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1.
The pattern of photosynthetic carbon metabolism was determined in tissue cultures of Portulaca oleracea. Four-carbon acids are the most heavily labeled photosynthetic products during short term exposure to 14CO 2, containing greater than 40% of the total radioactivity incorporated. Phosphoglyceric acid and sugars account for only 10% of the label after equal exposure times. Other features of the CO 2 assimilation pattern in Portulaca callus tissue include a relatively large percentage of label located in various minor products throughout the time course studied, and a greater incorporation of 14C into sugars in tissue cultures than occurs in leaves. Ultrastructurally, the chloroplasts and cells of the callus are like those in the mesophyll cells of Portulaca leaves. The requirement for Kranz anatomy for operation of functional C 4 physiology is discussed. 相似文献
2.
We did this work to discover the pathway of CO 2 fixation into sugars in the dark during gluconeogenesis by the cotyledons of 5-day-old seedlings of Cucurbita pepo L. We paid particular attention to the possibility of a contribution from ribulosebisphosphate carboxylase. The detailed distribution of 14C after exposure of excised cotyledons to 14CO 2 in the dark was determined in a series of pulse and chase experiments. After 4s in 14CO 2, 89% of the 14C fixed was in malate and aspartate. In longer exposures, and in chases in 12CO 2, label appeared in alanine, phosphoenolpyruvate, 3-phosphoglycerate and sugar phosphates, and accumulated in sugars. The transfer of label from C-4 acids to sugars was restricted by inhibition of phosphoenolpyruvate carboxykinase in vivo by 3-mercaptopicolinic acid. We conclude as follows. Initial fixation of CO 2 in the dark is almost entirely into phosphoenolpyruvate, probably via phosphoenolpyruvate carboxylase (EC 4.1.1.31) which we showed to be present in appreciable amounts. Incorporation into sugars occurs chiefly, if not completely, as a result of randomization of the carboxyl groups of the C-4 acids and subsequent conversion of the oxaloacetate to sugars via the accepted sequence for gluconeogenesis. Ribulosebisphosphate carboxylase appears to make very little contribution to sugar synthesis from fat. 相似文献
3.
The dependence of alfalfa ( Medicago sativa L.) root and nodule nonphotosynthetic CO 2 fixation on the supply of currently produced photosynthate and nodule nitrogenase activity was examined at various times after phloem-girdling and exposure of nodules to Ar:O 2. Phloemgirdling was effected 20 hours and exposure to Ar:O 2 was effected 2 to 3 hours before initiation of experiments. Nodule and root CO 2 fixation rates of phloem-girdled plants were reduced to 38 and 50%, respectively, of those of control plants. Exposure to Ar:O 2 decreased nodule CO 2 fixation rates to 45%, respiration rates to 55%, and nitrogenase activities to 51% of those of the controls. The products of nodule CO 2 fixation were exported through the xylem to the shoot mainly as amino acids within 30 to 60 minutes after exposure to 14CO 2. In contrast to nodules, roots exported very little radioactivity, and most of the 14C was exported as organic acids. The nonphotosynthetic CO 2 fixation rate of roots and nodules averaged 26% of the gross respiration rate, i.e. the sum of net respiration and nonphotosynthetic CO 2 assimilation. Nodules fixed CO 2 at a rate 5.6 times that of roots, but since nodules comprised a small portion of root system mass, roots accounted for 76% of the nodulated root system CO 2 fixation. The results of this study showed that exposure of nodules to Ar:O 2 reduced nodule-specific respiration and nitrogenase activity by similar amounts, and that phloem-girdling significantly reduced nodule CO 2 fixation, nitrogenase activity, nodule-specific respiration, and transport of 14C photoassimilate to nodules. These results indicate that nodule CO 2 fixation in alfalfa is associated with N assimilation. 相似文献
4.
