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1.
Potential competition between CO 2 and NO 2− photoassimilation for photogenerated reductant ( e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature `source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO 2 and/or NO 2− to produce the highest demand for reductant by CO 2 and/or NO 2− assimilatory processes (enzymes). Even in the presence of CO 2 fixation, NO 2− reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO 2 assimilation. Regardless of the magnitude of the incident light energy, CO 2 assimilation did not repress NO 2− reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO 2 as well as with inorganic nitrogen photoassimilation. 相似文献
2.
Light-dependent O 2 reduction concomitant with O 2 evolution, ATP formation, and NADP reduction were determined in isolated spinach ( Spinacia oleracea L. var. America) chloroplast lamellae fortified with NADP and ferredoxin. These reactions were investigated in the presence or absence of catalase, providing a tool to estimate the reduction of O 2 to H 2O 2 (Mehler reaction) concomitant with NADP reduction. In the presence of 250 micromolar O 2, O 2 photoreduction, simultaneous with NADP photoreduction, was dependent upon light intensity, ferredoxin, Mn 2+, NADP, and the extent of coupling of phosphorylation to electron flow. In the presence of an uncoupling concentration of NH4+, saturating light intensity (>500 watts/square meter), saturating ferredoxin (10 micromolarity) rate-limiting to saturating NADP (0.2-0.9 millimolarity), and Mn2+ (50-1000 micromolarity), the maxium rates of O2 reduction were 13-25 micromoles/milligram chlorophyll per hour, while concomitant rates of O2 evolution and NADP reduction were 69 to 96 and 134 to 192 micromoles/milligram chlorophyll per hour, respectively. Catalase did not affect the rate of NADPH or ATP formation but decreased the NADPH:O2 ratios from 2.3-2.8 to 1.9-2.1 in the presence of rate-limiting as well as saturating concentrations of NADP. Photosynthetic electron flow at a rate of 31 micromoles O2 evolved/milligram chlorophyll per hour was coupled to the synthesis of 91 micromoles ATP/milligram chlorophyll per hour, while the concomitant rate of O2 reduction was 0.6 micromoles/milligram chlorophyll per hour and was calculated to be associated with an apparent ATP formation of only 2 micromoles/milligram chlorophyll per hour. Thus, electron flow from H2O to O2 did not result in ATP formation significantly above that produced during NADP reduction. 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.
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
5.
Leaf area, chlorophyll content, net CO 2 photoassimilation, and the partitioning of fixed carbon between leaf sucrose and starch and soluble protein were examined in Glycine max (L) Merr. cv Williams grown under three different nitrogen regimes. One group (Nod+/+) was inoculated with Bradyrhizobium and watered daily with a nutrient solution containing 6 millimolar NH 4NO 3. A second set (Nod+/−) was inoculated and had N 2 fixation as its sole source of nitrogen. A third group (Nod −) was not inoculated and was watered daily with a nutrient solution containing 6 millimolar NH 4NO 3. The mean net micromole CO 2 uptake per square decimeter per hour of the most recently matured source leaves was similar among the three groups of plants, being about 310. Mean leaf area of the source leaves, monitored for net photosynthesis was also similar. However, the mean milligram of chlorophyll per square decimeter of Nod+/− test leaves was about 50% lower than the other groups' leaves and indicated nitrogen deficiency. Thus, Nod+/− utilized their chlorophyll more efficiently for photosynthetic CO 2 uptake than the plants of the other treatments. The ratio of foliar carbohydrate:protein content was high in Nod+/− but low in the plants from the other two treatments. This inverse relationship between foliar protein and carbohydrate content suggests that more fixed carbon is diverted to the synthesis of protein when nitrogen availability is high. It was also found that Nod+/− sequestered more storage protein in their paraveinal mesophyll than plants of the other treatments. This study indicates that when inorganic nitrogen regimes are used to control photosynthate partitioning, then both leaf carbohydrate and leaf protein must be considered as end products of carbon assimilate allocation. 相似文献
6.
