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1.
An experiment was conducted to investigate the reduction of endogenous NO 3−, which had been taken up by plants in darkness, during the course of the subsequent light period. Vegetative, nonnodulated soybean plants ( Glycine max [L]. Merrill, `Ransom') were exposed to 1.0 millimolar 15NO 3− for 12 hours in darkness and then returned to a solution containing 1.0 millimolar 14NO 3− for the 12 hours `chase' period in the light. Another set of plants was exposed to 15NO 3− during the light period to allow a direct comparison of contributions of substrate from the endogenous and exogenous sources. At the end of the 15NO 3− exposure in the dark, 70% of the absorbed 15NO 3− remained unreduced, and 83% of this unreduced NO 3− was retained in roots. The pool of endogenous 15NO 3− in roots was depleted at a steady rate during the initial 9 hours of light and was utilized almost exclusively in the formation of insoluble reduced-N in leaves. Unlabeled endogenous NO 3−, which had accumulated in the root prior to the previous dark period, also was depleted in the light. When exogenous 15NO 3− was supplied during the light period, the rate of assimilation progressively increased, reflecting an increased rate of uptake and decreased accumulation of NO 3− in the root tissue. The dark-absorbed endogenous NO 3− in the root was the primary source of substrate for whole-plant NO 3− reduction in the first 6 hours of the light period, and exogenous NO 3− was the primary source of substrate thereafter. It is concluded that retention of NO 3− in roots in darkness and its release in the following light period is an important whole-plant regulatory mechanism which serves to coordinate delivery of substrate with the maximal potential for NO 3− assimilation in photosynthetic tissues. 相似文献
2.
An experiment was conducted to investigate the relative changes in NO 3− assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants ( Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar 15NO 3− for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of 15NO 3− decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous 15NO 3− assimilation processes, which were distinctly different than those in 15NO 3− uptake. During the extended dark period, translocation of absorbed 15N out of the root to the shoot varied rhythmically. The adjustments were independent of 15NO 3− uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of 15NO 3− always was limited more than uptake. The assimilation of 15N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. The plants clearly retained a capacity for 15NO 3− reduction and synthesis of insoluble reduced- 15N even when 15NO 3− uptake was severely restricted and minimal carbohydrate reserves remained in the tissue. 相似文献
3.
In vivo NO 3− reduction in roots and shoots of intact barley ( Hordeum vulgare L. var Numar) seedlings was estimated in light and darkness. Seedlings were placed in darkness for 24 hours to make them carbohydrate-deficient. During darkness, the leaves lost 75% of their soluble carbohydrates, whereas the roots lost only 15%. Detached leaves from these plants reduced only 7% of the NO 3− absorbed in darkness. By contrast, detached roots from the seedlings reduced the same proportion of absorbed NO 3−, as did roots from normal light-grown plants. The rate of NO 3− reduction in the roots accounted for that found in the intact dark-treated carbohydrate-deficient seedlings. The rates of NO 3− reduction in roots of intact plants were the same for approximately 12 hours, both in light and darkness, after which the NO 3− reduction rate in roots of plants placed in darkness slowly declined. In the dark, approximately 40% of the NO 3− reduction occurred in the roots, whereas in light only 20% of the total NO 3− reduction occurred in roots. A lesser proportion was reduced in roots because the leaves reduced more nitrate in light than in darkness. 相似文献
4.
Nitrate reduction was studied as a function of carbohydrate concentration in detached primary leaves of barley ( Hordeum vulgare L. cv Numar) seedlings under aerobic conditions in light and darkness. Seedlings were grown either in continuous light for 8 days or under a regimen of 16-hour light and 8-hour dark for 8 to 15 days. Leaves of 8-day-old seedlings grown in continuous light accumulated 4 times more carbohydrates than leaves of plants grown under a light and dark regimen. When detached leaves from these seedlings were supplied with NO 3− in darkness, those with the higher levels of carbohydrates reduced a greater proportion of the NO 3− that was taken up. In darkness, added glucose increased the percentage of NO 3− reduced up to 2.6-fold depending on the endogenous carbohydrate status of the leaves. Both NO 3− reduction and carbohydrate content of the leaves increased with age. Fructose and sucrose also increased NO 3− reduction in darkness to the same extent as glucose. Krebs cycle intermediates, citrate and succinate, did not increase NO 3− reduction, whereas malate slightly stimulated it in darkness. In light, 73 to 90% of the NO3− taken up was reduced by the detached leaves; therefore, an exogenous supply of glucose had little additional effect on NO3− reduction. The results indicate that in darkness the rate of NO3− reduction in primary leaves of barley depends upon the availability of carbohydrates. 相似文献
5.
