Piaiit Physiol. (1968) 43, 1503-15,11

Ethylene, Plant Senescence and Abscission-Stanley P. Burg

Department of Biochemistry and Program in Cellular and Molecular Biology,University of Miami School of Medicine, 1600 N.W. 10th Avenue, Mianmi, Florida 33136

A bstract. Evidence supporting the hypothesis that ethylene is involved in the control ofsenescence and abscission is reviewed. The data indicate that ethylene causes abscissionin vivo by inhibiting auxin synthesis and transport or enhancing auxin destruction, thus loweringthe diffusible auxin level. Studies with isolated leaves and explants suggest that the gas alsomay influence abscission by accelerating senescence and through an action on plant cell walls.Freshly prepared explants produce ethylene at a rate which must be high enough to maximallyaffect the tissue and this may explain why these explants (stage I) cannot respond to appliedethylene.

In the period between 1860 and 1870 it wasreported oIn several occasions that certain trees(34, 49) and many varieties of plants (30) weredefoliated after accidental exposure to illuminatinggas. That ethylene contained in illuminating gascaused the leaf fall was suggested by the demonstra-tion in 1901 (58) that the olefin is the biologicallyactive conmpoinent of illuminating gas, and directlyproven by Doubt in 1917 (29). During the next 4decades the ability of ethylene to induce abscissionwas considered, as were most other actions of thegas. to be merely an interesting and remarkablecuriosity. However, it now has been establishedthat the gas is an endogenous regulator not onlv offruit ripening (12, 14), but also of vegetative (17,21. 24. 35, 36,48) and reproductive activities (16,23). Therefore, it is appropriate to inquire whetherethylene also controls or influences the natural ab-scission process.

Behavior of Whole Plants Exposed to Ethylene.Applied ethylene is most effective in causing oldleaves to abscise so that as plants age, progressivelyless of the gas is required to defoliate them (29).Similarly older leaves are more prone to abscise thanyoung ones when defoliants are applied (51). Thissusceptibility of old leaves is correlated with and infact may be due to their low auxin content (69),and thus it is not surprising that application ofNAA or IAA prevents ethylene from stimulatingabscission both in intact plants (28, 39) and explants(2, 5). W\e have noted that applied auxin (2,4-D)also prevents ethylene from inhibiting growth inlight grown pea plants, and deduce from this that

1 MIanv of the experiments described in this revieww-ere supported by research grant UI-00164-03 from theUnited States Public Health Service, National Center forUrban and Industrial Health, and were carried out dur-ing the tenure of NIH Career Research DevelopmentAward 5-K3-GM-6871.

ethylene might depress growtth by reducing thisplant's auxin content. Possibly a similar actioncauses ethylene induced abscission in vizo.

Effect of Ethylene otn Auxin Mletabolismii. XVhenethylene is applied to intact plants, the amount ofdiffusible auxin which can be recovered is consider-ably reduced (37, 50, 55, 68). There are several pos-sible explanations for this response: (i) Ethylenieinhibits polar transport: Althouglh ethylene has noeffect on auxin uptake or polar auxinl transportwhen it is applied to excised stem sections (1, 19,50, 55), it often inhibits transport ini vivo (19, 37,56, 57). The capacity of the transport system inthe stem of an etiolated pea plant is reduced by over90 % within 24 hours after ethvlene is applied,without any significant change in the velocity ofthe system (19). We could not detect a transportdisturbance in coleoptiles cut from Avena sativaseedlings which had been exposed to and respondedto ethylene during a 24 hour period; but the trans-port capacity of corn coleoptiles was reduced 30 %under the same conditions, indicating that the gasis able to disturb transport in this modified leaf(19). In all cases the inhibition is irreversible,persisting long after ethylene is removed. (ii)Ethylene enhatnces auxin destrutctioni: Morgan et al.(39, 56) found that ethylene fumigation enhancesIAA oxidase activity in cotton and several otherplants. Although this cannot account for the lowereddiffusible auxin level of pea and Avena seedlingsexposed to ethylene, because auxin destruction is notsignificantlv altered in these cases (17, 19, 56), itcould be an important factor in other tissues. Ithas been suggested that ethvlene induced IAA oxi-dase activity might mediate ethylene stimulated ab-scission in cotton (66), and this idea is supportedby the fact that phenols which stimulate IAA oxidaseactivity promote abscission of cotton explants,whereas those which inhibit IAA oxidase activitvretard abscission. (iii) Et1hVlene inihibits auixin

1503

PLANT PHYSIOLOGY

synthesis: Valdovinos ct al. (68) report that theconversion of labeled trvptophan to IAA by breisderived from liglht grown pea and Coleus plants ismuch reduced if the plants halve been treated with25 ppmii ethylene for 18 hlouirs.

