Increase In Zeaxanthin Research Articles

Photoprotective Role of Photosynthetic and Non-Photosynthetic Pigments in Phillyrea latifolia: Is Their "Antioxidant" Function Prominent in Leaves Exposed to Severe Summer Drought?

Carotenoids and phenylpropanoids play a dual role of limiting and countering photooxidative stress. We hypothesize that their “antioxidant” function is prominent in plants exposed to summer drought, when climatic conditions exacerbate the light stress. To test this, we conducted a field study on Phillyrea latifolia, a Mediterranean evergreen shrub, carrying out daily physiological and biochemical analyses in spring and summer. We also investigated the functional role of the major phenylpropanoids in different leaf tissues. Summer leaves underwent the most severe drought stress concomitantly with a reduction in radiation use efficiency upon being exposed to intense photooxidative stress, particularly during the central hours of the day. In parallel, a significant daily variation in both carotenoids and phenylpropanoids was observed. Our data suggest that the morning-to-midday increase in zeaxanthin derived from the hydroxylation of ß-carotene to sustain non-photochemical quenching and limit lipid peroxidation in thylakoid membranes. We observed substantial spring-to-summer and morning-to-midday increases in quercetin and luteolin derivatives, mostly in the leaf mesophyll. These findings highlight their importance as antioxidants, countering the drought-induced photooxidative stress. We concluded that seasonal and daily changes in photosynthetic and non-photosynthetic pigments may allow P. latifolia leaves to avoid irreversible photodamage and to cope successfully with the Mediterranean harsh climate.

Effect of the application of an enzymatic pretreatment on bioactive compounds ofCaryocar brasilienseCamb pulp oil

This study aimed to evaluate the application of an enzymatic pretreatment in obtaining pequi pulp oil by cold pressing. It was evaluated the yield and quality of the oil obtained mainly in relation to the bioactive compounds. There was an increase of approximately 22% in the process yield compared to the cold pressing without enzymatic pretreatment. Comparing the physicochemical characteristics of the obtained oils, it was observed an increase in the refractive index, acid value, unsaponifiable matter, total phenolics and carotenoid concentrations. The application of the enzymatic pretreatment significantly increased the amount of total carotenoids (about five times) in the oil and modified the composition of the carotenoids present, showing a significant increase in zeaxanthin, a carotenoid with high antioxidant potential. Furthermore, there was an increase in oil unsaturation, especially in the percentage of oleic acid. PRACTICAL APPLICATIONS: Industrial processes for the extraction of edible oil generally involve solvent extraction or mechanical processing. Enzyme‐assisted aqueous extraction is an environmentally cleaner alternative technology for oil extraction. Application of an enzymatic pretreatment in the cold pressing extraction of the pequi pulp oil was efficient presenting an increase in the yield and generating an oil with higher nutritional value, rich in bioactive compounds, which can be used by the food and pharmaceutical industries.

Effects of δ-aminolevulinic acid dehydratase silencing on the primary and secondary metabolisms of citrus.

δ‐aminolevulinic acid dehydratase (ALAD) is an important enzyme in tetrapyrrole synthesis. ALAD combines two δ‐aminolevulinic acid (δ‐ALA) molecules to form the pyrrole molecule, porphobilinogen, an important precursor for plant pigments involved in photosynthesis, respiration, and nutrient uptake. In this study, we investigated the effects of silencing of ALAD gene on citrus leaf pigments and metabolites. The ALAD enzyme was inhibited using virus‐induced gene silencing (VIGS) technology using citrus tristeza virus (CTV). δ‐ALA accumulated in citrus plants inoculated with the recombinant virus (CTV‐tALAD) to silence ALAD and resulted in discrete yellow spots (yellow islands) and necrosis in leaves and stems. The levels of chlorophylls, starch, sucrose, trans‐ and cis‐violaxanthin, and α‐ and β‐cryptoxanthin were reduced in CTV‐tALAD plants, whereas zeaxanthin was increased. The increase in zeaxanthin and the decrease in its precursors indicated that the reduction in chlorophylls resulted in light damage. Salicylic acid and jasmonic acid levels, as well as emission of (E)‐α‐bergamotene and (E)‐β‐farnesene, increased in CTV‐tALAD plants indicating these plants were under stress. Our results showed that silencing of ALAD induces stress in plants and that VIGS using mild CTV strains is a promising technique to study biological function of citrus genes.

