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1.
We have recently reported that the activity of maize leaf glycerate kinase [EC 2.7.1.31] is regulated in vivo by the light/dark transition, possibly involving the ferredoxin/thioredoxin mechanism, and that the stimulating effect of light can be mimicked in vitro by incubation of crude leaf extract with reducing compounds (LA Kleczkowski, DD Randall 1985 Plant Physiol 79: 274-277). In the present study it was found that the time course of thiol activation of the enzyme was substantially dependent on the presence of some low molecular weight inhibitor(s) of activation found both in leaf extracts and mesophyll chloroplasts. Activity of glycerate kinase from maize as well as wheat leaves increased upon greening of etiolated plants and was correlated with the development of photosynthetic apparatus in these species. The maize enzyme was strongly activated by thiols at all stages of development from etiolated to green seedlings. Thiol activation of glycerate kinase was observed for a number of C 4 plants, notably of the nicotinamide adenine dinucleotide phosphate-malic enzyme type, with the strongest effect found for the enzyme from leaf extracts of maize and sorghum (10- and 8-fold activation, respectively). Among the C 3 species tested, only the enzyme from soybean leaves was affected under the same conditions (1.6-fold activation). This finding was reflected by an apparent lack of cross-reactivity between the enzyme from maize leaves and antibodies raised against purified spinach leaf glycerate kinase. We suggest that, in addition to its role as a final step of photorespiration in leaves, glycerate kinase from C 4 species may serve as a part of the facilitative diffusion system for the intercellular transport of 3-phosphoglycerate. Simultaneous operation of both the passive and the facilitative diffusion mechanisms of 3-phosphoglycerate transport in C 4 plants is postulated. 相似文献
2.
Current models of C 3 photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO 2 and O 2, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO 2 and O 2, i.e., increasing O 2 stimulates assimilation and increasing CO 2 inhibits assimilation. We propose a model which explains reversed sensitivity to CO 2 and O 2 by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO 2, O 2, and light is demonstrated.Abbreviations PFD
photon flux density
- PGA
3-phosphoglycerate
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose-1,5-bisphosphate
- TPU
triose phosphate utilization 相似文献
3.
Glycerate kinase (EC 2.7.1.31) from maize ( Zea mays) leaves was shown to be regulated by light/dark transition. The enzyme more than doubled in activity after either the leaves or isolated mesophyll chloroplasts were illuminated with white light for 10 minutes. Rate of inactivation in the dark was faster in leaves than in the isolated chloroplast fraction. The stimulating effect of light could be mimicked in crude preparations by addition of 10 or 50 millimolar dithiothreitol or 100 millimolar 2-mercaptoethanol. The thiol treatment resulted in 8- to 10-fold activation of glycerate kinase, with the highest rates in the range of 27 to 30 micromoles per mg chlorophyll per hour. Activation was not accompanied by any changes in the apparent Mr value of glycerate kinase as determined by gel filtration ( Mr = 47,000). In contrast to maize glycerate kinase, the enzyme from spinach was not affected by either light or thiol exposure. Partially purified maize glycerate kinase was activated up to 3-fold upon incubation with a mixture of spinach thioredoxins m and f and 5 millimolar dithiothreitol. The thioredoxin and dithiothreitol-treated glycerate kinase could be further stimulated by addition of 2.5 millimolar ATP. The results suggest that glycerate kinase from maize leaves is capable of photoactivation by the ferredoxin/thioredoxin system. The synergistic effect of ATP and thioredoxins in activation of the enzyme supports the earlier expressed view that the ferredoxin/thioredoxin system functions jointly with effector metabolites in light-mediated regulation during photosynthesis. 相似文献
4.