Any or all of the above responses could accountfor the loss of diffusible auxin in ethylene treatedtissue and thus explain why the gas stimulatesabscission.

Epinasty vs. Albsci.ssion. Epinasty, unlike abscis-sion, is not prevented bv auxins; to the contrary highconcentrations of auxin inidutce epinasty by stimu-lating ethylene formation (39). Therefore, even ifthe endogenous auxin level were sustained at a highenough value to prevent abscission, leaves still oughtto become epinastic in the presence of ethylene.Doubt (29) and others (27, 40) report that exposureto 0.1 to 2 ppm ethiylene produces epinasty withoutcausing any leaves to fall, whereas the same treat-ment carried out witlh 2 to 10 ppm ethylene defoliatesall but the youngest leaves. These results suggestthat a low concentration of ethylene may induceepinasty without substantially reducing the auxinlevel, while a higher concentration could be requiredto lower the auxin level sufficiently to permit ab-scission. Such an interpretation would explain whyabscission in zivo seems to require more ethylenethan is needed to produce the same response inexplants, or other responses both in vivo and invitro (table I).

Production of Ethiylenie in Vivo. Ethylene isusually produced in those parts of the plant whichhave the highest auxin content, presumably becausethe production of the gas is stimulated by auxin(2,6,16,17,21.23,24,39,42,48,62). A high rateof ethylene evolution is associated with the scaleleaves and apical regions of the stem and root ofetiolated pea seedlings (21, 35, 48), also with theapex and youngest petiole of a light grown pea stem(fig 1); in contrast an older petiole produces muchless of the gas. Tlhuis throughout the etiolated peaplant the rate of ethylene production is correlated

4.1

0.7-

0.2

0.15

0.2

0.2

ETHYLENE PRODUCTION(m,uL. gm-1. hr.-I )

FIG. 1. Ethylene production by -arious parts of 14day old green pea plants. Rates -%vere determined for5 mm pieces of stem and petiolar tissue, or entire ten-drils and apical hooks. Tissue was incubated in a solu-tion containing 2 % sucrose and 50 msi potassium phos-phate buffer (pH 6.8) until the wound response hadsubsided (5 hr), and then 20 sections were sealed in50 ml Erlenme3-er flasks containing 8 ml of the same

solution to determine ethylene evolution during an addi-tional 18 hours.

Table I. Relative Sensitivities of Various Processes to Ethylene

Response Threshold

ppm0.01Inhibition of hook openinlg

Stem swelling andlbiibitionof stem growtth

Root swelling and inhibitionof root growvth

EpinastNInhibition of l-tteral I A.transport

Leaf fadingAbscission of explantsInhibition of blud gr"oxi-thFruit ripeningI

0.01

0.010.025-0.05

0.030.020.01

0.1-0.2

Ethy%lene coIncHalf-maxinmal

ppm0.1

1 These data are taken fronm refel-enices 2, 12 17, 21, 24, 27, 35, 48.

MNlaximalppm

1

0.2

0.25

0.3030.150.2

31

1

21-10

1 504

BURG-ETHYLENE, PLANT SENESCENCE, AND ABSCISSION

> -O- CASSIACOLEUS

LI N\ \ -A - GOSSYPIUM

Z -A- PHASEOLUS

500

a-

z

2 3DAYS

FIG. 2. Progressive changes in the amount of ethy-lene produced by freshly prepared explants (adapted fromref. 2). All rates are compared to that measured dur-ing an initial 6 hour period, which is considered to be100 % in each case.

