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

Exciton quenching by oxidized chlorophyll Z across the two adjacent monomers in a photosystem II core dimer

This study aimed to clarify (1) which pigment in a photosystem II (PSII) core complex is responsible for the 695-nm emission at 77 K and (2) the molecular basis for the oxidation-induced fluorescence quenching in PSII. Picosecond time-resolved fluorescence dynamics was compared between the dimeric and monomeric PSII with and without addition of an oxidant. The results indicated that the excitation-energy flow to the 695-nm-emitting chlorophyll (Chl) at 36 K and 77 K was hindered upon monomerization, clearly demonstrating significant exciton migration from the Chls on one monomer to the 695-nm-emitting pigment on the adjacent monomer. Oxidation of the redox-active Chl, which is named Chl_Z caused almost equal quenching of the 684-nm and 695-nm emission bands in the dimer, and lower quenching of the 695-nm band in the monomer. These results suggested two possible scenarios responsible for the 695-nm emission band: (A) Chl11-13 pair and the oxidized Chl_ZD1 work as the 695-nm emitting Chl and the quenching site, respectively, and (B) Chl29 and the oxidized Chl_ZD2 work as the 695-nm emitting Chl and the quenching site, respectively.

Ultrafast laser filament-induced fluorescence for detecting uranium stress in Chlamydomonas reinhardtii

Plants and other photosynthetic organisms have been suggested as potential pervasive biosensors for nuclear nonproliferation monitoring. We demonstrate that ultrafast laser filament-induced fluorescence of chlorophyll in the green alga Chlamydomonas reinhardtii is a promising method for remote, in-field detection of stress from exposure to nuclear materials. This method holds an advantage over broad-area surveillance, such as solar-induced fluorescence monitoring, when targeting excitation of a specific plant would improve the detectability, for example when local biota density is low. After exposing C. reinhardtii to uranium, we find that the concentration of chlorophyll a, chlorophyll fluorescence lifetime, and carotenoid content increase. The increased fluorescence lifetime signifies a decrease in non-photochemical quenching. The simultaneous increase in carotenoid content implies oxidative stress, further confirmed by the production of radical oxygen species evidence in the steady-state absorption spectrum. This is potentially a unique signature of uranium, as previous work finds that heavy metal stress generally increases non-photochemical quenching. We identify the temporal profile of the chlorophyll fluorescence to be a distinguishing feature between uranium-exposed and unexposed algae. Discrimination of uranium-exposed samples is possible at a distance of [Formula: see text]35 m with a single laser shot and a modest collection system, as determined through a combination of experiment and simulation of distance-scaled uncertainty in discriminating the temporal profiles. Illustrating the potential for remote detection, detection over 125 m would require 100 laser shots, commensurate with the detection time on the order of 1 s.

Desiccation-rehydration measurements in bryophytes: current status and future insights

Desiccation-rehydration experiments have been employed over the years to evaluate desiccation tolerance of bryophytes (Bryophyta, Marchantiophyta, and Anthocerotophyta). Researchers have applied a spectrum of protocols to induce desiccation and subsequent rehydration, and a wide variety of techniques have been used to study desiccation-dependent changes in bryophyte molecular, cellular, physiological, and structural traits, resulting in a multifaceted assortment of information that is challenging to synthesize. We analysed 337 desiccation-rehydration studies, providing information for 351 species, to identify the most frequent methods used, analyse the advances in desiccation studies over the years, and characterize the taxonomic representation of the species assessed. We observed certain similarities across methodologies, but the degree of convergence among the experimental protocols was surprisingly low. Out of 52 bryophyte orders, 40% have not been studied, and data are lacking for multiple remote or difficult to access locations. We conclude that for quantitative interspecific comparisons of desiccation tolerance, rigorous standardization of experimental protocols and measurement techniques, and simultaneous use of an array of experimental techniques are required for a mechanistic insight into the different traits modified in response to desiccation. New studies should also aim to fill gaps in taxonomic, ecological, and spatial coverage of bryophytes.

