Age-related Changes in Stomatal Response to Cytokinins and Abscisic Acid

Kinetin and zeatin(100 mmol m^–3)reversald the ABA-mediated(100mmol ^m-2)closure of stomata of young maize leaves but did notaffect stomatal apertures of these leaves when applied alone.As leaves aged, kinetin or zeatin alone promoted increased stomatalapertures, while abscisic acid (ABA) applied alone had a reducedeffect on stomata. Even with older leaves, cytokinins reversadthe effect of ABA on stomata. Maize, stomata, abscisic acid, kineusc, zeatin, Zea mays     

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     

Responses of Commelina communis L. Guard Cell Protoplasts to Abscisic Acid

Fitzsimons, P. J. and Weyers, J. D. B. 1987. Responses of Commelinacommunis L. guard cell protoplasts to abscisic acid.—J.exp. Bot. 38: 992–1001. Guard cell protoplasts (GCPs) isolated from the leaf epidermisof Commelina communis L. responded to abscisic acid (ABA) ina manner which was qualitatively and quantitatively similarto that of intact stomata. ABA inhibited swelling of GCPs underlow-CO₂ conditions and swollen GCPs responded to the hormoneby shrinking. Both the absolute volume decrease and the initialrate of shrinking were commensurate with the extent and ratesof solute loss computed for ABA-treated intact, open stomata.This indicates that GCPs represent a suitable experimental systemfor studies of ABA-mediated solute fluxes. A radiotracer equilibrationmethod was developed for the rapid estimation of GCP osmoticvolume changes. Using this technique it was found that, on average,82% of the reduction in solute content caused by ABA treatmentwas due to the loss of K⁺. It is envisaged that electroneutralitymight be maintained during ABA-induced shrinkage of GCPs bynet inward proton movement leading to acidification of the vacuole. Key words: Abscisic acid, Commelina communis L., guard cells, protoplasts     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant: I. Abscisic acid content, light-dark changes in the stomatal conductance and genetic analysis

The present study was conducted to characterize geneticallyand physiologically a spontaneous mutation in sunflower whichconfers a wilty phenotype. The wilting condition of the mutantis due to abnormal stomatal behaviour. The mutant stomata resistclosure in darkness. This abnormality is associated with lowlevels of endogenous abscisic acid (ABA). By artificially elevatingthe ABA content of the mutant plants by spray treatments with10^– and 10^– M solutions it proved possible to effecta phenotypic reversion of the mutant. It has, therefore, beenproposed that the primary effect of this spontaneous mutationis to reduce the level of ABA. The genetic analysis has shownthat the willy phenotype is due to recessive nuclear mutationat a single locus. Key words: ABA, Helianthus annuus L, stomatal conductance, wilty mutant     

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     

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     

Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies

The effects of abscisic acid (ABA) on photosynthesis in leavesof Helianthus annuus L. were compared with those in leaves ofVicia faba L. After the ABA treatment, the response of photosyntheticCO₂ assimilation rate, A, to calculated intercellular partialpressure of CO₂, P_i, (A(p_i) relationship) was markedly depressedin H. annuus. A less marked depression was also observed inV.faba. However, when the abaxial epidermes were removed fromthese leaves, neither the maximum rate nor the CO₂ responseof photosynthetic oxygen evolution was affected by the applicationof ABA. Starch-iodine tests revealed that photosynthesis was not uniformover the leaves of H. annuus treated with ABA. The starch contentwas diffferent in each bundle sheath extension compartment (thesmallest subdivision of mesophyll by veins with bundle sheathextensions, having an area of ca. 0.25 mm² and ca. 50 stomata).In some compartments, no starch was detected. The distributionof open stomata, examined using the silicone rubber impressiontechniques, was similar to the pattern of starch accumulation.In V.faba leaves, which lack bundle sheath extensions, distributionof starch was more homogeneous. These results indicate that the apparent non-stomatal inhibitionof photosynthesis by ABA deduced from the depression of A(pⁱ)relationship is an artifact which can be attributed to the non-uniformdistribution of transpiration and photosynthesis over the leaf.Intercellular gaseous environment in the ABA-treated leavesis discussed in relation to mesophyll anatomy. ¹ Present address: Department of Botany, Duke University, Durham,NC 27706, U.S.A. (Received September 30, 1987; Accepted January 13, 1988)     

