共查询到20条相似文献,搜索用时 15 毫秒
1.
M. S. Patel 《Journal of neurochemistry》1974,22(5):717-724
To evaluate the relative significance of CO2-fixing enzymes in the metabolism of rat brain, the subcellular distribution of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase, as well as the fixation of H14CO3? by the cytosol and the mitochondria was investigated. Pyruvate carboxylase and phosphoenol-pyruvate carboxykinase are mainly localized in the mitochondria whereas NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase are present in both the cytosol and the mitochondria. In the presence of pyruvate rat brain mitochondria fixed H14CO3? at a rate of about 170 nmol/g of tissue/min whereas these organelles fixed negligible amounts of H14CO3? in the presence of α-ketoglutarate or phosphoenolpyruvate. Rat brain cortex slices fixed H14CO3? at a rate of about 7 nmol/g of tissue/min and it was increased by two-fold when pyruvate was added to the incubation medium. The carboxylation of α-ketoglutarate and pyruvate by the reversal of the cytosolic NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase respectively was very low as compared to that by pyruvate carboxylase. The rate of carboxylation reaction of both NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase was only about 1/10th of that of decarboxylation reaction of the same enzyme. It is suggested that under physiological conditions these two enzymes do not play a significant role in CO2-fixation in the brain. In rat brain cytosol, citrate is largely metabolized to α-ketoglutarate by a sequential action of aconitate hydratase and NADP-isocitrate dehydrogenase. The operation of the citrate-cleavage pathway in rat brain cytosol is demonstrated. The data show that among four CO2-fixing enzymes, pyruvate carboxylase, an anaplerotic enzyme, plays the major role in CO2-fixation in the brain. 相似文献
2.
Abstract Results obtained with Hydrodictyon africanum, and data from the literature, show that most green algae of the chlorophyte type (e.g. Chlorella, Chlamydomonas, Hydrodictyon) differ in their photosynthetic C fixation characteristics from most green algae of the charophyte type (e.g. Spirogyra, Chara) and from C3 higher plants. The chlorophyte algae fix inorganic carbon by the photosynthetic carbon reduction cycle pathway, but have a low CO2 compensation point in 250 μM O2, a low inhibition of CO2 fixation from 10 μM CO2/250 μM O2 when compared with 10 μM CO2/zero O2, and a low half-saturation constant for CO2. These three characteristics are different from those of charophytes and C3 higher plants, and resemble those of C4 higher plants. It is suggested that these characteristics of chlorophyte algae are the result of a ‘CO2 concentrating mechanism’ which increases the CO2/O2 ratio at the site of ribulose bisphosphate carboxylase-oxygenase action in a similar way to that achieved by the C4?C3 acid cycle in C4 plants. In the chlorophyte algae, however, CO2 concentration probably involves active HCO3? transport at the inner membrane of the chloroplast envelope. Active HCO3? transport can occur at the plasmalemma of charophyte algae and submerged aquatic higher plants as well as chlorophyte algae, so it is unlikely to explain the differences between the two groups of aquatic green plants. Differences in the properties of ribulose bisphosphate carboxylase-oxygenase, and differences in CO2 production in the light, also seem inadequate to account for the different photosynthetic characteristics. The chlorophyte type of ‘C02 concentrating mechanism’ appears to be common in other classes of eukaryotic algae, and in cyanophytes. Some of the ‘advanced’ members of these eukaryotic algal classes (including the chlorophytes) may lack the mechanism, while some ‘primitive’ charophytes may retain the mechanism which their ancestors presumably possessed. 相似文献
3.
