Measurement of the Apoplastic Activity of K+ and Cl- in the Leaf Epidermis of Commelina communis in Relation to Stomatal Activity

Bowling, D. J. F. 1987. Measurement of the apoplastic activityof K⁺ and Cl^– in the leaf epidermis of Commelina communisin relation to stomatal activity.–J. exp. Bot. 38: 1351–1355. Ionic activity of K⁺ and Cl^– in the apoplast of the lowerepidermis of the leaf of Commelina communis was measured usingion selective micro-electrodes. Large gradients across the stomatalcomplex were observed which were related to stomatal aperture.On stomatal closure the activity of K⁺ and Cl^– in theapoplast of the guard cell rose from 3·0 mol m^–3to 100 mol m^–3 and 33 mol m^–3 respectively. It wasconcluded that the apoplast is an important pathway for iontransport between the cells. Key words: Stomata, ionic activity, leaves, apoplast     

Tentoxin Suppresses Stomatal Opening by Inhibiting Photophosphorylation

Tentoxin and, to a lesser extent, dihydrotentoxin (both at 10mmol m^–3) reduce stomatal opening in epidermal stripsof Commelina communis in the light but not in darkness. Thiseffect was significantly greater in normal air than in CO₂-freeair. Fusicoccin overcame the tentoxin effect. However, tentoxindid not inhibit stomatal opening in the light in epidermal stripsof Paphiopedilum harrisianum, a species which lacks guard cellchloroplasts. It is concluded that tentoxin exerts its actionon stomata not by an ionophorous effect in the plasmalemma ofguard cells but by the inhibition of photophosphorylation intheir chloroplasts. The effects of DCMU and tentoxin on guardcells are discussed in terms of their effects on chloroplastsand the extent to which energy is supplied from this organelleduring stomatal opening in the light. The results indicate thatneither photophosphorylation nor non-cyclic electron transportin guard cell chloroplasts are essential for stomatal opening. Key words: Commelina, epidermal strips, Paphiopedilum, photophosphorylation, stomata, tentoxin     

Involvement of Cyclic AMP in ABA- and Ca2--Mediated Signal Transduction of Stomatal Regulation in Vicia faba

The focus of this study is to investigate possible involvementof cyclic AMP in regulation of Vicia stomatal movements. Thepresence of 0.1 mM 8-Br-cAMP, a membrane-permeable analogueof cAMP, alone in the incubation medium did not affect stomatalopening in the light in leaf epidermal peel experiments. However,addition of 0.1 mM 8-Br-cAMP completely reversed exogenous ABA-and C^a2+-induced inhibition of stomatal opening. Consistentwith these results, patch-clamping experiments showed that intracellularaddition of 0.5 mM or 1 mM cAMP significantly reversed the inhibitionof whole-cell inward K⁺ currents by internally supplied 13 µMCa²⁺ or 10 µM ABA in stomatal guard cell protoplasts,respectively. Furthermore, intracellular addition of either10 µM prostaglandin E₁ (PGE1, an adenylate cyclase activator)or 1 mM 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesteraseinhibitor) mimicked the effect of exogenous cAMP on the removalof ABA- or Ca²⁺ inhibition of inward K⁺-current. These resultssuggest that a cAMP signaling pathway is involved in signaltransduction in stomatal regulation by interacting with ABAand Ca²⁺ signaling cascades. A hypothetical mechanism by whichcAMP may regulate K⁺ in stomatal guard cells is also discussed. (Received May 6, 1999; Accepted August 27, 1999)     

Characterization of the vacuolar-type H+-ATPase from guard cell protoplasts of Commelina

