Photoinhibition of Photosystem 11I

Home , Photoinhibition

Plant Physiol. (1992) 100, 1554-1561 Received for publication April 15, 1992 0032-0889/92/100/1 554/08/$01 .00/0 Accepted July 6, 1992 Photoinactivation of Catalase Occurs under Both High- and Low-Temperature Stress Conditions and Accompanies Photoinhibition of Photosystem 11I

Jurgen Feierabend*, Cornelia Schaan, and Birgit Hertwig Botanisches Institut, J. W. Goethe-Universitat, P.O. Box 11 19 32, D-6000 Frankfurt am Main, Federal Republic of Germany

ABSTRACT developed a multiplicity of antioxidative mechanisms (3, 27), Severe photoinactivation of catalase (EC 1.11.1.6) and a decline some components are obviously not fully protected from of variable fluorescence (FJ), indicating photoinhibition of photo- photodamage. Thus, the enzyme catalase is light sensitive in synthesis, were observed as rapid and specific symptoms in leaves vitro and suffers from photoinactivation with subsequent exposed to a high heat-shock temperature of 40°C as well as in degradation in intact leaves as well (7, 12, 31). To maintain leaves exposed to low chilling temperatures in white light of only a constant catalase level in light, its loss has to be continu- moderately high photosynthetic photon flux density of 520 ME m-2 ously compensated for by an adequate concomitant resyn- s-'. Other parameters, such as peroxidase (EC 1.11.1.7), glycolate thesis of the enzyme (12). When, however, light degradation oxidase (EC 1.1.3.1), glutathione reductase (EC 1.6.4.2), or the exceeds the capacity for repair (25) or when resynthesis is chlorophyll content, were hardly affected under these conditions. impaired by inhibitors (7) or by stress conditions such as low At a compatible temperature of 22°C, the applied light intensity of catalase is observed. did not induce severe photoinactivations. In darkness, exposures temperature (31), an apparent loss to high or low temperatures did not affect catalase levels. Also, The velocity of loss depends on light intensity. decline of F, in light was not related to temperature sensitivity in Another rapid symptom of light stress is photoinhibition darkness. The effective low-temperature ranges inducing photo- of photosynthesis, which similarly occurs either at excessively inactivation of catalase differed significantly for chilling-tolerant high irradiance or at only moderate light intensities when and chilling-sensitive plants. In leaves of rye (Secale cereale L.) plants are simultaneously exposed to additional stress con- and pea (Pisum sativum L.), photoinactivation occurred only below ditions. Photoinhibition was mostly related to a preferential 15°C, whereas inactivation occurred at 15°C in cucumber (Cucu- inactivation of PSII (4, 16-18). Photoinhibition is accom- mis sativus L.) and maize (Zea mays L.). The behavior of F, was panied by a decline of F,2, a parameter that is proportional similar, but the difference between chilling-sensitive and chilling- to the quantum yield of PSII, because excitation energy is tolerant plants was less striking. Whereas the catalase polypeptide, as heat inhibited reaction centers (17). Whereas although photoinactivated, was not cleaved at 0 to 4°C, the Dl dissipated by protein of photosystem 11 was greatly degraded during the low- conceptions about the primary events in photoinhibition are temperature treatment of rye leaves in light. Rye leaves did not still greatly diverging and the phenomenon may involve exhibit symptoms of any major general photodamage, even when multiple mechanisms occurring under different experimental they were totally depleted of catalase after photoinactivation at 0 conditions (4, 16, 17), long-term photoinhibition of PSII has to 4°C, and catalase recovered rapidly at normal temperature. In been related in various systems to a light-induced damage or cucumber leaves, the