PHYSIOL. PLANTARUM 70: 175.-182. Copenhagen 1W7

Rose leaf elasticity changes in response to mycorrhizalcolonization and drought acclimation

R. M. Aiige, K. A. Sehekel and R. L. Wample

Augd, R. M., Sehekel, K. A. and Wample. R, L. 1987. Rose leaf elasticity changes inresponse to mycorrhizal colonization and drought acclimation. - Physiol. Plantarum70: 175-182.

Tissue elasticity can affect plant response to drought, in terms of turgor maintenanceand water uptake from drying soils. The purpose of this study was to determine theeffect of myeorrhizal colonization and drought acclimation on rose (Rosa hybrida L.cv, Samantha) leaf elasticity. Bulk elasticity was characterized by the pressure-volume method using plots of the elastic modulus as a function of leaf turgor press-ure, total water potential and relative water content. The treatments, arranged in a2x3 factorial design, included acclimated and unacclimated plants, and either Glo-mus intraradices Schenck and Smith, Glomus deserticola Trappe, Bloss and Menge.or a non-mycorrhizal control. Plants with root mycorrhizal colonization showed re-duced leaf elasticity (i.e. higher elastic moduli) over a broad range of leaf water po-tential and water content. Both mycorrhizal colonization and acclimation facilitatedthe maintenance of positive values of turgor and elasticity at lower leaf water poten-tial and water content than in controls. Mycorrhizai infections may aid plants in ac-climating to water deficits through effects on leaf tissue elasticity.

Additional key words - Elastic modulus. Glomus, mycorrhizae, pressure-volume,Rosa hybrida, turgor, water relations.

R. M. Auge (reprint requests), K. A. Sehekel and R. L. Wample, Dept of Horticultureand Landscape Architecture, Washington State Univ., Pullman. WA 99164-6414,USA.

Introduction

It is believed that cell turgor plays an important role inthe transduction of cellular water status into physiolog-ical and biochemical terms (Zimmermann 1978). At aparticular water potential or water content, the turgorpressure (pressure potential) in higher plant tissues de-pends upon the osmotic potential and tissue elasticity(Turner and Jones 1980; Radin 1983). Turgor pressurechanges are coupled with voiume changes by the elasticproperties of the cell walls, described by the so-calledvolumetric elastic modulus, E (Zimmermann 1978).The bulk tissue elastic modulus is a useful parameter forrelating overall tissue turgor pressure changes tochanges in tissue water content.

Turgor maintenance in tissues subjected to water defi-cits is facilitated by osmotic adjustment (lUmer and

Received 18 August, 1986; revised 14 January, 1987

Physiol PlaMatum 70.1987

Jones 1980, Bowman and Roberts 1985). The fractionof total osmotic adjustment attributable to active soluteaccumulation varies with species and environmentalconditions, and may be small (Fereres et al. 1979, Joneset al. 1985). The amount of passive osmotic adjustmentdue to tissue water loss depends on the elasticity of thecell walls (Jones et at. 1985). Therefore, wall elasticitymay exert a substantial influence on total osmotic ad-justment.

Mycorrhizae can alter host response to drought stress(Nelsen and Safir 1982, Allen and Boosalis 1983, Busseand Ellis 1985, Auge et al. 1986c, Bildusas et al. 1986).One tnechanism for this alteration is an enhancement ofosmotic adjustment in mycorrhizal plants (Auge et al.1986b). A mycorrhizal effect on tissue elasticity couldfurther aid drought acclimation. The present papercharacterizes elasticity relationships in rose leaves, and

175

summarizes the effects of mycorrhizal colonization anddrought acclimation on bulk elasticity in leaves fromplants of similar size and phosphorus nutrition.

