Aquatic Botany 91 (2009) 181–186 Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot Effects of NaCl salinity on growth, morphology, photosynthesis and proline accumulation of Salvinia natans Arunothai Jampeetong a,*, Hans Brix b a b Department of Biology, Faculty of Science, Chiang Mai University, Meuang, Chiang Mai 50202, Thailand Department of Biological Sciences, Plant Biology, Aarhus University, Ole Worms Allé 1, 8000 Aarhus C, Denmark A R T I C L E I N F O A B S T R A C T Article history: Received 28 November 2008 Received in revised form 15 May 2009 Accepted 20 May 2009 Available online 29 May 2009 The effects of NaCl salinity on growth, morphology and photosynthesis of Salvinia natans (L.) All. were investigated by growing plants in a growth chamber at NaCl concentrations of 0, 50, 100 and 150 mM. The relative growth rates were high (ca. 0.3 d 1) at salinities up to 50 mM and decreased to less than 0.2 d 1 at higher salinities, but plants produced smaller and thicker leaves and had shorter stems and roots, probably imposed by the osmotic stress and lowered turgor pressure restricting cell expansion. Na+ concentrations in the plant tissue only increased three-fold, but uptake of K+ was reduced, resulting in very high Na+/K+ ratios at high salinities, indicating that S. natans lacks mechanisms to maintain ionic homeostasis in the cells. The contents of proline in the plant tissue increased at high salinity, but concentrations were very low (<0.1 mmol g 1 FW), indicating a limited capacity of S. natans to synthesize proline as a compatible compound. The potential photochemical efficiency of PSII (Fv/Fm) of S. natans remained unchanged at 50 mM NaCl but was reduced at higher salinities, and the photosynthetic capacity (ETRmax) was significantly reduced at 50 mM NaCl and higher. It is concluded that S. natans is a salt-sensitive species lacking physiological measures to cope with exposure to high NaCl salinity. At low salinities salts are taken up and accumulate in old leaves, and high growth rates are maintained because new leaves are produced at a higher rate than for plants not exposed to salt. ß 2009 Elsevier B.V. All rights reserved. Keywords: Constructed wetland ETRmax Free-floating macrophyte Fv/Fm-ratio Ionic imbalance Proline Salinity Salt stress Salvinia natans Water fern 1. Introduction Communities of aquatic plants are sensitive to changes in water salinity (Smith et al., 2009). In dry regions the salinity of inland water bodies can be high because evapotranspiration exceeds precipitation, and irrigation for agricultural production can lead to salt accumulation in irrigated soils and discharge of high salinity waters to rivers and downstream lakes (Rengasamy, 2006). Also intrusion of seawater into coastal freshwaters can occur as a consequence of extreme storm events and tidal influence, and the resulting high and often fluctuating salinities is an issue affecting aquatic plant survival in such areas. Salt-sensitive plants have reduced survival, growth and development when exposed to even low to moderate salinities, whereas salt-tolerant species are able to grow and reproduce even at oceanic salinities (Munns and Tester, 2008). High concentrations of salt impose both osmotic and ionic stresses on the plants which lead to several morphological and physiological changes. High concentrations of NaCl on the outside of the roots lower the water * Corresponding author. Tel.: +66 53 943346; fax: +66 53 892259. E-mail address: ajampeetong@yahoo.com (A. Jampeetong). 0304-3770/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2009.05.003 potential and make it harder for the roots to extract water, and high concentrations of Na+ and Cl inside the plant cells are inhibitory to many enzyme processes. Accumulation of toxic levels of NaCl in the cytoplasm must therefore be avoided. Plant adaptations to salinity include sequestration of salt ions in vacuoles and accumulation of ‘compatible compounds’, such as sugars, proline and glycinebetaine in the cytoplasm to balance the osmotic pressure (Hopkins, 1999). The intercellular water potential is thereby lowered below the external water potential allowing continued water uptake. However, different species of plants inherently possess different measures and different capacities of coping with exposure to high salinity, and salt stress responses and tolerance vary between species (Munns and Tester, 2008). Salvinia natans (L.) All. forms rapidly expanding mats of foliage on stagnant water bodies in tropical and subtropical regions. It has leaves in whorls of three of which two elliptic leaves float on the water surface and the third is heavily dissected and submerged, performing similar functions to roots (referred to as roots in this study). It is tolerant to high ammonium concentrations (Jampeetong and Brix, 2009), and its high growth rate and the ease of biomass harvesting makes it a prosperous candidate for use in constructed wetland systems for removal of nutrients from polluted waters. Salinity tolerance of this species has, to the 182 A. Jampeetong, H. Brix / Aquatic Botany 91 (2009) 181–186 authors knowledge, not been studied, but the sister species, Salvinia molesta, has been reported to be able to survive at salinities up to 20% of seawater (McFarland et al., 2004). We investigated the ability of S. natans to grow in saline waters and characterized the stress responses of the plant to NaCl salinity in order to increase our understanding of the distribution of this species in relation to salinity in nature. Specimens of S. natans were grown at four concentrations of NaCl and the growth, morphology, various cell constituents as well as photosynthetic performance were monitored to asses the salinity stress responses. The study provides new insight into how free-floating species respond to increased concentrations of NaCl salinity, and supply information that can be used to evaluate the potential applicability of S. natans for use in constructed wetland systems receiving saline polluted waters. exposures of gradually increasing actinic irradiances to the leaf using the fibreoptics and the leaf Clip Holder 2030-B. The plants were exposed to light in the growth cabinet for at least 1 h before the light-curve runs. Due to the short time of acclimation between the single measurements (20 s) it is possible that photosynthesis of the leaves was not in steady state at each irradiance. The LRC was fitted on the basic of the photosynthesis model of Eilers and Peeters (1988) and the apparent rate of photosynthetic electron transport of PSII (ETR) was obtained as ETR = Yield  PAR  0.5  0.84, where Yield = the overall photochemical quantum yield, PAR = the flux density of incident photosynthetically active radiation, the factor 0.5 assumes equal excitation of both PSII and PSI and the factor 0.84 assumes that 84% of the incident quanta are absorbed by the leaf. 2.4. Chlorophyll and carotenoid contents 2. Materials and methods 2.1. Experimental setup Salvinia natans (L.) All. was obtained from a commercial nursery for tropical aquatic plants in Denmark. The plants were incubated in a growth chamber at a temperature of 25 8C, 90% relative air humidity and a photon flux density of 445 mmol m 2 s 1 provided by metal halide bulbs in a 12 h light/12 h dark cycle. The growth medium was a 0.5 strength standard nitrogen-free nutrient solution (Smart and Barko, 1985) to which 0.1 mM P L 1 as KH2PO4, 0.5 mM N L 1 as (NH4)2SO4 and micronutrients were added. The pH of the growth medium was adjusted to 7.0. Fronds of Salvinia from the stock cultures were placed in 4-L containers (n = 5) in the growth chamber. The experimental treatments consisted of four levels of salinity (0, 50, 100, and 150 mM) prepared from nutrient solution and reagent grade NaCl (Merck). The salt concentration in the nutrient solution was gradually increased over a period of 5 d to the final treatment concentrations. The growth solutions were changed and the plants were cleaned gently by hand to remove epiphytic algae every second day. After 2 weeks, the plants were harvested, cleaned and their fresh weight (FW) measured. The plants were then separated into shoots and roots and freeze dried to constant weight. The relative growth rate (d 1) in each treatment was calculated by the formula: RGR = (ln W2 ln W1)/t, where W1 and W2 are the initial and final dry weight (DW, g), and t is the incubating time (d). 2.2. Plant morphology At the end of the second week, plants of all treatments were sampled for morphological studies. The leaf area, number of roots, root length, and the horizontal and vertical stem length of the youngest fully developed pair of leaves (the third and fourth set of leaves from the apex) of each plant were measured. The plants were then separated into leaves, roots, horizontal stem and vertical stem and their DW measured after freeze drying. 