Beneficial effects of silicon nutrition in alleviating salinity stress in hydroponically grown canola, Brassica napus L., plants

Abstract

Silicon (Si) is the second most abundant element in soil and effectively counteracts the effects of various abiotic stresses, such as drought, heavy metal toxicity and salinity, on plants. In the present study the ameliorating effects of Si nutrition supplied as 2 mmol L⁻¹ sodium silicate were investigated on hydroponically grown canola (Brassica napus L.) plants under salinity stress (i.e. 150 mmol L⁻¹ sodium chloride). Salinity decreased plant growth parameters such as tissue fresh and dry weights. These decreases were accompanied by increased lignin contents, Na⁺ ion accumulation, increased lipid peroxidation and decreased chlorophyll contents in plants. Silicon nutrition, however, enhanced plant growth parameters and led to the prevention of lignin and the Na⁺ accumulation in shoots, reduced levels of lipid peroxidation in the roots and higher levels of chlorophyll. As a result of salinity, catalase activity in the whole plant and both soluble and cell wall peroxidase activities in the shoots decreased. Silicon nutrition, however, increased the reactive oxygen species scavenging capacity of salt-stressed plants through increased catalase and cell wall peroxidase activities. Thus, silicon nutrition ameliorated the deleterious effects of salinity on the growth of canola plants through lower tissue Na⁺ contents, maintaining the membrane integrity of root cells as evidenced by reduced lipid peroxidation, increased reactive oxygen species scavenging capacity and reduced lignification.

Key words:

• Brassica napus
• lignin
• lipid peroxidation
• salinity
• silicon

Introduction

Salinity is a major factor limiting plant growth and crop productivity. Approximately one-third of the world’s irrigated lands already suffer from excess salinity (Szabolcs 1994). It is assumed that salt stress imbalances cellular ions, and this imbalance results in ion toxicity and osmotic stress, compromising plant growth and survival (Tester and Davenport 2003). Salt stress through an increase in reactive oxygen species (ROS), such as singlet oxygen, superoxide anion (O₂˙⁻), hydrogen peroxide (H₂O₂) and hydroxyl radical (OH˙), also leads to oxidative stress (Alscher et al. 1997; Mittler 2002; Neill et al. 2002). Reactive oxygen species can alter normal cellular metabolism through oxidative damage to lipids, proteins and nucleic acids (Alscher et al. 1997; Imlay 2003; McKersie and Leshem 1994). Reactive oxygen species scavenging enzymes, such as superoxide dismutase, convert superoxide anions to H₂O₂. Subsequently, hydrogen peroxide is scavenged by catalase and different classes of peroxidases (McKersie and Leshem 1994; Noctor and Foyer 1998). The activities of some ROS scavenging enzymes involved in lignin biosynthesis like cell wall bound peroxidases and some other enzymes related to lignification are affected by salinity (Gunes et al. 2007; Vaidyanathan et al. 2003; Wang et al. 1997). Salinity leads to increased activities of ROS scavenging enzymes like peroxidase in Chloris gayana (Rhodes grass) and Atriplex prostrata plants associated with greater tissue lignification (Ortega et al. 2006; Sánchez-Aguayo et al. 2004; Wang et al. 1997), which in turn restricts further plant growth.

Canola (Brassica napus L.) is an important crop for edible oil production (Francois 1994; Qasim et al. 2003). Recent efforts have focused on the use of canola oil to produce clean-burning fuel (i.e. biodiesel) and canola is currently the third most important crop after soybean and maize for biodiesel production (Vasudevan and Briggs 2008). Most studies on canola have reported a reduction in leaf area, shoot and root biomass under salinity and have compared the salt tolerance of this species with other species of Brassica (Ashraf and McNeilly 1990; Francois 1994; Qasim et al. 2003). It appears that canola plants are moderately salt tolerant (Ashraf and McNeilly 1990; Francois 1994). As salt affected areas in the world are increasing (Szabolcs 1994; Tester and Devenport 2003), the adoption of economic strategies to extend canola cultivation in saline areas through both the development of genetically engineered plants and other conventional methods like breeding is justified (Purty et al. 2008; Zhang et al. 2001). Although the exogenous application of compatible solutes, such as proline or glycine–betaine, may reduce the deleterious effects of salt stress on canola plants (Athar et al. 2009), the use of other non-costly ubiquitous compounds that result in more economic production is favored.

