Search

The ability to maintain the balance between the absorbed light energy and energy used in photosynthesis is a key factor of plant adaptation to variable environmental conditions. In this work, diurnal variations in photosynthesis, respiration, thermal energy dissipation, and the activity of the antioxidant system were studied in hoary plantain (Plantago media L.) growing on an open slope (sun plants) and under natural shading in the herbage (shade plants). The highest leaves net photosynthetic rate (Pn) was observed early in the morning and amounted to 2.6 and 9.2 μmol CO2/m2 s in shade and sun plants, respectively. In the daytime, the Pn values of sun plants decreased significantly (threefold) along with the decrease in stomatal conductance; changes of both parameters developed concurrently with the increase in insolation and air temperature. The Pn changes in leaves of shade plants were less pronounced and weakly dependent on stomatal conductance. The leaves of shade plants contained comparatively high amounts of soluble carbohydrates, whereas the sun plant leaves accumulated larger amounts of starch. In the daytime, nonphotochemical quenching (NPQ) of chlorophyll a fluorescence in photosystem II of sun plant leaves could be as large as 2.6 rel. units, which was four- to fivefold higher than NPQ in shade plants. In leaves of sun plants in the morning and evening hours, the ratio of cytochrome pathway (CP) and the alternative (AP) respiratory pathways was approximately 1.0, whereas this ratio decreased to 0.4 during the day, synchronously with an increase in NPQ. The CP/AP ratio in shade plant leaves remained constant throughout the diurnal cycle and equaled 1.4, indicating a comparatively high energy efficiency of respiration in shaded plants growing under the grass canopy. The leaves of sun plants featured an increased content of superoxide anion radical and hydrogen peroxide as well as the elevated activity of antioxidant enzymes (superoxide dismutase, guaiacol peroxidase) that control the accumulation of reactive oxygen species. The results emphasize the importance of coordinated changes in energy-dissipating processes and the activity of the antioxidant system for maintaining the energy and redox balance in phototrophic tissues during long-term adaptation of plants to environmental conditions, excessive insolation in particular. [ABSTRACT FROM AUTHOR]

The OJIPSMT pattern of chlorophyll (Chl) a fluorescence induction (FI) was obtained using Scenedesmus obliquus (Scenedesmus) microalgal cells exposed, after dark adaptation, to moderate and high light intensities (1200 and 1800 μmol photons m–2 s–1) in a time range from the microseconds to 10 minutes. The fast and, particularly, slow OJIPS(M)T stages of FI extending up to 100 s were quantitatively described by fitting a thylakoid membrane (TM) model. The calculations adequately reproduced the dynamics of P700 oxidoreduction transients. Parameters of TM processes were determined, and the transients of the TM model variables are shown in calculations. Dynamic stages of redox reactions in reaction centers (RCs) of photosystem II (PSII), the PQ/PQH2 quinone pool, the cytochrome b6 f complex (Cyt bf), and P700 of photosystem I (PSI) RCs were found to accompany the adaptation of the TM system to light. In silico analysis revealed the role that Cyt bf plays in regulating electron fluxes when bL/H hemes remain in a more reduced state at a high light intensity than at moderate illumination in an interval from 100 ms to 3 s. A dependence of the rate constants k42–49 on the level of reduction was revealed for non-radiative recombination of separated charges in the RC Phe–P680+ and Q P680+, together with a dependence of k42–49 on chloroplast illumination. Setting the dynamic rate constants kFNR(t) and kD-qE(t) made it possible to compare the generation ΔpHo-i(t) = pHS(t) – pHL(t), initiation of qE quenching at pHL|trigg = 6.7, activation of FNR reductase in the light, and achievement of stationary charge flows within 1–2 minutes of light induction. Fitting the model of isolated PSII to phytoplankton FI signals showed similar electron transfer parameters of PSII RCs for Scenedesmus and Chlorella monocultures and phytoplankton samples. Markers of the phytoplankton community state were an exception, differing between degrading and ecologically functional water bodies. The differences in markers included a reduced size of the antennas, an increased energy dissipation, and a decrease in pH in the lumen. [ABSTRACT FROM AUTHOR]

When dormant, perennial plants dwelling in the regions with pronounced seasonality of climate can withstand prolonged periods of harsh environmental conditions. The period of plant dormancy is commonly divided into pre-dormancy, endodormancy, and ecodormancy. During pre-dormancy, genetic, physiological, biochemical, and morphological rearrangements increasing stress resilience of the plant organism are completed. In the course of endodormancy, meristem cells cannot resume division even under favorable conditions. Environmental stimuli trigger dormancy release and the onset of ecodormancy when plant cell division and growth are restrained only by unfavorable environmental conditions. Frequent nowadays, weather fluctuations can lead to abnormal progression of dormancy. It results in the increased risk of damage to plants, especially crop plants, by adverse climatic conditions. This situation calls for the development of methods for noninvasive express monitoring of plant dormancy. Studies of the relationships between the dormancy status of plants and the functioning of their photosynthetic apparatus made possible the development of methods for monitoring of woody plant condition by recording the variable fluorescence of chlorophyll contained either in needles or in the endoderm of the shoots. This review briefly summarizes current knowledge about the mechanism of the dormancy induction and release. The functioning and regulation of the photosynthetic apparatus during winter dormancy as well as characteristic patterns of chlorophyll fluorescence induction in this period are considered. The difficulties of interpretation of chlorophyll fluorescence signals in the context of monitoring of plant dormancy are discussed together with its potential applications. [ABSTRACT FROM AUTHOR]

