The regulation of potassium absorption in barley roots

The dynamics of changes in K(+) influx across the plasmalemma and of internal K(+) concentrations [K(+)](1) of intact barley (Hordeum vulgare) roots were examined as the roots were converted from ;high-salt' to ;low-salt' roots. Following the transfer of plants grown in 0.5 mm CaSO(4) solutions plus various concentrations of KCl to 0.5 mm CaSO(4) solutions, influx rates increased and internal K(+) concentrations declined as a function of time and the initial K(+) status of the tissue. The relationship between plasmalemma influx and [K(+)](1) was examined over a wide range of [K(+)](1) values by growing intact plants in various concentrations of KCl. Plasmalemma influx was inversely correlated with the square of [K(+)](1). A model for the regulation of plasmalemma influx by [K(+)](1) is considered.     

Response-velocity of chloride absorption by barley roots to oligomycin

Chloride absorption by excised barley roots is significantlyinhibited within the first 2 min of incubation in oligomycin.A steady inhibited absorption rate is attained in the presenceof oligomycin after 10 min (Received February 18, 1970; )     

Root respiration associated with ammonium and nitrate absorption and assimilation by barley

We examined nitrate assimilation and root gas fluxes in a wild-type barley (Hordeum vulgare L. cv Steptoe), a mutant (nar1a) deficient in NADH nitrate reductase, and a mutant (nar1a;nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO₃⁻ induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a;nar7w mutant. These results indicate that, under NH₄⁺ nutrition, 14% of root carbon catabolism is coupled to NH₄⁺ absorption and assimilation and that, under NO₃⁻ nutrition, 5% of root carbon catabolism is coupled to NO₃⁻ absorption, 15% to NO₃⁻ assimilation, and 3% to NH₄⁺ assimilation. The additional energy requirements of NO₃⁻ assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH₄⁺ and NO₃⁻ absorption and assimilation constitute a significant portion of root respiration.     

The absorption of potassium and several organic compounds by barley roots: Effect of siduron

Absorption of ⁴²K by excised roots of barley (Hordeum vulgareL.) grown in 0 or 5 ppm siduron (l-(2-methylcyclohexyl)-3-phenylurea)was a linear function of time for at least 60 minutes with transportbeing unidirectional. Absorption of siduron was a function ofthe external concentration to the limits of its solubility (0.09mM). However, the siduron- ¹⁴C absorbed by roots grown in either0 or 5 ppm siduron was in a readily exchangeable form and desorptionfor 4 hr exchanged 80 % of the label. Glucose-¹⁴C, adenine-8¹⁴Cand leucine¹⁴C were actively absorbed with 70 to 85 % of thelabel being absorbed in 24 hr. Although roots grown in iduronabsorbed less ⁴²K, glucose-¹⁴C, adenine-¹⁴C and siduron-¹⁴C,and more leucine-¹⁴C than similar roots grown in water culture,it is probable that these differences were not large enoughto account for the noted reduction (60%) in root growth. (Received January 9, 1968; )     

Aldehyde oxidase isoforms and subunit composition in roots of barley as affected by ammonium and nitrate

Aldehyde oxidase (AO; EC 1.2.3.1) isoforms in roots of barley plants grown on ammonium or nitrate as nitrogen sources were studied. Roots of ammonium-grown barley plants exhibited considerable levels of AO2, AO3, and AO4 activities after native PAGE. Significantly lower AO2 and AO3 activity bands were observed in roots of plants grown on nitrate. When abscisic aldehyde was used as a substrate a strong response of the AO2 band was observed as well as a faint reaction of the AO3 band, but no activity of AO4 was observed using this substrate. The 160 and 145 kDa polypeptides were detected in ammonium grown plants. Root extracts of nitrate-fed plants revealed only a minor 145 kDa protein band and none of the 160 kDa subunit was detected. The assembly of the AO3 heterodimer requires the simultaneous presence of 160 and 145 kDa subunits. Subunit analysis of AO2 and AO4 revealed homodimeric composition of 160 and 145 kDa, respectively. Western blot analysis revealed changing AO subunits levels during germination and plant development. Differential expression of AO subunits (160 and 145 kDa) and subsequent formation of isoforms, which differ in substrate specificity, distribution and fulfil different enzymatic reactions, may constitute an important regulatory mechanism in the plant.     

