Transgressive physiological and transcriptomic responses to light stress in allopolyploid Glycine dolichocarpa (Leguminosae).

Allopolyploidy is often associated with increased photosynthetic capacity as well as enhanced stress tolerance. Excess light is a ubiquitous plant stress associated with photosynthetic light harvesting. We show that under chronic excess light, the capacity for non-photochemical quenching (NPQ(max)), a photoprotective mechanism, was higher in a recently formed natural allotetraploid (Glycine dolichocarpa, designated 'T2') than in its diploid progenitors (G. tomentella, 'D3'; and G. syndetika, 'D4'). This enhancement in NPQ(max) was due to an increase in energy-dependent quenching (qE) relative to D3, combined with an increase in zeaxanthin-dependent quenching (qZ) relative to D4. To explore the genetic basis for this phenotype, we profiled D3, D4 and T2 leaf transcriptomes and found that T2 overexpressed genes of the water-water cycle relative to both diploid progenitors, as well as genes involved in cyclic electron flow around photosystem I (CEF-PSI) and the xanthophyll cycle, relative to D4. Xanthophyll pigments have critical roles in NPQ, and the water-water cycle and CEF-PSI are non-photosynthetic electron transport pathways believed to facilitate NPQ formation. In the absence of CO(2), T2 also exhibited greater quantum yield of photosystem II than either diploid, indicating a greater capacity for non-photosynthetic electron transport. We postulate that, relative to its diploid progenitors, T2 is able to achieve higher NPQ(max) due to an increase in xanthophyll pigments coupled with enhanced electron flow through the water-water cycle and CEF-PSI.

The effects of gold and Co-adsorbed carbon on the adsorption and thermal decomposition of acetic acid on Pd{111}.

The growth and annealing behavior of ultrathin Au films on Pd{111} were monitored with scanning tunneling microscopy (STM) and medium energy ion scattering (MEIS). The adsorption of acetic acid on both clean and deliberately carbon-contaminated bimetallic surfaces was investigated with reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). We report that the surface chemistry of acetic acid is strongly modified by the presence of Au in the bimetallic surface which acts both to stabilize adsorbed acetate and to decrease the tendency of acetic acid to decompose on adsorption to produce adsorbed carbon. The adsorption of acetic acid at 300 K is found to cause measurable segregation of Pd to the surface for all surface compositions tested.

Excited-state dynamics in photosystem II: insights from the x-ray crystal structure.

The heart of oxygenic photosynthesis is photosystem II (PSII), a multisubunit protein complex that uses solar energy to drive the splitting of water and production of molecular oxygen. The effectiveness of the photochemical reaction center of PSII depends on the efficient transfer of excitation energy from the surrounding antenna chlorophylls. A kinetic model for PSII, based on the x-ray crystal structure coordinates of 37 antenna and reaction center pigment molecules, allows us to map the major energy transfer routes from the antenna chlorophylls to the reaction center chromophores. The model shows that energy transfer to the reaction center is slow compared with the rate of primary electron transport and depends on a few bridging chlorophyll molecules. This unexpected energetic isolation of the reaction center in PSII is similar to that found in the bacterial photosystem, conflicts with the established view of the photophysics of PSII, and may be a functional requirement for primary photochemistry in photosynthesis. In addition, the model predicts a value for the intrinsic photochemical rate constant that is 4 times that found in bacterial reaction centers.

Reaction centers of photosystem II with a chemically-modified pigment composition: exchange of pheophytins with 13(1)-deoxo-13(1)-hydroxy-pheophytin a.

