Potwh Fbrtilinnand Manuring, 102ndwlte. '

No. 81 1988

The role of potassium in drought tolerance' N.P. Sarena

Plant Physiologist Pulse Program Inter-,a: onat Crops Rrsea.-b ltlst,:~:e for the Emi.Arid Tropics (ICRISAT) Pat~nche,~ P O A P 562324 India

Summary

Water stress effect9 are severe in semi.arid enr!rgrqen:s vrhere drought5 are unpred~ctable and cause ins:ab,lin, in 'produclmn Eltons are be~ng made :c ,m- prove drought tolerance by plant breeding but 11 will be some rime before a wl. ution is found.

Studies of the role of potassium in water uptake and the regulation of 11s loss through stomata1 control In controlled e&pertmenls and f~s ld otud~es of veld response to potassium in limiting soil water e~viroirrents indicate that i t r a y improve water relationsof plants and water use etfic~ency and maintain y~elds un. der water s t r w . Funher study is needed of the effects of potassium in relation to drought in field experiments under diverse agroclimatic conditions on roils de. ficirnt and suffic~ent in potassium. Attention should be paid to the econorrics ol potassium reyMnse under drought conditions

1. Introduction

Drought is a complex and variable phenomenon and progress in drrcg,hl re- search has been slow, though, judging from !he number sf workshops and sym- posia held and bcoks published (Turner and Kramer 19e0. Paleg r! a1 1981, Musselland Sraples. 1979. Chnstiansen and Lewis 1982. Irr,. 1981). the sub lect has been receiving priority atlention Progrers via plan: breed~ng vrII be verr difficult (Arnon. 1980) and success is diH~cult In the near future

Improved crop, soil and water management, by cncreas;ng root penetraton. regulating transpiration, and lmproving water use efficiency can Increase and stabilize yields under drought conditions.

The uptake of nutrients IS one of the proem that IS affected by water stress Nutrient coolent in the leaf cell contributes to osmoregulation and is imponant in drought tolerance (Atman. 1 981).

This wper doa not pretend Io ba a comprehensive treatment of the role of polasrium, as there are a number of good reviews of the subject: it concentrates o n a h Jectdcasca.

' lpgad r C . 1 No. 114 4 (ICRISLT)

In an e d y review. Achilw (1961) distinguished between dirs* affects of potassium which either incrswe uptab of water or redun, its loss thrargh plants or create any other internal condition of drought rssislance and indirect effects when it increases yield without affecting water relations. It is the fonner which are of the greater interest to plant physiologists and whlch are discussad here. The et. facts of deficiency and suffic~ency of potasslum on a number of plant processes including stornatal movement and transp~ratlon l os , translocation of metab- olites, dry matter production and water use effic~ency have been extensively studled. However, thereare relatively few reports which deal wlth such effects on crops growing on sites where soil potassium is not at deficiency levels. Slnce so11 molsture content affects the mobility of potassium in the so11 and hence its avail- ability lo plants it IS not easy to say whether response to K fert~l~zec under dry condlt~ons are due to effects on so11 K ava~labillty or to improrernent of drought tolerance by the plant

2. Effixta on roo- and uptake of water

Baker and Weatherley ( 1 969) showed that water moverneqt from the outer cells of the root Into the xylem is regulated by potass.-m Menyei and S~rn~c ( 1 973) show4 rhat the quantity of water moved upward by root pressm In de- cap~rated sunflowers was :educed as K conccntratlon ~n the rootlng medurn was lowered Royaler (1 958) suggested that added K increased the permeab~l~ty of root cells towater The osmotic potential of root cells was lowered when K uptake was h ~ g h thus increasing water uptake (Mengel and Kirkby, 1980)

Adequate nutrient supply encourages rmt proiiferat~on and deeper penetration so that water can be taken up from depth wh~ch isobv~ously advantageous under drought conditions. Ebvard (1 981) In Illinois, found that maize roots penetrated 6 0 crn deeper when receiving K fertilizer, giving access tc an extra 10 crn of water Adequate nutrients decreased shmtlroat ratio as well as improving the water up- take potential of plants (Beringer and Trolldenier. 1978). but k g (1 972) found with wheat and peas a negligible effect of potassium content of leaves on shoot/ root ratios.

