The Uptake of Surcrose and its Conversion to Starch in Detached Ears of Wheat

Detached ears of wheat were cultured on solutions of ¹⁴C-sucroseand the distribution of carbon-14 in the ear was followed. Within 8 h radioactive sucrose was found in all the tissuesof the cultured ear, and considerable amounts of carbon-14 hadaccumulated in other ethanol-soluble compounds. Carbon-14 accumulatedrapidly in the starch deposited in the endosperm, but littlewas found in the starch of the pericarp, or in other materialinsoluble in ethanol in the vegetative organs. During 48 h the specific radioactivity of the sucrose in theendosperm increased in a hyperbolic pattern and was equal tothat of the starch produced. Carbon-14 in the glucose and fructoseaccumulated more slowly and in a Linear fashion. Experiments with sucrose containing carbon-14 in both moietiesequally, or in the fructosyl moiety exclusively, confirmed thatboth moieties are converted into starch and at about the samerate. As sucrose in endosperm provided with asymmetric sucroseretained a considerable degree of asymmetry, it seems as thoughinversion is not a necessary step in the transport of sucroseinto the grain. In ears provided with ¹⁴C-sucrose at 30 mg ml^–1 the rateof accumulation of ¹⁴C-sucrose in the culm, rachis, and floralorgans was about 0.6 times the value at 50 mg ml^–1. However,in the sucrose of the endosperm, and in the starch depositedthere, the rates of accumulation of carbon-14 from both levelswere identical. This finding supports the concept that the transportof sucrose is limited during the final stages of its passageinto the grain.     

The Role of Phloem Transport in the Translocation of Sucrose Along the Stem of Carnation Cut Flowers

The pathway and distribution of ¹⁴C-sugars in flower parts havebeen examined to find out in which tissue sugars are translocatedin the stem of the cut carnation; ¹⁴C-sucrose or ¹⁴C-glucosewas supplied at the base of the cut stem from a feeding solutionand the localization and chemical nature of the carbon-14 recoveredfrom flower parts were investigated. By reducing the rate oftranspiration it was found that the uptake of feeding solutionwas also reduced, but the distribution of absorbed ¹⁴C-sucrosein the flower parts was different from that which would be expectedif sucrose moved exclusively in the transpiration stream. Autoradiographsdemonstrated that ¹⁴C absorbed from the feeding solution as¹⁴C-sucrose appeared in both xylem and phloem but predominantlyin the latter; girdling failed to stop the translocation ofthe absorbed ¹⁴C-sucrose. Results of experiments with ¹⁴C-sucroseand ¹⁴C-glucose showed that sucrose was the mobile sugar andthat glucose was converted to sucrose before it was translocated.It was concluded that the translocation of sucrose absorbedfrom the feeding solution takes place both in xylem and phloemand is regulated by a mechanism involving the loading and translocationof sucrose in the phloem.     

Photorespiratory Amino Donors, Sucrose Synthesis and the Induction of CO2 Fixation in Barley Deficient in Glutamine Synthetase and/or Glutamate Synthase

Murray, A. J. S., Black well, R. D., Lea, P. J. and Joy, K.W. 1988. Photorespiratory amino donors, sucrose synthesis andthe induction of CO₂ fixation in barley deficient in glutaminesynthetase and/or glutamate synthase.—J. exp. Bot. 39:845–858. A number of mutants of barley have been produced which lackboth chloroplastic glutamine synthetase and ferredoxin-dependentglutamate synthase activities. The plants accumulated ammoniato the same extent as mutants deficient in only glutamine synthetasebut shared the gas-exchange characteristics of the glutamatesynthase deficient parent. These mutants have been used to demonstratedirectly the ability of alanine to ameliorate the dramatic dropin fixation rate normally exhibited by glutamate synthase deficientmutants on transfer to photorespiratory conditions. Immediatelyafter transfer to air, the mutants deficient in glutamate synthaseactivity demonstrated a reduced ability to incorporate ¹⁴C derivedfrom ¹⁴CO₂ into sucrose. This effect was, however, dependenton the previous induction of CO₂ fixation. Use of ¹⁴CO₂ revealedthat the induction phase of CO₂ fixation was altered in allthree mutants. Neither of the parents nor the double mutantaccumulated sucrose in air under conditions which promote sucroseaccumulation by the wild type. The implications of these resultsfor photosynthesis and the control of sucrose synthesis arediscussed. Key words: Photorespiratory barley mutant, amino donors, sucrose, GS, glutamate synthase.     

