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.     

Osmotic adaptation in halotolerant yeast, Debaryomyces nepalensis NCYC 3413: role of osmolytes and cation transport

Debaryomyces nepalensis NCYC 3413, a food spoiling yeast isolated from rotten apple, has been previously demonstrated as halotolerant yeast. In the present study, we assessed its growth, change in cell size, and measured the intracellular polyol and cations (Na⁺ or K⁺) accumulated during growth in the absence and presence of different concentrations of salts (NaCl and KCl). Cells could tolerate 2 M NaCl and KCl in defined medium. Scanning electron microscopic results showed linear decrease in mean cell diameter with increase in medium salinity. Cells accumulated high amounts of K⁺ during growth at high concentrations of KCl. However, it accumulated low amounts of Na⁺ and high amounts of K⁺ when grown in the presence of NaCl. Cells grown in the absence of salt showed rapid influx of Na⁺/K⁺ on incubation with high salt. On incubation with 2 M KCl, cells grown at 2 M NaCl showed an immediate efflux of Na⁺ and rapid uptake of K⁺ and vice versa. To withstand the salt stress, osmotic adjustment of intracellular cation was accompanied by intracellular accumulation of polyol (glycerol, arabitol, and sorbitol). Based on our result, we hypothesize that there exists a balanced efflux and synthesis of osmolytes when D. nepalensis was exposed to hypoosmotic and hyperosmotic stress conditions, respectively. Our findings suggest that D. nepalensis is an Na⁺ excluder yeast and it has an efficient transport system for sodium extrusion.     

Ionic and osmotic components of salt stress specifically modulate net ion fluxes from bean leaf mesophyll

Ionic mechanisms of salt stress perception were investigated by non‐invasive measurements of net H⁺, K⁺, Ca²⁺, Na⁺, and Cl^? fluxes from leaf mesophyll of broad bean (Vicia faba L.) plants using vibrating ion‐selective microelectrodes (the MIFE technique). Treatment with 90 m M NaCl led to a significant increase in the net K⁺ efflux and enhanced activity of the plasma membrane H⁺‐pump. Both these events were effectively prevented by high (10 m M ) Ca²⁺ concentrations in the bath. At the same time, no significant difference in the net Na⁺ flux has been found between low‐ and high‐calcium treatments. It is likely that plasma membrane K⁺ and H⁺ transporters, but not the VIC channels, play the key role in the amelioration of negative salt effects by Ca²⁺ in the bean mesophyll. Experiments with isotonic mannitol application showed that cell ionic responses to hyperosmotic treatment are highly stress‐specific. The most striking difference in response was shown by K⁺ fluxes, which varied from an increased net K⁺ efflux (NaCl treatment) to a net K⁺ influx (mannitol treatment). It is concluded that different ionic mechanisms are involved in the perception of the ‘ionic’ and ‘osmotic’ components of salt stress.     

High salinity tolerance in the stl2 mutation of Ceratopteris richardii is associated with enhanced K+ influx and loss

The roles of K⁺ uptake and loss in the salinity response of the wild type and the salt-tolerant mutant stl2 of Ceratopteris richardii were studied by measuring Rb⁺ influx and loss and the effects of Na⁺, Mg²⁺, Ca²⁺ and K⁺-transport inhibitors. In addition, electrophysiological responses were measured for both K⁺ and Rb⁺ and for the effects of Na⁺ and NH₄⁺ on subsequent K⁺-induced depolarizations. stl2 had a 26–40% higher uptake rate for Rb⁺ than the wild type at 0.5–10 mol m^?3 RbCl. Similarly, membrane depolarizations induced by both RbCl and KCl were consistently greater in stl2. In the presence of 0–180 mol m^?3 NaCl, stl2 maintained a consistently greater Rb⁺ influx than the wild type. stl2 retained a greater capacity for subsequent KCl-induced depolarization following exposure to NaCl. Five mol m^?3 Mg²⁺ decreased Rb⁺ uptake in stl2; however, additional Mg²⁺ up to 40 mol m^?3 did not affect Rb⁺ uptake further. Ca²⁺ supplementation resulted in a very minor decrease of Rb⁺ uptake that was similar in the two genotypes. Tetraethylammonium chloride and CsCl gave similar inhibition of Rb⁺ uptake in both genotypes, but NH₄Cl gave substantially greater inhibition in the wild type than in stl2. NH₄Cl resulted in a greater membrane depolarization in the wild type and the capacity for subsequent depolarization by KCl was markedly reduced. stl2 exhibited a higher Independent loss of Rb⁺ than the wild type, but, in the absence of external K⁺, loss of Rb⁺ was equivalent in the two genotypes. Since constitutive K⁺ contents are nearly identical, we conclude that high K⁺ influx and loss exact a metabolic cost that is reflected in the inhibition of gametophytic growth. Growth inhibition can be alleviated by reduced supplemental K⁺ or by treatments that slightly reduce K⁺ influx, such as moderate concentrations of Na⁺ or Mg²⁺. We propose that high throughput of K⁺ allows maintenance of cytosolic K⁺ under salt stress and that a high uptake rate for K⁺ results in a reduced capacity for the entrance and accumulation of alternative cations such as Na⁺ in the cytosol.     

