Excess Light Absorption Research Articles

Desiccation tolerance mechanisms of resurrection plant Selaginella pulvinata

The resurrection plant Selaginella pulvinata possesses an extraordinary ability to survive longterm desiccation, with the capacity to recover to normal growth after four years in a desiccated state. To understand the phenomenon, we investigated morphology, cytology, physiology, and performed PacBio single-molecule, real-time (SMRT) sequencing and Illumina RNA-sequencing of S. pulvinata during dehydration and rehydration. The full-length transcriptome dataset of S. puvinata consisted of 78,914 isoforms, and 75,819 isoforms (96.07%) were functionally annotated. The mapping rate of RNA-seq was 86.83–93.46% when utilizing the full-length transcriptome dataset as a reference. A total of 17,934 differentially expressed genes (DEGs) were identified during dehydration and rehydration. During dehydration, swollen chloroplasts and mitochondria, as well as vacuolation occupied space in desiccated cells, which cooperated with dense cytoplasm to inhibit the collapse and folding of desiccated cells. Accumulation and up-regulated gene expressions of osmotic agent and protective proteins contributed to the formation of vacuolation and dense cytoplasm. To reduce ROS damage during dehydration, two strategies were employed in S. pulvinata, i.e., reducing ROS production and scavenging ROS. Reducing exposure area, inactive PSII and down-regulated gene expression in photosynthesis decreased ROS production caused by excessive light absorption. Increased activities and up-regulated gene expressions in antioxidant enzyme system promoted ROS scavenging. ABA signaling pathway was hypothesized to play an important regulatory role in the desiccation tolerance of S. pulvinata due to significant ABA accumulation and up-regulated expressions of related genes during dehydration. Moreover, the comparative transcriptome analysis revealed that the pathways involved in dehydration and rehydration of the resurrection species S. pulvinata, S. lepidophylla and S. sellowii were consistent, indicating the similar desiccation tolerance mechanisms existing in the different resurrection species of Selaginella. Our study preliminarily revealed the desiccation tolerance mechanisms of S. pulvinata, which holds significant potential for inspiring biotechnological strategies aimed at enhancing drought tolerance in crops.

Simultaneous CAS9 editing of cpSRP43, LHCA6, and LHCA7 in Picochlorum celeri lowers chlorophyll levels and improves biomass productivity.

High cellular pigment levels in dense microalgal cultures contribute to excess light absorption. To improve photosynthetic yields in the marine microalga Picochlorum celeri, CAS9 gene editing was used to target the molecular chaperone cpSRP43. Depigmented strains (>50% lower chlorophyll) were generated, with proteomics showing attenuated levels of most light harvesting complex (LHC) proteins. Gene editing generated two types of cpSRP43 transformants with distinct lower pigment phenotypes: (i) a transformant (Δsrp43) with both cpSRP43 diploid alleles modified to encode non-functional polypeptides and (ii) a transformant (STR30309) with a 3nt in-frame insertion in one allele at the CAS9 cut site (non-functional second allele), leading to expression of a modified cpSRP43. STR30309 has more chlorophyll than Δsrp43 but substantially less than wild type. To further decrease light absorption by photosystem I in STR30309, CAS9 editing was used to stack in disruptions of both LHCA6 and LHCA7 to generate STR30843, which has higher (5-24%) productivities relative to wild type in solar-simulating bioreactors. Maximal productivities required frequent partial harvests throughout the day. For STR30843, exemplary diel bioreactor yields of ~50 g m-2day-1 were attained. Our results demonstrate diel productivity gains in P.celeri by lowering pigment levels.

Photosynthesis and Cell Growth Trigger Degradation of Phycobilisomes during Nitrogen Limitation.

