Antisense suppression of violaxanthin de-epoxidase in tobacco does not affect plant performance in controlled growth conditions

Violaxanthin de-epoxidase (VDE) catalyzes the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin in the xanthophyll cycle. Tobacco was transformed with an antisense VDE construct under control of the cauliflower mosaic virus 35S promoter to determine the effect of reduced levels of VDE on plant growth. Screening of 40 independent transformants revealed 18 antisense lines with reduced levels of VDE activity with two in particular (TAS32 and TAS39) having greater than 95% reduction in VDE activity. Northern analysis demonstrated that these transformants had greatly suppressed levels of VDE mRNA. De-epoxidation of violaxanthin was inhibited to such an extent that no zeaxanthin and only very low levels of antheraxanthin could be detected after exposure of leaves to high light (2000 μmol m⁻² s⁻¹ for 20 min) with no observable effect on levels of other carotenoids and chlorophyll. Non-photochemical quenching was greatly reduced in the antisense VDE tobacco, demonstrating that a significant level of the non-photochemical quenching in tobacco requires de-epoxidation of violaxanthin. Although the antisense plants demonstrated a greatly impaired de-epoxidation of violaxanthin, no effect on plant growth or photosynthetic rate was found when plants were grown at a photon flux density of 500 or 1000 μmol m⁻² s⁻¹ under controlled growth conditions as compared to wild-type tobacco. This revised version was published online in June 2006 with corrections to the Cover Date.     

Regulation of the xanthophyll cycle pool size in duckweed (Lemna minor) plants

Duckweed ( Lemna minor L.) plants grown under high light are characterized, when compared to low light acclimated plants, by a higher xanthophyll cycle (VAZ) pool content, but also by a higher proportion of photoconvertible violaxanthin and a superior ability to synthesize VAZ pigments. When duckweed plants were transferred to a high light environment a general response was the quick adjustment of the carotenoid composition, mainly xanthophyll cycle pigments. These changes resulted from a balance between a process of continuous light-independent carotenoid degradation and a light-induced accumulation. The use of norflurazon, an inhibitor of carotenogenesis, allowed us to demonstrate that the observed light induced increase of the VAZ pool was mainly caused by de novo synthesis through carotenogenesis. The extent of light-induced carotenogenesis was proportional to the light treatment and also to the operation of the VAZ cycle since it was partly abolished by treatments leading to a low activity of the VAZ cycle, such as low light, DTT or DCMU. These results suggest that not only the light itself, but also a mechanism triggered by a factor associated with the de-epoxidation state of the VAZ cycle controls carotenogenesis at some point before phytoene formation in the terpenoid biosynthesis pathway.     

Significance of the lipid phase in the dynamics and functions of the xanthophyll cycle as revealed by PsbS overexpression in tobacco and in-vitro de-epoxidation in monogalactosyldiacylglycerol micelles

The dynamics of the xanthophyll cycle relative to non-photochemical quenching (NPQ) were examined in tobacco plants overexpressing violaxanthin de-epoxidase (VDE), PsbS and PsbS+VDE for effects on NPQ and violaxanthin (V) de-epoxidation over a range of light intensities. Induction of de-epoxidation and NPQ increased in overexpressed VDE and PsbS plants, respectively. Surprisingly, under low light, overexpressing PsbS enhanced de-epoxidation in addition to NPQ. The effect was hypothesized as due to PsbS binding zeaxanthin (Z) or inducing the binding of Z within the quenching complex, thus shifting the equilibrium toward higher de-epoxidation states. Studies in model systems show that Z can stereospecifically inhibit VDE activity against violaxanthin. This effect, observed under conditions of limiting lipid concentration, was interpreted as product feedback inhibition. These results support the hypothesis that the capacity of the thylakoid lipid phase for xanthophylls is limited and modulates xanthophyll-cycle activity, in conjunction with the release of V and binding of Z by pigment-binding proteins. These modulating factors are incorporated into a lipid-matrix model that has elements of a signal transduction system wherein the light-generated protons are the signal, VDE the signal receptor, Z the secondary messenger, the lipid phase the transduction network, and Z-binding proteins the targets.     

Do the capacity and kinetics for modification of xanthophyll cycle pool size depend on growth irradiance in temperate trees?

