Modelling excitation energy transfer and trapping in the filamentous cyanobacterium Anabaena variabilis PCC 7120

Photosynthesis Research - The phycobilisome (PBS) serves as the major light-harvesting system, funnelling excitation energy to both photosystems (PS) in cyanobacteria and red algae. The picosecond...     

Mechanism of the light state transition in photosynthesis. I. Analysis of the kinetics of cytochrome f oxidation in State 1 and State 2 in the red alga, Porphyridium cruentum

The kinetics of photooxidation and reduction of cytochrome f were examined spectrophotometrically in the red alga Porphyridium cruentum in light State 1 and light State 2. Experiments were performed on intact cells that had been chemically fixed and stabilized in the light states. The cytochrome f turnover was measured during conditions of linear electron transport driven by both photosystems and during several cyclic reactions mediated by the long-wavelength Photosystem (PS) I. The data show that the rate of photooxidation of cytochrome f increased in State 2 when the cells were activated by subsaturating intensities of green light absorbed primarily by the phycobilisome. No differences in kinetics were found between algae in State 1 or State 2 when they were activated by light absorbed primarily by the chlorophyll of PS I. The results confirm that changes in energy distribution between the two photosystems occur as a result of the light state transition and verify that the redistribution of excitation results in the predicted changes in electron transport.     

State 1-state 2 adaptation in the cyanobacteria Synechocystis PCC 6714 wild type and Synechocystis PCC 6803 wild type and phycocyanin-less mutant

The mechanism of excitation energy distribution between the two photosystems (state transitions) is studied in Synechocystis 6714 wild type and in wild type and a mutant lacking phycocyanin of Synechocystis 6803. (i) Measurements of fluorescence transients and spectra demonstrate that state transitions in these cyanobacteria are controlled by changes in the efficiency of energy transfer from PS II to PS I (spillover) rather than by changes in association of the phycobilisomes to PS II (mobile antenna model). (ii) Ultrastructural study (freeze-fracture) shows that in the mutant the alignment of the PS II associated EF particles is prevalent in state 1 while the conversion to state 2 results in randomization of the EF particle distribution, as already observed in the wild type (Olive et al. 1986). In the mutant, the distance between the EF particle rows is smaller than in the wild type, probably because of the reduced size of the phycobilisomes. Since a parallel increase of spillover is not observed we suggest that the probability of excitation transfer between PS II units and between PS II and PS I depends on the mutual orientation of the photosystems rather than on their distance. (iii) Measurements of the redox state of the plastoquinone pool in state 1 obtained by PS I illumination and in state 2 obtained by various treatments (darkness, anaerobiosis and starvation) show that the plastoquinone pool is oxidized in state 1 and reduced in state 2 except in starved cells where it is still oxidized. In the latter case, no important decrease of ATP was observed. Thus, we propose that in Synechocystis the primary control of the state transitions is the redox state of a component of the cytochrome b ₆/f complex rather than that of the plastoquinone pool.Abbreviations DCCD dicyclohexylcarbodiimide - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - EF exoplasmic face - PQ plasto-quinone - PS photosystem - PBS phycobilisome     

Light Harvesting and State Transitions in Cyanobacteria

Cyanobacteria are oxygenic phototrophic prokaryotes and are considered to be the ancestors of chloroplasts. Their photosynthetic machinery is functionally equivalent in terms of primary photochemistry and photosynthetic electron transport. Fluorescence measurements and other techniques indicate that cyanobacteria, like plants, are capable of redirecting pathways of excitation energy transfer from light harvesting antennae to both photosystems. Cyanobacterial cells can reach two energetically different states, which are defined as “State 1” (obtained after preferential excitation of photosystem I) and “State 2” (preferential excitation of photosystem II). These states can be distinguished by static and time resolved fluorescence techniques. One of the most important conclusions reached so far is that the presence of both photosystems, as well as certain antenna components, are necessary for state transitions to occur. Spectroscopic evidence suggests that changes in the coupling state of the light harvesting antenna complexes (the phycobilisomes) to both photosystems occur during state transitions. The finding that the phycobilisome complexes are highly mobile on the surface of the thylakoid membrane (the mode of interaction with the thylakoid membrane is essentially unknown), has led to the proposal that they are in dynamic equilibrium with both photosystems and regulation of energy transfer is mediated by changes in affinity for either photosystem.     

