Photosynthetic activity of far-red light in green plants

We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, Q_A. The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8 ± 0.5 kJ mol⁻¹ and 12.5 ± 0.7 kJ mol⁻¹ have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol⁻¹ between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants.     

The long-wavelength limit of plant photosynthesis

It is a common knowledge that the photosynthesis efficiency drops rapidly under the long-wavelength light excitation above 680 nm. We discovered that in sunflower leaves attached to the plant the initial fall is replaced by an unexpected increase at much longer wavelengths, so that a detectable O(2) evolution is remained till 780 nm. The quantum yield of O(2) evolution at the local maximum at 745 nm reaches almost 20% of the yield at 650 nm. We conclude that extreme long-wavelength chlorophylls may be present in the intact photosystem II antenna system, similarly to photosystem I.     

Salt tolerance in crop plants monitored by chlorophyll fluorescence in vivo

The potential of measurements of chlorophyll fluorescence in vivo to detect cellular responses to salinity and degrees of salt stress in leaves was investigated for three crop plants. Sugar beet (Beta vulgaris L.) (salt tolerant), sunflower (Helianthus annuus L.) (moderately salt tolerant), and bean (Phaseolus Vulgaris L. cv Canadian Wonder) (salt intolerant) were grown in pots and watered with mineral nutrient solution containing 100 millimolar NaCl. The fast rise in variable chlorophyll fluorescence yield that is correlated with photoreduction of photosystem II acceptors increased in leaves of sugar beet plants treated with salt suggesting stimulation of photosystem II activity relative to photosystem I. In sunflower, this fast rise was depressed by approximately 25% and the subsequent slow rate of quenching of the chlorophyll fluorescence was stimulated. These differences were more marked in the older mature leaves indicating an increasing gradient of salt response down the plant. The salt effect in vivo was reversible since chloroplasts isolated from mature leaves of salt-treated and control sunflower plants gave similar photosystem II activities. Unlike in sugar beet and sunflower, leaves of salt-treated bean progressively lost chlorophyll. The rate of slow quenching of chlorophyll fluorescence decreased indicating development of a partial block after photosystem II and possible initial stimulation of photosystem II activity. With further loss of chlorophyll photosystem II activity declined. It was concluded that measurements of chlorophyll fluorescence in vivo can provide a rapid means of detecting salt stress in leaves, including instances where photosynthesis is reduced in the absence of visible symptoms. The possible application to screening for salt tolerance is discussed.     

Ultrafast primary processes in photosystem I of the cyanobacterium Synechocystis sp. PCC 6803.

Ultrafast primary processes in the trimeric photosystem I core antenna-reaction center complex of the cyanobacterium Synechocystis sp. PCC 6803 have been examined in pump-probe experiments with approximately 100 fs resolution. A global analysis of two-color profiles, excited at 660 nm and probed at 5 nm intervals from 650 to 730 nm, reveals 430 fs kinetics for spectral equilibration among bulk antenna chlorophylls. At least two lifetime components (2.0 and 6.5 ps in our analysis) are required to describe equilibration of bulk chlorophylls with far red-absorbing chlorophylls (>700 nm). Trapping at P700 occurs with 24-ps kinetics. The multiphasic bulk left arrow over right arrow red equilibration kinetics are intriguing, because prior steady-state spectral studies have suggested that the core antenna in Synechocystis sp. contains only one red-absorbing chlorophyll species (C708). The disperse kinetics may arise from inhomogeneous broadening in C708. The one-color optical anisotropy at 680 nm (near the red edge of the bulk antenna) decays with 590 fs kinetics; the corresponding anisotropy at 710 nm shows approximately 3.1 ps kinetics. The latter may signal equilibration among symmetry-equivalent red chlorophylls, bound to different monomers within trimeric photosystem I.     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

Small light-harvesting antenna does not protect from photoinhibition

High-light-induced decrease in photosystem II (PSII) electron transfer activity was studied in high- and low-light-grown pumpkin (Cucurbita pepo L.) plants in vivo and in vitro. The PSII light-harvesting antenna of the low-light leaves was estimated to be twice as big as that of the high-light leaves. The low-light leaves were more susceptible to photoinhibition in vivo. However, thylakoids isolated from these two plant materials were equally sensitive to photoinhibition when illuminated in the absence of external electron acceptors. Only the intensity of the photoinhibitory light and the chlorophyll concentration of the sample, not the size of the light-harvesting antenna, determined the rate of PSII photoinhibition in vitro. Because excitation of the reaction center and not only the antenna chlorophylls is a prerequisite for photoinhibition of PSII activity, independence of photoinhibition on antenna size provides support for the hypothesis (Schatz EH, Brock H, Holzwarth AR [1988] Biophys J 54: 397-405) that the excitations of the antenna chlorophylls are in equilibrium with the excitations of the reaction centers. Better tolerance of the high-light leaves in vivo was due to a more active repair process and more powerful protective mechanisms, including photosynthesis. Apparently, some protective mechanism of the high-light-grown plants is at least partially active at low temperature. The protective mechanisms do not appear to function in vitro.     

