全 文 ：Received 1 Dec. 2003 Accepted 23 Feb. 2004
Supported by the State Key Basic Research and Development Plan of China (G199011707) and the National Natural Science Foundation of
China (30170550, 30170175).
* Author for correspondence. E-mail: .
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (6): 691-697
Effects of Silicon on Rice Leaves Resistance to Ultraviolet-B
LI Wen-Bin, SHI Xin-Hui , WANG He*, ZHANG Fu-Suo
(Department of Plant Nutrition, College of Resources and Environment Science, China Agricultural University,
Key Laboratory of Plant-Interactions, MOE, Beijing 100094, China)
Abstract: In order to investigate the possible role of silicon in rice (Oryza sativa L.) leaves resistance
to ultraviolet-B (UV-B), experiments were conducted by using rice plants solution-cultured with or without
silicon supplementation. Results showed that under high UV-B irradiation the silicon-deficient leaves
exhibited obvious brown spots and strips of UV damage symptoms, but the silicon-treated leaves were not
affected. A 21% and 67% increase in soluble and insoluble UV-absorbing compounds was observed in the
epidermis of silicon-treated leaves, respectively. Furthermore, fluorescence microscopy revealed that a
great deal of insoluble UV-absorbing compounds was enriched in silicon bodies that were formed in the cell walls and
cell lumina of epidermis of silicon-treated leaves, whereas the insoluble UV-absorbing compounds were less in the
epidermis of non-silicon-treated leaves. Based on these results, it is concluded that the elevated UV resistance
of silicon-treated leaves is due to the increase of phenolic compounds in epidermis induced by silicon.
Key word: rice (Oryza sativa); ultraviolet-B; phenolic compounds; silicon
The reduction in stratospheric ozone layer and the re-
sulting increase in ultraviolet-B (UV-B) radiation on earth
have induced widespread public concern and become an
important scientific problem (Huang et al., 1998). In recent
years, considerable researches on UV-B resistance in plants
have been done (Janson et al., 1998; Brandt and Koch,
2003). UV-B damages proteins and DNA, affects genome
stability (Ishibashi et al., 2003), inhibits photosynthesis,
and reduces the yield (Nayak et al., 2003). In principle, two
different tolerance strategies occur in vascular plants:
screening of the internal tissues against the UV-B radiation
and repair of inflicted damage (Janson and Gaba, 1998;
Ishibashi et al., 2003). Both mechanisms complement each
other and both are apparently indispensable. Many re-
searches have proved that phenolic compounds are the
main substances to absorb and screen UV-B radiation in
the plant tissues. They are predominantly located in the
cuticle, cell wall, and vacuole of epidermal cells which pro-
tect the underlying mesophyll cells from UV-B injury (Krauss
et al., 1997; Hutzler et al., 1998; Schmitz-Hoerner and
Weissenbo, 2003). With the elevation of UV-B radiation,
the content of phenolic compounds in epidermal cells in-
creased (Carlos et al., 2001), so the epidermal cells are in-
dispensable barriers against ultraviolet stress. Rice is a typi-
cal silicon accumulator; the level of SiO2 in the leaves is as
high as 12% on a dry weight basis (Epstein, 1999; Rich-
mond and Sussman, 2003). The insoluble SiO2 bodies, de-
posited mainly in epidermal cells, are composed of 1-2
nanometer particles (Epstein, 1999). A recent report indi-
cated that plastic films containing nanometer SiO2 particles
have higher capacity to absorb UV irradiation because na-
nometer SiO2 particles possess super high absorption
surface, special optic absorbing quality and quantum ef-
fect (Mizutani and Nago, 1999). Another research demon-
strated that UV-B radiation promoted silicon deposition in
rice leaves (IRRI, 1991). So it is reasonable to ask whether
the nanometer SiO2 particles deposited in the epidermal
cells can enhance UV-B absorbing ability of rice leaves. If it
does, whether it is due to the accumulation of UV-absorb-
ing compounds induced by nanometer SiO2 particles in
epidermal cells or due to the SiO2 particle itself. To answer
these questions, a series of experiments were conducted
using rice plants solution-cultured with or without silicon
supplementation. The experimental results show that sili-
con can markedly enhance accumulation of phenolic com-
pounds in epidermal cells of rice leaves and therefore in-
crease the resistance to UV-B stress.
