The plasmalemma: seat of the type 2 mechanisms of ion absorption

Salt toleration by plants: enhancement with calcium

Bean plants subjected to a sodium chloride concentratioz about onetenth that of seawater for 1 week suffered no damage if the calcium concentration of the nutrient solution was 1 millimole per liter or higher, but at lower calcium concentrations damage was severe and apparently due to a massive breakthrough of sodium into the leaves.

Potassium transport in salt-stressed barley roots

Salinization of the medium inhibits both K(+) uptake by excised barley (Hordeum vulgare L.) roots and K(+) release from their stele, as measured by short-term (86)Rb uptake and xylem exudation, respectively. Although inhibition was not specific to chloride, mannitol caused a different response from that of inorganic sodium salts, indicating that inhibition was at least partly the result of an ion effect. In roots previously exposed to low levels of NaCl, NaCl stress directly affected stelar K(+) release, whereas in low-sodium roots stelar K(+) release was much less salt-sensitive than K(+) uptake.

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.

Multiphasic absorption of glucose and 3-o-methyl glucose by aged potato slices

The isotherm for glucose absorption by aged potato (Solanum tuberosum var. Russet Burbank) discs shows four distinct phases in the concentration ranges 1.0 to 75 mum, 75 mum to 1.5 mm, 1.5 to 15 mm, and 15 to 100 mm, respectively. Each segment of the multiphasic isotherm, when plotted reciprocally by the method of Lineweaver and Burk or of Hofstee, without regard for uptake in earlier phases, indicates absorption rate to be a hyperbolic function of concentration. The observations suggest that glucose uptake is carrier-mediated, and that the transport barrier undergoes a series of all-or-none transformations at critical external concentrations, yielding successive new and higher values for the parameters Km and V(max) 3-O-Methyl glucose, a nonmetabolizable analogue of glucose, shows the same multiphasic absorption isotherm, with Km values essentially similar to those for glucose uptake, and V(max) values somewhat lower than those for glucose absorption. Whereas the first three phases of the absorption isotherm are taken to reflect passage across the plasma membrane, the fourth phase may reflect kinetics of glucose or 3-O-methyl glucose transport to the vacuole.

Chloride accumulation by mung bean root tips: a low affinity active transport system at the plasmalemma

Net uptake of Cl(-) into root tips of mung bean (Phaseolus aureus) increases steadily with increasing external concentrations from 1 to 60 mm. Membrane potentials were measured to determine the equilibrium concentration of Cl(-) in the tissue which could be due to diffusion. This concentration was readily exceeded in both the relatively nonvacuolate tips (0 to 1 mm) and the vacuolate, mature upper sectons (1 to 11 mm) of the roots. The activity coefficient of both cytoplasmic and vacuolar Cl(-), measured with Cl(-) sensitive microelectrodes, was approximately the same as that of a pure KCl solution of the same concentration. It is concluded that the "second mechanism" of ion uptake involves a large increase in the rate of active transport at the plasmalemma as the external concentration is increased above 1 mm.

Plasmalemma: The Seat of Dual Mechanisms of Ion Absorption in Chlorella pyrenoidosa

Dual mechanisms of absorption of rubidium were demonstrated in a nonvacuolate unicellular alga, Chlorella pyrenoidosa, in both the light and the dark. The two mechanisms were sensitive to metabolic inhibitors. At high concentrations rubidium enhanced the respiration of Chlorella cells. The findings support the conclusion that the mechanisms of rubidium absorption in both the low and high concentration ranges are active processes and reside in the plasmalemma.

Gramicidin-D-stimulated influx of monovalent cations into plant roots

Gramicidin D and nigericin were found to stimulate K(+) influx into oat roots. Valinomycin and nonactin had little effect on K(+) influx. The region of the root most sensitive to gramicidin was the elongation zone. Monocot roots were more sensitive to gramicidin than dicot roots. At 0.2 mM KCl, gramicidin stimulated K(+) influx by 4- to 8fold over a 30-min absorption period. Although a gramicidin response is detectable within one minute, maximum stimulation occurred after a slight (approximately 2-min) lag period. The gramicidin effect was much greater at 0.2 mM KCl than at 20 mM KCl. Respiratory inhibitors reduced the gramicidin-stimulated K(+) influx by 50-80%. The results are discussed in terms of possible mechanisms of action of the various ionophores on ion transport in roots.

