Search

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., (© The Author(s) 2023. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Evergreen conifers growing in high-latitude regions must endure prolonged winters that are characterized by sub-zero temperatures combined with light, conditions that can cause significant photooxidative stress. Understanding overwintering mechanisms is crucial for addressing winter adversity in temperate forest ecosystems and enhancing the ability of conifers to adapt to climate change. This review synthesizes the current understanding of the photoprotective mechanisms that conifers employ to mitigate photooxidative stress, particularly non-photochemical "sustained quenching", the mechanism of which is hypothesized to be a recombination or deformation of the original mechanism employed by conifers in response to short-term low temperature and intense light stress in the past. Based on this hypothesis, scattered studies in this field are assembled and integrated into a complete mechanism of sustained quenching embedded in the adaptation process of plant physiology. It also reveals which parts of the whole system have been verified in conifers and which have only been verified in non-conifers, and proposes specific directions for future research. The functional implications of studies of non-coniferous plant species for the study of coniferous trees are also considered, as a wide range of plant responses lead to sustained quenching, even among different conifer species. In addition, the review highlights the challenges of measuring sustained quenching and discusses the application of ultrafast-time-resolved fluorescence and decay-associated spectra for the elucidation of photosynthetic principles., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Masson SAS. All rights reserved.)

High-throughput detection of plant environmental stresses is required for minimizing the reduction in crop yield. Environmental stresses in plants have primarily been validated by the measurements of photosynthesis with gas exchange and chlorophyll fluorescence, which involve complicated procedures. Remote sensing technologies that monitor leaf reflectance in intact plants enable real-time visualization of plant responses to environmental fluctuations. The photochemical reflectance index (PRI), one of the vegetation indices of spectral leaf reflectance, is related to changes in xanthophyll pigment composition. Xanthophyll dynamics are strongly correlated with plant stress because they contribute to the thermal dissipation of excess energy. However, an accurate assessment of plant stress based on PRI requires correction by baseline PRI (PRI o ) in the dark, which is difficult to obtain in the field. In this study, we propose a method to correct the PRI using NPQ T , which can be measured under light. By this method, we evaluated responses of excess light energy stress under drought in wild watermelon (Citrullus lanatus L.), a xerophyte. Demonstration on the farm, the stress behaviors were observed in maize (Zea mays L.). Furthermore, the stress status of plants and their recovery following re-watering were captured as visual information. These results suggest that the PRI is an excellent indicator of environmental stress and recovery in plants and could be used as a high-throughput stress detection tool in agriculture.

The fluorescence signals emitted by chlorophyll molecules of plants is a promising non-destructive indicator of plant physiology due to its close link to photosynthesis. In this work, a deep photophysical study of chlorophyll fluorescence was provided, to assess the sub-optimal illumination effects on three plant species: L. sativa, A. hybridus and S. dendroideum. In all the cases, low light (LL) treatment induced an increase in pigment content. Fluorescence ratios - corrected by light reabsorption processes - remained constant, which suggested that photosystems stoichiometry was conserved. For all species and treatments, quantum yields of photophysical decay remained around 0.2, which meant that the maximum possible photosynthesis efficiency was about 0.8. L. sativa (C3) acclimated to low light illumination, displayed a strong increase in the LHC size and a net decrease in the photosynthetic efficiency. A. hybridus (C4) was not appreciably stressed by the low light availability whereas S. dendroideum (CAM), decreased its antenna and augmented the quantum yield of primary photochemistry. A novel approach to describe NPQ relaxation kinetics was also presented here and used to calculate typical deactivation times and amplitudes for NPQ components. LL acclimated L. sativa presented a much larger deactivation time for its state-transition-related quenching than the other species. Comprehensive fluorescence analysis allowed a deep study of the changes in the light-dependent reactions of photosynthesis upon low light illumination treatment., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Key Message: M-type thioredoxins are required to regulate zeaxanthin epoxidase activity and to maintain the steady-state level of the proton motive force, thereby influencing NPQ properties under low-light conditions in Arabidopsis. Non-photochemical quenching (NPQ) helps protect photosynthetic organisms from photooxidative damage via the non-radiative dissipation of energy as heat. Energy-dependent quenching (qE) is a major constituent of NPQ. However, the mechanism underlying the regulation of qE is not well understood. In this study, we demonstrate that the m-type thioredoxins TRX-m1, TRX-m2, and TRX-m4 (TRX-ms) interact with the xanthophyll cycle enzyme zeaxanthin epoxidase (ZE) and are required for maintaining the redox-dependent stabilization of ZE by regulating its intermolecular disulfide bridges. Reduced ZE activity and accumulated zeaxanthin levels were observed under TRX-ms deficiency. Furthermore, concurrent deficiency of TRX-ms resulted in a significant increase in proton motive force (pmf) and acidification of the thylakoid lumen under low irradiance, perhaps due to the significantly reduced ATP synthase activity under TRX-ms deficiency. The increased pmf, combined with acidification of the thylakoid lumen and the accumulation of zeaxanthin, ultimately contribute to the elevated stable qE in VIGS-TRX-m2m4/m1 plants under low-light conditions. Taken together, these results indicate that TRX-ms are involved in regulating NPQ-dependent photoprotection in Arabidopsis.

In this study, we have compared the photosynthetic characteristics of two contrasting species of Tradescantia plants, T. fluminensis (shade-tolerant species), and T. sillamontana (light-resistant species), grown under the low light (LL, 50-125 µmol photons m -2  s -1 ) or high light (HL, 875-1000 µmol photons m -2  s -1 ) conditions during their entire growth period. For monitoring the functional state of photosynthetic apparatus (PSA), we measured chlorophyll (Chl) a emission fluorescence spectra and kinetics of light-induced changes in the heights of fluorescence peaks at 685 and 740 nm (F 685 and F 740 ). We also compared the light-induced oxidation of P 700 and assayed the composition of carotenoids in Tradescantia leaves grown under the LL and HL conditions. The analyses of slow induction of Chl a fluorescence (SIF) uncovered different traits in the LL- and HL-grown plants of ecologically contrasting Tradescantia species, which may have potential ecophysiological significance with respect to their tolerance to HL stress. The fluorometry and EPR studies of induction events in chloroplasts in situ demonstrated that acclimation of both Tradescantia species to HL conditions promoted faster responses of their PSA as compared to LL-grown plants. Acclimation of both species to HL also caused marked changes in the leaf anatomy and carotenoid composition (an increase in Violaxanthin + Antheraxantin + Zeaxanthin and Lutein pools), suggesting enhanced photoprotective capacity of the carotenoids in the plants grown in nature under high irradiance. Collectively, the results of the present work suggest that the mechanisms of long-term PSA photoprotection in Tradescantia are based predominantly on the light-induced remodeling of pigment-protein complexes in chloroplasts.

