The contribution of tonoplast and plasma membrane to the electrical properties of a higher-plant cell

Translocation of ¹⁴C-labelled assimilates down the petioles was studied in intact plants of Pelargonium zonale (L.) L'Hérit ex Ait. The central bundle of the petiole was dissected out and treated with solutions of various inhibitors. Whereas cytochalasin B had no effect on ¹⁴C-translocation, a distinct and localized inhibition was caused by CCCP (10^-7 M), antimycin (5×10^-5 M), atractylate (5×10^-5 M), and valinomycin (10^-5 M) without any significant change in the proportion of [¹⁴C]sucrose in the translocate. The inhibition of translocation is inferred both by accumulation of ¹⁴C distal to and a decrease in ¹⁴C concentration basal to the treated petiolar region. If valinomycin was fed into the transpiration stream by flapping the peripheral bundles of the petioles an increased labeling of sugar phosphates occurred in the ¹⁴C fed leaf. Plasmolysis tests indicated that whereas CCCP interfered with the semipermeability of phloem cell membranes, valinomycin had no such effect. The results with valinomycin suggest a compartmentation of potassium ions for the translocation process but are ambiguous as to whether or not a potassium pump is involved.Dedicated to Wilhelm Halbsguth, Kiel to his 65th birthday     

Transport 14C-markierter Assimilate im Phloem von Pelargonium zonale und Phaseolus vulgaris

Zusammenfassung 6–10 min nach Beginn der ¹⁴CO₂-Assimilation befindet sich ¹⁴C-Aktivität im Stiel des ¹⁴CO₂-exponierten Blattes belichteter Pflanzen von Pelargonium zonale und Phaseolus vulgaris. Die Verteilungsrichtung der Assimilate im Sproß scheint vor allem durch das Alter des ¹⁴C-assimilierenden Blattes bestimmt, jedoch nicht einseitig festgelegt zu sein; geringe, aber faßbare Mengen an ¹⁴C-Saccharose wereden innerhalb von 20 min (bei Phaseolus) und 180 min (bei Pelargonium) auch aus relativ jüngeren in ältere Blätter transportiert.Neben ¹⁴C-Saccharose wurden im Blattstiel-und Stengelgewebe stets markierte Zuckerphosphate, Hexosen und organische Säuren nachgewiesen. Stärke war nur zu einem verschwindend geringen Anteil an der Gesamtmarkierung der analysierten Transportstrecken beteiligt.

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Translocation of ¹⁴C-labelled assimilates in the phloem of Pelargonium zonale and Phaseolus vulgaris

Summary After the laminae of leaves of intact plants had been exposed to ¹⁴CO₂ the translocation of ¹⁴C-labelled assimilates across the petioles starts very quickly: 6 to 10 min later ¹⁴C-activity could be detected in the basal part of the petioles. The way of distribution within the plant seems to be influenced mainly by the age of the ¹⁴CO₂-assimilating leaf, however, but not in the sense of an unidirectional movement; little but distinct amounts of ¹⁴C were carried also from younger yet full expanded leaves down to older leaves, within 20 min (in Phaseolus) or 180 min (in Pelargonium). Besides sucrose, which was shown by paper chromatography to be the main form of assimilates translocated in these species, we identified sugar phosphates, hexoses, and some organic acids in the petiole and stem tissue as being radioactive also. In our experiments, the petiole segments did not contain any remarkable amount of ¹⁴C-labelled starch.

     

Translocation pathways in the petioles and stem between source and sink leaves of Populus deltoides Bartr. ex Marsh.

