Silicon can reduce levels of several important diseases of rice, including blast brown spot, sheath blight, leaf scald and grain discoloration. Levels of control are equal to that achieved by fungicides for disease such as blast and brown spot. Hence, the number of fungicide applications and rates can be reduced significantly. Residual activity of silicon was effective for disease control in the second year crop and was comparable to a first year silicon application or a full rate of a fungicide. Silicon enhanced performance of partially-resistant cultivars so that they were comparable to highly resistant cultivars for both blasts and sheath blight. These findings suggest that silicon could be employed in integrated disease management systems for reducing fungicide use and enhancing host plant resistance for the control of important rice diseases worldwide.
Silicon (Si) has been exploited for its prophylactic properties against plant disease for hundreds of years. Its role as a disease-preventing product has been well documented, but the mechanisms by which it exerts its beneficial properties in planta remain poorly understood. For a long time, the observation of a systematic accumulation of silica in cell walls and appositions occurring at pathogen penetration sites led to the conclusion that this parietal strengthening was responsible for the increased resistance of plants to diseases. However, recent evidence suggests that Si. would rather play an active role in reinforcing plant disease resistance by stimulating the expression of its natural defense reactions. Incidentally, in the cucumber (Cucumis sativus)-powdery mildew (Sphaerotheca fuliginea) system, this latter mechanism appears to be predominant, if not exclusive. A better understanding of this rather unique property of Si. could be exploited to optimize its use in agriculture and to help decipher how plants can be naturally stimulated to protect themselves against pathogens.
Silicification is reported in the Pteridophyta and the Spermatophyta, including gymnosperms and angiosperms. Dicotyledon families containing Si accumulators of considerable agricultural significance include the Fabaceae, Cucurbitaceae and Asteraceae. Among the monocotyledons, the Cyperaceae and Poaceae (Gramineae) are pre-eminent. Silica deposits, commonly called phytoliths, occur in cell walls, cell lumens or in extracellular locations. These deposits frequently possess a characteristic morphology revealing their tissue and taxonomic origin. Silicification occurs in roots and the shoot including leaves, culms and in grasses, most heavily in the inflorescence. Deposits occur in epidermal, strengthening, storage and vascular tissues. Biogenic silica structure is affected by ambient physico-chemical conditions mediated by tissue maturation, pH, ionic concentrations and cell wall structure, as illustrated by the results of a developmental study of silicification in wheat seedlings. Silicified tissues provide support and protection and may also sequester toxic metals, as illustrated by our recent work on the codeposition of aluminum with silicon in cereals and conifers. Some phytoliths have been implicated as carcinogens. Phytoliths are being increasingly used in archaeology as many retain their morphology in sediments.
Silicon (Si) has not been proven to be an essential element for higher plants, but its beneficial effects on growth have been reported in a wide variety of crops, including rice, wheat, barley, and cucumber. Si fertilizer is applied to crops in several countries for increased productivity and sustainable production. Plants take up Si in the form of silicic acid, which is transported to the shoot, and after loss of water, it is polymerized as silica gel on the surface of leaves and stems. Evidence is lacking concerning the physiological role of Si in plant metabolism. Since the beneficial effects of this element are apt to be observed in plants which accumulate Si, the silica gel deposited on the plant surface is thought to contribute to the beneficial effects of Si, which may be small under optimized growth conditions, but become obvious under stress conditions. In this review, the effects of Si under biotic stresses (disease and insect damage) and abiotic stresses including climate stresses (typhoon and cool summer damage), water deficiency stress, and mineral stresses (deficiency of P and excess of P, Na, Mn, N and Al) are discussed.
