The HAL1 function on Na+ homeostasis is maintained over time in salt-treated transgenic tomato plants, but the high reduction of Na+ in leaf is not associated with salt tolerance

To achieve a deeper knowledge on the function of HAL1 gene in tomato ( Solanum lycopersicum ) plants submitted to salt stress, in this study, we studied the growth and physiological responses to high salt stress of T3 transgenic plants (an azygous line without transgene and both homozygous and hemizygous lines for HAL1 ) proceeding from a primary transformant with a very high expression level of HAL1 gene. The homozygous plants for HAL1 gene did not increase their salt tolerance in spite of an earlier and higher reduction of the Na⁺ accumulation in leaves, being moreover the Na⁺ homeostasis maintained throughout the growth cycle. The greater ability of the homozygous line to regulate the Na⁺ transport to the shoot to long term was even shown in low accumulation of Na⁺ in fruits. By comparing the homozygous and hemizygous lines, a higher salt tolerance in the hemizygous line, with respect to the homozygous line, was observed on the basis of fruit yield. The Na⁺ homeostasis and osmotic homeostasis were also different in homozygous and hemizygous lines. Indeed, the Na⁺ accumulation rate in leaves was greater in hemizygous than in homozygous line after 35 days of 100 m M NaCl treatment and only at the end of growth cycle did the hemizygous line show leaf Na⁺ levels similar to those found in the homozygous line. With respect to the osmotic homeostasis, the main difference between lines was the different contribution of inorganic and organic solutes to the leaf osmotic balance. Taken together, these results suggest that the greater Na⁺ exclusion ability of the homozygous line overexpressing HAL1 induces a greater use of organic solutes for osmotic balance, which seems to have an energy cost and hence a growth penalty that reverts negatively on fruit yield.     

Glycinebetaine-induced modulation of antioxidant enzymes activities and ion accumulation in two wheat cultivars differing in salt tolerance

Modulation of water relations, activities of antioxidant enzymes and ion accumulation was assessed in the plants of two wheat cultivars S-24 (salt tolerant) and MH-97 (moderately salt sensitive) subjected to saline conditions and glycinebetaine (GB) applied foliarly. Different levels of GB, i.e., 0 (unsprayed), 50 and 100 mM (in 0.10% Tween-20 solution) were applied to the wheat plants at the vegetative growth stage. Leaf water potential, leaf osmotic potential and turgor potential were decreased due to salt stress. Salt stress increased the Na⁺ and Cl⁻ accumulation coupled with a decrease in K⁺ and Ca²⁺ in the leaves and roots of both cultivars thereby decreasing tissue K⁺/Na⁺ and Ca²⁺/Na⁺ ratios. Furthermore, salt stress decreased the activities of superoxide dismutase (SOD), whereas it increased the activities of catalase (CAT) and peroxidase (POD) in both wheat cultivars. However, accumulation of GB in the leaves of both wheat cultivars was consistently increased with an increase in concentration of exogenous GB application under both non-saline and saline conditions. Accumulation of Na⁺ was decreased with an increase in K⁺ accumulation upon a consistent increase in GB accumulation under salt stress conditions thereby resulting in better K⁺/Na⁺ and Ca²⁺/Na⁺ ratios in the leaves and roots. High accumulation of GB and K⁺ mainly contributed to osmotic adjustment, which is one of the factors known to be responsible for improving growth and yield under salt stress. The activities of all antioxidant enzymes, SOD, CAT and POD were enhanced by GB application in cv. MH-97 under saline conditions, whereas all these except SOD were reduced in cv. S-24. It is likely that both applied GB and intrinsic SOD scavenged ROS in the tolerant cultivar thereby resulting into low activities of CAT and POD enzymes under salt stress. In conclusion, the adverse effects of salt stress on wheat can be alleviated by the exogenous application of 100 mM GB by modulating activities of antioxidant enzymes and changes in water relations and ion homeostasis. Furthermore, effectiveness of GB application on regulation of activities of antioxidant enzymes was found to be cultivar-specific.     

