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Plant, Cell and Environment (1997) 20, 1193-1198
Salinity resistance and plant growth revisited
p. NEUMANN
Plant Phy.siology Lahoraloiy, Lowdennilk t'\icutiy of Agricultural Engineering, Tcctinion tIT, Haifa 32(K)0, tsract
ABSTRACT
Tliis iirlicle reconsiders a recent hypothesis coiiccrnin}» Ihcphysiolojiy of jjrovvtii inhibition by salinity and its relc-vaiice to the breeding of salt-resistant crops (Mnnns 1993,Plant, Cell and Environment 16, pp. 15-24). The hypothesisslates tliat the osmotic elTects of salinity on water avail-ability will strongly and equally inhibit the growth ofrelated species and varieties. The genotypic diversityneeded lor breedin;^ increased resistance lo growth inliibi-tion by sulinity is only expected lo appear aller week,s ormonths. Higher rates of sail acciimuliilion in more sensi-tive varieties then lead to accelerated leafsenescence. Thislurther inhibils new growlli, as C(>ni|)arecl with more resi,s-tant varieties. Accordingly, breeders aiming to increasecrop growth under salinity should locus efforts on manip-ulating genes which can decrease rales of salt accumula-tion. However, the osmolic inhibition of growth by salinilyappears lo involve regnlatory physio* ;icnl changes. Thus,some si^notypic diversity might be expected. Clear evi-dence is i)resented for genotypic diversity in early growthI espouses to salt or PECJ-indnced osmotic stress, in .severalspecies and varieties. The conclusion is thai developmentof plants with increased resistance to inhibition of growUiby the osmolic effects of external salinity (in addition toincreiised resistance lo salt accumulation) is bolh feasibleand desirable.
Key-words: breeding: diversity: gt-owlh: it-rigalioii: tiiecha-ttisitis: osmolic: r-fsistance: salinity: toxic: yield.
INTRODUCTION
Excessive soil salirtily can t-esult IVotu rialut-al pt-occsses, orIVom ct-op irrigation wilh saline itrigalioti waler utider poordt-airiage condilioirs. Excessive soil salir-rily occuts in matiysemi-atid to arid r-cgions of the world where it inhibits thegr-owth and yields of crop planls (Gt-eetiway & Mutiiis1980: Tatiji 1990). The physiology of plant t-esponses losalinity and Ihcir relation to salinity t-esislairce have beenmuch tcsear-ched and IVequcnlly reviewed in t-eccnt yeats(eg, Pasletnak & Pielro 1985: Cheeseman 1988: Liiuchli1990: Rengel 1992: Munrts 1993; Neumann 1995a), Otiecommon theme is Ihal plants with incteased salinity tesis-lancc are expected to tnaintain higher t-ates of gr-owlh thanless tcsislant planls, under equivalent levels of salinily.
Corrcspimdencc: P. Neumann. F<i.\: 972 4 S22152'): e-mail:agpelcrn @ leclumi.wlechnion.ac. il
Unlor-lututlcly, the irilt-(->ducliot-t of moi-c salitiily-tesistatilct-ops has bcet-t a t-elatively slow ptocess, and this fact hasgeticr-aled r-titich r-cceni debate atnongsl physiologists,br-eedet-s atid genetic cngineer-s aboul lhe physiologicaltnechanisms involved it-t salinily t-esislance atid the bestway lo pt-occed wilh fulut-e bt-eeditig cffor-ts (e.g. WynJones & Gotham 1986: Bkttn 1988: Noble & Rogers 1992;Munns 1993: Richatds 1993: Flowers & Yeo 1995:Bohnetl & Jensen 1996),
Clearly, lhe I'it-st step is lo idetilify pat-cnl platils withtt-aits which at-e hcr-ilable atid conlributc to salinily t-esis-laticc. One pt-obletn is that ihct-c is slill no clear consensuscoiicernitig the physiological trails thai at-e primarilyt-csponsiblc for gr-owlh itihibilion by salinity. However-,pr-omising ttaits cati be idctilified by: (a) sct-eettitig forappt-opriale diversity in t-cspotises lo salitiily among tnod-et-n cullivat-s or t-elaled wild species (e.g, Blum 1988:Flower-s & Yeo 1995): (b) lt-ealmctil wilh mutagetis ittot-der lo pt-odtrce r-tnttants which show hypet-sensitive ort-educed t-esponses to salinily as compat-cd with the wildtype (e.g. Zhartg el al. 1995: Wu ct at. 1996): (c) cngineet--ing tt-atisgcttic platits which expr-ess one or r-tior-e Ibt-cigngenes which ate expected to irtct-ease cellttlar t-csistance tosalinity (e.g. Bohnert & Jensen 1996).
