Plant Physiol. (1981) 68, 349-3540032-0889/81/68/0349/06/$00.50/0

The Role of Ethylene in the Senescence of Oat Leaves1Received for publication September 16, 1980 and in revised form January 26, 1981

SHIMON GEPSTEIN2 AND KENNETH V. THIMANNDepartment of Biology, Thimann Laboratories, University of California, Santa Cruz, California 95064

ABSTRACT

The evolution of ethylene, both from the endogenous source and fromadded 1-aminocyclopropane-1-carboxylic acid (ACC), has been foUlowed inclose relationship with the senescent loss of chlorophyll from seedling oatleaves. In white light, where chlorophyll loss is slow, the ethylene evolutionincreases slowly at first, but when the loss of chlorophyll becomes morerapid, ethylene evolution accelerates. CoCI2 inhibits this increase andcorrespondingly maintains the chlorophyll content, with an optimum con-centration of 10 micromolar. The rapid rate of chlorophyll loss in the darkis slightly decreased by 3-aminoethoxyvinyl glycine (AVG), by cobalt, andslightly stimulated by ACC. The slower chlorophyll loss in white light,however, is almost completely inhibited by silver ions, greatly decreased bycobalt and by AVG, and strongly increased by ACC. Since the chlorophyllloss is accompanied by proteolysis, it represents true senescence. Chloro-phyll loss in light is also strongly antagonized by C02, 1% CO2 givingalmost 50% chlorophyll maintenance in controls, while in the presence ofadded ACC or ethylene gas, the chlorophyll loss is 50% reversed by about3% CO2. The ethylene system in leaves is thus more sensitive to CO2 thanthat in fruits. Indoleacetic acid also clearly decreases the effect of ACC. Itis shown that kinetin, C02, Ag+, and indoleacetic acid, all of which opposethe effect of ethylene, nevertheless increase the evolution of ethylene bythe leaves, and it is suggested that ethylene evolution may, in manyinstances, mean that its hormonal metabolism is being prevented.

Abscisic acid somewhat increases ethylene evolution also, but its actionin promoting senescence in light is antagonized only partially by Ag+,C02+, or AVG. For this and a number of other reasons it is concluded thatethylene and abscisic acid both independently control leaf senescence inthe light.

The study of leaf senescence continually brings to light moreand more interactions. The earliest researches tended to concen-trate on the nucleic acids (1 1, 24, 43, 44), then proteolysis came tothe fore, together with the balance between proteolysis and proteinsynthesis, and the specific roles of individual amino acids (29 andsee 35). Comparison between the behavior of isolated leaves andthose attached to the plant then brought out the important partplayed by transport phenomena (22, 32, 38). The continuation ofstudies of the difference between senescence in darkness and inlight has recently introduced an apparently controlling functionfor stomatal aperture (21, 36, 37), a finding which helps to explainthe apparently time-limited and localized effect oflight in delayingsenescence (39). Since AbA accumulation is well known to causestomatal closure (e.g. 18), it was natural to look for a function ofthis hormone in senescence, and indeed it was reported early toincrease leaf senescence (6, 12, 27). Our own experiments (14)

' Supported in part by grant PCM 7683126 from the National ScienceFoundation, to K. V. Thimann.

2 Permanent address: Dept. of Biology, The Technion, Haifa, Israel.

have shown not only that applied AbA greatly accelerates senes-cence, but also that there is a large increase of AbA in the dark,especially at the time when senescence is most rapid. Whensenescence is greatly delayed by externally applied kinetin, or bywhite light, the amount of AbA in the leaves becomes barelydetectable. The AbA content also increases sharply in leavesattached to the plant when they begin to undergo normal senes-cence (14).Thus the data obtained would appear to put AbA in, or close

to, the position of controlling or initiating agent. However, thereis good reason to doubt that it can be the sole such agent. Onesuch reason is the early report that in the curing of tobacco leaves,ethylene hastens the yellowing stage (33), and causes increases ofrespiration and of ATP, along with the loss of Chl-the typicalsenescence syndrome (31). Another is the well-known role ofethylene in the maturing of fruit, a process which, in threerespects-namely loss of Chl, hydrolysis of polymers, and climac-teric rise of respiration-obviously has much in common with thesenescence of leaves (34). Against these considerations is ourearlier observation that Ethrel (2-chlorethyl-phosphonic acid),which is readily converted to ethylene in plant tissue, exerts verylittle effect in hastening the senescence of oat leaves in darkness(37). (Comment on this negative finding will be made below.) Therole of ethylene in oat leaf senescence therefore needed clarifica-tion.The work of Aharoni and Lieberman (3, 4) has made an