The metabolism of [ 14C]succinate and acetate was examined in leaf slices of winter wheat ( Triticum aestivum L. cv Frederick) in the dark and in the light (1000 micromoles per second per square meter photosynthetically active radiation). In the dark [1,4- 14C]succinate was rapidly taken up and metabolized into other organic acids, amino acids, and CO 2. An accumulation of radioactivity in the tricarboxylic acid cycle intermediates after 14CO 2 production became constant indicates that organic acid pools outside of the mitochondria were involved in the buildup of radioactivity. The continuous production of 14CO 2 over 2 hours indicates that, in the dark, the tricarboxylic acid cycle was the major route for succinate metabolism with CO 2 as the chief end product. In the light, under conditions that supported photorespiration, succinate uptake was 80% of the dark rate and large amounts of the label entered the organic and amino acids. While carbon dioxide contained much less radioactivity than in the dark, other products such as sugars, starch, glycerate, glycine, and serine were much more heavily labeled than in darkness. The fact that the same tricarboxylic acid cycle intermediates became labeled in the light in addition to other products which can acquire label by carboxylation reactions indicates that the tricarboxylic acid cycle operated in the light and that CO 2 was being released from the mitochondria and efficiently refixed. The amount of radioactivity accumulating in carboxylation products in the light was about 80% of the 14CO 2 release in the dark. This indicates that under these conditions, the tricarboxylic acid cycle in wheat leaf slices operates in the light at 80% of the rate occurring in the dark. 相似文献
5.
The metabolism of fixed 14CO 2 and the utilization of the C-4 carboxyl of malate and aspartate were examined during photosynthetic induction in Flaveria trinervia, a C 4 dicot of the NADP-malic enzyme subgroup. Pulse/chase experiments indicated that both malate and aspartate appeared to function directly in the C 4 cycle at all times during the induction period (examined after 30 seconds, 5 minutes and 20 minutes illumination). However, the rate of loss of 14C-label from the C-4 position of malate plus aspartate was relatively slow after 30 seconds of illumination, compared to treatments after 5 or 20 minutes of illumination. Similarly, the appearance of label in other photosynthetic products (e.g. 3-phosphoglycerate, sugar phosphates, alanine) during the chase periods was generally slower after only 30 seconds of leaf illumination, compared to that after 5 of 20 minutes illumination. This may be due to the lower rate of photosynthesis after 30 seconds illumination. The appearance of label in carbons 1→3 of each C 4 acid during the chase periods was relatively slow after either 30 seconds or 5 minutes illumination, while there was a relatively rapid accumulation of label in carbons 1→3 of both C 4 acids after 20 minutes illumination. Thus, while the turnover rate of the 14C-4 label in both C 4 acids increased only during the first 5 minutes of the induction period, only later during induction is there an increased rate of appearance of label in other carbon atoms of the C 4 acids. The implied source of 14C for labeling of the 1→3 positions of the C 4 acids is an apparent carbon flux from 3-phosphoglycerate of the reductive pentose phosphate pathway to phosphoenolpyruvate of the C 4 cycle. 相似文献
6.
In susceptible oat, toxaphene inhibits photosynthetic electron flow and concomitant ATP synthesis. Although the rate of 14CO 2 assimilation is apparently not affected markedly there is an increase in dry weight of leaves contacting the pesticide. The labelling patterns in leaf sections exposed to 14CO 2 are similar for both toxaphene-treated and untreated seedlings. However, if given a period in darkness before extraction it is evident that assimilation products in leaf sections from toxaphene-treated leaves remain as small M, materials, including substantial amounts of sugars, whereas in untreated controls these were converted to polymeric materials. In toxaphene-treated seedlings the translocation of assimilation products to the roots is decreased and sucrose accumulates in the leaves. 相似文献
7.
Leaf excision greatly affected the actual levels of 14C-assimilates in laminas and petioles of primary bean leaves ( Phaseolus vulgaris L.) following a transport period. However, it did not affect the percentage of starch in the insoluble residue; starch decreased from 20% of the insoluble residue after exposure to 14CO 2 to 3% after 5 hr in both attached and detached leaves. The transition from import to export of attached and detached leaves was at the same stage, i.e., when the cotyledons were 63 to 85% depleted. The composition of the 14C-assimilates in importing leaves was different from that in exporting leaves. In the former, only 5% of the soluble label was free sugar, while 74% was free sugar in the latter. The failure of importing leaves to export was not due to the labeled substances being nontransferable. Extracts from importing leaves applied to exporting leaves were exported; these extracts were high in amino acids and organic acids but low in free sugar. However, exporting leaves exposed to 14CO 2 appeared to export sugars more readily than amino acids. Cotyledon excision did not delay transition of leaves from import to export. Actually, excision seemed to enhance slightly the transition of the primary leaves from import to export. 相似文献
8.
