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.     

Estimation of the volumetric elastic modulus and membrane hydraulic conductivity of willow sieve tubes

Severed aphid stylets were used to follow the kinetics of sieve tube turgor and osmotic pressure (π) responses following step changes in water potential applied to the cambial surface of willow (Salix exigua Nutt.) bark strips. The kinetics of the turgor response were monitored with a pressure transducer. In separate experiments, the kinetics of the π response were followed by freezing point determinations on stylet exudate. The sieve tube volumetric elastic modulus in the bark strips was about 21 bars, but may be higher in intact stems. The membrane hydraulic conductivity was about 5 × 10⁻³ centimeters per second per bar; several factors make it difficult to estimate its value accurately. Differences in the turgor pressure (P) and π responses, as well as the relatively more rapid initial turgor response to a water potential (ψ) change, suggested a time-dependent component in sieve tube wall elasticity.

Our observations were generally not supportive of the idea that sieve tubes might osmoregulate. However, the bark strip system may not be suitable for addressing that question.

Separate measurements of ψ, P, and π demonstrate that the relationship predicted by the fundamental cell water potential equation, ψ = P − π, is applicable within experimental error (± 0.4 bar) to sieve tube water relations.

     

Distorted phytochrome action spectra in green plants

An evaluation was made of the extent which a Münch-type pressure flow mechanism (i.e., osmotically-generated pressure flow) might contribute to phloem transport in soybean. Estimates of sucrose concentrations in source (leaf) and sink (root) sieve tubes were obtained by a negativestaining procedure. Water potential measurements of the leaf and of the nutrient solution allowed calculation of the turgor pressures in source and sink sieve tubes. The turgor difference between source and sink sieve tubes was compared to that required to drive translocation at the observed velocity between the source and sink, as measured by [¹⁴C] photosynthate movement. Sieve-tube conductivity was calculated from the sieve-tube dimensions, assuming an essentially unobstructed pathway. In three experiments, the sucrose concentration was consistently higher in source sieve tubes (an average of 11.5%) than in sink sieve tubes (an average of 5.3%). The ratio of these values (2.3:1) agreed reasonably well with an earlier ratio for source/sink sieve tube concentrations of 1.8:1, obtained by quantitative microautoradiography. The resulting calculated turgor difference (an average of 4.1 bars) was adequate to drive a pressure flow mechanism at the observed translocation velocities (calculated to require a turgor difference of 1.2 to 4.6 bars). No other force need be presumed to be involved.This work was presented in part at a joint U.S.-Australian Conference on Transport and Transfer Processes in Plants, Canberra, Australia, December 15–20, 1975; see Fisher (1976)     

Estimation of Osmotic Gradients in Soybean Sieve Tubes by Quantitative Autoradiography: Qualified Support for the MUnch Hypothesis

An attempt was made to evaluate Münch's hypothesis of osmotically generated pressure flow in soybean (Glycine max L.) sieve tubes from velocity measurements and calculations of pressure potentials and sieve tube resistances. Pressure potential was estimated from values for water potentials and osmotic potential. Leaf water potential measurements were made by isopiestic thermocouple psychrometry, while the water potential of the nutrient solution was made with a vapor pressure osmometer. Osmotic potential was measured by first bringing the sucrose pools in the entire plant to the same specific radioactivity by steady-state-labeling of the shoot with constant specific radioactivity ¹⁴CO₂ for 5 to 8 hours. Sucrose concentrations in sieve tubes were calculated from the disintegration rate per unit volume in sieve elements as measured by absolute quantitative microautoradiography of freeze-substituted, Eponembedded source (leaf) and sink (root) tissues.     

Bidirectional translocation of sugars in sieve tubes of squash plants

Two streams of sugars moving in opposite directions in the petiole of a half-grown leaf were demonstrated by feeding tritiated glucose to a fully grown leaf of a squash plant (Cucurbita melopepo Bailey) and ¹⁴CO₂ to the half-grown one. Autoradiographic evidence indicates that the movement of both streams occurred within the same sieve tubes. The data do not fit the mass flow theory of translocation which requires unidirectional flow of sugar solution in the lumen of the sieve tube.     