Fruiting structures of a number of legumes including chickpea are known to carry out photosynthetic CO2 assimilation, but the pathway of CO2 fixation and particularly the role of phosphoenolpyruvate carboxylase (EC 4.1.1.31) in these tissues is not clear. Activities of some key enzymes of the Calvin cycle and C4 metabolism, rates of 14CO2 fixation in light and dark, and initial products of photosynthetic 14CO2 fixation were determined in podwall and seedcoat (fruiting structures) and their subtending leaf in chickpea (Cicer arietinum L.). Compared to activities of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) and other Calvin cycle enzyme, viz. NADP+-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13), NAD+-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12) and ribulose-5-phosphate kinase (EC 2.7.1.19), the levels of phosphoenolpyruvate carboxylase and other enzymes of C4 metabolism viz. NADP+-malate dehydrogenase (EC 1.1.1.82), NAD+-malate dehydrogenase (EC 1.1.1.37), NADP+ malic enzyme (EC 1.1.1.40), NAD+-malic enzyme (EC 1.1.1.39), glutamate oxaloacetate transaminase (EC 2.6.1.1) and glutamate pyruvate transaminase (EC 2.6.1.2), were generally much higher in podwall and seedcoat than in the leaf. Podwall and seedcoat fixed 14CO2 in light and dark at much higher rates than the leaf. Short-term assimilation of 14CO2 by illuminated fruiting structures produced malate as the major labelled product with less labelling in 3-phosphoglycerate, whereas the leaf showed a major incorporation into 3-phosphoglycerate. It seems likely that the fruiting structures of chickpea utilize phosphoenolpyruvate carboxylase for recapturing the respired carbon dioxide. 相似文献
4.
CHRISTER LARSSON 《Physiologia plantarum》1979,46(3):221-226
The Compartmentation of 14CO2 fixation and concomitant metabolism of l4C-iabelled products in a recombined system, composed of isolated intact spinach (Spinacia oleracea) chloroplasls and a ‘cytoplasm’ fraction, has been studied. Addition of ‘cytoplasm’ to chloroplasts fixing 14CO2 increased the label in hexoae monophosphates outside the chloroplasts at the expense of excreted dihydroxyacetone phosphate. The label in ammo acids was increased both inside and outside the chloroplasts. The results support the view that chloroplasls are not able to make 2-oxoacids for amino acid synthesis directly from fixed CO2, but have to co-operate with the cytoplasm and other organelles. The results also show that recombined systems can be useful for studies on the compartmen tation of carbon metabolism in pholosynthesizing plant tissues. 相似文献
5.
Immediate export in leaves of C3‐C4 intermediates were compared with their C3 and C4 relatives within the Panicum and Flaveria genera. At 35 Pa CO2, photosynthesis and export were highest in C4 species in each genera. Within the Panicum, photosynthesis and export in ‘type I’ C3‐C4 intermediates were greater than those in C3 species. However, ‘type I’ C3‐C4 intermediates exported a similar proportion of newly fixed 14C as did C4 species. Within the Flaveria, ‘type II’ C3‐C4 intermediate species had the lowest export rather than the C3 species. At ambient CO2, immediate export was strongly correlated with photosynthesis. However, at 90 Pa CO2, when photosynthesis and immediate export increased in all C3 and C3‐C4 intermediate species, proportionally less C was exported in all photosynthetic types than that at ambient CO2. All species accumulated starch and sugars at both CO2 levels. There was no correlation between immediate export and the pattern of 14C‐labelling into sugars and starch among the photosynthetic types within each genus. However, during CO2 enrichment, C4Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO2, whereas the C4Flaveria species accumulated only additional starch. 相似文献
6.
Inter-generational effects on the growth of Poa annua (L.) in ambient and elevated atmospheric CO2 conditions (350 and 550 μl l–1, respectively) were studied in two different experiments. Both experiments showed similar results. In a greenhouse experiment growth, measured as the numbers of tillers produced per week, was compared for plants grown from first and second generation seeds. Second generation seeds were obtained from plants grown for one whole generation in either ambient or elevated atmospheric CO2 (‘ambient’ and ‘elevated’ seeds, respectively). First generation plants and second generation ‘ambient’ plants did not respond to elevated CO2. Second generation ‘elevated’ plants produced significantly more tillers in elevated CO2. In the second experiment model terrestrial ecosystems growing in the Ecotron and which included Poa annua were used. Above-ground biomass after one and two generations of growth were compared. At the end of Generation 1 no difference was found in biomass production while at the end of Generation 2 biomass increased in elevated CO2 by 50%. The implications for climate change research are discussed. 相似文献
7.
