Table Of ContentThe Great Basin Naturalist
PuBLisiiKi) AT Phono, Uiaii, by
Bkicii \\i Young UxixKusnT
ISSN 0017-3614
Volume53 30June 1993 No. 2
Great Basin Natiinilist53(2). pp.UT-IOH
DEPLETION OF SOIL MOISTURE BY T\\ O COLD-DESERT BUN(:IK;RASSES
AND EFFECTS ON PIIOTOSYNTHETIC PERFORMANCE
Jay E. Andi'i-.son and Xancet' L. Toff
—
.\HSTIUC.T Tiiis .studycompared the abilities oft\v()cool-season hunciigrasses toextract moistnre from adr\ingsoil
andcompared plioto.s\iithetic and stoniatal responses ofthe twospecies as soil moistnre snpplieswere depleted. When
thrownin49-Lpotsinagreenhonse,Lcipniiscinereiisextractedmorewaterhomthesoilandmaintainedhighergasexchange
ratestolowerahsolnteamountsol'soilwaterthandidA^ropi/rondcsciioniin. Thesoilwatercontentatthelowerlimitof
extraction was ]().37c forL. ciiicrfiis ami lo.o'/f for.A. (Icsciionnn. Wlicn soil moisturewas expre.s.sedasextractablesoil
water there was little difference l)eti.\een the species in [)attcrn ofWafer use. Both species miiintained higii stomatal
conductances(g«)iuidphotosvntheticrates(A)untilextractablesoilmoisturewasreducedtoabout 15%. Forfield-grown
plantsunderseverewater.stress,AwashigherinL.clncrcusthaninA.dc.scilontiiiatcomparableleafwaterpotentials.Tiie
relationshipbetweenAandgwwassimilarforthetwospecies;thehigherAinL. cinereuswasaconsecjueuceofhighergw.
Thus,higher.A inL. cincrciisisachie\edthrough somesacrificeofwater-useefficieucv.
Kcij words:extract(IIlie -.ailicalcr. IctiJnatcrf)<)t<-uliid. slmiuitiilcoiidiicttiiicc. uritcr-usreffirirun/. Le\-mus cinereus.
Agropxrondesertonnu.
Plant species \di\ wide!) in their tolerance ComstockandEhleringer 1984, Ehleiingerand
ofseiLsonciIdroughtandinthenlechtinismsthey Cook 1984, DcLucia and Heckathoni 1989,
usetocopewithdecliningsuppliesofsoil mois- Cha\'es 1991) as well as changes in the diunial
ture. Somespecies tolerate seiisonaldrouglitIn patterns of giis exchaiige (Schul/e and Hall
maintaining high leafwater potentials through 1982,Tenhunenetal. 1987). Itclearl\-wouklbe
stomatal clostu'e (Turner 1979). Although the\- advantageous (orsuch species to niiiintiiin pho-
may niiiintain a liigh photo.synthetic capacitv; to.sMithetic rates as high as possible as soil and
low stomatal conductance will sexereh' restrict plantwaterpotentials decline.
carhon gain under prolonged drought. In con- .Asplantsc.xtractwaterfromadningsoil,the
trast, otherspecies allowtheirleafwaterpoten- ainoun( ()l plant-aMiilablewaterdecreasesexpo-
tials to drop as soil wat(^r potentials decline nciitialK with decrciising water potential (e.g.,
(Turner 1979). This enables the plant to con- SlatNcr 1967, Fig. 3.3). Conse(juentl\, the \'ol-
tinue to extract water from a dmny; soil, but tnneofwatergainedIn*aplantin dningagixen
decreases in leaf water pot(Mitial t\picall\ are \olunie ofsoil to -2.0 .MPaoxer that giiined in
accompaniedb\-decreasesinphoto.sxmtheticca- dning a soil to -1.5 MPa. for example, is .so
pacity and stomatal conductance (Jones 1973. small that it would se(Mn rather nciiliiiible in
.^DepartmentofBiologicalSciences,klalioStateUiii\ersit\.Pocatcllo,IdalioS.52()9.
