Carotenoid composition in sun and shade leaves of plants with different life forms

The carotenoid composition of sun leaves of nine species of annual crop plants (some with several varieties) was compared with sun and shade leaves of several other groups of plants, among those sun and shade leaves of several species of perennial shrubs and vines and deep-shade leaves of seven rainforest species. All sun leaves contained considerably greater amounts of the components of the xanthophyll cycle violaxanthin, antheraxanthin and zeaxanthin as well as of β-carotene than the shade leaves, as had previously been reported for a variety of other species by Thayer & Björkman (Photosynthesis Research, 1990, 23, 331–343). Therefore, high light specifically stimulated β,β-carotenoid synthesis. The sun leaves of these crop species did not contain α-carotene which was, however, present in large amounts in all shade leaves and in smaller amounts in sun leaves of three of the four species of perennial shrubs and vines. There was no difference in neoxanthin content on a chlorophyll basis between sun and shade leaves, and there was no consistent general difference in the lutein content between all sun and all shade leaves. The zeaxanthin (and antheraxanthin) content at peak irradiance and the xanthophyll cycle pool size were compared for sun leaves from the different groups of plants with different life forms and different metabolic activities. When growing in full sunlight the annual crop species and a perennial mesophyte had high rates of photosynthesis whereas the perennial shrubs and vines had relatively low photosynthesis rates. More zeaxanthin (and antheraxanthin) were accumulated at noon in full sunlight in those species with the lower photosynthesis rates. However, it was not such that those species also possessed the larger pools of violaxanthin + antheraxanthin + zeaxanthin. Instead, the xanthophyll cycle pools of sun leaves of the annual crop species and the perennial mesophyte were not smaller, and were even possibly larger, than those of sun leaves of the perennial shrubs and vines with low photosynthesis rates. This was so in spite of the fact that the crop species experienced much lesser degrees of excessive light at full sun than the shrubs and vines. Thus, many of the crop species converted only about 30–50% of their xanthophyll cycle pool to zeaxanthin at noon, whereas the shrubs and vines typically converted more than 80% of their pool into zeaxanthin. The crop species also had larger pools of β-carotene than the shrubs and vines but smaller pools of lutein than the majority of the latter species.     

Differences in the capacity for radiationless energy dissipation in the photochemical apparatus of green and blue-green algal lichens associated with differences in carotenoid composition

Green algal lichens, which were able to form zeaxanthin rapidly via the de-epoxidation of violaxanthin, exhibited a high capacity to dissipate excess excitation energy nonradiatively in the antenna chlorophyll as indicated by the development of strong nonphotochemical quenching of chlorophyll fluorescence (F_M, the maximum yield of fluorescence induced by pulses of saturating light) and, to a lesser extent, F_O (the yield of instantaneous fluorescence). Blue-green algal lichens which did not contain any zeaxanthin were incapable of such radiationless energy dissipation and were unable to maintain the acceptor of photosystem II in a low reduction state upon exposure to excessive photon flux densities (PFD). Furthermore, following treatment of the thalli with an inhibitor of the violaxanthin de-epoxidase, dithiothreitol, the response of green algal lichens to light became very similar to that of the blue-green algal lichens. Conversely, blue-green algal lichens which had accumulated some zeaxanthin following long-term exposure to higher PFDs exhibited a response to light which was intermediate between that of zeaxanthin-free blue-green algal lichens and zeaxanthin-containing green algal lichens. Zeaxanthin can apparently be formed in blue-green algal lichens (which lack the xanthophyll epoxides, i.e. violaxanthin and antheraxanthin) as part of the normal biosynthetic pathway which leads to a variety of oxygenated derivatives of β-carotene during exposure to high light over several days. We conclude that the pronounced difference in the capacity for photoprotective energy dissipation in the antenna chlorophyll between (zeaxanthin-containing0 green algal lichens and (zeaxanthin-free) blue-green algal lichens is related to the presence or absence of zeaxanthin, and that this difference can explain the greater susceptibility to high-light stress in lichens with blue-green phycobionts.     