involvement of ethylene in the senescence of tobacco leaf discsvirtually certain, and their data suggest a similar situation forleaves of sugar beet and pinto beans (5). Still more recentlyMorgan and Durham (23) have shown ethylene to control thesenescence of leaves of Melia. The recent identification of ACC3as the natural precursor of ethylene (2, 40), and the availabilityboth of this compound and of the inhibitor of ethylene biosyn-thesis, AVG, facilitated the present researches. The delay of leafsenescence by white light (39) has proved to be a key observationin this connection, for white light inhibits the evolution ofethyleneboth from oat seedling leaves and from mature tobacco leaf discs,by from 60 to 90%o (15). The extent of the inhibition exerted onthe endogenously produced ethylene is about the same as thatexerted on the much larger volume produced when ACC issupplied. This and other comparisons between light and dark havebeen valuable leads in the present analysis. As a result, it will beshown that ethylene indeed exerts a dominant role in the causationof senescence in oat leaves.

MATERIALS AND METHODS

Seeds of Avena sativa L., cv. Victory were sown in vermiculitemoistened with tap water and grown under continuous white light(200 ft-c at plant level) for 7 days at 25 C. Groups of eight of theapical 3-cm segments of the first leaves were floated on 10 ml of

3Abbreviations: ACC, I-aminocyclopropane-l-carboxylic acid; AVG,3-aminoethoxyvinyl glycine, or L-2-amino-4-(aminoethoxy)trans-3-buta-noic acid.

349 www.plantphysiol.orgon January 12, 2020 - Published by Downloaded from

Copyright © 1981 American Society of Plant Biologists. All rights reserved.

GEPSTEIN AND THIMANN

water or test solution in Petri dishes either in light (same as above)or in darkness at 25 C.The Chl and free a-amino nitrogen of each group were deter-

mined by extraction in boiling 80% alcohol and spectrophotometryas described earlier (34).For ethylene determination, a group of leaf segments was

introduced into 5-ml vials containing 0.4 ml water or solution andsealed with rubber serum caps. To avoid possible effects ofchangesin gas composition (C02, 02, C2H4) on leaf senescence in suchlong-term experiments (up to 9 days) the leaf segments wereallowed to accumulate ethylene for not more than 5 h. Duringthis period, the vials were held in darkness because of the inhibit-ing effect of light (15). Also, to ensure that the ethylene evolvedwas a function of the specific treatment and not simply a result ofthe ensuing senescence, the ethylene was determined after arelatively short time on the test solution, usually 48 h. An exceptionto this procedure is in the time course experiment of Figure 1. Atthat time 1 ml of air, withdrawn from the vial with a syringe, wasintroduced into the gas chromatograph (Perkin Elmer Sigma III,with flame ionization detector, using a 150 x 0.3 cm column ofPoropak R). The temperature of the column was 60 C throughoutand the flow rate 40 ml/min.ACC was obtained from Calbiochem, AVG from the Maag

Co., Poropak R from Waters and Associates, Milford, MA, andthe abrasive for scrubbing was aluminum oxide Borlon FFF fromSimonds Abrasive Co., Philadelphia.For the experiments of Figure 5 the zero CO2 concentration was

obtained by absorption in 10% KOH, and the higher concentra-tions by injection into the flasks. In the figures and tables, repre-sentative experiments are presented but the variation betweenthree or more experiments was not over 15%.

