ULTRASTRUCTURE OF THE MATURE UNGERMINATED RICE (ORYZA SATIVA) CARYOPSIS. THE CARYOPSIS COAT AND THE ALEURONE CELLS

The results of a light and electron microscopic study of the caryopsis coat and aleurone cells in ungerminated, unimbibed rice (Oryza sativa) caryopses are presented. Surrounding the rice grain is the caryopsis coat composed of the pericarp, seed coat and nucellar layers. The outermost layer, the pericarp, consists of crushed cells and is about 10 μm thick. The seed coat, interior to the pericarp, is one cell thick and has a thick cuticle. Between the seed coat cuticle and endosperm are the remains of the nucellus. The nucellus is about 2.5 μm thick and has a thick cuticle adjacent to the seed coat cuticle. Interior to the caryopsis coat is the aleurone layer of the endosperm. The aleurone completely surrounds the rice grain and is composed of two cell types—aleurone cells that surround the starchy endosperm and modified aleurone cells that surround the germ. The aleurone cells of the starchy endosperm contain many aleurone grains and lipid bodies around a centrally located nucleus. The modified aleurone cells lack aleurone grains, have fewer lipid bodies than the other aleurone cells, and contain filament bundles (fibrils). Plastids of aleurone cells exhibit a unique morphology in which the outer membranes invaginate to form tubules and vesicles within the plastid. Transfer aleurone cells are not observed in the mature rice caryopsis.     

A comparative study into the chemical constitution of cutins and suberins from Picea abies (L.) Karst., Quercus robur L., and Fagus sylvatica L.

The compositions of BF₃/CH₃OH depolymerisates of cutins and suberins from leaf and periderm samples from Picea abies [L.] Karst., Quercus robur L., and Fagus sylvatica L., respectively, were determined by quantitative capillary gas chromatography/mass spectroscopy. Long-chain monobasic, -hydroxymonobasic, dihydroxymonobasic, trihydroxymonobasic and epoxyhydroxymonobasic alkanoic acids constituted the major aliphatic monomers of leaf cutins. The total amounts of cutin monomers ranged from 629 mg · m^–2 (Fagus) to 1350 mg · m^–2 (Quercus). Cutin composition and amounts did not significantly differ between current year and three-year-old needles of Picea. Trans-esterification of periderm samples yielded a much greater variety of aliphatic monomers than obtained from cutins. In addition to the substance classes found with cutins, suberin depolymerisates also contained , -dibasic acids while dihydroxymonobasic acids were lacking. Depolymerisates from periderms taken from different locations on a Picea tree did not differ significantly in their relative composition. The results are discussed in terms of the distinctive characteristics of the aliphatic portions of cutins and suberins, respectively. Discriminant analysis is applied for formulating a quantitative and inarbitrary classification rule for cutins and suberins. The precision, statistical significance and robustness of this classification rule are tested by employing it to a large set of compositional data (70 plant species) from the literature. The relevance of data obtained by depolymerization methods for elucidating the physical structure of cutins and suberins in situ is evaluated.To whom correspondence should be addressedThe authors are indebted to Drs. J. Winkler and H. Krause (Laboratorium für Strukturchemie des Fachbereichs Chemie, Biologie und Geowissenschaften, Technische Universität München, Garching, FRG) for performing capillary gas chromatography-mass spectrometry and their valuable help in the identification of cutin and suberin constituents. The work was supported by grants from the Deutsche Forschungsgemeinschaft and the Bayerisches Staatsministerium für Unterricht, Kultus, Wissenschaft und Kunst.     

Composition,ultrastructure and function of the cutin- and suberin-containing layers in the leaf,fruit peel,juice-sac and inner seed coat of grapefruit (Citrus paradisi Macfed.)

