Fluorescence induction in the phycobilisome-containing cyanobacterium Synechococcus sp PCC 7942: Analysis of the slow fluorescence transient

At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at approximately 0.2-1 and M at approximately 100-500 s, with a minimum S at approximately 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with O denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS-->Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS-->PS I excitation transfer. Blocking the PBS-->PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K.Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobacterial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P-->T fluorescence decay, reminiscent of the typical P-->T fluorescence decay of higher plants and algae. A similar P-->T decay was also displayed by DCMU-treated Synechococcus cells at 2 degrees C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2-->1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state 1 and upon illumination they display an extended P-->T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1-->2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS-->PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Synechococcus becomes plant-like.

Studies on the induction of chlorophyll fluorescence in barley protoplasts. II. Resolution of fluorescence quenching by redox state and the transthylakoid pH gradient

During the chlorophyll fluorescence oscillation described in barley protoplasts (Quick & Horton, Proc. R. Soc. Lond . B 220, 361-370, 1984) the components which contribute to quenching have been quantified. Quenching due to oxidized Q ( q Q ) was measured either by DCMU addition or by light doubling and indicated an oscillation in the redox state of Q which was antiparallel to the rate of oxygen evolution but was approximately 15 s out of phase at a variety of light intensities and temperatures. An oscillation in the extent of energy-dependent quenching, q e , was observed in strong but not weak light. These results are discussed in terms of the mechanism of the changes in energy and redox states that can contribute additively but in differing proportions to the fluorescence oscillation.

Relations between electron transport and carbon assimilation; simultaneous measurement of chlorophyll fluorescence, transthylakoid pH gradient and O 2 evolution in isolated chloroplasts

An apparatus is described that allows simultaneous measurement of photosynthetic O 2 evolution, chlorophyll fluorescence and the trans­thylakoid pH gradient (∆pH) in isolated chloroplasts irradiated with light sufficient to saturate photosynthesis. In intact chloroplasts, quenching of chlorophyll fluorescence due to both oxidation of the primary electron acceptor of photosystem II (Q) and formation of ∆pH was seen. The relative proportions of the two kinds of quenching varied in response to ( a ) the light intensity, ( b ) the presence of phosphoglycerate and ( c ) whether or not the chloroplasts were in the induction period or in a period of linear photosynthetic O 2 evolution. In broken chloroplasts reconstituted for CO 2 fixation, transient changes in the rates of O 2 evolution, ∆pH, the redox state of Q and chlorophyll fluorescence were observed as a result of changes in ( a ) the availability of electron acceptor as determined by the additions of NADP and phosphoglycerate and ( b ) the ratio of ATP to ADP, as manipulated by addition of ribose 5-phosphate. The changes in chlorophyll fluorescence in this system could be manipulated to show a pattern very similar to that observed in leaves.

Some effects of changes in gas phase on the steady-state chlorophyll a fluorescence exhibited by illuminated leaves

Excursions in chlorophyll fluorescence were obtained under continuous illumination by changing the composition of the atmosphere surrounding pieces of spinach leaf. Removal of CO 2 caused a transient rise and a subsequent fall to a new quasi-steady-state level. This response could be interrupted at any time by readmission of air, and similar but smaller excursions could be initiated by decreasing the CO 2 concentration. The effect was then proportional to the decrease in CO 2 whereas the steady-state value was related to the final CO 2 concentration. The excursions promoted by the decrease in CO 2 concentration were enhanced if O 2 was simultaneously decreased from 20 to 2%. Responses to changes in O 2 concentration in the absence of CO 2 , to increases in CO 2 concentration alone, and to pulses of CO 2 and O 2 in a N 2 atmosphere, are also described. The results are discussed in relation to changes in NADPH and ATP brought about by photosynthetic carbon assimilation and its regulatory mechanisms.

