Diurnal change in the relationship between stomatal conductance and abscisic acid in the xylem sap of field-grown peach trees

To evaluate whether abscisic acid (ABA) in the xylem sap playsan important role in controlling stomatal aperture of field-grownPrunus persica trees under drought conditions, stomatal conductance(g) and xylem ABA concentrations were monitored both in irrigatedand non-irrigated trees, on two consecutive summer days (threetimes a day). Stomata1 conductance of non-irrigated trees hada morning maximum and declined afterwards. The changes in gduring the day, rather than resulting from variations in theconcentrations of ABA in the xylem sap or the delivery rateof this compound to the leaves, were associated with changesin the relationship between g and xylem ABA. The stomata ofwater-stressed trees opened during the first hours of the day,despite the occurrence of a high concentration of ABA in thexylem sap. However, stomatal responsiveness to ABA in the xylemwas enhanced throughout the day. As a result, a tight inverserelationship between g and the logarithm of xylem ABA concentrationwas found both at midday and in the afternoon. A similar relationshipbetween g and ABA was found when exogenous ABA was fed to leavesdetached from well-watered trees. These results indicate thatABA derived from the xylem may account for the differences ing observed between field-grown peach trees growing with differentsoil water availabilities. Several possible explanations forthe apparent low stomatal sensitivity to xylem ABA in the morning,are discussed, such as high leaf water potential, low temperatureand high cytokinin activity. Key words: Prunus persica L., stomata, xylem ABA, water deficits, root-to-shoot communication     

Physiology of the parasitic association between maize and witchweed (Striga hermonthica): is ABA involved?

Striga hermonthica (Del.) Benth. is an obligate hemiparasiticangiosperm which can cause severe losses of yield in cerealcrops in the semi-arid tropics. The effects of this parasiteon the growth and stomatal conductance of three varieties ofmaize (Zea mays L.) during the first 6 weeks of the associationhave been studied. From 24 d after planting (DAP), infectedplants were significantly shorter than uninfected controls.When the plants were harvested 45 DAP, infected plants had fewerfully expanded leaves, less leaf biomass and less pseudo-stembiomass than uninfected controls. However, the parasitized plantshad more root biomass and hence a higher root:shoot ratio thanuninfected controls. The stomatal conductance of infected hostswas severely inhibited by comparison with that in uninfectedplants. The possibility that abscisic acid (ABA) may be involved inthe regulation of the parasitic association was investigated.ABA concentrations in leaf tissue of maize (cv. Cargimontana)and S. hermonthica were determined by radioimmunoassay. Whilethere was a difference between cultivars in the extent of theresponse, the concentrations of ABA were significantly higherin infected maize plants than in the uninfected controls. InS. hermonthica, leaf tissue ABA concentration was found to bean order of magnitude higher than in the host leaf tissue. Detachedleaves of S. hermonthica which were dehydrated at room temperatureuntil they had lost 10–20% of their fresh weight containedthree times the ABA concentration of control leaves. This suggeststhat leaves of S. hermonthica can synthesize or re-mobilizeABA in response to water deficit. It is not yet known whetherthis contributes to the higher concentration in infected hosts,but the results suggest that ABA has a role in this parasiticassociation. Key words: Striga hermonthica, abscisic acid, growth, parasitic angiosperm, stomatal conductance     

The Effects of Cytokinins and ABA on Stomatal Behaviour of Maize and Commelina

Epidermal strips and leaf fragments of Commelina and leaf fragmentsof maize were incubated on solutions containing naturally-occurringor synthetic cytokinins and/or ABA. The effects of these treatmentson stomatal behaviour were assessed. Cytokinins alone did notpromote stomatal opening in either species but concentrationsof both zeatin and kinetin from 10^–3 to 10^–1 molm^–3 caused some reversal of ABA-stimulated closure ofmaize stomata. The reversal of the ABA effect increased withincreasing cytokinin concentration. Cytokinins had no effecton ABA-stimulated closure of Commelina stomata. When appliedalone, at high concentration (10^–1 mol m^–3), toCommelina epidermis or leaf pieces both zeatin and kinetin restrictedstomatal opening. Key words: ABA, Cytokinins, Stomata, Maize, Commelina     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant II. Water relations, stomatal conductance, abscisic acid content in leaves and xylem sap of plants subjected to water deficiency