Characteristics of the vacuolar-type (V-type) H⁺-ATPase fromguard cell protoplasts of Commelina communis L. were investigatedusing a linked enzyme assay and nitrate inhibition as a diagnosticindicator of the enzyme activity. ATPase activity was completelyinhibited by about 50 mol m^–3 nitrate and activity wasoptimal near pH 8.0. The temperature optimum for activity wasabout 37 C and an Arrhenius plot indicated changes in activationenergy for the ATPase at 15C and possibly at about 30 C. Theenzyme was stimulated by Cl^– while Ca²⁺ inhibited activity(l₅₀ = 1.5 mol m^–3). The apparent K_m (MgATP) was 0.62mol m^–3. Incubation of guard cell protoplasts for up to 5 h in 50 µMabscisic acid (ABA) or 25µM fusicoccin (FC) did not affectsubsequent ATPase activity. In vitro assays with FC or ABA alsodid not affect enzyme activity. Activity was not affected bylight or potassium ferricyanide, two factors which are knownto influence stomatal activity. Beticoline was a potent inhibitorof activity (l₅₀ = 50 µM) while DCCD was less effective(l₅₀ = 90µM). On chlorophyll, protein and protoplast bases, V-type ATPaseactivity was greater in guard cell protoplasts than mesophyllcell protoplasts by 66, 13.9 and 1.9, respectively. On atonoplast surface area basis the enzyme activity was 5.6 timeshigher in guard cell protoplasts than in mesophyll cell protoplasts Thus, although the characteristics of the V-type, H ⁺-ATPaseof GCP are very similar to those found in other cell types,rates of activity and probably tonoplast enzyme density aremuch greater in guard cell protoplasts than mesophyll cell protoplastsof C. communis which corresponds with the large and rapid ionfluxes across the tonoplast associated with stomatal movements Key words: Guard cell protoplasts, stomata, V-type H ⁺-ATPase     

Effects of KCI Concentration on Changes in Starch and Malate Levels in Abaxial and Adaxial Epidermis in Relation to Stomatal Opening

The effects of KCI concentration in the incubation medium onstomatal opening and starch and malate metabolism in isolatedabaxial and adaxial epidermis of Commelina communis were investigated.Increasing KCI produced progressively wide apertures, reducedstarch hydrolysis and malate synthesis, but failed to eliminatethe normal disparity in abaxial and adaxial opening. The fusicoccin-stimulatedwide opening was accompanied by a dramatic enhancement of malateformation and starch hydrolysis at zero KCI, but not at 200mol m^–3 KCI, indicating a controlling influence of KCIconcentration on carbohydrate and malate metabolism. Accordingly,malate and Cl^– appear to compete for K⁺, and the one assumesan increasingly decisive role as the other becomes limiting.However, starch hydrolysis and malate production must play acentral role in stomatal opening in intact leaves because theyare unlikely to have such high Cl^– regimes as 200 molm^–3, though not necessarily in isolated epidermis incubatedin media enriched with inorganic ions. The restricted adaxial opening is attributed to limited K⁺ accumulation,starch hydrolysis and malate production in the guard cells,but the primary cause is not known. Key words: Stomata, Abaxial/Adaxial Epidermis     

The Effects of Fusicoccin on Anion Fluxes in Isolated Guard Cells of Commelina communis L.

Clint, G. M. 1987. The effects of fusicoccin on anion fluxesin isolated guard cells of Commelina communis L.—J. exp.BoL 38: 863–876. The effects of 3?10^–2 mol m^–3 fusicoccin (FC) onbromide fluxes and contents in isolated guard cells of Commelinacommunis L. have been studied using K⁸²Br at pH 3?9 and pH 6?7.At pH 3?9 FC caused a reduction in both the influx and the effluxof ⁸²Br, whereas at pH 6?7 FC had no effect on the influx butcaused a transient increase in the efflux of ⁸²Br. There wasno obvious change in bromide content with FC treatment at eitherpH. The behaviour of the anion fluxes in response to FC suggeststhat FC does not act solely via a hyperpolarization at the plasmalemma.A redistribution of bromide between the intracellular compartmentssuggests that anion flux from the cytoplasm to the vacuole maybe stimulated by FC at pH 3?9. The failure of guard cells toincrease their anion content on treatment with FC despite anincrease in stomatal aperture and in cation content suggeststhat in FC-induced stomatal opening excess cation is balancedby organic acid synthesis within the guard cell. Key words: Fusicoccin, guard cells, ion fluxes, Commelina communis     

The Investigation of Stomatal Ionic Relations using Guard Cell Protoplasts: I. METHODOLOGY

