Ameliorating effect of triacontanol on salt stressederythrina variegata seedlings. Changes in growth,biomass, pigments and solute accumulation

Erythrina variegata Lam. seedlings were grown under low (100 mM NaCl) and high (250 mM NaCl) salinity. Seedlings exposed to high salinity for 10 d showed significant reduction in growth rate and biomass production while the root/shoot ratio increased. In contrast to pigment and protein contents, starch and saccharide contents increased in salt stressed seedlings. When the seedlings were subsequently sprayed with triacontanol (1 mg kg^-1) the salinity effect was partially ameliorated and growth, biomass, chlorophyll and carotenoid contents increased.     

Effect of 24-epibrassinolide on biomass, growth and free proline concentration in Spirulina platensis (Cyanophyta) under NaCl stress

In this study, biomass, growth and free proline concentration were investigated in Spirulina platensis treated with different concentrations of NaCl (50, 100, 150, and 200 mM) and 24-epibrassinolide (24-epiBL) hormone (0.5, 1.0, and 3.0 μM) over 5 days. As a result of analysing the cultures under salinity stress, it was determined that biomass and growth rate decreased significantly, while proline concentration increased considerably under salinity stress. The increase in the concentration of proline suggests a role in response to NaCl stress. Among the cultures treated with different concentrations of 24-epiBL, maximum growth was determined at the cultures at 1.0 μM 24-epiBL. Algal growth was also greater at the 0.5 and 3.0 μM concentrations of 24-epiBL with respect to control cultures. With respect to control, 24-epiBL affected growth rate and biomass positively, but proline concentration did not change. Among the cultures supplied with different combinations of NaCl and 24-epiBL, growth rate increased in 150/0.5 and 150/3.0 mM/μM concentrations, but was maximal for the culture containing 150/1.0 mM/μM combination. The increase in algal growth suggests a role for 24-epiBL in partially alleviated to NaCl stress. These results suggest that 24-epiBL may have a protective role for S. platensis reducing the inhibitor effects of salinity stress.     

Effect of NaCl-induced changes in growth,photosynthetic characteristics,water status and enzymatic antioxidant system of <Emphasis Type="Italic">Calligonum caput-medusae</Emphasis> seedlings

Calligonum caput-medusae is known to grow well when irrigated with water containing NaCl. The aim of this study was to investigate ecophysiological responses of C. caput-medusae to different NaCl concentrations. In our study, we examined the effect of 0, 50, 100, 200, and 400 mM NaCl. Our results demonstrated that maximum seedling growth occurred at 50 mM NaCl. Photosynthetic parameters, such as the photosynthetic pigment content and gas exchange parameters, correlated with growth response. High salinity (≥ 100 mM NaCl) resulted in a significant reduction of the plant growth. Similarly, marked declines in the pigment content, maximal efficiency of PSII photochemistry, net photosynthetic rate, transpiration rate, and stomatal conductance were also detected. However, intercellular CO2 concentration showed a biphasic response, decreasing with water containing less than 200 mM NaCl and increasing with NaCl concentration up to 400 mM. Water-use efficiency and intrinsic water-use efficiency exhibited the opposite response. The reduction of photosynthesis at the high NaCl concentration could be caused by nonstomatal factors. High salinity led also to a decrease in the relative water content and water potential. Correspondingly, an accumulation of soluble sugars and proline was also observed. Na⁺ and Cl^? concentrations increased in all tissues and K⁺ concentrations were maintained high during exposure to NaCl compared with the control. High salinity caused oxidative stress, which was evidenced by high malondialdehyde and hydrogen peroxide contents. In order to cope with oxidative stress, the activity of antioxidative enzymes increased to maximum after 50 mM NaCl treatment. The data reported in this study indicate that C. caput-medusae can be utilized in mild salinity-prone environments.     

Intrinsic water use efficiency controls the adaptation to high salinity in a semi-arid adapted plant,henna (Lawsonia inermis L.)

