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; )     

Photosynthesis, Respiration, and Carbon Assimilation in Water-Stressed Maize at Two Oxygen Concentrations

Photosynthesis decreased with decreasing leaf water potentialas a consequence of stomatal closure and possibly non-stimataleffects of severe stress. Assimilation ceased at c. 16x 10⁵Pa. Photo-respiration, in 21% O₂, was small in relation to assimilationin unstressed leaves and decreased as leaf water potential fellbut it was much larger in proportion to photosynthesis at severestress. Decreasing the O₂ content to 1.5% increased photosynthesisslightly and decreased photo-respiration but did not changethe stress at which assimilation stoped. Dark respiration wasinsensitive to both O₂ and stress. Less ¹⁴C accumulated in stressedleaves but in 21% O₂ a greater proportion of it was in aminoacids, particularly glycine and serine. 1.5% O₂ decreased the¹⁴C in glycine to 10% and in serine to 50% of their levels in21% O₂. In both O₂ concentrations the proportion of ¹⁴C in serineincreased only at the most severe stress. Gas exchange measurementsand changes in the ¹⁴C flux to glycine are interpreted as theresult of glycolate pathway metabolism increasing as a proportionof assimilation in stressed leaves in high O₂. The small absoluterate of photorespiration in high O₂ and at low leaf water potentialmay be due to slow rates of glycine decarbodylation as wellas efficient fixation of any CO₂ produced. Serine is synthesizedby an O₂-sensitive pathway and an O₂-insensitive pathway, whichis most active at severe stress. Synthesis of alanine competeswith that of glycine and serine for a common precursor suppliedby the photo-synthetic carbon reduction cycle. The relativespecific radioactivities of aspartate and alanine suggest thatthey are derived from a common precursor pool, probably pyruvatefrom 3-PGA. The amounts of 3-PGA, aspartate, malate, alanine,and sucrose decreased with increasing water stress as a consequenceof slower assimilation and pool filling. Other amino acids,glycine, serine, glutamate, and proline, accumulated at lowwater potential possibly due to increased synthesis and slowerrates of consumption. Changes in pool sizes, carbon fludes,and specific activities of metabolites are related to the mechanismof C₄ photosynthesis and current concepts of glycolate pathwaymetabolism.     

Effects of Certain Inhibitors on Photorespiration by Wheat Leaf Segments

The effect on the carbon metabolism of wheat leaf segments ofcertain inhibitors of photorespiration was studied. Sodium 2-hydroxy-3-butynoatesupplied for 40 min resulted in accumulation of ¹⁴C in glycolicacid with only a 7% inhibition of photosynthesis; when suppliedfor 90 min, photosynthesis was inhibited by 47%. When ¹⁴CO₂was replaced by 1000 vpm ¹²CO₂, radioactivity in glycine decreasedbut increased more rapidly in sucrose with less release of ¹⁴CO₂.Isonicotinyl hydrazide (INH) inhibited photosynthesis from ¹⁴CO₂by 50% and glycine replaced sucrose as the main product. When,after 15 min, ¹⁴CO₂ was replaced by 150 vpm ¹²CO₂, in the presenceof INH less ¹⁴CO₂ was released, ¹⁴CO in glycine decreased moreslowly, and less [¹⁴CO]sucrose accumulated. Glycidate (potassium2,3-epoxypropionate) at 2 mM had no effect on photosyntheticrate and little effect on carbon metabolism; 20 mM glycidateinhibited photosynthesis by 64% and resulted in less radioactivityin glycine, more in phosphate esters, and less ¹⁴CO₂ released.When photosynthesis was measured in 1000 vpm CO₂ the inhibitorsgave smaller effects on metabolism than during photosynthesisfrom 150 vpm ¹⁴CO₂ but 20 mM glycidate still resulted in a 42%inhibition of photosynthesis. When U- [¹⁴CO]glycerate was appliedto leaf segments in air with 320 vpm ¹⁴CO₂ the total uptakeof glycerate was not changed by the inhibitors. INH and glycidateboth decreased the amount of glycerate metabolised. More ¹⁴COaccumulated in glycine in the presence of INH and in phosphateesters and serine in the presence of glycidate. Hydroxybutynoateincreased the production of glycolate from glycerate but didnot affect the total amount of glycerate metabolised. Although all three inhibitors affected photorespiratory metabolismnone stimulated photosynthesis. The results are consistent withthe main release of CO₂ in photorespiration arising from theconversion of glycine to serine.     