The influence of stage of development (preflowering versus flowering) in nodulated and nonnodulated soybeans ( Glycine max [L.] Merr. cv. Wells) on partitioning of 14C into assimilates following exposure of a soybean leaf to 14CO 2 by both steady-state and pulse-labeling techniques was studied. Blades on the second fully expanded leaf from the stem apex were exposed to 14CO 2. Radioactive assimilates were extracted from source leaf blades, petioles, and stems (both the path up and path down from source leaf), were separated into neutral (sugars), basic (amino acids), and acidic (organic acids, sugar phosphates) fractions by ion exchange chromatography. The basic fraction was further resolved using thin layer chromatography and the percentage of radioactivity recovered in each amino acid was determined. 相似文献
9.
Partitioning and translocation of 14C-photosynthates were examined during flowering and seed maturation in soybean ( Glycine max [L.]Merr.) plants to quantify allocation to sugars, amino acids, organic acids, and starch and to study transport of C and N from leaves to reproductive sinks. The trifoliolate leaf at the eighth node was exposed to steady state levels of 14CO 2 for 2 hours, followed by immediate extraction and identification of radioactive assimilates in the fed leaf blade, tissues of the transport path ( e.g. petiole and stem), and fruits if they were present. About one-third of the total 14C recovered from the leaf blades was in starch until late pod-filling, after which the proportion dropped to 16%. Sugars comprised 70% to 86% of the recovered 14C from soluble assimilates of the source leaf, with highest proportions occurring during late flowering and early pod-filling. Amino acids accounted for 8% to 17% of the 14C recovered from the soluble fraction, and were most evident during early flowering and mid to late pod-filling. The 14C-organic acids comprised from 3% to 14% of the soluble 14C-assimilates in leaves. Petioles consistently contained a higher percentage of recovered radioactivity in sugars (87-97%) and a lower percentage in amino acids (3-12%) than did leaf blades. 14C-Amino acids in petioles attained their highest levels during mid and late pod-filling, while 14C-organic acids comprised 2% or less of the recovered radioactivity after pod initiation. The distribution of 14C-assimilates in the internode below the source leaf was similar to that found in petioles. A comparison of the above data to calculated C and N requirements for seed development suggests that 14C-amino acids derived from current photosynthesis and translocated from source leaves supply at least 12% to 48% of the seed N depending on the stage of pod-filling. 相似文献
10.
Isolated epidermis of Commelina communis L. and Tulipa gesneriana L. assimilated 14CO 2 into malic acid and its metabolites but not into sugars or their phosphates; epidermis could not reduce CO 2 by photosynthesis and therefore must be heterotrophic (Raschke and Dittrich, 1977). If, however, isolated epidermis of Commelina communis was placed on prelabelled mesophyll (obtained by an exposure to 14CO 2 for 10 min), radioactive sugars appeared in the epidermis, most likely by transfer from the mesophyll. Of the radioactivity in the epidermis, 60% was in sucrose, glucose, fructose, 3-phosphoglyceric acid and sugar phosphates. During a 10-min exposure to 14CO 2, epidermis in situ incorporated 16 times more radioactivity than isolated epidermal strips. Isolated epidermis of Commelina communis and Tulipa gesneriana took up 14C-labelled glucose-1-phosphate (without dephosphorylation), glucose, sucrose and maltose. These substances were transformed into other sugars and, simultaneously, into malic acid. Carbons-1 through-3 of malic acid in guard cells can thus be derived from sugars. Radioactivity appeared also in the hydrolysate of the ethanol-insoluble residue and in compounds of the tricarboxylic-acid cycle, including their transamination products. The hydrolysate contained glucose as the only radioactive compound. Radioactivity in the hydrolysate was therefore considered an indication of starch. Starch formation in the epidermis began within 5 min of exposure to glucose-1-phosphate. Autoradiograms of epidermal sections were blackened above the guard cells. Formation of starch from radioactive sugars therefore occurred predominantly in these cells. Epidermis of tulip consistently incorporated more 14C into malic and aspartic acids than that of Commelina communis (e.g. after a 4-h exposure to [ 14C]glucose in the dark, epidermis, with open stomata, of tulip contained 31% of its radioactivity in malate and aspartate, that of Commelina communis only 2%). The results of our experiments allow a merger of the old observations on the involvement of starch metabolism in stomatal movement with the more recent recognition of ion transfer and acid metabolism as causes of stomatal opening and closing.Abbreviation G-1-P
glucose-1-phosphate 相似文献
11.
