Salt accumulation and depletion in the root-zone of the halophyte Atriplex nummularia Lindl.: influence of salinity,leaf area and plant water use

Background and aims

Salt is known to accumulate in the root-zone of Na⁺ excluding glycophytes under saline conditions. We examined the effect of soil salinity on Na⁺ and Cl^? depletion or accumulation in the root-zone of the halophyte (Atriplex nummularia Lindl).

Methods

A pot experiment was conducted in soil to examine Na⁺ and Cl^? concentrations adjacent to roots at four initial NaCl treatments (20, 50, 200 or 400 mM NaCl in the soil solution). Plant water use was manipulated by leaving plants with all leaves intact, removing approximately 50 % of leaves, or removing all leaves. Daily evapotranspiration was replaced by watering undrained pots to weight with deionised water. After 35-38 days, samples were taken of the bulk soil and of soil loosely- and closely-adhering to the roots.

Results

In plants with leaves intact grown with 200 and 400 mM NaCl, average Na⁺ and Cl^? concentrations in the closely adhering soil were about twice the concentrations of the bulk soil. Ion accumulation increased with final leaf area and with cumulative transpiration over the duration of the trial. By contrast, in plants grown with the lowest salinity treatment (20 mM NaCl), Na⁺ and Cl^? concentrations decreased in the closely adhering soil with increasing leaf area and increasing cumulative water use.

Conclusions

Our data show that Na⁺ and Cl^? are depleted from the root-zone of A. nummularia at low salinity but accumulate in the root-zone at moderate to high salinity, and that the ions are drawn towards the plant in the transpiration stream.     

Effects of NO 3 − -N on the growth and salinity tolerance of Tamarix laxa Willd

The influence of NO ₃ ^? -N on growth and osmotic adjustment was studied in Tamarix laxa Willd., a halophyte with salt glands on its twigs. Seedlings of T. laxa Willd. were exposed to 1 mM (control) or 300 mM NaCl, with 0.05, 1 or 10 mM NO ₃ ^? -N for 24 days. The relative growth rate of seedlings at 300 mM NaCl was lower than that of control plants at all NO ₃ ^? -N levels, but the concentrations of organic N and total N in the twigs did not differ between the two NaCl treatments. Increasing NO ₃ ^? supply under 300 mM NaCl improved the growth of T. laxa, indicating that NO ₃ ^? played positive roles in improving salt resistance of the plant. The twigs of T. laxa Willd. accumulated mainly inorganic ions, especially Na⁺ and Cl^?, to lower osmotic potential (Ψs): the contributions of Na⁺ and Cl^? to Ψs were estimated at 31% and 27% respectively, at the highest levels of supply of both NaCl and NO ₃ ^? -N. The estimated contribution of NO ₃ ^? -N to Ψs was as high as 20% in the twigs in these conditions, indicating that NO ₃ ^? was also involved in osmotic adjustment in the twigs. Furthermore, increases in tissue NO ₃ ^? were accompanied by decreases in tissue Cl^? and proline under 300 mM NaCl. The estimated contribution of proline to Ψs declined as with NO ₃ ^? -N supply increased from 1 to 10 mM, while the contributions of nitrate to Ψs were enhanced under 300 mM NaCl. This suggested that higher accumulation of nitrate in the vacuole alleviated the effects of salinity stress on the plant by balancing the osmotic potential. In conclusion, NO ₃ ^? -N played both nutritional and osmotic roles in T. laxa Willd. in saline conditions.     

Assessment of the preventive effect of vermicompost on salinity resistance in tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis> cv. Ailsa Craig)

