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1.
Bundle sheath chloroplasts have been isolated from Zea mays leaves by a procedure involving enzymic digestion of mechanically prepared strands of bundle sheath cells followed by gentle breakage and filtration. The resulting crude chloroplast preparation was enriched by Percoll density layer centrifugation to yield intact chloroplasts (about 20 micrograms chlorophyll per 10-gram leaf tissue) with high metabolic activities. Based on activities of marker enzymes in the chloroplast and bundle sheath cell extracts, the chloroplasts were essentially free of contamination by other organelles and cytoplasmic material, and were generally about 70% intact. Chlorophyll a/b ratios were high (about 10). With appropriate substrates these chloroplasts displayed high rates of malate decarboxylation, measured as pyruvate formation, and CO 2 assimilation (maximum rates approximately 5 and 3 micromoles per minute per milligram chlorophyll, respectively). These activities were light dependent, linear for at least 20 minutes at 30°C, and displayed highest rates at pH 8.0. High metabolic rates were dependent on addition of an exogenous source of carbon to the photosynthetic carbon reduction cycle (3-phosphoglycerate or dihydroxyacetone phosphate) and a nucleotide (ATP, ADP, or AMP), as well as aspartate. Generally, neither malate decarboxylation nor CO 2 assimilation occurred substantially in the absence of the other activity indicating a close relationship between these processes. Presumably, NADPH required for the photosynthetic carbon reduction cycle is largely supplied during the decarboxylation of malate by NADP-malic enzyme. The results are discussed in relation to the role of bundle sheath chloroplasts in C 4 photosynthesis by species of the NADP-malic enzyme type. 相似文献
2.
The present studies provide the first measurements of the resistance to diffusive flux of metabolites between mesophyll and bundle sheath cells of C 4 plants. Species examined were Panicum miliaceum, Urochloa panicoides, Atriplex spongiosa, and Zea mays. Diffusive flux of metabolites into isolated bundle sheath cells was monitored by following their metabolic transformation. Evidence was obtained that the observed rapid fluxes occurred via functional plasmodesmata. Diffusion constants were determined from the rate of transformation of limiting concentrations of metabolites via cytosolic enzymes with high potential velocities and favorable equilibrium constants. Values on a leaf chlorophyll basis ranged between 1 and 5 micromoles per minute per milligram of chlorophyll per millimolar gradient depending on the molecular weight of the metabolite and the source of bundle sheath cells. Diffusion of metabolites into these cells was unaffected by a wide variety of compounds including respiratory inhibitors, monovalent and divalent cations, and plant hormones, but it was interrupted by treatments inducing cell plasmolysis. The molecular weight exclusion limit for permeation of compounds into bundle sheath cells was in the range of 850 to 900. These cells provide an ideal system for the quantitative study of plasmodesmatal function. 相似文献
3.
Mesophyll cells and bundle sheath strands were isolated rapidly from leaves of the C 4 species Digitaria pentzii Stent. (slenderstem digitgrass) by a chopping and differential filtration technique. Rates of CO 2 fixation in the light by mesophyll and bundle sheath cells without added exogenous substrates were 6.3 and 54.2 micromoles of CO 2 per milligram of chlorophyll per hour, respectively. The addition of pyruvate or phosphoenolpyruvate to the mesophyll cells increased the rates to 15.2 and 824.6 micromoles of CO 2 per milligram of chlorophyll per hour, respectively. The addition of ribose 5-phosphate increased the rate for bundle sheath cells to 106.8 micromoles of CO 2 per milligram of chlorophyll per hour. These rates are comparable to those reported for cells isolated by other methods. The Km(HCO 3−) for mesophyll cells was 0.9 m m; for bundle sheath cells it was 1.3 m m at low, and 40 m m at higher HCO 3− concentrations. After 2 hours of photosynthesis by mesophyll cells in 14CO 2 and phosphoenolpyruvate, 88% of the incorporated 14C was found in organic acids and 0.8% in carbohydrates; for bundle sheath cells incubated in ribose 5-phosphate and ATP, more than 58% of incorporated 14C was found in carbohydrates, mainly starch, and 32% in organic acids. These findings, together with the stimulation of CO 2 fixation by phosphoenolpyruvate for mesophyll cells and by ribose 5-phosphate plus ATP for bundle sheath cells, and the location of phosphoenolpyruvate and ribulose bisphosphate carboxylases in mesophyll and bundle sheath cells, respectively, are in accord with the scheme of C 4 photosynthesis which places the Calvin cycle in the bundle sheath and C 4 acid formation in mesophyll cells. 相似文献
4.