Excitation dynamics and heterogeneity of energy equilibration in the core antenna of photosystem I from the cyanobacterium Synechocystis sp. PCC 6803

Energy equilibration in the photosystem I core antenna from the cyanobacterium Synechocystis sp. PCC 6803 was studied using femtosecond transient absorption spectroscopy at 298 K. The photosystem I core particles were excited at 660, 693, and 710 nm with 150 fs spectrally narrow laser pulses (fwhm = 5 nm). Global analysis revealed three kinetic processes in the core antenna with lifetimes of 250-500 fs, 1.5-2.5 ps, and 20-30 ps. The first two components represent strongly excitation wavelength-dependent energy equilibration processes while the 20-30 ps phase reflects the trapping of energy by the reaction center. Excitation into the blue and red edge of the absorption band induces downhill and uphill energy flows, respectively, between different chlorophyll a spectral forms of the core. Excitation at 660 nm induces a 500 fs downhill equilibration process within the bulk of antenna while the selective excitation of long-wavelength-absorbing chlorophylls at 710 nm results in a 380 fs uphill energy transfer to the chlorophylls absorbing around 695-700 nm, presumably reaction center pigments. The 1.5-2.5 ps phases of downhill and uphill energy transfer are largely equivalent but opposite in direction, indicating energy equilibration between bulk antenna chlorophylls at 685 nm and spectral forms absorbing below 700 nm. Transient absorption spectra with excitation at 693 nm exhibit spectral evolution within approximately 2 ps of uphill energy transfer to major spectral forms at 680 nm and downhill energy transfer to red pigments at 705 nm. The 20-30 ps trapping component and P(700) photooxidation spectra derived from data on the 100 ps scale are largely excitation wavelength independent. An additional decay component of red pigments at 710 nm can be induced either by selective excitation of red pigments or by decreasing the temperature to 264 K. This component may represent one of the phases of energy transfer from inhomogeneously broadened red pigments to P(700). The data are discussed based on the available structural model of the photosystem I reaction center and its core antenna.     

Thermodynamic investigation into the mechanism of the chlorophyll fluorescence quenching in isolated photosystem II light-harvesting complexes

Chlorophyll fluorescence quenching can be stimulated in vitro in purified photosystem II antenna complexes. It has been shown to resemble nonphotochemical quenching observed in isolated chloroplasts and leaves in several important respects, providing a model system for study of the mechanism of photoprotective energy dissipation. The effect of temperature on the rate of quenching in trimeric and monomeric antenna complexes revealed the presence of two temperature-dependent processes with different activation energies, one between approximately 15 and 35 degrees C and another between approximately 40 and 60 degrees C. The temperature of the transition between the two phases was higher for trimers than for monomers. Throughout this temperature range, the quenching was almost completely reversible, the protein CD was unchanged, and pigment binding was maintained. The activation energy for the low temperature phase was consistent with local rearrangements of pigments within some of the protein domains, whereas the higher temperature phase seemed to arise from large scale conformational transitions. For both phases, there was a strong linear correlation between the quenching rate and the appearance of an absorption band at 685 nm. In addition, quenching was correlated with a loss of CD at approximately 495 nm from Lutein 1 and at 680 nm from chlorophylls a1 and a2, the terminal emitters. The results obtained indicate that quenching of chlorophyll fluorescence in antenna complexes is brought about by perturbation of the lutein 1/chlorophyll a1/chlorophyll a2 locus, forming a poorly fluorescing chlorophyll associate, either a dimer or an excimer.     