Adaptation to salinity of a semi-arid inhabitant plant, henna, is studied. The salt tolerance mechanisms are evaluated in the belief that gas exchange (water vapor and CO₂) should play a key role on its adaptation to salt stress because of the strong evaporation conditions and soil water deficit in its natural area of distribution. We grow henna plants hydroponically under controlled climate conditions and expose them to control (0 mM NaCl), and two levels of salinity; medium (75 mM NaCl) and high (150 mM NaCl). Relative growth rate (RGR), biomass production, whole plant and leaf structure and ultrastructure adaptation, gas exchange, chlorophyll fluorescence, nutrients location in leaf tissue and its balance in the plant are studied. RGR and total biomass decreased as NaCl concentration increased in the nutrient solution. At 75 mM NaCl root biomass was not affected by salinity and RGR reached similar values to control plants at the end of the experiment. At this salinity level henna plant responded to salinity decreasing shoot to root ratio, increasing leaf specific mass (LSM) and intrinsic water use efficiency (iWUE), and accumulating high concentrations of Na⁺ and Cl⁻ in leaves and root. At 150 mM NaCl growth was severely reduced but plants reached the reproductive phase. At this salinity level, no further decrease in shoot to root ratio or increase in LSM was observed, but plants increased iWUE, maintaining water status and leaf and root Na⁺ and Cl⁻ concentrations were lower than expected. Moreover, plants at 150 mM NaCl reallocated carbon to the root at the expense of the shoot. The effective PSII quantum yield [Y(II)] and the quantum yield of non-regulated energy dissipation [Y(NO)] were recovered over time of exposure to salinity. Overall, iWUE seems to be determinant in the adaptation of henna plant to high salinity level, when morphological adaptation fails.     

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.     

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.     

On the mechanism of salt tolerance

As glycerol was suggested as an osmotic agent in the salt tolerantDebaryomyces hansenii the concentrations of total, intracellular, and extracellular glycerol produced by this yeast was followed during growth in 4 mM, 0.68 M, and 2.7 M NaCl media. The total amount of glycerol was not directly proportional to biomass production but to the cultural salinity with maximum concentrations just prior to or at the beginning of the stationary phase. In all cultures the cells lost some glycerol to the media, at 2.7 M NaCl the extracellular glycerol even amounted maximally to 80% of the total. A distinct maximum of intracellular glycerol, related to dry weight or cell number, appeared during the log phase at all NaCl concentrations. As the intracellular calculated glycerol concentrations amounted to 0.2 M, 0.8 M, and 2.6 M in late log phase cells at 4 mM, 0.68 M, and 2.7 M NaCl, respectively, whereas the corresponding analysed values for the glycerol concentrations of the media were 0.7 mM, 2.5 mM, and 3.0 mM, glycerol contributes to the osmotic balance of the cells.During the course of growth all cultures showed a decreasing heat production related to cell substance produced, most pronounced at 2.7 M NaCl. At 2.7 M NaCl the total heat production amounted to-1690 kJ per mole glucose consumed in contrast to-1200 and-1130 kJ at 4 mM and 0.68 M NaCl, respectively. TheY _m -values were of an inverse order, being 129, 120, and 93 at 4 mM, 0.68 M, and 2.7 M NaCl, respectively.     

Effects of temperature,salinity and their interaction on growth of the benthic dinoflagellate Ostreopsis cf. ovata (Dinophyceae) from Japanese coastal waters

