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1.
Simultaneous measurements of CO(2) (CER) and O(2) (OER) exchange in roots and shoots of vegetative white lupin (Lupinus albus) were used to calculate the flow of reducing power to the synthesis of biomass that was more reduced per unit of carbon than carbohydrate. On a whole-plant basis, the diverted reductant utilization rate (DRUR which is: 4 x [CER + OER]) of shoot tissue was consistently higher than that of roots, and values obtained in the light were greater than those in the dark. An analysis of the biomass being synthesized over a 24-h period provided an estimate of whole-plant DRUR (3.5 mmol e(-) plant(-1) d(-1)), which was similar to that measured by gas exchange (3.2 mmol e(-) plant(-1) d(-1)). Given that nitrate reduction to ammonia makes up about 74% of whole-plant DRUR, root nitrate reduction in white lupin was estimated to account for less than 43% of whole-plant nitrate reduction. The approach developed here should offer a powerful tool for the noninvasive study of metabolic regulation in intact plants or plant organs. 相似文献
2.
Soybean ( Glycine max) yields high levels of both protein and oil, making it one of the most versatile and important crops in the world. Light has been implicated in the physiology of developing green seeds including soybeans but its roles are not quantitatively understood. We have determined the light levels reaching growing soybean embryos under field conditions and report detailed redox and energy balance analyses for them. Direct flux measurements and labeling patterns for multiple labeling experiments including [U‐ 13C 6]‐glucose, [U‐ 13C 5]‐glutamine, the combination of [U‐ 14C 12]‐sucrose + [U‐ 14C 6]‐glucose + [U‐ 14C 5]‐glutamine + [U‐ 14C 4]‐asparagine, or 14CO 2 labeling were performed at different light levels to give further insight into green embryo metabolism during seed filling and to develop and validate a flux map. Labeling patterns (protein amino acids, triacylglycerol fatty acids, starch, cell wall, protein glycan monomers, organic acids), uptake fluxes (glutamine, asparagine, sucrose, glucose), fluxes to biomass (protein amino acids, oil), and respiratory fluxes (CO 2, O 2) were established by a combination of gas chromatography‐mass spectrometry, 13C‐ and 1H‐NMR, scintillation counting, HPLC, gas chromatography‐flame ionization detection, C:N and amino acid analyses, and infrared gas analysis, yielding over 750 measurements of metabolism. Our results show: (i) that developing soybeans receive low but significant light levels that influence growth and metabolism; (ii) a role for light in generating ATP but not net reductant during seed filling; (iii) that flux through Rubisco contributes to carbon conversion efficiency through generation of 3‐phosphoglycerate; and (iv) a larger contribution of amino acid carbon to fatty acid synthesis than in other oilseeds analyzed to date. 相似文献
3.
CO 2 exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m ?2S ?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO 2 evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO 2 (g dry weight) ?1 S ?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m ?2 S ?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O 10 value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO 2 exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO 2 concentrations via CO 2 refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m ?2 S ?1. Young peach fruits responded to increasing ambient CO 2 concentrations with decreasing net CO 2 exchange rates in light, but more mature fruits did not respond to increases in ambient CO 2. Fruit CO 2 exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO 2 concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO 2 response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO 21 ?2. In CO 2 free air, fruit photosynthesis was dependent on CO 2 refixation since CO 2 uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO 2-free air and 370 μl CO 2 1 ?1 indicated that young peach fruits were apparently able to take up CO 2 from the external atmosphere. CO 2 uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO 2 refixation. 相似文献
4.
A mass spectrometric method combining 16O/ 18O and 12C/ 13C isotopes was used to quantify the unidirectional fluxes of O 2 and CO 2 during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O 2 uptake and CO 2 evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O 2 (61 micromoles of O 2 per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O 2 per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO 2 in darkness at a rate of 27 micromoles of CO 2 per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O 2 evolution and CO 2 fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phospho enolpyruvate carboxylase activity. Under light, CO 2 evolution by guard cell protoplasts was sharply decreased (37%), while O 2 uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO 2 assimilation and may be dissipated for other purposes such as ion uptake. 相似文献
5.