The products of C 14O 2 fixation by Chlamydomonas and Chlorella were studied under conditions most favorable for glycolate synthesis. The highest percentage of the C 14 was incorporated into glycolate in the pH range of 8 to 9. After 1 to 2 minutes as much as 40% of the C 14 was found in glycolate products and only a trace of C 14 was present as phosphoglycerate. Below pH 8 the rate of photosynthesis was much faster, but only a small percent of the C 14 was incorporated into glycolate in 1 or 2 minutes, while a high percent of the C 14 accumulated in phosphoglycerate. C 14 labeling of glycolate even at pH 8 or above did not occur at times shorter than 10 seconds. During the first seconds of photosynthesis, nearly all of the C 14 was found in phosphoglycerate and sugar phosphates. Thus glycolate appears to be formed after the phosphate esters of the photosynthetic carbon cycle. Washing Chlamydomonas with water 2 or 3 times resulted in the loss of most of their free phosphate. When a small aliquot of NaHC14O3 was added to washed algae in the absence of this buffering capacity, the pH of the algal medium became 8 or above and much of the fixed C14 accumulated in glycolate. 相似文献
12.
Large amounts of alanine are produced in the first few seconds of photosynthesis in Portulaca oleracea L. The normal precursor-product relationship (phosphoglyceric acid → pyruvate → alanine) does not appear to operate in this species since labeling in pyruvate precedes that in phosphoglyceric acid. Pulse-chase experiments show that the alanine is rapidly metabolized. After a 6-second pulse of 14CO 2, the percentage of 11C in alanine drops more than 30% in the first 10 seconds of a 12CO 2 chase period. The percentage of 14C in the other early-labeled photosynthetic products, aspartate and malate, also decreases during the 12CO 2 chase. The decrease of label in these compounds is concomitant with an increase in the labeling of sucrose and alanine, which in this case is formed via phosphoglyceric acid. Randomization of label within alanine increases gradually throughout the 2-minute chase. 相似文献
13.
Ribulose-1,5-bisphosphate carboxylase activity was found in endosperm of germinating castor bean seed Ricinus communis and was localized in proplastids. The endosperm carboxylase has been extensively purified and is composed of two different subunits. The molecular weights of the native carboxylase and its subunits were 560,000, 55,000, and 15,000 daltons, respectively. The Michaelis-Menten constants, Km, for the endosperm carboxylase with respect to ribulose 1,5-bisphosphate, bicarbonate, CO 2, and magnesium in millimolar are 0.54, 13.60, 0.92, and 0.57, respectively. The endosperm carboxylase was activated by Mg 2+ and HCO 3−. The preincubation of the carboxylase with 1 millimolar HCO 3− and 5 millimolar MgCl 2 resulted in activation by low and inhibition by high concentrations of 6-phosphogluconate. In studies of dark 14CO2 fixation by endosperm slices, [14C]malate and [14C]citrate were the predominantly labeled products after 30 seconds of exposure of the tissue to H14CO3−. In pulse-chase experiments, 87% of the label is malate, and citrate was transferred to sugars after a 60-minute chase with a small amount of the label appearing in the incubation medium as 14CO2. The minimal incorporation of the label from 14CO2 into phosphoglyceric acid indicated a lack of the endosperm ribulose-1,5-bisphosphate carboxylase participation in the endosperm's CO2 fixation system. The activities of key Calvin cycle enzymes were examined in the endosperms and cotyledons of dark-grown castor bean seedlings. Many of these autotrophic enzymes develop in the dark in these tissues. The synthesis of ribulose-1,5-bisphosphate carboxylase in the nonphotosynthetic endosperms is not repressed in the dark, and high levels of enzymic activity appear with germination. All of the Calvin cycle enzymes are present in the castor bean endosperm except NADP-linked glyceraldehyde 3-P dehydrogenase, and the absence of this dehydrogenase probably prevents the functioning of these series of reactions in dark CO2 fixation. 相似文献
14.
