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1.
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
2.
Preincubation of illuminated tobacco ( Nicotiana tabacum L.) leaf disks in glycidate (2,3-epoxypropionate) or glyoxylate inhibited photorespiration by about 40% as determined by the ratio of 14CO 2 evolved into CO 2-free air in light and in darkness. However, under identical preincubation conditions used for the light/dark 14C assays, the compounds failed to reduce photorespiration or stimulate net photosynthesis in tobacco leaf disks based on other CO 2 exchange parameters, including the CO 2 compensation concentration in 21% O 2, the inhibitory effect of 21% O 2 on net photosynthesis in 360 microliters per liter of CO 2 and the rate of net photosynthetic 14CO 2 uptake in air. The effects of both glycidate and glyoxylate on the 14C assay are inconsistent with other measures of photorespiratory CO2 exchange in tobacco leaf disks, and thus these data question the validity of the light to dark ratio of 14CO2 efflux as an assay for relative rates of photorespiration (Zelitch 1968, Plant Physiol 43: 1829-1837). The results of this study specifically indicate that neither glycidate nor glyoxylate reduces photorespiration or stimulates net photosynthesis by tobacco leaf disks under physiological conditions of pO2 and pCO2, contrary to previous reports. 相似文献
3.
An analysis of the kinetics of simultaneous photosynthesis and photorespiration at the end of a diffusion path is applied to observed net photosynthetic rate as a function of O 2 and CO 2 concentrations. The data of Ku and Edwards (Plant Physiol. 59: 991-999, 1977) from wheat ( Triticum aestivum L.) are analyzed in detail. Ku and Edwards, using an analysis that ignored diffusion resistance between the intercellular air space and fixation site, the competitive effect of CO 2 on photorespiration, and the actual concentrations of gases at the fixation site, concluded that: ( a) the affinity coefficient of the leaf for CO 2 was approximately 3.5 to 5 micromolar; ( b) this affinity coefficient is independent of temperature between 25 and 35 C; ( c) the effect of O 2 was independent of temperature over this range; and ( d) competition between CO 2 and O 2 is responsible for the major share of CO 2 loss from photosynthesis due to photorespiration. They suggest that using gas concentrations calculated as equilibium values in the liquid phase is very important in reaching these conclusions. By applying a more complete analysis to their data which includes diffusion in the cell, it is concluded that: ( a) the affinity coefficient of the leaf for CO 2 is 0.1 to 1.1 micromolar; ( b) the temperature dependence of this affinity coefficient cannot be determined from existing data, but there is no evidence to refute independent temperature effect on the two functions of ribulose-1,5-bisphosphate carboxylase-oxygenase being important in the regulation of whole leaf net photosynthesis; and ( c) the competitive interplay of CO 2 and O 2 at ribulose-1,5-bisphosphate carboxylase may under certain conditions lead to a stimulation of fixation by the Calvin cycle because of photorespiration. These conclusions are reached whether CO 2 and O 2 are expressed as dissolved concentrations or as gas concentrations in the intercellular air space. The relative merits of these two expressions of concentration are discussed. 相似文献
4.
The relationship between photosynthesis and photorespiration was determined in normal and 26 mutants of barley ( Hordeum vulgare L. var. Himalaya). The rate of apparent photosynthesis ranged from 1 to 30 milligrams of CO 2 per square decimeter per hour. The variation in rate of photosynthesis was due, in some cases, to differences in chlorophyll content, in others to stomatal resistance, and in still others to unknown factors; but no single factor accounted for the variation. Photorespiratory activity, as determined by the 14CO 2/ 12CO 2 technique, CO 2 evolution into CO 2-free air, and the response of photosynthesis to low and high O 2 concentrations, was positively and significantly correlated with photosynthesis. This supports the idea that the two processes are integrally and tightly coupled. There appears to be no competition between photosynthesis and photorespiration, and the probability of finding plants with high rates of photosynthesis and low rates of photorespiration measured under natural conditions, appears to be very low. 相似文献
5.