Water and nutrients, particularly phosphorus (P), are the two most limiting factors for dwarf bamboo growth in tropical and subtropical areas. Dwarf bamboo is highly sensitive to water stress and often causes severe P deficiency in its growing soils due to the characteristics of shallower roots and expeditious growth. However, little is known about its photoprotective response to soil water deficit and the underlying mechanisms regulated by P application. In this study, a completely randomized design with two factors of two water regimes (well-watered and water-stressed) and two P levels (with and without P application) was arranged to investigate this issue in dwarf bamboo (Fargesia rufa) plants. Water stress not only decreased water status and photochemical activity but also increased lipid peroxidation due to reactive oxygen species (ROS) accumulation irrespective of P application. In this case, thermal dissipation and antioxidative defense were promoted. Moreover, the role of the water-water cycle under this stress still could not be ignored because it accounted for a large proportion of total energy (J PSII ). P application significantly enhanced photochemical activity accompanied by increased chlorophyll content in water-stressed plants. Meanwhile, P application remarkably reduced thermal dissipation and hardly affected photorespiration and the water-water cycle under water stress. Although P application only enhanced ascorbate (AsA) level, ROS, particularly hydrogen peroxide (H 2 O 2 ), and lipid peroxidation were significantly reduced in water-stressed plants. Therefore, P application can improve the photosynthetic capacity by regulating the redistribution of energy absorbed by PSII antennae and independently activating of the H 2 O 2 -scavenging function of AsA to alleviate oxidative damage in F. rufa plants, thereby improving their survival under water stress conditions., (Copyright © 2017 Elsevier Masson SAS. All rights reserved.)

As the Arctic warms, tundra vegetation is becoming taller and more structurally complex, as tall deciduous shrubs become increasingly dominant. Emerging studies reveal that shrubs exhibit photosynthetic resource partitioning, akin to forests, that may need accounting for in the "big leaf" net ecosystem exchange models. We conducted a lab experiment on sun and shade leaves from S. pulchra shrubs to determine the influence of both constitutive (slowly changing bulk carotenoid and chlorophyll pools) and facultative (rapidly changing xanthophyll cycle) pigment pools on a suite of spectral vegetation indices, to devise a rapid means of estimating within canopy resource partitioning. We found that: (1) the PRI of dark-adapted shade leaves (PRIo) was double that of sun leaves, and that PRIo was sensitive to variation among sun and shade leaves in both xanthophyll cycle pool size (V + A + Z) (r (2) = 0.59) and Chla/b (r (2) = 0.64); (2) A corrected PRI (difference between dark and illuminated leaves, ΔPRI) was more sensitive to variation among sun and shade leaves in changes to the epoxidation state of their xanthophyll cycle pigments (dEPS) (r (2) = 0.78, RMSE = 0.007) compared to the uncorrected PRI of illuminated leaves (PRI) (r (2) = 0.34, RMSE = 0.02); and (3) the SR680 index was correlated with each of (V + A + Z), lutein, bulk carotenoids, (V + A + Z)/(Chla + b), and Chla/b (r (2) range = 0.52-0.69). We suggest that ΔPRI be employed as a proxy for facultative pigment dynamics, and the SR680 for the estimation of constitutive pigment pools. We contribute the first Arctic-specific information on disentangling PRI-pigment relationships, and offer insight into how spectral indices can assess resource partitioning within shrub tundra canopies.

In their natural environment plants and algae are exposed to rapidly changing light conditions and light intensities. Illumination with high light intensities has the potential to overexcite the photosynthetic pigments and the electron transport chain and thus induce the production of toxic reactive oxygen species (ROS). To prevent damage by the action of ROS, plants and algae have developed a multitude of photoprotection mechanisms. One of the most important protection mechanisms is the dissipation of excessive excitation energy as heat in the light-harvesting complexes of the photosystems. This process requires a structural change of the photosynthetic antenna complexes that are normally optimized with regard to efficient light-harvesting. Enhanced heat dissipation in the antenna systems is accompanied by a strong quenching of the chlorophyll a fluorescence and has thus been termed non-photochemical quenching of chlorophyll a fluorescence, NPQ. The general importance of NPQ for the photoprotection of plants and algae is documented by its wide distribution in the plant kingdom. In the present review we will summarize the present day knowledge about NPQ in higher plants and different algal groups with a special focus on the molecular mechanisms that lead to the structural rearrangements of the antenna complexes and enhanced heat dissipation. We will present the newest models for NPQ in higher plants and diatoms and will compare the features of NPQ in different algae with those of NPQ in higher plants. In addition, we will briefly address evolutionary aspects of NPQ, i.e. how the requirements of NPQ have changed during the transition of plants from the aquatic habitat to the land environment. We will conclude with a presentation of open questions regarding the mechanistic basis of NPQ and suggestions for future experiments that may serve to obtain this missing information., (Copyright © 2014 Elsevier GmbH. All rights reserved.)

In the present study the non-photochemical quenching (NPQ) of four biofilm-forming and two planktonic green algae was investigated by fluorescence measurements, determinations of the light-driven proton gradient and determination of the violaxanthin cycle activity by pigment analysis. It was observed that, despite the common need for efficient photoprotection, the structural basis of NPQ was heterogeneous in the different species. Three species, namely Chlorella saccharophila, Chlorella vulgaris and Bracteacoccus minor, exhibited a zeaxanthin-dependent NPQ, while in the three other species, Tetracystis aeria, Pedinomonas minor and Chlamydomonas reinhardtii violaxanthin de-epoxidation was absent or unrelated to the establishment of NPQ. Acclimation of the algae to high light conditions induced an increase of the NPQ activity, suggesting that a significant part of the overall NPQ was rather inducible than constitutively present in the green algae. Comparing the differences in the NPQ mechanisms with the phylogenetic position of the six algal species led to the conclusion that the NPQ heterogeneity observed in the present study was not related to the phylogeny of the algae but to the environmental selection pressure. Finally, the difference in the NPQ mechanisms in the different species is discussed within the frame of the current NPQ models., (Copyright © 2014 Elsevier GmbH. All rights reserved.)

Diatoms are especially important microorganisms because they constitute the larger group of microalgae. To survive the constant variations of the light environment, diatoms have developed mechanisms aiming at the dissipation of excess energy, such as the xanthophyll cycle and the non-photochemical chlorophyll (Chl) fluorescence quenching. This contribution is dedicated to the relaxation of the latter process when the adverse conditions cease. An original nonlinear regression analysis of the relaxation of non-photochemical Chl fluorescence quenching, qN, in diatoms is presented. It was used to obtain experimental evidence for the existence of three time-resolved components in the diatom Phaeodactylum tricornutum: qNf, qNi and qNs. qNf (s time-scale) and qNs (h time-scale) are exponential in shape. By contrast, qNi (min time-scale) is of sigmoidal nature and is dominant among the three components. The application of metabolic inhibitors (dithiothreitol, ammonium chloride, cadmium and diphenyleneiodonium chloride) allowed the identification of the mechanisms on which each component mostly relies. qNi is linked to the relaxation of the ΔpH gradient and the reversal of the xanthophyll cycle. qNs quantifies the stage of photoinhibition caused by the high light exposure, qNf seems to reflect fast conformational changes within thylakoid membranes in the vicinity of the photosystem II complexes.

