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1.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
2.
A computer-controlled multichannel data acquisition system was employed to obtain continuous measurements of net nitrate or chlorate uptake by roots of intact barley plants ( Hordeum vulgare cv Betzes) using nitrate-specific electrodes. Plants, previously grown in solutions maintained at 10 or 200 micromolar NO 3− (low N or high N conditions, respectively), were provided with 200 micromolar NO 3− or ClO 3− during the uptake period. Initial rates of NO 3− uptake were several times higher in low N plants than in high N plants. Within 10 min, uptake in the former plants declined to a new steady rate which was sustained for the remainder of the experiment. No such time-dependent changes were evident in the high N plants. Rates and patterns of net chlorate uptake exhibited almost identical dependence upon previous nitrate provision. NO 3− ( 36ClO 3−) influx, by contrast, appeared to be independent of NO 3− pretreatment prior to influx determination. Nitrate efflux, estimated by several different methods, was strongly correlated with internal nitrate concentration of the roots. 相似文献
3.
Evidence is presented that chlorate is an extremely good analog for nitrate during nitrate uptake by intact barley ( Hordeum vulgare cv. Fergus) roots. The depletion of ClO 3− or NO 3− from uptake media over 2 to 6 hours by seedlings was found to be dependent on combined NO 3− plus ClO 3− concentrations, and total anion uptake was equivalent at different NO 3−/ClO 3− ratios. After loading barley seedlings with 36ClO 3− for 6 hours, kinetic parameters were derived from the analysis of efflux of [ 36Cl] chlorate into unlabeled solution. On the basis of this analysis, the half times for exchange for the cytoplasmic and vacuolar phases were 17 minutes and 20 hours, respectively. 相似文献
4.
Short-term (10 minutes) measurements of plasmalemma NO 3− influx ( oc) into roots of intact barley plants were obtained using 13NO 3−. In plants grown for 4 days at various NO 3− levels (0.1, 0.2, 0.5 millimolar), oc was found to be independent of the level of NO 3− pretreatment. Similarly, pretreatment with Cl − had no effect upon plasmalemma 13NO 3− influx. Plants grown in the complete absence of 13NO 3− (in CaSO 4 solutions) subsequently revealed influx values which were more than 50% lower than for plants grown in NO 3−. Based upon the documented effects of NO 3− or Cl − pretreatments on net uptake of NO 3−, these observations suggest that negative feedback from vacuolar NO 3− and/or Cl − acts at the tonoplast but not at the plasmalemma. When included in the influx medium, 0.5 millimolar Cl − was without effect upon 13NO 3− influx, but NH 4+ caused approximately 50% reduction of influx at this concentration. 相似文献
5.
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
6.
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
7.
Experiments with intact plants of Lolium perenne previously grown with 14NO 3− revealed significant efflux of this isotopic species when the plants were transferred to solutions of highly enriched 15NO 3−. The exuded 14NO 3− was subsequently reabsorbed when the ambient solutions were not replaced. When they were frequently replaced, continual efflux of the 14NO 3− was observed. Influx of 15NO 3− was significantly greater than influx of 14NO 3− from solutions of identical NO 3− concentration. Transferring plants to 14NO 3− solutions after a six-hour period in 15NO 3− resulted in efflux of the latter. Presence of Mg 2+, rather than Ca 2+, in the ambient 15NO 3− solution resulted in a decidedly increased rate of 14NO 3− efflux and a slight but significant increase in 15NO 3− influx. Accordingly, net NO 3− influx was slightly depressed. A model in accordance with these observations is presented; its essential features include a passive bidirectional pathway, an active uptake mechanism, and a pathway for recycling of endogenous NO 3− within unstirred layers from the passive pathway to the active uptake site. 相似文献
8.
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
9.
We report here on an investigation of net nitrate and proton fluxes in root cells of maize ( Zea mays L.) seedlings grown without (noninduced) and with (induced) 0.1 millimolar nitrate. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, WJ Lucas [1987] Plant Physiol 84: 1177-1184) was utilized to quantify net ionic fluxes from the measurement of electrochemical potential gradients for NO 3− and H + within the unstirred layer at the root surface. The nitrate-inducibility, pH dependence, and concentration dependence of net NO 3− uptake correlated quite closely with the electrical response of maize roots to nitrate under the same experimental conditions (as described in PR McClure, LV Kochian, RM Spanswick, JE Shaff [1990] Plant Physiol 93: 281-289). Additionally, it was found that potential inhibitors of the plasmalemma H +-ATPase (vandate, diethylstilbestrol), which were shown to abolish the electrical response to NO 3− (in PR McClure, LV Kochian, RM Spanswick, JE Shaff [1990] Plant Physiol 93: 281-289), dramatically inhibited NO 3− absorption. These results strongly indicate that the NO 3− electrical response is due to the operation of a NO 3− transport system in the plasmalemma of maize root cells. Furthermore, the results from the H +-ATPase inhibitor studies indicate that the NO 3− transport system is linked to the H +-ATPase, presumably as a NO 3−/H + symport. This is further supported by the pH response of the NO 3− transport system (inhibition at alkaline pH values) and the change in net H + flux from a moderate efflux in the absence of NO 3−, to zero net H + flux after exposing the maize root to exogenous nitrate. Although these results can be explained by other interpretations, the simplest model that fits both the electrical responses and the NO 3−/H + flux data is a NO 3−/H + symport with a NO 3−:H + flux stoichiometry >1, whose operation results in the stimulation of the H +-ATPase due to the influx of protons through the cotransport system. 相似文献
10.
