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1.
Abstract Models developed to explain the biphasic response of CO 2 compensation concentration to O 2 concentration and the C 3-like carbon isotope discrimination in C 3-C 4 intermediate species are used to characterize quantitatively the steps necessary in the evolution of C 4 photosynthesis. The evolutionary stages are indicated by model outputs, CO 2 compensation concentration and δ 13C value. The transition from intermediate plants to C 4 plants requires the complete formation of C 4 cycle capacity, expressed by the models as transition from C 4 cycle limitation by phosphoenolpyruvate (PEP) regeneration rate to limitation by PEP carboxylase activity. Other steps refer to CO 2 leakage from bundle sheath cells, to further augmentations of C 4 cycle components, to the repression of ribulose-1,5-bisphos-phate carboxylase in the mesophyll cells, and to a decrease in the CO 2 affinity of the enzyme. Possibilities of extending the suggested approach to other physiological characteristics, and the adaptive significance of the steps envisaged, are discussed. 相似文献
2.
Abstract. The global uptake of CO 2 in photosynthesis is about 120 gigatons (Gt) of carbon per year. Virtually all passes through one enzyme, ribulose bisphosphate carboxylase/oxygenase (rubisco), which initiates both the photosynthetic carbon reduction, and photorespiratory carbon oxidation, cycles. Both CO 2 and O 2 are substrates; CO 2 also activates the enzyme. In C 3 plants, rubisco has a low catalytic activity, operates below its K m (CO 2), and is inhibited by O 2. Consequently, increases in the CO 2/O 2 ratio stimulate C 3 photosynthesis and inhibit photorespiration. CO 2 enrichment usually enhances the productivity of C 3 plants, but the effect is marginal in C 4 species. It also causes acclimation in various ways: anatomically, morphologically, physiologically or biochemically. So, CO 2 exerts secondary effects in growth regulation, probably at the molecular level, that are not predictable from its primary biochemical role in carboxylation. After an initial increase with CO 2 enrichment, net photosynthesis often declines. This is a common acclimation phenomenon, less so in field studies, that is ultimately mediated by a decline in rubisco activity, though the RuBP/P i-regeneration capacities of the plant may play a role. The decline is due to decreased rubisco protein, activation state, and/or specific activity, and it maintains the rubisco fixation and RuBP/P i regeneration capacities in balance. Carbohydrate accumulation is sometimes associated with reduced net photosynthesis, possibly causing feedback inhibition of the RuBP/P iregeneration capacities, or chloroplast disruption. As exemplified by field-grown soybeans and salt marsh species, a reduction in net photosynthesis and rubisco activity is not inevitable under CO 2 enrichment. Strong sinks or rapid translocation may avoid such acclimation responses. Over geological time, aquatic autotrophs and terrestrial C 4 and CAM plants have genetically adapted to a decline in the external CO 2/O 2 ratio, by the development of mechanisms to concentrate CO 2 internally; thus circumventing O 2 inhibition of rubisco. Here rubisco affinity for CO 2 is less, but its catalytic activity is greater, a situation compatible with a high-CO 2 internal environment. In aquatic autotrophs, the CO 2 concentrating mechanisms acclimate to the external CO 2, being suppressed at high-CO 2. It is unclear, whether a doubling in atmospheric CO 2 will be sufficient to cause a de-adaptive trend in the rubisco kinetics of future C 3 plants, producing higher catalytic activities. 相似文献
3.
Abstract Evidence is drawn from previous studies to argue that C 3—C 4 intermediate plants are evolutionary intermediates, evolving from fully-expressed C 3 plants towards fully-expressed C 4 plants. On the basis of this conclusion, C 3—C 4 intermediates are examined to elucidate possible patterns that have been followed during the evolution of C 4 photosynthesis. An hypothesis is proposed that the initial step in C 4-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO 2 using RuBP carboxylase. The CO 2-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 CO 2. As the activity of PEP carboxylase increased to higher values, other enzymes in the C 4-pathway are proposed to have increased in activity to facilitate the processing of the products of C 4-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C 4-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C 4 plants. Such metabolism would have limited benefit in terms of concentrating CO 2 at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C 4-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C 4 plants. Thus, during the initial stages of C 4-evolution it is proposed that improvements in photorespiratory CO 2-loss and their influence on increasing the rate of net CO 2 assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO 2-concentrating mechanism is proposed as the driving force during the later stages. 相似文献
4.
