Plant and Soil223: 153–160, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Leaf water characteristics and drought acclimation in sunflower genotypes

P. Maury, M. Berger, F. Mojayad and C. Planchon∗Ecole Nationale Sup´erieure Agronomique, Institut National Polytechnique, avenue de l’Agrobiopôle, BP 107, F-31326 Castanet Tolosan C´edex, France

Received 12 July 1999. Accepted in revised form 11 April 2000

Key words:drought acclimation,Helianthus annuusL., photosynthesis, pressure-volume curves, water relations,water stress

Abstract

The responses of leaf water parameters to drought were examined using three sunflower (Helianthus annuusL.)genotypes. Osmotic potential at full water saturation (π100), apoplastic water fraction (AWF) and bulk elasticmodulus (BEM) were determined by pressure-volume curve analysis on well watered or on water-stressed plants(−1.0 MPa< 91 < −1.5 MPa) previously drought-pretreated or not. The drought-pretreated plants were subjectedto a 7-day drought period (predawn leaf water potential reached−0.9 MPa) followed by 8 days of rewatering.In well watered plants, all genotypes in response to drought acclimation displayed a significantly decreasedπ100

associated with a decrease in the leaf water potential at the turgor-loss point (decrease in9tlp was between 0.15 and0.21 MPa, depending on the genotype). In two genotypes, drought acclimation affected the partitioning of waterbetween the apoplastic and symplastic fractions without any effect on the total amount of water in the leaves. Asa third genotype displayed no modification of AWF and BEM after drought acclimation, the decreasedπ100 wasonly due to the net accumulation of solutes and was consistent with the adjustment of the photochemical efficiencyobserved previously in this genotype in response to drought acclimation. In water-stressed plants, the osmoticadjustment (OA) can increase further beyond that observed in response to the drought pretreatment. However, themaintenance of photosynthetic rate and stomatal conductance at low leaf water potentials not only depends on theextent of osmotic adjustment, but also on the interaction between OA and AWF or BEM. Adaptative responses ofleaf water parameters to drought are thus quite contrasted in sunflower genotypes.

Abbreviations: Amax – net-CO2 assimilation rate at light saturation; AWF – apoplastic water fraction; BEM –bulk elastic modulus; DP – drought pretreated; NDP – not drought pretreated; DW – dry weight; gs – stomatalconductance; OA – osmotic adjustment;9 – water potential;91 – leaf water potential;9tlp – leaf water potentialat the turgor-loss point;π – leaf osmotic potential;π100 – osmotic potential at full water saturation; RNES

s –relative symplastic solute content determined by osmometry; RNPV

s – relative symplastic solute content determinedby pressure-volume technique; RWC – relative water content; RWCtlp – RWC at turgor-loss point; TW – turgidweight

Introduction

Plant acclimation to drought entails modifications ofcharacteristics necessary to sustain key physiologicalprocesses. Maintenance of leaf turgor in the face ofdecreasing soil moisture has been emphasized as animportant adaptation trait that contributes to drought

∗ FAX No: 562 193583. E-mail: planchon@ensat.fr

tolerance (Hsiao et al., 1976). Several reports suggestthat plant metabolic processes are in fact more sensit-ive to turgor and cell volume than to absolute water po-tential (reviewed by Jones and Corlett, 1992). Amongthe physiological mechanisms that act to maintain leafturgor pressure, decreased osmotic potential result-ing either from a decrease in osmotic water fractionor from an osmotic adjustment (net accumulation ofsolutes in the symplast) was pointed out (Jones and

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Turner, 1980). Changes in tissue elasticity in responseto drought, which modify the relationship betweenturgor pressure and cell volume, might contribute todrought tolerance, as observed in black spruce (Blakeet al., 1991), but not in sunflower (Jones and Turner,1980).

There is evidence that sunflower exhibits a largeintraspecific variation for the extent of osmotic ad-justment (OA) in response to water stress (Conroy etal., 1988; Chimenti and Hall, 1993, 1994); however,plants that rely on OA as a drought tolerance mechan-ism must maintain accumulated solutes during periodsof high water for the next drought (Morgan, 1984).Edwards and Dixon (1995) showed that OA was ashort term response to drought inThuja occidentalisL., in contrast to the earlier results of Jones and Turner(1980) who had reported that drought-pretreated sun-flower plants had lower osmotic potential than nonpretreated plants even 7 days after rewatering. Thisdifference may reflect species-dependent responses todrought.

