Sucrose Efflux from the Maize Scutellum

Sucrose efflux from maize scutellum slices was promoted by high pH and by K+, Na+ or Rb+. Incubation in mannose (which drastically reduces the ATP level) caused high rates of sucrose efflux only when KCl was present at pH 8. The effects of triphenylmethylphosphonium ion (TPMP+, a lipid soluble cation) on sucrose efflux were similar to those of mannose plus KCl. Mannose and TPMP+ caused release of stored sucrose into the cytoplasm, but pH8 and KCl (mannose) or pH 8 (TPMP+) in the bathing solution were necessary for rapid efflux of sucrose. Rb⁺ uptake took place during sucrose efflux. In mannose, rates of Rb⁺ uptake and sucrose efflux were low at pH 5.6 and high at pH 8.0, although the time courses for uptake and efflux were different. It is concluded that sucrose efflux is electrogenic and that it occurs as sucrose-H⁺ symport. A scheme for sucrose transport across plasmalemma and tonoplast is presented.     

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.     

Sucrose uptake by developing soybean cotyledons

Sucrose uptake by excised developing soybean cotyledons shows a biphasic dependence on sucrose concentration. At concentrations less than about 50 millimolar external sucrose, uptake can be described as a carrier-mediated process, with a K_m of 8 millimolar. At higher external sucrose concentrations, a linear dependence becomes apparent, which suggests the participation of a nonsaturable component in total uptake. Sucrose absorption is dependent on the presence of an electrochemical potential gradient for protons since agents interfering with the generation or maintenance of this gradient (NaN₃ or carbonylcyanide-m-chlorophenyl hydrazone) decrease sucrose transport to a level at or below that predicted from the operation of the noncarrier-mediated process alone. The saturable component of sucrose uptake is also sensitive to the sulfhydryl-modifying compounds N-ethylmaleimide and p-chloro-mercuribenzenesulfonate. The thiol-reducing agent diethioerythritol reverses fully the p-chloro-mercuri-benzenesulfonate inhibition, but not that of N-ethyl maleim de. Sucrose transport is sensitive to external pH, being decreased at high pH₀. Since sucrose-induced depolarization of the membrane potential and carrier-mediated sucrose influx show similar pH-dependence, inhibitor sensitivity, and values of K_m for sucrose, a sucrose/proton contransport process appears to operate in developing soybean cotyledon cells. Measurement of free space and intracellular sucrose concentrations in vivo suggests that the carrier-mediated process is fully saturated and that sucrose transport may be limiting for sucrose accumulation by the developing seed.     

Cytokinins and leaf development in sweet pepper (Capsicum annuum L.)

Stimulation of leaf expansion by an exogenous cytokinin was studied in isolated leaf discs of sweet pepper with emphasis on the assimilate utilization of the tissue. Leaf discs were floated on solutions containing sucrose and plant growth regulators. Benzyladenine (BA) promoted the area expansion rate of the leaf discs. Sucrose at 100 mM resulted in increased area expansion rate compared with 10 mM sucrose. However, the increased sucrose concentration had no influence on the effect of BA. Over a period of 24 h, treatment with BA did not result in any change of sucrose uptake nor of the partitioning of assimilated carbon in the leaf discs. Neither did BA treatment affect the activity of acid invertase (EC 3.2.1.26) or pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) in the leaf discs. We conclude that the observed promotion of leaf area expansion by exogenous BA is not mediated through the uptake of sucrose or the carbohydrate metabolism of the leaf tissue.Abbreviations BA N⁶-benzyladenine - GA₃ gibberellic acid - PPi-PFK pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) This study was supported by grants from the Danish Research Counsil (SJVF 13-4148 and 13-4547 to P.U. SJVF 13-4146 and 13-4494 to T.H.N.) and from The Research Center for Plant Biotechnology to P.U.     

Alkali Cation/Sucrose Co-transport in the Root Sink of Sugar Beet

The mechanism of sucrose transport into the vacuole of root parenchyma cells of sugar beet was investigated using discs of intact tissue. Active sucrose uptake was evident only at the tonoplast. Sucrose caused a transient 8.3 millivolts depolarization of the membrane potential, suggesting an ion co-transport mechanism. Sucrose also stimulated net proton efflux. Active (net) uptake of sucrose was strongly affected by factors that influence the alkali cation and proton gradients across biological membranes. Alkali cations (Na⁺ and K⁺) at 95 millimolar activity stimulated active uptake of sucrose 2.1- to 4-fold, whereas membrane-permeating anions inhibited active sucrose uptake. The pH optima for uptake was between 6.5 and 7.0, pH values slightly higher than those of the vacuole. The ionophores valinomycin, gramicidin D, and carbonyl cyanide m-chlorophenylhydrazone at 10 micromolar concentrations strongly inhibited active sucrose uptake. These data are consistent with the hypothesis that an alkali cation influx/proton efflux reaction is coupled to the active uptake of sucrose into the vacuole of parenchyma cells in the root sink of sugar beets.     

