[14C]Carbon-dioxide fixation by isolated leaf epidermes with stomata closed or open

Following small hypo-osmotic shocks, ion concentrations (Na⁺, K⁺, Cl^-) in Platymonas subcordiformis decreased; this was due mainly to an increase of cell volume. With larger hypo-osmotic stresses, the decrease of ion concentration continued and, additionally, extrusion of mannitol was observed. The ion and mannitol concentrations were not regained after 240 min. In contrast, following hyperosmotic shocks, the ion concentrations increased transitorily during the first 20–40 min. The same was true for K⁺ following small hyperosmotic stresses and for Na⁺ and — partially — Cl^- with larger shocks. Large hyperosmotic stresses caused permanent accumulation of mannitol, which levelled off after 60–80 min. Thus the transient increase of ions bridged the concentration gap until mannitol was accumulated to a high enough concentration to account for the osmotic adaptation of Platymonas, together with a basal level of the ions K⁺, Na⁺, Cl^-.Abbreviations PS photosynthesis - Resp respiration     

Sodium,potassium, chloride and proline concentrations of chloroplasts isolated from a halophyte,Mesembryanthemum crystallinum L.

Concentrations of four major solutes (Na⁺, K⁺, Cl^-, proline) were determined in isolated, intact chloroplasts from the halophyte Mesembryanthemum crystallinum L. following long-term exposure of plants to three levels of NaCl salinity in the rooting medium. Chloroplasts were obtained by gentle rupture of leaf protoplasts. There was either no or only small leakage of inorganic ions from the chloroplasts to the medium during three rapidly performed washing steps involving precipitation and re-suspension of chloroplast pellets. Increasing NaCl salinity of the rooting medium resulted in a rise of Na⁺ und Cl^- in the total leaf sap, up to approximately 500 and 400 mM, respectively, for plants grown at 400 mM NaCl. However, chloroplast levels of Na⁺ und Cl^- did not exceed 160–230 and 40–60 mM, respectively, based upon a chloroplast osmotic volume of 20–30 l per mg chlorophyll. At 20 mM NaCl in the rooting medium, the Na⁺/K⁺ ratio of the chloroplasts was about 1; at 400 mM NaCl the ratio was about 5. Growth at 400 mM NaCl led to markedly increased concentrations of proline in the leaf sap (8 mM) compared with the leaf sap of plants grown in culture solution without added NaCl (proline 0.25 mM). Although proline was fivefold more concentrated in the chloroplasts than in the total leaf sap of plants treated with 400 mM NaCl, the overall contribution of proline to the osmotic adjustment of chloroplasts was small. The capacity to limit chloroplast Cl^- concentrations under conditions of high external salinity was in contrast to an apparent affinity of chloroplasts for Cl^- under conditions of low Cl^- availability.Abbreviation Chl chlorophyll     

Partial Turgor Pressure Regulation in Chara canescens and its Implications for a Generalized Hypothesis of Salinity Response in Charophytes

Concentrations of ions and sucrose in the vacuolar sap of Chara canescens growing in an oligohaline lake (1.5 ‰) were estimated over the main growth period of the plants. During fructification vacuolar sap contained a mean of 41 mol m^?3 (range 10.2–61.8) sucrose. The mean turgor pressure was 239 mosmol kg^?1 (range 219–264). In long- and short-term experiments these plants were subjected to increasing salinities up to 22 ‰. When salinity was increased from 1.5 to 4.4 ‰ turgor pressure was restored to only 80 % of the initial value. This reduced level of turgor pressure was maintained up to a salinity of 22 ‰. The increase in vacuolar osmotic potential was due to the monovalent ions Na⁺, K⁺ and Cl^?. The relative amounts of Na⁺ and K⁺ participating in the regulation process were dependent on external salinity. The regulatory mechanisms observed in the brackish water species Ch. canescens are compared with those reported from freshwater and euryhaline species.     