Xanthophyll cycle components and capacity for non-radiative energy dissipation in sun and shade leaves ofLigustrum ovalifolium exposed to conditions limiting photosynthesis

The relationships between photosynthetic efficiency, non-radiative energy dissipation and carotenoid composition were studied in leaves ofLigustrum ovalifolium developed either under full sunlight or in the shade. Sun leaves contained a much greater pool of xanthophyll cycle components than shade leaves. The rate of non-radiative energy dissipation, measured as non-photochemical fluorescence quenching (NPQ), was strictly related to the deepoxidation state (DPS) of xanthophyll cycle components in both sun and shade leaves, indicating that zeaxanthin (Z) and antheraxanthin (A) are involved in the development of NPQ. Under extreme conditions of excessive energy, sun leaves showed higher maximum DPS than shade leaves. Therefore, sun leaves contained not only a greater pool of xanthophyll cycle components but also a higher proportion of violaxanthin (V) actually photoconvertible to A and Z, compared to shade leaves. Both these effects contributed to the higher NPQ in sun versus shade leaves. The amount of photoconvertible V was strongly related to chla/b ratio and inversely to leaf neoxanthin content. This evidence indicates that the amount of photoconvertible V may be dependent on the degree of thylakoid membrane appression and on the organization of chlorophyll-protein complexes, and possible explanations are discussed. Exposure to chilling temperatures caused a strong decline in the photon yield of photosynthesis and in the intrinsic efficiency of PS II photochemistry in sun leaves, but little effects in shade leaves. These effects were accompanied by increases in the pool of xanthophyll cycle components and in DPS, more pronounced in sun than in shade leaves. This corroborates the view that Z and A may play a photoprotective role under unfavorable conditions. In addition to the xanthophyll-related non-radiative energy dissipation, a slow relaxing component of NPQ, independent from A and Z concentrations, has been found in leaves exposed to low temperature and high light. This quenching component may be attributed either to other regulatory mechanism of PS II efficiency or to photoinactivation.Research supported by National Research Council of Italy, Special Project RAISA, Sub-Project 2, Paper N. 1587.     

Positive correlation between levels of retained zeaxanthin + antheraxanthin and degree of photoinhibition in shade leaves of Schefflera arboricola (Hayata) Merrill

Attached intact leaves of Schefflera arboricola grown at three different photon flux densities (PFDs) were subjected to 24-h exposures to a high PFD and subsequent recovery at a low PFD. While sun leaves showed virtually no sustained effects on photosystem II (PSII), shade-grown leaves exhibited pronounced photoinhibition of PSII that required several days at low PFD to recover. Upon transfer to high PFD, levels of nonphotochemical quenching in PSII as well as levels of zeaxanthin were initially low in shade leaves but continued to increase gradually during the 24-h exposure. The xanthophyll cycle pool size rose gradually during and also subsequent to the photoinhibitory treatment in shade leaves. Upon return to low PFD, a marked and extremely long-lasting retention of zeaxanthin and antheraxanthin was observed in shade but not sun leaves. During recovery, changes in the conversion state of the xanthophyll cycle therefore closely mirrored the slow increases in PSII efficiency. This novel report of a close association between zeaxanthin retention and lasting PSII depressions in these shade leaves clearly suggests a role for zeaxanthin in photoinhibition of shade leaves. In addition, there was a decrease in β-carotene levels, some decrease in chlorophyll, but no change in lutein and neoxanthin (all per leaf area) in the shade leaves during and subsequent to the photoinhibitory treatment. These data may be consistent with a degradation of a portion of core complexes but not of peripheral light-harvesting complexes. A possible conversion of β-carotene to form additional zeaxanthin is discussed. Received: 24 October 1997 / Accepted: 12 November 1997     

Zeaxanthin and the Heat Dissipation of Excess Light Energy in Nerium oleander Exposed to a Combination of High Light and Water Stress

