Low-affinity Na+ uptake in the halophyte Suaeda maritima

Na(+) uptake by plant roots has largely been explored using species that accumulate little Na(+) into their shoots. By way of contrast, the halophyte Suaeda maritima accumulates, without injury, concentrations of the order of 400 mM NaCl in its leaves. Here we report that cAMP and Ca(2+) (blockers of nonselective cation channels) and Li(+) (a competitive inhibitor of Na(+) uptake) did not have any significant effect on the uptake of Na(+) by the halophyte S. maritima when plants were in 25 or 150 mM NaCl (150 mM NaCl is near optimal for growth). However, the inhibitors of K(+) channels, TEA(+) (10 mM), Cs(+) (3 mM), and Ba(2+) (5 mM), significantly reduced the net uptake of Na(+) from 150 mM NaCl over 48 h, by 54%, 24%, and 29%, respectively. TEA(+) (10 mM), Cs(+) (3 mM), and Ba(2+) (1 mm) also significantly reduced (22)Na(+) influx (measured over 2 min in 150 mM external NaCl) by 47%, 30%, and 31%, respectively. In contrast to the situation in 150 mm NaCl, neither TEA(+) (1-10 mM) nor Cs(+) (0.5-10 mM) significantly reduced net Na(+) uptake or (22)Na(+) influx in 25 mM NaCl. Ba(2+) (at 5 mm) did significantly decrease net Na(+) uptake (by 47%) and (22)Na(+) influx (by 36% with 1 mM Ba(2+)) in 25 mM NaCl. K(+) (10 or 50 mM) had no effect on (22)Na(+) influx at concentrations below 75 mM NaCl, but the influx of (22)Na(+) was inhibited by 50 mM K(+) when the external concentration of NaCl was above 75 mM. The data suggest that neither nonselective cation channels nor a low-affinity cation transporter are major pathways for Na(+) entry into root cells. We propose that two distinct low-affinity Na(+) uptake pathways exist in S. maritima: Pathway 1 is insensitive to TEA(+) or Cs(+), but sensitive to Ba(2+) and mediates Na(+) uptake under low salinities (25 mM NaCl); pathway 2 is sensitive to TEA(+), Cs(+), and Ba(2+) and mediates Na(+) uptake under higher external salt concentrations (150 mM NaCl). Pathway 1 might be mediated by a high-affinity K transporter-type transporter and pathway 2 by an AKT1-type channel.     

Rubidium Transport in Membrane Vesicles from the Halophyte Suaeda maritima

The uptake and efflux of Rb⁺ by membrane vesicles isolated fromshoots of the halophyte Suaeda maritima have been investigated.Uptake came to an apparent equilibrium after 1 h and the initialrate of uptake was considerably slower than that reported forbacterial membrane vesicles Additions of ATP reduced both Rb⁺uptake and the half-time for loss in efflux experiments, althoughthis effect was not specific for ATP and probably was not associatedwith energy transfer The permeability coefficient for Rb⁺ wascalculated to be between 0 1 and 0 3 x 10^–2 cm s^–1.The value of membrane vesicles in ion transport studies in plantsis discussed. Suaeda maritima, seablite, halophyte, membrane vesicles, ion transport, rubidium     

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     

Salt tolerance in the halophyte Suaeda maritima L. Dum.

Osmotic potentials and individual epidermal cell turgor pressures were measured in the leaves of seedlings of Suaeda maritima growing over a range of salinities. Leaf osmotic potentials were lower (more negative) the higher the salt concentration of the solution and were lowest in the youngest leaves and stem apices, producing a gradient of osmotic potential towards the apex of the plant. Epidermal cell turgor pressures were of the order of 0.25 to 0.3 MPa in the youngest leaves measured, decreasing to under 0.05 MPa for the oldest leaves. This pattern of turgor pressure was largely unaffected by external salinity. Calculation of leaf water potential indicated that the gradient between young leaves and the external medium was not altered by salinity, but with older leaves, however, this gradient diminished from being the same as that for young leaves in the absence of NaCl, to under 30% of this value at 400 mM NaCl. These results are discussed in relation to the growth response of S. maritima.     