Sustained zeaxanthin-dependent thermal dissipation is induced by desiccation and low temperatures in the fern Polypodium virginianum

Desiccation and low temperatures inhibit photosynthetic carbon reduction and, in combination with light, result in severe oxidative stress, thus, tolerant organisms must utilize enhanced photoprotective mechanisms to prevent damaging reactions from occurring. We sought to characterize the desiccation tolerance of the fern Polypodium virginianum and to assess the role of the xanthophyll cycle and sustained forms of thermal dissipation in its response to desiccation, as well as to low temperatures during winter. Our results demonstrate that P. virginianum is desiccation-tolerant and that it increases its utilization of sustained forms of zexanthin (Z)-dependent thermal dissipation in response to desiccation and low temperatures during winter. Experiments with detached fronds were conducted in dark and natural light conditions and demonstrated that some dark-formation of Z occurs in this species. In addition, desiccation in the light resulted in more pronounced declines in maximal photochemical efficiency (F_v /F_m ) and higher Z levels than desiccation in the dark, indicating a substantial fraction of the sustained reduction in F_v /F_m is due to Z-dependent sustained dissipation. Recovery from desiccation and from low temperatures exhibited biphasic kinetics with a more rapid phase (1-4 h), which was accompanied by an increase in minimal fluorescence yield (F_o ) but no change in Z, and a slower phase (up to 24 h) correlating with reconversion of Z to violaxanthin. These data suggest that two mechanisms of sustained thermal dissipation occur in response to desiccation and low temperatures, possibly corresponding to sustained forms of the energy-dependent and zeaxanthin-dependent mechanisms of dynamic thermal dissipation.

Photosynthesis on the edge: photoinhibition, desiccation and freezing tolerance of Antarctic bryophytes

In Antarctica, multiple stresses (low temperatures, drought and excessive irradiance) hamper photosynthesis even in summer. We hypothesize that controlled inactivation of PSII reaction centres, a mechanism widely studied by pioneer work of Fred Chow and co-workers, may effectively guarantee functional photosynthesis under these conditions. Thus, we analysed the energy partitioning through photosystems in response to temperature in 15 bryophyte species presenting different worldwide distributions but all growing in Livingston Island, under controlled and field conditions. We additionally tested their tolerance to desiccation and freezing and compared those with their capability for sexual reproduction in Antarctica (as a proxy to overall fitness). Under field conditions, when irradiance rules air temperature by the warming of shoots (up to 20 °C under sunny days), a predominance of sustained photoinhibition beyond dynamic heat dissipation was observed at low temperatures. Antarctic endemic and polar species showed the largest increases of photoinhibition at low temperatures. On the contrary, the variation of thermal dissipation with temperature was not linked to species distribution. Instead, maximum non-photochemical quenching at 20 °C was related (strongly and positively) with desiccation tolerance, which also correlated with fertility in Antarctica, but not with freezing tolerance. Although all the analysed species tolerated - 20 °C when dry, the tolerance to freezing in hydrated state ranged from the exceptional ability of Schistidium rivulare (that survived for 14 months at - 80 °C) to the susceptibility of Bryum pseudotriquetrum (that died after 1 day at - 20 °C unless being desiccated before freezing).

Resistance of Antarctic moss Sanionia uncinata to photoinhibition: chlorophyll fluorescence analysis of samples from the western and eastern coasts of the Antarctic Peninsula

Interspecific differences in sensitivity of the Antarctic moss Sanionia uncinata from King George Island (KGI) and James Ross Island (JRI) to photoinhibitory treatment were studied in laboratory conditions using chlorophyll fluorescence techniques. Slow (Kautsky) and fast (OJIP) kinetics were used for the measurements. Samples were exposed to a short-term (60 min) photoinhibitory treatment (PIT, 2000 μmol·m^-2 ·s^-1 PAR). The photoinhibitory treatment (PIT) led to photoinhibition which was indicated by the decrease in F_V /F_M and Φ_PSII in KGI but not in JRI samples. However, this decrease was small and full recovery was reached 90 min after PIT termination. Non-photochemical quenching (NPQ) was activated during the PIT, and rapidly relaxed during recovery. Early stages of photoinhibition showed a drop in F_V /F_M and Φ_PSII to minimum values within the first 10 s of the PIT, with their subsequent increase apparent within fast (0-5 min PIT) and slow (5-50 min PIT) phases of adjustment. The PIT caused a decrease in the performance index (Pi_Abs), photosynthetic electron transport per reaction centre (RC) (ET₀ /RC). The PIT induced an increase in thermal dissipation per RC (DI₀ /RC), effectivity of thermal dissipation (Phi_D₀ ), absorption per RC (ABS/RC) and trapping rate per RC (TR₀ /RC). In conclusion, PIT led to only slight photoinhibition followed by fast recovery in S. uncinata from KGI and JRI, since F_V /F_M and Φ_PSII returned to pre-photoinhibitory conditions. Therefore, S. uncinata might be considered resistant to photoinhibition even in the wet state. The KGI samples showed higher resistance to photoinhibition than the JRI samples.