Cutin and suberin polymers from various anatomical regions of grapefruit were analyzed chemically and ultrastructurally. The leaf, fruit peel and juice-sac showed an amorphous cuticular layer. The cutin in the leaf was composed of 10,16-dihydroxy C₁₆ acid and its positional isomers as the major monomers whereas 16-hydroxy-10-oxo C₁₆ acid was a major component in the fruit peel. Juice-sac cutin, on the other hand, contained the dihydroxy C₁₆ acids, hydroxyoxo C₁₆ acids, hydroxyepoxy C₁₈ acids and trihydroxy C₁₈ acids. Ultrastructural examination of the inner seed coat showed that an amorphous cuticular layer encircled the entire seed except in the chalazal region which showed several layers of cells with lamellar suberin structure throughout the cell walls. Consistent with the ultrastructural assignment, the compositions of the aliphatic components of the polymers from the chalazal region and the non-chalazal region indicated the presence of suberin and cutin, respectively. The aliphatic portion of the polymer from the chalazal region of the inner seed coat contained C₁₆, C_18:1, C₂₂ and C₂₄ -hydroxy acids (46% combined total) and the corresponding dicarboxylic acids (43%) as the major components. -Hydroxy-9,10-epoxy C₁₈ acids and 9,10,18-trihydroxy C₁₈ acids were the major components (77%) of the polymer from the non-chalazal portion of the inner seed coat. The main portion and the chalazal region of the inner seed coat yielded 17 and 342 g/cm² of aliphatic monomers, respectively, and the diffusion resistance of these two portions of the inner seed coat were 62 and 192 sec/cm, respectively. The inner seed coat was shown to be the major moisture diffusion barrier influencing imbibition and germination.Scientific Paper No. 5649, Project 2001, College of Agriculture Research Center, Washington State University, Pullman, Washington 99164     

TRANSFER CELLS IN THE PLACENTAL PAD AND CARYOPSIS COAT OF PAPPOPHORUM SUBBULBOSUM ARECH. (POACEAE)

During caryopsis development the layers of the pericarp, integuments, and nucellus all contribute to the formation of the caryopsis coat. The coat consists of a layer of outer pericarp epidermal transfer cells, a collapsed and senescent layer of middle pericarp cells, and a discontinuous layer of inner pericarp epidermal transfer cells. The latter is not present across the placental pad. The integuments are present as a collapsed dense layer, the nucellus is discontinuous and cellular. The placental pad occurs at the ventral surface of the caryopsis, opposite the scutellum and coleorhiza. It consists of 15–20 collapsed cell layers, including the pigment strand and placental vascular bundle. From the inside several partially collapsed cell layers of the nucellar projection occur which contain transfer-cell walls. The middle dense layers, the pigment strand, consist of the middle pericarp remnant, plus the remains of the placental vascular bundle. The pericarp inner epidermis does not extend across the pad. The aleurone layer is a continuous uniseriate layer around the entire caryopsis except at the placental pad; here it is crushed and contains the remnant of a transfer-cell wall. The outer pericarp epidermis is a continuous layer of transfer cells across the pad. These cells contain membranous inclusions suggesting that they may be functional during germination.     

Floral morphology of the family compositae

Vernonia anthelmintica (Linn.) Willd., tribe Vernonieae, family Compositae has been studied. The cypsela has dimorphic pappus, 20 longitudinal ribs (10 primary and 10 secondary), and two types of hairs developing from single epidermal cells on surface. Certain features of vasculature of floret observed in this taxon have been considered as intermediate between relatively primitive and highly advanced conditions known in the family. Others elucidate further that the ovary is bilocular with axile placentation at base in which region the ovule is attached, and unilocular with parietal placentation in the remaining portion, and also that the ovule possibly belongs to one of the two carpels, which alone is fertile, and is an organ morphologically double in nature. The pappus has been regarded as a longitudinally dismembered calyx tube. The tissue of the ovule outside nucellus differentiates into an endothelium, a periendothelial zone, and an outer zone. In the mature seed the thickned outer epidermis and remmants of a few subjacent layers form the outer zone, and endothelium redued to a pellicle, the inner zone of the seed coat. The jacket layer of the endosperm persists between the seed coat and the dicotyledonous, orthorrhizal embryo. The pericarp development resembles that of most other Compositae in an early differentiation of the ovary wall into two zones, and later breakdown of a major part of the inner zone. But it is quite characteristic in other respects, especially, the peripheral chin of firovascular bundles which forms its main mechanical zone.     

Ovule and seed ontogeny inGnetum gnemon L.