Predicting Light Acclimation in Cyanobacteria from Nonphotochemical Quenching of Photosystem II Fluorescence,Which Reflects State Transitions in These Organisms

An important factor in photosynthetic ecophysiology is the light regime that a photobiont is acclimated to exploit. In a wide range of cyanobacteria and cyano-lichens, the easily measured fluorescence parameters, coefficient of nonphotochemical quenching of photosystem II variable fluorescence (qN) and nonphotochemical quenching, decline to a minimum near the acclimated growth light intensity. This characteristic pattern predicts the integrated light regime to which populations are acclimated, information that is particularly useful for cyanobacteria or cyano-lichens from habitats with highly variable light intensities. qN reflects processes that compete with photosystem II photochemistry for absorbed excitation energy. In cyanobacteria, we find no evidence for energy-dependent quenching mechanisms, which are the predominant components of qN in higher plants. Instead, in cyanobacteria, qN correlates closely with the excitation flow from the phycobilisome to photosystem I, indicating that qN reflects the state transition mechanism for equilibration of excitation from the phycobilisome to the two photosystems.     

ApcD is necessary for efficient energy transfer from phycobilisomes to photosystem I and helps to prevent photoinhibition in the cyanobacterium Synechococcus sp. PCC 7002

Phycobilisomes (PBS) are the major light-harvesting, protein-pigment complexes in cyanobacteria and red algae. PBS absorb and transfer light energy to photosystem (PS) II as well as PS I, and the distribution of light energy from PBS to the two photosystems is regulated by light conditions through a mechanism known as state transitions. In this study the quantum efficiency of excitation energy transfer from PBS to PS I in the cyanobacterium Synechococcus sp. PCC 7002 was determined, and the results showed that energy transfer from PBS to PS I is extremely efficient. The results further demonstrated that energy transfer from PBS to PS I occurred directly and that efficient energy transfer was dependent upon the allophycocyanin-B alpha subunit, ApcD. In the absence of ApcD, cells were unable to perform state transitions and were trapped in state 1. Action spectra showed that light energy transfer from PBS to PS I was severely impaired in the absence of ApcD. An apcD mutant grew more slowly than the wild type in light preferentially absorbed by phycobiliproteins and was more sensitive to high light intensity. On the other hand, a mutant lacking ApcF, which is required for efficient energy transfer from PBS to PS II, showed greater resistance to high light treatment. Therefore, state transitions in cyanobacteria have two roles: (1) they regulate light energy distribution between the two photosystems; and (2) they help to protect cells from the effects of light energy excess at high light intensities.     

Both spillover and light absorption cross-section changes are involved in the regulation of excitation energy distribution between the two photosystems during state transitions in wheat leaf

Weak red light-induced changes in chlorophyll fluorescence parameters and in the distribution of PS I and PS II in thylakoid membranes were measured in wheat leaves to investigate effective ways to alter the excitation energy distribution between the two photosystems during state transition in vivo. Both the chlorophyll fluorescence parameter Fm/Fo and F685/F735, the ratio of fluorescence yields of the two photosystems at low temperature (77 K), decreased when wheat leaves were illuminated by weak red light of 640 nm, however, Fm/Fo decreased to its minimum in a shorter time than F685/F735. When Photosystem (PS II) thylakoid (BBY) membranes from adequately dark-adapted leaves (control) and from red light-illuminated leaves were subjected to SDS-polyacrylamide gel electrophoresis under mildly denaturing conditions, PS I was almost absent in the control, but was present in the membranes from the leaves preilluminated with the weak red light. In consonance with this result, the content of Cu, measured by means of the energy dispersive X-ray microanalysis (EDX), increased in the central region, but decreased in the margin of the grana stacks from the leaves preilluminated by the red light as compared with the control. It is therefore suggested that: (i) both spillover and absorption cross-section changes are effective ways to alter the excitation energy distribution between the two photosystems during state transitions in vivo, and the change in spillover has a quicker response to the unbalanced light absorption of the two photosystems than the change in light absorption cross-section, and (ii) the migration of PS I towards the central region of grana stack during the transition to state 2 leads to the enhancement of excitation energy spillover from PS II to PS I.     