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.     

Ion‐dependent metabolic responses of Vicia faba L. to salt stress

Salt‐affected farmlands are increasingly burdened by chlorides, carbonates, and sulfates of sodium, calcium, and magnesium. Intriguingly, the underlying physiological processes are studied almost always under NaCl stress. Two faba bean cultivars were subjected to low‐ and high‐salt treatments of NaCl, Na₂SO₄, and KCl. Assimilation rate and leaf water vapor conductance were reduced to approximately 25–30% without biomass reduction after 7 days salt stress, but this did not cause severe carbon shortage. The equimolar treatments of Na⁺, K⁺, and Cl^? showed comparable accumulation patterns in leaves and roots, except for SO₄^2? which did not accumulate. To gain a detailed understanding of the effects caused by the tested ion combinations, we performed nontargeted gas chromatography–mass spectrometry‐based metabolite profiling. Metabolic responses to various salts were in part highly linearly correlated, but only a few metabolite responses were common to all salts and in both cultivars. At high salt concentrations, only myo‐inositol, allantoin, and glycerophosphoglycerol were highly significantly increased in roots under all tested conditions. We discovered several metabolic responses that were preferentially associated with the presence of Na⁺, K⁺, or Cl^?. For example, increases of leaf proline and decreases of leaf fumaric acid and malic acid were apparently associated with Cl^? accumulation.     

Effects of external NaCl on the growth of Atriplex amnicola and the ion relations and carbohydrate status of the leaves

Abstract Atriplex amnicola, was grown in nutrient solution cultures with concentrations of NaCl up to 750 mol m^?3. The growth optimum was at 25–50 mol m^?3 NaCl and growth was 10–15% of that value at 750 mol m^?3 NaCl. Sodium chloride at 200 mol m^?3 and higher reduced the rate of leaf extension and increased the time taken for a leaf to reach its maximal length. Concentrations of Na⁺, K⁺ and Mg²⁺ in leaves of different ages were investigated for plants grown at 25, 200 and 400 mol m^?3 NaCl. Although leaves of plants grown at 200 and 400 mol m^?3 NaCl had high Na⁺ concentrations at young developmental stages, much of this Na⁺ was located in the salt bladders. Leaves excluding bladders had low Na⁺ concentrations when young, but very high in Na⁺ when old. In contrast to Na⁺, K⁺ concentrations were similar in bladders and leaves excluding bladders. Concentrations of K⁺ were higher in the rapidly expanding than in the old leaves. At 400 mol m^?3 NaCl, the K⁺:Na⁺ ratios of the leaves excluding bladders were 0.4–0.6 and 0.1 for rapidly expanding and oldest leaves, respectively. The Na⁺ content in moles per leaf, excluding bladders, increased linearly with the age of the leaves; concurrent increases in succulence were closely correlated with the Na ⁺ concentration in the leaves excluding the bladders. Soluble sugars and starch in leaves, stems and buds were determined at dusk and dawn. There was a pronounced diurnal fluctation in concentrations of carbohydrates. During the night, most plant parts showed large decreases in starch and sugar. Concentrations of carbohydrates in most plant organs were similar for plants grown at 25 and 400 mol m^?3 NaCl. One notable exception was buds at dusk, where sugar and starch concentrations were 30–35% less in plants grown at 400 mol m^?3 NaCl than in plants grown at 25 mol m^?3 NaCl. The data indicate that the growth of A. amnicola at 400 mol m^?3 NaCl is not limited by the availability of photosynthate in the plant as a whole. However, there could have been a growth limitation due to inadequate organic solutes for osmotic regulation.     

