A change in the midpoint potential of the quinone QA in Photosystem II associated with photoactivation of oxygen evolution

AbstractThe effect of photoactivation (the assembly of the Mn cluster involved in oxygen evolution) in Photosystem II (PS II), on the redox midpoint potential of the primary quinone electron acceptor, QA, has been investigated. Measurements of the redox state of QA were performed using chlorophyll fluorescence. Cells of Scenedesmus obliquus were grown in the dark to obtain PS II lacking the oxygen-evolving complex. Growth in the light leads to photoactivation. The midpoint potential of QA was shifted, upon photoactivation, from + 110 mV to −80 mV. In cells of a low-fluorescence mutant, LF1, that is unable to assemble the oxygen-evolving complex but that has an otherwise normal PS II, the higher potential form of QA was found. NH2OH treatment of spinach PS II, which releases the Mn and thus inactivates the oxygen-evolving complex, causes an upshift of the redox potential of QA (Krieger and Weis (1992) Photosynthetica 27, 89–98). Oxygen evolution can be reconstituted by incubation in the light in the presence of MnCl2 and CaCl2. Such photoactivation caused the midpoint potential of QA to be shifted back from around +55 mV to lower potentials (−80 mV), typical for active PS II. The above results indicate that the state of the donor side of PS II has a direct influence on the properties of the acceptor side. It is suggested that the change from the high- to the low-potential form of QA may represent a mechanism for protection of PS II during the assembly of the O2-evolving enzyme

Acclimation of Photosynthesis to Changes in the Environment Results in Decreases of Oxidative Stress in Arabidopsis thaliana

From Frontiers via Jisc Publications RouterHistory: collection 2021, received 2021-03-22, accepted 2021-08-18, epub 2021-09-23Publication status: PublishedThe dynamic acclimation of photosynthesis plays an important role in increasing the fitness of a plant under variable light environments. Since acclimation is partially mediated by a glucose-6-phosphate/phosphate translocator 2 (GPT2), this study examined whether plants lacking GPT2, which consequently have defective acclimation to increases in light, are more susceptible to oxidative stress. To understand this mechanism, we used the model plant Arabidopsis thaliana [accession Wassilewskija-4 (Ws-4)] and compared it with mutants lacking GPT2. The plants were then grown at low light (LL) at 100 μmol m−2 s−1 for 7 weeks. For the acclimation experiments, a set of plants from LL was transferred to 400 μmol m−2 s−1 conditions for 7 days. Biochemical and physiological analyses showed that the gpt2 mutant plants had significantly greater activity for ascorbate peroxidase (APX), guiacol peroxidase (GPOX), and superoxide dismutase (SOD). Furthermore, the mutant plants had significantly lower maximum quantum yields of photosynthesis (Fv/Fm). A microarray analysis also showed that gpt2 plants exhibited a greater induction of stress-related genes relative to wild-type (WT) plants. We then concluded that photosynthetic acclimation to a higher intensity of light protects plants against oxidative stress

Shadowing reduced speech and alignment

This study examined whether listeners align to reduced speech. Participants were asked to shadow sentences from a casual speech corpus containing canonical and reduced targets. Participants’ productions showed alignment: durations of canonical targets were longer than durations of reduced targets; and participants often imitated the segment types (canonical versus reduced) in both targets. The effect sizes were similar to previous work on alignment. In addition, shadowed productions were overall longer in duration than the original stimuli and this effect was larger for reduced than canonical targets. A possible explanation for this finding is that listeners reconstruct canonical forms from reduced forms.peer-reviewe

Contrasting Responses to Stress Displayed by Tobacco Overexpressing an Algal Plastid Terminal Oxidase in the Chloroplast

The plastid terminal oxidase (PTOX) – an interfacial diiron carboxylate protein found in the thylakoid membranes of chloroplasts – oxidizes plastoquinol and reduces molecular oxygen to water. It is believed to play a physiologically important role in the response of some plant species to light and salt (NaCl) stress by diverting excess electrons to oxygen thereby protecting photosystem II (PSII) from photodamage. PTOX is therefore a candidate for engineering stress tolerance in crop plants. Previously, we used chloroplast transformation technology to over express PTOX1 from the green alga Chlamydomonas reinhardtii in tobacco (generating line Nt-PTOX-OE). Contrary to expectation, growth of Nt-PTOX-OE plants was more sensitive to light stress. Here we have examined in detail the effects of PTOX1 on photosynthesis in Nt-PTOX-OE tobacco plants grown at two different light intensities. Under ‘low light’ (50 μmol photons m–2 s–1) conditions, Nt-PTOX-OE and WT plants showed similar photosynthetic activities. In contrast, under ‘high light’ (125 μmol photons m–2 s–1) conditions, Nt-PTOX-OE showed less PSII activity than WT while photosystem I (PSI) activity was unaffected. Nt-PTOX-OE grown under high light also failed to increase the chlorophyll a/b ratio and the maximum rate of CO2 assimilation compared to low-light grown plants, suggesting a defect in acclimation. In contrast, Nt-PTOX-OE plants showed much better germination, root length, and shoot biomass accumulation than WT when exposed to high levels of NaCl and showed better recovery and less chlorophyll bleaching after NaCl stress when grown hydroponically. Overall, our results strengthen the link between PTOX and the resistance of plants to salt stress

