Non-photochemical quenching of chlorophyll fluorescence in Chlorella fusca acclimated to constant and dynamic light conditions

Non-photochemical quenching of chlorophyll fluorescence (NPQ) involves dissipation of light energy in the photosynthetic apparatus via a number of physiologically distinct processes. The relationships among NPQ, the (de)epoxidation state of the xanthophyll cycle pigments and state transitions was studied in the green alga Chlorella fusca, acquired from six differently light-acclimated continuous cultures. A 10 h light and 14 h darkness, periodicity was obeyed in all cultures. Three cultures received a high total daily irradiance, three others a low one. High and low irradiances were each dosed in three different modes at constant supply, with sine shape intensity modulation, or as a sine with superimposed oscillations. In the constant supply mode, but not for the sine and oscillating modes, high-light rendered a three-fold higher xantophyll cycle pigment content than low-light. Dynamic interconversion of xantophyll cycle pigments was restricted to high-light cultures. NPQ followed the kinetics of the light supply mode and was highest in high light cultures. In low-light cultures, NPQ correlated mainly to state transitions. These observations were supported by experiments with dithiothreithol-treated samples. The relative impact of xantophyll cycle operation and state transitions on NPQ in green algae from different light climates will be discussed with reference to higher plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fluorescence quenching in four unicellular algae with different light-harvesting and xanthophyll-cycle pigments

We examined the relationship between non-photochemical quenching (NPQ) and xanthophyll de-epoxidation in the unicellular algae Euglena gracilis, Ochromonas danica, Phaeodactylum tricornutum, and Dunaliella tertiolecta. Generally, low-light-grown algae had a smaller pool of xanthophyll-cycle pigments per chlorophyll than medium-light-grown grown cells, but they developed more NPQ during exposure to high light. Thus, lumen acidification was apparently lower in medium-light-grown cells in spite of the exposure to a photon flux density (PFD) three times the growth PFD. In darkness Dunaliella maintained a relatively large content of de-epoxidized xanthophylls, and NPQ developed without concomitant de-epoxidation in response to a 5-min exposure to high light. Violaxanthin de-epoxidation that occurred during longer exposures to light did not cause a further rise in NPQ in Dunaliella. In Ochromonas, NPQ and xanthophyll de-epoxidation increased simultaneously during a 15-min exposure to high light. A further rise in NPQ was not accompanied by xanthophyll de-epoxidation. In Phaeodactylum, the rise in NPQ and de-epoxidation were nearly linearly related during a 60-min exposure to high light. NPQ recovered quickly after darkening in these three algae and no significant photodamage occurred. In Euglena no xanthophyll-conversions and no quickly reversible NPQ occured in response to high light, suggesting that photodamage occurred. Dunaliella has similar light-harvesting and xanthophyll-cycle pigments as higher plants but the relationship between NPQ and DPS during the exposure to high light was different from the linear relationship that is commonly observed in plants. Conversely, Phaeodactylum, which has different light-harvesting and xanthophyll-cycle pigments, had a relationship similar to that in plants.     

Diurnal changes in the xanthophyll cycle pigments of freshwater algae correlate with the environmental hydrogen peroxide concentration rather than non-photochemical quenching