Under nitrogen (N)-limited conditions, the non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803) actively grows during early stages of starvation by performing photosynthesis but gradually stops the growth and eventually enters dormancy to withstand long-term N limitation. During N limitation, Synechocystis 6803 cells degrade the large light-harvesting antenna complex phycobilisomes (PBSs) presumably to avoid excess light absorption and to reallocate available N to essential functions for growth and survival. These two requirements may be driving forces for PBS degradation during N limitation, but how photosynthesis and cell growth affect PBS degradation remains unclear. To address this question, we examined involvements of photosynthesis and cell growth in PBS degradation during N limitation. Treatment of photosynthesis inhibitors and shading suppressed PBS degradation and caused non-bleaching of cells during N limitation. Limitations of photosynthesis after initial gene responses to N limitation suppressed PBS degradation, implying that photosynthesis affects PBS degradation in a post-translational manner. In addition, limitations of cell growth by inhibition of peptidoglycan and fatty acid biosynthesis, low growth temperature and phosphorous starvation suppressed PBS degradation during N limitation. Because decreased photosynthetic activity led to decreased cell growth, and vice versa, photosynthesis and cell growth would inseparably intertwine each other and affect PBS degradation during N limitation in a complex manner. Our data shed light on the coordination mechanisms among photosynthesis, cell growth and PBS degradation during N limitation.

Wide variation of winter-induced sustained thermal energy dissipation in conifers: a common-garden study.

Low temperature in winter depresses rates of photosynthesis, which, in evergreen plants, can exacerbate imbalances between light absorption and photochemical light use. Damage that could result from increased excess light absorption is minimized by the conversion of excitation energy to heat in a process known as energy dissipation, which involves the de-epoxidized carotenoids of the xanthophyll cycle. Overwintering evergreens employ sustained forms of energy dissipation observable even after lengthy periods of dark acclimation. Whereas most studies of photoprotective energy dissipation examine one or a small number of species; here, we measured the levels of sustained thermal energy dissipation of seventy conifer taxa growing outdoors under common-garden conditions at the Red Butte Garden in Salt Lake City, Utah, U.S.A. (forty nine taxa were also sampled for needle pigment content). We observed an extremely wide range of wintertime engagement of sustained energy dissipation; the percentage decrease in dark-acclimated photosystem II quantum efficiency from summer to winter ranged from 6 to 95%. Of the many pigment-based parameters measured, the magnitude of the seasonal decrease in quantum efficiency was most closely associated with the seasonal increase in zeaxanthin content expressed on a total chlorophyll basis, which explained only slightly more than one-third of the variation. We did not find evidence for a consistent wintertime decrease in needle chlorophyll content. Thus, the prevailing mechanism for winter decreases in solar-induced fluorescence emitted by evergreen forests may be decreases in fluorescence quantum yield, and wintertime deployment of sustained energy dissipation likely underlies this effect.

Differences in leaf anatomy, photosynthesis, and photoprotective strategies in the yellow-green leaf mutant and wild type of Rosa beggeriana Schrenk

Although the underlying mechanisms of chlorophyll biosynthesis have been intensively deciphered in model plants, leaf color mutants have not yet been fully studied in woody species. In this study, leaf anatomy, diurnal changes in photosynthesis and photoprotection were investigated in the yellow-green leaf (ygl) mutant of Rosa beggeriana. Chloroplast ultrastructure, pigment biosynthesis, and photosynthesis were impaired and led to photoinhibition at noon in the ygl mutant. The negative changes in leaf anatomy increased the risk of excess light absorption in the mutant. Moreover, the ygl mutant preferred to consume the excited energy through photorespiration rather than via heat dissipation. Finally, antioxidant defenses failed to scavenge reactive oxygen species and led to severe lipid peroxidation in the mutant. The results suggested that inhibition of photosynthesis in the ygl mutant of R. beggeriana was associated with the altered photoprotective strategies that involved in leaf anatomy, photorespiration, thermal dissipation, and antioxidant system.

LHCSR3 is a nonphotochemical quencher of both photosystems in Chlamydomonas reinhardtii.