Microautoradiography was used to follow the translocation pathways of ¹⁴C-labeled photosynthate from mature source leaves, through the stem, to immature sink leaves three nodes above. Translocation occurred in specific bundles of the midveins and petioles of both the source and sink leaves and in the interjacent internodes. When each of six major veins in the lamina of an exporting leaf was independently spot-fed ¹⁴CO₂, label was exported through specific bundles in the petiole associated with that vein. When the whole lamina of a mature source leaf was fed ¹⁴CO₂, export occurred through all bundles of the lamina, but acropetal export in the stem was confined to bundles serving certain immature sink leaves. Cross-transfer occurred within the stem via phloem bridges. Leaves approaching maturity translocated photosynthate bidirectionally in adjacent subsidiary bundles of the petiole. That is, petiolar bundles serving the lamina apex were exporting unlabeled photosynthate while those serving the lamina base were simultaneously importing labeled photosynthate. The petioles and midveins of maturing leaves were strong sinks for photosynthate, which was diverted from the export front to differentiating structural tissues. The data support the idea of bidirectional transport in adjacent bundles of the petiole and possibly in adjacent sieve tubes within an individual bundle.Abbreviations C central leaf trace - L left leaf trace - LPI leaf plastochron index - R right leaf trace     

The kinetics of sucrose concentration in the phloem of individual vascular bundles of the Ricinus communis seedling measured by nuclear magnetic resonance microimaging

The sucrose concentration was measured at 70-min intervals in the phloem of individual bundles of the hypocotyl of Ricinus seedlings by ¹H nuclear magnetic resonance (NMR) spectroscopic imaging. The sucrose concentration stayed fairly constant in all bundles for more than 7 h if the cotyledons were embedded in the endosperm or excised and incubated in 100 mM sucrose. If, however, the sucrose solution was replaced by sucrose-free buffer solution, the sucrose levels in the phloem decreased with a kinetic depending on the seedling: in some cases there was a smooth decline, in some a decline followed by a slight recovery and in some cases a clear-cut oscillation. The sucrose concentration was often not identical in the phloem of the individual bundles. The oscillations were larger in the phloem at the apex of the hypocotyl than in the phloem at the base of the hypocotyl. Cutting the petiole of one cotyledon led to a decrease in sucrose not only in the four bundles directly connected to the severed petiole but in all eight bundles of the hypocotyl. Cutting the petiole and dividing the vascular ring at the cotyledonary node and at the root crown did not prevent the decline of sucrose in all eight bundles. Therefore, a functional equilibration of translocated solutes between the eight bundles may occur within the 1-h measuring interval by radial diffusion through the parenchyma of the hypocotyl. Received 4 July 1997 / Accepted: 4 October 1997     

Characteristics of the phloem path: analysis and distribution of carbohydrates in the petiole of Cyclamen

Compartmentation fluxes of carbohydrates along the phloem path were analysed in the petiole of Cyclamen persicum (L.) Mill. Sucrose represented the dominant fraction (58-75% of soluble carbohydrates in the vascular symplast). Planteose (12-22%), glucose (3-8%) and fructose (3-13%) occurred in lower amounts (data from liquid chromatography, percentages of the total peak area). Starch was not detectable. Upon feeding leaves with ¹⁴CO2, 98% and 90% of radiolabel was recovered as sucrose in the vascular symplast after 3 h and 24 h, respectively. Thus, sucrose appeared to be the exclusive transport sugar in Cyclamen. Experiments with asymmetrically labelled sucrose revealed that there was no metabolism of translocated sucrose. Analysis of six consecutive petiole segments (each 2 cm in length) showed a homogeneous longitudinal distribution of these sugars differed markedly. On average, the sucrose concentration amounted to 4.7 and 0.4 mg g^-1 FM in the vascular apoplast and petiole parenchyma, respectively. Sucrose was unloaded with out hydrolysis and stored in the periphery of the phloem path. Planteose was identified as another storage saccharide. Sucrose synthesis by sucrose phosphate synthase occurred when isolated vascular bundles were incubated with [¹⁴C]glucose or [¹⁴C]fructose. These data suggest that the phloem path is characterized by both source and sink like activity.     

Translocation and incorporation of 14C into the petiole from different regions within developing cottonwood leaves

Summary The ability of a developing cottonwood (Populus deltoides Bartr.) leaf to export ¹⁴C-labeled assimilates begins at the lamina tip and progresses basipetally with increasing LPI. This progression indicates that portions of leaves function quasi-independently in their ability to export ¹⁴C-photosynthate. Although most of the exported radioactivity was recovered in the petiole as water-80% alcohol-soluble compounds, there was also substantial incorporation into the chloroform and insoluble fractions. This observation indicates that assimilates translocated from the lamina are used in structural development of the petiole. Freeze substitution and epoxy embedding were used to prepare microautoradiographs for localization of water-soluble compounds. Radioactivity was found in all cell types within specific subsidiary bundles of the petiole. However, radioactive assimilates appeared to move from the translocation pathway in the phloem toward active sinks in the walls of the expanding metaxylem cells. Translocation in the mature xylem vessels was not observed.     