An experiment was conducted to investigate alterations in uptake and assimilation of NO 3− by phosphorus-stressed plants. Young tobacco plants ( Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were deprived of an external phosphorus (P) supply for 12 days. On selected days, plants were exposed to 15NO 3− during the 12 hour light period to determine changes in NO 3− assimilation as the P deficiency progressed. Decreased whole-plant growth was evident after 3 days of P deprivation and became more pronounced with time, but root growth was unaffected until after day 6. Uptake of 15NO 3− per gram root dry weight and translocation of absorbed 15NO 3− out of the root were noticeably restricted in −P plants by day 3, and effects on both increased in severity with time. Whole-plant reduction of 15NO 3− and 15N incorporation into insoluble reduced-N in the shoot decreased after day 3. Although the P limitation was associated with a substantial accumulation of amino acids in the shoot, there was no indication of excessive accumulation of soluble reduced- 15N in the shoot during the 12 hour 15NO 3− exposure periods. The results indicate that alterations in NO 3− transport processes in the root system are the primary initial responses limiting synthesis of shoot protein in P-stressed plants. Elevated amino acid levels evidently are associated with enhanced degradation of protein rather than inhibition of concurrent protein synthesis. 相似文献
6.
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
7.
The effects of several photosynthetic inhibitors and uncouplers of oxidative phosphorylation on NO 3− and NO 2− assimilation were studied using detached barley ( Hordeum vulgare L. cv Numar) leaves in which only endogenous NO 3− or NO 2− were available for reduction. Uncouplers of oxidative phosphorylation greatly increased NO 3− reduction in both light and darkness, while photosynthetic inhibitors did not. The NO2− concentration in the control leaves was very low in both light and darkness; 98% or more of the NO2− formed from NO3− was further assimilated in control leaves. More NO2− accumulated in the leaves in light and darkness in the presence of photosynthetic inhibitors. Of this NO2−, 94% or more was further assimilated. It appears that metabolites, either external or internal to the chloroplast, capable of reducing NADP (which, in turn, could reduce ferredoxin via NADP reductase) might support NO2− reduction in darkness and light when photosynthetic electron flow is inhibited by photosynthetic inhibitors. Nitrite assimilation was much more sensitive to uncouplers in darkness than in light: in darkness, 74% or more of NO2− formed from NO3− was further assimilated, whereas in light, 95% or more of the NO2− was further assimilated. 相似文献
8.
The effect of the exogenous and endogenous NO 3− concentration on net uptake, influx, and efflux of NO 3− and on nitrate reductase activity (NRA) in roots was studied in Phaseolus vulgaris L. cv. Witte Krombek. After exposure to NO 3−, an apparent induction period of about 6 hours occurred regardless of the exogenous NO 3− level. A double reciprocal plot of the net uptake rate of induced plants versus exogenous NO 3− concentration yielded four distinct phases, each with simple Michaelis-Menten kinetics, and separated by sharp breaks at about 45, 80, and 480 micromoles per cubic decimeter. Influx was estimated as the accumulation of 15N after 1 hour exposure to 15NO3−. The isotherms for influx and net uptake were similar and corresponded to those for alkali cations and Cl−. Efflux of NO3− was a constant proportion of net uptake during initial NO3− supply and increased with exogenous NO3− concentration. No efflux occurred to a NO3−-free medium. The net uptake rate was negatively correlated with the NO3− content of roots. Nitrate efflux, but not influx, was influenced by endogenous NO3−. Variations between experiments, e.g. in NO3− status, affected the values of Km and Vmax in the various concentration phases. The concentrations at which phase transitions occurred, however, were constant both for influx and net uptake. The findings corroborate the contention that separate sites are responsible for uptake and transitions between phases. Beyond 100 micromoles per cubic decimeter, root NRA was not affected by exogenous NO3− indicating that NO3− uptake was not coupled to root NRA, at least not at high concentrations. 相似文献
9.
