Salt stress‐induced seedling growth inhibition coincides with differential distribution of serotonin and melatonin in sunflower seedling roots and cotyledons

Indoleamines regulate a variety of physiological functions during the growth, morphogenesis and stress‐induced responses in plants. Present investigations report the effect of NaCl stress on endogenous serotonin and melatonin accumulation and their differential spatial distribution in sunflower (Helianthus annuus) seedling roots and cotyledons using HPLC and immunohistochemical techniques, respectively. Exogenous serotonin and melatonin treatments lead to variable effect on hypocotyl elongation and root growth under NaCl stress. NaCl stress for 48 h increases endogenous serotonin and melatonin content in roots and cotyledons, thus indicating their involvement in salt‐induced long distance signaling from roots to cotyledons. Salt stress‐induced accumulation of serotonin and melatonin exhibits differential distribution in the vascular bundles and cortex in the differentiating zones of the primary roots, suggesting their compartmentalization in the growing region of roots. Serotonin and melatonin accumulation in oil body rich cells of salt‐treated seedling cotyledons correlates with longer retention of oil bodies in the cotyledons. Present investigations indicate the possible role of serotonin and melatonin in regulating root growth during salt stress in sunflower. Effect of exogenous serotonin and melatonin treatments (15 μM) on sunflower seedlings grown in the absence or presence of 120 mM NaCl substantiates their role on seedling growth. Auxin and serotonin biosynthesis are coupled to the common precursor tryptophan. Salt stress‐induced root growth inhibition, thus pertains to partial impairment of auxin functions caused by increased serotonin biosynthesis. In seedling cotyledons, NaCl stress modulates the activity of N‐acetylserotonin O‐methyltransferase (HIOMT; EC 2.1.1.4), the enzyme responsible for melatonin biosynthesis from N‐acetylserotonin.     

A novel fluorescence imaging approach to monitor salt stress‐induced modulation of ouabain‐sensitive ATPase activity in sunflower seedling roots

Seedlings exposed to salt stress are expected to show modulation of intracellular accumulation of sodium ions through a variety of mechanisms. Using a new methodology, this work demonstrates ouabain (OU)‐sensitive ATPase activity in the roots of sunflower seedlings subjected to salt stress (120 mM NaCl). 9‐Anthroylouabain (a derivative of ouabain known to inhibit Na⁺,K⁺‐ATPase activity in animal systems, EC 3.6.3.9) has been used as a probe to analyze OU‐sensitive ATPase activity in sunflower (Helianthus annuus) seedling roots by spectrofluorometric estimation and localization of its spatial distribution using confocal laser scanning microscopy. Salt stress for 48 h leads to a significant induction of OU‐sensitive ATPase activity in the meristematic region of the seedling roots. Calcium ions (10 mM) significantly inhibit enzyme activity and a parallel accumulation of sodium ions in the cytosol of the columella cells, epidermis and in the cells of the meristematic region of the roots is evident. As a rapid response to NaCl stress, the activity of OU‐sensitive ATPase gets localized in the nuclear membrane of root protoplasts and it gets inhibited after treatment with calcium ions. Nuclear membrane localization of the OU‐sensitive ATPase activity highlights a possible mechanism to efflux sodium ions from the nucleus. Thus, a correlation between OU‐sensitive ATPase activity, its modulation by calcium ions and accumulation of sodium ions in various regions of the seedling roots, has been demonstrated using a novel approach in a plant system.     

