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1.
Photosynthetic activities of bundle sheath cell strands isolated from several C 4 pathway species were examined. These included species that decarboxylate C 4 acids via either NADP-malic enzyme ( Zea mays, NADP-malic enzyme-type), NAD-malic enzyme ( Atriplex spongiosa and Panicum miliaceum, NAD-malic enzyme-type) or phosphoenolpyruvate carboxykinase ( Chloris gayana and Panicum maximum, phosphoenolpyruvate carboxykinase-type). Preparations from each of these species fixed 14CO 2 at rates ranging between 1.2 and 3.5 μmol min ?1 mg ?1 of chlorophyll, with more than 90% of the 14C being assimilated into Calvin cycle intermediates. With added HCO 3? the rate of light-dependent O 2 evolution ranged between 2 and 4 μmol min ?1 mg ?1 of chlorophyll for cells from NAD-malic enzyme-type and phosphoenolpyruvate carboxykinase-type species but with Z. mays cells there was no O 2 evolution detectable. Most of the 14CO 2 fixed by Z. mays cells provided with H 14CO 3? plus ribose 5-phosphate accumulated in the C-1 of 3-phosphoglycerate. However, 3-phosphoglycerate reduction was increased several fold when malate was also provided. Cells from all species rapidly decarboxylated C 4 acids under appropriate conditions, and the CO 2 released from the C-4 carboxyl was reassimilated via the Calvin cycle. Malate decarboxylation by Z. mays cells was dependent upon light and an endogenous or exogenous source of 3-phosphoglycerate. Bundle sheath cells of NAD-malic enzyme-type species rapidly decarboxylated [ 14C]malate when aspartate and 2-oxoglutarate were also provided, and [ 14C]aspartate was decarboxylated at similar rates when 2-oxoglutarate was added. Cells from phosphoenolpyruvate carboxykinase-type species decarboxylated [ 14C]aspartate when 2-oxoglutarate was added and they also catalyzed a slower decarboxylation of malate. Cells from NAD-malic enzyme-type and phosphoenolpyruvate carboxykinase-type species evolved O 2 in the light when C 4 acids were added. These results are discussed in relation to proposed mechanisms for photosynthetic metabolism in the bundle sheath cells of species utilizing C 4 pathway photosynthesis. 相似文献
2.
In the presence of purified nitrate reductase (NR) and 1 mM NADH, illuminated pea chloroplasts catalysed reduction of NO 3? to NH 3 with the concomitant evolution of O 2. The rates were slightly less than those for reduction of NO 2? to NH 3 and O 2, evolution by chloroplasts in the absence of NR and NADH ( ca 6 μg atoms N/mg Chl/hr). Illuminated chloroplasts quantitatively reduced 0.2 mM oxaloacetate (OAA) to malate. In the presence of an extrachloroplast malate-oxidizing system comprised of NAD-specific malate dehydrogenase (NAD-MDH), NAD, NR and NO 3?, illuminated chloroplasts supported OAA-dependent reduction of NO 3? to NH 3 with the evolution of O 2. The reaction did not proceed in the absence of any of these supplements or in the dark but malate could replace OAA. The results are consistent with the reduction of NO 3?by reducing equivalents from H 2O involving a malate/OAA shuttle. The ratios for O 2, evolved: C 4-acid supplied and N reduced: C 4-acid supplied in certain experiments imply recycling of the C 4-acids. 相似文献
3.
Oxygen inhibition of leaf slice photosynthesis in Panicum milioides increased from 20% to 30% at 21% O 2 in the presence of maleate, a phosphoenolpyruvate carboxylase inhibitor. The increased O 2 sensitivity was completely reversed by the addition of malate and aspartate, the stable products of the phosphoenolpyruvate carboxylase reaction. The C 4 acids, malate and aspartate, also reduced O 2 inhibition of photosynthesis by isolated bundle sheath strands, but not mesophyll protoplasts. Similarly, only bundle sheath strands exhibited an active C 4 acid-dependent O 2 evolution. Compartmentation of C 4 cycle enzymes, with pyruvate, Pi dikinase in the mesophyll and NAD-malic enzyme in the bundle sheath, was demonstrated. It is concluded that reduced photorespiration in P. milioides is due to a limited potential for C 4 photosynthesis permitting an increase in pCO 2 at the site of bundle sheath ribulosebisphosphate carboxylase. 相似文献
4.