Photosynthetic pigments are an integral and vital part of all photosynthetic machinery and are present in different types and abundances throughout the photosynthetic apparatus. Chlorophyll, carotenoids and phycobilins are the prime photosynthetic pigments which facilitate efficient light absorption in plants, algae, and cyanobacteria. The chlorophyll family plays a vital role in light harvesting by absorbing light at different wavelengths and allowing photosynthetic organisms to adapt to different environments, either in the long-term or during transient changes in light. Carotenoids play diverse roles in photosynthesis, including light capture and as crucial antioxidants to reduce photodamage and photoinhibition. In the marine habitat, phycobilins capture a wide spectrum of light and have allowed cyanobacteria and red algae to colonise deep waters where other frequencies of light are attenuated by the water column. In this review, we discuss the potential strategies that photosynthetic pigments provide, coupled with development of molecular biological techniques, to improve crop yields through enhanced light harvesting, increased photoprotection and improved photosynthetic efficiency. [ABSTRACT FROM AUTHOR]

Golden-leaf privet (Ligustrum × vicaryi) is widely used as a horticultural shrub because of its upper golden leaves, but its lower leaves are green. However, the putative mechanisms of its upper golden leaves and the leaf color changes in response to light shifts have not been well studied so far. Here, chlorophylls (Chl), carotenoids, and Chl precursors from both golden and green leaves grown in full sunlight (approximately 1200 μmol photons m−2 s−1 at noon) or low-light conditions (180 μmol m−2 s−1) were determined spectrophotometrically. In addition, their gas exchange parameters and Chl fluorescence were measured in situ. Metabolic flux analysis of chlorophyll intermediates indicated that the conversion of prochlorophyllide to chlorophyllide was significantly blocked in golden leaves when compared with green leaves. Green leaves showed higher photosynthetic capacity in low light than golden leaves, but golden leaves presented unexpectedly stronger photosynthetic capacity and lower reactive oxygen species accumulation under the high-light condition. Furthermore, golden leaves showed a higher level of nonphotochemical quenching (NPQ) after the light-to-dark shift and presented a stronger adaptive ability to a broad range of light environments. Higher NPQ values and less oxidative damage in golden leaves may be correlated with their higher carotenoid levels. The results imply that lower chlorophyll levels and higher carotenoid levels in canopy leaves may help privet plants acclimate better to illumination changes. This study demonstrates the key role of irradiance in generating the two types of Ligustrum × vicaryi leaves and sheds a light on cultivation of other ornamental foliage plants. [ABSTRACT FROM AUTHOR]

Dynamics of chlorophyll a fluorescence changes (including JIP test parameters) and photosynthetic pigments were studied in oat (Avena sativa L.) leaves of late summer sowing during autumnal cooling. As the average daily temperature decreased from 8 to 2°С, the chlorophyll content decreased by 40%, while the chlorophyll a/b ratio ranged between 2.2 and 2.4. The total pool of carotenoids and the relative content of β‑carotene, lutein, neoxanthin, and pigments of the violaxanthin cycle changed insignificantly. The relative number of QB-nonreducing reaction centers (RC) of photosystem II (PSII) and the quantum yield of energy-dependent component qE of nonphotochemical quenching in the PSII antenna (φNPQ parameter) were found to increase during the seasonal cooling. The values of Fv/Fm parameter characterizing the maximal photochemical efficiency of PSII complexes were 0.72–0.76. At temperatures around 0°C, Fv/Fm decreased to 0.62. At the same time, the nonregulated (light-independent) thermal dissipation (photoinhibitory component qI of nonphotochemical quenching) was enhanced, which was manifested in the increase in parameters φDo and DI0/RC. It is concluded that, under seasonal cooling, the PSII reaction centers partly lose photochemical activity and start functioning as dissipative centers of excitation energy. In a significant part of inactivated RC, the loss of QA-reduction was reversible, which implies the formation of photoinactivated forms of PSII complexes stabilized at low temperatures. The decrease in daily average temperatures to −3°С with nighttime low temperatures from −7 to −8°С led to plant death within 3 days. [ABSTRACT FROM AUTHOR]

Although the importance of nonphotochemical quenching (NPQ) on photosynthetic biomass production and crop yields is well established, the in vivo operation of the individual mechanisms contributing to overall NPQ is still a matter of controversy. In order to investigate the timescale and activation dynamics of specific quenching mechanisms, we have developed a technique called snapshot transient absorption (TA) spectroscopy, which can monitor molecular species involved in the quenching response with a time resolution of 30 s. Using intact thylakoid membrane samples, we show how conventional TA kinetic and spectral analyses enable the determination of the appropriate wavelength and time delay for snapshot TA experiments. As an example, we show how the chlorophyll-carotenoid charge transfer and excitation energy transfer mechanisms can be monitored based on signals corresponding to the carotenoid (Car) radical cation and Car S1 excited state absorption, respectively. The use of snapshot TA spectroscopy together with the previously reported fluorescence lifetime snapshot technique (Sylak-Glassman et al. in Photosynth Res 127:69–76, 2016) provides valuable information such as the concurrent appearance of specific quenching species and overall quenching of excited Chl. Furthermore, we show that the snapshot TA technique can be successfully applied to completely intact photosynthetic organisms such as live cells of Nannochloropsis. This demonstrates that the snapshot TA technique is a valuable method for tracking the dynamics of intact samples that evolve over time, such as the photosynthetic system in response to high-light exposure. [ABSTRACT FROM AUTHOR]