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
11.
In soybean ( Glycine max L. Merr. cv Kingsoy), NO 3− assimilation in leaves resulted in production and transport of malate to roots (B Touraine, N Grignon, C Grignon [1988] Plant Physiol 88: 605-612). This paper examines the significance of this phenomenon for the control of NO 3− uptake by roots. The net NO 3− uptake rate by roots of soybean plants was stimulated by the addition of K-malate to the external solution. It was decreased when phloem translocation was interrupted by hypocotyl girdling, and partially restored by malate addition to the medium, whereas glucose was ineffective. Introduction of K-malate into the transpiration stream using a split root system resulted in an enrichment of the phloem sap translocated back to the roots. This treatment resulted in an increase in both NO 3− uptake and C excretion rates by roots. These results suggest that NO 3− uptake by roots is dependent on the availability of shoot-borne, phloem-translocated malate. Shoot-to-root transport of malate stimulated NO 3− uptake, and excretion of HCO 3− ions was probably released by malate decarboxylation. NO 3− uptake rate increased when the supply of NO 3− to the shoot was increased, and decreased when the activity of nitrate reductase in the shoot was inhibited by WO 42−. We conclude that in situ, NO 3− reduction rate in the shoot may control NO 3− uptake rate in the roots via the translocation rate of malate in the phloem. 相似文献
12.
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
13.
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
14.
Chloride or nitrate decreased a pH gradient (measured as [ 14C]methylamine accumulation) in tonoplast-enriched vesicles. The ΔpH decrease was dependent on the anion concentration. These effects are independent of the anion-sensitive H +-ATPase of the tonoplast, since the pH gradient (acid inside) was imposed artificially using a pH jump or a K + gradient and nigericin. 4,4′-Diisothiocyano-2,2′-stilbene disulfonic acid partially prevented the decrease in pH gradient induced by Cl −. Two possible models to account for this anion-dependent decrease of ΔpH are: (a) H + loss is accompanied by Cl − or NO 3− efflux from the vesicles via H +/anion symport systems on the tonoplast and (b) H + loss is accompanied by Cl − or NO 3− uptake into the vesicles via H +/anion antiport systems. Depending on the requirements and conditions of the cell, these two systems would serve to either mobilize Cl − and NO 3− stored in the vacuole for use in the cytoplasm or to drive anions into the vacuole. Chloride or nitrate also decreased a pH gradient in fractions containing plasma membrane and Golgi, implying that these membranes may have similar H +-coupled anion transport systems. 相似文献
15.
The regulation of NO 3− assimilation by xylem flux of NO 3− was studied in illuminated excised leaves of soybean ( Glycine max L. Merr. cv Kingsoy). The supply of exogenous NO 3− at various concentrations via the transpiration stream indicated that the xylem flux of NO 3− was generally rate-limiting for NO 3− reduction. However, NO 3− assimilation rate was maintained within narrow limits as compared with the variations of the xylem flux of NO 3−. This was due to considerable remobilization and assimilation of previously stored endogenous NO 3− at low exogenous NO 3− delivery, and limitation of NO 3− reduction at high xylem flux of NO 3−, leading to a significant accumulation of exogenous NO 3−. The supply of 15NO 3− to the leaves via the xylem confirmed the labile nature of the NO 3− storage pool, since its half-time for exchange was close to 10 hours under steady state conditions. When the xylem flux of 15NO 3− increased, the proportion of the available NO 3− which was reduced decreased similarly from nearly 100% to less than 50% for both endogenous 14NO 3− and exogenous 15NO 3−. This supports the hypothesis that the assimilatory system does not distinguish between endogenous and exogenous NO 3− and that the limitation of NO 3− reduction affected equally the utilization of NO 3− from both sources. It is proposed that, in the soybean leaf, the NO 3− storage pool is particularly involved in the short-term control of NO 3− reduction. The dynamics of this pool results in a buffering of NO 3− reduction against the variations of the exogenous NO 3− delivery. 相似文献
16.
