Electron Transport In Photosystem Research Articles

Zinc nano and zinc ethylenediaminetetraacetic acid (EDTA) mediated water deficit stress alleviation in pearl millet (Pennisetum glaucum (L.) R. Br.): Photosystem II electron transport and pigment dynamics

Water stress adversely affects the photosynthetic apparatus and pigment composition in plants, leading to reduced yields and compromised plant health. Zinc (Zn) foliar spray in nano form presents a potential solution to ameliorate water deficit stress. We attempt here a detailed dissection of electron transport in Photosystem II (PSII) through studies on chlorophyll a fast fluorescence kinetics and non-photochemical quenching (NPQ) and pigment dynamics in response to water stress. We also investigated the possible changes in these processes and the regulation of it by Zn Nano and Zn Ethylenediaminetetraacetic acid (EDTA) foliar sprays that may lead to amelioration of stress. Our results indicated that water stress created a "traffic jam" like situation in the electron transport system of Photosystem II, leading to decreased photosynthetic efficiency. Treatments with water deficit stress + Zn Nano (particle size < 90 nm) spray with Zn concentration at 20 mg L−1 and water deficit stress + Zn EDTA spray (solid material size ∼ 100 µm) with Zn concentration at 240 mg L−1 effectively ameliorated water deficit stress by its action on flux ratio parameters viz., quantum yield for electron transport (φE0), probability of electron transport beyond QA (ψ0) and quantum yield of electron transport from QA⁻ to PS1 end electron acceptors (ϕR0) and also the specific fluxes and phenomenological fluxes. These treatments positively influenced chlorophyll content, and xanthophyll components, including violaxanthin, antheraxanthin, and zeaxanthin, and reduced NPQ and the de expoxidation state. Higher concentrations of Zn Nano foliar spray (water stress + Zn Nano spray Zn @ 30 mg L⁻¹) did not ameliorate water deficit stress as effectively as the lower concentrations, although this higher concentration was not in any way toxic. This lack of stress amelioration at higher concentrations of Zn Nano spray may be due to physiological limitations of elemental zinc action within the plant. Our findings suggest that Zn foliar sprays in Nano and EDTA form at optimum concentrations can significantly improve plant resilience to water stress.

Transcriptomics Reveals the Pathway for Increasing Brassica chinensis L. Yield under Foliar Application of Titanium Oxide Nanoparticles.

In this study, Brassica chinensis L seedlings after 6 weeks of soil cultivation were treated with foliar application of TiO2 NPs (20 mg/L) for different times. Transcriptomics analysis was employed to investigate the impact of TiO2 NPs on the physiology, growth, and yield of B. chinensis L. Results showed that TiO2 NPs' exposure significantly increased the biomass, total phosphorus, and catalase enzyme activity by 23.60, 23.72, and 44.01%, respectively, compared to the untreated ones (not bulk or ion).TiO2 NPs increased the leaf chlorophyll content by 4.9% and photosynthetic rate by 16.62%, which was attributed to the upregulated expression of seven genes (PetH, PetF, PsaF, PsbA, PsbB, PsbD, and Lhcb) associated with electron transport in photosystem I and light-harvesting in leaves. The water balance of B. chinensis was improved correlating with the altered expressions of 19 aquaporin genes (e.g., PIP2;1 and NIP6;1). The expressions of 58 genes related to plant hormone signaling and growth were dysregulated, with notable downregulations in GA20, SnRK2, and PP2C and upregulations of DELLAs, SAM, and ETR. Moreover, the 11 tricarboxylic acid cycle genes and 13 glycolysis genes appear to stimulate pathways involved in promoting the growth and physiology of B. chinensis. This research contributes valuable insights into new strategies for increasing the yield of B. chinensis.

Chlorophyll Fluorescence Imaging Reveals the Dynamics of Bentazon Action on Sunflower (Helianthus annuus L.) Plants

Fluorescence photography was used to evaluate the effects of bentazon, an herbicide that inhibits electron transport in photosystem II, on sunflower, taken as a model plant. We measured chlorophyll fluorescence to visualize the penetration, distribution, and effect of the herbicide on the plant. Our results showed that bentazon penetrates the leaf within 10–16 min, reaching full depth within 40 min. Also, we show that the herbicide is distributed along the conducting tissues of the leaf. The presence of necrotic spots, as well as the size of increased-fluorescence areas, were positively correlated to the concentration of herbicide. These findings suggest that fluorescence imaging may be a useful tool for observing the absorption and distribution of photosystem II-inhibiting herbicides in plants as an alternative to radioactive labeling in some cases.

