High Light Treatment Research Articles

Photoinhibition of photosynthesis in Antarctic lichen Usnea antarctica. I. Light intensity- and light duration-dependent changes in functioning of photosystem II

The paper deals with the differences in sensitivity of Antarctic lichen to photoinhibition. Thalli of Usnea antarctica were collected at the James Ross Island, Antarctica (57°52´57´´W, 63°48´02´´S) and transferred in dry state to the Czech Republic. After rewetting in a laboratory, they were exposed to 2 high light treatments: short-term (30 min), and long-term (6 h). In short-term treatment, the sample were exposed to 1000 and 2000 µmol m-2 s-1 of photosynthetically active radiation (PAR). In long-term experiment, PAR of 300, 600, and 1000 µmol m-2 s-1 were used. Photosynthetic efficiency of U. antarctica thalli was monitored by chlorophyll fluorescence parameters, potential (FV/FM) and actual (FPSII) quantum yield of photochemical processes in photosystem II in particular. In short-term treatments, the F0, FV and FM signals, as well as the values of FV/FM, and FPSII showed light-induced decrease, however substantial recovery after consequent 30 min. in dark. Longer exposition (60 min) to high light led to more pronounced decrease in chlorophyll fluorescence than after 30 min treatment, however dark recovery was faster in the thalli treated before for longer time (60 min). Long-term treatment by high light caused gradual decrease in FV/FM and FPSII with the time of exposition. The extent of the decrease was found light dose-dependent. The time course was biphasic for FV/FM but not for FPSII. The study showed that wet thalli of Usnea antarctica had high capacity of photoprotective mechanisms to cope well either with short- or long-term high light stress. This might be of particular importance in the field at the James Ross Island, particularly at the begining of growing season when melting water is available and, simultaneously, high light stress may happen on fully sunny days.

Multiple strategies to prevent oxidative stress in Arabidopsis plants lacking the malate valve enzyme NADP-malate dehydrogenase

The nuclear-encoded chloroplast NADP-dependent malate dehydrogenase (NADP-MDH) is a key enzyme controlling the malate valve, to allow the indirect export of reducing equivalents. Arabidopsis thaliana (L.) Heynh. T-DNA insertion mutants of NADP-MDH were used to assess the role of the light-activated NADP-MDH in a typical C3 plant. Surprisingly, even when exposed to high-light conditions in short days, nadp-mdh knockout mutants were phenotypically indistinguishable from the wild type. The photosynthetic performance and typical antioxidative systems, such as the Beck–Halliwell–Asada pathway, were barely affected in the mutants in response to high-light treatment. The reactive oxygen species levels remained low, indicating the apparent absence of oxidative stress, in the mutants. Further analysis revealed a novel combination of compensatory mechanisms in order to maintain redox homeostasis in the nadp-mdh plants under high-light conditions, particularly an increase in the NTRC/2-Cys peroxiredoxin (Prx) system in chloroplasts. There were indications of adjustments in extra-chloroplastic components of photorespiration and proline levels, which all could dissipate excess reducing equivalents, sustain photosynthesis, and prevent photoinhibition in nadp-mdh knockout plants. Such metabolic flexibility suggests that the malate valve acts in concert with other NADPH-consuming reactions to maintain a balanced redox state during photosynthesis under high-light stress in wild-type plants.

Does trait plasticity of three boreal nutrient-conserving species relate to their competitive ability?

