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.     

Salinity effects on polyphenol content and antioxidant activities in leaves of the halophyte Cakile maritima.

Cakile maritima is a local oilseed halophyte exhibiting potential for secondary metabolite production. In the present study, plant growth, leaf polyphenol content and antioxidant activity were comparatively analyzed in two C. maritima Tunisian accessions (Jerba and Tabarka, respectively sampled from arid and humid bioclimatic stages) under salt constraint. Three-week-old plants were subjected to 0, 100, and 400 mM NaCl for 28 days under glasshouse conditions. A significant variability in salt response was found between both accessions: while Tabarka growth (shoot biomass, leaf expansion) was significantly restricted at 100 and 400 mM NaCl, compared to the control, Jerba growth increased at 100mM before declining at 400 mM NaCl. The better behaviour of Jerba salt-challenged plants, compared to those of Tabarka, may be related to their higher polyphenol content (1.56- and 1.3-fold the control, at 100 and 400 mM NaCl respectively) and antioxidant activity (smaller IC(50) values for both 1,1-diphenyl-2-picrylhydrazyl and superoxide scavenging), associated with lower leaf MDA accumulation (ca. -66% of the control at 100mM NaCl). Taken together, our findings suggest that halophytes may be interesting for production of antioxidant compounds, and that the accession-dependent capacity to induce antioxidative mechanisms in response to salt, may result in a corresponding variability for growth sustainability.     

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.     

Differential response to salinity and water deficit stress in Polypogon monspeliensis (L.) Desf. provenances during germination

The effects of provenance, salinity (0, 100, 200, 300 and 400 mm NaCl) and water deficit (0, -0.6, -1.1, -1.6 and -2.1 MPa mannitol solutions) on germination success of Polypogon monspeliensis were investigated. Eight Tunisian provenances from different bioclimatic origins were considered. Seed mass varied significantly between populations. Germination percentage was significantly affected by provenance, salinity and their interaction. Even at 300 mm NaCl, germination percentage of Tabarka, Kelbia and Kebili ranged from ca. 60% to ca. 85%, whereas Monastir, Gabes and El Haouaria succeeded in germinating in 200 mm NaCl. The 300 mm NaCl treatment highly reduced germination of Monastir and El Haouaria, and inhibited that of Gabes. Soliman and El Jem were the least salt-tolerant provenances. The severity of water deficit impact on seed germination was also provenance-dependent, especially at osmotic potentials of -1.1 to -1.6 MPa. At -1.6 MPa, germination percentage of Tabarka, Monastir and Kebili was close to 80%, while that of Gabes, El Jem and Kelbia was 0%, 5% and 20%, respectively. Regardless of provenance, germination was strongly impaired at -2.1 MPa. The variability of stress tolerance in P. monspeliensis could be of practical significance in programmes aimed at restoring arid and salt-affected lands since it allows use of provenances that germinate and establish successfully under unfavourable conditions prevailing in such zones.     

Growth,Na, K,osmolyte accumulation and lipid membrane peroxidation of two provenances of Cakile maritima during water deficit stress and subsequent recovery

The effects of water deficit stress on growth, Na⁺, K⁺ and osmolyte accumulation in the halophyte species Cakile maritima were investigated. Two Tunisian provenances, Tabarka and Chaffar, belonging to different bioclimatic stages, humid and arid, respectively, were compared. After germination, thirty-day-old seedlings were cultivated for 4 weeks under optimal or limiting water supply, at 100% and 25% of field capacity (FC), respectively. A subset of stressed plants was thereafter rehydrated. The final harvest was carried out after 60 days of treatment. Upon water deficit stress, Chaffar provenance showed significantly lower reduction in biomass production, net CO₂ assimilation and stomatal conductance as well as of leaf water content. Leaf malondialdehyde (MDA) content was significantly increased in the two provenances but this effect was more pronounced in Tabarka plants than in Chaffar ones. Several criteria seem to be associated with the relative tolerance of Chaffar to water deficit: a slow growth rate, a greater ability to control photosynthetic gas exchange, a high ability to preferentially allocate photoassimilates to its roots, and a greater capacity for osmotic adjustment ensured by K⁺ and some compatible solutes such as proline and glycine betaine, but not soluble sugars. The superiority of Chaffar provenance also appeared at the level of its ability to recover after a severe water deficit stress (irrigation at 25% FC only during one month). The data suggest that compatible osmolytes (proline and glycine betaine) accumulated upon water deficit stress play important roles in this halophyte, being involved not only in osmotic adjustment but probably serving also in preservation of the structural and functional integrity at the cellular level during water deficit.     

