Uptake of Radioactivity by Aquatic Plants and Location in the Cells I. THE EFFECT OF pH ON THE STRONTIUM-90 AND YTTRIUM-90 UPTAKE BY THE GREEN ALGA ULVA LACTUCA AND THE EFFECT OF STABLE YTTRIUM ON YTTRIUM-90 UPTAKE

The uptake of strontium-90 and yttrium-90 by Ulva lactuca fromsea water was measured over a period of 179 days. In sea waterat normal pH (8.0) and without added carrier Ulva reached astrontium-90 equilibrium at 0.32 times the concentration inthe water within 3 hrs, but rapidly depleted the water of yttrium-90.Addition of 0.15 ppm of non-radioactive (stable) yttrium ledto the establishment of an yttrium-90 equilibrium within 3 hrs,with the Ulva having a concentration factor of 550. It was foundthat without added carrier the yttrium-90 remained on the outersurface of the plants, but when carrier was added the yttrium-90taken up mainly entered the cells and was laid down in the protoplasmicinclusions. The strontium-90 was located only in the protoplasmicinclusion. Uptake was investigated in a pH range 5.1 to 8.0.Progressive reductions of pH from 8.0 to 6.5 led to a smallincrease in the amount of strontium-90 uptake but to large increasein the yttrium-90 uptake. However, in the pH range 5.8 to 5.1the yttrium-90 uptake was greatly reduced. External pH below6.0 caused death of the alga within 5 days. The effect of pHon strontium uptake is thought to be due to changes in equilibriumin the carbonate-bicarbonate system. The effects of pH and stableyttrium concentration on yttrium-90 uptake are discussed interms of changes in the state of yttrium in the sea water, resultingin increased availability to the plant, and of changes in thenature of the plant surface resulting from its physiologicalresponse to lowered pH.     

The Uptake of Growth Substances: I. FACTORS CONTROLLING THE UPTAKE OF PHENOXYACETIC ACIEDS BY LEMNA MINOR

An analysis has been made of the factors controlling the uptakeof 2:4-dichloro-phenoxyacetic acid (2:4-D) and related compoundsby Lemna minor, grown under controlled conditions. All compoundswere labelled with ¹⁴C in the carboxyl group and the ¹⁴C inthe plants, after liquid combustion, assayed as barium carbonate. With 2:4-D the rate of entry from concentrations of 8 to 32mg./l. is maximal in the first 20 minutes, then falls progressivelyuntil the rate is zero between 1 and 2 hours and in the thirdphase (up to 24 hours) there is a net loss from the plants dueto an outward movement of the compound back into the externalsolution. When the chlorine substitutions are in the 2:6-positionthe course of uptake is similar, save that loss to the solutiontakes place later. For the 2-chloro-substitution, after theinitial phase of uptake there is some loss but this is followedby a period of recovery and uptake again proceeds, but slowly.In contrast, the course of uptake over 24 hours of phenoxyaceticacid is normal, i.e. there is a steady accumulation. When plants are pretreated with labelled 2:4-D for 30–60minutes and transferred to water or culture solution then over90 per cent. of the ¹⁴C is found in the external medium after4:5 hours and this release cannot be accounted for in termsof exchange processes since the addition of unlabelled compoundto the medium retards the rate of loss. This loss is slowedbut not stopped at 1.25°C. and up to 22.5°C. the rangeof the Q₁₀ is 1.6–1.9. Uptake in the first 30 minutes is temperature sensitive between7.5 and 30°C. (Q₁₀ 2.3–2.6) and in general is positivelyand curvilinearly related to the external concentration. Pretreatmentwith unlabelled compound up to 2 hours progressively depressesthe subsequent initial uptake of labeled growth regulator. Itis concluded that initially the rate of entry greatly exceedsthe rate of loss but that with time the ratio steadily diminishesto less than unity. Initial uptake of 2:4-D is markedly dependent on the pH of thesolution and closely but not completely correlated with theexternal concentration of undissociated molecules. On the otherhand, the outward movement is relatively unaffected by the externalpH. Combinations of concentration and time of exposure which bringabout loss to the solution need not cause any permanent retardationof growth. In fact, exposure for 1 hour to 8 mg./l. significantlyaccelerated growth in the next 8 days. These results are discussed in relation to previous findingsand it is concluded that the pattern of uptake is determinedon the one hand by the chemical structure of the growth substanceand on the other by specific physiological differences at celllevel.     

