Short- and long-term modulation of the lutein epoxide and violaxanthin cycles in two species of the Lauraceae: sweet bay laurel (Laurus nobilis L.) and avocado (Persea americana Mill.)

Short- and long-term responses of the violaxanthin (V) and lutein epoxide (Lx) cycles were studied in two species of Lauraceae: sweet bay laurel (Laurus nobilis L.) and avocado (Persea americana L.). The Lx content exceeded the V content in shade leaves of both species. Both Lx and V were de-epoxidised on illumination, but only V was fully restored by epoxidation in low light. Violaxanthin was preferentially de-epoxidised in low light in L. nobilis. This suggests that Lx accumulates with leaf ageing, partly because its conversion to lutein is limited in shade. After exposure to strong light, shade leaves of avocado readjusted the total pools of alpha- and beta-xanthophyll cycles by de novo synthesis of antheraxanthin, zeaxanthin and lutein. This occurred in parallel with a sustained depression of F(v)/F(m). In Persea indica, a closely related but low Lx species, F(v)/F(m) recovered faster after a similar light treatment, suggesting the involvement of the Lx cycle in sustained energy dissipation. Furthermore, the seasonal correlation between non-reversible Lx and V photoconversions and pre-dawn F(v)/F(m) in sun leaves of sweet bay supported the conclusion that the Lx cycle is involved in a slowly reversible downregulation of photosynthesis analogous to the V cycle.     

De Novo Synthesis and Degradation of Lx and V Cycle Pigments during Shade and Sun Acclimation in Avocado Leaves

