Phloem Unloading in Developing Leaves of Sugar Beet : I. Evidence for Pathway through the Symplast

Physiological and transport data are presented in support of a symplastic pathway of phloem unloading in importing leaves of Beta vulgaris L. (`Klein E multigerm'). The sulfhydryl reagent p-chloromercuribenzene sulfonic acid (PCMBS) at concentration of 10 millimolar inhibited uptake of exogenous [¹⁴C]sucrose by sink leaf tissue over sucrose concentrations of 0.1 to 5.0 millimolar. Inhibited uptake was 24% of controls. The same PCMBS treatment did not affect import of ¹⁴C-label into sink leaves during steady state labeling of a source leaf with ¹⁴CO₂. Lack of inhibition of import implies that sucrose did not pass through the free space during unloading. A passively transported xenobiotic sugar, l-[¹⁴C]glucose, imported by a sink leaf through the phloem, was evenly distributed throughout the leaf as seen by whole-leaf autoradiography. In contrast, l-[¹⁴C]glucose supplied to the apoplast through the cut petiole or into a vein of a sink leaf collected mainly in the vicinity of the major veins with little entering the mesophyll. These patterns are best explained by transport through the symplast from phloem to mesophyll.     

Phloem unloading in tobacco sink leaves: insensitivity to anoxia indicates a symplastic pathway

Phloem unloading in transition sink leaves of tobacco (Nicotiana tabacum L.) was analyzed by quantitative autoradiography. Detectable levels of labeled photoassimilates entered sink leaves approx. 1 h after source leaves were provided with ¹⁴CO₂. Samples of tissue were removed from sink leaves when label was first detected and further samples were taken at the end of an experimental phloem-unloading period. The amount of label in veins and in surrounding cells was determined by microdensitometry of autoradiographs using a microspectrophotometer. Photoassimilate unloaded from first-, second-and third-order veins but not from smaller veins. Import termination in individual veins was gradual. Import by the sink leaf was completely inhibited by exposing the sink leaf to anaerobic conditions, by placing the entire plant in the cold, or by steam-girdling the sink-leaf petiole. Phloem unloading was completely inhibited by cold; however, phloem unloading continued when the sink-leaf petiole was steam girdled or when the sink leaf was exposed to a N₂ atmosphere. Compartmental efflux-analysis indicated that only a small percentage of labeled nutrients was present in the free space after unloading from sink-leaf veins in a N₂ atmosphere. The results are consistent with passive symplastic transfer of photoassimilates from phloem to surrounding cells.Symbol VI radio of ¹⁴C in veins and interveinal tissue     

Response of Phloem Loading and Export to Rapid Changes in Sink Demand

Sink demand was abruptly changed for an illuminated sugar beet source leaf by shading the six to ten other source leaves. Export of recently assimilated, labeled material underwent a transient increase and then returned to a steady rate approximately equal to the pretreatment rate. Uncovering the darkened leaves caused a transient decrease in export of ¹⁴C; following recovery there was a gradual decline. It remains to be established whether export of unlabeled reserves occurs in response to increased sink demand. The possibility that phloem loading increases in response to decreased sieve tube turgor was tested. Phloem loading of exogenous ¹⁴C-sucrose increased when turgor in leaf cells was decreased by floating leaf discs on solutions with up to 1 M mannitol osmoticum. However, the increase appeared to be the result of plasmolysis of mesophyll cells possibly resulting from easier access to minor veins via the free space. Phloem loading in leaf discs continued undiminished even though sieve tube-companion cell sucrose concentration exceeded a calculated value of 1 M. Regulation of export to meet sink demand by a direct response of phloem loading to a turgor or concentration set point does not appear to occur. Phloem loading may be promoted by the influx of water which drives mass flow, increasing phloem loading in response to increased velocity of transport.     