The increase in net photosynthesis in M 4 progeny of an O 2-resistant tobacco ( Nicotiana tabacum) mutant relative to wild-type plants at 21 and 42% O 2 has been confirmed and further investigated. Self-pollination of an M 3 mutant produced M 4 progeny segregating high catalase phenotypes (average 40% greater than wild type) at a frequency of about 60%. The high catalase phenotype cosegregated precisely with O 2-resistant photosynthesis. About 25% of the F 1 progeny of reciprocal crosses between the same M 3 mutant and wild type had high catalase activity, whether the mutant was used as the maternal or paternal parent, indicating nuclear inheritance. In high-catalase mutants the activity of NADH-hydroxypyruvate reductase, another peroxisomal enzyme, was the same as wild type. The mutants released 15% less photorespiratory CO 2 as a percent of net photosynthesis in CO 2-free 21% O 2 and 36% less in CO 2-free 42% O 2 compared with wild type. The mutant leaf tissue also released less 14CO 2 per [1- 14C]glycolate metabolized than wild type in normal air, consistent with less photorespiration in the mutant. The O 2-resistant photosynthesis appears to be caused by a decrease in photorespiration especially under conditions of high O 2 where the stoichiometry of CO 2 release per glycolate metabolized is expected to be enhanced. The higher catalase activity in the mutant may decrease the nonenzymatic peroxidation of keto-acids such as hydroxypyruvate and glyoxylate by photorespiratory H 2O 2. 相似文献
6.
The hypothesis that net photosynthesis is diminished in many plant species because of a high rate of CO 2 evolution in the light has been tested further. High rates of CO 2 output in CO 2-free air in comparison with dark respiration were found in Chlamydomonas reinhardi, wheat leaves, tomato leaves, and to a lesser extent in Chlorella pyrenoidosa by means of the 14C-photorespiration assay. In tobacco leaves high photorespiration was characteristic of a standard variety, Havana Seed, and a possibly still higher rate was found in a yellow heterozygous mutant, JWB Mutant. However, the dark homozygous sibling of the latter, JWB Wild, had a low photorespiration for the tobacco species. The relative rates of photorespiration were in the same sequence when measured by the 14CO 2 released in normal air from leaf disks supplied with glycolate-1- 14C in the light. As would be predicted by the hypothesis, the maximal net rate of photosynthesis at 300 ppm of CO2 in the air in JWB Wild leaves was greater (24%) than in Havana Seed, while JWB Mutant had less CO2 uptake than the standard variety (21%). At 550 ppm of CO2 the differences in net photosynthesis were not as great between the 2 siblings as at 200 ppm. The relative leaf expansion rates of seedlings of the 3 tobacco varieties in a greenhouse had the same relationship as their rates of CO2 assimilation. Thus within the tobacco species, as in a comparison between tobacco and maize, low photorespiratory CO2 evolution was correlated with higher photosynthetic efficiency. Therefore it seems that increased CO2 uptake should be achieved by genetic interference with the process of photorespiration. 相似文献
7.
The occurrence of photorespiration in soybean ( Glycine max [L.] Merr.) leaf cells was demonstrated by the presence of an O 2-dependent CO 2 compensation concentration, a nonlinear time course for photosynthetic 14CO 2 uptake at low CO 2 and high O 2 concentrations, and an O 2 stimulation of glycine and serine synthesis which was reversed by high CO 2 concentration. The compensation concentration was a linear function of O 2 concentration and increased as temperature increased. At atmospheric CO 2 concentration, 21% O 2 inhibited photosynthesis at 25 C by 27%. Oxygen inhibition of photosynthesis was competitive with respect to CO 2 and increased with increasing temperature. The K m (CO 2) of photosynthesis was also temperature-dependent, increasing from 12 μ m CO 2 at 15 C to 38 μ m at 35 C. In contrast, the K i (O 2) was similar at all temperatures. Oxygen inhibition of photosynthesis was independent of irradiance except at 10 m m bicarbonate and 100% O 2, where inhibition decreased with increasing irradiance up to the point of light saturation of photosynthesis. Concomitant with increasing O 2 inhibition of photosynthesis was an increased incorporation of carbon into glycine and serine, intermediates of the photorespiratory pathway, and a decreased incorporation into starch. The effects of CO 2 and O 2 concentration and temperature on soybean cell photosynthesis and photorespiration provide further evidence that these processes are regulated by the kinetic properties of ribulose-1,5-diphosphate carboxylase with respect to CO 2 and O 2. 相似文献
8.