Carbon Import into Developing Ovules of Pisum sativum: The Role of the Water Relations of the Seed Coat

We tested the hypothesis that the transport of carbon to developingpea ovules is controlled by the water potential of the seedcoat, in both the short-term (minutes to hours) and long-term(days). At 14 d after anthesis, when the embryo just fills theseed coat, the osmotic pressure of seed coat apoplast solutionwas about 1 MPa (equivalent to 400 mOsmol kg^–1). Transportof carbon into perfused attached seed coats at this stage ofdevelopment was monitored with radioactive carbon-11. Aftera small (50 mOsmol kg^–1) increment in the osmotic pressureof the bathing solution, transport of carbon increased abruptly,but after about 100 min it returned towards pretreatment values.Therefore, although osmotic pressure in the sink apoplast initiallyaffected carbon import, as expected from the M     

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; )     

Studies on Translocation of Metabolites in the Xylem of Grapevine Shoots

Fluid extracted from grapevine shoots by suction contained upto 0.2 per cent (w/v) of sugar. The total sugar concentrationand the relative concentrations of sucrose, glucose, and fructosechanged along the length of a shoot. Whereas sucrose is absentfrom or present only in traces in fluid bleeding from cut stumps,it was found to be a major constituent in fluid extracted fromsections of wood by suction. ¹⁴C-glucose administered to thewood of an excised shoot moved up in the transpiration stream,moved laterally, into the bark, and was partly converted tosucrose and fructose. When ¹⁴CO₂ was supplied to a leaf of anintact vine, radioactive sugars were found in the fluid afterwardsextracted from the wood by suction. When a ring of bark wasremoved from the stem at a point above the ¹⁴CO₂-treated leafmost of the ¹⁴C in the bark, wood, and extracted fluid belowthe ring was in the form of sugar but ¹⁴C in tissues above thering was mainly in organic acids and amino acids. It is suggestedthat a barrier within the wood prevents diffusion of sugarsinto the transpiration stream, and that the fluid extractedfrom the wood by suction cannot be regarded as ascending sapsince it contains considerable amounts of non-moving material.     

Interconversions of 14C-labelled Sugars in Apple Tree Tissues12

Tissue pieces excised from orchard-grown apple trees duringa growing season exhibited different and changing capabilitiesof transferring ¹⁴C-label from sucrose, fructose and sorbitolto other soluble carbohydrates. All tissues incorporated fructose¹⁴C into sucrose but only leaves incorporated significant amountsof label from sucrose into sorbitol. As seeds developed andmatured, their ability to incorporate ¹⁴C from sorbitol andfrom fructose into sucrose increased. Sorbitol and sucrose arethe major translocated photosynthetic products of apple leavesbut whereas sorbitol appears to be an end-product of synthesis,sucrose may be considered as a substrate involved more directlyin carbohydrate utilization. Key words: Malus domestica, Apple, Carbohydrate interconversions     

Unsaponifiable Lipid Biosynthesis in the Seedling of Euphorbia lathyris L.: A Bypass via Alcoholic Fermentation

[1-¹⁴C]-ethanol supplied to the cotyledons of 9-d-old Euphorbialathyris seedlings was rapidly incorporated into unsaponifiablelipids, particularly into sterols, latex triterpenols and intothe triterpene ketones of the epicuticular wax. The [¹⁴C]-triterpenoidproduction from ethanol was hardly affected by sucrose in theexternal medium when sucrose uptake rates were low, but whenthe uptake rate was higher the [¹⁴C]-triterpenoid productionfrom [¹⁴C]-ethanol was greatly reduced. This observation isconsistent with the proposition that at high sucrose uptakerates, some sucrose is converted into ethanol, so that the incorporationof [¹⁴C]-ethanol into triterpenoids is reduced by competitionwith endogenously formed ethanol. A calculation based on theputative daily ethanol production in the cotyledons and thedaily triterpenoid production of seedlings indicates that about10 % of the triterpenoid synthesis in vivo may be from ethanol. Ethanol, Euphorbia lathyris, fermentation, seedling, triterpenoid biosynthesis     

Ethylene-Enhanced Transport of 14C-Labeled Metabolites in Rice Seedlings in Relation to Ethylene-Stimulated Coleoptile Growth

