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1.
A practical calculation procedure to correct the underestimatecaused by isotope dilution in 15N uptake experiments is described.The estimate is based on the experimental observation of 15Nincorporation into particuUtc organic matter but not on directmeasurement of isotope enrichment in the substrate. Conventionaldata sets of 15N uptake and an estimate of the ratio of theflux of regeneration and uptake can provide the informationneeded to correct for isotope dilution. Application of thismethod to 15NH4+ uptake data in the literature showed that theunderestimate is small in open ocean waters but sometimes substantialin coastal or estuarine waters. 相似文献
2.
Short-term effects of rhizosphere microorganisms on fe uptake from microbial siderophores by maize and oat 总被引:3,自引:2,他引:1
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Effects of rhizosphere microorganisms on Fe uptake by oat (Avena sativa) and maize (Zea mays) were studied in short-term (10 h) nutrient solution experiments. Fe was supplied either as microbial siderophores (pseudobactin [PSB] or ferrioxamine B [FOB]) or as phytosiderophores obtained as root exudates from barley (epi-3-hydroxy-mugineic acid [HMA]) under varied population densities of rhizosphere microorganisms (axenic, uninoculated, or inoculated with different microorganism cultures). When maize was grown under axenic conditions and supplied with FeHMA, Fe uptake rates were 100 to 300 times higher compared to those in plants supplied with Fe siderophores. Fe from both sources was taken up without the involvement of an extracellular reduction process. The supply of FeHMA enhanced both uptake rate and translocation rate to the shoot (more than 60% of the total uptake). However, increased density of microorganisms resulted in a decrease in Fe uptake rate (up to 65%), presumably due to microbial degradation of the FeHMA. In contrast, when FeFOB or FePSB was used as the Fe source, increased population density of microorganisms enhanced Fe uptake. The enhancement of Fe uptake resulted from the uptake of FeFOB and FePSB by microorganisms adhering to the rhizoplane or living in the free space of cortical cells. The microbial apoplastic Fe pool was not available for root to shoot transport or, thus, for utilization by the plants. These results, in addition to the low uptake rate under axenic conditions, are in contrast to earlier hypotheses suggesting the existence of a specific uptake system for Fe siderophores in higher plants. The bacterial siderophores PSB and FOB were inefficient as Fe sources for plants even when supplied by stem injection. It was concluded that microorganisms are involved in degradation processes of microbial siderophores, as well as in competition for Fe with higher plants. 相似文献
3.
Effects of salinity and light on organic carbon and nitrogen uptake in a hypersaline microbial mat 总被引:1,自引:0,他引:1
Utilization of dissolved organic matter (DOM) is thought to be the purview of heterotrophic microorganisms, but photoautotrophs can take up dissolved organic nitrogen (DON) and dissolved organic carbon (DOC). This study investigated DOC and DON uptake in a laminated cyanobacterial mat community from hypersaline Salt Pond (San Salvador, Bahamas). The total community uptake of (3)H-labeled substrates was measured in the light and in the dark and under conditions of high and low salinity. Salinity was the primary control of DOM uptake, with increased uptake occurring under low-salinity, 'freshened' conditions. DOC uptake was also enhanced in the light as compared with the dark and in samples incubated with the photosystem II inhibitor 3(3,4-dichlorophenyl)-1, 1-dimethylurea, suggesting a positive association between photosynthetic activity and DOC uptake. Microautoradiography revealed that some DOM uptake was attributed to cyanobacteria. Cyanobacteria DOM uptake was negatively correlated with that of smaller filamentous microorganisms, and DOM uptake by individual coccoid cells was negatively correlated with uptake by colonial coccoids. These patterns of activity suggest that Salt Pond microorganisms are engaged in resource partitioning, and DOM utilization may provide a metabolic boost to both heterotrophs and photoautrophs during periods of lowered salinity. 相似文献
4.
