Introduction

Among leguminous plants, common bean (Phaseolus vulgaris) is considered the most important cultivated legume in the world (Romero et al. 2013). For first time, it has been cultivated from almost 6000 B.C. in Central and South America (Peru and Mexico). Thereafter, it was transported by Spaniards and the Portuguese to Europe, Africa, and Asia (Wortmann 2006). Almost 30 and 50% from the world common bean production is achieved by Europe and Asia, respectively. The main European and Asian producers for common bean are Turkey and China with around 13 and 17% of the total world production, respectively (Singh 1999; Rubatzkey and Yamagucbi 1997). But, Brazil is the first world producer of common bean (Borém and Carneiro 1999). In the Middle East, common bean grows perfectly as the main leguminous crop in the nutrient-rich soils as found in Jordan, Lebanon, Syria, Iraq, Palestine, Sudan, and Egypt. However, common bean is sensitive for salinity, so a reduction in common bean yield is expected in salt-affected soils (with electrical conductivity above 2 dS m−1), which represent between 20 and 30% of the total common bean agricultural areas in the Middle East (Bayuelo-Jiménes et al. 2002).

The abundance of fresh water in terms of both quantity and quality is extremely difficult worldwide especially in those arid and semiarid regions (Assimakopoulou et al. 2015). Due to scarcity of water, population growth, and agricultural expansion, the use of low-quality water in agriculture became inevitable. However, there are many restrictions for using wastewater for agricultural purposes; therefore, water with high electrical conductivity values and/or Na+ levels can be applied if new techniques will be engineered to improve its quality and/or increase the efficiency of used water by cultivated plants (Paranychianakis and Chartzoulakis 2005). Salinity is one of the major limiting factors for maximizing and sustainability of agricultural production. Consequently, novel approaches and techniques are mandatory to ameliorate their detrimental consequences in order to guarantee a continuous supply of food and enhance the food security concept (Zhang et al. 2011). It is well known that specific ion toxicity (especially Na+ and Cl−) and decreasing soil water potential, via increasing osmotic pressure of soil solution, are the main impacts of salinity on plant growth (Sheldon et al. 2004). In order to uptake water from such saline soil, plants have to increase their osmotic pressure in root tissues either by collecting more salts (halophytes) or biosynthesizing solutes (glycophytes) (Orcutt and Nilsen 2000). Although these two strategies are too extreme, many plants use both two mechanisms to resist the high salt concentrations and face the adverse effects of high salt levels on cell physiology (Saneoka et al. 1999). However, the osmotic adjustment between soil solution and plant tissues in most cases results in diminishing plant growth and productivity because many metabolic compounds are consumed to maintain the osmotic regulation (Xu et al. 2000). Sodium (Na) is needed in low concentration for optimum plant growth, while in high concentration, it becomes toxic, resulting in many detrimental consequences on physiological processes in plants such as reducing the activity of nitrate reductase and blocking photosystem II (Orcutt and Nilsen 2000) by destroying chlorophyll (Krishnamurthy et al. 1987). Also, an osmotic stress in plant cells takes place as a consequence for the ion imbalance between Na+ and K+ when Na+ exists in plant tissues with high concentrations (Sivritepe et al. 2005; Khan and Panda 2008).

Silicon (Si) comes after oxygen in earth crust composition by 28.8% based on dry weight. Si is ubiquitous and exists in all living organisms including plants and humans (Farooq and Dietz 2015). The biological role of Si was firstly well known for unicellular (diatoms) and multicellular (sponges and corals) organisms, where it is needed for better the growth and development of cells (Carlisle 1997). Also, many higher plants, such as cereals, uptake Si in higher amounts than essential nutrients (Epstein 2009) and Si has a geochemical cycle between environment and plants. For instance, wheat seedlings showed considerable tendency towards absorption of Si from aqueous solution containing 0.5 mM Si during 84 days, where at the end of this period, no Si was detected in growth solution (Epstein 2009). Several studies reported that Si plays a crucial role to alleviate the stressful features of biotic (diseases and pests) and abiotic (salinity, drought, and metal toxicity) stresses via affecting the physical and chemical defense system of plants (Epstein 2009; Ma et al. 2011: Farooq and Dietz 2015). Provisioning crop with the adequate dose of Si is expected to expand the productivity and improve its quality. The expected benefits from application of Si come from enhancing water use efficiency in plants by reducing the evapotranspiration via the stomata, increasing the activity of some antioxidant enzymes, and supporting plants against diseases by diminishing their sensitivity towards the harmful organisms (Roohizadeh et al. 2015). Treating seeds with Si (whether as a traditional form or nanosilica (NS)) has positive effects on their germination as in tomato (Haghighi et al. 2012; Siddiqui and Al-Whaibi 2014), soybean (Li et al. 2004; Lu et al. 2002), Vicia faba (Roohizadeh et al. 2015), and sweet pepper (Tantawy et al. 2015). The importance of Si for improving plant growth was also reported by Roohizadeh et al. (2015) for V. faba, and this is attributed to increase the water use efficiency in plant (Romero-Aranda et al. 2006) and improve the competence of photosynthesis (Liang et al. 2003).

