The suberin lamella,a possible barrier to water movement from the veins to the mesophyll ofThemeda triandra forsk

Summary Precipitation of ferrous ions by ferricyanide in transpiring leaves ofThemeda triandra Forsk. produced crystalline deposits, which were visible with the light and electron microscope. Prussian blue crystals were formed within the lumina of the tracheary elements and the apoplast, or cell wall continuum of the vascular tissues and bundle-sheath cells. Little if any deposition was noted within the lignified secondary thickenings of the tracheary elements. The localization pattern suggests that the ferrous ions moved from the lumina of the tracheary elements via the exposed primary walls. Prussian blue crystals were abundant in the outer tangential and radial walls of the bundle-sheath cells. By contrast, crystals were lacking in the walls of neighbouring mesophyll cells, suggesting that the suberin lamella in the bundle-sheath walls effectively inhibited the apoplastic movement of ferrous ions and possibly may impede, or restrict the movement of water across the bundle-sheath/mesophyll interface.     

Loading and transport of assimilates in different maize leaf bundles

The loading and transport functions of vascular bundles in maize (Zea mays L.) leaf strips were investigated by microautoradiography after application of ¹⁴CO₂. The concentrations of ¹⁴C-contents in thin-walled sieve tubes of individual bundles in the loading and transport regions were determined by digital image analysis of silver-grain density over the sieve tubes and compared. In the loading region, relatively high concentrations of ¹⁴C-contents were found in the thin-walled sieve tubes of small bundles and in the small, thin-walled sieve tubes of the intermediate bundles; the concentration of ¹⁴C-label in large bundles was very low. In the transport region, at a transport distance of 2 cm, all of the small bundles contained ¹⁴C-assimilates, but generally less than the same bundles did in the loading region; by comparison, at that distance intermediate and large bundles contained two-to threefold more ¹⁴C-assimilates than the same bundles in the loading region. The lateral transfer of assimilates from smaller to larger bundles via transverse veins could be demonstrated directly in microautoradiographs. A reverse transport from larger to smaller bundles was not found. At a transport distance of 4 cm, all large and intermediate bundles were ¹⁴C-labeled, but many of the small bundles were not. Although all longitudinal bundles were able to transport ¹⁴C-asimilates longitudinally down the blade, it was the large bundles that were primarily involved with longitudinal transport and the small bundles that were primarily involved with loading.     

Ultrastructure of and plasmodesmatal frequency in mature leaves of sugarcane

Vascular bundles and contiguous tissues of leaf blades of sugarcane (Saccharum interspecific hybrid L62–96) were examined with light and transmission electron microscopes to determine their cellular composition and the frequency of plasmodesmata between the various cell combinations. The large vascular bundles typically are surrounded by two bundle sheaths, an outer chlorenchymatous bundle sheath and an inner mestome sheath. In addition to a chlorenchymatous bundle sheath, a partial mestome sheath borders the phloem of the intermediate vascular bundles, and at least some mestome-sheath cells border the phloem of the small vascular bundles. Both the walls of the chlorenchymatous bundlesheath cells and of the mestome-sheath cells possess suberin lamellae. The phloem of all small and intermediate vascular bundles contains both thick- and thin-walled sieve tubes. Only the thin-walled sieve tubes have companion cells, with which they are united symplastically by pore-plasmodesmata connections. Plasmodesmata are abundant at the Kranz mesophyll-cell-bundlesheath-cell interface associated with all sized bundles. Plasmodesmata are also abundant at the bundle-sheathcell-vascular-parenchyma-cell, vascular-parenchyma-cellvascular-parenchyma-cell, and mestome-sheath-cell-vascular-parenchyma-cell interfaces in small and intermediate bundles. The thin-walled sieve tubes and companion cells of the large vascular bundles are symplastically isolated from all other cell types of the leaf. The same condition is essentially present in the sieve-tube-companion-cell complexes of the small and intermediate vascular bundles. Although few plasmodesmata connect either the thin-walled sieve tubes or their companion cells to the mestome sheath of small and intermediate bundles, plasmodesmata are somewhat more numerous between the companion cells and vascular-parenchyma cells. The thick-walled sieve tubes are united with vascular-parenchyma cells by pore-plasmodesmata connections. The vascular-parenchyma cells, in turn, have numerous plasmodesmatal connections with the bundle-sheath cells.This study was supported by National Science Foundation grants DCB 87-01116 and DCB 90-01759 to R.F.E. and a University of Wisconsin-Madison Dean's Fellowship to K. R.-B. We also thank Claudia Lipke and Kandis Elliot for photographic and artistic assistance, respectively.     