The present study investigates potential differences in trait plasticity as an additional contributing mechanism explaining ericaceous shrub dominance during different periods after logging. Two ericaceous species (Kalmia angustifolia, Rhododendron groenlandicum), and black spruce plants (Picea mariana), were submitted to combinations of 3 light levels and 2 levels of nitrogen addition during a simulated growing season of 9 weeks under greenhouse conditions. Plant traits related to light (photosynthesis, leaf mass per unit of area, leaf and aboveground biomass allocation, foliar N concentration) and nutrient acquisition (root mass ratio, specific absorption ratio, absorption of 15N) were measured in response to the manipulation of these resources. The leaf mass per unit of area (LMA) of both ericaceous species was significantly reduced by shading; LMA of Rhododendron and Kalmia was, respectively, 54% and 31% higher in the highest light treatment. The LMA of black spruce was unchanged in response to light level. There were few differences among species in trait response to N addition. Black spruce was characterized by higher nutrient absorption rate and specific absorption rate at higher N levels compared to the 2 ericads. With the exception of LMA, plasticity to light was higher for Kalmia; in contrast, plasticity to N addition was higher for Rhododendron and black spruce. Finally, LMA appears to be a key trait explaining the competitive advantage of ericaceous species (especially Rhododendron) and potential encroachment on forested sites after disturbance.

Photoprotective function of betacyanin in leaves of Amaranthus cruentus L. under water stress

The photoprotective function of leaf betacyanin in water-stressed Amaranthus cruentus plants was examined by comparing leaves of two strains which differ significantly in the amount of betacyanin. At 0, 1, and 2 days after the imposed water stress, leaves were subjected to high-light (HL) treatment to assess their photosynthetic capacity and photoinhibition susceptibility. The water stress equally reduced leaf relative water content (RWC), gas-exchange rate and chlorophyll (Chl) contents in both leaves, indicating that the severity of water stress was comparable between the strains. Consequently, the extent of photoinhibition after the HL treatment increased in both strains as water stress developed; however, it was significantly greater in acyanic leaves than in betacyanic leaves, suggesting lower photoinhibition susceptibility in the betacyanic strain. The betacyanic leaves also exhibited approximately 30% higher values for photochemical quenching coefficient (qP) during the period of water stress despite the nonphotochemical quenching coefficient (qN) did not differ significantly between the strains. These results may be partially explained by the increased amount of leaf betacyanin under water stress. Moreover, a decrease in Chl content in betacyanic leaves might have enhanced light screening effect of betacyanin by increasing relative abundance of betacyanin to Chl molecule. In addition, reduced Chl content increased light penetrability of leaves. As a result, the extent of photoinhibition at the deeper tissue was exacerbated and the Chl fluorescence emitted from these tissues was more readily detected, facilitating assessment of photoinhibition at deeper tissues where the effect of betacyanic light screening is considered to be most apparent. Our results demonstrated that leaf betacyanin contributes to total photoprotective capacity of A. cruentus leaves by lowering excitation pressure on photosystem II (PSII) via attenuation of potentially harmful excess incident light under water stress.

ABOVE- AND BELOW-GROUND NUTRIENT TISSUE CONCENTRATION AND LEAF PIGMENT CHANGES IN PATAGONIAN WOODY SEEDLINGS GROWN UNDER LIGHT AND SOIL MOISTURE GRADIENTS

To understand the ecophysiology of natural regeneration above- and below-ground nutrient tissue concentration and leaf pigment changes in Nothofagus pumilio (lenga) seedlings grown in three light intensities (4%, 26% and 64% of natural irradiance) and two soil moisture levels (40–60% and 80–100% soil capacity) under greenhouse controlled conditions were evaluated. Carbon (C), nitrogen (N), phosphorus (P), magnesium (Mg), potassium (K), calcium (Ca) and pigment (chlorophylls and carotenoids) were measured on seedlings. Carbon, N, Mg, K and Ca increased in low light intensity and soil moisture treatments, while P decreased. Nutrients were higher in above- than in below-ground biomass. Chlorophylls were lower in high light treatments, while carotenoids increased their content. All pigments were greater in low soil moisture treatments. These changes are closely related to their photosynthetic plasticity and biomass compartmentalization. Plants growing in high light were more efficient to produce the same amount of plant biomass.