Relativistic quantum model of confinement and the current quark masses

We consider a relativistic quantum model of confined massive spinning quarks and antiquarks which describes leading Regge trajectories of mesons. The quarks are described by the Dirac equations and the gluon contribution is approximated by the Nambu-Goto straight-line string. The string tension and the current quark masses are the main parameters of the model. Additional parameters are phenomenological constants which approximate nonstring short-range contributions. Comparison of the measured meson masses with the model predictions allows one to determine the current quark masses (in MeV) to be $m_s = 227 \pm 5,~ m_c = 1440 \pm 10,~ m_b = 4715 \pm 20$. The chiral $SU_3$ model[23] makes it possible to estimate from here the $u$- and $d$-quark masses to be $m_u = 6.2 \pm 0.2$~ Mev and $m_d = 11.1 \pm 0.4$ Mev.Comment: 15 pages, LATEX, 2 tables. (submitted to Phys.Rev.D

Wikis, blogs and podcasts: a new generation of Web-based tools for virtual collaborative clinical practice and education

BACKGROUND: We have witnessed a rapid increase in the use of Web-based 'collaborationware' in recent years. These Web 2.0 applications, particularly wikis, blogs and podcasts, have been increasingly adopted by many online health-related professional and educational services. Because of their ease of use and rapidity of deployment, they offer the opportunity for powerful information sharing and ease of collaboration. Wikis are Web sites that can be edited by anyone who has access to them. The word 'blog' is a contraction of 'Web Log' – an online Web journal that can offer a resource rich multimedia environment. Podcasts are repositories of audio and video materials that can be "pushed" to subscribers, even without user intervention. These audio and video files can be downloaded to portable media players that can be taken anywhere, providing the potential for "anytime, anywhere" learning experiences (mobile learning). DISCUSSION: Wikis, blogs and podcasts are all relatively easy to use, which partly accounts for their proliferation. The fact that there are many free and Open Source versions of these tools may also be responsible for their explosive growth. Thus it would be relatively easy to implement any or all within a Health Professions' Educational Environment. Paradoxically, some of their disadvantages also relate to their openness and ease of use. With virtually anybody able to alter, edit or otherwise contribute to the collaborative Web pages, it can be problematic to gauge the reliability and accuracy of such resources. While arguably, the very process of collaboration leads to a Darwinian type 'survival of the fittest' content within a Web page, the veracity of these resources can be assured through careful monitoring, moderation, and operation of the collaborationware in a closed and secure digital environment. Empirical research is still needed to build our pedagogic evidence base about the different aspects of these tools in the context of medical/health education. SUMMARY AND CONCLUSION: If effectively deployed, wikis, blogs and podcasts could offer a way to enhance students', clinicians' and patients' learning experiences, and deepen levels of learners' engagement and collaboration within digital learning environments. Therefore, research should be conducted to determine the best ways to integrate these tools into existing e-Learning programmes for students, health professionals and patients, taking into account the different, but also overlapping, needs of these three audience classes and the opportunities of virtual collaboration between them. Of particular importance is research into novel integrative applications, to serve as the "glue" to bind the different forms of Web-based collaborationware synergistically in order to provide a coherent wholesome learning experience

Exploiting heterogeneous environments: does photosynthetic acclimation optimize carbon gain in fluctuating light?

Plants have evolved complex mechanisms to balance the efficient use of absorbed light energy in photosynthesis with the capacity to use that energy in assimilation, so avoiding potential damage from excess light. This is particularly important under natural light, which can vary according to weather, solar movement and canopy movement. Photosynthetic acclimation is the means by which plants alter their leaf composition and structure over time to enhance photosynthetic efficiency and productivity. However there is no empirical or theoretical basis for understanding how leaves track historic light levels to determine acclimation status, or whether they do this accurately. We hypothesized that in fluctuating light (varying in both intensity and frequency), the light-response characteristics of a leaf should adjust (dynamically acclimate) to maximize daily carbon gain. Using a framework of mathematical modelling based on light-response curves, we have analysed carbon-gain dynamics under various light patterns. The objective was to develop new tools to quantify the precision with which photosynthesis acclimates according to the environment in which plants exist and to test this tool on existing data. We found an inverse relationship between the optimal maximum photosynthetic capacity and the frequency of low to high light transitions. Using experimental data from the literature we were able to show that the observed patterns for acclimation were consistent with a strategy towards maximizing daily carbon gain. Refinement of the model will further determine the precision of acclimation