RESPONSE OF THE PHOTOSYNTHETIC APPARATUS OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) TO NITRATE,PHOSPHATE, OR IRON STARVATION1

The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient-starved cells, the ratios of light-harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The chl a-specific light absorpition coefficient (a*) and the functional absorption cross-section of PSII (σ) increased during nutrient starvation, consistent with reduction of intracellular self-shading (i.e. a reduction of the “package effect”) as cells became chlorotic. The light-harvesting complex proteins remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. The ratio of the xanthophylls cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photo-protective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ration of variable to maximum fluorescence (F_v/F_m) of dark-adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in F_v/F_m were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient-starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate-starved cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.     

<Emphasis Type="Italic">GmFtsH9</Emphasis> expression correlates with in vivo photosystem II function: chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient analysis and eQTL mapping in soybean

Filamentation temperature-sensitive H (FtsH) is an ATP-dependent zinc metalloprotease involved in diverse biological functions. There are 12 FtsH proteins in Arabidopsis, among which AtFtsH2 plays an important role in regulating the turnover of photosystem II (PSII) reaction center D1 protein and the development of the photosynthetic apparatus. Here, we have identified 11 FtsH genes in the soybean genome by a bioinformatics approach. These soybean FtsH genes corresponded to seven Arabidopsis FtsH genes, suggesting that the main characteristics of soybean FtsH genes were formed before the evolutionary split of soybean and Arabidopsis. Phylogenetic analyses allowed us to clone a soybean AtFtsH2-like gene designated as GmFtsH9. The predicted protein of GmFtsH9 consists of 690 amino acids and contains three typical FtsH proteins conserved domains. The expression level of GmFtsH9 was determined in a soybean recombinant inbred line population under a pot experiment conducted for measuring chlorophyll a fluorescence transient parameters, photosynthetic CO₂ fixation rate (P _N), and seed yield. Expression quantitative trait loci (eQTL) mapping revealed two trans-acting eQTLs for GmFtsH9. The significant correlation of gene expression level with chlorophyll a fluorescence transient parameters and the presence of overlapping eQTL (QTL) between gene expression level and chlorophyll a fluorescence transient parameters indicated that GmFtsH9 could be involved in regulating PSII function. These results further lead to the understanding of the mechanism underlying FtsH gene expression, and contribute to the development of marker-assisted selection breeding programs for modulating soybean FtsH gene expression.     

Alterations of the chlorophyll-protein pattern in chronically photoinhibited Chenopodium rubrum cells

When photoautotrophic Chenopodium rubrum L. culture cells were exposed to high photon flux densities for seven consecutive light periods a marked reduction in photochemical efficiency, chlorophyll (Chl) content and Chl a/b ratio occurred. These alterations were accompanied by distinct changes in the pigment and protein composition of the thylakoid membranes. In photosystem II (PSII) a reduction in the relative contents of proteins from the reaction center (D1 protein, D2 protein and Cyt b₅₅₉) and the inner antenna (CP43 and CP47) was observed. In agreement with the reduction in the Chl a/b ratio an increase in the relative content of the major light-harvesting complex of PSII (LHCII) could be demonstrated. The minor chlorophyll-proteins of PSII were only slightly affected but PSI (quantified as total complex) showed a reduction upon chronic photoinhibition. The changes in protein composition were accompanied by a drastic increase in the contents of lutein and the xanthophyll-cycle pigments and by a reduction in the β-carotene content. The effects on lutein and xanthophyll-cycle pigment content were most pronounced in stroma thylakoids. Here, an increase in LHCII (which harbours these pigments) was clearly detectable. Considering the pigment content of LHCII, the change in its apoprotein content was not large enough to explain the pigment changes.     