It has previously been shown that the long-term treatment of Arabidopsis thaliana with the chloroplast inhibitor lincomycin leads to photosynthetic membranes enriched in antennas, strongly reduced in photosystem II reaction centers (PSII) and with enhanced nonphotochemical quenching (NPQ) (Belgio et al. Biophys J 102:2761-2771, 2012). Here, a similar physiological response was found in the microalga Chromera velia grown under high light (HL). In comparison to cells acclimated to low light, HL cells displayed a severe re-organization of the photosynthetic membrane characterized by (1) a reduction of PSII but similar antenna content; (2) partial uncoupling of antennas from PSII; (3) enhanced NPQ. The decrease in the number of PSII represents a rather unusual acclimation response compared to other phototrophs, where a smaller PSII antenna size is more commonly found under high light. Despite the diminished PSII content, no net damage could be detected on the basis of the Photosynthesis versus irradiance curve and electron transport rates pointing at the excess capacity of PSII. We therefore concluded that the photoinhibition is minimized under high light by a lower PSII content and that cells are protected by NPQ in the antennas. [ABSTRACT FROM AUTHOR]

Cyanobacteria are photosynthetic prokaryotes and widely used for photosynthetic research as model organisms. Partly due to their prokaryotic nature, however, estimation of photosynthesis by chlorophyll fluorescence measurements is sometimes problematic in cyanobacteria. For example, plastoquinone pool is reduced in the dark-acclimated samples in many cyanobacterial species so that conventional protocol developed for land plants cannot be directly applied for cyanobacteria. Even for the estimation of the simplest chlorophyll fluorescence parameter, F / F , some additional protocol such as addition of DCMU or illumination of weak blue light is necessary. In this review, those problems in the measurements of chlorophyll fluorescence in cyanobacteria are introduced, and solutions to those problems are given. [ABSTRACT FROM AUTHOR]

Salt-tolerant grasses of warm sub-tropical ecosystems differ in their distribution patterns with respect to salinity and moisture regimes. Experiments were conducted on CO fixation and light harvesting processes of four halophytic C grasses grown under different levels of salinity (0, 200 and 400 mM NaCl) under ambient environmental conditions. Two species were from a high saline coastal marsh ( Aeluropus lagopoides and Sporobolus tremulus) and two were from a moderate saline sub-coastal draw-down tidal marsh ( Paspalum paspalodes and Paspalidium geminatum). Analyses of the carbon isotope ratios of leaf biomass in plants indicated that carbon assimilation was occurring by C photosynthesis in all species during growth under varying levels of salinity. In the coastal species, with increasing salinity, there was a parallel decrease in rates of CO fixation ( A), transpiration ( E) and stomatal conductance ( g ), with no effect on water use efficiency ( WUE). These species were adapted for photoprotection by an increase in the Mehler reaction with an increase in activity of PSII/CO fixed accompanied by high levels of antioxidant enzymes, superoxide dismutase and ascorbate peroxidase. The sub-coastal species P. paspalodes and P. geminatum had high levels of carotenoid pigments and non-photochemical quenching by the xanthophyll cycle. [ABSTRACT FROM AUTHOR]

Eustigmatophyte algae represent an interesting model system for the study of the regulation of the excitation energy flow due to their use of violaxanthin both as a major light-harvesting pigment and as the basis of xanthophyll cycle. Fluorescence induction kinetics was studied in an oleaginous marine alga Nannochloropsis oceanica. Nonphotochemical fluorescence quenching was analyzed in detail with respect to the state of the cellular xanthophyll pool. Two components of nonphotochemical fluorescence quenching (NPQ), both dependent on the presence of zeaxanthin, were clearly resolved, denoted as slow and fast NPQ based on kinetics of their formation. The slow component was shown to be in direct proportion to the amount of zeaxanthin, while the fast NPQ component was transiently induced in the presence of membrane potential on subsecond timescales. The applicability of these observations to other eustigmatophyte species is demonstrated by measurements of other representatives of this algal group, both marine and freshwater. [ABSTRACT FROM AUTHOR]

In higher plants, algae, and cyanobacteria, chlorophyll (Chl) a fluorescence induction (ChlFI) has a fast (under a second) increasing OJIP phase and a slow (few minutes) PS(M)T phase, where O is for origin, the minimum fluorescence, J and I for intermediate levels, P for peak, S for a semi-steady state, M for a maximum (which is sometimes missing), and T for the terminal steady-state level. We have used a photosynthesis model of Ebenhöh et al. (Philos Trans R Soc B, 2014, doi:) in an attempt to simulate the slow PS(M)T phase and to determine the origin of the S-M rise in Chlamydomonas ( C.) reinhardtii cells. Our experiments in silico show that a slow fluorescence S-M rise (as that observed, e.g., by Kodru et al. (Photosynth Res 125:219-231, 2015) can be simulated only if the photosynthetic samples are initially in a so-called 'state 2,' when the absorption cross section (CS) of Photosystem II (PSII) is lower than that of PSI, and Chl a fluorescence is low (see, e.g., a review by Papageorgiou and Govindjee (J Photochem Photobiol B 104:258-270, 2011). In this case, simulations show that illumination induces a state 2 (s2) to state 1 (s1) transition (qT), and a slow S-M rise in the simulated ChlFI curve, since the fluorescence yield is known to be higher in s1, when CS of PSII is larger than that of PSI. Additionally, we have analyzed how light intensity and several photosynthetic processes influence the degree of this qT, and thus the relative amplitude of the simulated S-M phase. A refinement of the photosynthesis model is, however, necessary in order to obtain a better fit of the simulation data with the measured ChlFI curves. [ABSTRACT FROM AUTHOR]

