Silica deposition in abaxial epidermis before the opening of leaf blades of Pleioblastus chino (Poaceae, Bambusoideae)

BACKGROUND AND AIMS Silica deposition is one of the important characteristics of the family Poaceae. The distribution, deposition process and physiology of silica in this family have been extensively investigated. Bamboos among members of Poaceae have leaves with a fairly long life span, and the leaves continuously accumulate silica in their tissues throughout their life, not only during the course of leaf opening, but also after opening. It has been revealed that the silica deposition process in relation to ageing of the bamboo leaf after opening differed depending on the cell types comprising the tissues. However, silica deposition has never been examined during the development and maturation periods of bamboo leaves. Hence, to clarify the silica deposition process in a developmental stage of the bamboo leaf, distribution of silica was observed in the abaxial epidermis before the opening of the leaf blades of Pleioblastus chino. METHODS: Abaxial epidermal tissues of leaves were examined using a scanning electron microscope equipped with an energy dispersive X-ray microanalyser. KEY RESULTS: Among seven cell types comprising the abaxial epidermis, three types of cells, guard cells, prickle hairs and silica cells, deposited silica conspicuously, and another four types, cork cells, long cells, micro hairs and subsidiary cells, deposited only a little silica. Among the former group of cell types, silica cells and guard cells deposited silica over their entire surfaces, while prickle hairs deposited silica only in the point-tips. Silica deposition was detected firstly in prickle hairs, and then in silica cells and guard cells. Only silica cells were assumed to deposit silica conspicuously before leaf opening but not conspicuously after opening. CONCLUSIONS: Cell types in leaf epidermis of bamboo are classified into three groups according to the silica deposition pattern. Silica deposition in silica cells may be positive as a part of the physiological activities of leaves.     

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     

Silica deposition in relation to ageing of leaf tissues in Sasa veitchii (Carriere) Rehder (Poaceae: Bambusoideae)

BACKGROUND AND AIMS: Silica deposition is one of the important characteristics of plants in the family Poaceae. There have been many investigations into the distribution, deposition and physiological functions of silica in this family. Two hypotheses on silica deposition have been proposed based on these studies. First, that silica deposition occurs passively as a result of water uptake by plants, and second, that silica deposition is controlled positively by plants. To test these two apparently contradictory hypotheses, silica deposition in relation to the ageing of leaf tissues in Sasa veitchii was investigated. METHODS: Tissues were examined using a light microscope and a scanning electron microscope equipped with an energy dispersive X-ray microanalyser. KEY RESULTS: The deposition process differed depending on cell type. In mesophyll tissue, fusoid cells deposited large amounts of silica depending on leaf age after maturation, while chlorenchyma cells deposited little. In epidermal tissue, comprised of eight cell types, only silica cells deposited large amounts of silica during the leaf's developmental process and none after maturation. Bulliform cells, micro-hairs and prickle hairs deposited silica densely and continuously after leaf maturation. Cork cells, guard cells, long cells and subsidiary cells deposited silica at low levels. CONCLUSIONS: The significance of these observations is discussed in relation to the two hypotheses proposed for silica deposition in Poaceae. The results of the present study clearly indicate that both hypotheses are compatible with each other dependent on cell types.     

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 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     

An Ultrastructural Study on the Development of Silicified Tissues in the Leaf Tip of Barley (Hordeum sativum Jess)

The development of silicified tissues in the tip of the fourthand fifth leaf of 7-week-old barley plants grown in nutrientsolution was investigated by transmission electron microscopy.The pattern of silicification was similar in the epidermal cells,sclerenchyma cells and intercellular spaces. Silica granulesfirst appeared around the periphery and eventually filled theentire cell lumen or space. The granules then polymerized toform a solid mass. The walls of the sclerenchyma cells alsobecame silicified but to a less extent than those of the epidermalcells, in which silicification sometimes occurred prior to thatin the lumen. The results are discussed in relation to the chemistry of silicaand possible mechanisms of silicification. Hordeum sativum Jess, barley, silica bodies, opalines, epidermal cells, sclerenchyma, ultrastructure     

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     

Epidermal Structure in the Ligule of Rice (Oryza sativa L.)

