Plant Physiol. (1969) 44, 1144-1149

Abscission: Role of Abscisic Acid L. E. Cracker and F. B. Abeles Plant Sciences Laboratories, Fort Detrick, Frederiok, Maryland 21701

Received March 31, 1969. Downloaded from https://academic.oup.com/plphys/article/44/8/1144/6090212 by guest on 29 September 2021 Abstract. Th2 effect of abscisic acid on cotton (Gossypium hirsutuin L. cv. Acala 4-42) and bean (Phaseolus vulgaris L. cv. Red Kidney) explants was 2-fold. It increased ethylene producticn from the explants, which was found to account for some of its ability to accelerate abscission. Absci ic acd also increased the activity of cellulase. Increased synthesis of cellulaso was not du to an increase in aging of the explants but rather was an effect of abscisic acid on the processes that lead to cellulase synthesis or activity.

The ability of a diffusible substance from cotton Reynolds Tobacco Conmpany and consi-ted of 47.3 % fruit to accelerate the abscission of cotton exlplants d,l-cis,trans abscisic acid (the natural isomer) and ultimately led to the discovery of what is now known ,523 % of the d.l-trans-trans isomer. All concentra- as abscisic acid, a plant hormone capable of regut- tions were based on the cis-trats isomer. Samples lating a number of plant processes (9, 16). The of dl-cis,tans isomer (SD 16108) were a gift of role of abscilc acid in abscission however, remains the Shell Development Company, and a number of uncer.ain. The substance promotes the abscission of preliminary experiments with tthis material indica-ted explants, but then so do a large numnber of other that there was no essential difference in the proRer- compounds, manv of which are not normal constitu- ties of tIe 2 preparations. ents of leaves (1). Evidence bo.h for (9) and Earlier papers from this laboratory have described against (15) a role for thiis hornmone in abscission Lhe trea:mncnt of explants with e.hylene anid CO., in has been pre ented. However the work of Dale and desiccators and gas collection bottles (4), the meas- Milford (15) has been criticizcd becau-e chromo- urement of ethylene by gas chromatography (8), the tographic behavior of tlheir compound differed from injection of actinomycin D (1 1 g//,l water) into the 8-inhibitor and did not pronlote abscission of the separation layers of explants (6), the determina- young cotton fruit. tion of celluilase ac.ivity by the lo s of viscosity of This paper presents a series of experinments de- sodiuim carboxymethyl cellulose (CMC) (4), and signed to elucidate the role of abscisic acid in the break strength measurements by a recording ab- abscission of isolated ab cission zone explants, taking scissor ( 14). advantage of improved techlniqtues for the measure- ment of cell separation (14) and the production of cell wall degrading enzymes (4). Results

Materials and Methods The effect of abscisic acid on break strength and ethylene production from explants is summmarized in Planit Material. The methods used to grow bean table I. The data are in agreement witlh earlier (Phaseolus viilgaris L. cv. Red Kidnev) and cotton observations (1) thlat increasing amounts of abscisic (Gossypium hirsutumii L. cv. Acala 4-42) plants, and acid cause explants to produce increasing amotunts of to prepare and store explants, have been described ethylene and that there was a close correlation be- earlier (1,6, 7). Each experimental datum with tween the loss of break strengLh and ethylene evolu- bean explants represen:s experiments repeated on 3 tion. different occasions witlh 3 sets of 10 explants. Cotton Table II presents the results of experiments in experiments were repeated twice, but since there are which explants treated with abscisic acid were ex- 2 separation layers per explant, each experimental po ed to 10 % CO.,. The data indicate that abscisic datum represents a total of 120 observations. acid increased ethylene evolution, but that the de- Application of Chemicals. Indole-3-acetic acid crease in break strength of cotton caused by abscisic (IAA) and abscisic acid were applied by placing a acid was only partiallv overcome by CO.. 50 ,ul drop of 1.5% agar containing the IAA or The data in table III show that ethylene was abscisic acid on the distal cut surface of an explant. unable to mnask the effect of abscisic acid on the IAA concentration was 5 X 10-5 M and abscisic break strength of cotton and bean explants. When acid, except for so-me preliminary concentration curve explants were treated with saturating levels of experiments, was 5 X 10-4 m. The abscisic acid ethylene and abscisic acid, an abscisic acid effect on used in these experiments was a gift of the R. J. break strength was still apparent. 1144 CRACKER AND ABELES-ABSCISSION: ROLE OF ABSCISIC ACID 1 145 Table I. Effcct )7.' Abscisic Acid Gn Break Strength anid Ethyl1ne Produection it Cotton and Beant Explanlts Explants were l)laced in 43-nmi gas collection bottles fitted witli rubber vaccine caps and kept at 230 with con- tinu_us 400 it-c flucrescenit light. Wound ethylene was flushed out of bottles 8 hr after excisi n. Break strength and etlhylenie productioni were measured at 28 hr. Cotton Bean Abscisic Break Break acid strength C2H4 strength C.H4 m g ppm g ppm 0 50a, 0.121' 174' 0.061' Downloaded from https://academic.oup.com/plphys/article/44/8/1144/6090212 by guest on 29 September 2021 5 x l0-6 47a,b 0.136' 163' 0.084a,b 5 X 10r 38b 0.136' 140a,b 0.072a,b 5 X 10-4 0c 0.316b 108b 0.096b Means having the same letter within a column are not significantly diiierent at the 5 % level.

