首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
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 NO3 -. Seedlings were labelled with 15NO3 - 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 NO3 - 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 15NO3 - 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   

2.
Net nitrate uptake, 36ClO?3/NO?3 influx and 36Cl? influx into Pisum sativum L. cv. Feltham First seedlings have been examined following growth in culture medium containing different combinations of chloride and nitrate. When young (6 days old) seedlings, that had been grown in the absence of N were used, nitrate accumulation stimulated net nitrate uptake and 36ClO?3/NO?3 influx (r2= 0.99) while chloride accumulation inhibited nitrate uptake and 36ClO?3/NO?3 influx (r2= 0.65). When nitrate was provided during growth there was no effect of chloride pretreatment on net nitrate uptake and there was little effect of total [NO?3+ Cl?]i on 36ClO?3/NO?3 influx (r2= 0.26). A direct effect of Cl? on 36ClO?3/NO?3 influx was only found when seedlings had been starved of N for more prolonged periods (14 days). When moderate chloride was supplied during growth, 36Cl? influx was insensitive to nitrate or chloride accumulated, but significantly correlated with loge [NO?3+ Cl?]i (r2= 0.75). When trace amounts of Cl? were supplied during growth 36Cl? influx was inhibited by (a) NO?3 in the external medium and (b) Cl? pretreatment, but was insensitive to NO?3 pretreatment. The sensitivity of 36Cl? influx to external nitrate was not found following Cl? pretreatment in the absence of nitrate. The possibility that there are two populations of chloride carriers which differ in their sensitivity to external nitrate is discussed. Tentative schematic models to account for the regulation of nitrate and chloride uptake are proposed in the context of current hypotheses for regulation of ion transport and control systems theory.  相似文献   

3.
The influence of CO2 on the assimilation of nitrate in intact corn seedlings was measured with 15N labelled nitrate, 24 and 48 h after the dark-grown seedlings were transferred to the light, either in normal air or in CO2-free air. During the first 24 h CO2 had no influence on nitrate reduction in intact seedlings. Experiments with detopped seedlings showed that during this period the roots were the only site of nitrate reduction. After 48 h seedlings grown in normal air had reduced more nitrate than detopped seedlings, and seedlings grown in CO2-free air had reduced the same amount of nitrate as detopped seedlings. During the whole 48 h period CO2 had no influence on the level of nitrate reductase of the leaves. It was concluded that in normal air corn leaves started to reduce nitrate after a lag period of 24 h and that in CO2-free air they were incapable of nitrate reduction.  相似文献   

4.
Seven-day-old maize seedlings grown in a nitrogen-free hydroponic culture were exposed for 48 h to 0, 100 and 300 μM trans-cinnamic, p-coumaric, ferulic, caffeic acids, umbelliferone and 200 μM KNO3. Net nitrate uptake was affected by trans-cinnamic, ferulic and p-coumaric acids in a concentration-dependent manner, and trans-cinnamic acid appeared to be the strongest inhibitor. Conversely, at low concentrations, caffeic acid stimulated net nitrate uptake while umbelliferone did not influence it. After 24 h of treatment, plasma membrane H+-ATPase activity significantly decreased in a concentration-dependent manner in response to trans-cinnamic, ferulic and p-coumaric acids, while umbelliferone and caffeic acid had no effect on H+-ATPase activity.  相似文献   

5.
The physiological consequences for NO3 utilization by the plant of underexpression and overexpression of nitrate reductase (NR) were investigated in nine transformants of Nicotiana tabacum and Nicotiana plumbaginifolia. The in vitro NR activities (NRAs) in both roots and leaves of low- and high-NR tobacco transformants ranged from 5–10% to 150–200%, respectively, of those measured in wild-type plants. The level of NR expression markedly affected the NO3 reduction efficiency in detached leaves and intact plants. In both species, 15NO3 reduction ranged from 15–45% of 15NO3 uptake in the low-NR plants, to 40–80% in the wild-type, and up to 95% in high-NR plants. In the high-NR genotypes, however, total 15NO3 assimilation was not significantly increased when compared with that in wild-type plants, because the higher 15NO3 reduction efficiency was offset by lower 15NO3 uptake by the roots. The inhibition of NO3 uptake appeared to be the result of negative feedback regulation of NO3 influx, and is interpreted as an adjustment of NO3 uptake to prevent excessive amino acid synthesis. In genotypes underexpressing NR, the low 15NO3 reduction efficiency also was generally associated with a decrease in net 15NO3 uptake as compared with the wild type. Thus, underexpression of NR resulted in an inhibition of reduced 15N synthesis in the plant, although the effect was much less pronounced than that expected from the very low NRAs. The restricted NO3 uptake in low-NR plants emphasizes the point that the products of NO3 assimilation are not the only factors responsible for down-regulation of the NO3 uptake system.  相似文献   

