De-epoxidation Of Xanthophyll Cycle Pigments Research Articles

Photoinhibition and Photoprotective Responses of a Brown Marine Macroalga Acclimated to Different Light and Nutrient Regimes.

Plants and brown algae avoid photoinhibition (decline in photosystem II efficiency, Fv/Fm) caused by excess light energy and oxidative stress through several photoprotective mechanisms, such as antioxidant xanthophyll production and heat dissipation. The heat dissipation can be measured as non-photochemical quenching (NPQ) and is strongly driven by de-epoxidation of xanthophyll cycle pigments (XCP). Although NPQ is known to increase under high light acclimation and nutrient-deficient conditions, a few studies have investigated the combined effects of the conditions on both NPQ and associated xanthophyll-to-chlorophyll (Chl) a ratio. The present study investigated the photosynthetic parameters of the brown alga Sargassum fusiforme acclimated to three irradiance levels combined with three nutrient levels. Elevated irradiance decreased Fv/Fm but increased NPQ, XCP/Chl a ratio, and fucoxanthin/Chl a ratio, suggesting the photoprotective role of antioxidant fucoxanthin in brown algae. Reduced nutrient availability increased NPQ but had no effect on the other variables, including XCP/Chl a ratio and its de-epoxidation state. The results indicate that NPQ can be used as a sensitive stress marker for nutrient deficiency, but cannot be used to estimate XCP pool size and state.

Reprint of “Photoprotection from anthocyanins and thermal energy dissipation in senescing red and green Sambucus canadensis peduncles”

Senescing photosynthetic tissues utilize a number of photoprotective mechanisms, including thermal energy dissipation involving the xanthophyll cycle, and often the screening of light energy by anthocyanins. The relative utilization of thermal energy dissipation when anthocyanins are present versus absent remains to be characterized. Using sun-acclimated Sambucus canadensis peduncles, which synthesize anthocyanins concomitant with fruit ripening, we compared pigment content (anthocyanins, carotenoids and chlorophylls) and chlorophyll fluorescence data (non-photochemical quenching [NPQ] and photochemical quantum yields of photosystem II, during and after prolonged light stress), before, during and after fruit ripening. Shade-acclimated peduncles, which remain green during senescence, were used for comparison. Additionally, we compared the xanthophyll cycle activity (i.e., constituents of the xanthophyll cycle) of sun- and shade-acclimated peduncles during fruit ripening. Before and during fruit ripening, when exposed to 1600 μmol photons m−2 s−1 white light, shade-acclimated peduncles had higher NPQ values and, during fruit ripening, greater proportions of de-epoxidized xanthophyll cycle pigments than did the sun-acclimated peduncles, although both types incurred similar levels of photoinactivation (i.e., reduction in ΦPSII). After the fruit had ripened and peduncles were at the later stages of senescence, the green peduncles were photoinactivated significantly more than were the red ones, following light stress. Collectively, our data indicate that green peduncles, at the early stages of senescence, compensate for their absence of anthocyanins by invoking higher levels of energy dissipation. However, at the later stages of senescence, the level of compensatory thermal energy dissipation in green peduncles was insufficient, and was associated with greater levels of photoinactivation, compared to red peduncles. We conclude that photoprotection by anthocyanins augments the protective functions of xanthophyll cycle pigments in S. canadensis peduncles, and is especially important for mitigating effects of light stress towards the later stages of senescence in high light environments.