Exploring the feasibility of a single-protoplast proteomic analysis

BackgroundRecent advances in high-resolution mass spectrometry have now enabled the study of proteomes at the single-cell level, offering the potential to unveil novel aspects of cellular processes. Remarkably, there has been no prior attempt to investigate single-plant cell proteomes. In this study, we aimed to explore the feasibility of conducting a proteomic analysis on individual protoplasts.FindingsAs a result, our analysis identified 978 proteins from the 180 protoplasts, aligning with well-known biological processes in plant leaves, such as photosynthetic electron transport in photosystem II. Employing the SCP package in the SCoPE2 workflow revealed a notable batch effect and extensive missing values in the data. Following correction, we observed the heterogeneity in single-protoplast proteome expression. Comparing the results of single-protoplast proteomics with those of bulk leaf proteomics, we noted that only a small fraction of bulk data was detected in the single-protoplast proteomics data, highlighting a technical limitation of the current single-cell proteomics method.ConclusionsIn summary, we demonstrated the feasibility of conducting a single-protoplast proteomic experiment, revealing heterogeneity in plant cellular proteome expression. This underscores the importance of analyzing a substantial number of plant cells to discern statistically significant changes in plant cell proteomes upon perturbation such as abscisic acid treatment in future studies. We anticipate that our study will contribute to advancing single-protoplast proteomics in the near future.

Toxic effect of ciprofloxacin on the photosynthesis reactions in microalga Scenedesmus quadricauda (Turp.) Bréb.

Ciprofloxacin (CIP) is widely used broad-spectrum antimicrobial drug of fluoroquinolone family. The widespread use of ciprofloxacin increases its release into the environment. Ciprofloxacin is detected in aquatic ecosystems potentially harming aquatic organisms. The CIP effect on photosynthetic organisms is not fully studied. In this study we examined the CIP effect on green freshwater microalgae Scenedesmus quadricauda (Turp.) Bréb. using chlorophyll fluorescence methods (JIP test parameters and rapid light curves). A significant decrease in the cell number was observed at ≥10 mg/L of сiprofloxacin in comparison with control. Analysis of chlorophyll fluorescence parameters obtained from OJIP transients revealed the changes in photosynthetic reactions in сiprofloxacin treatment. Ciprofloxacin was found to inhibit electron transport rate in photosystem II (PSII). The decrease in the quantum yield of electron transport in photosystem II (φEo) was accompanied by the decrease in performance index (PIABS) and an increase in energy dissipation (DI0/RC). Ciprofloxacin enhanced the photosensitivity of microalgae but did not inhibit the recovery of photosynthetic activity after the photooxidative stress. In this regard the effect of CIP differs from that of the well-known antibiotic chloramphenicol that inhibits the resynthesis of plastid proteins and, accordingly, the recovery of photosynthetic activity associated with the resynthesis of PSII protein D1. Among the fluorescence parameters, PIABS was found to be the most stress-specific; therefore it can be proposed to detect an early toxic CIP effect in microalgae.

Analysis of photosynthetic pigments pathway produced by CO2-toxicity-induced Scenedesmus obliquus

The coal-to-gas process produces carbon dioxide, which increases global warming, and its wastewater treatment generates sludge with high organic toxicity. Scenedesmus obliquus is a potential solution to such environmental problems, and photosynthetic pigments are the focus of this study. The optimal concentration of CO2 for the growth of Scenedesmus obliquus was found to be 30 % after increasing the concentration of CO2 (0.05 %–100 %). The accumulation of photosynthetic pigments during cultivation could reach 31.74 ± 1.33 mg/L, 11.21 ± 0.42 mg/L, and 5.59 ± 0.19 mg/L respectively, and the organic toxicity of sludge extract could be reduced by 44.97 %. Upregulation of A0A383VSL5, A0A383WMQ3, and A0A2Z4THB7 as photo systemic oxygen release proteins and propylene phosphate isomerase resulted in oxygen-evolving proteins in photosystem II, electron transport in photosystem I, and intermediates in carbon fixation. This is achieved by increasing the intracellular antennae protein and carbon fixation pathway, allowing Scenedesmus obliquus to both tolerate and fix CO2 and reduce the organic toxicity of sludge. These findings provide insights into the innovative strategy underlining the fixation of CO2, treatment and disposal of industrial residual sludge, and the enhancement of microalgal biomass production.

