Glycolate synthesis in a C3 C4 and intermediate photosynthetic plant type

Excised leaves of a C₃-photosynthetic type, Hordeum vulgare,a C₄-type, Panicum miliaceum, and an intermediate-type, Panicummilioides, were allowed to take up through their cut ends a1 mM solution of butyl hydroxybutynoate (BHB), an irreversibleinactivator of glycolate oxidase. After 30 to 60 min in BHB,extractable glycolate oxidase activity could not be detectedin the distal quarter of the leaf blades. Following this pretreatment,recovery of ¹⁴C-glycolate from ¹⁴CO₂ incorporated in a 10 minperiod was nearly maximal for each of the three plant types.Labeled glycolate was 51% of the total ¹⁴CO₂ incorporated forthe C₃-species, 36% for the intermediate-species, and 27% forthe C₄-species Increased labeling of glycolate was compensatedfor primarily by decreased labeling of the neutral and basicfractions for the C₃ and intermediate-type species. In the C₄-type,label decreased primarily in the neutral and insoluble fractions,but increased in the basic fraction. A lower rate of glycolatesynthesis is indicative of a lower rate of photorespirationand consistent with a lower O₂/CO₂ ratio present in the bundle-sheathcells of C₄-plants. We conclude that both decreased glycolatesynthesis and the refixation of photorespiratory-released CO₂are important in maintaining a lower rate of photorespirationin C₄-plants compared to C₃ plants. Intermediate glycolate synthesisin Panicum milioldes is consistent with its intermediate levelof O₂ inhibition of photosynthesis and intermediate rate ofphotorespiration. (Received May 6, 1978; )     

RESPIRATORY METABOLISM IN THE PHLOEM, XYLEM AND CAMBIUM OF CARROT ROOT

Comparisons of respiratory metabolism among the phloem, cambiumand xylem of mature carrot root were carried out and the followingresults were obtained. 1) The activity of O₂ uptake on a dry weight basis in the cambiumwas higher than that in the xylem. The phloem showed the lowestactivity. 2) RQ was close to unity in all the three tissues.3) The inhibition rate of O₂ uptake by malonate was considerablyhigh in the three tissues. The highest inhibition was observedin the cambium. 4) In the phloem and xylem the malonic inhibitionrate of ¹⁴CO₂ release from G-U-¹⁴C was not so high as comparedwith that of O₂ uptake. In the cambium, however, those of O₂uptake and ¹⁴CO₂ output were comparable. 5) Malonate treatmentdid not cause any significant change in RQ. 6) The C₆/C₁ ratiowas highest in the cambium and lowest in the phloem. From these it is concluded that the participation of the TCAcycle in respiration is great in all the three tissues of carrotroot, and among the three the cambium has the highest activityof the cycle. As for the PP pathway, the xylem and phloem havea higher activity than the cambium. (Received April 11, 1967; )     

The Influence of Leaf Development on the Expression of C4 Metabolism in Flaveria trinervia, a C4 Dicot

The initial products of ¹⁴CO₂ assimilation were determined understeady state illumination of leaves of Flaveria trinervia, aC₄ dicot of the NADP-mialic enzyme subgroup. Leaf age influencedthe partitioning of ¹⁴CO₂ between the C₄ cycle and the reductivepentose phosphate (RPP) pathway. An estimated 10 to 12%of theCO₂ entered the RPP pathway directly in leaves about 20% fullyexpanded, whereas CO₂ was apparently fixed entirely throughthe C₄ pathway in leaves 75% or more expanded. This partitioningpattern was attributed to the bundle sheath compartment in youngleaves having a relatively high conductance to CO₂ (i.e., beingsomewhat leaky). Of the initially labelled C₄ acids, the proportion that wasmalate, relative to aspartate, increased continuously duringleaf expansion (from 60 : 40 to 87 : 13 at full expansion).Concurrently, there was an increase in the whole leaf activityof NADP malate dehydrogenase and a decrease in the activitiesof aspartate and alanine aminotransferases. Low chlorophylla/b values were observed in young leaves, which may coincidewith an enhanced capacity for non-cyclic electron transportin the bundle sheath chloroplasts of such tissue. Both enhancedaspartate metabolism and direct fixation of CO₂ in the bundlesheath could provide a greater sink for utilization of photochemicallyderived NADPH in the bundle sheath of young leaves. Such metabolicchanges are discussed in relation to a possible decrease inCO₂ conductance of the bundle sheath during leaf development. (Received March 4, 1986; Accepted June 25, 1986)     

Characterization of the Type of Photosynthetic Carbon Dioxide Fixation in Jute (Corchorus olitorius L.)