The mechanism by which sucrose is transported into the inner spaces of immature internodal parenchyma tissue of sugarcane ( Saccharum officinarum L. var. H 49-5) was studied in short term experiments (15 to 300 seconds). Transport of sucrose, glucose, and fructose was each characterized by a V max of 1.3 μmoles/gram fresh weight·2 hours, and each of these three sugars mutually and competitively inhibited transport of the other two. When 14C-glucose was supplied exogenously, 14C-glucose 6-phosphate and 14C-glucose were the first labeled compounds to appear in the tissue; no 14C-sucrose was detected until after 60-second incubation. After 15-second incubation in 14C-sucrose, all intracellular radioactivity was in glucose, fructose, glucose 6-phosphate, and fructose 6-phosphate; trace amounts of 14C-sucrose were found after 30 seconds and after 5 minutes, 71% of the intracellular radioactivity was in sucrose. Although it was possible that sucrose was transported intact into the inner space and then immediately hydrolyzed, it was shown that the rate of hydrolysis under these conditions was too low to account for the rate of hexose accumulation. Pretreatment of the tissue with rabbit anti-invertase antiserum eliminated sucrose transport, but had no effect on glucose transport. Since the antibodies did not penetrate the plasmalemma, it was concluded that sucrose was hydrolyzed by an invertase in the free space prior to transport. The glucose and fructose moieties, or their phosphorylated derivatives, were then transported into the inner space and sucrose was resynthesized. No evidence for the involvement of sucrose phosphate in transport was found in these experiments. 相似文献
15.
Illuminated flag leaves of Triticum monococcum(2X), T. urartu(2X), T. dicoccum(4X), T. dicoccoides(4X), and T. aestivum(6X) were exposed to 14CO 2 for 10 seconds and subsequently allowed to continue photosynthesis in the ambient air for periods of up to 2 minutes. The relative distribution of 14C among water-soluble products in the leaves was similar for each species at each sampling time. After the 10-second pulse of 14CO 2, radioactivity was mainly in phosphate esters with less than 5% in C 4 acids. Subsequently, radioactivity increased in sucrose, glycine, and serine at the expense of that in phosphate esters. By 2 minutes, between 18% and 29% of the 14C was in glycine plus serine. The results suggest rapid photorespiration in all species and an absence of C 4 photosynthesis. 相似文献
16.
The uptake and metabolism of α-[5- 14C]ketoglutarate by phosphorus-deficient and full nutrient (control) lemon ( Citrus limon) leaves were studied over various time intervals. After 45 minutes in P-deficient leaves, the bulk of incorporated 14C appeared in organic acids and much less in amino acids, while in the control leaves, the 14C contents of organic and amino acids were equal. In P-deficient leaves, after longer incubation times the 14C content of organic acids and amino acids increased, while that of CO 2 and residue fractions remained low. In full nutrient leaves the 14C content of amino acids and organic acids decreased after longer incubation time and increased in the insoluble residue and CO 2. In full nutrient leaves the organic and amino acid metabolism were closely related and accompanied by protein synthesis and CO 2 release, while in P-deficient leaves an accelerating accumulation of arginine and citric acid was linked together with inhibition of protein synthesis and CO 2 liberation. 相似文献
17.