To determine the effects of vermicompost leachate (VCL) on resistance to salt stress in plants, young tomato seedlings (Solanum lycopersicum, cv. Ailsa Craig) were exposed to salinity (150 mM NaCl addition to nutrient solution) for 7 days after or during 6 mL L^??1 VCL application. Salt stress significantly decreased leaf fresh and dry weights, reduced leaf water content, significantly increased root and leaf Na⁺ concentrations, and decreased K⁺ concentrations. Salt stress decreased stomatal conductance (g_s), net photosynthesis (A), instantaneous transpiration (E), maximal efficiency of PSII photochemistry in the dark-adapted state (F_v/F_m), photochemical quenching (qP), and actual PSII photochemical efficiency (ΦPSII). VCL applied during salt stress increased leaf fresh weight and g_s, but did not reduce leaf osmotic potential, despite increased proline content in salt-treated plants. VCL reduced Na⁺ concentrations in leaves (by 21.4%), but increased them in roots (by 16.9%). VCL pre-treatment followed by salt stress was more efficient than VCL concomitant to salt stress, since VCL pre-treatment provided the greatest osmotic adjustment recorded, with maintenance of net photosynthesis and K⁺/Na⁺ ratios following salt stress. VCL pre-treatment also led to the highest proline content in leaves (50 µmol g^??1 FW) and the highest sugar content in roots (9.2 µmol g^??1 FW). Fluorescence-related parameters confirmed that VCL pre-treatment of salt-stressed plants showed higher PSII stability and efficiency compared to plants under concomitant VCL and salt stress. Therefore, VCL represents an efficient protective agent for improvement of salt-stress resistance in tomato.     

Contributions of inorganic ions,soluble carbohydrates,and multiatomic alcohols to water homeostasis in <Emphasis Type="Italic">Artemisia lerchiana</Emphasis> and <Emphasis Type="Italic">A. pauciflora</Emphasis>

Two morphological forms of wormwood Artemisia lerchiana (f. erecta and f. nutans) and A. pauciflora Web. (morphological form erecta) were grown on sand culture at a range of NaCl concentrations in the nutrient medium and then assayed for Na⁺, K⁺, and Cl^? content in various organs. In addition, the content of mono-, di-, and trisaccharides and multiatomic alcohols (mannitol and glycerol); water content; and organ biomass were determined. All plants examined showed high NaCl tolerance, comparable to that of halophytes. They were able to maintain high tissue hydration under conditions of salinity-induced growth suppression. The intracellular osmotic pressure in wormwood organs was mainly determined by the presence of Na⁺, K⁺, and Cl^?, as well as by mono-, di-, and trisaccharides, mannitol, and glycerol. The high content of Na⁺ and Cl^? in wormwood organs was also observed in the absence of salinity, which implies the ability of these organs to absorb ions from diluted NaCl solutions and accumulate ions in cells of their tissues. With the increase in salinity, the content of Na⁺ and Cl^? in roots and leaves increased to even higher levels. It is concluded that the ability of wormwood plants to absorb and accumulate inorganic ions provides for sustainable high intracellular osmotic pressure and, accordingly, low water potential under drought and salinity conditions. Growing plants under high salinity lowered the content of monosaccharides in parallel with accumulation of the trisaccharide raffinose. It is supposed that soluble carbohydrates and multiatomic alcohols are not only significant for osmoregulation but also perform a protective function in wormwood plants. The lower osmotic pressure in root cells compared to that in leaf cells of all plants examined was mainly due to the gradient distribution of K⁺ and Cl^? between roots and leaves. The two Artemisia species and two morphological forms of A. lerchiana did not differ appreciably in the ways of water balance regulation. It is found that different morphologies of two A. lerchiana forms are unrelated to variations in intracellular osmotic and turgor pressures.     

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.

     

Sodium and Chloride Sensitivity in Alfalfa (Medicago sativa L.): Growth,Photosynthesis, and Tissue Ion Regulation in Contrasting Genotypes

