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Assessing the influence of exogenous ethylene on electron transport and fluorescence quenching in leaves of Glycine max
I:,.D,,,..<.I.I a.d l,'~p,~b~<,¢loi/~.[a#{i. Prin/cll hi (heal lllil tin
\~d ~2. No. I. l'P 14{t 455. t!i!17
11119g }1177 92 ~5.111) ¢ I) I1(I t 1992 PUle{till
ASSESSING THE I N F L U E N C E OF E X O G E N O U S ETHYLENE ON ELECTRON TRANSPORT AND FLUORESCENCE QUENCHING IN LEAVES OF GLYCINE M A X S. D. WULLSCHLEGER, P. J. H A N S O N and C. A. G U N D E R S O N
Environmental Sciences I)ivision, Oak Ridge National l,al)oratory, P.O. Box 2008, Oak Ridge, TN 37831-6034, U.S.A.
(Received 13 Februarr 19{)2; acc@l~'di, revMed,/Drm 19 Mql, 1992) WU1.LSCHLEGER S. O., HANSON P. ,]. and GUNDERSON (',. A. A.~ses~ing lhe influence of exo,~enou.~ eltz~lene on electrm~ lramport and.fluorescence quenching i, [eave~ q/ Glycine max. ENVIRONMENTAL AND I';XPERIMENTAI~BOTANY32, 449 455, 1992. We conducted a series of modeling exercises designed to re-evaluate the light-r('sp,msc and C().,-resp(mse curves of TAYLOR and (;tSNOERSON IP[. I'hv~iol. 86, 85 92, 1988) and to examine further their conclusion flint ethylene-induced inhil~ilion of electron transport max contribute to reduced C()., assimilation in leaves of (;lyci,e max. By partitioning the response of (I()., assinlilation to either electron u'ansl)orl-limiled (w Rubisco-limited rates ofcarboxylati(m, we calculated thai the electron transport capacity (.7,,,,,) ofcdiylenc-treated leaves decreased by over 30'% lbllowing a 4-hr exposure to 10 Idil cthvlenc {tlld lloted that ethylene-induced reduclillllS ill ('.O, assimilation could I)e explained without a d e c r e a s e i l l Rubisco activity (I),,,..). ~ l e a s u r e l n e l l t s C,t ill PIT,0 ( : h i t t u o r e s c e n c c stlpported these observations alld indicated that tile ef~ciencv I)v which excitation ellCl'~y wits captured in PSI I
(i.e. (F,,,- b],}il:,,,)
was redtlced
l}om 0.~0 to
0.73 after
a 4-]lr exposure
Io
10 ldll ethylene. This
reduction was also accompanied I)t a 12°. decrease in steady-state photochemical quenchino (qi,). indit'atillg that a lower proportion of open or oxidized PSI I reaction (el/tcrs were particil)atino" in lighi-dependenl processes, l"Ali'cts o1" eihyh'nc Oll Chl tluores('encc were amplified at increased irradiant'e> Suggesi, iIl~ that l)holoinhil)ilion IIl~./y phiy a role in the elilvlene-induccd inhil)ition of (10, assimilation.
An i m p o r t a n t question r e g a r d i n g the photosynthetic response of plants to ethylene is whether |;()LIAR gas-exchange in some species is highly ethylene-induced changes in carbon assimilation sensitive to short-term applications of exogenous occur solely through p e r t u r b a t i o n s to the stomaethylene, with rates of CO2 assimilation decreastal phase of the CO._, exchange process, or whether ing 16,51 o tbllowing a 2- to 4-hr exposure. !~"10.,'_,.~0.<,1, these reductions might equally reflect s(nne (~UNDERSON and TAYLOR ~0, examined this inhialtered biochemical or photochemical probition of gas-exchange in Glycine ma~ and reported cesses. Early studies suggested that CO~ assimilthat an ethylene concentration of 0.35 #xl/1applied ation declined d u r i n g ethylene exposure because to whole plants fi-)r 4 hr was sufficient to elicit of decreasing stomatal conductance/1'I~' Howa h a l f m a x i m a l reduction in leaf photosynthesis. ever, while c h a n g i n g stoinatal c o n d u c t a n c e can However, despite the a p p a r e n t sensitivity of CO~ account tbr much of the ethylene response in some assimilation to ethylene, the physiological mechspecies, ~° other mechanisms c o n t r i b u t i n g to the anisms c o n t r i b u t i n g to this response r e m a i n uninhibition of foliar gas-exchange have been sugresolved. gested. TAYLOR and GUNDERSON 21 concluded 449 INTRODUCTION
450
S.D. WULLSCHLEGER el al.
that although ethylene-induced reductions in stomatal conductance were often greater than those of photosynthesis in Glycine max, the major effect of ethylene on CO~ assimilation was mediated not through stomatal closure, but rather through an unspecified effect of" ethylene on light-dependent processes as evidenced in their study by a 51~I{~ reduction in quantum-use efficiency. CHOE and WHANG 4 earlier observed that electron transport in isolated chloroplasts of Avena saliva was rapidly inhibited by ethylene and, hence, it follows that ethylene-induced reductions in electron transport may also contribute to decreased CO2 assimilation in intact plants, although such studies are lacking. Recent advances in understanding the biochemical regulation of CO~ assimilation '7'~> and in our ability to model these processes ~:') have greatly expanded the opportunity to address the mechanisms by which stressors affect plant gasexchange. Theretbre, we conducted a series of modeling exercises designed to re-evaluate the conclusions of TAYLOR and GUNDERSON21 concerning the ett~ct of ethylene on decreasing CO~ assimilation. Furthermore, we measured in vivo Chl fluorescence using the saturated-pulse method to document directly the potential impact of ethylene on electron transport capacity in leaves of Glycine max. MATERIALS AND M E T H O D S
Plant malerial and elhylene exposure Plants of Glycine max (L.) Merr, cv. Davis were grown from seed under greenhouse conditions in 4-.1 pots filled with Promix BX (Premier Brands, Inc.). Seedlings were watered daily and fertilized weekly with a liquid t~rtilizer (Peters Fertilizer, 20 20-20; W. R. Grace Co.). Studies were conducted 3-4 weeks after germination, when the second trifoliate was fully expanded. Seedlings were moved to the exposure system 2 hr befbre initiation of ethylene treatments. Ethylene exposures were conducted in a controlled exposure system as described previously. :2z: Air entering the chamber passed through charcoal and particle filters, and the humidity was increased with a cool-mist impeller humidifier. Radiant energy (~400-1000 /.tmol m ~ sec z PPFD) was provided by a 1000-W
high intensity discharge multivapor lamp and the intensity changed by raising or lowering the lamp above the exposure chambers. Ethylene was injected continuously into the chamber's inlet from a gas cylinder (1.5~!/~)v/v C~H2 in N2, Matheson Gas Products) and the concentration monitored with a flame ionization detector (Model 400 Hydrocarbon Analyzer, Beckman Instruments). Ethylene was maintained at 10/.tl/1 throughout a 4-hr exposure period.
