Ultrastructure and Differentiation of Protein-storing Cells in Secondary phloem of Hevea brasiliensis Stem

The ultrastructure and differentiation of the protein-storingcells in secondary phloem of terminal branchlets of Hevea brasiliensisMull. Arg. were studied using electron microscopy. The cellsare parenchyma tissue in the axial system and characterizedby the presence of a large amount of proteinaceous fibrils inthe central vacuole. The fibrils of the protein-storing cellsare straight, about 9 nm in diameter and arranged in a directionroughly parallel to the longitudinal axis of the stem. At theearly differentiation stage of the protein-storing cells, amass of proteinaceous fibrils appears in the cytoplasm, thenis separated from the peripheral cytoplasm by the endomembranesystem derived from endoplasmic reticulum, resulting in theformation of the central vacuole with the fibrils inside asthe vacuolar content The peripheral cytoplasm may continue toproduce proteinaceous fibrils with which the fibrils of thecentral vacuole is supplemented after the protein-storing cellsare formed. Hevea brasiliensis, storage protein, proteinaceous fibrils, vacuole, secondary phloem     

Laticifer Differentiation in Hevea brasiliensis: Induction by Exogenous Jasmonic Acid and Linolenic Acid

Laticifer differentiation of Hevea brasiliensis was investigatedby application of lanolin containing jasmonic acid (JA) or otherchemicals to the surface of young stems in epicormic shoots.The young stems had primary laticifers and no secondary laticifers.When applied to extending young stems, JA led to a significantincrease in primary laticifer number but did not induce secondarylaticifer differentiation. Secondary laticifer differentiationand a less significant increase in primary laticifer numberwere caused by JA application to the extended young stems. Theinduction of the secondary laticifers was dependent on the concentrationof JA applied. Cambium cell division leading to the formationof secondary phloem was not accelerated by JA treatment. Treatedbark tissues showed no visible changes except for the additionallaticifers, which were normal in ultrastructure. The secondarylaticifers were also induced by the application of linolenicacid, a precursor of JA biosynthesis. Abscisic acid, ethephonand salicylic acid had no detectable effect on laticifer differentiation.Copyright 2000 Annals of Botany Company Hevea brasiliensis, laticifer differentiation, jasmonic acid, linolenic acid, vascular cambium.     

Range of Hevea brasiliensis Pollen Dispersal Estimated by Esterase Isozyme Markers

A study was carried out to estimate the distance Hevea brasiliensispollen could be dispersed under natural conditions. Seeds werecollected at varying distances from the boundary between twoadjacent fields that were each planted with a pure stand ofa genetic clone. Esterase isozyme markers were used to determineif the seeds had been derived from self- or cross-pollination.The incidence of cross-pollination was then examined in relationto the distance from the inter-clonal boundary. A logarithmicmodel gave the best fit (r²=0.864) and suggested that pollencould travel distances in the order of 0.3 to 1.1 km. Copyright1999 Annals of Botany Company Hevea brasiliensis (rubber tree), pollen dispersal, cross-pollination, self-pollination, esterase isozymes.     

Metabolism of 2-Chloroethylphosphonic Acid (Ethephon) in Suspension Cultures of Hevea brasiliensis

Suspension cultures of Hevea brasiliensis cells can metabolizethe growth regulator 2-chloroethylphosphonic acid to producea number of compounds, one of which appears to be an acid-labileconjugate. The general metabolic pattern closely resembles thatfound previously using Hevea leaves. Evidence that the compoundsformed are not chromatographic artefacts of a type describedby other authors is presented.     