Effects of light intensities endogenous hormones of Bletilla striata during acclimatization

The effect of light intensities on the endogenous hormones of Bletilla striata during seedling acclimatization was studied in order to explain the mechanism and provide basic data for conservation and cultivation of this wild medicinal species. The samples was treated with three photosynthetic photon flux densities: 3 000, 6 000, 12 000 lx during the acclimatization stage to study the hormone dynamic changes of IAA, ABA, GA3. Endogenous IAA concentration was declined in all treatments and the lowest level in all treatments was the middle light intensity. A sharp increase of endogenous ABA concentration in a short period, the concentration was increased by 380.2% in middle light (320.8 ng x g(-1)). Endogenous GA3 concentration was declined especially in the high light treatment. At 10 days of acclimatization the GA3 concentration was decreased by 52.2% in the high light (473.6 ng x g(-1)). Endogenous hormones concentration could be rapidly changed by the light, and endogenous ABA concentration could be sharply increased when acclimatization in middle light intensities.

Glutaredoxin GRXS13 plays a key role in protection against photooxidative stress in Arabidopsis

Glutaredoxins (GRXs) belong to the antioxidant and signalling network involved in the cellular response to oxidative stress in bacterial and eukaryotic cells. In spite of the high number of GRX genes in plant genomes, the biological functions and physiological roles of most of them remain unknown. Here the functional characterization of the Arabidopsis GRXS13 gene (At1g03850), that codes for two CC-type GRX isoforms, is reported. The transcript variant coding for the GRXS13.2 isoform is predominantly expressed under basal conditions and is the isoform that is induced by photooxidative stress. Transgenic lines where the GRXS13 gene has been knocked down show increased basal levels of superoxide radicals and reduced plant growth. These lines also display reduced tolerance to methyl viologen (MeV) and high light (HL) treatments, both conditions of photooxidative stress characterized by increased production of superoxide ions. Consistently, lines overexpressing the GRXS13.2 variant show reduced MeV- and HL-induced damage. Alterations in GRXS13 expression also affect superoxide levels and the ascorbate/dehydroascorbate ratio after HL-induced stress. These results indicate that GRXS13 gene expression is critical for limiting basal and photooxidative stress-induced reactive oxygen species (ROS) production. Together, these results place GRXS13.2 as a member of the ROS-scavenging/antioxidant network that shows a particularly low functional redundancy in the Arabidopsis GRX family.

Physiological responses to flooding and light in two tree species native to the Amazonian floodplains

In Amazonian floodplains, plant survival is determined by adaptations and growth strategies to effectively capture sunlight and endure extended periods of waterlogging. By measuring gas exchange, quantum efficiency of photosystem 2 (PSII), and growth parameters, we investigated the combined effects of flooding gradients and light on two common evergreen floodplain tree species, the light-tolerant Cecropia latiloba and the shade-tolerant Pouteria glomerata. Individual plants were subjected to different combinations of light and flooding intensity in short-term and long-term experiments. Plants of C. latiloba lost all their leaves under total submersion treatments (plants flooded to apex and with reduced irradiance) and showed highest maximum assimilation rates ( A max) in not flooded, high light treatments (6.1 μmol CO 2 m −2 s −1). Individuals of P. glomerata showed similar patterns, with A max increasing from 1.9 μmol CO 2 m −2 s −1 under total flooding to 7.1 μmol CO 2 m −2 s −1 in not flooded, high light treatments. During the long-term flooding experiment, quantum efficiency of PSII ( F v/ F m) of C. latiloba was not affected by partial flooding. In contrast, in P. glomerata F v/ F m decreased to values below 0.73 after 120 days of total flooding. Moreover, total submergence led P. glomerata to reduce significantly light saturation point (LSP), as compared to C. latiloba. For both species morphological adjustments to long-term flooding, such as the production of adventitious roots, resulted in reduced total biomass, relative growth rate (RGR) and leaf mass ratio (LMR). Growth increase in C. latiloba seemed to be more limited by low-light than by flooding. Therefore, the predominant occurrence of this species is in open areas with high light intensities and high levels of inundation. In P. glomerata flooding induced high reductions of growth and photosynthesis, whereas light was not limiting. This species is more abundant in positions where irradiance is reduced and periods of submergence are slightly modest. We could show that the physiological requirements are directly responsible for the flooding ( C. latiloba) and shade ( P. glomerata) tolerance of the two species, which explains their local distribution in Amazonian floodplain forests.

