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Ascorbic acid (AsA), a water-soluble antioxidant, act as cofactor for enzymes that regulate photosynthesis, hormonal and antioxidant biosynthesis that enables the plants to alleviate the harmful effects of salinity stress. The study was aimed to investigate the ameliorative role of foliar application of 200 ppm ascorbic acid (AsA) in improving growth by changing antioxidant response, photosynthetic capacity and mineral nutrient status of two canola varieties (Dunkled and Cyclone) under salinity stress (200 mM NaCl). Salt stress reduced plant biomass, photosynthetic activity, and accumulation of macro and micronutrients such as K+, Ca2+and Zn2+ in both canola varieties, on the other hand accumulation of Na+ and Cl−was increased. The salinity-induced oxidative stress (H 2 O 2 and MDA) caused photoinhibition of PSII at the donor and acceptor ends. The salinity-induced reduction in efficiency of electron donation to PSII (Fv/Fo) and electron transport through PSII (Fm/Fo), especially in Cyclone, significantly reduced ETRII thus enhancing CEF around PSI and NPQ in both canola varieties that has been considered as photo-protective mechanism of plants. In addition, salt stress enhanced CEF around PSI. However, application of AsA improved the structural stability of PSII, linear electron transport and reduced donor end limitation of PSI activity by improving electron transfer from PSII to PSI. The application of AsA improved the stomatal conductance, inter-cellular CO 2 concentration and net CO 2 assimilation rate thereby resulting in improved consumption of extra-electrons in CO 2 fixation generated by photosynthetic electron transport. Exogenous AsA application reduced the oxidative stress and improved the antioxidant potential by managing extra-electrons produced in CO 2 fixation. All these physiological and biochemical changes due to AsA application helped the canola plants to improve growth and yield under salinity. Thus, the application of ascorbic acid has the potential to ameliorate the detrimental effects of NaCl stress on canola plants. [Display omitted] • Canola (Brassica napus L.) growth and yield is greatly reduced by salinity stress. • Photosynthetic efficiency was hampered by salinity-induced oxidative stress. • Foliar spray of ascorbic acid mitigated the toxic effects of salinity stress. • Antioxidant defense system was boosted after application of ascorbic acid. • Cyclic electron flow and non-photochemical quenching was enhanced as ascorbic acid was applied. [ABSTRACT FROM AUTHOR]

Monogalactosyldiacylglycerol (MGDG) is the main lipid constituent of thylakoids and a structural component of photosystems and photosynthesis-related proteo-lipid complexes in green tissues. Previously reported changes in MGDG abundance upon stress treatments are hypothesized to reflect mobilization of MGDG-based polyunsaturated lipid intermediates to maintain extraplastidial membrane integrity. While exchange of lipid intermediates between compartmental membranes is well documented, physiological consequences of mobilizing an essential thylakoid lipid, such as MGDG, for an alternative purpose are not well understood. Arabidopsis seedlings exposed to mild (50 mM) salt treatment displayed significantly increased abundance of both MGDG and the extraplastidial lipid, phosphatidylcholine (PC). Interestingly, similar increases in MGDG and PC were observed in Arabidopsis fad3 mutant seedlings defective in endoplasmic reticulum (ER)–localized linolenic acid formation, in which compensatory plastid-to-ER-directed mobilization of linolenic acid–containing intermediates takes place. The postulated (salt) or evident (fad3) plastid–ER exchange of intermediates concurred with altered thylakoid function according to parameters of photosynthetic performance. While salt treatment of wild-type seedlings inhibited photosynthetic parameters in a dose-dependent manner, interestingly, untreated fad3 mutants did not show overall reduced photosynthetic quantum yield. By contrast, we observed a reduction specifically of non-photochemical quenching (NPQ) under high light, representing only part of observed salt effects. The decreased NPQ in the fad3 mutant was accompanied by reduced activity of the xanthophyll cycle, leading to a reduced concentration of the NPQ-effective pigment zeaxanthin. The findings suggest that altered ER-located fatty acid unsaturation and ensuing inter-organellar compensation impacts on the function of specific thylakoid enzymes, rather than globally affecting thylakoid function. [ABSTRACT FROM AUTHOR]

Background Some Sacoglossa sea slugs steal and integrate chloroplasts derived from the algae they feed on into their cells where they continue to function photosynthetically, a process termed kleptoplasty. The stolen chloroplasts – kleptoplasts – can maintain their functionality up to several months and support animal metabolism. However, chloroplast longevity can vary depending on sea slug species and algal donor. In this study, we focused on Elysia viridis, a polyphagous species that is mostly found associated with the macroalga Codium tomentosum, but that was reported to eat other macroalgae, including Chaetomorpha sp. Methods We have investigated the changes in E. viridis physiology when provided with the two different food sources to evaluate to which extent the photosynthetic and photoprotective mechanisms of the algae chloroplasts matched those of the plastids once in the animal cells. To perform the study, we rely on the evaluation of chlorophyll a variable fluorescence to study the photophysiological state of the integrated kleptoplasts and high-performance liquid chromatography (HPLC) to study variations in the photosynthetic pigments. Results We observed that the photosynthetic efficiency of E. viridis is lower when fed with Chaetomorpha. Also, significant differences were observed in the non-photochemical quenching (NPQ) abilities of the sea slugs. While sea slugs fed with C. tomentosum react similarly to high-light stress as the alga, E. viridis hosting Chaetomorpha chloroplasts were unable to properly recover from photoinhibition or perform a functional xanthophyll cycle (XC). Conclusions Our results showed that, even if the sea slugs fed with the two algae show photosynthetic activities like the respective algal donors, not all the photoprotective mechanisms present in Chaetomorpha can be maintained in E. viridis. This indicates that the functionality of the kleptoplasts does not depend solely on their origin but also on the degree of compatibility with the animal species integrating them.

Orange carotenoid proteins (OCPs) are unique photoreceptors that are critical for cyanobacterial photoprotection. Upon exposure to blue-green light, OCPs are activated from a stable orange form, OCPO, to an active red form, OCPR, which binds to phycobilisomes (PBSs) and performs photoprotective non-photochemical quenching (NPQ). OCPs can be divided into three main families: the most abundant and best studied OCP1, and two others, OCP2 and OCP3, which have different activation and quenching properties and are yet underexplored. Crystal structures have been acquired for the three OCP clades, providing a glimpse into the conformational underpinnings of their light-absorption and energy dissipation attributes. Recently, the structure of the PBS-OCPR complex has been obtained allowing for an unprecedented insight into the photoprotective action of OCPs. Here, we review the latest findings in the field that have substantially improved our understanding of how cyanobacteria protect themselves from the toxic consequences of excess light absorption. Furthermore, current research is applying the structure of OCPs to bio-inspired optogenetic tools, to function as carotenoid delivery devices, as well as engineering the NPQ mechanism of cyanobacteria to enhance their photosynthetic biomass production.