Studies on the induction of chlorophyll fluorescence in barley protoplasts. I. Factors affecting the observation of oscillations in the yield of chlorophyll fluorescence and the rate of oxygen evolution

Chlorophyll fluorescence and oxygen evolution have been measured in suspensions of protoplasts isolated from barley leaves. Like the parent tissue, the protoplasts exhibit transients in both fluorescence and the rate of oxygen evolution upon illumination and before a steady state rate of photosynthesis is reached. The two signals are anti-parallel and slightly phase shifted with fluorescence changes preceeding alteration in the rate of oxygen evolution by 10-15 s. Fast ( t ≈ 15 s) and a slow ( t ≈ 2 min) fluorescence peaks were observed, the latter strongly resembling the oscillations described in leaves (Walker, Horton, Sivak & Quick, Photobiochem . Photobiophys . 5, 35 (1983)). The amplitude of the oscillation was most pronounced in strong light, and at high (more than 8.0) pH whereas the period was temperature dependent. Antimycin A was found to suppress the slow oscillation but to exaggerate the fast transient, whereas low ( < 0.1 μM) DCMU had the reverse effect. These results are discussed in terms of a proposed mechanism to account for these transients and it is suggested that cyclic electron flow may have an important role.

Slow secondary fluorescence kinetics associated with the onset of photosynthetic carbon assimilation in intact isolated chloroplasts

Experiments with carefully isolated, largely intact chloroplasts, capable of fast rates of CO 2 -dependent O 2 evolution, show that the fall in chlorophyll a fluorescence (from the early maxima reached immediately after illumination) is interrupted by a ‘shoulder’ which is associated with the exponential increase in the rate of O 2 evolution. The length of this induction period was increased by storage, by decreased temperature, by increased orthophosphate concentration in the assay medium or by the presence of D, L-glyceraldehyde. It could also be shortened by the addition of 3-phosphoglycerate or dihydroxyacetonephosphate. In each treatment the shoulder in fluorescence shifted so that the association with the period of exponential increase was maintained. When illumination was re-started after a short dark interval, induction was minimal and no shoulder could be discerned, but both the lag in the onset of O 2 evolution and the shoulder were restored when the chloroplasts were resuspended in fresh assay medium during the period of darkness. The relation between chlorophyll a fluorescence and the onset of photosynthetic carbon assimilation is discussed.

Effect of methyl viologen on slow secondary fluorescence kinetics associated with photosynthetic carbon assimilation in intact isolated chloroplasts

Methyl viologen in catalytic amounts induces pronounced secondary kinetics in fluorescence in intact isolated chloroplasts performing photosynthetic carbon assimilation. These transient increases in fluorescence and oscillations were associated with the induction phase of O 2 evolution in a similar manner to the transient ‘shoulder’ detected previously (Z. G. Cerović, M. N. Sivak and D. A. Walker, Proc . R . Soc . Lond . B 220, 327–338 (1984)). Experiments with the addition of antimycin A and gramicidin D demonstrated that methyl viologen induced an increased ATP production linked to pseudocyclic electron transport. The adjustment of ATP and NADPH production to meet the requirements of the reductive pentose phosphate pathway during induction is thought to be the cause of the detected transients and oscillations in fluorescence.

Fluorescence characteristics of photoautotrophic soybean cells

We report here the first measurements on chlorophyll (Chl) a fluorescence characteristics of photoautotrophic soybean cells (cell lines SB-P and SBI-P). The cell fluorescence is free from severe distortion problems encountered in higher plant leaves. Chl a fluorescence spectra at 77 K show, after correction for the spectral sensitivity of the photomultiplier and the emission monochromator, peaks at 688, 696 and 745 nm, representing antenna systems of photosystem II-CP43 and CP47, and photosystem I, respectively. Calculations, based on the complementary area over the Chl a fluorescence induction curve, indicated a ratio of 6 of the mobile plastoquinone (including QB) to the primary stable electron acceptor, the bound plastoquinone QA. A ratio of one between the secondary stable electron acceptor, bound plastoquinone QB, and its reduced form QB (-) was obtained by using a double flash technique. Owing to this ratio, the flash number dependence of the Chl a fluorescence showed a distinct period of four, implying a close relationship to the 'S' state of the oxygen evolution mechanism. Analysis of the QA (-) reoxidation kinetics showed (1) the halftime of each of the major decay components (∼ 300 μs fast and ∼ 30 ms slow) increases with the increase of diuron and atrazine concentrations; and (2) the amplitudes of the fast and the slow components change in a complementary fashion, the fast component disappearing at high concentrations of the inhibitors. This implies that the inhibitors used are able to totally displace QB. In intact soybean cells, the relative amplitude of the 30 ms to 300 μs component is higher (40:60) than that in spinach chloroplasts (30:70), implying a larger contribution of the centers with unbound QB. SB-P and SBI-P soybean cells display a slightly different sensitivity of QA (-) decay to inhibitors.