To examine the effects of ethylene on sugar transport from endospermsto coleoptiles in rice (Oryza sativa L. cv. Sasanishiki) seedlings,the contents of free sugars in the coleoptiles of explants fedcold glucose and the distributions of ¹⁴C-activities after feedingof ¹⁴C-glucose to the scutella were determined at various timesafter ethylene application. Changes in sucrose, glucose andfructose in the cold glucose-fed explants exposed to ethylenewere similar to those in the ethylene-treated intact seedlingshaving endosperms. Ethylene enhanced the transport of ¹⁴C-labeledmetabolites from the scutella to the coleoptiles. Most of the¹⁴C accumulated in the ethylene-treated coleoptiles were presentas neutral substances in the ethanol-soluble fraction. Regardlessof the presence or absence of ethylene, the incorporation of¹⁴C into sucrose preceded that into glucose and fructose. Theglucose and fructose moieties of ¹⁴C-sucrose in the coleoptileswere almost equally labeled, and the specific activities of¹⁴C-sucrose were higher than those of ¹⁴C-glucose and ¹⁴C-fructose.These results suggested that sucrose synthesized in the scutellawas exported to the coleoptiles, and cleaved there into glucoseand fructose. Ethylene may accelerate the transport of ¹⁴C-labeledmetabolites by activating sucrose cleavage in the coleoptiles. (Received July 1, 1985; Accepted September 17, 1985)     

Action Mechanism of Ethylene in the Control of Sugar Translocation in Relation to Rice Coleoptile Growth I. Sucrose Metabolism

The fate of ¹⁴C-glucose fed through scutella of rice (Oryzasativa L. cv. Sasanishiki) seedling explants was investigatedin relation to ethylene action on sugar translocation to growingcoleoptiles and leaves. In the scutellum, sucrose, UDPglucoseand F6P were rapidly labeled, and sucrose-phosphate synthaseactivity was higher than sucrose synthase activity. Radioactivesucrose soon appeared in both coleoptiles and leaves, and increasedrapidly. Its specific activity in both tissues became almostequal to that in the scutella. The specific activities of ¹⁴C-glucosein both coleoptiles and leaves changed almost in parallel tothose of ¹⁴C-fructose. These results suggest that sucrose wassynthesized in the scutellum and exported to the coleoptileand leaf, where it was cleaved to glucose and fructose. Ethylene slightly increased the specific activities of ¹⁴C-sucrosein all tissues, but markedly increased those of ^l4C-glucoseand -fructose only in the coleoptile. We assume that the ethyleneenhancement of sucrose transport from scutellum to the coleoptileresults from the activation of sucrose unloading in the growingcoleoptile where imported sucrose is cleaved into glucose andfructose. (Received May 25, 1987; Accepted October 30, 1987)     

Glutamine synthesis in germinating castor bean endosperm

The pathway of glutamine synthesis in germinating castor beanendosperm was investigated by feeding experiments with (2,3-¹⁴C)succinateand by determining enzyme activities related to pyruvate formationand utilization. ¹⁴C of (2,3-¹⁴C)succinate was rapidly and sequentiallyincorporated into amino acids in the following order: aspartateor alanine, glutamate and glutamine. ¹⁴CO₂ was slowly released,especially during the early hours of incubation. Fluorocitrateinhibited ¹⁴CO₂ release while aminooxyacetate stimulated itslightly. Fluorocitrate inhibited the incorporation of ¹⁴C intoglutamate and glutamine. Aminooxyacetate inhibited ¹⁴C incorporationinto aspartate, alanine, glutamate and glutamine. Glutaminesynthetase activity was detected in a soluble fraction. NAD-malicenzyme activity was detected in mitochondria by sucrose densitygradient centrifugation. Activities of pyruvate decarboxylaseand aldehyde dehydrogenasewere detected. Aldehyde dehydrogenasewas partially purified about 60-fold by ammonium sulfate fractionationand the DEAE-cellulose chromatography. The Km values of theenzyme were 0.71 miu for NAD and 0.43 mM for acetaldehyde. Basedon these results and properties of pyruvate kinase reportedpreviously (9), the metabolism of pyruvate in cytosol and mitochondriawas discussed in connection with glutamine synthesis in germinatingcastor bean endosperm. (Received August 25, 1978; )     

An Effect of Ethylene on the Distribution of 14C-sucrose from the Petals to Other Flower Parts in the Senescent Cut Inflorescence of Dianthus caryophyllus