decline of catalase after exposures to bright modification and subsequent depletion of the plastoquinone light at 0 to 4°C was accompanied by bleaching of chlorophyll, QB-binding reaction center protein Dl of PSII under condi- and the recovery observed at 25°C was slow and required several tions in which photodegradation exceeds its repair (4, 11, 17, days. Similar to the Dl protein of photosystem 11, catalase differs 18, 22, 26). Recovery from photoinhibition has been shown greatly from other proteins by its inactivation and high turnover in in various to protein synthesis in the light. Inasmuch as catalase and Di protein levels depend on con- higher plants require tinuous repair synthesis, preferential and rapid declines are gen- chloroplasts, which are the site of Dl protein synthesis (11, erally to be expected in light whenever translation is suppressed 14, 16, 18). by stress actions, such as heat or chilling, and recovery will reflect Because of their common property of rapid tumover, both the repair capacity of the plants. catalase and the Dl protein can be expected to be particularly sensitive to all conditions affecting the effiaency of protein synthesis in light. Loss of catalase activity due to photoinactivation at low temperature has been described (31). How- ever, on the basis of the above considerations, not only low While green plants are dependent on light, they are threat- temperature but various other stress conditions should be ened by its photooxidative actions. Although plant cells have equally effective in inducng a specific decline of catalase and photoinhibition of PSII whenever the capacity for repair ' Financial support by the Bundesminister fur Forschung und Technologie, Bonn, FRG, and by the Fonds der Chemischen Indus- 2 Abbreviations: F,, variable fluorescence: (F,, - F,)/F,,; F,, initial trie, Frankfurt, FRG, is greatly appreciated. fluorescence; F,,,, maximum fluorescence. 1554 TEMPERATURE-INDUCED PHOTOINACTIVATION OF CATALASE 1 555 synthesis is suppressed. Therefore, in the present investiga- cucumber or maize were used, and the homogenates were tion, we have also applied a high heat-shock temperature adjusted to a final volume of 5 mL. For pea, extracts were that is known to immediately block translation, except for adjusted to a final volume of 10 mL and prepared from three the synthesis of specific heat-shock proteins (30). Further- lateral leaflets. The homogenates were centrifuged for 5 min more, we have extended the analysis of the low-temperature at 7000g and 40C. The resulting supernatants were used for action by comparing the effective temperature ranges in the enzyme assays. chilling-tolerant and chilling-sensitive plants. Under all con- For immunoblotting, soluble supernatants and total mem- ditions, the behavior of catalase was compared to that of Fv brane fractions of total leaf extracts were prepared as previ- as an indicator of PSII performance and to that of other ously described (12). enzymes related to peroxide metabolism. PAGE MATERIALS AND METHODS Electrophoresis under denaturing conditions in the pres- Plant Material and Growing Conditions ence of 0.4% (w/v) SDS was performed on polyacrylamide slab gels (10 x 13 x 0.15 cm3) consisting of a 5% (w/v) Experiments were performed with leaves or leaf segments stacking gel and a 10 to 15% (w/v) resolving gel in the buffer of young plants of rye (Secale cereale L. cv Halo, F. v. Lochow system of Laemmli (19). Petkus GmbH, Bergen, FRG), pea (Pisum sativum L. cv Rhein- perle, local commercial source), cucumber (Cucumis sativus Preparation of Antiserum and Immunoblotting L. cv Corona, TS Seeds, Ambacht, Holland), and maize (Zea mays L. cv Inra Corn, Raiffeisen Frankfurt eG, FRG). Seeds Antisera and immunoblotting procedures were described of rye were surface sterilized by a 10-min vacuum infiltration previously (12). and soaking for about 30 min in a freshly prepared, filtered solution of 3% (w/v) calcium hypochlorite-chloride, thor- Radioactive Labeling oughly washed with demineralized H20 and grown for 6 d at 220C in glass-covered plastic boxes on filter paper Labeling of leaf segments with L-[35S]methionine, subse- (Schleicher & Schull, No. 598) moistened with H20. Peas, quent extraction, membrane preparation, and immunoprecip- after a 2-h presoaking in H20, were grown for 14 d at 27°C; itation with anti-catalase serum was performed as described after 5 d in the presence of H20, a modified Knop's nutrient in the preceding publication (12). solution (9) was applied. Cucumber seeds, after a 1-h presoaking in H20, were grown for 14 d at 27°C; after 6 d, a Fluorescence Measurements modified Knop's nutrient solution was applied. Maize seeds, Fluorescence induction was measured at 250C in leaf sec- after a 3-h presoaking in H20, were grown for 10 d at 27°C; tions dark adapted for 10 min with a Hansatech Ltd. LD2 after 5 d, a modified Knop's nutrient solution was applied. cuvette, equipped with a LS1 photodiode light source, fluo- Rye seedlings were kept in continuous light provided by rescence detector, and a TR1 transient recorder. Cucumber, fluorescent tubes (Osram L/36N/36 Natura and Philips TL maize, and pea leaf parts, as prepared for the experimental 47W/47 in altemating sequence) giving an incident PPFD of treatments (see above), were used. For rye, measurements 90 ,umol of photons m2 s-. Cucumber, maize, and pea were performed with 5-cm sections cut from the middle of plants were grown in 16 h of light provided by an Osram three primary leaves. Power-Star HQI-T 2000 W/D lamp giving an incident PPFD of 200 ,umol of photons m-2 S-1. Analytical Methods Experimental Treatments Total Chl was determined from 80% (v/v) acetone extracts according to the method of Arnon (1). Leaves or leaf segments were exposed for 24 h, or as Enzyme activities were assayed spectrophotometrically at specified in the text, to different temperatures and to a high 250C. The assays for catalase (EC 1.11.1.6), GSH reductase light intensity of approximately 520 Omol m-2 s-1 PPFD (EC 1.6.4.2), glycolate oxidase (EC 1.1.3.1), and guaiacol- provided by an Osram Power-Star HQI-T 100OW/D lamp in dependent peroxidase (EC 1.11.1.7) were previously de- combination with a KG1 heat filter (Schott & Genossen). For scribed (31). the high light exposures, rye seedlings, lateral leaflets of the Between two and four independent experiments were per- third, fully expanded leaf of pea seedlings, cucumber seg- formed, and SE values are indicated in the figures. ments 35 mm in diameter from the primary leaves, or 5-cm segments of maize from 1 cm below the tip of the second RESULTS leaf were laid flat on a filter paper moistened with H20. Photoinactivation at Heat-Shock Temperature Preparation of Cell-Free Extracts For exposures to a heat-shock temperature of 400C, 6-d- The leaf tissues were homogenized under ice-cold condi- old rye seedlings were used when the primary leaves were tions with mortar and pestle with acid-washed sea sand in almost mature. Under a PPFD of 520 jtmol m-2 s-', a 4-h 50 mm potassium phosphate buffer, pH 7.5. For each extrac- heat-shock treatment induced a marked decline in catalase tion, 10 primary leaves of rye or two leaf segments of activity in rye leaves, which persisted after return to a normal 1 556 FEIERABEND ET AL. Plant Physiol. Vol. 100, 1992 temperature (250C), and an even more striking decline of Fv, which recovered slightly during the next 24 h after return to 250C (Fig. 1). It was previously shown that the heat-shock exposure in light was accompanied by a substantial decline in the rate of synthesis, as well as in the amounts of both the catalase protein and the D1 protein of PSII (12). Incorporation 48 46 ...w

