Phellem Cells Research Articles

Anthocyanin synthesis in native and wound periderms of potato.

Skin color of red potatoes is due to accumulation of anthocyanins in the tuber periderm, a protective tissue that replaces the epidermis at an early stage of tuber development. The periderm consists of external layers of suberized phellem cells making up the skin, and internal layers of parenchyma-like phelloderm cells. Red pigmentation is an important marketing factor for red-skinned potatoes. However, injuries to the tuber surface, which are common in the potato industry, result in the development of a wound periderm that is devoid of the characteristic red coloration. To study the reason for these differences in anthocyanin accumulation, the expression level of anthocyanin biosynthesis genes and regulators was monitored in native and wound periderm using microarray analysis and quantitative polymerase chain reaction. We found significantly higher expression of the anthocyanin pathway in the phelloderm cells compared with the skin and tuber-flesh samples. However, in wound periderm, the anthocyanin pathway was strongly downregulated relative to the native periderm. This was true for two developmental stages of the native periderm--'immature', when the skin is prone to skinning injuries, and 'mature', following skin set--suggesting that anthocyanin synthesis continues postharvest. Wound-induced expression of steroidal glycoalkaloid glycosyltransferases, suberin-related 3-ketoacyl-CoA synthase and actin indicated that downregulation of the anthocyanin-specific pathway does not reflect global repression of the wound-periderm transcriptome. Loss of pigmentation may result from reduced expression of the Myb-bHLH-WD40 anthocyanin regulatory complex--a possible candidate might be the bHLH transcription factor JAF13.

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 12h, rapidly reached a maximum at about 18h and then declined to near zero at 48h. About 28% of the nuclei were EdU labeled at 18h reflecting the proportion of cells in S-phase of the cell cycle. During the ∼30h 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.

Ontogenesis and structure of periderm in. Acer negundo L. and x Fatshedera lizei Guillaum

The ontogenesis and structure of the periderm were investigated in two dicotyledonous plants: <em>Acer negundo</em> L. and x <em>Fatshedera lizei</em> Guillaum. In Fatshedera lizei periderm is produced by the monolayer of phellogen proper, whereas in <em>Acer negundo</em> sometimes cells in the immediate neighbourhood of phellogen proper, particularly the phelloderm participate in the formation of periderm. In Acer negundo at the boundary with the next annual phellem increment typical phelloid cells are present. Their thick walls contain pectins, cellulose and lignin, but do not contain suberin. The presence of phelloid was not detected in <em>Fatshedera lizei</em>. The walls of the phellem cells with horseshoe shaped thickenings of the inner tangential walls contain, beside pectins, cellulose and lignin, also a thin suberin layer directly on the primary walls. Thus, they are specifically shaped thickwalled cork cells. It is suggested to refer to the suberin layer jointly with the thick cellulose-lignin layer of the walls of thickwalled cork cells by the term "secondary wall". The term "tertiary wall (lamella)" is reserved for thin wall straw of the type of' "membranogenic stratum" and "warty stratum". In <em>Acer negundo</em> the structure of the suberin layers in the thick ceil walls of cork proper is cornplex.

Characterisation and fractioning of Tectona grandis bark in view of its valorisation as a biorefinery raw-material

The anatomy and chemical composition of Tectona grandis bark from mature trees in East Timor are described as well as the characterisation of fractionation by grinding and granulometric separation.Teak bark is composed of secondary phloem, periderm and a narrow rhytidome that included various periderms with phloem tissues between them. The layer of phellem cells in each periderm was thin. The phloem showed an orderly stratification with tangential bands of fibres in concentric rings that alternated with thin bands of axial parenchyma and sieve tube elements. Abundant prismatic calcium oxalate crystals were present.The bark fractured easily into clean particles. The yield of fines was low and 64.4% of the particles were over 2mm.The mean chemical composition of teak bark was: ash 18.5%, total extractives 10.7%, lignin 20.0% and suberin 1.9%. The polysaccharides, corresponding to approximately 47%, showed a predominance of glucose (60.3% of total neutral monosaccharides) and an important content of xylose (20.0%). The content of rhamnose was also comparatively high (4.9%). The content of soluble phenolics was 1.6%. Ash elemental composition showed the predominance of calcium, representing about 93% of the total inorganics, followed by potassium (4.8%) and magnesium (1.9%).Extractives were present preferentially in the fines, with about 30% more extractives than the coarse fraction. Lignin content and monosaccharide composition were similar in the different bark fractions. A difference between fractions was found in relation to suberin content which was lower in the fines: 0.6% and 3.5% in the fine and medium fractions, respectively.