Leaves of C 3 plants which exhibit a normal O 2 inhibition of CO 2 fixation at less than saturating light intensity were found to exhibit O 2-insensitive photosynthesis at high light. This behavior was observed in Phaseolus vulgaris L., Xanthium strumarium L., and Scrophularia desertorum (Shaw.) Munz. O 2-insensitive photosynthesis has been reported in nine other C 3 species and usually occurred when the intercellular CO 2 pressure was about double the normal pressure. A lack of O 2 inhibition of photosynthesis was always accompanied by a failure of increased CO 2 pressure to stimulate photosynthesis to the expected degree. O 2-insensitive photosynthesis also occurred after plants had been water stressed. Under such conditions, however, photosynthesis became O 2 and CO 2 insensitive at physiological CO 2 pressures. Postillumination CO 2 exchange kinetics showed that O 2 and CO 2 insensitivity was not the result of elimination of photorespiration. It is proposed that O2 and CO2 insensitivity occurs when the concentration of phosphate in the chloroplast stroma cannot be both high enough to allow photophosphorylation and low enough to allow starch and sucrose synthesis at the rates required by the rest of the photosynthetic component processes. Under these conditions, the energy diverted to photorespiration does not adversely affect the potential for CO2 assimilation. 相似文献
5.
The aim of this work was to study the pathway(s) of sugar phosphate metabolism in chloroplasts of the unicellular green alga, Dunaliella marina (Volvocales). Phosphofructokinase, detectable in crude cell extracts, copurifled with intact chloroplasts on sucrose density gradients. In isolated chloroplasts, phosphofructokinase activity displayed latency to the same degree as chloroplast marker enzymes. From the quantitative distribution of enzyme activities in fractionated cells, it is concluded that there is an exclusive localization of phosphofructokinase in chloroplasts. In addition, no separation into multiple forms could be achieved. For the study of regulatory properties, chloroplast phosphofructokinase was partially purified by ammonium sulfate fractionation followed by DEAE-cellulose chromatography. The pH optimum of the enzyme activity was 7.0 and was not altered with varying concentrations of substrates or low-molecular-weight effectors. Fructose 6-phosphate showed a sigmoidal saturation curve whose shape was further changed with varying protein concentrations of the preparation. The second substrate, ATP, gave a hyperbolic saturation curve with a Michaelis constant of 60 μm. At a Mg 2+ concentration of 2.5 mm, ATP concentrations exceeding 1 mm inhibited the enzyme in a positive cooperative manner. The same type of inhibition was observed with other phosphorylated intermediates of carbon metabolism, the most efficient being phospho enolpyruvate, glycolate 2-phosphate, glycerate 3-phosphate, and glycerate 2-phosphate. Inorganic phosphate was the only activator found for phosphofructokinase. With nonsaturating fructose 6-phosphate concentrations, Pi activated in a positive cooperative fashion, while no activation occurred with saturating fructose 6-phosphate concentrations. In the presence of either an activator or an inhibitor, the sigmoidal shape of the fructose 6-phosphate saturation curve was altered. Most notably, the activator Pi could relieve the inhibitory action of ATP, phospho enolpyruvate, glycerate 3-phosphate, glycerate 2-phosphate, and glycolate 2-phosphate. Based on these experimental findings, the regulatory properties of D. marina chloroplast phosphofructokinase are discussed with respect to its playing a key role in the regulation of chloroplast starch metabolism during a light/dark transition. All available evidence is compatible with the interpretation that phosphofructokinase is active only in the dark thus channeling starch degradation products into glycolysis. 相似文献
6.
Washed whole chloroplasts of Spinacia oleracea isolated and assayed in a tris (hydroxymethyl aminomethane)-HCl buffered sucrose solution exhibited low dark CO 2 fixing activity, whereas washed whole chloroplasts isolated in the same buffer but assayed in that buffer without sucrose exhibited much greater dark CO 2 fixing activity. The lowered activity could be attributed to the impermeability of the chloroplast membrane to ribose-5-phosphate or adenosine triphosphate. The preservation of the integrity of the chloroplast membrane, as reflected by its impermeability to either or both of the abovementioned compounds, was measured by the fixation of 14CO 2 into acid-stable products in the presence of ribose-5-phosphate and adenosine triphosphate by the whole chloroplast as compared with fixation by the chloroplast extract. An effect (i.e., apparent resistance to the passage of ribose-5-phosphate or adenosine-5-triphosphate into the chloroplast) similar to, but less pronounced than, that produced by the presence of sucrose in the isolation medium was observed upon the addition of MnCl 2 or CaCl 2 to the buffered sucrose isolation medium. The addition of KCl enhanced slightly the effect produced by addition of sucrose alone to the isolation medium. The presence of MgCl 2 in the isolation medium, however, either caused the chloroplasts to become leaky or more fragile since more of the activity of the carboxylative phase enzymes appeared in the cytoplasm. When a mixture of all of the metal ions was added to the buffered sucrose suspending medium, the chloroplasts exhibited the same response observed with MgCl 2 alone. The addition of ethylene diaminetetraacetate or dithiothreitol appeared to alter the permeability of the chloroplast membrane nonspecifically when the assay was conducted in the absence of sucrose. Specific activities (μmoles CO 2 fixed/mg chlorophyll × hr) as high as 329.6 have been observed for dark fixation by chloroplasts. The phosphoenolpyruvate carboxylase activity in the chloroplasts was only one-seventh that of ribulose diphosphate carboxylase. The phosphoenolpyruvate carboxylase activity in the cytoplasm was 5 times that of the chloroplasts. 相似文献
7.