The photorespiratory activity of mesophyll protoplasts of Nicotiana plumbaginifolia has been clearly demonstrated by the presence of a Warburg-effect, the occurrence of an important CO 2-sensitive O 2 uptake and the effect of some photorespiratory inhibitors on photosynthetic activity. At a nonsaturating dissolved inorganic carbon (DIC) concentration (0.1 millimolar), we observed that the rate of CO 2 fixation was 60% lower at 50% O 2 compared to that measured at 2% O 2. Using 18O 2 and mass spectrometry, we measured O 2 exchange as a function of light intensity and of DIC concentration. Oxygen uptake measured at the CO 2 compensation point (47.4 micromoles O 2 per hour per milligram chlorophyll) was three-fold higher than that measured at a saturating CO 2 concentration. Cyanide or iodoacetamide, inhibitors of the Calvin cycle, were found to reduce the O 2 uptake to the same extent as CO 2 saturation. We conclude from these results that the major part of the CO 2-sensitive O 2 uptake is due to photorespiration. Further, we investigated the effect on net photosynthesis of some inhibitors of the glycolate pathway. At CO 2 saturation (10 millimolar DIC), 5 millimolar aminoacetonitrile (AAN), and 1 millimolar aminooxyacetate (AOA) did not cause any significant decrease in net photosynthesis. However, when these two inhibitors were added under a period of active photorespiration (10 minutes at the CO 2 compensation point at 20% O 2), we observed a decrease in the rate of net photosynthesis at 10 millimolar DIC measured afterward (respectively, 18 and 29%). This inhibition did not appear at 2% O 2, but was stronger at 50% O 2 (40% for AAN and 47% for AOA). With 0.05 millimolar butyl 2-hydroxy-3-butynoate (BHB) or 0.5 millimolar l-methionine- dl-sulfoximine ( l-MSO), rates of net photosynthesis at 10 millimolar DIC were decreased by 10 to 15%. Additional decreases were observed after a period at the CO 2 compensation point at 20% O 2 (30% for BHB and 20% for l-MSO). From the sites of action of the four inhibitors tested, we suggest the inhibition of photosynthesis occurring after a period of active photorespiration to be due to the toxic accumulation of nonmetabolized phosphoglycolate. 相似文献
9.
The effect of O 2 on the CO 2 exchange of detached leaves of corn ( Zea mays), wheat ( Triticum vulgare), oats ( Avena sativa), barley ( Hordeum vulgare), timothy ( Phleum pratense) and cat-tail ( Typha angustifolia) was measured with a Clark oxygen electrode and infrared carbon dioxide analysers in both open and closed systems. Corn leaves did not produce CO2 in the light at any O2 concentration, as was shown by the zero CO2 compensation point and the absence of a CO2 burst in the first minute of darkness. The rate of photosynthesis was inhibited by O2 and the inhibition was not completely reversible. On the other hand, the steady rate of respiration after a few minutes in the dark was not affected by O2. These results were interpreted as indicating the absence of any measurable respiration during photosynthesis. Twelve different varieties of corn studied all responded to O2 in the same way. The other 5 monocotyledons studied did produce CO2 in the light. Moreover, the CO2 compensation point increased linearly with O2 indicating a stimulation of photorespiration. The implications of the lack of photorespiration in studies of primary productivity are discussed. 相似文献
10.
