Relative sensitivity of various spectral forms of photosynthetic pigments to leaf senescence in wheat (Triticum aestivum L.)

The change in the characteristics of the absorption spectrum of chloroplasts which were isolated from the mature and senescing primary wheat leaves, was examined at various wavelengths in which the photosynthetic pigments mostly absorb. Chlorophyll (Chl) a was observed to be relatively more sensitive to leaf senescence than Chl b and carotenoids. Furthermore, the various spectral in vivo forms of Chl a, did not degrade to a similar extent; the far red absorbing forms of Chl a including species that absorb maximally at 692 nm (Chl a-692), 700 nm (Chl a-700) and 708 nm (Chl a 708) were found to be extremely sensitive to senescence induced losses. Both attached and detached senscing primary wheat leaves exhibited nearly similar pattern in the loss of photosynthetic pigments which suggests that the loss in long wavelength absorbing forms of Chl a is a selective indicator of leaf senescence.     

Dichroism of transient absorbance changes in the red spectral region using oriented chloroplasts. II. P-700 absorbance changes

The light induced transient absorbance changes associated with the trap of photosystem I have been studied using magnetically oriented spinach chloroplasts and a polarized measuring beam. The ΔA spectra for the two polarizations parallel and perpendicular to the plane of the photosynthetic membranes have been recorded in the spectral range 630–850 nm.A dichroic ratio greater than two is observed both in the main band around 700 nm and in the radical cation band around 810 nm, leading to the conclusion that the far-red transition moment of the P-700 dimeric species is lying almost parallel to the membrane plane.Dichroic ratios smaller than one are reported in the 650–670 nm band of the ΔA spectrum. The possible attribution of this band to excitonic interactions in the dimer favors the hypothesis of a tilting out of the membrane plane of this transition. This finding ruled out an orientation parallel to the membrane plane of the two chlorophyll molecules constituting the P-700 phototrap.A small residual transient absorbance change is observed in the absence of artificial electron acceptor. Its spectrum shows significant differences as compared to the normal P-700 spectrum: the magnitude of the signal at 700 nm is only 15–25% of the normal signal, the half-band width of the band around 700 nm is nearly twice as large and the dichroic ratio in the band is only 1.5±0.1. In the presence of ferricyanide, this signal is still observed both for intact and osmotically broken chloroplasts, suggesting a heterogeneity in the population of traps in Photosystem I.     

Enhancement of photosynthesis in Sorghum bicolor by ultraviolet radiation

We assessed the influence of ultraviolet radiation (UV) on net photosynthetic CO₂ assimilation rate (P_n) in Sorghum bicolor, with particular attention to examining whether UV can enhance P_n via direct absorption of UV and absorption of UV‐induced blue fluorescence by photosynthetic pigments. A polychromatic UV response spectrum of leaves was constructed by measuring P_n under different UV supplements using filters that had sharp transmission cut‐offs from 280 to 382 nm, against a background of non‐saturating visible light. When the abaxial surface was irradiated, P_n averaged 4.6% higher with the UV supplement that cut‐off UV at 311 nm, compared to lower and higher UV wavelength supplements. This former supplement differed from higher wavelength supplements by primarily providing more UV between 320 and 350 nm. To assess the possibility of direct absorption of UV by photosynthetic pigments, we measured the absorbance of extracted chlorophylls. Chlorophyll a had absorbance peaks at 340 and 389 nm that were 49 and 72% of that at the sorét peak. Chlorophyll b had absorbance peaks at 315 and 346 nm that were both 35% of that at the sorét peak. Since the epidermis transmits some UV, the strong UV absorbance of chlorophyll implies a potential role for irradiance beyond the bounds of the conventionally defined photosynthetically active radiation waveband (400–700 nm). To assess the role of absorption of UV‐induced blue fluorescence, we measured the UV‐induced fluorescence excitation and emission spectra of leaves. Abaxial excitation peaked at 328 nm, while emission peaked at 446 nm. In this analysis, we used our abaxial fluorescence excitation spectrum and the UV photosynthetic inhibition spectrum of Caldwell et al. (1986) to weight the UV irradiance with each cut‐off filter, thereby estimating the potential contribution of UV‐induced blue fluorescence to photosynthesis and the inhibitory effects of UV irradiance on photosynthesis, respectively. With a non‐saturating visible background, we estimate that the absorption of UV‐induced blue fluorescence and the direct absorption of UV by photosynthetic pigments maximally enhanced photosynthesis by about 1% each with the UV supplement that cut‐off UV at 311 nm. We suggest that a portion of the incident UV on the S. bicolor leaves was used to drive photosynthesis.     