The influence of potassium content on the kinetics of potassium influx into excised ryegrass and barley roots

The influx of K⁺ into excised roots of barley (Hordeum vulgare L.) and ryegrass (Lolium multiflorum L.) previously grown with or without K⁺ was measured in K⁺ solutions ranging in concentration from 0.01 to 50 mM. In both species the K⁺ influx was lower in the roots with high K⁺ content. The extent of reduction by high internal [K⁺] decreased with external concentration above 1 mM. These results support the contention that at high external concentrations passive diffusion makes significant contributions to observed fluxes.     

Regulation of potassium absorption in barley roots: an allosteric model

Plasmalemma influx isotherms for K(+) were measured in the system I concentration range (0.01-0.32 mm), for barley (Hordeum vulgare L.) roots of varying internal K(+) concentration, and Km values for influx calculated. In plants grown for several days in CaSO(4) or in CaSO(4) plus KCl solutions, as well as in plants grown in CaSO(4) for several days and then rapidly loaded with KCl during a pretreatment period, Michaelis constant values were positively correlated with internal K(+) concentrations. Influx of K(+) is shown to be sigmoidally related to internal K(+) concentration and Hill plots of influx data give linear transformations with n = 4. This information is taken as support for an allosteric model for the regulation of K(+) influx in which the "carrier" is envisaged as possessing a single external binding site for K(+) as well as four internal sites for allosteric control of influx.     

Effect of sulfur and potassium on zinc absorption by barley

Summary Maximum uptake of Zn in barley (Hordeum vulgare L.) seedlings occurred from nutrient solutions containing SO₄−S at 3.5 ppm and K at 6 ppm. Decreased translocation of Zn from roots to tips was observed when plants were grown with lower levels of S and K. Cysteine substituted for SO₄-ion as a source of S in Zn absorption, and more Zn was absorbed with cysteine than with sulfate. The effect of K on Zn absorption seems to be influenced by S nutrition in plants.     

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).

     

Combination of ammonium nitrate, cerium chloride and potassium chloride salts improves Agrobacterium tumefaciens-mediated transformation of Nicotiana tabacum

The frequency of plant transformation can be improved by addition of various chemical into transformation media. In the past, we showed that exposure of tobacco, wheat and triticale explants to ammonium nitrate, cerium and lantanium chloride and potassium chloride resulted in an increase in the frequency of transformation. Here, we tested whether a combination of increased concentrations of the aforementioned salts yielded a higher transformation frequency. We found that exposure to 61.8 mM ammonium nitrate caused a 5.0-fold increase in transformation frequency, whereas exposure to 1.0 μM cerium chloride or 47.0 mM potassium chloride resulted in 1.2- and 2-fold increases, respectively. Exposure to 61.8 mM ammonium nitrate and 1.0 μM cerium chloride led to a 4.8-fold increase in transformation frequency, whereas exposure to 61.8 mM ammonium nitrate and 47.0 mM potassium chloride let to a 5.2-fold increase. Finally, exposure to 61.8 mM ammonium nitrate, 1.0 μM cerium chloride and 47.0 mM potassium chloride produced a 5.1-fold increase. The analysis of the intactness of T-DNA borders showed that plants exposed to ammonium nitrate and a combination of ammonium nitrate with other salts had the more intact right borders and the less intact left borders. The best results were observed when all three salts (ammonium nitrate, potassium chloride and cerium chloride) were used. Thus, we concluded that the addition of cerium chloride and potassium chloride does not substantially improve the transformation rate beyond the improvement observed upon treatment with 61.8 mM ammonium nitrate, but may slightly improve the intactness of T-DNA borders.     