RESULTSThe Parallel Between Ethylene Production and Senescence. As

described earlier (15), the evolution of ethylene by seedling oatleaves, although smaller per unit weight than that from maturetobacco leaf discs, can be measured satisfactorily by suitablylimiting the gas volume (see under "Materials and Methods").Figure 1 presents the time course of endogenous ethylene evolu-tion, and the corresponding time course of senescence in whitelight, as measured by Chl loss. Not only is the general parallelismhighly suggestive, but the period of most rapid senescence, i.e.from the 5th to the 7th day, coincides with the increased rate ofethylene evolution on just those 2 days. An increased rate ofethylene production with increasing senescence has been reportedmany times, notably by Jackson and Osborne (16), Beyer andMorgan (8), and, recently again, by Morgan and Durham (23).From these correlations, it was logical to observe the effect of

increasing ethylene production by treating the leaves with ACC,the natural ethylene precursor. Table I summarizes the changes inChl content, on the days of most rapid senescence, in the presenceofadded ACC. In the dark, the ACC does cause a modest decreasein Chl, but the small magnitude of the effect suggests that factorsother than ethylene are dominant. In the light, however, wheresenescence is slower, the effect of ACC is clear, the time of rapidloss of Chl having been brought forward by 1 full day. Thus, ACCdoes accelerate Chl loss.The Effect of Inhibiting Ethylene Formation or Ethylene Action.

If increased ethylene production hastens senescence, then de-creased ethylene production should delay it. Figure 2 shows suchan experiment with CoCl2, which Yu and Yang (41) found to bea powerful inhibitor of ethylene evolution. At 10 ,UM, the optimumconcentration, the ethylene evolved is inhibited by nearly 90%o,and the Chl content after 7 days is correspondingly tripled. Themoderate decrease in ethylene production at higher Co2" concen-tration may be due to a secondary effect.More complete studies with the three best-known inhibitors,

0CD

=(0

-c0..0

-

-E

LwJ

5

4

3

2

0 1 2 3 4 5 6 7 8 9

DaysFIG. l. Time course of the changes in Chl (above) and ethylene evo-

lution (below) during senescence of oat leaves on water in light.

Table I. Chl Contents of Oat Leaves Senescing in Darkness and in Light,as Influenced by the Presence ofACC

Chl expressed as Ae,o or as percentage of initial value (0.305).

Control Plus 0.2 mm ACC

A6w1 % A6wz %In darkness

After 3 days 0.080 27 0.043 14After 4 days 0.024 7.9 0.017 5.6

In lightAfter 6 days 0.140 46 0.045 15After 7 days 0.057 17 0.018 6.0

0020

.-ct 0.Io

0

0+2 -Co Concentration,/LM

FIG. 2. The effect of serial concentrations of applied CoCl2 on thesenescence of oat leaves after 7 days in light. ( ), Chl content; (---),rate of ethylene evolution.

350 Plant Physiol. Vol. 68, 1981

www.plantphysiol.orgon January 12, 2020 - Published by Downloaded from Copyright © 1981 American Society of Plant Biologists. All rights reserved.

ETHYLENE AND SENESCENCE OF OAT LEAVES

0 1 2 3 4Days

FIG. 3. Time course of the effects of ACC, and of three inhibitors ofethylene formation and action, on the Chl content ofoat leaves in darkness.C, control in water; ACC, I mM; AVG, 0.2 ms; CoCl2, 10 tsm; AgNO3, 60Mm applied for 30 min.

0.39-

0CD(0

_ 0.2

0.r00

(L) 0.1I

0 L0 " 4 5 6 7

DaysFIG. 4. Time course of the effects of the same reagents as in Figure 3

but in light. LC, light control.

cobalt, silver, and AVG are shown in Figures 3 and 4. In Figure3, senescence in darkness shows only a slight retardation, as inTable I. Figure 4 shows, however, that in white light, where thesenescence rate is slower, all three reagents exert powerful effects.Silver nitrate, 10 mg/l, was applied only for 30 min on account ofits cumulative toxicity. The Co2+ was used at 10 Mm as in Figure2. That the inhibition by the most specific reagent, AVG, is onlypartial here remains unexplained; it seems unlikely that it couldbe due to slow entry of the reagent into the ethylene-producingcells, in view of the ready entry of spermidine, a molecule ofcomparable size and polarity (13).

