Acclimation of potassium influx in rye (Secale cereale) to low root temperatures

The influx of K⁺(⁸⁶Rb⁺) into intact roots of rye (Secale cereale L. cv. Rheidal) exposed to a differential temperature (DT) between the root (8° C) and shoot (20° C) is initially reduced compared with warm-grown (WG) controls with both shoot and root maintained at 20° C. Over a period of 3 d, however, K⁺-influx rates into DT plants are restored to levels similar to or greater than those of the WG controls, the absolute rates of K⁺ influx being strongly dependent upon the shoot/root ratio. Acclimation in DT plants results in a reduction of K⁺ influx into the apical (0–2 cm) region of the seminal root which is associated with a compensatory increase in K⁺ influx into the more mature, basal regions of the root. Values of V _max and apparent K _m for K⁺ influx into DT plants were similar to those for WG plants at assay temperatures of 8° C and 20° C except for an increase in the apparent K _m at 8° C. The influx of K⁺ from solutions containing 0.6 mol·m^-3 K⁺ into both WG and DT plants was found to be linearly related to assay temperature over the range 2–27° C, and the temperature sensitivity of K⁺ influx to be dependent upon shoot/root ratio. At high shoot/root ratios, the ratio of K⁺ influx at 20° C:K⁺ influx at 8° C for WG plants approached a minimum value of 1.9 whereas that for DT plants approached unity indicating that K⁺ influx into DT plants has a large temperature-insensitive component. Additionally, when plants were grown in solutions of low potassium concentration, K⁺ influx into DT plants was consistently greater than that into WG plants, in spite of having a greater root potassium concentration ([K⁺]int). This result indicates some change in the regulation of K⁺ influx by [K⁺]int in plants exposed to low root temperatures. We suggest that K⁺ influx into rye seedlings exposed to low root temperatures is regulated by the increased demand placed on the root system by a proportionally larger shoot and that the acclimation of K⁺ influx to low temperatures may be the result of an increased hydraulic conductivity of the root system.Abbreviations DT differential temperature pretreatment - [K⁺]int root potassium concentration - [K⁺]ext potassium concentration of nutrient medium - WG warm-grown pretreatment     

A Model for the Regulation of K Influx, and Tissue Potassium Concentrations by Negative Feedback Effects upon Plasmalemma Influx

By means of a modified Michaelis-Menten equation for K⁺ influx, which includes terms for root and external K⁺ concentrations (root [K⁺] and [K⁺]₀, respectively) it is possible to predict the manner in which short-term (perturbation) fluxes of K⁺ into roots of barley plants (Hordeum vulgare cv Fergus) vary with root [K⁺] and [K⁺]₀. Influx values derived from this equation were used to predict changes of root and shoot [K⁺] and K⁺ absorption rates (as functions of time and [K⁺]₀) from a knowledge of K⁺ efflux, relative growth rates of roots and shoots, and the partitioning of absorbed K⁺ between these organs. A microcomputer program was employed to model these changes in low-salt plants following transfer to solutions in which [K⁺]₀ was maintained at values ranging from 5 to 1000 millimoles per cubic meter. The model was operated on the basis of 10 minute absorption periods which provided data for continuous `updating' of tissue [K⁺]. The simulations were undertaken for periods corresponding to 30 days. During this time the model accurately predicted the manner in which K⁺ influx and root and shoot [K⁺] gradually approach values which are essentially independent of [K⁺]₀. The computer program was also used to predict the outcome of changing various external and internal parameters of the proposed regulatory system. The results of these simulations are discussed in the context of current models for negative feedback control of ion fluxes.     

Measuring Fluxes of Mineral Nutrients and Toxicants in Plants with Radioactive Tracers

Unidirectional influx and efflux of nutrients and toxicants, and their resultant net fluxes, are central to the nutrition and toxicology of plants. Radioisotope tracing is a major technique used to measure such fluxes, both within plants, and between plants and their environments. Flux data obtained with radiotracer protocols can help elucidate the capacity, mechanism, regulation, and energetics of transport systems for specific mineral nutrients or toxicants, and can provide insight into compartmentation and turnover rates of subcellular mineral and metabolite pools. Here, we describe two major radioisotope protocols used in plant biology: direct influx (DI) and compartmental analysis by tracer efflux (CATE). We focus on flux measurement of potassium (K⁺) as a nutrient, and ammonia/ammonium (NH₃/NH₄⁺) as a toxicant, in intact seedlings of the model species barley (Hordeum vulgare L.). These protocols can be readily adapted to other experimental systems (e.g., different species, excised plant material, and other nutrients/toxicants). Advantages and limitations of these protocols are discussed.     

