Histological analysis of the maturation of native and wound periderm in potato (Solanum tuberosum L.) Tuber

Maturation of potato (Solanum tuberosum L.) tuber native and wound periderm and development of resistance to periderm abrasion were investigated utilizing cytological and histochemical techniques. Both native and wound periderm consist of three different tissues: phellem, phellogen and phelloderm. It was previously determined that the phellogen walls of immature native periderm are thin and prone to fracture during harvest, leading to periderm abrasion (excoriation). Phellogen walls thicken and become less susceptible to fracture upon maturation of the periderm, leading to resistance to excoriation. We now demonstrate that phellogen cells of immature wound periderm also have thin radial walls and that wound periderm abrasion is due to fracture of these walls. Maturation of the wound periderm is also associated with an increase in the thickness of the phellogen radial walls. Histological analysis with ruthenium red and hydroxylamine-FeCI2, which stain unesterified and highly methyl-esterified pectins, respectively, indicates that the phellogen cell walls of native and wound periderm differ significantly regardless of the stage of maturity. Results obtained by staining with ruthenium red and hydroxylamine-FeCI2 imply that phellogen cell walls of immature native periderm contain methyl-esterified pectin, but are lacking in unesterified (acidic) pectins. Maturation of native periderm is accompanied by an apparent increase in unesterified pectins in the walls of phellogen cells, which may allow for the strengthening of phellogen cell walls via calcium pectate formation. Histological staining of the phellogen walls of wound periderm, on the other hand, implies that these walls are deficient in pectins. Moreover, maturation of wound periderm is not accompanied by an increase in unesterified pectins in these walls. Since peroxidase is known to catalyse the cross-linking of cell wall polymers, we stained native and wound periderm for the presence of peroxidase utilizing guaiacol as a substrate. Peroxidase staining was strong in the phellogen walls of both immature and mature native periderm and we could not detect any differences in staining between them. Peroxidase staining was weak in the phellogen walls of immature wound periderm and was not detectably different in mature wound periderm. Peroxidase data imply that there are distinct differences between native and wound periderm, though our data do not indicate that changes in peroxidase activity are involved in the development of resistance to periderm abrasion that occurs upon maturation of the periderm. However, we cannot rule out the involvement in this process of peroxidase isozymes that have low affinity for the substrates utilized here.     

Molecular and cytological aspects of native periderm maturation in potato tubers

Mature native periderm that exhibits resistance to excoriation (RE) is the primary defense for potato tubers against abiotic and biotic challenges. However, little is known about the physiology of periderm maturation and associated gene expressions. In this study, periderm maturation events and associated gene expressions were determined in tubers of two diverse potato genotypes (NDTX4271-5R (ND) and Russet Burbank (RB); 2008 and 2009 crops) at four harvest maturities ranging from immature (non-senesced vines and low RE) to mature (senesced vines and high RE). Approximately 104 d after planting, the fine balance of accumulation and loss of periderm phellem cell layers showed signs of subsiding, indicating cessation of cell division by the phellogen. Phellogen radial cell walls thickened as periderm matured throughout the harvests, increasing RE/skin-set. In both genotypes, the cell cycle gene cyclin-dependent kinase B (StCDKB) rapidly down-regulated after the second harvest coinciding with apparent cessation of cell division. Expression patterns of genes encoding epidermal growth factor binding protein (StEBP) and cyclin-dependent kinase regulatory subunit (StCKS1At) were less indicative of phellogen inactivation and periderm maturation. Genes encoding the structural cell wall proteins extensin (StExt1) for ND and extensin-like (StExtlk) for ND and RB remained up-regulated respectively by the second harvest, suggesting involvement with completion of phellem cell accumulation and on-set of periderm maturation. The expression of genes encoding pectin methyl esterase (StPME), StExt1 and a cell wall strengthening “tyrosine-and lysine-rich protein” (StTLRP) increased in phellogen cells from later harvests of ND tubers, but were down regulated in RB tubers; this suggests roles in phellem cell generation and completion of delayed cell wall development in non-meristematic phellogen cells of ND, a red skinned phenotype. StCDKB and StPrePME genes were rapidly down-regulated by the third harvest for both genotypes. Collectively, these results suggest that down-regulation of these genes coordinates with on-set of periderm maturation and skin-set progression.     