Osmotic acclimation of the brackish water xanthophyceae,Vaucheria dichotoma (L.) MARTIUS: Inorganic ion composition and amino acids

The salinity tolerance ofVaucheria dichotoma, a siphonous Xanthophycean alga was investigated. The alga survived an external osmotic potential range between 74 and 1, 176 mOsmol (ca. 2.5 and 40.0 ppt. (parts per thousand]). Turgor pressure was regulated in salinities ranging from 74 to 441 mOsmol. With further increase of the salinity, turgor pressure decreased from 153 to 9 mOsmol (0.44 to 0.08 MPa). At 441 mOsmol salinity the major intracellular ions were present in the following concentrations (mM/l cell water): K⁺, 145; Na⁺; 90; sulphate, 91; Cl⁻, 91. Under the most severe salinity stress (1,176 mOsmol) the ionic concentration increased to (mM/l cell water): K⁺, 250; Na⁺, 75; sulphate, 35; Cl⁻, 351. The content of amino acids: alanine (Ala), threonine (Thr and glutamic acid (Glu) was lower, nerver exceeding 5–11 mM, however; the concentrations were positively correlated with salinity.     

Mechanisms of Salinity Control in Sea Bass

Sea bass can regulate the concentration of Na⁺, K⁺, and Cl^-, among other ions, in their blood, skin, gills, and kidney. Therefore, the salinity of the water does not have a great influence on their metabolism, and sea bass can live in both sea and freshwater in accordance with the salt concentration. Most salinity control occurs in the gills, primarily through the control of chloride cells present there. The concentration of ions in the blood is controlled by the cotransporter Na⁺ / K⁺ / 2Cl^- (NKCC) in the chloride cell, and the subunits of Na⁺ / K⁺ ATPase (NKA) function to maintain homeostasis. The expression of NKA is regulated by subunits of the protein FXYD, allowing the sea bass to survive in compliance with the salinity. In this way, it is possible for sea bass to live in sea and freshwater by controlling the salinity of its body using functions of various channels, proteins, and genes present in the chloride cells of sea bass. In this study, we investigated recent studies of salt control mechanisms in sea bass and their application.     

Ion movements induced by transcellular osmosis inNitella flexilis

Summary Movements of K⁺, Na⁺, and Cl^}- ions during transcellular osmosis were studied in internodal cells ofNitella flexilis. Much K⁺ was released from the endosmotic cell part, but only a little from the exosmotic cell part. The amount of K⁺ released depended on the osmotic gradient driving transcellular osmosis. Movement of Na⁺ was hardly detected. Cl^}- was released in nearly the same amounts as K⁺. Release of K⁺ from the endosmotic cell half was stimulated remarkably by lowering the temperature from 20 to 1 °C, and also by lowering the internal osmotic pressure but inhibited by raising it.The dependence of K⁺ release on osmotic gradient, internal osmotic pressure and temperature can be explained by their effects on membrane depolarization and membrane resistance (Hayama et al. 1978). We concluded thatP _K remained unchanged, whileP _Cl increased a great deal in the endosmotic cell part.     

Effect of salinity on osmotic adjustment characteristics of <Emphasis Type="Italic">Kandelia candel</Emphasis>

To elucidate the osmotic adjustment characteristics of mangrove plants, inorganic ion and organic solute contents of intermediate leaves were investigated in 3-month-old Kandelia candel (L.) Druce seedlings during 45 days of NaCl treatments (0, 200, and 500 mM NaCl). The contents of Na⁺, Cl⁻, total free amino acids, proline, total soluble sugars, pinitol and mannitol increased to different degree by salinity, whereas, K⁺ content decreased by salinity compared with control. NaCl treatment induced an increase of inorganic ion contribution while a decrease of organic solute contribution. It was concluded that accumulating a large amount of inorganic ions was used as the main osmotic adjustment mechanism under salinity treatment. However, accumulation of organic osmolytes might be considered to play much more important role in osmoregulation under severe salinity (500 mM NaCl) than under moderate salinity (200 mM NaCl), thus the damage caused by high toxic ions (Na⁺ and Cl⁻) concentration in K. candel leaves could be avoided.     