Model estimales oJ'eleclron Iransporl tale Electron transport rates for control and ethylene-treated leaves were derived from the data of TAYLOR and GUNDERSON(21) by two methods. First, they were estimated using selected gasexchange properties (Table l) and the following equation t~om VON CAEMMERER and FAR QUHAR~ 231
3', = 4 x C-72F~*- x ( A - R d ) C-F,
(1)
where J, is the rate of electron transport, C is the CO 2 partial pressure inside the chloroplast, F . is the CO.,-compensation point in the absence of non-photorespiratory respiration, A is the rate of CO2 assimilation, and Rd is the rate of CO,, evolution resulting ti'om processes other than photorespiration, c~' Because CO~ at the site of carboxylation is only marginally less than that in the substomatal cavity, :6~ we substituted Ci for chloroplast COe partial pressure. Additionally, we substituted the rate ot" dark respiration tbr Rd, and in the place of F . used F, the CO2 compensation point. Electron transport rates were also estimated from the light-response and CO2-response curves of TAYLOR and Gt, NDERSON.c2v'This was achieved by partitioning the response of CO2 assimilation in control and ethylene-treated leaves to either electron transport-limited or Rubisco-limited rates ofcarboxylation. (7'15 According to SAOE,"15: when photosynthetic electron transport limits the regeneration of RuBP, assimilation can be expressed by,
J Wi = 4.5 + I O . 5 ~ ( F ; / C )
(2)
where dr is the potential rate of electron transport
ETHYLENE INHIBITS ELECTRON TRANSPORT and is r~lated to the m a x i m u m rate of electron transport (i.e. J,,,~) according to the equation, J, ..... x l
J -
(3)
I+-2.1 x . ] ......
where I is that irradiance absorbed by a leat: " Similarly, when Rubisco activity limits car1)oxylation, assimilation can be described by,
rl~
Vc ...... x C =
C+ K,
x
(I + O/K,,)
,'4)
where f):...... is the m a x i m u m carboxylation velocity of tully activated Rubisco, 0 is the partial pressure o f ' O 2 in the stroma, and K, and K. are the Michaelis constants of Rubisco fbr CO., and oxygen, respectively. T h e light-response curves of TAYLOR and (JUNDFRSON :2t w e r e used in conjunction with Equations (2) and (3) to obtain initial estimates of.I, ..... tbr control and ethylene-treated leaves. Once these estimates were obtained, the CO~response curves of TAYLOR and GUNDERSON 21 were then used along with Equations (2), (3), and (4) to approximate D ...... and t() thrther improve estimates of m a x i m u m electron transport capacity. Curve fitting was accomplished using non-linear regression techniques. :1(~
. lna{vses ojchlorophyll jtuorescence ht vi~,o fluorescence of dark-adapted (30 rain: control and ethylene-treated leaves was determined by the saturated-pulse method ~17'~) using a PAM 101 Chl fluorometer (H. Walz, Effeltrich, Germany). Minimal fluorescence (F,,) or that fluorescence when all P S I I reaction centers are open was estimated using a 1.6 kHz pulsed-beain of weak irradiance (0.10 /tmol photons m -e sec i). Maximal fluorescence (F,,~) was determined by applying an 800 msec saturating pulse of irradiance (2550/~mol photons m .2 sec 1) to close transiently all reaction centers and to reduce thlh. the primary electron acceptor of PSII. T w o components of steady-state fluorescence were also calculated according to the equations Of S C H R F I B E R tel al., ~7 qp =
:1~ ;,,
and
q~" -
r F, I,,,
('~))'
where F, is variable fluorescence, (F,), is the satu-
451
rated level of variable fluorescence, and (F,),. is the maximal variable fluorescence which is calculated as the difference between F., and F,. ~7 Photochemical quenching (qp) was used to indi('ate tile fiaction of oxidized or open P S I I reaction centers, while non-photochemical quenching (q~) was used to indicate energy dissipation through pathwaxs not leading to photochemical events. The etficiency of excitation energy capture by PSII was calculated as (F,,,-F,,)/F,,, and the q u a n t u m yield of PSII approximated as qp × ( F , , - F,,',/F,,. ~ These latter two parameters correspond to the probability that Sill absorbed photon will reach a P S I I reaction center and the chance that an absorbed photon will resuh in a photochemical event, ee
Slalislical analysis Measurements of in vivo Chl fluorescence tbr control and ethylene-treated leaves were conducted on eight seedlings, randomly selected from a population of 20 plants. All fluorescence data were analyzed using the SAS statistical package ~: and treatment means separated using the Student's/-test. The interaction of ethylene and irradiance was analyzed using A N O V A procedures and treatment combinations deemed significantly difl'erent at the P - 0.05 level of probability. RESULTS AND DISCUSSION Electron transport rates (i.e, J,.) estimated t}om the gas-exchange data of TAYLOR and (}UNDERSON :21 w e r e 37(~0 lower in ethylene-treated leaves compared to leaves from untreated plants (Table 1). Similar results were obtained by partitioning the light-response and CO~-response curves of TAYLOR and GUNDERSON ~21 tO either electron transport-limited or RuMsco-limited rates of carboxylation, with ethylene decreasing model estimates of j ...... by 312~,, yet having no effect on the model parameter | ? ...... (Fig. 1). Because [~:...... remained unchanged tbllowing ethylene treatment, the initial slope of the CO.,response curve (i.e. carboxylation efficiency) was similar tbr both control and ethylene-treated leaves. Estimates of carboxylation efliciency based on our modeled results averaged 0.063 flmol m ~ sec i/xbar i which closely agrees with
S . D . WULLSCHLEGER et al.