Regulation of photosynthetic electron-transport in Phaseolus vulgaris L., as determined by room-temperature chlorophyll a fluorescence

The regulation of photosystem II (PSII) by light-, CO2-, and O2-dependent changes in the capacity for carbon metabolism was studied. Estimates of the rate of electron transport through PSII were made from gas-exchange data and from measurements of chlorophyll fluorescence. At subsaturating photon-flux density (PFD), the rate of electron transport was independent of O2 and CO2. Feedback on electron transport was observed under two conditions. At saturating PFD and low partial pressure of CO2, p(CO2), the rate of electron transport increased with p(CO2). However, at high p(CO2), switching from normal to low p(O2) did not affect the net rate of photosynthetic CO2 assimilation but the rate of electron-transport decreased by an amount related to the change in the rate of photorespiration. We interpret these effects as 1) regulation of ribulose-1,5-bisphosphatecarboxylase (RuBPCase, EC 4.1.1.39) activity to match the rate of electron transport at limiting PFD, 2) regulation of electron-transport rate to match the rate of RuBPCase at low p(CO2), and 3) regulation of the electron-transport rate to match the capacity for starch and sucrose synthesis at high p(CO2) and PFD. These studies provide evidence that PSII is regulated so that the capacity for electron transport is matched to the capacity for other processes required by photosynthesis, such as ribulose-bisphosphate carboxylation and starch and sucrose synthesis. We show that at least two mechanisms contribute to the regulation of PSII activity and that the relative engagement of these mechanisms varies with time following a step change in the capacity for ribulose-bisphosphate carboxylation and starch and sucrose synthesis. Finally, we take advantage of the relatively slow activation of deactivated RuBPCase in vivo to show that the activation level of this enzyme can limit the rate of electron transport as evidenced by increased feedback on PSII following a step change in p(CO2). As RuBPCase as activated, the feedback on PSII declined.

Temperature-induced alterations of in vivo chlorophyll a fluorescence induction in cucumber as affected by DCMU

Induction of chlorophyll a fluorescence and photosynthesis as affected by temperature were measured in cucumber leaf discs. Abrupt changes of the maximal variable fluorescence, Fv(p), and photosynthesis were observed around 9° and 21°C when the temperature was decreased from 30° to 0°C. The temperature-dependent maximal fluorescence of DCMU-treated leaf discs showed a single change around 21°C. Temperature-induced chlorophyll a fluorescence alterations are discussed in relation to electron transport activity of the two photosystems and photosynthetic activity of the cucumber leaf discs.

Observation and characterisation of a transient in the yield of chlorophyll fluorescence in intact spinach chloroplasts

A transient in chlorophyll fluorescence, which is associated with a transient in 9-aminoacridine fluorescence and a perturbation in the rate of oxygen evolution, has been observed in intact spinach chloroplasts. The results indicate that changes in the redox state of Q are, at least partially, responsible for the transient in chlorophyll fluorescence. The size of the transient is highly dependent upon the concentration of inorganic phosphate and upon the pH of the medium. The properties of the transient are consistent with the suggestion that it reflects changes in the levels of stromal intermediates during induction.

Sudden changes in the rate of photosynthetic oxygen evolution and chlorophyll fluorescence in intact isolated chloroplasts: the role of orthophosphate

Simultaneous "ripples" (sudden changes in rate) in CO2 dependent O2 evolution and associated chlorophyll a fluorescence were followed in isolated, largely intact, spinach chloroplasts. These ripples could only be observed under conditions in which the supply of inorganic phosphate was limiting. This limitation was achieved either by 1) omission of phosphate in the assay medium, 2) use of inhibitors of the phosphate translocator, or 3) the addition of triose phosphate, a competitive inhibitor of Pi for the same translocator.The possible relation of these ripples to the dampening oscillations that can be observed in leaves, leaf pieces, isolated cells and protoplasts, is discussed.