The role of cotyledons in the establishment of Suaeda physophora seedlings

The role of cotyledons in seedling establishment of the euhalophyte Suaeda physophora under non-saline and saline conditions (addition of 1 mM or 400 mM NaCl) was investigated. Survival and fresh and dry weights were greater for seedlings grown in the light (12-h light/12-h dark) than in the dark (24-h dark). The shading of cotyledons tended to decrease shoot height, shoot organic dry weight, number of leaves, and survival of seedlings regardless of NaCl treatment, but the effect of cotyledon shading was greater with 400 mM NaCl. Concentrations of Na⁺ were higher in cotyledons than in leaves, regardless of NaCl treatment. The K⁺/Na⁺ ratio was lower in cotyledons than in leaves for seedlings treated with 1 mM NaCl but not for seedlings treated with 400 mM NaCl. Addition of 400 mM NaCl decreased oxygen production in cotyledons but especially in leaves. These results are consistent with the hypothesis that, by generating oxygen via photosynthesis and by compartmentalizing Na⁺, cotyledons are crucial for the establishment of S. physophora seedlings in saline environments.     

Salt stress differentially regulates mobilisation of carbon and nitrogen reserves during seedling establishment of Pityrocarpa moniliformis

• Seedling establishment is a critical step in environment colonisation by higher plants that frequently occurs under adverse conditions. Thus, we carried out an integrated analysis of seedling growth, water status, ion accumulation, reserve mobilisation, metabolite partitioning and hydrolase activity during seedling establishment of the native Caatinga species Piptadenia moniliformis (Benth.) Luckow & R.W. Jobson under salinity.
• Two‐day‐old seedlings were cultivated in vitro for 4 days in water agar (control) or supplemented with 50 or 100 mm NaCl. Biochemical determinations were performed according to standard spectrophotometric protocols.
• We found that 100 mm NaCl stimulated starch degradation, amylase activity and soluble sugar accumulation, but limited storage protein hydrolysis in the cotyledons of P. moniliformis seedlings. Although Na⁺ accumulation in the seedling affected K⁺ partitioning between different organs, it was not possible to associate the salt‐induced changes in reserve mobilisation with Na⁺ toxicity, or water status, in the cotyledons. Remarkably, we found that starch content increased in the roots of P. moniliformis seedlings under 100 mm NaCl, probably in response to the toxic effects of Na⁺.
• The mobilisation of carbon and nitrogen reserves is independently regulated in P. moniliformis seedlings under salt stress. The salt‐induced delay in seedling establishment and the resulting changes in the source–sink relationship may lead to storage protein retention in the cotyledons. Possibly, the intensification of starch mobilisation in the cotyledons supported starch accumulation in the root as a potential mechanism to mitigate Na⁺ toxicity.

     

Glucose-6-phosphate dehydrogenase-dependent hydrogen peroxide production is involved in the regulation of plasma membrane H+-ATPase and Na+/H+ antiporter protein in salt-stressed callus from Carex moorcroftii

Glucose‐6‐phosphate dehydrogenase (G6PDH) is important for the activation of plant resistance to environmental stresses, and ion homeostasis is the physiological foundation for living cells. In this study, we investigated G6PDH roles in modulating ion homeostasis under salt stress in Carex moorcroftii callus. G6PDH activity increased to its maximum in 100 mM NaCl treatment and decreased with further increased NaCl concentrations. K⁺/Na⁺ ratio in 100 mM NaCl treatment did not exhibit significant difference compared with the control; however, in 300 mM NaCl treatment, it decreased. Low‐concentration NaCl (100 mM) stimulated plasma membrane (PM) H⁺‐ATPase and NADPH oxidase activities as well as Na⁺/H⁺ antiporter protein expression, whereas high‐concentration NaCl (300 mM) decreased their activity and expression. When G6PDH activity and expression were reduced by glycerol treatments, PM H⁺‐ATPase and NADPH oxidase activities, Na⁺/H⁺ antiporter protein level and K⁺/Na⁺ ratio dramatically decreased. Simultaneously, NaCl‐induced hydrogen peroxide (H₂O₂) accumulation was abolished. Exogenous application of H₂O₂ increased G6PDH, PM H⁺‐ATPase and NADPH oxidase activities, Na⁺/H⁺ antiporter protein expression and K⁺/Na⁺ ratio in the control and glycerol treatments. Diphenylene iodonium (DPI), the NADPH oxidase inhibitor, which counteracted NaCl‐induced H₂O₂ accumulation, decreased G6PDH, PM H⁺‐ATPase and NADPH oxidase activities, Na⁺/H⁺ antiporter protein level and K⁺/Na⁺ ratio. Western blot result showed that G6PDH expression was stimulated by NaCl and H₂O₂, and blocked by DPI. Taken together, G6PDH is involved in H₂O₂ accumulation under salt stress. H₂O₂, as a signal, upregulated PM H⁺‐ATPase activity and Na⁺/H⁺ antiporter protein level, which subsequently resulted in the enhanced K⁺/Na⁺ ratio. G6PDH played a central role in the process.     