Photosynthetic organisms prevent oxidative stress from light energy absorbed in excess through several photoprotective mechanisms. A major component is thermal dissipation of chlorophyll singlet excited states and is called nonphotochemical quenching (NPQ). NPQ is catalyzed in green algae by protein subunits called LHCSRs (Light Harvesting Complex Stress Related), homologous to the Light Harvesting Complexes (LHC), constituting the antenna system of both photosystem I (PSI) and PSII. We investigated the role of LHCSR1 and LHCSR3 in NPQ activation to verify whether these proteins are involved in thermal dissipation of PSI excitation energy, in addition to their well-known effect on PSII. To this aim, we measured the fluorescence emitted at 77 K by whole cells in a quenched or unquenched state, using green fluorescence protein as the internal standard. We show that NPQ activation by high light treatment in Chlamydomonas reinhardtii leads to energy quenching in both PSI and PSII antenna systems. By analyzing quenching properties of mutants affected on the expression of LHCSR1 or LHCSR3 gene products and/or state 1-state 2 transitions or zeaxanthin accumulation, namely, npq4, stt7, stt7 npq4, npq4 lhcsr1, lhcsr3-complemented npq4 lhcsr1 and npq1, we showed that PSI undergoes NPQ through quenching of the associated LHCII antenna. This quenching event is fast-reversible on switching the light off, is mainly related to LHCSR3 activity, and is dependent on thylakoid luminal pH. Moreover, PSI quenching could also be observed in the absence of zeaxanthin or STT7 kinase activity.

Rapid colour changes in Euglena sanguinea (Euglenophyceae) caused by internal lipid globule migration

ABSTRACTThe accumulation of red pigments, frequently carotenoids, under chronic stress is a response observed in diverse kinds of eukaryotic photoautotrophs. It is thought that red pigments protect the chlorophyll located underneath by a light-shielding mechanism. However, the synthesis or degradation of carotenoids is a slow process and this response is usually only observed when the stress is maintained over long periods of time. In contrast, rapid colour changes have been reported in the euglenophyte Euglena sanguinea. Here we study the ecophysiological process behind this phenomenon through chlorophyll fluorescence, and pigment, colour and ultrastructural analyses. Reddening in E. sanguinea was due to the presence of a large amount of free and esterified astaxanthin (representing 80% of the carotenoid pool). The process was highly dynamic, shifting from green to red in 8 min (and vice-versa in 20 min). This change was not due to de novo carotenogenesis, but to the relocation of cytoplasmic lipid globules where astaxanthin accumulates. Thus, red globules were observed to migrate from the centre of the cell to peripheral locations when exposed to high light. Globule migration seems to be so efficient that other classical photoprotective mechanisms are not operative in this species. Despite the presence and operation of the diadino-diatoxanthin cycle, non-photochemical quenching was almost undetectable. Since E. sanguinea forms extensive floating colonies, reddening can be observed at a much greater scale than at the cellular level and the mechanism described here is one of the fastest and most dramatic colour changes attributable to photosynthetic organisms at cell and landscape level. In conclusion E. sanguinea shows an extremely dynamic and efficient photoprotective mechanism, based more on organelle migration than on carotenoid biosynthesis, which prevents excess light absorption by chlorophylls reducing the need for other protective processes related to energy dissipation.

Strongly coupled CdS/graphene quantum dots nanohybrids for highly efficient photocatalytic hydrogen evolution: Unraveling the essential roles of graphene quantum dots

