Ecophysiological adaptations of coastal halophytes from foredunes and salt marshes

Ecophysiological strategies of coastal halophytes from foredunes and salt marshes are discussed. A comparison is made of the factors that limit growth in salt marshes and sand dunes. In salt marshes, zonation and succession are primarily governed by variation in soil salinity, which strongly depends on inundation with seawater. Results are described of experiments which aim at separating salinity and inundation effects on growth, osmotic and mineral relations in a comparison of salt-marsh halophytes. The growth response of plants cannot simply be correlated (and causally explained) with the concentration of Na, Cl, and K in the tissues. Also, the compatible osmotic solutes proline and methylated quaternary ammonium compounds may accumulate both in species with a positive response to increased salinity and in species with a growth reduction under seawater inundation. More likely inadequate adaptation of the plants water potential with these components is partly the cause of retarded growth. Disfunctioning of the plant in this respect may be at three levels: (a) total water potential of the plant, (b) (loss) of turgor pressure potential; (c) regulation at the cellular level. The ecological importance of some factors in seawater other than sodium chloride is considered. In coastal sand dunes airborne rather than soil salinity limits plant growth, together with the effects of abrasion, sand accretion, drought and the poor nutrient status of the dune sand. Adaptations of sand-dune species to these factors may consist of: large seeds with storage tissue germinating in the dark and seedling growth enough to emerge through the accreted sand. Aerial parts must be resistant to mechanical damage (high wind speed and abrasion), possibly by a sclerophyllous and tough structure. Efficient nutrient uptake, translocation and retranslocation seem to help survive sand-dune species in a nutrient-poor rooting medium.     

Water relations and osmotic adjustment in Lycopersicon esculentum and L. pennellii during short-term salt exposure and recovery

Cultivated tomato Lycopersicon esculentum (L.) Mill. cv. P-73 and its wild salt-tolerant relative L. pennellii (Correll) D'Arcy accession PE-47 growing on silica sand in a growth chamber were exposed to 0, 70, 140 and 210 m M NaCl nutrient solutions 35 days after sowing. The saline treatments were imposed for 4 days, after which the plants were rinsed with distilled water. Salinity in L. esculentum reduced leaf area and leaf and shoot dry weights. The reductions were more pronounced when sodium chloride was removed from the root medium. Reduction in leaf area and weight in L. pennellii was only observed after the recovery period. In both genotypes salinity induced a progressive reduction in leaf water potential and leaf conductance. During the recovery period leaf water potential (ψ₁) and leaf conductance (g₁) reached levels similar to those of control plants in wild and cultivated species, respectively. Leaf osmotic potential at full turgor (ψ_os) decreased in the salt treated plants of both genotypes, whereas the bulk modulus of elasticity was not affected by salinity. Leaf water potential at turgor loss point (ψ_tlp) and relative water content at turgor loss point (RWC_tlp) appeared to be controlled by leaf osmotic potential at full turgor (ψ_os) and by bulk modulus of elasticity, respectively. At lowest salinity, the wild species carried out the osmotic adjustment based almost exclusively on Cl⁻ and Na⁺, with a marked energy savings. Under highest salinity, this species accommodate the stress through a higher expenditure of energy due to the contribution of organic solutes to the osmotic adjustment. The domesticated species carried out the osmotic adjustment based always on an important contribution of organic solutes.     

Is zonation on coastal sand dunes determined primarily by sand burial or by salt spray? A test in New Zealand dunes

There has long been controversy on which environmental factor is the predominant determinant of community zonation on sand dunes. It is demonstrated here that, on a dune system in southern New Zealand, several environmental factors that could limit growth all vary along the sea-to-inland sand dune zonation: soil moisture, soil nutrients, wind exposure, sand burial, salt spray and soil salinity. Correlation of the responses of 30 species to experimental stress (burial, darkness, rooting-medium salinity and salt spray) with the zonation of the species in the field indicates that, in the four dune systems studied, sand burial and salt are both important, with salt generally being the more important. However, the relative importance of the factors differs between sites.     