S‐nitrosylation/denitrosylation as a regulatory mechanism of salt stress sensing in sunflower seedlings

Nitric oxide (NO) and various reactive nitrogen species produced in cells in normal growth conditions, and their enhanced production under stress conditions are responsible for a variety of biochemical aberrations. The present findings demonstrate that sunflower seedling roots exhibit high sensitivity to salt stress in terms of nitrite accumulation. A significant reduction in S‐nitrosoglutathione reductase (GSNOR) activity is evident in response to salt stress. Restoration of GSNOR activity with dithioerythritol shows that the enzyme is reversibly inhibited under conditions of 120 mM NaCl. Salt stress‐mediated S‐nitrosylation of cytosolic proteins was analyzed in roots and cotyledons using biotin‐switch assay. LC‐MS/MS analysis revealed opposite patterns of S‐nitrosylation in seedling cotyledons and roots. Salt stress enhances S‐nitrosylation of proteins in cotyledons, whereas roots exhibit denitrosylation of proteins. Highest number of proteins having undergone S‐nitrosylation belonged to the category of carbohydrate metabolism followed by other metabolic proteins. Of the total 61 proteins observed to be regulated by S‐nitrosylation, 17 are unique to cotyledons, 4 are unique to roots whereas 40 are common to both. Eighteen S‐nitrosylated proteins are being reported for the first time in plant systems, including pectinesterase, phospholipase d ‐alpha and calmodulin. Further physiological analysis of glyceraldehyde‐3‐phosphate dehydrogenase and monodehydroascorbate reductase showed that salt stress leads to a reversible inhibition of both these enzymes in cotyledons. However, seedling roots exhibit enhanced enzyme activity under salinity stress. These observations implicate the role of S‐nitrosylation and denitrosylation in NO signaling thereby regulating various enzyme activities under salinity stress in sunflower seedlings.     

Na+/H+ antiport activity in tonoplast vesicles isolated from sunflower roots induced by NaCl stress

Na⁺ transport across the tonoplast and its accumulation in the vacuoles is of crucial importance for plant adaptation to salinity. Mild and severe salt stress increased both ATP- and PP_i-dependent H⁺ transport in tonoplast vesicles from sunflower seedling roots, suggesting the possibility that a Na⁺/H⁺ antiport system could be operating in such vesicles under salt conditions (E. Ballesteros et al. 1996. Physiol. Plant. 97: 259–268). During a mild salt stress, Na⁺ was mainly accumulated in the roots. Under a more severe salt treatment, Na⁺ was equally distributed in shoots and roots. In contrast to what was observed with Na⁺, all the salt treatments reduced the shoot K⁺ content. Dissipation by Na⁺ of the H⁺ gradient generated by the tonoplast H⁺-ATPase, monitored as fluorescence quenching of acridine orange, was used to measure Na⁺/H⁺ exchange across tonoplast-enriched vesicles isolated by sucrose gradient centrifugation from sunflower (Helianthus annuus L.) roots treated for 3 days with different NaCl regimes. Salt treatments induced a Na⁺/H⁺ exchange activity, which displayed saturation kinetics for Na⁺ added to the assay medium. This activity was partially inhibited by 125 μM amiloride, a competitive inhibitor of Na⁺/H⁺ antiports. No Na⁺/H⁺ exchange was detected in vesicles from control roots. The activity was specific for Na⁺. since K⁺ added to the assay medium slightly dissipated H⁺ gradients and displayed non-saturating kinetics for all salt treatments. Apparent K_m for Na⁺/H⁺ exchange in tonoplast vesicles from 150 mM NaCl-treated roots was lower than that of 75 mM NaCl-treated roots, V_max remaining unchanged. The results suggest that the existence of a specific Na⁺/H⁺ exchange activity in tonoplast-enriched vesicle fractions, induced by salt stress, could represent an adaptative response in sunflower plants, moderately tolerant to salinity.     

