Photosynthesis Research 31: 49-56, 1992. (~) 1992 Kluwer Academic Publishers. Printed in the Netherlands.

Regular paper

Quantitative study of state 1-state 2 transitions in broken chloroplastscomparison to in-vivo properties Gur Braun & Shmuel Malkin Biochemistry Department, The Weizmann Institute of Science, Rehovot 76100, Israel Received 20 March 1991; accepted in revised form 25 October 1991

Key words: absorption cross-section, chlorophyll fluorescence, light distribution imbalance, light harvesting complex Abstract

A detailed quantitative study was conducted on state 1-state 2 transition and its reversal in broken chloroplasts by modulated fluorimetry. The characteristics of the transition obtained supported other previous in-vitro findings. More importantly, a very close quantitative similarity was obtained under suitable conditions to previous in-vivo studies, particularly in approaching a constancy of Fm/F 0 during the transition and the equality of the fractional change of these fluorescence parameters with the calculated light distribution fraction to PS II. This confirms that in broken chloroplasts too, the state transitions involve reciprocal changes in the absorption cross-sections of PS II and PS I.

Abbreviations: AMP-PNP-adenylylimidodiphosphate; L H C I I - l i g h t harvesting chlorophyll a/bprotein complex; M e V - methylviologen

Introduction

Previous experiments on state 1 to state 2 transition (Myers 1971, Fork and Satoh 1986, Williams and Allen 1987) in intact leaves showed clearly that the cross-sections of PS I and PS II for light 2 absorption change in a reciprocal complementary manner during the transition (Canaani and Malkin 1984, Canaani et al. 1984, Malkin et al. 1986). Photoacoustic experiments of modulated oxygen evolution indicated most convincingly, higher absorption cross-section for PS II and a smaller one for PSI in state 1 compared to state 2, while the sum of the absorption cross-sections (i.e., total cross-section) remained approximately constant (Canaani and Malkin 1984, Canaani et al. 1984). Modulated fluorimetry experiments indicated the same fractional changes in the parameters F m and F 0 which were also closely equal to the fractional change in light distribution to

PS II,/3, calculated independently (Malkin et al. 1986). Both photoacoustic and fluorescence measurements lead independently to the same conclusion. The model that stands behind these observations suggests a lateral movement of part of LHC-II between the two photosystems (when illumination conditions that cause state transitions are created) with final stronger associations to PS II or PS I, in states 1 or 2, respectively (Williams and Allen 1987). The different interactions in the different states result from LHC-II being either non-phosphorylated (state 1) or phosphorylated (state 2) (Williams and Allen 1987). Work on isolated thylakoid membranes (Horton and Black 1982) drew attention also to another effect of phosphorylation, namely that the negative charges created by LHC II phosphorylation weaken the effect of charge screening by the cations in the medium, causing partial

50 membrane destacking and resulting in sufficient lateral approach of at least part of PS II and PSI complexes to allow excitation energy-transfer between them. This will result in nearly equal rates of excitation ('spill-over'). Such effect mainly occurs when the concentration of the screening cations is much below saturation (e.g., around 1 mM of Mg+2). The fluorescence responses, particularly the ratio Fm/F 0, and the oxygen evolution responses would behave then differently, than in-vivo. Indeed, it was shown (Horton and Black 1982) that the F m level in phosphorylated membranes may be smaller by as much as 50-60% at about 1.5mM Mg ÷2, compared to control membranes. (cf. their Fig. 3). This is much more than the typical drop of less than about 20% in a typical in-vivo state 1-state 2 transition (Malkin et ah 1986). On the other hand, at saturating Mg ÷2 concentrations (5 mM or more), the change in F m w a s more moderate (about 25%) and thus, more typical (Horton and Black 1982). It became important to complement the above study by following directly the in-vitro state 1-state 2 transitions and particularly to measure both F m and F0, as well as the calculated imbalance, in order to check if and when the characteristics of the transition do approach those of the in-vivo ones. This becomes even more important in view of contradictory reports (Teller et ah 1984, Allen and Melis 1988) whether or not there is an increase of P S I activity upon the transition to state 2, as would be expected. Direct demonstration of state 1-state 2 transition in broken chloroplasts, using modulated fluorimetry, was made previously (Teller and Barber 1981), but it was not comprehensive enough to reach the above goal. It showed that the steady-state fluorescence (Fs) decreased with time in presence of light 2 upon addition of ATP, reflecting the transition to state 2. At the same time, the quenching of Fs by background far-red light became smaller. This experiment demonstrated particularly the requirement for ATP, while other aspects were not emphasized. In p a r t i c u l a r , F m was not measured so that it was impossible to obtain any quantitative information on the magnitude of the light distribution and the relation between its change and the changes in all fluorescence parameters.

