The effects of calcium sulphate on growth,membrane stability and nutrient uptake of tomato plants grown under salt stress

A pot experiment was carried out with tomato (Lycopersicon esculentum Mill.) cv. “Target F1” in a mixture of peat, perlite, and sand (1:1:1) to investigate the effects of supplementary calcium sulphate on plants grown at high NaCl concentration (75 mM). The treatments were: (i) control (C), nutrient solution alone; (ii) salt treatment (C + S), 75 mM NaCl; (iii) salt plus calcium treatment 1 (C + S + Ca1), 75 mM NaCl plus additional mixture of 2.5 mM CaSO₄ in nutrient solution; (iv) salt plus calcium treatment 2 (C + S + Ca2), 75 mM NaCl plus additional mixture of 5 mM CaSO₄ in nutrient solution. The plants grown under salt stress produced low dry matter, fruit weight, and relative water content than those grown in standard nutrient solution. Supplemental calcium sulphate added to nutrient solution containing salt significantly improved growth and physiological variables affected by salt stress (e.g. plant growth, fruit yield, and membrane permeability) and also increased leaf K⁺, Ca²⁺, and N in tomato plants. The effects of supplemental CaSO₄ in maintaining membrane permeability, increasing concentrations of Ca²⁺, N, and K⁺ and reducing concentration of Na⁺ (because of cation competition in root zone) in leaves could offer an economical and simple solution to tomato crop production problems caused by high salinity.     

Effects of salt stress on ion content,antioxidant enzymes and protein profile in different tissues of Broussonetia papyrifera

Although some plant responses to salinity have been characterized, the precise mechanisms by which salt stress damages plants are still poorly understood especially in woody plants. In the present study, the physiological and biochemical responses of Broussonetia papyrifera, a tree species of the family, Moraceae, to salinity were studied. In vitro-produced plantlets of B. papyrifera were treated with varying levels of NaCl (0, 50, 100 and 150 mM) in hydroponic culture. Changes in ion contents, accumulation of H₂O₂, as well as the activities and isoform profiles of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in the leaves, stems and roots were investigated. Under salt stress, there was higher Na⁺ accumulation in roots than in stems and leaves, and Ca^2 +, Mg^2 + and P^3 + content, as well as K⁺/Na⁺ ratio were affected. NaCl treatment induced an increase in H₂O₂ contents in the tissues of B. papyrifera. The work demonstrated that activities of antioxidant defense enzymes changed in parallel with the increased H₂O₂ and salinity appeared to be associated with differential regulation of distinct SOD and POD isoenzymes. Moreover, SDS-PAGE analysis of total proteins extracted from leaves and roots of control and NaCl-treated plantlets revealed that in the leaves salt stress was associated with decrease or disappearance of some protein bands, and induction of a new protein band after exposure to 100 and 150 mM NaCl. In contrast, NaCl stress had little effect on the protein pattern in the roots. In summary, these findings may provide insight into the mechanisms of the response of woody plants to salt stress.     

In vitro salinity tolerance of two pistachio rootstocks: Pistacia vera L. and P. atlantica Desf

Seedlings of Pistacia vera L. and Pistacia atlantica Desf. were cultured on hormone-free DKW medium supplemented with NaCl. The plants were subjected to low NaCl concentrations ranging from 0 to 80 mM for 45 days or to high salt concentrations (0, 131, and 158.5 mM for P. vera and 0, 131, and 240 mM for P. atlantica) for 25 days. Toxicity symptoms were recorded for seedlings exposed to low NaCl treatments. Plant growth, survival rates, mineral content, as well as proline and soluble sugar contents were determined and evaluated at the end of the culture period. The results indicated that low NaCl treatments yielded no instances of plant death in both species. At high salt conditions, however, significant mortality rates were noted for both species, being 22.86% at 240 mM NaCl for P. atlantica and 25.8% at 158.5 mM NaCl for P. vera. With regards to salinity effects, levels of 60 and 80 mM NaCl induced significant decreases of stem elongation and leaf number in the P. vera species. Salinities between 40 and 80 mM NaCl, however, induced a decrease in the root number of both species. The fresh weights of P. vera and P. atlantica also decreased significantly after 45 days of culture at NaCl concentrations between 40 and 80 mM and after 25 days of culture at 158.5 and 240 mM NaCl, respectively. The sodium and chloride uptake in plant organs seemed to be controlled more efficiently in P. atlantica than in P. vera. In both species, the K⁺ content was noted to undergo a significant decrease when salinity increased. While the K⁺/Na⁺ ratio was maintained above 2 at low NaCl treatments, it was sharply decreased at high NaCl conditions, suggesting a failure of K–Na selectivity mechanism. The Ca²⁺/Na⁺ ratio decreased significantly at 60 and 80 mM NaCl in P. vera and at 60 mM NaCl for P. atlantica. In both Pistacia species, high NaCl treatments (131–240 mM NaCl) induced a significant increase in proline content.     

