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.     

Sodium chloride stress induces nitric oxide accumulation in root tips and oil body surface accompanying slower oleosin degradation in sunflower seedlings

Present work highlights the involvement of endogenous nitric oxide (NO) in sodium chloride (NaCl)‐induced biochemical regulation of seedling growth in sunflower (Helianthus annuus L., cv. Morden). The growth response is dependent on NaCl concentration to which seedlings are exposed, they being tolerant to 40 mM NaCl and showing a reduction in extension growth at 120 mM NaCl. NaCl sensitivity of sunflower seedlings accompanies a fourfold increase in Na⁺/K⁺ ratio in roots (as compared to that in cotyledons) and rapid transport of Na⁺ to the cotyledons, thereby enhancing Na⁺/K⁺ ratio in cotyledons as well. A transient increase in endogenous NO content, primarily contributed by putative NOS activity in roots of 4‐day‐old seedlings subjected to NaCl stress and the relative reduction in Na⁺/K⁺ ratio after 4 days, indicates that NO regulates Na⁺ accumulation, probably by affecting the associated transporter proteins. Root tips exhibit an early and transient enhanced expression of 4,5‐diaminofluorescein diacetate (DAF‐2DA) positive NO signal in the presence of 120 mM NaCl. Oil bodies from 2‐day‐old seedling cotyledons exhibit enhanced localization of NO signal in response to 120 mM NaCl treatment, coinciding with a greater retention of the principal oil body membrane proteins, i.e. oleosins. Abolition of DAF positive fluorescence by the application of specific NO scavenger [2‐phenyl‐4,4,5,5‐tetramethyllimidazoline‐1‐oxyl‐3‐oxide (PTIO)] authenticates the presence of endogenous NO. These novel findings provide evidence for a possible protective role of NO during proteolytic degradation of oleosins prior to/accompanying lipolysis.     

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.     

Regulation of reserve mobilisation in sunflower during late seedling establishment in continuous darkness

• Reserve mobilisation, metabolite partitioning and reserve‐degrading enzyme activity were studied in sunflower seedlings cultivated in vitro under a 12‐h photoperiod or in the dark to investigate the involvement of source–sink relation and carbon starvation in the regulation of reserve mobilisation under continuous darkness.
• Reserves, metabolites and enzyme activity were determined with standard spectrophotometric methods.
• At the first 24 h of treatment (acclimation phase), darkness did not affect growth, but restricted carbon and nitrogen use, as indicated by sugar and amino acid accumulation in the different seedling parts. After 5 days of treatment (survival phase), extended darkness limited growth and retarded storage lipid mobilisation due to carbon starvation, as evidenced by the depletion of carbohydrates in cotyledons and hypocotyl, as well as the consumption of amino acids in hypocotyls and roots.
• Alterations in the source–sink relationship might have been a response to prolonged darkness, instead of a mechanism used to regulate reserve mobilisation, as these alterations cannot be associated with negative feedback mediated by metabolite accumulation. Storage lipid degradation depends, at least in part, on mechanisms that co‐ordinately regulate the activities of lipases and isocitrate lyase. Taking these results together, it is possible that reserve mobilisation in sunflower seedlings cultivated in the dark might be regulated by mechanisms that perceive the absence of light and predict carbon starvation, adjusting reserve use according to future energy demands to allow, at least in the short term, seedling survival.

     

Enhanced V-ATPase activity contributes to the improved salt tolerance of transgenic tobacco plants overexpressing vacuolar Na(+)/H (+) antiporter AtNHX1

AtNHX1, a vacuolar Na⁺/H⁺ antiporter gene from Arabidopsis thaliana, was introduced into tobacco genome via Agrobacterium tumefaciens-mediated transformation to evaluate the role of vacuolar energy providers in plants salt stress response. Compared to the wild-type plants, over-expression of AtNHX1 increased salt tolerance in the transgenic tobacco plants, allowing higher germination rates of seeds and successful seedling establishment in the presence of toxic concentrations of NaCl. More importantly, the induced Na⁺/H⁺ exchange activity in the transgenic plants was closely correlated to the enhanced activity of vacuolar H⁺-ATPase (V-ATPase) when exposed to 200 mM NaCl. In addition, inhibition of V-ATPase activity led to the malfunction of Na⁺/H⁺ exchange activity, placing V-ATPase as the dominant energy provider for the vacuolar Na⁺/H⁺ antiporter AtNHX1. V-ATPase and vacuolar Na⁺/H⁺ antiporter thus function in an additive or synergistic way. Simultaneous overexpression of V-ATPase and vacuolar Na⁺/H⁺ antiporter might be appropriate for producing plants with a higher salt tolerance ability.     

