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1.
Unidirectional O 2 fluxes were measured with 18O 2 in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O 2, O 2 uptake was maximum at 60 microliters per liter CO 2. At lower CO 2 concentrations, it was strongly inhibited, as was photosynthetic O 2 evolution. At 2% O 2, there remained a substantial O 2 uptake, even at high CO 2 level; the O 2 evolution was inhibited at CO 2 concentrations under 330 microliters per liter. The O 2 uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO 2. The behavior of the O 2 uptake and the study of the CO 2 compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis. 相似文献
2.
Light-dependent O 2 exchange was measured in intact, isolated soybean ( Glycine max. var. Williams) cells using isotopically labeled O 2 and a mass spectrometer. The dependence of O 2 exchange on O 2 and CO 2 was investigated at high light in coupled and uncoupled cells. With coupled cells at high O 2, O 2 evolution followed similar kinetics at high and low CO 2. Steady-state rates of O 2 uptake were insignificant at high CO 2, but progressively increased with decreasing CO 2. At low CO 2, steady-state rates of O 2 uptake were 50% to 70% of the maximum CO 2-supported rates of O 2 evolution. These high rates of O 2 uptake exceeded the maximum rate of O 2 reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O 2-uptake process ( i.e. photorespiration). Rates of O2 exchange in uncoupled cells were half-saturated at 7% to 8% O2. Initial rates (during induction) of O2 exchange in uninhibited cells were also half-saturated at 7% to 8% O2. In contrast, steady-state rates of O2 evolution and O2 uptake (at low CO2) were half-saturated at 18% to 20% O2. O2 uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O2 for photoreductant. These results suggest that two mechanisms (O2 reduction and photorespiration) are responsible for the light-dependent O2 uptake observed in uninhibited cells under CO2-limiting conditions. The relative contribution of each process to the rate of O2 uptake appears to be dependent on the O2 level. At high O2 concentrations (≥40%), photorespiration is the major O2-consuming process. At lower (ambient) O2 concentrations (≤20%), O2 reduction accounts for a significant portion of the total light-dependent O2 uptake. 相似文献
3.
Intact spinach ( Spinacia oleracea L.) chloroplasts, when pre-illuminated at 4 millimoles quanta per square meter per second for 8 minutes in a CO 2-free buffer at 21% O 2, showed a decrease (30-70%) in CO 2-dependent O 2 evolution and 14CO 2 uptake. This photoinhibition was observed only when the O 2 concentration and the quantum fluence rate were higher than 4% and 1 millimole per square meter per second, respectively. There was only a small decrease in the extent of photoinhibition when the CO 2 concentration was increased from 0 to 25 micromolar during the treatment, but photoinhibition was abolished when the CO 2 concentration was increased to 30 micromolar. Addition of small quantities of P-glycerate (40-200 micromolar) or glycerate (160 micromolar) was found to prevent photoinhibition. Other intermediates of the Calvin cycle (fructose-6-P, fructose-1,6-P, ribose-5-P, ribulose-5-P) also prevented photoinhibition to various extents. Oxaloacetate was not effective in preventing photoinhibition in these chloroplasts. The amount of O 2 evolved during treatments with 3-P-glycerate or glycerate was no more than 65% of that measured in the presence of low CO 2 concentrations (9-12 micromolar) which did not prevent photoinhibition. In all cases, the extent to which photoinhibition was prevented by these metabolites was not correlated to the amount of O 2 evolved during the photoinhibitory treatment. It is concluded that in these chloroplasts the prevention of the O 2-dependent photoinhibition of light saturated CO 2 fixation capacity is not linked to the dissipation of excitation energy via the photosynthetic electron transport nor to ATP utilization. The requirement of O 2 for photoinhibition of CO 2 fixation capacity in isolated chloroplasts may be explained by an effect of O 2 in allowing metabolic depletion of Calvin cycle intermediates. 相似文献
4.
A mass spectrometer with a membrane inlet was used to monitor light-driven O 2 evolution, O 2 uptake, and CO 2 uptake in suspensions of algae ( Scenedesmus obliquus). We observed the following. ( a) The rate of O 2 uptake, which, in the presence of iodoacetamide, replaces the uptake of CO 2, showed a distinct plateau (V max) beyond ~30% O 2 and was half-maximal at ~8% O 2. We concluded that this light-driven O 2 uptake process, which does not involve carbon compounds, is saturated at lower O 2 concentrations than are photorespiration and glycolate formation. ( b) In the absence of inhibitor, O 2 evolution was relatively unaffected by the presence or absence of CO 2. During the course of CO 2 depletion, electron flow to CO 2 was replaced by an equivalent flow to O 2. ( c) There was a distinct delay between the cessation of CO 2 uptake and the increase in O 2 uptake. We ascribe this delay to the transient utilization of another electron acceptor—possibly bicarbonate or another bound form of CO 2. 相似文献
5.