1 Materials and Methods
1.1 Plant materials
Rice hybrid cultivar (Ⅱ7#)(Oryza sativa L.) from
Changsha City in Hunan Province of China was used. Seeds
were germinated in quartz sand at 28 ℃,after 14 d, the
seedlings were transplanted into 2-L plastic pots filled
with nutrient solution that was prepared according to
Yoshida’s formula using pure water made by Millipore
Acta Botanica Sinica 植物学报 Vol.46 No.6 2004692
ultrapure water system (Yoshida et al., 1976). The composi-
tion of nutrient solution (pH 5.6) is as follows: 4.38 mmol/L
NH4NO3, 1.54 mmol/L K2SO4, 0.60 mmol/L KH2PO4, 1.0
mmol/L CaCl2, 3.27 mmol/L MgSO4, 0.04 mmol/L MnCl2, 0.2
mmol/L (NH4)6Mo7O24, 0.6 mmol/L ZnSO4, 0.6 mmol/L
CuSO4, 0.076 mmol/L H3BO3, 0.026 mmol/L FeSO4. The so-
lution was supplemented with or without 1.5 mmol/L silicic
acid. The growth chamber was 12 h light at 28 ℃and 12 h
darkness at 25 ℃. Air humidity was 60% R. H. (day) and
80% R. H. (night) in the chamber. Ambient photosynthetic
photon flux (400-700 nm) ranged from 600-1 200
mmol .m-2.s-1. After growing for three months, the first
fully developed leaf was taken for experiments.
1.2 Estimation of UV-B tolerance of rice leaves
According to Skaltst’s method (Skaltst et al., 1994), UV-
B irradiation was performed in a dark chamber at room tem-
perature by using a 10 W Philips UV fluorescent lamp that
was enveloped by a 0.13-mm cellulose acetate film to elimi-
nate the light with wavelength shorter than 280 nm. Before
UV-B treatment, the fresh leaves were cut into about 1-cm-
long sections and floated on distilled water in petri dishes.
The distance between the leaf sections and UV lamp was
10 cm. After exposure to UV-B irradiation for 30 h, the leaves
were checked for brown spots and strips of visible UV dam-
age symptoms under Olympus stereoscopic microscope
and Leica universal microscope. In order to take clearer
photographs of UV damage symptoms, the UV-B irradiated
leaves were extracted with a mixture of acetone and ethanol
(1:1,V/V) to remove the chlorophyll.
1.3 Histochemical localization of UV-absorbing com-
pounds
In situ localization of UV-absorbing compounds in rice
leaves: fresh hand-cut transverse sections of rice leaves
were sealed in 80% aqueous glycerin, and the yellow-green
autofluorescence of phenolic compounds in leaf cells were
examined under Leica universal microscope excited by ul-
traviolet light. Isolation and observation of silicon bodies
in leaf epidermis: in order to obtain epidermal peels from
rice leaves following Dayanadan et al. (1983), 1-cm-long
pieces of fresh leaves were scraped on abaxial sides to
remove most of the cells above the adaxial epidermis, then
the isolated adaxial epidermis were digested in H2SO4 con-
centrate for 4 d, after enough washing with distilled water,
the released pure silicon bodies were collected for examina-
tion of yellow-green autofluorescence of phenolic com-
pounds excited by UV light.