Contributions of arbuscular mycorrhizal fungi to growth,photosynthesis, root morphology and ionic balance of citrus seedlings under salt stress

A pot study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi (Glomus mosseae and Paraglomus occultum) and salt (NaCl) stress on growth, photosynthesis, root morphology and ionic balance of citrus (Citrus tangerine Hort. ex Tanaka) seedlings. Eighty-five-day-old seedlings were exposed to 100 mM NaCl for 60 days to induce salt stress. Mycorrhizal colonization of citrus seedlings was not affected by salinity when associated with P. occultum, but significantly decreased when with G. mosseae. Compared with the non-mycorrhizal controls, mycorrhizal seedlings generally had greater plant height, stem diameter, shoot, root and total plant biomass, photosynthetic rate, transpiration rate and stomatal conductance under the 0 and 100 mM NaCl stresses. Root length, root projected area and root surface area were also higher in the mycorrhizal than in the non-mycorrhizal seedlings, but higher root volume in seedlings with G. mosseae. Leaf Na⁺ concentrations were significantly decreased, but leaf K⁺ and Mg²⁺ concentrations and the K⁺/Na⁺ ratio were increased when seedlings with both G. mosseae and P. occultum. Under the salt stress, Na⁺ concentrations were increased but K⁺ concentrations decreased in the mycorrhizal seedlings. Under the salt stress, Ca²⁺ concentrations were increased in the seedlings with P. occultum or without AM fungi (AMF), but decreased with G. mosseae. Ratios of both Ca²⁺/Na⁺ and Mg²⁺/Na⁺ were also increased in seedlings with G. mosseae under the non-salinity stress, while only the Mg²⁺/Na⁺ ratio was increased in seedlings with P. occultum under the salt stress. Our results suggested that salt tolerance of citrus seedlings could be enhanced by associated AMF with better plant growth, root morphology, photosynthesis and ionic balance.     

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.     

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。     

Salt stress decreases seedling growth and development but increases quercetin and kaempferol content in Apocynum venetum

• Apocynum venetum L. is a traditional Chinese medicinal herb with great potential to treat angiocardiopathy. Its major medicinal constituents are flavonoids. However, the natural habitats of A. venetum are typically affected by salt stress, which can modify both biomass and accumulation of medicinal compounds.
• In this study, the effects of salt stress on growth and development of A. venetum, accumulation of flavonoids and expression patterns of genes involved in flavonoid biosynthesis were evaluated.
• In general, the growth and development of seedlings (seedling height, root length, leaf length, leaf width and seed germination) were inhibited by salt stress. Unlike typical halophytes, there was no optimal NaCl concentration range that promoted growth and development, but seedlings had an elevated DW/FW ratio under salt stress (induced by irrigation with 50, 100, 200 or 400 mm NaCl). Furthermore, quercetin and kaempferol were significantly accumulated in A. venetum seedlings under salt stress, resulting in a balanced content and reduced FW. Moreover, the expression of AvCHS, AvCHI and AvF3GT was inhibited by salt stress; however, AvF3’H, AvF3H and AvFLS, which are involved in the flavonol synthesis pathway, were up‐regulated under salt stress, consistent with a decrease in total flavonoids and an increase of flavonols (quercetin and kaempferol).
• In summary, cultivation of A. venetum in saline soils appeared to be feasible and improved the medicinal quality of A. venetum (quercetin and kaempferol accumulation under salt stress), thus this species can effectively utilize saline soil resources.

     

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.     

Source-sink regulation of cotyledonary reserve mobilization during cashew (Anacardium occidentale) seedling establishment under NaCl salinity