Effects of Oxygen on Metabolism by the Glycollate Pathway in Leaves

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

CO2-Fixation, Glycolate Formation, and Enzyme Activities of Photorespiration and Photosynthesis during Greening and Senescence of Sunflower Cotyledons

Time-courses of ¹⁴CO₂-fixation and of enzyme activities involvedin photorespiration and photosynthesis were determined duringthe life span of cotyledons from sunflower seedlings (Helianthusannuus L.). Glycolate formation in vivo was estimated from theresults of combined labelling and inhibitor experiments. NADPH-glyceraldehyde-3-phosphatedehydrogenase, NADPH-glyoxylate reductase and chlorophyll werewell correlated with the time-course of ¹⁴CO₂-fixation (photosynthesis).There was, however, a considerable discrepancy between the developmentalsequence of photosynthesis and that of both ribulose-l,5-bisphosphatecarboxylase and glycolate oxidase. Furthermore, time-coursesof glycolate oxidase activity in vitro and of glycolate formationin vivo differed significantly. Therefore, the use of glycolateoxidase as a marker for the activity of photorespiration ingreening sunflower cotyledons may be questionable. Results from¹⁴CO₂-labelling experiments with cotyledons treated with theglycolate oxidase inhibitor 2-hydroxy butynoic acid suggestthat glycolate formation relative to CO₂-fixation is reducedin senescent cotyledons. Key words: Development, glycolate oxidase, photorespiration, ribulose-l,5-bisphosphate carboxylase, oxygenase     

Photosynthesis and photorespiration in soybean [Glycine max (L.) Merr.] chronically exposed to elevated carbon dioxide and ozone

The effects of elevated carbon dioxide (CO₂ and ozone (O₃) onsoybean (Glycine max (L.) Merr.] photosynthesis and photorespiration-relatedparameters were determined periodically during the growing seasonby measurements of gas exchange, photorespiratory enzyme activitiesand amino acid levels. Plants were treated in open-top fieldchambers from emergence to harvest maturity with seasonal meanconcentrations of either 364 or 726 µmol mol^–1 CO₂in combination with either 19 or 73 nmol mol^–1 O₃ (12h daily averages). On average at growth CO₂ concentrations,net photosynthesis (A) increased 56% and photorespiration decreased36% in terminal mainstem leaves with CO₂ enrichment. Net photosynthesisand photorespiration were suppressed 30% and 41%, respectively,by elevated O₃ during late reproductive growth in the ambientCO₂ treatment, but not in the elevated CO₂ treatment. The ratioof photorespiration to A at growth CO₂ was decreased 61% byelevated CO₂ There was no statistically significant effect ofelevated O₃ on the ratio of photorespiration to A. Activitiesof glycolate oxidase, hydroxypyruvate reductase and catalasewere decreased 10–25% by elevated CO₂ and by 46–66%by elevated O₃ at late reproductive growth. The treatments hadno significant effect on total amino acid or glycine levels,although serine concentration was lower in the elevated CO₂and O₃ treatments at several sampling dates. The inhibitoryeffects of elevated O₃ on photorespiration-related parameterswere generally commensurate with the O₃-induced decline in A.The results suggest that elevated CO₂ could promote productivityboth through increased photoassimilation and suppressed photorespiration. Key words: Photorespiration, CO₂-enrichment, ozone, climate change, air pollution     

Ammonia Induces Starch Degradation in Chlorella Cells

When ammonia was added to cells of Chlorella which had fixed¹⁴CO₂ photo synthetically, ¹⁴C which had been incorporated intostarch was greatly decreased. A similar effect was observedwhen potassium nitrate and sodium nitrite were added. The ammonia-induceddecrease in ¹⁴C-starch was observed in all species of Chlorellatested. With cells of C. vulgaris 11h, most of the radioactivityin starch was recovered in sucrose, indicating that ammoniainduces the conversion of starch into sucrose. The percent of¹⁴C recovered in sucrose differed from species to species andpractically no recovery in sucrose was observed in C. pyrenoidosa.In most species tested, the enhancing effects of blue lightand ammonia on O₂ uptake as well as the ammonia effect on starchdegradation were greater in cells which had been starved inphosphate medium in the dark than in non-starved cells. In contrast,the enhancing effect of ammonia on dark CO₂ fixation was muchgreater in non-starved cells. C. pyrenoidosa was unique in thatblue light did not show any effect on its O₂ uptake. (Received August 15, 1984; Accepted November 16, 1984)     

