REGULAR ARTICLE

Effect of salinity on growth, ion accumulationand the roles of ions in osmotic adjustment of twopopulations of Suaeda salsa

Jie Song & Min Chen & Gu Feng & Yonghui Jia &

Baoshan Wang & Fusuo Zhang

Received: 8 December 2007 /Accepted: 29 May 2008 / Published online: 18 July 2008# Springer Science + Business Media B.V. 2008

Abstract The effect of salinity on growth, ionaccumulation and the roles of ions in osmoticadjustment of two populations of Suaeda salsa wereinvestigated. Seeds were collected from an intertidalzone or a saline inland zone in the Yellow River Deltain Shandong province, China. Seedlings were ex-posed to 10, 100, 200, 400 or 600 mM NaCl for 18days in a greenhouse. NO3

− concentration in the soilwhere S. salsa grows in an intertidal zone was muchlower than that for the second population, but leafNO3

− concentration was the same in the twopopulations under field conditions. When plants werecultured in a greenhouse under natural light condi-tions, S. salsa from the intertidal zone showed fewermain stem branches and lower relative shoot growthcompared to S. salsa from saline inland. Leaf Cl−

concentration of saline inland S. salsa was signifi-cantly higher than that of S. salsa from the intertidalzone, while the opposite was true for the concentra-

tion of NO3− in leaves of plants. For S. salsa from the

intertidal zone NO3− contributed more than Cl− to the

osmotic potential, whereas S. salsa from the salineinland exhibited a reverse relationship under salineconditions, indicating that NO3

− plays an importantosmotic role in S. salsa from the intertidal zone inhigh salinity. In conclusion, S. salsa from theintertidal zone may employ superior control of ionuptake and content than S. salsa from the salineinland zone. The two populations of Suaeda salsapresented different ability in chloride exclusion andnitrate accumulation. These characteristics may affectthe distributions of S. salsa in natural highly salineenvironments.

Keywords Euhalophyte . Cl− . NO3− .

Osmotic adjustment . Salinity . Suaeda salsa

Introduction

Over 800 million hectares of land are salt-affectedthroughout the world (Munns 2005). While almost allmodern crops lack the genetic basis for salt tolerance,halophytes can grow well in salty soils employingspecial biological strategies, e.g. excreting excessivesalt through glands (Ramadan 2001; Taleisnik andAnton 1988), compartmentalizing toxic ions intovacuoles (Zhao et al. 2003) or in the coat of seeds(Khan et al. 1985; Song et al. 2005), or by selectivelyexcluding toxic ions from the roots (Tester and

Plant Soil (2009) 314:133–141DOI 10.1007/s11104-008-9712-3

Responsible Editor: John McPherson Cheeseman.

J. Song :M. Chen :Y. Jia : B. WangCollege of Life Science, Shandong Normal University,Jinan 250014, People’s Republic of China

G. Feng (*) : F. ZhangCollege of Resource and Environmental Science,China Agricultural University,Beijing 100094, People’s Republic of Chinae-mail: fenggu@cau.edu.cn

Davenport 2003). While their potential has not beenrealized agronomically, halophytes have beenregarded as a possible source of vegetable, forage,and oilseed crops (Glenn and Brown 1999). Thereforea better understanding of how naturally adaptedhalophytes tokerate salty soils is essential to help tofocus the efforts of plant breeders and molecularbiologists working with conventional crop plants(Glenn and Brown 1999).

Salinity reduces plant growth through osmoticstress, ion toxicity, and subsequently through nutri-tional stress (Nublat et al. 2001; Short and Colmer1999). Regulation of the osmotic balance by anaccumulation of inorganic cations or anions in thevacuole is a most important biological feature ofhalophytes, particularly for leaf/shoot succulent hal-ophytes, to lower their osmotic potential to a degreethat the uptake of water from saline soils isaccomplished (Glenn and Brown 1999; Short andColmer 1999). Song et al. (2006) reported the osmoticrole of NO3

− was more important but that of Cl− wasless important in a euhalophyte, Suaeda physophorathan that in a xerophyte, Haloxylon persicum, undersaline conditions, and this characteristic may deter-mine the natural distribution of the two species intheir saline or arid environments. The nature of thesalt and its concentration in the soil may vary indifferent environments, such that a species found inmore than one environment may experience differentchallenges requiring different protective strategiesleading to osmotic adjustment. The relative successwith which species deal with the variability in saltstresses will determine their natural distribution, butthis aspect of the halophytic lifestyle and adaptationto different environments is little understood.