The photoprotective role of the universal violaxanthin cycle that interconverts violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z) is well established, but functions of the analogous conversions of lutein-5,6-epoxide (Lx) and lutein (L) in the selectively occurring Lx cycle are still unclear. We investigated carotenoid pools in Lx-rich leaves of avocado (Persea americana) during sun or shade acclimation at different developmental stages. During sun exposure of mature shade leaves, an unusual decrease in L preceded the deepoxidation of Lx to L and of V to A+Z. In addition to deepoxidation, de novo synthesis increased the L and A+Z pools. Epoxidation of L was exceptionally slow, requiring about 40 d in the shade to restore the Lx pool, and residual A+Z usually persisted overnight. In young shade leaves, the Lx cycle was reversed initially, with Lx accumulating in the sun and declining in the shade. De novo synthesis of xanthophylls did not affect α- and β-carotene pools on the first day, but during long-term acclimation α-carotene pools changed noticeably. Nonetheless, the total change in α- and β-branch carotenoid pools was equal. We discuss the implications for regulation of metabolic flux through the α- and β-branches of carotenoid biosynthesis and potential roles for L in photoprotection and Lx in energy transfer to photosystem II and explore physiological roles of both xanthophyll cycles as determinants of photosystem II efficiency.It has long been recognized that photosynthesis in plants must resolve two conflicting requirements, the need to ramp up maximum light-harvesting efficiency in dim light and to wind back to lower efficiency when light is in excess, in order to maintain high rates of growth and productivity in varying light environments (Björkman, 1981; Pearcy, 1990). A wealth of research has established that plants adjust through an array of morphological and molecular events that confer photoprotection, mitigate and repair photoinactivation of PSII, and facilitate acclimation of the photosynthetic apparatus over different time scales in response to variable light regimes in wild plants, crops, and algae (Osmond et al., 1999; Demmig-Adams et al., 2006). In the context of the light reactions, low light acclimation optimizes light harvesting and energy transfer to the photosystems, particularly PSII, via enlarged functional antennae, accumulation of accessory light-harvesting pigments, and down-regulation of unnecessary competing photoprotective processes. High light acclimation involves increased photoprotection and photorepair, downsized antennae, fewer photosystems, and sometimes changes in the PSI to PSII stoichiometry (Osmond et al., 1999; Förster et al., 2005). Along with their function in energy transfer to the photosynthetic reaction centers as accessory pigments to chlorophylls, the xanthophyll pigments violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z) play a central role in these transformations of the photosynthetic apparatus, especially in thermal energy dissipation and detoxification of reactive oxygen species.Two xanthophyll cycles are now known in terrestrial plants, the lutein epoxide cycle (Lx cycle) based on interconversions of lutein-5,6-epoxide (Lx) and lutein (L) synthesized from α-carotene (α-C), and the violaxanthin cycle (V cycle) based on the interconversions of V and A+Z synthesized from β-carotene (β-C; García-Plazaola et al., 2007). Presumably, both cycles are catalyzed by the same enzymes, violaxanthin epoxidase (VDE) for deepoxidation in high light and zeaxanthin epoxidase (ZE) for the reverse reactions in low light or darkness (Latowski et al., 2004). Rediscovery of the Lx cycle in the parasitic angiosperm Cuscuta reflexa (Bungard et al., 1999) has led to growing interest in differing manifestations of this cycle in terrestrial plants and its relationships to the apparently universal V cycle (Demmig-Adams, 1998). A complete Lx cycle seems to function on a daily basis in both C. reflexa and the mistletoe Amyema miquelii (Matsubara et al., 2001), even though Lx conversion to L is sometimes slower than V to A+Z and dark recovery of Lx is usually slower than that of V. Intriguingly, in shade leaves of Inga sapindoides, high concentrations of Lx were seemingly irreversibly converted to L on exposure to strong light, in marked contrast to the co-occurring, fully reversible V cycle (Matsubara et al., 2005). Similar responses have been found in other woody plants with long-lived leaves in deeply shaded canopies, including Mediterranean oaks (Quercus spp.; García-Plazaola et al., 2003), sweet bay laurel (Laurus nobilis), and avocado (Persea americana; Esteban et al., 2007, 2008). This response type is known as a truncated Lx cycle (García-Plazaola et al., 2007).The functions attributed to the Lx cycle were initially based on structural analogies between Lx and A and between L and Z (Bungard et al., 1999; Pogson and Rissler, 2000; Matsubara et al., 2001). With increased evidence for the possible role of L in photoprotection (Pogson et al., 1996, 1998; Lokstein et al., 2002; Dall''Osto et al., 2006), additional functional analogies emerged. Furthermore, recent in vitro reconstitution studies with light-harvesting complex proteins and purified pigments also support a spatial overlap of the cycles, as some pigment-binding sites can be occupied by either α- or β-xanthophylls (Matsubara et al., 2007). An attractive hypothesis is that photoconversion of Lx to L might sustain or enhance photoprotection associated with the V cycle (Demmig-Adams and Adams, 1992; Niyogi, 2000). In support of this view, it has been demonstrated in leaves of Quercus rubra and in leaflets of Inga marginata that increasing amounts of photoconverted L, which persist even when A and Z are epoxidized to V, were associated with faster engagement and higher levels of nonphotochemical quenching (NPQ) of chlorophyll fluorescence (García-Plazaola et al., 2003; Matsubara et al., 2008). Furthermore, evidence from mammalian eye research as well as from plants suggests that L also acts as a highly efficient reactive oxygen species scavenger (Kim et al., 2006; Johnson et al., 2007).Broader issues, such as the roles of short-term dynamics of the two cycles in relation to long-term processes of shade and sun acclimation and in relation to leaf development and age, are poorly understood. Nonfruiting shoots of avocado trees constitute a very suitable model system in which to address these issues. Long-lived leaves of shade-grown avocado contain some of the highest levels of Lx thus far recorded (Esteban et al., 2007; García-Plazaola et al., 2007) and have two to four flushes of leaf initiation per year that exhibit a form of delayed greening in which leaf expansion precedes increases in stomatal conductance, chlorophyll content, and CO₂ assimilation. Expanding leaves remain sinks for up to 1 month until they reach about 70% to 80% of full expansion (Schaffer et al., 1991), and stomata do not become fully functional until leaves attain 90% of full expansion (Scholefield and Kriedemann, 1979). However, shoots also retain old leaves through several flushes, and leaves from the previous season contribute significantly to total plant carbon gain (Liu et al., 2002), with photosynthesis rates up to 50% of those in new, fully expanded leaves (Heath et al., 2005). These properties offer an array of opportunities for new research into the concurrent operation of the two xanthophyll cycles.Since there have been very few studies of these complex responses, we carried out a series of short- and long-term light treatments that are likely to reflect what leaves may experience in natural environments, with the aim to gain further insight into the physiological relevance of the Lx and V cycles under those circumstances. Four types of acclimation experiments were undertaken in this study. First, short-term acclimation from shade to sun addressed fast responses to a drastic increase in the light environment, simulating a prolonged sun fleck in shaded mature leaves or exposure to a bright sunny day in young leaves that had emerged during a prolonged overcast (shaded) growth period. These experiments revealed an unexpected loss of L prior to deepoxidation of Lx and V and a reverse Lx cycle in young leaves. Second, long-term acclimation of sun leaves to prolonged shade simulated normal processes of shading by further growth of outer canopy leaves. These treatments established the very slow accumulation of Lx in avocado leaves. Third, sequential sun exposures of mature leaves over several days, followed by continuous shade, were applied to simulate successive prolonged sun flecks, mimicking stochastic canopy disturbance during severe weather events, which confirmed many responses in the above experiments, particularly the very slow epoxidation of L to Lx in prolonged shade. Fourth, long-term acclimation of young and mature leaves to sun was examined. These experiments simulated sudden changes to canopy architecture as experienced during pruning and extended our understanding of the comparative rates and magnitude of Lx and V cycle engagement. We discuss the short-and long-term kinetics of both cycles in avocado leaves of different ages during acclimation, with particular attention to the stoichiometric relationships between xanthophyll and carotenoid pools and changing PSII efficiency.     

