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1.
In C 4 plants carbonic anhydrase catalyzes the critical first step of C 4 photosynthesis, the hydration of CO 2 to bicarbonate. The maximum activity of this enzyme in C 4 leaf extracts, measured by H + production with saturating CO 2 and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C 3 leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO 2 concentrations was apparently just sufficient to prevent the rate of conversion of CO 2 to HCO 3− from limiting C 4 photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities. 相似文献
2.
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
3.
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
4.
The apicomplexan parasite Toxoplasma gondii does not possess complex I of the mitochondrial respiratory chain, but has two genes encoding rotenone-insensitive, non-proton pumping type-II NADH dehydrogenases (NDH2s). The absence of such “alternative” NADH dehydrogenases in the human host defines these enzymes as potential drug targets. TgNDH2-I and TgNDH2-II are constitutively expressed in tachyzoites and bradyzoites and are localized to the mitochondrion as shown by epitope tagging. Functional expression of TgNDH2-I in the yeast Yarrowia lipolytica as an internal enzyme, with the active site facing the mitochondrial matrix, permitted growth in the presence of the complex I inhibitor DQA. Bisubstrate kinetics of TgNDH2-I measured within Y. lipolytica mitochondrial membrane preparations were in accordance with a ping-pong mechanism. Using inhibition kinetics we demonstrate here that 1-hydroxy-2-alkyl-4(1)quinolones with long alkyl chains of C 12 (HDQ) and C 14 are high affinity inhibitors for TgNDH2-I, while compounds with shorter side chains (C 5 and C 6) displayed significantly higher IC 50 values. The efficiency of the various quinolone derivatives to inhibit TgNDH2-I enzyme activity mirrors their inhibitory potency in vivo, suggesting that a long acyl site chain is critical for the inhibitory potential of these compounds. 相似文献
5.
Balanced nutrition is fundamental to health and immunity. For herbivorous insects, nutrient-compositional shifts in host plants due to elevated atmospheric CO 2 concentrations and temperature may compromise this balance. Therefore, understanding their immune responses to such shifts is vital if we are to predict the outcomes of climate change for plant–herbivore–parasitoid and pathogen interactions. We tested the immune response of Paropsis atomaria Olivier (Coleoptera: Chrysomelidae) feeding on Eucalyptus tereticornis Sm. seedlings exposed to elevated CO 2 (640 μmol mol −1; C E) and temperature (ambient plus 4 °C; TE). Larvae were immune-challenged with a nylon monofilament in order to simulate parasitoid or pathogen attack without other effects of actual parasitism or pathology. The cellular ( in vivo melanisation) and humoral ( in vitro phenoloxidase PO activity) immune responses were assessed, and linked to changes in leaf chemistry. C E reduced foliar nitrogen (N) concentrations and increased C:N ratios and concentrations of total phenolics. The humoral response was reduced at C E. PO activity and haemolymph protein concentrations decreased at C E, while haemolymph protein concentrations were positively correlated with foliar N concentrations. However, the cellular response increased at C E and this was not correlated with any foliar traits. Immune parameters were not impacted by TE. Our study revealed that opposite cellular and humoral immune responses occurred as a result of plant-mediated effects at C E. In contrast, elevated temperatures within the tested range had minimal impact on immune responses. These complex interactions may alter the outcomes of parasitoid and pathogen attack in future climates. 相似文献
6.