Isolated reaction centers of photosystem II with an altered pigment content were obtained by chemical exchange of the native pheophytin a molecules with externally added 13(1)-deoxo-13(1)-hydroxy-pheophytin a. Judged from a comparison of the absorption spectra and photochemical activities of exchanged and control reaction centers, 70-80% of the pheophytin molecules active in charge separation are replaced by 13(1)-deoxo-13(1)-hydroxy-pheophytin a after double application of the exchange procedure. The new molecule at the active branch was not active photochemically. This appears to be the first stable preparation in which a redox active chromophore of the reaction center of photosystem II was modified by chemical substitution. The data are compatible with the presence of an active and inactive branch of cofactors, as in bacterial reaction centers. Possible applications of the 13(1)-deoxo-13(1)-hydroxy-pheophytin a-exchanged preparation to the spectral and functional analysis of native reaction centers of photosystem II are discussed.

Spectral and photochemical properties of borohydride-treated D1-D2-cytochrome b-559 complex of photosystem II.

The D1-D2-cytochrome b-559 reaction center complex of photosystem II with an altered pigment composition was prepared from the original complex by treatment with sodium borohydride (BH4-). The absorption spectra of the modified and original complexes were compared to each other and to the spectra of purified chlorophyll a and pheophytin a (Pheo a) treated with BH4- in methanolic solution. The results of these comparisons are consistent with the presence in the modified complex of an irreversibly reduced Pheo a molecule, most likely 13(1)-deoxo-13(1)-hydroxy-Pheo a, replacing one of the two native Pheo a molecules present in the original complex. Similar to the original preparation, the modified complex was capable of a steady-state photoaccumulation of Pheo- and P680+. It is concluded that the pheophytin a molecule which undergoes borohydride reduction is not involved in the primary charge separation and seems to represent a previously postulated photochemically inactive Pheo a molecule. The Qy and Qx transitions of this molecule were determined to be located at 5 degrees C at 679.5-680 nm and 542 nm, respectively.

Chemical platinization and its effect on excitation transfer dynamics and P700 photooxidation kinetics in isolated photosystem I.

Isolated photosystem I (PSI) reaction center/core antenna complexes (PSI-40) were platinized by reduction of [PtCl6]2- at 20 degrees C and neutral pH. PSI particles were visualized directly on a gold surface by scanning tunneling microscopy (STM) before and after platinization. STM results showed that PSI particles were monomeric and roughly ellipsoidal with major and minor axes of 6 and 5 nm, respectively. Platinization deposited approximately 1000 platinum atoms on each PSI particle and made the average size significantly larger (9 x 7 nm). In addition to direct STM visualization, the presence of metallic platinum on the PSI complexes was detected by its effect of actinic shading and electrostatic shielding on P700 photooxidation and P700+ reduction. The reaction centers (P700) in both platinized and nonplatinized PSI-40 were photooxidized by light and reduced by ascorbate repeatedly, although at somewhat slower rates in platinized PSI because of the presence of platinum. The effect of platinization on excitation transfer and trapping dynamics was examined by measuring picosecond fluorescence decay kinetics in PSI-40. The fluorescence decay kinetics in both platinized and control samples can be described as a sum of three exponential components. The dominant (amplitude 0.98) and photochemically limited excitation lifetime remained the same (16 ps) before and after platinization. The excitation transfer and trapping in platinized PSI-40 was essentially as efficient as that in the control (without platinization) PSI. The platinization also did not affect the intermediate-lifetime (400-600 ps) and long-lifetime (> 2500 ps) components, which likely are related to intrinsic electron transport and to functionally uncoupled chlorophylls, respectively. The amplitudes of these two components were exceptionally small in both of the samples. These results provide direct evidence that although platinization dramatically alters the photocatalytic properties of PSI, it does not alter the intrinsic excitation dynamics and initial electron transfer reactions in PSI.

The Effects of Excess Irradiance on Photosynthesis in the Marine Diatom Phaeodactylum tricornutum.