Natural ultraviolet radiation exposure alters photosynthetic biology and improves recovery from desiccation in a desert moss

Plants in dryland ecosystems experience extreme daily and seasonal fluctuations in light, temperature, and water availability. We used an in situ field experiment to uncover the effects of natural and reduced levels of ultraviolet radiation (UV) on maximum PSII quantum efficiency (Fv/Fm), relative abundance of photosynthetic pigments and antioxidants, and the transcriptome in the desiccation-tolerant desert moss Syntrichia caninervis. We tested the hypotheses that: (i) S. caninervis plants undergo sustained thermal quenching of light [non-photochemical quenching (NPQ)] while desiccated and after rehydration; (ii) a reduction of UV will result in improved recovery of Fv/Fm; but (iii) 1 year of UV removal will de-harden plants and increase vulnerability to UV damage, indicated by a reduction in Fv/Fm. All field-collected plants had extremely low Fv/Fm after initial rehydration but recovered over 8 d in lab-simulated winter conditions. UV-filtered plants had lower Fv/Fm during recovery, higher concentrations of photoprotective pigments and antioxidants such as zeaxanthin and tocopherols, and lower concentrations of neoxanthin and Chl b than plants exposed to near natural UV levels. Field-grown S. caninervis underwent sustained NPQ that took days to relax and for efficient photosynthesis to resume. Reduction of solar UV radiation adversely affected recovery of Fv/Fm following rehydration.

Is zeaxanthin needed for desiccation tolerance? Sustained forms of thermal dissipation in tolerant versus sensitive bryophytes

Desiccation tolerant (DT) plants engage and disengage sustained forms of energy dissipation in response to desiccation and rehydration. This project sought to characterize the role of zeaxanthin and thylakoid protein phosphorylation status in sustained energy dissipation during desiccation in bryophytes with varying DT. Tolerant (Polytrichum piliferum, Dicranum species, Calliergon stramineum) and sensitive (Grimmia species, Schistidium rivulare, Sphagnum species) moss were desiccated in darkness or natural light conditions for up to three weeks. Desiccation caused pronounced reductions in F_v /F_m in all cases which was enhanced by light exposure during desiccation. Desiccation in darkness resulted in no accumulation of Z in any species, however, in natural light conditions there was significant accumulation of Z in tolerant but not sensitive species. Desiccation in natural light, relative to darkness, resulted in more pronounced reductions in F_o in tolerant but not sensitive species. Recovery of F_v /F_m upon rehydration occurred in two phases, a rapid phase (minutes) and a slower phase (hours). Increased time of desiccation, and light exposure, resulted in a reduction in the rapid phase. Desiccation in light conditions resulted in some accumulation of the phosphorylated form of the major light harvesting trimer (LHCII). Data are consistent with two mechanisms of sustained quenching, neither of which requires Z. However, when desiccation occurs in natural light conditions, accumulation of Z likely contributes to one or both of the sustained forms of dissipation. Increases in LHCII phosphorylation during desiccation are consistent with increased connectivity between the photosystems. The absence of Z formation in sensitive species may contribute to their lack of desiccation tolerance.

