Reexamination of silicon effects on rice growth and production under field conditions using a low silicon mutant

Silicon (Si) is a beneficial element for healthy growth and high and sustainable production of rice, but the mode of action of the beneficial effects has not been well understood. We carried out field trials for four years at two different locations to re-examine the effects of Si on the growth and production of rice using a low silicon rice (lsi1) mutant. The mutant accumulated much lower Si at each growth stage compared with the wild-type rice (Oryza sativa L. cv Oochikara), but there was no difference in the accumulation of other nutrients including N, P, and K. Measurements at different growth stages showed that low Si in the mutant hardly affected the tiller number, chlorophyll content (SPAD value), and root growth. The plant height and shoot dry weight of the wild-type rice were slightly higher than those of the mutant at a later growth stage, but the difference was not significant between the two lines. However, grain yield was reduced by 79–98%, depending on year, due to a low Si accumulation in the mutant, which showed the largest effect of Si on rice production among all studies reported so far. Among the yield components, the percentage of filled spikelets was mostly affected, being only 13.9% of the wild-type rice in the mutant. The grain color of the mutant became brown because of excessive transpiration and infection of pathogens. These results indicate that Si increases rice yield mainly by enhancing the fertility of spikelets.     

Function of silica bodies in the epidermal system of rice (Oryza sativa L.): testing the window hypothesis

Silicon has been considered to be important for normal growthand development of the rice plant (Oryza sativa L.). To investigatethe physiological function of deposited silica in rice leaves,the hypothesis that silica bodies in the leaf epidermal systemmight act as a ‘window’ to facilitate the transmissionof light to photosynthetic mesophyll tissue was tested. Thesilica content of leaves increased with supplied silicon andwas closely correlated with the number of silica bodies perunit leaf area in the epidermal system. There was a significantdifference in silica deposition and formation of silica bodiesbetween Si-treated and non-treated leaves; silicon was polymerizedinside the silica cells and bulliform cells of the epidermis,in Si-treated leaves. Although the ‘windows’ wereonly formed in leaves with applied silicon, optical propertiesof leaf transmittance, reflectance and absorptance spectra inSi-treated and non-treated leaves were almost equal. Furthermore,light energy use efficiency and quantum yield of Si-treatedleaves were less than in leaves not containing silica bodies.Thus, silica bodies, at least based on the data, do not functionas windows in rice leaves. Key words: Silicon, window hypothesis, rice, optical property, quantum yield     

Function of silica bodies in the epidermal system of rice (Oryza sativa L.): testing the window hypothesis

Silicon has been considered to be important for normal growth and development of the rice plant (Oryza sativa L.). To investigate the physiological function of deposited silica in rice leaves, the hypothesis that silica bodies in the leaf epidermal system might act as a 'window' to facilitate the transmission of light to photosynthetic mesophyll tissue was tested. The silica content of leaves increased with supplied silicon and was closely correlated with the number of silica bodies per unit leaf area in the epidermal system. There was a significant difference in silica deposition and formation of silica bodies between Si-treated and non-treated leaves; silicon was polymerized inside the silica cells and bulliform cells of the epidermis, in Si-treated leaves. Although the 'windows' were only formed in leaves with applied silicon, optical properties of leaf transmittance, reflectance and absorptance spectra in Si-treated and non-treated leaves were almost equal. Furthermore, light energy use efficiency and quantum yield of Si-treated leaves were less than in leaves not containing silica bodies. Thus, silica bodies, at least based on the data, do not function as windows in rice leaves.     

Do lignification and silicification of the cell wall precede silicon deposition in the silica cell of the rice (Oryza sativa L.) leaf epidermis?

Aims

Rice is a well-known silica-accumulating plant. The dumbbell-shaped silica bodies in the silica cells in rice leaf epidermis are formed via biosilicification, but the underlying mechanisms are largely unknown.

Methods

Leaves at different developmental stages were collected to investigate silica cell differentiation by analyzing structures and silicon localization in the silica cells.