Table II. Effect of 10 % CO., ons Abscission of Cotton1 The ability of actinonimycin D to inhibit abscission and Bean Explants Treated With 5X10-4 M in a 10 ppmli ethylene gas phase lasts 4 hr after Abscisic Acid excisioni in cot'lon explants (Fig. 3A) and 6 hr in Explants were placed in 43-ml gas collection bottles beani explanits (Fig. 3B). Abscisic acid had no fitted with rubber vaccine caps and kept at 250 with inifltuenice oni the length of time explants retained full continu-us 400 ft-c fluorescent light. Wound ethylene sensitivity to actinomycin D but it again caused a was flushed cut of bottles 8 hr after excision. Break lower break strength once the inhibitory effect of strength and ethylene production were measured at 28 hr. actinomvcin D was lost. The cellulase content of explants in an atmos- Cotto;n Bean of 6 Break Break l)here 10 ppm ethylene increases aifter hr for Treatment strength C2H4 strength C2H4 9 pptmt 9 ppm Control 63a 0.192a 160' 0.076' j 0--°--O--C 9?9------e. ONROCONTROL Abscisic acid 16C 0.332' 92b 0.Ilo, b Ooo CO, 60a 0. 70' 180a 0.106 Abscisic acid 39' 0.241a 189' 0.146" ABSCISIC ACID °"' + CO, -A-I C Means having the same letter within a column 30. 00% are not significantly different at the 5 % level.

I0- 20[ The data in Fig. 1 slhow the effect of abscisic acid A)COTTON " on changes in the break strength of A) cotton and I1 B) bean explants stored in an atmosphere of 10 ppm z ethylene. w .o6 The break strength of cotton and bean ' explants in an ethylene atmosphere remains constaint I- -A- ~ ~ for about 16 hr and then decreases. The effect of (I 0 4 8 12 16 20 24 2 8 abscisic acid in cotton explants appears to shorten x the time requirLed for a loss of break strengtlh by 300 w about 2 hr. The data obtained with bean explants w -6v-(J-(0'oW-Qe -CONTROL indicated n1o effect oIn inductioni period but rather 10260 TVJZ an accelerated loss of break strength once abscission ABSCISIC ACID-O coI0 was started. IAA was able to prevent a loss of break strength 220 % i of explants wlhen applied shortly after excision (Fig. ox0o 2A and 2B). When the time of IAA application 180 was delayed by 5 hr for cotton (Fig. 2A) and 8 hr B) BEAN for bean (Fig. 2B) then its effect was reduced and 140. delaying the time of application even further caused a corresponding loss of effectiveness. As shown in 0 4 8 12 16 20 24 28 Fig. 2A and 2B, abscisic acid did not appear to have HOURS an effect on the of time IAA period was able to FIG. 1. Effect of abscisic acid on break strength of prevent a loss of break strength. However, abscisic A) cotton and B) bean explants. Gas phase was 10 acid caused a lower break strength once the aging ppm ethylene. Abscisic acid was applied at 0 hr. Break requirements were met, strength determinations made at times indicated. 1146 PLANT PHYSIOLOGY