Rice blast and grain discoloration are mainly responsible for significant losses in grain yield and quality both in upland and irrigated ecosystems in Brazil. Rice (Oryza sativa L.) planting in rotation with soybean in extensive, contiguous areas and high input technology provided a conductive environment to diseases which were hitherto unimportant, such as sheath blight in irrigated rice and take-all in upland rice. Even though varietal resistance constitutes a major component in rice disease management, it should be integrated with long-term benefits of silicon (Si) fertilization. A field study conducted with genotypes showing wide variability for grain discoloration and different rates of SiO2 showed promising results. Initial greenhouse inoculation tests are encouraging in controlling leaf blast at the vegetative phase with Si. The logical extension of firmly established existing concepts on Si and rice disease management should rely on multidisciplinary approach and inter-institutional collaboration. Extensive on-farm trials at hot spot locations for diseases will compliment the experimental results and increase the speed and efficacy in accomplishing the desired goals.
The predominant silicon (Si) compound in the soil solution is silicic acid, and the baseline condition for Si transport into and within a plant with no membrane channels or transporters which can move Si compounds is the movement of silicic acid across membranes by dissolving in the lipid phase of the membrane ('lipid solution' transport). Based on the best current estimates of 'lipid solution' permeability of membranes to silicic acid (∼10−10 m s−1), even the lowest Si contents in plants cannot be explained in terms of the soil solution silicic acid concentration and the lipid solution mechanism, and a component of silicic acid entry coupled to transpiratory water uptake is required. For Oryza (rice) and, under some conditions, Hordeum (barley), and Phaseolus (bean), active influx of silicic acid is needed to account for the observed silica content. Further work is needed as to the mechanism of active tranport of silicic acid following the lead of the characterization of Na+-coupled transport in a diatom, and on how silicic acid is coupled to water transport (involving aquaporins?), and on the phloem mobility of silicic acid.
Characteristics of an acceptable silicon (Si) source are: a high content of soluble-Si, physical properties conducive to mechanized application, ready availability, and low cost. Since Si is the second most abundant element in the earth's crust, finding sources of Si is easy. But, Si is always combined with other elements and most sources are insoluble. Responses of crops to soluble-Si applications in sands (largely SiO2) provide an example of the insolubility of one source. Slags, by-products from the processing of iron and alloy industries, have been utilized quite extensively. Their concentrations and solubility of Si and their contents of other elements vary widely. For a given source, solubility is indirectly related to particle size. A few sources are soluble, but too costly for general use. Potassium silicate is used in nutriculture for disease control in some high value crops. Sodium silicate and silica gel have also been used to supply Si in research and high value crops. Calcium silicates have emerged as the most important sources for soil applications. Of those, calcium meta-silicate (wollasonite, CaSiO3) has been the most effective source in many locations with low concentrations of soluble-Si in soils. Such a material, supplied as a slag by-product from the high temperature electric furnace production of elemental P, is applied extensively to Everglades mucks and associated sands planted to sugarcane and rice. Thermo-phosphate, a commercial fertilizer used in Brazil to supply P, Ca, and Mg, also supplies soluble-Si due to high temperature manufacturing process effects on its magnesium silicate ingredient.
Soil minerals and organic matter control physical and chemical soil properties. Silicon (Si) is a basic mineral formatting element. The aim of our investigation was to obtain information about the effect of Si fertilization on physical and chemical soil properties. Silicon fertilization has been reported to result in increased soil exchange capacity, improved water and air regimes, transformation of P-containing minerals and formation of alumosilicates and heavy metal silicates. All these effects are caused by the change in soil mineral composition that results from silicate addition (Si fertilizers) and/or formation of new clay minerals, which are characterized by high biogeochemical activity. They have large surface area and are able to adsorb water, phosphates, potassium (K), nitrogen (N), aluminum (Al), and heavy metals. Adsorption may occur as chemosorptions or physical sorption. Cations (Al, heavy metals) usually are chemosorbed on Si-rich surface and lose their mobility. Phosphates and N are weakly adsorbed and remain in plant-available form. Amorphous silica, montmorillonite, and vermiculite represent the newly-formed minerals. These minerals affect the soil composition, and physical and chemical properties. The amounts of amorphous silica, monosilicic acids, and polysilicic acids in the soil are closely related to each other. Monosilicic acids regulate chemical properties of the soil solution. Polysilicic acids have an effect on soil physical properties.
