The influence of calcium on potassium fluxes across the root of Ricinus communis

Summary The effect of calcium on the flux of potassium to the exudate of detached root systems of Ricinus communis has been investigated. Previous analyses have indicated the presence of a water dependent and a water independent flux of potassium which vary with the concentration of potassium in the bathing medium, in the presence of 0.1 mM CaCl₂. In the present study it has been observed that at a higher concentration of calcium in the bathing medium (2.5 mM CaCl₂) the water dependent flux of potassium is greatly reduced while the water independent flux is not affected. It is proposed that the differential effect of calcium on these two fluxes is a reflection of the degree of dependence of these fluxes on the permeability of the plasmamembranes within the root.     

Phloem transport,solute flux and the kinetics of sap exudation in Ricinus communis L.

The osmotic characteristics of phloem-sap exudation were examined in soil-grown and watercultured plants of Ricinus communis L. Prolonged exudation occurred from bark incisions in water-cultured plants. Fresh incisions caused large alterations in solute flux, but phloem-sap solute potential _s changed by less than ±8% over a period of 7 h. This was associated with a constancy in the levels of sucrose and K⁺, the principal solutes in the sap. Studies with foliar-applied tracers and leaf-excision experiments suggested that exudation was maintained by solute loading from mature leaves. A wide range of mass transfer values through the phloem was found, these being a function of exudation rate. We consider that the exudation process possesses essentially similar characteristics to phloem transport in the intact plant. The way in which bark incisions bring about large changes in solute flux is discussed in terms of the physical properties of the sieve-tube system.Abbreviations water potential - _s solute potential - _p pressure potential     

Active Transport of Ions across the Root of Ricinus communis

The electrical potential difference between the exuding sapof detopped castor oil plants and the external I mM KC1 bathingsolution has been measured, together with the concentrationsof potassium and chloride. A typical value for the electricalpotential of the sap with respect to the external solution was—56 mV, while the sap concentrations of potassium andchloride were 8 mM and 3.5 mM respectively. The total cationconcentration, made up of K, Na, Ca, and Mg, was about Io m.equiv.1.;the anion deficit is made up by SO₄, NO₃, and PO₄. A comparisonof the measured potential difference with the Nernst potentialsfor potassium and chloride suggests that the movement of potassiuminto the sap is a passive process while the movement of chlorideis an active process against the electrochemical potential gradient.It is suggested that the potassium and chloride ions pass throughthe diffusion barrier at which active transport takes placebefore they exchange with the other ions which appear in thesap. On the basis of this assumption it is shown that potassiumis close to passive equilibrium while chloride is accumulatedagainst an electrochemical potential difference of 110 mV or2, 500 cal.mole^–1.     

A mathematical model of solute coupled water transport in toad intestine incorporating recirculation of the actively transported solute

A mathematical model of an absorbing leaky epithelium is developed for analysis of solute coupled water transport. The non-charged driving solute diffuses into cells and is pumped from cells into the lateral intercellular space (lis). All membranes contain water channels with the solute passing those of tight junction and interspace basement membrane by convection-diffusion. With solute permeability of paracellular pathway large relative to paracellular water flow, the paracellular flux ratio of the solute (influx/outflux) is small (2-4) in agreement with experiments. The virtual solute concentration of fluid emerging from lis is then significantly larger than the concentration in lis. Thus, in absence of external driving forces the model generates isotonic transport provided a component of the solute flux emerging downstream lis is taken up by cells through the serosal membrane and pumped back into lis, i.e., the solute would have to be recirculated. With input variables from toad intestine (Nedergaard, S., E.H. Larsen, and H.H. Ussing, J. Membr. Biol. 168:241-251), computations predict that 60-80% of the pumped flux stems from serosal bath in agreement with the experimental estimate of the recirculation flux. Robust solutions are obtained with realistic concentrations and pressures of lis, and with the following features. Rate of fluid absorption is governed by the solute permeability of mucosal membrane. Maximum fluid flow is governed by density of pumps on lis-membranes. Energetic efficiency increases with hydraulic conductance of the pathway carrying water from mucosal solution into lis. Uphill water transport is accomplished, but with high hydraulic conductance of cell membranes strength of transport is obscured by water flow through cells. Anomalous solvent drag occurs when back flux of water through cells exceeds inward water flux between cells. Molecules moving along the paracellular pathway are driven by a translateral flow of water, i.e., the model generates pseudo-solvent drag. The associated flux-ratio equation is derived.     

