Mechanisms of drought-induced dissipation of excitation energy in sun- and shade-adapted drought-tolerant mosses studied by fluorescence yield change and global and target analysis of fluorescence decay kinetics.

Some mosses stay green and survive long even under desiccation. Dissipation mechanisms of excess excitation energy were studied in two drought-tolerant moss species adapted to contrasting niches: shade-adapted Rhytidiadelphus squarrosus and sun-adapted Rhytidium rugosum in the same family. (1) Under wet conditions, a light-induced nonphotochemical quenching (NPQ) mechanism decreased the yield of photosystem II (PSII) fluorescence in both species. The NPQ extent saturated at a lower illumination intensity in R. squarrosus, suggesting a larger PSII antenna size. (2) Desiccation reduced the fluorescence intensities giving significantly lower F 0 levels and shortened the overall fluorescence lifetimes in both R. squarrosus and R. rugosum, at room temperature. (3) At 77 K, desiccation strongly reduced the PSII fluorescence intensity. This reduction was smaller in R. squarrosus than in R. rugosum. (4) Global and target analysis indicated two different mechanisms of energy dissipation in PSII under desiccation: the energy dissipation to a desiccation-formed strong fluorescence quencher in the PSII core in sun-adapted R. rugosum (type-A quenching) and (5) the moderate energy dissipation in the light-harvesting complex/PSII in shade-adapted R. squarrosus (type-B quenching). The two mechanisms are consistent with the different ecological niches of the two mosses.

Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system.

Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars or as probes of the extreme environments in which low-mass white dwarfs form cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes.

Synthesis and structure of tricarbonyl(η(6)-arene)chromium complexes of phenyl and benzyl D-glycopyranosides.

A series of 15 glycoside-derived tricarbonyl(η(6)-arene)chromium complexes were prepared in 19-87% yield by heating fully acetylated or methylated aryl O-, S-, N- and C-glycosides of D-glucopyranose and D-mannopyranose with hexacarbonylchromium. All tricarbonylchromium complexes were fully characterized. The structures of nine crystalline complexes were determined by X-ray diffraction, revealing unusual intra- and intermolecular nonclassical hydrogen bonds.

David Alan Walker (1928-2012).

David Alan Walker, Emeritus Professor of Biology, University of Sheffield, UK and Fellow of the Royal Society, died on February 13, 2012. David had a marvelous 60 year career as a scientist, during which he was a researcher, mentor, valued colleague, and a prolific writer in the field of photosynthesis. His career was marked by creative breakthroughs in isolation and analysis of chloroplast metabolism in vitro and simple but valuable technical advances for measurement of photosynthesis in vivo that remain relevant on a global scale to production of crops and biofuels, as well as plant responses to climate change. We include here personal remembrances by the authors (GEE and UH), and by (in alphabetical order): Zoran Cerovic (France), Bob Furbank (Australia), Geoffrey Hind (USA), John Humby (UK), Agu Laisk (Estonia), Peter Lea (UK), Ross Lilley (Australia), Barry Osmond (Australia), Simon Robinson (Australia) and Charles Stirling (UK).

Conservation and dissipation of light energy in desiccation-tolerant photoautotrophs, two sides of the same coin.

Conservation of light energy in photosynthesis is possible only in hydrated photoautotrophs. It requires complex biochemistry and is limited in capacity. Charge separation in reaction centres of photosystem II initiates energy conservation but opens also the path to photooxidative damage. A main mechanism of photoprotection active in hydrated photoautotrophs is controlled by light. This is achieved by coupling light flux to the protonation of a special thylakoid protein which activates thermal energy dissipation. This mechanism facilitates the simultaneous occurrence of energy conservation and energy dissipation but cannot completely prevent damage by light. Continuous metabolic repair is required to compensate damage. More efficient photoprotection is needed by desiccation-tolerant photoautotrophs. Loss of water during desiccation activates ultra-fast energy dissipation in mosses and lichens. Desiccation-induced energy dissipation neither requires a protonation reaction nor light but photoprotection often increases when light is present during desiccation. Two different mechanisms contribute to photoprotection of desiccated photoautotrophs. One facilitates energy dissipation in the antenna of photosystem II which is faster than energy capture by functional reaction centres. When this is insufficient for full photoprotection, the other one permits energy dissipation in the reaction centres themselves.