Occurrence of the lutein-epoxide cycle in mistletoes of the Loranthaceae and Viscaceae

The lutein-epoxide cycle (Lx cycle) is an auxiliary xanthophyll cycle known to operate only in some higher-plant species. It occurs in parallel with the common violaxanthin cycle (V cycle) and involves the same epoxidation and de-epoxidation reactions as in the V cycle. In this study, the occurrence of the Lx cycle was investigated in the two major families of mistletoe, the Loranthaceae and the Viscaceae. In an attempt to find the limiting factor(s) for the occurrence of the Lx cycle, pigment profiles of mistletoes with and without the Lx cycle were compared. The availability of lutein as a substrate for the zeaxanthin epoxidase appeared not to be critical. This was supported by the absence of the Lx cycle in the transgenic Arabidopsis plant lutOE, in which synthesis of lutein was increased at the expense of V by overexpression of -cyclase, a key enzyme for lutein synthesis. Furthermore, analysis of pigment distribution within the mistletoe thylakoids excluded the possibility of different localizations for the Lx- and V-cycle pigments. From these findings, together with previous reports on the substrate specificity of the two enzymes in the V cycle, we propose that mutation to zeaxanthin epoxidase could have resulted in altered regulation and/or substrate specificity of the enzyme that gave rise to the parallel operation of two xanthophyll cycles in some plants. The distribution pattern of Lx in the mistletoe phylogeny inferred from 18S rRNA gene sequences also suggested that the occurrence of the Lx cycle is determined genetically. Possible molecular evolutionary processes that may have led to the operation of the Lx cycle in some mistletoes are discussed.Abbreviations A antheraxanthin - - and -Car - and -carotene - Chl chlorophyll - -DM dodecyl--d-maltoside - DPS de-epoxidation state of the violaxanthin cycle (= [A+Z]/[V+A+Z]) - Lut lutein - Lx lutein epoxide - Caro total carotenoid concentration - V violaxanthin - VAZ pool size of the violaxanthin cycle (= V+A+Z) - VDE violaxanthin de-epoxidase - Z zeaxanthin - ZE zeaxanthin epoxidase     