The response of w-1, a wilty sunflower (Helianthus annuus L.)mutant, to water stress is described in comparison with thecontrol line (W-1). Detached leaves of w-1 strongly dehydratedduring the first 30 min without significant changes in leafconductance, whereas W-1 responded rapidly to water loss byreducing stomatal aperture. After 2 h stress ABA increased slightlyin w-1, while W-1 leaves showed a 20-fold increase. When waterstress was imposed to potted plants by water withholding, w-1quickly dehydrated, and lost turgor, while W-1 maintained positiveturgor values for a longer period. Wild-type plants respondedto small changes in leaf water potential by accumulating ABAand by closing stomata, whereas in the mutant significant changesin ABA content and in stomatal conductance were found only atvery low water potentials. In another experiment in which waterwas withheld under high relative humidity, when soil water contentstarted to decrease W-1 rapidly closed stomata in the absenceof any change in leaf water status and the reduction in conductancewas paralleled by a rise in xylem sap ABA concentration. Bycontrast the mutant started to accumulate ABA in the xylem sapand to close stomata when soil water content and leaf waterpotential were dramatically reduced. The low endogenous ABAlevels and the inability to synthesize the hormone rapidly eitherin the leaves or in the roots seem to be responsible for thehigh sensitivity of w-1 to water stress. Key words: ABA, Helianthus annuus L, water relations, stomatal conductance, drought, wilty mutant     

The Influence of Prior Water Stress and Abscisic Acid Foliar Spraying on Stomatal Responses to CO2, IAA, ABA, and Calcium in Leaves of Solanum melongena

The influence of a water stress or foliar ABA spraying pretreatmenton stomatal responses to water loss, exogenous ABA, IAA, Ca²⁺,and CO₂ were studied using excised leaves of Solanum melongena.Both pretreatments increased stomatal sensitivity of water loss,in the presence and absence of CO₂, but decreased stomatal sensitivityto exogenous ABA. CO₂ greatly reduced the effect of exogenouslyapplied ABA. IAA decreased leaf diffusion resistance for controland ABA sprayed leaves, but did not influence the LDR of previouslywater-stressed leaves. CA²⁺ did not influence LDR of any leavesof any treatments. Key words: Water stress, stomatal response, pretreatments     

Metabolism of xylem-delivered ABA in relation to ABA flux and concentration in leaves of maize and Commelina communis

When detached maize leaves were fed with an ABA solution viathe xylem, the relationship between the relative stomatal inhibitionand ABA concentrations was similar under different humidityconditions, but the relationship between such inhibition andABA flux was different according to changes of humidity. Tounderstand whether such stomatal behaviour was related to theway through which xylem-delivered ABA was metabolized, detachedleaves of maize and Commelina were fed with tritium-labelled(³H)-ABA at concentrations similar to that found in xylem ofdroughted plants and it was found that xylem-delivered ABA wasmetabolized rapidly in both species. The half-life of ABA metabolism,calculated from the time-related ABA disappearance curve, was42 and 64 min for maize and Commelina, respectively. The veryshort half-life suggests that there is a large capacity in leavesto metabolize xylem-delivered ABA and that metabolism is a majorfactor in the control of ABA accumulation in leaves. When ABAwas fed at different fluxes, either through changing the feedingconcentrations or through manipulating the rates of leaf transpiration(i.e. the volume flux), ABA was metabolized at rates that wereproportional to the amount that was delivered. The absoluterate of ABA metabolism was, therefore, linearly related to theamount of ABA that had arrived. It was found that xylem-deliveredABA reached the epidermis of Commelina, and was metabolizedat the same pattern as that in mesophyll tissues, i.e. at asimilar half-life and at rates constantly related to the amountthat was delivered. The role of the rapid ABA metabolism wasdiscussed in the context of stomatal control by either concentrationor flux of xylem-carried ABA. Key words: Abscisic acid, ABA metabolism, xylem-delivered ABA, maize, Commelina     

A Model for the Interaction of Low Temperature, ABA, IAA, and CO2 in the Control of Stomatal Behaviour