PHOTOSYNTHETIC RESPONSE OF THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) TO ULTRAVIOLET RADIATION1

Solar ultraviolet radiation (UVA + UVB) impairs photosynthesis in marine algae. Canopy blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh are exposed to high levels of solar UV in the field. To determine the effects of UV radiation on photosynthesis in the giant kelp and to identify sites of UV damage, O₂ evolution, reaction center organization, light harvesting, and energy transfer efficiency were measured in canopy blades that had been exposed to elevated levels of UV in the laboratory. UV treatment reduced both the light-saturated rate and the light-limited rate of photosynthesis by 50% but produced no significant change in the rate of dark respiration. A significant impairment of photosystem II (PSII) reaction center function was observed, suggesting that PSII is a major site of damage in chromophytes. Reduced quantum efficiency of photosynthesis and loss of energy transfer from light-harvesting pigments (fucoxanthin, chlorophyll a, and chlorophyll c) to PSII indicate that the major light-harvesting complex of M. pyrifera, the fucoxanthin-chlorophyll protein complex (FCPC), was another site of UV damage. These measures provide the first evidence of a direct effect of UV radiation on specific sites in the photosynthetic apparatus of chromophytes and indicate that in situ fluorescence excitation analysis may be a simple means to detect UV stress in algae.     

Metabolic recovery of the Antarctic liverwort <Emphasis Type="Italic">Cephaloziella varians</Emphasis> during spring snowmelt

We measured the responses of pigments and chlorophyll a fluorescence parameters of the Antarctic leafy liverwort Cephaloziella varians to snowmelt during austral spring 2005 at Rothera Point on the western Antarctic Peninsula. Although no changes to the concentrations of UV-B photoprotective pigments were detected during snowmelt, chlorophyll and carotenoid concentrations and maximum photosystem (PS)II yield (F _v /F _m) were respectively 88, 60 and 144% higher in the tissues of the liverwort that had recently emerged from snow than in those under a 10 cm depth of snow. A laboratory experiment similarly showed that effective PSII yield increased rapidly within the first 45 min after plants sampled from under snow were removed to an illuminated growth cabinet. The pigmentation and PSII yields of plants during snowmelt were also compared with those of plants in January, during the middle of the growing season at Rothera Point. During snowmelt, plants had lower F _v /F _m values, chlorophyll a/b ratios and concentrations of UV-B photoprotective pigments and carotenoids than during mid-season, suggesting that although there is some recovery of PSII activity and increases in concentrations of photosynthetic pigments during snowmelt, the metabolism of C. varians is restricted during this period.     

Stress-induced degradation of the photosynthetic apparatus is accompanied by changes in thylakoid protein turnover and phosphorylation

Development of chlorosis and loss of PSII were compared in young spinach plants suffering under a combined magnesium and sulphur deficiency. Loss of chlorophyll could be detected already after the first week of deficiency and preceded any permanent functional inhibition of PSII as detected by changes in the chlorophyll fluorescence parameter F_v/F_m. A substantial decrease in F_v/F_m was observed only after the second week of deficiency. After 4 weeks, the plants had lost about 70% of their original chlorophyll content, but fluorescence data indicated that 80% of the existing PSII centers were still capable of initiating photosynthetic electron transport. The degradation of the photosynthetic apparatus without loss of PSII activity was due to changes in protein turnover, especially of the PSII D1 reaction center protein. Already by day 7 of deficiency, a 1.4-fold increase in D1 protein synthesis was observed measured as incorporation of ¹⁴C-leucine. Immunological determination by western-blotting did not reveal a change in D1 protein content. Thus, D1 protein was also degraded more rapidly. The increased turnover was high enough to prevent any loss or inhibition of PSII. After 3 weeks, D1 protein synthesis on a chlorophyll basis was further increased by a factor of 2. However, this was not enough to prevent a net loss of D1 protein of about 70%. Immunological determination revealed that together with the D1 protein also other polypeptides of PSII became degraded. This process prevented a large accumulation of photo-inactivated PSII centers. However, it initiated the breakdown of the other thylakoid proteins, especially of LHCII, resulting in the observed chlorosis. Together with the change in protein turnover and stability, a characteristic change in thylakoid protein phosphorylation was observed. In the deficient plants steady state phosphorylation of both LHCII and PSII proteins was increased in the dark. In the light phosphorylation of PSII proteins was stimulated and after 3 weeks of deficiency was even higher in the deficient leaves than in the control plants. In contrast, the phosphorylation level of LHCII decreased in the light and could hardly be detected after 3 weeks of deficiency. Phosphorylation of the reaction center polypeptides presumably increased their stability against proteolytic attack, whereas phosphorylated LHCII seems to be the substrate for proteolysis.     