Previous studies have shown, in some sunflowercultivars, higher net photosynthesis after rewateringthan in regularly watered plants, together with ahigher tolerance to a subsequent water stress (Conroyet al., 1988; Mojayad and Planchon, 1994). Toler-ance was characterized by delayed stomatal closure,in association with the maintenance of photochem-ical processes at low leaf water potentials (Mauryet al., 1996). As this response is not a general be-haviour in sunflower, the study reported below wasfocused on changes in tissue water relations charac-teristics which can play a decisive role in the adapt-ative response to drought. The pressure-volume (PV)technique (Tyree and Hammel, 1972) was used toanalyse the leaf water relations in well watered andin water-stressed plants of three sunflower genotypespreviously drought-pretreated or not. The response ofthe photosynthetic processes to the water stress anddrought pretreatment was also analysed.

Materials and methods

Plant material

The three sunflower (Helianthus annuusL.) genotypesinvestigated – the cultivated F1 hybrid (Viki) and itshomozygous parental lines (T57-female, T32-male)– were supplied by Maïs Adour, Mont de Marsan,France. Seeds were germinated in vermiculite and

seedlings were transplanted into plastic pots (14 cmdeep, 2.5 L volume; 1 plant per pot) containing equalvolumes of soil, sand and peat and placed in a con-trolled environment greenhouse. Plants of the threegenotypes were separated into two equal groups: thenon drought-pretreated (NDP) plants were regularlywatered to field capacity until bud formation (stageR3, Schneiter and Miller, 1981); the second groupwas drought-pretreated (DP). All the measurementswere carried out at stage R3, whether on the con-trol plants (well watered:9l=−0.60 MPa, standarderror=0.10), or on moderately water-stressed plants(−1.0 MPa< 9l < −1.5 MPa) previously drought-pretreated or not. At stage R3, the drought treatmentconsisted in withholding water supply.

Drought pretreatment at stage R1

DP plants were subjected to a 7-day drought periodat stage R1 (15 days before stage R3) and were thenregularly irrigated until they reached stage R3 (8 dayslater). The 7-day drought period consisted in dailyreplacing a decreasing fraction (50–0%) of the waterloss by evaporation or plant transpiration, until thepredawn9l of leaf 15 was about−0.9 MPa.

In situ gas exchange

Net CO2-assimilation rate at light saturation (Amax)and stomatal conductance (gs) of the 15th attached leafwere measured on plants at stage R3 with a portableLi-6200 (Li-Cor, Lincoln, NE, USA) photosynthesissystem (air temperature, 25±2 ◦C) between 10 00 and12 00 h. Measurements were made during a 30–45 speriod after a steady depletion of CO2 was detected(using a 4 l chamber and a flow rate over desiccant ad-justed to maintain a constant air humidity of 60±5%)under 500µmol m−2 s−1 photosynthetic photon flux.

Procedure for pressure-volume curves

Leaf 15 was cut from each of the plants and then,as soon as possible, cut again under distilled water.Leaves were allowed to reach full turgor by storingthem at 9◦C in a closed dark container with the peti-oles in distilled water. After a 3 h rehydration period,each leaf was weighed for determing turgid weight(TW) and wrapped in a humidified plastic bag to min-imise both temperature fluctuations and tissue waterloss (Wenkert et al., 1978), and inserted in the pressurechamber. After establishing the balancing pressure,chamber pressure was successively raised by 0.5–2.5

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MPa increments. Expressed sap was collected in pre-weighed small vials filled with cotton. After each sapcollection, the chamber pressure was slowly reducedto the previous balance pressure. The sap collector wasthen removed, sealed and the new balance pressuredetermined. The pressurisation rate was about 0.025MPa s−1 (Turner, 1981). After the PV measurements,the leaves were oven dried at 80◦C for at least 48 h andleaf dry weight (DW) was determined by weighing.Rehydration effects on osmotic potentials and otherleaf water relations parameters, as determined by PVcurves, have previously been reported (Kubiske andAbrams, 1990; Meinzer et al., 1986). Comparing PVcurves from hydrated and non-hydrated leaves showedthe hydration technique did not significantly alter thewater characteristics of sunflower genotypes studied(data not shown). Evans et al. (1990) predicted thatchanges due to rehydration would be less likely to oc-cur in plants of mesic environments and more likelyto occur in plants of arid environments. An importantconsideration is that the rehydration time must not betoo long, especially in plants adapted to water deficits,because the plants may lose solutes during rehydration(Jones and Turner, 1980).