The structure of the abaxial epidermis of the ligule of rice(Oryza sativa L.) as seen in the light and scanning electronmicroscope is described. Long cells, silica cells, cork cells,prickle hairs, microhairs and stomata-like structures were found.Reasons for considering these latter structures to be true stomataare given. Results of an X-ray analysis for silicon are presentedand the distribution of silica discussed in relation to itsfunctional role in the ligule. Oryza sativa L, rice, Gramineae, ligule, epidermis, scanning electron microscopy, X-ray analysis, silica, stomata     

Silicon distribution and accumulation in shoot tissue of the tropical forage grass Brachiaria brizantha

Silicon (Si) accumulation in organs and cells is one of the most prominent characteristics of plants of the family Poaceae. Many species from this family are used as forage plants for animal feeding. The present study investigates in Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. cv. Marandu: (1) the dry matter production and Si content in shoot due to soil Si fertilizations; (2) the Si distribution among shoot parts; and (3) the silica deposition and localization in leaves. Plants of B. brizantha cv. Marandu were grown under contrasting Si supplies in soil and nutrient solution. Silica deposition and distribution in grass leaf blades were observed using light microscope and scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDXS). Silicon concentration in the B. brizantha shoot increased according to the Si supply. Silicon in grass leaves decreased following the order: mature leaf blades > recently expanded leaf blades > non-expanded leaf blades. Silicon accumulates mainly on the upper (adaxial) epidermis of the grass leaf blades and, especially, on the bulliform cells. The Si distribution on adaxial leaf blade surface is non uniform and reflects a silica deposition exclusively on the cell wall of bulliform cells.     

Cellular Structure and Light Absorbtion in Leaves of Frithia pulchra (Mesembryanthemaceae)

In order to quantify the structural differences between celltypes of leaves from a ‘ window’ plant, an ultrastructuralmorphometric analysis was made of the epidermal, window andchlorenchyma tissues of Frithia pulchra. Epidermal cells arethe largest cells found in Frithia leaves and are characterizedby the presence of a thick outer tangential cell wall and numerousvacuolar inclusions. Epidermal tissue has an optical densityof 0.30. The transparent window tissue (i.e. optical density= 0.08) has a uniform ultrastructure throughout the length ofthe leaf. The vacuome comprises aproximately 97 per cent ofthe protoplasmic volume of window cells. Chlorenchyma cellspossess thin cell walls and are surrounded by numerous intercellularspaces. Cells of the apical chlorenchyma tissue possess approximately30 plastids per cell. These chloroplasts have an average individualvolume of 220 µm². Cells of the basal chlorenchyma tissuecontain chloroplasts that are five to six times smaller andmore numerous than those in cells of the apical chlorenchyma.The increased volume of chloroplasts in the apical comparedwith basal chlorenchyma cells (i.e. 31.4 and 20.2 per cent ofthe protoplasm, respectively) is positively correlated withtheir optical densities of 1.46 and 0.97, respectively. Frithia pulchra, stereology, leaf, light absorption, window plant     

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     

Intracellular Silica Deposition in Immature Leaves in Three Species of the Gramineae

