Ability of multicellular salt glands in <Emphasis Type="Italic">Tamarix</Emphasis> species to secrete Na<Superscript>+</Superscript> and K<Superscript>+</Superscript> selectively

The present study aimed to determine the mechanism of cation-selective secretion by multicellular salt glands. Using a hydroponic culture system, the secretion and accumulation of Na⁺ and K⁺ in Tamarix ramosissima and T. laxa under different salt stresses (NaCl, KCl and NaCl+KCl) were studied. Additionally, the effects of salt gland inhibitors (orthovanadate, Ba²⁺, ouabain, tetraethylammonium (TEA) and verapamil) on Na⁺ and K⁺ secretion and accumulation were examined. Treatment with NaCl (at 0–200 mmol L⁻¹ levels) significantly increased Na⁺ secretion, whereas KCl treatment (at 0–200 mmol L⁻¹ levels) significantly increased K⁺ secretion. The ratio of secretion to accumulation of Na⁺ was higher than that of K⁺. The changes in Na⁺ and K⁺ secretion differed after adding different ions into the single-salt solutions. Addition of NaCl to the KCl solution (at 100 mmol L⁻¹ level, respectively) led to a significant decrease in K⁺ secretion rate, whereas addition of KCl to the NaCl solution (at 100 mmol L⁻¹ level, respectively) had little impact on the Na⁺ secretion rate. These results indicated that Na+ secretion in Tamarix was highly selective. In addition, Na⁺ secretion was significantly inhibited by orthovanadate, ouabain, TEA and verapamil, and K⁺ secretion was significantly inhibited by ouabain, TEA and verapamil. The different impacts of orthovanadate on Na⁺ and K⁺ secretion might be the primary cause for the different Na⁺ and K⁺ secretion abilities of multicellular salt glands in Tamarix.     

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     

Growth,photosynthesis, and ion distribution in hydroponically cultured <Emphasis Type="Italic">Populus alba</Emphasis> L. cuttings grown under various salinity concentrations

Growth, photosynthesis, and Na⁺, K⁺, and Ca²⁺ distributions were investigated in 2-year-old hydroponically cultured Populus alba L. cuttings exposed to salt stresses (0, 0.85, 8.5, 17, and 85 mM NaCl in experiment 1 and 0, 50, 100, 150, and 200 mM in experiment 2) for 4 weeks in 2/5 Hoagland solution. Salt did not markedly inhibit height growth and diameter increment in 150 and 100 mM NaCl, respectively. The 85 mM NaCl treatment increased the dry weights of roots and total dry weight of plants, while 150 mM NaCl significantly reduced the dry weights of leaves, stems, and total plant weight. The decline in the photosynthetic rate lagged 2 weeks behind that of stomatal conductance in the 50 and 100 mM salt solutions. Different ions exhibited different distributions in different parts of the plant. Most Na⁺ ions were excluded and/or compartmentalized in roots at low and moderate salt stress (≤50 mM). K⁺ content in leaves increased with the increase in the salt concentration in the growth solutions.     

Changes in water relations,photosynthetic activity and proline accumulation in one-year-old olive trees (<Emphasis Type="Italic">Olea europaea</Emphasis> L. cv. Chemlali) in response to NaCl salinity

The comparative responses of young olive trees (Olea europaea L. cv “Chemlali”) to different NaCl salinity levels were investigated over 11 months. One-year-old own rooted plants were grown in 10-L pots containing sand and perlite mixture (1:3 v/v). Trees were subjected to three irrigation treatments: CP (control plants that were irrigated with fresh water); SS1 (salt stressed plants irrigated with water containing 100 mM NaCl) and SS2 plants (salt stressed plants irrigated with water containing 200 mM NaCl). Shoot elongation rate, relative water content, leaf water potential and net carbon dioxide exchange rates decreased significantly with increased NaCl salinity level. Under stressed conditions, the increase of Na⁺ and Cl⁻ ions in both leaves and roots was accompanied with that of proline and soluble sugars. The above results show that the accumulation of proline and sugars under stressed conditions could play a role in salt tolerance. The absence of toxicity symptoms under both stress treatments and the superior photosynthetic activity recorded in SS1-treated plants suggest that cv Chemlali is better able to acclimatize to 100 mM NaCl than at 200 mM NaCl. Our findings indicate that saline water containing 100 mM NaCl, the most available water in arid region in Tunisia, can be recommended for the irrigation of cv Chemlali in the arid south of Tunisia.     