Benthic dinoflagellates of the genus Ostreopsis produce palytoxin (PTX)‐like compounds. The worldwide distributed Ostreopsis ovata/O. cf. ovata is potentially responsible for outbreaks of human health problems around the coasts of tropical, subtropical, and temperate regions. The present study examined growth responses of an O. cf. ovata strain s0662 collected from coastal waters of Japan with 35 different combinations of temperature (15–35°C) and salinity (20–40) and discusses the bloom dynamics of the organism in Japanese coastal environments. The O. cf. ovata strain s0662 tolerated a wide range of temperature (17.5–35°C) and salinity (25–40). Results of a two‐way ANOVA showed significant effects of temperature‐salinity interaction on growth rates and biomass yields of the O. cf. ovata strain (F(24,70) > 127, P < 0.001). The strain showed a maximal growth rate (1.03 divisions day^?1) and biomass yield (240 relative fluorescence) at temperature 25°C and salinity 30. The high growth rates of over 1.0 division day^?1 were obtained in conditions of temperature 25–30°C and salinity 30–35, which indicates that strain s0662 prefers high temperature and salinity conditions. The growth rates of O. cf. ovata under the optimal conditions were higher than those of other benthic toxic‐dinoflagellates, Coolia monotis, Gambierdiscus toxicus, and Prorocentrum lima (Dinophyceae) previously reported. Taken together, we suggest that O. cf. ovata is able to grow faster than the other benthic dinoflagellates in waters of high temperature and salinity. The physiological feature probably confers an ecological advantage on O. cf. ovata in the bloom development during warmer seasons in Japan and may be responsible for outbreaks of PTX‐like poisoning in the region especially during the warmer seasons.     

OPTIMIZATION OF SALT CONCENTRATIONS FOR A HIGHER CAROTENOID PRODUCTION IN DUNALIELLA SALINA (CHLOROPHYCEAE)1

Dunaliella salina (Dunal) Teodor, when treated over 25 d with a wide range of NaCl salinities (0.6–4.5 M), showed its maximal growth potentialities at 1.5–3.0 M NaCl and was able to survive even at 4.5 M NaCl. Sodium concentrations increased significantly at the supraoptimal salinities, reaching up to 5 mmol · g^?1 dry weight (dwt) at 4.5 M NaCl. Interestingly, ability of D. salina to take up essential mineral nutrients was not impaired by increased salinity. As for growth, chl concentrations were maximal in the 1.5–3.0 M NaCl range. Interestingly, carotenoid concentrations increased with the increasing salinity. The highest values of total antioxidant activity (5.2–6.9 mg gallic acid equivalents [GAE] · g^?1 dwt), antiradical activity, and reducing power were measured at 1.5–3.0 M NaCl. As a whole, these results showed that at 1.5–3.0 M NaCl, D. salina produce appreciable antioxidant level. But, once it reaches its growth maximum, a salt addition up to 4.5 M could enhance its carotenoid yield.     

Effect of salt stress on photosynthesis and chlorophyll fluorescence in Medicago truncatula

In the present study, photosynthetic parameters including gas exchanges, pigment contents, and chlorophyll fluorescence, were compared in two contrasting local Medicago truncatula lines TN6.18 and TN8.20, in response to salt added to the nutrient solution. Plants were cultivated under symbiotic nitrogen fixation (SNF) after inoculation with a reference strain Sinorhizobium meliloti 2011, a very tolerant strain to salinity (700 mM NaCl), and grown in a controlled glasshouse. On one month old plants (with active SNF), salt treatment (75 mM NaCl) was gradually applied. Photosynthesis, assimilating pigments and chlorophyll fluorescence were monitored throughout the experiment during both short and long terms, compared to control (non-saline) conditions. A genotypic variation in salt tolerance was found; TN6.18 was the more sensitive to salinity. The relative tolerance of TN8.20 was concomitant with the highest photochemical quenching coefficient (q_P) affecting the maximum quantum yield of PSII (Y); the real quantum yield (?_exc) was the most affected in the sensitive line. Moreover, stomatal and PSII reaction centers activities differed clearly between the studied lines. We found that the effect of salinity on photosynthesis of M. truncatula was related to PSII activity reduction rather than to stomatal conductance limitation. Photosynthesis was reduced by the inhibition of CO₂ assimilation caused by PSII damage. This was clearly estimated by the Y, ?_exc and especially by the quantum yield of electron transport of PSII (Φ_PSII). Thus, on the basis of our results on the two local M. truncatula lines, we recommend the use of chlorophyll fluorescence as non-destructive screening method to discriminate susceptible and resistant legumes to salt stress.     

