Ecotoxicological assessment of cigarette butts on morphology and photosynthetic potential of Azolla pinnata.

Cigarette butts (CBs) have become the most ubiquitous form of anthropogenic litter globally. CBs contain various hazardous chemicals that persist in the environment for longer period. These substances are susceptible to leaching into the environment through waterways. The recent study was aimed to evaluate the effects of disposed CBs on the growth and development of Azolla pinnata, an aquatic plant. It was found that after a span of 6 days, the root length, surface area, number of fronds, and photosynthetic efficacy of plant were considerably diminished on the exposure of CBs (concentrations 0 to 40). The exposure of CBs led to a decrease in the FM, FV/F0, and φP0, in contrast, the φD0 increased in response to CBs concentration. Moreover, ABS/CSm, TR0/CSm, and ET0/CSm displayed a negative correlation with CB-induced chemical stress. The performance indices were also decreased (p-value ≤ 0.05) at the highest concentration of CBs. LD50 and LD90 represent the lethal dose, obtained value for LD50 is 20.30 CBs and LD90 is 35.26 CBs through probit analysis. Our results demonstrate that the CBs cause irreversible damage of photosynthetic machinery in plants and also reflect the efficacy of chlorophyll a fluorescence analysis and JIP test for assessing the toxicity of CBs in plants.

Salt stress effects on the photosynthetic electron transport chain in two chickpea lines differing in their salt stress tolerance.

The main objective of this study was to evaluate the effects of salt stress on the photosynthetic electron transport chain using two chickpea lines (Cicer arietinum L.) differing in their salt stress tolerance at the germination stage (AKN 87 and AKN 290). Two weeks after sowing, seedlings were exposed to salt stress for 2 weeks and irrigated with 200 ml of 200 mM NaCl every 2 days. The polyphasic OJIP fluorescence transient and the 820-nm transmission kinetics (photosystem I) were used to evaluate the effects of salt stress on the functionality of the photosynthetic electron transport chain. It was observed that a signature for salt stress was a combination of a higher J step (VJ), a smaller IP amplitude, and little or no effect on the primary quantum yield of PSII (φPo). We observed for AKN 290 a shorter leaf life cycle, which may represent a mechanism to cope with salt stress. For severely salt-stressed leaves, an inhibition of electron flow between the PQ pool and P700 was found. The data also suggest that the properties of electron flow beyond PSI are affected by salt stress.

Does Parmelina tiliacea lichen photosystem II survive at liquid nitrogen temperatures?

Parmelina tiliacea lichens kept in the wet and dry state were stored in liquid nitrogen for 1 week and the subsequent recovery of their photosynthetic apparatus was followed. The chlorophyll a fluorescence rise and the maximum quantum yield of primary photochemistry φPo (FV/FM) were analysed for this purpose. Storage of wet thalli for 1 week in liquid nitrogen led to an impairment of photosystem II and probably the photosynthetic apparatus as a whole, from which the thalli did not recover over time. Thalli exposed in the dry state thalli were far less affected by the treatment and recovered well. These results indicate that the thalli are extremely tolerant to liquid nitrogen temperatures only in the dry state.

Mitochondrial electron transport protects floating leaves of long leaf pondweed (Potamogeton nodosus Poir) against photoinhibition: comparison with submerged leaves.

Investigations were carried to unravel mechanism(s) for higher tolerance of floating over submerged leaves of long leaf pondweed (Potamogeton nodosus Poir) against photoinhibition. Chloroplasts from floating leaves showed ~5- and ~6.4-fold higher Photosystem (PS) I (reduced dichlorophenol-indophenol â†’ methyl viologen â†’ O2) and PS II (H2O â†’ parabenzoquine) activities over those from submerged leaves. The saturating rate (V max) of PS II activity of chloroplasts from floating and submerged leaves reached at ~600 and ~230 µmol photons m(-2) s(-1), respectively. Photosynthetic electron transport rate in floating leaves was over 5-fold higher than in submerged leaves. Further, floating leaves, as compared to submerged leaves, showed higher F v/F m (variable to maximum chlorophyll fluorescence, a reflection of PS II efficiency), as well as a higher potential to withstand photoinhibitory damage by high light (1,200 µmol photons m(-2) s(-1)). Cells of floating leaves had not only higher mitochondria to chloroplast ratio, but also showed many mitochondria in close vicinity of chloroplasts. Electron transport (NADH â†’ O2; succinate â†’ O2) in isolated mitochondria of floating leaves was sensitive to both cyanide (CN(-)) and salicylhydroxamic acid (SHAM), whereas those in submerged leaves were sensitive to CN(-), but virtually insensitive to SHAM, revealing the presence of alternative oxidase in mitochondria of floating, but not of submerged, leaves. Further, the potential of floating leaves to withstand photoinhibitory damage was significantly reduced in the presence of CN(-) and SHAM, individually and in combination. Our experimental results establish that floating leaves possess better photosynthetic efficiency and capacity to withstand photoinhibition compared to submerged leaves; and mitochondria play a pivotal role in protecting photosynthetic machinery of floating leaves against photoinhibition, most likely by oxidation of NAD(P)H and reduction of O2.