Is salinity tolerance related to Na accumulation in Upland cotton (Gossypium hirsutum) seedlings?

Physiological responses to salt stress were studied in two cotton cultivars previously selected on the basis of growth under salinity. Plants were grown in nutrient solutions under controlled conditions. In the first experiment, the genotypes were grown at different salt concentrations (0, 100 and 200 mt M NaCl) and growth rates, water contents and ion accumulation were determined. In a second experiment, both genotypes were grown at the same salt concentration (200 mt M NaCl). Dry matter partitioning in individual leaves, stem and roots, water contents, specific leaf area (SLA), ion accumulation (K⁺, Na⁺, Cl^–) and leaf water potentials were measured. Finally, an experiment with low salt levels (2.7 and 27 mt M NaCl) was run to compare K and Na⁺ uptake and distribution.There were no differences in growth between the cultivars in the absence of salt stress, whereas under stress genotype Z407 had higher leaf area and dry matter accumulation than P792. Leaf water potential and leaf water content were lower in cv P792 than in cv Z407. There were no significant differences in the levels of Cl^– accumulation between genotypes. The main feature of the tolerant genotype (Z407) was a higher accumulation of Na⁺ in leaves and an apparent capacity for K⁺ redistribution to younger leaves.We postulate that the higher tolerance in Z407 is the result of several traits such as a higher Na⁺ uptake and water content. Adaptation through adequate, but tightly controlled ion uptake, typical of some halophytes, matched with efficient ion compartmentation and redistribution, would result in an improved water uptake capacity under salt stress and lead to maintenance of higher growth rates.     

Effect of NaCl and KCl salts on the growth and solute accumulation of the halophyte Atriplex nummularia

Atriplex nummularia plants are able to grow well in the absence of significant amounts of Na⁺. Medium levels of salinity (100 mM NaCl or KCl) did not cause substantial inhibition of growth but increasing concentrations of salt induced a progressive decline in length and weight of the plants. This inhibition was significantly higher in KCl grown plants than in NaCl grown plants. In addition, although it has been proposed that both K⁺ and Na⁺ are involved in the osmotic adjustment of plants in response to high soil salinity, we show that Na⁺ ions contribute more efficiently than K⁺ ions to perform this function. Our results also indicate that most of the osmotic adjustment of the plant was due to the accumulation of inorganic ions. The strong inhibition of Rb⁺ transport caused by internal sodium suggests that this cation could be efficiently used by the plant and, as a consequence, the transport of other monovalent cations is down-regulated.     

Ability of multicellular salt glands in <Emphasis Type="Italic">Tamarix</Emphasis> species to secrete Na<Superscript>+</Superscript> and K<Superscript>+</Superscript> selectively

The present study aimed to determine the mechanism of cation-selective secretion by multicellular salt glands. Using a hydroponic culture system, the secretion and accumulation of Na⁺ and K⁺ in Tamarix ramosissima and T. laxa under different salt stresses (NaCl, KCl and NaCl+KCl) were studied. Additionally, the effects of salt gland inhibitors (orthovanadate, Ba²⁺, ouabain, tetraethylammonium (TEA) and verapamil) on Na⁺ and K⁺ secretion and accumulation were examined. Treatment with NaCl (at 0–200 mmol L⁻¹ levels) significantly increased Na⁺ secretion, whereas KCl treatment (at 0–200 mmol L⁻¹ levels) significantly increased K⁺ secretion. The ratio of secretion to accumulation of Na⁺ was higher than that of K⁺. The changes in Na⁺ and K⁺ secretion differed after adding different ions into the single-salt solutions. Addition of NaCl to the KCl solution (at 100 mmol L⁻¹ level, respectively) led to a significant decrease in K⁺ secretion rate, whereas addition of KCl to the NaCl solution (at 100 mmol L⁻¹ level, respectively) had little impact on the Na⁺ secretion rate. These results indicated that Na+ secretion in Tamarix was highly selective. In addition, Na⁺ secretion was significantly inhibited by orthovanadate, ouabain, TEA and verapamil, and K⁺ secretion was significantly inhibited by ouabain, TEA and verapamil. The different impacts of orthovanadate on Na⁺ and K⁺ secretion might be the primary cause for the different Na⁺ and K⁺ secretion abilities of multicellular salt glands in Tamarix.     