INFLUENCE OF ULTRAVIOLET RADIATION ON GROWTH AND PHOTOSYNTHESIS OF TWO COLD OCEAN DIATOMS1

The influence of chronic exposure to UV-B and UV-A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV-B. Fluorescence induction kinetics of photo-system II (PSII) revealed that UV-B caused lower F_v/F_m ratios and half-rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV-B—exposed populations, which also had the highest chlorophyll a content per cell. The UV-B—exposed cells were, however, more vulnerable to visible light-induced photoinhibition. Exposure to UV-A in the absence of UV-B had little effect on growth, fluorescence induction of PSII, or chlorophyll a contents but did have some inhibitory effects on carbon assimilation per chlorophyll a and per cell. The increased photosynthetic capacity of UV-B-exposed cells suggested some ability to compensate for damage to the photosynthetic apparatus.     

Distribution of Thylakoid Proteins between Stromal and Granal Lamellae in Spirodela : Dual Location of Photosystem II Components

We have quantified the lateral distribution of 12 thylakoid proteins of Spirodela oligorrhiza by immunoblot analysis of detergent-derived granal and stromal lamellae. The immunological, ultrastructural, cytochemical, and biophysical measurements each indicated the expected overall separation of photosystem II (PSII) and photosystem I (PSI) components; however, certain proteins were not completely localized to one lamellar fraction. The apoproteins of the light harvesting chlorophyll a/b complex, subunit 1 of PSI and the components of the PSII reaction center (the 32 kilodalton, D2, and cytochrome b₅₅₉ proteins) were dually located between granal and stromal lamellae. Proteins associated exclusively with one of the membrane types were: in granal lamellae, the 43 and 51 kilodalton PSII proteins, and in stromal lamellae, the α and β subunits of the proton ATPase.     

Isolation and characterization of oxygen-evolving thylakoid membranes and Photosystem II particles from a marine diatom Chaetoceros gracilis

Thylakoid membranes retaining high oxygen-evolving activity (about 250 μmol O₂/mg Chl/h) were prepared from a marine centric diatom, Chaetoceros gracilis, after disruption of the cells by freeze-thawing. We also succeeded in purification of Photosystem II (PSII) particles by differential centrifugation of the thylakoid membranes after treatment with 1% Triton X-100. The diatom PSII particles showed an oxygen-evolving activity of 850 and 1045 μmol O₂/mg Chl/h in the absence and presence of CaCl₂, respectively. The PSII particles contained fucoxanthin chlorophyll a/c-binding proteins in addition to main intrinsic proteins of CP47, CP43, D2, D1, cytochrome b559, and the antenna size was estimated to be 229 Chl a per 2 molecules of pheophytin. Five extrinsic proteins were stoichiometrically released from the diatom PSII particles by alkaline Tris-treatment. Among these five extrinsic proteins, four proteins were red algal-type extrinsic proteins, namely, PsbO, PsbQ', PsbV and PsbU, whereas the other one was a novel, hypothetical protein. This is the first report on isolation and characterization of diatom PSII particles that are highly active in oxygen evolution and retain the full set of extrinsic proteins including an unknown protein.     