1.0 b

U- -%.5 a U- t,0-

Figure 2. Comparison of the incorporation of L-[VS]methionine into catalase (a, Cat) or into the D1 protein of PS11 (b) in leaf sections of 6-d-old rye seedlings under heat-shock (HS) conditions in light of 520,umol m-2 s-' PPFD. C, Untreated control kept at 22°C; HS, 4- during the 4 h labeling period leaf sections were exposed to 40°C; HS + pretreatment (Pretr.), the C- during 4-h labeling period, leaf sections were exposed to 40°C, but 3 h before the labeling period, 1.0 plants had been exposed to a 15-min pretreatment at 400C in .6- darkness. Between pretreatment and the 4-h heat-shock labeling period, plants were kept at 25°C and a PPFD of 520 ,umol m-2 s-1. Shown are fluorograms after SDS-PAGE separations of (a) immu- noprecipitates obtained with anti-catalase serum or (b) total cell Z .0 membranes.

into the D1 protein was suppressed less than incorporation into catalase (Fig. 2). At a temperature of 250C, the PPFD .0 applied did not induce major changes. In darkness, the heat treatment did not result in a loss of catalase, and the decline in Fv was much smaller than under high light intensity (Fig. t.. 1). In addition, in darkness, not only a decrease in Fm but a. also a significant increase in F, contributed to the decline in .4- (Fm - F)/Fm,, whereas at 400C in light, the decline in this ratio resulted solely from a decrease of Fm. Similar to other :' 1.0 heat-shock events, a preadaptation could be induced by a short 15-min treatment at 400C in darkness, which markedly lowered the extent of the photoinactivation events observed during a later prolonged heat-shock and light treatment (Fig. 0 1). Labeling experiments with L-[35Sjmethionine showed that after a short pretreatment at 400C slightly higher rates of 0 4 12 incorporation into both catalase and the D1 protein were h Treatment maintained during a later, extended heat-shock period, as compared with an immediate exposure to 400C (Fig. 2). Figure 1. Changes in catalase, glycolate oxidase, and F, after ex- Photoinactivation of catalase and photoinhibition of pho- posure of 6-d-old rye seedlings to a 4-h heat-shock treatment of 40'C. *, Leaves before treatment; O, 4 h at 40°C and 520 ,umol tosynthesis were very specific effects of the 400C treatments. During the same experimental time, the Chl contents (not m-2 s-1 plus 24 h at 25°C and 520 ,mol m-2 s-'; , 4 h at 40°C in darkness plus 24 h at 25°C and 520 gmol m-2 s-1; 0, controls shown) of the leaves remained unchanged for the plant without heat-shock, 28 h at and 520 tmol m-2 s-1; A, 15 min at material described in Figure 1, and another peroxisomal 40°C in darkness plus 3 h at 25°C and 520 Amol m-2 s-I plus 4 h enzyme, glycolate oxidase, was not affected by any photo- at 40°C and 520 ,Amol m-2 s-1 plus 21 h at 25'C and 520 umol inactivation (Fig. 1). In later experiments with plants grown -2 -I m s. from another seed crop, gradual bleaching of Chl was ob- TEMPERATURE-INDUCED PHOTOINACTIVATION OF CATALASE 1 557 served during the high light exposure at 250C following the affected by the low-temperature exposures, independently of heat-shock treatment. the light conditions. In maize leaves, GSH reductase increased in light at temperatures above 100C (Fig. 4). Peroxi- Photoinactivation at Low Temperatures dase activity was not light sensitive. In pea leaflets and cucumber leaf discs, its activity increased during the treat- To assay and compare photoinactivation events in the low- ments when the temperatures were higher than 100C. These temperature range, seedlings or leaf sections of two chilling- increases are presumably related to the detachment of the tolerant plants, rye and pea, and of two chilling-sensitive leaves (15). However, at 70C, a loss of peroxidase occurred plants, maize and cucumber, were exposed for 24 h to a in the two chilling-sensitive species, maize and cucumber, PPFD of 520 ,umol m-2 s-' at different temperatures between both in light and in darkness (Fig. 4). These comparisons 7 and 250C. By this selection of species, each group included emphasize the high specificity of the chilling-induced pho- both a monocotyledonous and a dicotyledonous plant. In all toinactivation of catalase and of PSII. plants, quite marked photoinactivation of catalase and of In rye leaves, the fate of the catalase and of the Dl protein PSII, as monitored by the decline in FV, was observed at the was monitored by immunoblotting (Fig. 5). After a 24-h lowest temperature of 70C. The effective temperature ranges exposure of rye leaves to low temperature (0-40C) and a were, however, clearly different for the tolerant and sensitive PPFD of 520 umol m-2 s-1, the amount of the catalase subunit species, in particular, for catalase. In rye and pea, photoinac- polypeptide was not significantly changed (Fig. 5b), although tivation of catalase was seen only below 150C, whereas the the enzyme activity was very low (Fig. 3; also see figure 1 of loss of catalase was already quite marked between 15 and ref. 31). After termination of the cold treatment, when the 250C in maize and cucumber (Fig. 3). In all plants, FV grad- leaves were kept for 1 h at 220C and a PPFD of 90 zmol m-2 ually declined in light with decreasing temperature (Fig. 3). s- , a minor decline in the catalase polypeptide was detected. Also, for FV, the decline at 150C was greater in maize and Thereafter, both enzyme activity (31) and catalase protein cucumber than in rye and pea. However, the difference content recovered to normal levels. As documented before in between chilling-tolerant and chilling-sensitive plants was experiments with purified enzyme in vitro (7), the catalase not as pronounced as for the photoinactivation of catalase. polypeptide was also not cleaved during the course of pho- In darkness, the 24-h low-temperature exposures had no toinactivation in vivo. Proteolytic breakdown, which presum- effect on catalase. Only in cucumber leaf discs did F, decline ably removed the damaged enzyme under heat-shock con- at 70C in darkness. During the 24-h treatments at low tem- ditions (12), was apparently suppressed by the low temper- perature, photoinactivation of catalase and of photosynthesis ature. In contrast, the Dl protein was already markedly were not accompanied by any bleaching of Chl (Fig. 3). degraded during the 24-h light exposure at 0 to 40C (Fig. 5a). Of several other enzyme activities that have been com- Care was taken that at the end of the treatment the leaf tissue pared, glycolate oxidase and GSH reductase were hardly was immediately frozen in liquid N2 without any warming

