PhowsynthesisResearch 48: 247-254, 1996. (~) 1996KluwerAcademicPublishers. Printedin the Netherlands. Regular paper

Light-dependent modification of Photosystem II in spinach leaves K e v i n O x b o r o u g h 1,3, L a d i s l a v N e d b a l 1'4, R o g e r A. C h y l l a 1'5 & J o h n W h i t m a r s h 1,2,*

1Department of Plant Biology, University of lllinois, Urbana, IL 61801, USA; 2photosynthesis Research Unit, Agricultural Research Service/USDA, 1201 W. Gregory Drive, Urbana, IL 61801, USA; 3present address: Department of Biological and Chemical Sciences, University of Essex, Wivenhoe Park, Colchester, Essex C04 3SQ, UK; 4present address: Institute of Microbiology, Opatovicky mlyn, 37981 Trebon, Czech Republic; 5present address: Department of Biochemistry, University of Wisconsin, Madison, W153706, USA; *Corresponding author Received 13 November1995; accepted in revised form 23 January 1996

Key words: electrochromic shift, chlorophyll fluorescence, inactive Photosystem II, light adaptation

Abstract In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electrons to Photosystem I. Although these 'inactive' PS II centers are capable of reducing the primary quinone acceptor, QA, oxidation of Q~, occurs approximately 1000 times more slowly than at 'active' centers. Previous studies based on dark-adapted leaves show that minimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be significantly impaired by the presence of inactive centers. The objective of the work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity within leaves did not indicate any increase in the concentration of active PS II centers during light treatments between 10 s and 5 min, showing that inactive centers are not converted to active centers during light treatment. Light-induced modification of inactive PS II centers did occur, however, such that 75% of these centers were unable to sustain stable charge separation. In addition, the maximum yield of chlorophyll fluorescence associated with inactive PS II centers decreased substantially, despite the lack of any overall quenching of the maximum fluorescence yield. The effect of light treatment on inactive centers was reversed in the dark within 10-20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the photosynthetic membrane. One possibility is that inactive centers are converted into centers that quench fluorescence by formation of a radical, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed excitation energy is dissipated thermally, through electron cycling at the reaction center.

Abbreviations: AA518-absorbance change at 518 nm, reflecting the formation of an electric field across the thylakoid membrane; AAFL1 - amplitude of the fast (< 100 ms) phase ofAA518 induced by the first of two saturating, single-turnover flashes spaced 30 ms apart; AAFL2- amplitude of the fast (< 100 ms) phase of AA518 induced by the second of two saturating, single-turnover flashes spaced 50 ms apart; DCBQ-2,6-dichloro-p-benzoquinone; F o - yield of chlorophyll fluorescence when QA is fully oxidized; F m - yield of chlorophyll fluorescence when QA is fully reduced; F x - y i e l d of chlorophyll fluorescence when QA is fully reduced at inactive PS II centers, but fully oxidized at active PS II centers; Pheo-pheophytin; P 6 8 0 - t h e primary donor of Photosystem II; PPFD photosynthetic photon flux density; Q A - primary quinone acceptor of PS II; QB -secondary quinone acceptor of PS II