Influence of Excision and Aging upon K+ Influx into Barley Roots: Recovery or Enhancement? 1

The influx of K⁺ from ⁸⁶Rb-labeled solutions in the concentration range 0.008 to 0.2 mm into roots of intact plants and excised roots of barley plants (Hordeum vulgare [L.]) previously grown in 5 mm CaSO₄ (low K⁺ roots) or 0.5 mm CaSO₄ plus 5 mm KCl (high K⁺ roots) was measured. A consistent observation of these experiments was a substantial reduction of influx (usually by about 50%) following excision. The possible leakage of K⁺ into the medium and subsequent dilution of specific activity of labeled solutions was eliminated as an explanation for influx reduction in excised low K⁺ roots. Reduction of transpirational rates was also without effect upon influx into low K⁺ roots. Excision followed by 2 hours aging in 0.5 mm CaSO₄ solution revealed that influx values recovered within the 2 hours to the values obtained in intact roots. It is concluded that much of the literature which describes the enhancement of ion uptake following excision actually describes excision damage followed by recovery.     

Potassium ion influx measurements on cultured Chinese hamster cells exposed to 60-Hertz electromagnetic fields

Potassium ion influx was measured by monitoring ⁴²KCI uptake by Chinese hamster ovary (CHO) cells grown in suspension culture and exposed in the culture medium to 60–Hz electromagnetic fields up to 2.85 V/m. In the presence of the field CHO cells exhibited two components of uptake, the same as previously observed for those grown under normal conditions; both these components of influx were decreased when compared to sham-exposed cells. Although decreases were consistently observed in exposed cells when plotted as log_c of uptake, the differences between the means of the calculated fluxes of exposed and sham-exposed cells were quite small (on the order of 4–7%). When standard deviations were calculated, there was no significant difference between these means; however, when time-paired uptake data were analyzed, the differences were found to be statistically significant. Cells exposed only to the magnetic field exhibited similar small decreases in influx rates when compared to sham-exposed cells, suggesting that the reduction in K⁺ uptake could be attributed to the magnetic field. Additionally, intracellular K⁺ levels were measured over a prolonged exposure period (96 h), and no apparent differences in intracellular K⁺ levels were observed between field-exposed and shamexposed cultures. These results indicate that high-strength electric fields have a small effect on the rate of transport of potassium ions but no effect on long-term maintenance of intracellular K⁺.     

Cation fluxes in excised and intact roots in relation to specific and varietal differences

Summary In this short survey differences between species and varieties in the four major mechanisms that affect selective uptake of potassium and sodium to the plant within the root are considered. These include influx selectivity, K⁺/Na⁺ exchange at the plasmalemma, and selectivity at the tonoplast as well as at the symplasm-xylem boundary. The affinity of various plants for potassium influx in system 1 is rather uniform although varietal differences in barley have been observed. Differences are much more pronounced for sodium influx, for which Helianthus showed rather high and Fagopyrum rather low affinity. There is substantial variation between species in the efficiency of K⁺/Na⁺ exchange at the plasmalemma of cortical root cells; the three cereals Hordeum, Triticum, and Secale were highly efficient while K⁺/Na⁺ exchange in Atriplex, Helianthus and Allium was poor, even if the cytoplasmic sodium content was accounted for. Apparently there was no direct relation between salt tolerance and K⁺/Na⁺ exchange. The observed differences in the efficiency of K⁺-dependent sodium extrusion or K⁺/Na⁺ exchange were not due to the use of excised roots, they were observed also when roots of whole seedlings were investigated. At the tonoplast a 11 exchange of vacuolar potassium for sodium has been observed in roots of Hordeum. By this exchange sodium ions are removed from the symplasm and potassium ions are recovered from vacuoles and thus made available for transport to the shoot. Indications for specific differences in this exchange have been observed; the exchange appears to be more efficient in Helianthus than in Hordeum roots. More comparative studies are needed here. At the boundary between symplasm and xylem vessels selectivity can be set up during xylem release of cations and there are reports that suggest a preference for sodium (Lycopersicum cheesemanii, Solanum pennellii, and Suaeda) and for varietal differences amongst tomatoes. Selectivity at this boundary, the plasmalemma of the xylem parenchyma cells was described in this paper by the selectivity ratio of transport that relates the rates of xylem transport to the cytoplasmic sodium and potassium concentrations. Based on this ratioAtriplex hortensis was shown to discriminate for sodium during xylem release while there was little selectivity in Hordeum and possibly some discrimination in favour of K⁺ in Allium roots. The data are shortly discussed in relation to salt tolerance and to the breeding of salt-tolerant crop varieties.     