The facts of silicon (Si) in plant life are one thing; the concepts regarding Si in plant physiology are another thing altogether. Most terrestrial plants grow in media dominated by silicates, and the soil solution bathing roots contains Si at concentrations exceeding those of phosphorus (P) by roughly a factor of 100. Plants absorb the element, and their Si content is of the same order of magnitude as that of the macronutrient elements. The general plant physiological literature, however, is nearly devoid of Si. The reason for this marked discrepancy is the conclusion that Si is not an "essential" element because most plants can grow in nutrient solutions lacking Si in their formulation. Such Si-deprived plants are, however, experimental artifacts. They may differ from Si-replete plants in (i) chemical composition; (ii) structural features; (iii) mechanical strength; (iv) various aspects of growth, including yield; (v) enzyme activities; (vi) surface characteristics; (vii) disease resistance; (viii) pest resistance; (ix) metal toxicity resistance; (x) salt tolerance; (xi) water relations; (xii) cold hardiness; and probably additional features. The gap between plant physiological facts and plant physiological concepts must be closed. The facts of Si in plant life will not change; hence it is the concepts regarding the element that need revising.
In 1917, Onodera first showed that rice (Oryza sativa L.) plants affected by blast tended to contain less silicon than healthy ones. This was probably the first report that suggested an effect of silicon on blast resistance. Since then, many researchers have demonstrated that applying silicon to the soil causes higher silicon levels in rice and, as a consequence, an increase in blast resistance. Several hypotheses to explain this phenomenon were proposed up to 1950. Most importantly, the fact that silicon is mainly localized in the leaf surface supports the hypothesis that the silicon layer may act as a physical barrier against blast fungus penetration. However, this cause-effect relationship has not yet been fully accepted. Application of silicon to commercial rice paddy fields became popular after the effectiveness of readily available silicate slag on blast was demonstrated in 1952. The use of silicate slag reached a peak in the early 1970s. However, the amount of research has declined since the 1960s because the interest of most blast researchers has changed to investigating other blast countermeasures such as fungicides and host plant resistance.
The results are summarized of a number of glasshouse and field experiments conducted since 1970, in which the relative efficiencies of calcium metasilicate slag and calcium carbonate were compared. In four out of the five field trials, significant responses, ranging from 9 to 24 tons cane/ha, were obtained in both the calcium silicate slag and lime treatments. On average, the silicon-based treatments were 5% better than the lime treatments. In one trial where the ameliorants were incorporated to a depth of 65 cm, calcium silicate increased yield significantly (P>0.01), whilst the response to lime did not attain a level of statistical significance. All ameliorants caused a reduction in exchangeable Al in the soil and a reduction in manganese uptake. With treatments containing silica, the increased yields were associated with an increase in the silica concentration in the plant. Current research is focused on the association between silicon assimilation and host-plant resistance to the stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae). Recent evidence from a large scale pot trial in which sugarcane was treated with calcium silicate and artificially infested with E. saccharina at 9.5 months showed significant reductions of 33.7% in borer damage and 19.8% in borer mass. Scanning of leaf samples by near infra-red spectrometry (NIRS) suggests that up to 60% of the variation in E. saccharina resistance could be accounted for by the leaf silicon (R=0.60) content.