Background and Aims In photosynthetic organisms exposure to high light induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, H₂O₂ is involved in key signalling pathways in development and stress responses, although in excess it can induce damage. A ubiquitous response to high light is the induction of the xanthophyll cycle, but its role in algae is unclear as it is not always associated with NPQ induction. The aim of this study was to reveal how diurnal changes in the level of H₂O₂ are regulated in a freshwater algal community.Methods A natural freshwater community of algae in a temporary rainwater pool was studied, comprising photosynthetic Euglena species, benthic Navicula diatoms, Chlamydomonas and Chlorella species. Diurnal measurements were made of photosynthetic performance, concentrations of photosynthetic pigments and H₂O₂. The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related H₂O₂ responses to high light.Key Results NPQ was shown to prevent H₂O₂ release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic carbon excited by UV-B radiation was probably responsible for a part of the H₂O₂ produced in the water column. Concentrations of H₂O₂ peaked at 2 µm at midday and algae rapidly scavenged H₂O₂ rather than releasing it. A vertical H₂O₂ gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the violaxanthin and diadinoxanthin cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ.Conclusions The diurnal cycling of H₂O₂ was apparently modulated by the organisms in this freshwater algal community. Although the community showed flexibility in its levels of NPQ, the diurnal changes in xanthophylls correlated with H₂O₂ concentrations. Alternative NPQ mechanisms in algae involving proteins of the light-harvesting complex type and antioxidant protection of the thylakoid membrane by de-epoxidized carotenoids are discussed.     

Changes in the condition of photosynthetic apparatus of a diatom alga <Emphasis Type="Italic">Thallassiosira weisflogii</Emphasis> during photoadaptation and photodamage

Two populations of a diatom alga Thallassiosira weisflogii were grown at photon flux densities (PFD) of 0.8 and 8 μmol/(m² s). For both diatom populations, the recovery of chlorophyll fluorescence parameters (F ₀, F _m, F _v/F _m, and NPQ) was monitored after nondestructive irradiation by visible light at PFD of 40 μmol/(m² s) and after high-intensity irradiation by visible light (1000–4000 μmol/(m² s)). The exposure of diatoms to PFD of 40 μmol/(m² s)—higher than PFD used for algal growth but still nondamaging to photosynthetic apparatus—induced nonphotochemical quenching (NPQ), which was stronger in algae grown at higher PFD (8 μmol/(m² s)) than in algae grown at low light. After irradiation with high-intensity light, the recovery of chlorophyll fluorescence parameters was more pronounced in algae grown at elevated PFD level. During short-term irradiation of diatoms with high-intensity visible light (1000 μmol/(m² s)), a stronger NPQ was observed in the culture adapted to high irradiance. After the treatment of algae with dithiothreitol (an inhibitor of carotenoid deepoxidase in the diadinoxanthin cycle) or NH₄Cl (an agent abolishing the proton gradient at thylakoid membranes), a short exposure of algae to PFD of 40 μmol/(m² s) induced hardly any nonphotochemical quenching. The results indicate the dominant contribution of xanthophyll cycle carotenoids to energy-dependent quenching.     

Assessing photosynthetic efficiency in an experimental mangrove canopy using remote sensing and chlorophyll fluorescence

This study examined the ability of the photochemical reflectance index (PRI) to track changes in effective quantum yield (Δ F/F _m ′), non-photochemical quenching (NPQ), and the xanthophyll cycle de-epoxidation (DPS) in an experimental mangrove canopy. PRI was correlated with (Δ F/F _m ′) and NPQ over the 4-week measurement period and over the diurnal cycle. The normalised difference vegetation index (NDVI) was not correlated with any aspect of photochemical efficiency measured using chlorophyll fluorescence or xanthophyll pigments. This study demonstrated that photochemical adjustments were responsible for controlling the flow of energy through the photosynthetic apparatus in this mangrove forest canopy rather than canopy structural or chlorophyll adjustments.     

Operation of the xanthophyll cycle in the seagrass Zostera marina in response to variable irradiance

Changes in the photobiology and photosynthetic pigments of the seagrass Zostera marina from Chesapeake Bay (USA) were examined under a range of natural and manipulated irradiance regimes. Photosynthetic activity was assessed using chlorophyll-a fluorescence, and photosynthetic pigments were measured by HPLC. Large changes in the violaxanthin, zeaxanthin, and antheraxanthin content were concomitant with the modulation of non-photochemical quenching (NPQ). Photokinetics (F_v/F_m, rapid light curves (RLC), and non-photochemical quenching) varied as a result of oscillating irradiance and were highly correlated to xanthophyll pigment content. Zeaxanthin and antheraxanthin concentrations increased under elevated light conditions, while violaxanthin increased in darkened conditions. Unusually high concentrations of antheraxanthin were found in Z. marina under a wide range of light conditions, and this was associated with the partial conversion of violaxanthin to zeaxanthin. These results support the idea that xanthophyll intermediate pigments induce a photoprotective response during exposure to high irradiances in this seagrass.     

