Origin of Chlorophyll Fluorescence in Plants at 55–75°C ¶

The origin of heat-induced chlorophyll fluorescence rise that appears at about 55–60°C during linear heating of leaves, chloroplasts or thylakoids (especially with a reduced content of grana thylakoids) was studied. This fluorescence rise was earlier attributed to photosystem I (PSI) emission. Our data show that the fluorescence rise originates from chlorophyll a (Chl a ) molecules released from chlorophyll-containing protein complexes denaturing at 55–60°C. This conclusion results mainly from Chl a fluorescence lifetime measurements with barley leaves of different Chl a content and absorption and emission spectra measurements with barley leaves preheated to selected temperatures. These data, supported by measurements of liposomes with different Chl a /lipid ratios, suggest that the released Chl a is dissolved in lipids of thylakoid membranes and that with increasing Chl a content in the lipid phase, the released Chl a tends to form low-fluorescing aggregates. This is probably the reason for the suppressed fluorescence rise at 55–60°C and the decreasing fluorescence course at 60–75°C, which are observable during linear heating of plant material with a high Chl a /lipid ratio ( e.g. green leaves, grana thylakoids, isolated PSII particles).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74574/1/0031-8655_2003_0770068OOCFIP2.0.CO2.pd

Light absorption and scattering by high light-tolerant, fast-growing Chlorella vulgaris IPPAS C-1 cells

Algal cells are highly complex optical systems that can dynamically change their structure. Consequently, absorption and scattering properties of algae change, while the cells are acclimating to different light conditions or during growth and division in a cell cycle. This may be particularly important in algal species that can grow rapidly under very high-light such as Chlorella vulgaris IPPAS C-1 that is studied here. From cell transmittance measured conventionally and using integrating sphere, we evaluated absorption and scattering coefficients and cross sections per cell dry weight and chlorophyll content. This was done for asynchronous cell culture grown in low-light (LL; 220 μmol (photons) m−2 s−1) or high-light (HL; 1760 μmol (photons) m−2 s−1) light, as well as during cell cycle of synchronous culture grown in HL. During the cell cycle, we also determined cell ultrastructural organization by transmission electron microscopy, and correlated its parameters with absorption and scattering cross sections per cell dry weight. We found that the IPPAS C-1 cells of asynchronous culture scatter light more than other cells, however, internal organization of the cells that is decisive for scattering is less sensitive to HL and LL treatment than the cell pigment content that controls absorption. The light scattering and absorption were dynamically changed during cell cycle of synchronous cells grown in the HL. Changes in ratio of chloroplast to protoplast area, reflecting amount of scattering chloroplast membrane (outer, inner) interfaces, best correlated with changes in light scattering. We suggest that the increased light scattering by the HL-acclimated IPPAS C-1 cells might be responsible for increased HL resilience reported in the literature. Biotechnological aspect of this study is that the scattering and absorption properties of phytoplankton cells ought to be calibrated for each particular growth phase or irradiance to which the cells are acclimated

Evidence that cytochrome b(559) is involved in superoxide production in photosystem II: effect of synthetic short-chain plastoquinones in a cytochrome b(559) tobacco mutant

Light-induced production of superoxide (O(2)(•−)) in spinach PSII (photosystem II) membrane particles was studied using EPR spin-trapping spectroscopy. The presence of exogenous PQs (plastoquinones) with a different side-chain length (PQ-n, n isoprenoid units in the side-chain) enhanced O(2)(•−) production in the following order: PQ-1>PQ-2≫PQ-9. In PSII membrane particles isolated from the tobacco cyt (cytochrome) b(559) mutant which carries a single-point mutation in the β-subunit and also has a decreased amount of the α-subunit, the effect of PQ-1 was less than in the wild-type. The increase in LP (low-potential) cyt b(559) content, induced by the incubation of spinach PSII membrane particles at low pH, resulted in a significant increase in O(2)(•−) formation in the presence of PQ-1, whereas it had little effect on O(2)(•−) production in the absence of PQ-1. The enhancement of O(2)(•−) formation induced by PQ-1 was not abolished by DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. Under anaerobic conditions, dark oxidation of LP cyt b(559) increased, as pH was decreased. The presence of molecular oxygen significantly enhanced dark oxidation of LP cyt b(559). Based on these findings it is suggested that short-chain PQs stimulate O(2)(•−) production via a mechanism that involves electron transfer from Pheo(−) (pheophytin) to LP cyt b(559) and subsequent auto-oxidation of LP cyt b(559)