Different photoprotective strategies for white leaves between two co-occurring Actinidia species

At the outer canopy, the white leaves of Actinidia kolomikta can turn pink but they stay white in A. polygama. We hypothesized that the different leaf colors in the two Actinidia species may represent different photoprotection strategies. To test the hypothesis, leaf optical spectra, anatomy, chlorophyll a fluorescence, superoxide (O₂ ˙^- ) concentration, photosystem II photo-susceptibility, and expression of anthocyanin-related genes were investigated. On the adaxial side, light reflectance was the highest for white leaves of A. kolomikta, followed by its pink leaves and white leaves of A. polygama, and the absorptance for white leaves of A. kolomikta was the lowest. Chlorophyll and carotenoid content of white and pink leaves in A. kolomikta were significantly lower than those of A. polygama, while the relative anthocyanin content of pink leaves was the highest. Chloroplasts of palisade cells of white leaves in A. kolomikta were not well developed with a lower maximum quantum efficiency of PSII than the other types of leaves (pink leaves of A. kolomikta and white leaves of A. Polygama at the inner/outer canopy). After high light treatment from the abaxial surface, F_v /F_m decreased to a larger extent for white leaves of A. kolomikta than pink leaf and white leaves of A. polygama, and its non-photochemical quenching was also the lowest. White leaves of A. kolomikta showed higher O₂ ˙^- concentration compared to pink leaves under the same strong irradiance. The expression levels of anthocyanin biosynthetic genes in pink leaves were higher than in white leaves. These results indicate that white leaves of A. kolomikta apply a reflection strategy for photoprotection, while pink leaves resist photoinhibition via anthocyanin accumulation.

The Protective Role of Non-Photochemical Quenching in PSII Photo-Susceptibility: A Case Study in the Field

Non-photochemical quenching (NPQ) has been regarded as a safety valve to dissipate excess absorbed light energy not used for photochemistry. However, there exists no general consensus on the photoprotective role of NPQ. In the present study, we quantified the Photosystem II (PSII) photo-susceptibilities (mpi) in the presence of lincomycin, under red light given to five shade-acclimated tree species grown in the field. Photosynthetic energy partitioning theory was applied to investigate the relationships between mpi and each of the regulatory light-induced NPQ [Y(NPQ)], the quantum yield of the constitutive nonregulatory NPQ [Y(NO)] and the PSII photochemical yield in the light-adapted state [Y(PSII)] under different red irradiances. It was found that in the low to moderate irradiance range (50-800 μmol m-2 s-1) when the fraction of open reaction centers (qP) exceeded 0.4, mpi exhibited no association with Y(NPQ), Y(NO) and Y(PSII) across species. However, when qP < 0.4 (1,500 μmol m-2 s-1), there existed positive relationships between mpi and Y(NPQ) or Y(NO) but a negative relationship between mpi and Y(PSII). It is postulated that both Y(NPQ) and Y(NO) contain protective and damage components and that using only Y(NPQ) or Y(NO) metrics to identify the photo-susceptibility of a species is a risk. It seems that qP regulates the balance of the two components for each of Y(NPQ) and Y(NO). Under strong irradiance, when both protective Y(NPQ) and Y(NO) are saturated/depressed, the forward electron flow [i.e. Y(PSII)] acts as the last defense to resist photoinhibition.

Nitrogen deposition increases xylem hydraulic sensitivity but decreases stomatal sensitivity to water potential in two temperate deciduous tree species

Although the effects of nitrogen deposition on tree water relations are studied extensively, its impact on the relative sensitivities of stomatal and xylem hydraulic conductance to vapor pressure deficit and water potential is still poorly understood. This study investigated the effects of a 7-year N deposition treatment on the responses of leaf water relations and sensitivity of canopy stomatal conductance to vapor pressure deficit (VPD) and water potential, as well as the sensitivity of branch hydraulic conductance to water potential in a dominant tree species (Quercus wutaishanica) and an associated tree species (Acer mono) in a temperate forest. It was found that the N deposition increased stomatal sensitivity to VPD, decreased stomatal sensitivity to water potential, and increased the vulnerability of the hydraulic system to cavitation in both species. The standardized stomatal sensitivity to VPD, however, was not affected by the N deposition, indicating that the stomata maintained the ability to regulate the water balance under nitrogen deposition condition. Although the increased stomatal sensitivity to VPD could compensate the decreased stomatal sensitivity to water potential to some extent, the combined response would increase the percentage loss of hydraulic conductivity (PLC) when 50 % loss in stomatal conductance occurred, particularly in the dominant species Q. wutaishanica. The result indicates that N deposition would increase the risk of hydraulic failure in those species if the soil and/or air becomes drier under future climate change scenarios. The results of the study can have significant implications on the modelling of ecosystem vulnerability to drought under the scenario of atmospheric nitrogen deposition.