The nature of the process responsible for the stationary O 2 uptake occurring in the light under saturating CO 2 concentration in Chlamydomonas reinhardii has been investigated. For this purpose, a mass spectrometer with a membrane inlet system was used to measure O 2 uptake and evolution in the algal suspension. First, we observed that the O 2 uptake rate was constant (about 0.5 micromoles of O 2 per milligram chlorophyll per minute) during a light to dark transition and was not affected by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Salicylhydroxamic acid had no effect on O 2 uptake in the dark or in the light, but was found to have the same inhibitory effect either in the dark or in the light when added to cyanide-treated algae. The stimulation of the O 2 uptake rate due to the uncoupling effect of carbonyl cyanide m-chlorophenylhydrazone was about the same in the dark or in the light. From these results, we conclude that mitochondrial respiration is maintained during illumination and therefore is not inhibited by high ATP levels. Another conclusion is that in conditions where photorespiration is absent, no other light-dependent O 2 uptake process occurs. If Mehler reactions are involved, in Chlamydomonas, under conditions where both photosynthetic carbon oxidation and reduction cycles cannot operate (as in cyanide-treated algae), their occurrence in photosynthesizing algae either under saturating CO 2 concentration or at the CO 2 compensation point appears very unlikely. The comparison with the situation previously reported in Scenedesmus (R. J. Radmer and B. Kok 1976 Plant Physiol 58: 336-340) suggests that different O 2 uptake processes might be present in these two algal species. 相似文献
6.
This study examines the effect of antimycin A and nitrite on 14CO 2 fixation in intact chloroplasts isolated from spinach ( Spinacia oleracea L.) leaves. Antimycin A (2 micromolar) strongly inhibited CO 2 fixation but did not appear to inhibit or uncouple linear electron transport in intact chloroplasts. The addition of small quantities (40-100 micromolar) of nitrite or oxaloacetate, but not NH 4Cl, in the presence of antimycin A restored photosynthesis. Antimycin A inhibition, and the subsequent restoration of photosynthetic activities by nitrite or oxaloacetate, was observed over a wide range of CO 2 concentration, light intensity, and temperature. High O 2 concentration (up to 240 micromolar) did not appear to influence the extent of the inhibition by antimycin A, nor the subsequent restoration of photosynthetic activity by nitrite or oxaloacetate. Studies of O 2 exchanges during photosynthesis in cells and chloroplasts indicated that 2 micromolar antimycin A stimulated O 2 uptake by about 25% while net O 2 evolution was inhibited by 76%. O 2 uptake in chloroplasts in the presence of 2 micromolar antimycin A was 67% of total O 2 evolution. These results suggest that only a small proportion of the O 2 uptake measured was directly linked to ATP generation. The above evidence indicates that cyclic photophosphorylation is the predominant energy-balancing reaction during photosynthesis in intact chloroplasts. On the other hand, pseudocyclic O 2 uptake appears to play only a minimal role. 相似文献
7.
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
8.
Evolution of O 2 by illuminated intact detached leaves from barley ( Hordeum vulgare L. cv Athos) and pea ( Pisum sativum L. cv Lincoln) in a CO 2-saturating atmosphere was enhanced when KNO 3 (1-2.5 millimolar) had been previously supplied through the transpiration stream. The extra O 2 evolution observed after feeding KNO 3 increased with the light intensity, being maximal at near saturating photon flux densities and resulting in no changes in the initial slope of the O 2 versus light-intensity curve. No stimulation of O 2 evolution was otherwise observed after feeding KCl or NH 4Cl. The data indicate that nitrate assimilation uses photosynthetically generated reductant and stimulates the rate of non-cyclic electron flow by acting as a second electron-accepting assimilatory process in addition to CO 2 fixation. 相似文献
9.
