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 Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat 1. Partitioning During Transport to the Grain

When ¹⁴CO₂ was fed to flag leaf laminae at 20 d post-anthesis,the transport organs between the leaf and the grains containedappreciable ¹⁴C in glutamine, glutamate, serine, alanine, threonineand glycine. Smaller amounts of ¹⁴C were present in gamma-aminobutyricacid (GABA), aspartate and cysteine. Other amino acids whichwere labelled in the source leaf were not labelled in the transportorgans. The export of labelled glutamine, serine, glycine andthreonine from the source leaf was favoured in comparison tothe other amino acids mentioned. Threonine accumulated, andwas subsequently metabolised, in the rachis. [¹⁴C]GABA alsoaccumulated in the rachis. In the grains, the relative amountof soluble [¹⁴C]alanine increased with chase time. This wasprobably due to de novo synthesis and reflected the specialrole of alanine in grain nitrogen metabolism. Wheat, Triticum aestivum, ¹⁴CO₂, amino acids, transport, carbon metabolism     

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.     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

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.     

Glycolate Pathway and Serine Synthesis in Relation to CO2 Concentration in Tomato Leaves

Isotopic trapping of the carbon flowing through the glycolatepathway by exogenous glycolate, glycine and L-serine was investigatedduring ¹⁴CO₂ photosynthesis at different CO₂ concentrationsin tomato leaves. L-Serine markedly trapped the carbon flowingfrom ¹⁴CO₂. The amounts of ¹⁴C incorporated into serine decreasedat a high CO₂ concentration, but increased with an increasein the CO₂ concentration in the presence of exogenous serineduring 10-min photosynthesis in ¹⁴CO₂. When ¹⁴CO₂ was fed for5 to 40 sec at 1300 ppm CO₂ to tomato leaves which had beengiven L-serine, an increase in the accumulation of ¹⁴C-serinebegan after 20 sec, and the ¹⁴C-serine molecules formed at 20and 40 sec were labeled uniformly. In the presence of exogenousserine during 10-min photosynthesis in 1300 ppm CO₂, isonicotinicacid hydrazide increased the incorporation of ¹⁴CO₂ into glycinewith a corresponding decrease in the accumulation of ¹⁴C-serine,but it did not inhibit serine accumulation completely; an evidencefor that some serine was formed by a pathway other than theglycolate pathway. The effect of the CO₂ concentration on theglycolate pathway is discussed in terms of serine synthesisin the presence of exogenous serine. (Received June 1, 1981; Accepted September 30, 1981)     

Effect of L-Methionine Sulphoximine on the Products of Photosynthesis in Wheat (Triticum aestivum) Leaves

Methionine sulphoximine, an inhibitor of glutamine synthetase,caused ammonia accumulation in detached wheat leaves. The ratewas increased by increased oxygen in the atmosphere and by simultaneouslysupplying glycine or giving extra nitrate; it was decreasedby isonicotinyl hydrazide. Ammonia production was light-dependentand continued at a constant rate in air for at least 2 h. Photosynthesiswas progressively inhibited after the first hour; this inhibitionwas not because of increased stomatal resistance. Leaves suppliedwith 30 mol m^–3 ammonium chloride, without methioninesulphoximine, accumulated more ammonia than leaves treated withthe inhibitor but showed less inhibition of photosynthesis.The inhibitor decreased synthesis of [¹⁴C] amino acids from¹⁴CO₂ in the light but increased the synthesis of [¹⁴C] malateand, relatively, the incorporation of ¹⁴C into sugar phosphates.In the absence of inhibitor, nitrate increased and ammoniumion decreased synthesis of malate. Methionine sulphoximine,by causing a shortage of amino acids, probably inhibited photosynthesisin part by decreasing the recycling of carbon from the photorespiratorycycle back to the Calvin cycle. Key words: Photosynthetic ¹⁴CO₂ assimilation, Methionine sulphoximine, Detached wheat leaves     

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.     