Suaeda salsa L. (Chenopodiaceae) is a succulenthalophytic herb. Fresh branches of S. salsa are highlyvaluable as a vegetable, and the seeds can produceedible oil (Zhao et al. 2002). S. salsa occurs both insaline soils and in the intertidal zone (Liu 2006).Leaves of S. salsa from the saline inland have beenshown to accumulate high amounts of Na+ and Cl−

(Zhang and Zhao 1998), compartmentalizing theseions to vacuoles, which then lowers the osmoticpotential of cells under saline conditions (Zhao etal. 2003). We hypothesized that S. salsa in anintertidal zone would have a greater ability toregulate the nature of the ions that were taken upcompared with S. salsa on saline inland habitats, and

such trait might be related to their geographicdistribution. To test this hypothesis, the effects ofNaCl on growth, ion accumulation and the roles ofthese ions in osmotic adjustment for S. salsa fromthe intertidal zone and plants grown in saline inlandlocations were investigated.

Materials and methods

Plant material

Seeds of Suaeda salsa L. from either the intertidalzone or saline inland locations were collected fromthe respective habitats in the Yellow River Delta inShandong province, China, in October 2005. Dryseeds were stored in a refrigerator at <4°C beforebeing used.

Plant culture and NaCl treatments

Seeds of S. salsa from the intertidal zone or salineinland were sown in plastic pots. There were sixseedlings in each pot containing 2.5 kg river sand.The plants were cultured in a greenhouse undernatural light conditions. Nutrient solutions weresupplied every day with the following composition:3 mM NO3

−, 1 mM Ca2+, 1.2 mM K+, 1 mM Na+, 0.4mM Mg2+, 0.4 mM SO4

2−, 0.2 mM H2PO4−, 1 mM

Cl−, 45 μM Fe-EDTA, 23 μM H2BO3−,4.6 μM Mn2+,

0.16 μM Cu2+, 0.38 μM Zn2+, 0.28 μM MoO42−. The

pH was adjusted to 6.0±0.1 with NaOH and H2SO4.

The amount of 200 mL nutrient solution wassupplied every day; an amount that flushed thedrained pots. After pre-culture for 55 days, seedlingsof S. salsa from the intertidal zone or saline inlandwere treated with 10, 100, 200, 400 or 600 mM NaCl.The NaCl was dissolved in nutrient solution describedabove. To avoid osmotic shock, NaCl was appliedgradually by adding 50 mM NaCl per day. Thetemperature in the greenhouse was 31±3oC day/25±3°C night. The experiment was terminated at 18 daysafter final salinity concentrations were reached, andshoots and roots were harvested separately. Freshplant samples were frozen in liquid nitrogen tomeasure the osmotic potential and concentrations ofCl−, NO3

−. The concentrations of Na+, K+ weredetermined in dry samples. Three replicates were setfor each treatment.

134 Plant Soil (2009) 314:133–141

Measurements of development

The number and the increase in number of side branchesalong main stem of S. salsa from the intertidal zone orsaline inland locations were recorded at the beginningand at the end of the experiment. One plant (as onereplicate) was chosen among 6 plants in each pot, and3 replicates were set for each treatment.

Determination of plant growth

Shoot and root of plants were sampled at the beginningof treatments (3 pots with 6 plants each) and at the end ofthe experiment (3 pots with 6 plants each for eachtreatment). After washing the plant samples, fresh weight(FW) was measured. Then the dry weight (DW) wasmeasured after being dried at 80°C for three days. Watercontent (WC) was calculated as: (FW- DW)/DW. Meanvalue of 6 plants in each pot was used as one replicate,and three replicates were set for each treatment.