Main conclusion: Simultaneous genome editing of the two homeologousLCYeandZEPgenes ofNicotiana benthamianaresults in plants in which all xanthophylls are replaced by zeaxanthin. Plant carotenoids act both as photoreceptors and photoprotectants in photosynthesis and as precursors of apocarotenoids, which include signaling molecules such as abscisic acid (ABA). As dietary components, the xanthophylls lutein and zeaxanthin have photoprotective functions in the human macula. We developed transient and stable combinatorial genome editing methods, followed by direct LC–MS screening for zeaxanthin accumulation, for the simultaneous genome editing of the two homeologous Lycopene Epsilon Cyclase (LCYe) and the two Zeaxanthin Epoxidase (ZEP) genes present in the allopolyploid Nicotiana benthamiana genome. Editing of the four genes resulted in plants in which all leaf xanthophylls were substituted by zeaxanthin, but with different ABA levels and growth habits, depending on the severity of the ZEP1 mutation. In high-zeaxanthin lines, the abundance of the major photosystem II antenna LHCII was reduced with respect to wild-type plants and the LHCII trimeric state became unstable upon thylakoid solubilization. Consistent with the depletion in LHCII, edited plants underwent a compensatory increase in PSII/PSI ratios and a loss of the large-size PSII supercomplexes, while the level of PSI-LHCI supercomplex was unaffected. Reduced activity of the photoprotective mechanism NPQ was shown in high-zeaxanthin plants, while PSII photoinhibition was similar for all genotypes upon exposure to excess light, consistent with the antioxidant and photoprotective role of zeaxanthin in vivo. [ABSTRACT FROM AUTHOR]

Chlorophyll a fluorescence (ChlFa) parameters provide insight into the physiological and biochemical processes of plants and have been widely applied to monitor and evaluate the photochemical process and photosynthetic capacity of plants in a variety of environments. Recent advances in remote sensing provide new opportunities for the detection of ChlFa at large scales but demand further tremendous efforts. Among such efforts, application of the hyperspectral index is always possible, but the performance of hyperspectral indices in detecting ChlFa parameters under varying light conditions is much less investigated. The objective of this study is to investigate the performance of reported hyperspectral indices for tracking ChlFa parameters under different light conditions and to develop and evaluate novel spectral indices. Therefore, an experiment was conducted to simultaneously measure ChlFa parameters and spectral reflectance of sunlit and shaded leaves under varying light conditions, and 28 reported hyperspectral indices were examined for their performance in tracking the ChlFa parameters. Furthermore, we developed novel hyperspectral indices based on various spectral transformations. The results indicated that the maximum quantum efficiency of photosystem II (PSIImax), the cumulative quantum yield of photochemistry (ΦP), and the fraction of open reaction centers in photosystem II (qL) of sunlit leaves were significantly higher than those of shaded leaves, while the cumulative quantum yield of regulated thermal dissipation (ΦN) and fluorescence (ΦF) of shaded leaves was higher than that of sunlit leaves. Efficient tracing of ChlFa parameters could not be achieved from previously published spectral indices. In comparison, all ChlFa parameters were well quantified in shaded leaves when using novel hyperspectral indices, although the hyperspectral indices for tracing the non-photochemical quenching (NPQ) and ΦF were not stable, especially for sunlit leaves. Our findings justify the use of hyperspectral indices as a practical approach to estimating ChlFa parameters. However, caution should be used when using spectral indices to track ChlFa parameters based on the differences in sunlit and shaded leaves. [ABSTRACT FROM AUTHOR]

Main Conclusion: WHIRLY1 deficient barley plants surviving growth at high irradiance displayed increased non-radiative energy dissipation, enhanced contents of zeaxanthin and the flavonoid lutonarin, but no changes in α-tocopherol nor glutathione. Plants are able to acclimate to environmental conditions to optimize their functions. With the exception of obligate shade plants, they can adjust their photosynthetic apparatus and the morphology and anatomy of their leaves to irradiance. Barley (Hordeum vulgare L., cv. Golden Promise) plants with reduced abundance of the protein WHIRLY1 were recently shown to be unable to acclimatise important components of the photosynthetic apparatus to high light. Nevertheless, these plants did not show symptoms of photoinhibition. High-light (HL) grown WHIRLY1 knockdown plants showed clear signs of exposure to excessive irradiance such as a low epoxidation state of the violaxanthin cycle pigments and an early light saturation of electron transport. These responses were underlined by a very large xanthophyll cycle pool size and by an increased number of plastoglobules. Whereas zeaxanthin increased with HL stress, α-tocopherol, which is another lipophilic antioxidant, showed no response to excessive light. Also the content of the hydrophilic antioxidant glutathione showed no increase in W1 plants as compared to the wild type, whereas the flavone lutonarin was induced in W1 plants. HPLC analysis of removed epidermal tissue indicated that the largest part of lutonarin was presumably located in the mesophyll. Since lutonarin is a better antioxidant than saponarin, the major flavone present in barley leaves, it is concluded that lutonarin accumulated as a response to oxidative stress. It is also concluded that zeaxanthin and lutonarin may have served as antioxidants in the WHIRLY1 knockdown plants, contributing to their survival in HL despite their restricted HL acclimation. [ABSTRACT FROM AUTHOR]

Solar‐induced chlorophyll fluorescence (SIF) is a remotely sensed optical signal emitted during the light reactions of photosynthesis. The past two decades have witnessed an explosion in availability of SIF data at increasingly higher spatial and temporal resolutions, sparking applications in diverse research sectors (e.g., ecology, agriculture, hydrology, climate, and socioeconomics). These applications must deal with complexities caused by tremendous variations in scale and the impacts of interacting and superimposing plant physiology and three‐dimensional vegetation structure on the emission and scattering of SIF. At present, these complexities have not been overcome. To advance future research, the two companion reviews aim to (1) develop an analytical framework for inferring terrestrial vegetation structures and function that are tied to SIF emission, (2) synthesize progress and identify challenges in SIF research via the lens of multi‐sector applications, and (3) map out actionable solutions to tackle these challenges and offer our vision for research priorities over the next 5–10 years based on the proposed analytical framework. This paper is the first of the two companion reviews, and theory oriented. It introduces a theoretically rigorous yet practically applicable analytical framework. Guided by this framework, we offer theoretical perspectives on three overarching questions: (1) The forward (mechanism) question—How are the dynamics of SIF affected by terrestrial ecosystem structure and function? (2) The inference question: What aspects of terrestrial ecosystem structure, function, and service can be reliably inferred from remotely sensed SIF and how? (3) The innovation question: What innovations are needed to realize the full potential of SIF remote sensing for real‐world applications under climate change? The analytical framework elucidates that process complexity must be appreciated in inferring ecosystem structure and function from the observed SIF; this framework can serve as a diagnosis and inference tool for versatile applications across diverse spatial and temporal scales. [ABSTRACT FROM AUTHOR]

Magnesium is an essential macronutrient for plant photosynthesis, and in response to Mg deficiency, dicots appear more sensitive than monocots. Under Mg deficiency, we investigated the causes of differing photosynthetic sensitivities in a dicot and a monocot species. Rice (Oryza sativa L.) and cucumber (Cucumis sativus L.) were grown in hydroponic culture to explore their physiological responses to Mg deficiency stress. Both Mg-deficient rice and cucumber plants exhibited lower biomass, leaf area, Mg concentration, and chlorophyll content (Chl) compared with Mg-sufficient plants. However, a more marked decline in Chl and carotenoid content (Car) occurred in cucumber. A lower CO2 concentration in chloroplasts (Cc) was accompanied by a decrease in the maximum rate of electron transport (Jmax) and the maximum rate of ribulose 1,5-bisphosphate carboxylation (Vcmax), restricting CO2 utilization in Mgdeficient plants. Rice and cucumber photorespiration rate (Pr) increased under Mg deficiency. Additionally, for cucumber, Car and non-photochemical quenching (NPQ) were reduced under lower Mg supply. Meanwhile, cucumber Mg deficiency significantly increased the fraction of absorbed light energy dissipated by an additional quenching mechanism (Ff,D). Under Mg deficiency, suppressed photosynthesis was attributed to comprehensive restrictions of mesophyll conductance (gm), Jmax, and Vcmax. Cucumber was more sensitive to Mg deficiency than rice due to lower NPQ, higher rates of electron transport to alternative pathways, and subsequently, photooxidation damage. [ABSTRACT FROM AUTHOR]

The photosynthetic apparatus is a highly modular assembly of large pigment-binding proteins. Complexes called antennae can capture the sunlight and direct it from the periphery of two Photosystems (I, II) to the core reaction centers, where it is converted into chemical energy. The apparatus must cope with the natural light fluctuations that can become detrimental to the viability of the photosynthetic organism. Here we present an atomic scale view of the photoprotective mechanism that is activated on this line of defense by several photosynthetic organisms to avoid overexcitation upon excess illumination. We provide a complete macroscopic to microscopic picture with specific details on the conformations of the major antenna of Photosystem II that could be associated with the switch from the light-harvesting to the photoprotective state. This is achieved by combining insight from both experiments and all-atom simulations from our group and the literature in a perspective article. [ABSTRACT FROM AUTHOR]