Wounding coordinately induces cell wall protein, cell cycle and pectin methyl esterase genes involved in tuber closing layer and wound periderm development

Little is known about the coordinate induction of genes that may be involved in agriculturally important wound-healing events. In this study, wound-healing events were determined together with wound-induced expression profiles of selected cell cycle, cell wall protein, and pectin methyl esterase genes using two diverse potato genotypes and two harvests (NDTX4271-5R and Russet Burbank tubers; 2008 and 2009 harvests). By 5 d after wounding, the closing layer and a nascent phellogen had formed. Phellogen cell divisions generated phellem layers until cessation of cell division at 28 d after wounding for both genotypes and harvests. Cell cycle genes encoding epidermal growth factor binding protein (StEBP), cyclin-dependent kinase B (StCDKB) and cyclin-dependent kinase regulatory subunit (StCKS1At) were induced by 1 d after wounding; these expressions coordinated with related phellogen formation and the induction and cessation of phellem cell formation. Genes encoding the structural cell wall proteins extensin (StExt1) and extensin-like (StExtlk) were dramatically up-regulated by 1-5 d after wounding, suggesting involvement with closing layer and later phellem cell layer formation. Wounding up-regulated pectin methyl esterase genes (StPME and StPrePME); StPME expression increased during closing layer and phellem cell formation, whereas maximum expression of StPrePME occurred at 5-14 d after wounding, implicating involvement in later modifications for closing layer and phellem cell formation. The coordinate induction and expression profile of StTLRP, a gene encoding a cell wall strengthening "tyrosine-and lysine-rich protein," suggested a role in the formation of the closing layer followed by phellem cell generation and maturation. Collectively, the genes monitored were wound-inducible and their expression profiles markedly coordinated with closing layer formation and the index for phellogen layer meristematic activity during wound periderm development; results were more influenced by harvest than genotype. Importantly, StTLRP was the only gene examined that may be involved in phellogen cell wall thickening after cessation of phellogen cell division.     

Casparian bands occur in the periderm of Pelargonium hortorum stem and root

Background and Aims

Casparian bands are characteristic of the endodermis and exodermis of roots, but also occur infrequently in other plant organs, for example stems and leaves. To date, these structures have not been detected in phellem cells of a periderm. The aim of this study was to determine whether Casparian bands occur in phellem cells using tests that are known to detect Casparian bands in cells that also contain suberin lamellae. Both natural periderm and wound-induced structures were examined in shoots and roots.

Methods

Using Pelargonium hortorum as a candidate species, the following tests were conducted: (1) staining with berberine and counterstaining with aniline blue, (2) mounting sections in concentrated sulphuric acid and (3) investigating the permeability of the walls with berberine as an apoplastic, fluorescent tracer.

Key Results

(1) Berberine–aniline blue staining revealed a modification in the radial and transverse walls of mature phellem cells in both stems and roots. Three days after wounding through to the cortex of stems, the boundary zone cells (pre-existing, living cells nearest the wound) had developed vividly stained primary walls. By 17 d, staining of mature phellem cells of wound-induced periderm was similar to that of natural periderm. (2) Mature native phellem cells of stems resisted acid digestion. (3) Berberine was excluded from the anticlinal (radial and transverse) walls of mature phellem cells in stems and roots, and from the wound-induced boundary zone.

Conclusions

Casparian bands are present in mature phellem cells in both stems and roots of P. hortorum. It is proposed that Casparian bands act to retard water loss and pathogen entry through the primary cell walls of the phellem cells, thus contributing to the main functions of the periderm.     

