Derivation of Pressure-Volume Curves by a Non-Linear Regression Procedure and Determination of Apoplastic Water

Data from pressure-volume (PV) analysis may be submitted totransformation I [i.e. leaf water potential (₁) versus inverserelative water content (1/R)] or to transformation II (i.e.1/₁ versus R). This may cause an essential distortion of theerror structure especially in transformation II due to the relativelylarge range which is to be covered by the 1/₁ ratio. Similarly,logarithmic transformation of leaf turgor potential (_P) whenderiving the sensitivity factor of elasticity (ß)by linear regression from values of In _p and 1/R may distortthe error structure. In order to investigate the magnitude ofthe distortion effect on parameters derived from PV analysisby regression a non-linear regression procedure was comparedwith the common linear procedure when calculating _p from ßin the turgid region and leaf osmotic potential (_P) in boththe turgid and non-turgid region. As test plants we used fieldgrown species of spring barley (Hordeum distichum L., cvs Gunnarand Alis). The results show that transformations and applicationof linear regression procedures distort the error structureof _p more than the error structure of _', which was only slightlyaffected. However, we recommend the use of the non-linear procedurein both cases. Furthermore, from PV analysis, obtained by thermocouple hygrometryon living and killed leaf tissue, respectively, we derived themathematical basis for calculating the apoplastic water fraction(R_a). R_a was 0.15 at R= 1 and decreased with dehydration. The equations describing the relation between and R and between_p and R were extended to take into account the apoplastic waterfraction. Key words: Apoplastic water, distortion errors, non-linear regression, pressure-volume curves     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: II. GERMINATION AT REDUCED WATER POTENTIAL

Seed germination rates (GR =inverse of time to germination)are sensitive to genetic, environmental, and physiological factors.We have compared the GR of tomato (Lycopersicon esculentum Mill.)seeds of cultivar T5 to those of rapidly germinating L. esculentumgenotypes PI 341988 and PI 120256 over a range of water potential(). The influence of seed priming treatments and removal ofthe endosperm/testa cap enclosing the radicle tip on germinationat reduced were also assessed. Germination time-courses atdifferent 's were analysed according to a model that identifieda base, or minimum, allowing germination of a specific percentage(g) of the seed population (_b(g)), and a ‘hydrotime constant’(_H) indicating the rate of progress toward germination per MPa.h.The distribution of _b(g) determined by probit analysis was characterizedby a mean base (_b) and the standard deviation in _b among seeds(_b). The three derived parameters, _b, _b) and _H, were sufficientto predict the time-courses of germination of intact seeds atany . A normalized time-scale for comparing germination responsesto reduced is introduced. The time to germination at any (t_g())can be normalized to be equivalent to that observed in water(t_g(0)) according to the equation t_g(0)=[l–(/_b(g))]t_g().PI 341988 seeds were more tolerant of reduced and had a morerapid GR than T5 seeds due to both a lower _b and a smaller _H.The rapid germination of PI 120256, on the other hand, couldbe attributed entirely to a smaller _H. Seed priming (6 d in–1.2 MPa polyethylene glycol 8000 solution at 20 ?C followedby drying) increased GR at all >_b(g), but did not lower theminimum allowing germination; i.e. priming reduced _H withoutlowering _b. Removing the endosperm/testa cap (cut seeds) markedlyincreased GR and lowered the mean required to inhibit germinationby 0.7 to 0.9 MPa. However, this resulted primarily from downwardadjustment in _b during the incubation of cut seeds at low inthe test solutions. The difference in _b between intact and cutseeds incubated at high was much less (0.l MPa), indicatingthat at the time of radicle protrusion, the endosperm had weakenedto the point where it constituted only a small mechanical barrier.In the intact seed, endosperm weakening and the downward adjustmentin embryo _b ceased at < –0.6 MPa, while the reductionin _H associated with priming proceeded down to at least –1.2MPa. Based on these data and on the pressure required to pushthe embryos from the seeds at various times after imbibition,it appears that the primary effect of priming was to shortenthe time required for final endosperm weakening to occur. However,as priming increased GR even in cut seeds, priming effects onthe embryo may control the rate of endosperm weakening. Key words: tomato, Lycopersicon esculentum Mill., water potential, germination rate, seed priming, genetic variation     

The Meaning of Matric Potential

The commonly used equation, = P - + , which describes thepartitioning of plant water potential, , into components ofhydrostatic pressure, P, osmotic pressure, , and matric potential,, is misleading. The term , which is supposed to show the influenceof a solid phase on , is zero if a consistent definition ofpressure is used in the standard thermodynamic derivation. However,it can be usefully defined by = + _D, where _D is the osmoticpressure of the equilibrium dialysate of the system. The practicaland theoretical significance of this definition is discussed.     