Distribution and frequency of plasmodesmata in relation to photoassimilate pathways and phloem loading in the barley leaf

Large, intermediate, and small bundles and contiguous tissues of the leaf blade of Hordeum tvulgare L. ‘Morex’ were examined with the transmission electron microscope to determine their cellular composition and the distribution and frequency of the plasmodesmata between the various cell combinations. Plasmodesmata are abundant at the mesophyll/parenchymatous bundle sheath, parenchymatous bundle sheath/mestome sheath, and mestome sheath/vascular parenchyma cell interfaces. Within the bundles, plasmodesmata are also abundant between vascular parenchyma cells, which occupy most of the interface between the sieve tube-companion cell complexes and the mestome sheath. Other vascular parenchyma cells commonly separate the thick-walled sieve tubes from the sieve tube-companion cell complexes. Plasmodesmatal frequencies between all remaining cell combinations of the vascular tissues are very low, even between the thin-walled sieve tubes and their associated companion cells. Both the sieve tube-companion cell complexes and the thick-walled sieve tubes, which lack companion cells, are virtually isolated symplastically from the rest of the leaf. Data on plamodesmatal frequency between protophloem sieve tubes and other cell types in intermediate and large bundles indicate that they (and their associated companion cells, when present) are also isolated symplastically from the rest of the leaf. Collectively, these data indicate that both phloem loading and unloading in the barley leaf involve apoplastic mechanisms.     

Cytochemical localization of phosphatase activity in vascular bundles and contiguous tissues of the leaf ofZea mays L.

Summary The cytochemical localization of phosphatase activity has been carried out on small and intermediate vascular bundles and contiguous tissues of the leaf ofZea mays L. Similar localization patterns were obtained with the nucleoside triphosphates ATP, CTP, GTP, ITP, and UTP, and with ADP and -GP. Reaction product (lead deposits) was observed on the plasma membrane of all cell types. It was invariably heavier on the plasma membranes of the bundle-sheath cells, vascular-parenchyma cells, and the thin-walled sieve tubes and their associated companion cells than on those of the mesophyll cells. Within the bundles, the heaviest lead deposits frequently were found on the plasma membranes of the thin-walled sieve tubes and the least amount (often lacking) on those of the thick-walled sieve tubes. Formation of reaction product was suppressed by NaF, vanadate, and molybdate but not by PCMBS (p-chloromercuribenzene sulfonic acid). The results of the substrate-specificity and inhibitor-sensitivity studies indicate that a nonspecific acid phosphatase was probably responsible for the deposition of the reaction product and not the plasma membrane H⁺-ATPase. These results, in addition to an evaluation of the pertinent literature, lead us to conclude that H⁺-ATPase activity has yet to be demonstrated unequivocally in association with the plasma membrane of phloem cells with lead precipitation procedures. Nevertheless, the differences in amounts of reaction product generally associated with the plasma membranes of the thick- and thin-walled sieve tubes of the maize leaf indicate that the two types of sieve tube differ from one another physiologically.     

Ultrastructure,plasmodesmatal frequency,and solute concentration in green areas of variegated Coleus blumei Benth. leaves

The photosynthetic tissue of green portions of variegated Coleus blumei leaves consists primarily of palisade and spongy parenchyma cells as well as bundle-sheath cells. The moderate numbers of plasmodesmata connecting these cells may be sufficient to provide a symplastic pathway for assimilates moving toward the minor veins. The minor veins, however, are unusual in having two sets of phloem-loading cells which have little symplastic continuity with one another: one consisting of large, peripherally located intermediary cells, and a second set made up of smaller, usually more internal companion cells, both sets having their associated sieve-tube members. The intermediary cells are connected to vascular-parenchyma and bundle-sheath cells by unique branched plasmodesmata which are particularly abundant at the bundle-sheath interface. In addition, numerous plasmodesmata-pore connections occur between the intermediary cells and their associated sieve-tube members. Neither the intermediary cells nor their sieve-tube members plasmolyze when treated with 1.4 M mannitol, whereas mesophyll and vascular-parenchyma cells plasmolyze at 0.5 M and bundle-sheath cells at 0.6 M mannitol. By contrast, the companion cells and their associated sieve-tube members are symplastically isolated from the bundle-sheath cells and the sieve-tube-intermediary-cell complexes, and share few plasmodesmata with the vascular-parenchyma cells. Moreover, the companion cells plasmolyze at 1.1 M mannitol and their sieve tubes at 1.3 M. The intermediary-cell-sieve-tube complex thus appears to be structurally equipped to load assimilates entirely via the symplast, while the sieve-tube-companion-cell complex is probably loaded from the apoplast.Abbreviation ER endoplasmic reticulum     