Another pt-oblctn is ihal completely dilTet-cnt gr-owlhr-cspottses tnay be appt-opriale for dilTcietil types of tar-geteirvit-onments. For example, when plat-tls at-e gr-owri insemi-arid t-cgioiis, under low-it-iput conditions whet-e com-biiralions of soil salinily and lerrninal drought ateexpected, early onset of sall-itiduccd t-eductions in leafgt-owlh tales may r-cpt-esctil an adaptive t-csponso. Tints,eat-ly gt-owlh inhibition, by litniling lr-an,spir-alional lossesand pr-olotiging lhe availabilily of limited soil watcrr-eser-ves, could pt-olotig sur-vival during the dt-y summer-months lor long enough to allow allaintnctil of sotne tepr-o-duclivc yield (cf Richards 1993: Ncutnanti 1995b),
Cotivetsely, Ibrhigh-itipul croj-js ptodttced ttndei- itilermil-tet-tt ir-t-igation with ttiodcr-ately saline water, strrvival is tiottrsually in question artd yield is llic main concetn. In thiscase, early otisel of inhibilioti ofgr-owlh r-alcs by lhe build tipof soil salinity could diteclly limit plant size and yield,Mot-eovet-, ongoing irihibitiort of leaf gt-owth also dect-easesIhc volume of new leaf tissues into which excess salt iot-is(which ate ir-arispot-led IVom the soil to the leaves via lhett-anspit-atioti slt-eatn) can be safely accumulaled. Cotitinuoussail accutntrlalioti combit-ted with litniled ptoduclion of newleaf volume could then lead to eatlier build ttp of excess(toxic) levels of sail. This might fut-lher accelet-alc the otisetof leaf setiescencc atid necrosis (Munr-ts 1993).
© 1997 Blackwell Science Ltd 1193
1194 P. Neumann
Sail-induced inhibition of leaf gt-owlh can also slowcanopy closur-e during seedling establishmenl. This tnaydect-ease seedling water use efficiency, since tnot-c soilwater is lost via dir-ect evapor'ation IVom the soil surfacethan via the leaves (cf. Richards 1993; Van den Boogaat-dct al. 1996). Finally, salinily can tapidly inhibit rootgr-owth and hence capacity for uptake of waler and essen-tial tninetal nutrients from the soil (Ncutnann 1995a).
Clearly, ther-elbt-e, the dit-ect gt-owth inhibitory effects ofsalinity are undesit-able during high-input ctop production.This article ar-gues that effor-ts at inct-easing plant capacityto continuously tnaintain high r-ates of vegetative andreproductive growth under saline irrigation could helpitnpt-ove ct-op yields in setni-arid regions. It criticallyteconsiders a recent hypothesis (Munns 1993) which sug-gests that efforts at inct-easing salinity resistance should beuniquely focused on t-estricting the toxic effects on gt-owthof excessive salt accumulation within the plant,
WHICH PHYSIOLOGICAL MECHANISMSUNDERLAY VARIETAL DIFFERENCES INGROWTH INHIBITION BY SALINITY?
