Xylem-Phloem Exchange Via the Rays: the Undervalued Route of Transport

Journal of Experimental Botany, Vol. 41, No. 227, pp. 631-644, June 1990

REVIEW ARTICLE Xylem-Phloem Exchange Via the Rays: The Undervalued Route of Transport

AART J. E VAN BEL Transport Physiology Research Group, Botanical Laboratory, University of Utrecht, Lange Nieuwstraat 106, 3512 PN Utrecht, The Netherlands

Received 18 September 1989

ABSTRACT The radial solute exchange between xylem and phloem is an important determinant of the nutrient distnbution and C/N economy of seed plants. Nevertheless, our knowledge of the mechanism of xylem-phloem exchange is very limited. This paper aims to integrate anatomical and physiological data about xylem-phloem exchange and focuses on the mechanisms of radial transport. Electrophysiological mapping and intracellular injection of fluorescent dyes demonstrate that stem tissue is organized in symplastic subunits specialized in either longitudinal or lateral transport. Radial orientation, anatomical and physiological organization, strong metabolic activities, relatively negative membrane potentials, and high capacities of uptake make the rays the most likely routes for symplastic xylem-to-phloem transport. Parallel apoplastic transport may occur through radial canacules between the ray cells. The symplastic xylem-to-phloem transport includes a number of steps: (a) passage across the vessel/parenchyma interface, (b) transfer in the ray, and (c) delivery from the ray into the sieve tube/companion cell complex. Large contact pits in the vessel walls give access to the ray cells, by which the solutes are accumulated through substrate/proton co-transport. Dense pitting on the tangential walls and other ultrastructural features suggest preferential symplastic transport in the radial direction. Auxiliary endo/exocytosis is under dispute. The transport step from the ray to the sieve tube/companion cell complex remains to be elucidated. In phloem-to-xylem transport, the unloading from the phloem along the stem is essentially apoplastic, but under specific conditions it may be symplastic. Further transfer to the xylem probably occurs via the symplastic route through the ray. During radial transport, important metabolic interconversions take place into either transport or storage compounds. In many trees, the metabolism of the ray cells undergoes cyclic, annual fluctuations. Complex season-bound deposition/mobilization processes in the ray cells corresponding with seasonal changes in membrane permeability and cellular organization, reflect a master-system designed to co-ordinate the C/N economy throughout the year.

Key words: Xylem-phloem exchange, radial transport.

INTRODUCTION The work of Stout and Hoagland (1939) and Biddulph only for a few trees (Sauter, 1966a, b, 1971, 1980*, 1981a, and Markle (1944) established the phenomenon of solute b, c, 1982a, b, 1983, 1988). transfer from xylem to phloem and vice versa. However, Mineral uptake by roots, solute transport through the our knowledge of the exchange between xylem and xylem, phloem loading and unloading, and the associated phloem is still very limited. Apart from work on legumes long-distance transport and partitioning have been more (Pate, Layzell, and Atkins 1979a; Pate, Layzell, and appealing and rewarding subjects for transport physiolo- McNeil, 1979*; Pate, Atkins, Herridge, and Layzell, 1981; gists. Moreover, transport physiology of roots and leaves Layzell and LaRue, 1982; Pate, 1986) and Triticum (Lam- can be studied without perturbation or only with minor bers, Beilharz, Simpson, and Dalling, 1982; Simpson, damage to relatively large and structurally uniform Lambers, and Dalling, 1982, 1983) dealing with flux organs. Work on radial transport is technically more analyses of organic compounds, systematic documenta- difficult. It requires major surgery to separate the minute tion on xylem-phloem exchange of solutes in intact tissues responsible for xylem-phloem exchange from a herbaceous plants is lacking. The physiological character- bulk of other tissues with the risk of results which are not istics of the radial transfer have been described in depth very representative for the in vivo situation. Therefore,

© Oxford University Press 1990 632 Van Bel—Xylem-Phloem Exchange much knowledge on radial transport has been obtained 19796). In soybean, xylem-to-phloem transport was calcu- using indirect approaches. lated to be responsible for the import of 35% to 52% fruit This paper aims to link anatomical and physiological N (Layzell and LaRue, 1982). aspects of the xylem-phloem exchange studied at the Whole-plant studies in Lupinus (Pate et al., 1979a; b; cellular, tissue and whole-plant level in plant physiology, 1980) and Triticum (Simpson et al., 1982, 1983) revealed a horticulture and silviculture and focuses on the mechan- continuous cycling of nitrogen, mediated by xylem- isms of radial transport. Earlier reviews on radial trans- phloem exchange, and provided a much more dynamic port have laid more emphasis on the metabolism of ray impression of the nitrogen economy than appears from cells (H611, 1975) and the seasonal metabolic changes in mere analysis of plant parts. The key role of xylem- relation to radial flux rates in trees (Sauter, 1982/)). The phloem exchange in C/N cycling and budgetting in herba- present review will be restricted to the most elaborate ceous plants has been reviewed elsewhere (Pate et al., xylem-phloem exchange, the radial transport of solutes in 1980; Simpson, 1986; Pate, 1986). the dicotyledonous stem, with occasional reference to the Trees and, in particular, the deciduous trees of the transport events in conifers. The exchange of solutes in temperate zone, rely on xylem-phloem exchange more petioles and leaf veins is beyond the scope of the review, than any other group of plants, since radial transport is an though solute exchange between xylem and phloem is obligatory part of their survival strategy. Rays function as substantial in these organs (McNeil, Atkins, and Pate, storage organs, to or from which materials are radially 1979; Vogelmann, Dickson, and Larson, 1985; Simpson, imported or exported depending on the season. Seasonal 1986; Van der Schoot, 1989). fluctuations in storage and mobilization occur in decidu- As for the solutes transported, the paper will deal ous forest (e.g. oak-McLaughlin, McConathy, Barnes, primarily with the xylem-to-phloem transport of organic and Edwards, 1980; willow—Sauter, 1981a; 1988; N-compounds which are synthesized in the roots in large poplar—Bonicel, Haddad, and Gagnaire, 1987), in fruit quantities (Bollard, 1960; Pate, 1974) and with the (e.g. peach—Stassen, Stindt, Strydom, and Terblanche, phloem-to-xylem transport of photosynthates. 1981; apple—Titus and Kang, 1982; walnut—Deng, Weinbaum, and DeJong, 1989a; Deng, Weinbaum, IMPORTANCE OF XYLEM-PHLOEM DeJong, and Muraoka, 19896), trees and shrubs (e.g. EXCHANGE FOR SOLUTE kiwi—Ferguson, Eiseman, and Leonard, 1983) as well as PARTITIONING IN INTACT PLANTS in evergreen Gymnosperms (e.g. Picea—H611, 1984; It has become clear that xylem-to-phloem transport is an Pinus—Smith and Paul, 1988) and Angiosperms (e.g. important determinant of the carbon and nitrogen distri- lemon—Kato, 1986; olive—Drossopoulos and Niavis, bution within the plant (Pate et ai, 1980; Simpson, 1986). 