Manometric Measurement of Turgor Pressures in Laticiferous Phloem Tissues2

A manometric technique for the determination of turgor pressuresin laticiferous phloem tissues has given reproducible resultsin Hevea brasiliensis and a few other tree species. In Hevea,early morning pressures are in the range 7.9–15.0 atmospheres,falling during the day and recovering at night. These diurnalpressure changes are positively correlated with atmosphericrelative humidity and negatively correlated with changes intemperature, evaporation, leaf water deficit, and stomatal opening.They are not displayed by trees devoid of leaves. Thus the lossin turgor is probably the result of withdrawal of water fromthe phloem tissues under transpirational stress. Pressures at the base of the trunk normally exceed those atthe top, the gradient usually approximating to 1 atmosphere/10metres at night and rising up to six times this figure duringthe day. This increase probably reflects the development oftension gradients in the xylem during transpiration. A generalturgor gradient from base to crown does not preclude mass flowin sieve-tubes in the opposite direction provided that ratesof loading and unloading are such that a sufficient osmoticgradient is maintained in them in the required direction. No marked long-term effects of regular tapping on turgor pressurehave been noted in Hevea trees and there is no evidence forseasonal changes in turgor under our conditions.     

Stomatal Characteristics, Leaf Waxes, and Transpiration Rates of Theobroma cacao and Hevea brasiliensis Seedlings

Stomatal size and frequency as well as structure and distributionof leaf waxes were compared in seedlings of two varieties ofTheobroma cacao and three families of Hevea brasiliensis. Stomataof both species were located on the abaxial leaf surface only.The stomata of Theobroma averaged 45% shorter and 29% narrowerthan those of Hevea, but stomatal frequency was 44% higher inTheobroma. Stomatal size and frequency differed more among Heveafamilies than between Theobroma varieties. The structure ofthe leaf waxes of Theobroma and Hevea differed appreciably.It also varied greatly between the abaxial and adaxial leafsurfaces of Hevea, but only slightly in Theobroma. The structureof leaf waxes varied little between Theobroma varieties or amongHevea families. Leaf conductances and transpiration rates weremuch higher in Hevea than in Theobroma seedlings. The implicationsof water relations in interplanting of Theobroma and Hevea arediscussed. Cacao, Theobroma cacao, rubber, Hevea brasiliensis, stomatal size, stomatal frequency, leaf waxes, transpiration     

A Comparison of Abscission of Rubber (Hevea brasiliensis) Leaves Infected with Microcyclus ulei with Leaf Abscission Induced by Ethylene Treatment, Deblading and Senescence

Studies on the histology and on effects of growth substancesand phenols as well as changes in activities of pectinmethylesterase indicated that the mechanism of abscission of Hevealeaflets infected with Microcyclus ulei differed from the mechanismof abscission of debladed, ethylene treated and senescent leaves.An abscission layer which was formed during abscission of debladed,ethylene-treated and senescent leaves was absent during abscissionof heavily diseased leaves. The ratio of pectinmethyl esteraseactivities in tissues distal to the abscission zone to activitiesin tissues proximal to the zone decreased in debladed and ethylenetreated leaves but such decreases were not detected during abscissionof Hevea leaves infected with M. ulei. Hevea brasiliensis Muell. Arg., rubber, leaf abscission, Microcyclus ulei, ethylene, indol-3-ylacetic acid, kinetin     

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     

On the Nature of the Blocking of the Laticiferous System at the Leaf-base of Hevea brasiliensis

A method is described for the anatomical examination of laticiferoussystems. The arrangement of the latex tubes in Hevea brasiliensis isrecorded, and the occlusion of the system at the mature leaf-basesof this plant is shown to be due to the deposition of plugsof callose in the tubes.     

Effects of Growth Regulators on Polyamine Content and Peroxidase Activity in Hevea brasiliensis Callus

3, 4-dichlorophenoxyacetic acid (3,4-D) and benzylaminopurine(BAP) at 9 µM (control medium) was compared with 4.5,2.25, and 0.45 µM for ability to induce callogenesis andembryogenesis from seed explants of Hevea brasiliensis. Supplyingthese growth regulators at 4.5 µM for 20 d improved embryogenicpotential compared with the control medium (El Hadrami, Carronand d'Auzac, 1991, Annals of Botany 67, 511–515), sustainedputrescine, spermidine and spermine at a higher level throughoutof much of the culture period (40–70 d), and maintainedlow levels of peroxidase activity. In the control medium, poorcallus embryogenesis is considered a consequence of rapid ageingof tissues characterized by (i) acceleration of an early buttransient production of polyamines, which promoted embryogeniccapacity, and (ii) an early peak in peroxidase activity thatwas positively correlated with callus browning, one of the factorslimiting embryogenesis. Somatic embryogenesis, polyamines, peroxidase, Hevea brasiliensis, rubber-tree     

Structure of the Bark and Clonal Variability in Hevea brasiliensis Muell. Arg. (Willd. ex A. Juss.)

