Age-related differences in frost sensitivity of the photosynthetic apparatus of two Plagiomnium species

Shoots of two species of moss, Plagiomnium undulatum (Hedw.) Kop. and Plagiomnium affine (Funck) Kop., were subjected to freezing at various temperatures. After thawing, the activities of different photosynthetic reactions were determined in relation to the ages of the leaves. Analysis of the fast kinetics of chlorophyll-a fluorescence of individual leaves showed that young and old tissues were considerably less frost tolerant than mature ones. In principle, the pattern of freeze inactivation of photosynthetic reactions resembles that observed in higher plants. The decreases in the amplitude of F_v (variable fluorescence) and the ratio of F_v to F_m (maximum fluorescence) with increasing freezing stress reflect a progressive inactivation of photosystem II (PSII)-mediated electron transport, i.e. inhibition of photoreaction to photochemistry and-or electron donation to the photochemical reaction, and thus a decline in the potential photochemical efficiency of PSII. The insignificant change in the F₀ (constant fluorescence) level during progressive decline of F_v indicates that the excitation-energy transfer between antenna pigments and from those to reaction centres of PSII was little impaired by lethal freezing stress. Sugar analyses of various stem sections showed that ontogenetic variation in the frost tolerance of leaves cannot be attributed to differences in the cellular levels of sucrose, glucose and fructose.Abbreviations and Symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_m maximum fluorescence - F₀ constant (initial) fluorescence - F_v variable fluorescence     

两种生境条件下6种牧草叶绿素含量及荧光参数的比较

昆仑山前山牧场海拔较高, 策勒绿洲海拔相对较低, 两者生境差异较大。以昆仑山前山牧场和策勒绿洲边缘两种不同生境条件下生长的6种牧草: 冰草(Agropyron cristatum)、无芒雀麦(Bromus inermis)、矮生高羊茅(Festuca elata)、披碱草(Elymus dahuricus )、红豆草(Onobrychis pulchella)及和田大叶(Medicago sativa var. luxurians)为试验材料, 研究了不同生境条件下牧草叶片叶绿素含量及叶绿素荧光动力学参数的变化情况。结果显示: (1)在两种生境条件下, 昆仑山前山牧场生境生长的牧草叶绿素a、叶绿素b、总叶绿素的含量明显较高, 生长在策勒绿洲生境的牧草品种叶绿素a/b值较高; (2)昆仑山前山牧场生境牧草最大荧光、光系统II (PSII)最大光化学效率、PSII潜在活性和单位面积反应中心的数量的值明显高于策勒绿洲生境品种, 而初始荧光、单位反应中心吸收的光能、单位反应中心捕获的能量、单位反应中心耗散的能量、荧光诱导曲线初始斜率值则低于策勒绿洲生境品种。因此, 两种生境下环境因子发生了改变, 对牧草产生综合的胁迫作用; 策勒绿洲生境明显对牧草生长产生了抑制, 策勒绿洲生境牧草的色素含量降低以及PSII的机构遭到损坏, 导致反应中心一部分失活或裂解, 剩余有活性的反应中心的效率增加, 昆仑山生境则相对比较适宜牧草生长; 两种生境不同牧草叶绿素含量和叶绿素荧光参数的变化幅度不同。     

Photosynthetic properties of <Emphasis Type="Italic">Quercus</Emphasis> × <Emphasis Type="Italic">hispanica</Emphasis> Lam. and <Emphasis Type="Italic">Q. suber</Emphasis> L. under harsh Central European winter conditions

In search for new forestation tree species for future Central European climate conditions, Mediterranean evergreen oak taxa are investigated for their summer drought- and winter frost-hardiness. Here we report on the winter performance of the photosynthetic apparatus of Quercus × hispanica Lam. and its evergreen parental species Q. suber L. under extraordinary harsh winter conditions. Both taxa showed a strong decline of photosystem II (PSII) quantum efficiency (F_v/F_m) with a concomitant increase in the deepoxidation state (DES) of the xanthophyll pigments depending on (severe) frost events during winter, and these parameters significantly correlated with minimum air temperatures during periods of chronic photoinhibition at mid-winter, but not at the onset of winter in response to the first frost nights. F_v/F_m and DES correlated with each other in both taxa throughout the winter.     