Silicon (Si) is the second most abundant element in soil. It is also a major structural component of the cell walls in some monocotyledonous species, such as rice and barley, and readily accumulates as a compression-resisting element analogous to lignin (Inanaga and Okasaka 1996; Raven 1983). Although Si is not usually considered to be an essential element, it has been shown to be beneficial for plant growth and production (Epstein 1994; Liang et al. 2006). Silicon fertilizers are applied as slag, compost, rice straw, calcium and sodium silicates (Ma and Takahashi 2002). For example, the Si application rate in rice fields amounts to 2.5 t ha⁻¹ of calcium silicate slag with 20% Si content (Alvarez and Datnoff 2001) and 9 t ha⁻¹ in the form of rice chaff with 4% Si content (Ma and Takahashi 2002; Akbar Hossain et al. 2001). Silicon has been shown to ameliorate the adverse effects of salinity in several plants, including barley (Liang et al. 1996; Liang 1998), wheat (Ahmad et al. 1992), mesquite (Prosopis juliflora) (Bradbury and Ahmad 1990), spinach and tomato (Gunes et al. 2007). A reduction in the accumulation of sodium in the shoots of several plants, such as barley (Liang et al. 1996) and rice (Yeo et al. 1999), under salinity has been reported following the application of Si. It has been suggested that Si deposition in the exodermis and endodermis reduces sodium uptake under salinity through a reduction in apoplastic transport across the root (Gong et al. 2006; Yeo et al. 1999). The amelioration of salt toxicity effects by Si nutrition in maize (Moussa 2006), barley (Liang et al. 2003, 2006), cucumber (Zhu et al. 2004), spinach and tomato (Al-aghabary et al. 2004; Gunes et al. 2007) has been attributed to decreased oxidative damage.

To the best of our knowledge, no reports have examined the interaction of salinity and Si nutrition in canola or how the beneficial effects of Si in salt-stressed canola plants (if any) are exerted. Accordingly, in the present work, growth, changes in ion concentrations, phenolics, lignin and the activities of some ROS scavenging enzymes were studied in canola plants (Brassica napus L.) under salinity supplied with or without sodium silicate to examine the possibility of extending canola cultivation into saline soils.

Materials and methods

Plant culture and growth conditions

The experiments were conducted during 2006–2007 in a greenhouse at the Gorgan University of Agricultural Sciences and Natural Resources. Seeds of canola (Brassica napus L. cv. Hayola) were obtained from Iran Oilseeds Research Institute and after screening for uniform size and color, the seeds were sterilized with a 2.5% sodium hypochlorite solution. The seeds were incubated in a moistened paper towel and germinated in the dark at 25 ± 5°C for 48 h. Healthy seedlings of uniform size were selected for hydroponic culture. Each plant was supported in the centre of a grey foam lid and the roots were immersed in a 10 L black plastic container filled with continuously aerated Hoagland nutrient solution. The experiment was carried out in a factorial completely randomized design. Factor one was salinity, which was applied to the root medium as NaCl (0 and 150 mmol L⁻¹), and factor two was silicon nutrition, which was supplied as sodium silicate (0 and 2 mmol L⁻¹). Treatments were started 3 weeks after transplanting the seedlings to hydroponic culture with 25 mmol L⁻¹ NaCl and increased in three steps over the next 3 days to 100 mmol L⁻¹ to avoid osmotic shock. The pH of the nutrient solution was adjusted daily to remain at 6.0 ± 0.2 and the nutrient solution was refreshed weekly. During the experiment, the midday irradiance in the greenhouse was approximately 900 μmol photons m⁻² s⁻¹, the maximum and minimum air temperatures were 27°C and 18°C, respectively, and the mean relative humidity was 74%. Plants were harvested 25 days after starting the treatments and used to assess growth parameters and for chemical analyses. Samples from the stems (including petioles), leaves and roots were weighed, oven-dried for 3 days at 70°C, re-weighed and ground to determine the mineral contents. Fresh samples or deep-frozen samples were used for the biochemical assays.