Water dependent and water independent fluxes of potassium in exuding root systems of Ricinus communis

Summary The flux of water, , to the xylem of exuding root systems of Ricinus communis was controlled using a range of mannitol concentrations permitting the influence of this water flux on the potassium flux, f _K, to be studied. The relationship between and f _K thus obtained was investigated, for a number of external concentrations of potassium, Cm, supplied as potassium nitrate. An analysis of these data indicated the presence of a water dependent and a water independent f _K both of which varied with Cm. The water dependent f _K shows a parabolic relationship with Cm for Cm values <1 mM followed by a sharp inflection and decline at higher Cm values whereas the water independent f _K shows an hyperbolic relationship over the same range of Cm values.Uptake of potassium by exuding root systems was measured and shown to be dependent on the solute potential of the medium. The uptake was also shown to exhibit a dual absorption isotherm the kinetics of which indicate a low Km system (system 1) and a high Km system (system 2). The Km value obtained for system 1 is very similar to that obtained for the water independent f _K. It is postulated that the water independent f _K is contributed by that portion of f _K arriving in the stele via the cortical symplast and is directly dependent on Cm. The water dependent f _K is contributed by those ions moved across the root in response to centripetal water movement through the cortical cell walls.     

Molecular mechanisms of water and solute transport across archaebacterial lipid membranes

Archaebacteria thrive in environments characterized by anaeobiosis, saturated salt, and both high and low extremes of temperature and pH. The bulk of their membrane lipids are polar, characterized by the archaeal structural features typified by ether linkage of the glycerol backbone to isoprenoid chains of constant length, often fully saturated, and with sn-2,3 stereochemistry opposite that of glycerolipids of Bacteria and Eukarya. Also unique to these bacteria are macrocyclic archaeol and membrane spanning caldarchaeol lipids that are found in some extreme thermophiles and methanogens. To define the barrier function of archaebacterial membranes and to examine the effects of these unique structural features on permeabilities, we investigated the water, solute (urea and glycerol), proton, and ammonia permeability of liposomes formed by these lipids. Both the macrocyclic archaeol and caldarchaeol lipids reduced the water, ammonia, urea, and glycerol permeability of liposomes significantly (6-120-fold) compared with diphytanylphosphatidylcholine liposomes. The presence of the ether bond and phytanyl chains did not significantly affect these permeabilities. However, the apparent proton permeability was reduced 3-fold by the presence of an ether bond. The presence of macrocyclic archaeol and caldarchaeol structures further reduced apparent proton permeabilities by 10-17-fold. These results indicate that the limiting mobility of the midplane hydrocarbon region of the membranes formed by macrocyclic archaeol and caldarchaeol lipids play a significant role in reducing the permeability properties of the lipid membrane. In addition, it appears that substituting ether for ester bonds presents an additional barrier to proton flux.     

Effects of drought on nutrient and ABA transport in Ricinus communis

We studied the effects of variations of water flux through the plant, of diurnal variation of water flux, and of variation of vapour pressure deficit at the leaf on compensation pressure in the Passioura-type pressure chamber, the composition of the xylem sap and leaf conductance in Ricinus communis. The diurnal pattern of compensation pressure showed stress relaxation during the night hours, while stress increased during the day, when water limitation increased. Thus compensation pressure was a good measure of the momentary water status of the root throughout the day and during drought. The bulk soil water content at which predawn compensation pressure and abscisic acid concentration in the xylem sap increased and leaf conductance decreased, was high when the water usage of the plant was high. For all xylem sap constituents analysed, variations in concentrations during the day were larger than changes in mean concentrations with drought. Mean concentrations of phosphate and the pH of the xylem sap declined with drought, while nitrate concentration remained constant. When the measurement leaf was exposed to a different VPD from the rest of the plant, leaf conductance declined by 400mmol m^?2 s^?1 when compensation pressure increased by 1 MPa in all treatments. The compensation pressure needed to keep the shoot turgid, leaf conductance and the abscisic acid concentration in the xylem were linearly related. This was also the case when the highly dynamic development of stress was taken into account.     