Alfalfa (Medicago sativa L.) is a moderately salt-tolerant plant. This study was conducted to evaluate responses of two contrasting alfalfa genotypes (OMA-84-salt sensitive and OMA-285-salt-tolerant) to components (Na⁺, and/or Cl^?) of salt stress. Alfalfa genotypes were exposed to Na⁺???salts (without chloride), Cl^????salts (without sodium), and NaCl (sodium chloride) stresses with two concentrations (30 and 150 mM). The treatments, involving macronutrients, with the same osmotic potentials, were taken as control. Salt stress, irrespective of type and intensity, caused a significant reduction in plant biomass, physiological (net photosynthetic rate, photosystem II efficiency, chlorophyll fluorescence, water use efficiency, maximum yield of primary photochemistry, and electron transport rate), and shoot mineral (calcium, magnesium, phosphorus, and potassium) contents compared to control; however, this reduction was in the order of NaCl (150 mM)?>?Na⁺ (150 mM)?>?Cl^? (150 mM). The alfalfa genotype OMA-285 sustained growth under both types of salt stresses than the genotype OMA-84 due to less accumulation of Na⁺ and Cl^? ions, maintenance of higher K⁺/Na⁺ ratio, and better photosynthetic activities. In conclusion, salt stress caused a significant reduction in alfalfa growth, this reduction was more under NaCl stress and the effect was mainly additive. The alfalfa genotype OMA-285 sustained growth under salt stresses than the genotype OMA-84 due to ionic homeostasis. However, the tested genotypes were more sensitive to Na⁺ toxicity than the Cl^? toxicity, and the contrasting genotypes differed in tissue tolerance of high Na⁺ and Cl^?. Further research is needed to evaluate tissue tolerance in a diverse and large group of alfalfa genotypes to elucidate the general salt tolerance mechanism in alfalfa.

     

Indigenous salt-tolerant rhizobacterium <Emphasis Type="Italic">Pantoea dispersa</Emphasis> (PSB3) reduces sodium uptake and mitigates the effects of salt stress on growth and yield of chickpea

Salt stress has multiple damaging effects on plants including physiological damage, reduced growth, and productivity. Plant growth-promoting rhizobacteria (PGPR) are one of the valuable options to mitigate the negative effects of this stress. In the present study, native bacteria from chickpea’s rhizosphere were isolated, and checked for their salt tolerance and plant growth-promoting attributes (phosphate (P) solubilization, siderophores, indole-3-acetic acid (IAA) production, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase production). One isolate, subsequently identified as Pantoea dispersa, showed appreciable production of IAA (218.3 µg/ml) and siderophores (60.33% SU), P-solubilization (3.64 µg/ml) and ACC deaminase activity (207.45 nmol/mg/h) in the presence of 150 mM NaCl, under laboratory conditions. Salt stress in uninoculated chickpea (GPF2 cultivar) plants induced high accumulation of Na⁺ ions (3.86 mg g^?1 dw) in the leaves, along with significant reduction in K⁺ uptake, membrane integrity, chlorophyll concentration, and leaf water content, thus resulting in impaired growth of the plant and yield (pods and seeds) in a salt concentration-dependent manner. The damage due to salt stress was restored significantly in plants inoculated with P. dispersa. A significant improvement in biomass (32–34%), pods number (31–34.5%), seeds number (32–35.7%), pods weight (30–32.6%), and seeds weight (27–35%) per plant occurred in salt stress-affected plants, which was associated with significant reduction in Na⁺ uptake, reduced membrane damage, significantly improved leaf water content, chlorophyll content, and K⁺ uptake. This study suggests for the first time that native P. dispersa strain PSB3 can be used to alleviate the negative effects of salt stress on chickpea plants and holds the potential to be used as a biofertilizer.     

Ecophysiological response of Crambe maritima to airborne and soil-borne salinity

Background and Aims

There is a need to evaluate the salt tolerance of plant species that can be cultivated as crops under saline conditions. Crambe maritima is a coastal plant, usually occurring on the driftline, with potential use as a vegetable crop. The aim of this experiment was to determine the growth response of Crambe maritima to various levels of airborne and soil-borne salinity and the ecophysiological mechanisms underlying these responses.

Methods

In the greenhouse, plants were exposed to salt spray (400 mm NaCl) as well as to various levels of root-zone salinity (RZS) of 0, 50, 100, 200 and 300 mm NaCl during 40 d. The salt tolerance of Crambe maritima was assessed by the relative growth rate (RGR) and its components. To study possible salinity effects on the tissue and cellular level, the leaf succulence, tissue Na⁺ concentrations, Na⁺ : K⁺ ratio, net K⁺/Na⁺ selectivity, N, P, K⁺, Ca²⁺, Mg²⁺, proline, soluble sugar concentrations, osmotic potential, total phenolics and antioxidant capacity were measured.

