Introduction
Salinity is an increasing problem in many regions worldwide. Halophytes are plants that can adapt to the high salt concentrations of saline environments (Flowers et al., Reference Flowers, Troke and Yeo1977; Flowers and Yeo, Reference Flowers and Yeo1986; Munns and Tester, Reference Munns and Tester2008). These environments are normally dominated by NaCl, and the optimal NaCl concentration for the growth of halophytes in culture solutions ranges from 20 to 500 mM (Flowers et al., Reference Flowers, Troke and Yeo1977).
Salinity concentration at the surface of the soil changes over time due to continuous evaporation and occasional precipitation in natural saline environments (Tobe et al., Reference Tobe, Li and Omasa2000). Particularly in the saline soil areas of northern China, salinity and drought stress, due to high evaporation and low precipitation, markedly affect the seed germination of annual halophytes (Jin et al., Reference Jin, Zhang, Sun and Gao1999).
Germination is a crucial stage in the life cycle of halophytes as it determines whether a plant can establish itself successfully in a given condition (Bajji et al., Reference Bajji, Kinet and Lutts2002). Soil salinity inhibits the germination of seeds by creating an unfavourable osmotic potential external to the seed, which prevents water uptake (Welbaum et al., Reference Welbaum, Tissaoui and Bradford1990), and through the toxic effects of Na+ and Cl− ions on the germinating seeds (Khajeh-Hosseini et al., Reference Khajeh-Hosseini, Powell and Bingham2003). Although halophytes are salt tolerant, germination of their seeds is often inhibited by increased salinity; the best germination rate of halophytes is obtained under non-saline conditions and their germination rate decreases with an increase in salinity (Ungar, Reference Ungar1991; Khan and Gul, Reference Khan and Gul1998). For successful establishment of plants in saline environments, seeds must remain viable at high salinity and germinate when salinity in the growth medium decreases (Ungar, Reference Ungar1978, Reference Ungar, Kigel and Galili1995).
Seeds can be classified as having low or high vigour, depending on the degree of field emergence, particularly under unfavourable conditions. When seeds are exposed to stress, plant reproduction and crop yield are severely affected (Kranner et al., Reference Kranner, Minibayeva, Beckett and Seal2010), and seed characteristics such as size and mass are positively correlated with seed vigour. Germination tests provide essential information on the best conditions for germination in order to exploit full seed potential.
Seed vigour is an important factor that affects seedling establishment and crop growth and, ultimately, production rate, which provides valuable information for assessing seed quality. Many factors affect seed vigour. Seed size is an important characteristic for determining seed germination and seedling growth (Temme, Reference Temme1986; Wulff, Reference Wulff1986; Seiwa, Reference Seiwa2000). Variability in seed size may contribute to variability in seed germination. With increased seed size, higher germination and emergence rates were obtained in oat (Avena sativa; Willenborg et al., Reference Willenborg, Wildeman, Miller, Rossnagel and Shirtliffe2005); furthermore, larger seeds resulted in improved stand establishment and faster germination in field bindweed (Convolvulus arvensis; Tanveer et al., Reference Tanveer, Tasneem, Khaliq, Javaid and Chaudhry2013).
Seed storage compounds are the initial raw materials to support seed germination. Wheat seeds (Triticum aestivum L.) with high protein content will produce more vigorous seedlings and sometimes higher yields (Ries and Everson, Reference Ries and Everson1973). Seed proteins form nitrogen stores to supply the growing seedlings. Evidently, storage in seeds is related to the nutrient quality of the seeds (Shewry and Halford, Reference Shewry and Halford2002). Reduction in the accumulation of rice seed storage protein leads to changes in nutrient quality and storage organelle formation (Kawakatsu et al., Reference Kawakatsu, Hirose, Yasuda and Takaiwa2010). Lipids have also been shown to be important for seed germination or seedling establishment in Arabidopsis seeds (Kelly et al., Reference Kelly, Quettier, Shaw and Eastmond2011).