Light intensity and temperature independently impact all parts of the photosynthetic machinery in plants and algae. Yet to date, the vast majority of pulse amplitude modulated (PAM) chlorophyll a fluorescence measurements have been performed at well-defined light intensities, but rarely at well-defined temperatures. In this work, we show that PAM measurements performed at various temperatures produce vastly different results in the chlorophyte Chlorella vulgaris. Using a recently developed Phenoplate technique to map quantum yield of Photosystem II (Y(II)) and non-photochemical quenching (NPQ) as a function of temperature, we show that the fast-relaxing NPQ follows an inverse normal distribution with respect to temperature and appears insensitive to previous temperature acclimation. The slow-relaxing or residual NPQ after 5 minutes of dark recovery follows a normal distribution similar to Y(II) but with a peak in the higher temperature range. Surprisingly, higher slow- and fast-relaxing NPQ values were observed in high-light relative to low-light acclimated cultures. Y(II) values peaked at the adaptation temperature regardless of temperature or light acclimation. Our novel findings show the complete temperature working spectrum of Y(II) and how excess energy quenching is managed across a wide range of temperatures in the model microalgal species C. vulgaris. Finally, we draw attention to the fact that the effect of the temperature component in PAM measurements has been wildly underestimated, and results from experiments at room temperature can be misleading. [ABSTRACT FROM AUTHOR]

Non-photochemical quenching (NPQ), the dissipation of excess light energy as heat, has been long recognized as a major protective mechanism that minimizes the potential for oxidative damage to photosystem II (PSII) reaction centers. Two major positive contributors to NPQ are the carotenoid zeaxanthin, generated from violaxanthin by the enzyme violaxanthin de-epoxidase (VDE or NPQ1), and the thylakoid protein PsbS (NPQ4). The involvement of the lumenal Deg proteases in the repair of PSII from photoinhibition prompted us to further explore their possible role in other responses of Arabidopsis thaliana to high light. Here we show that upon exposure to high light, the single deg1 and the triple deg158 mutants display different levels and kinetics of NPQ, compared with the deg58 mutant and WT that behave alike. In response to high light, the two genotypes lacking Deg1 overaccumulate NPQ1 and NPQ4. After temporal inhibition of protein translation in vivo, the level of these two proteins in deg1 is higher than in WT. Together, the results suggest that Deg1 represents a new level of regulation of the NPQ process through adjusting the quantity of NPQ1 and NPQ4 proteins, probably through their proteolysis., (© 2025 The Author(s). The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd.)

In high light, the antenna system in oxygenic photosynthetic organisms switches to a photoprotective mode, dissipating excess energy in a process called non-photochemical quenching (NPQ). Diatoms exhibit very efficient NPQ, accompanied by a xanthophyll cycle in which diadinoxanthin is de-epoxidized into diatoxanthin. Diatoms accumulate pigments from this cycle in high light, and exhibit faster and more pronounced NPQ. The mechanisms underlying NPQ in diatoms remain unclear, but it can be mimicked by aggregation of their isolated light-harvesting complexes, FCP (fucoxanthin chlorophyll-a/c protein). We assess this model system by resonance Raman measurements of two peripheral FCPs, trimeric FCPa and nonameric FCPb, isolated from high- and low-light-adapted cells (LL,HL). Quenching is associated with a reorganisation of these proteins, affecting the conformation of their bound carotenoids, and in a manner which is highly dependent on the protein considered. FCPa from LL diatoms exhibits significant changes in diadinoxanthin structure, together with a smaller conformational change of at least one fucoxanthin. For these LL-FCPa, quenching is associated with consecutive events, displaying distinct spectral signatures, and its amplitude correlates with the planarity of the diadinoxanthin structure. HL-FCPa aggregation is associated with a change in planarity of a 515-nm-absorbing fucoxanthin, and, to a lesser extent, of diadinoxanthin. Finally, in FCPb, a blue-absorbing fucoxanthin is primarily affected. FCPs thus possess a plastic structure, undergoing several conformational changes upon aggregation, dependent upon their precise composition and structure. NPQ in diatoms may therefore arise from a combination of structural changes, dependent on the environment the cells are adapted to. • FCP antenna proteins isolated from diatoms exhibit energy quenching when aggregated • Resonance Raman reveals that quenching correlates with specific changes in conformation of three bound carotenoids • At least five different protein conformations are required to explain the spectral changes in FCPa • The observed changes are sensitive to the light environment the diatoms were grown under [ABSTRACT FROM AUTHOR]

• PRI is useful to detect plant stress, but is disturbed by noise sources in images. • We developed a new method to extract plant stress from hyperspectral images. • Our method eliminates non-leaf objects, edges, specular reflection, etc. • Extracted PRI values successfully distinguish stressed and non-stressed plants. Environmental stressors such as drought and heat often reduce plant productivity and yield. Early identification of stress responses can help to avoid yield losses. Although the photochemical reflectance index (PRI) has been widely investigated as a detection method for stress response, it is influenced not only by the stress status of the plant, but also by other factors, such as signals from non-leaf objects like soil and stems, self-shading of leaves, and specular reflections. Thus, PRI may not be reliable unless these confounding effects are addressed. In this study, we developed a new method to extract plant stress status information from high-resolution spectroscopic images. This new system filters pixels less affected by non-leaf objects, shaded leaves, and specular reflections. Additionally, it selects leaf pixels exposed to similar irradiances, which is crucial because PRI is a light-dependent variable. This new filtering method successfully distinguished drought stress in tomato plants. Thus, a robust framework for evaluating stress responses in plant production has been developed. [ABSTRACT FROM AUTHOR]

Plants employ an array of photoprotection mechanisms to alleviate the harmful effects of high light intensity. The violaxanthin cycle, which is associated with non-photochemical quenching (NPQ), involves violaxanthin de-epoxidase (VDE), and zeaxanthin epoxidase (ZEP) and is one of the most rapid and efficient mechanisms protecting plants under high light intensity. Woody bamboo is a class of economically and ecologically important evergreen grass species widely distributed in tropical and subtropical areas. However, the function of VDE in bamboo has not yet been elucidated. In this study, we found that high light intensity increased NPQ and stimulated the de-epoxidation of violaxanthin cycle components in moso bamboo (Phyllostachys edulis), whereas, samples treated with the VDE inhibitor (dithiothreitol) exhibited lower NPQ capacity, suggesting that violaxanthin cycle plays an important role in the photoprotection of bamboo. Further analysis showed that not only high light intensity but also extreme temperatures (4 and 42°C) and drought stress upregulated the expression of PeVDE in bamboo leaves, indicating that PeVDE is induced by multiple abiotic stresses. Overexpression of PeVDE under the control of the CaMV 35S promoter in Arabidopsis mutant npq1 mutant could rescue its NPQ, indicating that PeVDE functions in dissipating the excess absorbed light energy as thermal energy in bamboo. Moreover, compared with wildtype (Col-0) plants, the transgenic plants overexpressing PeVDE displayed enhanced photoprotection ability, higher NPQ capacity, slower decline in the maximum quantum yield of photosystem II (Fv/Fm) under high light intensity, and faster recovery under optimal conditions. These results suggest that PeVDE positively regulates the response to high light intensity in bamboo plants growing in the natural environment, which could improve their photoprotection ability through the violaxanthin cycle and NPQ. [ABSTRACT FROM AUTHOR]

