Effect of Abscisic Acid on Glucose Metabolism in Guard Cells of Vicia faba L.

The effects of abscisic acid (ABA) on the formation of osmoticallyactive solutes and on cell wall synthesis in guard cells wereexamined using sonicated abaxial epidermal strips of Vicia fabaL. incubated with ¹⁴C-glucose at pH 4 and 6. Radioactivity wasincorporated mainly into malate,sucrose, starch and cell-wallfractions. ¹⁴C- Glucose uptake by the guard cells was reducedwhen 1 µm ABA was added. Malate formation, which was moreactive at pH 6 than at 4, was inhibited by ABA at pH 6, butnot at pH 4. Conversion of ¹⁴C-glucose into ¹⁴C-sucrose wasstimulated by ABA at both pH values. Release of radio activesolutes (composed mainly of glucose and malate)from the guardcells into the medium was more active at pH 6 than at pH 4.ABA stimulated there lease at both pH values. Turnover of starchwas more remarkable when the pH value was 6. ABA inhibited thesynthesis of starch, but did not affect its degradation. Cell-wallsynthesis inthe guard cells was also inhibited by ABA, the inhibitionrate being greater at pH 4 than at pH 6.These results suggestthat ABA may have two different actions on stomatal movement:to changethe metabolic activities in the guard cells so as tolower the concentration of osmotically active solutes, and tochange the mechanical properties of cell walls by modulatingcell-wall metabolism. (Received September 7, 1987; Accepted November 30, 1987)     

Abscisic Acid-Induced Stomatal Closure in Vicia faba Epidermal Strips. Excretion of Solutes from Guard Cells and Increase in Elastic Modulus of Guard Cell Wall

The effects of abscisic acid (ABA) on the size of the apertureof stomata on epidermal strips of Vicia faba were studied inincubation media with different pH values. The osmotic potentialof guard cells, as determined by the limiting plasmolysis method,was higher at pH 4.0 than at pH 6.0, although the size of thestomatal apertures was almost identical at both pH values. AtpH 4.0, ABA effectively caused stomatal closure but had onlya small effect on the osmotic potential, whereas, at pH 6.0,ABA significantly increased the osmotic potential. ABA promotedthe efflux of Cl^– and malate from epidermal strips intothe incubation medium, an effect which was more marked at pH6.0, with a concomitant efflux of K⁺ to balance the charge onthe exported anions. From these results, it is suggested thatABA may cause an increase in the elastic modulus of the cellwalls of guard cells. ³ Present address: Nagano Prefectural Vegetable and OrnamentalCrops Experimental Station, 2206 Oomuro, Matsusiro-machi, Nagano381-12, Japan (Received September 30, 1986; Accepted January 9, 1987)     

Differential Stimulation of PEP-carboxylation in Guard Cells and Mesophyll Cells by Ammonium or Fusicoccin

Dark CO₂-fixation in guard cells of Vicia faba was much moresensitive to ammonium than in mesophyll cells. Addition of ammonium(5.0 mol m^–3; pH₀ 7.6) caused up to a 7-fold increasein dark CO₂-fixation rates in guard cell protoplasts (GCP),whereas in leaf slices, mesophyll cells, and mesophyll protoplaststhe increase was only about 1.4-fold. In both cell or tissuetypes, total CO₂-fixation rates were higher in the light (2–12-foldhigher in GCP and 28-fold in mesophyll); these rates were onlyslightly changed by ammonium treatment. However, separationof ¹⁴C-labelled products after fixation of CO₂ in the lightby GCP revealed a large ammonium-induced shift in carbon flowfrom starch and sugars to typical products of C₄-metabolism(mainly malate and aspartate). In contrast, in mesophyll cellsamino acid and malate labelling was only moderately increasedby ammonium at the expense of sucrose. The data suggest thatin vivo ammonium might facilitate stomatal opening and/or delaystomatal closing through an increased production of organicacids. Key words: PEP-carboxylation, guard cell protoplasts, ammonium, fusicoccin     

Effects of Osmotic Stress, Ionic Stress and IAA on the Cell-Membrane Resistance of Vigna Hypocotyls