Three different mechanisms of energy dissipation of a desiccation-tolerant moss serve one common purpose: to protect reaction centres against photo-oxidation.

Three different types of non-photochemical de-excitation of absorbed light energy protect photosystem II of the sun- and desiccation-tolerant moss Rhytidium rugosum against photo-oxidation. The first mechanism, which is light-induced in hydrated thalli, is sensitive to inhibition by dithiothreitol. It is controlled by the protonation of a thylakoid protein. Other mechanisms are activated by desiccation. One of them permits exciton migration towards a far-red band in the antenna pigments where fast thermal deactivation takes place. This mechanism appears to be similar to a mechanism detected before in desiccated lichens. A third mechanism is based on the reversible photo-accumulation of a radical that acts as a quencher of excitation energy in reaction centres of photosystem II. On the basis of absorption changes around 800 nm, the quencher is suggested to be an oxidized chlorophyll. The data show that desiccated moss is better protected against photo-oxidative damage than hydrated moss. Slow drying of moss thalli in the light increases photo-protection more than slow drying in darkness.

From horse thief to professor: confessions of a plant physiologist.

Can 50 years of research, performed between ignorance and the wish to know, and executed between hope, despair, satisfaction and pain, be compressed into an abstract? What has been done in more than 50 years may be expressed in four words: it was worth it. If I had another life, I would do it again. In the beginning of my career, life was an enigma. It still is. Molecular details of the workings of life had been largely unknown when I began. Now, at the end, I still wish to know details: how is light, master of life, manipulated to either support life, when photosynthesis is possible, or to protect it when light endangers it. What is the molecular and the physical nature of the biological mechanisms which control both, energy conservation and energy dissipation, in photosynthesis?

Photoprotection of reaction centers: thermal dissipation of absorbed light energy vs charge separation in lichens.

During desiccation, fluorescence emission and stable light-dependent charge separation in the reaction centers (RCs) of photosystem II (PSII) declined strongly in three different lichens: in Parmelia sulcata with an alga as the photobiont, in Peltigera neckeri with a cyanobacterium and in the tripartite lichen Lobaria pulmonaria. Most of the decline of fluorescence was caused by a decrease in the quantum efficiency of fluorescence emission. It indicated the activation of photoprotective thermal energy dissipation. Photochemical activity of the RCs was retained even after complete desiccation. It led to light-dependent absorption changes and found expression in reversible increases in fluorescence or in fluorescence quenching. Lowering the temperature changed the direction of fluorescence responses in P. sulcata. The observations are interpreted to show that reversible light-induced increases in fluorescence emission in desiccated lichens indicate the functionality of the RCs of PSII. Photoprotection is achieved by the drainage of light energy to dissipating centers outside the RCs before stable charge separation can take place. Reversible quenching of fluorescence by strong illumination is suggested to indicate the conversion of the RCs from energy conserving to energy dissipating units. This permits them to avoid photoinactivation. On hydration, re-conversion occurs to energy-conserving RCs.

Photoprotection of reaction centres in photosynthetic organisms: mechanisms of thermal energy dissipation in desiccated thalli of the lichen Lobaria pulmonaria.

*The photobionts of lichens have previously been shown to reversibly inactivate their photosystem II (PSII) upon desiccation, presumably as a photoprotective mechanism. The mechanism and the consequences of this process have been investigated in the green algal lichen Lobaria pulmonaria. *Lichen thalli were collected from a shaded and a sun-exposed site. The activation of PSII was followed by chlorophyll fluorescence measurements. *Inactivation of PSII, as indicated by the total loss of variable fluorescence, was accompanied by a strong decrease of basal fluorescence (F(0)). Sun-grown thalli, as well as thalli exposed to low irradiance during drying, showed a larger reduction of F(0) than shade-grown thalli or thalli desiccated in the dark. Desiccation increased phototolerance, which was positively correlated to enhanced quenching of F(0). Quenching of F(0) could be reversed by heating, and could be inhibited by glutaraldehyde but not by the uncoupler nigericin. *Activation of energy dissipation, apparent as F(0) quenching, is proposed to be based on an alteration in the conformation of a pigment protein complex. This permits thermal energy dissipation and gives considerable flexibility to photoprotection. Zeaxanthin formation apparently did not contribute to the enhancement of photoprotection by desiccation in the light. Light-induced absorbance changes indicated the involvement of chlorophyll and carotenoid cation radicals.