2.3. Chlorophyll fluorescence measurements Chl a fluorescence was measured using a pulse amplitude modulated fluorimeter (PAM-2000, Walz GmbH, Effeltrich, Germany). Maximum photochemical efficiencies were estimated by measuring Fv/Fm ratios on intact leaves within the growth cabinet. The plants were dark-acclimated for 1 h before measurements were taken in order to allow complete oxidation of the PSII reaction centres. Instant light response curves (LRC) were obtained using the light-curve programme of the PAM which applies a series of 20 s The content of Chl a, Chl b and total Chl a + b and total carotenoids (xanthophylls and carotenes) in the leaves of the plants was determined by UV–vis spectroscopy (Lichtenthaler, 1987). The freeze dried leaves were cut into small pieces from which subsamples of 5–10 mg were extracted with 8 ml 96% ethanol in the dark at room temperature for 24 h. The absorbance of the extracts was measured at 470.0, 648.6 and 664.2 nm wavelengths. 2.5. Water extractable NH4+ in the plant tissue The freeze dried plant material (leaves and roots) were ground finely in a mill grinder. 5–10 mg of dried plant material was extracted with 15 mL of Milli-Q water at 80 8C in a water bath for exactly 20 min. The concentrations of NH4+ were then analysed by a flow injection spectrophotometer (Quikchem Method no. 10107-06-3B; Lachat Instruments, Milwaukee, WI, USA). 2.6. CN analysis The total carbon and nitrogen content were analyzed in subsamples (ca. 2–3 mg DW) of leaves and roots using a CN analyzer (Na2000, Carlo Erba, Italy). 2.7. Proline analysis Leaves and roots were sampled after 2 weeks in the treatments for analysis of proline using the methodology described by Bates et al. (1973). The plant tissue was immediately after harvest frozen in liquid nitrogen. 0.2–0.5 g FW of the frozen material was ground to a fine powder in a pre-cooled mortar with liquid nitrogen. The powder was homogenized with 5 mL of 3% aqueous sulfosalicylic acid and then the homogenate was centrifuged at 14,000  g for 2 min. Two milliliters of acid-ninhydrin and 2 mL of glacial acetic acid were added into 2 mL of the homogenate in a test tube. The mixture was then incubated at 100 8C for 1 h, after which the reaction was stopped by placing the test tube in an ice bath. Four milliliters of toluene were added to each test tube and vortexed for 15–20 s. The organic and inorganic phases were separated, and the absorbance at 520 nm of the organic toluene phase containing the chromophore was used to quantity the amount of proline. Concentrations of proline in the plant tissue are expressed on a FW basis. 2.8. Mineral elements The concentrations of calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) were analysed on subsamples (ca. 150– 180 mg DW) of freeze dried plant materials which were ground A. Jampeetong, H. Brix / Aquatic Botany 91 (2009) 181–186 183 of squares was performed to identify treatment effects. Post hoc comparisons of treatment means were performed using Scheffe’s test at the 5% significance level. 3. Results 3.1. Growth and morphology Fig. 1. Effects of NaCl salinity on the relative growth rate (RGR, mean  SE, n = 5) of Salvinia natans. Different letters above columns indicate significant differences between treatments. finely in a mill grinder. The subsamples were digested in a microwave sample preparation system (Multiwave 3000, Anton Paar GmbH, Austria) using 4 mL concentrated HNO3 and 2 ml H2O2. The concentrations of the elements were analysed by ICPOES (Optima 2000 DV, Perkin Elmer Instruments Inc., CT, USA). Salt treatment significantly (P < 0.001) affected the growth of S. natans (Fig. 1). The relative growth rates (RGR) were ca. 0.3 d 1 at salinities up to 50 mM and decreased to less than 0.2 d 1 at salinities of 100 and 150 mM. The morphology of the plants was also significantly affected by the treatments (Fig. 2, Table 1). In all treatments with salt the plants produced significantly smaller leaves with shorter stems than in the control (Fig. 2a and b), and the specific leaf area (SLA) decreased with salinity (Fig. 2c). The plants produced fewer roots per frond when exposed to salinity (Table 1), and especially the length of roots was dramatically affected as root lengths in the 100 and 150 mM treatment were only a fifth of the length in the control (Fig. 2d). 