The Derivation of the Cell Wall Elasticity Function from the Cell Turgor Potential

An equation is derived expressing average turgor pressure ofa leaf (_p) as a function of relative water content (RWC). Basedon this derivation, the relationships of the bulk elastic modulus(_v) and both RWC and _p, are formulated and discussed. The bulkelastic modulus (_v) becomes zero for _p = 0, that is at the turgorloss point for the leaf. At full water saturation the valueof ev is proportional to the water saturation turgor potential_p(max). The factor relating _P and _v (structure coefficient ,Burstrom, Uhrstr?m and Olausson, 1970) changes only very littlefor values of _p, which are not too close to zero. An exampleis given for the calculation from experimental data of the turgorpressure function, the structure coefficient function, and the_v function. Key words: Cell wall, Turgor pressure, Bulk elastic modulus     

Alterations in the Components of Peanut Leaf Water Potential during Desiccation

Changes in components of leaf water potential during soil waterdeficits influence many physiological processes. Research resultsfocusing on these changes during desiccation of peanut (Arachishypogeae L.) leaves are apparently not available. The presentstudy was conducted to examine the relationships of leaf water_l, solute _s and turgor _p potentials, and percent relative watercontent (RWC) of peanut leaves during desiccation of detachedleaves and also during naturally occurring soil moisture deficitsin the field. The relationship of _p to _l and RWC was evaluated by calculating_p from differences in _l and _s determined by thermocouple psychrometryand by constructing pressure-volume (P-V) curves from the _land RWC measurements. Turgor potentials of ‘Early Bunch’and ‘Florunner’ leaves decreased to zero at _l of–1.2 to –1.3 MPa and RWC of 87%. There were no cultivardifferences in the _l at which _p became zero. P-V curves indicatedthat the error of measuring _s after freezing due to dilutionof the cellular constituents was small but resulted in artefactualnegative _p values. Random measurements on two dates of _l, _s, and calculation of_p from well-watered and water-stressed field plots consistingof several genotypes indicated that zero _p occurred at _l of–1.6 MPa. It was concluded that the relationships of _p,_l, _s, and RWC of peanut leaves were similar to leaves of othercrops and that these relationships conferred no unique droughtresistance mechanism to peanut.     

Physiological Responses to Drought of Lolium perenne L.: Measurement of, and Genetic Variation in, Water Potential, Solute Potential, Elasticity and Cell Hydration

Thomas, H. 1987. Physiological responses to drought of Loliumperenne L.: Measurement of, and genetic variation in, waterpotential, solute potential, elasticity and cell hydration.—J.exp. Bot. 38: 115–125. Clonally-replicated genotypes of Loiium perenne L. were grownin a controlled environment. Leaf water potential (_w) osmoticpotential (_s), turgor potential (_p = _w – _s), elasticity(E), leaf hydration (g water per g dry matter, H) and numberof green leaves per tiller (N_GL) were measured before and duringa 42 d drought treatment. A simplified method of estimating E (at _w < 1?0 MPa) usingonly six measurements was developed to permit a measurementrate of 8 leaves per hour. Measurement errors in all characterswere 3% or less. During drought, _w and _s (at _w = 0?5 MPa) decreased significantly,_p and E increased significantly, and H decreased slightly. Plantsize during drought was negatively correlated with _s, and ^Hand positively correlated with _p, osmotic adjustment, E andN_GL. Measurements made on the genotypes before draughting didnot give a reliable indication of their physiological conditionafter adaptation to drought. Genetically controlled variation (‘broad sense heritability’)of drought-adapted plants for E was 15%, _w 23%, _s, 34%, _p, 35%,H 34% and N_GL 64%. The possibilities for, and effectivenessof, divergent selection of genotypes with high and low expressionof the characters are discussed. Key words: Water relations, Lolium, genetic variation     

Abscisic Acid Accumulation and Water Relations of Four Cultivars of Phaseolus vulgaris L. under Drought