Detailed studies have been made on the structure of the barkof ten Hevea clones and the clonal variabilities with regardto the density and size of ray groups, density of laticifersper row per unit circumference of the tree, diameter of laticifersand the extent of connection between laticifers. Clonal variabilitywas highly significant with regard to the density of ray groups,ray height, ray width in the laticifer layer and the laticifercharacters. The influence of ray characters on the orientationof laticifers and thereby its quantity is discussed. The scopeof using anatomical parameters for clone identification is examined. Hevea brasiliensis, Para rubber tree, laticifers, bark (structure), anatomy, clonal variability, rubber     

A Complex Sub-cellular Component of Widespread Occurrence in Plant Latices

A survey of exudates from a number of latex-bearing plants revealsthe wide-spread occurrence of a complex subcellular componentapparently analogous to the ‘lutoid’ of Hevea brasiliensislatex. Under the conditions of our experiments this componentis usually associated with thread-like material.     

Stem anatomy and development of inter- and intraxylary phloem in Leptadenia pyrotechnica (Forssk.) Decne. (Asclepiadaceae)

Modification of external morphology and internal structure of plants is a key feature of their successful survival in extreme habitats. They adapt to arid habitats not only by modifying their leaves, but also show several modifications in their conducting system. Therefore, the present study is aimed to investigate the pattern of secondary growth in Leptadenia pyrotechnica (Forssk.) Decne., (Asclepiadaceae), one such species growing in Kachchh district, an arid region of Gujarat State. A single ring of vascular cambium, responsible for radial growth, divided bidirectionally and formed the secondary xylem centripetally and the phloem centrifugally. After a short period of secondary xylem differentiation, small arcs of cambium began to form secondary phloem centripetally instead of secondary xylem. After a short duration of such secondary phloem formation, these segments of cambium resumed their normal function to produce secondary xylem internally. Thus, the phloem strands became embedded within the secondary xylem and formed interxylary phloem islands. Such a recurrent behavior of the vascular cambium resulted in the formation of several patches of interxylary phloem islands. In thick stems the earlier formed non-conducting interxylary phloem showed heavy accumulation of callose on the sieve plates followed by their crushing in response to the addition of new sieve elements. Development of intraxylary phloem is also observed from the cells situated on the pith margin. As secondary growth progresses further, small arcs of internal cambium get initiated between the protoxylem and intraxylary phloem. In the secondary xylem, some of the vessels are exceptionally thick-walled, which may be associated with dry habitats in order to protect the vessel from collapsing during the dryer part of the year. The inter- and intraxylary phloem may also be an adaptive feature to prevent the sieve elements to become non-conducting during summer when the temperature is much higher.     

The Ultrastructure of the Alkaloidal Vesicles of Papaver somniferum latex

Papaver somniferum latex contains abundant small vesicles. Theirultrastructure was studied in tissue sections from adult plantsand in sections of sequential fractions of centrifuged latex.The vesicles were found to exist in two forms, the first witha smooth but progressively granulated outer membrane and thesecond, probably derived from the first, with adherent ‘cap-like’structures which in the heavier centrifuged fractions possesseda zonally-ordered interior. These vesicle fractions were active in synthesizing morphineand the name ‘alkaloidal vesicle’ is proposed forthem. Papaver somniferum latex also contains an organelle whichwas found to resemble a complex organelle present in the latexof Hevea brasiliensis. Its function is not yet known.     

Ethylene Production by Hevea brasiliensis Tissues Treated with Latex Yield-stirnulatory Compounds

Leaflets and pieces of immature stem of Hevea brasiliensis weretreated with a range of compounds, and the subsequent ratesof release of ethylene compared with the reported efficacy ofthe individual substances as stimulants of latex yield. Themost effective ethylene inducers were also the best stimulants.Amongst the 53 compounds tested, no compound which was not ayield stimulant induced the formation of significant amountsof ethylene. Of all the known stimulants tested, only acetylenewas neither converted to, nor induced the formation of ethylene.     