-20nFifthStreet..Apt.2C,Daxis.Cdifornia95616.
97
)
98 C;l\KAT 1^ASIN NATURALIST [Volume53
tenns of totiil carbon gain. Jordan and Millcr preliminaiygas-exchangedatafromfield-grown
(1980) and Jordan et al. (1983) estimated tliat plants (J. Anderson unpublished data) sug-
the additionalwatei"made availableto acrop as gestedthatL. cinercusplantsmaintainedhigluM-
aconsequence oflowering leafwaterpotential photosvntheticactivit)'and hadhigherstomatal
"afewbars"wouldsupporttranspirationonlyfor conductance at low leaf water potentials than
3 or 4 days in the absence of additional root didA. desei-fonintplants.Totestthosepossibili-
growth. Thus, there would seem to b(^ little ties,weconductedagreenhouseexperiment(1
advantage in making the necessar\'osmotic ad- to compare photosvnthetic and stomatal re-
justment and/orotherleafmodifications to tol- sponses of these two species to dning soil and
erate very low water potentials, and we might (2) to compare the lower limit ofextraction of
expectlittle diffenMice amongdrought-tolerant the twospecies. In addition, wecomparedpho-
species in their lower limit ofextraction ofsoil tosvnitheticcapacityandconductanceofthet\\'o
moistm-e. [Weuse Ritchies (1981) definitionof species underwaterstress imposednaturall)'in
the lower bmit of extraction: the amount of the field. Bothlaboratorvandfielddatasupport
water remaining in the soil when plant growth the hyjoothesis that, in comparison toA. dcser-
andactivit)'completelystop.] torhun, L. cinereiis plants deplete soil moisture
On the other hand, tolerance of \'en low reserves more completelv and maintain higher
plant walcr potentials ma\' offer adxantages photosynthetic rates as water supplies are di-
otherthan gainingmorewaterfromaparticular minished; but this is achiex'ed through some
volumeofsoil. Osmoticadjustment mavenable sacrifice inwater-use efficiencybyL. cinercus.
a plant to maintain turgor in growing roots
which,inturn,wouldenabletheplanttoexplore Methods
the soil for additiontil water reseives (Shaipe
and Da\i(\s 1979, Jordan et al. 1983, Westgate Plant Nhiterials
and Boyer 1985, Tuhkm- 1986). Drought-toler-
Lcijinus cinercus is a robust tussock grass
ant sagebrush {ArtcDiisUi trichnifata) plants of nati\etocolddesertsandlowermountiiinslopes
the Great Basin of North America mo\'e water throughout the Intennountiiin West of North
at night along a h\draulic gradient in the roots America. Itoccurs on alkalineorSiiline lowland
from deep in the soil to drier soil at shallow sites as well as nonsciline upland sites, often on
depths (Richards and Caldwell 1987, Caldwell deep soils (Young and Exans 198], \\alkerand
and Richards 1989). The water deposited in Brotherson 1982). Plantsusedinthisstudvwere
thoseshallowlaxerscanbeextractedthefollow- transplanted from a near monoculturiil natural
ing day to support carbon gain or other plnsi- stand at the Idalio National Engineering Labo-
ological activity-. Finally, Caldwell (1985) ratoi-y(INEL).Additionalinfoniiationconcern-
postulatedthat diying asoil toaxeiv lowwater ing the ecoplnsiolog\' of L. cinercus cm be
Tcohnetseentarmgauymebnetaswaanydofreesxuclltsudsiungggceosmtpetthaittosrisg.- fouAn<d^r(i)npiAinr(d)enr(sIocnseerttoaln.mi(1i9s93a)t.ussock grass na-
nificant but possibly subtle differences could tive to the steppes ofAsia; it is naturalized in
existinthelowerlimit(jfextractionofsoilwater western NorthAmerica,where ithasbeenused
among speci(>s. Indeed, 8inclair and Ludlow extensixely forrangeland rehabilitation. Stands
(1986)foundsmalldifferencesinHu^lowerlimit established bv seeding often persist as near
of extraction among four trojiical legumes monocultures (Marlette and Anderson 1986).