Assimilation of NO 3− and NH 4+ by perennial ryegrass ( Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH 4+, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO 3− had been reduced, whereas 97% of the NH 4+ had been assimilated. During the early stages (0 to 8 hours) of NO 3− uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO 3−, NH 4+ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO 3− uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate. 相似文献
17.
Roots of higher plants are usually exposed to varying spatial and temporal changes in concentrations of soil mineral nitrogen. A split root system was used to see how Lolium multiflorum Lam. roots adapt to such variations to cope with their N requirements. Plants were grown in hydroponic culture with their root system split in two spatially separated compartments allowing them to be fed with or without KNO 3. Net NO 3
- uptake, 15NO 3
- influx and root growth were studied in relation to time. Within less than 24 h following deprivation of KNO 3 to half the roots, the influx in NO 3
- fed roots was observed to increase (about 200% of the influx measured in plant uniformly NO 3
- supplied control plant) thereby compensating the whole plant for the lack of uptake by the N deprived roots. Due to the large NO 3
- concentrations in the roots, the NO 3
- efflux was also increased so that the net uptake rate increased only slightly (35% maximum) compared with the values obtained for control plants uniformly supplied with NO 3
-. This increase in net NO 3
- uptake rate was not sufficient to compensate the deficit in N uptake rate of the NO 3
- deprived split root in the short term. Over a longer period (>1 wk), root growth of the part of the root system locally supplied with NO 3
- was stimulated. An increase in root growth was mainly responsable for the greater uptake of nitrate in Lolium multiflorum so that it was able to fully compensate the deficit in N uptake rate of the NO 3
- deprived split root. 相似文献
18.
13NO 3− was used to investigate patterns of NO 3− influx into roots of barley plants ( Hordeum vulgare L. cv Klondike) previously grown with (`induced') or without (`uninduced') a source of external NO 3− ([NO 3−] 0). In both induced and uninduced plants, 13NO 3− influx was biphasic in the range from 0.005 to 50 moles per cubic meter [NO 3−] 0. In the low concentration range (<1 mole per cubic meter for induced plants and <0.3 mole per cubic meter for uninduced plants), influx was saturable and Vmax and Km values for influx either increased or decreased according to NO 3− pretreatment. By contrast, 13NO 3− influx in the high concentration range revealed a strictly linear concentration dependence. These fluxes appeared to be mediated by a constitutive, rather than an inducible, transport system. 相似文献
19.
Ricinus communis L. was used to test the Dijkshoorn-Ben Zioni hypothesis that NO 3− uptake by roots is regulated by NO 3− assimilation in the shoot. The fate of the electronegative charge arising from total assimilated NO 3− (and SO 42−) was followed in its distribution between organic anion accumulation and HCO 3− excretion into the nutrient solution. In plants adequately supplied with NO 3−, HCO 3− excretion accounted for about 47% of the anion charge, reflecting an excess nutrient anion over cation uptake. In vivo nitrate reductase assays revealed that the roots represented the site of about 44% of the total NO 3− reduction in the plants. To trace vascular transport of ionic and nitrogenous constituents within the plant, the composition of both xylem and phloem saps was thoroughly investigated. Detailed dry tissue and sap analyses revealed that only between 19 and 24% of the HCO 3− excretion could be accounted for from oxidative decarboxylation of shoot-borne organic anions produced in the NO 3− reduction process. The results obtained in this investigation may be interpreted as providing direct evidence for a minor importance of phloem transport of cation-organate for the regulation of intracellular pH and electroneutrality, thus practically eliminating the necessity for the Dijkshoorn-Ben Zioni recycling process. 相似文献
20.
Nitrate influx, efflux and net nitrate uptake were measured for the slow-growing Quercus suber L. (cork-oak) to estimate the N-uptake efficiency of its seedlings when grown with free access to nitrate. We hypothesise
that nitrate influx, an energetically costly process, is not very efficiently controlled so as to avoid losses through efflux,
because Q. suber has relatively high respiratory costs for ion uptake. Q. suber seedlings were grown in a growth room in hydroponics with 1 mM NO 3
-. Seedlings were labelled with 15NO 3
- in nutrient solution for 5 min to measure influx and for 2 h for net uptake. Efflux was calculated as the difference between
influx and net uptake. Measurements were made in the morning, afternoon and night. The site of nitrate reduction was estimated
from the ratio of NO 3
- to amino acids in the xylem sap; the observed ratio indicated that nitrate reduction occurred predominantly in the roots.
Nitrate influx was always much higher than net acquisition and both tended to be lower at night. High efflux occurred both
during the day and at night, although the proportion of 15NO 3
- taken up that was loss through efflux was proportionally higher during the night. Efflux was a significant fraction of influx.
We concluded that the acquisition system is energetically inefficient under the conditions tested.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
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