The Initial Uptake of Ions by Barley Roots: III. THE UPTAKE OF CATIONS

Experiments have been carried out to determine the extent towhich rubidium and strontium, recently absorbed by excised barleyroots, can be divided into discrete fractions, namely, ionswhich are extractable with water, those which are exchangeable,and those remaining in the tissue at the end of the experiment.The object of this study was to determine whether the behaviourof cations could be validly interpreted in terms of the electricaldouble-layer theory as had been previously demonstrated foranions. To facilitate the interpretation of the results obtained, comparableexperiments were carried out on a cation-exchange resin andon gelatin. The behaviour of strontium accorded qualitatively with the theoryproviding allowance was made for effects due to the hydrolysisof negative sites and for the presence of positive charges inthe roots. For rubidium, on the other hand, the situation was much morecomplex. Not only was there evidence even at 0.2°C for siteswhich retained a high specificity for this ion, but also movementof the ion due to physical diffusion was masked by concurrentmetabolic activity. It was concluded that a meaningful separation of the over-alluptake of rubidium into successive steps of exchange absorptionand metabolic accumulation cannot be made.     

Amino Acid Transport in Suspension-cultured Plant Cells: II. CHARACTERIZATION OF l-LEUCINE UPTAKE

l-Leucine (l-Leu) transport into suspension cultured Nicotiana tabacum L. cv. Wisconsin 38 cells has been investigated. Cells were batch-cultured and routinely assayed 3.5 to 4 days after subculturing. Uptake rates were measured over the concentration range of 10 micromolar to 150 millimolar. Kinetic analysis of the uptake rates indicated that uptake was multiphasic with three saturable phases and one unsaturable phase. The three saturable phases which occur in the concentration ranges of 10 to 40 micromolar, 50 to 100 micromolar, and 0.2 to 5.0 millimolar exhibited the following characteristics; (a) phases were energy-dependent as shown by 84 to 94% inhibition of uptake rates by metabolic inhibitors; (b) phases exhibited broad pH optima between 3.0 and 5.5; (c) phases showed stereospecificity for l-Leu; (d) over a 12-hour incubation period, phases concentrated l-Leu 43, 90, and 10 times when the initial l-Leu concentration was 20 micromolar, 100 micromolar, and 1.0 millimolar, respectively; (e) phases had K(m) values of 17.6 micromolar, 60.1 micromolar, and 1.38 millimolar, respectively; and (f) in the temperature range of 17 to 27 C phases had Q(10) values of 2.1, 1.4, and 1.4, respectively. l-Leu uptake in the three saturable phases was inhibited by a 20-fold higher concentration of 18 other amino acids; phenylalanine, alanine, and methionine were the most effective inhibitors, whereas aspartic acid, asparagine, histidine, and arginine were the least effective. The nonsaturable phase which was responsible for increases in the uptake rate above 5.0 mm appeared to be primarily diffusional since it was minimally influenced by metabolic inhibitors and had a Q(10) of 1.3.     

STUDIES ON PROTEIN UPTAKE BY ISOLATED TUMOR CELLS : I. Electron Microscopic Evidence of Ferritin Uptake by Ehrlich Ascites Tumor Cells

Ferritin, added to the incubation medium of ascites tumor cells, was used as an electron microscopic marker to study the uptake of large protein molecules by morphologically intact cells. A definite uptake could be detected after 1 hour of incubation in Tyrode bicarbonate solution containing 0.04 to 13.3 mg ferritin/ml. Ferritin was found in a variety of membrane-surrounded structures, suggesting that pinocytesis and related membrane movements are occurring under physiological conditions and can account for the penetration of intact macromolecules into isolated tumor cells. Supplementation of the medium with serum albumin (33 mg/ml) increased the average amount of ferritin per cell and per pinocytotic structure. Ferritin was strongly adsorbed by fragments of lysed cells, which were readily taken up by intact cells. Besides its role as carrier, this debris appeared to stimulate membrane movements. Only rare examples were found to suggest the release of ferritin from the pinocytotic structures into the cytoplasm. Thus, the disintegration of such structures cannot be considered an obvious step towards a rapid metabolic utilization of protein by the cell. Particles of colloidal gold presented to the cell under the same conditions were not taken up to any significant extent, thus providing good evidence for a selective ingestion of particles of comparable sizes.     