Young leaves of salt-depleted Aeluropus litoralis Parl. plants show CO2 fixation by the C3-carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO2 fixation by the C4-pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose-1,5-diphosphate carboxylase. 相似文献
8.
Abstract Evidence is drawn from previous studies to argue that C3—C4 intermediate plants are evolutionary intermediates, evolving from fully-expressed C3 plants towards fully-expressed C4 plants. On the basis of this conclusion, C3—C4 intermediates are examined to elucidate possible patterns that have been followed during the evolution of C4 photosynthesis. An hypothesis is proposed that the initial step in C4-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO2 using RuBP carboxylase. The CO2-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO2. As the activity of PEP carboxylase increased to higher values, other enzymes in the C4-pathway are proposed to have increased in activity to facilitate the processing of the products of C4-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C4-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C4 plants. Such metabolism would have limited benefit in terms of concentrating CO2 at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C4-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C4 plants. Thus, during the initial stages of C4-evolution it is proposed that improvements in photorespiratory CO2-loss and their influence on increasing the rate of net CO2 assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO2-concentrating mechanism is proposed as the driving force during the later stages. 相似文献
9.
Metabolism of Propane, n-Propylamine, and Propionate by Hydrocarbon-Utilizing Bacteria 总被引:11,自引:5,他引:6
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Studies were conducted on the oxidation and assimilation of various three-carbon compounds by a gram-positive rod isolated from soil and designated strain R-22. This organism can utilize propane, propionate, or n-propylamine as sole source of carbon and energy. Respiration rates, enzyme assays, and 14CO2 incorporation experiments suggest that propane is metabolized via methyl ketone formation; propionate and n-propylamine are metabolized via the methylmalonyl-succinate pathway. Isocitrate lyase activity was found in cells grown on acetate and was not present in cells grown on propionate or n-propylamine. 14CO2 was incorporated into pyruvate when propionate and n-propylamine were oxidized in the presence of NaAsO2, but insignificant radioactivity was found in pyruvate produced during the oxidation of propane and acetone. The n-propylamine dissimilatory mechanism was inducible in strain R-22, and amine dehydrogenase activity was detected in cells grown on n-propylamine. Radiorespirometer and 14CO2 incorporation studies with several propane-utilizing organisms indicate that the methylmalonyl-succinate pathway is the predominant one for the metabolism of propionate. 相似文献
10.
The Hurley Pasture Model was used to examine the short and long-term responses of grazed grasslands in the British uplands to a step increase from 350 to 700 μmol mol–1 CO2 concentration ([CO2]) with inputs of 5 or 100 kg N ha–1 y–1. In N-rich grassland, [CO2] doubling quickly increased net primary productivity (NPP), total carbon (Csys) and plant biomass by about 30%. By contrast, the N-poor grassland underwent a prolonged ‘transient’, when there was little response, but eventually NPP, Csys and plant biomass more than doubled. The ‘transient’ was due to N immobilization and severe depletion of the soil mineral N pool. The large long-term response was due to slow N accumulation, as a result of decreased leaching, decreased gaseous N losses and increased N2-fixation, which amplified the CO2 response much more in the N-poor than in the N-rich grassland. It was concluded that (i) ecosystems use extra carbon fixed at high [CO2] to acquire and retain nutrients, supporting the contention of Gifford et al. (1996 ), (ii) in the long term, and perhaps on the real timescale of increasing [CO2], the response (in NPP, Csys and plant biomass) of nutrient-poor ecosystems may be proportionately greater than that of nutrient-rich ones, (iii) short-term experiments on nutrient-poor ecosystems may observe only the transient responses, (iv) the speed of ecosystem responses may be limited by the rate of nutrient accumulation rather than by internal rate constants, and (v) ecosystem models must represent processes affecting nutrient acquisition and retention to be able to simulate likely real-world CO2 responses. 相似文献
11.