The effects of CO 2 concentration and illumination on net gas exchange and the pathway of 14CO 2 fixation in detached seeds from developing fruits of Lupinus albus (L.) have been studied. Increasing the CO2 concentration in the surrounding atmosphere (from 0.03 to 3.0% [v/v] in air) decreased CO2 efflux by detached seeds either exposed to the light flux equivalent to that transmitted by the pod wall (500 to 600 micro-Einsteins per square meter per second) in full sunlight or held in darkness. Above 1% CO2 detached seeds made a net gain of CO2 in the light (up to 0.4 milligrams of CO2 fixed per gram fresh weight per hour) but 14CO2 injected into the gas space of intact fruits (containing around 1.5% CO2 naturally) was fixed mainly by the pod and little by the seeds. Throughout development seeds contained ribulose-1,5-bisphosphate carboxylase activity (EC 4.1.1.39), especially in the embryo (up to 99 micromoles of CO2 fixed per gram fresh weight per hour) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) in both testa (up to 280 micromoles of CO2 fixed per gram fresh weight per hour) and embryo (up to 355 micromoles of CO2 fixed per gram fresh weight per hour). In kinetic experiments the most significant early formed product of 14CO2 fixation in both light and dark was malate but in the light phosphoglyceric acid and sugar phosphates were also rapidly labeled. 14CO2 fixation in the light was linked to the synthesis of sugars and amino acids but in the dark labeled sugars were not formed. 相似文献
6.
The short term effects of increased levels of CO 2 on gas exchange of leaves of bigtooth aspen ( Populus grandidentata Michx.) were studied at the University of Michigan Biological Station, Pellston, MI. Leaf gas exchange was measured in situ in the upper half of the canopy, 12 to 14 meters above ground. In 1900 microliters per liter CO 2, maximum CO 2 exchange rate (CER) in saturating light was increased by 151% relative to CER in 320 microliters per liter CO 2. The temperature optimum for CER shifted from 25°C in 320 microliters per liter CO 2 to 37°C in 1900 microliters per liter CO 2. In saturating light, increasing CO 2 level over the range 60 to 1900 microliters per liter increased CER, decreased stomatal conductance, and increased leaf water use efficiency. The initial slope of the CO 2 response curve of CER was not significantly different at 20 and 30°C leaf temperatures, although the slope did decline significantly during leaf senescence. In 1900 microliters per liter CO 2, CER increased with increasing light. The light saturation point and maximum CER were higher in 30°C than in 20°C, although there was little effect of temperature in low light. The experimental results are consistent with patterns seen in laboratory studies of other C 3 species and define the parameters required by some models of aspen CER in the field. 相似文献
7.
The effects of water stress and CO 2 enrichment on photosynthesis, assimilate export, and sucrose-P synthase activity were examined in field grown soybean plants. In general, leaves of plants grown in CO 2-enriched atmospheres (300 microliters per liter above unenriched control, which was 349 ± 12 microliters per liter between 0500 and 1900 hours EST over the entire season) had higher carbon exchange rates (CER) compared to plants grown at ambient CO 2, but similar rates of export and similar activities of sucrose-P synthase. On most sample dates, essentially all of the extra carbon fixed as a result of CO 2 enrichment was partitioned into starch. CO 2-enriched plants had lower transpiration rates and therefore had a higher water use efficiency (milligrams CO 2 fixed per gram H 2O transpired) per unit leaf area compared to nonenriched plants. Water stress reduced CER in nonenriched plants to a greater extent than in CO 2-enriched plants. As CER declined, stomatal resistance increased, but this was not the primary cause of the decrease in assimilation because internal CO 2 concentration remained relatively constant. Export of assimilates was less affected by water stress than was CER. When CERs were low as a result of the imposed stress, export was supported by mobilization of reserves (mainly starch). Export rate and leaf sucrose concentration were related in a curvilinear manner. When sucrose concentration was above about 12 milligrams per square decimeter, obtained with nonstressed plants at high CO 2, there was no significant increase in export rate. Assimilate export rate was also correlated positively with SPS activity and the quantitative relationship varied with CER. Thus, export rate was a function of both CER and carbon partitioning. 相似文献
8.