The present study investigated the interaction of growth irradiance (Q_int) with leaf capacity for and kinetics of adjustment of the pool size of xanthophyll cycle carotenoids (sum of violaxanthin, antheraxanthin and zeaxanthin; VAZ) and photosynthetic electron transport rate (J_max) after changes in leaf light environment. Individual leaves of lower‐canopy/lower photosynthetic capacity species Tilia cordata Mill. and upper canopy/higher photosynthetic capacity species Populus tremula L. were either illuminated by additional light of 500–800 µmol m^?2 s^?1 for 12 h photoperiod or enclosed in shade bags. The extra irradiance increased the total amount of light intercepted by two‐fold for the upper and 10–15‐fold for the lower canopy leaves, whereas the shade bags transmitted 45% of incident irradiance. In control leaves, VAZ/area, VAZ/Chl and J_max were positively associated with leaf growth irradiance (Q_int). After 11 d extra illumination, VAZ/Chl increased in all cases due to a strong reduction in foliar chlorophyll, but VAZ/area increased in the upper canopy leaves of both species, and remained constant or decreased in the lower canopy leaves of T. cordata. The slope for VAZ/area changes with cumulative extra irradiance was positively associated with Q_int only in T. cordata, but not in P. tremula. Nevertheless, all leaves of P. tremula increased VAZ/area more than the most responsive leaves of T. cordata. Shading reduced VAZ content only in P. tremula, but not in T. cordata, again demonstrating that P. tremula is a more responsive species. Compatible with the hypothesis of the role of VAZ in photoprotection, the rates of photosynthetic electron transport declined less in P. tremula than in T. cordata after the extra irradiance treatment. However, foliar chlorophyll contents of the exposed leaves declined significantly more in the upper canopy of P. tremula, which is not consistent with the suggestion that the leaves with the highest VAZ content are more resistant to photoinhibition. This study demonstrates that previous leaf light environment may significantly affect the adaptation capacity of foliage to altered light environment, and also that species differences in photosynthetic capacity and acclimation potentials importantly alter this interaction.     

Dynamics of chromophore binding to Lhc proteins in vivo and in vitro during operation of the xanthophyll cycle

Three plant xanthophylls are components of the xanthophyll cycle in which, upon exposure of leaves to high light, the enzyme violaxanthin de-epoxidase (VDE) transforms violaxanthin into zeaxanthin via the intermediate antheraxanthin. Previous work () showed that xanthophylls are bound to Lhc proteins and that substitution of violaxanthin with zeaxanthin induces conformational changes and fluorescence quenching by thermal dissipation. We have analyzed the efficiency of different Lhc proteins to exchange violaxanthin with zeaxanthin both in vivo and in vitro. Light stress of Zea mays leaves activates VDE, and the newly formed zeaxanthin is found primarily in CP26 and CP24, whereas other Lhc proteins show a lower exchange capacity. The de-epoxidation system has been reconstituted in vitro by using recombinant Lhc proteins, recombinant VDE, and monogalactosyl diacylglycerol (MGDG) to determine the intrinsic capacity for violaxanthin-to-zeaxanthin exchange of individual Lhc gene products. Again, CP26 was the most efficient in xanthophyll exchange. Biochemical and spectroscopic analysis of individual Lhc proteins after de-epoxidation in vitro showed that xanthophyll exchange occurs at the L2-binding site. Xanthophyll exchange depends on low pH, implying that access to the binding site is controlled by a conformational change via lumenal pH. These findings suggest that the xanthophyll cycle participates in a signal transduction system acting in the modulation of light harvesting versus thermal dissipation in the antenna system of higher plants.     

小麦紫黄质脱环氧化酶(VDE) cDNA的克隆及表达

应用RT-PCR方法从小麦(Triticum aestivum L. cv.Xiaoyan 54)中克隆了紫黄质脱环氧化酶(violaxanthinde-epoxidase,VDE) cDNA,预测的蛋白质序列与拟南芥(Arabidopsis thaliana)、水稻(Oryza sati va)有很高的同源性.Southern杂交结果表明,在小麦中可能存在3个拷贝的紫黄质脱环氧化酶基因.Northern杂交结果表明它在绿色叶片中特异表达,在黄化小麦幼苗变绿过程中其mRNA水平受光诱导,并且强光增加了其mRNA在成熟叶片中的表达.     