1.4 Measurement of relative levels of UV-absorbing com-
pounds in epidermis
Preparation of intact epidermal tissues: according to the
procedure of Dayanadan et al. (1983), intact adaxial epider-
mis was mechanically removed from fresh rice leaves by
gently scraping off the abaxial side. The intact epidermal
pieces free of perforations and mesophyll, determined by
microscopic examination, were used for the following
experiments.
Assessment of soluble UV-absorbing compounds: UV-
B absorbing compounds were extracted with acid methanol
mixture of MeOH-H2O-HCl (79:20:1, V/V/V) (Rozama et al.,
2001). Samples of fresh adaxial epidermis were treated with
above extraction solution at 4 ℃for 48 h, then centrifuged
(10 min, 4 000 g). The UV absorption spectra of the super-
natants were scanned over 200-400 nm bandwidth using
spectrophotometer (UV-2501, Shimazu Co., Japan). The larg-
est absorption peak was selected for measurement of ab-
sorption value of soluble UV-absorbing compounds. The
relative level of soluble UV-absorbing compounds in epi-
dermis was expressed as the UV absorption value of per
unit fresh sample weight. Every treatment was repeated
five times.
Assessment of insoluble UV-absorbing compounds:
according to Skaltsa et al. (1994), the above methanol ex-
tracted intact epidermis was gently pressed between two
slices of filter paper and dried at room temperature. The dry
epidermis was adhered to a black aluminium plate with a 4
mm2 hole; the aluminium plate was then put into the sample
shelf in the UV spectrophotometer for 190-500 nm band-
width scanning. For correction of the spectra and absorp-
tion value, an identical control of aluminium plate without
epidermis was inserted in the control light pathway. Ac-
cording to the obtained UV absorption spectra of epidermis,
the absorbance of insoluble UV-absorbing compounds in
epidermis was measured at 270 nm and 320 nm and calcu-
lated based on per unit leaf area. Every treatment was re-
peated five times.
2 Results
2.1 Effect of silicon on UV tolerance of rice leaves
Without UV-B irradiation both the silicon-treated (Fig.
1b) and non-treated leaves (Fig.1a) showed no necrotic
spots, however, after 30 h exposure to UV-B radiation,
the silicon-deficient leaves exhibited many brown spots
and strips that are typical UV damage symptoms in ac-
cordance with the previous reports (Caasi-Lit et al., 1997)
(Fig.1c). In contrast, there were almost no visible UV
damage symptoms in the silicon-fed leaves exposed to
the same level of UV-B radiation (Fig.1d), indicating that
silicon can remarkably enhance the UV tolerance of rice
leaves.
LI Wen-Bin et al.: Effects of Silicon on Rice Leaves Resistance to Ultraviolet-B 693
Acta Botanica Sinica 植物学报 Vol.46 No.6 2004694
2.2 Distribution of UV-B absorbing compounds in the
leaves
To clarify the mechanism of silicon induced UV resis-
tance of rice leaves, the distribution of UV-B absorbing
compounds in leaf tissues was examined. From the trans-
verse section of silicon-deficient leaf, it can be seen that
under UV excitation there is only faint yellow-green
autofluorescence in the epidermal cell walls (blank arrow in
Fig.1e) and inside bulliform cells (solid arrow in Fig.1e),
which means that the content of UV-absorbing compounds
in silicon-deficient leaves is less. In contrast, the epidermal
cell walls (blank arrow in Fig.1f) and bulliform cells (solid
arrow in Fig.1f) of silicon-treated leaves emitted strong yel-
low-green autofluorescence when excited by UV light,
which indicated that there was larger quantity of UV-ab-
sorbing compounds accumulated in the epidermis of sili-
con-treated leaves as compared to non-treated leaves.