The significance of glutathione for photoprotection at contrasting temperatures in the alpine plant species Soldanella alpina and Ranunculus glacialis

The significance of total glutathione content was investigated in two alpine plant species with highly differing antioxidative scavenging capacity. Leaves of Soldanella alpina and Ranunculus glacialis incubated for 48 h in the presence of buthionine-sulfoximine had 50% lower glutathione contents when compared with leaves incubated in water. The low leaf glutathione content was not compensated for by activation of other components involved in antioxidative protection or electron consumption. However, leaves with normal but not with low glutathione content increased their ascorbate content during high light (HL) treatment (S. alpina) or catalase activity at low temperature (LT) (R. glacialis), suggesting that the mere decline of the leaf glutathione content does not act as a signal to ameliorate antioxidative protection by alternative mechanisms. CO(2)-saturated oxygen evolution was not affected in glutathione-depleted leaves at various temperatures, except at 35°C, thereby increasing the high temperature (HT) sensitivity of both alpine species. Leaves with low and normal glutathione content were similarly resistant to photoinhibition and photodamage during HL treatment at ambient temperature in the presence and absence of paraquat or at LT. However, HL- and HT-induced photoinhibition increased in leaves with low compared to leaves with normal glutathione content, mainly because the recovery after heat inactivation was retarded in glutathione-depleted leaves. Differences in the response of photosystem II (PSII) activity and CO(2)-saturated photosynthesis suggest that PSII is not the primary target during HL inactivation at HT. The results are discussed with respect to the role of antioxidative protection as a safety valve for temperature extremes to which plants are not acclimated.

High Light Induced Disassembly of Photosystem II Supercomplexes in Arabidopsis Requires STN7-Dependent Phosphorylation of CP29

Photosynthetic oxidation of water and production of oxygen by photosystem II (PSII) in thylakoid membranes of plant chloroplasts is highly affected by changes in light intensities. To minimize damage imposed by excessive sunlight and sustain the photosynthetic activity PSII, organized in supercomplexes with its light harvesting antenna, undergoes conformational changes, disassembly and repair via not clearly understood mechanisms. We characterized the phosphoproteome of the thylakoid membranes from Arabidopsis thaliana wild type, stn7, stn8 and stn7stn8 mutant plants exposed to high light. The high light treatment of the wild type and stn8 caused specific increase in phosphorylation of Lhcb4.1 and Lhcb4.2 isoforms of the PSII linker protein CP29 at five different threonine residues. Phosphorylation of CP29 at four of these residues was not found in stn7 and stn7stn8 plants lacking the STN7 protein kinase. Blue native gel electrophoresis followed by immunological and mass spectrometric analyses of the membrane protein complexes revealed that the high light treatment of the wild type caused redistribution of CP29 from PSII supercomplexes to PSII dimers and monomers. A similar high-light-induced disassembly of the PSII supercomplexes occurred in stn8, but not in stn7 and stn7stn8. Transfer of the high-light-treated wild type plants to normal light relocated CP29 back to PSII supercomplexes. We postulate that disassembly of PSII supercomplexes in plants exposed to high light involves STN7-kinase-dependent phosphorylation of the linker protein CP29. Disruption of this adaptive mechanism can explain dramatically retarded growth of the stn7 and stn7stn8 mutants under fluctuating normal/high light conditions, as previously reported.

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.