One group of C 4, species utilize a NAD-malic enzyme to decarboxylate C 4 acids. This enzyme, together with a major isoenzyme of aspartate aminotransferase and a NAD-malate dehydrogenase, is localized in the mitochondria of the bundle sheath cells and the following pathway for C 4, acid decarboxylation has been proposed: aspartate → oxaloacetate → malate → CO 2 + pyruvate. The present study reports that mitochondria isolated from the bundle sheath cells of one of these species, Atriplex spongiosa, are capable of decarboxylating C 4, acids at rates between 5 and 8 μmol/min/mg chlorophyll. For maximum decarboxylating activities, these particles required aspartate, 2-oxoglutarate and phosphate as well as malate; in the absence of any one of these compounds, activity was reduced to 0.3–0.8 μmol/min/mg chlorophyll. Rates for C 4 acid decarboxylation were much greater than the respiratory activities of these particles, including the capacity to form citrate or to oxidize malate, succinate, pyruvate or 2-oxoglutarate (0.03–0.6 μmol/min/mg chlorophyll). A comparison of mitochondria prepared from leaves of various C 4, and C 3, species showed that only the mitochondria from the bundle sheath cells of plants with high NAD-malic enzyme have capacities for rapid C 4 acid decarboxylation. The effects of a variety of experimental conditions on C 4 acid decarboxylating activities are also reported. The role of these mitochondria in C 4 photosynthesis is discussed. 相似文献
5.
Recent work has suggested that the photosynthetic rate of certain C 4 species can be stimulated by increasing CO 2 concentration, [CO 2], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO 2 leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO 2]. Three C 4 species that differed in the reported degree of bundle sheath leakiness to CO 2, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO 2] of 350 μl l ?1 (ambient) or 700 μl l ?1 (elevated). Assimilation as a function of increasing [CO 2] at high photosynthetic photon flux density (PPFD, 1 600 μmol m ?2 s ?1) indicated that leaf photosynthesis was not saturated under current ambient [CO 2] for any of the three C 4 species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO 2] was light dependent for all three C 4 species. The stimulation of leaf photosynthesis at elevated [CO 2] was not associated with previously published values of CO 2 leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO 2] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C 4 species can respond directly to increased [CO 2] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves. 相似文献
6.
In C 4 grasses belonging to the NADP-malic enzyme-type subgroup, malate is considered to be the predominant C 4 acid metabolized during C 4 photosynthesis, and the bundle sheath cell chloroplasts contain very little photosystem-II (PSII) activity. The present studies showed that Flaveria bidentis (L.), an NADP-malic enzyme-type C 4 dicotyledon, had substantial PSII activity in bundle sheath cells and that malate and aspartate apparently contributed about equally to the transfer of CO 2 to bundle sheath cells. Preparations of bundle sheath cells and chloroplasts isolated from these cells evolved O 2 at rates between 1.5 and 2 mol · min –1 · mg –1 chlorophyll (Chl) in the light in response to adding either 3-phosphoglycerate plus HCO
3
–
or aspartate plus 2-oxoglutarate. Rates of more than 2 mol O 2 · min –1 · mg –1 Chl were recorded for cells provided with both sets of these substrates. With bundle sheath cell preparations the maximum rates of light-dependent CO 2 fixation and malate decarboxylation to pyruvate recorded were about 1.7 mol · min –1 · mg –1 Chl. Compared with NADP-malic enzyme-type grass species, F. bidentis bundle sheath cells contained much higher activities of NADP-malate dehydrogenase and of aspartate and alanine aminotransferases. Time-course and pulse-chase studies following the kinetics of radiolabelling of the C-4 carboxyl of C 4 acids from 14CO 2 indicated that the photosynthetically active pool of malate was about twice the size of the aspartate pool. However, there was strong evidence for a rapid flux of carbon through both these pools. Possible routes of aspartate metabolism and the relationship between this metabolism and PSII activity in bundle sheath cells are considered.Abbreviations DHAP
dihydroxyacetone phosphate
- NADP-ME(-type)
NADP-malic enzyme (type)
- NADP-MDH
NADP-malate dehydrogenase
- OAA
oxaloacetic acid
- 2-OG
2-oxoglutarate
- PEP
phosphoenolpyruvate
- PGA
3-phosphoglycerate
- Pi
orthophosphate
- Ru5P
ribulose 5-phosphate 相似文献
7.