The response of Phaeodactylum tricornutum to excess light was remarkably similar to that observed in higher plants and green algae and was characterized by complex changes in minimal fluorescence yields of fully dark-adapted samples and declines in maximum variable fluorescence levels and oxygen evolution rates. In our study the parallel decreases in the effective rate constant for photosystem II (PSII) photochemistry, the variable fluorescence yield of a dark-adapted sample, and light-limited O2 evolution rates after short (0-10 min) exposures to photoinhibitory conditions could not be attributed to damage or down-regulation of PSII reaction centers. Instead, these changes were consistent with the presence of nonphotochemical quenching of PSII excitation energy in the antennae. This quenching was analogous to that component of nonphotochemical quenching studied in higher plants that is associated with photoinhibition of photosynthesis and/or processes protecting against photoinhibition in that it did not relax readily in the dark and persisted in the absence of a bulk transthylakoid proton gradient. The quenching was most likely associated with photoprotective processes in the PSII antenna that reduced the extent of photoinhibitory damage, particularly after longer exposures. Our results suggest that a large population of damaged, slowly recovering PSII centers did not form in Phaeodactylum even after 60 min of exposure to excess actinic light.

Excited state dynamics in chlorophyll-based antennae: the role of transfer equilibrium.

We present computer simulations of excited state dynamics in models of PS I and PS II which are based upon known structural and spectral properties of the antennae. In particular, these models constrain the pigment binding sites to three-dimensional volumes determined from molecular properties of the antenna complexes. The simulations demonstrate that within a 10-30 ps after light absorption, rapid energy transfer among coupled antenna chlorophylls leads to a quasiequilibrium state in which the fraction of the excited state on any antenna chlorophyll, normalized to the total excited state remaining on the model, remains constant with time. We describe this quasiequilibrium state as a "transfer equilibrium" (TE) state because of its dependence on the rates of processes that couple excited state motion and quenching in the antenna as well as on the individual antenna site energies and temperature. The TE state is not a true equilibrium in that loss of the excited state primarily due to photochemistry (but also due to fluorescence, thermal emission, and intersystem crossing) continues once TE is established. Depending on the dynamics of the system, the normalized distribution of excited state at TE may differ substantially from the Boltzmann distribution (the state of the model at infinite time in the absence of any avenues for decay of excited state). The models predict lifetimes, equilibration times, and photochemical yields that are in agreement with experimental data and affirm trap-limited dynamics in both photosystems. The rapid occurrence of TE states implies that any excited state dynamics that depends on antenna structure and excitation wavelength must occur before the TE state is established. We demonstrate that the excited state distribution of the TE state is central to determining the excited state lifetime and quantum efficiency of photochemistry.

Photochemical and Nonphotochemical Fluorescence Quenching Processes in the Diatom Phaeodactylum tricornutum.

Nonphotochemical fluorescence quenching was found to exist in the dark-adapted state in the diatom Phaeodactylum tricornutum. Pretreatment of cells with the uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP) or with nigericin resulted in increases in dark-adapted minimum and maximum fluorescence yields. This suggests that a pH gradient exists across the thylakoid membrane in the dark, which serves to quench fluorescence levels nonphotochemically. The physiological processes involved in establishing this proton gradient were sensitive to anaerobiosis and antimycin A. Based on these results, it is likely that this energization of the thylakoid membrane is due in part to chlororespiration, which involves oxygen-dependent electron flow through the plastoquinone pool. Chlororespiration has been shown previously to occur in diatoms. In addition, we observed that cells treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea exhibited very strong nonphotochemical quenching when illuminated with actinic light. The rate and extent of this quenching were light-intensity dependent. This quenching was reversed upon addition of CCCP or nigericin and was thus due primarily to the establishment of a pH gradient across the thylakoid membrane. Preincubation of cells with CCCP or nigericin or antimycin A completely abolished this quenching. Cyclic electron transport processes around photosystem I may be involved in establishing this proton gradient across the thylakoid membrane under conditions where linear electron transport is inhibited. At steady state under normal physiological conditions, the qualitative changes in photochemical and nonphotochemical fluorescence quenching at increasing photon flux densities were similar to those in higher plants. However, important quantitative differences existed at limiting and saturating intensities. Dissimilarities in the factors that regulate fluorescence quenching mechanisms in these organisms may account for these differences.

Limitations of the pulse-modulated technique for measuring the fluorescence characteristics of algae.

