Energy Coupled Transport of Sorbitol and Other Sugars into the Protoplast Isolated from Apple Fruit Flesh

The uptake kinetics of sorbitol, sucrose, glucose and fructoseacross the plasma membrane using protoplasts isolated from applefruit flesh (Malus pumila Mill. var. domestica Schneid.) wasinvestigated. When sorbitol was taken up into the cell, PCMBS-sensitivesaturable transport was distinguishable from the diffusive transport.At a low sorbitol concentration, the saturable transport systemaccounted for more than 50% of the total uptake, whereas ata high concentration the diffusive transport system was moredominant. The saturable transport was suggested be a carrier-mediatedtransport system coupled with ATP because the system was inhibitedCCCP or orthovanadate. The K_m value for sorbitol was computedto be 3.6mM. A carrier-mediated transport system coupled withATP was also observed for glucose and fructose with correspondingK_m values of 5.0 and 2.5 mM. However, no saturable transportfor sucrose was observed over a range of 0.1 to 10 mM sucroseconcentration. The relationship among these sugar transportsystems across the plasma membrane, apoplastic unloading, andsugar accumulation vacuoles are discussed. ¹Present address: Laboratory of Horticulture, Faculty of Agriculture,Nagoya University, Chikusa, Nagoya 464, Japan. (Received April 8, 1988; Accepted June 8, 1988)     

An Hg-sensitive channel mediates the diffusional component of glucose transport in olive cells

In several organisms solute transport is mediated by the simultaneous operation of saturable and non-saturable (diffusion-like) uptake, but often the nature of the diffusive component remains elusive. The present work investigates the nature of the diffusive glucose transport in Olea europaea cell cultures. In this system, glucose uptake is mediated by a glucose-repressible, H⁺-dependent active saturable transport system that is superimposed on a diffusional component. The latter represents the major mode of uptake when high external glucose concentrations are provided. In glucose-sufficient cells, initial velocities of d- and l-[U-¹⁴C]glucose uptake were equal and obeyed linear concentration dependence up to 100 mM sugar. In sugar starved cells, where glucose transport is mediated by the saturable system, countertransport of the sugar pairs 3-O-methyl-d-glucose/d-[U-¹⁴C]glucose and 3-O-methyl-d-glucose/3-O-methyl-d-[U-¹⁴C]glucose was demonstrated. This countertransport was completely absent in glucose-sufficient cells, indicating that linear glucose uptake is not mediated by a typical sugar permease. The endocytic inhibitors wortmannin-A and NH₄Cl inhibited neither the linear component of d- and l-glucose uptake nor the absorption of the nonmetabolizable glucose analog 3-O-methyl-d-[U-¹⁴C]glucose, thus excluding the involvement of endocytic mediated glucose uptake. Furthermore, the formation of endocytic vesicles assessed with the marker FM1-43 proceeded at a very slow rate. Activation energies for glucose transport in glucose sufficient cells and plasma membrane vesicles were 7 and 4 kcal mol^− 1, respectively, lower than the value estimated for diffusion of glucose through the lipid bilayer of phosphatidylethanolamine liposomes (12 kcal mol^− 1). Mercury chloride inhibited both the linear component of sugar uptake in sugar sufficient cells and plasma membrane vesicles, and the incorporation of the fluorescent glucose analog 2-NBDG, suggesting protein-mediated transport. Diffusive uptake of glucose was inhibited by a drop in cytosolic pH and stimulated by the protein kinase inhibitor staurosporine. The data demonstrate that the low-affinity, high-capacity, diffusional component of glucose uptake occurs through a channel-like structure whose transport capacity may be regulated by intracellular protonation and phosphorylation/dephosphorylation.     

Sugar transport in isolated corn root protoplasts

Isolated corn (Zea mays L.) root protoplasts were used to study sucrose and hexose uptake. It is found that glucose was preferentially taken up by the protoplasts over sucrose and other hexoses. Glucose uptake showed a biphasic dependence on external glucose concentration with saturable (K_m of 7 millimolar) and linear components. In contrast, sucrose uptake only showed a linear kinetic curve. Sucrose and glucose uptake were linear over a minimum of 1 hour at pH 6.0 and 1 millimolar exogenous sugar concentration. Glucose uptake showed a sharp 42°C temperature optimum, while sucrose uptake showed a lower temperature sensitivity which did not reach a maximum below 50°C. Uptake of both sugars was sensitive to several metabolic inhibitors and external pH. Differences between sucrose and glucose uptake in two different sink tissue (i.e. protoplasts from corn roots and soybean cotyledons) are discussed.     

