Adaptation of the Euryhaline Charophyte Lamprothamnium papulosum to Brackish and Freshwater: Turgor Pressure and Vacuolar Solute Concentrations During Steady-State Culture and After Hypo-osmotic Treatment

The euryhaline charophyte Lamprothamnium papulosum (Wallr.)J. Gr. was adapted to media with decreasing salinities rangingfrom 550 to 0 mosmol kg^–1. Vegetative plants grown inmedia with osmotic pressures (⁰) in the range of 550 to 130mosmol kg^–1 maintained a constant turgor pressure () at309 + 7 mosmol kg^–1. The ions K+, Na+ and Cl–, werethe predominant solutes in the vacuole. Changes in their concentrationsaccount for the variation in internal osmotic pressure (¹) with,⁰. The divalent ions Mg²⁺, Ca²⁺ and were also present in significant amounts, but their concentrationsdid not alter with changes in, ⁰. In cells subjected to hypo-osmotic shock the regulation of was incomplete. The turgor pressure increased from 302 to 383mosmol kg^–1. The first rapid response to the sudden decreasein ⁰ was a loss of K⁺ and Cl^–. In contrast to the decreasein ionic concentrations an accumulation of sucrose occurredwhich could account for the increase of . The increase in sucroseconcentration started 24 to 48 h after the downshock and reachedits highest value after 3 to 4 weeks. The sucrose concentrationin the vacuole was up to 320 mol m^–3. During this timethe ionic content continued to decrease but did not counterbalancethe sucrose concentration sufficiently to regain the original. High sucrose levels accompanied by an enhanced were also observedduring the period of fructification (sexual reproduction: formationof antheridia and oogonia) in Lamprothamnium kept under conditionsof constant salinity. It is concluded that high sucrose content and elevated arecharacteristic of sexual reproduction in this charophyte. Lamprothamniumis able to tolerate different during various developmentalstages (e.g. vegetative and reproductive phases). Key words: Lamprothamnium papulosum, sucrose, turgor pressure     

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     

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     

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     

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     

Analysis of Pressure-Volume Curves of Leaves of Rosa hybrida cv. Sonia

By analysing the relationship between inverse water potential(^–1), and relative water content (RWC) measured on leavesof roses (Rosa hybrida cv. Sonia), grown soilless, it was foundthat a non-linear (NL) model was better suited than a linearmodel to reproduce values observed in the non-turgid region.To explain this apparent curvature, it is assumed that a reductionof the non-osmotic water fraction (A_p) takes place when decreases.Osmotic potentials () measured on fresh and frozen leaf discstend to support this hypothesis. A method for exploiting PVcurves, which takes into account the variation of A_p, is described.It delivers values for the turgor pressure (_p), the relativeosmotic water content, and the mean bulk volumetric elasticitycoefficient, lower than those given by the linear model. Onthe other hand, it gives higher estimates for A_p and for . Whenapplying the traditional model to obtain estimates for waterrelations characteristics of rose leaves, and comparing resultsfrom two distinct salinity treatments (electrical conductivitiesof 1·8 mS cm^–1 and 3·8 mS cm^–1, respectively),one deduces a significant reduction of at turgor-loss in thehigh salinity treatment. The NL method is, in addition, ablesimultaneously to reveal a reduction of and a significant increasein _p at RWC=100% this proves that soilless–grown roseplants are able to osmoregulate when subjected to a constantand relatively high degree of salinity. Key words: Apoplastic water, non-linear regression, pressure-volume curves, tissue-water relations     

Stomatal Behaviour and Water Relations of Chilled Phaseolus vulgaris L. and Pisum sativum L.