Glycine as a substrate for photorespiration

Substrates for photorespiration were examined by feeding ¹⁴Clabeled compounds to tobacco and corn leaf segments and by measuring¹⁴CO₂ evolution in light and darkness. CO₂ release in the darkwas rapid, but in light CO₂ release was slow due to refixationby photosynthesis. Carboxyl labeled glycine was more rapidlydecarboxylated than were glyoxylate, glycolate or serine. Hydroxypyridinemethanesulfonate, an inhibitor of glycolate oxidase, blocked CO₂ releasefrom glycolate but not from glycine. Isonicotynyl hydrazideblocked CO₂ release from both glycine and glycolate. DCMU blockedphotosynthetic refixation of the released CO₂, consequentlythe rates of CO₂ release in light and dark were about equal.It was concluded that CO₂ release during photo-respiration camefrom the conversion of 2 molecules of glycine to one serineand one CO₂. ¹⁴CO₂ release from glycine-l-¹⁴C in the dark or with DCMU inlight can be used as an assay for photorespiration ability. CO₂ release from glycine and glycolate by corn leaf segmentsin the dark proceeded at the rate of that in normal tobaccoleaf. This result, together with other work on O₂ exchange andenzymatic analysis, indicates that corn and other plants docarry on photorespiration, but it is not manifested by CO₂ releasein light. A yellow tobacco mutant, Consolation 402, had high rates ofphotorespiration by the ¹⁴CO₂ assay, nearly half (or more) asmany peroxisomes as chloroplasts, and high rates of CO₂ releasefrom glycine-l-¹⁴C or glycolate-l-¹⁴C. A common tobacco, BrightYellow, had lower rates of photorespiration, fewer visible peroxisomes,and slower decarboxylation of glycine and glycolate. The amount of ¹⁴CO₂ release from glycine-l-¹⁴C or glycolate-l-¹⁴Cincreased only slightly when the temperature was raised from25 to 35°C. ¹Parts of this work were abstracted at the Annual Meeting (April,1969) of Japanese Society of Plant Physiologists, Kanazawa ²Department of Biochemistry, Michigan State University, EastLansing, Michigan, U.S.A. (Received September 3, 1969; )     

The Effect of Isonicotinyl Hydrazide on the Photosynthetic Incorporation of Radioactive Carbon Dioxide into Ethanol-Soluble Compounds of Chlorella

The effect of 10^-2M. isonicotinyl hydrazide (isoniazid) on theincorporation of radioactive carbon dioxide by Chlorella duringphotosynthesis has been studied under steady-state conditionsat two carbon dioxide concentrations. Isoniazid treatment resultsin increased radioactivity in sucrose, glycollic acid, and glycineand decreased radioactivity in sugar monophosphates, serine,and alanine. An unidentified compound which is strongly radioactiveafter short-term exposures to ¹⁴CO₂ is present in isoniazid-treatedcells. It is suggested that isoniazid pre-dominantly inhibitsthe conversion of glycine to serine.     

Oxygen Exchange in Relation to Carbon Assimilation in Water-stressed Leaves During Photosynthesis

In a study on metabolic consumption of photosynthetic electronsand dissipation of excess light energy under water stress, O₂and CO₂ gas exchange was measured by mass spectrometry in tomatoplants using ¹⁸O₂ and ¹³CO₂. Under water stress, gross O₂ evolution(E_O), gross O₂ uptake (U_O), net CO₂ uptake (P_N), gross CO₂ uptake(TPS), and gross CO₂ evolution (E_C) declined. The ratio P_N/E_Ofell during stress, while the ratios U_O/E_O and E_C/TPS rose.Mitochondrial respiration in the light, which can be measureddirectly by ¹²CO₂ evolution during ¹³CO₂ uptake at 3000 µll^{–1 13}CO₂, is small in relation to gross CO₂ evolutionand CO₂ release from the glycolate pathway. It is concludedthat PSII, the Calvin cycle and mitochondrial respiration aredown-regulated under water stress. The percentages of photosyntheticelectrons dissipated by CO₂ assimilation, photorespiration andthe Mehler reaction were calculated: in control leaves morethan 50 % of the electrons were consumed in CO₂ assimilation,23 % in photorespiration and 13 % in the Mehler reaction. Undersevere stress the percentages of electrons dissipated by CO₂assimilation and the Mehler reaction declined while the percentageof electrons used in photorespiration doubled. The consumptionof electrons in photorespiration may reduce the likelihood ofdamage during water deficit.     

The Mechanism of Photosynthesis in the Tea Plant (Camellia sinensis L.)