1988a, b). A simple experimental system suffices to demonstrate the significant contribution of xylem-to-phloem transport to THE PRESUMPTIVE PATH OF solute partitioning between the metabolic sinks (Van Bel, XYLEM-PHLOEM EXCHANGE 1984). Comparison of the partitioning of the xylem trans- Organic N-compounds produced by the roots are taken port marker inulin carboxylic acid and the unnatural up from the xylem stream along the vascular path. amino acid analog a-aminoisobutyric acid allowed quan- Autoradiographs showed a heterogeneous distribution of tification of the xylem/phloem import ratio of each indi- 14C-labelled amino acids fed to detached shoots; most vidual leaf. For instance, all a-aminoisobutyric acid label was retained by the nodes (Pate, Gunning, and found in a mature leaf (leaf number 5) had been imported Milliken, 1970; McNeil et al., 1979). Likely reasons for via the xylem, whilst the apex (leaf number 13) acquired this are that the vessel surface is enlarged by vascular only 5% of this amino acid through the xylem. Appar- anastomoses in the node and that, with the exception of ently, the nutritional demands of the apical sink leaves legumes (Kuo, Pate, Rainbird, and Atkins, 1980), only were almost fully met by phloem import of solutes nodes contain xylem transfer cells, particularly in the originating from xylem-to-phloem transport in the stem. departing traces (Pate et al., 1970; Gunning, Pate, and More sophisticated modelling of xylem-to-phloem Green, 1970). The transfer cells are disproportionally transport of organic carbon and nitrogen in intact plants active in withdrawal from the vessels (Pate and O'Brien, has been produced by Pate and coworkers (Pate et al., 1968; Pate et al., 1970). In species without transfer cells in 1979a, b; McNeil et al., 1979; Pate et al., 1981; Layzell and the stem, unspecialized internodal xylem parenchyma cells LaRue, 1982). Collection of xylem and phloem sap from are also capable of appreciable uptake from the vessels Lupinus albus enabled the carbon and nitrogen fluxes to be (Van Bel, 19786). analysed. Xylem-to-phloem transport in the stem of Lupi- Ready transfer from xylem to phloem has been found nus was found to provide the shoot apices with 60%, the (Urquhart and Joy, 1982; Pate, Peoples, and Atkins, 1984; fruits with 70% and the nodulated roots with 23% of the Vogelmann et al., 1985). Solutes which are poorly nitrogen they received through the phloem (Pate et al.. absorbed by the xylem parenchyma hardly arrive in the Van Bel—Xylem-Phloem Exchange 633 phloem (Rainbird, Thome, and Hardy, 1984; Yoneyama, ray cells and the interface between ray and sieve tube- 1984; Vogelmann et al., 1985), which shows that escape companion cell complex will be examined in the following from the vessels and appearance in the phloem are sections. correlated. These observations are complemented by (micro)autoradiographs of stems through which 14C-amino THE STRUCTURAL FRAME OF RADIAL acids were translocated administered to the xylem vessels. TRANSPORT The pictures illustrate that the main avenue for radial The vessel/ray interface transfer from xylem to phloem is through the rays It is expected (Sauter, 19826) that solutes from the vessels (McNeil et al., 1979; Dickson, Vogelmann, and Larson, reach the ray cells via the contact pits. These are extremely 1985; Vogelmann et al., 1985). large pits between the ray cells and vessels. The thin, It is well-established that assimilates can escape from loosely woven or even partly open cellulosic network in the sieve tubes and cross to the xylem (Swanson and El the pit (Siau, 1984) may give easy access to water and Shishiny, 1958; Eschrich, 1966; Peel, 1967; Hardy and solutes. This claim is substantiated by the observation Possingham, 1969). Part of the evidence is autoradiogra- that the permeability of the secondary walls to lanthanum phic: 14C-photosynthate produced in the leaves and initi- ions was low, whereas the contact pit membranes exhib- ally transported by the phloem, rapidly shows up in the ited a very high permeability to lanthanum (Wisniewski, xylem (Webb and Gorham, 1965; Eschrich, 1966). Explicit Ashworth, and Schaffer, 1987a, b). Thus, the contact pits micro-autoradiographic evidence on the route of radial constitute the gates between vessel and apoplast of the movement is scarce (Ziegler, 1965; Langenfeld-Heyser, xylem parenchyma. 1987), but demonstrates unequivocally that the radial Before entering the symplast, the solutes must pass the track runs through the rays. protective layer, a peculiar deposit formed in the contact The rays are radial plates of parenchyma cells in the cells of many species after completion of the secondary vascular bundles of larger herbaceous and all woody wall. This tertiary layer is situated unilaterally against the dicotyledons. Their anatomical and physiological organi- paratracheal cell walls (e.g. Robinia; Czaninski, 1973) or zation seems to reflect specialization in xylem-phloem along the whole cell wall with a thicker deposit at the exchange (H611, 1975). If lateral transport occurs through tracheal side (e.g. Ly coper sicon; Mueller and Beckman, the rays, radial solute transport includes at least three 1984). The protective layer is as electron-dense as the steps: uptake from the xylem vessels by the contact ray primary pit walls and is also composed of primary pecto- cells, cell-to-cell transfer through the rays, and delivery cellulosic substances (Czaninski, 1977; Gregory, 1978; into the sieve tubes (Fig. 1). The structural and physiolog- Mueller and Beckman, 1984). The occasional transfer cell- ical features of the vessel/ray cell interface, the array of like projections of the protective layer at the contact pit

contact interparen- pit cnyma pits on the tangential walls

T"\ xylem vessel ray parenchyma sieve tube/ companion cell complex

FIG. 1. Simplified model of the presumptive radial route of xylem-phloem exchange. Solutes from the vessels move via the contact pits to the plasma membrane boundary of the ray symplast after having crossed the protective layer. After transmembrane transfer, cell-to-cell displacement via the plasmodesmata brings the solutes to the vicinity of the sieve tube/companion cell complex. The nature of the path between ray and sieve tube/companion cell complex is uncertain. Materials moving from sieve tube to vessels follow essentially the same route in the opposite direction. In parallel with the symplastic path, an apoplast continuum from vessel to sieve tube is available for the solutes not absorbed by the parenchymatous elements. 634 Van Bel—Xylem-Phloem Exchange areas (Wooding and Northcote, 1965; Mueller and Beck- tangential walls are in the proportion of about 5:5:8, man, 1984; El Mahjoub, LePicard, and Moreau, 1984; respectively (Van der Schoot and Van Bel, 1989ft). The Castro, 1985, 1986) may point to a special role of the frequency of plasmodesmatal contacts in the tangential contact pits in solute transfer. The wall extensions are walls of the ray parenchyma cells of poplar (8-0 plasmo- thought to intensify transmembrane solute transfer by desmata /urn"2 covering 1-98% of the wall surface; Sauter providing a larger membrane surface area for uptake and Kloth, 1986) are comparable to plasmodesmatal (Pate and Gunning, 1972). frequencies of other cells known to be engaged in short- Though the protective layer is highly water-permeable distance transport (Robards, 1976). (Wisniewski et al., 1987a, ft), one wonders why a protec- In trees, some radial rows (contact cell rows) contain tive layer occurs at all. The initial postulate (Schmid, cells with giant pits where the rays border the vessels; 1965) that the layer protects the paratracheal cells against other rows (isolation cell rows) lack cells with such lytic enzymes digesting the primary walls proved to be 'contact pits'. This differentiation in contact and isolation untenable (Czaninski, 1973). At present, the involvement cell rows (Braun, 1967) represents a division of labour. of the protective layer in the formation of tyloses (Foster, Only the contact cells sensu strictu are engaged in the 1967) is generally accepted. Recently, this view has been exchange of solutes with the vessels; the rest of the contact challenged by the suggestion that the layer may function cell row is utilized for storage (Braun, 1967, 1970). The primarily as a buffer against the diurnal oscillations of isolation cells are primarily designed for radial transport hydrostatic pressure in the vessels (Van Bel and Van der (Braun, 1967, 1970). Schoot, 1988). The primary pit walls may be too weak to For rapid radial transport, special adaptations may be withstand tensions generated under slightly negative hy- present at the plasmodesmatal junctions in the rays. dropotentials in the vessels. The protective layer may, Sauter (1982ft) speculated on a mechanism for rapid therefore, act to strengthen the pit membrane to prevent symplastic transfer by