Acta Physiologiae Plantarum

, Volume 33, Issue 3, pp 675–682

Silicon enhances photochemical efficiency and adjusts mineral nutrient absorption in Magnaporthe oryzae infected rice plants

Authors

  • Dan Gao
    • College of Resources and Environmental SciencesChina Agricultural University
    • Key Laboratory of Ecological Agriculture of Ministry of AgricultureSouth China Agricultural University
    • Key Laboratory of Ecological Agriculture of Ministry of AgricultureSouth China Agricultural University
  • Jining Chen
    • Key Laboratory of Ecological Agriculture of Ministry of AgricultureSouth China Agricultural University
  • Shiming Luo
    • Key Laboratory of Ecological Agriculture of Ministry of AgricultureSouth China Agricultural University
  • Rensen Zeng
    • Key Laboratory of Ecological Agriculture of Ministry of AgricultureSouth China Agricultural University
  • Jianyuan Yang
    • Institute of Plant Protection, Guangdong Academy of Agricultural Sciences
  • Xiaoyuan Zhu
    • Institute of Plant Protection, Guangdong Academy of Agricultural Sciences
Original Paper

DOI: 10.1007/s11738-010-0588-5

Cite this article as:
Gao, D., Cai, K., Chen, J. et al. Acta Physiol Plant (2011) 33: 675. doi:10.1007/s11738-010-0588-5
  • 281 Views

Abstract

Silicon (Si) has been verified to play an important role in enhancing plant resistance against pathogens, but the exact mechanisms remain unclear. Two near-isogenic lines of rice (Oryza sativa L.), CO39 (blast susceptible), and C101LAC (Pi-1) (blast resistant), were hydroponically grown to study the effects of exogenous silicon application on the changes of disease incidence, mineral nutrient concentrations, chlorophyll content, and photochemical efficiency in Magnaporthe oryzae infected rice plants. Si amendment in nutrient solution at a concentration of 2.0 mM significantly reduced the disease index of rice plants of CO39 and C101LAC (Pi-1). Silicon application alone had no effects on mineral nutrient contents, chlorophyll content, maximum/potential quantum efficiency (Fv/Fm), and the maximum primary yield (Fv/F0) of photochemistry of PS II in healthy rice leaves. M. oryzae inoculation significantly increased the content of K, Na, Ca, Mg, Fe, and reduced the value of Fv/F0 and Fv/Fm in rice leaves. However, Si treatment suppressed M. oryzae induced increase of mineral nutrient contents, and significantly increased Fv/F0 and Fv/Fm value compared with Si-deficient infected plants. These results suggest that silicon-enhanced resistance to rice blast is associated with an enhancement of photochemical efficiency and adjustment of mineral nutrient absorption in M. oryzae-infected rice plants.

Keywords

RiceBlastSiliconMagnaporthe oryzaeMineral nutrient contentsChlorophyll fluorescence

Introduction

Rice (Oryza sativa L.) blast disease, caused by the ascomycete fungus Magnaporthe oryzae, is one of the most serious and widespread diseases of rice in the world. Resistant varieties, fungicide application, biological control, agricultural practices such as genetic diversity, and mineral nutrition have been used to control this important disease (Wolfe 2000; Zhu et al. 2000; Walters and Bingham 2007; Ribot et al. 2008). However, genetic resistance can breakdown after a few years of intensive cultivation because of the complicated genetic diversity and adaptation of the fungus. Blast control through chemical fungicides will lead to the establishment of races of the pathogen resistant to these chemicals. Moreover, indiscriminate use of pesticides may result in serious negative effects to the environment. Therefore, it is important to develop alternative environmentally friendly strategies for management of plant diseases. Silicon (Si) fertilization may provide a viable method to reduce the negative effects of blast, especially where soils are low or limiting in plant available Si (Datnoff et al. 1997; Seebold et al. 2001).