It has been suggested that efficient water uptake from the soil and its transport upwards are more imponant than stornatal conductance in determining drought resistance by sorghum and cotton (Ackersdn and Krleg, 1977)

2. 1 Loss of water by transpiratron

Treatments which could reduce transpirational loss would be valuable In mois- ture stress. Early work reviewed by Achrtov (1 961) suggests that K supply reduces transpiration and increases water uptake and, further, Improves water use efficiency. A number of observations of reduced transpiration with potassium were mentioned in a review by Hoeher ( l97 1). Westermann (1 942) found with 12 s p i e s in water culture that potassium increased transpiration whereas cal. cium reduced it. In another experiment, he found that K reduced transpiration in plants with a restricted water supply. Rogeler(1958) with sunflower, wheat, bar- ley, maize and clover grown in water culture and in soil in pot experiments found plants lacking K to transpire much water and that the addition of a wll amount of K greatly reduced transpiration Brsg (1 972) found transpiration reduced when enough K was applied to imressa tissue K content (fig. 1) and recently. 80 Lslsan (1 981) showed that K reduced both day and night transpiration in Douglas fir (fig. 2). On the other hand. Achitov (loc. cit.) quotes repom of the rev- effect and

Chri-(l97q found cht though K increased transptratlm in pine, the plants survived drought bettw dw to efficient osmoregulation. It thus appem that there is some uncsnainty in thaw mattenand that the resuln were affected by the initial K level in the soil.

2 2 Srornaral regularron

Mengeland Kirkby(l981) say that K playra spectf~c role In most plant spectes In openlng and clostng of stomata - a role whlch cannot be played by any other catton Thts has been tnvesttgated and dtscussed by many workers Ftsher and Hsra (1 968). Humble and Raxhks (1 97 I ) , frolldenrer I1 97 1) Skogley ( 1 976) showed that barley well supplled wtth K closed ~ t s stomata In 5 mtnutes and so reduced water loss whtle K def~ctent barley requtred 45 mtnutes ( f ~ g 3)

3. Effect on plant turgor

Ustng Phaseolus vtrlgarrs In solutton culrure Arneke ( 1 98 1 I showed the effect of K on various aspec:s of water regulatior. ( f ~ g 41 He found that total water potenttal of upper and lower leaves reacted ditferenel+ to K plants well suppl~ed

l a Wheat 16 Peas

K conten? rng g '

Fig. 1 The transpiration rate (A) and stornatal opening (8) of Triricum aestivum and peas, grown in a nutrient solution of different potassium concentrattons The stornatal opening is expressad as the width 06 the broadest pan of the guard cells minus the same width of closed stomata. Abscissa. concentrations of potasslum in the shoot (mg per g fresh weight). Each dot represents the transpiration per leaf area of ten plants in one growth vessel or the mean value of ten stomata (after Brag

Fig 2 The rate of transp~ration (rngig needle dry matter and hour) In relarton to the N and K level (mg i 100 g needle dry maner) (after 80 Larsen, 1981)

having a higher water potential in the upper leaves than defic~ent plants, while the reverse held for the lower leaves. The mean osmotic potential of upper and lower leaves was lowered from - 7.62 to - 9.1 1 bar by increased K concentration The turgor potential of lower leave was not affected by K but that of younger leaves changed from 5.05 to 7.1 7 bar.

In hb work quoted above Bo bm found that overall drought resistance (the sum of avoidance and tdetanur) imresssd with increasing K level.

FIQ 3 K deflc~ency on barley results in a h~gP and prolonged transplrdtlon loss under hot wlndy condlt~ons (after Skoylek 1976)

4. Effects of dry matter production

Mengeland von Braunschwe~y ( 1 9 7 2 ) found that at low so11 rno~sture tenslon. about 1 7 mg ' 100 g exchangeable K was needpd to produce 6 g dry maner com pared wtth 46 rng at hlgh $011 molsture tenslop conclud~ng that h~gh* r levels of exchangeahle K are needed under srress cordlflons The effect ~sexpla~ned by the greater mobll~ty of so11 K under rnolst condlt~ons (fig 5) However ~t appears that there wasa large response to K even at f~eld caparlty sugyestlng that the sol1 was actually deflc~enr In K

5. Effects on maturity

Early maturntion is advantageous in drought avoidance In a review by Darsr (1980) it is stated that application of K to potassium deficient rnalze hastened silking and lengthened the gram filling period Effects on soybean were Incon- sistenl.

6. Effects on yield

There are many reports of yield response to K fenilizer on sods I& in K but there are few r8fwn.S of response under dry conditions. Indeed, Mengel and Kirkby (1 980) suggest that such responses occur when N and water are not lirn~ting. If the main effect of water shortage is to reduce availability of K in the soil. then K

I0 20 30 I

- 1 .