[Short-term outcomes of modified load-sharing rip-stop repair compared with suture-bridge repair for large rotator cuff tears]

Objective: To examine the short-term outcomes of modified load-sharing rip-stop repair (mLSRS) and suture-bridge repair (SB) for large rotator cuff tear. Methods: The clinical data of patients who underwent shoulder arthroscopy for large rotator cuff repair at Department of Orthopedic, China Academy of Chinese Medical Sciences Wang Jing Hospital from January 2017 to December 2018 were collected. Totally 111 patients (39 in mLSRS group and 72 in SB group) were included in the study. There were 45 males and 66 females, aged (58.0±6.9)years (range: 40 to 72 years).A total of 39 pairs of patients were successfully matched by propensity score matching. The pain of shoulder joint was assessed by visual analogy score (VAS) , the shoulder function was assessed by Constant-Murley score, the abduction strength of bilateral shoulder joint was assessed by 90˚ force assessment method of upper limb on the plane of scapula, and the healing of rotator cuff on the operative side was assessed by sugaya ultrasonic classification standard. The difference of short-term outcome between the two groups were compared using t test, χ(2) test, Mann-Whitney U test, repeated measures design anova and multivariate analysis of variance. Results: At the 4th week after surgery, the VAS of mLSRS group was significantly lower than SB group(1 (2) vs. 2 (2) , F=5.32, P=0.02). One year after surgery, Constant-Murley score of two groups were significantly higher than those before surgery, but there was no significant difference between two groups(89.1±5.7 vs. 87.6±7.3, t=1.00, P=0.32). Abductor strength of affected limbs were lower than that of the opposite side, but there were no statistically significant differences between two groups(4.28±1.61 vs. 4.54±1.64, t=-0.70, P=0.49). According to the Sugaya classification, 32 cases in mLSRS group were healed while 7 cases were not healed,30 cases in SB group were healed while 9 cases were not healed(χ(2)=0.32, P=0.58). Conclusion: The use of mLSRS fixation construct on large rotator cuff tear can reduce pain and the short-term outcome is reliable.

[Phylogenetic analysis of Echinococcus granulosus genotypes based on the GenBank database]

OBJECTIVE

To analyze the sequences of the cytochrome C oxidase subunit I (Cox1) gene of various Echinococcus granulosus genotypes that are currently recorded in the GenBank database, so as to investigate the genetic variation and differentiation of the E. granulosus genotypes across the world.

METHODS

The sequences of the Cox1 gene of various E. granulosus genotypes that are currently recorded in the GenBank database were collected, and the same sequences of the Cox1 gene identified from a region were excluded. The mutation sites among the Cox1 gene sequences were identified and a phylogenetic tree was created based on the Cox1 gene.

RESULTS

Transversion mutation was the predominant type of mutation in the Cox1 gene of E. granulosus. The same Cox1 gene sequence was found in E. granulosus G1, G6 and G7 genotypes isolated from various geographical locations across the world, with the corresponding GenBank accession numbers of KY766891, MH300971 and MH301007, respectively. Phylogenetic analysis revealed that E. granulosus G10 genotype had a remarkable geographical aggregation.

CONCLUSIONS

E. granulosus G1, G6 and G7 genotypes have primitive Cox1 gene sequences. There is a geographical aggregation of the E. granulosus G10 genotype in the phylogenetic tree, which has a tendency towards reproductive isolation.

Stomatal Sensitivity to Vapor Pressure Deficit and the Loss of Hydraulic Conductivity Are Coordinated in Populus euphratica, a Desert Phreatophyte Species

There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (K _l) via precise control over PLC under variable ΔΨ (water potential differential between soil and leaf) conditions to maintain the -m/b constant (-m: the sensitivity of stomatal conductance to VPD; b: reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD, stomatal conductance (g _s), g _s responses to VPD, mid-day minimum leaf water potential (Ψ _lmin), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the -m/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ ₅₀ (the xylem water potential with 50% loss of hydraulic conductivity) was -1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ ₅₀ and wood density/leaf specific hydraulic conductivity (k _l) and between specific hydraulic conductance sensitivity to water potential [d(k _s)/dln(-Ψ)] and specific hydraulic conductivity (k _s). A modified hydraulic model was applied to investigate the relationship between g _s and VPD under variable ΔΨ and K _l conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant -m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field.

