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1.
Pisum arvense plants were subjected to 5 days of nitrogen deprivation. Then, in the conditions that increased or decreased the root glutamine and asparagine pools, the uptake rates of 0.5 mM NH4 + and 0.5 mM K+ were examined. The plants supplied with 1 mM glutamine or asparagine took up ammonium and potassium at rates lower than those for the control plants. The uptake rates of NH4 + and K+ were not affected by 1 mM glutamate. When the plants were pre-treated with 100 μM methionine sulphoximine, an inhibitor of glutamine synthesis, the efflux of NH4 + from roots to ambient solution was enhanced. On the other hand, exposure of plants to methionine sulphoximine led to an increase in potassium uptake rate. The addition of asparagine, glutamine or glutamate into the incubation medium caused a decline in the rate of NH4 + uptake by plasma membrane vesicles isolated from roots of Pisum arvense, whereas on addition of methionine sulphoximine increased ammonium uptake. The results indicate that both NH4 + and K+ uptake appear to be similarly affected by glutamine and asparagine status in root cells. The research was supported by grant of KBN No. 6PO4C 068 08  相似文献   

2.
Previous data in Egeria densa leaves demonstrated a strong inhibitory effect of Cs+ on passive K+ influx and on K+-induced, ATP-dependent electrogenic proton extrusion. In this paper we analyzed, using the same material, the effects of Cs+ on ammonium (NH4+) and methylammonium (CH3NH3+) transport in order to elucidate whether a common transport system for K+ and NH4+ could be demonstrated. The effects of Cs+ on NH4+- and CH3NH3+-induced titratable H+ extrusion (–ΔH+) and on transmembrane electrical potential difference (Em) in E. densa leaves were analyzed in parallel. All experiments were run either in the absence or presence of fusicoccin, corresponding to low or high H+-ATPase activity and membrane hyperpolarization and leading, in this material, to respectively active or passive transport of K+. The results suggest the presence in E. densa leaves of two distinct pathways for NH4+ uptake: one in common with NH4+ and (with lower affinity) CH3NH3+, insensitive to Cs+, and a second system, operating at higher H+-ATPase activity and Em hyperpolarization, strongly inhibited by Cs+ and impermeable to CH3NH3+. In agreement with this hypothesis, Xenopus laevis oocytes injected with the KAT1 RNA of Arabidopsis thaliana were permeable to K+ and NH4+, but not to CH3NH3+.  相似文献   

3.
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   

4.
Abstract. H+ efflux from the trap lobes of Dionaea muscipula Ellis (Venus's flytrap) was measured in vitro. FC, IAA, and 2,4-D markedly increase the rate of H+ efflux within minutes of their addition to the incubation medium whereas ABA and DES cause the rate to decrease. Consequently, the H+ efflux mechanism of Dionaea is considered to be similar to the H+ extrusion pumps of other higher plants in this respect. However, the H+ extrusion mechanism of Dionaea may be unusual in that long-term exposure of the trap lobes to known secretion elicitors— bactopeptone, NH4+, Na +, urea, thiourea, glycine or xanthine—also causes a large increase in the steady-state rate of H+ efflux from the trap lobes. Since the observed H+ effluxes primarily correspond to the adaxial surface of the trap lobes and show similar time- and secretion elicitor-dependencies to the responses seen in situ, it appears that the H+ effluxes measured in vitro bear a direct relationship to those observed in the intact, actively secreting plant. Three of the secretion elicitors that were tested— K+, NH4+, and urea—have rapid effects on the rate of H+ extrusion in addition to their long-term effects. K+ and NH4+ cause a rapid acceleration of H+ efflux whereas urea causes a rapid deceleration or, at high external concentrations, reversal of the net flux. The effect of K+ is inferred to result from K+ -H+ exchange between the tissue and bathing medium. Studies with structural analogues of NH4+ and urea and inhibitors of the assimilation of reduced nitrogen suggest that the effects of NH4+ and urea result from the pH-perturbing consequences of their metabolism subsequent to their absorption. These effects are considered to be auxiliary to the elicitation of secretion. It is proposed that H+ efflux from the trap lobes is mediated by a K+-H+ exchange mechanism, the activity of which is modified by long-term exposure to secretion elicitors and/or short-term exposure to factors which alter the availability of endogenous H+ ions.  相似文献   