Chlorella pyrenoidosa was labelled by 14CO 2 and the nature and amount of excreted organic compounds in nutrient media of different osmotic pressure were determined after a 24 h period. The total rate of excretion of organic bound 14C was about 4 μg 14C per mg harvested algal dry matter or 1% of the total 14C content of the algae at the beginning of the excretion period. The main compounds found in the excretions were unidentified substances with a molecular weight higher than 700, amino acids, organic acids and sugars. The osmotic pressure of the nutrient medium did not affect the total amount of the organic excretions. However, the excreted amounts of some specific compounds differed in respect to the osmotic conditions of the nutrient medium. 相似文献
18.
Separated mesophyll cells from cotton ( Gossypium hirsutum var. Stoneville 1613 Glandless) were isolated with pectinase and mechanical agitation. The separated cells had rates of light-dependent CO 2 fixation between 50 to 100 μmoles CO 2 per mg chlorophyll per hour. The presence of Ca 2+ in the incubation medium did not significantly affect the type of photosynthetic products formed, but 2 m m Ca 2+ did cause a 50% decrease in the appearance of photosynthetic products in the incubation medium. The movement of all types of products (sugars, organic, and amino acids) out of the cells was reduced similarly by the Ca 2+. Light had no affect on the movement of products out of the cells, whereas 1 m m ethylenediaminetetra-acetate greatly increased the movement. The addition of 1.6 m m NH 4Cl to the cell suspensions caused a large increase in the amount of fixed 14C appearing in the amino acid fraction and a decrease in the sugar fraction. These metabolic changes in the cells were reflected in the movement of products out of the cells so that the incubation medium also contained a larger amount of label in amino acids and a smaller amount in sucrose. Although the cell plasma membrane restricted the movement of soluble products, it did not discriminate significantly between the types of products moved. 相似文献
19.
Partitioning and transport of recently fixed photosynthate was examined following 14CO 2 pulse-labeling of intact, attached leaves of Salvia splendens L. maintained in an atmosphere of 300 microliters per liter CO 2 and 20, 210, or 500 milliliters per liter O 2. Under conditions of increasing O 2 (210, 500 milliliters per liter), a smaller percentage of the recently fixed 14C in the leaf was allocated to starch, whereas a greater percentage of the fixed 14C appeared in amino acids, particularly serine. The increase in 14C in amino acids was reflected in material exported from source leaves. A higher percentage of 14C in serine, glycine, and glutamate was recovered in petiole extracts when source leaves were maintained under elevated O 2 levels. Although pool sizes of these amino acids were increased in both the leaves and petioles with increasing photorespiratory activity, no significant changes in either 14C distribution or concentration of transport sugars ( i.e. stachyose, sucrose, verbascose) were observed. The data indicate that, in addition to being recycled intracellularly into Calvin cycle intermediates, amino acids produced during photorespiration may also serve as transport metabolites, allowing the mobilization of both carbon and nitrogen from the leaf under conditions of limited photosynthesis. 相似文献
20.
The lipids of Acanthamoeba castellanii (Neff) consist of 52% neutral lipids and 48% polar lipids. Triglycerides account for 75% and free sterols for 17% of the neutral lipids. The major phospholipids are phosphatidylcholine (45%), phosphatidylethanolamine (33%), phosphatidylserine (10%), a phosphoinositide (6%), and diphosphatidylglycerol (4%). The phosphoinositide is unique in that it contains fatty acids, aldehyde, inositol, and phosphate in the ratio of 1.4:0.5:1.1, but it contains no glycerol. Sphingomyelin, cerebrosides, psychosine, and glycoglycerides were not detected, but small amounts of unidentified long chain bases and sugars are present. The rates of uptake of palmitate-1- 14C and of its incorporation into glycerides and phospholipids were not affected by the phagocytosis of polystyrene latex beads. Although phagocytosis usually decreased the uptake by amebas of phosphate- 32P, serine-U- 14C, and inositol-2- 3H, their subsequent incroporation into phospholipids was not demonstrably stimulated or inhibited by phagocytosis. Phagocytosis did seem to increase the incorporation into ameba phospholipids of phosphatidylcholine-1 ,2- 14C but not that of phosphatidylethanolamine-1 ,2- 14C. These experiments, in which the incorporation of radioactive precursors into total cell lipids was measured, do not, of course, eliminate the possibility that localized effects may occur. 相似文献
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