Ray-tracing analysis on the far-red induced light-capturing ability of kale

Far-red (FR, 700–800 nm) light has recently been used to regulate phytochrome-mediated morphological responses in vertical farms. Long-term morphological changes in different FR intensities are well known, but changes in photosynthetic capacity have not been sufficiently investigated. The objectives of this study were to quantitatively analyze the growth, photosynthetic responses, and light interception of kale (Brassica oleracea L. var. acephala) depending on the FR fraction. The plants were cultivated at a photosynthetic photon flux density (PPFD) of 240 μmol m−2 s−1 under red and white LEDs with FR LED (peak at 730 nm) for 16 h. The FR fractions determined as FR/R+FR were FR0.07 (control), FR0.28, FR0.60, and FR0.72. To evaluate the morphological and physiological responses in each FR fraction, the growth, chlorophyll content, chlorophyll fluorescence, P700 absorbance change, and photosynthetic rate were measured. Light interception was quantitatively calculated using 3D-scanned plant models with physiological changes and ray-tracing simulations. The dry weight of shoot and root, total petiole length, and leaf area increased with increasing FR fractions. As a result of ray-tracing analysis, the light interception and canopy photosynthetic rate increased with high FR fraction treatment. As the FR fraction increased, the effective quantum yield of photosystem II [Y(II)], the parameters of the JIP test that is related to quantum yield and efficiency for electron transport in photosystem II and photosystem performance indices decreased. Despite the decreased electron transport efficiency, the canopy photosynthetic rate increased in relation to the FR fraction. In conclusion, the increase in the FR fraction increased the yield of kale because of the synergistic improvement in morphological change and canopy photosynthesis.

Ajustes fotossintéticos e concentração de prolina provavelmente estão ligados à memória de estresse em soja exposta a seca recorrente

ABSTRACT Drought stress is the main abiotic factor limiting soybean yield. The memory of recurrent water stress can provide greater efficiency in minimizing the negative effects of drought. Thus, the aim of this work was to understand the temporal adjustments in photosynthesis presented by soybeans when exposed to recurrent drought at the beginning of the flowering and grain filling stages. The experiment was carried out in a randomized block design with five replications, consisting of four treatments: i) WS-R1 (moderate water deficit at the beginning of flowering), ii) WS-R5 (severe water deficit during grain filling), iii) WS-R1+R5 (moderate water deficit at early flowering and severe water deficit during grain filling), and iv) WW (well-watered condition). Severe stress caused reductions in gas exchange parameters and the relative water content, with increased initial fluorescence and water use efficiency. The plants from the WS-R5 and WS-R1+R5 treatments showed a reduction in the apparent rate of electron transport in photosystem II (PSII), photochemical quenching, and effective quantum yield of PSII, as well as increased nonphotochemical quenching values. Furthermore, the proline concentration in the leaves was higher in plants from the WS-R1+R5 treatment, contributing to the greater ability to maintain turgid cells compared to the WS-R5 plants. The photosynthetic adjustments related to faster isohydric responses and photoprotective mechanisms in soybean plants subjected to recurrent drought allowed the maintenance in the weight or number of grains compared to plants without water restriction, demonstrating the activation of efficient memory mechanisms of response to water stress.

