The Oxidation of 14C-labelled Glucose by Chlorella vulgaris: 2. The Fate of Radioactive Carbon

Most of the ¹⁴C added as glucose to carbohydrate-starved cellsof Chlorella Vulgaris can be recovered as alcohol-soluble compoundsor as polysaccharide. Only 5–I6 per cent., depending onthe position of ¹⁴C in the glucose supplied, is released ascarbon dioxide. Similar results were obtained with Chlorellapyrenoidosa and Ankistrodesmus. The labelled alcohol-solublecompounds in Chlorella vulgaris include amino-acids, particularlyglutamic acid, aspartic acid, and alanine, and, when glucose-I-¹⁴Cis metabolized, the amount of ¹⁴C recovered in these amino-acidsis about the same as that recovered as carbon dioxide. Degradationof the glucose incorporated into polysaccharide shown that theC₁ and C₆ atoms of glucose rapidly interchange when in the cells.The bearing of these results on attempts to estimate the relativeimportance of different pathways of glucose breakdown is discussed.     

Short-Term Fixation of 14Carbon by the Submerged Aquatic Angiosperm Potamogeton pectinatus

In ¹⁴C fixation experiments, 3-phosphoglyceric acid was thefirst product of carbon assimilation in the light in Potamogetonpectinatus. The pattern of early ¹⁴C-labelled compounds wasthe same over a range of pH values of the medium from 3.5 to8.1. Rates of ¹⁴C incorporation declined with increasing pHof the medium indicating that free CO₂ is the major exogenoussource of carbon for photosynthesis in Potamogeton pectinatus.     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

The Oxidation of 14C-labelled Glucose by Chlorella vulgaris: 1. The Time Course of 14CO2 Production

Glucose, either uniformly labelled with¹⁴C, or specificallylabelled in the I, 2, or 6 position, was added to C. vulgaris.Radio-active carbon dioxide was produced initially ten timesfaster from glucose-I-¹⁴C than from glucose-6-¹⁴C. This differencewas found with carbohydrate-starved cultures, exponentiallygrowing cultures, and cultures assimilating ammonia or nitraterapidly. A similar difference was also found with C. pyrenoidosaand Ankistrodesmus. 37 per cent. of the ¹⁴C added as glucose-¹-¹⁴Cto exponentially growing cells was recovered as carbon dioxidebut generally the recovery was less than this. Only 5 per cent.of ¹⁴C added as glucose-6-¹⁴C was recovered as carbon dioxide.The specific activity of the carbon dioxide produced was considerablylower than that of the carbon in the added glucose.     

Short-term Effects of Heat-girdles on Source Leaves of Vicia faba: Analysis of Phloem Loading and Carbon Partitioning Parameters

Petiole heat-girdle treatments (followed by a 5 min ¹⁴CO₂ assimilation)were performed on mature leaves of Vicia faba, in order to assesstheir effect on the partitioning of photo-assimilates to theminor vein phloem. Whole leaf autoradiographic evidence indicateda high leaf-to-leaf variation in the image intensity over theminor veins (relative to the mesophyll/epidermal background)in both control and heat-girdled groups of leaves. The averagedegree of minor vein labelling in heat-girdled leaves, however,was found to be significantly lower than that in controls. Comparativeassessment of vein labelling was based on microscopic densityreadings of silver grains over veinal and interveinal regionsin autoradiographic images. Investigations into the cause ofthis alteration in vein labelling indicated no involvement ofan inhibition of apoplasmic phloem loading, as both heat-girdledand control leaves of Vicia were shown to have comparable minorvein uptake of exogenously supplied ¹⁴C-sucrose. Heat-girdlingwas shown, however, to increase significantly the partitioningof recently fixed carbon into the insoluble (mainly starch)fraction relative to the ethanol-soluble fraction, within 12min of the treatment. We suggest that this carbon partitioningchange can primarily account for the change in vein labelling,since an increase in the insoluble fraction would result in(1) more ¹⁴C-activity remaining in the leaf mesophyll and (2)less ¹⁴C-activity going into the mesophyll export pool, andthus, less ¹⁴C-sucrose being transferred to the minor vein region.Additionally, although leaf export was completely halted inheat-girdled leaves, ¹⁴C-activity was found within the majorveins as far as the point of petiole heat-girdling (followinga 5 min assimilation and 4 h chase). Apparently, continued (butlimited) solution flow within the sieve elements is maintainedby transport pathway unloading within the treated leaves. Key words: Phloem loading, carbon partitioning, heat-girdle, Vicia faba     