Determination of sodium, potassium and silicon

Potassium and sodium concentrations were determined in the roots and shoots after digesting 100 mg powder of the oven-dried tissues in a mixture of concentrated nitric acid and perchloric acid (3:1; v/v) at 175°C. The potassium and sodium contents of the digested extracts were quantified using a flame photometer (model Jenway PFP7, Essex, UK). The Si in the plant tissues was determined spectrophotometrically after digestion of the plant material with H₂O₂ and NaOH in an autoclave as described by Elliot and Synder (1991).

Assay of enzymatic activity

Extracts for the enzymatic assays were prepared as described by Kar and Mishra (1976). Fresh leaf samples (0.05 g) were homogenized with 2 mL phosphate buffer (100 mmol L⁻¹, pH 6.8) and centrifuged at 17,000 g for 15 min. The clear supernatant was subsequently used as an enzyme source of catalase, soluble peroxidase and polyphenol oxidase.

The remaining 17,000 g pellets were washed four times with the extraction buffer until no peroxidase activity could be detected. The pellet from the last centrifugation step was mixed with 1 mol L⁻¹ NaCl solution and the mixture was centrifuged at 17,000 g for 20 min at 4°C. The supernatant containing cell wall proteins was saved. Soluble and cell wall peroxidase activities were determined using the guaiacol–peroxidase reaction. For the peroxidase assay, the 3 mL reaction mixture consisted of 25 mmol L⁻¹ phosphate buffer (pH 6.8), 20 mmol L⁻¹ guaiacol, 40 mmol L⁻¹ H₂O₂ and 10 μL of the enzyme extract. The reaction was initiated by the addition of H₂O₂ and changes in the absorbance at 470 nm were recorded for 2 min. Activity of peroxidase was calculated assuming an extinction coefficient of 26.6 (mmol L⁻¹)⁻¹ cm⁻¹ for tetraguaiacol (Chen et al. 2000; Kar and Mishra 1976). Enzyme activity was expressed as μmol H₂O₂ g⁻¹ fresh weight (FW) min⁻¹.

To assay catalase activity, 100 μL of the supernatant was added to a reaction mixture in a final volume of 3 mL that consisted of 50 mmol L⁻¹ phosphate buffer (pH 6.8) and 15 mmol L⁻¹ H₂O₂ as substrate. The decrease in the absorbance at 240 nm (owing to H₂O₂ destruction) was recorded for 2 min. Assuming an extinction coefficient of 40 (mmol L⁻¹)⁻¹ cm⁻¹ for H₂O₂, catalase activity was determined. Enzyme activity was expressed as μmol H₂O₂ decomposed g⁻¹ FW min⁻¹.

Polyphenol oxidase activity was determined in a 3 mL reaction mixture consisting of 25 mmol L⁻¹ phosphate buffer (pH 6.8), 10 mmol L⁻¹ pyrogallol and 100 μL of the enzyme extract. Changes in the absorbance of the solution at 420 nm were monitored for 2 min. Activity of polyphenol oxidase was expressed as the amount of purpurogallin formed, assuming an extinction coefficient of 2.47 (mmol L⁻¹)⁻¹ cm⁻¹ (Resende et al. 2002).