Ultraviolet-B protection of ascorbate and tocopherol in plants related with their function on the stability on carotenoid and phenylpropanoid compounds

Ascorbate and tocopherol are important hydrophilic or lipophilic antioxidants in plants, while their crucial roles in the antioxidant defense system under ultraviolet B radiation were not well understood. The mutants of Arabidopsis thaliana deficient in ascorbate (vtc1 and vtc2) or tocopherol (vte1) were used to analyze their physiological, biochemical and metabolic change in responses to Ultraviolet B radiation. Results showed that loss of either ascorbate or tocopherol caused reduction in phenylpropanoid and flavonol glycosides compounds, as well as reduction in superoxide dismutase activity and total cellular antioxidant capacity. This ultimately led to higher oxidative stress as well as lower levels of photosynthetic pigments (carotenoid and chlorophyll) and CO2 assimilation rate in the vtc1, vtc2, and vte1 mutants than the wild type under UV-B radiation, besides unstable early light-induced protein (ELIP1) in those mutants. On the other hand, the loss of tocopherol in vte1 mutants was compensated by the increase of zeaxanthin and anthocyanin contents, which armed vte1 mutants with higher heat dissipation capacity in PS II and higher antioxidative capacity than vtc mutants. Consequently the tolerance to UV-B radiation were much higher in vte1 mutant than in vtc mutants, furthermore, PS II function and light harvesting protein (LHCb1) abundance were reduced only in ascorbate-deficient mutant relative to wild type. Our results suggested that the ascorbate and tocopherol provided not only direct protective function against UV-B radiation but also indirect effects by influencing other protective system, in particular by affecting the stability of various carotenoid and phenylpropanoid compounds.

Composition of inorganic and organic nutrient sources influences phytoplankton community structure in the New River Estuary, North Carolina

The New River Estuary, NC, is a nutrient-sensitive, eutrophic water body that is prone to harmful algal blooms. High annual loading from the watershed of varying nutrient forms, including inorganic phosphorus and inorganic and organic nitrogen, may be linked to the persistence of algal blooms in the estuary. In order to evaluate phytoplankton response to nutrient inputs, a series of in situ nutrient addition experiments were carried out during June 2010 to July 2011 on water from an estuarine site known to support algal blooms. Estuarine water was enriched with nutrients consisting of individual and combined sources of dissolved inorganic nitrogen, orthophosphate, urea, and a natural dissolved organic nitrogen (DON) addition derived from upstream New River water. The combined inorganic N and P addition most frequently stimulated phytoplankton biomass production as total chlorophyll a. The responses of diagnostic (of major algal groups) photopigments were also evaluated. Significant increases in peridinin (dinoflagellates), chlorophyll b (chlorophytes), and myxoxanthophyll (cyanobacteria) were most frequently promoted by additions containing riverine DON. Significant increases in zeaxanthin (cyanobacteria) were more frequently promoted by inorganic nitrogen additions, while increases in fucoxanthin (diatoms) and alloxanthin (cryptophytes) were not promoted consistently by any one nutrient treatment. Evaluating the impact of varying nutrient forms on phytoplankton community dynamics is necessary in order to develop strategies to avoid long-term changes in community structure and larger-scale changes in ecosystem condition.

Shoot tissue pigment levels increase in ‘Florida Broadleaf’ mustard (Brassica juncea L.) microgreens following high light treatment

The xanthophyll cycle pigments [zeaxanthin (ZEA)+antheraxanthin (ANT)+violaxanthin (VIO)] in plants are vital for energy dissipation of excess absorbed light. It is possible to manipulate xanthophyll cycle pigment levels through exposure to light. The objective of this study was to increase ZEA concentrations in mustard (Brassica juncea L. ‘Florida Broadleaf’) microgreens through exposure to high light just prior to tissue harvest. Mustard microgreen plants were cultured on growing pads in a controlled environment under a 14h photoperiod under 275μmolphotonsm−2s−1. Upon emergence of the first true leaf, light treatments of: (1) 275μmolphotonsm−2s−1; and (2) 463μmolphotonsm−2s−1 were initiated. Microgreen plants were harvested after light treatments had accumulated 36h during the photoperiod. Shoot pigments were measured for pigments using HPLC separation. Significant decreases in chlorophyll a (P≤0.001) and b (P≤0.01) concentrations under the 463μmolphotonsm−2s−1 light treatment indicated high light stress had occurred. There were significant decreases in β-carotene (P≤0.01) and neoxanthin (P≤0.001) in the 463μmolphotonsm−2s−1 light treatment; however, lutein concentrations were unchanged. Increases in ZEA (P≤0.001), and ANT (P≤0.01) occurred under the 463μmolphotonsm−2s−1 light treatment. Increasing the ZEA levels in microgreens with a simple change in light management may be an effective way for producers to enhance the nutritional value of this increasingly popular specialty crop.