Phloem Loading in Coleus blumei in the Absence of Carrier-Mediated Uptake of Export Sugar from the Apoplast

Phloem loading in Coleus blumei Benth. leaves cannot be explained by carrier-mediated transport of export sugar from the apoplast into the sieve element-companion cell complex, the mechanism by which sucrose is thought to load in other species that have been studied in detail. Uptake profiles of the export sugars sucrose, raffinose, and stachyose into leaf discs were composed of two components, one saturable and the other not. Saturable (carrier-mediated) uptake of all three sugars was almost completely eliminated by the inhibitor p-chloromercuribenzenesulfonic acid (PCMBS). However, when PCMBS was introduced by transpiration into mature leaves it did not prevent accumulation of ¹⁴C-photosynthate in minor veins or translocation of labeled photosynthate from green to nonchlorophyllous regions of the leaf following exposure to ¹⁴CO₂. The efficacy of introducing inhibitor solutions in the transpiration stream was proven by observing saffranin O and calcofluor white movement in the minor veins and leaf apoplast. PCMBS introduced by transpiration completely inhibited phloem loading in tobacco leaves. Phloem loading in C. blumei was also studied in plasmolysis experiments. The carbohydrate content of leaves was lowered by keeping plants in the dark and then increased by exposing them to light. The solute level of intermediary cells increased in the light (phloem loading) in both PCMBS-treated and control tissues. A mechanism of symplastic phloem loading is proposed for species that translocate the raffinose series of oligosaccharides.     

Phloem Unloading in Sink Leaves of Nicotiana benthamiana: Comparison of a Fluorescent Solute with a Fluorescent Virus

Using noninvasive imaging techniques, we compared phloem unloading of the membrane-impermeant, fluorescent solute carboxyfluorescein (CF) with that of potato virus X expressing the gene for the green fluorescent protein. Although systemic virus transport took considerably longer to occur than did CF transport, unloading of both solute and virus occurred predominantly from the class III vein network, a highly branched veinal system found between class II veins. The minor veins (classes IV and V) played no role in solute or virus import but were shown to be functional in xylem transport at the time of import by labeling with Texas Red dextran. After virus exit from the class III phloem, the minor veins eventually became infected by cell-to-cell virus movement from the mesophyll. During the sink/source transition, phloem unloading of CF was inhibited from class III veins before the cessation of phloem import through them, suggesting a symplastic isolation of the phloem in class III veins before its involvement in export. The progression of the sink/source transition for carbon was unaffected by the presence of the virus in the sink leaf. However, the virus was unable to cross the sink/source boundary for carbon that was present at the time of viral entry, suggesting a limited capacity for cell-to-cell virus movement into the apical (source) region of the leaf. A functional model of the sink/source transition in Nicotiana benthamiana is presented. This model provides a framework for the analysis of solute and virus movement in leaves.     

Further Evidence of Apoplastic Unloading into the Stem of Bean: Identification of the Phloem Buffering Pool

Further evidence that sucrose is passively leaked from the sievetubes directly into the apoplast of bean stems and activelyreloaded is reported. The stem apoplast is identified as thepool of sucrose outside the sieve tubes which buffers againstsudden changes in sieve tube sucrose concentration, caused bysudden changes in sink demand or source supply. Key words: Sucrose transport, Phaseolus vulgaris, Apoplast, Phloem unloading     

Short-term Effects of Heat-girdles on Source Leaves of Vicia faba: Analysis of Phloem Loading and Carbon Partitioning Parameters

Petiole heat-girdle treatments (followed by a 5 min ¹⁴CO₂ assimilation)were performed on mature leaves of Vicia faba, in order to assesstheir effect on the partitioning of photo-assimilates to theminor vein phloem. Whole leaf autoradiographic evidence indicateda high leaf-to-leaf variation in the image intensity over theminor veins (relative to the mesophyll/epidermal background)in both control and heat-girdled groups of leaves. The averagedegree of minor vein labelling in heat-girdled leaves, however,was found to be significantly lower than that in controls. Comparativeassessment of vein labelling was based on microscopic densityreadings of silver grains over veinal and interveinal regionsin autoradiographic images. Investigations into the cause ofthis alteration in vein labelling indicated no involvement ofan inhibition of apoplasmic phloem loading, as both heat-girdledand control leaves of Vicia were shown to have comparable minorvein uptake of exogenously supplied ¹⁴C-sucrose. Heat-girdlingwas shown, however, to increase significantly the partitioningof recently fixed carbon into the insoluble (mainly starch)fraction relative to the ethanol-soluble fraction, within 12min of the treatment. We suggest that this carbon partitioningchange can primarily account for the change in vein labelling,since an increase in the insoluble fraction would result in(1) more ¹⁴C-activity remaining in the leaf mesophyll and (2)less ¹⁴C-activity going into the mesophyll export pool, andthus, less ¹⁴C-sucrose being transferred to the minor vein region.Additionally, although leaf export was completely halted inheat-girdled leaves, ¹⁴C-activity was found within the majorveins as far as the point of petiole heat-girdling (followinga 5 min assimilation and 4 h chase). Apparently, continued (butlimited) solution flow within the sieve elements is maintainedby transport pathway unloading within the treated leaves. Key words: Phloem loading, carbon partitioning, heat-girdle, Vicia faba     