248 Introduction Although most PS II reaction centers use light energy to drive the oxidation of water and the reduction of plastoquinone, a significant proportion are unable to transfer electrons to the plastoquinone pool at physiologically significant rates. Experiments using higher plants, algae, and cyanobacteria indicate that inactive PS II centers are a common feature of oxygenic organisms (Chylla et al. 1987; Chylla and Whitmarsh 1989; Chylla and Whitmarsh 1990a,b; Graan and Ort 1986; Henrysson and Sundby 1990; Lavergne and Leci 1993; Melis 1985; Nedbal et al. 1991; van Wijk et al. 1993). In vivo measurements show that as much as one-third of the Photosystem II complexes in spinach leaves are inactive (Chylla and Whitmarsh 1989). Because of technical limitations, virtually all measurements of inactive PS II centers have been made using dark-adapted material. Under these conditions, the capacity of inactive PS II centers for water oxidation seems unimpaired, as are the primary photochemical events leading to the reduction of QA (stable charge-separation). However, measurements of AA518 indicate that the reoxidation of QA at inactive PS II centers is approximately 1000 times slower than at active centers (e.g., Lee and Whitmarsh, 1989); exhibiting a 2 s half-time (Chylla et al. 1987; Chylla and Whitmarsh 1989, 1990). Measurements of the electrochromic shift (Chylla and Whitmarsh 1990) and flash-induced proton release due to water oxidation at different light intensities indicate that the antenna system serving each inactive PS II center is approximately half the size of that serving each active center: 110 and 230 chlorophyll molecules, respectively (Nedbal et al. 1991). Membrane fractionation studies indicate that stromal membranes are enriched in inactive PS II centers (Hendrysson and Sundby 1990), but inactive centers are present in granal membranes as well (Chylla 1990). These differences between the antenna size and membrane distribution of active and inactive PS II, are shared by PS IIa and PS IIt~ (Melis 1985). However, the inevitable question of whether or not inactive PS II centers and PS IIa represent the same sub-fraction of the PS II pool remains unanswered. Evidence exists to suggest that both the rate of QA oxidation in inactive PS II centers and the lightsaturated rate of oxygen evolution are accelerated by the addition of certain quinones (e.g., DCBQ) to thylakoid membrane preparations (Graan and Ort 1986; Hendrysson and Sundby 1990; Nedbal et al. 1991).

These data were explained in terms of an ability of these quinones to oxidize QA at inactive PS II centers at a much faster rate than plastoquinone. An alternative explanation has been put forward by Lavergne and Leci (1993), who suggest that the effect of DCBQ on both the rate of oxygen evolution and chlorophyll fluorescence yield arises from an enhancement of the rate at which QA is oxidized at a proportion of active PS II centers. It is proposed that the slow oxidation of QA at these centers results from their location within regions of the thylakoid membrane within which the oxidation ofplastoquinol is impaired. These apparently conflicting views will be discussed elsewhere (Whitmarsh et al., manuscript in preparation). In the context of this study, it is important to note that inactive PS II centers have been detected in leaves and isolated thylakoid membranes without the use of added quinones and, usually, after only two turnovers of reaction centers (e.g., Chylla et al. 1987; Chylla and Whitmarsh 1990a,b; Henrysson and Sundby 1990; Lee and Whitmarsh 1989; Melis 1985; Snel et al. 1992; van Wijk et al. 1992) and that the controversy surrounding the action of DCBQ has little impact on the conclusions summarized above. Why plants would devote resources for the construction of reaction center complexes that do not significantly contribute to energy transduction is unknown. Taking into account their relative concentration, activity and antenna size, it has been estimated that inactive PS II centers could reduce the quantum efficiency of oxygen evolution by approximately 10% (Nedbal and Whitmarsh 1992). A caveat to this conclusion is that the function of inactive PS II centers in light-adapted leaves is not known. Consequently, the possibility exists that these centers are modified in the light, such that their impact on the quantum yield of oxygen evolution is reduced. For example, it has been suggested that inactive PS II centers are modified by high light such that they become capable of electron transfer to the plastoquinone pool (Neale and Melis 1990); thereby acting as a reserve pool that partially compensates for normally active PS II centers under photoinhibitory conditions. One aim of this study was to determine whether such a conversion occurs in vivo. While our data do indicate that inactive PS II centers are modified by light, this modification does not appear to involve conversion to an active form. Our data reveal that, following exposure to light, stable charge-separation is inhibited within 50-75% of inactive PS II centers and the yield of variable chlorophyll fluorescence associated with these centers is decreased

249 by more than 50%. These data are consistent with, but not proof of, the-light-induced formation of cyclic electron transport within inactive PS II centers.

Materials and methods

Plants Spinach plants used for whole leaf measurements (Spinacia oleracea) were grown hydroponically from seeds (Hybrid 424, Ferry-Morse Seed Co., Mountain View, CA) in a controlled environment chamber as described elsewhere (Robinson and Portis 1988). Light was provided by a combination of VHO cool-white fluorescent and incandescent lamps which provided 400-600 #mol m -2 S - l PPFD.

Light-treatment of leaves Spinach leaves were dark-adapted for at least 2 h prior to light-treatment. Actinic illumination was provided by a 250 W tungsten/halogen lamp, filtered by a Melles-Griot heat reflecting mirror (03 MHG 007), and a red glass filter (Coming CS2-58). The abaxial surface of the leaf was exposed to a gentle flow of moist air throughout the illumination period.