grown in pots. Plantsusedinthisstud\ weretransplantc^dfrom
Anderson et al. (1987) compared the sea- seededstandsatthe INEL.Additional infonna-
sonal patterns of soil water extraction among tion about the ecophvsiolog\" of A. descrtonnn
fourdrought-tolerant,cold-de.sertsp(>cies.The\ can be found in Nowak and Caldwell (1984,
found little (hfference in the lower limit ofex- 1986) and Nowaketal. (1988).
traction among the fourspecies when grown in
monoculturesonacommonsoil;howexer, there Crreenhouse Studies
was some indication that the native bunchgrass Twenty-four pots were constnicted from
Lri/inii.s ciiwrciis (Scribn. & Meir.) A. Lfne pol\Ain\'l chloride pipes; each pot was 1 m tall
might be able toextract more water from a soil and 0.25 m in diameter The pots were filled
than could the introduced species Ag/-('/>///v)// witha 1:1:1 mixtureofBacctopottingsoil,sand,
dcsci-tonun (Fisch. ex Link) ,Schult. In addition and a clay-loam soil used in e.xperimentiil field
19931 Depletion of Soil MoistliU': by Hi nchcirassks 99
plotsattheINEL(seeFieldStudies).Themean A
(and standard error) dr\' mass of soil placed in 100
thepotswas44.4 ± 0.4kg.Atfieldcapacit\-that
volume of soil held 18.5 ± 0.1 L ofwater, or
39.7% water b\- xohnne. Donuant A. descr-
foniin and L. cineretis plants were collected
from standsatthe INELin Noxemberandheld 60
at5°C until 12plantsofeachspeciesweretrans-
planted into the 49-L pots in December. The
plants were placed in agreenhouse where thex' • •
receixednaturiilsunlightsupplementedbyfour 20 - A- desertoruni
i5()()-\\' metal halide lamps. The height ofthe • L- cinereus
lampswassetsothatphotos\Titheticphotonflux
density (PFD) at canopvheightwas 1500-2000
fxmolm~"s 'atmiddawThephotoperiodwas 13h.
Plants were fertilizedtwice aweekwith full
strength Hnakuranutrientsolution (Smith etal. o
Q_
1983).Aftertheplantsbecamewellestablished,
sixofeach species were iissigned randomly to a
well-watered(control)treatment,andtheother
sixwereassignedtoawater-stresstreatment.The
well-watered plants recei\ed nutrient solution O
CL
twice a week and distilled water once a week.
Water stress was induced b\'withholdingwater
from the plants for 50 da\s, after bringing the
soilwatercontenttofieldcapacih. Becauseofthe
large\'olumeofsoiliuidwaterinapot,waterstress
wasimposedgradually,simulatingsoildningthat 10 15 20 25 30 35
occursnaturallyunderfieldconditions. Time (doy)
The potswere weighede\eiy .'3-4 daws dur- Fio;. L (A)Cliangesintot;i]soilwatercontentwithtime
ing the dningperiod to detenuine the amount after withholding water from A<iro])t/r(»t (Icseiiontin and
ofwelterheldin the soil ofeach pot. Soil water Lcyiniis cincrciis plants (dashed Unes) and for irrigated
controls (solidlines)growingin49-Lpotsinagreenhouse.
content was ex-pressed in hvo ways. Percent of (B)SoilwaterpotentiiJ\s.timelortreatmentmeanscorre-
total soil water (TSW) was defined as (\'olume spondingtotliecloseds\inl)olsinA. SeeTable 1 forstatis-
of wi'ter in the soil) / (\olume ofwater at held ticalanuKses.
capacity) X 100. Extractable soil water (ESW;
seeRitchie 1981)wasexpressedasapercentage from pools on tlie floor resulting from the wa-
ofthe difference betx\'een the\()lumetricwater tering ofOtherplants. .Alterda\"36the amount
content at held capacitx and that when growth of wat(M' in the soil decreased to le\els slightK
ofthatspecieshadstopped(da\-50ofthedning lower than those on da\'29. Because growth of
period). Soilwaterpotentialwas lueiisuredwith indixiduals of both species had e.s.sentialK
single-junction, screen-caged psvchrometers stopped b\ da\ 33 and changes in soil water
R. D. Menill Specialt\ EfjuipnuMit, Logan, contentafterthatdatewcm'cnegligible,onK'data
(J.