Uptake of inorganic ions and compatible solutes in cultured mangrove cells during salt stress

Summary Callus of the mangrove plant, Sonneratia alba J. Smith, established from pistils of flower buds were cultured on solid Murashige and Skoog medium supplemented with 0 to 500 mM NaCl. Maximum growth was observed with 50 mM NaCl, and net growth of callus occurred for concentrations up to 200 mM NaCl. At 500 mM NaCl, growth of callus was completely inhibited, although a part of the tissue was still alive after 30 d. Cellular levels of Na⁺ and Cl⁻ were greatly increased by the treatment with NaCl. Uptake of K⁺ was also enhanced and was accompanied by increasing levels of Na⁺ and Cl⁻ so that the Na⁺/K⁺ ratio was almost constant (4.1–4.2) in callus grown with 50–200 mM NaCl. Levels of Mg²⁺ and Ca²⁺ were not changed significantly with 50–200 mM NaCl, whereas levels of free NH ₄ ⁺ , NO ₃ ⁻ and SO ₄ ²⁻ ions, which are convertible to organic compounds, were lowest in callus grown with 50 mM NaCl. The rate of conversion of ¹⁵NH ₄ ⁺ into macromolecules during 30 d culture with 0–100 mM NaCl did not vary greatly, but 200 mM NaCl reduced the biosynthesis of macromolecules from this ion. The highest rate of conversion of ¹⁵NO ₃ ⁻ into macromolecules was observed at 50 mM NaCl. Identification of compatible solutes with NMR-spectroscopy indicated that mannitol is the compatible solute for intact plants of Sonneratia alba, but no accumulation of mannitol was found in calluses, not even in those grown at high concentrations of NaCl.     

Relative importance of Na+, Cl−, and abscisic acid in NaCl induced inhibition of root growth of rice seedlings

The relative importance of endogenous abscisic acid (ABA), as well as Na⁺ and Cl^– in NaCl-induced responses related to growth in roots of rice seedlings were investigated. The increase in ammonium, proline and H₂O₂ levels, and cell wall peroxidase (POD) activity has been shown to be related to NaCl-inhibited root growth of rice seedlings. Increasing concentrations of NaCl from 50 to 150 mM progressively decreased root growth and increased both Na⁺ and Cl^–. Treatment with NaCl in the presence of 4,4-diisothiocyano-2,2-disulfonic acid (DIDS, a nonpermeating amino-reactive disulfonic acid known to inhibit the uptake of Cl^–) had less Cl^– level in roots than that in the absence of DIDS, but did not affect the levels of Na⁺, and responses related to growth in roots. Treatment with 50 mM Na-gluconate (the anion of which is not permeable to membrane) had similar Na⁺ level in roots as that with 100 mM NaCl. It was found that treatment with 50 mM Na-gluconate effected growth reduction and growth-related responses in roots in the same way as 100 mM NaCl. All these results suggest that Cl^– is not required for NaCl-induced responses in root of rice seedlings. Endogenous ABA level showed no increase in roots of rice seedlings exposed to 150 mM NaCl. It is unlikely that ABA is associated with NaCl-inhibited root growth of rice seedlings.     