Silicon is one of the most important elements for several plants, especially rice. The beneficial role of silicon in enhancing plant resistance to various biotic (i.e. pest attack and pathogen infection)and abiotic stress (i.e. drought, salinity, metal ion toxicities etc.) is particularly evident (Fauteux et al. 2005; Liang et al. 2007). Silicon deposits mainly in the cell lumen and cell wall in the form of SiO2·nH2O (Parry and Smithson 1964; Lanning and Eleuterius 1989). Plants take up silicon in the form of mono-silicic acid. There are numerous reports on Si suppressing plant diseases and pests, such as blast and sheath blight in rice (Datnoff et al. 1997; Seebold et al. 2001; Rodrigues et al. 2003; Cai et al. 2008), powdery mildew in cucumber, Arabidopsis in wheat (Menzies et al. 1991; Fauteux et al. 2006; Côté-Beaulieu et al. 2009), Eldana saccharina in sugarcane (Keeping and Meyer 2002), and rust in cowpea (Heath and Stumpf 1986). Evidence showed that silicon-mediated resistance to the pathogen is associated with the higher deposit of silicon in leaf and the activation of host defense response (Bowen et al. 1992; Kim et al. 2002; Rémus-Borel et al. 2005; Cai et al. 2008).

Photosynthetic traits such as gas exchange and chlorophyll fluorescence parameters have been considered to be very useful, non-invasive indicators to investigate the behavior of the photosynthetic apparatus under environmental stress, including pathogen attack (Lichtenthaler and Miehé 1997; Araus et al. 1998; Baker and Rosenqvist 2004; Bonfig et al. 2006). When plants were infected by pathogens, physiological and photosynthetic properties and growth of plants were negatively influenced (Chia and He 1999; Swiech et al. 2001; Guo et al. 2005). Most studies showed that pathogen infection led to a decrease in photosynthesis (Bastiaans 1991; Ogren and Evans 1992; Bastiaans and Roumen 1993; Chou et al. 2000; Berger et al. 2004) and modification or damage of the photosynthetic apparatus (Lichtenthaler and Miehé 1997) which might be the result of a down-regulation of photosynthesis or damage of the photosynthetic apparatus. Both of maximum/potential quantum efficiency (Fv/Fm) and the maximum primary yield (Fv/F0) of photochemistry of PS II are related to photosynthetic efficiency of plant leaves (Shangguan et al. 2000). Chlorophyll fluorescence analyses showed that the PSII photochemical efficiency is decreased by viral infection (van Kooten et al. 1990; Rahoutei et al. 2000; Bassanezi et al. 2002; Baker and Rosenqvist 2004; Bonfig et al. 2006).

Pathogen infection can also affect essential cation uptake and nutrient balance (Balasubramanian 1973; Bains and Jhooty 1984). However, little is known about Si effects on nutrient status and photochemical efficiency of pathogen infected plants. In this study, two rice near-isogenic lines differing in blast resistance were selected to investigate the effects of silicon on disease resistance of rice blast and the effects of silicon supply in combination with M. oryzae inoculation on chlorophyll fluorescence and mineral nutrient contents.

Materials and methods

Plant materials and growth conditions

Two rice near-isogenic lines (NILs) with different resistance to blast disease [CO39 (susceptible), C101LAC (Pi-1) (resistant)] were used throughout the experiments. Rice seeds were surface-sterilized with 10% H2O2 for 10 min and rinsed thoroughly with distilled water, then soaked with distilled water and germinated on moist filter paper for 48 h in Petri dishes. Twenty germinated seeds were transplanted in each polyethylene plastic pot (160 mm diameter × 130 mm height) that was filled with sterilized vermiculite and 200 ml of Hoagland’s nutrient solution. The components of nutrient solution were as follows: 5 mM KNO3, 2 mM MgSO4·7H2O, 1 mM KH2PO4, 46.25 μM H3BO3, 9.14 μM MnCl2·4H2O, 0.76 μM ZnSO4·7H2O, 0.32 μM CuSO4·5H2O, 0.07 μM (NH4)6Mo7O24·2H2O, 20 μM EDTA-Fe–Na. The final pH of solution after the addition of treatments was adjusted to 5.6 using 1 M KOH or HCl, and the nutrient solutions were renewed every day. Rice seedlings were grown in a growing chamber at 25/22°C (day/night) with a photoperiod of 13 h, a light intensity of 300 μmol m−2 s−1.