which solute vesicles are transferred bulging of the xylem parenchyma (Van Bel and Van der from cell to cell in the tangential pit areas of the ray cells. Schoot, 1988). Since endocytosis may contribute significantly to trans- Where the radial contact cells are absent, the vessels are membrane transfer in plant cells (Saxton and Breiden- often surrounded by a sheath of axial contact cells with bach, 1988; Steer, 1988), it is not inconceivable that large pits (Van der Schoot and Van Bel, 1989ft). The ratio endo/exocytotic events play an important role in the of axial to radial contact cells varies with the plant species radial trafficking of solutes. In tomato ray cells, plasma (e.g. in tomato the ratio is 65:35; Van der Schoot and Van membrane-originating vesicles were observed in the pit Bel, 1989ft). This ratio concurs with the observation that, areas (Van der Schoot, 1989). In developing xylem ele- in sugar maple, axial contact cells appear to have more ments, similar vesicles produced by invagination of the surface area contact with vessels than the ray parenchyma plasma membrane were thought to be associated with cell (Gregory, 1978). wall deposition (Robards, 1968). It is more likely, how- All 'contact elements' (Braun, 1970) or 'vessel-associ- ever, that the vesicles in mature ray cells are operating in ated cells' (Czaninski, 1977) are characterized by a dense cell-to-cell transfer, as their frequency was 30 times higher cytoplasm with well-developed endoplasmic reticulum in the pit areas than along the rest of the cell wall (Van der and numerous ribosomes, mitochondria and peroxisomes Schoot, 1989). and few, if any, amyloplasts (Czaninski, 1977). Together Radial intercellular canacules between the ray cells offer with the transfer cell-like protrusions in the broad pit a potential route for apoplastic flow driven by the transpi- areas (Wooding and Northcote, 1965; Mueller and Beck- ration (Preusser, Dietrichs, and Gottwald, 1961; Fengel, man, 1984; El Mahjoub et al., 1984; Castro, 1985, 1986), 1965; Van der Schoot and Van Bel, 1989ft). The canacules the structural features of the contact cells indicate high probably originate from fusion of schizogeneously- metabolic activities that could fuel massive transmem- formed intercellular spaces (Van der Schoot and Van Bel, brane transport. The homology in strategic location and 1989ft). The apoplastic component of lateral transport ultrastructure suggests a function for the vessel-associated may be significant. This supposition is corroborated by cells similar to that of companion cells in phloem trans- the radial movement of amino acids through the apoplast port (Czaninski, 1977). (Van Bel, 1974a, 1978ft). Lateral escape of alanine from xylem vessels was reduced to 20% when tomato stem The ray parts were wrapped in plastic to prevent transpiration Having arrived in the contact cells, the solutes may (Van Bel, 1974a). Potentially important exchange between move symplastically. The preferential direction of trans- radial apoplast and symplast is indicated by the many port will be radial, as the tangential walls of the ray blind pits in the ray parenchyma walls orientated towards parenchyma are perforated by numerous plasmodesmata, the canacules (Preusser et al., 1961; Fengel, 1965; Van der aggregated in pit fields (Barnett, 1981). In vascular rays of Schoot and Van Bel, 1989ft). The radial canacules may be tomato, the pit frequencies on the transverse, radial and functionally homologous to the radial tracheids in coni- Van Bel—Xylem-Phloem Exchange 635 fers, which are largely responsible for radial permeability the plasmodesmatal path and the electrical cell-cell conduc- (Liese and Bauch, 1967). tance (Overall and Gunning, 1982). The role of the cambial zone in the radial transport is Cell elements with similar membrane potentials were obscure. Perhaps, this tissue plays a special role in the often grouped together in the internodal tissue of tomato radial transport owing to its meristematic character. (Van der Schoot, 1989). This electrical clustering may reflect the existence of symplast domains in the stem, each The ray I sieve tube interface characterized by its specific electric potential. The ray cells Unlike the plasmodesmatal frequencies in the minor constitute cellular plates of relatively negative membrane vascular bundles of the exporting leaves (Gamalei, 1985, potentials ( — 50 to —70 mV) in a matrix of living fibres 1988; Van Bel, Van Kesteren, and Papenhuijzen, 1988), with membrane potentials between -20 and —30 mV the number of plasmodesmatal connections between the (Van der Schoot, 1989). These values are close to those of cells along the phloem transport path are poorly docu- wood ray cells in poplar, where 50% of the membrane mented. In contrast to the intense symplastic contact potentials was in the range between -50 and -90 mV between sieve tube and companion cell, few plasmodes- (Himpkamp, 1988). The rays in tomato likely provide a matal connections exist between the sieve tubes and physiologically insulated path for symplastic radial trans- phloem parenchyma (Esau, 1969, 1973; Kollmann, 1973; port (Van der Schoot, 1989) linking the paratracheal Hayes, Offler, and Patrick, 1985). parenchyma ( — 70 to — 100 mV) and the external phloem It is doubtful whether plasmodesmatal connections (-70 to -80 mV). Both appear to be axial symplast between the parenchymatous phloem elements are evenly domains on the basis of membrane potential clustering. distributed as some authors assume (Kollmann, 1973; The movement of intracellularly injected, membrane- Behnke, 1975). In Mimosa, the plasmodesmatal frequency impermeant fluorescent dyes labelling plasmodesmatal con- between phloem parenchyma and companion cells was nectivity supports the existence of axial symplast domains low compared to the frequency between the phloem in tomato stems such as the sieve tube/companion cell parenchyma cells (Esau, 1973). A similar plasmodesmatal complex (Van der Schoot and Van Bel, 1989c) and the constriction in the symplastic path from the sieve tube to paratracheal parenchyma (Van der Schoot and Van Bel, other phloem cells was found at the companion cell/ 1990). Plasmodesmatal connectivity in rays by radial move- phloem parenchyma interface in Phaseolus (Hayes et al., ment of fluorescent dyes has not been assessed satisfactorily 1985) and in Ricinus (A.J.E. Van Bel and R. Kempers, yet, since the lignified character of the walls and the unpublished results). Owing to the possible symplastic thinness of the cytoplasmic layer impeded intracellular isolation, the sieve tube/companion cell complex may act iontophoresis (Van der Schoot and Van Bel, 1990). as an independently functioning unit in dicotyledons. In conifers, intense plasmodesmatal contact was ob- PHYSIOLOGICAL EVIDENCE FOR served between the sieve cells and the Strasburger cells in PARALLEL TWO-WAY TRANSPORT the rays (Sauter, 1980a). These connections have a charac- FROM XYLEM TO PHLOEM ter similar to that of the sieve tube membrane/companion The vascular architecture and physiological organization cell contacts in Angiosperms (Sauter, Dorr, and Koll- indicate two parallel paths for transport from xylem to mann, 1976). Strasburger cells are, therefore, regarded as phloem: (a) an apoplastic path through the radial canacules, companion cell equivalents, though also on the basis of and (b) a symplastic path through an array of ray cells. many other ultrastructural features (Sauter et al., 1976). Irrespective of the route, the vessel solutes commence The plasmodesmatal coupling between sieve cells, Stras- their radial transport via the highly water-permeable burger cells, and the 'common' ray parenchyma elements contact pits. Thus the escape from the porous vessels (Sauter et al., 1976) suggests a direct symplastic transport (contact pits!) is primarily of a physical kind, namely route from sieve cell to ray in conifers. diffusion out of the moving solution into the adjoining free space. Subsequent active uptake by the xylem paren- THE RADIAL PATH AS A chyma cells maintains a diffusion gradient between vessel PHYSIOLOGICAL DOMAIN and the adjacent