Nanoscale science (nanotechnology) is the science of particles at nanoscale (1–100 nm in size); it describes the characteristics of nanosized materials and their behavior in different systems. Recently, nanotechnology has gained the attention of researchers in many disciplines of science although nanoparticles exist already with the beginning of the universe. Scientists expect that nanotechnology will play a crucial and vital role in many fields in our life such as in medicine, industry, agriculture, electronics, energy, and environment (Nair et al. 2010; Zhang et al. 2015). In agriculture, using nanoparticles is expected to improve the crop productivity by enhancing plant nutrition, precision farming, water use efficiency, crop protection against predators and diseases, innovative tools for pathogen detection, molecular biology, and environmental protection (Walker 2005). Few studies investigated the effect of NS on seed germination of some crops including common bean; however, no work was found to report the effect of NS on seed germination of common bean irrigated with saline water. So, the key point of this work is to investigate the ability of NS to ameliorate the seed germination and growth of common bean under elevated Na+ stresses.

Materials and methods

Germination experiment

The aim of this experiment is to evaluate the seed germination of common bean (P. vulgaris var. strabic) on different Na+ concentrations with and without different doses of NS. For this purpose, the experiment was conducted in complete randomized blocks with three replicates. A 15-cm-diameter sterilized Petri dishes were used as experimental units. A double-filter paper was placed in each Petri dish, and then 15 mL of each treatment was added to the Petri dish. Five Na+ concentrations, i.e., 1000, 2000, 3000, 4000, and 5000 mg L−1, were prepared from NaCl. Each Na+ treatment was treated with four doses of NS suspensions (0, 100, 200, and 300 mg L−1). Distilled water was used as a control (no Na+ stress). NS suspensions were prepared by suspending synthesized hydrophilic NS (Aerosil 300 produced by Evonik Industries, Germany) in distilled water. Information of used NS was as follows: specific surface area (270–330 m2 g−1), pH (3.7–4.5), and mean diameter (10 nm). Seeds were washed thoroughly with distilled water and then immersed in 10% hypochlorite solution for 5 min to clean the surface before experiment. Seeds of each treatment were first soaked in NS suspension (with the same concentration as applied in its NS treatment) for 4 h before transplanting to allow seeds to absorb more NS to activate the germination, and then 15 seeds were sown on the filter paper. All Petri dishes were sealed well to prevent filter paper drying and maintained moist and wet across the experiment period of 10 days. All Petri dishes were placed in a dark place with a room temperature of 23 ± 2 °C. A daily count of germinant was done from the onset of germination up to 10 days thereafter with a minimum length of 2-mm emergent radicle. At the end of experiment, wet and dry masses were taken for each replicate. Germination indicators were calculated as follows:

  1. a.

    Final germination percentage (FGP, %) was calculated as the percentage according to Ranal and Santana (2006) using the formula:

$$ FGP,\%=\left[\frac{TNG}{TNP}\right]\times 100 $$

where FGP, % is the final germination percentage, TNG is the total number of germinated seeds, and TNP is the total number of planted seeds.

  1. b.

    Mean germination time (MGT) was computed by the formula cited by Mauromicale and Licandro (2002) given below

$$ MGT=\sum \left(\frac{ni\times ti}{ni}\right) $$

where MGT is the mean germination time, ni is the number of germinated seeds on germination days, and ti is the number of days during the germination period (between 0 and 10 days).

The mean germination time was used to evaluate seedling emergence.

  1. c.

    Vigor index (VI) was calculated using the formula of Kharb et al. (1994), as follows:

$$ VI=\left[\frac{SDM\left(\mathrm{g}\right)\times GP}{100}\right] $$

where VI is the vigor index, SDM is the seedling dry mass (g), and GP is the germination, %.

  1. d.

    Germination speed (GS) was computed as described by Czabator (1962) using the formula presented below

$$ GS=\sum \left(\frac{ni}{ti}\right) $$

where GS is the germination speed and ni is the number of germinated seeds on germination day.

