Effects of Light/Dark on Anion Fluxes in Isolated Guard Cells of Commelina communis L.

The effects of light/dark on anion fluxes in isolated guardcells of Commelina communis L. have been studied, using ⁸²Brand ³⁶Cl. Transfer of open guard cells from light to dark hasno effect on the ⁸²Br influx, but produces a marked transientstimulation of ⁸²Br or ³⁶Cl efflux, similar to the effect ofsuch transfer on the ⁸⁶Rb fluxes, and to the effects on both⁸⁶Rb and ⁸²Br fluxes of adding ABA. On return of guard cellsto light, after the transient, there is a further reductionin Cl/Br efflux. It is argued that control of a specific processof ion extrusion is important in regulating the ability of guardcells to stay open. In three out of four batches of steady-statetissue labelled with ⁸²Br, the plasmalemma fluxes were highenough, relative to the tonoplast fluxes, for the efflux kineticsto be separable into two exponential components, allowing estimationof bromide contents in cytoplasm and vacuole (Q_c and Q_v), andfluxes at plasmalemma and tonoplast. With opening in light,Q_c increased by 3.9 ± 0.4 pmol mm^–2 µm^–1and Qy by 5.2 ± 0.6 pmol mm^–2 µm^–1(change in content per mm² of epidermis perµm change inaperture). Using rough estimates for the volumes of cytoplasmand vacuole these figures suggest that at 6.1 µm in thedark the concentrations were about 63 mol m^–3 in the cytoplasmand 35 mol m^–3 in the vacuole, rising to about 185 molm^–3 in the cytoplasm and 125 mol m^–3 in the vacuole,at 16.7 µm aperture in light. Neither increase can providean adequate increase in salt concentration to account for theosmotic change required, and some solute other than potassiumsalt must also be involved. In one experiment with 82Br andin the only experiment with 36Cl the plasmalemma flux was lower,and not high enough relative to the tonoplast flux to allowseparation of two phases in the efflux curves, and calculationof cytoplasmic and vacuolar contents and fluxes. The effectsof transfer from light to dark were, nevertheless, similar inboth types of tissue. Key words: Commelina communis L., Light/dark effects, Anion fluxes, Guard cells     

Effects of Light/Dark on Cation Fluxes in Guard Cells of Commelina communis L.

The effects of light/dark on cation fluxes in isolated guardcells of Commelina communis L. have been studied, using ⁸⁶RbCland ²²NaCl. Transfer to the dark has no effect on ⁸⁶Rb influx,but produces a marked transient stimulation of ⁸⁶Rb efflux,similar to that seen previously on adding ABA. The ⁸⁶Rb effluxfalls on return to light only during the period of stimulatedflux; after the transient, return to light has no effect onefflux. The ability to produce this transient stimulation ontransfer to the dark is recovered in a subsequent light period.In general, in Na-loaded cells, the stimulated efflux is notseen. and the cells do not close in the dark. The results arenot consistent with a simple permeability or potential change,but suggest a specific ion excretion activated by the transferto the dark. Key words: Commelina communis L., Light/dark effects, Cation flux, Guard cells     

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     

Ion Fluxes in 'Isolated' Guard Cells of Commelina communis L.

Ion fluxes have been measured in ‘isolated’ guardcells of Commelina communis L. using ⁸⁶RbCl and K⁸²Br, in epidermalstrips in which all cells other than guard cells have been killedby treatment at low pH. To avoid problems of slow free spaceexchange most fluxes have been measured at pH 3.9, at whichstomata open well in K(Rb) Cl(Br) and are stable for many hours.At pH 3.9 the intracellular ⁸⁶Rb exchanged as a single compartmentwith a half-time of 2–3 h, independent of external concentration(C_o). The influx of ⁸⁶Rb rose with concentration, to a V_maxof about 23 pmol mm^–2 h^–1. The efflux curve of ⁸²Brcould be well fitted by two exponential terms, with half-timesof 38 min (independent of C_o), and 5–35 h (falling withincreasing C_o). Bromide contents of cytoplasm and vacuole (Q_cand Q_v), and fluxes at plasmalemma and tonoplast, were calculatedfrom the efflux kinetics. Over C_o 20–60 mM, as the apertureincreased from 7 µm to 17 µm, the tonoplast flux(0.5–11.5 pmol mm^–2h^–1) was always much lessthan the plasmalemma flux (7–77 pmol mm^–2 h^–1).Q_c and Q_v both increased with aperture. The increase in Q_c of10.3 pmol mm^–2 µm^–1 is adequate to accountfor the osmotic changes required to change the aperture, aspreviously estimated. However, the change in vacuolar contentof only 5.9 pmol mm^–2 µm^–1 is much too smallto account for the osmotic changes required, or to balance thecytoplasmic changes. It appears therefore that increasing KBroutside not only increases the cytoplasmic salt content, andthe Br flux at the tonoplast, but also stimulates the vacuolaraccumulation of some other solute.     