- Figure 3. Changes of catalase activity, F,, and Catalse Fm Fo)/Fm Chxophyl the Chl contents in leaves or leaf sections from 150 rye, pea, maize, or cucumber after a 24-h exposure to different temperatures either at a 100 PPFD of 520 Mmol m-2 s-' (0) or in darkness (0). 50

100 a, a 50

100 Re 50 0 100 50 0 5 10 15 25 5 10 15 25 5 10 1S 25 tC ditn a 24 h teatment 1558 FEIERABEND ET AL. Plant Physiol. Vol. 100, 1992

Figure 4. Changes of the activities of peroxi- Peroddase Glyedate oAdase ktatioe redictase dase, glycolate oxidase, and GSH reductase in leaves or leaf sections from rye, pea, maize, or cucumber after a 24-h exposure to different temperatures either at 520 ktmol m-2 S-1 (0) or in darkness (0). 100

15 10 50 pea

o- 150 If. Ct> o

100

5 10 15 25 5 10 15 25 5 10 15 25 dbcin a 24h teatwet

before grinding and extraction. The results suggest that photocleavage was substantially contributing to the D1 protein breakdown under these conditions of long-term cold treatment. When the leaves were subsequently exposed to 220C, some further degradation was seen at only low light intensity or in darkness, particularly in the presence of chloramphenicol (Fig. 5a). In the absence of a photoinhibitory low-temperature pretreatment, decline in the D1 protein did not occur within 1 h after chloramphenicol application at low light or b in darkness (not shown). The observations suggest that part of the D1 protein still present at the end of the cold treatment was, nevertheless, photodamaged and could be degraded by a proteolytic or autoproteolytic process after return to a higher temperature, as described for isolated thylakoids (2, 4).

a Recovery from Photoinactivation at Low Temperature Because the striking loss of catalase in light under extreme temperature conditions must severely weaken the antioxidant defense mechanisms of leaves, it might be accompanied by an increase in general photooxidative damage and bleaching. Figure 5. Immunoblotting of total cell membranes (a) and total However, previous studies (31) have shown that green leaves soluble proteins (b) from rye leaves with anti-D1 serum (a) or anti- of chilling-tolerant rye do not exhibit major symptoms of catalase serum (b). Leaves of 6-d-old rye seedlings grown at a PPFD enhanced general photooxidative injury, either during a of 90 ,gmol m-2 5s1 and 22'C (untreated control, C) were exposed period of catalase depletion at low temperature or during to 0 to 4°C at a PPFD of 520 gmol m-2 S1 (0°) for 24 h and subsequent rapid recovery at normal temperature. Even in subsequently transferred to a temperature of 22'C for 1 h either at etiolated rye leaves, from which catalase activity was com- low light of 90 ,mol m-2 S1 (+1 h LL) or in darkness (+1 h D) in the presence (+CAP) or absence (-CAP) of 6.3 mm chlorampheni- pletely removed by a 24-h treatment at 0 to 40C in high light, col. Chloramphenicol was applied 1 h before the transfer from low greening was not impaired in bright light after return to to high temperature. Cat, Catalase. 220C, and catalase recovered rapidly (Fig. 6). In contrast, in TEMPERATURE-INDUCED PHOTOINACTIVATION OF CATALASE 1559