The distribution of carbon-14 in the flower parts of the cutcarnation inflorescence after feeding ¹⁴C-sucrose through thepetals was studied during natural ageing and after ethylenetreatment. Levels of ethylene which caused irreversible wiltingof petals also promoted an accelerated transfer of the radioactivesucrose to the nectar, gynaecium and stem. Since the nectarreceived a relatively large proportion of the radioactive carbon,the composition of the sugars in the nectar and the vascularizationof the nectary were investigated. Sucrose comprised about 85per cent of the nectar sugars and the balance was glucose andfructose. The vascular tissue closest to the nectary consistedof phloem elements; tracheary elements terminated deeper inthe receptacle and were surrounded by a ring of phloem. Thepercentage of solutes in the nectar was about 18 per cent andincreased when the flowering stems were placed in sucrose solutions;the solutes in the nectar were principally sugars. Taken togetherthe results show that the nectary can act as a sink for sucroseand, in the flower at least, that translocation of sucrose takesplace in the phloem. The results provide further evidence forthe hypothesis that ethylene promotes mobilization of substrateand an efflux of material from petals to the gynaecium, nectarand stem.     

Glycine as a Methyl Donor in Dimethyl-{beta}-Propiothetin Synthesis

The carbon-2 of glycine can be incorporated into the methylgroup of dimethyl-ß-propiothetin in Ulva lactuca.This conversion is stimulated by unlabelled methionine. Highconcentrations of unlabelled glycine inhibit the incorporationof either L-methionine^-35S or L-methionine-methyl^-14C into DMP.The specific activity of methionine, isolated from alga incubatedwith glycine-2-¹⁴C and a high concentration of unlabelled methionine,is too low to permit it to be an intermediate in glycine-2-¹⁴Cincorporation into DMP. The incorportaion of radioctivity fromL-methionine^-35S and L-methionine-methyl-³H into DMP indicatesthat while at least one methyl group is derived from methionine,other compounds can donate a portion of the second methyl group.It is concluded that glycine incorporation into the methyl groupof DMP is not via methionine.     

Ethylene production by wounded tissue of citrus fruit

Segments cut from young immature fruits and albedo discs excisedfrom both immature and mature fruits of Satsuma mandarin ormature fruits of Natsudaidai produced much ethylene during incubationat 26?C in the dark. Ethylene formation was markedly acceleratedby the application of abscisic acid but markedly delayed by3,5-dibromo-4-hydroxybenzoic acid. Both the stimulation andretardation decreased greatly during the course of incubation.Both compounds seem to be associated with the early stages ofethylene formation by wounded citrus fruit tissues. Albedo discs were fed ¹⁴C methionine labeled at one of threedifferent positions. Of the three radioanalogs (carbon-2, carbon-3and methyl carbon), the label at the 3 position was preferentiallyincorporated into ethylene. This agrees with the former observationthat ethylene is derived from carbon-3 and -4 of methionine.Incorporation of label into ethylene from L-[3-¹⁴C] methioninewas strongly inhibited by L-canaline, L-ethionine, 2,4-dinitrophenoland cycloheximide. Ethylene evolution was also strongly inhibitedby 2,4-dinitrophenol, KCN, NaN₃ and cycloheximide, but lesscompletely by L-canaline and L-ethionine. These results supportthe view that ATP and pyridoxal phosphate are utilized in activationof methionine to form ethylene. (Received October 25, 1977; )     

The Influence of Partial Pressure of Carbon Dioxide upon Carbon Metabolism in the Tomato Leaf

The incorporation of radioactive carbon into various photosyntheticproducts was investigated with tomato plants in atmospherescontaining between 40 and 1400 parts/10⁶ carbon dioxide. A significantlygreater proportion of ¹⁴C entered sucrose and alcohol-insolublematerial at high concentrations of carbon dioxide. Incorporationinto glycine and serine was significantly greater at lower carbon-dioxideconcentrations. The pool size of these intermediates was alsodetermined and it was concluded that in the presence of highpartial pressures of carbon dioxide the flow of carbon fromthe photosynthetic cycle through the C² pathway is decreased.     

Import of Sucrose and its Partitioning in the Synthesis of Galactomannan and Raffinose-oligosaccharides in the Developing Guar (Cyamopsis tetragonolobus) Seed