Magneto-optical measurements of the pigments in fully active photosystem II core complexes from plants

Preparation of a minimum PSII core complex from spinach is described, containing four Mn per reaction center (RC) and exhibiting high O2 evolving activity [approximately 4000 micromol of O2 (mg of chl)(-1) x h(-1)]. The complex consists of the CP47 and CP43 chlorophyll binding proteins, the RC D1/D2 pair, the cytochrome b559 subunits, and the Mn-stabilizing psbO (33 kDa) protein, all present in the same stoichiometric amounts found in the parent PSII membranes. Several small subunits are also present. The cyt b559 content is 1.0 per RC in core complexes and PSII membranes. The total chlorophyll content is 32 chl a and <1 chl b per RC, the lowest yet reported for any active PSII preparation. The core complex exhibits the characteristic EPR signals seen in the S2 state of higher plant PSII. A procedure for preparing low-temperature samples of very high optical quality is developed, allowing detailed optical studies in the S1 and S2 states of the system to be made. Optical absorption, CD, and MCD spectra reveal unprecedented detail, including a prominent, well-resolved feature at 683.5 nm (14630 cm(-1)) with a weaker partner at 187 cm(-1) to higher energy. On the basis of band intensity, CD, and MCD arguments, these features are identified as the exciton split components of P680 in an intact, active reaction center special pair. Comparisons are made with solubilized D1/D2/cyt b559 material and cyanobacterial PSII.     

Light-induced absorption spectra of the D1–D2–cytochrome b 559 complex of Photosystem II: Effect of methyl viologen concentration

The light-induced difference absorption spectra associated to the photo-accumulation of reduced pheophytin a were studied in the isolated D1–D2–Cyt b559 complex in the presence of variable methyl viologen concentrations and different illumination conditions under anaerobiosis. Depending on the methyl viologen/reaction centre ratio, the relative intensities of the spectral bands at 681.5±0.5, 667.0±0.5 and 542.5±0.5 nm were modified. The reduced pheophytin a located at the D1-branch of the complex absorbs at 681.7±0.5 nm, and at least two additional pigment species contribute to the Q_y band of the difference absorption spectra with maxima at 667.0±0.5 and 680.5±0.5 nm. We propose the additional species correspond to a peripheral chlorophyll a and the pheophytin a located at the D2-branch of the complex, respectively. The blue absorbing chlorophyll at 667 nm is susceptible to chemical redox changes with a midpoint reduction potential of +470 mV. The Q_x absorption bands of both pheophytins localised at the D2- and D1-branch of the D1–D2–Cyt b559 complex were at 540.7±0.5 and 542.9±0.5, respectively. The results indicated that the two pheophytin molecules can be photoreduced in the D1–D2–Cyt b559 complex in certain experimental conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.     

Mechanistic aspects of xanthophyll cycle-dependent photoprotection in higher plant chloroplasts and leaves

Higher plants must dissipate absorbed light energy that exceeds the photosynthetic capacity to avoid molecular damage to the pigments and proteins that comprise the photosynthetic apparatus. Described in this minireview is a current view of the biochemical, biophysical and bioenergetic aspects of the primary photoprotective mechanism responsible for dissipating excess excitation energy as heat from photosystem II (PSII). The photoprotective heat dissipation is measured as nonphotochemical quenching (NPQ) of the PSII chlorophyll a (Chl a) fluorescence. The NPQ mechanism is controlled by the trans-thylakoid membrane pH gradient (ΔpH) and the special xanthophyll cycle pigments. In the NPQ mechanism, the de-epoxidized endgroup moieties and the trans-thylakoid membrane orientations of antheraxanthin (A) and zeaxanthin (Z) strongly affect their interactions with protonated chlorophyll binding proteins (CPs) of the PSII inner antenna. The CP protonation sites and steps are influenced by proton domains sequestered within the proteo-lipid core of the thylakoid membrane. Xanthophyll cycle enrichment around the CPs may explain why changes in the peripheral PSII antenna size do not necessarily affect either the concentration of the xanthophyll cycle pigments on a per PSII unit basis or the NPQ mechanism. Recent time-resolved PSII Chi a fluorescence studies suggest the NPQ mechanism switches PSII units to an increased rate constant of heat dissipation in a series of steps that include xanthophyll de-epoxidation, CP-protonation and binding of the xanthophylls to the protonated CPs; the concerted process can be described with a simple two-step, pH-activation model. The xanthophyll cycle-dependent NPQ mechanism is profoundly influenced by temperatures suboptimal for photosynthesis via their effects on the trans-thylakoid membrane energy coupling system. Further, low temperature effects can be grouped into either short term (minutes to hours) or long term (days to seasonal) series of changes in the content and composition of the PSII pigment-proteins. This minireview concludes by briefly highlighting primary areas of future research interest regarding the NPQ mechanism.     