A comparative study of the tolerance of salt marsh plants to manganese

Summary Salicornia europaea, Puccinellia maritima, Triglochin maritima, Aster tripolium, Plantago maritima, Armeria maritima, Juncus gerardii andFestuca rubra, collected as seed from a salt marsh at Portaferry, County Down, were grown on saline (340 mM NaCl) and non saline nutrient solutions at five concentrations of manganese sulphate (0.025–10.0 mM). After an eight week growing period, shoot and root yields and the concentrations of sodium, potassium, calcium and manganese in the shoots were determined.Except forS. europaea the saline treatments had a strongly limiting effect on plant growth. Each of the species investigated showed a degree of tolerance to high concentrations of manganese which was similar to that of calcifuge species and plants characteristic of waterlogged sand dune slack communities, but which was very much greater than that ofArrhenatherum elatius a species usually excluded from acidic soils. There was little evidence to support the hypothesis that tolerance of high manganese concentrations was correlated with the position of the experimental plants in the salt marsh ecotone or that the manganese nutrition of halophytic and glycophytic marsh species differs. Whilst manganese uptake increased proportionally with solution manganese concentration, there were few other major effects of manganese on the balance of shoot cation concentrations in the plants investigated. Both antagonistic and synergistic effects of sodium on manganese uptake were recorded for different species.     

Effects of salinity on growth,membrane permeability and root hydraulic conductivity in three saltbush species

The species of the genus Atriplex have been introduced in West Asia and North Africa to determine their adaptability for use as fodder species. These halophytes are well adapted to extreme environmental conditions and may possess interesting properties for soil rehabilitation. The effect of NaCl stress on growth, water relation and mineral nutrition were investigated in three xero-halophyte species of Atriplex used for rehabilitation of arid steppe in Algeria. Atriplex halimus, Atriplex canescens and Atriplex nummularia, were cultivated in hydroponic conditions and treated with increasing doses of NaCl (0–300 mM). All species showed positive plant growth for low and moderate levels of salinity. A. halimus had higher dry weight production than A. nummularia and A. canescens in high salinity concentration. Increasing concentration of salinity induced decrease in chlorophyll content (Chl a and b) and root hydraulic conductivity (L₀) in all species, especially in A. canescens. All three species showed marked increase in electrolyte leakage across the salinity gradient. In addition all species were able to accumulate a large quantity of sodium (Na), chloride (Cl) and proline and to maintain higher relative water content, which was probably associated with a greater capacity for osmotic adjustment, whereas potassium (K) and calcium (Ca) decreased with increase salinity. The data suggest that salt tolerance strategies in all Atriplex species could involve a delicate balance among ion accumulation, osmotic adjustment, production of osmotica and maintenance of relative water content and growth.     

REGULATION OF NaCl IN JAUMEA CARNOSA (ASTERACEAE), A SALT MARSH SPECIES,AND ITS EFFECT ON LEAF SUCCULENCE

A salt marsh species, Jaumea carnosa, was used in hydroponic experiments to test the effects of increasing NaCl concentrations on leaf succulence and plant accumulations of K, Ca, Mg, Na and Cl. A nested experimental design was used with four salinity levels. Plants were grown in full Hoagland's solution plus different amounts of NaCl (0.0–1.2 osmoles). Leaf succulence was measured as percent water content as well as vertical elongation of mesophyll cells. There were no corresponding increases in leaf succulence with increasing concentrations of NaCl in the root zone. Plants receiving aerosol spray (40 mg/dm²/day) did not show significant increases in leaf succulence. Leaf succulence was significantly increased when the plants were removed from the NaCl solutions and placed in non-salinized Hoagland's solution. Osmotic concentrations of cell sap in leaf tissues showed significant increases as NaCl concentrations increased in the root zone. The concentrations of K, Ca and Mg were higher in plants grown without NaCl than in those grown with NaCl. The accumulations of K in the root tissues were always higher than those of the shoot tissues. Although there was a two-fold difference in NaCl concentrations at the highest levels, the concentrations of Na in the shoot tissues were relatively similar. The results of the Cl analyses of shoot tissues showed a similar pattern of regulation of uptake. This regulation of salt uptake may be important in preventing injury by limiting accumulations of salt in plant tissues when growing in soils of high osmotic potentials.     

