Studies on the effect of certain enzymic poisons on the metabolism of storage organs IV. Respiration and nitrogen assimilation by radish root slices in relation to feeding with KCN

At low and high concentrations, respectively, KCN stimulatedand inhibited the CO₂ output of radish root slices. Analysisof tissue slices showed that CO₂ production is nearly accountedfor by the loss in carbohydrates. After 48 hr, the content ofprotein-N was slightly changed in all the cyanide-treated tissues;whereas, that of soluble-N decreased markedly at high cyanideconcentrations and hardly changed at low concentrations. Leakageof soluble-N occurred at high concentrations of cyanide. The nitrogen compounds taken up were incorporated into the nitrogenpool of radish slices. KCN retarded uptake of nitrogen compounds,inhibited their utilization and led to excessive leakage ofsoluble-N from the tissues into the media. Increased CO₂ outputof slices in nitrogen media was largely accounted for by theloss in carbohydrates from the tissues. High concentrations of cyanide are thought to inhibit CO₂ outputthrough inhibition of the oxidase system and accompanying phosphorylations.The present results with low concentrations further substantiatethe uncoupling theory as a main factor involved in respiration. ¹Permanent address: Department of Botany, Faculty of Science,University of A'in Shams, Abbassia, Cairo, Egypt, U.A.R. (Received October 3, 1969; )     

Photosynthesis in Cell Suspension Culture of a C4 Plant, Gisekia pharnaceoides L.

Photomixotrophic cell suspension culture was established fromthe leaf derived callus cells of Gisekia pharnaceoides L., aC₄ dicotyledonous weed. The late log phase cells possessed shade-typecharacters such as low chlorophyll a/b ratio, less pronouncedO₂ evolution and CO₂ fixation, saturation of photosyntheticCO₂ fixation at low intensity. The chloroplasts from these cellscontained granal stacking with high degree of a very few granawhich are characterized by their wide and high degree of stackings. The predominant labelling of 3-phosphoglyceric acid and sugarphosphates (40% of the total ¹⁴C incorporated) during 5 s exposureto ¹⁴CO₂ in light and subsequent decrease in percentage of ¹⁴Cin these compounds with increase in exposure time indicatedthe operation of the C₃ pathway in these cells. The simultaneoussynthesis of malate (23% of the total ¹⁴C incorporated) is relatedto the much pronounced glycolytic and tricarboxylic acid cycleactivities in these cells. The initial proliferation of callimainly from the zones of vascular supplies in the leaf, highstarch content of the cells, presence of large starch grainsin all the chloroplasts, activities of Calvin cycle enzymes,heavy labelling of C₃ type intermediates and less labellingof aspartate as early photosynthates and rapid accumulationof radioactivity into starch during ¹⁴CO₂ assimilation indicatedthat most of the cells in photomixotrophic culture were derivedfrom bundle sheath cells or the leaf cells of Gisekia changetheir function under culture conditions. ¹Present address: Tropical Botanic Garden and Research Institute,Navaranga Road, Trivandrum 695 011, India. (Received January 29, 1982; Accepted June 4, 1983)     

Studies on organic acid metabolism and ethylene production during controlled atmosphere storage of apples {Mallus pumila MILLER, cv. Rolls)

Organic acid metabolism and ethylene formation during controlledatmosphere storage (CA-storage) of apples (Mallus pumila MILLER,cv Rolls) were studied. A higher titratable acidity was observedin apples during CA-storage as compared to those in air control.The incorporation of atmospheric ¹⁴CO₂ into malic acid was greaterin apples stored in the higher CO₂ concentration. The conversionof succinic acid-¹⁴C into fumaric acid-¹⁴C was slightly lessin the apple in modified high carbon dioxide atmosphere thanthose in air. O₂ uptake and CO₂ output by apple slices weremarkedly inhibited by the addition of succinic and malic acidsat a concentration higher than 25 mM. These factors seem to be the possible cause of a higher acidityof fruits stored in CA-condition. Ethylene production from wholefruits or tissue slices was markedly inhibited under CA-condition. The retardation of acid metabolism and the inhibition of ethyleneproduction of apples during CA-storage seem to be the importantfactors which help to maintain their storage quality. (Received March 18, 1970; )     

