Model for fluorescence quenching in light harvesting complex II in different aggregation states

Low-temperature (77 K) steady-state fluorescence emission spectroscopy and dynamic light scattering were applied to the main chlorophyll a/b protein light harvesting complex of photosystem II (LHC II) in different aggregation states to elucidate the mechanism of fluorescence quenching within LHC II oligomers. Evidences presented that LHC II oligomers are heterogeneous and consist of large and small particles with different fluorescence yield. At intermediate detergent concentrations the mean size of the small particles is similar to that of trimers, while the size of large particles is comparable to that of aggregated trimers without added detergent. It is suggested that in small particles and trimers the emitter is monomeric chlorophyll, whereas in large aggregates there is also another emitter, which is a poorly fluorescing chlorophyll associate. A model, describing populations of antenna chlorophyll molecules in small and large aggregates in their ground and first singlet excited states, is considered. The model enables us to obtain the ratio of the singlet excited-state lifetimes in small and large particles, the relative amount of chlorophyll molecules in large particles, and the amount of quenchers as a function of the degree of aggregation. These dependencies reveal that the quenching of the chl a fluorescence upon aggregation is due to the formation of large aggregates and the increasing of the amount of chlorophyll molecules forming these aggregates. As a consequence, the amount of quenchers, located in large aggregates, is increased, and their singlet excited-state lifetimes steeply decrease.     

Change in CO2 balance under a series of forestry activities in a cool-temperate mixed forest with dense undergrowth

To evaluate the effects on CO₂ exchange of clearcutting a mixed forest and replacing it with a plantation, 4.5 years of continuous eddy covariance measurements of CO₂ fluxes and soil respiration measurements were conducted in a conifer-broadleaf mixed forest in Hokkaido, Japan. The mixed forest was a weak carbon sink (net ecosystem exchange, −44 g C m⁻² yr⁻¹), and it became a large carbon source (569 g C m⁻² yr⁻¹) after clearcutting. However, the large emission in the harvest year rapidly decreased in the following 2 years (495 and 153 g C m⁻² yr⁻¹, respectively) as the gross primary production (GPP) increased, while the total ecosystem respiration (RE) remained relatively stable. The rapid increase in GPP was attributed to an increase in biomass and photosynthetic activity of Sasa dwarf bamboo, an understory species. Soil respiration increased in the 3 years following clearcutting, in the first year mainly owing to the change in the gap ratio of the forest, and in the following years because of increased root respiration by the bamboo. The ratio of soil respiration to RE increased from 44% in the forest to nearly 100% after clearcutting, and aboveground parts of the vegetation contributed little to the RE although the respiration chamber measurements showed heterogeneous soil condition after clearcutting.     

Habitat selection by the swamp wallaby (Wallabia bicolor) in relation to diel period,food and shelter

Patterns of resource selection by animals may be influenced by sex, and often change over a 24‐h period. We used a dry sclerophyll landscape managed for commercial timber production to investigate the effects of sex and diel period on habitat selection by the swamp wallaby (Wallabia bicolor). We predicted that selection would be (i) affected by both sex and diel period; and (ii) positively related to lateral cover during the day, but to food resources at night. Non‐metric multidimentional scaling indicated that some of the available habitats differed markedly with respect to visibility (an indicator of lateral cover), fern cover, forb cover, wallaby density and a forage quality index, providing the basis for non‐random habitat selection. At the landscape scale, wallabies showed strong selection for 5‐year‐old regenerating sites, selected against 10‐year‐old regenerating sites and unharvested forest, and avoided recently harvested (3–10 months post‐harvest) sites completely. At the scale of individual home ranges, a pooled male and female sample demonstrated selection for unharvested forest over recently harvested sites during both diurnal and nocturnal periods. A separate analysis showed that both sex and diel period influenced the selection of 5‐ and 10‐year‐old sites and the surrounding unharvested forest. Using a novel approach, we demonstrated that diurnal habitat selection by both sexes was negatively correlated with visibility, representing stronger selection for areas with more lateral cover. Nocturnal selection by females was positively correlated with values of a forage quality index, but this was not the case for males. We hypothesise that the observed patterns of selection were driven by the need to find food and avoid predators, but were also affected by the different reproductive strategies of males and females. Our results demonstrate the importance of incorporating factors such as sex and diel period into analyses of habitat selection.     