Terrestrial laser scanning (TLS) data allow spatially explicit (x, y, z) laser return intensities to be recorded throughout a plant canopy, which could considerably improve our understanding of how physiological processes vary in three-dimensional space. However, the utility of TLS data for the quantification of plant physiological properties remains largely unexplored. Here, we test whether the laser return intensity of green (532-nm) TLS correlates with changes in the de-epoxidation state of the xanthophyll cycle and photoprotective non-photochemical quenching (NPQ), and compare the ability of TLS to quantify these parameters with the passively measured photochemical reflectance index (PRI). We exposed leaves from five plant species to increasing light intensities to induce NPQ and de-epoxidation of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z). At each light intensity, the green laser return intensity (GLRI), narrowband spectral reflectance, chlorophyll fluorescence emission and xanthophyll cycle pigment composition were recorded. Strong relationships between both predictor variables (GLRI, PRI) and both explanatory variables (NPQ, xanthophyll cycle de-epoxidation) were observed. GLRI holds promise to provide detailed (mm) information about plant physiological status to improve our understanding of the patterns and mechanisms driving foliar photoprotection. We discuss the potential for scaling these laboratory data to three-dimensional canopy space., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)

In the present study, low concentrations of the very mild detergent n‐dodecyl‐α‐d‐maltoside in conjunction with sucrose gradient ultracentrifugation were used to prepare fucoxanthin chlorophyll protein (FCP) complexes of the centric diatom Thalassiosira pseudonana. Two main FCP fractions were observed in the sucrose gradients, one in the upper part and one at high sucrose concentrations in the lower part of the gradient. The first fraction was dominated by the 18 kDa FCP protein band in SDS‐gels. Since this fraction also contained other protein bands, it was designated as fraction enriched in FCP‐A complex. The second fraction contained mainly the 21 kDa FCP band, which is typical for the FCP‐B complex. Determination of the lipid composition showed that both FCP fractions contained monogalactosyl diacylglycerol as the main lipid followed by the second galactolipid of the thylakoid membrane, namely digalactosyl diacylglycerol. The negatively charged lipids sulfoquinovosyl diacylglycerol and phosphatidyl glycerol were also present in both fractions in pronounced concentrations. With respect to the pigment composition, the fraction enriched in FCP‐A contained a higher amount of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (Dt), whereas the FCP‐B fraction was characterized by a lower ratio of xanthophyll cycle pigments to the light‐harvesting pigment fucoxanthin. Protein analysis by mass spectrometry revealed that in both FCP fractions the xanthophyll cycle enzyme diadinoxanthin de‐epoxidase (DDE) was present. In addition, the analysis showed an enrichment of DDE in the fraction enriched in FCP‐A but only a very low amount of DDE in the FCP‐B fraction. In‐vitro de‐epoxidation assays, employing the isolated FCP complexes, were characterized by an inefficient conversion of DD to Dt. However, in line with the heterogeneous DDE distribution, the fraction enriched in FCP‐A showed a more pronounced DD de‐epoxidation compared with the FCP‐B. [ABSTRACT FROM AUTHOR]

The Photochemical Reflectance Index (PRI) and solar induced fluorescence (SIF) provide information on plant photosynthetic activity. PRI and SIF are both strongly influenced by irradiance, but uncertainties related to the interpretation of these light responses at large spatial scales remain, partly due to a shortage of suitable data from aircraft or satellite platforms. The goal of this study was to explore interpretations of the PRI- and SIF-light responses of trees owing to species, functional types (evergreen and deciduous) and season. Using airborne hyperspectral and ultraspectral imagery in a North American urban forest, we derived PRI, SIF, and albedo (an indicator of illumination) at the 1-m pixel level. We then quantified crown-level PRI and SIF light responses of ten different tree species at three time points from late-summer to autumnal senescence using hierarchical models. Our results confirmed that both PRI and SIF were strongly influenced by illumination with PRI decreasing and SIF increasing with illumination. Both slope and intercept of the PRI-albedo relationship changed with season, but the pattern varied among species and functional types. SIF values decreased during autumnal senescence for all species, but evergreen species exhibited less seasonal decline in the slope of SIF-albedo relationship compared to deciduous species. The PRI and SIF light responses derived from the airborne imagery offer complementary information on dynamic photosynthesis responses presumably due to varying canopy structure, pigmentation and photoprotection among species and functional types. From airborne platforms, PRI- and SIF-light responses can be used to explore the contrasting physiological responses of individual tree crowns, providing a spatially and temporally explicit view of dynamic plant traits related to photoregulation and a novel view of functional diversity for entire landscapes. • PRI and SIF light responses of trees were fitted using airborne data. • Both PRI and SIF were strongly influenced by illumination. • PRI and SIF light responses varied among species, functional types and season. • PRI and SIF light responses revealed dynamic photosynthetic characteristics. [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]

Lichens are unique phototrophic organisms whose physiology of stress tolerance attracts much attention. Parameters of photosynthetic and respiratory activities of the epiphytic large-leaved lichen Lobaria pulmonaria were investigated with an aim to reveal physiological responses to seasonal changes in environmental conditions. The highest accumulation of chlorophylls in thalli (2.3 mg/g dry wt) was noted in autumn (October); the amount of green pigments decreased 2.5 times in spring (April). The chlorophyll/carotenoid ratio varied from 3.1 to 4.4. The extent of deepoxidation of xanthophyll cycle pigments equaled 34% in winter but it was two times as low in summer. When L. pulmonaria thalli were hydrated and acclimated shortly under standard laboratory conditions, they exhibited a relatively high photochemical activity and were able to assimilate CO2 throughout the entire annual cycle. The rate of net CO2 uptake by thalli under optimal irradiance and temperature ranged from 3 to 5 µmol CO2/m2 s and the highest values were recorded in spring. No significant seasonal changes were observed in the total respiration rate of thalli. The proportions of various respiratory pathways were altered in spring and autumn, and metabolic heat production was accelerated due to the activation of an energetically low-efficient alternative respiratory pathway. The results provide evidence that the functional adaptation of photo- and mycobionts in the lichen is implicated in resistance of this complete system to seasonal changes in environmental conditions. [ABSTRACT FROM AUTHOR]

Zeaxanthin epoxidase (ZEP) plays important roles in plant growth and development due to its functions in abscisic acid (ABA) biosynthesis and in the xanthophyll cycle. Many studies have been exploring the roles of ZEP in seed germination and response to various environmental stresses. In this study, we reported that heterologously overexpressing the ZEP gene from Medicago sativa (MsZEP) in tobacco increased ABA content in tobacco mature seeds and negatively regulated seed germination. Enhanced ABA synthesis in seed embryo and delayed germination might also be related to the increased 9-cis-epoxycarotenoid dioxygenase (NCED6) expression. Moreover, we found that overexpression of MsZEP resulted in an increased expression of the SOMNUS gene but a decreased expression of the DNA ligase 6 gene (Lig6) suggesting that MsZEP might affect seed proteome and DNA integrity. Furthermore, enhanced chlorophyll content in transgenic tobacco seedlings overexpressing MsZEP might be due to its function in the xanthophyll cycle and ABA biosynthesis.

The identification of drought-tolerant olive tree genotypes has become an urgent requirement to develop sustainable agriculture in dry lands. However, physiological markers linking drought tolerance with mechanistic effects operating at the cellular level are still lacking, in particular under severe stress, despite the urgent need to develop these tools in the current frame of global change. In this context, 1-year-old olive plants growing in the greenhouse and with a high intra-specific variability (using various genotypes obtained either from cuttings or seeds) were evaluated for drought tolerance under severe stress. Growth, plant water status, net photosynthesis rates, chlorophyll contents and the extent of photo- and antioxidant defenses (including the de-epoxidation state of the xanthophyll cycle, and the contents of carotenoids and vitamin E) were evaluated under well-watered conditions and severe stress (by withholding water for 60 days). Plants were able to continue photosynthesizing under severe stress, even at very low leaf water potential of −4 to −6 MPa. This ability was achieved, at least in part, by the activation of photo- and antioxidant mechanisms, including not only increased xanthophyll cycle de-epoxidation, but also enhanced α-tocopherol contents. "Zarrazi" (obtained from seeds) and "Chemlali" (obtained from cuttings) showed better performance under severe water stress compared to the other genotypes, which was associated to their ability to trigger a higher antioxidant protection. It is concluded that (i) drought tolerance among the various genotypes tested is associated with antioxidant protection in olive trees, (ii) the extent of xanthophyll cycle de-epoxidation is strongly inversely related to photosynthetic rates, and (iii) vitamin E accumulation is sharply induced upon severe chlorophyll degradation. [ABSTRACT FROM AUTHOR]