temperature, before they started to recover. Recovery at 220C was relatively slow. About 4 d were needed until catalase and Chl had again reached their original levels. The cold m treatments of cucumber leaves were also accompanied by a cm small decline in Fv, which recovered immediately after the hi 10 GO 0.- end of the cold treatments. Although other enzyme activities EU were not markedly affected, the cold treatments were typi- 5 cally followed by a major increase of peroxidase activity in cucumber leaves (Fig. 7). 0 0 12 24 0 12 24 DISCUSSION h of illumation Cold-induced decline in catalase has been observed (8, 21, Figure 6. Changes in catalase activity and the total Chl contents in 23, 29, 31), and in rye leaves this was demonstrated to be leaves of dark-grown 6-d-old rye seedling after exposure to 90 Amol the result of photoinactivation (31). Results of our present m-2 s-' (solid line) or 520,mol m-2 s-' PPFD (dashed line) with comparative investigations support a more general under- (E) or without (0) a 24-h pretreatment at 0 to 4°C and 520 ,umol standing of this phenomenon. The occurrence of catalase m-2 s-' PPFD. photoinactivation is not specifically related to chilling temperatures. Because of the rapid turnover of this light-sensitive enzyme (12), loss of catalase through photoinactivation ap- normal catalase-containing etiolated rye leaves continuously pears to occur, even when light is not excessive, whenever grown at 220C, catalase suffered marked photoinactivation leaves are subjected to unfavorable stress conditions that upon transfer to light (Fig. 6). At the lower light intensity, suppress their ability to replace sufficiently the degraded photoinactivation was indicated by an extended lag phase, catalase. Both heat-shock and chilling temperatures were as observed earlier (6), and at the higher PPFD, a strong equally effective in inducing declines in catalase at a moderate transient decline in catalase activity occurred. light intensity. The decrease of protein and catalase synthesis, In cold-sensitive cucumber, prolonged cold exposures in which was documented in rye leaves at low (31) as well as bright light were followed by severe photooxidative bleaching at high temperature (12), appears to be sufficient to explain and necrosis of the tissue after return to normal conditions. the marked and specific decrease in the catalase level. Other To avoid lethal effects and to keep the tissue in a condition proteins or enzymes that are not light sensitive remain stable allowing further analysis, we applied two cold treatments of under such conditions. 2 h each to cucumber plants in bright light on two consecutive Catalase photoinactivation generally paralleled the occur- days. These treatments resulted in limited injury. The loss of rence of photoinhibition of PSII, as indicated by the decline catalase activity was accompanied by Chl degradation to a in F.. Similar to the behavior of catalase, photoinhibition is similar extent (Fig. 7). The declines of catalase as well as that known to occur not only under excessive but also under of Chl continued for about 1 d after return to a compatible moderate light conditions when leaves are exposed to either

Figure 7. Changes in the Chl content, F,, and catalase and peroxidase activities in leaves of .20 cucumber at 22°C and 520 ,umol m-2 S-1 PPFD ~15 after two exposures for 2 h each of whole 14- d-old cucumber plants to 0 to 4°C at 520 ,umol Fa m-2 s-' PPFD on two consecutive days. C, EU L0 Control values in the tissue before cold treat- 10 a ment. Space limitations of the growth cabinet did not allow plants to be assayed each day ' after the cold treatment in one series. Prelimi- Chlorophyll Peroxidase nary experiments have, however, provided evi- 0 dence that the recoveries of Chl and catalase 20 0.8 were not completed before day 4 after the cold treatment. 0.6 10 0.4 a0 . 0.2 =L Catatase F r

