Profiles of light absorption and chlorophyll within spinach leaves from chlorophyll fluorescence

Chlorophyll fluorescence was used to estimate profiles of absorbed light within chlorophyll solutions and leaves. For chlorophyll solutions, the intensity of the emitted fluorescence declined in a log–linear manner with the distance from the irradiated surface as predicted by Beer's law. The amount of fluorescence was proportional to chlorophyll concentration for chlorophyll solutions given epi‐illumination on a microscope slide. These relationships appeared to hold for more optically complex spinach leaves. The profile of chlorophyll fluorescence emitted by leaf cross sections given epi‐illumination corresponded to chlorophyll content measured in extracts of leaf paradermal sections. Thus epifluorescence was used to estimate relative chlorophyll content through leaf tissues. Fluorescence profiles across leaves depended on wavelength and orientation, reaching a peak at 50–70 µm depth. By infiltrating leaves with water, the pathlengthening due to scattering at the airspace : cell wall interfaces was calculated. Surprisingly, the palisade and spongy mesophyll had similar values for pathlengthening with the value being greatest for green light (550 > 650 > 450 nm). By combining fluorescence profiles with chlorophyll distribution across the leaf, the profile of the apparent extinction coefficient was calculated. The light profiles within spinach leaves could be well approximated by an apparent extinction coefficient and the Beer–Lambert/Bouguer laws. Light was absorbed at greater depths than predicted from fibre optic measurements, with 50% of blue and green light reaching 125 and 240 µm deep, respectively.     

The photostability of different chlorophyll forms in dark grown leaves of wheat

Formulae were developed for calculation of the relative amount of different pigment forms of dark grown leaves of wheat, present before and after photoreduction of the protochlorophyllide. Three pigment forms were calculated from in vivo absorption spectra: the photoreducible protochlorophyllide with absorption maximum at 650 nm and the two chlorophyll(ide) forms with absorption maximum at 684 nm and 673 nm, respectively. The formulae were used to study the changes of the pigment forms at repeated photoreduction of the protochlorophyllide, and at a repeated treatment involving photoreduction of the protochlorophyllide followed by partial photo-decomposition of the chlorophyllide formed. Five consecutive photoreductions and reaccumulations of protochlorophyllide were carried out by high intensity irradiations of one second (red light, 700 W m^-2) given at intervals of 3 h. The results show that the pool size of reaccumulated protochlorophyllide decreased sharply with the number of photoreductions performed. The absorption spectrum of the chlorophyllide formed at each photoreduction proceeded through the Shibata shift (transformation of the 684-form to the 673-form) and the late red-shift (transformation of the 673-form to other pigment form(s) in the dark). High intensity irradiation for ten minutes (red light, 700 W m^-2) immediately after each phototransformation caused a photodecomposition of about three quarters of the newly formed chlorophyllide (which was in the 684-form) while the earlier formed chlorophyll(ide) (in the 673-form) appeared not to be decomposed. This partial photodecomposition of the chlorophyllide had no effect on further accumulation of protochlorophyllide in the dark, and the absorption spectrum of the remaining chlorophyllide proceeded through the Shibata shift. The partial photodecomposition caused an inhibition of the late red-shift, and the accumulated chlorophyll(ide) remained in the 673-form.     

不同强度的红光和蓝光下菜豆叶片的荧光特性

光质和光强均是影响植物光合作用的重要外部因素,该文以菜豆(Phaseolus vulgaris)为材料,通过叶绿素荧光技术比较研究了菜豆叶片在不同光强的红光和蓝光下叶绿素荧光特性的变化规律。结果表明:随着红光和蓝光光强的增加,菜豆叶片的光适应下的最大光化学效率(Fv'/Fm')呈下降趋势,但与在红光下相比,蓝光下叶片的Fv'/Fm'值较高。随着蓝光光强的增加,菜豆叶片PSⅡ实际光化学效率(Y(Ⅱ))和光化学猝灭系数(q P和q L)先呈上升趋势之后逐渐趋于平稳;而随着红光光强的增加,以上参数呈下降趋势。随着红光和蓝光光强的增加,非光化学猝灭系数(NPQ)、相对电子传递速率(ETR)以及调节性能量耗散的量子产量Y(NPQ)均呈上升趋势,但与在红光下相比,蓝光下叶片NPQ和Y(NPQ)的值较低,而ETR值较高。非调节性能量耗散产量Y(NO)随着红光光强增加而呈上升趋势,而随着蓝光光强增加呈下降趋势。综上可见,随着光强的增加菜豆叶片的光化学效率呈降低趋势,但叶片在蓝光下的光化学吸收和利用效率高于红光。研究结果可为植物对光强和光质的响应提供一定的参考。     