Sucrose and Glucose Uptake into Beta vulgaris Leaf Tissues : A Case for General (Apoplastic) Retrieval Systems

Concentration curves for sugar and amino acid uptake by Beta vulgaris L. leaf tissues contained both a saturable and a linear component. Similarly shaped curves were obtained for influx of sucrose, glucose, and 3-O-methyl glucose by leaf discs, whole petiole slices, petiole segments containing pith tissue only, and petiole segments containing vascular bundles, although the tissues took up the various sugars via different proportions of saturable versus linear uptake. Two millimolar p-chloromercuribenzenesulfonic acid selectively inhibited the saturable component of sucrose uptake, but had almost no effect on the linear component. Uptake of glucose and 3-O-methyl glucose remained unaffected by p-chloromercuribenzenesulfonic acid treatment. Anoxia was found to inhibit the linear component of both sucrose and 3-O-methyl glucose influx, while the saturable component remained unaffected. The linear component of sucrose uptake was also competitively inhibited by maltose, as well as being selectively promoted by certain exposures to 5 millimolar N-ethylmaleimide, 2 micrograms per milliliter cycloheximide, and high levels of mannitol acting as osmoticum. These results support the proposal that the linear component is due to a process more complex than simple, or exchange, diffusion. It would also appear that the linear transport component utilizes a separate energy source than does the saturable component of sucrose influx.

Evidence for phloem loading from the apoplast was re-examined with respect to the present findings. Saturable sucrose uptake by minor vein tissues may represent retrieval of solute from the free space, which could explain the `apoplastic loading' phenomenon.

     

Transport of 2-aminoisobutyric acid in mesophyll protoplasts of Pisum sativum L. Application of silicone oil layer centrifugation

Uptake of 2-amino [1-¹⁴C]isobutyric acid by pea mesophyll protoplasts was investigated using silicone oil layer centrifugation. Uptake was expressed on the basis of the ³H₂O-space of the protoplast pellet. The ³H₂O-space can be used as a measure of protoplast volume, taking into consideration that about 10% of it is extracellular space. At concentrations in the range 10 μM–10 mM, the uptake of 2-aminoisobutyric acid (Aib) was linear with time for at least 1 h. At an external concentration of 10 μM, up to a 10-fold accumulation of Aib in the protoplasts was observed during 1 h of incubation. The concentration-dependence of the uptake rate conforms to the sum of a Michaelis-Menten term and a linear term. Large differences in uptake rates were found for different preparations of protoplasts, especially at low concentrations of Aib. This could be attributed to differences in the activity of the saturable component. Both transport components were strongly inhibited by 10 μM CCCP, even when transport was apparently downhill.     

Sulfur dioxide inhibits the sucrose carrier of the plant plasma membrane.

Plasma membrane vesicles were prepared by phase partition from a microsomal fraction of broad bean (Vicia faba L.) leaf. In order to study the effects of sodium sulfite on active uptake of sucrose, the vesicles were artificially energized by a transmembrane pH gradient (delta pH) and/or a transmembrane electrical gradient (delta psi). At 1 mM, sulfite strongly inhibited sucrose uptake but did not affect the two components of the proton motive force, delta pH (measured by dimethyloxazolidine dione) and delta psi (measured by tetraphenylphosphonium). Moreover, sulfite did not inhibit the proton-pumping ATPase of the plasma membrane vesicles. These data demonstrate that sulfite may inhibit transport of photoassimilates in plant by a direct inhibition of the sucrose carrier of the plasma membrane.     

Sugar uptake and proton release by protoplasts from the infected zone of Vicia faba L. nodules: evidence against apoplastic sugar supply of infected cells