外源褪黑素与钙离子互作对高温胁迫下黄瓜幼苗过氧化伤害的缓解效应

为了探明褪黑素(MT)和钙离子(Ca²⁺)在调控植物耐热性中是否存在互作关系,以黄瓜幼苗为试材,分析了内源MT和Ca²⁺对高温胁迫的响应;并通过叶面喷施100 μmol·L^-1 MT、10 mmol·L^-1 CaCl₂、3 mmol·L^-1乙二醇二乙醚二胺四乙酸(EGTA,Ca²⁺螯合剂)+100 μmol·L^-1 MT、0.05 mmol·L^-1氯丙嗪(钙调素拮抗剂,CPZ)+100 μmol·L^-1 MT、100 μmol·L^-1氯苯丙氨酸(p-CPA,MT合成抑制剂)+10 mmol·L^-1 CaCl₂和去离子水(H₂O),研究高温下(42/32 ℃)外源MT和Ca²⁺对黄瓜幼苗活性氧积累、抗氧化系统及热激转录因子(HSF)和热激蛋白(HSPs)等的影响。结果表明: 黄瓜幼苗内源MT和Ca²⁺均受高温胁迫诱导;外源MT可上调常温下钙调素蛋白(CaM)、钙依赖蛋白激酶(CDPK5)、钙调磷酸酶B类蛋白(CBL3)、CBL结合蛋白激酶(CIPK2)mRNA表达;CaCl₂处理的MT合成关键基因色氨酸脱羧酶(TDC)、5-羟色胺-N-乙酰转移酶(SNAT)和N-乙酰-5-羟色胺甲基转移酶(ASMT)水平也显著升高,MT含量快速增加。MT和CaCl₂可显著增强高温下黄瓜的抗氧化能力,减少活性氧(ROS)积累,同时上调HSF7、HSP70.1和HSP70.11 mRNA表达,从而减轻高温胁迫引起的过氧化伤害,植株热害症状明显减轻,热害指数和电解质渗漏率显著降低。加入EGTA和CPZ后,MT对黄瓜幼苗抗氧化能力和热激蛋白表达的促进效应明显减弱,Ca²⁺对高温下黄瓜幼苗过氧化伤害的缓解效应也被p-CPA逆转。可见,MT和Ca²⁺均可诱导黄瓜幼苗的耐热性,二者在热胁迫信号转导过程中存在互作关系。     

An Arabidopsis vasculature distributed metal tolerance protein facilitates xylem magnesium diffusion to shoots under high-magnesium environments

Magnesium(Mg²⁺) is an essential metal for plant growth; however, its over-accumulation in cells can be cytotoxic. The metal tolerance protein family(MTP) belongs to an ubiquitous family of cation diffusion facilitator(CDF) proteins that export divalent metal cations for metal homeostasis and tolerance in all organisms. We describe here the identification of MTP10 to be critical for xylem Mg homeostasis in Arabidopsis under high Mg²⁺conditions. The Arabidopsis plant contains...     

盐胁迫下外源褪黑素和Ca2+对甜瓜幼苗的缓解效应

以甜瓜品种‘羊角酥瓜’为试材,利用人工气候室控制环境条件(昼/夜25/18 ℃),研究盐胁迫条件下外源褪黑素(MT)和Ca²⁺对甜瓜幼苗根系和叶片中Cl^-、Na⁺、K⁺、Mg²⁺、Ca²⁺离子含量,Na⁺/K⁺、 Na⁺/Ca²⁺、Na⁺/Mg²⁺值,以及H⁺-ATP酶活性、渗透调节物质积累和细胞膜质过氧化的影响.结果表明: 与对照相比,盐胁迫处理显著抑制甜瓜幼苗生长,增加根系和叶片中Cl^-、Na⁺含量,降低K⁺、Mg²⁺、Ca²⁺含量.盐胁迫下,喷施外源MT或Ca²⁺处理均可以显著降低甜瓜根系和叶片中Cl^-、Na⁺含量,提高K⁺、Mg²⁺、Ca²⁺含量,植株体内Na⁺/K⁺、Na⁺/Ca²⁺和 Na⁺/Mg²⁺值下降;同时也提高了根系和叶片H⁺-ATP酶活性及叶片渗透调节物质的含量,降低盐胁迫对细胞膜的伤害,表现在甜瓜叶片相对电导率和丙二醛含量降低.总之,在盐胁迫条件下,外源MT、Ca²⁺单独和复配处理均可通过提高H⁺-ATP酶活性来降低盐害离子的含量,改善甜瓜幼苗中的离子平衡,同时增加渗透调节物质的含量,降低膜质过氧化水平,从而增强其对盐胁迫的适应性,其中MT和Ca²⁺复配处理时的效果更好.复配外施 MT 和Ca²⁺在诱导甜瓜幼苗提高耐盐方面具有协同增效作用.     