The ovule ofGnetum gnemon has three envelopes around the nucellus. The outer one forms two clear swellings at the lateral sides during the early developmental stages. The middle envelope also shows two swellings in many cases arranged decussately with respect to those of the outer one. All these swellings become obscure or disappear later. The inner envelope arises as an annular primordium and forms several manifest lobes at the stage of pollination. it develops two proliferating structures from its middle portion, viz., a flange and a micropyle-closing tissue. The three envelopes differentiatiate into the fleshy outer, sclerenchymatic middle and compressed inner layers of the seed coat, respectively. The inner one, however, remains restricted to the apical part of the seed owing to endochalazal growth. The outer envelope is derived from both dermal and subdermal cells of the ovule primordium and, therefore, is of dual origin. The middle and inner envelopes are subdermal in origin. The present study has cleared up some conflicting reports in the previous publications.     

Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.)

Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element — companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element — companion cell complexes and the endosperm, and this is discussed.     

Seed coat development in Leucospermum cordifolium (Knight) Fourcade (Proteaceae) and a clarification of the seed covering structures in Proteaceae

MANNING, J. C. & BRITS, G. J., 1993. Seed coat development in Leucospermum cordifolium (Knight) Fourcade (Proteaceae) and a clarification of the seed covering structures in Proteaceae . The development of the seed coat and pericarp is studied in Leucospermum cordifolium from ovule to mature seed. The ovule and seed are characterized by a tegmic pachychalaza. The pericarp is adnate to the integuments from anthesis and remains unthickened to maturity. The outer integument forms the seed coat and the seed is endotestal: the outer epidermis becomes tanniniferous and the inner epidermis develops into a crystalliferous palisade. The inner integument degenerates at an early stage. Examination of the literature reveals that the crystal palisade layer of the outer integument has been erroneously assumed to constitute an endocarp. This finding indicates that a re-interpretation of all published information on the seed coat in indehiscent Proteaceae is necessary before any speculations on the phylogenetic significance of the seed coat can be entertained.     

Water-soluble phenolic compounds in the coat control germination and peroxidase reactivation in <Emphasis Type="Italic">Triticum aestivum</Emphasis> seeds

In this work, we investigated the inhibitory effects of water-soluble phenolic compounds (WSPCs) in the coat of after-ripening wheat (Triticum aestivum L.) seeds on the processes of germination and peroxidase reactivation. Wheat bran has a WSPC content of 862.5 μg gallic acid equivalent g⁻¹ dry weight. When seeds were incubated in the water extract of bran, germination, peroxidase reactivation, and coleoptile and radicle growth were suppressed in a WSPC concentration-dependent manner. The inhibitory effects were significantly ameliorated by removing WSPCs from bran extract by treating with 1% insoluble polyvinylpolypyrrolidone. Pretreatment of seeds with 0.1% H₂O₂ reduced the WSPC content in the coat, which was confirmed using Fourier transform infrared microspectroscopy. With H₂O₂ pretreatment, seed germination, peroxidase reactivation, and post-germination seedling growth were significantly stimulated. Application of the known phenolics caffeic acid, feruic acid, or vanillin to the germination medium blocked seed germination and suppressed peroxidase reactivation. The results described here indicate that WSPCs act as endogenous inhibitors in the coat to control germination of Triticum aestivum seeds, and that inhibition of germination is at least partially caused by suppressing peroxidase reactivation.     

Development of plant cuticles: fine structure and cutin composition of Clivia miniata Reg. leaves

The fine structure and monomeric composition of the ester-cutin fraction (susceptible to BF₃/CH₃OH transesterification) of the adaxial leaf cuticle of Clivia miniata Reg. were studied in relation to leaf and cuticle development. Clivia leaves grow at their base such that cuticle and tissues increase in age from the base to the tip. The zone of maximum growth (cell expansion) was located between 1 and 4 cm from the base. During cell expansion, the projected surface area of the upper epidermal cells increased by a factor of nine. In the growth region the cuticle consists mainly of a polylamellate cuticle proper of 100–250 nm thickness. After cell expansion has ceased both the outer epidermal wall and the cuticle increase in thickness. Thickening of the cuticle is accomplished by interposition of a cuticular layer between the cuticle proper and the cell wall. The cuticular layer exhibits a reticulate fine structure and contributes most of the total mass of the cuticle at positions above 6 cm from the leaf base. The composition of ester cutin changed with the age of cuticles. In depolymerisates from young cuticles, 26 different monomers could be detected whereas in older ones their number decreased to 13. At all developmental stages, 9,16-/10,16-dihydroxyhexadecanoic acid (positional isomers not separated), 18-hydroxy-9-octadecenoic acid, 9,10,18-trihydroxyoctadecanoic acid and 9,10-epoxy-18-hydroxyoctadecanoic acid were most frequent with the epoxy alkanoic acid clearly predominating (47% at 16 cm). The results are discussed as to (i) the age dependence of cutin composition, (ii) the relationship between fine structure and composition, (iii) the composition of the cuticle proper, the cuticular layer and the non-depolymerizable cutin fraction, and (iv) the polymeric structure of cutin.Abbreviations CL cuticular layer - CP cuticle proper - MX cutin polymer matrix     