Sugar concentrations along and across the Ricinus communis L. hypocotyl measured by single cell sampling analysis

Single cell sap sampling and analysis were used to measure the longitudinal and radial distribution of sucrose, glucose and fructose in the apical cell division zone and in the basal, elongated zone of the Ricinus hypocotyl. Sucrose and hexose increased in concentration from the apex to the base of the seedling axis. In the cell division zone low hexose and sucrose concentrations prevailed in cortex and pith, with a slightly higher hexose concentration in pith cells. The sucrose concentrations in sieve tubes and in phloem were much higher than in the cortex and pith cells. In the basal zone of the hypocotyl high levels of sucrose in phloem, cortex and pith were found, therefore radial, diffusional sucrose flow away from the phloem was considered unlikely. It is proposed that radial flow of growth-water to the hypocotyl periphery together with the down-regulation of a sucrose transporter at the phloem leads to a preferential sucrose flow to the expanding cortex. The pith cells, which do not experience flow of growth-water, are probably insufficiently supplied with sucrose from the phloem resulting eventually in cell death as the plant grows. Shortage of sucrose supply, experimentally achieved by removal of the endosperm, led to sucrose hydrolysis in the pith. The sucrose levels in the other tissues decreased less. It appears that the hydrolysis to hexose was initiated to maintain the osmotic value in the pith cell sap. It is speculated that high hexose levels in the cells are indicative of insufficient sucrose supply via the phloem and that the pith cells are confronted with that situation during early seedling development.     

Active and Passive Transport of the Major Nutrient Ions across the Root of Ricinus communis

Mesurements have been made of the electrical potential differncebetween the exuding sap of detopped castor-oil plants and anutrient solution bathing the roots. The initial concentrationsof the major nutrient ions in the root medium were (m.equiv./I.):K 0.7, Ca 0.8, Mg 0.4, NO₂ 1.4, So₄ 0.4, and phosphate 0.2,when present, Na was at a level of 0.3 m.equiv./I. and Cl at0.1 m.equiv./I. The concentration of each of these ions in thesap was of the order of 10 times that in the external solution,was —50 mV. Comparison of the measured potential differencewith the Nernst potential calculated for each ion incicatesthat, under the experimental conditions used here, the influxof K, Na, Ca, and Mg is a passive process while the transportof No₃, Cl, SO₄, H₂PO₄ and HPO₄ is in each case an active processagainst the electrochemical potential gradient. There is no evidence that a K—Na pump is working in theseroots although, in the absence of estimates for flux ratios,an outward ‘pump’ is not ruled out for Na, or Caand Mg. K is the only ion which appears to be at equilibriumconcentration in the sap.     

Transient three-dimensional modeling of soil water and solute transport with simultaneous root growth,root water and nutrient uptake

A three-dimensional solute transport model was developed and linked to a three-dimensional transient model for soil water flow and root growth. The simulation domain is discretized into a grid of finite elements by which the soil physical properties are spatially distributed. Solute transport modeling includes passive and active nutrient uptake by roots as well as zero- and first-order source/sink terms. Root water uptake modeling accounts for matric and osmotic potential effects on water and passive nutrient uptake. Root age effects on root water and nutrient uptake activity have been included, as well as the influence of nutrient deficiency and ion toxicity on root growth. Examples illustrate simulations with different levels of model complexity, depending on the amount of information available to the user. At the simplest level, root growth is simulated as a function of mechanical soil strength only. Application of the intermediate level with root water and nutrient uptake simulates the influence of timing and amount of NO3 application on leaching. The most comprehensive level includes simulation of root and shoot growth as influenced by soil water and nutrient status, temperature, and dynamic allocation of assimilate to root and shoot.     

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.     