Precise measurements of the minimal fluorescence yield (F(o)) and maximal fluorescence yield (F(m)) of a dark-adapted sample are prerequisites for the quantification of other fluorescence parameters. The pulse amplitude-modulated chlorophyll fluorometer (PAM 101 Chlorophyll Fluorometer, Heinz Walz, Effeltrich, Germany) and saturating pulse technique have frequently been used in measuring F(o) and F(m) and in resolving the contributions of photochemical and nonphotochemical quenching to the total fluorescence yield. The extent to which instrument-dependent factors may affect the accurate measurement of F(o) and F(m) is addressed. It is shown that the increase in pulse amplitude-modulated measuring beam intensity at 1.6 and 100 kHz was nonlinear at higher light intensity settings. The implications of this for measurements of F(o) (1.6 kHz) and F(m) (100 kHz) are discussed. It is also demonstrated that underestimation of F(m) may result due to saturation of the PAM 101 photodiode by scattered infrared light associated with intense light pulses. In addition, it is shown how sample-dependent factors may affect measurements of F(o) and F(m) in samples with low chlorophyll concentrations, in particular, dilute algal suspensions of Phaeodactylum tricornutum and Chlamydomonas reinhardtii. A technique is presented for the accurate measurement of F(o) in algal suspensions (<8 mug chlorophyll a mL(-1)). The importance of examining the saturating pulse transient and F(m) level as a function of the damping setting, pulse width, and pulse intensity, and in the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea is discussed.

Femtosecond energy-transfer processes in the B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1.

The B800-to-B850 energy transfer time in the purified B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1 is determined to be 0.7 ps at room temperature. The electronic state dynamics of the principal carotenoid of this species, spheroidene, are examined, both in vivo and in vitro, by direct femtosecond time-resolved experiments and by fluorescence emission yield studies. Evidence is presented which suggests that carotenoid-to-bacteriochlorophyll energy transfer may occur directly from the initially excited carotenoid S2 state, as well as from the carotenoid S1 state. Further support for this conjecture is obtained from calculations of energy transfer rates from the carotenoid S2 state. Previous measurements of in vivo carotenoid and B800 dynamics are discussed in light of the new results, and currently unresolved issues are described.

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.

Photosynthesis at High Temperature in Tuber-Bearing Solanum Species : A Comparison between Accessions of Contrasting Heat Tolerance.

Differences in the photosynthetic performance between pairs of heat tolerant (HT) and heat sensitive (HS) accessions of tuber-bearing Solanum species were measured at 40 degrees C, after treating plants at 40/30 degrees C. After 1 to 9 days of heat treatment, both HT and HS accessions showed progressive inhibitory effects, primarily decreased rates of CO(2) fixation, and loss of leaf chlorophyll. These effects were most pronounced in the HS accessions. Stomatal conductivity and internal CO(2) concentrations were lower for both accessions at 40 degrees C especially for the HS accessions, suggesting that at ambient CO(2) concentrations, stomatal conductance was limiting CO(2) availability at the higher temperature. In the HT accessions, stomatal limitations were largely attributed to differences in vapor pressure deficit between 25 degrees and 40 degrees C, while the HS accessions exhibited significant nonstomatal limitations. The young expanding leaves of the HS accession showed some HT characteristics, while the oldest leaves showed severe senescence symptoms after 9 days at 40/30 degrees C. The data suggest that differences in heat sensitivity between HT and HS accessions are the result of accelerated senescence, chlorophyll loss, reduced stomatal conductance, and inhibition of dark reactions at high temperature.

Femtosecond dynamics of energy transfer in B800-850 light-harvesting complexes of Rhodobacter sphaeroides.