Partially Dissecting Electron Fluxes in Both Photosystems in Spinach Leaf Disks during Photosynthetic Induction

Photosynthetic induction, a gradual increase in photosynthetic rate on a transition from darkness or low light to high light, has ecological significance, impact on biomass accumulation in fluctuating light and relevance to photoprotection in strong light. However, the experimental quantification of the component electron fluxes in and around both photosystems during induction has been rare. Combining optimized chlorophyll fluorescence, the redox kinetics of P700 [primary electron donor in Photosystem I (PSI)] and membrane inlet mass spectrometry in the absence/presence of inhibitors/mediator, we partially estimated the components of electron fluxes in spinach leaf disks on transition from darkness to 1,000 �mol photons�m-2�s-1 for up to 10 min, obtaining the following findings: (i) the partitioning of energy between both photosystems did not change noticeably; (ii) in Photosystem II (PSII), the combined cyclic electron flow (CEF2) and charge recombination (CR2) to the ground state decreased gradually toward 0 in steady state; (iii) oxygen reduction by electrons from PSII, partly bypassing PSI, was small but measurable; (iv) cyclic electron flow around PSI (CEF1) peaked before becoming somewhat steady; (v) peak magnitudes of some of the electron fluxes, all probably photoprotective, were in the descending order: CEF1 > CEF2 + CR2 > chloroplast O2 uptake; and (vi) the chloroplast NADH dehydrogenase-like complex appeared to aid the antimycin A-sensitive CEF1. The results are important for fine-tuning in silico simulation of in vivo photosynthetic electron transport processes; such simulation is, in turn, necessary to probe partial processes in a complex network of interactions in response to environmental changes.

Self-powered diamond ultraviolet photodetector with a transparent Ag nanowire electrode

A novel diamond UV photodetector with a transparent Ag nanowire (NW) electrode was fabricated. A transparent Ag NW network was spin-coated on single crystalline diamond to form a Schottky contact, which had a barrier height of about 1.37 eV obtained by XPS measurements. Meanwhile, multilayered metals of Au/Pt/Ti were evaporated on the diamond sample to form ohmic contacts. Compared to a Au/Pt/Ti-Au/Pt/Ti diamond photodetector, the Au/Pt/Ti-Ag NW diamond photodetector exhibited relatively high responsivity under 220 nm (18 mA W^-1), high linear dynamic range (41 dB), and short response time (0.12 s for rise time, 0.56 s for decay time). In particular, the Au/Pt/Ti-Ag NW diamond photodetector displayed high photocurrent of about 200 pA at zero-bias, indicating its potential for creating self-powered devices.

Optimising the linear electron transport rate measured by chlorophyll a fluorescence to empirically match the gross rate of oxygen evolution in white light: towards improved estimation of the cyclic electron flux around photosystem I in leaves

The cyclic electron flux (CEF) around photosystem I (PSI) was discovered in isolated chloroplasts more than six decades ago, but its quantification has been hampered by the absence of net formation of a product or net consumption of a substrate. We estimated in vivo CEF in leaves as the difference (ΔFlux) between the total electron flux through PSI (ETR1) measured by a near infrared signal, and the linear electron flux through both photosystems by optimised measurement of chlorophyll a fluorescence (LEFfl). Chlorophyll fluorescence was excited by modulated green light from a light-emitting diode at an optimal average irradiance, and the fluorescence was detected at wavelengths >710nm. In this way, LEFfl matched the gross rate of oxygen evolution multiplied by 4 (LEFO2) in broad-spectrum white actinic irradiance up to half (spinach, poplar and rice) or one third (cotton) of full sunlight irradiance. This technique of estimating CEF can be applied to leaves attached to a plant.