452
Table 1. Gas analysis parameters reported by rI'AYLORand GUNDERSON':~1'for control and ethylene-treated soybean CO2 compensation Dark respiration point (#mol m ~ sec T) (/~bar)
Assimilation Treatment (/~mol m 2 see ') Control Ethylene
9.7 6.7
0.93 1.10
25 20
Chloroplast CO2 (/*bar)
Estimated electron transport rate (/~mol m ~ sec ')
265 230
46.0 28.8
Whole plants were exposed to either 0 (control) or 10/~1/1 ethylene for a 4-hr period. This intbrmation was used to estimate the influence of ethylene on electron transport rates according to Equation (1). that of 0.051 /~mol m-2 sec 1 /*bar-' given by TAYLOR and GUNDERSON. '21) In vivo fluorescence of dark-adapted control and ethylene-treated leaves supported the modeled results as presented above. Following a 4-hr exposure to ethylene, the efficiency by which excitation energy was captured in P S I I (i.e. (F,,,-F,,)/b~,~) was reduced from 0.80 for leaves of untreated plants to 0.73 tbr ethylene-treated leaves (Table 2). This reduction was also accompanied by a 12% decrease in photochemical quenching (qj,) indicating that a lower ORNL-DWG 91 M-7*?~I
16
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I 0
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i 0
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:~ 8
I
oi 0
0
O0 I~ 0
0 0
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, ~ ~
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200
=
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300
/
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400
Internal C 0 2 partial pressure (gbar)
FI6. 1. Modeled CO2-response curves tor (a) control and (b) ethylene-treated soybean. Gas exchange data of'TAYLOR and GUNDERSON{-~ were reanalyzed using non-linear regression procedures to estimate Rubiscolimited and electron transport-limited rates of carbon assimilation. Arrows indicate where the transition between Rubisco-limited and electron-limited assimilation occurred. Dashed lines are the polynomial regressions as determined by TAYLOR and GUNDERSON.(21
proportion of open or oxidized PSI 1 reaction centers were participating in light-dependent processes. Decreasing qp was additionally reflected by an increase in non-photochemical (qN) quenching from 0.36 tbr control leaves to 0.40 tbr leaves from ethylene-treated plants (data not shown). In combination, the observed reductions in both the efficiency by which energy was captured in ethylene-treated leaves and in photochemical quenching contributed to a decrease in the q u a n t u m yield of P S I I (i.e. qv x ( F i n - fo)/tc]n ) fi'om 0.68 to 0.54 (Table 2). Ethylene-induced efl~cts on electron transport and fluorescence quenching as observed in this study are consistent with earlier reports that ethylene disrupts the intrinsic capacity of" the mesophyll tissue to assimilate CO2, ~m'~'' but that this response is not necessarily via an eft~ct on Rubisco activity. ~<:~: Although some have argued that ethylene affects C O 2 assimilation only indirectly through stomatal or epinastic growth responses, '~4'~4''-'5 others have suggested that its influence is more direct and localized within the processes of light capture and/or utilization. ~4'2]: CHOE and WHANG'4 observed that chloroplasts isolated from Arena sativa were highly sensitive to ethylene, exhibiting symptoms of increased proteolysis, chloroplast deterioration, loss of thylakoid integrity, and loss of PSII activity within 24 hr after cthephon application. We extend these observations m leaves of Glycine max and note that while the impact of ethylene in our studies was more subtle than that reported by CHOE and WHANC, 4' ethylene had a marked influence on Chl fluorescence, indicating a perturbation of sufficient magnitude to decrease COe assimilation and to lower the efficiency by which energy is captured by PSII.
ETHYLENE INHIBITS ELECTRON TRANSPORT
453
Table 2. Chlor@hyll fluomcence parameters for dark-adapted control and ethylene-treated soybean
Treatment Control Ethylene Probability
/~'c,
Fm
(mV)
(mV)
(Fm - F,,)/P;,,
q~,
q,, x (Fro - E,)~Fro
220+_15 252_+16 0.024
1109+38 949_+77 0.010
0.80+0.01 0.73_+0.01 0.001
0.84+0.0l 0.74_+0.02 0.001
0.68_+0.01 0.54_+0.02 0.00 l
Whole plants were exposed to either 0 (control) or 10 #1/1 ethylene tbr a 4-hr period.
M a r k e d reductions in the photochemical efficiency of P S I I are often interpreted as resulting from photoinhibition. ~1,5)This type of d a m a g e arises when leaves are exposed to light in excess of that which can be dissipated through photosynthetic electron transport. !5i KITAJIMA and BUTLER c13) presented a model of light-energy dissipation in which they describe how photoinhibition should contribute to enhanced fluorescence from open reaction centers (b;,), while decreasing fluorescence from closed reaction centers (F,,). Similar patterns of fluorescence, that is increasing F,, and decreasing Fro, were observed tbr ethylene-treated leaves ('Fable 2) and, therefore, photoinhibition may be implicated in the response of Glycine max to ethylene. O u r initial observations that ethylene-induced reductions in qp and ( F m - Fo)/Fm were dependent upon the level ofirradiance during exposure (Fig. 2) would also support the possibility that photoinhibition plays a role in the ethylene-induced inhibition of CO2 assimilation. In the only other report of ethylene effects on properties related to electron transport, CnOE and W H A N G '4) observed that ethylene demonstrated a marked capacity to disrupt thylakoid integrity and PSII activity in chloroplasts isolated from Arena saliva, a response that occurred to some extent in darkness, but that increased in the light. Although ethylene-induced reductions in electron transport, as interred in our study from in vivo Chl fluorescence, increased with increasing light intensity and in a m a n n e r consistent with photoinhibition, these reductions were not of the magnitude anticipated. BJORKMAN(2 reported that tbr two mangrove species, the extent to which increasing irradiance decreased ( F m - F , , ) / F m strongly depended upon leaf orientation with inhibition being greatest for horizontally oriented
leaves. Given that we did not restrict leaf orientation during our exposure studies, and that epinastic leaf movement is a common response to ethylene in m a n y species, we might conclude that d a m a g e arising from ethylene-induced photoinhibition was partially minimized by changing leaf orientation. Ethylene-induced epinasty has been reported to reduce light interception by up to 60(~o in ethylene-treated Xanlhium slrumarium ~25 and, hence, this whole-plant response to ethylene
ORN~C~G9,~ras2 PPFD= 410 gruel m-2 sec-1 PPFD=980 I~mol m-2 sec-1 1.0 0.8 o- 0,6
Eft4 "~o 0.2
30
1.0
0.8 0.6 0.4 LL 0.2 0 Control
Ethylene
Control
Ethylene
FIG. 2. Photochemical quenching (panels a,b) and (F,,-P~,)/Fm (panels c,d) for dark-adapted leaves of control and ethylene-treated soybean. Two levels of irradiance were used during the exposure to examine whether these Chl fluorescence parameters would respond differently to ethylene as irradiance increased, thereby indicating the potential involvement of photoinhibition. Increasing the irradiance during ethylene exposure significantly decreased photochemical quenching (P = 0.04) relative to that of the low irradiance exposure, but had no effect on (F,y,- F,,)/F,~.
S . D . WULLSCHLEGER et al.