Seed heteromorphism or dimorphism is an adaptive mechanism in many xerophyte and halophyte species to an unfavourable environment during germination (Imbert, Reference Imbert2002). These adaptations are commonly associated with environments that are highly variable, either in time or space. Previous studies have shown that brown seeds of Suaeda salsa germinated faster and appeared more tolerant than black seeds under salinity or drought stress (Zhao et al., Reference Zhao, Song and Yin2004; Song et al., Reference Song, Fan, Zhao, Jia, Du and Wang2008). As an adaptation strategy, this may indicate that the dimorphic seeds of S. salsa have different responses to salt and drought, or combined stresses. However, there is little information on germination of dimorphic seeds of S. salsa grown under culture conditions of low and optimum sodium chloride (NaCl) concentrations.
Besides inherent factors, seed vigour is also influenced by environmental factors. For non-halophytes, salinity greatly affects seed germination of green gram cultivars (Misra and Dwivedi, Reference Misra and Dwivedi2004), and induces a reduction in germination rate and a delay in the initiation of germination and seedling establishment of durum wheat (Triticum durum; Almansouri et al., Reference Almansouri, Kinet and Lutts2001). Environmental conditions may affect seed quality during seed formation of cotton (Gossypium barbadense cv. Giza 86). High application of nitrogen fertilizer significantly increased seed weight, seed viability, seedling vigour and cold germination test performance, whereas application of foliar potassium and the plant growth retardant mepiquat chloride improved seedling vigour in the next growing season (Sawan et al., Reference Sawan, Fahmy and Yousef2009). In hybrid maize, the use of 165 kg ha− 1 nitrogen fertilizer and seven-day irrigation intervals during growth of the mother plant resulted in improved seed germination and seedling vigour indices (Farhadi et al., Reference Farhadi, Daneshyan, Hamidi, Rad and Valadabadi2014). However, little is known about effect of environmental factors on the seed quality of halophytes.
S. salsa is a leaf succulent annual herb belonging to the family Chenopodiaceae; it is one of the main halophytic species distributed widely in the saline soils of northern China (Zhao et al., Reference Zhao, Fan and Ungar2002). As its seeds contain approximately 30–40% edible oil, are rich in unsaturated fatty acids (Bai et al., Reference Bai, Liu, Li, Lieth and Mochtchenko2003) and its fresh branches are edible, this species has economic potential as a source of oil, food or fodder (Wang et al., Reference Wang, Lüttge and Ratajczak2001; Zhao et al., Reference Zhao, Fan and Ungar2002). S. salsa is adapted to, and can grow well in, saline soils, through its ability to hyper-accumulate Na+ and Cl− in its succulent leaves (Zhao et al., Reference Zhao, Fan, Song, Sun, Wang, Zhang and Ungar2005). For example, the optimum salt concentration for vegetative growth and photosynthesis of the euhalophyte S. salsa is 200 mM NaCl (Lu et al., Reference Lu, Qiu, Wang and Zhang2003; Qiu et al., Reference Qiu, Chen, Guo, Bao, Ma and Wang2007; Song et al., Reference Song, Chen, Feng, Jia, Wang and Zhang2009). To our knowledge, very few studies have been carried out on the effect of NaCl on seed quality of the euhalophyte S. salsa grown in the absence and presence of NaCl for its complete life cycle. Whether non-saline growth conditions affect seed germination and seed vigour of the halophyte S. salsa is still unknown.
In order to determine the seed quality generated from S. salsa plants under low and optimum NaCl concentration, S. salsa plants were grown under sand culture conditions, from seed germination to seed harvest. The main objective of the research was to evaluate the effects of low and optimum concentrations of NaCl application to the mother plant on seed production and seed quality of the euhalophyte S. salsa through observing the germination and vigour of seeds under the different levels of NaCl.