NDH-1 Is Important for Photosystem I Function of Synechocystis sp. Strain PCC 6803 under Environmental Stress Conditions

Cyanobacterial NDH-1 interacts with photosystem I (PSI) to form an NDH-1-PSI supercomplex. Here, we observed that absence of NDH-1 had little, if any, effect on the functional fractions of PSI under growth conditions, but significantly reduced the functional fractions of PSI when cells of Synechocystis sp. strain PCC 6803 were moved to conditions of multiple stresses. The significant reduction in NDH-1-dependent functional fraction of PSI was initiated after PSII activity was impaired. This finding is consistent with our observation that the functional fraction of PSI under growth conditions was rapidly and significantly decreased with increasing concentrations of DCMU, which rapidly and significantly suppressed PSII activity by blocking the transfer of electrons from QA to QB in the PSII reaction center. Furthermore, absence of NDH-1 resulted in the PSI limitation at the functionality of PSI itself but not its donor-side and acceptor-side under conditions of multiple stresses. This was supported by the result of a significant destabilization of the PSI complex in the absence of NDH-1 but the presence of multiple stresses. Based on the above results, we propose that NDH-1 is important for PSI function of Synechocystis sp. strain PCC 6803 mainly via maintaining stabilization of PSI under conditions of environmental stresses.

Changes in activities of both photosystems and the regulatory effect of cyclic electron flow in field-grown cotton (Gossypium hirsutum L) under water deficit

To clarify the influence of water deficit on the functionality of the photosynthetic apparatus of cotton plants, leaf gas exchange, chlorophyll a fluorescence, and P700 redox state were examined in field-grown cotton Gossypium hirsutum L. cv. Xinluzao 45. In addition, we measured changes in the P515 signal and analyzed the activity of ATP synthase and the trans-thylakoid proton gradient (ΔpH). With increasing water deficit, the net CO₂ assimilation rate (A_N) and stomatal conductance (g_s) significantly decreased, but the maximum quantum efficiency of PSII photochemistry (F_v/F_m) did not change. The photochemical activity of photosystem II (PSII) was reflected by the photochemical quenching coefficient (qP), quantum efficiency of photosystem II [Y(II)], and electron transport rate through PSII [ETR(II)], while the activity of photosystem I (PSI) was reflected by the quantum efficiency of photosystem I [Y(I)] and the electron transport rate through PSI [ETR(I)]. Both activities were maintained under mild water deficit, but were slightly decreased under moderate water deficit. Under moderate water deficit, cyclic electron flow (CEF), the fraction of absorbed light dissipated thermally via the ΔpH- and xanthophyll-regulated process [Y(NPQ)], and the fraction of P700 oxidized under a given set of conditions [Y(ND)] increased. Our results suggest that the activities of both photosystems are stable under mild water deficit and decrease only slightly under moderate water deficit. Moderate water deficit stimulates CEF, and the stimulation of CEF is essential for protecting PSI and PSII against photoinhibition.

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.

Multiple roles of oxygen in the photoinactivation and dynamic repair of Photosystem II in spinach leaves

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 CO2 ± O2. 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 τ, the time for an initial k r to decrease by half. At 1500 μmol photons m(-2) s(-1), oxygen decreased the initial k r but increased τ. Similarly, at even higher irradiance in air, there was a further decrease in the initial k r and increase in τ. 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 O2 on PS II functionality.

Photoinactivation of Photosystem II in wild-type and chlorophyll b-less barley leaves: which mechanism dominates depends on experimental circumstances

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.

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. ΔFlux = 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 ΔFlux existed at high irradiance, attributable to charge recombination and/or pseudo-cyclic electron flow. Therefore, in low-light-acclimated plants exposed to high light, ΔFlux has contributions from various paths of electron flow through PS I.

Broadband nonlinear optical response in multi-layer black phosphorus: an emerging infrared and mid-infrared optical material

Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, is a high-mobility layered semiconductor with direct band-gap determined by the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer). Therefore, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared (IR) and mid-IR part of the spectrum. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for infrared and mid-infrared opto-electronics. Herein, we experimentally verified its broadband and enhanced saturable absorption of multi-layer BP (with a thickness of ~10 nm) by wide-band Z-scan measurement technique, and anticipated that multi-layer BPs could be developed as another new type of two-dimensional saturable absorber with operation bandwidth ranging from the visible (400 nm) towards mid-IR (at least 1930 nm). Our results might suggest that ultra-thin multi-layer BP films could be potentially developed as broadband ultra-fast photonics devices, such as passive Q-switcher, mode-locker, optical switcher etc.

