Abundance of Cysteine Endopeptidase Dionain in Digestive Fluid of Venus Flytrap (Dionaea muscipula Ellis) Is Regulated by Different Stimuli from Prey through Jasmonates

The trap of the carnivorous plant Venus flytrap (Dionaea muscipula) catches prey by very rapid closure of its modified leaves. After the rapid closure secures the prey, repeated mechanical stimulation of trigger hairs by struggling prey and the generation of action potentials (APs) result in secretion of digestive fluid. Once the prey''s movement stops, the secretion is maintained by chemical stimuli released from digested prey. We investigated the effect of mechanical and chemical stimulation (NH₄Cl, KH₂PO₄, further N(Cl) and P(K) stimulation) on enzyme activities in digestive fluid. Activities of β-D-glucosidases and N-acetyl-β-D-glucosaminidases were not detected. Acid phosphatase activity was higher in N(Cl) stimulated traps while proteolytic activity was higher in both chemically induced traps in comparison to mechanical stimulation. This is in accordance with higher abundance of recently described enzyme cysteine endopeptidase dionain in digestive fluid of chemically induced traps. Mechanical stimulation induced high levels of cis-12-oxophytodienoic acid (cis-OPDA) but jasmonic acid (JA) and its isoleucine conjugate (JA-Ile) accumulated to higher level after chemical stimulation. The concentration of indole-3-acetic acid (IAA), salicylic acid (SA) and abscisic acid (ABA) did not change significantly. The external application of JA bypassed the mechanical and chemical stimulation and induced a high abundance of dionain and proteolytic activity in digestive fluid. These results document the role of jasmonates in regulation of proteolytic activity in response to different stimuli from captured prey. The double trigger mechanism in protein digestion is proposed.     

The dynamics of H+ efflux from the trap lobes of Dionaea muscipula Ellis (Venus's flytrap)

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.     

Trap closure and prey retention in Venus flytrap (Dionaea muscipula) temporarily reduces photosynthesis and stimulates respiration

Background and Aims

The carnivorous plant Venus flytrap (Dionaea muscipula) produces a rosette of leaves: each leaf is divided into a lower part called the lamina and an upper part, the trap, with sensory trigger hairs on the adaxial surface. The trap catches prey by very rapid closure, within a fraction of a second of the trigger hairs being touched twice. Generation of action potentials plays an important role in closure. Because electrical signals are involved in reduction of the photosynthetic rate in different plant species, we hypothesized that trap closure and subsequent movement of prey in the trap will result in transient downregulation of photosynthesis, thus representing the energetic costs of carnivory associated with an active trapping mechanism, which has not been previously described.

Methods

Traps were enclosed in a gas exchange cuvette and the trigger hairs irritated with thin wire, thus simulating insect capture and retention. Respiration rate was measured in darkness (R_D). In the light, net photosynthetic rate (A_N), stomatal conductance (g_s) and intercellular CO₂ concentration (c_i) were measured, combined with chlorophyll fluorescence imaging. Responses were monitored in the lamina and trap separately.

Key Results

Irritation of trigger hairs resulted in decreased A_N and increased R_D, not only immediately after trap closure but also during the subsequent period when prey retention was simulated in the closed trap. Stomatal conductance remained stable, indicating no stomatal limitation of A_N, so c_i increased. At the same time, the effective quantum yield of photosystem II (Φ_PSII) decreased transiently. The response was confined mainly to the digestive zone of the trap and was not observed in the lamina. Stopping mechanical irritation resulted in recovery of A_N, R_D and Φ_PSII.

Conclusions

We put forward the first experimental evidence for energetic demands and carbon costs during insect trapping and retention in carnivorous plants, providing a new insight into the cost/benefit model of carnivory.     