Characterization and expression of an mRNA encoding a wound-induced (Win) protein from ethylene-treated tomato leaf abscission zone tissue

A cDNA clone (TAB7) encoding a putative woundinduced (Win) proteinhas been isolated from a tomato (Lycopersicon esculentum Mill.cv. Ailsa Craig) leaf abscission zone cDNA library using a differentialscreening strategy. The clone has a high degree of homologyat the amino acid level to both the potato win1 and 2 genes,Hevea brasiliensis hevein and Nicotiana tabacum PR-4a and PR-4bproteins. The mRNA encoded by TAB7 is up-regulated within 12h of exposure to ethylene (10µl l^–1) and its expressionincreases steadily within the cells comprising the leaf abscissionzone and to a lesser extent in the adjacent non-zone tissue.This rise precedes the onset of cell separation. Southern analysisindicates that the mRNA is encoded by either a single gene ora small gene family. The role of the protein during abscissionis discussed. Key words: Lycopersicon esculentum, abscission zone, ethylene, tomato, wound-induced proteins     

Interxylary phloem: Diversity and functions

Interxylary phloem is here defined as strands or bands of phloem embedded within the secondary xylem of a stem or root of a plant that has a single vascular cambium. In this definition, interxylary phloem differs from intraxylary phloem, bicollateral bundles, pith bundles, and successive cambia. The inclusive but variously applied terms included phloem and internal phloem must be rejected. Histological aspects of interxylary phloem are reviewed and original data are presented. Topics covered include duration of interxylary phloem; relationship in abundance between sieve tubes in external phloem and interxylary phloem; distinctions between interxylary and intraxylary phloem; presence of parenchyma, fibers, and crystals in the interxylary phloem strands; development of cambia within interxylary phloem strands; three-dimensionalization and longevity of phloem, systematic distribution of interxylary phloem; physiological significance; and habital correlations. No single physiological phenomenon seems to explain all instances of interxylary phloem occurrence, but rapidity and volume of photosynthate transport seem implicated in most instances.     

Influence of Exogenous Hormones on Somatic Embryogenesis in Hevea brasiliensis

Generating somatic embryos from the inner teguments of Heveaseeds is difficult. Like other ligneous plants, the rubber-treeis generally considered to be recalcitrant with regard to somaticembryogenesis. In this paper we show that when conventionallyused amounts of both 3, 4-dichlorophenoxyacetic acid (3, 4-D)and benzylaminopurine (BAP) are reduced by 2–4 times andwhen the length of exposure to these hormones is also reduced,the embryogenic capacity of calli increases without affectingcallogenesis. The germination of several embryos and plantletswas obtained. We conclude that an excess of auxin and cytokininin the medium, even of a weak auxin such as 3, 4-D, and theprolonged exposure to these hormones suppress the embryogeniccapacity of the callus. Hevea brasiliensis, rubber-tree, somatic embryogenesis, hormones     

Phospholipid composition of the membrane of lutoids from Hevea brasiliensis latex

Intact lutoids were isolated from the latex of Hevea brasiliensis and purified on a sucrose density gradient. Lutoid membranes prepared by osmotic lysis are characterized by a high content of phosphatidic acid in which unsaturated and saturated fatty acids are present in equal proportions. Linolenic acid is absent. The results are discussed in relation to the biological role of lutoids in the latex vessels of Hevea brasiliensis.     

Phytophthora spp. from rubbertree plantations in Yunnan Province of China

Phytophthora spp. associated with leaf fall, stem canker and black stripe of rubbertree (Hevea brasiliensis Muell.-Arg.) in Xishuangbanna of Yunnan Province were isolated from leaves, bark, fruit and soil in the plantations. Of a total of 50 isolates, 42 were designated as P. citrophthora (R. E. Sm. & E. H. Sm.) Leonian, while the others were identified as P. palmivora (Butl.) Butl., P. colocasiae Rac. and P. cactorum (Leb. & Conn) Schroet. This appears to be the first report of P. citrophthora on Hevea brasiliensis.