Deposition and localization of lipid polyester in developing seeds of Brassica napus and Arabidopsis thaliana

Mature seeds of Arabidopsis thaliana and Brassica napus contain complex mixtures of aliphatic monomers derived from non-extractable lipid polyesters. Most of the monomers are deposited in the seed coat, and their compositions suggest the presence of both cutin and suberin layers. The location of these polyesters within the seed coat, and their contributions to permeability of the seed coat and other functional properties are unknown. Polyester deposition was followed over Brassica seed development and distinct temporal patterns of monomer accumulation were observed. Octadecadiene-1,18-dioate, the major leaf cutin monomer, was transiently deposited. In contrast, the saturated dicarboxylates maintained a constant level during seed desiccation, whereas the fatty alcohols and saturated omega-hydroxy fatty acids continually increased. Dissection and analysis of Brassica seed coats showed that suberization is not specific to the chalaza. Analysis of the Arabidopsis ap2-7 mutant suggested that suberin monomers are preferentially associated with the outer integument. Several Arabidopsis knockout mutant lines for genes involved in polyester biosynthesis (att1, fatB and gpat5) were examined for seed monomer load and composition. The variance in polyester monomers of these mutants is correlated with dye penetration assays. Furthermore, stable transgenic plants expressing promoter::YFP fusions showed ATT1 promoter activity in the inner integument, whereas GPAT5 promoter is active in the outer integument. Together, the Arabidopsis data indicated that there is a suberized layer associated with the outer integument and a cutin-like polyester layer associated with the inner seed coat.     

Structure and development of the ovule and seed in Aristolochiaceae, with particular reference to Saruma

All members of Aristolochiaceae have anatropous, bitegmic, crassinucellate ovules, which are endostomic except in Saruma and Asarum arifolium where ovules are amphistomic. The outer integument is two cell-layered and the inner integument is three cell-layered. The chalazal megaspore is the functional one. All these conditions appear to be plesiomorphic for the order Piperales, which consists of five families, Aristolochiaceae, Hydnoraceae, Lactoridaceae, Piperaceae and Saururaceae. The embryo sac in Aristolochiaceae is eight-nucleate and corresponds to the Polygonum type; a hypostase is frequently present in this family. The seed coat of Aristolochia s.l., Asarum, Saruma and some Thottea species consists primarily of a two cell-layered testa, and a three cell-layered tegmen. In some species the cells of the outer epidermis become radially elongated, forming reticulate wall thickenings. Cells of the inner layer of the testa have crystals and thickened inner walls. The three layers of the tegmen are tangentially elongated, and become cross fibres at maturity, as fibres of the outer and inner layers are parallel to the seed axis, whereas those of the middle layer are perpendicular to it. This type of seed coat anatomy is synapomorphic for Aristolochiaceae. In addition, the gross morphology of the seed and elaiosome histology are remarkably similar in Asarum and Saruma, thus supporting a sister-group relationship between them. Embryological and seed characters do not supply any synapomorphy that support a close relationship between Aristolochiaceae, Hydnoraceae and Lactoridaceae. Instead, some seed features such as the absence of seed appendages and the collapsed cells of endotesta may indicate a close relationship of Lactoris with Piperaceae plus Saururaceae, although this is the subject of further analysis.     