Clint, G. M. 1985. The investigation of stomatal ionic relationsusing guard cell protoplasts. 1. Methodology.—J exp. Bot.36: 1726–1738. A study was made of the methodology for the production and useof guard cell protoplasts in ion transport studies, with particularemphasis placed on the effects of the composition of the externalmedium on protoplast survival and performance. Addition of externalKCl to media during the production of guard cell protoplastsfrom Commelina communis L. was found to improve viability andto increase K⁺ content and physiological competence of the isolatedprotoplasts. Addition of low levels (20 x 10^–3 mol m^–3)CaCl₂ increased protoplast yield and the maintenance of viabilityin long-term incubation. Ambiguities and uncertainties werefound in the application of methods commonly used for the assessmentof viability of isolated protoplasts. Poor yields (despite highpercentage recoveries) together with difficulties in the assessmentof viability were considered to pose major potential problemsin the use of guard cell protoplasts in ion transport studies. Key words: Guard cell protoplasts, ion transport, Commelina communis     

Acid-Induced Stomatal Opening in Commelina communis and Vicia faba

The effects of H^$ and fusicoccin (FC) on stomatal opening inthe dark were investigated using epidermal strips of Commelinacommunis and Vicia faba cv. Ryosai Issun. Citrate-phosphatebuffer induced maximal opening of stomata at pH 3.0 when testedover the range of 2.7 to 5.0. HCl at 1 mM also induced stomatalopening without appreciable accumulation of K^$ in the guardcells. After 4 hr treatment with 10 µM FC, stomata openedwith concomitant accumulation of K^$ in the guard cells, although1–2 hr treatment caused opening without concomitant K^$increase. These results suggest that stomatal opening can be caused bysalt accumulation and/or changes of the physicochemical conditionsin the cell wall of the guard cells due to high acidity. ¹ Present address: Biological Laboratory, Faculty of Education,Nagasaki University, Nagasski 852, Japan. (Received April 30, 1982; Accepted July 17, 1982)     

Vanadate Sensitive ATPase and Phosphatase Activity in Guard Cell Protoplasts of Commelina

Flicker, M. D. and Willmer, C. M. 1986. Vanadate sensitive ATPaseand phosphatase activity in guard cell protoplasts of Commelina.—J.exp. Bot. 38: 642–648. Phosphatase activity was measured in extracts of guard cellprotoplasts of Commelina communis L. using the artificial substratep-nitrophenylphosphate. A pH optimum of 5.8 to 6.3 was determined.Ammonium molybdate (Ol mol m^–3) and sodium vanadate (1–0mol m^–3) gave almost complete inhibition of phosphataseactivity at pH 60. ATPase assays were, therefore, conductedin the presence of 0–2 mol m ^–3 molybdate and vanadatewas used as a specific inhibitor of plasmamembrane ATPase activity.Vanadate sensitive ATPase activity showed a pH optimum of 6.6and activity was stimulated by KC1. These properties are characteristicof plasmamembrane proton pumping ATPases in other systems andsuggest that proton extrusion in guard cells could be mediatedby a similar enzyme. The maximum ATPase activity is sufficientto account for all the proton flux observed during the stomatalopening response. Key words: ATPase, Commelina, guard cell protoplasts, phosphatase, vanadate     

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     

Studies on the Mechanisms of Action of the Antitranspirant Phenylmercuric Acetate, and its Penetration into the Mesophyll

Experiments have examined the effect of phenylmercuric acetate(PMA) on the guard cells of Commelina communis. In one series,PMA was supplied to the leaf surface; after different time intervalsthe epidermis was removed and the ability of the stomata toopen was determined. In the other series, different concentrationsof PMA were included in the medium used for inccubating epidermalstrips with which ion-stimulated stomatal opening was assayed.At concentrations of 10^-54 M and above the effect of PMA wassevere and the structural integrity of the guard cells was affected;they were unable to accumulate neutral red. At concentrationsarpound 10^-6 M the guard cells were less affected and PMA broughtabout a transient stimulation of stomatal opening by releasingsubsidiary-cell turgor pressure. A solution of 5 x 10^-4 M PMA applied to leaves reduced by halfthe photosynthetic ¹⁴CO₂ incorporation into C. communis mesophyll.In Zea mays it increased the CO₂ compensation point and alsothe resistance to diffusion in the gas phase (R_A, but therewas a proportionately greater increase in the apparent liquidphase resistance (R_t). This direct inhibition of mesophyll photosynthesisundermines one of the major objectives of applying anatitranspirants,and for this reason it is suggested that PMA is unsuitable forgeneral application to crops.     

Putative involvement of cytosolic Ca2+ and GTP-binding proteins in cyclic-GMP-mediated induction of stomatal opening by auxin in Commelina communis L.