Anatomy and Development of the Endodermis and Phellem ofQuercus suberL. Roots

Quercus suber L. has been investigated with special attention to the stem bark and its cork formation layer, but excluding the roots. Roots are the location of infection by pathogens such as Phytophthora cinnamomi responsible for the tree's sudden death. It is widely accepted that suberin establishes boundaries within tissues, serves as a barrier against free water and ion passage, and works as a shield against pathogen attacks. We followed the suberization of young secondary roots of cork oak. The first suberin deposition detectable by transmission electron microscopy (TEM) and neutral red (NR) was in the endoderm Casparian strips. Casparian strips are not detected by Sudan red 7B and Fluorol yellow (FY) that specifically stain lamellae suberin. Reaction to Sudan was verified in the endodermis and later on in phellem cells that resulted from the phellogen. Under TEM, the Sudan and FY-stained cells showed clear suberin lamellae while the newer formed phellem cells displayed a distinct NR signal compared to the outermost phellem cells. We concluded that suberin chemical components are arranged differently in the cell wall according to the physiological role or maturation stage of a given tissue.

The development of stomata and other epidermal cells on the rice leaves

In the leaves of rice (Oryza sativa), stomatal initials arose from two asymmetric cell divisions and a symmetric division. Guard mother cells (GMCs) and long cells in stomatal files (LCSs) were formed through the first asymmetric division of the precursor cell of GMCs. Subsidiary cells (SCs) were produced by the second asymmetric division of subsidiary mother cells or LCSs. Following SC formation, GMCs divided once symmetrically to generate guard cells and then differentiated terminally to form mature stomata. The developmental patterns of long cells, prickle hairs and short cells (phellem cells and silica cells) were also examined. Interestingly, we found that the different developmental stages of stomata and epidermal cells occurred in the similar location of immature leaves of the same phyllotaxis. In addition, two spacing patterns ("one stoma, one long cell" and "one short cell row") probably exist in rice leaves.

Identification of a putative molecular regulator of cork cambium

Background Cork oak (Quercus suber) is a long-living species of the Fagaceae family that highly contributes for the economy of several countries such as Portugal. It is the only plant species with a phellogen capable of a sustainable production of cork with properties suited for industry applications. Phellogen, or cork cambium, usually initiates in the subepidermis and differentiates during the first year of growth [1]. Its meristematic activity gives rise to phellem (cork) cells to the outside and to phelloderm to the inside. It is likely that genes involved in the regulation of other plant meristems such as shoot apical meristem and vascular cambium are also involved in the regulation of phellogen. Based on its role in other meristems, and on specific expression patterns in poplar stems, we hypothesize that SHORT-ROOT (SHR), a transcription factor from the GRAS family, may be involved in the regulation of phellogen. This gene has been well described in the Arabidopsis root where it plays a key role in the radial patterning and in regulating the specification of the root stem cell niche [2-4]. Recently, its involvement in the root vascular system to control patterning processes [5] and in the control of proliferative cell division in developing leaves [6]has also been reported. In this work we report the cloning of SHR from cork oak transcriptome and the characterization of SHR expression patterns in the stem with the aim of investigating its putative function in the phellogen.

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

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. 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. (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. 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.

Ultrastructural Observations Reveal the Presence of Channels between Cork Cells

The ultrastructure of phellem cells of Quercus suber L. (cork oak) and Calotropis procera (Ait) R. Br. were analyzed using electron transmission microscopy to determine the presence or absence of plasmodesmata (PD). Different types of Q. suber cork samples were studied: one year shoots; virgin cork (first periderm), reproduction cork (traumatic periderm), and wet cork. The channel structures of PD were found in all the samples crossing adjacent cell walls through the suberin layer of the secondary wall. Calotropis phellem also showed PD crossing the cell walls of adjacent cells but in fewer numbers compared to Q. suber. In one year stems of cork oak, it was possible to follow the physiologically active PD with ribosomic accumulation next to the aperture of the channel seen in the phellogen cells to the completely obstructed channels in the dead cells that characterize the phellem tissue.

