A Crystalline P-Protein in Gmelina arborea

In a light microscope study of the secondary phloem in Gmelinaarborea (Verbenaceae) many sieve elements were found to possessbar-shaped cytoplasmic inclusions of proteinaceous nature Itis suggested that these inclusions represent a type of crystallineP-protein not reported in the family Verbenaceae before Crystalline P-protein, phloem, sieve element inclusion     

Development and Structure of Phloem in the Petiole of Lagenaria siceraria (Mol.) Standl. and Momordica charantia L.

Light microscopic studies of the petioles of Lagenaria sicerariareveal that the external phloem of each bicollateral vascularbundle develops earlier than the internal phloem, and that thesieve elements of the external phloem are arranged in the outerand inner zones. Each sieve element of L. siceraria and Momordicacharantia is vertically associated with a maximum of six andtwo companion cells respectively. Discrete granular bodies seenin the cytoplasm of young sieve elements develop into globular,oval, or elongated slime bodies. Enlargement and fusion of slimebodies, and the subsequent dispersal of slime occur in the parietalcytoplasm. The dispersal of slime coincides with degradationof the nucleus and perforation of the pore sites. Before nucleardisorganization, the sieve-element nucleolus is extruded. Slimeafter its immediate dispersal appears amorphous and uniformlydistributed in the sieve elements. Plugs exhibit varying degreesof condensation of slime near the sieve plates. Certain maturesieve elements in the external phloem of L. siceraria have ovalbodies which we consider reaggregated or undispersed slime.Evidence has been obtained that a central cavity occurs in afew, almost mature, sieve elements wherein the cytoplasm includingthe slime is peripheral.     

Hyperplastic Phloem and Its Plastids in Spinach Infected with the Curly Top Virus

The hyperplastic growth induced in the phloem tissue by infectionwith the curly top virus was studied in minor veins of leavesof spinach, Spinacia oleracea L., by the use of the electronmicroscope. Proliferation of cells occurs in the phloem andin the parenchyma bordering the phloem. The arrangement of cellsis less orderly when hyperplasia occurs in older than in youngertissue but in both instances the majority of cells differentiateinto sieve elements. As in normal phloem, sieve element plastidshaving a ring of proteinaceous fibrils are a consistent featurein the hyperplastic phloem. Depending on the kind of cell inwhich hyperplasia is initiated, the plastids may originate fromyoung plastids similar to those in normal sieve elements orfrom more or less completely differentiated chloroplasts. Theprotoplasts of the hyperplastic sieve elements, including theplastids, degenerate during differentiation or after maturation.     

Localization of ATPase in Sink Tissues of Ricinus

Histochemical localization of ATPase was carried out on phloemtissues from vegetative and reproductive sinks of Ricinus communis,using lead precipitation procedures. Reaction products werelocalized mainly at the plasma membrane of the sieve elements,companion cells and phloem parenchyma cells. Activity was alsopresent in plasmodesmata, the tonoplast of companion cells anddispersed P-protein within the sieve element lumen. The resultsare discussed in relation to the possible involvement of a plasmamembrane ATPase in apoplastic and symplastic unloading fromthe phloem conducting tissues. ATPase, sink tissues, unloading, Ricinus communis     

Some Microscopical Observations of Functioning Sieve Tubes of Heracleum using Nomarski Optics

The study of isolated phloem in Heracleum has been extendedto intact functioning sieve tubes. Techniques of phloem dissectioncombined with Nomarski interference optics have been developedto permit useful visual observations, photomicrography, andciné photographs of sieve tubes which are apparentlynormal. In these preparations, plastids and organelles calledby us ‘marker particles’ are visible in rapid bouncingmotion, and the state of dispersal of these particles is relatedto the amount of damage done to the preparations. The movement of the marker particles and their subsequent fixationshows that they are apparently attached to or restrained byan invisible network in situ. The network is very sensitiveto disturbance and readily collapses around the sieve platesto form slime plugs upon damage to the sieve element. The markerparticles do not move through the cell nor across sieve platesin mature Heracleum. In young cells cyclosis is observable alongthe periphery and this suggests that a vacuole may then be present.In undamaged mature cells there was evidence neither of a vacuolenor of trans-cellular tubules of any size optically detectable. The motion of the particles was greater than Brownian movementand appeared to be under some physiological control. Their movementprobably indicated the presence of an operating transport phenomenon,either because solution was moving past them in the sieve tubeor because they were themselves attached to a contractile networkactively in pulsatory motion. Nearby companion and parenchymacells showed normal cytoplasmic streaming. Proposed mechanisms of translocation involving cytoplasmic streamingdo not seem to be applicable to the phloem of Heracleum. Themovement of the marker particles seemed to agree best with amechanism of ‘activated’ mass flow.     