Contribution of the Pericarp to the Amino Acids of the Seeds of Datura stramonium L.

Pericarps of Datura stramomum L. photosynthesizing in ¹⁴CO₂,have been shown to contribute amino acids (especially aspartate,threonine, serine, glutamate, glycine, alanine, phenylalanine,lysine, and histidine) to the protein-bound amino acids of theseed. This contribution is, however, a relatively minor onein comparison with the contribution made by photosyntheaizingleaves in the close proximity of the fruit.     

Nitrate Nutrition and Temperature Effects on Wheat: a Synthesis of Plant Growth and Nitrogen Uptake in Relation to Metabolic and Physiological Processes

Lawlor, D. W., Boyle, F. A., Keys, A. J., Kendall, A. C. andYoung, A. T. 1988. Nitrate nutrition and temperature effectson wheat: a synthesis of plant growth and nitrogen uptake inrelation to metabolic and physiological processes.—J.exp. Bot. 39: 329-343. Growth of spring wheat was measured in cool (13°C day/10°Cnight) or warm (23°C/18°C) temperatures, combined withlarge and small amounts of nitrate fertilizer. The rate of growthof dry matter was less at cool temperatures but total growthover the same period of development was slightly greater inthe cool than in the warm. Main-shoot and tiller leaves grewslower and, despite growing for a longer period, were shorterin the cool than in the warm. They had greater fresh and drymass and content of starch and fructosans per unit area. Coolconditions increased root dry mass, root to shoot ratio andnitrogen content in dry matter. Additional nitrate increasedleaf area of main shoots slightly but of tillers greatly; itincreased leaf and tiller dry matter and total plant dry mass.Additional nitrate decreased the proportion of dry matter inroots and in stems and the N content of dry matter in all plantparts. Regulation of growth by temperature, nitrate supply andthe rôle of photosynthesis and nitrogen uptake, is consideredin relation to the mechanisms of incorporation of carbon andnitrogen into biochemical constituents. It is concluded thattemperature regulates the rate of protein synthesis, which determinesplant growth rate. Nitrogen flux into the plant is not directlylinked to protein synthesis so that the content of NO^–₃and of amino acids is related both to growth and to conditionsgoverning NO^–₃ uptake and its reduction. When nitrogensupply is large, growth is limited by temperature, not NO^–₃.Inadequate nitrate supply decreases protein synthesis (and thereforegrowth) more than it decreases carbon assimilation, so thatorgans such as roots and stems increase in dry matter relativeto shoots and all tissues have smaller proportions of nitrogenin dry matter. Cool conditions, although decreasing the rateof protein synthesis, increase its duration and decrease thesize of leaves, so that the content of protein per unit leafarea is greater in cool than in warm grown leaves. Consequencesof changes in the balance of N and C supply and growth ratefor dry matter distribution in plants are discussed. Key words: Wheat, nitrate nutrition, temperature     

The Distribution of 14C-labelled Assimilates in Young Apple Trees as Influenced by Doses of Supplementary Nitrogen: II. Soluble Carbohydrates and Amino Acids

Methanol extracts of young MM.104 apple trees fed ¹⁴CO₂ viaa single leaf were fractionated to compare ¹⁴C activity in totalsoluble sugar and amino acid components. ¹⁴C activity in aminoacids increased after the supply of ammonium nitrate to thesoil in plants where ¹⁴C labelled carbohydrates were presentin the roots. Estimates of specific carbon activity gave lowervalues for the amino acid carbon than the general value fortotal soluble carbohydrates. The fractionation of subsequentmethanol extracts of MM.104 roots has shown that sucrose hadlower specific activity than other components. Although thelevels of activity would accord with sucrose being a substratefor amino acid synthesis, an alternative explanation for theobserved results involving a cyclical system for transferringnitrogen is postulated.     