The translocation of photosynthetic products from mesophyll into midrib in tobacco plant III. The transformation of translocated 14C-sugars in the veins

¹⁴C-U-sugars were introduced into tobacco plants through themesophyll, the veins of the first order of branching, and themidrib, and ¹⁴C-compounds in the veins and the midrib whichtranslocated towards the base of the midrib were traced duringthe period of 120 min after the ¹⁴C-sugar introductions. 1) When ¹⁴C-U-sucrose was introduced into the leaf, no matterwhat the means of feeding was, most of the ¹⁴C which translocatedbasipetally in the veins and the midrib was found in the formof sucrose. 2) When ¹⁴C-U-glucose or ¹⁴C-U-fructose was administered tothe leaf dirough the cut vein of the first order of branching,most of the ¹⁴C which translocated basipetally in the veinsand the midrib was found in the form of sucrose. 3) ¹⁴C-U-glucose or ¹⁴C-U-fructose injected into the vascularbundles of the midrib was translocated basipetally, as such,10 and 30 min after injection; and at 30 min, the amount ofthe ¹⁴C-sucrose in the midrib attained 9–22% of the 80%ethanol-soluble ¹⁴C in the midrib. 4) When ¹⁴C-U-glucose or ¹⁴C-U-fructose was supplied to themesophyll, the radioactivities of these hexoses were predominantin the first and second veins soon after application, then decreasingwith a concomitant increase in the radioactivity of the ¹⁴C-sucrose. From these results, it was inferred that in the veins of thefirst and second order of branching, glucose and fructose whichmoved from the mesophyll did not translocate as such, but wereutilized for the synthesis of sucrose available for translocationvia the midrib to the stem. ¹A part of this paper was presented at the Crop Science Societyof Japan, in April, 1969 (Received December 8, 1969; )     

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)     

Environmental Factors Affecting the Loss of Carbon from the Roots of Wheat and Barley Seedlings

The amounts of carbon released into soil from roots of wheatand barley seedlings grown under three environmental conditionsfor 3 weeks with shoots in constant specific activity ¹⁴CO₂are reported. This carbon loss was measured as respired ¹⁴CO₂from both the root and the accompanying microbial populationand as root derived ¹⁴C-labelled organic C compounds in thesoil. With a 16 h photoperiod, growth at 15 ?C constant or 18?C day/14 ?C night gave a loss of 33–40% of the totalnet fixed carbon (defined as ¹⁴C retained in the plant plus¹⁴C lost from the root). The proportion of ¹⁴C translocatedto the roots that was released into the soil did not changewith temperature, so carbon distribution within the plant musthave changed. With a 12 h photoperiod and a temperature regimeof 18 ?C/14 ?C carbon loss from the roots was decreased to 17–25%of the total fixed carbon. Key words: Cereals, Roots, Carbon loss     

14C-Translocation in Kalanchoe pinnata at two Different Stages of Development

Mayoral, M. L. and Medina, E. 1985. ¹⁴C-translocation in Kalanchoepinnata at two different stages of development.—J. exp.Bot. 36: 1405–1413 Translocation of ¹⁴C-compounds from mature leaves was measuredin plants of Kalanchoe pinnata to determine the interactionbetween plant age and CAM phase when CO² is taken up. Matureleaves of 4 and 12 month old plants were fed with ¹⁴CO² eitherduring CAM phase 1 (midnight) or at the beginning of CAM phase4 (early afternoon). Export of ¹⁴C activity from source leaves,and distribution of ¹⁴C activity in soluble and insoluble compoundswas measured both in source leaves and sink organs. In 4 monthold plants 4 d were needed to export 76% of total ¹⁴C activityincorporated during CAM phase 1, while leaves labelled at thebeginning of CAM phase 4 exported 44% of total ¹⁴C activityafter 4 h, and 80% after 24 h. In both cases the major fractionof total radioactivity translocated was found in the roots inthe form of neutral sugars. Differences in translocation patternsare due to distribution of ¹⁴C in the source leaves, 96 % of¹⁴C taken up during CAM phase 1 is found in the insoluble fractionat the end of the subsequent phase 3, while 93 % of total radioactivitytaken up at the beginning of phase 4 is found in the solublefraction at the end of this phase. In 12 month old plants labelledduring phase 1 very little translocation could be detected atthe end of phase 3, while only 20% of total radioactivity wastranslocated from leaves labelled during phase 4 and measured4 h later. ¹⁴C activity in the older leaves had a similar distributionin soluble and insoluble fractions as the one determined inthe younger plants. Ability to translocate carbon compoundsfrom source leaves during phase 3 was shown by loading matureleaves at dawn with ¹⁴C-sucrose. Here again, mature leaves ofyounger plants showed faster translocation of radioactivitythan those of older plants Key words: Kalanchoe, crassulacean acid metabolism, translocation, sink, source relationships     