- 2 - K , 0 1 m n v l K - - - K > 4 0 m n v l K

- 3 b

Mmn wslm of 12 r d u t e

-1.

- 5

-6 d w\ K

. *. * . * - - - - * C . - * - - . - \ -.--- -9.

. . \

id lli, K .

- 10 '

ber

Fig 4a Mean osmotlc potential ( e ~3 of leaves with two l e ~ c i s of potass~bm nutrttion (lower and upper leaves combined) (attar Arnekt?. 198 1 )

4 baf

7 .

6 . *. 0 vo K > 0 . . . *

-9- .

0 l

0 w, K I

- K , 0 . 1 mmol K - - - Kl 4,O m m d K

I* Mew values d 7 2 r c p l M t a

Fig. 46 M s a n turgor potential (0 yrJ of leaves with two levels of potassium nutrition ( l o w and uppac leaves combined) (after Ameke, 1981).

h g 5 Importance of so11 rnolsture and exchangeable potasslum In so11 for the growh of malze seedl~ngs (after Mengel und 8raunschwe:g 19 72)

t

8-

7 . r

6.-

5.9

D a - ; A - .

3

2

I

fertilizer would be expected to ameliorate the apparent shortage of soil K and to increase yield. Response to K would vary with the year. depend~ng on rainfall and this is saen in results reported by van der Paauw ( I 958).

Van der Paauwcompiled a report on fen~lizer response by potato wheat. grass and beans over the 15 years from 1935 10 1949 Crops differed In thelr respon- siveness to K and the optimum level differed with crop If there were more than 46 ralnless days aher planting, potato yield declined in proportion to the number of ra~nless days on the KO t r w t m n t but was maintained ~f the K supply was good (fig. 6 and 7). K uptake was reduced in dry years. Wheat was much less respon- sive than potatoar and here K increased yield only in dry years (fig. 8) Such effects were not evident with grass and beans.

-.

-- - r

1 I

0 1 6 2 0 2 4 2. 1

Water r r iwm (pFi

Fig. 6 Aelatron between the number ol rainless days (MayJuly) and the yield of potato tubers in the presence and absence of heavy appl;cat~ons of potask (380 and 0 kg K 2 0 per ha) (after van der Paauw. 1958)

Boulay (1 976) found that drought resistance in vines was increased by a heav., preplanting application of K. Bennger and Trolldenier ( 1 978) state that crops liberally treated with K remain turgid and green tor a longer t~me

Water stress also occurs it so11 water is excessive and Barber (! 967) recorded responses by maize to K under both dry and wet conditions In a 3 year experiment (table 1) bter ( 1 97 1 ) he found that K response by soybean depended on rainfall In the 12 weeks following planting (fig. 9). ~f rainfall was below 380 mm. response to K was nearly linear.

On soils of medium K availability in Ohlo, responre by maize in dry years was as much as 2700 kg/ha compared with only 198 kg1 ha In years w ~ t h optimum ra~nfall (Johnson. 1979). Hernando and Orihuel (1 977) found tomatoes to be more K responsive w ~ t h drier watering schedules

7. Water use efficiency Transpiration coefficient (water consumed/ D.M. produced) can be reduced

either because yield is increased or because water consumption IS reduced or both. The former seems to be the case in work reponed by Blancher er 81. ( 1 962)

150

l o o -

Fig. 7 Relation between the number of ratnless days (May-July) and the maxi mum increases in yields of potato tubers obtained afrer applicat~ons of potash (atter van der Paauw. 1958)

who found water use efftc~ency to be h ~ g h In lucerne when tlssue K contevt was 1 3% K but low when 11 was 0 8% or less but t h~s may have been due to restric tton of D M prcduct~on by K defic~ency ( f ~ g 10)

In Herwig s (1970) work quoted by Hoefner (1 97 1) lncreas~ng K ~ncreased water consumptton In the early stages of growth of oats but the greater arc umu latlcm of dry matter resulted in lower transp~ratlon coeHicients (table 2) However at harvest transplratlon coeff~c~ents were stmilar In both treatments Lower tran splration coefflctents In flax well supplied w ~ t h K were observed by Lrnser and Herwrg(1968) but thls was due to greater D M accumulat~on rather than reduced water consumptton (table 3) On the other hand h ~ g h K plants at low soil moisture consumed less water glvlng lower transpiration coeff~c~ents High K plants (2 6% K In tissue) at 40% fteld capacity produced much the same D M as low K plants (0 4%) at 80% F C The former also consumed less water Th~s, showlng that high tlssue K may reduce water consumptlon. IS interesting evidence