Über die Bildung von Kallose bei einer Hemmung des Transportes in den Siebröhren durch Cyanid

Zusammenfassung Durch vergleichende Untersuchung der Transporthemmung und der Kallosebildung in unbehandelten und cyanidvergifteten Zentralleitbündeln der Blattstiele vonPelargonium zonale wurde die Frage einer mechanischen Transporthemmung durch die Kallose geprüft. Cyanid regte in der verwendeten Konzentration die Kallosebildung bei einem Teil der Siebplatten deutlich an; eine Rückbildung in Leitbündeln, die nach einer Erholungszeit wieder Fluorescein leiteten, wurde nicht beobachtet, doch nahm mit dem Alter der Blattstiele die Kallosemenge auch ohne Vergiftung erheblich zu. Anhand der Ergebnisse wird eine rein mechanische Transporthemmung durch Cyanid infolge von Kallosebildung für unwahrscheinlich gehalten.

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Summary The problem of a mechanical inhibition of transport by callose formation was tested in comparative studies of the inhibition of transport and the formation of callose with untreated and cyanide-treated central bundles of the leaf petioles ofPelargonium zonale. Cyanide, in the applied concentration, clearly stimulated callose formation on many sieve plates. A disappearance of callose in bundles which, after a time of recovery, were able to transport again was not observed, but, even without poisoning, callose formation still increased markedly during the aging of the petioles. A mechanical inhibition of transport by cyanide-induced callose formation is, therefore, thought to be unlikely.

     

Translocation in Plants Possessing Supernumerary Phloem: II. THE INCLUDED PHLOEM OF BOUGAINVILLEA GLABRA CHOISY AND THE BICOLATERAL BUNDLES OF CUCUMBER (CUCUMIS SATIVUS L.)

The translocation path in the included phloem of Bougainvilleaand in the bicolateral bundles of cucumber was studied by exposingyoung branches to ¹⁴CO₂ and detecting the radioactive compoundsby autoradiography. In Bougainvillea, the structure and functionof the phloem system is comparatively uncomplicated and uniform.All phloem bundles, i. e. those which are located in parenchymatoustissue of the central zone and those embedded in the secondaryxylem, become labelled. Exogenous IAA was translocated in thebundles, but the exact mode of translocation was not ascertained.Apical dominance was not affected by girdling. The implicationof this fact is discussed with respect to the translocationof the auxin that determines the correlative inhibition involvedin apical dominance. In cucumber the inner phloem became labelled throughout theplant to a lesser extent than the outer phloem. However, inthe petiole of the assimilating leaf the intensity of the labelwas the same in both inner and outer phloem, although the innerphloem has fewer elements. Below the treated leaf the innerphloem translocated less than the outer phloem. Above this leafthe inner phloem was entirely unlabelled     

Complexity untwined: The structure and function of cucumber (Cucumis sativus L.) shoot phloem

Cucurbit phloem is complex, with large sieve tubes on both sides of the xylem (bicollateral phloem), and extrafascicular elements that form an intricate web linking the rest of the vasculature. Little is known of the physical interconnections between these networks or their functional specialization, largely because the extrafascicular phloem strands branch and turn at irregular angles. Here, export in the phloem from specific regions of the lamina of cucumber (Cucumis sativus L.) was mapped using carboxyfluorescein and ¹⁴C as mobile tracers. We also mapped vascular architecture by conventional microscopy and X-ray computed tomography using optimized whole-tissue staining procedures. Differential gene expression in the internal (IP) and external phloem (EP) was analyzed by laser-capture microdissection followed by RNA-sequencing. The vascular bundles of the lamina form a nexus at the petiole junction, emerging in a predictable pattern, each bundle conducting photoassimilate from a specific region of the blade. The vascular bundles of the stem interconnect at the node, facilitating lateral transport around the stem. Elements of the extrafascicular phloem traverse the stem and petiole obliquely, joining the IP and EP of adjacent bundles. Using pairwise comparisons and weighted gene coexpression network analysis, we found differences in gene expression patterns between the petiole and stem and between IP and EP, and we identified hub genes of tissue-specific modules. Genes related to transport were expressed primarily in the EP while those involved in cell differentiation and development as well as amino acid transport and metabolism were expressed mainly in the IP.     