Activities of photosynthetic and photorespiratory enzymes viz.,ribulose bisphosphate carboxylase, phosphoenol pyruvate carboxylaseand glycolate oxidase from jute (Corchorus olitorius L.; cv.JRO 632) leaves were compared with those from maize (C₄) andsunflower (C₃) leaves. The photosynthetic CO₂ fixation products,the release of ¹⁴CO₂ in light and dark following photosynthesisin ¹⁴CO₂, chlorophyll a: b ratio, gross leaf photosyntheticrate and dry matter production rate were also studied. The resultsshow that jute is a C₃ plant. Key words: Jute, Corchorus olitorius, C₃ photosynthesis     

Panicum milioides, a Gramineae plant having Kranz leaf anatomy without C4-photosynthesis

Light and electron microscopic observations of the leaf tissueof Panicum milioides showed that the bundle sheath cells containeda substantial number of chloroplasts and other organelles. Theradial arrangement of chlorenchymatous bundle sheath cells,designated as Kranz leaf anatomy, has been considered to bespecific to C₄ plants. However, photosynthetic ¹⁴CO₂ fixationand ¹⁴CO₂ pulse-and-chase experiments revealed that the reductivepentosephosphate pathway was the main route operating in leavesof P. milioides. The interveinal distance of the leaves wasintermediate between C₃and C₄Gramineae species. These resultsindicate that P. milioides is a natural plant species havingchracteristics intermediate between C₃ and C₄ types. (Received March 6, 1975; )     

Effects of Prolonged Darkness on the Sensitivity of Leaf Respiration to Carbon Dioxide Concentration in C3and C4Species

Predicting responses of plant and global carbon balance to theincreasing concentration of carbon dioxide in the atmosphererequires an understanding of the response of plant respirationto carbon dioxide concentration ([CO₂]). Direct effects of thecarbon dioxide concentration at which rates of respiration ofplant tissue are measured are quite variable and their effectsremain controversial. One possible source of variation in responsivenessis the energy status of the tissue, which could influence thecontrol coefficients of enzymes, such as cytochrome-c oxidase,whose activity is sensitive to [CO₂]. In this study we comparedresponses of respiration rate to [CO₂] over the range of 60to 1000 µmol mol^-1in fully expanded leaves of four C₃andfour C₄herbaceous species. Responses were measured near themiddle of the normal 10 h dark period, and also after another24 h of darkness. On average, rates of respiration were reducedabout 70% by the prolonged dark period, and leaf dry mass perunit area decreased about 30%. In all species studied, the relativedecrease in respiration rate with increasing [CO₂] was largerafter prolonged darkness. In the C₃species, rates measured at1000 µmol mol^-1CO₂averaged 0.89 of those measured at 60µmol mol^-1in the middle of the normal dark period, and0.70-times when measured after prolonged darkness. In the C₄species,rates measured at 1000 µmol mol^-1CO₂averaged 0.79 of thoseat 60 µmol mol^-1CO₂in the middle of the normal dark period,and 0.51-times when measured after prolonged darkness. In threeof the C₃species and one of the C₄species, the decrease in theabsolute respiration rate between 60 and 1000 µmol mol^-1CO₂wasessentially the same in the middle of the normal night periodand after prolonged darkness. In the other species, the decreasein the absolute rate of respiration with increase in [CO₂] wassubstantially less after prolonged darkness than in the middleof the normal night period. These results indicated that increasingthe [CO₂] at the time of measurement decreased respiration inall species examined, and that this effect was relatively largerin tissues in which the respiration rate was substrate-limited.The larger relative effect of [CO₂] on respiration in tissuesafter prolonged darkness is evidence against a controlling roleof cytochrome-c oxidase in the direct effects of [CO₂] on respiration.Copyright 2001 Annals of Botany Company Carbon dioxide, respiration, Abutilon theophrasti(L.), Amaranthus retroflexus(L.),Amaranthus hypochondriacus (L.), Datura stramonium(L.), Helianthus annuus(L.), Solanum melongena(L.), Sorghum bicolor(L. Moench), Zea mays     

