Intracellular potassium compartments in Nitella axillaris

Increased Uptake of Chelated Copper Ions by Lolium perenne Attributed to Amplified Membrane and Endodermal Damage

The contributions of mechanisms by which chelators influence metal translocation to plant shoot tissues are analyzed using a combination of numerical modelling and physical experiments. The model distinguishes between apoplastic and symplastic pathways of water and solute movement. It also includes the barrier effects of the endodermis and plasma membrane. Simulations are used to assess transport pathways for free and chelated metals, identifying mechanisms involved in chelate-enhanced phytoextraction. Hypothesized transport mechanisms and parameters specific to amendment treatments are estimated, with simulated results compared to experimental data. Parameter values for each amendment treatment are estimated based on literature and experimental values, and used for model calibration and simulation of amendment influences on solute transport pathways and mechanisms. Modeling indicates that chelation alters the pathways for Cu transport. For free ions, Cu transport to leaf tissue can be described using purely apoplastic or transcellular pathways. For strong chelators (ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA)), transport by the purely apoplastic pathway is insufficient to represent measured Cu transport to leaf tissue. Consistent with experimental observations, increased membrane permeability is required for simulating translocation in EDTA and DTPA treatments. Increasing the membrane permeability is key to enhancing phytoextraction efficiency.

Factors Affecting the Fluxes of Potassium and Chloride Ions in Nitella translucens

A more complete study of ionic concentrations and fluxes in the giant internodal cells of Nitella translucens has been made. The vacuolar concentrations were 76 mM K and 170 mM Cl. The content of the chloroplast layer was 135 mmicromoles K/cm(2) and 215 mmicromoles Cl/cm(2); in a layer 9 micro thick these correspond to concentrations of 150 mM K and 240 mM Cl. Such a high level of chloride requires active transport of chloride into the cytoplasm, either at the plasmalemma or at the membranes bounding the cytoplasmic particles; it cannot be achieved by active transport of chloride only at the tonoplast. With concentrations of 0.1 mM K and 1.3 mM Cl outside, the fluxes into the cytoplasm had mean values of 1.0 to 1.4 micromicromoles K/cm(2)sec. and 2.1 to 2.8 micromicromoles Cl/cm(2)sec.; the corresponding fluxes from the cytoplasm to the vacuole were about 110 micromicromoles K/cm(2)sec. and 175 micromicromoles Clcm(2)sec. The transfer of both potassium and chloride to the vacuole under different conditions appeared to be correlated with the uptake of chloride into the cytoplasm. It is suggested that two separate processes are involved in the active accumulation of salts in the vacuole-an active uptake of chloride in the cytoplasm and a subsequent transfer of salt to the vacuole. It may be that the second process involves the formation of small vesicles in the cytoplasm and their subsequent discharge into the central vacuole.

Ionic Relations of Nitella translucens

The ionic state of single internodal cells of a fresh water characean, Nitella translucens, has been studied. In mature cells the vacuolar concentrations were 78 mM K, 60 mM Na, and 151 mM Cl, compared with concentrations of 0.1 mM K, 1.0 mM Na, and 1.3 mM Cl in the bathing medium. The results suggest an active influx of potassium and an active efflux of sodium at the plasmalemma, and an active influx of chloride, probably at the tonoplast. The cation transport is inhibited by ouabain, and is more efficient in young cells; the chloride transport is insensitive to ouabain, and unaffected by age. Thus the two systems appear to be independent. It is suggested that the active fluxes are 0.5 to 0.6 micromicromoles K/cm(2) sec. inwards, and 0.45 micromicromoles Na/cm(2) sec. outwards. The passive influxes, 0.3 micromicromoles K/cm(2) sec. and 0.55 micromicromoles Na/cm(2)sec., give a value for the relative permeabilities of the plasmalemma, P(Na)/P(K), of 0.18. The absolute magnitudes of the permeabilities, compared with those derived from resistance measurements, suggest that potassium ions interact strongly in the membrane. The cation fluxes at the tonoplast are much higher than those at the plasmalemma. The active influx of chloride is 0.85 micromicromoles/cm(2) sec. in light, but only 0.052 micromicromoles/cm(2) sec. in the dark. The potassium influx is also reduced in the dark. Thus the energy for both active transport processes is closely geared to light-dependent metabolism, rather than to respiration.