The Initial Uptake of Ions by Barley Roots: I. UPTAKE OF ANIONS

Experiments have been carried out to examine the applicabilityof the Goüy–Chapman electrical double-layer theoryto the initial entry of anions into excised barley roots. Anattempt was made to separate the total quantity of labellediodide or sulphate ions which had entered the roots under varyingconditions into three fractions: the two fractions which couldbe successively eluted with water and extracted with an exchangingsolution and that which remained in the roots after these treatments.Except at a low temperature and a low hydrion concentration,the three fractions could not clearly be distinguished, andthere were indications of the presence of metabolically maintainedpositive sites. However, the effect of varying pH on the rateat which anions were eluted with water was consistent with thepresence in the roots of a system of pores, the walls of whichcarried a varying number of negative charges depending on thehydrion concentration. The effect of calcium ions on the magnitudeof the water extractable fraction also accorded qualitativelywith the double-layer theory.     

Rates of Photosynthesis in Characean Cells: I. PHOTOSYNTHETIC 14CO2 FIXATION BY NITELLA TRANSLUCENS

A study has been made of photosynthetic ¹⁴CO₂ fixation by isolated‘mature’ internodes of Nitella translucens. Experimentalconditions were similar to those used in studies of the ionicrelations of these cells. Maximum rates of photosynthesis were33–40µµmoles CO₂, fixed per cm² of surfacearea per second (equivalent to 12–15 /xmoles fixed permg chlorophyll per hour). ^l4CO₂ fixation was inhibited to thedark level by 3(3,4,dichlorophenyl)-1, 1-dimethylurea (at 0-6µM or 10µM) and by the uncoupler carbonyl cyanide-m-chlorophenylhydrazone(SµM). The presence of imidazole or ammonium sulphate(both of which uncouple ATP production in vitro) did not resultin an inhibition of 14CO2 fixation. These results are discussedin relation to published work on solute uptake by Nitella translucens.During photosynthesis there was rapid movement of ¹⁴C-labelledorganic compounds out of the chloroplasts. ¹⁴C-labelled sucrose,ammo-acids, and sugar phosphates were found in samples of vacuolarsap.     

Phosphorylated Compounds in Plants: II. THE ESTIMATION OF HEXOSE PHOSPHATES AND ADENOSINE PYROPHOSPHATES IN PLANT TISSUE 3

Modifications of the method of Slater for the estimation ofhexose phosphates and adenosine pyrophosphates in plant tissueare described. Triphosphatea of nucleosides other than adenosineinterfere with the estimation of adenosine pyrophosphates inplant tissue, but other interference is not serious if the concentrationof hexose phosphates or adenosine pyrophosphates is not lowerthan about 0.15 µmoles per ml. of tissue extract.     

The Potassium Relations of Lemna minor L.: II. THE MECHANISM OF POTASSIUM UPTAKE

The experiments described in this paper concern the nature ofthe K⁺-influx mechanism in Lemna minor L. It is establishedthat K⁺ influx is ‘site restricted’ and that theaffinity of the system involved is in close agreement with thatof the well-documented System I mechanisms that are found inmany plants (Epstein, 1972). Experiments with ATP and CCCP suggestthat K⁺ influx is an active process, that the influx machineryresides at the plasmalemma and that this machinery containsan ATPase activity. Membrane vesicles isolated from the plantsabsorb K⁺(Rb⁺) with an affinity comparable to that of the systeminvolved in physiological K⁺ influx.     