Response of antioxidant systems to NaCl stress in the halophyte Cakile maritima

The effects of salt stress on antioxidative activities were investigated in a coastal halophyte, Cakile maritima . Two Tunisian accessions, Jerba and Tabarka, were compared. Plants were subjected to 100, 200, or 400 m M NaCl for 20 days. Parameters of oxidative stress [malondialdehyde (MDA), electrolyte leakage (EL), and hydrogen peroxide (H₂O₂) concentration], activities of several enzymes [superoxide dismutase (SOD), catalase (CAT), peroxydase (POD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR)], and antioxidant molecules (ascorbate, ASC, and glutathione, GSH) were determined. Growth of Jerba plants was improved at 100 m M NaCl as compared to that of control. Tabarka growth was inhibited by salt at all NaCl concentrations. The relative salt tolerance of Jerba was associated with high antioxidant enzyme activities and glutathione content, together with low MDA content, EL, and H₂O₂ concentration. Lower antioxidant activities and higher MDA content, EL, and H₂O₂ concentration were found in Tabarka. As a whole, these data suggest that the capacity to limit oxidative damage is important for salt tolerance of C. maritima .     

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.     

Different antioxidant responses to salt stress in two different provenances of <Emphasis Type="Italic">Carthamus tinctorius</Emphasis> L.

Seedlings of two Tunisian Carthamus tinctorius L. provenances (Kairouan and Tazarka) differing in salt sensitivity were hydroponically grown at 0 and 50 mM NaCl over 21 days. Leaves of Kairouan (salt-sensitive) showed a 48% restriction in their growth at 50 mM NaCl although they accumulated less sodium than those of Tazarka (less salt-sensitive) that maintained an unchanged growth. Salt treatment induced oxidative stress in C. tinctorius and the effect was more pronounced in the leaves of the more salt sensitive provenance, Kairouan. Both provenances exhibited a stimulation of antioxidant enzyme activities with higher catalase (CAT) and superoxide dismutase (SOD) activities in Tazarka and higher peroxidase (POD) activity in Kairouan. But, it seems that antioxidant activities were more correlated with polyphenol content. Actually, leaves of Tazarka experienced higher polyphenol and antioxidant activity than Kairouan at 50 mM NaCl. Hence, moderate salinity (3 g NaCl L⁻¹) enhanced bioactive molecule yield in the less salt sensitive provenance, Tazarka. In addition, C. tinctorius was found rich in ascorbic acid, but the moderate salt stress enhanced its production only in the sensitive provenance.     

Does Seed Priming Induce Changes in the Levels of Some Endogenous Plant Hormones in Hexaploid Wheat Plants Under Salt Stress？

In order to assess whether salt tolerance could be Improved In spring wheat （Triticum aestivum L.）, the present study was performed by soaking the seeds of two cultlvars, namely MH-97 （salt sensitive） and Inqlab-91 （salt tolerant）, for 12 h In distilled water or 100 mol/m^3 CaCl2, KCI, or NaCI. Primed seeds from each treatment group and non-primed seeds were sown In a field In which NaCI salinity of 15 dS/m was developed. Priming of seeds with CaCl2, followed by priming with KCI and NaCI, was found to be effective In alleviating the adverse effects of salt stress on both wheat cultivars In terms of shoot fresh and dry weights and grain yield. Priming with CaCl2 alleviated the adverse effects of salt stress on hormonal balance In plants of both cultlvars. In MH-97 plants, CaCl2 pretreatment considerably reduced leaf absclslc acid （ABA） concentrations and Increased leaf free salicylic acid （SA） concentrations under both saline and non-saline conditions. In contrast, In the Inqlab-91 plant, CaCl2 Increased free Indoleacetic acid （IAA） and indolebutyrlc acid （IBA） content. However, priming of seeds with CaCl2 did not alter free polyamlne levels in either cultlvar, although spermldlne levels were considerably lower In plants raised from seeds treated with CaCl2 for both cultlvars under saline conditions. Priming with KCI Increased growth In Inqlab-91 plants, but not In MH-97 plants, under saline conditions. The salinity Induced reducUon In auxins （IAA and IBA） was alleviated by NaCI priming In both cultlvars under saline conditions. However, NaCI Increased leaf free ABA content and lowered leaf SA and putresclne levels In Inqlab-91 plants under saline conditions. In conclusion, although all three priming agents （I.e. CaCl2, KCI, and NaCI） were effective In alleviating the adverse effects of salt stress on wheat plants, their effects on altering the levels of different plant hormones were different In the two cuItlvars.     

Heterologous expression of Arabidopsis H+‐pyrophosphatase enhances salt tolerance in transgenic creeping bentgrass (Agrostis stolonifera L.)