Results

Exogenous silicon application increased both shoot and root biomass. When silicon was supplied, silica cells in the leaf epidermis developed gradually into a dumbbell-shape and became increasingly silicified as leaves aged. Silicon deposition in the silica cells was not completed until the leaf was fully expanded. Multiple lines of evidence suggest that lignification of silica cell walls precedes silicon deposition in the lumen of silica cells. The organized needle-like silica microstructures were formed by moulding the inner cell walls and filling up the lumen of the silica cell following leaf maturation.

Conclusions

Two processes were involved in silicon deposition: (1) the silica cell wall was lignified and silicified, and then (2) the silicon was deposited gradually in silica cells as leaves aged. Silica body formation was not completed until the leaf was fully mature.     

Silicon suppresses zinc uptake through down-regulating zinc transporter gene in rice

One of the beneficial effects of silicon (Si) is to improve nutrient imbalance including deficiency and excess of nutrients, however the molecular mechanisms underlying this effect are still poorly understood. In this study, we investigated the interaction between Si and zinc (Zn) in rice by using a mutant (lsi1) defective in Si uptake and its wild-type (WT, cv. Oochikara) at different Zn levels. High Zn inhibited the root elongation of both WT and lsi1 mutant, but Si did not alleviate this inhibition in both lines. By contrast, Si supply decreased Zn concentration in both the roots and shoots of the WT, but not in the lsi1 mutant. A short-term (24 h) labeling experiment with stable isotope ⁶⁷Zn showed that Si decreased ⁶⁷Zn uptake, but did not affect the root-to-shoot translocation and distribution ratio to different organs of ⁶⁷Zn in the WT. Furthermore, Si accumulated in the shoots, rather than Si in the external solution, is required for suppressing Zn uptake, but this was not caused by Si-decreased transpiration. A kinetic study showed that Si did not affect K_m value of root Zn uptake, but decreased V_max value in the WT. Analysis of genes related with Zn transport showed that among ZIP family genes, the expression of only OsZIP1 implicated in Zn uptake, was down-regulated by Si in the WT, but not in the lsi1 mutant. These results indicate that Si accumulated in the shoots suppresses the Zn uptake through down-regulating the transporter gene involved in Zn uptake in rice.     

Effects of silicon sources on its deposition, chlorophyll content, and disease and pest resistance in rice

Rice (Oryza sativa L.) was grown in pots with pyridine N-oxide (PNO), 4-morpholino pyridine N-oxide (MNO), and sodium meta silicate as the sources for silicon. Aliquots of these were added in fortnightly intervals to seedlings through anthesis stage. The plants were monitored for plant growth characteristics, chlorophyll content (SPAD values), photosystem 2 activity (variable to maximum fluorescence ratio of dark adapted leaves), and for blast and yellow stem borer resistance. Deposition of silica in the leaves was monitored by scanning electron microscopy and silicon mapping. PNO or MNO application resulted in significant silicon accumulation in leaf bundle sheath cells. Application of PNO and MNO imparted disease and pest resistance by increasing silicon uptake of rice plants.     

Characterization of the silicon uptake system and molecular mapping of the silicon transporter gene in rice

Rice (Oryza sativa L. cv Oochikara) is a typical silicon-accumulating plant, but the mechanism responsible for the high silicon uptake by the roots is poorly understood. We characterized the silicon uptake system in rice roots by using a low-silicon rice mutant (lsi1) and wild-type rice. A kinetic study showed that the concentration of silicon in the root symplastic solution increased with increasing silicon concentrations in the external solution but saturated at a higher concentration in both lines. There were no differences in the silicon concentration of the symplastic solution between the wild-type rice and the mutant. The form of soluble silicon in the root, xylem, and leaf identified by (29)Si-NMR was also the same in the two lines. However, the concentration of silicon in the xylem sap was much higher in the wild type than in the mutant. These results indicate that at least two transporters are involved in silicon transport from the external solution to the xylem and that the low-silicon rice mutant is defective in loading silicon into xylem rather than silicon uptake from external solution to cortical cells. To map the responsible gene, we performed a bulked segregant analysis by using both microsatellite and expressed sequence tag-based PCR markers. As a result, the gene was mapped to chromosome 2, flanked by microsatellite marker RM5303 and expressed sequence tag-based PCR marker E60168.     

