Fluorescence Induction Studies in Isolated Chloroplasts IV. The Inhibition of Electron Transfer from Primary to Secondary Electron Carriers of PS-II at Low Temperature and by DCMU

The effects of wide concentration ranges of NaCI, KC, and MgCl2 on femcyanide reduction and the fluorescence induction curve of isolated spinach (Spinacia oleraces) chloroplasts were investigated. Concentrations of the monovalent salts above 100 mM and MgC12 above 25 mm produced a decrease in the rate of ferricyanide reduction by thylakoids uncoupled with 2.5 mm NH4Cl which cannot be attributed to changes in the primary photochemical capacity of photosystem IL Salt-induced decreases in the effective concentration of the secondary electron acceptor of photosystem II, plastoquinone, reduce the capacity for secondary photochemistry of photosystem II and this could contribute to the reduction in ferricyanide reduction by uncoupled thylakoids at high salinities. The rate of ferricyanide reduction by coupled thylakoids is little affected by salinity changes, indicating that the rate-limiting phosphorylation mechanism in electron flow from water to ferricyanide in coupled thylakoids is salt-tolerant, whereas the rate-limiting reaction in uncoupled ferricyanide reduction is considerably affected by salinity changes. Salt-induced changes in the fluorescence induction curve are interpreted in terms of changes in the rate constants for excitation decay by radiationless transitions, exciton transfer from photosystem II chlorophylis to other associated chlorophyll species, and photochemistry.The ionic environment of the thylakoid membrane plays an important role in regulating many of the physicochemical reactions involved in the primary photosynthetic processes. Changes in both the conformational and energetic states of the thylakoid membrane can be induced by changes in the external cation concentration. Divalent cations can affect the degree of stacking of the thylakoids; stacking increases as the external cation concentration increases (10). Such changes in membrane stacking have been related to changes in energy transfer between PSII and PSI (16), although recently it has been shown that changes in stacking -can occur unrelated to changes in fluorescence yield and energy transfer (23). The majority of studies on cation regulation of the primary photosynthetic processes have been made with monovalent cation concentrations in the range 0 to 150 mm, and 0 to 20 mm for divalent cations. Both 100 mm monovalent and 5 mm divalent cations produce an increase in PSII activity at the expense of PSI due to a decrease in energy spillover from PSII to PSI (2). Changes in energy distribution within the thylakoid are thought to be related to cation-induced changes in membrane ultrastructure, which produce a spatial separation of PSI and PSII Chl matrices, thus decreasing the possibility of exciton migration from PSII to PSI (8,18). Cations can also affect the rate of electron flow through PSI, PSII, and both PSII and PSI in series (2) effects can often be attributed to changes in the distribution of energy within the photosynthetic apparatus (2). Coupling of phosphorylation to electron transport can also be cation-controlled (1...

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Effect of High Cation Concentrations on Photosystem II Activities

Baker¹

1978

“…In the presence of DCMU, only a fast phase of rise exists, reflecting the reduction of Q only. Without DCMU, there is in general a combination of fast and slow rise phases, which may be interpreted as reflecting, respectively, units in which Q is disconnected or connected to the plastoquinone pool (25 Fig. 1).…”

Section: Methodsmentioning

confidence: 99%

“…They are active with respect to the primary reaction, but inactive with respect to the electron transfer. The ratio of connected units to total units is given by the ratio of extent of the slow phase to extent of the total variable fluorescence (25 In our analysis of ADCMU we simply disregarded this slow phase, and normalized the extent of the fast phase only. This could introduce an error of about 10 to 15 % in the relative estimation of ADCMU.…”

Section: Methodsmentioning

confidence: 99%

Development of Photosystem II Complex during Greening of Chlamydomonas reinhardi y-1