There is genet-al agr-eement that whole-plant growthresponses to salinity at-e tnultigenic and that a better know-ledge of the underlying physiology is tequir-ed in ot-der tounderstand why some plant species and varieties at-e tnot-csalt-r-esistant than others. This is a eotnplex task since plantgt-owth r-esponses to salinity can var'y with: (1) the degr'eeof stress encountet-ed (tnild, tnodet-ate or sevet-e); (2) theplant ot-gan, variety or species which is investigated; (3)the plant developtnental stage and the dut-ation of theslt-ess. Mot-eover-, lhe gt-owlh inhibitot-y ellecls of salinitycan also be alTected by variation in the activity of calciutnor potassiutn ions in the saline t-oot mediutn (Lauchli 1990;Rengel 1992; Neumann 1995a: Wu e/«/. 1996),
Tht-ce physiological tnechanisms have often been con-sidet-ed to be pritnarily t-esponsible for the growth inhibi-tion induced by salinily: (I) tutgor pressut-e r-eductions inthe expanding tissues; (2) t-eductions in the photosystemactivity of leaf cells, and (3) dit-ect effects of accutnulatedsalt on critical tnetabolic steps in dividing and expandingcells. However, Munns (1993) ihor-oughly r'eviewed thesepossibilities and cited nurnerxurs experitnental findingswhich indicate that these particular mechanisms are not thepr'itnar-y factors responsible for growth inhibition undersaline conditions.
As an alternative, Munns hypothesized that plant gr-owthis initially inhibited (phase 1) by cellular t-esponses to theostnotic elTects of external salt, i,e, by responses to thedeer-eased availability of soil water. In a later, secondt-esponse (phase 2), gt-owth is fut-ther inhibited by the toxicelTects of excessive salt accumulation within the plant.According to this two-phase hypothesis, there is littlespecies or varietal diversity in the degree to which growthis inhibited by the osmotic compotienl ol salinity. Varietaldivet-sity in plant growth t-esponses is only expected toappear afler Iong-tertn exposut-e to salinity (i,e, when phase
2 commences, after weeks or mor-tlhs). Higher t-atcs of sailaecutnulation in the tnatute leaves of tnor-c sall-setisilivevarieties then lead to toxic effects, i,e, accelet-aled leafsenescence and/or nect-osis. The acceler-ated loss of viableleaf ar-ea, as a result of this senescence, should then reducephotosynthesis, and levels of essential hormones, logt-owth-limiting levels (i,c, new growth will be fur-ther-itihibited).
In suppott of the two-phase hypothesis, Munns el al.(1995) showed that diffetences in the degt-ee of gt-owthinhibition, atnong several varieties of wheat and barley,only appeared in phase 2, i,e, alter c. I tnonth of saliniza-tion, when differ-ential r-ates of leaf senescence wer-e initi-ated. These dilTer-cnces tended lo coincide wilh salt accu-mulation t-ates and with field r-atings of varietal salinityr-esistance, Sitnilar t-esults wet-e obtained in a cotnparisonof two maize cullivar-s known lo differ in salt uptake r-ates(Forttneier & Schubet-l 1995),
Munns el al. (1995) consideted sotne contr-adictor-ycases wher-e early (within days) varietal dilTct-ences ingr-owth r-esponses to salinity wet-e r-epor-ted. These casessuggested thai genolypic vat-iability also existed in theosmotic inhibition of gr-owth. However, they ar-gued that,in these cases, sail uptake was accelet-ated by high gt-owingtetnperatut-es; this further accelet-ated onset of leaf senes-cence and additional gt-owlh inhibition, in the tnor-e sensi-tive eultivars. In sumniar-y, the two-phase hypothesis sug-gests that any varietal diver-sily in plant gt-owth respotisesto salinity will only appear slowly and will be caused bygenotypic dilfet-ences in rates of salt accumulation.