The effect of leaf dehydration on photosynthetic O 2 exchange of potato ( Solanum tuberosum L., cv. Haig) leaf discs was examined using 18O 2 as a tracer and mass spectrometry. In normal air (350 μl·l ?1CO 2) and under an irradiance of 390 μmol photons·m ?2·s 1, a relative water deficit (RWD) of about 30% severely decreased net O 2 evolution and increased O 2 uptake by about 50%, thus indicating an enhancement of photorespiration. Increasing CO 2 concentrations diminished O 2 uptake and stimulated net O 2 evolution both in well-hydrated and in dehydrated (RWD of about 30%) leaves. Much higher CO 2 concentrations (up to 4%) were required to observe a complete effect of CO 2 in dehydrated leaves. The chloroplastic CO 2 concentration at the ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) level (C c) was calculated from O 2-exchange data in both well-hydrated and dehydrated leaves, assuming that the specificity factor of Rubisco was unaffected by desiccation. When plotting net O 2 photosynthesis as a function of C c, a similar relationship was obtained for well-hydrated and waterstressed leaf discs, thus showing that the main effect of water deficit is a decrease of the chloroplastic CO 2 concentration. At saturating CO 2 levels, the non-cyclic electron-transport rate, measured either as gross O 2 photosynthesis or as the chlorophyll fluorescence ratio (F m -F s)/F m, was insensitive to water deficit, provided RWD was below 40%. In this range of RWD, the decrease in gross O 2 photosynthesis observed in normal air was attributed to the inability of oxidative processes to sustain the maximal electron-flow rate at low chloroplastic CO 2 concentration. The maximal efficiency of photosystem II, estimated as the chlorophyll fluorescence ratio (F m -F 0)/F m measured in dark-adapted leaves, was not affected by water deficits up to 60%. 相似文献
11.
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
12.
Wheat was cultivated in a small phytotronic chamber. 18O 2 was used to measure the O 2 uptake by the plant, which was recorded simultaneously with the O 2 evolution, net CO 2 uptake, and transpiration. At normal atmospheric CO 2 concentration, photorespiration, measured as O 2 uptake, was as important as the net photosynthesis. The level of true O 2 evolution was independent of CO 2 concentration and stayed nearly equal to the sum of net CO 2 photosynthesis and O 2 uptake. We conclude that at a given light intensity, O 2 and CO 2 compete for the reducing power produced at constant rate by the light reactions of photosynthesis. 相似文献
13.
A normal appearing plant with a low rate of photorespiration (ratio of 14CO 2 released light/dark = 1.6) was found in an unselected tobacco ( Nicotiana tabacum) cultivar. The plant was self-pollinated, and further selections were made on several successive generations. Excised leaves from the progeny of the selections were examined for photorespiration and net CO 2 assimilation in normal air during photosynthesis. Similar measurements were made of plants derived from selfed parents with high rates of photorespiration (ratio of 14CO 2 released light/dark = 3.0 or greater). Efficient photosynthetic plants (greater than 22.0 mg of CO 2 dm −2 hr −1) with low rates of photorespiration produced a larger proportion of efficient progeny (about 25%) than did selfing inefficient plants (about 6%), but this proportion did not increase in successive generations. 相似文献
14.
In closed systems, the O 2 compensation point ( ΓO) was previously defined as the upper limit of O 2 level, at a given CO 2 level, above which plants cannot have positive carbon balance and survive. Studies with 18O 2 measure the actual O 2 uptake by photorespiration due to the dual function of Rubisco, the enzyme that fixes CO 2 and takes O 2 as an alternative substrate. One-step modelling of CO 2 and O 2 uptakes allows calculating a plant specificity factor ( Sp) as the sum of the biochemical specificity of Rubisco and a biophysical specificity, function of the resistance to CO 2 transfer from the atmosphere to Rubisco. The crossing points (Cx, Ox) are defined as CO 2 and O 2 concentrations for which O 2 and CO 2 uptakes are equal. It is observed that: (1) under the preindustrial atmosphere, photorespiration of C3 plants uses as much photochemical energy as photosynthesis, i.e. the Cx and Ox are equal or near the CO 2 and O 2 concentrations of that epoch; (2) contrarily to ΓC, a ΓO does not practically limit the plant growth, i.e. the plant net CO 2 balance is positive up to very high O 2 levels; (3) however, in a closed biosystem, ΓO exists; it is not the limit of plant growth, but the equilibrium point between photosynthesis and the opposite respiratory processes; (4) a reciprocal relationship exists between ΓO and ΓC, as unique functions of the respective CO 2 and O 2 concentrations and of Sp, this invalidates some results showing two different functions for ΓO and ΓC, and, consequently, the associated analyses related to greenhouse effects in the past; (5) the pre-industrial atmosphere levels of O 2 and CO 2 are the ΓO and ΓC of the global bio-system. They are equal to or near the values of Cx and Ox of C3 plants, the majority of land plants in preindustrial period. We assume that the crossing points represent favourable feedback conditions for the biosphere-atmosphere equilibrium and could result from co-evolution of plants-atmosphere-climate. We suggest that the evolution of Rubisco and associated pathways is directed by an optimisation between photosynthesis and photorespiration. 相似文献
15.
