water relation parameters in drougth

8/10/2019 Water Relation Parameters in Drougth

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deficit (Hsiao et al; 1985; Schulze, 1986). Thereis evidence that cell expansion depends on turgorpressure P (rather than total water potential) andthat the expansion rate is proportional to theexcess of P over a threshold value (Jones, 1986).Therefore, the maintenance of P over a particularthreshold seems essential for continuous plantgrowth. Despite the low water availability, thevegetative growth of the vines in Greececontinued until late summer. Probably theimmature still expanding leaves possess aphysiological mechanism, based on leaf waterrelations, which helps them to maintain a positivecell pressure, large enough to sustain itsenlargement. Turgor pressure can be maintainedby two distinct strategies: i) by lowering theosmotic potential ()through the production of

osmotically active solutes or ii) by increasingtheelasticity of cell walls. The first strategy leads tothe conservation of water in the tissue, whereasthe second strategy allows the maintenance of thesame turgor pressure with less water. There issome evidence that the type and relativeimportanceof these strategies vary among speciesor even within a species (Parker and Pallardy,1985), as well as among leaves of different ages(Patakas et al, unpublished).

There are also severalapproaches

to determineleaf water relations (Pearcy et al, 1991). A recentapproach is to deduce the water relations from theanalysis of the dependenceof leaf water potential()on relative water content (RWC) using livingtissue. This method - commonly referred aspressure/volume technique - was originallydeveloped by Scholander et al (1964) and hassubsequently been refined by Tyree and co-workers (Tyree and Karamanos, 1980; Tyree andJarvis, 1982). It leads to a detailed descriptionofchanges in P with the water content of the tissueas well as the calculation of the bulk modulus ofelasticity (ϵ)by the derivative of that function.The psychrometric technique is often used todetermine the diurnal changes in tissue osmoticpotential (Turner, 1981). Most studies on plantwater relations have been performed in matureleaves; much less information exists aboutchanges in the variables of water relations duringleaf ontogeny.

The objectivesof this study were to determine

parametersof water relations in

grapevinesleaves

using both the pressure/volume and thepsychrometric techniques as well as to evaluatethe dominant physiological strategy employed bythe leaves of different ages in order to adjust towater stress and/or sustain their growth.

MATERIAL AND METHODS

Ten-year old field grown grape vines (Vitis vinifera Lcv Roditis) grafted on 110 Richter (V rupestris x VBerlandieri) rootstocks nearby the experimentalstation of the University of Thessaloniki were used.Leaves were classified

accordingto their

agein the

following four groups: 18-day old still expandingimmature leaves (L1); 35-day old fully expandedmature leaves (L2); 80-day old fully expanded matureleaves (L3); 120-dayold fully expanded leaves withoutany signs of senescence (L4). In order to estimateexactly the leaf age, leaf appearance wassystematically recorded. Irrigation had not beenapplied and no rainfall was recorded for 5 weeksbefore and during the experimental period. Diurnalfield data concerning water relations and gas exchangeparameters were measured for the leaves of thedifferent groups. The experiment was repeated four

times duringsummer.

No significant differenceswere

obtained between measurements on different days andhence the following results represent measurements forone single day.

Predawn leaf water potential was measured on threemature leaves (L2) using the psychometric technique.Measurements were made at 10-day intervals startingin May (fig 1).

Of the 90 leaves tagged for each leaf age, sevenleaves were used to study diurnal changes in leafconductance with water vapour (Cw). This variablewas mesured approximatelyhourly from 0600 to 2000h using a steady-stateporometer (Li-1600).

Leaf water (y) as well as osmotic ()potentialswere measured hourly from 0600 to 1800 h on fiveleaves per age group using the psychrometrictechnique (Wilson et al, 1979). Three pairs of 6 mmdiscs were punched from each leaf. The first pair ofdiscs was used for the determination of leaf waterpotential. The discs were placed within seconds insample chambers which were kept in a controlledtemperature at about 22 C, shielded with apolystyrene box. Osmotic potential was measured onthe second pair of discs obtained from each leaf. Thesediscs were rapidly sealed in teflon tape and aluminiumfoil and frozen on dry ice. Then the sealed discs were


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thawed and placed in the chambers. Turgor potentialwas calculated as the difference between leaf waterpotential and osmotic potential.