Excitation energy transfer and carotenoid radical cation formation in light harvesting complexes — A theoretical perspective

Light harvesting complexes have been identified in all chlorophyll-based photosynthetic organisms. Their major function is the absorption of light and its transport to the reaction centers, however, they are also involved in excess energy quenching, the so-called non-photochemical quenching (NPQ). In particular, electron transfer and the resulting formation of carotenoid radical cations have recently been discovered to play an important role during NPQ in green plants. Here, the results of our theoretical investigations of carotenoid radical cation formation in the major light harvesting complex LHC-II of green plants are reported. The carotenoids violaxanthin, zeaxanthin and lutein are considered as potential quenchers. In agreement with experimental results, it is shown that zeaxanthin cannot quench isolated LHC-II complexes. Furthermore, subtle structural differences in the two lutein binding pockets lead to substantial differences in the excited state properties of the two luteins. In addition, the formation mechanism of carotenoid radical cations in light harvesting complexes LH2 and LH1 of purple bacteria is studied. Here, the energetic position of the S₁ state of the involved carotenoids neurosporene, spheroidene, spheroidenone and spirilloxanthin seems to determine the occurrence of radical cations in these LHCs upon photo-excitation. An elaborate pump-deplete-probe experiment is suggested to challenge the proposed mechanism.     

Identification of a specific fucoxanthin-chlorophyll protein in the light harvesting complex of photosystem I in the diatom Cyclotella meneghiniana

Thylakoids of the diatom Cyclotella meneghiniana were separated by discontinuous gradient centrifugation into photosystem (PS) I, PSII, and fucoxanthin-chlorophyll protein (FCP) fractions. FCPs are homologue to light harvesting complexes of higher plants with similar function in e.g. brown algae and diatoms. Still, it is unclear if FCP complexes are specifically associated with either PSI or PSII, or if FCP complexes function as one antenna for both photosystems. However, a trimeric FCP complex, FCPa, and a higher FCP oligomer, FCPb, have been described for C. meneghiniana, already. In this study, biochemical and spectroscopical evidences are provided that reveal a different subset of associated Fcp polypeptides within the isolated photosystem complexes. Whereas the PSII associated Fcp antenna resembles FCPa since it contains Fcp2 and Fcp6, at least three different Fcp polypeptides are associated with PSI. By re-solubilisation and a further purification step Fcp polypeptides were partially removed from PSI and both fractions were analysed again by biochemical and spectroscopical means, as well as by HPLC. Thereby a protein related to Fcp4 and a so far undescribed 17 kDa Fcp were found to be strongly coupled to PSI, whereas presumably Fcp5, a subunit of the FCPb complex, is only loosely bound to the PSI core. Thus, an association of FCPb and PSI is assumed.     

Differential turnover of the photosystem II reaction centre D1 protein in mesophyll and bundle sheath chloroplasts of maize

Photoinhibition is caused by an imbalance between the rates of the damage and repair cycle of photosystem II D1 protein in thylakoid membranes. The PSII repair processes include (i) disassembly of damaged PSII-LHCII supercomplexes and PSII core dimers into monomers, (ii) migration of the PSII monomers to the stroma regions of thylakoid membranes, (iii) dephosphorylation of the CP43, D1 and D2 subunits, (iv) degradation of damaged D1 protein, and (v) co-translational insertion of the newly synthesized D1 polypeptide and reassembly of functional PSII complex. Here, we studied the D1 turnover cycle in maize mesophyll and bundle sheath chloroplasts using a protein synthesis inhibitor, lincomycin. In both types of maize chloroplasts, PSII was found as the PSII-LHCII supercomplex, dimer and monomer. The PSII core and the LHCII proteins were phosphorylated in both types of chloroplasts in a light-dependent manner. The rate constants for photoinhibition measured for lincomycin-treated leaves were comparable to those reported for C3 plants, suggesting that the kinetics of the PSII photodamage is similar in C3 and C4 species. During the photoinhibitory treatment the D1 protein was dephosphorylated in both types of chloroplasts but it was rapidly degraded only in the bundle sheath chloroplasts. In mesophyll chloroplasts, PSII monomers accumulated and little degradation of D1 protein was observed. We postulate that the low content of the Deg1 enzyme observed in mesophyll chloroplasts isolated from moderate light grown maize may retard the D1 repair processes in this type of plastids.     