Albeit the remarkably high Ultraviolet B loads, high temperatures, and drought stress substantiate the need for efficient photoprotective strategies in Mediterranean high-mountain plants, these remain understudied. Considering the sensitivity of photosystems to extreme conditions, we evaluated an environmental gradient’s weight on the photoprotection of five high-mountain specialists from Central Spain. Diurnal and seasonal variations in chlorophyll, chlorophyll fluorescence, carotenoids, and xanthophylls in consecutive and climatically contrasting years were taken to evaluate the effect of the impending climate coarsening at the photosystem level. Our results revealed significant differences among species in the xanthophyll cycle functioning, acting either as a continuous photoprotective strategy enhancing photochemistry-steadiness; or prompted only to counteract the cumulative effects of atypically adverse conditions. The lutein cycle’s involvement is inferred from the high lutein content found in all species and elevations, acting as a sustained photoprotective strategy. These findings added to high de-epoxidation state (DEPS) and minor seasonal changes in the chlorophyll a/b ratio, infer the xanthophyll and Lutein cycles are crucial for upkeeping the photosystems’ optimal functioning in these plants heightening their photoprotective capacity during periods of more unfavorable conditions. Nevertheless, an atypically dry growing season’s detrimental effect infers the feasible surpassing of stress-thresholds and the precariousness of the communities’ functional diversity under climate change.

Plants that are tolerant of extreme low temperatures, and plants that are tolerant of desiccation, face similar physiological challenges, as low temperatures often result in extracellular ice formation resulting in desiccation of cells. Desiccation due to either low temperatures or water loss leads to severe mechanical as well as oxidative stress, and plants tolerant to either low temperatures or desiccation utilize similar mechanisms to cope with the extreme stress. Key mechanisms to cope with the mechanical stressors associated with cellular crowding include accumulation of both non-reducing sugars as well as LEA proteins. Key mechanisms to cope with oxidative stress include increases in (or the ability to maintain) antioxidant capacity, as well as increased usage of thermal energy dissipation. During both stressors major reductions in photochemical efficiency are observed that correlate with dark retention of the xanthophyll pigments zeaxanthin and antheraxanthin, suggesting sustained forms of thermal energy dissipation persist in these extreme stress scenarios to cope with the increased excitation pressure. The focus of this review is to highlight the shared physiological mechanisms used in both stress scenarios, with a focus on the mechanisms of photoprotection. [ABSTRACT FROM AUTHOR]

In changing light conditions, photosynthetic organisms develop different strategies to maintain a fine balance between light harvesting, photochemistry, and photoprotection. One of the most widespread photoprotective mechanisms consists in the dissipation of excess light energy in the form of heat in the photosystem II antenna, which participates to the Non Photochemical Quenching (NPQ) of chlorophyll fluorescence. It is tightly related to the reversible epoxidation of xanthophyll pigments, catalyzed by the two enzymes, the violaxanthin deepoxidase and the zeaxanthin epoxidase. In Phaeomonas sp. (Pinguiophyte, Stramenopiles), we show that the regulation of the heat dissipation process is different from that of the green lineage: the NPQ is strictly proportional to the amount of the xanthophyll pigment zeaxanthin and the xanthophyll cycle enzymes are differently regulated. The violaxanthin deepoxidase is already active in the dark, because of a low luminal pH, and the zeaxanthin epoxidase shows a maximal activity under moderate light conditions, being almost inactive in the dark and under high light. This light-dependency mirrors the one of NPQ: Phaeomonas sp. displays a large NPQ in the dark as well as under high light, which recovers under moderate light. Our results pinpoint zeaxanthin epoxidase activity as the prime regulator of NPQ in Phaeomonas sp. and therefore challenge the deepoxidase-regulated xanthophyll cycle dogma. [ABSTRACT FROM AUTHOR]

Key message: Subarctic plants in summer (subjected to continuous light) showed photosynthetic pigment contents mainly driven by PPFD (unrelated to day/night cycles) and a xanthophyll cycle responsiveness to PPFD exacerbated during night-times.Abstract: Composition and content of photosynthetic pigments is finely tuned by plants according to a subtle equilibrium between the absorbed and used energy by the photosynthetic apparatus. Subarctic and Arctic plants are subjected to extended periods of continuous light during summer. This condition represents a unique natural scenario to study the influence of light on pigment regulation and the presence of diurnal patterns potentially governed by circadian rhythms. Here, we examined the modulation of the photosynthetic apparatus in three naturally co-occurring woody species: mountain birch (Betula pubescens ssp. czerepanovii), alpine bearberry (Arctostaphylos alpina) and Scots pine (Pinus sylvestris) around the summer solstice, at 67 °N latitude. Plants were continuously exposed to solar radiation during the 3-day study period, although PPFD fluctuated, being lower during night-times. Photochemical efficiencies for a given PPFD were similar during daytime and night-time for the three species. In Scots pine, for a given PPFD, net assimilation was slightly higher during daytime than during night-time. Overall, the dynamism in pigment content was mainly driven by PPFD, and was generally unrelated to day/night cycles. Weak indications of potential circadian regulation were found over a few pigments only. Interestingly, the xanthophyll cycle was active at any time of the day in the three species but its responsiveness to PPFD was exacerbated during night-times. This was particularly evident for bearberry, which maintained a highly de-epoxidised state even at night-times. The results could indicate an incomplete acclimation to a 24-h photoperiod for these species, which have colonised subarctic latitudes only recently. [ABSTRACT FROM AUTHOR]

Abstract: Under natural conditions, light exposure for Mediterranean shrubs can be highly variable, especially during cloudy days or under a canopy, and can interfere with other environmental factors such as temperature and water availability. With the aim of decoupling the effect of radiation and temperature from water availability, we conducted an experiment where two perennial and three summer semi‐deciduous shrub species were subjected to different levels of irradiation. In order to follow plant responses to light exposure, we measured gas exchange, photosystem II photochemical efficiency, photosynthetic pigments and leaf mass area in spring and summer. Results showed that all study species presented a plastic response to different light conditions, and that light‐related traits varied in a coordinated manner. Summer semi‐deciduous species exhibited a more opportunistic response, with higher photosynthesis rates in full sun, but under shade conditions, the two strategies presented similar assimilation rates. Stomatal conductance did not show such a drastic response as photosynthetsis, being related to changes in WUE. Daily cycles of Fv/Fm revealed a slight photoinhibitory response during summer, mainly in perennial species. In all cases photosynthetic pigments adjusted to the radiation level; leaves had lower chlorophyll content, higher pool of xanthophylls and higher proportion of the de‐epoxydaded state of xanthophylls under sun conditions. Lutein content increased in relation to the xanthophyll pool under shade conditions. Our results evidenced that radiation is an important driving factor controlling morphological and physiological status of Mediterranean shrub species, independently of water availability. Summer semi‐deciduous species exhibit a set of traits with higher response variability, maximising their photosynthetic assimilation under different sun conditions. [ABSTRACT FROM AUTHOR]