The xanthophyll cycle and antioxidative defense system are enhanced in the wheat hybrid subjected to high light stress

Although the wheat hybrids have often shown higher grain yields, the physiological basis of the higher yields remains unknown. Previous studies suggest that tolerance to photoinhibition in the hybrid may be one of the physiological bases (Yang et al., 2006, Plant Sci 171:389-97). The objective of this study was to further investigate the possible mechanism responsible for tolerance to photoinhibition in the hybrid. Photosystem II (PSII) photochemistry, the xanthophyll cycle, and antioxidative defense system were compared between the hybrid and its parents subjected to high light stress (1500μmolm(-2)s(-1)). The analyses of oxygen-evolving activity, chlorophyll fluorescence, and protein blotting demonstrated that the higher tolerance in the hybrid than in its parents was associated with its higher tolerance of PSII to photoinhibition. High light induced an increase in non-photochemical quenching, and this increase was greater in the hybrid than in its parents. There were no differences in the pool size of the xanthophyll cycle between the hybrid and its parents. The content of violaxanthin decreased significantly, whereas the content of zeaxanthin+antherxanthin increased considerably during high light treatments. However, the decrease in violaxanthin content and the increase in zeaxanthin+antherxanthin content were greater in the hybrid than in its parents. High light resulted in a significant accumulation of H(2)O(2), O(2)(-) and catalytic Fe, and this accumulation was less in the hybrid than in its parents. High light induced a significant increase in the activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monodehydroascorbate reductase, and these increases were greater in the hybrid than its parents. These results suggest that the higher tolerance to photoinhibition in the hybrid may be associated with its higher capacity for antioxidative defense metabolism and the xanthophyll cycle.

UV-A Mediated Modulation of Photosynthetic Efficiency, Xanthophyll Cycle and Fatty Acid Production of Nannochloropsis

Nannochloropsis, a green microalga, is a source for commercially valuable compounds as extensively described and, in particular, is recognised as a good potential source of eicosapentaenoic acid (20:5ϖ3), an important polyunsaturated fatty acid for human consumption for prevention of several diseases. Climate change might include variation in the ultraviolet (UV) levels as one of the consequences derived from the anthropogenic activity. This paper shows the response of Nannochloropsis cultures exposed for 7 days to UV-A (320-400 nm) added to photosynthetically active radiation (PAR; 400-700 nm). Growth rates and photosynthetic activity were assessed to determine the impact of UV-A increased levels on the cell growth and basic metabolism activity. Xanthophyll pigments (zeaxanthin and violaxanthin), carotenoids (canthaxanthin and β-carotene) and polyunsaturated fatty acids (myristic, palmitic, palmitoleic, arachidonic and eicosapentaenoic acids) were measured for assessing the antioxidant response of the microalgae to added UV-A radiation to PAR. The results show that the modulated use of UV-A radiations can lead to increased growth rates, which are sustained in time by an increased light transduction activity. The expected antioxidant response to the incident UV-A radiation consisted of increases in zeaxanthin and β-carotene contents--synthesis of antioxidant carotenoids-and increases in the saturated fatty acids to polyunsaturated fatty acids ratio. The results suggest that modulated UV-A radiation can be used as a tool to stimulate value molecules accumulation in microalgae through an enhanced both light transduction process and antioxidant response, while sustaining cell growth.