Spexin (SPX) is a recently discovered endogenous peptide consisting of 14 amino acids. It was found that SPX, kisspeptin (KISS), and galanin (GAL) peptides belong to the same gene family and are also endogenous ligands of GAL2 and GAL3 receptors. The amino acid sequence of the SPX peptide is relatively conserved in vertebrates and invertebrates. The mRNA and protein of SPX are highly expressed both in peripheral organs and in the peripheral/central nervous system of mammals, birds, and fishes. Many biological roles of SPX has been found in nonmammal/mammals, including food intake, energy metabolism, reproduction, nociception, gastrointestinal motility, stress, and endocrine functions. This review collectively mentions the peptide structure of SPX, its receptors and distribution in tissues, and the biological activities of SPX on various organs. [ABSTRACT FROM AUTHOR]

African rice (Oryza glaberrima) has adapted to challenging environments and is a promising source of genetic variation. We analysed dynamics of photosynthesis and morphology in a reference set of 155 O. glaberrima accessions. Plants were grown in an agronomy glasshouse to late tillering stage. Photosynthesis induction from darkness and the decrease in low light was measured by gas exchange and chlorophyll fluorescence along with root and shoot biomass, stomatal density, and leaf area. Steady-state and kinetic responses were modelled. We describe extensive natural variation in O. glaberrima for steady-state, induction, and reduction responses of photosynthesis that has value for gene discovery and crop improvement. Principal component analyses indicated key clusters of plant biomass, kinetics of photosynthesis (CO2 assimilation, A), and photoprotection induction and reduction (measured by non-photochemical quenching, NPQ), consistent with diverse adaptation. Accessions also clustered according to countries with differing water availability, stomatal conductance (g s), A , and NPQ, indicating that dynamic photosynthesis has adaptive value in O. glaberrima. Kinetics of NPQ, A , and g s showed high correlation with biomass and leaf area. We conclude that dynamic photosynthetic traits and NPQ are important within O. glaberrima , and we highlight NPQ kinetics and NPQ under low light. [ABSTRACT FROM AUTHOR]

Plants grow in a variable environment in regard to soil water and light driving photochemical reactions. Light energy exceeding plant capability to use it for photochemical reactions must be dissipated by processes of non-photochemical quenching (NPQ). The aim of the study was to evaluate the impact of various components of NPQ on the response of Arabidopsis thaliana to fluctuating light and water availability. A laboratory experiment with Arabidopsis thaliana wild type (WT) and mutants npq1 and npq4 grown under optimum or reduced water availability was conducted. Dark-adapted plants were illuminated with fluctuating light (FL) of two intensities (55 and 530 μmol m−2 s−1) with each of the phases lasting for 20 s. The impact of water availability on the role of zeaxanthin and PsbS protein in NPQ induced at FL was analysed. The water deficit affected the dynamics of NPQ induced by FL. The lack of zeaxanthin or PsbS reduced plant capability to cope with FL. The synergy of both of these components was enhanced in regard to the amplitude of NPQ in the drought conditions. PsbS was shown as a component of primary importance in suiting plant response to FL under optimum and reduced water availability. [ABSTRACT FROM AUTHOR]

The aims of this study were to investigate otolith dysfunction, especially isolated otolith dysfunction (with preserved semicircular canal function) in persistent postural-perceptual dizziness (PPPD) patients. Twenty-one patients who had been diagnosed with PPPD were enrolled in this study. The subjects filled out questionnaires [the Dizziness Handicap Inventory (DHI) and the Niigata PPPD Questionnaire (NPQ)] and underwent vestibular evoked myogenic potential (VEMP) tests, video head-impulse tests (vHIT), and stabilometry. Among the 21 subjects with PPPD, 9 showed isolated otolith dysfunction, 4 exhibited both otolith dysfunction and semicircular canal dysfunction, and 2 demonstrated isolated semicircular canal dysfunction. Six subjects exhibited normal VEMP and vHIT results. Concerning the subjects' questionnaire scores and stabilometric parameters, there were no significant differences among subgroups when the subjects were classified according to their VEMP and vHIT results while stabilometric parameters obtained in PPPD subjects were significantly increased than published data of healthy subjects. As precipitating conditions for PPPD, vestibular neuritis was the most frequent and the second most was idiopathic otolithic vertigo. In conclusion, the majority of PPPD patients had otolith dysfunction, and most of them showed isolated otolith dysfunction. Idiopathic otolithic vertigo can be a precipitating factor of PPPD. While otolith dysfunction may be associated with initiation of PPPD symptoms, PPPD symptoms are also considered to be associated with other dysfunctions of the sensory processing system. [ABSTRACT FROM AUTHOR]

Purpose: The present research was done to evaluate the responses of ‘Yusef Khani’ (Y) and ‘Malas-e-Yazdi’ (M) Iranian pomegranates to salt stress under field conditions. Research method: Treatments included different salinized water (EC=1.05 as control, 4.61 and 7.46 dS m–1) and two commercially Iranian pomegranate varieties. Main findings: Interaction of salinity × variety showed the lowest chlorophyll (chl) and potassium (K) level, and the highest chloride (Cl) and sodium (Na) in M variety, under high level of salinity. Although the lowest non-photochemical quenching, and effective quantum yield of photochemical energy conversion in PSII observed in this variety under 7.46 dS m–1, however, basal quantum yield of non-photochemical processes in PSII increased. Accumulation of Na and Cl in leaf tissue increased with increasing salinity in both varieties. Moreover, lower accumulation of calcium (Ca), magnesium (Mg) and iron (Fe) observed in both varieties. More Na and Cl was obtained within leaves of M variety, compared with other one. We found that there were some differences between these varieties and ‘Yousef Khani’ was more tolerant to salinity compared with ‘Malas-e-Yazdi’. Limitations: It might be better to evaluate several varieties for salinity resistance, however, it was impossible to us. Originality/Value: Iran is the main source of genetic variability for pomegranate. There is a huge diversity within pomegranate germplasm that should be studied for salinity and drought resistance. Thus, here we conducted a research to find a salt resistant pomegranate.

The aims of this study were to investigate otolith dysfunction, especially isolated otolith dysfunction (with preserved semicircular canal function) in persistent postural-perceptual dizziness (PPPD) patients. Twenty-one patients who had been diagnosed with PPPD were enrolled in this study. The subjects filled out questionnaires [the Dizziness Handicap Inventory (DHI) and the Niigata PPPD Questionnaire (NPQ)] and underwent vestibular evoked myogenic potential (VEMP) tests, video head-impulse tests (vHIT), and stabilometry. Among the 21 subjects with PPPD, 9 showed isolated otolith dysfunction, 4 exhibited both otolith dysfunction and semicircular canal dysfunction, and 2 demonstrated isolated semicircular canal dysfunction. Six subjects exhibited normal VEMP and vHIT results. Concerning the subjects' questionnaire scores and stabilometric parameters, there were no significant differences among subgroups when the subjects were classified according to their VEMP and vHIT results while stabilometric parameters obtained in PPPD subjects were significantly increased than published data of healthy subjects. As precipitating conditions for PPPD, vestibular neuritis was the most frequent and the second most was idiopathic otolithic vertigo. In conclusion, the majority of PPPD patients had otolith dysfunction, and most of them showed isolated otolith dysfunction. Idiopathic otolithic vertigo can be a precipitating factor of PPPD. While otolith dysfunction may be associated with initiation of PPPD symptoms, PPPD symptoms are also considered to be associated with other dysfunctions of the sensory processing system.