INITIAL TWIG INJURY TO HIGH ELEVATION PICEA RUBENS IN EASTERN NORTH AMERICA CAUSED BY INADEQUATE PERIDERM FORMATION

Apical dieback is the predominant injury symptomatology associated with growth declines of high elevation Picea rubens Sarg, trees in North America. Histological observations of uninjured tissues and of initial injury to tissues were made to understand the mechanism of injury to this species. Observations were made of hundreds of grab samples of apparently uninjured tissues and of uninjured twigs from trees growing on mountains of the Adirondack Mt., NY; Mount Mansfield, VT; Mt. Mitchell, NC; and Clingman's Dome, TN, from March 1985 to April 1986. In the normal growth pattern of red spruce, three buds elongate from each twig terminus during spring. These buds expand into shoot increments during the growing season and three new buds will form at the tip of each of the three elongated increments. The timing of developmental events varied markedly among buds of individual trees and among trees. Bud break occurred between mid-June to the end of July. Most shoot elongation was completed and periderm formation began near the end of August or in early September. A phellogen, one cell thick, formed and a phellem layer developed from phellogen derivatives during autumn 1985. Many twig samples taken in October and November had produced only one, or at most two, phellem layers external to the phellogen during the relatively short growing season. In some samples, three or more phellem layers were present between November 1985 and March 1986. In some cases a distinct phellem was not developed at all. Usually a phelloderm one cell thick developed in autumn of the first year. Tissue necrosis occurred in twigs during their first overwintering period. Injured twigs with necrotic tissues had only one or two continuous or discontinuous phellem layers. In samples that exhibited initial injury, necrotic tissues consisted mostly of cortical cells and phloem subjacent to this meager periderm. Frequently, necrotic tissues developed initially near the bases of needles and at branch “nodes” (transition zone tissues with older twigs). In contrast, twigs of healthy appearance had two or more continuous phellem layers external to the phellogen.     

Anatomical and Histochemical Aspects of Bark Regeneration in Hevea brasiliensis

Natural rubber is obtained from the bark of Hevea brasiliensis.Both virgin and renewed bark are exploited for this purposeby a process known as tapping which involves controlled woundingand excision of bark tissues. The process of bark renewal andits nature and consequences thus assume importance. Anatomicaland histochemical changes encountered with tapping were thedeposition of lignin and suberin in the peripheral cells, enlargementof tray cells near the cut surface and the formation of a woundperiderm. In the course of development, the wound phellogenmade tangential continuity with the original phellogen in thevirgin bark and functioned as a single phellogen. Vascular cambialactivity was enhanced due to wound stimulus and the newly differentiatedsieve tubes and ray cells were larger in size. The first peridermwas functional for only a short period of time, after whicha new meristematic zone developed in the inner tissues. Virginand renewed bark differed in the proportion of soft and hardbark, amount and distribution of sclereids, tannin cells andcrystals.Copyright 1995, 1999 Academic Press Bark renewal, Hevea brasiliensis, histochemistry, laticifers, para rubber tree, wound periderm     

Developmental stages and fine structure of surface callus formed after debarking of living lime trees (Tilia sp.)

Wounding of trees by debarking during the vegetative period sometimes results in the formation of callus tissue which develops over the entire wound surface or on parts of it. This light and transmission electron microscopy study of living lime trees found that the formation of such a surface callus is subdivided into three stages. During the first stage, numerous cell divisions take place in regions where differentiating xylem remains at the wound surface after debarking. This young callus tissue consists of isodiametric parenchymatous cells. Cambium cells, sometimes also remaining at the wound surface, collapse and do not contribute to callus formation. During the second stage, cells in the callus undergo differentiation by forming a wound periderm with phellem, phellogen and phelloderm. In the third stage, a cambial zone develops between the wound periderm and the xylem tissue laid down prior to wounding. This process is initiated by anticlinal and periclinal divisions of a few callus cells only. Later this process extends tangentially to form a continuous belt of wound cambium. Subsequently, this cambium produces both wound xylem and wound phloem and thus contributes to further thickening.     