Seasonal Variation in Radial Growth and Phloem Activity of Terminalia ivorensis A. Chev

Radial growth in five Terminalia ivorensis trees has been recordedfrom dendrometer reading for a period of 12 months. The durationof the growing season was 7–9 months. Variation in annualradial increment between individual trees was observed to bedue both to differences in the length of the growing seasonand the rate of growth during that period. Seasonal changesin the diameter of sieve elements, and the extent of callosedeposition on the sieve plates have also been investigated.Sieve element diameters were smallest in the dry season, possiblybecause of shrinkage. The width of phloem tissue showing definitivecallose was fairly constant throughout the year, but the zonewith open pores on the sieve plates changed, being widest inSeptember, and narrowest in March when the trees were almostbare. There were two peaks of cambial activity, indicated byan increase in width of the ‘open pore zone’, onein April at the time of bud break, and a second in September. The sugar concentration of the phloem exudate obtained fromsmall cuts into the bark of the trees varied throughout theyear. Concentrations were highest in March, during the dry season,and lowest in May, when the young leaves were expanding. Terminalia ivorensis A. Chev., tropical timber tree, radial growth, callose, phloem exudate, phloem activity     

The Effect of Calcium Starvation on Assimilate Partitioning and Mineral Distribution of the Phloem

Populus plants were grown in a medium lacking calcium and exposedto ¹⁴CO₂. In contrast to plants in the complete nutrient medium,the percentage amount of ¹⁴C-assimilates increased in the leavesof calcium-deficient plants and decreased in the stem and theroots. When plants were grown without potassium or magnesiumno differences in the amount of ¹⁴C-label occurred in comparisonwith plants in the complete nutrient medium. Translocation wasrecorded by microautoradiography. It was observed that considerableamounts of labelled photoassimilates were unloaded from thephloem in the middle part of the stem in plants of the completenutrient medium. In contrast, during calcium starvation ¹⁴C-labelwas restricted to the phloem of the stem. In addition, the concentrationsof magnesium and phosphorus showed a remarkable increase instem sieve tubes of calcium-deficient plants. When sieve tubesof source leaves from Populus, barley and maize were comparedwith those of sink leaves, the latter showed higher calciumconcentrations. The results suggest that calcium is a necessaryfactor in the regulation of phloem translocation. Key words: Calcium deficiency, phloem translocation, sieve element loading and unloading, X-ray microanalysis     

The phloem, a miracle of ingenuity

This review deals with aspects of the cellular and molecular biology of the sieve element/companion cell complex, the functional unit of sieve tubes in angiosperms. It includes the following issues: (a) evolution of the sieve elements; (b) the specific structural outfit of sieve elements and its functional significance; (c) modes of cellular and molecular interaction between sieve element and companion cell; (d) plasmodesmal trafficking between sieve element and companion cell as the basis for macromolecular long‐distance signalling in the phloem; (e) diversity of sieve element/companion cell complexes in the respective phloem zones (collection phloem, transport phloem, release phloem); (f) deployment of carriers, pumps and channels on the plasma membrane of sieve element/companion cell complexes in various phloem zones; and (g) implications of the molecular‐cellular equipment of sieve element/companion cells complexes for mass flow of water and solutes in a whole‐plant frame.     

