The Role of Silicon in Plant
Susceptibility to Disease
PPWS 5204 (paper for "Plant Disease Management" course)
November 9, 1998
(See also "Silicon: The
Estranged Medium Element")
2. Role of Si in Reduction of Fungal Diseases
3. Role of Si in Amelioration of Diseases Caused
by Abiotic Stresses
5. Literature Cited
Silicon (Si) is the second most abundant element
in the earth’s crust and is also abundant in most soils (Marschner, 1995;
Epstein, 1994; Datnoff et al., 1997). It is readily taken up by
plants and is often present in relatively high concentrations in plant tissues
(Epstein, 1994). Silicon concentrations in plant tissues sometimes
even exceed the concentrations of Nitrogen and Potassium (Epstein, 1994).
Therefore, Si is often a major constituent of plant tissue, although it
is not considered to be an essential nutrient for terrestrial plants in
general (Epstein, 1994). No other presumably non-essential element
is present in such consistently high amounts in terrestrial plants (Epstein,
Among terrestrial plants, only the horsetails
(class Equisetaceae) have been conclusively shown to require Si
as an essential nutrient (Hoffman and Hillson, 1979; Chen and Lewin, 1969;
Epstein, 1994). Some plants, such as many dicots, are known as silicon nonaccumulators
and tend to have tissue concentrations of 0.5% or less (Marschner, 1995).
Other plants, such as the wetland grasses, are known as Si accumulators
because they tend to have relatively high concentrations (5% or higher)
of tissue Si (Epstein, 1994). However, within a given plant species
or cultivar, tissue levels of silicon vary in relation to soil Si availability
(Datnoff et al., 1991).
Silicon has been shown to be a beneficial element
for many, and, under certain conditions, perhaps most terrestrial plants
(Marschner, 1995; Epstein, 1994). The beneficial effects of adequate
Si include decreased susceptibility to fungal pathogens (and insects),
amelioration of abiotic stresses, and increased growth in some plants (Marschner,
1995; Epstein, 1994). Although Si has for centuries been used for
preventing disease in agriculture (often in the form of horsetail extracts),
we are only beginning to gain a better understanding of its possible role(s)
in plant physiology and in disease prevention (Belanger et al., 1995).
Role of Si in Reduction of
Fungal Diseases in Plants
One of the most thoroughly studied beneficial
effects of Si on plant health is its role in reducing susceptibility of
some plants to fungal diseases. This effect of Si has been particularly
well documented in rice and greenhouse cucumbers.
1. Silicon and Rice Diseases
Rice is considered to be a Si accumulator and
tends to actively accumulate Si to tissue concentrations of 5% or higher
(Epstein, 1994; Miyake and Takahashi, 1983). Relatively large amounts
of plant available Si appear to be very important for both robust growth
and fungal disease resistance of rice (Winslow, 1992; Datnoff et al.,
1997). Although many rice-growing soils initially contain significant
quantities of Si, repeated rice cropping can reduce Si levels to the point
that Si fertilization becomes beneficial for growth and disease resistance
(Datnoff et al., 1997). Furthermore, in some parts of the world, rice
is grown on highly weathered soils (e.g. Oxisols and Ultisols), very sandy
soils (e.g. Entisols), or highly organic soils (Histosols) that are often
initially low in soluble Si (Datnoff et al., 1997). Common Si fertilizers
include calcium silicate slag (CaAl2Si2O8), calcium silicate (CaSiO3) and
sodium metasilicate (NaSiO3) (Tisdale et al., 1993).
In the Florida Everglades, rice is extensively
grown on organic Histosols with low levels of plant-available Si (Datnoff
et al., 1991). Numerous studies have shown that disease resistance
of rice increases in response to Si fertilization of these soils (Datnoff
et al., 1997). Neck blast and brown spot are two major fungal diseases
limiting rice production in Florida (Datnoff et al., 1991). Datnoff
et al. (1991) found that adding Si fertilizer (calcium silicate slag) to
Everglades Histosols reduced the amount of neck blast by 73-86% and reduced
the amount of brown spot by 58-75% during the 1987 and 1988 growing
seasons. Remarkably, this degree of disease control was not significantly
different from that achieved by fungicides such as benomyl (Datnoff et
al., 1997). Rice yields also increased by 56-88% with Si fertilization,
an effect that was at least partly due to the decrease in disease (Datnoff
et al., 1991).