Relative growth rate (RGR) was calculated usingthe following equation (Botella et al. 1997):

RGR ¼ LnW2� LnW1ð Þ= t2� t1ð ÞWhere: W1 and W2 represent shoot dry weight at thebeginning of treatments and at the end of theexperiment, respectively, and t1 (first treatment day)and t2 (final treatment day), time in days.

Determination of osmotic potential

Frozen leaves were allowed to thaw in a syringe andthe liquid squeezed from the plant tissues wasanalyzed using a freezing point osmometer (Fiske210, Advanced Instruments INC., Massachusetts,USA) to determine the ic value (the value readingfrom the instrument). The osmotic potential ofindividual solutes was calculated as: Ψs = -icRT,where R is the universal gas constant and T is thetemperature in degrees Kelvin (Zhao et al. 2003).Three replicates were set for each treatment.

Determination of inorganic ions

Frozen leaves were extracted with boiling distilled water,the solution was filtered, and the Cl− and NO3

−

concentrations were determined. NO3− concentration

was determined by a colorimetric method (Cataldo et al.1975) (UV-120-02 Spectrophotometer, Shimadzu,

Kyoto, Japan), and Cl− concentration was determinedby titration with 0.03 mM AgNO3, with 5% K2CrO4 asindicator. A 40-mg dry sample was processed in amuffle stove and the ash dissolved with concentratednitric acid and then set to a volume of 50 mL withdeionized water. The concentrations of Na+ and K+ weredetermined by flame photometer (Flame Photometer410, Sherwood Scientific Ltd, Cambridge, UK). Threereplicates were used for each treatment. The Ψs of Na+,K+, Cl− or NO3

− was calculated as: Ψs = -nRT/V, wheren is the number of solute molecules; R is the universalgas constant; T is the temperature in degrees Kelvin; andV is the volume in litres. Osmotic coefficients of ions intissue water were assumed to equal 1 (Song et al. 2006).

Determination of NO3− concentration in leaves

of plants in field condition

S. salsa in the intertidal zone (N37°25′; E118°58′) orsaline inland zone (N37°20′; E118°36′) were collectedin the Yellow River Delta in Shandong, the middle-eastprovince of China in June, 2006. Seven plants ofeach population were collected randomly. Watercontent in leaves was determined as described above.Leaves of each plant were frozen with liquid nitrogen,and then NO3

− concentration was determined asdescribed above.

Soil sample collection and determination of watercontent and ion content

Soil samples (0–30 mm under soil surface) around theroot environment of each plant were also taken. Soilsamples were dried in an oven for two days at 105°C.Soil water contents were determined as WC = (FW-DW)/FW×100%. Nitrate in fresh soil was extractedusing a 0.01 mMCaCl2 solution. Samples were filteredusing filter paper to remove soil particles. The clearextract was stored in a refrigerator at <4°C beforebeing used. Nitrate-N was determined by a colorimet-ric method with an autoanalyzer (AutoAnalyzer3Digital Colorimeter, Bran+Luebbe Ltd., Germany).

Statistical analysis

Data under filed conditions, and data of concentra-tions of ions and the number and increase in numberof side branches at the same salinity level betweentwo populations were subjected to a one-way

Plant Soil (2009) 314:133–141 135

ANOVA, and data of growth, water content, osmoticpotential, and the estimated contribution of ions toosmotic potential in leaves of two S. salsa populationswere subjected to a two-way ANOVA using theSAS™ software (SAS Institute Inc. 1989). Multiplecomparisons of means of concentrations of ions andthe number and increase in number of side branchesbetween different salinity treatments within a popula-tion were performed using Duncan’s test at the 0.05significance level (all tests were performed with SPSSVersion 12.0 for Windows).

Results

Water content and concentrations of NO3− in soils

and in leaves of plants in field condition

Soil and leaf water content in S. salsa from anintertidal zone were higher than that in S. salsa fromsaline inland. NO3

− concentration in soil where S.salsa occurs in an intertidal zone was much lowerthan that in soil where S. salsa occurs in saline inland,but leaf NO3

− concentration was the same in the twopopulations in field condition (Table 1).