Materials and methods

Broken chloroplasts were isolated from market lettuce and stored at a liquid nitrogen temperature (Farkas and Malkin 1979, Braun and Malkin 1990). The standard reaction mixture contained 20mM HEPES, pH7.3-7.4, 1/~M gramicidin D, 200/xM MeV as an electron acceptor and variable concentrations of NaCI and MgCI 2. The chloroplasts were suspended in the dark for at least 2 min prior to each experiment with total final chlorophyll concentration not exceeding 5/zgm1-1. Gramicidin-D was included in the reaction mixture to ensure that the chloroplasts were initially in a state of a large imbalance between PS II and PSI in light 2, closest to the in-vivo state 1 (Braun and Malkin 1990). In addition, this facilitated light-limiting conditions for electron transport. For the transition from state 1 to state 2, aliquots of ATP stock solution were added during illumination. All materials were of analytic grade. Biochemicals were purchased from Sigma. Modulated chlorophyll-a fluorescence was measured at room temperature as described previously (Braun and Malkin 1990). The modulated exciting light was nearly monochromatic (480 mm, bandwidth 5 nm) having an intensity in the light limiting range ( 0 . 5 n E c m -2 s-l). The fluorescence, isolated by a 683 nm interference filter, was detected with a silicon photodiode, processed by a lock-in amplifier and its amplitude registered on a strip chart recorder. Two types of non-modulated actinic irradiation (background light) were used: 1. A blue broad-band light (about 400-600 nm) isolated with Coming 4-96 glass (120nE cm -2 s-l), serving to saturate electron transport and thus, to cause the fluorescence to approach the F m level. 2. Far-red light (light 1) isolated with a 720 nm interference filter (30 nE cm -2 s -1), to oxidize (i.e., open) PS II reaction centers and thus to cause the modulated fluorescence to drop to a low level defined here as F 0. The modulated fluorescence level in the absence of any background light, F s, is situated always between F 0 and F m. The light activity distribution coefficients to PSII (/3) and P S I (a), for the common case

51 /3 > a, are related to chlorophyll fluorescence parameters as follows: First, the degree of openness of PS II reaction centers, f, is calculated from the experimental values of Fs, F 0 and F m (Malkin et al. 1986):

0.8

•~

• SO/ 0.6

f = (F m - Fs)/(F m -- Fo). m.

0.4 0.2

(for fl larger or equal to a - hence it is assumed that PS II reaction centers are all open). Thus, the ratio of photoactivities PS II/PS I is: ~la

If there is no waste of light energy (e.g., a + /3 = 1), it follows also that:

1

I , 2

1 4

I I 6 8 mM MgCl z

I I0

The effect of MgCl2 and NaC1 on the imbalance term 1) in broken chloroplasts. The standard reaction medium contained either 10mM NaCl (open circles) or 210 mM NaCl (closed circles) and the MgC12concentration was varied. Fo, FS and Fm were measured as described in Materials and methods (cf. also Fig. 2).

a=f/(l+f).

Our results were expressed frequently in terms of (/3/a) - 1, i.e., the relative deviation from full balance between the two photosystems (Braun and Malkin 1990): -

I 0

Fig. 1. (fl/a-

= 1/f .

/3=1/(1+f);

\210 me NaCl

f-

-? The need of balance in electron flow between the two photosystems in the steady-state, when no additional light is added requires:

o

= (1 -f)/f

.

In this case, no assumption about the waste of light energy is needed and the contribution of each photosystem alone to the imbalance is not attempted.