Chloroplasts isolated from synchronous cultures of the unicellular green alga Chlamydomonas reinhardii, SAG 11-32/b (−), fix CO 2 at rates between 25 and 50 micromoles per milligram chlorophyll per hour. The upper value is approximately half of the rate of the intact cell. During storage in the dark on ice, the chloroplast preparation loses 30 to 50% of its CO2 fixing capability per hour. Under reducing conditions (+ 1 millimolar dithiothreitol), this loss of activity is about twice as fast. The same reducing conditions stimulate CO2 fixation in the light. High concentrations of inorganic phosphate (>2 millimolar) inhibit CO2 fixation. This inhibition is overcome by the addition of glycerate 3-phosphate. It is concluded that chloroplasts from C. reinhardii possess a higher plant type phosphate translocator. With respect to dependency upon light intensity, pH and Mg2+ concentration, the results were similar to that reported for chloroplasts from higher plants. However, in contrast to higher plant chloroplasts, maximum CO2 fixation is observed at the relatively low osmotic concentration of 0.12 molar mannitol in the reaction buffer. 相似文献
7.
Isolated spinach ( Spinacia oleracea L. var. Bloomsdale) leaf protoplasts reduced nitrate at rates of 9 micromoles per milligram chlorophyll per hour in light with a 3- to 4-fold stimulation in the presence of HCO 3−. A similar stimulation of nitrate reduction in the absence of CO 2 fixation was obtained by the addition of malate, oxaloacetate (OAA), phospho-3-glyceric acid (PGA), or dihydroxyacetone phosphate (DHAP). Stimulation by malate and DHAP was light-independent, while the PGA and OAA effect was light-dependent. Nitrate reduction was found to be coupled to the cytoplasmic oxidation of DHAP or malate. The PGA/DHAP and OAA/malate shuttle across the chloroplast envelope has been demonstrated to support CO 2 fixation and/or nitrate reduction. The leaf protoplasts readily assimilated nitrate into amino-N in a stoichiometric relationship. 相似文献
8.
Soybean ( Glycine max L. Merr. cv Bragg) was grown throughout its life cycle at 330, 450, and 800 microliters CO 2 per liter in outdoor controlled-environment chambers under solar irradiance. Leaf ribulose-1,5-bisphosphate carboxylase (RuBPCase) activities and ribulose-1,5-bisphosphate (RuBP) levels were measured at selected times after planting. Growth under the high CO 2 levels reduced the extractable RuBPCase activity by up to 22%, but increased the daytime RuBP levels by up to 20%. Diurnal measurements of RuBPCase (expressed in micromoles CO2 per milligram chlorophyll per hour) showed that the enzyme values were low (230) when sampled before sunrise, even when activated in vitro with saturating HCO3− and Mg2+, but increased to 590 during the day as the solar quantum irradiance (photosynthetically active radiation or PAR, in micromoles per square meter per second) rose to 600. The nonactivated RuBPCase values, which averaged 20% lower than the corresponding HCO3− and Mg2+-activated values, increased in a similar manner with increasing solar PAR. The per cent RuBPCase activation (the ratio of nonactivated to maximum-activated values) increased from 40% before dawn to 80% during the day. Leaf RuBP levels (expressed in nanomoles per milligram chlorophyll) were close to zero before sunrise but increased to a maximum of 220 as the solar PAR rose beyond 1200. In a chamber kept dark throughout the morning, leaf RuBPCase activities and RuBP levels remained at the predawn values. Upon removal of the cover at noon, the HCO3− and Mg2+-activated RuBPCase values and the RuBP levels rose to 465 and 122, respectively, after only 5 minutes of leaf exposure to solar PAR at 1500. These results indicate that, in soybean leaves, light may exert a regulatory effect on extractable RuBPCase in addition to the well-established activation by CO2 and Mg2+. 相似文献
9.