The biosynthetic pathway of carotenoids in the astaxanthin-producing green alga Chlorella zofingiensis

The carotenoid composition of the astaxanthin-producing green alga Chlorella zofingiensis was investigated using high-performance liquid chromatography. Astaxanthin, adonixanthin, and zeaxanthin are the major carotenoids in this alga. The pigment pattern was characterized during the accumulation period, and in response to diphenylamine (DPA), an inhibitor of carotenoid biosynthesis. An increase in zeaxanthin followed by a decrease in xanthophyll was seen after the induction of astaxanthin biosynthesis by glucose. This biphasic kinetics of zeaxanthin was parallel to the marked increase in adonixanthin (from 0 mg g(-1) to 0.21 mg g(-1)) and astaxanthin (from 0.05 mg g(-1) to 0.35 mg g(-1)) and decrease of beta-carotene (from 0.30 mg g(-1) to 0.03 mg g(-1)). More importantly, unlike the Haematococcus alga, in which there was a high beta-carotene accumulation after DPA treatment, C. zofingiensis showed an accumulation of extra zeaxanthin instead of beta-carotene after treatment of the cells with DPA. All these results observed in vivo studies corroborate the observations in vitro studies at the enzyme level that zeaxanthin is a substrate for the carotenoid oxygenase in C. zofingiensis. It is suggested that zeaxanthin might be an important intermediate and not an end product of the biosynthetic pathway of astaxanthin. Therefore, a new pathway for astaxanthin formation by C. zofingiensis, which is different from that of the other astaxanthin-producing microorganisms, is proposed. An understanding of the astaxanthin biosynthetic pathway may yield important information toward the optimization of astaxanthin production, especially for the improvement of astaxanthin through genetic manipulations.

Pigment dynamics during cold-induced photoinhibition of Acacia melanoxylon.

Long-term acclimation of photo- and pigment-chemistry was investigated in a naturally-regenerating stand of Acacia melanoxylon R.Br. ex Ait. A pronounced decrease in photochemical efficiency of A. melanoxylon saplings was observed between autumn and winter in both thinned and unthinned treatments, but the decrease was more severe in the thinned treatment. Associated pigment changes in the unthinned treatment included a decrease in total chlorophyll content and a rise in chlorophyll a : b. Similar acclimation occurred in the thinned treatment with additional increase in zeaxanthin per unit chlorophyll observed. Saplings in the thinned treatment were exposed to lower minimum temperatures, more hours of frost and higher light intensities in the mid- to lower-crown. Growth chamber studies of the short-term acclimation of photo- and pigment-chemistry were conducted in a low / high light and cold / warm temperature factorial experiment. Photochemical efficiency and quantum yield adjusted within one day and then remained constant for 10 d in response to the imposed treatments. Chlorophyll concentration had decreased in all treatments by day 2 in the growth chambers, and subsequently increased in warm, but not in cold, treatments, irrespective of light level by day 10 in the growth chambers. The concentration of lutein-5,6-epoxide decreased in response to the cold-high light treatment and increased in response to other treatments by day 10 in the growth chambers, consistent with a function in sustained photoprotection in leaves of shade-adapted species. Our experiments indicated that A. melanoxylon is susceptible to cold-induced photoinhibition under cool temperatures (2-8°C) and moderate light intensities (450 μmol m-2 s-1).

Interplay between photochemical activities and pigment composition in an outdoor culture of Haematococcus pluvialis during the shift from the green to red stage

The transfer of laboratory cultures of H. pluvialis to high irradiance outdoors caused a substantial decline in the maximum quantum yield of photosystem II (PSII), from 0.65 in the morning to 0.45 at midday, as measured by the ratio of variable to maximum fluorescence yields (Fv/Fm), and a steep rise in non-photochemical quenching (NPQ). Chlorophyll fluorescence induction curves of morning samples showed a clear I-step, reflecting a certain PSII heterogeneity. Single turnover flash measurements on samples taken from the outdoor photobioreactor in the middle of day showed an increase in the reoxidation time constant of the reduced plastoquinone QA−, i.e., the time required for electron transfer from the primary plastoquinone acceptor of PSII QA− to the secondary plastoquinone acceptor QB. Photosynthesis rates were almost constant during the day. Along with the increase in non-photochemical quenching, there was a slight increase in zeaxanthin and antheraxanthin contents and decrease in violaxanthin, showing the presence of an operative xanthophyll cycle in this microalga. A marked increase of secondary carotenoids was found at the end of the first day of exposure to sunlight, mainly astaxanthin monoester, which reached 15.5% of the total carotenoid content. Though cells turned reddish during the second day, the decline in the fluorescence parameter Fv/Fm in the middle of the day was less than during the first day, and there was no further increase in the value for NPQ. Similar behaviour was observed during the third day when the culture was fully red. After four days of exposure to sunlight, the dry weight reached 800 mg L−1 and the concentration of secondary carotenoids (81% astaxanthin monoester) reached 4.4% dry weight.