Cytometric aspects of the leaves of the halophyte Suaeda maritima

An analysis of the frequency of occurrence of chloroplasts, mitochondria and microbodies in leaf cells of the halophyte Sueda maritima (L.) Dum, showed a remarkable increase (almost a doubling) in the number of mitochondria per cell section when plants were grown in saline (340 mol m⁻³ NaCl) as opposed to non-saline conditions. Chloroplast and microbody volume per cell increased under saline conditions, but number per cell was insensitive to salt. Chloroplasts contained a greater number of starch grains in plants grown under saline as compared to non-saline conditions and, under both conditions, were closely associated with the cell wall adjacent to the intercellular spaces. The implications for growth, photosynthesis and respiration are discussed.     

The effect of salt on protein synthesis in the halophyte Suaeda maritima

Summary An amino acid-incorporating microsomal fraction has been isolated from the leaves of the halophyte Suaeda maritima and the characteristics of the incorporation described. There were no differences in the properties of the microsomes isolated from plants grown in saline and non-saline conditions. The incorporation was severely inhibited by high concentrations of sodium or potassium ions. The results are discussed in relation to the mechanism of salt tolerance in halophytes and the localization of salt in the cells.     

Oxygen dynamics in a salt-marsh soil and in Suaeda maritima during tidal submergence

Habitats occupied by many halophytes are not only saline, but are also prone to flooding and yet surprisingly few studies have evaluated submergence tolerance in halophytes. Sediment, floodwater, and intra-plant O₂ dynamics were evaluated during tidal submergence for the leaf-succulent halophyte Suaeda maritima (L.) Dum. For S. maritima growing in soil just above the mud flat in a UK salt marsh, the soil was only moderately hypoxic just prior to tidal inundation, presumably owing to drainage and O₂ entry facilitated by frequent, large cracks. O₂ declined to very low levels following high tide. By contrast, mud flat sediment remained waterlogged, lacked cracks, and was anoxic. Plant O₂ dynamics were investigated using field-collected plants in sediment blocks transported to a controlled-submergence system in a glasshouse. Submergence during night-time resulted in anoxia within leaves, whereas during day-time O₂ was produced by underwater photosynthesis. The thin lateral roots of S. maritima presumably access some O₂ from hypoxic sediments, but could also experience transient episodes of severe hypoxia/anoxia, especially as any internal O₂ movement from shoots would be small owing to the low gas-filled porosity in roots. Fermentative metabolism to lactate, producing some ATP in the absence of O₂, might contribute to tolerance of transient O₂ deficits. Lactate was high in root tissues, whereas ethanol production (tissue and incubation medium contents) was low, both in comparison with values reported for other species. Our findings demonstrate the importance of tolerance to transient waterlogging and submergence for the halophyte S. maritima growing in a tidal salt marsh.     

X-ray microanalysis of ion distribution within root cortical cells of the halophyte Suaeda maritima (L.) Dum.

The ion content of compartments within cortical cells of mature roots of the halophyte Suaeda maritima grown at 200 mol·m^-3 NaCl has been studied by X-ray microanalysis of freeze-substituted thin sections. Sodium and Cl were found in the vacuoles at about four-times the concentration in the cytoplasm or cell walls, whereas K was more concentrated in the cell walls and cytoplasm than in vacuoles. The vacuolar Na concentration was 12- to 13-times higher than that of K. The Na concentration of cell walls of cortical cells was about 95 mol·m^-3 of analysed volume. The cytoplasmic K concentration within the mature cortical cells was estimated to be 55 mol·m^-3 of analysed volume.     

A procedure for isolating vacuoles from leaves of the halophyte Suaeda maritima

Abstract. A method is described here for isolating protoplasts and vacuoles from leaves of the halophyte Suacda maritima. Integrity of the protoplasts and vacuoles was tested by staining and shown to be more than 75%, while use of biochemical markers, staining and light microscopy suggested a high degree of purity of the vacuoles. Phosphatase and NADH cytochrome- c -reductase were associated with vacuoles; phosphatase showed an eight-fold enrichment and NADH cytochrome- c -reductase a 3.5-fold enrichment relative to protoplasts. The vacuoles contained only 15% of the protein in protoplasts.     