The 22-kDa PsbS protein of Photosystem II is involved in nonphotochemical quenching (NPQ) of chlorophyll fluorescence. Genome-wide analysis of the expression pattern in PsbS knockout (KO) rice plants showed that a lack of this protein led to changes in the transcript levels of 406 genes, presumably a result of superoxide produced in the chloroplasts. The top Gene Ontology categories, in which expression was the most differential, included 'Immune response', 'Response to jasmonic acid', and 'MAPK cascade'. From those genes, we randomly selected nine that were up-regulated. Our microarray results were confirmed by quantitative RT-PCR analysis. The KO and PsbS RNAi (knockdown) plants were more resistant to pathogens Magnaporthe oryzae PO6-6 and Xanthomonas oryzae pv. oryzae than either the wild-type plants or PsbS-overexpressing transgenic line. These findings suggest that superoxide production might be the reason that these plants have greater pathogen resistance to fungal and bacterial pathogens in the absence of energy-dependent NPQ. For example, a high level of cell wall lignification in the KO mutants was possibly due to enhanced superoxide production. Our data indicate that certain abiotic stress-induced reactive oxygen species can promote specific signaling pathways, which then activate a defense mechanism against biotic stress in PsbS-KO rice plants. [ABSTRACT FROM AUTHOR]

Nannochloropsis is an eukaryotic alga of the phylum Heterokonta, originating from a secondary endosymbiotic event. In this work, we investigated how the photosynthetic apparatus responds to growth in different light regimes in Nannochloropsis gaditana. We found that intense illumination induces the decrease of both photosystem I and II contents and their respective antenna sizes. Cells grown in high light showed a larger capacity for electron transport, with enhanced cyclic electron transport around photosystem I, contributing to photoprotection from excess illumination. Even when exposed to excess light intensities for several days, N. gaditana cells did not activate constitutive responses such as nonphotochemical quenching and the xanthophyll cycle. These photoprotection mechanisms in N. gaditana thus play a role in acclimation to fast changes in illumination within a time range of minutes, while regulation of the electron flow capacity represents a long-term response to prolonged exposure to excess light. [ABSTRACT FROM AUTHOR]

Effects of knockout of the gene encoding α-carbonic anhydrase 4 (α-CA4) on growth and photosynthesis of Thale cress ( Arabidopsis thaliana (L.) Heynh., var. Columbia) were investigated. The shoot weight of mutant plants was found to be higher than in wild-type plants, and the leaves of mutants were enriched in starch content. The electron microscopy study revealed a considerable increase in the number and size of starch grains in chloroplasts of mutant plants. Comparison of wild-type and mutant plant leaves in terms of chlorophyll a fluorescence, coefficient of photochemical fluorescence quenching, effective quantum yield of photosystem II reaction, and nonphotochemical fluorescence quenching under steady-state illumination and saturating CO content in the air led to the proposal that α-CA4 participates in the development of nonphotochemical energy dissipation by accelerating the supply of protons for activation of violaxanthin deepoxidase and for structural changes in the light-harvesting complexes. [ABSTRACT FROM AUTHOR]

In this work, we have investigated how the luminescence characteristics and the total content of antioxidants in leaves of strawberry ( Fragaria × ananassa Duch.) change after infestation of plants by spider mites ( Tetranychus atlanticus McGregor). At early stages of plant injury, the chlorophyll concentration remained unchanged. However, the total amount of antioxidants decreased and the intensity of high-temperature thermoluminescence band (60-100°C) increased, indicating degradation of membrane lipids caused by oxidative stress. In infested leaves, the light-induced rise of nonphotochemical quenching of chlorophyll fluorescence was slower, while the relaxation of nonphotochemical quenching in the dark was faster than in control leaf samples. These data may indicate that the thylakoid membranes in infested leaves were more permeable to ions than in control leaves. [ABSTRACT FROM AUTHOR]

Electron transport in photosystem II (PSII) and photosystem I (PSI) was estimated in terms of chlorophyll fluorescence and changes in P700 redox, respectively, in the unicellular green alga Dunaliella salina in the presence or absence of a nitrogen source in the culture medium. In a nitrogen-containing medium, the quantum yield of PSII (Φ) and that in PSI (Φ) were at the same level in low light, but cyclic electron transport around photosystem I (CET-PSI) was induced under high light as estimated from an increase in Φ/Φ. High light might further enhance the rate of electron transport in PSI by inducing the state 2 transition, in which the distribution of light energy is shifted to PSI at the expense of PSII. Nitrogen deficiency resulted in a decrease in Φ and an increase in Φ. As a consequence, the rate of CET-PSI was expected to increase. The high CET-PSI under N deficiency was probably associated with a high level of energy quenching ( qE) formation in PSII. [ABSTRACT FROM AUTHOR]

Photosynthetic properties and photosystem stoichiometry of in vitro-grown juvenile, adult, and rejuvenated Sequoia sempervirens shoots were characterized. 77K fluorescence analysis indicated that photosystem II/photosystem I ratios were highest for chloroplasts in adult shoots. Photosynthetic oxygen evolution rates (on the same chlorophyll bases) were also slightly higher for chloroplasts in adult shoots. Our results suggested a significant alteration of photosystem stoichiometry in chloroplasts of Sequoia sempervirens shoots during phase change. In addition, chlorophyll a fluorescence analysis showed that juvenile, adult, and rejuvenated shoots showed virtually identical maximal quantum efficiencies of photosystem II (FV/FM). Nonphotochemical quenching (NPQ) during actinic light illumination, however, was significantly enhanced for AS76 adult shoots. The differences in photosynthetic properties and photosystem stoichiometry among juvenile, adult, and rejuvenated shoots may reflect adjustments in the photosynthetic apparatus to acclimate to distinct physiological states of Sequoia sempervirens during phase changes. [ABSTRACT FROM AUTHOR]