Concomitant measurements of relative watercontent (RWC) were made on the third pair of discsobtained from the same leaf that was used for the

determination of water potential components. The leafdiscs were weightedquickly in a saturated atmosphereand their fresh weights (FW) were recorded.

Afterwards, leaf discs were placed in distilled waterfor 10 h and the new weight was taken as turgidweight (TW). The discs were then dried at 80 C andtheir dry weight(DW) was measured.

From FW, DW and TW, the relative water content(RWC) of leaves at different times of the day could becalculated by the formula:

Measurements of leaf water isotherms were madeusing the pressure chamber, which also enabledcalculation of leaf turgor pressure (P) and the bulkmodulus of elasticity (ϵmax) (Koide et al, 1991). Fiveleaves per age group were hydrated to near maximumturgor by immersing their cut ends in distilled water,covering the leaves with black plastic bags and leavingthem in the dark for 10 h. The water potential of theleaf when it was put into the pressure chamber wasbetween -0.015 and -0.03 MPa. The pressure in thechamber was increased in steps of 0.2-0.3 MPa overthe balance pressure (Koide et al, 1991).

Water potential, turgor pressure and osmoticpotential ()were all plotted against the relativesymplasmic water loss (RSWL). This was calculatedas loss of water expressed as percentage of thesymplasmic water content at full turgor (Wo), ie,

where Wo is calculated from the intercept of thestraight part of the pressure volume curve - relating1/vs RWC - with the abscissa (RWC) (Turner,1981) and W is the actual weight of symplasmic waterafter pressurization.

The bulk modulus of elasticity (ϵ)was calculatedfrom:

The value of ϵobtained near maximum turgor (ϵmax)was compared for leaves of different ages.

Changes in symplasmic water content as apercentage of the total water content were alsomonitored in all leaves by calculating the symplasmicwater fraction (SWF) (Pavlik, 1984) by:

Data were analyzed using one-way analysis ofvariance and the LSD test (Snedecor and Cochran,1980).

RESULTS

Predawn leaf water potential declinedcontinuously from May to August (fig 1). Therelationship between RWC and water potential inall ages of leaves is shown in figure 2. Highervalues of RWC were found in the early morning(0600 h) and lower values in the early afternoon(1500 h). RWC as well as leaf water potentialseemed to decrease in all leaves during themorning. Immature leaves (L1) showed lowerRWC values as measured at 0600 h than themature and the old leaves (L2, L3, L4) indicating


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that these reached at a lower value of leaf

hydration during the night. The decrease in RWCtended to be smaller in old leaves (L4) and it wasgreater in immature leaves (L1). Furthermore,immature leaves exhibited lower values of RWCfor the same value of

.Leaf water

potentialat

0600 h was nearly the same in all leaves at about-0.6 MPa. It dropped to about -1.4 MPa in bothmature and immature leaves and to -1.1 MPa inold leaves at 1500 h.

Porometer measurements indicated that oldleaves (L4) had lower stomatal conductance thanmature (L2, L3) and immature leaves (L1)regardless of the time of the day (fig 3a). Thestomatal closure seemed to start at higher valuesof (-0.85 MPa) in the old leaves than in themature leaves (-1.3 MPa). Stomatal closureresulted in the reduction of transpiration in allleaves studied (fig 3b).

The symplasmic water fraction (SWF) as apercentage of the total water content, decreasedwith leaf age from 78% in the immature leaves

(L1) to approximately 62% in old leaves (L4)(table I).

The analysis of water potential isotherms (fig4) showed that statistically significant changes inboth and its components, P and ,occurredduring leaf ontogeny. In particular, the osmotic

potential at full turgor (ie, o, at symplasmicwater loss = 0) decreased with the leaf age from-1.35 MPa in immature leaves (L1) to -1.7 MPa

in (L4). Also the relative symplasmic water loss atincipient plasmolysis (ie, at P = 0) decreasedduring leaf development from 17% in immatureleaves to 11.2% in old ones. Furthermore, thechanges of dP vs dRSWL seemed to be lower inthe immature than the mature and old leaves.Thus, for a 5% water loss the dP was 1.38-0.8 =0.58 MPa in (L1) in comparison to 1.5-0.64 =0.86 MPa in (L2), 1.6-0.59 = 1.01 MPa in (L3)and 1.7-0.51 = 1.19 MPa in (L4).