The embryology ofStegnospermataceae,with a discussion on its status,affinities and systematic position

The embryology ofStegnosperma halimifolium andS. watsonii has been studied in detail. The tapetum is of the secretory type and its cells become multinucleate. Simultaneous cytokinesis in the pollen mother cells follows meiosis. The ripe pollen grains are 3-celled. The ovule is crassinucellate, bitegmic and amphitropous, with the micropyle formed by the inner integument alone. The female archesporium is one celled, and the parietal tissue 3–5 layered. The embryo sac development conforms to thePolygonum type. A central strand, 6 or 7 cells thick, differentiates inside the nucellus and extends from the base of the embryo sac to the chalazal region. The endosperm is nuclear. The embryogeny conforms to the Caryophyllad type. The seed coat is formed by the outer epidermis of the outer integument and the inner epidermis of the inner integument. Based on this evidence and other data, the status of the genus as an independent family,Stegnospermataceae (Stegnospermaceae) is confirmed. Apparently, it forms a connecting link betweenPhytolaccaceae andCaryophyllaceae.     

Ultrastructure of the “fringe-layer”, the innermost epidermis of cotton seed coats

Summary The inner epidermis of the inner integument of cotton seed coats (fringe-layer) and the cuticles between this cell layer and the nucellus were examined in the light and electron microscope at different times of their development. The cells of the fringe-layer contain only small vacuoles and their cytoplasm is densely packed with organelles and free and membrane-bound polysomes. The lateral walls contain many plasmodesmata. At the time when the fruit capsules stop growing, the fringe-cells produce a cell wall labyrinth, resembling that of transfer cells. The cell wall labyrinth is restricted to the lateral walls. The differentiated state of the fringe-cells is short-lived. At about the time of elaboration of the cell wall labyrinth most of them become progressively vacuolated, lignify, and lose their cytoplasmic constituents. The development of the fringe-layer is well correlated with other developmental events in the inner integument, but not with the filling of embryo and endosperm with reserve substances.At anthesis, the fringe-layer and nucellus are covered by a thin cuticle proper of about 20 nm. After anthesis, the nucellar cells start to produce a cuticular layer of considerable, but variable, thickness (0.25–2.5 m), containing a polysaccharide network.In drying seeds the cells of the fringe-layer disrupt. The thin outer tangential wall remains attached to the seed coat. The rest of the cell, together with the cuticles and the collapsed cells of the nucellus, form a protective layer around embryo and endosperm, remaining attached to the seed coat at the chalazal end.     

Aliphatic composition of cutin from inner seed coat of apple

Lithium aluminum deuteride reduction released aliphatic monomers from the inner seed coat fraction but not from the outer seed coat fraction of mature apples. These monomers were identified by GC/MS and the results indicate that the inner coat of apple seed contains a cutin polymer with the major monomer acids being 18-hydroxyoctadec-9-enoic (31%), 9,10-epoxy-18-hydroxyoctadecanoic (28%) and 9,10,18-trihydroxyoctadecanoic (20 %). The monomer composition of this seed coat cuticular polymer was very similar in seeds taken from freshly harvested fruit and in those taken from fruit which had been stored at 4° for 6 months.     

Early Development of the Seed Coat of Soybean (Glycine max)

Although the development of the soybean ovule has been fairlywell studied, knowledge of the sequence of events in the seedcoat during the first 3 weeks after flowering is incomplete.The goal of the present study was to document, using light microscopy,the early development of the soybean seed coat with respectto changes in structure and histochemistry. At anthesis, theseed coat consists of an outer layer of cuboidal epidermal cellssurrounding several layers of undifferentiated parenchyma (whichtogether constitute the outer integument), and an inner layerof cuboidal endothelial cells (the inner integument). At 3 dpost anthesis (dpa), the inner integument has expanded to includethree to five layers of relatively large cells with thick, heavily-stainingcell walls immediately adjacent to the endothelium. By 18 dpa,the outer integument has developed into a complex of tissuescomprised of an inner layer of thick-walled parenchyma, an outerlayer of thin-walled parenchyma containing vascular tissue whichhas grown down from the lateral vascular bundles in the hilumregion, a hypodermis of hourglass cells, and palisade layer(epidermis). The thick-walled parenchyma of the inner integumenthas become completely stretched and compressed, leaving a single,deeply staining wall layer directly above the endothelium. At21 dpa, the outermost cells of the endosperm have begun to compressthe endothelium. At 45 dpa (physiological maturity) the seedcoat retains only the palisade layer, hourglass cells, and afew layers of thin-walled parenchyma. The innermost layer ofthe endosperm, the aleurone layer, adheres to the inside ofthe seed coat. This knowledge will be invaluable in future studiesof manipulation of gene expression in the seed coat to modifyseed or seed coat characteristics. Copyright 1999 Annals ofBotany Company Soybean, Glycine max, seed coat, development, aleurone.     