apoplast continuum. Horwitz' calcula- Electrophysiological mapping of the membrane potentials tions (1958) predicted a linear relationship between the of the cells in the conducting bundles of tomato internodes logarithm of the solute concentration in the vessels and (where 90% of the xylem elements are alive; Van der Schoot the distance of solute movement as result of first-order and Van Bel, 1989a) suggests that the ray constitutes a kinetics (with a diffusion constant or a first order chemical symplast domain. The term symplast domain was coined by reaction-rate constant). Erwee and Goodwin (1985) and denotes a functional unit of Perfusing 14C-amino acids through tomato excised cells created by symplastic isolation. A relation between internodes indeed produced a logarithmic relationship symplastic discontinuity and electrical disparity logically between the escape and the length of the stem (Van Bel, follows from the linear relationship between the diameter of 1974a, b; Van Bel, Mostert, and Borstlap, 1979). Escape 636 Van Bel—Xylem-Phloem Exchange from the xylem vessels obeyed biphasic saturation kinetics contact cells. In this context, the excessively high ATP- plus a diffusional component (Van Bel et al., 1979). It is concentrations in the xylem parenchyma of tomato may not clear whether the biphasic active uptake from the be significant (Van Bel et al., 1981). vessels is due to carrier systems with different saturation The proton-motive force as the drive of carrier-medi- kinetics (see for a review, Reinhold and Kaplan, 1984) or ated amino acid uptake from the vessels was discovered by to the heterogeneous distribution of substrate in the perfusing buffered solutions with and without organic apoplast from which the solutes are absorbed (Ehwald, solutes through excised internodes (Van Bel and Her- Sammler, and Goring, 1973). mans, 1977; Van Bel and Van Erven, 19796). Substrate/ The most logical explanation for the escape kinetics proton cotransport matches the pH-dependence of the seems that the biphasic kinetics represents active uptake escape from the vessels (Van Bel and Hermans, 1977), as by the xylem parenchyma cells and the diffusional compo- A pH is one of the components of the proton-motive nent reflects solute movement through the apoplast. This force. The sequence of the pH-optima was in the order of interpretation is consistent with the parallel functioning of the pi (isoelectric point) of the amino acids. According to apoplastic and symplastic radial transport. Circumstantial Kinraide and Etherton (1980), acidic amino acids need to evidence for an apoplastic pathway in herbs are the be associated with two protons, whereas neutral amino immense apparent free space of the xylem path, much acids would only need one proton. Under physiological larger than the volume of the vessels (Van Bel, 1978a) and conditions, basic amino acids are always positively the 5-fold increase of lateral transfer induced by transpira- charged and, therefore, depend only on A ¥ for crossing tion (Van Bel, 1974a). the plasma membrane. The fewer protons the amino acids The basal driving force for radial flow of water in the need to be transported, the higher is the pH for optimal direction of the stem periphery, however, may be the high uptake, as with increasing pH, A pH decreasingly and A W osmolarity of the sieve tube contents. This could also be increasingly contribute to the proton-motive force (Van true for tree branches and stems, where stomata are Bel et al., 1981). Selective sugar uptake from the xylem absent. Phloem-to-xylem transport, therefore, probably vessels is likely to be driven by the proton-motive force as follows a one-way track through the radial symplast, as well, in view of its pH-dependent carrier kinetics and the driving force for apoplastic radial mass flow of water sensitivity to metabolic inhibitors (Van Bel, 1976; Van Bel is in the opposite direction. et al., 1979; Sauter, 1981a, 6, 1983; Himpkamp, 1988). The promotive effect of light on amino acid uptake by XYLEM-TO-PHLOEM TRANSPORT internode discs (Van Bel and Van Erven, 1979a) and Transfer from vessel to contact cell excised internodal xylem of tomato (Van Bel and Van der Selectivity of the amino acid escape from the vessels was Schoot, 1980; Van Bel et al., 1981) was ascribed to an observed (Van Die and Vonk, 1967; Hill-Cottingham and enhanced proton-motive force as a result of ATP-produc- Lloyd-Jones, 1968), before a mechanism of transport was tion probably via photophosphorylation (Van Bel et al., established. Later, carrier-mediated uptake by the contact 1981). Appreciable photosynthetic activity was observed cells was found for many amino acids (Van Bel and Van in the chloroplast-rich rays of Fouquieria (Nedoff, Ting, der Schoot, 1980; Van Bel, Van Leeuwenkamp, and Van and Lord, 1985), Fagus (Larcher, Lutz, Nagele, and der Schoot, 1981). Acidic amino acids are remarkably Bodner, 1988) and Pinus (Langenfeld-Heyser, 1989). This poorly absorbed (Van Bel, 19786; Vogelmann et al., assigns ray chloroplasts a function in the xylem-phloem 1985). It is not clear how many carrier types are involved exchange in assisting to maintain a proton-motive force in the amino acid uptake from the vessels. Experiments sufficient to keep the bulk of organic solutes within the with transport mutants of tobacco indicate that only two radial symplast. different carrier systems (one for the acidic, one for the Complex H + /K+ exchanges accompany substrate up- other amino acids) are responsible for amino acid uptake take from the vessels (Van Bel and Van Erven, 1979a, 6), in plants (Borstlap, Schuurmans, and Bourgin, 1987). probably as result of H+/K+ charge compensations. Internodal cell clusters with high membrane potentials, Potassium ions are released into the xylem stream when a like paratracheal parenchyma and rays, simultaneously substrate is taken up through proton symport (Van Bel display a high metabolic activity (Van der Schoot and Van and Van Erven, 19796). In turn, the potassium concentra- Bel, 1990). Furthermore, micrographs of stems treated tion of the xylem sap influences the rate of amino acid with tetrazolium chloride indicating metabolic activity uptake (Van Bel and Van Erven, 1979a, 6; Van Bel and (Van der Schoot and Van Bel, 19896) have a strong Van der Schoot, 1980). K+-concentrations lower than resemblance to micro-autoradiographs of stems fed with 2-0 mol m~3 stimulate amino acid uptake in comparison 14C-labelled amino acids (Dickson et al., 1985; Vogel- with potassium-free controls, whereas K +-concentrations, mann et al., 1985). The positive correlation between higher than 20 mol m"3, do the reverse (Van Bel and Van metabolic activity, membrane potential and uptake rate is der Schoot, 1980). In tomato, a high potassium concentra- suggestive of high rates of active uptake into the vessel tion (30 mol m"3) in the vessels decreased amino acid Van Bel—Xylem-Phloem Exchange 637 withdrawal from the vessels and consequently diminished The kinetics of sucrose uptake by isolated phloem the xylem-to-phloem transport and the supply of the bundles of celery (Daie, 1987) and Cyclamen (Grimm, apical leaves with organic N (Van der Schoot, 1989). The personal communication) demonstrate carrier-mediated 3 inhibitory action of potassium is ascribed to lowering the uptake. The uptake parameters are close: Km 5-0 mol m"" 3 diffusion potential resulting in a decreased active uptake and 15 ftmol h "' (gFW) ~' for celery and Km 5-2 mol m " of organic substrates (Komor, Rotter, and Tanner, 1977; and 4-2 ^mol h"1 (gFW)"1 for Cyclamen. Van Bel and Koops, 1985; Takeda, Senda, Ozeki, and Komamine, 1988). PHLOEM-TO-XYLEM TRANSPORT Potassium also plays a role in the exchange of cations Transfer from sieve tube to ray attached to the negatively charged groups in the vessel The release of solutes from the sieve tube to the stem walls. This exchange interferes with the uptake of basic apoplast is controlled by the same pump/leak system that amino acids (Hill-Cottingham, and Lloyd-Jones, 1972; dictates phloem loading (Minchin and Thorpe, 1987). The Van Bel, 1978/)) and is relevant to several fruit