The Export and Distribution of 14C-labelled Assimilates from each Leaf on the Shoot of Lolium temulentum during Reproductive and Vegetative Growth

In both reproductive and vegetative plants of Lolium temulentumL., the export of ¹⁴C-labelled assimilates from each healthyleaf on the main shoot to terminal meristem, stem, tillers,and roots was measured each time a new leaf was expanded, fora period of 5 to 6 weeks. Some labelled assimilates moved fromeach leaf on the main shoot to every meristem in the same shoot,as well as to the tops and roots of adjacent organically attachedtillers. The terminal meristem of the reproductive shoot, which includedthe developing inflorescence, received 70–80 per centof the carbon assimilated by the emerged portion of the growingleaf, 15–25 per cent of the carbon assimilated by thetwo youngest expanded leaves, and 5–10 per cent of thatfrom each of the older leaves. A similar pattern of carbon supplyto the terminal meristem was found in vegetative shoots, exceptthat older leaves on young vegetative shoots supplied even lessof their carbon to the terminal meristem. The general conclusionis that developing leaves at the tip of the shoot receive aboutthe same proportion of carbon from each leaf as does a developinginflorescence. Young expanded leaves provided most labelled assimilates forstem growth; during both reproductive and vegetative growth,expanded leaves increased their export of labelled carbon tostem, and exported less of their ¹⁴C to roots and sometimesto tillers. In these reproductive and vegetative shoots, grown in a constantexternal environment, the major changes in the pattern of distributionof labelled assimilates appeared to be the result of increasedmeristematic activity in stem internodes; the development ofan inflorescence had no obvious direct effect on the carboneconomy of shoots.     

The Uptake of Amino-Acids and their Incorporation into the Proteins of Excised Barley Embryos

Methods are described by which barley embryos may be excisedand grown under sterile conditions on a medium containing sucrose,minerals, and a complete mixture of amino-acids. Growth underthese conditions was comparable with that of intact seedlingsand the uptake of sugar and amino-acid could be studied withoutdisturbing the metabolic steady state. Purified preparationsof the embryo proteins have been made and the constituent amino-acidsseparated. ¹⁴C labelling in these amino-acids was determinedby a new gas-scintillation method. In an isotopic competitionexperiment embryos were grown in ¹⁴C-sucrose with nitrate oran amino-acid mixture as nitrogen source. The presence of exogenousamino-acids suppressed the incorporation of carbon from carbohydrateinto amino-acid residues of the embryo protein. The degree ofsuppression varied, being undetectable for glutamic acid butalmost complete for lysine and leucine; it appeared to be relatedto the length of the synthetic pathway from carbohydrate tothe amino-acid. The evidence suggested that amino-acids areprotein precursors, and this conclusion was confirmed in furtherexperiments in which ¹⁴C-aspartic acid, -glutamine, -proline,-leucine, or -lysine were supplied singly in a complete mixtureof amino-acids. The ¹⁴C was found predominantly in the amino-acidresidue of protein corresponding to the ¹⁴C-amino-acid supplied,with smaller amounts in other amino-acids of the same or relatedfamilies. Aspartic acid and glutamine yielded appreciable quantitiesof respiratory carbon dioxide, although the contribution wassmall compared to that of sucrose. Little carbon was lost ascarbon dioxide from leucine or lysine. The results are discussedin relation to the role of amino-acids in protein synthesis,and to the existence of feedback control in the amino-acid metabolismof higher plants.     