小麦紫黄质脱环氧化酶(VDE)cDNA的克隆及表达(英文)

应用RT-PCR方法从小麦(Triticum aestivum L.cv.Xiaoyan 54)中克隆了紫黄质脱环氧化酶(violaxanthinde-epoxidase,VDE)cDNA,预测的蛋白质序列与拟南芥(Arabidopsis thaliana)、水稻(Oryza sativa)有很高的同源性。Southern杂交结果表明,在小麦中可能存在3个拷贝的紫黄质脱环氧化酶基因。Northern杂交结果表明它在绿色叶片中特异表达,在黄化小麦幼苗变绿过程中其mRNA水平受光诱导,并且强光增加了其mRNA在成熟叶片中的表达。     

Seasonal reversibility of acclimation to irradiance in leaves of common box (Buxus sempervirens L.) in a deciduous forest

Understorey shade plants are seasonally exposed to dramatic changes in light conditions in deciduous forests related with the dynamics of the overstorey leaf phenology. These transitions are commonly followed by changes in herb plant communities, but shade-tolerant evergreen species must be able to adapt to changing light conditions. In this work we checked the photoprotective responses of evergreen species to acclimate to the shady summer environment and reversibly de-acclimate to a more illuminated environment after leaf fall on deciduous overstoreys. For that purpose we have followed the process of light acclimation in leaves of common box (Buxus sempervirens) during the winter to spring transition, which decrease irradiance in the understorey, and conversely during the transition from summer to autumn. Four parameters indicative of the structure and degree of acclimation of the photosynthetic apparatus have been studied: chlorophyll a/b ratio which is supposed to be inversely proportional to the antenna size, α/β-carotene which increases in shade acclimated leaves and the pools of α-tocopherol and xanthophyll cycle pigments (VAZ) which are two of the main photoprotection mechanisms in plants. Among these parameters, chlorophyll a/b ratio and VAZ pool responded finely to changes in irradiance indicating that modifications in the light harvesting size and photoprotective capacity contribute to the continuous acclimation and de-acclimation of long-lived evergreen leaves.     

EFFECTS OF TEMPERATURE AND LIGHT TREATMENT ON VIOLAXANTHIN DE-EPOXIDASE ACTIVITY AND XANTHOPHYLL CYCLE-DEPENDENT ENERGY DISSIPATION IN WHEAT LEAVES

 为了探讨温度和光强是如何影响离体紫黄质脱环氧化酶(VDE)活性, 阐明依赖叶黄素循环的热耗散与VDE活性关系, 该文以小麦(Triticum aestivum)为材料, 研究了不同光强(200、500、900和1 200 μmol&;#8226;m^–2&;#8226;s^–1)和不同温度(4、25、38和45 ℃) 交叉处理对小麦叶片VDE活性以及依赖叶黄素循环热耗散能力的影响。结果表明: 小麦叶片VDE活性在30 ℃最高, 说明30 ℃是小麦叶片VDE体外条件下的最适温度; 不同光强处理下小麦叶片VDE活性基本一致。与室温(25 ℃)处理的叶片相比, 低温(4 ℃)处理的叶片VDE活力没有明显下降, 而高温(45 ℃)处理则导致了叶片VDE活性急剧下降。小麦叶片热耗散(NPQ)以及依赖叶黄素循环的热耗散(qE)均随着处理光强的增加不断上升, 而qE/NPQ则随光强增加略微下降, 在1 200 μmol&;#8226;m^–2&;#8226;s^–1光强条件下qE/NPQ则急剧下降。该研究揭示VDE活性与依赖叶黄素循环热耗散能力的指标qE/NPQ的变化有一定的相关性, 但不完全一致。并针对此问题进行了讨论。     

Xanthophyll Cycle Pigment Localization and Dynamics during Exposure to Low Temperatures and Light Stress in Vinca major

The distribution of xanthophyll cycle pigments (violaxanthin plus antheraxanthin plus zeaxanthin [VAZ]) among photosynthetic pigment-protein complexes was examined in Vinca major before, during, and subsequent to a photoinhibitory treatment at low temperature. Four pigment-protein complexes were isolated: the core of photosystem (PS) II, the major light-harvesting complex (LHC) protein of PSII (LHCII), the minor light-harvesting proteins (CPs) of PSII (CP29, CP26, and CP24), and PSI with its LHC proteins (PSI-LHCI). In isolated thylakoids 80% of VAZ was bound to protein independently of the de-epoxidation state and was found in all complexes. Plants grown outside in natural sunlight had higher levels of VAZ (expressed per chlorophyll), compared with plants grown in low light in the laboratory, and the additional VAZ was mainly bound to the major LHCII complex, apparently in an acid-labile site. The extent of de-epoxidation of VAZ in high light and the rate of reconversion of Z plus A to V following 2.5 h of recovery were greatest in the free-pigment fraction and varied among the pigment-protein complexes. Photoinhibition caused increases in VAZ, particularly in low-light-acclimated leaves. The data suggest that the photoinhibitory treatment caused an enrichment in VAZ bound to the minor CPs caused by de novo synthesis of the pigments and/or a redistribution of VAZ from the major LHCII complex.     

