Quantification of non-Q B -reducing centers in leaves using a far-red pre-illumination

An alternative approach to quantification of the contribution of non-QB-reducing centers to Chl a fluorescence induction curve is proposed. The experimental protocol consists of a far-red pre-illumination followed by a strong red pulse to determine the fluorescence rise kinetics. The far-red pre-illumination induces an increase in the initial fluorescence level (F25 μs) that saturates at low light intensities indicating that no light intensity-dependent accumulation of QA− occurs. Far-red light-dose response curves for the F25 μs-increase give no indication of superimposed period-4 oscillations. F25 μs-dark-adaptation kinetics following a far-red pre-pulse, reveal two components: a faster one with a half-time of a few seconds and a slower component with a half-time of around 100 s. The faster phase is due to the non-QB-reducing centers that re-open by recombination between QA− and the S-states on the donor side. The slower phase is due to the recombination between QB− and the donor side in active PS II reaction centers. The pre-illumination-induced increase of the F25 μs-level represents about 4-5% of the variable fluorescence for pea leaves (∼2.5% equilibrium effect and 1.8-3.0% non-QB-reducing centers). For the other plant species tested these values were very similar. The implications of these values will be discusse

A non-invasive assay of the plastoquinone pool redox state based on the OJIP-transient

The plastoquinone (PQ) pool of the photosynthetic electron transport chain becomes reduced under anaerobic conditions. Here, anaerobiosis was used as a tool to manipulate the PQ-pool redox state in darkness and to study the effects of the PQ-redox state on the Chl-a fluorescence (OJIP) kinetics in pea leaves (Pisum sativum L.). It is shown that the FJ (fluorescence intensity at 3ms) is linearly related to the area above the OJ-phase (first 3ms) representing the reduction of the acceptor side of photosystem II (PSII) and FJ is also linearly related to the area above the JI-phase (3-30ms) that parallels the reduction of the PQ-pool. This means that FJ depends on the availability of oxidized PQ-molecules bound to the QB-site. The linear relationships between FJ and the two areas indicate that FJ is not sensitive to energy transfer between PSII-antennae (connectivity). It is further shown that a ∼94% reduced PQ-pool is in equilibrium with a ∼19% reduction of QA (primary quinone acceptor of PSII). The non-linear relationship between the initial fluorescence value (F20μs) and the area above the OJ-phase supports the idea that F20μs is sensitive to connectivity. This is reinforced by the observation that this non-linearity can be overcome by transforming the F20μs-values into [QA −]-values. Based on the FJ-value of the OJIP-transient, a simple method for the quantification of the redox state of the PQ-pool is propose

Heat stress and the photosynthetic electron transport chain of the lichen Parmelina tiliacea (Hoffm.) Ach. in the dry and the wet state: differences and similarities with the heat stress response of higher plants

Thalli of the foliose lichen species Parmelina tiliacea were studied to determine responses of the photosynthetic apparatus to high temperatures in the dry and wet state. The speed with which dry thalli were activated by water following a 24h exposure at different temperatures decreased as the temperature was increased. But even following a 24h exposure to 50°C the fluorescence induction kinetics OJIP reflecting the reduction kinetics of the photosynthetic electron transport chain had completely recovered within 128min. Exposure of dry thalli to 50°C for 24h did not induce a K-peak in the fluorescence rise suggesting that the oxygen evolving complex had remained intact. This contrasted strongly with wet thalli were submergence for 40s in water of 45°C inactivated most of the photosystem II reaction centres. In wet thalli, following the destruction of the Mn-cluster, the donation rate to photosystem II by alternative donors (e.g. ascorbate) was lower than in higher plants. This is associated with the near absence of a secondary rise peak (~1s) normally observed in higher plants. Analysing the 820nm and prompt fluorescence transients suggested that the M-peak (occurs around 2-5s) in heat-treated wet lichen thalli is related to cyclic electron transport around photosystem I. Normally, heat stress in lichen thalli leads to desiccation and as consequence lichens may lack the heat-stress-tolerance-increasing mechanisms observed in higher plants. Wet lichen thalli may, therefore, represent an attractive reference system for the evaluation of processes related with heat stress in higher plant

Photosynthetic electron transport activity in heat-treated barley leaves: The role of internal alternative electron donors to photosystem II

AbstractElectron transport processes were investigated in barley leaves in which the oxygen-evolution was fully inhibited by a heat pulse (48 °C, 40 s). Under these circumstances, the K peak (∼F400 μs) appears in the chl a fluorescence (OJIP) transient reflecting partial QA reduction, which is due to a stable charge separation resulting from the donation of one electron by tyrozine Z. Following the K peak additional fluorescence increase (indicating QA− accumulation) occurs in the 0.2–2 s time range. Using simultaneous chl a fluorescence and 820 nm transmission measurements it is demonstrated that this QA− accumulation is due to naturally occurring alternative electron sources that donate electrons to the donor side of photosystem II. Chl a fluorescence data obtained with 5-ms light pulses (double flashes spaced 2.3–500 ms apart, and trains of several hundred flashes spaced by 100 or 200 ms) show that the electron donation occurs from a large pool with t1/2 ∼30 ms. This alternative electron donor is most probably ascorbate

Evidence for a fluorescence yield change driven by a light-induced conformational change within photosystem II during the fast chlorophyll a fluorescence rise

AbstractExperiments were carried out to identify a process co-determining with QA the fluorescence rise between F0 and FM. With 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU), the fluorescence rise is sigmoidal, in its absence it is not. Lowering the temperature to −10°C the sigmoidicity is lost. It is shown that the sigmoidicity is due to the kinetic overlap between the reduction kinetics of QA and a second process; an overlap that disappears at low temperature because the temperature dependences of the two processes differ. This second process can still relax at −60°C where recombination between QA− and the donor side of photosystem (PS) II is blocked. This suggests that it is not a redox reaction but a conformational change can explain the data. Without DCMU, a reduced photosynthetic electron transport chain (ETC) is a pre-condition for reaching the FM. About 40% of the variable fluorescence relaxes in 100ms. Re-induction while the ETC is still reduced takes a few ms and this is a photochemical process. The fact that the process can relax and be re-induced in the absence of changes in the redox state of the plastoquinone (PQ) pool implies that it is unrelated to the QB-occupancy state and PQ-pool quenching. In both +/−DCMU the process studied represents ~30% of the fluorescence rise. The presented observations are best described within a conformational protein relaxation concept. In untreated leaves we assume that conformational changes are only induced when QA is reduced and relax rapidly on re-oxidation. This would explain the relationship between the fluorescence rise and the ETC-reduction