In the chilling sensitive (C.S.) species Phaseolus vulgarisit was found that at 22 ?C ABA induced stomatal closure butthis effect was dependent on the presence of CO₂. In the absenceof CO₂ the effect of ABA was completely lost. In contrast toABA, the effect of IAA at 22 ?C was to increase stomatal openingas the IAA concentration increased from 10^–2 to 10 molm^–3, and this effect was dependent upon the presence ofCO₂. However, at 5 ?C the action of ABA was reversed and itwas found to induce stomatal opening when fed via the transpirationstream in excised leaves. Similarly, the CO₂ response characteristicswere reversed at low temperatures as removal of CO₂ from theatmosphere caused stomatal closure. However, the effect of IAAat 5 ?C in the presence of CO₂ and with or without ABA was toincrease stomatal aperture with increasing IAA concentration.Significantly, ABA was found to have no effect upon aperturein the presence of CO₂ when IAA was added. The interactive effectsof ABA, IAA, CO₂ and low temperature are discussed in relationto a model proposed by the authors. Key words: IAA, ABA, CO₂, Stomata     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant: III. Phenotypic interaction in reciprocal grafts from wilty mutant and wild-type plants

The present study was conducted to evaluate phenotypic interactionin reciprocal grafts between wilty (w-1) sunflower mutant andnormal (W-1) plants. The w-1 genotype is a ‘leaky’ABA-deficient mutant, characterized by high stomatal conductance,in both light and dark conditions, and high transpiration rate. In well-watered conditions, mutant scions grafted on to normalrootstock (w-1/W-1) showed higher leaf relative water content,leaf water potential and ABA levels than those of control grafts(w-1/w-1). In addition, detached leaves of w-1/W-1 exhibitedlower water loss than w-1/w-1 grafts, while mutant rootstockdid not affect the transpiration rate of detached W-1 leaves.When drought stress was imposed to potted plants by withholdingwater, the mutant scions grafted on to normal roots showed apartial phenotypic reversion. A rapid stomatal closure and arise in ABA levels in response to a small decrease in leaf waterpotential was observed. By contrast, in w-1/w-1 grafts significantreductions in stomatal conductance and ABA accumulation weredetected only in conjunction with a severe water deficit. W-1scions on mutant stocks (W-1/w-1) maintained the normal phenotypeof control wild-type grafts (W1/W-1). Key words: ABA, grafting, Helianthus annuus, stomatal conductance, water relations, wilty mutant     

The role of abscisic acid and water relations in drought responses of subterranean clover

The role of water relations and abscisic acid (ABA) in the responsesto drought were studied in a mediterranean forage crop, Trifoliumsubterraneum L. under field conditions. Soil and plant waterstatus, leaf gas exchange parameters, and xylem sap ABA contentwere determined at different times during a long-term soil dryingepisode in irrigated and droughted plants. The diurnal time-coursesof these parameters were also measured at the end of a droughtperiod. In response to soil drying stomatal conductance (g) was reducedearly to 50% that of irrigated plants before any substantialchange in water potential was detected. A close logarithmicregression between photosynthesis rate (A) and g was present.For the first weeks of drought the decline in A was less pronouncedthan in g, thus increasing water use efficiency. Stomatal conductanceduring diurnal time-courses showed no consistent relationshipswith respect to etther ABA or leaf water potential. Throughoutthe experimental period dependence of g on leaf water statuswas evident from the tight correlation (r²=0.88, P<0.01)achieved between stomatal conductance and midday water potential,but the correlation was also high when comparing g with respectto ABA content in xylem sap (r=0.83, P<0.001). However, thestomata from drought acclimated plants were apparently moresensitive to xylem ABA content. For similar xylem ABA concentrationsstomatal conductance was significantly higher in irrigated thanin waterstressed plants. Key words: Drought, stomatal conductance, water potential, abscisic acid     

Does Xylem Sap ABA Control the Stomatal Behaviour of Water-Stressed Sycamore (Acer pseudoplatanus L.) Seedlings?