We compared C 3 and CAM (crassulacean acid metabolism) states in Mesembryanthemum crystallinum, a facultative CAM species, with respect to the involvement of phosphoenolpyruvate carboxylase (PEPC) and nitrogen metabolismrelated enzymes in plant response to Botrytis cinerea infection. The enzyme activities were monitored both in pathogeninoculated 2 nd leaf pair and non-inoculated 3 rd leaf pair. The control activities of most studied enzymes were dependent on the mode of photosynthesis. Compared to C 3 plants, those performing CAM exhibited higher PEPC, nitrate reductase (NR), and deaminating glutamate dehydrogenase (NAD-GDH) activities but lower glutamine synthetase (GS) and alanine aminotransferase (ALT) activities. Regardless of the mode of photosynthetic carbon assimilation, the plants responded to infection with enhancement of PEPC and inhibition of NR activities in the inoculated leaves. Whereas the activity of GS remained unaffected, those of all glutamate-yielding enzymes, namely ferredoxin-dependent glutamate synthase (Fd-GOGAT), aspartate aminotransferase (AST), ALT, and aminating glutamate dehydrogenase (NADHGDH) were altered after infection. However, the time-course and extent of the observed changes differed in C 3 and CAM plants. In general, CAM plants responded to infection with an earlier increase in PEPC and Fd-GOGAT activities as well as later inhibition of NR activity. Contrary to C 3 plants, in those performing CAM the activities of PEPC, Fd-GOGAT, NADH-GDH, and AST in the non-inoculated 3 rd leaf pair were similarly influenced by infection as in leaves directly inoculated with the pathogen. This implies that the local infection induced an alteration of carbon/nitrogen status in healthy upper leaves. This reprogramming resulting from changes in PEPC and nitrogen metabolism-related enzymes was C 3- and CAM-specific. 相似文献
7.
Ribulose bisphosphate carboxylase/oxygenase (RuBPCase) from several plants had substantially greater activity in extracts from lightexposed leaves than dark leaves, even when the extracts were incubated in vitro with saturating HCO 3− and Mg 2+ concentrations. This occurred in Glycine max, Lycopersicon esculentum, Nicotiana tabacum, Panicum bisulcatum, and P. hylaeicum (C 3); P. maximum (C 4 phosphoenolpyruvate carboxykinase); P. milioides (C 3/C 4); and Bromelia pinguin and Ananas comosus (Crassulacean acid metabolism). Little or no difference between light and dark leaf extracts of RuBPCase was observed in Triticum aestivum (C 3); P. miliaceum (C 4 NAD malic enzyme); Zea mays and Sorghum bicolor (C 4 NADP malic enzyme); Moricandia arvensis (C 3/C 4); and Hydrilla verticillata (submersed aquatic macrophyte). It is concluded that, in many plants, especially Crassulacean acid metabolism and C 3 species, a large fraction of ribulose-1,5-bisphosphate carboxylase/oxygenase in the dark is in an inactivatable state that cannot respond to CO 2 and Mg 2+ activation, but which can be converted to an activatable state upon exposure of the leaf to light. 相似文献
8.
Ecotypic differences in the photosynthetic carbon metabolism of Mollugo verticillata were studied. Variations in C 3 and C 4 cycle activity are apparently due to differences in the activities of enzymes associated with each pathway. Compared to C 4 plants, the activities of C 4 pathway enzymes were generally lower in M. verticillata, with the exception of the decarboxylase enzyme, NAD malic enzyme. The combined total carboxylase enzyme activity of M. verticillata was greater than that of C 3 plants, possibly accounting for the high photosynthetic rates of this species. Unlike either C 3 or C 4 plants, ribulose bisphosphate carboxylase was present in both mesophyll and bundle sheath cell chloroplasts in M. verticillata. The localization of this enzyme in both cells in this plant, in conjunction with an efficient C 4 acid decarboxylation mechanism most likely localized in bundle sheath cell mitochondria, may account for intermediate photorespiration levels previously observed in this species. 相似文献
9.