Effects of salinity on uptake and distribution of Na+, Cl- and K+ in two wheat cultivars

Plants of two wheat (Triticum aestivum L.) cultivars differing in salt tolerance were grown in sand with nutrient solutions. 35-d-old plants were subjected to 5 levels of salinity created by adding NaCl, CaCl₂ and Na₂SO₄. Growth reduction caused by salinity was accompanied by increased Na⁺ and Cl^- concentrations, Na⁺/K⁺ ratio, and decreased concentration of K⁺. The salt tolerant cv. Kharchia 65 showed better ionic regulation. Salinity up to 15.7 dS m^-1 induced increased uptake of Na⁺ and Cl^- but higher levels of salinity were not accompanied by further increase in uptake of these ions. Observed increases in Na⁺ and Cl^- concentrations at higher salinities seemed to be the consequence of reduction in growth. Uptake of K⁺ was decreased; more in salt sensitive cultivar. This was also accompanied by differences in its distribution.     

Steady state osmotic adaptation inUlva lactuca

The effects of hyper- and hypo-saline stresses on the levels of various inorganic and organic solutes inUlva lactuca have been recorded. Hypoosmotic stress decreased the tissue concentration of K⁺, Na⁺ and Cl^- while hyper-osmotic stress caused a transient increase in Na⁺ and a stable accumulation of K⁺ and Cl^-. The tissue content of -dimethylsulphoniopropionate (-dimethylpropiothetin) responded to changes in salinity. The time course of hypersaline stress showed the -dimethylsulophoniopropionate concentration rose as the Na⁺ level fell. The levels of free sugars and amino acids, including proline, were relatively low in this alga and did not appear to be important in osmotic adjustment. The possibility that tertiary sulphonium dipolar ions have an analogous role in some algae to glycinebetaine and possibly other quaternary nitrogen compounds in higher plants as cytoplasmic osmotica is discussed briefly.Abbreviations DMSP -dimenthylsulphomiopropionate - AFT apparent free space - TLE thin layer electrophoresis - NPS ninhydrin positive substances - TLC thin layer chromatography     

OSMOTIC ADJUSTMENT AND DYNAMIC CHANGES IN DISTRIBUTION OF LOW MOLECULAR WEIGHT SOLUTES BETWEEN CELLULAR COMPARTMENTS OF CARROT AND BEET ROOT CELLS EXPOSED TO SALINITY

Carrot cells (Daucus carota L.) in suspension culture exposed to medium containing 150 mM NaCl plasmolyzed immediately and deplasmolyzed within 35 to 40 hr. Three days after exposure to NaCl the cells resumed proliferation. Accommodation to salinity and renewal of growth was accompanied by absorption of Na⁺ from the external medium. On completion of deplasmolysis, K⁺ concentration in the cytosol doubled and Na⁺ concentration approximated that of K⁺. The vacuolar K⁺ concentration was practically unchanged while Na⁺ accumulated to a concentration double that of K⁺. Cl^−- accumulation started later and eventually exceeded that of Na⁺ plus K⁺. Malate was redistributed during accommodation to salinity and eventually returned to its initial level. Amino acid content in the cytosol increased fivefold, while in the vacuole it remained unchanged. These results show that: 1) recovery from osmotic shock requires absorption of easily penetrating solute, mainly Na⁺; 2) distribution of solutes, absorbed or synthesized in cells exposed to salinity, is a dynamic process; 3) cells could grow and proliferate in high NaCl content in the cytosol; 4) red beet root cells grown in the presence of NaCl contain higher cytoplasmic Na⁺ than K⁺; and 5) during adjustment to salinity small spherical carrot cells survive the osmotic shock and do not show any detectable damage.     