Coastal plant and soil relationships along the southwestern coast of South Korea

We studied how plant species distribution was regulated by the relationships between vegetation and soil factors on the southwestern coast of South Korea. Vegetation was classified using two-way indicator species analysis (TWINSPAN), thereby producing four vegetation groups that were linked to three habitat types. Two ordination techniques —; detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) —; were applied to examine the relationships between vegetation and 12 edaphic factors, including soil pH, water and osmotic potentials, moisture content, electrical conductivity, Cl and Na⁺ contents, total Kjeldahl nitrogen, and contents of organic matter, sand, silt, and clay. Results were similar for both types of evaluations. According to DCA and CCA, the 23 communities tended to cluster into three types: salt swamp, salt marsh, and sand dune. The first two canonical axes accounted for 14.9% of the community-soil factor relationship among communities. As identified via CCA, the main gradients were soil-water relations and soil texture.     

Adaptations enhancing survival and establishment of seedlings on coastal dune systems

Studies on the survival and establishment of seedlings in coastal and lacustrine sand dune systems suggest that nutrient deficiency, lack of moisture, sand accretion, salt spray and predation are probably the most important limiting factors. Seedlings employ both avoidance and tolerance strategies to with-stand the different stresses. For example, seedling recruitment of dune species coincided with periods of high moisture availability and occurred in years with high well distributed rainfall. The seedlings exhibited rapid vertical elongation of roots, avoided moisture stress by leaf rolling, dense hair on leaves, fleshy cotyledonary leaves, and by growing in the shade of nurse plants. A certain proportion of seedlings of all species survived partial burial and showed stimulation of leaf growth, total leaf area, number of tillers and total dry matter. Seedlings of most species could not survive complete burial, however, some species did grow through the sand deposit by an increase in the number of nodes and length of internodes. Plants responded to salt spray by inhibition of seed germination, developing resistance, increase in leaf thickness and reduced uptake of Na and Cl ions. Several characters such as higher vigour of seedlings, larger seed size and seed polymorphism were also of adaptive significance to the species.     

The physiological ecology of strandline plants

Factors which affect the vegetation of strandlines on sandy shores were investigated with particular attention to the growth of four annual species Atriplex glabriuscula, Atriplex laciniata, Cakile maritima and Salsola kali which are widely distributed around the coasts of North-West Europe. The response of these species to increases in salinity and sand accretion are reported from laboratory experiments. Field observations were also made of photosynthesis and the availability of, and the response to, nitrogen. The results are used to discuss the adaptation of these plants to growth on strandlines in an attempt to explain their restriction to this habitat.     

盐胁迫下外源ABA对玉米幼苗耐盐性的影响

利用不同浓度外源ＡＢＡ 可以提高生长在盐渍条件下玉米（Ｚｅａ ｍ ａｙｓ Ｌ．）幼苗的耐盐能力,增加幼苗的干重。其原因是由于幼苗渗透势的降低和渗透调节能力的增大,地上部分拒盐能力的提高,摄取的大量Ｎａ＋ 积累在根部。同时发现外源ＡＢＡ 增大幼苗渗透势和渗透调节能力是幼苗有机渗透物质增加的结果。讨论了玉米地上部分拒盐的机理     

Effect of Exogenous ABA on the Salt Tolerance of Corn Seedlings Under Salt Stress

The exogenous ABA in different concentrations was used to spray Zea mays L. (cv. Luyu 11) seedlings under different salinity. The growth status, osomotic potential and osmotic regulatory activity, the contents of Na+ and K+, soluble sugars and proline were determined at different time after treatment. The results were shown as follows: The exogenous ABA gave an increase in salt resistance and dry weight of corn seedlings under different salinity, this increase was as a result of inhanced osmotic adjustment activity and exclusion of sodium ions from the shoot through the action of exogenous ABA, resulting an accumulation of sodium in the roots. Moreover, the decrease of osmotic potential and the increase of osmotic regulatory activity were for reasons of enhancement of organic osmotors, instead inorganic, e.g. Na+ and K+ osmotors. The mechanism of sodium exclusion from the shoot was also discussed.     