Effect of salinity on plant growth

Up to 400 mM NaCl had no adverse effect on shootand root dry weight of seedlings of S. salsa fromeither the intertidal zone or saline inland, except thatroot dry weight was lower in 400 mM NaCl than thatin 10 mM NaCl for S. salsa from the intertidal zone.Shoot dry weight of seedlings of S. salsa from salineinland was significantly higher than that of S. salsafrom the intertidal zone in all NaCl concentrationsused (Table 2). This was also found for shoot and root

relative growth rates (RGR) for S. salsa from eitherthe intertidal zone or saline inland (Table 2).

Salinity had no adverse effect on water content(WC) in leaves of S. salsa from the intertidal zone,while leaf water content of S. salsa from saline inlandlocations was reduced in the 400 and 600 mM NaCltreatments. Water content in leaves of S. salsa fromsaline inland was higher than that of S. salsa from theintertidal zone in all NaCl concentrations (Table 2).

Effect of salinity on plant development

The final number, and the increase in number, of sidebranches along the main stem of S. salsa from thesaline inland was much higher than that of S. salsafrom the intertidal zone (Fig. 1a,b). Salinity had noadverse effect on the number (Fig. 1a) and theincrease in number (Fig. 1b) of side branches in mainstem of S. salsa from either intertidal zone or salineinland, except that at 600 mM NaCl the increase innumber of side branches in main stem of S. salsafrom saline inland was reduced (P<0.05) (Fig. 1b).

Effect of salinity on concentrations of ions

Concentrations of Na+ increased with increasedexternal application of NaCl in S. salsa plants fromeither the intertidal zone or saline inland. Concen-trations of Na+ in leaves of S. salsa from theintertidal zone were the same as those in leaves ofS. salsa from the saline inland, except that it washigher at 10 mM NaCl in leaves of S. salsa from theintertidal zone than that in leaves of S. salsa fromsaline inland (Fig. 2a).

Concentrations of Cl− increased with increasedexternal application of NaCl in S. salsa from eitherthe intertidal zone or saline inland. Concentrations of

Table 1 Water content and concentrations of NO3− in soils and in leaves of plants in S. salsa from an intertidal zone or saline inland

in field condition

Origin of population Soil Leaves of plants

Water content (%) NO3− concentration

(mmol l−1 soil solution)Water content(ml g−1 DW)

NO3− concentration

(mol l−1 tissue water)

Intertidal zone 22.8±0.69a 0.32±0.08b 11.5±1.10a 0.19±0.03aSaline inland 17.9±1.30b 1.12±0.30a 10.4±1.07b 0.20±0.05a

Means in a column that have a different letter are significantly different at P<0.05; date are means ±SE, n=7.

136 Plant Soil (2009) 314:133–141

Cl− in leaves of S. salsa from the intertidal zone werelower than those of S. salsa from saline inland in therange from 10 to 600 mM NaCl (Fig. 2b), i.e.concentrations of Cl− in leaves of S. salsa from theintertidal zone were 79, 60, 64, 66 and 71% in 10,100, 200, 400 and 600 mM NaCl, respectively, of thatin leaves of S. salsa from saline inland (Fig. 2b).

The concentration of K+ decreased along theincrease of NaCl in S. salsa from either the intertidalzone or saline inland. However, the concentration ofK+ in leaves of S. salsa from the intertidal zone wasmuch higher than that of S. salsa from saline inland at600 mM NaCl (Fig. 2c).

The concentration of NO3− in the leaves of S. salsa

from the intertidal zone was much higher than theconcentration in S. salsa from saline inland at 10 to600 mM NaCl, i.e. concentrations of NO3

− in leavesof S. salsa from the intertidal zone were 3.6, 3.6, 3.2,3.3 and 3.4 times in 10, 100, 200, 400 and 600 mMNaCl, respectively, of that in the leaves of S. salsafrom saline inland (Fig. 2d). The concentration of

NO3− decreased when NaCl increased to 200 mM

NaCl in S. salsa from the intertidal zone, and itdecreased at 400 and 600 mM NaCl in S. salsa fromsaline inland (Fig. 2d).