Photoprotective responses of an ice algal community in Saroma-Ko Lagoon, Hokkaido, Japan

In polar seas, ice algal communities can acclimate to extremely low light conditions. Reduced acclimation to shade in ice algal communities, as a result of shortened ice seasons at the lower latitude limits of sea ice distribution, has been suggested as advantageous for avoiding strong photoinhibition when cells are released into high light levels at the water’s surface. Thermal dissipation of excess energy by xanthophyll cycle pigments in the de-epoxidated state may occur in ice algal communities released from retreating sea ice. A light exposure experiment was conducted on ice algal communities obtained from sea ice at Saroma-Ko Lagoon in Hokkaido, Japan. Photoprotective responses to direct sunlight were examined through non-photochemical quenching (NPQ) of chlorophyll fluorescence and xanthophyll pigments. De-epoxidation of diadinoxanthin (DD) to diatoxanthin (DT) occurred rapidly, and NPQ showed a dynamic response to high light exposure. The linear relationship between the ratio of DT to chlorophyll a and NPQ followed a steeper slope than previously observed for mesophilic diatoms. The steeper slope could be explained by an apparent increase in DT for the mesophilic diatoms and induction of NPQ in response to low temperatures only in the ice algal communities. Enhanced production of DT in mesophilic diatoms could be the result of de-epoxidation of DD plus de novo synthesis, and the enhancement of NPQ might be caused by low temperature stress in the ice algae. Although the response of NPQ might be related to temperature, NPQ independent of DT synthesis should also be studied.     

Ferredoxin-quinone reductase benefits cyclic electron flow around photosystem 1 in tobacco leaves upon exposure to chilling stress under low irradiance

The function of chloroplast ferredoxin quinone reductase (FQR)-dependent flow was examined by comparing a wild type tobacco and a tobacco transformant (ΔndhB) in which the ndhB gene had been disrupted with their antimycin A (AA)-fed leaves upon exposure to chilling temperature (4 °C) under low irradiance (100 μmol m⁻² s⁻¹ photon flux density). During the chilling stress, the maximum photochemical efficiency of photosystem (PS) 2 (F_v/F_m) decreased markedly in both the controls and AA-fed leaves, and P700⁺ was also lower in AA-fed leaves than in the controls, implying that FQR-dependent cyclic electron flow around PS1 functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under such stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (q_f) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), formed the difference between AA-fed leaves and controls. The lower NPQ in AA-fed leaves might be related to an inefficient proton gradient across thylakoid membranes (ΔpH) because of inhibiting an FQR-dependent cyclic electron flow around PS1 at chilling temperature under low irradiance.     

Diversity in Xanthophyll Cycle Pigments Content and Related Nonphotochemical Quenching (NPQ) Among Microalgae: Implications for Growth Strategy and Ecology