PSII photochemistry, thermal energy dissipation, and the xanthophyll cycle in Kalanchoë daigremontiana exposed to a combination of water stress and high light

Kalanchoë daigremontiana, a CAM plant grown in a greenhouse, was subjected to severe water stress. The changes in photosystem II (PSII) photochemistry were investigated in water‐stressed leaves. To separate water stress effects from photoinhibition, water stress was imposed at low irradiance (daily peak PFD 150 μmol m^?2 s^?1). There were no significant changes in the maximal efficiency of PSII photochemistry (F_v/F_m), the traditional fluorescence induction kinetics (OIP) and the polyphasic fluorescence induction kinetics (OJIP), suggesting that water stress had no direct effects on the primary PSII photochemistry in dark‐adapted leaves. However, PSII photochemistry in light‐adapted leaves was modified in water‐stressed plants. This was shown by the decrease in the actual PSII efficiency (Φ_PSII), the efficiency of excitation energy capture by open PSII centres (F_v′/F_m′), and photochemical quenching (q_P), as well as a significant increase in non‐photochemical quenching (NPQ) in particular at high PFDs. In addition, photoinhibition and the xanthophyll cycle were investigated in water‐stressed leaves when exposed to 50% full sunlight and full sunlight. At midday, water stress induced a substantial decrease in F_v/F_m which was reversible. Such a decrease was greater at higher irradiance. Similar results were observed in Φ_PSII, q_P, and F_v′/F_m′. On the other hand, water stress induced a significant increase in NPQ and the level of zeaxanthin via the de‐epoxidation of violaxanthin and their increases were greater at higher irradiance. The results suggest that water stress led to increased susceptibility to photoinhibition which was attributed to a photoprotective process but not to a photodamage process. Such a photoprotection was associated with the enhanced formation of zeaxanthin via de‐epoxidation of violaxanthin. The results also suggest that thermal dissipation of excess energy associated with the xanthophyll cycle may be an important adaptive mechanism to help protect the photosynthetic apparatus from photoinhibitory damage for CAM plants normally growing in arid and semi‐arid areas where they are subjected to a combination of water stress and high light.     

Response of chlorophyll fluorescence parameters to drought stress in sugar beet seedlings

We compared the parameters of chlorophyll fluorescence between two sugar beet (Beta vulgaris L.) species differing in drought tolerance. Our results indicated that there were different responses to the drought stress of these sugar beet species. In drought-tolerant sugar beet, the F ₀ increased slightly, while qN increased substantially, indicating that these plants can protect PSII reaction centers from the damage. F _v/F _m and qP decreased slightly during the initial period of drought stress; this suggests that there is a slight impact of drought stress on the openness of PSII reaction centers, and thus the plants did not suffer seriously. This was further shown by the decreased Yield and electron transfer rate. The parameters of chlorophyll fluorescence were stable and can be used as an important indicator for sugar beet seedlings in the early drought tolerance.     

Seasonal Patterns of Photosystem 2 Photochemical Efficiency in Evergreen Sclerophylls and Drought Semi-Deciduous Shrubs under Mediterranean Field Conditions

Photochemical efficiency of photosystem 2 (PS2), assessed from in situ chlorophyll (Chl) fluorescence measurements, was seasonally monitored in five evergreen sclerophyll and five malacophyllous drought semi-deciduous species, co-occurring in the same Mediterranean field site. In evergreen sclerophylls, a considerable drop in the variable (F_v) to maximum (F_m) Chl fluorescence ratio coincided with the lowest winter temperatures, indicating low PS2 efficiency during this period. Summer drought caused a comparatively slight decrease in F_v/F_m and only in three of the five evergreen sclerophyll species tested. In drought semi-deciduous shrubs, the winter drop in F_v/F_m was much less conspicuous. During the summer, and in spite of the severe and prolonged desiccation of their malacophyllous leaves, F_v/F_m was maintained high and only in one species the PS2 efficiency was transiently suppressed, when the leaf relative water content became lower than 30 %. Thus evergreen sclerophylls are more prone to photoinhibition by low winter temperatures, while the sensitivity of drought semi-deciduals depends on the extent and duration of summer drought. This revised version was published online in September 2006 with corrections to the Cover Date.     