Considering the effect of silicon (Si) in reducing the blast symptoms on wheat in a scenario where the losses in the photosynthetic capacity of the infected plants is lowered, this study investigated the ability of using the incident light, the chloroplastidic pigments (chlorophylls and carotenoids) alterations and the possible role of carotenoids on the process of light dissipation on wheat plants non-supplied (-Si) or supplied (+Si) with Si and inoculated or not with Pyricularia oryzae . For + Si plants, blast severity was reduced compared to -Si plants. Reductions in the concentration of photosynthetic pigments (total chlorophyll, violanxanthin + antheraxanthin + zeaxanthin, β-carotene and lutein) were greater for inoculated -Si plants than for inoculated + Si ones. The α-carotene concentration increased for inoculated -Si and +Si plants in comparison to non-inoculated plants limiting, therefore, lutein production. Higher functional damage to the photosystem II (PSII) was noticed for inoculated -Si plants with reductions in the values of maximum quantum quenching, photochemical yield of PSII and electron transport rate, but higher values for quenching non-photochemical. This finding also contributed to reductions in the values of light saturated rate photosynthesis and light saturation point for -Si plants which was attenuated for inoculated + Si plants. Increase in dark respiration values occurred for inoculated plants than for non-inoculated ones. The Si supply to wheat plants, besides reducing blast severity, contributed to their better photosynthetic performance. Moreover, inoculated + Si plants coped with drastic losses of light energy dissipation processes (fluorescence and heat) by increasing the concentration of carotenoids which helped to maintain the structural and functional viability of the photosynthetic machinery minimizing, therefore, lipid peroxidation and the production of reactive oxygen species. [ABSTRACT FROM AUTHOR]

Trehalose was supplied to wheat (Triticum aestivum L.) seedlings just before a high temperature (40 °C) treatment and some physiological parameters were measured during the heat stress and recovery. The application of trehalose decreased the net photosynthetic rate (PN) of wheat seedlings under the heat stress, but to a small extent increased the dry mass (DM) and leaf water content (LWC) after recovery from the heat stress. The trehalose-induced decrease in PN under the heat stress was not associated with a stomatal response. The heat stress slightly decreased the maximal efficiency of photosystem II (PS II) photochemistry (the variable to maximum chlorophyll a fluorescence ratio, Fv/Fm) similarly in the trehalose treated or non-treated plants. Under the heat stress, the actual efficiency of PS II photochemistry (ΦPSII) and the efficiency of excitation energy capture by open reaction centers (Fv'/Fm') were lower in the trehalose-pretreated seedlings, whereas they were higher after the recovery. The patterns of changes in nonphotochemical quenching (NPQ) were contrary to those of ϕPS II and Fv'/Fm'. The chlorophyll content was lower, whereas the β-carotene content and the degree of de-epoxidation (DEPS) of xanthophyll cycle pigments were higher in the trehalose-pretreated wheat seedlings under the heat stress. These results suggest that exogenous trehalose partially promotes recovery of wheat by the increase of NPQ, β-carotene content, and DEPS.

The goal of this study was to explore the relationships between the photochemical reflectance index (PRI) at the leaf level and pigment pools, focusing on the constitutive role of pigments in influencing PRI over seasonal or ontogenetic time frames. The purpose was to re-evaluate the role of PRI as an indicator of seasonally shifting pigment (chlorophyll, carotenoid and anthocyanin) contents, and hence photosynthetic activity, across a range of tree and crop species. We studied natural vegetation - three tree species (maple, chestnut and beech) and two managed irrigated and rainfed crop species (maize and soybean), contrasting in photosynthetic pathway and leaf structure, and having wide variation of pigment content and composition. In anthocyanin-free leaves, PRI related to both total chlorophyll (Chl) and carotenoid (Car) contents, however, much closer relationships were found between PRI and Car to Chl ratio (Car/Chl). The sensitivity of PRI to Car/Chl varied widely in tree species with the degree of secondary carotenoids. In crop leaves where the Car vs. Chl relationship was very close, the slopes of PRI vs. Car/Chl relationships for maize and soybean were almost identical. PRI vs. Car/Chl relationships for leaves of different tree species formed a significant, uniform relationship with PRI. Two crops also formed a significant, uniform PRI vs. Car/Chl relationship with a slope half the value found for trees. In anthocyanin-containing leaves, PRI did not clearly relate to any pigment content because reflectance values at both PRI wavebands are affected by anthocyanin content. The findings of a strong link between leaf level PRI and Car/Chl over seasonal and ontogenetic time spans supports recent findings calling for a more careful evaluation of the relationship between PRI and either LUE or photosynthetic activity. In particular, studies that contrast short-term (e.g. diurnal) vs. long-term (e.g. seasonal) pigment, PRI, and photosynthetic responses in contrasting vegetation types are needed. [ABSTRACT FROM AUTHOR]

The goal of this study was to explore the relationships between stand-level photochemical reflectance index (PRI) and canopy structure/pigment pools, as well as light use efficiency (LUE) of photosynthetically active vegetation focusing on seasonal or ontogenetic time frames. PRI was originally designed as a means of assessing the xanthophyll cycle and LUE over short (e.g. diurnal) time frames, and few studies have explored the drivers of PRI over longer, seasonal time frames, particularly in crops having different photosynthetic pathways or canopy structures. Consequently, our purpose was to understand and quantify the drivers of PRI responses over seasonal time scales for two crops, maize (C 4 ) and soybean (C 3 ), contrasting in photosynthetic pathway, leaf structure and canopy architecture. In both crops, PRI was very closely related to green LAI (R 2 > 0.90) and stand chlorophyll (Chl) content (R 2 > 0.93). The slopes of the relationships in different phenological stages, vegetative and reproductive, were substantially different (3-fold smaller in the vegetative stage). The main cause of this disparity was the high PRI value of soil/residue background. While PRI was a sensitive indicator of the changes in stand green LAI and stand Chl content over the full growing season, it was not sensitive to LUE; LUE explained below 12% of PRI variation in maize and 19% in soybean. Unlike leaf-level PRI, stand-level PRI was not clearly related to the Car/Chl ratio, presumably because the large changes in canopy structure (affecting stand Chl and green LAI) had a dominant influence on PRI over this time frame. The strong relationship between PRI and stand Chl content as well as between PRI and Chl-related vegetation index over a growing cycle allowed us to subtract the stand Chl content effect from measured PRI to reveal the component of PRI most likely related to periods of stress. However, for accurate subtraction of the Chl effect from long-term PRI records, thoughtful study of uncertainties related to “natural” variation of PRI-stand Chl relationships, and stand Chl content estimation for different varieties of the same species and for different species is required. The findings of a strong link between stand-level PRI and stand green LAI and Chl content and the lack of a clear relationship between PRI and LUE over seasonal and ontogenetic time spans suggest the need for a more careful evaluation of the relationship between PRI and either LUE or photosynthetic activity. In particular, studies that contrast short-term (e.g. diurnal) vs. long-term (e.g. seasonal) pigment, PRI, and photosynthetic responses in contrasting vegetation types are needed to clarify the different mechanisms involved at different temporal and spatial scales. These findings have important implications for attempts to monitor photosynthetic phenology from remote sensing, many of which have relied on PRI as an indicator of photosynthetic activity. [ABSTRACT FROM AUTHOR]