盐胁迫下外源褪黑素和Ca2+对甜瓜幼苗的缓解效应

以甜瓜品种‘羊角酥瓜’为试材,利用人工气候室控制环境条件(昼/夜25/18 ℃),研究盐胁迫条件下外源褪黑素(MT)和Ca²⁺对甜瓜幼苗根系和叶片中Cl^-、Na⁺、K⁺、Mg²⁺、Ca²⁺离子含量,Na⁺/K⁺、 Na⁺/Ca²⁺、Na⁺/Mg²⁺值,以及H⁺-ATP酶活性、渗透调节物质积累和细胞膜质过氧化的影响.结果表明: 与对照相比,盐胁迫处理显著抑制甜瓜幼苗生长,增加根系和叶片中Cl^-、Na⁺含量,降低K⁺、Mg²⁺、Ca²⁺含量.盐胁迫下,喷施外源MT或Ca²⁺处理均可以显著降低甜瓜根系和叶片中Cl^-、Na⁺含量,提高K⁺、Mg²⁺、Ca²⁺含量,植株体内Na⁺/K⁺、Na⁺/Ca²⁺和 Na⁺/Mg²⁺值下降;同时也提高了根系和叶片H⁺-ATP酶活性及叶片渗透调节物质的含量,降低盐胁迫对细胞膜的伤害,表现在甜瓜叶片相对电导率和丙二醛含量降低.总之,在盐胁迫条件下,外源MT、Ca²⁺单独和复配处理均可通过提高H⁺-ATP酶活性来降低盐害离子的含量,改善甜瓜幼苗中的离子平衡,同时增加渗透调节物质的含量,降低膜质过氧化水平,从而增强其对盐胁迫的适应性,其中MT和Ca²⁺复配处理时的效果更好.复配外施 MT 和Ca²⁺在诱导甜瓜幼苗提高耐盐方面具有协同增效作用.     

Salinity response of a freshwater charophyte, Chara vulgaris

Abstract. Chara vulgaris L. growing in an oligohaline lake was adapted to laboratory conditions and subjected to long-term salinity treatments ranging from 0 to 350 mol m ³ NaCl added to the lake water (40–680 mosmol kg ¹). Osmotic potential and concentration of the main osmotically active solutes (K⁺, Na⁺, Mg²⁺, Cl and sucrose) in the vacuolar sap of the central internodal cells were estimated. C. vulgaris did regulate turgor but incompletely. Turgor decreased from 335 mosmol kg ¹ under control conditions to 52–111 mosmol kg ¹ at 350 mol m ³ NaCl. The enhancement of πⁱ was achieved by increase in both ions and sucrose. Sterile and fertile plants differed in their response to osmotic stress. In sterile plants, the ions accounted for about 87% of the vacuolar osmotic potential. The increase of πⁱ under osmotic stress was exclusively due to an accumulation of Na⁺ and Cl^-. In fertile plants, sucrose accounted for about 35% of πⁱ and ions for about 51% Under osmotic stress, sucrose content increased together with the ionic content of Na⁺ and Cl^-.     

Quantifying the three main components of salinity tolerance in cereals

Salinity stress is a major factor inhibiting cereal yield throughout the world. Tolerance to salinity stress can be considered to contain three main components: Na⁺ exclusion, tolerance to Na⁺ in the tissues and osmotic tolerance. To date, most experimental work on salinity tolerance in cereals has focused on Na⁺ exclusion due in part to its ease of measurement. It has become apparent, however, that Na⁺ exclusion is not the sole mechanism for salinity tolerance in cereals, and research needs to expand to study osmotic tolerance and tissue tolerance. Here, we develop assays for high throughput quantification of Na⁺ exclusion, Na⁺ tissue tolerance and osmotic tolerance in 12 Triticum monococcum accessions, mainly using commercially available image capture and analysis equipment. We show that different lines use different combinations of the three tolerance mechanisms to increase their total salinity tolerance, with a positive correlation observed between a plant's total salinity tolerance and the sum of its proficiency in Na⁺ exclusion, osmotic tolerance and tissue tolerance. The assays developed in this study can be easily adapted for other cereals and used in high throughput, forward genetic experiments to elucidate the molecular basis of these components of salinity tolerance.     