Measurement of gradients of absorbed light in spinach leaves from chlorophyll fluorescence profiles

Profiles of chlorophyll fluorescence were measured in spinach leaves irradiated with monochromatic light. The characteristics of the profiles within the mesophyll were determined by the optical properties of the leaf tissue and the spectral quality of the actinic light. When leaves were infiltrated with 10^?4M DCMU [3‐(3,4‐dichlorophenyl)‐1, 1‐dimethyl‐urea] or water, treatments that minimized light scattering, irradiation with 2000 μmol m^?2 s^?1 green light produced broad Gaussian‐shaped fluorescence profiles that spanned most of the mesophyll. Profiles for chlorophyll fluorescence in the red (680 ± 16 nm) and far red (λ > 710 nm) were similar except that there was elevated red fluorescence near the adaxial leaf surface relative to far red fluorescence. Fluorescence profiles were narrower in non‐infiltrated leaf samples where light scattering increased the light gradient. The fluorescence profile was broader when the leaf was irradiated on its adaxial versus abaxial surface due to the contrasting optical properties of the palisade and spongy mesophyll. Irradiation with blue, red and green monochromatic light produced profiles that peaked 50, 100 and 150 μm, respectively, beneath the irradiated surface. These results are consistent with previous measurements of the light gradient in spinach and they agree qualitatively with measurements of carbon fixation under monochromatic blue, red and green light. These results suggest that chlorophyll fluorescence profiles may be used to estimate the distribution of quanta that are absorbed within the leaf for photosynthesis.     

Differential regulation of the accumulation of the light-harvesting chlorophyll a/b complex and ribulose bisphosphate carboxylase/oxygenase in greening pea leaves

The photoregulation of chloroplast development in pea leaves has been studied by reference to three polypeptides and their mRNAs. The polypeptides were the large subunit (LSU) and the small subunit (SSU) of ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO), and the light-harvesting chlorophyll a/b protein (LHCP). The polypeptides were assayed by a sensitive radioimmune assay, and the mRNAs were assayed by hybridization to cloned DNA probes. LSU, LSU mRNA, and LHCP mRNA were detectable in etiolated seedlings but LHCP, SSU, and SSU mRNA were at or below the limit of detection. During the first 48 hr of de-etiolation under continuous white light, the mRNAs for LSU, SSU, and LHCP increased in concentration per apical bud by about 40-fold, at least 200-fold, and about 25-fold, respectively, while the total RNA content per apical bud increased only 3.5-fold. In the same period, the LSU, SSU, and LHCP contents per bud increased at least 60-, 100-, and 200-fold, respectively. The LHCP increased steadily in concentration during de-etiolation, whereas the accumulation LSU, SSU, and SSU mRNA showed a 24-hr lag. The accumulation of SSU, SSU mRNA, and LHCP mRNA showed classical red/far-red reversibility, indicating the involvement of phytochrome in the regulatory mechanism. LSU and LSU mRNA were induced equally well by red and far-red light. The LHCP failed to accumulate except under continuous illumination. These results indicate that the accumulation of SSU is controlled largely through the steady-state level of its mRNA, which is in turn almost totally dependent on light as an inducer and on phytochrome as one of the photoreceptors. The accumulation of LSU is largely but not totally determined by the level of its mRNA, which appears to be under strong photoregulation, which has yet to be shown to involve phytochrome. Phytochrome is involved in the regulation of LHCP mRNA levels but substantial levels of the mRNA also occur in the dark. LHCP accumulation is not primarily governed by the levels of LHCP mRNA but by posttranslational stabilization in which chlorophyll synthesis plays a necessary but not sufficient role.     