Light increased the initial rate and the extent of glycerate uptake by intact isolated chloroplasts. Half-maximum stimulation occurred with 10 to 20 watts per square meter of red light. Preillumination of chloroplasts enhanced uptake in a subsequent dark period. The light effect was abolished by DCMU and also by uncoupling agents such as nigericin and carbonyl cyanide p-trifluoromethoxyphenyl hydrazone. Arsenate and phlorizin only inhibited glycerate uptake to the extent that metabolism in the chloroplast was decreased by insufficient ATP. The concentration of glycerate accumulated in the chloroplast stroma was not significantly decreased. Chloroplasts isolated from young pea shoots (Pisum sativum, L. cv Massey Gem) were depleted of ATP by incubation with inorganic pyrophosphate or with ATP analogs. These treatments also decreased metabolism of glycerate but the actual concentration of glycerate accumulated in the chloroplast stroma was not decreased. The results indicate that glycerate uptake is driven by ion gradients established across the chloroplast envelope in the light. ATP is not involved in the transport of glycerate into chloroplasts, being required only for the subsequent metabolism of glycerate in the chloroplast stroma. It is proposed that glycerate transport may be coupled to the proton gradient established in the light across the chloroplast envelope. 相似文献
8.
The capacity of ribulose-1,5-bisphosphate carboxylase to bind reversibly chloroplast metabolites which are the substrates for both thylakoid and stromal enzymes was assessed using spinach chloroplasts and chloroplast extracts and with pure wheat ribulose-1,5-bisphosphate carboxylase. Measurements of the rate of coupled electron flow to methyl viologen in ‘leaky’ chloroplasts (which retained the chloroplast envelope and stromal enzymes but which were permeable to metabolites) and also with broken chloroplasts and washed thylakoids were used to study the effects of binding ADP and inorganic phopshate to ribulose-1,5-bisphosphate carboxylase. The presence of ribulose-1,5-bisphosphate carboxylase significantly altered the values obtained for apparent Km for inorganic phosphate and ADP of coupled electron transport. The Km (P i) in washed thylakoids was 60–80 μM, in ‘leaky’ chloroplasts it was increased to 180–200 μM, while in ‘leaky’ chloroplasts preincubated with KCN and ribulose 1,5-bisphosphate the value was decreased to 40–50 μM. Similarly, the Km (ADP) of coupled electron transport in washed thylakoids was 60–70 μM, in ‘leaky’ chloroplasts it was 130–150 μM and with ‘leaky’ chloroplasts incubated in the presence of KCN and ribulose 1,5-bisphosphate a value of 45–50 μM was obtained. The ability of ribulose 1,5-bisphosphate carboxylase to reduce the levels of free glycerate 3-phosphate in the absence of ribulose 1,5-bisphosphate was examined using a chloroplast extract system by varying the concentrations of stromal protein or purified ribulose 1,5-bisphosphate carboxylase. The effect of binding glycerate 3-phosphate to ribulose-1,5-bisphosphate carboxylase on glycerate 3-phosphate reduction was to reduce both the rate an the amount of NADPH oxidation for a given amount of glycerate 3-phosphate added. The addition of ribulose 1,5-bisphosphate reinitiated NADPH oxidation but ATP or NADPH did not. Incubation of purified ribulose-1,5-bisphosphate carboxylase with carboxyarabinitolbisphosphate completely inhibited the catalytic activity of the enzyme and decreased inhibition of glycerate-3-phosphate reduction. Two binding sites with different affinities for glycerate 3-phosphate were observed with pure ribulose-1,5-bisphosphate carboxylase. 相似文献
9.