Translocation and accumulation of translocate in the sugar beet petiole

Accumulation of translocate during steady-state labeling of photosynthate was measured in the source leaf petioles of sugar beet (Beta vulgaris L. monogerm hybrid). During an 8-hr period, 2.7% of the translocate or 0.38 μg carbon/min was accumulated per cm petiole. Material was stored mainly as sucrose and as compounds insoluble in 80% ethanol. The minimum peak velocity of translocation approached an average of 54 cm/hr as the specific activity of the ¹⁴CO₂ pulse was progressively increased. The ratio of cross sectional area required for translocation to actual sieve tube area in the petiole was 1.2. A regression analysis of translocation rate versus sieve tube cross sectional area yielded a coefficient of 0.76. The specific mass transfer rate in the petiole was 1.4 g/hr cm² phloem or 4.8 g/hr cm² sieve tube. Histoautoradiographic studies indicated that translocation occurs through the area of phloem occupied by sieve tubes and companion cells while storage occurs in these cells plus cambium and phloem parenchyma cells. The ability of the petiole to act as a sink for translocate is consistent with the concept that storage along path tissue serves to buffer sucrose concentration in the translocate during periods of fluctuating assimilation.     

Scaling phloem transport: information transmission

Sieve tubes are primarily responsible for the movement of solutes over long distances, but they also conduct information about the osmotic state of the system. Using a previously developed dimensionless model of phloem transport, the mechanism behind the sieve tube's capacity to rapidly transmit pressure/concentration waves in response to local changes in either membrane solute exchange or the magnitude and axial gradient of apoplastic water potential is demonstrated. These wave fronts can move several orders of magnitude faster than the solution itself when the sieve tube's axial pressure drop is relatively small. Unlike the axial concentration drop, the axial pressure drop at steady state is independent of the apoplastic water potential gradient. As such, the regulation of whole‐sieve tube turgor could play a vital role in controlling membrane solute exchange throughout the translocation pathway, making turgor a reliable source of information for communicating change in system state.     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Kinetics of C-photosynthate translocation in morning glory vines

The movement of ¹⁴C-photosynthate in morning glory (Ipomea nil Roth, cu. Scarlet O'Hara) vines 2 to 5 meters long was followed by labeling a lone mature leaf with ¹⁴CO₂ and monitoring the arrival rate of tracer at expanding sink leaves on branches along the stem. To a first approximation, the kinetic behavior of the translocation profiles resembled that which would be expected from movement at a single velocity (“plug flow”) without tracer loss from the translocation stream. There was no consistent indication of a velocity gradient along the vine length. The profile moved along the vine as a distinct asymmetrical peak which changes shape only slowly. The spatial distribution of tracer along the vine reasonably matched that predicted on the basis of the arrival kinetics at a sink, assuming plug flow with no tracer loss. These observations are in marked contrast to the kinetic behavior of any mechanism describable by diffusion equations.

However, a progressive change in profile shape (a symmetrical widening) was observed, indicating a range of translocation velocities. A minimum of at least two factors must have contributed to the observed velocity gradient: the exchange of ¹⁴C between sieve elements and companion cells (demonstrated by microautoradiography) and the range of velocities in the several hundred sieve tubes which carried the translocation stream. Possible effects of these two factors on profile spreading were investigated by means of numerical models. The models are necessarily incomplete, due principally to uncertainties about the exchange rate between sieve elements and companion cells and the degree of functional connectivity between sieve tubes of different conductivities. However, most of the observed profile spreading may be reasonably attributed to the combined effects of those two factors.

The mass average velocity of translocation (calculated from the mean times of ¹⁴C arrival at successive sink leaves) was about 75% of the maximum velocity (calculated from the times of initial detection at the same sink leaves), which was usually between 0.6 and 1 cm min⁻¹. Owing to tracer exchange between sieve elements and companion cells, the mass average velocity of tracer in the sieve tubes was probably closer to 86% of the maximum velocity, a figure which agreed with a predicted velocity distribution based on calculated sieve tube conductivities and the size distribution of functional sieve tubes.