5.
Root growth in higher plants is sensitive to excess ammonium (NH4+). Our study shows that contact of NH4+ with the primary root tip is both necessary and sufficient to the development of arrested root growth under NH4+ nutrition in Arabidopsis. We show that cell elongation and not cell division is the principal target in the NH4+ inhibition of primary root growth. Mutant and expression analyses using DR5:GUS revealed that the growth inhibition is furthermore independent of auxin and ethylene signalling. NH4+ fluxes along the primary root, measured using the Scanning Ion‐selective Electrode Technique, revealed a significant stimulation of NH4+ efflux at the elongation zone following treatment with elevated NH4+, coincident with the inhibition of root elongation. Stimulation of NH4+ efflux and inhibition of cell expansion were significantly more pronounced in the NH4+‐hypersensitive mutant vtc1‐1, deficient in the enzyme GDP‐mannose pyrophosphorylase (GMPase). We conclude that both restricted transmembrane NH4+ fluxes and proper functioning of GMPase in roots are critical to minimizing the severity of the NH4+ toxicity response in Arabidopsis.  相似文献   

6.
Wheat plants grown during 10 days in the absence of N were pretreated with 1.0 eq m-3 of methionine, asparagine or glutamine and/or 1.0 eq m-3 MSX4 or 0.17 eq m-3 DON. Net NH4 + uptake was measured both in the presence or in the absence of the amino acid or enzyme inhibitor used in the pretreatment. The effect of met, asn and gln on net K+ uptake was also studied using K+-depleted plants. Changes in the contents of root free NH4 +, asn, gln and the activities of GS, PEP-carboxylase, NAD+-GDH and NADH-GDH were determined. Net NH4 + uptake in gln and asn pretreated plants was markedly, and sometimes completely suppressed provided uptake was measured in the presence of the amides. On the other hand, the met pretreated plants absorbed only 35% less NH4 + than the control. When NH4 + uptake was measured in the absence of the amino acids, only those plants pretreated with asn showed a marked suppression of net uptake during the first 120 min. None of the 3 amino acids tested significantly inhibited K+ uptake. Free NH4 + concentration in roots of N-starved plants increased after 4 h incubation with gln, asn or MSX in the absence of external NH4 +. Nevertheless, no correlation was observed between root NH4 + concentration and the extent of net NH4 + uptake suppression. The inhibitory effect exerted by asn decreased when it was supplied together with MSX or DON. Pretreatments with gln or asn in the absence of external NH4 + significantly increased the level of asn in the roots, while that of gln remained unchanged. It is concluded that asn and gln specifically suppress net NH4 + uptake in wheat, although it is not clear wether they act only from the root exterior, or through an endogenous pool exhibiting fast turn-over.Abbreviations AUR ammonium uptake rate - DON 6-diazo-5-oxo-L-norleucine - GDH glutamic dehydrogenase - GOGAT oxoglutarate- glutamine aminotransferase - GS glutamine synthetase - MSX L-methionine sulfoximine - PEP phosphoenolpyruvate - PVPP polyvinylpolypyrrolidone  相似文献   

7.
Wang  Guoying  Li  Chunjian  Zhang  Fusuo 《Plant and Soil》2003,256(1):169-178
NH4 +-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO3 , NH4NO3, NH4 + or NH4 ++6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO3 -N by NH4 +-N (NH4NO3) did not change growth, transpiration, uptake and flow of water and K+ compared with the NO3 -N treatment. However, NH4 +-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO3 -N and NH4NO3 treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (13C) compared with the NO3 treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K+ uptake to 10%. Under NH4 + nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K+ currently taken up, indicating massive mobilisation of K+ from lower leaves and even the roots. Phloem retranslocation of K+ from the shoot and cycling through the root contributed 67% to the xylem transport of K+, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K+) caused by NH4 +-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH4 +-fed plants are discussed.  相似文献   

8.
The effects of internally applied 1 mM vanadate on the Na+ efflux in dialysed squid axons were found to depend on the presence of external K+. In K+-free artificial sea water, vanadate did not produce any change in the rate of Na+ efflux, whereas in the presence of 10 mM K+ the Na+ efflux was reduced to values even lower than those observed in the absence of K+ (inversion of the K+-free effect). In vanadate-poisoned axons, K+ and NH4+ at low concentrations activated Na+ efflux, but at high concentrations both cations were inhibitory. However, NH4+ was always a better activator and a poorer inhibitor than K+.  相似文献   

