Photosynthesizing Leaves and Nodulated Roots as Donors of Carbon to Protein of the Shoot of the Field Pea (Pisum arvense L.)

In Pisum arvense, the amides and amino-acids normally suppliedto the shoot in the transpiration stream transfer carbon toprotein largely throught the amino-acids, aspartic acid (+asparagine),glutarnic acid (+glutamine), threonine, lysine, arginine, andproline. Carbon from carbon dioxide enters the protein of photosynthesizingtissues through an essentially complementary set of amino-acidsincluding glycine, alanine, serine, valine, and the aromaticamino-acids tyrosine, phenylalnine, and histidine. Young tissuesof the shoot synthesize certain amino-acids de novo by metabolismof sugars supplied from photosynthesizing leaves. Each mature leaf on a shoot contributes carbon to current synthesisof protein at the shoot apex. Sucrose accounts for more than90 per cent of the labelled carbon leaving any age of leaf whichhas been fed with ¹⁴CO₂. Upper leaves supply labelled assimilatesdirectly to the shoot apex, and the radiocarbon from these assimilatesis subsequently incorporated into a wide range of amino-acidunits of protein. The majority of the labelled assimilates exportedfrom a lower leaf move downwards to the root and nodules and,in consequence, the amino-acids and amides associated with rootmetabolism are strongly represented among the compounds eventuallylabelled in the apical region of the shoot.     

Storage Pools and Turnover Systems in Growing and Non-growing Cells: Experiments with 14C-Sucrose, 14C-Glutamine, and 14C-Asparagine

¹⁴C-labelled sucrose, glutamine, and asparagine have been suppliedto aseptically cultured carrot explants that either grew rapidlyby cell division or, by contrast, only slowly by cell expansion.The radioactive substrates were supplied in a brief ‘pulse’followed by a much longer period during which the tissues weresupplied with ¹²C-substrates. The passage of ¹⁴C through thevarious soluble compounds of the tissue and into the proteinwas followed. Alternatively, the ¹⁴C-labelled compound was suppliedthroughout the entire period of an experiment while the tissuealso received ¹²C-sucrose. The pulse-labelling experiments demonstrateturnover and the fate of the breakdown products, as well asthe emphasis placed on this kind of metabolism by cells at differentlevels of activity in their growth. The long-term labellingexperiments show the different ways in which carbon from varioussources may be used and how these pathways are affected by growth.The amount of ¹⁴C present in the various free (ethanol soluble)and combined (ethanol insoluble, acid-hydrolysable compounds—proteins)was determined, as well as the specific activity of the carbonin each compound. The fate of ¹⁴C supplied as sucrose had muchin common with ¹⁴C supplied as glutamine, with respect to theease with which it entered both the protein being synthesizedand the carbon dioxide evolved, but it was very different from¹⁴C supplied as asparagine. To interpret these data, compartmentsor pools of metabolites are postulated in the organized cell;exogenous ¹⁴C-sucrose and ¹⁴C-glutamine readily furnish carbonfor pools of amino-acids en route to protein, which are protectedfrom both the stored compounds and those which arise after proteinbreakdown. However, exogenous ¹⁴C-asparagine enters, is accumulated,and persists in the pool of stored compounds which also receivethe nitrogen-rich substances that arise from protein breakdown.The kinetic data and the specific activities of the carbon inits various forms require that protein breakdown and re-synthesisoccur concomitantly, that the stimulus to grow, exerted by coconutmilk, accentuates protein synthesis and also the pace of itsturnover, that some respired carbon dioxide arises from protein,and that this moiety of the respiration is increased by thecoconut-milk stimulus as it accentuates the pace of cyclicalturnover. In similar experiments with free cells from differentplants, the same general conclusions apply, but the rates ofturnover of protein are greater in free cells than in tissueexplants. Some specific differences, however, exist. Cells ofArachis, the only legume investigated, permit ¹⁴C-asparagineto contribute, like ¹⁴C-glutamine, to both protein synthesisand respired ¹⁴CO₂; it is not merely segregated in a storagepool. Thus, by virtue of their organization, plant cells maintainthe same substances simultaneously in distinct phases or compartments,where they play distinctive roles, without mingling. Geneticsendows each cell with the information that makes its biochemicalreactions feasible; the organization of the cells determineshow far the feasible becomes practised in cells in any givensituation.     

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.     