Upon termination of watering of plants of Nerium oleander exposed to high light, photochemical efficiency became reduced as leaf water content decreased. Evidence is presented that this type of photoinhibition reflects to a substantial degree radiationless dissipation of excitation energy, probably mediated by the carotenoid zeaxanthin. During the imposition of water stress, the zeaxanthin content of leaves increased at the expense of violaxanthin and β-carotene as a water deficit developed over a period of several days. The increase in zeaxanthin content was linearly related to an increase in the rate of radiationless energy dissipation in the antenna chlorophyll as calculated from the characteristics of chlorophyll a fluorescence measured with a pulse amplitude modulated fluorometer at room temperature. The increase in the rate of radiationless dissipation was also linearly related to a decrease in PSII photochemical efficiency as indicated by the ratio of variable to maximum fluorescence. Leaves of well-watered shade plants of N. oleander exposed to strong light showed a similar increase in zeaxanthin content as sun leaves of the same species subjected to drought in strong light. Shade leaves possessed the same capacity as sun leaves to form zeaxanthin at the expense of both violaxanthin and β-carotene. The resistance of this species to the destructive effects of excess light appears to be related to interconversions between β-carotene and the three carotenoids of the xanthophyll cycle.     

Relationships between carotenoid composition and growth habit in British plant species

The pigment composition of leaves from a number of different plant species collected from field sites in the region of Sheffield, UK, have been compared using high-performance liquid chromatography. Expression of pigment content per unit leaf area was dominated by variation in the total leaf chlorophyll. Neither chlorophyll per unit area nor the chlorophyll a/b ratio were found to be correlated with the habitat from which the plants originated. When the amounts of different carotenoids were expressed relative to the total carotenoid pool, it was found that whilst neither total carotene (α- +β-carotene) nor neoxanthin correlated with ability to grow in shade, the leaf content of both lutein and the total xanthophyll cycle carotenoids (zeaxanthin, anther-axanthin and violaxanthin) did, with lutein content being high in shade species and xanthophyll cycle intermediates low. There was a strong negative correlation between the relative amounts of each of these groups of carotenoids. The ratio of lutein to xanthophyll cycle carotenoids was strongly correlated to an index of shade tolerance.     

The carotenoid zeaxanthin and ‘high-energy-state quenching’ of chlorophyll fluorescence

The possibility that zeaxanthin mediates the dissipation of an excess of excitation energy in the antenna chlorophyll of the photochemical apparatus has been tested through the use of an inhibitor of violaxanthin de-epoxidation, dithiothreitol (DTT), as well as through the comparison of two closely related organisms (green and blue-green algal lichens), one of which (blue-green algal lichen) naturally lacks the xanthophyll cycle. In spinach leaves, DTT inhibited a major component of the rapidly relaxing high-energy-state quenching' of chlorophyll fluorescence, which was associated with a quenching of the level of initial fluorescence (F₀) and exhibited a close correlation with the zeaxanthin content of leaves when fluorescence quenching was expressed as the rate constant for radiationless energy dissipation in the antenna chlorophyll. Green algal lichens, which possess the xanthophyll cycle, exhibited the same type of fluorescence quenching as that observed in leaves. Two groups of blue-green algal lichens were used for a comparison with these green algal lichens. A group of zeaxanthin-free blue-green algal lichens did not exhibit the type of chlorophyll fluorescence quenching indicative of energy dissipation in the pigment bed. In contrast, a group of blue-green algal lichens which had formed zeaxanthin slowly through reactions other than the xanthophyll cycle, did show a very similar response to that of leaves and green algal lichens. Fluorescence quenching indicative of radiationless energy dissipation in the antenna chlorophyll was the predominant component of high-energy-state quenching in spinach leaves under conditions allowing for high rates of steady-state photosynthesis. A second, but distinctly different type of high-energy-state quenching of chlorophyll fluorescence, which was not inhibited by DTT (i.e., it was zeaxanthin independent) and which is possibly associated with the photosystem II reaction center, occurred in addition to that associated with zeaxanthin in leaves under a range of conditions which were less favorable for linear photosynthetic electron flow. In intact chloroplasts isolated from (zeaxanthin-free) spinach leaves a combination of these two types of rapidly reversible fluorescence quenching occurred under all conditions examined.Abbreviations DTT dithiothreitol - F₀ (or F₀) yield of instantaneous fluorescence at open PS II reaction centers in the dark (or during actinic illumination) - F_M (or F_M) yield of maximum fluorescence induced by a saturation pulse of light in the dark (or during actinic illumination) - F_V (or F_V) yield of variable fluorescence induced by a saturating pulse of light in the dark (or during actinic illumination) - k _D rate constant for radiationless energy dissipation in the antenna chlorophyll - SV Stern-Volmer equation - PFD photon flux density - PS I photosystem I - PS II photosystem II - Q_A acceptor of photosystem II - q_N coefficient of nonphotochemical chlorophyll fluorescence quenching - q_P coefficient of photochemical chlorophyll fluorescence quenching     