It have been recognized that the coupling of graphene quantum dots (GQDs) with semiconductor photocatalysts endow the resulting nanocomposites with enhanced photocatalytic performances, however, the essential roles of GQDs have not been clearly revealed yet. Herein, we report that a high efficiency of the photocatalytic H2 evolution was achieved using strongly coupled nanohybrids of CdS with GQDs (CdS/GQDs) as visible-light-driven photocatalysts. CdS/GQDs nanohybrids were synthesized by a facile hydrothermal method in which the crystallization of CdS precursor and coupling of GQDs could be accomplished in one-step. GQDs are firmly decorated on the surface of CdS nanoparticles, forming “dot-on-particle” heterodimer structures. GQDs have no significant influence on the crystallite structure of CdS but render the nanohybrids with strong light absorption at the wavelength beyond the band edge of CdS. Under visible light irradiation (≥420nm), CdS/GQDs nanohybrids reach the highest H2 production rate of 95.4μmol·h−1, about 2.7 times higher than that of pure CdS nanoparticles, at GQDs content of 1.0wt%, and the apparent quantum efficiency (AQE) was determined to be 4.2% at 420nm. Incident light-wavelength dependent experiments reveal that the light absorption of CdS dominated the performance of nanohybrids, and the excess light absorption coming from GQDs hardly contributes to the observed higher activity. Photocurrent response, steady-state and time-resolved PL, and EIS measurements suggest that the high activity of CdS/GQDs is attributed predominantly to the graphene-like nature of GQDs, which can act as an efficient electron acceptor to induce an efficient charge separation. This work clearly reveals that GQDs mainly played a role of electron acceptor instead of a photosensitizer in enhancing the photocatalytic H2 evolution performances of CdS/GQDs nanohybrids, which offers a new insight to understand the essential roles of GQDs in semiconductor/GQDs nanohybrids for efficient solar energy conversion applications.

Distinct ecophysiological strategies of widespread and endemic species from the megadiverse campo rupestre

The Espinhaço Mountain Range in eastern Brazil hosts a megadiverse and endemic tropical flora. The main vegetation is the campo rupestre, which harbors several endemic plant species. In this study, we sought to compare photosynthetic and leaf nutrient-use strategies in widespread and endemic species that allow its adjustments to high light and impoverished soils in campo rupestre. Three species with contrasting levels of endemism were selected: Coccoloba cereifera, an endangered micro-endemic species from Serra do Cipó; Lavoisiera cordata, vulnerable and endemic to Espinhaço range; and Vochysia thyrsoidea, a common widespread species from the Cerrado domain. During the dry season, we performed field measurements of chlorophyll a fluorescence and pigment content to evaluate the photosynthetic efficiency of these species. We also assessed soil fertility, nutrient status and leaf anatomy. Widespread and endemic species in campo rupestre showed distinct strategies to cope with high light levels and nutrient-impoverished soils. The micro-endemic species C. cereifera showed high nutrient use efficiency, which enables it to colonize impoverished sandy soils. This species and L. cordata minimize excessive light absorption at midday through shoot architecture as well as present efficient non-photochemical energy dissipation during high light level exposures. The high effective quantum yield and electron transport rate of the widespread species V. thyrsoidea may account in part for its wide distribution under extreme environmental conditions in Neotropical sites. The occurrence of widespread and endemic species in this megadiverse mountain environment might be attributed to distinct physiological strategies of species to cope with high light levels and impoverished soils.

Rice (Oryza sativa L.) with reduced chlorophyll content exhibit higher photosynthetic rate and efficiency, improved canopy light distribution, and greater yields than normally pigmented plants

The net photosynthesis and photosynthetic N use efficiency (PNUE) of a rice mutant with reduced chlorophyll synthesis were much higher than those of its wild type (WT). To understand the physiological mechanism, we conducted a model analysis and pot and field experiments under different N rates. Both the model analysis and the experiments showed that low chlorophyll content did not impede the development of chloroplast in the mutant; the reduced chlorophyll content and high expression levels of chlorophyll binding protein genes suggested the smaller light-harvesting chlorophyll antenna size, and higher expression levels of thylakoid membrane protein genes indicated high potential for chloroplast electron transport in the mutant; the small antenna size avoided excessive light absorption, and increased PSII efficiency, resulting in high electron transport rate in the mutant. The higher Rubisco content in the mutant ensured a higher Rubisco capacity for carboxylation. The high CO2 conductivity of stomata and mesophyll ensured availability of CO2 at the site of carboxylation. All these contributed to the increased photosynthetic rate in the mutant. Leaf N content was found higher in the mutant than in the WT, however, the extent to which the photosynthetic rate was increased was much larger than that of leaf N content, leading to improved PNUE in mutant. Furthermore vertical light distribution was improved in the canopy of the mutant, as greater transmittance of sunlight within the canopy was observed in the density experiment in the field. The mutant showed yield advantages considering its shorter growth duration, indicating the higher photosynthetic capacity and improved canopy light distribution with reduced chlorophyll synthesis. The mutant plant could potentially be used to improve photosynthetic efficiency.