Utah) placedat soil ck-pthsof150.350,550,and for 33 da\s are included in most anaKs(\s pre-
750 nun in three pots per species containing sentedhere.
water-stressedplants Psychrometricoutputwas Rate ofelongation ol expanding lca\es was
.
monitored ever)' 3—4 days using a model NT-3 usedasan indexofgrowth rate. Leafelongation
nanoNoltmeter (Decagon De\ic(\s Inc., Pull- rate (LER)oftheNoungestleafwasdetennined
man,Washington). 1)\' measuring its length at two times and di\id-
On day33 there was a slight increase in the ingthedifferencein lengthiwthetimeintenaJ.
weight of pots in the water-stress treatment Length measurementswere madeontwovege-
(Fig. 1),butitwas notuntilwenotedasubstan- tati\e tillers perpot on three consecuti\e davs
tial incrcixse in weight of some pots on day 36 eachweek. Leafelongation rates reported here
that we reiilized water had entered some pots were averajiedo\er48 h.
100 Great Basin Natur.\list [^olunie oo
Table 1 GeneralUnearmodelsan;il\sisofsoilwatercontentexpressedaspercentoftotalsoilwater.soilwatercontent
expressedasextractablesoilwater,elongationrateofthexonngestleafonatiller,andsoilwaterpotentialforLeymitscinereii.s
andAf^ropyron dcsertonim plantsgrowingin 49-Lpots inaglasslionse. Foreachdejjendent\ariable, nitiineffectswere
aKva\rincluded in the model, hnt ifinteraction terms were not significant, they were excluded from the model. All
independent xariables in each model were treated as classification \ariables. There were two le\els ofspecies (SP; A.
desertorwnandL. cinereus). Foralldependent\ariabiesinparta,thereweret\vole\'elsofstress(STR;well-wateredand
water-stressed)and 10davs(DAY)afterwaterwaswithheldfromthethewater-stressedplants (1,4,S. 12, 16, 19,22,26,
29,33).Forsoilwaterpotentialinpartb,therewerefourlevelsofsoildepth(DP: 150,350..550,750nun)and9da\s(DAY;
4.8, 12. 16. 19,22,26.29,33).
(a) Effect: Species Stress Da\ SP X STR SP X DAY STR x DAY SP x STR x DAY
Totalsoilwater P< .(X)01 P< .0001 P< .0001 F< .0002 P< .0001
n =228
E.\tractablesoilwater P< .0001 P< .0001 P< .0001
n =228
Leafelongationrate P< .0001 P< .()(X)1 P< .0001 P< .0001 P < .0002 P< .0001 P< .0003
n =25.5
(b) Effect: Species Depth Da\ SP x DP SP x D.\Y DP x DAY SP x DP x D-\Y
Soilwaterpotenti.il P< .002 P< .0001 P< .0001
n = 204
n.s.=notsienificant.
Field Studies of2 kPa. The concentration ofCOo inside the
The field .stiidie.s were conducted at the cu\'ettewas330-340|jlLL~\ Lightwasproxided
INEL Experimentiil Field Station where b\ a 15()-\\' cpiartz halogen projector bulb.
MeiLsurements were made periodicalK
monoculturesofA. desertoniin andL. cinereus
were established b\" transplanting mature indi- throughoutthedningperiodbetween()9()()and
viduiilsfromneiii'ln standstoexperimentalplots 1600 hours; TS\\' in the pot was detennined
having a homogeneous soil to adepth of2.4 m inmiediateK'aftergiis-exchiingemeasurements.