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     

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)     

Water relations and abscisic acid levels of watermelon as affected by rooting volume restriction

Rooting volume restriction (RVR) reduces shoot growth of plantsprovided with sufficient water or nutrients. The effects ofRVR on water status, abscisic acid (ABA) levels in leaves, roots,or xylem sap from detopped plants of watermelon [Citrullus lanatus(Thunb.) Matsum. and Nakai ‘StarBrite’] seedlingswere evaluated with five rooting volumes (18, 26, 36, 46, or80 cm3). Shoot water potential increased with increasing rootingvolume, with no difference between plants from 18 and 26 cm³cells or between plants from 36 and 46 cm³ cells. Stomatal conductancewas not consistently affected by RVR; at 10 and 20 DAE, stomatalconductance in plants grown in 36 cm³ cells was higher thanthat of plants grown in any other cell volume. Severe RVR (18and/or 26 cm³) tended to produce plants with higher ABA levelsin roots (15 DAE only), xylem sap (all dates), and leaves (5and 10 DAE). Plants grown in 18 and 26 cm³ cells had higherroot ABA levels than those from 46 and 80 cm³ cells at 15 DAE.Plants grown in 18 cm³ cells had the highest xylem sap ABA levelat all dates, but ABA levels did not differ among plants grownin the other cell volumes. Plants grown in 18 cm³ cells at 5DAE and 18 and 26 cm³ cells at 10 DAE also had higher leaf ABAlevels than those from other rooting volumes. The results suggestthat ABA may act as a signal for reduced growth of plants underRVR conditions. Key words: Abscisic acid, ABA, root signals, root volume restriction, water relations     

ABA-induced ion efflux in stomatal guard cells: multiple actions of ABA inside and outside the cell

Abscisic acid (ABA) induces a transient stimulation of ⁸⁶Rb⁺ efflux from isolated guard cells of Commelina communis L. The form of the efflux transients produced in suboptimal conditions (low concentrations of ABA and/or high external pH at which ABA will penetrate poorly) has been compared with the full transient. In suboptimal conditions the stimulation of efflux is both delayed and reduced. The pH-dependence of the delay before initiation of the efflux transient suggests that a threshold internal concentration of ABA is required. However in suboptimal conditions even when the threshold internal concentration is reached and a transient is triggered, the degree of stimulation is reduced, an effect which also appears to depend on internal ABA. It is suggested that the differences reflect activation of different numbers of tonoplast ion channels for release of vacuolar ions. By contrast, the same end-state seems to be reached in optimal and suboptimal conditions, but after different times. The relative efflux stimulation during the efflux transient tracks the declining ion content; both the peak and the end of the transient are reached at the same ion content, but at different times. It is suggested that this reflects an ABA-induced change in the set-point of a regulated ion channel which is sensitive to ion content, perhaps a stretch-activated channel. This effect is independent of external concentration in the range 0.1–10 µM, and pH 6 and pH 8 are equally effective, suggesting an external site of action. Thus the results suggest multiple actions of ABA, involving both internal and external receptors. Regulation of both tonoplast ion channels by internal ABA, and of a regulated channel responsive to ion content by external ABA are suggested.     