Effect of Glomus mosseae on chlorophyll content, chlorophyll fluorescence parameters, and chloroplast ultrastructure of beach plum (Prunus maritima) under NaCl stress

The present study was undertaken to investigate the effect of Glomus mosseae on chlorophyll (Chl) content, Chl fluorescence parameters and chloroplast ultrastructure of beach plum seedlings under 2% NaCl stress. The results showed that compared to control, both Chl a and Chl b contents of NaCl + G. mosseae treatment were significantly lower during the salt stress, while Chl a/b ratio increased significantly. The increase of minimal fluorescence of darkadapted state (F₀), and the decrease of maximal fluorescence of dark-adapted state (F_m) and variable fluorescence (F_v) values were inhibited. The maximum quantum yield of PSII photochemistry (F_v/F_m), the maximum energy transformation potential of PSII photochemistry (F_v/F₀) and the effective quantum yield of PSII photochemistry (??_PSII) increased significantly, especially the latter two variables. The values of the photochemical quenching coefficient (q_P) and the nonphotochemical quenching (NPQ) were similar between G. mosseae inoculation and noninoculation. It could be concluded that G. mosseae inoculation could protect the photosystem II (PSII) of beach plum, enhance the efficiency of primary light energy conversion and improve the primitive response of photosynthesis under salinity stress. Meanwhile, G. mosseae inoculation was beneficial to maintain the integrity of thylakoid membrane and to protect the structure and function of chloroplast, which suggested that G. mosseae can alleviate the damage of NaCl stress to chloroplast.     

Gender, mediterranean drought, and seasonality: photosystem II photochemistry in Pistacia lentiscus L.

In this work, photosystem II (PSII) photochemistry, leaf water potential, and pigment contents of male and female Pistacia lentiscus L. were investigated during a seasonal cycle at three different, arid locations: superior semiarid, inferior semiarid, and arid. The results showed that the gender, season, and the site conditions interacted to influence the quantum yield and pigment contents in P. lentiscus. Predawn leaf water status was determined only by the site and season. The annual patterns of PSII maximum quantum efficiency (F_v/F_m) were characterized by a suboptimal activity during the winter, especially, populations with the more negative water potential exhibited a lower chlorophyll (Chl) a content and chronic photoinhibition irrespective of a gender. We also demonstrated that both photochemical or nonphotochemical mechanisms were involved to avoid the photoinhibition and both of them depended on the season. This plasticity of photosynthetic machinery was accompanied by changes in carotenoids and Chl balance. In the spring, the female F_v/F_m ratio was significantly higher than in male individuals, when the sexual dimorphism occurred during the fruiting stage, regardless of site conditions. P. lentiscus sex-ratio in Mediterranean areas, where precipitations exceeded 500 mm, was potentially female-biased. Among the fluorescence parameters investigated, nonphotochemical quenching coefficient appeared as the most useful one and a correlation was found between Chl a content and F_v/F_m. These results suggest that functional ecology studies would be possible on a large scale through light reflectance analysis.     

Leaf structure and patterns of photoinhibition in two neotropical palms in clearings and forest understory during the dry season

We studied the leaf structural, water status, and fast fluorescence responses of two palms, Socratea exorrhiza and Scheelea zonensis, under natural dry season conditions in a clearing (high light [HL] palms) and the forest understory (low light [LL] palms) on Barro Colorado Island, Panama. HL-Socratea leaves were more shade-adapted, less xeromorphic, and more strongly affected by drought than HL-Scheelea. F_v/F_m (the ratio of variable to maximum chlorophyll fluorescence) and t_½ (the half-rise time of F_m) was lower in HL-leaves of both species, indicating photoinhibition. In HL-Scheelea, the light-induced reduction of F_v/F_m was much less than in HL-Socratea, and F_v/F_m recovered completely overnight. Patterns of relative water content, specific leaf dry weight, stable carbon isotope composition, and leaf conductance suggest that increased drought resistance in Scheelea reduces susceptibility to photoinhibition. An increase in F_o indicated the inactivation of PSII reaction centers in HL-Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL-Socratea are consistent with photoinhibition. Under LL, the species showed no appreciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of F_v/F_m in HL-plants relative to LL-plants on both leaf surfaces, particularly on the lower surface, due to a decrease of F_m in both surfaces and an increase in F., of lower surface. For both species, F_o for the lower surfaces of HL-plants was higher and t_½ was markedly lower than for the upper surface, as is typical for shade-adapted leaves. Xeromorphic leaf structure may reduce susceptibility to photoinhibition during the dry season. Drought-enhanced photoinhibition could limit the ability of some species to exploit treefall gaps.     