Xanthophyll cycle-related nonphotochemical quenching (NPQ), which is present in most photoautotrophs, allows dissipation of excess light energy. Xanthophyll cycle-related NPQ depends principally on xanthophyll cycle pigments composition and their effective involvement in NPQ. Xanthophyll cycle-related NPQ is tightly controlled by environmental conditions in a species-/strain-specific manner. These features are especially relevant in microalgae living in a complex and highly variable environment. The goal of this study was to perform a comparative assessment of NPQ ecophysiologies across microalgal taxa in order to underline the specific involvement of NPQ in growth adaptations and strategies. We used both published results and data acquired in our laboratory to understand the relationships between growth conditions (irradiance, temperature, and nutrient availability), xanthophyll cycle composition, and xanthophyll cycle pigments quenching efficiency in microalgae from various taxa. We found that in diadinoxanthin-containing species, the xanthophyll cycle pigment pool is controlled by energy pressure in all species. At any given energy pressure, however, the diatoxanthin content is higher in diatoms than in other diadinoxanthin-containing species. XC pigments quenching efficiency is species-specific and decreases with acclimation to higher irradiances. We found a clear link between the natural light environment of species/ecotypes and quenching efficiency amplitude. The presence of diatoxanthin or zeaxanthin at steady state in all species examined at moderate and high irradiances suggests that cells maintain a light-harvesting capacity in excess to cope with potential decrease in light intensity.     

Effects of Cold-Hardening on Chilling-Induced Photoinhibition of Photosynthesis and on Xanthophyll Cycle Pigments in Sweet Pepper

Two cultivars of Capsicum annuum L. were acclimated for 5 d at sub-optimal temperature (14 °C) and irradiance of 250 µmol m^–2 s^–1. This cold-hardening resulted in some reduction in the extent of photoinhibition during an 8 h exposure to high irradiance at 4 °C. Obvious differences were observed between non-hardened leaves (NHL) and cold-hardened leaves (CHL) in the recovery under low irradiance at room temperature. The CHL of both cultivars recovered faster than NHL, especially during the initial fast phase of recovery. Compared with NHL, the total content of carotenoids (Cars), based on chlorophyll, Chl (a+b), and the proportions of xanthophyll cycle pigments referred to total Cars increased in CHL, mainly due to an increase of violaxanthin (V) + antheraxanthin (A) + zeaxanthin (Z) content per mol Chl (a+b). Faster development and a higher non-photochemical quenching (NPQ) of Chl fluorescence, related to a stronger deepoxidation of the larger xanthophyll cycle pool in NHL, could act as a major defence mechanism to reduce the formation of reactive oxygen species during severe chilling. This is suggested by higher content of Z or Z+A in photoinhibition as well as by its rapid decline during the initial fast phase of recovery. In contrast to the chilling-sensitive cv. 0004, the chilling-tolerant cv. 1141 did more easily acclimate its photosynthetic apparatus to low temperatures.     

Duration of irradiation rather than quantity and frequency of high irradiance inhibits photosynthetic processes in the lichen <Emphasis Type="Italic">Lasallia pustulata</Emphasis>

Lichen thalli were exposed to 4 regimes differing in irradiance and duration of irradiation. Photosynthetic efficiency of thalli was monitored by chlorophyll fluorescence parameters and xanthophyll cycle analysis. Maximal quantum yield of photosystem 2 (F_V/F_M) decreased gradually with time in long-term treatment. The effect of additional short-term high irradiance (HI) treatment applied each 24 h was not significant. Nevertheless, short-term HI applied repeatedly on thalli kept in the dark led to a significant decrease of F_V/F_M. Non-photochemical quenching recorded during the long-term treatment corresponded to the content of zeaxanthin (Z). In short-term treatment, however, proportion of Z (and antheraxanthin) to total amount of xanthophyll cycle pigments recovered to the initial values every 24 h after each repeated short-term HI event in thalli kept in dark. Thus duration of irradiation rather than irradiance and frequency of HI events is important for a decrease in primary photosynthetic processes in wet thalli of Lasallia pustulata. Rapidly responding photoprotective mechanisms, such as conversion of xanthophyll cycle pigments, are involved mainly in short-term irradiation events, even at HI.     

