INTRODUCTION The Photoinactivation of Photosystem II in Leaves: A Personal Perspective (original) (raw)
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The photoinactivation of photosystem II in leaves: A personal perspective
2001
a, a parameter that describes how effectively photoinactivated PS II units protect their functional neighbours; Car, carotenoids; pH, transthylakoid pH difference; D1 protein, psbA gene product in the PS II reaction centre; f, functional fraction of PS II; F v /F m , the ratio of variable to maximum chlorophyll a fluorescence; k d , rate coefficient for degradation of D1 protein; k i and k r , rate coefficient for photoinactivation and repair of PS II, respectively; NADP + , oxidized nicotinamide adenine dinucleotide phosphate; P680, the primary electron donor in the PS II reaction centre; Ph, pheophytin; PS, photosystem; Q A , first quinone acceptor of an electron in PS II; R s , the gross rate of D1 protein synthesis.
Photoinactivation of Photosystem II in leaves
Photosynthesis Research, 2005
Photoinactivation of Photosystem II (PS II), the light-induced loss of ability to evolve oxygen, inevitably occurs under any light environment in nature, counteracted by repair. Under certain conditions, the extent of photoinactivation of PS II depends on the photon exposure (light dosage, x), rather than the irradiance or duration of illumination per se, thus obeying the law of reciprocity of irradiance and duration of illumination, namely, that equal photon exposure produces an equal effect. If the probability of photoinactivation (p) of PS II is directly proportional to an increment in photon exposure (p=kDx, where k is the probability per unit photon exposure), it can be deduced that the number of active PS II complexes decreases exponentially as a function of photon exposure: N=N o exp()kx). Further, since a photon exposure is usually achieved by varying the illumination time (t) at constant irradiance (I), N=N o exp()kI t), i.e., N decreases exponentially with time, with a rate coefficient of photoinactivation kI, where the product kI is obviously directly proportional to I. Given that N=N o exp()kx), the quantum yield of photoinactivation of PS II can be defined as )dN/dx=kN, which varies with the number of active PS II complexes remaining. Typically, the quantum yield of photoinactivation of PS II is ca. 0.1 lmol PS II per mol photons at low photon exposure when repair is inhibited. That is, when about 10 7 photons have been received by leaf tissue, one PS II complex is inactivated. Some species such as grapevine have a much lower quantum yield of photoinactivation of PS II, even at a chilling temperature. Examination of the longer-term time course of photoinactivation of PS II in capsicum leaves reveals that the decrease in N deviates from a single-exponential decay when the majority of the PS II complexes are inactivated in the absence of repair. This can be attributed to the formation of strong quenchers in severely photoinactivated PS II complexes which are able to dissipate excitation energy efficiently and to protect the remaining active neighbours against damage by light.
The time course of photoinactivation of photosystem II in leaves revisited
Photosynthesis Research, 2012
Since photosystem II (PS II) performs the demanding function of water oxidation using light energy, it is susceptible to photoinactivation during photosynthesis. The time course of photoinactivation of PS II yields useful information about the process. Depending on how PS II function is assayed, however, the time course seems to differ. Here, we revisit this problem by using two additional assays: (1) the quantum yield of oxygen evolution in limiting, continuous light and (2) the flash-induced cumulative delivery of PS II electrons to the oxidized primary donor (P700 ? ) in PS
Physiologia Plantarum, 2011
Photosystem II (PS II) is photoinactivated during photosynthesis, requiring repair to maintain full function during the day. What is the mechanism(s) of the initial events that lead to photoinactivation of PS II? Two hypotheses have been put forward. The 'excess-energy hypothesis' states that excess energy absorbed by chlorophyll (Chl), neither utilized in photosynthesis nor dissipated harmlessly in non-photochemical quenching, leads to PS II photoinactivation; the 'Mn hypothesis' (also termed the two-step hypothesis) states that light absorption by the Mn cluster in PS II is the primary effect that leads to dissociation of Mn, followed by damage to the reaction centre by light absorption by Chl. Observations from various studies support one or the other hypothesis, but each hypothesis alone cannot explain all the observations. We propose that both mechanisms operate in the leaf, with the relative contribution from each mechanism depending on growth conditions or plant species. Indeed, in a single system, namely, the interior of a leaf, we could observe one or the other mechanism at work, depending on the location within the tissue. There is no reason to expect the two mechanisms to be mutually exclusive.