Examining the photoprotection hypothesis for adaxial foliar anthocyanin accumulation by revisiting comparisons of green- and red-leafed varieties of coleus (Solenostemon scutellarioides)

Although plants rely on light to drive energy production via photosynthesis, excess light can be harmful. Plants have evolved photoprotective mechanisms to mitigate this threat, including thermal energy dissipation, the most common form of which involves de-epoxidized constituents of the xanthophyll cycle facilitating the conversion of excess excitation energy to heat. A role in photoprotection has also been proposed for red anthocyanins when they accumulate near the adaxial leaf surface. Here, we compared the response to experimental light stress of a red-leafed (anthocyanin rich) and a green-leafed variety of coleus [Solenostemon scutellarioides (L.) Codd], examining chlorophyll fluorescence emission and pigment composition. After experimentally imposed intense white light, red- and green-leafed coleus exhibited manifestations of light stress (decreased photosystem II quantum efficiency) of a similar magnitude. This, considered alone, could be interpreted as evidence that anthocyanins do not serve a photoprotective role. However, during excess light exposure, the green-leafed variety employed a greater level of thermal energy dissipation and possessed correspondingly higher xanthophyll cycle pool sizes and de-epoxidation states. During exposure to red light, which anthocyanins absorb very poorly, levels of thermal energy dissipation did not differ between coleus varieties. Taken together, our findings suggest that adaxial anthocyanins minimize stress associated with excess light absorption and that the green-leafed variety of coleus compensated for its much lower levels of adaxial anthocyanins by invoking higher levels of energy dissipation. Thus, anthocyanin accumulation should be considered alongside the suite of photoprotective mechanisms employed by photosynthetic tissues.

An irradiation density dependent energy relaxation in plant photosystem II antenna assembly

Plant photosystem II (PSII) is a multicomponent pigment-protein complex that harvests sunlight via pigments photoexcitation, and converts light energy into chemical energy. Against high light induced photodamage, excess light absorption of antenna pigments triggers the operation of photoprotection mechanism in plant PSII. Non-photochemical energy relaxation as a major photoprotection way is essentially correlated to the excess light absorption. Here we investigate the energy relaxation of plant PSII complexes with varying incident light density, by performing steady-state and transient chlorophyll fluorescence measurements of the grana membranes (called as BBY), functional moiety PSII reaction center and isolated light-harvesting complex LHCII under excess light irradiation. Based on the chlorophyll fluorescence decays of these samples, it is found that an irradiation density dependent energy relaxation occurs in the LHCII assemblies, especially in the antenna assembly of PSII supercomplexes in grana membrane, when irradiation increases to somewhat higher density levels. Correspondingly, the average chlorophyll fluorescence lifetime of the highly isolated BBY fragments gradually decreases from ~1680 to ~1360ps with increasing the irradiation density from 6.1×109 to 5.5×1010 photon cm−2 pulse−1. Analysis of the relation of fluorescence decay change to the aggregation extent of LHCIIs suggests that a dense arrangement of trimeric LHCIIs is likely the structural base for the occurrence of this irradiation density dependent energy relaxation. Once altering the irradiation density, this energy relaxation is quickly reversible, implying that it may play an important role in photoprotection of plant PSII.

Simultaneous Effect of Temperature and Irradiance on Growth and Okadaic Acid Production from the Marine Dinoflagellate Prorocentrum belizeanum