Ammonium-Stimulated Root Hair Branching is Enhanced by Methyl Jasmonate and Suppressed by Ethylene in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Root hair development is orchestrated by nutritional factors and plant hormones. We investigated the action of ammonium (NH₄⁺) and its interactions with methyl jasmonate (MeJA) and ethylene in Arabidopsis root hair growth. The formation of root hair branches was dramatically stimulated in media containing 1.25 to 20 mM NH₄⁺ at pH values of 4.0 to 6.5. The NH₄⁺-treated root hairs showed a very short tip growth stage and swells on the sides that indicated the emergence of branches. MeJA (0.08 to 10 μM) worked in synergism with NH₄⁺ to enhance hair branching. In contrast, ethylene had an antagonistic effect; the stimulation of hair branching by NH₄⁺ was suppressed by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and was diminished in ethylene-overproducing mutant eto1-1 seedlings. Moreover, the application of Ag⁺, an ethylene inhibitor, reduced the ACC-induced inhibition of NH₄⁺-stimulated hair branching and restored NH₄⁺-stimulated hair branching in eto1-1 seedlings. Thus, the actions of jasmonate and ethylene appear to be dependent on nutritional conditions such as available nitrogen.     

Root growth inhibition by NH4+ in Arabidopsis is mediated by the root tip and is linked to NH4+ efflux and GMPase activity

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.     

Regulation by fixed nitrogen of host-symbiont recognition in the Rhizobium-clover symbiosis

Either NO₃⁻ (16 millimolar) or NH₄⁺ (1 millimolar) completely inhibited infection and nodulation of white clover seedlings (Trifoliin repens) inoculated with Rhizobium trifolii. The binding of R. trifolii to root hairs and the immunologically detectable levels of the plant lectin, trifoliin, on the root hair surface had parallel declining slopes as the concentration of either NO₃⁻ or NH₄⁺ was increased in the rooting medium. This supports the role of trifoliin in binding R. trifolii to clover root hairs. Agglutination of R. trifolii by trifoliin from seeds was not inhibited by these levels of NO₃⁻ or NH₄⁺. The results suggest that these fixed N ions may play important roles in regulating an early recognition process in the Rhizobium-clover symbiosis, namely the accumulation of high numbers of infective R. trifolii cells on clover root hairs.     

Human SLC4A11 Is a Novel NH3/H+ Co-transporter

SLC4A11 has been proposed to be an electrogenic membrane transporter, permeable to Na⁺, H⁺ (OH⁻), bicarbonate, borate, and NH₄⁺. Recent studies indicate, however, that neither bicarbonate or borate is a substrate. Here, we examined potential NH₄⁺, Na⁺, and H⁺ contributions to electrogenic ion transport through SLC4A11 stably expressed in Na⁺/H⁺ exchanger-deficient PS120 fibroblasts. Inward currents observed during exposure to NH₄Cl were determined by the [NH₃]_o, not [NH₄⁺]_o, and current amplitudes varied with the [H⁺] gradient. These currents were relatively unaffected by removal of Na⁺, K⁺, or Cl⁻ from the bath but could be reduced by inclusion of NH₄Cl in the pipette solution. Bath pH changes alone did not generate significant currents through SLC4A11, except immediately following exposure to NH₄Cl. Reversal potential shifts in response to changing [NH₃]_o and pH_o suggested an NH₃/H⁺-coupled transport mode for SLC4A11. Proton flux through SLC4A11 in the absence of ammonia was relatively small, suggesting that ammonia transport is of more physiological relevance. Methylammonia produced currents similar to NH₃ but with reduced amplitude. Estimated stoichiometry of SLC4A11 transport was 1:2 (NH₃/H⁺). NH₃-dependent currents were insensitive to 10 μm ethyl-isopropyl amiloride or 100 μm 4,4′- diisothiocyanatostilbene-2,2′-disulfonic acid. We propose that SLC4A11 is an NH₃/2H⁺ co-transporter exhibiting unique characteristics.     