We have investigated the effect of 2-hydroxy-3-butynoic acid (HBA) and its methyl ester (MeHBA) on photosynthesis and pigment formation in Zea mays, a C 4 photosynthesis-type plant. In the presence of the specific inhibitor of glycollate oxidase, assimilation of CO 2 was decreased significantly. Labelling patterns showed accumulation of glycollate, though not so marked as in C 3 photosynthesis-type plants, and marked decreases in incorporation into glycine, serine and particularly glycerate. This inhibition was specific for the S(+) enantiomers of HBA and MeHBA. In greening maize R, S-MeHBA inhibited formation of chloroplast pigments and this effect could be shown to be due to the S(+) enantiomer; of a range of metabolises tested only supplementations with serine or pyruvate were partly effective in restoring greening. 相似文献
10.
The photosynthetic carbon reduction cycle intermediates can be divided into three classes according to their effects on the rate of photosynthetic CO 2 evolution by whole spinach ( Spinacia oleracea) chloroplasts and on their ability to affect reversal of certain inhibitors (nigericin, arsenate, arsenite, iodoacetate, antimycin A) of photosynthesis: class I (maximal): fructose 1, 6-diphosphate, dihydroxyacetone phosphate, glyceraldehyde-3-phosphate, ribose-5-phosphate; class 2 (slight): glucose 6-phosphate, fructose 6-phosphate, ribulose-1, 5-diphosphate; class 3 (variable): glycerate 3-phosphate. While class 1 compounds influence the photosynthetic rate, they do not lower the Michaelis constant of the chloroplast for bicarbonate or affect strongly other photosynthetic properties such as the isotopic distribution pattern. It was concluded that the class 1 compounds influence the chloroplast by not only supplying components to the carbon cycle but also by activating or stabilizing a structural component of the chloroplast. 相似文献
11.
Photorespiration rates in tissue cultures of a C 4 plant, Portulaca oleracea, were compared to those in tissue cultures of a C 3 plant, Streptanthus tortuosus. The C 4 plant tissue cultures have one-half to one-third the photorespiration rate of the C 3 plant tissue cultures and respond to varying O 2 concentrations in a manner typical of C 4 plants. The results suggest that the lack of detectable photorespiration in C 4 plants is not related to leaf anatomy. 相似文献
12.
These studies demonstrated that CO 2 rather than HCO 3− is the inorganic carbon metabolite produced by the C 4 acid decarboxylases involved in C 4 photosynthesis (chloroplast located NADP malic enzyme, mitochondrial NAD malic enzyme, and cytosolic phosphoenolpyruvate [PEP] carboxykinase). The effect of varying CO 2 or HCO 3− as a substrate for the carboxylation reaction catalyzed by these enzymes or as inhibitors of the decarboxylation reaction was also determined. The K mCO 2 was 1.1 millimolar for NADP malic enzyme and 2.5 millimolar for PEP carboxykinase. For these two enzymes the velocity in the carboxylating direction was substantially less than for the decarboxylating direction even with CO 2 concentrations at the upper end of the range of expected cellular levels. Activity of NAD malic enzyme in the carboxylating direction was undetectable. The decarboxylation reaction of all three enzymes was inhibited by added HCO 3−. For NADP malic enzyme CO 2 was shown to be the inhibitory species but PEP carboxykinase and NAD malic enzyme were apparently inhibited about equally by CO 2 and HCO 3−. 相似文献
13.
