Fast Fluorescence Induction Research Articles

Phytoplankton Photophysiology Utilities: A Python Toolbox for the Standardization of Processing Active Chlorophyll-a Fluorescence Data

The uptake and application of single turnover chlorophyll fluorometers to the study of phytoplankton ecosystem status and microbial functions has grown considerably in the last two decades. However, standardization of measurement protocols, processing of fluorescence transients and quality control of derived photosynthetic parameters is still lacking and makes community goals of large global databases of high-quality data unrealistic. We introduce the Python package Phytoplankton Photophysiology Utilities (PPU), an adaptable and open-source interface between Fast Repetition Rate and Fluorescence Induction and Relaxation instruments and python. The PPU package includes a variety of functions for the loading, processing and quality control of single turnover fluorescence transients from many commercially available instruments. PPU provides the user with greater flexibility in the application of the Kolber-Prasil-Falkowski model; tools for plotting, quality control, correcting instrument biases and high-throughput processing with ease; and a greater appreciation for the uncertainties in derived photosynthetic parameters. Using data from three research cruises across different biogeochemical regimes, we provide example applications of PPU to fit raw active chlorophyll-a fluorescence data from three commercial instruments and demonstrate tools which help to reduce uncertainties in the final fitted parameters.

Photosynthetic monitoring techniques indicate maximum glycogen accumulation in nitrogen-limited Synechocystis sp. PCC 6803 culture

Chlorophyll fluorescence and oxygen evolution are rapid and non-invasive monitoring techniques to obtain information about the photosynthesis performance of microalgae cultures. These methods may be employed for optimizing the biomass productivity as well as indicate increased production of some valuable compounds.In this work photosynthesis monitoring techniques – saturation pulse analysis of fluorescence quenching, fast fluorescence induction kinetics and photosynthetic oxygen production/respiration – were employed for monitoring of the physiological state of the nitrogen-limited culture of Synechocystis sp. PCC 6803. The correlation between changes of photosynthetic activities, growth and glycogen accumulation was examined in these cultures. The aim was to determine proper point for biomass harvest comparing the changes of photosynthetic activities and the kinetics of glycogen accumulation. In a 4-day trial the highest glycogen accumulation in biomass was found on day 2 showing an inflection point which was accompanied by the reversal of the chlorophyll fluorescence variables, namely maximal photochemical efficiency of PSII Fv/Fm, relative electron transport rate rETR, photochemical efficiency α and photosynthesis saturating irradiance Ik. The rapid increase of cell respiration after day 2 also indicated the relation of these changes with onset of glycogen catabolism.Our laboratory experiments revealed that the three photosynthetic monitoring techniques provided good indication of the physiological changes leading to high glycogen accumulation in biomass of the nitrogen-limited Synechocystis culture. In this way the glycogen accumulation can be manipulated. The photosynthesis measurements can indicate the right time to harvest the culture rich in glycogen in biotechnological applications.

Photobiochemical changes in Chlorella g120 culture during trophic conversion (metabolic pathway shift) from heterotrophic to phototrophic growth regime

Physiological and photobiochemical changes and growth in the heterotrophic strain Chlorella vulgaris g120 were studied during trophic conversion from heterotrophic to phototrophic growth regime. After the exposure of the Chlorella g120 culture to light, it revealed a significant activity of the electron transport (450–700 μmol e− m−2 s−1 as measured by chlorophyll fluorescence) and high PSII photochemical yield Fv/Fm between 0.7 and 0.8. Fast fluorescence induction kinetics showed that PSII electron acceptors in the plastoquinone pool remained partly oxidized, indicating no downregulation of PSII electron transport. The data further revealed that high photobiochemical activity is lost in futile (protective) processes of non-photochemical quenching and respiration which indicate that surplus energy is dissipated in these processes. Pigment analysis showed low chlorophyll content − 0.35–1.15% as compared with exclusively phototrophic strain Chlorella vulgaris R-117. Nevertheless, the carotenoid content in g120 was relatively high − 0.20–0.33% of dry weight which resulted in a considerably high ratio of carotenoid/chlorophyll − 0.31–0.44. This strain probably does not possess the fully competent photosynthetic apparatus and can only partially adapt to phototrophy. We show that the heterotrophically grown g120 strain can undergo metabolic shift from heterotrophic to phototrophic growth regime. It might be an interesting strain from biotechnological point of view as a source of carotenoids, especially lutein.