Lateral transport of ions into the xylem of corn roots: I. Kinetics and energetics

A technique is described for study of the kinetics of lateral transport of ions across single roots of corn, Zea mays, in short term experiments under steady state conditions. The kinetics of chloride transfer to the vessels reflected the kinetics of absorption of chloride by the root cells. Efflux from the root vacuoles contributed to only a small extent to transport of chloride into the exudate. Lateral transport of chloride was inhibited by bromide at chloride concentrations in the ranges of both mechanisms 1 and 2 in a manner implicating competition. The uncoupler carbonylcyanide m-chlorophenylhydrazone used at 1 mum caused transfer of chloride to cease almost immediately at both low and high concentrations of chloride. Oligomycin depressed transport of chloride to the vessels within 10 to 15 minutes after application at 2 micrograms per milliliter. Inhibition by oligomycin was 75% at 0.5 mm chloride and 55% at 5 mm.It is concluded that lateral transport of chloride across corn roots is mediated by the dual mechanisms of ion absorption which reside in the plasmalemma. Transfer of chloride is inhibited by bromide and depends upon ATP as energy source. Chloride moves from the plasmalemma, the site of carriermediated absorption, to the xylem vessels by way of the symplasm. There is no evidence in these experiments that lateral transport of chloride in corn roots is governed by diffusion at any concentrations of chloride used in these experiments.

Correlation between ion fluxes and ion-stimulated adenosine triphosphatase activity of plant roots

The energy-dependent influx of Rb(+) into excised roots of corn, wheat, and barley has been determined and compared to the Rb(+)-stimulated ATPase activity of membrane fractions obtained from root homogenates of these species. The external Rb(+) concentrations studied were in the range of 1 to 50 mm. The ratio of Rb(+) influx/Rb(+)-stimulated ATPase was approximately 0.85 and was nearly constant for all the species and Rb(+) concentrations studied. The correlation coefficient for Rb(+) influx versus Rb(+)-activated ATPase was 0.94. The results support the concept that ATP is the energy source for ion transport in roots and that an ATPase participates in the energy transduction process involved in energy-dependent ion transport.

Oligomycin effect on ion absorption by excised barley roots and on their ATP content

Chloride absorption by excised barley roots from dilute solutions is more oligomycin-sensitive than its absorption from more concentrated solutions and than K(+) and Na(+) absorption from dilute as well as concentrated solutions. Oligomycin decreased the ATP content of excised barley roots. The mode of oligomycin interference with ion absorption by plant cells is discussed.

[The influx of K(+) ions in leaves of Elodea densa, dependence on light, potassium concentration, and temperature]