Similar thermal breadth of two temperate coral species from the Mediterranean Sea and two tropical coral species from the Great Barrier Reef

Temperate organisms are generally exposed to a more variable and cooler climate than tropical organisms, and are therefore expected to have broader thermal tolerance and a different thermal performance curve. This study investigated these hypotheses by comparing the thermal performance of two common tropical coral species found in the Great Barrier Reef with the two most common temperate coral species from the Mediterranean Sea. Photosynthesis rates, dark respiration rates, maximum PSII quantum yield (F_v/F_m) and electron transport rates (rETR_m) were measured on coral fragments exposed to an acute temperature increase and decrease up to 5 °C above and below the average environmental seawater temperature. Dark respiration rates and F_v/F_m increased linearly with temperature, suggesting broad thermal tolerance. For photosynthesis and rETR_m, the performance breadths were surprisingly similar between the tropical and temperate species. However, the thermal optimum for performance was generally below the local average temperature, and only coincided with the prevailing environmental temperature for one of the tropical species. The broad thermal tolerance for photosynthesis displayed in this study supports previous observations that corals can survive short periods of abnormally warm temperatures and suggests that corals adopt thermal generalist strategies to cope with temperature variation in the environment. Nevertheless, current mean temperatures are 10–30% above the thermal optimum for the species studied here, demonstrating that conditions are already pushing the boundaries of coral thermal tolerance.

     

The interplay of anthocyanin biosynthesis and chlorophyll catabolism in senescing leaves and the question of photosystem II photoprotection

Fully exposed, senescing leaves of Cornus sanguinea and Parthenocissus quinquefolia display during autumn considerable variation in both anthocyanin and chlorophyll (Chl) concentrations. They were used in this study to test the hypothesis that anthocyanins may have a photoprotective function against photosystem II (PSII) photoinhibitory damage. The hypothesis could not be confirmed with field sampled leaves since maximum photochemical efficiency (F_v/F_m) of PSII was negatively correlated to anthocyanin concentration and the possible effects of anthocyanins were also confounded by a decrease in F_v/F_m with Chl loss. However, after short-term laboratory photoinhibitory trials, the percent decrease of F_v/F_m was independent of Chl concentration. In this case, a slight alleviation of PSII damage with increasing anthocyanins was observed in P. quinquefolia, while a similar trend in C. sanguinea was not statistically significant. It is inferred that the assumed photoprotection, if addressed to PSII, may be of limited advantage and only under adverse environmental conditions.     

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation

CO₂ assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol^-1 of ozone (5 days, 5 h day^-1). Gas-exchange measurements revealed a reduction in CO₂ assimilation and stomatal conductance, accompanied by an increase in the intercellular CO₂ concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F₀ remained unchanged, while F_m and the F_v/F_m ratio decreased after O₃ fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor Q_A. In the mature leaves the photochemical quenching (q_p) was significantly lowered by the pollutant, but this was not the case in the young leaves where q_p did not change. In both mature and young ozonated pumpkin leaves, the development of non-photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between F_v/F_m and the de-epoxidation state of dark-adapted leaves. Decreases in the F_v/F_m ratio are generally attributed to damage to the PSII reaction centre, apart from the down-regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the F_v/F_m ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the F_v/F_m was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.     

Seasonal changes in photosystem II organisation and pigment composition in Pinus sylvestris