16, I02 -- mapcam m K n prwtr rnm lo* runhll

Fig 9 The less the ramtall for the 12.week period aher plan111ig the greater the percent yield Increase of soybeans from K on a low K so11 (atler Barber 1 97 1)

Fig. 10 Relationship betrveen K content in lucerne leaves and transpiration/g dry matter produced (after Blancher et a/., 1962)

1 1

Itseemswellatabl~shd that potasslum IS Important In stomata1 regulaticn and has effects on wter potenttal but most of the data come from short term experl mma In pots or nutrlent solut~crr and as Ber~nyer and Trolldenrer (1 978) say relat~onshtps ~ t a b l ~ s h e d under controlled condttlons may have l~t t le relevarrce to fidd cond~t~ons Effects are dlfficulr to Interpret beuuse crops are more sens~tlve to water stressat cenatc stages than at others It does seem to be ~mporta~i t to col lect more data horn held experiments and 11 IS Important that sirch experrmcnts should cover ~ t k w ~ t h low. med~um and htgh K Further work should ~nclude full growth analysts and assessment of the economics of treatment

Looking ahead: sum up

Raum trorncamrdkd pot and mlm mhum m ~ l m a n n n matlynot @rcsbl* to luld mndnms Funha B g a m t n K m In th I d d n n a w v on

dltteront 4 r#es Z

mnb lavr nd htgh n orch.oyeable K ' Clors monmcg cd $011 and cl~mstd essenrt.1

1 MI IAD CGR 2 Dn, mrnef producfmn 3 Watw urr and wata u n eU\r~ency 4 Marurtty 5 Y~dd and y~eld componenh L--

-Y R . m * d h c m w r m b r m v r t r a r ~ p e t . . h & * ~ l r t n p l m u . P o P I h R n l . u

w. m. Humur. 151h wm. lnmrwrr rml Po tah Ina~tu le Few ~Srnuodnd) !-I 5 (1961)

#rrr R.C. Kneg 0 R S t w w .nd nommmud t+afm o l m m use In conm mm and smghum

%I Ryvd 60. 8-53 (1977)

W W . T h l rill- ol p o ~ s u u n cm ffw ampononn d The m r e i polcnt~ei and on the ,ale d pr& d P% rulgrn~. P o w Rev* Subpct 13 lkgum~nouscmps 51h rulfe Na 7 1981

Inlamaltanal Potah Inst!tute. Ban ( S r ~ l ~ d ) 1-5 (1 98 1 I

A m I B r d m g f w haghe yields h d i n a l o r t t e w r l ,m the !ST xsr!on In Phyr to~og~<.~I 1som1i of

rrog prcQw1,utty Ptocbed~ngrof the 155th collagu~um 01 the In lb fnat~~~nal Polash I ns t i t ~ l e helo ,n '&a- gentng.n 'The Nc rMands 77-81 119801

8akn 0 A W e a r I ~ ~ y P E &at- and a u t e t r a n w m by erudmg r m l t r s t e c i ~f Rc,r<.s < o n r n ~ r . s J E ~ D Bot 20 485-596 1 1 9691

Bar- 5 A Raanfall and irrprse Bcnv crops w rh olant 'occ A ? (11 6 t u~o red I~LI~P Y C ~ W P ~ 'A L

1980) '1 9633

B a r n 5 4 % r b e d ? 4 o - e ~ n b ~ o f n t l ~ z r r r .olatton & n n c r o p s r ~ l h p l a r ~ r ' ~ x ~ 5 ' 2 ) 9 -1 1 Iub ,?I%!

I r on Nemwr IN i 1980! , '971)

& & b v W Po!asu *I ,n,slarcr Jr 'a .,pne n 'a recsrr-su M t d *t,ccl* ,utcr 'qt,,.'.lc! '.or*. br ' r<)er H

( 1461 ) Beqg J E ~ n d T u r - ~ r N C ,19:n', Croura:ertlelctt, A 6 . Al;r,: 2 9 ' 5 ' 2" j f 9?51

B z r m p Y Vvaler and '?flsllre, i.se 'a, fwd ;coducli,-n n 3 r d and scrnt dr I zvre , , r 2-6 t .q .~ -1 ? r . !