Changes in gas-exchange rates during the life span of the leaves of rice ( Oryza sativa L.) were analyzed quantitatively by measuring changes in the carboxylation/oxygenation and regeneration of ribulose 1,5-bisphosphate (RuBP) at photon fluence rates of 2000 (saturating) and 500 (subsaturating) μmol quanta·m -2·s -1 under ambient air conditions. The RuBP levels were always higher than the active-site concentrations of RuBP carboxylase (EC 4.1.1.39), irrespective of the irradiance supplied. Analysis of the CO 2-assimilation rate as a function of intercellular CO 2 concentration indicated that RuBP regeneration does not limit CO 2 assimilation. The estimated RuBP-carboxylase/oxygenase activity in vivo was linearly correlated to the rate of CO 2 assimilation at each level of irradiance. This enzyme activity was just enough to account for the rate of CO 2 assimilation at the saturating irradiance and was 35% more than the rate of CO 2 assimilation at the subsaturating irradiance. Analysis of the assimilation rate at subsaturating irradiance as a function of intercellular CO 2 concentration indicated that a limitation caused by enzyme activation comes into play. The results indicate that the rate of CO 2 assimilation in rice leaves under ambient air conditions is limited during their entire life span by the RuBP-carboxylation/oxygenation capacity. 相似文献
5.
Effects of growth light intensity on the temperature dependence of CO 2 assimilation rate were studied in tobacco ( Nicotiana tabacum) because growth light intensity alters nitrogen allocation between photosynthetic components. Leaf nitrogen, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) contents increased with increasing growth light intensity, but the cyt f/Rubisco ratio was unaltered. Mesophyll conductance to CO 2 diffusion ( gm) measured with carbon isotope discrimination increased with growth light intensity but not with measuring light intensity. The responses of CO 2 assimilation rate to chloroplast CO 2 concentration ( Cc) at different light intensities and temperatures were used to estimate the maximum carboxylation rate of Rubisco ( Vcmax) and the chloroplast electron transport rate ( J). Maximum electron transport rates were linearly related to cyt f content at any given temperature (e.g. 115 and 179 µmol electrons mol ?1 cyt f s ?1 at 25 and 40 °C, respectively). The chloroplast CO 2 concentration ( Ctrans) at which the transition from RuBP carboxylation to RuBP regeneration limitation occurred increased with leaf temperature and was independent of growth light intensity, consistent with the constant ratio of cyt f/Rubisco. In tobacco, CO 2 assimilation rate at 380 µmol mol ?1 CO 2 concentration and high light was limited by RuBP carboxylation above 32 °C and by RuBP regeneration below 32 °C. 相似文献
6.
Ribulose-1,5-bisphosphate (RuBP) pool size was determined at regular intervals during the growing season to understand the effects of tropospheric ozone concentrations, elevated atmospheric carbon dioxide concentrations and their interactions on the photosynthetic limitation by RuBP regeneration. Soybean ( Glycine max [L.] Merr. cv. Essex) was grown from seed to maturity in open-top field chambers in charcoal-filtered air (CF) either without (22 nmol O 3 mol ?1) or with added O 3 (83 nmol mol ?1) at ambient (AA, 369 μmol CO 2 mol ?1) or elevated CO 2 (710 μmol mol ?1). The RuBP pool size generally declined with plant age in all treatments when expressed on a unit leaf area and in all treatments but CF-AA when expressed per unit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) binding site. Although O 3 in ambient CO 2 generally reduced the RuBP pool per unit leaf area, it did not change the RuBP pool per unit Rubisco binding site. Elevated CO 2, in CF or O 3-fumigated air, generally had no significant effect on RuBP pool size, thus mitigating the negative O 3 effect. The RuBP pools were below 2 mol mol ?1 binding site in all treatments for most of the season, indicating limiting RuBP regeneration capacity. These low RuBP pools resulted in increased RuBP regeneration via faster RuBP turnover, but only in CF air and during vegetative and flowering stages at elevated CO 2. Also, the low RuBP pool sizes did not always reflect RuBP consumption rates or the RuBP regeneration limitation relative to potential carboxylation (%RuBP). Rather, %RuBP increased linearly with decrease in the RuBP pool turnover time. These data suggest that amelioration of damage from O 3 by elevated atmospheric CO 2 to the RuBP regeneration may be in response to changes in the Rubisco carboxylation. 相似文献
7.