Simultaneous measurement of water flow velocity and solute transport in xylem and phloem of adult plants of Ricinus communis over a daily time course by nuclear magnetic resonance spectrometry

A new method for simultaneously quantifying rates of flow in xylem and phloem using the FLASH imaging capabilities of nuclear magnetic resonance (NMR) spectrometry was applied in this study. The method has a time resolution of up to 4 min (for the xylem) and was used to measure the velocity of flows in phloem and xylem for periods of several hours to days. For the first time, diurnal time course measurements of flow velocities and apparent volume flows in phloem and xylem in the hypocotyl of 40‐d‐old Ricinus communis L were obtained. Additional data on gas exchange and the chemical composition of leaves, xylem and phloem sap were used to assess the role of leaves as sinks for xylem sap and sources for phloem. The velocity in the phloem (0·250 ± 0·004 mm s^?1) was constant over a full day and not notably affected by the light/dark cycle. Sucrose was loaded into the phloem and transported at night, owing to degradation of starch accumulated during the day. Concentrations of solutes in the phloem were generally less during the night than during the day but varied little within either the day or night. In contrast to the phloem, flow velocities in the xylem were about 1·6‐fold higher in the light (0·401 ± 0·004 mm s^?1) than in the dark (0·255 ± 0·003 mm s^?1) and volume flow varied commensurately. Larger delays were observed in changes to xylem flow velocity with variation in light than in gas exchange. The relative rates of solute transport during day and night were estimated on the basis of relative flow and solute concentrations in xylem and phloem. In general, changes in relative flow rates were compensated for by changes in solute concentration during the daily light/dark cycle. However, the major solutes (K⁺, NO₃^?) varied appreciably in relative concentrations. Hence the regulation of loading into transport systems seems to be more important to the overall process of solute transport than do changes in mass flow. Due to transport behaviour, the chemical composition of leaves varied during the day only with regard to starch and soluble carbohydrates.     

Characterization of amino-acid transport in Ricinus communis roots using isolated membrane vesicles

The mechanism and specificity of amino-acid transport at the plasma membrane of Ricinus communis L. roots was investigated using membrane vesicles isolated by phase partitioning. The transport of glutamine, isoleucine, glutamic acid and aspartic acid was driven by both a pH gradient and a membrane potential (internally alkaline and negative), created artificially across the plasma membrane. This is consistent with transport via a proton symport. In contrast, the transport of the basic amino acids, lysine and arginine, was driven by a negative internal membrane potential but not by a pH gradient, suggesting that these amino acids may be taken up via a voltage-driven uniport. The energized uptake of all of the amino acids tested showed a saturable phase, consistent with carrier-mediated transport. In addition, the membrane-potential-driven transport of all the amino acids was greater at pH 5.5 than at pH 7.5, which suggests that there could be a direct pH effect on the carrier. Several amino-acid carriers could be resolved, based on competition studies: a carrier with a high affinity for a range of neutral amino acids (apart from asparagine) but with a low affinity for basic and acidic amino acids; a carrier which has a high affinity for a range of neutral amino acids except isoleucine and valine, but with a low affinity for basic and acidic amino acids; and a carrier which has a higher affinity for basic and some neutral amino acids but has a lower affinity for acidic amino acids. The existence of a separate carrier for acidic amino acids is discussed.Abbreviations PM plasma membrane - TPP⁺ tetraphenylphosphonium ion - pH pH gradient - membrane potential This work was supported by the Agricultural and Food Research Council and The Royal Society. We would like to thank Mrs. Sue Nelson for help with some of the membrane preparations.     

Hydrophilic solute transport across rat alveolar epithelium

Diffusional fluxes of a series of hydrophilic nonelectrolytes (molecular radii ranging from 0.15 to 0.57 nm) were measured across the alveolocapillary barrier in the isolated perfused fluid-filled rat lung. Radiolabeled solutes were lavaged into the distal air spaces of isolated Ringer-perfused lungs, and apparent permeability-surface area products were calculated from the rates of isotope appearance in the recirculating perfusate. These data were used to estimate theoretical equivalent pore radii in the alveolar epithelium, with the assumption of diffusive flow through water-filled cylindrical pores. The alveolar epithelium is best characterized by two pore populations, with small pores (radius 0.5 nm) occupying 98.7% of total pore area and larger pores (radius 3.4 nm) occupying 1.3% of total pore area. Net water flow out of the alveolar space was measured by including an impermeant solute (dextran) in the lavage fluid and measuring its concentration in the alveolar space as a function of time. Under control conditions, net water flow averaged 167 nl/s. When 24 microM terbutaline was added to the perfusate, net water flow increased significantly to 350 nl/s (P less than 0.001). Terbutaline had no effect on the fluxes of either glycerol (which traverses the small pore pathway) or sucrose (which traverses the large pore pathway). These findings indicate that the intact mammalian alveolar epithelium is complex and highly resistant to the flow of solutes and water.     