(see Anderson et J. 1987 for details). The soil Forgiis-e.xchangemeasurementsinthefield,
consisted of26% sand, 54% silt, and 20% clay leaftemperaturewas24°C,PFDwas 1900jxmol
and had a bulk densit\' of 1.28 g cm'^^l The m'"s"^ or greater, v was 2.3 kPa, and CO2 con-
measurementsreportedhereweremadeduring centration inside the cuvette was 335 ± 5 |xL
the tliird growing season after the plants were L '. Lightwas from sunlightora 150-\A'projec-
transplanted. torlamp. MeasurementsweremadeinJuneand
Gas Exchange and PlantWater eiU"l\" lul\-between 0830 and 1500 hours.
Potential Measurements Leafwaterpotential (i|;) was measuredwith
a pressure chamber (PMS Instruments Co.,
Net photosynthesis (A), transpiration (E), Corvallis, Oregon) immediatelv after gas-
and leafconductance to watervapor (g,v) were exchange measurements on the same leaf.
measured on the youngest, fullv expanded
leaves,oronthepenultimateleafonatillerafter Calculations and Statistical .\nal\ses
inflorescences dexeloped, with an open, com-
pensatinggiLs-cxchange SNStemwhich has been A, E, andg,, (leafplus boundaiA la\er) were
previousl)'described(Nowtiketid. 1988,Toftet calculated according to CJaemmerer and Far-
al. 1989). Gas-exchimge measurements of (juhar(1981). Ambientatmosphericpressureat
greenhouse-grown plants were made at a leaf the INELisabout85kPa(fieldmeasurements)
temperatureof25°C,aPFDof1900-2000ixmol andat Pocatello, Idiilio, is about86 kPa(green-
m"-s'\andaleaf-to-iiirwatervaporgradient(v) house mecisurements). Statisticiil analvseswere
1993] Depletion oe Soie Moisture hv Buxchcha.sses 101
L. cinereus j
0.8 -
0.4
0.2 -
0.0
Great Basin Natur\list [\blunie53
102
Table2.AnalysesofcwarianceofphotosMitliesis.leafcoiKluctaiicetowater\apor.andintercellularCO2concentration
forlA'wiutscine'reusandA^ropijnmdesertonim plantsgrownin49-Lpotsinaglasshouse (see Fig. 3).Thechissification
variableineachmode!wasspecies(SP):A.desertarum(Agde)andL.ciiwreiis(Leci).Thecovariate(X)w-asatransformation
wofastoXtal=sosielcwa't(eTrS(WTS+W)C.).exwphreersesesdeca"s'piesrtcheenitmoefrtsheesweactaenrtcf-uonnctetnitonatafnidelCdciaspaacciotn\s.tTanhte,gdeenetreirimlifnoerdmIonfittheerattriaonnsfaonrdmagit\i'oenn
hereforeachspt>cies.
Constant
(a) Effect: Species TSW SP X TSW Agde Leci Agdt Leci
PhotosMithesis F<.()04 f<.()001 P<.004 -26.0 -7.8 .86 .83
»i = 16
Conductance P<.()4 P<.()(X)1 •26.5 -13.9 .83 .83
n = 16
biterc-elluliir[CO2] P< .002 P< .005 P< .004 -27.1 -19.5 .67 .86
H = 16
The relatioii.ship l)eh\een A org,, andT.S^^' water per unit ot carbon gained than did A.
forboth .species is shown in Figure 3. To facih- (lcseiioni))L\\hichreflectstheadditionalcostof
tate statistical aiuiKses, numerous modelswere waterpaidto achieve higherA.
fittedtothedatab\ linearregressionandtrans- Aandg„ ofplantssampledinthe fieldinlate
fonnationofthedependentand/orindependent springandearK' summerwere positi\el\'coire-
\ariabl(\ The model that consistentK pnnided latedwithij;measuredconcurrentlv(Fig.4).For
the best lit consideringall datasetswas: bothspecies,Awasmorecloselycorrelatedwith
Y = bo + b, sec ' (TSW + C) wv|;etrheanhiwgahserg„f,oranL.d ctihiewicvoini-seltahtainonfocroeAf.ficclieesnctr-s
where Y is net photosynthesis or leafconduc- fonini. Atthe time fieldsamplingwas initiated,
tance,sec ' istheinxerse secantfunction,C is a L.cincreusplantswereconsiderabK morewater
constiuit,b(i isthe intercept, andbi is the slope. stressed than were A. desei'tonim plants be-
Tiie constants were determined iterati\el\- b\ cause ofdifferences in ES\\" in the plots at the
substituting \alues for them until the highest beginning of the growing sciLson (data not
coefficientofdetermination (K~) was obtained. shown).Thisdifferenceisreflectedbythehigh-
Valines ofC and R' are gi\en in Table 2 for the est v|i N'alues recorded for the two species (Fig.