Conifers of the boreal zone encounter considerable combined stress of low temperature and high light during winter, when photosynthetic consumption of excitation energy is blocked. In the evergreen Pinus sylvestris L. these stresses coincided with major seasonal changes in photosystem II (PSII) organisation and pigment composition. The earliest changes occurred in September, before any freezing stress, with initial losses of chlorophyll, the D1-protein of the PSII reaction centre and of PSII light-harvesting-complex (LHC II) proteins. In October there was a transient increase in F₀, resulting from detachment of the light-harvesting antennae as reaction centres lost D1. The D1-protein content eventually decreased to 90%, reaching a minimum by December, but PSII photochemical efficiency [variable fluorescence (F_v)/maximum fluorescence (F_m)] did not reach the winter minimum until mid-February. The carotenoid composition varied seasonally with a twofold increase in lutein and the carotenoids of the xanthophyll cycle during winter, while the epoxidation state of the xanthophylls decreased from 0.9 to 0.1 from October to January. The loss of chlorophyll was complete by October and during winter much of the remaining chlorophyll was reorganised in aggregates of specific polypeptide composition, which apparently efficiently quench excitation energy through non-radiative dissipation. The timing of the autumn and winter changes indicated that xanthophyll de-epoxidation correlates with winter quenching of chlorophyll fluorescence while the drop in photochemical efficiency relates more to loss of D1-protein. In April and May recovery of the photochemistry of PSII, protein synthesis, pigment rearrangements and zeaxanthin epoxidation occurred concomitantly. Indoor recovery of photosynthesis in winter-stressed branches under favourable conditions was completed within 3 d, with rapid increases in F₀, the epoxidation state of the xanthophylls and in light-harvesting polypeptides, followed by recovery of D1-protein content and F_v/F_m, all without net increase in chlorophyll. The fall and winter reorganisation allow Pinus sylvestris to maintain a large stock of chlorophyll in a quenched, photoprotected state, allowing rapid recovery of photosynthesis in spring.Abbreviations Elips early light-induced proteins - EPS epoxidation state - F₀ instantaneous fluorescence - F_m maximum fluorescence - F_v variable fluorescence - LHC II light-harvesting complex of PSII - LiDS lithium dodecyl sulfate This research was supported by the Swedish Natural Science Research Council. We wish to thank Dr. Adrian Clarke¹ (Department of Plant Physiology, University of Umeå, Sweden) for advice on electrophoresis, valuable discussion and providing antibodies. Dr. Stefan Jansson¹ and Dr. Torill Hundal (Department for Biochemistry, University of Stockholm, Sweden) provided antibodies. Jan Karlsson¹ helped with the HPLC, Dr. Marianna Krol gave advice on green gels and Dr. Vaughan Hurry (Cooperative Research Centre for Plant Sciences, Australian National University, Canberra, Australia) provided valuable discussion.     

Metabolic recovery of the Antarctic liverwort <Emphasis Type="Italic">Cephaloziella varians</Emphasis> during spring snowmelt

We measured the responses of pigments and chlorophyll a fluorescence parameters of the Antarctic leafy liverwort Cephaloziella varians to snowmelt during austral spring 2005 at Rothera Point on the western Antarctic Peninsula. Although no changes to the concentrations of UV-B photoprotective pigments were detected during snowmelt, chlorophyll and carotenoid concentrations and maximum photosystem (PS)II yield (F _v /F _m) were respectively 88, 60 and 144% higher in the tissues of the liverwort that had recently emerged from snow than in those under a 10 cm depth of snow. A laboratory experiment similarly showed that effective PSII yield increased rapidly within the first 45 min after plants sampled from under snow were removed to an illuminated growth cabinet. The pigmentation and PSII yields of plants during snowmelt were also compared with those of plants in January, during the middle of the growing season at Rothera Point. During snowmelt, plants had lower F _v /F _m values, chlorophyll a/b ratios and concentrations of UV-B photoprotective pigments and carotenoids than during mid-season, suggesting that although there is some recovery of PSII activity and increases in concentrations of photosynthetic pigments during snowmelt, the metabolism of C. varians is restricted during this period.     

Genotypic differences in the responses of gas exchange,chlorophyll fluorescence,and antioxidant enzymes to aluminum stress in <Emphasis Type="Italic">Festuca arundinacea</Emphasis>

In this study, the gas exchange, chlorophyll fluorescence, and antioxidant activity in eight tall fescue cultivars were investigated under aluminum stress. The results showed that the net photosynthetic rate (P _N) and stomatal conductance (g _s) were decreased, while the intercellular CO₂ concentration (C_i) was stable or increased under Al stress conditions. The efficiency of excitation capture by open PSII reaction centers (F′_v/F′_m), the maximum quantum yield of PSII photochemistry (F _v/F _m), the quantum yield of PSII electron transport (Φ_PSII), and the photochemical quenching (qP) were also decreased after Al stress, while the non-photochemical quenching (NPQ) was increased. Moreover, Al stress increased the antioxidant activities and MDA contents in each tall fescue cultivars. However, there was a lot genotype differences between the Al-tolerant and Al-sensitive cultivars. Cv. Barrington was the most sensitive cultivar and cv. Crossfire 2 was the most tolerant cultivar. The excessive excitation energy could not be dissipated efficiently by antenna pigments, and reactive oxygen species could not be scavenged efficiently, thereby resulting in membrane lipid peroxidation in cv. Barrington under Al stress conditions.     

