Exogenous proline effects on water relations and ions contents in leaves and roots of young olive

The ability of exogenous compatible solutes, such as proline, to counteract salt inhibitory effects was investigated in 2-year-old olive trees (Olea europaea L. cv. Chemlali) subjected to different saline water irrigation levels supplied or not with exogenous proline. Leaf water relations [relative water content (RWC), water potential], photosynthetic activity, leaf chlorophyll content, and starch contents were measured in young and old leaves. Salt ions (Na⁺, K⁺, and Ca²⁺), proline and soluble sugars contents were determined in leaf and root tissues. Supplementary proline significantly mitigated the adverse effects of salinity via the improvement of photosynthetic activity (Pn), RWC, chlorophyll and carotenoid, and starch contents. Pn of young leaves in the presence of 25 mM proline was at 1.18 and 1.38 times higher than the values recorded under moderate (SS1) and high salinity (SS2) treatments, respectively. Further, the proline supply seems to have a more important relaxing effect on the photosynthetic chain in young than in old leaves of salt-stressed olive plants. The differential pattern of proline content between young and old leaves suggests that there would be a difference between these tissues in distinguishing between the proline taken from the growing media and that produced as a result of salinity stress. Besides, the large reduction in Na⁺ accumulation in leaves and roots in the presence of proline could be due to its interference in osmotic adjustment process and/or its dilution by proline supply. Moreover, the lower accumulation of Na⁺ in proline-treated plants, compared to their corresponding salinity treatment, displayed the improved effect of proline on the ability of roots to exclude the salt ions from the xylem sap flowing to the shoot, and thus better growth rates.     

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

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

Salicylic acid mitigates salinity stress by improving antioxidant defence system and enhances vincristine and vinblastine alkaloids production in periwinkle [<Emphasis Type="Italic">Catharanthus roseus</Emphasis> (L.) G. Don]

A pot experiment was conducted to find out whether the foliar spray of salicylic acid (SA) could successfully ameliorate the adverse effects of salinity stress on periwinkle. Thirty-day-old plants were supplied with Control; 0 mM NaCl + 10⁻⁵ M SA (T₁); 50 mM NaCl + 0 SA (T₂); 100 mM NaCl + 0 SA (T₃); 150 mM NaCl + 0 SA (T₄); 50 mM NaCl + 10⁻⁵ M SA (T₅); 100 mM NaCl + 10⁻⁵ M SA (T₆); 150 mM NaCl + 10⁻⁵ M SA (T₇). The plants were sampled 90 days after sowing to assess the effect of SA on stressed and unstressed plants. Salt stress significantly reduced the growth attributes including plant height, leaf-area index, shoot and root fresh weights, shoot and root dry weights. Increasing NaCl concentrations led to a gradual decrease in photosynthetic parameters and activities of nitrate reductase and carbonic anhydrase. Ascorbic acid, total alkaloids and antioxidants enzymes superoxide dismutase, catalase and peroxidase also declined in NaCl-treated plants. The plants, undergoing NaCl stress, exhibited a significant increase in electrolyte leakage and proline content. Foliar application of SA (10⁻⁵ M) reduced the damaging effect of salinity on plant growth and accelerated the restoration of growth processes. It not only improved the growth parameters but also reversed the effects of salinity. Total alkaloid content was improved by SA application both in unstressed and stressed plants. The highest level of total alkaloid content recorded in leaves of SA-treated stressed plants was 11.1%. Foliar spray of SA overcame the adverse effect of salinity by improving the content of vincristine (14.0%) and vinblastine (14.6%) in plants treated with 100 M NaCl.     