Genetic and biochemical evidence for involvement of HOTHEAD in the biosynthesis of long-chain α-,ω-dicarboxylic fatty acids and formation of extracellular matrix

In plants, extracellular matrix polymers built from polysaccharides and cuticular lipids have structural and protective functions. The cuticle is found to be ten times thinner in Arabidopsis thaliana (L.) Heynh than in many other plants, and there is evidence that it is unusual in having a high content of α-,ω-dicarboxylic fatty acids (FAs) in its polyesters. We designated the new organ fusion mutant hth-12 after it appeared to be allelic to adhesion of calyx edges (ace) and hothead (hth), upon molecular cloning of the gene by transposon tagging. This mutant is deficient in its ability to oxidize long-chain ω-hydroxy FAs to ω-oxo FAs, which results in leaf polyesters in decreased α-,ω-dicarboxylic FAs and increased ω-hydroxy FAs. These chemical phenotypes lead to disorder of the cuticle membrane structure in hth-12. ACE/HTH is a single-domain protein showing sequence similarity to long-chain FA ω-alcohol dehydrogenases from Candida species, and we hypothesize that it may catalyze the next step after cytochrome P450 FA ω-hydroxylases in the ω-oxidation pathway. We show that ACE/HTH is specifically expressed in epidermal cells. It appears very likely therefore that the changes in the amount of α-,ω-dicarboxylic FAs in hth-12 reflect the different composition of cuticular polyesters. The ACE/HTH gene is also expressed in root epidermal cells which do not form a polyester membrane on the exterior surface, thereby making it possible that the end products of the pathway, α-,ω-dicarboxylic FAs, are generally required for the cross-linking that ensures the integrity of the outer epidermal cell wall.     

Embryology ofEriocaulon setaceum (Eriocaulaceae)

Eriocaulon setaceum can be characterized by: young microsporangium wall with epidermis, endothecium (with fibrous thickenings), and glandular tapetum (uninucleate cells); pollen grains 3-celled, spiraperturate; embryo sac development according to the Polygonum type and with antipodal cyst; endosperm nuclear; embryo small, with incipient differentiation into cotyledonary and epicotylary loci; seed coat mainly from the inner layers of the integuments; pericarp 2-layered and membranous. Embryologically, theEriocaulaceae are nearer to theXyridaceae than to otherFarinosae. Their elevation to the rank of an order,Eriocaulales, therefore appears justified.     

Distribution of Cadmium,Lead, Nickel,and Strontium in Imbibing Maize Caryopses

Seed germination is tolerant to heavy metals apparently because the seed coat is impermeable to metal ions. However, it is not clear whether the seed coat is a universal barrier for all metals. In addition, depending on their physical and chemical properties, a distribution of various metals may differ within an imbibing caryopsis, and therefore they produce dissimilar effects on seed germination. The toxic effects of Cd(NO₃)₂, Pb(NO₃)₂, Ni(NO₃)₂, and Sr(NO₃)₂ were estimated from the germination rates of maize (Zea mays L.) caryopses following two-day incubation with these salts. The distribution of heavy metals and Sr was studied by histochemical methods based on the formation of colored complexes with dithizone (Cd and Pb), dimethylglyoxyme (Ni), and sodium rhodizonate (Sr). Although the metals under study did not affect maize radicle protrusion, they inhibited seed germination in the following order: Cd > Ni ≈ Pb > Sr. Cd and Pb accumulated mainly in the seed coat cells, but Sr and Ni in the embryo cells and in the cells of endosperm (Sr) and scutellum (Ni). Although Cd was found only in the seed coat, it was the strongest inhibitor of seed germination. Apparently, due to high toxicity, Cd exerted its inhibitory effect at the concentrations too low for histochemical assay. In spite of easy translocation across the seed coat of imbibing caryopses, Sr did not considerably inhibit radicle protrusion and seed germination, apparently because of its low toxicity and predominant localization in the apoplast of embryo and endosperm cells.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 635–640.Original Russian Text Copyright © 2005 by Seregin, Kozhevnikova.