In vivo cytochrome and alternative pathway respiration in leaves of Arabidopsis thaliana plants with altered alternative oxidase under different light conditions

The in vivo activity of the alternative pathway (ν(alt)) has been studied using the oxygen isotope fractionation method in leaves of Arabidopsis thaliana modified for the expression of the AtAOX1a gene by anti-sense (AS-12) or overexpression (XX-2). Under non-stressful conditions, ν(alt) was similar in all plant lines regardless of its different alternative pathway capacities (V(alt)). Total leaf respiration (V(t)) and V(alt) were directly related to growth light conditions while electron partitioning between the cytochrome pathway (CP) and alternative pathway (AP) was unchanged by light levels. Interestingly, the AP functioned at full capacity in anti-sense plants under both growth light conditions. The role of the AP in response to a high light stress induced by short-term high light treatment (HLT) was also studied. In wild type and XX-2, both CP and AP rates increased proportionally after HLT while in AS-12, where the AP was unable to increase its rate, the CP accommodated all the increase in respiration. The results obtained under high light stress suggest that flexibility in the response of the mitochondrial electron transport chain is involved in sustaining photosynthetic rates in response to this stress while the saturated AP in AS-12 plants may contribute to the observed increase in photoinhibition.

Photosynthetic response ofNereocystis luetkeana(Phaeophyta) to high light

SUMMARYPhotosynthetic response to high light was determined for Bull kelp,Nereocystis luetkeana(K. Mertens) Postels and Ruprecht in order to understand how this species is affected by short‐term fluctuations in irradiance. Exposure ofN. luetkeanablades to high intensity photosynthetically active radiation (1000 µmol photons m−2s–1) caused increased non‐photochemical quenching of fluorescence and higher de‐epoxidation ratios for xanthophyll pigments indicating that energy‐quenching xanthophylls were used to protect blades against photoinhibition. Despite initiation of these photoprotective mechanisms, maximum photochemical efficiency of photosystem II (Fv/Fm) decreased 40% in response to a 60 min exposure to 1000 µmol photons m−2s–1photosynthetically active radiation indicating that photoinhibition had occurred. Light‐saturated rates of oxygen evolution were not changed significantly by the high light treatment. Recovery of maximum photochemical efficiency of photosystem II to within 8% of initial values occurred after a 300‐min dim light period. Younger sections of the blades were slightly more susceptible to high light damage than older sections. Middle sections of the blades were more prone to light‐induced damage at water temperatures of 7°C or 18°C, as compared to 13°C. Exposure to biologically effective ultraviolet‐B radiation (UV‐Bbe) (up to 4.5 kJ m–2 day–1) in photoinhibitory light conditions did not significantly affect light‐induced damage to photosystem II.

A Small Zinc Finger Thylakoid Protein Plays a Role in Maintenance of Photosystem II in Arabidopsis thaliana

This work identifies LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1), a Zn finger protein that shows disulfide isomerase activity, interacts with the photosystem II (PSII) core complex, and may act in repair of photodamaged PSII complexes. Two mutants of an unannotated small Zn finger containing a thylakoid membrane protein of Arabidopsis thaliana (At1g75690; LQY1) were found to have a lower quantum yield of PSII photochemistry and reduced PSII electron transport rate following high-light treatment. The mutants dissipate more excess excitation energy via nonphotochemical pathways than wild type, and they also display elevated accumulation of reactive oxygen species under high light. After high-light treatment, the mutants have less PSII-light-harvesting complex II supercomplex than wild-type plants. Analysis of thylakoid membrane protein complexes showed that wild-type LQY1 protein comigrates with the PSII core monomer and the CP43-less PSII monomer (a marker for ongoing PSII repair and reassembly). PSII repair and reassembly involve the breakage and formation of disulfide bonds among PSII proteins. Interestingly, the recombinant LQY1 protein demonstrates a protein disulfide isomerase activity. LQY1 is more abundant in stroma-exposed thylakoids, where key steps of PSII repair and reassembly take place. The absence of the LQY1 protein accelerates turnover and synthesis of PSII reaction center protein D1. These results suggest that the LQY1 protein may be involved in maintaining PSII activity under high light by regulating repair and reassembly of PSII complexes.