Effects of NaCl salinity on growth, morphology, photosynthesis and proline accumulation of Salvinia natans

The effects of NaCl salinity on growth, morphology and photosynthesis of Salvinia natans (L.) All. were investigated by growing plants in a growth chamber at NaCl concentrations of 0, 50, 100 and 150 mM. The relative growth rates were high (ca. 0.3 d⁻¹) at salinities up to 50 mM and decreased to less than 0.2 d⁻¹ at higher salinities, but plants produced smaller and thicker leaves and had shorter stems and roots, probably imposed by the osmotic stress and lowered turgor pressure restricting cell expansion. Na⁺ concentrations in the plant tissue only increased three-fold, but uptake of K⁺ was reduced, resulting in very high Na⁺/K⁺ ratios at high salinities, indicating that S. natans lacks mechanisms to maintain ionic homeostasis in the cells. The contents of proline in the plant tissue increased at high salinity, but concentrations were very low (<0.1 μmol g⁻¹ FW), indicating a limited capacity of S. natans to synthesize proline as a compatible compound. The potential photochemical efficiency of PSII (F_v/F_m) of S. natans remained unchanged at 50 mM NaCl but was reduced at higher salinities, and the photosynthetic capacity (ETR_max) was significantly reduced at 50 mM NaCl and higher. It is concluded that S. natans is a salt-sensitive species lacking physiological measures to cope with exposure to high NaCl salinity. At low salinities salts are taken up and accumulate in old leaves, and high growth rates are maintained because new leaves are produced at a higher rate than for plants not exposed to salt.     

Salt-Induced Hypodermal Transfer Cells in Roots of Prosopis farcta and Ion Distribution within Young Plants

Prosopis farcta was grown on hydroculture with additions of 0.5, 10, 50, and 100 mM NaCl and without salt treatment. In plants from a 0.5 mM NaCl treatment, Cl^? was taken up into stems and leaves, but Na⁺ was withheld from the shoot. At 10 mM NaCl, shoot K⁺ concentration was below that of the control; Na⁺ and Cl^? were taken up to stems and cotyledons in nearly equimolar amounts. However, in the leaves, Na⁺ concentrations were only half of those of Cl^?. With increasing salt stress, Na⁺ and Cl^? were transported to the shoot, but kept at relatively low levels in the roots. Na⁺/ K⁺ ratios in roots did not increase proportionally to those in the solution. At an external Na⁺/K⁺ of > 5 and a root Na⁺/K⁺ of >1 (10 mM NaCl treatment), K⁺ selectivity was induced which rose exponentially with increasing salt stress; and cell wall protuberances were discovered in the hypodermis at the zone of side root formation. These transfer cells were found neither in roots from the 0.5 mM NaCl treatment nor in the controls. Their possible role in the Na⁺/K⁺ selectivity of the roots of Prosopis farcta is discussed.     

Effects of caecal ligation and saline acclimation on plasma concentration and organ mass in male and female Pekin ducks,Anas platyrhynchos

