Ion Absorption and Retention by Chlorella pyrenoidosa. III. Selective Accumulation of Rubidium, Potassium and Sodium

The selective preference of Chlorella pyrenoidosa for alkali metal cations was found to have the order Rb > K > > > Na. It was demonstrated that a cation of higher preference can replace in the cell cations of lower preference by an ion interchange process.

The replacement of Na from the cell by K or Rb occurred against high external Na concentrations up to 450 meq/1 Na.

It is suggested that the structural selectivity of the Chlorella cell may be amplified by a chromatographic type repetitive selection process, driven by metabolically dependent unsymmetric shape changes of cellular membranes.

     

Rubidium and Potassium Absorption by Bean-leaf Slices Compared to Sodium Absorption

At salt concentrations of 0.1 mM as well as of 5.0 mM, the ²²Na⁺ absorption capacity of bean (Phaseolus vulgaris L. cv. ‘Brittle Wax’) leaf tissue increased during the period of leaf expansion and decreased rapidly after leaf maturation. The absorption capacity for ⁸⁶Rb⁺ and ⁴²K⁺ was highest in very young leaves and decreased continuously in expanding and in mature leaves. The ⁸⁶Rb⁺ absorption capacity of mature leaves was not increased by detopping the plants; this senescence-retarding treatment more than doubled ²Na⁺ absorption. The absorption of ²²Na⁺ by bean-leaf slices was not enhanced by light, whereas ⁸⁶Rb⁺ and ⁴²K⁺ absorption was much affected. Previously absorbed ⁸⁶Rb⁺ and ⁴²K⁺ were more available for exchange than ²²Na⁺.     

Ion selectivity of colicin E1: II. Permeability to organic cations

Channels formed by colicin E1 in planar lipid bilayers have large diameters and conduct both cations and anions. The rates at which ions are transported, however, are relatively slow, and the relative anion-to-cation selectivity is modulated over a wide range by the pH of the bathing solutions. We have examined the permeability of these channels to cationic probes having a variety of sizes, shapes, and charge distributions. All of the monovalent probes were found to be permeant, establishing a minimum diameter at the narrowest part of the pore of approximately 9 A. In contrast to this behavior, all of the polyvalent organic cations were shown to be impermeant. This simple exclusionary rule is interpreted as evidence that, when steric restrictions require partial dehydration of an ion, the structure of the channel is able to provide a substitute electrostatic environment for only one charged group at time.     

Extrusion of Sodium Ions by Barley Roots: II. Localization of the Extrusion Mechanism and its Relation to Long-distance Sodium Ion Transport

An active sodium effux hasbeen demonstrated in excised barleyroots. Isolated cortical and stelar tissues from the same materialdo not show any evidence of such an efflux mechanism. When intactbarley plantswere allowed to absorb ²²Na active sodium extrusionwas not observed, although the percentage of ²²Na transportedto the shoot was stimulated 20 to 40 percent by ouabain at 5x 10^–4 M. Absorption and long-distance transport of phosphatewas unaffected by ouabain at this concentration. Interpretationof these data lead to the conclusion.that the sodium-effluxpump in barley roots is located at or near the endodermis externalto the Casparian band. This pump thus provides a mechanism wherebysodium can be partially excluded from the shoots of barley plants.     

Mechanisms of Sodium and Rubidium Uptake by Excised Barley Roots

Accumulation of sodium and rubidium by excised barley roots was investigated. The concentration isotherm yielded one absorption shoulder. Nevertheless, it is suggested that two mechanisms take part in the uptake of sodium and rubidium: One non-metabolic mechanism with an apparent participation at low external salt concentrations (< 1 mM) and at high concentrations (> 20 mM). Such a mechanism is almost unaffected by low temperature conditions and by metabolic inhibitors. Rubidium possesses a high affinity toward this non-metabolic system. The second mechanism is sensitive to metabolic inhibitors and to low temperature conditions. It dominates at intermediate external concentrations (1–20 mM). Sodium possesses high affinity towards this mechanism. The two mechanisms operate in a parallel manner beyond a diffusion barrier (= plasmamembrane) surrounding the cells. It is assumed that both the metabolic and the non-metabolic mechanisms operate in the entire concentration spectrum, but their relative contribution to the total uptake varies at different ranges.     