Benthic marine dioflagellate microalgae belonging to the genus Prorocentrum are a major source of okadaic acid (OA), OA analogues and polyketides. However, dinoflagellates produce these valuable toxins and bioactives in tiny quantities, and they grow slowly compared to other commercially used microalgae. This hinders evaluation in possible large-scale applications. The careful selection of producer species is therefore crucial for success in a hypothetical scale-up of culture, as are appropriate environmental conditions for optimal growth. A clone of the marine toxic dinoflagellate P. belizeanum was studied in vitro to evaluate its capacities to grow and produce OA as an indicator of general polyketide toxin production under the simultaneous influence of temperature (T) and irradiance (I0). Three temperatures and four irradiance levels were tested (18, 25 and 28 °C; 20, 40, 80 and 120 µE·m−2·s−1), and the response variables measured were concentration of cells, maximum photochemical yield of photosystem II (PSII), pigments and OA. Experiments were conducted in T-flasks, since their parallelepipedal geometry proved ideal to ensure optically thin cultures, which are essential for reliable modeling of growth-irradiance curves. The net maximum specific growth rate (µm) was 0.204 day−1 at 25 °C and 40 µE·m−2·s−1. Photo-inhibition was observed at I0 > 40 μEm−2s−1, leading to culture death at 120 µE·m−2·s−1 and 28 °C. Cells at I0 ≥ 80 µE·m−2·s−1 were photoinhibited irrespective of the temperature assayed. A mechanistic model for µm-I0 curves and another empirical model for relating µm-T satisfactorily interpreted the growth kinetics obtained. ANOVA for responses of PSII maximum photochemical yield and pigment profile has demonstrated that P. belizeanum is extremely light sensitive. The pool of photoprotective pigments (diadinoxanthin and dinoxanthin) and peridinin was not able to regulate the excessive light-absorption at high I0-T. OA synthesis in cells was decoupled from optimal growth conditions, as OA overproduction was observed at high temperatures and when both temperature and irradiance were low. T-flask culture observations were consistent with preliminary assays outdoors.

Simultaneous growth and neutral lipid accumulation in microalgae

In this paper the hypothesis was tested whether TAG accumulation serves as an energy sink when microalgae are exposed to an energy imbalance caused by nutrient limitation. In our continuous culture system, excess light absorption and growth-limiting nitrogen supply rates were combined, which resulted in accumulation of TAG (from 1.5% to 12.4% w/w) in visible lipid bodies in Neochloris oleoabundans, while cell replication was sustained. A fourfold increase in TAG productivity showed that TAG indeed served as an energy sink. However, the bulk of excess energy was dissipated leading to a significantly reduced biomass productivity and yield of biomass on light. This demonstrates that when aiming at industrial TAG production, sustaining efficient light energy use under nutrient stress is an important trait to look for in potential production organisms.

Molecular Factors Controlling Photosynthetic Light Harvesting by Carotenoids

Carotenoids are naturally occurring pigments that absorb light in the spectral region in which the sun irradiates maximally. These molecules transfer this energy to chlorophylls, initiating the primary photochemical events of photosynthesis. Carotenoids also regulate the flow of energy within the photosynthetic apparatus and protect it from photoinduced damage caused by excess light absorption. To carry out these functions in nature, carotenoids are bound in discrete pigment-protein complexes in the proximity of chlorophylls. A few three-dimensional structures of these carotenoid complexes have been determined by X-ray crystallography. Thus, the stage is set for attempting to correlate the structural information with the spectroscopic properties of carotenoids to understand the molecular mechanism(s) of their function in photosynthetic systems. In this Account, we summarize current spectroscopic data describing the excited state energies and ultrafast dynamics of purified carotenoids in solution and bound in light-harvesting complexes from purple bacteria, marine algae, and green plants. Many of these complexes can be modified using mutagenesis or pigment exchange which facilitates the elucidation of correlations between structure and function. We describe the structural and electronic factors controlling the function of carotenoids as energy donors. We also discuss unresolved issues related to the nature of spectroscopically dark excited states, which could play a role in light harvesting. To illustrate the interplay between structural determinations and spectroscopic investigations that exemplifies work in the field, we describe the spectroscopic properties of four light-harvesting complexes whose structures have been determined to atomic resolution. The first, the LH2 complex from the purple bacterium Rhodopseudomonas acidophila, contains the carotenoid rhodopin glucoside. The second is the LHCII trimeric complex from higher plants which uses the carotenoids lutein, neoxanthin, and violaxanthin to transfer energy to chlorophyll. The third, the peridinin-chlorophyll-protein (PCP) from the dinoflagellate Amphidinium carterae, is the only known complex in which the bound carotenoid (peridinin) pigments outnumber the chlorophylls. The last is xanthorhodopsin from the eubacterium Salinibacter ruber. This complex contains the carotenoid salinixanthin, which transfers energy to a retinal chromophore. The carotenoids in these pigment-protein complexes transfer energy with high efficiency by optimizing both the distance and orientation of the carotenoid donor and chlorophyll acceptor molecules. Importantly, the versatility and robustness of carotenoids in these light-harvesting pigment-protein complexes have led to their incorporation in the design and synthesis of nanoscale antenna systems. In these bioinspired systems, researchers are seeking to improve the light capture and use of energy from the solar emission spectrum.