Silicon in Imperata cylindrica (L.) P. Beauv: content,distribution, and ultrastructure

Silicon concentration, distribution, and ultrastructure of silicon deposits in the Poaceae Imperata cylindrica (L.) P. Beauv. have been studied. This grass, known for its medicinal uses and also for Fe hyperaccumulation and biomineralization capacities, showed a concentration of silicon of 13,705?±?9,607 mg/kg dry weight. Silicon was found as an important constituent of cell walls of the epidermis of the whole plant. Silica deposits were found in silica bodies, endodermis, and different cells with silicon-collapsed lumen as bulliforms, cortical, and sclerenchyma cells. Transmission electron microscope observations of these deposits revealed an amorphous material of an ultrastructure similar to that previously reported in silica bodies of other Poaceae.     

Contribution of leaf nitrogen to photosynthetic gas exchange in contrasting rice (Oryza sativa L.) cultivars during the grain-filling period

Photosynthetic parameters and leaf carbon isotope composition (??¹³C) in contrasting rice genotypes in relation to supplemental nitrogen (N) application and water management during the grain-filling period were compared. The changes in stomatal conductance (g _s) and ratio of intercellular to ambient CO₂ mole fraction (C _i/C _a) depended on the leaf nitrogen concentration (leaf N) in both ??Hinohikari?? (temperate japonica genotype) and ??IR36?? (indica genotype). In ??Hinohikari??, ??¹³C reflects photosynthetic gas exchange during the grain-filling period, which is indicated by the significant response of ??¹³C to leaf N. In contrast, in ??IR36?? ??¹³C did not depend on leaf N. This varietal difference in ??¹³C to leaf N can be attributed to a difference in the timing of leaf senescence. In ??IR36??, leaf N and photosynthetic parameters decreased more rapidly, indicating earlier senescence and a shorter grain-filling period in comparison with ??Hinohikari??. The significant increase in shoot dry mass in ??Hinohikari?? resulting from supplemental N application, compared with nonsignificant effect observed in ??IR36??, suggests that the timing of senescence in relation to the grainfilling period has a preponderant influence on productivity.     

Silicon-Mediated Resistance in a Susceptible Rice Variety to the Rice Leaf Folder,Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae)

The rice leaf folder, Cnaphalocrocis medinalis (Guenée), is one of the most destructive rice pests in Asian countries. Rice varieties resistant to the rice leaf folder are generally characterized by high silicon content. In this study, silicon amendment, at 0.16 and 0.32 g Si/kg soil, enhanced resistance of a susceptible rice variety to the rice leaf folder. Silicon addition to rice plants at both the low and high rates significantly extended larval development and reduced larval survival rate and pupation rate in the rice leaf folder. When applied at the high rate, silicon amendment reduced third-instars’ weight gain and pupal weight. Altogether, intrinsic rate of increase, finite rate of increase and net reproduction rate of the rice leaf folder population were all reduced at both the low and high silicon addition rates. Although the third instars consumed more in silicon-amended treatments, C:N ratio in rice leaves was significantly increased and food conversion efficiencies were reduced due to increased silicon concentration in rice leaves. Our results indicate that reduced food quality and food conversion efficiencies resulted from silicon addition account for the enhanced resistance in the susceptible rice variety to the rice leaf folder.     