Ion Distribution in Salt-stressed Mature Zea mays Roots in Relation to Ultrastructure and Retention of Sodium

Ultrastructural features and the distribution of soluble ionshave been examined in mature roots of Zea mays plants grownin both NaCl and Na₂SO₄ salinities. When the plants were grown in either salt, the Na concentrationincreased proximally along the root with a concomitant declinein the K concentration. Both trends were reversed in the shoot. X-ray microanalysis of deep-frozen, fully hydrated specimensshowed that in salt-treated roots Na, and Cl, or S were distributedabout stoichiometrically in the cortex and endodermis. Na wasusually less concentrated than the anion in the lumens of thevessels, but was concentrated markedly relative to either Clor S in the adjoining xylem parenchyma cells. In the older, proximal parts of seminal roots of plants grownboth without salt (controls) and in the presence of either NaClor Na₂SO₄, wall developments occurred in xylem parenchyma cellsat the half-bordered pits in which the cell wall became markedlythicker and possessed a loosely packed fibrillar structure.These structures were not comparable with the transfer-celltype of protuberances reported in the roots of other species. In the xylem parenchyma of plants grown in the presence of Na₂SO₄there were dramatic increases in the quantities of rough endoplasmicreticulum, ribosomes, and mitochondria relative both to controlsand NaCl treatments. The results are discussed in relation to the possible functionof the xylem parenchyma of the mature root in the reabsorptionof Na from the xylem sap, which may mitigate adverse effectsof salinity in salt-sensitive glycophytes.     

Alkalinity-induced Aggregation in Chlorella vulgaris II. Changes in the Cell Wall during the Cell Cycle

In a Chlorella culture growing synchronously at pH 6.3 undera 12 hr light-12 hr dark regime, autospores are released duringthe dark period of the cycle by mechanical rupture of the cellwall. When the culture is made in an alkaline medium (pH 9.5)during the first 8 hr of the light period, the release of autosporesis prevented, and at the end of the cycle, the cells are enlarged.It was found that the diurnal fluctuations in cellulase specificactivity and in polysaccharide content, which normally takeplace at pH 6.3, do not occur at alkaline pH. The concentrationof polysaccharides, as measured by the periodic acid-Schiffreagent, and cellulase activity are almost doubled. Peak levelsare reached at the 8th hour of the cell cycle rather than theusual 4th hour and persist during the second part of the cellcycle, whereas they normally decrease. It is suggested thatunder alkaline conditions, the diurnal increase in cellulaseactivity is amplified and prolonged, resulting in increasedflexibility of the mother cell wall. As a result, the cell wallis stretched, rather than ruptured, during the development ofdaughter autospores, which are thus prevented from being released. ¹ Present address: Research & Development Authority, Ben-GurionUniversity of the Negev, P.O. Box 1025, Beer-Sheva 84110, Israel. (Received June 16, 1981; Accepted October 22, 1981)     

The Influence of External Concentration on Sodium Ion Absorption by Carrot Root Tissue 4

The influence of external concentration on sodium ion absorptionby slowly and rapidly growing carrot root tissue cultures hasbeen examined. In conflict with the observations of Stewardand Millar (1954) it is observed that absorption by the twokinds of cultures is affected similarly by external concentration.The discrepancy between the two investigations is discussedand an alternative interpretation of the results of Stewardand Millar is proposed.     

Uptake and Distribution of Sodium and Potassium by Corn Seedlings : II. Ion Transport within the Mesocotyl

In this paper, uptake and distribution of sodium and potassium within the mesocotyl are considered in excised, 8-day-old corn (Zea mays L.) seedlings supplied with label via the transpiration stream. The stele and cortex were dissected following uptake and analyzed separately. At equal concentrations, sodium uptake by the stele was much more rapid than potassium uptake, and sodium was preferentially retained within the stele. Transport of sodium to the cortex halted when the supply of ions in the transpiration stream was interrupted. Potassium would not substitute for sodium in restoring this transport but neither did it compete with sodium for transport to the cortex. In the presence of continued sodium supply, transport was temperature sensitive.