Scanning ion‐selective electrode technique and X‐ray microanalysis provide direct evidence of contrasting Na+ transport ability from root to shoot in salt‐sensitive cucumber and salt‐tolerant pumpkin under NaCl stress

Grafting onto salt‐tolerant pumpkin rootstock can increase cucumber salt tolerance. Previous studies have suggested that this can be attributed to pumpkin roots with higher capacity to limit the transport of Na⁺ to the shoot than cucumber roots. However, the mechanism remains unclear. This study investigated the transport of Na⁺ in salt‐tolerant pumpkin and salt‐sensitive cucumber plants under high (200 mM) or moderate (90 mM) NaCl stress. Scanning ion‐selective electrode technique showed that pumpkin roots exhibited a higher capacity to extrude Na⁺, and a correspondingly increased H⁺ influx under 200 or 90 mM NaCl stress. The 200 mM NaCl induced Na⁺/H⁺ exchange in the root was inhibited by amiloride (a Na⁺/H⁺ antiporter inhibitor) or vanadate [a plasma membrane (PM) H⁺‐ATPase inhibitor], indicating that Na⁺ exclusion in salt stressed pumpkin and cucumber roots was the result of an active Na⁺/H⁺ antiporter across the PM, and the Na⁺/H⁺ antiporter system in salt stressed pumpkin roots was sufficient to exclude Na⁺. X‐ray microanalysis showed higher Na⁺ in the cortex, but lower Na⁺ in the stele of pumpkin roots than that in cucumber roots under 90 mM NaCl stress, suggesting that the highly vacuolated root cortical cells of pumpkin roots could sequester more Na⁺, limit the radial transport of Na⁺ to the stele and thus restrict the transport of Na⁺ to the shoot. These results provide direct evidence for pumpkin roots with higher capacity to limit the transport of Na⁺ to the shoot than cucumber roots.     

Tolerance of submerged germinating rice to 50–200 mM NaCl in aerated solution

This paper concerns tolerance to 50–200 mM NaCl of submerged rice (Oryza sativa cv. Amaroo) during germination and the first 138–186 h of development in aerated solution. Rice was able to germinate and the seedlings even tolerated exposure to 200 mM NaCl, albeit with severe growth restrictions. After return to 0.3 mM NaCl, growth increased, indicating that even at 200 mM NaCl there was no irreparable injury. Osmotic adjustment was achieved by using Na⁺ and Cl^– as the major osmotica. At 200 mM NaCl commenced at sowing, the shoot Na⁺ and Cl^– concentrations between 50–110 h were about 210 and 260 mM, respectively, i.e. above the external concentration. Thus, there was a high tissue tolerance to NaCl. The internal concentrations declined subsequently, concurrent with a decline in growth. At 50–200 mM NaCl, the contributions from ions to π_sap were 81–92% in roots and 62–74% in shoots. The assessed turgor pressures at 200 mM NaCl were 0.33 MPa in shoots and 0.15 MPa in roots, compared to 0.62 and 0.43 MPa at 0.3 mM NaCl. In the General Discussion section, we compare the different responses of submerged seedlings to the responses of transpiring rice plants, reported in the literature, and suggest that the submerged system is useful to evaluate effects of NaCl on turgor pressure and particularly to establish whether there are specific effects of Na⁺ and Cl^– in tissues.     

Evidence for Hormonal Regulation of the Selectivity of Ion Uptake by Plant Cells

Kinetin exerted opposing effects on the uptake of K⁺ and of Na⁺ by leaf discs of Helianthus annuus: The absorption of K⁺ was stimulated and that of Na+ was inhibited. The K⁺/Na⁺ ratio in kinetin-treated discs was 80–100 % higher than in control tissue. Kinetin also promoted K⁺ uptake by detached cotyledons which had been removed from light-grown seedlings. On the other hand, no clear effect on the absorption of Na⁺ by these cotyledons could be established. Benzyladenine brought about a significant elevation in the K⁺/Na⁺ ratio in attached cotyledons. It is concluded that cytokinins bring about a change in the selectivity of the cells of sunflower leaves and cotyledons towards K⁺ and Na⁺, such that the ‘affinity’ of the cells for pottasium is increased, as compared to the ‘affinity’ for sodium. The possible significance of such a change in selectivity is discussed.     