Nitrogen-containing Compounds in the Shoot System of Pisum arvense L.: II. The Significance of Amino-acids and Amides Released from Nodulated Roots

A pulse-labelling technique is described for applying specificradio substrates to detached shoots via the transpiration stream.Unlabelled bleeding sap is used as a carrier for the substratesand as a chaser during a further period of assimilation. Thesubstrates studied are ¹⁴C-U-aspartic acid, ¹⁴C-U-asparagine,¹⁴C-U-gluta-mic acid, ¹⁴C-U-glutamine, ¹⁴C-homoserine, ¹⁴C-₂-glycine,and metabolically labelled samples of bleeding sap. All substratescontribute carbon to chloroform-soluble and water-soluble substances,and to the protein and other insoluble constituents of the shoot.Each substrate makes a characteristic contribution of carbonto the soluble reserves and protein of the shoot. Members ofthe aspartate and glutamate families of amino-acids dominateamong the products labelled during assimilation of the fiveradiosubstrates which are regular constituents of the sap. Resultsare contrasted with feeding ¹⁴C-₂-glycine, an amino-acid normallyabsent from the bleeding sap.     

Effects of Chloramphenicol on the Uptake and Incorporation of Amino-acids by Carrot Root Tissue

The uptake of uniformly labelled L-glutamic acid-¹⁴C, glycine-¹⁴C,and L-proline-¹⁴C by carrot root tissue is inhibited by chloramphenicolat a concentration of 2 g./l. L-glutamic acid absorption isaffected to a much greater extent than is the uptake of theother two amino-acids. It is shown that the ¹⁴C from the labelledamino-acids is incorporated into protein of the carrot slices,and that the amount of incorporation from glycine and L-prolineis about six times as large as that from L-glutamic acid undercomparable conditions. Almost all the ¹⁴C detected in the proteinafter feeding labelled L-proline or glycine remains in the amino-acidsupplied; in the case of L-glutamic acid some of the label istransferred to other amino-acid residues, but most of it remainsin glutamic acid. Chloramphenicol is found to inhibit ¹⁴C-incorporationfrom L-proline and glycine in short-term, but not in long-term,experiments whereas incorporation from L-glutamic acid is inhibitedin both long- and short-term experiments. Chloramphenicol alsoinhibits incorporation of ¹⁴C from L-glutamic acid into proteinsof preparations of isolated subcellular particles from carrotroot tissue. Under some conditions, ¹⁴C incorporation into proteinproceeds for some time after the slices have been removed fromthe radioactive solution. Such incorporation is not inhibitedsignificantly by chloramphenicol when ¹⁴C-labelled L-prolineis supplied, but it is reduced by about 50 per cent. when glycineand L-glutamic acid are used. It is shown that chloramphenicolinhibits net protein synthesis in carrot root slices suspendedin aerated solutions. It is concluded that amino-acid incorporationinto the protein of carrot slices proceeds by at least two mechanisms,one of which is related to protein synthesis and is sensitiveto chloramphenicol and the other is unrelated to protein synthesisand insensitive to the drug. Most of the L-proline and glycineincorporation into carrot root tissue protein seems to occurby the chloramphenicol-insensitive mechanism while much of theL-glutamic acid-¹⁴C incorporation seems to take place by thechloramphenicol-sensitive one.     

The Metabolism of Labelled Lysine and Pipecolic Acid by Acacia Phyllodes

C¹⁴-lysine and C¹⁴- and H³-pipecolic acids have been used tostudy the metabolism of the lysine family of amino-acids inAcacia, which contains ₄-hydroxypipecolic acid as a characteristiccomponent of the soluble-nitrogen fraction. Degradation of C¹⁴-lysinewas rapid and was far more extensive than that observed earlierin higher plants. Pipecolic acid was the major radioactive productin short-term experiments. After longer metabolic periods, radioactivitywas distributed over a wide range of amino-acids, organic acids,and sugars. A tentative metabolic scheme is produced to explainthese observations concerning lysine degradation. The distributionof radioactivity in the amino-acids of the protein present inthe phyllodes was determined 24 hours after supplying C¹⁴-lysine.Specific activities of free and bound amino-acids are comparedat this time. Hydroxyproline forms a notable component of thephyllode protein. Pipecolic acid degradation has been demonstrated for the firsttime in a biological system. The breakdown pathway was studiedin Acacia phyllodes using H²-pipecolic acid. Substances tentativelyidentified as '-piperdine-2-carboxylic acid and -amino--hydroxycaproicacid were amongst the early degradation products. Ultimately,lysine and -aminoadipic acid became labelled. In contrast tothe experiments with C organic acids and sugars did not becomeradioactive. The explanation of this finding is probably tobe found in the ease with which H³ atoms present in certainchemical groupings may undergo exchange with normal hydrogenatoms of water molecules. The biosynthesis of 4-hydroxypipecolic acid probably involvesa direct hydroxylation of the parent imino-acid.     