• Leaf position is a determinant of abiotic stress-induced senescence and abscission. • Lower position's leaf with less than 20 SPAD value reduces F v /F m and abscise. • F v /F m in lower leaf shows oxidative stress level by hypoxia and intense light. • Adventitious root formation under hypoxia is inhibited by intense light stress. • Prediction of daily maximum photosynthetic rate by SPAD value and F v /F m in Capsicum. Oxidants produced by roots in hypoxic soil and stress caused by intense light are two causes of reduced photosynthetic capacity and efficiency. Moreover, they promote the formation of leaf abscission zones, which has detrimental effects on crop growth. It remains unclear how leaf position, an endogenous factor in nonstress-induced senescence, affects leaf senescence caused by oxidative stress. Thus, we used Capsicum plants to examine how leaf senescence and abscission changed photosynthetic capacity and efficiency at five leaf positions. We analyzed the association between photosynthetic changes and leaf abscission. Our results demonstrated that hypoxia resulted in lower root biomass, lower water uptake in leaves, and lower leaf carotenoid content. Moreover, the interaction between hypoxia and intense light stress causes lower carotenoid and chlorophyll content, suggesting that leaf oxidative stress levels affect the degree of photoprotection conferred by the presence of carotenoids. We also found a significant reduction in the maximum quantum yield of photosystem II under hypoxia conditions, especially in leaves at lower positions on the plant. Finally, we observed that the combined stress of hypoxia and intense light induced abscission at these lower leaf positions. Our results suggested that the level of oxidative damage in respective leaf caused by hypoxia and high light stress is significantly affected by leaf position, and stress interactions cause abscission in the lower leaves near the roots. [ABSTRACT FROM AUTHOR]

Semi-arid plant species cope with excess of solar radiation with morphological and physiological adaptations that assure their survival when other abiotic stressors interact. At the leaf level, sun and shade plants may differ in the set of traits that regulate environmental stressors. Here, we evaluated if leaf-level physiological seasonal response of Mediterranean scrub species ( Myrtus communis , Halimium halimifolium , Rosmarinus officinalis, and Cistus salvifolius ) depended on light availability conditions. We aimed to determine which of these responses prevailed independently of the marked seasonality of Mediterranean climate, to define a leaf-level strategy in the scrub community. Thirty six leaf response variables – involving gas exchange, water status, photosystem II photochemical efficiency, photosynthetic pigments and leaf structure – were seasonally measured in sun exposed and shaded plants under field conditions. Physiological responses showed a common pattern throughout the year, in spite of the marked seasonality of the Mediterranean climate and of species-specific differences in the response to light intensity. Variables related to light use, CO 2 assimilation, leaf pigment content, and LMA (leaf mass area) presented differences that were consistent throughout the year, although autumn was the season with greater contrast between sun and shade plants. Our data suggest that in Mediterranean scrub shade plants the lutein pool could have an important role in the photoprotection of the photosynthetic tissues. There was a negative linear correlation between the ratio lutein/total chlorophylls and the majority of leaf level variables. The combined effect of abiotic stress factors (light and drought or light and cold) was variable-specific, in some cases enhancing differences between sun and shade plants, while in others leading to unified strategies in all scrub species. [ABSTRACT FROM AUTHOR]

Shrubs often form the understorey in Mediterranean oak woodlands. These shrubs are exposed to recurrent water deficits, but how they will respond to predicted future exacerbation of drought is not yet understood. The ecophysiology of the shrub Cistus salvifolius L. was studied over the summer of 2005, which was during a heat-wave superimposed on the most severe drought in the Iberian Peninsula in the last 140 years. Branch water potential fell drastically during the summer, accompanied by stomatal closure and downregulation of PSII, with a concomitant loss of chlorophyll in the leaves. A parallel increase in the ratio of light-dissipating to light-capturing pigments and the proportion of xanthophyll cycle pigments in the de-epoxidated state, along with alterations in the structure of the light harvesting complex, may have reduced the potential for damage to leaves. Substantial increases in leaf tocopherol content during high radiation may have reduced damage from free radicals. Following autumn rains, leaves of the same shrubs showed physiological recovery, indicating the resilience of this Mediterranean species, for which an extremely dry hydrological year with 45% less rainfall than average, did not prevent healthy leaf functioning in response to renewed soil moisture availability. Climate change is expected to exacerbate summer drought in Mediterranean environments. We analysed the response of a rockrose species to a heat wave superimposed on severe drought, discovering that a combination of physiological and biochemical adjustments allowed survival, with recovery following autumn rains. The shrub's capacity to withstand substantial seasonal variability may also buffer the impact of significant inter-annual variation in water availability, indicating potential resilience to climate change in the understorey of Mediterranean oak woodlands. [ABSTRACT FROM AUTHOR]

Trehalose was supplied to wheat ( Triticum aestivum L.) seedlings just before a high temperature (40 °C) treatment and some physiological parameters were measured during the heat stress and recovery. The application of trehalose decreased the net photosynthetic rate (P) of wheat seedlings under the heat stress, but to a small extent increased the dry mass (DM) and leaf water content (LWC) after recovery from the heat stress. The trehalose-induced decrease in PN under the heat stress was not associated with a stomatal response. The heat stress slightly decreased the maximal efficiency of photosystem II (PS II) photochemistry (the variable to maximum chlorophyll a fluorescence ratio, F/F) similarly in the trehalose treated or non-treated plants. Under the heat stress, the actual efficiency of PS II photochemistry (Φ) and the efficiency of excitation energy capture by open reaction centers (F′/F′) were lower in the trehalose-pretreated seedlings, whereas they were higher after the recovery. The patterns of changes in nonphotochemical quenching (NPQ) were contrary to those of ϕ and F′/F′. The chlorophyll content was lower, whereas the β-carotene content and the degree of de-epoxidation (DEPS) of xanthophyll cycle pigments were higher in the trehalose-pretreated wheat seedlings under the heat stress. These results suggest that exogenous trehalose partially promotes recovery of wheat by the increase of NPQ, β-carotene content, and DEPS. [ABSTRACT FROM AUTHOR]

Adaptive reactions of the pigment system in assimilating shoots of evergreen dwarf shrub Ephedra monosperma J.G. Gmel. ex C.A. Mey. were studied under natural conditions of Central Yakutia. Seasonal changes in the content and ratio of green and yellow pigments were revealed; their relation to the stage of plant development and formation of cold tolerance was shown. The decrease in chlorophyll content started in September when the natural photoperiod became shorter and the air temperature lower; the chlorophyll concentration in winter was 30% lower than in summer. The content of β-carotene decreased twofold. The xanthophyll cycle pigments increased in content and deepoxidation level by a factor of 1.7 and 3.6, respectively. In peripheral cells of assimilating parenchyma, accumulation of a secondary carotenoid, rhodoxanthin was noted. In the period of active plant growth (from June to August), rhodoxanthin was absent, while its concentration in shoots in winter was 75 μg/g dry wt. It is concluded that changes in the pigment pool reflect structural and functional reorganization of photosynthetic machinery and are an indispensable part of the intricate process of plant hardening. Activation of energy-dissipating and antioxidant pigment systems, together with accumulation of the light-screening secondary carotenoid rhodoxanthin, promote the retention of photosynthetic apparatus and the survival of Ephedra monosperma plants under extreme conditions of cryolithozone of Yakutia. [ABSTRACT FROM AUTHOR]