Symbiotic dinitrogen fixation of legume nodules is fuelled by phloem-imported carbohydrates. These have to pass several cell layers to reach cells infected with Rhizobium bacteroids. It is unclear whether apoplastic steps are involved in carbohyd-rate translocation within the nodule. Protoplasts were isolated from the infected and uninfected cells of the central tissue of Vicia faba nodules using a recently developed protocol. These protoplasts were used to elucidate pathways for sugar transport in this tissue. Both types of protoplasts released protons into the medium. Acidification was inhibited by vanadate and erythrosin B. However, it was stimulated by fusicoccin only in uninfected cells. A symport of sugars with protons can therefore be energized in both cell types. Uptake of 14C-labelled sugars was determined using a phthalate centrifugation technique. Uninfected protoplasts accumulated glucose through high-affinity H+/glucose-symport that was not competitively inhibited by fructose or sucrose. Uninfected protoplasts also absorbed sucrose with biphasic kinetics. At 0.1, 1, and 10 mM sucrose, uptake was inhibited by CCCP. Fusicoccin did not stimulate the linear phase of sucrose uptake. Glucose inhibited sucrose uptake nearly completely. This was not related to sucrose cleavage in the medium because sucrose was absorbed at a much higher rate than glucose, and glucose concentration did not increase in sucrose-containing protoplast suspensions. By contrast with uninfected protoplasts, infected cells did not show transporter-mediated glucose or sucrose uptake. The findings underline a role of uninfected cells in sugar translocation. Infected cells are not apoplastically supplied with sugars and possibly depend on uninfected cells for carbon supply.     

Carrier-mediated uptake of glyphosate in broad bean (Vicia faba) via a phosphate transporter

The possibility that the herbicide glyphosate (N-phosphonomethylglycine) may be taken up in plant cells via a phosphate transporter of the plasma membrane was investigated using protoplasts of broad bean leaves ( Vicia faba L.). Phosphonoformic acid, a powerful inhibitor of phosphate transport in animal cells, was first demonstrated to be a competitive inhibitor of phosphate uptake inbroad bean protoplasts. Glyphosate was able to inhibit phosphate uptake into the protoplasts, and to protect partially the phosphate transporter from inhibition by phosphonoformic acid. Concentration dependence studies showed that glyphosate uptake exhibited a saturable phase at low glyphosate concentrations (0. 5 to 3 μ M ), superimposed by a linear uptake at higher concentrations (up to 100 μ M ). Inhibition of glyphosate uptake by para -chloromercuribenzene sulphonic acid, sodium azide and carbonyl-cyanide- m -chlorophenylhydrazone was much stronger at 1 than at 100 μ M glyphosate. Kinetics indicated that the saturable component of glyphosate transport was competitively inhibited by either phosphate or phosphonoformic acid. It is concluded that glyphosate can be absorbed via a phosphate transporter of the plasma membrane     

Isolation and sugar uptake characteristics of protoplasts from maize endosperm suspension cultures

The isolation and sugar uptake characteristics of protoplasts from maize ( Zea mays L.) endosperm-derived suspension cultures are described. In contrast with protoplasts from intact developing endosperm, which by virtue of their large size and high starch content are too fragile for sugar uptake experiments, suspension cultures yielded protoplasts capable of withstanding the necessary handling and centrifugations. Intactness of the protoplasts was demonstrated by dye exclusion or accumulation and latency of malate dehydrogenase activity. Uptake of radioactivity from [³H]-inulin did not increase with time, but that from [¹⁴C]-sugars increased over a wide range of external concentrations. Kinetics of fructose, glucose and sucrose uptake were biphasic, and the saturable components of uptake were eliminated by p -chloromercuribenzene sulfonate (PCMBS). Rates of uptake of sucrose and 1'-fluorosucrose were similar, confirming that hydrolysis by cell wall invertase contributes to sucrose uptake by the suspension cultures. The isolation of protoplasts from this tissue source will enable experimental access to plasma membrane sugar carriers which may exist in the intact maize endosperm.     

Sugar retrieval by coats of developing seeds of Phaseolus vulgaris L. and Vicia faba L

Influxes of glucose, fructose and sucrose were characterised for coat cells of developing seeds of Phaseolus vulgaris L. and Vicia faba L. by monitoring uptake of [(14)C]sugars into excised seed-coat halves and two different protoplast populations derived from seed coats. Sugar influxes by the two populations of protoplasts were similar for each sugar species [sucrose > (fructose approximately glucose)] and hexoses competed with sucrose. Concentration-dependent influxes of all three sugars by excised seed coats could be described by a simple directly proportional relationship between concentration ([S]) and uptake rate (v) in the physiological range of sugar concentrations (v approximately A.[S]). Alternatively, with the exception of fructose influx by Vicia, all could be fitted to a Michaelis-Menten relationship, as could sucrose uptake by Vicia protoplasts. Apparent K(m) values were high ( approximately 100-500 mM) compared with those reported for other systems. Sucrose transport was distinct from glucose and fructose transport in both species. Sugar influx was decreased by p-chloromercuribenzenesulfonic acid, carbonylcyanide m-chlorophenylhydrazone and erythrosin B. These responses are consistent with sugar/H(+) symport acting to retrieve photoassimilates leaked to the apoplasm during post-sieve element transport within seed coats.     