The cell-membrane resistance (R_m) of Vigna hypocotyls was examined,and the effects of osmotic stress, ionic stress and IAA on R_mwere investigated. R_m decreased by 64 to 77% under osmotic stressin the presence of absorbable solutes (40 mM sorbitol, 15 mMKC1, 30 mM sucrose; or 40 mM sorbitol, 15 mM KC1, 30 mM sucroseplus 10^–4 M IAA) or under ionic stress (50 mM NaCl or50 mM KC1). R_m was not changed by perfusion with 10^–4M IAA. Therefore, the hyper-polarizations of the membrane potentialobserved in both cases should be ascribed totally to the activationof the electrogenic proton pump. Although R_m showed an increaseof 1.6 fold when the hypocotyls were subjected to osmotic stress(100 mM sorbitol or 100 mM sorbitol plus 10^–4 M IAA),83.6% or 92.4% of the hyperpolarization of the membrane potential(V_px was also the result of the activation of the pump. Theresults, calculated on the basis of the current source model,support the viewpoint that the hyperpolarization of the cellmembrane potential of Vigna hypocotyls under osmotic stress,ionic stress or in the presence of IAA is an expression of theactivation of the proton pump, and is not caused by an increasein R_m. ¹ Present address: Researchers and Planners of Natural Environment, Yotsugi Bldg. (2F), 1-5-4 Horinouchi, Suginami-Ku, Tokyo,166 Japan ² Present address: Graduate School of Integrated Science, YokohamaCity University, 22-2 Seto, Kanazawa-Ku, Yokohama, 236 Japan (Received February 14, 1991; Accepted July 24, 1991)     

Effect of Abscisic Acid on Cell-Wall Metabolism in Guard Cells of Vicia faba L.

Cell-wall synthesis in guard cells of Vicia faba L. was examinedusing sonicated epidermal strips incubated with [¹⁴C]glucose.The cell walls of the guard cells incorporated [¹⁴C]glucoseat a lower level in the dark than in the light. Stomatal aperturein the epidermal strips was reduced by application of 1 µmabscisic acid (ABA) in the light but not in the dark. The ABAtreatment reduced the incorporation of [¹⁴C]glucose into thecell walls especially in the light. Fractionation of the labeledcell-wall components revealed that ABA inhibited the synthesisof pectic substances and cellulose, but did not affect hemicellulosesynthesis. Microautoradiographs of the cell-wall fraction ofthe epidermal strips showed that a large amount of radioactivitywas distributed at both ends of the guard cells in the absenceof ABA and that removal of pectic substances from the cell-wallfraction resulted in uniform distribution of the radioactivityin the cell walls of the guard cells. These results indicatedthat the synthesis of pectic substances was active at both endsof the guard cells and was inhibited by ABA. Measurement ofspecific activities of neutral sugars in the guard-cell wallsshowed that polymers composed of galactose underwent activeturnover and that synthesis of glucans was inhibited by ABA.These results revealed a strong correlation between the stomatalmovement and the synthesis of pectic substances and cellulosein the guard cells, suggesting that the cell-wall metabolismin the guard cells may play a role in the regulation of stomatalmovement. (Received October 9, 1987; Accepted March 9, 1988)     

New fava bean guard cell signaling mutant impaired in ABA-induced stomatal closure

We isolated a mutant from Vicia faba L. cv. House Ryousai. Itwilts easily under strong light and high temperature conditions,suggesting that its stomatal movement may be disturbed. We determinedresponses of mutant guard cells to some environmental stimuli.Mutant guard cells demonstrated an impaired ability to respondto ABA in 0.1 mM CaCl₂ and stomata did not close in thepresence of up to 1 mM ABA, whereas wild-type stomata closedwhen exposed to 10 µM ABA. Elevating external Ca²⁺caused a similar degree of stomatal closure in the wild typeand the mutant. A high concentration of CO₂ (700 µlliter^–1) induced stomatal closure in the wild type, butnot in the mutant. On the basis of these results, we proposethe working hypothesis that the mutation occurs in the regiondownstream of CO₂ and ABA sensing and in the region upstreamof Ca²⁺ elevation. The mutant is named fia (fava bean impairedin ABA-induced stomatal closure). ³ Corresponding author: E-mail, smoiwai{at}agri.kagoshima-u.ac.jp;Fax, +81-99-285-8556.     