Pathway of phloem loading in the C3 tropical orchid hybrid Oncidium Goldiana

The apoplast of mature leaves of the tropical orchid OncidiumGoldiana was perfused with 0.5 mM p-chloromercuribenzenesulphonicacid (PCMBS) via the transpiration stream in order to test themode of phloem loading. The efficacy of introducing PCMBS byperfusion was shown by saffranin O dye movement in the veinsand leaf apoplast in control experiments. Photoassimilate exportas the result of phloem loading was measured by collection of¹⁴CO₂-derived photoassimilates from the basal cut-ends of intactleaves. Phloem loading and translocation of photoassimilates was inhibitedby 89% in leaves perfused with PCMBS for 1 h. The effect ofPCMBS on leaf photosynthesis was minimal. The amount of radiocarbonfixed by PCMBS-treated leaves averaged 89% of control leavesperfused with distilled water. A negative correlation betweenthe total amount of photoassimilate exuded and the calculatedconcentration of PCMBS in the leaf apoplast was also observed.The results indicate that phloem loading in Oncidium Goldianaoccurs via the apoplastic pathway. Key words: Phloem loading, apoplast, PCMBS, tropical orchid     

A CO2 Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and 14CO2/12CO2 Exchange

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger,J. 1985. A CO₂ concentrating system in leaves of higher C₃-plantspredicted by a model based on RuBP carboxylase/oxygenase kineticsand ¹⁴CO₂/¹²CO₂ exchange.–J. exp. Bot. 36: 1542–1550. A model is presented which compares the ratio of the two activitiesof the enzyme nbulose bisphosphate carboxylase/oxygenase asdetermined in vitro with the ratio of photosynthesis to photorespirationin leaves as determined from differential ¹⁴CO₂/¹²CO₂ uptakeor from CO₂ compensation concentration. Discrepancies betweenmeasurements made in vitro and in vivo are attributed to theeffect of a CO₂ concentrating system in the leaf cells. Interferencefrom dark respiration is discussed. A CO₂ concentrating systemis postulated which is efficient mainly at low temperature andlow CO₂ concentration. Key words: —Photosynthesis, photorespiration, ribulose bisphosphate carboxylase/oxygenase     

Preparation of Radioactive Starch, Glucose and Fructose from C14O2

Radioactive starch, glucose and fructose have been preparedfrom tobacco leaves after assimilation of C¹⁴O₂. The apparatusused for photosynthesis consisted of a shallow Perspex leafchamber connected to a closed gas system, in which C¹⁴O₂ wasgenerated from BaC¹⁴O₂. Six leaves, area 14 to 18 sq. dm. whenexposed to bright sunlight with an initial CO₂ concentrationof 8 to 10 per cent., assimilated 3.35 g. of C¹⁴O₂ in 8 to 10hours. At least 80 per cent. of the C¹⁴O₂ supplied appearedin the leaves as starch and sugar and over 80 per cent. of theradioactivity was accounted for in these carbohydrates. Thespecific activity per m. atom of carbon of the isolated productswas 85 to 90 per cent. of that of the C¹⁴O₂. Small amounts ofradioactive carbon were also incorporated in the leaf proteinand in the celluose, hemicellulose and polyuronides.     