This short review describes how solid-state NMR has provided a mechanistic and electronic picture of pigment-protein and pigment-pigment interactions in photosynthetic antenna complexes. NMR results on purple bacterial antenna complexes show how the packing of the protein and the pigments inside the light-harvesting oligomers induces mutual conformational stress. The protein scaffold produces deformation and electrostatic polarization of the BChl macrocycles and leads to a partial electronic charge transfer between the BChls and their coordinating histidines, which can tune the light-harvesting function. In chlorosome antennae assemblies, the NMR template structure reveals how the chromophores can direct their self-assembly into higher macrostructures which, in turn, tune the light-harvesting properties of the individual molecules by controlling their disorder, structural deformation, and electronic polarization without the need for a protein scaffold. These results pave the way for addressing the next challenge, which is to resolve the functional conformational dynamics of the lhc antennae of oxygenic species that allows them to switch between light-emitting and light-energy dissipating states. [ABSTRACT FROM AUTHOR]

The xanthophylls of the light-harvesting complexes of photosystem II (LHCII), zeaxanthin, and lutein are thought to be essential for non-photochemical quenching (NPQ). NPQ is a process of photoprotective energy dissipation in photosystem II (PSII). The major rapidly reversible component of NPQ, qE, is activated by the transmembrane proton gradient, and involves the quenching of antenna chlorophyll excited states by the xanthophylls lutein and zeaxanthin. Using diaminodurene (DAD), a mediator of cyclic electron flow around photosystem I, to enhance ΔpH we demonstrate that qE can still be formed in the absence of lutein and light-induced formation of zeaxanthin in chloroplasts derived from the normally qE-deficient lut2npq1 mutant of Arabidopsis. The qE induced by high ΔpH in lut2npq1 chloroplasts quenched the level of fluorescence when all PSII reaction centers were in the open state ( F state), protected PSII reaction centers from photoinhibition, was sensitive to the uncoupler nigericin, and was accompanied by absorption changes in the 410-565 nm region. Titrations show the ΔpH threshold for activation of qE in lut2npq1 chloroplasts lies outside the normal physiological range and is highly cooperative. Comparison of quenching in isolated trimeric (LHCII) and monomeric (CP26) light-harvesting complexes from lut2npq1 plants revealed a similarly shifted pH dependency compared with wild-type LHCII. The implications for the roles of lutein and zeaxanthin as direct quenchers of excitation energy are discussed. Furthermore, we argue that the control over the proton-antenna association constant, p K, occurs via influence of xanthophyll structure on the interconnected phenomena of light-harvesting antenna reorganization/aggregation and hydrophobicity. [ABSTRACT FROM AUTHOR]

The operation of photosynthetic energy-dissipating processes is commonly characterized by measuring the light response of the nonphotochemical quenching (NPQ) of chlorophyll fluorescence, or NPQ versus E curves. This study proposes a mathematical model for the quantitative description of the generic NPQ versus E curve. The model is an adaptation of the Hill equation and is based on the close dependence of NPQ on the xanthophyll cycle (XC). The model was tested on NPQ versus E curves measured in the plant Arabidopsis thaliana and the diatom Nitzschia palea, representing the two main types of XC, the violaxanthin-antheraxanthin-zeaxanthin (VAZ) type and the diadinoxanthin-diatoxanthin (DD-DT) type, respectively. The model was also fitted to a large number of published light curves, covering the widest possible range of XC types, taxa, growth conditions, and experimental protocol of curve generation. The model provided a very good fit to experimental and published data, coping with the large variability in curve characteristics. The model was further used to quantitatively compare the light responses of NPQ and of PSII electron transport rate, ETR, through the use of indices combining parameters of the models describing the two types of light-response curves. Their application to experimental and published data showed a systematic large delay of the buildup of NPQ relatively to the saturation of photochemistry. It was found that when ETR reaches saturation, NPQ is on average still below one fifth of its maximum attainable level, which is only reached at irradiances about three times higher. It was also found that organisms having the DD-DT type of XC appeared to be able to start operating the XC at lower irradiances than those of the VAZ type. [ABSTRACT FROM AUTHOR]

Plants dissipate excess absorbed light energy as heat to protect themselves from photo-oxidative stress. The Arabidopsis thaliana npq6 mutant affected in thermal dissipation was identified by its partial defect in the induction of nonphotochemical quenching of chlorophyll fluorescence (NPQ) by excess light. Positional cloning revealed that npq6 contains a frameshift mutation caused by a single base-pair deletion in the At5g43050 gene, which encodes a member of the hypothetical chloroplast open reading frame 20 (YCF20) family of proteins with unknown function(s). The YCF20 protein family is mostly conserved in oxygenic photosynthetic organisms including cyanobacteria, eukaryotic algae, and plants. Amino acid sequence comparison identified two other genes in Arabidopsis that encode similar proteins to NPQ6: At1g65420 and At3g56830. These three Arabidopsis proteins have functional chloroplast-targeting transit peptides. Using reverse genetics, a mutant with a T-DNA insertion within the At1g65420 gene was identified and shown to exhibit a low NPQ phenotype similar to that of npq6; therefore, At1g65420 was named NPQ7. In contrast, a knockdown mutant in the At3g56830 gene with lower transcript levels showed wild-type levels of NPQ. The npq6 npq7 double mutant had an additive NPQ defect, indicating that the YCF20 family members in Arabidopsis have overlapping functions affecting thermal dissipation. [ABSTRACT FROM AUTHOR]