Some analytical assays for the determination of bioactivity of exotic fruits

Introduction – The consumption of new exotic fruits, with their high nutritional and sensory value, has significantly increased in the past few years. Among the tropical fruits durian (Durio zibethinus Murr.) is less known than mango (Mangifera indica L.) and avocado (Persea americana). It has been shown that durian, mango and avocado possessed high nutritional and bioactive properties, but these data were determined using different methods. In order to obtain reliable results we investigated samples of durian, mango and avocado of the same stage of ripeness and unified methods were used for determination of the antioxidant potential. As far as we know, no results of such comparative investigation of three tropical fruits (durian, mango and avocado) and the use of such tests for phytochemical control have been published. Objective – Lyophilised durian, mango and avocado samples harvested in 2008 in Thailand and Israel were investigated. Methodology – The contents of crude protein, fat, carbohydrate, dietary fibre, total polyphenols, flavonoids, tannins and flavanols were determined by elemental analysis and UV spectroscopy. The presence of polyphenols (flavonoids and phenolic acids) in the investigated samples was studied by Fourier transform infrared (FT‐IR) spectroscopy and three‐dimensional fluorometry. Four complementary radical scavenging assays were used for antioxidant determination: ferric reducing antioxidant power (FRAP), 2, 2‐azino‐bis (3‐ethyl‐benzothiazoline‐6‐sulfonic acid) diamonium salt (ABTS^?+), 1‐diphenyl‐2‐picrylhydrazyl method (DPPH) and cupric reducing antioxidant capacity (CUPRAC). Chemometrical processing was used for statistical comparison of the fruits. Results – All spectrometric measurements were highly correlated. The contents of total fibre, proteins and fats were significantly higher (p < 0.05) in avocado, and carbohydrates were significantly lower in avocado (p > 0.05) than in the two other fruits. The wavelength numbers of FTIR spectra for three investigated fruits were in the same range (1700–600 cm^?1) as for catechin and gallic acid, used as standards. One main peak could be easily observed at the approximate location of ex/em 275/305 nm and the other one at ex/em 350/430 nm in the methanol polyphenol extracts of investigated fruits in three‐dimensional fluorescence, in contour and cross fluorescence maps. Similarity was found between durian, mango and avocado in polyphenols (9.88 ± 1.0, 12.06 ± 1.3 and 10.69 ± 1.1, mg gallic acid equivalents/g dry weight, d.w.), and in antioxidant assays such as CUPRAC (27.46 ± 2.7, 40.45 ± 4.1 and 36.29 ± 3.7, µM Trolox equivalent (TE)/g d.w.) and FRAP (23.22 ± 2.0, 34.62 ± 3.4 and 18.47 ± 1.9, µM TE/g d.w.), respectively. The multisample median test between all possible pairs of groups is a Tukey–HSD type comparison and denotes the different groups in a case when a pair‐wise test is significant and its q statistical value is greater than the table q parameter. The multisample median test of FRAP values were chosen from the compared fruits triplets as similar or homogenous subsets durian and avocado. Conclusion – Nutritional and bioactive values of durian are comparable with these indices in mango and avocado. These fruits contain high, comparable quantities of basic nutritional and antioxidant compounds, and possess high antioxidant potentials. All fruits show a high level of correlation between the contents of phenolic compounds and the antioxidant potential. The methods used (three‐dimensional fluorescence, FTIR spectroscopy, radical scavenging assays) are suitable for bioactivity determination of these fruits. In order to receive best results, a combination of these fruits has to be included in the diet. The methods used are applicable for bioactivity determination in phytochemical analysis in general. Copyright © 2010 John Wiley & Sons, Ltd.     

A SMALLER AND IMPAIRED XANTHOPHYLL CYCLE MAKES THE DEEP SEA MACROALGAE LAMINARIA ABYSSALIS (PHAEOPHYCEAE) HIGHLY SENSITIVE TO DAYLIGHT WHEN COMPARED WITH SHALLOW WATER LAMINARIA DIGITATA1

Pigment composition, fluorescence parameters, and oxygen evolution of the deep water Laminaria abyssalis Oliveira and of the shallow water L. digitata Lamoroux were determined in response to high irradiances. This was performed in the presence and absence of an inhibitor of violaxanthin de‐epoxidase (dithiothreitol) or an inhibitor of the chloroplast‐encoded protein synthesis (chloramphenicol). Photochemical quenching in L. digitata was almost 3‐fold that seen in L. abyssalis, whereas both nonphotochemical quenching and PSII photochemical yield were doubled. Laminaria digitata possessed a xanthophyll‐cycle pool nearly double that of L. abyssalis. After photoinhibitory treatment, L. digitata displayed substantial violaxanthin de‐epoxidation, whereas in L. abyssalis de‐epoxidation only took place in limited amounts. Both species were able to fully recover their epoxidation status after transfer back to dim light. Overnight incubation with dithiothreitol fully blocked de‐epoxidation in both species, and both displayed similar fluorescence properties. Chloramphenicol caused no change in their fluorescence parameters. With high light treatment, L. abyssalis was completely and irreversibly inhibited both in the presence and absence of inhibitors, whereas L. digitata showed 60% inhibition of its photosynthetic activity and full recovery in the absence of inhibitors. In the presence of dithiothreitol, L. digitata did not recover to the preillumination conditions and chloramphenicol delayed the recovery of the oxygen evolution activity. We suggest that the xanthophyll cycle is the main mechanism of photoprotection of these Laminaria species and that the higher susceptibility of L. abyssalis to photoinhibition may be due to its limited de‐epoxidation capacity and reduced xanthophyll‐cycle pool size.     