Photoprotection of green plants: a mechanism of ultra-fast thermal energy dissipation in desiccated lichens.

In order to survive sunlight in the absence of water, desiccation-tolerant green plants need to be protected against photooxidation. During drying of the chlorolichen Cladonia rangiformis and the cyanolichen Peltigera neckeri, chlorophyll fluorescence decreased and stable light-dependent charge separation in reaction centers of the photosynthetic apparatus was lost. The presence of light during desiccation increased loss of fluorescence in the chlorolichen more than that in the cyanolichen. Heating of desiccated Cladonia thalli, but not of Peltigera thalli, increased fluorescence emission more after the lichen had been dried in the light than after drying in darkness. Activation of zeaxanthin-dependent energy dissipation by protonation of the PsbS protein of thylakoid membranes was not responsible for the increased loss of chlorophyll fluorescence by the chlorolichen during drying in the light. Glutaraldehyde inhibited loss of chlorophyll fluorescence during drying. Desiccation-induced loss of chlorophyll fluorescence and of light-dependent charge separation are interpreted to indicate activation of a highly effective mechanism of photoprotection in the lichens. Activation is based on desiccation-induced conformational changes of a pigment-protein complex. Absorbed light energy is converted into heat within a picosecond or femtosecond time domain. When present during desiccation, light interacts with the structural changes of the protein providing increased photoprotection. Energy dissipation is inactivated and structural changes are reversed when water becomes available again. Reversibility of ultra-fast thermal dissipation of light energy avoids photo-damage in the absence of water and facilitates the use of light for photosynthesis almost as soon as water becomes available.

Activation of mechanisms of photoprotection by desiccation and by light: poikilohydric photoautotrophs.

Mechanisms of protection against photo-oxidation in selected desiccation-tolerant lichens and mosses have been investigated by measuring loss of light absorption during desiccation and chlorophyll fluorescence as indicators of photoprotection. Apparent absorption (1-T) spectra measured in the reflectance mode revealed stronger absorption of photosynthetic pigments in hydrated than in desiccated organisms, but differences were pronounced only in a cyanolichen, less so in some chlorolichens, and even less in mosses. Since the amplitude of chlorophyll fluorescence is a product of (1-T) light absorption by chlorophyll and quantum yield of fluorescence, and since fluorescence is inversely related to thermal energy dissipation, when chemical fluorescence quenching is negligible, fluorescence measurements were used to measure changes in energy dissipation. Preincubation of the hydrated organisms and desiccation in darkness excluded the contribution of mechanisms of energy dissipation to photoprotection which are dependent on the presence of zeaxanthin or on the light-dependent formation of a quencher of fluorescence within the reaction centre of photosystem II. Fast drying in darkness or in very low light was less effective in decreasing chlorophyll fluorescence than slow drying. Heating the desiccated organisms increased fluorescence by inactivating the mechanism responsible for fluorescence quenching. Glutaraldehyde inhibited fluorescence quenching during desiccation. Prolonged exposure of a desiccated moss or a desiccated lichen to very strong light caused more photo-induced damage after fast drying than after slow drying. The photo-oxidative nature of damage was emphasized by the observation that irreversible loss of fluorescence was larger in air than in a nitrogen atmosphere. It is concluded from these observations that desiccation-induced conformational changes of a chlorophyll protein complex result in the fast radiationless dissipation of absorbed light energy. This mechanism of photoprotection is more effective in preventing photo-oxidative damage than other mechanisms of energy dissipation which require light for activation such as zeaxanthin-dependent energy dissipation or quencher formation within the reaction centre of photosystem II.

Thermal energy dissipation in reaction centres and in the antenna of photosystem II protects desiccated poikilohydric mosses against photo-oxidation.