Abbreviations - DW, leaf dry weight; £, bulk leaf elastic mod-ulus; f „,, the maximum value of E; E,y E near full turgor; FW,leaf intermediate fresh weight; g, leaf conductance; K, slope ofthe linear portion of the E-W. relationship at low turgors;PPFD, pholosynthetic photon flux density (400-700 nm); P-V,pressure-volume; RWC, relative leaf water content; RWC*,the RWC at which £ became 0; SW, leaf saturated weight; VA,vesicular-arbuscular; W,,, relative osmotic water content;WSD. leaf water saturation deficit; f bulk leaf turgor press-ure; V (EJ, W. at £„; f^, bulk leaf water potential;

J, *k.f at £•„;*:,. bulk leaf osmotic potential; ^ ( E J£„.

minations, allowed to dry on the laboratory bench, andincremental water losses derived by weighing.

Balance points were observed through a microscopemounted above the pressure chamber (SoilmoistureEquip. Corp., Santa Barbara, CA), and appropriateP-V precautions observed (Kikuta et al. 1985). In com-puting P-V parameters, no attempt was made to correctfor pcKsible cavitations occtming during leaf dehydra-tion.

The bulk tissue elastic modulus, E, was calculated asthe slope of the relationship between bulk leaf turgorpressure (Wp) and relative osmotic water fraction (W^)for every increment in tissue water loss (Zimmermann1978, Jones and Turner 1980, Jones et al. 1985):

E = d'^'/(dV/V„,) (1)

Materials and methods

Bulk elastic moduhis derivation

Tissue elasticity was characterized with the pressure-volume (P-V) approach (Tyree and Hammel 1972, Rich-ter et al. 1979), using the bench top drying procedure(Robichaux 1984, Ritchie and Roden 1985) on leavessampled as described earlier (Auge et al. 1986b).Leaves were removed from the chamber between deter-

where V is the cumulative volume of water evaporatedduring the P-V determination, and V^, represents thetotal volume of osmotic (symplastic) water in the leaf.Vp was calculated from the measurements of leaf waterpotential and the osmotic potential obtained from theP-V regression. W^ was calculated as:

(2)

Tab. 1, Summary of the bulk leaf elasticity for non-mycorrhizal and myccrrhizal rose plants, which were unacclimated or ac-chmated to drought stress. Given for each treatment are: the initial slope K of the £-turgor pressure relationship; £„, the maxi-mum bulk elastic modulus; £j, E near full turgor; ^JEJ, the turgor pressure at E^; Wj^(E^), the osmotic potential at £„,;V^^EJ, the total leaf water potential at £„; and RWC?, the value of the RWC as £ became zero (turgor loss point). Values aremeans of 6 replicates, with ± SE listed beneath each mean (except for K, in which regression coefficient is listed; see text for de-tails). Linear contrast indicate non-significance (NS), or significance at the 5% (*), 1% ( " ) or 0.1% (**•) level.

Treatment

Non-mycorrhizal

G. intraradices

G. deserticola

Linear contrasts

Unacclimated

Acclimated

Unacclimated

Acclimated

Unacclimated

Acclimated

Non-mycorrhizal vs mycorrhizalUnacclimated vs acclimatedG. deserticola vs G. intraradicesMycorrhizae x acclimation

K

19.5I = 0.98

15.6r = 0.98

8.7r = 0.85

9.1r = 0.91

10.0r = 0.93

7.9r = 0.98

-

(MPa)

8.01.29.10.8

7.41.1

9.11.6

10.71.1

9.70.9

NSNSNSNS

(MPa)

2.20.6

1.30.3

3.60.6

2.90.5

3.21,0

2.50.5

*

NSNSNS

'VJEJ(UTa)

0.350.02

0.540.07

0.580.10

0.690.11

0.740.08

0.990.07

NS

(MPa)

-1.300.05

-1.560.06

-1.600.10

-1.760.11

-1.620.10

-1.900.06

NSNS

(MPaJ

-0.960.05

-1.010.07

-1.020.13

-1.070.09

-0.870.05

-0.920.07

NSNSNSNS

RWC"

88.40.7

86.00.9

82.02.0

83.02.0

81.42.0

82.21.1

NSNSNS

176 Pbysiol. PlantanimTO, 1987

Fig. 1. Relationship betweenthe leaf elastic modulus, E,and turgor pressure, ^'p,for unacclimated andacclimated rose plants. Eachpoint is the mean of 3measurements, SE for Eranged from 0.1 to 2.7 MPa,and for % from 0.00 to 0,08MPa. o, non-mycorrhizalplants (N); A, Glomusintraradices-cdkmu^d plants(1); • , Glorruts deserticola-colonized plants (D).