Sugar utilization by yeast during fermentation

Summary When glucose and fructose are fermented separately, the uptake profiles indicate that both sugars are utilized at similar rates. However, when fermentations are conducted in media containing an equal concentration of glucose and fructose, glucose is utilized at approximately twice the rate of fructose. The preferential uptake of glucose also occurred when sucrose, which was first rapidly hydrolyzed into glucose and fructose by the action of the enzyme invertase, was employed as a substrate. Similar results were observed in the fermentation of brewer's wort and wort containing 30% sucrose and 30% glucose as adjuncts. In addition, the high levels of glucose in the wort exerted severe catabolite repression on maltose utilization in theSaccharmyces uvarum (carlsbergensis) brewing strain. Kinetic analysis of glucose and fructose uptake inSaccharomyces cerevisiae revealed aK _m of 1.6 mM for glucose and 20 mM for fructose. Thus, the yeast strain has a higher affinity for glucose than fructose. Growth on glucose or fructose had no repressible effect on the uptake of either sugar. In addition, glucose inhibited fructose uptake by 60% and likewise fructose inhibited, glucose uptake by 40%. These results indicate that glucose and fructose share the same membrane transport components.     

Sugar transport in leaf discs of Phaseolus coccinius

Kinetic profiles for sucrose, glucose and 3-OMG glucose were determined in leaf discs of Phaseolus coccinius L. (cv. Scarlet). All three sugars exhibited identical uptake kinetics. At sugar concentrations below 25 m M , transport was due to an active, carrier-mediated transport system. A linear component was the dominant mode of uptake at sugar concentrations above 25 m M . Sucrose and glucose carriers were specific for these sugars, since no uptake inhibition was observed from competing sugars. Sucrose was not hydrolyzed by leaf tissue because the label in asymetrically labeled sucrose was not randomized. Furthermore, no label was present in hexose fractions when tissue was incubated with [⁸⁴C]-sucrose. Therefore, [¹⁴C]-sucrose uptake did not reflect hexose uptake.
Both saturable and linear components of uptake contribute significantly to total uptake rates. The former, however, is more important when apoplastic sugar concentrations are low. The molecular nature of the linear component is not well understood but accounts for most of the uptake at high sugar concentrations.     

Sugar uptake by protoplasts isolated from tomato fruit tissues during various stages of fruit growth

The uptake of radioactive glucose and sucrose by protoplasts isolated from pericarp and placenta tissues of tomato ( Lycopersicon esculentum cv. Counter) fruit was investigated in relation to the dry matter accumulation rates of these tissues. Uptake of glucose by protoplasts isolated from pericarp tissue was highest in fruit of around 20 g fresh weight or 25 days after anthesis. Sucrose uptake by pericarp protoplasts was lower than that of glucose and did not show a peak of uptake. The maximum rate of glucose uptake by protoplasts from the pericarp was at the time when the tomato fruit was accumulating dry matter at the highest rate. Glucose uptake by placenta protoplasts was lower and at a similar level as sucrose.
Protoplast uptake of glucose, but not of sucrose, was partially inhibited by (1) p -chloromercuribenzene sulphonic acid, a sulphydryl group modifier; (2) erythrosin B, an H⁺-ATPase inhibitor; and (3) valinomycin, a K⁺-ionophore, suggesting that membrane transport of glucose by tomato fruit sink cells may be a carrier-mediated, energy-dependent process.
The main route of carbohydrate accumulation by tomato fruit during the period of rapid fruit growth may be by cleavage of sucrose by apoplastic acid invertase prior to hexose transport across the plasma membrane.     

Sucrose uptake and partitioning in discs derived from source versus sink potato tubers