Revival of a resurrection plant correlates with its antioxidant status.

Myrothamnus flabellifolia, a short woody shrub from southern Africa, can survive severe desiccation of its vegetative organs. We studied mechanisms protecting this plant from oxidative damage during desiccation for 2 weeks, 4 and 8 months, and also during subsequent rehydration. This plant retains high concentrations of chlorophyll during desiccation, and these chlorophyll molecules are probably a source for potentially harmful singlet oxygen production. Desiccation triggered substantial increases in zeaxanthin and redox shifts of the antioxidants glutathione and ascorbate towards their oxidised forms. Simultaneously, the concentrations of violaxanthin, beta-carotene, ascorbate, alpha-tocopherol, and glutathione reductase activity progressively decreased. Antheraxanthin, gamma-tocopherol, lutein, neoxanthin and glucose-6-phosphate dehydrogenase displayed less pronounced changes in response to desiccation. Even after 4 months of desiccation, Myrothamnus flabellifolia recovered rapidly upon rehydration. Re-watering induced formation of ascorbate and glutathione, simultaneous reduction of their oxidised forms, and rapid production of alpha-tocopherol and of various carotenoids. Only after 8 months of desiccation did the antioxidant system of M. flabellifolia break down; 3 weeks after the onset of rehydration, these plants abscised their leaves, but even then they were still able to recover and develop new ones. Ascorbate, beta-carotene and alpha-tocopherol were totally depleted after 8 months of desiccation and did not recover upon rehydration; glutathione was partly maintained, but only in the oxidised form. We present a model demonstrating which parts of antioxidant pathways break down as oxidative stress becomes detrimental and we discuss some potential implications of our results for the genetic modification of crop plants to improve their drought tolerance.

Light perception and the role of the xanthophyll cycle in blue-light-dependent chloroplast movements in lemna trisulca L

In most higher plants, chloroplasts move towards the periclinal cell walls in weak blue light (WBL) to increase light harvesting for photosynthesis, and towards the anticlinal walls as an escape reaction, thus avoiding photo-damage in strong blue light (SBL). The photo- receptor(s) triggering these responses have not yet been identified. In this study, the role of zeaxanthin as a blue-light photoreceptor in chloroplast movements was investigated. Time-lapse 3D confocal imaging in Lemna trisulca showed that individual chloroplasts responded to local illumination when one half of the cell was treated with light of different intensity or spectral quality to that received by the other half, or was maintained in darkness. Thus the complete signal perception, transduction and effector system has a high degree of spatial resolution and is consistent with localization of part of the transduction chain in the chloroplasts. Turnover of xanthophylls was determined using HPLC, and a parallel increase was observed between zeaxanthin and chloroplast movements in SBL. Ascorbate stimulated both a transient increase in zeaxanthin levels and chloroplast movement to profile in physiological darkness. Conversely, dithiothreitol blocked zeaxanthin production and responses to SBL and, to a lesser extent, WBL. Norflurazon preferentially inhibited SBL-dependent chloroplast movements. Increases in zeaxanthin were also observed in strong red light (SRL) when no directional chloroplast movements occurred. Thus it appears that a combination of zeaxanthin and blue light is required to trigger responses. Blue light can cause cis-trans isomerization of xanthophylls, thus photo-isomerization may be a critical link in the signal transduction pathway.

Flavonoid and carotenoid pigments in flower tissue of Sandersonia aurantiaca (Hook.)

Luteolin, cryptoxanthin and zeaxanthin were detected as the major pigments present in mature flowers of Sandersonia aurantiaca (Hook.). Both the flavonoids and carotenoid pigments were examined at four stages of flower development from green flower bud to senescent flower prior to tepal necrosis. The major component of the orange colour of the Sandersonia flowers is due to the presence of the carotenoids zeaxanthin and cryptoxanthin, both predominantly present in the esterified form. The main change during development is the loss of chlorophyll, β-carotene and lutein, and the increase in zeaxanthin and cryptoxanthin as the flower matures from a green bud to an open flower. The major flavonoids present were luteolin and luteolin 7- O-glucoside. Flavonoid concentration decreased over the course of development to a mature flower but there was an increase in content per flower. The flavonoids were present in greater quantities than the carotenoids. However, as they absorb mostly in the UV region, they are not believed to contribute significantly to the observed colour.

