The Role of Silicon in Plant Susceptibility to Disease

Chad Husby
PPWS 5204 (paper for "Plant Disease Management" course)

November 9, 1998


(See also "Silicon:  The Estranged Medium Element")

Contents:
1. Introduction
2. Role of Si in Reduction of Fungal Diseases in Plants
3. Role of Si in Amelioration of Diseases Caused by Abiotic Stresses
4. Conclusions
5. Literature Cited




Introduction

    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, 1994).  

    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, 1983).

    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 settings.

3.    Possible Mechanisms Through Which Si Affects Disease Susceptibility

    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 al., 1997).

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.

Conclusions

    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.

Literature Cited

• Belanger, R. R., P. A. Bowen, D. L. Ehret, and J. G. Menzies. 1995. Soluble silicon: its role in crop and disease management of greenhouse crops. Plant Disease. 79(4):329-336.

• Chen, C. H. and J. C. Lewin. 1969. Silicon as a nutrient for Equisetum arvense. Canadian Journal of Botany. 47:125-131.

• Cherif, M., A. Asselin, and R. R. Belanger. 1994. Defence response induced by soluble silicon in cucumber roots infected by Pythium spp. Phytopathology. 84(3):236-242.

• Cherif, M. and R. R. Belanger. 1992. Use of potassium silicate amendments in recirculating nutrient solutions to suppress Pythium ultimum on Long English Cucumber. Plant Disease. 76(10):1008-1011.

• Cherif, M, N. Benhamou, J. G. Menzies, and R. R. Belanger. 1992. Silicon induced resistance in cucumber plants against Pythium ultimum . Physiological and Molecular Plant Pathology. 41:411-425.

• Cherif, M, J. G. Menzies, D. L. Ehret, C. Bogdanoff, and R. R. Belanger. 1994. Yield of cucumber infected with Pythium aphanidermatum when grown with soluble silicon. HortScience. 29(8):896-897.

• Datnoff, L. E., C. W. Deren, and G. H. Snyder. 1997. Silicon fertilization for disease management of rice in Florida. Crop Protection. 16(6):525-531.

• Datnoff, L. E., R. N. Raid, G. H. Snyder, and D. B. Jones. 1991. Effect of calcium silicate on blast and brown spot intensities and yields of rice. Plant Disease. 75(7):729-732.

• Deren, C. W., L. E. Datnoff, G. H. Snyder, and F. G. Martin. 1994. Silicon concentration, disease response, and yield components of rice genotypes grown on flooded organic histosols. Crop Science. 34:733-737.

• Epstein, E. 1994. The anomaly of silicon in plant biology. Proceedings of the National Academy of Sciences USA. 91:11-17.

• Hoffman, F. M. and C. J. Hillson.  1979. Effects of silicon on the life cycle of Equisetum hyemale L. Botanical Gazette. 140(2):127-132.

• Marschner, H. 1995. Mineral Nutrition of Higher Plants. Academic Press, London.

• Menzies, J. G., D. L. Ehret, A. D. M. Glass, T. Helmer, C. Koch, and F. Seywerd. 1991a. Effects of soluble silicon on the parasitic fitness of Sphaerotheca fuliginea on Cucumis sativus. Phytopathology. 81:84-88.

• Menzies, J. G., D. L. Ehret, A. D. M. Glass, and A. L. Samuels. 1991b. The influence of silicon on cytological interactions between Sphaerotheca fuliginea and Cucumis sativus.  Physiological and Molecular Plant Pathology. 39:403-414.

• Menzies, J. G. and R. R. Belanger. 1996. Recent advances in cultural management of diseases of greenhouse crops. Canadian Journal of Plant Pathology. 18:186-193.

• Miyake, Y. and E. Takahashi. 1983. Effect of silicon on the growth of solution-cultured cucumber plant. Soil Science and Plant Nutrition. 1983. 29(1):71-83.

• Samuels, A. L., A. D. M. Glass, D. L. Ehret, and J. G. Menzies. 1993. The effects of silicon supplementation on cucumber fruit: changes in surface characteristics. Annals of Botany. 72:433-440.

• Tisdale, S. L.; W. J. Nelson; J. D. Beaton. 1993. Soil Fertility and Fertilizers. Macmillan Publishing Company, New York.

• Winslow, M. D. 1992. Silicon, disease resistance, and yield of rice genotypes under upland cultural conditions. Crop Science. 32:1208-1213.


If you have any comments or questions, please contact the author, Chad Husby ( chad.husby@fiu.edu or husby.1@osu.edu )

© Chad E. Husby 2002

Last modified August 11, 2002

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