Effects of silicon on H+-ATPase and H+-PPase activity,fatty acid composition and fluidity of tonoplast vesicles from roots of salt-stressed barley (Hordeum vulgare L.)

We investigated the effects of silicon (Si) on time-dependent changes in root tonoplast H⁺-ATPase and H⁺-PPase activities, membrane fatty acid compositions and tonoplast fluidity in two barley (Hordeum vulgare L.) cultivars differing in salt tolerance. Plants were grown in NaCl-free (control) and NaCl-supplied (60 and 120 mM, respectively) nutrient solutions with or without 1.0 mM Si. Plant roots were harvested to isolate tonoplast vesicles for assay of H⁺-ATPase and H⁺-PPase activities at days 2, 4, and 6 after treatment in the first experiment and for analysis of membrane fatty acid composition and fluidity at day 4 after treatment in the second experiment. The results showed that tonoplast H⁺-ATPase and H⁺-PPase activities in roots of salt-treated plants increased at day 2, which was more obvious at 60 mM NaCl in the salt-tolerant cultivar than in the salt-sensitive cultivar, and then decreased at day 4 and onward. These enzyme activities decreased consistently from days 2 to 6 for treatment with 120 mM NaCl. However, inclusion of 1.0 mM Si significantly enhanced both H⁺-ATPase and H⁺-PPase activities in roots of salt stressed barley, which was irrespective of NaCl level or cultivar used. The ratio of unsaturated to saturated fatty acids (U/S) increased under salt stress for both cultivars. Addition of Si to salt treatment increased the ratio of U/S in salt-tolerant cultivar but it did not in salt-sensitive cultivar compared to non-Si-amended salt treatment. Salt treatment decreased tonoplast fluidity of roots of barley significantly compared with control treatment. However, root tonoplast fluidity was significantly lower in the Si-amended salt treatment than in the non-Si-amended salt treatment. These results were in line with the previous findings that Si could help increase antioxidative defense and reduce membrane lipid oxidative damage in barley under salt stress. The possible mechanisms involved in Si-enhanced salt tolerance were discussed with respect to cell membrane integrity, stability and function in barley.     

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.     

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

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

Effect of NaCl on growth and Cd accumulation of halophyte Spartina alterniflora under CdCl2 stress

A study quantifying the effect of NaCl on growth and Cd accumulation of Spartina alterniflora subjected to Cd stress was conducted. Seedlings were cultivated in the presence of 1 or 3 mM Cd alone, or combined with NaCl (50 or 100 mM). The results showed that NaCl magnified the phytotoxicity of moderate Cd stress (1 mM Cd) on plants due to reduced levels of plant biomass, plant height, and chlorophyll a + b, while no synergistic effects were recorded under severe Cd stress (3 mM Cd). Proline and Ca^2 + accumulated along with additional NaCl under moderate Cd stress, instead of reduced or unchanged levels under severe Cd stress owing to different adoption strategies caused by NaCl under different Cd stresses. NaCl reduced the oxidative stress in Cd-treated plants through increasing levels of antioxidative enzymes (catalase (CAT) and peroxidase (POD)) under moderate Cd stress. With NaCl addition, Cd^2 + contents in S. alterniflora increased and reduced under moderate and severe Cd stress, respectively. However, total Cd^2 + amounts increased with increasing NaCl concentration due to biological dilution. NaCl improved the increase of Cd^2 + translocation factor (TF) under moderate Cd stress, indicating that NaCl might improve Cd^2 + uptake and translocation from roots to shoots, and enhance the phytoextraction of S. alterniflora on Cd; while phytostabilization of Cd under severe Cd stress may be possible due to the reduced TF. Thus, NaCl alleviated phytotoxicity caused by Cd stress through improved management of osmotic solutes and oxidative status, and affected Cd accumulations in S. alterniflora differently under moderate and severe Cd stresses.     