Expression pattern and physiological roles of Plastid Terminal Oxidase (PTOX) in wild and cultivated barley genotypes under drought stress

Plastid terminal oxidase (PTOX) is a plastid-localized protein that acts as plastoquinol oxidase. The objective of this study was to determine the role of plastid terminal oxidase in response to drought stress and examine the differential response of PTOX pathway in both cultivated (Yousef and Morocco) and tolerant wild barley (spantaneum; HS) genotypes. Plants were subjected to different levels of soil water availability including control, mild and severe drought stress. Decrease in leaf relative water content as a result of progressive drought stressed led to decrease in stomatal conductance and increased enzymatic antioxidant machinery in all genotypes. Drought stress did not affect the PTOX transcript and protein level in cultivated genotypes Yousef and Morocco. However, PTOX gene expression and protein content significantly increased in HS genotype under severe drought. Assessment of PTOX activity via fast fluorescence induction curve in presence of PTOX specific inhibitor revealed that PTOX could efficiently oxidize plastoquinon pool and acts as alternative electron pathway in HS genotype. Results indicated that the role of PTOX in stress response was more significant in the wild barley than the cultivated barley varieties, where the emphasis has been on the desirable agronomic traits.

An optimized protocol for the preparation of oxygen-evolving thylakoid membranes from Cyclotella meneghiniana provides a tool for the investigation of diatom plastidic electron transport

BackgroundThe preparation of functional thylakoid membranes from diatoms with a silica cell wall is still a largely unsolved challenge. Therefore, an optimized protocol for the isolation of oxygen evolving thylakoid membranes of the centric diatom Cyclotella meneghiniana has been developed. The buffer used for the disruption of the cells was supplemented with polyethylene glycol based on its stabilizing effect on plastidic membranes. Disruption of the silica cell walls was performed in a French Pressure cell and subsequent linear sorbitol density gradient centrifugation was used to isolate the thylakoid membrane fraction.ResultsSpectroscopic characterization of the thylakoids by absorption and 77 K fluorescence spectroscopy showed that the photosynthetic pigment protein complexes in the isolated thylakoid membranes were intact. This was supported by oxygen evolution measurements which demonstrated high electron transport rates in the presence of the artificial electron acceptor DCQB. High photosynthetic activity of photosystem II was corroborated by the results of fast fluorescence induction measurements. In addition to PSII and linear electron transport, indications for a chlororespiratory electron transport were observed in the isolated thylakoid membranes. Photosynthetic electron transport also resulted in the establishment of a proton gradient as evidenced by the quenching of 9-amino-acridine fluorescence. Because of their ability to build-up a light-driven proton gradient, de-epoxidation of diadinoxanthin to diatoxanthin and diatoxanthin-dependent non-photochemical quenching of chlorophyll fluorescence could be observed for the first time in isolated thylakoid membranes of diatoms. However, the ∆pH, diadinoxanthin de-epoxidation and diatoxanthin-dependent NPQ were weak compared to intact diatom cells or isolated thylakoids of higher plants.ConclusionsThe present protocol resulted in thylakoids with a high electron transport capacity. These thylakoids can thus be used for experiments addressing various aspects of the photosynthetic electron transport by, e.g., employing artificial electron donors and acceptors which do not penetrate the diatom cell wall. In addition, the present isolation protocol yields diatom thylakoids with the potential for xanthophyll cycle and non-photochemical quenching measurements. However, the preparation has to be further refined before these important topics can be addressed systematically.

Photoprotective capacity of non-photochemical quenching in plants acclimated to different light intensities.

Arabidopsis plants grown at low light were exposed to a gradually increasing actinic light routine. This method allows for the discerning of the photoprotective component of NPQ, pNPQ and photoinhibition. They exhibited lower values of Photosystem II (PSII) yield in comparison to high-light grown plants, and higher calculated dark fluorescence level (F'o calc.) than the measured one (F'o act.). As a result, in low-light grown plants, the values of qP measured in the dark appeared higher than 1. Normally, F'o act. and F'o calc. match well at moderate light intensities but F'o act. becomes higher at increasing intensities due to reaction centre (RCII) damage; this indicates the onset of photoinhibition. To explain the unusual increase of qP in the dark in low-light grown plants, we have undertaken an analysis of PSII antenna size using biochemical and spectroscopic approaches. Sucrose gradient separation of thylakoid membrane complexes and fast fluorescence induction experiments illustrated that the relative PSII cross section does not increase appreciably with the rise in PSII antenna size in the low-light grown plants. This suggests that part of the increased LHCII antenna is less efficiently coupled to the RCII. A model based upon the existence of an uncoupled population LHCII is proposed to explain the discrepancies in calculated and measured values of F'o.