Urea based osmoregulation and endocrine control in elasmobranch fish with special reference to euryhalinity

Since the landmark contributions of Homer Smith and co-workers in the 1930s there has been a considerable advance in our knowledge regarding the osmoregulatory strategy of elasmobranch fish. Smith recognised that urea was retained in the body fluids as part of the ‘osmoregulatory ballast’ of elasmobranch fish so that body fluid osmolality is raised to a level that is iso- or slightly hyper-osmotic to that of the surrounding medium. From studies at that time he also postulated that many marine dwelling elasmobranchs were not capable of adaptation to dilute environments. However, more recent investigations have demonstrated that, at least in some species, this may not be the case. Gradual acclimation of marine dwelling elasmobranchs to varying environmental salinities under laboratory conditions has demonstrated that these fish do have the capacity to acclimate to changes in salinity through independent regulation of Na⁺, Cl⁻ and urea levels. This suggests that many of the presumed stenohaline marine elasmobranchs could in fact be described as partially euryhaline. The contributions of Thomas Thorson in the 1970s demonstrated the osmoregulatory strategy of a fully euryhaline elasmobranch, the bull shark, Carcharhinus leucas, and more recent investigations have examined the mechanisms behind this strategy in the euryhaline elasmobranch, Dasyatis sabina. Both partially euryhaline and fully euryhaline species utilise the same physiological processes to control urea, Na⁺ and Cl⁻ levels within the body fluids. The role of the gills, kidney, liver, rectal gland and drinking process is discussed in relation to the endocrine control of urea, Na⁺ and Cl⁻ levels as elasmobranchs acclimate to different environmental salinities.     

Effect of Salinity upon Cell Membrane Potential in the Marine Halophyte, Salicornia bigelovii Torr

The electrophysiology of root cells of the marine halophyte, Salicornia bigelovii Torr., has been investigated. Cellular concentrations of K⁺, Cl⁻, and Na⁺ and resulting cell membrane potentials were determined as functions of time and exposure to dilutions of artificial seawater. Treatment of these data by the Nernst criterion suggests that Cl⁻ is actively transported into these root cells, but that active transport need not be invoked to explain the accumulation of Na⁺ at all salinities investigated nor for K⁺ at moderate to high salinities. In low environmental salinity, the cell electropotential of Salicornia root cells was found to respond to inhibitors in a fashion similar to that observed in glycophytes; in high environmental salinity, root cell membrane potential appears to be insensitive to bathing salinity and m-chlorocarbonylcyanide phenylhydrazone induces membrane hyperpolarization, in contrast to the response of glycophytes to such treatments. The fact that measured membrane potentials exceed diffusion potentials for Na⁺, K⁺, and Cl⁻ and the observation of a rapid depolarization by CO in the dark suggests an electrogenic component in Salicornia root cell membrane potentials.     

Mercury and osmoregulation in the euryhaline crab,Eriocheir sinensis

This review will focus on cases where it might be possible that the toxicity of the heavy metal mercury results from an interaction with osmoregulatory mechanisms. It is shown that mercury-induced impairment of osmoregulatory capability in the sense of severe modifications of the blood osmotic concentration is more pronounced in brachyuran decapod species adapted to dilute waters. The rationale for considering these effects is based on a comparison between mercury effects on three species of decapod crustaceans exhibiting various degrees of osmoregulation capability: the strong regulator crab Eriocheir sinensis, the weak regulator Carcinus maenas, both of them being euryhaline, and the stenohaline osmoconformer Cancer pagurus. It is established that a synergistic effect exists between salinity and HgCl₂ toxicity in euryhaline species which are hyperregulators in dilute media, that is, E. sinensis and C. maenas only. Depth study of E. sinensis as a model demonstrates that Na⁺ and Cl⁻ permeabilities of the gill epithelium is affected by mercury, as well as the Na⁺ and Cl⁻ active transport processes located at the same level. Evidences are brought showing that mercury drastically disturbs the Na⁺/K⁺ pump and the Cl⁻ channels located in the serosal baso-lateral membranes of the posterior gills.     

