Introduction

Chrysanthemum (Dendranthema grandiflora) is one of the most important cut flowers and pot plants grown in many parts of the world. Chrysanthemum is commonly cultivated in soil, and thus easy to be exposed to salinization caused by continuous cropping in plastic houses. Salinization causes reduction in production quality and yields. Therefore, increasing salinity tolerance in chrysanthemum is necessary for high productivity. Because plastic houses were increased and new reclaimed lands were constructed in South Korea, breeding of salt-tolerant crops and strategy for overcoming the negative effects occurring in salinized soil have been required recently. For this reason, an understanding of mechanisms providing plants with salinity tolerance is important in itself.

Salt stress reduces water potential and causes ion imbalance or disturbances in ion homeostasis and toxicity, and affects all the major processes such as growth, photosynthesis, protein synthesis, energy, and lipid metabolism. Salinity is considered as a major factor in restricting plant growth and marketable crop productivity (Parida and Das 2005). It is estimated that about a third of the world’s cultivated land is affected by salinity (Perez-Alfocea et al. 1996). The growth suppression by salinization occurs in all plants, but their tolerance levels vary widely among different plant species. Salinity can be reduced with reclamation, water and drainage, but the cost of engineering and management is very high. Increasing costs for water and energy emphasize the need for an alternative strategy (Shannon 1984). Supplementary Si , an alternative strategy, could be attempted in saline soil to overcome the negative effects of salinity on the plant growth and yield.

Si is one of the most beneficial elements for several plants although it is officially not considered as an essential nutrient for plant. Si is deposited in the leaf, stem, and hull in the form of amorphous silica gel (SiO₂ ·nH₂O ) and soluble silicic acid (SiOH₄ ) (Epstein 1994). In monocots, several plants were demonstrated to be improved in production, disease resistance and abiotic stress tolerance by Si supplement (Ma et al. 2001). Especially, rice (Oryza sativa L.) is well known to accumulate most effectively Si due to the presence of a transporter for xylem loading (Mitani and Ma 2005). Accordingly, high content of Si in rice tissues contributes to enhance disease resistance and reduce sodium uptake under salt stress and the toxicity of heavy metals. Also, the beneficial effects of Si on plant growth, biotic and abiotic stresses have been observed in a wide variety of plant species (Ma et al. 2001). For example for abiotic stress, Si has been shown to ameliorate the adverse effects of salinity on plants as follows: salt tolerance in wheat and barley (Ahmad et al. 1992; Liang et al. 1996), photosynthetic activity and the ultrastructure of leaf cells in barley (Liang 1998) and membrane integrity in the leaves of barley (Liang et al. 1996). However, very little is known about the beneficial effects of Si on salt tolerance of the dicots. Especially, there is no report related to salt tolerance of floriculture crops like chrysanthemum.

This experiment was conducted to study the effectiveness of Si on ameliorating the deleterious effects of salinity in chrysanthemum and to investigate a possible role of silicon deposition in leaf for enhancement of salt tolerance in chrysanthemum.

Materials and Methods

Plant culture and treatments

Dendranthema grandiflorum cv. ‘Iwakunohakusen’ is a popular cultivar commercially cultivated in greenhouses. Rooted cuttings were purchased from a seedling farm and transplanted into round pots (15 cm in diameter) containing soil mixture of upland soil : leaf mold : sand (3 : 3 : 4, v : v : v). The plants were cultivated in a greenhouse with drip irrigation system, and fertilized with nutrient solution (20N – 20P - 20K Fertilant, Planta Co., Germany). To give salt stress to the plants, 100 mM NaCl (salt, S) was added into drenching solution. Potassium silicate (K₂SiO₃ , KSi ) and silicate fertilizer (Humaxilchito, Saturn Biotech. Co., Korea, SiF) were used as Si sources. Supplementation of NaCl and Si sources were conducted by drenching into soil weekly at 250 mL/pot and total treatment frequency was 10 times. K₂SiO₃ was supplied at concentration of 0.36 mL･L^-1 (100 mg·L^-1SiO₂ ) and SiF at concentration recommended by a producer, respectively (Table 1). Pure Si concentrations of two Si sources were determined colorimetrically (Hallmark et al. 1982) and resulting in almost similar concentration of 51 - 56 mg･L^-1). Control plants were applied with an equivalent amount of ground water. The experimental design consisted of a control (nutrient solution with ground water drenching) and four treatments (0.36 mL･L^-1K₂SiO₃ , 100 mM NaCl , 100 mM NaCl + 0.36 mL･L^-1K₂SiO₃ , and 100 mM NaCl + 0.5 mL･L^-1K₂SiO₃ ). Each treatment was replicated three times and each replicate included five pots.

