Salt tolerance in the halophyte Salicornia dolichostachya Moss: Growth,morphology and physiology

Salinization of agricultural land is an increasing problem. Because of their high tolerance to salinity, Salicornia spp. could become models to study salt tolerance; they also represent promising saline crops. The salinity-growth response curve for Salicornia dolichostachya Moss was evaluated at 12 salt concentrations in a hydroponic study in a greenhouse and at 5 different seawater dilutions in an outside setting. Salt concentrations ranged between 0 mM and 500 mM NaCl (≈seawater salinity). Plants were grown for six weeks and morphological and physiological adaptations in different tissues were evaluated.S. dolichostachya had its growth optimum at 300 mM NaCl in the root medium, independent of the basis on which growth was expressed. The relative growth rate (RGR) in the greenhouse experiment was comparable with RGR-values in the outdoor growth experiment. Leaf succulence and stem diameter had the highest values at the growth optimum (300 mM NaCl). Carbon isotope discrimination (δ¹³C) decreased upon salinity. S. dolichostachya maintained a lower leaf sap osmotic potential relative to the external solution over the entire salinity range, this was mainly accomplished by accumulation of Na⁺ and Cl⁻. Glycine betaine concentrations did not significantly differ between the treatments. Na⁺:K⁺-ratio and K⁺-selectivity in the shoots increased with increasing salinity, both showed variation between expanding and expanded shoot tissue. We conclude that S. dolichostachya was highly salt tolerant and showed salt requirement for optimal growth. Future growth experiments should be done under standardized conditions and more work at the tissue and cellular level needs to be done to identify the underlying mechanisms of salt tolerance.     

Morphological and physiological responses of Scaevola sericea (Goodeniaceae) seedlings to salt spray and substrate salinity

The effects of substrate salinity and salt spray upon seedlings of Scaevola sericea were examined in this study. Three levels of substrate salinity: 0.0 ppt, 3.0 ppt, and 10.0 ppt were examined in conjunction with three levels of salt spray: zero, medium (200 mg m^-2mdd^-1), and high (1200–1500 mg-m^-2mdd^-1). Leaf surface area, root to shoot ratio, as well as leaf, stem, and root mass decreased significantly (P 0.05) with increasing substrate salinity. Biomass accumulation was very low at 10.0 ppt substrate salinity, suggesting that higher levels of substrate salinity cannot be tolerated by the seedlings. Salt spray had a substantial effect on several of these variables, however its effects were less pronounced than those of substrate salinity. Cell sap osmolarity, leaf thickness, and leaf specific mass increased significantly (P 0.05) with both increasing substrate salinity and salt spray levels. Leaf carbon isotope ratios (δ^l3C) became more positive with increasing salinity, indicating an enhancement of the intrinsic water use efficiency of the seedlings at higher salinities. Scaevola sericea is one of the dominant plants found at the leading edge of strand communities in the Hawaiian archipelago and throughout much of the tropical Pacific. Since substrate salinity and salt spray increase with proximity to the ocean, the two factors may act together to limit the seaward expansion of S. sericea in coastal habitats.     

Hypersalinity effects on leaf ultrastructure and physiology in the mangrove Avicennia marina

The effects of hypersalinity on leaf ultrastructure and physiology in the mangrove, Avicennia marina, were investigated by comparing leaves of adult trees growing naturally in the field under seawater and hypersalinity conditions in Richards Bay, South Africa. We tested the hypothesis that hypersalinity has a deleterious effect on membranes and cellular organelles such as chloroplasts and mitochondria, which would impact negatively physiological processes, such as ion and water relations, and photosynthetic performance. Soil ψ and soil salinity were −2.96 ± 0.07 MPa and 35 ± 2.8 psu in the seawater salinity site, compared to −5.91 ± 0.42 MPa and 58 ± 3.6 psu respectively, in the hypersaline site. In the hypersaline site, leaves were smaller and thicker, with thicker cuticles, while chloroplasts, mitochondria and nuclei exhibited swelling and disintegration, compared to those at seawater salinity. Multivesicular structures and vesicles, observed in vacuoles, chloroplasts, mitochondria, and along cell walls and plasma membranes, were more abundant in leaves from the hypersaline than the seawater site, and were probably indicative of greater plant salt uptake in the former site. Leaf concentrations of total chlorophyll and chlorophylls a and b were lower in trees from the hypersaline site by 33%, 29%, and 45% respectively, compared to those at seawater salinity. Midday minimum xylem ψ was −3.82 ± 0.33 MPa in the seawater site and −6.47 ± 0.45 MPa in the hypersaline site. In the hypersaline site, the concentration of leaf Na⁺ was 40% higher, while those of K⁺, Ca²⁺, and Mg²⁺ were lower by 45%, 44%, and 54% respectively, than those in the seawater site. CO₂ exchange and the intrinsic photochemical efficiency of PS II were significantly lower in trees from the hypersaline site by 48 and 19% respectively. The ultrastructural evidence supported the physiological data that A. marina trees in the hypersaline site are under extreme salinity stress and that this species is growing there at the upper limit of its salt tolerance.     