Pigment apparatus in <Emphasis Type="Italic">Ajuga reptans</Emphasis> plants as affected by adaptation to light growth conditions

Mechanisms of adaptation of the photosynthetic apparatus at the level of pigment complex in a shade-tolerant bugle plant (Ajuga reptans L.) grown at full solar irradiation in an open plot were studied. In “sun” plants, the content of photosynthetic pigments decreased markedly as compared to “shade” plants grown under a forest canopy at 5–10% of the full solar irradiation. In leaves of sun plants, the portion of β-carotene and lutein in the carotenoid spectrum was higher than in shade plant leaves, antheraxanthin and zeaxanthin were present, and de-epoxidation of violaxanthin was by an order of magnitude higher in sun plant leaves reaching 40%. The data obtained indicate the role of the violaxanthin cycle in the protection of photosynthetic apparatus in a shade-tolerant plant against destruction under excessive irradiation.     

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Summary Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO₂ partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO₂ partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.     

Characterization of pigment apparatus in winter-green and summer-green leaves of a shade-tolerant plant <Emphasis Type="Italic">Ajuga reptans</Emphasis>

Comparative study was performed to assess the content and proportions of photosynthetic pigments and the violaxanthin cycle (VXC) activity in winter-green and summer-green leaves of bugleweed (Ajuga reptans L.) plants grown in shaded (photosynthetically active radiation, PAR 150 μmol/(m² s)) and sunny (PAR 1200 μmol/(m² s)) habitats in the Botanic Garden of Jagiellonian University (Krakow, Poland). In overwintered and newly formed leaves of shade plants, the content of green and yellow pigments was two times higher than in leaves of sun plants. The shade plants were distinguished by accumulation of β-carotene, while lutein was predominant in leaves of sun plants. Under the action of strong light (2000 μmol/(m²s)), the level of violaxanthin deepoxidation in winter-green leaves of shade and sun plants increased five- to sixfold, whereas it changed insignificantly in summer-green leaves of shade plants. It is concluded that, in a shadetolerant species A. reptans, the photosynthetic apparatus of winter-green leaves in sun and shade plants and of summer-green leaves in sun plants is protected against excess insolation by high activity of VXC. The carotenoids of summer-green leaves in shade plants are supposed to function mainly as light-harvesting pigments.     

Zeaxanthin and non‐photochemical quenching in sun and shade leaves of C3 and C4 plants

The relationships between non‐radiative energy dissipation and the carotenoid content, especially the xanthophyll cycle components, were studied in sun and shade leaves of several plants possessing C₃ ( Hedera helix and Laurus nobilis ) or C₄ ( Zea mays and Sorghum bicolor ) photosynthetic pathways. Sun‐shade acclimation caused marked changes in the organisation and function of photosynthetic apparatus, including significant variation in carotenoid content and composition. The contents of zanthophyll cycle pigments were higher in sun than in shade leaves in all species, but this difference was considerably greater in C₃ than in C₄ plants. The proportion of photoconvertible violaxanthin, that is the amount of violaxanthin (V) which can actually be de‐epoxidised to zeaxanthin, was much greater in sun than in shade leaves. The amount of photoconvertible V was always linearly dependent on the chlorophyll a/b ratio, although the slope of the relationship varied especially between C₃ and C₄ species. The leaf zeaxanthin and antheraxanthin contents were correlated with non‐radiative energy dissipation in all species under different light environments. These relationships were curvilinear and variable between sun and shade leaves and between C₃ and C₄ species. Hence, the dissipation of excess energy does not appear to be univocally dependent on zeaxanthin content and other photoprotective mechanisms may be involved under high irradiance stress. Such mechanisms appear largely variable between C₃ and C₄ species according to their photosynthetic characteristics.     