CIES t vmp? r i L r 26 %over ,br - ' ?r emhef 19 79 Srtlyhar I s b r a F e :#?iW E Curt nqrr Ger

map c~deral Qewb l c fer~!re Inte," $1 cmai dcl E-qra~r Ch!mkqt,es ?'' sp ,798 ' ,

&r,rger H T r c I ' l ~ ~ e , f Inflbenrr l f p8:!,3br,ucr ?,,rf,,8,,r, 8:" !he re?l,rr.rc ' r . rnr,.S.-'el !a< , ! re \ , Ir

P r ~ e & \ * g r :>I 1P.s ' 1th L O I ~ O Q U I U ~ of rbe n~erna~,o,lai Pnfdsh !ntt I L Y 8crr S w t r ~ ~ l , ~ , I! 1 R9 - 2 ? 1 ('979,

Blancre, P Sf'der R Chat,-on; C ' n ierm~,ory rn l rc ' almcr.dl~or- hyrfr i:,.r ,I+% pla,>r-\ A:) ! . , < , r %r (Pars)) :3 9 s : 10 ( q ~ c l e d lrom Bernger ~ r . 6 T,>l'ler.tec I q:R, ;! 9b2!

80 b r r r r r J brought ,e.l#staPrd and wale, .eldl,on$ of :onten .n relat8nr lo 'he nt, lre-t \t.rf.i5 uolarl

R w e w Sutlec! 22 Fores:r, 28rn sulfa No 11 1981 ~nrerrdl~ondl 00ldsh 'n,,l,lul* kt*? 5" ':?I

land 1-5 (19811

Brag H The , n l l u m c of potassttim on 'he transp~raf!on rate and rtarnaral ,>p~mng ,r. r rrr ,~bm .+el! w,rn

and Pisum Y l i v u r n Phyglol Plan! 26 25LS-257 ( 1 9721

C h r t r t e w L The eHeci of mnrgat,,c nutrtenls on water economy and ha,il~nes,, ,I ronIvr5 I1 The illn.'

of vawtvg polar5iun and r ~ l r ~ r n contcntr $on water $tr-; and drought harr),nr(s 01 1 x 1 grown Ptnus

sylrestrrs 1 and P ~ e d dbres L Katsl Isedl'lrgs Studla Forestalla Su.*.,,.a Nu 136 5-20 ~ t m k b c ~ l n

(Quoted from k ' n g e r and Trollden,er 1978) 11 976) - Chris:,ar.wn M N L w q s C F Breeding planrs tor less favourable ervwonments John W.lsv and Zons

N w York 459 pp ( 1 982)

Darst 8 Etlecqs of polassfun on cr9p rnatul lh Irr Potasslum lo( agrtculr~,re A 5#tu*r.on rna ivs~r 139-1 5 0 Potash 2nd Pho$phate Inrtal,~te of Indta 1980 2801 Buford Hv+ Y E Aflar.:a GA 30329

op '39-1 5 0 119801

Edward ! V Potasr 'eri8l8ze. arrd mncmased !oierJnce 10 $Ires$ Agrlutem 1 No 1 11 98 1, Cdrada l < i~ . r :

Ird fro<rr P,~lash Qcvlevr Sub,ecr 23 Plant Pr,,lcr'ton and $an1 dlewase Nr. 5 1 9821 1-6 ( ' 982, F,s,llcr R A HSIM 7 C Stomatai OWnlny .- #Colaled rpidermal cellsot VKia fa& I1 Rerpotw 1c KC: con

cerlrdt~or and !he .ole o l po!ass#ur~ absorpr~or. Plan? Phv%lol 43 1953-58 ,1968,

Hmdr;do V Onhue1 6 impoflance of mteracltor ol K . lrr~gatlon o r rornaro (crops ptmee,l,nr)r ;I the

Inrernat~onal Senmar o r Sol1 environrnel ! and lenzl~ly managenel*' ,n Inlens~vr 3gr'c.ulrurr: The h , e l y 01 :he Scence of % t i and Manure Japan 181 -: 9 1 (quoled from Velwn 'N L 19801 11 9 1 7 )

Herwrg K Unpublqshcd data (quoted from Hofner 197 1) ( 1970)

H o f r n W Influence01 polassturn on wale, economy In Proceed~ngsol !he 81h colloquturn Internarmnal Polash Inrt~rule k - e 5w!rnland 125-1 29 (1 971)

Humwe G 0 Raxhke K Sl0maU.I W n o n g auantttat~vely related lo polasswm (ransporf Plant Phr<<ol 48. 447-453 11971)

16/101

~rrq m a , ~ r d ~ d o o p m p m m n l l a ~ h ~ mmmoh.m on m Itnunmad R a R o a ~ 3 1 Irmnum. La Br*x t h r l i p p ~ . Mby b-8 lPB?