Materials and methods
Seed material
Seeds of S. salsa were collected from the Yellow River Delta (37°20′N; 118°36′E) in Shandong, the middle-eastern province of China. The saline land where S. salsa grows has Na+ and Cl− concentrations of 2.4 and 2.0 g (kg dry soil)− 1, respectively (Liu, Reference Liu2006). After 6 months of refrigerated storage ( < 4°C), seeds were sown in plastic buckets (26 cm diameter, 30 cm height) with drainage holes and filled with rinsed river sand. The plants were irrigated twice a day, in the morning and late afternoon, with 1 mM NaCl (control) or 200 mM NaCl (six pots for each concentration of NaCl) dissolved in Hoagland nutrient solution. In the 200 mM NaCl treatments, the chance of osmotic shock was reduced by stepping up in 50 mM increments every 12 h until final concentrations (200 mM) were achieved. The pH of all solutions was adjusted to 6.2 ± 0.1 with 1 M potassium hydroxide (KOH) and sulphuric acid (H2SO4). NaCl treatments were carried out throughout the whole plant life cycle. The plants were grown in a greenhouse under natural light conditions at Shandong Normal University; the temperature was 28 ± 3/23 ± 3°C (day/night) with a relative humidity of 60/80% (day/night).
Seeds were hand-harvested from plants cultured with 1 or 200 mM NaCl and then cultured under the same conditions as the mother plant. That is, the seeds from the 1 mM NaCl (control) continued treatment at 1 mM NaCl (control); the seeds from the 200 mM NaCl treatment continued at 200 mM NaCl. The third generation of seeds was obtained as the seed source for future germination and other experiments, as described below.
Germination percentage and germination potential
Five NaCl concentrations (0, 25, 50,100 and 150 mM) were used to determine the salt tolerance of the various seed samples. NaCl was dissolved in 1/5 Hoagland solution and 0 mM NaCl was used as the control. Hoagland solution (1/5) had the following composition: 1 mM Ca(NO3)2, 1 mM KNO3, 0.4 mM MgSO4, 0.2 mM KH2PO4, 10 μM Fe-EDTA, 23 μM H3BO3, 4.55 μM MnCl2, 0.16 μM CuSO4, 0.38 μM ZnSO4 and 0.06 μM Na2MoO4. The pH was adjusted to 6.2 ± 0.1 with 1 M KOH and H2SO4.
Each treatment consisted of four replicates of 80 seeds. The criterion for germination was visible radicle protrusion (Bewley and Black, Reference Bewley and Black1994). First, seeds of S. salsa were surface sterilized by soaking in 6% sodium hypochlorite for 15 min, followed by five rinses with distilled water, before the start of each germination trial. Twenty uniformly sized and undamaged seeds were placed on two layers of filter paper, which was moistened with 5 ml of NaCl solution in a Petri dish (90 mm in diameter). The experiment was carried out in darkness in a growth chamber (RXZ-500, Ningbo, China). The temperature was maintained at 20 and 25°C during night and day, respectively, with a 12-h photoperiod. Daily germination rate was measured and NaCl solutions were replaced every 2 d to keep the NaCl concentration unchanged. Final germination rate and germination potential were calculated after 9 d of incubation using the following formulas:
$$\begin{eqnarray} Germination\ percentage\ (\%) = (sum\ of\ the\ number\ of\ seeds\ germinated/\quad \quad the\ total\ number\ of\ seeds\ tested)\times 100 \end{eqnarray}$$
$$\begin{eqnarray} Germination\ potential\ (\%) = (sum\ of\ the\ number\ of\ seeds\ germinated\ \ \quad when\ seed\ germination\ reached\ its\ peak/\ \quad the\ total\ number\ of\ seeds\ tested)\times 100 \end{eqnarray}$$
For the germination index and seed vigour index, five normal seedlings from each replicate were selected at random, and root length was measured. Germination index and seed vigour index were then calculated using the following formulas:
$$\begin{eqnarray} Germination\ index\ (GI) = \sum ( G _{ t }/ D _{ t }) \end{eqnarray}$$
where G t is the number of seeds germinated at day t and D t is the corresponding days of germination.
$$\begin{eqnarray} Seed\ vigour\ index\ (VI) = GI\times ( S /100) \end{eqnarray}$$
where GI is the germination index and S is the mean root length of seedlings after 9 d of germination (Corchete and Guerra, Reference Corchete and Guerra1986).
After 9 d, ungerminated seeds in each treatment were rinsed three times with distilled water and then incubated in new Petri dishes with distilled water for 6 d to determine total germination percentage, using the following formula:
$$\begin{eqnarray} Total\ germination\ percentage\ (\%) = ( A / C )\times 100 \end{eqnarray}$$
where A is the sum of the number of seeds germinated in NaCl solutions plus those that were recovered to germinate in distilled water, and C is the total number of seeds tested (Khan and Ungar, Reference Khan and Ungar1984).