Transcriptomic profiling of heat-stress response in potato periderm

Potato (Solanum tuberosum L.) periderm is composed of the meristematic phellogen that gives rise to an external layer of suberized phellem cells (the skin) and the internal parenchyma-like phelloderm. The continuous addition of new skin layers and the sloughing of old surface layers during tuber maturation results in smooth, shiny skin. However, smooth-skin varieties frequently develop unsightly russeting in response to high soil temperatures. Microscopic observation of microtubers exposed to high temperatures (37 degrees C) suggested heat-enhanced development and accumulation of suberized skin-cell layers. To identify the genes involved in the periderm response to heat stress, skin and phelloderm samples collected separately from immature tubers exposed to high soil temperatures (33 degrees C) and controls were subjected to transcriptome profiling using a potato cDNA array. As expected, the major functional group that was differentially expressed in both skin and phelloderm consisted of stress-related genes; however, while the major up-regulated phelloderm genes coded for heat-shock proteins, many of the skin's most up-regulated sequences were similar to genes involved in the development of protective/symbiotic membranes during plant-microbe interactions. The primary activities regulated by differentially expressed peridermal transcription factors were response to stress (33%) and cell proliferation and differentiation (28%), possibly reflecting the major processes occurring in the heat-treated periderm and implying the integrated activity of the stress response and tissue development. Accumulating data suggest that the periderm, a defensive tissue, responds to heat stress by enhancing the production and accumulation of periderm/skin layers to create a thick protective cover. Skin russeting may be an indirect outcome; upon continued expansion of the tuber, the inflexible skin cracks while new layers are produced below it, resulting in a rough skin texture.

The development and characteristics of periderm and rhytidome in Eucalyptus marginata

To understand the pathway used by Phytophthora cinnamomi Rands to penetrate the bark of jarrah, the present study describes unwounded periderm and rhytidome development. Periderm formation is described from its initiation in 4-week-old seedlings to the formation of rhytidome in saplings. Periderm in young seedlings consists of a single type of phellem, namely thin-walled suberised cells. In older seedlings where multiple layers of periderm have formed, layers of thick-walled lignified phellem cells in compacted bands alternate with thin-walled suberised cells. Rhytidome formation in older lignotuberous seedlings and in sapling jarrah occurs through the isolation of secondary phloem by periderm. The rhytidome consists of expanded and partially disintegrated secondary phloem tissue sandwiched between layers of phellem cells. Localised periderm formation beneath stomata results in the formation of lenticels, which are ephemeral features. Superficial periderms occur at sites of leaf and shoot abscission, and of lateral shoot emergence. Concealed axillary shoots lack cuticle on emergence. As the trees age, the internal production of lignified and suberised periderm and rhytidome results in an impenetrable barrier to invasion by P. cinnamomi. However, external sites including lenticels and leaf and shoot abscission and emergence areas, all provide points of ingress in unwounded stems.

Potato skin proteome is enriched with plant defence components

Periderm is a tissue of secondary origin that replaces damaged epidermis. It can be found in underground plant organs, as an above-ground tissue of woody species (cork), and as a wound-healing tissue. Its outer layers are composed of phellem cells with suberized walls that constitute a protective barrier, preventing pathogen invasion and fluid loss. In potato, a model for periderm studies, periderm tissue replaces the epidermis early in tuber development and the suberized phellems constitute the tuber's skin. To identify factors involved in phellem/skin development and that play a role in its defensive characteristics, two-dimensional gel electrophoresis was used to compare the skin and parenchymatic flesh proteomes of young developing tubers. Proteins exhibiting differentially high signal intensity in the skin were sorted by functional categories. As expected, the differential skin proteome was enriched in proteins whose activity is characteristic of actively dividing tissues such as cell proliferation, C1 metabolism, and the oxidative respiratory chain. Interestingly, the major functional category consisted of proteins (63%) involved in plant defence responses to biotic and abiotic stresses. This group included three isozymes of caffeoyl-CoA O-methyltransferase and five isozymes of peroxidase that may play a role in suberization processes. The differential expression of these proteins in the skin was further verified by RT-PCR of their corresponding transcripts in skin and tuber flesh samples. The results presented here shed light on the early events in skin development and further expand the concept of the periderm as a protective tissue containing an array of plant defence components.