Gas Exchange and Carbohydrate and Nitrogen Concentrations in Leaves of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) at Different Carbon Dioxide Concentrations and Temperatures

Pascopyrum smithii (C₃) andBouteloua gracilis (C₄) are importantforage grasses native to the Colorado shortgrass steppe. Thisstudy investigated photosynthetic responses of these grassesto long-term CO₂enrichment and temperature in relation to leafnonstructural carbohydrate (TNC) and [N]. Glasshouse-grown seedlingswere transferred to growth chambers and grown for 49 d at twoCO₂concentrations (380 and 750 µmol mol^-1) at 20 and 35°C, and two additional temperatures (25 and 30 °C) at750 µmol mol^-1CO₂. Leaf CO₂exchange rate (CER) was measuredat a plant's respective growth temperature and at two CO₂concentrationsof approx. 380 and 700 µmol mol^-1. Long-term CO₂enrichmentstimulated CER in both species, although the response was greaterin the C₃,P. smithii . Doubling the [CO₂] from 380 to 750 µmolmol^-1stimulated CER ofP. smithii slightly more in plants grownand measured at 30 °C compared to plants grown at 20, 25or 35 °C. CO₂-enriched plants sometimes exhibited lowerCER when compared to ambient-grown controls measured at thesame [CO₂], indicating photosynthetic acclimation to CO₂growthregime. InP. smithii , such reductions in CER were associatedwith increases in TNC and specific leaf mass, reductions inleaf [N] and, in one instance, a reduction in leaf conductancecompared to controls. InB. gracilis , photosynthetic acclimationwas observed more often, but significant changes in leaf metabolitelevels from growth at different [CO₂] were generally less evident.Temperatures considered optimal for growth (C₃: 20 °C; C₄:35 °C) sometimes led to CO₂-induced accumulations of TNCin both species, with starch accumulating in the leaves of bothspecies, and fructans accumulating only inP. smithii. Photosynthesisof both species is likely to be enhanced in future CO₂-enrichedand warmer environments, although responses will sometimes beattenuated by acclimation. Acclimation; blue grama (Bouteloua gracilis (H.B.K.) Lag ex Steud.); leaf nitrogen concentration; nonstructural carbohydrates; photosynthesis; western wheatgrass (Pascopyrum smithii (Rydb.) Love)     

Phosphorus responses of C3 and C4 species

An hypothesis was formulated that phosphorus (P) partitioningin tissues of C₄ leaves would permit C₄ plants to resist P deficiencybetter than C₃ plants. To test this hypothesis, 12 C₃, C₄, andC₃–C₄ intermediate species were grown at adequate, deficient,and severely deficient P supply in a solid-phase-buffered sandculture system to characterize photosynthetic and growth responses.Species differed considerably in response to P stress. The growthof C₃ species was more sensitive to P supply than C₄ species,but C₃ and C₄ species had similar photosynthetic P use efficiency,and C₄ species did not have low leaf P content, contrary toour hypothesis. In fact, leaf photosynthetic rates were notcorrelated with growth responses. Moncots had lower leaf P contentand better maintenance of leaf production under P stress thandicots, because of greater inhibition of branching (dicots)than of tillering (monocots). The most P efficient species inthis survey was Brachiaria, a C₄ monocot that increased rootbiomass allocation under stress while maintaining P allocationto the shoot. It is concluded that C₄ species are not inherentlymore P efficient than C₃ species, but that monocots are moreP efficient than dicots, because of contrasting P and biomassallocation under stress. Key words: Phosphorus deficiency, C₃ plants, C₄ plants, growth response     

(Na+-K+)-stimulated ATPase in Leaves of C4 Plants: Possible Involvement in Active Transport of C4 Acids

Leaves of three C₄ plants, Setaria italica, Pennisetum typhoides,and Amaranthus paniculatus possessed five- to ten-fold higheractivities of a (Na⁺-K⁺)-dependent ATPase than those of twoC₃ plants, Oryza sativa and Rumex vesicarius. Na⁺-K⁺ ATPasefrom leaves of Amarathus exhibited an optimal pH of 7?5 andan optimal temperature of 35 ?C. It required 40 mM K⁺ and 80mM Na⁺ for maximal activity. Ouabain partially inhibited (Na⁺-K⁺)-dependentATPase activity in leaves of C₄ plants. Ouabain also blockedthe movement of label from initially formed C₄ acids into endproducts in leaves of only C₄ plants, Setaria and Amaranthusbut not in a C₃ plant, Rumex. We propose that Na⁺-K⁺ ATPasemay mediate transfer of energy during active transport of C₄acids from mesophyll into the bundle sheath.     