The Uptake of Growth Substances: XIV. PATTERNS OF UPTAKE BY LEMNA MINOR OF PHENOXYACETIC AND BENZOIC ACIDS FOLLOWING PROGRESSIVE CHLORINATION

The interrelationships between chemical structure and patternsof uptake by Lemna minor have been examined for (a) phenoxyaceticacid and its 2-, 4-, 2,4-, 2,6-, 3,5-, 2,5-, 2, 4, 5- and 2,4,6-chloroderivatives and (b) benzoic acid and its 2-, 2,4-, 2,5-, 2,3,6-chloro-and 3,6-dichloro-2-methoxy derivatives. All compounds contained¹⁴C in the carboxyl group. The plants from a clonal populationwere grown at a constant temperature and continuously illuminated. With progressive chlorination of phenoxyacetic acid, uptakeis enhanced, so that by 6 h there is a fourfold difference betweenthe monochloro- and trichloro-derivatives. In complete contrast,chlorination of benzoic acid greatly suppresses uptake and thedifferences associated with the degree of chlorination are smaller. Arising out of previous studies, the effects of adding streptomycin,synthalin, and cetyltri-methylammoniumbromide on the courseof uptake of individual members of the two series have beenexamined. Each of the additions can induce positive, negative,or null changes in the pattern of uptake, but the nature ofthe response is also dependent on the properties of the compound. These findings are discussed in relation to prior studies concerning(a) penetration into the leaves of Phaseolus vulgaris, (b) uptakeby excised segments of etiolated stems, and (c) changes in physico-chemicalproperties resulting from progressive chlorination. Many of the complexities still remain to be resolved but itseems clear that adsorption by Borne membrane system involvingthe carboxyl group of the entering acid and the positively chargedquaternary ammonium group of alpha-lecithin cannot be restrictedto compounds which are physiologically active as auxins or herbicides.     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

The Effect of Micro-organisms on the Absorption of Inorganic Nutrients by Intact Plants: II. UPTAKE AND UTILIZATION OF PHOSPHATE BY BARLELY PLANTS GROWN UNDER STERILE AND NON-STERILE CONDITIONS

The uptake of phosphate by barley plants grown under sterileand non-sterile conditions has been compared from solutionsranging in concentration between 0.001 to 10.0 ppm P. If specialprecautions are not taken to exclude microorganisms their activityon or in plant roots greatly modifies the absorption and utilizationof phosphate in plants, especially when the external concentrationis low. More phosphate is incorporated into nucleic acids, phospholipids,and phosphoproteins in the roots; at 0.001 ppm P these compoundsaccount for over 60 per cent of the phosphate absorbed comparedwith about 15 per cent in plants grown under sterile conditionsand even at 1.0 ppm P there is a twofold difference. The immobilizationof phosphate in the roots reduces its transfer to the shoots;at 0.001 ppm P, 2 as compared with 20 per cent of the phosphateabsorbed is transferred to the shoots. This effect decreaseswith increasing concentration but small differences are stillapparent up to about 0.5 ppm P. A further effect of micro-organisms is apparent when plantsare transferred from dilute solutions to phosphate-free solutions.Under non-sterile conditons there is an almost tenfold increasein the loss to these solutions of previously absorbed phosphate. It is evident that conclusions on the mechanism of absorptionof phosphate, based on experiments in which the actions of micro-organismshave been ignored, require reinvestigation.     

Inorganic Carbon Uptake during Photosynthesis : II. Uptake by Isolated Asparagus Mesophyll Cells during Isotopic Disequilibrium

The species of inorganic carbon (CO₂ or HCO₃⁻) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO₂ or HCO₃⁻ transport, during steady state photosynthesis, is achieved by monitoring the changes (by ¹⁴C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO₂ or HCO₃⁻ transport) and experimental time-courses of ¹⁴C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO₂, rather than HCO₃⁻, is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO₂ is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO₃⁻ transport, as the incorporation of ¹⁴C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO₂ uptake alone. The contribution of HCO₃⁻ to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO₃⁻ concentration. The evidence for direct HCO₃⁻ transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of ¹⁴C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO₂, which is partially alleviated by a high extracellular concentration of HCO₃⁻.     