A Zea mays callus culture containing chlorophyll was established and grown photomixotrophically. Cell chloroplast structure, and pigment and soluble protein contents were examined. Expression of some key enzymes of C 4 carbon metabolism was compared with that of etiolated (heterotrophic) and green photoautotrophic leaves. Chlorophyll content of the callus was 15–20% that of green leaves. Soluble protein content of callus was half that of leaf cells. Electron microscopic observations showed that green callus cells contained only typical granal chloroplasts. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.38) activities in green callus were ca 30% those of green leaves but 2–3 times higher than in etiolated leaves. Quantitative enzyme protein determination, using antibodies specific to maize leaf Rubisco showed that the chloroplastic carboxylase represented about 7% of total soluble protein in green callus, in parallel to its low chlorophyll content. The specific activity of Rubisco in callus and leaves was unchanged. Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activity in green callus was about 20% that of green leaves and similar to that measured in etiolated leaves. Apparent K m (PEP) values (0.08 m M) for PEPC isolated from green callus and etiolated leaves were very different from values (0.5 m M) obtained with PEPC from green leaves. These kinetic characteristics together with the absence of inhibition by malate and activation by glucose-6-phosphate suggest that the properties of PEPC isolated from green callus and etiolated maize leaves are very similar to those of PEPPC from C 3 plants. Using PEPC antibodies specific to green maize leaf enzyme, immunotitration of PEPC preparations containing identical enzyme units allowed complete precipitation of the green leaf enzyme with increasing antibody volumes. In contrast, 60–70% of the activity of PEPC from etiolated and green callus was inhibited, suggesting low affinity for the maize green leaf PEPC antiserum (typical C 4 form). Ouchterlony double diffusion tests revealed only partial recognition of PEPC in green callus and etiolated leaves. NAD-malate dehydrogenase (NAD-MDH, EC 1.1.1.37) activity in callus was 2 and 3 times higher, respectively, than in etiolated and green leaves. NADP-malic enzyme (NADP-ME, EC 1.1.1.40) activity in callus cultures was much lower than in green leaves. All our data support the hypothesis that cultures of fully dedifferentiated chlorophyllous tissues of Zea mays possess a C 3-like metabolism. 相似文献
10.
The pyruvate, phosphate dikinase activity (PPD, EC 2.7.9.1) associated with crude extracts of leaf tissue of some C 3 and C 4 plants was determined by phosphoenolpyruvate plus PPi-dependent phosphorylation of AMP. The PPD activity of all C 4 plants examined was > 15 nmol/mg protein/min. Several factors contributed to the underestimation of PPD activity in crude extracts of at least some species. Significant PPD activity (> 0.15 nmol/mg protein/min) was not detected in the majority of C 3 species but several C 3 species and the two CAM species studied exhibited activity in the range 0.4–4 nmol/mg protein/min while the C 3 species Avena sativa showed activity up to 8 nmol/mg protein/min. The oat leaf enzyme was partially purified; it exhibited properties similar to those of partially purified PPD from maize. Leaf extracts of the orchids Cymbidium canaliculatum and C. madidum contained high levels of PPD activity similar to the majority of C 4 plants. PPD activity has also been shown in other previously unstudied species. 相似文献
11.
The 18O content of CO 2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO 2 and 18O-enriched leaf water and retrodiffusion of most of this CO 2 back to the atmosphere, leaves effectively discriminate against 18O during photosynthesis. Discrimination against 18O ( Δ 18O) is expected to be lower in C 4 plants because of low ci and hence low retrodiffusing CO 2 flux. C 4 plants also generally show lower levels of carbonic anhydrase (CA) activities than C 3 plants. Low CA may limit the extent of 18O exchange and further reduce Δ 18O. We investigated CO 2–H 2O isotopic equilibrium in plants with naturally low CA activity, including two C 4 ( Zea mays, Sorghum bicolor) and one C 3 ( Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ 18O in C 4, as well as in some C 3, plants. Variations in CA activity and in the extent of CO 2–H 2O isotopic equilibrium ( θ eq) estimated from on-line measurements of Δ 18O showed large range of 0–100% isotopic equilibrium ( θ eq = 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO 2 concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ 18O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ eq < 1 (incomplete isotopic equilibrium) on 18O of atmospheric CO 2 can be similar for C 3 and C 4 plants and in both cases it increases with natural enrichment of 18O in leaf water. 相似文献
12.