Sucrose transport into the phloem of Ricinus communis L. seedlings as measured by the analysis of sieve-tube sap

Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2–3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes. The concentration which was achieved by loading and the time course could also be documented. This study concentrated on the loading of sucrose because it is the major naturally translocated sieve-tube compound. The sucrose concentration of sieve-tube sap was approx. 300 mM when the cotyledons were buried in the endosperm. When the cotyledons were excised from the endosperm and incubated in buffer, the sucrose concentration decreased gradually to 80–100 mM. This sucrose level was maintained for several hours by starch breakdown. Incubation of the excised cotyledons in sucrose caused the sucrose concentration in the sieve tubes to rise from 80 to 400 mM, depending on the sucrose concentration in the medium. Thus the sucrose concentration in the sieve tubes could be manipulated over a wide range. The transfer of labelled sucrose to the sieve-tube sap took 10 min; full isotope equilibration was finally reached after 2 h. An increase of K⁺ in the medium or in the sieve tubes did not change the sucrose concentration in the sievetube sap. Similarly the experimentally induced change of sucrose concentration in the sieve tubes did not affect the K⁺ concentration in the exudate. High concentrations of K⁺, however, strongly reduced the flow rate of exudation. Similar results were obtained with Na⁺ (data not shown). The minimum translocation speed in the sieve tubes in vivo was calculated from the growth increment of the seedling to be 1.03 m·h^-1, a value, which on average was also obtained for the exudation system with the endosperm attached. This comparison of the in-vivo rate of phloem transport and the exudation rate from cut hypocotyls indicates that sink control of phloem transport in the seedlings of that particular age was small, if there was any at all, and that the results from the experimental exudation system were probably not falsified by removal of the sink tissues.Abbreviations PTS 3-hydroxy-5,8, 10-pyrenetrisulfonate     

A Developmental Analysis of the Enolase Isozymes from Ricinus communis

Enolase activity was measured in clarified homogenates of various tissues during the life cycle of the castor oil plant (Ricinus communis L. cv Baker 296). The proportions of total activity due to the plastid and cytosolic isozymes were determined after separation by ion-exchange chromatography. The contribution of the plastid isozyme varied from more than 30% of the total at the midpoint of endosperm development to less than 1% in mature leaves and roots. During endosperm development, enolase activity increased to a peak coincident with the maximum rate of storage lipid accumulation, then decreased to nearly undetectable levels in the mature seed. Plastid enolase protein, measured using an enzyme-linked immunosorbent assay, increased in parallel with the increase in activity but decreased less rapidly and was still easily detectable in mature seeds.     

Structure and analysis of phospholipase D gene from Ricinus communis L

Phospholipase D (PLD; EC 3.1.4.4) has been proposed to play a pivotal role in various cellular processes, but molecular understanding of this enzyme is rather limited. This report describes the nucleotide sequence, structure, and genomic organization of a PLD gene from castor bean (Ricinus communis L. cv. Hale). The PLD gene was isolated from a castor bean genomic library using the PLD cDNA as a hybridization probe. Sequence comparison with the PLD cDNA revealed that the PLD gene consisted of four exons and three introns, one of which interrupts the 5-untranslated region. Southern blot analysis indicated that the cloned PLD gene was present as a single-copy gene, and yet there were other PLD or PLD-related sequences in the castor bean genome.     