Growth promotion and an increase in cell wall extensibility by silicon in rice and some other Poaceae seedlings

The effect of silicon on organ growth and its mechanisms of action were studied in rice (Oryza sativa L. cv. Koshihikari), oat (Avena sativa L. cv. Victory), and wheat (Triticum aestivum L. cv. Daichino-Minori) seedlings grown in the dark. Applying silicon in the form of silicic acid to these seedlings via culture solution resulted in growth promotion of third (rice) or second (oat and wheat) leaves. The optimal concentration of silicon was 5–10 mM. No growth promotion was observed in early organs, such as coleoptiles or first leaves. In silicon-treated rice third leaves, the epidermal cell length increased, especially in the basal regions, without any effect on the number of cells, showing that silicon promoted cell elongation but not cell division. Silicon also increased the cell wall extensibility significantly in the basal regions of rice third leaves. These results indicate that silicon stimulates growth of rice and some other Poaceae leaves by increasing cell wall extensibility. Received: July 31, 2001 / Accepted: September 18, 2001     

Effects of Slag-Based Silicon Fertilizer on Rice Growth and Brown-Spot Resistance

It is well documented that slag-based silicon fertilizers have beneficial effects on the growth and disease resistance of rice. However, their effects vary greatly with sources of slag and are closely related to availability of silicon (Si) in these materials. To date, few researches have been done to compare the differences in plant performance and disease resistance between different slag-based silicon fertilizers applied at the same rate of plant-available Si. In the present study both steel and iron slags were chosen to investigate their effects on rice growth and disease resistance under greenhouse conditions. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the effects of slags on ultrastructural changes in leaves of rice naturally infected by Bipolaris oryaze, the causal agent of brown spot. The results showed that both slag-based Si fertilizers tested significantly increased rice growth and yield, but decreased brown spot incidence, with steel slag showing a stronger effect than iron slag. The results of SEM analysis showed that application of slags led to more pronounced cell silicification in rice leaves, more silica cells, and more pronounced and larger papilla as well. The results of TEM analysis showed that mesophyll cells of slag-untreated rice leaf were disorganized, with colonization of the fungus (Bipolaris oryzae), including chloroplast degradation and cell wall alterations. The application of slag maintained mesophyll cells relatively intact and increased the thickness of silicon layer. It can be concluded that applying slag-based fertilizer to Si-deficient paddy soil is necessary for improving both rice productivity and brown spot resistance. The immobile silicon deposited in host cell walls and papillae sites is the first physical barrier for fungal penetration, while the soluble Si in the cytoplasm enhances physiological or induced resistance to fungal colonization.     

Silicification in sorghum (Sorghum bicolor) cultivars with different drought tolerance

Sorghum belongs to a group of economically important, silicon accumulating plants. X-ray microanalysis coupled with environmental scanning electron microscopy (ESEM) of fresh root endodermal and leaf epidermal samples confirms histological and cultivar specificity of silicification. In sorghum roots, silicon is accumulated mostly in endodermal cells. Specialized silica aggregates are formed predominantly in a single row in the form of wall outgrowths on the inner tangential endodermal walls. The density of silica aggregates per square mm of inner tangential endodermal cell wall is around 2700 and there is no significant difference in the cultivars with different content of silicon in roots. In the leaf epidermis, silicon deposits were present in the outer walls of all cells, with the highest concentration in specialized idioblasts termed 'silica cells'. These cells are dumb-bell shaped in sorghum. In both the root endodermis and leaf epidermis, silicification was higher in a drought tolerant cultivar Gadambalia compared with drought sensitive cultivar Tabat. Silicon content per dry mass was higher in leaves than in roots in both cultivars. The values for cv. Gadambalia in roots and leaves are 3.5 and 4.1% Si, respectively, and for cv. Tabat 2.2 and 3.3%. However, based on X-ray microanalysis the amount of Si deposited in endodermal cell walls in drought tolerant cultivar (unlike the drought susceptible cultivar) is higher than that deposited in the leaf epidermis. The high root endodermal silicification might be related to a higher drought resistance.     