with a high auxin content (59) and low IAA oxidaseactivity (33). \VNhen the IAA content is artificiallyaltered by applying growth hormone, the rate ofethylene production continually reflects the level offree IAA. For example, ethylene production by pea

roots (24) increases abruptly within 15 to 30 minutesafter IAA is applied, reaches a maximum coincidentwith free IAA within a few hours, declines, andstops in about 10 hours when all IAA has been

._ 3

.-

ECP

Et 2z0

0

0

a.

w Iz

LI

20 40 60

HOU RS

FIG. 3. Progressive changes in the absolute rate atwvhich ethylene is produced bY freshly prepared beanexplants (adapted from ref. 62, assuming the tissue tocontain 80 % w-ater on a fr wt basis).

destroyed or converted to indoleacetyl aspartate (8).The magnitude of the stimulation which auxincauses also depends upon the age of the tissue.Thus the capacity of bean petiole explants to produceethylene when NAA is applied decreases with time(6), and we find that while the threshold concen-

tration of IAA needed to elicit ethylene productionin various parts of the etiolated pea stem is fairlyconstant, the magnitude of the response at each IAAconcentration is inversely related to the age of thetissue.

Behavior of Explanits E-Txposed to Ethylene.Explants derived from Phzaseoluts and other plantsproduce ethylene at a maximal rate immediatelyafter they are excised, and then the rate declines(2 and fig 2). Ethylene production per gram freshweight of bean explants can be calculated frompublished data (62), and is presented in figture 3.These values only become meaningful when we

inquire "how low a rate of ethylene productionsuffices to cause abscission"?

Relationship Between Ethzylcne Production Rate,Applied Ethylene, and the Internal Ethylene Content.Gas exchange in plant and animal tissue is regulatedby Fick's law. At equilibrium the rate of ethyileneproduction equals the rate at wlhich the gas escapes

from the tissue, and this in turn is proportional toa number of constants (surface area, volume, dif-fusivity of ethvlene, etc.) times the concentrationgradient forcing ethylene from the tissue into theambient air (15). Thus:rate of production=rate of escape=K (C i,-Cont)where K is a constant wvhere Cin the concentration ofethylene within the tissue, C-,.t the concentration inthe ambient air, and Cin Cout is proportional tothe concentration gradient. The action of ethylenedepends only on the internal concentration (CiC),and if a tissue is well aerated (Cout = 0) the internalconcentration is directly proportional to the rate ofethylene production. \Vhen a tissue is not wellaerated or if ethylene is applied, the internal contentis increased by exactly the amount of gas appliedor accumulated; so that (Cin- Cout) always equalsthe internal concentration of well aerated tissue.

It is unfortunate that many studies on ethyleneproduction simplv measure the concentration ofethylene in the gas phase above tissues sealed inclosed containers. Obviously if sufficient ethyleneaccumulates it will be the dominant factor deter-mining the internal concentration, but this seldomis the case. For example, by direct measurementit can be shown that a gram of apple tissue has tobe sealed in a 100 ml bottle for 200 to 1000 hoursin order to dotuble its internal concentration relativeto that of well aerated tissue (12, 15). By indirectmethods (see below) it can be calculated that theinternal concentration of ethylene in a gram ofvegetative tissue would double under the same con-

ditions duiring about 40 hours confinement. It makesno difference what the absolute rate of ethyleneevolution is in such a case; all that matters is the

RUBINSTEIN AND ABELES (1964, 1965)

1505

PLANT PHYSIOLOGY

relative amount of tissue and size of the bottle.Therefore measurements of ppnm ethylene in the gasphase vastly underestimate the internal ethyleneconcentration and are not directly proportional toit under normal experimental conditions.