Acclimation of barley to changes in light intensity: chlorophyll organization

Barley seedlings (Hordeum vulgare L. cv. Boone) were grown at 20°C with a 16h/8h light/dark cycle of either high (H) intensity (550 mole m^-2 s^-1) or low (L) intensity (55 mole m^-2 s^-1) white light. Plants were transferred from high to low (H L) or low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod and a 3 cm apical segment removed for analysis. H control plants had greater chlorophyll (Chl) per leaf area and higher Chl a/b ratios than L controls. Analysis of Chl-protein complexes revealed that H and L plants had the same percentage of total Chl (62–65%) associated with Photosystem II (PS II), but that the organization of Chl within PS II was different. H plants contained lower levels of light-harvesting complex (LHC-II) and higher levels of the PS II complex CPa compared with L plants. Leaf Chl content and Chl organization within PS II were sensitive to changes in light intensity. In H L plants, leaf Chl content decreased, Chl a/b ratio decreased, and a redistribution of Chl from CPa to LHC-II occurred during acclimation to low light. Acclimation of L H plants to high light involved an increase in leaf Chl content, an increase in Chl a/b ratio, and a decrease in LHC-II. In contrast, the level of photosystem I related Chl-protein complexes (CP1 + CP1a) was similar in all light treatments. The light acclimation process occurred slowly over a period of 6 to 8 d in H L and L H plants.Abbreviations DMF dimethylformamide - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L control plants grown under low light intensity - L H plants transferred from low to high light intensity Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11989 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.     

Ultra-structural and functional changes in the chloroplasts of detached barley leaves senescing under dark and light conditions

Changes in the chloroplast ultra-structure and photochemical function were studied in detached barley (Hordeum vulgare L. cv. Akcent) leaf segments senescing in darkness or in continuous white light of moderate intensity (90 mumol m-2 s-1) for 5 days. A rate of senescence-induced chlorophyll degradation was similar in the dark- and light-senescing segments. The Chl a/b ratio was almost unchanged in the dark-senescing segments, whereas in the light-senescing segments an increase in this ratio was observed indicating a preferential degradation of light-harvesting complexes of photosystem II. A higher level of thylakoid disorganisation (especially of granal membranes) and a very high lipid peroxidation were observed in the light-senescing segments. In spite of these findings, both the maximal and actual photochemical quantum yields of the photosystem II were highly maintained in comparison with the dark-senescing segments.     

Light-induced increase in initial chlorophyll fluorescence Fo level and the reversible inactivation of PS II reaction centers in soybean leaves

With a portable PAM-2000 fluorometer it was observed that responses of initial chlorophyll fluorescence F_o level to strong light were different in various plant species examined. When the photochemical efficiency of Photosystem II, F_v/F_m, declined, F_o increased significantly in leaves of some plants such as soybean and cotton, while F_o decreased remarkably in other plants such as wheat and barley. In order to explore the mechanism of the increase in F_o in soybean leaves, the change in D1 protein amount and effects of lincomycin and far-red light on these fluorescence parameters were observed by SDS–PAGE combined with gel scanning and chlorophyll fluorescence analysis. The following results were obtained. (1) The amount of inactive PS II reaction centers increased under strong light and decreased during subsequent dark recovery [Hong and Xu (1997) Chinese Sci Bull 42(8): 684–689]. (2) No net loss of D1 protein occurred after strong light treatment. (3) Lincomycin taken up through petioles following strong light treatment had no significant effect on D1 protein level and the decay of F_o in the dark. (4) Far-red light applied after strong light treatment could largely attenuate the increase in F_o and accelerate F_o decay in the dark. Based on these results, it is deduced that the increase in F_o under strong light is mainly due to reversible inactivation of part of PS II reaction centers, rather than the net loss of D1 protein and that reversible inactivation of PS II is prevalent in some plants.     

The detection,isolation and characterization of a light-harvesting complex which is specifically associated with Photosystem I

10% of the chlorophyll associated with a ‘native’ Photosystem (PS) I complex (110 chlorophylls/P-700) is chlorophyll (Chl) b. The Chl b is associated with a specific PS I antenna complex which we designate as LHC-I (i.e., a light-harvesting complex serving PS I). When the native PS I complex is degraded to the core complex by LHC-I extraction, there is a parallel loss of Chl b, fluorescence at 735 nm, together with 647 and 686 nm circular dichroism spectral properties, as well as a group of polypeptides of 24-19 kDa. In this paper we present a method by which the LHC-I complex can be dissociated from the native PS I. The isolated LHC-I contains significant amounts of Chl b (Chl $\text{a}\text{b ? 3.7}$). The long-wavelength fluorescence at 730 nm and circular dichroism signal at 686 nm observed in native PS I are maintained in this isolated complex. This isolated fraction also contains the low molecular weight polypeptides lost in the preparation of PS I core complex. We conclude that we have isolated the PS I antenna in an intact state and discuss its in vivo function.     