Enhanced tolerance of photosynthesis against high temperature damage in salt-adapted halophyte Atriplex centralasiatica plants

Thermotolerance of photosynthesis in salt‐adapted Atriplex centralasiatica plants (100–400 mm NaCl) was evaluated in this study after detached leaves and whole plants were exposed to high temperature stress (30–48 °C) either in the dark or under high light (1200 mol m^?2 s^?1). In parallel with the decrease in stomatal conductance, intercellular CO₂ concentration and CO₂ assimilation rate decreased significantly with increasing salt concentration. There was no change in the maximal efficiency of PSII photochemistry (F_v/F_m) with increasing salt concentration, suggesting that there was no damage to PSII in salt‐adapted plants. On the other hand, there was a striking difference in the response of PSII and CO₂ assimilation capacity to heat stress in non‐salt‐adapted and salt‐adapted leaves. Leaves from salt‐adapted plants maintained significantly higher F_v/F_m values than those from non‐salt‐adapted leaves at temperatures higher than 42 °C. The F_v/F_m differences between non‐salt‐adapted and salt‐adapted plants persisted for at least 24 h following heat stress. Leaves from salt‐adapted plants also maintained a higher net CO₂ assimilation rate than those in non‐salt‐adapted plants at temperatures higher than 42 °C. This increased thermotolerance was independent of the degree of salinity since no significant changes in F_v/F_m and net CO₂ assimilation rate were observed among the plants treated with different concentrations of NaCl. The increased thermotolerance of PSII induced by salinity was still evident when heat treatments were carried out under high light. Given that photosynthesis is considered to be the physiological process most sensitive to high temperature damage, increased thermotolerance of photosynthesis may be of significance since A. centralasiatica, a typical halophyte, grows in the high salinity regions in the north of China, where the temperature in the summer is often as high as 45 °C.     

Two components of onset and recovery during photoinhibition of Ulva rotundata

Short-term (up to 5 h) transfers of shade-adapted (100 mol · m^–2 · s^–1) clonal tissue of the marine macroalga Ulva rotundata Blid. (Chlorophyta) to higher irradiances (1700, 850, and 350 mol · m^–2 · s^–1) led to photoinhibition of room-temperature chlorophyll fluorescence and O₂ evolution. The ratio of variable to maximum (F_v/F_m) and variable (F_v) fluorescence, and quantum yield () declined with increasing irradiance and duration of exposure. This decline could be resolved into two components, consistent with the separation of photoinhibition into energy-dissipative processes (photoprotection) and damage to photosystem II (PSII) by excess excitation. The first component, a rapid decrease in F_v/F_m and in F_v, corresponds to an increase in initial (F_o) fluorescence and is highly sensitive to 1 mM chloramphenicol. This component is rapidly reversible under dim (40 mol · m^–2 · s^–1) light, but is less reversible with increasing duration of exposure, and may reflect damage to PSII. The second (after 1 h exposure) component, a slower decline in F_v/F_m and F_v with declining F_o, appears to be associated with the photoprotective interconversion of violaxanthin to zeaxanthin and is sensitive to dithiothreitol. The accumulation of zeaxanthin in U. rotundata is very slow, and may account for the predominance of increases in F_o at high irradiances.Abbreviations and Symbols CAP chloramphenicol - DTT dithiothreitol - F_o, F_m, F_v initial, maximum, and variable fluorescence - quantum yield - PFD photon flux density - PSII photosystem II To whom correspondence should be addressedWe are grateful to O. Björkman and S. Thayer, Carnegie Institution of Washington, Stanford, Cal., USA, for analysis of xanthophyll pigments reported here. This research was supported by National Science Foundation grant OCE-8812157 to C.B.O. and J.R. Support for G.L. was provided by a NSF-CNRS (Centre National de la Recherche Scientifique) exchange fellowship.     