Influence of light intensity and its spectral composition on photosynthetic activity of cucumber Cucumis sativus under fusarium wilt

The responses of cucumber chloroplasts of the Kustovoi variety formed under lighting of different intensity (6000 and 11 000 lx) or under LED lighting with a predominance of red light and far red light to infection with fungus Fusarium oxysporum were studied. The amount of chlorophylls and carotenoids in chloroplasts formed at low light increased in 72 h after infection, and at high light a significant increase in pigment catabolism was observed. Under fusarium wilt, the violaxanthin cycle was not involved in the conditions of the studied light range, and the photochemical activity of chloroplasts did not depend much on the level of illumination. The predominance of red light or far red light caused an increase in both chlorophylls and carotenoids content in terms of the dry mass of the leaf compared to their content in plants grown on white light. Infection on white light and red light caused an increase in the total content of chlorophylls and carotenoids, and a decrease in these parameters relative to healthy plants was noted on far red light. Fusarium wilt led to a decrease in photochemical activity and electron transport of photosystem I (by 20 %) in chloroplasts on red light and far red light. A decrease in the functional efficiency of photosystem II was also observed, most likely due to irreversible changes in pigment-protein complexes with two types of LED lighting. Various mechanisms of the response of cucumber chloroplasts to pathogen infection are discussed, depending on the light conditions of the formation of photosynthetic membranes.

High-resolution dissection of photosystem II electron transport reveals differential response to water deficit and heat stress in isolation and combination in pearl millet [Pennisetum glaucum (L.) R. Br.

Heat and Water Deficit Stress (WDS) tend to impede and restrict the efficiency of photosynthesis, chlorophyll fluorescence, and maximum photochemical quantum yield in plants based on their characteristic ability to interfere with the electron transport system in photosystem II. Dissection of the electron transport pathway in Photosystem II (PSII) under water deficit and Heat Stress (HS) can be insightful in gaining knowledge on the various attributes of the photosynthetic performance of a plant. We attempt a high-resolution dissection of electron transport in PSII with studies on chlorophyll a fast fluorescence kinetics and non-photochemical quenching (NPQ) as a response to and recovery from these stresses in pearl millet [Pennisetum glaucum (L.) R. Br.] in isolation and combination. In this study, we bring out the mechanisms by which both heat and water stress, in isolation and in combination, affect the photosynthetic electron transport in Photosystem II. Our results indicate that oxygen evolution complex (OEC) damage is the primary effect of heat stress and is not seen with the same intensity in the water-stressed plants. Low exciton absorption flux in heat stress and combined stress was seen due to OEC damage, and this caused an electron transport traffic jam in the donor side of PS II. Both the specific energy flux model and the phenomenological flux model developed from the derived values in our study show that water deficit stress in combination with heat stress has a much stronger effect than the stresses in isolation on the overall electron transport pathway of the PS II in pearl millet plants.

Overexpression of OHPs in Neopyropia yezoensis (Rhodophyta) reveals their possible physiological roles

The red macroalga Neopyropia yezoensis is a model organism, and an economically important seaweed widely cultured in eastern Asian countries. Transgenic research on N. yezoensis is difficult and limited to reporter genes and selection markers due to the specificities of this species. In this study, plasmids were constructed to overexpress six one helix protein (OHP) coding genes in N. yezoensis, and four transgenic lines with high expression levels of the target genes were obtained. The photosynthetic parameters were determined and it was found that the overexpression of OHPs altered electron transport in photosystem I and photosystem II, which was indicated by the increased 1-qL values and decreased P700+ reduction rates. The results also showed that the non-photochemical quenching (NPQ) kinetics in N. yezoensis could not be stimulated by high light (HL) treatment. Two of the four transgenic lines showed stimulated NPQ, and they did not exhibit an obvious increased ability to resist to HL, which was shown by the decreased Fv/Fm values. The increased H2O2 content and decreased peroxidase (POD) activity were considered the possible results of the overexpressed OHPs, and they were responsible for the enhanced NPQ and HL sensitivity in those two transgenic lines.