A mechanical isolation procedure was developed to study the respiratory properties of mitochondria from the mesophyll and bundle sheath tissue of Panicum miliaceum, a NAD-malic enzyme C 4 plant. A mesophyll fraction and a bundle sheath fraction were obtained from young leaves by differential mechanical treatment. The purity of both fractions was about 80%, based on analysis of the cross-contamination of ribulose bisphosphate carboxylase activity and phosphoenolpyruvate carboxylase activity. Mitochondria were isolated from the two fractions by differential centrifugation and Percoll density gradient centrifugation. The enrichment of mitochondria relative to chloroplast material was about 75-fold in both preparations. Both types of mitochondria oxidized NADH and succinate with respiratory control. Malate oxidation in mesophyll mitochondria was sensitive to KCN and showed good respiratory control. In bundle sheath mitochondria, malate oxidation was largely insensitive to KCN and showed no respiratory control. The oxidation was strongly inhibited by salicylhydroxamic acid, showing that the alternative oxidase was involved. The bundle sheath mitochondria of this type of C4 species contribute to C4 photosynthesis through decarboxylation of malate. Malate oxidation linked to an uncoupled, alternative pathway may allow decarboxylation to proceed without the restraints which might occur via coupled electron flow through the cytochrome chain. 相似文献
8.
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
9.
Panicum milioides, a naturally occurring species with C 4-like Kranz leaf anatomy, is intermediate between C 3 and C 4 plants with respect to photorespiration and the associated oxygen inhibition of photosynthesis. This paper presents direct evidence for a limited degree of C 4 photosynthesis in this C 3-C 4 intermediate species based on: 1. (a) the appearance of 24% of the total 14C fixed following 4 s photosynthesis in 14CO2-air by excised leaves in malate and aspartate and the complete transfer of label from the C4 acids to Calvin cycle intermediates within a 15 s chase in 12CO2-air; 2. (b) pyruvate- or alanine-enhanced light-dependent CO2 fixation and pyruvate stimulation of oxaloacetate- or 3-phosphoglycerate-dependent O2 evolution by illuminated mesophyll protoplasts, but not bundle sheath strands; and 3. (c) NAD-malic enzyme-dependent decarboxylation of C4 acids at the C-4 carboxyl position, C4 acid-dependent O2 evolution, and 14CO2 donation from [4-14C]C4 acids to Calvin cycle intermediates during photosynthesis by bundle sheath strands, but not mesophyll protoplasts.
However, P. milioides differs from C4 plants in that the activity of the C4 cycle enzymes is only 15 to 30% of a C4 Panicum species and the Calvin cycle and phosphoenolpyruvate carboxylase are present in both cell types. From these and related studies (Rathnam, C.K.M. and Chollet, R. (1979) Arch. Biochem. Biophys. 193, 346–354; (1978) Biochem. Biophys. Res. Commun. 85, 801–808) we conclude that reduced photorespiration in P. milioides is due to a limited degree of NAD-malic enzyme-type C4 photosynthesis permitting an increase in pCO2 at the site of bundle sheath, but not mesophyll, ribulosebisphosphate carboxylase-oxygenase. 相似文献
10.
The combined effects of O 2 on net rates of photosynthesis, photosystem II activity, steady‐state pool size of key metabolites of photosynthetic metabolism in the C 4 pathway, C 3 pathway and C 2 photorespiratory cycle and on growth were evaluated in the C 4 species Amaranthus edulis and the C 3 species Flaveria pringlei. Increasing O 2 reduced net CO 2 assimilation in F. pringlei due to an increased flux of C through the photorespiratory pathway. However, in A. edulis increasing O 2 up to 5–10% stimulated photosynthesis. Analysis of the pool size of key metabolites in A. edulis suggests that while there is some O 2 dependent photorespiration, O 2 is required for maximizing C 4 cycle activity to concentrate CO 2 in bundle sheath cells. Therefore, the response of net photosynthesis to O 2 in C 4 plants may result from the balance of these two opposing effects. Under 21 versus 5% O 2, growth of A. edulis was stimulated about 30% whereas that of F. pringlei was inhibited about 40%. 相似文献
11.
A theoretical model of the composition of the inorganic carbon pool generated in C 4 leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO 2 and O 2 in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO 2 concentration of 70 micromolar during steady state photosynthesis in a typical C 4 plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO 2. Under these circumstances the flux of carbon through the C 4 acid cycle would have to exceed the net rate of CO 2 assimilation by 15.5%. With the calculated O 2 concentration of 0.44 millimolar, the potential photorespiratory CO 2 loss in bundle sheath cells would be about 3% of CO 2 assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO 3−, the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO 2 and O 2 diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C 4 plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C 4 acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate. 相似文献
12.