Analysis of pressure-volume curves

Bulk leaf 15 water relations characteristics were de-termined from PV curves (Tyree and Hammel, 1972).For each leaf, the data were plotted as the recip-rocal of balance pressure vs relative water content(RWC) and estimates of PV parameters were obtainedas described below. Five repetitions were made foreach genotype∗ pretreatment∗ treatment combina-tion. Estimates of leaf osmotic potential at full turgor(π100) and at turgor-loss point (9tlp), RWC at turgor-loss point (RWCtlp), and apoplastic water fraction(AWF) were obtained as described by Wilson et al.(1980). Bulk elastic modulus (BEM) was calculatedas BEM= −π100 ∗ (100− AWF)/(100− RWCtlp)

(Groom and Lamont, 1997). Symplastic solute con-tent (NsPV) was calculated from the relation: NsPV =(−π100∗ (TW−DW)∗ ((100−AWF)/100))/(R∗T),where R is the universal gas constant andT is theabsolute leaf temperature. Relative symplastic solutecontent (RNsPV) was calculated as NsPV divided byleaf dry weight (DW).

Osmometry of leaf sap

Symplastic solute contents were also measured byosmometry of leaf 16 sap (NsES) in order to com-

pare them with the values obtained from PV analysis(NsPV). The leaves were previously rehydrated as inthe PV curves procedure in order to determine theturgid weight (TW). Then, each turgid leaf was cutinto two equal pieces. One part of the leaf tissue wasused for the determination of the dry weight (DW).On the other part, NsES was measured by freezingthe leaf tissue in liquid nitrogen, squeezing sap outin a syringe; after centrifugation at 4000g for 10min the osmolarity (2, number of moles of osmot-ically active particles per kilogram of water) of theextracted sap was determined with a Roebling freez-ing point micro-osmometer at 25◦C calibrated with0 and 300 mOsmol NaCl solutions. The symplasticsolute content corrected for the apoplastic water wascalculated from the following relationship, assumingthe same AWF in leaves 15 and 16 of the same plant:NsES= 2 ∗ (TW− DW) ∗ ((100− AWF)/100).

Environmental conditions

The plants were grown exclusively under natural lightin a greenhouse (daylength approximately 14 h; 1600µmol m−2 s−1 maximum photosynthetic photon fluxat midday, overall integrated mid value estimated at500µmol m−2 s−1). The photosynthetic photon flux(PPF) was measured by a Li-Cor Quantum Sensor (Li-190 SEB, Li-Cor Inc., Lincoln, NE). The minimumair temperature was 15◦C and the maximum air tem-perature was maintained under 30◦C during the dayby coolers. Relative humidity was between 40 and90%. Plants were fertilized weekly using 100 mL ofHoagland’s solution (Hoagland and Arnon, 1950).

Data analysis

Three-way factorial analyses of variance (ANOVA)were used in order to determine the statistical signific-ance of changes that occurred in leaf water parametersin response to (i) genotype, (ii) drought pretreat-ment, and (iii) water stress. Means were calculated onfive replications for each genotype∗ pretreatment∗treatment combination and compared by LSD (LeastSignificant Difference) at the 0.05 confidence level.ANOVA residuals were used for the calculation ofthe 5% LSD, this was done under the assumption ofhomogeneity of variances (Levene test).

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Table 1. Net CO2-assimilation rate at light saturationAmax (µmol CO2 m−2 s−1) and stomatal conductance gs (molH2O m−2 s−1) in well watered (91=−0.60 MPa) and in water-stressed (−1.0 MPa< 91 < −1.5 MPa) plants ofthree sunflower genotypes drought-pretreated (DP) or not pretreated (NDP).∗ and ∗∗ significantly different from thenon drought-pretreated plants atp=0.05 andp=0.01, respectively (n=5)

Genotypes and Amax gs Amax gs

acclimation (µmol CO2 m−2 s−1) (mol H2O m−2 s−1) (µmol CO2 m−2 s−1) (mol H2O m−2 s−1)

Control plants Water-stressed plants

(91=−0.60 MPa) (−1.0 MPa< 91 < −1.5 MPa)

T32 NDP 18.9 0.49 1.8 0.06

DP 15.4∗ 0.22∗∗ 4.5 0.01

T57 NDP 19.1 0.80 6.9 0.20

DP 19.7 0.71 10.2 0.26

Viki NDP 18.5 0.70 2.7 0.11

DP 21.5 0.62 11.5∗∗ 0.26∗

LSD (5%,n=5) 3.4 0.14 3.4 0.14

Results

Drought pretreatment effect on net photosynthesisand stomatal conductance during a subsequent waterstress

Net CO2-assimilation rate at light saturation (Amax)per unit leaf area of the three genotypes at stageR3 was similar for the well watered (9l = −0.60MPa) non drought-pretreated plants (Table 1). In wellwatered plants, the drought pretreatment decreasedAmax in T32, in association with a reduction of sto-matal conductance. At low leaf water potentials (−1.0MPa< 9l < −1.5 MPa), the drought pretreatmentinduced a better maintenance of photosynthesis inViki.