Characterization of potato genotypes by chlorophyll fluorescence during plant aging in a Mediterranean environment

Field experiments were conducted in Sicily (south Italy) during two seasons to characterize by chlorophyll (Chl) fluorescence four genotypes (Spunta, Sieglinde, Daytona, and Ninfa) of potato (Solanum tuberosum L.) for off-season production during plant aging and to analyse the possible relation between Chl parameters and tuber yield. Chl fluorescence parameters [initial fluorescence (F₀), maximum fluorescence (F_m), F_v/F_m, time in which maximal fluorescence occurs (T_max)] gained from Kautsky kinetics and Chl content were measured weekly, from 5^th to 6^th leaf appearance to beginning of plant senescence in the first season and to full plant senescence in the second season. F₀ and F_v/F_m were the most reliable Chl fluorescence parameters for the definition of genotypic differences while Chl content and T_max were the most reliable Chl parameters to predict plant aging. Tuber yield was highly correlated with Chl content, T_max, F₀, and F_m.     

Juniper Shade Enables Terricolous Lichens and Mosses to Maintain High Photochemical Efficiency in a Semiarid Temperate Sand Grassland

On a semiarid sand grassland (Festucetum vaginatae) colonised by juniper (Juniperus communis L.) shrubs terricolous lichens and mosses segregate strongly between microhabitats: certain species grow in the open grassland, others almost exclusively in the shade of junipers. The contrasting irradiances of these microhabitats influence much the metabolism of these organisms, and thus affect their small-scale distribution. This was confirmed by determining the efficiency of photochemical energy conversion by measuring chlorophyll a fluorescence parameters. In the open grassland maximum photochemical efficiency of photosystem 2 (PS2, F_v/F_m) declined from the humid spring to the hot and dry summer in all species, and this was caused by an increase in base fluorescence (F₀), but not by the decrease in fluorescence maximum (F_m). In summer, mosses and lichens growing in the open grassland generally possessed lower F_v/F_m than cryptogams growing in the shade cast by juniper shrubs. Thus mosses and lichens in the open grassland suffer lasting reduction in photochemical efficiency in summer, which is avoided in the shade of junipers. Juniper shrubs indeed influence the composition and small-scale spatial pattern of sympatric terricolous lichen and moss communities by—among others—providing a shelter against high light in summer.     

Xanthophyll cycle and light stress in nature: uniform response to excess direct sunlight among higher plant species

Photosystem II (PS II) efficiency, nonphotochemical fluorescence quenching, and xanthophyll cycle composition were determined in situ in the natural environment at midday in (i) a range of differently angled sun leaves ofEuonymus kiautschovicus Loesener and (ii) in sun leaves of a wide range of different plant species, including trees, shrubs, and herbs. Very different degrees of light stress were experienced by these leaves (i) in response to different levels of incident photon flux densities at similar photosynthetic capacities amongEuonymus leaves and (ii) as a result of very different photosynthetic capacities among species at similar incident photon flux densities (that were equivalent to full sunlight). ForEuonymus as well as the interspecific comparison all data fell on one single, close relationship for changes in intrinsic PSII efficiency, nonphotochemical fluorescence quenching, or the levels of zeaxanthin + antheraxanthin in leaves, respectively, as a function of the actual level of light stress. Thus, the same conversion state of the xanthophyll cycle and the same level of energy dissipation were observed for a given degree of light stress independent of species or conditions causing the light stress. Since all increases in thermal energy dissipation were associated with increases in the levels of zeaxanthin + antheraxanthin in these leaves, there was thus no indication of any form of xanthophyll cycle-independent energy dissipation in any of the twenty-four species or varieties of plants examined in their natural environment. It is also concluded that transient diurnal changes in intrinsic PSII efficiency in nature are caused by changes in the efficiency with which excitation energy is delivered from the antennae to PSII centers, and are thus likely to be purely photoprotective. Consequently, the possibility of quantifying the allocation of absorbed light into PSII photochemistry versus energy dissipation in the antennae from changes in intrinsic PSII efficiency is explored.Abbreviations A antheraxanthin - F actual level of fluorescence - F_a, F _o minimal fluorescence in the absence, presence of thylakoid energization - F_m, F _m maximal fluorescence in the absence, presence of thylakoid energization - F_m, - F)/F _m actual PSII efficiency ( = percent of absorbed light utilized in PSII photochemistry) - F_v/F_m, F _v /F_m/ PSII efficiency of open centers in the absence, presence of thylakoid energization - NPQ nonphotochemical fluorescence quenching - F_m/F _m - 1; q_p quenching coefficient for photochemical quenching - V violaxanthin - Z zeaxanthin