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1.
Ulva, a common green seaweed, performs at the biochemical level as a typical C 3 plant. Over 90% of label was found in glycerate 3-phosphate following a 3 second 14C pulse in the light, and the label was subsequently transferred to sugars. Also, the level of ribulose-1,5-bisphosphate carboxylase activity in crude extracts was about 10 times higher than that of phosphoenolpyruvate carboxylase. Concerning gas exchange, photosynthetic rates of Ulva showed no O 2 sensitivity, indicating that photorespiratory CO 2 losses are repressed as in C 4 plants. This apparent anomaly could be explained by the efficient HCO 3− uptake system of Ulva which might concentrate CO 2 to the chloroplasts, thus suppressing the oxygenase activity of ribulose-1,5-bisphosphate carboxylase. 相似文献
2.
The exchange properties of the activator CO 2 of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO 2 on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO 2. The exchange kinetics of the activator CO 2 were monitored by activation of the enzyme to steady state in the presence of 12CO 2, followed by addition of 14CO 2 and determination of the amount of labeled CO 2 trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants ( Kobs) for exchange with both the purified enzyme (0.45 min −1) and in illuminated chloroplasts (0.18 min −1) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO 2 was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation. 相似文献
3.
Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) rapidly extracted from leaves of wheat ( Triticum aestivum) and purified activated RuBPCO were incubated in the presence and absence of 20 millimolar HCO 3− and changes in activation state were followed. Rapid inactivation occurred in the presence, but not in the absence, of HCO 3−. Effects of CO 2 concentration and pH during preincubation before assay on activation state of RuBPCO were investigated in equilibrium studies. Twenty percent inactivation occurred at high CO 2 concentration if pH was high, but not if it was low, suggesting that RuBPCO was inactivated by HCO 3−. The inactivation by HCO 3− was more rapid than the dissociation of activating CO 2 in CO 2-free buffer (both in the presence of 20 millimolar MgCl 2), suggesting that HCO 3− was bound to the active enzyme complex. The dissociation of inactivating HCO 3− from the enzyme was slow enough that inhibition could be demonstrated in experiments with HCO 3− treatments during preincubation and constant conditions during assay. Inorganic phosphate did not seem to interfere with the binding of HCO 3−. 相似文献
4.
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
5.
Summary Activated carboxylase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), as well as photosynthetic rates were measured for 42 species of freshwater and marine macrophytes. While the carboxylase activity varied greatly among the species investigated (0.2–12.5 mol CO 2 mg –1 chlorophyll min –1), the submersed freshwater plants showed significantly lower activities than emergent, floating leaved or secondary submersed forms. The variability in photosynthetic rates correlated with the carboxylase activity only for the marine macroalgae, and their photosynthesis to carboxylase activity ratios were close to 1. These plants also had a consistently high inorganic carbon transport capability, and it is suggested that ribulose-1,5-bisphosphate carboxylase/oxygenase activity is an important internal factor regulating the photosynthetic capacity within this plant group where, apparently, the internal CO 2 concentration is high and photorespiration is suppressed. Among the freshwater forms, it appears that their much lower inorganic carbon transport ability, rather than their carboxylase activity, limits the photosynthetic process. 相似文献
6.
A new method is presented for measurement of the CO 2/O 2 specificity factor of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The [ 14C]3-phosphoglycerate (PGA) from the Rubisco carboxylase reaction and its dilution by the Rubisco oxygenase reaction was monitored by directly measuring the specific radioactivity of PGA. 14CO 2 fixation with Rubisco occurred under two reaction conditions: carboxylase with oxygenase with 40 micromolar CO 2 in O 2-saturated water and carboxylase only with 160 micromolar CO 2 under N 2. Detection of the specific radioactivity used the amount of PGA as obtained from the peak area, which was determined by pulsed amperometry following separation by high-performance anion exchange chromatography and the radioactive counts of the [ 14C]PGA in the same peak. The specificity factor of Rubisco from spinach ( Spinacia oleracea L.) (93 ± 4), from the green alga Chlamydomonas reinhardtii (66 ± 1), and from the photosynthetic bacterium Rhodospirillum rubrum (13) were comparable with the published values measured by different methods. 相似文献
7.
