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Journal of soil science and plant nutrition

versión On-line ISSN 0718-9516

J. Soil Sci. Plant Nutr. vol.15 no.4 Temuco dic. 2015  Epub 18-Oct-2015

http://dx.doi.org/10.4067/S0718-95162015005000073 

 

Exogenous silicon nutrition ameliorates salt-induced stress by improving growth and efficiency of PSII in Oryza sativa L. cultivars

 

M. Mahdieh1, N. Habibollahi1, M.R. Amirjani1, M. H. Abnosi1, M. Ghorbanpour2,*

1Department of Biology, Faculty of Science, Arak University, 38156 8-8349, Arak, Iran.

2Department of Medicinal Plants, Faculty of Agriculture and Natural Resources, Arak University, Arak,  38156-8-8349 Iran. *Corresponding author: m-ghorbanpour@araku.ac.ir

 


Abstract

Nutrient management of plants is the most practical and easiest way of combating salt stress. The effect of silicon (Si) nutrition on salt stress symptoms was investigated in hydroponically grown rice seedlings. Seeds from Khazar (salt sensitive) and Zayandehrood (salt tolerant) cultivars of rice were exposed to 0 and 100 mM NaCl in the absence and presence of silicon (3 mM) using sodium silicate. Plant growth parameters, sodium (Na+) and potassium (K+) concentrations, Si accumulation, chlorophyll content and efficiency of PSII (Fv/Fm) were determined in 25 days old seedlings exposed to salinity for 96 h. The results showed that salt stress generally inhibited seedling growth and reduced photosynthesis efficiency. However, the addition of Si significantly decreased shoot Na+ concentration, but increased Si uptake. Additionally, in salinized plants, the addition of Si increased Fv/Fm in Khazar cultivar. It could be concluded that Na+ concentration, Si accumulation and Fv/ Fm ratio play key role in salinity stress tolerance. Application of Si, however, alleviated to some extent detrimental effects of salinity stress by improving growth and physiological performance of both cultivars under saline conditions.

Keywords: Oryza sativa, salinity stress, PSII (Fv/Fm), silicon, Na+, K+

 


 

1. Introduction

Soil salinity is conspicuous in arid and semi-arid areas, affecting 2 million km2 of agricultural land and 30-50% of the irrigated land of our planet (Flowers et al., 1986). It has been estimated that more than 20% of all cultivated lands around the world containing levels of salts high enough to cause salt stress on crop plants (Flowers and Yeo, 1995). Soil salinity inhibits the growth of cultivated plants, and this effect is influenced by function of plant species, salinity degree as well as the ionic composition of the soil solution (Roy et al., 1993). The deleterious effects of salinity on plant growth are associated with; low osmotic potential of soil solution (water stress), nutritional imbalance, specific ion effect (salt stress) (Parvaiz and Satyawati, 2008). Survival and growth under saline environments are the result of adaptive processes such as ion transport and compartmentalisation, synthesis and accumulation of organic solutes, bearing to an osmotic adjustment (Fougere et al., 1991).

Rice (Oryza sativa) is the primary staple food for more than two billion people in Asia, Africa and Latin America. Of the total calories consumed globally, 23% are supplied by rice (Khush, 2001). Also, rice is a typical plant that shows active uptake of silicon (Si) and is known to be a Si accumulator.

Silicon is the second most abundant element in the earth's crust, which accumulates in plants at a rate similar to those of macro-elements nutrients such as Ca2+, P and Mg2+ (Epstein, 2001). The beneficial effects of Si also characteristically vary with the growth conditions. The effects are usually expressed more clearly when plants are under various abiotic and biotic stresses (Ma et al., 2001). It has been reported that Si alleviates the effects of abiotic stresses such as manganese, aluminum and heavy metal toxicities, and salinity, drought, chilling and freezing stresses (Ma et al., 2004). Moreover, it has been suggested that Si affects plant growth under stress conditions by affecting a variety of processes, for example mitigation of specific ion influence of salt (Tahir et al., 2006). The effect of Si on plant growth is dose- and crop specific (Ali et al., 2009). Si is the only element that does not damage plants when accumulated in excess due to its non-dissociative properties at physiological pH and polymerization (Ma et al., 2001).