Compartmentation in Vicia faba Leaves: II. Kinetics of C-Sucrose Redistribution among Individual Tissues following Pulse Labeling

Leaflets of Vicia faba L. were pulse labeled with ¹⁴CO₂ and the kinetics of ¹⁴C-sucrose redistribution among individual tissues was followed. Sucrose specific activity in the whole leaf peaked about 15 minutes after labeling and declined with a half-time of about 80 minutes. In one experiment, leaflet discs taken at various times during the ¹²CO₂ chase were quick frozen, freeze-substituted, and embedded in plastic. The tissue was sectioned paradermally and sections of palisade parenchyma, of spongy parenchyma, and of spongy parenchyma that contained veins were collected. Water extracts from these sections were assayed for sucrose specific activity. Sucrose specific activity in the palisade parenchyma was higher than that of the spongy parenchyma and reached a maximum in both tissues 9 to 15 minutes after labeling. Sucrose specific activity initially declined rapidly in the palisade parenchyma followed by a period during which little or no loss occurred. Sucrose specific activity in sections containing veins peaked at 15 minutes with a maximum value substantially higher than either mesophyll tissue, indicating that recently synthesized sucrose was preferentially exported from the mesophyll. Decline of activity in these sections containing veins continued for the remainder of the experiment. Sucrose specific activity in lower epidermal peels peaked several minutes after that of the whole leaflet and remained lower. Sucrose specific activity in upper epidermal peels was variable (probably due to contamination), but the limited data suggest that the sucrose specific activity there reached somewhat higher values than those of the lower epidermis. The experiments indicate that each leaf tissue contains a kinetically identifiable sucrose pool (which we refer to as “histological compartmentation”), and that further compartmentation may occur at the intracellular level. A simulation of leaf sucrose compartmentation is presented.     

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.     

Release of Sucrose from Vicia faba L. Leaf Discs

The release of sucrose from leaf discs of Vicia faba L. to a bathing medium was studied for evidence of a relationship between this release and mesophyll export of photosynthate in vivo. Sucrose was released specifically over hexoses and represented over 85% of total photosynthate released. The sucrose appeared to be derived from the mesophyll tissue directly and release did not require concurrent photosynthesis. The data indicated two separate channels for sucrose release. The first was sensitive to inhibition by 1 millimolar p-chloromercuribenzenesulfonic acid and the second was promoted by lowering the Ca²⁺ concentration below 0.1 millimolar. Flow through both channels was about equal when tissue that had been actively photosynthesizing for several hours was used. The rate of release was not dependent on the extracellular pH, but was inhibited by 10 micromolar carbonylcyanide p-trifluromethoxyphenylhydrazone. Lowering the Ca²⁺ concentration below 0.1 millimolar or raising the K⁺ concentration above 100 millimolar stimulated sucrose release. The stimulation by high K⁺ was not reversed by adding Ca²⁺. The data supported the postulate that Ca²⁺ removal or K⁺ addition changed the permeability of the mesophyll plasma membrane to sucrose.     

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     

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     

Effect of Exogenously Supplied Foliar Potassium on Phloem Loading in Beta vulgaris L

The effect of foliar application of K⁺ on processes associated with phloem loading was investigated in source leaves of sugar beet (Beta vulgaris L.). KCI was supplied exogenously at concentrations of up to 100 millimolar in the solution bathing the abraded upper epidermis of source leaves. K⁺ added at concentrations below 30 millimolar generally promoted the rate of export of material derived from ¹⁴CO₂ but not from exogenously applied [¹⁴C]sucrose. Paralleling promotion of export, the level of material derived from photosynthesis, which was released into the bathing solution, also increased in response to addition of K⁺ to the free space. Net photosynthetic rate was not affected. K⁺ at 5 and 15 millimolar concentrations did not stimulate uptake of [¹⁴C]sucrose into source leaf discs.     