Changes in photosynthetic pigments and chlorophyll-<Emphasis Type="Italic">a</Emphasis> fluorescence attributes of sweet-forage and grain sorghum cultivars under salt stress

Water shortage leads to a low quality of water, especially saline water in most parts of agricultural regions. This experiment was designed to determine the effects of saline irrigation on sorghum as a moderately salt-tolerant crop. To study salinity effects on photosynthetic pigment attributes including the chlorophyll content and chlorophyll fluorescence, an experiment was performed in a climate-controlled greenhouse at two vegetative and reproductive stages. The experimental design was factorial based on a completely randomized design with five NaCl concentrations (control, 50, 100, 150, and 200 mM), two grain and sweet-forage sorghum cultivars (Kimia and Pegah, respectively) and four replications. According to the experimental data, there were no significant differences between two grain and sweet-forage cultivars. Except for 100 and 150 mM NaCl, salinity significantly decreased the chlorophyll index and pigment contents of the leaf, while it increased the chlorophyll-a fluorescence characteristics. Although salinity reduced photosynthetic pigments and the crop yield, either grain or sweet-forage cultivars could significantly control the effect of salinity between 100 and 150 mM NaCl at both developmental stages, showing the possibility of using saline water in sorghum cultivation up to 150 mM NaCl.     

Combined effect of salinity and pH on lipid content and fatty acid composition of Tisochrysis lutea

The haptophyte microalga Tisochrysis lutea was heterotrophically grown in F2 medium with different combinations of pH and salinity. Growth, oil content and fatty acids (FAs) profile were determined under each set of conditions. The salinity was adjusted using NaCl at concentrations of 0.4, 0.6, 0.8, or 1.0 M, while pH was adjusted at 7, 8, or 9, and heterotrophic growth was performed using organic carbon in the form of sugar cane industry waste (CM). Fatty acid methyl esters (FAMEs) were identified by gas chromatography. The results showed that pH of 8.0 was the optimal for dry weight and oil production, regardless of the salinity level. At pH 8.0, growth at a salinity of 0.4 M NaCl was optimal for biomass accumulation (1.185 g L^-1). Under these conditions, the maximum growth rate was 0.055 g L^-1 d^-1, with a doubling time of 17.5 h and a degree of multiplication of 2.198. Oil content was maximal (34.87%) when the salinity was 0.4 M and the pH was 9.0. The ratio of saturated to unsaturated FAs was affected by the pH value and salinity, in that unsaturated FAs increased to 58.09% of the total FAs, considerably greater than the value of 40.59% obtained for the control (0.4 M NaCl and pH 8.0).     

Polyhydric alcohol production and intracellular amino acid pool in relation to halotolerance of the yeast Debaryomyces hansenii

Changes in polyol production and the intracellular amino acid pool were followed during the growth cycle of Debaryomyces hansenii in 4 mM and 2.7 M NaCl media. The intracellular levels of polyols were markedly enhanced by high salinity, the dominant solutes being glycerol in log phase cells and arabinitol in stationary phase cells. At low salinity arabinitol was the most prominent intracellular solute throughout the growth cycle. There were no major changes in the composition of the total amino acid pool with changes in cultural salinity. The amount of total free amino acids related to cell dry weight was 15–50% lower in cells cultured in 2.7 M NaCl as compared to 4 mM NaCl media.After subtraction of contributions from intracellular polyols the calculated cellular C/N ratio was found to be unaffected by cultural age and salinity during the late log and early stationary phase. On prolonged incubation of stationary phase cells, this ratio decreased, particularly at high salinity. The sensitivity of cells towards exposure to high salinity was measured in terms of the length of the lag phase after transference to 2.7 M NaCl media. This lag phase decreased with increasing intracellular polyol concentrations. At a given polyol content, stationary phase cells were considerably less sensitive than were log phase cells.When cultured at high salinity the mutant strain, 26-2b, grew more slowly and retained less of the total polyol produced during the early growth stages than did the wildtype. Exogenously supplied mannitol, arabinitol, and glycerol stimulated the growth of the mutant in saline media. Erythritol was without effect.Abbreviations GLC gas-liquid chromatography - TCA trichloroacetic acid     

The ATP pool in relation to the production of glycerol and heat during growth of the halotolerant yeast Debaryomyces hansenii