Steady-state and transient polarized absorption spectroscopy of photosytem I complexes from the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus

Core antenna and reaction centre of photosytem I (PS I) complexes from the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus have been characterized by steady-state polarized absorption spectroscopy, including linear dichroism (LD) and circular dichroism (CD). CD spectra and the second derivatives of measured 77 K CD spectra reveal the spectral components found in the polarized absorption spectra indicating the excitonic origin of the spectral forms of chlorophyll in the PS I complexes. The CD bands at 669-670(+), 673(+), 680(−), 683-685(−), 696-697(−), and 711(−) nm are a common feature of used PSI complexes. The 77 K CD spectra of the trimeric PS I complexes exhibit also low amplitude components around 736 nm for A. platensis and 720 nm for T. elongatus attributed to red-most chlorophylls. The LD measurements indicate that the transition dipole moments of the red-most states are oriented parallel to the membrane plane. The formation of P700⁺A₁⁻ or ³P700 was monitored by time-resolved difference absorbance and LD spectroscopy to elucidate the spectral properties of the PS I reaction centre. The difference spectra give strong evidence for the delocalization of the excited singlet states in the reaction centre. Therefore, P700 cannot be considered as a dimer but should be regarded as a multimer of the six nearly equally coupled reaction centre chlorophylls in accordance with structure-based calculations. On the basis of the results presented in this work and earlier work in the literature it is concluded that the triplet state is localized most likely on P_A, whereas the cation is localized most likely on P_B.     

The use of photothermal radiometry in assessing leaf photosynthesis: I. General properties and correlation of energy storage to P700 redox state

Energy storage measurements by modulated photothermal radiometry (PTR) were carried out on intact leaves to assess the value of the PTR method for photosynthesis research. In particular, correlations to the redox state of P₇₀₀ under various conditions were examined. PTR monitors modulated light conversion to heat by sensing the resulting modulated infra-red radiation emitted from the leaf. It is, therefore, a complementary method to photoacoustics for estimating energy storage and its time variation, particularly under controlled leaf atmosphere.With modulated light-1 (>690 nm) the energy storage approached zero and P₇₀₀ was maximally oxidized. When background light of shorter wavelength (<690 nm-light-2) was added, energy storage momentarily increased (a manifestation of Emerson enhancement) while P₇₀₀ was reduced. The values of both parameters varied as a function of the background light intensity, keeping a mutual linear relationship. Following the initial change, there was a slow reversal transient of P₇₀₀ oxidation with a parallel decrease in energy storage. Temporal correlation to P₇₀₀ redox state after dark adaptation was observed also for the energy storage measured in modulated light 2 when combined with background actinic light of medium intensity (about 50 W m²). Under these circumstances P₇₀₀ was almost totally oxidized initially and then gradually reduced while energy storage was initially low and then increased parallel to P₇₀₀ reduction.A comparison between the maximum energy storage in modulated light 1, enhanced by background light 2, to the energy storage with short wavelength light (where light tends to be more evenly distributed) indicates a comparable contribution to energy storage from each active photosystem. The above experiments indicate that energy storage contribution from PS I is directly related to the extent of openness of its reaction-centers.While some aspects of the data call for more experimentation, these experiments already establish PTR as a valuable method to monitor photosynthetic energy storage activity in vivo, particularly when used simultaneously with other non-invasive methods.Abbreviations ES energy storage - light 1 or light 2 light of spectral distribution which favors absorption in PS I or PS II, resp. - PTR photothermal radiometry - P₇₀₀ the primary donor in PS I reaction center     

Nanosecond flash studies of the absorption spectrum of the Photosystem I primary acceptor Ao

Photosystem I particles devoid of the secondary electron acceptor A₁ were studied by nanosecond flash absorption. The primary radical pair (P-700⁺, A₀ ^-) decays with a half-time of 35 ns. The difference spectrum was measured (400–870 nm). After subtraction of the P-700⁺/P-700 difference spectrum, the A₀ ^-/A₀ was obtained. It includes bleachings centered at 690 and 430 nm, and broad positive bands in the near infra-red and the blue-green. This spectrum is consistent with A₀ being chlorophyll a absorbing at 690 nm.     