A morphometric analysis of the phloem-unloading pathway in developing tobacco leaves

A morphometric analysis of developing leaves of Nicotiana tabacum L. was conducted to determine whether imported photoassimilates could be unloaded by symplastic transport and whether interruption of symplastic transport could account for termination of import. Five classes of veins were recognized, based on numbers of cells in transverse section. Photoassimilate is unloaded primarily from Class III veins in tissue nearing the end of the sink phase of development. Smaller veins (Class IV and V) do not transport or unload photoassimilate in sink tissue because the sieve elements of these veins are immature until after the tissue stops importing. In Class III veins the sieve element-companion cell (SE-CC) complexes are surrounded by phloem parenchyma which abuts the bundle sheath. Along the most obvious unloading route, from SE-CC complex to phloem parenchyma to bundle sheath to mesophyll cells, the frequency of plasmodesmata at each interface increases. To determine whether this pattern of plasmodesmatal contact is consistent with symplastic unloading we first demonstrated, by derivation from Fick's law that the rate of diffusion from a compartment is proportional to a number N which is equal to the ratio of surface area to volume of the compartment multiplied by the frequency of pores (plasmodesmata) which connect it to the next compartment. N was calculated for each compartment within the vein which has the SE-CC complex as its center, and was shown to be statistically the same in all cases except one. These observations are consistent with a symplastic unloading route. As the leaf tissue matures and stops importing, plasmodesmatal frequency along the unloading route decreases and contact area between cells also decreases as intercellular spaces enlarge. As a result, the number of plasmodesmata between the SE-CC complex and the first layer of mesophyll cells declines in nonimporting tissue to 34% of the number found in importing tissue, indicating that loss of symplastic continuity between the phloem and surrounding cells plays a role in termination of photoassimilate unloading.Abbreviation SE-CC sieve element-companion cell     

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     

Distribution of imported 14C in developing leaves of eastern cottonwood according to phyllotaxy

Summary Individual leaves of eastern cottonwood (Populus deltoides Bartr.), representing an ontogenetic series from leaf plastochron index (LPI) 3.0 to 8.0, were fed ¹⁴CO₂ and harvested after 2–24 h. Importing leaves from LPI-1.0 through 8.0 on each plant were sectioned into 9 parts, and each part was quantitatively assayed for ¹⁴C activity. The highest level of ¹⁴C import was by leaves from LPI 1.0 to 3.0, irrespective of source-leaf age. ¹⁴C was translocated preferentially to either the right or left lamina-half depending on the position of the importing leaf in the phyllotactic sequence and its stage of development. For example, import was high when the importing leaf and the source leaf had two vascular bundles in common, moderately high with one bundle in common, and low with no bundles in common. The distribution of ¹⁴C within young importing leaves was highest in the lamina tip and decreased toward the base. With increasing leaf age, incorporation declined in the lamina tip and increased in the base.It may be concluded that each cottonwood leaf progresses through a continuum of importing and exporting stages as its lamina expands. The photosynthate imported by a given leaf is compartmentalized, with different exporting leaves supplying photosynthate to rather restricted regions of the lamina. Such localization within the importing leaf depends on its vascular connections with each of the exporting leaves, and these are predictable from a knowledge of the phyllotaxy.Plant Physiologists.     

Source and sink leaf metabolism in relation to Phloem translocation: carbon partitioning and enzymology

The import-export transition in sugar beet leaves (Beta vulgaris) occurred at 40 to 50% leaf expansion and was characterized by loss in assimilate import and increase in photosynthesis. The metabolism and partitioning of assimilated and translocated C were determined during leaf development and related to the translocation status of the leaf. The import stage was characterized by C derived from either ¹⁴C-translocate or ¹⁴C-photosynthate being incorporated into protein and structural carbohydrates. Marked changes in the C partitioning were temporally correlated with the import-export conversion. Exporting leaves did not hydrolyze accumulated sucrose and the C derived from CO₂ fixation was preferentially incorporated into sucrose. Both source and sink leaves contained similar levels of acid invertase and sucrose synthetase activities (sucrose hydrolysis) while sucrose phosphate synthetase (sucrose synthesis) was detected only in exporting leaves. The results are discussed in terms of intracellular compartmentation of sucrose and sucrose-metabolizing enzymes in source and sink leaves.     