A quantitative relationslhip betwreen internalethylene conteint and the rate of ethylene productionhas been derived bv direct measurement in the caseof fruit tissues (12, 15), anid indirectly with vegeta-tive tissue ('14, 16, 17, 24). A few ppm and allhigher concenitratiolns of ethylene applied to etio-lated pea stem sections cause maximal swelling andinhibition of elongation, and if the external IAAconcentratioln is raised to a level that induces etlhyleneproduction at not less tlhani 5 mA,lgm- 1hr- , naximalswelling and inhlibitioni of elonigation result fromendogenouslylproduced gas (17). U'nder these con-ditions the tissuie becomes ninresponsix'e to appliedgas. In other words pea tissule behaves as thotughit cointainls a few ppmii ethylene henl it lproduces thegas at a rate of 5 mngulmgm'-1'r-'. Identical valueshave beeln obtained with sunflower stenm tissue (17).anid pea roots ( 24). Similarly the lhook of anletiolated black Valentine beal)plant is previente(lfrom opening in red light by 1 ppm applied ethylene(table I), ancd normally remains, almost completelyclosed when it produces ethylene at a rate of3.4 m,uJ.gm-' hr-1 (48). In each case a rate ofethylene production in the raange between 3 andS m1.dgm-l hrl causes the tissuie to respond com-pletelx- to its endogenouis gas, anld since abscissionin explants is maximally stinmulated by the sameamount of ethylene requiredI to affect hook closureand the swelling responise ( table I), it follows thata rate of ethylene production between 3 anid 5npl-gJgm-1hr-1 shoufld be comiipletely effective inacceleratinig abscission. A5s freshly prepared beanexplants produce 3 miud of ethylene per gm-1 hr-I(fig 3) they mnust be optimlall- stimulated.

Sigulificaiice of Enidogcniouts Ethylente Produictioniin1 Explanits. Rubinisteini andl Leopold (63) definedstage I explants as those whose longevity is extendedb)y ap)plication of auixinls. \Vhenl the explant agesin the absence of applied auxin for 6 to 12 hours itenters a new phase, stage II, during wvhich appliedauxins, amino acids anid etlhy-lene stimulate abscission(fig 4A refs 2,26). The idea that a certain agingprocess m1ust occur before petioles become sensitiveto ethylene implies that the gas does not acceleratestage I, and this hypothesi-s is sulpported b- (latashowinig that applied ethylene fails to cause chloro-phyll breakdown, protein hydrolysis, and(I loss in dryweight of bean pulvinar tislsue (v5). Osborne (43)holds anl opposing v:iew, that ethvlene enhancessenescence in the petiole just as it does in certainother tisstues (23, 29, 53, 70). and results obtainedby Dijkman anid Burg (fig 5 and table II) supportthis interpretation. Ethylene enhances degreeningof isolated Avena leaves; about 0.3 ppm is halfeffective, anid a 3 to 6 houir exp)osure to 2 ppmn andall higher concentrations prodtuces a maximal effect.

IOc

z0C/)U)C-)Inm 5C

zIE0 :

0(I)0zw LLJzWa

HE

2 3

DAYS

Fic. 4. (UIpper) Effect of 0.25 ppm ethyclee appYlicdat -arious times (indicated by arrows) oni the absci.;iouof freshly prepared cottoln explants (adapted from ret.2). Control explan-ts did not abscise dluring the couirseof the experimnent. (Low-er) Ethylene produlctioni Vycontrol explants. A high rate of ethx ceue productionduiring the firsit 6 to 12 holurs makes the tisste apipearto be inseulsitive to applied ethylene at that time.

100

800

o, _

0

60

01 0 10 100 1000

ETHYLENE CONC (ppm)

FIG. 5. Fading of 5 day old Az'cna sati7w leax esincubated in 10 ml of x-water in the presence of -arioulsconcentrations of ethylene. Chlorophyll is measuired asthe optical density (OD) at 660 m,u of a 50 ml ethanolicextract obtained from 1 gram of leaves. Arrox- indicateshalf-maximal effect. Data of Dijknlmai an(d Burg.

) GOSSYPIUM ,'°|ABELES, 1967) /PI

) ~~~~~~~~~~~~~~~I /I/fND

. I -

C

3

2

1506

BURG-ETHYLEN-E PLANT SENESCENCE, AND ABSCISS1ON

Table I1. Fading of Avena sativa Lcaz'cs Exposed to2 ppuii Ethliene for Various Periods of Timiie

Expostire

hr

0

0-30-60-240-120

I Data of Dijkimain anid Bi

OD at 660 m,u per g fr w-tafter 120 hrl

0.590.650.400.400.40

\VThv then does ethylene fail to enhlance stage I inexplants? Probably this simiply reflects the factthat explants initially contain a maximally stimula-tory amount of ethylene, makinig it impossible todemonstrate any response to ethylene applied at thattime. During the transition from stage I to stageII the ethylene production decreases, perhaps dueto loss of auxin, and consequently the explants be-come responsive to applied gas.