The formation of solid, discrete deposits of opaline silicawithin the cell lumen of leaf tissues is reported in speciesrepresenting three subfamilies of the Gramineae; the preparationof a silica-minimal nutrient solution is discussed. Opal phytolithnumbers are related to tissue age and to two external silicaconcentrations for tiller leaves over a period of 32 days followingbud initiation. Variations in silica deposition patterns among the individualleaves of a homologous series on the shoot apex are relatedto differential growth-rates. During ontogeny, deposition occursin an ordered sequence of cell types related to the basipetalmaturation gradient within the leaf. Initial deposition wasdetected in silica cells (idioblasts) of imbricated, bud leavesof Sieglingia decumbens, 1 mm long; phytoliths are confinedto these cells in expanding, basal portions of the leaf. Phytolithcounts/sq mm of epidermis vary with the degree of long-cellexpansion. The error is reduced by expressing silicificationas the number of phytoliths/100 silica cells. Post-expansiondeposition was initiated in epidermal long cells of the leafblade tip. The higher silica concentration resulted in a morerapid utilization of available deposition sites and larger phytoliths(P = 0.001). A passive cell influx of silicic acid is discussedin relation to cytodifferentiation.     

Relationships between photosynthetic activity and silica accumulation with ages of leaf in Sasa veitchii (Poaceae, Bambusoideae)

BACKGROUND AND AIMS: Bamboos have long-lived, evergreen leaves that continue to accumulate silica throughout their life. Silica accumulation has been suggested to suppress their photosynthetic activity. However, nitrogen content per unit leaf area (N(area)), an important determinant of maximum photosynthetic capacity per unit leaf area (P(max)), decreases as leaves age and senescence. In many species, P(max) decreases in parallel with the leaf nitrogen content. It is hypothesized that if silica accumulation affects photosynthesis, then P(max) would decrease faster than N(area), leading to a decrease in photosynthetic rate per unit leaf nitrogen (photosynthetic nitrogen use efficiency, PNUE) with increasing silica content in leaves. METHODS: The hypothesis was tested in leaves of Sasa veitchii, which have a life span of 2 years and accumulate silica up to 41 % of dry mass. Seasonal changes in P(max), stomatal conductance, N(area) and silica content were measured for leaves of different ages. KEY RESULTS: Although P(max) and PNUE were negatively related with silica content across leaves of different ages, the relationship between PNUE and silica differed depending on leaf age. In second-year leaves, PNUE was almost constant although there was a large increase in silica content, suggesting that leaf nitrogen was a primary factor determining the variation in P(max) and that silica accumulation did not affect photosynthesis. PNUE was strongly and negatively correlated with silica content in third-year leaves, suggesting that silica accumulation affected photosynthesis of older leaves. CONCLUSIONS: Silica accumulation in long-lived leaves of bamboo did not affect photosynthesis when the silica concentration of a leaf was less than 25 % of dry mass. Silica may be actively transported to epidermal cells rather than chlorenchyma cells, avoiding inhibition of CO2 diffusion from the intercellular space to chloroplasts. However, in older leaves with a larger silica content, silica was also deposited in chlorenchyma cells, which may relate to the decrease in PNUE.     

An Ultrastructural Study on the Developmental Phases and Silicification of the Glumes of Phalaris canariensis L.

In the glume of Phalaris canariensis L. silicon deposition takesplace in the macrohairs, papillae, prickle hairs and silicacells of the abaxial epidermis before panicle emergence. Early in their development the macrohairs have large vacuolesand thin walls. At maturity the walls become thickened and aremajor sites of silica deposition. Dry ashing reveals a helicalpattern within the hair walls. Distinct papillae and prickle hairs were first observed oneweek before panicle emergence. Here silicification was initiallyconfined to outer tangential walls, but by two weeks after emergencetheir cytoplasmic contents had broken down, and the lumina werefilled with siliceous granules. Cork-silica twin cells werealso present in the abaxial epidermis. By panicle emergencethe silica cells were infilled, but the cork cells retainedtheir cytoplasmic contents. The long cells of the abaxial epidermiswere initially thin walled, but thickening occurred in the outertangential wall, this being complete by one week after emergence.These cells remained relatively unsilicified throughout. After panicle emergence the adaxial epidermal cells, and theirassociated parenchyma cell layers began to lose their cellularcontents and collapse. This process was complete two weeks afteremergence when the collapsed walls formed a thin internal layerbetween the two epidermi. Electron opaque granular material,containing several elements, but predominantly calcium, waspresent between the collapsed cell walls. The results are comparedwith those for the lemma, and silica deposition mechanisms arealso discussed. Phalaris canariensis L., canary grass, silicification, trichome, glume, ultrastructure     