454
m a y be an effective m e c h a n i s m by which plants avoid excess i r r a d i a n c e and minimize photoinhibitory d a m a g e . This would not, however, c o n t r a d i c t the observation that ethylene has a direct influence on reducing single-leaf CO~ assimilation (~i or our findings that ethylene a p p a r e n t l y reduces electron t r a n s p o r t c a p a c i t y in Glycine max. Finally, our studies d o c u m e n t that decreasing CO2 assimilation in intact leaves of Glycine max following ethylene exposure was associated with a significant i m p a c t on Chl fluorescence, i n d i c a t i n g that ethylene interferes with some c o m p o n e n t of the electron t r a n s p o r t p a t h w a y . W h e t h e r ethylene binds to the thylakoid m e m b r a n e and subsequently blocks electron transport, or whether the effect is much less specific reflecting a general deterioration of the chloroplast is currently unknown. However, regardless of its m o d e of action, based on our results it would a p p e a r that ethylene exerts a substantial effect on electron transport and sufficiently impacts the photosynthetic a p p a r a t u s leading to reductions in C O 2 assimilation.
Acknowledgments We acknowledge with appreciation manuscript reviews by S. B. McLaughlin, G. E. Taylor, Jr, and R. J. Norby. The research was sponsored by the Ecological Research Division, OffÉce of Heahh and Environmental Research, U.S. Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Publication No. 3882, Environmental Sciences Division, Oak Ridge National Laboratory. The senior author was supported in part by an appointment to the Alexander Hollaender Distinguished Postdoctoral Fellowship Program sponsored by the U.S. Department of Energy, Office of Health and Environmental Research, and administered by Oak Ridge Associated Universities.
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REFERENCES
1. BAKER N. R. (1991) A possible role for photosystem II in environmental perturbations of photosynthesis. Physiologia Pl. 81, 563-570. 2. BJORKMANO. (1987) High-irradiance stress in higher plants and interaction with other stress lectors. Pages 11 18 in J. BIGGINS,ed. Progress in photosynthesis research, Vol. 4. Martinus Nijhofl, Dordrecht. 3. BROOKSA. and FARQUnAR G. D. (1985) Effect of temperature on the CO2/O~ specificity ofribulose-
14. 15.
16.
1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the light. Planta 165, 397 406. CHOE H. T. and WHANG M. (1986) Effects of ethephon on aging and photosynthetic activity in isolated chloroplasts. PI. Physiol. 80, 305--309. DEMMm B., WINTER K., KRUCER A. and CZYGAN F. C. (1987) Photoinhibition and zeaxanthin formation in intact leaves. A possible role of the xanthophyll cycle in the dissipation of excess light energy. Pl. Physiol. 84, 218 224. FARQUHARG. D. and VON CAEMMERERS. (1982) Modelling of photosynthetic responses to environmental conditions. Pages 549 587 in O. L. LANGE, P. S. NOBEL, C. B. OSMOND and H. ZIEGLER, eds Encyclopedia of plant physiology (New Series), Vol. 12B; Physiological plant ecology II. Springer, Berlin. FARQUHAR(~. D., VON CAEMMERERS. and BERRY J. A. (1980) A biochemical model of photosynthetic CO 2 assimilation in leaves of C:~ species. Planta 149, 78--90. GENTRY B., BRIANTAISJ. M. and BAKER N. R. (1989) The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence. Biochim. biophys. Acla 990, 87 92. GOVINDARAJANA. G. and POOVAIABB. W. (1982) Effect of root zone carbon dioxide enrichment on ethylene inhibition of carbon assimilation in potato plants. Physiologia Pl. 55, 465 469. GUNDERSONC. A. and TAYLOR G. E., JR (1988) Kinetics of inhibition of foliar gas exchange by exogenous ethylene: an ultrasensitive response. New Phytol. 110, 517 524. GUNDERSONC. A. and TAYLOR G. E.,JR (1991) Ethylene directly inhibits foliar gas exchange in Glycine max. PI. Physiol. 95, 337 339. KAYSS.J. and PALLASJ. E.,JR (1980) Inhibition of photosynthesis by ethylene. Nature 285, 56 57. KITAJIMAM. and BUTLERW. L. (1975) Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone. Biochim. biophys. Acta 376, 105 --115. PALLASJ. E.,JR and KAYS S.J. (1982) Inhibition of photosynthesis by ethylene a stomatal effect. PI. Physiol. 70, 598 601. SAGE R. F. (1990) A model describing the regulation of ribulose-l,5-bisphosphate carboxylase, electron transport, and triose phosphate use in response to light intensity and CO2 in C:~ plants. Pl. Physiol. 94, 1728-1734. SAS INSTITUTE (1985) SAS user's guide, statistics. SAS Institute, Cary, NC.
ETHYLENE INHIBITS ELECTRON TRANSPORT 17. SCHREIBERU., SCHLIWAU. and BILGERW. (1986) Continuous recording of photochemical and nonphotochemical fluorescence quenching with a new type of modulated fluorometer. Photosynth. Res. 10, 51 --62. 18. S~UIF.RS. A., TAYLOR G. E., JR, SELVIDGEW. J. and GUNDERSON C. A. (1985) Effect of ethylene and related hydrocarbons on carbon assimilation and transpiration in herbaceous and woody plants. Envir. Sci. Technol. 19, 432-437. 19. STUHLFAUTHT., SCHEUERMANNR. and FocK H. P. (1990) Light energy dissipation under water stress conditions. Pl. Physiol. 92, 1053-1061. 20. TAYLOR G. E., JR and GUNDERSONC. A. (1986) The response offoliar gas exchange to exogenously applied ethylene. Pl. Physiol. 82, 653 657. 21. TAYLORG. E., JR and GUNDERSONC. A. (1988) Physiological site of ethylene effects on carbon
22.
23.
24.
25.
455
dioxide assimilation in Glycine max L. Merr. PI. Physiol. 86, 85 92. VAN KOOTENO., MEURS C. and VAN Loon L. C. (1990) Photosynthetic electron transport in tobacco leaves infected with tobacco mosaic virus. Physiologia PI. 80, 446-452. voy CAEMMERERS. and FAROUHAR G. D. (1981) Some relationships between the biochemistry o[ photosynthesis and the gas exchange of leaves. Planta 153, 376 387. WOODROWL. and GRODZINSKIB. (1989) An evaluation of the effects of ethylene on carbon assimilation in lycopersicon esculentum Mill. J. exp. Bot. 40, 361 368. WOODROWL., Jlao J., TSUJITAM. J. and GRODZINSI
\~d ~2. No. I. l'P 14{t 455. t!i!17
11119g }1177 92 ~5.111) ¢ I) I1(I t 1992 PUle{till
ASSESSING THE I N F L U E N C E OF E X O G E N O U S ETHYLENE ON ELECTRON TRANSPORT AND FLUORESCENCE QUENCHING IN LEAVES OF GLYCINE M A X S. D. WULLSCHLEGER, P. J. H A N S O N and C. A. G U N D E R S O N
Environmental Sciences I)ivision, Oak Ridge National l,al)oratory, P.O. Box 2008, Oak Ridge, TN 37831-6034, U.S.A.