Role of malate synthesis mediated by phosphoenolpyruvate carboxylase in guard cells in the regulation of stomatal movement

To clarify the pathway and role of malate synthesis in guard cells, epidermal strips isolated from Vicia faba L. leaflets were treated with 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP), a specific inhibitor of phosphoenolpyruvate carboxylase (PEPC). When dark-closed stomata were illuminated, malate accumulated in guard cells and stomata opened; these were inhibited by 60% and 30%, respectively, by 5 mM DCDP treatment. When light-opened stomata were treated with DCDP, both malate level in guard cells and stomatal aperture decreased. Treatment with 5 mM DCDP partially inhibited CO2 incorporation into malate in guard cells. Treatment with mannitol at 0.4 M (osmotic stress) in the light increased malate level in guard cells and closed stomata. DCDP treatment decreased both malate level and stomatal aperture under stressed condition. These results show that malate synthesis in the light under both non-stressed and stressed conditions is dependent on PEPC activity. The extent of the decrease in malate level by DCDP treatment was larger under stressed condition than under nonstressed condition, suggesting that osmotic stress may enhance the activity of this pathway of malate synthesis which is induced by light. Role of malate synthesis in guard cells is discussed.     

Effect of CO2 on Volume Change of Guard Cell Protoplast from Vicia faba L

Guard cell protoplasts (GCP) were isolated from epidermal stripsof Vicia faba L. by enzymatic digestion. The presence of non-osmoticvolume in the protoplast was suggested by the relationship betweenprotoplast volume and the mannitol concentration of the suspendingmedium. Light illumination caused swelling of GCP only whenKCl was present in the suspending medium. Dark treatment causedshrinking of GCP irrespective of the presence of 10 mM KCl.In the presence of 10 µM abscisic acid (ABA), GCP shrank.Light-induced swelling was suppressed at concentrations of ambientCO₂ higher than that in normal air. Promotion of swelling wasnot always observed at lower CO₂ concentration. These volumechange responses to light, ABA and CO₂ suggest that GCP retainsits physiological activity as a guard cell. The osmotic contributionof K⁺ to volume increase was lower than expected. Ambient CO₂seems to have some effect on the contribution of K⁺ to osmoregulationof GCP. (Received January 30, 1982; Accepted June 25, 1982)     

Bl?tenbildung bei Lemna paucicostata 6746 durch kombinierte Anwendung von Abscisins?ure und GCG

To a certain extent the flowering of Lemna paucicostata 6746is evoked in continuous light by application of abscisic acid(ABA) and CCC. Moreover, the action of the combined substancesappears in two separate concentration ranges. In the range ofABA 2?10^–9 M/CCC 10^–7–10^–6 M floweringis initiated without inhibition of vegetative growth and proceedsonly in the presence of high intensity light and sucrose. Acombination of ABA 2?10^–5 M/CCC 10^–3M simultaneouslycauses a strong inhibition of frond multiplication. Here theeffect can be observed also under low intensity light conditionsand without sucrose in the medium. A range with flower inhibitingactivity lies between the two flower promoting concentrationranges. (Received November 16, 1972; )     

Responses of Stomata in Epidermal Strips of Vicia fabato Carbon Dioxide and Growth Hormones when Incubated on Potassium Chloride and Potassium Iminodiacetate

The effect of various K⁺ levels in combination with Cl^− or iminodiacetate (IDA^{& minus;}) on stomatal responsesin isolated epidermal strips of Vicia faba L. were examinedin order to determine the role of malate during guard cell movements.Responses of guard cells to ABA, kinetin, and varying CO₂ levelswere similar when epidermal strips were floated on KCL or KIDAat 10 mM; such responses were typical in that ABA caused closure,kinetin stimulated opening in ambient air, and apertures weregreater in CO₂-free than ambient air. Maximal stomatal openingwas observed in both ambient and CO₂-free air with KCL at 100mM. The transfer of epidermal strips from 100 mM KCL to solutionsof 100 mM KCL supplemented with ABA or kinetin did not bringabout changes in stomatal aperture. KCL at 100 mM supporteda greater degree of stomatal opening than did 100 mM KIDA irrespectiveof the CO₂ content of the air. In CO₂-free air transfer of epidermalstrips from 100 mM KIDA to solutions containing 100 mM KIDAsupplemented with ABA or kinetin caused little change in stomatalaperture, whereas, in ambient air, the same treatments resultedin stomatal opening. The results are discussed in relation tothe role of malate during guard cell movements.     