It has been observed in this laboratory (28) that the very slowloss of Chl in light can be accelerated up to the rate in darknessby withholding CO2. Table II shows that this increased rate ofChl loss in C02-free air is prevented by the three reagents usedabove. But, more strikingly, study of a series of concentrations ofCO2 (Fig. 5) shows that even as little as 1% CO2 prevents most ofthe Chl loss resulting from 3 days in light. Apparently, high CO2concentrations such as 10% are needed only when ethylene isacting on certain fruits, and with leaves a partial preservation ofthe Chl is given by 1% CO2 even when an added ethylene sourceis present (Fig. 5). These data make clear that at least a good part

Table II. The Effect ofEthylene Inhibitors on the Chl Content of OatLeaves Senescing in Light in the Absence of CO2

Chl as As6o or as percentage of initial value (0.320). Values after 4 daysin light.

Treatment ChlorophyllA6w0 % Initial

Light control 0.085 27AVG, 0.2 mM 0.170 54Co2+, 1O AM 0.190 60Ag+, 10 mg/l for 30 min 0.200 63Light control in aire (0.03% C02) 80

a From other experiments.

0.3

0to(0

- 0.2

cL.0h.r0 01

00 l 15

% CO2FIG. 5. Chl loss from oat leaves after 3 days in light as affected by CO2

concentration. C, control; C2H4, ethylene gas 100,ulll. Other concentrationsas in Figures 3 and 4. The CO2 concentrations, including the zero value,were obtained as shown under "Materials and Methods."

of the normal senescence of oat leaves is controlled by ethylene.The Interaction of Ethylene with Other Hormones. It is 20 years

since Osborn and Halloway (25, 26) reported that 2,4-D retardedthe loss of Chl and protein in leaves of Prunus and Euonymus thatwere undergoing autumnal senescence. The greater potency ofkinetin in this respect seems to have caused a shift of interest awayfrom the effect ofauxins. However, numerous workers have shownthat IAA can greatly increase ethylene evolution, both in leavesand in the hypocotyls and epicotyls of seedlings (e.g. 10, 19). Suchan increase would be expected to promote senescence. On thecontrary, Table III shows that IAA, at the very concentrationsthat increase the evolution of ethylene, clearly retards Chl loss. Inthese experiments, the ethylene was allowed to accumulate onlyfor the first 2 days because of the risk that its increasing concen-tration could stimulate further ethylene production autocatalyti-cally. In other experiments, the increased Chl loss caused by ACCcould also be prevented, in part, by IAA.A comparable experiment with kinetin is presented in Table IV.

In the dark, where Chl loss was prevented completely at bothkinetin concentrations, ethylene production is scarcely changed,but in the light, where Chl is lost more slowly, kinetin causes anincrease of over 700o in ethylene evolution. Thus, although theaction of ethylene on senescence is clear, the evolution of ethylenehas a less simple relationship to senescence. In Table IV, IAA isseen to promote as in Table III, but only in the dark, at least inthe 2 days used.The data raise the question of whether the effect of AbA could

be indirect, by way of ethylene. Our present experiments give only

Ag4

Co2coV2

AVG

L.C.

ACC0

Plant Physiol. Vol. 68, 1981 351

-IL-

www.plantphysiol.orgon January 12, 2020 - Published by Downloaded from Copyright © 1981 American Society of Plant Biologists. All rights reserved.

GEPSTEIN AND THIMANN

Table III. Chl Contents and Ethylene Evolution of Oat Leaves Senescing in Darkness or in Light as Influencedby IAA

After 3 Days in the Dark After 5 Days in Light

Treatment Ethylene' Ethylene'Chlorophyll on 2nd day Chlorophyll on 2nd day

A6w0 % initial nl/g.h Aco % initial nl/g.hControl 0.095 30 2.00 0.260 81 0.97IAA, 1 ZtM 0.135 42 2.00 0.300 93 0.97IAA, 10 M 0.168 53 3.50 0.320 100 1.10IAA, 100 uM (0.1 mM) 0.220 70 6.0 0.315 99 6.0

Ethylene measured for 5 h.

Table IV. Chl Contents and Ethylene Evolution of Oat Leaves Senescing in Darkness or in Light as Influencedby Kinetin

After 3 Days in Dark After 5 Days in Light

Treatment Ethylene' EthyleneaChlorophyll on 2nd day Chlorophyll on 2nd day

A6w % initial nl/g.h A6wo % initial nl/g.hControl 0.095 29 2.0 0.260 74 0.97Kinetin, 3 mg/l 0.350 100 2.0 0.362 103 4.0Kinetin, 30 mg/l 0.321 91 2.5 0.310 88 7.5Kinetin, 3 mg/l + IAA, 10 uM 0.355 100 5.5 0.363 103 4.0

a Ethylene measured for 5 h.