The uptake of sucrose into isolated discs cut from sink (growing) and source (sprouting) potato (Solanum tuberosum L.) tuber tissue was studied. The uptake of sucrose into sink-tuber discs demonstrated biphasic kinetics. The large saturable component was inhibited by incubation of the discs with p-chloromercuribenzene sulfonic acid (PCMBS) whilst both the saturable and linear components were inhibited by carbonyl cyanide m-chlorophenylhydrazone (CCCP). By contrast, in source-tuber discs, the linear component represented the majority of sucrose taken up, the saturable component playing only a minor role. In source discs, only the saturable component of uptake was inhibited by either PCMBS or CCCP. A large proportion (up to 25%) of sucrose taken up into sink-tuber discs was converted to starch but as the tubers aged the proportion of sucrose converted to starch decreased to the level found in source-tuber discs (approx. 3%). By contrast with sink-tuber discs (see Oparka and Wright, 1988b, Planta 175, 520–526) sucrose uptake into source discs was insensitive to turgor and demonstrated an uptake pattern similar to that of CCCP-treated sink tissue. It is proposed that exogenous sucrose is taken into the storage parenchyma of sink-tuber discs by both a carrier-mediated and a diffusional process. By contrast, uptake into the storage parenchyma of source-tuber discs appears to be essentially diffusional. The turgor sensitivity of sucrose uptake into sink-tissue discs may be mediated via the plasmalemma H⁺-ATPase. As the tuber ages the sucrose-uptake activity decreases and the capacity of the storage parenchyma to synthesise starch is lost. The data are discussed in relation to the in-vivo mechanisms of sucrose transport in storage tissues.     

Transport of sugars across the plasma membrane of beetroot protoplasts

Protoplasts isolated from beetroot tissue took up glucose preferentially whereas sucrose was transported more slowly. The ¹⁴C-label from [¹⁴C]glucose and [¹⁴C]sucrose taken up by the cells could be detected rapidly in phosphate esters and, after feeding of [¹⁴C]glucose was found also in sucrose. The temperature-dependent uptake process (activation energy E_A about 50 kJ · mol^–1) seems to be carrier mediated as indicated by its substrate saturation and, for glucose, by competition experiments which revealed positions C₁, C₅ and C₆ of the D-glucose molecule as important for effective uptake. The apparent K_m(20° C) for glucose (3-O-methylglucose) was about 1 mM whereas for sucrose a significantly lower apparent affinity was determined (K_m about 10 mM). When higher concentrations of glucose (5 mM) or sucrose (20 mM) were administered, the uptake process followed first-order kinetics. Carrier-mediated transport was inhibited by N,N-dicyclohexylcarbodiimide, Na-orthovanadate, p–chloromercuribenzenesulfonic acid, and by uncouplers and ionophores. The uptake system exhibited a distinct pH optimum at pH 5.0. The results indicate that generation of a proton gradient is a prerequisite for sugar uptake across the plasma membrane. Protoplasts from the bundle regions in the hypocotyl take up glucose at higher rates than those derived from bundle-free regions. The results favour the idea that apoplastic transport of assimilates en route of unloading might be restricted to distinct areas within the storage organ (i.e. the bundle region) whereas distribution in the storage parenchyma is symplastic.Abbreviations CCCP Carbonylcyanide m–chlorophenylhydrazone - DCCD N,N-dicyclohexylcarbodiimide - DOG deoxyglucose - Mes 2-(N-morpholino)ethanesulfonic acid - 3-OMG 3-O-methylglucose - PCMBS p–chloromercuribenzenesulfonic acid - SDS Sodium dodecyl sulfate - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Influence of cell turgor on sucrose partitioning in potato tuber storage tissues

Sucrose uptake and partitioning in potato (Solanum tuberosum L.) tuber discs were examined under a range of mannitol and ethylene-glycol concentrations. Mannitol caused the same changes in turgor over a wide range of incubation periods (90 min-6 h), indicating that it did not penetrate the tissue. In comparison, ethylene glycol reduced turgor losses but did not eliminate them, even after 6 h. Between 100 mM and 300 mM mannitol, turgor fell by 350 kPa, compared with 35 kPa in ethylene glycol. Uptake experiments in mannitol alone showed that total sucrose uptake was strongly correlated with both osmotic potential and with turgor potential. In subsequent experiments sucrose uptake and partitioning were examined after 3 h equilibration in 100 mM and 300 mM concentrations of mannitol and ethylene glycol. Total sucrose uptake and the conversion of sucrose to starch were enhanced greatly only at 300 mM mannitol, indicating an effect of turgor, rather than osmotic potential on sucrose partitioning. The inhibitors p-chloromercuribenzenesulfonic acid and carbonylcyanide m-chlorophenylhydrazone (CCCP) both reduced sucrose uptake, but in quite different ways. p-Chloromercuribenzenesulfonic acid reduced total sucrose uptake but did not affect the partitioning of sucrose to starch. By contrast, CCCP inhibited total uptake and virtually eliminated the conversion of sucrose to starch. Despite this, sucrose uptake in the presence of CCCP continued to increase as the mannitol concentration increased, indicating an increase in passive transport at higher mannitol concentrations. Increased sucrose uptake above 400 mM mannitol was shown to be the result of uptake into the free space. The data show that starch synthesis is optimised at low but positive turgors and the relation between sucrose partitioning and the changing diurnal water relations of the tuber are discussed.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - PCMBS p-chloromercuribenzenesulfonic acid     