Effect of divalent and other ions on potassium uptake by barley roots

Summary Potassium uptake as affected by the presence of divalent and other ions was studied from KCl solutions to which other salts were added. The study also includes the effect of the initial Ca-content of roots on the uptake of K⁺ from KCl and KCl + CaCl₂ solutions. The effect of time on K-uptake from electrolyte mixtures containing Mg-ions was also included. The experimental results seem to fit well a model of K⁺-uptake based on diffusion of ions in mixed electrolytes.     

Measurement of net fluxes of ammonium and nitrate at the surface of barley roots using ion-selective microelectrodes

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.     

Mechanisms of potassium absorption by higher plant roots

Potassium, as a plant macronutrient, is accumulated in plant cells from relatively dilute soil solutions and is indispensable for many vital processes. Studies characterising potassium uptake by roots stretch back over many decades. However, it is only with the introduction of modern electrophysiological and molecular techniques that investigations have been possible at a molecular level. Such approaches have confirmed the existence of discrete high and low affinity uptake systems at the root plasma membrane and have greatly enhanced our understanding of the underlying molecular nature of these uptake systems.
High affinity K⁺ uptake from micromolar external K⁺ levels is coupled to H⁺ transport as demonstrated independently by patch clamping of single root protoplasts and by studying the transport system after expression in Xenopus oocytes . The measured coupling ratio between the two ions is 1:1 and is sufficient to account for an accumulation ratio in excess of 10⁶, a value which encompasses experimental observations on K⁺ accumulation.
Low affinity K⁺ uptake activates at relatively high external K⁺ levels in the millimolar range and is 'passive' i.e. down the electrochemical gradient for potassium. In two higher plant species single cell inward potassium currents have been identified which are associated with low affinity potassium uptake. Furthermore, specific ion channels which underlie these potassium influxes and form a major constituent of the low affinity potassium uptake pathway have been identified and characterised.     

Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, K _m and I _max (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in I _max (up to four times), without concomitant change in K _m, which remained between 5 and 7 M. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the K _m (i.e. an increased apparent affinity for potassium) from 53 M to 11 M, without alteration to I _max. After longer periods of potassium starvation, I _max increased (about two times) while the K _m remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.Symbol I _max the maximum rate of absorption at saturating concentrations     

The influence of ammonium on nitrate reduction in wheat seedlings

Summary Ammonium markedly inhibited nitrate absorption by nitrogenstarved wheat seedlings but did not decrease the proportion of absorbed nitrate that was reduced. Seedlings high in nitrate (absorbed prior to the experimental periods) reduced similar amounts of this nitrate regardless of whether or not ammonium was present and being absorbed during the period of measurement. Ammonium or products of ammonium assimilation did not interfere with the induction, stability, or activity of nitrate reductase. Consequently, the hypothesis that ammonium depresses nitrate uptake indirectly by inhibiting nitrate reduction is rejected, and it is suggested that the ammonium effect is directly on the nitrateuptake process.Paper No. 2800 of the Journal Series of the North Carolina State University Agricultural Experiment Station. These investigations were supported by the U.S. Atomic Energy Commission, Contract No. AT-(40-1)-2410.     

Influence of potassium chloride on alcohol absorption

Simultaneous intragastric administration of large doses of KCl (430 mg/kg and 860 mg/kg) with ethanol (4 g/kg) significantly reduces blood alcohol levels and diminishes manifestations of alcohol intoxication in rats. It was shown with parenteral administration of alcohol that the effect is not related to an acceleration of alcohol metabolism. Analysis of alcohol concentrations of gastric and intestinal content as well as $\text{in situ}$ studies with animals whose stomachs were ligated at the pylorus revealed that KCl interferes with the absorption of alcohol through inhibition of gastric absorption and gastric emptying. The finding that equimolal concentrations of NaCl were unable to duplicate the described effects characterizes them as specific actions of the potassium ion.