Iron is a cofactor of photosystems and electron carriers in the photosynthetic electron transport chain. Low concentrations of dissolved iron are, therefore, the predominant factor that limits the growth of phototrophs in large parts of the open sea like the Southern Ocean and the North Pacific, resulting in “high nutrient–low chlorophyll” (HNLC) areas. Diatoms are among the most abundant microalgae in HNLC zones. Besides efficient iron uptake mechanisms, efficient photoprotection might be one of the key traits enabling them to outcompete other algae in HNLC regions. In diatoms, Lhcx proteins play a crucial role in one of the main photoprotective mechanisms, the energy-dependent fluorescence quenching (qE). The expression of Lhcx proteins is strongly influenced by various environmental triggers. We show that Lhcx2 responds specifically and in a very sensitive manner to iron limitation in the diatom Phaeodactylum tricornutum on the same timescale as the known iron-regulated genes ISIP1 and CCHH11. By comparing Lhcx2 knockout lines with wild type cells, we reveal that a strongly increased qE under iron limitation is based on the upregulation of Lhcx2. Other observed iron acclimation phenotypes in P. tricornutum include a massively reduced chlorophyll a content/cell, a changed ratio of light harvesting and photoprotective pigments per chlorophyll a, a decreased amount of photosystem II and photosystem I cores, an increased functional photosystem II absorption cross section, and decoupled antenna complexes. H2O2 formation at photosystem I induced by high light is lowered in iron-limited cells, while the amount of total reactive oxygen species is rather increased. Our data indicate a possible reduction in singlet oxygen by Lhcx2-based qE, while the other iron acclimation phenotype parameters monitored are not affected by the amount of Lhcx2 and qE.

Xanthophyll cycles (XC) have proven to be major contributors to photoacclimation for many organisms. This work describes a light-driven XC operating in the chlorophyte Chlamydomonas reinhardtii and involving the xanthophylls Lutein (L) and Loroxanthin (Lo). Pigments were quantified during a switch from high to low light (LL) and at different time points from cells grown in Day/Night cycle. Trimeric LHCII was purified from cells acclimated to high or LL and their pigment content and spectroscopic properties were characterized. The Lo/(L + Lo) ratio in the cells varies by a factor of 10 between cells grown in low or high light (HL) leading to a change in the Lo/(L + Lo) ratio in trimeric LHCII from .5 in low light to .07 in HL. Trimeric LhcbMs binding Loroxanthin have 5 ± 1% higher excitation energy (EE) transfer (EET) from carotenoid to Chlorophyll as well as higher thermo- and photostability than trimeric LhcbMs that only bind Lutein. The Loroxanthin cycle operates on long time scales (hours to days) and likely evolved as a shade adaptation. It has many similarities with the Lutein-epoxide – Lutein cycle (LLx) of plants.

Iron is a cofactor of photosystems and electron carriers in the photosynthetic electron transport chain. Low concentrations of dissolved iron are, therefore, the predominant factor that limits the growth of phototrophs in large parts of the open sea like the Southern Ocean and the North Pacific, resulting in "high nutrient–low chlorophyll" (HNLC) areas. Diatoms are among the most abundant microalgae in HNLC zones. Besides efficient iron uptake mechanisms, efficient photoprotection might be one of the key traits enabling them to outcompete other algae in HNLC regions. In diatoms, Lhcx proteins play a crucial role in one of the main photoprotective mechanisms, the energy-dependent fluorescence quenching (qE). The expression of Lhcx proteins is strongly influenced by various environmental triggers. We show that Lhcx2 responds specifically and in a very sensitive manner to iron limitation in the diatom Phaeodactylum tricornutum on the same timescale as the known iron-regulated genes ISIP1 and CCHH11. By comparing Lhcx2 knockout lines with wild type cells, we reveal that a strongly increased qE under iron limitation is based on the upregulation of Lhcx2. Other observed iron acclimation phenotypes in P. tricornutum include a massively reduced chlorophyll a content/cell, a changed ratio of light harvesting and photoprotective pigments per chlorophyll a , a decreased amount of photosystem II and photosystem I cores, an increased functional photosystem II absorption cross section, and decoupled antenna complexes. H2O2 formation at photosystem I induced by high light is lowered in iron-limited cells, while the amount of total reactive oxygen species is rather increased. Our data indicate a possible reduction in singlet oxygen by Lhcx2-based qE, while the other iron acclimation phenotype parameters monitored are not affected by the amount of Lhcx2 and qE. [ABSTRACT FROM AUTHOR]

Xanthophyll cycles (XC) have proven to be major contributors to photoacclimation for many organisms. This work describes a light-driven XC operating in the chlorophyte Chlamydomonas reinhardtii and involving the xanthophylls Lutein (L) and Loroxanthin (Lo). Pigments were quantified during a switch from high to low light (LL) and at different time points from cells grown in Day/Night cycle. Trimeric LHCII was purified from cells acclimated to high or LL and their pigment content and spectroscopic properties were characterized. The Lo/(L + Lo) ratio in the cells varies by a factor of 10 between cells grown in low or high light (HL) leading to a change in the Lo/(L + Lo) ratio in trimeric LHCII from.5 in low light to.07 in HL. Trimeric LhcbMs binding Loroxanthin have 5 ± 1% higher excitation energy (EE) transfer (EET) from carotenoid to Chlorophyll as well as higher thermo- and photostability than trimeric LhcbMs that only bind Lutein. The Loroxanthin cycle operates on long time scales (hours to days) and likely evolved as a shade adaptation. It has many similarities with the Lutein-epoxide – Lutein cycle (LLx) of plants. [ABSTRACT FROM AUTHOR]

• New method for high throughput phenotyping. • New method for screening of chemical stressors' impact on photochemistry. • Temperature deactivates key parts of photochemistry in algae. • NPQ responses in select conditions of temperature, light and phosphate availability. • Activation of photoprotection is strongly temperature dependent. Rapid light curves are one of the most widely used methods for assessing the physiological state of photosynthetic organisms. While the method has been applied in a range of physiological studies over the last 20 years, little progress has been made in adapting it for the new age of multi-parametric phenotyping. In order to advance research that is aimed at evaluating the physiological impact of multiple factors, the Phenoplate was developed: a simultaneous assessment of temperature and light gradients. It was used to measure rapid light curves of three marine microalgae across a temperature gradient and altered phosphate availability. The results revealed that activation of photoprotective mechanisms occurred with high efficiency at lower temperatures, and relaxation of photoprotection was negatively impacted above a certain temperature threshold in Tetraselmis sp. It was observed that Thalassiosira pseudonana and Nannochloropsis oceanica exhibited two unique delayed non-photochemical quenching signatures: in combinations of low light with low temperature, and darkness with high temperature, respectively. These findings demonstrate that the Phenoplate approach can be used as a rapid and simple tool to gain insight into the photobiology of microalgae. [ABSTRACT FROM AUTHOR]

Abstract pH homeostasis in the chloroplast is crucial for the control of photosynthesis and other metabolic processes in plants. Recently, nuclear‐encoded Day‐Length‐dependent Delayed Greening1 (DLDG1) and Fluctuating‐Light Acclimation Protein1 (FLAP1) that are required for the light‐inducible optimization of plastidial pH in Arabidopsis thaliana were identified. DLDG1 and FLAP1 homologs are specifically conserved in oxygenic phototrophs, and a DLDG1 homolog, Ycf10, is encoded in the chloroplast genome in plant cells. However, the function of Ycf10 and its physiological significance are unknown. To address this, we constructed ycf10 tobacco Nicotiana tabacum mutants and characterized their phenotypes. The ycf10 tobacco mutants grown under continuous‐light conditions showed a pale‐green phenotype only in developing leaves, and it was suppressed in short‐day conditions. The ycf10 mutants also induced excessive non‐photochemical quenching (NPQ) compared with those in the wild‐type at the induction stage of photosynthesis. These phenotypes resemble those of Arabidopsis dldg1 mutants, suggesting that they have similar functions. However, there are distinct differences between the two mutant phenotypes: The highly induced NPQ in tobacco ycf10 and the Arabidopsis dldg1 mutants are diminished and enhanced, respectively, with increasing duration of the fluctuating actinic‐light illumination. Ycf10 and DLDG1 were previously shown to localize in chloroplast envelope‐membranes, suggesting that Ycf10 and DLDG1 differentially control H+ exchange across these membranes in a light‐dependent manner to control photosynthesis.