Photosynthetic Assimilation of 14CO2 by Waterstressed Sunflower Leaves at Two O2 Concentrations and the Specific Activity of Products

Attached leaves of sunflower (Helianthus annuus L. var. Mennonite)with water potentials of –5 to –18 ? 10⁵ Pa, wereexposed for different times to 300 vpm CO₂ containing ¹⁴CO₂and 21 or 1.5% O₂. ¹⁴C accumulated linearly with time in bothO₂ concentrations and at all stresses. 3-Phosphoglyceric acidwas saturated with ¹⁴C after 10 min in unstressed plants atboth O₂ concentrations but with increasing stress the rate ofaccumulation and the specific activity decreased. With decreasingleaf water potential there was accumulation of radioactivityin the glycolate pathway intermediates glycine and serine. Otheramino acids contained a slightly larger proportion of assimilatedcarbon as water potential decreased. The specific activitiesof all compounds were smaller with stress. In contrast to theamino acids less radioactivity accumulated in sugars, organicacids, and sugar phosphates and their specific activities decreasedwith stress. The radioactive labelling patterns and specificactivity measurements are interpreted as showing increased carbonflux in the glycolate pathway and inhibition of the metabolismof serine to sucrose. These changes are related to previousresults showing that with stress photo respiration increasesas a proportion of photosynthesis. Lowering the O₂ concentrationto 1.5% decreased the accumulation of radioactivity in glycineand stopped photorespiration. It increased the amount of radioactivityin serine and sucrose but did not greatly change specific activities.Oxygen effects were independent of water stress. Glycolate pathwaymetabolism is discussed in relation to photorespiration andthe effects of water stress.     

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; )     

Water Stress Induced Changes in the Amounts of Some Photosynthetic Assimilation Products and Respiratory Metabolites of Sunflower Leaves

Sunflower leaves, water-stressed under controlled conditions,contained greater amounts of amino acids as their water potentialdecreased, with glycine, serine, and glutamate increasing morethan alanine and aspartate. Proline accumulated only at severestress. Low O₂ concentration altered the amounts of amino acids,principally decreasing the amount of glycine and increasingserine. The changes in total pool size are related to previousresults on the accumulation of 14C and the specific activityof products. Photorespiration was large under water stress,where leaves accumulated carbon in glycine of low specific activity,and in 21% O₂, where both total amount and specific activityof glycine was greater than in 1.5% O₂. This suggests that thereare two pools of glycine, one controlled by O₂ and closely relatedto photosynthesis, the other non-photosynthetic and affectedby water stress. The organic acids suocinate, citrate, and fumarate increasedat small leaf-water potentials. Sucrose decreased in amountwith stress and was absent at the most severe stress; therewas less glucose and fructose. The amount of carbon lost fromsugars was similar to the amount accumulated in amino acidstogether with the carbon lost in respiration. It is concluded that stress decreased the flux of carbon fromphotosynthesis for the synthesis of amino acids and sugars butmore carbon from stored materials, principally sucrose, wasused in the production of organic acids and amino acids.     