Photosynthetic and Photoinhibition Behavior of Two Field-Grown Grapevine Cultivars under Multiple Summer Stresses

The identification of drought-resistant genotypes of <i>Vitis vinifera</i> that can optimize their water use is attracting increasing interest, especially in areas where it is difficult to extend irrigation and that are undergoing a progressive shift toward subtropicalization. This study was carried out in 2003 (a year characterized by severe drought conditions) and 2004 (characterized by absence of drought conditions) to evaluate the changes in morphostructural characteristics and physiological behavior of two red grapevine cultivars widely cultivated in Italy: Sangiovese and Montepulciano. Unlike 2004, data recorded during 2003 showed that, contrary to Montepulciano vines, the vertical shoot-positioned canopy of Sangiovese vines was divided into two different zones. The basal zone was close to the fruiting zone, featured low carbon gain, and was characterized by 57% of leaves affected by chlorosis and necrosis, while the remaining leaves were affected by chronic photoinhibition (Fv/Fm &lt;0.50). The upper zone was characterized by high photosynthetic activity and water-use efficiency. The changes in leaf inclination and lower leaf absorption and the greater leaf transmittance in Sangiovese leaves appeared to help in the avoidance of excessive light and heat absorption. Irrespective of their positions, the leaves of Montepulciano promptly closed their stomata and saved water, while reducing their physiological activity. Results indicate that grapevine strategies in response to multiple and severe summer stress conditions are a function of genotype, leaf age, and leaf position along the shoot. Unlike Montepulciano, under drought conditions, Sangiovese can be considered to be well adapted, since it can optimize the whole-vine carbon gain.

Energetic cost of photoinhibition in corals

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 313:1-12 (2006) - doi:10.3354/meps313001 Energetic cost of photoinhibition in corals Mia O. Hoogenboom*, Kenneth R. N. Anthony, Sean R. Connolly ARC Centre of Excellence for Coral Reef Studies, and School of Marine Biology and Aquaculture, James Cook University, Townsville, Queensland 4811, Australia *Email: mia.hoogenboom@jcu.edu.au ABSTRACT: Photoinhibition may constitute an energetic cost for photosynthetic organisms through damage to the photosynthetic apparatus, or by increased metabolism due to damage avoidance or repair. For several species of scleractinian corals, fluorescence techniques have revealed a significant reduction in photochemical efficiency of symbiotic dinoflagellates within coral tissue in response to excess light absorption. To date, it has been unclear whether or not photoinhibition has a negative impact on energy budgets in corals. We simultaneously quantified the effect of exposure to excessive light on net rates of photosynthesis and on fluorescence-derived photochemistry. We acclimated colonies of the reef-building coral Turbinaria mesenterina to 3 different irradiance regimes in the laboratory. The corals were then exposed to light levels up to 10 times higher than their acclimation irradiance and assayed for rates of photosynthesis and photochemical yields. Results indicated that daily costs of photoinhibition are negligible. Reduced net rates of photosynthesis in the afternoon, compared to the morning, were predominantly due to enhanced afternoon rates of dark respiration. However, photoacclimation to high light levels reduces daily energy acquisition in the long term, primarily due to decreased chlorophyll concentrations. Therefore, although changes in the photosynthetic activity of symbiotic dinoflagellates over a diurnal irradiance cycle do not cause a measurable decline in net oxygen evolution for coral colonies, repeated exposure to excessive irradiance can reduce energy acquisition per unit surface area, and hence influence the upper limit of the depth distribution of scleractinian corals. KEY WORDS: Photoinhibition · Respirometry · PAM fluorescence · Scleractinian coral · Energy budget · Photosynthetic efficiency · Turbinaria mesenterina Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 313. Online publication date: May 11, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