Silicon and M. oryzae treatments

Four treatments were set up for both CO39 (susceptible) and C101LAC (Pi-1) (resistant) NILs: (1) no silicon addition and no inoculation with M. oryzae (Si–Mg−); (2) 2.0 mM silicon addition but no inoculation (Si+Mg−); (3) no silicon addition but inoculation with M. oryzae (Si–Mg+); (4) 2.0 mM silicon addition plus M. oryzae inoculation (Si+Mg+). Treatments for each rice line were arranged in a randomized complete block design with five replications and 20 plants per treatment. The silicon was added as potassium silicate (K2SiO3) to the Hoagland’s nutrient solution 15 days after transplanting. In the silicon-deficient treatment, potassium chloride (KCl) was used to replenish potassium. Each replication has 20 rice plants.

Inoculation with M. oryzae and disease assessment

M. oryzae strain 98–288a was used to inoculate rice seedlings of the two rice lines 28 days after transplanting (four-leaf stage). The whole rice plants were sprayed with a suspension of conidia (20 ml suspension per 20 seedlings, containing 5 × 105 conidia/ml). The non-inoculated plants were sprayed with the same amount of sterile water. Inoculated and non-inoculated rice seedlings were kept in a moist chamber at 25°C for 24 h and then transferred to the glasshouse. Disease development was investigated 7 days after inoculation. Infected leaves were scored with a rating (r) of 0–9, denoting proportions of blast disease over the whole leaf area (IRRI 2002). Disease index was calculated according to the following equation:
$$ {\text{Disease}}\,{\text{index}}\,\left( \% \right) = \left[ {\sum {\left( {r \times n_{r} } \right)/\left( {9 \times N_{r} } \right)} } \right] \times 100 $$
where r = rating value, nr = number of infected leaves with a rating of r, and Nr = total number of leaves tested.

Chlorophyll content measurements

Fully expanded leaves (the second and third leaves) were collected 7 days after M. oryzae inoculation to determine chlorophyll content. Leaf discs were ground to a fine powder and extracted with 10 ml of 80% acetone (v/v). The homogenate was centrifuged at 4,000g at 4°C for 10 min, and the supernatant was separated and used for the chlorophyll assay. The amounts of chlorophyll a and b were determined spectrophotometrically, by reading the absorbance at 663 and 645 nm. The chlorophyll contents were expressed as unit’s mg per gram-fresh weight (mg/g FW) and calculated by using the extinction coefficients and the equations given by Porra et al. (1989).

Chlorophyll fluorescence measurements

The third fully expanded leaves 7 days after pathogen inoculation were selected to measure chlorophyll fluorescence parameters Fv/F0 and Fv/Fm. Fluorescence values were measured using chlorophyll fluorometer (OS-30P, Opti-Sciences, U.S.) after a 20-min dark period in ambient conditions in the laboratory. Minimal (F0), maximal (Fm) and variable fluorescence (Fv) levels were recorded, and then the maximum quantum efficiency of PSII photochemistry (Fv/Fm) and the basal quantum yield (Fv/F0) were calculated.

Si content determination

The Si contents of rice leaves were determined by the colorimetric molybdenum blue method (Van der Vorm 1987). Briefly, rice leaves (0.3 g) were ashed in porcelain crucibles for 3 h at 550°C, the ash was dissolved in 1.3% hydrogen fluoride (HF), and then the Si concentrations in the solutions were measured by the colorimetric molybdenum blue method at 811 nm with a spectrophotometer (PGENERAL TU-1901 UV–VIS, Beijing, China).

Mineral nutrient contents determination

Fully developed leaves were sampled 7 days after M. oryzae inoculation to determine mineral nutrient content, including Na, K, Ca, Mg, and Fe. Leaf samples (0.3 g) were dried at 65°C for 48 h and dry weight (DW) was determined. Dried samples were ground by hands with a glass rod in test tubes. Five milliliters of dense chlorine acid and 15 ml nitric acid were added and incubated overnight at room temperature, then digested at 180°C using a heating block, and finally diluted to a volume of 50 ml with distilled deionized water. The content of Na, K, Ca, Mg, and Fe was determined by Atomic Absorption Spectrometer(SpectrAA220FS, Varian, US).