In a study of the halotolerant yeast Debarymyces hansenii cultured in 4 mM and 2.7 M NaCl the intracellular ATP pool, the heat production, the oxygen uptake, and, in the high culture salinity also, the intracellular glycerol concentration were found to be correlated. The intracellular ATP in the 2.7 M NaCl culture had a constant concentration of 3.5 mM ATP during the second half of the lag phase, while in 4 mM NaCl it rose to a maximum of 3.1 mM during the late log phase. The intracellular glycerol concentration in 2.7 M NaCl was about 1.3M during the entire exponential growth phase. Sine the glycerol concentration of the medium was not more than 0.23 mM, glycerol must contribute to the osmotic balance of the cells in high salinity. The corresponding maximum values for the 4 mM NaCl culture were 0.16 M and 0.08 mM. The experimental enthalpy changes were approximately the same for the two salinities, viz. about-1200 kJ per mole consumed glucose. The Y _m-values for the 4 mM and 2.7 M NaCl cultures were 91 and 59, respectively, the difference being a consequence of the decreased efficiency of growth in high salinity.Abbreviations CFU colony-forming units - PCA perchloric acid - TCA trichloroacetic acid     

Salinization of the soil solution decreases the further accumulation of salt in the root zone of the halophyte Atriplex nummularia Lindl. growing above shallow saline groundwater

Water use by plants in landscapes with shallow saline groundwater may lead to the accumulation of salt in the root zone. We examined the accumulation of Na⁺ and Cl^? around the roots of the halophyte Atriplex nummularia Lindl. and the impacts of this increasing salinity for stomatal conductance, water use and growth. Plants were grown in columns filled with a sand–clay mixture and connected at the bottom to reservoirs containing 20, 200 or 400 mM NaCl. At 21 d, Na⁺ and Cl^? concentrations in the soil solution were affected by the salinity of the groundwater, height above the water table and the root fresh mass density at various soil depths (P  < 0.001). However, by day 35, the groundwater salinity and height above the water table remained significant factors, but the root fresh mass density was no longer significant. Regression of data from the 200 and 400 mM NaCl treatments showed that the rate of Na⁺ accumulation in the soil increased until the Na⁺ concentration reached ~250 mM within the root zone; subsequent decreases in accumulation were associated with decreases in stomatal conductance. Salinization of the soil solution therefore had a feedback effect on further salinization within the root zone.     

Effects of Salinity on Growth, Water Relations and Ion Accumulation of the Subtropical Perennial Halophyte, Atriplex griffithii var. stocksii

The effects of salinity on growth, water relations, glycinebetainecontent, and ion accumulation in the perennial halophyte Atriplexgriffithii var. stocksii were determined. The following questionswere addressed: (1) What effect does salinity have on growthresponses at different ages? (2) Is A. griffithii an ion accumulator?(3) Does A. griffithii accumulate glycinebetaine in responseto salinity? Atriplex griffithii plants were grown in pots at0, 90, 180 and 360  m M NaCl in sand culture in a plantgrowth chamber and plants were harvested after 30, 60 and 90d. Plant total dry weight was significantly inhibited at 360m M NaCl. Root growth showed a substantial promotion at 90 mM NaCl. The water potential and osmotic potential of shootsbecame more negative with increasing salinity and time of growth.The Na⁺and Cl^-content in both shoots and roots increased withincreases in salinity. Increased treatment levels of NaCl induceddecreases in Ca⁺, K⁺and Mg²⁺in plants. Atriplex griffithii accumulateda large quantity of ions, with the ash content reaching 39%of the dry weight in leaves. Inorganic ion accumulation is significantin osmotic adjustment and facilitates water uptake along a soil-plantgradient. Glycinebetaine concentration was low in roots, andin stems it increased with increases in salinity. Total amountsof glycinebetaine in leaves increased with increases in salinity,and its concentration increased substantially at 360 m M NaCl.Copyright 2000 Annals of Botany Company Atriplex griffithii, glycinebetaine, growth, ions, water relations.     