Phloem loading in Vicia faba leaves: Effect of N-ethylmaleimide and parachloromercuribenzenesulfonic acid on H+ extrusion,K+ and sucrose uptake

The effects of a penetrating (NEM) and a non-penetrating (PCMBS) sulfhydryl-specific reagent on proton extrusion, ⁸⁶Rb and [U-¹⁴C]sucrose uptake by Vicia faba leaves have been studied. Proton extrusion was strongly or completely inhibited by 0.1 mM NEM. ⁸⁶Rb and [U-¹⁴C]sucrose uptake were markedly reduced by NEM concentrations equal to or higher than 0.5 mM. Under our experimental conditions, PCMBS (1 mM) exerted a strong inhibition on [¹⁴C]sucrose uptake but did not inhibit proton extrusion and ⁸⁶Rb uptake. The sensitivity of phloem loading to PCMBS is thought to be a consequence of sugar-carrier blockage and not of inhibition of the proton pump.Abbreviations CCCP carbonylcyanide-m-chlorophenylhydrazone - DES diethylstilbestrol - DCCD dicyclohexylcarbodiimide - FC Fusicoccin - NEM N-ethylmaleimide - PCMBS p-chloromercuribenzenesulfonic acid     

Characterization of Hexose Transporter for Facilitated Diffusion of the Tonoplast Vesicles from Pear Fruit

Tonoplast vesicles were prepared from the flesh tissue of maturepear fruit. Sugar uptakes into the vesicles determined by twodifferent methods, the membrane and the gel filtration methods,were quite similar. The uptake was highest for glucose and subsequently,in order, for fructose, sucrose and sorbitol. It was not stimulatedby addition of ATP, although the vesicles could create a protongradient. However, the uptakes were significantly inhibitedby p-chloromercuribenzene sulphonate (PCMBS, SH-reagent andinhibitor of sugar transporter). Further, the PCMBS-sensitiveuptakes of glucose and fructose saturated with their increasedconcentrations. Thus, these PCMBS-sensitive uptakes are mediatedby the transporter of facilitated diffusion. The uptakes ofglucose or fructose each had two K_m values. K_m values for glucosewere 0.35 and 18 mM, and those for fructose were 1.6 and 25raM. The uptake of 0.2 mM glucose was inhibited by 2 mM fructoseand that of 2 mM fructose was inhibited by 2 mM glucose, butneither was inhibited by sucrose or sorbitol. O-methyl-glucose(OMG) also inhibited both the glucose and fructose uptakes.Therefore, the same transporter may mediate both glucose andfructose uptakes at lower concentrations; this hexose transportsystem differed from the sucrose and sorbitol transport systems. ¹Research Fellow of the Japan Society for the Promotion of Science. ²Present address: Faculty of Agriculture, Tohoku University,1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai, 981 Japan.     

Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on the transmembrane potential difference

Sodium sulfite (Na₂SO₃) decreased uptake from 1 m M sucrose by the parenchyma and by the veins of leaves of broadbean ( Vicia faba L. cv. Aguadulce). The decrease depended on the concentration of the pollutant and the duration of pretreatment. The inhibition was non-competitive. Sulfite affected the transmembrane potential difference (PD) of the leaf tissues. The short-term response obeyed an 'all or nothing' law. At 0.1 m M and above, sulfite led to a quick depolarization of one-third of the initial potential after a lag phase of about 5 min; for concentrations lower than 0.1 m M , sulfite did not affect the potential. By contrast, the long-term effect of Na₂SO₃ on the transmembrane PD strongly depended on its concentration. After 2–12 h of pretreatmemt there was no effect at 10 μ M , a weak effect at 0.1 m M , and then increasing depolarization as the pollutant concentration increased. The inhibitory effect of Na₂SO₃ on sucrose uptake is thus, at least partly, due to its effect on a component of the proton-motive force. ΔΨ. However, the lack of correlation noticed with 0.1 m M Na₂SO₃ between the effect on sucrose uptake and the long-term effect on transmembrane PD suggests numerous sites of sulfite action.