Other analytical methods

Soluble protein was quantified spectrophotometrically at 595 nm using the Bradford (1976) method after extracting plant material with 0.1 mol L⁻¹ phosphate buffer at pH 6.8 (Kar and Mishra 1976). For chlorophyll determination, the tissue was briefly ground in cold 80% acetone solution and the absorbance was measured at 645 and 663 nm using the method of Arnon (1949). The chlorophyll a and chlorophyll b contents were calculated accordingly.

Phenolic compounds were extracted with ethanol as described by Fukoda et al. (2003) and quantified according to Lavid et al. (2001). Plant phenolic extracts (50 μL) were diluted with 3 mL of H₂O. To this, 100 μL of 50 mmol L⁻¹ FeCl₃ in 0.1 N HCl plus 100 μL of 0.008 mol L⁻¹ K₃Fe(CN)₆ were added and incubated at ambient temperature for 20 min. The absorbance at 720 nm was determined against the reagent blank mixture with ethanol rather than plant extract. Using gallic acid, a standard curve was constructed to estimate phenolics.

The extent of tissue lipid peroxidation was determined as described by Heath and Packer (1968) and expressed as the amount of malondialdehyde (MDA) equivalents produced. Plant tissue was homogenized with 0.1% (w/v) trichloroacetic acid (TCA) and centrifuged at 6000 g for 15 min. The obtained supernatant (250 μL) was mixed with 2 mL thiobarbituric acid reagent (0.25% thiobarbituric acid (TBA) in 10% TCA). After heating for 30 min at 95°C in a water bath, the mixture was cooled and centrifuged at 6000 g for 10 min. The absorbance was recorded at 532 nm and corrected for non-specific absorbance at 600 nm and 440 nm (Hodges et al. 1999).

Lignin was extracted with ethanolic HCl (absolute ethanol : 1 mol L⁻¹ HCl; 1:1; v/v) after pre-extracting the plant tissues three times with 50% methanol at 60°C to remove any phenolic compounds (Zimmer 1999). The lignin content was determined spectrophotometrically at 488 nm using phloroglucinol according to Zimmer (1999).

Table 1 Effect of salinity (control versus 150 mmol L−1 NaCl) on the fresh and dry weights of canola plants grown for 25 days with or without 2 mmol L−1 supplementary silicon as Na2SiO3 in the nutrient solution

Treatments	Fresh weight (g plant^−1)			Dry weight (g plant^−1)
	Shoot	Root	Total	Shoot	Root	Total
Control	81.18 ± 5.31a	15.99 ± 0.65a	97.18 ± 5.68a	5.94 ± 0.55a	1.16 ± 0.11a	7.10 ± 0.65a
Si	77.69 ± 4.96a	13.73 ± 0.42ab	91.43 ± 5.35a	5.40 ± 0.28ab	1.16 ± 0.07a	6.42 ± 0.34a
NaCl	25.99 ± 6.39c	6.92 ± 1.28c	32.91 ± 7.64c	1.8 ± 0.51c	0.40 ± 0.07b	2.27 ± 0.57c
Si + NaCl	52.56 ± 4.94b	12.04 ± 0.85b	64.61 ± 4.92b	3.98 ± 0.35b	0.61 ± 0.05b	4.59 ± 0.37b
LSD 0.05	17.73	2.82	19.53	1.42	0.253	1.65
Data are mean ± standard error (n = 4). Means followed by the same letter within each column are not significantly different according to a least significant difference (LSD) test (P < 0.05).

Table 2 Na+, K+ and Si concentrations in the roots and shoots of canola plants grown for 25 days at 150 mmol L−1 NaCl with or without 2 mmol L−1 supplementary silicon as Na2SiO3 in the nutrient solution

