Silica and Ash in Seeds of Cultivated Grains and Native Plants

Silica and ash contents and the depositional patterns of opalinesilica have been determined in the seeds of 31 plant species.Included were 13 monocotyledons, eight dicotyledons and theseeds of eight common cereal grains. The cereal grains, exceptfor Oryza sativa L. (3.2%) and Avena sativa L. (1.4%), werequite low in silica. The silica in these cereals was in thelemma. In seeds with high silica content it often makes up morethan 50% of the ash. Silica in seeds occurs largely in the outercoating of the seed. Dicotyledon seeds tend to have less silicathan those of monocotyledons. Energy-dispersive X-ray analysisshows that the distribution of the element silicon is clearlyrelated to certain epidermal structures such as ridges, raisedareas, trichomes and hairs. It also occurs in cell walls. Membersof a specific plant family tend to have very similar silicadepositional patterns in their seeds. Small amounts of K, S,Cl and Ca are also found in seeds. Light-microscopy studiesshow that the silica in the lemma of seeds such as Oryza sativaL. is deposited in cellular sheet-like structures with crenateedges. Silica in seeds also occurs in fibres and in other cellularstructures (silica cells) that become phytoliths. Seeds, epidermis, seed coat, silica and ash content, scanning electron microscopy, energy-dispersive X-ray analysis, silica depositional patterns, trichomes     

Genotypic Variation in the Thickness of Silica Deposition on Flowering Rice Spikelets

Silicon may play an important role in regulating the transpirationrate of rice (Oryza sativa L.), particularly cuticular transpiration.The control of cuticular transpiration is important in ricespikelets because water stress at anthesis may severely disruptfertility and grain yield. Data on the quantitative variationamong rice cultivars in the thickness of the silica layer ofthe flowering spikelet were obtained in order to assess thepotential for genetic selection for silica layer thickness asa potential means for increasing spikelet resistance to waterloss. Flowering spikelets were collected from 17 genotypes and thicknessmeasurements were made of the major anatomical layers in crosssection. The silica layer of the lemma varied from 42 to 177µm among cultivars. Similar variation was observed inthe palea. Differences among cultivars were also found in thethickness of the epidermis and sclerenchyma-parenchyma layer. The total cross-sectional thickness of the lemma and palea averagedslightly over 100 µm. Total thickness was not significantlycorrelated with the length, width or product of length width.Thickness of silica deposition was significantly correlatedwith spikelet length width suggesting that these easily measuredparameters may be useful in screening for silica thickness. Oryza sativa L., rice, panicle, silicon, lemma, palea, husk     

Storage-protein Deposition in the Developing Rice Caryopsis in Relation to the Transport Tissues

The developing caryopsis of rice (Oryza sativa L.) was examinedhistologically at successive stages of grain-filling in orderto identify the factors which determine the distribution ofstorage protein in the endosperm, and which terminate the depositionof endosperm protein. The storage protein was deposited at theperiphery of the endosperm, and this distribution was apparentlycaused by the radial pattern of cell development in the endosperm,and by the proximity of the peripheral endosperm cells to thenucellar epidermis. The nucellar epidermis directly surroundsthe endosperm and functions as the pathway for amino acid transportto the endosperm. During the later stages of caryopsis developmentthe nucellar epidermis became compressed by being ‘sandwiched’between the expanding endosperm and the rigid hull (the tightlylocked palea and lemma) which encloses the caryopsis. It isproposed that this compression of the nucellar epidermis blocksthe supply of amino acids to the endosperm and thereby terminatesthe deposition of storage protein in the rice grain. Oryza sativa, rice, caryopsis (development), endosperm, grain filling, nucellar epidermis, storage protein     

Structural Differentiation of the Nucellar Epidermis in the Caryopsis of Rice (Oryza sativa)

In the developing caryopsis of rice (Oryza sativa L.) the nucellarepidermis forms a uniseriate layer through which assimilatesare transported to the endosperm. An anatomical study demonstratedthat the nucellar epidermal cells are fusiform in shape andare hexagonally packed. The anticlinal walls of the nucellarepidermis are characterized by ribs of wall-thickening whichare orientated radially with respect to the caryopsis. The wall-thickeningsappear to be cellulosic primary walls, as indicated by theirstaining with Calcofluor and periodic acid-Schiff's reagent.It is proposed that the geometry of the nucellar cells and theribs of wall-thickening are structural adaptations to resistthe compressional force which is placed on the nucellar epidermisduring the latter stages of grain filling. Oryza sativa, rice, caryopsis, grain filling, nucellar epidermis, wall-thickening     