Ontogeny of the vascular bundles and contiguous tissues in the maize leaf blade

The patterns of initiation and early development of the minor and major veins in the flat portion of the leaf blade of maize (Zea mays L.) follow similar patterns. The veins and their associated bundle sheath cells commonly arise from cell assemblages derived from a single cell lineage, or longitudinal file of cells, rather than from two “half vein units” derived from different cell lineages. In addition, apparently, none of the vascular cells derived from the procambium is directly related ontogenetically to a bundle sheath cell. In veins derived from larger cell assemblages, the lateral bundle sheath cells are more closely related ontogenetically to the mesophyll cells, which are derived from the ground meristem, than to the vascular cells, which are derived from procambium. The bundle sheath cells, accordingly, are interpreted as being ground meristem in origin.     

Localization of antioxidant enzymes in the cellular compartments of sorghum leaves

The localization of antioxidant enzymes between the mesophyll and bundle sheath cells were determined in sorghum (Sorghum vulgare L.) leaves. The activity of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR) were assayed in whole leaf, mesophyll and bundle sheath fractions of sorghum leaves subjected to water-limited conditions. Drought was imposed by withholding water and the plants were maintained at different water potentials ranging from 0.5–2.0 MPa. The purity of the isolates was tested using the marker enzymes like RuBPcase and PEPcase. GR was mostly localized in mesophyll fraction, while SOD, APX and peroxidase were located in bundle sheath cells. Catalase was found to be equally distributed between the two cell types. Under water stress conditions, most of the SOD activity was found in the bundle sheath tissues. Little or no activity of the enzymes CAT, APX or POD was found in the mesophyll extracts when exposed to water stress. GR activity increased when exposed to low water regimes. From this study, it is clear that antioxidants are differentially distributed between the mesophyll and bundle sheath cells in sorghum leaves. Under water stress conditions, the mesophyll cells showed less damage from oxidative stress when compared to the bundle sheath cells. This is critical for determining the sensitivity of sorghum to extreme climatic conditions.     

Changes in the density of microtubular networks in mesophyll cells and mesophyll derived protoplasts of Nicotiana and Triticum during leaf development

Changes in the density of microtubular mesh-works were analysed in mesophyll cells and mesophyll derived protoplasts of Nicotiana tabacum L. and Triticum aestivum L. during leaf development. The main purpose of this study was to test whether the low density, if not lack, of microtubular networks recently described in protoplasts that had been isolated from fully differentiated mesophyll cells happened during protoplast isolation or whether the loss of microtubules actually occurred during differentiation of the leaf tissue. Immunofluorescence microscopy showed that the density of the microtubular cytoskeleton in the leaf tissue decreased steadily after cessation of cell growth in both species. Nevertheless, in Triticum microtubule disappearance was swifter and occurred along a gradient from the base to tip of the leaf, a phenomenon reflecting the differences in the ontogeny between the dicotyledonous Nicotiana and the mono-cotyledonous Triticum leaves. Protein extraction from leaf tissues and Western blot analysis indicated that in both species the disappearance of microtubules was the result of a degradation of tubulin and not only due to a depolymerisation into tubulin subunits. When the cell walls were removed from live cells and the protoplasts released, the original patterns of the microtubules became obscured and, particularly in differentiated cells, the integrity and density of the microtubule strands deteriorated. The potential application of the density of the microtubular cytoskeleton as a marker in studies on differentiation and dedifferentiation in mesophyll cells and protoplasts is discussed.We wish to thank Silke Heichel for excellent technical assistance. We also express our thanks to the group of A.M. Lambert at CNRS, Strasbourg, France, for advice during establishment of our Western blot system. The work was supported by a grant of the German Ministry of Science and Technology (BMFT).     