Sampling and determination of water content, and salinity tolerance from biomass

When the chrysanthemum bloomed, the plants were harvested and separated into root and shoot. Cutting area of shoot was immediately put in water for 2 h. Fresh and dry weights of shoots were recorded for 9 - 12 randomly selected samples per treatment. Harvested roots were washed to remove all precipitates on the surfaces before being weighed to record fresh weight of root. Dry weight was measured after drying the samples for 72 h at 70°C. Water content of plant was calculated as the difference between fresh weight and dry weight of plant, which was obtained for shoot and root, respectively. Salinity tolerance was calculated as ratio of treatment to control in dry matter and described as percentage.

Determination of chlorophyll content and electrolyte leakage, and assessment of leaf damage by leafminer larvae

The fourth or fifth leaves from shoot apex were removed from one plant randomly selected for each replicate, and used for assays of chlorophyll content and electrolyte leakage. Before used for the assays, leaf samples were washed with distilled water to remove surface contamination. Each leaf was punched with a puncher for the assay of electrolyte leakage. Membrane permeability (EC, %) was measured for disc-shaped part of the leaf samples by using an electrical conductivity method as described by Reezi et al. (2009) Chlorophyll content was determined by using the part left by punching leaf samples. Each sample (1 g) was homogenized with 5 mL of acetone (80%, v/v) using pestle and mortar and filtered through a filter paper (Whatman, No. 2). The absorbance was measured with UV/visible spectrophotometer at 663 and 645 nm and chlorophyll contents were calculated using the equations used by Tuna et al. (2008). To assess pest resistance, the numbers of damaged leaves by Phytomyza albiceps and total leaves were counted for 15 plants per treatment.

Determination of chemical properties and macronutrient and Si contents in soil

Soil samples were collected twice before treatment of Si and salinity and after harvesting the plants. They were air dried and then passed through a 2 mm mesh of sieve. To investigate chemical characteristics of soil samples, pH, EC, and NaCl concentration were measured at distilled water : solid soil (5 : 1, v/v). Soil pH was measured using pH meter, and electric conductivity (EC) and NaCl concentration were measured by an electric conductivity meter. EC value was corrected by multiplying the measured value by five. Soil Si content in the form of SiO₂ was determined colorimetrically after extraction by 1 N sodium acetate buffer (Hallmark et al. 1982).

The contents of N, P and K in each soil sample were measured in the forms of NO₃-N , available P₂O₅ , and available K, which were determined colorimetrically at 525, 880 and 470 nm by reaction with each developing agent after extracting 3 g of soil sample with 30 mL extraction solution, respectively. This procedure was based on analysis manual of national institute of agricultural science and technology (NIAST) (Lim 2000).

Determination of macronutrient and silicon contents in plant tissues

Plants harvested from three replicates were divided into leaf and root and washed with distilled water to remove any dust on leaf and root surfaces, soaked in 0.5 M HCl for 20 s, followed by three to four rinses in distilled water and then dried at 70°C for 3 d. The dried leaves and roots were ground to powder using a motorized, stainless steel grinder, divided into three replicates, and stored in polyethylene bottles.