Results and discussion

Figure 1 describes the dependence of the imbalance term in an initial state following the dilution of the chloroplasts in the reaction medium as a function of MgC12 concentration, both in the presence or in the absence of a high (200 mM) NaCl concentration. Evidently, there was no specific dependence of the imbalance on MgCl 2 concentration except for that required to yield stacked membranes. When the membranes were already stacked by the high NaC1 concentration,

the imbalance was independent of MgCI 2 concentration albeit a somewhat higher imbalance was consistently obtained with low NaC1. Starting with a high level of light distribution imbalance in the presence of 5 mM MgC12 (termed 'state 1'), a transition to a lower level of imbalance (termed 'state 2') was initiated by the addition of 0.2 mM ATP to an illuminated chloroplast suspension with light 2 (Fig. 2). This was characterized by the appreciable decrease of F s (e.g., by 41% after 30 min) while F 0 was relatively less affected (e.g., by 20% after 30min), hence the concomitant decrease in the extent of the quenching by far-red light. This experiment complements the one reported by Teller and Barber (1981) in that it also shows the F m level. With stacked membranes (in presence of 100 or 200mM NaCl) but in the total absence of MgCl2, no transition occurred upon ATP addition. However, when the ATP (0.2mM) was supplemented with an equimolar amount of MgCl2, the transition to state 2 was resumed, indicating a minimum requirement of Mg ÷2, presumably in forming an A T P - M g ÷2 complex, which is probably the substrate for the kinase activity.

52

(a) Fm. . . .

10

mM

NoCI

5

mM

MgCl !

Fro*- . . . .

f'--~Fs*_

u

c

FS-- F

_. F,."

....

/

o u) 0

"-'---n

ff

,-r "Io

_o

Fo

S... o ._

"0 0

i

I

I

2 min

I

I

20 min

I

I

I min

111

I i

II II

tDJ'-"

,Io

I

I

20 rain Imin

I

B min

llll

m..oct

I mM

MgCI 2

F. .........

---Fro"

o c 0 LL '10 ¢P o

Fo---

---Fs' 7"~J

0 ~E

- ~ - ~ PO"

+

Imin 48t ,nm

?-Smin

I111

Imin t l h r .

III {

Imin

8min

I111

Fig. 2. Time course of fluorescence changes related to 'state 1'-'state 2' transitions in broken chloroplasts, at different illumination conditions and ATP addition. This figure also demonstrates the determination of F 0 and Fm. The standard reaction mixtures contained also 10 mM NaCI plus 5 mM MgCI 2 (Fig. 2a) or 110 mM NaC1 plus 1 mM MgCI 2 in Fig. 2b. Final ATP was added to a final concentration of 0.2 mM. Wavy arrows denote the turn-on (upward) and off (downward) of the modulated light. Thick arrows denote similarly the turn-on and off of the strong background light for F m determination and thin arrows denote likewise the turn-on and off of the far-red light for F 0 determination. F ' , F'~ and F 0 designate the fluorescence parameters for 'state 2'. F", F" and F~ are the fluorescence parameters obtained after the reversal to 'state 1'. The star superscript denotes those fluorescence parameters (with their level marked by dashed lines), which were obtained after 15 rain in a control experiment with 0.2 mM A M P - P N P added instead of ATP under the same conditions.

The experiments of Figs. 2a and 2b are independent in that they were made on two unrelated chloroplast preparations. They represent two separate examples differing in that a very appreciable decrease in F m is noted in Fig. 2a

while only a moderate decrease occurs in Fig. 2b. Since the transition time was quite long (about 30 min; this is probably because the invitro conditions were not as optimal as the invivo ones, and because of a possible partial

53 depletion of some essential components), one may expect also that (irrelevant) aging effects will be superimposed, affecting F m greatly. We have repeated a very large number of experiments like those shown in Fig. 2 in a variety of samples and noticed indeed aging effects in the majority of cases. One criterion for aging was an appreciable decline in F m in a control experiment, without the addition of ATP or with the addition of an inactive analogue of ATP like AMP-PNP. The experiment of Fig. 2a belongs to the majority of cases where aging was very noticeable, while Fig. 2b represents a minority which almost did not exhibit any aging. A recovery back to state 1 ought to occur in the darkness or with the addition of far-red light (Canaani and Malkin 1984, Canaani et al. 1984). With the preparation of Fig. 2a, only a partial recovery of fluorescence parameters occurred under illumination with both the modulated 480nm and the superimposed non-modulated far-red lights, as shown. A similar pattern of recovery occurred in the dark (not shown). The recovery of F m w a s particularly very small. This is consistent with the occurrence of considerable aging. On the other hand, with the preparation of Fig. 2b, a complete recovery of F0, F s and F m was accomplished in the dark. This indicates the relative absence of aging effects in the preparation of Fig. 2b. Under illumination with both lights, only a partial recovery of fluorescence parameters was obtained. This was true for the experiment of Fig. 2b as well as for all other experiments. It is to be noted that the different ionic media did not play any essential role in the difference obtained between the two experiments. Similar results were obtained with the preparation of Fig. 2a when the medium contained 100mM NaCI and 1 mM MgCI 2. In the last case, a slightly higher rate of transition towards state 2 was obtained (not shown). The experiment of Fig. 2b is therefore most suitable for numerical comparisons. In this experiment, the relative decreas~ in F m , 30min after ATP addition, was about 13%, the relative decrease in F 0 was about 9% and the relative decrease in the calculated /3 (dropping from about 0.6 in state 1 to about 0.52) was about 13.5%. It appears that F0, F m in particular and/3