Wheat ( Triticum aestivum L.) was grown under CO 2 partial pressures of 36 and 70 Pa with two N-application regimes. Responses of photosynthesis to varying CO 2 partial pressure were fitted to estimate the maximal carboxylation rate and the nonphotorespiratory respiration rate in flag and preceding leaves. The maximal carboxylation rate was proportional to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content, and the light-saturated photosynthetic rate at 70 Pa CO 2 was proportional to the thylakoid ATP-synthase content. Potential photosynthetic rates at 70 Pa CO 2 were calculated and compared with the observed values to estimate excess investment in Rubisco. The excess was greater in leaves grown with high N application than in those grown with low N application and declined as the leaves senesced. The fraction of Rubisco that was estimated to be in excess was strongly dependent on leaf N content, increasing from approximately 5% in leaves with 1 g N m −2 to approximately 40% in leaves with 2 g N m −2. Growth at elevated CO 2 usually decreased the excess somewhat but only as a consequence of a general reduction in leaf N, since relationships between the amount of components and N content were unaffected by CO 2. We conclude that there is scope for improving the N-use efficiency of C 3 crop species under elevated CO 2 conditions. 相似文献
8.
This study investigated inorganic carbon accumulation in relation to photosynthesis in the marine dinoflagellate Prorocentrum micans. Measurement of the internal inorganic carbon pool showed a 10-fold accumulation in relation to external dissolved inorganic carbon (DIC). Dextran-bound sulfonamide (DBS), which inhibited extracellular carbonic anhydrase, caused more than 95% inhibition of DIC accumulation and photosynthesis. We used real-time imaging of living cells with confocal laser scanning microscopy and a fluorescent pH indicator dye to measure transient pH changes in relation to inorganic carbon availability. When steady-state photosynthesizing cells were DIC limited, the chloroplast pH decreased from 8.3 to 6.9 and cytosolic pH decreased from 7.7 to 7.1. Re-addition of HCO 3− led to a rapid re-establishment of the steady-state pH values abolished by DBS. The addition of DBS to photosynthesizing cells under steady-state conditions resulted in a transient increase in intracellular pH, with photosynthesis maintained for 6 s, the amount of time needed for depletion of the intracellular inorganic carbon pool. These results demonstrate the key role of extracellular carbonic anhydrase in facilitating the availability of CO 2 at the exofacial surface of the plasma membrane necessary to maintain the photosynthetic rate. The need for a CO 2-concentrating mechanism at ambient CO 2 concentrations may reflect the difference in the specificity factor of ribulose-1,5 bisphosphate carboxylase/oxygenase in dinoflagellates compared with other algal phyla. 相似文献
9.
Carbon isotope fractionation by structurally and catalytically distinct ribulose-1,5-bisphosphate carboxylases from one eucaryotic and four procaryotic organisms has been measured under nitrogen. The average fractionation for 40 experiments was −34.1 ‰ with respect to the δ 13C of the dissolved CO 2 used, although average fractionations for each enzyme varied slightly: spinach carboxylase, −36.5 ‰; Hydrogenomonas eutropha, −38.7 ‰; Agmenellum quadruplicatum, −32.2 ‰; Rhodospirillum rubrum, −32.1 ‰; Rhodopseudomonas sphaeroides peak I carboxylase, −31.4 ‰; and R. sphaeroides peak II carboxylase, −28.3 ‰. The carbon isotope fractionation value was largely independent of method of enzyme preparation, purity, or reaction temperature, but in the case of spinach ribulose-1,5-bisphosphate carboxylase fractionation, changing the metal cofactor used for enzyme activation had a distinct effect on the fractionation value. The fractionation value of −36.5 ‰ with Mg 2+ as activator shifted to −29.9 ‰ with Ni 2+ as activator and to −41.7 ‰ with Mn 2+ as activator. These dramatic metal effects on carbon isotope fractionation may be useful in examining the catalytic site of the enzyme. 相似文献
10.