Summary The intestinal caeca reabsorb urinary sodium chloride (NaCl) and water (Rice and Skadhauge 1982). Free water may be generated if the reabsorbed NaCl is secreted via salt gland secretion (Schmidt-Nielsen et al. 1958). Therefore ceacal ligation should (a) reduce hingut NaCl and water reabsorption, (b) enhance the increase in plasma osmolality during saline acclimation, and (c) affect drakes more than ducks. Twelve Pekin drakes and 13 Pekin ducks, Anas platyrhynchos, were caecally ligated or sham operated before acclimation to 450 mmol · 1 NaCl. Body mass, hematocrit, plasma osmolality, and inonic concentrations of plasma, cloacal fluid, and salt gland secretion were measured after each increase in drinking water salinity. Osmoregulatory organ masses were determined. Caecal ligation did not effect plasma osmolality or ion concentrations of plasma, cloacal fluid, or salt gland secretion, but reduced salt gland size in ducks. Drakes and ducks drinking fresh water had the same hematocrit, plasma osmolality, and plasma concentrations of Na⁺ and Cl^–. In both sexes exposure to 75 mmol · 1^-1 NaCl significantly decreased plasma [Na⁺] and doubled cloacal fluid [Na⁺]. Exposure to 450 mmol · 1^-1 NaCl decreased body mass and increased hematocrit, plasma [Na⁺], [Cl^–], and plasma osmolality (more in drakes than in ducks); cloacal fluid osmolality nearly doubled compared to freshwater-adapted ducks, due mainly to osmolytes other than Na⁺ and Cl^–. The [Cl^–] in salt gland secretion only slightly exceeded drinking water [Cl^–].Abbreviations AVT antiduretic hormone - CF cloacal fluid - ECFV extraoellular fluid volume - FW freshwater acclimated - Hct hematocrit - MDWE mean daily water flux - [Na ⁺]_cf cloacal fluid sodium concentration - [Na ⁺]_pl plasma sodium concentration - Osm _cf cloacal fluid osmolality - Osm _pl plasma osmolality - SGS salt gland secretion - TBW total body water     

Osmotic adjustment,water relations and growth attributes of the xero-halophyte <Emphasis Type="Italic">Reaumuria vermiculata</Emphasis> L. (Tamaricaceae) in response to salt stress

Reaumuria vermiculata (L.), a perennial dwarf shrub in the family of Tamaricaceae, is a salt-secreting xero-halophyte found widely in arid areas of Tunisia. In the present study, physiological attributes of R. vermiculata were investigated under salt stress. Four-month-old plants were subjected to various salinity levels (0, 100, 200, 300, 400 or 600 mM NaCl) for 30 days under greenhouse conditions. Results showed that plants grew optimally when treated with standard nutrient solution without NaCl supply. However, increasing osmolality of nutrient solutions caused a significant reduction in biomass production and relative growth rate. This reduction was more pronounced in roots than in shoots. In addition, this species was able to maintain its shoot water content at 30% of the control even when subjected to the highest salt level, whereas root water content seemed to be unaffected by salt. Shoot water potential declined significantly as osmotic potential of watering solutions was lowered and the more negative values were reached at 600 mM NaCl (−3.4 MPa). Concentrations of Na⁺ and Cl⁻ in the shoots of R. vermiculata were markedly increased with increasing osmolality of nutrient solutions, whereas concentration of K⁺ was not affected by NaCl supply. Salt excretion is an efficient mechanism of Na⁺ exclusion from the shoots of this species exhibiting high K⁺/Na⁺ selectivity ratio over a wide range of NaCl salinity. Proline accumulation in shoots was significantly increased with increase in salt level and may play a role in osmoregulation.     

Ethylene and nitric oxide are involved in maintaining ion homeostasis in <Emphasis Type="Italic">Arabidopsis</Emphasis> callus under salt stress

In the present study, the role of ethylene in nitric oxide (NO)-mediated protection by modulating ion homeostasis in Arabidopsis callus under salt stress was investigated. Results showed that the ethylene-insensitive mutant etr1-3 was more sensitive to salt stress than the wild type (WT). Under 100 mM NaCl, etr1-3 callus displayed a greater electrolyte leakage and Na⁺/K⁺ ratio but a lower plasma membrane (PM) H⁺-ATPase activity compared to WT callus. Application of exogenous 1-aminocyclopropane-1-carboxylic acid (ACC, an ethylene precursor) or sodium nitroprusside (SNP, a NO donor) alleviated NaCl-induced injury by maintaining a lower Na⁺/K⁺ ratio and an increased PM H⁺-ATPase activity in WT callus but not in etr1-3 callus. The SNP actions in NaCl stress were attenuated by a specific NO scavenger or an ethylene biosynthesis inhibitor in WT callus. Under 100 mM NaCl, the NO accumulation and ethylene emission appeared at early time, and NO production greatly stimulated ethylene emission in WT callus. In addition, ethylene induced the expression of PM H⁺-ATPase genes under salt stress. The recovery experiment showed that NaCl-induced injury was reversible, as signaled by the similar recovery of Na⁺/K⁺ ratio and PM H⁺-ATPase activity in WT callus. Taken together, the results indicate that ethylene and NO cooperate in stimulating PM H⁺-ATPase activity to modulate ion homeostasis for salt tolerance, and ethylene may be a part of the downstream signal molecular in NO action.     