3.2. Chlorophyll and total carotenoids 2.9. Statistics All statistics were carried out using the software Statgraphics Plus ver. 4.1 (Manugistics, Inc., MD, USA). Data were tested for normal distribution and variance homogeneity using Cochran’s Ctest and if necessary log-transformed to ensure homogeneity of variance. One-way analysis of variance (ANOVA) using type III sum The contents of chlorophylls in the plants were significantly affected by salinity (Table 2) and decreased from 10.7 mg g 1 DW total Chl a + b in the control to 4.3–4.7 mg g 1 DW for plants grown at 100–150 mM salinity. The content of total carotenoids in the plants was not significantly different between treatments, but the Chl a + b/carotenoid-ratio decreased significantly with salinity. Fig. 2. Effects of NaCl salinity on (a) leaf area, (b) horizontal stem length, (c) specific leaf area, SLA, and (d) root length of S. natans (mean  SE, n = 5). Different letters above columns indicate significant differences between treatments. Table 1 Leaf weight, root weight, number of roots, horizontal stem weight and vertical stem length of Salvinia natans (mean  SE, n = 5) grown at different NaCl concentrations and results of ANOVA (F-ratios). Different letter superscripts between columns indicate significant differences between treatments. NaCl concentration 0 mM 50 mM 100 mM 150 mM F-ratio ** *** P < 0.01. P < 0.001. Leaf weight (mg DW) 6.86  0.49 5.80  0.49 8.54  0.49 9.26  0.49 10.23*** a a b b Root weight (mg DW) 2.17  0.16 1.75  0.68 0.56  0.16 0.48  0.11 5.45** b b a a Number of roots 23.2  1.0 18.9  0.8 15.2  0.4 18.2  1.1 9.08*** b a a a Horizontal stem weight (mg DW) 0.44  0.01 0.27  0.02 0.26  0.07 0.22  0.02 6.90** b a a a Vertical stem length (mm) 4.0  0.3 1.0  0.2 1.0  0.2 1.0  0.2 39.44*** b a a a A. Jampeetong, H. Brix / Aquatic Botany 91 (2009) 181–186 184 Table 2 Concentrations of chlorophylls and total carotenoids (xanthophylls and carotenes) in S. natans (mean  SE, n = 5) grown at different NaCl concentrations and results of ANOVA (F-ratios). Different letter superscripts between columns indicate significant differences between treatments (P < 0.001). NaCl concentration 0 mM 50 mM 100 mM 150 mM F-ratio *** Chl a (mg g 1 DW) 7.57  0.29 5.55  0.23 3.08  0.26 3.29  0.35 54.27 *** c b a a Chl b (mg g Chl a/b ratio 1 3.08  0.14 1.57  0.10 1.22  0.12 1.39  0.16 39.12 *** Total Chl a + b (mg g 1 DW) DW) c b a a 2.46  0.03 2.98  0.04 2.57  0.06 2.41  0.06 abc d c a *** 25.01 10.66  0.43 7.42  0.33 4.30  0.38 4.67  0.50 49.15 *** c b a a Total carotenoids (mg g 1 DW) Chl a + b/total carotenoid ratio 0.97  0.06 0.87  0.08 0.76  0.05 0.72  0.04 11.51  0.95 9.35  1.22 5.69  0.38 6.42  0.47 3.14 b b a a *** 10.35 P < 0.001. 3.3. Water extractable NH4+ in the plant tissue The concentration of water extractable NH4+ in the plant tissues was significantly affected by the salinity treatments (Table 3). In the leaves, the concentration of NH4+ was highest at the highest salinities, but in the roots the concentration decreased with salinity and was highest in the control. 3.4. Total carbon and nitrogen The total C content was higher in leaves than in roots, and the content increased significantly with salinity in both leaves and roots (Table 3). The content of total N in leaves decreased from 5.8% DW in the control treatment to 2.8–3.1% DW at the high salinity treatments, but the contents in the roots (3.6–3.9% DW) were less affected by the salinity treatments (Table 3). The C:N- ratio increased with salinity most in the leaves due to the low N content. 3.5. Cations The salinity treatments significantly affected ion uptake and concentrations of cations in the tissues (Fig. 3). The concentration of K in the plant tissue was especially affected as concentrations were more than 10 times lower in the highest salinity treatment compared with the control treatment (Fig. 3a). The Na concentrations increased as expected with the Na concentrations in the growth medium, but not proportionally, and in roots the Na concentration was lower at 150 mM salinity than at 50 and 100 mM salinity (Fig. 3b). Calcium and magnesium concentrations in leaves were lower at 50 and 100 mM salinities compared with the control, but at 150 mM salinity the concentrations were again higher (Fig. 3c Fig. 3. Effects of NaCl salinity on the concentrations