Genotypic response to internal water deficit in nondrought-pretreated plants

The three genotypes submitted to dehydration in thepressure chamber lost turgor at similar leaf water po-tentials (9tlp ca.−0.9 MPa, Table 2A). However, thethree genotypes lost turgor at different relative wa-ter content values (RWCtlp, Table 2A). The genotypeT57, which displayed the lowest value of bulk elasticmodulus (less rigid cell wall), maintained turgor at sig-nificant lower values of RWC. The water stress at stageR3 decreased9tlp and RWCtlp values. The genotypeT57 displayed again the lowest value of relative water

content at turgor loss, which may indicate a higher de-hydration tolerance. This characteristic was associatedwith a higher level of photosynthesis in water-stressedplants in comparison with the two other genotypes.

Drought pretreatment effect on leaf watercharacteristics of well watered plants (91 = −0.60MPa)

When the plants had been drought-pretreated at stageR1, turgor-loss was observed for lower9tlp values inthe three genotypes. The decrease in9tlp was between0.15 and 0.21 MPa, depending on the genotype (Table2A). In response to the drought pretreatment, all geno-types displayed a decrease in osmotic potential at fullturgor (π100, Table 2A). Changes in bulk elastic mod-ulus (BEM) and in apoplastic water fraction (AWF)were also induced by the drought pretreatment, butdiffered between the three genotypes. As a result,BEM became similar in the three drought-pretreatedgenotypes. In T32 and T57, the drought pretreat-ment affected the AWF without any effect on thetotal amount of water in the leaves (expressed by theamount of water at full turgor per unit leaf area). De-pending on the homozygous lines, contrasting effectsof the drought pretreatment on AWF were observed.In Viki, no change in AWF was observed in re-sponse to the drought pretreatment. After turgor loss,the linear relation between 1/91 and RWC supported

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Table 2. Leaf water characteristics in well watered (91 = −0.60 MPa, Table 2A) and in water-stressed (−1.0 MPa< 91 < −1.5MPa, Table 2B) plants of three sunflower genotypes drought-pretreated (DP) or not pretreated (NDP). Leaf water potential atturgor-loss point (9tlp), relative water content at turgor-loss point (RWCtlp), osmotic potential at full turgor (π100), bulk elasticmodulus (BEM) and apoplastic water fraction (AWF) were determined by pressure-volume curve analysis. The amount of waterat full turgor per unit leaf aera was calculed as (TW-DW/S) with S leaf area, TW and DW, turgid and dry weights of the leafrespectively.∗ and∗∗ significantly different from the non drought-pretreated plants atp=0.05 andp=0.01, respectively (n=5)

Genotypes and 9tlp RWCtlp π100 BEM AWF (TW-DW)/S

acclimation (−MPa) (%) (−MPa) (MPa) (%) (g/m2)

A - Control plants (91 = −0.60 MPa)

T32 NDP 0.90 92.9 0.79 6.79 38.7 260.8

DP 1.06∗ 88.7∗ 0.88∗ 4.80∗ 29.9∗ 276.6

T57 NDP 0.93 90.4 0.79 5.45 31.1 256.8

DP 1.16∗ 90.0 0.96∗ 5.72 41.1∗ 272.4

Viki NDP 0.96 91.8 0.84 6.75 35.1 239.2

DP 1.11∗ 89.1∗ 0.92∗ 5.80 35.9 249.5

B - Water-stressed plants (−1.0 MPa< 91 < −1.5 MPa)

T32 NDP 1.04 90.9 0.90 6.95 33.7 333.6

DP 1.12 89.6 0.94 5.98 34.4 322.7

T57 NDP 1.12 88.5 0.90 4.83 38.6 278.9

DP 1.36∗∗ 90.8∗ 1.15∗∗ 7.81∗∗ 38.9 303.3

Viki NDP 1.12 90.8 0.95 5.80 35.5 298.8

DP 1.37∗∗ 87.6∗ 1.10∗∗ 5.90 35.4 281.3

LSD (5%,n=5) 0.10 2.3 0.06 1.50 8.1 39.0

Table 3. Relative symplastic solute content in well watered (91 = −0.60 MPa) and in water-stressed (−1.0 MPa< 91 < −1.5MPa) plants of three sunflower genotypes drought-pretreated (DP) or not pretreated (NDP). Relative symplastic solute content (RNS,in mmol/g of dry weight) as determined by the expressed sap (RNES

S corrected by AWF) or the pressure-volume technique (RNPVS ).