The Arabidopsis vacuolar H⁺‐pyrophosphatase (AVP1), when over‐expressed in transgenic (TG) plants, regulates root and shoot development via facilitation of auxin flux, and enhances plant resistance to salt and drought stresses. Here, we report that TG perennial creeping bentgrass plants over‐expressing AVP1 exhibited improved resistance to salinity than wild‐type (WT) controls. Compared to WT plants, TGs grew well in the presence of 100 mm NaCl, and exhibited higher tolerance and faster recovery from damages from exposure to 200 and 300 mm NaCl. The improved performance of the TG plants was associated with higher relative water content (RWC), higher Na⁺ uptake and lower solute leakage in leaf tissues, and with higher concentrations of Na⁺, K⁺, Cl^‐ and total phosphorus in root tissues. Under salt stress, proline content was increased in both WT and TG plants, but more significantly in TGs. Moreover, TG plants exhibited greater biomass production than WT controls under both normal and elevated salinity conditions. When subjected to salt stress, fresh (FW) and dry weights (DW) of both leaves and roots decreased more significantly in WT than in TG plants. Our results demonstrated the great potential of genetic manipulation of vacuolar H⁺‐pyrophosphatase expression in TG perennial species for improvement of plant abiotic stress resistance.     

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.     

Salt tolerance of Beta macrocarpa is associated with efficient osmotic adjustment and increased apoplastic water content

The chenopod Beta macrocarpa Guss (wild Swiss chard) is known for its salt tolerance, but the mechanisms involved are still debated. In order to elucidate the processes involved, we grew wild Swiss chard exposed to three salinity levels (0, 100 and 200 mm NaCl) for 45 days, and determined several physiological parameters at the end of this time. All plants survived despite reductions in growth, photosynthesis and stomatal conductance in plants exposed to salinity (100 and 200 mm NaCl). As expected, the negative effects of salinity were more pronounced at 200 mm than at 100 mm NaCl: (i) leaf apoplastic water content was maintained or increased despite a significant reduction in leaf water potential, revealing the halophytic character of B. macrocarpa; (ii) osmotic adjustment occurred, which presumably enhanced the driving force for water extraction from soil, and avoided toxic build up of Na⁺ and Cl^– in the mesophyll apoplast of leaves. Osmotic adjustment mainly occurred through accumulation of inorganic ions and to a lesser extent soluble sugars; proline was not implicated in osmotic adjustment. Overall, two important mechanisms of salt tolerance in B. macrocarpa were identified: osmotic and apoplastic water adjustment.     

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.     

盐胁迫对不同种源麻栎叶片光合特征的影响

采用5 个不同盐浓度处理,研究了盐胁迫对山东费县、安徽滁州和四川万源3个种源地麻栎叶片光合特征的影响.结果表明：随着盐浓度的增加,3个种源的麻栎叶片叶绿素a、叶绿素b、净光合速率（P_n）均呈下降趋势,其中叶绿素a在山东种源中降幅最大,下降了21.5%～36.1%,四川种源降幅最小,在3.5%～19.8%之间.所有种源对照组的P_n均成“单峰”曲线,未出现“午休”现象;高盐浓度处理下则有明显的午休现象,经通径分析,P_n主要受饱和水汽压差和叶温的影响,气孔导度、胞间CO₂浓度和光强是次要因子.不同种源对照处理的麻栎叶片光合特征受原种源地环境因子的影响较大.麻栎的耐盐胁迫能力大小为：四川万源种源>安徽滁州种源>山东费县种源.     

Silicon increases salt tolerance by influencing the two-phase growth response to salinity in wheat (Triticum aestivum L.)

Salt usually stresses plants in two ways, osmotic stress and ion toxicity. Plant responds to salinity in two distinct phases through time. It is known that silicon (Si) could alleviate salt stress by decreasing the Na⁺ accumulated in the leaf. In order to determine the function of Si in the two-phase growth response (osmotic and ion toxicity) to salinity, we selected the wheat cultivar “Changwu 134” out of 10 wheat cultivars, and confirmed that it responds to salinity in two distinct phases through time. The fresh weight, leaf area, and leaf Na⁺ concentration were measured during 31 days of 120 mM NaCl supplemented with 1 mM Si treatment. The results revealed that the growth of plants under salinity conditions both with and without Si application were in accordance with the two-phase growth model. Si alleviated the salt stress in the both two-phase growth, but the alleviative effects were more pronounced in the osmotic stress phase than ion toxicity phase. These results clearly showed that Si can enhance plant salt tolerance by alleviating the salt-induced osmotic stress.     