A feruloyl transferase involved in the biosynthesis of suberin and suberin‐associated wax is required for maturation and sealing properties of potato periderm

Suberin and waxes embedded in the suberin polymer are key compounds in the control of transpiration in the tuber periderm of potato (Solanum tuberosum). Suberin is a cell‐wall biopolymer with aliphatic and aromatic domains. The aliphatic suberin consists of a fatty acid polyester with esterified ferulic acid, which is thought to play an important role in cross‐linking to the aromatic domain. In potato, ferulic acid esters are also the main components of periderm wax. How these ferulate esters contribute to the periderm water barrier remains unknown. Here we report on a potato gene encoding a fatty ω‐hydroxyacid/fatty alcohol hydroxycinnamoyl transferase (FHT), and study its molecular and physiological relevance in the tuber periderm by means of a reverse genetic approach. In FHT RNAi periderm, the suberin and its associated wax contained much smaller amounts of ferulate esters, in agreement with the in vitro ability of the FHT enzyme to conjugate ferulic acid with ω‐hydroxyacid and fatty alcohols. FHT down‐regulation did not affect the typical suberin lamellar ultrastructure but had significant effects on the anatomy, sealing properties and maturation of the periderm. The tuber skin became thicker and russeted, water loss was greatly increased, and maturation was prevented. FHT deficiency also induced accumulation of the hydroxycinnamic acid amides feruloyl and caffeoyl putrescine in the periderm. We discuss these results in relation to the role attributed to ferulates in suberin molecular architecture and periderm impermeability.     

Initial Anatomical Changes Associated with Tuber Formation on Single-node Potato (Solanum tuberosum L.) Cuttings

One-node potato (Solanum tuberosum L. cv. ‘Katahdin’)cuttings were used to study early anatomical changes associatedwith tuberization. Starch deposition and the percentage frequencyof cells in mitosis increased in the medullary region of thebud within 1 d after cutting, whereas increases in average cellsize were not detected until 4 d after cutting. Starch depositionand mitosis were the earliest detectable changes in anatomyassociated with tuber initiation. Potato, Solanum tuberosum L., tuber initiation, cuttings, cell enlargement, mitosis, starch deposition     

盾叶薯蓣根状茎的发育解剖学和组织化学研究

盾叶薯蓣根状茎顶端的生长点由鳞片包被,其衍生细胞分化为原表皮、基本分生组织和散生的原形成层束,以后分化为表皮、基本组织和散生的维管束构成的初生结构。根状茎顶端下方的原表皮内存在初生增厚分生组织,其细胞不断向内分裂和其衍生细胞的体积增大使根状茎能够迅速增粗。分化完成的根状茎主要由周皮、基本组织和散生的维管束构成。周皮由木栓层、木栓形成层和栓内层组成;基本组织由薄壁细胞组成;维管束属于有限维管束。薯蓣皂甙主要存在于基本组织薄壁细胞中。原分生组织和原形成层不含薯蓣皂甙,维管束的木质部和韧皮部中的韧皮纤维也无薯蓣皂甙的分布,韧皮部的生活细胞和维管束鞘细胞有薯蓣皂甙的积累。近顶端的基本分生组织细胞内薯蓣皂甙不形成液滴,随着细胞分裂逐渐停止,细胞内开始形成含薯蓣皂甙的液滴,反映皂甙是在成熟细胞内积累。其中,有小型维管束分布的基本组织中薯蓣皂甙的积累与分布最丰富,两年生根状茎中薯蓣皂甙的含量比一年生的高。     