In-Phase Cycling of Photosynthesis and Conductance at Saturating Carbon Dioxide Pressure Induced by Increases in Water Vapour Pressure Deficit

Bunce, J. A. 1987. In-phase cycling of photosynthesis and conductanceat saturating carbon dioxide pressure induced by increases inwater vapour pressure deficit.—J. exp. Bot. 38: 1413–1420. The leaf to air water vapour deficit was increased suddenlyfrom about 1·0 to 2·5 IcPa for single leaves ofsoybean (Glycine max L. Merr.) plants held at 30 °C, 2·0mmol m ^–2 s^–1 photosynthetic photon flux density(PPFD) and carbon dioxide pressures saturating to photosynthesis.After a lag of about 10 min, photosynthetic rate and stomatalconductance to water vapour began to decrease, and then cycledin phase with each other. The period of the cydes was about20 min. During these cycles the substomatal carbon dioxide pressurewas constant in the majority of leaves examined, and was alwaysabove saturation for photosynthesis. Epidermal impressions showedthat most stomata changed in aperture during the cycles, andthat very few were ever fully closed. Water potential measuredon excised discs changed by at most 0·1 MPa from theminima to the maxima in transpiration rate. In contrast, forleaves of sunflower (Helianthus animus L.) grown at low PPFD,the increase in VPD led to leaf wilting and decreased photosynthesis,followed by recovery of turgor and photosynthesis as stomatalconductance began to decrease. In these leaves photosynthesisand conductance then cycled approximately 180° out of phase.It is suggested that in soybeans decreased leaf conductanceinduced by high VPD provided a signal which decreased the rateof photosynthesis at carbon dioxide saturation by a mechanismthat was not related to a water deficit in the mesophyll. Key words: Photosynthesis, stomatal conductance, cycling, vapour pressure deficit     

Stomatal Functioning of In Vitro and Greenhouse Apple Leaves in Darkness, Mannitol, ABA, and CO2

Leaves from in vitro and greenhouse cultured plants of Malusdomestica (Borkh.) cv. Mark were subjected to 4 h of darkness;4 h of 1 M mannitol induced water stress; 1 h of 10^–4M to 10^–7 M cis-trans abscisic acid (ABA) treatment; 1h of 0.12% atmospheric CO₂. Stomatal closure was determinedby microscopic examination of leaf imprints. In all treatments,less than 5% of the stomata from leaves of in vitro culturedplants were closed. The diameter of open stomata on leaves fromin vitro culture remained at 8 µm. In contrast, an averageof 96% of the stomata on leaves of greenhouse grown plants wereclosed after 4 h in darkness; 56% after 4 h of mannitol inducedwater stress; 90% after 1 h of 10^–4 M ABA treatment; 61%after 1 h in an atmosphere of 0.12% CO₂. Stomata of in vitroapple leaves did not seem to have a closure mechanism, but acquiredone during acclimatization to the greenhouse environment. Thelack of stomatal closure in in vitro plants was the main causeof rapid water loss during transfer to low relative humidity.     