Measurement of AA518 in leaves A kinetic optical spectrophotometer, similar to that described elsewhere (Chylla etal. 1987), was used to measure AA518 induced by saturating, single-turnover flashes. Red actinic flashes were provided by a Xenon flash lamp (FX-200, EG&G, Salem, MA), filtered by a Melles-Griot heat reflecting mirror (03 MHG 007) and a red glass filter (Coming CS 2-58). The measuring light and actinic flashes were transmitted through a bifurcated light guide positioned close to the adaxial surface of the leaf. Data was acquired at a bandwidth of 10 kHz. Data acquisition and the triggering of shutters was achieved using a DOS-based computer equipped with a CIO-DAS16Jr A-to-D card and a CIO-CTR10 counter/timer card (Computer Boards Inc., Mansfield, MA 02048). Measurements from different leaves were added to improve the signal to noise ratio.

Measurement of chlorophyll fluorescence from leaves Saturating pulses of 12,000/~mol m -2 s -1 white light (for measurement of Fm) were provided from a tung-

sten/halogen lamp filtered by a Melles-Griot heatreflecting mirror (03 MHG 007). Moist air was blown across the abaxial surface of the leaf during periods of continuous illumination. Saturating, single-turnover flashes were provided by a Xenon flash lamp (FX-200, EG&G, Salem, MA) filtered by a Melles-Griot heatreflecting mirror (03 MHG 007). Both lamps were connected to branches of a Walz F100 quadfurcated fiber-optic (Walz GmbH, Effeltrich, Germany). The other two branches of the fiber optic were connected to the emitter/detector unit of a Walz PAM 100 fluorometer. The common end of the fiber optic was positioned 4 mm from the adaxial leaf surface. Uniblitz shutters were located between the leaf and fiber optic (leaf shutter) and in front of the tungsten/halogen lamp (lamp shutter) used to provide both actinic illumination and light-saturating pulses. Unless otherwise specified, the protocol used for measurement of Fo, Fx and Fm was as follows: (1) A baseline was recorded over 25 ms with the PAM fluorometer at the 1.6 kHz setting. (2) The leaf shutter was opened and Fo determined over 25 ms. (3) A saturating, single-turnover flash was delivered to the leaf surface and, simultaneously, the fluorometer was switched to the 100 kHz mode (increasing the measuring beam intensity from less than 0.5 #mol m -2 s - l to approximately 20 #mol m -2 s - l ) . Fx was determined by extrapolating the pseudo-linear phase of the decay of chlorophyll fluorescence back to the time of the flash. (5) The lamp shutter was opened, and Fm determined over 250 ms. Data were acquired at a bandwidth of 5 kHz for measurement of Fo and Fx and 500 Hz for measurement of Fm.

Results

Active and inactive PS H reaction centers in leaves Two independent methods were used to analyze the effect of preillumination on PS II activity in leaves: measurements of the flash-induced electrochromic shift, and measurements of the relative chlorophyll fluorescence yield. The flash-induced electrochromic shift is an absorbance change caused by the formation of a transmembrane electric field (reviewed by Witt 1979) and provides a quantitative measure of reaction center turnover. Here we determined changes in the relative proportions of active centers (PS II + PS I) and inactive PS II centers capable of stable chargeseparation by measuring the absorbance change at 518 nm induced by two saturating, single-turnover flashes

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Flash Figure 1. The application of a saturating, single-turnover flash to a dark-adapted spinach leaf induces an absorbance increase at 518 nm (AA518). The amplitude of the fast phase of the AA518 induced by the first flash (AAFLI)is proportional to charge-separation at all reaction centers (PS II + PS I). Because QA is reoxidized very slowly at inactive PS II centers, most of these centers remain incapable of stable charge-separation at the time of the second flash, 30 ms later. Consequently, the contribution of inactive PS II centers to the AA518 induced by the second flash (AAFL2)is negligible. The difference between AAFLl and AAFL2 (demonstrated in the lower half of the figure) therefore reflects the number of inactive PS II centers.