Efficient Heat Dissipation and Cyclic Electron Flow Confer Daily Air Exposure Tolerance in the Intertidal Seagrass Halophila beccarii Asch

Seagrasses inhabiting the intertidal zone experience periodically repeated cycles of air exposure and rehydration. However, little is known about the photoprotective mechanisms in photosystem (PS)II and PSI, as well as changes in carbon utilization upon air exposure. The photoprotective processes upon air exposure in Halophila beccarii Asch., an endangered seagrass species, were examined using the Dual-PAM-100 and non-invasive micro-test technology. The results showed that air exposure enhanced non-photochemical quenching (NPQ) in both PSII and PSI, with a maximum increase in NPQ and Y(ND) (which represents the fraction of overall P700 that is oxidized in a given state) of 23 and 57%, respectively, resulting in intensive thermal energy dissipation of excess optical energy. Moreover, cyclic electron transport driven by PSI (CEF) was upregulated, reflected by a 50 and 22% increase in CEF and maximum electron transport rate in PSI to compensate for the abolished linear electron transport with significant decreases in pmf_LEF (the proton motive force [pmf]) attributable solely to proton translocation by linear electron flow [LEF]). Additionally, H⁺ fluxes in mesophyll cells decreased steadily with increased air exposure time, exhibiting a maximum decrease of six-fold, indicating air exposure modified carbon utilization by decreasing the proton pump influxes. These findings indicate that efficient heat dissipation and CEF confer daily air exposure tolerance to the intertidal seagrass H. beccarii and provide new insights into the photoprotective mechanisms of intertidal seagrasses. This study also helps explain the extensive distribution of H. beccarii in intertidal zones.

Survey of the occurrence of desiccation-induced quenching of basal fluorescence in 28 species of green microalgae

Desiccation-induced chlorophyll fluorescence quenching seems to be an indispensable part of desiccation resistance in the surveyed 28 green microalgal species. Lichens are desiccation tolerant meta-organisms. In the desiccated state photosynthesis is inhibited rendering the photobionts potentially sensitive to photoinhibition. As a photoprotective mechanism, strong non-radiative dissipation of absorbed light leading to quenching of chlorophyll fluorescence has been proposed. Desiccation-induced quenching affects not only variable fluorescence, but also the so-called basal fluorescence, F₀. This phenomenon is well-known for intact lichens and some free living aero-terrestrial algae, but it was often absent in isolated lichen algae. Therefore, a thorough screening for the appearance of desiccation-induced quenching was undertaken with 13 different aero-terrestrial microalgal species and lichen photobionts. They were compared with 15 aquatic green microalgal species, among them also three marine species. We asked the following questions: Do isolated lichen algae show desiccation-induced quenching? Are aero-terrestrial algae different in this respect to aquatic algae and is the potential for desiccation-induced quenching coupled to desiccation tolerance? How variable is desiccation-induced quenching among species? Most of the aero-terrestrial algae, including all lichen photobionts, showed desiccation-induced quenching, although highly variable in extent, whereas most of the aquatic algae did not. All algae displaying quenching were also desiccation tolerant, whereas all algae unable to perform desiccation-induced quenching were desiccation intolerant. Desiccation-induced fluorescence quenching seems to be an indispensable part of desiccation resistance in the investigated species.

A tribute to Ulrich Heber (1930-2016) for his contribution to photosynthesis research: understanding the interplay between photosynthetic primary reactions, metabolism and the environment

The dynamic and efficient coordination of primary photosynthetic reactions with leaf energization and metabolism under a wide range of environmental conditions is a fundamental property of plants involving processes at all functional levels. The present historical perspective covers 60 years of research aiming to understand the underlying mechanisms, linking major breakthroughs to current progress. It centers on the contributions of Ulrich Heber who had pioneered novel concepts, fundamental methods, and mechanistic understanding of photosynthesis. An important first step was the development of non-aqueous preparation of chloroplasts allowing the investigation of chloroplast metabolites ex vivo (meaning that the obtained results reflect the in vivo situation). Later on, intact chloroplasts, retaining their functional envelope membranes, were isolated in aqueous media to investigate compartmentation and exchange of metabolites between chloroplasts and external medium. These studies elucidated metabolic interaction between chloroplasts and cytoplasm during photosynthesis. Experiments with isolated intact chloroplasts clarified that oxygenation of ribulose-1.5-bisphosphate generates glycolate in photorespiration. The development of non-invasive optical methods enabled researchers identifying mechanisms that balance electron flow in the photosynthetic electron transport system avoiding its over-reduction. Recording chlorophyll a (Chl a) fluorescence allowed one to monitor, among other parameters, thermal energy dissipation by means of 'nonphotochemical quenching' of the excited state of Chl a. Furthermore, studies both in vivo and in vitro led to basic understanding of the biochemical mechanisms of freezing damage and frost tolerance of plant leaves, to SO₂ tolerance of tree leaves and dehydrating lichens and mosses.