This research was designed to investigate the role of Si (0 and 3 mM) in alleviating salt stress (NaCl at 0 and 100 mM) on growth parameters, ions uptake in tissues, chlorophyll pigment contents and chlorophyll fluorescence value and relationship between the Si accumulation and salt tolerance in two Iranian local rice cultivars.

2. Materials and Methods

2.1. Plant materials and growth conditions

Seeds of two rice cultivars namely Khazar and Zayandehrood, were surface sterilized by immersion in 0.5% (v/v) commercial sodium hypochloride (NaOCl) for 10 min, rinsed three times and then sterilized with 70% (v/v) ethanol for 5 min. After washing with distilled water, the seeds were sterilized again with 0.2% (w/v) mercuric chloride (HgCl2) for 10 min followed by thorough soaking in water overnight at 25 °C in the dark. The seeds were then transferred to a net floated on 0.5 mm calcium chloride (CaCl2) solution in a plastic container. 7-day old seedlings were transplanted to a plastic pot (3 L) containing half-strength Hoagland nutrient solution (Hoagland, 1950) in three replicates (n=3). The solution was renewed every 4 days and amended with or without Si (3 mM) as sodium silicate. Silicon was added when seedlings were transferred to Hoagland solution. The final pH was adjusted to 5.5-6.5 using 1 N NaOH or HNO3. The plants were grown under 12 h photoperiod (supplied by Philips fluorescent light tubs) and 28 °C temperature in day. For salt stress induction, 25 days old plants were exposed to 100 mM NaCl for 4 days. Thereafter, plants were harvested and subsequently growth parameters and ion analysis were measured.

2.2. Growth parameters measurement

The growth period was ended at 30 days after germination. Then, harvested plants were separated into shoots and roots. Subsequently, shoot and root lengths were measured and thereafter oven-dried to a constant dry weight for 3 days at 75 °C and then weighed.

2.3. Sodium (Na+) and potassium (K+) determination

For analysis of Na+ and K+ in reference seedlings, 200 mg of the oven dried material was ashed in a muffle furnace at 550 °C for 8 h. The ashes were digested with 1 ml of 2.5N HCl. After appropriate dilutions, the filtrate was assayed for Sodium (Na+) and potassium (K+) concentrations using flame photometer (Corning 410, UK).

2.4. Si analysis

The dried shoot samples were ground in a Wiley mill built into fine powder. Subsequently, the samples (0.1 g) were then digested in a mixture of 3 mL of HNO3 (62%), 3 mL of hydrogen peroxide (30%), and 2 mL of hydrofluoric acid (46%) by microwaving, and the digested samples were diluted to 25 mL with 4% boric acid. The Si concentration in shoot was determined by the colorimetric molybdenum blue method at 600 nm according to the method of Ma et al. (2007) and expressed based on dry weight.

2.5. Determination of chlorophyll content

Dry leaf subsample (0.1 g) was ground into fine powder and extracted with 10 mL of 80% acetone (v/v). The homogenate was filtered and the supernatant was used for the chlorophyll content measurement. The amounts of chlorophyll a and b were determined spectrophotometrically (PG Instrument LTD T80+UV/VIS), by reading the absorbance at 663 and 645 nm, respectively. The chlorophyll content results are expressed as milligram per gram-fresh weight and calculated by using the extinction coefficients as described by Arnon (1949).

2.6. Measurement of chlorophyll fluorescence

Maximum quantum efficiency of PSII (Fv/Fm) after a 30-min dark period for dark adaption in seedlings was measured with mini PAM (Walz, 2000).

2.7. Statistical analysis

The experiment was factorial based on a completely randomized deign (CRD), and each treatment consisted of three replications. All data of the experiment were subjected to analysis using One-Way ANOVA with computer SPSS software (Version 11) followed by Duncan,s Multiple Range Test (DMRT) at P = 0.05 significance level. Data were represented by mean ± standard deviation (±SD).