Photoautotrophic fatty acid production of a highly CO 2‐tolerant green alga Chlorococcum littorale in the presence of inorganic carbon at 295 K and light intensity of 170 µmol‐photon m ?2 s ?1 was investigated. CO 2 concentration in the bubbling gas was adjusted by mixing pure gas components of CO 2 and N 2 to avoid photorespiration and β‐oxidation of fatty acids under O 2 surrounding conditions. Maximum content of total fatty acid showed pH‐dependence after nitrate depletion of the culture media and increased with the corresponding inorganic carbon ratio. Namely, [HCO 3?]/([CO 2]+ n[ ]) ratio in the culture media was found to be a controlling factor for photoautotrophic fatty acid production after the nitrate limitation. At a CO 2 concentration of 5% (vol/vol) and a pH of 6.7, the fatty acid content was 47.8 wt % (dry basis) at its maximum that is comparable with land plant seed oils. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1053–1057, 2015 相似文献
9.
A closed system consisting of an assimilation chamber furnished with a membrane inlet from the liquid phase connected to a mass spectrometer was used to measure O 2 evolution and uptake by Chlamydomonas reinhardtii cells grown in ambient (0.034% CO 2) or CO 2-enriched (5% CO 2) air. At pH = 6.9, 28°C and concentrations of dissolved inorganic carbon (DIC) saturating for photosynthesis, O 2 uptake in the light (U o) equaled O 2 production (E o) at the light compensation point (15 micromoles photons per square meter per second). E o and U o increased with increasing photon fluence rate (PFR) but were not rate saturated at 600 micromoles photons per square meter per second, while net O 2 exchange reached a saturation level near 500 micromoles photons per square meter per second which was nearly the same for both, CO 2-grown and air-grown cells. Comparison of the U o/E o ratios between air-grown and CO 2-grown C. reinhardtii showed higher values for air-grown cells at light intensities higher than light compensation. For both, air-grown and CO 2-grown algae the rates of mitochondrial O 2 uptake in the dark measured immediately before and 5 minutes after illumination were much lower than U o at PFR saturating for net photosynthesis. We conclude that noncyclic electron flow from water to NADP + and pseudocyclic electron flow via photosystem I to O 2 both significantly contribute to O 2 exchange in the light. In contrast, mitochondrial respiration and photosynthetic carbon oxidation cycle are regarded as minor O 2 consuming reactions in the light in both, air-grown and CO 2-grown cells. It is suggested that the “extra” O 2 uptake by air-grown algae provides ATP required for the energy dependent CO 2/HCO 3− concentrating mechanism known to be present in these cells. 相似文献
10.
Phaseolus vulgaris L. leaves were subjected to various light, CO 2, and O 2 levels and abscisic acid, then given a 10 minute pulse of 14CO 2 followed by a 5 minute chase with unlabeled CO 2. After the chase period, very little label remained in the ionic fractions (presumed to be mostly carbon reduction and carbon oxidation cycle intermediates and amino acids) except at low CO 2 partial pressure. Most label was found in the neutral, alcohol soluble fraction (presumed sucrose) or in the insoluble fraction digestable by amyloglucosidase. Sucrose formation was linearly related to assimilation rate (slope = 0.35). Starch formation increased linearly with assimilation rate (slope = 0.56) but did not occur if the assimilation rate was below 4 micromoles per square meter per second. Neither abscisic acid, nor high CO 2 in combination with low O 2 (thought to disrupt control of carbon metabolism) caused significant perturbations of the sucrose/starch formation ratio. These studies indicate that the pathways for starch and sucrose synthesis both are controlled by the rate of net CO 2 assimilation, with sucrose the preferred product at very low assimilation rates. 相似文献
11.