The supply of photosynthates by leaves for reproductive development in cotton ( Gossypium hirsutum L.) has been extensively studied. However, the contribution of assimilates derived from the fruiting forms themselves is inconclusive. Field experiments were conducted to document the photosynthetic and respiratory activity of cotton leaves, bracts, and capsule walls from anthesis to fruit maturity. Bracts achieved peak photosynthetic rates of 2.1 micromoles per square meter per second compared with 16.5 micromoles per square meter per second for the subtending leaf. However, unlike the subtending leaf, the bracts did not show a dramatic decline in photosynthesis with increased age, nor was their photosynthesis as sensitive as leaves to low light and water-deficit stress. The capsule wall was only a minor site of 14CO 2 fixation from the ambient atmosphere. Dark respiration by the developing fruit averaged −18.7 micromoles per square meter per second for 6 days after anthesis and declined to −2.7 micromoles per square meter per second after 40 days. Respiratory loss of CO 2 was maximal at −158 micromoles CO 2 per fruit per hour at 20 days anthesis. Diurnal patterns of dark respiration for the fruit were age dependent and closely correlated with stomatal conductance of the capsule wall. Stomata on the capsule wall of young fruit were functional, but lost this capacity with increasing age. Labeled 14CO 2 injected into the fruit interior was rapidly assimilated by the capsule wall in the light but not in the dark, while fiber and seed together fixed significant amounts of 14CO 2 in both the light and dark. These data suggest that cotton fruiting forms, although sites of significant respiratory CO 2 loss, do serve a vital role in the recycling of internal CO 2 and therein, function as important sources of assimilate for reproductive development. 相似文献
10.
The role of NO 3− and NO 2− in the induction of nitrite reductase (NiR) activity in detached leaves of 8-day-old barley ( Hordeum vulgare L.) seedlings was investigated. Barley leaves contained 6 to 8 micromoles NO 2−/gram fresh weight × hour of endogenous NiR activity when grown in N-free solutions. Supply of both NO 2− and NO 3− induced the enzyme activity above the endogenous levels (5 and 10 times, respectively at 10 millimolar NO 2− and NO 3− over a 24 hour period). In NO 3−-supplied leaves, NiR induction occurred at an ambient NO 3− concentration of as low as 0.05 millimolar; however, no NiR induction was found in leaves supplied with NO 2− until the ambient NO 2− concentration was 0.5 millimolar. Nitrate accumulated in NO 2−-fed leaves. The amount of NO 3− accumulating in NO 2−-fed leaves induced similar levels of NiR as did equivalent amounts of NO 3− accumulating in NO 3−-fed leaves. Induction of NiR in NO 2−-fed leaves was not seen until NO 3− was detectable (30 nanomoles/gram fresh weight) in the leaves. The internal concentrations of NO 3−, irrespective of N source, were highly correlated with the levels of NiR induced. When the reduction of NO 3− to NO 2− was inhibited by WO 42−, the induction of NiR was inhibited only partially. The results indicate that in barley leaves NiR is induced by NO 3− directly, i.e. without being reduced to NO 2−, and that absorbed NO 2− induces the enzyme activity indirectly after being oxidized to NO 3− within the leaf. 相似文献
11.
Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH 3, NO 3−, or N 2. Expressed as a function of relative growth rate, steady state cellular P contents and PO 4 assimilation rates did not vary with N-source. However, N-source did alter the maximal PO 4-limited growth rate achieved by the cultures: the NO 3− and N 2 cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH 3 grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO 3−-grown cells were the smallest (627 ± 34 micromoles C · 10 −9 cells), while NH 3-grown cells were largest (900 ± 44 micromoles C · 10 −9 cells) and N 2-fixing cells were intermediate (726 ± 48 micromoles C · 10 −9 cells) in size. In the NO 3−-and N 2-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH 3-grown cells. Heterocysts were absent in NH 3-grown cultures but were present in both the N 2 and NO 3− cultures. In the NO 3−-grown cultures C 2H 2 reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N 2-fixing cultures the acetylene:N 2 ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal. Compared with NH3, the assimilation of NO3− and N2 resulted either in a decrease in cellular C (NO3− and N2 cultures) or in a lower maximal growth rate (N2 culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO3− or N2 compared with NH3. 相似文献
12.
Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH 4+, NO 2−, NO 3−) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO 2 release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH 4+ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O 2 consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO 3− and NO 2− assimilation resulted in a larger stimulation of TCA cycle CO 2 release than did NH 4+, but a much smaller stimulation of mitochondrial O 2 consumption. NH 4+ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO 3− and NO 2− assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH 4+, NO 3− and NO 2− assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO 3− and NO 2− assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O 2 consumption during NO 3− and NO 2− assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO 3− and NO 2− reduction. 相似文献
13.