The xanthophyll cycle in green algae (chlorophyta): its role in the photosynthetic apparatus

Light-dependent conversion of violaxanthin to zeaxanthin, the so-called xanthophyll cycle, was shown to serve as a major, short-term light acclimation mechanism in higher plants. The role of xanthophylls in thermal dissipation of surplus excitation energy was deduced from the linear relationship between zeaxanthin formation and the magnitude of non-photochemical quenching. Unlike in higher plants, the role of the xanthophyll cycle in green algae (Chlorophyta) is ambiguous, since its contribution to energy dissipation can significantly vary among species. Here, we have studied the role of the xanthophyll cycle in the adaptation of several species of green algae (Chlorella, Scenedesmus, Haematococcus, Chlorococcum, Spongiochloris) to high irradiance. The xanthophyll cycle has been found functional in all tested organisms; however its contribution to non-photochemical quenching is not as significant as in higher plants. This conclusion is supported by three facts: (i) in green algae the content of zeaxanthin normalized per chlorophyll was significantly lower than that reported from higher plants, (ii) antheraxanthin + zeaxanthin content displayed different diel kinetics from NPQ and (iii) in green algae there was no such linear relationship between NPQ and Ax + Zx, as found in higher plants. We assume that microalgae rely on other dissipation mechanism(s), which operate along with xanthophyll cycle-dependent quenching.     

PHOTOPHYSIOLOGICAL PROPERTIES OF THE MARINE PICOEUKARYOTE PICOCHLORUM RCC 237 (TREBOUXIOPHYCEAE,CHLOROPHYTA)1

The photophysiological properties of strain RCC 237 belonging to the marine picoplanktonic genus Picochlorum, first described by Henley et al., were investigated under different photon flux densities (PFD), ranging from 40 to 400 μmol photons· m^?2·s^?1, mainly focusing on the development of the xanthophyll cycle and its relationship with the nonphotochemical quenching of fluorescence (NPQ). The functioning of the xanthophyll cycle and its photoprotective role was investigated by applying a progressive increase of PFD and using dithiotreitol and norflurazon to block specific enzymatic reactions in order to study in depth the relationship between xanthophyll cycle and NPQ. These two processes were significantly related only during the gradually increasing light periods and not during stable light periods, where NPQ and zeaxanthin were decoupled. This result reveals that NPQ is a photoprotective process developed by algae only when cells are experiencing increasing PFD or in response to stressful light variations, for instance after a sudden light shift. Results showed that the photobiological properties of Picochlorum strain RCC 237 seem to be well related to the surface water characteristics, as it is able to maintain its photosynthetic characteristics under different PFDs and to quickly activate the xanthophyll cycle under high light.     

PHOTOSYNTHESIS IN THE DEEP BLUE SEA: PHOTOPHYSIOLOGICAL RESPONSES OF PHAEODACTYLUM TRICORNUTUM TO DYNAMIC AND STATIC IRRADIANCES

The kinetics of xanthophyll-cycle pigment switching and fluorescence quenching dynamics in the marine diatom, Phaeodactylum tricornutum were determined in the context of dynamic and static growth light. Cultures were grown in a modified photobioreactor capable of producing dynamic light fields which exhibited attenuation characteristics similar to that of water; these cultures were pre-acclimated to high and low, static and dynamic, growth-light regimes for at least three days, and then examined under high, static and dynamic light. Pigment pools varied markedly. The two static light cultures had pigment complements that were very similar to “traditional” high and low-light static cultures. The dynamic-light grown cultures had pigment complements, which were very similar to each other but different from the static-grown cultures. The maximum xanthophyll-cycle pigment de-epoxidation state attainable under saturating light was equal for all four treatments. Induction of fluorescence quenching was significantly faster in the static-grown cultures, while xanthophyll-cycle de-epoxidation rates did not show as much variation. Minimum irradiances for xanthophyll-cycle induction were correlated to average growth irradiance. Taken as a whole, the results from this work suggest that dynamic light-grown phytoplankton have a unique photosynthetic functionality that is different from static light-grown phytoplankton. The significance of these observations in the context of realistic light fields, and the photosynthetic response capabilities of algae grown under them will be discussed.     