Silicon Improves the Tolerance of Wheat Seedlings to Ultraviolet-B Stress

Enhanced ultraviolet-B (UV-B) irradiation is one of the most important abiotic stresses that could influence the growth and physiological traits of plants. In this work, we reported the effects of silicon on the growth and physiological characteristics of wheat seedlings (Triticum aestivum L. cv Hengmai5229) subject to UV-B stress. Treatments with silicon significantly increased total biomass and chlorophyll (a + b) content, and reduced malondialdehyde (MDA) content and the rate of superoxide radical (O₂⁻) production in wheat seedlings subjected to UV-B stress. Silicon treatments also induced an increased in soluble sugar, anthocyanins, and flavonoid content. Leaf silicon concentration increased with the increasing of silicon supply to soil. Positive correlations were found in leaf silicon concentration with total biomass, chlorophyll (a + b), proline, and soluble protein content, respectively. MDA content and the rate of O₂⁻ production were negatively correlated with leaf silicon concentration in seedlings. The results demonstrated that silicon alleviated the damage caused by UV-B on wheat seedlings to some extent by the increase in antioxidant compounds content and leaf silicon concentration.     

Defense Responses in Rice Induced by Silicon Amendment against Infestation by the Leaf Folder Cnaphalocrocis medinalis

Silicon (Si) amendment to plants can confer enhanced resistance to herbivores. In the present study, the physiological and cytological mechanisms underlying the enhanced resistance of plants with Si addition were investigated for one of the most destructive rice pests in Asian countries, the rice leaf folder, Cnaphalocrocis medinalis (Guenée). Activities of defense-related enzymes, superoxide dismutase, peroxidase, catalase, phenylalanine ammonia-lyase, and polyphenol oxidase, and concentrations of malondialdehyde and soluble protein in leaves were measured in rice plants with or without leaf folder infestation and with or without Si amendment at 0.32 g Si/kg soil. Silicon amendment significantly reduced leaf folder larval survival. Silicon addition alone did not change activities of defense-related enzymes and malondialdehyde concentration in rice leaves. With leaf folder infestation, activities of the defense-related enzymes increased and malondialdehyde concentration decreased in plants amended with Si. Soluble protein content increased with Si addition when the plants were not infested, but was reduced more in the infested plants with Si amendment than in those without Si addition. Regardless of leaf folder infestation, Si amendment significantly increased leaf Si content through increases in the number and width of silica cells. Our results show that Si addition enhances rice resistance to the leaf folder through priming the feeding stress defense system, reduction in soluble protein content and cell silicification of rice leaves.     

盐生境下硅对坪用高羊茅生物学特性的影响

干旱半干旱区草坪绿地的长期灌溉容易引发土壤次生盐渍化,提高草坪草在盐生境下的生长发育能力是应对土壤次生盐渍化的主要途径之一.采用盆栽试验研究了盐生境下硅对坪用高羊茅(Festuca arundinaea)生物学特性的影响.结果表明,向盐生境土壤中添加不同浓度硅均提前高羊茅出苗时间2d,加快出苗速率,增加出苗总数,提高保苗率,且总出苗率和保苗率随着硅浓度增加而显著增大(P＜0.05),这说明盐生境下向土壤添加硅改善了幼苗完全死亡的现象.盐生境下硅显著增加了高羊茅叶长,株高和分蘖数,但对叶宽影响不显著,说明硅能够促进高羊茅生长,但对草坪绿地的质地影响不大.虽然硅能显著增加高羊茅总生物量(P＜0.05),但较低浓度时促进其茎叶生长,高浓度促进其根系生长.硅在高羊茅体内的沉积量随施入硅增加而增大,但其茎叶和根系内硅含量不超过3.0％,且根系内硅含量约为茎叶内硅含量的2倍.结果显示,硅提高了坪用高羊茅在盐生境下的适应能力,这为以后草坪绿地管理中应对土壤潜在盐渍化的问题提供了一定的科学依据.     

Silicon decreases chloride transport in rice (Oryza sativa L.) in saline conditions