6.
In the present study, we examined the effects of long- and short-term hypoxia on net uptake and transport of phosphorus to shoots of pond pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine, and the influence aerenchyma formation might have in maintenance of P uptake and transport. Seedlings were grown under aerobic (250 μM O2) or hypoxic (≤50 μM O2) solution conditions for 5.3 weeks in continuously flowing solution culture containing 100 μM P. Intact seedlings were then labeled with 32P for up to 24 h to determine how short- and long-term hypoxic solution conditions affected rates of unidirectional influx and the accumulation of 32P in roots and shoots. Seedlings in the long-term hypoxic treatment were grown for 5.3 weeks in hypoxic solution and also labeled in hypoxic uptake solution. The short-term hypoxic treatments included a 24-h hypoxic pretreatment followed by time in labeled hypoxic uptake solution for seedlings grown under aerobic or hypoxic conditions; in the latter case, diffusion of atmospheric O2 entry into stem and root collar lenticels was blocked, thus removing any influence that aerenchyma formation might have had on enhancing O2 concentrations of root tissue. Although unidirectional influx rates of 32P in roots of seedlings grown under long-term hypoxic conditions were 1.4 times those of aerobically grown seedlings, accumulation of 32P in roots was similar after 24 h in labeled uptake solution. These results suggest that 32P efflux was also higher under hypoxic conditions. Higher shoot/root fresh weight ratios and lower shoot P concentrations in seedlings grown under hypoxic solution conditions suggest that the “shoot P demand” per unit root should be high. Yet accumulation of 32P in shoots was reduced by 50% after 24 h in hypoxic uptake solution. Both short-term hypoxic treatments decreased accumulation of 32P in roots by more than 50%. Short-term hypoxia decreased shoot accumulation in seedlings grown under aerobic and hypoxic conditions by 84 and 50%. respectively. Short- and long-term hypoxic conditions increased the percentage of root 32P in the nucleic acid and chelated-P pools, resulting in a significantly smaller percentage of 32P in the soluble inorganic phosphate (pi) pool, the pool available for transport to the shoot. However, a reduction in pool size or in labeling of the pool available for transport cannot fully account for the large reduction in accumulation of 32P in shoots, particularly in the short-term hypoxic treatment of aerobically grown seedlings. Our results suggest that both influx and transport of 32P to shoots of pond pine seedlings are O2-dependent processes, and that the transport of 32P to shoots may be more sensitive to hypoxic solution conditions than influx at the cortical and epidermal plasmalemma, with aerenchyma formation supporting a substantial amount of both 32P uptake and transport.  相似文献   

7.
The mechanism of nitrate uptake for assimilation in procaryotes is not known. We used the radioactive isotope, 13N as NO3 -, to study this process in a prevalent soil bacterium, Pseudomonas fluorescens. Cultures grown on ammonium sulfate or ammonium nitrate failed to take up labeled nitrate, indicating ammonium repressed synthesis of the assimilatory enzymes. Cultures grown on nitrite or under ammonium limitation had measurable nitrate reductase activity, indicating that the assimilatory enzymes need not be induced by nitrate. In cultures with an active nitrate reductase, the form of 13N internally was ammonium and amino acids; the amino acid labeling pattern indicated that 13NO3 - was assimilated via glutamine synthetase and glutamate synthase. Cultures grown on tungstate to inactivate the reductase concentrated NO3 - at least sixfold. Chlorate had no effect on nitrate transport or assimilation, nor on reduction in cell-free extracts. Ammonium inhibited nitrate uptake in cells with and without active nitrate reductases, but had no effect on cell-free nitrate reduction, indicating the site of inhibition was nitrate transport into the cytoplasm. Nitrate assimilation in cells grown on nitrate and nitrate uptake into cells grown with tungstate on nitrite both followed Michaelis-Menten kinetics with similar K mvalues, 7 M. Both azide and cyanide inhibited nitrate assimilation. Our findings suggest that Pseudomonas fluorescens can take up nitrate via active transport and that nitrate assimilation is both inhibited and repressed by ammonium.  相似文献   