The fast and slow kinetics of chlorophyll a fluorescence induction in plants, algae and cyanobacteria: a viewpoint

The light-induced/dark-reversible changes in the chlorophyll (Chl) a fluorescence of photosynthetic cells and membranes in the μs-to-several min time window (fluorescence induction, FI; or Kautsky transient) reflect quantum yield changes (quenching/de-quenching) as well as changes in the number of Chls a in photosystem II (PS II; state transitions). Both relate to excitation trapping in PS II and the ensuing photosynthetic electron transport (PSET), and to secondary PSET effects, such as ion translocation across thylakoid membranes and filling or depletion of post-PS II and post-PS I pools of metabolites. In addition, high actinic light doses may depress Chl a fluorescence irreversibly (photoinhibitory lowering; q(I)). FI has been studied quite extensively in plants an algae (less so in cyanobacteria) as it affords a low resolution panoramic view of the photosynthesis process. Total FI comprises two transients, a fast initial (OPS; for Origin, Peak, Steady state) and a second slower transient (SMT; for Steady state, Maximum, Terminal state), whose details are characteristically different in eukaryotic (plants and algae) and prokaryotic (cyanobacteria) oxygenic photosynthetic organisms. In the former, maximal fluorescence output occurs at peak P, with peak M lying much lower or being absent, in which case the PSMT phases are replaced by a monotonous PT fluorescence decay. In contrast, in phycobilisome (PBS)-containing cyanobacteria maximal fluorescence occurs at M which lies much higher than peak P. It will be argued that this difference is caused by a fluorescence lowering trend (state 1 → 2 transition) that dominates the FI pattern of plants and algae, and correspondingly by a fluorescence increasing trend (state 2 → 1 transition) that dominates the FI of PBS-containing cyanobacteria. Characteristically, however, the FI pattern of the PBS-minus cyanobacterium Acaryochloris marina resembles the FI patterns of algae and plants and not of the PBS-containing cyanobacteria.     

Regulation of cyanobacterial photosynthesis determined from variable fluorescence yields of photosystem II

Abstract The cyanobacteria Fremyella diplosiphon 7601 and Synechocystis 6701 were grown in continuous cultures with monochromatic red light (680 nm). The distribution of light energy over photosystem I and II was determined from changes in PS II fluorescence at 685 nm. In both organisms, wavelengths absorbed primarily by chlorophyll a caused the high fluorescent state of PS II (State 1), while wavelengths absorbed by the phycobilisome led to low PS II fluorescence (State 2). Superimposing continuous light 2 on the excitation light yielded State 2 fluorescence patterns for Synechocystis 6701, while F. diplosiphon 7601 showed fluorescence patterns similar to state 1 → 2 transitions and changes in fluorescence yield were related to the intensity of the background light. Some ecological implications of energy (re)distribution in cyanobacterial photosynthesis are discussed.     

Photosynthetic pigment localization and thylakoid membrane morphology are altered in Synechocystis 6803 phycobilisome mutants

Cyanobacteria are oxygenic photosynthetic prokaryotes that are the progenitors of the chloroplasts of algae and plants. These organisms harvest light using large membrane-extrinsic phycobilisome antenna in addition to membrane-bound chlorophyll-containing proteins. Similar to eukaryotic photosynthetic organisms, cyanobacteria possess thylakoid membranes that house photosystem (PS) I and PSII, which drive the oxidation of water and the reduction of NADP+, respectively. While thylakoid morphology has been studied in some strains of cyanobacteria, the global distribution of PSI and PSII within the thylakoid membrane and the corresponding location of the light-harvesting phycobilisomes are not known in detail, and such information is required to understand the functioning of cyanobacterial photosynthesis on a larger scale. Here, we have addressed this question using a combination of electron microscopy and hyperspectral confocal fluorescence microscopy in wild-type Synechocystis species PCC 6803 and a series of mutants in which phycobilisomes are progressively truncated. We show that as the phycobilisome antenna is diminished, large-scale changes in thylakoid morphology are observed, accompanied by increased physical segregation of the two photosystems. Finally, we quantified the emission intensities originating from the two photosystems in vivo on a per cell basis to show that the PSI:PSII ratio is progressively decreased in the mutants. This results from both an increase in the amount of photosystem II and a decrease in the photosystem I concentration. We propose that these changes are an adaptive strategy that allows cells to balance the light absorption capabilities of photosystems I and II under light-limiting conditions.     