Regulated alterations in redox and energetic status are the key mediators of salinity tolerance in the halophyte <Emphasis Type="Italic">Sesuvium portulacastrum</Emphasis> (L.) L

The present work addresses the importance of antioxidant, redox and energetic parameters in regulating salt-tolerance in Sesuvium portulacastrum. Experiments were conducted on 45 days old plants subjected to 250 and 1,000 mM NaCl stress for 2–8 days. Plants showed no significant change in growth parameters (shoot length, dry weight, and water content) at 250 mM NaCl as compared to control. However, growth of plants was significantly affected at 1,000 mM NaCl. The differential growth behaviour could be attributed to a greater decline in the energetic parameters (in terms of ratios of NADP/NADPH and ATP/ADP) at 1,000 mM NaCl than at 250 mM NaCl. The osmotic stress imposed to plants at 250 mM NaCl was presumably balanced by the accumulation of sodium ions (Na⁺), an energetically favorable process, and did not require an increased synthesis of proline. In contrast, to counter osmotic stress at 1,000 mM NaCl, plants accumulated Na⁺ as well as proline and were, therefore, energetically stressed. Further, the response of enzymatic and molecular antioxidants at 1,000 mM was either close to or even lower than that at 250 mM, which resulted in oxidative damage at 1,000 mM, particularly on longer durations. In conclusion, it is suggested that altered redox and energetic status of the plants could play a key role in mediating the tolerance of Sesuvium under salinity stress.     

Biochemical,physiological and growth changes in response to salinity in callus cultures of <Emphasis Type="Italic">Sesuvium portulacastrum</Emphasis> L.

In vitro-grown cells of Sesuvium portulacastrum L., an important ‘salt accumulator’ mangrove associate, were incubated on a medium containing different levels of salt, including 0, 100, 200, or 400 mM NaCl, in order to evaluate biochemical, physiological, and growth responses. A significant decrease in callus growth, water status, and cell membrane damage was observed under salt stress. Osmotic adjustment was revealed by the accumulation of inorganic ions, such as sodium (Na⁺), and organic osmolytes (proline, glycine betaine, and total soluble sugars) in NaCl-treated calli compared to control. However, accretion of osmolytes and inorganic ions did not support growth of calli under NaCl stress conditions. The observed reduced growth rate in calli subjected to stress, up to 200 mM NaCl, was coupled with lower catalase and ascorbate peroxidase activities and with a significantly higher superoxide dismutase activity. These findings suggested that S. portulacastrum cell cultures exhibited higher osmotic adjustment to salt stress.     

Response of barley grains to the interactive e.ect of salinity and salicylic acid

Effect of grain soaking presowing in 1 mM salicylic acid (SA) and NaCl (0, 50, 100, 150 and 200 mM) on barley (Hordeum vulgare cv Gerbel) was studied. Increasing of NaCl level reduced the germination percentage, the growth parameters (fresh and dry weight), potassium, calcium, phosphorus and insoluble sugars content in both shoots and roots of 15-day old seedlings. Leaf relative water content (RWC) and the photosynthetic pigments (Chl a, b and carotenoids) contents also decreased with increasing NaCl concentration. On the other hand, Na, soluble sugars, soluble proteins, free amino acids including proline content and lipid peroxidation level and peroxidase activity were increased in the two plant organs with increasing of NaCl level. Electrolyte leakage from plant leaves was found to increase with salinity level. SA-pretreatment increased the RWC, fresh and dry weights, water, photosynthetic pigments, insolube saccharides, phosphorus content and peroxidase activity in the stressed seedlings. On the contrary, Na⁺, soluble proteins content, lipid peroxidation level, electrolyte leakage were markedly reduced under salt stress with SA than without. Under stress conditions, SA-pretreated plants exhibited less Ca²⁺ and more accumulation of K⁺, and soluble sugars in roots at the expense of these contents in the plant shoots. Exogenous application (Grain soaking presowing) of SA appeared to induce preadaptive response to salt stress leading to promoting protective reactions to the photosynthetic pigments and maintain the membranes integrity in barley plants, which reflected in improving the plant growth.     

Effect of NaCl stress on rice physiological properties

Salinity is a major yield-reducing factor in coastal and arid irrigated rice production systems. Rice seedlings (Oryza sativa cv. Tarom Atri) were exposed to different NaCl concentrations for 8 days after germination. Plants height, fresh and dry weight, relative water content, pigment and carbohydrate content, photosynthetic efficiency and lipid peroxidase and antioxidant enzyme activity of rice seedlings grown under salt stress were investigated. Seedling grown under 25and 50 mM salt were shorter than the control. They could, however, develop their secondary leaves. Seedlings grown in the nutrient solution supplied with 100 and 200 mM extra salt could not develop their secondary leaves. Fresh weight ofseedlings grown under salt stress reduced up to 42.2% of the non-treated seedlings. Chlorophylls and carotenoids contents decreased significantly in the salt-treated seedlings. Carotenoid contents in NaCl-treated seedlings were decreased to 39.3%. No significant changes occurred in the photochemical efficiency of control and stressed plants. Increasing concentrations of NaCl resulted in increase and decrease of Na⁺ and K⁺ ions, respectively. NaCl salinity caused an increase in both peroxide content and lipid peroxidation. Seedlings which recovered for 24 h showed lower peroxide and malondialdehyde content.     