Characterization of the LhcSR Gene Under Light and Temperature Stress in the Green Alga Ulva linza

As a green-tide-forming macroalga, Ulva linza is distributed worldwide and therefore subject to various environmental stresses. The LHCSR (also known as LI818 in green alga and LHCX in diatoms) protein is a stress-related member of the LHC family that plays an important role in photo-protective mechanism, which has been only found in algae. In this study, we cloned full-length cDNA sequence encoding the LhcSR gene from U. linza and analyzed its expression in response to different temperature and illumination gradients. The results showed that high light (HL) could enhance expression of LhcSR and that the expression level peaked at 3 h under HL. Similarly, the expression of LhcSR could also be induced by low temperature (LT). However, the expression patterns of LhcSR were quite different in response to LT and HL treatment. Specifically, the maximum gene expression under LT was much higher (11.8-fold) than under HL (5.4-fold) when compared to the expression under normal conditions. The upregulated expression of LhcSR lasted for 12 h under LT, but 3 h under HL. These data suggest that the LhcSR gene is involved in photoprotection in U. linza, and the results suggest a stronger link to LT. In addition, the discrepancy in expression under HL and LT was consistent with the ecological features of this alga, which only thrives during the cold season (featured as LT and low light).

Light-mediated 15N fractionation in Caribbean gorgonian octocorals: implications for pollution monitoring

The stable nitrogen isotope ratio (δ15N) of coral tissue is a useful recorder of anthropogenic pollution in tropical marine ecosystems. However, little is known of the natural environmentally induced fractionations that affect our interpretation of coral δ15N values. In symbiotic scleractinians, light affects metabolic fractionation of N during photosynthesis, which may confound the identification of N pollution between sites of varied depth or turbidity. Given the superiority of octocorals for δ15N studies, our goal was to quantify the effect of light on gorgonian δ15N in the context of monitoring N pollution sources. Using field collections, we show that δ15N declined by 1.4‰ over 20 m depth in two species of gorgonians, the common sea fan, Gorgonia ventalina, and the slimy sea plume, Pseudopterogorgia americana. An 8-week laboratory experiment with P. americana showed that light, not temperature causes this variation, whereby the lowest fractionation of the N source was observed in the highest light treatment. Finally, we used a yearlong reciprocal depth transplant experiment to quantify the time frame over which δ15N changes in G. ventalina as a function of light regime. Over the year, δ15N was unchanged and increased slightly in the deep control colonies and shallow colonies transplanted to the deep site, respectively. Within 6 months, colonies transplanted from deep to shallow became enriched by 0.8‰, mirroring the enrichment observed in the shallow controls, which was likely due to the combined effect of an increase in the source δ15N and reduced fractionation. We conclude that light affects gorgonian δ15N fractionation and should be considered in sampling designs for N pollution monitoring. However, these fractionations are small relative to differences observed between natural and anthropogenic N sources.

Effects of temperature and light on the progression of black band disease on the reef coral, Montipora hispida

Understanding environmental drivers of black band disease (BBD), a virulent disease affecting corals worldwide, is critical to managing coral populations. Field monitoring studies have implicated seasonally elevated temperature and light as drivers of annual BBD outbreaks on the Great Barrier Reef, but do not distinguish their relative impacts. Here, we compare progression of BBD lesions on Montipora hispida among three controlled temperature (28.0, 29.0, 30.5°C) and two controlled light treatments (170, 440 μmol m−2 s−1) within normal seasonal ranges at the site. BBD progression rates were greatest (5.2 mm d−1) in the 30.5°C/high-light treatment and least (3.2 mm d−1) in the 28°C/low-light treatment. High light significantly enhanced BBD progression, whereas increases in disease progression under high temperatures were not statistically significant, identifying the greater role of light in driving BBD dynamics within the temperature range examined. Greater BBD progression during daytime compared with nighttime (by 2.2–3.6-fold across temperature and light treatments) corroborates our conclusion that light is the pre-eminent factor driving BBD progression at typical summer temperatures. Decreased photochemical efficiency of algal endosymbionts in the high-temperature/high-light treatments suggests that compromised health of the coral holobiont contributes to enhanced disease progression, highlighting the complexity of disease dynamics in host–pathogen systems responding to environmental changes.