Range of photosynthetic control of postillumination P700+ reduction rate in sunflower leaves

The kinetics of the postillumination reduction of P700⁺ which reflects the rate constant for plastoquinol (PQH₂) oxidation was recorded in sunflower leaves at different photon absorption densities (PAD), CO₂ and O₂ concentrations. The P700 oxidation state was calculated from the leaf transmittance at 830 nm logged at 50 s intervals. The P700⁺ dark reduction kinetics were fitted with two exponents with time constants of 6.5 and about 45 ms at atmospheric CO₂ and O₂ concentrations. The time constant of the fast component, which is the major contributor to the linear electron transport rate (ETR), did not change over the range of PADs of 14.5 to 134 nmol cm^-2 s^-1 in 21% O₂, but it increased up to 40 ms under severe limitation of ETR at low O₂ and CO₂. The acceptor side of Photosystem I (PS I) became reduced in correlation with the downregulation of the PQH₂ oxidation rate constant. It is concluded that thylakoid pH-related downregulation of the PQH₂ oxidation rate constant (photosynthetic control) is not present under normal atmospheric conditions but appears under severe limitation of the availability of electron acceptors. The measured range of photosynthetic control fits with the maximum variation of ETR under natural stress in C₃ plants. Increasing the carboxylase/oxygenase specificity would lead to higher reduction of the PS I acceptor side under stress.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - C_w cell-wall CO₂ concentration, M - ETR electron transport rate - Fd ferredoxin - FNR ferredoxin-NADP reductase - FRL far-red light - PC plastocyanin - PAD photon absorption density nmol cm^-2 s^-1 - PFD photon flux density nmol cm^-2 s^-1 - PS I Photosystem I complex - PQ plastoquinon - PQH₂ plastoquinol - PS II Photosystem II complex - P700 Photosystem I donor pigment, reduced - S830 830 nm signal (D830, difference of S830 from the dark level) - WL white light - Y_l maximum quantum yield of PS I electron transport, rel. un     

Spectrophotometric titrations of ferricytochrome C with ferrohexacyanide in the pH range 5 to 7

Spectrophotometric titrations were conducted on the system horse heart ferricytochromec plus ferrohexacyanide in the pH range 5 to 7 and at temperatures 8, 18, 22 and 28°C. A difference extinction coefficient for reducedvs. oxidized cytochromec at 550 nm of 21 mmol^–1cm^–1 was used in part of the evaluations. On the assumption that only one electron-transferlinked proton dissociation is effective for both ferro- and ferricytochromec in this pH range, various possible models are developed with only three conforming with the experimental pH dependence of the spectrophotometric equilibrium constant. The data conform best to a model with protonic dissociation constants between pH 5 and 7 such that the reduced cytochromec species is at least a factor of 3 more acidic than the one for oxidized cytochromec (with pK_H 6). This interpretation holds least for the data at 22°C, which points to a structural rearrangement at about this temperature (Czerlinski and Bracokova, 1973; Zabinski and Czerlinski, 1974; Zabinski, et al., 1974). While the extinction coefficient of ferrocytochromec shows no significant change with pH and temperature, the one for ferricytochromec does: it is about 5% larger at pH 5 than at pH 7 (550 nm). Graphs for the absorption change of ferricytochromec (pH 7 as reference) document the details over the wavelength range 500 to 750 nm.     

A single step separation of PS 1, PS 2 and chlorophyll-antenna particles from spinach chloroplasts

After solubilization of photosynthetic membranes by digitonin, three main protein pigment complexes were isolated by electrophoresis with deoxycholate as detergent.The band with the slowest mobility, fraction 1, had PS 1 activity and was devoid of PS 2 activity. This fraction was four times enriched in P700 when compared with chloroplasts. Fraction 1 had little chl b, a long wavelength absorption maximum in the red, a maximum of low temperature emission fluorescence at 730nm, and a circular dichroism spectrum characteristic of PS 1 enriched fraction.Fraction 2 exhibited a PS 2 activity and no PS 1 activity. It was enriched five times in PS 2 reaction centre and had little chl b and carotenoids. The absorption maximum was at 674 nm and the low temperature fluorescence emission maximum was at 700 nm. Fraction 2 might be useful PS 2 enriched particle because of the great stability of this fraction with regard to photochemical activity and also rapidity and simplicity of its preparation.Fraction 3, which had the fastest migration, was devoid of photochemical activities; It was rich in chl b and had the fluorescence and the circular dichroism spectrum characteristic of an antenna complex.Abbreviations PS 1 (2) photosystem 1 (2) - chl chlorophyll - car carotenoid - Q primary plastoquinone electron acceptor - P700 primary electron donor of PS 1 - P680 primary electron donor of PS 2 - K₃Fe(CN)₆ potassium ferricyanide - DCMU dichlorophenyldimethylurea - DCPIP dichlorophenolindophenol - DPC diphenyl-carbazide     