Larqué-Saavedra, A., Rodriguez, M. T., Trejo, C. andNava, T. 1985. Abscisic acid accumulation and water relationsof four cultivars of Phaseolus vulgaris L. under drought.—J.exp. Bot 36: 1787–1792. Plants of four cultivars of Phaseolus vulgaris L. differingin drought resistance were grown in pots under greenhouse conditionsand prior to flowering water was withheld from the pots untilthe mid-day transpiration rate reached values below 1.0 µgH₂O cm^–2 s^–1 (designated the ‘drought’stage). At this point leaves were harvested on 3 or 4 occasionsover 24 h to determine the abscisic acid (ABA) concentration,total water potential (), solute potential (₁) and turgor potential(_p). Results showed that values of , ₁, and _p differed between cultivarswhen they reached the ‘drought’ stage. The stomatalsensitivity to changes in and _p, was as follows: Michoacán12A3 > Negro 150 Cacahuate 72 > Flor de Mayo. These datacorrelated well with the pattern of drought resistance reportedfor the cultivars. ABA accumulation at the ‘drought’ stage differedbetween cultivars at each sampling time, but overall differencesin ABA level between cultivars were not significant. ABA levelsdid not, therefore, correlate with the drought resistance propertiesreported for the cultivars. Results are discussed in relationto and hour of the day when bean samples were taken for ABAanalysis. Key words: Phaseolus vulgaris L., drought resistance, abscisic acid     

Water Movement from Soil to Root Investigated through Simultaneous Measurement of Soil and Stem Water Potential in Potted Trees

Legge, N. J. 1985. Water movement from soil to root investigatedthrough simultaneous measurement of soil and stem water potentialin potted trees.—J. exp. Bot. 36: 1583–1589. Osmotic tensiometers implanted in the stems of three mountainash (Eucalyptus regnans F. Muell.) saplings growing in largeplastic bins recorded stem water potential, _w, while soil waterpotential, _w, was simultaneously recorded by instruments nearthe trees' roots and in the surrounding root-free soil Earlyin a drying cycle, with the soil still wet, the diurnal variationin ₁, was often slight, despite diurnal variations in _u approaching2.0 M Pa. Late in a drying cycle the diurnal fluctuations in₁, and _u were very similar although changes in ₁, still laggedup to 1.5 h behind changes in _u. ₁values at this time occasionallyreached –3.0 MPa with no apparent damage to the treesWatering the bins in daytime led to a response in ₁, valueswithin about 5 min, whereas _u, values did not respond for afurther 20 min. _u values then rose rapidly but after only 1h began to decline again, while ₁, values remained at or nearsaturation for the rest of the day. Water uptake hypotheseswhich attribute an important role to a soil-root interface resistanceare not supported by these data Key words: —Soil water potential, penrhizal gradients     

An Error in the Calibration of Xylem-water Potential against Leaf-water Potential

An error occurs in the calibration of xylem pressure potential() against leaf-water potential () when the calibration is madeusing plant material in which the water stress has been inducedartificially after excision. The impostion of water stress afterexcision affects the determination more than it affects , consequentlythe relationship between these two indices of water stress isaltered. Care should be exercised to ensure that identical proceduresare adopted during . calibrations and during susbsequent fieldmeasurements of with the pressure-chamber apparatus.     

Analysis of the diurnal change in osmotic potential in leaves of Fraxinus excelsior L.

The daily cycle of the transpiration rate, stomatal conductance,water potential, and the concentration of the main osmoticumidentified in ash leaves, malate and mannitol, were monitoredin a field on the Isere river plain. On sunny days, the stomatalconductance tends to remain close to its maximun value allowinga high transpiration rate and diurnal variations in leaf waterpotential, _w, which may fall as low as –2 MPa at solarnoon. These variations of _w are closely correlated with changesin malate, mannitol and the concentration of the well-knownosmoticum K⁺, which agree with the involvement of an osmoticadjustment to counteract the evaporative demand during daylighthours. How malate, mannitol and K⁺ contribute to the osmoticadjustment was analysed subsequently by comparing the solutepotential _s, evaluated by the Boyle-Van't Hoff relation, tothe osmotic potential measured by thermocouple psychrometry.These experiments have led us to suspect some errors in themeasurement of , presumably due to experimental artefacts andthe ability of Ca ²⁺ , present in high levels in leaves, toform chelates with malate once the cells have been decompartmentedby freezing and thawing. Since significant changes of Ca²⁺ alsooccurred during the diurnal variations of _w, the possible mechanismsby which Ca²⁺ may be implicated in controlling the water statusof the tree are discussed. Key words: Fraxinus excelsior L, osmotic adjustment, thermocouple psychrometry, malate, calcium     