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 Incorporation of 14CO2 into Green Peas in the Dark

Shelled green peas (ripening seeds of Pisum sativum var. Onward)were kept in the dark for 2 or 3 days in an atmosphere, eitherof air or 10 per cent carbon dioxide in air, containing ¹⁴CO₂.Samples were removed at intervals, the acids of the T.C.A.C.extracted, estimated, and their content of ¹⁴C measured. Incorporationof ¹⁴C was mainly into the carboxyl carbon atoms of the acidswhich, with the exception of citrate, rose in specific activityrapidly for the first 4–6 h and then levelled off androse slowly for the rest of the experiment. The final valuesattained, again with the exception of C-6 of citrate, were muchbelow that of the tissue carbon dioxide. The cause of this failureto equilibrate with tissue carbon dioxide is discussed. Twomain carboxylation reactions are proposed, one from pyruvate(or other three carbon acid) to malate and the other from -oxoglutarateto oxalsuccinate and so to C-6 of citrate. The value of thevelocity constant of the first of these reactions was shownto be markedly decreased by increase in tissue carbon dioxidewhile that for the second carboxylation was unaffected. ¹⁴Cmoved into soluble amino-acids rapidly at first and then moreslowly; protein received ¹⁴C at a high rate throughout the experimentsmainly by isotopic exchange reactions. Calculations were madeof the rate of movement of carbon in the T.C.A.C. which wouldhave been required to give the observed changes in specificactivity of the metabolites but no scheme tested fitted allthe results satisfactorily.     

Synthesis of Citrulline and Arginine in Chlorella pyrenoidosa

The distribution of radioactivity in Chlorella during dark ¹⁴CO₂fixation was investigated either (a) in normal cells with andwithout added ammonium chloride, or (b) in nitrogen-starvedcells supplied with intermediates of the Krebs-Henseleit ureacycle. In the control experiments almost all the activity was presentin compounds of or associated with, the tricarboxylic acid cycle. The amino-acids citrulline and arginine became radioactive onlyin the presence of ammonia or ornithine where initially theycomprised 40–60 per cent. of the total activity, reactionsof the Krebs–Henseleit urea cycle being implicated intheir formation. No evidence could be found for a complete ureacycle. Unidentified compounds deriving their radioactivity fromthe C₄ carbon of citrulline and/or arginine were detected andformed up to 40 per cent. of the total ¹⁴CO₂ incorporated after25 min.     

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.     

Adaptation of Amino-acid Metabolism in Protein-depleted Rats

MCFARLANE and von Holt¹ showed that the oxidative degradation of two essential amino-acids (leucine and phenylalanine), given intraperitoneally, was decreased in rats suffering from protein-calorie deficiency, whereas that of two non-essential amino-acids (glutamate and alanine) was not greatly affected. They suggested that during protein deficiency, adaptive mechanisms came into operation resulting in the conservation of the essential carbon skeleton of the essential amino-acids. To investigate further these adaptive changes, I administered both ¹⁴C amino-acids and ¹⁵N-yeast protein to normally fed and protein-depleted rats and investigated the fate of both the carbon and nitrogen moieties.     

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.     

Effects of Carbon Dioxide on Metabolism by the Glycollate Pathway in Leaves

When solutions of [¹⁴C]glycollate, glycine, serine, glycerate,or glucose were supplied to segments of wheat leaves throughtheir cut bases in the light, most of the ¹⁴C was incorporatedinto sucrose in air but in CO₂-free air less sucrose was made.The synthesis of sucrose was decreased because metabolism ofserine was partly blocked. Sucrose synthesis from glucose andglycerate in CO₂-free air was decreased but to a smaller extent;relatively more CO₂ was evolved and serine accumulated. Theeffects of DCMU and light of different wavelengths on metabolismby leaves of L-[U-¹⁴C]serine confirmed that simultaneous photosyntheticassimilation of carbon was necessary for the conversion of serineto sucrose. Of various products of photosynthesis fed exogenouslyto the leaves -keto acids were the most effective in promotingphotosynthesis of sucrose and release of ¹⁴CO₂ from ¹⁴C-labelledserine. This suggests that in CO₂-free air the metabolism ofserine may be limited by a shortage of -keto acid acceptorsfor the amino group. In CO₂-free air added glucose stimulatedproduction of CO₂ and sucrose from D-[U-¹⁴C]- glycerate andno competitive effects were evident even though glucose is convertedrapidly to sucrose under these conditions. In addition to asupply of keto acid, photosynthesis may also provide substratesthat can be degraded and provide energy in the cytoplasm forthe conversion of glycerate to sugar and phosphates and sucrose.     