Seasonal differences have been observed in the ability of desiccated mosses to dissipate absorbed light energy harmlessly into heat. During the dry summer season desiccation-tolerant mosses were more protected against photo-oxidative damage in the dry state than during the more humid winter season. Investigation of the differences revealed that phototolerance could be acquired or lost even under laboratory conditions. When a desiccated poikilohydric moss such as Rhytidiadelphus squarrosus is in the photosensitive state, the primary quinone, Q(A), in the reaction centre of photosystem II is readily reduced even by low intensity illumination as indicated by reversibly increased chlorophyll fluorescence. No such reduction is observed even under strong illumination in desiccated mosses after phototolerance has been acquired. In this state, reductive charge stabilization is replaced by energy dissipation. As a consequence, chlorophyll fluorescence is quenched. Different mechanisms are responsible for quenching. One is based on the presence of zeaxanthin provided drying occurs in the light. This mechanism is known to be controlled by a protonation reaction which is based on proton-coupled electron transport while the moss is still hydrated. Another mechanism which also requires light for activation, but no protonation, is activated during desiccation. While water is slowly lost, fluorescence is quenched. In this situation, an absorption band formed at 800 nm in the light is stabilized. It loses reversibility on darkening. Comparable kinetics of fluorescence quenching and 800 nm signals as well as the linear relationship between non-photochemical fluorescence quenching (NPQ) and loss of stable charge separation in photosystem II reaction centres suggested that desiccation-induced quenching is a property of photosystem II reaction centres. During desiccation, quenchers accumulate which are stable in the absence of water but revert to non-quenching molecular species on hydration. Together with zeaxanthin-dependent energy dissipation, desiccation-induced thermal energy dissipation protects desiccated poikilohydric mosses against photo-oxidation, ensuring survival during drought periods.

Conservation and dissipation of light energy as complementary processes: homoiohydric and poikilohydric autotrophs.

The relationship between photosynthetic energy conservation and thermal dissipation of light energy is considered, with emphasis on organisms which tolerate full desiccation without suffering photo-oxidative damage in strong light. As soon as water becomes available to dry poikilohydric organisms, they resume photosynthetic water oxidation. Only excess light is then thermally dissipated in mosses and chlorolichens by a mechanism depending on the protonation of a thylakoid protein and availability of zeaxanthin. Upon desiccation, another mechanism is activated which requires neither protonation nor zeaxanthin although the zeaxanthin-dependent mechanism of energy dissipation remains active, provided desiccation occurs in the light. Increased thermal energy dissipation under desiccation finds expression in the loss of variable, and in the quenching of, basal chlorophyll fluorescence. Spectroscopical analysis revealed the activity of photosystem II reaction centres in the absence of water. Oxidized beta-carotene (Car+) and reduced chlorophyll (Chl-), perhaps ChlD1 next to P680 within the D1 subunit, accumulates reversibly under very strong illumination. Although recombination between Car+ and Chl- is too slow to contribute significantly to thermal energy dissipation, a much faster reaction such as the recombination between P680+ and the neighbouring Chl- is suggested to form the molecular basis of desiccation-induced energy dissipation in photosystem II reaction centres. Thermal dissipation of absorbed light energy within a picosecond time domain deactivates excited singlet chlorophyll, thereby preventing triplet accumulation and the consequent photo-oxidative damage by singlet oxygen.

Photochemical reactions of chlorophyll in dehydrated photosystem II: two chlorophyll forms (680 and 700 nm).

Lichens and phototolerant poikilohydric mosses differ from spinach leaves, fern fronds or photosensitive mosses in that they show strongly decreased Fo chlorophyll fluorescence after drying. This desiccation-induced fluorescence loss is rapidly reversible under rehydration. Fluorescence emission from Photosystem II at 685 nm was decreased more strongly by dehydration than 720 nm emission. Reaction centers of Photosystem II lose activity on dehydration and regain it on hydration. Heating of desiccated lichens increased Fo chlorophyll fluorescence. The activation energy for the reversible part of the temperature-dependent fluorescence increase was 0.045 eV, which corresponds to the energy difference between the 680 and 697 nm absorption bands. In desiccated chlorolichens such as Parmelia sulcata, heating induces the appearance of positive variable fluorescence related to the reversible reduction of QA due to overcoming the energy barrier. This is interpreted to provide information on the mechanism of photoprotection: energy is dissipated by changing Chl680 or P680 into a chlorophyll form, which absorbs at 700 nm and emits light at 720 nm (Chl-720 or P680(700)) with a low quantum yield. Dissipation of light energy in this trap is activated by desiccation.