Stomatal aperture, photosythesis and water fluxes in mesophyll cells as affected by the abscission of leaves. Simultaneous measurements of gas exchange, light scattering and chlorphyll fluorescence

Carbon dioxide exchange, transpiration, chlorophyll fluorescence and light scattering of leaves of Lycopersicom esculentum, Helianthus annuus and Arbutus unedo were measured simultaneously before and after abscission of leaves. Scattering of a weak green measuring beam was used to monitor water fluxes across the thylakoid membranes of the mesophyll. When leaves were cut under water, stomata initially closed partially and then occasionally exhibited distinct regulatory oscillations. As stomata closed, light scattering decreased indicating water influx into the mesophyll. Stomatal oscillations were accompanied, with small but noticeable phase shifts, by oscillations of water fluxes at the thylakoid level. These fluxes could be distinguished from the water fluxes accompanying light-dependent ion pumping across the thylakoids by the concomitant chlorophyll fluorescence signals. The latter record energy-dependent ion fluxes in addition to redox changes of the electron-transport chain. As stomata closed partially after cutting a leaf under water, photosynthesis decreased. In Arbutus unedo and Helianthus annuus leaves, transient stomatal closure was insufficient to account for transient inhibition of photosynthesis which appeared to be brought about by transfer of an inhibitory solute through the petiole into the mesophyll. This solute also stimulated respiration in the dark. When leaves were cut in air, stomata opened transiently (Iwanoff effect) before wilting enforced closure. Photosynthesis followed the stomatal responses, increasing during opening and decreasing during closure.

Chlorophyll a fluorescence and light-scattering kinetics displayed by leaves during induction of photosynthesis

Light-scattering, which can be taken as an indicator of the transthylakoid proton-gradient, and chlorophyll a fluorescence, have been followed simultaneously during re-illumination of spinach leaves at different energy fluence rates and carbon dioxide concentrations. The slow fluorescence transient ("M peak"), which has been associated with photosynthetic induction, was observed in air only at the lower fluence rates used. Data are presented that indicate that M peaks in chlorophyll fluorescence kinetics can only be observed if there is also a simultaneous transient in light-scattering and that these transients are observed when the dark period is relatively long, fluence rate relatively low, and CO2 concentration relatively high.The results are discussed in relation to the varying demands on ATP by carbon assimilation during induction of photosynthesis at different carbon dioxide concentrations and the manner in which these variations influence the quenching of chlorophyll a fluorescence.

The relationship between carbon dioxide fixation and chlorophyll a fluorescence during induction of photosynthesis in maize leaves at different temperatures and carbon dioxide concentrations

The rate of CO2 fixation (Fc) and 680 nm chlorophyll fluorescence emission (F680) were measured simultaneously during induction of photosynthesis in Zea mays L. leaves under varying experimental conditions in order to assess the validity of fluorescence as an indicator of in vivo photosynthetic carbon assimilation. Z. mays leaves showed typical 'Kautsky' fluorescence induction curves consisting of a fast rise in emission (O to P) followed by a slow quenching via a major transient (S-M) to a steady-state (T). After an initial lag, net CO2 assimilation commenced at a point corresponding to the onset of the S-M transient on the F680 induction curve. Subsequently, Fc and F680 always arrived at a steady-state simultaneously. Decreasing the dark-adaption period increased the rate of induction of both parameters. Alteration of leaf temperature produced anti-parallel changes in induction characteristics of Fc and F680. Reducing the CO2 level to below that required for saturation of photosynthesis also produced anti-parallel changes during induction, however, at CO2 concentrations tenfold greater than the atmospheric level the rate of F680 quenching from P to T was appreciably reduced without a similar change in the induction of Fc. Removal of CO2 at steady-state produced only a small increase in F680 and a correspondingly small decrease in F680 occurred when CO2 was re-introduced. The complex relationship between chlorophyll fluorescence and carbon assimilation in vivo is discussed and the applicability of fluorescence as an indicator of carbon assimilation is considered.