(Received 13 Februarr 19{)2; acc@l~'di, revMed,/Drm 19 Mql, 1992) WU1.LSCHLEGER S. O., HANSON P. ,]. and GUNDERSON (',. A. A.~ses~ing lhe influence of exo,~enou.~ eltz~lene on electrm~ lramport and.fluorescence quenching i, [eave~ q/ Glycine max. ENVIRONMENTAL AND I';XPERIMENTAI~BOTANY32, 449 455, 1992. We conducted a series of modeling exercises designed to re-evaluate the light-r('sp,msc and C().,-resp(mse curves of TAYLOR and (;tSNOERSON IP[. I'hv~iol. 86, 85 92, 1988) and to examine further their conclusion flint ethylene-induced inhil~ilion of electron transport max contribute to reduced C()., assimilation in leaves of (;lyci,e max. By partitioning the response of (I()., assinlilation to either electron u'ansl)orl-limiled (w Rubisco-limited rates ofcarboxylati(m, we calculated thai the electron transport capacity (.7,,,,,) ofcdiylenc-treated leaves decreased by over 30'% lbllowing a 4-hr exposure to 10 Idil cthvlenc {tlld lloted that ethylene-induced reduclillllS ill ('.O, assimilation could I)e explained without a d e c r e a s e i l l Rubisco activity (I),,,..). ~ l e a s u r e l n e l l t s C,t ill PIT,0 ( : h i t t u o r e s c e n c c stlpported these observations alld indicated that tile ef~ciencv I)v which excitation ellCl'~y wits captured in PSI I
(i.e. (F,,,- b],}il:,,,)
was redtlced
l}om 0.~0 to
0.73 after
a 4-]lr exposure
Io
10 ldll ethylene. This
reduction was also accompanied I)t a 12°. decrease in steady-state photochemical quenchino (qi,). indit'atillg that a lower proportion of open or oxidized PSI I reaction (el/tcrs were particil)atino" in lighi-dependenl processes, l"Ali'cts o1" eihyh'nc Oll Chl tluores('encc were amplified at increased irradiant'e> Suggesi, iIl~ that l)holoinhil)ilion IIl~./y phiy a role in the elilvlene-induccd inhil)ition of (10, assimilation.
An i m p o r t a n t question r e g a r d i n g the photosynthetic response of plants to ethylene is whether |;()LIAR gas-exchange in some species is highly ethylene-induced changes in carbon assimilation sensitive to short-term applications of exogenous occur solely through p e r t u r b a t i o n s to the stomaethylene, with rates of CO2 assimilation decreastal phase of the CO._, exchange process, or whether ing 16,51 o tbllowing a 2- to 4-hr exposure. !~"10.,'_,.~0.<,1, these reductions might equally reflect s(nne (~UNDERSON and TAYLOR ~0, examined this inhialtered biochemical or photochemical probition of gas-exchange in Glycine ma~ and reported cesses. Early studies suggested that CO~ assimilthat an ethylene concentration of 0.35 #xl/1applied ation declined d u r i n g ethylene exposure because to whole plants fi-)r 4 hr was sufficient to elicit of decreasing stomatal conductance/1'I~' Howa h a l f m a x i m a l reduction in leaf photosynthesis. ever, while c h a n g i n g stoinatal c o n d u c t a n c e can However, despite the a p p a r e n t sensitivity of CO~ account tbr much of the ethylene response in some assimilation to ethylene, the physiological mechspecies, ~° other mechanisms c o n t r i b u t i n g to the anisms c o n t r i b u t i n g to this response r e m a i n uninhibition of foliar gas-exchange have been sugresolved. gested. TAYLOR and GUNDERSON 21 concluded 449 INTRODUCTION
450
S.D. WULLSCHLEGER el al.
that although ethylene-induced reductions in stomatal conductance were often greater than those of photosynthesis in Glycine max, the major effect of ethylene on CO~ assimilation was mediated not through stomatal closure, but rather through an unspecified effect of" ethylene on light-dependent processes as evidenced in their study by a 51~I{~ reduction in quantum-use efficiency. CHOE and WHANG 4 earlier observed that electron transport in isolated chloroplasts of Avena saliva was rapidly inhibited by ethylene and, hence, it follows that ethylene-induced reductions in electron transport may also contribute to decreased CO2 assimilation in intact plants, although such studies are lacking. Recent advances in understanding the biochemical regulation of CO~ assimilation '7'~> and in our ability to model these processes ~:') have greatly expanded the opportunity to address the mechanisms by which stressors affect plant gasexchange. Theretbre, we conducted a series of modeling exercises designed to re-evaluate the conclusions of TAYLOR and GUNDERSON21 concerning the ett~ct of ethylene on decreasing CO~ assimilation. Furthermore, we measured in vivo Chl fluorescence using the saturated-pulse method to document directly the potential impact of ethylene on electron transport capacity in leaves of Glycine max. MATERIALS AND M E T H O D S
Plant malerial and elhylene exposure Plants of Glycine max (L.) Merr, cv. Davis were grown from seed under greenhouse conditions in 4-.1 pots filled with Promix BX (Premier Brands, Inc.). Seedlings were watered daily and fertilized weekly with a liquid t~rtilizer (Peters Fertilizer, 20 20-20; W. R. Grace Co.). Studies were conducted 3-4 weeks after germination, when the second trifoliate was fully expanded. Seedlings were moved to the exposure system 2 hr befbre initiation of ethylene treatments. Ethylene exposures were conducted in a controlled exposure system as described previously. :2z: Air entering the chamber passed through charcoal and particle filters, and the humidity was increased with a cool-mist impeller humidifier. Radiant energy (~400-1000 /.tmol m ~ sec z PPFD) was provided by a 1000-W
high intensity discharge multivapor lamp and the intensity changed by raising or lowering the lamp above the exposure chambers. Ethylene was injected continuously into the chamber's inlet from a gas cylinder (1.5~!/~)v/v C~H2 in N2, Matheson Gas Products) and the concentration monitored with a flame ionization detector (Model 400 Hydrocarbon Analyzer, Beckman Instruments). Ethylene was maintained at 10/.tl/1 throughout a 4-hr exposure period.