Characterization of Spring Wheat Genotypes Differing in Drought-Induced Abscisic Acid Accumulation: II. LEAF WATER RELATIONS

Aspects of the water relations of spring wheat (Triticum aestivumL.) are described for cultivars Highbury (low ABA) and TW269/9(high ABA), and low and high ABA accumulating F⁶selections derivedfrom a cross between them. In a pot experiment, pressure-volume (P-V) curves were constructedfor main stem leaf four (MSL4) of well-watered plants of Highburyand TW269/9. Estimates of solute potential (²) from these curveswere similar for the two cultivars, but varied with the timeof sampling and the time allowed for hydration in dim light. In a field experiment with four low and four high ABA F⁶lines,P-V curves for flag leaves from both droughted and irrigatedplants gave at both zero turgor (^p) and zero water potential(¹) which differed with degree of stress, sampling time andgenotype. ¹was strongly dependent on the initial_L of the leafand was reduced on average by c. 0.4 MPa per MPa decline ininitial L.5, was lower (more negative) by c. 0.1-MPa in theafternoon than in the morning. Overall, was also 0.1 MPa lowerin low ABA lines than in high ABA lines. In another field experiment, flag leaves of five low and fivehigh ABA F⁶lines were sampled over a 4 week period from droughtedplots and _L and 5, measured (the latter by osmometry with expressedsap). For these leaves 5, at zero p or zero _L was consistentlylower by 0.3–0.5 MPa than estimates of 5, from the P-Vcurves with flag leaves. However, data for the low ABA lineswere again lower (by c. 0.1 MPa) than those for high ABA lines. The consequences of these differences in 1 are discussed inrelation to the stimulation of ABA accumulation in low and highABA selections. Key words: Water potential, Solute potential, P-V curves, Wheat (Triticum aestivum), Drought stress     

Alternative Methods of Analysing Water Potential Isotherms: Some Cautions and Clarifications: I. THE IMPACT OF NON-IDEALITY AND OF SOME EXPERIMENTAL ERRORS

In recent years alternative ways have been proposed to transformmeasurements of leaf water potential, , and relative water content,R^*, in order to derive values of osmotic pressure at full turgidityin leaves and shoots, ^o(when 0). Two types of transformationsare usually considered: 1/ versus R^* and versus 1/R^*, and linearregression is used to fit the data in the region where turgoris thought to be zero. It appears that when ^o is estimated bylinear extrapolation of 1/Psi; versus R^* then apoplastic watermight not influence the accuracy of ^o but when the versus \/R^*transformation is used apoplastic water causes an underestimateof ^o. We examine the accuracy of the estimate of ^o obtainedfrom the two transformations when there are random errors in, systematic errors in , and when the osmotic solutions arenon-ideal. The 1/ versus R^* transformation generally producesthe best estimate of ⁰ by linear extrapolation.     

Genotypic Differences in Leaf Water Relations between Brassica juncea and B. napus

Various kinds of measurement of tissue water status were madeseveral times during water stress and recovery in Brassica juncea(cv Canadian Black) and B napus (cv Drakkar) Unstressed plantsof the two species had similar leaf water potentials (_w), solute(_s) and turgor potentials (_p) Values of relative water content(RWC) and the slope of the linear relationship between _p andRWC (_p/RWC) were greater in B napus than in B juncea Statistical correlations of pooled data for the watered andstressed treatments differentiated the relationships among RWC,_w and its components in the two species The major statisticaldifference was that _p/RWC was related to RWC in B napus andto _w and _s in B juncea A decline in _p/RWC with decreasing _sin B juncea may be a mechanism for maintaining _p at low soilwater potentials through maintenance of more elastic cell walls. Brassica juncea, Brassica napus, osmotic adjustment, tissue elasticity, water relations     

Respiration and germination rates of tomato seeds at suboptimal temperatures and reduced water potentials