It has been proposed (5,26) that auxinis preventabscission by delaying the aginig process in thepetiole. Of course IAA is not unique in possessingthe ability to delay senescence; kinins (61) fre-quently have this property anid GA also may producethe samie effect (32). Perhaps these plant hormonesare like many animal hormones in this respect. forthe latter characteristicallv function not onlv toregulate a specific biochemical event but also tomaintaini their target tissue, thus preventing atrophyand death. If senescence must accompainy or pre-cede abscission, should not kinins and gibberellinsalso prevent abscission in at least some cases?Gibberellins accelerate abscission (24, 46) buit kininsreplace atuxin in the bean explant test (2), delavingabscission and preventing ethylene or proximallyapplied auxin from stimulating abscission. Reversalof ethvlene action by kinetin has also been demon-strated in 2 other instances: ethylene inhibits peabud growth but kinetin overrides the effect (21),and we have noted that Ave;ia leaves exposed toethylene and kinetin remain green mutlclh longer thanuntreated leaves or those induced to fade withethylene, although not quite as long as those exposedto kinetin alone. Thus in general the facts areconsistent with the hypothesis that ethylene acceler-ates a certain aging process (stage I) which pre-cedes the abscission event. Chatterjee and Leopold(26) and Jacobs (45) have presented evidence whichindicates that this aging phenomenon also occursin -vivo. Older petioles behave as thouglh they havenearly completed stage I, and in addition have a

reduced capacity to be stimulated to abscise whenauxin is applied proximally (26). Does ethyleneparticipate in this in vivo transition from stage Ito stage II? Perhaps it does, for these older petiolesin several respects resemble ethy lene treated tissue;thev have a reduiced capacity but a normal velocity

of auxin transport (in Coleuis-jacobs: Plant Physiol.43: 1480-95) and a lowered auxini content. Ifthey act like older portions of stemiis, appliedauxins should be only marginally effective in stimu-lating their ethvlene production, and this mightexplain why it is difficult to stimulate abscission inthese petioles by proximal auxin applicatioin (seebelow).

Stimu2llation1 of Abscissiont with GA anid Auxitius.The extensive studies of Jacobs et al. (45) haveindicated that correlative events enhancing abscissionmay be mediated by auxin transported from adjacentor distant leaves and the stern apex to the proximalside of an abscission zone wlhich previously experi-enced a decline in distally supplied auxin. Pertinentto this problem, but particularlv difficult to explain,is the observation of Biggs and Leopold (10) thatlow concentrations of proximally applied auxinstimulate abscission. This stimulation only developsafter the tissue has passed into stage II (25, 63) soclearly the proximal treatment does not prevent thestage I to II transition. Once a petiole has agedand entered stage II it is stimulated to abscise byeither distally or proximally supplied auxin (63).Biggs and Leopold (10) believed the stimulation ofabscission by low concentrationis of proximally ap-plied auxin, and its retardation by higlh concentra-tions, to be a reflection of the two phase actionwhich aux,in has on many other processes. Thisexplanation is not consistent with recent evidencesuggesting that the inhibitory phase of the growtlresponse curve often is due to ethylene formation(17, 21, 24, 42); for on these grounds it would bepredicted that high concentrations of auxin oughtto accelerate, not inhibit abscission. Abeles (2)has advanced an alternative explanation; he suggeststhat because of the strict polarity of transport in thepetiole a low concentration of auxin applied prox-imally fails to reach the abscission zone in sufficientquantity to arrest senescence. It does, however,stimtulate ethylene production. and this ethylene mayin turn enhance abscission after the transition fromstage I to II is completed. This attractive idea issupported by his finding that various treatmentswhiclh facilitate diffusion of auxin from the proximalend to the abscission zone. such as shortening thepetiolar stump, reduce the ability of auxin to stimu-late abscission (2). Presumably under these con-ditions enough auxin arrives in the abscission zoneat an early time to prevent the transition from1stage I to stage II. A similar explanation for theabscission inducing action of a variety of compounds,including GA and certain amino acids, has also beenadvanced (2, 6). However, there are several rea-sons for seriously questioning this explanation of theaction of proximally applied auxin. The lowest con-centration of auxin capable of stimulating abscissionis that which causes a just perceptible increase inethylene evolution (6, 17, 21), and it is unlikely thatso slight an increase in ethylene production couldsignificantly alter the ethylene content of an ab-