Ultraviolet difference spectroscopic analysis of the saccharide-binding properties of Ricinus communis agglutinin

The nature of the binding of saccharides to Ricinus communis agglutinin was studied by ultraviolet difference spectroscopy. Upon binding of galactose and galactose-containing saccharides, R. communis agglutinin displayed difference spectra with an extreme maximum at 291-293 nm and a smaller maximum at 284-285 nm. Such difference spectra suggest that the environment of a tryptophan residue located at or near the saccharide-binding site of R. communis agglutinin is being changed by an interaction between a tryptophan residue and the bound saccharides. The value of the difference spectra (delta epsilon) increased upon progressive addition of saccharide until the saccharide binding site was saturated with ligand. From the increase in delta epsilon at 291-293 nm, the association constants were obtained for the R. communis agglutinin-saccharide interaction over the temperature range 5-35 degrees C and various pH values. The results clearly demonstrate that the association constants are nearly equal in the range of pH 5-8, but decrease beyond the above pH range and with elevation of temperature. From the thermodynamic parameters for the binding of various saccharides to R. communis agglutinin, we suggest that there exists a subsite structure in the saccharide-binding site of the R. communis agglutinin molecule.     

Purification and characterization of Ricinus communis invertase

An invertase from Ricinus communis leaves was purified 4,400-fold. The preparation was homogeneous by criteria of gel electrophoresis, gel permeation, adsorption, and ionic exchange chromatography. One optimum pH at 3.5 was observed with crude invertase; however, purified preparations showed two optima, at pH 3.5 and 5.5. Addition of bovine serum albumin restored one maximum at pH 3.5 and elicited a 30% activation of the invertase. The effect was caused by many other proteins and by heparin, dextran sulfate, and polyvinylpyrrolidone. Fructose, fructose 1,6-diphosphate, maleic, trans-aconitic, malic, and ascorbic acids were simple competitive inhibitors of the purified enzyme. Glucose was a noncompetitive inhibitor. The activation by proteins suppressed these inhibitory effects. The minimum concentration of activator necessary to reach the maximal activation or "point of optimal activation" was always reached at a concentration of 1 X 10(-6) M, independently of the nature of the activator, when 8.6 X 10(-12) mol of enzyme were used. Apparent molecular weight determinations of the enzyme in the presence and absence of activator and molecular weight determinations based on determinations of the point of optimal activation suggested that the purified enzyme is a heptamer (Mr of 77,900, Stokes radius 32 A, frictional ration f/fo 1.1, partial specific volume 0.749 ml/g) and that the activated form is a trimer consisting of two enzyme subunits and one activator molecule. The activation was lost by dilution of the trimer. The enzyme subunit, as isolated by gel filtration in the presence of sodium dodecyl sulfate (Mr 11,000) was inactive but quickly regained activity upon removal of sodium dodecyl sulfate.     

Effects of air pollutants on proton and sucrose transport at the plasma membrane of Ricinus communis

The effects of the air pollutants O₃, SO₂ and NO₂ on aspects of sucrose/proton cotransport across the plasma membrane of Ricinus communis plants have been investigated. The H⁺-ATPase hydrolytic activity in cotyledon plasma membrane vesicles purified by phase partitioning showed small stimulations by Na₂SO₃ or NaNO₃ added separately or together to the assay medium. ATPase activity from plants pretreated by fumigation with SO₂ or O₃ also showed an increase, the effect of O₃ being quite marked. Plasma membrane H⁺-pumping in KI-treated microsomal fractions and medium acidification by intact cotyledons both showed small decreases in the presence of Na₂SO₃ or NaNO₂. Both Na₂SO₃ and NaNO₂ at high concentrations (2 mol m^–3) had significant effects on sucrose uptake by intact cotyledons, although sucrose efflux was unaffected. No significant effects on sucrose uptake or efflux by intact cotyledons were observed in plants pretreated by fumigation with SO₂ or O₃. Proton-coupled sucrose transport in isolated plasma membrane vesicles was inhibited in the presence of Na₂SO₃ or NaNO₂. However, both pollutants also significantly inhibited the uptake of acetate by the vesicles, indicating a dissipation of the pH gradient across the membrane. It was concluded that no specific aspect of the sucrose/proton cotransport mechanism was damaged by these air pollutants, and that the effects of these pollutants on carbohydrate partitioning are more likely to be due to general effects on membrane integrity or on other aspects such as leaf carbohydrate metabolism.