Photochemical properties in flag leaves of a super-high-yielding hybrid rice and a traditional hybrid rice (Oryza sativa L.) probed by chlorophyll a fluorescence transient.

Chlorophyll a fluorescence of flag leaves in a super-high-yielding hybrid rice (Oryza sativa L.) LYPJ, and a traditional hybrid rice SY63 cultivar with lower grain yield, which were grown in the field, were investigated from emergence through senescence of flag leaves. As the flag leaf matured, there was an increasing trend in photosynthetic parameters such as quantum efficiency of primary photochemistry ([Formula: see text] Po) and efficiency of electron transport from PS II to PS I (Ψ Eo). The overall photosynthetic performance index (PIABS) was significantly higher in the high-yielding LYPJ compared to SY63 during the entire reproductive stage of the plant, the same to MDA content. However, [Formula: see text] Po(=F V/F M), an indicator of the primary photochemistry of the flag leaf, did not display significant changes with leaf age and was not significantly different between the two cultivars, suggesting that PIABS is a more sensitive parameter than [Formula: see text] Po (=F V/F M) during leaf age for distinguishing between cultivars differing in yield.

Drought-induced modifications of photosynthetic electron transport in intact leaves: analysis and use of neural networks as a tool for a rapid non-invasive estimation.

Water deficit is one of the most important environmental factors limiting sustainable crop yields and it requires a reliable tool for fast and precise quantification. In this work we use simultaneously recorded signals of photoinduced prompt fluorescence (PF) and delayed fluorescence (DF) as well as modulated reflection (MR) of light at 820nm for analysis of the changes in the photosynthetic activity in detached bean leaves during drying. Depending on the severity of the water deficit we identify different changes in the primary photosynthetic processes. When the relative water content (RWC) is decreased to 60% there is a parallel decrease in the ratio between the rate of excitation trapping in the Photosystem (PS) II reaction center and the rate of reoxidation of reduced PSII acceptors. A further decrease of RWC to 20% suppresses the electron transfer from the reduced plastoquinone pool to the PSI reaction center. At RWC below values 15%, the reoxidation of the photoreduced primary quinone acceptor of PSII, Q(A)(-), is inhibited and at less than 5%, the primary photochemical reactions in PSI and II are inactivated. Using the collected sets of PF, DF and MR signals, we construct and train an artificial neural network, capable of recognizing the RWC in a series of "unknown" samples with a correlation between calculated and gravimetrically determined RWC values of about R(2)≈0.98. Our results demonstrate that this is a reliable method for determination of RWC in detached leaves and after further development it could be used for quantifying of drought stress of crop plants in situ. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

The energy flux theory 35 years later: formulations and applications.

Several models have been proposed for the energetic behavior of the photosynthetic apparatus and a variety of experimental techniques are nowadays available to determine parameters that can quantify this behavior. The Energy Flux Theory (EFT) developed by Strasser 35 years ago provides a straightforward way to formulate any possible energetic communication between any complex arrangement of interconnected pigment systems and any energy transduction by these systems. We here revisit the EFT, starting from the basic general definitions and equations and presenting applications in formulating the energy distribution in photosystem (PS) II units with variable connectivity, as originally derived, where certain simplifications were adopted. We then proceed to the derivation of equations for a PSII model of higher complexity, which corresponds, from the formalistic point of view, to the later formulated and now broadly accepted exciton-radical-pair model. We also compare the formulations derived with the EFT with those obtained, by different approaches, in the classic papers on energetic connectivity. Moreover, we apply the EFT for the evaluation of the excitation energy distribution between PSII and PSI and the distinction between state transitions and PSII to PSI excitation energy migration. Our analysis demonstrates that the EFT is a powerful approach for the formulation of any possible model, at any complexity level, even of models that may be proposed in the future, with the advantage that any possible energetic communication or energy transduction can be easily formulated mathematically by trivial algebraic equations. Moreover, the biophysical parameters introduced by the EFT and applicable for any possible model can be linked with obtainable experimental signals, provided that the theoretical resolution of the model does not go beyond the experimental resolution.