温室白粉虱取食行为的刺探电位(EPG)研究

该文揭示了温室白粉虱Trialeurodes vaporariorum Westwood成虫及幼虫的刺探电位波形与其刺探、取食、产卵行为之间的对应关系,并讨论了这项技术在研究植物抗虫机理方面的应用价值。在成虫,A波、C波分别代表刺探的开始和进行过程;F波代表刺探过程中遇到机械障碍;G波表示吸食木质部导管汁液;E(pd)的(1)和(2)分别表示与取食韧皮部筛管汁液有关的两种行为;粉虱的产卵波形分为两种亚波,分别由Ovi-I和Ovi-II表示,各自代表产卵时的两种行为：产卵器接触并划破叶表皮及卵柄插入叶组织。在幼虫,H波代表吸食筛管液,而L波则表示在筛管细胞内的一种非吸食行为。幼虫蜕皮时先拔出口针,新龄期的幼虫将其口针重新刺入叶组织。     

Observations on Companion Cells and Specialized Phloem Parenchyma Cells of Nelumbo nucifera Gaertn

The phloem of very young petioles of Nelumbo nucifera Gaertn.(Nelumbium speciosum Willd.) was studied with the light microscope.The elongated, mature sieve elements contain slime, plugs, strands,and numerous plastids. Some sieve elements remain nucleatedfor a brief period even after the sieve plates are well developed.The companion cells numbering 8–14 undergo disintegrationbefore the elongation of the ontogenetically related sieve elementis completed. They are uninucleate to begin with but later becomebinucleate and finally degenerate and obliterate. The variousstages in their ontogeny and disintegration are described. Ofthe very few specialized phloem parenchyma cells present, someare associated with sieve elements. They have slime body-likestructures, and plastid-like bodies which group together andeventually disintegrate.     

Plasmatic Filaments and Particles in Mature Sieve Elements of Heracleum sphondylium under the Electron Microscope

By using dimethyl sulphoxide in addition to glutaraldehyde onthe phloem of Heracleum a speedier fixative for phloem sieveelements has been developed, less disruptive to plasmatic filaments(p. f.)- Strands of p. f. under the electron microscope seemto run from sieve element to element, axially through the luminaand the sieve-plate pores. Some of these axial strands giveprofiles which suggest that they exist as tubular structuresin which a series of swellings, 20 to 60 ran in diameter, canbe detected as if microperistalsis were passing along them.Marker particles of five different types were noted, often attachedto wisps of plasmatic filaments. Many of the p. f. seemed tohave a helical substructure.     

The Accumulation of Translocated Material at the Sieve Plate of Dioscorea sansibarensis Pax

The aggregation of radioactive translocates at the sieve fieldsof the sieve plates of Dioscorea sansibarensis Pax is demonstratedby means of tissue autoradiography. The relevance of this findingto current theories of phloem transport is discussed.     

Tension in the Phloem?

The hydrostatic pressure in sieve tubes may probably be estimatedreasonably accurately from experimental measurements of theosmolarity of the sieve tube sap and of the tension in the xylem.The possibility of the existence of phloem tension is advanced.     

The Phloem of Nelumbo nucifera Gaertn

In common with other aquatic angiosperms, Nelumbo nucifera Gaertn.has a relatively strongly developed phloem tissue. The vascularsystem consists of discrete collateral bundles in which no cambiumdevelops and the phloem and xylem are separated by a narrowlayer of parenchyma cells. The phloem consists of sieve elements,companion cells, and phloem parenchyma cells. The sieve elementshave transverse end walls with simple sieve plates. The cellsattain considerable width in the late phloem (metaphloem). Thecompanion cells are in vertical strands. In the early phloem(protophloem) of large bundles the sieve tubes and companioncells are eventually obliterated. The parenchyma cells alsoform vertical strands which may contain tannin cells. Some parenchymacells and companion cells are binucleate. The sieve elementsshow ultrastructural features common for these cells in dicotyledons.At maturity, they lack nuclei, ribosomes, and tonoplasts, butretain a plasmalemma, mitochondria, and plastids. The latterare poorly differentiated and form starch. The endoplasmic reticulumis in part stacked, in part it forms a network next to the plasmalemma.The P-protein occurs in two forms. One consists of tubules notassembled in any specific type of array. The other, possiblycomposed of much extended tubules, is assembled in crystallineaggregates which are retained as such in mature cells. The sieveplate pores are lined with callose and plasmalemma. The lateralwalls are relatively thin and the nacreous layer varies in degreeof distinctness.     