Conditions for optimal CO 2 fixation and malate decarboxylation by isolated bundle sheath chloroplasts from Zea mays were examined. The relative rates of these processes varied according to the photosynthetic carbon reduction cycle intermediate provided. Highest rates of malate decarboxylation, measured as pyruvate formation, were seen in the presence of 3-phosphoglycerate, while carbon fixation was highest in the presence of dihydroxyacetone phosphate; only low rates were measured with added ribose-5-phosphate. Chloroplasts exhibited a distinct phosphate requirement and this was optimal at a level of 2 millimolar inorganic phosphate in the presence of 2.5 millimolar 3-phosphoglycerate, dihydroxyacetone phosphate, or ribose-5-phosphate. Malate decarboxylation and CO 2 fixation were stimulated by additions of AMP, ADP, or ATP with half-maximal stimulation occurring at external adenylate concentrations of about 0.15 millimolar. High concentrations (>1 millimolar) of AMP were inhibitory. Aspartate included in the incubation medium stimulated malate decarboxylation and CO 2 assimilation. In the presence of aspartate, the apparent Michaelis constant (malate) for malate decarboxylation to pyruvate by chloroplasts decreased from 6 to 0.67 millimolar while the calculated Vmax for this process increased from 1.3 to 3.3 micromoles per milligram chlorophyll. Aspartate itself was not metabolized. It was concluded that the processes mediating the transport of phosphate, 3-phosphoglycerate, and dihydroxyacetone phosphate transport on the one hand, and also of malate might differ from those previously described for chloroplasts from C 3 plants. 相似文献
5.
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
6.
The reductive carboxylic acid cycle, the autotrophic pathway of CO 2 assimilation in prokaryotes (photosynthetic and nonphotosynthetic autotrophic bacteria), was investigated in Chlamydomonas reinhardtii F-60, an algal mutant lacking a complete photosynthetic carbon reduction pathway (C 3) due to a deficiency in phosphoribulokinase. Evidence was obtained consistent with the presence of the reductive carboxylic acid cycle in F-60. This conclusion is based on the fact that: (a) acetate approximately doubled CO 2 fixation in whole cells (4 micromoles per milligram chlorophyll per hour) and in chloroplasts (32 nanomoles per milligram chlorophyll per hour); and (b) pyruvate synthase, α-ketoglutarate synthase, and ATP-citrate lyase, three indicators of the cycle, were found in cell-free extracts. 相似文献
7.
Intact mesophyll and bundle sheath chloroplasts wee isolated from the NADP-malic enzyme type C 4 plants maize, sorghum (monocots), and Flaveria trinervia (dicot) using enzymic digestion and mechanical isolation techniques. Bundle sheath chloroplasts of this C 4 subgroup tend to be agranal and were previously reported to be deficient in photosystem II activity. However, following injection of intact bundle sheath chloroplasts into hypotonic medium, thylakoids had high Hill reaction activity, similar to that of mesophyll chloroplasts with the Hill oxidants dichlorophenolindophenol, p-benzoquinone, and ferricyanide (approximately 200 to 300 micromoles O 2 evolved per mg chlorophyll per hour). In comparison to that of mesophyll chloroplasts, the Hill reaction activity of bundle sheath chloroplasts of maize and sorghum was labile and lost activity during assay. Bundle sheath chloroplasts of maize also exhibited some capacity for 3-phosphoglycerate dependent O 2 evolution (29 to 58 micromoles O 2 evolved per milligram chlorophyll per hour). Both the mesophyll and bundle sheath chloroplasts were equally effective in light dependent scavenging of hydrogen peroxide. The results suggest that both chloroplast types have noncyclic electron transport and the enzymology to reduce hydrogen peroxide to water. The activities of ascorbate peroxidase from these chloroplast types was consistent with their capacity to scavenge hydrogen peroxide. 相似文献
8.