Red shift in the spectrum of a chlorophyll species is essential for the drought-induced dissipation of excess light energy in a poikilohydric moss, Bryum argenteum

Some mosses are extremely tolerant of drought stress. Their high drought tolerance relies on their ability to effectively dissipate absorbed light energy to heat under dry conditions. The energy dissipation mechanism in a drought-tolerant moss, Bryum argenteum, has been investigated using low-temperature picosecond time-resolved fluorescence spectroscopy. The results are compared between moss thalli samples harvested in Antarctica and in Japan. Both samples show almost the same quenching properties, suggesting an identical drought tolerance mechanism for the same species with two completely different habitats. A global target analysis was applied to a large set of data on the fluorescence-quenching dynamics for the 430-nm (chlorophyll-a selective) and 460-nm (chlorophyll-b and carotenoid selective) excitations in the temperature region from 5 to 77 K. This analysis strongly suggested that the quencher is formed in the major peripheral antenna of photosystem II, whose emission spectrum is significantly broadened and red-shifted in its quenched form. Two emission components at around 717 and 725 nm were assigned to photosystem I (PS I). The former component at around 717 nm is mildly quenched and probably bound to the PS I core complex, while the latter at around 725 nm is probably bound to the light-harvesting complex. The dehydration treatment caused a blue shift of the PS I emission peak via reduction of the exciton energy flow to the pigment responsible for the 725 nm band.

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.

Terrestrial adaptation of green algae Klebsormidium and Zygnema (Charophyta) involves diversity in photosynthetic traits but not in CO2 acquisition

The basal streptophyte Klebsormidium and the advanced Zygnema show adaptation to terrestrialization. Differences are found in photoprotection and resistance to short-term light changes, but not in CO ₂ acquisition. Streptophyte green algae colonized land about 450-500 million years ago giving origin to terrestrial plants. We aim to understand how their physiological adaptations are linked to the ecological conditions (light, water and CO₂) characterizing modern terrestrial habitats. A new Klebsormidium isolate from a strongly acidic environment of a former copper mine (Schwarzwand, Austria) is investigated, in comparison to Klebsormidium cf. flaccidum and Zygnema sp. We show that these genera possess different photosynthetic traits and water requirements. Particularly, the Klebsormidium species displayed a higher photoprotection capacity, concluded from non-photochemical quenching (NPQ) and higher tolerance to high light intensity than Zygnema. However, Klebsormidium suffered from photoinhibition when the light intensity in the environment increased rapidly, indicating that NPQ is involved in photoprotection against strong and stable irradiance. Klebsormidium was also highly resistant to cellular water loss (dehydration) under low light. On the other hand, exposure to relatively high light intensity during dehydration caused a harmful over-reduction of the electron transport chain, leading to PSII damages and impairing the ability to recover after rehydration. Thus, we suggest that dehydration is a selective force shaping the adaptation of this species towards low light. Contrary to the photosynthetic characteristics, the inorganic carbon (C _i ) acquisition was equivalent between Klebsormidium and Zygnema. Despite their different habitats and restriction to hydro-terrestrial environment, the three organisms showed similar use of CO₂ and HCO₃^- as source of C_i for photosynthesis, pointing out a similar adaptation of their CO₂-concentrating mechanisms to terrestrial life.