curves shown in Figure 3. 4).Asaconsequence,thehighestratesofAwere
.\nalyses ofcoxaricUice were perfonned for lowerforL.cincreusthanforA. dcsoionmi and
each gas-exchange variable using species iis a were considerabK lower than the maximum A
cliissification variable and the inverse secant ofL. cincreus obsenedin the greenhouse (Fig.
transfonnation ofTSW as the conariate (Table 3)orforwell-wateredplantsinthefield(Ander-
2). /.. cinereu.s had higher A and g„ than A. son et al. 1993). That most L. cincreus plants
(le.sertonim atboth high and lowle\els ofTSW were stressed while some A. desertonim were
(Fig.3,Table2).Wliensoilwateravailabilit\-was notmayaccountforthehighercorrelationcoef-
expressed in relative tenns, i.e., ESW, the re- ficientsforL.cincreus. DatainFigure4indicate
spon.ses of the two species were .similar. Both that A and g,v were generally lower in se\'erel\'
niiiintainedhighAandg,, untile.xtractablewater stressed A. dcserfonim than in L. ci)icreus at
content reachedabcxit 15%; markedreductions comparable \\i. which is consistentwitli lindings
of A and g« occurred at lower lexels of ESW from the greenhouse study
(diita not shown). The relationshipbetvveen.A.andgu issimiku"
Intercellular CO^ concentrations (q) were for the tvvo species when grown either in the
higherin leaves ofL. ciiwrciis than inA. deser- fieldoro;reenIiouse (Fio;. 5). This a&iin demon-
tonim o\er awide range ofsoil watercontents strates that the higher A in L. cincreus was a
(Fig. 3, Table 2). Therefore, undercomparable conse(juence ofhigher rather than a higher
g,,
atmospheric conditions, L. cinereiis lost more photosyntheticcapacit): Field-grownplantshad
19931 Depletion of Soil Moistlhl hv Bl nchciusses 103
A. desertorum r=0.63
30 # L. cinereus r=0.88
25
en
I •
• • •:
10
<
Great Basin Naturalist [Volume53
104
nt'io;liboriiigChn/sotJtaninns nanseosus slinibs. first 3 weeks ofthe diying period (Fig. 2) also
T1k> size- aucl lower water-use efficiency of /.. may reflect greater osmotic adjustment in L.
chwremarelikelydisadvtuitageous,however,on cinercus.
sileswhere totiil wateravailableoverthe grow- Previous studiesha\'e showni that leafexpan-
ing season is more limited. Moitalit)' ofL. cin- sion frequenth'is more sensiti\'e towaiterstress
creus plants was greater than that of A. than isphoto.svTithesis (BeggandTurner 1976).
(h'sciioni))i onourexperimental fieldplotsdur- As reported here for two perenniiil tussock
ing a se\ere drought in 1987-88 (unpublished grasses, Kuang et al. (1990) found that LER in
obserxations). wheat and lupin was reduced almost immedi-
In contrast to L. cinereiis, A. (h'sviionim ately after withholding water. They demon-
establisheswellandthriveson shallowsoilsand strated that LER decreased in response to
ven- arid sites (Rogler and Lorenz 1983). Its drying soil even when leaf turgor was mtiin-
successonsuchsitesprobablyreflectsasmaller tained, and they suggested that leaf growth,
size at maturity (even underwell-watered con- stomatal conductance, andosmotic adjustment
ditions), the abilit\- to withstand prolonged are all controlled b)- the balance ofleaf plnto-
drought, andprolificproduction of\dable seeds honnonesas influencedbyhormonesproduced
(Hull and Klomp 1967, MarletteandAnderson intheroots. Giventhefrequentobsen'ationthat
1986, Pyke 1990). Ccildwell and his colleagues A and gw are closeh' coirelated (e.g.. Fig. 5), it
at Utah State Uni\'ersit\' have shown that A. would seem reasonable to add photo.s\nthetic
cicseiiontiii is a \igorous competitor for water capacit\'to the list.