The influence of monovalent cations upon influx and efflux of Ca2+ in barley roots

The effects of monovalent cations, inhibitors of metabolism dinitrophenol (DNP), carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), and KCN and temperature variations upon Ca²⁺ fluxes in intact roots of barley (Hordeum vulgare L. cv. Fergus and Herta) seedlings were investigated. ⁴⁵Ca²⁺ influx was depressed in CaSO₄-grown (low-salt) plants by the presence of NH₄⁺, K⁺, or Na⁺ in the uptake medium. In contrast Ca²⁺ influx was slightly increased by Li⁺. In low-salt roots pretreated with KCN and in roots preloaded with K⁺ (high-K⁺ plants), the presence of K⁺ in the medium had no significant effect on Ca²⁺ influx, while in roots preloaded with Na⁺, the presence of K⁺ in the medium depressed Ca²⁺ influx. In absolute terms, Ca²⁺ influx was significantly greater in high-salt (both K⁺ or Na⁺ preloaded) than in low-salt roots.Patterns of ⁴⁵Ca²⁺ efflux in the absence and in the presence of K⁺, NH₄⁺, or Li⁺ in the external medium showed that these monovalent cations caused stimulation of ⁴⁵Ca²⁺ efflux both from the cytoplasmic and vacuolar phases.It was noted that these modifications of Ca²⁺ fluxes by monovalent cations are transient and characteristic of a transitional stage of cation uptake by low-salt roots. We conclude that, together with stimulated active H⁺ efflux (another characteristic of this transitional stage), modifications of Ca²⁺ fluxes during monovalent cation uptake by low-salt roots is a response directed towards the maintenance of electrical neutrality.Determination of net fluxes revealed that the plants were close to Ca²⁺ flux equilibrium in the growth medium (0.5 mM CaSO₄). Transfer of these plants to 0.5 mM CaSO₄ + 0.25 mM K₂SO₄ caused a net release of CA²⁺ into the external medium.     

The relationship between Rb+ influx and K+-stimulated Mg2+-ATPase activity in oat roots of different K+ status flexible coupling?

The relationship between Rb⁺ influx and microsomal ATPase activity stimulated by K⁺ and Mg²⁺+ K⁺ was investigated for roots of 7-day-old seedlings of oat (Avena sativa L., cv. Brighton). Different concentrations of K⁺ in the roots, K⁺_root were produced by cultivating plants in complete nutrient solutions of different dilutions and dFifferent K⁺ concentrations at various temperatures. Experiments were performed in both light and darkness. The range of the influx/ATPase ratios was large with a factor of 5 or more between the highest and lowest values. In most cases, the highest ratios were obtained at low K⁺_root and at high temperatures, and the lowest at high K⁺_root and at low temperatures. At high temperatures (20 and 25°C) in the light, the influx/ATPase ratio was constant, independently of K⁺_root, if K⁺ in the medium was kept constant but the bulk of the nutrient solution diluted. If K⁺ was varied and the other components of the medium kept constant, the normal relation of decreasing influx/ATPase ratio at increasing K⁺_root was found; thus, Rb⁺ influx appears regulated by both the internal and external potassium conditions. Also in darkness, at 15°C and with K ⁺ in the medium varied, the influx/ATPase ratio was independent of K⁺_root but in the corresponding light experiments, ratio and K⁺_root had the normal, inverse relationship. The difference between light and dark conditions appears to indicate that growth rate is of importance for the relationship between energy input and transport. Our data lead to the concept of “flexible coupling” between transducers) of energy and ion carrier. Without excluding other possibilities, this may be one of the mechanisms for ecological adaptation to variations in the root medium.     