Effects of salinity and nitric oxide donor sodium nitroprusside (SNP) on development and salt secretion of salt glands of Limonium bicolor

NO, as a signaling molecule, is involved in abiotic stresses. Limonium bicolor seedlings were treated with 200 mM NaCl combined with 0.05 mM SNP for 20 days to study the effects of NO on development and salt-secretion rates of salt glands. It was shown that the total number of salt glands on adaxial surfaces under condition of 200 mM NaCl containing 0.05 mM SNP treatment increased significantly compared with that under 200 mM NaCl treatment. Na⁺ secretion rate per leaf under 200 mM NaCl containing 0.05 mM SNP was significantly higher than that under 200 mM NaCl without SNP. However, there was no significant difference in salt-secretion rate of individual salt glands between 200 mM NaCl containing 0.05 mM SNP treatment and 200 mM NaCl treatment. Although there was no significant difference in salt-secretion rate of individual glands, Na⁺ concentration in the leaves treated with 200 mM NaCl solution containing SNP was significantly lower than that treated with 200 mM NaCl solution. Treatment with 200 mM NaCl solution containing SNP caused a remarkable increase in Na⁺ concentration in salt glands. Obviously, the efficiency of the secretion process per gland was enhanced by adding SNP to NaCl. The results showed NO may enhance the salt secretion by inducing more dermatogen cells to develop into salt glands and by enhancing the efficiency of the secretion process per gland.     

Contribution of <Emphasis Type="Italic">Glomus intraradices</Emphasis> inoculation to nutrient acquisition and mitigation of ionic imbalance in NaCl-stressed <Emphasis Type="Italic">Trigonella foenum-graecum</Emphasis>

The study aimed to investigate the effects of an AM fungus (Glomus intraradices Schenck and Smith) on mineral acquisition in fenugreek (Trigonella foenum-graecum) plants under different levels of salinity. Mycorrhizal (M) and non-mycorrhizal (NM) fenugreek plants were subjected to four levels of NaCl salinity (0, 50, 100, and 200 mM NaCl). Plant tissues were analyzed for different mineral nutrients. Leaf senescence (chlorophyll concentration and membrane permeability) and lipid peroxidation were also assessed. Under salt stress, M plants showed better growth, lower leaf senescence, and decreased lipid peroxidation as compared to NM plants. Salt stress adversely affected root nodulation and uptake of NPK. This effect was attenuated in mycorrhizal plants. Presence of the AM fungus prevented excess uptake of Na⁺ with increase in NaCl in the soil. It also imparted a regulatory effect on the translocation of Na⁺ ions to shoots thereby maintaining lower Na⁺ shoot:root ratios as compared to NM plants. Mycorrhizal colonization helped the host plant to overcome Na⁺-induced Ca²⁺ and K⁺ deficiencies. M plants maintained favorable K⁺:Na⁺, Ca²⁺:Na⁺, and Ca²⁺:Mg²⁺ ratios in their tissues. Concentrations of Cu, Fe, and Zn²⁺ decreased with increase in intensity of salinity stress. However, at each NaCl level, M plants had higher concentration of Cu, Fe, Mn²⁺, and Zn²⁺ as compared to NM plants. M plants showed reduced electrolyte leakage in leaves as compared to NM plants. The study suggests that AM fungi contribute to alleviation of salt stress by mitigation of NaCl-induced ionic imbalance thus maintaining a favorable nutrient profile and integrity of the plasma membrane.     

Effects of salt stress on the growth,ion content,stomatal behaviour and photosynthetic capacity of a salt-sensitive species,Phaseolus vulgaris L.