Ethylene and 1 -Aminocyclopropane-1-Carboxylic Acid Production in flacca, a Wilty Mutant of Tomato, Subjected to Water Deficiency and Pre-treatment with Abscisic Acid

Neill, S. J., McGaw, B. A. and Horgan, R. 1986. Ethylene and1-aminocyclopropane-l-carboxylic acid production in flacca,a wilty mutant of tomato, subjected to water deficiency andpretreatment with abscisic acid —J. exp. Bot. 37: 535–541. Plants of Lycoperstcon esculentum Mill. cv. Ailsa Craig wildtype and flacca (flc) were sprayed daily with H²O or 2?10^–2mol m^–3 abscisic acid (ABA). ABA treatment effected apartial phenotypic reversion of flc shoots; leaf areas wereincreased and transpiration rates decreased. Leaf expansionof wild type shoots was inhibited by ABA. Indoleacetic acid (IAA), ABA and l-aminocyclopropane-l-carboxylicacid (ACC) concentrations were determined by combined gas chromatography-massspectrometry using deuterium-labelled internal standards ABAtreatment for 30 d resulted in greatly elevated internal ABAlevels, increasing from 1?0 to 4?3 and from 0?45 to 4?9 nmolg^–1 fr. wt. in wild type and flc leaves respectively.Endogenous IAA and ACC concentrations were much lower than thoseof ABA. IAA content ranged from 0?05 to 0?1 nmol g^–1 andACC content from 0?07 to 0?24 nmol g^–1 Ethylene emanationrates were similar for wild type and flc shoots. Wilting of detached leaves induced a substantial increase inethylene and ACC accumulation in all plants, regardless of treatmentor type. Ethylene and ACC levels were no greater in flc leavescompared to the wild type. ABA pretreatment did not preventthe wilting-induced increase in ACC and ethylene synthesis. Key words: ABA, ACC, ethylene, wilting, wilty mutants     

Abscisic Acid Content and Effects During Dehydration of Detached Leaves of Desiccation Tolerant Plants

Borya nitida is an angiospcrm whose detached leaves developcomplete tolerance to dehydration when they are equilibratedto air of 96% r.h. This treatment causes leaves to yellow aschlorophyll is destroyed, and abscisic acid contents increaseseveral-fold. Exogenous ABA (at 0.038–0.38 mol m^–3)promoted desiccation tolerance (a) in leaves undergoing toleranceinduction at 96% r.h., (b) only slightly during rapid dryingat rates which are normally injurious, and (c) considerablyin turgid tissue treated with ABA 48 h before rapid drying. ABA content also increased with intense water stress in Myrothamnusflabellifolia, a desiccation tolerant angiosperm which, unlikeBorya, retains most of its chlorophyll when dehydrated. Preliminaryincubation in ABA of detached leaves of this ‘resurrectionplant’ also promoted survival during rapid drying. Theability of ABA to substitute for the normal induction periodsuggests that this hormone participates in the development ofdesiccation tolerance. Key words: Abscisic acid, ABA, Drought tolerance, Resurrection plant     

Factors Affecting the Biosynthesis of Abscisic Acid

Incorporation of labelled mevalonate into abscisic acid (ABA)has been demonstrated in the cotyledons of mature avocado seeds,embryos and endosperms of developing wheat seeds, and avocadostems. The increase in ABA concentration on wilting parallelsthe increased incorporation of [2^–14C)mevalonate intoABA in avocado leaves and stems, suggesting that the increasein ABA content occurs by synthesis rather than by release froma stored precursor. Incorporation of [2^–14C]mevalonateby avocado mesocarp segments is unaffected by an 18 per centwater loss. The ABA content of roots was hardly affected bya 30 per cent water loss, indicating that the wilt-activatedmechanism is not fully operative in these tissues. Submerged Ceratophyllum plants and submerged parts of Callitricheshoots show a twofold increase in ABA content on wilting whereasthe aerial rosettes of the latter plant show a sixfold increase.This suggests that the occurrence of the wilt-induced mechanismis affected by previous growth conditions as well as by themorphology of the tissue.     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

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)     

Abscisic Acid Translocation and Influence of Water Stress on Grain Abscisic Acid Content