Concurrent measurements of leaf gas exchange and on-line 13C discrimination were used to evaluate the CO 2 conductance to diffusion from the stomatal cavity to the sites of carboxylation within the chloroplast (internal conductance; gi). When photon irradiance was varied it appeared that gi and/or the discrimination accompanying carboxylation also varied. Despite this problem, gi, was estimated for leaves of peach ( Prunus persica), grapefruit ( Citrus paradisi), lemon ( C. limon) and macadamia (Macadamia integrifolia) at saturating photon irradiance. Estimates for leaves of C. paradisi, C. limon and M. integrifolia were considerably lower than those previously reported for well-nourished herbaceous plants and ranged from 1.1 to2.2μmol CO 2 m ?2 s ?1 Pa ?1, whilst P. persica had a mean value of 3.5 μmol CO 2 m ?2 s ?1 Pa ?1. At an ambient CO 2 partial pressure of 33Pa, estimates of chloroplastic partial pressure of CO 2 ( Cc) using measurements of CO 2 assimilation rate (A) and calculated values of gi, and of partial pressure of CO 2 in the stomatal cavity ( Cst) were as low as 11.2 Pa for C. limon and as high as 17.8Pa for peach. In vivo maximum rubisco activities ( Vmax) were also determined from estimates of Cc. This calculation showed that for a given leaf nitrogen concentration (area basis) C. paradisi and C. limon leaves had a lower Vmax than P. persica, with C. paradisi and C. limon estimated to have only 10% of leaf nitrogen present as rubisco. Therefore, low CO 2 assimilation rates despite high leaf nitrogen concentrations in leaves of the evergreen species examined were explained not only by a low C c but also by a relatively low proportion of leaf nitrogen being used for photosynthesis. We also show that simple one-dimensional equations describing the relationship between leaf internal conductance from stomatal cavities to the sites of carboxylation and carbon isotope discrimination (Δ) can lead to errors in the estimate of gi. Potential effects of heterogeneity in stomatal aperture on carbon isotope discrimination may be particularly important and may lead to a dependence of gi upon CO 2 assimilation rate. It is shown that for any concurrent measurement of A and Δ, the estimate of Cc is an overestimate of the correct photosynthetic capacity-weighted value, but this error is probably less than 1.0 Pa. 相似文献
8.
The carbon isotope discrimination ratio of the floral parts,leaves, and stems of barley and oat plants were measured todetermine if net CO 2 fixation by PEP carboxylase (describedin these tissues by other workers) makes a significant contributionto total carbon fixation in these tissues. The 13C values rangedfrom 26.6 to 29.6% and are within the range normallyexpected for plants with the C 3 pathway of photosynthesis inwhich autotrophic CO 2 fixation proceeds via RuBP carboxylase.We conclude that PEP carboxylase does not make a substantialcontribution to autotrophic CO 2 fixation in the floral partsof these C 3 plants. 相似文献
9.
Changes in net photosynthetic rate (P N), stomatal conductance (g s), intercellular CO 2 concentrations (C i), transpiration rate (E) and water use efficiency (WUE) were measured in Plantago major L. plants grown under sufficient soil water supply or under soil water stress conditions. The plants had high P N in a wide range of soil water potential and temperature regimes. Soil water had little effect on P N under ambient CO 2 concentrations, which was explained by a high carboxylation rate, but increased the dark respiration rate. Carboxylation activity at low C i depended on RuBP regeneration, whereas at high C i it depended on the phosphate regeneration rate. The g s and E values were low in plants under stress as compared to the controls that resulted in an increase of WUE. The results obtained show that Plantago major plants have different ways of adaptation to soil water deficit conditions. 相似文献
10.