On the mutual orientation of pigments in Photosystem I particles from green plants

The mutual orientation of pigments in Photosystem I reaction centers from spinach is evaluated by polarized photochemistry. The photoinduced linear dichroism of the absorption changes of chlorophyll a₁ at 701 nm is studied as function of the excitation wavelength. The Photosystem I reaction center particles contain about 100 and if depleted about 40 chlorophylls, respectively. To prevent their rapid Brownian rotation they were immobilized on DEAE-Sephadex.The excitation spectrum of the linear dichroism reveals a high degree of order between the long axis of β-carotene and the Q_y transition moments of those chlorophyll a molecules absorbing at the red end of the spectrum. The latter are the most endangered ones for destructive oxidation via their triplet state. Hence, the location of β-carotene in close proximity to and in parallel with these chlorophylls seems to be most favourable for the protective role of β-carotene within the antennae system I. It is observed that the dichroic ratio of the absorption changes of chlorophyll a₁ does not exceed a figure of $\text{4}\text{3}$, which characterizes a circularly degenerate system, even at far red excitation (724 nm). This will hit selectively those few chlorophyll a molecules with their peak absorption at about 700 nm (including the photooxidizable dimer). We conclude, if the dimer is the only species peaking at 700 nm then the two chlorophyll a within the dimer have their y-axes oriented perpendicular to each other. If there are some antennae in addition to the dimer, the y-axes of all chlorophyll-a peaking at 700 nm form a star which accounts for the circular degeneracy of absorption.     

Orientation of reaction center and antenna chromophores in the photosynthetic membrane of rhodopseudomonas viridis

Whole cells of Rhodopseudomonas viridis were oriented in a magnetic field. The degree of orientation of the cells was determined by using a photoselection technique. In order to deduce the orientation of the antennae and chromophores of the reaction centers with respect to the membrane plane, we performed linear dichroism measurements of absolute spectra and light induced difference spectra linked to states P⁺I and PI^? on oriented cells. These measurements lead to the following conclusions:The antennae bacteriochlorophyll molecular plane is nearly perpendicular to the membrane. The Q_y and Q_x transitions moments of these molecules make respectively angles of 20 and 70°ith the membrane plane. The antenna carotenoid molecules make an angle of 45°ith the membrane.The primary electron donor possesses two transition moments centered respectively at 970 and 850 nm. The 970 nm transition moment is parallel to the membrane plane, the 850 nm transition is tilted out of the plane. Upon photooxidation of this primary electron donor, a monomer-like absorption band appears at 805 nm. Its transition makes an angle smaller than 25° with the membrane. The photooxidation of the dimer also induces an absorption band shift for the two other bacteriochlorophyll molecules of the reaction center. The absorption band shifts of the two bacteriochlorophyll molecules occur in opposite direction.One bacteriopheophytin molecule is photoreduced in state PI^?. This photoreduction induces an absorption band shift for only one bacteriochlorophyll molecule. Finally, the geometry of the dimeric primary donor seems to be affected by the presence of a negative charge in the reaction center.     