Anatomical and histochemical characteristics of Japanese birch (Tohoku) plantlets infected with the Inonotus obliquus IO-U1 strain

Host-pathogen interactions were investigated on a Japanese birch (Betula platyphylla var. japonica, Tohoku) plantlet after infection with a canker-rot fungus, the Inonotus obliquus IO-U1 strain. For a time-course study, intact, wounded, and infected plantlets were collected from 2 h to 30 days after treatment. Notable changes were observed morphologically in the treated portion of wounded and infected plantlets. Phenolics first deposited at the cut margin and subsequently in vessels after 4 h of infection. Their deposition extended to other xylem elements, the cortex, and the pith with an increase in the infection period. Phenolics deposition was extensive at 10 days post-inoculation (dpi), when most of the cells were entirely filled with phenolics. A necrophylactic periderm (NP) was formed at the junction of the original periderm with a layer of 2-4 new phellem cells at 30 dpi. Based on the results obtained, phenolics deposition and NP formation are considered to occur as infection-induced responses in Tohoku birch plantlets infected with I. obliquus IO-U1.

Anatomy of Brazilian Cereeae (subfamily Cactoideae, Cactaceae): Arrojadoa Britton & Rose, Stephanocereus A. Berger and Brasilicereus Backeberg

(Anatomy of Brazilian Cereeae (subfamily Cactoideae, Cactaceae): Arrojadoa Britton & Rose, Stephanocereus A. Berger wâBrasilicereus Backeberg). Arrojadoa, Stephanocereus and Brasilicereus are endemic Brazilian Cereeae, occurring along the Espinhaço Range, in the campos rupestres, cerrados and caatingas, from northern Minas Gerais to southern Bahia. The genera are columnar, erect to semi-erect cacti, except for one species, A bahiensis, which is globose. This study describes the anatomy of dermal, fundamental and vascular systems, aiming to find diagnostic characters for the genera and species. Basal portions of stems were sectioned transversely and longitudinally, and stained with Astrablue and Safranin. The species share a uniseriate epidermis, with thick cuticle; well developed collenchymatic hypodermis, containing prismatic crystals; cortex with numerous mucilage cells, druses and vascular bundles; outside cortex as a palisade parenchyma; periderm composed of lignified cork cells alternating with suberized cells; pheloderm consisting of a few layers of thin-walled cells; phloem composed of solitary or multiple of two to three sieve tube elements, companion cells, axial and radial parenchyma; secondary xylem with solitary to multiple vessels, with simple perforation plates and alternate bordered to semi-bordered pits; axial parenchyma scanty vasicentric to incomplete; libriform septate fibres; large rays. Unlignified parenchyma is seen in the secondary xylem, varying from a few cells to bands among axial and radial elements. The following are considered diagnostic characters: the shape of lignified phellem cells, cubic to radially elongate, which individualizes S. leucostele; an underdeveloped hypodermis and the occurrence of sclereids in the cortex are exclusive to Brasilicereus markgrqfii.

The involvement of ethylene in wound-induced suberization of potato tuber (Solanum tuberosum L.): a critical assessment

Abstract The determination of hormonal requirements for wound-induced suberization is important in developing new approaches and future postharvest technologies to control various wound-related disease and defect problems in potato tuber (Solanum tuberosum L.). Although, the hormone ethylene has been shown to be involved in various kinds of plant stress, including certain wound responses, its role in suberization had not been determined. The role of ethylene in wound-induced suberization of potato tuber was examined over a 9-day wound-healing period, using a variety of ethylene biosynthesis and action inhibitors. Ethylene evolution was stimulated by wounding and reached a maximum 2–3 days after tuber wounding, and then gradually declined. The competitive inhibitor of ethylene action, 2,5-norbornadiene, had no effect on the wound-induced accumulation of suberin polyphenolic(s) (SPP). Similarly, the ethylene antagonist 1-methylcyclopropene (1-MCP) had no apparent effect on wound-induced accumulation of SPPs or on the accumulation of suberin polyaliphatic(s) (SPA). Treatment of tubers with ethylene, applied either before or after wounding, had no effect on the induction or accumulation of either suberin biopolymer. Treatment with the ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), inhibited wound-induced ethylene production by ca. 90%, but did not affect wound-induced suberization. Collectively, these results indicate that, although increased ethylene evolution is part of the tuber wound response, ethylene is not required for wound-induced suberization of the closing layer (suberization of existing cells at the wound surface) during the first 2–4 days of wound-healing or subsequent suberization of phellem cells (between 4 and 9 days) created by the wound-induced formation of the phellogen. These results are important in determining the mechanisms regulating suberization and in assuring that wound-induced suberization is not inhibited with the application of new technologies that effectively control various ethylene mediated processes in vegetables and fruit.