Cloning and functional analysis of the K+ transporter, PhaHAK2, from salt-sensitive and salt-tolerant reed plants

We isolated PhaHAK2 cDNAs from salt-tolerant and salt-sensitive reed plants. PhaHAK2 belongs to group II by phylogenetic analysis, and was predicted to be a high-affinity plasma membrane K⁺ transporter. Yeast transformed with the PhaHAK2-u from salt-sensitive reed plants (Phragmites australis) had a decreased ability to take up K⁺ in the presence of NaCl and showed a higher Na⁺ permeability than yeast transformed with PhaHAK2-n or PhaHAK2-e from two salt-tolerant reed plants. These results suggest a possibility that the continuous K⁺ uptake by PhaHAK2 and maintenance of high K⁺/Na⁺ ratio under salt stress condition is one of the causes of the salt-tolerance in reed plants.     

Ecosystem changes in Galápagos highlands by the invasive tree Cinchona pubescens

Background and aims

Salinity is an increasing problem for agricultural production worldwide. Understanding how Na⁺ enters plants is important if reducing Na⁺ influx, a key component of the regulation of Na⁺ accumulation in plants and improving salt tolerance of crop plants, is to be achieved. Our previous work indicated that two distinct low-affinity Na⁺ uptake pathways exist in the halophyte Suaeda maritima. Here, we report the external NaCl concentration at which uptake switches from pathway 1 to pathway 2 and the kinetics of the interaction between external K⁺ concentration and Na⁺ uptake and accumulation in S. maritima in order to determine the roles of K⁺ transporters or channels in low-affinity Na⁺ uptake.

Methods

Na⁺ influx, Na⁺ and K⁺ accumulations in S. maritima exposed to various concentrations of NaCl (0–200 mM) were analyzed in the absence and presence of the inhibitors TEA and Ba⁺ (5 mM TEA or 3 mM Ba²⁺) or KCl (0, 10 or 50 mM).

Results

Our earlier proposal was confirmed and extended that there are two distinct low-affinity Na⁺ uptake pathways in S. maritima: pathway 1 might be mediated by a HKT-type transporter under low salinity conditions and pathway 2 by an AKT1-type channel or a KUP/HAK/KT type transporter under high salinity conditions. The external NaCl concentration at which two distinct low-affinity Na⁺ uptake switches from pathway 1 to pathway 2, the ‘turning point’, is between 90 and 95 mM. Over a short period (12 h) of Na⁺ and K⁺ treatments, a low concentration of K⁺ (10 mM) facilitated Na⁺ uptake by S. maritima under high salinity (100–200 mM NaCl), whether or not the plants had been subjected to a longer (3 d) period of K⁺ starvation. The kinetics suggests that low concentration of K⁺ (10 mM) might activate AKT1-type channels or KUP/HAK/KT-type transporters under high salinity (100–200 mM NaCl).

Conclusions

The turning-point of external NaCl concentrations for the two low-affinity Na⁺ uptake pathways in Suaeda maritima is between 90 and 95 mM. A low concentration of K⁺ (10 mM) might activate AKT1 or KUP/HAK/KT and facilitate Na⁺ uptake under high salinity (100–200 mM NaCl). The kinetics of K⁺ on Na⁺ uptake and accumulation in S maritima are also consistent with there being two low-affinity Na⁺ uptake pathways.     