Influence of thylakoid membrane lipids on the structure and function of the plant photosystem II core complex.

MGDG leads to a dimerization of isolated, monomeric PSII core complexes. SQDG and PG induce a detachment of CP43 from the PSII core, thereby disturbing the intrinsic PSII electron transport. The influence of the four thylakoid membrane lipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) on the structure and function of isolated monomeric photosystem (PS) II core complexes was investigated. Incubation with the negatively charged lipids SQDG and PG led to a loss of the long-wavelength 77 K fluorescence emission at 693 nm that is associated with the inner antenna proteins. The neutral galactolipids DGDG and MGDG had no or only minor effects on the fluorescence emission spectra of the PSII core complexes, respectively. Pigment analysis, absorption and 77 K fluorescence excitation spectroscopy showed that incubation with SQDG and PG led to an exposure of chlorophyll molecules to the surrounding medium followed by conversion to pheophytin under acidic conditions. Size-exclusion chromatography and polypeptide analysis corroborated the findings of the spectroscopic measurements and pigment analysis. They showed that the negatively charged lipid SQDG led to a dissociation of the inner antenna protein CP43 and the 27- and 25-kDa apoproteins of the light-harvesting complex II, that were also associated with a part of the PSII core complexes used in the present study. Incubation of PSII core complexes with MGDG, on the other hand, induced an almost complete dimerization of the monomeric PSII. Measurements of the fast PSII fluorescence induction demonstrated that MGDG and DGDG only had a minor influence on the reduction kinetics of plastoquinone QA and the artificial PSII electron acceptor 2,5-dimethyl-p-benzoquinone (DMBQ). SQDG and, to a lesser extent, PG perturbed the intrinsic PSII electron transport significantly.

Effects of Mild and Severe Drought Stress on Photosynthetic Efficiency in Tolerant and Susceptible Barley (Hordeum vulgare L.) Genotypes

AbstractDrought stress is one of the most important abiotic factors which adversely affect growth, metabolism and yield of crops worldwide. The objective of this study was to determine the effects of drought stress on photosynthesis in barley and examine the differential responses of photosynthetic apparatus in relatively tolerant (Yousof) and susceptible (Morocco) barley genotypes. Plants were subjected to different levels of soil water availability including control, mild and severe drought stress. In both genotypes, drought stress led to decrease in chlorophylls, β‐carotene and stomatal conductance accompanied by decrease in CO2 assimilation rate. Significant increase in α‐tochoperol content was only observed in Yousof cultivar under drought stress. Initial slope and plateau phase of CO2 response curve of drought‐stressed plants as well as polyphasic chlorophyll a fluorescence transient curve (OJIP test) and fast fluorescence induction kinetics were influenced by drought stress. These parameters were more affected in Morocco cultivar by drought stress compare with Yousof. Drought stress also resulted in reduction of D1 protein content in both genotypes and accelerated photoinhibition process. Based on our results, stomatal conductance is the main factor limiting photosynthesis in Yousof genotype under mild drought stress. However, in Morocco, in addition to stomatal limitation, damage to photosystem II (PSII), reduced electron transport and carboxylation efficiencies were important parts of limitation in photosynthesis. Severe drought stress resulted in structural and biochemical impairment of light‐dependent reactions as well as carboxylation process of photosynthesis in both genotypes. Alpha‐tocopherol showed an important protective role against drought stress in Yousof cultivar as a relatively drought‐tolerant cultivar.

Effects of drought stress on the evergreen Quercus ilex L., the deciduous Q. robur L. and their hybrid Q. × turneri Willd.

Five-year-old trees of deciduous Quercus robur L., evergreen Q. ilex L., and their semideciduous hybrid, Q. × turneri Willd. (var. pseudoturneri), growing in pots, were subjected to drought stress by withholding water for 18–22 days, until leaf water potentials decreased below −2 MPa. Gas-exchange rates, oxygen evolution, and modulated chlorophyll (Chl) fluorescence measurements revealed that by strong stomata closure and declining photosynthetic capacity down to approximately 50%, all three taxa responded with strongly reduced photosynthesis rates. In Q. robur, photochemical quenching of the drought-stressed plants was much lower than in nonstressed controls. Dissection of the occurring events in the photosynthetic electron transport chain by fast Chl fluorescence induction analysis with the JIP-test were discussed.