LHCII organization and thylakoid lipids affect the sensitivity of the photosynthetic apparatus to high-light treatment.

Pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution were used to investigate the role of the different amount and organization of light-harvesting complexes of photosystem II (LHCII) in four pea species on the susceptibility of the photosynthetic apparatus to high-light treatment. In this work we analyzed the thylakoid membrane lipid composition of the studied pea plants. A relationship between the structural organization of LHCII proteins, the amount of the main lipid classes and the sensitivity of the photosynthetic apparatus to high-light treatment was found. The results reveal that the photosynthetic apparatus, enriched in oligomeric forms of LHCII concomitant with decreased amount of anionic lipids and increased content of the monogalactosyldiacylglycerol (MGDG), is less sensitive to high light. Our data also suggest that the degree of LHCII oligomerization, as well as the lipid composition do not influence the degree of recovery of the PSII photochemistry after excess light exposure.

Molecular cloning and characterization of 4-hydroxyphenylpyruvate dioxygenase gene from Lactuca sativa

Vitamin E has been found to be associated with an important antioxidant property in mammals and plants. In photosynthetic organisms, the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD; E.C. 1.13.11.27) plays an important role in the vitamin E biosynthetic pathway. The full-length cDNA encoding HPPD was isolated from Lactuca sativa L. by rapid amplification of cDNA ends (RACE). The cDNA, designated as LsHPPD, was 1743 base pairs (bp) long containing an open reading frame (ORF) of 1338 bp encoding a protein of 446 amino acids. Sequence analysis indicated that LsHPPD shared high identity with HPPD from Medicago truncatula L. Real-time fluorescent quantitative PCR (qPCR) analysis revealed that LsHPPD was preferentially expressed in mature leaves compared with other tissues and that the LsHPPD expression was sensitive to high light and drought stress treatments. Transient expression of LsHPPD via agroinfiltration resulted in 12-fold increase in LsHPPD mRNA expression level and 4-fold enhancement in α-tocopherol content compared with the negative control. A decrease in chlorophyll content and inhibition of photosystem II were observed during stress treatments and agroinfiltration.

Cyclic electron flow around PSI functions in the photoinhibited rice leaves

We investigated the regulation mechanism of cyclic electron flow around photosystem I (CEF-PSI) in the rice leaves which suffered from photosystem II (PSII) photoinhibition. High-light (HL) treatment [2000 µmol photon m–2 s–1 at 0% carbon dioxide (CO2), 2% oxygen (O2) and 25°C] of rice leaves decreased both the maximum quantum efficiency of PSII (Fv/Fm) and the light-dependent O2-evolution rate [V(O2)]. High-light treatment did not affect the relative electron flux in PSI [Φ(PSI) × PFD]. In non-treated leaves, increasing in the photon flux density (PFD) enhanced V(O2), Φ(PSI) × PFD and the ratio of oxidized P700 to total P700 [(P700+)/(P700)total]. Φ(PSI) × PFD continued to increase even after the saturation of V(O2) against PFD. These results suggested that the electrons not used for the major electron sink, photosynthetic carbon reduction-cycle, did turnover in PSI, that is, CEF-PSI functioned at a higher PFD. High-light treatments did not affect the activity of CEF-PSI and increased (P700+)/(P700)total in the lower PFDs, compared to non-treated leaves. The activity of CEF-PSI depends on the amount of oxidized PQ. Photoinhibition of PSII suppressed electron influx from PSII to photosynthetic linear electron transport. The enhanced (P700+)/(P700)total suggested the increase in the ratio of oxidized plastoquinone (PQ) to total PQ, which supported the activity of CEF-PSI in the photoinhibited leaves.