spaced 30 m s apart. D u r i n g the first flash, all reaction centers (active a n d inactive PS II + PS I) u n d e r g o rapid charge separation, generating an electric field across the thylakoid m e m b r a n e . The formation of this field causes a shift in the absorption spectrum of certain carotenoid and chlorophyll b molecules, which can be m o n i t o r e d by an a b s o r b a n c e increase centered at 518 n m (AA518). The electrochromic shift measured as the AA518 n m is proportional to the n u m b e r of centers u n d e r g o i n g charge-separation d u r i n g the flash. D u r i n g the 30 ms interval before the second flash, the QA in active PS II centers is reoxidized and P700 + in PS I centers is rereduced, e n a b l i n g them to under-

go a second charge transfer reaction. Since inactive PS II centers are re-oxidized slowly, with a half-time of 1.5-2 s (Chylla et al. 1987), most of these centers r e m a i n closed at the time of the second flash. Consequently, the extent of the AA518 i n d u c e d by the second flash only represents charge-separation at active PS II centers + P S I centers (referred to collectively as active centers within the text) and the difference b e t w e e n the AA518 i n d u c e d by the first flash (AAFLI) and the second flash (AAFL2) is attributed to stable charge-separation at inactive PS II centers (Chylla and W h i t m a r s h 1990). This difference is s h o w n by the trace in Figure 1, which shows the AA518 i n d u c e d by two saturating, single-turnover flashes applied 30 ms apart to a dark-adapted spinach leaf. The yield of chlorophyll fluorescence from plants is highly sensitive to the redox-state of QA, b e i n g at a m i n i m u m (Fo) w h e n QA is fully oxidized and at a m a x i m u m (Fm) w h e n QA is fully reduced. We used chlorophyll fluorescence to m o n i t o r the reoxidation kinetics of QA after the application of a saturating, singleturnover flash. It was shown previously that a fraction of PS II reaction centers exhibit slow QA reoxidation kinetics (e.g., Chylla and W h i t m a r s h 1987, 1990b;

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Effect o f illumination on active and inactive PS H centers in leaves Lavergne and Trissl 1995; Melis 1985) and that the slow decay in fluorescence yield attributable to this reoxidation follows the same kinetics as the recovery o f AAFLt -- AAFL2 (Nedbal and Whitmarsh 1992). These data show that the slow fluorescence decay is due to the oxidation o f QA at inactive PS II centers. The m a x i m u m fluorescence yield from inactive PS II centers (Fx) was determined by applying a saturating, single-turnover flash to a leaf illuminated with a 'measuring b e a m ' o f sufficiently low intensity to leave QA in the oxidized state at virtually all PS II centers (at the Fo fluorescence level). Following the flash (see Figure 2), oxidation o f QA at active PS II centers occurs within a few ms (and, consequently, is not detected by the measuring system). Conversely, complete oxidation of QA at inactive PS II centers requires approximately 1 min. Fx can therefore be estimated by extrapolating the pseudo-linear portion o f the decay back to the time of the flash. Within dark adapted spinach leaves, it was found that the variable fluorescence yield from inactive PS II centers (Fx - Fo) contributed 12-15% of the total variable fluorescence (Fm - Fo), consistent with the small antenna size o f inactive centers (Chylla and Whitmarsh 1990a; Nedbal et al. 1991).

Illumination o f spinach leaves has a significant impact on the ability of inactive PS II centers to undergo stable charge-separation. This is evident from the data presented in Figure 3, which show PS II activity monitored by the two flash AA518 method described above, at various times after 10 s illumination with 900 # m o l m - 2 s - ] red light. The amplitude o f AAFL2 (active PS II + PS I) is the same before and after illumination, indicating that active PS II centers are unaffected by the light-treatment. However, the amplitude of A A ~ (total PS II + PS I) is approximately 15% lower after 10 s illumination of continuous illumination, suggesting that approximately 75% of inactive PS II centers are incapable of stable charge separation following light exposure. This effect is reversed in the dark within 1 5 20 min. Illuminating leaves for a period o f between 10 s, 1 min, or 5 min gave very similar results (Figure 4). Illuminating leaves with 20 min o f red or blue light also produced the same effect (Chylla 1990). These data show that inactive PS II centers are modified by light, but are not converted to an active form. Although a decrease in AAFLl -- AAFL2 would be expected if a proportion o f the inactive PS II center pool were con-