Leaf Xanthophyll content and composition in sun and shade determined by HPLC

As a part of our investigations to test the hypothesis that zeaxanthin formed by reversible de-epoxidation of violaxanthin serves to dissipate any excessive and potentially harmful excitation energy we determined the influence of light climate on the size of the xanthophyll cycle pool (violaxanthin + antheraxanthin + zeaxanthin) in leaves of a number of species of higher plants. The maximum amount of zeaxanthin that can be formed by de-epoxidation of violaxanthin and antheraxanthin is determined by the pool size of the xanthophyll cycle. To quantitate the individual leaf carotenoids a rapid, sensitive and accurate HPLC method was developed using a non-endcapped Zorbax ODS column, giving baseline separation of lutein and zeaxanthin as well as of other carotenoids and Chl a and b.The size of the xanthophyll cycle pool, both on a basis of light-intercepting leaf area and of light-harvesting chlorophyll, was ca. four times greater in sun-grown leaves of a group of ten sun tolerant species than in shade-grown leaves in a group of nine shade tolerant species. In contrast there were no marked or consistent differences between the two groups in the content of the other major leaf xanthophylls, lutein and neoxanthin. Also, in each of four species examined the xanthophyll pool size increased with an increase in the amount of light available during leaf development whereas there was little change in the content of the other xanthophylls. However, the -carotene/-carotene ratio decreased and little or no -carotene was detected in sun-grown leaves. Among shade-grown leaves the -carotene/-carotene ratio was considerably higher in species deemed to be umbrophilic than in species deemed to be heliophilic.The percentage of the xanthophyll cycle pool present as violaxanthin (di-epoxy-zeaxanthin) at solar noon was 96–100% for shade-grown plants and 4–53% for sun-grown plants with zeaxanthin accounting for most of the balance. The percentage of zeaxanthin in leaves exposed to midday solar radiation was higher in those with low than in those with high photosynthetic capacity.The results are consistent with the hypothesis that the xanthophyll cycle is involved in the regulation of energy dissipation in the pigment bed, thereby preventing a buildup of excessive excitation energy at the reaction centers.Abbreviations A antheraxanthin - C -carotene - C -carotene - EPS epoxidation state (V+0.5A)/(V+A+Z) - L lutein - N neoxanthin - PFD photon flux density - V violaxanthin - Z zeaxanthin C.I.W.-D.P.B. Publiation No. 1035     

Photosynthesis and Photoprotection in Overwintering Plants

Abstract: Seasonal differences in the capacity of photosynthetic electron transport, leaf pigment composition, xanthophyll cycle characteristics and chlorophyll fluorescence emission were investigated in two biennial mesophytes ( Malva neglecta and Verbascum thapsus ) that grow in full sunlight, and in leaves/needles of sun and shade populations of several broad-leafed evergreens and conifers (Vinca minor, Euonymus kiautschovicus, Mahonia repens, Pseudotsuga menziesii [Douglas fir], and Pinus ponderosa). Both mesophytic species maintained or upregulated photosynthetic capacity in the winter and exhibited no upregulation of photoprotection. In contrast, photosynthetic capacity was downregulated in sun leaves/needles of V. minor, Douglas fir, and Ponderosa pine, and even in shade needles of Douglas fir. Interestingly, photosynthetic capacity was upregulated during the winter in shade leaves/needles of V. minor, Ponderosa pine and Euonymus kiautschovicus. Nocturnal retention of zeaxanthin and antheraxanthin, and their sustained engagement in a state primed for energy dissipation, were observed largely in the leaves/needles of sun-exposed evergreen species during winter. Factors that may contribute to these differing responses to winter stress, including chloroplast redox state, the relative levels of source and sink activity at the whole plant level, and apoplastic versus symplastic phloem loading, are discussed.     

Survey of Thermal Energy Dissipation and Pigment Composition in Sun and Shade Leaves

A survey was conducted of the magnitude of energy dissipationin photosystem II (expressed as nonphotochemi-cal quenchingof chlorophyll fluorescence, NPQ) as well as leaf carotenoidcomposition of a wide range of different plant species growingin deep shade and/or full sun. Consistently higher levels ofthe reversible component of NPQ as well as higher degrees ofrapidly attainable de-epoxida-tion of the xanthophyll cycle(VAZ) pool were observed in sun leaves compared to deep shadeleaves. It is concluded that these altered features of the xanthophyllcycle allowed sun leaves to dissipate excess energy more effectivelyover the short term. In addition to the rapid increase in reversibleNPQ, shade leaves exhibited a slow further, and sustained, increasein NPQ. In contrast to these deep shade leaves experimentallyexposed to high PFDs, understory leaves experiencing highlyvariable PFD in their natural environment appeared to be ableto dissipate excess excitation energy adequately via xanthophyllcycle-dependent thermal dissipation. Furthermore, very consistenttrends across plant species were observed for changes in carotenoidcomposition (pools of carotenes, VAZ, and other xantho-phylls)in response to light environment, as long as it is assumed thatin some species rß-carotene can be replaced by     