The Permeability of the Guard Cell Plasma Membrane and Tonoplast

Uptake experiments and efflux compartmental analysis of planthormones, osmotica and toxins using ‘isolated’ guardcells of Valerianella locusta and guard cell protoplasts (GCP)of Vicia faba were performed in order to study the permeabilityproperties of guard cell plasma membrane and tonoplast. Theplasma membrane of guard cells exhibits a higher permeabilitythan plasma membranes of mesophyll cells for most solutes investigated.The permeability coefficients (P_s calculated for the guard cellplasma membranes are also significantly higher than the P_s valuesfor the guard cell tonoplast. This applies also for protonatedABA. We suppose that the high permeability for ABAH could bepart of the target cell properties. A Collander analysis demonstratesa linear correlation between P_s, values and the ratio K_r/M_r^1,5for both plasma membrane (r = 0.87) and for the tonoplast (r=0.93). Because of deviations from the observed correlations,the permeation of some solutes (ABA, GA, IAA through the tonoplast;methylamine through the plasma membrane) seems to be facilitatedby an additional transport mechanism. The Collander analysisof the plasma membrane of GCP shows very similar results tothe analysis of the plasma membrane of ‘isolated’guard cells, indicating that isolation of protoplasts does notalter the permeability of the guard cell plasma membrane. Key words: Permeability coefficient, guard cells, plasma membrane, tonoplast     

Carbon Dioxide Fixation by Guard Cell Protoplasts of Commelina communis

Biochemical studies of epidermal tissue may not reflect metabolismof the guard cells which represent less than 5% of the tissuevolume. Pure samples of guard cell protoplasts of Commelinacommunis were therefore used to investigate CO₂ fixation ratesand ¹⁴C-labelling patterns of metabolites in the light and thedark. Qualitatively, results were similar in most respects tothose obtained in a previous study (Schnabl, 1980) for guardcell protoplasts of Vicia faba. CO₂ fixation rates by guardcell protoplasts of C. communis were the same in the light andthe dark but about 50 times lower than the values Schnabl obtainedfor V.faba. The ¹⁴C-labelling pattern of metabolites in C. communiswas also similar in the light and the dark: over 60% of thetotal fixed was in malate with only 1% in sugar phosphates.Label was also detected in starch, aspartate, glutamate andcitrate but not in glycollate as previously recorded in V. fabaguard cell protoplasts. The results confirm the view that the reductive pentose phosphatepathway does not occur in guard cells of C. communis. Key words: CO₂ fixation, Guard cell protoplasts, Stomata     

Metabolic Alterations Associated With Abscisic Acid-Induced Frost Hardiness in Bromegrass Suspension Culture Cells

Metabolic alterations associated with the induction of freezingtolerance by abscisic acid (ABA) were characterized by chemicalanalysis and by [U-^l4C]sucrose partitioning into cellular constituentsin bromegrass (Bromus inermis Leyss cv. Manchar) cell suspensioncultures. ABA caused a significant elevation in dry matter,particularly in the fraction insoluble in 85% ethanol, thatwas highly correlated with enhanced frost tolerance. Cell walls,the largest component of the insoluble fraction, increased significantlyas frost tolerance increased throughout the ABA treatment period.ABA stimulated total [¹⁴C]sucrose uptake by cells from 7% onday 1 to 97% on day 7 compared to control cells. Partitioningstudies detected a significant increase in ¹⁴CO₂ evolution at3, 5 and 7 days after ABA treatment and a significantly higherincorporation of [¹⁴C]sucrose into the ethanol insoluble fractionafter 5 and 7 days of treatment. Organic acid depletion in ABA-treatedcells was also highly correlated with the increase in hardiness.The concentration of total sugars was higher in ABA-treatedcells. The results indicate that most of the metabolic changesduring ABA-induced acclimation were similar to changes reportedfor cells acclimated in response to low temperature. ¹Oregon Agricultural Experiment Station Technical Paper No.9052 ²Present address: Department of Horticulture, University ofSaskatchewan, Saskatoon, Sask. Canada S7N 0W0 (Received November 1, 1989; Accepted March 13, 1990)     

Effects of Calcium and Abscisic Acid on Volume Changes of Guard Cell Protoplasts of Commelina