trees, passive character of the delivery is indicated by enhanced where the basic amino acid arginine is the main organic appearance of ' 'C in the stem apoplast in the presence of N-carrier in xylem transport (Bollard, 1960). />-chloromercuribenzenesulphonic acid (Minchin, Ryan, and Thorpe, 1984). However, this criterion should be used Transfer from ray to sieve tube with caution, as /J-chloromercuribenzenesulphonic acid How materials move from the ray to the sieve tube is has been reported to block the sucrose carrier in the unknown. The fragmentary anatomical data is not suffici- uptake (Delrot, Despeghel, and Bonnemain, 1980) or the ent to support any proposal on ray-to-sieve tube transfer. release of sucrose (Anderson, 1986) in leaves. Circumstan- A major objection against symplastic transfer from ray to tial evidence for photosynthate unloading into the apo- sieve tube seems that the solutes have to move against a plast is that plasmolytic disruption of the plasmodesmata concentration gradient (Warmbrodt, 1987). One might did not inhibit the release of photosynthate (Minchin and speculate, therefore, that an apoplastic step is required to Thorpe, 1984; Hayes, Patrick, and Offler, 1987). enable accumulation into the sieve tubes. On the other It is questionable whether passive photosynthate release hand, transport against a concentration gradient may not along the stem is a general phenomenon, as phloem be incompatible with symplastic transfer (for arguments, unloading induced by the stem parasite Cuscuta was see Van Bel et al., 1988). under metabolic control (Wolswinkel, 1974). Treatment The dramatic drop in the membrane potential between with 2,4-dinitrophenol and washing stem segments in sieve tube/companion cell complex (- 90 to - 100 mV) and media of 0 °C strongly reduced phloem unloading. phloem parenchyma (- 40 to - 50 mV) in vascular bundles Rates of phloem unloading to the apoplast in stems of tomato is indicative of the electrical isolation of the sieve have been measured by following efflux of uC-labelled tube/companion cell complex from the adjacent cells (Van substrate (Minchin and Thorpe, 1984; Minchin et al., der Schoot and Van Bel, 1989c). After injection of fluores- 1984) or by changes in the apoplastic pool size of sugars cent dye into the sieve tube or companion cell, the probe (Hayes and Patrick, 1985; Hayes et al., 1987). Given the remained within the complex (Van der Schoot and Van Bel, concentrations of 3 to 100 mol m~3 sucrose in the apo- 1989c). Both findings are compatible with symplastic dis- plast (Patrick and Turvey, 1981; Minchin and Thorpe, continuity between sieve tube/companion cell complex and 1984; Hayes and Patrick, 1985) and 500 mol m"3 in the phloem parenchyma (Esau, 1973; Hayes et al., 1985) which sieve tubes, the passive efflux was computed to be between is consistent with solute retrieval by the sieve tube/compan- 4 and 5 pmol cirr2 s"1 (Patrick, 1990). The passive ion cell complex from the apoplast. release then accounts for 60 to 70% of the delivery from Early work with Ricinus petioles provided the first the sieve tubes (Hayes et al., 1985). This indicates the evidence for a sucrose/proton symport into the sieve tubes existence of a parallel symplastic route of unloading. along the translocation path (Malek and Baker, 1977). When the prevailing source/sink ratio favours net assimi- Direct evidence for proton-driven uptake is the sucrose- late storage in stems, unloading indeed seems to switch to induced depolarization of the sieve tube plasma mem- a symplastic unloading route (Hayes et al., 1987). brane (Wright and Fisher, 1981; Van der Schoot and Van Patrick (1990) speculated that the hydrodynamic plas- Bel, 1989c). The proton-motive force of the sieve tube/ modesmatal properties change with the turgor relations companion cell complex is thought to be substantial due between sieve tube and surrounding phloem parenchyma. to the steep outside-inside pH gradient (Giaquinta, 1983 Pressure-regulated plasmodesmatal valves (Zawadzki and and references therein). The electrogenic component of Fensom, 1986; Cote, Thain, and Fensom, 1987) between the membrane potential (about - 70 mV; Wright and the sieve tube/companion cell complex and phloem paren- Fisher, 1981; Van der Schoot and Van Bel, 1989c) is chyma would be consistent with seasonal apoplastic/sym- generated by ATP-ases at the expense of ATP in the sieve plastic transport shifts in sieve tube unloading along the tube/companion cell complex (Peel, 1987). path. Alternatively, temporal closure of the plasmodesmata 638 Van Bel—Xylem-Phloem Exchange may be obtained by plugging with osmiophilic substances release mechanism for amino acids changing with the time (Kwiatkowski and Maszewski, 1985). Occlusion of the of the year. plasmodesmata could be performed by Ca2+-dependent callose synthesis (Waldmann, Jeblick, and Kauss, 1988). CELL-TO-CELL TRANSFER IN THE RAY Reversible closure mechanisms would be of the utmost SYMPLAST importance for unloading in tree stems, where source/sink On the basis of the arguments put forward in the previous metabolism is strongly related to seasonal rhythms. Ac- sections, it is plausible that most of the xylem-to-phloem cording to this line of reasoning, one might anticipate that transport and all phloem-to-xylem transport goes through day/night oscillations in the hydrostatic potential of the the ray symplast. Symplastic transport accounted for the stem tissue also influence the release from the sieve tubes observed flux rates of sugar from phloem to the ray cells and the intercellular traffic in the rays. Beside the opening in Populus rather than cell-to-cell transfer via the apoplast status of the plasmodesmata, hydropotentials may steer (Sauter and Kloth, 1986). This conclusion was reached membrane transport by sieve tube/companion cell com- through calculations on starch deposition in the rays and plexes (Patrick, 1984; Wolswinkel, 1985) by affecting the the correspondingly required glucose supply (Sauter and membrane potential (Li and Delrot, 1987). Kloth, 1986). In addition, the linear increase of starch in the ray tissue over many weeks (Sauter, 1982a) is compat- Transfer from ray to vessel ible with a structural limitation (i.e. the plasmodesmatal In spring, release from ray cells of trees into the vessels gates). involves massive delivery of amino acids and sugars The broad margins of error in their calculations (Sauter deposited in the ray parenchyma during the previous and Kloth, 1986), however, do not allow assessment of the months (Sauter, 1980*, 1981a, c, 1982a, 1983, 1988). nature of the symplastic transport. According to Ziegler During the rest of the year, only minute amounts of (1961), the transport velocities in rays are incompatible assimilates arrive in the vessels (Sauter, 19806, 1981a, with the classic concept of diffusional transfer. Sauter 1982a, 1988). The retrieval systems of the ray cells appear hypothesized that vesicles, aggregated at the pit fields of to be so well-developed (Van Bel, 1976; Himpkamp, 1988) ray cells (Sauter, 19826; Van der Schoot, 1989), are that hardly any sugars can escape under appropriate involved in the intensification of solute cell-to-cell trans- temperature conditions (Sauter, 19816, 1983; Vogelmann fer. It is not clear whether the vesicles originate from the et al., 1985). The same probably applies to amino acids endoplasmic reticulum (Sauter, 19826) or the plasma (Hill-Cottingham and Lloyd-Jones, 1968; Van Bel, 19786; membrane (Van der Schoot, 1989). Possibly, at least two Dickson et al., 1985). Whilst resorption by the contact ray types of vesicles occur: one (from the endoplasmic reticu- cells undoubtedly represents a substrate/proton co-uptake lum) containing enzymes engaged in the endo/exocytosis (Van Bel and Van Erven, 1979a; Himpkamp, 1988), the of metabolites enclosed by the second type (from the nature of the release is less evident. plasma membrane). A major complication in the assessment of the leakage The polarity of the phosphatase activity—at the cam- mechanism of sucrose is that sucrose is hydrolysed by bium-facing tangential walls during mobilization, at the