Treatments	Na^+ (mg g^−1 dry weight)		K^+ (mg g^−1 dry weight)		Si (mg g^−1 dry weight)
	Shoot	Root	Shoot	Root	Shoot	Root
Control	2.86 ± 1.03c	1.19 ± 0.78b	49.1 ± 2.13a	53.8 ± 2.11a	0.22 ± 0.17b	0.61 ± 0.18c
Si	2.84 ± 1.91c	1.84 ± 0.74b	44.5 ± 2.04a	55.7 ± 4.54a	0.68 ± 0.11a	1.33 ± 0.21a
NaCl	35.01 ± 1.37a	18.28 ± 1.55 a	14.3 ± 0.59b	24.8 ± 3.20b	0.38 ± 0.01b	0.55 ± 0.12c
Si + NaCl	15.13 ± 0.33b	15.35 ± 0.57 a	16.5 ± 0.98b	24.4 ± 4.95b	0.56 ± 0.04a	0.91 ± 0.16b
LSD 0.05	5.55	3.23	5.17	12.63	0.20	0.29
Data are mean ± standard error (n = 4). Means followed by the same letter within each column are not significantly different according to a least significant difference (LSD) test (P < 0.05).

Statistical analyses

Statistical analyses of the data were carried out using SAS statistical software (SAS Institute 2004). All data were subjected to ANOVA and a comparison of the means was carried out using a least significant difference test.

Results

Growth

Salinity led to a significant reduction in both the fresh and dry weights of the plants. The total dry weight of the plants decreased by approximately 68% as a result of salinity (Table 1). The dry weight of the plants improved significantly under saline conditions when Si was added to the medium; however, it was still lower than the control plants. Silicon alone (i.e. under non-saline conditions) did not increase the dry matter. Under salinity, plants grown without Si had approximately 50% lower total dry weight compared with plants fed with 2 mmol L⁻¹ Si. The decline in shoot dry weight was greater than the decline in root dry weight under salinity and consequently the ratio of shoot to root decreased more than 60%. Silicon nutrition partly recovered this ratio.

Na⁺, K⁺ and Si contents of the tissues

The concentration of Na⁺ increased significantly under salinity in both shoots and roots (Table 2). Silicon nutrition decreased the accumulation of Na⁺ in shoots under salinity, that is, in the 2 mmol L⁻¹ Si treatment the Na⁺ content was approximately 56% less compared with plants grown without supplemental Si.

The concentration of K⁺ decreased in both roots and shoots, particularly in the shoots in plants under salinity (Table 2). The tissue K⁺ content was not affected by Si nutrition. A significant increase in Si concentration in both roots and shoots occurred when Si was supplied to the plants (Table 2). Salinity did not affect the Si concentration in the shoots; however, the content of Si in Si-supplied roots grown in a non-saline condition was 46% greater than the content in roots grown under salinity.

Figure 1 Effect of salinity (control versus 150 mmol L−1 NaCl) on the malondialdehyde concentrations as a measure of lipid peroxidation in the (a) shoots and (b) roots of canola plants grown for 25 days with or without supplementary silicon. Error bars represent the standard error. FW, fresh weight. Different small letters on histograms represent statistically significant differences at P < 0.05.

Figure 2 Effect of salinity (control versus 150 mmol L−1 NaCl) on the activity of (a,b) catalase, (c,d) perosidase, (e,f) cell wall perosidase and (g,h) polyphenol oxidase in the roots (a,c,e,g) and shoots (b,d,f,h) of canola plants grown for 25 days with or without supplementary silicon. Error bars represent the standard error. Different small letters on histograms represent statistically significant differences at P < 0.05.

Biochemical factors

Lipid peroxidation increased markedly under salinity (Fig. 1). In the 150 mmol L⁻¹ NaCl treatment, it increased approximately twofold in both roots and shoots compared with the control plants. A significant reduction in lipid peroxidation occurred only in the roots of salt-treated plants following Si application.