Intraspecific variation in the response of Themeda triandra to defoliation: the effect of time of recovery and growth rates on compensatory growth

Summary The response to a single defoliation was studied on three clones of Themeda triandra collected in the short, mid, and tall grassland regions of the Serengeti National Park (Tanzania). These sites represent a gradient of decreasing grazing intensity. Growth, allocation pattern, and several morphometric traits were monitored during an 80-day period. Clipped plants of the short and medium clones fully compensated for the reduction of biomass, while plants of the tall clone showed overcompensation. During the first two weeks after clipping, clipped plants showed lower relative growth rates than unclipped ones, whereas the opposite was observed later on. Clipped plants compensated for the removal of leaf area by producing new leaves with lower specific weights and higher nitrogen content. They also produced more, smaller tillers. Although clipped plants mobilized nonstructural carbohydrates from roots and crowns, this did not account for a significant amount of growth. Relative growth rates of unclipped plants of the short clone were higher. The relative growth rate of the short clone diminished less after clipping, but also exhibited the lowest increase later. The tall clone was the most negatively affected early, but showed the highest compensation later. Compared to the other clones, the short ecotype showed many of the characteristics that defoliation induced in each individual of any clone: higher allocation to leaf area production, higher relative growth rate, higher number but smaller size of tillers, and lower leaf specific weights.     

Distribution and structure of the plasmodesmata in mesophyll and bundle-sheath cells of Zea mays L.

In leaf blades of Zea mays L. plasmodesmata between mesophyll cells are aggregated in numerous thickened portions of the walls. The plasmodesmata are unbranched and all are characterized by the presence of electron-dense structures, called sphincters by us, near both ends of the plasmodesmatal canal. The sphincters surround the desmotubule and occlude the cytoplasmic annulus where they occur. Plasmodesmata between mesophyll and bundle-sheath cells are aggregated in primary pit-fields and are constricted by a wide suberin lamella on the sheath-cell side of the wall. Each plasmodesma contains a sphincter on the mesophyll-cell side of the wall. The outer tangential and radial walls of the sheath cells exhibit a continuous suberin lamella. However, on the inner tangential wall only the sites of plasmodesmatal aggregates are consistently suberized. Apparently the movement of photosynthetic intermediates between mesophyll and sheath cells is restricted largely or entirely to the plasmodesmata (symplastic pathway) and transpirational water movement to the cell walls (apoplastic pathway).Abbreviation ER endoplasmic reticulum     

Changes in the cellular localization of cytosolic glutamine synthetase protein in vascular bundles of rice leaves at various stages of development

Cellular localization of cytosolic glutamine synthetase (GS1; EC 6.3.1.2) in vascular bundles of leaf blades of rice (Oryza sativa L.), at the stage at which leaf blades 6 (the lowest position) to 10 were fully expanded, was investigated immunocytologically with an affinity-purified anti-GS1 immunoglobulin G. Strong signals for GS1 protein were detected in companion cells of large vascular bundles when blades 6–8 were tested. Signals for GS1 were also observed in vascular-parenchyma cells of both large and small vascular bundles. The results further support our hypothesis that GS1 is important for the export of leaf nitrogen from senescing leaves. The signals in companion cells were less striking in the younger green leaves and were hardly detected in the non-green portion of the 11th blade. In the non-green blades, strong signals for GS1 protein were detected in sclerenchyma and xylemparenchyma cells. When total GS extracts prepared from the 6th,10th, and the non-green 11th blades were subjected to anion-exchange chromatography, the activity of GS1 was clearly separated from that of chloroplastic GS, indicating that GS1 proteins detected in the vascular tissues were able to synthesize glutamine. The function of GS1 detected in the developing leaves is discussed.Abbreviations Fd-GOGAT ferredoxin-dependent glutamate synthase - GS1 cytosolic glutamine synthetase - GS2 plastidic glutamine synthetase - IgG immunoglobulin G     

Microautoradiographic studies of phloem loading and transport in the leaf of Zea mays L.