Pigment organization in the photosynthetic apparatus of Roseiflexus castenholzii

The light-harvesting-reaction center (LHRC) complex from the chlorosome-lacking filamentous anoxygenic phototroph (FAP), Roseiflexus castenholzii (R. castenholzii) was purified and characterized for overall pigment organization. The LHRC is a single complex that is comprised of light harvesting (LH) and reaction center (RC) polypeptides as well as an attached c-type cytochrome. The dominant carotenoid found in the LHRC is keto-γ-carotene, which transfers excitation to the long wavelength antenna band with 35% efficiency. Linear dichroism and fluorescence polarization measurements indicate that the long wavelength antenna pigments absorbing around 880 nm are perpendicular to the membrane plane, with the corresponding Q_y transition dipoles in the plane of the membrane. The antenna pigments absorbing around 800 nm, as well as the bound carotenoid, are oriented at a large angle with respect to the membrane. The antenna pigments spectroscopically resemble the well-studied LH2 complex from purple bacteria, however the close association with the RC makes the light harvesting component of this complex functionally more like LH1.     

Crop planting dates: an analysis of global patterns

Aim To assemble a data set of global crop planting and harvesting dates for 19 major crops, explore spatial relationships between planting date and climate for two of them, and compare our analysis with a review of the literature on factors that drive decisions on planting dates. Location Global. Methods We digitized and georeferenced existing data on crop planting and harvesting dates from six sources. We then examined relationships between planting dates and temperature, precipitation and potential evapotranspiration using 30‐year average climatologies from the Climatic Research Unit, University of East Anglia (CRU CL 2.0). Results We present global planting date patterns for maize, spring wheat and winter wheat (our full, publicly available data set contains planting and harvesting dates for 19 major crops). Maize planting in the northern mid‐latitudes generally occurs in April and May. Daily average air temperatures are usually c. 12–17 °C at the time of maize planting in these regions, although soil moisture often determines planting date more directly than does temperature. Maize planting dates vary more widely in tropical regions. Spring wheat is usually planted at cooler temperatures than maize, between c. 8 and 14 °C in temperate regions. Winter wheat is generally planted in September and October in the northern mid‐latitudes. Main conclusions In temperate regions, spatial patterns of maize and spring wheat planting dates can be predicted reasonably well by assuming a fixed temperature at planting. However, planting dates in lower latitudes and planting dates of winter wheat are more difficult to predict from climate alone. In part this is because planting dates may be chosen to ensure a favourable climate during a critical growth stage, such as flowering, rather than to ensure an optimal climate early in the crop's growth. The lack of predictability is also due to the pervasive influence of technological and socio‐economic factors on planting dates.     

On photoprotective mechanisms of carotenoids in light harvesting complex

Carotenoids in light harvesting complex (LHC) play an important role in preventing plants photodamage caused by excess light. Non-photochemical quenching (NPQ) is an important mechanism adopted by plants to deal with high light intensity and the major component is referred to as energy dependent quenching (qE). Despite numerous studies have been devoted to investigating the site and mechanism of qE, there are still much debate on these topics. In this article, we discussed the possible site and underlying mechanism of qE based on the structural similarity of carotenoids. Moreover, being as good antioxidants, carotenoids’ potential protective effects against LHC photo-oxidation by quenching active oxygen species or triplet excited state chlorophyll are also discussed.