The Incorporation of 14C into the Protein of Particles Isolated from Plant Cells

The incorporation of ¹⁴C from labelled fructose, succinate,urea, and proline, by particulate preparations from dormantand tissue-cultured carrot cells, is examined. It is shown that¹⁴C is incorporated readily from proline, and less readily fromfructose. No significant incorporation occurs from succinateor urea. No differences are noted between the two kinds of preparation.It is concluded that the incorporation of ¹⁴C does not dependon prior transfer of the label to carbon dioxide followed byfixation of carbon dioxide, since the particles do not incorporate¹⁴C from supplied carbon dioxide. Incorporation of ¹⁴C by various fractions of dormant carrottissue is examined, and it is established that the greatestincorporation per mg. nitrogen occurs in particles isolatedat 10,000 g. A total cell homogenate fails completely to incorporate¹⁴C from proline into protein, and this may be due to suppressionof the activity of the particles by a constituent of the supernatantliquid. The presence of coconut milk reduces the incorporationof ¹⁴C from proline by particles sedimented at 10,000 g, andaddition of a protein hydrolysate reduces it further. Hydroxy-prolinedoes not appear to compete with proline for incorporation, andin this respect the paniculate preparations contrast with wholecells. Particles from carrot tissue are shown to be more active inincorporating ¹⁴C from proline than are particles extractedby the same procedure from red beet roots, potato tubers, andskunk cabbage inflorescences. They are, however, considerablyless active than a mitochondrial preparation from rat liver. It is demonstrated by paper chromatography that the bulk ofthe ¹⁴C incorporated in the particles from carrot cells remainsin proline and there is little or no conversion of proline tohydroxyproline in the preparations. The nature of the particlesemployed in this investigation is discussed, and their metabolismconsidered, in relation to the structure and activity of wholecells.     

Taxol-induced alteration of intracellular amino-acid profile related to human cervical carcinoma HeLa cell death

The anti-tumor action of Taxol was investigated in the changes of amino-acids involved in tumor cell survival. By tracing the intracellular amino-acid profiles of HeLa cells treated with non-conditioned and three conditioned media (Taxol, l-alanine, and Taxol + l-alanine), it was observed that an alteration of amino-acid metabolism participates in Taxol-induced death of HeLa cells. The contents of 18 out of 21 detected amino-acids are 5–95% and the ones of lysine and methionine are 158 and 117% of the corresponding contents in the control after treatment with Taxol for 24 h, respectively. Addition of l-alanine inhibited cell apoptosis upon Taxol treatment by partially blocking the increase of lysine and methionine and reversing decrease trend of alanine, glycine, and glutamic acid. These results suggest that interference of amino-acid metabolism might be an important mechanism of Taxol cytotoxicity.     

Sucrose Breakdown in Relation to Fruit Growth of Acid Lime (Citrus aurantifolia)

The physiological capacity for sucrose breakdown in developingjuice sac cells of acid limes was estimated by assaying theactivity of the three enzymes of sucrose catabolism in additionto vacuolar acid hydrolysis. The maximum potential rates ofsucrose breakdown were compared with the observed rates of carbonutilization. Highest potential rates of sucrose breakdown (28.621mmol cm^–3 per hydrated active space d^–1) occurredat the initial stages of fruit development where carbon utilizationwas highest. As the fruit developed, the potential rates ofsucrose breakdown and carbon utilization declined to very lowlevels. At 80% of development, vacuolar acid hydrolysis becamethe only physiological mechanism for sucrose breakdown. Therelatively low amounts of sucrose hydrolysed by acid hydrolysisat this time were just sufficient to account for the measuredcarbon demands. The results suggest that carbon supplied bythis distinct sucrose catabolizing system is able to provideadequate levels of carbon skeletons for the observed levelsof respiration and dry weight deposition early in development,but becomes a limiting factor for growth in the later stages. Key words: Vacuolar acid hydrolysis, Citrus aurantifolia     