Effects of silicon on photosynthesis,water relations and nutrient uptake of <Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> under NaCl stress

A greenhouse experiment was conducted to investigate the effects of silicon application on Phaseolus vulgaris L. under two levels of salt stress (30 and 60 mM NaCl in the irrigation water). Salinity significantly reduced growth, stomatal conductance and net photosynthetic rate, and increased Na⁺ and Cl⁻ content mainly in roots. Silicon application enhanced growth of salt stressed plants, significantly reduced Na⁺ content especially in leaves and counterbalanced the effects of NaCl on gas exchange; the effect was more evident at 30 mM NaCl. Cl⁻ content in shoots and roots was not significantly modified by silicon application; the drop in K⁺ content caused by salinity was partially counterbalanced by silicon, especially in roots.     

Enhanced salt tolerance of tomatoes by exogenous salicylic acid applied through rooting medium

In Tunisia, like in the other countries of the Mediterranean, tomato is ranked among the important vegetables in the economic sphere. Tunisia ranks as the first consumer of this vegetable in the world. However, tomatoes are exposed to multiple environmental stresses. In particular, salinity is the most stressful limiting factor to productivity. Salt tolerance of the tomato is susceptible to be ameliorated by genetic and physiologic ways. Salicylic acid (SA), a plant phenolic, is now considered as a hormone-like endogenous regulator, and its role in the defense mechanisms against biotic and abiotic stressors has been well documented. So, the aim of this study was to investigate the impact of exogenous application of SA (0.01 mM) on growth, nutritional behavior, and some metabolic parameters (total chlorophyll, soluble sugars, proline, and proteins) of tomato plants cv. Moneymaker exposed to NaCl (100 mM). Our results showed that the application of 0.01 mM SA to tomato plants via root drenching attenuate the depressive effect of salinity on plants. This amelioration results in stimulation of growth and development of plant. Under stress conditions, SA-treated plants exhibited more accumulation of photosynthetic pigments and K(+) contents. Thus, SA induces an increase in soluble sugars in roots and leaves; also, we noted the increase of proteins only in roots. Overall, the adverse effects of salt stress tomato plants were alleviated by the exogenous application of SA at vegetative stage, which upregulated nutrition and the accumulation of some organic solutes and osmoprotectors such sugars, proline, and proteins. So salicylic acid can be greatly used to enhance salt tolerance of tomato plants.     

Effects of salt stress in relation to osmotic adjustment on sugarcane (<Emphasis Type="Italic">Saccharum officinarum</Emphasis> L.) callus cultures

In vitro responses of embryogenic sugarcane (Saccharum officinarum L.; cv. CoC-671) calli stressed with different levels of NaCl (0.0, 42.8, 85.6, 128.3, 171.1, 213.9 or 256.7 mM) were studied. The results showed that a significant decrease in callus growth and cell viability occurred with ≥85.6 mM NaCl. Higher amounts of free proline and glycine betaine were accumulated in NaCl-stressed calli. Although the leached and retained Na⁺ contents increased, the retained K⁺ content decreased with increasing levels of NaCl. Such a mechanism implies that sugarcane can be considered as a Na⁺-excluder. The accumulation of salt ions and osmolytes could play an important role in osmotic adjustment in sugarcane cells under salt stress.     