of (a) K, (b) Na, (c) Ca, (d) Mg, (e) the Na/K-ratio, and (f) the proline concentration in roots (dark column) and leaves (grey column) of S. natans (mean  SE, n = 5). Different letters above columns indicate significant differences between treatments. A. Jampeetong, H. Brix / Aquatic Botany 91 (2009) 181–186 185 Table 3 Water extractable NH4+ and total C and N contents in leaves and roots of S. natans (mean  SE, n = 5) grown at different NaCl concentrations and results of ANOVA (F-ratios). Different letter superscripts between columns indicate significant differences between treatments (P < 0.001). NaCl concentration (by weight) 0 mM 50 mM 100 mM 150 mM F-ratio *** Water extractable NH4+-N (mmol g Leaves DW) Roots b 0.06  0.002 0.04  0.0011 a 0.07  0.003 c 0.07  0.002 c 50.7*** 1 Leaves c 0.14  0.01 0.05  0.002 0.04  0.001 0.03  0.002 61. 9*** Total C (% DW) b ab a 42.7  0.1 43.5  0.1 44.9  0.1 44.7  0.1 Total N (% DW) Roots a b d c 210.96*** 38.7  0.2 41.7  0.2 42.8  0.1 43.4  0.1 171.7*** Leaves a b c d 5.81  0.03 4.19  0.05 2.84  0.02 3.07  0.02 1634.2*** C:N-ratio Roots d c a b 3.93  0.03 3.64  0.06 3.57  0.04 3.75  0.03 11.4*** Leaves c ab a b 7.3  0.1 10.4  0.1 15.8  0.1 14.6  0.1 1833.7*** Roots a b d c 9.8  0.1 11.5  0.2 12.0  0.1 11.5  0.1 a b c bc 60.1*** P < 0.001. Fig. 4. Effects of NaCl salinity on (a) the Fv/Fm ratio and (b) the maximum electron transport rate (ETRmax) of S. natans (mean  SE, n = 5). Different letters above columns indicate significant differences between treatments. and d). In roots Ca and Mg concentrations were less affected; only the Ca concentration at 150 mM salinity was higher than in the other treatments. The Na/K-ratio in the plant tissues in the 150 mM salinity treatment was higher by a factor of 28 in the leaves and 37 in the roots compared to the Na/K-ratio in the control plants (Fig. 3e). 3.6. Proline The concentrations of proline in the plant tissues were generally very low, <0.1 mmol g 1 FW, but the levels were significantly affected by salinity (Fig. 3f). The concentrations of proline were about three times higher in the plants grown at 150 mM salinity compared to the control. 3.7. Chlorophyll fluorescence and maximum electron transport rate (ETRmax) The Fv/Fm ratio of the plants grown at 50 mM salinity was the same as that of the control plants, but at 100 and 150 mM salinity the Fv/Fm ratios were significantly lower (Fig. 4a). The ETRmax was significantly affected by the salinity treatments even at 50 mM salinity (Fig. 4b). For plants grown at 100 and 150 mM salinity the ETRmax were two and five times lower, respectively, than the ETRmax of the control plants. 4. Discussion High NaCl concentrations in the growth medium of plants generate primary and secondary effects that negatively affect plant growth and development. Primary effects are ionic toxicity and osmotic stress. Ionic toxicity occurs because high concentrations of Na+ and Cl in the cytoplasm of cells disturb several biochemical and physiological processes, and osmotic stress is induced by the lowering of the water potential causing turgor reduction and cellular water loss. Secondary effects of NaCl stress include inhibition of K+ uptake, membrane dysfunction and generation of reactive oxygen species in the cells (Rout and Shaw, 2001; Ghoulam et al., 2002; Agarwal and Pandey, 2004; Upadhyay and Panda, 2005). The NaCl solutions evidently imposed osmotic stress in S. natans at concentrations of 50 mM and higher. Even though the RGR was not significantly reduced at 50 mM NaCl, the leaves were smaller and thicker, and the stems and roots shorter at this salinity. These morphological changes were probably caused by the reductions in turgor pressure within the cells that restricted cell expansion. Ionic toxicity of Na+ and Cl generally occurs at concentrations in the cytoplasm exceeding 100 mM where most enzymes start to become inhibited (Munns, 2002). Ionic toxicity, due to Na+ and Cl accumulation in the cytoplasm, did not seem to be of major importance for S. natans, as the difference in Na+ concentration in the tissues between plants grown at 50 mM NaCl, where RGR was not affected, and 100 and 150 mM NaCl, where RGR was strongly reduced, was only minor. Rather, disturbance of the K+ acquisition resulting in very high Na+/K+-ratios in the tissues were presumably a main factor responsible for the salt injury, as has also