The biological function of anthocyanins in leaves has been strongly discussed, but the hypothesis of a screening function is favored by most authors. For an evaluation of the function as ...photoprotective pigments, a quantification of their screening of the mesophyll is important. Here, chlorophyll fluorescence excitation of leaves of a red and a green variety of Berberis thunbergii was used to estimate the extent of screening by anthocyanins at 545 nm and over the whole photosynthetically active wavelength range. Growth at high light (430 µmol m⁻² s⁻¹) resulted in 90% screening at 545 nm corresponding to 40–50% screening over the whole wavelength range, depending on the light source. The concomitant reduction of photosynthetic quantum yield was of the same size as the calculated reduction of light reaching the chloroplasts. The induction of anthocyanins in the red variety also enhanced the epoxidation state of the violaxanthin cycle under growth conditions, indicating that red leaves were suffering less from excessive irradiance. Pool sizes of violaxanthin cycle carotenoids indicated a shade acclimation of the light harvesting complexes in red leaves. The observed reduction of internal light in anthocyanic leaves has by necessity a photoprotective effect.
At temperate latitudes environmental factors such as irradiance, including ultraviolet-B radiation (UV-B, 280–315 nm), temperature and day length vary widely over the course of a year in a concerted ...way. In the present study physiological acclimation of photoprotection, growth and development of the model organism Arabidopsis thaliana were correlated to these strongly but gradually changing conditions in a one year field study. Plants were sown in the field avoiding any manipulation (and abrupt change) during their life. Developmental rate was strongly dependent on prevailing temperature. Moderate signs of light stress in form of photoinhibition at photosystem II were significantly related to solar irradiances while amount of DNA damage was low and not correlated to UV-B irradiance. Although all the markers were hypothesized to primarily react to radiation, multiple regression analysis showed at least a similarly strong influence of temperature as that of light. Especially for the classical UV screening compounds a positive correlation to UV-B radiation during the course of the year was absent, whereas there was a significant negative correlation between temperature and quercetin content. The sum of violaxanthin cycle pigments was correlated to both, irradiance and temperature, but with opposite sign. Epidermal UV-B transmittance was also much better related to air temperature than to UV-B irradiance. The data show that under natural conditions temperature has at least a similar importance for photoprotective acclimation and partially also for photosensitivity as solar irradiance.
•Growth and phenology of Arabidopsis thaliana was followed over a year in the field.•UV absorbing pigments were significantly correlated to temperature but not to UV-B.•Only moderate photoinhibition and DNA damage were found.
Photosynthetic electron flow, driven by photosystem I and II, provides chemical energy for carbon fixation. In addition to a linear mode a second cyclic route exists, which only involves photosystem ...I. The exact contributions of linear and cyclic transport are still a matter of debate. Here, we describe the development of a method that allows quantification of electron flow in absolute terms through photosystem I in a photosynthetic organism for the first time. Specific in-vivo protocols allowed to discern the redox states of plastocyanin, P700 and the FeS-clusters including ferredoxin at the acceptor site of PSI in the cyanobacterium Synechocystis sp. PCC 6803 with the near-infrared spectrometer Dual-KLAS/NIR. P700 absorbance changes determined with the Dual-KLAS/NIR correlated linearly with direct determinations of PSI concentrations using EPR. Dark-interval relaxation kinetics measurements (DIRKPSI) were applied to determine electron flow through PSI. Counting electrons from hydrogen oxidation as electron donor to photosystem I in parallel to DIRKPSI measurements confirmed the validity of the method. Electron flow determination by classical PSI yield measurements overestimates electron flow at low light intensities and saturates earlier compared to DIRKPSI. Combination of DIRKPSI with oxygen evolution measurements yielded a proportion of 35% of surplus electrons passing PSI compared to PSII. We attribute these electrons to cyclic electron transport, which is twice as high as assumed for plants. Counting electrons flowing through the photosystems allowed determination of the number of quanta required for photosynthesis to 11 per oxygen produced, which is close to published values.
•Absolute quantification of electron flow through PSI•Proportions of linear and cyclic electron transport determined•Cyclic electron transport makes up 35% of electron flow through PSI.•These proportions of linear and cyclic transport require 11 quanta per O2 produced.