     

A Mathematical Treatment of Munch's Pressure-Flow Hypothesis of Phloem Translocation

The steady state solutions of two mathematical models are used to evaluate Münch's pressure-flow hypothesis of phloem translocation. The models assume a continuous active loading and unloading of translocate but differ in the site of loading and unloading and the route of water to the sieve tube. The dimensions of the translocation system taken are the average observed values for sugar beet and are intended to simulate translocation from a mature source leaf to an expanding sink leaf. The volume flow rate of solution along the sieve tube, water flow rate into the sieve tube, hydrostatic pressure, and concentration of sucrose in the sieve tube are obtained from a numerical computer solution of the models. The mass transfer rate, velocity of translocation, and osmotic and hydrostatic pressures are consistent with empirical findings. Owing to the resistance to water flow offered by the lateral membranes, the hydrostatic pressure generated by the osmotic pressure can be considerably less than would be predicted by the solute concentration. These models suggest that translocation at observed rates and velocities can be driven by a water potential difference between the sieve tube and surrounding tissue and are consistent with the pressure-flow hypothesis of translocation.     

Phosphate cycles in energy crop systems with emphasis on the availability of different phosphate fractions in the soil

The growing cells of hydroponic maize roots expand at constant turgor pressure (0.48 MPa) both when grown in low-(0.5 mol m^-3 CaCl₂) or full-nutrient (Hoagland's) solution and also when seedlings are stressed osmotically (0.96 MPa mannitol). Cell osmotic pressure decreases by 0.1–0.2 MPa during expansion. Despite this, total solute influx largely matches the continuously-varying volume expansion-rate of each cell. K⁺ in the non-osmotically stressed roots is a significant exception-its concentration dropping by 50% regardless of the presence or absence of K⁺ in the nutrient medium. This corresponds to the drop in osmotic pressure. Nitrate appears to replace Cl^- in the Hoagland-grown cells.Analogous insensitivity of solute gradients to external solutes is observed in the radial distribution of water and solutes in the cortex 12 mm from the tip. Uniform turgor and osmotic pressures are accompanied by opposite gradients of K⁺ and Cl^-, outwards, and hexoses and amino acids, inwards, for plants grown in either 0.5 mol m^-3 CaCl₂ or Hoagland's solution (with negligible Cl^-). K⁺ and Cl^- levels within both gradients were slightly higher when the ions were available in the medium. The gradients themselves are independent of the direction of solute supply. In CaCl₂ solution all other nutrients must come from the stele, in Hoagland's solution inorganic solutes are available in the medium.24 h after osmotic stress, turgor pressure is recovered at all points in each gradient by osmotic adjustment using organic solutes. Remarkably, K⁺ and Cl^- levels hardly change, despite their ready availability. Hexoses are responsible for some 50% of the adjustment with mannitol for a further 30%. Some 20% of the final osmotic pressure remains to be accounted for. Proline and sucrose are not significantly involved. Under all conditions a standing water potential step of 0.2 MPa between the rhizodermis and its hydroponic medium was found. We suggest that this is due to solute leakage.Abbreviations EDX energy dispersive X-ray microanalysis - water potential - 11-1 cell osmotic pressure - P turgor pressure     

Aphid stylectomy reveals an osmotic step between sieve tube and cortical cells in barley roots

The aim of the present study was to quantify osmotic pressuresdirectly in the translocation pathway, from leaf to growingroot tip, in order to understand the forces driving solutesfrom a source to a sink. Solutes move through the translocationpathway down an osmotically derived turgor gradient. Accordinglyaphid stylectomy and single cell sampling techniques have beencombined to examine the osmotic pressure of root phloem andgrowing root cells. Sieve tube sap was obtained from shootsand, for the first time, roots of barley seedlings using aphidstylectomy. Vacuolar sap was also obtained from a variety ofcells in leaf and root tissues using single cell sampling methods.Osmotic pressure of sieve tube sap from roots and shoots wasmeasured at high temporal resolution (within min) and over longperiods of time (up to 24 h). Osmotic pressure did not changesignificantly in the minutes immediately following excision,suggesting that confidence can be placed in the assumption thatstylet exudate is representative of sieve tube sap in vivo.There were no differences in the osmotic pressure of sieve tubesap from shoots (1.240.26 MPa, n = 10) or roots (1.420.15MPa, n = 13). However, osmotic pressure of sap from root corticalcells (0.710.09, n = 12) was about 0.7 MPa lower than thatof the sieve elements from roots, this difference may be maintainedby consumption of incoming solutes at the root tip. Resultsare discussed in the context of pressure driven flow in thephloem and symplastic contact between root tip cells and sievetube. It is hoped that the approach described here will provideimportant insights into the nature of the relationship betweenroot cell extension and assimilate supply through the phloem. Key words: Phloem, sieve tube, aphid, root, barley, osmotic pressure, translocation     