The movement of foliar applied [1-¹⁴C]abscisic acid (ABA) inwheat plants (Triticum aestivum L., cv. Kolibri) was investigatedat two stages of grain development (1000 grains, weight 19 and24 g dry matter). [1–¹⁴C]ABA seemed to be readily translocated within 12h into the developing grains as well as in other plant parts.A subsequent rapid metabolism took place leading to a decreasedactivity of the ABA-containing chromatogram fraction in theyounger plants 48 h after application. The metabolism seemodto be less intensive in the older grains, where the activityrunning with the ABA increased over 64 h. Treating the leaves of barley plants (Hordeum vulgare, L., cv.Union) 2 weeks after anthesis with a gentle stream of warm air(36° C) resulted in a significant increase in the ABA contentof all parts of the ear. The results mentioned above indicatethat this may be partially due to translocation from other partsof the plant such as the leaves.     

Long Distance Transport of Abscisic Acid in NaCI-Treated Intact Plants of Lupinus albus

Plants of Lupinus albus were grown for 51 d under control (1.1mol m^–3 NaCl) and saline (40 mol m^–3 NaCl) conditions.Plants were harvested and changes of carbon, nitrogen and abscisicacid (ABA) contents of individual organs were determined 41d and 51 d after germination. In the period between the twoharvests xylem and phloem saps were collected and respirationand photosynthesis of individual organs were measured. Usingflows of carbon, C/ABA ratios and increments of ABA flows ofABA in phloem and xylem and rates of biosynthesis and degradationof ABA were calculated. Both under control and saline conditionsnet biosynthesis occurred in the root, the basal strata of leavesand in the inflorescence. Metabolic degradation of ABA tookplace in the stem internodes and apical leaf strata. Salt stress increased xylem transport of ABA up to 10-fold andphloem transport to the root up to 5-fold relative to that ofthe controls. A considerable amount of ABA in the xylem saporiginated from biosynthesis in the roots, i.e. 55% in salt-treatedand smaller than 28% in control plants. The remaining part ofABA in the xylem sap originated from the shoot: it was translocatedin the phloem from fully differentiated leaves towards the rootand from there it was recirculated back to the aerial partsof the plant. The data suggest that ABA may serve as a hormonalstress signal from the root system. Key words: Lupinus albus, salt stress, abscisic acid, long distance transport     

Ineffectiveness of Abscisic Acid in Stomatal Closure of Yellow Lupin, Lupinus luteus var. Weiko III

Stomata of yellow lupin leaves are remarkably insensitive toabscisic acid (ABA). Stomatal resistance was monitored usingboth a viscous now porometer and a diffusion porometer. Resultswere confirmed with scanning electron microscopy. When exogenousABA solutions were supplied via petioles, 10^–6 M solutionshad no effect on stomatal resistance. Upper (adaxial) stomatawere not affected by 10^–5 M ABA but lower stomata showed3-fold more resistance after 2 h. Stomata of both surfaces closedafter 30 min in 10^–4 M ABA. Isolated epidermal peels of lupin leaves were floated on ABAsolutions yet upper surface peels showed no stomatal closingin 10^–4 M ABA, while lower surface stomata closed to abarely significant extent. Stomata of intact leaves were not very sensitive to darkness,showing at most a doubling in resistance after 6 h darkness.Complete stomatal closure, however, was readily produced bywilting leaves. Hence, lupin stomata are physically capableof closing. Endogenous ABA levels of water-stressed leaves increased approximately10-fold, which corresponds to concentrations below 10 µMABA. It is concluded that ABA is unlikely to play a role incontrolling short-term stomatal response of lupins.     