Our previous study has demonstrated that both RuBP carboxylation limitation and RuBP regeneration limitation exist simultaneously in rice grown under free-air CO 2 enrichment (FACE, about 200 μmol mol −1 above the ambient air CO 2 concentration) conditions [G.-Y. Chen, Z.-H. Yong, Y. Liao, D.-Y. Zhang, Y. Chen, H.-B. Zhang, J. Chen, J.-G. Zhu, D.-Q. Xu, Photosynthetic acclimation in rice leaves to free-air CO 2 enrichment related to both ribulose-1,5-bisphosphate carboxylase limitation and ribulose-1,5-bisphosphate regeneration limitation. Plant Cell Physiol. 46 (2005) 1036–1045]. To explore the mechanism for forming of RuBP regeneration limitation, we conducted the gas exchange measurements and some biochemical analyses in FACE-treated and ambient rice plants. Net CO 2 assimilation rate ( Anet) in FACE leaves was remarkably lower than that in ambient leaves when measured at the same CO 2 concentration, indicating that photosynthetic acclimation to elevated CO 2 occurred. In the meantime the maximum electron transport rate (ETR) ( Jmax), maximum carboxylation rate ( Vcmax) in vivo, and RuBP contents decreased significantly in FACE leaves. The whole chain electron transport rate and photophosphorylation rate reduced significantly while ETR of photosystem II (PSII) did not significantly decrease and ETR of photosystem I (PSI) was significantly increased in the chloroplasts from FACE leaves. Further, the amount of cytochrome (Cyt) f protein, a key component localized between PSII and PSI, was remarkably declined in FACE leaves. It appears that during photosynthetic acclimation the decline in the Cyt f amount is an important cause for the decreased RuBP regeneration capacity by decreasing the whole chain electron transport in FACE leaves. 相似文献
11.
Photosynthesis in two cultivars of Triticum aestivum was compared with photosynthesis in two lines having the same nuclear genomes but with cytoplasms derived from T. boeoticum. The in-vitro specific activity of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) isolated from lines with T. boeoticum cytoplasm was only 71% of that of normal T. aestivum. By contrast, the RuBPCase activities calculated from the CO 2-assimilation rate at low partial pressures of CO 2, p(CO 2), were the same for all lines for a given RuBPCase content. This indicates that both types of RuBPCase have the same turnover numbers in-vivo of 27.5 mol CO 2·(mol enzyme) –1·s –1 (23°). The rate of CO 2 assimilation measured at normal p(CO 2), p
a
=340 bar, and high irradiance could be quantitatively predicted from the amount of RuBPCase protein. The maximum rate of RuBP regeneration could also predict the rate of CO 2 assimilation at normal ambient conditions. Therefore, the maximum capacities for RuBP carboxylation and RuBP regeneration appear to be well-balanced for normal ambient conditions. As photosynthetic capacity declined with increasing leaf age, the capacities for RuBP carboxylation and RuBP regeneration declined in parallel.Abbreviations PAR
photosynthetically active radiation
- RuBP(Case)
ribulose-1,5-bisphosphate (carboxylase) 相似文献
12.
The capacity for photosynthesis is often affected when plants are grown in air with elevated CO 2 partial pressure. We grew Phaseolus vulgaris L. in 35 and 65 Pa CO 2 and measured photosynthetic parameters. When assayed at the growth CO 2 level, photosynthesis was equal in the two CO 2 treatments. The maximum rate of ribulose-1,5-bisphosphate (RuBP) consumption was lower in plants grown at 65 Pa, but the CO 2 partial pressure at which the maximum occurred was higher in the high-CO 2-grown plants, indicating acclimation to high CO 2. The acclimation of RuBP consumption to CO 2 involved a reduction of the activity of RuBP carboxylase which resulted from reduced carbamylation, not a loss of protein. The rate of RuBP consumption declined with CO 2 when the CO 2 partial pressure was above 50Pa in plants grown under both CO 2 levels. This was caused by feedback inhibition as judged by a lack of response to removing O 2 from the air stream. The rate of photosynthesis at high CO 2 was lower in the high-CO 2-grown plants and this was correlated with reduced activity of sucrose-phosphate synthase. This is only the second report of O 2-insensitive photosynthesis under growth conditions for plants grown in high CO 2. 相似文献
13.