Photosynthetic CO 2 fixation rates in leaves and intact chloroplasts of spinach measured at 18°–20° C are substantially decreased by pretreatment at temperatures exceeding 20° C. Mild heating which causes 80% inhibition of CO 2 fixation does not affect phosphoglyceroacid reduction and causes increases in the ATP/ADP ratio and the light-induced transthylakoid proton gradient. The inactivation of the CO 2 fixation is completely reversible with half-times of recovery in the order of 15–20 min. Comparison of steady-state patterns of 14C labeled Calvin cycle intermediates of heat-treated and control samples reveals a large increase in the ribulose-1,5-bisphosphate/phosphoglyceroacid ratio and a large decrease in the phosphoglyceroacid/triosephosphate ratio. It is concluded that inactivation of CO 2 fixation occurring at elevated temperatures is caused by inhibition of the ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). Measurements of light-induced light scattering changes of thylakoids and of the light-induced electrochromic absorption shift show that these signals are affected by mild heating in a way which is strictly correlated with the inactivation of the CO 2 fixation. It is proposed that the function of the ribulose-1,5-bisphosphate carboxylase in vivo requires a form of activation that involves properties of the thylakoid membrane which are affected by the heat treatment. The fact that these changes in thylakoid membrane properties and of ribulose-1,5-bisphosphate carboxylase activity are already affected at elevated temperatures which can still be considered physiological, and the reversible nature of these changes, suggest that they may play a role in temperature regulation of the overall photosynthetic process.Abbreviations 9-AA
9-aminoacridine
- DMO
5,5-dimethyloxazolidine-2,4-dione
- FBP
fructose-1,6-bisphosphate
- HEPES
N-2-hydroxyethylpiperazine N-2-ethane sulfonic acid
- HMP
hexose monophosphates
- PGA
3-phosphoglycerate
- PMP
pentose monophosphates
- RuBP
ribulose-1,5-bisphosphate
- SBP
seduheptulose-1,7-bisphosphate
- TP
triose monophosphates 相似文献
12.
Soybean ( Glycine max L. Merr. cv Bragg) was grown throughout its life cycle at 330, 450, and 800 microliters CO 2 per liter in outdoor controlled-environment chambers under solar irradiance. Leaf ribulose-1,5-bisphosphate carboxylase (RuBPCase) activities and ribulose-1,5-bisphosphate (RuBP) levels were measured at selected times after planting. Growth under the high CO 2 levels reduced the extractable RuBPCase activity by up to 22%, but increased the daytime RuBP levels by up to 20%. Diurnal measurements of RuBPCase (expressed in micromoles CO2 per milligram chlorophyll per hour) showed that the enzyme values were low (230) when sampled before sunrise, even when activated in vitro with saturating HCO3− and Mg2+, but increased to 590 during the day as the solar quantum irradiance (photosynthetically active radiation or PAR, in micromoles per square meter per second) rose to 600. The nonactivated RuBPCase values, which averaged 20% lower than the corresponding HCO3− and Mg2+-activated values, increased in a similar manner with increasing solar PAR. The per cent RuBPCase activation (the ratio of nonactivated to maximum-activated values) increased from 40% before dawn to 80% during the day. Leaf RuBP levels (expressed in nanomoles per milligram chlorophyll) were close to zero before sunrise but increased to a maximum of 220 as the solar PAR rose beyond 1200. In a chamber kept dark throughout the morning, leaf RuBPCase activities and RuBP levels remained at the predawn values. Upon removal of the cover at noon, the HCO3− and Mg2+-activated RuBPCase values and the RuBP levels rose to 465 and 122, respectively, after only 5 minutes of leaf exposure to solar PAR at 1500. These results indicate that, in soybean leaves, light may exert a regulatory effect on extractable RuBPCase in addition to the well-established activation by CO2 and Mg2+. 相似文献
13.
High CO 2 concentrations (HC) in air induce partial deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO, EC 4.1.1.39). Under saturating irradiance, increase in [CO 2] to 1 200 cm 3 m –3 reduces the concentration of operating carboxylation centres by 20–30 %. At a further increase in [CO 2], the activity remained on the same level. Under limiting irradiance, the lowest activity was reached at 600 cm 3(CO 2) m –3. The presence of oxygen diminished deactivation, but O 2 failed to stimulate reactivation under high CO 2. Conditions that favour oxygenation of ribulose-1,5-bisphosphate (RuBP) facilitated reactivation. Even HC did not act as an inhibitor. HC induces deactivation of RuBPCO by increasing the concentration of free reaction centres devoid of the substrate, which are more vulnerable to inhibition than the centres filled with substrates or products. 相似文献
14.