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0–150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl^- concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K⁺ concentration, although to a lesser degree. Increases in leaf Na⁺ concentration occurred only at the higher external NaCl concentrations (100 mM). Increases in leaf Cl^- were primarily balanced by increases in K⁺ and Na⁺. X-ray microanalysis of leaf cells from salinized plants showed that Cl^- concentration was high in both the cell vacuole and chloroplast-cytoplasm (250–300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO₂ concentration (C _i) was reduced by up to 30%. Salinization of plants was found to alter the ¹³C value of leaves of Phaseolus by up to 5 and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO₂ concentration. Salt stress also brought about a reduction in photosynthetic CO₂ fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C _i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl^- concentrations. The quantum yield for net CO₂ uptake was also reduced by salt stress.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - RuBP ribulose-1,5-bisphosphate - ¹³C ratio of ¹³C to ¹²C relative to standard limestone     

Influence of ionic interactions on essential oil and phenolic diterpene composition of Dalmatian sage (Salvia officinalis L.)

The potential of four essential cations (K⁺, Ca²⁺, Mg²⁺ and Fe²⁺) to alleviate salt toxicity was studied in sage (Salvia officinalis L.) plants grown in pots. Two concentrations of the following chloride salts: KCl, CaCl₂, MgCl₂ and FeCl_3, were used together with 100 mM NaCl to study the effects of these nutrients on plant growth, leaf essential oils (EOs) and phenolic diterpenes composition. The sage plants accumulated Na⁺ in their leaves (includers); this has affected secondary metabolites’ biosynthesis. Treatment with 100 mM NaCl slightly decreased borneol and viridiflorol, while increased manool concentrations. Addition of KCl, CaCl₂ and MgCl₂ increased considerably in a dose-dependent manner the oxygen-containing monoterpenes (1.8-cineole, camphor, β-thujone and borneol) in 100 mM NaCl-treated sage. Whereas, the contents of viridiflorol decreased further with the addition of KCl in 100 mM NaCl-treated sage. Our results suggest that the changes in EOs composition were more related to K⁺ and Ca²⁺ availability than to Na⁺ toxicity. Furthermore, treatment with NaCl decreased by 50% carnosic acid (CA), a potent antioxidant, content in the leaves. K⁺ and Ca²⁺ promoted the accumulation of CA and its methoxylated form (MCA) in the leaves. The concentration of CA was positively correlated with leaf K⁺ (r = 0.56, P = 0.01) and Ca²⁺ (r = 0.44, P = 0.05) contents. It appears that different salt applications in combination with NaCl treatments had a profound effect on EOs and phenolic diterpene composition in sage. Therefore, ionic interactions may be carefully considered in the cultivation of this species to get the desired concentrations of these secondary metabolites in leaf extracts.     

Regeneration of plantlets of guava (<Emphasis Type="Italic">Psidium guajava</Emphasis> L.) from somatic embryos developed under salt-stress condition

The present study demonstrates the regeneration of plantlets of guava (Psidium guajava L.) from somatic embryos developed under salt-stress conditions. With increasing concentrations of NaCl in induction medium (MS + 4.52 μM 2,4-d + 5% sucrose) from 0 to 200 mM, the number of somatic embryos per responsive explant decreased. Somatic embryos induced on 0–100 mM NaCl containing medium developed into torpedo stages, whereas, the development of somatic embryos that differentiated on 150 and 200 mM NaCl-supplemented medium was arrested prior to torpedo stage and did not undergo maturation phase. Somatic embryos that developed on NaCl-containing medium, showed better germination in the presence of NaCl as compared with those developed on medium without NaCl. The effect of increasing salt-stress was also investigated on plant growth, chlorophyll and carotenoids, Na⁺ and K⁺, and proline and glycine betaine accumulation in in vitro grown plantlets. The level of Na⁺ in leaves increased with increasing concentrations of NaCl in the medium. Accumulation of free proline and glycine betaine in leaves significantly increased with increasing salinity. The results suggest that accumulation of proline and glycine betaine may be important for osmotic adjustment in guava under salinity stress.     