Physiological effects of short acute UVB treatments in Chenopodium quinoa Willd

Increased ultraviolet B (UVB) radiation due to global change can affect plant growth and metabolism. Here, we evaluated the capacity of quinoa to resist under short acute UVB irradiation. Quinoa was daily exposed for 30 or 60 min to 1.69 W m−2 UVB. The results showed that 30 min exposure in 9 d-course did not cause severe alterations on photosynthetic pigments and flavonoids, but a significant increase of antioxidant capacity was observed. Otherwise, 60 min UVB in 5 d-course reduced almost all these parameters except for an increase in the de-epoxidation of xanthophyll cycle pigments and led to the death of the plants. Further studies of gas exchange and fluorescence measurements showed that 30 min UVB dramatically decrease stomatal conductance, probably associated to reactive oxygen species (ROS) production. Inhibition of photosynthetic electron transport was also observed, which could be a response to reduce ROS. Otherwise, irreversible damage to the photosynthetic apparatus was found with 60 min UVB probably due to severe ROS overproduction that decompensates the redox balance inducing UVB non-specific signaling. Moreover, 60 min UVB compromised Rubisco carboxylase activity and photosynthetic electron transport. Overall, these data suggest that quinoa modulates different response mechanisms depending on the UVB irradiation dosage.

Photoprotection from anthocyanins and thermal energy dissipation in senescing red and green Sambucus canadensis peduncles

Senescing photosynthetic tissues utilize a number of photoprotective mechanisms, including thermal energy dissipation involving the xanthophyll cycle, and often the screening of light energy by anthocyanins. The relative utilization of thermal energy dissipation when anthocyanins are present versus absent remains to be characterized. Using sun-acclimated Sambucus canadensis peduncles, which synthesize anthocyanins concomitant with fruit ripening, we compared pigment content (anthocyanins, carotenoids and chlorophylls) and chlorophyll fluorescence data (non-photochemical quenching [NPQ] and photochemical quantum yields of photosystem II, during and after prolonged light stress), before, during and after fruit ripening. Shade-acclimated peduncles, which remain green during senescence, were used for comparison. Additionally, we compared the xanthophyll cycle activity (i.e., constituents of the xanthophyll cycle) of sun- and shade-acclimated peduncles during fruit ripening. Before and during fruit ripening, when exposed to 1600 μmol photons m−2 s−1 white light, shade-acclimated peduncles had higher NPQ values and, during fruit ripening, greater proportions of de-epoxidized xanthophyll cycle pigments than did the sun-acclimated peduncles, although both types incurred similar levels of photoinactivation (i.e., reduction in ΦPSII). After the fruit had ripened and peduncles were at the later stages of senescence, the green peduncles were photoinactivated significantly more than were the red ones, following light stress. Collectively, our data indicate that green peduncles, at the early stages of senescence, compensate for their absence of anthocyanins by invoking higher levels of energy dissipation. However, at the later stages of senescence, the level of compensatory thermal energy dissipation in green peduncles was insufficient, and was associated with greater levels of photoinactivation, compared to red peduncles. We conclude that photoprotection by anthocyanins augments the protective functions of xanthophyll cycle pigments in S. canadensis peduncles, and is especially important for mitigating effects of light stress towards the later stages of senescence in high light environments.

Xanthophyll Cycle Activity in Two Prominent Arctic Shrub Species

ABSTRACTWhen the capacity for photosynthesis is constrained by unfavorable growing conditions, excess absorbed light is safely lost from leaves via thermal energy dissipation—a photoprotective mechanism ubiquitous among higher plants. The relatively low irradiance conditions yet stressful growing environment of the arctic tundra suggest contrasting hypotheses regarding the necessity for plant investment in photoprotection. To examine these hypotheses, the photoprotective pigments of the xanthophyll cycle were investigated in conjunction with non-photochemical quenching (NPQ) of chlorophyll fluorescence emission in two dominant arctic shrub species, Salix pulchra and Betula nana. The xanthophyll cycle pool sizes of S. pulchra leaves were substantially higher than those reported in most other higher plant species, whereas B. nana leaves maintain modestly high xanthophyll cycle pool sizes. In addition, high retention of de-epoxidized xanthophyll cycle pigments in both species and saturation of xanthophyll cycle conversion at low-light intensities were observed and associated with high levels of NPQ. The xanthophyll cycle leaf pigment pools reported are among the first published for arctic plants and support the hypothesis that foliar xanthophyll cycle activity is greater in environments prone to harsher growing conditions.