Studies on the effect of certain enzymic poisons on the metabolism of storage organs III. Uptake and utilization of sucrose by radish root slices in presence of iodoacetate

Iodoacetate greatly retarded the uptake of sucrose and slightlyaffected its inversion by radish root slices. Carbohydrate content of the samples decreased substantiallyboth in water and in iodoacetate. Feeding with sucrose led tomarked accumulation of carbohydrates and supplemental additionof iodoacetate induced less accumulation of carbohydrates. Iodoacetate caused exudation of nitrogen fractions into theculture media. Protein synthesis via amino acids seems to beoperative in iodoacetate treated slices. It is also suggestedthat nitrate-N, in presence of sucrose, is converted into peptidesand proteins. Addition of iodoacetate to sucrose media inhibitedthis pathway of protein synthesis. Both sucrose and iodoacetate (4 x 10^–4M) stimulated theCO₂ output whereas 25 x 10^–4M iodoacetate did not changethe CO₂ output when compared with that of controls. Sucroseand iodoacetate (4 x 10^–4 M) when joined together maskedthe accelerating effect of each other. ¹Present address: Department of Botany, Faculty of Science,University of A'in Shams, Abbassia, Cairo, Egypt, U. A. R. (Received November 6, 1968; )     

Environmental Influences on Carbon Recycling in a Terrestrial CAM Bromeliad, Bromelia humilis Jacq.

The effects of night-time temperature, leaf-to-air vapour pressuredeficit (VPD) and water stress on CO₂ recycling in Bromeliahumilis Jacq. grown under two light and nitrogen regimes wereinvestigated. At night-time temperatures above 30°C, integratednet dark CO₂ uptake was severely reduced and CO₂ for malatesynthesis was mainly derived from dark respiration. At 35°C,up to 84% of the CO₂ liberated by dark respiration was refixedinto malic acid. Below 30 °C only nitrogen deficient plantsshowed significant recycling. No significant differences wereobserved between high and low light grown plants in CO₂ recycling.A doubling of leaf-to-air VPD from 7-46 Pa kPa^–1 to 15.49Pa kPa^–1 resulted in a 2- to 20-fold decrease in leafconductance and about 50 to 65% reduction in integrated darkCO₂ uptake. However, about twice as much CO₂ was recycled atthe higher VPD as in the lower. Ten days of water stress resultedin 80 to 100% recycling of respiratory CO₂. Under high VPD andwater stress treatments, the amount of water potentially savedthrough recycling of CO₂ reached 2- to 6-fold of the actualtranspiration. In general, nitrogen deficient plants had higherper cent recycling of respiratory CO₂ in response to high night-timetemperature, increased VPD or water stress. The results emphasizethe ecological relevance of carbon recycling in CAM plants. Key words: Bromelia humilis, CAM, PPFD, dark respiration, temperature, VPD, water stress     

STUDIES ON THE RE-ASSIMILATION OF RESPIRATORY CO2 IN ILLUMINATED LEAVES

Experiments were performed, using rice, barley and Hydrangealeaves, to examine the re-assimilation of respiratory ¹⁴CO₂while photosynthesis is going on in an open air flow system. It was found that the leaves which had assimilated ¹⁴CO₂ beforehandevolved, when kept under photosynthesizing conditions, threeto four tenths (variable according to plant species and externalconditions) of the amount of ¹⁴CO₂ to be produced in the dark.Such an incomplete re-utilization of ¹⁴CO₂ was observed alsoin spinach leaf homogenate as well as in the leaves which hadpreviously absorbed ¹⁴C-glucose. The ¹⁴CO₂ output in rice leaves was found to be acceleratedby the light of high intensity. A possibility of light stimulationon the respiration was suggested. (Received October 7, 1961; )     