青岛近海及其临近海域冬季微微型浮游植物的分布 总被引:2,自引:0,他引:2
微微型浮游植物(0.2~2.0 μm) 是海水中最小的自养浮游生物, 在世界各海域广泛分布, 并在海洋有机物质循环中起着非常重要的作用.利用荧光显微技术对青岛近海及其邻近海域冬季微微型浮游植物丰度进行了调查,研究了微微型浮游植物的空间变化和昼夜变化的特征, 并分析了微微型浮游植物丰度与环境因子的相关性. 结果显示, 冬季该海域以富含藻红素(Phycoerythrin-rich, PE)的聚球藻(Synechococcus, Syn)细胞占优势,微微型真核藻类(Picoeukaryote, Euk)次之,而富含藻蓝素(Phycocyanin-rich, PC)的聚球藻细胞数量很低, 未发现原绿球藻(Prochlorococcus, Pro)的存在.Syn 的变化范围为8.97×103~1.95×105 cells·ml-1, 平均4.67×104 cells·ml-1; Euk的变化范围为1.95×102~1.01×104 cells·ml-1, 平均2.39×103 cells·ml-1. Syn丰度在胶南以南海域出现高值区域, 在即墨海域和崂山东南海域出现低值区域. Euk丰度在日照海域出现高值区域; 崂山海域为低值区域; 各水层Syn和Euk丰度均无明显差异(P﹥0.05). 对胶州湾中部连续站4层水体的24 h昼夜连续变化进行观测发现, Syn、Euk丰度都有明显昼夜波动.相关性分析表明: Syn与温度、 电导率呈正相关, 与溶氧浓度呈显著负相关; Euk与盐度和溶氧浓度呈显著负相关. 微微型浮游植物对总浮游植物生物量的贡献约为20%. 相似文献
5.
Algal 14C and total carbon metabolisms. 1. Models to account for the physiological processes of respiration and recycling 总被引:1,自引:0,他引:1
A consistent set of equations has been written to describe thenet rate of algal 14CO2 uptake (and where appropriate respirationand photosynthesis) which take into account separately complicationsdue to respiration of the labelled photosynthetic products andthe recycling of respiratory CO2. Written specifically intothe equations is the concept of new and oldcarbon, the coefficient q is used in the respiration model toallow for the differential respiration of organic material fromthe new and old carbon pools. Analyticalintegrals have been found for respiration and recycling models,and the behaviour of the models studied over periods of 12 h(i.e. up to 70% of the intrinsic generation time). The rateconstant for respiration has a greater effect on the behaviourof the recycling than the respiration model. Over short timecourses (up to 30% of the intrinsic generation time), the effectsof respiration and recycling on net 14CO2 uptake are quite distinct,especially at high P/R ratios, and not complicated by assumptionsover the value of q. Although the value of q will have a time-dependentsecondary effect on the modelled total carbon-specific respirationrate, this was found not to give rise to major problems of interpretation.Beyond 50% of the intrinsic generation time, the separate treatmentof respiration and recycling in the models becomes less satisfactory.It was concluded that the present equations, which are not constrainedby mass balance considerations, would not be appropriate fora model that combines the two processes. The pattern of recyclingat low P/R values is identified as one of the major uncertaintiesin producing models of 14C uptake. The effect of the releaseof dissolved organic material can be anticipated in a generalway. The models have been used to define an experimental strategyto establish the separate effects of respiration and recyclingon the time course of net 14C uptake. The initial rates givethe clearest resolution of the two processes and it would appearthat with photosynthetic rates in the region of 1 day1,incubation periods up to 36 h would be suitable to determinethe importance of recycling in controlling net 14C uptake. Withthe present models, only in the absence of recycling could theeffect of respiration be studied and the value of q established. 相似文献
6.
The cotyledons of Euphorbia lathyris L. take up sucrose andamino acids from the endosperm. The interaction between theuptake of sucrose and that of amino acids by cotyledons of intactseedlings was investigated. Sucrose (100 mol m3) reducedvaline uptake to 75% of the control rate; the active uptakecomponent of valine uptake was reduced from 45 to 25 % of thetotal uptake rate. In a reverse experiment, 100 mol m3valine inhibited sucrose uptake by 25%. At 500 mol m3sucrose, valine uptake was completely restored to the controlrate, whereas high valine concentrations failed to restore sucroseuptake. The stimulation of valine uptake by sucrose is linkedto the role of sucrose as a primary respiratory substrate. Whenthe cotyledons were bathed in sucrose concentrations rangingfrom 0 to 100 mol m3 (these concentrations are non-saturatingwith respect to sucrose uptake), a constant 1.8% of the sucrosetaken up was respired. The Km of the concentration-dependentsucrose oxidation (44±6 mol m3) agreed reasonablywell with that for sucrose uptake (29±6 mol m3).When the external sucrose concentration was increased from 100to 600 mol m3, the sucrose uptake increased by 30% again,while sucrose oxidation was increased by 300%. This increasewas not due to an increased engagement of the alternative (cyanide-resistant)pathway for respiration. Alternative pathway, Euphorbia lathyris L., fermentation, seedling, sucrose uptake, valine uptake 相似文献
7.