Calcium ions contracted guard cell protoplasts (GCP) of Commelinacommunis L., being particularly effective within the concentrationrange of 0 to 0.2 mol m^–3. Abscisic acid (ABA) in thepresence of EGTA, which chelates free Ca²⁺ in the medium, contractedGCP to a similar extent to Ca²⁺ alone or Ca²⁺ and ABA together.Similarly, ABA in the absence of free Ca²⁺ (i.e. an ABA/EGTAtreatment) inhibited K⁺-induced swelling of contracted GCP,as did Ca²⁺ alone or ABA and Ca²⁺ together. Lanthanum, a Ca²⁺channel blocker, prevented the contraction of GCP by Ca²⁺ buthad no effect if ABA was also present with Ca²⁺. The inhibitionof swelling of GCP by Ca²⁺ was also prevented by the presenceof lanthanum or verapamil (another Ca²⁺ channel blocker). These results indicate that Ca²⁺ and ABA can act independentlyof each other in contracting swollen GCP and in preventing K⁺-inducedswelling of contracted GCP of C. communis. If swelling and contractionof GCP are equivalent to stomatal opening and closure, respectively,the results do not support the hypothesis that ABA opens Ca²⁺channels in the plasma membrane of guard cells allowing Ca²⁺to enter the cells and, as a second messenger, to set in motionclosing processes. Key words: Abscisic acid, calcium, guard cell protoplasts, stomata     

Cellular osmotic phenomena during stomatal movements ofCommelina communis

Summary The accuracy of most of the published values for guard cell osmotic pressures is disputed and it is considered that many values are grossly in error. Since most of the values were obtained from incipient plasmolysis experiments limitations of the technique were investigated. It was concluded that it is not possible to use the incipient plasmolysis method for accurately determining guard cell osmotic pressures since all concentrations of plasmolytica (concentrations down to 0.1 M sucrose or calcium nitrate were used) bring about incipient plasmolysis depending on the period of time the tissue is immersed in the plasmolytica. In other words, the concentration of a plasmolyticum at which incipient plasmolysis occurs continues to decrease as the plasmolysing time increases. Furthermore, the time taken for incipient plasmolysis to occur varies according to the solutes in the plasmolyticum and the extent of stomatal aperture.A reason for the changing values of guard cell osmotic pressures was the loss of K⁺, and to a lesser extent, Cl^–, Ca²⁺ and Na⁺, and sugars and organic acids from the tissue during exposure to graded concentrations of plasmolytica (sucrose and calcium nitrate). A good correlation between loss of solutes from the epidermal tissue and decrease in guard cell osmotic pressure was not observed, however.Histochemical tests for K⁺ support the view that leakage of K⁺ from the guard cells occurs while the tissue is immersed in the plasmolytica except when high concentrations of sucrose (2.0 M) and calcium nitrate (greater than 1.0 M) were used and then leakage was minimal. However, these high concentrations of plasmolytica caused cell damage.The osmotic relationships of the various cell types within the epidermis ofCommelina communis were investigated during stomatal movements. Although absolute values for the osmotic pressures of the various cell types could not be evaluated it was apparent from the rates of changes of the osmotic pressures that when stomata closed guard cell osmotic pressures decreased while epidermal and subsidiary cell osmotic pressures increased to almost the same values as the guard cells.     

Regulation of Mg,K-ATPase Activity in Guard Cells of Commelina communis by Phytochrome and ABA

The microsomal fraction obtained from guard cell protoplastswas assayed for ATPase activity at pH 6.5. Triton X-100 didnot affect this stimulation of activity of ATPase by K⁺ up to5 mM, but the detergent abolished the stimulatory effect ofK⁺ at higher concentrations. The ATPase activity was inhibitedby N,N-dicyclohexylcarbodiimide and ABA. Irradiation with redlight enhanced the ATPase activity more than did irradiationwith far-red light. ABA and irradiation with red or far-redlight were effective only in the presence of K⁺. These resultssuggest the possibility that the ATPase activity is modulatedonly indirectly by light and ABA. (Received January 9, 1989; Accepted July 10, 1989)     

Osmoregulation in Rice Coleoptile Elongation as Promoted by Cooperation between IAA and Ethylene