Phloem loading in cucumber: combined symplastic and apoplastic strategies

Phloem loading, as the first step of transporting photoassimilates from mesophyll cells to sieve element‐companion cell complex, creates a driving force for long‐distance nutrient transport. Three loading strategies have been proposed: passive symplastic loading, apoplastic loading and symplastic transfer followed by polymer‐trapping of stachyose and raffinose. Although individual species are generally referred to as using a single phloem loading mechanism, it has been suggested that some plants may use more than one, i.e. ‘mixed loading’. Here, by using a combination of electron microscopy, reverse genetics and ¹⁴C labeling, loading strategies were studied in cucumber, a polymer‐trapping loading species. The results indicate that intermediary cells (ICs), which mediate polymer‐trapping, and ordinary companion cells, which mediate apoplastic loading, were mainly found in the fifth and third order veins, respectively. Accordingly, a cucumber galactinol synthase gene (CsGolS1) and a sucrose transporter gene (CsSUT2) were expressed mainly in the fifth/third and the third order veins, respectively. Immunolocalization analysis indicated that CsGolS1 was localized in companion cells (CCs) while CsSUT2 was in CCs and sieve elements (SEs). Suppressing CsGolS1 significantly decreased the stachyose level and increased sucrose content, while suppressing CsSUT2 decreased the sucrose level and increased the stachyose content in leaves. After ¹⁴CO₂ labeling, [¹⁴C]sucrose export increased and [¹⁴C]stachyose export reduced from petioles in CsGolS1i plants, but [¹⁴C]sucrose export decreased and [¹⁴C]stachyose export increased into petioles in CsSUT2i plants. Similar results were also observed after pre‐treating the CsGolS1i leaves with PCMBS (transporter inhibitor). These results demonstrate that cucumber phloem loading depends on both polymer‐trapping and apoplastic loading strategies.     

Sink Effect of Cytokinin in Detached Leaves Is Not Caused by Structural Rearrangements of Phloem

The sink effect of cytokinin is manifested as a decrease in source capacity and the induction of sink activity in the phytohormone-treated region of a mature excised leaf. In order to find out whether this effect was due to the direct action of cytokinin on the phloem structure, two types of phloem terminals were examined. In pumpkin (Cucurbita pepo L.) leaves, the phloem terminals are open; i.e., they are linked to mesophyll by numerous symplastic connections, which are located in narrow areas called plasmodesmal pit fields. In broad bean (Vicia faba L.) leaves, the phloem terminals belong to the closed type and have no symplastic links with mesophyll. The electron microscopic study of terminal phloem did not reveal any structural changes in the companion cells, which could account for the suppression of assimilate export. The treatment of leaves with cytokinin neither disturbed the structure of plasmodesmal pit fields in pumpkin leaves nor eliminated the wall protuberances (the ingrowths promoting phloem loading) in bean leaves. No evidence was obtained that the cytokinin-induced import of assimilates in mature leaves is caused by the recovery of meristematic activity, i.e., by either formation of new phloem terminals having immature sieve elements capable of unloading or by the development of new sieve elements within the existing veins. Cytokinin did not induce de novo formation of phloem elements. Structural characteristics of the leaf phloem, such as the number of branching orders in the venation pattern, the number of vein endings per areole, the number of areoles per leaf, the area of one areole, and the number of sieve elements per bundle remained unaltered. It is concluded that the sink effect of cytokinin in excised leaves cannot be determined by alteration of the phloem structure.     

Companion-Cell Specific Localization of Sucrose Synthase in Zones of Phloem Loading and Unloading

An immunohistochemical approach was used in maize (Zea mays) and citrus (Citrus paradisi) to address the previously noted association between sucrose synthase and vascular bundles and to determine the localization of the low but detectable levels of sucrose synthase that remain in leaves after the import-export transition. Sucrose synthase protein was immunolocalized at the light microscope level using paraffin sections reacted with rabbit sucrose synthase polyclonal antisera and gold-conjugated goat anti-rabbit immunoglobulin G. Immunolabel was specifically observed in phloem companion cells of minor and intermediate veins in mature leaves of both species. Similar localization was apparent in the midrib of mature citrus leaves, with additional labeling in selected files of phloem parenchyma cells. A clear companion-cell specificity was evident in the phloem unloading zone of citrus fruit, where high activity of sucrose synthase has been demonstrated in vascular bundles during periods of rapid import. Sucrose synthase protein was not associated with adjacent cells surrounding the vascular strands in this tissue. The companion-cell specificity of sucrose synthase in phloem of both importing and exporting structures of these diverse species implies that this may be a widespread association and underscores its potential importance to the physiology of vascular bundles.     