In order to further investigate the distribution of phe-
nolic compounds in silicon bodies, epidermis separated from
silicon-treated leaves were digested in H2SO4 concentrate
to obtain pure silicon bodies. The sheet-like silicon bodies
from epidermal cell outer walls (Fig.1g) and the fan-shaped
silicon bodies from bulliform cells (Fig.1i) showed intense
yellow-green autofluorescence respectively (Fig.1h, j) when
excited by UV light, which proved that silicon bodies
contained many UV-absorbing compounds. Phenolic com-
pounds may be precipitated together with silicon or en-
riched in silicon bodies during the development of silicified
epidermis. There are no silicon bodies formed in the non-
silicon treated leaves. Above results indicate that silicon
treatment promotes the accumulation of phenolic com-
pounds in rice leaf epidermis.
2.3 Quantity of UV-B absorbing compounds in rice epi-
dermis
Figure 2A shows absorption spectra property of soluble
phenolic compounds in epidermis. There are two distinct
peaks at 270 and 320 nm, showing that the UV absorbing
compounds probably are flavonoids (Shi and Di, 2000).
Except for quantitative change in absorption value, the UV
absorption spectra of soluble phenolic compounds from
silicon-treated and non-treated leaves did not show quali-
tative difference. Compared with silicon deficient leaves,
the relative level of soluble phenolic compounds in the
epidermis of silicon-treated leaves increased by 17% at 270
nm and by 21% at 320 nm (Fig.2B). The absorbance differ-
ence between these two treatments was significant at 5%
level.
Figure 3A presents the absorption spectra of insoluble
UV-absorbing compounds in epidermal cells, which shows
strong absorbance between 200 and 380 nm. The relative
Fig.1. a. A portion of silicon-deficient rice leaf without UV-irradiation, × 116; b. A portion of silicon-treated rice leaf without UV-
irradiation, ×116; c. Brown spots and strips of UV damage symptoms on silicon-deficient rice leaf (reflective light photograph), ×116,
inset is a close-up view of brown strips (arrow) (transmitting light photograph), ×700; d. No UV damage symptoms on silicon-treated
leaf; e. Faint yellow-green autofluorescence in epidermal cell outer wall (blank arrow) and inside bulliform cells (solid arrow) of silicon-
deficient leaf, ×1 400; f. Strong yellow-green autofluorescence in epidermal cell outer wall (blank arrow) and inside bulliform cells (solid
arrow) of silicon-treated leaves, × 1 400; g. A sheet-like silicon body from epidermal cell wall of silicon-treated leaves (dark field
photograph), × 1 600; h. Autofluorescence emitted from the sheet-like silicon body in Fig.1g excited by UV light, × 1 600; i. A fan-
shaped silicon body from epidermal bulliform cells (dark field photograph), ×1 800; j. Autofluorescence emitted from the fan-shaped
silicon body in Fig.1i excited by UV light.
←
Fig.2. Soluble phenolic compounds in rice adaxial epidermis of Si-treated and non-treated leaves. A. Absorption spectra. B. Relative
absorbance value.
LI Wen-Bin et al.: Effects of Silicon on Rice Leaves Resistance to Ultraviolet-B 695
level of insoluble UV-absorbing compounds in epidermal
cells of silicon-treated leaves was about 65% higher than
that in the silicon-deficient leaves (Fig.3B). The difference
was significant at 1% level.
Above quantitative data indicate that silicon treatment
promotes the accumulation of both soluble and insoluble
UV-absorbing compounds in rice leaf epidermis, and the
latter is more enhanced.
3 Discussion
In recent years, considerable studies indicate that sili-
con is a beneficial element for higher plants. Silicon has
been suggested to have a positive effect on reproduction,
alleviation of metal toxicity and nutrient imbalance, provi-
sion of structural rigidity, and increased resistance to fun-
gal diseases (Epstein, 1999; Richmond and Sussman, 2003;
Rodrigues et al., 2003). However, research data on the pos-
sible role of silicon in UV-B resistance is not available. Many
reports indicate that in rice plant phenolic compounds are
the main effective UV-protectants, since those components
are mostly localized in the epidermal cells of the surface of
the leaves and possess the higher absorption of UV radia-
tion (Markham et al., 1998; Ishibashi et al., 2003). In accor-
dance with above results, the important findings of our
study were that silicon promotes the accumulation of UV-B
absorbing compounds in epidermal cells, so it remarkably
enhanced the UV resistance of rice leaves. The soluble and
insoluble UV-absorbing compounds in leaf epidermis in-
creased by 21% and 65% respectively (Fig.2B, 3B) when
silicon was included in nutrient solution. Insoluble UV-ab-
sorbing compounds are tightly bound to silicon bodies
and could not be extracted by acidic methanol.