Physiological and biochemical traits involved in the genotypic variability to salt tolerance of Tunisian Cakile maritima

Cakile maritima (family: Brassicaceae) was collected from three provenances belonging to different bioclimatic stages (humid, semi arid and arid) in Tunisia to study their eco‐physiological and biochemical responses to salinity. Seedlings were cultivated on inert sand for 20 days under NaCl treatments (0, 100, 200, 400 mm NaCl). Plant response to salinity was provenance‐ and salt‐dependent. At 100 mm NaCl, growth parameters (leaf biomass, area, number per plant and relative growth rate) were improved in plants from Jerba (originating from arid bioclimatic stage) compared with the control, while growth was reduced in those from Tabarka (from humid area). High salt levels (400 mm NaCl) decreased the plant growth in the three provenances, but plants in Tabarka were the most salt sensitive. The relative salt tolerance of plants from Jerba and Bekalta provenances was associated with low levels of malondialdehyde as well as of electrolyte leakage and endoproteolytic activity. Salt reduced leaf hydration, the decrease in water content being dose‐dependent and more pronounced in Tabarka. Increase in salinity led to significant increase in leaf succulence and decrease in leaf water potential, especially in Jerba plants. The plants from the latter displayed the highest leaf levels of Na⁺ and Cl^?, proline, soluble carbohydrates, soluble proteins, and polyphenols. Overall, the higher salt tolerance of plants from Jerba provenance, and to a lower extent of those from Bekalta, may be partly related to their better capacity for osmotic adjustment and to limit oxidative damage when salt‐challenged.     

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.     

In vitro evaluation of some high yield potato (Solanum tuberosum L.) cultivars under imposition of salinity at the cellular and organ levels

Salinity and drought stress, which combines a lack of water and sodium toxicity, are more of the problems faced by plants and agricultural crops in newly reclaimed lands. Therefore, the direction of our research is to produce salinity-tolerant plants to increase the productivity of crops under conditions of salt stress. Potato callus was studied using different concentrations of NaCl (0.0, 50, 75, 100, 125, 150 and 200 mM). Shoot induction was obtained from callus treated with MS medium containing 4.0 and 5.0 mg l^?1 TDZ + 0.5 mg l^?1 GA3 with NaCl up to 125 mM and 150 mM for Rosetta and Victoria, respectively. When plantlets were cultured on MS medium containing 3.0 mg l^?1 kinetin and 1.0 mg l^-1paclobutrazol (PBZ) with 80 or 90 g l^?1 sucrose after two months gave a good microtuber per explant of Rosetta and Victoria cultivar which gave number of microtuber/plantlet (1.85) and (2.40) when plantlets treated with 125 mM and 150 mM NaCl of Rosetta and Victoria cultivar, respectively. In general, the results were shown in each treatment of NaCl and that amounts of proline at 125 and 150 mMNaCl were significantly more than 0.0, 50, 75 and 100 mM NaCl. This result is related to the role of proline in the osmotic adjustment of a higher concentration of salinity. The results showed that the amounts of sodium increased with increasing the salt concentration, but the amount of potassium decreased and also increased the Na+/K+ ratio with increasing the salt concentration. This research is important for in vitro potato plant regeneration, which requires optimization before genetic transformation can be achieved.     

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.     

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

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

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

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

Carbohydrate and Proline Contents in Leaves, Roots, and Apices of Salt-Tolerant and Salt-Sensitive Wheat Cultivars1

Intra-specific variations in nonstructural carbohydrates and free proline were determined in leaves, apices, roots, and maturing seeds of two salt-tolerant cultivars (CR and Kharchia-65) and one salt-sensitive cv. Ghods of spring wheat (Triticum aestivum L.) grown in sand culture at various levels of salinity (0, 100, 200, and 300 mM NaCl and CaCl₂ at 5 : 1 molar ratio) under controlled environmental conditions. The levels of leaf, apex, and root ethanol-soluble carbohydrates, fructans, starch, and proline increased in line with elevating level of salinity in all three cultivars under investigation. The contents of proline, soluble and insoluble carbohydrates in the apex increased to levels exceeding those in the leaves and roots. Soluble carbohydrate content of salt-sensitive cv. Ghods was higher in the leaves, apices, and roots and lower in the maturing seeds than in the other cultivars at all levels of salinity except at 300 mM. The results show considerable variation in the amount of soluble, insoluble sugars, and proline among plant tissues and wheat genotypes in response to salinity. Higher soluble carbohydrates and fructan in leaves, roots and maturing seeds of stressed plants indicate that their accumulation may help plant to tolerate salinity. Salt-sensitive cv. Ghods accumulated less soluble sugars in the maturing seeds and higher soluble sugars in the apices, which might be used as an indicator in screening wheat genotypes for salinity tolerance.     