Physiological integration enhanced the tolerance of Cynodon dactylon to flooding

Many flooding-tolerant species are clonal plants; however, the effects of physiological integration on plant responses to flooding have received limited attention. We hypothesise that flooding can trigger changes in metabolism of carbohydrates and ROS (reactive oxygen species) in clonal plants, and that physiological integration can ameliorate the adverse effects of stress, subsequently restoring the growth of flooded ramets. In the present study, we conducted a factorial experiment combining flooding to apical ramets and stolon severing (preventing physiological integration) between apical and basal ramets of Cynodon dactylon, which is a stoloniferous perennial grass with considerable flooding tolerance. Flooding-induced responses including decreased root biomass, accumulation of soluble sugar and starch, as well as increased activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in apical ramets. Physiological integration relieved growth inhibition, carbohydrate accumulation and induction of antioxidant enzyme activity in stressed ramets, as expected, without any observable cost in unstressed ramets. We speculate that relief of flooding stress in clonal plants may rely on oxidising power and electron acceptors transferred between ramets through physiological integration.

A novel P700 redox kinetics probe for rapid, non-intrusive and whole-tissue determination of photosystem II functionality, and the stoichiometry of the two photosystems in vivo

We sought a rapid, non-intrusive, whole-tissue measure of the functional photosystem II (PS II) content in leaves. Summation of electrons, delivered by a single-turnover flash to P700(+) (oxidized PS I primary donor) in continuous background far-red light, gave a parameter S in absorbance units after taking into account an experimentally determined basal electron flux that affects P700 redox kinetics. S was linearly correlated with the functional PS II content measured by the O(2) yield per single-turnover repetitive flash in Arabidopsis thaliana expressing an antisense construct to the PsbO (manganese-stabilizing protein in PS II) proteins of PS II (PsbO mutants). The ratio of S to z(max) (total PS I content in absorbance units) was comparable to the PS II/PS I reaction-center ratio in wild-type A. thaliana and in control Spinacea oleracea. Both S and S/z(max) decreased in photoinhibited spinach leaf discs. The whole-tissue functional PS II content and the PS II/photosystem I (PS I) ratio can be non-intrusively monitored by S and S/z(max), respectively, using a quick P700 absorbance protocol compatible with modern P700 instruments.

[Decomposition of herbaceous species in reservoir riparian region of Three Gorges Reservoir under flooding condition]

A total of 10 annuals and perennials of herbaceous species were investigated in reservoir riparian region of Three Gorges Reservoir. The correlations between the plants' nutrient release rate and the substrate composition and structural matter were studied under flooding condition. The decomposition rates of different species differed substantially, with the maximum of Alternanthera philoxeroides (decomposition rate constant k = 0.0228 d(-1)) and the minimum of Microstegium vimineum (k = 0.0029 d(-1)). There was no significant difference in k between annuals and perennials. There was no significant difference in nitrogen and phosphorus contents between annuals and perennials. Paspalum paspaloides and Bidens pilosa released more nutrients into the water than the other species. A. philoxeroides had a higher potential to release nitrogen while it had little effect on water phosphorus compared with the other species. Total N, P contents in the water were negatively correlated with the plants' decomposition rate, initial C content, C:N ratio, lignin:N ratio, and positively correlated with initial contents of K, Ca and N in plants.

Whole-tissue determination of the rate coefficients of photoinactivation and repair of photosystem II in cotton leaf discs based on flash-induced P700 redox kinetics

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 .

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 150μM antimycin A did not affect LEFO2, but decreased ΔFlux 10-fold. ΔFlux was practically negligible below 350μmolphotonsm-2s-1, but increased linearly above it. The following results were obtained at 980μmolphotonsm-2s-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.

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.

Quantifying and monitoring functional photosystem II and the stoichiometry of the two photosystems in leaf segments: approaches and approximations

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. (2) 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 over-estimate the content of functional PS II if mitochondrial respiration is lower in flash illumination than in darkness. (3) The cumulative delivery of electrons from PS II to P700(+) (oxidized primary donor in PS I) after a flash is added to steady background far-red light is a whole-tissue measurement, such that a single linear correlation with functional PS II applies to leaves of all plant species investigated so far. However, the magnitude obtained in a simple analysis (with the signal normalized to the maximum photo-oxidizable P700 signal), which should equal the ratio of PS II to PS I centers, was too small to match the independently-obtained photosystem stoichiometry. Further, an under-estimation of functional PS II content could occur if some electrons were intercepted before reaching PS I. (4) The electrochromic signal from leaf segments appears to reliably quantify the photosystem stoichiometry, either by progressively photoinactivating PS II or suppressing PS I via photo-oxidation of a known fraction of the P700 with steady far-red light. Together, these approaches have the potential for quantitatively probing PS II in vivo in leaf segments, with prospects for application of the latter two approaches in the field.