Potassium activation of Na+-dependent leucine transport in brush-border membrane vesicles from rat jejunum

Na⁺-dependent leucine uptake was greater in potassium loaded brush-border membrane vesicles compared with controls. This effect was not mediated by an electrical potential difference, since it was still present in voltage-clamped conditions. Inhibition experiments indicate the same Na⁺-dependent leucine transport activity in the presence or in the absence of potassium. The affinity of sodium for the cotransporter was identical at 10 or 100 mM potassium. Leucine kinetics at different potassium concentrations showed a maximum 2.4-fold increase in V_max, while K_m was unaffected. The secondary plots of the kinetic results were not linear. This kinetic behaviour suggests that K⁺ acts as a non-essential activator of Na⁺-dependent leucine cotransport. A charge compensation of sodium-leucine influx is most probably a component of the potassium effect in the presence of valinomycin.     

Complexity of potassium acquisition: How much flows through channels?

The involvement of potassium (K⁺)-selective, Shaker-type channels, particularly AKT1, in primary K⁺ acquisition in roots of higher plants has long been of interest, particularly in the context of low-affinity K⁺ uptake, at high K⁺ concentrations, as well as uptake from low-K⁺ media under ammonium (NH₄⁺) stress. We recently demonstrated that K⁺ channels cannot mediate K⁺ acquisition in roots of intact barley (Hordeum vulgare L.) seedlings at low (22.5 µM) external K⁺ concentrations ([K⁺]_ext) and in the presence of high (10 mM) external NH₄⁺, while the model species Arabidopsis thaliana L. utilizes channels under comparable conditions. However, when external NH₄⁺ was suddenly withdrawn, a thermodynamic shift to passive (channel-mediated) K⁺ influx was observed in barley and both species demonstrated immediate and dramatic stimulations in K⁺ influx, illustrating a hitherto unexplored magnitude and rapidity of K⁺-uptake capacity and plasticity. Here, we expand on our previous work by offering further characterization of channel-mediated K⁺ fluxes in intact barley, with particular focus on anion effects, root respiration and pharmacological sensitivity and highlight key additions to the current model of K⁺ acquisition.     

Potassium-dependent changes in the expression of membrane-associated proteins in barley roots : I. Correlations with k(rb) influx and root k concentration

Barley (Hordeum vulgare L. cv Halcyon) seedlings which had been grown in full strength complete inorganic nutrient media (containing 6 millimolar K⁺) had high internal K⁺ concentrations and low values of K⁺ (⁸⁶Rb⁺) influx when influx was measured from solutions containing 100 micromolar K⁺. Transfer of these plants to solutions lacking K⁺ resulted in significant reductions of root and shoot K⁺ concentrations and values of K⁺ (⁸⁶Rb⁺) influx increased by greater than 10-fold within 3 days. When plants treated in this way were returned to complete solutions, containing K⁺, the changes induced by K⁺ deprivation were reversed. Parallel studies of microsomal membranes by means of SDS-PAGE demonstrated that the expression of a group of polypeptides increased or decreased in parallel with changes of K⁺ (⁸⁶Rb⁺) influx. Most prominent of these were 45 and 34 kilodalton polypeptides which specifically responded to K⁺ status of the barley plants; their expression was not enhanced by N or P deprivation. The 45 kilodalton polypeptide was susceptible to degradation by a membrane associated protease when microsomes were washed in buffer containing 0.2 millimolar PMSF. This loss was prevented by increasing PMSF concentration to 2 millimolar.     

Regulation of ammonium and potassium uptake by glutamine and asparagine in Pisum arvense plants

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     

Fluxes of h and k in corn roots : characterization and stoichiometries using ion-selective microelectrodes