PHOTOSYNTHESIS OF GLYCINE AND SERINE IN GREEN PLANTS

1. The effects of "carbonyl" reagents on the photosyntheticin-corporation of ¹⁴CO₂ into the assimilation products of tobaccoand spinach leaves were studied. The presence of "carbonyl"reagents causes an increase in the ratio of ¹⁴CO₂ incorporatedin glycine and a decrease in serine. The incorporation of ¹⁴Cfrom glycolate-1-¹⁴C and glycolaldehyde-2-¹⁴C into glycine andserine was also affected by "carbonyl" reagents, as in the caseof ¹⁴CO₂-experiment. 2. The feeding experiments of glycine-1-¹⁴C and serine-1-¹⁴Cin the presence and in the absence of "carbonyl" reagents revealedthat these reagents inhibit the conversion of glycine to serine. 3. The results obtained above, together with the effects ofthiols on ¹⁴CO₂ incorporation presented in this paper, supportthe assumption that glycine and serine are formed via glycolateand glyoxylate during photosynthesis in green plants. 4. Comparison of ¹⁴C incorporation in malate from ¹⁴CO₂, glycolate-1-¹⁴C,glycine-1-¹⁴C and serine-1-¹⁴C in the presence and in the absenceof "carbonyl" reagents suggested the occurrence of the pathwayof the malate formation via glycolate and glyoxylate, not passingthrough glycine and serine, during photosynthesis. ¹ A part of this paper was presented at the Symposium on "Nitrogenand Plant" by the Japanese Society of Plant Physiologists, inOctober, 1963 ² Present address: Radiation Center of Osaka Prefecture, Sakai,Osaka     

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; )     

The Transport of Photosynthetic Products from the Chloroplasts of Tobacco Leaves

Tobacco leaves were fractionated by the non-aqueous method afterphotesynthesis for short periods in ¹⁴CO₂ and with or withoutsubsequent photosynthesis in ¹²CO₂ or respiration in the dark.Phosphate esters became labelled only slowly in the non-chloroplastparts of the leaf; glycine and serine which became rapidly labelledin the non-chloroplast leaf fraction appear to be concernedin the transport of newly assimilated carbon from the chloroplast.     

The influence of salinity on the utilization of root anaplerotic carbon and nitrogen metabolism in tomato seedlings

In hydroponically grown Lycopersicon esculentum (L.) Mill. cv.F144 the site of NO₃^– reduction and assimilation withinthe plant was shifted from the shoot to the root by salinity.Uptake of NO₃^– from the root solution was strongly inhibitedby salinization. Consequently, NO₃^– concentrations inthe leaf, stem and root tissues as well as the nitrate reductaseactivities of the leaves were lower in salinized than in controlplants. Lower NO₃^–, but higher reduced-N, concentrationswere observed in the xylem sap as a result of the enhanced participationof the root in NO₃^– reduction in salinized plants. Lowerstem K⁺ concentrations and leaf malate concentrations were foundin salinized compared to control plants which indicates reducedfunctioning of the K⁺–shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determinedby supplying a pulse of NaH¹⁴CO₃ followed by extraction andseparation of the labelled products on ion exchange resins.The rate of H¹⁴CO₃^– incorporation was c. 2-fold higherin control than in salinized plants. In salinized plants theproducts of H¹⁴CO₃^– incorporation within the roots werediverted into amino acids, while the control plants divertedrelatively more ¹⁴C into organic acids. Products of inorganiccarbon incorporation in the roots of salinized plants providean anaplerotic source of carbon for assimilation of reducedNO₃^– into amino acids, while in control plants the productswere predominantly organic acids as part of mechanisms to maintainionic balance in the cells and in the xylem sap. Key words: Tomato, nitrate, PEPc, respiration, salinity     

Arginine metabolism by pumpkin seedlings. Separation of plant extracts by ion exchange resins

Arginine-U-¹⁴C was injected into the cotyledons of 7-day oldpumpkin seedlings. At most, 24% of the administered ¹⁴C wastransported to the axis tissue. The amounts of arginine incorporatedinto cotyledonary protein suggests that turnover was occurringat a rapid rate. Arginine was extensively metabolized, and after96 hr 50% of the administered ¹⁴C had been released as ¹⁴CO₂.The remaining label was primarily in unmetabolized arginine,protein or transported to the axis tissue with little labelin other amino acids. The results suggest that the carbon fromarginine is incorporated into protein or catabolized to CO₂while the carbon for new amino acid skeletons is derived fromsugar. A simple, reproducible method for the quantitative fractionationof plant extracts or hydrolysates of insoluble plant materialinto basic amino acids, acidic amino acids, neutral amino acids,organic acids and sugars was reported. (Received September 10, 1968; )