Simultaneous application of salicylic acid and calcium improves salt tolerance in two contrasting tomato (Solanum lycopersicum) cultivars

Soil salinity is one of the most important environmental factors responsible for serious agricultural problems. Tomato salt tolerance may be improved by genetic selection and by the use of adapted physiological tools. The aim of this study was to investigate the impact of exogenous application of salicylic acid (SA 0.01 mM) and calcium sulphate (CaSO₄ 5 mM), singly or in combination, on plant growth, photosynthetic pigments, nutritional behaviour and some metabolic parameters (total chlorophyll, carotenoids, soluble sugars, proline and lipid peroxidation) of two tomato cultivars (cv. Super Marmande and cv. Red River) exposed to salt stress (100 mM NaCl). Application of 100 mM NaCl reduced plant growth, total chlorophyll and carotenoid contents. Salt stress also induced an accumulation of Na⁺, a decrease in K⁺ and Ca^2 + concentration and root sugar level, an increase in malondialdehyde (MDA) and proline concentration. Deleterious impact of salinity was related to modification in ion content rather than modification in the plant water status. Exogenous application of SA or Ca alone improved plant behaviour in the presence of NaCl. Nevertheless, the best results in terms of growth, photosynthetic pigment concentrations and mineral nutrition (limitation of Na⁺ accumulation and maintenance of K⁺ and Ca^2 + content) were obtained in response to the combined SA + Ca treatment. Although the involved physiological parameters varied depending on the considered cultivar, our results suggest that Ca^2 + and SA may interact to reduce the stress experienced by the plant in the presence of NaCl.     

Effect of high salinity on Atriplex portulacoides: Growth,leaf water relations and solute accumulation in relation with osmotic adjustment

Atriplex (Halimione) portulacoides is a halophyte with potential interest for saline soil reclamation and phytoremediation. Here, we assess the impact of salinity reaching up to two-fold seawater concentration (0–1000 mM NaCl) on the plant growth, leaf water status and ion uptake and we evaluate the contribution of inorganic and organic solutes to the osmotic adjustment process. A. portulacoides growth was optimal at 200 mM NaCl but higher salinities (especially 800 and 1000 mM NaCl) significantly reduced plant growth. Na⁺ and Cl⁻ contents increased upon salt exposure especially in the leaves compared to the roots. Interestingly, no salt-induced toxicity symptoms were observed and leaf water content was maintained even at the highest salinity level. Furthermore, leaf succulence and high instantaneous water use efficiency (WUE_i) under high salinity significantly contributed to maintain leaf water status of this species. Leaf pressure–volume curves showed that salt-challenged plants adjusted osmotically by lowering osmotic potential at full turgor (Ψ_π¹⁰⁰) along with a decrease in leaf cell elasticity (values of volumetric modulus elasticity (ε) increased). As a whole, our findings indicate that A. portulacoides is characterized by a high plasticity in terms of salt-response. Preserving leaf hydration and efficiently using Na⁺ for the osmotic adjustment especially at high salinities (800–1000 mM NaCl), likely through its compartmentalization in leaf vacuoles, are key determinants of such a performance. The selective absorption of K⁺ over Na⁺ in concomitance with an increase in the K⁺ use efficiency also accounted for the overall plant salt tolerance.     