50 50s LI-:j- - - e U) b- CON TROL ABSCISIC 40 'O 0 40'ACID 0-CONTROL

ABSCISIC ACID- S " 0 30 Uf) 30' 0 - 4 A) COTTON S Downloaded from https://academic.oup.com/plphys/article/44/8/1144/6090212 by guest on 29 September 2021 0 20O 20, CD O 2 4 6 8 10 12 I z 10o A) COTTON w C, tr. U1) 2 4 6 8 10 12 300j z w w an Uf) 260 CONTROL U) X ABSCISIC ACID \t

ct 0 220- w 'I

U1) I80 ~ ~0 a: I '0 N 4\ 140 . B) BEAN 0 2 4 6 8 10 12 TIME ACTINOMYCIN D INJECTED (HRS)

PFIG. 3. Effect of abscisic acid oIn the ability of ac- 0 2 4 6 8 10 12 tinonmycin D to inhibit abscission in A) cotton and B) TIME OF IAA APPLICATION (HRS) beaui explants. Gas phase was 10 ppm ethy lene Ab- scisic acid and control agar droplets applied at 0 hr and FIG. 2. Effect of abscisic acid oni the ability of IAA left cn tlhroughout the cJurs2 of the experiment. Actin^- to retard abscission of A) cotton and B) bean explants. mycini D injected into explants at the times indicated. Gas phase was 10 ppm ethylene. Abscisic acid or plain agar controls added at 0 hr. Abscisic acid and control agar were carefully wiped off with moist tissue and replaced wvith IAA at times indicated. previous work oin abscission in this laboratory with cotton andl bleans ha(1s shown inhibition of a'bscission wvithl CO., ( 5). Hlowever, abscisic acid's ability to cottoni (ig. 4A) and 12 hr (Fig. 4B) for bean. accelercate abscissioni couild still be (duie to ethylene Abscisic acid did not decrease the lag period before stimulationi, siuce otlher cases of the inability of C( cellulase appeared in cotton explants and it increased to reverse ethylene effects have been reported (2, 8). celltulase activity over controls only after 14 hr. It did, however. encourage us to measure the effect

However, in bean explants abscisic acid did appear of abccisic acid in 10 ppmi ethylene. which we have to decrease the lag period as well as increasing fotun(d ealrlier to represenit a saturating concentration cellulase activity. of the gas for abscission (5). TIhlle Observation (table H I) that abscisic acid Discussion still had l)a onioyive effect on al sci ssion in the presence of saturating levels of ethl lene substantiates