Table V. Chl Contents of Oat Leaves after 4 Days in Light in Presence ofAbA and Ethylene Inhibitors

Leaves scrubbed. AgNO3 supplied for 30 min only. AVG was 0.2 mM.

Treatment Chlorophyll

A6w6 % initialLight Control 0.25 71AbA, I mg/l 0.12 34AbA, I mg/l + Ag+ 0.23 66AbA, I mg/l + AVG 0.20 57AbA, 2 mg/l 0.07 20AbA, 2 mg/l + Ag+ 0.17 49AbA, 2 mg/l + AVG 0.16 46

a partial answer to this question. Table V shows that the Chl losscaused by AbA at 1 mg/l in light is largely offset by AVG andalmost completely by silver ions. At 2 mg/I, the effects of AVGand Ag+ are the same in absolute amounts but less in proportion.Neither reagent brings the Chl content up to that of the lightcontrols. Thus, part of the effect of AbA, but clearly not all,evidently rests on its increasing ethylene production. (It should benoted that the AbA concentrations used in this experiment aremuch lower than those used previously, because of gentle scrub-bing of the leaves with abrasive; this lowers the effective concen-tration by about a power of 10.) Such a moderate increase inethylene production could be directly demonstrated. Leaves onthe 1st day in light evolved 0.75 nl/g-h ethylene while in AbA 30mg/l (unscrubbed) the value was 1.6. On the 2nd day, the lightcontrols evolved 0.97 nl/g.h and in AbA 2.50. Since this increasewas clear in only 1 day it is not likely that it was the result ofincreasing senescence.

In all the above, both the data and the text have concerned onlythe loss of Chl. To be sure that true senescence, and not merelybleaching, is involved, the free amino nitrogen must be seen toincrease and the increase to be prevented when Chl loss is pre-vented. An example of such data is shown in Table VI. The AbAhas almost doubled the amino nitrogen while the Co2' and Ag+

Table VI. Changes in Free Amino Acids and Chl after 4 Days in Lightwith AbA and Selected Inhibitors

CoCl2 was 10 UM and AgNO3 was 60 ,iM for 30 min.

Treatment Chlorophyll Free Amino Nitro-

A660 A570Initial Values 0.35 0.25Light control 0.36 0.35AbA, 30 mg/l 0.16 0.67AbA, + Co2+ 0.23 0.34AbA + Ag+ 0.20 0.40Co2+ alone 0.36 0.20Ag+ alone 0.30 0.19Ethephon 0.27 0.40

have held the value down to about the initial. As in Table V, theethylene inhibitors clearly antagonize the AbA effect in part.

DISCUSSION

Where ethylene measurements are concerned, the influence ofwounding is always a problem. The long and narrow oat leavespresent a minimum of wounded tissue, but even so some reagentsthat promote or inhibit senescence can be seen to enter preferen-tially at the cut surfaces. To promote entry of reagents, a lightscrubbing with fine abrasive has often been practiced, and ishighly effective, the external concentration of AbA necessary fora given effect on senescence being reduced from 30 iLg/g withintact leaves to about I ,ug/g after scrubbing. But the output ofwound ethylene is clearly increased, and we have also found thatthe respiratory rate is greatly increased. The effects of woundingand scrubbing must therefore be carefully watched, and in com-paring our data with those of other workers using other materialsand procedures, such differences in treatment must be taken intoaccount. The use of scrubbing in experiments of quite differentkinds, e.g. on auxin action, may need to be specifically controlled.The ready reversal of the action of ACC (i.e. of ethylene) by

CO2 provides firm support for the initiatory role of ethylene in

352 Plant Physiol. Vol. 68,1981

www.plantphysiol.orgon January 12, 2020 - Published by Downloaded from Copyright © 1981 American Society of Plant Biologists. All rights reserved.