Inductive responses of some organic metabolites for osmotic homeostasis in peanut (Arachis hypogaea L.) seedlings during salt stress

The salt tolerance of peanut (Arachis hypogaea L.) seedlings was evaluated by analyzing growth, nutrient uptake, electrolyte leakage, lipid peroxidation and alterations in levels of some organic metabolites under NaCl stress. The plant height, leaf area and plant biomass decreased significantly in salt-treated seedlings as compared with control. The relative water content (RWC %) of leaf decreased by 16 % at high concentrations of NaCl. There was an increase in the lipid peroxidation level and decrease in the electrolyte leakage at high concentrations of NaCl. The total free amino acid and proline contents of leaf increased by 5.5- and 43-folds, respectively in 150 mM NaCl-treated plants as compared with control. Total sugar and starch content increased significantly at high concentrations of NaCl. Chl a, Chl b, total chlorophyll and carotenoid contents decreased significantly at high salinity. Na⁺ contents of leaf, stem and root increased in dose-dependent manner. K⁺ content remained unaffected in leaf and root and decreased in stem by salinity. The results from present study reveal that the peanut plants have an efficient adaptive mechanism to tolerate high salinity by maintaining adequate leaf water status associated with growth restriction. In order to circumvent the stress resulting from high salinity, the levels of some organic metabolites such as total free amino acids, proline, total sugars and starch were elevated. The elevated levels of the organic metabolites may possibly have some role in maintenance of osmotic homeostasis, nutrient uptake and adequate tissue water status in peanut seedlings under high-salinity conditions.     

Effects of heterogeneous salinity on growth,water uptake,and tissue ion concentrations of alfalfa

Aims

Soil salinity varies greatly in the plant rhizosphere. The effect of nonuniform salinity on the growth and physiology response of alfalfa plants was determined to improve understanding of salt stress tolerance mechanisms of alfalfa.

Methods

Plant growth, predawn leaf water potential, water uptake, and tissue ionic content were studied in alfalfa plants grown hydroponically for 9 days using a split-root system, with uniform salinity or horizontally nonuniform salinity treatments (0/S, 75/S, and 150/S corresponding to 0, 75, and 150 mM NaCl on the low salt side, respectively).

Results

Compared with uniform high salinity, 0/S and 75/S treatments significantly increased the alfalfa shoot dry mass and stem extension rate. Compensatory water uptake by low salt roots of 0/S and 75/S treatments was observed. However, decreased leaf Na⁺ concentration, increased leaf K⁺/Na⁺, and compensatory growth of roots on the low salt side were observed only following the 0/S treatment.

Conclusions

Nonuniform salinity dose not enhance plant growth once a threshold NaCl concentration in low salinity growth medium has been reached. Compensation of water uptake from the low-salt root zone and regulation of K⁺/Na⁺ homeostasis in low salt root play more important role than regulation of leaf ions in enhancing alfalfa growth under nonuniform salinity.

     

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.     

Ability of leaf mesophyll to retain potassium correlates with salinity tolerance in wheat and barley

This work investigated the importance of the ability of leaf mesophyll cells to control K⁺ flux across the plasma membrane as a trait conferring tissue tolerance mechanism in plants grown under saline conditions. Four wheat (Triticum aestivum and Triticum turgidum) and four barley (Hordeum vulgare) genotypes contrasting in their salinity tolerance were grown under glasshouse conditions. Seven to 10‐day‐old leaves were excised, and net K⁺ and H⁺ fluxes were measured from either epidermal or mesophyll cells upon acute 100 mM treatment (mimicking plant failure to restrict Na⁺ delivery to the shoot) using non‐invasive microelectrode ion flux estimation (the MIFE) system. To enable net ion flux measurements from leaf epidermal cells, removal of epicuticular waxes was trialed with organic solvents. A series of methodological experiments was conducted to test the efficiency of different methods of wax removal, and the impact of experimental procedures on cell viability, in order to optimize the method. A strong positive correlation was found between plants' ability to retain K⁺ in salt‐treated leaves and their salinity tolerance, in both wheat and especially barley. The observed effects were related to the ionic but not osmotic component of salt stress. Pharmacological experiments have suggested that voltage‐gated K⁺‐permeable channels mediate K⁺ retention in leaf mesophyll upon elevated NaCl levels in the apoplast. It is concluded that MIFE measurements of NaCl‐induced K⁺ fluxes from leaf mesophyll may be used as an efficient screening tool for breeding in cereals for salinity tissue tolerance.     

Salt tolerance of prickly pear cactus (Opuntia ficus-indica)

In view of the need to exploit saline water resources in agriculture in arid zones, we investigated the salt tolerance of Opuntia ficus-indica in plants growing in solution culture. Salt (NaCl) was added in concentrations ranging from 5 (control) to 200 mol m^-3. Cladode growth was sensitive to salinity, being 60% of the control at 50 mol m^-3 NaCl. The root-to-stem ratio decreased significantly only at 200 mol m^-3. Various other parameters were studied, such as water content, Na, K and Cl content, osmotic pressure, and CO₂ uptake. Of these parameters the decreases in cladode water content and CO₂ uptake were related to the decrease in cladode growth. Raised salinity increased cladode osmotic pressure, which was associated with tissue dehydration. We concluded that osmotic adjustment does not occur in prickly pear under salt stress.