Characteristics of C 4 photosynthesis were examined in young, mid-age, and mature leaves of Flaveria trinervia (an NADP-malic enzyme-type C 4 dicot). The turnover of [4- 14C] (malate plus aspartate) following a pulse with 14CO 2 was similar in leaves of different ages (apparent half-time of 18-25 seconds). However, the rate of 14CO 2 incorporation in mid-age leaves was about 1.5-fold higher than in young leaves, and about 2.5-fold higher than in mature leaves. The rate of 14CO 2 fixation was proportional to the total active pool of malate plus aspartate but was not correlated with the total photosynthetically derived inorganic carbon pool. The leaf's ability to concentrate inorganic carbon photosynthetically declined during leaf expansion, from 29 down to 7 nanomoles per milligram chlorophyll. Similarly, the active aspartate pool also declined during leaf expansion, from about 123 down to 20 nanomoles per milligram chlorophyll. Enhanced metabolism of aspartate to CO 2 and pyruvate in young leaves is suggested to facilitate the maintenance of high CO 2 levels in bundle sheath cells which are thought to have a higher conductance to CO 2. 相似文献
9.
We sought to characterize the inorganic carbon pool (CO 2 plus HCO 3−) formed in the leaves of C 4 plants when C 4 acids derived from CO 2 assimilation in mesophyll cells are decarboxylated in bundle sheath cells. The size and kinetics of labeling of this pool was determined in six species representative of the three metabolic subgroups of C 4 plants. The kinetics of labeling of the inorganic carbon pool of leaves photosynthesizing under steady state conditions in 14CO 2 closely paralleled those for the C-4 carboxyl of C 4 acids for all species tested. The inorganic carbon pool size, determined from its 14C content at radioactivity saturation, ranged between 15 and 97 nanomoles per milligram of leaf chlorophyll, giving estimated concentrations in bundle sheath cells of between 160 and 990 micromolar. The size of the pool decreased, together with photosynthesis, as light was reduced from 900 to 95 microeinsteins per square meter per second or as external CO 2 was reduced from 400 to 98 microliters per liter. A model is developed which suggests that the inorganic carbon pool existing in the bundle sheath cells of C 4 plants during steady state photosynthesis will comprise largely of CO 2; that is, CO 2 will only partially equlibrate with bicarbonate. This predominance of CO 2 is believed to be vital for the proper functioning of the C 4 pathway. 相似文献
10.
Photosynthetically active bundle sheath strands capable of assimilating up to 8 micromoles CO 2 per milligram chlorophyll per hour have been isolated from fully expanded leaves of Zea mays L. Mesophyll cell contamination of the preparations was negligible, as evidenced by light and electron microscopy and by a high ratio of chlorophyll a to chlorophyll b in the strands. Ribose 5-phosphate markedly stimulated the rate of photosynthetic 14CO 2 fixation by the isolated strands. In contrast, both pyruvate and phosphoenolpyruvate had a comparatively small stimulatory effect on bundle sheath 14CO 2 fixation. After 5 minutes of photosynthesis in 14C-bicarbonate, 95% of the incorporated 14C was found in compounds other than C 4-dicarboxylic acids, most notably in 3-phosphoglycerate and sugar phosphates. A similar distribution of 14C was observed in the presence of exogenous ribose 5-phosphate. Extracts of bundle sheath strands contained high specific activities of “malic” enzyme, phosphoglycolate phosphatase, hydroxypyruvate reductase, and ribulose 1,5-diphosphate carboxylase, whereas the specific activities of NADP +-malate dehydrogenase and phosphopyruvate carboxylase were extremely low. These results indicate that the Calvin cycle occurs in the bundle sheath cells of maize. 相似文献
11.