The Incorporation of 14C-Proline into the Proteins of Growing Cells: I. EVIDENCE OF SYNTHESIS IN DIFFERENT PROTEINS AND CELLULAR COMPONENTS

¹⁴C-proline was supplied to aerated potato disks, in which celldivision was occurring, and also to rapidly growing potato carrotexplants. It was absorbed and incorporated into all the subcellularprotein fractions examined, including the electrophoreticallydistinguishable fractions of the soluble protein of the potatodisks and explants. The ¹⁴C-proline was partially convertedto ¹⁴C- hydroxyproline in all the protein fractions, exceptfor one of the soluble protein fractions of potato explantsand the soluble proteins of one set of potato disks. Most ofthe ¹⁴C-proline and ¹⁴C-hydroxyproline contained in the tissuewas found in the soluble protein and also in the cellular fragmentsobtained by centrifugation at 500 g. The relative importanceof the soluble protein in the incorporation of ¹⁴C-proline andits conversion to ¹⁴C-hydroxyproline was greatest over a shortperiod of a few hours of contact with the ¹⁴C-proline supplied.Over a longer period (70 hours) the cellular fragments (500g) had become the most important and contained over 40 per cent.of the total ¹⁴C, and more than 60 per cent. of the ¹⁴C-hydroxyproline,in the protein of the tissues. In the soluble fraction of potatoexplants, seven protein bands were distinguishable on electrophoresis.A different but characteristic value of the ratio ¹⁴C-hydroxyprolineto ¹⁴C-proline was associated with each protein band, exceptfor the one region where ¹⁴C-hydroxyproline did not occur. Thebasic proteins (i.e. those moving towards the cathode) werethe most active in the incorporation of ¹⁴C-proline and itsconversion to ¹⁴C-hydroxyproline. The rather general distributionof the ¹⁴C-hydroxyproline is noted and the possible siginificanceof the basic proteins and the proteins associated with the cellularfragments (500 g) is considered in relation to the growth, celldivision, and cell wall formations which occurs in the rapidlygrowing tissue cultures.     

The Use of C14-Proline by Growing Cell: Its Conversion to Protein and Hydroxyproline

C¹⁴-proline is readily absorbed by growing tissue cultures ofcarrot root phloem and of potato tuber in experiments carriedout under aseptic conditions. The C¹⁴-proline rapidly entersinto the protein of the tissue, appearing there in as shorta period as 15 minutes, and, thereafter, the amount incorporatedinto the protein bears a linear relation to time. Virtuallyall the C¹⁴ appears in the protein hydrolysate in the form ofproline and hydroxyproline. It is shown that the conversionfrom proline to hydroxyproline occurs after the C¹⁴-prolineis combined into the protein and that this conversion proceedsprogressively with time. The ratio of C¹⁴ as proline to C¹⁴as hydroxyproline declined progressively from a value of 4.0after 30 minutes of contact and seemed to become stabilizedeventually at 0.7. C¹⁴-hydroxyproline, which can be absorbedby the tissue, seems not to be incorporated into the proteinas such. The protein moiety which contains the C¹⁴-hydroxyprolinefrom C¹⁴-proline represents a stable protein which is not metabolizedand whose carbon does not ‘turn over’. This inertprotein seems to be characteristic of cells which are in rapiddivision under the influence of coconut milk or are synthesizingprotein in response to other stimuli such as the events at acut tissue surface. The protein in question seems to be presentmainly in the cytoplasm rather than in its paniculate inclusions.These results are compatible with earlier views which requirethat part of the protein in the cell ‘turns over’its carbon, whereas another part does not do so.     

Changes in Steady-State Photosynthetic 14C-Incorporation into cellular Metabolites of Chlorellax pyrenoidosa during Transitions from High to Low Irradiance

Changes in the levels of ¹⁴C-labelled metabolites were monitoredin Chlorella pyrenoidosa cells during a transition from highto low irradiance, i.e., from 700 to 430 µmol quanta (400–700nm) m^–2 s^–1. Chlorella cells assimilated ¹⁴CO₂ photosynthetically(steady-state ¹⁴C-labelling) for 12 min at the high irradianceand then 10 min at the low irradiance. With the transition tolow light, the level of ¹⁴C-labelled ribulose 1,5- bisphosphate(RuBP) did not decrease, even though the rate of total ¹⁴C-incorporationdecreased by 80%. The data suggest that RuBP carboxylase deactivatesrapidly (within 1 or 2 min) on exposure to low light, causingRuBP pool sizes to be maintained (or even increased) in spiteof a decreased rate of RuBP regeneration. There was also evidenceof light modulation of other enzymes, including some enzymesinvolved in sucrose synthesis. The rate of sucrose synthesisdecreased with decrease in light intensity while the level ofuridine diphosphoglucose increased, but within a few minutes,both returned to their former levels. ¹Present address: Chemical Biodynamics, Lawrence Berkeley Laboratory,Building 3, 1 Cyclotron Road, Berkeley, CA 94720, U.S.A. (Received March 8, 1986; Accepted June 25, 1986)     

