The time course of photoinactivation of photosystem II in leaves revisited

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.The support of this study by an Australian Research Council Grant (DP1093827) awarded to W.S.C., a China Scholarship Council Fellowship to J.K., JSPS Postdoctoral Fellowships for Research Abroad (21-674) to R.O. and a Knowledge Innovation Programme of the Chinese Academy of Sciences Grant (KSCX2-EW-J-1) to D.-Y.F. is gratefully acknowledged

Operation of dual mechanisms that both lead to photoinactivation of Photosystem II in leaves by visible light

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.This work was supported by a JSPS Research Fellowship for Young Scientists (18-8553 to R. O.); a JSPS Postdoctoral Fellowships for Research Abroad (to R. O.); an Australian Research Council (DP1093827 to W. S. C.); a China Scholarship Council Fellowship (to J. K.) and a Grant-in-Aid for Challenging Exploratory Research (21657007 to I. T.)

Quantification of cyclic electron flow in spinach leaf discs

We quantified the photosynthetic cyclic electron flux (CEF) around Photosystem I as the difference between the total electron flux through PS I (ETR1) and the linear electron flux through both photosystems. Both measurements were made in the whole tissue of spinach leaf discs illuminated in the same geometry and in CO2-enriched air to suppress photorespiration. (1) CEF was negligibly small below 300 μmol photons m-2 s-1. Above this irradiance, CEF increased approximately linearly up to the highest irradiance used (1900 μmol photons m-2 s-1). (2) CEF at a fixed irradiance of 980 μmol m-2 s-1 increased by a factor of almost 3 as the temperature was increased from 5°C to 40°C. It did not decline, even when the linear electron flux decreased at high temperatures. (3) Antimycin A, at a high concentration, decreased CEF to about 10% of the control value without affecting the linear electron flux. This method appears to be reliable for quantifyin! g CEF non-intrusively. By contrast, estimation of the linear electron flux from chlorophyll fluorescence over-estimated CEF in the above treatments.A China Scholarship Council fellowship to (JK) and grants from the Australian Research Council to WSC (DP1093827)and MRB (Centre of Excellence in Plant Energy Biology)supported this work

Partially dissecting the steady-state electron fluxes in Photosystem I in wild-type and pgr5 and ndh mutants of Arabidopsis

Cyclic electron flux (CEF) around Photosystem I (PS I) is difficult to quantify. We obtained the linear electron flux (LEFO2) through both photosystems and the total electron flux through PS I (ETR1) in Arabidopsis in CO2-enriched air. Delta Flux = ETR1 - LEFO2 is an upper estimate of CEF, which consists of two components, an antimycin A-sensitive, PGR5 (proton gradient regulation 5 protein)-dependent component and an insensitive component facilitated by a chloroplastic nicotinamide adenine dinucleotide dehydrogenase-like complex (NDH). Using wild type as well as pgr5 and ndh mutants, we observed that (1) 40% of the absorbed light was partitioned to PS I; (2) at high irradiance a substantial antimycin A-sensitive CEF occurred in the wild type and the ndh mutant; (3) at low irradiance a sizable antimycin A-sensitive CEF occurred in the wild type but not in the ndh mutant, suggesting an enhancing effect of NDH in low light; and (4) in the pgr5 mutant, and the wild type and ndh mutant treated with antimycin A, a residual Delta Flux existed at high irradiance, attributable to charge recombination and/or pseudo-cyclic electron flow. Therefore, in low-light-acclimated plants exposed to high light, Delta Flux has contributions from various paths of electron flow through PS I

Response of Carbon-Fixing Bacteria to Patchy Degradation of the Alpine Meadow in the Source Zone of the Yellow River, West China

This study aims to enlighten our understanding of the distribution of soil carbon-fixing bacteria (cbbL-harboring bacteria) and their community diversity in differently degraded patches at three altitudes. MiSeq high-throughput sequencing technology was used to analyze the soil carbon-fixing bacteria community diversity of degraded patches and healthy meadow at three altitudes. Redundancy analysis (RDA) and structural equation model (SEM) were used to analyze the correlation and influence path between environmental factors and carbon-fixing bacteria. The results showed that degradation reduced the relative abundance of Proteobacteria from 99.67% to 95.57%. Sulfurifustis, Cupriavidus, and Alkalispirillum were the dominant genera at the three altitudes. Hydrogenophaga and Ectothiorhodospira changed significantly with altitude. RDA results confirmed that available phosphorus (AP) was strongly and positively correlated with Proteobacteria. AP and total nitrogen (TN) were strongly and positively correlated with Hydrogenophaga. Grass coverage and sedge aboveground biomass were strongly and positively correlated with Sulfurifustis and Ectothiorhodospira, respectively. Elevation adversely affected the relative abundance of dominant carbon-fixing bacteria and diversity index by reducing the coverage of grass and soil volumetric moisture content (SVMC) indirectly, and also had a direct positive impact on the Chao1 index (path coefficient = 0.800). Therefore, increasing the content of nitrogen, phosphorus and SVMC and vegetation coverage, especially sedge and grass, will be conducive to the recovery of the diversity of soil carbon-fixing bacteria and improve the soil autotrophic microbial carbon sequestration potential in degraded meadows, especially in high-altitude areas