Model estimales oJ'eleclron Iransporl tale Electron transport rates for control and ethylene-treated leaves were derived from the data of TAYLOR and GUNDERSON(21) by two methods. First, they were estimated using selected gasexchange properties (Table l) and the following equation t~om VON CAEMMERER and FAR QUHAR~ 231
3', = 4 x C-72F~*- x ( A - R d ) C-F,
(1)
where J, is the rate of electron transport, C is the CO 2 partial pressure inside the chloroplast, F . is the CO.,-compensation point in the absence of non-photorespiratory respiration, A is the rate of CO2 assimilation, and Rd is the rate of CO,, evolution resulting ti'om processes other than photorespiration, c~' Because CO~ at the site of carboxylation is only marginally less than that in the substomatal cavity, :6~ we substituted Ci for chloroplast COe partial pressure. Additionally, we substituted the rate ot" dark respiration tbr Rd, and in the place of F . used F, the CO2 compensation point. Electron transport rates were also estimated from the light-response and CO2-response curves of TAYLOR and Gt, NDERSON.c2v'This was achieved by partitioning the response of CO2 assimilation in control and ethylene-treated leaves to either electron transport-limited or Rubisco-limited rates ofcarboxylation. (7'15 According to SAOE,"15: when photosynthetic electron transport limits the regeneration of RuBP, assimilation can be expressed by,
J Wi = 4.5 + I O . 5 ~ ( F ; / C )
(2)
where dr is the potential rate of electron transport
ETHYLENE INHIBITS ELECTRON TRANSPORT and is r~lated to the m a x i m u m rate of electron transport (i.e. J,,,~) according to the equation, J, ..... x l
J -
(3)
I+-2.1 x . ] ......
where I is that irradiance absorbed by a leat: " Similarly, when Rubisco activity limits car1)oxylation, assimilation can be described by,
rl~
Vc ...... x C =
C+ K,
x
(I + O/K,,)
,'4)
where f):...... is the m a x i m u m carboxylation velocity of tully activated Rubisco, 0 is the partial pressure o f ' O 2 in the stroma, and K, and K. are the Michaelis constants of Rubisco fbr CO., and oxygen, respectively. T h e light-response curves of TAYLOR and (JUNDFRSON :2t w e r e used in conjunction with Equations (2) and (3) to obtain initial estimates of.I, ..... tbr control and ethylene-treated leaves. Once these estimates were obtained, the CO~response curves of TAYLOR and GUNDERSON 21 were then used along with Equations (2), (3), and (4) to approximate D ...... and t() thrther improve estimates of m a x i m u m electron transport capacity. Curve fitting was accomplished using non-linear regression techniques. :1(~
. lna{vses ojchlorophyll jtuorescence ht vi~,o fluorescence of dark-adapted (30 rain: control and ethylene-treated leaves was determined by the saturated-pulse method ~17'~) using a PAM 101 Chl fluorometer (H. Walz, Effeltrich, Germany). Minimal fluorescence (F,,) or that fluorescence when all P S I I reaction centers are open was estimated using a 1.6 kHz pulsed-beain of weak irradiance (0.10 /tmol photons m -e sec i). Maximal fluorescence (F,,~) was determined by applying an 800 msec saturating pulse of irradiance (2550/~mol photons m .2 sec 1) to close transiently all reaction centers and to reduce thlh. the primary electron acceptor of PSII. T w o components of steady-state fluorescence were also calculated according to the equations Of S C H R F I B E R tel al., ~7 qp =
:1~ ;,,
and
q~" -
r F, I,,,
('~))'
where F, is variable fluorescence, (F,), is the satu-
451
rated level of variable fluorescence, and (F,),. is the maximal variable fluorescence which is calculated as the difference between F., and F,. ~7 Photochemical quenching (qp) was used to indi('ate tile fiaction of oxidized or open P S I I reaction centers, while non-photochemical quenching (q~) was used to indicate energy dissipation through pathwaxs not leading to photochemical events. The etficiency of excitation energy capture by PSII was calculated as (F,,,-F,,)/F,,, and the q u a n t u m yield of PSII approximated as qp × ( F , , - F,,',/F,,. ~ These latter two parameters correspond to the probability that Sill absorbed photon will reach a P S I I reaction center and the chance that an absorbed photon will resuh in a photochemical event, ee
Slalislical analysis Measurements of in vivo Chl fluorescence tbr control and ethylene-treated leaves were conducted on eight seedlings, randomly selected from a population of 20 plants. All fluorescence data were analyzed using the SAS statistical package ~: and treatment means separated using the Student's/-test. The interaction of ethylene and irradiance was analyzed using A N O V A procedures and treatment combinations deemed significantly difl'erent at the P - 0.05 level of probability. RESULTS AND DISCUSSION Electron transport rates (i.e, J,.) estimated t}om the gas-exchange data of TAYLOR and (}UNDERSON :21 w e r e 37(~0 lower in ethylene-treated leaves compared to leaves from untreated plants (Table 1). Similar results were obtained by partitioning the light-response and CO~-response curves of TAYLOR and GUNDERSON ~21 tO either electron transport-limited or RuMsco-limited rates of carboxylation, with ethylene decreasing model estimates of j ...... by 312~,, yet having no effect on the model parameter | ? ...... (Fig. 1). Because [~:...... remained unchanged tbllowing ethylene treatment, the initial slope of the CO.,response curve (i.e. carboxylation efficiency) was similar tbr both control and ethylene-treated leaves. Estimates of carboxylation efliciency based on our modeled results averaged 0.063 flmol m ~ sec i/xbar i which closely agrees with
S . D . WULLSCHLEGER et al.