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

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

Lutein from deepoxidation of lutein epoxide replaces zeaxanthin to sustain an enhanced capacity for nonphotochemical chlorophyll fluorescence quenching in avocado shade leaves in the dark

Leaves of avocado (Persea americana) that develop and persist in deep shade canopies have very low rates of photosynthesis but contain high concentrations of lutein epoxide (Lx) that are partially deepoxidized to lutein (L) after 1 h of exposure to 120 to 350 μmol photons m(-2) s(-1), increasing the total L pool by 5% to 10% (ΔL). Deepoxidation of Lx to L was near stoichiometric and similar in kinetics to deepoxidation of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z). Although the V pool was restored by epoxidation of A and Z overnight, the Lx pool was not. Depending on leaf age and pretreatment, the pool of ΔL persisted for up to 72 h in the dark. Metabolism of ΔL did not involve epoxidation to Lx. These contrasting kinetics enabled us to differentiate three states of the capacity for nonphotochemical chlorophyll fluorescence quenching (NPQ) in attached and detached leaves: ΔpH dependent (NPQ(ΔpH)) before deepoxidation; after deepoxidation in the presence of ΔL, A, and Z (NPQ(ΔLAZ)); and after epoxidation of A+Z but with residual ΔL (NPQ(ΔL)). The capacity of both NPQ(ΔLAZ) and NPQ(ΔL) was similar and 45% larger than NPQ(ΔpH), but dark relaxation of NPQ(ΔLAZ) was slower. The enhanced capacity for NPQ was lost after metabolism of ΔL. The near equivalence of NPQ(ΔLAZ) and NPQ(ΔL) provides compelling evidence that the small dynamic pool ΔL replaces A+Z in avocado to "lock in" enhanced NPQ. The results are discussed in relation to data obtained with other Lx-rich species and in mutants of Arabidopsis (Arabidopsis thaliana) with increased L pools.     

Issue Information ‐ Referees List

Rapid developments in remote‐sensing of vegetation and high‐throughput precision plant phenotyping promise a range of real‐life applications using leaf optical properties for non‐destructive assessment of plant performance. Use of leaf optical properties for assessing plant performance requires the ability to use photosynthetic pigments as proxies for physiological properties and the ability to detect these pigments fast, reliably and at low cost. We describe a simple and cost‐effective protocol for the rapid analysis of chlorophylls, carotenoids and tocopherols using high‐performance liquid chromatography (HPLC). Many existing methods are based on the expensive solvent acetonitrile, take a long time or do not include lutein epoxide and α‐carotene. We aimed to develop an HPLC method which separates all major chlorophylls and carotenoids as well as lutein epoxide, α‐carotene and α‐tocopherol. Using a C₃₀‐column and a mobile phase with a gradient of methanol, methyl‐tert‐butyl‐ether (MTBE) and water, our method separates the above pigments and isoprenoids within 28 min. The broad applicability of our method is demonstrated using samples from various plant species and tissue types, e.g. leaves of Arabidopsis and avocado plants, several deciduous and conifer tree species, various crops, stems of parasitic dodder, fruit of tomato, roots of carrots and Chlorella algae. In comparison to previous methods, our method is very affordable, fast and versatile and can be used to analyze all major photosynthetic pigments that contribute to changes in leaf optical properties and which are of interest in most ecophysiological studies.     

Photosystem II energy use,non-photochemical quenching and the xanthophyll cycle in Sorghumbicolor grown under drought and free-air CO2 enrichment(FACE) conditions

The present study was carried out to test the hypothesis thatelevated atmospheric CO₂ (Ca) will alleviate over‐excitationof the C₄ photosynthetic apparatus and decrease non‐photochemicalquenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescencewas monitored in Sorghum bicolor plants grown under a free‐aircarbon‐dioxide enrichment (FACE) by water‐stress (Dry) experiment.Under Dry conditions elevated Ca increased the quantum yield ofphotosystem II (φPSII) throughout the day throughincreases in both photochemical quenching coefficient (q_p)and the efficiency with which absorbed quanta are transferred toopen PSII reaction centres (F_v′/F_m′).However, in the well‐watered plants (Wets) FACE enhanced φPSIIonly at midday and was entirely attributed to changes in F_v′/F_m′_. Underfield conditions, decreases in φPSII under Dry treatmentsand ambient Ca corresponded to increases in NPQ but the de‐epoxidation stateof the xanthophyll pool (DPS) showed no effects. Water‐stress didnot lead to long‐term damage to the photosynthetic apparatus asindicated by φPSII and carbon assimilation measuredafter removal of stress conditions. We conclude that elevated Caenhances photochemical light energy usage in C₄ photosynthesisduring drought and/or midday conditions. Additionally,NPQ protects against photo‐inhibition and photodamage. However,NPQ and the xanthophyll cycle were affected differently by elevatedCa and water‐stress.     