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

Formation of photosystem II reaction centers that work as energy sinks in lichen symbiotic Trebouxiophyceae microalgae

Lichens are poikilohydric symbiotic organisms that can survive in the absence of water. Photosynthesis must be highly regulated in these organisms, which live under continuous desiccation-rehydration cycles, to avoid photooxidative damage. Analysis of chlorophyll a fluorescence induction curves in the lichen microalgae of the Trebouxiophyceae Asterochloris erici and in Trebouxia jamesii (TR1) and Trebouxia sp. (TR9) phycobionts, isolated from the lichen Ramalina farinacea, shows differences with higher plants. In the presence of the photosynthetic electron transport inhibitor DCMU, the kinetics of Q(A) reduction is related to variable fluorescence by a sigmoidal function that approaches a horizontal asymptote. An excellent fit to these curves was obtained by applying a model based on the following assumptions: (1) after closure, the reaction centers (RCs) can be converted into "energy sink" centers (sRCs); (2) the probability of energy leaving the sRCs is very low or zero and (3) energy is not transferred from the antenna of PSII units with sRCs to other PSII units. The formation of sRCs units is also induced by repetitive light saturating pulses or at the transition from dark to light and probably requires the accumulation of reduced Q(A), as well as structural changes in the reaction centers of PSII. This type of energy sink would provide a very efficient way to protect symbiotic microalgae against abrupt changes in light intensity.

Harvesting Far-Red Light by Chlorophyll f in Photosystems I and II of Unicellular Cyanobacterium strain KC1

Cells of a unicellular cyanobacterium strain KC1, which were collected from Japanese fresh water Lake Biwa, formed chlorophyll (Chl) f at 6.7%, Chl a' at 2.0% and pheophytin a at 0.96% with respect to Chl a after growth under 740 nm light. The far-red-acclimated cells (Fr cells) formed extra absorption bands of Chl f at 715 nm in addition to the major Chl a band. Fluorescence lifetimes were measured. The 405-nm laser flash, which excites mainly Chl a in photosystem I (PSI), induced a fast energy transfer to multiple fluorescence bands at 720-760 and 805 nm of Chl f at 77 K in Fr cells with almost no PSI-red-Chl a band. The 630-nm laser flash, which mainly excited photosystem II (PSII) through phycocyanin, revealed fast energy transfer to another set of Chl f bands at 720-770 and 810 nm as well as to the 694-nm Chl a fluorescence band. The 694-nm band did not transfer excitation energy to Chl f. Therefore, Chl a in PSI, and phycocyanin in PSII of Fr cells transferred excitation energy to different sets of Chl f molecules. Multiple Chl f forms, thus, seem to work as the far-red antenna both in PSI and PSII. A variety of cyanobacterial species, phylogenically distant from each other, seems to use a Chl f antenna in far-red environments, such as under dense biomats, in colonies, or under far-red LED light.

Chlorophyll b in angiosperms: Functions in photosynthesis, signaling and ontogenetic regulation

Chlorophyll b (Chlb) is an antenna chlorophyll. The binding of Chlb by antenna proteins is crucial for the correct assembly of the antenna complexes in thylakoid membranes. Since the levels of the proteins of major and minor antenna are affected to different extents by Chlb binding, the availability of Chlb influences the composition and the size of antenna complexes which in turn determine the supramolecular organization of the thylakoid membranes in grana. Therefore, Chlb synthesis levels have a major impact on lateral mobility and diffusion of membrane molecules, and thus affect not only light harvesting and thermal energy dissipation processes, but also linear electron transport and repair processes in grana. Furthermore, in angiosperms Chlb synthesis affects plant functions beyond chloroplasts. First, the stability of pigment-protein complexes in the antennae, which depends on Chlb, is an important factor in the regulation of plant ontogenesis, and Chlb levels were recently shown to influence plant ontogenetic signaling. Second, the amounts of minor antenna proteins in chloroplasts, which depend on the availability of Chlb, were recently shown to affect ABA levels and signaling in plants. These mechanisms can be examined in mutants where Chlb synthesis is reduced or abolished. The dramatic effects caused by the lack of Chlb on plant productivity are interpreted in this review in light of the pleiotropic effects on photosynthesis and signaling, and the potential to manipulate Chlb biosynthesis for the improvement of crop production is discussed.