452
Table 1. Gas analysis parameters reported by rI'AYLORand GUNDERSON':~1'for control and ethylene-treated soybean CO2 compensation Dark respiration point (#mol m ~ sec T) (/~bar)
Assimilation Treatment (/~mol m 2 see ') Control Ethylene
9.7 6.7
0.93 1.10
25 20
Chloroplast CO2 (/*bar)
Estimated electron transport rate (/~mol m ~ sec ')
265 230
46.0 28.8
Whole plants were exposed to either 0 (control) or 10/~1/1 ethylene for a 4-hr period. This intbrmation was used to estimate the influence of ethylene on electron transport rates according to Equation (1). that of 0.051 /~mol m-2 sec 1 /*bar-' given by TAYLOR and GUNDERSON. '21) In vivo fluorescence of dark-adapted control and ethylene-treated leaves supported the modeled results as presented above. Following a 4-hr exposure to ethylene, the efficiency by which excitation energy was captured in P S I I (i.e. (F,,,-F,,)/b~,~) was reduced from 0.80 for leaves of untreated plants to 0.73 tbr ethylene-treated leaves (Table 2). This reduction was also accompanied by a 12% decrease in photochemical quenching (qj,) indicating that a lower ORNL-DWG 91 M-7*?~I
16
(a) I
I
l
I
I
_
Lg8
-
oO ~
_~,
~
I 0
-
I
i 0
~--/
8
o
p
~thXI
:~ 8
I
oi 0
0
O0 I~ 0
0 0
_
, ~ ~
~ . ; ~ ;
~ l
~12
I
°%°o0 °o
~4
0
~1
~ . ~
I
w
I
I
I
i
I
0
0
O~
0 0
I
I
4 I ~
~b
0
100
I c~lfc~ -I
i
-
I
200
=
VCrm3x 34.2
-I
300
/
I
400
Internal C 0 2 partial pressure (gbar)
FI6. 1. Modeled CO2-response curves tor (a) control and (b) ethylene-treated soybean. Gas exchange data of'TAYLOR and GUNDERSON{-~ were reanalyzed using non-linear regression procedures to estimate Rubiscolimited and electron transport-limited rates of carbon assimilation. Arrows indicate where the transition between Rubisco-limited and electron-limited assimilation occurred. Dashed lines are the polynomial regressions as determined by TAYLOR and GUNDERSON.(21
proportion of open or oxidized PSI 1 reaction centers were participating in light-dependent processes. Decreasing qp was additionally reflected by an increase in non-photochemical (qN) quenching from 0.36 tbr control leaves to 0.40 tbr leaves from ethylene-treated plants (data not shown). In combination, the observed reductions in both the efficiency by which energy was captured in ethylene-treated leaves and in photochemical quenching contributed to a decrease in the q u a n t u m yield of P S I I (i.e. qv x ( F i n - fo)/tc]n ) fi'om 0.68 to 0.54 (Table 2). Ethylene-induced efl~cts on electron transport and fluorescence quenching as observed in this study are consistent with earlier reports that ethylene disrupts the intrinsic capacity of" the mesophyll tissue to assimilate CO2, ~m'~'' but that this response is not necessarily via an eft~ct on Rubisco activity. ~<:~: Although some have argued that ethylene affects C O 2 assimilation only indirectly through stomatal or epinastic growth responses, '~4'~4''-'5 others have suggested that its influence is more direct and localized within the processes of light capture and/or utilization. ~4'2]: CHOE and WHANG'4 observed that chloroplasts isolated from Arena sativa were highly sensitive to ethylene, exhibiting symptoms of increased proteolysis, chloroplast deterioration, loss of thylakoid integrity, and loss of PSII activity within 24 hr after cthephon application. We extend these observations m leaves of Glycine max and note that while the impact of ethylene in our studies was more subtle than that reported by CHOE and WHANC, 4' ethylene had a marked influence on Chl fluorescence, indicating a perturbation of sufficient magnitude to decrease COe assimilation and to lower the efficiency by which energy is captured by PSII.
ETHYLENE INHIBITS ELECTRON TRANSPORT
453
Table 2. Chlor@hyll fluomcence parameters for dark-adapted control and ethylene-treated soybean
Treatment Control Ethylene Probability
/~'c,
Fm
(mV)
(mV)
(Fm - F,,)/P;,,
q~,
q,, x (Fro - E,)~Fro
220+_15 252_+16 0.024
1109+38 949_+77 0.010
0.80+0.01 0.73_+0.01 0.001
0.84+0.0l 0.74_+0.02 0.001
0.68_+0.01 0.54_+0.02 0.00 l
Whole plants were exposed to either 0 (control) or 10 #1/1 ethylene tbr a 4-hr period.
M a r k e d reductions in the photochemical efficiency of P S I I are often interpreted as resulting from photoinhibition. ~1,5)This type of d a m a g e arises when leaves are exposed to light in excess of that which can be dissipated through photosynthetic electron transport. !5i KITAJIMA and BUTLER c13) presented a model of light-energy dissipation in which they describe how photoinhibition should contribute to enhanced fluorescence from open reaction centers (b;,), while decreasing fluorescence from closed reaction centers (F,,). Similar patterns of fluorescence, that is increasing F,, and decreasing Fro, were observed tbr ethylene-treated leaves ('Fable 2) and, therefore, photoinhibition may be implicated in the response of Glycine max to ethylene. O u r initial observations that ethylene-induced reductions in qp and ( F m - Fo)/Fm were dependent upon the level ofirradiance during exposure (Fig. 2) would also support the possibility that photoinhibition plays a role in the ethylene-induced inhibition of CO2 assimilation. In the only other report of ethylene effects on properties related to electron transport, CnOE and W H A N G '4) observed that ethylene demonstrated a marked capacity to disrupt thylakoid integrity and PSII activity in chloroplasts isolated from Arena saliva, a response that occurred to some extent in darkness, but that increased in the light. Although ethylene-induced reductions in electron transport, as interred in our study from in vivo Chl fluorescence, increased with increasing light intensity and in a m a n n e r consistent with photoinhibition, these reductions were not of the magnitude anticipated. BJORKMAN(2 reported that tbr two mangrove species, the extent to which increasing irradiance decreased ( F m - F , , ) / F m strongly depended upon leaf orientation with inhibition being greatest for horizontally oriented
leaves. Given that we did not restrict leaf orientation during our exposure studies, and that epinastic leaf movement is a common response to ethylene in m a n y species, we might conclude that d a m a g e arising from ethylene-induced photoinhibition was partially minimized by changing leaf orientation. Ethylene-induced epinasty has been reported to reduce light interception by up to 60(~o in ethylene-treated Xanlhium slrumarium ~25 and, hence, this whole-plant response to ethylene
ORN~C~G9,~ras2 PPFD= 410 gruel m-2 sec-1 PPFD=980 I~mol m-2 sec-1 1.0 0.8 o- 0,6
Eft4 "~o 0.2
30
1.0
0.8 0.6 0.4 LL 0.2 0 Control
Ethylene
Control
Ethylene
FIG. 2. Photochemical quenching (panels a,b) and (F,,-P~,)/Fm (panels c,d) for dark-adapted leaves of control and ethylene-treated soybean. Two levels of irradiance were used during the exposure to examine whether these Chl fluorescence parameters would respond differently to ethylene as irradiance increased, thereby indicating the potential involvement of photoinhibition. Increasing the irradiance during ethylene exposure significantly decreased photochemical quenching (P = 0.04) relative to that of the low irradiance exposure, but had no effect on (F,y,- F,,)/F,~.
S . D . WULLSCHLEGER et al.