Important photosynthetic contribution from the non-foliar green organs in cotton at the late growth stage

Non-foliar green organs are recognized as important carbon sources after leaves. However, the contribution of each organ to total yield has not been comprehensively studied in relation to the time-course of changes in surface area and photosynthetic activity of different organs at different growth stages. We studied the contribution of leaves, main stem, bracts and capsule wall in cotton by measuring their time-course of surface area development, O(2) evolution capacity and photosynthetic enzyme activity. Because of the early senescence of leaves, non-foliar organs increased their surface area up to 38.2% of total at late growth stage. Bracts and capsule wall showed less ontogenetic decrease in O(2) evolution capacity per area and photosynthetic enzyme activity than leaves at the late growth stage. The total capacity for O(2) evolution of stalks and bolls (bracts plus capsule wall) was 12.7 and 23.7% (total ca. 36.4%), respectively, as estimated by multiplying their surface area by their O(2) evolution capacity per area. We also kept the bolls (from 15 days after anthesis) or main stem (at the early full bolling stage) in darkness for comparison with non-darkened controls. Darkening the bolls and main stem reduced the boll weight by 24.1 and 9%, respectively, and the seed weight by 35.9 and 16.3%, respectively. We conclude that non-foliar organs significantly contribute to the yield at the late growth stage.

The trade-off between safety and efficiency in hydraulic architecture in 31 woody species in a karst area

Karst topography is a special landscape shaped by the dissolution of one or more layers of soluble bedrock, usually carbonate rock such as limestone or dolomite. Due to subterranean drainage, overland flow, extraction of water by plants and evapotranspiration, there may be very limited surface water. The hydraulic architecture that plants use to adapt to karst topography is very interesting, but few systematic reports exist. The karst area in southwestern China is unique when compared with other karst areas at similar latitudes, because of its abundant precipitation, with rainfall concentrated in the growing season. In theory, resistance to water-stress-induced cavitation via air seeding should be accompanied by decreased pore hydraulic conductivity and stem hydraulic conductivity. However, evidence for such trade-offs across species is ambiguous. We measured the hydraulic structure and foliar stable carbon isotope ratios of 31 karst woody plants at three locations in Guizhou Province, China, to evaluate the functional coordination between resistance to cavitation and specific conductivity. We also applied phylogenetically independent contrast (PIC) analysis in situations where the inter-species correlations of functional traits may be biased on the potential similarity of closely related species. The average xylem tension measurement, at which 50% of hydraulic conductivity of the plants was lost (Ψ(50)), was only -1.27 MPa. Stem Ψ(50) was positively associated with specific conductance (K(s)) (P < 0.05) and leaf specific conductance (K(l)) (P < 0.05). However, the PIC correlation for both relationships was not statistically significant. δ(13)C was positively related to K(l) in both the traditional cross-species correlation analysis and the corresponding PIC correlations (P < 0.05). The Huber value (sapwood area:leaf area ratio) was negatively correlated with K(s) in both the traditional cross-species correlation and the corresponding PIC correlations (P < 0.01). The characteristics of hydraulic architecture measured in this study showed that karst plants in China are not highly cavitation-resistant species. This study also supports the idea that there may not be an evolutionary trade-off between resistance to cavitation and specific conductivity in woody plants. Whole-plant hydraulic adjustment may decouple the trade-off relationship between safety and efficiency at the branch level.

How does cyclic electron flow alleviate photoinhibition in Arabidopsis?

Cyclic electron flow (CEF) around photosystem I has a role in avoiding photoinhibition of photosystem II (PSII), which occurs under conditions in which the rate of photodamage to PSII exceeds the rate of its repair. However, the molecular mechanism underlying how CEF contributes to photoprotection is not yet well understood. We examined the effect of impairment of CEF and thermal energy dissipation (qE) on photoinhibition using CEF (pgr5) and qE (npq1 and npq4) mutants of Arabidopsis (Arabidopsis thaliana) exposed to strong light. Impairment of CEF by mutation of pgr5 suppressed qE and accelerated photoinhibition. We found that impairment of qE, by mutations of pgr5, npq1, and npq4, caused inhibition of the repair of photodamaged PSII at the step of the de novo synthesis of the D1 protein. In the presence of the chloroplast protein synthesis inhibitor chloramphenicol, impairment of CEF, but not impairment of qE, accelerated photoinhibition, and a similar effect was obtained when leaves were infiltrated with the protonophore nigericin. These results suggest that CEF-dependent generation of DeltapH across the thylakoid membrane helps to alleviate photoinhibition by at least two different photoprotection mechanisms: one is linked to qE generation and prevents the inhibition of the repair of photodamaged PSII at the step of protein synthesis, and the other is independent of qE and suppresses photodamage to PSII.