We report femtosecond transient absorption studies of energy transfer dynamics in the B800-850 light-harvesting complex (LHC) of Rhodobacter sphaeroides 2.4.1. For complexes solubilized in lauryldimethylamine-N-oxide (LDAO), the carotenoid to bacteriochlorophyll (Bchl) B800 and carotenoid to Bchl B850 energy transfer times are 0.34 and 0.20 ps, respectively. The B800 to B850 energy transfer time is 2.5 ps. For complexes treated with lithium dodecyl sulfate (LDS), a carotenoid to B850 energy transfer time of less than or equal to 0.2 ps is seen, and a portion of the total carotenoid population is decoupled from Bchl. In both LDAO-solubilized and LDS-treated complexes an intensity-dependent picosecond decay component of the excited B850 population is ascribed to excitation annihilation within minimal units of the LHC.

Detection of photosynthetic energy storage in a photosystem I reaction center preparation by photoacoustic spectroscopy.

Thermal emission and photochemical energy storage were examined in photosystem I reaction center/core antenna complexes (about 40 Chl a/P700) using photoacoustic spectroscopy. Satisfactory signals could only be obtained from samples bound to hydroxyapatite and all samples had a low signal-to-noise ratio compared to either PS I or PS II in thylakoid membranes. The energy storage signal was saturated at low intensity (half saturation at 1.5 W m(-2)) and predicted a photochemical quantum yield of >90%. Exogenous donors and acceptors had no effect on the signal amplitudes indicating that energy storage is the result of charge separation between endogenous components. Fe(CN)6 (-3) oxidation of P700 and dithionite-induced reduction of acceptors FA-FB inhibited energy storage. These data are compatible with the hypothesis that energy storage in PS I arises from charge separation between P700 and Fe-S centers FA-FB that is stable on the time scale of the photoacoustic modulation. High intensity background light (160 W m(-2)) caused an irreversible loss of energy storage and correlated with a decrease in oxidizable P700; both are probably the result of high light-induced photoinhibition. By analogy to the low fluorescence yield of PS I, the low signal-to-noise ratio in these preparations is attributed to the short lifetime of Chl singlet excited states in PS I-40 and its indirect effect on the yield of thermal emission.

Measurement of the fractional oxygenation of leghemoglobin in intact detached pea nodules by reflectance spectroscopy.

A method is presented for the rapid measurement of the spectral properties of detached nodules of pea (Pisum sativum L. cv "Sparkle") by diffuse reflectance spectroscopy. After correcting the spectra for surface light scattering, the spectrum of leghemoglobin is obtained. From this, the fractional oxygenation of leghemoglobin and the internal O(2) concentration can be calculated. With this method, we determined internal O(2) while measuring nitrogenase activity (C(2)H(2)) in detached pea nodules over a range of external O(2) concentrations. Nitrogenase activity was maximum when leghemoglobin was 25% oxygenated, corresponding to a calculated free O(2) concentration of 45 nanomolar in infected cells. Advantages of this method over previous methods which employed transmitted light are: (a) many nodules can be assayed simultaneously, (b) nitrogenase activity (C(2)H(2)) can be determined at the same time as spectra are recorded, and (c) spectra can be obtained from nodules submerged in buffer containing metabolic effectors.

Antenna structure and excitation dynamics in photosystem I. II. Studies with mutants of Chlamydomonas reinhardtii lacking photosystem II.

Using time-resolved single photon counting, fluorescence decay in photosystem I (PS I) was analyzed in mutant strains of Chlamydomonas reinhardtii that lack photosystem II. Two strains are compared: one with a wild-type PS I core antenna (120 chlorophyll a/P700) and a second showing an apparent reduction in core antenna size (60 chlorophyll a/P700). These data were calculated from the lifetimes of core antenna excited states (75 and 45 ps, respectively) and from pigment stoichiometries. Fluorescence decay in wild type PS I is composed of two components: a fast 75-ps decay that represents the photochemically limited lifetime of excited states in the core antenna, and a minor (less than 10%) 300-800 ps component that has spectral characteristics of both peripheral and core antenna pigments. Temporal and spectral properties of the fast PS I decay indicate that (a) excitations are nearly equilibrated among the range of spectral forms present in the PS I core antenna, (b) an average excitation visits a representative distribution of core antenna spectral forms on all pigment-binding subunits regardless of the origin of the excitation, (c) reduction in core antenna size does not alter the range of antenna spectral forms present, and (d) transfer from peripheral antennae to the PS I core complex is rapid (less than 5 ps).