IAA-induced elongation of rice (Oryza sativa L. cv. Sasanishiki)coleoptiles is regulated by cooperation between IAA and ethyleneproduced in response to IAA. However, the presence of some solutes,such as K^$, Na^$, Rb^$, glucose and sucrose, in the incubationmedia was found to be indispensable for this cooperation. Withoutthose solutes, the IAA-induced elongation was not sustainedover a long time period. IAA caused increases in both the osmoticpotentials of the coleoptile cells and the extensibility oftheir cell wall. In epidermal cells of IAA-treated coleoptiles,the osmotic potential increased from –0.87 to –0.62MPa during a 4-h incubation with 1 mM KCl. Moreover, IAA promotedthe uptake of K^$ or Na^$ from the media into the coleoptiles.However, these effects of IAA were partially prevented by aminoethoxyvinylglycine(AVG), and all the AVG effects were completely nullified byethylene applied simultaneously and exogenously. Both IAA andethylene did not affect the wall yield stress. These resultssuggested that the long-term elongation induced by IAA in ricecoleoptile segments results from inhibiting increases in osmoticpotentials of their cells. The maintenance by IAA of low osmoticpotentials may be partly due to the promotive action of ethyleneproduced in response to IAA on the solute uptake from the media. (Received July 6, 1983; Accepted February 15, 1984)     

Chilling Injury in Cotton {Gossypium hirsutum L.): Light Requirement for the Reduction of Injury and for the Protective Effect of Abscisic Acid

Pretreatment by darkness increased chilling (4°C) injuryin whole cotton (Gossypium hirsutum L.) seedlings and isolatedcotyledonary tissue. Addition of sucrose in the dark periodprevented the effect of darkness. Application of the photosyntheticinhibitor DCMU in light simulated the effect of darkness. ABA(10^–5 M) decreased chilling injury when applied in lightas a pretreatment before the onset of chilling. The same pretreatmentin darkness was almost ineffective, unless sucrose was added.ABA applied in light together with DCMU was ineffective in decreasingchilling injury. Lower light intensity resulted in increasedchilling injury and a decreased effect of ABA in the preventionof chilling injury. The antimicrotubular drug colchicine increased the chillinginjury. Pretreatment with ABA in light decreased the chillingand colchicine injury while the same pretreatment in darknesswas ineffective. These results suggest that a deficiency of a photosyntheticproduct increases the chilling sensitivity of the tissue. ABAapparently increases chilling resistance through a metabolicprocess which depends on photosynthetic activity. ³ Incumbent of the Seagram Chair in Plant Sciences (Received November 20, 1980; Accepted January 31, 1981)     

The Photometabolism of Acetate by Chlorella pyrenoidosa

Chlorella pyrenoidosa can utilize sodium acetate as a carbonsource for growth in the light. Growth proceeds under aerobicconditions both in the presence and in the absence of carbondioxide, but under anaerobic conditions only in its presence.The assimilation of acetate does not result from oxidation tocarbon dioxide followed by photosynthetic fixation because theproducts of ¹⁴C-acetate assimilation are different from theproducts of ¹⁴CO₂ fixation in the presence of unlabelled acetate. In aerobic conditions 10-⁶ M DCMU induces a pattern of acetateassimilation in the light similar to that in the dark. Thus,in the presence of DCMU in the light, less acetate carbon isincorporated into cells, particularly into lipids, polysaccharide,and protein, and more is released as carbon dioxide than inits absence. The effect of 4 x 10^-3 M MFA on acetate assimilationin the presence of 10^-6 M DCMU is the same in light and dark.Acetate assimilation is unaffected by desaspidine and sodiumbisulphite. The mean generation time of C. pyrenoidosa growing on acetatein the light under aerobic conditions is 20 hours. When 10^-5M DCMU is added the mean generation time is 60 hours, the sameas that for Chlorella growing on acetate in the dark. The activityof the enzymes of the glyoxylate cycle, isocitrate lyase (E.C.4.1.3.1.)and malate synthetase (E.C.4.1.3.2.) is repressed in the light,but activity of both enzymes increases markedly when DCMU isadded.     

Effects of Fusicoccin in 'Isolated' Guard Cells of Commelina communis L

The effects of 3 ? 10^–2 mol m^–3 FC on rubidium fluxesand contents in isolated guard cells of Commelina communis L.have been studied using ⁸⁶RbCl. Fusicoccin causes a marked stimulationof influx and an immediate, apparently irreversible, decreasein efflux of ⁸⁶Rb. The effect on influx is usually more importantin determining the new net flux of Rb. Observed fluxes differmarkedly from those predicted by the Goldman-Hodgkin-Katz equation,suggesting that FC does not act solely via an effect upon theplasmalemma potential. Fusicoccin appears to have a more directeffect upon the ion movements associated with changes in stomatalaperture than either ABA or transfer to the dark. Observed changesin Rb content cannot account fully for the osmotic changes associatedwith aperture increase. Key words: Fusicoccin, Guard cells, Ion fluxes, Commelina communis