Statistical analysis of the data

All the data were expressed as the means ± standard error (SE) of five replicates and analyzed by ANOVA using SPSS software(version 13.0; SPSS Inc., Chicago, USA) computer program, statistical differences among treatments within the same rice line were determined using the least significant difference (LSD) at a 0.05 probability level.

Results

Inhibitory effect of Si on rice blast

The disease index of rice blast significantly decreased after Si application to both rice near-isogenic lines compared with non-Si-treated control (Fig. 1). Application of 2.0 mM silicon reduced disease index by 45.1% for CO39 (susceptible) and 55.6% for C101LAC (Pi-1) (resistant), respectively.
https://static-content.springer.com/image/art%3A10.1007%2Fs11738-010-0588-5/MediaObjects/11738_2010_588_Fig1_HTML.gif
Fig. 1

Effects of silicon application and M. oryzae inoculation on disease index (%) of blast in rice near-isogenic lines CO39 (susceptible) and C101LAC (Pi-1) (resistant). Si+ and Si− indicate treatment with or without 2.0 mM silicon in culture solution, respectively. Values are means ± standard error from five replicates (n = 5). Different letters denote statistical difference using a least significant difference test (P < 0.05)

Chlorophyll content and chlorophyll fluorescence

Inoculation with M. oryzae significantly reduced chlorophyll content in the leaves of both rice lines (Fig. 2). Silicon addition did not significantly affect chlorophyll content in both M. oryzae infected and non-infected leaves.
https://static-content.springer.com/image/art%3A10.1007%2Fs11738-010-0588-5/MediaObjects/11738_2010_588_Fig2_HTML.gif
Fig. 2

Effects of silicon application and M. oryzae inoculation on chlorophyll content in the leaves of two rice isogenic lines CO39 (susceptible) and C101LAC (Pi-1) (resistant). Si+ and Si− indicate treatments with and without 2.0 mM silicon in culture solution, respectively. Mg+ and Mg− indicate treatments with and without M. oryzae inoculation, respectively. Values are means ± standard error from five replicates (n = 5). Significant differences (P < 0.05 using least significant difference test) among different treatments for the same rice line are indicated by different letters above the bars

Maximal quantum yield of PS II (Fv/Fm) showed no significant difference for non-inoculated rice plants regardless of silicon treatment (Fig. 3). Fv/Fm showed 8.15 and 9.99% reduction for CO39 and C101LAC (Pi-1), respectively, after M. oryzae inoculation. Si application to M. oryzae inoculated plants significantly increased the ratio of Fv/Fm compared with non-Si-treated infected plants.
https://static-content.springer.com/image/art%3A10.1007%2Fs11738-010-0588-5/MediaObjects/11738_2010_588_Fig3_HTML.gif
Fig. 3

Effects of silicon application and M. oryzae inoculation on chlorophyll fluorescence parameter Fv/F0 and Fv/Fm in the leaves of two rice isogenic lines CO39 (susceptible) and C101LAC (Pi-1) (resistant). Si+ and Si− indicate treatments with and without 2.0 mM silicon in culture solution, respectively. Mg+ and Mg− indicate treatments with and without M. oryzae inoculation, respectively. Values are means ± standard error from five replicates (n = 5). Significant differences (P < 0.05 using least significant difference test) among different treatments for the same rice line are indicated by different letters above the bars

The change in basal quantum yield (Fv/F0) with different treatments was similar to Fv/Fm (Fig. 3). In non-inoculated rice plants, Si amendment did not affect the ratio of Fv/F0. On the contrary, M. oryzae infection significantly decreased the value of Fv/F0. However, Si application increased the value of Fv/F0 by 20.99% for CO39 and 29.09% for C101LAC (Pi-1), respectively, in inoculated rice plants. There was no significant Fv/F0 difference between Si treatment and non-Si treatment in the non-inoculated plants.