Summary: Photosynthetic organisms in nature often experience light fluctuations. While low light conditions limit the energy uptake by algae, light absorption exceeding the maximal rate of photosynthesis may go along with enhanced formation of potentially toxic reactive oxygen species. To preempt high light‐induced photodamage, photosynthetic organisms evolved numerous photoprotective mechanisms. Among these, energy‐dependent fluorescence quenching (qE) provides a rapid mechanism to dissipate thermally the excessively absorbed energy. Diatoms thrive in all aquatic environments and thus belong to the most important primary producers on earth. qE in diatoms is provided by a concerted action of Lhcx proteins and the xanthophyll cycle pigment diatoxanthin. While the exact Lhcx activation mechanism of diatom qE is unknown, two lumen‐exposed acidic amino acids within Lhcx proteins were proposed to function as regulatory switches upon light‐induced lumenal acidification. By introducing a modified Lhcx1 lacking these amino acids into a Phaeodactylum tricornutum Lhcx1‐null qE knockout line, we demonstrate that qE is unaffected by these two amino acids. Based on sequence comparisons with Lhcx4, being incapable of providing qE, we perform domain swap experiments of Lhcx4 with Lhcx1 and identify two peptide motifs involved in conferring qE. Within one of these motifs, we identify a tryptophan residue with a major influence on qE establishment. This tryptophan residue is located in close proximity to the diadinoxanthin/diatoxanthin‐binding site based on the recently revealed diatom Lhc crystal structure. Our findings provide a structural explanation for the intimate link of Lhcx and diatoxanthin in providing qE in diatoms. Significance Statement: Together with a functional xanthophyll cycle, Lhcx proteins confer rapid photoprotection (energy‐dependent fluorescence quenching) in diatoms in response to excess light. By domain swap experiments, we identify essential parts of the Lhcx proteins that are mandatory to fulfill this role and identify a putative link between Lhcx proteins and the xanthophyll cycle in triggering energy‐dependent fluorescence quenching. [ABSTRACT FROM AUTHOR]

pH homeostasis in the chloroplast is crucial for the control of photosynthesis and other metabolic processes in plants. Recently, nuclear‐encoded Day‐Length‐dependent Delayed Greening1 (DLDG1) and Fluctuating‐Light Acclimation Protein1 (FLAP1) that are required for the light‐inducible optimization of plastidial pH in Arabidopsis thaliana were identified. DLDG1 and FLAP1 homologs are specifically conserved in oxygenic phototrophs, and a DLDG1 homolog, Ycf10, is encoded in the chloroplast genome in plant cells. However, the function of Ycf10 and its physiological significance are unknown. To address this, we constructed ycf10 tobacco Nicotiana tabacum mutants and characterized their phenotypes. The ycf10 tobacco mutants grown under continuous‐light conditions showed a pale‐green phenotype only in developing leaves, and it was suppressed in short‐day conditions. The ycf10 mutants also induced excessive non‐photochemical quenching (NPQ) compared with those in the wild‐type at the induction stage of photosynthesis. These phenotypes resemble those of Arabidopsis dldg1 mutants, suggesting that they have similar functions. However, there are distinct differences between the two mutant phenotypes: The highly induced NPQ in tobacco ycf10 and the Arabidopsis dldg1 mutants are diminished and enhanced, respectively, with increasing duration of the fluctuating actinic‐light illumination. Ycf10 and DLDG1 were previously shown to localize in chloroplast envelope‐membranes, suggesting that Ycf10 and DLDG1 differentially control H+ exchange across these membranes in a light‐dependent manner to control photosynthesis. [ABSTRACT FROM AUTHOR]

Supplementary lighting of specific wavelengths can be used for inducing morphological and physiological responses in different crops, ultimately improving yield and quality. Based on this approach, new greenhouse covering materials are being developed in order to improve the use of sunlight in horticulture. These new-generation greenhouse coverings may incorporate light spectrum modulation agents or fluorescent additives which convert solar UV radiation into visible light. In this work, we tested the agronomical and physiological response of lettuce grown under a greenhouse covered with poly-methyl-methacrylate (PPMA) panels doped with a blend of the rare-earth inorganic material with a photo-luminescent effect. The doped greenhouse elicited a 36% increase in lettuce yield compared to the undoped greenhouse. Chlorophyll and carotenoid content, as well as antioxidant activity and ascorbic acid content, were not affected by greenhouse cover, but the doped panels induced a 22% reduction in total phenolics and a 14% increase in nitrate content in leaves. The greenhouse covering materials also affected the photochemistry of photosynthesis, as the daily fluctuations in both the effective quantum yield (ΦPSII) and the electron transport rate (ETR) were attenuated under the doped greenhouse. Non-photochemical quenching (NPQ) was closely related to the light environment in all experimental conditions, with the highest values at 14:00 h. Our results showed that the red-supplemented light spectrum under the doped greenhouse cover contributed to increased plant growth and yield, with a corresponding effect on the physiology of photosynthesis.

Key message: Decreased absorptance and increased singlet oxygen production may cause photoinhibition of both PSII and PSI in birch leaves during autumn senescence; however, photosynthetic electron transfer stays functional until late senescence. During autumn senescence, deciduous trees degrade chlorophyll and may synthesize flavonols. We measured photosynthetic parameters, epidermal flavonols, singlet oxygen production in vivo and photoinhibition of the photosystems (PSII and PSI) from green and senescing silver birch (Betula pendula) leaves. Chlorophyll a fluorescence and P700 absorbance measurements showed that the amounts of both photosystems decreased throughout autumn senescence, but the remaining PSII units stayed functional until ~ 90% of leaf chlorophyll was degraded. An increase in the chlorophyll a to b ratio, a decrease in > 700 nm absorbance and a blue shift of the PSI fluorescence peak at 77 K suggest that light-harvesting complex I was first degraded during senescence, followed by light-harvesting complex II and finally the photosystems. Senescing leaves produced more singlet oxygen than green leaves, possibly because low light absorption by senescing leaves allows high flux of incident light per photosystem. Senescing leaves also induced less non-photochemical quenching, which may contribute to increased singlet oxygen production. Faster photoinhibition of both photosystems in senescing than in green leaves, under high light, was most probably caused by low absorption of light and rapid singlet oxygen production. However, senescing leaves maintained the capacity to recover from photoinhibition of PSII. Amounts of epidermal flavonols and singlet oxygen correlated neither in green nor in senescing leaves of silver birch. Moreover, Arabidopsis thaliana mutants, incapable of synthesizing flavonols, were not more susceptible to photoinhibition of PSII or PSI than wild type plants; screening of chlorophyll absorption by flavonols was, however, small in A. thaliana. These results suggest that flavonols do not protect against photoinhibition or singlet oxygen production in chloroplasts. [ABSTRACT FROM AUTHOR]

Photosynthetic rates vary depending on growth conditions, even within species. Remote sensing techniques have a great potential to predict the photosynthetic rates of leaves with different characteristics. Here, we demonstrate that the photosynthetic rates of leaves acclimated to different light and nutrient conditions can be estimated based on the chlorophyll fluorescence (ChlF), the photochemical reflectance index (PRI), and a chlorophyll index. Chenopodium album plants were grown under different light and nutrient conditions. PRI, ChlF parameters, and CO2/H2O gas exchange rates of leaves were simultaneously determined under the various light and CO2 conditions. PRI was used to assess non-photochemical quenching (NPQ), but the relationship between NPQ and PRI was weakened when the data on leaves grown under different conditions were pooled, because PRI in darkness ( PRI 0 ) changed with the leaf pigment composition. Among 15 pigment indices, we found that NDVI green , a reflectance index related to the leaf chlorophyll content, had the best correlation with PRI 0 ( r 2 = 0.89 ) across the studied leaves, and the correction of PRI by NDVI green improved the predictability of NPQ ( r 2 = 0.82 ). Using the steady-state ChlF, the NPQ estimated from PRI and NDVI green , and the stomatal conductance coefficient, we calculated the CO2 assimilation rates, which were strongly correlated with the actual rates (RMSE = 4.85 μ mol m - 2 s - 1 ), irrespective of growth conditions. Our approach has the potential to contribute to a more accurate estimation of photosynthetic rates in remote sensing. However, further studies on species variations and connecting with radiative transfer models are needed to demonstrate this at the canopy scale. [ABSTRACT FROM AUTHOR]