Differently oriented chlorophylls in Mesotaenium caldariorum detected by microphotometrical dichroism measurements in vivo

The chloroplasts of individual cells of Mesotaenium caldariorum were examined microphotometrically under non-polarized and polarized measuring light. The measurement with non-polarized light showed different absorption bands of the thylakoids depending on the position of their surface with respect to the incident light beam: in the edge position, the absorption bands lie at 672 nm, in the face position at 678 nm. From this difference in absorption maxima, we conclude that the molecules related to the sub-bands at the two wavelengths are oriented differently. The Q_y transition of the molecules which absorb light at 678 nm must be oriented parallel to the face of the thylakoids (fraction I), while that of the molecules absorbing at 672 nm is oriented perpendicular to the face (fraction II). Measurement with polarized light leads to the same conclusion that two fractions of differently oriented chlorophylls exist: In the edge position, a very large difference between E_∥ and E_⊥ (dichroism) was found in red light, with a maximum of E_∥ lying at 675 nm and a maximum of E_⊥ at 670 nm, with a shoulder at 650 nm. In the blue region, especially in the Soret band zone, the chloroplast showed a negative dichroism in the edge position, which changes over to positive values when the wavelength exceeds 450 nm. In the face position no dichroism in red or blue light could be detected. Comparison of the ‘edge position dichroism’ in red light with that in blue light justifies the supposition that the chlorin planes of the chlorophyll molecules may be oriented perpendicular or parallel to the thylakoid face, in the case of perpendicular orientation with the Q_y transitions of fraction II and the x-transitions (B_x, Q_x) of fraction I projecting out of the plane, and for parallel orientation with all transition moments lying parallel to the plane (fraction I). The relative dichroism, $\text{(E}{}_{\mathrm{\parallel }}\text{? E}{}_{\mathrm{\perp }}\text{)}\text{(E}{}_{\mathrm{\parallel }}\text{+ E}{}_{\mathrm{\perp }}\text{)}$, measured at the edge position amounts to 0.34 (i.e., 34% of the total absorption) at 680 nm. These data probably do not reflect the total quantity of oriented chlorophyll because from the opposite orientations of the Q_y transition moments of fraction I and II pigment a partial quenching of the measurable dichroism results. The red light absorption bands of the two chlorophylls oriented in an opposite manner (fractions I and II) correspond to the known bands of Photosystem I and II.     

UV‐radiation effects on photosynthesis and photoprotection in gametophytic and sporophytic stages of the freshwater red alga Kumanoa ambigua (Rhodophyta,Batrachospermales)

The aim of this study was to analyze the photosynthetic performance of gametophytic and sporophytic (‘Chantransia’) stages of Kumanoa ambigua in culture under UV radiation. We hypothesized that both life history stages of K. ambigua would exhibit different photosynthetic responses to UVR exposure. Experiments were performed under three conditions: (i) photosynthetically active radiation (PAR) only (400–700 nm), P control; (ii) PAR + UVA (320–700 nm), PA treatment; and (iii) PAR + UVA + UVB (280–700 nm), PAB treatment. The photosynthetic parameters were measured as in vivo chlorophyll a fluorescence. Differences were found between life stages, observing higher values of NPQ and effective quantum yields (ΔF/F_m′) under UVA and PAR in gametophytes compared to sporophytes. One type of mycosporine‐like amino acid (MAA) was detected in the gametophyte in all treatments, but not in the ‘Chantransia’ stage. The increased photosynthetic performance for some parameters and the presence of MAA in gametophyte suggest that it is less sensitive to UV radiation, particularly UVA, in comparison to sporophyte under culture conditions. This approach is relevant for a better understanding of the adaptation and physiological acclimation of freshwater Rhodophyta to varying light climates in terms of global changes.     

MICROENVIRONMENTAL CONTROL OF PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN AN EPILITHIC CYANOBACTERIAL BIOFILM1