Identification and Characterization of Colletotrichum spp. affecting Fruit after Harvest in Brazil

Colletotrichum spp. cause anthracnose in various fruits post‐harvest and are a particularly important problem in tropical and subtropical fruits. The disease in fruits of avocado, guava, papaya, mango and passion fruit has been reported to be caused by C. gloeosporioides, and in banana by C. musae. In subtropical and temperate crops such apple, grape, peach and kiwi, the disease is caused by C. acutatum. The variation in pathogenic, morphological, cultural and molecular characteristics of Brazilian isolates of Colletotrichum acutatum Simmonds and isolates from post‐harvest decays of avocado, banana, guava, papaya, mango and passion fruit was evaluated. The fruits were inoculated with mycelium of C. acutatum, Colletotrichum spp. and C. musae on a disc of potato dextrose agar. The morphological, cultural and molecular characteristics studied were conidia morphology, colony growth at different temperatures, colony coloration and PCR with primers CaInt2 and ITS4 for C. acutatum and CgInt and ITS4 for C. gloeosporioides. C. acutatum was pathogenic to avocado, guava, papaya, mango and passion fruit, but it was not pathogenic to banana. The morphological, cultural and molecular studies indicated that the avocado, papaya, mango and passion fruit isolates were C. gloeosporioides. The natural guava isolate was identified as C. acutatum, which had not been found previously to produce anthracnose symptoms on guava in Brazil.     

The photobiology of Heterosigma akashiwo. Photoacclimation,diurnal periodicity,and its ability to rapidly exploit exposure to high light

Periodic and seasonal exposure to high light is a common occurrence for many near‐shore and estuarine phytoplankton. Rapid acclimatization to shifts in light may provide an axis by which some species of phytoplankton can outcompete other microalgae. Patterns of photoacclimation and photosynthetic capacity in the raphidophyte Heterosigma akashiwo (Hada) Hada ex Hara et Chihara isolated from the mid‐Atlantic of the United States were followed in continuous cultures at low‐ and high‐light intensities, followed by reciprocal shifts to the opposite light level. The maximum quantum yield (F_v/F_m) as well as the photosynthetic cross‐section (σ_PSII) of photosystem II was higher in high‐light cultures compared to low‐light cultures. Significant diurnal variability in photochemistry and photoprotection was noted at both light levels, and high‐light‐acclimated cultures displayed greater variability in photoprotective pathways. When shifted from low to high light, there was only a slight and temporary decline in maximum quantum yield, while cell specific growth more than doubled within 24 h. Rapid acclimation to high light was facilitated by short‐term photoprotection (nonphotochemical quenching), reduced PSII reaction center connectivity, and electron transport. Short‐term increases in de‐epoxidated xanthophyll pigments contributed to nonphotochemical protection, but lagged behind initial increases in nonphotochemical quenching and were not the primary pathway of photoprotection in this alga. By 48 h, photochemistry of cultures shifted from low to high light resembled long‐term high‐light‐acclimated cultures. This isolate of H. akashiwo appears well poised to exploit rapid shifts in light by using unique cellular adjustments in light harvesting and photochemistry.     

Effects of ultraviolet (UV) exclusion on the seasonal concentration of photosynthetic and UV-screening pigments in Scots pine needles