Osmoregulation in Cotton Fiber: Accumulation of Potassium and Malate during Growth

Kinetics and osmoregulation of cotton (Gossypium hirsutum L.) fiber growth (primarily extension) have been studied. Growth is dependent on turgor pressure in the fiber. It is inhibited when a decrease in the water potential of the culture medium due to an addition of Carbowax 6000, equals the turgor pressure of the fiber. Potassium and malate accumulate in the fiber and reach peak levels when the growth rate is highest. Maximum concentrations of potassium and malate reached in the fiber can account for over 50% of the osmotic potential of the fiber. As growth slows down, levels of potassium and malate decrease and turgor pressure declines. Cotton ovules are capable of fixing H¹⁴CO₃⁻ in the dark, predominantly into malate. Fiber growth is inhibited by the absence of potassium and/or atmospheric CO₂. We suggest that potassium and malate act as osmoregulatory solutes and that malate, at least in part, arises from dark CO₂ fixation reactions.     

A Working Model of a Sieve Tube

A working model of a sieve tube is described, based upon thePressure-Flow Hypothesis. The flow of solution in the sievetube is envisaged as being due to the joint influence of anaxial turgor pressure gradient and a lateral water potentialdifference which causes an intake of water along the tube. Inthis second respect the model differs from that originally suggestedby Münch. The model comprises a length of dialysis tubing rendered semi-permeable,incorporating capillary resistances at regular intervals. Asucrose solution is pumped into one end and collected at theother, the whole being submerged in water. Turgor pressures and concentrations along the model were recordedat intervals; thus the approach to the steady state was followed.In the steady state ¹⁴C-sucrose was introduced to the solutionbeing pumped and its approach to a steady distribution in themodel was followed. Important conclusions reached are that the Munch Pressure-FlowHypothesis implies a pressure profile convex upwards, a velocityincrease down the phloem, and an exponential fall in tracerconcentration without its lateral leakage.     

Gradients of turgor, osmotic pressure, and water potential in the cortex of the hypocotyl of growing ricinus seedlings : effects of the supply of water from the xylem and of solutes from the Phloem

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe. Because the osmotic pressure of individual cells (πⁱ) was also determined, the water potential (ψ) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of well-watered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor ≈ πⁱ and ψ ≈ 0 across the cortex, there were also no gradients of ψ across the tissue. Gradients of cell turgor and πⁱ increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cutting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCl₂ solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.     

Long Distance Transport in Macrocystis integrifolia: II. Tracer Experiments with C and P

Discs from mature regions of Macrocystis blades picked up significantly more [³²P]phosphate from the ambient medium than similar discs from young meristematic regions, and this uptake was higher in light than in darkness. Double-labeling experiments with NaH¹⁴CO₃ and [³²P]phosphate, using intact fronds as well as cut frond segments, indicated that ³²P was translocated from mature blades to sink regions at velocities of 25 to 45 centimeters per hour, velocities comparable to ¹⁴C translocation velocity in the same material. There was a slight delay in transport of ³²P which may be due to a delay in loading or to a high metabolism of ³²P in the transporting channels. Histoautoradiography of stipe segments in the translocation pathway indicated that transport of label occurred in the peripheral parts of medulla. An analysis of ³²P-labeled compounds in the fed blade and in the sieve tube sap, collected from basal cut ends of stipes, indicated major differences in labeling patterns. In the blade, a high proportion of ³²P was recovered as inorganic phosphate and relatively small amounts were found in hexose mono- and diphosphates, UDPG and ATP. In the sieve tube sap, however, only a small amount of ³²P was present as inorganic phosphate, a large proportion was found in hexose mono- and diphosphates, and appreciable amounts were present in ATP and UDPG.     