Biochemical indicators of salt stress in Plantago maritima: Implications for environmental stress assessment

Our study is focused on native spontaneous species of saline ecosystems Plantago maritima. Plants were cultivated at several salt concentrations (0, 50, 100, 200, 300, 400 and 500 mM NaCl) in a glass greenhouse under semi-controlled conditions. Growth parameters, water parameters and ionic status were determined and they were used as criteria to assess the response of P. maritima under a salinity gradient. Catalase, guaiacaol and ascobate peroxidase activities, total protein and proline were also determined. Our results show that P. maritima is a facultative halophyte capable of expressing its maximum growth potential at relatively low concentrations of salt (less than 3 g l⁻¹ NaCl). At high doses of salt (concentrations > 200 mM), the decrease in the growth of P. maritima is associated to a decrease in the uptake of K⁺. There is a disruption of the water intake of their organs and therefore results an invasion of the cytoplasm by Na⁺ toxic ion. However, stressed plants use K⁺ more sparingly. They invest especially in the production of biomass expressed by the dry weight of the shoots, and they use Na⁺ and proline for osmotic adjustment. The halophyte studied is able to accumulate high levels of proline in response to increasing salt concentration. The accumulation of the amino compound, mainly in roots, is interpreted as an indicator of salt tolerance. Additionally, a significant correlation between the tolerance of the plants to salinity and the activity of several antioxidant enzymes has been observed. Hence, we suggest the possibility of using these activities as a biochemical indicator for salt tolerance in P. maritima. Our study points out two types of biomarkers of salt exposure: enzymatic biomarkers in the leaves and proline content in the roots. Both did show very good correlation with salt exposure, and thus may be considered good biomarkers of exposure with a very good dose–response relationship.     

The combined effects of gibberellic acid and salinity on some antioxidant enzyme activities,plant growth parameters and nutritional status in maize plants

The combined effects of salt stress and gibberellic acid (GA₃) on plant growth and nutritional status of maize (Zea mays L. cv., DK 647 F1) were studied in a pot experiment. Treatments were (1) control (C): nutrient solution alone, (2) salt stress (S): 100 mM NaCl, (3) S + GA1: 100 mM NaCl and 50 ppm GA₃ and (4) S + GA2: 100 mM NaCl and 100 ppm GA₃. Salt stress (S) was found to reduce the total dry matter, chlorophyll content, relative water content (RWC), but to increase proline accumulation, superoxide dismutase (SOD; EC 1.15.1.1), peroxidase (POD; EC 1.11.1.7) and polyphenol oxidase (PPO; 1.10.3.1) enzyme activities and electrolyte leakage. GA₃ treatments overcame to variable extents the adverse effects of NaCl stress on the above physiological parameters. GA₃ treatments reduced the activities of enzyme in the salt-stressed plants. Salt stress reduced some macro and micronutrient concentrations but exogenous application of GA₃ increased these to levels of control treatment. Foliar application of GA₃ counteracted some of the adverse effects of NaCl salinity with the accumulation of proline which maintained membrane permeability and increased macro and micronutrient levels.     

On the mechanism of salt tolerance in olive (Olea europaea L.) under low- or high-Ca2+ supply