Effect of salinity on osmotic potentialand the estimated contribution of ions to Ψs

Salinity decreased the osmotic potential in leaves of S.salsa from either the intertidal zone or saline inland,especially for S. salsa from saline inland (Table 3).

The estimated contribution of Na+ to Ψs (CNa)increased when NaCl increased to 400 mM NaCl forS. salsa from both the intertidal zone and saline inland(Table 2). CNa in leaves of S. salsa from the intertidalzone was varied between 1.2 and 1.3 times across therange of NaCl concentrations, of that in leaves of S.salsa from saline inland (Table 3).

The estimated contribution of Cl− to Ψs (CCl)increased when NaCl increased to 200 mM NaCl forS. salsa from both locations (Table 3). CCl in leaves of

Table 2 The effects of NaCl on dry weight (DW), relative growth rate (RGR) of shoot and root, and water content(WC)in leaves of S.salsa from the intertidal zone or saline inland

Origin of population NaCl (mM) DW (mg plant−1) RGR (mg g−1day−1) WC (ml g−1 DW)

Shoot Root Shoot Root

Intertidal zone 10 205.5±8b1 58.6±1.4b 49.4±1.3b 57.4±3.9b 8.8±0.2b100 255.1±13b 63.9±2.9a 56.6±1.7a 62.9±1.5a 9.9±0.4a200 268.4±25a 67.3±2.9a 58.2±3.2a 64.7±1.4a 10.0±0.7a400 225.6±3b 54.2±2.3b 52.5±0.4b 57.5±1.4b 8.8±0.5b600 177.5±13c 29.7±2.3c 44.5±2.4c 37.4±2.5c 8.2±0.3bMean2 226.4b3 84.8b 52.2 b 56.0 b 9.1b

Saline inland 10 641.8±15b 122.0±3.8a 71.6±0.8b 84.5±1.1a 12.6±0.1ab100 690.7±36a 133.4±9.9a 74.0±1.7a 87.4±2.4a 13.6±0.9a200 702.3±28a 121.0±8.5a 74.5±1.3a 84.2±2.4a 11.6±0.5bc400 635.2±22bc 106.3±9.3b 71.2±1.1b 79.8±3.0b 10.6±0.7c600 590.7±22c 72.7±2.1c 68.8±1.2c 67.2±1.0c 10.4±1.8cMean 652.1a 110.1a 72.0a 80.6 a 11.8a

Analysis of Variance (F-Values)Origin of population 3492.4*** 712.6*** 1047.4*** 812.3*** 88.0***Salinity 25.5*** 64.7*** 32.7*** 92.7*** 9.8***

Plants were treated with a range of NaCl for 18 days1Within each column, values with the same letter are not significantly different at P<0.05 across NaCl levels for S. salsa from theintertidal zone or saline inland; data are means ±SE, n=32 Mean value for S. salsa from the intertidal zone or saline inland3Mean values with same letters are not significantly different at P<0.05 between S. salsa from the intertidal zone and S. salsa fromsaline inland

*** P<0.001

Plant Soil (2009) 314:133–141 137

S. salsa from the intertidal zone was 76, 73, 78, 79and 90% in 10, 100, 200, 400 and 600 mM NaCl,respectively, of that in leaves of S. salsa from salineinland (Table 3).

The estimated contribution of K+ to Ψs (CK)decreased with the increase of NaCl in S. salsa fromboth habitats, especially in 600 mM NaCl for S. salsafrom saline inland (Table 3). CK in leaves of S. salsafrom the intertidal zone was varied between 1.0 and1.4 times across the range of NaCl concentrations, ofthat in leaves of S. salsa from saline inland (Table 3).