Glycolate synthesis was inhibited 40–50% in illuminated tobacco leaf disks, which have rapid rates of photorespiration, when floated on 20 mm potassium glycidate (2,3-epoxypropionate), an epoxide similar in structure to glycolate. The inhibitor also decreased the release of photorespiratory CO 2 about 40%, and the specificity of glycidate was demonstrated by the 40–50% increase in rate of photosynthetic CO 2 uptake observed in its presence. The importance of glycolate synthesis and metabolism in the production of photorespiratory CO 2 and the role of glycolate in diminishing net photosynthesis in species with rapid rates of photorespiration was thus further confirmed. L-(or 2 S)-Glycidate was slightly more active than DL-glycidate, but glycidate was more effective as a specific inhibitor in leaf tissue than several other epoxide analogs of glycolate examined. The products of photosynthetic 14O 2 fixation after 3 or 4 min of uptake were proportionately altered in the presence of glycidate, and the specific radioactivity of the [ 14C]glycolate produced was closer to that of the 14CO 2 supplied. Glycidate inhibited glycolate synthesis in tobacco leaf disks irreversibly, since the degree of inhibition was the same for at least 2 hr after the inhibitor solution was removed. Glycidate also blocked glycolate synthesis in maize leaf disks, tissue with low rates of photorespiration, but large increases in net photosynthesis were not observed in maize with glycidate, because glycolate synthesis is normally only about 10% as rapid in maize as in tobacco. The demonstration of increases in net photosynthesis of 40–50% when glycolate synthesis (and photorespiration) is blocked with glycidate indicates in an independent manner that the biochemical or genetic control of photorespiration should permit large increases in plant productivity in plant species possessing rapid rates of photorespiration. 相似文献
16.
Experiments are described further indicating that O 2-resistant photosynthesis observed in a tobacco ( Nicotiana tabacum) mutant with enhanced catalase activity is associated with decreased photorespiration under conditions of high photorespiration relative to net photosynthesis. The effects on net photosynthesis of (a) increasing O 2 concentrations from 1% to 42% at low CO 2 (250 microliters CO 2 per liter), and (b) of increasing O 2 concentrations from 21% to 42% at high CO 2 (500 microliters CO 2 per liter) were investigated in M 6 progeny of mutant and wild-type leaf discs. The mutant displayed a progressive increase in net photosynthesis relative to wild type with increasing O 2 and the faster rate at 42% O 2 was completely reversed on returning to 21% O 2. The photosynthetic rate by the mutant was similar to wild type in 21% and 42% O 2 at 500 microliters CO 2 per liter, and a faster rate by the mutant was restored on returning to 250 microliters CO 2 per liter. The results are consistent with a lowered release of photorespiratory CO 2 by the mutant because greater catalase activity inhibits the chemical decarboxylation of α-keto acids by peroxisomal H 2O 2. Higher catalase activity was observed in the tip and middle regions of expanding leaves than in the basal area. On successive selfing of mutant plants with enhanced catalase activity, the percent of plants with this phenotype increased from 60% in M 4 progeny to 85% in M 6 progeny. An increase was also observed in the percent of plants with especially high catalase activity (averaging 1.54 times wild type) on successive selfings suggesting that homozygosity for enhanced catalase activity was being approached. 相似文献
17.