Leaf diffusion resistance interpreted as stomatal resistance,leaf water potential (_w), solute potential (_s) and leaf turgorpotential (_p) of the chilling sensitive species Phaseolus vulgariswere determined during chilling at 4 °C in the light. Bothchill-hardened and non-hardened plants were used. For comparison,the chilling resistant species Pisum sativum was also used. The results for chilled P. sativum were similar to those obtainedfor chill-hardened P. vulgaris plants receiving a chilling treatment.In both cases a reduction in stomatal aperture and the maintenanceof a positive leaf turgor were the responses to chilling. Leavesof chilled but non-hardened P. vulgaris plants were found tomaintain open stomata throughout the chilling treatment despitea severe wilt developing after 7 h at 4 °C. This was incontrast to the chill-resistant P. sativum. which showed a rapidclosing and subsequent re-opening of the stomata to a new reducedaperture. During the first 12 h of chilling _wof P. vulgaris leaves changedfrom –0.47 MPa to –1.24 MPa. On more prolonged chilling_w tended to return to pre-chilling values. In addition. _p decreasedfrom 0.42 MPa to zero after only 9 h of chilling, and remainedat this value for the remainder of the chilling period, _s, changedrapidly from –0.89 MPa to –1.35 MPa in the first7.5 h, and after 9 h. _w and _s, were equal, i.e. zero _p. In contrast,the chilling resistant plant P. sativum maintained a positive_p throughout the chilling period, and there was little differencebetween values of _w, and _s in control and chilled leaves. Key words: Chilling, Stomata, ater relations, Phaseolus vulgaris, Pisum sativum     

Phosphate and Membrane Electropotentials in Trifolium repens L.

In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (E_vo) when KH₂PO₄ was addedto phosphate-free media, but these were of the same magnitudeas the controls (K²SO⁴ and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK⁺ calculated using the Goldman equation. There was no significantdepolarization on adding H₃PO₄ to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H⁺–H₂PO₄ symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for E_vo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedE_vo (mean effect = 9 mV). A significant difference in E_vo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (E_vo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport     

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.     

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     

Uptake and Accumulation of {alpha}-Naphthalene Acetic Acid by Cell Suspensions of Galium mollugo L.

Fidgeon, C. and Wilson, G. 1988. Uptake and accumulation ofa-naphthalene acetic acid by cell suspensions of Galium mollugoL.—J. exp. Bot. 39: 241-249. Galium mollugo cell suspensions require -NAA for continued growthand cell division. The kinetics of -NAA uptake from the medium(B₅) by Galium cells was assessed using 1-¹⁴C -NAA in a standardratio of cells to medium (0.25 g: 2.5 cm³). It was found thatthe uptake of -NAA was rapid, over 90% being taken up within4 h. Cells which had accumulated -NAA for 4 h or more did notrelease it back into the medium. It was found that Galium cellsaccumulated -NAA against a significant concentration gradient;suggesting the participation of an active component in the uptakemechanism. The effect of free-space and surface adsorption onthe uptake of -NAA was determined by means of a repeated washtechnique. These two factors were found to be of importanceonly during the first hour of uptake. Neither dead cells norplasmolysed cells absorbed -NAA. It is clear that, in the normal growth cycle, Galium cells cantake up the available -NAA within 3 or 4 h of inoculation andthat this can stimulate a cell division response of 3-4 generationsover the subsequent 14 d. Key words: Galium, cell suspension, -naphthalene acetic acid     

Corrigendum

Light response curves for (•) gross ¹⁶O₂ evolution, and() CO² uptake in 210 mmol mol^–1 O₂ with 900–1000µbar CO₂ or () in air by leaves of Hirschfeldia incana.The difference between (•) and () or () was quantitativelyequivalent to the measured ¹⁸O₂ uptake. The areas under thecurves are labelled to identify regions of assimilatory andnon-assimilatory electron flow redrawn from data of Canvin etal. (1980). It should be noted that the data and the labelling of the figureaxes are correct as printed.     

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     

The Effect of Hydroxymethanesulphonate on Photosynthesis in Chlorella pyrenoidosa

Photosynthesis under conditions known to favour glycollate excretionby algae did not result in glycollate excretion in a strainof Chlorella pyrenoidosa unless an inhibitor of glycollate oxidase,-hydroxypyridin-2yl-methane sulphonate (-HPMS), was present.This inhibitor increased the total amount of glycollate presentin the supernatant from the cells during photosynthetic carbondioxide fixation and gave accumulation of ¹⁴C in glycollateduring ¹⁴CO₂ fixation under conditions favouring glycollatesynthesis. At pH 8.3 -HPMS did not stimulate photosynthetic¹⁴CO₂ fixation in C. pyrenoidosa as occurs with some algae.Photoassimilation of acetate was inhibited by -HPMS, and thiswas shown to result from acetyl-CoA synthetase inhibition by-HPMS.     