An easily reversible structural change underlies mechanisms enabling desert crust cyanobacteria to survive desiccation

Biological desert sand crusts are the foundation of desert ecosystems, stabilizing the sands and allowing colonization by higher order organisms. The first colonizers of the desert sands are cyanobacteria. Facing the harsh conditions of the desert, these organisms must withstand frequent desiccation-hydration cycles, combined with high light intensities. Here, we characterize structural and functional modifications to the photosynthetic apparatus that enable a cyanobacterium, Leptolyngbya sp., to thrive under these conditions. Using multiple in vivo spectroscopic and imaging techniques, we identified two complementary mechanisms for dissipating absorbed energy in the desiccated state. The first mechanism involves the reorganization of the phycobilisome antenna system, increasing excitonic coupling between antenna components. This provides better energy dissipation in the antenna rather than directed exciton transfer to the reaction center. The second mechanism is driven by constriction of the thylakoid lumen which limits diffusion of plastocyanin to P700. The accumulation of P700(+) not only prevents light-induced charge separation but also efficiently quenches excitation energy. These protection mechanisms employ existing components of the photosynthetic apparatus, forming two distinct functional modes. Small changes in the structure of the thylakoid membranes are sufficient for quenching of all absorbed energy in the desiccated state, protecting the photosynthetic apparatus from photoinhibitory damage. These changes can be easily reversed upon rehydration, returning the system to its high photosynthetic quantum efficiency.

Water redistribution determines photosynthetic responses to warming and drying in two polar mosses

Predicting impacts of climate change requires an understanding of the sensitivity of species to temperature, including conflated changes in humidity. Physiological responses to temperature and clump-to-air vapour pressure difference (VPD) were compared in two Antarctic moss species, Ceratodon purpureus (Hedw.) Brid. and Schistidium antarctici (Cardot) L.I. Savicz & Smirnova. Temperatures from 8 to 24°C had no significant effects on photosynthesis or recovery from drying, whereas high VPD accelerated drying. In Schistidium, which lacks internal conduction structures, shoots dried more slowly than the clump, and photosynthesis ceased at high shoot relative water content (RWC), behaviour consistent with a strategy of drought avoidance although desiccation tolerant. In contrast, shoots of Ceratodon have a central vascular core, but dried more rapidly than the clump. These results imply that cavitation of the hydroid strand enables hydraulic isolation of extremities during rapid drying, effectively slowing water loss from the clump. Ceratodon maintained photosynthetic activity during drying to lower shoot RWC than Schistidium, consistent with a strategy of drought tolerance. These ecophysiological characteristics may provide a functional explanation for the differential distribution of Schistidium and Ceratodon along moisture gradients in Antarctica. Thus, predicting responses of non-vascular vegetation to climate change at high latitudes requires greater focus on VPD and hydraulics than temperature.

Novel type of red-shifted chlorophyll a antenna complex from Chromera velia. I. Physiological relevance and functional connection to photosystems

Chromera velia is an alveolate alga associated with scleractinian corals. Here we present detailed work on chromatic adaptation in C. velia cultured under either blue or red light. Growth of C. velia under red light induced the accumulation of a light harvesting antenna complex exhibiting unusual spectroscopic properties with red-shifted absorption and atypical 710nm fluorescence emission at room temperature. Due to these characteristic features the complex was designated "Red-shifted Chromera light harvesting complex" (Red-CLH complex). Its detailed biochemical survey is described in the accompanying paper (Bina et al. 2013, this issue). Here, we show that the accumulation of Red-CLH complex under red light represents a slow acclimation process (days) that is reversible with much faster kinetics (hours) under blue light. This chromatic adaptation allows C. velia to maintain all important parameters of photosynthesis constant under both light colors. We further demonstrated that the C. velia Red-CLH complex is assembled from a 17kDa antenna protein and is functionally connected to photosystem II as it shows variability of chlorophyll fluorescence. Red-CLH also serves as an additional locus for non-photochemical quenching. Although overall rates of oxygen evolution and carbon fixation were similar for both blue and red light conditions, the presence of Red-CLH in C. velia cells increases the light harvesting potential of photosystem II, which manifested as a doubled oxygen evolution rate at illumination above 695nm. This data demonstrates a remarkable long-term remodeling of C. velia light-harvesting system according to light quality and suggests physiological significance of 'red' antenna complexes.