the idea is 'that enhanced ethylene production only a The data in table I present evideince in favor of plart of the exl)lana1tion of the labscisic acid iieclhanismil the view that enhanced ethylene production plays a of action. role in the ability of abscisic acid to accelerate Fig. ; outlilles somlie of the processes tlhought to abscission of isolated abscission zone explants. How- occulr dur1 inig ascission an(ld (lelilneates the abscission ever, the data in tables II and III show that enhanced process ilto anumbera. of (liscrete stages or steps. ethylene production'is only a part of the explanation A review discussing this schemie in greater detail has of why abscisic acid stimulated abscission. been published earlier (3). The experiments pre- Taible II shows that only a part of the abscisic sented in Figs. 1 thrtu 4 wvere designed to distinguish acid effect on cotton coWld be overcome by CO2, a whether abscisic acid accelerated the aging process competitive inhibitor of ethylene action (5, 11). All (Stage 1) or the induiction of cell wall degrading CRACKER AND ABELES-ABSCISSION: ROLE OF ABSCISIC ACID 1147 enzymes (Stage 2). All of the experinments reported 0TT% OF ETCESSES OCCRRITS DCELrC ARACESSEOSI OP ABSCCSSE% T0'E EXPLANTS in Figs. I tihru 4 wvere doiie in the presence of 10 ppm S'f.VUE- ethylene. This was done to inisure that the effects of ACTNG PERIED- INDUCCT OF CELL iALL_ CELL SEPARATI abscisic acid and otlher treatments performed were 'DECRA'DING eSZY!IES ABILITY OF AUJXIN TO BLOCK ABSCISSION LOST I.SSCRESED SENSEIVIYETCELFENF^O EXCISIOS OF ASsCISSIOT ZONE CXPLANTS ACTIEATION OF AKSC1SS104. GETEOL Table III. Stimiuflationi, of 4bscissionilvby 5X10-4 bI WSS OF ACINC. EETARDANTS AA) RNA SYTTES1 FROM DISTAL TISSUME .4bsciszc .4cid antd 10 ppm11 Ethylene PRRCEECAD EYITHFESCs 14ITIATIO' WOt4D PHYSIOU),-,' CELLUL'ASF Explants were placed in desiccators c3ntaining air INCREASED ETtYLLDNEPRODRUCTIONTES or CALLOSE DECOMPflS ITON ethylene and ke)pt at 250 with continuous 400 ft-c ACTIVATION OF SENESCENCE ACVI iluorescent ligfht for the times indicated. FOLsUTIOf OV TN'LOSES MOBILIZATION CENO%F Downloaded from https://academic.oup.com/plphys/article/44/8/1144/6090212 by guest on 29 September 2021 P. I BONSI C LEASES Break strength CHLAOROYEYLLASES PROTEASES Ireatment Cotton (16 hr) Beans (26 hr) ARSCISIC ACID EMCT .6J ~~g LNCREASED ElHLENE YRODtCTION Control 122 158 INCREASED CELUASE ACTIVM L Abscisic acid 79 115 C,H. 37 52a FIG. 5. Outline of processes occurring during abscis- Abscisic acid ±- C2H4 60 44' sion of abscission zone explants. rf A .s :ka Means-_Irsigniticantly_. ditterentrr ,- _,at to0I A %- level.s- AllIII other not due to an increase in ethylene production. It means xvithin a column significantly different at 5 0 vas assumed that 10 saturated level.\a sue la 0pmehlnppm ethylene auae anyn requiremiient the explants had for the gas (5). One way of measuring the effect of abscisic acid +'°r * . , , on aginig is to determine the effect of abscisic acid oIn the time reqtuired for a loss of break strength. Since an aging period is required before cell wall CONTROL degrading enzymes start acting, an increased rate of -IO, aging should appear as an earlier loss in break ABSCISIC ACID i'' strength in explants treated with abscisic acid. The u -20-\0B A dataa lossobtainedof breakwithlstrengthcotton may(Fig.have1A)occurredindicate 2thathr \ o sooner in abscisic-acid-treated explants. Abscisic -30\ acid lhad no effect on the time required for a loss of 0 A) COTTON break strength in bean explants (Fig. iB) but, 'I- -40 rather, caused a more rapid loss of break strength i-n 0 2 4 6 8 10 IP 14 once the process was initiated. 0 _ We believe that the available data suggest that U) _1__I_It_-______5 the major function of IAA in abscission is to prevent .-* CONTROL aging of explants. As shown in Figs. 2A and 2B, z 0CONTROL ¢0 o\ " IAA can be added to explants for a fixed period of w * '4) time after excision and still prevent a loss of break CD -lo10 . z strength measured some hours later. If abscisic acid 41 \~ (clid accelerate aging, it should be able to shorten the -20 A time that IAA is still able to keep the explants in ABSCISIC ACID their original unaged state. If the role of abscisic -30r \ acid is to accelerate aging, we would have expected that IAA would have lost the ability to block abscission sooner in treated explants. However, the B) BEAN (lata in Figs. 2A and 2B indicate that the length of BBEAN Stage 1, measured as the ability of IAA to retard O 2 4 6 8 10 12 14 abscission, was the same in control and abscisic- HOURS acid-treated explants. FIG. 4. Effect of abscisic acid on the production of The completion of Stage 1 also sets into motion celllulase from separation layers of A) cotton and B) the (legradation of chlorophyll as well as other cellu- beain explants. Gas phase was W0 ppm ethylene. Ab- lar constituents (7, 24). We found that abscisic acid scissic acid and control agar drops applied to explants had no effect on chlorophyll degradation in bean at 0I hr and left on throughout the course of the experi- and cotton explan-tsusing methods described earlier mernt. Explants were harvested and frozen at times in- (7). However, other workers (10, 16) have ob- dicEated until analyzed for cellulase. The cellulase was that abscisic acid worer s of hloro- inctubated with 1.5 % sodium carboxymethyl cellulose served that abscisic acid increased the loss of chioro- in 0.05 M