Carrier-Mediated ABA Uptake by Suspension-Cultured Phaseolus coccineus L. Cells: Stereospecificity and Inhibition by Ionones and ABA Esters

Astle, M. and Rubery, P. 1987. Carrier-mediated ABA uptake bysuspension-cultured Phaseolus coccineus L. cells: Stereospecificityand inhibition by ionones and ABA esters.—J. exp. Bot.38: 150–163. The substrate for the abscisic acid (ABA) carrier in Phaseoluscoccineus L. suspension-cultured cells is shown to be the (S)ABAenantiomer, K_m = 1?0 mmol m^–3. The methyl (MeABA) andphenyl (PheABA) esters of ABA inhibit carrier-mediated uptakeof ABA with half-maximal inhibition achieved at about 7?0 mmolm^–3 and 10 mmol m^–3 respectively: with (S)MeABAthis value is decreased to about 2?0 mmol m^–3. There isno demethylation of radioactive MeABA by the cells during 5min incubations. Although MeABA reversibly inhibits the ABAcarrier, it is not a transport substrate: association of radioactiveMeABA with living cells is unaffected by non-radioactive MeABAor ABA and, by comparison with frozen-and-thawed cells, it isshown that the radioactivity remains extracellular. It is proposedthat MeABA binds to the carrier to form an abortive complexthat is not translocated. The terpenoid ABA analogue LAB 144143also inhibits carrier-mediated ABA uptake. At concentrationsup to about 20 mmol m^–3 - and ß-ionone specificallyinhibit the ABA carrier with the half-maximal effect at about0?6 mmol m^–3 ß-ionone. However, at higher iononeconcentrations, the uptake of ABA, indol-3-yl acetic acid andof 5,5-dimethyloxazolidine-2,4-dione (DMO) are all stimulated:this may reflect general permeabilization of the membrane toweak acids by ionone. Key words: Uptake carrier, abscisic acid, methyl and phenyl esters of ABA, ionone, Phaseolus coccineus L. suspension culture     

Effect of Phenolic Compounds on ABA-induced Changes in K+ Concentration of Guard Cells and in Epidermal Diffusive Resistance

The study of the structure-activity relationship of phenoliccompounds in reversing the ABA-effect on stomata led us to investigatethe changes in K⁺ concentrations in guard cells and in the epidermaldiffusive resistance of leaves, after treatment with ABA andphenolics. The amount of potassium localized in guard cells usually correspondsto stomatal aperture in different treatments. Umbelliferone,however, permits stomatal opening without retention of potassiumin the guard cells, which is an exception. The effect of phenolicsin retaining K⁺ in epidermal peels is matched by recorded epidermaldiffusive resistance changes in the leaves.Although flavonoidsand some other phenolics behave differently showing recoveryin epidermal peels with K⁺ in guard cells, epidermal diffusiveresistance is not recovered. Key words: Epidermal diffusive resistance, K⁺, ABA, phenolics, stomata     

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     

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     

Effects of Abscisic Acid on Nucleic Acid Synthesis and the Induction of Nitrate Reductase in Lemna polyrhiza

Abscisic acid (ABA) at low concentrations brings about the formationof turions (dormant fronds) in Lemna polyrhiza within 3 to 5days after application. The incorporation of ³H-thymidine intoDNA, separated by polyacrylamide-gel electrophoresis, is inhibitedby 80 to 90 per cent within 1 to 3 h of ABA application. Theincorporation of ¹⁴C-orotate and ³²P into RNA is not inhibiteduntil 3 to 9 h after ABA application, but 70 per cent inhibitionis reached after 24 h on 10^–5 M ABA. There is little inhibitionof ¹⁴C-leucine incorporation into protein until 2 to 3 daysafter application of ABA. The capacity of nitrate to inducenitrate reductase in cultures previously grown on nitrate-freemedium is not affected by ABA even up to 3 days after application.The results are discussed in relation to the mode of actionof ABA.     

Effect of abscisic acid on elongation and kinetin-induced tuberization of isolated stolons of Solarium tuberosum L.