change in an approximately parallel manner. In the experiment of Fig. 2a, where F m suffered a considerable aging effect, F m decreased by about 36% after 30 min. If we considered the drop of F m in the control, the percent decrease of F m in the transition to state 2 relative to the proper control was about 21%. /3 decreased during 30min from about 0.715 to 0.615 (a drop of about 14%) while F 0 changed by about 22%. During recovery under illumination with both lights, F 0 increased by about 15% and/3 by about 14%. The recovery of F m w a s very small (about 4.5%) due to the aging. Therefore, it also appears that aging affected F 0 too to some extent. More significantly, it turns out that/3 can still be calculated from F~, F 0 and F m despite the aging effect on their individual values and it appears from this calculation that there is a complete reversal of the imbalance despite the aging. Such calculations of/3 were repeated in other samples and one can conclude that the reversal in /3 is generally complete either in darkness or when far-red light is superimposed on the modulated light 2. In the following, we nevertheless have selected only those experiments in which aging effects were relatively small (not more than about 15% drop in F m after 15 min in a control experiment). Aging was random and could not be controlled. Parallel changes of F0, F m and/3 (when there are only minimal aging effects), are reflected in the following Figs. 3 and 4. Figure 3 describes a comparison between the Fm/F0 ratio obtained in the initial 'state 1' to the ratio obtained 15 min after the addition of 2mM ATP (Fig. 3a) or 0.2 mM ATP (Fig. 3b) as a function of M g C I 2 concentration. In Fig. 3a there are differences in this ratio between states 1 and 2, mostly pronounced at the low Mg +2 concentration range except at very low concentrations, where the extent of the state 1-state 2 transition itself was very small. With 0.2mM ATP (Fig. 3b), the differences between the two curves tended to disappear, except perhaps for a small range of concentrations around 1 mM MgC12. In both cases, as the MgC12 concentrations increased, the ratio F m / F 0 tended to approach similar values for both states 1 and 2, although being always larger by about 10-15% in the initial 'state 1', a difference which is probably caused by some aging.

54 I

l

'

I

'

5.4~ (a)

2.8 0 LL 2.E E 2.4 LL 2.2

I

'

o

I

'

o

I

'

I

s,ote ,

I

_

'

I

'

I

(b)

'

i

'

o

I

'

i

s~ot, ,

-

•

C

I'-I

1.8

II

,

0

I

-

z mM

ATP

,

I

2

,

4

I

,

6

I

8

ATP

,

,

I0 0

I

2

,

I

4

,

I

6

,

I

8

,

I

I0

mM MgCI 2

Fig. 3. The dependence of FJF0

o n M g C I 2 concentration in the initial 'state 1' (open circles) and in a state approaching 'state 2' (closed circles), obtained 15 min after the addition of either 2 mM ATP (Fig. 3a) or 0.2 mM ATP (Fig. 3b). The standard reaction mixture contained also 10 mM NaCl and with different MgCl2 concentrations. Other experimental details are as in Fig. 2.

'

~

I

'

I

'

1

'

I

'

I

5o

z

4o

?.\.

3o

-

z IJJ

~

z

I o

20 10 o

,

0

I

2

,

I

,

I

4 6 mM MgCI2

,

I

8

,

I

I0

Fig. 4. The percent change in fluorescence parameters Fm

(closed circles), F 0 (open circles) and the calculated /3 (triangles) between the initial 'state 1' and 15 min after the addition of 2mM ATP under light 2 illumination, as a function of the MgCI2 concentration. The standard reaction medium was supplemented with 10 mM NaC1 with different MgCI2 concentrations. Figure 4 shows the fractional change in the decrease of each of the p a r a m e t e r s Fm, F 0 and/3, after the addition of 2 m M A T P under illumination, as a function of MgCI 2 concentrations. T h e r e is a p r o n o u n c e d p e a k for the percent extent of the decrease in Fro, c o m p a r e d to the quite m o d e r a t e and shallow ones for the decrease in F 0 and/3. This p e a k in the decrease of