Kinetics and dynamics for light state transition in cyanobacterium Spirulina platensis cells

Light state transition in oxygenic organisms was defined as the ability to equalize the excitation of the two photosystems for maximal photosynthetic efficiency. In cyanobacteria, extensive researches on state transition have continuously provided new knowledge in the past decades but the molecular mechanism and physiological significance are still ambiguous. In this work, kinetics and dynamics of the transition from state 1 to state 2 in cyanobacterium Spirulina platensis cells were studied at different intensity of orange light from 10 to 120 μmol m(-2) s(-1). It was revealed that the state transition worked constantly independent of light intensity while the rates varied. The synchronous fluorescence kinetics for phycobilisome (PBS) and photosystem components indicated that the state transition was entirely regulated by "mobile PBS", and continuously changed fluorescence amplitudes suggested a series of intermediate states were involved between state 1 and state 2. The dynamic property of PBS movement during the state transition was revealed by (1,0) distribution of photo-linkable PBSs, indicating a collective movement of all PBSs. The results suggest that state transition in cyanobacteria possesses not only physiological but also photochemical significance.     

Sensing environmental temperature change through imbalances between energy supply and energy consumption: Redox state of photosystem II

A basic requirement of all photosynthetic organisms is a balance between overall energy supply through temperature-independent photochemical reactions and energy consumption through the temperature-dependent biochemical reactions of photosynthetic electron transport and contiguous metabolic pathways. Since the turnover of photosystem II (PSII) reaction centers is a limiting step in the conversion of light energy into ATP and NADPH, any energy imbalance may be sensed through modulation of the redox state of PSII. This can be estimated in vivo by chlorophyll a fluorescence as changes in the redox state of PSII, or photosystem II excitation pressure, which reflects changes in the redox poise of intersystem electron transport carriers. Through comparisons of photosynthetic adjustment, we show that growth at low temperature mimics growth at high light. We conclude that terrestrial plants, green algae and cyanobacteria do not respond to changes in growth temperature or growth irradiance per se, but rather, respond to changes in the redox state of intersystem electron transport as reflected by changes in PSII excitation pressure, We suggest that this chloroplastic redox sensing mechanism may be an important component for sensing abiotic stresses in general. Thus, in addition to its role in energy transduction, the chloroplast may also be considered a primary sensor of environmental change through a redox sensing/signalling mechanism that acts synergistically with other signal transduction pathways to elicit the appropriate molecular and physiological responses.     

Nitrite regulates distribution of excitation energy between the two photosystems by causing state transition.

The mechanism of distribution of absorbed excitation energy between the two photosystems in the presence of nitrite has been investigated in spinach (Spinacia oleracea L.) thylakoid membranes. Nitrite inhibited PS II activity (H(2)O --> DCPIP reaction) and enhanced PS I activity (DCPIPH(2) --> MV reaction). Nitrite decreased the F(v)/F(m) ratio measured at room temperature and increased the F(730)/F(685) ratio measured at low temperature (77 K). These results suggested that nitrite caused a decrease in the excitation energy available to PS II and transferred more energy to PS I by the mechanism of state transition. Measurement of fluorescence excitation spectra at 77 K showed that nitrite increased the absorption cross-section of PS I antenna at the expense of chlorophyll b and LHC II. Based on these observations we have suggested a role of nitrite in causing state transition.     