ORGANIZATION AND ONTOGENY OF THE VASCULAR SYSTEM IN THE PETIOLE OF EASTERN COTTONWOOD

The topologic arrangement of petiolar bundles varies within the length of the cottonwood petiole. Each petiolar bundle is formed by the subdivision and aggregation of acropetally differentiating subsidiary bundles in a predictable pattern. The subsidiary bundles provide vascular continuity between the stem and specific portions of the leaf lamina. Spot-labeling of individual veins with ¹⁴CO₂, freeze substitution, and microautoradiography were used to establish the relation between the secondary veins of the lamina and the vasculature of the petiole. Within the petiole vasculature each subsidiary bundle was continuous with a specific portion of the lamina and seemed to have a separate function. Subsidiary bundles continuous with the central leaf trace were closely related functionally to the tip region of the lamina, while the subsidiary bundles continuous with the lateral leaf traces were functionally related to the middle and basal portions of the lamina.     

An investigation of bidirectional translocation in the Phloem

Patterns of (14) CO(2) , assimilate movement in Vicia jaba plants having 7 nodes were studied. Bidirectional translocation occurred throughout most of the stem length when tracer was applied to leaves of various ages. To determine whether this bidirectional translocation occurs within single sieve tubes, a O.1 % solution of the fluorescent dye K-fluorescein was applied to a lightly scraped area on the stem in the middle of a young internode. After one hour the dye was present short distances above and below the treated area. Free-hand sections of the internode showed the dye to be localized in the traces of the larger leaves below tbe treated area and in the traces of the younger leaves above the treated area. The dye was never present in the same bundle both above and below the treated area, indicating that each bundle and sieve tube translocated the dye in only one direction. These results were confirmed using Phaseolus vulgaris, Vinca rosea, and Pelargonium hortum. A similar study in which petioles of young Ecballium elaterium leaves were treated showed that usually the phloem of one bundle translocated the dye in only one direction but in some cases the external phloem of the bicollateral bundles carried the dye toward the stem while the internal phloem carried the dye toward the blade. When longer time intervals were used in all these experiments, the dye sometimes appeared in the same phloem areas both above and below the treated area. This is explained by a lateral transfer of tracer within the phloem, either through secondary phloem or through bundle anastomoses at the nodes.     

EDTA-assisted phytoextraction of lead and cadmium by Pelargonium cultivars grown on spiked soil

The aim of this study was to assess EDTA-assisted Pb and Cd phytoextraction potential of locally grown Pelargonium hortorum and Pelargonium zonale. Plants were exposed to different levels of Pb (0–1500?mg kg^?1) and Cd (0–150?mg kg^?1) in the absence or presence of EDTA (0–5?mmol kg^?1). P. hortorum and P. zonale accumulated 50.9% and 42.2% higher amount of Pb in shoots at 1500?mg kg^?1 Pb upon addition of 5?mmol kg^?1 EDTA. Plant dry biomass decreased 46.8% and 64.3% for P. hortorum and P. zonale, respectively at the combination of 1500?mg kg^?1 Pb and 5?mmol kg^?1 EDTA. In Cd and EDTA-treated groups, P. hortorum and P. zonale accumulated 2.7 and 1.6-folds more Cd in shoots at 4 and 2?mmol kg^?1 EDTA, respectively, in 150?mg Cd kg^?1 treatment. Plant dry biomass of P. hortorum and P. zonale was reduced by 46.3% and 71.3%, respectively, in soil having 150?mg Cd kg^?1 combined with 5?mmol kg^?1 EDTA. Translocation factor and enrichment factor of both plant cultivars at all treatment levels were >1. Overall, the performance of P. hortorum was better than that of P. zonale for EDTA-assisted phytoextraction of Pb and Cd.     