The effects of ultraviolet (UV) radiation on the photosynthetic and UV‐screening pigments in needles of Scots pine (Pinus sylvestris L.) saplings were studied in a UV‐exclusion field chamber experiment in northern Finland (67°N) during 2001–2002. The chambers held filters that excluded both UVB and UVA, only UVB, transmitted all UV, or lacked filters. Analyses of control needles (no filter and polyethene filter) showed that the first changes to occur in spring (end of April) was an abrupt increase in the epoxidation state (EPS) of the xanthophyll cycle pigments, likely in relation with the beginning of the photosynthetic activity. The concentration of chlorophyll, lutein, neoxanthin, α‐carotene, β‐carotene, and the size of the xanthophyll cycle pool (violaxanthin+antheraxanthin+zeaxanthin=VAZ) changed only later when needles reached their summer photosynthesis state. Exclusion of UV radiation significantly affected the xanthophyll cycle but not the other photosynthetic pigments analysed. Interestingly, the effects on xanthophylls were dependent on the sampling date. Under UVA/B‐exclusion, the EPS was increased and VAZ pool size was unchanged in April, whereas EPS remained unchanged and the VAZ pool size was reduced in May and June. The existence of two sustained and active antenna modes during winter and summer could be an explanation for the specific UV‐exclusion effect in the different season. A high‐performance liquid chromatography analysis of soluble phenolics showed that the exclusion of UVA/B radiation caused a significant effect on five compounds out of 46 studied, without affecting the concentration of the total soluble phenolics. Under UVA/B‐exclusion, the concentration of three of them (secoisolariciresinol‐glucopyranoside, two unknown) was reduced while the concentration of dicoumaroyl‐astragalin and pinosylvin monomethylether was increased compared with both controls separately. In general, the exclusion of UVA/B caused a stronger effect than the exclusion of UVB on both photosynthetic and UV screening pigments. The effects of UV radiation on xanthophyll cycle pigments were season‐specific and detectable only under stressful spring conditions (freezing temperatures and high irradiance due to snow reflection). The effect on the xanthophyll cycle could be a direct consequence of UV treatments, or an indirect consequence of the changed flavonoid composition, or a combination of both.     

Cytosolic monoterpene biosynthesis is supported by plastid‐generated geranyl diphosphate substrate in transgenic tomato fruits

Geranyl diphosphate (GPP), the precursor of most monoterpenes, is synthesized in plastids from dimethylallyl diphosphate and isopentenyl diphosphate by GPP synthases (GPPSs). In heterodimeric GPPSs, a non‐catalytic small subunit (GPPS‐SSU) interacts with a catalytic large subunit, such as geranylgeranyl diphosphate synthase, and determines its product specificity. Here, snapdragon (Antirrhinum majus) GPPS‐SSU was over‐expressed in tomato fruits under the control of the fruit ripening‐specific polygalacturonase promoter to divert the metabolic flux from carotenoid formation towards GPP and monoterpene biosynthesis. Transgenic tomato fruits produced monoterpenes, including geraniol, geranial, neral, citronellol and citronellal, while exhibiting reduced carotenoid content. Co‐expression of the Ocimum basilicum geraniol synthase (GES) gene with snapdragon GPPS‐SSU led to a more than threefold increase in monoterpene formation in tomato fruits relative to the parental GES line, indicating that the produced GPP can be used by plastidic monoterpene synthases. Co‐expression of snapdragon GPPS‐SSU with the O. basilicum α–zingiberene synthase (ZIS) gene encoding a cytosolic terpene synthase that has been shown to possess both sesqui‐ and monoterpene synthase activities resulted in increased levels of ZIS‐derived monoterpene products compared to fruits expressing ZIS alone. These results suggest that re‐direction of the metabolic flux towards GPP in plastids also increases the cytosolic pool of GPP available for monoterpene synthesis in this compartment via GPP export from plastids.     

The role of frugivorous birds in fruit removal and seed germination of the invasive alien Cotoneaster franchetii in central Argentina

Many invasive plant species have fleshy fruits that are eaten by native frugivorous birds which disperse their seeds and may facilitate their germination, playing an important role in plant invasion success. The fleshy‐fruited shrub Cotoneaster franchetii (Rosaceae) is an important invasive alien in the mountainous regions of central Argentina. To determine the role of avian frugivorous in fruit removal of this species, we conducted a frugivore exclusion experiment including bagged and unbagged branches in 75 plants of C. franchetii. At the end of the dispersal period, we compared the percentage of missing fruits (removed by birds + naturally dropped) in unbagged branches with the percentage of naturally dropped fruits in bagged branches. To assess whether any mechanism acting on seeds during their passage through bird guts (de‐inhibition by pulp removal and/or seed scarification) affects seed germination of this species, we compared percentage and speed of germination among seeds obtained from faeces of the native frugivorous Turdus chiguanco, from manually de‐pulped fruits, and from intact fruits. The percentage of missing fruits per shrub in unbagged branches was significantly higher than the percentage of naturally dropped fruits in bagged branches, suggesting that frugivorous birds play an important role in fruit removal of C. franchetii in the study area. Seeds from bird faeces and from manually de‐pulped fruits germinated in higher percentage and faster than seeds from intact fruits. Germination percentage and speed of seeds from manually de‐pulped fruits were significantly higher than those of gut‐passed seeds. These results indicate that T. chiguanco increases and accelerates seed germination of C. franchetii through pulp removal, but not through seed scarification. Overall, our findings indicate that native frugivorous birds facilitate the dispersal and germination success of C. franchetii, likely playing an important role in its invasion throughout the mountainous region of central Argentina.     