l1507

PLANT PIIYSIOLOGY

scission zoIie s;everal mm d(li.anll. For exaimpleethylene produced in the biook regioll of etiolatedpea p)lants keeps the lhook frolmi openiing (35) anlflvet nlot enioulgh ethylene reaches the subapical zonea fexx mm away to cause any percel)tible s5wellillg.Similarlv ethylene causes anl intense inhibition ofgrow tl oni the lowx er side of a r oo. durinig thegeotrop)ic responise, bltt only a sliglht iniibitioni onthe upper si(le (24) and lateral transport of 10 1rm1AA throu-gh a pea stemii gi-es rise to ethylene pro-duction on the low-er side of the stemii. comopletelypreventing lateral transport there hut nlot in theupper si'de (17)I. .\ll of these exampl)les ind(licatethat ethvlene acts very close to its site of lpro(dlctionin vegetative tissue. Aikotlher seriouLs problem isthe fin-ding that the same amount of NA.\ and24-1 is pre.sent in the abs.,cis.sio- zonlle anld subj1'acenttissuie 4 or 24 hours after application, regardlless ofwhether it is introduced lproximailly or (Iistallv(47, 63) hence there is nlo correlatioln betxweeni theautxin content of the abscission zonte anld the r-e-sultant resl)oi-se, as tlis explanationi lpretl)l)oses.Finally, it shiould be noted that it takes less than1 I M NAA applied proximally to stimulate abscis-SiOln, whereas imiore thali 100 ttm NAAi\ ist beiVpplied cit/icr proximally or distally to retar(l thleprocess. Apparently miutclh mior-e auxini is requiiredto prevent abscission tlhani is nieededl to stimutlateethylene prodtiction; thierefor-e ani explanation basedsolelv onl these 2 factors caniniot explain why lowconicentratiolis of distally supplied autixin fail tostimiuilate abscission.

l[1ecc/anism of lit/i lciic cActioii. Ho i,[acI iNActionms Docs Et/i /1cue Elxcrt.t Ethylene causes a1multiplicity of effects, but almost withiout exceptionthiere is a remarkable uniformity in the anmotint ofgaS Wx hich miutst be applied to produce a threshiold,half-maximial and comiiplete respollse (table T).Altlhotuglh the concelntr-ationl del)endlence curve needInot be a maeastire of the affinity of a regulator forits -eceptor site. it probal)ly is in thlis case becauseit is possible by meanis of this ctirve to demolis'-ratecomlpetitive ihliibition betweeln ethylene anldi CO..(18). This being the case wve canl interpret theuiiformitv in dose response curves to mieain that allof these ostensibly different effccts are contr-olle(dby reccptor sites having closely similar affiniities foiethlvlene. In addition, in the case of at least 6differeint respionses (ilncluidilng explant abscission)varioul1s analoguies of ethy-lene have the same relativeefficac in each of the test systems, and in these andlall other cases examinied, CO., is a comlpetitiveitlhibitor of ethylenie actioin (18). These resultsstiggest that a single receptor site i.s involved inmost actions of the gas.i. alid lacking evidence to thecontrary wNe can mnlake a simplifxying assunmption thatthis. receptor catalyzes a single initial event. Thesittiation may be likiened to that of the phytochromeconlVersion system. where onle hasic chlange leads toa dixversity of effects which (lepeid mainly oni thenaturl-e of the tissuie.

Certaiin properties of the receptor site can lieinferred from its molecular specificity. The require-ments for ethylene action are similar to those whichhlave been established for miietal biniding to unsatu-rated comipouiiinds, and in fact it lhas beein rel)ortedthat the biological activity of the various substitutedolefines closely- l)parallels their iffinity for silver ion(18 in a(l(lition, 1ox\ concentrations of CO 1re'l)lace ethylene in all its funictionis, and( since th-ieactiOnl of CO typically involves mietal binding, wehav e prolpose(l that of ethylene to be l)redica-te(l onthe Sa-lmle effect (18).