Key Results

Salt spray did not affect the RGR of Crambe maritima. However, leaf thickness and leaf succulence increased with salt spray. Root zone salinities up to 100 mm NaCl did not affect growth. However, at 200 mm NaCl RZS the RGR was reduced by 41 % compared with the control and by 56 % at 300 mm NaCl RZS. The reduced RGR with increasing RZS was largely due to the reduced specific leaf area, which was caused by increased leaf succulence as well as by increased leaf dry matter content. No changes in unit leaf rate were observed but increased RZS resulted in increased Na⁺ and proline concentrations, reduced K⁺, Ca²⁺ and Mg²⁺ concentrations, lower osmotic potential and increased antioxidant capacity. Proline concentrations of the leaves correlated strongly (r = 0·95) with RZS concentrations and not with plant growth.

Conclusions

Based on its growth response, Crambe maritima can be classified as a salt spray tolerant plant that is sensitive to root zone salinities exceeding 100 mm NaCl.     

Sodium,potassium, chloride and proline concentrations of chloroplasts isolated from a halophyte,Mesembryanthemum crystallinum L.

Concentrations of four major solutes (Na⁺, K⁺, Cl^-, proline) were determined in isolated, intact chloroplasts from the halophyte Mesembryanthemum crystallinum L. following long-term exposure of plants to three levels of NaCl salinity in the rooting medium. Chloroplasts were obtained by gentle rupture of leaf protoplasts. There was either no or only small leakage of inorganic ions from the chloroplasts to the medium during three rapidly performed washing steps involving precipitation and re-suspension of chloroplast pellets. Increasing NaCl salinity of the rooting medium resulted in a rise of Na⁺ und Cl^- in the total leaf sap, up to approximately 500 and 400 mM, respectively, for plants grown at 400 mM NaCl. However, chloroplast levels of Na⁺ und Cl^- did not exceed 160–230 and 40–60 mM, respectively, based upon a chloroplast osmotic volume of 20–30 l per mg chlorophyll. At 20 mM NaCl in the rooting medium, the Na⁺/K⁺ ratio of the chloroplasts was about 1; at 400 mM NaCl the ratio was about 5. Growth at 400 mM NaCl led to markedly increased concentrations of proline in the leaf sap (8 mM) compared with the leaf sap of plants grown in culture solution without added NaCl (proline 0.25 mM). Although proline was fivefold more concentrated in the chloroplasts than in the total leaf sap of plants treated with 400 mM NaCl, the overall contribution of proline to the osmotic adjustment of chloroplasts was small. The capacity to limit chloroplast Cl^- concentrations under conditions of high external salinity was in contrast to an apparent affinity of chloroplasts for Cl^- under conditions of low Cl^- availability.Abbreviation Chl chlorophyll     

Effect of salinity on osmotic adjustment,proline accumulation and possible role of ornithine-δ-aminotransferase in proline biosynthesis in <Emphasis Type="Italic">Cakile maritima</Emphasis>

The short time response to salt stress was studied in Cakile maritima. Plants were exposed to different salt concentrations (0, 100, 200 and 400 mM NaCl) and harvested after 4, 24, 72 and 168 h of treatment. Before harvesting plants, tissue hydration, osmotic potential, inorganic and organic solute contents, and ornithine-δ-aminotransferase activity were measured. Plants of C. maritima maintained turgor and tissue hydration at low osmotic potential mainly at 400 mM NaCl. The results showed that, in leaves and stems, Na⁺ content increased significantly after the first 4 h of treatment. However, in roots, the increase of Na⁺ content remained relatively unchanged with increasing salt. The K⁺ content decreased sharply at 200 and 400 mM NaCl with treatment duration. This decrease was more pronounced in roots. The content of proline and amino acids increased with increasing salinity and treatment duration. These results indicated that the accumulation of inorganic and organic compounds was a central adaptive mechanism by which C. maritima maintained intracellular ionic balance under saline conditions. However, their percentage contribution to total osmotic adjustment varies from organ to organ; for example, Na⁺ accumulation mainly contributes in osmotic adjustment of stem tissue (60%). Proline contribution to osmotic adjustment reached 36% in roots. In all organs, proline as well as δ-OAT activity increased with salt concentration and treatment duration. Under normal growth conditions, δ-OAT is mainly involved in the mobilization of nitrogen required for plant growth. However, the highly significant positive correlation between proline and δ-OAT activity under salt-stress conditions suggests that ornithine pathway contributed to proline synthesis.     