Growth experiment

This experiment was carried out to clarify the role of NS to improve the growth of common bean seedlings irrigated with highly concentrated Na+ solutions for 3 weeks. Therefore, a plastic pot (5 cm × 5 cm × 8 cm) was filled with 115 g of nursery mixture comprising of sand and peat moss at 1:1 ratio with a saturation percentage of 80%. Seeds of common bean were washed thoroughly with distilled water before soaking in NS suspensions for 4 h according to their treatments before sowing for the same purpose as mentioned above. One seed per each pot was covered by a 1-cm layer of growth mixture, and nine pots per each treatment were used. The pots were irrigated with different Na+ solutions (1000, 2000, 3000, 4000, and 5000 mg Na+ L−1) and different NS levels (0, 100, 200, and 300 mg L−1). All pots were kept wet at almost 65% from its saturation percentage during the experiment period. The pots were placed in greenhouse with an average daily temperature of 23 ± 2 °C. At the end of the experiment, shoot and root lengths as well as their fresh and dry masses were measured. Na, potassium (K), and Si contents in common bean plants were also analyzed after the harvest.

Perpetration of plant materials

After harvesting, plants were divided into shoots and roots. The plant samples were washed thoroughly with 0.1 M HCl to ensure that any adhered materials are totally removed, rinsed in deionized water, and then left to air-dry. Clean samples were dried for 2 days at 65 °C in a forced air oven, grinded by a stainless steel grinder, and then sieved pass through a 60-mesh screen. Finally, samples were kept in plastic bags for further chemical analysis.

Determination of Na, K, and Si contents

A 0.5-g oven-dry plant sample was placed in a 50-mL conical flask and digested with 2.5 mL concentrated sulfuric acid (H2SO4, 95–97%, 1.84 kg L−1; Merck) on a hot plate at approximately 270 °C. Then repeatedly, small quantities of H2O2 were added until the digest remained clear. The samples were left to cool and then transferred and diluted to 50 mL with ultra-pure water in a volumetric flask. Total sodium and potassium were determined according to Cottenie (1980) using an atomic absorption spectrophotometer (AAS; Perkin Elmer 3300) with a detection limit of 100 ppb. For Si determination, 0.05 g of the grinded plant sample was placed into a 50-mL polyethylene tube and 5 mL of 100% NaOH was added and then shaken to mix thoroughly. The capped tube was moved in an autoclave and heated for 30 min and then allowed to cool at room temperature. Two milliliters of H2O2 was added to the tube and reheated in the autoclave for additional 30 min. After cooling, the tube was brought to 50 mL using distilled water. After digestion, 0.1 mL of digestion mixture was transferred to a new tube and 10 mL of distilled water was added. Then, each tube received 0.25 mL of the 1:1 mixture of HCl and H2O. Then, ammonium molybdate solution (0.5 mL, 100 g L−1, pH 7.0) was added and tubes were shaken and allowed to stand for 10 min. Tartaric acid (0.5 mL, 200 g L−1) was added and mixed thoroughly by shaking tubes and left to stand for additional 3 min. Sodium bisulfate (0.7 mL, 50 g/400 mL) was added and mixed. The blue color that developed was measured between 10 and 30 min at 650 nm. The standard curve analysis was done using known Si concentrations (Frantz et al. 2008).

Statistical analysis

Data analysis was performed using Microsoft Excel 2010 (mean values and standard deviation) from two individual experiments. All data were analyzed statistically by the XLSTAT software package. Experiments were set up in a completely randomized design with three repeats for each treatment. When a significant difference was observed between treatments, multiple comparisons were made by Fisher’s test. Significant differences were accepted at the p level < 0.05.

Results

Germination of common bean seeds

Final germination percentage

Results of the FGP (%) of common bean seeds germinated under increasing Na+ concentrations with and without NS are depicted in Fig. 1a. Increasing Na+ stress drastically affected the FGP of common bean. The FGP decreased to 57.7% at 5000 mg Na+ L−1 compared to 82.3% for control (zero Na+) when no NS was applied. All NS doses, i.e., 100, 200, and 300 mg L−1, significantly improved FGP, recording higher values than untreated seeds at all Na+ concentrations. For example, at 5000 mg Na+ L−1, FGP increased by 4.0, 12.1, and 19.7% at 100, 200, and 300 mg L−1 NS, respectively.