Suboptimal temperature (T) affects germination rates (reciprocalof time to radicle emergence) on a thermal time basis; thatis, the T in excess of a base or minimum temperature multipliedby the time to a given per cent germination [t_g) is a constant.Respiration rates are also sensitive to T, and proportionalrelationships are often found between respiration rates andgermination rates. Reduced water potential () delays seed germinationon a hydrotime basis (i.e. the in excess of a base water potentialmultiplied by t_g is a constant). It was tested whether respirationrates prior to radicle emergence vary in proportion to T and as expected from the thermal and hydrotime models. Respirationrates (C0₂ evolution) of cold-tolerant, rapidly germinating(PI 341988) and cold-sensitive, more slowly germinating (T5)tomato [Lycopersicon esculentum Mill.) seeds were evaluatedover a range of T and conditions. For both genotypes, respirationrates until the beginning of radicle emergence were relatedto T on a thermal time basis and increased approximately linearlywith above -2.0 MPa, consistent with the hydrotime model. Respirationrates were uniquely related to germination rates, regardlessof whether germination timing was affected by T, , or genotype.However, germination timing was unaffected when respirationrates were manipulated by varying 0₂ partial pressure. Thus,while both germination and respiration rates vary with T and consistent with thermal and hydrotime models of biologicaltime, respiration rates per se were not the limiting factorin germination timing of tomato seeds. Key words: Lycopersicon esculentum Mill., tomato, germination, respiration, temperature, water potential     

Water Relations of Chromium VI Treated Bush Bean Plants (Phaseolus vulgaris L. cv. Contender) under both Normal and Water Stress Conditions

The effect of Chromium VI on leaf water potential (^w), solutepotential (^a), turgor potential (^p) and relative water content(RWC) of primary and first trifoliatc leaves of Phaseolus vulgarisL. was studied under normal growth conditions and during anartificially induced water stress period in order to establishthe possible influence of this heavy metal on the water stressresistance of plants. Plants were grown on perlite with nutrientsolution containing 0, 1•0, 2•5, 5•0 or 10•0µg cm^–3 Cr as Na₂Cr₂O₇.2H₂O. The effect of Cr onwater relations was highly concentration dependent, and primaryand first trifoliate leaves were affected differently. The growthreducing concentrations of Cr (2•5, 5•0 and 10•0µg cm^–3) generally decreased _s and _w and increased_p in primary leaves. The 1•0 µg cm^–3 Cr treatmentdid not affect growth, but altered water relations substantially:in primary leaves _w and _p were increased and _s decreased, whilein trifoliate leaves the effect was the opposite. All Cr treatedplants resisted water stress for longer than control plants.The higher water stress resistance may be due to the lower _sand to the increased cell wall elasticity observed in Cr VItreated plants. Key words: Phaseolus vulgaris, Chromium VI, water stress, Richter plot     

A Transient Stimulation of Net Photosynthesis of Rye Leaves by {alpha}-Hydroxy-2-pyridinemethanesulphonic Acid ({alpha}-HPMS) due to Inhibition of Photorespiratory CO2 Release

The effects of -hydroxy-2-pyridinemethanesulphonic acid (-HPMS)upon net photosynthesis (P_n, the CO₂ compensation point (),post-lower illumination burst of CO₂ (PLIB) and post-lower temperatureburst of CO₂ (PLTB) in detached rye (Secale cereale L.) leaveswere investigated. At low concentrations ( 0.5 mol m^–3),-HPMS initially stimulated P_n and decreased the magnitude ofboth PLIB and PLTB. The decreased at all concentrations of-HPMS (0.05–5.0 mol m^–3. The effects of -HPMS onP_n and were time-dependent and, after a few minutes, the P_nwas inhibited while values increased considerably. At a higherconcentration (5.0 mol m ^–3), the transient effects of-HPMS were shorter () or not observed at all (P_n. Both PLIBand PLTB, when expressed in relation to P_n, increased at higherlevels of this compound. Similar data with respect to the effectsof -HPMS on PLIB and PLTB were found for leaves of dandelion(Taraxacum officinale L.). The results suggest that -HPMS may stimulate P_n by inhibitingphotorespiration, as originally suggested by Zelitch (1966),but only at low concentrations and over a short time span. Thedecrease of PLIB and PLTB values at low -HPMS levels is consistentwith these processes being a residual activity of the glycolatepathway. Key words: CO₂ compensation point, -hydroxy-2-pyridinemethanesulphonic acid, photorespiration, photosynthesis     

Interspecific Grafts Demonstrate Root System Control of Leaf Water Status in Water-Stressed Phaseolus