The long-term toxic elTects of sail accumulation ar-eundoubtedly of great impor-tance to both vegetative andr-epr-oductive development (cf Khatun & Flowers 1995),Moreover, the two-phase hypothesis has focused attetitionon leaf senescence r-ate, a whole-plant trail which is easy lomeasur-e and which tnay act as a useful phenotypic mar-ker-tbr varietal divet-sity in salt aecutnulation r-ates. However-,this hypothesis also claitns that salt-induced t-eductions inleaf growth will be sitnilar in salt-sensilive and sall-r-esis-lant varieties, during lhe fir-st weeks or months after salin-ization, when only the ostnotic phase of gt-owth inhibitionis opet-ative. Thus, selectioti and bteeding lor incteasedresistance should be focused exclusively on r-ales of saltaccumulation within the leaves. This is a lar--r'eaching con-clusion since, if it is correct, it dooms to lailur-e the processof selection and br-eeding lor incr-eases in salinity resis-tance on the basis of early varietal diffetences in osmoti-cally induced gt-owth responses.
In this context, it is itnpot-latit to r-emember that theostnolie effects of salinity on water availabilily and gt-owlhmay be maintained thrxjughout plant development. Thus,even if new varieties with t-educed t-ates of salt aecutnula-tion and delayed leaf senescence cati be pr-oduced, theirlong-term rates of gt-owth under saline conditions couldr-etnain inhibited. Reductions in bolh osmotic and toxiclitnitations to gt-owth tnay thet-elot-e be tequir-ed in order loobtain significant incr-eases in r-esistance to gr-owth inhibi-tion by salinity. Breeding lor resistance to osmotic inhibi-
© 1997 Blackwell Scieitce Ltd, Plant. Cell and Enviromncnl, 20, I 193-1 198
Salinity resistance and ptant growth 1195
tion of gtowth will depetui on (he idenlificatioti of parentalvarieties showitig diversity in growth itihibitory lesponsesto ostnotic stress. However, such diversity is specificallyexcluded by Ihe two-phase hypoihesi.s.
IS THERE REALLY NO VARIETAL DIVERSITYIN OSMOTICALLY INDUCED GROWTHRESPONSES TO SALINITY?
In this sectioti I will first argue that the respotises of gfow-itig tissues to ostnotic stress are highly regulated, so thatsome genetic diversity in (his trait tnight be expected. 1 willthen teview several reports which clearly indicate thai vari-etal diversity, i.e, genotypic differences in osmoticallyinduced growth itthibition by salinity, does exist in a num-ber of species.
EARLY GROWTH RESPONSES TOMODERATE SALINITY ARE HIGHLYREGULATED
The effects of moderate (tioti-lethal) external salitiity ongrowth are rapidly established aud can be expected to con-f inue for as long as an excess of salts are present in the t ootzone. What are the specific inechanistns by which sail-induced decreases iti soil water avai\abvHly (i.e. tnove nega-tive soil waler potentials) inhibit plant growth?
Plant growth is ultimately the direct result of massiveand rapid expansion of the young cells produced by meris-feinafic divisions. Moreover, cell expansion in both tootsand leaves can be inhibited by salinity (Kurth et al. 1986;Zidan et al. 1990; Neumann 1995a). Much progress hasf">een made in idetitifying atid measuring the biophysicalparameters wliicli directly control rates of cell expansion.The ptocess depends on an inwatdly directed water poten-tial gradient which generates turgor pressure and cell wateruptake (i.e. volutne inctease). The cell expatision ptocessalso depends on the hydraulic conductivity of the wateruptake pathway, up(ake of" so/utes (o tnaintaiu ostuoticpotentials, and the yielding of the surrounding cell walls(e.g. Cosgrove 1993; Neumanti 1995b). Each of llte.scparameters tnay iti turn be tegulated by tnetabolic activities(e.g. o.smolic adjii.stmeiit, Bernstein 1961; Blutn 1988);
salinity could inhibit growth by directly or indirectlyan'ecting any of tlietn.
However, ostnotic potetUial gradients, hydtaufic con-ductivity and cell turgor pressures in the cell expansionzone, al the tip of growing cereal roots, becatne equivalentto control levels after 2 d of exposure to mckierate salinity,even though growth remained inhibited (cf. Pritchatd1994: Neutnann et al. 1994). Sitnilarly, the long-term inhi-bilioti of cereal leaf growth by salinity did not appear lo becaused by decteased turgor in the cell expansion zone ofgrowing cereal leaves (e.g. Thiel et al. 1988; Cramer 1992;Munns 1993).