Excitation transport and trapping on spectrally disordered lattices.

It is widely assumed that the decay of fluorescence in photosynthetic systems can be described as a sum of exponential components and that the amplitude of each component is directly related to the absorption cross-section of the antenna pigments coupled to the fluorescing species. We present exact calculations of excited state decay in two-dimensional regular lattices of different geometries containing multiple spectral forms of antenna pigments. We illustrate by these calculations that there is no simple relation between the decay amplitudes (and resulting time-resolved excitation spectra) and the steady-state absorption spectra. Only in the limit that the electronic excitations reach a rapid equilibrium among all antenna spectral forms does the excitation spectrum depend uniquely on the spectral features of the array. Using the simulations in conjunction with our recent fluorescence studies, we examine excitation transport and trapping dynamics in photosystem I and the limitations imposed by the finite time resolution in single photon counting experiments. In particular, we show that rising components, associated with excitation transfer among different spectral forms, with lifetimes <20 ps would be undetected in a typical photon counting experiment.

Antenna structure and excitation dynamics in photosystem I. I. Studies of detergent-isolated photosystem I preparations using time-resolved fluorescence analysis.

The temporal and spectral properties of fluorescence decay in isolated photosystem I (PS I) preparations from algae and higher plants were measured using time-correlated single photon counting. Excitations in the PS I core antenna decay with lifetimes of 15-40 ps and 5-6 ns. The fast decay results from efficient photochemical quenching by P700, whereas the slow decay is attributed to core antenna complexes lacking a trap. Samples containing core and peripheral antenna complexes exhibited an additional intermediate lifetime (150-350 ps) decay. The PS I core antenna is composed of several spectral forms of chlorophyll a that are not temporally resolved in the decays. Analysis of the temporal and spectral properties of the decays provides a description of the composition, structure, and dynamics of energy transfer and trapping reactions in PS I. The core antenna size dependence of the spectral properties and the contributions of the spectral forms to the time-resolved decays show that energy is not concentrated in the longest wavelength absorbing pigments but is nearly homogenized among the spectral forms. These data suggest that the "funnel" description of antenna structure and energy transfer (Seely, G. R. 1973. J. Theor. Biol. 40:189-199) may not be applicable to the PS I core antenna.

Antenna size dependence of fluorescence decay in the core antenna of photosystem I: estimates of charge separation and energy transfer rates.

We have examined the photophysics of energy migration and trapping in photosystem I by investigating the spectral and temporal properties of the fluorescence from the core antenna chlorophylls as a function of the antenna size. Time-correlated single photon counting was used to determine the fluorescence lifetimes in the isolated P700 chlorophyll a-protein complex and in a mutant of Chlamydomonas reinhardtii that lacks the photosystem II reaction center complex. The fluorescence decay in both types of sample is dominated by a fast (15-45 psec) component that is attributed to the lifetime of excitations in the photosystem I core antenna. These excitations decay primarily by an efficient photochemical quenching on P700. The measured lifetimes show a linear relationship to the core antenna size. A linear dependence of the excitation lifetime on antenna size was predicted previously in a lattice model for excitation migration and trapping in arrays of photosynthetic pigments [Pearlstein, R.M. (1982) Photochem. Photobiol. 35, 835-844]. Based on this model, our data predict a time constant for photochemical charge separation in the photosystem I reaction center of 2.8 +/- 0.7 or 3.4 +/- 0.7 psec, assuming monomeric or dimeric P700, respectively. The predicted average single-step transfer time for excitation transfer between core antenna pigments is 0.21 +/- 0.04 psec. Under these conditions, excitation migration in photosystem I is near the diffusion limit, with each excitation making an average of 2.4 visits to the reaction center before photoconversion.