Measurement of seed protein content
The seeds of S. salsa were washed three times using purified water, after which proteins were extracted using a modified protocol described by Shen et al. (Reference Shen, Jing and Kuang2003). Four replicates of dried seeds (0.1 g samples) were ground with a pestle and mortar to a powder under liquid nitrogen. The powder was transferred to a 2-ml centrifuge tube containing 1 ml extraction buffer [20 mM Tris–HCl, pH 7.5, 250 mM mannitol, 10 mM EDTA, 1 mM phenylmethanesulphonyl fluoride (PMSF), 1 mM dithiothreitol (DTT) and 1% Triton X-100]. After mixing and a 30-min incubation, the mixture was centrifuged (15,000 g, 4°C, 20 min) twice. The supernatant was collected and the protein concentration measured using the Bradford method with bovine serum albumin as the standard (Bradford, Reference Bradford1976).
Measurement of soluble sugar and starch content
Four dried-seed replicates (0.1 g samples) were ground with a pestle and mortar to a powder under liquid nitrogen. Each powder was transferred to a 10-ml glass tube and extracted with 5 ml of 80% ethanol at 80°C for 30 min, followed by two extractions with 2 ml of 80% ethanol. The supernatants were combined and soluble sugar content was determined with anthrone reagent, using glucose as the standard (Yemm and Willis, Reference Yemm and Willis1954).
The remaining pellets were dried at 45°C to remove the ethanol, and boiled for 10 min with 3 ml double-distilled water in 10-ml centrifuge tubes. Subsequently, the samples were cooled to room temperature, and 4 ml perchloric acid (HClO4) was added to decompose the starch. Starch in the paste was hydrolysed for 15 min, and the glucose content was determined at 630 nm as described previously (Yemm and Willis, Reference Yemm and Willis1954). Starch content was calculated using the formula:
$$\begin{eqnarray} Starch\ content\ (\%) = G \times 0.9/DW\times 100 \end{eqnarray}$$
where G is the glucose content of the samples after acid hydrolysis, the constant 0.9 is the theoretical factor to convert glucose to starch, and DW is the dry weight (g).
Measurement of total lipid content
For lipid extraction, four replicate samples of dried seeds (0.1 g) were weighed accurately and frozen in liquid nitrogen for a few minutes; the frozen samples were then ground to a fine powder in a mortar, and the powder was transferred to 10-ml glass tubes for lipid extraction. Lipids were extracted using a mixture of hexane and isopropanol (3:2, v:v) to determine total lipids gravimetrically (Hara and Radin, Reference Hara and Radin1978).
Statistical analysis
Each experiment described above was performed randomly with four replicates, the statistical results were presented as means ± SD. Data were analysed using SPSS, version 17 one-way analysis of variance (ANOVA) statistical software packages (SPSS Inc., Chicago, Illinois, USA). Different letters indicate significant differences in the mean (at P ≤ 0.05) from Duncan's tests.
Results
NaCl markedly increases seed size and weight
To investigate the effect of NaCl on the seeds of S. salsa, seed size and thousand-seed weight of the black and brown seeds were examined. Compared with the control, the seed size and seed weight were significantly augmented by salt (200 mM NaCl). The latter treatment resulted in significantly bigger seeds than those of the control (Fig. 1A); the length and thickness of black seeds and brown seeds were 125.96%, 128.00% and 151.26%, 129.33% of the control, respectively (Fig. 1B), and individual black seed and brown seed weights were 228.19% and 173.73% of the control, respectively (Fig. 1C).
Figure 1 Size (A, B) and 1000-seed weight (C) of black and brown seeds of Suaeda salsa from mother plants grown under conditions with 1 and 200 mM NaCl. Values of seed size are means ± SD (n= 100) and values of seed weight are means ± SD (n= 5). Different letters indicate significant differences at the level of 0.05 according to Duncan's test.