Flaveria cronquistii (C 3), F. chloraefolia (C 3-C 4), F. floridana (C 3-C 4), F. pubescens (C 3-C 4), F. anomala (C 3-C 4), F. linearis (C 3-C 4), F. brownii (C 4), F. palmeri (C 4), F. trinervia (C 4) and F. australasica (C 4), comprising 10 out of the 21 known species of the genus Flaveria (Asteraceae), were included in a comparative study of the kinetic and regulatory properties of green leaf phosphoenolpyruvate (PEP) carboxylase. At least three kinetically distinct enzyme-forms were identified on the basis of their affinities for PEP and the degree of allosterism with respect to this substrate. The kinetic properties of PEP carboxylase of most of the species seemingly were modified in vivo depending on the growth conditions of the plants. Km(PEP free)-values of the enzyme from the five C 3-C 4 intermediate species ranged from 6 micromolar ( F. chloraefolia, low light-grown) to 38 micromolar ( F. pubescens, high light-grown). In contrast, the Km for PEP of PEP carboxylase from the C 3 species F. cronquistii (13 micromolar) apparently was not influenced by growth conditions. The response of the enzyme from the C 3 and C 3-C 4 species was hyperbolic in all cases. A second isoform with a lower affinity for PEP (88-100 micromolar), but also hyperbolic kinetics was found in the C 4 species F. brownii, whereas in the three other C 4 species examined a PEP carboxylase with a still lower affinity for PEP (187-221 micromolar) and sigmoidal kinetics was present. These isozyme-related kinetic data were supported by analyses of the elution behavior of the enzyme during anion-exchange chromatography on DEAE-Trisacryl M. The results are discussed with respect to the evolution of C 4 photosynthesis in the Flaveria genus. 相似文献
14.
Various properties of phosphoenolpyruvate carboxylases were compared in leaf preparations from C 3-C 4 intermediate, C 3, and C 4Panicum species. Values of Vmax in micromoles per milligram chlorophyll per hour at pH 8.3 were 57 to 75 for the enzyme from Panicum milioides, Panicum schenckii, and Panicum decipiens (all C 3-C 4). The values for Panicum laxum (C 3) and Panicum prionitis (C 4) were 20 to 40 and 952 to 1374, respectively. The Vmax values did not change at pH 7.3 except for the C 4 value, which increased about 24%. At pH 8.3, the phosphoenolpyruvate carboxylases from C 3 and C 3-C 4 species had slightly higher Km HCO 3− and lower K′ phosphoenolpyruvate values than did the C 4 enzyme. With each species at pH 7.3, all K′ phosphoenolpyruvate values were 2- to 4-fold greater. 相似文献
15.
C 2H 4 disrupts the internal membranes of the chloroplast and induces an increase in chlorophyllase activity in degreening calamondin [x Citrofortunella mitis (Blanco) Ingram and Moore] fruit. Whether the loss of chlorophyll in the peel is causally related to breakdown of the chloroplast and/or chlorophyllase activity is not readily apparent. Chlorophyllase levels were inversely related to chlorophyll content, but electron micrographs also showed that internal membranes of the chloroplasts were disrupted simultaneously with the decrease in chlorophyll content. Silver, a potent inhibitor of C 2H 4-mediated effects, retarded the loss of chlorophyll in calamondin rind, reduced the C 2H 4-induced increase in chlorophyllase level, and prevented the disruption of the chloroplast membranes. The results do not permit the proposal of a mechanism of C 2H 4 metabolism in the degreening of calamondin fruit. 相似文献
16.
This study was undertaken to examine the degree of Kranz anatomy development in the species intermediate to C 3 and C 4 types (C 3-C 4) in Panicum, Neurachne, Flaveria, and Moricandia. In each genus, C 3 and/or C 4 species were used for comparison. Leaf transections from each species were examined by light and transmission electron microscopy. The percentages of leaf photosynthetic cell profiles partitioned to bundle sheaths were higher in C 4 than in C 3 species, while C 3-C 4 species tended to be in between. However, percentages for C 3-C 4 species in Moricandia and some C 3-C 4Flaveria species were not greater than C 3. When expressed on a cell profile area basis, C 3-C 4 species partitioned more photosynthetic tissue to bundle sheaths than C 3 species in Moricandia, but not in Flaveria. Neurachne minor S. T. Blake (C 3-C 4) partitioned a very small portion of cell profile area to the inner bundle sheaths (5%) compared to Neurachne munroi F. Muell (C 4) (21%). The percentage of organelles partitioned to bundle sheaths was much greater in C 3-C 4 than in C 3 species. The average C 3 percentages for mitochondria plus peroxisomes were 19, 8, and 19.5% for Neurachne, Flaveria, and Moricandia, respectively, compared to 41, 29, and 46.5% for the C 3-C 4 species. The CO 2 compensation concentration was negatively related to the partitioning of tissue to bundle sheaths and to the percentage of organelles in bundle sheaths. It is concluded that all of the C 3-C 4 species examined have developed some degree of Kranz anatomy and that this altered anatomy is involved in their reduced apparent photorespiration. 相似文献
17.