Changes in Violaxanthin Deepoxidase Activity and Unsaturation of Thylakoid Membrane Lipids in Indica and Japonica RiceUnder Chill and Strong Light

To explore the differences of sesitivities to chill and strong light in indica and japonica rice (Oryza sativa), the changes in unsaturation of thylakoid membrane lipids and xanthophyll cycle were studied under Chill condition and strong light. The contents of unsaturated fatty acids of thylakoid membrane lipids decreased and that of the saturated ones increased with the time of Chill- and strong lighttreatment, resulting in the reduction of the index of unsaturation of fatty acids (IUFA). The activities of violaxanthin deepoxidase (VDE), a key enzyme of xanthophyll cycle, also reduced. The content of violaxanthin (V) increased, and the contents of antheraxanthin (A) and zeaxanthin (Z) decreased, the ratio of (A+Z)/(A+Z+V) decreased correspondingly. Arrhenius analysis showed that VDE was sensitive to both chill and unsaturation level of thylakoid membrane lipids. Correlation analysis showed that there was distinctly positive relationships between IUFA of thylakoid membrane lipids and the activity of VDE, Fv/Fm, and D1 protein content. Lower IUFA values, less fluidity and stability of thylakoid membrane lipids, lower VDE activity and (A+Z)/(A+Z+V) ratio were found in indica rice cv. Shanyou 63 than in japonica rice cv. 9516 under chill and strong light.     

Leaf orientation and the response of the xanthophyll cycle to incident light

Summary Leaves from two species, Euonymus kiautschovicus and Arctostaphylos uva-ursi, with a variety of different orientations and exposures, were examined in the field with regard to the xanthophyll cycle (the interconversion of three carotenoids in the chloroplast thylakoid membranes). East-, south-, and west-facing leaves of E. kiautschovicus were sampled throughout the day and all exhibited a pronounced and progressive conversion of violaxanthin to zeaxanthin, followed by a reconversion of zeaxanthin to violaxanthin later in the day. Maximal levels of zeaxanthin and minimal levels of violaxanthin were observed at the time when each leaf (orientation) received the maximum incident light, which was in the morning in east-facing, midday in southfacing, and in the afternoon in west-facing leaves. A very slight degree of hysteresis in the removal of zeaxanthin compared to its formation with regard to incident light was observed. Leaves with a broader range of orientations were sampled from A. uva-ursi prior to sunrise and at midday. All of the examined pigments (carotenoids and chlorophylls) increased somewhat per unit leaf area with increasing total daily photon receipt. The sum of the carotenoids involved in the xanthophyll cycle, violaxanthin + antheraxanthin + zeaxanthin, increased more strongly with increasing growth light than any other pigment. In addition, the amounts of zeaxanthin present at midday also increased markedly with increasing total daily photon receipt. The percentage of the xanthophyll cycle that was converted to zeaxanthin (and antheraxanthin) at peak irradiance was very large (approximately 80%) in the leaves of both E. kiautschovicus and A. uva-ursi. The daily changes in the components of the xanthophyll cycle that paralleled the daily changes in incident light in the leaves of E. kiautschovicus, and the increasing levels of the xanthophyll cycle components with total daily photon receipt in the leaves of A. uva-ursi, are both consistent with the involvement of zeaxanthin (i.e. the xanthophyll cycle) in the photoprotection of the photosynthetic apparatus against damage due to excessive light.Abbreviations A antheraxanthin - EPS epoxidation state of the xanthophyll cycle=(V+0.5A)/(V+A+Z) - PFD photon flux density (400–700 nm) - PFD_i photon flux density incident upon the upper leaf surface - T_air air temperature - T_L leaf temperature - V violaxanthin - Z zeaxanthin     

A Structural Basis for the pH-Dependent Xanthophyll Cycle in Arabidopsis thaliana