Dodds WK Martí E Tank JL Pontius J Hamilton SK Grimm NB Bowden WB McDowell WH Peterson BJ Valett HM Webster JR Gregory S 《Oecologia》2004,140(3):458-467
Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH4+ uptake rates were measured in eight streams using short-term 15N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams. 相似文献
8.
The regulation of ammonia uptake was investigated in internodalcells of the freshwater alga Chara australis. Ammonia uptakewas estimated by monitoring (i) its depletion from the bathingsolution, (ii) the uptake of radiolabelled methylamine, an analogueof ammonia, and (iii) depletion of ammonia in the unstirredlayer with the microelectrode ion-flux estimation technique(MIFE). Distribution of methylamine (14CH3NH3+) between thevacuole and cytoplasm was estimated with efflux analysis. Whencells were bathed continuously in solutions containing ammoniaor methylamine, the uptake rates of both amines decreased over12 to 48 h despite the continuing existence of a large electrochemicalgradient favouring influx of the NH+4 and CH3NH+4 cations. Treatmentwith 1.0 to 10.0 mM MSX, an inhibitor of glutamine synthetase,caused the internal ammonia concentration to rise and reducedthe subsequent uptake of ammonia and methylamine by up to 70%within 2 h. These results suggest that the permease facilitatingNH+4/CH3NH+4 influx is under feedback or kinetic regulationfrom either internal ammonia or an intermediate of nitrogenassimilation. Treatment with metabolic inhibitors (CCCP, azide and DCMU) andsome weak acids (DMO and butyric acid) for 30 to 60 min inhibitedmethylamine uptake, but the changes in the electrical potentialdifference across the plasma membrane could not account forthe magnitude of inhibition. The rate of cytopiasmic streaming,which is an indicator of the cellular ATP concentration in Chara,was inhibited by many of these treatments. However, under certainconditions of external pH and concentration, butyric acid couldreversibly inhibit ammonia and methylamine uptake without affectingcytoplasmic streaming, demonstrating that a decrease in cytoplasmicATP concentration was not responsible for the inhibition. Theeffect of butyric acid was rapid, causing a 60% inhibition ofuptake in 15 min. We conclude that weak acids can inhibit theNH+4/CH3NH+4 permease by acidifying the cytoplasm and suggestthat this may also explain the effects of the metabolic inhibitorson ammonia and methylamine uptake. Key words: Ammonia, methylamine, uptake, regulation, Chara 相似文献
9.
Protoplasts from growing regions of etiolated pea stems takeup glucose more rapidly than fructose when supplied for briefperiods at low concentrations. The uptake of the two hexosesis differentially inhibited by galactoac and by reagents thatcurtail ATP synthesis, and uptake of one hexose is not preventedby the other, even at a 100-fold excess. Sucrose uptake is muchslower than that of either hexose and is correlated with theappearance of invertase activity in the medium. Label from [14C-glc]-sucroseis taken up more rapidly than from [14C-fru]-sucrose. It isconcluded that these cells take up supplied sucrose only afterhydrolysis to hexoses, which are then absorbed by differentcarrier-mediated processes. Key words: Glucose, fructose, invertase, pea, protoplast, sucrose 相似文献
10.
Development and decline of cation uptake capacity in discs takenfrom the vascular and parenchyma rings of storage tissue ofred table beet (Beta vulgaris L.) were observed during 12 dof ageing. Uptake capacity for Na+ and Rb+ showed a steady risereaching maximums by the fourth to fifth days of ageing. Thereafter,there was a steady decline in the uptake rates. Vascular ringtissues were able to develop a greater uptake capacity for bothNa+ and Rb+ than the tissues of parenchyma rings. This difference,which was more pronounced for Rb+ than for Na+ uptake, is attributedto a combination of variations in cell density and differencesin the acquisition and retention of the cation uptake capacity.Respiration of tissue discs showed no significant rise duringageing, nor were there significant differences in the respirationof vascular and parenchyma tissues. Vascular tissues containedsignificantly more betacyanin than parenchyma tissues; and theyretained their pigment, as well as their acquired cation uptakecapacity, for a longer period during the ageing process. Key words: Cation uptake, Red beet, Vascular rings, Ageing 相似文献
11.