The effect of changes in Ca²⁺/Na⁺ ratios at the root zone has been reported in Olea europaea, a species mostly cultivated in calcareous soils. Plants were exposed to low (2.0 mM, low-Ca) or high-Ca²⁺ supply (9.0 mM, high-Ca) and supplied with 0 or 200 mM NaCl. Measurements were performed on water relations, gas exchange and photosynthetic performances, ion fluxes at whole-plant and leaf level, Na⁺ allocation at organismal level, the elemental and soluble carbohydrate concentration in the leaf. Most parameters were also measured during a period of relief from salinity stress, as Olea europaea suffers from fluctuating root zone NaCl concentrations over the whole growing season. High-Ca²⁺ supply decreased stomatal conductance, especially during the first two weeks of treatment. In response to salinity stress (i) leaf turgor potential was more severely depressed in high-Ca than in low-Ca plants, whereas net CO₂ assimilation rate and relative growth rate were unaffected by root zone Ca²⁺ concentrations (ii) high-Ca plants had a markedly superior ability to both exclude Na⁺ from the shoot and to selectively transport K⁺ over Na⁺ than low-Ca plants; (iii) both CO₂ carboxylation efficiency and maximal efficiency of PSII photochemistry (F_v/F_m) were significantly smaller in low-Ca than in high-Ca plants, likely as a result of a greater accumulation of toxic ions. Consistently, when osmotic stress was relieved by supplying plants with good quality water (relief period), both photosynthetic (+44%) and growth rates (+65%) recovered to a markedly superior degree in high-Ca than in low-Ca plants which had been previously treated with 200 mM NaCl. We conclude that (1) high-Ca²⁺ supply expose olive leaves to a more severe dehydration, but allowed to restrict both the entry and the allocation of potentially toxic ions to sensitive shoot organs; (2) a transient restriction of water-mass flow to the shoot during salinization may be of relatively minor significance in Olea europaea, which is very tolerant to drought; (3) overall salt tolerance in Olea europaea, as in most evergreen sclerophylls inhabiting Mediterranean areas, tightly depends upon the ability to reduce water uptake and transpiration during the dry/warm period and to recover photosynthetic and growth rates when low-salinity flood water is available. Therefore, data from the present experiment allow conclude that an increase in root zone Ca²⁺ concentration enhances tolerance to salinity stress in olive plants.     

Salinity-induced physiological and proteomic changes in Anabaena doliolum

This study is the first to offer information on salinity-induced inhibition of physiological variables, changes in proteome, and induction of glycolate metabolism in Anabaena doliolum. A significant reduction in O₂-evolution, carbon fixation, chlorophyll and NADPH/NADH level and increase in intracellular Na⁺ and respiration were observed following 150 mM NaCl treatment for 1 and 24 h. Interestingly, ATP content registered significant decrease after 1 h and recovery after 24 h treatment of 150 mM NaCl. Two-dimensional gel electrophoresis and MALDI-TOF MS detected a set of six proteins showing significant reproducible alterations, and homology with iron superoxide dismutase, superoxide dismutase (imported), phycocyanin alpha chain, elongation factor-Tu (EF-Tu), ribulose 1,5-bisphosphate carboxylase/oxygenase and phosphoribulokinase of Nostoc PCC7120. Increased RuBisCO and decreased carbon fixation suggested operation of glycolate metabolism. This was confirmed by accumulation of free and phospho-glyceric acid, increase in glycolate oxidase activity, glycine, serine and ammonium contents. Since peroxide generated in this pathway cannot be scavenged due to sensitivity of catalase to NaCl the organism fails to acclimatize under salt stress.     

Alleviating the adverse effects of NaCl stress in maize seedlings by pretreating seeds with salicylic acid and 24-epibrassinolide

The effects of bio-regulators salicylic acid (SA) and 24-epibrassinolide (EBL) as seed soaking treatment on the growth traits, content of photosynthetic pigments, proline, relative water content (RWC), electrolyte leakage percent (EC%), antioxidative enzymes and leaf anatomy of Zea mays L. seedlings grown under 60 or 120 mM NaCl saline stress were studied. A greenhouse experiment was performed in a completely randomized design with nine treatments [control (treated with tap water); 60 mM NaCl; 120 mM NaCl; 10^− 4 M SA; 60 mM NaCl + 10^− 4 M SA; 120 mM NaCl + 10^− 4 M SA; 10 μM EBL; 60 mM NaCl + 10 μMEBL or 120 mM NaCl + 10 μM EBL] each with four replicates. The results indicated that NaCl stress significantly reduced plant growth traits, leaf photosynthetic pigment, soluble sugars, RWC%, and activities of catalase (CAT), peroxidase (POX) as well as leaf anatomy. However, the application of SA or EBL mitigated the toxic effects of NaCl stress on maize seedlings and considerably improved growth traits, photosynthetic pigments, proline, RWC%, CAT and POX enzyme activities as well as leaf anatomy. This study highlights the potential ameliorative effects of SA or EBL in mitigating the phytotoxicity of NaCl stress in seeds and growing seedlings.     