Calculation of absolute photosystem I absorption cross-sections from P700 photo-oxidation kinetics

A procedure is described which permits determination of the absolute absorption cross-section of a photosynthetic unit from the kinetics of reaction center photo-oxidation under weak, continuous actinic illumination. The method was first tested on a simple model compound of known absorption cross-section. We then applied the technique to absorption cross-section and functional antenna size measurements in photosystem I (PS I). A kinetic model is presented that can be used to fit P700 photo-oxidation measurements and extract the effective photochemical rate constant. The procedure is shown to properly correct for sample scattering and for the presence of heterogeneous absorbers (pigments not functionally coupled to P700). The relevance of these corrections to comparisons of antenna size using techniques that measure relative absorption cross-sections is discussed. Measurements on pea thylakoids in the presence and absence of 5 mM MgCl₂ show a 45% increase in PS I absorption cross-section in unstacked thylakoids. Analysis of detergent-isolated native PS I preparations (200 chlorophyll a+b/P700) clearly indicate that the preparation contains a broad distribution of antenna sizes. Finally, we confirm that Chlamydomonas reinhardtii strain LM3-A4d contains a PS I core antenna complex which binds only 60 chlorophyll a/P700, about half the functional size of the wild type complex. Limitations associated with calculation of functional antenna size from cross-section measurements are also discussed.Abbreviations PS photosystem - PS I-200 detergent-isolated photosystem I preparation containing about 200 Chl a+b/P700 - A _xxx absorbance at xxx nm - absolute absorption cross-section - I _a rate of light absorption - In _o incident actinic light intensity - _p quantum yield of photochemistry - k _eff effective rate constant for P700 photo-oxidation measured under conditions of limiting actinic intensity - k _r rate constant for P700⁺ reduction     

The action of lipases on chloroplast membranes I. The release of plastocyanin from galactolipase-treated thylakoid membranes

Bean chloroplasts treated with galactolipase (lipolytic acyl hydrolase) isolated from bean leaves showed an inhibition of photosystem I activity as measured by methyl viologen-mediated oxygen uptake and NADP⁺ photoreduction. This inhibition was partially reversed by exogenous plastocyanin added to galactolipase-treated thylakoid membranes. Galactolipase released substantial amounts of endogenous plastocyanin (about 40%) from bean chloroplasts. The results are discussed with regard to the localization of plastocyanin in thylakoid membranes.Abbreviations chlf chlorophyll - DCMU 3-(2,4-dichlorophenyl)-1,1-dimethylurea - DGDG digalactosyldiacylglycerol - MGDG monogalactosyldiacylglycerol - MV methyl viologen - NADP⁺ nicotinamide dinucleotide phosphate - PC phosphatidylcholine - PG phosphatidylglycerol - PE phosphatidylethanolamine - PI phosphatidylinositol - SQDG sulphoquinovosyldiacylglycerol - SDS sodium dodecyl sulphate - TMPD N,N,N,N-tetramethyl-p-phenylenediamine - Tricine N-Tris-(hydroxymethyl)-methylglycine - Tris Tris-(hydroxymethyl)-aminomethane     

The occurrence of rapidly reversible non-photochemical quenching of chlorophyll a fluorescence in cyanobacteria

Cyanobacteria have previously been considered to differ fundamentally from plants and algae in their regulation of light harvesting. We show here that in fact the ecologically important marine prochlorophyte, Prochlorococcus, is capable of forming rapidly reversible non-photochemical quenching of chlorophyll a fluorescence (NPQf or qE) as are freshwater cyanobacteria when they employ the iron stress induced chlorophyll-based antenna, IsiA. For Prochlorococcus, the capacity for NPQf is greater in high light-adapted strains, except during iron starvation which allows for increased quenching in low light-adapted strains. NPQf formation in freshwater cyanobacteria is accompanied by deep Fo quenching which increases with prolonged iron starvation.     

Estimation of the light distribution between photosystems I and II in intact wheat leaves by fluorescence and photoacoustic measurements

Usisng intact leaves, the extent of the decrease in chlorophyll a fluorescenece caused by the addition of continuous 710 nm light superimposed on modulated (20 Hz) 550 nm light was used to determine the distribution of this absorbed light between photosystems I () and II (). The F_o and F_m levels, which defined the total variable fluorescenece, were taken as equal to those obtained with excess 710 nm light and with saturating blue-green light, respectively.An analogous procedure was used with a photoacoustic detector, saturating white light defining a base line for oxygen yield, the levels with an without 710 nm light being used to define and respectively.The two methods gave similar values for the distribution of light between the two photosystems for the experimental conditions used, averaging 0.55 for a range of Triticum genotypes and Brachypodium sylvaticum grown in high or low light.     