We report here on an experimental system that utilizes ion-selective microelectrodes to measure the electrochemical potential gradients for H⁺ and K⁺ ions within the unstirred layer near the root surface of both intact 4-day-old corn seedlings and corn root segments. Analysis of the steady state H⁺ and K⁺ electrochemical potential gradients provided a simultaneous measure of the fluxes crossing a localized region of the root surface. Net K⁺ influx values obtained by this method were compared with unidirectional K⁺ (⁸⁶Rb⁺) influx kinetic data; at any particular K⁺ concentration, similar values were obtained by either technique. The ionspecific microelectrode system was then used to investigate the association between net H⁺ efflux and net K⁺ influx. Although the computed H⁺:K⁺ stoichiometry is dependent upon the choice of diffusion coefficients, the values obtained were extremely variable, and net K⁺ influx rarely appeared to be charge-balanced by H⁺ efflux. In contrast to earlier studies, we found the cortical membrane potential to be highly K⁺ sensitive within the micromolar K⁺ concentration range. Simultaneous measurements of membrane potential and K⁺ influx, as a function of K⁺ concentration, revealed similar K_m values for the depolarization of the potential (K_m 6-9 micromolar K⁺) and net K⁺ influx (K_m 4-7 micromolar K⁺). These data suggest that K⁺ may enter corn roots via a K⁺-H⁺ cotransport system rather than a K⁺/H⁺ antiporter.     

The α1 Na+−K+ pump of the Dahl salt-sensitive rat exhibits altered Na+ modulation of K+ transport in red blood cells

The properties of the α1 Na⁺-K⁺ pump were compared in Dahl salt-sensitive (DS) and salt-resistant (DR) strains by measuring ouabain-sensitive luxes (mmol/liter cell x hr = FU, Mean ± se) in red blood cells (RBCs) and varying internal ( _i ) and external ( _o ) Na⁺ and K⁺ concentrations. Kinetic parameters of several modes of operation, i.e., Na⁺/ K⁺, K⁺/K⁺, Na⁺/Na⁺ exchanges, were characterized and analyzed for curve-fitting using the Enzfitter computer program. In unidirectional flux studies (n=12 rats of each strain) into fresh cells incubated in 140 mm Na⁺ + 5 mm K⁺, ouabain-sensitive K⁺ influx was substantially lower in the DS than in DR RBCs, while ouabain-sensitive Na⁺ efflux and Na _i were similar in both strains. Thus, the coupling ratio between unidirectional Na⁺∶K⁺ fluxes was significantly higher in DS than in DR cells at similar RBC Na⁺ content. In the presence of 140 mm Na _o , activation of ouabain-sensitive K⁺ influx by K _o had a lower K _m and V _max in DS as estimated by the Garay equation (N=2.70 ± 0.33, K _m 0.74 ± 0.09 mm; V _max 2.87 ± 0.09 FU) than in DR rats (N=1.23 ± 0.36, K _m 2.31 ± 0.16 mm; v _max 5.70 ± 0.52 FU). However, the two kinetic parameters were similar following Na _o removal. The activation of ouabain-sensitive K⁺ influx by Na _i had significantly lower V _max in DS (9.3 ± 0.4 FU) than in DR (14.5 ± 0.6 FU) RBCs but similar K _m. These data suggest that the low K⁺ influx in DS cells is caused by a defect in modulation by Na _o and Na _i . Na⁺ efflux showed no differences in Na _i activation or trans effects by Na _o and K _o , thus accounting for the different Na⁺∶K⁺ coupling ratio in the Dahl strains. Further evidence for the differences in the coupling of ouabain-sensitive fluxes was found in studies of net Na⁺ and K⁺ fluxes, where the net ouabain-sensitive Na⁺ losses showed similar magnitudes in the two Dahl strains while the net ouabainsensitive K⁺ gains were significantly greater in the DR than the DS RBCs. Ouabain-sensitive Na⁺ influx and K⁺ efflux were also measured in these rat RBCs. The inhibition of ouabain-sensitive Na⁺ influx by K _o was fully competitive for the DS but not for the DR pumps. Thus, for DR pumps, K _o could activate higher K⁺ influx in DR pumps without a complete inhibition of ouabain-sensitive Na⁺ influx. This behavior is consistent with K _o interaction with distinct Na⁺ and K⁺ transport sites. In addition, the inhibition of K⁺ efflux by Na, was different between Dahl strains. Ouabain-sensitive K⁺ efflux at Na _i level of 4.6 mmol/liter cell, was significantly higher in DS (3.86 ± 0.67 FU) than in DR (0.86 ± 0.14 FU) due to a threefold higher K₅₀ for Na _i -inhibition 9.66 ± 0.41 vs. 3.09 ± 0.11 mmol/liter cell. This finding indicates that Na⁺ modulation of K⁺ transport is altered at both sides of the membrane. The dissociation of Na⁺ modulatory sites of K⁺ transport from Na⁺ transport sites observed in RBCs of Dahl strains suggests that K⁺ transport by the Na⁺-K⁺ pump is controlled by Na⁺ allosteric sites different from the Na⁺ transport sites. The alterations in K⁺ transport may be related to the amino acid substitution (Leu/Gln₂₇₆) reported for the cDNA of the α1 subunit of the Na⁺-K⁺ pump in the DS strain or to post-translational modifications during RBC maturation. These studies were supported by the following grants: NIH (HL-35664, HL-42120, HL-18318, HL-39267, HL-01967). J.R.R. is a Ford Foundation Predoctoral Fellow. A preliminary report of this work was presented at the International Conference on the Na⁺-K⁺ pump and 44th Annual Meeting of the Society of General Physiologists held at Woods Hole, MA, September 5–9, 1990, and published as an abstract in the J. Gen. Physiol. 96:70a, 1990.     