Seedling establishment is a critical process to crop productivity, especially under saline conditions. This work was carried out to investigate the hypothesis that reserve mobilization is coordinated with salt-induced inhibition of seedling growth due to changes in source-sink relations. To test this hypothesis, cashew nuts (Anacardium occidentale) were sown in vermiculite irrigated daily with distilled water (control) or 50mM NaCl and they were evaluated at discrete developmental stages from the seed germination until the whole seedling establishment. The salt treatment coordinately delayed the seedling growth and the cotyledonary reserve mobilization. However, these effects were more pronounced at late seedling establishment than in earlier stages. The storage protein mobilization was affected by salt stress before the lipid and starch breakdown. The globulin fraction represented the most important storage proteins of cashew cotyledons, and its mobilization was markedly delayed by NaCl along the seedling establishment. Free amino acids were mostly retained in the cotyledons of salt-treated seedlings when the mobilization of storage proteins, lipids and starch was strongly delayed. Proline was not considerably accumulated in the cotyledons of cashew seedlings as a response to NaCl salinity. According to these results it is noteworthy that the salt-induced inhibition of seedling growth is narrowly coordinated with the delay of reserve mobilization and the accumulation of hydrolysis products in cotyledons. Also, it was evidenced that free amino acids, especially those related to nitrogen transport, are potential signals involved in the regulation of storage protein hydrolysis during cashew seedling establishment under NaCl salinity.     

Comparison of the effects of NaCl and KCl at the roots on seedling growth,cell death and the size,frequency and secretion rate of salt glands in leaves of <Emphasis Type="Italic">Limonium sinense</Emphasis>

Twenty days’ exposure to 50 or 100 mM NaCl in the rooting medium substantially increased fresh and dry weights of seedling shoots of the recretohalophyte Limonium sinense while 200 or 300 mM were increasingly inhibitory. KCl treatment was only slightly stimulating (50 mM) or strongly inhibitory (100–300 mM). Lesser effects on leaf area were also seen. Diameter of foliar salt glands was significantly larger than that of controls in 100 and 200 mM NaCl with the effect being reversed at higher concentrations. Gland enlargement was also observed in the presence of 100 mM KCl, while larger concentrations reduced gland size. Generally, gland diameter was larger in the presence of NaCl than in KCl. NaCl and KCl also increased gland number per leaf and secretion rate per gland. At 100 and 200 mM NaCl or KCl, Na⁺ secretion per leaf from NaCl-treated plants exceeded K⁺ secretion rate from KCl-treated plants while at 200 mM, Na⁺ secretion per gland was significantly higher for Na⁺ than for K⁺. Evidence of cell death in leaves of salt-treated plants using Evans blue staining indicates that release of cell contents through loss of membrane integrity contributed to the secretion values. We conclude that the greater tolerance of L. sinenseto to NaCl compared to KCl is linked to the more effective secretion of Na⁺ than of K⁺ and, in turn, to a greater stimulation of salt gland formation and activity and larger gland diameter.     

ZxAKT1 is essential for K+ uptake and K+/Na+ homeostasis in the succulent xerophyte Zygophyllum xanthoxylum

The inward‐rectifying K⁺ channel AKT1 constitutes an important pathway for K⁺ acquisition in plant roots. In glycophytes, excessive accumulation of Na⁺ is accompanied by K⁺ deficiency under salt stress. However, in the succulent xerophyte Zygophyllum xanthoxylum, which exhibits excellent adaptability to adverse environments, K⁺ concentration remains at a relatively constant level despite increased levels of Na⁺ under salinity and drought conditions. In this study, the contribution of ZxAKT1 to maintaining K⁺ and Na⁺ homeostasis in Z. xanthoxylum was investigated. Expression of ZxAKT1 rescued the K⁺‐uptake‐defective phenotype of yeast strain CY162, suppressed the salt‐sensitive phenotype of yeast strain G19, and complemented the low‐K⁺‐sensitive phenotype of Arabidopsis akt1 mutant, indicating that ZxAKT1 functions as an inward‐rectifying K⁺ channel. ZxAKT1 was predominantly expressed in roots, and was induced under high concentrations of either KCl or NaCl. By using RNA interference technique, we found that ZxAKT1‐silenced plants exhibited stunted growth compared to wild‐type Z. xanthoxylum. Further experiments showed that ZxAKT1‐silenced plants exhibited a significant decline in net uptake of K⁺ and Na⁺, resulting in decreased concentrations of K⁺ and Na⁺, as compared to wild‐type Z. xanthoxylum grown under 50 mm NaCl. Compared with wild‐type, the expression levels of genes encoding several transporters/channels related to K⁺/Na⁺ homeostasis, including ZxSKOR, ZxNHX, ZxSOS1 and ZxHKT1;1, were reduced in various tissues of a ZxAKT1‐silenced line. These findings suggest that ZxAKT1 not only plays a crucial role in K⁺ uptake but also functions in modulating Na⁺ uptake and transport systems in Z. xanthoxylum, thereby affecting its normal growth.     