The rate of CO 2 assimilation and levels of metabolites of the C 4 cycle and reductive pentose phosphate pathway in attached leaves of maize ( Zea mays L.) were measured over a range of light intensity from 0 to 1,900 microEinsteins per square meter per second under a saturated CO 2 concentration of 350 microliters per liter and a limiting CO 2 concentration of 133 microliters per liter. The level of ribulose 1,5-bisphosphate (RuBP) stayed almost constant (around 60 nanomoles per milligram chlorophyll [Chl]) from low to high light intensities under 350 microliters per liter. Levels of 3-phosphoglycerate (PGA) increased from 100 to 650 nanomoles per milligram Chl under 350 microliters per liter CO 2 with increasing light intensity. The calculated RuBP concentration of 6 millimolar (corresponded to 60 nanomoles per milligram Chl) was about two times above the estimated RuBP binding-site concentration on ribulose bisphosphate carboxylase-oxygenase (Rubisco) of ~2.6 millimolar in maize bundle sheath chloroplasts in the light. The ratio of RuBP/PGA increased with decreasing light intensity under 350 microliters per liter CO 2. These results suggest that RuBP carboxylation is under control of light intensity possibly due to a limited supply of CO 2 to Rubisco through the C 4 cycle whose activity is highly dependent on light intensity. Pyruvate level increased with increasing light intensity as long as photosynthesis rate increased. A positive relationship between levels of PGA and those of pyruvate during steady-state photosynthesis under various conditions suggests that an elevated concentration of PGA increases the carbon input into the C 4 cycle through the conversion of PGA to PEP and consequently the level of total intermediates of the C 4 cycle can be raised to mediate higher photosynthesis rate. 相似文献
12.
A system has been developed for the isolation of photosynthetically active chloroplasts from leaves of Populus deltoides. A high proportion of the chloroplasts appeared intact. The maximum rates of different photosynthetic processes were as follows: CO 2 fixation 3.5 micromoles per milligram chlorophyll per hour, noncyclic ATP synthesis 10 micromoles per milligram chlorophyll per hour, and cyclic ATP synthesis 300 micromoles per milligram chlorophyll per hour. 相似文献
13.
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
14.
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
15.
Young expanding spinach leaves exposed to 14CO 2 under physiological conditions for up to 20 minutes assimilated CO 2 into lipids at a mean rate of 7.6 micromoles per milligram chlorophyll per hour following a lag period of 5 minutes. Label entered into all parts of the lipid molecule and only 28% of the 14C fixed into lipids was found in the fatty acid moieties, i.e. fatty acids were synthesized from CO 2in vivo at a mean rate of 2.1 micromoles per milligram chlorophyll per hour. Intact spinach chloroplasts isolated from these leaves incorporated H 14CO 3 into fatty acids at a maximal rate of 0.6 micromole per milligram chlorophyll per hour, but were unable to synthesize either the polar moieties of their lipids or polyunsaturated fatty acids. Since isolated chloroplasts will only synthesize fatty acids at rates similar to the one obtained with intact leaves in vivo if acetate is used as a precursor, it is suggested that acetate derived from leaf mitochondria is the physiological fatty acid precursor. 相似文献
16.