Determination of the rate of K movement through potassium channels in isolated rat heart and liver mitochondria

Both ATP-regulated (mitoK_ATP) and large conductance calcium-activated (mitoBK_Ca) potassium channels have been proposed to regulate mitochondrial K⁺ influx and matrix volume and to mediate cardiac ischaemic preconditioning (IP). However, the specificity of the pharmacological agents used in these studies and the mechanisms underlying their effects on IP remain controversial. Here we used increasing concentrations of K⁺-ionophore (valinomycin) to stimulate respiration by rat liver and heart mitochondria in the presence of the K⁺/H⁺ exchanger nigericin. This allowed rates of valinomycin-induced K⁺ influx to be determined whilst parallel measurements of light scattering (A₅₂₀) and matrix volume (³H₂O and [¹⁴C]-sucrose) enabled rates of K⁺ influx to be correlated with increases in matrix volume. Light scattering readily detected an increase in K⁺ influx of < 5 nmol K⁺ min^− 1 per mg protein corresponding to < 2% mitochondrial matrix volume increase. In agreement with earlier data no light-scattering changes were observed in response to any mitoK_ATP channel openers or blockers. However, the mitoBK_Ca opener NS1619 (10-50 µM) did decrease light scattering slightly, but this was also seen in K⁺-free medium and was accompanied by uncoupling. Contrary to prediction, the mitoBK_Ca blocker paxilline (10-50 µM) decreased rather than increased light scattering, and it also slightly uncoupled respiration. Our data argue against the presence of significant activities of either the mitoK_ATP or the mitoBK_Ca channel in rat liver and heart mitochondria and provide further evidence that preconditioning induced by pharmacological openers of these channels is more likely to involve alternative mechanisms.     

Aluminum and plant age effects on adsorption of cations in the Donnan free space of ryegrass roots

Four cultivars of ryegrass (Lolium multiflorum Lam. cvs. Gulf, Marshall, Urbana, and Wilo) were grown in nutrient solution (pH 4.2) at two Al levels (0 and 74 μM). Cations were desorbed from the Donnan free space of roots of 15-, 23-, and 35-day-old plants using BaCl₂, BaCl₂-triethanolamine, NH₄OAc, and KCl. The amounts of desorbed Ca²⁺ and K⁺ decreased, while desorption of Mn²⁺ and Na⁺ increased with plant age. Differences between 15- and 35-day-old plants, but not between 15- and 23-day-old, were significant. Aluminum considerably decreased the amount of desorbed divalent cations (Ca²⁺, Mg²⁺) and increased the amount of desorbed K⁺ and Na⁺. Ability to resist these changes appeared to be one of the mechanisms determining Al tolerance of ryegrass cultivars.     

Maintenance of photosynthesis at low leaf water potential in wheat : role of potassium status and irrigation history