The effect of sodium chloride on the Ca2+, K+ and Na+ concentrations of the seed coat and embryo of Acacia tortilis and A. coriacea

The uptake of Na⁺ and the loss of Ca²⁺ and K⁺ by seeds of Acacia tortilis (Forsk.) Hayne (salt tolerant) and A. coriacea DC. (salt sensitive) were determined after 24 h soaking in 250 mol m^-1,3 NaCl or in distilled water. Na⁺ uptake was higher by the seed coat than by the embryo of both species and higher by A. coriacea than by A. tortilis. The greater Na⁺ uptake by A. coriacea was associated with greater Ca and K⁺ leakage. The Na⁺ concentration of solution imbibed by the embryo of both species was lower than the Na⁺ concentration in the external solution, indicating an exclusion of Na⁺. When A. tortilis and A. coriacea seeds were treated with a series of NaCl concentration (0–400 mol m^-1,3), the exclusion mechanism was particularly clear with A. tortilis at lower concentrations (50 and 150 mol m^-1,3) of NaCl. In contrast, the seed coat of both species accumulated Na⁺. Thus the seed coat may play an important role in ion exchange. These results show that it is important to consider the seed coat and embryo separately rather than the whole seed when considering ion exchange in relation to salinity tolerance.     

Effects of salinity on uptake and distribution of Na+, Cl- and K+ in two wheat cultivars

Plants of two wheat (Triticum aestivum L.) cultivars differing in salt tolerance were grown in sand with nutrient solutions. 35-d-old plants were subjected to 5 levels of salinity created by adding NaCl, CaCl₂ and Na₂SO₄. Growth reduction caused by salinity was accompanied by increased Na⁺ and Cl^- concentrations, Na⁺/K⁺ ratio, and decreased concentration of K⁺. The salt tolerant cv. Kharchia 65 showed better ionic regulation. Salinity up to 15.7 dS m^-1 induced increased uptake of Na⁺ and Cl^- but higher levels of salinity were not accompanied by further increase in uptake of these ions. Observed increases in Na⁺ and Cl^- concentrations at higher salinities seemed to be the consequence of reduction in growth. Uptake of K⁺ was decreased; more in salt sensitive cultivar. This was also accompanied by differences in its distribution.     

Relation between permeability to potassium and sodium ions and fusicoccin-stimulated hydrogen-ion efflux in barley roots

Summary Measurements are described of fusicoccin (FC)-stimulated H⁺ efflux in barley (Hordeum vulgare L.) roots when K⁺ and Na⁺ concentrations were varied. In low-salt roots H⁺ efflux was stimulated in both 5 mM KCl and NaCl. In salt-saturated roots H⁺ efflux was stimulated more effectively in KCl than in NaCl solution. The stimulation of H⁺ efflux thus is parallel with the selectivity of these different root preparations for K⁺ and Na⁺ and with estimates of permeability ratios (P _Na/P _K) determined from electrical measurements. It is suggested that the results support electrogenic coupling between FC-stimulated H⁺ efflux and cation uptake.     

The K+/H+ antiporter LeNHX2 increases salt tolerance by improving K+ homeostasis in transgenic tomato

The endosomal LeNHX2 ion transporter exchanges H⁺ with K⁺ and, to lesser extent, Na⁺. Here, we investigated the response to NaCl supply and K⁺ deprivation in transgenic tomato (Solanum lycopersicum L.) overexpressing LeNHX2 and show that transformed tomato plants grew better in saline conditions than untransformed controls, whereas in the absence of K⁺ the opposite was found. Analysis of mineral composition showed a higher K⁺ content in roots, shoots and xylem sap of transgenic plants and no differences in Na⁺ content between transgenic and untransformed plants grown either in the presence or the absence of 120 mm NaCl. Transgenic plants showed higher Na⁺/H⁺ and, above all, K⁺/H⁺ transport activity in root intracellular membrane vesicles. Under K⁺ limiting conditions, transgenic plants enhanced root expression of the high‐affinity K⁺ uptake system HAK5 compared to untransformed controls. Furthermore, tomato overexpressing LeNHX2 showed twofold higher K⁺ depletion rates and half cytosolic K⁺ activity than untransformed controls. Under NaCl stress, transgenic plants showed higher uptake velocity for K⁺ and lower cytosolic K⁺ activity than untransformed plants. These results indicate the fundamental role of K⁺ homeostasis in the better performance of LeNHX2 overexpressing tomato under NaCl stress.     