The role of arbuscular mycorrhizal fungi in alleviating salt stress in Medicago sativa L. var. icon

Medicago sativa L. is the most important forage crop in arid and semi-arid areas, where increased salinity is a major factor limiting plant growth and crop productivity. The role of arbuscular mycorrhizal (AM) fungus Glomus viscosum H.T. Nicolson strain A6 in protecting alfalfa plants from salt stress, induced by sodium chloride (NaCl), was studied in two ways. Firstly, the root systems of 3-month old M. sativa plants, both mycorrhizal (AM+) and non-mycorrhizal (non-AM) (M. sativa L. var. icon), were placed in solutions of increasing salt concentrations (0, 50, 100, 150, 200 mM NaCl) to study the wilting response. G. viscosum improved the tolerance to salinity stress and the benefit was expressed in terms of the time required to reach the T4 stage in the wilting experiment. Secondly, to evaluate the ability of the Glomus-alfalfa symbiosis to tolerate salt, a pot experiment was set up in a glasshouse in which 3-month old alfalfa plants (M. sativa var. icon) were grown in a peat substratum at three salinity levels (0, 100, 150 mM NaCl). The AM symbiosis stimulated plant height, leaf area, root density, fresh and dry plant weight under saline conditions. Furthermore, proline accumulation was higher in mycorrhizal M. sativa plants than in non-mycorrhizal plants under conditions of salt stress. These and other results indicated that the micropropagated selected clone of M. sativa var. icon, when in symbiosis with G. viscosum H.T. Nicolson strain A6, exhibited better growth and physiological activities under saline conditions than non-AM plants. The AM+ plants also had lower sodium and chloride concentrations in tissues than non-AM plants.     

Maintenance of photosynthesis and the antioxidant defence systems have key roles for survival of Halopeplis perfoliata (Amaranthaceae) in a saline environment

• Coastal salt marsh plants employ various combinations of morphological and physiological adaptations to survive under saline conditions. Little information is available on salinity tolerance mechanisms of Halopeplis perfoliata, a C3 stem succulent halophyte.
• We investigated the growth, photosynthesis and antioxidant defence mechanisms of H. perfoliata under saline conditions (0, 150, 300 and 600 mM NaCl) in an open greenhouse.
• Optimal shoot succulence, projected shoot area and relative growth rate were obtained in the low (150 mm NaCl) salinity treatment, while growth was inhibited at the highest salinity (600 mm NaCl). The CO₂ compensation point and carbon isotope composition of biomass confirmed C₃ photosynthesis. Increases in salinity did not affect the photosynthetic pigment content or maximum quantum efficiency of PSII of H. perfoliata. Assimilation of CO₂ (A) also remained unaffected by salinity. A modest effect on some gas exchange and photochemistry parameters was observed at 600 mm NaCl. With increasing salinity, there was a continual increase in respiration, suggesting utilisation of energy to cope with saline conditions. Under 300 and 600 mm NaCl, there was an increase in H₂O₂ and MDA with a concomitant rise in AsA, GR content and CAT activity.
• Hence, H. perfoliata appears to be an obligate halophyte that can grow up to seawater salinities by modulating photosynthetic gas exchange, photochemistry and the antioxidant defence systems.

     

INFLUENCE OF SALINITY AND SULFATE ON THE TOXICITY OF CHROMIUM(VI) TO THE ESTUARINE DIATOM THALASSIOSIRA PSEUDONANA1

The acute toxicity of Cr(VI) to the diatom Thalassiosira pseudonana (Hasle and Heimdal) clone 3H was determined in artificial media of 3.2 and 0.32 ppt salinity and with variations of sulfate concentration in the media independent of salinity. Inhibitory concentrations of Cr(VI) ranged from 6.6 μM for growth rate and 4.9 μM for cell yield at 3.2 ppt salinity and 2.8 μM sulfate to 0.04 μM for growth rate and 0.02 μM for cell yield at 0.32 ppt salinity and 0.019 mM sulfate. The inhibition by Cr(VI) was a function of the ratio of Cr(VI) to sulfate. Inhibition occurred when-this ratio exceeded about 500:1. It is suggested that the mechanism for the toxicity of Cr(VI) to diatoms and perhaps other aquatic organisms involves a site at which sulfate and chromate compete.