∗ and∗∗ significantly different from the non drought-pretreated plants atp=0.05 andp=0.01, respectively (n=5)

Genotypes and RNPVS RNES

S RNPVS RNES

Sacclimation

Control plants Water-stressed plants

(91 = −0.60 MPa) (−1.0 MPa< 91 < −1.5 MPa)

T32 NDP 1.58 1.21 1.81 1.32

DP 2.01∗ 1.39∗ 2.00 1.43

T57 NDP 1.56 1.27 1.53 1.07

DP 1.73 1.15 1.93∗∗ 1.26∗Viki NDP 1.58 1.10 1.69 1.22

DP 1.78∗ 1.25 1.95∗ 1.24

LSD (5%,n=5) 0.20 0.17 0.20 0.17

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the assumption of the constancy of AWF during leafdehydration in the pressure chamber.

In response to the drought pretreatment, T32 andViki displayed an increase in relative symplastic solutecontent (RNsPV, Table 3); this result was furthercorroborated by the expressed sap method (RNsES,Table 3). Irrespective of the genotype and treatment(well watered or water-stressed plants), the values ofthe relative symplastic solute content determined bythe expressed sap method were lower than those ob-tained by the pressure-volume technique (1.23 vs 1.76mmol/g of dry weight). The expressed sap method ap-peared to be less sensitive and, as a result, the increasein relative symplastic solute content was not signi-ficant for Viki. Nevertheless, the Pearson correlationcoefficient between the two methods was 0.742∗∗.

Drought pretreatment effect on leaf watercharacteristics of water-stressed plants (−1.0MPa< 91 < −1.5 MPa)

When the water-stressed plants had been previouslydrought pretreated, turgor-loss was observed for signi-ficantly lower9tlp values in T57 and Viki genotypes(9tlp, Table 2B). The ca. 0.25 MPa decrease in9tlpwas associated with a similar decrease in osmotic po-tential at full turgor (π100, Table 2B). An increase inrelative symplastic solute content (RNsPV, Table 3)was also observed for these two genotypes in responseto drought pretreatment, although this increase wasdetected by the expressed sap method for T57 only.The osmotic adjustment observed was associated witha decrease in the relative water content at turgor-loss(RWCtlp) in Viki but not in T57. The increase in bulkelastic modulus (more rigid cell wall) observed in T57in response to the drought pretreatment might accountfor the higher RWCtlp values. In T32, no signific-ant changes in leaf water parameters were observedfor water-stressed plants in response to the droughtpretreatment.

Discussion

In this study, the three sunflower genotypes displayedsignificant differences in their adaptative response todrought. These differences were observed at both thephotosynthetic and water relations levels. Sunflowerplants rewatered after experiencing mild drought con-ditions and subsequently exposed to water deficit canexhibit a higher drought tolerance of their photo-synthetic activity. This behaviour has already been

described in sunflower (Conroy et al., 1988; Mat-thews and Boyer, 1984) and cotton (Wise et al., 1992).Nevertheless, the genotypes analysed here displayeda contrasting photosynthetic response to a droughtpretreatment. This phenomenon was observed withplants grown under natural light in a greenhouse andis in agreement with previously obtained results undercontrolled growth chamber conditions (Mojayad andPlanchon, 1994; Maury et al., 1996). The results cor-roborated the involvement of a genotypic-dependentadaptation to drought pretreatment. OA is consideredas being important for the maintenance of photo-synthetic activity during dehydration (Conroy et al.,1988), through its role in turgor maintenance (Jonesand Turner 1980). The three genotypes displayed OAin response to the drought pretreatment and an ad-ditional OA was observed in T57 and Viki duringthe subsequent water stress. The cumulative OA ingenotype T57 was higher than that in Viki (0.36 vs0.26 MPa). However, in contrast to Viki no signific-ant changes of net CO2-assimilation rate and stomatalconductance at low leaf water potentials were ob-served in T57 in response to the drought pretreatment.The extent of osmotic adjustment does not appear tobe the most important determining factor in photosyn-thetic and stomatal conductance maintenance at lowleaf water potentials, as observed by Girma and Krieg(1992) in sorghum.