Cytoplasmic inheritance and ultrastructure of the male generative cell of higher plants

Summary The ultrastructure of the generative cell of the pollen grain has been studied in two different plants: Mirabilis jalapa L., where variegation is transmitted only by the egg cell, and Pelargonium zonale Ait., where variegation can be transmitted also by the pollen grain. It was found that only in Pelargonium zonale Ait. does the male generative cell possesses a great number of proplastids.     

ANATOMICAL FEATURES OF METAPHLOEM IN STEMS OF SABAL,COCOS AND TWO OTHER PALMS

Sieve tubes in metaphloem of palm stems function throughout the life of the plant and merit close investigation. A stem of Sabal palmetto estimated to be 50 years old was sampled extensively. Variation in length of sieve-tube elements throughout this stem was measured and is discussed. In the metaphloem of individual vascular bundles companion cells are not sharply differentiated from other phloem parenchyma cells. Definitive callose deposits and slime are normally absent from mature sieve tubes, even in fixed material. Otherwise no conspicuous structural features which might account for the longevity of sieve tubes can be discerned. Occlusion of phloem strands after leaf fall is initially by callose deposition on sieve plates followed immediately by tylosoid formation. Similar sampling of Cocos nucifera, Washingtonia robusta and to a lesser extent Archontophoenix alexandrae confirmed these results except for quantitative differences.     

Autoradiographic and biochemical evidence for the systemic translocation of systemin in tomato plants

The movement of systemin, the 18-amino-acid polypeptide inducer of proteinase inhibitors in tomato (Lycopersicon esculentum L.) plants, was investigated in young tomato plants following the application of [¹⁴C]systemin to wounds on the surface of leaves. Wholeleaf autoradiographic analyses revealed that [¹⁴C]systemin was distributed throughout the wounded leaf within 30 min, and then during the next several hours was transported to the petiole, to the main stem, and to the upper leaves. The movement of [¹⁴C]systemin was similar to the movement of [¹⁴C]sucrose when applied to leaf wounds, except that sucrose was slightly more mobile than systemin. Analyses of the radioactivity in the petiole phloem exudates at intervals over a 5-h period following the application of [¹⁴C]systemin to a wound demonstrated that intact [¹⁴C]systemin was present in the phloem over the entire time, indicating that the polypeptide was either stable for long periods in the phloem or was being continually loaded into the phloem from the source leaf. The translocation pathway of systemin was also investigated at the cellular level, using light microscopy and autoradiography. Within 15 min after application of [³H]systemin to a wound on a terminal leaflet, it was found distributed throughout the wounded leaf and was primarily concentrated in the xylem and phloem tissues within the leaf veins. After 30 min, the radioactivity was found mainly associated with vascular strands of phloem tissue in the petiole and, at 90 min, label was found in the phloem of the main stem. Altogether, these and previous results support a role for systemin as a systemic wound signal in tomato plants.The authors acknowledge the Washington State University Electron Microscope Center and staff for their technical advice and collaboration. We also thank Greg Wichelns for growing our plants and Dr. Steven Doares for providing [³H]systemin. This research was supported in part by the Washington State College of Agriculture and Home Economics Project No. 1791 and National Science Foundation grants IBN 9117795 and IBN 9104542     

Peroxidase assay in plants: Interference by ascorbic acid and endogenous inhibitors in Sedum and Pelargonium enzyme extracts

Sedum album and Pelargonium zonale extracts do not show any peroxidase activity. Both extracts provoke a lag phase in the horse-radish peroxidase-catalyzed oxidation of guaiacol by H₂O₂. Preincubation of Sedum album extract with ascorbate oxidase eliminated completely the lag phase. Ascorbic acid has been identified as the substance responsible for this lag phase by reacting with a coloured intermediary product of the analytical reaction. In the Pelargonium zonale extract, the lag phase seems to be due to competitive inhibitors of peroxidase, which are of a phenolic nature.     