Microautoradiographs showed that [¹⁴C]sucrose taken up in the xylem of small and intermediate (longitudinal) vascular bundles of Zea mays leaf strips was quickly accumulated by vascular parenchyma cells abutting the vessels. The first sieve tubes to exhibit ¹⁴C-labeling during the [¹⁴C]sucrose experiments were thick-walled sieve tubes contiguous to the more heavily labeled vascular parenchyma cells. (These two cell types typically have numerous plasmodesmatal connections.) With increasing [¹⁴C]sucrose feeding periods, greater proportions of thick- and thin-walled sieve tubes became labeled, but few of the labeled thin-walled sieve tubes were associated with labeled companion cells. (Only the thin-walled sieve tubes are associated with companion cells.) When portions of leaf strips were exposed to ¹⁴CO₂ for 5 min, the vascular parenchyma cells-regardless of their location in relation to the vessels or sieve tubes-were the most consistently labeled cells of small and intermediate bundles, and label (¹⁴C-photosynthate) appeared in a greater proportion of thin-walled sieve tubes than thick-walled sieve tubes. After a 5-min chase with ¹²CO₂, the thin-walled sieve tubes were more heavily labeled than any other cell type of the leaf. After a 10-min chase with ¹²CO₂, the thin-walled sieve tubes were even more heavily labeled. The companion cells generally were less heavily labeled than their associated thin-walled sieve tubes. Although all of the thick-walled sieve tubes were labeled in portions of leaf strips fed ¹⁴CO₂ for 5 min and given a 10-min ¹²CO₂ chase, only five of 72 vascular bundles below the ¹⁴CO₂-exposed portions contained labeled thick-walled sieve tubes. Moreover, the few labeled thick-walledsieve tubes of the transport region always abutted ¹⁴C-labeled vascular parenchyma cells. The results of this study indicate that (1) the vascular parenchyma cells are able to retrieve at least sucrose from the vessels and transfer it to the thick-walled sieve tubes, (2) the thick-walled sieve tubes are not involved in long-distance transport, and (3) the thin-walled sieve tubes are capable themselves of accumulating sucrose and photosynthates from the apoplast, without the companion cells serving as intermediary cells.     

Ultrastructural features of flavonoid accumulation in leaf cells ofChrysosplenium americanum

Summary The intracellular localization of partially O-methylated flavonol glucosides was studied inChrysosplenium americanum leaves using caffeine as stabilizing and visualizing reagent. Electron microscopic observations and chromatographic data indicated that intracellular flavonoids were found to accumulate mainly within the walls of epidermal and mesophyll cells. Various membrane profiles and associated vesicles appeared to be involved in the packaging and channelling of the electron-dense material towards the cell wall. There was no evidence to suggest the participation of chloroplasts in these processes. The significance of flavonoid accumulation in cell walls was discussed in relation to the nature of these compounds and their possible ecophysiological role in this plant.Abbreviations ER endoplasmic reticulum - HPLC high-performance liquid chromatography - TLC thin-layer chromatography     

Protease-sensitive anchoring of microfilament bundles provides tracks for cytoplasmic streaming inVallisneria

Summary In mesophyll cells of the aquatic angiospermVallisneria gigantea Graebner, the endoplasm streams rotationally along the cell walls normal to the leaf surface in situ. Bundles of microfilaments anchored in the ectoplasm serve as tracks for the cytoplasmic streaming. In single mesophyll cells isolated by enzymatic digestion, hypertonic treatment induces abnormal streaming concomitant with plasmolysis, specifically at one or both of the shorter sides of an approximate rectangle. The disorderly arrangement of microfilaments in such cells has been confirmed by fluorescence microscopy of cells stained with FITC-phalloidin. While inhibitors of proteases added to the enzyme solution used for isolation of cells suppress the disturbance of rotational streaming, exogenously applied protease promotes it. The results suggest that bundles of microfilaments in the ectoplasm are stabilized by protease-sensitive factor(s) in the presence of the cell wall.     

A novel proline-rich glycoprotein associated with the extracellular matrix of vascular bundles of Brassica petioles

A panel of monoclonal antibodies (MAC204, MAC236, MAC265) which recognise extracellular matrix glycoproteins implicated in plant-microbe interactions has been used to study glycoprotein antigens in petioles of turnip (Brassica campestris L.). While MAC204 recognised two glycoproteins (gp120 and gp45) with apparent M_r 120 000 and 45 000 in petiole extracts made with 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris) buffer containing sodium dodecyl sulfate, MAC236 recognised gp120 but not gp45, and MAC265 gave no or only weak reactivity. Tissue dissection studies established that gp120 was predominantly associated with the vascular bundle whereas gp45 was largely associated with the pith. This was consistent with results from tissue prints probed with MAC204 and MAC236 which also suggested a vascular localisation for gp120. Immunoelectronmicroscopy showed that MAC204 and MAC236 both labelled three-way junctions between cells of the phloem and sclerid fibres. Both gp120 and gp45 were shown to carry epitopes in common with known hy`droxyproline-rich glycoproteins. Unlike gp45, gp120 could be extracted from petioles with Tris buffer alone and then isolated from this extract by trichloroacetic acid treatment (which left gp120 soluble), followed by size-exclusion and ion-exchange chromatography. Amino acid analysis revealed gp120 to be a novel glycoprotein, particularly rich in proline, lysine, valine and threonine but relatively poor in hydroxyproline. The most abundant sugars were arabinose and galactose. The potential role of this very basic cell surface glycoprotein in plant defence against microbes is discussed. Received: 25 November 1996 / Accepted: 12 December 1996     

Metabolism of phosphoenolpyruvate in the C4 cycle during photosynthesis in the phosphoenolpyruvate-carboxykinase C4 grass Spartina anglica Hubb.