Si concentration was determined with 0.1 g of ground samples by the colorimetric molybdenum blue method after autoclave-induced digestion as described by Frantz et al. (2008). Soil nutrients of Na , Ca , K , and P were determined with 0.5 g of ground samples by inductively coupled plasma spectrophotometer (ICPS) after digestion with perchloric acid sulfuric acid at the ratio of 9 : 1 (v : v). Nutrient contents are expressed on the basis of dry weight.

Statistical analysis

Data were analyzed using SPSS program. Statistically different groups were determined by Tukey’s multiple range test for data related to biomass of which sample sizes for every treatment was not equal (P ≤ 0.05). Data related the assays of soil and plant were analyzed by Duncan’s multiple test because there was no difference in sample sizes for all treatment (P ≤ 0.05). The resulting P-value of Levene's test was more than 0.05. Relationships between individual variables were examined using simple linear regressions with 95% confidence interval. Matched pair t-test was used to examine salt stress effect on soil and plants with or without Si supplement.

Results

Effects of Si on biomass and salinity tolerance of chrysanthemum under salt stress

Fresh and dry weights of chrysanthemum plants were decreased by salinity treatment but recovered significantly by Si treatment (Table 2). The biomass decreased by salinity treatment was recovered in some degree when Si was applied together with NaCl , and especially in root recovered to control level. Water content was also significantly different between KSi and salinity single treatment as shown in Table 2. Si supplement (KSi ) increased water content by 17% and 16% in shoot and root, respectively, as compared to those of control. However, salinity treatment reduced water content by 13% in shoot and 21% in root. Water content reduced by salinity was recovered to higher level than control in root by Si supplement (S + KSi and S + SiF ). Salinity tolerance under salinity treatment was 70 - 78% in chrysanthemum plants, while Si supplies with salt (S + KSi and S + SiF ) improved it up to over 90% in shoot and 100% in root.

Effects of silicon on macronutrients contents in pot-soil and plant leaf under salt stress

Soil analyses after the harvest showed big differences between soil groups with or without exposure to salinity in chemical properties such as NaCl concentration, EC and pH (Fig. 1). Salinity treatment (100 mM NaCl ) caused abrupt salt accumulation in pot soil increasing NaCl concentration from 5.6% to 16.3% during the cultivation. This salt accumulation was reduced by 7.4 - 12.3% with Si supplement (S + KSi and S + SiF ). Also, EC responded similarly with the changes of NaCl concentration by combination of salinity (0 and 100 mM) with supplementary Si (0 and 1.8 mM) (Fig. 1B). However, pH showed different change from those of NaCl and EC (Fig. 1C) increasing when Si was applied together with NaCl . Differences in the contents of macronutrients of pot soil between before the treatments and after the harvest were shown in Fig. 2A. Changes of nutrient contents such as NO₃-N , P₂O₅ and K were significantly different among the treatments respectively (P < 0.05). NO₃-N was increased in all treatments (KSi , S + KSi and S + SiF ) except for salinity treatment, especially with remarkable adsorption in Si supplement (KSi ). Meanwhile, P₂O₅ was decreased by Si supplements (KSi , S + KSi and S + SiF ) regardless of exposure to salinity. In all treatments, much larger K was decreased by considerable desorption in soil with the largest significant decrease in soil supplied with Si plus NaCl (S + KSi and S + SiF ) (P < 0.05).

Changes of NO₃-N , P₂O₅ and K contents were represented as relative values to control in Fig. 2A, and plotted respectively for the salinity levels relative to NaCl concentration of control (Fig. 2B). There was no linear relationship between changes of NO₃-N , P₂O₅ or K content and salinity level in soil (r² = 0.12). However, there was a linear relationship between each nutrient change and salinity level under salinity treatment. Desorption of the nutrients was decreased as salinity level increased under salt stress. Relative change of nutrient content for the treatments was the biggest in K and followed by P₂O₅ and NO₃-N . As compared to those of control, Si supplement without NaCl increased NO₃-N and K in soil, while it decreased P₂O₅ . Unlike the chemical properties in soil, salt stress alone did not significantly affect macronutrients contents of soil. In contrast, Si supplements with salinity significantly decreased most of the macronutrients contents in soil differently from Si single treatment. As commented in Fig. 2A, changes of P₂O₅ content were related to changes of SiO₂ content in soil. There was an inverse linear relationship at a very high level between changes of SiO₂ and P₂O₅ contents (r² = 0.83, Fig. 2C), but not linear relationships between changes of SiO₂ and the other nutrients (r² = 0.1 and 0.3 for NO₃-N and K , respectively).