Tissue Partitioning During Leaf Development in Ornamentally-grown Frithia pulchra (Mesembryanthemaceae), a 'Window Plant'

Young (i.e. 5-mm long) leaves of the window-plant Frithia pulchra(Mesembryanthemaceae) allocate approx, 21 % of their volumeto epidermis, 49 % to chlorenchyma, and 29 % to window tissue.By the time leaves are 25 mm long, the relative volumes of epidermisand chlorenchyma decrease to approx, 7 and 27 % respectively.During the same period, the relative volume of window tissueincreases from 29 to 66 %. The relative volumes of epidermis,window, and chlorenchyma tissues do not change as leaf lengthincreases from 25 to 57 mm. These results indicate that earlystages of leaf development in F. pulchra involve preferentialreallocations of volume to different tissues, whereas laterstages of leaf development involve uniform expansion of allof the leaf's tissues (i.e. the relative volumes of tissuesdo not change). The relative volumes of epidermis and windoware always largest in the lower third of a leaf. The relativevolume of chlorenchyma is largest in the upper third of youngleaves but becomes constant in the upper two thirds of leavesduring later stages of development. These results, indicatingthat leaves and tissues of F. pulchra are asymmetric and developpolarly, are discussed relative to corresponding studies ofcellular size and leaf structure. Frithia pulchra, leaf development, Mesembryanthemaceae, stereology, window plant     

Silica Deposition in Some C3 and C4 Species of Grasses, Sedges and Composites in the USA

We report new information on silica deposition in 15 plant species,including nine grasses, two sedges and four composites. Thesilica depositional patterns found in seven of the grass speciesindicate that they are C₄ plants. However the festucoid grassCortaderia selloana is a C₃ plant with long leaf trichomes andoval silica structures in the leaves. In contrast the panicoidC₄ grasses Chasmathium latifolium, Chasmathium sessiflorum,Imperata cylindrica, Panicum repens, Panicum commutatum andSetaria magna, all produce dumb-bell-shaped silica structuresin the leaves. The chloridoid grasses Spartina patens and Spartinacynosuroides have saddle-shaped structures and no dumb-bellor oval shaped ones. The sedges Rhynchospora plumosa and Scirpuscyperinus were found to have oval phytoliths and may be C₃ plants.Our examination of these and other grasses strongly suggeststhat C₄ grasses tend to produce the same type of silica cells.Grasses and sedges with C₃ type photosynthesis tend to produceoval silica structures. The composite Grindelia squarrosa andsunflowers Helianthus angustifolia, Helianthus atrorubens andHelianthus tuberosus absorb relatively small amounts of siliconand larger amounts of calcium, where both elements deposit inleaf trichomes. We found no clear indicator for the C₃ sunflowersor C₄ types in the Asteraceae. Helianthus tuberosus leaves havemany trichomes on the adaxial surface. These trichomes havea higher concentration of silica than the surrounding leaf surface.Helianthus tuberosus leaves had much higher ash and silica contentsthan those of Helianthus angustifolia and Helianthus atrorubens.The composite Grindelia squarrosa has a usual deposition ofsilica in the basal cells around the guard cells. Silica depositionoften reflects the surface features of a leaf. An exceptionis Scripus cyperinus where the silica structures are deep inthe tissue and do not reflect the surface configurations. Theinforescence of Setaria magna had a 14.64 silica content. Thetufts of white, silky hairs characteristic of Imperata cylindricainflorescence have no silica. C₃ and C₄ plants, silica and ash content, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, spectra of elements in plants, trichomes, silica fibres, phytoliths     

Histological Changes of the Peanut (Arachis hypogaea) Gynophore and Fruit Surface During Development, and their Potential Significance for Nutrient Uptake