Differences in efficient metabolite management and nutrient metabolic regulation between wild and cultivated barley grown at high salinity

Physiological and biochemical responses of Hordeum maritimum and H. vulgare to salt stress were studied over a 60‐h period. Growth at increasing salinity levels (0, 100, 200 and 300 mM NaCl) was assessed in hydroponic culture. H. maritimum was shown to be a true halophyte via its typical behaviour at high salinity. Shoot growth of cultivated barley was gradually reduced with increasing salinity, whereas that of wild barley was enhanced at 100 and 200 mm NaCl then slightly reduced at 300 mM NaCl. The higher salt tolerance of H. maritimum as compared to H. vulgare was due to its higher capacity to maintain cell turgor under severe salinity. Furthermore, H. maritimum exhibited fine regulation of Na⁺ transport from roots to shoots and, unlike H. vulgare, it accumulated less Na⁺ in shoots than in roots. In addition, H. maritimum can accumulate more Na⁺ than K⁺ in both roots and shoots without the appearance of toxicity symptoms, indicating that Na⁺ was well compartmentalized within cells and substituted K⁺ in osmotic adjustment. The higher degree of salt tolerance of H. maritimum is further demonstrated by its economic strategy: at moderate salt treatment (100 mm NaCl), it used inorganic solutes (such as Na⁺) for osmotic adjustment and kept organic solutes and a large part of the K⁺ for metabolic activities. Indeed, K⁺ use efficiency in H. maritimum was about twofold that in H. vulgare; the former started to use organic solutes as osmotica only at high salinity (200 and 300 mm NaCl). These results suggest that the differences in salt tolerance between H. maritimum and H. vulgare are partly due to (i) differences in control of Na⁺ transport from roots to shoots, and (ii) H. maritimum uses Na⁺ as an osmoticum instead of K⁺ and organic solutes. These factors are differently reflected in growth.     

Comparative salt tolerance analysis between Arabidopsis thaliana and Thellungiella halophila, with special emphasis on K(+)/Na(+) selectivity and proline accumulation

The eco-physiology of salt tolerance, with an emphasis on K⁺ nutrition and proline accumulation, was investigated in the halophyte Thellungiella halophila and in both wild type and eskimo-1 mutant of the glycophyte Arabidopsis thaliana, which differ in their proline accumulation capacity. Plants cultivated in inert sand were challenged for 3 weeks with up to 500 mM NaCl. Low salinity significantly decreased A. thaliana growth, whereas growth restriction was significant only at salt concentrations equal to or exceeding 300 mM NaCl in T. halophila. Na⁺ content generally increased with the amount of salt added in the culture medium in both species, but T. halophila showed an ability to control Na⁺ accumulation in shoots. The analysis of the relationship between water and Na⁺ contents suggested an apoplastic sodium accumulation in both species; this trait was more pronounced in A. thaliana than in T. halophila. The better NaCl tolerance in the latter was associated with a better K⁺ supply, resulting in higher K⁺/Na⁺ ratios. It was also noteworthy that, despite highly accumulating proline, the A. thaliana eskimo-1 mutant was the most salt-sensitive species. Taken together, our findings indicate that salt tolerance may be partly linked to the plants’ ability to control Na⁺ influx and to ensure appropriate K⁺ nutrition, but is not linked to proline accumulation.     

NaCl胁迫对圆柏幼苗生长和离子吸收及分配的影响

以当年生圆柏幼苗为实验材料,采用温室调控盆栽土培法研究了不同浓度NaCl(0、100、200、300mmol·L-1)胁迫21d对其生长情况及不同器官(根、茎、叶)中K~+、Na~+、Ca~(2+)和Mg~(2+)的吸收和分配的影响,以探讨圆柏幼苗对盐环境的生长适应性及耐盐机制。结果表明:(1)随着NaCl胁迫浓度的增加,圆柏幼苗生长,包括株高、地径、相对生长量以及生物量的积累均呈下降趋势,而其根冠比却增加。(2)在各浓度NaCl胁迫处理下,圆柏幼苗根、茎、叶中Na~+含量较对照均显著增加,而且叶中Na~+含量显著高于茎和根,叶中Na~+含量是根中的5倍。(3)随着NaCl胁迫浓度的升高,圆柏幼苗各器官中K~+、Ca~(2+)和Mg~(2+)含量以及K~+/Na~+、Ca~(2+)/Na~+及Mg~(2+)/Na~+比值均呈下降趋势。(4)在NaCl胁迫条件下,圆柏幼苗根系离子吸收选择性系数SK,Na、SCa,Na、SMg,Na显著提高,茎、叶离子转运选择性系数SCa,Na、SMg,Na则逐渐降低,叶中离子转运选择性系数SK,Na则随着NaCl胁迫浓度的升高显著降低,大量Na~+进入地上部,减缓了盐胁迫对根系的伤害。研究认为,圆柏幼苗的盐适应机制主要是通过根系的补偿生长效应及茎、叶对Na~+的聚积作用来实现的,同时也与根对K~+、Ca~(2+)、Mg~(2+)的选择性运输能力增强和茎、叶稳定的K~+、Ca~(2+)、Mg~(2+)的选择性运输能力有关。     

