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1.
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   

2.
Ammonium, nitrate, and proton fluxes along the maize root   总被引:10,自引:0,他引:10  
Ion-selective microelectrodes were used to measure NH4+, NO3 and H+ fluxes along the primary root of maize seedlings. Plants were exposed to nutrient solutions containing NH4+, NO3 or both ions. Nitrogen fluxes along the root varied substantially among the different treatments. Net NH4+ and NO3 uptake and H+ extrusion were low at the very apex of the root and generally increased in the more basal regions. In the absence of nitrogen or in the presence of NO3 alone, net H+ uptake (and root surface alkalinization) occurred at the root tip (0–1 mm), whereas net H+ extrusion occurred in all other regions. In the presence of NH4+ alone, a dramatic increase in net H+ extrusion was detected in all regions except for the region 6–11 mm from the apex. In contrast, when NO3 alone was supplied, net H+ extrusion was depressed at all locations except for the tip (0–1 mm). When both NH4+ and NO3 were supplied, NO3 uptake was suppressed at all locations while net H+ extrusion was increased relative to NO3 alone. The capacities to absorb NH4+ and NO3 at the tip were similar, as indicated by flux rates when NH4+ or NO3 were supplied as sole sources, but when supplied together, net NO3 uptake was half that of net NH4+ uptake, indicating that NH4+ may satisfy the nitrogen requirements of the poorly vascularized apical tissue in the most energy-efficient way. The high spatial resolution of the measurements enabled us to establish that acidification in the root expansion zone is maintained regardless of nitrogen source.  相似文献   

3.
4.
Knowledge of the preferred source of N for Eucalyptus nitens will lead to improved fertiliser management practices in plantations. Ion selective microelectrodes were used non-invasively to measure simultaneously net fluxes of NH4 +, NO3 and H+ along the tap root of solution-cultured E. nitens. Measurements were conducted in solutions containing 100 m NH4NO3. The pattern of fluxes was such that there was a large influx of NH4 +, a smaller influx of NO3 and large H+ efflux. The ratio of these fluxes was constant, according to the ratio 3:1:–6 (NH4 +:NO3 :H+). Within the region 20–60 mm from the root apex of E. nitens seedlings there was spatial and temporal variation in fluxes but flux patterns remained constant. Root hair density did not affect fluxes nor did proximity to lateral roots. Variation was less than that found in previous studies of localised root fluxes using similar high-resolution measurement techniques. It was concluded that small-scale spatial variation in fluxes may have confounded previous studies. There were associations between fluxes of all three ions, the strongest associations being between NH4 + and H+, and NH4 + and NO3 . Overall, these results are consistent with NH4 + being the preferred source N for E. nitens.  相似文献   

5.
To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH4+ and NO3? uptake from mixed sources, NH4+ and NO3? were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH4+-fed axis than in the NO3?-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH4+-fed axis can be accounted for by the retransport of CH2O equivalents from the root back to the shoot with amino acids produced by NH4+ assimilation. Uptake of NH4+ or NO3? by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH4+ and NO3? were supplied to separate halves of the split-root system, the cumulative NH4+ to NO3? uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH4+ to NO3? uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH4+ and NO3? uptake by roots. For whole roots supplied with both NH4+ and NO3?, the restriction in uptake of NO3? may involve a stimulation of NO3? efflux rather than an inhibition of NO3? influx. While only the net uptake of NH4+ and NO3? was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH4+, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH4+ was supplied with NO3? to whole-root systems or separately to an axis of the split-root system. In the case of NO3?, however, the number of net efflux events increased from 8 when NO3? was supplied to a separate axis of the split-root system to between 19 and 24 when NO3? was supplied with NH4+ to whole-root systems.  相似文献   

6.
Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH4 + as its major inorganic nitrogen source. This study investigates the effects of NH4 + and NO3 ? on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH4 + preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH4 + and NO3 ? supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH4 +-preferring than Yangdao6. NH4 + alleviated Al toxicity in two cultivars compared with NO3 ?. Both NH4 +-Al supply and pretreatment with NH4 + reduced Al accumulation in roots and root tips compared with NO3 ?. NH4 + decreased but NO3 ? increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO3 ? concentration enhanced Al accumulation in root tips but increasing the NH4 + concentration had the opposite effect. These results show NH4 + alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO3 ?, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH4 +, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH4 + is suggested.  相似文献   