Abstract: Diatoms are a major group of microalgae whose photosynthetic productivity supports a substantial part of the aquatic primary production. In their natural environment they have to cope with strong fluctuations of the light climate which can be harmful for photosynthesis. In order to prevent the damage of their photosynthetic machinery, diatoms use fast regulatory processes among which the non-photochemical quenching of chlorophyll a fluorescence (NPQ) is one of the most important. In a previous work, we highlighted differences in the kinetics and extent of NPQ between diatom species/strains originating from different aquatic habitats. We proposed that the NPQ differences observed between strains/species could potentially participate to their ecophysiological adaptation to the light environment of their respective natural habitat. In order to better understand the molecular bases of such differences, we compared the NPQ features of four strains/species of diatoms known for their NPQ discrepancy. We could identify new spectroscopic fingerprints concomitant to NPQ and the related xanthophyll cycle. These fingerprints helped us propose a molecular explanation for the NPQ differences observed between the diatom species/strains examined. The present work further strengthens the potential role of NPQ in the ecophysiology of diatoms. [Copyright &y& Elsevier]

Abstract: Aureococcus anophagefferens is a picoplanktonic microalga that is very well adapted to growth at low nutrient and low light levels, causing devastating blooms (“brown tides”) in estuarine waters. To study the factors involved in long-term acclimation to different light intensities, cells were acclimated for a number of generations to growth under low light (20μmolphotonsm−2 s−1), medium light (60 or 90μmolphotonsm−2 s−1) and high light (200μmolphotonsm−2 s−1), and were analyzed for their contents of xanthophyll cycle carotenoids (the D pool), fucoxanthin and its derivatives (the F pool), Chls c 2 and c 3, and fucoxanthin Chl a/c polypeptides (FCPs). Higher growth light intensities resulted in increased steady state levels of both diadinoxanthin and diatoxanthin. However, it also resulted in the conversion of a significant fraction of fucoxanthin to 19′-butanoyloxyfucoxanthin without a change in the total F pool. The increase in 19′-butanoyloxyfucoxanthin was paralleled by a decrease in the effective antenna size, determined from the slope of the change in F0 as a function of increasing light intensity. Transfer of acclimated cultures to a higher light intensity showed that the conversion of fucoxanthin to its derivative was a relatively slow process (time-frame of hours). We suggest the replacement of fucoxanthin with the bulkier 19′-butanoyloxyfucoxanthin results in a decrease in the light-harvesting efficiency of the FCP antenna and is part of the long-term acclimative response to growth at higher light intensities. [Copyright &y& Elsevier]

Abstract: Fucoxanthin-chlorophyll complexes (FCP) from the centric diatom Cyclotella meneghiniana were isolated and the trimeric FCPa complex was reconstituted into liposomes at different lipid to Chl a ratios. The fluorescence yield of the complexes in different environments was calculated from room temperature fluorescence emission spectra and compared to the aggregated state of FCPa. FCPa surrounded by high amounts of lipids resembled detergent solubilised complexes and with decreasing lipid levels, i.e. in a situation where protein contacts were increasingly favoured, the fluorescence yield of FCPa gradually decreased. In addition, the yield displayed a strong pH-dependency in case of lower lipid contents. The further reduction in fluorescence yield brought about by the conversion of diadinoxanthin to diatoxanthin was pH independent and only depended on the amount of diatoxanthin synthesised. The implications of these data for non-photochemical quenching in centric diatoms are discussed. [Copyright &y& Elsevier]

Abstract: The effects of salt stress and leaf canopy position on mechanisms of photo- and antioxidant protection, including changes in photosynthetic pigments and low-molecular-weight chloroplastic antioxidants, were examined in sage (Salvia officinalis L.) plants exposed to two doses of NaCl (50mM and 100mM) for 5weeks. Sage appeared to be a moderately salt-resistant glycophyte. The addition of 100mM NaCl to the nutrient solution reduced total leaf biomass, the number of leaves, leaf water potential, net photosynthesis, stomatal conductance and chlorophyll levels. However, the malondialdehyde levels, which indicate the extent of lipid peroxidation, did not increase in plants treated with either 50mM or 100mM NaCl, relative to controls. In the plants treated with 100mM NaCl, the accumulation of Na+ in the leaves occurred in parallel with a drastic reduction in the net CO2 assimilation rates, but also with the activation of mechanisms of photo- and antioxidant protection, including xanthophyll cycle de-epoxidation and the accumulation of tocopherols and phenolic diterpenes. Furthermore, we examined the extent to which canopy position determines the photoprotective demand and antioxidant protection of leaves, and how salinity affects this demand. The lower leaves showed a lower photoprotective demand than the upper leaves. However, the former showed higher lipid peroxidation than the latter under salt stress, which suggests that the lower, older leaves suffer from greater photo-oxidative stress than the upper, younger ones, despite being located in areas with a lower photoprotective demand within the canopy. We concluded that leaf position in the canopy should be carefully considered in studies aimed at unravelling mechanisms of salt-stress tolerance in plants. [Copyright &y& Elsevier]

In the pursuit of knowledge on the biological behavior of Brazilian Atlantic Forest tree species, this study evaluated the susceptibility of the light-demanding species, Schinus terebinthifolia Raddi., Pseudobombax grandiflorum (Cav.) A. Robyns and Joannesia princeps Vell., and of the shade-tolerant species, Hymenaea courbaril L. var. stilbocarpa and Lecythis pisonis Camb, to photoinhibition and acclimation capacity. These species were first cultivated under two irradiance conditions, I (20% direct sunlight radiation) and I (all-sky or direct sunlight) and then transferred from I to I. The effects of the sudden increase in light radiation intensity on photosynthetic activity were then evaluated through chlorophyll (Chl) fluorescence imaging, HPLC xanthophylls analysis, and cell membrane lipid peroxidation measurements. Light-demanding species were found to present a higher photochemical efficiency and higher acclimation capacity under high light irradiance than shade-tolerant species. The higher photoinhibition tolerance observed in light-demanding species was associated to their higher capacity for photochemical dissipation and dissipation of excess excitation energy via the xanthophyll cycle, leading to a lower ROS generation. The obtained results suggested that a knowledge of acclimation capacity, by means of Chl fluorescence imaging yields, is a useful indicator of species successional grouping. [ABSTRACT FROM AUTHOR]

Abstract: Photoprotection of photosystem II (PSII) is essential to avoid the light-induced damage of the photosynthetic apparatus due to the formation of reactive oxygen species (=photo-oxidative stress) under excess light. Carotenoids are known to play a crucial role in these processes based on their property to deactivate triplet chlorophyll (3Chl⁎) and singlet oxygen (1O2 ⁎). Xanthophylls are further assumed to be involved either directly or indirectly in the non-photochemical quenching (NPQ) of excess light energy in the antenna of PSII. This review gives an overview on recent progress in the understanding of the photoprotective role of the xanthophylls zeaxanthin (which is formed in the light in the so-called xanthophyll cycle) and lutein with emphasis on the NPQ processes associated with PSII of higher plants. The current knowledge supports the view that the photoprotective role of Lut is predominantly restricted to its function in the deactivation of 3Chl⁎, while zeaxanthin is the major player in the deactivation of excited singlet Chl (1Chl⁎) and thus in NPQ (non-photochemical quenching). Additionally, zeaxanthin serves important functions as an antioxidant in the lipid phase of the membrane and is likely to act as a key component in the memory of the chloroplast with respect to preceding photo-oxidative stress. This article is part of a Special Issue entitled: Photosystem II. [Copyright &y& Elsevier]