Movement of 14C- and 11C-labelled Assimilate in Wheat Leaves: the Effect of IAA

The rate of translocation of naturally-loaded, radiolabelledassimilate has been studied in leaves of wheat treated withIAA. The velocity was measured directly by following the movementof ¹¹C-labelled material along the leaf. The kinetics of translocationwere also estimated by using a two-compartment model to calculatethe rate constant of disappearance of ¹⁴C from the fed area.IAA was applied at three different sites on the leaf and atvarious times up to 24 h before ¹⁴CO₂ feeding. No effects ofIAA were observed on: (1) direction and velocity of transport;(2) loading of assimilate into the phloem; or (3) the site andkinetics of unloading. These results are discussed with referenceto the movement of IAA along the transport path and the useof different tissues as experimental systems.     

Photosynthetic Capacity and Nitrogen Use Efficiency of Maize, Wheat, and Rice: A Comparison Between C3 and C4 Photosynthesis

The C₃ species wheat and rice and the C₄ species maize weregrown for 2–3 weeks in controlled environment growth chambersat 20 or 30 °C day and 15 °C night temperatures. CO₂assimilation rates (at 20 and 30 °C) and several leaf parametersincluding total nitrogen, soluble protein, and RuBP carboxylaseprotein were determined. When the assimilation rates under atmosphericCO₂ and O₂ levels were expressed on a total nitrogen basis (=nitrogen use efficiency), the C₄ species maize had a greaternitrogen use efficiency than either of the two C₃ species examined,regardless of the combination of temperatures used for growthor measurement of CO₂ assimilation. Maize is also shown to makemore efficient use of its soluble protein and RuBP carboxylaseprotein than either wheat or rice when measurements are madeat 320 parts 10^–6 CO₂ and 21% O₂. Atmospheric CO₂ enrichmentduring CO₂ assimilation measurements increased the nitrogenuse efficiency in the C₃ species. In one treatment (wheat grownand measured at 20 °C), CO₂ assimilation under saturatingCO₂ showed nitrogen, soluble protein, and RuBP carboxylase proteinuse efficiencies equal to or greater than that of the C₄ species. These data indicate that C4 species may make more efficientuse of their nitrogen, soluble protein, and RuBP carboxylaseprotein than C₃ species under atmospheric CO₂ conditions. Thismay be due in part to the C₄ cycle and CO₂-concentrating mechanismin C₄ photosynthesis.     

Effect of the age of tobacco leaves on photosynthesis and photorespiration

Relationships among the activities of enzymes related to photosynthesisand photorespiration, and ¹⁴CO₂ photosynthetic products wereinvestigated with individual tobacco leaves attached to thestalk from the bottom to the top. P-glycolate phosphatase ofthe chloroplasts and glycolate oxidase of the peroxisomes hadtheir maximum activities in the 25th leaf from the dicotyledons.Maximum photorespiration was similarly distributed. The highestratio of serine-¹⁴C to glycine-¹⁴C in the photosynthesates andmaximum glycolate formation were also observed in the 25th leaf.Glutamateglyoxylate aminotransferase, serine hydroxymethyltransferaseand glycine decarboxylase were more active in the upper leaves.RuDP carboxylase had nearly constant activity in all leaves,except for the youngest in which activity decreased. MaximumCO₂ photosynthesis and enzyme activity for the C₄ dicarboxylicacid cycle occurred in the upper, youngest leaf. Distributionof photosynthetic CO² fixation among the leaves did not coincidewith RuDP carboxylase activity. The photosynthetic capacityappeared to be better related to the distribution pattern forenzymes of the C₄ dicarboxylic acid pathway, i.e. PEP carboxylase,pyruvate Pi dikinase and 3-PGA phosphatase in the upper leaves.The results suggest that the C₄ dicarboxylic acid pathway participates,to some extent, in photosynthesis in young leaves of tobacco,a dicotyledonous plant. ¹This work was reported at the Annual Meeting (1970) of theJapanese Plant Physiologists in Kobe. ²The Central Research Institute, Japan Monopoly Corporation1-28-3, Nishishinagawa, Shinagawaku, Tokyo, 141 Japan. (Received November 2, 1972; )     

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.     