Effects of malachite green on biochemistry and photosystem II photochemistry of Eichhornia crassipes.

Malachite green (MG) is a common synthetic dye that raises environmental concerns. This study reveals that MG has inhibitory effects on the biochemistry and physiology of Eichhornia crassipes . Effects of different concentrations of MG on ROS-scavenging enzymes, α-amylase, proline, chlorophyll pigments, and various photosynthetic parameters of E. crassipes were investigated. Chlorophyll fluorescence analysis coupled with the JIP test showed the inhibitory effects of MG on biochemistry and photosynthetic potential depended on concentration and time. Up to 2days of MG exposure, α-amylase and proline were upregulated with increasing MG concentration. When exposure time and concentration increased, all the parameters initially increased, then sharply declined. Chlorophyll content decreased with exposure time and concentration. Due to the slowing down of electron transport on the donor side brought on by MG exposure, P680+ builds up. According to an analysis of E. crassipes PSII activity, exposure to MG raises the proportion of inactive PSII reaction centres and active PSII centres. After increasing the exposure period (2, 4, and 6days) and MG concentration (50, 100, 150, and 200mgL-1 ), it decreased the absorption efficiency electron transport potential, maximal quantum yield of primary photochemistry, and the quantum yield of electron transport. These modifications led to a decline in the entire photosynthesis performance. The current research suggests that MG has detrimental effects on plants; therefore, the need for stringent regulations to prevent the release of dye-containing effluents into aquatic environments.

Sunlight-induced repair of photosystem II in moss Semibarbula orientalis under submergence stress.

Lower plants such as bryophytes often encounter submergence stress, even in low precipitation conditions. Our study aimed to understand the mechanism of submergence tolerance to withstand this frequent stress in moss (Semibarbula orientalis ) during the day and at night. These findings emphasise that light plays a crucial role in photoreactivation of PSII in S. orientalis , which indicates that light not only fuels photosynthesis but also aids in repairing the photosynthetic machinery in plants. Submergence negatively affects photosynthesis parameters such as specific and phenomenological fluxes, density of functional PSII reaction centres (RC/CS), photochemical and non-photochemical quenching (Kp and Kn), quantum yields (ϕP0 , ϕE0 , ϕD0 ), primary and secondary photochemistry, performance indices (PIcs and PIabs), etc. Excessive antenna size caused photoinhibition at the PSII acceptor side, reducing the plastoquinone pool through the formation of PSII triplets and reactive oxygen species (ROS). This ROS-induced protein and PSII damage triggered the initiation of the repair cycle in presence of sunlight, eventually leading to the resumption of PSII activity. However, ROS production was regulated by antioxidants like superoxide dismutase (SOD) and catalase (CAT) activity. The rapid recovery of RS/CS observed specifically under sunlight conditions emphasises the vital role of light in enabling the assembly of essential units, such as the D1 protein of PSII, during stress in S. orientalis . Overall, light is instrumental in restoring the photosynthetic potential in S. orientalis growing under submergence stress. Additionally, it was observed that plants subjected to submergence stress during daylight hours rapidly recover their photosynthetic performance. However, submergence stress during the night requires a comparatively longer period for the restoration of photosynthesis in the moss S. orientalis .

Heat stress and the photosynthetic electron transport chain of the lichen Parmelina tiliacea (Hoffm.) Ach. in the dry and the wet state: differences and similarities with the heat stress response of higher plants.