Conditions are described for isolating functional phycobilisome-thylakoid vesicles from the red alga Porphyridium cruentum. Phycobilisome-thylakoid vesicles were prepared by brief sonication and centrifugation in a medium containing 0.5 molar sucrose, 0.5 molar potassium phosphate, and 0.3 molar sodium citrate (pH 7.0). They required ferricyanide as an oxidant and had O 2 evolution rates (about 450 micromoles O 2 per hour per milligram chlorophyll) higher than whole cells (about 250 micromoles O 2 per hour per milligram chlorophyll). Energy transfer to photosystem II chlorophyll was evident from a high F695 nanometer (−196 C) emission peak. Preparations could be stored for over 24 hours and were considerably more stable than those from the cyanobacterium Anabaena variabilis (Katoh T, E Gantt 1979 Biochim Biophys Acta 546: 383-393). In electron micrographs of negatively stained material, the active thylakoid vesicles were found covered by closely spaced phycobilisomes on their external surface. The phycobilisome number in negatively stained vesicles was 450 per square micrometer, which was in the same range as the 400 per square micrometer observed in surface sections. A cell containing 1.5 × 10 −6 micrograms phycoerythrin and 1.3 × 10 −6 micrograms chlorophyll was found to contain 5 to 7 × 10 5 phycobilisomes on a thylakoid area of 1.1 to 1.6 × 10 3 square micrometers. 相似文献
10.
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
11.
A problem often encountered when assaying mesophyll cell isolates prepared from mature soybean leaves, was that of poor reproducibility in rates of net 14CO 2 photoassimilation and NO 2
– photoreduction. It was known that soybean source leaves repeatedly displayed their most active net CO 2 photoassimilation in the period from attainment of maximal leaf area to approximately two to five days subsequent to that point. Advantage was taken of the fact that when soybean leaflets of each leaf reach their maximal area they also have reached their maximal leaf length from base to tip. This facilitates a more rapid determination of the point in time in which leaflet areas had reached A max. Soybean plants ( Glycine max cv. Williams) were propagated in the growth chamber with a 12 h light-12 h dark cycle, 25C, 65% RH, and 700 microeinsteins per meter squared per second. At 24 d post-emergence, the third leaf (numbered acropetally from the unifoliates) of each plant had just attained maximum leaflet areas (110 cm 2) and lengths (13 cm). For this study, leaf mesophyll cells were enzymatically isolated, using commercially prepared pectinase, from leaflet sets of leaves selected from each of the second, third, and fourth leaf positions. Maximal rates of net 14CO 2 photoassimilation (with 5 mM HCO 3
–) for the second, third and fourth leaf (leaflet) isolates were, respectively, 27.0, 57.0, and 41.7 mol 14CO 2 assimilated per milligram chlorophyll per hour; simultaneously maximal rates of NO
inf2
sup–
photoreduction (1 mM NO
inf2
sup–
) were, respectively, 4.4, 8.1, and 0.0 mol NO
inf2
sup–
reduced per milligram chlorophyll per hour. These studies made it clear that in order repeatedly to attain reproducible maximal rates of leaf cell isolate net 14CO 2 photoassimilation and NO
inf2
sup–
photoreduction, it always was necessary to select the newest, fully expanded leaves (e.g. leaf number 3) for cell isolation. Leaves from several plants only were pooled if they were excised from identically the same node on each of the plants.Abbreviations A max -
maximum leaflet (trifoliolate) area attained during ontogeny
- CO 2 -
CO 2 gas dissolved in solution
- HCO
inf3
sup–
-
bicarbonate
- L max -
maximum leaf blade length (midvein) attained during ontogeny
- NiRase -
chloroplast nitrite reductase (reduced ferredoxin)
- NiPR -
nitrite photoreduction
- PE -
post-emergence
- Pn -
net CO 2 photoassimilation (for leaflets and mesophyll cell isolates)
- PPRC -
pentose phosphate reductive cycle 相似文献
12.