Phloem Loading of Sucrose: pH Dependence and Selectivity

Autoradiographic, plasmolysis, and ¹⁴C-metabolite distribution studies indicate that the majority of exogenously supplied ¹⁴C-sucrose enters the phloem directly from the apoplast in source leaf discs of Beta vulgaris. Phloem loading of sucrose is pH-dependent, being markedly inhibited at an apoplast pH of 8 compared to pH 5. Kinetic analyses indicate that the apparent K_m of the loading process increases at the alkaline pH while the maximum velocity, V_max, is pH-independent. The pH dependence of sucrose loading into source leaf discs translates to phloem loading in and translocation of sucrose from intact source leaves. Studies using asymmetrically labeled sucrose ¹⁴C-fructosyl-sucrose, show that sucrose is accumulated intact from the apoplast and not hydrolyzed to its hexose moieties by invertase prior to uptake. The results are discussed in terms of sucrose loading being coupled to the co-transport of protons (and membrane potential) in a manner consistent with the chemiosmotic hypothesis of nonelectrolyte transport.     

Characterization of the uptake of sucrose and glucose by isolated seed coat halves of developing pea seeds. Evidence that a sugar facilitator with diffusional kinetics is involved in seed coat unloading

Uptake of ¹⁴C-labelled sucrose and glucose by isolated seed coat halves of pea (Pisum sativum L. cv. Marzia) seeds was measured in the concentration range <0.1 μM to 100 mM. The initial influx of sucrose was strictly proportional to the external concentration, with a coefficient of proportionality (k) of 6.2 μmol·(g FW)^?1·min^?1·M^?1. Sucrose influx was not affected by 10 μM carbonylcyanide m-chlorophenylhydrazone (CCCP), but it was inhibited by 40% in the presence of 2.5 mM p-chloromercuribenzenesulfonic acid (PCMBS). Influx with diffusional kinetics was also observed for glucose (k = 4.8 μmol·(g FW)^?1·min ^?1·M ^?1) and mannitol (k = 5.1 μmol·(g FW)^?1·min^?1·M^?1). For glucose an additional saturable system was found (K_m = 0.26 mM, V _max = 4.2 nmol·(g FW)^?1·min^?1), which appeared to be completely inhibited by CCCP and partly by PCMBS. In contrast to the diffusional pathway, uptake by this saturable system was slightly pH-dependent, with an optimum at pH 5.5. The influx of sucrose appears to be by the same pathway as the efflux of endogenous sucrose, which was inhibited by 36% in the presence of 2.5 mM PCMBS (De Jong A, Wolswinkel P, 1995, Physiol Plant 94: 78–86). It is argued that passive transport may be the only mechanism for sucrose transport through the plasma membrane of seed coat parenchyma cells. The estimated permeability coefficient of the plasma membrane for sucrose (P = 3.5·10^?7 cm·s^?1) is more than 1 × 10⁶-fold higher than that reported for artificial lipid membranes. This relatively high permeability is hypothesized to result from pore-forming proteins that allow the diffusion of sucrose. Furthermore, it is shown that a sucrose gradient across the plasma membrane of the seed coat parenchyma of only 22 mM will suffice to result in the net efflux of sucrose which is required to feed the embryo.     

Changes of pH of solutions during perfusion through stem segments: further evidence for hydrogel regulation of xylem hydraulic properties?

Changes in hydraulic conductivity (K_h) and pH were measured in stem segments of laurel (Laurus nobilis L.) during perfusion with iso-osmotic solutions of KCl, NaCl and sucrose. Sucrose had no effect on K_h while 100 mM NaCl or KCl induced up to 22 and 35 % increase of K_h with respect to deionized water, respectively. Increases in K_h were accompanied by a sharp drop in pH from 6.0 (inlet solution) to 5.0 (outlet solution). The same effect was observed with both KCl and NaCl solutions but not in the case of sucrose. Also, similar changes of K_h and pH were observed for stems killed after immersion in hot water. Our results might provide further evidence for ion-mediated regulation of xylem hydraulic conductivity based on the hydrogel properties of pectins at the pit membrane level.     

Contribution à l’étude de làabsorption du saccharose par des tissus de tubercule de pomme de terre adulte (Solatium tuberosum L.) modalités et influence des phytohormones

Sugar uptake by potato tuber discs was studied. Discs were used “fresh” or after a 24-h ageing period. It was shown that ageing increases (by 3 to 4 times) the rate of glucose and sucrose uptake. Sucrose uptake by fresh tissues was insensitive to the presence of glucose or fructose while a competitive effect was observed after ageing. This indicates the development of an invertase activity, which was inhibited by tris-buffer. Sucrose and glucose uptake by aged discs was dependent on cellular metabolism as shown by the sensitivity to low temperature and metabolic inhibitors (NaCN, DNP, CCCP). Involvement of thiol groups was demonstrated by the inhibition with NEM and PCMBS. Orthovanadate, which decreases phosphate uptake by 85 % (Poderet al. 1986) did not produce any effect on glucose and sucrose uptake by aged tissues. Fusicoccin produced only a slight stimulation (15 %). These results argue in favour of the involvement of specific ATPases in ion and sugar uptake. No involvement of a redox system was observed. ABA and BAP inhibited the uptake induced by ageing but had no effect on the endogenous sugar content. BAP would act by its effect on the amount of ATP while ABA would act at the membrane level. The results are discussed in relation to the mechanisms of transfer of glucosyl groups and to the transport of sucrose by the symplasmic pathway. Reçule     