Two populations of a diatom alga Thallassiosira weisflogii were grown at photon flux densities (PFD) of 0.8 and 8 µmol/(m2 s). For both diatom populations, the recovery of chlorophyll fluorescence parameters ( F0, Fm, Fv/ Fm, and NPQ) was monitored after nondestructive irradiation by visible light at PFD of 40 µmol/(m2 s) and after high-intensity irradiation by visible light (1000–4000 µmol/(m2 s)). The exposure of diatoms to PFD of 40 µmol/(m2 s)—higher than PFD used for algal growth but still nondamaging to photosynthetic apparatus—induced nonphotochemical quenching (NPQ), which was stronger in algae grown at higher PFD (8 µmol/(m2 s)) than in algae grown at low light. After irradiation with high-intensity light, the recovery of chlorophyll fluorescence parameters was more pronounced in algae grown at elevated PFD level. During short-term irradiation of diatoms with high-intensity visible light (1000 µmol/(m2 s)), a stronger NPQ was observed in the culture adapted to high irradiance. After the treatment of algae with dithiothreitol (an inhibitor of carotenoid deepoxidase in the diadinoxanthin cycle) or NH4Cl (an agent abolishing the proton gradient at thylakoid membranes), a short exposure of algae to PFD of 40 µmol/(m2 s) induced hardly any nonphotochemical quenching. The results indicate the dominant contribution of xanthophyll cycle carotenoids to energy-dependent quenching. [ABSTRACT FROM AUTHOR]

Plants dissipate excess excitation energy as heat by non-photochemical quenching (NPQ). NPQ has been thought to resemble in vitro aggregation quenching of the major antenna complex, light harvesting complex of photosystem II (LHC-II). Both processes are widely believed to involve a conformational change that creates a quenching centre of two neighbouring pigments within the complex. Using recombinant LHC-II lacking the pigments implicated in quenching, we show that they have no particular role. Single crystals of LHC-II emit strong, orientation-dependent fluorescence with an emission maximum at 680 nm. The average lifetime of the main 680 nm crystal emission at 100 K is 1.31 ns, but only 0.39 ns for LHC-II aggregates under identical conditions. The strong emission and comparatively long fluorescence lifetimes of single LHC-II crystals indicate that the complex is unquenched, and that therefore the crystal structure shows the active, energy-transmitting state of LHC-II. We conclude that quenching of excitation energy in the light-harvesting antenna is due to the molecular interaction with external pigments in vitro or other pigment–protein complexes such as PsbS in vivo, and does not require a conformational change within the complex. [ABSTRACT FROM AUTHOR]

There are at least two photosynthetic cyclic electron transport (CET) pathways in most C3 plants: the NAD(P)H dehydrogenase (NDH)-dependent pathway and a pathway dependent upon putative ferredoxin:plastoquinone oxidoreductase (FQR) activity. While the NDH complex has been identified, and shown to play a role in photosynthesis, especially under stress conditions, less is known about the machinery of FQR-dependent CET. Recent studies indicate that FQR-dependent CET is dependent upon PGR5, a small protein of unknown function. In a previous study we found that overexpression of PGR5 causes alterations in growth and development associated with decreased chloroplast development and a transient increase in nonphotochemical quenching (NPQ) after the shift from dark to light. In the current study we examine the spatiotemporal expression pattern of PGR5, and the effects of overexpression of PGR5 in Arabidopsis under a host of light and stress conditions. To investigate the conserved function of PGR5, we cloned PGR5 from a species which apparently lacks NDH, loblolly pine, and overexpressed it in Arabidopsis. Although greening of cotyledons was severely delayed in overexpressing lines under low light, mature plants survived exposure to high light and drought stress better than wild-type. In addition, PSI was more resistant to high light in the PGR5 overexpressors than in wild-type plants, while PSII was more sensitive to this stress. These complex responses corresponded to alterations in linear and cyclic electron transfer, suggesting that over-accumulation of PGR5 induces pleiotropic effects, probably via elevated CET. We conclude that PGR5 has a developmentally-regulated, conserved role in mediating CET. [ABSTRACT FROM AUTHOR]

Effects of inhibitors and cofactors of cyclic and noncyclic electron transport on nonphotochemical quenching of chlorophyll fluorescence induced by action potential (AP) was investigated in Chara corallina cells. Under control conditions, energy-dependent quenching (q E) develops upon the increase in photosynthetically active radiation (PAR); it also arises and reversibly disappears after AP generation at moderate irradiances. The treatment of cells with diuron (DCMU) completely eliminated q E established at high irradiances and q E induced by AP generation. The activation of cyclic electron transport by DCMU in combination with phenazine methosulfate restored q E at high irradiances but did not restore q E imposed after AP generation. The presence in the medium of a PSI acceptor, methyl viologen at concentrations from 100 μM to 0.83 mM had no effect on fluorescence and photosynthetic activity of chloroplasts until the application of a single excitatory stimulus. Once a single AP was generated in the presence of methyl viologen, it induced irreversible q E at a wide range of irradiances, which indicates the AP-triggered redirection of a part of electron flow from the main pathway to the artificial acceptor. It is concluded that AP generation opens access for permeation of a divalent cation methyl viologen from the medium to the chloroplast stroma across two membrane barriers, the plasmalemma and the inner membrane of the chloroplast envelope. [ABSTRACT FROM AUTHOR]