The interaction of low water potential effects on photosynthesis, and leaf K⁺ levels in wheat (Triticum aestivum L.) plants was studied. Plants were grown at three K⁺ fertilization levels; 0.2, 2, and 6 millimolar. With well watered plants, 2 millimolar K⁺ supported maximal photosynthetic rates; 0.2 millimolar K⁺ was inhibitory, and 6 millimolar K⁺ was superoptimal (i.e. rates were no greater than at 2 millimolar K⁺). Photosynthesis was monitored at high (930 parts per million) and low (330 parts per million) external CO₂ throughout a series of water stress cycles. Plants subjected to one stress cycle were considered nonacclimated; plants subjected to two successive cycles were considered acclimated during the second cycle. Sensitivity of photosynthesis to declining leaf water potential was affected by K⁺ status; 6 millimolar K⁺ plants were less sensitive, and 0.2 millimolar K⁺ plants were more sensitive than 2 millimolar K⁺ plants to declining water potential. This occurred with nonacclimated and acclimated plants at both high and low assay CO₂. It was concluded that the K⁺ effect on photosynthesis under stress was not mediated by treatment effects on stomatal resistance. Differences between the K⁺ treatments were much less pronounced, however, when photosynthesis of nonacclimated and acclimated plants was plotted at a function of declining relative water content during the stress cycles. These results suggest that K⁺ effects on the relationship between relative water content and water potential in stressed plants was primarily responsible for the bulk of the K⁺-protective effect on photosynthesis in stressed plants. In vitro experiments with chloroplasts and protoplasts isolated from 2 millimolar K⁺ and 6 millimolar K⁺ plants indicated that upon dehydration, K⁺ efflux from the chloroplast stroma into the cytoplasm is less pronounced in 6 millimolar K⁺ protoplasts.     

Potassium fluxes and ouabain binding in growing,density-inhibited and rous sarcoma virus-transformed chicken embryo cells

Potassium fluxes, ouabain binding, and Na⁺ and K⁺ intracellular concentrations were determined for cultures of growing normal, density-inhibited and Rous sarcoma virus-transformed chicken embryo fibroblasts. No significant differences in K⁺ influx or ouabain binding were detected between growing normal cells and Rous sarcoma virus-transformed cells; however, ouabain binding and ouabain-sensitive K⁺ influx were 1.5- to 1.8-fold lower in density-inhibited cells. Thus, potassium influx in this system can be classified as a growth-related, but not transformation-specific change. As determined by both flame photometry and radioisotopic (⁴²K) equilibration, growing normal and density-inhibited cells had similar potassium contents, whereas transformed cells exhibited 1.4-fold higher potassium levels. Sodium ion levels, as measured by flame photometry, were also 2- to 4.5-fold higher in transformed than normal or density-inhibited cells. Complementary studies of potassium efflux showed a 1.3- to 1.5-fold higher rate (based on the percentage of pool exiting the cell) in growing normal versus density-inhibited or transformed fibroblasts. Because of the larger potassium pool in transformed cells, efflux based on absolute number of potassium ions is similar in normal and transformed chicken embryo fibroblasts.     

Potassium transport in two tomato species : lycopersicon esculentum and lycopersicon cheesmanii

The commercial tomato, lycopersicon esculentum Mill. cv Walter, and its wild relative, Lycopersicon cheesmanii ssp. minor (Hook.) C.H. Mull., were grown for 30 days under controlled conditions and in solution culture varying in its content of Na⁺ and K⁺. Subsequently, ⁸⁶Rb-labeled K⁺ uptake and distribution were studied. From all solutions, `Walter' consistently absorbed <sup>86</sup>Rb-labeled K<sup>+</sup> at a higher rate in micromoles per gram fresh weight per 30 minutes than L. cheesmanii. L. cheesmanii distributed a greater proportion of the absorbed K<sup>+</sup> from its root to its shoot. When 0.6 millimolar NaNO<sub>3</sub> replaced 0.6 millimolar KNO<sub>3</sub> in the pretreatment solution, intact plants of both genotypes followed a similar pattern as when they were pretreated with K<sup>+</sup> only, but a greater percentage of the absorbed K<sup>+</sup> remained in the roots. Leaf slices of L. cheesmanii plants deprived of K<sup>+</sup> for 6 days showed a greater rate of K<sup>+</sup> uptake than did slices from `Walter' plants pretreated the same way. Stem slices of L. cheesmanii, however, had a lower uptake rate than did those of `Walter'. Both leaf and stem slices of `Walter' plants, pretreated 6 days with 0.6 millimolar NaNO₃ substituting for 0.6 millimolar KNO₃ in their growth medium, had greater rates of ⁸⁶Rb-labeled K⁺ uptake from 0.5 and 20 millimolar KCl solutions than did slices of L. cheesmanii. These marked differences in patterns of ion uptake and translocation indicate that these genotypes of tomato have evolved different mechanisms to deal with K⁺ and Na⁺ in their environments.