Changes in respiratory metabolism in tissue cultures of carrot root

Changes in respiratory metabolism accompanied by callus formationin cultured explants of carrot root were followed and the followingresults were obtained. 1) When the explant was cultured on amedium containing kinetin and 2,4-D, active cell division occurredand resulted in callus formation by the 9th–12th days.2) Fresh weight remarkably increased after a lag-time of about5 days. Changes in protein content on fresh weight basis weresimilar to changes in fresh weight. 3) Respiration rate increasedduring the first few days, when growth could not be distinctlymeasured. Accompanying the rise in respiration, the C₆/C₁ ratioalso increased. As callus developed, the respiratory rate andC₆/C₁ ratio gradually decreased and RQ, became higher than unity.4) Alcohol dehydrogenase activity increased between the 4thand 9th days after culture. 5) When sub-cultured callus tissuewas fed with G-U-¹⁴C, some radioactivity was detected in thealcohol of the tissues. 6) These results suggest that duringthe first 4–6 days after culture the activity of the EMBDEN-MEYERHOF-PARNAS-TCApathway was remarkably increased and, as callus developed, therelative participation of the pentose phosphate pathway graduallyincreased and simultaneously alcohol fermentation occurred. (Received December 13, 1968; )     

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 (     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 3. Time-course Study on 13C Incorporation into Soluble Metabolites and Respiratory Evolution of 13CO2 from Roots and Nodules

Well-nodulated soya bean (Glycine max L.) plants were allowedto assimilate ¹³CO₂ for 10 h in the light, under steady-stateconditions in which CO₂ concentration and ¹³C abundance wereboth strictly controlled at constant levels. The respiratoryevolution of ¹³CO₂ from roots and nodules and ¹³C incorporationinto various metabolic fractions were measured during the ¹³CO₂feeding and subsequent 48 h chase period. CO₂ respired from nodules was much more rapidly labelled with¹³C than that from roots. The level of labelling (percentageof carbon currently assimilated during the ¹³COM₂ feeding period)of CO₂ respired from nodules reached a maximum of about 87 percent after 4 h of steady-state ^l3CO₂ assimilation and thereafterremained fairly constant. The absolute amount of labelled carbonevolved by the respiration of the nodules during the 10 h ¹³CO₂feeding period was 1·5-fold that of root respiration.These results demonstrated that the currently assimilated (labelled)carbon was preferentially used to support nodule respiration,while root respiration relied considerably on earlier (non-labelled)carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilatedcarbon in all tissues of the plants, since the levels of labellingaccounted for 86–91 per cent at the end of the ¹³CO₂ feeding.In the nodules, the kinetics and levels of sucrose labellingwere in fairly good agreement with those of respired CO₂, whilein the roots, the level of labelling of respired CO₂ was significantlylower than that of sucrose. Succinate and malate were highly labelled in both roots andnodules but they were labelled much more slowly than sucroseand respired CO₂. The kinetics and levels of labelling of theseKrebs cycle intermediates resembled those of major amino acidswhich are derived directly from Krebs cycle intermediates. Itis suggested that large fractions of organic acids in noduleswere physically separate from the respiration site. Glycine max L., Soya bean, ¹³CO₂ assimilation, respiratory evolution of ¹³CO₂, carbon metabolism in root nodules     

Studies on the effect of certain enzymic poisons on the metabolism of storage organs II. Differential effects of iodoacetate on the respiratory metabolism and permeability barriers of radish root slices