When exposed to light, the cells of characean algae produce intermittent regions of H+ extrusion and H+ absorption, featuring different photosynthetic activities. Methods for local measurements of outer pH, O2 content, and photochemical activity of photosystem II (PSII) were applied to examine microscopic regions of Chara coralline Klein ex Willd. internodes. The results show that the functional spatial heterogeneity of these excitable cells is controlled not only by light but also by electric excitation of the plasma membrane. Generation of a single action potential (AP) induced a reversible transition to the state with homogenous pH distribution and had different effects on photosynthesis in cell regions producing alkaline and acid zones. The effective quantum yield of PSII primary processes and the maximal chlorophyll fluorescence decreased after AP in the alkaline cell regions but were almost unaffected in the acidic cell regions. The suppression of photosynthesis after AP was also evident in the decrease of photosynthetic O2 evolution. The results provide evidence that electric signals arising at the plasmalemma are transmitted to the level of thylakoid membranes. The effects of electric excitation on fluorescence and the quantum yield of PSII photochemistry were best pronounced at low light intensities and low level of nonphotochemical quenching. The sensitivity of chlorophyll fluorescence in resting and excited cells to light intensity and protonophores indicates that the AP-induced fluorescence changes derive from the increase in pH gradient at the thylakoid membrane. The temporal elimination of alkaline zones and inhibition of photosynthesis apparently arise from parallel operational sequences that have a common initial stage. A possible role of cytosolic Ca2+ rise in the mechanism of photosynthesis suppression after electric excitation of the plasma membrane is discussed. [ABSTRACT FROM AUTHOR]

Capillary microelectrodes and pulse amplitude-modulated microfluorometry were used to study light-triggered changes in cell membrane potential, chlorophyll fluorescence, and photochemical yield of PSII in chloroplasts of a hornwort Anthoceros sp. The action potential was generated by illuminating the plant sample for a few seconds. It was accompanied by a reversible decrease in quantum efficiency of PSII and by nonphotochemical quenching of fluorescence that continued as long as 10 min after the light stimulus. The presence of ammonium ions (2 mM) enhanced the amplitude and prolonged the duration of dark changes of fluorescence parameters in accordance with the reported increase in duration and amplitude of the light-triggered action potential in the presence of NH. A rapid retardation of PSII activity within the first seconds of illumination was also evident from absorbance changes at 810 nm reflecting the redox conversions of chlorophyll P700. The PSII-dependent stage of reduction in the induction curves of P700 absorbance was strongly suppressed, and the amplitudes of signals induced by white and far-red light (717 nm) differed insignificantly. It is concluded that a short-term irradiation triggers the generation of ΔpH at the thylakoid membranes, which is accompanied by inhibition of the plasma membrane H+ pump and by reversible inactivation of PSII due to increased thermal dissipation of chlorophyll excitations. [ABSTRACT FROM AUTHOR]

When cells of the green alga Chlorella vulgaris Beij. are transferred from growth at 5 °C and an irradiance of 150 μmol photons m-2 s-1 to 27 °C and the same irradiance, they undergo what is normally considered a high-light to low-light phenotypic change. This involves a 3-fold increase in cellular chlorophyll content with a concomitant increase in light-harvesting complex polypeptide levels. This process appears to occur in response to the cellular capacity to utilize the products of photosynthesis, with the redox state of the plastoquinone pool sensing the cellular energy balance. The phenotypic adjustment can be enhanced or blocked using chemical inhibitors that modulate the redox state of the plastoquinone pool. The functional changes in the photosynthetic apparatus that occurred during the high-light to low-light acclimation were examined with special consideration paid to the paradox that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated cells, with non-functional photosystem II (PSII), accumulate light-harvesting polypeptides. At the structural and basic functional levels, the light-harvesting complex of the cells treated with DCMU was indistinguishable from that of the untreated, control cells. To examine how PSII was protected in the DCMU-treated cells, we measured the content of xanthophyll-cycle pigments. It appeared that a zeaxanthin-dependent nonphotochemical quenching process was involved in PSII protection during greening in the presence of DCMU. Metabolic inhibitors of mitochondrial respiration were used to examine how the change in cellular energy balance regulates the greening process. Apparently, the mitochondrion acts to supply energy to the chloroplast during greening, and inhibition of mitochondrial respiration diminishes chlorophyll accumulation apparently through an increase in the redox state of the plastoquinone pool. [ABSTRACT FROM AUTHOR]

The response of the photosynthetic apparatus to high irradiance illumination (440–2200 W/m2) was studied in the diatom Thallassiosira weisflogii by fluorescence methods. Changes in the photosynthetic apparatus were monitored by measuring characteristics of chlorophyll fluorescence F0, Fm, Fv / Fm, and qN for several hours after illumination of the alga with high-intensity light. Incubation of the alga with 2 mM DTT, an inhibitor of de-epoxidase of carotenoids in the diadinoxanthin cycle, led to a decrease in the nonphotochemical quenching of chlorophyll fluorescence and a drop in the Fv / Fm ratio, a characteristic that reflects the quantum efficiency of the functioning of the photosynthetic apparatus. Light-induced absorption changes associated with transformations of carotenoids of diadinoxanthin cycle were recorded in vivo in algal suspensions in the absence and in the presence of DTT. Using the microfluorometric method, we measured cell distribution over the efficiency of the primary processes of photosynthesis ( Fv / Fm) after illumination. We found cells with a high tolerance of their photosynthetic apparatus to photooxidative damage. The relatively high tolerance of a portion of the cell population to high-light illumination can be related to light-induced transformation of carotenoids and to the functioning of other protective systems of the photosynthetic apparatus in diatoms. [ABSTRACT FROM AUTHOR]