Strategies providing success in a variable habitat: III. Dynamic control of photosynthesis in Cladophora glomerata

Diurnal patterns of photosynthesis were studied in July and April populations of Cladophora glomerata (L.) Kütz. from open and from shaded sites. Summer samples exposed to full sunlight showed decreased efficiency of open photosystem II at noon, and only slight differences were found between samples that had grown at open or at shaded sites. Electron transport rate was limited at highest fluence rates in shade plants, and non‐photochemical quenching (NPQ) revealed faster regulation in samples from open sites. Daily course of de‐epoxidation was not linearly correlated with the course of NPQ. The comparison of samples from open and from shaded sites revealed a higher capacity of thermal energy dissipation and an increase in the total amount of xanthophyll‐cycle pigments (21%) in samples from open sites. In April, down‐regulation of the efficiency of open photosystem II was related to lower water temperature, and hence, increased excitation pressure. In April the pool size of xanthophyll‐cycle pigments was increased by 21% in comparison with summer and suggested higher levels of thermal energy dissipation via de‐epoxidized xanthophylls. In both, summer and spring the amount of xanthophyll‐cycle pigments was 20% higher in samples from open sites. Acclimation of C. glomerata to growth light conditions was further shown by experimental induction of NPQ, indicating NPQ increases of 23%, and increases of 77% in the reversible component of NPQ in open site samples. The effect of temperature on photosynthetic rate was non‐linear, and different optimum temperatures of electron transport rate and oxygen evolution were exhibited.     

Mechanistic aspects of xanthophyll cycle-dependent photoprotection in higher plant chloroplasts and leaves

Higher plants must dissipate absorbed light energy that exceeds the photosynthetic capacity to avoid molecular damage to the pigments and proteins that comprise the photosynthetic apparatus. Described in this minireview is a current view of the biochemical, biophysical and bioenergetic aspects of the primary photoprotective mechanism responsible for dissipating excess excitation energy as heat from photosystem II (PSII). The photoprotective heat dissipation is measured as nonphotochemical quenching (NPQ) of the PSII chlorophyll a (Chl a) fluorescence. The NPQ mechanism is controlled by the trans-thylakoid membrane pH gradient (ΔpH) and the special xanthophyll cycle pigments. In the NPQ mechanism, the de-epoxidized endgroup moieties and the trans-thylakoid membrane orientations of antheraxanthin (A) and zeaxanthin (Z) strongly affect their interactions with protonated chlorophyll binding proteins (CPs) of the PSII inner antenna. The CP protonation sites and steps are influenced by proton domains sequestered within the proteo-lipid core of the thylakoid membrane. Xanthophyll cycle enrichment around the CPs may explain why changes in the peripheral PSII antenna size do not necessarily affect either the concentration of the xanthophyll cycle pigments on a per PSII unit basis or the NPQ mechanism. Recent time-resolved PSII Chi a fluorescence studies suggest the NPQ mechanism switches PSII units to an increased rate constant of heat dissipation in a series of steps that include xanthophyll de-epoxidation, CP-protonation and binding of the xanthophylls to the protonated CPs; the concerted process can be described with a simple two-step, pH-activation model. The xanthophyll cycle-dependent NPQ mechanism is profoundly influenced by temperatures suboptimal for photosynthesis via their effects on the trans-thylakoid membrane energy coupling system. Further, low temperature effects can be grouped into either short term (minutes to hours) or long term (days to seasonal) series of changes in the content and composition of the PSII pigment-proteins. This minireview concludes by briefly highlighting primary areas of future research interest regarding the NPQ mechanism.     