The addition of glyoxylate to tobacco ( Nicotiana tabacum) leaf discs inhibited glycolate synthesis and photorespiration and increased net photosynthetic 14CO 2 fixation. This inhibition of photorespiration was investigated further by studying the effect of glyoxylate on the stimulation of photosynthesis that occurs when the atmospheric O 2 level was decreased from 21 to 3% (the Warburg effect). The Warburg effect is usually ascribed to the increased glycolate synthesis and metabolism that occurs at higher O 2 concentrations. Photosynthesis in control discs increased from 59.1 to 94.7 micromoles of CO 2 per gram fresh weight per hour (a 60% increase) when the O 2 level was lowered from 21 to 3%, while the rate for discs floated on 15 millimolar glyoxylate increased only from 82.0 to 99.7 micromoles of CO 2 per gram fresh weight per hour (a 22% increase). The decrease in the O 2 sensitivity of photosynthesis in the presence of glyoxylate was explained by changes in the rate of glycolate synthesis under the same conditions. The rate of metabolism of the added glyoxylate by tobacco leaf discs was about 1.35 micromoles per gram fresh weight per hour and was not dependent on the O2 concentration in the atmosphere. This rate of metabolism is about 10% the amount of stimulation in the rate of CO2 fixation caused by the glyoxylate treatment on a molar carbon basis. Glyoxylate (10 millimolar) had no effect on the carboxylase/oxygenase activity of isolated ribulose diphosphate carboxylase. Although the biochemical mechanism by which glyoxylate inhibits glycolate synthesis and photorespiration and thereby decreases the Warburg effect is still uncertain, these results show that cellular metabolites can regulate the extent of the Warburg effect. 相似文献
18.
All measurements of photorespiration and gross photosynthesis in leaves, whether using isotopes or not, are underestimated because of the recycling of O 2 or CO 2. On the basis of a simple diffusion model, we propose a method for the calculation of the recycling and the corresponding underestimation of the measurements. This procedure can be applied when the stomatal resistance is known, and allows for a correction of certain results in the literature. It is found that measurements of the photorespiratory CO 2 release are usually underestimated by 20 to 100%, which sets the estimated rate of CO 2 photorespired at 30 to 50% of the net photosynthesis in C3 plants under normal conditions. In water stress studies, the correction of the photorespiration is still more important (1.5-3.3) because the stomata are closed more. Analysis of the diffusion of O 2 shows that its recycling is low and that the underestimation of photorespiration with 18O 2 is negligible. 相似文献
19.
The effects of gas phase O 2 concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO 2 fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO 2 and 25 to 30°C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O 2 levels relative to 1% O 2 when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O 2 levels only at saturating irradiance. Simultaneous measurements of CO 2 and H 2O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO 2 fixation and O 2-dependent, dissipative processes such as photorespiration as a function of leaf internal CO 2 concentration. Changes in the in vivo CO 2:O 2 `specificity factor' ( Ksp) with increasing irradiance are examined. The magnitude Ksp was found to decline from a value of 85 at moderate irradiance to 68 at very low light, and to 72 at saturating photon flux rates. The results are discussed in terms of the applicability of the ribulose bisphosphate carboxylase/oxygenase enzyme model to photosynthesis in vivo. 相似文献
20.
The weedy species Parthenium hysterophorus (Asteraceae) possesses a Kranz-like leaf anatomy. The bundle sheath cells are thick-walled and contain numerous granal chloroplasts, prominent mitochondria, and peroxisomes, all largely arranged in a centripetal position. Both mesophyll and bundle sheath chloroplasts accumulate starch. P. hysterophorus exhibits reduced photorespiration as indicated by a moderately low CO 2 compensation concentration (20-25 microliters per liter at 30°C and 21% O 2) and by a reduced sensitivity of net photosynthesis to 21% O 2. In contrast, the related C 3 species P. incanum and P. argentatum (guayule) lack Kranz anatomy, have higher CO 2 compensation concentrations (about 55 microliters per liter), and show a greater inhibition of photosynthesis by 21% O 2. Furthermore, in P. hysterophorus the CO 2 compensation concentration is relatively less sensitive to changes in O 2 concentrations and shows a biphasic response to changing O 2, with a transition point at about 11% O 2. Based on these results, P. hysterophorus is classified as a C 3-C 4 intermediate. The activities of diagnostic enzymes of C 4 photosynthesis in P. hysterophorus were very low, comparable to those observed in the C 3 species P. incanum ( e.g. phosphoenolpyruvate carboxylase activity of 10-29 micromoles per milligram of chlorophyll per hour). Exposures of leaves of each species to 14CO 2 (for 8 seconds) in the light resulted in 3-phosphoglycerate and sugar phosphates being the predominant initial 14C products (77-84%), with ≤4% of the 14C-label in malate plus aspartate. These results indicate that in the C 3-C 4 intermediate P. hysterophorus, the reduction in leaf photorespiration cannot be attributed to C 4 photosynthesis. 相似文献
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