Energy transfer and distribution in the red alga Porphyra perforata studied using picosecond fluorescence spectroscopy

The detailed process of excitation transfer among the antenna pigments of the red alga Porphyra perforata was investigated by measuring time-resolved fluorescence emission spectra using a single-photon timing system with picosecond resolution. The fluorescence decay kinetics of intact thalli at room temperature revealed wavelength-dependent multi-component chlorophyll a fluorescence emission. Our analysis attributes the majority of chlorophyll a fluorescence to excitation originating in the antennae of PS II reaction centers and emitted with maximum intensities at 680 and 740 nm. Each of these fluorescence bands was characterized by two kinetic decay components, with lifetimes of 340-380 and 1700-2000 ps and amplitudes varying with wavelength and the photochemical state of the PS II reaction centers. In addition, a small contribution to the long-wavelength fluorescence band is proposed to arise from chlorophyll a antennae coupled to PS I. This component displays fast decay kinetics with a lifetime of approx. 150 ps. Desiccation of the thalli dramatically increases the contribution of this fast decay component.     

State transitions,photosystem stoichiometry adjustment and non-photochemical quenching in cyanobacterial cells acclimated to light absorbed by photosystem I or photosystem II

Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F relative fluorescence intensity at emission wavelength nm - F _o fluorescence intensity when all PS II traps are open - light 1 light absorbed preferentially by PS I - light 2 light absorbed preferentially by PS II - PBS phycobilisome - PS photosystem     

Three different mechanisms of energy dissipation of a desiccation-tolerant moss serve one common purpose: to protect reaction centres against photo-oxidation

Three different types of non-photochemical de-excitation of absorbed light energy protect photosystem II of the sun- and desiccation-tolerant moss Rhytidium rugosum against photo-oxidation. The first mechanism, which is light-induced in hydrated thalli, is sensitive to inhibition by dithiothreitol. It is controlled by the protonation of a thylakoid protein. Other mechanisms are activated by desiccation. One of them permits exciton migration towards a far-red band in the antenna pigments where fast thermal deactivation takes place. This mechanism appears to be similar to a mechanism detected before in desiccated lichens. A third mechanism is based on the reversible photo-accumulation of a radical that acts as a quencher of excitation energy in reaction centres of photosystem II. On the basis of absorption changes around 800 nm, the quencher is suggested to be an oxidized chlorophyll. The data show that desiccated moss is better protected against photo-oxidative damage than hydrated moss. Slow drying of moss thalli in the light increases photo-protection more than slow drying in darkness.     

Alteration of photosystem II properties with non-photochemical excitation quenching

Oxygen yield from single turnover flashes and multiple turnover pulses was measured in sunflower leaves differently pre-illuminated to induce either 'energy-dependent type' non-photochemical excitation quenching (qE) or reversible, inhibitory type non-photochemical quenching (qI). A zirconium O2 analyser, combined with a flexible gas system, was used for these measurements. Oxygen yield from saturating single turnover flashes was the equivalent of 1.3-2.0 micromole(-) m(-2) in leaves pre-adapted to low light. It did not decrease when qE quenching was induced by a 1 min exposure to saturating light, but it decreased when pre-illumination was extended to 30-60 min. Oxygen evolution from saturating multiple turnover pulses behaved similarly: it did not decrease with the rapidly induced qE but decreased considerably when exposure to saturating light was extended or O2 concentration was decreased to 0.4%. Parallel recording of chlorophyll fluorescence and O2 evolution during multiple turnover pulses, interpreted with the help of a mathematical model of photosystem II (PS II) electron transport, revealed PS II donor and acceptor side resistances. These experiments showed that PS II properties depend on the type of non-photochemical quenching present. The rapidly induced and rapidly reversible qE type (photoprotective) quenching does not induce changes in the number of active PS II or in the PS II maximum turnover rate, thus confirming the antenna mechanism of qE. The more slowly induced but still reversible qE type quenching (photoinactivation) induced a decrease in the number of active PS II and in the maximum PS II turnover rate. Modelling showed that, mainly, the acceptor side resistance of PS II increased in parallel with the reversible qI.     

Red-shifted chlorophyll a bands allow uphill energy transfer to photosystem II reaction centers in an aerial green alga,Prasiola crispa,harvested in Antarctica

An aerial green alga, Prasiola crispa (Lightf.) Menegh, which is known to form large colonies in Antarctic habitats, is subject to severe environmental stresses due to low temperature, draught and strong sunlight in summer. A considerable light-absorption by long-wavelength chlorophylls (LWC) at around 710 nm, which seem to consist of chlorophyll a, was detected in thallus of P. crispa harvested at a terrestrial environment in Antarctica. Absorption level at 710 nm against that at 680 nm was correlated with fluorescence emission intensity at 713 nm at room temperature and the 77 K fluorescence emission band from LWC was found to be emitted at 735 nm. We demonstrated that the LWC efficiently transfer excitation energy to photosystem II (PSII) reaction center from measurements of action spectra of photosynthetic oxygen evolution and P700 photo-oxidation. The global quantum yield of PSII excitation in thallus by far-red light was shown to be as high as by orange light, and the excitation balance between PSII and PSI was almost same in the two light sources. It is thus proposed that the LWC increase the photosynthetic productivity in the lower parts of overlapping thalli and contribute to the predominance of alga in the severe environment.     