Effect of Abscisic Acid on Sorbitol Uptake in Growing Apple Fruits

Levels of abscisic acid (ABA) in the fruit flesh of developingapples (cv. Golden Delicious) were measured by electron capturegas chromatography. ABA content of the tissue, calculated ona fresh weight basis, decreased at a constant rate from 200µg g^–1 in young fruit to 10 µg g^–1 inolder fruit and then increased when the ripening process commenced.On a whole fruit basis, the ABA level increased during the initialphase of fruit growth, remained constant during the linear growthphase and increased again when fruits started to ripen. During fruit development the ABA content correlated with therate of sorbitol uptake, when measured after discs of fruittissue were incubated in [¹⁴C]sorbitol. Sorbitol uptake washigh during the initial growth phase and declined at a constantrate during fruit development. ABA present in the incubation medium stimulated sorbitol uptakeinto fruit tissue at concentrations higher than 10^–8 M,whereas indolyl-3-acetic acid had no effect on uptake. When comparing sorbitol uptake in different zones of young fruit,it was found that uptake was higher in discs of outer fruitlayers than in discs from inner fruit zones. Key words: Pyrus malus, Apple, Abscisic acid, Sorbitol     

Extrastomatal Control of Transpiration in Leaves of Yellow Lupin (Lupinus luteus var. Weiko III) after Drought or Abscisic Acid Treatment

Stornata of leaves of non-stressed yellow lupin plants wereclosed with phenylmercuric acetate (PMA) and viscous flow resistanceand apparent diffusive resistance were measured for both leafsurfaces. Viscous flow resistance was proportional to the ythpower of apparent diffusive resistance with y = 1.19 ±0.10 for the upper surface and y = 1.58 ± 0.11 for thelower surface, i.e. viscous flow resistance changed faster thanapparent diffusive resistance. However, in three separate experiments where lupin plants werewatered with 10^–4 M abscisic acid (ABA) for 1 week, they values were between 0.29 and 0.59 for the upper surface andbetween 0.18 and 1.00 for the lower surface. Similarly, drought-hardened plants showed y values between 0.45and 0.52 for the upper surface, and between 0.43 and 0.88 forthe lower surface. Both ABA. and drought-induced changes iny values were statistically different from PMA results at the1% confidence level. Upon rewatering, drought-stressed plants and ABA-treated plantsrecovered within 2 and 5 d respectively. Thus with both treatments there was a considerable increasein apparent diffusive resistance (and thus a decrease in transpiration)which could not be accounted for by a comparable increase inviscous flow resistance (i.e. stomatal closure). These results suggest that drought stress and ABA treatmentcause an extrastomatal resistance to transpiration in the leavesof yellow lupin plants.     

The Metabolism of Abscisic Acid

The light-catalysed isomerization of (+)-abscisic acid (ABA)to its trans isomer during isolation from leaves was monitoredby the addition of (±)-[2-¹⁴C]ABA to the extraction medium.(+)Trans-abscisic acid (t-ABA) was found to occur naturallyin rose (Rosa arvensis) leaves at 20µg/kg fresh weight;(+)-ABA was present at 594µg/kg. (±)-[2-¹⁴D]Trans-abscisicacid was not isomerized enzymically to ABA in tomato shoots. (±)-Abscisic acid was converted by tomato shoots to awater-soluble neutral product, ‘Metabolite B’, whichwas identified as abscisyl-ß-D-glucopyranoside. When(±)-[2-¹⁴C]trans-abscisic acid in an equimolar mixturewith (±)-[2-¹⁴C}ABA was fed to tomato shoots it was convertedto its glucose ester 10 times faster than was ABA. Trans-abscisyl-ß-D-glucopyrano8ide only was formedfrom (±)-[2-¹⁴C]t-ABA in experiments lasting up to 30h. Glucosyl abscisate was formed slowly from ABA and the freeacid fraction contained an excess of the unnatural (–).ABAas did the ABA released from abscisyl-ß-D-glucopyranosideby alkaline hydrolysis. The (+).ABA appeared to be the solesource of the acidic ‘Metabolite C" previously noted. The concentrations of endogenous (+)-, (+)-[2-¹⁴C]-, and (–)-[2-¹⁴C]ABAremaining as free acid, and also in the hydrolysate of abscisyl-ß-D-glucopyranoside,were measured by the ORD, UV absorption, and scintillation spectrometryof highly purified extracts of ABA from tomato shoots whichhad been supplied with racemic [2-^l4C]ABA.