Bundle sheath strands capable of assimilating up to 68 μmoles CO 2 per mg chlorophyll per hr in the dark have been isolated from fully expanded leaves of Zea mays L. This dark CO 2-fixing system is dependent on exogenous ribose-5-phosphate, ADP or ATP, and Mg 2+ for maximum activity. The principal product of dark fixation in this system is 3-phosphoglycerate, indicating that the CO 2-fixing reaction is mediated by ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). The rate of dark CO 2 uptake in the strands in the presence of saturating levels of ribose-5-phosphate plus ADP is inhibited by oxygen. The inhibitory effect of oxygen is rapidly and completely reversible, and is relieved by increased levels of CO 2. Glycolate is synthesized in this dark system in the presence of [U- 14C]ribose-5-phosphate, ADP, oxygen, and an inhibitor of glycolate oxidase (EC 1.1.3.1). Glycolate formation is completely abolished by heating the strands, and the rate of glycolate synthesis is markedly reduced by either lowering the oxygen tension or increasing the level of CO 2.These results, obtained with intact cells in the absence of light, indicate that the direct inhibitory effect of oxygen on photosynthesis is associated with photosynthetic carbon metabolism, probably at the level of ribulose-1,5-bisphosphate carboxylase, and not with photophosphorylation or photosynthetic electron transport. Furthermore, the findings indicate that the synthesis of glycolate from exogenous substrate can readily occur in the absence of photosynthetic electron transport, an observation consistent with the ribulose-1, 5-bisphosphate “oxygenase” scheme for glycolate formation during photosynthesis. 相似文献
15.
Photosynthetic CO 2 assimilation, transpiration, ribulose-1,5-bisphosphate carboxylase (RuBPCase), and soluble protein were reduced in leaves of water-deficit (stress) `Valencia' orange ( Citrus sinensis [L.] Osbeck). Maximum photosynthetic CO 2 assimilation and transpiration, which occurred before midday for both control and stressed plants, was 58 and 40%, respectively, for the stress (−2.0 megapascals leaf water potential) as compared to the control (−0.6 megapascals leaf water potential). As water deficit became more severe in the afternoon, with water potential of −3.1 megapascals for the stressed leaves vs. −1.1 megapascals for control leaves, stressed-leaf transpiration declined and photosynthetic CO 2 assimilation rapidly dropped to zero. Water deficit decreased both activation and total activity of RuBPCase. Activation of the enzyme was about 62% (of fully activated enzyme in vitro) for the stress, compared to 80% for the control. Water deficit reduced RuBPCase initial activity by 40% and HCO 3−/Mg 2+-saturated activity by 22%. However, RuBPCase for both stressed and control leaves were similar in Kcat (25 moles CO 2 per mole enzyme per second) and Km for CO 2 (18.9 micromolar). Concentrations of RuBPCase and soluble protein of stressed leaves averaged 80 and 85%, respectively, of control leaves. Thus, reductions in activation and concentration of RuBPCase in Valencia orange leaves contributed to reductions in enzyme activities during water-deficit periods. Declines in leaf photosynthesis, soluble protein, and RuBPCase activation and concentration due to water deficit were, however, recoverable at 5 days after rewatering. 相似文献
16.
When Ribulose- 1,5-bisphosphate carboxylase/oxygenase was purified from spinach leaves ( Spinacia oleracea) using precipitation with polyethylene glycol and MgCl 2 followed by DEAE cellulose chromatography, 75% of phosphoribulokinase and 7% of phosphoriboisomerase activities copurified with ribulose- 1,5-bisphosphate carboxylase/oxygenase. This enzyme preparation showed ribose-5-phosphate and ribulose-5-phosphate dependent carboxylase and oxygenase activities which were nearly equivalent to its corresponding ribulose- 1,5-bisphosphate dependent activity. The ribose-5-phosphate and ribulose-5-phosphate dependent reaction rates were stable and linear for much longer time periods than the ribulose- 1,5-bisphosphate dependent rates. When sucrose gradients were used to purify ribulose- 1,5-bisphosphate carboxylase/oxygenase from crude stromal extracts, phosphoribulokinase was found to cosediment with ribulose- 1,5-bisphosphate carboxylase. Under these conditions most of the phosphoriboisomerase activity remained with the slower sedimenting proteins. Ammonium sulfate precipitation resulted in separation of the ribulose- 1,5-bisphosphate carboxylase peak from phosphoribulokinase peak. Crude extracts of peas Pisum sativum and spinach contained 0.725 to 0.730 milligram of phosphoribulokinase per milligram of chlorophyll, respectively, based on an enzyme-linked immunosorbent assay. 相似文献
17.