Specific Effects of Kinetin on the Uptake of Monovalent Cations by Sunflower Cotyledons

Kinetin promoted the uptake of K⁺ and Rb⁺ into detached sunflower cotyledons. This action was concomitant with an acceleration of growth. A slighter promotion of Li⁺ uptake was also noted, but there was no consistent influence on that of Na⁺. A small inhibitory effect on NH₄⁺₄ uptake was apparent when the latter was computed per average weight of sample during the course of incubation. Light also promoted the growth of the cotyledons, but depressed their capacity to absorb potassium. The action of kinetin on cotyledons removed from 5–7 day old seedlings was weaker than on those removed from 2–4 day old seedlings with regard to growth but stronger with regards to K⁺ uptake. When K⁺ uptake by cotyledons taken from 7-day old seedlings was followed with time the kinetin effect was already detectable within a few hours, but it became more pronounced after 10 hours' incubation. Kinetin did not accelerate growth or K⁺ uptake in hypo-cotyl segments. IAA, which was previously shown to promote these processes in hypocotyl segments, inhibited them in cotyledons. A working hypothesis is suggested according to which endogenous auxins and cytokinins regulate the absorption of K⁺ in shoot cells of the intact plant in a manner similar to that in which they act in excised tissues and in this way affect the distribution and redistribution of K⁺ in the shoot; and that they are among the factors which determine the selectivity of ion uptake in the intact plant.     

Kinetic studies of a (Na++ K++ Mg2+)ATPase in sugar beet roots. III. A proposed model for the (Na++ K+) activation and its significance for field properties

A microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase preparation from sugar beet roots was used. The activation by simultaneous addition of Na⁺ and K⁺ at different levels was examined in terms of steady state kinetics. The observed data can be summarized in the following way: 1. The apparent affinity between the enzyme and the substrate MgATP depends on the ratio between Na⁺ and K⁺. At low Na⁺ concentration (below 5 mM), the apparent K_m decreases with increasing concentrations of K⁺ (1–20 mM). At 5 mM Na⁺, the K⁺ level does not change the apparent K_m, while at Na⁺ levels above 10 mM, the apparent K_m between enzyme and substrate increases with increasing concentration of K⁺. 2. When the MgATP concentration is kept constant, homotropic cooperativity (concerning one type of ligand) and heterotropic cooperativity (concerning different types of ligands) exist in the activation by Na⁺ and K⁺. The Na⁺ binding is cooperative with different K_m values and Hill coefficients (n) in the presence of low and high concentration of K⁺. At low Na⁺ level (< 5 mM). a negative cooperativity exists for Na⁺ (n_Na < 1) which is more pronounced in the presence of high [K⁺]. When the concentration of Na⁺ is raised the negative cooperativity disappears and turns into a positive one (n_Na > 1). Only K⁺ binding in the presence of low [Na⁺] shows cooperativity with a Hill coefficient that reflects changes from negative to positive homotropic cooperativity with increasing concentrations of K⁺ (n_K < 1 → n_K > 1). In the presence of [Na⁺] > 10 mM, the changes in n_k are insignificant. 3. A model is proposed in which one or two different K sites and one or two Na sites control the catalytic activity, with multiple interactions between Na⁺, K⁺ and MgATP. 4. In the presence of Na⁺ (< 10 mM), K⁺ is probably bound to two K sites, one of which translocates K⁺ through the membrane by an antiport Na⁺/K⁺ mechanism. This could be connected with an elevated K⁺ uptake in the presence of Na⁺ and could therefore explain some field properties of sugar beets.     