Measurement of carbon isotope discrimination (Δ) of organic plant material integrates the combination of C 4 and C 3 carboxylation processes during the phases of CAM through dark and light periods. These processes are tempered by environmental conditions which regulate CAM activity at the molecular, biochemical and ecological level. The factors contributing to short-term changes in Δ are discussed in terms of the day-night changes in metabolite pools and integration via on-line, instantaneous discrimination techniques. Thus, the isotope signature of newly fixed carbon in malic acid reflects the balance between diffusion and carboxylation limitation together with direct and indirect effects of respiratory metabolism. Leakage of CO 2 during decarboxylation leads to greater discrimination being expressed than is predicted from existing models. Over the timescales of seasonal growth and productivity, most constitutive CAM and C 3-CAM intermediate plants show little variation in Δ (2–4‰). The changes induced by developmental and environmental signals and genetic regulation of CAM are compared for stem and leaf succulents. The role of CAM as a potentially highly productive photosynthetic pathway is contrasted with the induction of CAM as a maintenance mechanism in response to environmental stresses. Analyses of Δ have already contributed much to our understanding of the distribution and regulation of CAM, and in turn can also be used to analyse phylogenetic relationships and the origins of CAM as determined from palaoecological evidence. 相似文献
14.
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
15.
Background and AimsElucidation of the mechanisms by which plants adapt to elevated CO 2 is needed; however, most studies of the mechanisms investigated the response of plants adapted to current atmospheric CO 2. The rapid respiration rate of cotton ( Gossypium hirsutum) fruits (bolls) produces a concentrated CO 2 microenvironment around the bolls and bracts. It has been observed that the intercellular CO 2 concentration of a whole fruit (bract and boll) ranges from 500 to 1300 µmol mol −1 depending on the irradiance, even in ambient air. Arguably, this CO 2 microenvironment has existed for at least 1·1 million years since the appearance of tetraploid cotton. Therefore, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO 2 will indicate how plants will adapt to future increased atmospheric CO 2 concentration. Specifically, it is hypothesized that with elevated CO 2 the capacity to regenerate ribulose-1,5-bisphosphate (RuBP) will increase relative to RuBP carboxylation. MethodsTo test this hypothesis, the morphological and physiological traits of bracts and leaves of cotton were measured, including stomatal density, gas exchange and protein contents. Key resultsCompared with leaves, bracts showed significantly lower stomatal conductance which resulted in a significantly higher water use efficiency. Both gas exchange and protein content showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio ( Jmax/ Vcmax) in bracts than in leaves. ConclusionsThese results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO 2 requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO 2. 相似文献
16.
Statistical analysis of Km (CO 2) values of ribulose-1,5-bisphosphate (RuBP) carboxylase from 35 C 4 grass species shows that the mean value for PEP-carboxykinase (PCK) type C 4 species (41.4±s.e. 2.2 μM CO 2) is significantly different from that of NAD-malic enzyme (NAD-ME) type species (55.3±3.1 μM CO 2) or NADP-malic enzyme (NADP-ME type species (52.5±s.e. 2.0μM CO 2). These C 4 type differences remain detectable within both the eu-panicoid and chloridoid grass subfamilies. By contrast, no between-subfamily differences were found within C 4 types. Variation in Km (CO 2) values of RuBP carboxylase may be related to in vivo differences in CO 2 concentration at the enzyme site, mediated perhaps by differences in CO 2-leakiness of C 4 leaf ‘photosynthetic carbon reduction’ (PCR or ‘Kranz’) tissue. 相似文献
17.
Incubation of the submersed aquatic macrophyte, Hydrilla verticillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO 2/1) and low (6 to 8 μl CO 2/l) CO 2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C 3 and C 4 species, respectively. Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO 3 ?) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg 2+) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively. Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C 4 acid inhibition and protection were not reliable methods to distinguish C 3 and C 4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from hight Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C 3 and C 4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C 4 type. 相似文献
18.