Plants adjust their photosynthetic activity to changing light conditions. A central regulation of photosynthesis depends on the xanthophyll cycle, in which the carotenoid violaxanthin is converted into zeaxanthin in strong light, thus activating the dissipation of the excess absorbed energy as heat and the scavenging of reactive oxygen species. Violaxanthin deepoxidase (VDE), the enzyme responsible for zeaxanthin synthesis, is activated by the acidification of the thylakoid lumen when photosynthetic electron transport exceeds the capacity of assimilatory reactions: at neutral pH, VDE is a soluble and inactive enzyme, whereas at acidic pH, it attaches to the thylakoid membrane where it binds its violaxanthin substrate. VDE also uses ascorbate as a cosubstrate with a pH-dependent K_m that may reflect a preference for ascorbic acid. We determined the structures of the central lipocalin domain of VDE (VDE_cd) at acidic and neutral pH. At neutral pH, VDE_cd is monomeric with its active site occluded within a lipocalin barrel. Upon acidification, the barrel opens up and the enzyme appears as a dimer. A channel linking the two active sites of the dimer can harbor the entire carotenoid substrate and thus may permit the parallel deepoxidation of the two violaxanthin β-ionone rings, making VDE an elegant example of the adaptation of an asymmetric enzyme to its symmetric substrate.     

Differential down-regulation of zeaxanthin epoxidation in two rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars with different chilling sensitivities

When the leaf segments of rice (Oryza sativa L.) plants were subjected to chilling in the moderate light, zeaxanthin (Zx) formation was faster in a chilling-tolerant Dongjin-byeo (DJ) than in a chilling-sensitive IR841. Although the rate of Zx formation was accelerated by the treatment of 5 mM salicylaldoxime, an inhibitor of Zx epoxidase (ZE), there was almost no changes in DJ. A similar result was observed when leaf segments were treated with 50 mM sodium fluoride, a potent inhibitor of chloroplast phosphatase. The slow Zx epoxidation in IR841 during light-chilling was confirmed in leaf segments treated with 10 mM dithiothreitol, an inhibitor of violaxanthin de-epoxidase (VDE). However, the differences between the two cultivars were not observed at 25oC. These results suggest that compared with IR841 the higher rate of Zx formation in DJ is not due to the higher VDE activity in DJ but is due to more rapid down-regulation of ZE in DJ, possibly by its phosphorylation. Compared with DJ, IR841 accumulated more superoxide with PSI inactivation during light-chilling, which eliminates the possibility of increased ZE down-regulation in DJ leaves by photo-oxidation. In vitro study with alkaline phosphatase supports the idea of down-regulation of ZE by phosphorylation under light-chilling condition. We propose that this reversible down-regulation of Zx epoxidation possibly by the phosphorylation of ZE is an important regulation mechanism of violaxanthin cycle that confers chilling tolerance of a rice cultivar under chilling stress in the light with moderate intensities.     

Functional roles of the major chloroplast lipids in the violaxanthin cycle

Monogalactosyldiacylglyceride (MGDG) and digalactosyldiacylglyceride (DGDG) are the major membrane lipids of chloroplasts. The question of the specialized functions of these unique lipids has received limited attention. One function is to support violaxanthin de-epoxidase (VDE) activity, an enzyme of the violaxanthin cycle. To understand better the properties of this system, the effects of galactolipids and phosphatidylcholines on VDE activity were examined by two independent methods. The results show that the micelle-forming lipid (MGDG) and bilayer forming lipids (DGDG and phosphatidylcholines) support VDE activity differently. MGDG supported rapid and complete de-epoxidation starting at a threshold lipid concentration (10 μM) coincident with complete solubilization of violaxanthin. In contrast, DGDG supported slow but nevertheless complete to nearly complete de-epoxidation at a lower lipid concentration (6.7 μM) that did not completely solubilize violaxanthin. Phosphotidylcholines showed similar effects as DGDG except that de-epoxidation was incomplete. Since VDE requires solubilized violaxanthin, aggregated violaxanthin in DGDG at low concentration must become solubilized as de-epoxidation proceeds. High lipid concentrations had lower activity possibly due to formation of multilayered structures (liposomes) that restrict accessibility of violaxanthin to VDE. MGDG micelles do not present such restrictions. The results indicate VDE operates throughout the lipid phase of the single bilayer thylakoid membrane and is not limited to putative MGDG micelle domains. Additionally, the results also explain the differential partitioning of violaxanthin between the envelope and thylakoid as due to the relative solubilities of violaxanthin and zeaxanthin in MGDG, DGDG and phospholipids. The violaxanthin cycle is hypothesized to be a linked system of the thylakoid and envelope for signal transduction of light stress.     