ETHYLENE AND SENESCENCE OF OAT LEAVES

senescence. The data on oat leaves agree closely with earlierreports on materials other than fruits. Thus, Burg and Burg (10)found that 1.5% CO2 gave about 50% reversal of the ethyleneeffect on pea stems, and Sisler (30) estimated that about the samelevel of CO2 would give 50%o reversal of the ethylene effect ontobacco leaf segments. It is evident that the ethylene system inleaves is more sensitive to CO2 than that in typical fruits.The conclusion from the present experiments that ethylene

greatly promotes senescence in light, and slightly also promotes itin darkness, contrasts with our previous conclusion that ethyleneexerts little or no effect. However, those experiments using Ethrelwere limited to senescence in darkness, and Figure 3 shows thathere the effect, though real, is in fact very small. But whether thiscan explain the observation by Brady et al. (9) that ethylene doesnot hasten the senescence of seedling wheat leaves is not clear,especially as AbA did promote senescence of these leaves, as itdoes in oats (14).An important question is whether the effects of AbA and

ethylene are interrelated. In an earlier paper (14) it has beenshown that: (a) AbA strongly accelerates senescence, especially inthe light; (b) AbA accumulates in oat leaves in the dark; (c) inwhite light, or with kinetin in the dark, this accumulation is almostwholly prevented; (d) both in detached leaves floating on waterand in leaves naturally senescing on the plant, the AbA contentincreases sharply just at the time of Chl loss. AbA is thus certainlyone causative factor in senescence.

Nevertheless, if AbA could be shown to promote senescencethrough increasing the ethylene production this would be anattractive simplification, since it does increase ethylene evolution,in days 1 and 2 in light, to somewhat more than double. But it isnegated by the following five facts: (a) the data of Table V showonly a partial antagonism by AVG or Ag+; a similar very partialeffect is seen in Table VI; (b) leaves in AbA begin to lose Chlearly in day 2, while ACC does not increase Chl loss until day 3or 4; (c) AbA can show its effect even in presence of a saturatinglevel of ACC. Thus, in our experiments, 2 mm ACC, a saturatinglevel, increased Chl loss after 4 days in light by 25%, but withAbA 30 mg/l added the Chl loss was 67%; (d) under conditionswhere ethylene exerts no obvious effect on Chl loss or proteinbreakdown in wheat leaves (9), AbA is reported to have had aclear-cut effect. (e) at least with IAA and kinetin, as noted below,there is no obvious relationship, or sometimes even an inverseone, between ethylene evolution and effect on senescence. Thelarge inhibitions in Figure 4 seem to show that in light, ethyleneis in major control. But in general, ethylene production is sosensitive to modest stresses-wounding (1, 42), scrubbing (above),water-logging (3, 20), etc., that it seems safest to ascribe therelatively small increase in ethylene caused by AbA to anotherkind of stress, namely "AbA-induced stress."The action of kinetin raises a question in the opposite sense,

namely as to whether its prevention of senescence could be due toits inhibiting ethylene production. But this possibility is clearlydisproved in Table IV, for in the dark, where kinetin inhibitssenescence maximally, the production of ethylene is unaffected.In the light, kinetin actually increases ethylene production yet stillsomewhat inhibits senescence. Thus, the action of kinetin onsenescence is either independent of, or even inversely related to,its effect on ethylene evolution.The role of IAA is even stranger, for at just the concentrations

at which it delays Chl loss, Table III shows that it greatly increasesethylene evolution. Figure 2 of reference 4 shows striking syner-gism of IAA with kinetin in ethylene production in the dark. Yet,in other plant reactions, it functions in the opposite sense, for itantagonizes the cytokinins in apical dominance, and antagonizesethylene in abscission. In the specific context of senescence, itseems to be almost a general rule that reagents that retard orinhibit senescence actually promote the evolution of ethylene; for

example, CO2 or AgNO3 increase ethylene evolution by a factorof 3 (3), which is confirmed in our experiments. The ethylenereleased by ACC in our experiments was increased by a factor of6 in presence of 10%o C02. Very much larger increases are causedby synergisms between three or more such agents (3). It is sug-gested that if ethylene is prevented from undergoing whatevermetabolic reaction is needed for it to promote senescence, it isevolved instead. Similar possibilities, based on a feedback mech-anism controlling ethylene production, have been discussed byAharoni et al. (3) and by Beyer (7). However, the limited corre-lation between ['4C]ethylene and the evolved "CO2 in the latterexperiments may well be explained by formation of ethylene oxide(17). The same apparent opposition between the hormonal actionof ethylene and its evolution appears not to hold in fruits, forthere generally it both causes senescence and is simultaneouslyevolved. Whether AbA independently promotes senescence infruits, as in leaves, remains to be determined.