8.
Abstract. Nitrate uptake into Chara corallina cells at different external pH (pHo) after different NO3 pretreatment conditions has been investigated. Following NO3 pretreatment (0.2 mol m−3 NO3) there was little effect of pHo on subsequent net NO3 uptake into Chara cells. After N deprivation (2 mmol m−3 NO3) there was a pronounced effect of pHo on nitrate uptake, the maximum rate occurring at pHo 4.7. There was no consistent relationship between OH efflux and NO3 uptake in short term experiments (< 1 h). NO3 efflux was also sensitive to pHo, the maximum rate occurring at ∼ pHo 5.0. An inhibitor of the proton pump, DES, immediately stimulated NO3 uptake into cells pretreated with NO3 and prevented the time-dependent decrease in NO3, uptake into Chara cells that had been previously treated with low N (2 mmol m−3 NO3). NO3 efflux was found to be very sensitive to DES with Ki= 0.7 mmol m−3. At the optimum pHo for NO3 uptake the effect of DES on membrane potential (ψm) were slight, and only apparent after 30 min. The results are interpreted in context of current models relating NO3 uptake and H+ pump activity. A new model for NO3 uptake is proposed which involves NO3/NO3 exchange at steady state.  相似文献   

9.
Root morpho-topology and net nitrate uptake of two citrus seedlings, Volkamer Lemon and Carrizo Citrange, grown at two nitrogen supplies (NO3-N 5 M and 1000 M, respectively) were studied. Root morphological and topological parameters were gauged by an image-specific analysis system (WinRHIZO). Net nitrate uptake was estimated using the nitrate depletion method. The main findings showed that Carrizo seedlings had a dichotomous branching root system characterized by high root tip numbers and long 2nd order lateral roots. Conversely, Volkamer root systems had a herringbone structure with a long tap root and 1st order lateral root. Nitrate treatment did not seem to affect the pattern of the two genotypes, except for the 2nd order lateral roots (Carrizo more than Volkamer) and root/shoot ratio and root mass ratio (Volkamer more than Carrizo) that were significantly different at low nitrate supply. Nitrate treatments induced a diverse net nitrate uptake regulation between citrus rootstocks. Indeed, at low nitrate supply, Carrizo showed a more efficient nitrate acquisition process in terms of: 1) higher net nitrate uptake maximum of the inducible high affinity transport system or full induction (A), (2) higher cumulative nitrate uptake (At) and (3) lower t1 parameter defined as the half time of the net nitrate uptake rate of the inducible transport system during the induction phase, compared to Volkamer. Conversely, at the high nitrate level, only the genotypical difference of the t1 parameter was maintained. The results suggested that, at the low nitrate level, the morphological root traits such as higher 2nd order lateral roots and greater root tip numbers of the Carrizo compared with Volkamer seedlings, enhance the capacity to absorb nitrate from nutrient solution.  相似文献   

10.
Kinetic parameters for NH4+ and NO3? uptake were measured in intact roots of Lolium perenne and actively N2-fixing Trifolium repens. Simultaneously, net H+ fluxes between the roots and the root medium were recorded, as were the net photosynthetic rate and transpiration of the leaves. A Michaelis–Menten-type high-affinity system operated in the concentration range up to about 500 mmol m?3 NO3? or NH4+. In L. perenne, the Vmax of this system was 9–11 and 13–14 μmol g?1 root FW h?1 for NO3? and NH4+, respectively. The corresponding values in T. repens were 5–7 and 2 μmol g?1 root FW h?1. The Km for NH4+ uptake was much lower in L. perenne than in T. repens (c. 40 compared with 170 mmol m?3), while Km values for NO3? absorption were roughly similar (around 130 mmol m?3) in the two species. There were no indications of a significant efflux component in the net uptake of the two ions. The translocation rate to the shoots of nitrogen derived from absorbed NO3?-N was higher in T. repens than in L. perenne, while the opposite was the case for nitrogen absorbed as NH4+. Trifolium repens had higher rates of transpiration and net photosynthesis than L. perenne. Measurements of net H+ fluxes between roots and nutrient solution showed that L. perenne absorbing NO3? had a net uptake of H+, while L. perenne with access to NH4+ and T. repens, with access to NO3? or NH4+, in all cases acidified the nutrient solution. Within the individual combinations of plant species and inorganic N form, the net H+ fluxes varied only a little with external N concentration and, hence, with the absorption rate of inorganic N. Based on assessment of the net H+ fluxes in T. repens, nitrogen absorption rate via N2 fixation was similar to that of inorganic N and was not down-regulated by exposure to inorganic N for 2 h. It is concluded that L. perenne will have a competitive advantage over T. repens with respect to inorganic N acquisition.  相似文献   