\What is the nature of the initial event cataly-zedbv ethylvene after it has attached to its miietallicreceptor? The folloN-inig theories wvill be consideredbriefly: i) enilanicemiienit of niemibranie permeability.ii) initeraction with 1 \A, iii ) induction of RNAmid protein synthesis. and iv ) effects on the lplantcell wall.

Eiihaiiceeoic;it of cllenbraiic Pernicability. Thleidea that ethylene affects celilular perineabilit\ lprob-ably arose becaLuse of the high solubilitv of the gasin lipid. Carbon monooxide because of its very lowdipole also is extremely soluble in lipid; vet low-concentrations in the range x\01ich mimiiic etf lveneactioin in plants clause (leath in hulians in a mii ttterof hours not by anx action in a lipid phase, bit bvbindlilng to haemioglobill. Similarl-. CO binds tocvtochromie oxidase not because of its fat solubilitvbuit beeawise of its affinity (KT) for the recel)tor.x-whicil perhlapls 1v coincidence is the samie as itsaiffinitv Kin for the ethylene recel)tor site ( 18).Aniothier analogue, vinyl fluoride, is hipghly effectiVebiologically but infinitely less soluible than etflvlenein lipid, beilng eqluallv soluble in lipid and water.Because of these observations, and thle fact thatethylene is a verv water soluble gas (it hais the sirnewvater solubilitv as CO and is 10 timiles as solubleas 0., in xvater'), it is not possible froml the plhsicvalproperties of the gas to determine wvhethier itsbiological action occurs in a lipiid phase or elsewherein the cell. Claims of enhlnanice(d permeability dueto ethylene application have beenl publishied (7. 38),aand the idea sul)ported by reports of a progressiveincrease in lealkage (durinig the ripeninig of frtlits(9, 64, 65), but in the author's opinion there is littleevidenice to support this poinlt of viexv. The stucdiesoni xater perimeability ( 7, 38) xxere carried out ulid(ler-conditiolnS causiilng a net floxx non-diffusive) ofxvater adloug an osmotic pressure gradienit. and tileh-e-fore are likelv to reflect changes in solute colntenltand xWall Dlasticity ratlher than alltered \vater permie-,bility. especially in experiments lastinlg many honirs(7). It has not been possible to repeat certa,in ofthese .tudies 38). Aloreover ethylene docs iioteInlihlace p)ernieablilitv Nxvlenl it affects pea tisstie ( 22),and biologicallx- active concentrations of ethylenehave lCo effect on mitochiondrial permi-eadbility (52 ).allthlougil colnceIntr-ationls in the narcotic r-angie areequally (ldaiilgin- to both animiial and( lpllnt ilito-chondrial. The d(ata on fruit leakage ofteli does not