9.
Net electrolyte efflux from suspension-cultured tobacco cells undergoing the hypersensitive reaction to Pseudomonas syringae pv. pisi resulted from a specific efflux of K+ which was accompanied by an equimolar net influx of H+. These fluxes began 60 to 90 minutes after inoculation of tobacco cells with bacteria, reached maximum rates of 6 to 9 micromoles per gram fresh weight tobacco cells per hour within 2.5 to 3 hours, and dropped below 4 micromoles per gram per hour within 5 hours. Tobacco cells lost approximately 35% of total K+ during this period, and average cellular pH declined by approximately 0.75 pH unit. These events were accompanied by a 30% decrease in cellular ATP. K+ and H+ fluxes were inhibited by the protonophore (p-trifluoromethoxy)carbonyl cyanide phenylhydrazone and by increasing the K+ concentration of the external solution. Tobacco leaf discs inoculated with the bacterium also exhibited a specific net K+ efflux and H+ influx. These results suggest that induction of the hypersensitive reaction in tobacco proceeds through the activation of a passive plasmalemma K+/H+ exchange mechanism. It is hypothesized that activation of this exchange is a major contributing factor in hypersensitive plant cell death.  相似文献   

10.
Effects of ammonium on the photosynthetic recovery of Nostoc flagelliforme Berk. et M. A. Curtis were assayed when being rehydrated in low‐K+ or high‐K+ medium. Its photosynthetic recovery was K+ limited after 3 years of dry storage. The potassium absorption of N. flagelliforme reached the maximum after 3 h rehydration in low‐K+ medium but at 5 min in high‐K+ medium. The K+ content of N. flagelliforme rehydrated in high‐K+ medium was much higher than that in low‐K+ medium. The maximal PSII quantum yield (Fv/Fm) value of N. flagelliforme decreased significantly when samples were rehydrated in low‐K+ medium treated with 5 mM NH4Cl. However, the treatment of 20 mM NH4Cl had little effect on its Fv/Fm value in high‐K+ medium. The relative Fv/Fm 24 h EC50 (concentration at which 50% inhibition occurred) value of NH4+ in high‐K+ medium (64.35 mM) was much higher than that in low‐K+ medium (22.17 mM). This finding indicated that high K+ could alleviate the inhibitory action of NH4+ upon the photosynthetic recovery of N. flagelliforme during rehydration. In the presence of 10 mM tetraethylammonium chloride (TEACl), the relative Fv/Fm 24 h EC50 value of NH4+ was increased to 46.34 and 70.78 mM, respectively, in low‐K+ and high‐K+ media. This observation suggested that NH4+ entered into N. flagelliforme cells via the K+ channel. Furthermore, NH4+ could decrease K+ absorption in high‐K+ medium.  相似文献   

11.
Summary Representative arable soils from Hesse were investigated for their contents of fixed NH4 + and EUF-extractable potassium in the rooting zone. Alluvial soils were found to be rich in fixed ammonium and low in EUF-extractable potassium, while soils of basaltic origin were low in fixed ammonium and rich in EUF-extractable potassium. A negative correlation (r=0.79*) was found between fixed NH4 + and EUF-extractable soil K+. The content of fixed NH4 + in the soil profile showed considerable and significant changes during the growing season, which finding is supposed to be due to NH4 + uptake by the crop.  相似文献   

12.
We present the first characterization of K+ optimization of N uptake and metabolism in an NH4+‐tolerant species, tropical lowland rice (cv. IR‐72). 13N radiotracing showed that increased K+ supply reduces futile NH4+ cycling at the plasma membrane, diminishing the excessive rates of both unidirectional influx and efflux. Pharmacological testing showed that low‐affinity NH4+ influx may be mediated by both K+ and non‐selective cation channels. Suppression of NH4+ influx by K+ occurred within minutes of increasing K+ supply. Increased K+ reduced free [NH4+] in roots and shoots by 50–75%. Plant biomass was maximized on 10 mm NH4+ and 5 mm K+, with growth 160% higher than 10 mm NO3‐grown plants, and 220% higher than plants grown at 10 mm NH4+ and 0.1 mm K+. Unlike in NH4+‐sensitive barley, growth optimization was not attributed to a reduced energy cost of futile NH4+ cycling at the plasma membrane. Activities of the key enzymes glutamine synthetase and phosphoenolpyruvate carboxylase (PEPC) were strongly stimulated by elevated K+, mirroring plant growth and protein content. Improved plant performance through optimization of K+ and NH4+ is likely to be of substantial agronomic significance in the world's foremost crop species.  相似文献   