The effect of cyanide on ammonia and urea metabolism was studiedwith intact cells of Chlorella ellipsoidea Gerneck, a greenalga which apparently lacks urease. Ammonia uptake was inhibited more readily by cyanide than wasurea uptake. Urea uptake was stimulated by lower concentrationsof cyanide. The addition of cyanide caused the formation ofammonia from some cellular nitrogenous compounds. In the presenceof exogenously added urea, the molar ratio of ammonia accumulatedin the medium to urea taken up exceeded 2.0 as the cyanide concentrationincreased. However, the molar ratio of ammonia actually producedfrom urea nitrogen to urea taken up was less than 1.35 at anyconcentration of cyanide tested. In the presence of higher concentrationsof cyanide, the rate of incorporation of 15N into amino acidsfrom 15N-urea was higher than that from 15N-ammonium sulfate. The results suggest that Chlorella ellipsoidea possesses a pathwaythrough which urea nitrogen is assimilated directly withouta preliminary breakdown to ammonia. (Received October 18, 1976; ) 相似文献
12.
Apparent uptake and transport of H232PO4 from nutrientsolutions containing 100 mmol m3 phosphate were characterizedasfunctions of time, concentration and pH in ryegrass seedlings.On a log/log plot, concentration versus uptake to the root resolvedintotwo linear phases, suggesting a change in uptake mechanism orefflux at the break. These results were compared with thosefor 32P uptake and transport in solutions containing Al rangingfrom 0185 mmol m3. Al addition depressed pH, butbecauseuptake of P was unaffected by pH below 50, noadjustments were attempted. Uptake time-courses revealed clearlythe usualinitial adsorption shoulder in the uptake curve, increasingwith Al concentration up to 37 mmol m3. Beyond about2 h, P uptaketo the root became linear, at rates increasingwith external Al concentration up to 37 mmol m3. Concentrationsof Al muchabove 100 mmol m3 were toxic. Al treatmentsdid not affect P transport to the shoot and absorbed Al wasconfined to the root.The quantities of P and Al taken up intothe root indicated storage in cortex cell vacuoles, lockingup significant amounts of P.Experiments with tillering plantsshowed similar characteristics to those with seedlings. Sequesteringof P with Al within the rootcortex cells was evident, particularlyin plants which had been grown in nutrient containing Al fromsoon after germination. Aland P solution chemistry is discussedin the context of this work and the consequences of effectson P uptake for the economy ofphosphate poor upland soils wereconsidered. Key words: Phosphate, aluminium, adsorption, uptake, Lolium perenne L 相似文献
13.
Function and contribution of the root tip in the induction of NO3- uptake along the barley root axis
The seminal roots of N-free-grown barley seedlings were ableto take up NO3 immediately upon initial exposure; theuptake rate in the tip was half of that in the older root zones(middle and base). A lag of 60 min was required in all rootzones before the uptake rates started to increase during inductionwith external NO3. This increase could be prevented bythe addition of pFPA; we thus assume that additional NO3transport proteins were synthesized during NO3 induction.During the time-course of NO3 induction different uptakerates were measured in morphologically different regions ofthe tip (1 mm segments) indicating a regulation of NO3induction on a narrow local scale. In NO3 grown plants, NO3 uptake as well as NO3content increased basipetally along the root axis concomitantlywith increasing vacuolization of the cells. Although NO3uptake into the tip was only half of that into the older rootzones, this NO3 uptake was very important for the entireroot. Firstly, it provided the substrate for protein biosynthesisin the meristematic region: nitrate reductase activity and totalsoluble protein were highest in the first apical mm of the tip.Secondly, 3% of the NO3 taken up by the tip was foundin the base where it induced NO3 uptake: NO3 wastranslocated almost exclusively basipetally and as little as20nmolg1 root fr. wt. translocated from the tip weresufficient for acceleration of NO3 induction in the rootbase of N-free-grown plants. This clearly shows that the inductionof NO3 uptake does not depend exclusively on the availabilityof external NO3, but can be mediated also with internallytranslocated NO3.The root tip, therefore, may be consideredthe NO3 sensing region of the root. Key words: Barley, Hordeum vulgare L, internal NO3, NO3 uptake, root zones 相似文献
14.
