the need to assess chamber effects along with treatmenteffects (CO₂) when using open-top chambers.Copyright 1994, 1999Academic Press Andropogon gerardii, elevated CO₂, Salvia pitcheri, stomatal density, tallgrass prairie     

Carbohydrate Metabolism of Rice Seedlings Grown in Oxygen Deficient Solution

Atwell, B. J. and Greenway, H. 1987. Carbohydrate metabolismof rice seedlings grown in oxygen deficient solution.—J.exp. Bot. 38: 466–478. Rice seedlings (Oryza sativa L.) were grown in the dark forup to 4 d in solutions containing various concentrations ofO₂. The rate of depletion of the endosperm was most rapid inaerated solution (0·25 mol O₂ m^–3), largely dueto the inhibition of growth of seedlings at very low O₂ concentrations.Earlier suggestions that there is a deficit of sugars for growthand energy generation in O₂ deficient coleoptiles were tested. Coleoptiles, shaking in aerated solution, respired about one-thirdof the endogenous sugars to CO₂ and incorporated the rest intostructural compounds. In contrast, the proportion of carbonwhich went to growth in anoxia was very low. Consistent withthese results, endogenous sugar levels were generally highestat low O₂ concentrations. Even so, coleoptiles grown and testedas low as 0·03 mol O₂ m^–3 showed appreciable metabolismof exogenous ¹⁴C-glucose to CO₂, soluble and insoluble compounds,suggesting that a minimal O₂ supply was sufficient to sustainsome growth. Furthermore, glucose feeding caused little or norise in O₂ uptake or tissue sugar levels. Similarly, the specificactivity of the evolved CO₂ was not markedly different in coleoptilesgrowing at 0·03 and 0·25 mol O₂ m^–3 Further evidence was obtained to show that endogenous substrateswere adequate for growth and respiration at both low and highO₂ concentrations. Exogenous glucose and malate did not stimulateO₂ uptake at any stage of growth in aerated coleoptiles. Therewas sufficient endogenous substrate to sustain a 35–45%rise in O₂ uptake induced by uncoupling and enrichment withO₂. Exogenous glucose did not stimulate growth of intact seedlingsat any O₂ concentration. Key words: Rice seedlings, carbohydrate metabolism, oxygen deficient solution     

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     

Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

Wheat plants were grown in a controlled environment with daytemperatures of 18 ?C and with 500 µ Einsteins m^–28^–1 of photosynthetically active radiation for 16 h. Beforeanthesis and 2 to 3 weeks after, rates of net photosynthesiswere measured for leaves in 2 or 21% O₂ containing 350 vpm CO₂at 13, 18, 23, and 28 ?C and with 500 µEinsteins m^–2s^–1 of photosynthetically active radiation. Also, underthe same conditions of light intensity and temperature, therates of efflux of CO₂ into CO₂-free air were measured and,for mature flag leaves 3 to 4 weeks after anthesis, gross andnet photosynthesis from air containing 320 vpm ¹⁴CO₂ of specificactivity 39?7 nCi µmol^–1. When the O₂ concentration was decreased from 21 to 2% (v/v)the rate of net photosynthesis increased by 32 per cent at thelowest temperature and 54 per cent at the highest temperature.Efflux of CO₂ into CO₂-free air ranged from 38 per cent of netphotosynthesis at 13 ?C to 86 per cent at 28 ?C. Gross photosynthesis,measured by the ¹⁴C assimilated during 40 s, was greater thannet photosynthesis by some 10 per cent at 13 ?C and 17 per centat 28 ?C. These data indicate that photorespiration was relativelygreater at higher temperatures.