Ammonia,tetraethylammonium, barium and amiloride induce metamorphosis in the marine hydroid Hydractinia

Summary In Hydractinia metamorphosis from the swimming larval stage to the sessile polyp stage has been found to be inducible by several agents, including Li⁺, K⁺, Cs⁺, Rb⁺, diacylglycerol (DG), tetradecanoyl-phorbol-acetate (TPA) and some other tumour-promoting phorbol esters. Induction is antagonized by ouabain and compounds which are able to increase the internal level of S-adenosylmethionine (SAM). Based on the finding that Hydractinia larvae contain such compounds in a stored form, including N-methylpicolinic acid, N-methylnicotinic acid and N-trimethylglycine, as well as on the results of experiments with antagonists of SAM production and transmethylation, it has been argued that regulation of the internal SAM level plays a key role in the control of metamorphosis. However, it remains to be clarified whether the inducing agents act by decreasing the SAM level or by via different pathways. In the present study, substances chemically related to the substances known to induce or inhibit metamorphosis were tested for their metamorphosis-inducing abilities. Some were found to be effective, including NH₄ ⁺, methylamine, tetraethylammonium ions (TEA⁺), ethanolamine, Ba²⁺, Sr²⁺ and the diuretic, amiloride. It is of particular interest that in many organisms TPA and DG increase cytoplasmic pH while amiloride prevents a rise in pH_i. Several of the substances known to trigger metamorphosis may increase the internal NH₄ ⁺ concentration by hindering the export of the constantly produced NH₄ ⁺ through K⁺ channels or through the Na⁺-H⁺ antiport. Treatment with Cs⁺ for 1 h increases the internal level of NH₄ ⁺. Produced and applied ammonia, as well as applied methylamine and ethanolamine, may act by accepting methyl groups, thus reducing the SAM level.     

AMMONIUM STIMULATION OF DARK CARBON FIXATION AS AN INDICATOR OF NITROGEN DEFICIENCY IN PHYTOPLANKTON: POTENTIAL ERRORS CAUSED BY AMMONIUM-OXIDIZING BACTERIA1

Stimulation of dark fixation of carbon by NH₄⁺ is often used as an indicator of phytoplankton N deficiency. This assay is based on the influence of available NH₄⁺ on anaplerotic CO₂ fixation by algae. However, carbon fixation by chemoautotrophic NH₄⁺-oxidizing bacteria may also be stimulated by NH₄⁺ enrichment, a process that can mask the algal response in natural communities. NH₄⁺ addition enhanced dark carbon fixation up to 300%, relative to unamended controls, in organisms collected on a 0.7-μm retention filter in oligotrophic Flathead Lake, Montana, but the effect was not detectable in the presence of nitrapyrin, an inhibitor of NH₄⁺-oxidizing bacteria. Dark carbon fixation was enhanced with addition of NH₄⁺ in organisms retained on 2-μm filters (which should allow passage of most bacteria). NH₄⁺ stimulated dark carbon fixation in N-deficient axenic cultures of Chlamydomonas reinhardtii Dang but not in N-replete cultures in both the presence and absence of nitrapyrin. Application of nitrapyrin or size fractionation treatments, to separate the processes of dark carbon fixation by nitrifiers and phytoplankton, may improve the efficacy of assays using NH₄⁺ stimulation of dark carbon fixation to specifically indicate N deficiency in natural algal communities.     

Ion complexation in nonactin,monactin, and dinactin: A Raman spectroscopic study

The ability of the macrotetrolide nactins to complex selectivity with a wide variety of cations makes these ionophorous antibiotics important model systems for the study of biologic ionic transport. We report a Raman spectroscopic investigation of the Na⁺, K⁺, Rb⁺, Cs⁺, Tl⁺, NH₄⁺, NH₃OH⁺, C(NH₂)₃⁺, and Ba⁺⁺ complexes of nonactin, monactin, and dinactin in 4:1 (v/v) CH₃OH/CHCl₃ and in the solid state. The nactins display characteristic spectral changes upon complexation, some of which are specific for a given cation. In the K⁺, Rb⁺, Cs⁺, NH₃OH⁺, and C(NH₂)₃⁺ complexes, which are apparently isosteric, the ester carbonyl stretch frequency is found to be linearly proportional to the cation–carbonyl electrostatic interaction energy, as calculated from a simplified model. Deviations for the Na⁺, NH₄⁺, Tl⁺, and Ba⁺⁺ complexes are interpreted as arising from additional nonelectrostatic interactions. Additional information is obtained from other spectral regions and from measurements of depolarization ratios. Spectra of the nactin complexes differ from each other more in the solid state than in solution, reflecting the effects of crystalline contact forces.     