The net CO 2 assimilation by leaves of maize ( Zea mays L. cv. Adonis) plants subjected to slow or rapid dehydration decreased without changes in the total extractable activities of phospho enolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (ME). The phosphorylation state of PEPC extracted from leaves after 2–3 h of exposure to light was not affected by water deficit, either. Moreover, when plants which had been slowly dehydrated to a leaf relative water content of about 60% were rehydrated, the net CO 2 assimilation by leaves increased very rapidly without any changes in the activities of MDH, ME and PEPC or phosphorylation state of PEPC. The net CO 2-dependent O 2 evolution of a non-wilted leaf measured with an oxygen electrode decreased as CO 2 concentration increased and was totally inhibited when the CO 2 concentration was about 10%. Nevertheless, high CO 2 concentrations (5–10%) counteracted most of the inhibitory effect of water deficit that developed during a slow dehydration but only counteracted a little of the inhibitory effect that developed during a rapid dehydration. In contrast to what could be observed during a rapidly developing water deficit, inhibition of leaf photosynthesis by cis-abscisic acid could be alleviated by high CO 2 concentrations. These results indicate that the inhibition of leaf net CO 2 uptake brought about by water deficit is mainly due to stomatal closure when a maize plant is dehydrated slowly while it is mainly due to inhibition of non-stomatal processes when a plant is rapidly dehydrated. The photosynthetic apparatus of maize leaves appears to be as resistant to drought as that of C 3 plants. The non-stomatal inhibition observed in rapidly dehydrated leaves might be the result of either a down-regulation of the photosynthetic enzymes by changes in metabolite pool sizes or restricted plasmodesmatal transport between mesophyll and bundle-sheath cells. 相似文献
13.
Miscanthus has been proposed as a promising crop for phytoremediation due to its high biomass yield and remarkable adaptability to different environments. However, little is known about the resistance of Miscanthus spp. to cadmium (Cd). To determine any differences in resistance of Miscanthus to Cd, we examined plant growth, net photosynthetic rate (Pn), activities of anti-oxidant and C 4 photosynthetic enzymes, concentrations of Cd in leaves and roots, and observed the chloroplast structure in three Miscanthus species treated with 0, 10, 50, 100 or 200 μM Cd in solutions. Miscanthus sinensis showed more sensitivity to Cd, including sharp decreases in growth, Pn, PEPC activity and damage to chloroplast structure, and the highest H 2O 2 and Cd concentrations in leaves and roots after Cd treatments. Miscanthus sacchariflorus showed higher resistance to Cd and better growth, had the highest Pn and phosphoenolpyruvate carboxylase (PEPC) activities and integrative chloroplast structure and the lowest hydrogen peroxide (H 2O 2) and leaf and root Cd concentrations. The results could play an important role in understanding the mechanisms of Cd tolerance in plants and in application of phytoremediation. 相似文献
14.
Soybean ( Glycine max L. Merrill cv `Bragg') plants were grown in pots at six elevated atmospheric CO 2 concentrations and two watering regimes in open top field chambers to characterize leaf xylem potential, stomatal resistance and conductance, transpiration, and carbohydrate contents of the leaves in response to CO 2 enrichment and water stress conditions. Groups of plants at each CO 2 concentration were subjected to water stress by withholding irrigation for 4 days during the pod-filling stage. Under well watered conditions, the stomatal conductance of the plants decreased with increasing CO2 concentration. Therefore, although leaf area per plant was greater in the high CO2 treatments, the rate of water loss per plant decreased with CO2 enrichment. After 4 days without irrigation, plants in lower CO2 treatments showed greater leaf tissue damage, lower leaf water potential, and higher stomatal resistance than high CO2 plants. Stomatal closure occurred at lower leaf water potentials for the low CO2 grown plants than the high CO2 grown plants. Significantly greater starch concentrations were found in leaves of high CO2 plants, and the reductions in leaf starch and increases in soluble sugars due to water stress were greater for low CO2 plants. The results showed that even though greater growth was observed at high atmospheric CO2 concentrations, lower rates of water use delayed and, thereby, prevented the onset of severe water stress under conditions of low moisture availability. 相似文献
15.