The classical method for determining total silicon (Si) content of various materials has been conversion of insoluble silicates into sodium silicate through high temperature fusion with sodium hydroxide, or other sodic bases. The Si can then be determined by a variety of methods, including gravimetric, colorimetric, and absorption/emission spectrometry. Silicon also has been determined gravimetrically in plant tissue as the residue after acid digestion. We have developed a simple, inexpensive, and rapid method for solublizing Si in plant tissue that facilitates analysis of a large number of samples. When analyzing soils and fertilizers, a method for gauging the plant-available Si, rather than total Si, generally is desired. A number of soil-test methods have been developed. Some require extended incubation periods, fieldmoist soil, or other procedures that inhibit adoption by routine soil-testing laboratories. Silicon extracted by acetic acid has been correlated to Si uptake by rice (Oryza sativa L.) and rice grain yield. Using this method, the Everglades Soil Testing Laboratory analyses nearly five thousand samples annually. Since Si fertilizer sources differ in Si content and Si solubility, analytical methods have been developed for predicting their relative ability to provide plant-available Si. We use a column leaching method based on Si elution in Tris buffer (pH 7) for the evaluation of potential Si soil amendments. However, greenhouse and field evaluations are essential for making final determinations.
During the 1950s, the first field trials of sodium silicate on rice grown in marine deposit soils sgowed little effect on yield, although furnace slags gave some effect for most paddy soils. However, since 1960, ground wollastonite has been found to be most effective in improving growth of rice under a balanced supply of nitrogen (N), phosphorus (P), and potassium (K) in more than 90% of Korean paddy soils containing less than 130 mg kg of available SiO2 in the top soil. Intensive studies on models for fertility management of paddy soils based on soil tests, including available silica, were started for sustainable rice production in the 1970s. These approaches and models may also be used for various upland crops of grass species such as maize, wheat, and barley. In the future, the use of those models should also be tested for environmental hazards due to the emission of greenhouse gases, such as nitrous oxide or methane, either from the soil or through the plants. After World War II, the Republic of Korea suffered from a shortage of staple rice grain. Agronomists tried every effort to improve unit area rice production under limited agricultural land area and fertilizer supply. The use of silicon (Si) for increased rice production was started as a measure for increasing food grain production. Since 1977, it has contributed to Korea gaining self-sufficiency in rice production. The use of silicate fertilizer is still a common practice for paddy soil fertility management, having started in the early 1970s with the government-subsidized supply of 400,000 Mg yearly of silicate fertilizer. The Republic of Korea and Japan, having used more than sufficient chemical fertilizers for sufficient rice production in their limited land area for the last several decades, have much experience to share with other countries in the use of Si. This chapter introduces and summarizes Korean experiences of the last several decades and suggests future research aims.
A lack of knowledge about the role of silicon (Si) in horticultural crops became apparent with the change to soilless growing media in the glasshouse industry in the Netherlands. It was found that in these systems the Si contents in plant tissue were significantly lower in comparison with crops grown in soil. Investigations were carried out on the effects of Si application in soilless culture. With cucumber (Cucumis sativus), melon (Cucumis melo), courgette (Solanum melongena), strawberry (Fragaria ananassa), bean (Phaseolus vulgaris), and rose (Rosa), the Si contents were increased as a result of the addition of Si into the root environment. However, the uptake was almost negligible in tomato (Lycopersicon esculentum), sweet pepper (Capsicum frutescens cv. ‘grossu’), lettuce (Lactuca sativa), gerbera (Gerbera sp.), and carnation (Dianthus caryophyllus). Results showed that cucumber, rose, and courgette could benefit from enhanced Si concentration in the root environment, since total yield was increased and powdery mildew was suppressed. Despite a minor uptake of Si in lettuce, it was found that Si uptake affected the Mn distribution, thereby, alleviating Mn toxicity in the plant. Initially, severe problems with blocking of the irrigation system occurred due to instability of Si sources. These were solved by the introduction of potassium metasilicate. The use of si colloids was found to be less effective.
A fundamental understanding of aqueous silicate chemistry is a prerequisite to unraveling silicon's (Si) role in living systems. Owing primarily to the advent of 29Si nuclear magnetic resonance (NMR) spectroscopy, the depth of that understanding has increased dramatically over the past two decades. By comparison, details of the biochemistry of Si are sparse. Although several proteins and amino acids believed to be associated with silicates have been isolated, no organosilicon compounds have so far been identified under physiological conditions. Nevertheless, hypervalent Si complexes have very recently been shown to form in the presence of aliphatic polyols and polyol acids, such as mannitol and saccharic acid, respectively. Such simple aliphatic hydrocarbons may play a crucial role in the uptake, transport, and deposition of Si in nature. The objective of this paper is to provide an overview of the speciation, equilibria, and chemical exchange kinetics of Si in aqueous environments, along with an examination of applicable methods of chemical analysis.