Directional and non‐directional vegetation changes in a temperate salt marsh in relation to biotic and abiotic factors

Abstract. The effects of reduction and cessation of sheep grazing on salt‐marsh vegetation were studied on a formerly intensively grazed salt marsh in northern Germany. Plant species cover was recorded in 45 permanent plots from 1992 to 2000. In 1995, physical and chemical soil parameters were analysed. Results of Redundancy Analysis (RDA) indicated that salinity and the depth of anoxic conditions below the surface were the most important soil factors related to the spatial vegetation pattern. Furthermore, plant species distribution was influenced by present and past grazing intensity, by soil grain size and nitrogen content. Vegetation changes over 9 yr were analysed by non‐linear regression. The cover of Aster tripolium, Atriplex portulacoides, and, to a lesser extent, Artemisia maritima and Elymus athericus increased due to reduced grazing pressure, whereas the cover of Salicornia europaea decreased. After a strong increase in the first years Aster decreased 2 to 6 yr after abandonment. In the mid salt‐marsh zone Puccinellia maritima was replaced by Festuca rubra. The cover of Puccinellia, Festuca, Suaeda maritima, Glaux maritima and Salicornia fluctuated strongly, probably due to differences in weather conditions and inundation frequency. Species richness per 4 m² generally increased while vegetation evenness decreased during the study period. Only in the high salt marsh abandoned for 9 yr did the number of species decrease slightly. Thus far, cessation of grazing did not lead to large‐scale dominance of single plant species.     

Physiological Comparisons among Four Related Bromus Species with Varying Ecological Amplitude: Polyamine and Aromatic Amine Composition in Response to Salt Spray and Drought

Abstract: We examined the amine composition in response to experimental osmotic or salt treatments of populations from four related Bromus species in relation to the environment of their origin. We studied populations from (1) a sand dune protected from salt spray, and (2) a sand dune subject to salt spray. The study of amine composition and contents, the first in Bromus, revealed a significant effect of origin for Spd and slightly for Spm, and higher levels were found in offspring of plants originating from an environment subject to sea spray. In all populations and species, plants subjected to osmotic and salt treatments showed significant decreases in Put, Spd, Spm and Dop levels. This was accompanied by significant increases in the oxidation product Dap and the amplitude of this response coincided with the presence of salt spray observed in the environment of origin of seeds, except for populations of B. rigidus. Interspecific differences in amine composition and contents were also detected. Higher levels of Tyr and Dop were found in B. sterilis and B. diandrus, respectively. For Dap and Tyr, B. rigidus, the species with the narrowest ecological amplitude in Brittany, showed the lowest response to either treatment. Our study of amine variations in Bromus populations and species illustrate the links between the amplitude of amine response to experimental salt or osmotic treatments and (1) the exposure of populations to sea spray; and (2) the ecological amplitude of Bromus species. Finally, experimental stress resulted in a strong decrease of interspecific variability in relation to amine metabolism in these four species.     

Fungal Assemblages Associated with Roots of Halophytic and Non-halophytic Plant Species Vary Differentially Along a Salinity Gradient

Structure of fungal communities is known to be influenced by host plants and environmental conditions. However, in most cases, the dynamics of these variation patterns are poorly understood. In this work, we compared richness, diversity, and composition between assemblages of endophytic and rhizospheric fungi associated to roots of two plants with different lifestyles: the halophyte Inula crithmoides and the non-halophyte I. viscosa (syn. Dittrichia viscosa L.), along a spatially short salinity gradient. Roots and rhizospheric soil from these plants were collected at three points between a salt marsh and a sand dune, and fungi were isolated and characterized by ITS rDNA sequencing. Isolates were classified in a total of 90 operational taxonomic units (OTUs), belonging to 17 fungal orders within Ascomycota and Basidiomycota. Species composition of endophytic and soil communities significantly differed across samples. Endophyte communities of I. crithmoides and I. viscosa were only similar in the intermediate zone between the salt marsh and the dune, and while the latter displayed a single, generalist association of endophytes, I. crithmoides harbored different assemblages along the gradient, adapted to the specific soil conditions. In the lower salt marsh, root assemblages were strongly dominated by a single dark septate sterile fungus, also prevalent in other neighboring salt marshes. Interestingly, although its occurrence was positively correlated to soil salinity, in vitro assays revealed a strong inhibition of its growth by salts. Our results suggest that host lifestyle and soil characteristics have a strong effect on endophytic fungi and that environmental stress may entail tight plant-fungus relationships for adaptation to unfavorable conditions.     