Stress and use of herbicides in field crops

When herbicides are combined under natural conditions or applied in stress conditions (drought, for instance), the efficiency of their action may decrease which results in considerable yield losses. The reason thereto is that another herbicide or stressor can trigger the adaptation mechanism in weed plants, and they survive, resulting in weed infestation. In particular, it applies to such herbicides as acetyl-CoA-carboxylase inhibitors or so- called graminicides, which are effective only for grass weeds control. The efficiency of this groups of herbi- cides is heavily dependent on the environment and often decreases when combined with herbicides, efficient against dicotyledon weeds. It turns out that this occurs due to the fact their final phytotoxicity is not determined at the level of the site of action (herbicide target) but depends on the stage of herbicide-induced pathogenesis – processes, occurring due to the interaction between the herbicide and its site of action. The stress response of the weeds may make its contribution into herbicide-induced pathogenesis. Plants are known to respond to the action of various abiotic stressors in the form of non-specific stress response and depending on the intensity and duration of the stressor’s action, a plant either adapts or dies. At present there are sufficient data, demonstrating that programmed cell death (PCD) is involved in the herbicide-induced pathogenesis. Reactive oxygen species (ROS) induce PCD in specific classes of herbicides. The participation of ROS and PCD in herbicide-induced pathogenesis allows for targeted effects on the phytotoxic action of herbicides, for instance, via combined application of herbicides with possible PCD inducers and prooxidants. The confirmation of the role of non- specific response in the development of phytotoxic action of herbicides is found in the phenomena of cross- adaptation (activation of antioxidant defense) and cross-synergism (activation of oxidative stress) under the application of herbicides. Based on our own research and literature data, the importance of cross-adaptation and cross-synergism in applying herbicides in drought conditions and to determine the nature of the interac- tion in herbicide complexes is discussed. In particular, the review discusses the reduction of phytotoxicity of the ACCase herbicides due to the phenomenon of cross-adaptation in drought conditions and in combination with herbicides, which are acetolactate synthase inhibitors. The results of investigations were presented about the reduction of antagonism in the mixtures of herbicides, which are ACCase and ALS inhibitors, because of the use of substances with prooxidant properties, as well as the inhibitor of the antioxidant enzyme superoxide dismutase. On the other hand, we analyzed the possibility of increasing the phytotoxic effect of herbicides, ACCase inhibitors, in combination with herbicides with prooxidant properties – inhibitors of electron transport in Photosystem 2 (FS 2) chloroplasts and protoporphyrinogen oxidase (PROTOX) inhibitors. It became the foundation for the elaboration of efficient herbicide compositions for wheat and onion fields. While combining herbicides, the issue of synergism is becoming relevant due to the problem of the spread of target-site resis- tance, since, to prevent this type of resistance, it is necessary to combine herbicides with different mechanisms of phytotoxicity. The presented data demonstrate that the increased activity of antioxidant defense systems, which is the result of a long process of evolutionary adaptation of weeds to the action of abiotic stressors, is an element of non-target-site-based resistance to herbicides. Possible ways to prevent the negative impact of non-specific stress response on the efficiency of herbicides, as well as the prospects of the chemical method of weeds control are discussed.

PsbX maintains efficient electron transport in Photosystem II and reduces susceptibility to high light in Synechocystis sp. PCC 6803

PsbX is a 4.1 kDa intrinsic Photosystem II (PS II) protein, found together with the low-molecular-weight proteins, PsbY and PsbJ, in proximity to cytochrome b559. The function of PsbX is not yet fully characterized but PsbX may play a role in the exchange of the secondary plastoquinone electron acceptor QB with the quinone pool in the thylakoid membrane. To study the role of PsbX, we have constructed a PsbX-lacking strain of Synechocystis sp. PCC 6803. Our studies indicate that the absence of PsbX causes sensitivity to high light and impairs electron transport within PS II. In addition to a change in the QB-binding pocket, PsbX-lacking cells exhibited sensitivity to sodium formate, suggesting altered binding of the bicarbonate ligand to the non-heme iron between the sequential plastoquinone electron acceptors QA and QB. Experiments using 35S-methionine revealed high-light-treated PsbX-lacking cells restore PS II activity during recovery under low light by an increase in the turnover of PS II-associated core proteins. These labeling experiments indicate the recovery after exposure to high light requires both selective removal and replacement of the D1 protein and de novo PS II assembly.