apoplastic acid invertase and can be resorbed in the form opposite walls during deposition—suggests involvement of hexoses (Sauter, 19816, c, 1983). The nature of sucrose in the transport of solutes. Sauter (19826) advanced leakage in trees probably varies with the season (Sauter, strong arguments in support of a key role of acid phos- 1988). Passive leakage in winter may result from mem- phatase in the mobilization of starch. However, the brane damage due to repetitive freezing and thawing permanently high phosphatase activity at the contact pits (Sauter, 19806). Sucrose leakage induced by 2 °C pro- (Sauter, 19666, 1967), even when starch is completely ceeded more vigorously over a few days than in the 20 °C- dissolved in the contact cells, indicates that the phospha- controls (Sauter, 19806, 1982a). The leakage, however, tase also may be involved in transmembrane transfer. was partly inhibited by p-chloromercuribenzoate and Phosphatase activity at the contact area has been associ- NaF indicating simultaneous facilitated exodiffusion via ated with active sucrose secretion into the xylem vessel in the sucrose carriers (Sauter, 19806, 1982a). The conspicu- spring (Sauter, 19826). ous, well-timed burst of sucrose release in mid-spring was The frequent irregularities in the polarity of the phos- ascribed to active release of sucrose (Sauter, 1988). In that phatase activity (Sauter and Braun, 1968) have been period, exocytotic sucrose release may be required to attributed to differential sink polarities in individual ray attain a sufficient delivery into tree vessels (Sauter, 19826). sections (Sauter, 19826). This phenomenon, however, On the basis of comparison with amino acid transport could also reflect a division of labour between the rays: in other tissues, it was concluded (Sauter, 1981a) that the some of them could be specialized in xylem-to-phloem amino acid efflux rates were unlikely to be caused merely transport, others in phloem-to-xylem transport. Alterna- by exodiffusion. In parallel with the proposals for sugar tively, one part of a ray could be designed for xylem-to- delivery, it might be worthwhile to consider a differential phloem transport, the other part for transport in the Van Bel—Xylem-Phloem Exchange 639 opposite direction. The phosphatase test may be useful to filled last, is the preferential route of radial transfer, which detect such a differential transport through rays. functions best when the starch obstacles have been re- moved. METABOLIC EVENTS IN THE RADIAL Starch synthesis/breakdown in spring was temperature- PATH dependent: starch was mobilized at temperatures lower Solute transformation in the radial path determines the than 5 °C and higher than 10 °C and deposited between quantity and species of compounds available for transfer 5 °C and 10 °C (Sauter, 1967). The mobilization of protein and, hence, can be considered as one of the basic determi- probably is also temperature-dependent. In contrast to nants of xylem-to-phloem exchange. Only aspects of starch, protein shows a two-peak mobilization in spring: metabolism relevant to xylem/phloem exchange will be the first one corresponding with the blossoming of the briefly discussed here. male catkins, the second with the sprouting of the vegeta- After feeding 14C-compounds to the xylem vessels of tive buds (Sauter, 1981a). The spatial and temporal detached shoots, tracheal sap was obtained by vacuum distribution of fat and protein deposition/mobilization in extraction and phloem sap by spontaneous bleeding (Pate, trees is an interesting target for future studies, as our Sharkey, and Lewis, 1975; Sharkey and Pate, 1975). In knowledge about this subject is very restricted. Spartium, asparagine passed largely unchanged to the Apart from the balance between storage and mobiliza- phloem whereas the 14C from aspartic acid or glutamine tion, membrane permeability of the ray cells is season- appeared in the phloem sap attached to other amino acids dependent. Sucrose uptake from the xylem vessels of Salix or in a number of non-amino compounds (Pate et al., is subject to seasonal changes (Sauter, 1983). During 1975). In Lupinus, certain 14C-labelled compounds (val- summer, this uptake is about four times higher than in ine, asparagine, threonine, serine, citrulline, glutamine) mid-winter. Similar seasonal patterns were observed for were transferred rapidly in an unchanged form from glucose uptake by the xylem ray cells of Populus (Himp- xylem to phloem. Of others (glycine, methionine, aspartic kamp, 1988). The latter study analysed the factors acid, homoserine, glutamic acid and gamma-aminobu- bringing about the seasonal changes in transmembrane tyric acid), the bulk appeared in the phloem in a variety of flux. The pH-dependence of uptake indicated the involve- compounds (Sharkey and Pate, 1975). Rapid incorpora- ment of sugar/proton cotransport solely between April tion of threonine and alanine into protein was observed in and September. From October onwards, uptake kinetics the rays of Populus (Vogelmann et al., 1985). Whereas display a constant diffusional component starting to alanine was fully transformed, part of the threonine decrease in April, until the diffusional flux rate is five moved unchanged to the phloem. times lower in July than in December. Active uptake is In trees, storage/mobilization and long-distance trans- negligible from December to March. In April, an active port closely correspond according to annual cycles (see for uptake component rapidly develops, the Vm of which is a review, Sauter, 1982/?). The availability of amino acids twenty times higher between May and September than in and sugars for radial transport is seasonally dependent as the mid-winter period. The A"m of the active uptake illustrated by inversely related seasonal fluctuations in free remains constant throughout the year (Himpkamp, 1988). metabolites in xylem and phloem and in protein, fat and The passive permeability of the plasma membrane starch deposits in the rays (McLaughlin et al., 1980; seemingly is maximal during winter and early spring. Stassen et al., 1981; Titus and Kang, 1982; Sauter, 1982*; Solutes which are mobilized during this period, leak en Ferguson et al., 1983; H611, 1984; Kato, 1986; Bonicel et masse into the vessels, the rate of which is accentuated by al., 1987; Smith and Paul, 1988; Drossopoulos and Niavis, the low level of active retrieval (Sauter, 19826). 1988a, 6; Deng et al., 1989a, 6). The balance between The differential temperature effects in autumn and storage and mobilization undergoes seasonal changes spring on enzyme activities involved in the deposition/mo- dependent on factors which will be highlighted here bilization of starch (Sauter, 19826) and the annual change briefly. of polarity of the phosphatase activity in the ray cells (Sauter, 19666, 1967; Sauter and Braun, 1968) underline DETERMINANTS OF SEASONAL that radial transport is a process dependent on numerous, FLUCTUATIONS IN RADIAL TRANSPORT partly unknown metabolic determinants. Sauter (1966a, 19826) studied in detail the seasonal fluctuations of starch metabolism and distinguished six stages MODEL OF XYLEM-TO-PHLOEM of starch synthesis/breakdown in the xylem of Populus. TRANSPORT The breakdown of starch followed a specific scenario Distribution of 14C-amino acids in response to various (Sauter, 1966a). Starch deposition in summer and autumn manipulations allowed assessment of some determinants followed the reverse order (Sauter, 1966a). The sequence of xylem-to-phloem transport in herbaceous plants (Van of events suggests that the pathway via contact cell and Bel, 1984; Van der Schoot, 1989). These experiments isolation cell row, the elements being cleared first and exploited the pecularity that, in detached shoots, the ray 640 Van Bel—Xylem-Phloem Exchange system acts as a valve between the communicating vessels, CONCLUSIONS xylem and phloem. It was concluded that xylem-to- In conclusion, the determinants of xylem-phloem ex- phloem transport (a) occurs along the whole vascular path change are: with preference for the nodes, (b) is inversely related to the (1) the flow rate in the transport