NaCl stress reduces germination of black seeds but does not affect the brown seeds
Seed size plays a major role in germination and seedling establishment, particularly under environmental stress, such as salinity. Both black and brown seeds of S. salsa grown under the 200 mM NaCl treatment were significantly bigger than those grown under the 1 mM NaCl treatment. We examined the response of germination percentage and germination potential of black and brown seeds to different concentrations of NaCl and the total germination percentage after NaCl removal. For the black seeds, both germination percentage and germination potential decreased progressively as the level of salinity increased. However, at different NaCl concentrations (0, 25, 50, 100 and 150 mM), the germination percentage and germination potential of the black seeds from the 200 mM NaCl treatment was significantly higher than those of the control; these were 126.67%, 154.16%, 155.31%, 140.43%, 233.33% and 135.18%, 137.50%, 160.46%, 156.41%, 335.29% of the control, respectively (Fig. 2A, B). Surprisingly, for the brown seeds, there was no significant difference in the germination rate and germination potential between the seeds from the mother plants of the 1 and 200 mM NaCl treatments, except for germination potential in the 150 mM NaCl treatment. When the ungerminated seeds were transferred to distilled water after the 9-day exposure to salinity, the total germination percentage of the S. salsa seeds was obtained (Fig. 2C). The result showed nearly 100% germination of the ungerminated black and brown seeds from the mother plants of the 200 mM NaCl treatment and a slight reduction in germination of the ungerminated black seeds from the maternal control plants (Fig. 2C).
Figure 2 Effect of different concentrations of NaCl on the germination percentage (A), germination potential (B), total germination percentage (C), germination index (D) and seed vigour index (E) of the black and brown seeds of Suaeda salsa from mother plants grown under conditions with 1 and 200 mM NaCl. Values are means ± SD (n =4). Different letters represent significant differences at the level of 0.05 according to Duncan's test.
NaCl improves germination and seed vigour indices
The germination index reflects the speed and uniformity of germination. The germination index of both black and brown seeds from the mother plants of the control was significantly lower than that of the seeds from the mother plants of the 200 mM NaCl treatment. Furthermore, the germination index of brown seeds from the mother plants of both the control and the 200 mM NaCl treatment was significantly higher than that of the black seeds (Fig. 2D). At 150 mM NaCl, the germination indexes of black seeds and brown seeds from the mother plants of the 200 mM NaCl treatment were 311.32% and 176.13% of the control, respectively. At 0 mM NaCl the germination indexes of black seeds and brown seeds from the mother plants of the 200 mM NaCl treatment were 154.86% and 154.30% of the control, respectively (Fig. 2D). These results indicate that the seeds from the mother plants of the 200 mM NaCl treatment had a higher germination index and salt tolerance as compared to the seeds from the control plants.
The seed vigour index represents the germination capacity and growing tendency of seedlings. Similar results in vigour index of the seeds from the mother plants of the control and the 200 mM NaCl treatment were obtained under different concentrations of NaCl (Fig. 2E). At 150 mM NaCl, the vigour indexes of black seeds and brown seeds from the mother plants of the 200 mM NaCl treatment were 993.50% and 234.63% of the control, respectively. At 0 mM NaCl, the vigour indexes of black seeds and brown seeds from the mother plants of the 200 mM NaCl treatment were 360.50% and 275.83% of the control, respectively (Fig. 2E). These findings further indicate that the seeds from the mother plants of the 200 mM NaCl treatment had a higher seed vigour index than those from the control plants.
Total seed storage compounds
Seed proteins are stored nitrogen sources for germinating seedlings. As shown in Fig. 3A, the total seed protein content in the brown seeds was much higher than in the black seeds. Furthermore, the protein content of the black and brown seeds from the mother plants of the 200 mM NaCl treatment was significantly higher than that from the control (111.46% and 113.06% of the control, respectively; Fig. 3A). The total soluble sugar content of the brown seeds was also higher than that of the black seeds, whereas the starch content of the brown seeds was lower than that of the black seeds. However, both the soluble sugar and starch contents of the seeds from the mother plants of the 200 mM NaCl treatment were significantly higher than those from the control; this was 130.50% and 155.28% (black seeds) and 104.06% and 149.66% (brown seeds) of the control, respectively (Fig. 3B, D). The total lipid content of the brown seeds was significantly higher than that of the black seeds from the mother plants at the same growth condition of 1 or 200 mM NaCl; moreover, the lipid content in both the black and brown seeds from the mother plants of the 200 mM NaCl treatment was significantly higher than that of the seeds from the control (128.74% and 127.76% of the control, respectively; Fig. 3C).