Two different mechanisms cooperate in the desiccation-induced excited state quenching in Parmelia lichen

The highly efficient desiccation-induced quenching in the poikilohydric lichen Parmelia sulcata has been studied by ultrafast fluorescence spectroscopy at room temperature (r.t.) and cryogenic temperatures in order to elucidate the quenching mechanism(s) and kinetic reaction models. Analysis of the r.t. data by kinetic target analysis reveals that two different quenching mechanisms contribute to the protection of photosystem II (PS II). The first mechanism is a direct quenching of the PS II antenna and is related to the characteristic F740 nm fluorescence band. Based on the temperature dependence of its spectra and the kinetics, this mechanism is proposed to reflect the formation of a fluorescent (F740) chlorophyll-chlorophyll charge-transfer state. It is discussed in relation to a similar fluorescence band and quenching mechanism observed in light-induced nonphotochemical quenching in higher plants. The second and more efficient quenching process (providing more than 70% of the total PS II quenching) is shown to involve an efficient spillover (energy transfer) from PS II to PS I which can be prevented by a short glutaraldehyde treatment. Desiccation causes a thylakoid-membrane rearrangement which brings into direct contact the PS II and PS I units. The energy transferred to PS I in the spillover process is then quenched highly efficiently in PS I due to the formation of a long-lived P700(+) state in the dried state in the light. As a consequence, both PS II and PS I are protected very efficiently against photodestruction. This dual quenching mechanism is supported by the low temperature kinetics data.

Evidence for the absence of enzymatic reactions in the glassy state. A case study of xanthophyll cycle pigments in the desiccation-tolerant moss Syntrichia ruralis

Desiccation-tolerant plants are able to withstand dehydration and resume normal metabolic functions upon rehydration. These plants can be dehydrated until their cytoplasm enters a 'glassy state' in which molecular mobility is severely reduced. In desiccation-tolerant seeds, longevity can be enhanced by drying and lowering storage temperature. In these conditions, they still deteriorate slowly, but it is not known if deteriorative processes include enzyme activity. The storage stability of photosynthetic organisms is less studied, and no reports are available on the glassy state in photosynthetic tissues. Here, the desiccation-tolerant moss Syntrichia ruralis was dehydrated at either 75% or <5% relative humidity, resulting in slow (SD) or rapid desiccation (RD), respectively, and different residual water content of the desiccated tissues. The molecular mobility within dry mosses was assessed through dynamic mechanical thermal analysis, showing that at room temperature only rapidly desiccated samples entered the glassy state, whereas slowly desiccated samples were in a 'rubbery' state. Violaxanthin cycle activity, accumulation of plastoglobules, and reorganization of thylakoids were observed upon SD, but not upon RD. Violaxanthin cycle activity critically depends on the activity of violaxanthin de-epoxidase (VDE). Hence, it is proposed that enzymatic activity occurred in the rubbery state (after SD), and that in the glassy state (after RD) no VDE activity was possible. Furthermore, evidence is provided that zeaxanthin has some role in recovery apparently independent of its role in non-photochemical quenching of chlorophyll fluorescence.

Dissipation of excess excitation energy by drought-induced nonphotochemical quenching in two species of drought-tolerant moss: desiccation-induced acceleration of photosystem II fluorescence decay

Drought-tolerant mosses survive with their green color intact even after long periods of dehydration that would kill ordinary plants. The mechanism of dissipation of excitation energy under drought stress was studied in two species of drought-tolerant moss, Rhytidium rugosum and Ceratodon purpureus. They showed severe quenching of photosystem II chlorophyll fluorescence (PSII) after being dehydrated in the dark. Quenching was induced by the acceleration of the fluorescence decay rate. This drought-induced nonphotochemical quenching (designated d-NPQ) was fully reversed by rehydration. Global analysis of fluorescence decay at 77 K indicated rapid 46 ps transfer of excitation energy from the 680-690 nm PSII bands to a 710 nm band, and to 740-760 nm bands. The latter bands decayed to the ground state with the same time constant showing the rapid dissipation of excitation energy into heat. The quenching by d-NPQ in dry moss was stronger than that by PSII charge separation or nonphotochemical quenching (NPQ), which operates under hydrating conditions. Drought-tolerant mosses, thus, dissipate excess excitation energy into heat. The d-NPQ mechanism in moss resembles that reported in lichens, suggesting their common origin.

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.

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?