To investigate whether cyclic GMP (cGMP) would mediate, in an intracellular Ca²⁺ -dependent manner, coupling of auxin to stomatal opening, the stomatal opening responses to the auxin indolyl-3-butyric acid (IBA) and to the cGMP membrane-permeable derivative 8-bromoguanosine 3^′,5^′-cyclic monophosphate (8-Br-cGMP) were compared in epidermal strips of Commelina communis. In this comparison were studied possible effects of intracellular Ca²⁺ modulators, GTP-binding protein (G-protein) modulators and selective inhibitors of enzymatic reactions which use or generate cGMP. The stomatal response to IBA was almost similarly reversed by the Ca²⁺ buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N^′,N^′-tetraacetic acid (BAPTA), the intracellular Ca²⁺-release inhibitors ruthenium red and procaine, the inactive cGMP analog Rp-8-bromoguanosine 3^′,5^′-cyclic monophosphorothioate (Rp-8-Br-cGMPS), the inhibitor of cGMP-producing guanylyl cyclase LY 83583, the G-protein inhibitor mas17 and the G-protein antagonist pGlu-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH₂. Comparison with stomatal opening in response to 8-Br-cGMP, which was almost completely suppressed by either BAPTA, ruthenium red, procaine or Rp-8-Br-cGMPS, strongly suggests that cGMP acts downstream of G-protein activation as a second messenger for IBA signal transduction and that the cGMP pathway likely depends on cytosolic Ca²⁺signaling. Received: 8 November 1997 / Accepted: 6 March 1998     

Calcium Activation of Potassium Channels in the Plasmalemma of Eremosphaera viridis

The role of cytoplasmic calcium activity in activation of K⁺-channelsin the unicellular green alga Eremosphaera viridis has beenstudied. As reported previously, after a ‘light off’signal a voltage independent opening of K⁺-channels in the plasmalemmais observed. This effect is indicated by a transient polarization(TP) with a simultaneous increase of the membrane conductance.TPs can also be triggered by different treatments, which allowinvestigations within a ‘short-circuited’ signalchain. (i) After incubation with EGTA a single extended TP canbe released by a sudden increase of the external calcium concentration.The Ca²⁺-channel inhibitors nifedipine (10 ^–2 mol m^–3)and verapamil (5 ? 10^–2 mol m^–3) suppress the releaseof this TP. (ii) In the presence of external calcium the additionof the ionophore A23187 [GenBank] (10–3 mol m^–3) causes anextremely prolonged TP. (iii) Low external concentrations ofbarium (10^–2 mol m^–3) induce repetitive TPs in thepresence of external calcium. In this case the Ca²⁺-channelinhibitors are less effective. (iv) Strontium (0.1–1.0mol m^–3) is able to trigger repetitive TPs even withoutexternal calcium. Whereas barium may stimulate a calcium influx,strontium can serve as a substitute for calcium to induce anopening of K⁺-channels. These results indicate strongly a Ca²⁺-dependentand voltage-independent activation of K⁺-channels in the plasmalemmaof Eremosphaera. The participation of cytoplasmic calcium inthe signal transduction chain after a ‘light off’signal is discussed. Key words: Ca²⁺-dependent K⁺-channels, Ca²⁺-channel effectors, A23187, transient membrane potential, Eremosphaera     

Ion-Stimulated Stomatal Opening Induced by Preillumination in Epidermal Strips of Commelina communis

The effects of preillumination were investigated on ion-stimulated stomatal opening of epidermal strips isolated from Commelina communis L. leaves, which are dark-starved 24 hours or more. The rate and the extent of ion-stimulated stomatal openings were increased by preexposure of epidermal strips to light. The evidences are interpreted as indicating that the energy induced by preillumination can be conserved in guard cells for considerable time periods and then used for a delayed stomatal opening in the presence of higher concentration of potassium or sodium ions. Action spectrum showed two peaks, one in blue and one in the red light region. The ratio of the blue peak to the red peak is 1.2; which is the smallest reported value in action spectra of stomatal movements. 3-(4-chlorophenyl)-1,-1-Dimethylurea suppressed the ion-stimulated stomatal opening induced by the preillumination. We conclude that the photosynthetic electron transport system, containing photosystem II, in guard cell chloroplasts is a basic system of energy acquirement for stomatal opening.     