Physiological, metabolic, and stomatal adjustments in response to salt stress in Jatropha curcas

Salinity is a major issue affecting photosynthesis and crop production worldwide. High salinity induces both osmotic and ionic stress in plant tissues as a result of complex interactions among morphological, physiological, and biochemical processes. Salinity, in turn, can provoke inactivation of some enzymes in the Calvin-Benson cycle and therefore affect the fine adjustment of electron transport in photosystem I and carbon related reactions. Here, we used three contrasting Jatropha curcas genotypes namely CNPAE183 (considered tolerant to salinity), CNPAE218 (sensible), and JCAL171 (intermediate) to understand salinity responses. By performing a long-term (12 months) experiment in land conditions, we investigated distinct mechanisms used by J. curcas to cope with threatening salinity effects by analyzing gas exchange, mineral nutrition and metabolic responses. First, our results highlighted the plasticity of stomatal development and density in J. curcas under salt stress. It also demonstrated that the CNPAE183 presented higher salt-tolerance whereas CNPAE218 displayed a more sensitive salt-tolerance response. Our results also revealed that both tolerance and sensitivity to salinity were connected with an extensive metabolite reprogramming in the Calvin-Benson cycle and Tricarboxylic Acid cycle intermediates with significant changes in amino acids and organic acids. Collectively, these results indicate that the CNPAE183 and CNPAE218 genotypes demonstrated certain characteristics of salt-tolerant-like and salt-sensitive-like genotypes, respectively. Overall, our results highlight the significance of metabolites associated with salt responses and further provide a useful selection criterion in during screening for salt tolerance in J. curcas in breeding programmes.

Metaproteomics reveals the structural and functional diversity of Dermatocarpon miniatum (L.) W. Mann. Microbiota

Metaproteomics is a strategy to understand the taxonomy, functionality and metabolic pathways of the microbial communities. The relationship among the symbiotic microbiota in the entire lichen thallus, Dermatocarpon miniatum, was evaluated using the metaproteomic approach. Proteomic profiling using one-dimensional SDS-PAGE followed by LC-MS/MS analysis resulted in a total of 138 identified proteins via Mascot search against UniRef100 and Swiss-Prot databases. In addition to the fungal and algal partners, D. miniatum proteome encompasses proteins from prokaryotes, which is a multifarious community mainly dominated by cyanobacteria and proteobacteria. While proteins assigned to fungus were the most abundant (55 %), followed by protists (16 %), bacterial (13 %), plant (11 %), and viral (1 %) origin, whereas 4 % remained undefined. Various proteins were assigned to the different lichen symbionts by using Gene Ontology (GO) terms, e.g. fungal proteins involved in the oxidation-reduction process, protein folding and glycolytic process, while protists and bacterial proteins were involved in photosynthetic electron transport in photosystem II (PS II), ATP synthesis coupled proton transport, and carbon fixation. The presence of bacterial communities extended the traditional concept of fungal-algal lichen symbiotic interaction.

Effect of Cadmium Ions on Some Biophysical Parameters and Ultrastructure of Ankistrodesmus sp. В-11 Cells

Effects of low concentrations of cadmium ions on growth, photosynthesis, and cell ultrastructure of microalga Ankistrodesmus sp. B-11 were investigated. The addition of cadmium to growth medium at concentrations of 0.005–0.02 mg/L led to a significant decrease in the number of Ankistrodesmus sp. B-11 cells relatively to that in the untreated culture. The addition of cadmium at concentrations >0.05 mg/L completely stopped cell growth. Cadmium ions induced ultrastructural changes in the arrangement of thylakoids within the stroma, the detachment of thylakoid membranes with the formation of void interthylakoid spaces, and a significant increase in vacuolization of microalgal cells. Simultaneous measurements of fluorescence induction curves and redox transformations of photosystem I components on a microsecond time scale by means of a M‑PEA-2 fluorometer revealed that cadmium ions inhibit electron transport in photosystem II (PSII). The quantum yield of electron transport in PSII (φEo) and the performance index (PIABS) were found to decrease; the photoreduction of P700 pigment was decelerated, while energy dissipation (DI0/RC) and ΔpH-dependent nonphotochemical quenching (qE) increased significantly under the action of cadmium. The performance index (PIABS) was the most sensitive parameter; it can be used for the detection of early toxic effects of cadmium ions on algae.