Fig. 1
figure 1

Germination parameters of common bean seeds under different concentrations of Na+, when nanosilica was applied, aiming to ameliorate the detrimental effects of Na+ salinity. a Final germination percentage. b Mean germination time. c Vigor index. d Germination speed. Nanosilica showed a considerable capacity to improve germination parameters under increasing doses of Na+ up to 5000 mg L−1

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Mean germination time

Although NS did not show any obvious effect on the MGT of common bean seeds, it was clear that NS doses led to a decrease of MGT at low Na+ concentrations up to 2000 mg L−1 (Fig. 1b). The shortest MGT (5.24 days) among all treatments was found at 300 mg L−1 NS when no Na+ was present in the germination medium. While MGT at the treatment of 4000 mg Na+ L−1 was 6.43 days without NS, MGT was diminished to 5.83 days when 300 mg L−1 of NS was added at the same Na+ level. However, 2000 mg L−1 of Na+ showed a considerable effect on the MGT of common bean seeds under 0, 100, and 200 mg L−1 NS, recording 5.48, 5.32, and 5.50 days, respectively, compared to 5.90 days when no Na+ and NS were used.

Vigor index

The calculated values of VI of the germinated seeds of common bean are graphically represented in Fig. 1c. High Na+ concentrations showed a negative impact on VI. The VI values were dramatically reduced with increasing Na+ concentrations, particularly when the germination medium was free from NS. The application of 4000 mg L−1 Na+ minimized the VI by 52.7% compared to control (no Na+ and no NS). However, this VI value can be enhanced by 14.5, 47.4, and 144.6% at 100, 200, and 300 mg L−1 NS, respectively. Also, NS proved its importance in low Na+ concentrations, since it increased the VI from 5.05 to 10.98 at 300 mg L−1 NS under 1000 mg Na+ L−1 stress. The highest VI value was 14.58 at 300 mg L−1 NS with zero Na+.

Germination speed

Figure 1d denotes the data of GS when common bean seeds were germinated on Na+ stress with and without NS. Significantly increasing Na+ levels decreased the GS of seeds by almost 32.4% at a level of 5000 mg Na+ L−1 compared to control seeds (no Na+ and no NS). NS demonstrated great ability to alleviate the detrimental effects of Na+ on seed germination, where all NS doses increased GS values whether in the presence or absence of Na+. When the germination medium was free of Na+, GS increased from 17.3 to 24.6 germinant day−1 after adding 300 mg L−1 of NS. The same behavior was found when NS was applied to seeds treated with 1000, 2000, 3000, 4000, and 5000 mg Na+ L−1. Three hundred milligrams per liter of NS increased GS by 22.6% at the treatment of 5000 mg Na+ L−1 compared to the treatment that received zero NS at the same level of Na+.

Wet and dry masses

Results of both wet and dry masses of germinated seeds (after 10 days) are found in Table 1. High Na+ concentrations had a negative effect on the seed germination of common bean and then on the wet mass of seedlings. A decrease of 10% was recorded in the wet mass of seedling grown on 5000 mg Na+ L−1 against control. However, this detrimental effect was diminished and ameliorated by adding NS to the growth medium by 100, 200, or 300 mg L−1, where all NS doses resulted in almost higher wet masses for seedlings than control. Treating seeds with 100 mg L−1 NS at 5000 mg L−1 Na+ increased the wet mass of seedling by 53.3%, while an increase of 14.5% was calculated at 300 mg L−1 NS. Likewise, dry mass of seedling was considerably affected by high concentrations of Na+, as values of dry mass decreased from 0.089 g seedling−1 (control) to 0.052 g seedling−1 (at 3000 mg L−1 of Na+). Specific ion toxicity of Na+ on dry mass of common bean seedlings was alleviated by NS. All seedlings which treated with NS recorded higher dry masses than control. For example, at 4000 mg Na+ L−1 enrichment, the growth medium with 300 mg L−1 NS increased the dry mass from 0.056 g seedling−1 (control) to 0.116 g seedling−1 (Table 1).

Table 1 Wet and dry masses of the common bean seedling (at 10 days after germination) germinated in the presence of nanosilica on high Na+ concentrations
Full size table