11.
The distribution of NO3? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO3?, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 mM NH4+ and 0.5, 1.5 or 10 mM NO3?. Xylem transport of NO3? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO3? reduction at all 3 levels of NO3? nutrition. However, the contribution of the shoots to the whole plant NO3? reduction increased with increasing external NO3? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO3?, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO3? translocation from roots to shoots. It is proposed that the lack of NO3? export to the shoots at low NO3? uptake rate resulted from a competition between NO3? reduction in the root epidermis/cortex and NO3? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO3? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO3? assimilation, principally occurring in the roots, was mainly incorporated in the shoots.  相似文献   

12.
The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15mM total amino acids) in a complete cell-culture media containing 1.0 mM NO3 - strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 mM mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 mM Ca(NO3)2 or 1 mM CaCl2. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake.  相似文献   

13.
Summary Wheat (Triticum vulgare L., cv. Blueboy) seedlings, grown with 0.25, 1.0 and 15 mM nitrate in complete nutrient solutions, were transferred 10 days after germination to 1.0 mM K15NO3 (99 A% 15N) plus 0.1 mM CaSO4 at pH 6.0. The solutions were replaced periodically over a 6-h period (5 mW cm-2; 23°). Changes in the [15N]- and [14N]nitrate in the solution were determined by nitrate reductase and mass-spectrometric procedures and potassium by flame photometry. Influx of [15N]nitrate was depressed in plants grown at 1.0 mM nitrate relative to those grown at 0.25 mM, but there was no appreciably difference in [14N]nitrate efflux. Prior growth at 15 mM further restricted [15N]nitrate influx which, together with a substantial increase in [14N]nitrate efflux, resulted in no net nitrate uptake during the course of the experiment. Efflux of [14N]nitrate occurred to solutions containing no nitrate but it was significantly enhanced upon exposure to [15N]nitrate in the external solution. Influx of [15N]nitrate was more restricted at 5°, relative to 23°, than was [14N]nitrate efflux. The nitrate concentrations of the root tissue immediately before exposure to the K15NO3 solutions did not give a precise indication of the subsequent [15N]nitrate influx rates nor of the [14N]nitrate efflux rates. Net K+ uptake was related to the magnitude of the net nitrate uptake, not to the initial K+ concentration in the roots. The data are interpreted as indicating that [15N]nitrate influx and [14N]nitrate efflux are largely independent processes, subject to different controls, and that net nitrate uptake provides the driving force for net potassium uptake.Paper No. 4884 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, NC, USA. This investigation was supported in part by the U.S. Energy Research and Development Administration, Contract No. AT-(40-1)-2410  相似文献   

14.
Barley seedlings (Hordeum vulgare L. cv. California Mariout) grown hydroponically for 14-19 d without addition of NaCl were used for describing the effects of salt application on net nitrate uptake and for the calculation of kinetic parameters. The addition of NaCl, KCl, CaCl2, and Na2SO4 to the uptake solution in the experiments led to similar inhibition of nitrate uptake, only at low and very high salt concentrations were ion-specific effects found. The same decrease in nitrate uptake can also be achieved by sorbitol or betaine at corresponding osmolalities. Thus it was concluded that the inhibition of uptake was caused mainly by the osmotic effects of salts. Differences in the mechanisms of inhibition were detected between the two systems of nitrate uptake (high affinity system: HATS, and low affinity system: LATS). The HATS was inhibited non-competitively by NaCl, an apparent Ki of 60 mol m-3 was calculated using a Dixon-plot. Fitting an equation assuming a non-competitively inhibited HATS by computer program to the raw data resulted in an apparent Ki of about 37 mol m-3. In contrast, the LATS was affected in a complex way: up to 60 mol m-3 NaCl the affinity was increased, which led to a stimulation of nitrate uptake at low nitrate concentrations (<2 mol m-3). An inhibition of the LATS became obvious at concentrations above 3 mol m-3 nitrate (for all applied salt concentrations) or with 100 mol m-3 NaCl (throughout the whole nitrate range). Related plots of the data pointed to a competitive effect.Key words: Hordeum vulgare L., net nitrate uptake, high affinity transport system (HATS), low affinity transport system (LATS), salt, inhibition, apparent kinetic parameters.   相似文献   