Hyper-osmotic stress induces volume change and calcium transients in chondrocytes by transmembrane, phospholipid, and G-protein pathways

Mechanical compression of cartilage is associated with a rise in the interstitial osmotic pressure, which can alter cell volume and activate volume recovery pathways. One of the early events implicated in regulatory volume changes and mechanotransduction is an increase of intracellular calcium ion ([Ca²⁺]_i). In this study, we tested the hypothesis that osmotic stress initiates intracellular Ca²⁺ signaling in chondrocytes. Using laser scanning microscopy and digital image processing, [Ca²⁺]_i and cell volume were monitored in chondrocytes exposed to hyper-osmotic solutions. Control experiments showed that exposure to hyper-osmotic solution caused significant decreases in cell volume as well as transient increases in [Ca²⁺]_i. The initial peak in [Ca²⁺]_i was generally followed by decaying oscillations. Pretreatment with gadolinium, a non-specific blocker of mechanosensitive ion channels, inhibited this [Ca²⁺]_i increase. Calcium-free media eliminated [Ca²⁺]_i increases in all cases. Pretreatment with U73122, thapsigargin, or heparin (blockers of the inositol phosphate pathway), or pertussis toxin (a blocker of G-proteins) significantly decreased the percentage of cells responding to osmotic stress and nearly abolished all oscillations. Cell volume decreased with hyper-osmotic stress and recovered towards baseline levels throughout the duration of the control experiments. The peak volume change with 550 mOsm osmotic stress, as well as the percent recovery of cell volume, was dependent on [Ca²⁺]_i. These findings indicate that osmotic stress causes significant volume change in chondrocytes and may activate an intracellular second messenger signal by inducing transient increases in [Ca²⁺]_i.     

Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na⁺/Cl^- content were measured on leaves of one-year-old sweet orange ( Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO₃) were used to separate the effects of Cl⁻ and Na⁺ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl⁻ in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na⁺ concentrations of up to 478 m M in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca²⁺ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl⁻ and Na⁺ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.     

Effect of Ca channel blockers on glycerol levels in Dunaliella tertiolecta under hypoosmotic stress

The role of Ca²⁺ in glycerol dissimilation under hypoosmotic stress in the halotolerant alga Dunaliella tertiolecta was investigated using a pharmacological approach. A stretch-activated Ca²⁺ channel blocker, GdCl₃, inhibited glycerol dissimilation under hypoosmotic stress. However, addition of voltage-dependent Ca²⁺ channel blockers and inhibitors of mitochondrial and endoplasmic reticulum Ca²⁺ channels did not affect the glycerol dissimilation under hypoosmotic stress. The results of the present study suggest that the influx of Ca²⁺ from the extracellular space via the stretch-activated Ca²⁺ channels localized in the plasma membrane is required for the transduction of osmotic signal of D. tertiolecta.     

Arabidopsis mutants with reduced response to NaCl and osmotic stress

We isolated 6 mutant lines of Arabidopsis thaliana that expressed reduced sensitivity to salt and osmotic stress during germination. All 6 lines cum recessive mutations in a single gene, designated reduced salt sensitivity (rss), linked to the ADH marker on chromosome 1. The rss mutants are less sensitive than wild type for NaCl and osmotic stress inhibition of germination, tolerating approximately 150 mM higher concentrations of NaCl and about 250 mM higher concentrations of sorbitol in the media. Germination assays on media containing various salts indicate that the rss mutations reduce sensitivity lo Na⁺ and Rh⁺ but also, to a much lesser degree, to K⁺ and Cs^s+. However, the rss mutation does not improve plant growth when plantlets are transferred to high salt or high osmotic pressure media after germination. The rss plantlets accumulate praline to a significantly lesser degree than wild type when they are exposed to either salt or osmotic stress. Thus, the rss mutants differ from wild type both at germination and during vegetative growth indicating that the rss mutations are pleiotropic and might affect perception of solutes or some aspect of stress-induced signaling. The rss mutations do not alter ABA sensitivity and therefore probably do not affect ABA-mediated signaling.     