Photosynthesis was examined in leaves of Flaveria brownii A. M. Powell, grown under either 14% or 100% full sunlight. In leaves of high light grown plants, the CO 2 compensation point and the inhibition of photosynthesis by 21% O 2 were significantly lower, while activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and various C 4 cycle enzymes were considerably higher than those in leaves grown in low light. Both the CO 2 compensation point and the degree of O 2 inhibition of apparent photosynthesis were relatively insensitive to the light intensity used during measurements with plants from either growth conditions. Partitioning of atmospheric CO 2 between Rubisco of the C 3 pathway and phosphoenolpyruvate carboxylase of the C 4 cycle was determined by exposing leaves to 14CO 2 for 3 to 16 seconds, and extrapolating the labeling curves of initial products to zero time. Results indicated that ~94% of the CO 2 was fixed by the C 4 cycle in high light grown plants, versus ~78% in low light grown plants. Thus, growth of F. brownii in high light increased the expressed level of C 4 photosynthesis. Consistent with the carbon partitioning patterns, photosynthetic enzyme activities (on a chlorophyll basis) in protoplasts from leaves of high light grown plants showed a more C 4-like pattern of compartmentation. Pyruvate, Pi dikinase and phosphoenolpyruvate carboxylase were more enriched in the mesophyll cells, while NADP-malic enzyme and ribulose 1,5-bisphosphate carboxylase/oxygenase were relatively more abundant in the bundle sheath cells of high light than of low light grown plants. Thus, these results indicate that F. brownii has plasticity in its utilization of different pathways of carbon assimilation, depending on the light conditions during growth. 相似文献
16.
Four species of the genus Flaveria, namely F. anomala, F. linearis, F. pubescens, and F. ramosissima, were identified as intermediate C 3-C 4 plants based on leaf anatomy, photosynthetic CO 2 compensation point, O 2 inhibition of photosynthesis, and activities of C 4 enzymes. F. anomala and F. ramosissima exhibit a distinct Kranz-like leaf anatomy, similar to that of the C 4 species F. trinervia, while the other C 3-C 4 intermediate Flaveria species possess a less differentiated Kranz-like leaf anatomy. Photosynthetic CO 2 compensation points of these intermediates at 30°C were very low relative to those of C 3 plants, ranging from 7 to 14 microliters per liter. In contrast to C 3 plants, net photosynthesis by the intermediates was not sensitive to O 2 concentrations below 5% and decreased relatively slowly with increasing O 2 concentration. Under similar conditions, the percentage inhibition of photosynthesis by 21% O 2 varied from 20% to 25% in the intermediates compared with 28% in Lycopersicon esculentum, a typical C 3 species. The inhibition of carboxylation efficiency by 21% O 2 varied from 17% for F. ramosissima to 46% for F. anomala and were intermediate between the C 4 (2% for F. trinervia) and C 3 (53% for L. esculentum) values. The intermediate Flaveria species, especially F. ramosissima, have substantial activities of the C 4 enzymes, phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, NADP-malic enzyme, and NADP-malate dehydrogenase, indicating potential for C 4 photosynthesis. It appears that these Flaveria species may be true biochemical C 3-C 4 intermediates. 相似文献
17.
The aim of this work was to investigate how light regulates the activity of phosphoenolpyruvate carboxylase in vivo in C 4 plants. The properties of phosphoenolpyruvate carboxylase were investigated in extracts which were rapidly prepared (in less than 30 seconds) from darkened and illuminated leaves of Zea mays. Illumination resulted in a significant decrease in the S 0.5(phosphoenolpyruvate) but there was no change in Vmax. The form of the enzyme from illuminated leaves was less sensitive to malate inhibition than was the form from darkened leaves. At low concentrations of phosphoenolpyruvate, the activity of the enzyme was strongly stimulated by glucose-6-phosphate, fructose-6-phosphate, triose-phosphate, alanine, serine, and glycine and was inhibited by organic acids. The enzyme was assayed in mixtures of metabolites at concentrations believed to be present in the mesophyll cytosol in the light and in the dark. It displayed low activity in a simulated `dark' cytosol and high activity in a simulated `light' cytosol, but activities were different for the enzyme from darkened compared to illuminated leaves. 相似文献
18.
