Mechanistic aspects of the inhibition of photosynthesis by light = [Mechanistische aspekten van de remming van de fotosynthese door licht]

The photosynthetic apparatus is sensitive to excess light. This phenomenon is called photoinhibition. It affects specifically photosystem II (PSII) and is related in some way to the turnover of the DI protein, a central component of PSII.Measurements performed with plant systems that were photoinhibited under in vivo conditions give evidence for the conclusion that the photoinactivation site is localized on the acceptor side of PSII. Several mechanisms have been postulated to explain the inactivation process. In Chapter 3, one of these mechanisms is treated more extensively. The protonation of the secondary electron acceptor, Q B , is as yet badly understood. It is hypothesized (Chapter 3) that the acceptor side of (PSII) shows carbonic anhydrase activity. A C0 2 and a H 2 0 molecule can bind to the non-heme iron and can react with each other to form bicarbonate and to release a proton. The theory postulates that during a photoinhibitory treatment the probability that bicarbonate/C0 2 disappears from its binding site increases. It is further argued that this loss is irreversible.Silicomolybdate is an electron acceptor that is able to accept electrons from the non-heme iron. Binding of SiMo to its acceptor site causes displacement of bicarbonate/CO 2 . In Chapter 4 the interaction between SiMo, bicarbonate/C0 2 and (PSII) was analyzed. Information was obtained on the binding site of SiMo, and the binding characteristics of both SiMo and bicarbonate/CO 2 . The characterization of SiMo binding was necessary to be able to use the compound for photoinhibition studies.In Chapter 5 it was established that in pea thylakoids the inactivation site of photoinhibition is indeed located on the acceptor side of PSII. Further it was observed that the donor side is also inactivated though at a much slower rate. Photoinactivation of both donor and acceptor side are light dose dependent. Displacement studies of bicarbonate/CO 2 with nitric oxide (NO) and SiMo indicated that the displacement of bicarbonate is irreversible. As expected, the addition of bicarbonate does not give any lasting protection against photoinhibition. The pH- dependence of acceptor side inactivation corresponds with theoretical considerations of bicarbonate/CO 2 behavior: an increased sensitivity towards photoinhibition below pH 7 and a maximum difference between the rate of donor and acceptor side inactivation around pH 6.4. These observations support the theory that bicarbonate release is responsible for the photoinactivation of PSII.In Chapter 6 a site-directed mutant of Synechocystis sp. PCC 6803 was used to find support for our hypothesis. This mutant is mutated in the binding environment of the non-heme iron. It is four times more sensitive to photoinhibition than a reference strain. One of the main effects of the mutation is a ten times higher sensitivity to formate (formate displaces bicarbonate). This indicates that bicarbonate is more loosely bound to PSII. in this mutant. This may explain the increased sensitivity to photoinhibition and in that case this result supports our hypothesis.In Chapter 7 the effects of photoinhibition on the regulation of photosynthetic electron transport were studied. A combination of photoacoustic and fluorescence spectroscopy was used. A small population of (PSII) reaction centers was found that does not produce oxygen, but does fluoresce. The fluorescence data were corrected for these inactive centers. Initially, the fraction of reduced PSII reaction centers increases as a consequence of the photoinhibitory treatment (photochemical quenching, q P decreases). Possibly this change is brought about by dephosphorylation of the antenna complex. A more severe photoinhibitory treatment causes an oxidation of the (PSII) reaction centers (q P , increases). Other components of the electron transport chain, apart from (PSII) are hardly affected by a photoinhibitory treatment and therefore, the demand for electrons remains at the same level. As a consequence the still active PSII reaction centers can work progressively more efficient. The decline of energetic quenching, q E , during a photoinhibitory treatment could almost entirely be explained by a decline of the available number of (PSII) reaction centers. A small part of the decline of q E has other causes, possibly an increase of the lumen pH as a consequence of a lower proton excretion into the lumen or an increased proton permeability of the thylakoid membrane. The fluorescence data also indicated that the recovery rate of photoinhibition depends on the rate of ATP synthesized by linear electron transport. Cyclic electron transport is not able to compensate for the lost capacity of linear electron transport to induce ATP synthesis during a photoinhibitory treatment.In conclusion, the effects of a moderate photoinhibitory treatment in pea leaves can be explained by dephosphorylation of the antenna system. The effects of a severe photoinhibitory treatment are caused by a progressive inactivation of PSII. Indications were collected supporting the hypothesis that bicarbonate/CO 2 release is the trigger leading to the inactivation of PSII