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.     

The Role of C2H4 in anaerobic induction of cocklebur seed germination

Non-dormant small cocklebur seeds (Xanthium pennsylvanicum Wallr.)are potentiated to germinate, if they are subjected to anaerobiccondition for certain time periods after being sufficientlypre-soaked under aerobic conditions. This is termed "anaerobicinduction" of seed germination. Such induction was slightlyinhibited by CO₂ applied during anaerobiosis, but markedly promotedby C²H⁴ Thus, C²H⁴ can exert its action even in anaerobiosis,but does not enhance the fermentative CO₂ evolution. No actualanaerobic induction occurred when over 1? O₂ was present, evenif C²H⁴ had been applied. Therefore, anaerobic induction seemsto be due to a concerted action of some anaerobically proceedingevents and the anaerobically produced C²H⁴. (Received May 31, 1976; )     

Antagonism Between Malate and Ethylene in the Regulation of Avena sativa Coleoptile Elongation

Etiolated Avena sativa L. coleoptile sections were used to determinethe influence of C₂H₄ on in vivo and in vitro rates of CO₂ fixation,and to measure the influence of various permutations of C₂H₄,CO₂, and malate on growth. Whereas 1 mM malate or 320 µI^-1 CO₂ stimulated growth by approximately 100 per cent, inhibitionof growth by 10-8 µ I_-1 C₂H₄ was substantial only in thepresence of malate or CO₂ The increase in growth rate in responseto these two agents was eliminated by the simultaneous applicationof C₂H₄. The in vivo rate of dark [¹⁴C]bicarbonate fixationand in vitro enzymic assays of fixation were not measurablyinhibited by C₂H₄. These results are discussed in the lightof evidence which indicates that CO₂-stimulated growth is mediatedby dark fixation. The data do not support the view that C₂H₄inhibition of growth results from an inhibition of fixation,but suggests that C₂H₄ may inhibit some step in the processby which malate stimulates growth.     

Ethylene Evolution from Bracts and Leaves of Poinsettia, Euphorbia pulcherrima Willd

Woodrow, L. and Grodzinski, B. 1987. Ethylene evolution trombracts and leaves ol Poinsettia, Euphorbia pulcherrima Willd.—J.exp. Bot. 38: 2024–2032. Ethylene release from fully expanded, red and white bracts andleaves of poinsettia, Euphorbia pulcherrima Willd., was compared.On a laminar (area) basis leaves contained about 50 times morechlorophyll and demonstrated 10 times the photosynthetic rateof the bracts. Both tissues contained starch, however, solublecarbohydrate in the bracts consisted primarily of reducing hexoseswhile the leaves contained mainly sucrose for translocation.The total free alpha-amino nitrogen content of the bract tissuewas twice that of the leaf tissue. The leaves contained moreACC (1-aminocyclopropane-1-carboxylic acid) and produced proportionallymore endogenous C²H⁴ than either the red or white bracts. ACC-stimulated₂H₄ release was also greatest from the green tissue indicatingthat the EFE (ethylene forming enzyme) was most active in theleaves. The specific activity of the ¹⁴C₂H₄/¹²C₂H₄ releasedfrom [2,3-¹⁴C]ACC confirmed ACC as the primary precursor ofC₂H₄ in this tissue. Ethylene release from the non-photosynthetic,bract tissue was not markedly affected by alterations in CO₂or light conditions. In green leaf tissue endogeneous ethylenerelease increased from 1·5 to 6·0 pmol C₂H₄ cm^–2h^–1 while ACC-stimulated ethylene release increased from10 to 35 pmol C₂H₄ cm^2– h^1– as the CO₂ partial pressureincreased from 100 to 1 200 µbar. Key words: Poinsettia, ethylene, bracts