Uptake of 14C-urea by Thalassiosira pseudonana and Skeletonemacostatum grown in batch culture with NO2 and NO3as nitrogen sources was measured under three conditions: pre-depletion(when nitrogenous nutrient was present in the culture mediumat saturating concentrations), at-depletion (when nitrogenousnutrient could no longer be detected), and several hours post-depletion.Vmax-urea, the initial instantaneous uptake rate, remained constantunder all three conditions, and was in excess of uptake ratesrequired for cellular doubling. Variations in uptake under thethree conditions were observed, as functions of the length oftime over which uptake was observed and the growth rate of theculture. The maximum instantaneous uptake rate was not differentfor the three conditions; variations in uptake were due to theperiod of time over which the maximum uptake rate was maintained.The ability of cells to take up urea rapidly, even when adequatelynourished by NO2 and NO3, could be of significancein a low and variable urea-nutrient regime in the natural environment. 相似文献
15.
Richardson Tammi L.; Cullen John J.; Kelley Dan E.; Lewis Marlon R. 《Journal of plankton research》1998,20(2):219-241
We present a numerical model of nutrient uptake and photosynthesisduring migrations of the marine diatom Rhizosolenia that wasdeveloped to estimate fluxes of carbon and nitrogen due to thesemigrations in the open ocean. The predicted specific rate ofincrease of Rhizosolenia was 0.110.15 day1, whereas thetotal time for one migration cycle ranged between 3 and 5 days.Using published estimates of Rhizosolenia abundance, we estimatethat new primary production due to Rhizosolenia migrations rangesbetween 0.018 and 0.033 mmol N m2 day1. Thesevalues represent up to 17% of new production due to turbulentdiffusive fluxes of nitrate into the euphotic zone and are ofthe same order of magnitude as new production due to nitrogenfixation in tropical oceans. Large-scale contributions of Rhizosoleniato oceanic new production are limited by their relatively lowstanding crop. Variations in the formulation of losses withdepth greatly affected gross and net fluxes of carbon and nitrogen.Better characterization of losses of Rhizosolenia and improvedestimates of its abundance will help determine more accuratelythe contributions of Rhizosolenia to global biogeochemical cycles.
1Present address: Department of Agricultural and EnvironmentalScience, The Queen's University of Belfast New forge Lane, BelfastBT95PX, UK 相似文献
16.
Net copper uptake by cellulose discs, isolated root cell walls,and by live and dead roots of whole ryegrass seedlings, werestudied using 64Cu as a tracer. Uptake by cellulose discs stoppedafter around 10 h while uptake by isolated root cell walls continuedfor up to 50 h. An initial fast phase of uptake consisting predominantlyof cell wall adsorption was similar in live and dead tissuefor up to 19 h. A slower phase of uptake continued for up to50 h, greater in live than in dead tissue, the slower phaseof uptake in live tissue consisting of both a living and a deadcomponent. Based on these results, an alternative to the desorptionmethod for estimating the apoplastic contribution to total copperuptake is presented. Time-course studies with seedlings givena variety of growing solution/uptake solution regimes, and therelationship between copper uptake and external copper concentration,for short (4.8 h) and long (42.4 h) term uptakes, suggest thatdiffering contributions of cell wall adsorption and symplasmicabsorption may be responsible for differing effects of externalcopper concentration on uptake being expressed by the same tissue.Water flux had little effect on total uptake of copper althougha possible effect on absorption could not be ruled out. Key words: Copper uptake, cell wall adsorption, ryegrass seedlings 相似文献
17.
Reduction and uptake of methylene blue by human erythrocytes 总被引:1,自引:0,他引:1
A thiazine dye reductase has been described in endothelial cells that reduces methylene blue (MB), allowing its uptake into cells. Because a different mechanism of MB uptake in human erythrocytes has been proposed, we measured MB uptake and reduction in this cell type. Oxidized MB (MB+) stimulated reduction of extracellular ferricyanide in a time- and concentration-dependent manner, reflecting extracellular reduction of the dye. Reduced MB was then taken up by the cells and partially oxidized to MB+. Both forms were retained against a concentration gradient, and their redox cycling induced an oxidant stress in the cells. Whereas concentrations of MB+ <5 µM selectively oxidized NAD(P)H, higher concentrations also oxidized both glutathione (GSH) and ascorbate, especially in the absence of D-glucose. MB+-stimulated ferricyanide reduction was inhibited by thiol reagents with different mechanisms of action. Phenylarsine oxide, which is selective for vicinal dithiols in proteins, inhibited MB+-dependent ferricyanide reduction more strongly than it decreased cell GSH and pentose phosphate cycle activity, and it did not affect cellular NADPH. Open erythrocyte ghost membranes facilitated saturable NAD(P)H oxidation by MB+, which was abolished by pretreating ghosts with low concentrations of trypsin and phenylarsine oxide. These results show that erythrocytes sequentially reduce and take up MB+, that both reduced and oxidized forms of the dye are concentrated in cells, and that the thiazine dye reductase activity initially responsible for MB+ reduction may correspond to MB+-dependent NAD(P)H reductase activity in erythrocyte ghosts. thiazine dyes; ascorbic acid; ferricyanide; phenylarsine oxide; oxidant stress; redox cycling 相似文献
18.