Kinetics and spectra of photo-induced changes in the absorption of pigment-protein complexes of photosystem 1 in a picosecond range

The energy transfer from the light-harvesting antenna chlorophylls to the reaction center molecules and subsequent charge separation were investigated using a difference picosecond spectrophotometer with selective excitation. The objects were the pigment-protein complexes of photosystem 1 (Chl/P700 = 60) isolated from bean leaves. The difference absorption spectra of the excited states of light-harvesting antenna chlorophylls and the P700 photooxidation were measured. It was shown that the excited states of antenna chlorophylls were generated within 10 ps and deactivated with three-component kinetics: tau 1 = 20--45 ps, tau 2 = 100--300 ps, tau 3 greater than 500 ps. The process of the P700 photooxidation induced by the 650 nm exciting pulse was approximately monoexponential with tau equal to 15--30 ps. It is established that the P700 photooxidation is due to the efficient transfer of excitation energy from antenna chlorophylls to reaction centers.     

Optimization and evolution of light harvesting in photosynthesis: the role of antenna chlorophyll conserved between photosystem II and photosystem I

The efficiency of oxygenic photosynthesis depends on the presence of core antenna chlorophyll closely associated with the photochemical reaction centers of both photosystem II (PSII) and photosystem I (PSI). Although the number and overall arrangement of these chlorophylls in PSII and PSI differ, structural comparison reveals a cluster of 26 conserved chlorophylls in nearly identical positions and orientations. To explore the role of these conserved chlorophylls within PSII and PSI we studied the influence of their orientation on the efficiency of photochemistry in computer simulations. We found that the native orientations of the conserved chlorophylls were not optimal for light harvesting in either photosystem. However, PSII and PSI each contain two highly orientationally optimized antenna chlorophylls, located close to their respective reaction centers, in positions unique to each photosystem. In both photosystems the orientation of these optimized bridging chlorophylls had a much larger impact on photochemical efficiency than the orientation of any of the conserved chlorophylls. The differential optimization of antenna chlorophyll is discussed in the context of competing selection pressures for the evolution of light harvesting in photosynthesis.     

Chlorophyll triplet states associated with photosystem II of thylakoids

The analysis of FDMR thylakoid spectra, determined at multiple emission wavelengths, by a global decomposition technique, has revealed the presence of three previously undescribed triplet populations at emission wavelengths characteristic of Photosystem II chlorophyll/protein complexes. Their zero-field splitting parameters have been determined in order to compare them with the well-studied PSII recombination triplet state. None of these triplets have the zero-field splitting parameters characteristic of the recombination triplet and are therefore probably not generated directly in the reaction center. On the basis of their microwave-induced emission spectra, it is suggested that two are probably generated in the core complex(es) while the third may be generated in the external antenna. These triplets are formed under nonreducing redox conditions, when the recombination triplet is undetectable. It is suggested that they may be involved in the photoinhibitory damage of Photosystem II. The triplet-minus-singlet spectrum associated with the recombination triplet state has been determined for thylakoids after reduction of the secondary acceptors. Its main peak is at 685 nm, slightly red shifted with respect to earlier reports, with a weak signal, of opposite sign at approximately 675 nm. The 685 nm peak indicates that at cryogenic temperatures, the triplet is located on the long-wavelength chlorophyll state present in the reaction center complex of Photosystem II (D1.D2.Cytb(559) complex). From the absence of a clear structure in the 680 nm absorption region, this long-wavelength absorbing state does not appear to be strongly coupled to P(680), though it must be associated with one of the "inner core" pigments recently identified in the photosystem II crystallographic structure [Zouni et al. (2001) Nature 408, 739-743].     