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
13.
A mass spectrometric method combining 16O/ 18O and 12C/ 13C isotopes was used to quantify the unidirectional fluxes of O 2 and CO 2 during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O 2 uptake and CO 2 evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O 2 (61 micromoles of O 2 per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O 2 per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO 2 in darkness at a rate of 27 micromoles of CO 2 per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O 2 evolution and CO 2 fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phospho enolpyruvate carboxylase activity. Under light, CO 2 evolution by guard cell protoplasts was sharply decreased (37%), while O 2 uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO 2 assimilation and may be dissipated for other purposes such as ion uptake. 相似文献
14.
An experiment was conducted to determine the extent that NO 3− taken up in the dark was assimilated and utilized differently by plants than NO 3− taken up in the light. Vegetative, nonnodulated soybean plants ( Glycine max L. Merrill, `Ransom') were exposed to 15NO 3− throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing 14NO 3−, with plants sampled subsequently at each light/dark transition over 3 days. The rates of 15NO 3− absorption were nearly equal in the light and dark (8.42 and 7.93 micromoles per hour, respectively); however, the whole-plant rate of 15NO 3− reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). The lower rate of reduction in the dark was associated with both substantial retention of absorbed 15NO 3− in roots and decreased efficiency of reduction of 15NO 3− in the shoot. The rate of incorporation of 15N into the insoluble reduced-N fraction of roots in darkness (1.10 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant 15NO 3− reduction in darkness. A large portion of the 15NO3− retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of 15NO3− acquired during the light period. Taking into account reduction of NO3− from all endogenous pools, it was apparent that plant reduction in a given light period (~13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO3− (8.42 micromoles per hour) during that period. The primary source of substrate for NO3− reduction in the dark was exogenous NO3− being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO3− in the light occurred in the root system. 相似文献
15.
Aerobic and anaerobic chloroplastic respiration was monitored by measuring 14CO 2 evolution from [ 14C]glucose in the darkened spinach ( Spinacia oleracea) chloroplast and by estimating the conversion of fructose 1,6-bisphosphate to glycerate 3-phosphate in the darkened spinach chloroplast in air with O 2 or in N 2 with nitrite or oxaloacetate as electron acceptors. The pathway of 14CO 2 evolution from labeled glucose in the absence and presence of the inhibitors iodoacetamide and glycolate 2-phosphate under air or N 2 were those expected from the oxidative pentose phosphate cycle and glycolysis. Of the electron acceptors, O 2 was the best (2.4 nanomoles CO 2 per milligram chlorophyll per hour), followed by nitrite and oxaloacetate. With respect to glycerate 3-phosphate formation from fructose 1,6-bisphosphate, methylene blue increased the aerobic rate from 3.7 to 5.4 micromoles per milligram chlorophyll per hour. A rate of 4.8 micromoles per milligram chlorophyll per hour was observed under N 2 with nitrite and oxaloacetate. 相似文献
16.
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
17.
Photosynthetic properties of photoautotrophic suspensions cultured in a minimal growth medium have been evaluated to determine whether changes have occurred in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenol-pyruvate (PEP) carboxylase activity, chlorophyll content, or culture growth. Five photoautotrophic lines Amaranthus powellii, Datura innoxia, Glycine max, Gossypium hirsutum, and a Nicotiana tabacum-Nicotiana glutinosa fusion hybrid were grown in a medium without organic carbon other than phytohormones, and without vitamins. These photoautotrophic lines had total Rubisco activities ranging from 85 to 266 micromoles CO 2 fixed per milligram chlorophyll hour −1, with percent activation of Rubisco ranging from 16 to 53%. Inclusion of protease inhibitors in the homogenization buffer did not result in higher Rubisco activity. PEP carboxylase activity for cells cultured in minimal medium was found to range from 16 to 146 micromoles CO 2 per milligram chlorophyll hour −1, with no higher activity in the C 4Amaranthus cells compared with PEP carboxylase activity in the C 3 species assayed. Rubisco-to-PEP carboxylase ratios ranged from 2.2 to 1 up to 9.4 to 1. Chlorophyll contents increased in all but the Nicotiana cell line, and all of the photoautotrophic culture lines were capable of growth in vitamin-free medium with the exception of SB-P, which requires thiamine. 相似文献
18.