The leaf anatomy and certain photosynthetic properties of nitrate- and ammonia-grown plants of Moricandia arvensis (L.) DC., a species previously reported to be a C 3-C 4 intermediate, were investigated. Nitrate-grown plants had a high level of malate in the leaves while ammonia-grown plants had low levels of malate. In young leaves of nitrate-grown plants, there was a diurnal fluctuation of malate content, increasing during the day and decreasing during the night. Titratable acidity remained low in leaves of both nitrate- and ammonia-grown plants. In nitrate-grown plants, the activity of phosphoenolpyruvate (PEP) carboxylase was about 2-fold higher than in ammonia-grown plants, the latter having activity typical of C3 species. Also, in nitrate-grown plants, the ratio of activities of ribulose 1,5-bisphosphate (RuBP) carboxylase/PEP carboxylase was lower than in ammonia-grown plants. Nitrate reductase activities were higher in nitrate- than in ammonia-grown plants and the greatest activity was found in younger leaves. With nitrate-grown plants, during a pulse-chase experiment the label in malate, as a percentage of the total labeled products, increased from about 7% after a 10-second pulse with 14CO2 up to 17% during a 5-minute chase with 12CO2. The pattern of 14C labeling in various metabolites suggests the primary carboxylation is through RuBP carboxylase with a secondary carboxylation through PEP carboxylase. In similar experiments, with ammonia-grown plants, the percentage label in malate was only 0% to 4% with no increase in malate labeling during the chase period. The CO2 compensation point was lower in nitrate-grown than ammonia-grown plants. There was no evidence of Kranz-like anatomy in either the nitrate or ammonia-grown plants. Mitochondria of bundle-sheath cells were strikingly positioned along the inner tangential wall. This might allow the chloroplasts of these cells to fix the mitochondrial photorespired CO2 more effectively and contribute to the low CO2 compensation point in the species. Chloroplasts of bundle-sheath cells and contiguous mesophyll cells were similar in size and structure in plants grown on different media, although chloroplast thylakoids and stromata of the ammonia-grown plants stained more intensely than those of nitrate-grown plants. In addition, irregular clusters of phytoferritin particles occurred in the chloroplasts of the ammonia-grown plants. The results indicate that the substantial activity of PEP carboxylase, incorporation of CO2 into malate, the high malate content, and in part the relatively low CO2 compensation point in Moricandia arvensis may be accounted for by metabolism of nitrate rather than by a state of C3-C4 intermediacy. 相似文献
19.
Incubation of the submersed aquatic macrophyte, Hydrilla vertieillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO 2/l) and low (6 to 8 μl CO 2/l) CO 2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C 3 and C 4 species, respectively. Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO 3 -) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg 2+) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively. Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C 4 acid inhibition and protection were not reliable methods to distinguish C 3 and C 4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from high Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C 3 and C 4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C 4 type. 相似文献
20.
The relationship between the gas-exchange characteristics of attached leaves of Zea mays L. and the contents of photosynthetic intermediates was examined at different intercellular partial pressure of CO 2 and at different irradiances at a constant intercellular partial pressure of CO 2. (i) The behaviour of the pools of the C 4-cycle intermediates, phosphoenolpyruvate and pyruvate, provides evidence for light regulation of their consumption. However, light regulation of phosphoenolpyruvate carboxylase does not influence the assimilation rate at limiting intercellular partial pressures of CO 2. (ii) A close correlation between the pools of phosphoenolpyruvate and glycerate-3-phosphate exists under many different flux conditions, consistent with the notion that the pools of C 4 and C 3 cycles are connected via the interconversion of glycerate-3-phosphate and phosphoenolpyruvate. (iii) The ratio of triose-phosphate to glycerate-3-phosphate is used as an indicator of the availability of ATP and NADPH. Changes of this ratio with CO 2 and with irradiance are compared with results obtained in C 3 leaves and indicate that the mechanism of regulation of carbon assimilation by light in leaves of C 4 plants may differ from that in C 3 plants. (iv) The behaviour of the ribulose-1,5-bisphosphate pool with CO 2 and irradiance is contrasted with the behaviour of these pools measured in leaves of C 3 plants.Abbreviations
P
i
intercellular CO 2 pressure
- RuBP
ribulose-1,5-bisphosphate
- PEP
phosphoenolpyruvate
- triose-P
triose phosphates
- PGA
glycerate-3-phosphate 相似文献
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