Quantitative ion localization within Suaeda maritima leaf mesophyll cells

Grown under saline conditions, Suaeda maritima accumulates Na⁺ and Cl^- into its leaves, where individual mesophyll cells behave differently in their compartmentation of these ions. Measurements of ion concentrations within selected subcellular compartments show that freeze-substitution with dry sectioning is a valuable preparative technique for analytical electron microscopy of highly vacuolate plant material. Using this approach, absolute estimates were made of Na⁺, K⁺ and Cl^- concentrations in the cytoplasm, cell walls, chloroplasts and vacuoles of leaf mesophyll cells.Abbreviation TAEM transmission analytical electron microscopy     

Relations between intracellular ion activities and extracellular osmolarity inNecturus gallbladder epithelium

Summary The interactions between ion and water fluxes have an important bearing on osmoregulation and transepithelial water transport in epithelial cells. Some of these interactions were investigated using ion-selective microelectrodes in theNecturus gallbladder. The intracellular activities of K⁺ and Cl^– in epithelial cells change when the epithelium is adapted to transport in solutions of a low osmolarity. In order to achieve new steady states at low osmolarities, cells lost K⁺, Cl^– and some unidentified anions. Surprisingly, the apparent K⁺ concentration remained high: at an external osmolartity of 64 mOsm the intracellular K⁺ concentration averaged 95mm. This imbalance was sensitive to anoxia and ouabain. The effects of abrupt changes in the external osmolarities on the intracellular activities of Na⁺, K⁺ and Cl^– were also investigated. The gradients were effectuated by mannitol. The initial relative rates of change of the intracellular activities of Na⁺ and Cl^– were equal. The data were consistent with Na⁺ and Cl^– ions initially remaining inside the cell and a cell membraneL _p of 10^–3 cm sec^–1 osm^–1, which is close to the values determine by Spring and co-workers (K.R. Spring, A. Hope & B.-E. Persson, 1981.In: Water Transport Across Epithelia. Alfred Benzon Symposium 15. pp. 190–200. Munskgaard, Copenhagen). The initial rate of change of the intracellular activity of K⁺ was only 0.1–0.2 times the change observed in Na⁺ and Cl^– activities, and suggests that K⁺ ions leave the cell during the osmotically induced H₂O efflux and enter with an induced H₂O influx. The coupling is between 98 and 102 mmoles liter^–1. Various explanations for the anomalous behavior of intracellular K⁺ ions are considered. A discussion of the apparent coupling between K⁺ and H₂O, observed in nonsteady states, and its effects on the distribution of K⁺ and H₂O across the cell membrane in the steady states, is presented.     

Effects of Inhibitors of Protein and Nucleic Acid Synthesis on the Development of Ion Uptake Mechanisms in Beetroot Slices (Beta vulgaris)

Mechanisms for the uptake of K⁺, Na⁺ and Cl^- develop sequentially in thin slices of beetroot tissue washed under aerobic conditions. Actinomycin D inhibited or prevented the development of K⁺, Na⁺ and Cl^- uptake mechanisms when added to freshly cut slices, but had no effect on net ion uptake when added after the development of the ion uptake mechanisms. The use of puromycin as a specific inhibitor of protein synthesis was unsatisfactory as it caused leakage of pigments and excessive loss of ions from the disks. Cycloheximide prevented the development of ion uptake mechanisms when added at the start of the experiment, but when added after the development of ion uptake mechanisms its inhibitory effect did not become apparent until after a certain time interval which varied from 3 hours for Cl^- to 25 hours for K⁺ uptake in the same experiment. p-Fluorophenylalanine caused an appreciable shortening of the time required for the development of Na⁺ and K⁺ uptake capabilities, while it completely prevented the development of a Cl^- uptake mechanism. p-Fluorophenylalanine-induced early uptake of Na⁺ and K⁺, however, was followed by periods of net leakage of these ions. It is suggested that the development of ion uptake mechanisms depends on the production of m-RNA, which appears to be relatively stable after its synthesis. The synthesis and decay characteristics of specific proteins required for the ion uptake mechanisms appear to differ for each ion species.     