C 0 1 2 3 4 C 0 1 2 3 4 coldcoltratmnttreatmenttreatment days after co ld cold treatment treatmentdays after cold 1560 FEIERABEND ET AL. Plant Physiol. Vol. 100, 1992 heat (20) or chilling temperatures (24). In accord with the capacity for repair of both catalase and Dl, although mech- behavior of photoinhibition (13, 28) photoinactivation of anisms of improved protection against photodegradation may catalase was also not restricted to chilling-sensitive plants also be involved. The limited and preliminary experiences and not related to the susceptibility of the plants to chilling with recovery after photoinactivation at low temperature injury in darkness. Photoinactivation of both catalase and would be in accord with the hypothesis (10) that repair PSII appeared to represent quite general symptoms of light processes are slower in a chilling-sensitive plant, such as sensitivity and early photodamage in leaves. cucumber, than in a chilling-tolerant plant, such as rye. We previously presented evidence that photoinactivation Correspondingly, in rye leaves, periods of catalase deficiency of catalase and of PSII are not causally interrelated but were short, as compared to cucumber, after cold exposures represent independent, although mostly concomitant, events in light. In addition, alternative antioxidants increased, H202 (31). The parallel occurrence of both symptoms seems to be did not accumulate, and major photooxidative injury was not a consequence of the rapid turnover in light that is common observed in rye (31). Further elucidation may indicate to both catalase (12) and the reaction center protein Dl of whether bleaching and photooxidative damage after cold PSII (18, 22). Depletion of the latter will result in photoinhi- exposures of cucumber leaves are related to the prolonged bition (4, 11, 18). The decline in FV at 400C (Fig. 1) was as decline in catalase. Cucumber is the only plant for which an rapid as expected from our estimation of the half-life of the increase of the H202 content has been described after an Dl protein at this PPFD in rye leaves (about 90 min). The experimental depletion of catalase (23). disappearance of the Dl protein under these conditions has been documented (12). The increase in FV (Fig. 1) and of the ACKNOWLEDGMENTS Dl protein level at normal temperature after heat-shock was, We thank Claudia Prinz, Barbara Kramer, and Christel van Oijen however, not as rapid as expected from the fast recovery of for technical assistance. We are grateful to Dr. Johanningmeier, Dl protein synthesis (12), suggesting that the total response Bochum, FRG, for providing the antiserum against the Dl protein. was more complex and involved an increased turnover of Dl after a heat-shock treatment. At low-temperature photoinhi- LITERATURE CITED bition was also accompanied by a depletion of the content of 1. Arnon DJ (1949) Copper enzymes in isolated chloroplasts. Plant functional Dl. At approximately 0°C, the Dl protein was Physiol 24: 1-15 largely eliminated by photocleavage in our experiments with 2. Aro EM, Hundal T, Carlberg I, Andersson B (1990) In vitro rye, whereas the inactivated catalase was not cleaved and its studies on light-induced inhibition of photosystem II and D1- proteolytic breakdown retarded at low temperature. The pos- protein degradation at low temperatures. Biochim Biophys Acta 1019: 269-275 sibility of a direct photocleavage of the Dl protein mediated 3. Asada K, Takahashi M (1987) Production and scavenging by the production of triplet Chl, singlet oxygen, or oxygen of active oxygen in photosynthesis. In DJ Kyle, CB Osmond, radicals has been discussed (4) but was not observed with CJ Amtzen, eds, Photoinhibition. Elsevier, New York, pp isolated thylakoids in vitro, even at 20C (2). In isolated 227-287 Dl was 4. Barber J, Andersson B (1992) Too much of a good thing: light thylakoids, the photodamaged protein degraded by can be bad for photosynthesis. Trends Biochem Sci 17: 61-66 a subsequent endoproteolytic process that appears to be due 5. Feierabend J (1970) Proteinsynthese und Enzymbildung in to an autoproteolytic activity of the reaction center itself (4). Keimlingen bei niedrigen Wachstumstemperaturen und ihre Greer et al. (11) presented evidence that in barley leaves Beziehungen zum Cytokininhaushalt. Z Pflanzenphysiol 62: photoinhibition at low temperature was a consequence of the 70-82 6. Feierabend J, Beevers H (1972) Developmental studies on mi- strong inhibition of repair processes. Photoinactivated reac- crobodies in wheat leaves. I. Conditions influencing enzyme tion centers of PSII appear to be transformed in such a way development. Plant Physiol 49: 28-32 that they mediate thermal energy dissipation and serve as 7. Feierabend J, Engel S (1986) Photoinactivation of catalase in a means of protection from excess excitation energy (11, vitro and in leaves. Arch Biochem Biophys 251: 567-576 8. Feierabend J, Kemmerich P (1983) Mode of interference of 16, 17). chlorosis-inducing herbicides with peroxisomal enzyme ac- Further elucidation is needed for why chilling-tolerant tivities. Physiol Plant 57: 346-351 plants in the range of 150C are less light sensitive than 9. Feierabend J, Schrader-Reichhardt U (1976) Biochemical dif- susceptible species. This may involve more effective antioxi- ferentiation of plastids and other organelles in rye leaves with dative protection or a greater capacity of repair as suggested a high-temperature-induced deficiency of plastid ribosomes. Planta 129: 133-145 by Greer (10). The rate of protein formation in leaves of rye, 10. 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