UV radiation promotes flavonoid biosynthesis, while negatively affecting the biosynthesis and the de-epoxidation of xanthophylls: Consequence for photoprotection?

There is evidence that UV radiation may detrimentally affect the biosynthesis of carotenoids, particularly de-epoxided xanthophylls, while strongly promoting phenylpropanoid, particularly flavonoid biosynthesis in a range of taxa. Here we tested the hypothesis that mesophyll flavonoids might protect chloroplasts from UV-induced photo-oxidative damage, by partially compensating for the UV-induced depression of xanthophyll biosynthesis. To test this hypothesis we grew two members of the Oleaceae family, Ligustrum vulgare L. and Phillyrea latifolia L., under either partial shading or fully exposed to sunlight, in the presence or in the absence of UV radiation. The examined species, which display very similar flavonoid composition, largely differ in their ability to limit the transmission of UV and visible light through the leaf and, hence, in the accumulation of flavonoids in mesophyll cells. We conducted measurements of photosynthesis, chlorophyll a fluorescence kinetics, the concentrations of individual carotenoids and phenylpropanoids at the level of whole-leaf, as well as the content of epidermal flavonoids. We also performed multispectral fluorescence micro-imaging to unveil the intra-cellular distribution of flavonoids in mesophyll cells. UV radiation decreased the concentration of carotenoids, particularly of xanthophylls, while greatly promoting the accumulation of flavonoids in palisade parenchyma cells. These effects were much greater in L. vulgare than in P. latifolia. UV radiation significantly inhibited the de-epoxidation of xanthophyll cycle pigments, while enhancing the concentration of luteolin, and particularly of quercetin glycosides. Flavonoids accumulated in the vacuole and the chloroplasts in palisade cells proximal to the adaxial epidermis. We hypothesize that flavonoids might complement the photo-protective functions of xanthophylls in the chloroplasts of mesophyll cells exposed to the greatest doses of UV radiation. However, UV radiation might result in adaxial mesophyll cells being less effective in dissipating the excess of radiant energy, e.g., by decreasing their capacity of thermal dissipation of excess visible light in the chloroplast.

The xanthophyll cycle and NPQ in diverse desert and aquatic green algae

It has long been suspected that photoprotective mechanisms in green algae are similar to those in seed plants. However, exceptions have recently surfaced among aquatic and marine green algae in several taxonomic classes. Green algae are highly diverse genetically, falling into 13 named classes, and they are diverse ecologically, with many lineages including members from freshwater, marine, and terrestrial habitats. Genetically similar species living in dramatically different environments are potentially a rich source of information about variations in photoprotective function. Using aquatic and desert-derived species from three classes of green algae, we examined the induction of photoprotection under high light, exploring the relationship between nonphotochemical quenching and the xanthophyll cycle. In liquid culture, behavior of aquatic Entransia fimbriata (Klebsormidiophyceae) generally matched patterns observed in seed plants. Nonphotochemical quenching was lowest after overnight dark adaptation, increased with light intensity, and the extent of nonphotochemical quenching correlated with the extent of deepoxidation of xanthophyll cycle pigments. In contrast, overnight dark adaptation did not minimize nonphotochemical quenching in the other species studied: desert Klebsormidium sp. (Klebsormidiophyceae), desert and aquatic Cylindrocystis sp. (Zygnematophyceae), and desert Stichococcus sp. (Trebouxiophyceae). Instead, exposure to low light reduced nonphotochemical quenching below dark-adapted levels. De-epoxidation of xanthophyll cycle pigments paralleled light-induced changes in nonphotochemical quenching for species within Klebsormidiophyceae and Trebouxiophyceae, but not Zygnematophyceae. Inhibition of violaxanthin-zeaxanthin conversion by dithiothreitol reduced high-light-associated nonphotochemical quenching in all species (Zygnematophyceae the least), indicating that zeaxanthin can contribute to photoprotection as in seed plants but to different extents depending on taxon or lineage.