7.
Net fluxes of NH4+ and NO3 into roots of 7-day-old barley (Hordeum vulgare L. cv Prato) seedlings varied both with position along the root axis and with time. These variations were not consistent between replicate plants; different roots showed unique temporal and spatial patterns of uptake. Axial scans of NH4+ and NO3 net fluxes were conducted along the apical 7 centimeters of seminal roots of intact barley seedlings in solution culture using ion-selective microelectrodes in the unstirred layer immediately external to the root surface. Theoretically derived relationships between uptake and concentration gradients, combined with experimental observations of the conditions existing in our experimental system, permitted evaluation of the contribution of bulk water flow to ion movement in the unstirred layer, as well as a measure of the spatial resolution of the microelectrode flux estimation technique. Finally, a method was adopted to assess the accuracy of this technique.  相似文献   

8.
Neutral carrier-based liquid membrane ion-selective microelectrodes for NH4+ and NO3 were developed and used to investigate inorganic nitrogen acquisition in two varieties of barley, Hordeum vulgare L. cv Olli and H. vulgare L. cv Prato, originating in cold and warm climates, respectively. In the present paper, the methods used in the fabrication of ammonium- and nitrate-selective microelectrodes are described, and their application in the study of inorganic nitrogen uptake is demonstrated. Net ionic fluxes of NH4+ and NO3 were measured in the unstirred layer of solution immediately external to the root surface. The preference for the uptake of a particular ionic form was examined by measuring the net flux of the predominant form of inorganic nitrogen, with and without the alternative ion in solution. Net flux of NH4+ into the cold-adapted variety remained unchanged when equimolar concentrations (200 micromolar) of NH4+ and NO3 were present. Similarly, net flux of NO3 into the warm-adapted variety was not affected when NH4+ was also present in solution. The high temporal and spatial resolution afforded by ammonium- and nitrate-selective microelectrodes permits a detailed examination of inorganic nitrogen acquisition and its component ionic interactions.  相似文献   

9.
Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O2 m?2 h?1), ammonium (NH4 +), nitrate (NO3 ?), dissolved organic nitrogen, dinitrogen gas (N2), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH4 +), NO3 ? and N2 fluxes (annual means +14.6, +15.9 and 44.6 μmol N m?2 h?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O2 m?2 h?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH4 + flux which may have masked reductions in NH4 + flux associated with BMA assimilation during the light. Peaks in net N2 fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO3 ? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O2 m?2 h?1.  相似文献   

10.
Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO3 or NH4+, the solutions being maintained at pH 5.5. In NO3-fed plants excess nutrient anion over cation uptake was equivalent to net OH efflux, and the total charge from NO3 and SO42− reduction equated to the sum of organic anion accumulation plus net OH efflux. In NH4+-fed plants a large H+ efflux was recorded in close agreement with excess cation over anion uptake. This H+ efflux equated to the sum of net cation (NH4+ minus SO42−) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO3 that is taken up and reduced in NO3-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH4+-fed plants absorbed NH4+ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO3-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients.  相似文献   

11.
The effect of NO3 ?:NH4 + ratio (14:1, 9:6, 7.5:7.5, 1:14, total 15 mmol/L N) in the nutrient solution on biomass, root morphology, and C and N metabolism parameter in hydroponically grown oilseed rape (Brassica napus L.) was evaluated. The dry weights of leaves and roots were significantly largest at the equal NO3 ?:NH4 + ratio (7.5:7.5) compared with those of high NO3 ?:NH4 + ratio (14:1) or low NO3 ?:NH4 + ratio (1:14). Additionally, low NO3 ?:NH4 + ratio (1:14) reduced total root length and root surface area compared with the equal NO3 ?:NH4 + ratio (7.5:7.5), while high NO3 ?:NH4 + ratio (14:1) did not show any significant effect on root morphology except average diameter. The maximum of chlorophyll a, chlorophyll b and carotenoid were obtained under 7.5:7.5 treatment, whereas the maximum of the leaf net photosynthetic (P n), stomatal conductance (G s) and transpiration rate (T r) were increased with increase in NH4 + concentration in the nutrient solution. The activity of nitrate reductase (NR) showed a significant difference at different NO3 ?:NH4 + ratios and ranged 9:6 > 7.5:7.5 > 14:1 > 1:14, whereas the range of soluble sugar and soluble protein was 7.5:7.5 > 1:14 > 9:6 > 14:1. Our study reveals that oilseed rape growth is greater under 7.5:7.5 treatment than that under three other treatments. Oilseed rape growth at high or low NO3 ?:NH4 + ratios was inhibited by decreased pigments, NR activity, soluble sugar, and soluble protein, whereas subdued root growth should be apprehended considerate under high NH4 + condition.  相似文献   