Silicon can alleviate salt damage to plants, although the mechanism(s) still remains to be elucidated. In this paper, we report the effect of silicon on chloride transport in rice (Oryza sativa L.) seedlings in saline conditions. In the absence of salinity, silicon enhanced the growth of shoots, but not roots in three cultivars (cv. GR4, IR36, and CSR10). Salinity reduced the growth of both shoots and roots in all three genotypes. In saline conditions, addition of silicon to the culture solution again improved the growth of shoots, but not of roots. Under these saline conditions, the concentrations of chloride in the shoot were markedly decreased by adding silicon and the ratio of K⁺/Cl⁻ was significantly increased, while the concentration of chloride in the roots was unchanged. The decrease in chloride concentration in the shoot was correlated with the decrease in transpirational bypass flow in rice, as shown by the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). Addition of silicon increased the net photosynthetic rate, stomata conductance, and transpiration of salt-stressed plants in cv. IR36, indicating that the reduction of chloride (and sodium) uptake by silicon was not through a reduction in transpiration rate. Silicon addition also increased the instantaneous water use efficiency of salt-stressed plants, while it did not change the relative growth rate of shoots. The results suggest that silicon addition decreased transpirational bypass flow in the roots, and therefore decreased the transport of chloride to the shoot.     

ELECTRON PROBE ANALYSIS OF SILICON AND OTHER ELEMENTS IN LEAF EPIDERMAL CELLS OF THE RICE PLANT (ORYZA SATIVA L.)

By means of electron probe analysis, the effects of significant amounts of accumulation of silicon on the accumulation of calcium, potassium, magnesium, manganese, phosphorous, iron, and sodium in the silica cells of rice leaves are described. The silica cells of both the surfaces of the leaf blade and leaf sheath were studied. Silicon accumulation in the silica cells appears to decrease the amount of accumulation of potassium on both the surfaces of the leaf blade and sheath. The effect of significant amounts of silicon accumulation on the accumulation of other elements in a particular cell varies in different organs or on different surfaces of the organ of the same plant. Magnesium, manganese, iron, and phosphorus could not be detected in the adaxial epidermis of the leaf sheath and magnesium and iron in the adaxial epidermis of the leaf blade. Manganese, magnesium, and phosphorus were not detected in the abaxial epidermis of the leaf blade nor iron in the abaxial epidermis of the leaf sheath. Sodium was not revealed in either surface of the leaf blade and leaf sheath. Possible mechanisms for the effects of silicon accumulation on the accumulation of these elements in rice leaf epidermal cells are discussed.     

Augmenting drought tolerance in sorghum by silicon nutrition

Silicon (Si) distribution and accumulation in plant organs is widely reported as beneficial to overcome biotic and abiotic stresses. The investigation on distribution of Si in plant organs under water stress conditions was studied through experiments conducted at Arid Agriculture University, Pakistan during 2007–2009. Treatments (Si₂₀₀: 200 ml l^?1 of potassium silicate and Si₀: control or absence of silicon) were replicated thrice with two sorghum cultivars; SPV462 (drought susceptible) and Johar-1 (drought tolerant) screened using osmotic media of PEG-6000 (?4.0, ?6.0, ?8.0 and ?10.0 MPa). The results exhibited increased leaf water potential, leaf area index, SPAD chlorophyll with increased silicon concentration in leaves and roots of drought-tolerant genotype as compared to SPV462. Johar-1 exhibited maximum values for net assimilation and relative growth rate under silicon treatment in comparison to silicon absence. Similarly, maintenance of transpiration rate, because of accumulation of silicon in leaves resulted in optimum leaf water potential and optimum growth of crop.     

黄瓜中硅的生理功能及转运机制研究进展

硅是植物体的重要组成部分,尽管硅尚未被列为植物生长的必需元素,但它在促进植物生长发育、提高作物对非生物逆境（干旱、盐分和重金属等）和生物逆境（病虫害）抗性等方面都具有重要作用。硅不仅能改善植株对矿质营养的吸收,提高作物产量和品质,而且能沉积在叶片及叶鞘表皮细胞,形成硅化细胞和角质双硅层结构,增强寄主植物细胞壁的机械强度和稳固性,从而增强植物对真菌侵入和扩展的抵御能力,提高植物对金属离子毒害的抗性、缓解盐胁迫、增强抗高低温和抗紫外线辐射等。本文在植物硅素营养和转运机制研究的基础上,对硅素营养在黄瓜中生长发育、抗逆和吸收转运机制等方面的效应做了相关综述,并展望了黄瓜中硅研究的未来发展。