Silicon fertilization has also been shown to
significantly decrease fungal disease susceptibility of rice grown on
the highly-weathered and Si deficient upland mineral soils of West Africa
(Winslow, 1992). Rice yields on these soils increased considerably
(48%) in response to Si fertilization with sodium metasilicate (Winslow,
1992). A similar pattern has been found on highly weathered savannah
soils in Columbia (Datnoff et al., 1997).
The importance of genotypic (i.e. cultivar) variation
in Si content of rice was investigated by Deren et al. (1994) on Everglades
Histosols. Both among and within cultivar genotypes, these investigators
found a significant negative correlation between Si concentration and
disease severity (Deren et al., 1994). This correlation further corroborates
the important role silicon plays in disease susceptibility of rice.
However, the correlation was much stronger within genotypes than among
genotypes (Deren et al., 1994). This shows that genotypic factors
beyond those affecting Si concentration were also important in determining
disease susceptibility (Deren et al., 1994). Similar genotypic trends
were found by Winslow (1992) on Si deficient mineral soils in West Africa.
2. Silicon and Greenhouse Cucumber Diseases
As with rice, fungal disease resistance of greenhouse
grown cucumber has been shown to increase substantially in response to
Si fertilization (Belanger et al., 1995; Menzies and Belanger, 1996).
Unlike rice, cucumbers take up Si passively (Miyake and Takahashi, 1983).
However, cucumbers can reach high Si tissue concentrations (even as high
as grasses) if the Si concentration of the medium is high (Miyake and Takahashi,
Greenhouse cucumbers are often grown in nutrient
solutions (hydroponically) without added Si (Belanger et al., 1995).
This provides an excellent setting for investigating the effects of Si
on cucumber growth and disease susceptibility under controlled conditions.
The effects of Si fertilization on greenhouse cucumber infections by
powdery mildew and Pythium spp. have been studied very thoroughly,
although several other cucumber fungal diseases have also been shown to
decrease in response to Si fertilization (Menzies and Belanger, 1996).
Many investigators have established the effectiveness
of Si fertilization in reducing cucumber susceptibility to powdery mildew
(Menzies and Belanger, 1996). In an especially thorough and well-controlled
set of experiments, Menzies et al. (1991a) investigated the effects of
different rates of Si fertilization (with potassium silicate) on powdery
mildew severity. Cucmber leaves were inoculated with known conidia
concentrations (Menzies et al., 1991a). The investigators found
that Si fertilization reduced the leaf area covered by powdery mildew by
as much as 98% (Menzies et al., 1991a). Nutrient solution concentrations
of 100 ppm or more SiO2 were found to produce optimum disease reduction
(Menzies et al., 1991a).
Si fertilization has also been shown to decrease
greenhouse cucumber susceptibility to Pythium crown and root rots
(Menzies and Belanger, 1996). Cherif and Belanger (1992) found
that nutrient solution concentrations of 100 or 200 ppm SiO2 significantly
reduced root mortality, root decay, and yield losses on plants inoculated
with Pythium ultimum. Furthermore, in Si (potassium silicate) fertilized
and Pythium-inoculated plants, root dry weights and number of
fruits (especially high-grade fruits) were significantly higher than for
Pythium -inoculated plants without Si fertilization (Cherif and
Belanger, 1992). Interestingly, the inoculated cucumbers receiving
Si fertilization were as productive as the uninoculated controls in this
study (Cherif and Belanger, 1992). However, among the uninoculated
(disease free) plants, those receiving Si fertilization did not differ significantly
in productivity from those not receiving Si fertilization (Cherif and Belanger,
1992). This seems to indicate that Si improved cucumber health and
productivity solely in the presence of the pathogen. Experiments carried
out by Cherif et al. (1994) produced similar results with the root pathogen
Pythium aphanidermatum .
