Influence of electrostatic screening by cations on energy coupling between Photosystem II reaction centres and the light-harvesting chlorophyll ab protein complex II

Low temperature and high light dependent dynamic photoprotective strategies in Arabidopsis thaliana

Arabidopsis thaliana has been recognized as a chilling tolerant species based on analysis of resistance to low temperature stress, however, the mechanisms involved in this tolerance are not yet clarified. The low temperature-induced effects are exacerbated when plants are exposed to low temperatures in the presence of high light irradiance but the experimental data on the impact of light intensity during cold stress and its influence during recovery from stress are rather limited. The main objective of this study was to re-examine the photosynthetic responses of A. thaliana plants to short term (6 days) low temperature stress (12/10°C) under optimal (150 μmol m^-2 s^-1 ) and high light (500 μmol m^-2 s^-1 ) intensity and the subsequent recovery from the stress. Simultaneous measurements of the in vivo and in vitro functional performance of both photosystem II (PSII) and photosystem I (PSI), as well as, net photosynthesis, low temperature (77 K) chlorophyll fluorescence and immunoblot analysis of the relative abundance of PSII and PSI reaction center proteins were used to evaluate the role of light in the development of possible protective mechanisms during low temperature stress and the consequent recovery from exposure to low temperature and different light intensities. The results presented clearly suggest that Arabidopsis plants can employ a number of highly dynamic photoprotective strategies depending on the light intensity. These strategies include one based on LHCII quenching and two other quenching mechanisms localized within the PSII and PSI reaction centers, which are all expressed to different extent depending on the severity of the photoinhibitory treatments under low temperature stress conditions.

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of FV/FM on excitation wavelength

The F(0) and F(M) level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F(V)/F(M) (=(F(M) - F(0))/F (M)) have been derived: within each of the three spectra of F(V)/F(M), statistically meaningful variations were detected. Also, at distinct wavelength regions, the (V)/F(M) differed significantly between leaf types. From spectra of F(V)/F (M), excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F(V)/F(M) between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F(V)/F(M) at these wavelengths. All three leaves exhibited low values of F(V)/F(M) around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550-650 nm region, the F(V)/F(M) in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C(4) species, than in barley, a C(3) species. Finally, low values of F(V)/F(M) at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F(V)/F(M) ratio to derive information on spectral absorption properties of PS I and PS II is discussed.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

The effect of outer antenna complexes on the photochemical trapping rate in barley thylakoid Photosystem II

We have investigated the previous suggestions in the literature that the outer antenna of Photosystem II of barley does not influence the effective photosystem primary photochemical trapping rate. It is shown by steady state fluorescence measurements at the F(0) fluorescence level of wild type and the chlorina f2 mutant, using the chlorophyll b fluorescence as a marker, that the outer antenna is thermally equilibrated with the core pigments, at room temperature, under conditions of photochemical trapping. This is in contrast with the conclusions of the earlier studies in which it was suggested that energy was transferred rapidly and irreversibly from the outer antenna to the Photosystem II core. Furthermore, the effective trapping time, determined by single photon counting, time-resolved measurements, was shown to increase from 0.17+/-0.017 ns in the chlorina Photosystem II core to a value within the range 0.42+/-0.036-0.47+/-0.044 ns for the wild-type Photosystem II with the outer antenna system. This 2.5-2.8-fold increase in the effective trapping time is, however, significantly less than that expected for a thermalized system. The data can be explained in terms of the outer antenna increasing the primary charge separation rate by about 50%.

Peroxidative reactions attenuate oxygen effect on spectroscopic properties of isolated chloroplasts

Steady-state absorption and fluorescence excitation spectra have been measured at 25 degrees C in order to elucidate the differences between isolated chloroplasts from pea (chilling-sensitive plant) and bean (chilling-tolerant plant) and their response to oxygen treatment. A weaker light harvesting in bean in comparison with pea chloroplasts is related to higher free fatty acids level and extended peroxidation activities of bean chloroplasts. Peroxidation of free fatty acids in bean chloroplasts results in an accumulation of oxygenated forms of fatty acids demonstrated by a large negative band around 400 nm in absorption difference spectra, while the excitation spectra are not significantly altered. Similar changes have been observed in the lipase-treated pea chloroplasts. In contrast, in both pea and bean chloroplasts exhibiting no peroxidation due to antimycin A treatment, oxygen induces a pronounced absorbance increase in the regions around 435, 470 and 674 nm indicating the chloroplast swelling. A decline of chlorophyll fluorescence excitation caused by oxygen, may result from a decrease in energy transfer from antennae complexes to chlorophyll species emitting at both 680 and 740 nm. The oxygen-induced changes are partially reversed upon restoration of anaerobic conditions. The presented data show for the first time, that in contrast to pea chloroplasts the peroxidation abolishes an oxygen-induced decrease in light harvesting in bean chloroplasts, i.e., a chilling-sensitive plant.

Light-induced fluorescence quenching in the light-harvesting chlorophyll a/b protein complex

Irradiation of the principal photosystem II light-harvesting chlorophyll-protein antenna complex, LHC II, with high light intensities brings about a pronounced quenching of the chlorophyll fluorescence. Illumination of isolated thylakoids with high light intensities generates the formation of quenching centres within LHC II in vivo, as demonstrated by fluorescence excitation spectroscopy. In the isolated complex it is demonstrated that the light-induced fluorescence quenching: a) shows a partial, biphasic reversibility in the dark; b) is approximately proportional to the light intensity; c) is almost independent of temperature in the range 0-30°C; d) is substantially insensitive to protein modifying reagents and treatments; e) occurs in the absence of oxygen. A possible physiological importance of the phenomenon is discussed in terms of a mechanism capable of dissipating excess excitation energy within the photosystem II antenna.

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385-398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.