Foliar applied fullerol differentially improves salt tolerance in wheat through ion compartmentalization,osmotic adjustments and regulation of enzymatic antioxidants

Earlier we reported that seed pre-treatment with PHF promoted early seedling growth and salinity tolerance in wheat. As a way forward, experiments were conducted to investigate whether and to what extent foliar spray of fullerol could influence growth and physio-biochemical responses in salt stressed wheat. In a control experiment, seeds were sown in sand filled pots (500 g) under control and 150 mM NaCl stress. After 15 days, foliar spray of fullerol at 0, 10, 40, 80 and 120 nM concentration was applied and the data for various morpho-biochemical attributes recorded after 2 weeks. Fullerol caused improvements in shoot growth attributes while had least effect on root growth traits. Increase in total chlorophyll while reduction in Car/Chl ratio was evident under salinity in response to fullerol spray. Only 40 and 80 nM spray treatments improved antioxidant activities and reduced H2O2 contents while MDA contents which increased due to salt stress, remained unaffected by foliar spray. Fullerol spray also improved sugars, proline and free amino acids under salinity. During second experiment under natural conditions, 60 day old plants grown in sand filled pots (10 kg) under 0 and 150 mM NaCl were foliar sprayed with selected concentrations (0, 40 and 80 nM) of fullerol. Salinity inhibited gas exchange and grain yield attributes while fullerol-sprayed plants exhibited recovery. Fullerol spray resulted in high root and shoot K+ and shoot Ca2+ contents. Also, increase in shoot and root P, while lesser shoot Na+ was recorded due to 80 nM spray under salt stress. Overall, 40 and 80 nM fullerol spray improved photosynthetic activity, osmolytes accumulation and altered tissue ion compartmentalization which contributed to improvement in grain yield attributes under salinity.     

Does salt stress constrain spatial distribution of dune building grasses Ammophila arenaria and Elytrichia juncea on the beach?

Rising sea levels threaten coastal safety by increasing the risk of flooding. Coastal dunes provide a natural form of coastal protection. Understanding drivers that constrain early development of dunes is necessary to assess whether dune development may keep pace with sea‐level rise. In this study, we explored to what extent salt stress experienced by dune building plant species constrains their spatial distribution at the Dutch sandy coast. We conducted a field transplantation experiment and a glasshouse experiment with two dune building grasses Ammophila arenaria and Elytrigia juncea. In the field, we measured salinity and monitored growth of transplanted grasses in four vegetation zones: (I) nonvegetated beach, (II) E. juncea occurring, (III) both species co‐occurring, and (IV) A. arenaria dominant. In the glasshouse, we subjected the two species to six soil salinity treatments, with and without salt spray. We monitored biomass, photosynthesis, leaf sodium, and nutrient concentrations over a growing season. The vegetation zones were weakly associated with summer soil salinity; zone I and II were significantly more saline than zones III and IV. Ammophila arenaria performed equally (zone II) or better (zones III, IV) than E. juncea, suggesting soil salinity did not limit species performance. Both species showed severe winter mortality. In the glasshouse, A. arenaria biomass decreased linearly with soil salinity, presumably as a result of osmotic stress. Elytrigia juncea showed a nonlinear response to soil salinity with an optimum at 0.75% soil salinity. Our findings suggest that soil salinity stress either takes place in winter, or that development of vegetated dunes is less sensitive to soil salinity than hitherto expected.     

Effects of NaCl on water relations and cell wall elasticity and composition of red-osier dogwood (Cornus stolonifera) seedlings

Red-osier dogwood ( Cornus stolonifera Michx, Syn. Cornus sericea ), a species relatively well adapted to moderately saline conditions compared with other boreal species, was used to test the effects of NaCl on plant water relations, cell wall elasticity, and cell wall composition of seedlings. Three month-old seedlings were treated hydroponically with 0, 25, and 50 m m NaCl for 21 days. The osmotic potential at full turgor, osmotic potential at turgor loss, pressure potential at full turgor, and relative water content at turgor loss of red-osier dogwood shoot tissue were not significantly affected by the NaCl treatments. Cell wall elasticity of the shoot tissues did not change following NaCl treatments, suggesting that elastic adjustment did not play a role in the adaptation mechanism. Hemicellulose content of the cell wall increased in salt treated seedlings. The primary sugar found in the cell wall hemicellulose fraction was xylose. In the pectin fraction arabinose and galacturonic acid were the main sugars. Sodium chloride stress did not alter the sugar composition of the hemicellulose fraction; however, NaCl did increase the amount of rhamnose in the pectin fraction. The results of this study suggest that at moderate salinity red-osier dogwood does not make any osmotic or elastic adjustments in the shoot tissue, but some changes in the cell wall composition do occur. These changes could contribute to the decrease in growth recorded in red-osier dogwood during NaCl stress.     