Silicon content

Si supply significantly increased leaf Si content for both two rice lines. Si application led to 138.37 and 211.60% increases in Si content in the leaves of non-inoculated plants of CO39 and C101LAC (Pi-1), and 144.75 and 169.73% increases in Si content in the leaves of inoculated plants, respectively (Fig. 4). There was no significant difference in Si content between the two rice lines under different treatments.
https://static-content.springer.com/image/art%3A10.1007%2Fs11738-010-0588-5/MediaObjects/11738_2010_588_Fig4_HTML.gif
Fig. 4

Effects of silicon application and M. oryzae inoculation on Si content in the leaves of two rice isogenic lines CO39 (susceptible) and C101LAC (Pi-1) (resistant). Si+ and Si− indicate treatments with and without 2.0 mM silicon in culture solution, respectively. Mg+ and Mg− indicate treatments with and without M. oryzae inoculation, respectively. Values are means ± standard error from five replicates (n = 5). Significant differences (P < 0.05 using least significant difference test) among different treatments for the same rice line are indicated by different letters above the bars

Mineral nutrient contents

Concentrations of inorganic elements including K, Na, Ca, Mg, and Fe were significantly higher in leaves of M. oryzae infected plants for both susceptible (CO39) and resistant line C101LAC (Pi-1) ( Table 1). No significant difference was observed in mineral nutrient contents of rice leaves between Si-treated and non-Si-treated plants in the absence of M. oryzae inoculation. However, concentrations of K, Na, Ca, Mg, and Fe in M. oryzae inoculated leaves were significantly reduced by Si application, as compared with those in inoculated but non-Si treated plants. There was no significant difference in mineral nutrient contents between the two rice lines for those treatments.
Table 1

Effects of silicon application and M. oryzae inoculation on mineral nutrient contents in leaves of rice isogenic lines CO39 (susceptible) and C101LAC (Pi-1) (resistant)

Rice line

Treatment

K (g/kg)

Na (g/kg)

Ca (g/kg)

Mg (g/kg)

Fe (mg/kg)

CO39

Si–Mg−

23.6 ± 0.7b

1.1 ± 0.05c

1.05 ± 0.43b

2.55 ± 0.17b

112.2 ± 1.70b

Si+Mg−

20.8 ± 1.5b

1.0 ± 0.05c

0.95 ± 0.48b

2.45 ± 0.10b

97.5 ± 10.8b

Si–Mg+

40.7 ± 4.6a

3.0 ± 0.2a

3.71 ± 0.40a

5.03 ± 0.80a

158.2 ± 10.8a

Si+Mg+

25.9 ± 1.3b

2.1 ± 0.1b

1.82 ± 0.40b

3.22 ± 0.15b

122.9 ± 2.2b

C101LAC (Pi-1)

Si–Mg−

34.8 ± 0.7ab

1.5 ± 0.2c

2.03 ± 0.21b

3.27 ± 0.40b

113.9 ± 3.7b

Si+Mg−

28.1 ± 1.2b

1.4 ± 0.1c

1.77 ± 0.16b

2.86 ± 0.31b

99.4 ± 6.8b

Si–Mg+

45.3 ± 6.8a

3.0 ± 0.3a

3.84 ± 0.56a

5.38 ± 0.76a

172.0 ± 6.8a

Si+Mg+

40.6 ± 2.6a

2.3 ± 0.2b

2.31 ± 0.57b

4.13 ± 0.37b

137.9 ± 11.1b

Si+ and Si− represent treatments with and without 2.0 mM silicon in culture solution, respectively. Mg+ and Mg− represent treatments with and without M. oryzae inoculation, respectively

Values are means ± standard error from five replicates (n = 5)

Different letters denotes statistical difference in the same column and same rice line by using a least significant difference test (P < 0.05)