Growth-promoting Activity of an Aqueous Extract of Soybean Seeds for Some Microorganisms

The metabolic fates of the carbon skeletons of leucine, lysine, and threonine were studied in growing rats on the diets containing graded levels of protein calorie percentages (10, 20, 30, and 40PC%) by use of either gluten or zein at 4100 kcal of metabolizable energy per kg of diets. In growth experiment for 21 days, body weight gain, food intake, and body fat increased at higher PC% in the gluten diets, but rats given zein did not maintain their initial weight even at 40PC%. The concentration of plasma free lysine remained low with the zein diets, but plasma threonine increased at 10 and 20PC% in the gluten and zein diets, respectively. Plasma leucine increased as the protein level increased either dietary protein. More than 70% of ¹⁴C was incorporated into body protein 12 h after an intraperitoneal injection of labeled lysine in all groups, but little ¹⁴CO₂ was expired in rats on the gluten and zein diets. About 79% of ¹⁴C-threonine was incorporated into body protein in rats given the gluten and zein diets at 10PC%, but the values were gradually decreased with increasing the dietary protein levels. Some 40–50% of ¹⁴C-leucine was incorporated into the body protein in rats given the gluten diets, and the values for the zein diets were extensively decreased in the higher PC% groups where the expired ¹⁴CO₂ was inversely increased to a great extent. These results showed that, when a specific amino acid was limiting or deficient in the diet, the major portion of the labeled amino acid was utilized for body protein synthesis and little was oxidized to carbon dioxide, whereas the oxidative degradation of essential amino acid other than limiting one was increased and the efficiency of the amino acid utilization was relatively decreased.     

The Effect of Cycloheximide (Actidione) on Protein and Nucleic Acid Synthesis by Chlorella

Incorporation of ¹⁴C-phenylalanine, ¹⁴C-carbon dioxide, ¹⁴C-glucose,and ¹⁴C-glycine into the protein of Chlorella is inhibited bycycloheximide. A concentration of 2.5 µg per ml inhibitsincorporation by about 80 per cent; increasing the concentrationup to 10 µg per ml does not increase the degree of inhibition.The incorporation of ¹⁴C-adenine into ribonucleic acid (RNA)and deoxyribonucleic acid (DNA), and of ¹⁴C-glucose into polysaccharideis also inhibited. Unlike inhibition of protein synthesis, thatof nucleic acid and polysaccharide synthesis is observed onlyafter some delay. The delay is shortest for DNA synthesis andlongest for polysaccharide synthesis. Inhibition of ¹⁴C-glycineincorporation into DNA and RNA follows a similar pattern tothat obtained with ¹⁴C-adenine but the delay is much shorter.Cycloheximide also inhibits the formation of isocitrate lyasc(isocitrate-glyoxylate lyase, EC 4.1.3.1 [EC] ) when autotrophicallygrown cells are supplied with acetate.     

Myo-inositol Biosynthesis and Galactose Utilization by Wild Carrot (Daucus carota L. var. carota) Suspension Cultures

Wild carrot (Daucus carota var. carota) cell suspensions (63–120µm in diameter) were grown on a mineral salt medium containingdifferent carbon sources in the presence (10 mM) and absenceof myo-inositol. The data obtained after 14 and 21 days of growthshow that an external supply of myo-inositol is not essentialfor growth and development of wild carrot embryos. A linearrelationship was found between growth (d. wt) and embryo numberin the presence and absence of myo-inositol. Standard stock cell suspensions never exposed to exogenous myo-inositoland grown in the absence of 2, 4-D with glucose or galactoseas the carbon source synthesized radioactive myo-inositol whenexposed to D-[1–¹⁴C]glucose or D-[1–¹⁴C]galactose.Gas chromatographic analyses revealed the presence of myo-inositolin the bulk tissue grown in the presence of 2.25 µM 2,4-D with glucose, galactose, fructose or mannose as the solecarbohydrate. We could not detect any component indicating anisomer or a methylated derivative of an inositol in the tissueextracts. Stock cultures were maintained (with 2, 4-D) successfully forat least three successive sub-cultures on D-galactose as thesole carbohydrate. The growth achieved over this culture periodshowed that wild carrot cells used by us could quickly adaptto grow on D-galactose as rapidly as they grow on sucrose. Daucus carota L., wild carrot, suspension cultures, myo-inositol, galactose     

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.