Leaves of the tea plant photosynthesizing in ¹⁴CO₂ incorporatedmuch radioactivity into intermediates of the glycolate pathwayand little into C₄ acids. Increased O₂ in the atmosphere decreasedphotosynthesis, stimulated photorespiration, and increased theCO₂ compensation point. In air the rate of photorespirationwas 19% of net photosynthesis. These observations indicate aC₃ rather than a C₄ mechanism of photosynthesis.     

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; )     

Biosynthetic mechanism of glycolate in Chromatium IV. Glycolate-glycine transformation

The metabolic transformation of glycolate to glycine occurringin photosynthesizing cells of Chromatium was investigated bythe radioisotopic technique and by amino acid analysis. By analyzingthe distribution of radiocarbon upon feeding [1-¹⁴C] glycolate,[2-¹⁴C] glyoxylate and [1-¹⁴C] glycine to bacterial cells, itwas demonstrated that glycolate is converted to glycinc viaglyoxylate, and both glycolate and glycine are excreted extracellularly.Although the formation of serine was barely detected by theabove two techniques in both N₂ and O₂ atmospheres, it was foundthat ¹⁴CO₂ is evolved quite markedly from both [1-¹⁴C] glycolateand [1-¹⁴C] glycine fed to the Chromatium cells. Analyticalresults of transient changes in amino acid compositions underatmospheric changes of N₂O₂ and by the addition of exogenousglycolate in N₂ confirm the notion that glycolate is convertedto glycine. Acidic amino acids (glutamic acid and aspartic acid)appear to take part in glycine formation as amino donors. Theformation of glycine from glycolate in a N₂ atmosphere suggeststhat an unknown glycolate dehydrogenation reaction may operatein the overall process. ¹ This is paper XXXVII in the series ‘Structure and Functionof Chloroplast Proteins’. Paper XXXVI is ref. (5). Theresearch was supported in part by grants from the Ministry ofEducation of Japan (No. 111912), the Toray Science Foundation(Tokyo) and the Naito Science Foundation (Tokyo). (Received July 14, 1976; )     

Biosynthetic mechanism of glycolate in Chromatium I. Glycolate pathway

The pattern of radioactivity distribution in several amino acidsof Chromatium cells exposed to ¹⁴CO₂ was determined. By transferringthe bacterial cells from an atmosphere of nitrogen to oxygenthere occurred a transient decrease of ¹⁴CO₂ incorporation intoaspartate and glutamate, whereas that into glycine showed aprominent increase. The labeling of both serine and alaninedid not show a marked change under such conditions. The, activitiesof glycolate oxidase and glycolate dehydrogenase in crude extractsof the bacterial cells were very low. The formation of glycolic acid only occurred during the oxidativemetabolism of Chromatium cells grown on bicarbonate as a C source,being negligibly small in bacteria under nitrogen or after growthon malate or acetate. The activities of both ribulose- 1,5-bisphosphateoxygenase and phosphoglycolate phosphatase in the extract preparedfrom the bicarbonate-grown bacterial cells were very low andapparently could not account for the glycolic acid formationthrough these enzymic reactions. Metabolic patterns of glycolicacid in Chromatium are discussed in relation to the photorespiratoryphenomenon. (Received February 24, 1975; )     

Biosynthetic mechanism of glycolate in Chromatium 6. Glycolate formation and metabolism under low O21

Under low O₂ (0.05 mM O₂), there was no measurable excretionof glycolate or glycine by Chromatium cells, unlike the caseof their incubation under high (0.7 mM) O₂ However, upon additionof non-radioactive glycolate and glycine to the suspension medium,there occurred a measurable incorporation of ¹⁴CO₂ into thesecompounds, which were then excreted extracellularly; the totalradioactivities measured were approximately 15% of the totalCO₂ fixed photosynthetically. This phenomenon could be as cribedto the dilution of the intracellular pools by the compoundsadded. The results indicate that under low O₂ the glycolatemolecules produced are metabolically further transformed inthe bacterial cells. The incorporation of ¹⁴CO₂ into the extracellularglycolate fraction was maximal at 0.3 mM glycolate in both highand low O₂. Presumably, glycolate formed in the bacterial cellsunder both the high and low O₂ is metabolized in a similar manner,although the excess glycolate and glycine molecules are rapidlyexcreted. During glycolate metabolism CO₂ was evolved from anisonicotinylhydrazide-sensitive reaction, suggesting that thepathway <glycolate glycine . CO₂ was similar in green plants.The results thus indicate that studies on glycolate and glycinemetabolism in the anaerobic bacterium, Chromatium, provide auseful model system for elucidating the mechanism of photorespirationin green plants. (Received May 19, 1978; )     