Mass spectrometric measurements of dissolved free 13CO 2 were used to monitor CO 2 uptake by air grown (low CO 2) cells and protoplasts from the green alga Chlamydomonas reinhardtii. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of external carbonic anhydrase reduced the 13CO 2 concentration in the medium to close to zero. Similar results were obtained with low CO 2 cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO 2 uptake revealed that the removal of CO 2 from the medium in the light was due to selective and active CO 2 transport rather than uptake of total dissolved inorganic carbon. In the light, low CO 2 cells and protoplasts incubated with carbonic anhydrase took up CO 2 at an apparently low rate which reflected the uptake of total dissolved inorganic carbon. No net CO 2 uptake occurred in the dark. Measurement of chlorophyll a fluorescence yield with low CO 2 cells and washed protoplasts showed that variable fluorescence was mainly influenced by energy quenching which was reciprocally related to photosynthetic activity with its highest value at the CO 2 compensation point. During the linear uptake of CO 2, low CO 2 cells and protoplasts incubated with carbonic anhydrase showed similar rates of net O 2 evolution (102 and 108 micromoles per milligram of chlorophyll per hour, respectively). The rate of net O 2 evolution (83 micromoles per milligram of chlorophyll per hour) with washed protoplasts was 20 to 30% lower during the period of rapid CO 2 uptake and decreased to a still lower value of 46 micromoles per milligram of chlorophyll per hour when most of the free CO 2 had been removed from the medium. The addition of carbonic anhydrase at this point resulted in more than a doubling of the rate of O 2 evolution. These results show low CO 2 cells of Chlamydomonas are able to transport both CO 2 and HCO 3− but CO 2 is preferentially removed from the medium. The external carbonic anhydrase is important in the supply to the cells of free CO 2 from the dehydration of HCO 3−. 相似文献
17.
Previously, C Baysdorfer and JM Robinson (1985 Plant Physiol 77: 318-320) demonstrated that, in a reconstituted spinach chloroplast system, NADP photoreduction functioning at most maximal rate and reductant demand, was the successful competitor with NO 2− photoreduction for reduced ferredoxin. This resulted in a repression of NO 2− reduction until all NADP available had been almost totally reduced. Further experiments, employing isolated, intact spinach leaf plastids and soybean leaf mesophyll cells, were conducted to examine competition for reductant between CO 2 and NO 2− photoassimilation, in situ. In isolated, intact plastid preparations, regardless of whether the demand for reductant by CO 2 photoassimilation was high (5 millimolar `CO 2') with rates of CO 2 fixation in the range 40 to 90 micromoles CO 2 fixed per hour per milligram chlorophyll, low (0.5 millimolar `CO 2') with rates in the range 5 to 8 micromoles CO 2 per hour per milligram chlorophyll, or zero (no `CO 2'), NO 2− photoreduction displayed equal rates in the range of 8 to 22 micromoles per hour per milligram chlorophyll. In the absence of `CO 2', but in the presence of saturating white light, 3-phosphoglycerate photoreduction at rates of 82 to 127 micromoles per hour per milligram chlorophyll did not repress, and occasionally stimulated concomitant rates of NO 2− reduction which ranged from 23.4 to 38.5. Conversely, in plastid preparations, NO 2− at levels of 50 to 100 micromolar, stimulated plastid CO 2 fixation when `CO 2' was saturating with respect to carboxylation. Further, levels of NO 2− in the range 250 to 2500 micromolar, stimulated soybean leaf mesophyll cell net CO 2 fixation as much as 1.5-fold if `CO 2' was saturating with respect to CO 2 fixation. It appeared likely that, in high light in vivo, CO 2 and NO 2− photoassimilatory processes are not forced to intercompete for reduced ferredoxin in the intact chloroplast. 相似文献
18.