Seedling growth

Vegetative parameters

Length of shoot and root as well as their dry masses were measured at the end of the experiment (3 weeks) as shown in Table 2. Shoot length of common bean seedling was negatively affected with increasing Na+ levels. At a concentration of 5000 mg Na+ L−1, the seedlings could not keep growing; therefore, no data was recorded at 5000 mg Na+ L−1. In the absence of NS, the treatment of 2000 mg L−1 of Na+ recorded the highest value of shoot length (12.7 cm); meanwhile, control seedling had a shoot length of 7.3 cm. Although seedlings at 5000 mg Na+ L−1 did not succeed to grow, they started to grow when NS was added to the growth medium even by 100, 200, and 300 mg L−1. Seedlings treated with different doses of NS had a higher shoot length than untreated ones. For instance, seedling grown at 4000 mg Na+ L−1 had a shoot length of 12.0 cm, which increased to 13.3 cm when NS was applied to its growth medium by 300 mg L−1. Likewise, root length was higher for seedlings treated with different NS doses (100, 200, and 300 mg L−1) compared to untreated seedlings. Also, at the highest Na+ concentration (5000 mg L−1), seedlings were able to grow only when they enriched with NS, achieving the longest root length at 300 mg L−1 NS (Table 2). However, the longest root length was observed at 300 mg L−1 NS when seedlings did not expose to Na+ stress (control).

Table 2 Effect of nano-silica on vegetative parameters of common bean seedlings grown in the presence of elevated Na+ concentrations
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While low Na+ concentrations (up to 2000 mg L−1) enhanced the shoot part dry mass of common bean seedling, higher concentrations (above 2000 mg L−1) decreased shoot dry mass up to 4000 mg L−1. Above 4000 mg L−1, Na+ seedlings did not grow, which means there was a missing value (Table 2). From the data presented in Table 2, it could be concluded that NS demonstrated its importance for enhancing the growth of common bean seedlings whether under abiotic stress such as high Na+ concentrations or even in normal conditions. Treating the seedlings with different doses of NS increased values of shoot dry mass against control seedlings (untreated with NS). Untreated seedling with NS at 4000 mg Na+ L−1 had a shoot dry mass of 0.27 g seedling−1, but the treated one with 300 mg L−1 NS recorded a shoot dry mass of 0.56 g seedling−1 at the same Na+ concentration. In addition, NS improved the shoot dry mass of seedlings grown on the Na+-free growth medium, where shoot dry mass was 0.21 g seedling−1 (at zero NS) and increased to 0.98 g seedling−1 (at 300 mg L−1 NS). On the other hand, root dry mass was more sensitive to Na+ concentrations. Root dry mass drastically decreased with increasing Na+ concentrations. Control seedling had a root dry mass of 0.06 g, while this value decreased to 0.01 g at 4000 mg Na+ L−1 apart from adding NS (Table 2). Clearly, NS enhanced the root dry mass comparing with seedlings that received zero NS. Adding NS by 300 mg L−1 increased the root dry mass of seedling grown on 4000 mg Na+ L−1 by almost fourfold (Table 2).

Chemical analysis of common bean seedlings

Potassium content

Results of measured K content are depicted in Fig. 2a. The highest K content (8.7%) was measured at 1000 mg Na+ L−1, and then it started to decrease gradually with increasing Na+ concentration up to 4000 mg L−1, recording 6.7%. The gradual application of NS resulted in a gradual increase of K content in common bean seedlings. For instance, at zero Na+ (control), K content was 7.0% when no NS was used but adding NS with 100, 200, and 300 mg L−1 led to an increase of K content by 9.6, 11.0, and 11.6%, respectively. Among all NS doses, 300 mg L−1 recorded the highest K content under all Na+ concentrations.

Fig. 2
figure 2

Effect of different Na+ levels and three doses of nanosilica on the elemental analysis of common bean seedlings grown for 3 weeks. a Potassium content. b Sodium content. c Potassium/sodium ratio. d Silicon content

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Sodium content

Common bean seedlings absorbed more Na from its growth medium fortified with elevated concentrations of Na+, and thus, Na content increased with increasing Na+ doses as shown in Fig. 2b. Among the three NS doses, it was found that the seedlings treated with 100 mg L−1 NS had the highest Na content, while the seedlings treated with 300 mg L−1 NS contained the lowest Na content. The highest Na content (2.1%) was found at 5000 mg Na+ L−1 with 200 mg L−1 NS, and the lowest Na content was measured at zero Na+ and 300 mg L−1 NS.

Potassium/sodium ratio

Data of the calculated potassium-to-sodium (K/Na) ratio are shown in Fig. 2c. The K/Na ratio was calculated based on K and Na contents in common bean seedlings; therefore, it has the same tendency. Increasing Na+ levels (without using NS) in the growth medium resulted in the decreasing K/Na ratio, where the highest value of the K/Na ratio (29.4) was found at 1000 mg Na+ L−1 and then decreased to be 6.8 at 4000 mg Na+ L−1. When NS was added to seedlings, the K/Na ratio significantly increased to reach 149.8 at zero Na+ with 300 mg L−1 NS. Although NS doses increased the K/Na ratio, increasing Na+ levels resulted in a drastic reduction in the K/Na ratio (Fig. 2c).