The aim of this work was to investigate the fate of phosphoenolpyruvate (PEP) produced by decarboxylation of oxaloacetate during photosynthesis in the bundle sheaths of leaves of the PEP-carboxykinase C₄ grass Spartina anglica Hubb. Mesophyll protoplasts and bundle sheath cells were separated enzymically and used to investigate activities and distributions of putative enzymes of the C₄ cycle and the photosynthetic carbon metabolism of bundle sheath cells. The results indicate that neither conversion of PEP to pyruvate nor its conversion to 3-phosphoglycerate can account for all of the carbon flux through the C₄ cycle during photosynthesis. It is likely, therefore, either that PEP moves directly from bundle sheath to mesophyll or that more than one pathway of regeneration of PEP is involved in the C₄ cycle in this plant.Abbreviations Chl chlorophyll - PEP phosphoenolpyruvate - Pi phosphate - RuBP ribulose-1,5-bisphosphate     

Morphology and ultrastructure of the gravity-sensitive leaf sheath base of the grass Echinochloa colonum L

When a flowering stalk of Echinochloa colonum is held horizontally, growth is initiated in the lower side of each leaf sheath base, restoring the inflorescence to an upright position. Changes in the gravity vector are perceived by specialised statolithcontaining tissue which is associated with each of the symmetrically-arranged vascular bundles within the leaf sheath bases. The morphological and ultrastructural features of these gravity-sensitive regions have been examined by light and electron microscopy. Each statocyte cell contains a large central vacuole with a thin lining of cytoplasm. Up to 50 spherical starch statoliths lie along the lowermost side of the cells and these sediment readily following geotropic stimulation. Statoliths are found in contact with the plasmalemma, or may be prevented from touching it by bands of microtubules. Dictyosomes and mitochondria are numerous, but endoplasmic reticulum is sparse. The nuclei tend to remain at the original apex of each cell. Statocytes of the leaf sheath base are compared and contrasted with those of the root tip.Abbreviations GMA glycol methacrylate - PAS periodic acid-Schiff's reagent - ER endoplasmic reticulum     

Stimulation of sugar exit from leaf tissues ofVicia faba L.

After removal of the lower epidermis, leaf discs ofVicia faba L. were loaded with either [¹⁴C]sucrose or [³H]3-O-methylglucose (3-O-MeG). The exit of preloaded sucrose was strongly stimulated when sucrose was present in the bathing medium, and the exit of 3-O-MeG was also markedly increased in the presence of 3-O-MeG. This specific stimulation exhibited single saturation dependence on the external concentration of sugar (K _m=9 mM for sucrose, 5 mM for 3-O-MeG), and was sensitive to low temperature, uncouplers and thiol reagents. Sucrose exit was never affected by 3-O-MeG in the bathing medium. Sucrose did not affect the exit of 3-O-MeG in fresh discs, but promoted this exit in discs previously aged for 12 h, indicating partial external hydrolysis of sucrose in the latter tissues. Ageing also dramatically increased the exit of 3-O-MeG induced by 3-O-MeG but had no effect on the exit of sucrose induced by sucrose. The ability of 53 compounds (pentoses, hexoses, hexose-phosphates, polyols, di- and trisaccharides, phenyl- and nitrophenyl-derivatives, sweeteners) to interact with the sucrose carrier and with the hexose carrier was tested. Sucrose, maltose, -phenylglucoside andp-nitrophenyl--glucoside interacted with the sucrose carrier.d-glucose,d-xylose,d-fucose,d-galactose,d-mannose, 3-O-MeG and 2-deoxyglucose interacted with the hexose carrier.Abbreviations CCCP carbonylcyanide-m-chlorophenylhydrazone - EGTA ethylene glycol-bis(-aminoethyl ether)-N,N,N,N-tetraacetic acid - 3-O-MeG 3-O-methylglucose - PCMBS p-chloromercuribenzenesulphonic acid