Violaxanthin Cycle Pigment Contents in Potato and Tobacco Plants with Genetically Reduced Photosynthetic Capacity

The influence of photosynthetic activity on the light-dependent adaptation of the pool size of the violaxanthin cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) was studied in leaves of wild-type and transgenic potato (Solanum tuberosum L.) and tobacco (Nicotiana tabacum L.) plants. The genetically manipulated plants expressed an antisense mRNA coding for the chloroplastic fructose-bisphosphatase. Chl fluorescence quenching analysis revealed that the transformed plants exhibited a greatly impaired electron transport capacity. Light-limited and light-saturated non-photochemical quenching was strongly enhanced in the mRNA antisense potato plants. After 7 d of adaptation at various high photosynthetic photon flux densities (PPFDs), the violaxanthin cycle pool size increased, with a progressive elevation in PPFD. The pool size was higher for transgenic potatoes than for wild-type plants at all PPFDs. This difference vanished when pool size was correlated with the PPFD in excess of photosynthesis, as indicated by the epoxidation state of the violaxanthin cycle. Contrasting results were obtained for tobacco; in this species, photosynthetic activity did not affect the pool size. We conclude that regulatory mechanisms exist in potato, by which photosynthetic activity can influence the violaxanthin cycle pool size. Furthermore, evidence is provided that this adaptation of the pool size may contribute to an improved photoprotection of the photosynthetic apparatus under high-light conditions. However, tobacco plants seem to regulate their pool size independently of photosynthetic activity.     

Phenolic compounds and carotenoids during acclimation of spring barley and its mutant <Emphasis Type="Italic">Chlorina f2</Emphasis> from high to low irradiance

We examined the dynamics of phenolic compounds (PheCs) and carotenoids (Cars) in the leaves of wild type (WT) spring barley (Hordeum vulgare L.) and its mutant lacking chlorophyll b Chlorina f2 (Clo f2) grown from seeds at high irradiance (8 d at 1 000 μmol m^-2 s^-1; HI) during 9 d of acclimation to low irradiance (50 μmol m^-2 s^-1; LI). Our results show that a leaf epidermal flavonoid UV-shielding index remained rather constant after transfer of plants from HIto LIconditions and that it was significantly lower in Clo f2 compared to WTplants. This suggests that HIpretreated plants can be well protected against excessive UVfor at least 9 d, as supported also by the constant absorbance of leaf PheCs extracts in the UV-A region (at 335 nm). In contrast, absorbance in the UV-B region (at 270 nm) was reduced, particularly during the initial days of LItreatment, indicating specific changes in PheC profile. High-performance liquid chromatography of soluble PheCs revealed stable content of the major PheC saponarin during LIacclimation, whereas luteolin and feruloylquinic acid content decreased, particularly in WTplants. We also observed a pronounced decrease in Car relative content, particularly a reduction in the xanthophyll cycle pigments (violaxanthin, antheraxanthin, and zeaxanthin, VAZ) pool and diminution of their de-epoxidation state (DEPS) in dark-adapted leaves. As both VAZ and DEPS were higher in HI-acclimated Clo f2 plants than they were in WTplants, the presence of a significant VAZ pool within the lipid phase of thylakoid membrane is indicated. That can contribute to antioxidant capacity particularly in Clo f2 plants. We can therefore conclude that there is a tendency to retain the PheCs responsible for UVshielding during LIacclimation. Meanwhile, the accumulation of both PheCs and zeaxanthin serving as effective antioxidants is considerably downregulated within 9 d.     

Seasonal changes in xanthophyll composition and photosynthesis of cork oak (Quercus suber L.) leaves under mediterranean climate

Seasonal changes in pigment composition of sun and shade leavesof cork oak (Quercus suber) were studied under field conditionsin Portugal. Expanding leaves showed a high concentration ofxanthophyll cycle components, violaxanthin, antheraxanthin andzeaxanthin. The pool of violaxanthin plus antheraxanthin pluszeaxanthin (V+A+Z) varied greatly between the seasons, beinghigher at the end of summer and in winter when photosynthesiswas limited by water stress and cold, respectively. The sizeof V+A+Z pool was associated to synthesis of zeaxanthin in responseto an excess of light. In sun leaves, midday A+Z relative contentwas positively correlated with the V+A+Z pool, whereas in shadeleaves A+Z decreased with leaf ageing. In both leaf types A+Zwas positively correlated with the non-photochemical quenching(NPQ) of chlorophyll a fluorescence. However, in winter NPQdid not change significantly throughout the day, whereas the(A+Z)/(V+A+Z) increased following the typical daily trend observedin other seasons. Key words: Chlorophyll fluorescence, pigments, Quercus suber, thermal dissipation, xanthophylls