Phellogen Regeneration in Injured Peach Tree Bark

Injury to peach bark phellogen leads to the generation of newtissues and the re-establishment of meristematic continuity.Two types of tissue changes after wounding were identified andquantified in bark of seven peach clones: (1) cell wall modifications(lignification and suberization) of tissues present at the timeof wounding, and (2) generation of the new phellogen and itsderivatives. Tissue responses were quantified with a microscopephotometer using selective histochemistry and autofluorescenceto detect lignin and suberin deposition over time. Suberin continuitywas re-established via suberin deposition in a layer of cells,present at the time of wounding, approximately 800 µminternal to the wound surface. Phellogen continuity was re-establishedimmediately internal to and abutting the suberized tissue. Thenew phellogen gave rise to suberized phellem which, in its outwardexpansion, crushed the suberized boundary zone tissue formedearlier. All injured peach clones produced the same sequenceof tissue changes, although timing and degree of response variedwith clone and time of year. Differentiation, impervious tissue, lignin, Prunus persica (L.) Batsch, suberin, wounding     

Initial Anatomical Changes Associated with Tuber Formation on Single-node Potato (Solanum tuberosum L.) Cuttings: A Re-evaluation

Cell division and cell expansion during early stages of tuberdevelopment were studied using developing axillary buds on single-leafcuttings from potato (Solanum tuberosum L.). Cuttings takenfrom plants induced to form tubers, by short day (SD) treatment,were compared with cuttings from non-induced (long day, LD)plants. In the apical zone of the buds, cell division occurredfrom the first day after cutting, in both LD and SD cuttings.The planes of these divisions were transverse, associated withelongation of the buds. At day 5, a new orientation of celldivision was observed in the subapical zone of SD cuttings only.These divisions were longitudinal, associated with radial growth.Cell expansion occurred in both SD and LD cuttings, and wasnot uniquely related to the onset of tuber formation. Copyright1999 Annals of Botany Company Solanum tuberosum L., potato, tuber formation, cell division, cell expansion.     

Cell Cycling and the Fate of Potato Buds

The cell cycling characteristics of the regions of the apicalmeristems of underground shoots and buds of Solanum tuberosumL. were investigated by stathmokinesis and labelling. The apicaldomes of orthotropic shoots produce cells at twice the elementalrate of those of stolons, and their youngest leaf primordiaat twelve times the rate. Changing the fate of stolons so thatthey will become orthotropic by decapitating the tuber sproutsthat bear them results within 24 h in a general increase incell production especially in the leaf primordia. Axillary buds on tuber sprouts induced to become precursorsof orthotropic shoots instead of stolons undergo a spectacularincrease in cell division within 24 h in all regions, especiallyin the primordia and bud anlagen where the rate increases 20-foldor more. The summit is slower to react than other regions, but,by 24 h, its rate of cell division increases 11-fold and itis contributing 14 cells per day to the flanks from its 80 cells. In all the axillary buds the rate of mitosis in the summit ishalf that of the flanks of the apical dome, but, in both stolonsand orthotropic underground shoots, the rate is higher in thesummit than in the flanks or rib meristem. The results are discussed in relation to what is known of cellcycling changes after floral evocation. Solanum tuberosum L., potato, fate, cell division, apical meristem, stathmokinesis     