Necrophylactic periderm formation in the roots of western larch and Douglas‐fir trees infected with Armillaria ostoyae. I. The response to abiotic wounding in non‐infected roots

SummaryThe sequence of events leading to necrophylactic periderm formation was studied throughout the year following the abiotic wounding of the non‐infected roots of 10‐ and 27‐year‐old western larch (Larix occidentalis) and 11‐ and 25‐year‐old Douglas‐fir (Pseudotsuga menziesii) trees that were infected with Armillaria ostoyae. The sequence was the same for both ages and species of trees. Wound repair was more rapid in the summer compared with the spring and autumn. Following cell hypertrophy, a zone of lignified impervious tissue was in the initial stages of formation within 10 days of wounding in the summer and 14 days in the spring or autumn. The new phellogen produced a layer of phellem three to four rows of cells thick after 20 days in the summer or 40 days in the spring. Modified cells abutting the inner boundary of the impervious zone frequently developed thick lignified abaxial walls and thin suberized adaxial walls. A typical exophylactic periderm in healthy root bark tissue of both western larch and Douglas‐fir consisted of stone phellem one to four rows of cells thick and a layer of thin‐walled phellem three to six rows of cells thick in western larch and two to three rows thick in Douglas‐fir, a single row of phellogen cells and one to three rows of phelloderm cells. Mature thin‐walled phellem cells had pigmented contents, red in western larch and light brown in Douglas‐fir. In response to wounding, 27‐year‐old western larch and 25‐year‐old Douglas‐fir developed necrophylactic periderms with annual bands of phellem. The bands included a layer of phellem that was six to 12 and nine to 15 rows of cells thicker than the layer of phellem observed in the respective naturally developed exophylactic periderms. Fifty days following wounding in the summer, stone phellem, one to three rows of cells thick, was observed in the necrophylactic periderm of 10‐year‐old trees. When fully developed, the necrophylactic periderm in 27‐year‐old western larch also had a layer of stone phellem three to five rows of cells thick in each band. Stone phellem development was only sporadic in 25‐year‐old Douglas‐fir. Wounds in the winter showed no signs of activity associated with repair until dormancy broke in the spring.

THE PERIDERM DEVELOPMENT IN QUERCUS SUBER

In the cork oak (Quercus suber L.), the phellogen differentiates during the first year of growth in the cell layer immediately under the epidermis and divides to form 3–6 suberized phellem cells. Division of the phellogen only occurs after suberization of the previous divided cell. During the first four years of growth, the phellem cells have tannin-filled lumens and it is only in the 5th to 7th years that they acquire the characteristics of ʻadultʼ cork cells with empty lumens and thin suberized walls. The lenticels are formed by the lenticular phellogen, which differentiates under the stomata and has a high meristematic activity. In this region, the cells are unsuberized, with a loose arrangement and intercellular voids, constituting the filling or complementary tissue. After three years, the lenticels appear as small protuberances that soon become conspicuous. Inclusions of sclerenchymatous nodules and isolated sclereids occur occasionally mostly in the vicinity of, or in, the lenticels.