Measurement of turgor pressure and its gradient in the Phloem of oak

A direct method is described for measuring the pressure in secondary phloem sieve tubes of oak trees. One end of a 26-gauge stainless steel tube was shaped such that when it penetrated the outer bark and transected a few sieve elements, it was stopped by the xylem so that small openings in the end allowed phloem sap to enter the tube. The other end of the stainless tube (phloem needle) was joined to a long glass capillary sealed at its other end to form a manometer for measuring phloem sap pressure. A method for measuring the average osmotic and turgor pressures in cells of leaves is also described. Phloem turgor pressures varied greatly in a series of phloem punctures around the trunk at 1.5 and at 6.3 meters. The variation in turgor pressure was always greater than the variation in osmotic pressure. In a series of turgor pressures arranged in descending order, the values in a sequence for the upper level was usually a little (0-3 atm) larger than the values for the lower level. These results may suggest that translocation of assimilate is favored by a small turgor pressure gradient, but they do more to emphasize the complications in measuring gradients in an elastic low resistance distribution system composed of contiguous longitudinal conduits. The results also imply that the sieve tubes are inflated with assimilate fluid under high pressure which can readily move longitudinally and with less pressure drop than would be necessary if the sieve tubes were rigid.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.     

Long Distance Transport in Macrocystis integrifolia: I. Translocation of C-labeled Assimilates

Long distance transport of ¹⁴C-labeled photoassimilate was studied in Macrocystis integrifolia Bory. Movement of label followed the source-sink relationship; mature blades closer to the holdfast with young 2° and 3° fronds transported mostly to the base, those closer to the frond apex transported mostly to the apex, and those in intermediate positions transported both acropetally and basipetally. The velocity of movement of ¹⁴C as computed both from study of intact fronds and exudate was in the range of 35 to 72 centimeters per hour and these estimates are on the low side. The composition of the translocate as determined from intact fronds was the same as that determined from exudate analysis; furthermore, this composition was nearly identical with that of the photosynthate (40 to 50% mannitol and 40 to 50% amino acids). From these data we conclude that the exudate represents the sieve tube sap and that there is little if any selectivity exercised in the loading and translocation of photoassimilate. An analysis of translocated label in the growing apex is presented and indicates that the synthesis of polymeric compounds such as laminaran, alginate, cellulose, lipids, and “protein” occurs in situ from the transported mannitol and amino acids. Detailed data on chemical composition of sieve tube sap from M. integrifolia and M. pyrifera (L.) C.A. Agardh are given and compared with the sieve tube sap from higher plants. Finally, we show that stipe segments, 60 to 100 centimeters long with three to six attached blades, are useful for translocation studies in Macrocystis.     

The effect of cell turgor on sugar uptake in strawberry fruit cortex tissue

A reduction in cell turgor has been shown to stimulate sugar uptake in several plant sink tissues and it may regulate the import of assimilate into the sink apoplast, as well as maintain cell turgor. To determine whether cell turgor influences sugar uptake by strawberry (Fragaria x ananassa Duch. cv. Brighton) fruit cortex tissue, disks were cut from greenhouse-grown primary fruit at the green-white stage of development and placed in buffered incubation solutions containing either mannitol or ethylene glycol as an osmoticum. Cell turgor of fruit disks was calculated from the difference between the water potential of bathing solution and tissue solute potential after incubation at various osmolarities. Cell turgor increased when tissue disks were placed into mannitol incubation solutions more dilute than the water potential of fresh tissue (about 415 mOsmol kg^?1). The rate of uptake of [¹⁴C]-sucrose or [¹⁴C]-glucose decreased as osmolarity of the incubation solution increased, i.e. as cell turgor declined. Cell turgor and the rate of [¹⁴C]-sucrose uptake were unaffected when rapidly permeating ethylene glycol was used as an osmoticum. A decrease in cell turgor reduced both the V_max of the saturable (carrier mediated) kinetic component of sucrose uptake, and the slope of the linear (diffusional) component. The sulfhydryl binding reagent p-chloromercuibenzenesulfonic acid, an inhibitor of the plasma membrane sucrose carrier, strongly inhibited only the saturable component of sucrose uptake. Increased uptake of the nonmetabolizable sugar, O-methyl-glucose, at high turgor was similar to that of glucose, indicating that carrier activity was influenced by cell turgor, not cell metabolism. Turgor did not influence efflux of [¹⁴C]-sucrose from disks and had no effect on cell viability. Strawberry fruit cells do not possess a sugar uptake system that is stimulated by a reduction in turgor.