potass;ium phosphate buffer, pH 7, for 16 hr pliyll in other experimental systems. at 40o.X We have shown earlier t-hat actinomycin D 1148118rA'LN r PHYSIOLOGY had the ethylene production of the blocked RN N svnthlesis (20) and atbscission1 (6)) trees iio effect oni RN-A synthesis wvas essential to the leaves. Cooper et al. (13), however, found that suggesting that did D nwas defoliatin,g conceintrations' of ahscisic acid pro- cell separation proce.s;seed germination (22), have been synthesized. his loss of effectiveness iancv (16). inhibition inhibition of a-aniylase (12, 19), promotion of canll be thouhght to represent the onset of Stage -. If al)scisic acid accelerates the completion of Stage plienvialalnine aninionia-l-yase (25), floral morpho- Downloaded from https://academic.oup.com/plphys/article/44/8/1144/6090212 by guest on 29 September 2021 it slhouldl shorten the time for actinomycin D to genesis ( 16, 17, 23), or inhibition of growth (16, 21). tlleni increased ethylene its effectiveness. rThe data in F igs. 3A and 31 In fact, we reported earlier that lose of indicate that abscisic aci(I didl not shorten the time productioni (lid nlot account for the inhibition to block abscis- growth of excised soybean hvpocotyls by abscisic before actinomvcin D lost its ability aicid (21). a s i thie earlier figuires. SiOnl comlilpletely. How\'evr, the data presented here that abscisic acid di(d catuse lower break strengths oInce WVe conlcltude froml the actioni of ahscisic acid on abscission of isolated Stage 1 was completed. It shoultd be pointed out was 2-fold. First, it accelerated ethylene that thlere \\,as a 1-hir differelnce in tlle (lurationi of explants and, second, it increased cellulase activity. Stage 1 as measured by the auxin-induced 1lig and p)roduction acid has no effect on the actinomycin D-ind(uced lag in bean explalnts. This WVe believe that abscisic it increases the activity differeince was (lue to the fact that these aginig of explants hut r-ather primliarily such control over cellulase is exl)eriments wvere done at different times. If experi- of celilaulse. \NVhether at the level of RNA-protein syilthesis or of enzyme on actinomy'cin D-ilnduced miients atlxini-ind(uticed atlnd to be deter- retardation of abscission were (loIne simultaiineouisl\ release-activatioin-degradatioln remailns iminedl. with 1 sample of explants tllen thle duritions of Stage I were fouind to he similar. An experimeit (lesigne(d to measture the effect of Acknowledgment abscisic acid oIn Stage 2 is shown in Fig. 4. XVe have sho\ni that abscisic aicid accelerated tlle loss of We gratefully acknowledge the technical assistance break strenigth of cotton nd bean explants (Fig. 1) of AMr. Gerald Leather. Thaniks are also due to R. J. and that incl-ease in cellulase activity precededl the Reynolds Tobacco Company and the Shell Development loss of explant break strengtlh (14). If in addition Company for their gifts of abscisic acid. to abscisic acid's ability to increase ethylene pro- dutction, it lhas a second effect of increasing cellillase Literature Cited Ictivity, then tllis shouild be observable wvith the sensitive iscomietric techni(Iques uise(l to measure 1. ABELES, F. B. 1967. NIechanism of action of ab- celluilase activity. Cotton explants (Fig. 4A) treated scission1 accelerators. Ph11ysiol. Plantarun1 20: 442- did hlave greater cellil1ase activity, 54. with abscisic atcid ethylene but the dlifferenice \\as small iuntil 14 lhr. B1ean ex- 2. ABEi.ES, 1. B. 1968. Herbicide-induced of the gas in sublethal doses of also had productioni: Role plants, (Fig. 4B) trealted witlh abscisic acid 16: cottoll explanits., 2,4-D. Weed Science 498-500. greater cellutlase activ itv btit. uinlike 3. ABE-LES, F. B. 1968. Role of RNA and proteill the change in cellulase activitv occurred sooner thanl synithesis in ahscission. Planlt Physiol. 43: 1577- in thle conitrols. The cellulase data obtained fronm 86. bean explanits suiggest that abscisic acid shortened 4. .A sELE.S F. B. 1969. Abscission: Role of cellulase. 1lhe ind(ui1ction fol cellulase because it decreasedI I'lant P'hysiol. 44: 447-52. lperiodl H. 1968. Ab- the time required for the initial appearance of cellu- 5. ABELEs. F. B. AxN E. GAHAGAN. scission TIlhe role of ethlvlelne, ethllle allnalogues, lase activit!. carbon (lioxi(le anid oxygen. Planit Physiol. 43: 1255-58. Conclusions 6. A\B:LES, F. I. ANi) R. h. HoINi. 1967. Abscissioxi Role of protein synthesis. Ann. N. V. Acad. Sci. 144: 367-73. Tlle action of abscisic acid in accelerating explant 7. ALBELES, F. B., R. E. Hol.M, ANI) H. E. (GAIIAGAN. 1967. Abscission: The role of aging. Planit Phv- abscissionimay or may not be applicable to its effects leaves (13, 16, 18). Spray siol. 42: 1351-56. on the abscission of intact 8. AB3ELES. F. B. AND J. LONSKI. 1969. Stimulation applications result in contact of abscisic acid with of lettuce seed and germination by ethylene. Plant the blade as well as the separation layer, and ultimate Physiol. 44: 227-80. control of abscission may rest on modification of 9. ADDICOTT, F. T., H. R. CARNS, J. L. LyoN, 0. E. blade physiology. (The same would also hold true SMNITI, AND J. L. McMEANS. 1964. On the phy- for intact fruits.). Hartmann et al. (18) found that siology of abscisins. In Regulateurs de la Crois- concentrations of abscisic acid that defoliated olive sance vegetale. C. N. R. S. Paris. p 687-703. CRACKER AND ABE3LES-ABSCISSION: ROLE OF ABSCISIC ACID 1149