Thalli of the foliose lichen species Parmelina tiliacea were studied to determine responses of the photosynthetic apparatus to high temperatures in the dry and wet state. The speed with which dry thalli were activated by water following a 24 h exposure at different temperatures decreased as the temperature was increased. But even following a 24 h exposure to 50 °C the fluorescence induction kinetics OJIP reflecting the reduction kinetics of the photosynthetic electron transport chain had completely recovered within 128 min. Exposure of dry thalli to 50 °C for 24 h did not induce a K-peak in the fluorescence rise suggesting that the oxygen evolving complex had remained intact. This contrasted strongly with wet thalli were submergence for 40 s in water of 45 °C inactivated most of the photosystem II reaction centres. In wet thalli, following the destruction of the Mn-cluster, the donation rate to photosystem II by alternative donors (e.g. ascorbate) was lower than in higher plants. This is associated with the near absence of a secondary rise peak (~1 s) normally observed in higher plants. Analysing the 820 nm and prompt fluorescence transients suggested that the M-peak (occurs around 2-5 s) in heat-treated wet lichen thalli is related to cyclic electron transport around photosystem I. Normally, heat stress in lichen thalli leads to desiccation and as consequence lichens may lack the heat-stress-tolerance-increasing mechanisms observed in higher plants. Wet lichen thalli may, therefore, represent an attractive reference system for the evaluation of processes related with heat stress in higher plants.

Experimental in vivo measurements of light emission in plants: a perspective dedicated to David Walker.

This review is dedicated to David Walker (1928-2012), a pioneer in the field of photosynthesis and chlorophyll fluorescence. We begin this review by presenting the history of light emission studies, from the ancient times. Light emission from plants is of several kinds: prompt fluorescence (PF), delayed fluorescence (DF), thermoluminescence, and phosphorescence. In this article, we focus on PF and DF. Chlorophyll a fluorescence measurements have been used for more than 80 years to study photosynthesis, particularly photosystem II (PSII) since 1961. This technique has become a regular trusted probe in agricultural and biological research. Many measured and calculated parameters are good biomarkers or indicators of plant tolerance to different abiotic and biotic stressors. This would never have been possible without the rapid development of new fluorometers. To date, most of these instruments are based mainly on two different operational principles for measuring variable chlorophyll a fluorescence: (1) a PF signal produced following a pulse-amplitude-modulated excitation and (2) a PF signal emitted during a strong continuous actinic excitation. In addition to fluorometers, other instruments have been developed to measure additional signals, such as DF, originating from PSII, and light-induced absorbance changes due to the photooxidation of P700, from PSI, measured as the absorption decrease (photobleaching) at about 705 nm, or increase at 820 nm. In this review, the technical and theoretical basis of newly developed instruments, allowing for simultaneous measurement of the PF and the DF as well as other parameters is discussed. Special emphasis has been given to a description of comparative measurements on PF and DF. However, DF has been discussed in greater details, since it is much less used and less known than PF, but has a great potential to provide useful qualitative new information on the back reactions of PSII electron transfer. A review concerning the history of fluorometers is also presented.

The IP amplitude of the fluorescence rise OJIP is sensitive to changes in the photosystem I content of leaves: a study on plants exposed to magnesium and sulfate deficiencies, drought stress and salt stress.

The hypothesis that changes in the IP amplitude of the fluorescence transient OJIP reflect changes in leaf photosystem I (PSI) content was tested using mineral-deficient sugar beet plants. Young sugar beet plants (Beta vulgaris) were grown hydroponically on nutrient solutions containing either 1 mM or no Mg(2+) and 2.1 µM to 1.88 mM SO(4)(2-) for 4 weeks. During this period two leaf pairs were followed: the already developed second leaf pair and the third leaf pair that was budding at the start of the treatment. The IP amplitude [ΔF(IP) (fluorescence amplitude of the I-to-P-rise) and its relative contribution to the fluorescence rise: ΔV(IP) (amplitude of the relative variable fluorescence of the I-to-P-rise = relative contribution of the I-to-P-rise to the OJIP-rise)] and the amplitude of the transmission change at 820 nm (difference between all plastocyanin and the primary electron donor of photosystems I oxidized and reduced, respectively) relative to the total transmission signal (ΔI(max) /I(tot)) were determined as a function of the treatment time. Correlating the transmission and the two fluorescence parameters yielded approximately linear relationships in both cases. For the least severely affected leaves the parameter ΔV(IP) correlated considerably better with ΔI(max) /I(tot) than ΔF(IP) indicating that it is the ratio PSII:PSI that counts. To show that the relationship also holds for other plants and treatments, data from salt- and drought-stressed plants of barley, chickpea and pea are shown. The relationship between ΔV(IP) and PSI content was confirmed by western blot analysis using an antibody against psaD. The good correlations between ΔI(max) /I(tot) and ΔF(IP) and ΔV(IP) , respectively, suggest that changes in the IP amplitude can be used as semi-quantitative indicators for (relative) changes in the PSI content of the leaf.