The dP changedependson the bulk modulus ofelasticity (ϵ)of cell walls for a given symplasmicwater loss. Under the same conditions, dP will behigher in cells with rigid cell walls (ie, highervalue of ϵ)than in cells with more elastic cellwalls because

where dV/V is the relative changein volume.

The ϵ valueat full turgor (ϵmax) increasedsignificantly with leaf age from about 12.7 MPain the immature leaves to about 22.5 and 25 MPain (L2) and (L3), respectively(table I). Old leaves(L4) showed even higher ϵmax values of 28 MPa.

Relationships among leaf water potential at theincipient plasmolysis (ip), minimum diurnal leafwater potential (min) as measured at 1500 h andleaf age show that the (min) tended to divergefrom (ip) as leaves matured (fig 5). This suggests


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that in older leaves turgor pressure is maintainedat a higher level than in immature leaves. In fact,it can be seen (figs 4 and 5) that in the old leaves(L4), turgor pressure was maintained at a higherlevel (0.7 MPa) than in the immature leaves (0.1MPa).

The results obtained from the psychometrictechnique (fig 6) compared to those from the

pressure/volume technique (fig 4) indicateda

greater decrease in osmotic potential ()for thesame changes in .Furthermore, the diurnaldecrease of in relation to water loss tended to be

greater in immature and mature leaves than in theold leaves (fig 6). A more meaningfuldescriptionof osmotic changerequires a distinction betweenthe changes due to removal of water from thetissue symplasm and those due to activeaccumulation of solutes. The theoretical equationrelating osmotic potential ()and water volumeVwis:

where (ns) is the number of moles of solutes. Ifthere is no osmotic adjustment ns remains

constant. Comparing the first and last point of in figure 6 and the corresponding point of RWCthe net increase of solutes in leaf cells can becalculated (Noitsakis and Tsiouvaras, 1990).From table II it can be seen that the calculated netincrease of solutes in cells was significantlyhigher in the mature and immature leaves than inthe old leaves.

DISCUSSION

Approximately 31-34% of the water in matureleaves is located outside the plasmalemma. Thevalues reported for a number of speciesusing thepressure/volume technique have all been in therange 5-45% (Vos and Oyartzun, 1988; Andersenet al, 1991). Values of the apoplasmic and of the

symplasmic fractionseem to

be highly dependenton leaf maturity. Mature and old leaves showedsignificantly lower values compared to immatureleaves both in the symplasmic water fraction andin the fraction of water extracted to reach turgorloss point (table I), suggesting that water moved


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out from the symplasm into the apoplasm, ie thecell wall, as leaves matured. Such a changein theallocation of water increases the concentration ofsolutes in the symplasm and therefore results in adecrease in even without changes in the netsolutes amount (Nunes et al, 1989). Thissuggestion explainsthe lower values in and atfull turgor and along the whole range of tissuedehydration to turgor loss point obtained inmature and old leaves (fig 4). Similar results werereportedby Lakso et al (1984) and Salleo and LoGullo (1990). Of greater interest are the measureddiurnal changes in which were quite large inmature and immature leaves (fig 6). Thiscontrasts with the small changespredictedby anypressure/volumemeasurement, presumablydue tothe higher rates in tissue dehydration.Reductionin the degree of osmoregulation in relation to theincrease in the rate of tissue dehydration has alsobeen reported by Turner and Jones 1980. On theother hand the psychrometric technique is knownto be subjected to errors owing to the dilution ofthe apoplasmic with the symplasmic fractionresulting in an overestimation of osmoticpotential (Koide et al, 1991). This disadvantagecan be overcome with the estimation of the netsolute increase in cells. This can be calculated

using the data provided by the psychrometric

technique (Hsiaoet al, 1985; Noitsakis and

Tsiouvaras, 1990). Net solute increase refers tothe active solute accumulation, which determinesthe degree of osmoregulationin leaf cells. Fromour results it can be seen that the diurnalreduction in osmotic potential was partly due toactive accumulation of solutes (table II). Similarresults concerning short term osmotic regulationhave also been reported by Turner and Jones(1980). The active accumulation of solutes in oldleaves was very low. These results indicated that

grapevines generally are capable ofosmoregulationbut that this capacityis reduced inold leaves. Such regulationin immature leavescontributes to the maintenance of mid-day growthrates (Bressan et al, 1990). An identical process isapparently occurring in mature leaves.