Low and high concentrations of iodoacetate stimulated and inhibitedthe CO₂ production, respectively. Soluble sugars in tissue slicesdecreased sharply in high iodoacetate and showed little decreasein low concentrations of the inhibitor. The same pattern ofchanges was observed with soluble nitrogen fractions. Polysaccharidesand proteins were subjected to slight, if any changes. Leakage of different metabolites from tissue slices was operativein low and high concentrations of iodoacetate; the magnitudeof leakage being increased with an increase in concentrationof the inhibitor. That low concentrations of iodoacetate may have increased CO₂output through increased accessibility of substrates to enzymesin the cytoplasm, was suggested. In high concentrations, iodoacetatewas thought to have, possibly, a general effect on cell permeabilityand enzymes and by so doing to have led to increased leakageand inhibition of respiration. ¹Present address: Department of Botany, Faculty of Science,University of Khartoum, Khartoum, Sudan (Received March 1, 1968; )     

Studies in the Respiratory and Carbohydrate Metabolism of Plant Tissues1: XVII. THE EFFECT OF IODOACETATE ON THE CO2 OUTPUT, PATHWAYS OF GLUCOSE RESPIRATION AND CONTENTS OF PHOSPHATE ESTERS IN STRAWBERRY LEAVES

When specifically labelled glucose was fed to strawberry leaves,the C₆/C₁, quotient (rate of release of ¹⁴CO₂ from glucose-6-¹⁴C/rateof release of ¹⁴CO₂ from glucose-1¹⁴C ranged from 0.27 to 0.35in leaves in water and from 0.46 to 0.96 in leaves fed withiodoacetate. These quotients indicate that both the glycolyticand the pentose phosphate pathways participate in the respirationof strawberry leaves, with a greater contribution from the formerin the iodoacetate increased CO₂ output. Concurrently with the increase of CO² output in iodoacetate,the contents of glucose-6-phosphate (G6P), fructose-6 (F6P)and fructose-1,6-diphosphate (FDP) increased greatly; therewas a smaller increase of phosphoenol-pyruvate (PEP). The increasein the CO₂ output in iodoacetate may be explained solely onthe basis that the increases of G6P and FDP accelerate the ratesrespectively of the pentose phosphate pathway and of glycolysisand traffic into the tricarboxylic acid cycle. The increasein content of G6P and FDP is attributed to an increase in theaccessibility of enzymes and substrates caused by iodoacetate.Alternatively the increased CO₂ output in iodoacetate may bepartly due to uncoupling of oxidative phosphorylation.     

The Flux of 14C-Labelled Photosynthate through Soyabean Root Nodules during N2 Fixation

Experiments are described which examine the flux of photosyntheticassimilates from leaves to nodules of soyabean during N₂ fixation.The first part, where the respiratory efflux of ¹⁴CO₂ by noduleswas used as a means of assessing the import of labelled photosynthatefrom leaves, shows that most ¹⁴CO₂ loss from nodulated rootsis due to the metabolic activity of nodules. Much less photosynthatewas imported by nodules if the metabolic activity associatedwith N₂ fixation was inhibited by low O₂ concentration. The second part describes the chemical fate of current photosynthateas it is utilized by nodules. Labelled material was detectedin nodules within c.15 min of supplying ¹⁴CO₂ to the leaf. Thisrose to a maximum at c.70 min before declining by 85% withinthe following 4 h. Most (80%) ¹⁴carbon imported by nodules waseither lost as respiratory ¹⁴CO₂ or re-exported as productsof N₂ fixation. Ten per cent of imported carbon was found asstructural material and 10% as starch. Of the ¹⁴C soluble in ethanol, most was found in the neutralfraction (80% declining to 50% as sucrose) with smaller amountsas amino acids, organic acids (each category rising from 10%to 20%) and phosphate esters (<5%). Comparison of the distribution of ¹⁴C among amino acids, amidesand ureides in the nodules with that of xylem exudates indicatedthat selected compounds were exported from nodules. The ¹⁴Cdata indicate that c.80% of the nitrogen exported from noduleswas in the form of ureides (mainly allantoic acid) and only10–12% as amides. Key words: Nodules, ¹⁴C-photosynthate, Respiration, Carbon flux     

Maintenance and Growth Components of Carbon Dioxide Efflux from Growing Pea Fruits