10. ASPINALL, D., L. G. PALEG, AND F. T. ADDICOTT. i(;. HARTMANxN H. T., A. J. HESLOP, A-ND J. WHISLER. 1967. Abscisin II and some hormone-regulated 1968. Chemical induction of fruit abscission in plant responses. Australian J. Biol. Sci. 20: olives. Calif. Agr. 22: 14-16. 869-82. 19. HEMBERG, T. 1967. Abscisin II as an inhibitor of 11. BURG, S. P. AND E. A. BUTRG. 1967. Molecular a-amylase. Acta Chem. Scand. 21: 1665-66. requirements for the biological activity of ethylene. 20. HOL-M, R. E. ANDF B. ABELES. 1967. Abscis- Plant Physiol. 42: 144-52. sion: The role of RNA synthesis. Plant Physiol. 12. CHRISPEELS. M. J. AND) J. E. VARNER. 1967. Hor- 42: 1094-1102. monal control of enzyme synthesis: On the mode of action of gibberellic acid and abscisin in aleu- 2 1. HOLM, R. E. AND F. B. ABELES. 1968. The role of ethylene in 2,4-D induced growth inhibition. rone layers of barley. Plant Physiol. 42: 1003-16. Downloaded from https://academic.oup.com/plphys/article/44/8/1144/6090212 by guest on 29 September 2021 Planta 78: 293-304. 13. COOPER. WV. C., G. K. RASMUSSEN, B. J. ROGERS, P. C. RK1FCE. AND WV. H. HENRY. 1968. CoIntrol 22. 1KiIAx, A. A. 1968. Inhibition of gibberellic acid- of abscissiol in agricultural crops anid its physio- iniduced germiniation by abscisic acid and reversal logical basis. Plant Physiol. 43: 1560-76. 1)w cvtokinins. Plant Physiol. 43: 1463-65. AND B. ABELES. 14. CRA\KER. TL. E. F. 1969. Abscis- 23. NITSC 1, C. ANI) 1. P. NrTrscH. 1967. The ind(luc- sion. Quantitative with a recording measuremenit tiOln of flowerilig in -vitr)o in steml seg'gilments ot abscissor. Plant Physiol. 44: 1139-43. l'luitibago indic(r It. The productioni of re- 1 5. D)ALE, J. E AND G. F. MILFORD. 1964. The role productive buds. Planta 72: 37 1-84. of endogenous growth substances in the fruiting of upland cottoIn. New Phytologist 64: 28-37. 24. SCOTT, P. C. AND A. C. LEOPOLDI. 1966. A\bscission 41: 16. EL-ANTABLY, H. M. M., P. F. WAREING, AND J. as a miobilization phenomenon. Plant Physiol. HILL-IAN. 1966. Some physiological responses 826-30. to d, I abscisin (dormin). Planta 73: 74-90. 25. WALTON, D. C. AND E. SONDHEIMER. 1963. Ef- 17. EVANS, L. T. 1966. Abscisin II: Inhibitory ef- fect of abscisin II on phenylalanine ammonia-lyase fect on flower induction in a long-day plant. Sci- activity in excised bean axes. Plant Physiol. 43: ence 151: 107-08. 462-69.