Older leaves (L4) showed higher RWC valuescompared to mature and immature leaves, whichcan be attributed to the significantly lowertranspiration rates due to their lower stomatalconductance (fig 3a and b). Similar results were

reportedfor various

plant species(Dufrene and

Saugier, 1993). The reduction in stomatalconductance with leaf age has generally beenattributed to either an insufficient functioning ofstomatal apparatus relating to age, and/or to achange in stomatal physiology (Sandanam et al,

1981). In our results leaf conductance andtranspiration rate decreased rapidly from matureto old leaves. This rapid reduction might beattributed to the fact that the age-dependentreduction in stomatal conductance could beaccelerated under water stress conditions

(Thomas and Stoddart, 1980). In addition,stomatal closure started at higher values of inold leaves restricting even more water losses.Therefore, it could be assumed that in old leaves,water saving owing to very low transpiration ratesseems to contribute to the turgor maintenanceunder water stress conditions.

It will also be expected that the higher RWCvalues in mature leaves (L2, L3) compared toimmature leaves (L1) (fig 2) were due to their

abilityto better control their water balance

bylow

stomatal conductance and consequently by lowtranspiration. However, mature leaves exhibitedhigher stomatal conductance and hence highertranspiration rates than immature leaves (fig 3aand b). Higher transpiration rates in mature leavesresulted in a lower leaf water potential in the earlyafternoon, and this response might be explainedby the greater changes in ddue probably to thegreater change in dP and/or d.In most leaves,about 80% of the water potential changesbetween full turgor and incipient plasmolysishave been attributed to a change in P (Cheung etal, 1975). Since the capacity for osmoregulationwas almost the same in mature and immatureleaves (table II), the greater changes in dinmature leaves could be attributed to the higherchanges in dP in relation to water loss. Assumingthat the elasticity of cell walls determines the rateof change in turgor pressure in relation to waterloss, the higher values of ϵ inmature leaves areresponsible for the greater changes in dP inrelation to relative symplasmic water loss. Bulk

modulus of elasticity (ϵ)and thus cell wallrigidity, increased almost 2-fold from anintermediate stage of leaf development (L1) toleaf maturity (L2, L3). This considerable increaseof ϵ inrelation to leaf maturity could beinterpreted as a physiological mechanismconferring on mature vine leaves the capability tochange rapidly their water potential (high d)inresponse to small leaf water losses (low dRWC).The high values of ϵ,observed in the matureleaves of vines, mean a more rigid cell wall and

consequently easyloss of

turgorunder water

stress conditions. At this stage of leaf ontogeny,when the leaf expansionhas been completed, thisphysiological mechanism might be considered asmore efficient to face the typical short-term waterstress conditions occurring in the Mediterranean


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region through the large diurnal changes in airtemperature and because of poor soil wateravailability. In addition, the drastic decrease in in mature leaves could also help to maintain wateruptake from drying soils without undergoing alarge tissue water deficit. These characteristics ofwater relations of mature leaves should favourproduction under drought conditions (Abrams andMenges, 1992).

Conversely, immature vine leaves arecharacterized by more elastic cell walls. Lower ϵin immature leaves resulted in the maintenance ofa positive cell turgor at lower values of leaf watercontent than in mature leaves. The maintenanceof turgor in still expanding leaves is veryimportant for plant growth(Boyer, 1988; Matsudaand Rayan, 1990). It seems, that the higher cell

wall elasticity of immature leaves is an effectivestrategy that enables the vines to maintain apositive pressure in cells, large enough to sustainenlargement and thus plant growth under mildwater stress conditions.

REFERENCES

Abrams D, MengesS ( 1992) Leaf ageing and plateaueffects on seasonal pressure volume relationships inthree sclerophyllous Quercus species in southeastern USA. Funct Ecol 6, 353-360

Andersen N, Jensen R, Losch R (1991) Derivation ofpressure volume curves by a non-linear regressionprocedure and determination of apoplasticwater. JExp Bot 42, 159-65

Boyer S (1988) Cell enlargementand growth-endorsedwater potentials. Physiol Plant 73, 311-316

Bressan R, Nelson D, Iraki N (1990) Reduced cellexpansion and changes in cell walls of plant cells

adapted to NaCl. In: Environmental Injury to Plants(F Katterman, ed), Academic press, 137-169

Cheung S, Tyree T, Dainty J (1975) Water relationsparameters on single leaves obtained in a pressurebomb and some ecological interpretations. Can JBot, 53, 1342-46