Invasion of maritime chaparral by the introduced succulentCarpobrotus edulis

Invasion by the alien succulent,Carpobrotus edulis, has become a common occurrence after fire in maritime chaparral in coastal California, USA. We studied post-burnCarpobrotus establishment in chaparral that lackedCarpobrotus plants before the fire and compared seedbank and field populations in adjacent burned and unburned stands.Carpobrotus seeds were abundant in deer scat and in the soil before burning. Burning did not enhance germination: many seeds were apparently killed by fire and seed bank cores taken after fire revealed no germinable seeds. Laboratory tests showed that temperatures over 105°C for five minutes killedCarpobrotus seeds. In a field experiment involving use of herbivore exclosures, we found that herbivory was an important source of mortality for seedlings in both burned and unburned chaparral. All seedlings, however, died outside of the burn regardless of the presence of cages. Establishment there is apparently limited by factors affecting plant physiology. In the burned area, seedlings that escaped herbivory grew very rapidly. Overall, it appears that herbivory limited seedling establishment in both burned and unburned sites but that the post-burn soil environment supportedCarpobrotus growth in excess of herbivore use, thus promoting establishment.     

Growth and mineral nutrition of six rapid-cycling Brassica species in response to seawater salinity

The growth of six rapid-cycling lines of Brassica species, B. napus, B. campestris, B. nigra, B. juncea, B. oleracea and B. carinata was inhibited by seawater salinity. Based on the change in dry matter reduction relative to the control at varying concentrations of salts (4, 8 and 12 dS m^-1), the relative salt tolerance of these species was evaluated. B. napus and B. carinata were the most tolerant and most sensitive species, respectively, while the other four species were moderately tolerant. The influence of seawater on the concentrations of 12 elements including macronutrients and micronutrients in the shoots of these Brassica plants was characterized to determine the relationship between nutritional disturbance and relative salt tolerance. It was found that seawater salinity had a significant effect on the concentrations of Ca, Mg, K, Cl, Na and total N in the shoots of these plants but only the change in Ca concentration was significantly related to the relative salt tolerance of these six rapid-cycling Brassica species according to a rank analysis of the data. This finding indicates that Ca may play a regulatory role in the responses of Brassica species to saline conditions.     

Biodiversity and Structure of Macroinvertebrate Communities Along a Small Permanent Salinity Gradient (Meurthe River,France)

Changes in the macroinvertebrate community were investigated over 10 months at four sites along a 19 km salinity gradient (0.21–2.60 g l⁻¹) in a sixth-order stream, the Meurthe River, northeastern France. Abiotic characteristics other than salinity were similar between the sites. Macroinvertebrate taxonomic richness decreased by 30% downstream of the 1.4 g l⁻¹ sites while diversity, evenness or total abundance of taxa did not change along the gradient. In terms of functioning, a slight change in relative abundances of invertebrate feeding groups followed the salinity gradient. Eight invertebrate assemblages occurred within specific salinity distributions were identified. The exotics Gammarus tigrinus, Dreissena polymorpha, Corbicula fluminalis and Corophium curvispinum, were more abundant at the highest salinity site. These results suggest that rising salinity concentrations drastically affect the species composition, including favouring exotic species.     