The Influence of Plant Water Deficit on Distribution of 14C-labelled Assimilates in Cacao Seedlings

The relationship between plant water status and distributionof ¹⁴C-labelled assimilates in cacao (Theobroma cacao L.) wasevaluated after ¹⁴CO₂ pulse labelling leaves of seedlings subjectedto varying levels of water deficiency. The proportion of ¹⁴Cexported by source leaves was strongly affected by seedlingwater status. An increasing proportion of labelled assimilatesremained in source leaves at both 24-h and 72-h harvests aswater stress intensity increased. Water stress reduced the distributionof exported label to leaves and to the expanding flush in particularbut increased the proportion of label in stems and roots. Theresults suggest that current photoassimilates may be temporarilystored in source leaves and stems of cacao seedlings duringperiods of plant water deficit. The stress-induced changes inpartitioning of labelled carbon were in concordance with changesin shoot to root biomass ratios, which was likely due to greaterreduction in growth of above-ground organs to that of roots. Theobroma cacao L, assimilate partitioning, cacao, ¹⁴C-photoassimilate, water stress, water potential     

Long-term Partitioning, Storage and Re-mobilization of 14C Assimilated by Lolium perenne (cv. Melle)

The youngest fully expanded leaves of single tillers of vegetativeperennial ryegrass plants were exposed to ¹⁴CO₂. Thereafter,quantitative and fractional analysis of the partitioning, storageand re-mobilization after defoliation of the ¹⁴C-labelled assimilatewas sequentially conducted over a 22 d period. In undefoliated plants, most ¹⁴C reached its final destinationwithin 5–6 of feeding. Forty per cent of assimilated ¹⁴Cwas subsequently lost through respiration, while 13.5, 8.5 and34 per cent remained in roots, stem bases and tops respectively.At least some ¹⁴C was distributed to tillers throughout theplant, but secondary tillers subtended by the fed tiller madethe greatest demand on ¹⁴C translocated from the fed tiller. A small, but significant portion of ¹⁴C was invested into longterm storage in undefoliated plants, four per cent of the totalassimilated still being present in a labile chemical form inroots and stem bases 22 d after feeding. In plants that wereseverely defoliated 4 d after feeding, depletion of reserve¹⁴C was observed relative to undefoliated plants. The depletiontook place from stem bases, not roots, and both low and highmolecular weight storage compounds were involved. A portionof the depleted ¹⁴C was incorporated into new growth after defoliation. Lolium perenne, perennial ryegrass, assimilate partitioning, storage, re-mobilization, defoliation     

Short-term Products of 14C-acetate Assimilation by Chlorella pyrenoidosa in the Light

The kinetics of ¹⁴C-2-acetate assimilation by Chlorella pyrenoidosain the light were examined. Under aerobic conditions the primaryproduct of acetate assimilation was succinic acid which, afterten seconds, contained over 60 per cent of the ¹⁴C incorporatedby the cells. The percentage of the total ¹⁴C in succinate fellwith time, while that in citrate and glutamate increased. After1800 sec over 60 per cent of ¹⁴C was present in two compounds,glutamic acid and an unknown compound (X). Glucose-6-phosphate,fructose-6-phosphate, phosphoglyceric acid and phosphoenolpyruvicacid became labelled after 60 sec but together never containedmore than one per cent of the total ¹⁴C incorporated. Underanaerobic conditions succinate was still the primary productof acetate assimilation, and the absence of carbon dioxide resultedin a decrease in ¹⁴C incorporation into compound X. The patternof acetate assimilation in acetate grown and acetate adaptedChlorella was very similar to that in photo-autotrophicallygrown Chlorella. In the presence of 10^–6M DCMU, succinicacid was the primary product of acetate assimilation, but therewas an early Incorporation of ¹⁴C into glutamate, aspartate,and malate. 4 x10^–3M MFA did not effect the early incorporationof ¹⁴C into succinic acid, but resulted in accumulation of ¹⁴Cin citrate and a decreased amount in glutamate and in compound X.     