Carbon dioxide efflux from 5- to 20-day-old pea fruits was measuredfor plants grown in controlled environment at 15 °C and600 µmol s^–1 m^–2 photon flux density in a16 h photoperiod. The rate of CO₂ output per fruit increasedquickly from 0.005 to 0.018 mg CO₂ min^–1 during fruitelongation and subsequently more slowly to 0.030 mg CO₂ min^–1as the fruits inflated. On a d. wt basis the rate was highest,0.175 mg CO₂ g^–1 min^–1, in the youngest fruits anddeclined curvilinearly with increasing fruit weight to 0.02mg CO₂ g^–1 min^–1. Separation of maintenance andgrowth components was achieved by starvation methods and bymultiple regression analysis. From the latter method estimatesof the maintenance coefficient declined hyperbolically from150±8.7 mg carbohydrate g^–1 d. wt day^–1 inthe very young fruits (0.05 g) to 10.4±0.36 mg carbohydrateg^–1 d. wt day^–1 in older fruits (2.0 g). On a nitrogenbasis maintenance costs decreased from 2240 to 310 mg carbohydrateg^–1 nitrogen day^–1 while nitrogen concentrationfell from 6.7 to 3 per cent d. wt. A simple linear relationshipbetween maintenance cost per unit d. wt and nitrogen concentrationwas not observed. A growth coefficient of 50±6.7 mg carbohydrate g^–1growth (equivalent to a conversion efficiency, Y_G, of 0.95)was estimated for all fruits examined. The overall efficiency, Y, increased from a mean of 0.70 to0.85 during fruit elongation and subsequently declined to 0.80.For a given fruit weight, efficiency increased asymptoticallywith relative growth rate; both asymptote and slope of the relationshipincreased as the fruits grew. Pisum sativum L., garden pea, legume fruit, carbon dioxide efflux, maintenance respiration, growth respiration     

Characterization of Photosynthetic 14Carbon Assimilation by Potamogeton lucens L.

Photosynthetic assimilation of exogenous ¹⁴CO₂ and H¹⁴CO₃^–by the aquatic angiosperm Potamogeton lucens L. is reported.Equivalent maximum rates of assimilation (1.5 µmol s^–1m^–2) were obtained in the presence of saturating levelsof ¹⁴CO₂ (1.0 mol m^–3, pH 5.3) or H¹⁴CO₃^– (1.5 molm^–3, pH, 9.2). Under subsaturating ¹⁴CO₂ levels, bothgaseous diffusion and H¹⁴CO₃^– transport were shown tooperate simultaneously, such that maximal photosynthetic rateswere established. An induction lag of approximately 3 min was observed when exogenous¹⁴CO₂ was assimilated. A longer lag of approximately 12 minwas required, however, before linear assimilation rates wereestablished when H¹⁴CO₃ acted as the carbon source. The light-activatedH¹⁴CO₃^– transport system was found to be quite labile.A brief (5 min) dark treatment returned the system to the inactivestate. Bicarbonate transport was shown to be competitively inhibitedby CO₃^2–ions. The possibility is discussed that this formof inhibition may be common to many HCO₃^– assimilators. Preliminary polar cation transport studies (from lower to upperleaf surface) indicated an almost exact one to one relationshipbetween the rates of Na⁺ influx and efflux and H¹⁴CO₃^–assimilation. The possible relationship(s) between these transportprocesses and the requirement for electrical neutrality is brieflydiscussed.     