Sucrose release from soybean leaf slices

The release of photosynthate from leaf slices of soybean [ Glycine max (L.) Merr. cv. Ransom II], to a bathing medium was studied to ascertain how p -chloromercuribenzenesulfonic acid (PCMBS) can both stimulate and inhibit sucrose release. Soybean leaf slices released photosynthate to a bathing medium at a rate that was approximately linear with time. The photosynthate released was about 20% ionic and 80% non-ionic, and sucrose represented about 75% of the total. Removal of Ca²⁺ from the medium increased the rate of release of all fractions, but amino acid release showed the largest increase. Sucrose was released at a rate estimated to be about 20% of the normal transport rate in intact leaves. The rate of sucrose uptake from 5 m M sucrose into soybean leaf slices was optimum at pH 6.3, and the rate of sucrose release was lowest at the same pH. However, sucrose uptake was found to be insignificant during release experiments. Sucrose release, but not amino acid release, was inhibited 75% by 1 m M PCMBS.
The data support two components of sucrose release in leaves. The first is insensitive to the addition of PCMBS. This component probably represents leakage from phloem tissue. The second component is inhibited by PCMBS and probably represents release from the mesophyll. By comparing sucrose release from leaf slices of 12 different species of plants, 2 groups were found. In the first group, sucrose release was inhibited between 60 and 80% by PCMBS, and in the second group between 0% and 40%. The difference in the two groups can be explained by a relative difference in the size of the two components of sucrose release for each species.     

Trends in carbohydrate depletion, respiratory carbon loss, and assimilate export from soybean leaves at night

To evaluate assimilate export from soybean (Glycine max [L.] Merrill) leaves at night, rates of respiratory CO₂ loss, specific leaf weight loss, starch mobilization, and changes in sucrose concentration were measured during a 10-hour dark period in leaves of pod-bearing `Amsoy 71' and `Wells II' plants in a controlled environment. Lateral leaflets were removed at various times between 2200 hours (beginning dark period) and 0800 hours (ending dark period) for dry weight determination and carbohydrate analyses. Respiratory CO₂ loss was measured throughout the 10-hour dark period. Rate of export was estimated from the rate of loss in specific leaf weight and rate of CO₂ efflux. Rate of assimilate export was not constant. Rate of export was relatively low during the beginning of the dark period, peaked during the middle of the dark period, and then decreased to near zero by the end of darkness. Rate of assimilate export was associated with rate of starch mobilization and amount of starch reserves available for export. Leaves of Amsoy 71 had a higher maximum export rate in conjunction with a greater total change in starch concentration than did leaves of Wells II. Sucrose concentration rapidly declined during the first hour of darkness and then remained constant throughout the rest of the night in leaves of both cultivars. Rate of assimilate export was not associated with leaf sucrose concentration.     

Interaction of cell turgor and hormones on sucrose uptake in isolated Phloem of celery

Phloem tissue isolated from celery (Apium graveolens L.) was used to investigate the regulation of sucrose uptake by turgor (manipulated by 50-400 milliosomolal solutions of polyethylene glycol) and hormones indoleacetic acid (IAA) and gibberillic acid (GA₃). Sucrose uptake was enhanced under low cellular turgor (increase in the V_max). Furthermore, enhancement of sucrose uptake was due to a net increase in influx rates since sucrose efflux was not affected by cell turgor. Manipulations of cell turgor had no effect on 3-O-methyl glucose uptake. When 20 millimolar buffer was present in uptake solutions, low turgor-induced effects were observed only at low pH range (4.5-5.5). However, the effect was extended to higher external pH (up to 7.5) when buffer was omitted from uptake solutions. A novel interaction between cellular turgor and hormone treatments was observed, in that GA₃ (10 micromolar) and IAA (0.1-100 micromolar) enhanced sucrose uptake only at moderate turgor levels. The hormones elicited little or no response on sucrose uptake under conditions of low or high cell turgor. Low cell turgor, IAA, GA₃, and fusicoccin caused acidification by isolated phloem segments in a buffer-free solution. It is suggested that enhanced sucrose uptake in response to low turgor and/or hormones was mediated through the plasmalemma H⁺-ATPase and most likely occurred at the site of loading.     

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.