- J W K r d p r r o * d e * r h n d h h r v s s Bmrcmpsmthp(ntlmdb3(puond I m m N J I o n W L 1986(1979)

n h m g K 2wum-m- nmchn h n h m g . T-mm und NLhotoHIrwp~n bm Lnr u n w tmmdaa Bauckwhcqwg nru miartan W w p . b e uod Kal~dungung W r &dm 10 F&IQ& 2 2 Fdgm 1 - 1 2 l ~ r d t r r n B r 1 g a ~ T t r d l d c o r a 1978) (19681

M-r C Rdltlomh~ps bemmm the uDWIe d nmmnts wta m d the ,001 sptan A ~ ~ c u l t u r J ymld pMnt~alr In contwntal d ~ m r t a Procssd~nva ol IIW 16m colloqu~um d Nw lntwnat~wal Powh Insto IM B W ' S m M 77-96 11981)

M-U nn Bnulp-hmp 1 t Theoffact drol l mstunupwr lhr dvadlabfl!Iy o( K and n ~ n f l w m dn

t h g a r r h o f p u n ~ n u ~ n p b n Soil SCI 1i4(2) 142-148,1972)

M.ngll U K~rkby E A Potassum an crw produnon Adwncn n 4grnomv 33 59- 1 10 ( 19801

M m U S8rnmr R Effm of parwum SUDPIV on rhe acfoprtsl t r a n r w 01 wata IPNQ~PT -on$ ma ammo a c ~ b young dsop~ra td arntbwa d m n [ H d ~ i h u s a n n d P h ~ d d Plant I 8 232-236 (1973)

M d i H Sopre R C Snsu PkWogy tn crop @mu Prassdangs d tho Intansclonal Cmlerarxe on S a a Phy%olqn In c rw @en5 bva T h o i n m Inst~tulb lor plant research 197 1 (1 979)

Msm W 1 Inter-6 ol p o w l u m n t h moasrurs and twnwature In Potau,t~m to# 4rrul tura a vcuatlon anylrs T09-121 P O W and phowhare Inn~tute Atlanta Gborgta (1980)

P.lql L G LLlprdlO T h Phyl~olog~ and bloct'mm~slrr, of draughl ra~suncr n plants Acadnnlc P r m Auaral~a 492 pp (1981)

P m w M G Ion upoks In T h Phyl~dogv and bschem~sy d drougbt raqstancs tn plurtr Aadmrc Rar Aunralu 71-96 (1 981)

R q p k W Fn~ologica Rntsnu (Phvuologv ot plmn dm1 phyuolqy) (quoted from khtlov M 1961) (1058).

Yogby Eul 0 Poemurn In M m u ~ -1s and crop requaammts Mmtans Ag~rultural Eapsrnmertb tmn S u t m fbsnmh R w 88 (qwtod horn Msm W L 1980) (1 976)

S e 1 0 Bdrdmga F R Introdunmn Tho h u m e o n ~ q w s d draqht and crop rnwrch pr or1 t m far Ih t r allmatmn In Ph-y aml Bahomstw d drought rn8rum;~n aw @ants kademlc Prar Aumalia 1-1 3 (1 981)

Trddldm~sr G R k n t -1s of h ~ n f i m e d po(.ptum on Monuul opolng and d a ~ n g In Pro cssd~ngaot the 8th colloqu~um of In tunatmI P m h In.hNte Ibme %~nerllnd I 30-1 33 ( 1 97 1)

Tumw N C Knmn. P J Addputmn d plants to wnef and hph tempararrire s t f w Procesdtnp. ot 'a) sminar M d from Novernbf 6 lo 10 1978 11 IIW b m s p m Inst~tute of Washlngron Oqxrtrrunt d Prnt B~ology S m n f ~ d Cdl~tam~a 482 po (1980)

vsn dr Paarm F R&IIM be- fhc polah r e q u ~ r w n n of uopr and meI.orol~~cat cond~t~onr Plan1 nd Sail IX (3) 2 5 L 2 8 8 (1 958)

W-n E Jahm Wru Bat 90 335-181 (pooled from Achltov M 19811 (1 9423