Factors affecting the population dynamics of Suaeda maritima at initial stages of development

This study describes life history and population dynamics of the annual halophyte Suaeda maritima, at initial stages of seedling development, in salt-marshes of the Mont-Saint-Michel Bay. Effects of salinity on seed germination and seedling development were studied both in the laboratory and in the field. Salinity was measured as sediment electrical conductivity. Experiments in the laboratory showed that Suaeda maritima was relatively tolerant to salinity during the germination stage. In the field, germination occurred when soil salinity was minimal. Almost all seeds germinated in the field; this may explain the absence of a persistent seed bank. Salinity did not seem to affect the growth of seedlings either. Mortality in the field was attributed to physical factors such as anoxia during temporary immersion, burial by sediment and tidal uprooting. The intensity of these effects varied according to the location of Suaeda maritima seedlings: in hollows, along the edge of tidal creeks, in gaps among dense vegetation cover, or in pattern with Puccinellia maritima. Perennial vegetation did not restrict Suaeda maritima germination. In contrast, perennials such as Puccinellia maritima and Halimione portulacoides limited the development of Suaeda maritima seedlings since only a small number of this annual emerged beneath their canopies. The fate of seedlings depended to a great extent on the permeability of the perennial canopy to light. Thus, disturbance resulting in bare patches inside the perennial vegetation seemed essential for the development of this annual species. Suaeda maritima can grow in the presence of Puccinellia maritima so long as the latter present as an open matrix. Suaeda maritima may also benefit from protection against desiccation and tidal action where this occurs.     

Nitrogen nutrition in the estuarine zone: the case of Suaeda maritima var. macrocarpa

Suaeda maritima L. var. macrocarpa is a halophytic species distributed in the lower parts of salt marshes of the French coasts. The influence of salinity on nitrogen nutrition and on levels of the key enzymes involved in nitrogen assimilation is analyzed by growing Suaeda under experimental conditions. Use of ¹⁵N-labelled NO₃ ^- and NH₄ ⁺ shows that both ions are effective sources of inorganic nitrogen for Suaeda. The plant is found to use NH₄ ⁺ ions with a good yield, chiefly at high salinities (up to 130 mM). Nitrate reduction and ammonium assimilation by the glutamine synthetase/glutamate synthase pathway occurs mainly in leaves when Suaeda is grown at optimal saline conditions (130 mM NaCl). Absence of NaCl creates less favourable conditions and lowers the activity of nitrate reductase and glutamine synthetase but leads to an important activity of glutamate dehydrogenase in roots. This enzyme could play a major role under suboptimal environmental conditions (i.e., absence of NaCl for Suaeda maritima).Part of this paper is taken from a thesis that was submitted by J. P. Billard in fulfillment of the Doctorat d'Etat degree at the University of Caen, France.     

Salt Tolerance in the Halophyte Suaeda maritima L. Dum.: Intracellular Compartmentation of Ions

The time-course of exchange of sodium and potassium ions fromroot and leaf material of the halophyte Suaeda maritima hasbeen followed and the data analysed according to the phenomenologyof efflux, or compartmental, analysis. Sodium ions were exchangedmuch more slowly (c. 4 times) from the vacuoles of leaf cellsof plants grown in sodium chloride than were potassium ionsfrom the vacuoles of leaf cells of plants grown either in similarconcentrations of potassium chloride or in low concentrationsof potassium. In plants grown in sodium chloride, sodium ionswere exchanged 9 times more slowly from the vacuoles of leafcells than from the vacuoles of root cells. The concentration of sodium ions in the cytoplasm of leaf cellsof plants growing in 340 mol m^–3 sodium chloride was estimatedto be 165 mol m^–3 when the average concentration in theleaf tissue was about 600 mol m^–3. As measured by movement from mature to developing leaves inintact plants; there was less in vivo retranslocation of ²²Naand ³⁶CI in plants growing in sodium chloride than there wasof ⁸⁶Rb in plants growing either in potassium chloride or innon-saline conditions. The results are discussed in terms of the concept and energeticsof compartmentation of ions in the cells of halophytes.     