10.0

8.0

O10.0

UJ S.0

2.0

0.0

Acclimated

Unacclimated

0.0 0.4 0.8 1.2 1.6

Leaf turgor potential (MPa)

To determine V^,, the inverse of the balance pressure(P ') (y axis) was plotted against V (x axis) and a leastsquares linear regression was fitted to the linear seg-ment of the curve. This line represented the relationshipbetween the inverse of the leaf osmotic potential andthe volume expressed, V, with the x intercept itidicating

Leaf water saturation deficit (WSD) was calculated as(Kramer 1983):

WSD = 100 (SW - FW)/(SW - DW) (3)

where SW, FW and DW were the saturated weight, in-termediate fresh weight and dry weight, respectively, ofleaves used in P-V relations. Leaf relative water content(RWC) = 100 - WSD.

As £ is generally not constant at all leaf turgor press-ures ("Pp), water potentials (Vt,,) and relative watercontents (RWC); (Cheung et al. 1976, Jones et al.1985), E was characterized using plots of £ as a functionof each of these variables. In addition, the followingparameters were recorded: K, the initial slope (at lowturgors) of the f-Vp relation (Roberts et a!. 1981); £„ ,the maximum value of £ obtained from each replicate's

P-V plot; £i, the bulk elastic modulus near full turgor;Wp(£,), <?,(£„) and *,eaf(£^). the turgor pressure, os-motic potential and bulk leaf water potential, respec-tively, at £„; and RWC, the value of leaf RWC as Ebecame 0 (the turgor loss point). K was derived by re-gressing the means in the linear portion of the E-^^plots in Fig. 1.

nant culture, inoculatiaii procedures and growth roomcooditiQiis

Rosa hybrida L. cv. Samantha plants were grown in agreenhouse in calcined montmoriltonite clay (Turface;IMCore, Mundelein, IL) with one of three vesicular-arbusctilar (VA) mycorrbizal inocula originally incorpo-rated at a rate of 1 inoculum;4 Turface (v/v). Inoculumof both Glomtis deserticola Trappe, Bioss and Mengeand Glomus intraradices Schenck and Smith consistedof fresh soil and mycorrhizal root pieces of Glycine max(L.) Merr cv. Maple Amber and ROM hybrida L. cv. So-nia, grown in sand. The third inoculum, a control, wasan autoclaved mixture (1;1, v/v) of the above two irvoc-ula. All plants received appropriate inoculum water ex-tracts (final sieve 25 ^m) to establish the microflora as-

Physiol. PUuiUtruM TO, 1987 177

(A

10.0

8.0

6.0

4.0

2.0

10.0

••— 8.0CD

JS[jj 6.0

4.0

2.0

0.0

-1 r

Acclimated

Unacclimated

Fig. 2, Relationship between the leaf elasticmodulus and relative water content, RWC,for unacclimated and acclimated rose plams.Each point is the mean of 3 measurements.SE for £ ranged from 0.1 to 2.7 MPa, and forRWC from 0.1 to 1.6%. D, non-mycorrhizalplants (N); A, Glomus intraradices-colo-nized plants (I); • , Gtomtis deserticoia-colo-nized plants (D),

76 80 84 88 92 B6 100

Leaf relative water content (%)

sociated with each inoculum (Linderman and Hendrix1982).

At seven months, plants were transplanted into 25 cmpots, and at nine months plants of similar size weremoved into a controlled environment growth room fordrought acclimation and water relations studies.Growth room PPFD ranged from 290 to 350 junolm"^ s ', with a 14 h photoperiod. Day/night tempera-ture and relative humidity were 22/]7°C and 40/90%, re-spectively. Plants were watered daily throughout the ex-periment, and every other day received 10.4 mAf N and3.1 mM K (as Peter's 15-0-15 soluble fertilizer; W. R.Grace & Co., Fogelsville, PA). Control and mycorrhizalplants received weekly 3.0 and 0.7 mAf P, respectively.