The composition and spectral properties of three different forms of light-harvesting complex II

Different aggregates of LHC II play a very important role in regulating the light absorption and excitation energy transfer of plant. Trimeric LHC II was purified from spinach thylakoid membrane. In order to obtain the dimeric and monomeric LHC II, the trimer was treated with the mixture of 2% OGP and 10 μg/mL PLA₂, then loaded onto the sucrose density gradient in the presence of 0.06% triton X-100. The LHC II trimer, dimer and monomer isolated by sucrose density gradient all contained three polypeptides with molecular weight of 29, 28 and 26 kd respectively. The pigment composition showed much difference in the content of Chl b and xanthophyll among three forms of LHC II. To study the light capture and excitation energy transfer in different forms of LHC II, the absorption and fluorescence spectra were analyzed. The results clearly showed that the efficiency of energy absorption and transfer was different in the three kinds of LHC II, the highest for trimeric LHC II, intermediate for dimeric LHC II, and the lowest for monomeric LHC II. It was suggested that there might be a physiological homeostasis of different aggregates of LHC II in plants, which is significant for the plant self-regulating upon exposure to variable light environment.     

The composition and spectral properties of three different forms of light-harvesting complex II

Light-harvesting pigment-protein complexes arrayed in the thylakoid membrane serve as antenna to capture light energy and deliver it to photosynthetic reaction centers. The antenna complex of photosystem II (LHC II) is the most abundant pigment-protein complex in green plants. LHC II contains a set of polypeptides encoded by nuclear genes belonging to Lhcb family, of which, LHCB1, LHCB2 and LHCB3, encoded by Lhcb13, assemble to form heterotrimer on thylakoid membrane. The LHC II tr…     

The light stress-induced protein ELIP2 is a regulator of chlorophyll synthesis in Arabidopsis thaliana

The early light-induced proteins (ELIPs) belong to the multigenic family of pigment-binding light-harvesting complexes. ELIPs accumulate transiently and are believed to play a protective role in plants exposed to high levels of light. Constitutive expression of the ELIP2 gene in Arabidopsis resulted in a marked reduction of the pigment content of the chloroplasts, both in mature leaves and during greening of etiolated seedlings. The chlorophyll loss was associated with a decrease in the number of photosystems in the thylakoid membranes, but the photosystems present were fully assembled and functional. A detailed analysis of the chlorophyll-synthesizing pathway indicated that ELIP2 accumulation downregulated the level and activity of two important regulatory steps: 5-aminolevulinate synthesis and Mg-protoporphyrin IX (Mg-Proto IX) chelatase activity. The contents of glutamyl tRNA reductase and Mg chelatase subunits CHLH and CHLI were lowered in response to ELIP2 accumulation. In contrast, ferrochelatase activity was not affected and the inhibition of Heme synthesis was null or very moderate. As a result of reduced metabolic flow from 5-aminolevulinic acid, the steady state levels of various chlorophyll precursors (from protoporphyrin IX to protochlorophyllide) were strongly reduced in the ELIP2 overexpressors. Taken together, our results indicate that the physiological function of ELIPs could be related to the regulation of chlorophyll concentration in thylakoids. This seems to occur through an inhibition of the entire chlorophyll biosynthesis pathway from the initial precursor of tetrapyrroles, 5-aminolevulinic acid. We suggest that ELIPs work as chlorophyll sensors that modulate chlorophyll synthesis to prevent accumulation of free chlorophyll, and hence prevent photooxidative stress.     

The relationship between non-photochemical quenching of chlorophyll fluorescence and the rate of photosystem 2 photochemistry in leaves

It has been suggested previously that non-photochemical quenching of chlorophyll fluorescence is associated with a decrease in the rate of photosystem 2 (PS 2) photochemistry. In this study analyses of fluorescence yield changes, induced by flashes in leaves exhibiting different amounts of non-photochemical quenching of fluorescence, are made to determine the effect of non-photochemical excitation energy quenching processes on the rate of PS 2 photochemistry. It is demonstrated that both the high-energy state and the more slowly relaxing components of non-photochemical quenching reduce the rate of PS 2 photochemistry. Flash dosage response curves for fluorescence yield show that non-photochemical quenching processes effectively decrease the relative effective absorption cross-section for PS 2 photochemistry. It is suggested that non-photochemical quenching processes exert an effect on the rate of PS 2 photochemistry by increasing the dissipation of excitation energy by non-radiative processes in the pigment matrices of PS 2, which consequently results in a decrease in the efficiency of delivery of excitation energy for PS 2 photochemistry.     