Silicon content

Values of Si content are presented in Fig. 2d. However, Si content in common bean seedlings increased gradually with increasing Na+ and NS doses. Apart from NS treatments, Si content increased from 5.0% (at control) to 5.9% (at 4000 mg Na+ L−1). Adding NS pushed the seedlings to accumulate more Si in their tissues as results shown. At 4000 mg Na+ L−1, Si content increased from 5.9 to 8.2% at 0 and 300 mg L−1 NS, respectively.

Discussion

Salinity, whether related to soil or irrigation water, is one of the main limitations to maximize the agricultural production as a consequence for increasing the osmotic pressure of the soil solution and thus more stress on plant to absorb the water. In addition, specific toxicity of elevated sodium ion which ultimately leads to lower productivity therefore cannot fulfill food security concept (ensure save and continuous food supply). Salinity tolerance mainly relies on plant cultivar, and thus, genetic variability exists between cultivars (Asmare and Ambo 2013). However, many amendments can be utilized to reduce the adverse effect of salinity on germination and plant growth.

Current experiments strengthened the facts concerning the detrimental effects of elevated salinity especially those derived from Na+. Drastically, high Na+ concentrations reduced the FGP, VI, and GS of common bean seeds as well as wet and dry masses of the germinant. FGP dropped from 82.3 to 57.7% when Na+ concentration increased to 5000 mg L−1 versus control. Also, VI decreased sharply from 7.4 for control to 3.5 at the treatment of 4000 mg Na+ L−1. Similarly, the lowest GS (11.7 germinant day−1) was found when Na+ in the growth medium was 5000 mg L−1, while the highest GS (17.3 germinant day−1) was noticed at control. Wet mass of common bean germinant reduced by 10.4% when the Na+ level increased to 5000 mg L−1 compared to control (distilled water). This may indicate that increasing Na+ decreased relative water content in seedlings. The lowest dry mass was measured at a level of 3000 mg L−1 of Na+, while the highest dry mass was recorded for control. These results were in harmony with those obtained by other authors. For instance, Mena et al. (2015) reported that seed germination of common bean was reduced by increasing NaCl above 8775 mg L−1. Also, high NaCl concentrations postponed the initiation of seed germination of common bean and induced a reduction in its FGP (Cokkizgin 2012). In the same context, Kaymakanova (2009) stated the negative effect of NaCl on seed germination of common bean. Many other studies reported similar results concerning an adverse effect of NaCl on the FGP of common bean (Bayuelo-Jiménes et al. 2002; Meot-Duros and Magné 2008; Rahman et al. 2008). However, the delay in starting the germination process and diminishing the germination parameters such as FGP, VI, GS, wet mass, and dry mass could be attributed to inhibition of water absorption by seeds a result of increasing the osmotic potential of soil solution and/or ionic effect in which Na+ and Cl− concentrate in plant tissues, causing an inequity in the uptake of nutrients and poisonous effect (Mena et al. 2015; Shokohifard et al. 1989). To overcome the previously discussed adverse effect of elevated Na+ concentrations on the germination of common bean, seeds were treated with different doses of NS (100, 200, and 300 mg L−1). All NS doses proved its great ability to induce the germination of common bean seeds, in which values of all measured germination parameters of treated seeds with NS under increasing Na+ were higher than those of untreated seeds at the same level of Na+. Among all NS doses, the treatment of 300 mg L−1 was the best. For instance, at the highest Na+ concentration (5000 mg L−1), FGP, VI, and GS of the treated seeds with 300 mg L−1 of NS were 69.0%, 7.7, and 14.3 germinant day−1 in comparison with untreated seeds which recorded 57.7%, 4.3, and 11.7 germinant day−1, respectively. Similar results were cited by many other researchers for common bean and/or different crops. In two different studies, NS induced the seed germination of tomato, achieving an increase of 22.2 and 70.3% when applied by 8 and 5 g L−1, respectively (Siddiqui and Al-Whaibi 2014; Lu et al. 2015). An increase of 11% for the FGP of maize seeds was stated by Yuvakkumar et al. (2011), when seeds were treated by 300 mg kg−1 of NS. Qados (2015) reported that using NS by a dose of 2 mM increased the seed germination of faba bean (V. faba L.) by 47.7% compared to control. These results demonstrate the importance of treating seeds with NS to enhance its germination, and no toxicity symptoms were noticed. In this current study, all NS doses induced MGT positively compared to untreated seeds, meaning seeds needed a shorter time to germinate completely compared to seeds treated with Na+ only. Our present data are supported by findings reported earlier by Suriyaprabha et al. (2012), who stated that the MGT of maize seeds was induced positively by treating seeds with NS, but MGT decreased when seeds were treated with Na2SiO3. Treating tall wheatgrass seeds with 40 mg L−1 of NS fastens the germination, decreasing the MGT to be 5.23 days as the shortest period for complete germination (Azimi et al. 2014). Similar results were cited by other authors (Sadeghi et al. 2012; Cokkizgin 2012; Ologundudu et al. 2013). The importance of VI comes not only from being a vital tool to assess the fertility of seeds but also from being a key function for seedling growth after germination (Marcos-Filho 2015). In this study, despite high Na+ levels sharply weaken the vigorousness of common bean seedlings after germination as values of VI displayed, strikingly, NS with 100, 200, and 300 mg L−1 increased VI as a function to strengthen the seedling growth. Our results agree with results presented by Azimi et al. (2014), who said that an increase of 120% of VI was measured after adding NS to seeds of tall wheatgrass by 40 mg L−1. Also, a similar result was reported by Lu et al. (2015), who mentioned that 7 g L−1 of NS recorded the highest value of VI regarding control. The number of germinated seeds per day throughout the entire germination period is known as germination speed (GS). Although all NS doses under Na+ treatments recorded higher GS values than seeds treated only with Na+, a dose of 300 mg L−1 NS had the fastest GS. Similarly, the GS of Thymus kotschyanus treated with NS under laboratory conditions was higher than that of control (Khalaki et al. 2016). One of the main detrimental effects of salinity is reduction of the vegetative growth. The obtained data from the current experiment reveals this impact. Increasing Na+ concentrations diminished the growth, in which wet and dry masses of seedlings decreased drastically. NS showed a considerable capacity to ameliorate this negative impact of Na+-derived salinity. Strikingly, both wet and dry masses remarkably were increased after adding NS to common bean seeds germinated on highly concentrated Na+ solutions especially a dose of 100 mg L−1 NS. Our data were in harmony with those cited by Siddiqui and Al-Whaibi (2014), who stated that fresh and dry masses of tomato were induced by NS. A similar effect was noticed on maize after amending it with NS (Suriyaprabha et al. 2012). Contrary, Lu et al. (2015) reported that no significant differences were measured for fresh and dry weights of treated tomato seeds with NS versus control. However, our results emphasize that NS is able to improve the seed germination of common bean whether under an abiotic stressor such as high Na+ concentration or apart from such stressors as in control. This phenomenon could be attributed to biochemical changes inside the seed initiated by NS which crosses the seed coat faster and easier than traditional silica based on its very small size. Similar explanation was introduced by Zheng et al. (2005) who said that nanotitania induced seed germination of spinach based on its nanosize. One of the biochemical changes that assumed it takes place inside the seed after nanoparticles penetrate the seed coat is eliminating the free radicals that inhibit the germination. NS was found to alert some of redox reactions inside the seed by activating catalase and superoxide dismutase enzymes that play a vital role in producing superoxide ion radical, so removing faster the free radicals, and thus accelerate the seed germination (Gengmao et al. 2015). Furthermore, NS activates some other biochemical reactions that inhibit production of abscisic acid while it increases secretion of gibberellin and thus it helps in releasing the seed dormancy (Yuvakkumar et al. 2011).