Drought acclimation procedure

For acclimation, plants were allowed to dry until leafconductance (g) declined to 1.0 mm s ' , and then wererewatered. Four such cycles were repeated on 6 rep-licates of each of the non-mycorrhizal and mycorrhizaltreatments, for a total acclimation time of 17-20 days.Unacclin:iated treatments were watered daily, with g re-maining above 5.6 mm s"'. Leaf conductance was deter-

mined with a porometer (Delta-T Devices, Cambridge,England) as previously described (Auge et al. 1986a).Fertilization was discontinued during the acclimationperiod.

Colonization and phosphorus fcveis

Roots recovered from three soil cores from each plantwere cleared in 10% NaOH (w/v), stained with chlor-azol black E (Brundrett et al. 1984) and mycorrhizalcolonization quantified as described earlier (Biermannand Linderman 1981, Auge et al. 1986a). P content oflyophilized leaves was assayed immediately before andafter the drought-acclimation cycles, by the vanadate-molybdate-yellow method, on samples (4—8 per treat-ment) dry-ashed with magnesium nitrate and digested innitric acid (Chapman and Pratt 1%1).

Statistics

A 2x3 factorial, completely randomized design wasused, with two preconditioning treatments (unaccli-mated or drought-acclimated) atid three mycoTThizaltreatments (G. deserticola, G. intraradices or a non-mycorrhizai control). P-V curves were determined for 6

178 Physiol. Plantarum 70. 1987

Fig. 3. Relationship betweenthe leaf elastic modulus andtotal water potential, *̂jea(,for unacclimated andacclimated rose plants. Eachpoint is the mean of 3measurements, SE for Eranged from O.C to 2.1 MPa,and for *,,., from O.OO to0,08 MPa. • , non-mycorrhizal plants (N); A,Glomus intraradices-colonized plants (1); • ,Glomus deserticola-coloaizedplants (D).

10.0

8.0

6.0

(0Cu

(0

4If)

I

10.0

8.0

4.0

2.0

0.0

Acclimated

Unacclimated

-2.8 -2.4 -2.0 -1.6 -1.2 -0.8 -0.4

Leaf water potential (MPa)

plants per treatment, Univariate analyses of variancewith specific linear contrasts were performed to partit-ion the variance into the main effects and the inter-action between the two factors (Steele and Torrie 1980).Four contrasts, each involving more than two treat-ments, are listed in Tab. 1, as are SE for each mean.

Results

The relationships between £ and leaf W^. RWC and•Pfca, are shown in Figs 1, 2 and 3. Leaf £ of mycorrhizalplants was greater than in non-mycorrhizal plants overmuch of the £-RWC and £-W|^( plots, particularly inthe drought range (Figs 2 and 3). Below W^^, of -1.0MPa and RWC of about 92%, £ values were alwayshighest for the mycorrhizal plants, in both the unac-climated and acclimated treatments. When plotted as afunction of fp , £ was greatest at higher turgors in themycorrhizal plants (Fig. 1). Both mycorrhizal and ac-climation treatttients permitted the attainment of higherturgor pressures at full tissue saturation (Fig. 1).

Maximum values of £ did not differ appreciably inmagnitude among treatments (Tab. 1). £„ was devel-oped in both mycorrhizal treatments at turgor pressuresapproximately 0.2 to 0.4 MPa greater than in non-my-corrhizal treatments, regardless of acclimation (Tab, 1,Fig. 1). As indicated by linear contrast, 4'p(£^) wasgreater in G. d«ersco/fl-colonized roses than in G. in-traradices-co\oniisd roses (Tab. 1). Acclimated plantsalso had significantly higher *l'p(£J (Tab. 1). Treat-ment effects on W,(£^) corresponded to effects on4',(£^) (Tab. 1). Those plants with higher turgors at £„were also plants with lower W, at £„,. llie result wasthat W||,j((£^) remained relatively unaffected by accli-mation or mycorrhizal colonization. £„ was attained ata fie., of ahout -1.0 MPa and a RWC of about 93% inall treatments (Tab. 1, Figs 2 and 3).