Physiological and proteomic analysis of salinity tolerance of the halotolerant cyanobacterium <Emphasis Type="Italic">Anabaena</Emphasis> sp

The halotolerant cyanobacterium Anabaena sp was grown under NaCl concentration of 0, 170 and 515 mM and physiological and proteomic analysis was performed. At 515 mM NaCl the cyanobacterium showed reduced photosynthetic activities and significant increase in soluble sugar content, proline and SOD activity. On the other hand Anabaena sp grown at 170 mM NaCl showed optimal growth, photosynthetic activities and comparatively low soluble sugar content, proline accumulation and SOD activity. The intracellular Na⁺ content of the cells increased both at 170 and 515 mM NaCl. In contrast, the K⁺ content of the cyanobacterium Anabaena sp remained stable in response to growth at identical concentration of NaCl. While cells grown at 170 mM NaCl showed highest intracellular K⁺/Na⁺ ratio, salinity level of 515 mM NaCl resulted in reduced ratio of K⁺/Na⁺. Proteomic analysis revealed 50 salt-responsive proteins in the cyanobacterium Anabaena sp under salt treatment compared with control. Ten protein spots were subjected to MALDI-TOF–MS/MS analysis and the identified proteins are involved in photosynthesis, protein folding, cell organization and energy metabolism. Differential expression of proteins related to photosynthesis, energy metabolism was observed in Anabaena sp grown at 170 mM NaCl. At 170 mM NaCl increased expression of photosynthesis related proteins and effective osmotic adjustment through increased antioxidant enzymes and modulation of intracellular ions contributed to better salinity tolerance and optimal growth. On the contrary, increased intracellular Na⁺ content coupled with down regulation of photosynthetic and energy related proteins resulted in reduced growth at 515 mM NaCl. Therefore reduced growth at 515 mM NaCl could be due to accumulation of Na⁺ ions and requirement to maintain higher organic osmolytes and antioxidants which is energy intensive. The results thus show that the basis of salt tolerance is different when the halotolerant cyanobacterium Anabaena sp is grown under low and high salinity levels.     

The ecological adaptability of <Emphasis Type="Italic">Phragmites australis</Emphasis> to interactive effects of water level and salt stress in the Yellow River Delta

Soil salinity and waterlogging are two major environmental problems in estuarine wetlands. To prevent the typical wetland plants from degradation by soil salinization and salt waterlogging and more effectively use the plants to provide wetland ecosystem services, we examined the ecological adaptability of Phragmites australis, a characteristic plant species in the Yellow River Delta, to the interactive effects of water level and salt stress. The results showed that P. australis adapts to salt and water table stressed environments through slowing down the growth rate, maintaining the tiller number, and adjusting the biomass allocation of different organs. The highest plant height and the largest leaf area were at 0 cm water table treatment; the 0.5 % NaCl treatment increased the aboveground biomass; higher water table increased the fibrous root biomass allocation, but largely decreased the leaf biomass. The exclusion of toxic inorganic ions such as Na⁺ and Cl^? and the accumulation of organic solutes are also important mechanisms to aid survival in saline wetlands. On average 35.1 % of Cl^? and 53.9 % of Na⁺ accumulated in belowground organs. The study could provide fundamental guidance for wetland restoration projects and wetland sustainable use in coastal zones such as the Yellow River Delta.     

Salt-induced changes in photosynthetic activity and growth in a potential medicinal plant Bishop’s weed (<Emphasis Type="Italic">Ammi majus</Emphasis> L.)