Ninety-five percent of Australia's sugarcane is grown on the narrow coastal plain that stretches along the east coast of Queensland with rainfall ranging from 700 to 4500 mm. The most northern part of this area has a unique combination of landforms and climate (very high annual rainfall) that give rise to a specific range of soils. These soils have been under sugarcane production for up to 130 years, and apart from low levels of soluble soil Si resulting from natural weathering and leaching processes, there is evidence of declining Si levels following long-term sugarcane production. Although significant responses attributed to Si were observed in studies in the early 1970's following the application of cement, these studies were confounded by the influence of associated cations (eg. calcium) accompanying the Si source. Recent research has identified specific Si responses and delineated soils having sub-optimal levels of Si. Future research will focus on developing response functions on a range of soil types identified as Si responsive, and on assessment of the efficacy of silicate containing materials.
Despite the prolific research conducted in crop production and other aspects of silicon (Si) application, little is known about the potential economic benefits of its use in agriculture. Although some physical benefits obtained are impressive, the relative high cost of the material could make Si application unprofitable in some areas of the world. The purpose of this chapter is to show an economic analysis of two crops and one rotation: rice ( Oryza sativa ), sugarcane ( Saccharum officinarum ), and a rice-sugarcane rotation. The first case demonstrates the potential economic benefits taking into account the research conducted and the areas where rice is grown or could be grown. The second case does the same for sugarcane. The last case pertains to a specific rice-sugarcane rotation in Florida. These three cases seem to indicate that Si has a tremendous potential for increasing farm revenue.
Research in Canada on the significance of silicon (Si) to higher plants has focused on four main areas: 1) the nature and location of Si deposition in organs and cells of higher plants, 2) the chemistry of Si in biological systems, 3) the role of Si in plant-fungal pathogen interactions and 4) the use of diatomaceous earth and silica aerogels to control insects in post-harvest products. All four provide new and important information which improves our understanding and perhaps, exploitation of Si in agriculture.
The silicon (Si) concentration in plants can affect their health and productivity. Several experiments evaluated the variability of Si concentration in an array of rice (Oryza sativa) and sugarcane (Saccharum officinarum) genotypes grown under Si-limiting conditions. In all tests, significant variation for Si concentration among genotypes was identified, and ranking of genotypes over environments was fairly stable. Indica rices appeared to be less efficient at acquiring Si than japonicas. There were significant negative correlations between Si concentration and plant disease development. Silicon did not increase leaf photosynthesis but did increase sexual fertility and grain set.
Integrated management of six macronutrients: nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg) as well as the seven micronutrients iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), molybdenum (Mo), and chloride (Cl) are the ones that most agronomists only consider as essential for sustainable crop yields. However, under special crop/soil agriculture conditions there are some “non-essential” elements, like silicon (Si) that will enhance crop yield by promoting several desirable plant physiological processes. Due to the desilication process, Si in the soil is continuously lost as a result of leaching process. Subtropical and tropical soils are generally low in plant-available Si and would benefit from Si fertilization. Silicon content in some regions might be limited to sustainable crop production. The need for proper Si management to increase yield and sustain crop productivity appears to be necessary in temperate as well in tropical countries. In addition, Si diminution in the soil can occur in intensive cultivation practices and continuous monoculture of high-yielding cultivars. As a result, these soils are generally low in plant-available Si (Juo and Sanchez, 1986; Foy, 1992). Rice and sugarcane grown in rotation on organic and sandy soils have shown positive agronomic responses to pre-plant applications of calcium silicate slag (Anderson, 1991).