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.     

Anatomical changes induced by increasing NaCl salinity in three fodder shrubs, <Emphasis Type="Italic">Nitraria retusa</Emphasis>, <Emphasis Type="Italic">Atriplex halimus</Emphasis> and <Emphasis Type="Italic">Medicago arborea</Emphasis>

Nitraria retusa and Atriplex halimus (xero-halophytes) plants were grown in the range 0–800 mM NaCl while Medicago arborea (glycophyte) in 0–300 mM NaCl. Plants were harvested after 120 days of salt-treatment. The present study was designed to study the effect of salinity on root, stem and leaf anatomy, water relationship, and plant growth in greenhouse conditions. Salinity induced anatomical changes in the roots, stems and leaves. The cuticle and epidermis of N. retusa and A. halimus stems were unaffected by salinity. However, root anatomical parameters (root cross section area, cortex thickness and stele to root area ratio), and stem anatomical parameters (stem cross section area and cortex area) were promoted at 100–200 mM NaCl. Indicating that low to moderate salinity had a stimulating effect on root and stem growth of these xero-halophytic species. At higher salinities, root and stem structures were altered significantly, and their percentages of reduction were higher in A. halimus than in N. retusa whereas, in M. arborea, they were strongly altered as salinity rose. NaCl (100–300 mM) reduced leaf water content by 21.2–56.2% and specific leaf area by 51–88.1%, while increased leaf anatomical parameters in M. arborea (e.g. increased thickness of upper and lower epidermis, palisade and spongy mesophyll, entire lamina, and increased palisade to spongy mesophyll ratio). Similar results were evidenced in A. halimus leaves with salinity exceeding 100 mM NaCl. Leaves of N. retusa were thinner in salt-stressed plants while epidermis thickness and water content was unaffected by salinity. The size of xylem vessel was unchanged under salinity in the leaf’s main vein of the three species while we have increased number in M. arborea leaf main vein in the range of 200–300 mM NaCl. A longer distance between leaf vascular bundle, a reduced size and increased number of xylem vessel especially in stem than in root vascular system was evidenced in M. arborea treated plants and only at (400–800 mM) in the xero-halophytic species. The effects of NaCl toxicity on leaf, stem and root ultrastructure are discussed in relation to the degree of salt resistance of these three species. Our results suggest that both N. retusa and A. halimus show high tolerance to salinity while M. arborea was considered as a salt tolerant species.     

Physiological and antioxidant responses of <Emphasis Type="Italic">Mentha pulegium</Emphasis> (Pennyroyal) to salt stress

Mentha pulegium L. is a medicinal and aromatic plant belonging to the Labiatae family present in the humid to the arid bioclimatic regions of Tunisia. We studied the effect of different salt concentrations on plant growth, mineral composition and antioxidant responses. Physiological and biochemical parameters were assessed in the plant organs after 2 weeks of salt treatment with 25, 50, 75 and 100 mM NaCl. Results showed that, growth was reduced even by 25 mM, and salt effect was more pronounced in shoots (leaves and stems) than in roots. This growth decrease was accompanied by a restriction in tissue hydration and K⁺ uptake, as well as an increase in Na⁺ levels in all organs. Considering the response of antioxidant enzymes to salt, leaves and roots reacted differently to saline conditions. Leaf and root guaiacol peroxidase activity showed an increase by different concentration of NaCl, but superoxide dismutase activity in the same organs showed a slight modification in NaCl-treated leaves and roots. Moreover, polyphenol contents and antioxidant activity were analysed in M. pulegium leaves and roots under salt constraint. The analysis showed an increase of total polyphenol content (2.41–8.17 mg gallic acid equivalent g⁻¹ dry weight) in leaves. However, methanol extract of leaves at 100 mM NaCl displayed the highest DPPH· scavenging ability with the lowest IC₅₀ value (0.27 μg ml⁻¹) in comparison with control which exhibited IC₅₀ equal to 0.79 μg ml⁻¹.