Sycamore seedlings were grown with their root systems dividedequally between two containers. Water was withheld from onecontainer while the other container was kept well-watered. Effectsof soil drying on stomatal behaviour, shoot water status, andabscisic acid (ABA) concentration in roots, xylem sap and leaveswere evaluated. At 3 d, root ABA in the drying container increased significantly,while the root ABA in the unstressed container of the same plantsdid not differ from that of the control. The increase in rootABA was associated with the increase in xylem sap ABA and withthe decrease in stomatal conductance without any significantperturbation in shoot water status. At 7 d, despite the continuous increase in root ABA concentration,xylem sap ABA showed a marked decline when soil water contentwas depleted below 013 g g^–1. This reduction in xylemsap ABA coincided with a partial recovery of stomatal conductance.The results indicate that xylem sap ABA is a function of rootABA as well as the flow rate of water from roots to shoots,and that this ABA can be a sensitive indicator to the shootof the effect of soil drying. Key words: Acer pseudoplatanus L., soil drying, stomatal behaviour, xylem sap ABA     

Changes in Stomatal Conductance in Intact Ageing Wheat Leaves in Response to Abscisic Acid

The characteristics of ABA-induced changes in the fluxes ofCO₂ and water vapour from whole leaves of spring wheat (Triticumaestivum cv. Wembley) were examined. Aqueous solutions of ABAwere supplied via the transpiration stream to intact leavesof different ages mounted within a gas exchange cuvette. ABA caused a reduction in stomatal conductance (g) that wasproportional to the concentration in the solution fed to theleaf. For the maintenance of a reduction in g there was a requirementfor a continual supply of ABA. At concentrations greater than10^–2 mol m^–3 ABA reduced g by at least 50% of thecontrol value, while 1.0 mol m^–3 closed stomata within2 h. Concentrations as low as 10^–3 mol m^–3 produceda 20% reduction in g. As leaves aged they became less responsiveto applied ABA. The possibility that the stomatal response may change aftera leaf has previously experienced a pulse of ABA was exploredby repeating the exposure of a leaf to 10^–2 mol m^–3ABA. The first pulse of ABA produced a greater reduction ing than a subsequent exposure the following day. This declinein response of g to ABA on repeated exposure was maintainedwith leaves of different ages. The characteristics of the stomatal response to ABA are discussedin the context of what is known about the location of receptorsfor the hormone. It seems likely that a failure to respond toABA that has previously accumulated in the guard cells shouldbe viewed by means of maximizing the sensitivity to the currentsupply of ABA. It is suggested that the smaller response ofthe stomata of older leaves to ABA makes them more susceptibleto water stress, so that they can act as sensors for decliningwater potentials to give early protection to younger, metabolicallyactive leaves. Key words: Abscisic acid, leaf age, stomatal conductance, Triticum aestivum     

Stomata of Micropropagated DelphiniumPlants Respond to ABA, CO2, Light and Water Potential, but Fail to Close Fully

Morphological and physiological characteristics of micropropagatedplants of Delphinium cv. Princess Caroline were studied. Leavesproduced in vitro showed poor control of water loss which appearsto result from restricted responses by stomata and not frompoor cuticular development. Stomata of leaves produced in vitrowere larger and more frequent than those produced during acclimatization.Despite the fact that stomata from isolated epidermis of leavesproduced in vitro reduced their apertures when exposed to turgor-reducingtreatments, they did not close fully. This, together with highstomatal frequencies might explain the poor control of waterloss shown by intact leaves produced in culture when exposedto dry air. While leaves from acclimatized plants showed almostcomplete closure with ABA, low water potentials, darkness andCO₂, stomata from leaves produced in vitro reduced their apertureswhen exposed to those factors, but only to a limit. Therefore,stomata from leaves cultured in vitro seem to be partially functional,but some physiological or anatomical alteration prevents themfrom closing fully. Stomata from leaves produced in vitro wereparticularly insensitive to ABA which appears to be partly associatedwith the high cytokinin concentration in the culture medium.In the long-term, this stomatal insensitivity to ABA might contributeto plant losses when micropropagated plantlets are transferredto soil. Key words: Micropropagation, stomatal physiology, dehydration, PEG, ABA, BAP, darkness, CO₂, Delphinium     

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.     