Import of sucrose and its transformation to galactomannan andraffinose-oligosaccharides have been studied in the developingguar seed. The amount of galactomannan gradually increased withthe ageing of the seed. During the entire period of pod development,sucrose constituted the major portion of the free sugars inthe seed (both endosperm and cotyledons) as well as in the podwall. Besides myo-inositol, the free sugars detected in thedeveloping endosperm and cotyledons were glucose, fructose,raffinose and stachyose. Some compounds, possibly glycosides(RG values higher than that of fructose), were also detectedin the endosperm. In the later stages of seed development, therelative proportion of raffinose in the free sugars increased,reaching 50% of the total free sugars in 77-d-old cotyledons.With pod maturity, the activities of soluble acid and boundacid invertases in the pod wall increased manifold with a concomitantdecline in the non-reducing sugar content. These enzymes seemto be involved in the mobilization of sucrose from this fruitingstructure into the seed. An increased synthesis of raffinose-oligosaccharidesboth in the endosperm and cotyledons was associated with highactivities of soluble acid invertase (pH 4.8) and sucrose-UDPglucosyl transferase in these tissues. Feeding uniformly labelled¹⁴C-sugars to the detached intact pods as well as to the isolatedendosperm and cotyledons resulted in labelling of all endogenousfree sugars and galactomannan. The uptake and incorporationinto galactomannan of ¹⁴C was stimulated by Co²⁺, Mn²⁺ and Mg²⁺.Except for mannose, a major proportion of the ¹⁴C from glucose,fructose and sucrose appeared in sucrose in both endosperm andcotyledons indicating a fast reconstitution of sucrose in situ.Based on the present results, a possible mode of transformationof sucrose to galactomannan and raffinose-oligosaccharides hasbeen proposed. Key words: Sucrose, galactomannan, raffinose-oligosaccharides, invertase, sucrose-UDP glucosyl transferase, ¹⁴C-incorporation, guar seed     

The Path of Carbon in Photosynthesis: XIII. pH EFFECTS IN C14O2 FIXATION BY SCENEDESMUS

The rates of the photosynthetic and dark fixations of C¹⁴O₂in Scenedesmus have been compared in dilute phosphate buffersranging from pH 1.6 to pH 11.4 and the amounts of carbon incorporatedinto the various products have been determined by means of theradiochromatographic method. In photosynthesis, an acid medium favours early incorporationof C¹⁴ into sucrose, polysaccharides, and the three-carbon compoundsalanine and serine. Fixation into the four-carbon compoundsmalic and aspartic acids is enhanced in an alkaline medium.Kinetic experiments at several pH values suggest that severalpaths may be available for carbon dioxide assimilation. A tentative correlation of the results with the pH optima ofsome enzymes and resultant effects upon concentrations of intermediatesis presented.     

Photosynthetic carbon metabolism in wild, primitive and cultivated forms of wheat at three levels of ploidy: Role of the glycolate pathway

¹⁴CO₂ assimilation was studied with diploid, tetraploid, hexaploidspecies of the genera Triticum and their wild relatives Aegilops.Attached mature leaves of 3–4 weekold plants were allowedto undergo photosynthesis under air at ambient temperature.The pattern of distribution of ¹⁴C was notably similar in Triticumand Aegilops species whatever the level of ploidy. Sucrose wasthe sink for photosynthetic carbon. ¹⁴C for sucrose synthesis was supplied either through the glycolatepathway by glycolate, the product of the photorespiration orby the Calvin cycle intermediates exported into the cytoplasm.Depending on the species, the glycolate pathway provided 40to 75%of the sucrose ¹⁴C. The higher labeling of sucrose was associated with the greaterparticipation of the glycolate pathway in the wild diploid (DD)A. squarrosa and in the cultivated hexaploid (AABBDD) T. aestivum.The results suggest that the expression of the male D genomeis dominant over the female AB genome in T. aestivum. In T. aestivum under ambient conditions lowering (low temperature)or hindering (1% O₂ ) photorespiration, sucrose labeling decreased,but serine and glycine labeling was favoured. We propose thatin wheat leaves, the role of photorespiration is to drain artof the carbon exported from the chloroplast as glycolate, towardssucrose synthesis. (Received March 16, 1979; )     

Carbon and Nitrogen Sources for Protein Synthesis and Growth of Sugar-beet Leaves

Protein synthesis in very young leaves utilizes carbon fromphotosynthesis and from translocated sucrose, and nitrogen translocatedin both xylem and phloem. The carbon of young leaf protein isderived mainly from assimilated CO₂, while translocated sucrosecontributes proportionately more of its carbon to insolublecarbohydrate. Most protein amino-acids become labelled from¹⁴CO₂, glutamate being the notable exception. Glutamine or glutamateis synthesized from sucrose in roots, and is translocated toyoung leaves. It is suggested that a small but significant proportionof the nitrogen requirement of the young leaf is translocatedfrom roots as glutamine, in the phloem. Inorganic nitrogen istranslocated in xylem.