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     

Silicon enhances growth independent of silica deposition in a low-silica rice mutant, lsi1

To examine whether silica bodies are essential for silicon-enhanced growth of rice seedlings, we investigated the response of rice, Oryza sativa L., to silicon treatment. Silicic acid treatment markedly enhanced the SPAD (soil plant analytical development) values of leaf blades and the growth and development of leaves and lateral roots in cvs. Hinohikari and Oochikara, and a low-silicon mutant, lsi1. Combination of ethanol–benzene displacement and staining with crystal violet lactone enabled more detailed histochemical analysis to visualize silica bodies in the epidermis under bright-field microscopy. Supply of silicon induced the development of motor cells and silica bodies in epidermal cells in Hinohikari and Oochikara but not or marginal in lsi1. X-ray analytical microscopy detected silicon specifically in the leaf sheath, the outermost part of the stem, and the leaf blade midrib, suggesting that silicon is distributed to tissues involved in maintaining rigidity of the plant to prevent lodging, rather than being passively deposited in growing tissues. Silicon supplied at high dose accumulated in all rice seedlings and enhanced growth and SPAD values with or without silica body formation. Silicon accumulated in the cell wall may play an important physiological role different from that played by the silica deposited in the motor cell and silica bodies.     

A Transmission and Cryo-Scanning Electron Microscopy Study of the Formation of Aerenchyma (Cortical Gas-Filled Space) in Adventitious Roots of Rice (Oryza sativa)

The initiation and development ofaerenchyma in adventitiousroots of rice (Oryza sativa) was studied in tissuc up to approximately36-h-old using transmission electron microscopy (TEM) and cryoscanningelectron microscopy (cryo-SEM). Aerenchyma resulting from selective cortical cell collapse isa naturally occurring feature of rice roots. Evidence of somecortical cell disruption was noticeable by TEM in cells thatwere approximately 6-h-old; it became more advanced as the cellsaged. At 12 h, early stages of cell wall breakdown and lossof cell turgidity were seen. Complete collapse of columns ofcells had occurred by 24 h. In tissue that was 36-h-old, thesenescent cytoplasm of many remaining cells began to disperse.The visual evidence suggests that cell collapse was the resultof autolysis. This pattern of cortical degeneration in ricewas dissimilar to that reported elsewhere for Zea roots grownin an oxygen depleted environment. Cryo-SEM revealed the occurrence of small structures withinthe cortex with the external appearance of miniature, intactcells which are not preserved during conventional SEM preparativeprocedures. Key words: Rice (Oryza sativa L.), roots, aerenchyma, ultrastructure     

Structure and Function in the Grass Ligule: Optical and Electron Microscopy of the Membranous Ligule of Lolium temulentum L.

Aspects of the structure and ultrastructure of the membranousligule of mature leaves of Lolium temulentum L. are described.In transverse section the ligule was lens-shaped and wedge-shapedin longitudinal section, 6 or 7 cells wide near the base and1 or 2 cells wide at the edges. Two uniseriate epidermes encloseda chlorenchymatous mesophyll tissue of varying thicknesses;both epidermes were continuous with the leaf adaxial epidermis.The cells comprising these three issues all appeared like typicalgrass epidermal long cells; elongate papillate cells were presentat the edges. No stomata, trichomes, intercellular spaces orvascular tissue were found in the ligule. A marked polarizationof ultrastructural complexity existed from the large-vacuolateabaxial epidermis to the ‘densely cytoplasmic’ small-vacuolateadaxial epidermis. Cells of the latter tissue contained numerousmitochondria, hypersecretory dictyosomes and abundant strandsof rough endoplasmic reticulum. Fluorescence microscopy providedevidence for the accumulation of a polysaccharide-containingmaterial within the periplasmic space next to the outer tangentialwall of adaxial epidermal cells. The ligule is considered tobe a highly organized and differentiated leaf organ with a pholosyntheticmesophyll and an adaxial epidermis active in the synthesis ofprotein and polysaccharide. Darnel, fluorescence microscopy, ligule, Lolium temulentum L., Poaceae, ultrastructure     