Irrungen, Wirrungen? The Mehler reaction in relation to cyclic electron transport in C3 plants.

Plants not only evolve but also reduce oxygen in photosynthesis. Considerable oxygen uptake occurs during photorespiration of C3 plants. Controversies exist on whether direct oxygen reduction in the Mehler reaction together with associated electron transport is also a major sink of electrons when leaves are exposed to sunlight. Here, preference is given to the view that it is not. Whereas photorespiration consumes ATP, the Mehler reaction does not. In isolated chloroplasts photosynthesizing in the presence of saturating bicarbonate, the Mehler reaction is suppressed. In the water - water cycle of leaves, which includes the Mehler reaction, water is oxidized and electrons flow through Photosystems II and I to oxygen producing water. The known properties of coupled electron transport suggest that the water - water cycle cannot act as an efficient electron sink. Rather, by contributing to thylakoid acidification it plays a role in the control of Photosystem II activity. Cyclic electron transport competes with the Mehler reaction for electrons. Both pathways can help to defray possible ATP deficiencies in the chloroplast stroma, but play a more important role by making intrathylakoid protein protonation possible. This is a necessary step for the dissipation of excess excitation energy as heat. Linear electron flow to oxygen relieves the inhibition of cyclic electron transport, which is observed under excessive reduction of intersystem electron carriers. In turn, cyclic electron transport replaces functions of the linear pathway in the control of Photosystem II when oxygen reduction is decreased at low temperatures or, experimentally, when the oxygen concentration of the gas phase is low. Thus, cyclic electron flow acts in flexible relationship with the water-water cycle to control Photosystem II activity.

Stomatal uptake of SO2 , NOx and O3 by spruce crowns (Picea abies) and canopy damage in Central Europe.

The stomatal uptake of SO2 , NO2 and O3 by Norway spruce canopies (Picea abies (L.) Karst.) has been integrated at six sites in central Germany (Königstein in the Taunus mountains. Witzenhausen, Grebenau, Frankenberg, Fürth in the Odenwald mountains and Biebergemünd in the Spessart mountains), Results are based on 788000 half-hourly available sets of field data on air pollution and site meteorology measured since 1984 (45 site years). Data on stomatal water conductance are available from statistical response functions obtained in the field at all times of day and seasons of the year. From this, stomatal conductance was calculated fur different atmospheric tract gases. Statistical response functions are presented which allow the separate estimation of annual stomatal net uptake of trace gases in the field if only (i) measured annual means of SO2 , NO2 , and O3 , pollution and (ii) the length of the trunk growth period (defined by temperature) are known. The following specific annual doses of stomatal trace-gas net uptake in the field (µmol m-2 total needle surface d-1 annual trunk growth period/(nPa Pa-1 ) annual mean of trace gas concentration) are obtained; SO2 : (0.157 ± 0.011) µmol m-2 d-1 /(nPa SO2 , Pa-1 ), NOx : (0.4774 ± 0.034) µmol m-2 d-1 /(nPa NO2 Pa-1 ), O3 : (0.474 ± 0.034) µmol m-2 d-1 /(nPa O3 Pa-1 ). There is an apparent NO2 , compensation point at 7 nPa Pa-1 in the field if NOx (=NO2 , + NO) fluxes are integrated; the individual NO2 , compensation point equals c 3.2 nPa Pa-1 . Additionally, a statistical response function is presented, which estimates actual stomatal water transpiration rates in the field on the basis of measured water vapour pressure differences. VPD (mPa Pa-1 ). Complex canopy effects were calculated on the basis of available data using an effective leaf area which is 0×32 times the morphological leaf area index. Results are consistent (i) with available water balance data, (ii) with observed SO2 -dependent sulphate accumulation rates in spruce needles and (iii) with observed epicuticular SO2 , deposition rates. Pollution data are compared with observed damage of spruce canopies in the field. There was a statistically significant correlation between observed damage to spruce canopies.