The gas exchange of spinach plants, salt-stressed by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar, was studied 3 weeks after starting NaCl treatment. Photosynthesis became light saturated at 1100 to 1400 micromoles per square meter per second in salt-treated plants and at approximately 2000 micromoles per square meter per second in control plants. Photosynthetic capacity of the mesophyll measured as a function of intercellular partial pressure of CO 2 at the light intensity prevailing during growth and at light saturation were both decreased in the salttreated plants. The CO 2 compensation points and relative enhancements of photosynthesis at low O 2 were not affected by salinity. The lower photosynthetic rates in salt-treated leaves at 450 micromoles per square meter per second were associated with a 70% reduction in stomatal conductance and low intercellular CO 2 (219 microbars; cf. 285 microbars for controls). Increasing photon flux density to light saturation extended the linear portions of the CO 2 response curves, increased stomatal conductances, increased intercellular CO 2 in the salt-treated plants, but lowered it in controls, and accentuated differences in photosynthetic rate (area basis) between the treatments. Leaves from salt-treated plants were thicker but contained about 73% of the chlorophyll per unit area of control plants. When photosynthetic rates were expressed on a chlorophyll basis there was no difference in initial slope of assimilation versus intercellular CO2 between treatments. Photosynthetic rates (chlorophyll basis) at light saturation differed only by 20% which was also observed earlier with isolated, intact chloroplasts (Robinson et al. 1983 Plant Physiol 73: 238-242). Measurement of carbon isotope ratio revealed less discrimination against 13C with salt treatment and confirmed the persistence of low intercellular partial pressures of CO2 during plant growth. The development of a thicker leaf with less chlorophyll per unit area during salt treatment permitted stomatal conductance and intercellular partial pressure of CO2 to decline without restricting photosynthesis and had the benefit of greatly increasing water use efficiency. 相似文献
19.
The effects of nitrate and ammonium addition on net and gross photosynthesis, CO 2 efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO 2 efflux in the light. The nitrate and ammmonium induced rates of CO 2 efflux were 31.0 and 33.8 micromoles CO 2 per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO 2 was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279). 相似文献
20.
The effects of phosphoenolpyruvate (PEP), inorganic phosphate (Pi), and ATP on 3-phosphoglycerate (PGA)-dependent O 2 evolution by chloroplasts of Digitaria sanguinalis (L.) Scop. (crabgrass) were evaluated relative to possible mechanisms of PEP transport by the C 4 mesophyll chloroplast. Crude and Percoll purified chloroplast preparations exhibited rates of PGA-dependent O 2 evolution in the range of 90 to 135 micromoles O 2 per milligram chlorophyll per hour, and up to 180 micromoles O 2 per milligram chlorophyll per hour at optimal Pi concentrations (approximately 0.2 millimolar at 9 millimolar PGA). Higher concentrations of Pi were inhibitory. PEP inhibited O 2 evolution (up to 70%) in both chloroplast preparations when the PEP to PGA ratio was high ( i.e. 9 millimolar PEP to 0.36 millimolar PGA). Usually no inhibition was seen when the PEP to PGA ratio was less than 2. PEP acted as a competitive inhibitor and, at a concentration of 9 millimolar, increased the apparent K m (PGA) from 0.15 to 0.53 millimolar in Percoll purified chloroplasts. A low concentration of PGA and high ratio of PEP to PGA, which are considered unphysiological, were required to detect any inhibition of O 2 evolution by PEP. Similar results were obtained from crude versus Percoll purified preparations. Neither the addition of Pi nor ATP could overcome PEP inhibition. As PEP inhibition was competitive with respect to PGA concentration, and as addition of ATP or Pi could not prevent PEP inhibition of PGA-dependent O 2 evolution, the inhibition was not due to PEP exchange of adenylates or Pi out of the chloroplast. Analysis of the effect of Pi and PEP, separately and in combination, on PGA-dependent O 2 evolution suggests interactions between PEP, Pi, and PGA on the same translocator in the C 4 mesophyll chloroplast. C 3 spinach chloroplasts were also found to be sensitive to PEP, but to a lesser extent than crabgrass chloroplasts. The apparent K i values (PEP) were 3 and 21 millimolar for crabgrass and spinach, respectively. 相似文献
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