The estimated contribution of NO3− to Ψs (CNO3)

decreased with a increase of NaCl in S. salsa fromboth the intertidal zone and saline inland, especiallyfor S. salsa from saline inland (Table 3). CNO3 inleaves of S. salsa from the intertidal zone was 3.5,4.4, 3.9, 4.0 and 4.3 times in 10, 100, 200, 400 and600 mM NaCl, respectively, of that in leaves of S.salsa from saline inland (Table 3).

Discussion

The ability to regulate Na+ or Cl− uptake andtransport to the shoot is crucial for salt resistance inplants (Greenway and Munns 1980; Tester andDavenport 2003; Warwick and Halloran 1992). Undernatural condition Na+ and Cl− concentrations arehigher in sheaths than in leaf blades of, for example,Diplachne fusca (L.) Beauv., indicating that thespecies has the capacity to sequester high levels ofNa+ and Cl− in the sheath away from the leaf blade aswell as maintaining a high selectivity for K+ over Na+

(Warwick and Halloran 1991). Most plants canexclude about 98% of the salt in the soil solution,and only allow transport of a small amount throughthe xylem to the shoot (Munns, 2005). Salt-resistanttree species Melaleuca cuticularis and Casuarinaobesa were found to be more effective in excludingNa+ and Cl− than less salt-resistant tree speciesBanksia attenuate (Carter et al. 2006). Our resultsindicate that there was no difference in the ability ofS. salsa populations from the intertidal zone andsaline inland to exclude Na+ (Fig. 2a), while S. salsafrom the intertidal zone may be better at regulatingCl− uptake within a non-toxic level under highlysaline conditions (Fig. 2b). Cl− in expanded leaves isassociated with chlorosis and death, and these injuriesoccur even when Na+ is low in the leaves (Greenwayand Munns 1980). The content of Na+ and Cl− in thesoil where S. salsa occurs in the saline inland zone is2.4 and 2.0 g kg−1 dry soil, respectively, but is 4.5 and3.3 g kg−1 dry soil, respectively, in the soil whereS. salsa occurs in the intertidal zone (Liu 2006).Therefore, S. salsa from the intertidal zone may havea greater ability to regulate Cl− to a lower level thanthe second population.

Salinity decreases NO3− uptake in both non-halo-

phytes, e.g. Hordeum vulgare (Aslam et al. 1984) andRicinus communis (Peuke et al. 1996), and halophytes,e.g. Plantago maritima L. (Rubinigg et al. 2003), andthis has been attributed to a competition between NO3

−

and Cl− for certain transport systems, which areproposed to play significant roles in uptake or thexylem loading of NO3

− and Cl− (Bottacin et al. 1985;Cerezo et al. 1997; Köhler and Raschke 2000). Theprecise mechanisms that lead to more NO3

− and lessCl− accumulation in the leaves of S. salsa from theintertidal zone compared to S. salsa from saline inlandremain to be determined.

NaCl (mM)

Num

ber

of s

ide

bran

ches

alo

ng m

ain

stem

Incr

ease

in n

umbe

r of

side

bra

nche

s al

ong

mai

n st

em

aa a a

a

b bb

bb

0

20

5

10

15

20

25

30

a

aa

a

a

bb

b

b b

0

5

10

15

(a)

(b)

10 100 200 400 600

From saline inland From intertidal zone

Fig. 1 The number (a) and increase in number (b) of sidebranches along main stem of S. salsa from the intertidal zone orsaline inland at the end of the experiment. Plants were treatedwith a range of NaCl for 18 days. Means within the same NaClconcentration having different letters are significantly differentat P<0.05. Vertical bars represent standard errors (n=3)

138 Plant Soil (2009) 314:133–141

When NO3− absorption exceeds reduction, NO3

−

accumulates in the vacuole (Blom-Zandstra andLampe, 1983). Mott and Steward (1972) assumedthat the accumulation of NO3

− could function inosmotic adjustment. However, Stienstra (1986)showed that NO3

− did not have a specific functionin osmotic adjustment in Aster tripolium L. grown ina nutrient solution with either a continuous or anintermittent NO3

− supply. In field conditions, it seemsthat S. salsa from an intertidal zone has a high abilityto accumulate NO3

− even under low soil NO3−

concentration (Table 1). In the greenhouse experi-ment, leaves of S. salsa from the intertidal zoneaccumulated more NO3

− but less Cl− than the S. salsapopulation from the inland location (Fig. 2b,d). Adecrease in the contribution of Cl− to the osmoticpotential in leaves of S. salsa from the intertidal zonewas almost exactly compensated by an increase inNO3

− (Table 2). Therefore, it seems likely that NO3−

plays an important osmotic role in S. salsa from theintertidal zone in high salinity conditions.