Formation of Nodular Structures on Rice Seedlings by Rhizobia

Nodular structures have been induced on rice roots by treatingthe roots of 2-d-old seedlings with a cell wall degrading enzymemixture consisting of 1.0% cellulase YC, 0.1% pectolyase Y23,and 8% mannitol, followed by inoculation with rhizobia in thepresence of polyethylene glycol. Rhizobia were located bothwithin the degenerating cytoplasm of cells and between the cellsof spherical and elongate nodular structures. Although nitrogenaseactivity in such structures was at the limit of sensitivityof the assay procedure, this first report of the induction ofnodular structures on rice by rhizobia opens up the possibilityof extending effective nodulation to non-legumes, includingcereals, by facilitating the entry of rhizobia through cellwall degradation coupled with polyethylene glycol treatment. Key words: Rice (Oryza sativa L.), Rhizobium, cell wall degrading enzymes, polyethylene glycol, nodular structures, acetylene reduction     

Silicon Deposition in the Roots, Culm and Leaf of Phalaris canariensis L

Silicon deposition in the roots, culm and leaf of canary grass(Phalaris canariensis L.) was investigated using light microscopy,scanning electron microscopy and electron probe microanalysis. In adventitious roots grown in solution silicon was concentratedin four endodermal walls. Silicon was not detected in the endodermisof aerial adventitious roots, but was present in the epidermisand outer cortical cell layers. Silicon deposition in the culm mainly took place in the epidermis,and particularly in epidermal papillae. The silica deposition pattern in the leaf was typical of thesub-group Festucoideae. The leaf blade showed deposits in costalprickle hairs and wavy rods, but few intercostal deposits. Inthe ligule deposition was confined to isolated groups of pricklehairs on the abaxial surface. The major sites of silica depositionin the leaf sheath were the stomatal subsidiary cells, papillaeand intercostal idioblasts. Prickle hairs were much less commonin the sheath than the blade, and costal wavy rods appearedto be absent in the sheath. Phalaris canariensis L., canary grass, silicification, root, culm, leaf, electron probe microanalysis     

Structure and Function in the Grass Ligule: the Newly-initiated Ligule of Lolium temulentum L., a Light and Electron Microscope Study

Initiation and early development of the membranous ligule ofLolium temulentum L. was studied by light and transmission electronmicroscopy. The ligule appeared to be derived solely from theleaf adaxial epidermis and at this stage had a structure andultrastructure typical of a meristematic tissue. Cells of thefuture collar region of the leaf appear to be initiated at thesame time as the ligule. The siting of the ligule upon the leafis briefly discussed. Darnel, ligule (initiation), Lolium temulentum L., Poaceae, ultrastructure     

Biochemical Basis of Different Sensitivity to Methotrexate in Daucus carota and Oryza sativa Cell Cultures

The biochemical basis of the different sensitivity to methotrexateof Daucus carota and Oryza sativa cell cultures has been investigated.Carrot cells have a dihydrofolate reductase activity about tentimes higher than rice cells. In addition, they show a loweruptake rate of the inhibitor. No relevant differences have beenfound in the K_m value for the dihydrofolate of the two enzymesand in the degree of inhibition of their activity by methotrexate. Key words: Dihydrofolate reductase, Methotrexate resistance, Plant cell suspension cultures, Oryza sativa, Daucus carota     

Distribution of Silicified Cells in the Leaf Blades of Pleioblastus chino(Franchet et Savatier) Makino (Bambusoideae)

Distribution of silicified cells in the leaf blades of Pleioblastuschino was investigated using a light microscope and a scanningelectron microscope equipped with an energy dispersive X-raymicroanalyser. The most dense accumulation of silica was foundin epidermal tissues. Little silica was deposited in vascularbundles and chlorenchyma, while more was deposited in bundlesheath and fusoid cells. In the epidermis, silica density andfrequency of silicified cells differed depending on cell type,although silica deposition was observed in most cell types.Heavy deposition was found in silica cells, bulliform cells,micro hairs and prickle hairs. Silica cells were the cell typemost frequently silicified (96.9–99.7%) in the adaxialand abaxial epidermis. Silica may be deposited as leaf tissuesage.Copyright 2000 Annals of Botany Company Pleioblastus chino(Franchet et Savatier) Makino, bamboo, silicified cells, leaf blade, epidermis, chlorenchyma, silica, clearing method, freeze-fracturing, freeze-drying, light microscopy, scanning electron microscopy, X-ray microanalysis     

The Distribution of Silicon Deposits in the Fronds of Pteridium aquilinum L.

