Salt Tolerance in Two Suaeda Species: Seed Germination and Physiological Responses
The main aim of this study is to detect the effects of NaCl, NaHCO3 and sea salt on seed germination, seedling growth and seedling cation contents of two Suaeda species. Three germination experiments of S. corniculata and S. salsa seeds were conducted in growth chambers. The seeds were placed at three types of salt at concentrations: 0, 25, 50, 75, 100% seawater; 0, 100, 200, 300, 400, 500 mM NaCl and 0, 100, 200, 300, 400, 500 mM NaHCO3, separately. The two species varied in their salt tolerance for germination rates and percentages and showed higher germination percentage at higher salt stress (500 mM NaCl, NaHCO3 and 100% seawater). Some un-germinated seeds were recovered after being transferred to distilled water. The Na+ content in seedlings increased with the increase in stress intensity. While K+ content and K+/Na+ ratio decreased under NaCl and NaHCO3 stress. K+ content increased in seawater treatment while reaching higher salt concentrations, due to the extra K+ in seawater, but there was no significant difference among the treatments with varied seawater concentrations (p<0.001). These results suggested that S. corniculata and S. salsa could be used as pioneer plants for ecological recovery and exploitation of saline and sodic soils.
Received: March 15, 2010;
Accepted: May 28, 2010;
Published: June 19, 2010
Over 800 million hectares of land are affected by salinity throughout the world
(Munns, 2005). Saline soils formed when evaporation greatly
exceeds precipitation for at least part of the year and where salts are present
in moderate to high amounts in the parent material of the soil or with a saline
water table at shallow depth. Apart from their high salt content saline soils
show a considerable diversity in their hydrological, physical and chemical properties.
Saline soils may be calcium dominated, sodium dominated or magnesium dominated
with a subsequent tendency towards structural degradation (depending on the
presence or absence of calcium) (http://www.fao.org/AG/agL/agll/prosoil/saline.htm).
Under extreme climatic conditions (low rainfall, high evaporation) salts which
are present in soil solution can precipitate at the surface in various forms
such as white efflorescence, salt crusts and so on (http://www.fao.org/AG/agL/agll/prosoil/
Salt accumulation may limit plant growth to salt tolerant plants (halophytes)
only. Tolerance also varies with stages of their life cycle, which could be
expressed as: (1) the ability to tolerate high salinity without loosing viability
while stored in the soil (seed bank), (2) the ability to germinate at high salinities
and (3) the ability to complete its life cycle at high salinities (Khan
and Gul, 2002). Ravindran et al. (2007) have
demonstrated that soil salinity levels, as measured by Electrical Conductivity
(EC), can be reduced by the cultivation of halophytes which are able to accumulate
salts in their plant tissues on soils affected by salinity. Among them, Suaeda
corniculata is a halophyte which often occurs in sodic soils in plains,
such as Songnen Plain, where sodium chloride (NaCl) and sodium bicarbonate (NaHCO3)
are the most abundant salts (Wang et al., 2007).
Leaves of S. corniculata from saline-sodic inland are green during the
whole growing period (http://www.efloras.org/florataxon.aspx?flora_id=2
and taxon_id=200006943). S. salsa is a typical halophyte that naturally
occurs in highly saline soils in coastal saline wetland (Wang
et al., 2004). S. salsa in costal wetland is usually submerged
completely in seawater and the salt concentrations in the soil are always high
during seed germination and at the seedling stage (Song
et al., 2008). Leaves and stems of S. salsa in this area are
red-violet during the whole growth period, Fresh branches of S. salsa
are very valuable as a vegetable and the seeds can produce edible oil (Zhao
et al., 2002). These two species (Chenopodiaceae) are common in saline
soils, acquisition and maintenance of high water content may be the strategies
these species use to accumulate Na+ which may substitute for an osmoticum
or regulate internal ion concentrations under saline conditions.