Fm, however, was traced to be an artifact due to the depletion of Mg +2 by chelation to A T E Indeed, the concentration of Mg ÷2 at the p e a k corresponds to the concentration of ATP, as verified by other experiments where the A T P concentration varied. Also, at this concentration, the drop in the fluorescence was much m o r e rapid than in a typical transition to state 2. F u r t h e r m o r e , a similar drop in F m was observed with A T P analogues, A D P and A M P - P N P , which at higher Mg ÷2 did not cause any specific effect. As in Fig. 3, the difference between the curves for Fm, F 0 a n d / 3 diminishes considerably and they approach each other as the MgCI 2 concentration is increased further. Again, the curve for F m deviated a little upward, presumably due to aging. H o w e v e r , one should note the close proximity of the curves for F 0 a n d / 3 at a wide range of Mg ÷2 concentrations. T w o conditions are c o m p a r e d in Fig. 5 for the effect of 0.2 m M ATP. A t low (1 m M ) Mg ÷2, the a p p a r e n t transition to state 2 is totally reversed relatively rapidly by addition of m o r e (5 m M ) Mg ÷2. This reflects the same effect as observed by H o r t o n and Black (1982), interpreted as an insufficient screening of the extra negative charges on L H C - I I by Mg ÷2, and hence, partial destacking of the membranes. The same effect can also be traced in Fig. 3b when the points in 'state 1' and in 'state 2' at 0 . 8 m M MgCI 2 are c o m p a r e d . A t such MgCI 2 concentration, the transition from 'state 1' to 'state 2' lowers Fm/F 0 ratio by m o r e than 50%. On the other hand, at

55 //'I

,

//

g

i~

o

/ ~

5 mM MgCl 2

1

III t

,\

-),'"

ATPN

II ~

.... ", :

. . . . . .

g LL I mM MgCl 2

,

_a :E

i

t t +MgCl2

+MgCI2 D I IOmin

11

rl

tl

480nm

Fig. 5. Comparison between the effects of low (1 mM - solid curve) and saturating (5 mM - dashed curve) Mg ÷2 concentration on the time course of the steady-state fluorescence, F, and F 0 and the effect of further Mg +2 addition in 'state 2'. The standard reaction mixture was supplemented with 10 mM NaCI and with either 1 mM or 5 mM MgCI 2. ATP was added to a final concentration of 0.2 mM. Other details are as in Fig. 2.

closely saturating (5mM) Mg ÷2 concentration, the apparent transition to state 2 is almost not affected by addition of more Mg ÷2 and represents more closely the same transition as the in-vivo one. The reverse dark transition depended consistently on Mg ÷2 concentration. In Fig. 6 we explored this dependence in the presence of high NaCI concentration. An experimental measure for the reverse transition was defined by the 'percent reversal' equal t o 100 X (/31d -- /32)/ (/31-/32), where/31 and/32 denote respectively, the values o f / 3 for the initial state and 15 min after ATP addition, and/31d denotes the/3 value obtained after 30 min incubation in the dark, in the reverse transition. Incidentally, in Fig. 6 the imbalance reversed to even a higher value than that existed initially. Figure 6 indicates a requirement of Mg ÷2, saturated at about 0.5 mM, probably to satisfy the phosphatase activity. Phosphatase involvement is consistent with the effect of 20 mM Na2MoO 4, a phosphatase inhibitor, which resulted in a suppression of about 60% of the reverse transition in saturation. A similar effect was observed with 10mM NaF (not shown). One of the quantitative measures for the state transitions used in the present study is the change in /3, calculated by assuming a +/3 = 1.

IO0 .,9.0 0

./

2 o

O-+Nn2MoO4

c

8 o rt

o I

I

1

I

0.5

I

1.5

2

mM MgCI 2

Fig. 6. The effect of MgCI2 concentration on the reverse 'state 2 to state 1' transition in the dark. The standard reaction medium was supplemented with 110 mM NaCI and the MgC12 concentration was varied. Fluorescence parameters were measured before and 15 min after 0.2 mM ATP addition under light 2 illumination. Then, the samples were darkened for 30 min either in the absence (closed circles) or the presence (open circles) of 20mM Na2MoO4 and the fluorescence parameters were measured again. The percent reversal to the initial state was calculated from the/3 values as described in the text.