Responses of the saltmarsh rush Juncus kraussii to salinity and waterlogging

Juncus kraussii Hochst., an important saltmarsh macrophyte, is intensively harvested for many commercially orientated products and current populations are under threat of overexploitation. In saline, intertidal mud banks, this species occurs on higher ground, suggesting that it is adapted to lower salinities and less frequent inundation. The objectives of this study were to determine biomass accumulation, as well as morphological and physiological adaptations of J. kraussii to salinity and waterlogging stresses. Plants collected from the field were subjected to 0.2, 10, 30, 50 and 70% seawater under drained or flooded conditions for three months. Measurements were made of biomass accumulation, CO₂ exchange, chlorophyll fluorescence, ion and water relations. Furthermore, seed germination responses to a range of salinities were investigated. Total dry biomass accumulation, as well as the number and height of culms, decreased with increase in salinity under both flooded and drained conditions. Generally, CO₂ exchange, stomatal conductance, Photosystem II (PSII) quantum yield and electron transport rate (ETR) through PSII declined with increase in salinity in both the flooded and drained treatments. Predawn and midday ψ in culms decreased with increase in salinity, being lower under drained than flooded conditions. Inorganic solute concentrations in culms increased with increase in salinity, with Na⁺ and Cl⁻ being the predominant ions. Na⁺/K⁺ ratios in culms increased significantly with increase in salinity. Proline concentrations in roots and culms, which increased with salinity, were considerably higher under drained than flooded conditions. Germination was best at salinities less than 20% seawater and decreased significantly with further increase in salinity to 110% seawater. Transfer of ungerminated salt-treated seeds to distilled water stimulated germination. This study has demonstrated that J. kraussii is a highly salt and flood tolerant species, being able to grow and survive in salinities up to 70% seawater, under both drained and flooded conditions. Maximal growth occurred at low salinities (<10% seawater) under flooded condition.     

Effect of oxygen availability and salinity on EARLY LIFE HISTORY STAGES OF SALT MARSH PLANTS. I. Different germination strategies of Spartina alterniflora and Phragmites australis (Poaceae)

Gradients in oxygen availability and salinity are among the most important environmental parameters influencing zonation in salt marsh communities. The combined effects of oxygen and salinity on the germination of two salt marsh grasses, Spartina alterniflora and Phragmites australis, were studied in growth chamber experiments. Germination of both species was initiated by emergence of the shoot and completed by root emergence. Percentage S. alterniflora germination was reduced at high salinity (40 g NaCl/L) and in decreased oxygen (5 and 2.5%). In 0% oxygen shoots emerged, but roots did not. P. australis germination was reduced at a lower salinity (25 g NaCl/L) than S. alterniflora, and inhibited at 40 g NaCl/L and in anoxia. However, a combination of hypoxia (10 and 5% O2) and moderate salinity (5 and 10 g NaCl/L) increased P. australis germination. When bare areas in the salt marsh are colonized, the different germination responses of these two species to combinations of oxygen and salt concentrations are important in establishing their initial zonation. In high salinity wetlands S. alterniflora populates the lower marsh and P. australis occupies the high marsh at the upland boundary.     

Photosynthetic activity and leaf antioxidative responses of Atriplex portulacoides subjected to extreme salinity

Responses of Atriplex portulacoides upon 40-day-long exposure to salinity (0?C1,000?mM NaCl) were investigated. Mother plants originated from a sabkha located in a semi-arid region of Tunisia. The plant relative growth rate and leaf expansion increased significantly at 200?mM NaCl but decreased at higher salinities. Interestingly, the plants survived salinity as high as 1,000?mM NaCl without displaying salt-induced toxicity symptoms. Despite significant increase in leaf Na⁺ and Cl^? concentrations upon salt treatment, no significant effect on leaf relative water content was registered. Chlorophyll contents and the gas exchange parameters showed a significant stimulation at the optimal salinity (200?mM NaCl) followed by a decline at higher salinities. Extreme salinity hardly impacted the maximal efficiency of photosystem II photochemistry (F _v/F _m), but a marked decrease in the relative quantum yield of photosystem II (??_PSII) was observed, along with a significant increase in non-photochemical quenching (NPQ). Leaf malondialdehyde and carotenoid contents were generally unaffected following salt exposure, whereas those of anthocyanins, polyphenols, and proline increased significantly, being maximal at 1,000?mM NaCl. Leaf superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), and glutathione reductase (EC 1.6.4.2) activities were significantly stimulated by salinity, whereas catalase (EC 1.11.1.6) activity was maximal in the 0?C400?mM NaCl range. As a whole, protecting the photosynthetic machinery from salt-induced photodamage together with the sustained antioxidant activity may account for the performance of A. portulacoides under high salinity.     