Elver Invasion, Population Structure and Growth of Marbled eels Anguilla marmorata in a Tropical River on Réunion Island in the Indian Ocean

Winter skates, Leucoraja ocellata, exposed to 80% and 50% seawater (SW) exhibited rapid and significant weight gains followed by a slight recovery to new steady state levels within 8 days. Skates were acclimated at each salinity (100% SW [N = 16], 80% SW [N = 8], 50% SW [N = 8]), anesthetized (MS222) and bled from the caudal vein. In 100% SW, skate plasma (930mOsm/kg) was slightly hyperosmotic to the external medium (922mOsm/kg). Plasma osmolality decreased with seawater dilution, but became increasingly hyperosmotic to the bathing media. The environmental dilutions resulted in significant, but disproportionate changes in plasma Cl^–, P^–, Na⁺, Ca⁺, Mg⁺, trimethylamine oxide (TMAO) and urea concentrations. Mean corpuscular [Hb] and milliliter RBC water measurements suggest that skate red cells swelled less at each dilution than predicted for a passive erythrocyte osmometer. Concentrations of the major RBC solutes K⁺, urea, TMAO and Cl^– decreased by 8, 25, 5 and 21%, respectively in 80% SW. In 50% SW, K⁺, urea, TMAO and Cl^– concentrations decreased by 9, 47, 36 and 15%, respectively. Quantitatively, the other measured intracellular electrolytes (Mg⁺, Na⁺, P^– and Ca⁺) also exhibited disproportionate changes in concentration. Our results indicate that L. ocellata is a euryhaline elasmobranch that can tolerate significant reduction in the external salinity through the release of both ions and urea from the extracellular compartments while retaining electrolytes at the expense of urea in the intracellular compartment.     

The relationship between sodium,potassium, and chloride in amphibian muscle

The Na,⁺ Cl^-, and K⁺ content of toad plasma and the sartorius muscle has been determined. Although the Na⁺ and Cl^- level of the muscles in the living animal varied greatly (0 to 38.0 m.eq. per kg., and 0 to 31.8 m.eq. per kg. respectively) the K⁺ level was subject to a smaller variation (76.5 to 136 m.eq. per kg.). There was a direct relationship between Na⁺ and Cl^-, which was independent of the K⁺ level. There is a closely related gain of Na⁺ and Cl^- when muscle is soaked in normal Ringer. These gains are not related to the K⁺ loss, frequently found on soaking. The relationship between the three ions was studied in a large series of 124 muscles in normal Ringer. As found in vivo, there was a correlation between Na⁺ and Cl.^- This correlation was independent of K⁺ content, except when this was abnormally low. Alteration of the external NaCl level produced concomitant changes in the internal levels of these ions. Alteration of the external KCl level produced an increase in internal Cl^- similar to that found with high NaCl solutions, but the amount of K⁺ entering the cell was approximately one-third of the external increase. Removal of K⁺ from the external solution did not result in a loss of K⁺ from the cell, although there was an adequate amount of Cl^- present to accompany it. The results cannot be reconciled with either a Donnan concept for the accumulation of K⁺, or a linked carrier system. A theory is proposed to account for the ionic differentiation within the cell. The K⁺ is assumed to be adsorbed onto an ordered intracellular phase. The normal metabolic functioning of the cell is necessary to maintain the specificity of the adsorption sites. There is another intracellular phase, which lacks the structural specificity for K⁺, and which contains Na⁺, Cl^-, and K⁺ in equilibrium with the external solution. The dimensions of the free intracellular phase will vary from cell to cell, but it will be smaller in the intact animal, and will increase on soaking in normal Ringer, until it is approximately one-third of the total cellular volume. The increase in this phase may be ascribed to a decrease in the energy available to maintain the ordered phase.     