The Influence of Leaf Water Status and ABA on Leaf Growth and Stomata of Phaseolus Seedlings with Hypoxic Roots

The objective of this study was to assess the relative rolesof leaf water status and root-sourced signals in mediating beanleaf responses to root hypoxia. To do so, the roots of beanplants under varied VPD (0.95 kPa to 0.25 KPa) were made hypoxic.Under all conditions, leaf growth rates and stomatal conductanceswere reduced. There was a transitory decline in leaf water potentialat high VPD which accounted for the initial reduction in leafgrowth rates and stomatal conductance. At low VPD, no waterdeficits were detected. Leaf growth inhibition and reduced stomatalconductance under low VPD treatments were unrelated to leafwater status and must be induced by some other factor. In vitrogrowth of leaf discs was reduced by xylem sap collected fromhypoxic roots. Exogenously applied ABA, at high concentrationsin KCl and sucrose, or at low concentrations diluted in xylemsap from aerated plants, inhibited in vitro growth of leaf discs.Applications of ABA in the transpiration stream reduced stomatalconductance.     

Inhibition of Light-Stimulated Leaf Expansion by Abscisic Acid

Abscisic acid (ABA) applied to intact bean (Phaseolus vulgaris)leaves or to isolated leaf discs inhibits light-stimulated cellenlargement This effect may be obtained with 10^–4 molm^–3 ABA, but is more significant at higher concentrations.The inhibition of disc expansion by ABA is greater for discsprovided with an external supply of sucrose than for discs providedwith KC1, and may be completely overcome by increasing the KC1concentration externally to 50 mol m^–3. Decreased growthrate of ABA-treated tissue is not correlated with loss of solutesfrom growing cells, but is correlated with a decrease in cellwall extensibility. ABA does not prevent light-stimulated acidificationof the leaf surface, and stimulates the acidification of theexternal solution by leaf pieces. However, the capacity of thecell walls to undergo acid-induced wall loosening is diminishedby ABA-treatment. The possibility that ABA acts directly byinhibiting growth processes at the cellular level, or indirectlyby causing stomatal closure, is discussed. Key words: Phaseolus vulgaris, ABA, Inhibition, Leaf expansion     

Senescence-Associated Changes during Long-day-induced Leaf Senescence and the Nature of the Graft-transmissible Senescence Substance in Kleinia articulata

Schwabe, W. W. and Kulkarni, V. J. 1987. Senescence-associatedchanges during long-day-induced leaf senescence and the natureof the graft-transmissible senescence substance in Kleinia articulata.— J. exp. Bot. 38: 1741–1755. The long-day-induced senescence in Kleinia articulata leaveswas characterized by a loss in fresh and dry weight, in therate of leaf expansion and progressive loss of chlorophyll inthe detached rooted leaves. Ultrastructural examination of mesophyllcells of leaves from plants grown in continuous light showedthat osmiophilic globules accumulating in the chloroplasts werethe first visible sign of senescence in the organdies. Thesefirst signs of senescence could be detected in very young leavesof plants in continuous light, even before the leaves had expanded.Attempts were made to study the cause of this photoperiodicsenescence which, from previous work, appeared to involve agraft-transmissible substance. Leaves in continuous light showed reduced stomatal opening andextracts from them had very much higher activity in the Commelinastomatal closure assay (ABA-like activity ?) compared with non-senescingleaves grown in short days (8 h). However, even if all the activitywere due to ABA, this on its own does not appear to be the senescencesubstance because a much longer exposure to continuous lightwas required to induce irreversible senescence than to reachmaximum stomatal closure promoting activity in the bioassay.Moreover, severe water stress (high ABA?) did not lead to senescenceunless combined with continuous light or ethylene treatment.It is postulated that while ABA may play an important role inKleinia leaf senescence its lethal effect may not be realizedunless ethylene-induced membrane changes may synergisticallyassist. Key words: Leaf senescence, ABA, Daylength, stomatal movement, Kleinia     

Changes in stomatal movements in isolated epidermal strips of a CAM plant, Notonia grandiflora DC

The diuranl rhythm of the stomatal aperture size was investigatedusing isolated epidermal strips from a CAM plant, Notonia grandiflora.Maximal stomatal opening occurred at pH 6.5. The nocturnal stomatalopening was stimulated by KCl which produced stomatal closurein light. The stimulatory effect of KCl on stomatal openingin drakness was suppressed by ABA. (Received January 24, 1978; )     