Photosynthetic Nitrogen Metabolism in High and Low CO2-adapted Scenedesmus: I. INORGANIC CARBON-DEPENDENT O2 EVOLUTION, NITRATE UTILIZATION AND NITROGEN RECYCLING

Larsson, M., Larsson, C.-M. and Guerrero, M. G. 1985. Photosyntheticnitrogen metabolism in high and low CO₂-adapted Scenedesmus.I. Inorganic carbon-dependent O2 evolution, nitrate utilizationand nitrogen recycling.—J. exp Bot. 36: 1373–1386 Scenedesmus obtusiusculus Chod. was grown on an inorganic mediumflushed with either air or air supplemented with 3% CO₂. Inair-grown cells, O₂ evolution dependent on low, but not high,HCO^–₃ concentrations was strongly inhibited by the carbonicanhydrase inhibitor acetazolamide. Cells grown with 3% CO² exhibitedlow rates of O2 evolution at low external inorganic C; however,after 30 min in air O2 evolution rates at low inorganic C approachedthose of air-grown cells. These results are compatible withthe view that Scenedesmus develops a ‘CO² concentratingmechanism’ in air, with carbonic anhydrase as an importantconstituent When 3% CO²-grown cells were subjected to air-level of CO²,just a transient decline in NO^–₃ utilization was observed,but in the presence of acetazolamide the rate of the processdecreased drastically in response to the decrease in the CO²level. In CO²-free air NO₃^– was taken up at high ratesbut in a deregulated manner, leading to release of NH₄⁺. A portionof the NO₃^– taken up in the absence of CO² was apparentlyassimilated Cellular nitrate reductase (NR) activity initially decreasedbut subsequently recovered after a transition from 3% CO² toair. In the presence of acetazolamide, a persistent decreasein NR activity was observed. Cellular glutamine synthetase (GS)activity increased after transition from 3% CO² to air, theactivity increase being unaffected by acetazolamide. NH₄⁺ releaseto the medium in the presence of L-methionine-D, L-sulphoximine(MSO) transiently increased in 3% CO²-grown cells in responseto a transfer to air. MSO-induced NH₄⁺ release was in fact higherin air-grown cells than in 3% CO²-grown cells. Glycollate wasinitially released after transition from 3% CO² to air, butthere was no difference in glycollate release between MSO-treatedand untreated cells. In air-adapted Scenedesmus, N recyclingseems to be of minor importance in comparison to primary N assimilation Key words: CO²-fixation, N recycling, nitrate uptake, Scenedesmus     

Carbon isotope discrimination in photosynthetic bark

We developed and tested a theoretical model describing carbon isotope discrimination during photosynthesis in tree bark. Bark photosynthesis reduces losses of respired CO₂ from the underlying stem. As a consequence, the isotopic composition of source CO₂ and the CO₂ concentration around the chloroplasts are quite different from those of photosynthesizing leaves. We found three lines of evidence that bark photosynthesis discriminates against ¹³C. First, in bark of Populus tremuloides, the '¹³C of CO₂ efflux increased from -24.2‰ in darkness to -15.8‰ in the light. In Pinus monticola, the '¹³C of CO₂ efflux increased from -27.7‰ in darkness to -10.2‰ in the light. Observed increases in '¹³C were generally in good agreement with predictions from the theoretical model. Second, we found that '¹³C of dark-respired CO₂ decreased following 2-3 h of illumination (P<0.01 for Populus tremuloides, P<0.001 for Pinus monticola). These decreases suggest that refixed photosynthate rapidly mixes into the respiratory substrate pool. Third, a field experiment demonstrated that bark photosynthesis influenced whole-tissue '¹³C. Long-term light exclusion caused a localized increase in the '¹³C of whole bark and current-year wood in branches of P. monticola (P<0.001 and P<0.0001, respectively). Thus bark photosynthesis was shown to discriminate against ¹³C and create a pool of photosynthate isotopically lighter than the dark respiratory pool in all three experiments. Failure to account for discrimination during bark photosynthesis could interfere with interpretation of the '¹³C in woody tissues or in woody-tissue respiration.     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Long chain fatty acid synthesis in developing castor bean seeds IV. The synthetic system in proplastids