Of course, a vigorous seed gives a vigorous seedling. Low concentrations of Na+ below 2000 mg L−1 were necessary for the better growth of seedlings, and this is reflected in values of measured vegetative parameters. Otherwise, high Na+ concentrations above 2000 mg L−1 showed an adverse effect on shoot and root lengths as well as dry masses of shoot and root parts through diminishing seedling growth. An amendment growth medium with NS was enough to recover the health of seedlings and induce their growth under elevated Na+ concentrations. NS doses, particularly a dose of 300 mg L−1, enhanced the growth of common bean seedlings in which seedlings were able to grow well under a concentration of 5000 mg L−1 of Na+. In the treatment of 5000 mg L−1 of Na+ without adding NS, seedlings failed to grow, which means there are missing data for this treatment. Our results agreed with those reported by Mena et al. (2015), who stated that NaCl concentrations above 200 mM declined considerably the growth of common bean seedlings. Likewise, Kaymakanova and Stoeva (2008) reported that 100 mM NaCl dramatically reduced the growth of three cultivars of P. vulgaris. Also, it was cited that the growth of some Vigna unguiculata L. cultivars was decreased by 200 mM of NaCl (Mahamadou et al. 2013). Abdul Qados (2010) said that the plant height of V. faba (L.) was increased by low concentrations of NaCl, while high concentrations caused a considerable decline. In harmony with the results of the current study, Khajeh-Hosseini et al. (2003) reported that seedling length significantly decreased with increasing salinity level. Gama et al. (2007) suggested that increasing NaCl above 100 mM detrimentally declined the root length and root dry weight of common bean. Enhancing plant growth under a low concentration of salinity could be a consequence for osmotic adjustment caused by low NaCl levels (Abdul Qados 2010). Also, Munns (2002) assumed that salinity increases the fresh weight of shoot because plant tends to enlarge the volume of sap vacuoles to collect much water in order to dissolve the accumulated salt ions in plant tissues as a means to resist high salinity. There are many suggested mechanisms for how salinity adversely affects the plant growth. An et al. (2014) suggested that salinity increases cell wall rigidity via increased cellulose and decreased pectin across root zones particularly in some soybean cultivars. A physiological drought mechanism was introduced by Barrios et al. (1998) as a result of high salinity in the growth medium of plant. Salinity also was found to inhibit plant growth by diminishing photosynthesis, modification in protein synthesis by altering enzyme activities, decline carbohydrate content, and obstruct growth hormones (Mazher et al. 2007).