The role of antioxidant enzymes in photoprotection

The enzymatic component of the antioxidant system is discussed as one of the defensive mechanisms providing protection against excessive light absorption in plants. We present an analysis of attempts to improve stress tolerance by means of the creation of transgenic plants with elevated antioxidant enzyme activities and conclude that the effect of such transgenic manipulation strongly depends on the manner in which the stress is imposed. The following factors may diminish the differences in photosynthetic performance between transgenic plants and wild type under field conditions: effective functioning of the thermal dissipation mechanisms providing a primary line of defense against excessive light, long-term adjustments of the antioxidant system and other photoprotective mechanisms, the relatively low level of control over electron transport exerted by the Water-Water cycle, especially under warm conditions, and a decrease in the content of the transgenic product during leaf aging.

Adsorption of Thermomonospora fusca E5 and Trichoderma reesei cellobiohydrolase I cellulases on synthetic surfaces.

The interfacial behavior of Thermomonosporafusca E5 and Trichoderma reesei cellobiohydrolase I (CBHI) cellulases were studied at synthetic surfaces. For this purpose, colloidal silica and polystyrene particles were used to prepare cellulase-particle suspensions that could be analyzed by solution-phase techniques. Circular dichroism spectroscopy of each cellulase, alone as well as in suspension with silica, was used to determine whether structural changes occurred on adsorption. Changes in spectra were observed for CBHI, but not for E5. Gel-permeation chromatography of the cellulase-particle suspensions showed that neither cellulase binds to silica, suggesting that changes in spectra for CBHI were a result of solution-phase phenomena. Microfiltration of cellulase-polystyrene suspensions showed that both cellulases bind to polystyrene. However, circular dichroism experiments with polysterene proved unworkable, owing to excessive light absorption by the polystyrene. Adsorption kinetics of each cellulase were recorded, in situ, at hydrophilic and silanized, hydrophobic silica surfaces using ellipsometry. Ellipsometric data recorded for each cellulase at hydrophilic silica showed insignificant adsorption. Binding did occur between each cellulase and silanized silica, most likely mediated through hydrophobic associations. Adsorption in this case was irreversible to dilution.

Influence of Ca(2+) on the thylakoid lumen violaxanthin de-epoxidase activity through Ca(2+) gating of H(+) flux at the CF(o) H(+) channel.

Part of the chloroplast photoprotection response to excess light absorption involves formation of zeaxanthin (and antheraxanthin) via the violaxanthin deepoxidase enzyme, the activity of which requires lumen acidity near or below pH 6.0. Clearly, the violaxanthin de-epoxidase activity is strongly regulated because at equivalent energization levels (including the parameters of H(+) accumulation and ATP formation rates), there can be either low or high violaxanthin de-epoxidase enzyme activity. This work shows that the factor or factors responsible for regulating the violaxanthin deepoxidase correlate directly with those which regulate the expression of membrane-localized or delocalized proton gradient (Delta[Formula: see text] (H+)) energy coupling. The most clearly identified factor regulating switching between localized and delocalized energy coupling modes is Ca(2+) binding to the lumen side of the thylakoid membrane; in particular, Ca(2+) binding to the 8 kDA subunit III of the CF(o) H(+) channel. The activity of violaxanthin deepoxidase in pea (Pisum sativa) and spinach (Spinacea oleracea) thylakoids is shown here to be strongly correlated with conditions known from previous work to displace Ca(2+) from the CF(o) H(+) channel and thus to modulate the extent of lumenal acidification while maintaining a fairly constant rate of ATP formation.