454
m a y be an effective m e c h a n i s m by which plants avoid excess i r r a d i a n c e and minimize photoinhibitory d a m a g e . This would not, however, c o n t r a d i c t the observation that ethylene has a direct influence on reducing single-leaf CO~ assimilation (~i or our findings that ethylene a p p a r e n t l y reduces electron t r a n s p o r t c a p a c i t y in Glycine max. Finally, our studies d o c u m e n t that decreasing CO2 assimilation in intact leaves of Glycine max following ethylene exposure was associated with a significant i m p a c t on Chl fluorescence, i n d i c a t i n g that ethylene interferes with some c o m p o n e n t of the electron t r a n s p o r t p a t h w a y . W h e t h e r ethylene binds to the thylakoid m e m b r a n e and subsequently blocks electron transport, or whether the effect is much less specific reflecting a general deterioration of the chloroplast is currently unknown. However, regardless of its m o d e of action, based on our results it would a p p e a r that ethylene exerts a substantial effect on electron transport and sufficiently impacts the photosynthetic a p p a r a t u s leading to reductions in C O 2 assimilation.
Acknowledgments We acknowledge with appreciation manuscript reviews by S. B. McLaughlin, G. E. Taylor, Jr, and R. J. Norby. The research was sponsored by the Ecological Research Division, OffÉce of Heahh and Environmental Research, U.S. Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Publication No. 3882, Environmental Sciences Division, Oak Ridge National Laboratory. The senior author was supported in part by an appointment to the Alexander Hollaender Distinguished Postdoctoral Fellowship Program sponsored by the U.S. Department of Energy, Office of Health and Environmental Research, and administered by Oak Ridge Associated Universities.
4. 5.
6.
7.
8.
9.
10.
11.
12.
13.
REFERENCES
1. BAKER N. R. (1991) A possible role for photosystem II in environmental perturbations of photosynthesis. Physiologia Pl. 81, 563-570. 2. BJORKMANO. (1987) High-irradiance stress in higher plants and interaction with other stress lectors. Pages 11 18 in J. BIGGINS,ed. Progress in photosynthesis research, Vol. 4. Martinus Nijhofl, Dordrecht. 3. BROOKSA. and FARQUnAR G. D. (1985) Effect of temperature on the CO2/O~ specificity ofribulose-
14. 15.
16.
1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the light. Planta 165, 397 406. CHOE H. T. and WHANG M. (1986) Effects of ethephon on aging and photosynthetic activity in isolated chloroplasts. PI. Physiol. 80, 305--309. DEMMm B., WINTER K., KRUCER A. and CZYGAN F. C. (1987) Photoinhibition and zeaxanthin formation in intact leaves. A possible role of the xanthophyll cycle in the dissipation of excess light energy. Pl. Physiol. 84, 218 224. FARQUHARG. D. and VON CAEMMERERS. (1982) Modelling of photosynthetic responses to environmental conditions. Pages 549 587 in O. L. LANGE, P. S. NOBEL, C. B. OSMOND and H. ZIEGLER, eds Encyclopedia of plant physiology (New Series), Vol. 12B; Physiological plant ecology II. Springer, Berlin. FARQUHAR(~. D., VON CAEMMERERS. and BERRY J. A. (1980) A biochemical model of photosynthetic CO 2 assimilation in leaves of C:~ species. Planta 149, 78--90. GENTRY B., BRIANTAISJ. M. and BAKER N. R. (1989) The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence. Biochim. biophys. Acla 990, 87 92. GOVINDARAJANA. G. and POOVAIABB. W. (1982) Effect of root zone carbon dioxide enrichment on ethylene inhibition of carbon assimilation in potato plants. Physiologia Pl. 55, 465 469. GUNDERSONC. A. and TAYLOR G. E., JR (1988) Kinetics of inhibition of foliar gas exchange by exogenous ethylene: an ultrasensitive response. New Phytol. 110, 517 524. GUNDERSONC. A. and TAYLOR G. E.,JR (1991) Ethylene directly inhibits foliar gas exchange in Glycine max. PI. Physiol. 95, 337 339. KAYSS.J. and PALLASJ. E.,JR (1980) Inhibition of photosynthesis by ethylene. Nature 285, 56 57. KITAJIMAM. and BUTLERW. L. (1975) Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone. Biochim. biophys. Acta 376, 105 --115. PALLASJ. E.,JR and KAYS S.J. (1982) Inhibition of photosynthesis by ethylene a stomatal effect. PI. Physiol. 70, 598 601. SAGE R. F. (1990) A model describing the regulation of ribulose-l,5-bisphosphate carboxylase, electron transport, and triose phosphate use in response to light intensity and CO2 in C:~ plants. Pl. Physiol. 94, 1728-1734. SAS INSTITUTE (1985) SAS user's guide, statistics. SAS Institute, Cary, NC.
ETHYLENE INHIBITS ELECTRON TRANSPORT 17. SCHREIBERU., SCHLIWAU. and BILGERW. (1986) Continuous recording of photochemical and nonphotochemical fluorescence quenching with a new type of modulated fluorometer. Photosynth. Res. 10, 51 --62. 18. S~UIF.RS. A., TAYLOR G. E., JR, SELVIDGEW. J. and GUNDERSON C. A. (1985) Effect of ethylene and related hydrocarbons on carbon assimilation and transpiration in herbaceous and woody plants. Envir. Sci. Technol. 19, 432-437. 19. STUHLFAUTHT., SCHEUERMANNR. and FocK H. P. (1990) Light energy dissipation under water stress conditions. Pl. Physiol. 92, 1053-1061. 20. TAYLOR G. E., JR and GUNDERSONC. A. (1986) The response offoliar gas exchange to exogenously applied ethylene. Pl. Physiol. 82, 653 657. 21. TAYLORG. E., JR and GUNDERSONC. A. (1988) Physiological site of ethylene effects on carbon
22.
23.
24.
25.
455
dioxide assimilation in Glycine max L. Merr. PI. Physiol. 86, 85 92. VAN KOOTENO., MEURS C. and VAN Loon L. C. (1990) Photosynthetic electron transport in tobacco leaves infected with tobacco mosaic virus. Physiologia PI. 80, 446-452. voy CAEMMERERS. and FAROUHAR G. D. (1981) Some relationships between the biochemistry o[ photosynthesis and the gas exchange of leaves. Planta 153, 376 387. WOODROWL. and GRODZINSKIB. (1989) An evaluation of the effects of ethylene on carbon assimilation in lycopersicon esculentum Mill. J. exp. Bot. 40, 361 368. WOODROWL., Jlao J., TSUJITAM. J. and GRODZINSI