The proplastid fraction containing no cytosol and mitochondrionwas isolated from developing castor bean endosperm by stepwisesucrose density centrifugation. This fraction possesses thecapacity to synthesize LFAs from [u-14C]sucrose, [u-¹⁴C]-glucose,[u-¹⁴C]G-1-P, [u-¹⁴C]G-6-P, [2-¹⁴C]pyruvate and [1-¹⁴C]acetate.Little was incorporated from [1-¹⁴C]pyruvate into LFAs, butmuch into ¹⁴CO_a. Addition of cytosol to the proplastid fractiondid not enhance the LFA synthesis. From these data, the wholepath from sucrose to LFAs through glycolytic path and pyruvatedecarboxylation seems to be located within the proplastid indeveloping castor bean endosperm. The difference in utilizationof substrates indicates that the rate of LFA synthesis in castorbean proplastids is limited at a step between sucrose and hexosephosphate. In addition, experiments with CO₂ output and LFAsynthesis from [1-¹⁴C]glucose, [6-¹⁴C]glucose and [u-¹⁴C]G-6-Pstrongly suggest that the path flow branches actively throughG-6-P to the pentose phosphate path and little through acetylCoAto the TCA cycle. (Received May 12, 1975; )     

Photosynthetic carbon metabolism in photoautotrophically and photomixotrophically cultured tobacco cells

Labeling patterns of light and dark ¹⁴CO₂-fixation in photoautotrophicallyand photomixotrophically cultured tobacco cells were determined.During short term ¹⁴CO₂ fixation under light, malate(C₃–C₃carboxylation) was heavily labeled as were phosphoglyceric acidand sugar phosphates(C₁–C₅ carboxylation). Dark fixationcould not account for this high ¹⁴CO₂ incorporation into theC₄ compounds linked to PEPCase. Two carboxylation pathways linkedto the RuBPCase and PEPCase were indicated in ¹⁴CO₂-fixationin light in photoautotrophically and photomixotrophically culturedcells. (Received October 25, 1979; )     

Role of carbonic anhydrase in photosynthetic CO2 fixation in Chlorella

Chlorella vulgaris 11h cells grown in air enriched with 4% CO₂(high-CO² cells) had carbonic anhydrase (CA) activity whichwas 20 to 90 times lower than that of algal cells grown in ordinaryair (containing 0.04% CO₂, low-CO₂ cells). The CO₂ concentrationduring growth did not affect either ribulose 1,5-bisphosphate(RuBP) carboxylase activity or its Km for CO₂. When high-CO₂ cells were transferred to low CO₂ conditions,CA activity increased without a lag period, and this increasewas accompanied by an increase in the rate of photosynthetic¹⁴CO₂ fixation under ¹⁴CO₂-limiting conditions. On the otherhand, CA activity as well as the rate of photosynthetic ¹⁴CO₂fixation at low ¹⁴CO₂ concentrations decreased when low-CO₂cells were transferred to high CO₂ conditions. Diamox, an inhibitor of CA, at 0.1 mM did not affect photosynthesisof low-CO₂ cells at high CO₂ concentration (0.5%). Diamox inhibitedphotosynthesis only under low CO₂ concentrations, and the lowerthe CO₂ concentration, the greater was the inhibition. Consequently,the CO₂ concentration at which the rate of photosynthesis attainedone-half its maximum rate (Km) greatly increased in the presenceof this inhibitor. When CO₂ concentration was higher than 1%, the photosyntheticrate in low-CO₂ cells decreased, while that in high-CO₂ cellsincreased. Fractionation of the low-CO₂ cells in non-aqueous medium bydensity showed that CA was fractionated in a manner similarto the distribution of chlorophyll and RuBP carboxylase. These observations indicate that CA enhances photosynthesisunder CO₂-limiting conditions, but inhibits it at CO₂ concentrationshigher than a certain level. The mechanism underlying the aboveregulatory functions of CA is discussed. ¹This work was reported at the International Symposium on PhotosyntheticCO₂-Assimilation and Photorespiration, Sofia, August, 1977 (18).Requests for reprints should be addressed to S. Miyachi, RadioisotopeCentre, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan. (Received December 11, 1978; )