Obstacles in the quantification of the cyclic electron flux around Photosystem I in leaves of C3 plants

Sixty years ago Arnon and co-workers discovered photophosphorylation driven by a cyclic electron flux (CEF) around Photosystem I. Since then understanding the physiological roles and the regulation of CEF has progressed, mainly via genetic approaches. One basic problem remains, however: quantifying CEF in the absence of a net product. Quantification of CEF under physiological conditions is a crucial prerequisite for investigating the physiological roles of CEF. Here we summarize current progress in methods of CEF quantification in leaves and, in some cases, in isolated thylakoids, of C3 plants. Evidently, all present methods have their own shortcomings. We conclude that to quantify CEF in vivo, the best way currently is to measure the electron flux through PS I (ETR1) and that through PS II and PS I in series (ETR2) for the whole leaf tissue under identical conditions. The difference between ETR1 and ETR2 is an upper estimate of CEF, mainly consisting, in C3 plants, of a major PGR5-PGRL1-dependent CEF component and a minor chloroplast NDH-dependent component, where PGR5 stands for Proton Gradient Regulation 5 protein, PGRL1 for PGR5-like photosynthesis phenotype 1, and NDH for Chloroplast NADH dehydrogenase-like complex. These two CEF components can be separated by the use of antimycin A to inhibit the former (major) component. Membrane inlet mass spectrometry utilizing stable oxygen isotopes provides a reliable estimation of ETR2, whilst ETR1 can be estimated from a method based on the photochemical yield of PS I, Y(I). However, some issues for the recommended method remain unresolved

Estimation of the steady-state cyclic electron flux around PSI in spinach leaf discs in white light, CO2-enriched air and other varied conditions

Cyclic electron flux (CEF) around PSI is essential for efficient photosynthesis and aids photoprotection, especially in stressful conditions, but the difficulty in quantifying CEF is non-trivial. The total electron flux through PSI (ETR1) and the linear electron flux (LEFO2) through both photosystems in spinach leaf discs were estimated from the photochemical yield of PSI and the gross oxygen evolution rate, respectively, in CO2-enriched air. Delta Flux = ETR1 - LEFO2 is an upper estimate of CEF. Infiltration of leaf discs with 150 mu M antimycin A did not affect LEFO2, but decreased DFlux 10-fold. Delta Flux was practically negligible below 350 mu mol photons m(-2) s(-1), but increased linearly above it. The following results were obtained at 980 mmol photons m(-2) s(-1). Delta Flux increased 3-fold as the temperature increased from 5 degrees C to 40 degrees C. It did not decline at high temperature, even when LEFO2 decreased. Delta Flux increased by 80% as the relative water content of leaf discs decreased from 100 to 40%, when LEFO2 decreased 2-fold. The method of using Delta Flux as a non-intrusive upper estimate of steady-state CEF in leaf tissue appears reasonable when photorespiration is suppressed

Estimation of the steady-state cyclic electron flux around PSI in spinach leaf discs in white light,CO2-enriched air and other varied conditions

Cyclic electron flux (CEF) around PSI is essential for efficient photosynthesis and aids photoprotection, especially in stressful conditions, but the difficulty in quantifying CEF is non-trivial. The total electron flux through PSI (ETR1) and the linear electron flux (LEFO2) through both photosystems in spinach leaf discs were estimated from the photochemical yield of PSI and the gross oxygen evolution rate, respectively, in CO2-enriched air. ΔFlux = ETR1 – LEFO2 is an upper estimate of CEF. Infiltration of leaf discs with 150 μM antimycin A did not affect LEFO2, but decreased ΔFlux 10-fold. ΔFlux was practically negligible below 350 μmol photons m-2 s-1, but increased linearly above it. The following results were obtained at 980 μmol photons m-2 s-1. ΔFlux increased 3-fold as the temperature increased from 5°C to 40°C. It did not decline at high temperature, even when LEFO2 decreased. ΔFlux increased by 80% as the relative water content of leaf discs decreased from 100 to 40%, when LEFO2 decreased 2-fold. The method of using ΔFlux as a non-intrusive upper estimate of steady-state CEF in leaf tissue appears reasonable when photorespiration is suppressed.Grants from the Australian Research Council to WSC (DP1093827)