Seed germination is the initial and most crucial stage in the life cycle of
plants (Grime and Campbell, 1991). Different abiotic
factors such as temperature, soil salinity, photoperiod and soil moisture affect
germination of halophytes (Noe and Zedler, 2000; Khan,
2003; Guan et al., 2009). However, the effect
of soil salinity seems to dominate over all other factors in saline areas (Keiffer
and Ungar, 1997; Heidari, 2009). The ability for
seeds to recover in non-saline solution following transfer from saline solution
also plays an important role in saline areas, which has been reported for several
halophytes (Ungar, 1978; Khan and
Ungar, 1984; Hanslin and Eggen, 2005).
High Na+ tissue content has often been considered as the major factor
responsible for salt toxicity in plants. It is conventionally assumed that an
ability to exclude Na+ correlates with plant salt tolerance (Munns
and James, 2003; Garthwaite et al., 2005).
However, the importance of maintaining an optimal K+/Na+
ratio for plant salt tolerance is hardly surprising and is well discussed in
literatures (Cuin et al., 2003; Chen
et al., 2007). It is also obvious that such an optimal ratio can
be maintained by either restricting Na+ accumulation in plant tissues
or by preventing K+ loss from the cells (Chen
et al., 2007).
The aim of this study were (1) to investigate the seed germination of two Suaeda species treated with NaCl, NaHCO3 and the sea salts, (2) to discuss the physiological responses of Suaeda species to different salts at germination stage.
MATERIALS AND METHODS
Seed collection site: Seeds of S. corniculata were obtained from
Songnen Plain (44°3'N, 124°3'E) in 2008, where the soils are highly
saline. Seeds of S. salsa were collected from the coastal wetland in
Yellow River Delta (37°35'-38°12'N, 118°33'-119°20'E) in the
autumn of 2008. The seeds were stored in paper bags at room temperature (20±3°C)
until the experiment in January 2009.
Germination experiments: This study was carried out at Laboratory of
Coastal Wetland Ecology, Yantai Institute of Coastal Zone Research, Chinese
Academy of Sciences, China from January till May in 2009. Seeds were surface-sterilized
in 1% sodium hypochlorite solution for 15 min, rinsed in distilled water and
dried before the experiment. The seeds were germinated in petri dishes (10 cm
diameter) containing two layers of filter paper with 12 mL of test solution.
Germination tests were carried out in growth chambers (BSG-800, Shanghai, China)
with a 12 h photoperiod (Sylvania cool white fluorescent lamps, 200 μmol
m-2 sec-1, 400-700 nm, 25/15°C). To examine the effects
of different salts, we conducted six salinity concentrations (0, 100, 200, 300,
400, 500 mM NaCl), six alkalinity concentrations (0, 100, 200, 300, 400, 500
mM NaHCO3) and five sea water concentrations (0, 25, 50, 75 and 100%
seawater). A completely randomized design was used in the germination tests.
Four replicates of 50 seeds each were used for each treatment. The seeds were
considered to have germinated after radicle emergence. Germination was recorded
daily for 10 days (Mokhberdoran et al., 2009).
Methods of germination expression: The rate of germination was estimated
using a modified Timsons index of germination velocity = ΣG/t, where
G is the percentage of seed germination at one-day intervals and t is the total
germination period (Khan and Ungar, 1984). The maximum
value possible for our data using this index was 100 (i.e., 1000/10). The greater
the value, the more rapid is the germination. All seeds from the previous germination
tests that did not germinate after 10 days in different salt treatments, were
placed in new Petri dishes with filter paper moistened with distilled water
and incubated under the same conditions for an additional 10 days to study the
recovery of germination. The recovery percentage was determined by the following
where, a is the total number of seeds germinated after being transferred to
distilled water, b is the total number of seeds germinated in saline solution
and c is the total number of seeds used (modified from Khan
and Gulzar, 2003).