A Comparison of the Uptake and Translocation of Some Organic Herbicides and a Systemic Fungicide by Barley: II. RELATIONSHIP BETWEEN UPTAKE BY ROOTS AND TRANSLOCATION TO SHOOTS

The relationship between uptake by barley roots and translocationto shoots has been examined under different conditions for sixherbicides and a systemic fungicide (four triazines, diuron,2,4-dichlorophenoxyacetic acid and ethirimol). For all thesecompounds a large proportion of the material taken up by rootsdoes not appear to move readily to the shoots. Elution withwater of the roots of intact plants which had previously beenplaced in labelled solutions of these compounds enabled thematerial released to be separated into three fractions differingin their rates of diffusion out of the roots. There was reasonablygood correlation between the concentration of the most readilydiffusible fraction in the roots and the concentration in thetranspiration stream. The results obtained were consistent withthe postulate that lipophilic compounds can diffuse into thevacuoles of the cortical cells where they are available fortransport to the shoots, whereas for lipophobic compounds materialreaching the shoots originates largely from the free space inthe roots.     

Uptake of Phosphate by White Clover: II. THE EFFECT OF PH ON THE ELECTROGENIC PHOSPHATE PUMP

The effect of pH on the electrogenic phosphate pump in whiteclover roots was investigated. It was found that the pump operatedbetween pH 3.5 and 8.0 with an optimum at 4.3. External pH changesaffected phosphate uptake and PD to the same extent so thata constant relationship between them was maintained. The activityof the pump declined when the carbohydrate supply was cut offby excision of the root or defoliation of the plant but couldbe restored by lowering the pH around the root. It was concludedthat the electrogenic phosphate pump could be driven by a pHgradient across the cell membrane. However, there did not appearto be a direct relationship between the operation of the pumpand extrusion of H⁺ by the root.     

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: II. GROWTH OF LOLIUMPERENNEL. AT CONSTANT PHOSPHATE CONCENTRATIONS

The effect of phosphate concentration in flowing solution cultureat a range between 0.04 and 32 mmol m^–3 P on the growthof perennial ryegrass was studied in two experiments, each lastingabout 45 d after sowing. Phosphorus contents of seedlings wereaffected by the concentration in solution within about 5 d fromgermination, and dry weight differences were first observedat about 6 d after this. The rate of uptake of phosphate byseedlings was affected by the concentration in solution beforethe root fresh weight or root/emdash shoot ratio had changed.Young plants (less than 4 weeks old) were more sensitive tophosphate concentration in solution than older ones. In conditionsof high rate of growth, older plants required a solution concentrationbetween 0.1 and 0.4 mmol m^–3 P to achieve maximum potentialgrowth rate, whereas for plants of similar age but less dryweight, 0.04 mmol m^–3 P was adequate. Towards the endof the experimental period, plants growing at a nominal solutionconcentration of 0.04 mmol m^–3 P were able to obtain phosphatefrom a solution of about 0.01 mmol m^–3P. Phosphate toxicity was not observed, nor were there visual symptoms(other than reduced growth) of phosphate deficiency in plantswhose growth was limited by phosphate concentration in solution. Key words: Lolium perenne, Phosphate uptake     

Transport of Amino Acids in Barley Leaf Tissue: II. THE KINETICS OF UPTAKE OF AN UNNATURAL ANALOGUE

A detailed analysis is presented of the relationship betweenexternal concentration and the rate of uptake of an amino-acidanalogue, -aminoisobutyric acid, by barley leaf strips. Theresults are compatible with the operation of specific uptakemechanisms in the case of both freshly cut and ‘aged’tissue. Determination of the fraction of uptake susceptibleof inhibition by a competing amino acid suggested that a paralleldiffusion pathway, if any, plays only a minor role. Indicationsfor a dual amino-acid uptake mechanism were obtained. The principaleffect of ageing appears to be exerted on uptake in the lowerconcentration range.     