The effect of NaCl on proline accumulation in potato seedlings and calli

The effects of salt stress were studied on the accumulation and metabolism of proline and its correlation with Na⁺ and K⁺ content in shoots and callus tissue of four potato cultivars, viz., Agria, Kennebec (relatively salt tolerant), Diamant and Ajax (relatively salt sensitive). Na⁺ and proline contents increased in all cultivars under salt stress. However, K⁺ and protein contents decreased in response to NaCl treatments. The activities of enzymes involved in proline metabolism, Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and proline dehydrogenase (ProDH) increased and decreased, respectively, in response to elevated NaCl concentrations. The changes of P5CS and ProDH activities in more salt sensitive cultivars (Diamant, Ajax) were more than those in the tolerant ones. Then the stimulation of synthesis in combination with a partially increase of protein proteolysis, a decrease in proline utilization and inhibition of oxidation resulted in high proline contents in seedlings and calli under salt stress. In callus tissue, reduced growth and cell size may be partially responsible for high proline accumulation in response to high NaCl levels. However, although the basic proline contents in the seedlings of more salt tolerant cultivars were higher than the sensitive ones, a clear relationship was not generally observed between accumulation of proline and salt tolerance in potato.     

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.     

Effect of exogenous ammonium gluconate on growth,ion flux and antioxidant enzymes of maize (Zea Mays L.) seedlings under NaCl stress

• Ammonium gluconate (AG) provides both an organic carbon source and a nitrogen source, which can positively improve soil fertility and delay soil degradation.
• We investigated the underlying mechanisms of both NH₄⁺‐ and C₆H₁₁O₇^?‐mediated resistance to high salt concentrations in maize (Zea mays L.), and how they relate to antioxidant cellular machinery, root system architecture, root activity and lignin content in roots.
• Seedlings treated with AG maintained lower Na⁺ content, higher chlorophyll content, higher CAT and POD activity, compared with those without AG and ammonium carbonate (AC). The total size of the root system, primary root length and number of lateral roots detected on the primary root treated with AG decreased compared with those not treated with AG at the same NaCl concentration. However, average root diameter and root activity when treated with AG were significantly higher than roots without AG at the same NaCl concentration. Furthermore, total size of the root system, primary root length and number of lateral roots detected on primary rootsof seedlings treated with AG were higher than those treated with AC at the same NaCl concentration.
• These results suggested that AG may be a good organic fertiliser under salt stress by decreasing Na⁺ content and increasing chlorophyll content, activity of antioxidant enzymes, root diameter and root activity in maize seedlings.

     

Effects of NaCl stress on seed germination and seedling development of Brassica insularis Moris (Brassicaceae)

• Brassica insularis is a protected plant that grows on both coastal and inland cliffs in the western Mediterranean Basin. The objective of this study was to test if any variability exists in the salt stress response during seed germination and seedling development in this species relative to its provenance habitat.
• Variability among three populations in the salt stress effects on seed germination and recovery under different temperatures was evaluated. The effect of nebulisation of a salt solution on seedling development was evaluated between populations growing at different distances from the sea.
• Seeds of B. insularis could germinate at NaCl concentrations up to 200 mm . Seed viability was negatively affected by salt, and recovery ability decreased with increasing temperature or salinity. Inter‐population variability was detected in salt response during the seed germination phase, as well as in seedling salt spray tolerance. The inland population seedlings had drastically decreased survival and life span and failed to survive to the end of the experiment. In contrast, at least 90% of the coastal seedlings survived, even when sprayed at the highest frequency with salt solution.
• This study allowed investigation of two natural factors, soil salinity and marine aerosols, widely present in the B. insularis habitat, and provided the first insights into ecology of this protected species and its distribution in the Mediterranean. These results might be useful in understanding the actual distributions of other species with the same ecology that experience these same abiotic parameters.