Elemental analyses in leaf tissue showed remarkable differences between salinity treatment (S ) and Si supplements with salinity treatment (S + KSi and S + SiF ) for K and P nutrient c ontents (Table 3 ). K and P contents were significantly reduced by salinity single treatment but increased by Si supplement under salt stress (P < 0.05). Meanwhile, for leaf Na content, there was no significant difference among all the treatments (P = 0.34).

Effect of Si on av. SiO2 content of soil and Si uptake and accumulation of chrysanthemum plants under salt stress

Total Si supplied to pot soil during the cultivation was calculated from the concentration of each Si source and described in Table 1. Supplementary Si significantly increased Si content in the form of av. SiO2 in pot soil as compared to that of water-treated control. However, Si content of soil under salt stress was slightly different from what we expected. Single treatment of salinity also increased soil Si content, and beside Si supplement with salt stress significantly increased more than single treatment of Si (Fig. 3). As shown in Fig. 3C, salinity treatment (100 mM NaCl ) showed a significant increase in Si content of soil regardless of Si supplement (P < 0.001; P < 0.01).

Si content in root tissues of chrysanthemum plants without exposure to salt stress was 1.4-fold higher than in root tissues exposed to salt stress. As shown in linear regression lines of Fig. 3A and B, root Si content was not linearly related to av. SiO₂ content of soil (r² = 0.36) but closely related to salinity level of soil (r² = 0.93). There was an inverse relationship showing that Si content in root was decreased with increasing salinity level in soil (Fig. 3B). Consequently, Si supplement with salinity treatment did not recover the ability of roots to take up Si .

In contrast to root Si content, leaf Si content was directly related to av. SiO₂ content of soil rather than salinity level of soil (r² = 0.76 for av. SiO₂ content; r² = 0.1 for salinity level) (Fig. 3A and B). Leaf Si content significantly increased when Si supplemented under salt stress (P < 0.05, Fig. 3C). Salt stress increased significantly leaf Si content when Si was supplied (P < 0.05, Fig. 3C). Without Si supplement, leaf Si content was not affected by salt stress. As shown in Fig. 3A and 3B, the gap between Si contents of root and leaf became closer with SiO₂ content and salinity level in soil. This gap could be evaluated in terms of transport rate of Si from root to leaf corresponding to xylem loading rate of Si (Si leaf / Si root). Xylem loading rate of Si increased relating to both salt concentration (r² = 0.66) and the SiO₂ content (r² = 0.77) in pot soil to a certain extent. Exposure to salt stress increased xylem loading rate of Si by 51% and 32% in chrysanthemum plants with and without Si supplement, respectively. Also, Si supplement increased by 27% and 6% with and without salt stress, respectively.

Effect of Si on the chlorophyll and Ca contents, membrane integrity and pest (leafminer larvae) resistance under salt stress

Chlorophyll and Ca contents in leaf were directly related with leaf Si contents among the single treatments (S, C and KSi ), while electrolyte leakage and damage incidence by leafminer larvae were inversely related to them (Fig. 4). Si single treatment (KSi ) had positive effects on leaf, significantly increasing chlorophyll and Ca content and decreasing electrolyte leakage in chrysanthemum leaf. Contrary to this, salt single treatment had negative effects on leaf lowering slightly chlorophyll and Ca content with increase of electrolyte leakage and leaf damage by pest. As shown in root Si content above, Si supplement under salt stress did not recover all of them to the level of Si single treatment, still remaining the adverse effects of salt stress. However, the damage incidence of leaf with 32.6% increase by salt stress was decreased below the level of control (20 - 39% decrease of salinized plant).