After two weeks of moderate N restriction, growth of 3-week-old Zea mays L. plants was less than half that of the control and aspartate and malate levels in the leaves were severely suppressed (45 and 65% decrease, respectively). Since in NADP malic enzyme type C 4 plants, such as maize, malate and aspartate are intermediates in the C 4 photosynthetic pathway, the operation of the latter was investigated. Moderate nitrogen deficiency had only a small effect on the rate of photosynthesis (20% decrease) measured under 1000 umol m ?2 s ?1 irradiance. 14CO 2 pulse- 12CO 2 chase experiments combined with measurements of in vitro photosynthetic enzyme activities demonstrated the operation of a typical C 4 photosynthetic pathway in N-restricted plants. The turnover rates of malate and aspartate molecules involved in the C 4 cycle were determined by the loss of label in the carbon 4 moiety of these molecules during the chase period. It is shown that N restriction did not alter the turnover of malate but greatly accelerated that of aspartate. The amounts of malate and aspartate moving through photosynthetically active pools were estimated using a kinetic model. For malate, the size of this pool appeared to be only slightly diminished whereas for aspartate the size of the corresponding pool decreased by a factor of 3. It is proposed that under moderate NO 3? deficiency, despite deviations in malate metabolism leading to a pronounced decrease in the size of its cellular pool, a large amount of malate remained in the operation of the C 4 pathway. By contrast, the participation of aspartate in the operation of the C 4 pathway was greatly reduced. 相似文献
13.
Despite mounting evidence showing that C 4 plants can accumulate more biomass at elevated CO 2 partial pressure (p(CO 2)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C 4 plants to elevated p(CO 2) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO 2) can stimulate the growth of both well-watered and water-stressed C 4 plants. First, through enhanced leaf CO 2 assimilation rates due to increased intercellular p(CO 2). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO 2 assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO 2 fixation in the bundle sheath or the presence of C 3-like photosynthesis in young C 4 leaves are unlikely explanations for the high CO 2-responsiveness of C 4 photosynthesis. The interactions between elevated p(CO 2), leaf temperature and shoot water relations on the growth and photosynthesis of C 4 plants are identified as key areas needing urgent research. 相似文献
14.
Summary The leaf anatomy was investigated with respect to the arrangement of cells involved in photosynthesis. The full-grown leaf has one vascular bundle consisting mainly of phioem cells. In similarity to terrestrial C 4 plants the vascular bundle is surrounded by mesophyll bundle sheath cells. However, in contrast to C 4 plants, these cells do not contain chlorophyll or starch in Ceratophyllum. The early products in photosynthesis (10 seconds 14C labelling) were analyzed. Although no complete separation of all radioactivity in the plant extracts was reached, it was clear that malate was the major labelled component, indicating C 4 activity in the plants. No light saturation could be proven in Ceratophyllum in several stages of post-dormancy in a statistically significant way, although a tendency to light saturation was observed at intensities higher than 36 Wm –2. The photosynthetic activity was only slightly depressed by enhancement of the O 2 concentration in the medium. 相似文献
15.
C 3 photosynthesis is an inefficient process, because the enzyme that lies at the heart of the Benson–Calvin cycle, ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) is itself a very inefficient enzyme. The oxygenase activity of Rubisco is an unavoidable side reaction that is a consequence of its reaction mechanism. The product of oxygenation, glycollate 2-P, has to be retrieved by photorespiration, a process which results in the loss of a quarter of the carbon that was originally present in glycollate 2-P. Photorespiration therefore reduces carbon gain. Purely in terms of carbon economy, there is, therefore, a strong selection pressure on plants to reduce the rate of photorespiration so as to increase carbon gain, but it also improves water- and nitrogen-use efficiency. Possibilities for the manipulation of plants to decrease the amount of photorespiration include the introduction of improved Rubisco from other species, reconfiguring photorespiration, or introducing carbon-concentrating mechanisms, such as inorganic carbon transporters, carboxysomes or pyrenoids, or engineering a full C 4 Kranz pathway using the existing evolutionary progression in C 3–C 4 intermediates as a blueprint. Possible routes and progress to suppressing photorespiration by introducing C 4 photosynthesis in C 3 crop plants will be discussed, including whether single cell C 4 photosynthesis is feasible, how the evolution of C 3–C 4 intermediates can be used as a blueprint for engineering C 4 photosynthesis, which pathway for the C 4 cycle might be introduced and the extent to which processes and structures in C 3 plant might require optimisation. 相似文献
16.