Simultaneous in vivo recording of prompt and delayed fluorescence and 820-nm reflection changes during drying and after rehydration of the resurrection plant Haberlea rhodopensis.

A new instrument (M-PEA), which measures simultaneously kinetics of prompt fluorescence (PF), delayed fluorescence (DF) and modulated light reflection at 820nm (MR), was used to screen dark-adapted leaves of the resurrection plant Haberlea rhodopensis during their progressive drying, down to 1% relative water content (RWC), and after their re-watering. This is the first investigation using M-PEA, which employs alternations of actinic light (627-nm peak, 5000 micromol photons m(-2) s(-1)) and dark intervals, where PF-MR and DF kinetics are respectively recorded, with the added advantages: (a) all kinetics are recorded with high time resolution (starting from 0.01 ms), (b) the dark intervals' duration can be as short as 0.1 ms, (c) actinic illumination can be interrupted at different times during the PF transient (recorded up to 300 s), with the earliest interruption at 0.3 ms. Analysis of the simultaneous measurements at different water-content-states of H. rhodopensis leaves allowed the comparison and correlation of complementary information on the structure/function of the photosynthetic machinery, which is not destroyed but only inactivated (reversibly) at different degrees; the comparison and correlation helped also to test current interpretations of each signal and advance their understanding. Our results suggest that the desiccation tolerance of the photosynthetic machinery in H. rhodopensis is mainly based on mechanism(s) that lead to inactivation of photosystem II reaction centres (transformation to heat sinks), triggered already by a small RWC decrease.

Photoprotection of reaction centers: thermal dissipation of absorbed light energy vs charge separation in lichens.

During desiccation, fluorescence emission and stable light-dependent charge separation in the reaction centers (RCs) of photosystem II (PSII) declined strongly in three different lichens: in Parmelia sulcata with an alga as the photobiont, in Peltigera neckeri with a cyanobacterium and in the tripartite lichen Lobaria pulmonaria. Most of the decline of fluorescence was caused by a decrease in the quantum efficiency of fluorescence emission. It indicated the activation of photoprotective thermal energy dissipation. Photochemical activity of the RCs was retained even after complete desiccation. It led to light-dependent absorption changes and found expression in reversible increases in fluorescence or in fluorescence quenching. Lowering the temperature changed the direction of fluorescence responses in P. sulcata. The observations are interpreted to show that reversible light-induced increases in fluorescence emission in desiccated lichens indicate the functionality of the RCs of PSII. Photoprotection is achieved by the drainage of light energy to dissipating centers outside the RCs before stable charge separation can take place. Reversible quenching of fluorescence by strong illumination is suggested to indicate the conversion of the RCs from energy conserving to energy dissipating units. This permits them to avoid photoinactivation. On hydration, re-conversion occurs to energy-conserving RCs.

Delayed fluorescence in photosynthesis.

Photosynthesis is a very efficient photochemical process. Nevertheless, plants emit some of the absorbed energy as light quanta. This luminescence is emitted, predominantly, by excited chlorophyll a molecules in the light-harvesting antenna, associated with Photosystem II (PS II) reaction centers. The emission that occurs before the utilization of the excitation energy in the primary photochemical reaction is called prompt fluorescence. Light emission can also be observed from repopulated excited chlorophylls as a result of recombination of the charge pairs. In this case, some time-dependent redox reactions occur before the excitation of the chlorophyll. This delays the light emission and provides the name for this phenomenon-delayed fluorescence (DF), or delayed light emission (DLE). The DF intensity is a decreasing polyphasic function of the time after illumination, which reflects the kinetics of electron transport reactions both on the (electron) donor and the (electron) acceptor sides of PS II. Two main experimental approaches are used for DF measurements: (a) recording of the DF decay in the dark after a single turnover flash or after continuous light excitation and (b) recording of the DF intensity during light adaptation of the photosynthesizing samples (induction curves), following a period of darkness. In this paper we review historical data on DF research and recent advances in the understanding of the relation between the delayed fluorescence and specific reactions in PS II. An experimental method for simultaneous recording of the induction transients of prompt and delayed chlorophyll fluorescence and decay curves of DF in the millisecond time domain is discussed.