Discussion

The majority of researches related to the beneficial effects of supplementary Si have been focused on food crops, and numerous results have been reported (Liang et al. 2007; Ma et al. 2001). Data reported here show the beneficial effects of Si on chrysanthemum plant under saline conditions as observed on the other species under salt stress (Liang et al. 2007). However, the beneficial effects of Si were not such large as those in wheat, barley, sugarcane and tomato (Ashraf et al. 2010; Liang et al. 1996; Romero-Aranda et al. 2006; Tuna et al. 2008).

In saline condition, most of plant species showed a significant increase of Na in leaf with growth inhibition (Matoh et al. 1986; Romero-Aranda et al. 2006; Tuna et al. 2008). However, Na content of salinized chrysanthemum plants was not increased even though the plant growth was inhibited similarly with other species ( Table 3 ). It can be explained by assuming that chrysanthemum plant has an ability to exclude Na effectively so as to maintain optimal level of Na in the plant tissues. This assumption on chrysanthemum is supported by the previous report that Na accumulation in plant tissues by salinity varied depending on the plant species or cultivar and the external salinity level (Sivritepe et al. 2005). As a result, Na content in chrysanthemum plant was not affected by external Na concentration, and thus Na toxicity is not a factor responsible for the biomass reduction by salt stress. Meanwhile, data presented here showed that salt stress inhibited Si uptake by root from soil medium and reduced nutrients such as K and P in leaf. Thus, nutrient imbalance of chrysanthemum plants due to decrease of root activity is considered as a factor in charge of biomass reduction of chrysanthemum plants by salt stress. In addition, salinity tolerance of chrysanthemum was estimated at 70 - 80%, which is very close to Janz classified into a genotype with high salinity tolerance in bread wheat (Genc et al. 2007). In chrysanthemum plant, these low Na accumulation and high salinity tolerance coincide with the previous proposal that the level of Na⁺ exclusion in salt-stressed plant tissues has been related to salinity tolerance (Munns et al. 2006).

With salt-induced deleterious effect on plant growth, it has also been reported that Si addition under salt stress helps the plant growth by decreasing salt toxicity with Na content reduction in the shoot of rice, wheat and barley (Liang 1999; Tuna et al. 2008; Yeo et al. 1999). And it was suggested that Si deposition in cell walls of roots reduces the translocation of salts to the shoots. However, no reduction of Na content in leaf was also observed when chrysanthemum plants were treated with Si under salinity stress. Rather, such a double treatment induced slightly higher leaf Na content (P > 0.05) than salinity single treatment similarly with tomato plants (Romero-Aranda et al. 2006). In chrysanthemum plant, the alleviation of salt-induced adverse effects by Si supplement cannot be explained by decrease of Na toxicity via the mechanism mentioned above. Double treatment of NaCl plus Si increased contents of K, P, and Si in leaf which had been decreased by salt stress but did not affect Na content (Table 3 and Fig. 3). Consequently, these results suggest that added Si increases the biomass of salt-stressed plants via mechanism improving nutrient balance by enhancing nutrient uptake.

In addition, water status of plant could also play an important part in the beneficial effects of Si under saline condition because biomass reduction by salt stress is due to the osmotic effect of salts (Parida and Das 2005). The beneficial effect of Si has been related to the depression of excessive loss of water via transpiration inhibition (Savant et al. 1999) or deposition of silicate crystals including water molecules in leaves and stems (Matoh et al. 1986). They both can play a role to reduce water loss through the cuticles. Ability to retain water in plant is reflected in water content and biomass of plant. Water content decreased by salt stress was slightly increased in the shoot, and recovered to the control level or higher especially in root by double treatment of Si and salinity. It is expected that Si deposition in leaf could have reduced water loss by retaining water molecule (Fig. 3), which is supported by the result on leaf Si content. Also, salinity tolerance is closely associated with water content via osmotic adjustment and ion dilution effect by water retained in tissues (Parida and Das 2005). Our data showed that chrysanthemum plants supplied with Si under salt stress had higher salinity tolerance with higher water content than salinized plants. Thus, Si supplement under salt stress can enhance salinity tolerance improving water status in chrysanthemum plant.