Penetration of faba bean sieve elements by pea aphid does not trigger forisome dispersal

Immediately after their stylets penetrate a phloem sieve element, aphids inject saliva into the sieve element for approximately 30–60 s before they begin to ingest phloem sap. This salivation period is recorded as waveform E1 in electrical penetration graph (EPG) monitoring of aphid feeding behavior. It has been hypothesized that the function of this initial period of phloem salivation is to reverse or prevent plugging of the sieve element by one of the plant's phloem defenses: formation of P‐protein plugs or callose synthesis in the sieve pores that connect adjacent sieve elements. This hypothesis was tested using the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), and faba bean, Vicia faba L. (Fabaceae), as a model system, and the results do not support the hypothesis. In legumes, such as faba bean, P‐protein plugs in sieve elements are formed by dispersal of proteinaceous bodies called forisomes. Contrary to the hypothesis, the great majority of sieve element penetrations by pea aphid stylets do not trigger forisome dispersal. Thirteen sieve elements were cryofixed early in phloem phase before the aphids could complete their salivation period and the forisomes were not dispersed in any of the 13 samples. However, in these samples, the aphids completed on average a little over half of their normal E1 salivation period before they were cryofixed. Thus, it is possible that sieve element penetration triggered forisome dispersal in these samples but the abbreviated period of salivation was still sufficient to reverse dispersal. To rule out this possibility, 17 sieve elements were cryofixed during R‐pds, which are an EPG waveform associated with sieve element penetration but without the characteristic E1 salivation that occurs during phloem phase. In 16 of the 17 samples, the forisomes were not dispersed. Thus, faba bean sieve elements usually do not form P‐protein plugs in response to penetration by pea aphid stylets. Consequently, the characteristic E1 salivation that occurs at the start of each phloem phase does not seem to be necessary to prevent a plugging response because penetration of sieve elements during R‐pds does not trigger forisome dispersal despite the absence of E1 salivation. Furthermore, as P‐protein plugs do not normally form in response to sieve element penetration, E1 salivation that occurs at the start of each phloem phase is not a response to development of a P‐protein plug. Thus, the E1 salivation period at the beginning of the phloem phase appears to have function(s) unrelated to phloem sealing.     

Seasonal Changes in the Structure of the Secondary Phloem of Grewia tiliaefolia, a Deciduous Tree from India

Structure of the secondary phloem of Grewia tillaefolia Roxb.was studied in samples of bark collected at monthly intervalsfrom forest populations of Gujarat in western India. The secondaryphloem in this species is vertically storied and the axial elementsoccur as alternate tangential bands of fibres and sieve elementsproduced in succession. On average, two to four bands of fibresand corresponding bands of sieve elements are produced in ayear. The sieve elements function for more than one season anddifferent phloem increments are separated by terminal zonesmade up of very narrow sieve elements which mature just beforeand immediately after the period of dormancy. The tree becomesleafless about eight to ten weeks preceding the spring equinox.Cambial activity, phloem differentiation and phloem functionare suspended during this period. Differentiation of phloembegins after bud break which occurs in April, and continuesuntil January, but most of the phloem is produced between Julyand September when the rainy season is well advanced. The widthof the conducting zone is maximal at the end of the period ofgrowth when the tree is in full leaf. Inactivation of sieveelements, apparently by callose plugging the sieve plates, beginswith leaf abscission. The sieve elements produced in the precedingseason, just before dormancy is imposed resume function in thefollowing growing season and the older elements die. Companioncells and axial parenchyma cells surrounding sieve elementsappear to have s significant role during senescence of the conductingelements. The development and activity of the secondary phloemseem to be related to other developmental phenomena occurringwithin the tree.     

Comparative Structure of Companion Cells and Phloem Parenchyma Cells in Mimosa pudica L.