Photochemical and spectral properies of the photosystem I reaction centers-enriched particles]

The photosystem I reaction centers-enriched particles (RCP) having the ratio of P700/chlorophyll of 1 : 20--1 :30 were prepared from stroma lamellae photosystem subchloroplast fragments by extraction with water-saturated ether. The EPR signals and light-induced absorption changes indicate that the reaction centers are active in the particles. The spectral properties of the RCP were characterized. The main red absorption maximum is located at 673--674 nm with a "shoulder" at about 684--688 nm. A comparative analysis of the subchloroplast fragments and RCP absorption and fluorescence spectra suggests that the chl. a forms absorbed in the region of 680--695 nm are more closely associated with the reaction center. The most long-wavelength absorbing chl. a forms with maxima around 730--740 nm in the low temperature fluorescence spectra were not found in the RCP.     

Energy transfer and charge separation in photosystem I: P700 oxidation upon selective excitation of the long-wavelength antenna chlorophylls of Synechococcus elongatus.

Photosystem I of the cyanobacterium Synechococcus elongatus contains two spectral pools of chlorophylls called C-708 and C-719 that absorb at longer wavelengths than the primary electron donor P700. We investigated the relative quantum yields of photochemical charge separation and fluorescence as a function of excitation wavelength and temperature in trimeric and monomeric photosystem I complexes of this cyanobacterium. The monomeric complexes are characterized by a reduced content of the C-719 spectral form. At room temperature, an analysis of the wavelength dependence of P700 oxidation indicated that all absorbed light, even of wavelengths of up to 750 nm, has the same probability of resulting in a stable P700 photooxidation. Upon cooling from 295 K to 5 K, the nonselectively excited steady-state emission increased by 11- and 16-fold in the trimeric and monomeric complexes, respectively, whereas the quantum yield of P700 oxidation decreased 2.2- and 1.7-fold. Fluorescence excitation spectra at 5 K indicate that the fluorescence quantum yield further increases upon scanning of the excitation wavelength from 690 nm to 710 nm, whereas the quantum yield of P700 oxidation decreases significantly upon excitation at wavelengths longer than 700 nm. Based on these findings, we conclude that at 5 K the excited state is not equilibrated over the antenna before charge separation occurs, and that approximately 50% of the excitations reach P700 before they become irreversibly trapped on one of the long-wavelength antenna pigments. Possible spatial organizations of the long-wavelength antenna pigments in the three-dimensional structure of photosystem I are discussed.     

Nuclear pea mutants deficient in chlorophyll b and the major polypeptide of the light-harvesting chlorophyll a/b protein of photosystem II

Eight chlorophyll b deficient nuclear mutants of pea (Pisum sativum L.) have been characterized by low temperature fluorescence emission spectra of their leaves and by the ultrastructure, photochemical activities and polypeptide compositions of the thylakoid membranes. The room temperature fluorescence induction kinetics of leaves and isolated thylakoids have also been recorded. In addition, the effects of Mg²⁺ on the fluorescence kinetics of the membranes have been investigated. The mutants are all deficient in the major polypeptide of the light-harvesting chlorophyll a/b protein of photosystem II. The low temperature fluorescence emission spectra of aurea-5106, xantha-5371 and –5820 show little or no fluorescence around 730 nm (photosystem I fluorescence), but possess maxima at 685 and 695 nm (photosystem II fluorescence). These three mutants have low photosystem II activities, but significant photosystem I activities. The long-wavelength fluorescence maximum is reduced for three other mutants. The Mg²⁺ effect on the variable component of the room temperature fluorescence (685 nm) induction kinetics is reduced in all mutants, and completely absent in aurea-5106 and xantha-5820. The thylakoid membranes of these 2 mutants are appressed pairwise in 2-disc grana of large diameter. Chlorotica-1-206A and–130A have significant long-wavelength maxima in the fluorescence spectra and show the largest Mg²⁺ enhancement of the variable part of the fluorescence kinetics. These two mutants have rather normally structured chloroplast membranes, though the stroma regions are reduced. The four remaining mutants are in several respects of an intermediate type.Abbreviations Chl chlorophyll - CPI Chi-protein complex I, F_o, F_v - F_m parameters of room temperature chlorophyll fluorescence induction kinetics - F685, F695 and F-1 components of low temperature chlorophyll emission with maximum at 685, 695 and ca 735 nm, respectively - PSI photosystem I - PSII photosystem II - LHCI and LHCII light-harvesting chlorophyll a/b complexes associated with PSI and PSII, respectively - SDS sodium dodecyl sulfate     

What is beta-carotene doing in the photosystem II reaction centre?

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.