A comparison between yellow-green and green cultivars of four vegetable species in pigments, ascorbate, photosynthesis, energy dissipation, and photoinhibition

Yellow-green foliage cultivars of four vegetables grown outdoors, i.e., Chinese mustard (Brassica rapa), Chinese kale (Brassica oleracea var. alboglabra), sweet potato (Ipomoea batatas) and Chinese amaranth (Amaranthus tricolor), had lower chlorophyll (Chl) (a+b) (29-36% of green cultivars of the same species), total carotenoids (46-62%) and ascorbate (72-90%) contents per leaf area. Furthermore, yellow-green cultivars had smaller photosystem II (PSII) antenna size (65-70%) and lower photosynthetic capacity (52-63%), but higher Chl a/b (107-156%) and from low (60%) to high (129%) ratios of de-epoxidized xanthophyll cycle pigments per Chl a content. Potential quantum efficiency of PSII (Fv/Fm) of all overnight dark-adapted leaves was ca. 0.8, with no significant difference between yellow-green and green cultivars of the same species. However, yellow-green cultivars displayed a higher degree of photoinhibition (lower Fv/Fm after illumination) when they were exposed to high irradiance. Although vegetables used in this study are of either temperate or tropical origin and include both C3 and C4 plants, data from all cultivars combined revealed that Fv/Fm after illumination still showed a significant positive linear regression with xanthophyll cycledependent energy quenching (qE) and a negative linear regression with photoinhibitory quenching (qI). Fv/Fm was, however, not correlated with nonphotochemical quenching (NPQ). Yet, a higher degree of photoinhibition in yellow-green cultivars could recover during the night darkness period, suggesting that the repair of PSII in yellow-green cultivars would allow them to grow normally in the field.

Dehydration-mediated activation of the xanthophyll cycle in darkness: is it related to desiccation tolerance?

The development of desiccation tolerance by vegetative tissues was an important step in the plants' conquest of land. To counteract the oxidative stress generated under these conditions the xanthophyll cycle plays a key role. Recent reports have shown that desiccation itself induces de-epoxidation of xanthophyll cycle pigments, even in darkness. The aim of the present work was to study whether this trait is a common response of all desiccation-tolerant plants. The xanthophyll cycle activity and the maximal photochemical efficiency of PS II (F(v)/F(m)) as well as β-carotene and α-tocopherol contents were compared during slow and rapid desiccation and subsequent rehydration in six species pairs (with one desiccation-sensitive and one desiccation-tolerant species each) belonging to different taxa. Xanthophyll cycle pigments were de-epoxidised in darkness concomitantly with a decrease in F(v)/F(m) during slow dehydration in all the desiccation-tolerant species and in most of the desiccation-sensitive ones. De-epoxidation was reverted in darkness by re-watering in parallel with the recovery of the initial F(v)/F(m). The stability of the β-carotene pool confirmed that its hydroxylation did not contribute to zeaxanthin formation. The α-tocopherol content of most of the species did not change during dehydration. Because it is a common mechanism present in all the desiccation-tolerant taxa and in some desiccation-sensitive species, and considering its role in antioxidant processes and in excess energy dissipation, the induction of the de-epoxidation of xanthophyll cycle pigments upon dehydration in the dark could be understood as a desiccation tolerance-related response maintained from the ancestral clades in the initial steps of land occupation by plants.

Ultraviolet Radiation Reduces the Photoprotective Capacity of the Marine Diatom Phaeodactylum tricornutum (Bacillariophyceae, Heterokontophyta)