I 5t(11

BURG-ETHYLENE, PLANT SENESCENCE, AND ABSCISSION

15 \ ~ v rv;,I..LIM I cErI,

C,)~~~~~(1

312

UA.z

CD

z

4

-5

-10

-15

~~~2 3

SUCROSE CONC (M)FIG. 6. Fresh weight changes in green alnd fully ripe

Gros Mfichel banana disks (1 cm diam X 1 mm thick)floated for 60 minutes on solutions conitaining variousconcentrations of sucrose. In each case nlo change inweight occurred when the solution containied a concen-trationi of sucrose having a tonicity closely similar tothat of the expressed juice of the fruit. -Measurementsafter 120 minuXtes, and studies with manniiitol (0-0.7 NI)gave identical results. The data indicate that at least40 % of the water contained in the ripe fruit must residein ani osmotic volume.

reflect changes in membrane permeability but ratherthe total solute available for leakage (22), especiallysugar and malate which increase during the climac-teric. The concept of 100 % free space in bananasat the time of the climacteric maximum (65) is notcompatible with the observation that tissue fromclimlacteric banana fruit has normal osmotic proper-

ties (fig 6) and hence a substantial osmllotic volume.In anv event the reported increase in free space

occurs several days before ethylene prodtuction beginsin this frtuit (13) and hence could not be caused byethylene. Finally it has been demonstrated thatcertain types of ripe fruit disks most nearly resemblethe intact fruit wlhen their integrity is maintainedby a solution of moderate tonicitV, and under theseconditions they do not leak (22). Thus there islittle evidence to support the popular view thatethvlene mav act to alter cellular permeability.

Interaction of Ethylente with IAA. Several ef-fects of ethylene on IAA metabolism have beenestablished although they do not necessarily occur

in all types of plant tissue; the gas instantly andreversiblv inhibits lateral auxin movement (17).progressively anid irreversibly inhlibits the capacityof the polar auxin transport system (19, 56, 57)induces I XA oxidase activity (39); apparently re-

tards auxin synthesis (68); and lowers the diffusibleauxin level (37, 50, 55, 68). This last mentionedeffect, which may result from several of the otheractions, probably is a primary factor causing ab-scission in vivo.

Iniduction of RNA and Protein Synthesis. En-hancement of RNA and protein synthesis afterethylene application has been reported for explants(3, 4, 41) and other tissues (42). It is not clearwhether these changes are due to the direct actionof the gas, or whether they naturally occur duringabscission and simply reflect the fact that abscissionhas been accelerated by ethylene. It has been re-ported that at the time of 50 % abscission the rateof incorporation of label into RNA and the type ofRNA formed seems to be the same in bean explantsregardless of whether abscission has been hastenedby ethylene (3). Studies with inhibitors of proteinand RNA synthesis cannot resolve this problembecause they onlv indicate whether protein andRNA synthesis are required for abscission and notwhether they are needed for the initial steps inethylene action. It may be significant that in onecase, the inhibition of lateral auxin transport byethylene, it has been shown that cycloheximide failsto prevent ethylene action when it inhibits growthby 50 % (20). This finding implies that ethyleneaction, at least in this instance, does not requireprotein synthesis.

Effect of Ethylenie on the Cell Wall. Ethylenecauses growing cells in most roots and many stemsimmediately to reduce their rate of elongation andexpand instead in a radial direction (17, 21, 24).As the normal predominantly longitudinal directionof expansion is thought to be imposed on cells bythe restricting influence of radially deposited micro-fibrils, this observation suggests that ethylene some-how alters the structure of the cell wall to allowradial expansion. Ethylene also is highly effectivein stimulating root hair development (24), possiblyexplainiing why high concentrations of auxin havethe same action; this again implies an effect ofethvlene on the cell wall. The rate of incorporationof 14C-glucose into pea cell walls is slightly reducedbv ethylene (11). but most of the effect can beascribed to a small decrease in the rate of gluicoseuptake. 'When viewed through crossedl polaroids,the swollen cells display a characteristic pattern oflongitudinal banding in the walls (21) wrhiclh isindistinguishable from that observed in the samecells induced to swell by benzimidazole (60). Inthe latter case the banding is due to newly depositedlong,itudinally oriented ilcrofibrils. so presumablythese also are formed in etlivlene treated cells.

Changes in cell wall metabolism caused byeth-lene are likely to have significanice for theabscission process since dissolution of walls anid thlemiddle lamella are an important aspect of it. Dturinigabscission celltulase increases in the abscission zone(43), and this change is prevented by IAA andstimulated by ethvlene. To the contrarv, IA.N ii-

1 509

PLANT PHYSIOLOGY

dcices cellulase activity in the subapical zonie of peaswhen it causes swelling (31), but it should be notedthat in this experiment the concentration of TAAused was so high that it undoubtedly stimulatedextensive ethylene evolution, and it is clear that thisethylene and not the applied TAA causes the swellingin peas (17). The exact role which cellulase mightplay in the swelling reaction is not obvious sincethe total wall dry veight and rate of increase invall dry wTeight is not altered bv etlhvlene appliedto pea stem sections (17), but in the case of abscis-sion a hyvdrolytic process cotuld be extremely im-portalnt.

Evidence has been reviewed which stuggests aninvolvement of cthylene in the conitrol of senescenceand abscission. Applied gas probably acts to lowerthe l0vel of diffusible auxin by inhibitinig auxinsynthesis anid transport, and enlhalncing- auxin de-struction. Stubsequently ethylenie may acceleratesenescence and eventually abscissioni itself, possiblythrotugh its action on plant cell Nvalls.

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