The Potassium Requirement for Growth and Embryogenesis in Wild Carrot Suspension Cultures

A requirement for potassium for growth and forembryogenesis in suspension cultures of wild carrot (Daucus carota L.) was demonstrated. The concentration of K⁺ required for maximal growth (1 mM) was less than that required for maximal embryogenesis (20 mM). Neither Na⁺ nor NH₄⁺ could replace K⁺. Ammonium ion enhanced embryogenesis when K⁺ was present at suboptimal levels greater than 1 mM. Nitrogen sources strongly influenced growth and embryogenesis, but the effects of nitrogen were separable from those of K⁺. Subline differences were noted. Subline CSC-29 produced nearly half the maximum embryo number in 1 mM K⁺ while CSC-31 produced no embryos at that K⁺ concentration. Growth of CSC-29 was slightly repressed by Na⁺, but no more than by similarconcentrations of K⁺. Growth of CSC-31 in 1 mM K⁺ was strongly repressed by Na⁺. Embryogenesis in CSC-29 was unaffected by Na⁺. In CSC-31, Na⁺ repressed embryogenesis at lower concentrations of K⁺.     

NaCl胁迫对流苏幼苗生长、钠钾离子分布及渗透调节物质的影响

以2年生的流苏播种苗为材料,采用不同浓度(50、100、200、300 mmol·L^-1)NaCl溶液进行胁迫处理,研究盐胁迫对流苏的生长、Na^+和K^+分布格局、渗透调节物质的影响,以明确其耐盐阈值。结果表明:(1)随着NaCl胁迫浓度的增加,流苏幼苗生长量逐渐降低,盐害指数升高、存活率下降;幼苗耐盐阈值为98.693 mmol·L^-1(0.577%W/V)。(2)随着NaCl胁迫浓度的增加,流苏幼苗各器官中的Na^+含量持续增加,并在浓度为50 mmol·L^-1时表现为根>叶>茎,在其余各处理组表现为叶>根>茎;幼苗根、叶中的K^+含量表现为先增后减的变化趋势,茎中K^+含量总体表现为下降趋势,且器官中K^+含量表现为根>叶>茎;幼苗根部到茎部和茎部到叶部的离子选择性运输能力、各器官中的K^+/Na^+比值均呈下降趋势。(3)随着NaCl浓度的增加,流苏幼苗叶片可溶性糖、可溶性蛋白含量总体呈上升趋势,其脯氨酸含量呈先上升后下降的趋势。研究发现,流苏幼苗根系可通过对Na^+的吸收和累积来阻止其向地上部运输进而避免盐害发生;叶片和茎中通过提高对K^+的选择性吸收和累积,从而增大K^+/Na^+比值以减缓盐分对其生理代谢的伤害。     

Increased Na+ and Cl− accumulation induced by NaCl salinity inhibits cotyledonary reserve mobilization and alters the source-sink relationship in establishing dwarf cashew seedlings

We studied the NaCl-induced changes in cotyledons and the embryonic axis of establishing dwarf cashew (Anacardium occidentale) seedlings. The salt stress reduced the growth of dwarf cashew seedlings, and this response was related to the inhibition of cotyledonary reserve depletion. Lipid mobilization was inhibited by NaCl due to reduced lipase activity in the emerging and establishing seedlings. Additionally, there was reduced transient starch accumulation in the cotyledons of the salt-stressed seedlings that was associated with lower starch synthase activity at the early developmental stages and inhibited amylolytic and starch phosphorylase activities at the established seedling stage. The NaCl-induced changes in lipid and starch metabolism influenced the soluble sugar content in the cotyledons. Protein mobilization was inhibited by NaCl, and we observed the accumulation of amino acids and the inhibition of proteolytic activity in the cotyledons of the salt-stressed established seedlings. Salinity significantly reduced the free amino acid and reducing sugar contents in the embryonic axes of both emerged and established seedlings, whereas the non-reducing sugar content was affected by this stress only in the established seedlings. The Na⁺ and Cl^? contents progressively increased in the cotyledons and embryonic axis of the seedlings as the salinity increased. We conclude that salt stress inhibits dwarf cashew seedling establishment by inhibiting the mobilization of reserves, an inhibition that was related to increased Na⁺ and Cl^? accumulation in the cotyledons. Additionally, these toxic ions reduced the sink strength of the embryonic axis with regard to the products of cotyledonary reserve mobilization.     