Salt tolerance of the annual halophyte <Emphasis Type="Italic">Cakile maritima</Emphasis> as affected by the provenance and the developmental stage

Though halophytes are naturally adapted to salinity, their salt-tolerance limits are greatly influenced by their provenance and developmental stage. In the present study, physio-biochemical responses of two Tunisian ecotypes of the oilseed coastal halophyte Cakile maritima (Brassicaceae) to salinity (0–400 mM NaCl) were monitored during germination and vegetative growth stages. Tabarka and Jerba seeds were collected from humid or arid climatic areas, respectively. Plant response to salinity appeared to depend on the ecotype and salinity levels. Increasing salinity inhibited germination process. Jerba seeds were found to be more salt tolerant than the Tabarka ones. At the autotrophic stage of growth and under salt-free conditions, Jerba was less productive than Tabarka (in terms of dry matter accumulation), but plant biomass production and leaf expansion (area and number) of the former ecotype were progressively improved by 100 mM NaCl, as compared to the control. In contrast, at the same salt concentration, these parameters decreased under increasing salinity in Tabarka (salt sensitive). Leaf chlorophyll content was reduced at severe salinity, but this effect was more conspicuous in the sensitive Tabarka plants. Na⁺ contents in the Jerba and Tabarka leaves collected from the 400 mM NaCl-treated plants were 17- and 12-fold higher than in the respective controls. This effect was accompanied by a significant reduction in the leaf K⁺, Mg²⁺ and Ca²⁺ contents, especially in the salt-treated Tabarka. A significant accumulation of proline and soluble carbohydrates in leaves was found during the period of intensive leaf growth. These organic compounds likely play a role in leaf osmotic adjustment and in protection of membrane stability at severe salinity.     

Differences in efficient metabolite management and nutrient metabolic regulation between wild and cultivated barley grown at high salinity

Physiological and biochemical responses of Hordeum maritimum and H. vulgare to salt stress were studied over a 60‐h period. Growth at increasing salinity levels (0, 100, 200 and 300 mM NaCl) was assessed in hydroponic culture. H. maritimum was shown to be a true halophyte via its typical behaviour at high salinity. Shoot growth of cultivated barley was gradually reduced with increasing salinity, whereas that of wild barley was enhanced at 100 and 200 mm NaCl then slightly reduced at 300 mM NaCl. The higher salt tolerance of H. maritimum as compared to H. vulgare was due to its higher capacity to maintain cell turgor under severe salinity. Furthermore, H. maritimum exhibited fine regulation of Na⁺ transport from roots to shoots and, unlike H. vulgare, it accumulated less Na⁺ in shoots than in roots. In addition, H. maritimum can accumulate more Na⁺ than K⁺ in both roots and shoots without the appearance of toxicity symptoms, indicating that Na⁺ was well compartmentalized within cells and substituted K⁺ in osmotic adjustment. The higher degree of salt tolerance of H. maritimum is further demonstrated by its economic strategy: at moderate salt treatment (100 mm NaCl), it used inorganic solutes (such as Na⁺) for osmotic adjustment and kept organic solutes and a large part of the K⁺ for metabolic activities. Indeed, K⁺ use efficiency in H. maritimum was about twofold that in H. vulgare; the former started to use organic solutes as osmotica only at high salinity (200 and 300 mm NaCl). These results suggest that the differences in salt tolerance between H. maritimum and H. vulgare are partly due to (i) differences in control of Na⁺ transport from roots to shoots, and (ii) H. maritimum uses Na⁺ as an osmoticum instead of K⁺ and organic solutes. These factors are differently reflected in growth.     