Relationship between the photosynthetic activity and the performance of Cakile maritima after long-term salt treatment

Cakile maritima is a halophyte with potential for ecological, economical and medicinal uses. We address here the impact of salinity on its growth, photosynthesis and seed quality. Whole plant growth rate and shoot development were stimulated at moderate salinity (100–200 m M NaCl) and inhibited at higher salt concentrations. Although diminished in the presence of salt, potassium and calcium uptake per unit of root biomass was maintained at relatively high value, while nutrient-use efficiency (NUE) was improved in salt-treated plants. Chl and carotenoid concentrations decreased at extreme salinities, but anthocyanin concentration continuously grew with salinity. Net photosynthetic rate (A), stomatal conductance, maximum quantum efficiency of PSII and quantum yield were stimulated in the 100–200 m M NaCl range. Higher salinity adversely affected gas exchange and changed PSII functional characteristics, resulting in a reduction of A per leaf area unit. This phenomenon was associated with increased non-photochemical quenching. Harvest index, silique number and seeds per fruit valve were maximal at 100 m M NaCl. Despite the decreasing salt accumulation gradient from the vegetative to the reproductive organs, high salinities were detrimental for the seed viability and increased the proportion of empty siliques. Overall, the salt-induced changes in the plant photosynthetic activity resulted into analogous responses at the vegetative and reproductive stages. The enhancement of NUE, the absence of pigment degradation, the reduction of water loss and the concomitant PSII protection from photodamage through thermal dissipation of excess excitation significantly accounted for Cakile survival capacity at high salinity.     

Isoosmotic regulation of cotton and peanut at saline concentrations of k and na

Peanut (Arachis hypogaea L.) and cotton (Gossypium hirsutum) plants were grown for 4 weeks in saline, isoosmotic rooting substrates with different proportions of K and Na. Isoosmotic media did not affect growth (except at the highest external K concentrations) or estimates of intracellular osmotic pressure in expanding leaves (i.e. osmotic pressure of leaf sap and intracellular osmotic pressure as calculated from pressure-volume curves). In expanded leaves, an increase in the proportion of external K increased sap osmotic pressure. The sum of [K+Na+Cl] in the sap of expanding and expanded leaves accounted for the effect of isoosmotic media on the concentration of osmolytes with high electrical conductance, so the difference between sap osmotic pressure and [K+Na+Cl] accounted for the concetration of osmolytes with low conductance. In expanding leaves, an increase in the proportion of external K increased [K+Na+Cl] and decreased the concentration of osmolytes with low conductance. In expanded leaves, an increase in the proportion of external K increased [K+Na+Cl] to approximately the same extent as sap osmotic pressure. Isoosmotic regulation was apparent in expanding leaves but not evident in expanded leaves. This suggests a turgor homeostat which can influence the concentration of organic solutes in expanding leaves but cannot control the import of inorganic solutes from a rooting medium nor the total production of organic solutes in plants with a low sink:source ratio.     

Osmotic regulation in the marine alga, Codium decorticatum. I. Regulation of turgor pressure by control of ionic composition.

Codium decorticatum regulates its internal ionic composition and osmotic pressure in response to changes in external salinity. Over a salinity range of 23 to 37% (675 to 1120 mosmol/kg) Codium maintains a constant turgor pressure of 95 mosmol/kg (2.3 atm), observed as a constant difference between internal and external osmotic pressures. The changes in internal osmotic pressure are due to changes in intracellular inorganic ions. At 30 0/00 salinity the major intracellular ions are present in the following concentrations (mmol/kg cell H20): K+, 295; Na+, 255; Cl-, 450. At different salinities intracellular ion concentrations remain in constant proportion to the external ion concentrations, and thus the equilibrium potentials are approximately constant. The potential difference between the vacuole and seawater (-76 mV), whici is predominantly a K+ diffusion potential, is also constant with changing salinity. Comparison of the equilibrium potentials with the vacuole potential suggests that Cl- is actively absorbed and Na+ actively extruded, whereas K+ may be passively distributed between the vacuole and seawater. Turgor pressure does not change with environmental hydrostatic pressure, and increasing the external osmotic pressure with raffinose elicits a response similar to that obtained by increasing the salinity. These two results suggest that the stimulus for turgor regulation is a change in turgor pressure rather than a change in internal hydrostatic pressure or ion concentrations.