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     

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     

Carbon Isotope Discrimination and the CO2 Accumulating Mechanism in Chlorella emersonii

The relationship between the CO₂ accumulating mechanism andcarbon isotope discrimination has been investigated in the unicellulargreen alga Chlorella emersonii. Growth of Chlorella at highC0₂ levels (5%) which repress the activity of the CO₂ accumulatingmechanism results in more negative ¹³C values. The data presentedin this paper suggest that it is possible to induce the accumulatingmechanism by nitrogen limitation as well as by carbon limitation.Activity of the accumulating mechanism, irrespective of whetherit is induced by C or by N limitation, is accompanied by ¹³Cvalues considerably less negative than those of cells whichdo not possess such a mechanism. It is suggested that the CO₂accumulating mechanism results in essentially a closed systemin which the inherent isotope discrimination by RuBP carboxylaseis not expressed.     

Potassium Transport Across the Membranes of Chara: II.42K FLUXES AND THE ELECTRICAL CURRENT AS A FUNCTION OF MEMBRANE VOLTAGE

Smith, J. R. 1987. Potassium transport across the membranesof Chara. II. ⁴²K fluxes and the electrical current as a functionof membrane voltage.—J. exp. Bot. 38: 752–777. The current required to clamp the trans-membrane voltage ofinternodal cells of Chara australis at different levels wasmeasured simultaneously with either the ⁴²K influx or efflux.Examination of the voltage-dependence of the ratio of the electricalcurrent to the unidirectional tracer fluxes yielded no evidenceof any amplification of the electrical driving force on theK⁺ ions. There was thus no evidence for the interaction of K⁺ions with themselves or any other species during their passageacross the membrane. These measurements allow the determinationof , the fraction of the electrical current carried by K⁺ ions.When the external [K⁺] = 10 mol m^–3, the average valueof was 0?85 for V_m > –125 mV and 07?5 for V_m <–150 mV. When the external [K⁺] = 0?1 mol m^–3, was 0?6 for V_m < –80 mV and 0?1 for V_m > –250mV. It was also found that the conductance associated with K⁺transport was inhibited by hyperpolarization. Key words: Potassium, conductance, flux-ratio     

The Relationship Between Transpiration Rate, Water Potential, and Resistances to Water Movement in Sunflower (Helianthus annuus L.)

The effects of transpiration rate on the vertical gradientsof leaf and stem xylem water potential ( and ) were examinedusing hydroponic sunflower plants. Transpiration was variedby stepwise alterations of environmental conditions. The gradientsof and were relatively small (2.3 and 0.8 x 10⁵ Pa m^–1)when transpiration rates approached zero, but increased sharplyto 5.4 and 2.3 x 10⁵ Pa m^–1 as transpiration increased.However, the gradients were independent of transpiration ratesabove 0.4 g dm^–2 h^–1 owing to variability of theplant resistance. The gradients of _I were usually less thanhalf those of _I. ₁ in individual leaves remained constant over a wide range oftranspiration rates (0.4—2.4 g dm^–2 h^–1) andeach leaf possessed a characteristic plateau value related toits elevation. _I responded similarly but was approximately 2.0x 10⁵ Pa higher than _I at the same elevation. Identical resultswere obtained regardless of the procedure employed to vary transpiration. The drop in water potential between stem and leaf implies thatthe leaf resistance is appreciable. This was confirmed usingrapidly transpiring excised leaves freely supplied with water._I increased by 2.0–2.5 x 10⁵ Pa following removal of theroot resistance but remained 2 x 10⁵ Pa lower than similar excisedleaves in darkness. Furthermore, ^I in excised leaves remainedconstant over a wide range of transporting rates, demonstratingthat the leaf resistance is also variable. The results are discussed in relation to previous reports.     

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