Circadian Rhythm of Stomatal Movements in Epidermal Strips

Measurements were made of changes in stomatal pore widths inepidermal strips of leaves ofVicia faba and Commelina communis.Strips were incubated in dilute KCI solutions (1 and 10 molm^–3) flowing through a perfusion chamber on the stageof a microscope and kept for 4 d in continuous light. Circadianrhythms of stomatal apertures were detected in both species.Although the amplitude was small it was statistically significant.It is concluded that at least partof the mechanism for the stomatalrhythm resides in the epidermis, probably in the guard cells. Key words: Cireadian rhythm, epidermal strips, stomata     

Changing Responses of Stomata to Abscisic Acid and CO2 as Leaves and Plants Age

Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changingresponses of stomata to abscisic acid and CO₂ as leaves andplants age.—J. exp. Bot. 39: 401–410. Stomatal conductances were measured in ageing leaves of Commelinacommunis L. as plants developed; stomatal responses to CO₂ andabscisic acid (ABA) in epidermal strips of C. communis takenfrom ageing leaves of developing plants and in epidermal stripsfrom the same-aged leaves (the first fully-expanded leaf) ofdeveloping plants were also monitored. Stomatal behaviour wascorrelated with parallel measurements of photosynthesis andleaf ABA concentrations. Stomatal conductance in intact leavesdecreased from a maximum of 0-9 cm s^{– 1} at full leaf expansionto zero about 30 d later when leaves were very senescent. Conductancesdeclined more slowly with age in unshaded leaves. Photosynthesisof leaf slices also declined with age from a maximum at fullleaf expansion until about 30 d later when no O₂ exchange wasdetectable. Exogenously applied ABA (0.1 mol m^{– 3}) didnot affect respiration or photosynthesis. In epidermal stripstaken from ageing leaves the widest stomatal apertures occurredabout 10 d after full leaf expansion (just before floweringbegan) and then decreased with age; this decrease was less dramaticin unshaded leaves. The inhibitory effects of ABA on stomatalopening in epidermal strips decreased as leaves aged and wasgreater in the presence of CO₂ than in its absence. When leaveswere almost fully-senescent stomata were still able to open.At this stage, guard cells remained healthy-looking with greenchloroplasts while mesophyll cells were senescing and theirchloroplasts were yellow. Similar data were obtained for stomatain epidermal strips taken from the same-aged leaves of ageingplants. The inhibitory effects of ABA on stomatal opening alsodecreased with plant age. In ageing leaves both free and conjugated ABA concentrationsremained low before increasing dramatically about 30 d afterfull leaf expansion when senescence was well advanced. Concentrationsof free and conjugated ABA remained similar to each other atall times. It is concluded that the restriction of stomatal movements inintact leaves as the leaves and plants age is due mainly toa fall in photosynthetic capacity of the leaves which affectsintracellular CO₂ levels rather than to an inherent inabilityof the stomata to function normally. Since stomatal aperturein epidermal strips declines with plant and leaf age and stomatabecome less responsive to ABA (while endogenous leaf ABA levelsremain fairly constant until leaf senescence) it is suggestedthat some signal, other than ABA, is transmitted from the leafor other parts of the plant to the stomata and influences theirbehaviour. Key words: Abscisic acid, CO₂, Commelina, leaf age, senescence, stomatal sensitivity     

Effects of Cations on the Cytoplasmic pH of Chara corallina

Smith, F. A. and Gibson, J.–L. 1985. Effects of cationson the cytoplasmic pH of Chara corallina.—J.exp. Bot.36: 1331–1340 Removal of external Ca²⁺ from cells of Chara corallina lowersthe cytoplasmic pH, as determined by the intracellular distributionof the weak acid 5,5–dimethyloxazolidine2–,4–dione(DM0), when the external pH is below about 60. This effect isreversed, at least partially, by addition of the following cationsto Ca²⁺-free solutions: tetraethylammonium (TEA⁺) and Na⁺ at5 or 10 mol m^-3, Li⁺ and Cs⁺ (10 mol m^-3), or Mg²⁺, Mn²⁺ andLa³⁺ (02 or 05 mol m^-3). Under the same conditions, increasesin pH sometimes, but not always, occur in the presence of 10mol m^-3 K⁺ or Rb⁺ The results are discussed in relation to the major transportprocesses that determine pH and the electric potential differenceacross the plasma membrane, namely fluxes of H⁺ and of K⁺. Thesimplest explanation of the effects of the various cations testedin this study is that they primarily affect pHi_c via changesin influx of H⁺ but direct effects on the H⁺ pump or on K⁺ fluxesmay also be involved Key words: Chara corallina, cytoplasmic pH, cations, H⁺transport     

Stomatal Responses to Applied ABA and CO2 in Epidermis Detached from Well-Watered and Water-Stressed Plants of Commelina communis L.