and soil nutrients (reviewed by Dobrowolski et The relationship bet^veen g„ orA andTS\V
id. 1990). Itscompetitiveabilit}'reflects, atleast (Fig. 3) indicates aclose couplingbetween leaf
in part, theproduction of thin roots thatenable gasexchangeandsoilwatercontent,ashasbeen
it toextractwaterrapidK from the soil (Eissen- reportedforanumberofherbaceousandwoodx
stat and Caldwell 1988).' speciesgrowdnginavarietyofsoils (Cxollanetal.
It seems probable that the abilit)' ofL. ciii- 1985, Turner et al. 1985, SincUiir and Ludlow
ereustoreducesoilwat(M"contenttolowerlevels 1986, Henson et al. 1989). Turner et al. (1985)
than tho.se of soil supportingA. deso'foniDi re- and Gollan et al. (1985) demonstrated that al-
flects lower osmotic potentiiils in leaves of L. though g,v and ^ often were coiTclated, the na-
cinercus. Weattemptedtoestimateosmoticpo- ture of the relationship was dependent upon
tentiiil ofgreenhouse-grown plants from pres- environmental conditions and the rate of soil
sure-volume cun'es, but leaves of L. cincreiis dning.Thus,thex"foundnounicjue relationship
were so brittle that we were unable to obttiin between g„ and ijy and postulated that g,, and A
reliabledata. C>onciuTentmeasurementsofleaf are controlled by the lexel of water in the soil
relativewatercontent (RWC) andwaterpoten- ratherthanin theleaf. Subsecjuentstudieshave
tialoffield-grownplantsshowedthat,foragixen confinned that hypothesis, showing that the
R\\'(], L. cinercus liad k)wer i|i than didA. dc- roots "sense" water avciilability or some related
sctionnn (datanotshown; P< .04b\'analysisof parameterin thesoil andtransmitsignalstothe
covariance). This difference between species leaves that control theirbehavior (Gollan et al.
could arise from a greater degree of osmotic 1986, Masle and Passioura 1987, Passioura
adjustment (lower osmotic potential at a given 1988, Henson et al. 1989, Zhang and Daxies
HWC) by L. cinercus compared to A. clcscr- 1989, 1991, Tardieu et al. 1991).'Our data are
tonini. consistent with this model; for both species, g,^
Kuang et al. (1990) postulate that the fac- and Aw-ere closelyrelatedto soil watercontent
tor(s) tliat cau.ses a reduction in LER as the .soil (Fig.3). Ratherthan showingacause-and-effect
dries also induces osmotic adjustment in the relationship, the coirelations between g„ and i(/
leax'es.Their results showthat the proportional (Fig. 4) likelyreflect co-xariation in response to
changein LEHperunitosmoticadjustmentwas declining .soil moistiu'c supplies.
much greater in lupin tlian in wheat, whicli In conclusion, this stuth shows that there
suggests that LER in a species with greater may be small but significant differences in the
osmotic adjustment might be less .sen.sitive to extenttowhichcold-desertspeciescandiyasoil
the "stress" imposed by dning soil. Thus, the profile. Such differences may be important in
observation that LER wus' reduced relatively competiti\'einteractions(Caldwell 1985). Com-
lessinL.cinercusthaninA.dcscrionnnoverthe paredwithA. desertonini, L. cinercusmaintiiins
1993] Dkplktiox ok Soil Moistukk by Bi;\(:ii(;ha.ssks 105
liie;lKM-pliotosMitheticratesassoilmoisturesup- field stud\ ol hvoaridland tussockgrasses. Oecologia
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