Photosynthetic properties of photoautotrophic suspensions cultured in a minimal growth medium have been evaluated to determine whether changes have occurred in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenol-pyruvate (PEP) carboxylase activity, chlorophyll content, or culture growth. Five photoautotrophic lines Amaranthus powellii, Datura innoxia, Glycine max, Gossypium hirsutum, and a Nicotiana tabacum-Nicotiana glutinosa fusion hybrid were grown in a medium without organic carbon other than phytohormones, and without vitamins. These photoautotrophic lines had total Rubisco activities ranging from 85 to 266 micromoles CO 2 fixed per milligram chlorophyll hour −1, with percent activation of Rubisco ranging from 16 to 53%. Inclusion of protease inhibitors in the homogenization buffer did not result in higher Rubisco activity. PEP carboxylase activity for cells cultured in minimal medium was found to range from 16 to 146 micromoles CO 2 per milligram chlorophyll hour −1, with no higher activity in the C 4Amaranthus cells compared with PEP carboxylase activity in the C 3 species assayed. Rubisco-to-PEP carboxylase ratios ranged from 2.2 to 1 up to 9.4 to 1. Chlorophyll contents increased in all but the Nicotiana cell line, and all of the photoautotrophic culture lines were capable of growth in vitamin-free medium with the exception of SB-P, which requires thiamine. 相似文献
18.
Evidence of an inorganic carbon concentrating system in a marine macroalga is provided here. Based on an O 2 technique, supported by determinations of inorganic carbon concentrations, of experimental media (as well as compensation points) using infrared gas analysis, it was found that Ulva fasciata maintained intracellular inorganic carbon levels of 2.3 to 6.0 millimolar at bulk medium concentrations ranging from 0.02 to 1.5 millimolar. Bicarbonate seemed to be the preferred carbon form taken up at all inorganic carbon levels. It was found that ribulose-1,5-bisphosphate carboxylase/oxygenase from Ulva had a Km(CO 2) of 70 micromolar and saturated at about 250 micromolar CO 2. Assuming a cytoplasmic pH of 7.2 (as measured for another Ulva species, P Lundberg et al. [1988] Plant Physiol 89: 1380-1387), and given the high activity of internal carbonic anhydrase (S Beer, A Israel [1990] Plant Cell Environ [in press]) and the here measured internal inorganic carbon level, it was concluded that internal CO 2 in Ulva could, at ambient external inorganic carbon concentrations, be maintained at a high enough level to saturate ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation. It is suggested that this suppresses photorespiration and optimizes net photosynthetic production in an alga representing a large group of marine plants faced with limiting external CO 2 concentrations in nature. 相似文献
19.
Two strains of marine Synechococcus possessed a much greater potential for photorespiration than other marine algae we have studied. This conclusion was based
on the following physiological and biochemical characteristics: a) a light-dependent O 2 inhibition of photosynthetic CO 2 assimilation at atmospheric O 2 concentrations. The degree of inhibition was dependent on the relative concentrations of dissolved O 2 and CO 2, being greatest at 100% O 2 with no extra bicarbonate added to the medium; b) actively photosynthesizing cells had high levels of ribulose-1,5-bisphosphate
carboxylase compared with phosphoenolpyruvate carboxylase; ribulose-1,5-bisphosphate oxygenase activities were three times
greater than ribulose-1,5-bisphosphate carboxylase activities; c) cells photosynthesizing in 21% O 2, showed significant 14C-labelling of phosphoglycolate and glycolate and the percentage of total carbon-14 incorporated into these two compounds
increased when the O 2 concentration was 100%; d) at 100% O 2, there was a post-illumination enhanced rate of O 2 consumption, which was three times greater than dark respiration, and the rate declined with increasing bicarbonate concentrations.
The inhibitory effect of O 2 on photosynthesis did not appear to be solely due to photorespiration, since O 2 inhibition of photosynthetic O 2 evolution was much greater than that of photosynthetic CO 2 assimilation. Also, O 2 inhibition of photosynthetic O 2 evolution declined only slightly with decreasing light intensities, while the inhibition of CO 2 assimilation declined rapidly with decreasing light intensity. 相似文献
20.
The enzymic fractionation of the stable carbon isotopes of CO 2 (Δco 2) was determined using a purified preparation of ribulose-1,5-bisphosphate (RuBP) carboxylase isolated from cotton (a C 3 plant) leaves. The bicarbonate concentration in the reaction mixture saturated the enzyme and furnished an infinite pool of 12CO 2 and 13CO 2 for enzyme fractionation. The RuBP was 96 to 98% pure. The phosphoglycerate synthesized in the reaction mixtures was purified free of RuBP, phosphoglycolate, and other phosphate esters by column chromatography on Dowex 1-Cl − resin. The average Δco 2 value of −27.1% was determined from five separate experiments. A discussion of the isotope fractionation associated with photosynthetic CO 2 fixation in plants shows that the enzymic fractionation of stable carbon isotopes of CO 2 by RuBP carboxylase is of major importance in determining the δ 13C values of C 3 plants. 相似文献
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