• The reduced LHCSR levels in mutants correlate well with enhanced singlet oxygen production and level of PSII photoinhibition under high light. • The acclimation to HL decreases PSII Quantum Yield irrespective from photodamage/repair rate implying that reduced PSII activity is a strategy to cope with excess light. • Consistent with (2), the light use efficiency of WT and LHCSR mutants is very similar under a wide range of irradiances and light/dark periodicity. • We conclude that modulation of LHCSR level, as a target in algae domestication, is ineffective strategy towards enhancing biomass productivity. Unicellular green algae, a promising source for renewable biofuels, produce lipid-rich biomass from light and CO 2. Productivity in photo-bioreactors is affected by inhomogeneous light distribution from high cell pigment causing heat dissipation of light energy absorbed in excess and shading of the deep layers. Contrasting reports have been published on the relation between photoprotective energy dissipation and productivity. Here, we have re-investigated the relation between energy quenching (qE) activity, photodamage and light use efficiency by comparing WT and two Chlamydomonas reinhardtii strains differing for their complement in LHCSR proteins, which catalyse dissipation of excitation energy in excess (qE). Strains were analysed for ROS production, protein composition, rate of photodamage and productivity assessed under wide light and CO 2 conditions. The strain lacking LHCSR1 and knocked down in LHCSR3, thus depleted in qE, produced O 2 at significantly higher rate under high light, accompanied by enhanced singlet oxygen release and PSII photodamage. However, biomass productivity of WT was delayed in respect for mutant strains under intermittent light conditions only, implying that PSII activity was not the limiting factor under excess light. Contrary to previous proposals, domestication of Chlamydomonas for carbon assimilation rate in photo-bioreactors by down-regulation of photoprotective energy dissipation was ineffective in increasing algal biomass productivity. [ABSTRACT FROM AUTHOR]

Microalgae can respond to long-term increases in light intensity by altering the concentration of photosynthetic complexes. Under active growth, the ability of Chlamydomonas reinhardtii to acclimate to excess light is dependent on cell division to reduce the concentration of photosynthetic complexes. But, in batch culture, cells eventually reach stationary phase where their ability to divide is limited; this should impact their capacity to undergo photoacclimation. Our goal is to dissect excess-light responses as cells approach stationary phase and to determine how the strategies of photoacclimation differ compared to cells in the exponential-growth phase. In this study, cultures exited exponential growth and transitioned into a declining growth phase (DGP), where cells continued a slow rate of growth for the next seven days in both low (LL) and high-light (HL). During this period, both cultures experience a conditional senescence-related decline in chlorophyll levels. Under HL, however, the senescing cultures have a rapid decline in PSII reaction centres, maintain a stable concentration of LHCII antenna, rapidly increase LHCSR levels, and have a sustained increase in Fo/Fm. Collectively this implies that the remaining antenna act as pH-dependent, quenching centres, presumably to protect the senescing chloroplast against HL. We discovered that acclimating to HL post-exponential phase involves active degradation that is intertwined with the normal senescence process that allowed for a limited rate of cell division. Image 1 • There is an age-dependent decline in chlorophyll. • Senescing cultures with gas exchange maintain a limited growth that facilities acclimation. • Senescing cultures react to HL by degrading PSII and maintaining LHCII antenna. • Antennae, possibly detached from PSII, form pH-dependent quenching centres under HL. • Autophagy has a role in acclimating to HL. [ABSTRACT FROM AUTHOR]

This study was conducted to observe the seasonal variation trend in leaf chlorophyll a fluorescence of Butea monosperma tree belonging to Fabaceae family. Chlorophyll fluorescence parameters taken for the present study were NPQ = (FM/FM −1) (non-photochemical quenching of chlorophyll fluorescence), Fv/Fm = Fm–Fo/Fm (maximum photochemical quantum yield of photosystem II) and qL = qp.F0ʹ/F' (coefficient of photochemical fluorescence quenching). Chlorophyll a fluorescence of Butea monosperma was measured with the help of JUNIOR‐PAM, Chlorophyll Fluorometer, Heinz Walz GmbH, Germany in December (winter), May (summer) and August (rainy) seasons and also on day 1, day 10 and day 20 of December, May and August. The measurement revealed that the above three fluorescence parameters showed a seasonal variation trend, wherein NPQ and qL followed a similar variation trend while Fv/Fm showed a different trend. NPQ and qL values were maximum in the summer season followed by winter and rainy seasons, while Fv/Fm value was maximum in the rainy season followed by winter and summer seasons. The variations in chlorophyll a fluorescence in different seasons are due to different environmental conditions causing a different degree of stress to Butea monosperma plant in different seasons. [ABSTRACT FROM AUTHOR]

Plants are subject to dramatic fluctuations in the intensity of sunlight throughout the day. When the photosynthetic machinery is exposed to high light, photons are absorbed in excess, potentially leading to oxidative damage of its delicate membrane components. A photoprotective molecular process called non-photochemical quenching (NPQ) is the fastest response carried out in the thylakoid membranes to harmlessly dissipate excess light energy. Despite having been intensely studied, the site and mechanism of this essential regulatory process are still debated. Here, we show that the main NPQ component called energy-dependent quenching (qE) is present in plants with photosynthetic membranes largely enriched in the major trimeric light-harvesting complex (LHC) II, while being deprived of all minor LHCs and most photosystem core proteins. This fast and reversible quenching depends upon thylakoid lumen acidification (ΔpH). Enhancing ΔpH amplifies the extent of the quenching and restores qE in the membranes lacking PSII subunit S protein (PsbS), whereas the carotenoid zeaxanthin modulates the kinetics and amplitude of the quenching. These findings highlight the self-regulatory properties of the photosynthetic light-harvesting membranes in vivo , where the ability to switch reversibly between the harvesting and dissipative states is an intrinsic property of the major LHCII. [ABSTRACT FROM AUTHOR]

Drought is a major cause of losses in crop yield. Under field conditions, plants exposed to drought are usually also experiencing rapid changes in light intensity. Accordingly, plants need to acclimate to both, drought and light stress. Two crucial mechanisms in plant acclimation to changes in light conditions comprise thylakoid protein phosphorylation and dissipation of light energy as heat by non‐photochemical quenching (NPQ). Here, we analyzed the acclimation efficacy of two different wheat varieties, by applying fluctuating light for analysis of plants, which had been subjected to a slowly developing drought stress as it usually occurs in the field. This novel approach allowed us to distinguish four drought phases, which are critical for grain yield, and to discover acclimatory responses which are independent of photodamage. In short‐term, under fluctuating light, the slowdown of NPQ relaxation adjusts the photosynthetic activity to the reduced metabolic capacity. In long‐term, the photosynthetic machinery acquires a drought‐specific configuration by changing the PSII‐LHCII phosphorylation pattern together with protein stoichiometry. Therefore, the fine‐tuning of NPQ relaxation and PSII‐LHCII phosphorylation pattern represent promising traits for future crop breeding strategies. Elucidating the regulation of photosynthesis under the combination of abiotic stress and fluctuating light is crucial for understanding the plant physiology in realistic conditions and for crop improvement. Wheat copes with drought by reconfiguring the photosynthetic machinery and by slowing down the NPQ relaxation in fluctuating light before photodamage becomes detectable. [ABSTRACT FROM AUTHOR]