The phloem of Mimosa pudica L. furnishes an example of definablediversification of the parenchymatic members of the tissue intocompanion cells and parenchyma cells. The companion cells havedense protoplasts which contain the typical organelles of plantcells, including chloroplasts and many ribosomes. The sieveelements and companion cells are interconnected by numerousbranched plasmodesmata. The companion cells degenerate whenthe associated sieve elements cease to function. The parenchymacells have less dense protoplasts than the companion cells.In many parenchyma cells the rough endoplasmic reticulum assumesa tubular form, and bundles of microfilaments are present. Thecytoplasmic ribosomes occur in groups apparently held togetherby fibrils. Chloroplasts, mitochondria (some are exceptionallylong), dictyosomes, microbodies, and microtubules are the othercell components. Whether the parenchyma cells are ontogeneticallyrelated to the sieve elements or not, they do not degeneratewhen the sieve element ceases to function.     

Unusual Network of Internal Phloem in the Pod Mesocarp of Cowpea [Vigna unguiculata (L.) Walp. (Fabaceae)]

A mycelium-like network of internal phloem was observed in theinner mesocarp of the lateral pod walls of the fruit of certaingenotypes of cowpea [Vigna unguiculata (L.) Walp.] In the cultivarVita 3, the network consists of single, or rarely double, strandsof sieve elements and associated phloem parenchyma, orientedmainly parallel with the fibres of the adjacent endocarp, andstretching marginally beyond the sheets of fibres to connectabove and below with the outermost phloem of the longitudinalstrands of the dorsal and ventral sutures of the fruit. Theinternal phloem network does not relate conformationally to,or interconnect with the conventional (xylem+phloem) vasculatureof the mid mesocarp of the pod wall. In Vita 3, sieve elementsdifferentiate in the internal phloem after those in the majorveins of the pod, but before the presumptive endocarp fibrescommence wall thickening. The pod walls of twenty-one otherspecies of legumes proved negative for internal phloem, whileof nine varied genotypes of cowpea examined, six proved positive,three negative for the trait. Presence of internal phloem incowpea is not always associated with presence of endocarp fibresor necessarily with large fruits with large seeds. Possiblefunctions suggested for the phloem network are to provide assimilatesfor fibre wall thickening or to transport solutes to or fromsites of temporary storage in the fleshy inner layers of thepod wall. Internal phloem, legume fruit, translocation, mesocarp, pod wall, Vigna unguiculata, cowpea     

Developmental Features of the Primary Phloem in Phaseolus vulgaris L.

Certain developmental features of the primary phloem were examinedin Phaseolus vulgaris L., chiefly by the use of the pulvinusat the base of the petiole. The cells included in the studywere the sieve element, the companion cell, and the tannin cell.In the sieve element, the sieve plate shows the usual sequenceof conversion of plasmodesmatal canals into pores. The endoplasmicreticulum, which appears as flat cisternae associated with ribosomesin younger cells, later becomes in part stacked and in partaligned parallel with the walls as a network. The stacked ERprecedes the anastornosing parietal ER in time of development,but the parietal ER persists longer. Of the two forms of P-proteincharacteristic of a number of Fabaceae, the crystalline bodyappears considerably in advance of the body composed of tubules.Neither form of P-protein disperses completely in the maturecell, although the crystalline protein may spread out into aggregatesof fine fibrils. The companion cells show the typical denseprotoplasts and branched plasrnodesmatal connections with thesieve elements. The vacuome of these cells is dispersed intonumerous small vacuoles, many of which appear to be concernedwith autophagic digestion of protoplasmic material. The tannincells have large vacuoles in which the tannin material is located.The cells form vertical series in which the end walls becomeperforated.     

Elasticity of Phloem Tissues1

Linear displacement transducers and a pressure transducer wereused to simultaneously monitor pressure reduction and shrinkageassociated with phloem exudation following incision of the barkof Fraxinus americana L. The shrinkage associated with the pressurereduction indicated that the sieve tubes had elastic walls.Mathematical models of a mass flow system of phloem transporthave assumed rigid walls for the sieve tubes, but results demonstratedthat the dimensional changes associated with pressure potentialchanges would produce significant differences in volume flowrates if the pressure gradient were maintained at a constantvalue, and thus indicate a requirement for modification of themodels to accommodate elastic cell walls.