State 1-state 2 transition in leaves and its association with ATP-induced chlorophyll fluorescence quenching

By using chlorophyll fluorescence, a study has been made of changes in spillover of excitation energy from Photosystem (PS) II to PS I associated with the State 1–State 2 transition in intact pea and barley leaves and in isolated envelope-free chloroplasts treated with ATP. (1) In pea leaves, illumination with light preferentially absorbed by PS II (Light 2) led to a condition of maximum spillover (state 2) while light preferentially absorbed by PS I induced minimum spillover condition (State 1) as judged from the redox state of Q and low-temperature emission spectra. The State 1–State 2 transitions took several minutes to occur, with the time increasing when the temperature was lowered from 19 to 6°C. (2) In contrast to the wild type, leaves of a chlorophyll b-less mutant barley did not exhibit a State 1–State 2 transition, suggesting the involvement of the light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ complex in spillover changes in higher plants. (3) Spillover in isolated pea chloroplasts was increased by treatment with ATP either (a) in Light 2 in the absence of an electron acceptor or (b) in the dark in the presence of NADPH and ferredoxin. These observations can be interpreted in terms of the model that a more reduced state of plastoquinone activates the protein kinase which catalyzes phosphorylation of the light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ complex (Allen, J.F., Bennett, J., Steinback, K.E. and Arntzen, C.J. (1981). Nature 291, 25–29). This process was found to be very temperature sensitive. (4) Pea chloroplasts illuminated in the presence of ATP seemed to exhibit a slight decrease in the degree of thylakoid stacking, and an increased intermixing of the two photosystems. (5) The possible mechanism by which protein phosphorylation regulates the State 1–State 2 changes in intact leaves is presented in terms of changes in the spatial relationship of two photosystems resulting from alteration in membrane organization.     

Assembly of LH2 light-harvesting complexes in Rhodopseudomonas palustris cells illuminated by blue and red light

We investigated the formation of the B800-850 complex in cells of the bacterium Rhodopseudomonas palustris AB illuminated by red and blue light under anaerobic growth conditions. Under red illumination, the B800-850 complex was assembled with a reduced absorption band at 850 nm. The results of re-electrophoresis of the B800-850 complex and oxidation in the presence of potassium iridate suggest its heterogeneity. It may be a mixture of two complexes (B800 and B800-850). The B800-850 complex lacks the capacity for conformational transitions if assembled under blue illumination. Accordingly, the light-harvesting complex assembled in the blue light contains polypeptides that are not synthesized under normal conditions or at increased or decreased light intensities. The mechanism of regulation of the synthesis of the polypeptides of light-harvesting the B800-850 complex and its dependence on the spectral composition of the light is discussed.     

On the long-wavelength spectral forms of chlorophyll a in Photosystem I: Spectroscopic and immunological investigations on a greening mutant of the green alga Scenedesmus obliquus

The origin of the long-wavelength chlorophyll (Chl) absorption (_peak > 680 nm) and fluorescence emission (_peak > 685 nm) has been investigated on Scenedesmus mutants (C-2A-series, lacking the ability to synthesize chlorophyll in the dark) grown at 0.3 (LL), 10 (ML) and 240 µE s^–1 m^–2(HL). LL cells are arrested in an early greening state; consequently, Chl availability determines the phenotype. LL thylakoids are totally lacking long-wavelength Chl; nonetheless, PS I and PS II are fully functional. Gel electrophoresis and Western blots indicate that four out of seven resolved LHC polypeptides seem to require a high Chl availability for assembly of functional chlorophyll-protein complexes. The PS I core-complex of ML and HL thylakoids contains long-wavelength chlorophylls, but in the PS I core-complex of LL thylakoids these pigments are lacking. We conclude that long-wavelength pigments are only present in the PS I core in the case of high Chl availability. The following hypothesis is discussed: Chl availability determines not only the LHC polypeptide pattern, but also the number of bound Chl molecules per individual pigment-protein complex. Chl-binding at non-obligatory, peripheral sites of the pigment-protein complex results in long-wavelength Chl. In the case of low Chl availability, these sites are not occupied and, therefore, the long-wavelength Chl is absent.