The photosynthetic performance of an epilithic cyano-bacterial biofilm was studied in relation to the in situ light field by the use of combined microsensor measurements of O₂, photosynthesis, and spectral scalar irradiance. The high density of the dominant filamentous cyanobacteria (Oscillatoria sp.) embedded in a matrix of exopolymers and bacteria resulted in a photic zone of < 0.7 mm. At the biofilm surface, the prevailing irradiance and spectral composition were significantly different from the incident light. Multiple scattering led to an intensity maximum for photic light (400–700 nm) of ca. 120% of incident quantum irradiance at the biofilm surface. At the bottom of the euphotic zone in the biofilm, light was attenuated strongly to < 5–10% of the incident surface irradiance. Strong spectral signals from chlorophyll a (440 and 675 nm) and phycobilins (phycoerythrin 540–570 nm, phycocyanin 615–625 nm) were observed as distinct maxima in the scalar irradiance attenuation spectra in the upper 0.0–0.5 mm of the biofilm. The action spectrum for photosynthesis in the cyanobacterial layer revealed peak photosynthetic activity at absorption wavelengths of phycobilins, whereas only low photosynthesis rates were induced by light absorption of carotenoids (450–550 nm). Respiration rates in light- and dark-incubated biofilms were determined using simple flux calculations on measured O₂ concentration profiles and photosynthetic rates. A significantly higher areal O₂ consumption was found in illuminated biofilms than in dark-incubated biofilms. Although photorespiration accounted for part of the increase, the enhanced areal O₂ consumption of illuminated biofilms could also be ascribed to a deeper oxygen penetration in light as well as an enhanced volumetric O₂ respiration in and below the photic zone. Gross photosynthesis was largely unaffected by increasing flow velocities, whereas the O₂ flux out of the photic zone, that is, net photosynthesis, increased with flow velocity. Consequently, the amount of produced O₂ consumed within the biofilm decreased with increasing flow velocity. Our data indicated a close coupling of photosynthesis and respiration in biofilms, where the dissolved inorganic carbon requirement of the photo-synthetic population may largely be covered by the respiration of closely associated populations of heterotrophic bacteria consuming a significant part of the photosynthetically produced oxygen and organic carbon.     

Photosystem I charge separation in the absence of centers A and B. I. Optical characterization of center ‘A2’ and evidence for its association with a 64-kDa peptide

The flash-induced absorption transient at 698 nm in a Photosystem I subchloroplast particle showed the following characteristics after addition of 0.25–2.0% lithium dodecyl sulfate (LDS). (i) The 30-ms transient corresponding to the P-700⁺ P-430⁻ backreaction was replaced by a 1.2-ms transient. (ii) The amplitude of the transient did not change immediately after LDS addition, but decayed with a half-life of 10 min at pH 8.5. (iii) Methyl viologen had no effect on the magnitude or kinetics of the transient, indicating that it cannot accept an electron from this component. (iv) The difference spectrum of the transient from 400 nm to 500 nm was characteristic of an iron-sulfur protein. (v) The transient followed first-order Arrhenius behavior between 298 K and 225 K with an activation energy of 13.3 kJ/mol; between 225 K and 77 K, the 85-ms half-time remained temperature-invariant. These properties suggest that the LDS-induced absorption transient corresponds to the P-700⁺ A⁻₂ change recombination seen in the absence of a reduced electron-acceptor system. In the presence of LDS, the reaction-center complex was dissociated, allowing removal of the smaller peptides from the 64-kDa P-700-containing protein. With prolonged incubation, the iron-sulfur clusters were destroyed through conversion of the labile sulfide to zero-valence sulfur. About 35% of the zero-valence sulfur was found associated with the 64-kDa protein under conditions that allowed separation of the small peptides. We interpret the long lifetime of the P-700⁺ A⁻₂ transient after LDS addition and the association of zero-valence sulfur with a 64-kDa protein to indicate that A₂ is closely associated with, and perhaps integral with, the P-700-containing protein.     

Absorption and fluorescence spectra of chlorophyll-proteins isolated from Euglena gracilis

A spectroscopic study of chlorophyll-protein complexes isolated from Euglena gracilis membranes was carried out to gain information about the state of chlorophyll in vivo and energy transfer in photosynthesis. The membranes were dissociated by Triton X-100 and separated into fractions by sucrose gradient centrifugation and hydroxyapatite chromatography. Four different types of chlorophyll-protein complexes were distinguished from each other and from detergent-solubilized chlorophyll in these fractions by examination of their absorption, fluorescence excitation (400–500 nm) and emission spectra at low temperature. These types were: (1). A mixture of antenna chlorophyll a- and chlorophyll ab-proteins with an absorption maximum at 669 and emission at 682 nm; (2) a P-700-chlorophyll a-protein (chlorophyll: P-700 = 30 : 1), termed CPI with an absorption maximum at 676 nm and emission maxima at 698 and 718 nm; (3) a second chlorophyll a-protein (CPI-2) less enriched in P-700, with an absorption maximum at 676 nm and emission maxima at 680, 722 and 731 nm; (4) a third chlorophyll a-protein (CPa₁) with no P-700, absorption maxima at 670 and 683 nm, and an unusually sharp emission maximum at 687 nm. Treatment of CPa₁ with sodium dodecyl sulfate drastically altered its spectroscopic properties indicating that at least some chlorophyll-proteins isolated with this detergent are partially denatured. The results suggest that the complex absorption spectra of chlorophyll in vivo are caused by varying proportions of different chlorophyll-protein complexes, each with different groups of chlorophyll molecules bound to it and making up a unique entity in terms of electronic transitions.     