Stomatal Responses of Pearl Millet (Pennisetum americanum (L.) Leeke) Genotypes, in Relation to Abscisic Acid and Water Stress

Stomatal responses to water stress and to applied (±)-abscisicacid (ABA) were examined in genotypes of pearl millet (Pennisetumamericanum (L.) Leeke) known to differ in amounts of endogenousABA accumulating during drought. In both a pot and a field experiment,Serere 39, a genotype with a high capacity to accumulate ABA,showed a higher stomatal sensitivity to water stress than didthe ‘low’ ABA accumulator, BJ 104. In the fieldexperiment, a third genotype, B282, accumulating least amountsof ABA, also had the lowest stomatal sensitivity to water stress. There were no significant differences between these genotypesin stomatal response to applied (±)-ABA, or in the relationshipsbetween leaf conductance and levels of endogenous ABA. It isconcluded that the differences in accumulation of endogenousABA by these genotypes of pearl millet are of functional significance,and that endogenous ABA generated during a water stress whichdevelops over days or weeks mediates stomatal responses to suchstress.     

Osmotic Stress Temporarily Reverses the Inhibitions of Photosynthesis and Stomatal Conductance by Abscisic Acid--Evidence that Abscisic Acid Induces a Localized Closure of Stomata in Intact, Detached Leaves

Ward, D. A. and Drake, B. G. 1988. Osmotic stress temporarilyreverses the inhibitions of photosynthesis and stomatal conductanceby abscisic acid—evidence that abscisic acid induces alocalized closure of stomata in intact, detached leaves.—J.exp. Bot 39: 147–155. The influence of osmotic stress on whole leaf gas exchange wasmonitored in detached leaves of Glycine max supplied with anexogenous concentration (10^–5 mol dm^–3) of ±abscisicacid (ABA) sufficient to inhibit net photosynthesis and stomatalconductance by 60% and 70%, respectively, under a saturatingirradiance and normal air. Raising the osmotic (sorbitol) concentrationof the ABA solutions feeding leaves elicited rapid and synchronousreversals of the ABA-dependent inhibitions of net photosynthesisand conductance. These reversals reached a peak simultaneously,after which photosynthesis and conductance declined. The magnitudeof the transient stimulations at peak height was dependent uponthe sorbitol concentration of the ABA feeding solution, althoughthe time-course of the transients (half time, 4–6 min)was similar for the different osmotic concentrations applied.Irrespective of transient size the relative changes of photosynthesisand conductance were comparable; consequently the calculatedpartial pressure of CO₂ in the substomatal space (Ci) remainedrelatively constant during the transient phase. In contrastto the ABA-treated leaves, elevating the osmotic concentrationof the distilled water supply feeding control leaves stimulatedconductance to a much greater relative extent than photosynthesis.The co-stimulations of photosynthesis and conductance inducedin ABA-treated leaves by osmotic shock were not due to a restrictionin the transpirational uptake of ABA and occurred irrespectiveof the source osmoticum applied. These data are consistent with the hypothesis that the ABA-dependentinhibition of photosynthesis at constant Ci is an artifact causedby the spatially heterogeneous closure of stomata in responseto ABA. Alternative explanations for the responses are, however,considered. Key words: Abscisic acid, photosynthesis, osmotic stress, Glycine max, stomatal conductance     

Antitranspirant Activity in Xylem Sap of Maize Plants

Xylem sap from unwatered maize plants was collected and testedfor antitranspirant activity. Two assays were used. These werea transpiration assay with detached wheat leaves and a stomatalbio-assay involving the direct microscopic observation of epidermisof Commelina communis. The reduction in transpiration of detached wheat leaves promotedby xylem sap could be duplicated almost exactly by the applicationof solutions of ABA of equivalent concentration to that foundin the xylem sap. Removal of virtually all the ABA from thexylem sap, using an immunoaffinity column, removed virtuallyall the antitranspirant activity in both assays. These results are discussed in the context of other resultswhich suggest the presence of as-yet unidentified inhibitorsin the xylem sap of unwatered plants. We suggest that with maize plants at least, stomatal responsesto soil drying can be entirely explained by enhanced concentrationof ABA in the xylem stream. Key words: Antitranspirant activity, ABA, ABA bio-assay, xylem sap