Halophytes accumulate and compartmentalizelarge amounts of Na+ and Cl− in the vacuole to lowerthe osmotic potential, which enables them to expendless energy than non-halophytes on the synthesis ofother compounds that act osmotically. Halophytesaccumulate a relatively small amount of low molec-ular weight organic compounds to balance theosmotic pressure in the cytoplasm (Hasegawa et al.2000; Zhao et al. 2003). When NO3

− is the source ofN, the ATP requirement for the synthesis or accumu-lation of solutes in leaves is 3.5 for Na+, 34 formannitol, 41 for proline, 50 for glycine betaine, andabout 52 for sucrose (Munns, 2002). Therefore, theuse of inorganic ions is efficient and significantlymore economical than the synthesis of compatibleorganic solutes for plants under salt stress (Greenwayand Munns 1983). The estimated contribution of

b

aa a

b

a

aa a

a

0.00

0.05

0.10

0.15

0.20

b

a

a

aa

a

aa

a

a

0.0

0.2

0.4

0.6

0.8N

a+ c

once

ntra

tion

(mol

l-1 ti

ssue

wat

er)

K+

con

cent

ratio

n(m

ol l-1

tiss

ue w

ater

)

Cl-

con

cent

ratio

n(m

ol l-1

tiss

ue w

ater

)N

O3-

conc

entr

atio

n(m

ol l-1

tiss

ue w

ater

)

(a)

(d)

b b b b b

aa

a a a

0.00

0.05

0.10

0.15

a

a

a

a

a

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b

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0.2

0.4

0.6

0.8

(b)

(c) (d)

10 100 200 400 600 10 100 200 400 600

NaCl (mM) NaCl (mM)

From saline inland From intertidal zone

Fig. 2 The effect of NaCl on the concentrations of Na+ (a), Cl−

(b), K+ (c) and NO3− (d) in leaves of S. salsa from the intertidal

zone or saline inland. Plants were treated with a range of NaCl

for 18 days. Means within the same NaCl concentration havingsame letter are not significantly different at P<0.05. Verticalbars represent standard errors (n=3)

Plant Soil (2009) 314:133–141 139

amino acid and proline to Ψs was less than 1% and0.5%, respectively, both in leaves of S. salsa in 500mM NaCl (Zhang and Zhao 1998) and in leaves of,for example, S. physophora in 300 mM NaCl (Song etal, 2006). Therefore, a high concentration of NO3

−,which has a low energy demand for osmotic adjust-ment, accumulated in leaves of S. salsa from theintertidal zone may be advantageous characteristic forthis population to succeed in highly saline conditions.

Na+ competes with K+ for intracellular influx sincethese cations are transported by common proteins.Na+ also can lead to damage of (plasma) membranesand, thus, subsequently increase K+ efflux fromintracellular stores. Importantly as well, K+ is anessential co-factor for many enzymes, but Na+ is notand cannot replace K+ (Hasegawa et al. 2000). Thepresent results (Fig. 2) indicate that S. salsa from theintertidal zone possess a much more selective mech-anism for the uptake (or retention) of K+ over Na+, afeature that could be a significant mediator of ionhomeostasis under high salinity conditions.