镉胁迫对吊兰及银边吊兰生长及镉富集特性的影响

选择吊兰和银边吊兰为试验材料,采用水培法研究其在不同Cd²⁺处理浓度(0、20、80、200 μmol·L^-1)下生长及生理特性的变化。结果表明: 20 μmol·L^-1镉对两种吊兰的影响较小,单叶面积、总叶面积、叶绿素(Chl)a含量、总叶绿素[Chl (a+b)]含量、类胡萝卜素含量、Chl a/Chl b值、胞间二氧化碳浓度(C_i)和蒸腾速率(T_r)与对照(CK)基本无显著差异;80 μmol·L^-1镉胁迫下两种吊兰叶片初始荧光(F_o)和非光化学淬灭系数(NPQ)升至最高水平;200 μmol·L^-1镉胁迫下,两种吊兰生物量、叶绿素含量、最大净光合速率(P_n)、气孔导度(g_s)、最大光化学量子产量(F_v/F_m)、实际光化学量子产量Y_(II)、转移系数(TF)以及各部分生物量均降至最低水平,而两种吊兰的过氧化物酶(POD)、抗坏血酸过氧化物酶(APX)及过氧化氢酶(CAT)活性和银边吊兰的丙二醛(MDA)含量均有不同程度的增加。随着镉处理浓度的增加,两种吊兰各器官Cd含量持续升高,且主要富集在根部;吊兰各器官Cd含量及富集系数(BCF)在胁迫处理下均高于银边吊兰。研究表明,两种吊兰对镉具有一定的耐性,其中吊兰对Cd的耐受能力强于银边吊兰,可考虑作为绿化植物用于修复镉污染水体或土壤。     

The Arabidopsis phosphatase PP2C49 negatively regulates salt tolerance through inhibition of AtHKT1;1

Type 2C protein phosphatases (PP2Cs) are the largest protein phosphatase family. PP2Cs dephosphorylate substrates for signaling in Arabidopsis, but the functions of most PP2Cs remain unknown. Here, we characterized PP2C49 (AT3G62260, a Group G PP2C), which regulates Na⁺ distribution under salt stress and is localized to the cytoplasm and nucleus. PP2C49 was highly expressed in root vascular tissues and its disruption enhanced plant tolerance to salt stress. Compared with wild type, the pp2c49 mutant contained more Na⁺ in roots but less Na⁺ in shoots and xylem sap, suggesting that PP2C49 regulates shoot Na⁺ extrusion. Reciprocal grafting revealed a root‐based mechanism underlying the salt tolerance of pp2c49. Systemic Na⁺ distribution largely depends on AtHKT1;1 and loss of function of AtHKT1;1 in the pp2c49 background overrode the salt tolerance of pp2c49, resulting in salt sensitivity. Furthermore, compared with plants overexpressing PP2C49 in the wild‐type background, plants overexpressing PP2C49 in the athtk1;1 mutant background were sensitive to salt, like the athtk1;1 mutants. Moreover, protein–protein interaction and two‐voltage clamping assays demonstrated that PP2C49 physically interacts with AtHKT1;1 and inhibits the Na⁺ permeability of AtHKT1;1. This study reveals that PP2C49 negatively regulates AtHKT1;1 activity and thus determines systemic Na⁺ allocation during salt stress.     

Relationship between tomato fruit growth and fruit osmotic potential under salinity

To investigate the relationship between fruit growth and fruit osmotic potential (Ψ_s) in salty conditions, a sensitive tomato cultivar (Lycopersicon esculentum Mill.) and a tolerant accession of the wild species Lycopersicon pimpinellifolium Mill. were grown in a greenhouse with 0 and 70 mM NaCl, and the growth of the fruit studied from 15 to 70 days after anthesis (DAA). L. pimpinellifolium did not reduce significantly fruit weight in salty conditions throughout the growth period, whereas L. esculentum fruit weights decreased significantly with salinity from 45 DAA. L. esculentum fruit fresh weight reductions resulted from both less dry matter and water accumulation, although the fruit water content was affected by salinity before the fruit weight. In both species, fruit osmotic potential (Ψ_s) decreased significantly with salinity during the rapid fruit growth phase, although the changes were different. Thus, fruits from L. pimpinellifolium salt treated plants showed a Ψ_s reduction at the beginning (15 DAA) twice as high as that found in L. esculentum. As the advanced growth stage (from 15 to 55 DAA), the Ψ_s reduction percentages induced by salinity were quite similar in L. pimpinellifolium fruits, while increased in L. esculentum. Under saline conditions, the solutes contributing to reduce the fruit Ψ_s during the first 55 DAA were the inorganic solutes in both species, while in the ripe fruits they were hexoses. L. esculentum fruits accumulated K⁺ as the main osmoticum in salty conditions, while L. pimpinellifolium fruits were able to use not only K⁺ but also the Na⁺ provided by the salt.     