Metabolism of Exogenous Malonate by Coffee Leaf Tissue

When [l-¹⁴C]-malonate was supplied to discs cut from matureleaves of Coffea arabica, ¹⁴CO₂ was released (approximately12% of the total CO₂ respired) and organic acids of the Krebscycle, uronic acids, sugars and amino acids became radioactive.There was no incorporation of MC into either lipids or phenoliccompounds. The formation of glucose from malonate has not beenobserved in other studies with plant tissues. The synthesisof labelled glucose together with an active pentose phosphatepathway that is stimulated by malonate explains the accumulationof radioactive phosphogluconate in the leaf discs. Tentativeproposals are made for pathways to account for the results obtained. Key words: Coffee leaves, Malonate metabolism, Pentose phosphate pathway     

Measurements of 14C Incorporation by Illuminated Intact Leaves of Coffee Plants from Gas Mixtures Containing 14CO2

A study was made of the incorporation of ¹⁴C by intact leavesof Coffea arabica (cultivars Mundo Novo, Catuai, 1130–13,and H 6586–2) and Coffea canephora (cultivar Guarini)supplied with gas mixtures containing ¹⁴CO₂ under controlledconditions. Samples of the leaves were combusted and the ¹⁴Cin the CO₂ produced measured using a liquid scintillation counter.The results were used to estimate photosynthetic rates. Theeffects of changing the partial pressures of O₂ and CO₂ on thephotosynthetic rate were studied and estimates made of the CO₂compensation point and photorespiration. The data obtained show differences between the mean net photosyntheticrates of the C. arabica cultivars (6·14 mg CO₂ dm^–2h^–1) and the mean rate for the C. canephora cultivar (3·96mg CO₂ dm^–2 h^–1). The cultivar of the latter speciesphotorespired more rapidly than the cultivar Catuai of C. arabica.Rates of photosynthesis in coffee measured using the ¹⁴CO₂ methodwere similar to rates obtained by others using an infrared gasanalyser. The ¹⁴CO₂ method proved to be reliable for photosyntheticmeasurements and the apparatus is suitable for use in fieldconditions.     

Verification of 14C and O2 derived primary organic production measurements using an enclosed ecosystem

The stimulus for these experiments came from a recent seriesof papers which have suggested that the ¹⁴C technique may underestimateprimary production by as much as 10-fold. We evolved the followingstrategy to attempt to verify the ¹⁴C technique in nearshorewaters: (i) to examine the validity of in vitro (i.e., bottleincubation) measurements by comparing observed in situ oxygenchanges in a large enclosed natural ecosystem against thosedetermined in vitro and if no evidence of containment effectswere indicated, (ii) compare ¹⁴C and oxygen measurements insimultaneous in vitro incubations. The first step essentiallytests the containment problem, the second the physiologicaland calibration problems of the ¹⁴C technique. The first experimentwas run with nitrate as the main source of inorganic nitrogen,the second with ammonia. PQs for converting the ¹⁴C measurementsto oxygen values were calculated from the equation PQ = PQ_c+ 2/(C/NO₃) where PQ_c is the ‘carbon PQ’ (takenas 1.25) and (C/NO₃) is the molar carbon to nitrate assimilationratio. Although there appear to be some minor residual problemsin the interpretation of the data when nitrate was the dominantnitrogen source, the overriding conclusions were: first, thatthe close agreement between the changes in in situ and in vitrodissolved oxygen concentration during the photoperiod gave noevidence for any notable containment effect upon photosynthesis.Secondly, the in vitro rates of ¹⁴CO₂-determined photosyntheticproduction and gross photosynthetic oxygen production agreed,within the precision of the two techniques. The experiment furtherdemonstrated the need to determine soluble as well as particulateorganic production and to pay attention to the potential effectof the nitrogen nutrient upon the PQ. Thus it was concludedthat our data give no evidence for marked errors in the ¹⁴C-techniqueof measuring primary organic production for coastal waters. ^{Present address: Department of Marine Microbiology, Instituteof Botany, University of Gothenburg, Carl Skottsbergs Gata 22,S-413 19 Gothenburg, Sweden.}