Phloem Unloading within the Seed coat of Pisum sativum Observed using Surgically Modified Seeds

Phloem unloading in pea seed coats was observed by removingthe embryos from developing seeds and washing the attached coatswith a weakly buffered solution. The quantity of labelled photosynthateappearing in the washing solution varied immediately when thesolute concentration was changed, and is shown to be an osmoticresponse. This response is predicted by the Münch theoryof phloem transport with concentration dependent unloading.Respiratory inhibitors and the sulphydryl modifying reagentPCMBS had a slow effect upon the washout of tracer, which arrivedwithin the seed coat prior to inhibitor application, but completelystopped any washout of tracer arriving after its application.This time-course suggests that the inhibitors were not directlyinhibiting unloading, but preventing further tracer from enteringthe region of unloading within the seed coat. Phloem unloadingwithin the seed coats of Pisum appears to be passive and notdependent upon a PCMBS-sensitive carrier. Key words: Pisum sativum, seeds, phloem unloading     

Efflux of sucrose from minor veins of tobacco leaves

Mature leaves import limited amounts of nutrient when darkened for prolonged periods. We tested the hypothesis that import is restricted by the apoplast-phloem loading mechanism, ie., as sucrose exits the phloem of minor veins it is retrieved by the same tissue, thus depriving the mesophyll of nutrient. When single, attached, mature leaves of tobacco (Nicotiana tabacum L.) plants were darkened, starch disappeared from the mesophyll cells, indicating that the supply of solute to the mesophyll was limited. Starch was synthesized in mesophyll cells of darkened tissue when sucrose was applied to the apoplast at 0.1–0.3 mM concentration. Efflux from minor veins was studied by incubating leaf discs on [¹⁴C]sucrose to load the minor veins and then measuring subsequent ¹⁴C release. Efflux was rapid for the first hour and continued at a gradually decreasing rate for over 13 h. Net efflux increased when loading was inhibited by p-chloromercuribenzene-sulfonic acid, anoxia, isotope-trapping, or reduction of the pH gradient. Neither light nor potassium had a significant effect on the rate of labeled sucrose release. The site of labeled sucrose release was investigated by measuring efflux from discs in which sucrose had previously been loaded preferentially by either the minor veins or mesophyll cells. Efflux occurred primarily from minor veins.Abbreviations Mes 2(N-morpholino)ethanesulfonic acid - Mops 3(N-morpholino)propanesulfonic acid - PCMBS p-chloromercuribenzenesulfonic acid - SE-CC sieve element-companion cell complex     

Seed Coat Unloading in Pisum sativum--Osmotic Effects in Attached versus Excised Empty Ovules

Experiments were undertaken with embryo-less ovules of Pisumsativum to study the influence of apoplastic osmolality on seedcoat import and seed coat unloading.¹¹CO₂ pulse labelling alongwith collimated monitoring of plant tissues were used with attachedovules to measure continuously and simultaneously total podimport, import into a modified ovule and photo-assimilate washoutfrom the seed coat of the ovule into a flow-through bathingsolution.Our results indicated that seed coat import was immediatelyaffected by a change in the applied bathing solution osmolality,with a decrease in osmolality lowering seed coat import andan increase in osmolality increasing import. ¹¹C-photo-assimilatewashout from attached ovules was found to respond in a similarmanner to the apoplastic osmolality. However, the osmotic effecton ¹¹C-washout was a delayed response and it appears that themajority of this observed response was due to the alterationin seed coat tracer import. Further experiments with ¹⁴C-labelled,excised seed coat halves (i.e. no further import) supportedthis hypothesis by demonstrating that seed coat unloading (measuredas ¹⁴C-photo-assimilate washout) was actually enhanced at alow solution osmolality. PCMBS had no effect on seed coat importor washout in attached, modified ovules, suggesting that photo-assimilateunloading from seed coats of Pisum does not involve a carrierprotein. Studies of the spatial distribution of imported ¹⁴Cin Pisum seed coats further suggest that this unloading, intothe apoplast, occurs from non-phloem cell types, and that themovement of photo-assimilates from the sieve elements to theterminal unloading site occurs via symplastic transport. Key words: Pisum sativum, seed coat, seed coat unloading, phloem unloading     