12.
In order to assess the actual role of ectomycorrhizae in ion uptake by the ectomycorrhizal root system, we used a microelectrode ion flux estimation methodology that provided access to local values of net fluxes. This made it possible to investigate the heterogeneity of ion fluxes along the different types of roots of Pinus pinaster associated or not with ectomycorrhizal species. We compared two fungi able to grow with nitrate in pure culture, Rhizopogon roseolus and Hebeloma cylindrosporum, the former having a positive effect on host tree shoot growth (c. +30%) and the latter a negative effect (c.? 30%). In non‐mycorrhizal plants (control), NO3 was taken up at higher rates by the short roots than by the long ones, whereas K+ uptake occurred mainly in growing apices of long roots. In mycorrhizal plants, H. cylindrosporum did not modify K+ uptake and even decreased NO3 uptake at the level of ectomycorrhizal short roots, whereas R. roseolus strongly increased K+ and NO3 fluxes at the level of ectomycorrhizal short roots without any modification of the fluxes measured along the fungus‐free long roots. The measurement of ion influxes at the surface of the ectomycorrhizal roots can provide a way to reveal actual effects of mycorrhizal association on ion transport in relation to mycorrhizal efficiency in natural conditions.  相似文献   

13.
Nitrate fertilization has been shown to increase Zn hyperaccumulation by Noccaea caerulescens (Prayon) (formerly Thlaspi caerulescens). However, it is unknown whether this increased hyperaccumulation is a direct result of NO3 ? nutrition or due to changes in rhizosphere pH as a result of NO3 ? uptake. This paper investigated the mechanism of NO3 ?-enhanced Zn hyperaccumulation in N. caerulescens by assessing the response of Zn uptake to N form and solution pH. Plants were grown in nutrient solution with 300 μM Zn and supplied with either (NH4)2SO4, NH4NO3 or Ca(NO3)2. The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO3 ?-fed plants than in NH4 +-fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO3 ?, whereas the Zn concentration in the roots was mainly enhanced by pH 6.5. Nitrate increased Zn uptake in the roots at pH 6.5 and increased apoplastic Zn at pH 4.5. Zinc and Ca co-increased and was found co-localized in leaf cells of NO3 ?-fed plants. We conclude that NO3 ? directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.  相似文献   

14.
Abstract. Profiles of self-generated ion currents associated with the growing primary root tips of intact Hordeum vulgare L. and Trifolium repens L. (nonnodulated) seedlings were measured using a highly sensitive vibrating electrode in media containing NH+4 or NO-3, and compared to control roots growing in nitrogen free media. Under these three nutrient regimes, positive current entered the root at regions corresponding to the meristematic tissues and main elongation zones of root tips and left from the mature root tissues. Mapping the surface of the roots with a pH-sensitive microelectrode revealed regions of external alkalinity where positive electrical current entered the root, and external acidity where positive current exited. The correlation between pH-profile and the pattern of ion current generation in these experiments suggests that H+ ions were responsible for carrying the bulk of the root-generated current. Assimilation of NHJ results in net H+ extrusion while assimilation of NO-3, results in net OH-3 efflux. Growth on NH+4, as compared to growth on NO-3, stimulated the magnitude of the electrical current but did not affect significantly the growth rate of the roots. However, despite the differing stresses on internal pH regulation that arise due to growth on the two exogenous forms of combined nitrogen, the current profiles were qualitatively similar under the different conditions that were examined. The role of the circulating proton current is not yet known; however, the constancy of the current profile under different nutrient regimes sustains the hypothesis that the current may have a role in the regulation of root polarity.  相似文献   

15.
16.
The effect of pH on nitrate and ammonium uptake in the high‐affinity transport system and low‐affinity transport system ranges was compared in two conifers and one crop species. Many conifers grow on acidic soils, thus their preference for ammonium vs nitrate uptake can differ from that of crop plants, and the effect of pH on nitrogen (N) uptake may differ. Proton, ammonium and nitrate net fluxes were measured at seedling root tips and 5, 10, 20 and 30 mm from the tips using a non‐invasive microelectrode ion flux measurement system in solutions of 50 or 1500 µM NH4NO3 at pH 4 and 7. In Glycine max and Pinus contorta, efflux of protons was observed at pH 7 while pH 4 resulted in net proton uptake in some root regions. Pseudotsuga menziesii roots consistently showed proton efflux behind the root tip, and thus appear better adapted to maintain proton efflux in acid soils. P. menziesii's ability to maintain ammonium uptake at low pH may relate to its ability to maintain proton efflux. In all three species, net nitrate uptake was greatest at neutral pH. Net ammonium uptake in G. max and net nitrate uptake in P. menziesii were greatly reduced at pH 4, particularly at high N concentration, thus N concentration should be considered when determining optimum pH for N uptake. In P. menziesii and G. max, net N uptake was greater in 1500 than 50 µM NH4NO3 solution, but flux profiles of all ions varied among species.  相似文献   