In leaf tissues of chrysanthemum plant, pigments related to photosynthesis, membrane integrity and pest resistance were affected by foliar Si deposition among single treatments (C, KSi and S) resulting in a significant increase as the previous results that Si supplement increased chlorophyll contents and membrane integrity in rose and wheat (Reezi et al. 2009; Tuna et al. 2008), improved photosynthesis rates in barley (Liang 1998) and enhanced resistance against biotic stress in chrysanthemum (Jeong et al. 2012). However, Si supplement under salt stress did not recovered membrane permeability and chlorophyll content in chrysanthemum unlike the species mentioned above although leaf Si content increased. In the previous reports, Si added to salt treatment recovered membrane permeability and chlorophyll content to control level or higher in rose, wheat and barley leaves depending on salinity level, Si concentration supplied to plants, and plant cultivar (Liang 1998; Reezi et al. 2009; Tuna et al. 2008). Si or NaCl concentration supplied to chrysanthemum plant could not have been optimal condition to recover the function and structure of cell wall or plasma membrane. In chrysanthemum, the impact of salt stress (100 mM NaCl ) may override Si treatment (1.8 mM Si ). By analysis using simple linear regressions, it was found that chlorophyll content and membrane integrity were closely related to leaf Si content among single treatments except for double treatments of NaCl and Si. These different responses in single or double treatments were similar to those of Ca content. In chlorophyll content and membrane permeability, such a similarity with Ca supports the previous reports that Ca plays an important role in preserving the structure and function of cell walls and plasma membrane (Bauer et al. 2011). These results indicate that Ca , in preference to Si , is directly involved in mechanism increasing membrane integrity and chlorophyll content. Meanwhile, Si supplement under salt stress significantly improved pest resistance that had decreased by salt stress. Therefore, such different pattern in response to leaf Si content suggest that pest resistance is directly mediated by Si deposition in leaf differently from the mechanism recovering membrane integrity or chlorophyll content.

As known that monosilicic and polysilicic acids can regulate chemical and physical properties of the soil (Matichenkov and Bocharnikova 2001), Si supplied with salinity decreased NaCl concentration and EC of soil that were increased by salinity treatment (Fig. 1). Salt accumulated in soil by single treatment of NaCl significantly modified the chemical properties but did not significantly the macronutrient contents. When Si was added to NaCl treatment, significant decrease of macronutrients such as P and K occurred in soil (Fig. 2), while the leaf K and P contents decreased by salinity treatment, significantly increased (Table 3). For leaf K content, similar results were obtained by (Tuna et al. 2008) and (Ashraf et al. 2010) who reported that Si addition very significantly enhanced the potassium content in shoots of salt-stressed plants. The increased K and P in leaf have something in common with the decreased adsorption of P and K in soil when Si added to salinity treatment. Therefore, these results indicate that Si supplement improves nutrient balance increasing K and P availability via amendment of saline soil under salt stress. Meanwhile, it is known that potassium uptake and transport of p lant is active p rocess by A T P-driven H⁺ pump in the plasma membrane (Marschner 1995). Liang (1999) reported that H⁺-ATPase activity was increased by Si in salt stressed barley plants. In the chrysanthemum plants grown at high salinity, leaf K content may be enhanced via the mechanism that added Si stimulates the activation of H⁺-ATPase in the membrane as supposed in wheat plants by (Tuna et al. 2008).