The excitability of plant cells: with a special emphasis on characean internodal cells

This review describes the basic principles of electrophysiology using the generation of an action potential in characean internodal cells as a pedagogical tool. Electrophysiology has proven to be a powerful tool in understanding animal physiology and development, yet it has been virtually neglected in the study of plant physiology and development. This review is, in essence, a written account of my personal journey over the past five years to understand the basic principles of electrophysiology so that I can apply them to the study of plant physiology and development. My formal background is in classical botany and cell biology. I have learned electrophysiology by reading many books on physics written for the lay person and by talking informally with many patient biophysicists. I have written this review for the botanist who is unfamiliar with the basics of membrane biology but would like to know that she or he can become familiar with the latest information without much effort. I also wrote it for the neurophysiologist who is proficient in membrane biology but knows little about plant biology (but may want to teach one lecture on "plant action potentials"). And lastly, I wrote this for people interested in the history of science and how the studies of electrical and chemical communication in physiology and development progressed in the botanical and zoological disciplines.

Compartments and Fluxes of K, NA, and CL in Avena Coleoptile Cells

By the compartmental analysis method of MacRobbie and Dainty, and Pitman, estimates of K(+), Na(+), and Cl(-) concentrations and fluxes were obtained for the cytoplasm and vacuole of coleoptile cells of oat, Avena sativa L. cv. Victory. Double labeling was used in experiments with (42)K plus (22)Na and with (42)K plus (36)Cl in a complete nutrient solution. At the plasmalemma, according to the Ussing-Teorell flux ratio equation, Na(+) is pumped out and Cl(-) is actively transported inward. The results with K(+) are less conclusive, but it is probably pumped in. At the tonoplast there is an active inward transport of Na(+) and probably of K(+), but the status of Cl(-) is uncertain, depending upon whether there is an electrical potential difference between the cytoplasm and vacuole. The results suggest that ion selectivity resides mostly in the plasmalemma. Possible errors in the estimates and interpretations are discussed.

Potassium transport in Neurospora. Evidence for a multisite carrier at high pH

At low extracellular pH (4-6), net uptake of potassium by Neurospora is a simple exponential process which obeys Michaelis kinetics as a function of [K](o). At high pH, however, potassium uptake becomes considerably more complex, and can be resolved into two distinct exponential components. The fast component (time constant = 1.2 min) is matched quantitatively by a rapid loss of sodium; it is attributed to ion exchange within the cell wall, since it is comparatively insensitive to low temperature and metabolic inhibitors. By contrast, the slower component (time constant = 10.9 min) is inhibited markedly at 0 degrees C and by CN and deoxycorticosterone, and is thought to represent carrier-mediated transport of potassium across the cell membrane. This transport process exhibits sigmoid kinetics as a function of [K](o); the data can be fitted satisfactorily by two different two-site models (one involving a carrier site and a modifier site, the other an allosteric model). Either of these models could also accommodate the simple Michaelis kinetics at low pH.

Active and passive transport of potassium in cells of excised pea epicotyls

The Ussing-Theorell equation, which provides a fundamental test for the independent passive movement of ions under conditions of nonequilibrium, has been used to assess the active and passive components of K(+) uptake by segments of pea epicotyl (Pisum sativum L. cultivar Alaska), incubated for 24 hours in both 1-fold and 10-fold concentrations of a complete nutrient solution. Measurements of the rates at which (42)K diffused out of the segments provided data from which were estimated the K(+) content of, and the fluxes to and from, the nonfree space compartments, interpreted as being cytoplasm and vacuole. For this analysis the serial model of MacRobbie and Dainty and Pitman for the spatial arrangement of cell compartments was used. On the basis of these values, and measurements of electrical potential across the cell membranes, the vacuolar K(+) concentration was found to be fairly close to that expected as a result of passive diffusion between the cytoplasm and vacuole provided that no potential exists across the tonoplast. Cytoplasmic K(+) concentration, however, was much too high in both treatments to be accounted for in passive terms. It was concluded, therefore, that, on the basis of the model, the high ratio of influx to efflux was maintained in the cells by an active K(+) pump located at the plasmalemma. There is some reason to question the applicability of this model for flux analysis to the conditions of high net influx as encountered here; nonetheless, it provides a first approach to an over-all flux analysis in pea stem tissue.