Antioxidants and xanthophyll cycle-dependent energy dissipation in Cucurbita pepo L. and Vinca major L. acclimated to four growth PPFDs in the field

The acclimation of photochemistry, xanthophyll cycle-dependent energy dissipation, and antioxidants was characterized in leaves of Cucurbita pepo L. and Vinca major L. that developed under photosynthetic photon flux densities (PPFDs) ranging from deep shade to full sunlight in the field. The predominant acclimatory response of leaf pigment composition was an increase in the xanthophyll cycle pool size with increasing growth PPFD. In both species, the estimated rate of thermal energy dissipation at midday increased with increasing PPFD and midday levels of zeaxanthin and antheraxanthin per chlorophyll were closely correlated with the levels of non-photochemical fluorescence quenching under all growth PPFD regimes. However, at full sunlight there appeared to be considerably higher levels of xanthophyll cycle dependent energy dissipation in V. major compared with pumpkin while estimated rates of photochemistry exhibited the reverse trend. Leaf activities of the antioxidant enzymes ascorbate peroxidase and superoxide dismutase, as well as ascorbate content, increased with increasing growth PPFD in both plant species. Activities/contents were higher under 100% full sunlight and increased more strongly from intermediate growth PPFDs to 100% full sunlight in V. major than in C. pepo. These patterns of acclimation are similar to those exhibited by xanthophyll cycle-dependent energy dissipation. The patterns of acclimation of glutathione reductase are discussed in the context of the multiple roles for reduced glutathione. Catalase acclimated in a manner consistent with its role in scavenging H2O2 generated via photorespiration and/or mitochondrial respiration. Leaf -tocopherol did not exhibit growth PPFD-dependent trends.     

The P450-type carotene hydroxylase PuCHY1 from Porphyra suggests the evolution of carotenoid metabolism in red algae

Carotene hydroxylases catalyze the hydroxylation of a-and b-carotene hydrocarbons into xanthophylls. In red algae, b-carotene is a ubiquitously distributed carotenoid, and hydroxylated carotenoids such...     

Modulation of PsbS and flexible vs sustained energy dissipation by light environment in different species

Contrasting acclimation strategies of photosynthesis and photoprotection were identified for annual mesophytes (spinach, pumpkin, and Arabidopsis ) vs the tropical evergreen Monstera deliciosa . The annual species utilized full sunlight for photosynthesis to a much greater extent than the evergreen species. Conversely, the evergreen species exhibited a greater capacity for photoprotective thermal energy dissipation as well as a greater expression of the PsbS protein in full sun than the annual species. In all species, the majority of thermal energy dissipation [assessed as non-photochemical fluorescence quenching (NPQ)] was the flexible, ΔpH-dependent form of NPQ over the entire range of growth light environments. However, in response to a transfer of shade-grown plants to high light, the evergreen species exhibited a high level of sustained thermal dissipation (qI), but the annual species did not. This sustained energy dissipation in the evergreen species was not ΔpH-dependent nor did the low level of PsbS in shade leaves increase upon transfer to high light for several days. Sustained ΔpH-independent NPQ was correlated (a) initially, with sustained D1 protein phosphorylation and xanthophyll cycle arrest and (b) subsequently, with an accumulation over several days of PsbS-related one-helix proteins and newly synthesized zeaxanthin and lutein.     