Cation ratios

Disease resistance of crops is believed to be related to cation ratios (Bains and Jhooty 1984). Here, we compared the effects of silicon application and M. oryzae inoculation on mineral nutrient ratio in rice leaves (Table 2). M. oryzae infection significantly increased Ca/K ratio by 106.8 and 46.6%, (Ca + Mg)/K by 43.3 and 33.6%, (Ca + Mg)/(Na + K) ratio by 106.8 and 46.6% for CO39 and C101LAC (Pi-1), respectively. More increase in these cation ratios was observed in the susceptible line than that in the resistant line. In contrast, silicon application led to decrease in cation ratios. Ca/K ratio was decreased by 30.0 and 49.1%, (Ca + Mg)/K ratio by 10.3 and 27.7%, and (Ca + Mg)/(Na + K) ratio by 11.1 and 27.3% after silicon application to rice plants of CO39 and C101LAC (Pi-1), respectively. More decrease in these cation ratios was found in the susceptible line than that in the resistant line.
Table 2

Effects of silicon application and Magnaporthe oryzae inoculation on mineral nutrient ratio in the leaves of two rice isogenic lines CO39 (susceptible) and C101LAC(Pi-1) (resistant)

Rice line

Treatment

Ca/K

(Ca + Mg)/K

(Ca + Mg)/(K + Na)

CO39

Si–Mg−

0.044 ± 0.002b

0.150 ± 0.002b

0.145 ± 0.008b

Si+Mg−

0.046 ± 0.001b

0.163 ± 0.003b

0.156 ± 0.006b

Si–Mg+

0.091 ± 0.002a

0.215 ± 0.008a

0.200 ± 0.009a

Si+Mg+

0.070 ± 0.003b

0.195 ± 0.004b

0.180 ± 0.003b

C101LAC (Pi-1)

Si–Mg−

0.058 ± 0.001b

0.152 ± 0.002b

0.146 ± 0.008b

Si+Mg−

0.063 ± 0.001b

0.164 ± 0.003b

0.157 ± 0.005b

Si–Mg+

0.085 ± 0.003a

0.203 ± 0.007a

0.191 ± 0.007a

Si+Mg+

0.057 ± 0.002b

0.159 ± 0.008b

0.150 ± 0.009b

Si+ and Si− indicate treatment with or without 2 mM silicon in culture solution; Mg+ and Mg− indicate treatments with and without M. oryzae inoculation, respectively

Values are means ± standard error from five replicates (n = 5)

Different letters denote statistical difference in the same column and same rice line by using a least significant difference test (P < 0.05)

Discussion

Rice plants amended with Si exhibited an enhanced resistance against infection by M. oryzae for both susceptible and resistant rice NILs (Fig. 1). Previous studies have showed that the possible mechanisms of Si enhancing plant resistance to disease are characterized by two aspects. One is that Si accumulation and deposition in epidermal cell walls beneath the cuticle, forming a cuticle-Si double layer in the leaf blade play a role in preventing fungal penetration (Samuels et al. 1991; Kim et al. 2002; Cai et al. 2008; Hayasaka et al. 2008). Another is that Si can induce host defense responses through increasing the activities of defense-related enzymes such as POD, PPO, PAL, etc., and increasing the accumulation of antifungal compounds such as phenolics and phytoalexins (Fawe et al. 1998; Rodrigues et al. 2004; Rémus-Borel et al. 2005; Cai et al. 2008). The detailed mechanisms of Si-mediating pathogen resistance were reviewed by Fauteux et al. (2005) and Cai et al. (2009).

Measurements of chlorophyll fluorescence parameters provide useful information about photosystem II (PSII) activity and changes in photosynthetic metabolism of diseased leaves (Schnabel et al. 1998). In the literature, there are no available data on the relationship between Si and photochemical efficiency of PSII. Our results showed that pathogen infection significantly reduced chlorophyll fluorescence parameters including Fv/Fm and Fv/F0, but silicon application significantly increased these parameters in infected plants (Fig. 3). However, the increase of fluorescence parameters did not have the same magnitude as the decrease of disease index. For example, the increase on Fv/F0 was 20.99 and 29.09% (Fig. 3) when the decrease magnitude of disease index was 45.06% and 55.56% for CO39 and C101LAC (Pi-1), respectively (Fig. 1). Bassanezi et al. (2002) found that there was no change in electron transport capacity and generation of ATP and NADPH in apparently healthy areas of diseased leaves, but decreases in chlorophyll fluorescence emission occurred on visibly lesioned areas for bean rust, angular leaf spot, or anthracnose. Rahoutei et al. (2000) showed that Nicotiana benthamiana Gray plants infected with pepper mild mottle virus (PMMoV) and Paprika mild mottle virus (PaMMoV) resulted in the reduction of photosynthetic electron transport in photosystem II (PSII) in both symptomatic and asymptomatic leaves of virus-infected plants. Bonfig et al. (2006) reported that the maximum PSII quantum yield, effective PSII quantum yield, and nonphotochemical quenching were decreased in Arabidopsis leaves infected with either a virulent or an avirulent strain of Pseudomonas syringae.