Although most studies of cucumber responses to
Si fertilization have been carried out in greenhouse solution culture,
it is noteworthy that Si fertilization has also been shown to significantly
decrease Fusarium wilt of cucumber grown in soil (Belanger et
al., 1995). Therefore, it appears that Si fertilization of cucumber
may have important disease control benefits in a variety of agricultural
3. Possible Mechanisms Through Which Si Affects
Much of the work on possible mechanisms through
which Si affects disease susceptibility has been done on cucumber, but
the insights gained from these investigations may also apply to other
plants. Soluble Si taken up by plants tends to accumulate in the apoplast,
particularly in epidermal cell walls (Epstein, 1994; Marschner, 1995, Tisdale
et al., 1993; Samuels et al., 1993). This observation has led many
investigators to hypothesize that Si inhibits fungal disease by physically
inhibiting fungal germ tube penetration of the epidermis (Datnoff et al.,
1997; Belanger et al., 1995). However, subsequent investigators have
found that only the trichome bases on the cucumber epidermis tend to become
silicified (Belanger et al., 1995; Samuels et al., 1993).
Yet, Si has been observed to accumulate around
fungal hyphae and infection pegs in infected host plant cells (Datnoff
et al., 1997; Belanger et al., 1995). Many investigators have shown
that phenolic materials and chitinases also rapidly accumulate in these
infected host cells (Menzies et al., 1991b; Cherif et al., 1994; Belanger
et al., 1995). In fact, infected cells of Si-amended cucumber plants
accumulate phenolic materials much more quickly than infected cells of
unamended plants (Cherif et al., 1992; Menzies et al. 1991b). Si
amended plants have also been shown to have a significantly higher percentage
of infected cells which accumulate phenolics (Cherif et al., 1992; Menzies
et al. 1991b). Fungal hyphae penetrating the phenolic-laden cells
of Si amended plants were found to be seriously damaged by the accumulated
phenolics (Cherif et al., 1992). These phenolics were also conclusively
shown to be fungitoxic (Cherif et al. 1994). Therefore, it appears
likely Si fertilization reduces disease susceptibility primarily by stimulating
host-plant defenses (Belanger et al., 1995; Datnoff et al., 1997).
However, it is still possible that already silicified epidermal cells may
play some role in disease inhibition (Belanger et al., 1995; Datnoff et
Role of Si in Amelioration
of Diseases Caused by Abiotic Stresses
In addition to inhibiting fungal diseases, silicon
has also been shown to ameliorate certain mineral imbalances and other
diseases caused by abiotic stresses in plants (Marschner, 1995; Epstein,
1994). Several studies have found that Si can reduce or prevent
manganese (Mn) and iron (Fe) toxicity and may also have beneficial effects
on aluminum (Al) toxicity (Marschner, 1995; Tisdale et al., 1993).
Si does not seem to affect Mn uptake, but rather Mn distribution in plant
tissues (Marschern,1995). When Si levels in tissue are low, Mn
tends to be distributed nonhomogeneously and accumulates to toxic levels
in spots in leaves (Marschner, 1995). However, sufficient levels
of Si seem to cause Mn to be more evenly distributed in plant tissue,
thereby preventing toxic levels of this element from accumulating spots
(foci) in leaves (Marschner, 1995). Furthermore, Si has been shown
to alleviate an otherwise detrimental nutrient imbalance between zinc and
phosphorus (Marschner, 1995; Epstein, 1994).
Silicon can reduce salinity stress and reduce
transpiration in plants (Epstein, 1994; Marschner, 1995; Tisdale et al.,
1993). Furthermore, in sugarcane, there is evidence that Si may play
an important role in protecting leaves from ultraviolet radiation damage
by filtering out the harmful ultraviolet rays (Tisdale et al., 1993).
Thus, silicon has been shown to ameliorate abiotic stresses in several
ways and more such effects may be discovered.
Although silicon is not considered to be an essential
nutrient for most terrestrial plants, it is beneficial to many plants.
Si has the potential to significantly decrease the susceptibility of certain
plants to both biotic and abiotic diseases. Furthermore, in plants
such as rice, Si fertilization may even increase growth and yield in addition
to reducing disease severity. However, we are only now beginning
to better understand the role of Si in plant health and disease.
As we learn more about the importance of silicon in plant physiology, we
may find more ways to use this important element for improving plant health
and disease resistance.
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