Ascodichaena rugosa . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Ascodichaena rugosa . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. DISEASE: Although generally not regarded as pathogenic, there are some indications that Ascodichaena rugosa can act as a parasite. An ultrastructural study (using German material on beech) showed that hyphae of the fungus were distributed solely in phellem cells; cell-to-cell invasion was effected by penetration hyphae, causing no extensive dissolution of the cork wall; finger-like hyphae in the most recently formed layer of cork cells were interpreted as haustoria, suggesting a parasitic relationship; areas of bark infected by the fungus showed increased production of cork cells (BUTIN & PARAMESWARAN, 1980). On oak, the fungus colonizes cork cells and parasitizes the last separated phellem cells by forming finger-like haustoria. A full description of this disease and symptoms was provided by BUTIN (1981 a ). HOSTS: Acer pseudoplatanus (bark), Carpinus betulus (bark), Castanea sativa (twig), Fagus grandifolia (bark), F. crenata (branch), F. orientalis, F. sylvatica (bark, branch, dead attached twig, dead fallen twig, living bark, living twig, root, trunk, wood), F . sp. (bark), Nothofagus pumilio (twig), Quercus ilex (bark), Q. petraea (bark, dead attached twig, dead fallen bark, dead fallen twig, dead twig, living twig), Q. pubescens (twig), Q. robur (bark, dead attached twig, dead fallen twig, living twig), Q. sessilis (dead attached twig), Q. suber (twig), Quercus sp. (attached wood, bark, branch, dead attached twig, dead fallen twig, living twig, trunk, twig). GEOGRAPHICAL DISTRIBUTION: ASIA: Pakistan, Japan. NORTH AMERICA: Canada (Ontario, Québec), USA (Kansas, Maine, New Hampshire, New York). SOUTH AMERICA: Chile. EUROPE: Austria, Finland, France, Republic of Georgia, Germany, Great Britain, Greece, Ireland, Italy, Poland, Slovak Republic, Sweden, Ukraine. TRANSMISSION: By snails, which graze the stromata, and excrete viable spores (BUTIN, 198l a ).

Genetic and temporal variation in wound healing after abscission of peach leaves in relation to peach canker disease

Leaf scars from 13 peach cultivars and clones with varying susceptibility to peach canker disease were collected during the period November 1986 through May 1987 and were examined histologically. Leaf scar sections were examined with light and fluorescence microscopy for tissue changes related to wound healing, including depth and width of the polysaccharide-impregnated zone, the timing and tissue depth at which the ligno-suberized layer occurred, the thickness and numbers of cells in the ligno-suberized layer, the longitudinal length of the ligno-suberized layer and whether or not the tissue was complete in longitudinal section through the leaf scar, and the presence and numbers of phellem cells in the wound periderm and whether or not the periderm was complete across the longitudinal section of the leaf scar. The percentage of leaf scars with a complete polysaccharide-impregnated zone varied by date with 94–96% of the leaf scars exhibiting the zone in January and April. The thickness of the polysaccharide-impregnated zone also varied by date, reaching its thickest in January and April. Percentages of lignified leaf scars were mostly similar among cultivars on four of the five dates; however on 1 May, there was considerable variation among cultivars with percent lignification ranging from 0 to 100%. The proportion of leaf scars with lignification was not significantly correlated with the field susceptibility rankings for peach canker disease at any of the five dates. The proportion of leaf scars with complete ligno-suberized layers varied by date only, with the highest values observed on 12 January and 14 May. The lowest values was observed on 1 May, and this was significantly less than that observed on 7 November and 10 April. The highest percentage of leaf scars possessing periderm occurred on 12 January (74%), with all the other dates not significantly different from each other (ranging from 29 to 40%). Analysis of cultivar across all dates combined indicated that Candor generally showed the highest percentages of leaf scars with periderm (64%), and this was significantly higher than Sunhaven (24%), Earlired (14%), and V68101 (13%), which were not different from each other. The proportion of leaf scars with periderm was not significantly correlated with the field susceptibility rankings for peach canker disease at any of the five dates. Key words: Prunus, wounding, Leucostoma.

Structure and identification of root bark of Quercus robur L.

The root bark structure of Quercus robur L. was analysed at different stages of root development and compared to the structure of stem bark. Root bark thickness varied considerably between different roots. Sclereid quantity decreased with increasing distance from the stem, which means it increased with age. Visible growth increments diminished with increasing distance from the stem. In lateral roots crystal quantity decreased with increasing distance from the stem. In lateral roots secondary phloem fibre length, sieve tube member length, and sieve tube diameter showed no regular trend. There were only a few basic structural differences between root and stem bark. The zone of cell differentiation (cell expansion, lignification) was wider in root bark; sieve tube collapse was delayed. In lateral root bark fewer sclereids were formed. The first-formed periderm often originated from deeper cell layers. Thus, primary elements were lacking after periderm formation. In root bark the phellem cell walls were of equal thickness. Thus, phellem lacked visible growth increments. Root bark phellem cells were slightly larger. The root phelloderm was more distinct. The secondary phloem fibres were slightly shorter than those in stem bark. Sieve tube members of stem and root bark were of similar length and diameter. The qualitative bark anatomical characters of oak root bark are suitable for root identifications. Due to minor structural differences between root and stem bark the characters must be used with care.