Photosynthesis rates of detached Panicum miliaceum leaves were measured, by either CO 2 assimilation or oxygen evolution, over a wide range of CO 2 concentrations before and after supplying the phosphoenolpyruvate (PEP) carboxylase inhibitor, 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (DCDP). At a concentration of CO 2 near ambient, net photosynthesis was completely inhibited by DCDP, but could be largely restored by elevating the CO 2 concentration to about 0.8% (v/v) and above. Inhibition of isolated PEP carboxylase by DCDP was not competitive with respect to HCO 3−, indicating that the recovery was not due to reversal of enzyme inhibition. The kinetics of 14C-incorporation from 14CO 2 into early labeled products indicated that photosynthesis in DCDP-treated P. miliaceum leaves at 1% (v/v) CO 2 occurs predominantly by direct CO 2 fixation by ribulose 1,5-bisphosphate carboxylase. From the photosynthesis rates of DCDP-treated leaves at elevated CO 2 concentrations, permeability coefficients for CO 2 flux into bundle sheath cells were determined for a range of C 4 species. These values (6-21 micromoles per minute per milligram chlorophyll per millimolar, or 0.0016-0.0056 centimeter per second) were found to be about 100-fold lower than published values for mesophyll cells of C 3 plants. These results support the concept that a CO 2 permeability barrier exists to allow the development of high CO 2 concentrations in bundle sheath cells during C 4 photosynthesis. 相似文献
17.
Evidence is presented contrary to the suggestion that C 4 plants grow larger at elevated CO 2 because the C 4 pathway of young C 4 leaves has C 3-like characteristics, making their photosynthesis O 2 sensitive and responsive to high CO 2. We combined PAM fluorescence with gas exchange measurements to examine the O 2 dependence of photosynthesis in young and mature leaves of Panicum antidotale (C 4, NADP-ME) and P. coloratum (C 4, NAD-ME), at an intercellular CO 2 concentration of 5 Pa. P. laxum (C 3) was used for comparison. The young C 4 leaves had CO 2 and light response curves typical of C 4 photosynthesis. When the O 2 concentration was gradually increased between 2 and 40%, CO 2 assimilation rates ( A) of both mature and young C 4 leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO 2 assimilation ( ΦPSII/ ΦCO2) increased more in young (up to 31%) than mature (up to 10%) C 4 leaves. A of C 3 leaves decreased by 1·3 and ΦPSII/ ΦCO2 increased by 9-fold, over the same range of O 2 concentrations. Larger increases in electron transport requirements in young, relative to mature, C 4 leaves at low CO 2 are indicative of greater O 2 sensitivity of photorespiration. Photosynthesis modelling showed that young C 4 leaves have lower bundle sheath CO 2 concentration, brought about by higher bundle sheath conductance relative to the activity of the C 4 and C 3 cycles and/or lower ratio of activities of the C 4 to C 3 cycles. 相似文献
18.
C 4 photosynthesis is nature’s most efficient answer to the dual activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the resulting loss of CO 2 by photorespiration. Gly decarboxylase ( GDC) is the key component of photorespiratory CO 2 release in plants and is active in all photosynthetic tissues of C 3 plants, but only in the bundle sheath cells of C 4 plants. The restriction of GDC to the bundle sheath is assumed to be an essential and early step in the evolution of C 4 photosynthesis, leading to a photorespiratory CO 2 concentrating mechanism. In this study, we analyzed how the P-protein of GDC (GLDP) became restricted to the bundle sheath during the transition from C 3 to C 4 photosynthesis in the genus Flaveria. We found that C 3
Flaveria species already contain a bundle sheath–expressed GLDP gene in addition to a ubiquitously expressed second gene, which became a pseudogene in C 4
Flaveria species. Analyses of C 3-C 4 intermediate Flaveria species revealed that the photorespiratory CO 2 pump was not established in one single step, but gradually. The knowledge gained by this study sheds light on the early steps in C 4 evolution. 相似文献
19.
There is continuing controversy over whether a degree of C 4 photosynthetic metabolism exists in ears of C 3 cereals. In this context, CO 2 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 CO 2 compensation concentration at 210 mmol mol ?1 O 2 in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O 2 dependence of the CO 2 compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C 3 photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with 14CO 2 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 CO 2 mainly by the Calvin (C 3) cycle, with little fixation of 14CO 2 into the C 4 acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C 3 cereals C 4 photosynthetic metabolism is nil. 相似文献
20.
Carbon isotope discrimination in C 3–C 4 intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO 2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO 2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO 2 fixation occurring in the bundle sheath, the extent of bundle-sheath leakiness and the contribution which C 4-cycle activity makes to the CO 2 pool there. In most instances, carbon isotope discrimination in C 3–C 4 intermediates is C 3-like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO 2 into C 4-acids. In C 3–C 4 intermediates that refix photorespiratory CO 2 alone, it is possible for carbon isotope discrimination to be greater than in C 3-species, particularly at low CO 2 pressures or at high leaf temperatures. Short-term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C 3-C 4 intermediates and their hybrids as has recently been done for C 3 and C 4 species. 相似文献
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