Contributions of inorganic ions,soluble carbohydrates,and multiatomic alcohols to water homeostasis in <Emphasis Type="Italic">Artemisia lerchiana</Emphasis> and <Emphasis Type="Italic">A. pauciflora</Emphasis>

Two morphological forms of wormwood Artemisia lerchiana (f. erecta and f. nutans) and A. pauciflora Web. (morphological form erecta) were grown on sand culture at a range of NaCl concentrations in the nutrient medium and then assayed for Na⁺, K⁺, and Cl^? content in various organs. In addition, the content of mono-, di-, and trisaccharides and multiatomic alcohols (mannitol and glycerol); water content; and organ biomass were determined. All plants examined showed high NaCl tolerance, comparable to that of halophytes. They were able to maintain high tissue hydration under conditions of salinity-induced growth suppression. The intracellular osmotic pressure in wormwood organs was mainly determined by the presence of Na⁺, K⁺, and Cl^?, as well as by mono-, di-, and trisaccharides, mannitol, and glycerol. The high content of Na⁺ and Cl^? in wormwood organs was also observed in the absence of salinity, which implies the ability of these organs to absorb ions from diluted NaCl solutions and accumulate ions in cells of their tissues. With the increase in salinity, the content of Na⁺ and Cl^? in roots and leaves increased to even higher levels. It is concluded that the ability of wormwood plants to absorb and accumulate inorganic ions provides for sustainable high intracellular osmotic pressure and, accordingly, low water potential under drought and salinity conditions. Growing plants under high salinity lowered the content of monosaccharides in parallel with accumulation of the trisaccharide raffinose. It is supposed that soluble carbohydrates and multiatomic alcohols are not only significant for osmoregulation but also perform a protective function in wormwood plants. The lower osmotic pressure in root cells compared to that in leaf cells of all plants examined was mainly due to the gradient distribution of K⁺ and Cl^? between roots and leaves. The two Artemisia species and two morphological forms of A. lerchiana did not differ appreciably in the ways of water balance regulation. It is found that different morphologies of two A. lerchiana forms are unrelated to variations in intracellular osmotic and turgor pressures.     

Interactions among Ca2+, Na+ and K+ in salinity toxicity: quantitative resolution of multiple toxic and ameliorative effects

Root elongation by wheat seedlings (Triticum aestivum L. cv. Scout 66) was not inhibited by NaCl or KCl up to 130 mM in culture solutions or by high Na⁺ (2 mg g^-1 FW) or K⁺ (4 mg g^-1 FW) in the root tissue, provided that [Ca²⁺]>2 mM in the rooting medium. At [NaCl], [KCl], or [mannitol] >250 mOs, root elongation was progressively inhibited, irrespective of high [Ca²⁺]. In contrast, shoot elongation was sensitive to any diminution of water potential, and Ca²⁺ alleviated the toxicity only weakly. At solute concentrations <250 mOs, the following interactions were observed. Ca²⁺ alleviated Na⁺ and K⁺ toxicity to roots by at least three separate mechanisms. K⁺ was more toxic to roots than Na⁺, but Na⁺ was more toxic to shoots. Low levels of K⁺ relieved Na⁺ toxicity, but low levels of Na⁺ enhanced K⁺ toxicity. Tissue concentrations of Na⁺ were reduced by Ca²⁺ and K⁺ in the rooting medium, and tissue concentrations of K⁺ were enhanced by Ca²⁺ and Na⁺. Several hypotheses relating to salinity toxicity can be evaluated, at least for wheat seedlings. The osmoticant hypotheses (salinity intoxication occurs because of diminished water potential) is true for shoots at all salinity levels, but is true for roots only at high salinity. The Ca²⁺-displacement hypothesis (Na⁺ is toxic because it displaced Ca²⁺ from the cell surface) is correct, but often of minor importance. The K⁺-depletion hypothesis (Na⁺ is toxic because it causes a loss of K⁺ from plant tissues) is false. The Cl^--toxicity hypothesis (the apparent toxicity of Na⁺ is induced by associated Cl^-) is false. The results indicate that, apart from osmotic effects, high levels of Na⁺ in the rooting medium and in the tissues are not toxic unless Ca²⁺ is also deficient, a condition probably leading to inadequate compartmentation and excessive cytoplasmic accumulation. This study related growth to ion activities at plasma-membrane surfaces. These activities were computed by a Gouy-Chapman-Stern model then incorporated into non-linear growth models for growth versus toxicants and ameliorants.Key words: Calcium, potassium, salinity, sodium, toxicity