The CO 2- and H 2O-exchange rates between soybean canopies and the atmosphere were measured in three mobile chambers (4 m 3). Each chamber stopped at 8 or 9 plots (3.1-m 2 ground area) every 25 min. Diurnal and seasonal CO 2-exchange rates (CER) of 13 soybean ( Glycine max (L.) Merr.) cultivars are summarized here. The oldest two cultivars, released in 1927 and 1932, had the lowest CER values. The CER usually decreased in the afternoon (23.4 vs 27.8 mol CO 2 m -2 s -1 at 1.6 mmol photons m -2 s -1), except shortly after rainfall. During a drought, these reductions occurred earlier in the day and were more pronounced. We present evidence for a nonstomatal component of the CO 2 flux-reaction system causing CER reductions during a water stress. Daytime CER values were not correlated with temperature (24–34° C), but nighttime values were (15–25° C, r=0.85,* n=41). 相似文献
12.
The mass transfer rate of 14C-sucrose translocation from sugar beet ( Beta vulgaris, L.) leaves was measured over a range of net photosynthesis rates from 0 to 60 milligrams of CO 2 decimeters −2 hour −1 under varying conditions of light intensity, CO 2 concentration, and O 2 concentration. The resulting rate of translocation of labeled photosynthate into total sink tissue was a linear function (slope = 0.18) of the net photosynthesis rate of the source leaf regardless of light intensity (2000, 3700, or 7200 foot-candles), O 2 concentration (21% or 1% O 2), or CO 2 concentration (900 microliters/liter of CO 2 to compensation concentration). These data support the theory that the mass transfer rate of translocation under conditions of sufficient sink demand is limited by the net photosynthesis rate or more specifically by sucrose synthesis and this limitation is independent of light intensity per se. The rate of translocation was not saturated even at net photosynthesis rates four times greater than the rate occurring at 300 microliters/liter of CO 2, 21% O 2, and saturating light intensity. 相似文献
13.
Although there is now a considerable literature on the inhibition of leaf respiration (CO 2 evolution) by light, little is known about the effect of other environmental conditions on day respiratory metabolism. In particular, CO 2 and O 2 mole fractions are assumed to cause changes in the tricarboxylic acid pathway (TCAP) but the amplitude and even the direction of such changes are still a matter of debate. Here, we took advantage of isotopic techniques, new simple equations and instant freeze sampling to follow respiratory metabolism in illuminated cocklebur leaves ( Xanthium strumarium L.) under different CO 2/O 2 conditions. Gas exchange coupled to online isotopic analysis showed that CO 2 evolved by leaves in the light came from ‘old’ carbon skeletons and there was a slight decrease in 13C natural abundance when [CO 2] increased. This suggested the involvement of enzymatic steps fractionating more strongly against 13C and thus increasingly limiting for the metabolic respiratory flux as [CO 2] increased. Isotopic labelling with 13C 2‐2,4‐citrate lead to 13C‐enriched Glu and 2‐oxoglutarate (2OG), clearly demonstrating poor metabolism of citrate by the TCAP. There was a clear relationship between the ribulose‐1,5‐bisphosphate oxygenation‐to‐carboxylation ratio ( vo/ vc) and the 13C commitment to 2OG, demonstrating that 2OG and Glu synthesis via the TCAP is positively influenced by photorespiration. 相似文献
14.