13.
Abstract Changes in the net uptake rate of K+ and in the average tissue concentration of K+ were measured over 14 d in response to changes in root temperature with oilseed rape (Brassica napus L. cv. Bien venu) and barley (Hordeum vulgare L. cv. Atem). Plants were grown in flowing nutrient solutions containing 2.5 mmol m?3 K+ and were acclimatized over 49 d (rape) or 28 d (barley) to low root temperature (5°C) prior to steady–state treatments at root temperatures between 3 °C and 25 °C, with common air temperature. Uptake of K+ was monitored continuously over 14 d and nitrogen was supplied as NH4++ NO?3 or NH+4 or NO?3. Unit absorption rates of K+ increased with time and with root temperature up to Day 4 or 5 following the change in root temperature. Thereafter they usually approached steady-state, with Q10? 2.0 between 7 °C and 17°C, although rates became similar between 7 °C and 13°C. Uptake of K+ by rape plants was invariably greater under NO?3 nutrition compared with NH+4. The percentage K+ in the plant dry matter increased with temperature from 2% at 3 °C to 4% at 25 °C in rape, but there was less effect of temperature on the average concentrations of K+ in the plant fresh weight or plant water content. Concentrations of K+ in the leaf water fraction of rape plants decreased with increasing root temperature, but in barley they increased with increasing root temperature. Concentrations of K+ in the root water fraction were relatively stable with respect to root temperature. The results are discussed in terms of compensatory changes in K+ uptake following a change in root temperature and the relationships between growth, shoot: root ratio and K+ composition of the plant.  相似文献   

14.
A purified pectate lyase isozyme derived from Erwinia chrysanthemi induced rapid net K+ efflux and H+ influx in suspension-cultured tobacco cells. Comparable fluxes of other ions (Na+, Cl) were not observed. The K+ efflux/H+ influx response began within 15 minutes after addition of enzyme to cell suspensions and continued for approximately 1 hour after which cells resumed the net H+ efflux exhibited prior to enzyme treatment. The response was not prolonged by a second enzyme dose 1 hour after the first. The K+/H+ response was characterized by saturation at low enzymic activity (2 × 10−3 units per milliliter), and inhibition by the protonophore, carbonyl cyanide m-chlorophenylhydrazone, and was not associated with membrane leakiness caused by structural cell wall damage. The total K+ loss and H+ uptake induced by enzyme was one-fourth to one-third that induced by Pseudomonas syringae pv. pisi and did not reduce cell viability. These results indicate that pectate lyase induces a K+ efflux/H+ influx response in tobacco similar to but of shorter duration than that induced by P. syringae pv. pisi during the hypersensitive response. Pectate lyase or other cell wall degrading enzymes may therefore influence the induction of hypersensitivity.  相似文献   

15.
Acid-base regulation during ammonium assimilation in Hydrodictyon africanum   总被引:1,自引:1,他引:0  
Abstract The acid-base balance during ammonium (used to mean NH 4+ and/or NH3) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m?3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H+ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H+ accumulate in the medium per organic N synthesized. Granted a permeability (PNH3) of some 10?3 cm s?1, and a finite [NH3] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH 4+. This means that most of the H + appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H+ accumulates in the medium equivalent to any NH3 entry by requilibration from exogenous NH 4+. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO3+grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO3+in H. africanum. Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO3+-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm?3) the net influx was all attributable to NH4+influx via a uniporter, probably partly short-circuited by a passive NH3 efflux due to intrinsic membrane permeability to NH3. The net ammonium influx was in all cases associated with H+ accumulation in the medium. (1.3-1.7 H + per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated. Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H+ fluxes.  相似文献   

16.
NH4+ and K+ uptake experiments have been conducted with 3 ectomycorrhizal fungi, originating from Douglas fir (Pseudotsuga menziesii (Mirb.] Franco) stands. At concentrations up to 250 μM, uptake of both NH4+ and K+ follow Michaelis-Menten kinetics. Laccaria bicolor (Maire) P. D. Orton, Lactarius rufus (Scop.) Fr. and Lactarius hepaticus Plowr. ap. Boud. exhibit Km values for NH4+ uptake of 6, 35, and 55 μM, respectively, and Km values for K+ uptake of 24, 18, and 96 μM, respectively. Addition of 100 μM NH4+ raises the Km of K+ uptake by L. bicolor to 35 μM, while the Vmax remains unchanged. It is argued that the increase of Km is possibly caused by depolarization of the plasma membrane. It is not due to a competitive inhibition of K+ by NH4+ since the apparent inhibitor constant is much higher than the Km, for NH4+ uptake. The possibility that NH4+ and K+ are taken up by the same carrier can be excluded. The Km, values for K+ uptake in the two other fungi are not significantly affected by 100 μM NH4+. Except for a direct effect of NH4+ on influx of K+ into the cells, there may also be an indirect effect after prolonged incubation of the cells in the presence of 100 μM NH4+.  相似文献   