Catalase and various peroxidases scavenge tissue hydrogen peroxide and in this way control the severity of oxidative stress. Catalase activity decreased in both the roots and shoots under salt treatment; however, Si nutrition led to a significant increase in the activity of this enzyme in salt-treated plants (Fig. 2), so that in the 150 mmol L⁻¹ NaCl treatment Si-fed plants had 3.4-fold and 5.1-fold greater catalase activity in the roots and shoots, respectively, compared with plants grown without Si. In the salt treatment, soluble peroxidase activity decreased significantly in the shoots, whereas in the roots it increased to a great extent (Fig. 2). Silicon nutrition could not bring about any significant changes in soluble peroxidase activity. In the absence of Si nutrition, cell wall peroxidase activity decreased in the shoots, but increased in the roots in salt-treated plants. Silicon nutrition, however, increased the activity of this enzyme in the shoots and decreased it in the roots. Polyphenol oxidase activity did not change significantly with either salinity or silicon nutrition (Fig. 2).

Phenolics are one of the most important groups of secondary metabolites with anti-oxidative properties. The amount of soluble phenolics increased significantly in the NaCl treatment in both roots and shoots (Fig. 3). Silicon nutrition did not have any significant effect on phenolics in the roots and shoots. Salinity significantly increased the tissue lignin content in the absence of Si (Fig. 3). The lignin content in the shoots and roots of salt-treated plants was 43% and 34% greater, respectively, when compared with the controls. However, Si application decreased the lignin contents of both shoots and roots under salinity.

Leaf and root soluble proteins did not change significantly with NaCl nutrition. Silicon nutrition induced a marked decrease in leaf soluble proteins only in roots under NaCl treatment (Fig. 4). The leaf chlorophyll content decreased significantly in the salt treatment. The contents of chlorophyll a and chlorophyll b were 53% and 41% less, respectively, in salt-treated plants compared with control plants (Fig. 5). Under salinity, Si nutrition could recover the leaf chlorophyll content of the plants so that the contents of chlorophyll a and chlorophyll b in the Si-fed plants were 85% and 50% more, respectively, compared with plants grown without Si.

Figure 3 (a,b) Phenolics and (c,d) lignin contents in the roots (b,d) and shoots (a,c) of canola plants grown for 25 days under salinity with or without supplementary silicon. Error bars represent the standard error. DW, dry weight. Different small letters on histograms represent statistically significant differences at P < 0.05.

Figure 4 Soluble protein contents of the (a) shoots and (b) roots of canola plants grown for 25 days under salinity with or without supplementary silicon. Error bars represent the standard error. DW, dry weight. Different small letters on histograms represent statistically significant differences at P < 0.05.

Discussion

Salinity led to toxicity in the canola plants as evidenced by a drastic reduction in growth, greater amounts of Na⁺ and decreased K⁺ contents (Table 1,2). Reductions in growth under salinity have been reported in canola plants (Ashraf and McNeilly 1990; Francois 1994; Qasim et al. 2003). Silicon nutrition, however, decreased the Na⁺ concentration in canola shoots under salinity (Table 2), as has been reported in several other plants under saline conditions (Liang et al. 2006; Ma 2004). This effect may result from Si deposition in the cell walls and endodermis, which reduces Na⁺ uptake through a reduction in apoplastic transport across the root and its translocation to the shoots (Gong et al. 2006).