We examined the importance and the mechanisms of the root systems'effect on leaf water status in two bean species: Phaseolus vulgarisL. cv. Redcloud (Pv) and P. acutifolius Gray MN cultivatedaccession 258/78 (Pa). Pa maintains a higher leaf water potential(₁) than Pv. We used reciprocal grafts between the two species.We grew four plants (one of each graft combination) in one potso they experienced the same soil water potential. Shoot genotypedetermined ₁ of well-watered plants. Root genotype determined₁ of the most stressed plants. Stressed Pa root systems increased₁ of Pv shoots by 0·1 MPa over Pv shoots on Pv roots.Pa roots did not maintain by affecting stomatal conductancenor by simply having more dry weight. Pa roots may have greaterhydraulic conductivity than Pv roots. Key words: Phaseolus acutifolius, Phaseolus vulgaris, leaf water potential, root-shoot communication     

Integrated likelihood functions for non-Bayesian inference

Consider a model with parameter = (, ), where is the parameterof interest, and let L(, ) denote the likelihood function. Oneapproach to likelihood inference for is to use an integratedlikelihood function, in which is eliminated from L(, ) by integratingwith respect to a density function (|). The goal of this paperis to consider the problem of selecting (|) so that the resultingintegrated likelihood function is useful for non-Bayesian likelihoodinference. The desirable properties of an integrated likelihoodfunction are analyzed and these suggest that (|) should be chosenby finding a nuisance parameter that is unrelated to and thentaking the prior density for to be independent of . Such anunrelated parameter is constructed and the resulting integratedlikelihood is shown to be closely related to the modified profilelikelihood.     

Water Relations of Potato Leaves. II. Pressure--Volume Analysis and Inferences About the Constancy of the Apoplastic Fraction

Water relations characteristics of potato (Solanum tuberosumL. cv. Bintje) leaves were determined from pressure—volumeanalysis using a pressure chamber. Turgor was 077 MPa and thebulk volumetric modulus of elasticity 81 MPa at full turgidity;turgor loss occurred when water potential () had declined to–087 MPa at a relative water content (RWC) of 0912;the apoplastic water fraction (A) was 0235. As is usually found,there was a linear relation between 1/ and RWC beyond turgorloss. This finding supports the assumptions of the constancyof A during leaf dehydration. Beyond turgor loss the difference between and [measured afterfreezing and thawing (_d)] was about 01 MPa. This differencedid not increase as the leaf water content decreased. This resultcontradicts the constancy of A. It was concluded from calculations with a simple model of leafdehydration that analysis of the relation between and _d providesmore insight in the changes in the apoplastic fraction thanthe relation between 1/ and RWC. Research on the size of theapoplastic fraction and its changes with water potential wouldcomplement current understanding of leaf water relations. Solanum tuberosum, L., water potential, pressure chamber, osmotic potential, pressure potential, relative water content, apoplast, symplast     

Mycorrhizal influence on hydraulic and hormonal factors implicated in the control of stomatal conductance during drought

During drying, mycorrhizal plants often maintain higher stomatalconductance (g_s) than similarly-sized and -nourished non-mycorrhizalplants, but the mechanism of mycorrhizal influence remains unclear.Several hydraulic and non-hydraulic factors previously implicatedin control of stomatal behavior during drought were measured,to learn which are affected when roots of cowpea (Vigna unguiculata[L.] Walp. cv. White Acre) are extensively colonized by Glomusintraradices Schenck and Smith isolate UT143. At low soil watercontents (), mycorrhizal plants maintained higher g_s, transpirationand shoot water potential () than non-mycorrhizal plants. Thesehigher foliar water status characters were associated with lowerxylemsap abscisic acid concentrations ([ABA]) and lower ABAfluxes to leaves in mycorrhizal plants at low soil . Stomatalconductance was most closely correlated with xylem-sap [ABA],ABA flux to leaves and shoot . Stomatal conductance was notcorrelated with xylemsap concentrations of calcium or zeatinriboside equivalents, or with xylem-sap pH, nor were these xylem-sapconstituents affected by mycorrhizal symbiosis. Stomata of mycorrhizaland non-mycorhizal leaves showed similar sensitivities to ABA,whether leaves were intact or detached. It is concluded thatmycorrhizal fungi probably increased the capability of rootsystems to scavenge water in drier soil, resulting in less strainto foliage and hence higher g_s, and shoot at particular soil. Key words: Abscisic acid, cytokinins, Glomus intraradices