Silicon improves salinity tolerance in wheat plants

Durum wheat (Triticum durum cv. Gediz-75) and bread wheat (Triticum aestivum cv. Izmir-85) were grown in a complete nutrient solution in a growth room to investigate effect of silicone supplied to the nutrient solution on plants grown at salt stress. The experiment was a 2 × 2 factorial arrangement with two levels of NaCl in nutrient solution, 0 and 100 mM, and two levels of silicone (Si) in nutrient solution, 0.25 and 0.50 mM, as Na₂SiO₃. The plants grown at 100 mM NaCl produced less dry matter and chlorophyll content than those without NaCl. Supplementary Si at both 0.25 and 0.5 mM ameliorated the negative effects of salinity on plant dry matter and chlorophyll content. Membrane permeability and proline content in leaves increased with addition of 100 mM NaCl and these increases were decreased with Si treatments. Sodium (Na) concentration in plant tissues increased in both leaves and roots of plants in the high NaCl treatment and Si treatments lowered significantly the concentrations of Na in both leaves and roots. Bread wheat was more tolerant to salinity than durum wheat. The accumulation of Na in roots indicates a possible mechanism whereby bread wheat copes with salinity in the rooting medium and/or may indicate the existence of an inhibition mechanism of Na transport to leaves. Concentrations of both Ca and K were lower in the plants grown at high NaCl than in those in the control treatment and these two element concentrations were increased by Si treatments in both shoots and roots but remained lower than control values in most cases.     

Exogenous nitric oxide improves seed germination in wheat against mitochondrial oxidative damage induced by high salinity

Effects of exogenous nitric oxide (NO) on starch degradation, oxidation in mitochondria and K⁺/Na⁺ accumulation during seed germination of wheat were investigated under a high salinity level. Seeds of winter wheat (Triticum aestivum L., cv. Huaimai 17) were pre-soaked with 0 mM or 0.1 mM of sodium nitroprusside (SNP, as nitric oxide donor) for 20 h just before germination under 300 mM NaCl. At 300 mM NaCl, exogenous NO increased germination rate and weights of coleoptile and radicle, but decreased seed weight. Exogenous NO also enhanced seed respiration rate and ATP synthesis. In addition, seed starch content decreased while soluble sugar content increased by exogenous NO pre-treatment, which was in accordance with the improved amylase activities in the germinating seeds. Exogenous NO increased the activities of superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6); whereas decreased the contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and superoxide anions (O₂^??) release rate in the mitochondria. Exogenous NO also decreased Na⁺ concentration while increased K⁺ concentration in the seeds thereby maintained a balance between K⁺ and Na⁺ during germination under salt stress. It is concluded that exogenous NO treatment on wheat seeds may be a good option to improve seed germination and crop establishment under saline conditions.     

Low stomatal density and reduced transpiration facilitate strawberry adaptation to salinity

Water and soil salinization are major constraints to agricultural productions because plant adaptation to hyperosmotic environments is generally associated to reduced growth and ultimately yield loss. Understanding the physiological/molecular mechanisms that link adaptation and growth is one of the greatest challenges in plant stress research since it would allow us to better define strategies to improve crop salt tolerance. In this study we attempted to establish a functional link between morphological and physiological traits in strawberry in order to identify margins to “uncouple” plant growth and stress adaptation. Two strawberry cultivars, Elsanta and Elsinore, were grown under 0, 10, 20 and 40 mM NaCl. Upon salinization Elsanta plants maintained a larger and more functional leaf area compared to Elsinore plants, which were irreversibly damaged at 40 mM NaCl. The tolerance of Elsanta was correlated with a constitutive reduced transpirational flux due to low stomatal density (173 vs. 234 stomata mm⁻² in Elsanta and Elsinore, respectively), which turned out to be critical to pre-adapt plants to the oncoming stress. The reduced transpiration rate of Elsanta (14.7 g H₂O plant⁻¹ h⁻¹) respect to Elsinore (17.7 g H₂O plant⁻¹ h⁻¹) most likely delayed the accumulation of toxic ions into the leaves, preserved tissues dehydration and consented to adjust more effectively to the hyperosmotic environment. Although we cannot rule out the contribution of other physiological and molecular mechanisms to the relatively higher tolerance of Elsanta, here we demonstrate that low stomatal density may be beneficial for cultivars prescribed to be used in marginal environments in terms of salinity and/or drought.     