In contrast, salinity-induced reductions in growth haveoften been related to measured decreases in the plasticextensibility of the growing cell walls in root and leafexpansion zones (Cramer 1992; Pritchard et al. 1993;Neutnann 1993; Neutiianti el al. 1994). Eor example, theplasticity of expanding cell walls in maize seedling leavesdeereased within minutes of addition of growth inhibitorylevels of NaCl or non-ionic ostnolytes (Neutnann 1993;Chazen & Neumann 1994). The NaCl-iiiduced reductionsin leaf plasticity could be maintained for weeks and wereonly reversed after stress retnoval. Sitnilai'reductions inthe plasticity of cell walls in leaf expansion zones wereinduced by gfowlh inhibitory nutrient deficiencies, or bygrowth inhibilory levels of abscisic acid (Kutschera &Schopfer 1986; Snir & Neutnann 1997). Eitially, itiducedchanges in cell wall plasticity appear to be metabolicallytegulated and closely related to gtowth tates (Cleland1971; Taiz 1984; Rayle & Cleland 1992; Co.sgrove 1993).Il therefore seems that Ihe inhibitioti of plant growth,which is induced by the osmotic effects of external salinity,involves regulated cellular events. Although it may not bea cotninon occurrence, some genotypic variability iti theostnoticaify induced inhibition of growth by safinity tnigfitIhereibi'e be expected.
Genotypic variability in growth responses toosmotic stress
Titble 1 summarizes several reports showing that salittitytreattnents, or treatnients with the non-ionic osmolyte PEG(polyetliyletie glycol), can fesult in varietal (i.e. genotypic)
Plant
Bra.ssica, shootArabidopsis, root
and shootMaize, sboolMaize, root and shoot
Wheat, root and slioolWheat, shoot
Rice, root and shoot
Trealmeiil
80 mol 111 ' NaCl23 mol ni ' NaCl
-80 mol ni ' NaCIl70molnr'NaCl(orPEiG)
20% PEG~ UK) mol nr'NaCl
50 mol t ir ' NaCl
Days
1
91
47
U)
Reference
Hc&CnmKv(!993)Wu et al. (1996)
Cratncr ('/<;/. (1994)Mladcnova(l990)
I?kim(l98())Kingsbury <'/((/. (1984)
Moons <'/(i/. (1995)
Table 1, Reports of varietal dilTcrcnc'cs inroot or shoot growth responses So osmoticsttess, as caused fty salinity or PEG additionsto the root medium. Days refer to time toappcat ancc of differences in growth amongtwo or more varieties. T<i facilif;i(ccomparison ol" t)ic salinity treatments, theyhave all been expressed in units ofeoneentniiion
© 1997 Blackwell Seience Ltd, Plant. Cell atid Environiiiem. 20, 1193-1198
1196 P. Neumann
involving sail loxicils
i'or example early v^ariclal dH'tcrcrc-;[)(>ttscs U) saiinii\ wcvc dcK'ctod la
Bras'vica varieties. Dcspile ihe dilTcrc3 J. lc\'els ol' Na and Cl in the lca\cs
were fhe same (He &: Caunc] 19'-}}K Mteaves in ihese iwu vaiielic^ of B
ous!} shown lo respond lo drouL'hi stilerences in growUi. iluis. dilTereniiaf'pcareci Ut be assoeuilcci with, redue
abilit)' rallier [han laie- of sail aceunui
Olher reports also intlicaic ihal 'v
erovvih respon^cs lo salinitv can ocouiic!t\'. For e\ani(>le. Wu f/ ai i 1 y4(M ix
siiive Aratiidopsis niiitanu which s'nehanyes exeejil Ihat rool and Icai ii
niore sensitive (o mhibifinn in X j ( l -
B\ eotnparison with die wiUi wpc io
iealK itihihiled bv external ^oirau
However, this inhibinoti aispeaser. uinduced delleiciicy in potas^iuui a,\a
excess aeeumuiatinn of soijnnn :op\
C t ' a m e r ci ill- (l^'-U) repe>ii>'U Ldfl
)i k'iil'cic'iiViiiUDn njlcs oi nhu/c rk.r^
oC salini/alion. fnipoiianil} ihr - i>.v-
^enstlive l(> eariy _^no\\ih unnhUi-jnLitcd significanrh ie - -^ochiiin Ui, n V.