外生菌根真菌通过调节离子平衡提高蒙古栎耐盐性

为探究盐胁迫对蒙古栎生长的影响以及外生菌根真菌(ECMF)对蒙古栎离子平衡的调节作用,对蒙古栎幼苗接种4种ECMF(铆钉菇、褐环乳牛肝菌、厚环粘盖牛肝菌和美味牛肝菌)后,以1年生非菌根化与菌根化幼苗为试验材料,进行36 d的NaCl胁迫(0、100、200、300 mmol·L^-1)处理,分析幼苗的菌根特征、生长量、叶伤害症状、叶片电解质渗透率及含水量、根茎叶离子含量的变化特征。结果表明: 4种ECMF均能与蒙古栎建立共生体系,菌根化幼苗的根系较非菌根化幼苗粗壮。盐胁迫下,蒙古栎幼苗的生长受到抑制并出现焦叶症状,其叶片质膜损伤和失水程度随盐胁迫浓度升高而加重。低盐胁迫时(100 mmol·L^-1),蒙古栎优先将Na⁺积累在根和茎中,中高浓度盐胁迫下(200～300 mmol·L^-1),根成为积累Na⁺的首要器官。ECMF通过增加根部的Na⁺水平和减少茎、叶的Na⁺积累,加强对K⁺和Ca²⁺的吸收以提高K⁺/Na⁺和Ca²⁺/Na⁺,进而调节蒙古栎的离子平衡。4种ECMF对蒙古栎盐毒害的缓解作用存在差异,铆钉菇作用效果最好,褐环乳牛肝菌次之,厚环粘盖牛肝菌和美味牛肝菌的作用相对较小。     

Seagrass-salinity interactions: Physiological mechanisms used by submersed marine angiosperms for a life at sea

Due to the nature of coastal and estuarine systems, seagrasses must be able to tolerate short-term salinity fluctuations including both hyposaline and hypersaline conditions. Salt tolerance can be achieved, in part, through vacuolar ion sequestering (mostly Na⁺, K⁺, and Cl⁻) and cytosolic osmolyte accumulation (K⁺ and organic osmolytes), with differences in cellular ion levels attributed to selective ion flux and ion partitioning between the cytoplasm and vacuole (with lower cytoplasmic-to-vacuolar ratios favoring higher cellular Na⁺ concentrations). The hydrophilic nature of organic compounds such as organic acids, soluble carbohydrates, and free amino acids allow them to serve as osmoprotectants and low-molecular-weight chaperones which diminishes the inhibitory effects of potentially harmful ions on metabolic processes. Nevertheless, some carbohydrate studies on seagrasses have shown decreased soluble sugar content with increased salinities. During salt stress, carbohydrates are likely converted to other organic compounds that would better facilitate osmotic adjustment in these plants. This is further supported by observed decreases in sucrose-P synthase (a key enzyme involved in sucrose synthesis) activities in seagrass exposed to higher salinities. While modifications in ion flux and organic solute levels often follow changes in environmental salinities, these adjustments are relatively slow (hours to days). Therefore, the initial response to sudden salinity change will include rapid alterations in turgor pressure driven by water flux in the direction of the osmotic gradient. The rate of water movement depends largely on the hydraulic conductivity of the plasmalemma and the elastic properties of the cell wall (bulk elastic modulus; Є). Observations on cell wall elasticity indicate that some seagrasses maintain fairly rigid walls (high Є values), thereby limiting the amount of water influx during hypoosmotic stress. Although high Є would be beneficial to open-water coastal plants living in relatively stable saline environments, in estuaries where salinities fluctuate considerably over shorter intervals, high Є could promote flaccid cells with no turgor pressure during hyperosmotic conditions. Hypo- and hyperosmotic conditions also inhibit photosynthesis in seagrasses. Decreases in photosynthesis have been attributed to declines in chlorophyll content, changes in chloroplast ultrastructure, disruptions of electron flow through photosystems, and inhibitions of key photosynthetic enzymes. The uptake of nutrients can also be strongly influenced by salinity. High affinity Na⁺-dependent nutrient transport systems (for NO₃⁻, H₂PO₄⁻, and HPO₄⁻²) which benefit from the inwardly driving force for Na⁺ have been observed in seagrasses. Nitrate reductase, the key enzyme involved in nitrate reduction/ assimilation, also has elevated activities at higher salinities which would agree with Na⁺-dependent NO₃⁻ transport. While our basic understanding of how seagrasses survive in saline environments is increasing, it still lags well behind marine algae and terrestrial halophytes. It is likely that further investigations will reveal unique physiological adaptations that have not been observed in other plants.     