Migrations Orientées et Phytohormones — Valeur de la Feuille Détachée comme Matériel Expérimental

Directed Transport and Hormones — Value of Isolated Leaf as Experimental Material. Local application of aqueous phytohormonal solution (0.1 ml of various concentrations, from 50 to 0.025 mg-l^?1, of IAA, 2,4-D, NAA, BAP, GA₃) on isolated leaves influences the direction of ion transport. All tested hormones are efficient but the action depends upon the ionic species [positive action with ³²P, ³⁵S, ³⁶Rb (K), but null with ⁴⁵Ca or ⁴⁶Cl]. The movement of ions occurs through the phloem tissue and is not a cell-to-cell transfer. Competition between different parts of the leaf is demonstrated (a) between petiole which appears to be an important attractive centre and other parts of the leaf; (b) in a depetioloted leaf, between different centres induced by different phytohormones. In this respect, GA₃ appears to be the most efficient hormone in the tested material — leaves of Pelargonium zonale (L.) Aiton. The effects on directional transport are discussed in relation to other processes controlled by hormones, for instance growth, senescence, protein synthesis. A direct relation of directional transport with growth or delay of senescence may be discarded. The value of isolated leaves as simplified experimental systems is underlined by the fact that weak hormonal doses are efficient (0.025 mg.l for GA₃ and 0.5 mg.¹ for 2,4-D) as compared to the higher doses used in other plant systems.     

VASCULARIZATION OF DEVELOPING LEAVES OF GLEDITSIA TRIACANTHOS L. I. THE NODE,RACHIS, AND RACHILLAE

Leaves of Gleditsia triacanthos L. are served by three leaf traces that subdivide in the node to produce subsidiary bundles. The subsidiary bundles differentiate basipetally in the stem and acropetally in the petiole using the original leaf trace bundles (those that developed acropetally) as templates for their development. Within the pulvinus, the acropetal bundle components merge to form the rachis vasculature consisting of a semicircular arc and a ventral chord; several small bundles diverge to form ventral ridge bundles. Mixing of bundles occurs during vascularization of the lateral rachillae axes. Each diverging rachilla axis receives bundles from the semicircular arc, the ventral chord, and a ridge bundle in a relatively reproducible and predictable pattern. During this process the main rachis vasculature is gradually depleted, but the ridge bundles are reconstituted following divergence of each rachilla pair. The distal rachilla pair is vascularized by a bilateral partitioning of the entire rachis vasculature; a remnant of the central leaf trace terminates in a subulate terminal appendage. Vascularization of the bipinnate G. triacanthos leaf is compared to that of the simple Populus deltoides leaf.     

The phloem necrosis virus disease of tea in Ceylon; further characterization of necrosis in the leaf

Phloem necrosis due to the virus disease of that name in tea is fully described as it affects the leaf, on which diagnosis is chiefly based. Originating in the protophloem, it may extend inwards to the metaphloem and outwards to the pericycle, causing the breakdown and discoloration of the cells and their eventual death and obliteration, cell enlargement, and the production of new thin-walled cells by hyperplasia. The condition is termed 'true necrosis' to distinguish it from the non-pathogenic 'false necrosis', of unknown cause, which originates typically in the metaphloem but may have the same histological effects except for the absence of hyperplasia. In the petiole, the visual distinction between 'true' and 'false' necrosis on the basis of their position as seen in a transverse section of the bundle is relatively easy, and the continued use of this method of diagnosis is recommended. No such distinction can reliably be made in the midrib, where 'false' necrosis often occurs in the same position as 'true' necrosis, i.e. immediately within the pericycle. This is interpreted ontogenetically in terms of the smaller total width of the phloem in the midrib as compared with the petiole bundle; it effectively prevents the use of the midrib for diagnosis. The observations are discussed in terms of the inherent properties of the phloem, as affected by viruses and other agencies reported to have caused necrosis, among plants in general.     

Phloem loading in peach: Symplastic or apoplastic?

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ³H-inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of ¹⁴C-labelled source leaves with 2 mMp-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.