Biogenic isoprene substantially affects atmospheric chemistry, but it is not known how or why many plants, especially trees, make isoprene. We fed 13CO2 to leaves of Quercus rubra and monitored the incorporation of 13C into isoprene by mass spectrometry. After feeding 13CO2 for 9 min we found all possible labelling patterns from completely unlabelled to fully labelled isoprene. By 18 min, 84% of the carbon atoms in isoprene were 13C. Labelling of the last 20% of the carbon atoms was much slower than labelling of the first 80%. The rate of labelling of isoprene was similar to that reported for phosphoglyceric acid indicating that there is a close linkage between the carbon source for isoprene synthesis and the photosynthetic carbon reduction pathway. 相似文献
12.
We assessed the potential of using 14C contents of soil respired CO2 to calculate the contributions of heterotrophic and autotrophic respiration to total soil respiration. The partitioning of these fluxes is of utmost importance to evaluate implications of environmental change on soil carbon cycling and sequestration. At three girdled forest stands in Sweden and Germany, where the tree root (autotrophic) respiration had been eliminated, we measured both flux rates and 14C contents of soil respired CO2 in girdled and control plots in the summers of 2001 or 2002. At all stands, CO2 flux rates were slightly higher in the control plots, whereas the 14C contents of respired CO2 tended to be higher in the girdled plots. This was expected and confirmed that heterotrophically respired CO2 cycles more slowly through the forest ecosystem than autotrophically respired CO2. On the basis of these data, the contributions of hetero‐ and autotrophic respiration to total soil respiration were calculated using two separate approaches (i.e. based on flux rates or 14C). Fractions of heterotrophic respiration ranged from 53% to 87%. Values calculated by both approaches did not differ significantly from each other. Finally, we compared the 14C contents of soil respired CO2 in the girdled plots with the 14C contents of heterotrophically respired CO2 calculated by three different 14C models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the 14C content of CO2 respired in the girdled plots to be lower than ‘true’ heterotrophically respired CO2 in an undisturbed plot. Nevertheless, we argue that measurements and modeling of 14C can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed). 相似文献
13.
M. U. F. KIRSCHBAUM 《Plant, cell & environment》1994,17(6):747-754
The atmospheric CO2 concentration has increased from the pre-industrial concentration of about 280 μmol mol−1 to its present concentration of over 350 μmol mol−1, and continues to increase. As the rate of photosynthesis in C3 plants is strongly dependent on CO2 concentration, this should have a marked effect on photosynthesis, and hence on plant growth and productivity. The magnitude of photo-synthetic responses can be calculated based on the well-developed theory of photosynthetic response to intercellular CO2 concentration. A simple biochemically based model of photosynthesis was coupled to a model of stomatal conductance to calculate photosynthetic responses to ambient CO2 concentration. In the combined model, photosynthesis was much more responsive to CO2 at high than at low temperatures. At 350 μmol mol−1, photosynthesis at 35°C reached 51% of the rate that would have been possible with non-limiting CO2, whereas at 5°C, 77% of the CO2 non-limited rate was attained. Relative CO2 sensitivity also became smaller at elevated CO2, as CO2 concentration increased towards saturation. As photosynthesis was far from being saturated at the current ambient CO2 concentration, considerable further gains in photosynthesis were predicted through continuing increases in CO2 concentration. The strong interaction with temperature also leads to photosynthesis in different global regions experiencing very different sensitivities to increasing CO2 concentrations. 相似文献
14.
An inexpensive system for exposing plants in the field to elevated concentrations of CO2 总被引:5,自引:4,他引:1
An inexpensive, potentially mobile field exposure system is described which may be easily constructed by a small workshop. It may be operated as an open-top with a frustrum or covered with a polycarbonate ‘lid’. The system is cost-effective for CO2 exposure work because the small size allows provision of CO2-enriched atmospheres over prolonged periods at relatively low cost. A preliminary assessment of the chambers has been made and concentrations can be maintained at ±6% for a target atmosphere of 680 cm3 m?3 CO2 under normal operating conditions. Other chamber environmental conditions are reported. 相似文献
15.