By labeling first with ²²Na for 2 hours and subsequently with ²⁴Na for up to 21 hours, three sodium pools were identified within the stele. The first was rapidly transportable to the cortex. The second equilibrated rapidly with the first but was not itself directly available for transport. We postulate that these represent the stelar symplasm and apoplasm, respectively. A third pool was not transported and probably represents sequestration within the vacuoles of some cell type. Transport of label acquired during the initial 2 hours proceeded with a half-time of approximately 10 hours with 10 millimolar sodium present during the redistribution period, and with a half-time of approximately 30 hours at 1 millimolar sodium.

A working model is presented which explains these characteristics and supplies approachable questions for subsequent study.

     

The Absorption and Accumulation of Salts by Living Plant Cells: IX The Absorption of Rubidium bromide by Potato Discs

Chemical methods for the quantitative determination of rubidiumin aqueous plant extracts and saps were investigated but abandonedin favour of a spectrographic technique which is fully described. The results of quantitative determinations of the rubidium andbromide absorbed by potato discs from solutions of rubidiumbromide, and the effects upon this absorption of oxygen, oftime, and of the specific surface of the discs are described.The effects of these variables upon the accumulation of bromidewhich were previously described have been confirmed. All thevariables which affect bromide absorption affect the uptakeof rubidium in a similar manner. The absorption of rubidium is of two distinct kinds. The firstphase in the absorption process is a relatively rapid uptakeof rubidium unaccompanied by bromide. This process is unaffectedby oxygen, is not confined to the surface cells, and ceasesafter a short time. The second phase occupies a protracted periodduring which both rubidium and bromide are absorbed in equivalentamounts. During this phase the absorption is confined to a fewlayers of cells at the surface and is determined by oxygen concentrationin the manner already described for the bromide ion. The two types of absorption are described as ‘inducedabsorption’ and as ‘primary absorption’ inthe sense used earlier. The former is merely a property of thesubstances in the tissue, the latter is a process which tendsto increase the free energy of the tissue, therefore demandsthat work should be done, and is a property of the organizedliving cell. The relationships of the two types of absorption process totime have been described and they can be expressed in termsof equations. The effect of the surface and thickness of discs upon the absorptionof bromide and rubidium is interpreted quantitatively. The distance from the surface of the discs at which ‘primaryabsorption’ ceases is the same for rubidium and bromide,and coincides with the values previously determined for thedepth of the layer of tissue which exhibits enhanced respiration. The ‘induced absorption’ of rubidium may be observedin discs killed by alcohol. The effect of concentration uponthis process is similar to the adsorption isotherm. The effectof concentration upon the absorption of bromide by living discsis much less conspicuous and the requirements of the absorptionisotherm are not rigidly met. The effect of concentration alsoinvolves the factor of surface and thickness of disc. Unequal absorption of the bromide and rubidium arises from thesuperimposed effect of two distinct processes one of which causesthe absorption of rubidium only and the other tends to causeequal uptake of rubidium and bromide.     

Variations in Uptake of Sodium and Rubidium Along Barley Roots

Variations in uptake of sodium and rubidium were ob) served along the apical 8 cm segments of barley roots. Dif) ferent patterns of uptake were obtained for low- and for high-salt roots. The validity of using batches of entire barley roots for kinetic analysis of ion uptake is questioned.     

Studies on the Cell Wall of Chlorella V. Comparison of the Cell Wall Chemical Compositions in Strains of Chlorella ellipsoidea

Cell walls of four strains of Chlorella ellipsoidea (IAM C-27,C-87, C-102 and C-183) were compared as to their chemical compositions.Many differences were found: (1) The sugar composition of alkali-soluble cell walls differedin quantity as well as quality with glucuronic acid being foundonly in C-27 and C-87. (2) In alkali-insoluble cell walls glucosamine was found onlyin C-27. The other three strains contained mainly glucose. (3) The amino acid compositions of the alkali-insoluble cellwalls markedly differed among the four strains. The cell wallof C-102 contained more amino acids than carbohydrates, butC-27 and C-87 contained extremely little amino acid. In addition to the variation in cell wall composition, the opticalanisotropy findings also differed for these cell walls of Chlorellastrains which had been grouped as the same species. (Received August 16, 1983; Accepted December 27, 1983)