It was reported that some gramineous crops could not
effectively screen UV-B through epidermal cells (Bilger et
al., 1997), so our research may open a new route to improve
UV-B resistance of plants by applying silicon fertilizer. The
mechanism of silicon induced phenolic accumulation in leaf
epidermis may be that silicon stimulates de nono synthesis
of phenolic compounds; It is also possible that the insoluble
nanometer SiO2 particles, which is deposited in epidermis,
promote the enrichment of constitutional phenols in epi-
dermal cells by their super high adsorption surface, since it
was previous reported that one gram SiO2 particles with a
diameter of 20 nanometers possess adsorption surface of
400 square meters (Wang and Naser, 1994).
Some authors have postulated that silicon bodies in the
leaf epidermal system might act as a “window” to facilitate
the transmission of light to photosynthetic mesophyll tis-
sue (Kaufman et al., 1979), but our research result as well
as other author’s report (Agarie et al., 1996) does not sup-
port this theory. From the UV absorption spectra of epi-
dermis (Fig.4), it can be seen that there was no difference
between silicon-treated and non-treated epidermises in
transmission of photosynthetically active light with wave-
length higher than 500 nm, however, silicon-treated epider-
mis shows stronger UV absorbing ability than non-treated
epidermis, so it is conceivable that the silicon bodies in rice
leaf epidermis may function as “UV filter” not an open
“window”.
In addition to its direct damages to big biological
molecules, UV radiation also triggers the production of ac-
tive oxygen radicals and results in oxidative stress (Jin et
al., 2001). Previous report indicated that flavonoids and
other phenolics in Arabidopsis reduce UV-induced injury
by preventing the accumulation of active oxygen species
(Bieza and Lois, 2001). Therefore, except the UV filtering
effect of UV-absorbing compounds in the epidermis of sili-
con-treated rice leaves, the enrichment of these phenolics
Fig.3. Insoluble UV-absorbing compounds in rice adaxial epidermis. A. Absorption spectra. B. Relative absorbance value.
Acta Botanica Sinica 植物学报 Vol.46 No.6 2004696
would also contribute to consume more active oxygen
species.
Recently, more and more evidence has shown that sili-
con is closely associated with phenolic metabolism. Indeed,
our present study shows that silicon enhances the accu-
mulation of phenolic compounds in rice leaf epidermis; this
is similar to many other reports. Kidd et al. (2001) found
that the silicon-treated maize roots release fifteen times more
phenolics than untreated maize plants. These flavonoid-
phenolics (i.e. catechin and quercetin) have a strong Al-
chelating ability and may provide heavy-metal tolerance in
plants. Silicon also mediated accumulation of flavonoid
phytoalexins in cucumber leaves and increased its resis-
tance to fungal diseases (Fawe et al., 1998). Based on the
infrared absorption spectrum of siliceous cell wall from rice,
Inanaga et al. (1995) suggested a combination between
aromatic ring and Si-OH. Through the use of electron-en-
ergy-loss spectroscopy (EELS) in the study of the cell walls
of beech roots and leaves, researchers showed that silica
was closely associated with polyphenolic substances
(Watteau and Villemin , 2001). Furthermore, by using ge-
netic analysis method, Piperno et al. (2002) and Dorweiler
et al. (1997) revealed that in cucumber and maize silification
and lignification processes are genetically correlated.
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