Dufrene E, Saugier B (1993) Gas exchange of oil palmin relation to light, vapour pressure deficit,temperature and leaf age. Funct Ecol 7, 97-104

Hsiao T, Silk W, Jing J (1985) Leaf growth and waterdeficits: Biophysical effects. In: Control of LeafGrowth, Society for Experimental Biology (NRBaker, WJ Davies, CK Ong, eds), Seminar series27, Cambridge University press, Cambridge,239-266

Jones H ( 1986) Plants and Microclimate. CambridgeUniversityPress, Cambridge,213-223

Koide R, Robichaux R, Morse S, Smith C (1991) Plantwater status, hydraulic resistance and capacitance.In: Plant Physiological Ecology (R Pearcy, JEhleringer, H Mooney, P Runder eds), Chapmanand Hall, London, 161-183

Lakso A, Geyer A, Carpenter S (1984) Seasonalosmotic relations in apple leaves of different ages. J

Am Soc Hortic Sci 109, 544-547

Matsuda K, Rayan A (1990) Anatomy: A key factorregulatingplant tissue response to water stress. In:Environmental Injury to Plants (F Katterman eds),

Academic press, 63-87

Noitsakis B, Tsiouvaras C (1990) Seasonal changes incomponents of leaf water potential and leaf area

growth rate in kermes oak. Acta Oecologica 11,419-427

Nunes M, Catarino F, Pinto E (1989) Strategies foracclimation to seasonal drought in Ceratonia siliqualeaves. Physiol Plant 77, 150-56

Parker W, Pallardy S (1985) Genotypic variation intissue water relations of leaves and roots of blackwall nut (Juglansnigra) seedlings.Physiol Plant 64,105-110

Pavlik M (1984) Seasonal changes of osmotic

pressure, symplasmicwater content

and tissueelasticity in the blades of dune grasses growinginsitu along the coast of Oregon.Plant Cell Environ7, 531-9

Pearcy R, Ehleringer J, Mooney H, Runder P (1991)Plant Physiological Ecology. Chapman and Hall,London

Saleo S, LoGullo M (1990) Sclerophylly and plantwater relations in three Mediterranean Quercusspecies. Ann Bot 65, 259-270

Sandanam S, Gee G, Mapa R (1981) Leaf waterdiffusion resistance in clonal tea (Camellia sinensisL): Effects of water stress, leaf age and clones. AnnBot 47, 339-349

Scholander P, Hammel H, HemmingsenE, BradstreetE (1964) Hydrostatic pressure and osmoticpotentials in leaves of mangroves and some otherplants. Proceedings of the National Academy ofSciences, USA 51, 119-25

Schulze E (1986) Whole-plant responses to drought.In: Plant Growth and Salinity (N Turner, Passioura

J, eds),CSIRO

Australia,127-141

Snedecor G, Cochran W (1980) One wayclassifications; analysis of variance. In: StatisticalMethods (G Snedecor, Cochran W, eds), TheIOWA State UniversityPress, 205-238


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Thomas H, Stoddart J (1980) Leaf senescence. AnnRev Plant Physiol 31, 83-111

Turner N (1981) Techniques and experimentalapproaches for the measurement of plant waterstatus. Plant and Soil 58, 339-366

Turner N, JonesM

(1980) Turgor maintenance byosmotic adjustment: A review and evaluation. In: Adaptation of Plants to Water and High

TemperatureStress (N Turner, P Kramer, eds) JohnWiley& Sons, 87-103

Tyree M, Karamanos A (1980) Water stress as anecologicalfactor. In: Plants and their AtmosphericEnvironment (J Grace, E Ford, P Jarvis, eds),Blackwell, Oxford, 237-261

Tyree M, Jarvis P (1982) Water in tissues and cells. In:Encyclopaedia of Plant Physiology (O Lange, PNobel, C Osmond, H Zeiger, eds), New Series, Vol.12B, Springer-Verlag, Berlin, 35-77

Vos J, Oyartzun J (1988) Water relations of potatoleaves II. Pressure volume analysis and inferencesabout constancy of the apoplasticfraction. Ann Bot62, 449-54

Wilson J, Fisher M, Schulze E, Dolby G, Ludlow M(1979) Comparison between pressure-volume anddew point-hygrometry techniques for determiningwater relations characteristics of grass and legumeleaves. Oecologia 41, 77-88

water relation parameters in drougth

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