Non-photochemical quenching (NPQ) in photosynthetic organisms provides the necessary photoprotection that allows them to cope with largely and quickly varying light intensities. It involves deactivation of excited states mainly at the level of the antenna complexes of photosystem II using still largely unknown molecular mechanisms. In higher plants the main contribution to NPQ is the so-called qE-quenching, which can be switched on and off in a few seconds. This quenching mechanism is affected by the low pH-induced activation of the small membrane protein PsbS which interacts with the major light-harvesting complex of photosystem II (LHCII). We are reporting here on a mechanistic study of the PsbS-induced LHCII quenching using ultrafast time-resolved chlorophyll (Chl) fluorescence. It is shown that the PsbS/LHCII interaction in reconstituted proteoliposomes induces highly effective and specific quenching of the LHCII excitation by a factor ≥ 20 via Chl–Chl charge-transfer (CT) state intermediates which are weakly fluorescent. Their characteristics are very broad fluorescence bands pronouncedly red-shifted from the typical unquenched LHCII fluorescence maximum. The observation of PsbS-induced Chl–Chl CT-state emission from LHCII in the reconstituted proteoliposomes is highly reminiscent of the in vivo quenching situation and also of LHCII quenching in vitro in aggregated LHCII, indicating a similar quenching mechanism in all those situations. The PsbS mutant lacking the two proton sensing Glu residues induced significant, but much smaller, quenching than wild type. Added zeaxanthin had only minor effects on the yield of quenching in the proteoliposomes. Overall our study shows that PsbS co-reconstituted with LHCII in liposomes represents an excellent in vitro model system with characteristics that are reflecting closely the in vivo qE-quenching situation. [ABSTRACT FROM AUTHOR]

Non‐photochemical quenching (NPQ) is an important photoprotective mechanism in plants, which dissipates excess energy and further protects the photosynthetic apparatus under high light stress. NPQ can be dissected into a number of components: qE, qZ, and qI. In general, NPQ is catalyzed by two independent mechanisms, with the faster‐activated quenching catalyzed by the monomeric light‐harvesting complex (LHCII) proteins and the slowly activated quenching catalyzed by LHCII trimers, both processes depending on zeaxanthin but to different extent. Here, we studied the NPQ of the intertidal green macroalga, Ulva prolifera, and found that the NPQ of U. prolifera lack the faster‐activated quenching, and showed much greater sensitivity to dithiothreitol (DTT) than to dicyclohexylcarbodiimide (DCCD). Further results suggested that the monomeric LHC proteins in U. prolifera included only CP29 and CP26, but lacked CP24, unlike Arabidopsis thaliana and the moss Physcomitrella patens. Moreover, the expression levels of CP26 increased significantly following exposure to high light, but the concentrations of the two important photoprotective proteins (PsbS and light‐harvesting complex stress‐related [LhcSR]) did not change upon the same conditions. Analysis of the xanthophyll cycle pigments showed that, upon exposure to high light, zeaxanthin synthesis in U. prolifera was gradual and much slower than that in P. patens, and could effectively be inhibited by DTT. Based on these results, we speculate the enhancement of CP26 and slow zeaxanthin accumulation provide an atypical NPQ, making this green macroalga well adapted to the intertidal environments. [ABSTRACT FROM AUTHOR]

Intermittent water availability characterizes the canopy habitat, but few studies have focused on how C3 epiphytic bromeliads deal with drought. In this context, we investigated how water deficits affect the photosynthetic responses of the epiphytic bromeliad Vriesea gigantea regarding its physiological and anatomical traits that can minimize the effects of stomatal closure. In a controlled experiment in which bromeliads were submitted to 21 days of drought, we demonstrated a reduction in the leaf water content followed by strong reductions in net CO2 exchange and the efficiency of the photochemical system. However, there were increases in the yield of non-photochemical quenching and the activities of hydrophilic antioxidants. We observed substomatal chambers connected with air channels reaching the chlorophyllous parenchyma. Our findings indicate that the low net CO2 exchange and the energy imbalance possibly increased the cyclic transport of electrons and activated the thermal dissipation of energy to avoid damage to the photosynthetic apparatus. Additionally, the aeration channels may passively store CO2 to facilitate its re-assimilation. Because most epiphytic bromeliads are C3 plants and drought is frequent in the canopy, we speculate that some attributes of V. gigantea may occur in other C3 species, favouring their radiation in the epiphytic environment. [ABSTRACT FROM AUTHOR]

Diatoms, a major group of algae, account for about a quarter of the global primary production on Earth. These photosynthetic organisms face significant challenges due to light intensity variations in their underwater habitat. To avoid photodamage, they have developed very efficient non-photochemical quenching (NPQ) mechanisms. These mechanisms originate in their light-harvesting antenna – the fucoxanthin–chlorophyll protein (FCP) complexes. Spectroscopic studies of NPQ in vivo are often hindered by strongly overlapping signals from the photosystems and their antennae. Fortunately, in vitro FCP aggregates constitute a useful model system to study fluorescence (FL) quenching in diatoms. In this work, we present streak-camera FL measurements on FCPa and FCPb complexes, isolated from a centric diatom Cyclotella meneghiniana , and their aggregates. We find that spectra of non-aggregated FCP are dominated by a single fluorescing species, but the FL spectra of FCP aggregates additionally contain contributions from a redshifted emissive state. We relate this red state to a charge transfer state between chlorophyll c and chlorophyll a molecules. The FL quenching, on the other hand, is due to an additional dark state that involves incoherent energy transfer to the fucoxanthin carotenoids. Overall, the global picture of energy transfer and quenching in FCP aggregates is very similar to that of major light-harvesting complexes in higher plants (LHCII), but microscopic details between FCPs and LHCIIs differ significantly. [Display omitted] • Time-resolved fluorescence from aggregated and non-aggregated FCPs is presented. • Two emissive states are needed to describe the aggregated data. • Red emission is from a microscopic charge transfer state between Chl a and Chl c. • Quenching is due to a dark state, possibly related to energy transfer to fucoxanthin. [ABSTRACT FROM AUTHOR]

Abstract Background The preparation of functional thylakoid membranes from diatoms with a silica cell wall is still a largely unsolved challenge. Therefore, an optimized protocol for the isolation of oxygen evolving thylakoid membranes of the centric diatom Cyclotella meneghiniana has been developed. The buffer used for the disruption of the cells was supplemented with polyethylene glycol based on its stabilizing effect on plastidic membranes. Disruption of the silica cell walls was performed in a French Pressure cell and subsequent linear sorbitol density gradient centrifugation was used to isolate the thylakoid membrane fraction. Results Spectroscopic characterization of the thylakoids by absorption and 77 K fluorescence spectroscopy showed that the photosynthetic pigment protein complexes in the isolated thylakoid membranes were intact. This was supported by oxygen evolution measurements which demonstrated high electron transport rates in the presence of the artificial electron acceptor DCQB. High photosynthetic activity of photosystem II was corroborated by the results of fast fluorescence induction measurements. In addition to PSII and linear electron transport, indications for a chlororespiratory electron transport were observed in the isolated thylakoid membranes. Photosynthetic electron transport also resulted in the establishment of a proton gradient as evidenced by the quenching of 9-amino-acridine fluorescence. Because of their ability to build-up a light-driven proton gradient, de-epoxidation of diadinoxanthin to diatoxanthin and diatoxanthin-dependent non-photochemical quenching of chlorophyll fluorescence could be observed for the first time in isolated thylakoid membranes of diatoms. However, the ∆pH, diadinoxanthin de-epoxidation and diatoxanthin-dependent NPQ were weak compared to intact diatom cells or isolated thylakoids of higher plants. Conclusions The present protocol resulted in thylakoids with a high electron transport capacity. These thylakoids can thus be used for experiments addressing various aspects of the photosynthetic electron transport by, e.g., employing artificial electron donors and acceptors which do not penetrate the diatom cell wall. In addition, the present isolation protocol yields diatom thylakoids with the potential for xanthophyll cycle and non-photochemical quenching measurements. However, the preparation has to be further refined before these important topics can be addressed systematically.