This study demonstrates that UV radiation (UVR) reduces the photoprotective capacity of the diatom Phaeodactylum tricornutum by affecting xanthophyll cycle (XC) activity. The short-term reduction of photosystem II (PSII) maximum efficiency of charge separation (F(v)/F(m)) in cells exposed to UVR could be explained mainly by a reduced photoprotective capacity under this condition. Phaeodactylum tricornutum cells acclimated to two different photosynthetically active radiation (PAR) intensities, high light (HL, 200 micromol quanta m(-2) s(-1)) and low light (LL, 50 micromol quanta m(-2) s(-1)), were exposed to saturating irradiance (1100 micromol quanta m(-2) s(-1)) in the presence (PAR + UVR) and absence of UVR (PAR). HL cells exhibited a greater reduction in F(v)/F(m) in PAR + UVR when compared with the PAR treatment that was related to a reduction in the de-epoxidation of XC pigments. In contrast, in LL cells, UVR did not considerably affect XC de-epoxidation even though the reduction in F(v)/F(m) was greater than in HL cells. The negative effect of UVR on photoprotection was more pronounced in HL cells because they synthesized more XC pigments than LL cells. This was confirmed when XC activity was blocked with dithiothreitol and when PSII repair was inhibited with chloramphenicol (CAP). The differential reduction of F(v)/F(m) between PAR + UVR and PAR treatments disappeared when XC was blocked in HL cells. A higher reduction and an incomplete recovery of F(v)/F(m) were observed in cells incubated with CAP in the presence of UVR. Such responses confirm that UVR had a negative effect on photoprotective mechanisms causing an enhancement of damage by PAR, especially in HL-acclimated cells in which heat dissipation is important for PSII regulation.

The higher resistance to chilling stress in adaxial side of Rumex K-1 leaves is accompanied with higher photochemical and non-photochemical quenching

Responses of two sides of Rumex K-1 leaves to chilling stress (5 °C, photon flux density of 100 µmol m-2 s-1) were studied by using gas exchange, chlorophyll (Chl) fluorescence, and spectrum reflectance techniques. The Chl and carotenoid contents in the two sides were not affected by chilling treatment, and both were higher in the adaxial side. The maximum quantum yield of photosystem (PS) 2 and fraction of functional PS1 in the abaxial side decreased more markedly than those in the adaxial side during the chilling treatment, indicating that the abaxial side was damaged more significantly than the adaxial side. Before chilling, there were no obvious differences in actual photochemical efficiency of PS2, photosynthesis, and photorespiration between two sides of the leaves. Under chilling stress, the actual photochemical efficiency of PS2, photosynthesis, and photorespiration all declined more significantly in the abaxial side, which was partly attributed to lower carboxylation efficiency in the abaxial side than that in the adaxial side. Non-photochemical quenching was higher in the adaxial side, though the de-epoxidation of xanthophyll cycle pigments' pool on basis of Chl was higher in the abaxial side. Both the slower decrease in the photochemical quenching and the higher non-photochemical quenching may account for the higher resistance to chilling stress in the adaxial side of Rumex K-1 leaves.

Evidence for a rebinding of antheraxanthin to the light-harvesting complex during the epoxidation reaction of the violaxanthin cycle

In the present study, we investigated the epoxidation reaction of the violaxanthin (Vx) cycle in intact cells of Chlorella vulgaris. Our results show that the overall epoxidation is slightly slower in darkness compared to the epoxidation during high light (HL) illumination. The calculation of the rate constants of the two epoxidation steps revealed that, for both conditions, the first epoxidation step from zeaxanthin (Zx) to antheraxanthin (Ax) is faster than the second epoxidation step from Ax to Vx. However, the most noteworthy result of our present study is that Ax, which is transiently formed during the epoxidation reaction, participates in non-photochemical quenching of chlorophyll fluorescence (NPQ). A correlation between NPQ and the de-epoxidized xanthophyll cycle pigments during the time-course of the epoxidation reaction can only be achieved when NPQ is plotted versus the sum of Zx and Ax. The accumulation of significant amounts of Ax during the epoxidation reaction further indicates that Ax-dependent quenching proceeds with a similar efficiency compared to the Zx-mediated NPQ. As the xanthophyll-dependent NPQ relies on the presence of de-epoxidized xanthophylls in the PS II antenna, Ax-dependent NPQ is only possible under the assumption that Ax rebinds to the light-harvesting complex (LHC) II during the epoxidation reaction.