Distribution of Chlorophyll between the Two Photoreactions in Photosynthesis of the Blue-Green Alga Anacystis nidulans Grown at Two Different Light Intensities

Anacystis nidulans was grown in white light of two different intensities, 7 and 50 W ·m^?2. The in vivo pigmentations of the two cultures were compared. The ratio phycocyanin/chlorophyll a was 0.96 for cells grown at 7 W · m^?2 and 0.37 for cells grown at 50 W · m^?2. Phycocyanin-free photosynthetic lamellae (PSI-particles) were prepared, using French press treatment and fractionated centrifugation. Algae grown in the irradiance of 50 W · m^?2 showed a chlorophyll a/P700 ratio of 260, while algae grown at 7 W · m^?2 had a value of 140. Corresponding PSI-particles showed values of 122 and 109 respectively. Light-induced absorption difference spectra measured between 400–450nm indicated different ratios between cytochrome f and P700 in the two algal cultures. Enhancement studies of photosynthetic oxygen evolution were carried out. When a background beam of 691 nm was superimposed upon a signal beam of 625 nm, good enhancement was observed for both cultures. With the wavelengths 675 and 691 nm together a pronounced enhancement could be detected only in algae grown at the higher light level. Absorption spectra recorded on whole cells at 77°K revealed a small shift of the main red chlorophyll a absorption peak caused by light intensity. It is proposed that the reduction of the phycocyanin/chlorophyll a ratio in high light-grown cells is accompanied by an increased energy distribution by chlorophyll a into PSII.     

Physiological Characteristics of Photosynthesis and Respiration in Stems of Populus tremuloides Michx

The physiological responses of 6- to 8-year-old aspen (Populus tremuloides Michx.) stems to temperature, light, and CO₂ concentration were investigated in the field throughout the year using infrared CO₂ analysis. Light response studies showed that the rate of gross photosynthesis was linear from 0 to 400 ft-c (0 to 1.6 mw/cm² of 400-700 nm) with light saturation being reached between 800 to 1400 ft-c (3.2 to 5.6 mw/cm² of 400-700 nm). At this light intensity, the respiratory CO₂ loss was reduced to 10 to 15% of dark rates. Net photosynthetic CO₂ uptake was not observed even at intensities as high as 3400 ft-c (13.6 mw/cm² of 400-700 nm). The light response curve was similar for both winter and summer stems.     

Radiationless transitions as a protection mechanism against photoinhibition in higher plants and a red alga

Exposure of the red alga Porphyra perforata or leaves of Phytolacca americana and Echinodorus sp. to white light equivalent to full sunlight for short periods induced large decreases of variable fluorescence measured at 695 nm at 77K. This change was not produced by photoinhibition but rather appeared to result from an inorease in the rate constant of radiationless transition in the reaction centers of photosystem II. It is proposed that this increase is related to the formation of the high energy state which serves as a photoprotective mechanism in plants.     

Spectroscopic properties of the reaction center and of the 47 kDa chlorophyll protein of Photosystem II

The D1-D2-cytochrome b-559 reaction center complex and the 47 kDa antenna chlorophyll protein isolated from spinach Photosystem II were characterized by means of low temperature absorption and fluorescence spectroscopy. The low temperature absorption spectrum of the D1-D2-cytochrome b-559 complex showed two bands in the Q_y region located at 670 and 680 nm. On the basis of its absorption maximum and orientation the latter component may be attributed at least in part to P-680, the primary electron donor of Photosystem II. The 47 kDa antenna complex showed absorption bands at 660, 668 and 677 nm and a minor component at 690 nm. The latter transition appeared to be associated with the characteristic low temperature 695 nm fluorescence band of Photosystem II. The 695 nm emission band was absent in the D1-D2 complex, which indicates that it does not originate from the reaction center pheophytin, as earlier proposed. The transition dipole responsible for the main fluorescence at 684 nm from this complex had a parallel orientation with respect to the membrane plane in the native structure. The reaction center preparation contains two spectrally distinct carotenoids with different orientations.