Epidermal strips from either well-watered or water-stressedplants of Commelina communis L. were subjected to a range ofABA concentrations (10^–6–10^–3 mol m^–3)in the presence (330 parts 10^–6 in air) or virtual absence(3 parts 10–6 in air) of CO₂. The stomatal response toCO₂ was greater in epidermis from water-stressed plants, althoughthere was a distinct CO₂ response in epidermis from well-wateredplants. Additions of ABA via the incubation medium had littleeffect on the relative CO₂ response. Stomata responded to ABAboth in the presence and virtual absence of CO₂, but the relativeresponse to ABA was greatest in the high CO₂ treatment. Whenwell-watered plants were sprayed with a 10^–1 mol m^–3ABA solution 1 d prior to use, the stomatal response of detachedepidermis to both CO₂ and ABA was very similar to that of epidermisdetached from water-stressed leaves. It is hypothesized thata prolonged exposure to ABA is necessary before there is anymodification of the CO₂ response of stomata.     

Responses of the stomata of Aster tripolium to calcium and sodium ions in relation to salinity tolerance

In previous work, the stomata of the maritime halophyte Astertripolium L. were shown to close when NaCl concentrations risein the vicinity of the guard cells. Further studies have nowrevealed important effects of calcium on the ionic responsesof the stomata. When the guard cells were presented with KCl,Ca²⁺ suppressed opening in a manner similar to that which hasbecome familiar in other species such as Commelina communisL. However, in the presence of NaCl, Ca²⁺ had the opposite effect,reducing the closing response to NaCl. This pattern of behaviouris discussed in relation to known salt effects on membranes,but the underlying physiological basis remains obscure. A previous study led to the hypothesis that the closing responseof the stomata to Na⁺ ions may make an important contributionto the salinity tolerance of this species. Here we report thatincreasing supplies of Ca²⁺ ions reduce the effect of salinityon stomatal conductance in the whole plant as well as in theisolated epidermis. This finding is consistent with the wellestablished role of calcium in increasing resistance to salinity:in the presence of high calcium the plant can tolerate a greatersalt intake, and hence there is a reduced need for transpirationto be restricted by partial stomatal closure. Key words: Sodium, calcium, Aster tripolium, stomata, salinity tolerance     

The Flux and Distribution of Xylem Sap Calcium to Adaxial and Abaxial Epidermal Tissue 8

Inherent differences in the responses of stomata on abaxialand adaxial epidermal surfaces of leaves of Commelina communishave previously been suggested to be due to differences in theconcentrations of apoplastic Ca. Adaxial stomata have also beenreported to be more sensitive than abaxial stomata to appliedabscisic acid (ABA). The aims of these experiments were to determinethe validity of these conclusions and to see if xylem sap Cahas a role in determining the response of stomata to ABA. It can be shown from measurements of relative stomatal resistance(determined with a viscous flow porometer) and stomatal conductancethat stomata were more open in plants grown on 8-0 mol m^–3Ca, than with those grown on 2-0 mol m^–3 Ca. When attachedleaves were fed with ABA via the transpiration stream neitherthe extent nor the rate with which conductance declined wasdependent on Ca nutrition. The extent of Ca accumulation within both epidermes was relatedto the concentration of Ca in the rhizosphere and in the xylemsap. It did not, however, appear to reflect the apparent differencesin the flux of the transpiration stream between the two epidermes.Plants growing at the lower Ca concentration accumulated proportionallymore epidermal Ca relative to Ca in xylem sap. The evidencepresented suggests that Ca movement from the xylem to the epidermiscannot be simply described by a mass flow model, and that thedistribution of Ca is not an adequate explanation of the differencesin the behaviour of adaxial and abaxial stomata. The potentialrole for changes in xylem sap Ca to act as a regulator of stomatalbehaviour are discussed. Key words: Abscisic acid, calcium, Commelina communis L., stomatal conductance