Effects of road salts on groundwater and surface water dynamics of sodium and chloride in an urban restored stream

Road salts are a growing environmental concern in urban watersheds. We examined groundwater (GW) and surface water (SW) dynamics of Na⁺ and Cl^? in Minebank Run (MBR), an urban stream in Maryland, USA. We observed an increasing salinity trend in this restored stream. Current baseflow salinity does not exceed water quality recommendations, but rapid “first flush” storm flow was approximately one-third that of seawater. Comparisons between the upstream and downstream study reaches suggest that a major interstate highway is the primary road salt source. A heavily used road parallels most of MBR and was an additional source to GW concentrations, especially the downstream right bank. A baseflow synoptic survey identified zones of increased salinity. Downstream piezometer wells exhibited increases in salt concentrations and there was evidence that Na⁺ is exchanging Ca²⁺ and Mg²⁺ on soils. SW salt concentrations were generally elevated above GW concentrations. Salinity levels persisted at MBR throughout the year and were above background levels at Bynum Run, a nearby reference stream not bisected by a major highway, suggesting that GW is a long-term reservoir for accumulating road salts. Chronic salinity levels may be high enough to damage vegetation and salinity peaks could impact other biota. Beneficial uses and green infrastructure investments may be at risk from salinity driven degradation. Therefore, road salt may represent an environmental risk that could affect aquatic biota and limit the effectiveness of costly resource management and restoration efforts.     

Saline groundwater as an aquaculture medium: physiological studies on the red drum, Sciaenops ocellatus

Physiological responses of the euryhaline red drum, Sciaenops ocellatus, to chloride salt addition, low salinity, and high sulfate concentration were measured. Survival was increased by addition of calcium chloride (CaCl₂) or magnesium chloride (MgCl₂) to dilute artificial seawater (0.2 ppt salinity). Although survival and routine metabolic rates were greater in MgCl₂ treatments, growth and feed efficiency were greater in CaCl₂ treatments. Marginal metabolic scope increased when CaCl₂ or MgCl₂ were added to dilute artificial seawater. There was a strong positive linear relationship (p=0.0001, r=0.91) between fish survival and salinity of artificial seawater dilutions over the salinity range 0.1 to 3.0 ppt. Monovalent ion concentrations in red drum plasma varied; whereas, divalent ion concentrations were relatively constant. Survival and growth were not affected by high sulfate concentrations (2000 mg l^-1) in 3.0 ppt artificial seawater supplemented with either sodium sulfate or magnesium sulfate. Routine metabolic rate and marginal metabolic scope of red drum exposed to high sulfate concentrations were slightly, but not significantly, lower than those of red drum in 3 ppt artificial seawater.     

Effect of salinity on seed germination and seedling growth of the halophyte Suaeda japonica Makino

Seed germination and seedling growth of the annual halophyte species Suaeda japonica Makino were investigated in response to variable salinity of sediment pore water. The germination percentage of S. japonica’s soft brown seeds, which are dominant among dimorphic seeds, decreased with an increase in salinity, although germination was still observed at 1200‐mM NaCl concentration. The germination percentage and germination speed observed in April were higher than those observed in December when treated with sediment water with 400–1200 mM of NaCl concentrations. These data suggest that S. japonica seedlings could be established on sediments that experience high temperatures. Germination recovery of S. japonica seeds transferred from 600‐mM NaCl containing sediment (seawater equivalent) was lowest among 0–1200‐mM NaCl treatments, implying the low tolerance of seawater conditions of S. japonica seeds. Seeds germinated in 900‐ to 1200‐mM NaCl medium showed poor growth, but survived, in hypersaline conditions, and exhibited improvement in growth upon transfer to lower salinity.     