It is unclear if the relative content of NO 3− and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO 3− in the root. Experiments were conducted to determine whether NO 3− and reduced N in xylem exudate of vegetative, nonnodulated soybean plants ( Glycine max [L.] Merr., `Ransom') originated from exogenous recently absorbed 15NO 3− or from endogenous 14N pools. Plants either were decapitated and exposed to 15NO 3− solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to 15NO 3− in the dark and in the light. Considerable amounts of 14NO 3− and reduced 14N were transported into the xylem, but almost all of the 15N was present as 15NO 3−. Dissimilar changes in transport of 14NO 3−, reduced 14N and 15NO 3− during the 2 hours of sap collection resulted in large variability over time in the percentage of total N in the exudate which was reduced N. Over a 20-minute period the rate of 15N transport into the xylem of decapitated plants was only 21 to 36% of the 15N delivered to the shoot of intact plants. Based on the proportion of total 15N which was found as reduced 15N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed 15NO 3− was reduced in the root. This was much less than the 38 to 59% which would have been predicted from the relative content of total NO 3− and total reduced N in the xylem exudate. 相似文献
14.
Using 13NO 3−, effects of various NO 3− pretreatments upon NO 3− influx were studied in intact roots of barley ( Hordeum vulgare L. cv Klondike). Prior exposure of roots to NO 3− increased NO 3− influx and net NO 3− uptake. This `induction' of NO 3− uptake was dependent both on time and external NO 3− concentration ([NO 3−]). During induction influx was positively correlated with root [NO 3−]. In the postinduction period, however, NO 3− influx declined as root [NO 3−] increased. It is suggested that induction and negative feedback regulation are independent processes: Induction appears to depend upon some critical cytoplasmic [NO 3−]; removal of external NO 3− caused a reduction of 13NO 3− influx even though mean root [NO 3−] remained high. It is proposed that cytoplasmic [NO 3−] is depleted rapidly under these conditions resulting in `deinduction' of the NO 3− transport system. Beyond 50 micromoles per gram [NO 3−], 13NO 3− influx was negatively correlated with root [NO 3−]. However, it is unclear whether root [NO 3−] per se or some product(s) of NO 3− assimilation are responsible for the negative feedback effects. 相似文献
15.
Changes in the concentrations of NH 4+ and amides during the growth of suspension cultures of rose ( Rosa cv. Paul's Scarlet) cells were examined. When cells were grown in medium possessing only NO 3− as a nitrogen source, the concentrations of NH 4+ and amides increased to 4.0 × 10 −1 and 5.9 micromoles per gram fresh weight, respectively. The amounts of both constituents declined during the later stages of growth. When a trace amount of NH 4+ was added to the NO 3− base starting medium, the concentration of NH 4+ in the cells was increased to 7.0 × 10 −1 micromoles per gram fresh weight. 相似文献
16.
Uptake of NO 3− by nonnodulated soybean plants ( Glycine max L. Merr. cv Ransom) growing in flowing hydroponic culture at 22 and 14°C root temperatures was measured daily during a 31-day growth period. Ion chromatography was used to determine removal of NO 3− from solution during each 24-hour period. At both root-zone temperatures, rate of NO 3− uptake per plant oscillated with a periodicity of 3 to 5 days. The rate of NO 3− uptake per plant was consistently lower at 14°C than 22°C. The lower rate of NO 3− uptake at 14°C during the initial 5 to 10 days was caused by reduced uptake rates per gram root dry weight, but with time uptake rates per gram root became equal at 14 and 22°C. Thereafter, the continued reduction in rate of NO 3− uptake per plant at 14°C was attributable to slower root growth. 相似文献
17.
Growth chamber studies with soybeans ( Glycine max [L.] Merr.) were designed to determine the relative limitations of NO 3−, NADH, and nitrate reductase (NR) per se on nitrate metabolism as affected by light and temperature. Three NR enzyme assays (+NO 3−in vivo, −NO 3−in vivo, and in vitro) were compared. NR activity decreased with all assays when plants were exposed to dark. Addition of NO 3− to the in vivo NR assay medium increased activity (over that of the −NO 3−in vivo assay) at all sampling periods of a normal day-night sequence (14 hr-30 C day; 10 hr-20 C night), indicating that NO 3− was rate-limiting. The stimulation of in vivo NR activity by NO 3− was not seen in plants exposed to extended dark periods at elevated temperatures (16 hr-30 C), indicating that under those conditions, NO 3− was not the limiting factor. Under the latter condition, in vitro NR activity was appreciable (19 μmol NO 2− [g fresh weight, hr] −1) suggesting that enzyme level per se was not the limiting factor and that reductant energy might be limiting. 相似文献
18.
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
19.
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
20.
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
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