Unraveling ferulate role in suberin and periderm biology by reverse genetics

Plant cell walls are dramatically affected by suberin deposition, becoming an impermeable barrier to water and pathogens. Suberin is a complex layered heteropolymer that comprises both a poly(aliphatic) and a poly(aromatic) lignin-like domain. Current structural models for suberin attribute the crosslinking of aliphatic and aromatic domains within the typical lamellar ultrastructure of the polymer to esterified ferulate. BAHD feruloyl transferases involved in suberin biosynthesis have been recently characterized in Arabidopsis and potato (Solanum tuberosum). In defective mutants, suberin, even lacks most of the esterified ferulate, but maintains the typical lamellar ultrastructure. However, suberized tissues display increased water permeability, in spite of exhibiting a similar lipid load to wild type. Therefore, the role of ferulate in suberin needs to be reconsidered. Moreover, silencing the feruloyl transferase in potato turns the typical smooth skin of cv. Desirée into a rough scabbed skin distinctive of Russet varieties and impairs the normal skin maturation that confers resistance to skinning. Concomitantly to these changes, the skin of silenced potatoes shows an altered profile of soluble phenolics with the emergence of conjugated polyamines.Key words: BAHD feruloyl acyltransferases, ferulate, periderm, potato tuber skin, suberin, suberized tissues, waxRecently published reverse genetic studies in Arabidopsis and potato identified two orthologous genes that encode a BAHD feruloyl transferase acting on aliphatics and showed that deficiency in these enzymes produces a decrease in suberin-associated ferulic acid. These results, here discussed, signify an important advance in suberin biochemistry and ultrastructure, providing a valuable new insight into the organization of the suberized tissues and their role in the control of water vapour loss.     

六盘山鸡爪大黄根蒽醌类化合物组织化学定位的研究

采用组织化学方法研究了六盘山鸡爪大黄根蒽醌类化合物的组织化学定位特征及贮藏和积累的规律。结果表明：蒽醌类化合物在根内的贮藏是多位点的,在根周皮的木栓层和栓内层、次生维管组织的维管射线和根中央的部分木薄壁细胞内不同程度地贮藏和积累了一定数量的蒽醌类化合物,次生木质部的木射线和次生韧皮部的韧皮射线是主要贮藏和积累的部位,早期形成的维管射线中蒽醌类化合物的含量较晚期形成的射线含量高。     

PHLOEM OF SPHENOPHYLLUM

The phloem of most fossil plants, including that of Sphenophyllum, is very poorly known. Sphenophyllum was a relatively small type of fossil arthrophyte with jointed stems bearing whorls of leaves ranging in form from wedge or fan-shaped to bifid, to linear. The aerial stem systems of the plant exhibited determinate growth involving progressive reduction in the dimensions of the stem primary bodies, fewer leaves per whorl, and smaller and simpler leaves distally. The primary phloem occurs in three areas alternating in position with the arms of the triarch centrally placed primary xylem. Cells of the primary phloem, presumably sieve elements, are axially elongate with horizontal to slightly tapered end walls. In larger stems with abundant secondary xylem and secondary cortex or periderm, a zone of secondary phloem occurs whose structure varies in the three areas opposite the arms of the primary xylem, as opposed to the three areas lying opposite the concave sides of the primary xylem. The axial system of the secondary phloem consists of vertical series of sieve elements with horizontal end walls. In the areas opposite the protoxylem the parenchyma is present as a prominent ray system showing dilation peripherally. Sieve elements in the areas opposite the protoxylem arms have relatively small diameters. In the areas between the protoxylem poles the secondary phloem sieve elements have large diameters and are less obviously in radial files, while the parenchyma resembles that of the secondary xylem in these areas in that it consists of strands of cells extending both radially and tangentially. An actively meristematic vascular cambium has not been found, indicating that this layer changed histologically after the cessation of growth in the determinate aerial stem systems and was replaced by a post-meristematic parenchyma sheath made up of axially elongate parenchyma lacking cells indicative of being either fusiform or ray initials. A phellogen arose early in development in a tissue believed to represent pericycle and produced tissue comparable to phellem externally. Normally, derivatives of the phellogen underwent one division prior to the maturation of the cells. Concentric bands of cells with dark contents apparently represent secretory tissue in the periderm and cell arrangements indicate that a single persistent phellogen was present. Sphenophyllum is compared with other arthrophytes as to phloem structure and is at present the best documented example of a plant with a functionally bifacial vascular cambium in any exclusively non-seed group of vascular plants.     