The Uptake of Growth Substances: II. THE ABSORPTION AND ACCUMULATION OF 2:3:5-TRIIODOBENZOIC ACID BY THE ROOT AND FROND OF LEMNA MINOR

The absorption by Lemna minor of 2 : 3 : 5-triiodobenzoic acid(TIBA), labelled in the 2 position with iodine-131, has beeninvestigated. Under constant conditions of light (300 foot candles)and temperature (25?C.) the course of uptake by the whole plantis dependent on the concentration of TIBA in the culture solution(pH 5.I). Up to circa 1 mg./l. a high rate of uptake in thefirst 30 minutes is followed by a slower but steady accumulationduring the next 4.5 hours. When the concentration is increasedto 5 mg./1. uptake, at first rapid, falls off progressivelyso that after 1-2 hours the content of the tissues starts todecrease because TIBA moves out in to the external solutionfaster than it enters. The magnitude of this reversal from apositive to a negative rate of uptake,.partitularly for theroots, is even more pronounced at 10 mg/l. When plants, first placed in labelled solutions for 2 to 2.5hours are transferred to culture solution the loss of the labelledcompound to the external solution, especially from the roots,is very rapid : the decrease in root content may reach 90 percent. in the first 80 minutes. The initial rate of egress islittle affected either by the hydrogen ion concentration orthe presence of unlabelled TIBA in the external solution. Between5, 15, and 25? C. the temperature quotients range from 1.29to 1.70 for the root and 1.66-1.37 for the frond. Comparable experiments on the initial phase of uptake duringthe first 2o to 30 minutes demonstrate that (a) the rates aregreatly dependent on the external pH i.e. uptake is largelyin the molecular form and (b) between 5 and 15?C. the ratesare not temperature dependent but between 15 and 25?C. theyare: the quotients for roots and frond are 2.6 and 2.0. Comparisons of the course of uptake of normal plants and plantspreviously frozen in liquid air showed that at both I? and 25?C.'pre-frozen' fronds continue to accumulate TIBA and may containat the end of 5 hours 8 times as much as control fronds. Incontrast, the uptake by the pre-frozen roots is depressed. Theaddition of high (30-50 mg./l.) but not low (10 mg./1.) concentrationsof indole-3-acetic acid (IAA) to the external solution alsocauses a striking accumulation of TIBA in the frond but uptakeby the roots is not changed. Lastly, the presence of TIBA in the external solution whilenot affecting the uptake of cerium-144. in the first 2 hourssubsequently depresses uptake by the root but not by the frond. These findings are compared with the similar pattern of uptakefor z,4-dichlorophenoxyacetic acid described in the previouspaper. The nature of the processes controlling the inward andoutward flow of TIBA, their location within the cell and theirbearing on the physiological actions of auxins are discussed.     

Molecular Markers for Ion Compartmentation in Cells of Higher Plants: II. LIPID COMPOSITION OF THE TONOPLAST OF THE HALOPHYTE SUAEDA MARITIMA (L.) DUM.

Vacuoles of high purity were isolated from the leaves of thehalophyte Suaeda maritima (L.) Dum. The relative compositionsof phospholipids, phytosterols, and fatty acids in the tonoplastmembrane were determined and membrane fluidity was assessedby electron spin resonance. The characteristics of the tonoplastwere consistent with minimizing passive permeability to NaCI.The phospholipid: protein ratio (1.1: 1.0) was higher than thatrecorded in other membrane preparations, including vacuolesfrom beetroot storage material, commensurate with the low density(1.05 g cm^–3) of the S. maritima tonoplast. The tonoplastfatty acids were highly saturated and dominated by n-hexadecanoicacid and n-octadecanoic acid. Phytosterols identified by gaschromatography were cholesterol, campesterol, stigmasterol,and ß-sitosterol. Cholesterol was a trace percentagein protoplasts, but comprised 30% of the tonoplast sterols.Semi-quantitative analysis by chromatography on silica gel revealedan enrichment in the tonoplast of glycolipid which was not accountedfor as chloroplast contamination. The fluidity of the tonoplast,determined by electron spin resonance, was lower than the protoplasts,consistent with the high degree of saturation of the fatty acidchains. The relevance of the lipid composition of the tonoplastto its central role in ion compartmentation within the halophytecell is discussed. Key words: Ion compartmentation, membrane lipids, salinity, vacuole