Leaf CO 2 compensation points and stable hydrogen, oxygen and carbon isotope ratios were determined for Panicum species including C 3/C 4 intermediate photosynthesis plants, hybrids between C 3/C 4 intermediates and C 3 plants, C 3 and C 4 plants in the Panicum genus as well as several other C 3 and C 4 plants. C 3 plants had the highest compensation points, followed by hybrids, C 3/C 4 intermediates, and C 4 plants. δ 13C values of cellulose nitrate and saponifiable lipids from C 4 plants were about 10‰ higher than those observed for cellulose nitrate and saponifiable lipids of C 3/C 4 intermediates, hybrids, and C 3 plants. Oxygen isotope ratios of cellulose as well as those of leaf water were similar for all plants. There was substantial variability in the δD values of cellulose nitrate among the plants studied. In contrast, such variability was not observed in δD values of water distilled from the leaves, nor in the δD values of the saponifiable lipids. Variability in δD values of cellulose nitrate from C 3/C 4 intermediates, hybrids, C 3, and C 4 plants is due to fractionations occurring during biochemical reactions specific to leaf carbohydrate metabolism. 相似文献
18.
The sensitivity of stomatal conductance to changes of CO 2 concentration and leaf-air vapor pressure difference (VPD) was compared between two C 3 and two C 4 grass species. There was no evidence that stomata of the C 4 species were more sensitive to CO 2 than stomata of the C 3 species. The sensitivity of stomatal conductance to CO 2 change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO 2 concentration, C i/C a, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C i/C a was 0.7 and 0.5 (approximately) in C 3 and C 4 species, respectively. Transpiration efficiency (net CO 2 assimilation rate/transpiration rate) was larger in the C 4 species than in the C 3 species at current atmospheric CO 2 concentrations, but the relative increase due to high CO 2 was larger in the C 3 than in the C 4 species. 相似文献
19.
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
20.
Spinach chloroplasts were used to study the relationship between photosynthetic CO 2 fixation and temperature from 30 to −15°C. In saturating light and high concentrations of CO 2, the temperature coefficients (Q 10) above 20°C were less than 2 in the intact chloroplast. Below 15°C, the Q 10 values were greater than 2 and gradually increased with decreasing (down to 0°C) temperature to approximately 4.4. Photosynthesis responded similarly to temperature in a reconstituted chloroplast preparation fortified with ribose 5-phosphate. In the intact chloroplast, temperature did not alter the Q 10 value in low light and high CO 2. Elevating the temperature to 25°C after photosynthesizing at −15°C (46 minutes) or 0°C (17 minutes) restored the temperature-depressed photosynthetic rate without a lag in the intact chloroplast to the rate of a chloroplast continually at 25°C. At 0°C, the intact chloroplast photosynthetic rate responded slightly to the inorganic phosphate concentration (0.1-1.0 millimolar) and to pH (7.0-8.6). Relative to 25°C, the levels of ribulose 1,5-bisphosphate and glycerate 3-phosphate were increased 1300 and 200%, respectively, whereas glycolate decreased 57% during intact chloroplast photosynthesis at 0°C. Chilling temperature impeded the transport of photosynthetic intermediates from the stromal compartment to the external medium. Ethylene glycol was shown to be an appropriate additive to prevent freezing of the reaction mixture down to −15°C for photosynthetic CO 2 assimilation. 相似文献
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