Plasticity in sunflower leaf and cell growth under high salinity

A group of sunflower lines that exhibit a range of leaf Na ⁺ concentrations under high salinity was used to explore whether the responses to the osmotic and ionic components of salinity can be distinguished in leaf expansion kinetics analysis. It was expected that at the initial stages of the salt treatment, leaf expansion kinetics changes would be dominated by responses to the osmotic component of salinity, and that later on, ion inclusion would impose further kinetics changes. It was also expected that differential leaf Na ⁺ accumulation would be reflected in specific changes in cell division and expansion rates. Plants of four sunflower lines were gradually treated with a relatively high (130 mm NaCl) salt treatment. Leaf expansion kinetics curves were compared in leaves that were formed before, during and after the initiation of the salt treatment. Leaf areas were smaller in salt‐treated plants, but the analysis of growth curves did not reveal differences that could be attributed to differential Na⁺ accumulation, since similar changes in leaf expansion kinetics were observed in lines with different magnitudes of salt accumulation. Nevertheless, in a high leaf Na⁺‐including line, cell divisions were affected earlier, resulting in leaves with proportionally fewer cells than in a Na⁺‐excluding line. A distinct change in leaf epidermal pavement shape caused by salinity is reported for the first time. Mature pavement cells in leaves of control plants exhibited typical lobed, jigsaw‐puzzle shape, whereas in treated plants, they tended to retain closer‐to‐circular shapes and a lower number of lobes.     

Osmotic adjustment in cyanobacteria from hypersaline environments

The intracellular concentrations of the monovalent inorganic cations K⁺ and Na⁺, low molecular weight carbohydrates and quaternary ammonium compounds have been determined for 4 strains of cyanobacteria (Aphanothece halophytica, Coccochloris elabens, Dactylococcopsis salina and Synechocystis DUN52) originally isolated from hypersaline habitats (i.e. habitats with a salinity greater than that of seawater) over a range of external salt concentration (from 50% to 400% seawater). Intracellular cation levels (Na⁺ and K⁺) were determined to be within the range 80–320 mmol · dm^-3 (cell volume), showing only minor changes in response to salinity. Intracellular carbohydrates were found to comprise a negligible component of the intracellular osmotic potential [at 2–19 mmol · dm^-3 (cell volume)], throughout the salinity range. Quaternary ammonium compounds, however, were recorded in osmotically significant quantities [up to 1,640 mmol · dm^-3 (cell volume)] in these strains, showing major variation in response to salinity. Thus Synechocystis DUN 52 showed an increase in quaternary ammonium compounds in the oder of 1,200 mmol · dm^-3 between 50% and 400% seawater medium, accounting for a significant proportion of the change in external osmotic potential.Examination of intact cells and cell extracts using ¹³C and ¹H nuclear magnetic resonance (NMR) spectroscopy confirmed the presence of the quaternary ammonium compound glycine betaine as the major osmoticum in the 4 strains; no other compounds were detected during NMR assays. These results suggest a common mechanism of osmotic adjustment, involving quaternary ammonium compounds, in cyanobacteria from hypersaline environments.     

Tolerance of Hybrid Poplar (Populus) Trees Irrigated with Varied Levels of Salt,Selenium, and Boron

Agricultural drainage waters and industrial effluents often consist of waste waters laden with salts, boron (B), selenium (Se), molybdenum (Mo), and other contaminants. However, increasing shortages of high-quality water in arid and semiarid regions and increasing demands to maintain the water quality in rivers, lakes, streams, and groundwater have made water reuse an imperative. Trees have been viewed as potential candidates for wastewater reuse because of their capacities for high evapotranspiration, high growth rates, and abilities to accumulate salts and specific ions in a marketable product that is not biologically hazardous. Clones of eight hybrid poplar (Populus spp.) crosses were tested for salt tolerance and ion uptake characteristics in a sand culture study in Riverside, CA. After hardwood cuttings were planted and established under nonsaline conditions, young saplings were treated with artificial waste waters containing different levels of salts, Se, and B. High salt concentrations reduced growth and led to leaf damage and shedding; however, Se and B had no detrimental effect on growth. Salinity affected Se and B accumulation patterns in leaves. A significant degree of genetic variation in salt tolerance was noted among the clones. The salinity at which dry weight was reduced ranged from about 3.3 to about 7.6 dS m^-1 depending on clone, and the relative decrease in dry weight yield with increasing salinity varied among clones and ranged from about 10 to 15% per dS m^-1. This would indicate that poplars, whereas certainly more salt tolerant than avocado trees, are significantly less salt tolerant than eucalyptus. Leaf C1 concentrations increased in relation to the C1 concentrations in the irrigation waters, but also were subject to clonal variation. Salt tolerance in poplar was generally related to C1 in the leaves and stems but was also influenced by growth and vigor characteristics, as well as the allometric relationships between leaves and stems that influenced the sinks in which ions could accumulate before reaching toxic levels.     