The occurrence of silicon in mature fronds of Pteridium aquilinumwas investigated using light and scanning electron microscopyand X-ray micro analysis. The heaviest deposits were locatedin the outer tangential walls of the epidermis of the petiole,but the upper epidermal cells of the costal regions of the laminaealso accumulated small quantities. No discrete bodies were associatedwith these deposits. However, the mid-ribs of the laminae exhibitedsome long siliceous fibres associated with the luminae of thestrengthening tissues. The significance of these accumulationsin relation to deposition mechanisms and possible carcinogeniceffects are discussed. Pteridium aquilinum L., bracken, frond, microanalysis, silica     

Aerenchyma (Gas-space) Formation in Adventitious Roots of Rice (Oryza sativa L.) is not Controlled by Ethylene or Small Partial Pressures of Oxygen

Jackson, M. B., Fenning, T. M., and Jenkins, W. 1985 Aerenchyma(gas-space) formation in adventitious roots of rice (Oryza sativaL.) is not controlled by ethylene or small partial pressuresof oxygen.—J. exp. Bot. 36: 1566–1572. The extent of gas-filled voids (aerenchyma) within the cortexof adventitious roots of vegetative rice plants (Oryza sativaL. cv. RB3) was estimated microscopically from transverse sectionswith the aid of a computer-linked digitizer drawing board. Gas-spacewas detectable in 1-d-old tissue and increased in extent withage. After 7 d, approximately 70% of the cortex had degeneratedto form aerenchyma. The extent of the voids in 1-4-d-old tissuewas not increased by stagnant, poorly-aerated external environmentscharacterized by sub-ambient oxygen partial pressures and accumulationsof carbon dioxide and ethylene. Treatment with small oxygenpartial pressures, or with carbon dioxide or ethylene appliedin vigorously stirred nutrient solution also failed to promotethe formation of cortical gas-space. Furthermore, ethylene productionby rice roots was slowed by small oxygen partial pressures typicalof stagnant conditions. Silver nitrate, an inhibitor of ethylene action, did not retardgas-space formation; similarly when endogenous ethylene productionwas inhibited by the application of aminoethoxyvinylglycine(A VG), aerenchyma development continued unabated. Cobalt chloride,another presumed inhibitor of ethylene biosynthesis, did notimpair formation of the gas in rice roots nor did it decreasethe extent of aerenchyma even if A VG was supplied simultaneously.These results contrast with those obtained earlier using rootsof Zea mays L. We conclude that in rice, aerenchyma forms speedily even inwell-aerated environments as an integral part of ordinary rootdevelopment There seems to be little or no requirement for ethyleneas a stimulus in stagnant root-environments where aerenchymais likely to increase the probability of survival. Key words: Rice (Oryza sativa L.), ethylene, flooding, aeration, aerenchyma, environmental stress     

Silica and Ash in Tissues of Some Coastal Plants

Ash and silica contents and their depositional patterns in differenttissues of 44 Mississippi coastal plants were determined. Silicacontent of dried plants varied from no more than a trace inChenopodium album L. leaves to 7.37 per cent in Zizanopsis miliacea(Michx) Doell & Aschers leaves. Ash content varied from2.50 per cent in Lythrum lineare L. stems to 28.24 per centin Borrichia frutescens (L.) DC leaves. Plants in the same familytend to be alike in their ability to absorb or not absorb silica.Poaceae and Cyperaceae had consistently high concentrationsof silica. In contrast, the Asteraceae studied had very lowsilica contents but often had high contents of other minerals.Dicotyledonous plants studied had consistently lower silicacontents than the monocotyledons. Plants growing in salt watercontained considerable sodium chloride. Spectra were obtainedfor major elements in four different plants. Energy-dispersiveX-ray analysis shows that distribution of the element siliconis clearly related to certain epidermal structures such as guardcells, ridges, dumb-bells and balls that appear in electronmicrographs. Silica was deposited differently in each type ofplant studied. In many of the plants silica was deposited inrows of irregular-shaped particles running lengthwise of theleaf and in guard cells. In others, like Zizanopsis miliacea(Michx) Doell & Aschers, the deposit was sheet-like. Zizaniaaquatica L. not only had a sheet-like deposit, but the depositwas ridged and there were rows of dumb-bell-shaped silica cells.Related plants had similar structures. Euchlaena mexicana Schrad.,Tripsacum dactyloides (L.) L and Manisuris rugosa (Nutt.) Kuntzeall had irregular phytoliths similar to those in Zea mays L. coastal plants, marsh plants, ash content, silica deposition, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, X-ray diffraction patterns, spectra of elements in plants     

Coleoptile Senescence in Rice (Oryza sativa L.)