Artemisinin Inhibits Chloroplast Electron Transport Activity: Mode of Action

Artemisinin, a secondary metabolite produced in Artemisia plant species, besides having antimalarial properties is also phytotoxic. Although, the phytotoxic activity of the compound has been long recognized, no information is available on the mechanism of action of the compound on photosynthetic activity of the plant. In this report, we have evaluated the effect of artemisinin on photoelectron transport activity of chloroplast thylakoid membrane. The inhibitory effect of the compound, under in vitro condition, was pronounced in loosely and fully coupled thylakoids; being strong in the former. The extent of inhibition was drastically reduced in the presence of uncouplers like ammonium chloride or gramicidin; a characteristic feature described for energy transfer inhibitors. The compound, on the other hand, when applied to plants (in vivo), behaved as a potent inhibitor of photosynthetic electron transport. The major site of its action was identified to be the QB; the secondary quinone moiety of photosystemII complex. Analysis of photoreduction kinetics of para-benzoquinone and duroquinone suggest that the inhibition leads to formation of low pool of plastoquinol, which becomes limiting for electron flow through photosystemI. Further it was ascertained that the in vivo inhibitory effect appeared as a consequence of the formation of an unidentified artemisinin-metabolite rather than by the interaction of the compound per se. The putative metabolite of artemisinin is highly reactive in instituting the inhibition of photosynthetic electron flow eventually reducing the plant growth

Frequently asked questions about chlorophyll fluorescence, the sequel

[EN] Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122: 121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additionalChl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F-V/F-M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge fromdifferent Chl a fluorescence analysis domains, yielding in several cases new insights.Kalaji, H.; Schansker, G.; Brestic, M.; Bussotti, F.; Calatayud, A.; Ferroni, L.; Goltsev, V.... (2017). Frequently asked questions about chlorophyll fluorescence, the sequel. 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Mechanism of photoinhibition in pea chloroplasts effects of irradiance level and pH.

AbstractThe photosynthetic apparatus can be damaged by light energy in the process of photoinhibition. The target of this photoinhibition is mainly photosystem II (PSII). The mechanism leading to photoinhibitory damage is not yet known. We have characterized photoinhibition by measuring the photoinactivation of electron transport rates using the electron acceptors silicomolybdate and ferricyanide at different irradiance levels and different pH values. The effects of light on the donor side of PSII were measured with silicomolybdate, the effects on the acceptor side were measured with ferricyanide. We observed that photoinactivation of both donor and acceptor side of PSII are light dose-dependent, donor and acceptor side inactivation being independent processes. The donor side of PSII is less sensitive to photoinhibition than the acceptor side. The difference in pH dependence of donor and acceptor side photoinactivation leads us to propose that light-induced release of bicarbonate from PSII is a primary event leading to photoinhibition. In addition, we report that a photoinhibitory treatment increases the proton permeability of thylakoid membranes. This increase seems to be related to the presence of inactivated PSII reaction centers. It is suggested that radicals formed by inactivated PSII reaction centers causing lipid peroxidation are responsible

Performance of active Photosystem II centers in photoinhibited pea leaves

Effects of photoinhibition on photosynthesis in pea (Pisum sativum L.) leaves were investigated by studying the relationship between the severity of a photoinhibitory treatment (measured as F_v/F_m) and several photoacoustic and chlorophyll a fluorescence parameters. Because of the observed linear relationship between the decline of F_v/F_m and the potential oxygen evolution rate determined by the photoacoustic method, the parameter F_v/F_m was used as an indicator for the severity of photoinhibition. Our analysis revealed that part of the Photosystem II (PS II) reaction centers is inactive in oxygen evolution and is also less sensitive to photoinhibition. Correcting the parameter q_P (fraction of open PS II reaction centers) for inactive PS II centers unveiled a strong increase of q_P in severely inhibited pea leaves, indicating that the inactivated active centers do no longer contribute to q_P and that photoinhibition has an all or none effect on PS II centers. Analysis of q_E (energy quenching) demonstrated its initial increase possibly associated with dephosphorylation of LHC II. Analysis of q_I (photoinhibition dependent quenching) showed that the half-time of recovery of q_I increases steeply below an F_v/F_m of 0.65. This increase of the relaxation half-time corresponds with a decrease of the electron transport rate J and tentatively indicates that the supply of ATP, needed for the recovery, starts to decrease. The data indicate the necessity of correcting for inactive centers in order to make valuable conclusions about effects of photoinhibition on photosynthetic parameters