3. Results

3.1. Growth parameters

Results showed that salt stress significantly (P < 0.05) decreased root and shoot lengths, and their fresh and dry weights in both cultivars as compared to non-saline conditions (Table 1). Shoot length of the both cultivars, Zayandehrood and Khazar, with 100 mM NaCl and without Si reduced by 52.5% and 64.6%, respectively. However, when seedlings were grown under salinity stress with Si nutrition, the reduction percentage of shoot length in Zayandehrood and Khazar were 47.8% and 61.8%, respectively. Similarly, salt treatment alone decreased root length of Zayandehrood and Khazar by 20.5% and 34.9%, respectively, but the reduction percentage was 18.4% when Si was applied to salt-treated plants of Zayandehrood cultivar. However, application of Si under salinized and non-salinized conditions did not significantly affect the root dry matter of both khazar and Zayandehrood cultivars (Table 1). Comparing cultivars, under both optimal and saline environment, Zayandehrood represented better performance compared to Khazar based on growth indices and biomass accumulation. On the other hand, Khazar seems to be susceptible to saline environment due to more biomass reduction than Zayandehrood.

Table 1. Variations of growth parameters of the rice cultivars, Zayandehrood and Khazar, in the absence (-) or presence (+) of silicon (3 mM)

Data represents means ± standard deviation (±SD) of three replications. Values with the same lower case letter within a parameter are not significantly different at P = 0.05 significance level based on Duncan’s multiple range test.

3.2. Ions uptake

Data analysis revealed that salt stress increased the shoot concentration of Na+ inboth cultivars (Figure 1 a). Shoot concentration of K+ did not significantly change in response to salt stress induction as compared to non-saline conditions without added Si (Figure 1 b).

Si-deprived Zayandehrood plants had significantly lower shoot K+ concentration under saline conditions than that of Khazar.

Figure 1. Effects of salt stress (100 mM NaCl) on tissue element concentrations (a: shoot-Na+ concentration; b: shoot-K+ concentration; c: root-Si concentration; d: shoot-Si concentration) of rice seedlings in the absence (-) or presence (+) of Si. Bars with the same lower case letter within a parameter are not significantly different at P = 0.05 based on Duncan’s Multiple Range Test.

3.3. Si concentration

Data showed that Si concentration in shoot of Zayandehrood cultivar increased with addition of Si under saline and non-saline treatment (Figure 1 c, d). However, application of Si did not significantly affect the root Si concentration of the cultivars under normal and stress conditions. Comparing cultivars, there was no significant difference in the Si concentration of plant organs in both cultivars without Si application. Supply of Si significantly increased shoots Si concentration of Zayandehrood than that of Khazar.

3.4. Chlorophyll fluorescence parameter and chlorophyll content

The data (Figure 2) revealed that salt stress significantly (P < 0.05) decreased the chlorophyll a, b and total chlorophyll contents when compared to control non-saline conditions in Zayandehrood, although, the differences were not significant in Khazar cultivar. The value of chlorophyll a content in +salt/-Si condition was decreased by 27% and 34% in Khazar and Zayandehrood leaves, respectively. In control, Si application significantly decreased chlorophyll contents of both cultiv to plants treated with NaCl but without Si significantly decreased chlorophyll content of Zayandehrood, however, there was no significant diars. Under non-saline conditions and Si application, the value of chlorophyll a content was decreased by 48% and 45% in Khazar and Zayandehrood, respectively. Supply of Si to salinized plants comparedfference in chlorophylla content of Khazar cultivar supplied with or without exogenous application of Si. Similar results were observed for chlorophyll b content.