Flavonoid synthesis is predominantly regulated at the transcriptional level through the MYB–basic helix-loop-helix (bHLH)–WD40 (MBW) (MYB: transcription factor of the myeloblastosis protein ...family,WD40: tanscription factor with a short structural motif of 40 amino acids which terminates in an aspartic acid–tryptophan dipeptide) complex, and responds to both environmental and developmental stimuli. Although the developmental regulation of flavonoid accumulation in Arabidopsis thaliana has been examined in great detail, the response of the flavonoid synthesis pathway to abiotic stress (particularly low temperature) remains unclear.
A screen of a Dissociation element (Ds) transposon-induced mutation collection identified two lines which exhibited an altered profile of phenylpropanoid accumulation following exposure to low-temperature stress. One of the mutated genes (BRASSINOSTEROID ENHANCED EXPRESSION1 (BEE1)) encoded a brassinosteroid enhanced expression transcription factor, while the other (G2-LIKE FLAVONOID REGULATOR (GFR)) encoded a G2-like flavonoid regulator. Phenylpropanoid-targeted analysis was performed using high-performance LC-MS, and gene expression analysis using quantitative reverse transcription–PCR.
In both mutants, the accumulation of quercetins and scopolin was reduced under lowtemperature growing conditions, whereas that of anthocyanin was increased. BEE1 and GFR were both shown to negatively regulate anthocyanin accumulation by inhibiting anthocyanin synthesis genes via the suppression of the bHLH (TRANSPARENT TESTA8 (TT8) and GLABROUS3 (GL3)) and/or the MYB (PRODUCTION OF ANTHOCYANIN PIGMENTS2 (PAP2)) components of the MBW complex.
Our results provide new insight into the regulatory control of phenylpropanoid metabolism at low temperatures, and reveal that BEE1 and GFR act as important components of the signal transduction chain.
•Epidermal physode layer of Fucus species absorbs in UV wavelength range.•Epidermal physode layer effectively screens internal chloroplasts against UV-B.•UV-B transmittance through physode layer is ...correlated to UV-B-induced damage of PSII.•UV protection of Fucus species is more than sufficient for natural sunlight.
Phlorotannins are believed to provide UV protection in brown algae. In Fucus species, phlorotannin-rich vesicles (“physodes”) form an “epidermal physode layer”, which might represent the anatomical structure responsible for UV protection. However, this layer has been neglected so far; instead, phlorotannin contents of entire thalli were studied in UV exposure experiments.
We employed fluorometric methods to investigate in which wavelength region and to which degree the epidermal physode layer of two Fucus species reduces the transmittance of UV radiation. For comparison, we also studied Saccharina latissima, a brown algal species without epidermal physode layer.
We could show that the epidermal physode layer absorbs throughout the entire UV wavelength range and reduces the epidermal UV-B transmittance into the thallus strongly (15% transmitted in F. vesiculosus vs. 80–100% in S. latissima). UV-B transmittances varied with growth depth and the variations correlated with photosystem II (PSII) stress responses upon artificial UV-B illumination. We found that the UV-B protection is more than sufficient for natural UV-B levels and of rather constitutive nature.
In conclusion, the epidermal physode layer appears to be the anatomical feature exerting UV protection by phlorotannins in Fucus species and might thus be a core adaptation that enables Fucales the colonization of the intertidal habitat.
Pendulous lichens dominate canopies of boreal forests, with dark
Bryoria
species in the upper canopy vs. light
Alectoria
and
Usnea
species in lower canopy. These genera offer important ecosystem ...services such as winter forage for reindeer and caribou. The mechanism behind this niche separation is poorly understood. We tested the hypothesis that species-specific sunscreening fungal pigments protect underlying symbiotic algae differently against high light, and thus shape the vertical canopy gradient of epiphytes. Three pale species with the reflecting pigment usnic acid (
Alectoria sarmentosa
,
Usnea dasypoga
,
U. longissima
) and three with dark, absorbing melanins (
Bryoria capillaris
,
B. fremontii
,
B. fuscescens
) were compared. We subjected the lichens to desiccation stress with and without light, and assessed their performance with chlorophyll fluorescence. Desiccation alone only affected
U. longissima
. By contrast, light in combination with desiccation caused photoinhibitory damage in all species. Usnic lichens were significantly more susceptible to light during desiccation than melanic ones. Thus, melanin is a more efficient light-screening pigment than usnic acid. Thereby, the vertical gradient of pendulous lichens in forest canopies is consistent with a shift in type and functioning of sunscreening pigments, from high-light-tolerant
Bryoria
in the upper to susceptible
Alectoria
and
Usnea
in the lower canopy.