Histological changes in gynophores and fruits of Arachis hypogaeaL.cv. White Spanish were examined, utilizing scanning electronmicroscopy as well as light microscopy. The epidermis of theabove ground parts of gynophores is characterized by the presenceof stomata, lenticels and multicellular trichomes. Below groundportions of the same plant organ exhibit unicellular root-hair-likestructures. These protuberances of the epidermal cells can reacha very high density and length (up to 0.75mm) . Identical structurescan be found on the developing pod and are most prominent atthe reproductive stages R5-R6. In later developmental stagesthe hairs degenerate and the presence of large lenticels becomesthe obvious external feature of the pod. It is suggested thatthe substantial increase in surface area due to the hairs maywell be an anatomical adaptation for nutrient and water uptake. Arachis hypogaea, peanut fruit development, nutrient uptake     

Identification of a 41 kDa Protein Embedded in the Biosilica of Scales and Bristles Isolatedfrom Mallomonas splendens (Synurophyceae, Ochrophyta)

Cells of the photosynthetic protist Mallomonas splendens (Synurophyceae, Ochrophyta) are encased within a highly patterned wall or scale case that consists of silicified scales and bristles. In an effort to understand the mechanisms that unicellular protists utilize to produce elaborate, mineralized structures of great complexity and hierarchical structure, we identified and characterized a 41 kDa protein from purified scales/bristles isolated from M. splendens (SP41 for Scale Protein of 41 kDa). A cDNA encoding this protein was isolated and sequence analysis indicated that it is a novel protein. Polyclonal antibodies were generated against bacterially expressed SP41 and used to localize the protein throughout scale and bristle morphogenesis. Immunoelectron microscopy confirmed the biochemical data that SP41 is a component of mature scales and bristles, the protein localizing to silicified components of the purified extracellular matrix. During scale and bristle biogenesis within the cell, SP41 is deposited into a specialized Silica Deposition Vesicle (SDV) concomitant with silica deposition, a highly regulated event during scale and bristle formation. These results argue for SP41 playing a role in morphogenesis and/or silicification within the SDV during scale and bristle biogenesis.     

Structure and Function of Silica Bodies in the Epidermal System of Grass Shoots

Silica (SiO₂.nH₂O) is deposited in large quantities in the shootsystems of grasses. In the leaf epidermal system, it is incorporatedinto the cell wall matrix, primarily of outer epidermal walls,and within the lumena of some types of epidermal cells. This biogenic silica can be stained specifically with methylred, crystal violet lactone, and silver amine chromate. At theultrastructural level, the silica in lumens of silica cells,bulliform cells and long epidermal cells is made up of rodsabout 2.5 µm in length and 0.4µm in width. Ultimateparticles in the rods range from 1 to 2 nm in diameter. In contrast,silica in the cell wall matrix of trichomes and outer wallsof long epidermal cells is not rod-shaped, but rather, formsroughly spherical masses. Detailed analyses are presented on the frequencies of occurrenceof the different types of epidermal cells that contain silicain the leaves of representative C₃ and C₄ grasses. The C₄ grasseshave higher frequencies of bulliform cell clusters, silica cells,and long epidermal cells, whereas the C₃ grasses have higherfrequencies of trichomes. No correlation was found in the frequencyof occurrence of silica bodies in bulliform cells for C₃ grassesas compared with C₄ grasses. Of all the grasses examined, Coix,Oryza, and Eleusine had the highest densities of such bodies,and some taxa had no silica bodies apparent in their bulliformcells. The idea that silica bodies in bulliform cells and silica cellsmight act as "windows’ and trichomes might function as‘light pipes’ to facilitate light transmission throughthe epidermal system to photosynthetic mesophyll tissue belowwas tested. The experimental data presented do not support eitherof these hypotheses. C₂ and C₄ grasses, biogenic silica, light pipes, window hypothesis, silica staining, silica ultrastructure