commonly been found for both terrestrial and aquatic plants (Lutts et al., 1996a; Ashraf and Sultana, 2000; Rout and Shaw, 2001; Pagter et al., 2009). Na+ competes with K+ for uptake into cells, particularly when the external concentrations of Na+ are substantially higher than that of K+, and the ability to maintain Na+/K+ homeostasis in the cells is crucial for the salt tolerance of plants. Our results indicate that S. natans lacks mechanisms to maintain homeostasis as the high concentrations of Na+ in the growth solution created a deficiency of K+, as has also been shown in other studies (Colmer et al., 1996; Lutts et al., 1996b; Ghoulam et al., 2002). Compartmentalization of Na+ and Cl into the vacuole, and the accumulation of organic solutes, such as sugars, and amino acids, that do not inhibit metabolic processes, in the cytoplasm, is a common mechanism of maintaining intercellular homeostasis. Proline is one of the so-called ‘compatible compounds’ that are commonly found in high concentrations when plants are exposed to salt stress (Rout and Shaw, 1998; Dluzniewska et al., 2007; Wang et al., 2007; Pagter et al., 2009). In the present study the content of proline, even though it increased at high salinity, was 186 A. Jampeetong, H. Brix / Aquatic Botany 91 (2009) 181–186 very low (<0.1 mmol g 1 FW) indicating that S. natans has a limited capacity to sequester Na+ and Cl in the vacuoles and to synthesize proline as a compatible compound. After 1-week growth at high salinities the older leaves became first yellow green and then brown indicating marked injury of old leaves. Similar responses have been found for other aquatic macrophytes exposed to saline conditions (Haller et al., 1974; Rout and Shaw, 2001; Upadhyay and Panda, 2005). Chlorosis is a common response of salinity stress (Bourgeais-Chaillou et al., 1992; Lutts et al., 1996b; Rout and Shaw, 1998; Parida and Das, 2005; Jamil et al., 2007) that can also limit the photosynthetic efficiency. The Fv/Fm ratio of S. natans remained unchanged at 50 mM NaCl even though the plants contained less Chl at this salinity compared with the control plants. This shows that Chl content per se does not affect the photochemical efficiency of PSII, as has also been found in other studies (Li and Nothnagel, 1989). However, the photosynthetic capacity measured as ETRmax decreased when S. natans were exposed to NaCl salinity, and the lower Chl a + b/carotenoid-ratios at high salinity also indicate stress and damage to the photosynthetic apparatus. Salinity is known to affect NH4+ uptake and amount of rubisco (BourgeaisChaillou et al., 1992). The reduced contents of N in the plant tissues may have been the result of impeded uptake and assimilation of NH4+ as Na+ is known to competitively inhibit NH4+ uptake (Bradley and Morris, 1991). The impeded uptake of K+ at high salinities might also have affected stomatal control and hence photosynthesis (Yeo et al., 1985; Navarro et al., 2007). S. natans belongs to a group of small free-floating species together with Azolla and Lemna spp., whereas species such as Eichhornia crassipes and Pistia stratiotes are significantly larger. Haller et al. (1974) found that the larger species of free-floating plants were more salt tolerant than the smaller species. The small species have very simple roots with no secondary growth or branching (Lemna), or no roots but a highly dissected submerged leaf that has similar functions to roots (Salvinia). Larger species like E. crassipes have thicker and more differentiated root tissue with a well-developed endodermis and several cell layers separating the vascular bundle from the surrounding solution which may improve their possibility of controlling ion uptake (Julien et al., 2002; Mahmood et al., 2005). For the small species, the direct exposure of the lower side of the thin leaves to the saline solution may also play a role for their low salt tolerance. In conclusion, S. natans is a salt-sensitive species lacking efficient measures to cope with exposure to high salinity. The growth rate of the plant is not affected at NaCl salinities up to 50 mM but the fronds becomes much smaller and thicker and the roots shorter and fewer. The salts that unavoidably are taken up even at low salinities accumulate in the old leaves, eventually to toxic levels where the leaves die. 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