Inhibition by phosphate of light-state transitions in cyanobacterial cells

Light-state transitions in cyanobacteria are a rapid physiological adaptation mechanism which changes the distribution of excitation energy absorbed by the light-harvesting complexes between Photosystem II and Photosystem I. State transitions in two cyanobacterial species are shown to be inhibited by buffers containing 0.2–0.4 M phosphate. Both the state 1 and the state 2 transition are inhibited, so that cells may be locked in the state to which they were adapted before the addition of phosphate. The inhibition of the state 1 transition is due to inhibition of photosynthetic electron transport. However, the inhibition of the state 2 transition is probably due to a direct effect on the biochemical signal transduction pathway. The implications for the biochemical mechanism of state transitions are discussed.Abbreviations Chl chlorophyll - F fluorescence - HEPES N-[2-hydroxyethyl]piperazine-N-[2-ethanesulfonic acid] - P_i phosphate - PS photosystem     

Electron Transport Regulates Cellular Differentiation in the Filamentous Cyanobacterium Calothrix

Differentiation of the filamentous cyanobacteria Calothrix sp strains PCC 7601 and PCC 7504 is regulated by light spectral quality. Vegetative filaments differentiate motile, gas-vacuolated hormogonia after transfer to fresh medium and incubation under red light. Hormogonia are transient and give rise to vegetative filaments, or to heterocystous filaments if fixed nitrogen is lacking. If incubated under green light after transfer to fresh medium, vegetative filaments do not differentiate hormogonia but may produce heterocysts directly, even in the presence of combined nitrogen. We used inhibitors of thylakoid electron transport (3-[3,4-dichlorophenyl]-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) to show that the opposing effects of red and green light on cell differentiation arise through differential excitations of photosystems I and II. Red light excitation of photosystem I oxidizes the plastoquinone pool, stimulating differentiation of hormogonia and inhibiting heterocyst differentiation. Conversely, net reduction of plastoquinone by green light excitation of photosystem II inhibits differentiation of hormogonia and stimulates heterocyst differentiation. This photoperception mechanism is distinct from the light regulation of complementary chromatic adaptation of phycobilisome constituents. Although complementary chromatic adaptation operates independently of the photocontrol of cellular differentiation, these two regulatory processes are linked, because the general expression of phycobiliprotein genes is transiently repressed during hormogonium differentiation. In addition, absorbance by phycobilisomes largely determines the light wavelengths that excite photosystem II, and thus the wavelengths that can imbalance electron transport.     

Photoinhibition induced alterations in energy transfer process in phycobilisomes of PS II in the cyanobacterium, Spirulina platensis

Exposure of algae or plants to irradiance from above the light saturation point of photosynthesis is known as high light stress. This high light stress induces various responses including photoinhibition of the photosynthetic apparatus. The degree of photoinhibition could be clearly determined by measuring the parameters such as absorption and fluorescence of chromoproteins. In cyanobacteria and red algae, most of the photosystem (PS) II associated light harvesting is performed by a membrane attached complex called the phycobilisome (PBS). The effects of high intensity light (1000-4000 micromol photons m(-2) s(-1)) on excitation energy transfer from PBSs to PS II in a cyanobacterium Spirulina platensis were studied by measuring room temperature PC fluorescence emission spectra. High light (3000 micromol photons m(-2) s(-1)) stress had a significant effect on PC fluorescence emission spectra. On the other hand, light stress induced an increase in the ratio of PC fluorescence intensity of PBS indicating that light stress inhibits excitation energy transfer from PBS to PS II. The high light treatment to 3000 micromol photons m(-2) s(-1) caused disappearance of 31.5 kDa linker polypeptide which is known to link PC discs together. In addition we observed the similar decrease in the other polypeptide contents. Our data concludes that the Spirulina cells upon light treatment causes alterations in the phycobiliproteins (PBPs) and affects the energy transfer process within the PBSs.     