Isolation and characterization of the oxygen-evolving photosystem II complex from the economical red alga <Emphasis Type="Italic">Bangia fusco-purpurea</Emphasis>

The highly pure and active photosystem II (PSII) complex was isolated from Bangia fusco-purpurea (Dillw) Lyngb., an important economic red alga in China, through two steps of sucrose density gradient ultracentrifugation and characterized by the room absorption and fluorescence emission spectra, DCIP (2,6-dichloroindophenol) reduction, and oxygen evolution rates. The PSII complex from B. fusco-purpurea had the characteristic absorption peaks of chlorophyll (Chl) a (436 and 676 nm) and typical fluorescence emission peak at 685 nm (Ex = 436 nm). Moreover, the acquired PSII complex displayed high oxygen evolution (139 μmol O₂/(mg Chl h) in the presence of 2.5 mM 2,6-dimethybenzoqinone as an artificial acceptor and was active in photoreduction of DCIP (2,6-dichloroindophenol) by DPC (1,5-diphenylcarbazide) at 163 U/(mg Chl a h). SDS-PAGE also suggested that the purified PSII complex contained four intrinsic proteins (D1, D2, CP43, and CP47) and four extrinsic proteins (33-kD protein, 20-kD protein, cyt c-550, and 14-kD protein).     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

Cytochrome b-559 may function to protect photosystem II from photoinhibition

Although cytochrome b-559 is an integral component of the photosystem II complex (PSII), its function is unknown. Because cytochrome b-559 has been shown to be both photooxidized and photoreduced in PSII, one of several proposals is that it mediates cyclic electron transfer around PSII, possibly as a protective mechanism. We have used electron paramagnetic resonance spectroscopy to investigate the pathway of photooxidation of cytochrome b-559 in PSII and have shown that it proceeds via photooxidation of chlorophyll. We propose that this photooxidation of chlorophyll is the first step in the photoinhibition of PSII. The unique susceptibility of PSII to photoinhibition is probably due to the fact that it is the only reaction center in photosynthesis which generates an oxidant with a reduction potential high enough to oxidize chlorophyll. We propose that the function of cytochrome b-559 is to mediate cyclic electron transfer to rereduce photooxidized chlorophyll and protect PSII from photoinhibition. We also suggest that the chlorophyll(s) which are susceptible to photooxidation are analogous to the monomer chlorophylls found in the bacterial photosynthetic reaction center complex.     

Isolation and Characterization of an Oxygen Evolving Photosystem 2 Core Complex from the Thermophilic Cyanobacterium Mastigocladus laminosus