Light microscopic examination of leaf cross-sections showed that Flaveria brownii A. M. Powell exhibits Kranz anatomy, in which distinct, chloroplast-containing bundle sheath cells are surrounded by two types of mesophyll cells. Smaller mesophyll cells containing many chloroplasts are arranged around the bundle sheath cells. Larger, spongy mesophyll cells, having fewer chloroplasts, are located between the smaller mesophyll cells and the epidermis. F. brownii has very low CO 2 compensation points at different O 2 levels, which is typical of C 4 plants, yet it does show about 4% inhibition of net photosynthesis by 21% O 2 at 30°C. Protoplasts of the three photosynthetic leaf cell types were isolated according to relative differences in their buoyant densities. On a chlorophyll basis, the activities of phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase (carboxylation phase of C 4 pathway) were highest in the larger mesophyll protoplasts, intermediate in the smaller mesophyll protoplasts, and lowest, but still present, in the bundle sheath protoplasts. In contrast, activities of ribulose 1,5-bisphosphate carboxylase, other C 3 cycle enzymes, and NADP-malic enzyme showed a reverse gradation, although there were significant activities of these enzymes in mesophyll cells. As indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the banding pattern of certain polypeptides of the total soluble proteins from the three cell types also supported the distribution pattern obtained by activity assays of these enzymes. Analysis of initial 14C products in whole leaves and extrapolation of pulse-labeling curves to zero time indicated that about 80% of the CO 2 is fixed into C 4 acids (malate and aspartate), whereas about 20% of the CO 2 directly enters the C 3 cycle. This is consistent with the high activity of enzymes for CO 2 fixation by the C 4 pathway and the substantial activity of enzymes of the C 3 cycle in the mesophyll cells. Therefore, F. brownii appears to have some capacity for C 3 photosynthesis in the mesophyll cells and should be considered a C 4-like species. 相似文献
19.
Measurements of leaf gas exchange were conducted in situ for the C 3-C 4 intermediate plant Flaveria floridana. Leaves exhibited measurable CO 2 assimilation at atmospheric CO 2 concentrations as low as 20 μmol/mol. This result demonstrates that the low CO 2 compensation points observed in past studies of greenhouse-grown C 3-C 4 intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO 2 concentrations as low as 112 μmol/mol. The existence of near-maximum photosynthesis rates at such low intercellular CO 2 concentrations is interpreted as evidence for the existence of a CO 2-concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO 2 concentration as the leaf-to-air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C 3 plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C 3 species ( Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C 3-C 4 intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C 3 plants. 相似文献
20.
NADP:malic enzyme from corn ( Zea mays L.) leaves was purified by conventional techniques to apparent homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antibodies raised against this protein in rabbits were purified, coupled covalently to protein A-Sepharose CL-4B, and used as an immunoaffinity resin to purify the NADP:malic enzymes of the C 3 plants spinach ( Spinacia oleracea L.) and wheat ( Triticum aestivum L.), of the Crassulacean acid metabolism (CAM) plant Bryophyllum daigremontianum R. Hamed et Perr. de la Bathie and the C 4 plants corn, sugarcane ( Saccharum officinarum L.), and Portulaca grandiflora L. Such procedures yielded homogeneous protein preparations with a single protein band, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, except for P. grandiflora L. with two bands. The specific activities of the purified proteins ranged between 56 and 91 units (milligrams per protein). NADP:malic enzyme represented up to 1% of the total soluble protein in C 4 plants, 0.5% in the CAM plant, and less than 0.01% in C 3 plants. In immunotitration tests involving immunoprecipitation and immunoinhibition of activity by an antiserum against the corn leaf enzyme, the NADP:malic enzymes of corn and sugarcane showed virtually full identity of epitopes, while the NADP:malic enzymes of the C 3 and CAM plants exhibited a cross-reaction of one-twentieth and one-fourth by these tests, respectively. The NADP:malic enzyme of P. grandiflora exhibited characteristics more closely related to the enzymes of C 3 and CAM plants than to those of C 4 plants. 相似文献
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