channels. The rate of flow rate in the vessels, (c) is dependent on the solute mass flow in the vessels is related to the transpiration, concentration in the vessels, (d) is affected by the K + - in sieve tubes to the difference in loading and unload- concentration in the vessels, (e) has an appreciable sym- ing rates at the extremes of the phloem system; plastic component as shown by the differential, reductive (2) the substrate concentration in the long-distance trans- effect of metabolic inhibitors, (f) is dependent on storage in the ray parenchyma, as high amino acid pools prevent port channels, which in turn is controlled by various uptake by the ray symplast and subsequent phloem factors; loading. These parameters are used to construct a (3) the substrate-specific carrier-mediated uptake in contact semi-quantitative two-way model of xylem-to-phloem cells, ray cells, and sieve tube/companion cell complexes; transport (Fig. 2). Essential for this model is that the (4) the radial transport velocities through the symplastic proportion between apoplastic and symplastic radial and apoplastic paths; transport to the phloem in the stem changes with the (5) the metabolic conversions in the symplastic path endogenous and environmental conditions. including deposition and mobilization of the reserves. The deposition/mobilization balance is dependent on The results conform with partitioning of 14C-materials seasonal changes in metabolic activity, enzyme activi- applied to the base of Populus shoots (Vogelmann et ai, ties, organelle positioning, membrane permeability 1985). The acidic amino acids, poorly absorbed by the and retrieval capacity of the vascular symplast. xylem parenchyma, arrived in the mature leaves via the xylem, while threonine, accumulated and partly metabol- ACKNOWLEDGEMENTS ized by the ray cells, was transported to the apical leaves The author is indebted to Drs John Patrick, Michael via the phloem (Vogelmann et al., 1985). Alanine and Erwee, and Chris van der Schoot for critically reading the sucrose were completely retained by the xylem paren- manuscript. chyma of poplar. LITERATURE CITED ANDERSON, J. M., 1986. Sucrose release from soybean leaf slices. Physwlogia plantarum, 66, 319-27. BARNETT, J. R., 1981. Secondary xylem cell development. In Xylem cell development. Ed. J. R. Barnett. Castle House Publications Ltd. Tunbridge Wells, UK. Pp. 47-95. BEHNKE, H. D., 1975. Companion cells and transfer cells. In Phloem transport. Eds S. Aronoff, J. Dainty, P. R. Gorham, L. M. Srivastava and C. A. Swanson. Plenum Press, New York, London. Pp. 153-75. BIDDULPH, O., and MARKLE, J., 1944. Translocation of radio- phosphorus in the phloem of the cotton plant. American Journal of Botany, 31, 65-70. BOLLARD, E. G., I960. Transport in xylem. Annual Review of Plant Physiology, 11, 141-66. BONICEL, A., HADDAD, G., and GAGNAIRE, J., 1987. Seasonal variations of starch and major soluble sugars in different organs of young poplars. Plant Physiology and Biochemistry, ray cells with plasmodesmata ( = ) 25,451-9. BORSTLAP, A. C, SCHUURMANS, J., and BOURGIN, J.-P., 1985. Amino-acid-transport mutant of Nicotiana tabacum L. Planta, 166, 141-4. BRAUN, H. J., 1967. Entwicklung und Bau der Holzstrahlen FIG 2. Tentative model of the two-way xylem-to-phloem transport unter dem Aspekt der Kontakt-Isolations-Differenzierung ge- (Van der Schoot, 1989). Symplastic path. (1) water and solute escape genuber dem Hydrosystem. I.Das Prinnp der Kontakt-Isola- from the vessel via the contact pits, (2) membrane passage into the ray tions DifTerenzierung. Holzforschung, 21, 33-7. symplast domain, (3) exchange between cytoplasm and vacuoles or 1970. Funktionelle Histologie der sekundaren Sprossachse. storage metabolism, (4) transfer from cell to cell via plasmodesmata, (5) I.Das Holz. Handbuch der Pflanzenanatomie 9,1. Gebruder displacement to the sieve tube/companion cell complex, either symplastic Borntraeger, Berlin Stuttgart. or apoplastic. Apoplastic path. (1) diffusion of solutes not captured by the ray cells in the direction of the sieve tube, (2) partial withdrawal by CASTRO, M. A., 1985. Structure of the vessel-parenchyma pit the cells along the radial apoplast. (6) arrival in the sieve tube/compan- membrane in some species of Lauraceae. IA WA Bulletin, n.s. ion cell complex The diffusion gradient in the ray apoplast depends on 6, 35-8. the vessel concentration C, and the uptake rates at '2' and '6'. respec- 1986. Vessel-parenchyma pit membranes in Cucurbita tively. maxima Duch. Ibid. 7, 151-4. Van Bel—Xylem-Phloem Exchange 641

C(3TE, R., THAIN, J. F., and FENSOM, D. S, 1987. Increase in GREGORY, R. A., 1978. Living elements of the conducting electrical resistance of plasmodesmata of Chara induced by secondary xylem of sugar maple (Acer saccharum Marsh.). applied pressure gradient across nodes. Canadian Journal of IA WA Bulletin, 4, 65-9. Botany, 65, 509-11. GUNNING, B. E. S., PATE, J. S., and GREEN, L. W., 1970. Transfer CZANINSKI, Y., 1973. Observations sur une nouvelle couche cells in the vascular system of stems: taxonomy associated parietale dans les cellules associees aux vaisseaux du robinier with nodes and structure. Protoplasma, 71, 141-71. et du sycomore. Protoplasma, 77, 211-19. HARDY, P. J., and POSSINGHAM, J. V., 1969. Studies on the 1977. Vessel-associated cells. IAWA Bulletin, 3, 51-5. translocation of metabolites in the xylem of grapevine shoots. DAIE, J., 1987. Sucrose uptake in isolated phloem of celery' is a Journal of Experimental Botany, 20, 325-35. single saturable system. Planta, 171, 474-82. HAYFS, P. M., OFFLER, C. E., and PATRJCK, J. W., 1985. Cellular DELROT, S., DESPEGHEL, J.-P., and BONNEMAIN, J.-L., 1980. structures, plasma membrane surface areas and plasmodesma- Phloem loading in Viciafaba leaves: effects of N-ethylmaleim- tal frequencies of the stem of Phaseolus vulgaris L. in relation to ide and ^-chloromercuribenzenesulfonic acid on H +-extru- radial photosynthate transfer. Annals of Botany, 56, 125-38. sion, K+ and sucrose uptake. Ibid. 149, 144-8. and PATRICK, J. W., 1985. Photosynthate transport in DENG, X., WEINBAUM, S. A., and DEJONG, T. M., 1989a. Use of stems of Phaseolus vulgaris L. treated with gibberellic acid, labeled nitrogen to monitor transition in nitrogen dependence indole-acetic acid or kinetin. Effects at the site of hormone from storage to current-year uptake in mature walnut trees. application. Planta, 166, 371-9. Trees, 3, 11-16. and OFFLER, C. E., 1987. The cellular pathway of and MURAOKA, T. T., 1989ft. 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Nachweis und Charaktensierung eines tissues of olive tree (Olea europea L.). I. Nitrogenous com- H+/Glucose-Cotransportsystems in den Holzstrahlenzellen pounds. Annals of Botany, 62, 313-20. von Populus balsamifera L. (Salicaceae) und seine jahreszeit- 1988ft. Seasonal changes of the metabolites in the liche Veranderung. PhD-dissertation, Christian Albrecht leaves, bark and xylem tissues of olive tree {Olea europea L.). Universitat, Kiel, Federal Republic Germany. II. Carbohydrates. Ibid. 62, 321-7. H6LL, W., 1975. Radial transport in rays. In Encyclopedia of EHWALD, R., SAMMLER, P., and GORING, H., 1973. Die Bedeu- Plant physiology, transport in plants I, phloem transport. Eds tung der Diffusion im 'freien Raum' fur die Konzentrations- M. H. Zimmermann and J. A. Milburn. Springer-Verlag, abhangigkeit der Aufnahme von Zuckern und Ionen durch Berlin, Heidelberg, New York. Pp. 432-50. pflanzliche Gewebe. Biochemie und Phvsiologie der Pflanzen, 1984. Seasonal fluctuation of reserve materials in the 164, 596-613. trunkwood of spruce [Picea abies (L.)Karst.]