Salt tolerance inNitellopsis obtusa

Summary The mechanism of salt tolerance was studied using isolated internodal cells of the charophyteNitellopsis obtusa grown in fresh water. When 100 mM NaCl was added to artificial pond water (0.1 mM each of NaCl, KC1, CaCl₂), no cell survived for more than one day. Within the first 30 minutes, membrane potential (E_m) depolarized and membrane resistance (R_m) decreased markedly. Simultaneously, cytoplasmic Na⁺ increased and K⁺ decreased greatly. At steady state the increase in Na⁺ content was roughly equal to the decrease in K⁺ content. The Cl^– content of the cytoplasm did not change. These results suggest that Na⁺ enters the cytoplasm by exchange with cytoplasmic K⁺. Both the entry of Na⁺ and the exit of K⁺ are assumed to be passive and the latter being caused by membrane depolarization. Vacuolar K⁺, Na⁺, and Cl^– remained virtually constant, suggesting that rapid influx of Na⁺ from the cytoplasm did not occur.In 100 mM NaCl containing 10 mM CaCl₂, membrane depolarization, membrane resistance decrease and changes in cytoplasmic [Na⁺] and [K⁺] did not occur, and cells survived for many days. When cells treated with 100 mM NaCl were transferred within 1 hour to 100 mM NaCl containing 10 mM CaCl₂, Em decreased, Rm increased, cytoplasmic Na⁺ and K⁺ returned to their initial levels, and cells survived. Two possible mechanisms for the role of Ca²⁺ in salt tolerance inNitellopsis are discussed; one a reduction in plasmalemma permeability to Na⁺ and the other a stimulation of active Na⁺-extrusion.     

Alleviation of phosphorus deficiency stress by moderate salinity in the halophyte <Emphasis Type="Italic">Hordeum maritimum</Emphasis> L.

Hordeum maritimum (Poacea) is a facultative halophyte potentially useful for forage production in saline zones. Here, we assessed whether moderate NaCl-salinity can modify the plant response to phosphorus (P) shortage. Plants were cultivated for 55 days under low or sufficient P supply (5 or 60 μmol plant⁻¹ week⁻¹ KH₂PO₄, respectively), with or without 100 mM NaCl. When individually applied, salinity and P deficiency significantly restricted whole-plant growth, with a more marked effect of the latter stress. Plants subjected to P deficiency showed a significant increase in root growth (as length and dry weight) and root/shoot DW ratio. Enhanced root growth and elongation presumably correspond to the well-known root adaptive response to mineral deficiency. However, leaf relative water content, leaf P concentration, and leaf gas exchange parameters were significantly restricted. The interactive effects of salinity and P deficiency were not added one to another neither on whole plant biomass nor on plant nutrient uptake. Indeed, 100 mM NaCl-addition to P-deficient plants significantly restored the plant growth and improved CO₂ assimilation rate, root growth, K⁺/Na⁺ ratio and leaf proline and soluble sugar concentrations. It also significantly enhanced leaf total antioxidant capacity and leaf anthocyanin concentration. This was associated with significantly lower leaf osmotic potential, leaf Na⁺ and malondialdehyde (MDA) concentration. Taken together, these results suggest that mild salinity may mitigate the adverse effects of phosphorus deficiency on H. maritimum by notably improving the plant photosynthetic activity, the osmotic adjustment capacity, the selective absorption of K⁺ over Na⁺ and antioxidant defence.     

Effects of NaCl or Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> salinity on plant growth,ion content and photosynthetic activity in <Emphasis Type="Italic">Ocimum basilicum</Emphasis> L.

Basil (Ocimum basilicum L., cultivar Genovese) plants were grown in Hoagland solution with or without 50 mM NaCl or 25 mM Na₂SO₄. After 15 days of treatment, Na₂SO₄ slowed growth of plants as indicated by root, stem and leaf dry weight, root length, shoot height and leaf area, and the effects were major of those induced by NaCl. Photosynthetic response was decreased more by chloride salinity than by sulphate. No effects in both treatments on leaf chlorophyll content, maximal efficiency of PSII photochemistry (F _v/F _m) and electron transport rate (ETR) were recorded. Therefore, an excess of energy following the limitation to CO₂ photoassimilation and a down regulation of PSII photochemistry was monitored under NaCl, which displays mechanisms that play a role in avoiding PSII photodamage able to dissipate this excess energy. Ionic composition (Na⁺, K⁺, Ca²⁺, and Mg²⁺) was affected to the same extent under both types of salinity, thus together with an increase in leaves Cl⁻, and roots SO₄ ²⁻ in NaCl and Na₂SO₄-treated plants, respectively, may have resulted in the observed growth retardation (for Na₂SO₄ treatment) and photosynthesis activity inhibition (for NaCl treatment), suggesting that those effects seem to have been due to the anionic component of the salts.