Separation of light-induced linear, cyclic and stroma-sourced electron fluxes to P700+ in cucumber leaf discs after pre-illumination at a chilling temperature

Pre-illumination of cucumber leaf discs at 4 degrees C with low-irradiance white light (i) led to a marked decrease in the extent of photo-oxidation of P700 (the special chlorophyll pair in the PSI reaction center) in actinic light at room temperature and (ii) hastened the post-illumination re-reduction of P700+. Quantifying the linear, cyclic and stroma-sourced electron fluxes to P700+ in two actinic light regimes, we found that there was no increase in cyclic or linear electron fluxes to account for these changes. Rather, we observed a decrease in the maximum extent of P700 photo-oxidation assayed by a strong flash superimposed on continuous, background light of wavelength 723 nm, which we interpret to represent a loss of stable charge separation in PSI due to enhanced charge recombination as a result of the pre-illumination treatment. The funneling of electrons towards fewer non-damaged PSI complexes could explain the hastened post-illumination re-reduction of P700+, aided by a slight increase in a stroma-sourced electron flux after prolonged pre-illumination at 4 degrees C. Quantifying the separate fluxes to P700+ helps to elucidate the effects of chilling of cucumber leaf discs in the light and the reasons for the hastened post-illumination re-reduction of P700+.

[Effects of waterlogging on ecophysiological characteristics of Salix variegate seedlings]

Salix variegate, a widely distributed species along the riverbank in Three Gorges Reservoir area, plays an important role in soil conservation and riverbank stabilization. Waterlogging from April to May was simulated in 2006 to test the adapting ability and mechanism of S. variegate seedlings to water environment change resulted from the construction of Three Gorges Project. The results showed that under waterlogging, all of the S. variegate seedlings could survive, but their maximum photosynthesis (Pmax), transpiration rate (Tr), water use efficiency (WUE), and stomatal conductance (Gs) decreased significantly with the prolongation of waterlogging. In the early period of waterlogging (20 days), these indices changed little, with the Pmax and Tr decreased by 18.5% and 2.2%, respectively, compared with the control. After 30 days, Pmax and Tr decreased by 53.4% and 23.7%, respectively, and then, kept stable. The contents of chlorophyll and carotenoid and their ratio decreased significantly with the prolongation of waterlogging, while the ratio of chlorophyll a to b increased significantly. The ratio of chlorophyll to carotenoid ranged in 4.873-6.883, and that of chlorophyll a to b ranged in 2.855-3.912. Adventitious roots were developed after waterlogging, which would benefit the oxygen supply for the plants. It was suggested that S. variegate had good adaptability to waterlogging, and could be used as a pioneer species in the restoration of degraded water-level fluctuation zone.

Quantification of cyclic electron flow around Photosystem I in spinach leaves during photosynthetic induction

The variation of the rate of cyclic electron transport around Photosystem I (PS I) during photosynthetic induction was investigated by illuminating dark-adapted spinach leaf discs with red + far-red actinic light for a varied duration, followed by abruptly turning off the light. The post-illumination re-reduction kinetics of P700+, the oxidized form of the photoactive chlorophyll of the reaction centre of PS I (normalized to the total P700 content), was well described by the sum of three negative exponential terms. The analysis gave a light-induced total electron flux from which the linear electron flux through PS II and PS I could be subtracted, yielding a cyclic electron flux. Our results show that the cyclic electron flux was small in the very early phase of photosynthetic induction, rose to a maximum at about 30 s of illumination, and declined subsequently to <10% of the total electron flux in the steady state. Further, this cyclic electron flow, largely responsible for the fast and intermediate exponential decays, was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, suggesting an important role of redox poising of the cyclic components for optimal function. Significantly, our results demonstrate that analysis of the post-illumination re-reduction kinetics of P700+ allows the quantification of the cyclic electron flux in intact leaves by a relatively straightforward method.

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.