A change in /3 thus reflects automatically a reciprocal change in a. The near agreement between the changes in /3 and F 0 (Fig. 4) agrees

56 with the concept of reciprocal changes in the absorption cross-sections of PS II and PS I during the state transitions (cf. Malkin et al. 1986), which supports the previous in-vivo observations on intact leaves (Canaani et al. 1984, Malkin et al. 1986). There are indeed other literature reports which demonstrate more directly an increase in PSI activity upon the transition to state 2 (Farchaus et al. 1982, Horton and Lee 1984, Teller et al. 1984, Deleplaire and Wollman 1985, Larsson et al. 1986). It is not clear what are the reasons behind the failure to observe such increased P S I activity reported also (Haworth and Melis 1982, Deng and Melis 1986, Allen and Melis 1988). To summarize, it is demonstrated here that with saturating cationic levels, the state 1-state 2 transitions in broken chloroplasts approach the in-vivo characteristics, namely, the transitions result from reciprocal changes in the absorption cross sections of PS II and PSI with a minimum requirement of Mg ÷2 to support the kinase and phosphatase activities. References Allen JF and Melis A (1988) The rate of P-700 photooxidation under continuous illumination is independent of state 1-state 2 transitions in the green alga Scenedesmus obliquus. Biochim Biophys Acta 933:95-106 Braun G and Malkin S (1990) Regulation of the imbalance in light excitation between Photosystem II and Photosystem I by cations and by the energized state of the thylakoid membrane. Biochim Biophys Acta 1017:79-90 Canaani O and Malkin S (1984) Distribution of light excitation in intact leaf between the two photosystems of photosynthesis: Changes in absorption cross-sections following State 1-State 2 transitions, Biochim Biophys Acta 766: 513-524 Canaani O, Barber J and Malkin S (1984) Evidence that phosphorylation and dephosphorylation regulate the distribution of excitation energy between the two photosystems of photosynthesis in-vivo: Photoacoustic and fluorimetric study on an intact leaf. Proc Natl Acad Sci USA 81:1614-1618

Deleplaire P and Wollman F-A (1985) Correlations between fluorescence and phosphorylation changes in thylakoid membranes of Chlamydornonas reinhardtii in-vivo: A kinetic analysis. Biochim Biophys Acta 809:277-283 Deng X and Melis A (1986) Phosphorylation of the lightharvesting complex II in higher plant chloroplasts: Effect on Photosystem II and Photosystem I absorption crosssection. Photobiochem Photobiophys 13:41-52 Farchaus JW, Widger WQR, Cramer WA and Dilley RA (1982) Kinase-induced changes in electron transport rates of spinach chloronlasts. Arch Biochem Biophys 217: 362367 Farkas DL and Malkin S (1979) Cold storage of isolated Class-C chloroplasts. Plant Physiol 64:942-947 Fork DC and Satoh K (1986) The control by state transitions of distribution of excitation energy in photosynthesis. Ann Rev Plant Physiol 37:335-361 Haworth P and Melis A (1983) Phosphorylation of chloroplast thylakoid membrane proteins does not increase the absorption cross-section of PS I. FEBS Lett 160:277-280 Horton P and Black MT (1982) On the nature of the fluorescence decrease due to phosphorylation of chloroplast membrane proteins. Biochim Biophys Acta 680: 2227 Horton P and Lee P (1984) Phosphorylation of chloroplast thylakoids decreases the maximum capacity of Photosystem II electron transport. Biochim Biophys Acta 767: 563-567 Larsson UK, Orgen E, Oquist G and Andersson B (1986) Electron transport and fluorescence studies on the functional interaction between phospho-LHC II and PSI in isolated stroma lamellae vesicles. Photochem Photobiophys 13:29-39 Malkin S, Teller A and Barber J (1986) Quantitative analysis of State 1-State 2 transitions in intact leaves using modulated fluorimetry - evidence for changes in the absorption cross-section of the two photosystems during state transitions. Biochim Biophys Acta 848:48-57 Myers J (1971) Enhancement studies in photosynthesis. Ann Rev Plant Physiol 22:289-312 Telfer A and Barber J (1981) ATP-dependent State 1-State 2 changes in isolated pea thylakoids. FEBS Lett 129: 161-165 Teller A, Bottin H, Barber J and Mathis P (1984) The effect of magnesium and phosphorylation of light-harvesting chlorophyll a / b - protein on the yield of P-700 photooxidation in pea chloroplasts. Biochim Biophys Acta 764: 324330 Williams WP and Allen JF (1987) State 1/State 2 changes in higher plants and algae. Photosynth Res 13:19-45