Sixty seven-days-old plants of Ammi majus L. were subjected for 46 d to sand culture at varying concentrations of NaCl, i.e. 0 (control), 40, 80, 120, and 160 mM. Increasing salt concentrations caused a significant reduction in fresh and dry masses of both shoots and roots as well as seed yield. However, the adverse effect of salt was more pronounced on seed yield than biomass production at the vegetative stage. Calculated 50 % reduction in shoot dry mass occurred at 156 mM (ca.15.6 mS cm^?1), whereas that in seed yield was at 104 mM (ca.10.4 mS cm^?1). As in most glycophytes, Na⁺ and Cl^? in both shoots and roots increased, whereas K⁺ and Ca²⁺ decreased consistently with the successive increase in salt level of the growth medium. Plants of A. majusmaintained markedly higher K⁺/Na⁺ ratios in the shoots than those in the roots, and the ratio remained more than 1 even at the highest external salt level (160 mM). Net photosynthetic (P_N) and transpiration (E) rates remained unaffected at increasing NaCl, and thus these attributes had a negative association with salt tolerance of A. majus. Proline content in the shoots increased markedly at the higher concentrations of salt. Essential oil content in the seed decreased consistently with increase in external salt level. Overall, A. majusis a moderately salt tolerant crop whose response to salinity is associated with maintenance of high shoot K⁺/Na⁺ ratio and accumulation of proline in shoots, but P_N had a negative association with the salt tolerance of this crop.     

Modulation of Na<Superscript>+</Superscript>/Mg<Superscript>2+</Superscript> exchanger stoichiometry ratio by Cl<Superscript>−</Superscript> ions in basolateral rat liver plasma membrane vesicles

The Na⁺/Mg²⁺ exchanger represents the main Mg²⁺ extrusion mechanism operating in mammalian cells including hepatocytes. We have previously reported that this exchanger, located in the basolateral domain of the hepatocyte, promotes the extrusion of intravesicular trapped Mg²⁺ for extravesicular Na⁺ with ratio 1. This electrogenic exchange is supported by the accumulation of tetraphenyl-phosphonium within the vesicles at the time when Mg²⁺ efflux occurs. In this present study, the role of extra- and intra-vesicular Cl^? on the Na⁺/Mg²⁺ exchange ratio was investigated. The results reported here suggest that Cl^? ions are not required for the Na⁺ to Mg²⁺ exchange to occur, but the stoichiometry ratio of the exchanger switches from electrogenic (1Na _in ⁺ :1 Mg _out ²⁺ ) in the presence of intravesicular Cl^? to electroneutral (2Na _in ⁺ :1 Mg _out ²⁺ ) in their absence. In basolateral liver plasma membrane vesicles loaded with MgCl₂ labeled with ³⁶Cl^?, a small but significant Cl^? efflux (~30 nmol Cl^?/mg protein/1 min) is observed following addition of NaCl or Na-isethionate to the extravesicular medium. Both Cl^? and Mg²⁺ effluxes are inhibited by imipramine but not by amiloride, DIDS, niflumic acid, bumetanide, or furosemide. In vesicles loaded with Mg-gluconate and stimulated by Na-isethionate, an electroneutral Mg²⁺ extrusion is observed. Taken together, these results suggest that the Na⁺/Mg²⁺ exchanger can operate irrespective of the absence or the presence of Cl^? in the extracellular or intracellular environment. Changes in trans-cellular Cl^? content, however, can affect the modus operandi of the Na⁺/Mg²⁺ exchanger, and consequently impact "cellular" Na⁺ and Mg²⁺ homeostasis as well as the hepatocyte membrane potential.     