LITERATURE CITED

1. ABELES FB 1973 Stress ethylene. In Ethylene in Plant Biology. Academic Press,New York/London, pp 87-102

2. ADAMS DO, SF YANG 1979 Ethylene biosynthesis: identification of I-aminocy-clopropane-l-carboxylic acid as an intermediate in the conversion of methio-nine to ethylene. Proc Natl Acad Sci USA 76: 170-174

3. AHARONI N, JD ANDERSON, M LIEBERMAN 1979 Production and action ofethylene in senescing leaf discs. Effects of IAA, kinetin, silver ion and CO2.Plant Physiol 64: 805-809

4. AHARONI N, M LIEBERMAN 1979 Ethylene as a regulator of senescence in tobaccoleaf discs. Plant Physiol 64: 801-804

5. AHARONI N, M LIEBERMAN, HD SISLER 1979 Patterns of ethylene production insenescing leaves. Plant Physiol 64: 796-800

6. ASPINALL D, LG PALEY, FT ADDICOTT1r%7 Abscisin II and some hormone-regulated plant responses. Austr J Biol Sci 20: 869-882

7. BEYER E 1979 "C2H4 metabolism in cotton during leaf abscission. Plant Physiol64: 97 1-974

8. BEYER EM JR, PW MORGAN 1971 Abscission: the role of ethylene modificationof auxin transport. Plant Physiol 48: 208-212

9. BRADY CJ, NS ScoTT, R MUNNS 1974 The interaction of water stress with thesenescence pattern of leaves. In RL Bieleski, AR Ferguson MM Cresswell. eds,Mechanisms of Regulation of Plant Growth, Bulletin 12 of the Royal Societyof New Zealand, pp 403-409

10. BURG SP, EA BURG 1966 The interaction between auxin and ethylene and itsrole in plant growth. Proc Natl Acad Sci USA 55: 262-269

11. DYER TA, DJ OSBORNE 1971 Leaf nucleic acids. II. Metabolism during senescenceand effect of kinetin. J Exp Bot 22: 552-560

12. EL-ANTABLY HMM, PF WAREING, J HILLMAN 1967 Some physiological re-sponses to DL-abscisin (Dormin). Planta 73: 74-90

13. GALSTON AW, A ALTMAN, R KAUR-SAWHNEY 1978 Polyamines, ribonucleaseand the improvement of oat leaf protoplasts. Plant Sci Lett 11: 69-79

14. GEPSTEIN S, KV THIMANN 1980 Changes in the abscisic acid content of oat leavesduring senescence. Proc Natl Acad Sci USA 77: 2050-2053

15. GEPSTEIN S, KV THIMANN 1980 The effect of light on the production of ethylenefrom I-aminocyclopropane-l-carboxylic acid by leaves. Planta 149: 196-199

16. JACKSON MB, DJ OSBORNE 1972 Abscisic acid, auxin and ethylene in explantabscission. J Exp Bot 23: 849-862

17. JERIE PH, MA HALL 1978 The identification of ethylene oxide as a majormetabolite of ethylene. Proc R Soc Lond B 200: 87-94

18. JONES RJ, TA MANSFIELD 1970 Suppression of stomatal opening in leaves treatedwith abscisic acid. J Exp Bot 21: 714-719

19. KANG BG, W NEWCOMB, SP BURG 1971 Mechanism of auxin-induced ethyleneproduction. Plant Physiol 47: 504-509

20. KAWASE M 1971 Causes of centrifugal root promotion. Physiol Plant 25: 64-7021. KURAISHI S, F ISHIKAWA 1977 Relationship between transportation and amino

acid accumulation in Brassica leaf discs treated with cytokinins and fusicoccin.Plant Cell Physiol 18: 1273-1279