Iron deficiency-induced changes in the photosynthetic pigment composition of field-grown pear (Pyrus communis L) leaves

In this work we characterize the changes induced by iron deficiency in the pigment composition of pear (Pyrus communis L.) leaves grown under high light intensities in field conditions in Spain. Iron deficiency induced decreases in neoxanthin and β-carotene concomitantly with decreases in chlorophyll a, whereas lutein and carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. The chlorophyll a/chlorophyll b ratio increased in response to iron deficiency. The carotenoids within the xanthophyll cycle in iron-deficient and in iron-sufficient (control) leaves underwent epoxidations and de-epoxidations in response to ambient light conditions. In control leaves dark-adapted for several hours, most of the xanthophyll cycle pigment pool was in the epoxidated form vio-laxanthin, whereas iron-deficient leaves had significant amounts of zeaxanthin. Iron-deficient leaves also exhibited an increased non-photochemical quenching, supporting the possibility of a role for pigments within the xanthophyll cycle in photoprotection.     

Photosynthetic limitations and volatile and non‐volatile isoprenoids in the poikilochlorophyllous resurrection plant Xerophyta humilis during dehydration and rehydration

We investigated the photosynthetic limitations occurring during dehydration and rehydration of Xerophyta humilis, a poikilochlorophyllous resurrection plant, and whether volatile and non‐volatile isoprenoids might be involved in desiccation tolerance. Photosynthesis declined rapidly after dehydration below 85% relative water content (RWC). Raising intercellular CO₂ concentrations during desiccation suggest that the main photosynthetic limitation was photochemical, affecting energy‐dependent RuBP regeneration. Imaging fluorescence confirmed that both the number of photosystem II (PSII) functional reaction centres and their efficiency were impaired under progressive dehydration, and revealed the occurrence of heterogeneous photosynthesis during desiccation, being the basal leaf area more resistant to the stress. Full recovery in photosynthetic parameters occurred on rehydration, confirming that photosynthetic limitations were fully reversible and that no permanent damage occurred. During desiccation, zeaxanthin and lutein increased only when photosynthesis had ceased, implying that these isoprenoids do not directly scavenge reactive oxygen species, but rather protect photosynthetic membranes from damage and consequent denaturation. X. humilis was found to emit isoprene, a volatile isoprenoid that acts as a membrane strengthener in plants. Isoprene emission was stimulated by drought and peaked at 80% RWC. We surmise that isoprene and non‐volatile isoprenoids cooperate in reducing membrane damage in X. humilis, isoprene being effective when desiccation is moderate while non‐volatile isoprenoids operate when water deficit is more extreme.     

Metabolism in orange fruits is driven by photooxidative stress in the leaves

In plants, stress signals propagate to trigger distant responses and thus stress acclimation in non‐exposed organs. We tested here the hypothesis that leaves submitted to photooxidative stress may influence the metabolism of nearby fruits and thus quality criteria. Leaves of orange trees (Citrus sinensis (L.) Osbeck cv. ‘Navelate’) were acclimated to shade for 1 week and then submitted to full (FL) and medium light (ML) conditions. As expected, photoinhibition was detected in leaves of both FL and ML treatments as revealed by stress indicators (F_v/F_m, Performance Index) for at least 99 h after treatments. In the fruits near the stressed leaves, we then determined the activities of enzymes related to oxidative stress, superoxide dismutase, catalase and the enzymes of the ascorbate (AA)/glutathione cycle, as well as the contents in sugars, organic acids and carotenoids. Ascorbate peroxidase and monodehydroascorbate reductase activities in the pulp of fruits were dramatically higher in both treatments when compared to the control. AA and total sugars were not affected by the photooxidative stress. However, the FL treatment resulted in a 16% increase in total organic acids, with succinic acid being the major contributor, a shift towards less glucose + fructose and more sucrose, and a 15% increase in total carotenoids, with cis‐violaxanthin being the major contributor. Our observations strongly suggest the existence of a signal generated in leaves in consequence of photooxidative stress, transmitted to nearby fruits. Exploiting such a signal by agronomic means promises exciting perspectives in managing quality criteria in fruits accumulating carotenoids.