Although plant biologists do not look at silicon (Si) as an essential nutrient for plant growth yet, Si is beneficial for many crops and many plants uptake it by amounts higher than many other essential nutrients (Epstein 2009). Despite the fact that Si is the second abundant element in the earth crust, the addition of Si to plants is urgent especially in acidic soils because Si is depleted in these soils by the action of weathering (Korndörfer and Lepsch 2001). In this present research, NS actually showed an amazing ability to overcome the detrimental effects of high Na+ concentrations. NS induced the seedling growth resulting in higher values for all measured parameters against control. Our data showed that NS caused a reduction in Na uptake by plants, and this could be the reason for why NS induced the growth. All NS doses decreased Na uptake, but the amazing result is that Na uptake was drastically declined with increasing NS doses; therefore, a dose of 300 mg L−1 NS resulted in the lowest content of Na in plant tissues. In the same time, NS increased the uptake of K resulting in high values of K/Na ratio particularly under saline condition. Our results are supported by those results reported by Tahir et al. (2006). Gengmao et al. (2015) mentioned that based on its special physiochemical characteristics, Si is the only element which can increase the resistance of plants to abiotic stresses such as salinity and drought. Similarly, Yeo et al. (1999) stated that silicate improved the growth and photosynthesis of rice grown under salinity conditions. They referred this amelioration in growth and photosynthesis to a decline in Na uptake by silicate amendment and also because silicon sustained the integrity of plasma membrane via reticence of the separation of plasma membrane from plasmolysis. The reduction in Na uptake as they mentioned came from reducing sodium transport and the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), not by decreasing transpiration rate where silica increased the stomatal conductance. From all facts mentioned above, it can be concluded that Si is important for plants that grow under salinity circumstances. More Si in plant tissues is expected to improve the growth by first reducing Na uptake and thus decreasing toxicity of Na+. Second, Si deposits beneath the cuticle layer in leaves causing a reduction in transpiration rate; therefore, it maintains high relative water content in leaves which makes the plant not to uptake more water under high osmotic potential in soil around the root system. So, engineering silica in nanosize which makes more silica can easily cross the cell wall passively is crucial in order to induce the tolerance of plants to abiotic stresses.

Conclusion

The present study highlights the effect of elevated Na+ concentrations on seed germination and growth of common bean as well as tests the possible use of engineered NS to ameliorate the detrimental effects of Na+-derived salinity. All germination parameters were negatively affected with increasing Na+ levels particularly above 1000 mg L−1. Values of final germination percentage, vigor index, and germination speed as well as wet and dry masses of seedlings were significantly lower than those of control. In addition, this adverse effect of Na+ continued the same on the growth development of seedlings, in which seedlings could not grow above 4000 mg L−1 of Na+. However, amelioration of this adverse effect of salinity strongly correlated to NS. Treating seeds with NS improved their growth under salinity circumstances, recording higher values for all germination parameters and initiating better the germination process. Using NS by the lowest dose, i.e., 100 mg L−1, not only improved the growth of seedlings but also overcame the negative effect of the treatment of 5000 mg Na+ L−1, where seedlings treated with NS under this level of Na+ were able to survive and grow well during the experiment period compared to seedlings treated only with salt. Among all NS doses, the dose of 300 mg L−1 was the best. This study wanted to demonstrate the great capacity of NS to alleviate the detrimental impacts of high Na+-derived salinity on common bean.