15.
Malagoli  M.  Dal Canal  A.  Quaggiotti  S.  Pegoraro  P.  Bottacin  A. 《Plant and Soil》2000,221(1):1-3
In forest soils, ammonium is usually the predominant form of inorganic nitrogen. However, the capacity of trees to utilize both NO3 - and NH3 + may provide greater flexibility in responding to changes of nitrogen supply from the environment. Such capacity has been studied in seedlings of Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill.) grown in the presence or absence of either nitrate or ammonium. Nitrate-induced plants showed a higher nitrate uptake rate than non-induced plants; this difference was almost negligible after 24 h of exposure to NO3 -. Ammonium uptake in both species was consistently higher than that of nitrate, regardless of prior nitrogen provision. In both nutrient conditions, larch showed a more efficient transport system in comparison with Scots pine, with higher ammonium and nitrate uptake rates in both induced and non-induced plants. This was consistent also with the activity of nitrate reductase, measured in vivo in roots and leaves. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

16.
In vivo 15N and 14N nuclear magnetic resonance spectroscopy was used to investigate the assimilation of nitrate and ammonium in seedlings of Norway spruce (Picea abies [L.] Karst.). The main objective was to study accumulation of free NH+4 and examine to what extent the nitrogen source affects the composition of the free amino acid pools in roots, stems and needles. NH+4 concentrations in plants growing in the presence of 0.5–50 mM ammonium were quantified using 14N NMR. The NH+4 values in tissues ranged from 6 to 46 μmol (g fresh weight)?1. with highest concentrations in roots and needles. The tissue NH+4 peaked at 5.0 mM NH+4 in the medium. and failed to increase when NH+4 in the medium was increased to 50 mM, indicating metabolic control of the concentration of this cation in tissues. The 14N NMR spectra were used to estimate pH of the NH+4 storage pools. Based on the pH sensitivity of the quintet of 14NH+4 resonance, we suggest that the pH of the ammonium storage compartments in the roots and stems should be 3.7–3.8, and in needles 3.4–3.5, representing extremely low pH values of the tissue. 15N from nitrate or ammonium was first incorporated into the amide group of glutamine and then into α-amino groups, confirming that the glutamine synthetase/ glutamate synthase cycle is the major route of nitrogen assimilation into amino acids and thus plays a role in lowering the levels of NH+4 in the cytoplasm. NH+4 can also be assimilated in roots in plants growing in darkness. The main 15N-labelled amino acids were glutamine. arginine and alanine. Almost no 15N signals from needles were observed. Double labelling (δN + w, wN) of arginine is consistent with the operation of the ornithine cycle, and enrichment indicates that this cycle is a major sink of newly assimilated nitrogen. Nitrogen assimilation in roots in the presence of added methionine sulphoximine and glutamate indicated the catabolic action of glutamate dehydrogenase. The 15N NMR spectra of plants grown on 15N-urea showed a marked increase in the labelling of ammonium and glutamine. indicating high urease activity. Amino acids were also quantified using high pressure liquid chromatography. Arginine was found to be an important transport form of nitrogen in the stem.  相似文献   

17.
The effect of nitrate availability on characteristics of the nitrate assimilatory system was investigated in N-limited barley (Hordeum valgare L. cv. Golf), grown with the seminal root system split into initially equal-sized halves. The cultures were continuously supplied with nitrate-N at a relative addition rate (RA) of 0.09 day?1, which resulted in relative growth rates (RG) that were ca 85% of those observed under surplus nitrate nutrition. The total N addition was divided between the subroots in ratios of 100:0, 80:20, 70:30, 60:40, and 50:50. For comparison, standard cultures were grown at RAs ranging from 0.03 to 0.18 day?1. Initially, biomass and N partitioning to the subroots responded strongly and proportionally to the nitrate distribution ratio. After 12-14 days no further effect was observed. The Vmax for net nitrate uptake and in vitro nitrate reductase (NR) activity were measured in acclimated plants, i.e., after > 14 days under a certain nitrate regime. In subroots fed from 20 to 100% of the total N addition, Vmax for net nitrate uptake increased slightly, whereas NR activity was unaffected. Uptake and NR activities were insignificant in the 0%-subroot. Uneven nitrate supply to individual subroots had negligible effect on the whole-plant ability for nitrate uptake, and the relative Vmax (unit N taken up per unit N in whole plant tissue and time) remained about 7-fold in excess of the demand set by growth. Balancing nitrate concentrations (the resulting external nitrate concentrations at a certain RA) generally ranged between 2 and 10 μM at growth-limiting RA, both when predicted from uptake kinetics and when actually measured. When comparing split root and standard cultures when acclimated, it appears that uptake and NR activities in roots respond more strongly to over-all nitrate availability than to nitrate availability to individual subroots.  相似文献   