S. VON CAEMMERER V. QUINN N. C. HANCOCK G. D. PRICE R. T. FURBANK & M. LUDWIG 《Plant, cell & environment》2004,27(6):697-703
Carbonic anhydrase (CA, EC 4.2.1.1) catalyses the first reaction in the C4 photosynthetic pathway, the conversion of atmospheric CO2 to bicarbonate in the mesophyll cytosol. To examine the importance of the enzyme to the functioning of the C4 photosynthetic pathway, Flaveria bidentis (L.) Kuntze, a C4 dicot, was genetically transformed with an antisense construct in which the cDNA encoding a putative cytosolic CA (CA3) was placed under the control of a constitutive promoter. Some of the primary transformants had impaired CO2 assimilation rates and required high CO2 for growth. The T1 progeny of four primary transformants were used to examine the quantitative relationship between leaf CA activity and CO2 assimilation rate. CA activity was determined in leaf extracts with a mass spectrometric technique that measured the rate of 18O exchange from doubly labelled 13C18O2. Steady‐state CO2 assimilation rates were unaffected by a decrease in CA activity until CA activity was less than 20% of wild type when they decreased steeply. Transformants with less than 10% of wild‐type CA activity had very low CO2 assimilation rates and grew poorly at ambient CO2 partial pressure. Reduction in CA activity also increased the CO2 partial pressure required to saturate CO2 assimilation rates. The present data show that CA activity is essential for the functioning of the C4 photosynthetic pathway. 相似文献
16.
JACK FARINEAU 《Physiologia plantarum》1975,33(4):310-315
The role of malate decarboxylation as a source of CO2 and NADPH for the evolution of photosynthesis of isolated maize bundle-sheath strands has been investigated. The bundle-sheath cells were supplied with malate plus NADP, in the presence of intermediates of the Calvin cycle to increase the rate of CO2 fixation. The effects of malate addition on the rate of 3 phospoglycerate synthesis, with non-saturating concentrations of bicarbonate, can be explained by an increase of the cellular pool of CO2 in the cells due to malate decarboxylation. The CO2 reacting with RuDP to give phosphoglycerate corresponds effectively to carbon atom 4 of malate. Malate addition produces an enhancement of the rate of CO2 incorporation which is much more important when the reducing power is the limiting process for the evolution of the Calvin cycle (with phosphoglycerate as added substrate and/or in the presence of DCMU. These results demonstrate the utilization of NADPH produced by malate decarboxylation for the regeneration of RuDP. NADPH can also reverse the reaction of malate decarboxylation and gives rise to a synthesis of malate by carboxylation of pyruvate. In contrast, the pattern of 14C distribution among compounds is not strongly modified by malate addition. This result suggests that PGA reduction in the whole leaf must occur also in mesophyll cells to allow correctregeneration of the reduced compounds of the photosynthetic cycle. 相似文献
17.
NO3?-dependent O2 in synchronous Scenedesmus obtusiusculus Chod. in the absence of CO2 is stoichiometric with NH4+ excretion, indicating a close coupling of NO3? reduction to non-cyclic electron flow. Also in the presence of CO2, NO3? stimulates O2 evolution as manifested by an increase in the O2/CO2 ratio from 0.96 to 1.11. This quotient was increased to 1.36 by addition of NO2?, without competitive interaction with CO2 fixation, indicating that the capacity for non-cyclic electron transport at saturating light is non-limiting for simultaneous reduction of NO3? and CO2 at high rates. During incubation with NO3?+ CO2, no NH4+ is released to the outer medium, whereas during incubation with NO2?+ CO2, excess NH4+ is formed and excreted. NO3? uptake is stimulated by CO2, and this stimulation is also significant when the cellular energy metabolism is restricted by moderate concentrations of carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, whereas NO3? uptake in the absence of CO2 is severely inhibited by the uncoupler. Also under energy-restricted conditions NO3? uptake is not competitive with CO2 fixation. Antimycin A is inhibitory for NO3? uptake in the absence of CO2, and there is no enhancement of NO3? uptake by CO2 in the presence of antimycin A. It is assumed that the energy demand for NO3? uptake is met by energy fixed as triosephosphates in the Calvin cycle. Antimycin A possibly affects the transfer of reduced triose phosphates from the chloroplast to the cytoplasm. Active carbon metabolism also seems to exert a control effect on NO3? assimilation, inducing complete incorporation of all NO3? taken up into amino acids. This control effect is not functional when NO2? is the nitrogen source. Active carbon metabolism thus seems to be essential both for provision of energy for NO3? uptake and for regulation of the process. 相似文献
18.