Potential for fast chlorophyllafluorescence measurement in bryophyte ecophysiology

The aim of the study was to analyse if the measurement of fast fluorescence induction kinetics in bryophyte samples in field conditions could be used for characterizing the photochemistry of photosynthesis in bryophytes. Bryophyte samples were collected in five different habitats of the boreo-nemoral zone growing on various substrates. Twenty-four species were epigeic, six epilithic, ten epiphytic, three epixylic, and six semi-aquatic or aquatic. Extremely high variation was found for fluorescence parameters between bryophyte samples. Performance Index showed the highest variability, reaching 160% in the case of epiphytic bryophytes. There were statistically significant differences for mean values of Fv/Fm, RC/ABS, and Fv/F0 between epigeic and epiphytic bryophyte samples as well as between epiphytic and semi-aquatic & aquatic samples. For Performance Index, a significant difference was observed only between epiphytic and epigeic bryophytes. It was concluded that bryophytes display a low intensity of the photochemistry of photosynthesis even in relatively wet habitats. In general, measurement of fast fluorescence induction kinetics in field conditions could be a rapid and efficient tool to obtain quantitative data useful for ecophysiological studies.

Remodeling of tobacco thylakoids by over-expression of maize plastidial transglutaminase

Transglutaminases (TGases, EC 2.3.2.13) are intra- and extra-cellular enzymes that catalyze post-translational modification of proteins by establishing ɛ-(γ-glutamyl) links and covalent conjugation of polyamines. In chloroplast it is well established that TGases specifically polyaminylate the light-harvesting antenna of Photosystem (PS) II (LHCII, CP29, CP26, CP24) and therefore a role in photosynthesis has been hypothesised (Della Mea et al. [23] and refs therein). However, the role of TGases in chloroplast is not yet fully understood. Here we report the effect of the over-expression of maize ( Zea mays) chloroplast TGase in tobacco ( Nicotiana tabacum var. Petit Havana) chloroplasts. The transglutaminase activity in over-expressers was increased 4 times in comparison to the wild-type tobacco plants, which in turn increased the thylakoid associated polyamines about 90%. Functional comparison between Wt tobacco and tgz over-expressers is shown in terms of fast fluorescence induction kinetics, non-photochemical quenching of the singlet excited state of chlorophyll a and antenna heterogeneity of PSII. Both in vivo probing and electron microscopy studies verified thylakoid remodeling. PSII antenna heterogeneity in vivo changes in the over-expressers to a great extent, with an increase of the centers located in grana-appressed regions (PSIIα) at the expense of centers located mainly in stroma thylakoids (PSIIβ). A major increase in the granum size (i.e. increase of the number of stacked layers) with a concomitant decrease of stroma thylakoids is reported for the TGase over-expressers.

Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity

Fast and slow chlorophyll fluorescence induction curves at high and low actinic visible light, post-illumination changes in fluorescence yield and reflectance changes at 820 nm induced by far-red light were used to characterize the state of PSII and PSI and their electron transport capabilities in chlorophyllous twig cortices of Eleagnus angustifolius L., while corresponding leaves served as controls. Twigs displayed low dark-adapted PSII photochemical efficiencies and particularly low linear electron transport rates when illuminated. In addition, their PSII population was characterized by a high proportion of inactive, non-Q(B)-reducing centers and an incomplete quenching of fluorescence during the slow induction phase. It is suggested that PSII in twigs is an inefficient electron donor to PSI and/or the reductive pentose phosphate cycle. Yet, in spite of this apparent PSII deficiency, pools of intermediate electron carriers and potential PSI activity were more than sufficient to support the observed linear electron transport rates. Moreover, the rate of PSI reduction upon far-red/dark transitions and the magnitude of fluorescence yield increase upon white light/dark transitions were compatible with an efficient electron flow to PSI from stromal donors in the absence of PSII activity. We conclude that corticular chlorenchyma may be actively engaged in cyclic at the expense of a linear electron flow and discuss the possible physiological significance of this finding in conjunction with the particular microenvironmental conditions encountered within twigs.