Differences in the susceptibility to light stress in two lichens forming a phycosymbiodeme,one partner possessing and one lacking the xanthophyll cycle

Summary The effect of high light levels on the two partners of a Pseudocyphellaria phycosymbiodeme (Pseudocyphellaria rufovirescens, with a green phycobiont, and P. murrayi with a blue-green phycobiont), which naturally occurs in deep shade, was examined and found to differ between the partners. Green algae can rapidly accumulate zeaxanthin, which we suggest is involved in photoprotection, through the xanthophyll cycle. Blue-green algae lack this cycle, and P. murrayi did not contain or form any zeaxanthin under our experimental conditions. Upon illumination, the thallus lobes with green algae exhibited strong nonphotochemical fluorescence quenching indicative of the radiationless dissipation of excess excitation energy, whereas thallus lobes with blue-green algae did not possess this capacity. The reduction state of photosystem II was higher by approximately 30% at each PFD beyond the light-limiting range in the blue-green algal partner compared with the green algal partner. Furthermore, a 2-h exposure to high light levels resulted in large reductions in the efficiency of photosynthetic energy conversion which were rapidly reversible in the lichen with green algae, but were long-lasting in the lichen with blue-green algae. Changes in fluorescence characteristics indicated that the cause of the depression in photosynthetic energy conversion was a reversible increase in radiationless dissipation in the green algal partner and photoinhibitory damage in the blue-green algal partner. These findings represent further evidence that zeaxanthin is involved in the photoprotective dissipation of excessive excitation energy in photosynthetic membranes. The difference in the capacity for rapid zeaxanthin formation between the two partners of the Pseudocyphellaria phycosymbiodeme may be important in the habitat selection of the two species when living separate from one another.Abbreviations F _O yield of instantaneous fluorescence - F _M maximum yield of fluorescence induced by pulses of saturating light - F _V yield of variable fluorescence (F _M -F_O) induced by pulses of saturating light - PFD photon flux density (400–700 nm) - PS II photosystem II - q _NP coefficient for nonphotochemical fluorescence quenching - q _P (or 1-q _P ) coefficient for photochemical fluorescence quenching     

Photoprotection mechanisms in European beech (Fagus sylvatica L.) seedlings from diverse climatic origins

This study shows that beech leaves adapt to their light environment by inducing dramatic changes to antioxidant systems and pigment composition. Thus, ascorbate, tocopherol, glutathione, β-carotene and xanthophyll cycle pigments are much more concentrated in sun leaves, while α-carotene is much less concentrated than in shade leaves. These characteristics were used to identify the inherent potential of beech cotyledons from three contrasting climatic origins to tolerate light stress. The antioxidant content was initially different in the three provenances tested, but these initial differences tended to reduce with leaf ageing. The higher antioxidant and de-epoxidized xanthophyll content found in developing cotyledons indicated a superior potential for tolerance to photo-oxidative damage in those plants collected from the stressful climate of the Pyrenees. Nevertheless under an experimental high irradiation treatment no differences in light stress tolerance were observed between provenances. Received: 31 May 1999 / Accepted: 16 November 1999     

Why is it better to produce coffee seedlings in full sunlight than in the shade? A morphophysiological approach

The coffee plant is native to shaded environments and its seedlings are often produced in shaded nurseries. However, some nursery managers, in an effort to improve the acclimation of seedlings to field conditions after transplantation, produce seedlings in full sun exposure. In this study, the morphological and physiological parameters of arabica coffee (Coffea arabica) seedlings produced in full sun (T1) and in shade (T2) were examined. The biomass accumulation and relative growth rate of T1 and T2 seedlings were similar. The T1 seedlings had less biomass allocation to shoots, a lower leaf mass ratio and a lower leaf area ratio; however, they had a greater net assimilation rate (rate of increase in plant mass per unit leaf area), which was associated with a greater net photosynthetic rate. There were no alterations in the concentrations of total chlorophylls or in the chlorophyll a/b ratio when comparing T1 and T2 seedlings. No indications of photoinhibition or photooxidative damage were observed in the T1 plants, which were shown to have a more robust antioxidant system than the T2 plants. Seedlings transferred from shade to full sun (T3) were not capable of utilising the incident extra light to fix CO₂. These seedlings showed a remarkable nocturnal retention of zeaxanthin and a significantly increased deepoxidation state of the xanthophyll cycle, even at predawn, but the activity of antioxidant enzymes was lower than in the T1 and T2 plants. Despite the acclimation capacity of T3 seedlings to the new light environment, they exhibited chronic photoinhibition and considerable photooxidative damage throughout the seven days following the transfer to full sun exposure. We further discuss the practical implications of producing coffee seedlings in full sunlight and under shade.