Salt sensitivity and low discrimination between potassium and sodium in bean plants

Bean plants (Phaseolus vulgaris) were very sensitive to moderate concentrations of NaCl, showing a dramatic decrease in their K⁺ content in the presence of this salt. Increasing the KCl content of the nutrient medium released the inhibitory effect of NaCl by increasing the K⁺ content of the plants. Likewise moderate concentrations of KCl were toxic for bean plants because they produced a large K⁺ loading. NaCl partially released this toxicity by inhibiting the K⁺ loading. When compared to the moderately salt tolerant sunflower plants (Helianthus annuus), bean plants showed a lower capacity to discriminate between K⁺ and Na⁺, at high Na⁺ levels, and an uncontrolled K⁺ uptake at moderate concentrations of K⁺. It is concluded that this low capacity of discrimination of the K⁺ uptake system of bean plants in presence of Na⁺ can account for by the NaCl sensitivity of bean plants.     

Nitric oxide enhances salt tolerance in maize seedlings through increasing activities of proton-pump and Na+/H+ antiport in the tonoplast

Nitric oxide (NO), an endogenous signaling molecule in animals and plants, mediates responses to abiotic and biotic stresses. Our previous work demonstrated that 100 μM sodium nitroprusside (SNP, an NO donor) treatment of maize seedlings increased K⁺ accumulation in roots, leaves and sheathes, while decreasing Na⁺ accumulation (Zhang et al. in J Plant Physiol Mol Biol 30:455–459, 2004b). Here we investigate how NO regulates Na⁺, K⁺ ion homeostasis in maize. Pre-treatment with 100 μM SNP for 2 days improved later growth of maize plants under 100 mM NaCl stress, as indicated by increased dry matter accumulation, increased chlorophyll content, and decreased membrane leakage from leaf cells. An NO scavenger, methylene blue (MB-1), blocked the effect of SNP. These results indicated that SNP-derived NO enhanced maize tolerance to salt stress. Further analysis showed that NaCl induced a transient increase in the NO level in maize leaves. Both NO and NaCl treatment stimulated vacuolar H⁺-ATPase and H⁺-PPase activities, resulting in increased H⁺-translocation and Na⁺/H⁺ exchange. NaCl-induced H⁺-ATPase and H⁺-PPase activities were diminished by MB-1. 1-Butanol, an inhibitor of phosphatidic acid (PA) production by phospholipase D (PLD), reduced NaCl- and NO-induced H⁺-ATPase activation. In contrast, applied PA stimulated H⁺-ATPase activity. These results suggest that NO acts as a signal molecule in the NaCl response by increasing the activities of vacuolar H⁺-ATPase and H⁺-PPase, which provide the driving force for Na⁺/H⁺ exchange. PLD and PA play an important role in this process.     

Salt-Induced Hypodermal Transfer Cells in Roots of Prosopis farcta and Ion Distribution within Young Plants

Prosopis farcta was grown on hydroculture with additions of 0.5, 10, 50, and 100 mM NaCl and without salt treatment. In plants from a 0.5 mM NaCl treatment, Cl^? was taken up into stems and leaves, but Na⁺ was withheld from the shoot. At 10 mM NaCl, shoot K⁺ concentration was below that of the control; Na⁺ and Cl^? were taken up to stems and cotyledons in nearly equimolar amounts. However, in the leaves, Na⁺ concentrations were only half of those of Cl^?. With increasing salt stress, Na⁺ and Cl^? were transported to the shoot, but kept at relatively low levels in the roots. Na⁺/ K⁺ ratios in roots did not increase proportionally to those in the solution. At an external Na⁺/K⁺ of > 5 and a root Na⁺/K⁺ of >1 (10 mM NaCl treatment), K⁺ selectivity was induced which rose exponentially with increasing salt stress; and cell wall protuberances were discovered in the hypodermis at the zone of side root formation. These transfer cells were found neither in roots from the 0.5 mM NaCl treatment nor in the controls. Their possible role in the Na⁺/K⁺ selectivity of the roots of Prosopis farcta is discussed.