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light–dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB), 2,6-dichloro- p-benzoquinone (DCBQ), and tetramethyl- p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence ( F o) and to markedly retard the light-induced rise of variable fluorescence ( F v). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F o level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F v at low concentration, while F o was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F o level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).

Estimation of the energetic connectivity of PS II centres in plants using the fluorescence rise O–J–I–P: Fitting of experimental data to three different PS II models

In presence of DCMU, the sigmoidicity of Chl a fluorescence induction curve is correlated to the energetic connectivity (grouping) between photosynthetic units PS II. We are using the experimental data of the fluorescence transients without DCMU (i.e. the O–J–I–P fast fluorescence induction curves) to calculate the overall probability ‘ p’ of PS II connectivity of the samples (e.g. leaves or chloroplasts). The values obtained using the same method on DCMU treated samples give very close results. Beside the practical advantage of the quantitative determination of the grouping probability on plants from O–J–I–P fluorescence transients, we also present a possible role of the regulation of the connectivity process in photosynthesis as a response to external perturbations. A theoretical discussion based on numerical simulation and fitting of the experimental data, shows three different possible approaches of the fluorescence signal. All are deduced from the same reaction scheme of the acceptor part of PS II centre, involving the primary acceptor pheophytine (Ph), the two bound quinones (Q A and Q B), and the plastoquinone pool. The fraction of closed reaction centres B( t), responsible for the fluorescence transient, is calculated using different definitions as distinct combinations of the redox components for every case. The fluorescence rise of two models is based on the conventional assumption, that the presence of Q A − determines the high fluorescence state, and one model is based solely on the photochemical charge separation in the reaction centre complex. The transformation of a fluorescence induction curve into a kinetic of the fraction of closed reaction centres is a precondition for the simulation and fitting of biochemical models with experimental data.

Identification of Two Mechanisms of Atrazine Resistance in Setaria faberi and Setaria viridis Biotypes

Abstract Two resistant (R) biotypes of Setaria faberi and S. viridis have been detected in maize fields continuously treated with triazine herbicides. The R biotypes of S. faberi and S. viridis were 10.0 and 6.5 times, respectively, more resistant to atrazine than their susceptible (S) biotypes. Both R biotype plastids had less affinity for atrazine than the S biotypes based on chlorophyll fluorescence and Hill reaction assays. As expected, they exhibited cross-resistance to a wide variety of photosystem II inhibitors. The hierarchy of resistance level of these two biotypes was chloro- s -triazines ⪢ methoxy- s -triazines > methyltio- s -triazines ≥ as-triazines. There was no difference in the absorption and translocation of [ 14 C]atrazine in R and S biotypes. Fast fluorescence induction curves showed that atrazine inhibited whole-leaf photosynthesis in S biotypes and they later recovered slight fluorescence activity in both populations. All biotypes produced the same pattern of metabolites, but the R biotypes detoxified the atrazine to conjugate-atrazine at a higher rate than the S biotypes (R > 75% > S). The five wild-type Setaria spp. found in Spain showed differences in detoxification of atrazine. The hierarchy of detoxifying level of atrazine of these Setaria spp. was S. verticilata ≃ S. adherens ⪢ S. faberi ≃ S. viridis >⪢ S. glauca .

Photosynthetic parameters as early indicators of ozone injury in apple leaves

Ozone may affect leaf photosynthesis even before visible symptoms become apparent. This study had the objective to test several parameters of chlorophyll fluorescence and leaf gas exchange for their usefulness as indicators of latent ozone injury in the field. Container‐grown apple trees (Malus domestica Borkh. cv. Golden Delicious) were exposed to four different ozone levels in open‐top chambers. Identical leaves were analyzed in fixed‐time intervals for the characteristics of fast fluorescence induction kinetics in vivo. By using high‐time resolution, characteristic parameters describing the early photochemical events could be calculated according to the JIP‐test. Parameters responsive to the different ozone treatments showed clear dependence on the accumulated ozone dose. Ozone exposure immediately preceding the measurements was more important for the extent of the physiological effects than the total accumulated ozone dose. The most sensitive parameters were the turnover number N (indicating how many times QA has to be reduced for full reduction of all acceptors; positively correlated to ozone dose) and D0, the density of reaction centres per leaf area (negatively correlated to ozone dose). Most parameters analyzed showed clearer responses to ozone on the adaxial than on the abaxial surface of the leaf. Changes in the parameter N were better correlated to ozone doses with low cut‐offs (AOT00 and AOT20), whereas changes in D0 and in the specific electron fluxes per reaction centre were mainly influenced by ozone doses with high cut‐offs (AOT80 and AOT100). Leaf gas exchange analyses revealed a higher ozone sensitivity in carboxylation efficiency than in light utilization efficiency and in the rate of light‐saturated net photosynthesis. All ozone‐induced photosynthetic effects were observed in leaves showing no sign of visible leaf injury. This study identified fluorescence parameters that could be useful for rapid monitoring and early detection of latent leaf injury by ozone.