Differential response of antioxidant enzymes to salinity stress in two varieties of Azolla (Azolla pinnata and Azolla filiculoides)

The activity and modulation of antioxidant components were comparatively analyzed in two varieties of Azolla (Azolla pinnata and Azolla filiculoides) under different concentrations of NaCl. The total superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity increased in A. pinnata, whereas both enzyme activities decreased in A. filiculoides. The plants of A. pinnata exposed to 30 mM NaCl contained a lower amount of Na⁺ ions and showed a lower electrolyte leakage than A. filiculoides. Our studies indicate a differential response of antioxidant enzymes in relation to salt tolerance in Azolla plants. On the basis of our comparative analysis, A. pinnata has been ranked salt tolerant as compared with A. filiculoides, which is salt sensitive.     

Enhanced tolerance to salinity following cellular acclimation to increasing NaCl levels in Medicago truncatula

Salinity is a major abiotic stress that limits plant productivity. Plants respond to salinity by switching on a coordinated set of physiological and molecular responses that can result in acclimation. Medicago truncatula is an important model legume species, thus understanding salt stress responses and acclimation in this species is of both fundamental and applied interest. The aim of this work was to test whether acclimation could enhance NaCl tolerance in calli of M. truncatula. A new protocol is described incorporating multi-step up acclimation over 0–350 mM exogenous NaCl. By the end of the experiment, calli were tolerant to 150 mM and competent for embryogenesis at 100 mM NaCl. Positive and negative linear relationships between Na⁺ and K⁺ uptake and exogenous NaCl concentration intercepted at 160 mM suggesting a Na⁺/K⁺ homeostasis. Proline level peaked at 100/150 mM whilst highest osmolarity and lowest water content occurred at 250/350 mM NaCl. The concentration of water soluble sugars was positively related to 0–250 mM NaCl whilst callus growth and embryogenesis occurred regardless of endoreduplication. Expression of genes linked to growth (WEE1), in vitro embryogenesis (SERK), salt tolerance (SOS1), proline synthesis (P5CS) and ploidy level (CCS52 and WEE1) peaked at 100/150 mM NaCl. Hence, these genes and various physiological traits except sugar levels, served as useful markers of NaCl tolerance. To our knowledge, this is the first report of a multi-step acclimation conferring tolerance to 150 mM NaCl in leaf-derived calli of M. truncatula.     

Changes in membrane-associated H+-ATPase activities and amounts in young grape plants during the cross adaptation to temperature stresses

Proton pumps make a critical contribution to the physiology of plants, although it remains unclear whether or not membrane-associated H⁺-ATPase is involved in the cross adaptation to different temperature stresses. This experiment investigated the changes in membrane-associated H⁺-ATPase activities that associated with chilling-treated plants after heat acclimation (HA, 38 °C/10 h) and with heat-treated plants after cold acclimation (CA, 8 °C/2.5 days) in annual young grape plants (Vitis vinifera L. cv. Jingxiu) using biochemical and electron microscopic cytochemical assay methods in which cerium trichloride (CeCl₃) precipitation was adopted. The results indicated that plasma membrane H⁺-ATPase activity increased as a result of both pretreatments, while V-type and F-type H⁺-ATPase activity hardly changed. Under subsequent cross temperature stresses, however, the three H⁺-ATPase types did maintain higher activity levels than that of the control. This finding suggests that either a HA or CA pretreatment may promote stability in membrane-associated H⁺-ATPase. A western-blotting assay of the plasma membrane H⁺-ATPase (P-H⁺-ATPase) indicated that the immuno-signal intensity of a 100 kDa peptide was visibly stronger in the HA and CA pretreated plants than in the control both before and after stress. This suggests that the HA- or CA-induced P-type H⁺-ATPase activation can be partly attributed to a new synthesis of the enzyme protein. Further, the results also suggested that membrane-associated H⁺-ATPase was involved in the HA-induced chilling resistance and the CA-induced thermo-tolerance in grape plants and that they had a similar regulating mechanism.