Photosynthesis Research, 2015
Oxygen effects have long been ambiguous: exacerbating, being indifferent to, or ameliorating the net photoinactivation of Photosystem II (PS II). We scrutinized the time course of PS II photoinactivation (characterized by rate coefficient k i) in the absence of repair, or when recovery (characterized by k r) occurred simultaneously in CO 2 ± O 2. Oxygen exacerbated photoinactivation per se, but alleviated it by mediating the utilization of electrons. With repair permitted, the gradual net loss of functional PS II during illumination of leaves was better described phenomenologically by introducing s, the time for an initial k r to decrease by half. At 1500 lmol photons m-2 s-1 , oxygen decreased the initial k r but increased s. Similarly, at even higher irradiance in air, there was a further decrease in the initial k r and increase in s. These observations are consistent with an empirical model that (1) oxygen increased k i via oxidative stress but decreased it by mediating the utilization of electrons; and (2) reactive oxygen species stimulated the degradation of photodamaged D1 protein in PS II (characterized by k d), but inhibited the de novo synthesis of D1 (characterized by k s), and that the balance between these effects determines the net effect of O 2 on PS II functionality.
Photosynthesis Research, 2012
Given its unique function in light-induced water oxidation and its susceptibility to photoinactivation during photosynthesis, photosystem II (PS II) is often the focus of studies of photosynthetic structure and function, particularly in environmental stress conditions. Here we review four approaches for quantifying or monitoring PS II functionality or the stoichiometry of the two photosystems in leaf segments, scrutinizing the approximations in each approach. (1) Chlorophyll fluorescence parameters are convenient to derive, but the information-rich signal suffers from the localized nature of its detection in leaf tissue. The gross O 2 yield per single-turnover flash in CO 2 -enriched air is a more direct measurement of the functional content, assuming that each functional PS II evolves one O 2 molecule after four flashes. However, the gross O 2 yield per single-turnover flash (multiplied by four) could overestimate the content of functional PS II if mitochondrial respiration is lower in flash illumination than in darkness.
Photodamage to photosystem II - primary and secondary events
Journal of Photochemistry and Photobiology B: Biology, 1992
High light stress results in a reduction in the photosynthetic capacity of plants. This photoinhibition is targeted to photosystem II and seems to be an inevitable consequence of the complicated redox chemistry involved in the light-driven water-plastoquinone oxidoreduction reaction. Photoinactivation leads to irreversible damage of the reaction centre of photosystem II, in particular the Dl protein.
Planta, 1992
The obligate shade plant, Tradescantia albiflora Kunth grown at 50 ~tmol photons 9 m -2 s -1 and Pisum sativum L. acclimated to two photon fluence rates, 50 and 300 pmol. m -2. s -1, were exposed to photoinhibitory light conditions of 1700 pmol 9 m -2 9 s-1 for 4 h at 22 ~ C. Photosynthesis was assayed by measurement of CO2saturated Oz evolution, and photosystem II (PSII) was assayed using modulated chlorophyll fluorescence and flash-yield determinations of functional reaction centres. Tradescantia was most sensitive to photoinhibition, while pea grown at 300 ~tmol-m -2-s -~ was most resistant, with pea grown at 50 ~tmol 9 m-2. s-1 showing an intermediate sensitivity. A very good correlation was found between the decrease of functional PSII reaction centres and both the inhibition of photosynthesis and PSII photochemistry. Photoinhibition caused a decline in the maximum quantum yield for PSII electron transport as determined by the product of photochemical quenching (qp) and the yield of open PSII reaction centres as given by the steady-state fluorescence ratio, F'F~, according to Genty et al. (1989, Biochim. Biophys. Acta 990, 81-92). The decrease in the quantum yield for PSII electron transport was fully accounted for by a decrease in F'vF~, since qp at a given photon fluence rate was similar for photoinhibited and noninhibited plants. Under lightsaturating conditions, the quantum yield of PSII electron transport was similar in photoinhibited and noninhibited plants.