Maximal quantum yield of PS II (Fv/Fm) for Zayandehrood leaves showed no significant difference under both saline and normal conditions with or without Si application, but in Khazar, salt stress significantly decreased the ratio of Fv/Fm where Si was not added. Under salt environment without Si, the value of Fv/Fm was decreased by 2.5% and 1.3% in Khazar and Zayandehrood leaves, respectively. Moreover, under non-saline conditions with Si, the value of Fv/Fm was decreased up to 3.8% and 1.7% in Khazar and Zayandehrood leaves, respectively. However, under saline conditions plus Si, the value of Fv/Fm showed no significant difference in both cultivars than that of control non-saline conditions without Si (Figure 2.d). Also, under saline conditions without Si the difference between saline and non-saline treatments is not significant for Zayandehrood, while the difference is significant for the Khazar cultivar.

Figure 2. Effects of salt stress (100 mM NaCl) on photosynthetic parameter (a: chl a content; b: chl b content; c: Total chl content; d: (Fv/Fm) of rice seedlings in the absence (−) or presence (+) of Si (3 mM). Bars with the same lower case letter within a parameter are not significantly different, at P = 0.05 based on Duncan,s Multiple Range Test.

4. Discussion

Rice as a so-called glycophyte plant is very sensitive to salt stress particularly at the seedling stage, with height, root length and dry matter affected significantly by salinity. Also, cellular Na+ toxicity is the prevalent ion toxicity happens by salt stress, causing inhibition of a diversity of processes including K+ absorption (Rains and Epstein, 1965). A preserved basically salt tolerance mechanism intervened by high affinity K+ (HKT) transporters that absorb Na+ into the xylem parenchyma cell from the xylem vessel and precipitates K+ discharge into the xylem vessel (Hauser and Horie, 2010). Briefly, there are two major kinds of salt tolerance mechanisms; those reducing the salt entrance to the plant system particularly in photosynthetic tissues, and those reducing the salt concentration in the cytoplasm part (Munns, 2002).

Our findings in current study showed ameliorative effects of Si application on growth parameters and physiological performance of rice cultivars under saline conditions. It has been suggested that the beneficial effects of Si in salinized plants could be attributed to the reduction of shoot Na+ concentration (Liang, 1999; Yeo et al., 1999). Also, it has been reported that deposition of Si in the root cell wall decreased the salt transportation to the aerial parts. Also, it has been suggested that Na+ concentration increased in salt treated plants (Dkhil and Denden, 2010), and supply of Si reduced the adsorption of Na+ through the transpirational bypass flow (Yeo et al., 1999). Silicon inhibited the Na+ transportation to aerial parts or by its effect on transpiration movement or by making a complex with Na+. In present study, shoot Na+ concentration was markedly decreased especially when salinized plants were treated with Si, which support the hypothesis that ameliorative effects of Si application could be associated with the reduction of salt accumulation in the above ground plant parts.

Also, our study showed that plants treated with Si accumulated significantly more Si in their shoot than roots (Figure 1c, d). This is a typical response for Si-accumulating plants like rice, which is in agreement with a recent demonstration that Si uptake in rice is an active process (Nwugo and Huerta, 2008).

Moreover, the concentration and accumulation of Si was variable between both cultivars. Nakata et al. (2008) reported a high accumulation of Si in wild-type plant but a low Si accumulation in the leaves of the mutant lsi1 of rice. They also observed pest resistance in the leaves of the wild-type plant, but not in the mutant.

Growth reduction is generally observed in plants exposed to salinity stress. This may be partly due to lower water potential in cells which, in turn, causes stomatal closure and limits CO2 assimilation (Pattanagul and Thitisaksakul, 2008). Root length, however, showed a marked difference in response to salinity stress. This probably reflects the maintenance or even induction of the root elongation at low water potential, which can be considered as an adaptive response to drought and salinity stresses (Perez-Alfocea and Balibrea, 1996).

It has also been reported that the reduced seedling root and shoot growth under salt stress could be attributed to excessive accumulation of Na+ within plant body followed by reduction of enzymatic processes and protein synthesis (Tester and Davenport, 2003).