£[ was increased by an average of 75% by mycorrhizalcolonization (Tab. 1, Fig, 1), and K values in mycorrhi-zal plants were approximately half that of non-mycor-rhizal plants (Tab. 1). RWC, unaffected by acclima-tion, was significantly affected by colonization by bothfungi (Tab. 1). Overali, mycorrhizae decreased RWC"

Phvsioi. Plant3rum 70. t 179

3

£ao

1

1.8

1.4

1.2

1.0

0.6

0.6

0.4

0.2

0

1.2

1.0

0.6

0.6

0.4

0.2

0

- ^ s ^

'• \\ \ •

D ^e \

1 1 11 1 1 1

V \c \

• \ \

N V-v1 1 ° ^̂ 1̂ °

AccBm«»<i

-

-

-

-

•~ . M ,

1 1 11 1 1

UncccUiiMted

-

-

-

-

":—f ,0 4 a 12 16 20 24

Water saturation deficit (X)Fig. 4. Relationship between leaf turgor pressure, fp , and wa-ter saturation deficit, WSD, for unacclimated and acclimatedrose plants. WSD = 100 - RWC. Each point is the mean of 3measurements, SE for f ranged from 0.00 to 0,24 MPa, andfor WSD from 0 1 to 1.6%. • , non-mycorrhizal plants (N); A,Glomus inrrarffrfic^j-colonizcd plants; • , Glomus deserticola-colonized plants. Tlie curve for the mycorrhizal plants (M) wasfitted to data points from both Glomus species.

(which also represents the turgor loss point) by 39%.Because turgor was maintained at lower RWC, positivevalues of £ were also maintained at these lower RWC inmycorrhizal plants (Fig. 2). Regardless of acclimation,W, was greater at any particular level of WSD whenplants were mycorrhizal (Fig. 4).

Generally, leaves from plants colonized by eitherGlomus species displayed similar values for elasticityparameters (Tab. 1, Figs 1 and 2). Colonization levelswere 53 to 66% for unacclimated mycorrhizal plants,and 76 to 83% for acclimated mycorrhizal plants.

Plants from all treatments had adequate phosphoruslevels. Leaves of non-mycorrhizai, G. intraradices-colo-t]ized and G. dcjertico/o-colonized plants had 0.3,0.2-0.3 and 0.2% P, respectively. Drought acclimationhad no effect on leaf P content.

Disciission

ColonizatiotJ of rose plants by both Glomus species af-fected leaves by generally decreasing elasticity (i.e. in-

creasing the elastic modulus), particularly in the lowerranges of I*,,,, and RWC. Although low £ and E, values(corresponding to flexible cell walls) have been corre-lated with drought-adaptation in Dubautia (Robichaux1984, Robichaux and Canfield 1985) and may providecells with a high resistance to short-term water stress(Zimmermann 1978), a review of recent literature sug-gests that an increase in tissue rigidity often occurs in re-sponse to water deficit stress and may offer physiolog-ical and ecological advantages (Cheung et al. 1975,Jones and Turner 1978, Zimmermann 1978, Jones et al.1980, Monson and Smith 1982, Bowman and Roberts1985). Tissues with rigid cell walls undergo a greater de-crease in Wp per unit decrease in water content relativeto more elastic walls. This can reduce tissue water defi-cits associated with decreases in plant W,,,,, mostlythrough decreases in fp (Cheung et al. 1975, Zimmer-mann 1978, Monson and Smith 1982). This effect mayalso facihtate continued water uptake from drying soils(Bowman and Roberts 1985). Hence, in terms of anelasticity response, mycorrhizal colonization may offeran advantage to plants growing in dry soils. The aboveresults for mycorrhizal plants are consistent with the ob-servation that mycorrhizal hosts may have thicker cellwalls than non-mycorrhizal plants, through increasedlignification and production of other polysaccharides(Dehne and Schonbeck 1979).