Measurement of physiological indices: All seedlings were harvested after 10 days of salt treatment. The seedlings were first washed with distilled water three times and then dried at 60°C to constant weight. Dry samples of plant material (50 mg) were treated with 10 mL deionized water at 100°C for 1 h and the extract was taken to determine free Na+ and K+ concentrations. Inductively coupled plasma atomic emission spectrometry (ICP-AES) were used for the determination of sodium (Na), potassium (K) concentrations. K+/Na+ ratio was then calculated.
Data analysis: Germination data were arcsine transformed before the Analysis of Variance (ANOVA). The data were analysed using SPSS 11.5 (SPSS Inc., Chicago, IL, USA). Experimental data were subjected to one way analysis of variance and the means were separated by the Least Significant Difference (LSD). For the germination data, significance was tested at the 1% level, for the Na and K concentrations, significance was tested at the 0.1% level.
Final germination: The final germination of S. corniculata and
S. salsa were significantly affected by NaCl, NaHCO3, or seawater
(Table 1) (p<0.01). They both remained unaffected up to
200 mM NaCl and NaHCO3 treatments. In non-saline control treatment,
the germination percentages were 80.5 and 91% for S. corniculata and
S. salsa, respectively. At the highest NaCl concentration, the percentage
of germination was no less than 40%. Moreover, the germination percentage of
both S. corniculata and S. salsa were higher than 60% in all NaHCO3
treatments. Only 38% seeds of S. salsa germinated in comparison to 57.5%
germination of S. corniculata in 100% seawater treatment (Table
Germination rate: The highest germination rate was observed in non-saline controls and it decreased with the increase of salinity. S. salsa seeds showed higher germination ability than S. corniculata seeds in all treatments except for those treated with 50, 75 and 100% seawater (Table 1).
For the two species, the rate of germination were affected significantly by NaCl treatment and all remained unaffected up to 200 mM NaHCO3 treatments (Table 1) (p<0.01).
Germination recovery: After 10 days of salinity treatment, seeds were transferred to distilled water to determine the recovery of germination. The results presented the un-germinated seeds from all salt treatments recovered. The germination recovery ranged from 5.63% in 500 mM NaHCO3 (but note the original high germination of 64.5%) to 30.43% in 400 mM NaCl for S. corniculata and from 16.22% in 200 mM NaHCO3 to 43.55% in 100% seawater for S. salsa (Table 1).
Physiological indices: Na+ concentrations of the two species
increased with increasing NaCl (p<0.001). The overall increasing trends were
similar for S. corniculata and S. salsa, except in the treatment
of 300 mM NaCl, where Na+ concentrations in S. salsa seedlings
were higher than in S. corniculata seedlings (Fig. 1a-f).
In NaHCO3 solutions, Na+ concentrations of S. corniculata
seedlings increased with increasing NaHCO3 (p<0.001).
||Germination percentage, rate of germination (Mean±SE,
n = 4) at different levels of different salt solutions using modified Timsons
index (Khan and Ungar, 1984) and germination recovery
using recovery formula (modified from Khan and Gulzar,
|Different letters indicate significant differences from different
concentrations in same salt treatment (p<0.01)
||Effects of NaCl, NaHCO3 and seawater on concentrations
of Na+, K+ (mmol kg-1 dry mass) and K+/Na+
ratio in the seedlings of S. corniculata and S. salsa. Bars
represent±SE (n = 3). Different letters indicate significant differences
from each other (p<0.001)
However, in S. salsa seedlings the concentrations of Na+
decreased with the salinity increased up to 200 mM (Fig. 1).
In seawater treatments, Na+ concentrations of S. salsa seedlings
increased significantly from 0 to 100% seawater (p<0.001), while they were
not significantly different in S. corniculata seedlings treated with
50, 75 and 100% seawater (Fig. 1).