The flux of glycolate through the C 2 pathway in Chlamydomonas reinhardtii was estimated after inhibition of the pathway with aminooxyacetate (AOA) or aminoacetonitrile (AAN) by measurement of the accumulation of glycolate and glycine. Cells grown photoautotrophically in air excreted little glycolate except in the presence of 2 m m AOA when they excreted 5 micromoles glycolate per hour per milligram clorophyll. Cells grown on high CO 2 (1-5%) when transferred to air produced three times as much glycolate, with half of the glycolate metabolized and half excreted. The lower amount of glycolate produced by the air-grown cells reflects the presence of a CO 2 concentrating mechanism which raises the internal CO 2 level and decreases the ribulose-1,5-bisP oxygenase reaction for glycolate production. Despite the presence of the CO 2 concentrating mechanism, there was still a significant amount of glycolate produced and metabolized by air-grown Chlamydomonas. The capacity of these cells to metabolize between 5 and 10 micromoles of glycolate per hour per milligram chlorophyll was confirmed by measuring the biphasic uptake of added labeled glycolate. The initial rapid (<10 seconds) phase represented uptake of glycolate; the slow phase represented the metabolism of glycolate. The rates of glycolate metabolism were in agreement with those determined using the C 2-cycle inhibitors during CO 2 fixation. 相似文献
19.
Nitrate uptake by roots of cowpea ( Vigna unguiculata) was measured using 15NO 3−, and the energy cost to the root was estimated by respirometry. Roots of 8-day-old cowpea seedlings respired 0.6 to 0.8 milligram CO 2 per plant per hour for growth and maintenance. Adding 10 millimolar NO 3− to the root medium increased respiration by 20 to 30% during the following 6 hours. This increase was not observed if the shoots were in the dark. Removal of NO 3− from the root medium slowed the increase of root respiration. The ratios of additional respiration to the total nitrogen uptake and reduced nitrogen content in roots were 0.4 gram C per gram N and 2.3 grams C per gram N, respectively. The latter value is close to theoretical estimates of nitrate assimilation, and is similar to estimates of 1 to 4 grams C per gram N for the respiratory cost of symbiotic N 2 fixation. 相似文献
20.
It was hypothesized previously that an O 2 inhibition of NO 2− photoreduction would reflect a competition between O 2 and NO 2− for electrons from ferredoxin at the site of plastid nitrite reductase. In order to test this in vivo, intact spinach ( Spinacia oleracea L.) leaf chloroplast and mesophyll cell isolates held in high light were aerated with streams of 20% O 2/80% N 2 (250 micromolar O 2 in aqueous solution) or, alternatively, streams of 100% N 2. Bicarbonate plus CO 2 and NO 2− were supplied to reaction mixtures at levels just sufficient to promote maximal assimilations of CO 2 and NO 2−. In chloroplast isolates, there was a 9 to 30% O 2 inhibition of NO 2− reduction while there were high rates of CO 2 fixation. In spinach and soybean ( Glycine max) leaf cell isolates, NO 2− photoreduction rates were 10 to 55% inhibited by O 2 at near ambient levels. It is possible that O 2 may compete, albeit weakly, with NO 2− (nitrite reductase) for equivalents derived from reduced ferredoxin. Also, O 2 may oxidize sulfhydryl groups on nitrite reductase which are involved in substrate binding and/or activation. 相似文献
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