The shoot fresh weight was 296 g/plant for S. salsaon saline inland, while it was only 77 g/plant forS. salsa in an intertidal zone (Liu 2006), and the

present results (Table 2) were consistent with theprevious reporting data obtained by Liu (2006) in afield investigation. This relatively lower shoot growthrate (Table 2) seems, at least in part, to result from thelower number of side branches along the main stem ofS. salsa from the intertidal zone compared with thenumber in S. salsa from saline inland (Fig. 1). Highwater content in leaves or stems is an adaptative featurefor halophytes to regulate internal ion concentrations(Short and Colmer 1999). Stem water content was lessthan 6 mL g−1 dry weight in 600 mM NaCl for thehalophyte Halosarcia pergranulata subsp. pergranu-lata (Short and Colmer 1999). Leaf water content wasless than 8 mL g−1 dry weight in 300 mM NaCl for theeuhalophyte Suaeda physophora (Song et al. 2006).High leaf water content for euhalophyte S. salsa (Table 2)may constitute a strategy in this species to regulateinternal ion concentrations under saline conditions.

In conclusion, S. salsa from the intertidal zone mayemploy superior mechanisms at regulating the uptakeof ions from saline soils compared with S. salsa fromsaline inland habitats. The increase contribution ofNO3

−, but reduced contribution of Cl−, to the osmoticadjustment for S. salsa from the intertidal zone may

Table 3 The effects of NaCl on the osmotic potential (ψs), and the estimated contribution of Na+(CNa), Cl−(CCl), K

+(CK)and NO3−

(CNO3) to osmotic potential in leaves of S. salsa from the intertidal zone or saline inland

Origin of population NaCl (mM) Ψs (-MPa) CNa (%) CCl (%) CK (%) CNO3 (%)

Intertidal zone 10 1.5±0.02e1 36.8±0.8dc 21.6±0.6d 18.9±0.9a 15.5±1.2a100 2.1±0.01d 36.1±2.1d 20.4±0.6d 10.4±0.1 b 10.0±0.2b200 2.3±0.12c 38.6±1.0c 25.8±1.7c 8.3±0.7c 8.2±0.5c400 2.8±0.11b 41.7±1.9b 29.0±2.3b 6.5±0.3 d 6.9±0.5d600 3.0±0.07a 45.5±0.5a 34.5±1.7a 2.4±0e 6.2±0.3dMean2 2.4b3 39.7a 26.2b 9.3a 9.3a

Saline inland 10 1.5±0.10d 31.1±0.9b 28.2±2.0c 18.0±0.4a 4.4±0.3a100 2.6±0.20c 27.0±0.7c 27.8±0.8c 7.9±0.6b 2.3±0.2b200 2.9±0.14c 33.5±2.5ab 33.0±1.4b 6.2±0.3c 2.1±0.1b400 3.3±0.08b 35.4±1.1a 36.6±0.8a 4.9±0.2d 1.7±0c600 3.8±0.11a 36.4±3.2 a 38.5±1.4a 1.8±0.1e 1.4±0.2dMean 2.8a 32.7b 32.8a 7.8b 2.4b

Analysis of Variance (F-Values)Origin of population 125.3*** 124.3*** 152.2*** 82.1*** 1578.9***Salinity 250.2*** 27.1*** 76.8*** 1045.6*** 146.2***

Plants were treated with a range of NaCl for 18 days1Within each column, values with the same letter are not significantly different at P<0.05 across NaCl levels for S. salsa from theintertidal zone or saline inland; data are means±SE, n=32 Mean value for S. salsa from the intertidal zone or saline inland3Mean values with the same letters are not significantly different at P<0.05 between S. salsa from the intertidal zone and S. salsafrom saline inland

*** P<0.001

140 Plant Soil (2009) 314:133–141

be an adaptation in this population to persist in highlysaline conditions and colonize the intertidal zone. Thecombination of morphological, physiological andbiochemical characteristics may affect the distributionof different S. salsa populations in diverse naturalsaline environments.

Acknowledgments We are thankful to Professor Hans Bohnertand Dr. Lindsey Atkinson for their critical reading and revisionof the manuscript. Financial support from the Foundationof Excellent Young Scientists of Shandong Province(2006BS06002), and the State High Technological Researchand Development Plan of China (2007AA091701) is also greatlyappreciated.

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