Abstract: Photosynthetic functions have been investigated in ozone stressed (200ppb, 5h) Melissa officinalis plants at the end of fumigation and 24 and 48h after. Plants exhibited foliar injury and membrane permeability was significantly increased, indicating that there was membrane damage. After the end of treatment, CO2 fixation capacity decreased and this lasted during the recovery period (until a maximum of −63% when compared to controls). These strong negative effects on photosynthetic ability were observed to be due both to stomatal and mesophyllic limitations, since stomatal conductance decreased (−23%) and intercellular CO2 concentration significantly increased (+41%). Reduction in PSII efficiency is evidenced by (i) decrease of F v/F 0 (−11.4%), indicating a partial inhibition at PSII donor side; (ii) significant correlation between the apparent electron transport rate through PSII and photosynthetic activity, suggesting that the O3-induced effects are well established, as demonstrated by the development of leaf necrosis; (iii) increase in electrons required to fix one molecule of CO2, showing a decrease in activity of photosynthetic enzymes and their ability to fix CO2 in the presence of O3; (iv) decrease of q L, resulting in an increase in the PSII excitation pressure. On the other hand, a regulatory adjustment of PSII efficiency was highlighted by (i) higher value of q NP, abling to counteract the negative effects of O3 at chloroplast level because of their capacity to dissipate the excess of excitation energy; (ii) increase of the xanthophyll cycle pool size and DEPS index, showing a marked activation of photoprotective mechanisms. This represents an active response that M. officinalis initiates to cope with increased oxidative load. [Copyright &y& Elsevier]

Abstract: Plants use a small part of the total absorbed light energy for net carboxylation, while the remaining amount is dissipated via alternative pathways involving thermal processes, fluorescence and non-carboxylation photochemistry in order to limit the formation of reactive oxygen species (ROS) and other photooxidative risks. The commonly used analysis of the Photosystem II (PSII) fluorescence signals gives qualitative information about absorbed light energy management by plants, but it is difficult to appreciate the relative contribution of each pathway in energy partitioning. This study reports the application of quenching partitioning through a chlorophyll fluorescence approach performed on peach leaves subjected to three different light intensities for four durations of exposure in absence of recovery from photo-damage. This methodology was compared with the P700 redox kinetic method for determining the functional PSII fraction in leaves. In the absence of recovery processes the active PSII concentration decayed with an increase in photon exposure (the product of irradiance and the time of exposure), following an exponential pattern according to the reciprocity law. The photoprotective thermal dissipation (Φ NPQ ) was proportional to irradiance up to 30min of photoinhibitory treatment. Afterwards Φ NPQ was limited by the increasing competition for the absorbed energy re-emitted by the inactive PSII (Φ NF ). Φ NF increased with the photon exposure dissipating up to 70% of the total incoming energy. The energy funnelled to photochemistry (Φ PSII ) decreased with increasing exposure time or intensity, becoming zero after 120min of photoinhibitory treatment at the maximum irradiance (2100μmolphotonm−2 s−1). The relation between the fraction of energy dissipated by the inactive PSII (derived from the quenching partitioning) and the inactive PSII fraction (measured with the P700 redox kinetic method) was linear. The quenching partitioning through light-modulated chlorophyll fluorescence is a useful tool to analyse plant energy management and gives also a reasonable estimation of the active PSII fraction. This methodology can easily be used in the field as measurements are rapid, non-destructive and detection devices are portable. [Copyright &y& Elsevier]

Abstract: We monitored chlorophyll (Chl) fluorescence, pigment concentration and the de-epoxidation state of the xanthophyll cycle (DPS [1] DPS is defined as the ratio of (Z+A) to (V+A+Z), where Z, A and V stand for concentrations of zeaxanthin, antheraxanthin and violaxanthin, respectively. 1 ) in two warm temperate broad-leaved evergreen species (Quercus myrsinaefolia and Machilus thunbergii). Reduction of the maximal quantum yield of Photosystem II (PSII) (calculated from Fv/Fm, variable to maximal Chl a fluorescence) and retention of a high DPS were observed in both species in the winter, and can be interpreted as acclimation to winter. In particular, the acclimation of PSII in these species can be chiefly attributed to thermal dissipation, which is correlated with the retention of high zeaxanthin. Furthermore, we attempted to divide the fate of the absorbed light energy by the PSII antennae into three components: (i) PSII photochemistry (represented by its quantum yield, ΦPSII), (ii) dissipation by down-regulation via non-photochemical quenching (ΦNPQ) and (iii) other non-photochemical processes (ΦONP). The estimated energy allocation of the absorbed light indicated that the proportion of ΦPSII decreased, whereas that of ΦNPQ+ΦONP increased during winter. This result suggests that the excess energy absorbed in the PSII complexes is safely dissipated from the PSII antennae. Based on these results, we conclude that thermal dissipation from the PSII antennae plays an important role in two overwintering broad-leaved evergreen trees growing in Japan. [Copyright &y& Elsevier]

Abstract: Mechanisms of photoprotection and antioxidant protection, including changes in chlorophylls, xanthophyll cycle components and levels of low-molecular-weight chloroplastic antioxidants (lutein, β-carotene and α-tocopherol) were studied together with levels of malondialdehyde, a product of lipid peroxidation, in the response of rosemary (Rosmarinus officinalis L.) plants to salt stress. Plants were exposed to increasing NaCl concentrations (50, 100 and 150mM) for 6 weeks, and two concentrations of the following chloride salts: KCl, CaCl2, MgCl2 and FeCl3, were used together with 100mM NaCl to explore the extent to which these salts can alter the mechanisms of photoprotection, antioxidant protection and malondialdehyde accumulation in leaves. Increasing concentrations of NaCl decreased leaf water contents and photosynthetic pigment levels, while the contents of α-tocopherol and malondialdehyde increased, but with completely different kinetics. α-Tocopherol levels increased in a dose-dependent manner as stress progressed, while malondialdehyde levels increased at the highest dose (150mM NaCl) but only during early phases of stress. Furthermore, although the addition of chloride salts to NaCl-treated plants apparently improved leaf physiological status, in terms of water and chlorophyll contents, plants showed an increased photoprotective demand and increased oxidative stress, particularly in FeCl3-treated plants. It is concluded that (i) rosemary plants can withstand moderate doses of NaCl in the medium (at least 150mM NaCl for 6 weeks), (ii) oxidative stress may be a mechanism for protecting plants from moderate doses of salt stress rather than causing damage to plants, and (iii) the addition of chloride salts to NaCl-treated plants may dramatically increase the photoprotective demand and oxidative stress of leaves, while plant growth is not negatively affected. [Copyright &y& Elsevier]

The article discusses a study regarding non-photochemical chlorophyll fluorescence quenching (NPQ) and xanthophyll cycle responses resulting from photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in certain coastal phytoplanktons from Potter Cove on King George Island in Antarctica. The study involving Chaetoceros brevis and Phaeocystis antarctica cultures aims to determine whether photoinhibition plays a role in taxon dominance and moderate phytoplankton biomass in the region during summer. Results show that high light acclimation has led to significant NPQs and a decrease in excess PAR and UVR effects on photosynthesis. Another finding indicates that P. antarctica was more dependent on chloroplast protein synthesis under protein synthesis inhibition.

The article discusses research that aims to measure the photosynthetic traits and biomass composition of various western Canadian spring wheat cultivars in order to determine which would be optimum for the production of ethanol. The use of chlorophyll fluorescence and leaf spectral reflectance and carbon dioxide (CO2) gas exchange is mentioned. According to the study, the McKenzie cultivar has higher cellulose content, which suggested that the plant may be more desirable for the production of ethanol.