Mechanism of the Seismonastic Reaction in Mimosa pudica

The efflux of K(+) from the pulvinar cells of Mimosa pudica was shown to increase substantially during the seismonastic reaction. This result is shown to indicate a decrease in sigma (reflection coefficient) of pulvinar cell membrane for potassium salts which could account for the pulvinar cell turgor decrease during the seismonastic reaction.Membrane potentials and concentrations of Ca(2+), K(+), Cl(-), S, and P were measured in the top and bottom halves of pulvini. Pulvinar cells showed a large negative membrane potential, cell vacuole relative to external solution, but no significant difference in membrane potential could be detected between upper and lower pulvinar cells. A large difference in K(+) and Cl(-) concentration between top and bottom pulvinar halves was evident in reactive pulvini but not in unreactive pulvini. The effect of K(+) concentration on plant growth and leaf reactivity was also investigated.

Short term influx as a measure of influx across the plasmalemma

It is shown that the influx of tracer to a plant cell must be measured for less than one third of the half-time for exchange of cytoplasmic tracer, and any subsequent wash must be negligibly brief, if the initial influx measured over a finite period is to be a good estimate of the plasmalemma influx. Complications due to lack of knowledge of the cytoplasmic exchange rate constant and to extracellular contents make it difficult to make such an estimate from influx measurements alone. The use of influx measurements is further discussed.

[Investigations on the kinetics of ion uptake into the cytoplasm of mnium leaf cells by means of microautoradiography and electron probe X-ray microanalyser]

Methods of quantitative microautoradiography are described. The kinetics of ion uptake into the cytoplasm (including the plastids and other organelles) were investigated by means of microautoradiography and electron probe X-ray microanalyser.The results of microautoradiography show that under our experimental conditions at the end of the experiments the cytoplasm contains much more label per unit of volume than the vacuoles. The kinetics of this filling - up of the cytoplasm correspond to the system-1 isotherm of ion uptake. It must be concluded from the literature that system-1 reflects the process filling up the cytoplasm (=transport through the plasmalemma). The kinetic data reported here describe directly the actual filling - up of the cytoplasmic phase. For this reason the coincidence of kinetic features such as the hyperbolic shape of the rate versus concentration curves in both instances is of interest for the general model of ion uptake into cells of higher plants. The time course of the filling of the cytoplasm is dependent on the external concentration.Similar kinetic data are obtained with the electron probe X-ray microanalyser.The usefulness of a parallel application of microautoradiographic and electron probe X-ray microanalyser techniques in kinetic studies of transport is discussed.

Ion transport in Hydrodictyon africanum

The concentrations of K, Na, and Cl in the cytoplasm and vacuole, the tracer fluxes of these ions into and out of the cenocyte, and the electrical potential difference between bathing solution and vacuole and cytoplasm, have been measured in Hydrodictyon africanum. If the ions were acted on solely by passive electrochemical forces, a net efflux of K and Cl and a net influx of Na would be expected. Tracer fluxes indicate a net influx of K and Cl and efflux of Na in the light; these net fluxes are consequently active, with an obligate link to metabolism. The effects of darkness and low temperature indicate that most of the tracer K and Cl influx and Na efflux are linked to metabolism, while the corresponding tracer fluxes in the direction of the free energy gradient are not. Ouabain specifically inhibits the metabolically linked portions of tracer K influx and Na efflux. Alterations in the external K concentration have similar effects on metabolically mediated K influx and Na efflux. It would appear that K influx and Na efflux are linked, at least in the light.

Perspective on Function of Free Space in Ion Uptake by Roots

An observed effect of longitudinal flow rate through corn roots on phosphate transport is shown to be inconsistent with diffusion theory. R. C. Smith's results confirm rather the existence of a diffusion barrier between the xylem and the free space external to the central cylinder of the root.