17.
18.
Withania somnifera is an important medicinal plant that contains withanolides as bioactive compounds. We have investigated the effects of macroelements and nitrogen source in hairy roots of W. somnifera with the aim of optimizing the production of biomass and withanolide A content. The effects of the macroelements NH4NO3, KNO3, CaCl2, MgSO4 and KH2PO4 at concentrations of 0, 0.5, 1.0, 1.5 and 2.0× strengths and of nitrogen source [NH4 +/NO3 ? (0.00/18.80, 7.19/18.80, 14.38/18.80, 21.57/18.80, 28.75/18.80, 14.38/0.00, 14.38/9.40, 14.38/18.80, 14.38/28.20 and 14.38/37.60 mM)] in Murashige and Skoog medium were evaluated for biomass and withanolide A production. The highest accumulation of biomass (139.42 g l?1 FW and 13.11 g l?1 DW) was recorded in the medium with 2.0× concentration of KH2PO4, and the highest production of withanolide A was recorded with 2.0× KNO3 (15.27 mg g?1 DW). The NH4 +/NO3 ? ratio also influenced root growth and withanolide A production, with both parameters being larger when the NO3 ? concentration was higher than that of NH4 +. Maximum biomass growth (148.17 g l?1 FW and 14.79 g l?1 DW) was achieved at NH4 +/NO3 ? ratio of 14.38/37.60 mM, while withanolide A production was greatest (14.68 mg g?1 DW) when the NH4 +/NO3 ? ratio was 0.00/18.80 mM. The results are useful for the large scale cultivation of Withania hairy root culture for the production of withanolide A.  相似文献   

19.
Most measurements of nutrient uptake use either hydroponic systems or soil-grown roots that have been disturbed by excavation. The first objective of this study was to test how root excavation affects nitrate uptake. Rates of NO3? uptake by mycorrhizal loblolly pine (Pinus taeda L.) seedlings were measured in intact sand-filled columns, hydroponics, and disturbed sand-filled columns. Total nitrate uptake in intact sand-filled columns was higher than in disturbed columns, indicating that disturbance lowers uptake. Transferring plants from the sand-filled columns to hydroponics had little effect on NO3? uptake beyond delaying uptake for an hour. The second objective of this study was to determine whether NH4+, Ca2+, Mg2+ and K+ uptake could be studied using sand-filled columns, since previous studies had tested this method only for nitrate uptake. Uptake rates of NH4+ and K+ were positive, while Ca2+ and Mg2+ uptake rates were negative in intact sand-filled columns, indicating that net efflux may occur even without physical disturbance to the root system. The sand-filled column approach has some limitations, but holds promise for conducting nutrient uptake studies with minimal disturbance to the root system.  相似文献   

20.
The green-tide macroalga, Ulva prolifera, was tested in the laboratory to determine its nutrient uptake and photosynthesis under different conditions. In the nutrient concentration experiments U. prolifera showed a saturated uptake for nitrate but an escalating uptake in the tested range for phosphorus. Both N/P and NO3 ?/NH4 + ratios influenced nutrient uptake significantly (p?<?0.05) while the PSII quantum yield [Y(II)] (p?>?0.05) remained unaffected. The maximum N uptake rate (33.9?±?0.8 μmol g?1 DW h?1) and P uptake rate (11.1?±?4.7) was detected at N/P ratios of 7.5 and 2.2, respectively. U. prolifera preferred NH4 +-N to NO3 ?-N when the NO3 ?-N/NH4 +-N ratio was less than 2.2 (p?<?0.05). But between ratios of 2.2 and 12.9, the uptake of NO3 ?-N surpassed that of NH4 +-N. In the temperature experiments, the highest N uptake rate and [Y(II)] were observed at 20 °C, while the lowest rates were detected at 5 °C. P uptake rates were correlated with increasing temperature.  相似文献   

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