The capacity of intact chloroplasts to synthesize long chain fatty acids from acetate depends on the stroma pH in Spinacia oleracea, U. S. hybrid 424. The pH optimum is close to 8.5. Lowering of the stroma pH leads to a reduction of acetate incorporation but does not suffice to eliminate fatty acid synthesis completely. Chain elongation from palmitic to oleic acid shows the same pH dependence. Fatty acid synthesis is activated in the dark upon the simultaneous addition of dihydroxyacetone phosphate and orthophosphate supplying ATP and oxaloacetate for reoxidation of NADPH in the stroma. Under these conditions both dark fatty acid synthesis and synthesis of oleate from palmitate show the same pH dependence as in the light. Dark fatty acid synthesis is further stimulated by increasing the stromal Mg 2+ concentration with the ionophore A 23187. In contrast to CO 2 fixation, dark fatty acid synthesis is considerably reduced by dithiothreitol (DTT). This observation may be due to an acetyl-CoA deficiency, caused by a nonenzymic acylation of DTT, and a competition for ATP between DTT-activated CO 2 fixation and fatty acid synthesis. Because d,l-glyceraldehyde as inhibitor of CO 2 fixation compensates the DTT effect on dark fatty acid synthesis, reducing equivalents may be involved in the light dependence of acetate activation. 相似文献
15.
The nature of the different processes of O 2 uptake involved in the light in the red macroalga Chondrus crispus Stackhouse ( Rhodophyta, Gigartinales) was investigated. At limiting CO 2, INH (2.5 mM) did not alter the O 2 uptake rate. Glycolate was not excreted and did not accumulate within the cells. KCN reduced the rate of O 2 uptake in the light by 76% at limiting CO 2 and by 43% at saturating CO 2, but caused > 95% inhibition of O 2 evolution. DCMU (5 μM) totally blocked the photosynthetic electron transport chain, but allowed a residual O 2 uptake of 3.0±0.6 μmol O 2 .h ?1.g ?1 FW, irrespective of the CO 2 concentration. In saturating CO 2, a high light intensity pretreatment significantly stimulated the rate of O 2 uptake compared to net O 2 evolution, suggesting the persistence, in the light, of mitochondrial respiration. Irrespective of the CO 2 concentration, the optimum temperature for O 2 evolution was 17°C whereas dark O 2 uptake increased linearly with temperature. In contrast, O 2 uptake in the light showed an optimum at 17°C in limiting CO 2, and 21–25° C in saturating CO 2; its Q 10 was 2.4 at limiting CO 2, a value close to that of RuBP oxygenase, and 3.1 at saturating CO 2, a value close to that of dark respiration. It is concluded that: 1) mitochondrial respiration and Mehler reaction are both involved at all CO 2 concentrations, 2) RuBP oxygenase activity cannot account for more than 45%, and Mehler reaction for less than 20%, of the total O 2 uptake observed in the light at limiting CO 2. 相似文献
16.
A procedure for estimating biomass during batch fermentation from on-line gas analysis is presented. First, the respiratory quotient was used to determine the fraction of the total oxygen utilization rate required for cell maintenance and growth versus product synthesis. The modified oxygen utilization rate was then used to estimate biomass on-line by integrating the oxygen balance for cell synthesis-maintenance. The method is illustrated for the case of L-lysine synthesis by Corynebacterium glutamicum.List of Symbols
CER mmol CO 2/l · h
carbon dioxide evolution rate
-
M
O
2/ x mmol O 2/h · g cells
maintenance coefficient
-
OUR mmol O 2/l · h
oxygen utilization rate
-
OUR
X
mmol O 2/l · h
OUR fraction for cell maintenance and growth
-
RQ mmol CO 2/mmol O 2
respiratory quotient( CER/OUR)
-
X g cells/l
biomass concentration
-
Y
X/O 2
yield coefficients 相似文献
17.