Methyl jasmonate regulated diploid and tetraploid black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) tolerance to salt stress

Methyl jasmonate (MeJA) is an essential and promising plant growth regulation factor that can improve plant development and growth. Here, we explored the mechanism by which MeJA regulates the tolerance of black locust (Robinia pseudoacacia L.) to salt stress. In this study, diploid and tetraploid R. pseudoacacia were subjected to three treatments: 500 mM NaCl; 100 μM MeJA; and 500 mM NaCl and 100 μM MeJA, and the changes in plant growth, endogenous MeJA levels and the anti-oxidative metabolism of leaves were investigated. The results showed that salt stress significantly inhibited plant growth and induced the accumulation of Na⁺ and Cl^? ions, malondialdehyde (MDA) content and reactive oxygen species. However, these adverse effects could be alleviated by applying MeJA, which was followed by a marked increase in the activities of antioxidant enzymes. In addition, some genes encoding several antioxidant enzymes were also up-regulated. Simultaneously, the endogenous MeJA content in MeJA-treated plants was lower than in salt-treated plants. It is noteworthy that tetraploids always possessed higher salt tolerance and obtained greater positive effects from MeJA than diploids. These results suggested that MeJA might play a protective role in defense responses, enabling diploid and tetraploid black locust, especially tetraploid, to better tolerate the adverse effects of salt stress.     

Differential growth and yield responses of salt-tolerant and susceptible rice cultivars to individual (Na<Superscript>+</Superscript> and Cl<Superscript>−</Superscript>) and additive stress effects of NaCl

Negative impacts exerted by sodium (Na⁺) and chloride (Cl^?) ions individually as well their possible additive effects (under NaCl) were evaluated on growth and yield reductions in rice, besides investigating whether salt-tolerant genotypes respond differentially than their sensitive counterparts. Though both Na⁺ and Cl^? ions get accumulated in plant tissues under NaCl stress, most research has historically been aimed to decipher harmful effects induced by Na⁺ ions. Accordingly, physiological and molecular mechanisms involved in Cl^? toxicity are not clearly understood in crop plants. To address these issues, 65-day-old plants of two rice cultivars, Panvel-3 (tolerant) and Sahyadri-3 (sensitive) were subjected to Cl^?, Na⁺ and NaCl (each with 100 mM concentration and electrical conductivity of ≈10 dS m^?1) stress using soil-based systems. Stress conditions were maintained till harvesting of mature (128-day-old) plants. All three treatments induced substantial antagonistic effects on growth, dry mass, yield components (number of grains per panicle, length, width, thickness and weight of grain, along with the percentage of grains filled) and overall crop yield, with greater impacts under NaCl than its constituent ions. Salinity treatments caused an imbalance in reducing sugars, protein, starch and proline contents, with the greatest magnitude under NaCl. A negative correlation between Cl^?/Na⁺ accumulation and crop yield was witnessed, with higher severity on the sensitive cultivar. The overall magnitude of toxicity was observed highest in NaCl followed by Na⁺ and Cl^?, respectively, suggesting additive effects of constituent ions under NaCl. Both cultivars responded similarly; however, the tolerant cultivar, unlike the sensitive one, kept Na⁺:K⁺ ratio <1.0 and accumulated proline in response to salinity treatments used in this study.     

<Emphasis Type="Italic">OsNHX2</Emphasis>, an Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporter gene,can enhance salt tolerance in rice plants through more effective accumulation of toxic Na<Superscript>+</Superscript> in leaf mesophyll and bundle sheath cells

The Na⁺/H⁺ antiporters play an important role in salt tolerance in plants. However, the functions of OsNHXs in rice except OsNHX1 have not been well studied. Using the gain- and loss-of-function strategies, we studied the potential role of OsNHX2 in salt tolerance in rice. Overexpression of OsNHX2 (OsNHX2-OE) in rice showed the significant tolerance to salt stress than wild-type plants and OsNHX2 knockdown transgenic plants (OsNHX2-KD). Under salt treatments of 300-mM NaCl for 5 days, the plant fresh weights, relative water percentages, shoot heights, Na⁺ contents, K⁺ contents, and K⁺/Na⁺ ratios in leaves of OsNHX2-OE transgenic plants were higher than those in wild-type plants, while no differences were detected in roots. K⁺/Na⁺ ratios in rice leaf mesophyll cells and bundle sheath cells were higher in OsNHX2-OE transgenic plants than in wild-type plants and OsNHX2-KD transgenic plants. Our data indicate that OsNHX2 plays an important role in salt stress based on leaf mesophyll cells and bundle sheath cells and can be served in genetically engineering crop plants with enhanced salt tolerance.