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ABSTRACT: The dark-relaxation kinetics of variable fluorescence, Fv, in intact green leaves of Pisum stativum L. and Dolichos lablab L. were analyzed using modulated fluorometers. Fast (t1/2 = 1 s) and slow (t1/2 = 7–8 s) phases in fv dark-decay kinetics were observed; the rate and the relative contribution of each phase in total relaxation depended upon the fluence rate of the actinic light and the point in the induction curve at which the actinic light was switched off. The rate of the slow phase was accelerated markedly by illumination with far-red light; the slow phase was abolished by methyl viologen. The halftime of the fast phase of Fv dark decay decreased from 250 ms in dark-adapted leaves to 12–15 ms upon adaptation to red light which is absorbed by PSII. The analysis of the effect of far-red light, which is absorbed mainly by PSI, on Fv dark decay indicates that the slow phase develops when a fraction of QA
– (the primary stable electron acceptor of PSII) cannot transfer electrons to PSI because of limitation on the availability of P700+ (the primary electron donor of PSI). After prolonged illumination of dark-adapted leaves in red (PSII-absorbed) light, a transient. Fv rise appears which is prevented by far-red (PSI-absorbed) light. This transient fv rise reflects the accumulation of QA
– in the dark. The observation of this transient Fv rise even in the presence of the uncoupler carbonylcyanide m-chlorophenyl hydrazone (CCCP) indicates that a mechanism other than ATP-driven back-transfer of electrons to QA may be responsible for the phenomenon. It is suggested that the fast phase in Fv dark-decay kinetics represents the reoxidation of QA
– by the electron-transport chain to PSI, whereas the slow phase is likely to be related to the interaction of QA
– with the donor side of PSII.
Planta 03/1992; 187(1):122-127. · 3.00 Impact Factor
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ABSTRACT: After preheating of Amaranthus chloroplasts at elevated temperatures (up to 45C), the chlorophyll a fluorescence level under low excitation light rises as compared to control (unheated) as observed earlier in other chloroplasts (Schreiber U and Armond PA (1978) Biochim Biophys Acta 502: 138–151). This elevation of heat induced fluorescence yield is quenched by addition of 0.1 mM potassium ferricyanide, suggesting that with mild heat stress the primary electron acceptor of photosystem II is more easily reduced than the unheated samples. Furthermore, the level of fluorescence attained after illumination of dithionite-treated samples is independent of preheating (up to 45C). Thus, these experiments indicate that the heat induced rise of fluorescence level at low light can not be due to changes in the elevation in the true constant F0 level, that must by definition, be independent of the concentration of QA. It is supposed that the increase in the fluorescence level by weak modulated light is either partly associated with dark reduction of QA due to exposure of chloroplasts to elevated temperature or due to temperature induced fluorescence rise in the so called inactive photosystem II centre where QA are not connected to plastoquinone pool. In the presence of dichlorophenyldimethylurea the fluorescence level triggered by weak modulated light increases at alkaline pH, both in control and heat stressed chloroplasts. This result suggests that the alkaline pH accelerates electron donation from secondary electron donor of photosystem II to QA both in control and heat stressed samples. Thus the increase in fluorescence level probed by weak modulated light due to preheating is not solely linked to increase in true F0 level, but largely associated with the shift in the redox state of QA, the primary stable electron acceptor of photosystem II.
Photosynthesis Research 12/1989; 23(1):81-87. · 3.24 Impact Factor
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ABSTRACT: Heat-stress-induced photosynthetic electron transport and emission properties were studied in the cyanobacterium Spirulina platensis. Heat treatment of intact cells up to 50 °C did not cause major changes in the absorption and emission properties of both chlorophyll a and phycocyanin. However, above 50 °C, there was a specific bleaching of phycobiliproteins and an uncoupling of energy transfer in phycobilisomes. Heat stress also reduced the extent and slowed down the decay kinetics of light-induced quenching of the long wavelength emission band which has been shown to be associated with the redox state of P 700, the primary donor of photosystem I. Electron transport activities measured in intact cells showed a decline in the photosystem II mediated Hill activity and an increase in the photosystem I activity with increasing temperatures. However, isolated thylakoid membranes did not exhibit heat-induced stimulation in photosystem I activity. This indicates that the enhancement of photosystem I activity in intact cells is mostly due to increased permeability of cells for the entry of acceptors and donors upon heat treatment. However, mild heat treatments induced damage at the plastoquinone pool, as indicated by the inhibition in the durohydroquinone to methylviologen intersystem electron flow. These results suggest that unlike higher plants, the thylakoid membranes of the cyanobacterium Spirulina platensis do not show heat-induced stimulation in photosystem I activity. We argue that the lack of heat-induced stimulation in photosystem I activity in this cyanobacterium may arise as a result of the absence of light harvesting chlorophyll a/b complex and also variations in the membrane lipid organizations.
Journal of Photochemistry and Photobiology B: Biology.
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ABSTRACT: Changes in the fluorescence yield of chlorophyll were monitored to investigate the effect of HgCl2 on the cyanobacteria Spirulina platensis and Anacystis nidulans; weak modulated light, high intensity actinic light and additional strong sources of illumination were used. Depending on the concentration of HgCl2, three distinct types of change in the fluorescence yield of chlorophyll a were observed. At low concentrations (1.5 μM), HgCl2 behaves in a similar manner to diuron in that it increases the fluorescence intensity F0 in weak modulated light. This increase may be due to blockage of electron flow on the reducing side of photosystem II. At slightly increased levels of mercury (3 μM), the quenching of the variable fluorescence of chlorophyll suggests a decrease in electron flow on the donor side of photosystem II. This effect becomes much more evident in the presence of diuron. At sufficiently high concentrations (18 μM), a pronounced quenching of the chlorophyll fluorescence is observed, which may be due to both the blocking of photosystem II on the donor side and structural changes in the antenna pigments. These multiple effects of mercury are also observed in intact cells of Anacystis and in spheroplasts prepared from Anacystis. In the latter, the effects of mercury saturate at lower concentrations than those observed in intact cells. The effect of mercury cannot be reversed by ethylenediaminetetraacetic acid, suggesting that mercury binds with the pigment—protein complexes of the cyanobacteria.
Journal of Photochemistry and Photobiology B: Biology. 6(4):373-380.
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ABSTRACT: The effect of reduction in leaf water content on the complete chlorophyll α fluorescence transient of Dolichos leaf was monitored using a kinetic fluorescence monitor. Specific and sequential alterations in the profile of the fluorescence transient was observed as leaf water content was progressively decreased. Of all the characteristic, the PS 1 M 1 S 2 M 1 T (cf. Govindjee and Papageorgiou 1971) points of the fluorescence induction curve, the M 2 peak which is linked to CO 2 fixation potential of leaf, was extremely sensitive to desiccation. The extent of PS, phase of the transient diminished almost proportionally to leaf water content until the water content approached about 30%. The effect of strong additional illumination administered during the fluorescence transient suggests that the quenching of fluorescence due to desiccation is mostly linked to the redox-level of Q A ; the stable electron acceptor of photosytem II, and not to the changes in ΔpH formation or ‘state changes’. Rewatering of desiccated leaves restored the fluorescence transient except in case of severe desiccation. The monitoring of fluorescence transient thus offers a diagnostic tool to assess the nature and the level of impairments due to loss in leaf water content.