Fluorescence induction in the phycobilisome-containing cyanobacterium Synechococcus sp PCC 7942: analysis of the slow fluorescence transient

At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at approximately 0.2-1 and M at approximately 100-500 s, with a minimum S at approximately 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with O denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS-->Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS-->PS I excitation transfer. Blocking the PBS-->PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K.Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobacterial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P-->T fluorescence decay, reminiscent of the typical P-->T fluorescence decay of higher plants and algae. A similar P-->T decay was also displayed by DCMU-treated Synechococcus cells at 2 degrees C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2-->1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state 1 and upon illumination they display an extended P-->T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1-->2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS-->PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Synechococcus becomes plant-like.     

Changes in the antenna size of Photosystem I and Photosystem II in Synechococcus sp. strain PCC 7942 grown in the presence of SANDOZ 9785 — a Photosystem II inhibitor

SANDOZ 9785, also known as BASF 13.338, is a pyridazinone derivative that inhibits Photosystem II (PS II) activity leading to an imbalance in the rate of electron transport through the photosystems. Synechococcus sp. strain PCC 7942 cells grown in the presence of sublethal concentration of SANDOZ 9785 (SAN 9785) for 48 hours exhibited a 20% decrease in Chl a per cell. However, no changes were observed in the content of phycocyanin per cell, the size of the phycobilisomes or in the PS II:PS I ratio. From an estimate of PS II electron transport rate under varying light intensities and spectral qualities and analysis of room temperature Chl a fluorescence induction, it was deduced that growth of Synechococcus PCC 7942 in the presence of SAN 9785 leads to a redistribution of excitation energy in favour of PS II. Though the redistribution appears to be primarily caused by changes affecting the Chl a antenna of PS II, the extent of energetic coupling between phycobilisomes and PS II is also enhanced in SAN 9785 grown Synechococcus PCC 7942 cells. There was a reduction in the effective size of PS I antenna based on measurement of P700 photooxidation kinetics. These results indicate that when PS II is partially inhibited, the structure of photosynthetic apparatus alters to redistribute the excitation energy in favour of PS II so that the efficiency of utilization of light energy by the two photosystems is optimized. Our results suggest that under the conditions used, drastic structural changes are not essential for redistribution of excitation energy between the photosystems.Abbreviations APC Allophycocyanin - Chl a chlorophyll a - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophyenyl)-1,1-dimethyl urea - DCIP 2,6-dichlorophenolindophenol - F_o fluorescence when all the reaction centres are open - f_m fluorescence yield when all the reaction centres are closed - F_v variable chlorophyll fluorescence - HEPES N-2-Hydroxyethylpiperazine-N-2-ethanesulphonic Acid - I₅₀ concentration that causes 50% inhibition in activity - MV methyl viologen - pBQ para benzoquinone - PBS phycobilisome - PC phycocyanin - PS I, PS II Photosystem I, Photosystem II - P700 reaction centre Chl a of PS I - SAN 9785 SANDOZ 9785 i.e. 4-chloro-5-dimethylamino-2-phenyl-3 (2H) pyridazinone, also known as BASF 13.338     

Photoacoustic studies on the dependence of state transitions on grana stacking

Photoacoustic detection of oxygen evolution and Emerson enhancement in state 1 and state 2 were compared in a tobacco wild type and mutant (Su/su) deficient in chlorophyll. The mutant shows smaller changes in the distribution of excitation energy between the two photosystems than the wild type. Analysis of Emerson enhancement saturation curves indicates that in the mutant which is deficient in grana partitions and shows less stacking, state 1-state 2 transitions reflect changes in the yield of energy transfer from PS II to PS I (spillover). On the other hand, the wild type containing large grana shows changes in absorption cross-sections of the two photosystems upon state transitions. NaF, a specific phosphatase inhibitor, blocks the transition to state 1, indicating that LHC II phosphorylation has a role in excitation energy regulation in both the mutant as well as the wild type. It is demonstrated that N-ethylmaleimide, a specific sulfhydryl reagent, blocks the transition to state 2, suggesting that a disulfide-sulfhydryl redox couple activates the LHC II kinase in vivo.Abbreviations LHC II light harvesting chlorophyll a/b pigment protein complex of PS II - LHC II-P phosphorylated complex - NEM N-ethylmaleimide