Oxygen inhibition of photosynthesis and CO 2 evolution during photorespiration were compared in high CO 2-grown and air-grown Chlorella pyrenoidosa, using the artificial leaf technique at pH 5.0. High CO 2 cells, in contrast to air-grown cells, exhibited a marked inhibition of photosynthesis by O 2, which appeared to be competitive and similar in magnitude to that in higher C 3 plants. With increasing time after transfer to air, the photosynthetic rate in high CO 2 cells increased while the O 2 effect declined. Photorespiration, measured as the difference between 14CO 2 and 12CO 2 uptake, was much greater and sensitive to O 2 in high CO 2 cells. Some CO 2 evolution was also present in air-grown algae; however, it did not appear to be sensitive to O 2. True photosynthesis was not affected by O 2 in either case. The data indicate that the difference between high CO 2 and air-grown algae could be attributed to the magnitude of CO 2 evolution. This conclusion is discussed with reference to the oxygenase reaction and the control of photorespiration in algae. 相似文献
10.
Photosynthetic CO 2 and O 2 exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using 18O 2 to trace O 2 uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18°C, 330 microliters per liter CO 2, 21% O 2) the net photosynthetic rate was around 40 micromoles CO 2 per gram dry weight per hour in H. cupressiforme and 50 micromoles CO 2 per gram dry weight per hour in D. scoparium. The CO 2 compensation point lay between 45 and 55 microliters per liter CO 2 and the enhancement of net photosynthesis by 3% O 2versus 21% O 2 was 40 to 45%. The ratio of O 2 uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO 2 showed a high apparent Km (CO 2) even in nonsaturating light. On the other hand, O 2 uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O 2 concentration (saturating level as low as 30% O 2), and by 65% by decreasing the CO 2 concentration to the compensation point. Although O 2 is a competitive inhibitor of CO 2 uptake it could not replace CO 2 completely as an electron acceptor, and electron flow, expressed as gross O 2 production, was inhibited by both high O 2 and low CO 2 levels. At high CO 2, O 2 uptake was 70% lower than the maximum at the CO 2 compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction. 相似文献
11.
Net CO 2 exchange was monitored through a dark-light-dark transition, under 2% and 21% O 2 in the presence and absence of CO 2, in Chlamydomonas reinhardtii wild type and the high-CO 2-requiring mutant ca-1-12-1C. Upon illumination at 350 l/l CO 2, ca-1-12-1C cell exhibited a large decrease in net CO 2 uptake following an initial surge of CO 2 uptake. Net CO 2 uptake subsequently attained a steady-state rate substantially lower than the maximum. A large, O 2-enchanced post-illumination burst of CO 2 efflux was observed after a 10-min illumination period, corresponding to a minimum in the net CO 2 uptake rate. A smaller, but O 2-insensitive post-illumination burst was observed following a 30-min illumination period, when net CO 2 uptake was at a steady-state rate. These post-illumination bursts appeared to reflect the release of an intracellular pool of inorganic carbon, which was much larger following the initial surge of net CO 2 uptake than during the subsequent steady-state CO 2 uptake period.With the mutant in CO 2-free gas, O 2-stimulated, net CO 2 efflux was observed in the light, and a small, O 2-dependent post-illumination burst was observed. With wild-type cells no CO 2 efflux was observed in the light in CO 2-free gas under either 2% or 21% O 2, but a small, O 2-dependent post-illumination burst was observed. These results were interpreted as indicating that photorespiratory rates were similar in the mutant and wild-type cells in the absence of CO 2, but that the wild-type cells were better able to scavenge the photorespiratory CO 2. 相似文献
12.
An open system associated with an infrared gas analyzer was employed to study transients in CO 2 exchange generated upon darkening preilluminated leaf discs of tobacco ( Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO 2 exchange rate. A computer was used to evaluate parameters of the leaf CO 2 release rate to provide an estimate of the initial rate of postillumination CO 2 evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O 2, the decline in rate of CO 2 evolution upon darkening followed first order kinetics. Initial rates of CO 2 evolution following darkening were relatively independent of the prior ambient CO 2 concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO 2 concentration at 21% O 2. Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO 2 fixation rate at 32°C and high irradiance. The rapid initial CO 2 evolution observed upon darkening at 21% O 2 was absent in 3% O 2. Rates of photorespiration under normal atmospheric concentrations of CO 2 and O 2 as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C 3 leaf tissue. 相似文献
13.