Drought stress effects on photosystem I content and photosystem II thermotolerance analyzed using Chl a fluorescence kinetics in barley varieties differing in their drought tolerance.

Drought stress has multiple effects on the photosynthetic system. Here, we show that a decrease of the relative contribution of the I-P phase, DeltaV(IP) = -V(I) = (F(M)-F(I))/(F(M)- F(o)), to the fluorescence transient OJIP is observed in 10 drought-stressed barley and 9 chickpea varieties. The extent of the I-P loss in the barley varieties depended on their drought tolerance. The relative loss of the I-P phase seems to be related to a loss of photosystem (PS) I reaction centers as determined by 820-nm transmission measurements. In the second part of this study, the interaction of drought and heat stress in two barley varieties (the drought tolerant variety Aït Baha and the drought sensitive variety Lannaceur) was studied using a new approach. Heat stress was induced by exposing the plant leaves to temperatures of 25-45 degrees C and the inactivation of the O(2)-evolving complex (OEC) was followed measuring chlorophyll a (Chl a) fluorescence using a protocol consisting of two 5-ms pulses spaced 2.3 ms apart. In active reaction centers, the dark interval is long enough to allow the OEC to recover from the first pulse; whereas in heat-inactivated reaction centers it is not. In the latter category of reaction centers, no further fluorescence rise is induced by the second pulse. Lannaceur, under well-watered conditions, was more heat tolerant than Aït Baha. However, this difference was lost following drought stress. Drought stress considerably increased the thermostability of PS II of both varieties.

Photosynthetic electron transport activity in heat-treated barley leaves: the role of internal alternative electron donors to photosystem II.

Electron transport processes were investigated in barley leaves in which the oxygen-evolution was fully inhibited by a heat pulse (48 degrees C, 40 s). Under these circumstances, the K peak (approximately F(400 micros)) appears in the chl a fluorescence (OJIP) transient reflecting partial Q(A) reduction, which is due to a stable charge separation resulting from the donation of one electron by tyrozine Z. Following the K peak additional fluorescence increase (indicating Q(A)(-) accumulation) occurs in the 0.2-2 s time range. Using simultaneous chl a fluorescence and 820 nm transmission measurements it is demonstrated that this Q(A)(-) accumulation is due to naturally occurring alternative electron sources that donate electrons to the donor side of photosystem II. Chl a fluorescence data obtained with 5-ms light pulses (double flashes spaced 2.3-500 ms apart, and trains of several hundred flashes spaced by 100 or 200 ms) show that the electron donation occurs from a large pool with t(1/2) approximately 30 ms. This alternative electron donor is most probably ascorbate.

Dark recovery of the Chl a fluorescence transient (OJIP) after light adaptation: the qT-component of non-photochemical quenching is related to an activated photosystem I acceptor side.

The dark recovery kinetics of the Chl a fluorescence transient (OJIP) after 15 min light adaptation were studied and interpreted with the help of simultaneously measured 820 nm transmission. The kinetics of the changes in the shape of the OJIP transient were related to the kinetics of the qE and qT components of non-photochemical quenching. The dark-relaxation of the qE coincided with a general increase of the fluorescence yield. Light adaptation caused the disappearance of the IP-phase (20-200 ms) of the OJIP-transient. The qT correlated with the recovery of the IP-phase and with a recovery of the re-reduction of P700(+) and oxidized plastocyanin in the 20-200 ms time-range as derived from 820 nm transmission measurements. On the basis of these observations, the qT is interpreted to represent the inactivation kinetics of ferredoxin-NADP(+)-reductase (FNR). The activation state of FNR affects the fluorescence yield via its effect on the electron flow. The qT therefore represents a form of photochemical quenching. Increasing the light intensity of the probe pulse from 1800 to 15000 mumol photons m(-2) s(-1) did not qualitatively change the results. The presented observations imply that in light-adapted leaves, it is not possible to 'close' all reaction centers with a strong light pulse. This supports the hypothesis that in addition to Q(A) a second modulator of the fluorescence yield located on the acceptor side of photosystem II (e.g., the occupancy of the Q(B)-site) is needed to explain these results. Besides, some of our results indicate that in pea leaves state 2 to 1 transitions may contribute to the qI-phase.