The current work showed that the Si application ameliorated the adverse effects of salinity by increasing root and shoot lengths and fresh and dry weights in Si-containing solution in comparison to other treatment where Si was not supplemented. These results are supported by Yeo et al. (1990) who observed the similar results in rice crop only under saline conditions. It has also been reported that exogenously applied Si increased the growth of a number of monocot and dicot species under salt free conditions (Adatia and Besford, 1986). The other possible mechanisms responsible for better crop growth in the presence of Si under stressful conditions might be the prevention of loss of water from aerial parts of plant by keeping the water status maintained by the plant (Takahashi et al., 1990). As a result plants maintained the photosynthetic activity to increase dry matter production (Agurie et al., 1992). Another ameliorative effect of Si could be related to the hydrophilic nature of silicone. SiO2-nH2O deposition could help to keep water, to dilute salts and to protect tissues from physiological drought (Romero-Aranda et al., 2006).

Root to shoot ratio (Table 1) calculated on dry weight basis was lower in Si-enriched plants as compared to Si-depleted plants which indicated the facilitation of shoot growth over root growth and higher photosynthetic rate resulting into higher dry matter production. The current results showed that the root growth inhibition caused by NaCl was not overcome by the sodium silicate and non-significant differences were observed among different levels of Si.

Wang et al. (1997) reported a high accumulation of Na+ in organs of Atriplex prostrate when plants grown in saline solutions, while the concentration of K+, Ca2+ and Mg2+ decreased, and subsequently reduced the photosynthesis. It is well known that in molecular structure of chlorophyll, the Mg atom have an important role (Wang et al., 1997). It may be that Mg deficiency in NaCl treatment resulted in low chlorophyll content (Reezi et al., 2009). Savvas et al. (2009) reported that the inclusion of Si (1 mM) in the salinized nutrient solution mitigated the salinity-associated suppression on growth and yield of Zucchini squash due to restriction of net photosynthesis. Liang et al. (1996; 1999) reported that added Si decreased the permeability of plasma membrane of leaf cells. Si supply only increased Si concentration in rice leaves, whereas reduced K+, Ca2+ and Mg2+ concentration under drought stress conditions in rice (Chen et al., 2010) and wheat seedlings (Pei et al., 2010). It has also been reported that high Si concentration could not alleviate salt stress effects on leaf area and water content of salinized cut rose (Rosa xhybrida L.) plants (Reezi et al., 2009). Our results are in a good line with the findings of Tahira et al. (2015) who reported that silicon application under saline conditions resulted in significant differences in many physiological traits, and they concluded that silicon was useful in alleviating salt-induced deleterious effects in okra (Abelmoschus esculentus) at early growth stage.

The Fv/Fm ratio (maximum photochemical efficiency of PSII) has been shown to be reliable indicator of plant stresses (Ashraf, 1999). For the most severe salt stress, Fv/Fm decreases (Jiang et al., 2006). The chlorophyll a (Siringami et al., 2009) and chlorophyll b (Demiral and Turkan, 2006) contents in salt stressed seedlings of rice were positively correlated with Fv/Fm ratio. Kaufman et al. (1979) proposed a "window hypothesis" for Si by suggesting that Si in the form of silica bodies deposited in leaf epidermal cells could act as a "window" that enhances light-use efficiency by facilitating the transmission of light to the photosynthetic mesophyll tissue. Thus, the observed Si induced increase in Fv/Fm in salted rice plants, can be broadly interpreted as an increase in the application of light energy towards sustaining photochemical reactions, which denotes an increase in light-use-efficiency (Nwugo and Huerta, 2008). Alaghabary et al. (2004) showed that Si decreased salt-induced production of hydrogen peroxide (H2O2) and improved photochemical efficiency of PSII in salinised tomato plants.

5. Conclusions

From the results obtained, it can be concluded that Na+ concentration, Si accumulation and Fv/ Fm ratio play key role in salt tolerance of rice plant, and Si application significantly decreased Na+ and K+ concentration in shoot tissue as well. Between the cultivars Zayandehrood revealed better growth performance than Khazar under salinity stress environment and approved its salt tolerance mostly due to improving growth parameters and physiological performance.

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