Photoheterotrophic growth of cell suspensions of Nicotiana tabacum L. (cv. Xanthi) in organic culture medium enriched in sucrose (30 g per liter) showed a classical sigmoid growth curve. The cells developed functional chloroplast structures during the exponential growth phase, when their chlorophyll content increased steadily. A limited drop (30%) in the chlorophyll amount and structural changes of the plastids (starch accumulation) were observed during the lag phase. The measurements of photosynthetic capacities (O 2 evolution and CO 2 fixation) during the growth cycle revealed changes in the photosynthetic ratio (O 2/CO 2), which was near 1 during the lag and stationary phases and near 2 during exponential growth. During exponential growth there was also a rapid NO 3? uptake. Analysis of label distribution among the products of 14CO 2 fixation showed that both CO 2 assimilation pathways, linked to the ribulose-biphosphate carboxylase (the autotrophic pathway) and to phosphoenolpyruvate carboxylase (the non-autotrophic pathway) were operative with an important increase of the capacity of the latter during the exponential growth phase. Maximum rate of oxygen evolution, either endogenous or with p-benzoquinone as Hill reagent, as well as the increased CO 2 Fixation capacity via the non-autotrophic pathway during the exponential phase were concomitant with a high cyanide inhibited O 2 uptake. 相似文献
14.
Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O 2 evolution, dissolved inorganic carbon accumulation, and photosynthetic CO 2 fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO 2-concentrating process. This COS inhibition can be attributed to inhibition of external HCO 3− conversion to CO 2 and OH − by the carbonic anhydrase component of an active CO 2 pump. At a low pH of 5 to 6, COS stimulated O 2 evolution during photosynthesis by algae with low CO 2 in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO 2. Although COS had less effect on HCO 3− accumulation at pH 9 by a HCO 3− pump in Scenedesmus, COS reduced O 2 evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO 2 and H 2S, its inhibition of the CO 2 pump activity and photosynthesis is not accurate, when measured by O 2 evolution, by NaH 14CO 3 accumulation, or by 14CO 2 fixation. 相似文献
15.
Vacuolated and nonvacuolated root tissues of Zea mays were exposed to low water potentials by addition of mannitol or glycerol. Temporary increases were observed for O 2 uptake, but CO 2 evolution remained steady. This increase in O 2 uptake ceased after 15 minutes. Further treatment induced decreases in respiration, with similar reductions in O 2 uptake and CO 2 evolution. 相似文献
16.
The postillumination transient of CO 2 exchange and its relation to photorespiration has been examined in leaf discs from tobacco ( Nicotiana tabacum) and maize ( Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O 2 in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO 2 exchange equivalents ( i.e. nanomoles CO 2) versus time were derived from the analyzer responses of individual transients. In tobacco (C 3), subtraction of the time course of cumulative CO 2 exchange under photorespiratory conditions (21 or 43% O 2) from that obtained under nonphotorespiratory conditions (1% O 2) revealed the presence of an O 2-dependent and CO 2-reversible component within the first 60 seconds following darkening. This component was absent in maize (C 4) and at low external O 2:CO 2 ratios ( i.e. <100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O 2. This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO 2 evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration. 相似文献
17.
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
18.
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
19.
A mass spectrometer was used to simultaneously follow the time course of photosynthetic O 2 evolution and CO 2 depletion of the medium by cells of the cyanobacterium Synechococcus leopoliensis UTEX 625. Analysis of the data indicated that both CO 2 and HCO 3− were simultaneously and continuously transported by the cells as a source of substrate for photosynthesis. Initiation of HCO 3− transport by Na + addition had no effect on ongoing CO 2 transport. This result is interpreted to indicate that the CO 2 and HCO 3− transport systems are separate and distinctly different transport systems. Measurement of CO 2-dependent photosynthesis indicated that CO 2 uptake involved active transport and that diffusion played only a minor role in CO 2 acquisition in cyanobacteria. 相似文献
20.
Photosynthesis and light O 2-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO 2 and O 2 gas exchange measurements. The amount of CO 2 which was fixed during a complete day-night cycle was equal to the amount of total net O 2 evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O 2-evolution and of net light atmospheric CO 2-uptake give the rates of malate-decarboxylation-dependent CO 2 assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O 2 evolved/net CO 2 fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO 2 for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO 2 assimilation deduced from net O 2-evolution was 3 times higher than the maximum rate of atmospheric CO 2-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO 2-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO 2-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO 2 fixed during the last hour of the light period was stored as malate. Light O 2-uptake determined with 18O 2 showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O 2-uptake increased progressively; during the following part of the light period, the rate of O 2-uptake was 3.5 times higher than the maximum rate of CO 2-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO 2-free atmosphere or in continuous illumination, the rate of O 2-uptake was higher than in the control. This supports the hypothesis that the low rate of O 2-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO 2 concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O 2 and CO 2 gas exchange suggest that at the end of the light period the intracellular CO 2 concentration was very low. We propose that the high ratio of O 2-uptake/CO 2-fixation is principally caused by the stimulation of photorespiration during this period. 相似文献
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