Sodium transport in filamentous nitrogen fixing cyanobacteria

Two filamentous, nitrogen fixing cyanobacteria were examined for their salt tolerance and sodium (Na⁺) transport.Anabaena torulosa, a saline form, grew efficiently and fixed nitrogen even at 150 mM salt (NaCl) concentration while,Anabaena L-31, a fresh water cyanobacterium, failed to grow beyond 35 mM NaCl.Anabaena torulosa showed a rapidly saturating kinetics of Na⁺ transport with a high affinity for Na⁺ (K _m, 0.3 mM).Anabaena L-31 had a much lower affinity for Na⁺ (K_m, 2.8 mM) thanAnabaena torulosa and the pattern of uptake was somewhat different. BothAnabaena spp. exhibited an active Na⁺ extrusion which seems to be mediated by a Na⁺-K⁺ ATPase and aided by oxidative phosphorylation.Anabaena L-31 was found to retain much more intracellular Na⁺ thanAnabaena torulosa. The results suggest that the saline form tolerates high Na⁺ concentrations by curtailing its influx and also by an efficient Na⁺ extrusion, although these alone may not entirely account for its success in saline environment.     

Disruption of <Emphasis Type="Italic">RCI2A</Emphasis> leads to over-accumulation of Na<Superscript>+</Superscript> and increased salt sensitivity in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> plants

For plant salt tolerance, it is important to regulate the uptake and accumulation of Na⁺ ions. The yeast pmp3 mutant which lacks PMP3 gene accumulates excess Na⁺ ions in the cell and shows increased Na⁺ sensitivity. Although the function of PMP3 is not fully understood, it is proposed that PMP3 contributes to the restriction of Na⁺ uptake and consequently salt tolerance in yeasts. In this paper, we have investigated whether the lack of RCI2A gene, homologous to PMP3 gene, causes a salt sensitive phenotype in Arabidopsis (Arabidopsis thaliana (L.) Heynh.) plants; and to thereby indicate the physiological role of RCI2A in higher plants. Two T-DNA insertional mutants of RCI2A were identified. Although the growth of rci2a mutants was comparable with that of wild type under normal conditions, high NaCl treatment caused increased accumulation of Na⁺ and more reduction of the growth of roots and shoots of rci2a mutants than that of wild type. Undifferentiated callus cultures regenerated from rci2a mutants also accumulated more Na⁺ than that from wild type under high NaCl treatment. Furthermore, when wild-type and rci2a plants were treated with NaCl, NaNO₃, Na₂SO₄, KCl, KNO₃, K₂SO₄ or LiCl, the rci2a mutants showed more reduction of shoot growth than wild type. Under treatments of tetramethylammonium chloride, CaCl₂, MgCl₂, mannitol or sorbitol, the growth reduction was comparable between wild-type and rci2a plants. These results suggested that RCI2A plays a role directly or indirectly for avoiding over-accumulation of excess Na⁺ and K⁺ ions in plants, and contributes to salt tolerance.     

Salt Tolerance in Aquatic Macrophytes: Ionic Relation and Interaction

Effects of seawater salinity (SWS) and pure NaCl on the intracellular contents of Na⁺, K⁺, Mg²⁺, Ca²⁺, chlorophylls (Chl) and carotenoids (Car) were studied in three submerged aquatic macrophytes, Hydrilla verticillata, Najas indica and Najas gramenia, which differed in their tolerance to salinity. NaCl resulted in significant increase in Chl/Car ratio in the salt-sensitive H. verticillata and moderately salt-tolerant N. indica, but not in the salt-tolerant N. gramenia. SWS treatment did not result in any significant change in the ratio. The intracellular content of Na⁺ increased significantly in all the test plants upon exposure to both NaCl and SWS. The content of K⁺ decreased significantly in these plants upon salinity treatment, except in N. gramenia. The contents of Ca²⁺ and Mg²⁺ decreased significantly upon NaCl treatment and remained unchanged or increased upon SWS treatment. No relationship between salt tolerance and K⁺/Na⁺ ratio was observed. The maintenance of a minimal level of K⁺ was observed to be the most probable requirement of salt tolerance in aquatic macrophytes.