Effect of Humic Substances on the Growth of Microalgal Cultures

Treatment with humic substances (preparation Biomin) considerably increased viability of algae during long storage (6 months) in collection. The effect was greater with green algae (Scenedesmus and Chlorella) than with blue-green algae (Nostoc and Anabaena). In intensive cultures, lower Biomin concentrations (1–10 mg/l) increased the biomass production, protein, chlorophyll and carotenoid content, as well as -esterase and glutamate dehydrogenase activities, whereas the lipid and carbohydrate content was higher at 100 mg/l Biomin. A high Biomin concentration (1 g/l) exerted an inhibition. Biomin supply to the cultures in outdoor installation of Shetlik type was associated with a moderate stimulation of algal biomass accumulation. It was concluded that Biomin might be helpful in the improvement of algal viability and growth.__________From Fiziologiya Rastenii, Vol. 52, No. 3, 2005, pp. 463–466.Original English Text Copyright © 2005 by Pouneva.This article was submitted by the author in English.     

A two-component nonphotochemical fluorescence quenching in eustigmatophyte algae

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

High salt stress in the upper part of floating mats of Ulva prolifera,a species that causes green tides,enhances non‐photochemical quenching

Salt stress is a major abiotic stress factor that can induce many adverse effects on photosynthetic organisms. Plants and algae have developed several mechanisms that help them respond to adverse environments. Non‐photochemical quenching (NPQ) is one of these mechanisms. The thalli of algae in the intertidal zone that are attached to rocks can be subjected to salt stress for a short period of time due to the rise and fall of the tide. Ulva prolifera causes green tides and can form floating mats when green tides occur and the upper part of the thalli is subjected to high salt stress for a long period of time. In this study, we compared the Ulva prolifera photosynthetic activities and NPQ kinetics when it is subjected to different salinities over various periods of time. Thalli exposed to a salinity of 90 for 4 d showed enhanced NPQ, and photosynthetic activities decreased from 60 min after exposure up to 4 d. This indicated that the induction of NPQ in Ulva prolifera under salt stress was closely related to the stressing extent and stressing time. The enhanced NPQ in the treated samples exposed for 4 d may explain why the upper layer of the floating mats formed by Ulva prolifera thalli were able to survive in the harsh environment. Further inhibitor experiments demonstrated that the enhanced NPQ was xanthophyll cycle and transthylakoid proton gradient‐dependent. However, photosystem II subunit S and light‐harvesting complex stress‐related protein didn't over accumulate and may not be responsible for the enhanced NPQ.     

Thermal Dissipation of Excess Light inArabidopsis Leaves is Inhibited after Gamma-irradiation

To elucidate the effect of ionizing radiation on the non-photochemical quenching (NPQ) oir chlorophyll fluorescence, we analyzed the buildup and release of NPQ inArabidopsis wild-type (WT) andnpq1- 2 mutant plants after gamma-irradiation. Thenpqi- 2 mutant cannot normally induce the buildup of NPQ by a mutation in the violaxamthin de-epoxidase gene. A dose of 50 Gy h for 4 h significantly suppressed such buildup in the mutant and, more noticeably, in the WT. Both the initial rise and maximum level of NPQ were gradually inhibited after gamma-irradiation. In contrast, the release of NPQ and the maximum photochemical efficiency (Fv/Fm) of Photosystem II were largely unaffected in either genotype. This inhibition of NPQ buildup could be partly attributable to a significant decrease in the content of carotenoids, including xanthophyll pigments. Moreover, inhibition that was dependent on the xanthophyll cycle substantially enhanced the sensitivity of irradiated leaves to a photoinhibitory illumination of 800 |imol photons m 2 s"1. The difference in Fv/Fm values between the WT andnpq1- 2 under that photoinhibitory level of illumination was much smaller in the irradiated leaves than in the control. However, NPQ inhibition did not cause a significant difference in efficiency between WT and mutant when both were treated with UV-B irradiance of 2.4 W m 2. Therefore, we suggest that a significant decrease in carotenoid content after gamma-irradiation should partially contribute to the enhanced sensitivity of irradiated plants, at least to high-ligtil photoinhibition. This is accomplished by suppressing the thermal dissipation of excess light absorbed by photosynthetic pigments.