When light absorption by a plant exceeds its capacity for light utilization, photosynthetic light harvesting is rapidly downregulated by photoprotective thermal dissipation, which is measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). To address the involvement of specific xanthophyll pigments in NPQ, we have analyzed mutants affecting xanthophyll metabolism in Arabidopsis thaliana. An npq1 lut2 double mutant was constructed, which lacks both zeaxanthin and lutein due to defects in the violaxanthin de-epoxidase and lycopene ∈-cyclase genes. The npq1 lut2 strain had normal Photosystem II efficiency and nearly wild-type concentrations of functional Photosystem II reaction centers, but the rapidly reversible component of NPQ was completely inhibited. Despite the defects in xanthophyll composition and NPQ, the npq1 lut2 mutant exhibited a remarkable ability to tolerate high light. [ABSTRACT FROM AUTHOR]

High light stress induced not only a sustained form of xanthophyll cycle-dependent energy dissipation but also sustained thylakoid protein phosphorylation. The effect of protein phosphatase inhibitors (fluoride and molybdate ions) on recovery from a 1-h exposure to a high PFD was examined in leaf discs of Parthenocissus quinquefolia (Virginia creeper). Inhibition of protein dephosphorylation induced zeaxanthin retention and sustained energy dissipation (NPQ) upon return to low PFD for recovery, but had no significant effects on pigment and Chl fluorescence characteristics under high light exposure. In addition, whole plants of Monstera deliciosa and spinach grown at low to moderate PFDs were transferred to high PFDs, and thylakoid protein phosphorylation pattern (assessed with anti-phosphothreonine antibody) as well as pigment and Chl fluorescence characteristics were examined over several days. A correlation was obtained between dark-sustained D1/D2 phosphorylation and dark-sustained zeaxanthin retention and maintenance of PS II in a state primed for energy dissipation in both species. The degree of these dark-sustained phenomena was more pronounced in M. deliciosa compared with spinach. Moreover, M. deliciosa but not spinach plants showed unusual phosphorylation patterns of Lhcb proteins with pronounced dark-sustained Lhcb phosphorylation even under low PFD growth conditions. Subsequent to the transfer to a high PFD, dark-sustained Lhcb protein phosphorylation was further enhanced. Thus, phosphorylation patterns of D1/D2 and Lhcb proteins differed from each other as well as among plant species. The results presented here suggest an association between dark-sustained D1/D2 phosphorylation and sustained retention of zeaxanthin and energy dissipation (NPQ) in light-stressed, and particularly ‘photoinhibited’, leaves. Functional implications of these observations are discussed. [ABSTRACT FROM AUTHOR]

We have isolated very high light resistant nuclear mutants ( VHLR) in Chlamydomonas reinhardtii, that grow in 1500–2000 μmol photons m−2 s−1 (VHL) lethal to wildtype. Four nonallelic mutants have been characterized in terms of Photosystem II (PS II) function, nonphotochemical quenching (NPQ) and xanthophyll pigments in relation to acclimation and survival under light stress. In one class of VHLR mutants isolated from wild type ( S4 and S9), VHL resistance was accompanied by slower PS II electron transfer, reduced connectivity between PS II centers and decreased PS II efficiency. These lesions in PS II function were already present in the herbicide resistant D1 mutant A251L ( L*) from which another class of VHLR mutants ( L4 and L30) were isolated, confirming that optimal PS II function was not critical for survival in very high light. Survival of all four VHLR mutants was independent of CO2 availability, whereas photoprotective processes were not. The de-epoxidation state (DPS) of the xanthophyll cycle pigments in high light (HL, 600 μmol photons m−2 s−1) was strongly depressed when all genotypes were grown in 5% CO2. In S4 and S9 grown in air under HL and VHL, high DPS was well correlated with high NPQ. However when the same genotypes were grown in 5% CO2, high DPS did not result in high NPQ, probably because high photosynthetic rates decreased thylakoid ΔpH. Although high NPQ lowered the reduction state of PS II in air compared to 5% CO2 at HL in wildtype, S4 and S9, this did not occur during growth of S4 and S9 in VHL. L* and VHLR mutants L4 and L30, also showed high DPS with low NPQ when grown air or 5% CO2, possibly because they were unable to maintain sufficiently high ΔpH due to constitutively impaired PS II electron transport. Although dissipation of excess photon energy through NPQ may contribute to VHL resistance, there is little evidence that the different genes conferring the VHLR phenotype affect this form of photoprotection. Rather, the decline of chlorophyll per biomass in all VHLR mutants grown under VHL suggests these genes may be involved in regulating antenna components and photosystem stoichiometries. [ABSTRACT FROM AUTHOR]

The light-harvesting protein complexes (Lhc) play key roles in the processes of light absorption and protection in diatoms. However, different Lhc protein carries out distinct function in photosynthesis. For now, roles of many Lhc proteins in light acclimation are largely unknown. Here, function of Lhcx3 in marine diatom Phaeodactylum tricornutum was examined by using reverse genetic technologies. The overexpression of Lhcx3 led to increased diadinoxanthin + diatoxanthin content and elevated non-photochemical fluorescence quenching (NPQ) while knockdown of Lhcx3 reduced NPQ level. In addition, the expression of Lhcx3 could be induced by blue light but not by red light. After addition of the photosynthetic inhibitor, upregulation of Lhcx3 transcript in high light could be inhibited by NH4Cl, but not by DCMU (3-(3,4-dichlorophenyl)-l,l-dim ethylurea). In contrast, DCMU addition increased expression of Lhcx3 in high light. In combination with changes of NPQ after addition of inhibitor, we concluded that the Lhcx3 played key roles in high light acclimation of diatoms. This finding will provide new clues for genetic improvement of P. tricornutum with an aim to cultivate new strains with high growth rate. [ABSTRACT FROM AUTHOR]