Responses of two field-grown coffee species to drought and re-hydration

The gas exchange, parameters of chlorophyll fluorescence, contents of pigments, and activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), as well as lipid peroxidation were investigated in two field-grown coffee species, Coffea arabica and C. liberica, exposed to drought and re-hydration. Drought caused a more pronounced inhibition of net photosynthetic rate in C. liberica compared to C. arabica. The de-epoxidation of xanthophyll cycle pigments at midday estimated by leaf reflectance was much higher in C. arabica than in C. liberica, but no significant change was found in response to drought. Under moderate drought, the activities of SOD and APX increased significantly only in C. arabica. The maximum photochemical efficiency of photosystem 2, PS2 (Fv/Fm) at predawn did not change and there was no lipid peroxidation during this time. Under severe drought Fv/Fm decreased and initial fluorescence (F0) increased for both species, and SOD activity increased, APX activity remained relatively high, and malondialdehyde (MDA) accumulated in C. arabica, while APX decreased in C. liberica. The photosynthetic apparatus of C. arabica was completely recovered after 5 d of re-irrigation as indicated by the restoration of Fv/Fm to the control values. A lack of recovery upon rewatering of C. liberica indicated irreversible damage to PS2. Hence compared to C. liberica, C. arabica possesses a higher desiccation-induced antioxidative protection and higher portion of the total pigment pool used in photoprotection, which might aid alleviating photoinhibitory damage during desiccation and photosynthesis recovery when favourable conditions are restored.

Influence of Elevated Air Temperature on the Relationship between Photochemical Reflectance Index (PRI) and Photosynthetic Light Use Efficiency of Poplar Leaves

To clarify the application of the PRI to evaluation of photosynthetic light use efficiency under the varied air temperature, we investigated experimentally the PRI and photosynthesis in the leaves of 4-month-old poplar clones (Populus x Canadensis clone I45/51) which were exposed to 5 levels of air temperature (20, 25, 30, 35 and 40 ºC) and 4 levels of light intensities (250, 600, 1100 and 1700 µmol m-2 s-1 PPFD). Irrespective of air temperature treatment, increasing light intensity reduced the PRI, epoxidation state in the xanthophyll cycle (EPS) and quantum yield of photosystem II (ΔF/Fm’). The elevating air temperature significantly increased the PRI without clear changes of EPS, indicating that temperature-induced increase of the PRI could not be explained by only the de-epoxidation of xanthophyll cycle pigments. Consequently, the PRI positively correlated with EPS and ΔF/Fm’, but the regression lines differed among temperature treatments. The high air temperature above 30 ºC reduced light use efficiency of net photosynthetic rate (LUEN) at the light intensities of 250 and 1700 µmol m-2 s-1 PPFD, accompanied with lowered stomatal conductance and elevated respiration rate. Therefore, the PRI did not correlate to LUEN of poplar leaves in two higher temperature treatments. On the other hand, in all temperature treatments, the PRI positively correlated to the light use efficiency of gross photosynthetic rate (LUEG) which took into account of CO2 loss caused by increasing respiration. Although foliar PRI is increased by the elevating temperature, we conclude that the PRI can qualitatively trace the photosynthetic light use efficiencies both in photosystem II and gross photosynthetic rate in even heat-stressed poplar leaves.

Seasonal patterns of reflectance indices, carotenoid pigments and photosynthesis of evergreen chaparral species.