18.
Interactive effects of K+ and N (principally NH4+) on plant growth and ion uptake were investigated using hydroponically grown rice (Oryza sativa L. cv. M202) seedlings by varying the availability of NH4+ or NO3? and K+ during an 18d growth period, a 3d pretreatment period and during flux measurements. Plants grew best in media containing 100 mmol m?3 NH4+ and 200mmolm?3 K+ (N100/K200), followed by N2/K200 < N100/K2 < N2/K2. 86Rb+(K+) fluxes were increased by exposure to N during the 18 d growth period and the 3 d of pretreatment, but decreased by the presence of NH4+ during flux measurements. This inhibition was a function of prior N/K provision and the [NH4+]0 present during flux determinations. NH4+ was least inhibitory to 86Rb+(K+) influx in high-N/low-K plants. Pretreatments with K+ failed to stimulate NH4+ uptake, and the presence of K+ in the uptake solutions reduced NH4+ fluxes only in high-N/low-K plants.  相似文献   

19.
We examined the effects o long-term hypoxic growth conditions on net uptake and transport of P to shoots of pond Pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine. Seedlings were grown under aerobic orhypoxic solution conditions for 4–5 weeks in continuously flowing solution culture containing 100 μM P. Short – and long-term 32P. experiments were then concluded with intact seedlings to determine rates of 32P influx, efflux and net transport to the shoot. Shoot fresh weight/root fresh weight ratios were significantly higher under hypoxic gorwth conditions, reflecting the larger reduction in root growth than shoot growth, despite extensive aerechyma formation in roots. Estimates for the unidirectional influx of 32P in aerobic and hypoxic seedlings were 1.43 and 3.20 μmol P (gFW root)?1 h?1, respectively. However, 32P accumulation between the two treatments became similar within 8 h, suggesting that efflux was also higer in seedlings from the hypoxic treatment. Indeed in a separate experiment, hypoxic growth conditions increased efflux by over 60%. Transport of 32P to shoots was significantly reduced under hypoxic growth conditions, despite higher root P concentrations and lower shoot P concentrations. After 48 h, 32P accumulation in roots was similar between the two treatments. Yet total accumulation of seedling 32P decrcased by 31% under the hypoxic treatment, largely because of reduced transport of 32p to the shoot. The lower accumulation of 32 by shoots of seedlings in the hypoxic treatment may be the result of a direct inhibition on the transport process in O2-defident tissues, but could also reflect a slower turnover or labeling of the ool available for transport. Indeed, the percentage of total 32P in. roots present in the soluble P. (or transportable form of P) was about 33% lower in seedlings from the hypoxic treatment, probably reflecting increased assimilation into organic compounds as well as chelation with iron. Our results suggest that P transport to the shoots of acclimated seedlings may be more sensitive to hypoxic solution conditions than influx at the root Plasmalemma.  相似文献   

20.
Nitrate uptake and assimilation were examined in intact 18 days old wheat (Triticum aestivum, cv Capitole) seedlings either permanently grown on nitrate (high-N seedlings) or N-stressed by transfer to an 0 N-solution for the final 7 days (low-N seedlings). The N-stressed seedlings were characterized by a lower organic N content (2.5 mg instead of 4.9 mg per seedling) and an increased root dry weight.The seedlings received 15NO3K for 7 h in the light. Nitrate uptake was 2.8 times higher in low-N than in high-N seedlings. The assimilation rate was 35 and 16 μmol NO3?·h?1· g?1 dry weight respectively. Partitioning of NO3? to reduction and assimilation was the very same in both kinds of seedlings. The results support the view that 50 % of the nitrate reduction in Triticum aestivum, cv Capitole could be achieved in the roots.The present observations are interpreted as evidence that factors closely associated with the seedling N-status may have a major role in regulating NO3? uptake and assimilation. In low-N seedlings, the high amount of carbohydrates in roots may add its stimulus to the specific inducing effect of nitrate whereas in high-N seedlings, excess of nitrate or amino-acids may set the pace by negative feedback control.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号