Excess light-induced violaxanthin cycle activity in Nannochloropsis gaditana (Eustigmatophyceae): effects of exposure time and temperature

Violaxanthin cycle activity in Nannochloropsis gaditana has been investigated using high-performance liquid chromatography (HPLC) to determine the changes in pigment content on exposure to high irradiance (2000 μmol m−2 s−1). Within the range 20–40°C, the effects of exposure time and temperature on the activity of the cycle have also been ascertained. Violaxanthin cycle activity was clearly shown by changes in pigment content accompanied by a concurrent decrease in the epoxidation state (EPS). From the de-epoxidation time course, the de-epoxidation rate was found to be slower than in other plant systems. In addition, the extent of de-epoxidation was dependent on both exposure time and temperature. De-epoxidation occurred faster and more extensively at 20°C (the temperature at which the experimental cultures were grown) than at 25°C (optimal growth temperature) or 35°C. Thus, after exposure to excess light for 240 min at these temperatures, violaxanthin decreased by about 28%, 6%, and 21 % of the initial values, respectively. At 40°C, up to 78% of the violaxanthin pool was de-epoxidated, which would apparently reflect the de-epoxidation limit for this alga. As a result of increasing both the temperature within the range from 20°C to 35°C and the period of exposure to light, there was a similar increase in both zeaxanthin and antheraxanthin contents, but zeaxanthin formation was greater at the higher temperatures. The increase in zeaxanthin was more notable at 40°C than at any other temperature. It is proposed that these findings could be a result of a two-step process rather than a continuous sequence of violaxanthin de-epoxidation. In cultures exposed to 25°C and 35°C, zeaxanthin epoxidation to form violaxanthin in low light conditions (5–10 μmol m−2 s−1) was faster than in the culture exposed to 20°C, which was consistent with the more extensive de-epoxidation observed at the latter temperature. At 40°C violaxanthin did not recover, and zeaxanthin content increased during the period of recovery under low light.

Oxidative Damage in Pea Plants Exposed to Water Deficit or Paraquat1

Abstract The application of a moderate water deficit (water potential of −1.3 MPa) to pea (Pisum sativum L. cv Lincoln) leaves led to a 75% inhibition of photosynthesis and to increases in zeaxanthin, malondialdehyde, oxidized proteins, and mitochondrial, cytosolic, and chloroplastic superoxide dismutase activities. Severe water deficit (−1.9 MPa) almost completely inhibited photosynthesis, decreased chlorophylls, β-carotene, neoxanthin, and lutein, and caused further conversion of violaxanthin to zeaxanthin, suggesting damage to the photosynthetic apparatus. There were consistent decreases in antioxidants and pyridine nucleotides, and accumulation of catalytic Fe, malondialdehyde, and oxidized proteins. Paraquat (PQ) treatment led to similar major decreases in photosynthesis, water content, proteins, and most antioxidants, and induced the accumulation of zeaxanthin and damaged proteins. PQ decreased markedly ascorbate, NADPH, ascorbate peroxidase, and chloroplastic Fe-superoxide dismutase activity, and caused major increases in oxidized glutathione, NAD+, NADH, and catalytic Fe. It is concluded that, in cv Lincoln, the increase in catalytic Fe and the lowering of antioxidant protection may be involved in the oxidative damage caused by severe water deficit and PQ, but not necessarily in the incipient stress induced by moderate water deficit. Results also indicate that the tolerance to water deficit in terms of oxidative damage largely depends on the legume cultivar.

Accumulation of Zeaxanthin in Abscisic Acid-Deficient Mutants of Arabidopsis Does Not Affect Chlorophyll Fluorescence Quenching or Sensitivity to Photoinhibition in Vivo.

Abscisic acid (ABA)-deficient mutants of Arabidopsis do not synthesize the epoxy-xanthophylls antheraxanthin, violaxanthin, or neoxanthin. However, thylakoid membranes from these mutants contain 3-fold more zeaxanthin than wild-type plants. This increase in zeaxanthin occurs as a stoichiometric replacement of the missing violaxanthin and neoxanthin within the pigment-protein complexes of both photosystem I and photosystem II (PSII). The retention of zeaxanthin in the dark by ABA-deficient mutants sensitizes the leaves to the development of nonphotochemical quenching (NPQ) during the first 2 to 4 min following a dark-light transition. However, the increase in pool size does not result in any increase in steady-state NPQ. When we exposed wild-type and ABA-deficient mutants leaves to twice growth irradiance, the mutants developed lower maximal NPQ but suffered similar photoinhibition to wildtype, measured both as a decline in the ratio of variable to maximal fluorescence and as a loss of functional PSII centers from oxygen flash yield measurements. These results suggest that only a few of the zeaxanthin molecules present within the light-harvesting antenna of PSII may be involved in NPQ and neither the accumulation of a large pool of zeaxanthin within the antenna of PSII nor an increase in conversion of violaxanthin to zeaxanthin will necessarily enhance photoprotective energy dissipation.