Stereological Analysis of Leaf Cells of the Halophyte Suaeda maritima (L.) Dum

A stereological analysis of cells from the mesophyll, vascularbundles and central parenchyma in leaves of the halophyte Suaedamaritima (L.) Dum is described. The volume fraction of vacuolewas greater in plants grown under saline conditions when comparedwith those under non-saline conditions, and there was a concurrentincrease in the surface density of the tonoplast. The volumefraction of chloroplasts and cell wall fell under saline conditions,while that of the intercellular spaces increased. Salinizationof the growth medium was accompanied by a large increase inthe fraction of the cell volume occupied by the mitochondria:the increase in surface density of mitochondrial membranes wassome 72% when averaged over all the cell types analysed. Key words: Salt tolerance, Halophyte, Suaeda maritima, Stereology     

High phenotypic plasticity of Suaeda maritima observed under hypoxic conditions in relation to its physiological basis

Background and Aims

Phenotypic plasticity, the potential of specific traits of a genotype to respond to different environmental conditions, is an important adaptive mechanism for minimizing potentially adverse effects of environmental fluctuations in space and time. Suaeda maritima shows morphologically different forms on high and low areas of the same salt marsh. Our aims were to examine whether these phenotypic differences occurred as a result of plastic responses to the environment. Soil redox state, indicative of oxygen supply, was examined as a factor causing the observed morphological and physiological differences.

Methods

Reciprocal transplantation of seedlings was carried out between high and low marsh sites on a salt marsh and in simulated tidal-flow tanks in a glasshouse. Plants from the same seed source were grown in aerated or hypoxic solution, and roots were assayed for lactate dehydrogenase (LDH) and alcohol dehydrogenase, and changes in their proteome.

Key Results

Transplanted (away) seedlings and those that remained in their home position developed the morphology characteristic of the home or away site. Shoot Na⁺, Cl⁻ and K⁺ concentrations were significantly different in plants in the high and low marsh sites, but with no significant difference between home and away plants at each site. High LDH activity in roots of plants grown in aeration and in hypoxia indicated pre-adaptation to fluctuating root aeration and could be a factor in the phenotypic plasticity and growth of S. maritima over the full tidal range of the salt marsh environment. Twenty-six proteins were upregulated under hypoxic conditions.

Conclusions

Plasticity of morphological traits for growth form at extremes of the soil oxygenation spectrum of the tidal salt marsh did not correlate with the lack of physiological plasticity in the constitutively high LDH found in the roots.     

Effect of NaCl salinity in vivo and in vitro on ribonuclease activity in the halophyte Suaeda maritima

Ribonuclease (EC 2.7.7.17) activity in the obligate halophyte Suaeda maritima (L.) Dum. var. macrocarpa Moq. was studied in relation to salinity (increasing concentrations of NaCl) of incubation and growth media. In vitro, the addition of 50 to 400 m M NaCl did not affect ribonuclease activity. This result, which was also found for Phaseolus vulgaris , indicates that the hydrolase is insensitive to high saline concentrations. The subcellular distribution of RNase activity did not change significantly with the salinity of the medium or with the age of the plant. The microsomal ribonuclease activity expressed on a fresh weight basis represented in every case less than 6% of the total activity. After 23 days of culture, the absence of salt stimulated the activity of soluble ribonuclease in aerial parts of Suaeda ; inversely, the capacity of the enzyme was lower under optimal saline conditions (130 m M NaCl). This was also evidenced by transfer of whole plants from a non-saline to a saline medium. Such a saline shock caused a decrease followed by a stabilization of the capacity of ribonuclease from Suaeda . The influx of NaCl in the tissues lowered the activity of the hydrolase.     

La glutamine synthètase du Suaeda maritima. Action in vivo et in vitro du NaCl

The glutamine synthetase of Suaeda maritima. In vivo and in vitro action of NaCl Glutamine synthetase (GS; EC 6.3.1.2) was isolated and characterized from roots and aerial parts of the halophyte Suaeda maritima (L.) Dum. var. macrocarpa Moq. Km values of GS were identical in both types of organ and unchanged by the salinity in the medium. Addition of NaCl in the culturing solution increased the specific activity of the enzyme especially in the aerial parts, where GS is more abundant. This increase was all the more pronounced if the plant-salt contact period was extended (between 21 and 45 days). In vitro the addition of 0 to 500 m M of salt did not affect the activity of GS at satured substrate concentrations. At low glutamate concentrations in combination with 300 m M NaCl or more, a slight competitive inhibition was observed, never over 18%. – The remarkable insensibility of GS to salinity in vitro and the stimulating effect of NaCl in vivo on the synthesis of the leaf enzyme indicates that GS plays a fundamental part in the assimilation of NH4⁺ in the halophyte Suaeda maritima var. macrocarpa.     