Sucrose Transport and Phloem Unloading in Stem of Vicia faba: Possible Involvement of a Sucrose Carrier and Osmotic Regulation

Stems of Vicia faba plants were used to study phloem unloading because they are hollow and have a simple anatomical structure that facilitates access to the unloading site. After pulse labeling of a source leaf with ¹⁴CO₂, stem sections were cut and the efflux characteristics of ¹⁴C-labeled sugars into various buffered solutions were determined. Radiolabeled sucrose was shown to remain localized in the phloem and adjacent phloem parenchyma tissues after a 2-hour chase. Therefore, sucrose leakage from stem segments prepared following a 75-minute chase period was assumed to be characteristic of phloem unloading. The efflux of ¹⁴C assimilates from the phloem was enhanced by 1 millimolar p-chloromercuribenzene sulfonic acid (PCMBS) and by 5 micromolar carbonyl cyanide m-chlorophenly hydrazone (CCCP). However, PCMBS inhibited and CCCP enhanced general leakage of nonradioactive sugars from the stem segments. Sucrose at concentrations of 50 millimolar in the free space increased efflux of [¹⁴C]sucrose, presumably through an exchange mechanism. This exchange was inhibited by PCMBS and abolished by 0.2 molar mannitol. Increasing the osmotic concentration of the efflux medium with mannitol reduced [¹⁴C]sucrose efflux. However, this inhibition seems not to be specific to sucrose unloading since leakage of total sugars, nonlabeled sucrose, glucose, and amino acids from the bulk of the tissue was reduced in a similar manner. The data suggest that phloem unloading in cut stem segments is consistent with passive efflux of sucrose from the phloem to the apoplast and that sucrose exchange via a membrane carrier may be involved. This is consistent with the known conductive function of the stem tissues, and contrasts with the apparent nature and function of unloading in developing seeds.     

Phloem loading and unloading of sugars and amino acids

In terrestrial higher plants, phloem transport delivers most nutrients required for growth and storage processes. Some 90% of plant biomass, transported as sugars and amino nitrogen (N) compounds in a bulk flow of solution, is propelled though the phloem by osmotically generated hydrostatic pressure differences between source (net nutrient export) and sink (net nutrient import) ends of phloem paths. Source loading and sink unloading of sugars, amino N compounds and potassium largely account for phloem sap osmotic concentrations and hence pressure differences. A symplasmic component is characteristic of most loading and unloading pathways which, in some circumstances, may be interrupted by an apoplasmic step. Raffinose series sugars appear to be loaded symplasmically. However, sucrose, and probably certain amino acids, are loaded into minor veins from source leaf apoplasms by proton symporters localized to plasma membranes of their sieve element/companion cell (se/cc) complexes. Sucrose transporters, with complementary kinetic properties, are conceived to function as membrane transporter complexes that respond to alterations in source/sink balance. In contrast, symplasmic unloading is common for many sink types. Intervention of an apoplasmic step, distal from importing phloem, is reserved for special situations. Effluxers that release sucrose and amino acids to the surrounding apoplasm in phloem loading and unloading are yet to be cloned. The physiological behaviour of effluxers is consistent with facilitated membrane transport that can be energy coupled. Roles of sucrose and amino acid transporters in phloem unloading remain to be discovered along with mechanisms regulating symplasmic transport. The latter is hypothesized to exert significant control over phloem unloading and, in some circumstances, phloem loading.     

Phloem Pressure Differences and C-Assimilate Translocation in Ecballium elaterium

The role of phloem turgor pressure in ¹⁴C-assimilate translocation in Ecballium elaterium A. Rich was studied. The direction of translocation was manipulated by two methods: darkening, or defoliation, of the upper or lower halves of the shoots. After 24 hours of labeled assimilate movement, sieve tube turgor levels were measured with the phloem needle technique. Distribution of label, determined by autoradiography and counting, revealed a direct correlation between the direction of assimilate transport and the pressure difference. Phloem turgor levels always decreased in the stem of darkened shoots; this resulted in greater pressure differences in the stem between the source leaf receiving ¹⁴CO₂ and treated regions.