In addition, change of P₂O₅ was related to SiO₂ content, but not to salinity level in soil (Fig. 2B and C). The change decreased with increasing changes of SiO₂ content (Fig. 2C), which implies that Si supplement decrease phosphorus adsorption by soil resulting in enhancing phosphorus availability. Inverse relationship between them in soil, as shown in Fig. 2C, also suggests that SiO₂ promotes desorption of P₂O₅ by mechanism of competition adsorption between available P and Si as postulated in a previous report (Brown and Mahler 1987).

In the present study on Si content in leaf tissues, Si supplement affected more Si deposition in the plants under salt stress than the plants without salt stress unlike the results obtained in wheat and tomato plan (Romero-Aranda et al. 2006; Tuna et al. 2008). Leaf Si content was more related to SiO₂ content than salinity level in soil during the cultivation (Fig. 3A). Contrary to the result in leaf, Si uptake by root was inhibited by salt stress regardless of Si supplement. Under the salt stress, decrease of root activity in Si uptake could contribute to the increased SiO₂ content in soil.

Si deposition in shoot varies considerably among plant species, ranging from 0.1% to 10% of the dry weight (Epstein 1994). Most dicots show low Si accumulation because they passively transport Si mainly allowing the silicic acid to diffuse into their roots, stems and leaves. It was previously reported that the Si content of shoot was 4.5 to 5.0-fold and 11 to 38-fold higher than those of root in bread wheat and sugarcane respectively when grown with Si supply (Ashraf et al. 2010; Tuna et al. 2008). In the present experiments on chrysanthemum plants, shoot (leaf) Si content was 0.4-fold lower rather than that of root. As stated above, there was enormous difference in Si_leaf / Si_root between the dicot plants and the monocot plants. It has been known that there are two steps of the radial transport from the external soil solution to the root and the release of Si from root to the xylem (Mitani and Ma 2005). Recently, the importance of xylem loading has been recognized as a step leading to a high accumulation of Si in the shoot. Based on small amount of Si in both of root and leaf in chrysanthemum plants, it seems that root Si adsorbed passively by radial transport is transported into xylem passively with the water flow, and then distributed to the leaf according to their transpiration stream.

The process that Si released from root is transported into the xylem was evaluated on the basis of leaf to root Si contents ratio of chrysanthemum plants and estimated as xylem loading rate of Si . With low Si content in root (0.15 - 0.21% DW), the low xylem loading rate of Si, ranging from 0.39 to 0.62 (Si _leaf / Si _root < 1.0), may be attributed to the lack of transporters for xylem loading in chrysanthemum. Among horticultural crops, chrysanthemum plants accumulated much less Si (0.085% DW) than zinnia (1.51% DW) and gerbera (0.51% DW) plants when supplemented with the same Si source (Kamenidou et al. 2009; Kamenidou et al. 2010). Xylem loading rate of Si was enhanced more by salt stress (+ 32%) than Si supplement (+ 6%), and further interplay Si supplement with salt stress improved it up to 51%. These results indicate that salt stress inhibits Si uptake by root, while it promotes Si transport from root to leaf increasing diffusion rate. Possibly, Si accumulation enhanced by both salt stress and Si supplement may correlate with high transpiration and high metabolic rates in chrysanthemum plants.

Conclusion

Under salt stress, reduction in growth of chrysanthemum plants was due to physiological drought and nutrient imbalances of potassium and phosphorus rather than sodium toxicity in plant. However, salt-stressed chrysanthemum plants did not show such a remarkable reduction in biomass as other plant species previously studied, still exhibiting low Na content. This result indicates that chrysanthemum is characterized as the plant tolerant to salt stress. With very low Si deposition in leaf even by Si supplement, Si supplement under salt stress had ameliorative effects on salt-induced deleterious effects by increasing water content, salinity tolerance, and pest resistance with improving nutrient balance in chrysanthemum plants via mechanism amending soil by Si . These results also demonstrated the possibility that in saline area Si supplementation could increase the productivity of marketable cut flowers of chrysanthemum plant.