A novel purification procedure was developed for the isolation of oxygen evolving photosystem 2 (PS2) from Mastigocladus laminosus. The isolation procedure involves dodecyl maltoside extraction followed by column chromatography using anion exchange resins. The isolated PS2 reaction center (RC) was analyzed for its biochemical and biophysical characteristics. Analysis by SDS polyacrylamide gel electrophoresis revealed that the complex contained five intrinsic membrane proteins (CP 47, CP 43, D1, D2, and cyt b ₅₅₉) and at least three low molecular mass proteins. The complex exhibited high rates of oxygen evolution [333 mmol(O₂) kg^–1(Chl) s^–1] in the presence of 2.5 mM 2,6-dimethylbenzoquinone (DMBQ) as an artificial electron acceptor. The red chlorophyll a absorption peak of this complex was observed at 673.5±0.2 nm. The isolated PS2 core complex was free of photosystem 1 as inferred from its SDS-PAGE and fluorescence spectrum. The electron transfer properties of the Mastigocladus cells and the purified PS2 core complex were further probed by measuring thermoluminescence signals, which indicated the presence of a primary quinone electron acceptor (Q_A) in the purified PS2 core complex.     

The energy distribution between the photosystems and light-induced changes in the stoichiometry of system I and II reaction centers in the chlorophyll b-containing alga Mantoniella squamata (Prasinophyceae)

The chlorophyll b-containing alga Mantoniella squamata was analyzed with respect to its capacity to balance the energy distribution from the light-harvesting antenna to photosystem I or photosystem II. It was shown, that this alga is unable to alter the absorption cross section of the two photosystems in terms of short-time regulations (state transitions). The energy absorbed by the LHC, which contains 60% of total photosynthetic pigments, is transferred to both photosystems without any preference. The stoichiometry of the two photosystems is found to be extremely unequal and variable during light adaptation. In high light, the molar ratio of P-680 per P-700 is found to be two, whereas under low light conditions this ratio accounts to nearly four. This very unbalanced stoichiometry of the reaction centers gives some new insights into the concept of the photosynthetic unit as well as in the importance of the regulation of the energy distribution. It is assumed that the high concentration of photosystem II can be understood as a mechanism to prevent the overexcitation of photosystem I. In addition, the changes im membrane protein pattern are not accompanied by variations in the ratio of appressed to nonappressed membranes as probed by ultrastructural analysis. It is suggested that the thylakoids are organized like a homogenous pigment bed. The lack of state transitions can be interpreted as a consequence of this unusual membrane morphology.Abbreviations Chl chlorophyll - CPa chlorophyll a-protein of PSII - CPl P-700 chlorophyll a-protein - CPD Chlorophyll packing density index - cyt f cytochrome f - FP free pigments - LHC light-harvesting complex - P_max light saturated photosynthetic rates per chlorophyll - n number of experiments - PQ plastoquinone - PS photosystem - PSU photosynthetic unit - Q_E non-photochemical quenching - Q_Q photochemical quenching     

Photosynthesis and chlorophyllafluorescence during flag leaf senescence of field-grown wheat plants

The characteristics of photosynthetic gas exchange, chlorophyll a fluorescence, and xanthophyll cycle pigments during flag leaf senescence of field-grown wheat plants were investigated. With senescence progressing, the light-saturated net CO₂ assimilation rate expressed either on a basis of leaf area or chlorophyll decreased significantly. The apparent quantum yield of net photosynthesis decreased when expressed on a leaf area basis but increased when expressed on a chlorophyll basis. The maximal efficiency of PSII photochemistry decreased very little while actual PSII efficiency, photochemical quenching, and the efficiency of excitation capture by open PSII centers decreased considerably. At the same time, non-photochemical quenching increased significantly. A substantial decrease in the contents of violaxanthin and zeaxanthin, but a slight decrease in the content of antheraxanthin were observed. However, the de-epoxidation status of the xanthophyll cycle was positively correlated with progressive senescence. This increase was due mainly to a smaller decrease in zeaxanthin than in violaxanthin. Our results suggest that PSII apparatus remained functional, but a down-regulation of PSII occurred under the steady state of photosynthesis in senescent flag leaves. Such a down-regulation was associated with the closure of PSII centers and an enhanced xanthophyll cycle-related thermal dissipation in the PSII antennae.