Figure 3 Total seed protein (A), soluble sugar (B), seed lipid (C) and seed starch (D) content of black and brown seeds of Suaeda salsa from mother plants grown under conditions with 1 and 200 mM NaCl. Values are means ± SD (n= 4). Different letters represent significant differences at the level of 0.05 according to Duncan's test.
Discussion
Environmental conditions of the parental plant during reproductive growth affect the performance of the seeds. In hybrid sweet pepper (Capsicum annuumL. cv. ‘Hazera’ 1195), seeds developed at higher temperatures (in summer) displayed a lower percentage of seedling emergence than seeds developed under lower temperatures (in winter) (Xu and Kafkafi, Reference Xu and Kafkafi2003). However, Arabidopsis thaliana seeds from the warm parental environment showed faster germination rates than those from the cold parental environment (Blödner et al., Reference Blödner, Goebel, Feussner, Gatz and Polle2007). But for Leymus chinensis the increased temperature significantly reduced the number of flowering plants and seed number per square meter, and reduced the number of germinating seeds per unit area (Gao et al., Reference Gao, Wang, Zhang, Dong and Guo2012). Water during reproductive growth also influenced seed yield. Plants of the alpine perennial Ranunculus adoneus, growing at sites where snow melts earlier, produce larger seeds with higher survival and greater probability of emergence than seeds from plants at later-melting sites (Santon and Galen, Reference Santon and Galen1997). Water stress during reproductive growth resulted in a drastic reduction of seed yields of rapeseed (Brassica napus L.) (Ahmadi and Bahrani Reference Ahmadi and Bahrani2009) whereas in Plantago ovata and Nigella sativa, the lowest seed yield was observed when irrigation was stopped at the blooming stage; however, the oil content of the seeds was not reduced (Bannayan et al., Reference Bannayan, Nadjafi, Azizi, Tabrizi and Rastgoo2008).
Soil salinity is a major constraint to the economic exploitation of land for agriculture and forestry, and an increase in arid and semi-arid regions is one of the most severe environmental factors limiting the productivity of agricultural crops (McWilliam, Reference McWilliam1986). The saline environment of the parental plant may affect seed quality. For example, in the salt-tolerant species Iris hexagona, seeds produced on maternal plants growing at high salinity germinated earlier and in greater numbers than seeds from low-salinity plants (Van Zandt and Mopper, Reference Van Zandt and Mopper2004). In the present study, the saline concentrations in which the euhalophyte S. salsa grows were shown to markedly affect their seed quality. Compared with control plants, the black and brown seeds from mother plants grown in the 200 mM NaCl treatment displayed significantly higher germination parameters, such as germination percentage, germination potential, germination index and seed vigour index, except for the brown seed germination percentage and germination potential in the range of 0–100 mM NaCl. What may be the reasons for the difference between the germination ability of S. salsa from the control and 200 mM NaCl-treated plants? Our results indicate that a certain concentration of NaCl enhances seed vitality and seed quality during seed formation, that is to say, NaCl is a beneficial factor in seed development of the euhalophyte S. salsa.