eiv. Thu-^. greater nifiihiiion oi ieal p
i.i ]">ropor[ionaiel} higher r.iie-- :n
O'Regan ('/ ill i i'^y^i reported on ea
2R)\v[h of iv\o mai /e vanenes siih^
1 fl d. Here too, early geiKMspic d u i e i ,
presnniablv indepcisdeni o. s Ji
a[>pearcd io be rekiied io diileieiie-, -- i-
x*. ilimul
icei- in leaf gfiivvihter onh 3 il) in Iwu
nccs HI unnvtii al'lefof the Iwi! varieliesoivovei. (ieveiopiiiy
^icu luit! been previ-icss w hh sitnilar dif-
jl uri-nMh respotiseshons in water avail-
iialuHi in ihe leaves.aHetal dilTereiiees in
wulioiu siidiuni to \ -oL'iled sa]l-h\persen-owed no phenoiypie
oudi were 20 times
-hao in ihe wUd type.
n? j^rowih was speeif-
n -^alts wish in 1 d.
^ be related \o a sail-
nulaiion, raiiierthan
^ aricta) ililTcrenees
--. \\ idnn > h and '-> d
p which was more
\\ \,i!inil\ aecumu-c ••ah-re'-istani \'ari-
rw'ih uas uo\ tviateti
Kalt aeeutntilation.
ly dilTerenees in llie
ied io droughl lor
aces in grovMh were
aceiniiulation and
lesponse to reduced
Miadenovii M99()t reponcn e,a'=\ ^ieni>[ypie dilTerenees
in roo! and leal L'row [h lespiaise-- ,o saliniiv ni two otJier
iuai/e variclies. "Hie inowini; icnipeiatui'c {lK-2.i ' C) was
lot high a.nd the difiereiice'^ \U;T..' csialihshed in 24 !i:moreover, siiuiiar diMcienees in le.ii i>rowlh eotild also be
iiidnecd hy ireauiicnt wuh :so v^-..v,'o\h. I'l-Xs solutions. ThePJ:G polymers u,scd are iion IOUK and do noi
peneiraie plant roots Yhc\ do. h{-\\
efieet (ef. Cha/cn ci uL h^>^: and--eenis !ii<el) Shas liiese rapivIK e^Li
enees in mai /c !e,af grovsifi were
respotises to \\w. ostnoiie efleets oldian dilTerences in salt aeeunnnalion |i) the leave--.
Kingshury a dl. (<984i reporsed '^n \arieia! tiilTerenees
:i"i cai'iv grovvUi responses lo salinity in wheat leaves.Despite ihe nsc orii igii grovvnig lenipcratiires. Uiese eariy
dilTerenees were iioi related n' dilTcrenees m levels of saltAccumalMtofs. or k> i.MU:icm:c\ in wMcr rekftiojis. Theauthors sugges^xi ihat grovMh nsndit have been alTceted hy
varietal dilTerenees in ttic ahiiii} .>!' Hie leaf cells !o detox-dv salt hy eomp;H'tinciUah/aiion Hin\'e\xT. an additional
nossihilifv i thai ihese licrKsivpes UdleRxi iii their
t. Itave at!> iherein). Tlius, iihed \arielal dilTer-
icti hv viifTereniiaf) and N a d . rather
to osniotieallv indneed lednetiotis in water availahilitv.