Kinetics and localization of wound-induced DNA biosynthesis in potato tuber

Tuber wounding induces a cascade of biological responses that are involved in processes required to heal and protect surviving plant tissues. Little is known about the coordination of these processes, including essential wound-induced DNA synthesis, yet they play critical roles in maintaining marketability of the harvested crop and tubers cut for seed. A sensitive “Click-iT EdU Assay” employing incorporation of the thymidine analog, 5-ethynyl-2′-deoxyuridine (EdU), in conjunction with 4′,6-diamindino-2-phenylindole (DAPI) counter labeling, was employed to objectively identify and determine the time course and spatial distribution of tuber nuclei that were wound-induced to enter S-phase of the cell cycle. Both labeling procedures are rapid and sensitive in situ. Following wounding, EdU incorporation (indicating DNA synthesis) was not detectable until after 12 h, rapidly reached a maximum at about 18 h and then declined to near zero at 48 h. About 28% of the nuclei were EdU labeled at 18 h reflecting the proportion of cells in S-phase of the cell cycle. During the ∼30 h in which induced cells were progressing through S-phase, de novo DNA synthesis extended 7–8 cell layers below the wound surface. Cessation of nuclear DNA synthesis occurred about 4 d prior to completion of wound closing layer formation. Initiation of wound periderm development followed at 7 d, i.e. about 5 d after cessation of nuclear DNA biosynthesis; at this time the phellogen developed and meristematic activity was detected via the production of new phellem cells. Collectively, these results provide new insight into the coordination of wound-induced nucleic acid synthesis with associated tuber wound-healing processes.     

Water permeability of periderm membranes isolated enzymatically from potato tubers (Solanum tuberosum L.)

The fine structure and water permeability of potato tuber periderm have been studied. Periderm membranes (PM) were isolated enzymatically using pectinase and cellulase. They were composed of, about six layers of phellem cells arranged in radial rows. The walls of phellem cells consist of cellulosic primary and tertiary walls and suberized secondary walls which are lamellated. Middle lamellae and primary walls contain lignin. Since the PM did not disintegrate during enzymatic isolation it appears that lignin also extends into the secondary suberized walls. The water permeability of PM was low, ranging from 1–3·10^-10 m s^-1. This low water permeability developed only during storage of tubers in air. Periderm membranes from freshly harvested tubers had a relatively high permeability. The low permeability of PM from stored tubers is attributed to soluble lipids associated with suberin since: (1) extraction of soluble lipids from PM increased permeability by more than 100-fold, (2) a phase transition of soluble lipids was observed between 46 and 51° C, and (3) only the permeability of PM decreased during storage while the permeability of extracted PM remained unchanged. Evidence is presented that two pathways for water movement exist in parallel. Pathway 1 is represented by middle lamellae and primary walls extending in radial direction across the membranes. This pathway has a relatively high specific permeability. Pathway 2 is represented by a polylaminated structure made up of tangential walls of phellem cells which are orientated normal to the direction of water flow. This pathway has a low specific permeability because of the properties of secondary walls incrusted with soluble lipids. It is calculated that about 10% of the water flows across pathway 1 and 90% across pathway 2 which has a volume fraction of 0.995.     

Fine structure of isolated and non-isolated potato tuber periderm

Cell walls of the periderm of native potato tuber (Solanum tuberosum L. cv. Primura) consist of a primary wall, a suberized secondary wall and a tertiary wall. With a mixture of pectinase and cellulase intact periderm membranes can be isolated. Isolation does not affect fine structure. It is suggested that the lignin in the middle lamellae and primary walls prevents the enzymes from digesting pectinaceous materials and cellulose. In specimens fixed with OsO₄, the suberized walls appear as alternating electrondense and electron-lucent lamellae. This lamellar architecture is not altered by extraction with chloroform. Therefore, the current view that the electronlucent lamellae consist of soluble lipids (waxes) can no longer be maintained. It is argued that the lamellation is a property of the suberin itself, and the suberized wall consists of alternating layers of suberins differing in polarity. A hypothesis of suberin assembly from sub-units is advanced and the subunits are shown for the first time.