Effect of environmental factors on seed germination and emergence of Lepidium vesicarium

Lepidium vesicarium is a weed species with a wide distribution in the rangelands and dry‐land farming in East Azarbaijan, Iran. The experiments were undertaken to assay the effects of light, temperature, pH, osmotic potential, NaCl concentration and burial depth on seed germination and emergence of L. vesicarium. Germination was maintained at high levels (> 80%) over a wide day/night temperature range (10/5 to 30/20°C), but a severe reduction in the germination rate of L. vesicarium was found below 20/10°C. Germination of L. vesicarium was influenced by different light/dark regimes, as the germination rate was highest at 16 h light for the all treatments (0, 8, 12, 16 and 24 h light). Germination was 92–95% over a wide range of pH (2‐10). Germination was >50% at a water potential of ?0.7 MPa and salinity of 21 dS/m, indicating that drought and salt conditions have a minimal impact on seed germination. With increasing burial depth from 0 to 2 cm, the number of days required for 50% emergence increased and no germination was observed at burial depths deeper than 3 cm. This suggests that L. vesicarium would become troublesome in the rangelands and for growers in reduced‐tillage cropping systems. The ability to emerge from shallow depths, coupled with tolerance of a wide pH range, drought and salinity at germination, should be taken into account when managing this weed species.     

Spatio‐temporal variation of salt marsh seedling establishment in relation to the abiotic and biotic environment

Contrary to our expectations, soil salinity and moisture explained little of the spatial variation in plant establishment in the upper intertidal marsh of three southern California wetlands, but did explain the timing of germination. Seedlings of 27 species were identified in 1996 and 1997. The seedlings were abundant (maximum densities of 2143/m² in 1996 and 1819/m² in 1997) and predominantly annual species. CCAs quantified the spatial variation in seedling density that could be explained by three groups of predictor variables: (1) perennial plant cover, elevation and soil texture (16% of variation), (2) wetland identity (14% of variation) and (3) surface soil salinity and moisture (2% of variation). Increasing the spatial scale of analysis changed the variables that best predicted patterns of species densities. Timing of germination depended on surface soil salinity and, to a lesser extent, soil moisture. Germination occurred after salinity had dropped below a threshold or, in some cases, after moisture had increased above a critical level. Between 32% and 92% of the seedlings were exotic and most of these occurred at lower soil salinity than native species. However, Parapholis incurva and Mesembryanthemum nodiflorum were found in the same environments as the native species. In 1997, the year of a strong El Niño/Southern Oscillation event with high rainfall and sea levels, the elevation distribution of species narrowed and densities of P. incurva and other exotic species decreased but densities of native and rare species did not change. The ‘regeneration niche’ of wetland plant communities includes the effects of multiple abiotic and biotic factors on both the spatial and temporal variations in plant establishment.     

Photosynthesis and photosystem 2 efficiency of two salt-adapted halophytic seashore <Emphasis Type="Italic">Cakile maritima</Emphasis> ecotypes