Effect of Oxygen and Carbon Dioxide on Photorespiratory Flux Determined from Glycine Accumulation in a Mutant of Arabidopsis thaliana

Exposure to atmospheric conditions which promote photorespirationstrongly inhibits photosynthesis in a mutant of Arabidopsislacking mitochondrial serine transhydroxymethylase activity,and glycine accumulates as a stable end-product of photorespiratorycarbon and nitrogen flow. By providing exogenous serine andammonia to leaves of the mutant, wild-type photosynthesis ratescan be temporarily maintained in the absence of photorespiratoryCO₂ evolution. In these circumstances, the rate of glycine accumulationprovides a direct measure of photorespiratory flux which isnot complicated by the efflux and refixation of photorespiredCO₂, the dilution of radioactive label by endogenous metabolicpools, or non-specific effects of metabolic inhibitors. At thestandard atmospheric concentration of CO₂, the rate of glycineaccumulation in the mutant was proportional to the oxygen concentration,amounting to 53% of the rate of gross CO₂-fixation at 21% O₂.At normal levels of O₂, glycine accumulation was maximal atabout 475 µl CO₂1^–1 and was reduced at higher orlower CO₂ concentrations, being almost abolished at 3000µ1CO₂1^–1. These observations are discussed in the contextof a model of photorespiration based on the properties of ribulose1, 5-bisphosphate carboxylase/oxygenase, and in relation tothe results of previous attempts to measure photorespiration.Preliminary evidence from ¹⁴CO₂-labelling experiments whichsuggests a non-photorespiratory pathway of serine synthesisis also presented. Key words: Arabidopsis mutant, Photorespiration, Serine transhydroxymethylase     

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

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

Effects of temperature and CO2 concentration on photosynthetic CO2 fixation by Chlorella

The rates of photosynthetic ¹⁴CO₂ fixation by Chlorella vulgarisllh, grown under high CO₂, were determined between 4 to 37°Cwith air containing from 300 to 13,000 ppm ¹⁴CO₂. When the CO₂level was increased, both the rate of photosynthesis and theoptimum temperature for maximum photosynthesis increased. Themaximum photosynthetic rate was reached at 12°C with 300ppm ^l4CO₂. Among the photosynthetic products fromed at 300 ppm ¹⁴CO₂, glycolatedecreased greatly when the temperature was raised from 20 to30°C. At 3,000 ppm ¹⁴CO₂ an insignificant amount of glycolatewas formed at all temperatures, whereas ¹⁴C-incorporation intothe insoluble fraction, sucrose, and the lipid fraction wassignificantly higher than at 300 ppm ¹⁴CO₂. The ¹⁴C in sucrosewas greatly increased and the radioactivity in the insolublefraction decreased when the temperature was raised from 28 to36°C. (Received April 8, 1980; )     

Stimulation of Malate Synthesis by Glycolate in Tomato Leaves in Relation to CO2 Concentration

The mechanism by which malate synthesis from CO2 is increasedunder low concentrations of CO₂ was investigated in C₃ plants.A number of metabolites were administered to illuminated tomatoleaves, and their effects on the incorporation of ¹⁴CO₂ intomalate were determined. Compared with water as a control, glycolate,glyoxylate, D,L-glycerate, glycine, phosphoglycolate and L-serineincreased malate synthesis by factors of 6.8, 3.8, 3.3, 2.5,2.3 and 2.2, respectively. The effect of exogenous glycolateon malate synthesis from CO₂ was dependent on its concentrationup to 100 mu, but was independent of ambient CO₂ concentration.The feeding of l-¹⁴C-glycolate in the light indicated that glycolatestimulated the carbon flow from CO₂ to malate. The analysis of the products of ¹⁴CO₂ fixation in illuminatedleaves supplied with glycolate showed increases in malate andsugar and decreases in serine and phosphate esters. However,this stimulated malate synthesis ceased when malonate was suppliedsimultaneously with glycolate. Treatment with glycolate didnot affect the dark ¹⁴CO₂-fixation, but increased the ¹⁴C-malatesynthesis, with a corresponding decrease in ¹⁴C-aspartate and14^C-glutamate. These results suggest that exogenous glycolateactivates malate dehydrogenase in leaves, and that the increasedglycolate formation at low CO₂ concentrations is associatedwith the increased malate synthesis from CO₂. (Received January 12, 1981; Accepted May 20, 1981)     

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.