A dip in the chlorophyll fluorescence induction at 0.2-2 s in Trebouxia-possessing lichens reflects a fast reoxidation of photosystem I. A comparison with higher plants.

An unusual dip (compared to higher plant behaviour under comparable light conditions) in chlorophyll fluorescence induction (FI) at about 0.2-2 s was observed for thalli of several lichen species having Trebouxia species (the most common symbiotic green algae) as their native photobionts and for Trebouxia species cultured separately in nutrient solution. This dip appears after the usual O(J)IP transient at a wide range of excitation light intensities (100-1800 micromol photons m(-2) s(-1)). Simultaneous measurements of FI and 820-nm transmission kinetics (I(820)) with lichen thalli showed that the decreasing part of the fluorescence dip (0.2-0.4 s) is accompanied by a decrease of I(820), i.e., by a reoxidation of electron carriers at photosystem I (PSI), while the subsequent increasing part (0.4-2 s) of the dip is not paralleled by the change in I(820). These results were compared with that measured with pea leaves-representatives of higher plants. In pea, PSI started to reoxidize after 2-s excitation. The simultaneous measurements performed with thalli treated with methylviologen (MV), an efficient electron acceptor from PSI, revealed that the narrow P peak in FI of Trebouxia-possessing lichens (i.e., the I-P-dip phase) gradually disappeared with prolonged MV treatment. Thus, the P peak behaves in a similar way as in higher plants where it reflects a traffic jam of electrons induced by a transient block at the acceptor side of PSI. The increasing part of the dip in FI remained unaffected by the addition of MV. We have found that the fluorescence dip is insensitive to antimycin A, rotenone (inhibitors of cyclic electron flow around PSI), and propyl gallate (an inhibitor of plastid terminal oxidase). The 2-h treatment with 5 microM nigericin, an ionophore effectively dissipating the pH-gradient across the thylakoid membrane, did not lead to significant changes either in FI nor I(820) kinetics. On the basis of the presented results, we suggest that the decreasing part of the fluorescence dip in FI of Trebouxia-lichens reflects the activation of ferredoxin-NADP(+)-oxidoreductase or Mehler-peroxidase reaction leading to the fast reoxidation of electron carriers in thylakoid membranes. The increasing part of the dip probably reflects a transient reduction of plastoquinone (PQ) pool that is not associated with cyclic electron flow around PSI. Possible causes of this MV-insensitive PQ reduction are discussed.

The role of low soil temperature in the inhibition of growth and PSII function during dark chilling in soybean genotypes of contrasting tolerance.

Dark chilling affects growth and yield of warm-climate crops such as soybean [Glycine max (L.) Merr.]. Several studies have investigated chilling-stress effects on photosynthesis and other aspects of metabolism, but none have compared effects of whole-plant chilling (WPC; shoots and roots) with that of aboveground chilling in legumes. This is important because low root temperatures might induce additional constraints, such as inhibition of N(2) fixation, thereby aggravating chilling-stress symptoms. Effects of dark chilling on PSII, shoot growth, leaf ureide content and photosynthetic capacity were studied in two soybean genotypes, Highveld Top (chilling tolerant) and PAN809 (chilling sensitive), in experiments comparing effects of WPC with that of shoot chilling (SC). Both treatments inhibited shoot growth in PAN809 but not Highveld Top. Also, WPC in PAN809 caused a decrease in leaf ureide content followed by severe chlorosis and alterations in O-J-I-P fluorescence-rise kinetics, distinct from SC. A noteworthy difference was the appearance of a Delta K peak in the O-J-I-P fluorescence rise in response to WPC. These genotypic and treatment differences also reflected in the degree of inhibition of CO(2) assimilation rates. The appearance of a Delta K peak, coupled with growth inhibition, reduced ureide content, chlorosis and lower CO(2) assimilation rates, provides mechanistic information about how WPC might have aggravated chilling-stress symptoms in PAN809. We introduce a model explaining how chilling soil temperatures might trigger N-limitation in sensitive genotypes and how characteristic changes in O-J-I-P fluorescence-rise kinetics are linked to changes in carbon and nitrogen metabolism.