Malkin¹,

Shochat²,

Ohad³

1976

The relative content of organized pigment, active centers, and acceptor pools of photosystem II and their interconnection dunng the development of the photosynthetic membranes of Chlamydomonas reinhardi y-1 have been measured using the fluorescence induction technique. The degree of connectivity and efficiency of the developing system has been assessed also from measurements of maximal rates, quantum yield, and flash yield of 2,6-dichlorophenolindophenol photoreduction using H20 as the electron donor. The results obtained indicate that the process of membrane development in this organism consists of two phases: an initial phase of reorganization and connection between pre-existing components, and a second phase of actual accumulation of newly formed, complete, and active units. The ratio of active centers to Chl remains practically constant throughout the process while the degree of connectivity between the active center and the plastoquinone pool was doubled during the early phase of the greening. In addition the degree of connectivity between the plastoquinone pool and the rest of the electron transport chain increases as demonstrated by a 10-to 20-fold rise in the quantum yield and a 10-fold rise in the maximal rate and the flash yield. The ratio of light harvesting Chl to active centers remains apparently constant during the second phase of the greening as indicated by light saturation experiments and by the constancy of the apparent photosynthetic unit size. Electron donation from H20 seems to develop slower than the activity of the rest of the complex as demonstrated by measurements of 2,6-dichlorophenolindophenol photoreduction using 1,5-diphenylcarbazide as the electron donor. The value of all the above parameters which remain constant during the second phase of the greening are comparable to those obtained with membranes of light-grown cells.The process of chloroplast membrane biogenesis has been the subject of extensive work carried out in many laboratories. The results obtained so far have been reviewed recently (5,18,28). The process of assembly of components and membrane development can be summarized as follows. The chloroplast membranes grow by progressive insertion of newly formed components within pre-existing membranes. The addition of the new components can occur simultaneously or stepwise, and in the second case can result in continuous rearrangements and organizational changes within the membrane, detected at the ultrastructural level (17) and accompanied by drastic changes in photosynthetic activity (12,27,37 (10).The development of 02 evolution activity during greening of a dark-grown Chlorella mutant has been described by Herron and Mauzerall (15) and measurements of both activation of 02 evolution and variable fluorescence during greening of another Chlorella mutant have been reported by Dubertret and Joliot (7). The data obtained by both groups indicate that in these mutants active centers connected to the electron transport chain are present in the membrane at the very beginning...

“…ATP formation was assayed as described (2). Chl fluorescence induction and ATP-driven reverse electron flow (18) were measured in the apparatus constructed by Malkin (15).…”

mentioning

confidence: 99%

Evidence for a Block between Plastoquinone and Cytochrome f in a Photosynthetic Mutant of Lemna with Abnormal Flowering Behavior

Shahak¹,

Posner²,

Avron³

1976

Mutant strain 1073 of Lemna perpusilla is conduded to be blocked between plastoquinone and cytochromef in the photosynthetic electron transport system. The location of the block is based on the following observations of activities in chloroplasts isolated from the mutant and wild-type plants. (a) Relative to wild type, electron flow rates from water to ferricyanide, 2,6-dichlorophenol indophenol or NADP were very low in the mutant, but rates of photosystem I-dependent electron flow and cydic phosphorylation were high. Chloroplasts were prepared from Lemna and lettuce as previously described (2), except for the addition of 0.5 mg/ml of BSA to the grinding and washing media and 10 mg/ml to the resuspension medium (7). Chl concentration was determined according to Arnon (1). The reduction of ferricyanide, DCPIP, or NADP was measured in a Cary 14 spectrophotometer with an attachment to reduce scattering effects. Oxygen uptake was measured with a Yellow Springs Instrument Clark type 02 electrode. ATP formation was assayed as described (2). Chl fluorescence induction and ATP-driven reverse electron flow (18) RESULTSThe ability of mutant 1073 to photoreduce several electron acceptors in reactions requiring excitation of photosystem II, photosystem I, or both is given in Table I. Electron transport from water to ferricyanide, DCPIP, or NADP was very poor in the mutant chloroplasts relative to the WT or to lettuce. The rates of ferricyanide reduction in the WT or lettuce chloroplasts were stimulated 2-to 3-fold by the addition of the uncoupler methylamine, whereas the rate in the mutant chloroplasts was unaffected, indicating that the block was not located in the energy-converting system. In the photosystem I-sensitized electron transport from reduced DAD to 02 (9), the mutant was about as effective as the WT. Consistent with the above results, mutant chloroplasts were inactive in photophosphorylation coupled to electron flow from water to ferricyanide but were active in PMS-catalyzed cyclic photophosphorylation. The major block was therefore in the electron transport system close to photosystem II. To determine whether the block was located before, at, or after photosystem II, Chl fluorescence induction was followed. Figure 1 shows that the WT and mutant had essentially identical fluorescence induction curves, which were similarly affected by the addition of DCMU. The dark recovery of the variable fluorescence yield was also similar in the wild type and mutant (not shown). Since the rise in the variable fluorescence yield upon illumination is thought to be due to the electron transport system from water to and including the "pool" of electron acceptors (14), mainly plastoquinone, it is clear from the latter observa-577 www.plantphysiol.org on May 8, 2018 -Published by Downloaded from