SVEN MARHAN ELLEN KANDELER STEFANIE REIN† REAS FANGMEIER† PASCAL A. NIKLAUS‡ 《Global Change Biology》2010,16(1):469-483
Increased plant productivity under elevated atmospheric CO2 concentrations might increase soil carbon (C) inputs and storage, which would constitute an important negative feedback on the ongoing atmospheric CO2 rise. However, elevated CO2 often also leads to increased soil moisture, which could accelerate the decomposition of soil organic matter, thus counteracting the positive effects via C cycling. We investigated soil C sequestration responses to 5 years of elevated CO2 treatment in a temperate spring wheat agroecosystem. The application of 13C‐depleted CO2 to the elevated CO2 plots enabled us to partition soil C into recently fixed C (Cnew) and pre‐experimental C (Cold) by 13C/12C mass balance. Gross C inputs to soils associated with Cnew accumulation and the decomposition of Cold were then simulated using the Rothamsted C model ‘RothC.’ We also ran simulations with a modified RothC version that was driven directly by measured soil moisture and temperature data instead of the original water balance equation that required potential evaporation and precipitation as input. The model accurately reproduced the measured Cnew in bulk soil and microbial biomass C. Assuming equal soil moisture in both ambient and elevated CO2, simulation results indicated that elevated CO2 soils accumulated an extra ~40–50 g C m?2 relative to ambient CO2 soils over the 5 year treatment period. However, when accounting for the increased soil moisture under elevated CO2 that we observed, a faster decomposition of Cold resulted; this extra C loss under elevated CO2 resulted in a negative net effect on total soil C of ~30 g C m?2 relative to ambient conditions. The present study therefore demonstrates that positive effects of elevated CO2 on soil C due to extra soil C inputs can be more than compensated by negative effects of elevated CO2 via the hydrological cycle. 相似文献
19.
There is continuing controversy over whether a degree of C4 photosynthetic metabolism exists in ears of C3 cereals. In this context, CO2 exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf.) and two six-rowed barley (Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO2 compensation concentration at 210 mmol mol?1 O2 in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O2 dependence of the CO2 compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C3 photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with 14CO2 during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO2 mainly by the Calvin (C3) cycle, with little fixation of 14CO2 into the C4 acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C3 cereals C4 photosynthetic metabolism is nil. 相似文献
20.
Uptake of CO2 by aquatic vegetation 总被引:15,自引:10,他引:5
Abstract Photosynthesis by aquatic plants based on the supply of CO2 from air-equilibrated solutions may be limited by the low diffusion coefficient of CO2 in water. For plants in which the transport of CO2 from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO2 supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically-augmented CO2 levels, including the direct use of sediment CO2. Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO2 supply via the operation of biophysical CO2 concentrating mechanisms which are based on active transport of HCO?3, CO2 or H+ at the plasmalemma, and use bulk-phase HCO?3 or CO2 as the C source. A final group of aquatic plants use biochemical CO2 concentrating mechanisms based on auxiliary carboxylation by PEPc: C4-like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO2 supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low [CO2] and low [CO2]: [CO2]. In addition to overcoming restrictions on CO2 supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons. 相似文献