Salt Tolerance in the Halophyte Suaeda maritima (L.) Dum.: Interaction between Aluminium and Salinity

Growth of Suaeda maritima was stimulated by low aluminium concentrationsin saline solution culture with an increase in the number andextent of lateral roots. Under non-saline conditions the sameAl concentrations inhibited growth and led to an abnormal lateralroot initiation. Increasing the level of Al led to growth inhibitionunder both culture conditions. Salinity reduced the uptake of A1 into plant tissue, and therewas no evidence that A1 was tolerated internally. Although short-term³²P influx was increased by A1 there were no long-term effectsof significance on levels of Na, K, Ca nor P in the shoots. The results are discussed in relation to proposed mechanismsof A1 toxicity and the interaction between A1 and salt toxicities.An explanation is proposed for both stimulatory and inhibitoryeffects of A1 as a quantitative expression of a single primaryeffect upon the root system.     

The role of spatio‐temporal heterogeneity in the establishment and maintenance of Suaeda maritima in salt marshes

Abstract. The effects of disturbance and microtopography on the organization and dynamics of plant communities were studied in a European salt marsh located in the Bay of Mont St. Michel, France. The existence of seed trapping mechanisms was also tested. The study took place in the lower and middle marsh plant communities dominated by the perennials Puccinellia maritima and Halimione portulacoides, respectively and associated with the annual Suaeda maritima. Three treatments were used in series of plots placed in each community: (1) vegetation removal and root destruction to a depth of 10 cm and refilling, (2) non‐remnant herbicide treatment without vegetation removal and (3) creation of depressions (20 cm deep). These treatments were compared with adjacent control plots. The first year of the experiment showed that the perennials facilitated the establishment of Suaeda by trapping its seeds. Estimation of cover, density and biomass over 5 yr following the disturbances showed that in the first 2 yr Suaeda dominated the disturbed plots. Thereafter Suaeda was gradually eliminated by competitive exclusion after ca. 3 yr in the zone originally dominated by Puccinellia maritima and after 4 yr in the zone occupied by Halimione portulacoides. Depressions constituted refuge habitats for Suaeda by limiting competition with the perennials but also led to a high risk of mortality with temporal fluctuations in density. Despite a period of investigation limited to 5 yr, our study demonstrated that natural disturbances of various types occurred and influenced the dynamics of Suaeda, Halimione and Puccinellia. We deduced that natural disturbances and microtopography are responsible for the maintenance of the habitat in a state of non‐equilibrium by favouring the establishment of both spatial and temporal environmental heterogeneity. These conditions appear to be particularly favourable for the maintenance of annual species such as Suaeda maritima.     

The effect of combined salinity and waterlogging on the halophyte Suaeda maritima: The role of antioxidants

Suaeda maritima is a halophytic plant and its habitat is salt marsh. In order to adapt to saline or waterlogged conditions, plants have evolved mechanisms that include antioxidant protection. However, the combined effect of salinity and waterlogging on antioxidants in S. maritima is unknown. The aim of this study was therefore to investigate the effect of saline-flooding on levels of glutathione and phenolic compounds (antioxidants) and the correlation between their concentration and activity in S. maritima shoots grown in their natural habitat and in a glasshouse.Shoots were collected from two different elevations (and so different degrees of flooding) of a salt marsh while other plants were grown in half strength seawater in the controlled conditions of a glasshouse for 8 weeks (drained and flooded). Shoot samples were used to measure dry weight, glutathione and its reduction state, malonyldialdehyde content (MDA), polyphenol content, superoxide anion and 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) scavenging activity in the shoots of S. maritima.Growth of S. maritima was greater in plants growing on the high marsh than at a lower elevation and in drained medium as opposed to waterlogged conditions in the glasshouse. Waterlogging caused an increase in glutathione and its reduction state. The glutathione half-cell redox potential (E_GSSG/2GSH) was more negative in plant shoots grown under waterlogged conditions than in plants grown under normal conditions. Higher DPPH and superoxide anion scavenging activity was associated with high antioxidant concentrations (glutathione and polyphenols).Conclusions. Under saline-flooded conditions in the field and in the glasshouse, plants produced a higher concentration of antioxidants than under drained conditions. These result indicated that antioxidant molecules play an important role in S. maritima plants under combined salinity and waterlogging stress.