K+ concentrations of S. corniculata seedlings decreased significantly with increasing salinity in both NaCl (p<0.001 and NaHCO3 (p<0.001) treatments (Fig. 1). For S. salsa seedlings, decrease in K+ concentrations was not significant when treated with NaCl (p<0.001). In seawater solutions, K+ concentrations were significantly higher than control (0% seawater), but there was no significant difference among the treatment with different seawater concentrations, which was the case for both S. corniculata and S. salsa seedlings (Fig. 1).
The effects of 3 types of salts on seed germination: S. corniculata
and S. salsa are typical halophytes which naturally occur in inland saline-sodic
plains and coastal saline wetland respectively. For both species, increase in
salinity caused a decrease in germination percentage and delayed the germination,
but they showed high salt tolerance to NaCl, NaHCO3 and seawater,
as quantified by the percentage of germination (Table 1).
At 500 mM NaCl, the germination percentage of S. corniculata reached
44.5% and S. salsa reached 50%, this indicated that the two species can
establish in most saline environments. This observation was supported by other
studies on Arthrocnemum indicum (Khan and Gul, 1998),
Salicornia rubra (Khan et al., 2000) and
Suaeda japonica (Yokoishi and Tanimoto, 1994).
These reports indicate that S. corniculata and S. salsa seeds
could be classified as one of the most salt tolerant species during germination.
The pH range of NaHCO3 in this study was 8.4 - 9 and the germination
percentages of the two species were still high: S. corniculata 64.5%
and S. salsa 68% at 500 mM NaHCO3 (Table 1).
In sodic soils such as Songnen plain, sodium bicarbonate (NaHCO3)
is second most abundant salt next only to sodium chloride (NaCl), high germination
percentage makes it promising to use these plants to rebuild the sodic land
under serious threat of degradation. Furthermore, seeds of S. corniculata
and S. salsa are highly salt tolerant and can germinate at 100% seawater
salinity. This may follow the same rule as combined salt effect because in seawater
a lot of ions were included (Duan et al., 2003).
Germination rate decreased with the increasing of salinity as expected (Table
1). When the two species were compared, the effect of salinity on S.
corniculata was higher than that on S. salsa (Table
1). The recovery experiment showed that seeds exposed to different NaCl,
NaHCO3 and seawater concentrations for a short period (10 days) could
germinate after being transferred to distilled water (Table 1).
This is supported by Ungar (1995), who reported that tolerance
of seeds to salinity should be interpreted at two levels: (1) the ability to
germinate at saline conditions and (2) the ability to germinate at a non-saline
condition after exposure to high salinity. In this study the two levels of tolerance
of seeds to salinity were observed.
The effects of 3 types of salts on physiological indices: Halophytes
were reported to accumulate large amounts of Na+ under salt stress
and simultaneously inhibit K+ absorption, which results in the increase
of Na+ content and the decrease of K+ content (Shi
and Wang, 2005; Yang et al., 2007). Similar
phenomenon is observed in this study, results shown in Fig. 1a
and c indicated that with increasing stress intensity the
Na+ content in seedlings increased, while K+ content and
K+/Na+ ratio decreased under NaCl and NaHCO3
stress, but the K+/Na+ ratio were not significantly different
among 100, 200 and 300 mM NaCl and NaHCO3 treatments (Fig.
1e). Total accumulation of Na+ and K+ was higher under
NaCl stress than that under NaHCO3 stress for both species. This
indicated that the negative effects of HCO3- were higher
than Cl-. In seawater treatment, the K+ content increased
with the increasing of seawater (Fig. 1d), due to the presence
of K+ in seawater.
Authors are grateful for support from the Chinese Academy of Sciences (Grant No. kzcx2-yw-223); the National Natural Science Foundation of China ( 30770412 and 40873062); the National Science and Technology Supporting Program of China (Grant No. 2006BAC01A13); the CAS/SAFEA international partnership program for creation research team; the 100 Talents Program of the Chinese Academy of Sciences and the Science and Technology Planning Program of Shandong Province (No. 2008GG20005006, 2008GG3NS07005 and 2005GD42060003).
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