The time course of photoinactivation of photosystem II in leaves revisited

College of Animal Science & Technology, North-West Agriculture and Forestry University, Yangling, 712100, Shaanxi, China.
Photosynthesis Research (Impact Factor: 3.5). 05/2012; 113(1-3):157-64. DOI: 10.1007/s11120-012-9743-8
Source: PubMed


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 I measured as a 'P700 kinetics area'. The P700 kinetics area is based on the fact that the two photosystems function in series: when P700 is completely photo-oxidized by a flash added to continuous far-red light, electrons delivered from PS II to PS I by the flash tend to re-reduce P700(+) transiently to an extent depending on the PS II functionality, while the far-red light photo-oxidizes P700 back to the steady-state concentration. The quantum yield of oxygen evolution in limiting, continuous light indeed decreased in a way that deviated from a single-negative exponential. However, measurement of the quantum yield of oxygen in limiting light may be complicated by changes in mitochondrial respiration between darkness and limiting light. Similarly, an assay based on chlorophyll fluorescence may be complicated by the varying depth in leaf tissue from which the signal is detected after progressive photoinactivation of PS II. On the other hand, the P700 kinetics area appears to be a reasonable assay, which is a measure of functional PS II in the whole leaf tissue and independent of changes in mitochondrial respiration. The P700 kinetics area decreased in a single-negative exponential fashion during progressive photoinactivation of PS II in a number of plant species, at least at functional PS II contents ≥6 % of the initial value, in agreement with the conclusion of Sarvikas et al. (Photosynth Res 103:7-17, 2010). That is, the single-negative-exponential time course does not provide evidence for photoprotection of functional PS II complexes by photoinactivated, connected neighbours.

Download full-text


Available from: Wah Soon Chow, Jul 11, 2014
  • Source
    • ") is taken to represent the fraction f = 1.0 of functional PS II. The exponential decrease (Kou et al. 2012) of f from the value 1.0 "
    [Show abstract] [Hide abstract]
    ABSTRACT: Action spectra of photoinactivation of Photosystem II (PS II) in wild-type and chlorophyll b-less barley leaf segments were obtained. Photoinactivation of PS II was monitored by the delivery of electrons from PS II to PS I following single-turnover flashes superimposed on continuous far-red light, the time course of photoinactivation yielding a rate coefficient k i. Susceptibility of PS II to photoinactivation was quantified as the ratio of k i to the moderate irradiance (I) of light at each selected wavelength. k i/I was very much higher in blue light than in red light. The experimental conditions permitted little excess light energy absorbed by chlorophyll (not utilized in photochemical conversion or dissipated in controlled photoprotection) that could lead to photoinactivation of PS II. Therefore, direct absorption of light by Mn in PS II, rather than by chlorophyll, was more likely to have initiated the much more severe photoinactivation in blue light than in red light. Mutant leaves were ca. 1.5-fold more susceptible to photoinactivation than the wild type. Neither the excess-energy mechanism nor the Mn mechanism can explain this difference. Instead, the much lower chlorophyll content of mutant leaves could have exerted an exacerbating effect, possibly partly due to less mutual shading of chloroplasts in the mutant leaves. In general, which mechanism dominates depends on the experimental conditions.
    Full-text · Article · Jun 2015 · Photosynthesis Research
  • Source
    • "The " dip " in the oxidation level of P700 induced by the single-turnover flash reflects the number of electrons that arrive from PSII per flash (Losciale et al., 2008); therefore, the integrated transient flow of electrons from PSII that arrive at P700 + after a flash is essentially given by the area between the dipping curve and the horizontal line corresponding to the steady state. This P700 kinetics area (being normalized to the total photooxidizable P700) can indicate the ratio of PSII to PSI reaction centers on an arbitrary scale (Kou et al., 2012). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Excess light can have a negative impact on photosynthesis, thus plants have evolved many different ways to adapt to different light conditions to both optimize energy use and avoid damage caused by excess light. Analysis of the snowy cotyledon 4 (sco4) mutant revealed a mutation in a chloroplast-targeted protein which shares limited homology with CaaX-type-endopeptidases. The SCO4 protein possesses an important function in photosynthesis and development, with point mutations rendering the seedlings and adult plants susceptible to photo-oxidative stress. The sco4 mutation impairs acclimation of chloroplasts and their photosystems to excess light, evidenced in a reduction in PS I function, decreased linear electron transfer, yet increased non-photochemical quenching. SCO4 is localized to the chloroplasts and suggests the existence of an unreported type of protein modification within this organelle. Phylogenetic and yeast complementation analyses of SCO4-like proteins reveals that SCO4 is a member of a unknown group of higher plant-specific proteinases quite distinct from the well described CaaX-type endopeptidases RCE1 and STE24 and lacks canonical CaaX activity. Therefore, we hypothesize that SCO4 is a novel endopeptidase required for critical protein modifications within chloroplasts, influencing the function of proteins involved in photosynthesis required for tolerance to excess light.
    Full-text · Article · Aug 2013 · Plant physiology
  • Source
    • "Photoinactivation and repair of PS II are best characterized by their rate coefficients. The rate coefficient of photoinactivation k i can be obtained from the first-order time course of the loss of functional PS II in the absence of repair (e.g. in the presence of lincomycin, Tyystjärvi and Aro 1996; Kou et al. 2012); once obtained, it can be multiplied by the concentration of functional PS II to give the rate of photoinactivation. The rate coefficient of repair k r can deduced from the parallel photoinactivation (with k i separately determined in the absence of repair under otherwise identical conditions) and recovery processes that occur in the presence of repair; once obtained, it can be multiplied by the concentration of non-functional PS II to give the rate of recovery. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Using radioactively labelled amino acids to investigate repair of photoinactivated photosystem II (PS II) gives only a relative rate of repair, while using chlorophyll fluorescence parameters yields a repair rate coefficient for an undefined, variable location within the leaf tissue. Here, we report on a whole-tissue determination of the rate coefficient of photoinactivation k i , and that of repair k r in cotton leaf discs. The method assays functional PS II via a P700 kinetics area associated with PS I, as induced by a single-turnover, saturating flash superimposed on continuous background far-red light. The P700 kinetics area, directly proportional to the oxygen yield per single-turnover, saturating flash, was used to obtain both k i and k r . The value of k i , directly proportional to irradiance, was slightly higher when CO2 diffusion into the abaxial surface (richer in stomata) was blocked by contact with water. The value of k r , sizable in darkness, changed in the light depending on which surface was blocked by contact with water. When the abaxial surface was blocked, k r first peaked at moderate irradiance and then decreased at high irradiance. When the adaxial surface was blocked, k r first increased at low irradiance, then plateaued, before increasing markedly at high irradiance. At the highest irradiance, k r differed by an order of magnitude between the two orientations, attributable to different extents of oxidative stress affecting repair (Nishiyama et al., EMBO J 20: 5587-5594, 2001). The method is a whole-tissue, convenient determination of the rate coefficient of photoinactivation k i and that of repair k r .
    Full-text · Article · Apr 2013 · Photosynthesis Research
Show more