17.
Utilization of nitrogen in the form of either nitrate (NO 3 ? ) or ammonium (NH 4 + ) ions may affect the carbohydrate metabolism and energy budget of plants. Recent studies showed that greater expenses of NO 3 ? to NH 4 + reduction mostly occur in the roots and during darkness. Fertilization of corn with 15N-labeled nitrate and ammonium, combined with pulse labeling of plants in a 14CO2 atmosphere at the V6 and V8 growth stages, allowed us to evaluate the effect of N form on the CO2 efflux from soil. NH 4 + oxidation was inhibited by adding dicyandiamide. In respect to ammonium, nitrate addition increased root-derived CO2 efflux from corn by 2.6 times at stage V6 and by 1.8 times at stage V8. The time of peak 14CO2 efflux from soil also differed between two growing stages: at V6, efflux peaked only on the second day after pulse labeling, while at V8 this occurred within the first 6 h. The strong effect of NO 3 ? and NH 4 + on root respiration requires considering the N form in the soil and the nitrate reduction site location in a plant when modeling soil respiration changes and when separately estimating individual CO2 sources that contribute to the total soil CO2 efflux.  相似文献   

18.
The effects of metabolic and protein synthesis inhibitors on NH4 + uptake by Pisum arvense plants at low (0.05 mM) and high (1 mM) external ammonium concentration were studied. In short-time experiments cycloheximide decreased the ammonium uptake rate at low level of NH4 + and increased the absorption of NH4 + from uptake medium containing high ammonium concentration. Arsenate and azide supplied into uptake solutions at low ammonium concentration strongly decreased or completely suppressed the NH4 + uptake rate, respectively. When the experiments were carried out at high level of ammonium only azide decreased the uptake rate of NH4 + and arsenate stimulated this process. Dinitrophenol very strongly repressed the uptake rate of NH4 + at both ammonium concentrations. After removing dinitrophenol from both solutions, neither at low nor high external ammonium level the recovery of NH4 + uptake rate was achieved within 150 min or 3 h, respectively. The recovery of NH4 + uptake rate after removing azide was observed within 90 min and 3 h at low and high ammonium concentrations, respectively. The regulation of NH4 + uptake by some inhibitors at low external ammonium level was investigated using plasma membrane vesicles isolated from roots by two-phase partitioning. Orthovanadate completely suppressed the uptake of NH4 + by vesicles and quinacrine decreased the NH4 + uptake which 55 suggests that ammonium uptake depends on activities of plasma membrane-bound enzymes. On the other hand, it was found that dinitrophenol completely reduced the NH4 + uptake by vesicles. The various effects of inhibitors on ammonium uptake dependent on external ammonium concentration suggest the action of different ammonium transport systems in Pisum arvense roots. The ammonium transport into root cells at low NH4 + level requires energy and synthesis of protein in the cytoplasm. The research was supported by grant of KBN No. 6PO4C 068 08  相似文献   

19.
20.
The influence of NH4+, in the external medium, on fluxes of NO3 and K+ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH4+ was without effect on NO3 (36ClO3) influx whereas inhibition of net uptake appeared to be a function of previous NO3 provision. Plants grown at 10 micromolar NO3 were sensitive to external NH4+ when uptake was measured in 100 micromolar NO3. By contrast, NO3 uptake (from 100 micromolar NO3) by plants previously grown at this concentration was not reduced by NH4+ treatment. Plants pretreated for 2 days with 5 millimolar NO3 showed net efflux of NO3 when roots were transferred to 100 micromolar NO3. This efflux was stimulated in the presence of NH4+. NH4+ also stimulated NO3 efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO3 occur via effects upon efflux. By contrast to the situation for NO3, net K+ uptake and influx of 36Rb+-labeled K+ was inhibited by NH4+ regardless of the nutrient history of the plants. Inhibition of net K+ uptake reached its maximum value within 2 minutes of NH4+ addition. It is concluded that the latter ion exerts a direct effect upon K+ influx.  相似文献   

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