Values of £ varied with "Pp changes in all treatments(Fig. 1), an effect reported previously for many species(Cheung et al. 1976, Zimmermann 1978, Roberts et al.1981, Robichaux 1984, Bowman and Roberts 1985,Jones et al. 1985). Roberts et al. (1981) have describedfour general response classes for tbe relationship be-tween £ and Wp. Unacclimated, acclimated, mycorrhi-zal and non-mycorrhizal roses all displayed a "type 111"response; an initial increase of £ with increasing turgor,then a peak in £ at some intermediate turgor, followedby a decrease in £ with further increase in turgor(Fig. 1). These plots resemble those in a drought-adapted Dubautia species (Robichaux 1984, Robichauxand Canfield 1985). Similar plots exemplified the £-RWC and £-W|e,( relationships, as well. As was the casefor the type III response in Acer rubrum (Roberts et al.1981) and in the dry-habitat Dubautia (Robichaux 1984,Robichaux and Canfield 1985), the initial increases inthe £-"I'p plots for rose were linear. These initial slopes(K) showed that non-mycorrhizal leaves were twice asresponsive in elasticity changes with changes in Vp, inboth unacclimated and acclimated plants (Tab. 1).

The significance of the different £ response types canbe illustrated by plots of turgor as a function of watersaturation deficit (WSD); (Roberts et al. 1981, Rob-ichaux and Canfield 1985). Figure 4, which shows a sig-moidal relation between Wp and WSD in rose, closelyresembles the type III response reported by Roberts etai.; a slow loss of turgor at low water deficits preceeds abreak in the response with rapid loss of turgoi at inter-mediate water deficits, and finally atiother break in the

180 Hiysiol, Ptmtanun 70. 1987

response to again a slow loss of turgor at the larger defi-cits. Turgor response patterns are important because oftheir effect on turgor-sensitive processes, and also be-cause of the large contribution of the "Pp component toWij,,, which in turn regulates the water potential gra-dient responsible for water uptake (Roberts et al. 1981),At most WSD, turgor was greater in mycorrhizal plantsthan in non-mycorrhizal plants, again suggesting the ac-climative value of mycorrhizal colonization, especiallyto short-term water deficits (Fig. 4, lower graph).Greater W^ at a particular RWC has also been demon-strated to occur in response to drought in desert species(Monson and Smith 1982) and in the droughtadaptedspecies Dubautia ciliotata and D. menziesii (Robichaux1984, Robichaux and Canfield 1985).

Cheung et al. (1975) found that the difference be-tween the Wj at full and zero turgor for a given tissuetended to be smaller when cells have more rigid walls.The reverse was observed in mycorrhizal rose leaves.Plants colonized by both Gtomus species had the great-est difference between, as well as the lowest values for,W, at full and zero turgor (Auge et al. 1986b). My-corrhizal plants also tended to have stiffer walls (higher£ values). As Bowman and Roberts (1985) note, thiscombination of characteristics, a high £ and a low Vjj,may allow decreases in W^^,, to occur, facilitating wateruptake and maintaining positive turgor at low soil mois-tures. Mycorrhizal roses have been shown, in fact, tomaintain turgor at lower soil water potential than un-colonized roses (Auge et al. 1986b).

The functional dependency of the elastic properties ofbulk tissue on tissue water status is complex (Roberts etal. 1981) and a decision as to the exact function of elas-ticity changes in the regulation of plant water relationscannot be made with any certainty at the present time(Zimmermann 1978). Nevertheless, mycorrhizal colo-nization of rose roots does influence the elasticity ofleaves, and this influence, indicative of enhanced ac-climation to drought stress, is in accord with previousstudies on rose plant water relations and mycorrhizae(Auge et al. 1986a,b,c).

Acknowledgements - The senior author gratefully acknowl-edges the immense support of Ms. Kay Peterson. Scientific pa-per No. 7527, Agriculture Research Center, Washington StateUniv.

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