This study examined the ability of the Photochemical Reflectance Index (PRI) to track seasonal variations in carotenoid pigments and photosynthetic activity of mature evergreen chaparral shrubs. Our results confirm that PRI scales with photosystem two (PSII) photochemical efficiency across species and seasons, as demonstrated by PRI's strong correlation with de-epoxidized (photoprotective) xanthophyll cycle pigment levels (normalized to chlorophyll) and with the chlorophyll fluorescence index, ΔF/Fm'. PRI and carotenoid pigment levels (de-epoxidized xanthophyll cycle pigments normalized to chlorophyll or total carotenoid pigments normalized to chlorophyll) were correlated with seasonal fluctuations in midday net CO2 uptake of top-canopy leaves. By contrast, chlorophyll levels (as measured by the Chlorophyll Index) were not as strongly linked to photosynthetic activity, particularly when all species were considered together. Likewise, the Normalized Difference Vegetation Index (NDVI, an index of canopy greenness) did not correlate with net CO2 uptake. Canopy NDVI also did not correlate with canopy PRI, demonstrating that these indices were largely independent over the temporal and spatial scales of this study. Together, these patterns provide evidence for coordinated regulation of carotenoid pigments, PSII electron transport, and carboxylation across seasons and indicate that physiological adjustments are more important than structural ones in modifying CO2-fixation capacity during periods of photosynthetic down-regulation for these evergreen species. The strong correlation between PRI of whole canopies and PRI of top-canopy leaves suggests that the canopy can be treated as a "big leaf" in terms of this reflectance index and that PRI can be used in "scalable" models. This along with the links between carotenoid pigments, PSII photochemical efficiency and carboxylation across species and seasons supports the use of optical assays of pigment levels and PSII activity in CO2 flux models to derive photosynthetic rates.

Effect of elevated CO2 concentration on acclimation of tobacco plantlets to ex vitro conditions

Nicotiana tabacum L. plants grown in vitro were transferred to ex vitro conditions and grown for 28 d in a greenhouse under normal CO 2 concentration (C, 330 μmol mol -1 ) or elevated CO 2 concentration (E, 1000 μmol mol -1 ). Stomatal conductances of abaxial and adaxial epidermes measured under optimal conditions were not significantly affected by growth under E, but the stomatal regulation of gas exchange was better. Leaf photosynthetic rate (A) of elevated CO 2 plants was similar to that of control plants when both were measured under normal CO 2 , but higher when both were measured under elevated CO 2 . The A of elevated CO 2 plants was much higher than the A of control plants when measured under their respective growth CO 2 concentration, which resulted in their higher growth rate. Chlorophyll a and b contents, and activities of whole electron transport chain and of photosystem (PS) II were not markedly affected by growth under E, and the maximum efficiency of PSII measured as the ratio of variable to maximum fluorescence was even slightly increased. Hence no down-regulation of photosynthesis occurred in transplanted plants grown for 4 weeks under E. The contents of β-carotene and of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) were lower in E plants. The degree of de-epoxidation of xanthophyll cycle pigments was not changed or was even lower after transfer to ex vitro conditions, which indicated that no photoinhibition occurred. Therefore, CO 2 enrichment can improve acclimation of in vitro-grown plantlets to ex vitro conditions.

Epoxidation of zeaxanthin and antheraxanthin reverses non-photochemical quenching of photosystem II chlorophyll a fluorescence in the presence of trans-thylakoid ΔpH

The xanthophyll cycle apparently aids the photoprotection of photosystem II by regulating the nonradiative dissipation of excess absorbed light energy as heat. However, it is a controversial question whether the resulting nonphotochemical quenching is soley dependent on xanthophyll cycle activity or not. The xanthophyll cycle consists of two enzymic reactions, namely deepoxidation of the diepoxide violaxanthin to the epoxide-free zeaxanthin and the much slower, reverse process of epoxidation. While deepoxidation requires a transthylakoid pH gradient (ΔpH), epoxidation can proceed irrespective of a ΔpH. Herein, we compared the extent and kinetics of deepoxidation and epoxidation to the changes in fluorescence in the presence of a light-induced thylakoid ΔpH. We show that epoxidation reverses fluorescence quenching without affecting thylakoid ΔpH. These results suggest that epoxidase activity reverses quenching by removing deepoxidized xanthophyll cycle pigments from quenching complexes and converting them to a nonquenching form. The transmembrane organization of the xanthophyll cycle influences the localization and the availability of deepoxidized xanthophylls is to support nonphotochemical quenching capacity. The results confirm the view that rapidly reversible nonphotochemical quenching is dependent on deepoxidized xanthophyll.