22. LEWINGTON RJ, M TALBOT, EW SIMON 1967 The yellowing of attached anddetached cucumber cotyledons. J Exp Bot 18: 526-534

23. MORGAN PW, JI DURHAM 1980 Ethylene production and leaflet abscission inMelia azedarach L. Plant Physiol 66: 88-92

24. OSBORNE DJ 1967 Hormonal regulation of leaf senescence. Symp Soc Exp Biol21: 179-213

25. OSBORNE DJ, M HALLAWAY 1960 Auxin control of protein levels in detachedautumn leaves. Nature 188: 240-241

26. OSBORNE DJ, M HALLAWAY 1964 The auxin 2,4-D as a regulator of proteinsynthesis and senescence in detached leaves of Prunus. New Phytol 63: 334-346

27. SANKHLA N, D SANKHLA 1968 Abscisin-II-kinetin interaction of leaf senescence.Experientia 24: 294-295

28. SATLER SO, KV THIMANN 1980 Treatments that delay loss of chlorophyll in

Plant Physiol. Vol. 68, 1981 353

www.plantphysiol.orgon January 12, 2020 - Published by Downloaded from Copyright © 1981 American Society of Plant Biologists. All rights reserved.

354 GEPSTEIN A

darkness accelerate it in light. Plant Physiol 65: S-6229. SHIBAOKA H, KV THLmNN 1970 Antagonisms between kinetin and amino acids:

Experiments on the mode of action of cytokinins. Plant Physiol 46: 212-22030. SISLER EC 1980 Role of carbon dioxide in reversing ethylene action in tobacco

leaves. Tob Sci 24: 53-5531. SISLER EC, A PIAN 1973 Effects of ethylene and cyclic olefins on tobacco leaves.

Tob Sci 17: 68-7232. SPENCER PW, JS TITUs 1973 Apple leaf senescence: leaf disc compared to

attached leaf. Plant Physiol 51: 89-9233. STEFFENS GL, JG ALPHIN, ZT FoRD 1970 Ripening tobacco with the ethylene-

releasing agent 2-chloroethylphosphonic acid. Beitr Tabakforsch 5: 262-26534. TETLEY RM, KV THIMANN 1974 The metabolism ofoat leaves during senescence.

I. Respiration, carbohydrate metabolism and the action of cytokinins. PlantPhysiol 54: 294-303

35. THIMANN KV 1978 Senescence. Bot Mag Tokyo (Special Issue No 1): 17-4336. THIMANN KV, SO SATLER 1979 Relation between leaf senescence and stomatal

closure: Senescence in light. Proc Natl Acad Sci USA 76: 2295-2298

.NID THIMANN Plant Physiol. Vol. 68, 1981

37. THIMANN KV, SO SATLER 1979 Relation between senescence and stomatalopening: Senescence in darkness. Proc Natl Acad Sci USA 76: 2770-2773

38. THIMANN KV, RM TETLEY, TRAN VAN THANH 1974 The metabolism of oatleaves during senescence. II. Senescence in leaves attached to the plant. PlantPhysiol 54: 858-862

39. THIMANN KV, RM TETLEY, BM KRIVAK 1977 Metabolism of oat leaves duringsenescence. V. Senescence in light. Plant Physiol 59: 448-454

40. HOSHII H, A WATANABE, H IMASEKI 1980 Biosynthesis ofauxin-induced ethylenein mung bean hypocotyls. Plant Cell Physiol 21: 279-291

41. Yu Y-B, SF YANG 1979 Auxin-induced ethylene production and its inhibitionby aminoethoxyvinylglycine and cobalt ion. Plant Physiol 64: 1074-1077

42. Yu Y-B, SF YANG 1980 Biosynthesis of wound ethylene. Plant Physiol 66: 281-825

43. WOLLGIEHN R 1967 Nucleic acid and protein metabolism of excised leaves.Symp Soc Exp Biol 21: 231-266

44. WOOLHOUSE JW 1967 The nature of senescence in plants. Symp Soc Exp Biol21: 179-213

_s_ -

www.plantphysiol.orgon January 12, 2020 - Published by Downloaded from Copyright © 1981 American Society of Plant Biologists. All rights reserved.