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in nonphotochemical quenching and a decrease in photochemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of QB-non-reducing PSII reaction centres and a decrease in the rate of QA reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant of thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was downregulated by an increase in the proportion of QB-non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO 2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO 2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO 2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in non-photochemical quenching and a decrease in photochemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of Q B -non-reducing PSII reaction centres and a decrease in the rate of Q A reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant of thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was downregulated by an increase in the proportion of Q B -non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.

Inhibition of quantum yield of PS II electron transport in Spirulina platensis by osmotic stress may be explained mainly by an increase in the proportion of the QB-non-reducing PS II reaction centres

Modulated chlorophyll fluorescence and fluorescence induction kinetics were used to evaluate the PS II photochemistry in Spirulina platensis exposed to osmotic stress (0–0.8 M mannitol). Osmotic stress decreased the efficiency of excitation energy capture by open PS II reaction centres (Fv′/Fm′) and more significantly, decreased photochemical quenching (qP). Osmotic stress also decreased the maximal efficiency of PS II photochemistry (Fv/Fm). There was no significant change in non-photochemical quenching (qN), indicating that the decreased Fv′/Fm′ was not due to an increase in qN. Analyses of the fast fluorescence induction kinetics indicated that osmotic stress caused a significant increase in the proportion of the QB-non-reducing PS II reaction centres. Based on the results in this study, we suggest that a substantial increase in the proportion of the QB-non-reducing PS II reaction centres may be responsible for the decrease in qP and Fv′/Fm′, of which both resulted in the decrease in the quantum yield of PS II electron transport (ΦPSII ).

Resistance to Substituted Urea Herbicides inLolium rigidumBiotypes

Two populations ofLolium rigidum(R1and R2) from a monoculture of winter wheat were treated with diclofop-methyl, whereas another population ofL. rigidum(R3) was treated with diclofop-methyl plus chlorotoluron or diclofop-methyl plus isoproturon. The R1biotype is resistant to chlorotoluron and isoproturon (ED50R1/ED50Svalues of 6.5 and 4.2, respectively), while the R3also exhibits resistance to chlorotoluron and isoproturon (ED50R3/ED50Svalues of 9.26 and 5.53, respectively). The diclofop-methyl-resistant R2biotype was similarly sensitive to both substituted ureas, as was the susceptible (S) biotype. The resistance mechanism in the R1and R3biotypes is not due to reduced target site affinity for herbicides. There was no difference in the absorption and translocation of [14C]chlorotoluron in the resistant and susceptible biotypes. Fast fluorescence induction curves showed that substituted ureas inhibited whole-leaf photosynthesis in all biotypes. The R1and R3biotypes later recovered fluorescence activity at a slightly slower rate in the R1biotype than in the R3biotype. The R3population was also tolerant to benzthiazuron, buturon, fenuron, methabenzthiazuron, metobromuron, metoxuron, and monolinuron, with tolerance at a lower level to chloroxuron, diuron, and linuron. This biotype was susceptible to chlorobromuron. The R1biotype was tolerant to benzthiazuron and metoxuron and was less tolerant to buturon, fenuron, methabenzthiazuron, and metobromuron, while this biotype was susceptible to chlorobromuron, chloroxuron, diuron, linuron, and monolinuron.L. rigidumbiotypes produced the same pattern of metabolites, but the R1and R3resistant biotypes detoxified the chlorotoluron to conjugated chlorotoluron at a higher rate than the R2and S susceptible biotypes. In all biotypes treated with 1-aminobenzotriazole, a cytochrome P450 inhibitor, N-dealkylation was not affected, while ring methyl hydroxylation was strongly inhibited. These data suggested that the resistance mechanism in both biotypes is due to chlorotoluron metabolism via alkyl oxidation, and this is greater in R3than R1. The cytochrome P450 enzyme could be involved in this oxidation reaction.