Melting mountainous snowfields are populated by extremophilic microorganisms. An alga causing orange snow above timberline in the High Tatra Mountains (Poland) was characterised using multiple ...methods examining its ultrastructure, genetics, life cycle, photosynthesis and ecophysiology. Based on light and electron microscopy and ITS2 rDNA, the species was identified as
(Chlorophyceae). Recently, the taxon was described from Japan. However, cellular adaptations to its harsh environment and details about the life cycle were so far unknown. In this study, the snow surface population consisted of egg-shaped cysts containing large numbers of lipid bodies filled presumably with the secondary carotenoid astaxanthin. The outer, spiked cell wall was shed during cell maturation. Before this developmental step, the cysts resembled a different snow alga,
. The remaining, long-lasting smooth cell wall showed a striking UV-induced blue autofluorescence, indicating the presence of short wavelengths absorbing, protective compounds, potentially sporopollenin containing polyphenolic components. Applying a chlorophyll fluorescence assay on intact cells, a significant UV-A and UV-B screening capability of about 30 and 50%, respectively, was measured. Moreover, intracellular secondary carotenoids were responsible for a reduction of blue-green light absorbed by chloroplasts by about 50%. These results revealed the high capacity of cysts to reduce the impact of harmful UV and high visible irradiation to the chloroplast and nucleus when exposed at alpine snow surfaces during melting. Consistently, the observed photosynthetic performance of photosystem II (evaluated by fluorometry) showed no decline up to 2100 μmol photons m
s
. Cysts accumulated high contents of polyunsaturated fatty acids (about 60% of fatty acids), which are advantageous at low temperatures. In the course of this study,
was found also in Slovakia, Italy, Greece and the United States, indicating a widespread distribution in the Northern Hemisphere.
Epidermally located UV-B absorbing hydroxycinnamic acid derivatives and flavonoids serve as a screen against potentially damaging UV-B (280-315 nm) radiation in higher plants. We investigated the ...effect of low temperature on epidermal screening as assessed by a chlorophyll fluorescence technique. The epidermal UV-transmittance of greenhouse-grown Vicia faba plants was strongly dependent on growth temperatures between 21 and 9 degrees C, with significant differences already between 21 and 18 degrees C. There was a good correlation between epidermal UV-A and UV-B absorbance and the absorbance of whole leaf extracts at the respective wavelengths. Whereas in Oxyria digyna and Rumex longifolius no temperature dependence of epidermal transmittance could be detected, it was confirmed for seven other crop plant species, including summer and winter varieties, and for Arabidopsis thaliana. Dicotyledoneous plants showed a stronger response than monocotyledoneous ones. In all investigated species, the response in the UV-A spectral region was similar to that in the UV-B, suggesting that flavonoids were the responsible compounds. In V. faba, mature leaves did not respond with a change in epidermal transmittance upon transfer from warm to cool conditions or vice versa, whereas developing leaves did acclimate to the new conditions. We conclude that temperature is an important determinant of the acclimation of epidermal UV transmittance to environmental conditions in many plant species. The potential adaptive value of this response is discussed.
• Induction of the pigments parietin and melanin by specific wavelength ranges of solar radiation was studied in two widespread foliose lichens: the ubiquitous suntolerant Xanthoria parietina, from ...which the orange parietin can be extracted in living thalli by acetone, and naturally melanin-free shade-adapted specimens of the old forest species Lobaria pulmonaria. • Parietin-free X. parietina and melanin-free L. pulmonaria were exposed to solar radiation in the field under filter foils transmitting or absorbing natural UV-radiation (< 400 nm). Synthesis of both pigments depended on the presence of UV-radiation. • A growth chamber experiment showed that the induction of parietin and melanin synthesis required the presence of UV-B (280-320 nm). UV-A (320-400 nm) induced a very small synthesis only, whereas photosynthetically active radiation (PAR) was not inductive. Melanins and parietin were only synthesized in thalli being hydrated during UV-exposures, suggesting that induction required active metabolism. • The photosystem II (PSII) of X. parietina was not adversely affected by any applied radiation treatment, regardless of the hydration status during exposure, whereas L. pulmonaria was susceptible both to high PAR- and UV-irradiances in the desiccated state.
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