Scenedesmus cells grown on high CO 2, when adapted to air levels of CO 2 for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO 2 from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O 2 evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO 2 pump with K 0.5 for O 2 evolution of less than 5 micromolar CO 2 was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO 3− concentrating process between pH 8 to 10 lowered the K 0.5 (DIC) from 7300 micromolar HCO 3− in high CO 2 grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO 3− pump was inhibited by vanadate (K i of 150 micromolar), as if it involved an ATPase linked HCO 3− transporter. The CO 2 pump was formed on low CO 2 by high-CO 2 grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO 3− pump was partially activated on low CO 2 within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO 3− pump at pH 9 had requirements similar to the activation of the CO 2 pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO 2-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO 2 pump and in some algae an additional vanadate-sensitive plasmalemma HCO 3− transporter for a HCO 3− pump. 相似文献
18.
Wild-type Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were evaluated by noninvasive in situ methods for their capacity for net CO 2 assimilation, true rates of photosynthetic O 2 evolution (determined from chlorophyll fluorescence measurements of photosystem II), partitioning of photosynthate into sucrose and starch, and plant growth. Compared with wild-type plants, the starch mutants showed reduced photosynthetic capacity, with the largest reduction occurring in mutant TL25 subjected to high light and increased CO 2 partial pressure. The extent of stimulation of CO 2 assimilation by increasing CO 2 or by reducing O 2 partial pressure was significantly less for the starch mutants than for wild-type plants. Under high light and moderate to high levels of CO 2, the rates of CO 2 assimilation and O 2 evolution and the percentage inhibition of photosynthesis by low O 2 were higher for the wild type than for the mutants. The relative rates of 14CO 2 incorporation into starch under high light and high CO 2 followed the patterns of photosynthetic capacity, with TL46 showing 31% to 40% of the starch-labeling rates of the wild type and TL25 showing less than 14% incorporation. Overall, there were significant correlations between the rates of starch synthesis and CO 2 assimilation and between the rates of starch synthesis and cumulative leaf area. These results indicate that leaf starch plays an important role as a transient reserve, the synthesis of which can ameliorate any potential reduction in photosynthesis caused by feedback regulation. 相似文献
19.
Mass spectrometric measurements of dissolved free 13CO 2 were used to monitor CO 2 uptake by air grown (low CO 2) cells and protoplasts from the green alga Chlamydomonas reinhardtii. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of external carbonic anhydrase reduced the 13CO 2 concentration in the medium to close to zero. Similar results were obtained with low CO 2 cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO 2 uptake revealed that the removal of CO 2 from the medium in the light was due to selective and active CO 2 transport rather than uptake of total dissolved inorganic carbon. In the light, low CO 2 cells and protoplasts incubated with carbonic anhydrase took up CO 2 at an apparently low rate which reflected the uptake of total dissolved inorganic carbon. No net CO 2 uptake occurred in the dark. Measurement of chlorophyll a fluorescence yield with low CO 2 cells and washed protoplasts showed that variable fluorescence was mainly influenced by energy quenching which was reciprocally related to photosynthetic activity with its highest value at the CO 2 compensation point. During the linear uptake of CO 2, low CO 2 cells and protoplasts incubated with carbonic anhydrase showed similar rates of net O 2 evolution (102 and 108 micromoles per milligram of chlorophyll per hour, respectively). The rate of net O 2 evolution (83 micromoles per milligram of chlorophyll per hour) with washed protoplasts was 20 to 30% lower during the period of rapid CO 2 uptake and decreased to a still lower value of 46 micromoles per milligram of chlorophyll per hour when most of the free CO 2 had been removed from the medium. The addition of carbonic anhydrase at this point resulted in more than a doubling of the rate of O 2 evolution. These results show low CO 2 cells of Chlamydomonas are able to transport both CO 2 and HCO 3− but CO 2 is preferentially removed from the medium. The external carbonic anhydrase is important in the supply to the cells of free CO 2 from the dehydration of HCO 3−. 相似文献
20.
Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO 2 and O 2 concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO 2 is suggested to explain the discrepancy. The model suggests that resistance to CO 2 fixation is mainly in the carboxylation reactions, not in CO 2 transport. Using the steady state model, the kinetics of 14C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of 14C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO 2 released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration. 相似文献
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