Influence of exogenously supplied potassium and sodium salts on the abscisic acid-induced proline accumulation in barley leaf segments

A stimulation of the abscisic acid (ABA)-induced increase in proline was observed in leaf segments of barley ( Hordeum vulgare L. cv. Georgie) if K⁺ or Na⁺ were supplied in the external medium as salts of monovalent anions such as NO₃, Br, Cr and I, but not when sulphate or phosphate were used. To a lesser extent, the effect was evident also with RbCl, but it did not occur when chlorides of Li⁺. Cs⁺, NH₄⁺, Mg:⁺ and Ca²⁺ were used. Both KC1 and NaCl in the concentration range 2–100 m M influence the ABA-dependent proline accumulation to the same extent; the increase induced was about 100% at 10 m M , and reached a maximum between 60 and 100 m M. The effect is not due to the osmotic activity of the salts and does not seem to depend on changes in K⁺ and Na⁺ levels within the leaf tissue, but it is somehow linked to their external concentration. The existence of a specific interaction between ABA and K⁺ or Na⁺, possibly at the cell membrane level, is proposed.     

Responses of apical ion fluxes to NaCl stress in Elaeagnus angustifolia seedlings

Aims Elaeagnus angustifolia is one of the most salt-tolerant species. The objective of this study was to understand the mechanisms of ion transporation in E. angustifolia exposed to different salt concentrations through manipulations of K⁺/Na⁺ homeostasis.
Methods Seedlings of two variants of the species, Yinchuan provenance (YC, salt-sensitive type) and the Alaer provenance (ALE, salt-tolerant type), were treated with three different NaCl application modes, and the ion fluxes in the apical regions were measured using non-invasive micro-test technology (NMT). In mode 1, Na⁺ and K⁺ fluxes were measured after 150 mmol·L^-1 NaCl stress lasted for 24 h. In mode 2, K⁺ and H⁺ fluxes were quantified with a transient stimulation of NaCl solution. In mode 3, Amiloride (Na⁺/H⁺ antiporters inhibitor) and tetraethylammonium (TEA, K⁺ channel inhibitor) was used to treat apical regions of E. angustifolia seedlings after NaCl stress for 24 h, respectively.
Important findings Under NaCl stress for 24 h, net effluxes of Na⁺ and K⁺ were increased significantly. The net Na⁺ effluxes of YC provenance seedlings (720 pmol·cm^-2•s^-1) were lower than that of ALE provenance (912 pmol·cm^-2·s^-1), but the net K⁺ efflux was higher in YC provenance. Under the instantaneous NaCl stimulation, net K⁺ efflux was remarkably increased, with the net K⁺ efflux of YC provenance always higher than that of ALE provenance. Interestingly, H⁺ at the apical regions was found from influx to efflux, with the net H⁺ efflux of ALE provenance greater than that of the YC provenance. Under the NaCl and NaCl + Amiloride treatment, the net Na⁺ efflux of ALE provenance seedlings was higher than that of YC provenance, while the net K⁺ efflux was less in ALE provenance seedlings. On the other hand, the differences in net Na⁺ and K⁺ effluxes were insignificant between the two provenances under the control group and NaCl + TEA treatment. In conclusion, NaCl stress caused Na⁺ accumulation and K⁺ outflows of E. angustifolia seedlings; The E. angustifolia seedlings utilize Na⁺/H⁺ antiporters to reduce Na⁺ accumulation by excretion; and the maintenance of K⁺/Na⁺ homeostasis in salt-tolerant E. angustifolia provenance seedlings roots accounted for a greater Na⁺ extrusion and a lower K⁺ efflux under NaCl stress. Results from this study provide a theoretical basis for further exploring salt-tolerant E. angustifolia germplasm resource.