The effects of salinity (0–400 mM NaCl, marked S₀, S₁₀₀, S₂₀₀, and S₄₀₀) on growth, photosynthesis, photosystem 2 (PS2) efficiency, ion relations, and pigment contents were studied in two seashore Cakile maritima ecotypes (Tabarka and Jerba, respectively, sampled from humid and arid bioclimatic areas). Growth of Jerba plants was improved at S₁₀₀ as compared to S₀. Tabarka growth was inhibited by salinity at all NaCl concentrations. Leaf sodium and chloride concentrations increased with medium salinity and were higher in Jerba than in Tabarka plants. Chlorophyll content, net photosynthetic rate, stomatal conductance (g _s), and intracellular CO₂ concentration were stimulated at moderate salinity (S₁₀₀) in Jerba plants and inhibited at higher salt concentrations in both ecotypes: g _s was the most reduced parameter. The maximum quantum efficiency of PS2 (F_v/F_m), quantum yield, linear electron transport rate, and efficiency of excitation energy capture by open PS2 reaction centres showed no significant changes with increasing salt concentration in Jerba plant and were decreased in Tabarka subjected to S₄₀₀. However, the efficiency of dissipation of excess photon energy in the PS2 antenna was maintained in Jerba and was increased in Tabarka plants challenged with S₄₀₀. Hence the relative salt tolerance of Jerba was associated with a better ability to use Na⁺ and Cl⁻ for osmotic adjustment, the absence of pigment degradation, and the concomitant PS2 protection from photodamage.     

Enhanced tolerance of photosynthesis against high temperature damage in salt-adapted halophyte Atriplex centralasiatica plants

Thermotolerance of photosynthesis in salt‐adapted Atriplex centralasiatica plants (100–400 mm NaCl) was evaluated in this study after detached leaves and whole plants were exposed to high temperature stress (30–48 °C) either in the dark or under high light (1200 mol m^?2 s^?1). In parallel with the decrease in stomatal conductance, intercellular CO₂ concentration and CO₂ assimilation rate decreased significantly with increasing salt concentration. There was no change in the maximal efficiency of PSII photochemistry (F_v/F_m) with increasing salt concentration, suggesting that there was no damage to PSII in salt‐adapted plants. On the other hand, there was a striking difference in the response of PSII and CO₂ assimilation capacity to heat stress in non‐salt‐adapted and salt‐adapted leaves. Leaves from salt‐adapted plants maintained significantly higher F_v/F_m values than those from non‐salt‐adapted leaves at temperatures higher than 42 °C. The F_v/F_m differences between non‐salt‐adapted and salt‐adapted plants persisted for at least 24 h following heat stress. Leaves from salt‐adapted plants also maintained a higher net CO₂ assimilation rate than those in non‐salt‐adapted plants at temperatures higher than 42 °C. This increased thermotolerance was independent of the degree of salinity since no significant changes in F_v/F_m and net CO₂ assimilation rate were observed among the plants treated with different concentrations of NaCl. The increased thermotolerance of PSII induced by salinity was still evident when heat treatments were carried out under high light. Given that photosynthesis is considered to be the physiological process most sensitive to high temperature damage, increased thermotolerance of photosynthesis may be of significance since A. centralasiatica, a typical halophyte, grows in the high salinity regions in the north of China, where the temperature in the summer is often as high as 45 °C.     

Growth and ion accumulation of two rapid-cycling Brassica species differing in salt tolerance

The response of two rapid-cycling Brassica species differing in tolerance to seawater salinity was studied over a period of 24 days. In response to 8 dS m⁻¹ salinity, the two Brassica species showed clear differences in the changes in relative growth rate (RGR), net assimilation rate (NAR) and leaf area ratio (LAR). The RGR of B. napus was slightly reduced by salinity, wheareas the RGR of B. carinata was largely reduced in the early stages of salinization. LAR of B. napus was affected by salinity in the later stages of growth and significantly correlated with the reduction in RGR. On the other hand, the NAR of B. carinata was decreased by salinity, corresponding to the decrease of the RGR of B. carinata. The NAR of B. napus was not significantly affected by salinity according to analysis of covariance. The shoot concentrations of Na, Mg and Cl increased while the concentrations of K and Ca decreased sharply during the first 5 days of salinization; subsequently, all ion concentrations remained relatively constant. The concentrations of Na, K, Ca, Mg and Cl in the root were similarly affected by salinity. There were no significant differences of ion concentrations between species that could be related to the differences in salt tolerance. Thus, the differences in salt tolerance between species can not be related to differences in specific-ion effects, but may be related to some factor that reduces the NAR of B. carinata during the early stages of growth.