. Journal of Plant EL MAHJOUB, M., LEPICARD, D., and MOREAU, M., 1984. Origin Physiology, 117, 355-62. of tyloses in melon (Cucumis melo L.) in response to a vascular HORWITZ, L., 1958. Some simplified mathematical treatments of Fusarium. IAWA Bulletin, n.s. 5, 307-11. translocation in plants. Plant Physiology, 33, 81-93. ERWEE, M. G., and GOODWIN, P. B., 1985. Symplast domains in KATO, T , 1986. Nitrogen metabolism and utilization in Citrus. extrastelar tissues of Egeria densa Planch. Planta, 163, 9-19. Horticultural Reviews, 8, 181-216. ESAU, K., 1969. The phloem. In Handbuch der Pflanzenanatomie KINRAIDE, T. B., and ETHERTON, B., 1980. Electrical evidence for V,2. Gebruder Borntraeger, Berlin Stuttgart. Pp. 116-34. different mechanisms of uptake for basic, neutral, and acidic 1973. Comparative study of companion cells and phloem amino acids in oat coleoptiles. Plant Physiology, 65, 1085-9. parenchyma cells in Mimosa pudica L. Annals of Botany, 37, KOLLMANN, R., 1973. Cytologie des Phloems. In Grundlagen der 625-32. Cvtologie. Eds G. Hirsch, H. Ruska and P. Sitte. Gustav ESCHRICH, W., 1966. Translokation '^C-markierter Assimilate Fischer Verlag Jena. Pp. 479-504. im Licht und im Dunkeln bei Viciafaba. Planta, 70, 99-124. KOMOR, E., ROTTER, M., and TANNER, W., 1977. A proton- FENGEL, D., 1965. Elektronenmikroskopische Beitrage zum cotransport system in a higher plant: sucrose transport in Feinbau des Buchenholzes (Fagus syhatica L.)—Erste Mitteil- Ricinus communis. Plant Science Letters, 9, 153-62. ung: Untersuchungen an Markstrahl-Parenchymzellen. Holz Kuo, J., PATE, J. S., RAINBIRD, R. M., and ATKINS, C. A., 1980. Roh- und Werksloff, 23, 257-63 Internodes of grain legumes—new location for xylem paren- FERGUSON, A. R., EISEMAN, J. A., and LEONARD, J. A., 1983. chyma cells. Protoplasma, 104, 181-5. Xylem sap from Actinidia chinensis: seasonal changes in KWIATKOWSKI, M., and MASZEWSKI, J., 1985. Changes in ultra- composition. Annals of Botany, 51, 823-33. structure of plasmodesmata during spermatogenesis in Chara FOSTER, R. C, 1967. Fine structure of tyloses in three species of the vulgaris L.. Planta, 166, 46-50. Myrtaceae. Australian Journal of Biological Sciences, 15, 25-34. LAMBERS, H., SIMPSON, R. J., BEILHARZ, V. C, and DALLING, M. GAMALEI, Y. V., 1985. Characteristics of phloem loading in J., 1982. Growth and translocation of C and N in wheat woody and herbaceous plants. Soviet Plant Phystologv, 32, (Triticum aestivum) grown with a split root system. Physiolo- 656-65. gia plantarum, 56, 421-9. 1988. The taxonomical distribution of the leaf minor vein LANGENFELD-HEYSER, R., 1987. Distribution of leaf assimilates types. Botancheskii Zhournal, 73, 1662-72. in the stem of Picea abies L. Trees, 1, 97-103. GIAQUINTA, R. T., 1983. Phloem loading of sucrose. Annual 1989. CO2-fixation in stem slices of Picea abies (L.) Karst: Review of Plant Physiology, 34, 347-87. microautoradiographic studies. Ibid. 3, 24-32. 642 Van Bel—Xylem-Phloem Exchange

LARCHER, W., LUTZ, W., NAGELE, H., and BODNER, M., 1988. compounds of the shoot of a higher plant. Planta, 78, Photosynthetic functioning and ultrastructure of chloroplasts 60-71. in stem tissues of Fagus sylvatica. Journal of Plant Physiology, — PEOPLES, M. B., and ATKINS, C. A., 1984. Spontaneous 132,731-7. phloem bleeding from cryopunctured fruits of a ureide- LAYZELL, D. B., and LARUE, T. A., 1982. Modelling C and N producing legume. Plant Physiology, 74, 499-505. transport to developing soybean fruits. Plant Physiology, 70, SHARKEY, P. J., and LEWIS, O. A. M., 1975. Xylem to 1290-8. phloem transfer of solutes in fruiting shoots of legumes Li, Z.-S., and DELROT, S., 1987. Osmotic dependence of the studied by a phloem bleeding technique. Planta, 122, 11-26 transmembrane potential difference of broadbean mesocarp PATRICK, J. W., 1984. Photosynthate unloading from seed coats cells. Ibid. 84, 895-9. of Phaseolus vulgaris L. Control by tissue water relations. LIESE, W., and BAUCH, J., 1967. On the anatomical causes of the Journal of Plant Physiology, 115, 297-310. refractory behaviour of spruce and douglas fir. Journal of the 1990. Sieve element unloading: cellular pathway, mechan- Institute of Wood Science, 19, 3-14. ism and control. Physiologia plantarum, 78, 298-308. MALEK, F., and BAKER, D. A., 1977. Proton co-transport of -andTuRVEY, P. M., 1981. The pathway of radial transfer of sugars in phloem loading. Planta, 135, 297-9. photosynthate in decapitated stems of Phaseolus vulgaris L. MCLAUGHUN, S. B., MCCONATHY, R. K., BARNES, R. L., and Annals of Botany, 47, 611-21. EDWARDS, N. T., 1980. Seasonal changes in energy allocation PEEL, A. J., 1967. Demonstration of solute movement from the by white oak (Quercus alba). Canadian Journal of Forestry extracambial tissue into the xylem stream of willow. Ibid. 18, Research, 10, 379-88. 600-6. MCNEIL, D. L., ATKINS, C. A., and PATE, J S., 1979. Uptake 1987. Energy relations of solute loading in sieve elements of and utilization of xylem-borne amino acids by shoot organs of willow. Planta, 172, 209-13. a legume Plant Physiology, 63, 1076-81. PREUSSER, H.-J., DIETRICHS, H. H., and GOTTWALD, H., 1961. MINCHIN, P. E. H., RYAN, K. G., and THORPE, M. R., 1984. Elektronenmikroskopische Untersuchungen an ultradiinnen Further evidence of apoplastic unloading into the stem of Schnitten des Markstrahlenparenchyms der Rotbuche—Fagus bean: identification of the phloem buffering pool. Journal of svlvatica L. Holzforschung 15, 65-75. Experimental Botany, 35, 1744-53. RAINBIRD, R. M., THORNE, J. H., and HARDY, R. W. F., 1984. and THORPE, M. R., 1984. Apoplastic phloem unloading in Role of amides, amino acids, and ureides in the nutrition of the stem of bean. Ibid. 35, 538-50. developing soybean seeds. Plant Physiology, 74, 329-34 1987. Measurement of unloading and reloading of REINHOLD, L., and KAPLAN, A., 1984. Membrane transport of photo-assimilate within the stem of bean. Ibid. 38, 211-20. sugars and amino acids. Annual Review of Plant Physiology, MUELLER, W. C, and BECKMAN, C. H., 1984. Ultrastructure of 53, 45-83. the cell wall of vessel contact cells in the xylem of tomato ROBARDS, A. W , 1968. On the ultrastructure of differentiating stems. Annals of Botany, 53, 107-14. secondary xylem in willow. Protoplasma, 65, 449-64. NEDOFF, J. A., TING, J. P., and LORD, E. M., 1985. Structure and 1976. Plasmodesmata in higher plants. In Intercellular function of the green stem tissue in ocotillo (Fouquieria communication in plants: studies on plasmodesmata. Eds B. E. splendens). American Journal of Botany, 72, 143-51. S Gunning and A. W. Robards. Springer-Verlag, Berlin, OVERALL, R. L., and GUNNING, B. E. S., 1982. Intercellular Heidelberg, New York. Pp. 15-57. coupling in Azolla roots. I. Electrical coupling Protoplasma, SAUTER, J. J., 1966a. Untersuchungen zur Physiologie der 111, 151-60. Pappelholzstrahlen. I. Jahresperiodischer Verlauf der PATE, J. S., 1974. Uptake, assimilation and transport of nitrogen Starkespeicherung im Holzstrahlparenchym. Zeitschrift fiir compounds by plants. Soil Biology and Biochemistry, 5, Pflanzenphysiologie, 55, 246-58. 109-19. 19666. Untersuchungen zur Physiologie der Pappelholz- 1986. Xylem-to-phloem transfer—Vital component of the strahlen. II. Jahresperiodische Anderungen der Phosphata- nitrogen-partitioning system of a nodulated legume. In seaktivitat im Holzstrahlparenchym und ihre mogliche Bedeu- Phloem transport. Eds J. Cronshaw, W. J. Lucas and R. T. tung fur den KohlenhydratstofTwechsel und den aktiven Giaquinta. Alan R. Liss, New York, Pp. 445-62. Assimilattransport. Ibid. 55, 349-62. -ATKINS, C. A., HERRIDGE, D. F., and LAYZELL, D. B., 1981. 1967. Der Einfluss verschiedener Temperaturen auf die Synthesis, storage, and utilization of amino compounds in Reservestarke in parenchymatischen Geweben von Baum- white lupin (Lupinus albus L.). Plant Physiology, 67, 37-42. sprossachsen. Ibid. 56, 339-52. and GUNNING, B. E. S., 1972. Transfer cells. Annual Review 1971. Respiratory and phosphatase activities in contact of Plant Physiology, 23, 173-96. cells of wood rays and their possible roles in sugar secretion and MILLIKEN, F. F., 1970. Function of transfer cells Ibid. 67, 135-45. in the nodal regions of stems, particularly in relation to the 1980a. The Strasburger cells—equivalents of the compan- nutrition of young seedlings. Protoplasma, 71, 313-34. ion cells. Berichte der Deutschen Botanischen Gesellschaft, 93, LAYZELL, D. B., and ATKINS, C. A., 1979a. Economy of 29-42. carbon and nitrogen in a nodulated and non-nodulated (NO3- 19806. Seasonal variation of sucrose content in the xylem grown)legume. 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