Carotenoid composition and its relationship to chlorophyll and D1 protein during leaf development in a normally senescing cultivar and a stay-green mutant of Festuca pratensis

Chlorophyll and carotenoid were measured with reference to the developmental gradient along the expanding fourth leaf of Rossa, a normally yellowing cultivar of Festuca pratensis, and of a mutant genotype Bf993 which is unable to degrade pigments and associated thylakoid proteins during foliar senescence. Although pigment levels were highest in the central, mature portion of the leaf, there were nevertheless readily detectable amounts of chlorophyll and carotenoid even in basal tissues enclosed in the sheaths of older leaves. Carotenoid compositions were the same in all the tissues except the senescent tissue at the leaf tip. Here chlorophyll to carotenoid ratio decreased in Rossa but along the young leaf tissue of this genotype and throughout the leaf of Bf993 this ratio was constant. Amounts of D1 protein, associated with the Photosystem II reaction centre, were correspondingly retained to a somewhat greater extent in senescing tissue of Bf993 but surprisingly, given the expression of the genetic lesion in a delayed senescence, amounts of the protein were lower in young tissue from the stay-green mutant than in the normal genotype. When the leaf of Rossa was allowed to senesce until the entire blade was yellow there were marked changes in carotenoid composition, and chlorophyll degradation products accumulated. Any increase in zeaxanthin was slight and there was no evidence to suggest operation of a xanthophyll cycle.

Changes of fluorescence and xanthophyll pigments during dehydration in the resurrection plantSelaginella lepidophylla in low and medium light intensities.

The changes in photosynthetic efficiency and photosynthetic pigments during dehydration of the resurrection plantSelaginella lepidophylla (from the Chiuhahuan desert, S.W. Texas, USA) were examined under different light conditions. Changes in the photosynthetic efficiency were deduced from chlorophyll a fluorescence measurements (Fo, Fm, and Fv) and pigment changes were measured by HPLC analysis. A small decrease in Fv/Fm was seen in hydrated stems in high light (650 μmol photons·m-2·s-1) but not in low light (50 μmol photons·m-2·s-1). However, a pronounced decline in Fv/Fm was observed during dehydration in both light treatments, after one to two hours of dehydration. A rise in Fo was observed only after six to ten hours of dehydration. Concomitant with the decrease in photosynthetic efficiency during dehydration a rise in the xanthophyll zeaxanthin was observed, even in low-light treatments. The increase in zeaxanthin can be related to previously observed photoprotective non-photochemical quenching of fluorescence in dehydrating stems ofS. lepidophylla. We hypothesize that under dehydrating conditions even low light levels become excessive and zeaxanthin-related photoprotection is engaged. We speculate that these processes, as well as stem curling and self shading (Eickmeier et al. 1992), serve to minimize photoinhibitory damage toS. lepidophylla during the process of dehydration.

Changes in Carotenoid Composition and Photosynthesis in Sorghum under High Light and Salt Stresses

Summary The effect of high light stress and salt stress, individually and in combination, was investigated in the leaves of Sorghum bicolor (1.). High light treatment at 5°C and 20°C caused an increase in zeaxanthin and a decrease in violaxanthin contents. Decreases in β-carotene were also observed, which was followed by increased formation of 5,6-epoxide-β-carotene. Violaxanthin + antheraxanthin + zeaxanthin (total) was not the same in control and stressed plants. Salt stress (no photoinhibition) also resulted in the formation of zeaxanthin but it was significantly less than that observed in the high light stressed plants. Neoxanthin content showed a slight decrease, while 5,6-epoxide lutein increased under salt stress. Changes in the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) were reversed during recovery after photoinhibition. The CO2 assimilation rate and fluorescence Fv/Fm ratio were measured in intact leaves and isolated chloroplasts, respectively, from control and stressed plants. Formation of zeaxanthin and degradation of β-carotene are apparently involved in protection against photoinhibition.