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Illumination time (s) verted to an active state by light-treatment, such a change would result in an increase in AAFL2, rather than the decrease in AAFL1 observed in light-adapted leaves. The effect of illuminating a leaf with 900 #mol m -2 s -1 red light for 10 s or 1 min on the variable fluorescence yield from inactive PS II centers (Fx Fo) and total PS II centers (Fm - Fo) is shown in Figure 5. Following either light treatment, Fx - Fo (Figure 5, upper panel) decreased to less than 60% of the dark-adapted level. This effect was reversed in the dark. Increasing the period of illumination to 20 min did not increase the maximum quenching of Fx - Fo. However, a lag-phase that increased with longer illumination periods was observed for the dark recovery o f F x - Fo. In contrast, Figure 4 shows that the lag-phase observed for recovery of the electrochromic shift in inactive centers does not appear to depend on the illumination period. It is noteworthy that Fm - Fo was virtually unaffected by illumination (Figure 5, lower panel). This

Figure 6. Kinetics of the increase in chlorophyll fluorescence induced by 1.8 /~mol m -2 s - l , within a detached spinach leaf, shown on linear (top panel) and semi-log (bottom panel) plots. The top trace in each panel is after 2 h dark-adaptation, while the bottom trace is after 10 s illumination at 900 #mol m -2 s - 1 plus 2 min darkadaptation. Each trace represents the mean of 10 measurements.

is a key point, because under constant illumination the variable yield of chlorophyll fluorescence may be lowered by one or more 'non-photochemical' processes which act independently of the redox-state of QA (reviewed by Demmig-Adams 1990; Krause and Weis 1991). If the illumination period is short (a few min) and no stress is placed upon the plant, this quenching is normally reversed within one min dark-recovery. In our experiments the fluorescence yield at Fo was not changed by light treatment. Because the oxidation of QA at inactive PS II centers is very slow, even low intensity illumination is sufficient to cause the accumulation of QA at inactive PS II centers. The upper panel of Figure 6 shows the effect

253 of 1.8 #mol m - 2 s - I, before and after 10 s illumination at 900 #mol m -2 s - I followed by 2 min dark-recovery. The lower panel of Figure 6 shows iterative first-order curve-fits to these data. Both traces appear first-order, indicating a lack of connectivity among inactive PS II centers (discussed by Chylla and Whitmarsh 1989).

Discussion Illumination of spinach leaves with continuous saturating light modifies the performance of inactive PS II centers. The data presented in Figures 3 and 4 show that after exposure periods of 10 s or longer, inactive PS II centers are unable to undergo stable charge separation induced by a single turnover flash. This effect is revealed by a decrease of approximately 75% in the amplitude of the fast phase of AA518 due to inactive PS II centers. The fact that the AA518 due to active centers was not altered by light exposure, shows that the inactive were not converted to active PS II centers. As noted in the introduction, it was previously suggested that inactive PS II centers are changed by high light such that they become capable of electron transfer to the plastoquinone pool (Neale and Melis 1990). Our results show that for spinach leaves exposed to saturating light, light treatment does not increase the contribution of inactive PS II centers to linear electron transport. The simplest explanation for the decrease in the contribution of inactive PS II centers to the AA518 after light treatment is that stable charge-separation is prevented at a majority of the inactive PS II center pool. To explain this we considered the possibility that QA is stabilized in inactive centers. However, this straightforward interpretation is complicated by the observation that light treatment causes the variable fluorescence due to inactive centers to decrease concomitant with the decrease in the electrochromic shift (e.g., Figure 5). Although a decrease in (Fx Fo), the variable fluorescence due to inactive centers, would be expected if QA were stabilized in inactive PS II centers, this would occur through an increase in Fo, rather than the decrease in Fx seen in Figures 5 and 6. We also considered the possibility that the decrease in the variable fluorescence due to inactive centers is brought about through an increase in non-photochemical quenching of chlorophyll fluorescence within the light-harvesting system associated with inactive PS II centers (Demmig-Adams 1990; Krause and Weis 1991). A strong argument against