Seed size and weight affect seed quality (Willenborg et al., Reference Willenborg, Wildeman, Miller, Rossnagel and Shirtliffe2005; Tanveer et al., Reference Tanveer, Tasneem, Khaliq, Javaid and Chaudhry2013), and many factors affect seed size and weight on the maternal plant. As demonstrated by Sawan et al. (Reference Sawan, Fahmy and Yousef2009) for cotton, an increase in nitrogen and potassium application significantly increased the seed weight. In non-halophytes, the reproductive growth is limited in saline soils (Munns and Termaat, Reference Munns and Termaat1986; Munns et al., Reference Munns, James and Läuchli2006). Salt stress significantly decreased the seed weight per head and 1000-achene weight of sunflower hybrids (Di Caterina et al., Reference Di Caterina, Giuliani, Rotunno, De Caro and Flagella2007). As in salt-tolerant species, the individual seed weight and quality decreased with increasing levels of salinity in rye (Secale cerealeL.) (Francois et al., Reference Francois, Donovan, Lorenz and Maas1989). However, in the present experiment, 200 mM NaCl in the growth medium markedly improved the seed quality, including seed size and seed weight, which possibly contributes to a good germination performance under high salinity. As described for Convolvulus arvensis, larger seeds result in a higher germination percentage and germination index than small seeds (Tanveer et al., Reference Tanveer, Tasneem, Khaliq, Javaid and Chaudhry2013), and in sunflower (Helianthus annuus L.) seed size had a significant effect (P ≤ 0.01) on germination percentage (Farahani et al., Reference Farahani, Moaveni and Maroufi2011).
Seed dimorphism and polymorphism are well known for several halophytes (Khan and Ungar, Reference Khan and Ungar1984; Khan et al., Reference Khan, Gul and Weber2001; Li et al., Reference Li, Liu, Khan and Yamaguchi2005). Heavy seeds of Primula species, for example, germinated in greater numbers and more quickly than light seeds (Tremayne and Richards, Reference Tremayne and Richards2000). In the natural habitat of S. salsa, brown seeds are more salt resistant than black seeds, and germinate more rapidly than black seeds (Song et al., Reference Song, Fan, Zhao, Jia, Du and Wang2008). In the present study, the brown seeds were bigger and heavier than the black ones and this may be one of the reasons why the germination percentage for brown seeds was greater than that for the black seeds at all NaCl concentrations. Besides seed size and weight, seed vigour was also related to seed storage compounds. The bigger seeds of wheat cultivars had higher levels of protein than the smaller seeds (Ries and Everson, Reference Ries and Everson1973; Ries et al., Reference Ries, Ayers, Wert and Everson1976) and reduced seed storage protein contents led to altered storage organelle formation in rice (Kawakatsu et al., Reference Kawakatsu, Hirose, Yasuda and Takaiwa2010). Salinity (50 mM NaCl) markedly reduced the protein and starch contents of chickpea seeds (Cicer arietinum L.) (Murumkar and Chavan, Reference Murumkar and Chavan1986). However, in the present study the bigger seeds displayed higher contents of storage compounds, as well as better germination parameters. These results suggest that the higher vigour of seeds from the mother plants of the 200 mM NaCl treatment is due to the higher content of seed storage compounds.
Seed tolerance to salinity should be considered at two levels (Prado et al., Reference Prado, Boero, Gallardo and Gonzalez2000): (1) the ability to germinate under high salinity, and (2) the ability to recover and germinate following the removal of the saline conditions. In this study, the S. salsa seeds from the mother plants grown at 200 mM NaCl showed a favourable performance, both in germination parameters and recovery at 0–150 mM NaCl concentrations, as compared to seeds from the 1 mM NaCl treatment. As suggested by Ungar (Reference Ungar1978, Reference Ungar, Kigel and Galili1995), for successful establishment of plants in saline environments seeds must remain viable at high salinity and germinate when salinity in the growth medium decreases. Taking our results together, euhalophytes such as S. salsa need a high level of salt in their growth environment to produce high-quality seeds for faster germination and population establishment in high saline soils, and in this sense, low levels of salt represent a ‘stress’ condition for halophytes.
The present study investigated, for the first time, the effect of very low and optimum levels of NaCl on seed production and seed quality of the euhalophyte S. salsa. Seed germination characteristics, seed quality and seed storage compound contents from mother plants grown under 200 mM NaCl throughout a whole life cycle were markedly higher than those from the control plants (grown in 1 mM NaCl). These results suggest that a certain concentration of NaCl in the growth environment is favourable for seed development in euhalophytes, which possibly contributes to high seed germination and population establishment in high-salt environments.
Financial support
This work was supported by the NSFC (National Natural Science Research Foundation of China, project no. 31070158) and the Natural Science Research Foundation of Shandong Province (ZR2014CZ002).
Conflict of interest
None.