The three genotypes displayed an OA at least 8days after stress relief whether or not they displayedchanges in cell wall elasticity and in the apoplasticwater fraction (AWF). Effects of changes in relativesymplasmic solute content on water relations werealso evaluated by osmometry. As previously observedby Suáres et al. (1998), in comparison with PV ana-lysis lower values of relative symplastic solute contentwere obtained by osmometry after correction for AWF.The solutes involved in the different phases of the cell– vacuole, cytoplasm, organelles and cell wall – aregeneraly totally different (Tyree and Jarvis, 1982) andcan interact in a complex manner once the cells havebeen decompartmentalised by freezing and thawing(Marigo and Peltier, 1996). Although some paramet-ers derived from PV curves show variability, even inhomogeneous material (Richter, 1997), this methoddoes not suffer from dilution errors (Turner, 1981).In the T57 genotype, the decrease in osmotic po-tential at full turgor could arise from an increase inAWF as the amount of water per leaf area was notchanged. Calculations suggest that the passive concen-tration of solutes in the symplastic volume due to a

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10% increase in AWF could account for 50% of thereduction of the osmotic potential at full turgor. Thisphenomenon was also observed in celery in responseto salt stress (Pardossi et al., 1998). In contrast, inthe T32 genotype, the active osmotic adjustment (netsolute accumulation) was counteracted by a decreasein AWF and was associated with an increase in cellwall elasticity. Solute accumulation and increased tis-sue elasticity can act synergistically on the symplasticvolume, with the former providing an increased gradi-ent for influx of water and the latter allowing a greatervolume of water in the symplast at a given leaf wa-ter potential (Saliendra and Meinzer, 1991). Saliendraand Meinzer (1991) suggested that the adaptative sig-nificance of symplastic volume maintenance may liein the postponement of reaching a lethal symplas-mic volume. When there is no change in cell wallelasticity as observed in the Viki genotype, soluteaccumulation affects maximum turgor to a greater ex-tent. Osmotic adjustment and turgor maintenance arenot directly related and changes in the elastic mod-ulus or in apoplastic/symplasmic water fractions canalso delay turgor loss (Tyree and Jarvis, 1982). Tol-erance to internal water deficit had been characterizedby turgor loss at lower relative water content (RWC),promoting the maintenance of chloroplast functioningduring dehydration (Gupta and Berkowitz, 1987; Ran-ney et al., 1991). Differences in tolerance to internalwater deficit (expressed by RWCtlp values) were ob-served among the three genotypes studied. However,the water stress pretreatment modified the genotypictolerance to internal water deficit.

It has been shown here that cell wall elasticitycan increase in response to the drought pretreatment.An increase in cell wall elasticity can reduce fluc-tuations of9 in the dehydrating leaf (Zimmermannand Steudle, 1978). InAvicennia germinans, the in-crease in cell wall elasticity was combined with OAand allowed leaf water uptake and turgor mainten-ance through a large range of soil9 (Suárez et al.,1998). However, it was observed that the increase incell wall elasticity was lost and can even decrease dur-ing a subsequent water deficit. Such a decrease duringdrought stress has been previously reported in sun-flower (Chimenti and Hall, 1994). A decrease in cellwall elasticity results in a more pronounced loweringof leaf water potential for the same water loss and thusenhances the difference in water potential between soiland leaf. However, this phenomenon does not res-ult in an overall increase in water uptake over a 24h (day/night) cycle for non-succulent species as the

water potential difference between soil and plant israpidly neutralized during the night (Schulte, 1992).

Our results suggest in addition to cell wall elasti-city water partitioning between symplastic and apo-plastic fractions can be modified in water stress-relieved sunflower plants. The response of these plantsto a subsequent water stress can be widely differentfrom that of plants which have not previously experi-enced a drought stress. The maintenance of turgor atlower water contents and potentials, which is favour-able to photosynthetic processes, depends not onlyon OA but also on the interaction between these wa-ter parameters. Variations in AWF and in cell wallelasticity can thus counteract the effect of osmotic ad-justment on turgor. A more complete evaluation ofthe genotypic ability to tolerate internal water defi-cit must take into account the conditioning effects ofacclimation on these parameters.

Acknowledgements

This work was supported by grants from the ‘CentreTechnique Interprofessionnel des Oléagineux Métro-politains’ (CETIOM), Paris.

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