this explanation is that there is no quenching of Fm following 10 s or 1 min light treatment in Figure 5. Quenching of Fm was observed after light treatments of longer than 5 min, but even this was reversed more quickly than the quenching of Fx, indicating that different mechanisms are involved in the two types of quenching. Fluorescence quenching in inactive PS II during light treatment can be explained if the connectivity between the pigment beds associated with inactive and active PS II centers were increased, allowing for energy transfer from closed, inactive PS II centers to open, active PS II centers. This is an attractive idea, not only because it would explain the observed effects of light-treatment on Fx and Fm, but it would decrease the impact of inactive PS II centers on the quantum yield of PS II photochemistry. It also fits well with the observed maximum effect of light-treatment on quenching of Fx - Fo (about 45% in Figure 5) since energy-transfer among individual PS II reaction centers is inherently inefficient and, even if energy-transfer occurred from all of the inactive PS II centers, would not result in total quenching of chlorophyll fluorescence from these centers. Due to the observation that the fluorescence rise between Fo and Fx still appears to be first-order after light-treatment (Figure 6), we have to conclude that any increase in connectivity between the inactive and active PS II center pools is not paralleled by any increase in connectivity among inactive PS II centers. Light treatment of isolated thylakoid membranes gave results similar to those observed in leaves, a decrease in the contribution of inactive PS II centers to AA518 and a decrease in the variable fluorescence due to inactive centers (data not shown). However, there were two important differences. The lightinduced modification of the inactive PS II centers was not reversible in the dark, and some photoinhibition of active PS II centers was observed. Although an increase in energy transfer between the inactive and active PS II center pools can account for the effect of light treatment on chlorophyll fluorescence within spinach leaves (Figure 5), it does not account for the parallel effect on charge-separation at inactive PS II centers, as measured by AA518 (Figures 3 and 4). Consequently, an explanation of the effect of light treatment on both the yield of chlorophyll fluorescence from inactive PS II centers and the number of inactive PS II centers capable of stable charge-separation requires either an alternative to the suggestions made above, or the stimulation of two separate processes by light treatment, which are reversed with similar kinet-

254 ics during dark-recovery. O n e plausible explanation for both the l i g h t - i n d u c e d d e c r e a s e in the variable fluoresc e n c e and in the e l e c t r o c h r o m i c shift due to inactive PS II centers in spinach leaves is that light treatment stimulates the f o r m a t i o n o f a cyclic p a t h w a y at inactive PS II centers. S u c h a putative electron cycle w o u l d h a v e to be rapid to a c c o u n t for the decreased extent o f the e l e c t r o c h r o m i c shift and the q u e n c h i n g o f variable fluorescence o b s e r v e d u n d e r our e x p e r i m e n t a l conditions. W h e t h e r such a c y c l e c o u l d b e c o m e i n v o l v e d in q u e n c h i n g excitation e n e r g y to help protect active PS II centers f r o m e x c e s s light remains to be investigated. This w o u l d require c o n n e c t i v i t y b e t w e e n the active and i n a c t i v e PS II centers, for w h i c h there is no evid e n c e f r o m the data presented here. A n o t h e r possible e x p l a n a t i o n is that in i l l u m i n a t e d leaves, inactive PS II centers are c o n v e r t e d into a r e d o x state that q u e n c h es f l u o r e s c e n c e but does not support p h o t o c h e m i s t r y (e.g., a r e d o x state c o n t a i n i n g P h e o - or P680+).

Acknowledgements This w o r k was supported in part by the National R e s e a r c h Initiative C o m p e t i t i v e Grants P r o g r a m o f the U S D e p a r t m e n t o f A g r i c u l t u r e (#88-37130-3366).

References Chylla RA (1990) Observation and characterization of inactive Photosystem II reaction centers. PhD Thesis. University of Illinois, Urbana, IL Chylla RA and Whitmarsh J (1989) Inactive Photosystem II complexes in leaves: Turnover rate and quantitation. Plant Physiol 90:765-772 Chylla RA and Whitmarsh J (1990a) Light saturation response of inactive Photosystem II reaction centers in spinach. Photosynth Res 25:39-48 Chylla RA and Whitmarsh J (1990b) Measurement of the complete oxidation kinetics of QA in spinach leaves using flash fluorescence. In: Baltscheffsky M (ed) Current Research in Photosynthesis, Vol 1, pp 383-386. Kluwer Academic Publishers, Dordrecht, The Netherlands Chylla RA, Garab G and Whitmarsh J (1987) Evidence for slow turnover in a fraction of Photosystem II complexes in thylakoid membranes. Biochim Biophys Acta 894:562-571

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Light-dependent modification of Photosystem II in spinach leaves.

In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electr...
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