Williams Peter J.le B.; Robinson Carol; Sondergaard Morten; Jespersen Anne-Mette; Bentley Tracy L.; Lefevre Dominique; Richardson Katherine; Riemann Bo 《Journal of plankton research》1996,18(10):1961-1974
Three sets of comparisons of net and gross inorganic carbonassimilation and 14C uptake were made with an axenic cultureof Skdetonema costatum. The comparisons showed that in the physiologicalwindow studied (1020% of the intrinsic generation timeand gross photosynthesis/respiration ratios of 23), 14Cuptake into the paniculate plus the dissolved fractions approximatedto net photosynthesis. Rate constants derived from the chemicallydetermined changes were used to parameterize models that accountedfor the respiration of photosynthetic products and for the recyclingof respiratory CO14. The conclusion drawn was that over thetime scale studied, the 14C technique was measuring net photosynthesis,consistent with essentially 100% recycling of respiratory CO2.The study has shown that we now possess the basis to make arigorous analysis of net, gross CC4 fixation and net 14C uptake,and forms the first step in the development of algorithms forthe interpretation of 14C field observations. 相似文献
19.
The Role of Phloem Transport in the Translocation of Sucrose Along the Stem of Carnation Cut Flowers
The pathway and distribution of 14C-sugars in flower parts havebeen examined to find out in which tissue sugars are translocatedin the stem of the cut carnation; 14C-sucrose or 14C-glucosewas supplied at the base of the cut stem from a feeding solutionand the localization and chemical nature of the carbon-14 recoveredfrom flower parts were investigated. By reducing the rate oftranspiration it was found that the uptake of feeding solutionwas also reduced, but the distribution of absorbed 14C-sucrosein the flower parts was different from that which would be expectedif sucrose moved exclusively in the transpiration stream. Autoradiographsdemonstrated that 14C absorbed from the feeding solution as14C-sucrose appeared in both xylem and phloem but predominantlyin the latter; girdling failed to stop the translocation ofthe absorbed 14C-sucrose. Results of experiments with 14C-sucroseand 14C-glucose showed that sucrose was the mobile sugar andthat glucose was converted to sucrose before it was translocated.It was concluded that the translocation of sucrose absorbedfrom the feeding solution takes place both in xylem and phloemand is regulated by a mechanism involving the loading and translocationof sucrose in the phloem. 相似文献
20.
Arredondo M Muñoz P Mura CV Nùñez MT 《American journal of physiology. Cell physiology》2003,284(6):C1525-C1530
Despite important advancesin the understanding of copper secretion and excretion, the molecularcomponents of intestinal copper absorption remain a mystery. DMT1, alsoknown as Nramp2 and DCT1, is the transporter responsible for intestinaliron uptake. Electrophysiological evidence suggests that DMT1 can alsobe a copper transporter. Thus we examined the potential role of DMT1 asa copper transporter in intestinal Caco-2 cells. Treatment of cellswith a DMT1 antisense oligonucleotide resulted in 80 and 48%inhibition of iron and copper uptake, respectively. Cells incorporatedconsiderable amounts of copper as Cu1+, whereasCu2+ transport was about 10-fold lower. Cu1+inhibited apical Fe2+ transport. Fe2+, but notFe3+, effectively inhibited Cu1+ uptake. Theiron content of the cells influenced both copper and iron uptake. Cellswith low iron content transported fourfold more iron and threefold morecopper than cells with high iron content. These results demonstratethat DMT1 is a physiologically relevant Cu1+ transporter inintestinal cells, indicating that intestinal absorption of copper andiron are intertwined. 相似文献