Work by Btiim et ni i IM8(I) [)iovides sitme sitp[}ori for thisaltcrtianve explanation. They sereeiied a range of wheat
CLiliivars for early differenees in leaf gnwvth inhibitioti. \'o\-
iov\ nig additioiis o'i VWG solutions to the rools. [:ari> vat i
etal diffetenees in gri.>wll! res]iot!se ap(>eateii after only 4 d :again, these liilTeienees in lealgrovvth res|)otises io exter-nal Pl^Ci were eleail\ related to osinoiie elTeels and unte-
iated losal i eflecls,
Astam (7 III. ( 19')3l fount! that exposure i'''i riee plants in
saiiniiy siress t t i ronly 10 cKkirfug earl} vcizclaine tlcief-optiicnl resulted ti! subscqueni rediieiions tn vegetative and
reprotluetivc yields. Moreover., thete were sigtnfieanl vari-etal dilferetiees in the degree i^\ re'-[K!nse. I'bis led Aslani('/ (li to eoneliitle that ear!) vegetative gnnvth responses
ean be usetl a> a tratt for i-apis.i seleetioi; o\ salt-resistantvarieties.
.V]!.n)iis ;'/ al. i 1993) argued iha! ihese eatj'ly el'ieeis may
havL- been a. eonscquetice of tiigh growing tetiiperatnres,
which inereasetl sothum uptake rales anti Lteeelerated dif-lereniial leaf seneseenee ni the different varieties.
However, Moons vi ai [ 199."^! reported on varietal (iiiVer-
enecs in the grovvih oC root-, and leaves, for a salt sensitive
and IW'O salt-resistant varieties of rice. v\'ithin 10 i.! of r(X)t
exposmv io 50 mol m " N a ( T hiiportaiuls, no other visible
>ym|itonis o!' stress (e.g. s^dt aeeelerated leaf seneseenee)
\ ere ohserx ed. i e. osmotic resptuises eoidd have been
insoKcd,
FinaH}'. recenf nr^ esligaiioiis in oui l:ibora(or\' i'urlhcr
sup|iori lite existence of genotv pie \ar tabhly in !he growili
responses of nee leaves to osmotic su r s s . Seedlings of two
riee varieiies were treated v\iili iso-ostnoiie solutions of
NaC1 or V\.i\. In both treaiinents, varietal differeiK'es in
leaf growih rates appeared alier only 24 \\ {well belbt-e the
onset oi atiy leal seneseeiiee symptonisi . Moreover, thelespDnsL's )i> i'fXi ueaujieni weie ecrtainlv unudated to
toxie eiTects o\' N a d aini appear to have heeti osrnotiealiy
indtteed (/,. l.u and P. Nenniann. nnpublisheti lesnlts).
CONCLUSIONS
Complex physuilogieal changes such as cell wall and
osnunie adjustments are invohed in the early inhibinon ofgrowth in expanding plant lissues exposed to sismotiestress, l l i is suggcsis tliat. al least in theory, some geiunypie
diversity in varietal growih responses lo osmolie stressni(gh( he e\[iciieil. Sc\'Ci-d\ reporl^ o\' mcAsurcd Jiversilv
in early ([iliase 1) grciwtli responses io osmotie effects olsahniiy. iti a vvide variety o^ piani .sfieeies. eonfirtii this
e\pcelation and therefore eontrachei a eenlra! tenet of iheuvo [ihase hypothesis (4' salinity resisianee (Munns 199.^:Muniis Cl (li 1995).
!hu^. m addilion to altcnipnng lo rcdnee salt aeeurnula
Eion, bleeders shotiid also loetts some aiteiuiou on effortsto itietease ca|-)aeil> for eell e\|iat!sion and henee growth,undei conditions of moi.ier:iie osnioiie suess. Despite the
contfLtr) predielioiis in Munns (1993). raj>Kl screeningtests., based on identifviii'..; eariv tiiflerences in
, H (uu.i Ehu
Salinity resistance and plant growth 1197
i*csp{n)scs IO saliniu or VlXu shouki he tisclui for thispurpose (cf, BUim I*->88|, The iiittiKlucUon oC increasedcapacity lo tiiainUiin growth despilc ostnoiic stress couldhe especially hcneliciiil lV>r l\igh-\npyi evops grown underinterniitletil irrigation wilh inoderatei) siiimc water.
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