1. The influx of potassium ions in leaves of Elodea densa during short periods of time was measured using (42)K and (86)Rb as tracers. The K(+) influx was linear with time (Fig. 1) without a contribution by Donnan adsorption even in 1 min experiments. 2. Light increased the K(+) influx in air by a factor of up to 30-50 compared to dark/air. Light-induction of the K(+) influx is similar to the light-induction of photosynthesis except for the initial O2 outburst. The half-time of induction, however, is somewhat larger for K(+) influx than for photosynthesis (Fig.2). 3. The isotherms of K(+) influx exhibit the dual mechanism documented for many other species (Figs. 3 and 4). 4. Similar dual isotherms of K(+) influx are obtained in dark/air, light/air, and light/N2, suggesting similar transport mechanisms in light and dark (Figs. 3 and 4). 5. Using (86)Rb as a tracer for K(+), lower values of influx are obtained than with (42)K, the preference for (42)K being higher at low concentrations (Figs. 5,6). However, the light-stimulation (Fig. 5) and the effect of inhibitors on K(+) influx (Table 4) are also found with (86)Rb, indicating that it may be used for such measurements. 6. A change of temperature results in a dual Arrhenius plot (Fig. 7) of K(+) influx with two different apparent activation energies in the light as well as in the dark. The values of E app in the range of strong dependence on temperature are almost equal in light and dark. 7. The causes of the increased K(+) influx in the light are discussed. The influx is inhibited by uncoupling agents and inhibitors of the energy transfer (Table 3) suggesting a dependence on ATP production. On the basis of the carrier concept and using the equations of coenzyme kinetics, a change of the apparent K m (') and V max (') values caused by light can be predicted in the direction found experimentally (Fig. 8). However, the necessary rise of ATP concentration in the light is higher than can be anticipated in vivo. The increase of K(+) influx in the light is therefore attributed additionally to a) a hyperpolarization of the vacuolar potential in the light and b) a possible increase of the K(+) permeability in the light; further there may be c) a K(+) influx linked to ATP at a higher stoichiometry than 1/1 and/or d) an influx coupled to the light-stimulated Cl(-) influx.

Potassium Fluxes during Potassium Absorption by Intact Barley Plants of Increasing Potassium Content

The presence of previously absorbed K in plants caused a marked reduction in the short term influx of (86)Rb-labeled K into roots of barley seedlings. The influx values agreed with net K absorption rates into intact plants, thus suggesting that K efflux was negligible in comparison with influx.Earlier interpretations of a large K efflux component from excised roots approaching equilibrium K concentrations are considered to be due to an underestimation of net K absorption rates resulting from xylem exudation as the K status of the roots increased.

Water dependent and water independent fluxes of potassium in exuding root systems of Ricinus communis

The flux of water, [Formula: see text], to the xylem of exuding root systems of Ricinus communis was controlled using a range of mannitol concentrations permitting the influence of this water flux on the potassium flux, f K, to be studied. The relationship between [Formula: see text] and f K thus obtained was investigated, for a number of external concentrations of potassium, Cm, supplied as potassium nitrate. An analysis of these data indicated the presence of a water dependent and a water independent f K both of which varied with Cm. The water dependent f K shows a parabolic relationship with Cm for Cm values <1 mM followed by a sharp inflection and decline at higher Cm values whereas the water independent f K shows an hyperbolic relationship over the same range of Cm values.Uptake of potassium by exuding root systems was measured and shown to be dependent on the solute potential of the medium. The uptake was also shown to exhibit a dual absorption isotherm the kinetics of which indicate a low Km system (system 1) and a high Km system (system 2). The Km value obtained for system 1 is very similar to that obtained for the water independent f K. It is postulated that the water independent f K is contributed by that portion of f K arriving in the stele via the cortical symplast and is directly dependent on Cm. The water dependent f K is contributed by those ions moved across the root in response to centripetal water movement through the cortical cell walls.

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.

Sodium and potassium absorption by bean stem tissue

The effect of various periods of pretreatment in CaSO(4) solutions (aging) on the absorption of Na and K by bean stem slices was investigated. Freshly sliced tissue absorbed Na over the entire range of concentrations studied (0.02-50 mm). Potassium absorption by fresh tissue was nil at concentrations below 0.5 mm but at higher concentrations was similar to that of Na. When tissue was aged by aerating slices for 20 hr in 0.5 mm CaSO(4), K absorption was substantial over the entire range (0.01-50 mm), with evidence of a dual mechanism of absorption, whereas Na absorption was nil at concentrations below 0.2 mm. The formation of K-absorbing capacity with aging, and the loss of Na-absorbing capacity at low concentrations, were temperature-dependent and did not result from significant changes in rates of efflux of either ion. The absorption of Na by fresh tissue and K by aged tissue was sensitive to antimetabolites, with K uptake the more sensitive. Benzyladenine, an analog of kinetin, suppressed the formation of K-absorbing capability in aged tissue but did not prevent the loss of Naabsorbing capacity. Possible mechanisms for this alteration in ion-specificity of transport mechanisms are discussed.