The response of isolated stolons cultured in vitro, to abscisicacid (ABA) has been studied in the presence and absence of kinetin(6-furfurylaminopurine). ABA alone in concentrations from 7.5x 10^–4 mM to 7.5 x 10^–2 mM, inhibited stolon elongationbut failed to promote tuber initiation. In the presence of kinetin,ABA at concentrations of 3.0 x 10^–2 and 7.5 x 10^–2mM markedly inhibited kinetin-induced tuber initiation and stolonelongation, but at 7.5 x 10^–4 and 7.5 x 10^–3 mMABA did not prevent tuber initiation. When stolons were incubated on a medium containing kinetin andlater transferred to one containing ABA with or without kinetin,the inhibitory effect of ABA decreased appreciably as the timeof incubation on kinetin is increased. The results are discussed in relation to the role of ABA inthe inhibition of nucleic acid and protein synthesis and theinteraction with cytokinins and the possible effect of ABA onkinetin uptake, transport and accumulation at the locus of action. (Received February 26, 1969; )     

Exogenous ABA as a Modulator of the Response of Sorghum to High Salinity

Treatment of Sorghum bicolor (L.) Moench, cv. 610, with abscisicacid (ABA) during the first week of sahnization with 150 molm^–3 NaCl induced enhancement of growth and acceleratedadaptation to high salinity (300 mol m^–3 NaCl) Adaptationis defined as the development of the ability of the plant tosurvive, grow, and set seeds upon exposure to a NaCl concentrationwhich is lethal for the unadapted plant In the absence of ABAthe saline pretreatment requires 20 d for the development ofadaptation (Amzallag et al., 1990), with ABA treatment the sameresult is achieved within approximately one week The exposureof the plants to non-lethal salinity (150 mol m^–3 NaCl)apparently triggers a transient sensitivity to ABA lasting forabout 8 to 10 d following the beginning of sahnization Thisperiod coincides with an increase in leaf PEP carboxylase activitywhich seems to occur faster if the plants are treated with ABA.Exogenous ABA-induced enhancement of growth and control of shootNa⁺ concentration, occur at a lower ABA concentration (10 mmolm^–3) than the induction of adaptation to salinity whichoc curs at 40 mmol m^–3 or above. The lowered shoot Na⁺concentration which is induced by a low ABA concentration isnot sufficient to induce survival of the plants in high salinity(300 mol m^–3 NaCl). Key words: Growth, adaptation to salinity, ABA     

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)     

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)     

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     

The Stability of Conjugated Abscisic Acid during Wilting

Tomato and silverbeet shoots that had been fed (±)-[2-¹⁴C]abscisicacid (ABA) some days previously did not hydrolyse their alkali-saponifiable[¹⁴C]ABA fraction when they wilted. The content of alkali-saponifiableABA in untreated plants did not fall on wilting so the endogenousconjugate is not hydrolysed either. In tomato and silverbeet,therefore, conjugation of ABA is irreversible.     

Induction of Cold Hardiness by Salt Stress Involves Synthesis of Cold- and Abscisic Acid-Responsive Proteins in Potato (Solanum commersonii Dun)

Salt stress has been found to increase frost tolerance in someherbaceous species. In an attempt to understand the molecularbasis of the frost tolerance induced by salt stress, the effectof salt (100 mM NaCl) on total proteins in stem-cultured potatoplantlets (Solarium commersonn Dun) was analyzed on two-dimensionalgels. Nine salt-induced proteins were identified after 24 hsalt treatment, at which time cold hardiness increased by threedegrees. Direct comparisons of the proteins with those inducedby cold- and abscisic acid(ABA)-treatments revealed that fiveof the salt-induced proteins were also induced by cold(4°/2°C)-treatmentand seven were also induced by ABA(40µM)-treatment. Threeproteins (M_rr/pls 13/7.0, 27/6.6 and 48/6.9) were induciblein both cold- and ABA-treatments in association with frost hardening.After 6 h salt treatment, endogenous ABA levels in plantletleaves showed a transient six-fold increase before cold hardinessdeveloped. The results suggest that salt-induction of cold hardinessinvolves the synthesis of cold and ABA-responsive proteins andthe alteration of protein synthesis is mediated by ABA elevatedupon salt stress. This study also suggests that a subset ofproteins induced by cold and ABA-treatments are related to saltstress. ¹Scientific Journal Series Paper No. 20787 of the MinnesotaAgricultural Experimental Station, St. Paul, MN 55108, U.S.A. ²Present address: Department of Biochemistry Willard Hall, KansasState University, Manhattan, KS 66506, U.S.A. ³Present address: Via G.B. Marino 13, 80125-Napoli, Italy ⁴Present address: Institute of Biological Chemistry, WashingtonState Univ., Pullman, WA 99164, U.S.A.