Optimization and elucidation of interactions between ammonium, nitrate and phosphate inCentella asiatica cell culture using response surface methodology

The effects of macronutrients (NO₃ ⁻, NH₄ ⁺ and PO₄ ³⁻) on cell growth and triterpenoids production inCentella asiatica cell suspension cultures were analyzed using the Box-Behnken response surface model experimental design. In screening and optimization experiments, PO₄ ³⁻ as a single factor significantly influenced cell growth where increasing the phosphate level from 0.1 to 2.4 or 2.6 mM, elevated cell growth from 3.9 to 14–16 g/L. The optimum values predicted from the response surface model are 5.05 mM NH₄ ⁺, 15.0 mM NO₃ ⁻ and 2.6 mM PO₄ ³⁻, yielding 16.0 g/L cell dry weight with 99% fitness to the experimental data. While the NH₄ ⁺-NO₃ ⁻ interaction influenced cell growth positively in the optimization experiment, NH₄ ⁺ and NO₃ ⁻ as single factors; and interactions of NO₃ ⁻-PO₄ ³⁻, NH₄ ⁺-PO₄ ³⁻ and NH₄ ⁺-NO₃ ⁻ were all negative in the screening experiment. Cell growth and the final pH level were positively affected by PO₄ ³⁻, but negatively affected by NH₄ ⁺ and NH₄ ⁺-PO₄ ³⁻ interactions. The different effects of factors and their interactions on cell growth and final pH are influenced by a broad or narrow range of macronutrient concentrations. The productions of triterpenoids however were lower than 4 mg/g cell dry weight.     

Regulation of Na,K-ATPase Transport Activity by Protein Kinase C

Considerable evidence indicates that the renal Na⁺,K⁺-ATPase is regulated through phosphorylation/dephosphorylation reactions by kinases and phosphatases stimulated by hormones and second messengers. Recently, it has been reported that amino acids close to the NH₂-terminal end of the Na⁺,K⁺-ATPase α-subunit are phosphorylated by protein kinase C (PKC) without apparent effect of this phosphorylation on Na⁺,K⁺-ATPase activity. To determine whether the α-subunit NH₂-terminus is involved in the regulation of Na⁺,K⁺-ATPase activity by PKC, we have expressed the wild-type rodent Na⁺,K⁺-ATPase α-subunit and a mutant of this protein that lacks the first thirty-one amino acids at the NH₂-terminal end in opossum kidney (OK) cells. Transfected cells expressed the ouabain-resistant phenotype characteristic of rodent kidney cells. The presence of the α-subunit NH₂-terminal segment was not necessary to express the maximal Na⁺,K⁺-ATPase activity in cell membranes, and the sensitivity to ouabain and level of ouabain-sensitive Rb⁺-transport in intact cells were the same in cells transfected with the wild-type rodent α1 and the NH₂-deletion mutant cDNAs. Activation of PKC by phorbol 12-myristate 13-acetate increased the Na⁺,K⁺-ATPase mediated Rb⁺-uptake and reduced the intracellular Na⁺ concentration of cells transfected with wild-type α1 cDNA. In contrast, these effects were not observed in cells expressing the NH₂-deletion mutant of the α-subunit. Treatment with phorbol ester appears to affect specifically the Na⁺,K⁺-ATPase activity and no evidence was observed that other proteins involved in Na⁺-transport were affected. These results indicate that amino acid(s) located at the α-subunit NH₂-terminus participate in the regulation of the Na⁺,K⁺-ATPase activity by PKC. Received: 10 July 1996/Revised: 19 September 1996     

Kinetics of NH4+ and K+ uptake by ectomycorrhizal fungi. Effect of NH4+ on K+ uptake

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 K_m values for NH₄⁺ uptake of 6, 35, and 55 μM, respectively, and K_m values for K⁺ uptake of 24, 18, and 96 μM, respectively. Addition of 100 μM NH₄⁺ raises the K_m of K⁺ uptake by L. bicolor to 35 μM, while the V_max remains unchanged. It is argued that the increase of K_m 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 K_m, for NH₄⁺ uptake. The possibility that NH₄⁺ and K⁺ are taken up by the same carrier can be excluded. The K_m, 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⁺.     

Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings

The effect of exogenous NH₄⁺ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO₃⁺ in the presence of 0, 1 or 10 mM NH₄⁺, Exogenous NH₄⁺ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO₃⁺, In contrast, the induction of NiRA was inhibited by NH₄⁺ at all NO₃⁺ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH₄⁺ Pre-treatment with NH₄⁺ had no effect on the subsequent induction of NRA in the absence of additional NH₄⁺ whereas the induction of NiRA in NH₄⁺-pretreated roots was inhibited in the absence of NH₄⁺ At 10 mM NO₃⁺ L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH₄⁺ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH₄⁺ supply. During the postinduction phase, exogenous NH₄⁺ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH₃⁺ whereas, NiRA was unaffected by NH₄⁺ at either substrate concentration. The results indicate that exogenous NH₄⁺ regulates the induction of NRA in roots by limiting the availability of NO₃⁺. Conversely, it has a direct effect, independent of the availability of NO₃⁺, on the induction of NiRA. The lack of an NH₄⁺ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme.     

K+-independent effects of valinomycin in photosynthetic systems

With chromatophores ofRhodospirillum rubrum, valinomycin inhibited electron transport in the presence or absence of K⁺. NH₄Cl had no effect on photophosphorylation but uncoupled with valinomycin present. ATPase activity was stimulated by NH₄Cl plus valinomycin but not by either alone. K⁺ partially reversed the inhibition of phosphorylation and the stimulation of ATPase by valinomycin plus NH₄Cl.With chloroplasts, valinomycin inhibited coupled but not basal electron transport. The inhibition was only partially reversed by uncouplers. Valinomycin stimulated the light-activated Mg²⁺-dependent ATPase similar to several uncouplers such as quinacrine, methylamine, and S-13. In addition, valinomycin inhibited delayed light emission and stimulated the H⁺/e^– ratio. These contrasting activities in chloroplasts are not easily explained.Contribution number 389 of the Charles F. Kettering Research Laboratory.     

Ammonium and Methylammonium Transport in Egeria densa Leaves in Conditions of Different H+ Pump Activity

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 (E_m) 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 E_m 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⁺.     

Effect of ammonium on nitrate utilization by roots of dwarf bean

The effect of exogenous NH₄⁺ on NO₃⁻ uptake and in vivo NO₃⁻ reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO₃⁻ uptake. Pretreatment with NH₄Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO₃⁻ uptake.

When NH₄⁺ was given at the start of NO₃⁻ nutrition, the time pattern of NO₃⁻ uptake was the same as in plants receiving no NH₄⁺. After 6 hours, however, the NO₃⁻ uptake rate (NUR) and root NRA were inhibited by NH₄⁺ to a maximum of 45% and 60%, respectively.

The response of the NUR of NO₃⁻-induced plants depended on the NH₄Cl concentration. Below 1 millimolar NH₄⁺, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH₄⁺ at 2 millimolar and above caused a rapid and transient stimulation of NO₃⁻ uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO₃⁻-induced plants, root NRA responded less and more slowly to NH₄⁺ than did NUR.

Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH₄⁺ inhibition of the NUR of NO₃⁻-induced plants. We conclude that repression of the NUR by NH₄⁺ depends on NH₄⁺ assimilation. The repression by NH₄⁺ was least at the lowest and highest NH₄⁺ levels tested (0.04 and 25 millimolar).

     

The influence of ammonium and chloride on potassium and nitrate absorption by barley roots depends on time of exposure and cultivar

Net uptakes of K⁺ and NO₃⁻ were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K⁺ and NO₃⁻ uptake in the absence of NH₄⁺ or Cl⁻. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH₄⁺, K⁺, Cl⁻, or NO₃⁻ affected the cultivars very differently: (a) the presence of NH₄⁺ as NH₄Cl stimulated net NO₃⁻ uptake in Prato barley but inhibited net NO₃⁻ uptake in Olli barley; (b) Cl⁻ inhibited net NO₃⁻ uptake in Prato but had little effect in Olli; and (c) NH₄⁺ as (NH₄)₂SO₄ inhibited net K⁺ uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl⁻ initially inhibited net K⁺ uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH₄⁺ and Cl⁻ uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.