Owing to decreased activity of ROS scavenging enzymes, such as catalase in the whole plant and peroxidases in the shoots, under salinity (Fig. 2), cell free radical metabolism might be shifted toward the production of more ROS. Catalase and peroxidases are the two major ROS scavenging enzymes and a decline in their activities under salinity has been reported by several investigators (Hertwig et al. 1992), which ultimately leads to oxidative stress (Alscher et al. 1997; Mittler 2002). Increased lipid peroxidation in the salt-treated canola plants (Fig. 1) and their reduced growth might have resulted from alterations to the activities of these ROS-scavenging enzymes. Increased lipid peroxidation under salinity has been reported in other plants (Vaidyanathan et al. 2003), and represents membrane oxidative damage and dysfunction (Imlay 2003; Mittler 2002). Silicon nutrition significantly increased catalase activity in the whole plant and cell wall peroxidase in the shoots. Higher catalase activity under salinity following Si application has already been reported in tomato and barley (Al-aghabary et al. 2004; Liang et al. 2003). It appears that under salinity Si-supplied canola plants use ROS scavenging metabolic pathways more efficiently and this might be related to the lower toxicity resulting from reduced Na⁺ accumulation. Greater activities of other ROS scavenging enzymes, such as superoxide dismutase (SOD), peroxidases, glutathione reductase (GR) and dehydro-ascorbate reductase (DHAR), have been reported by Zhu et al. (2004) in salt-stressed cucumber plants following Si application. The significant reduction in lipid peroxidation that was observed particularly in the roots of salt-grown canola plants following Si application may relate to changes in the activities of these anti-oxidative enzymes. The ROS scavenging capacity might also be improved by Si nutrition through other mechanisms, such as non-enzymatic anti-oxidants (Gunes et al. 2007), which prevent oxidative damage to membranes, and it may account for the reduced lipid peroxidation observed in the canola plants.

Figure 5 Effect of salinity (control versus 150 mmol L−1 NaCl) on the (a) chlorophyll a and (b) chlorophyll b contents of canola plants grown for 25 days with or without supplementary silicon. Error bars represent the standard error. DW, dry weight. Different small letters on histograms represent statistically significant differences at P < 0.05.

Salinity led to a reduction in leaf-soluble proteins and chlorophyll a and chlorophyll b in canola plants (Fig. 5). These reductions can be attributed to the oxidative stress generated by salinity as several investigators have reported increased susceptibility of proteins to proteolysis and chlorophyll depletion following salinity-induced oxidative stress (Bhattacharjee 2005; Blokhina et al. 2003). Silicon nutrition can recover the chlorophyll content of canola plants under salinity, which suggests that it plays a role in the suppression of oxidative stress. Added Si has also been shown to improve the chlorophyll content of tomato and barley under salt stress (Al-aghabary et al. 2004; Liang 1998).

Salinity increased phenolics and the lignin content in canola plants (Fig. 3). An increase in phenolics might result from alterations to plant secondary metabolism following oxidative stress (Schutzendubel and Polle 2002). Greater lignin deposition in response to salinity has already been reported in rice and tomato (Sánchez-Aguayo et al. 2004; SeungGon et al. 2004). The reduction in growth of the canola plants under salinity might have resulted from the increased lignin and phenolics and may show harmful effects of salinity and/or plant response to salinity, which brings about greater selectivity and a reduced uptake of harmful ions via the apoplastic pathway as supposed by Sánchez-Aguayo et al. (2004). Cell wall loosening and plant growth would be impaired as a result of lignification. The lignin contents declined in the canola plants, particularly in the shoots, following the application of Si. Partial substitution of lignin by Si or the formation of Si–polyphenol complexes (Maksimovićet al. 2007) in the cell walls may facilitate cell wall loosening and cell extension and promote the growth of plants under stress conditions. Silicon deposition has been shown to increase cell-wall extensibility in sorghum roots (Hattori et al. 2003). Thus, the results of the present work may reveal another unknown beneficial effect of Si nutrition in salt-stressed plants, that is, the prevention of tissue lignifications, which allows further plant growth under saline conditions.

Despite the non-essentiality of Si for dicotyledonous plants, the data reported in the present study show new aspects of the beneficial effects of Si on plants grown under saline conditions. The application of Si prevents Na⁺ accumulation in canola plants. This reduced Na⁺ accumulation improves the plant ROS scavenging capacity (our data; Al-aghabary et al. 2004; Gunes et al. 2007; Liang et al. 2003, 2006; Moussa 2006), accompanied by reduced lipid peroxidation. The reduced lignification of Si-supplied canola plants grown under saline conditions found in the present study is another consequence of the restricted Na⁺ accumulation that allows further plant growth.

Acknowledgment

We thank GUASNR Deputy of Research and Office of Higher Education for financial support to A. Hashemi in the form of grants for MSc research projects.