Pathogen infection can also affect mineral nutrition uptake and nutrient balance. But no information is available on the effects of pathogen infection and silicon application on mineral nutrition contents in the plant tissue. Uptake of essential cations including K+, Ca2+, Mg2+ etc. had been reported to be reduced while Na+ content increased in various species in salinity stress (Grieve and Fujiyama 1987; Awad et al. 1990; Lee et al. 2007). Wang and Han (2007) reported that silicon supply increased K+ content of alfalfa plants, but no significant impact on Mg2+, Na+ and Fe3+ content as compared with the NaCl treatment. The present study showed that higher mineral nutrition contents including K, Ca, Mg, Na, and Fe in rice leaves were found in M. oryzae infected plants compared with healthy plants. However, Si supply significantly reduced the above mineral nutrition contents in both rice lines with M. oryzae inoculation (Table 1), and the mineral contents of nutrition contents in Si-treated inoculated rice leaves was close to the nutrition content level of control plant without Si supply and M. oryzae infection. These results demonstrated that silicon application could alleviate mineral nutrition imbalance resulting from pathogen attack and maintain a normal level of these mineral nutrition contents. Similar results that Si addition reduced Ca2+ content were observed in the shoots and leaves of rice and alfalfa plants under salt stress (Ma and Takahashi 1993; Wang and Han 2007), suggesting that Si amendment may result in a low transpiration rate.

Cation ratios were found to be related to plant disease resistance (Bains and Jhooty 1984). Higher ratios of (Ca + Mg)/(Na + K) or (Ca + Mg)/K are related to crop resistance against downy mildew and root-knot diseases (Balasubramanian 1973; Bains and Jhooty 1984). Balasubramanian (1973) found that yellowing due to downy mildew of sorghum was related to high (Na + K)/(Ca + Mg) or low Ca/K ratio. However, Bains and Jhooty (1984) reported that the (Ca + Mg)/K ratio of melon tissues infected with P. cubensis did not differ from that of the adjoining healthy tissues. In the present study, M. oryzae infection resulted in higher Ca/K, (Ca + Mg)/K, (Ca + Mg)/(Na + K) ratio; silicon application significantly reduced these ratio in infected plants (Table 2). One interesting thing is that M. oryzae infection led to more increase in cation ratios in the susceptible line, and Si treatment led to more decrease in susceptible line, suggesting that Si application displays more influence on cation ratio in susceptible line than those in resistant line. The effect of mineral nutrient ratio on disease resistance was probably related to an overall changed biochemical environment in the host cells. Further studies are still needed to determine the relationship between inorganic nutrient ratios and pathogen resistance of plants.

As shown earlier, Si could increase the value of chlorophyll fluorescence parameters and adjust mineral nutrition uptake and nutrient ratios in infected rice plants, but it had no effect on those variables measured in non-inoculated rice plants. These findings are consistent with other published reports suggesting that the Si effects become manifest only when plants grow under stress (abiotic and biotic) environments (Watanabe et al. 2004; Fauteux et al. 2006),

In conclusion, this study suggests that the increasing resistance of rice plants to M. oryzae pathogen by Si supply is associated with an enhancement of photochemical efficiency and adjustment of mineral nutrition uptake in infected rice plants.

Acknowledgments

This study was financially supported by grants from the National Key Basic Research Funds of China (2006CB1002006), Natural Science Foundation (31070396) and the earmarked fund for Modern Agro-industry Technology Research System.

Copyright information

© Franciszek Górski Institute of Plant Physiology, Polish Academy of Sciences, Kraków 2010