We investigated the cellular events associated with cell deathin the coleoptile of rice plants (Oryza sativa L.). Seeds germinatedunder submergence produced coleoptiles that were more elongatedthan those grown under aerobic conditions. Transfer of seedlingsto aerobic conditions was associated with coleoptile opening(i.e. splitting) due to death of specific cells in the sideof the organ. Another type of cell death occurred in the formationof lysigenous aerenchyma. Senescence of the coleoptile was alsonoted, during which discolouration of the chlorophyll and tissuebrowning were apparent. DNA fragmentation was observed by deoxynucleotidyltransferase-mediateddUTP nick end labelling (TUNEL) assay, and further confirmedby the appearance of oligonucleosomal DNA ladders in senescentcoleoptile cells. Two nucleases (Nuc-a and Nuc-b) were detectedby in-gel-assay from proteins isolated from coleoptiles. Nuc-a,commonly observed in three cell death phases required eitherCa²⁺or Mg²⁺, whereas Nuc-b which appeared during senescencerequired both Ca²⁺and Mg²⁺. Both nucleases were strongly inhibitedby Zn²⁺. Copyright 2000 Annals of Botany Company Aerenchyma, rice, cell death, coleoptile, fragmentation, nuclease, Oryza sativa, senescence, split, submergence, TUNEL     

Photosynthesis and Transpiration in Rice as Influenced by Soil Moisture and Air Humidity

The paper describes the effect of soil moisture content andair humidity on CO₂ exchange (P_N), CO₂ diffusion resistance(Cr) and transpiration (E) in four varieties of japonica rice(Oryza sativa L.). A decrease in soil moisture content reducedthe rate of photosynthesis to a varying degree in the varieties.Reduction in photosynthesis was attributed to increase in Cr.The effect of low soil moisture on photosynthesis and CO₂ diffusionwas further intensified by decrease in air humidity. By maintaininga high humidity in the air around the leaves however, the effectof soil moisture deficiency was reduced considerably, exceptin Rikuto Norin 21 which was very sensitive to soil-moisturedeficiency alone. Dryness of the air enhanced the transpirationrate, although the increase was relatively less in the plantsfacing a simultaneous water crisis at the root surface. In plantsgrowing under flooded conditions, a decrease in air humiditycaused a slight depression in P_N despite the simultaneous decreasein Cr. Oryza sativa L., rice, photosynthesis, transpiration, diffusion resistance, soil moisture, air humidity     

Utilization of Stored Nitrate during the Anaerobic Germination of Rice Seeds

The presence of nitrate in the seed and its utilization duringanaerobic germination of rice (Oryza sativa L.) were studied.The results indicate that nitrate was reduced and assimilatedby the coleoptile for 11 days during anaerobic growth aftergermination. The importance of the anoxic utilization of nitrateis discussed. (Received June 19, 1992; Accepted January 4, 1993)     

Silica and Ash Content and Depositional Patterns in Tissues of Mature Zea mays L. Plants

Ash and silica content and their depositional patterns in differenttissues of the mature corn plant (Zea mays L.) were determined.Ash and silica were highest in the leaf blades (up to 16.6 and10.9 per cent, respectively) followed by the leaf sheath, tassel,roots, stem epidermis and pith, and ear husk. The percentageof ash as silica was also highest in the leaves. Silica wasextremely low in the kernels. The upper stem epidermis and pithcontained nearly twice the silica content as did the lower portion.The patterns of ash and silica distribution were similar inplants grown in two different areas of Kansas, but were in lowerconcentration in the leaves and leaf sheaths from the area withlower soluble silica in the soil. Silica was deposited in theepidermis in a continuous matrix with cell walls showing serratedinterlocking margins in both leaves and stem. Rows of lobedphytoliths of denser silica were found in the epidermis as wellas highly silicified guard cells and trichomes. The silica matrixof the epidermis appears smooth on the outer surface and porousor spongy on the inner surface. Zea mays L. Corn, maize, ash content, silica deposition, scanning electron microscopy