Photorespiration Coupled With CO2 Assimilation Protects Photosystem I From Photoinhibition Under Moderate Poly(Ethylene Glycol)-Induced Osmotic Stress in Rice.

Photorespiration coupled with CO2 assimilation is thought to act as a defense system against photoinhibition caused by osmotic stress. In the present study, we examined whether such a mechanism is operative for the protection of photosystem I (PSI) in rice (Oryza sativa L.) including transgenic plants with decreased and increased Rubisco content (RBCS-antisense and RBCS-sense plants, respectively). All plants were hydroponically grown and moderate osmotic stress was imposed using hydroponic culture solutions containing poly(ethylene glycol) (PEG) at 16% or 20% (w/v) for 2 d. In wild-type plants, the rates of CO2 assimilation (A) were significantly decreased by the PEG treatment, whereas the photorespiration activity estimated from the rates of electron transport in photosystem II (PSII) and A were not affected. The maximal quantum efficiency of PSII (F v/F m) and the maximal activity of PSI (P m) were also not affected. In RBCS-antisense plants, A and the estimated photorespiration activity were considerably lower than those in wild-type plants in the presence or absence of the PEG treatment. P m and both F v/F m and P m decreased in the 16% PEG-treated and 20% PEG-treated RBCS-antisense plants, respectively. Thus, the decrease in Rubisco content led to the photoinhibition of PSI and PSII, indicating the importance of photorespiration coupled with CO2 assimilation for the protection of PSI from moderate PEG-induced osmotic stress. It was also shown that PSI was more sensitive to osmotic stress than PSII. In the PEG-treated wild-type and RBCS-antisense plants, osmotic-stress responses of the photosynthetic electron transport reactions upstream of PSI led to the oxidation of P700, which is thought to prevent PSI from over-reduction. Although such a defense system operated, it was not sufficient for the protection of PSI in RBCS-antisense plants. In addition, there were no large differences in the parameters measured between wild-type and RBCS-sense plants, as overproduction of Rubisco did not increase photorespiration activity.

Pigment mutants of the green microalga Chlamydomonas reinhardtii: morphological properties and photosynthetic performance

Pigment mutants CC-124y-1, СС-124у-2, and СС-124р-2 of the green microalga Chlamydomonasreinhardtii Dangeard were obtained by means of UV-induced mutagenesis. Morphological propertiesand photosynthetic activity of these mutants were examined. The mutants displayed a lowered contentof chlorophyll b and carotenoids. Analysis of fluorescence induction curves revealed the decreased ratesof electron transport in photosystem II (PSII), the increase in the fraction of QB-nonreducing centers, andthe enhancement of nonphotochemical fluorescence quenching. The mutations had no direct impact onoxidation of pigment P700 in PSI and on the decline of delayed fluorescence. The parameters of induction curves of prompt and delayed fluorescence are proposed for use in early diagnostics of UV-inducedmutagenesis. The pigment mutants obtained can be applied in biomonitoring studies.Key words: Chlamydomonas reinhardtii, chlorophyll a fluorescence, delayed fluorescence, JIP test,photosynthesis.

Effects of solar irradiation and night-time temperature on the thinning efficacy of metamitron (Brevis®) in apple

Trials with metamitron (Brevis®) were conducted at different phenological stages of apple fruit (7-18 mm fruit diameter) in the 2014-2016 seasons. Within each season, the same dosages were used at all application dates for all tested cultivars. On the basis of the obtained results a model was developed in order to estimate the efficacy of Brevis® at the established treatment. Metamitron causes a measurable reduction of electron transport in photosystem II for about 10 days, leading to a reduced photosynthetic output. In addition when elevated night-time temperatures (>10°C) occur a further fruit abscission can be enhanced.

Exogenous Melatonin Confers Salt Stress Tolerance to Watermelon by Improving Photosynthesis and Redox Homeostasis.

Melatonin, a pleiotropic signal molecule, has been shown to play important roles in the regulation of plant growth, development, and responses to environmental stresses. Since a few species have been investigated to unveil the effect of exogenous melatonin on salt stress, the underlying mechanism of melatonin-mediated salt stress tolerance in other plant species still remains largely unknown. In this study, the effects of melatonin on leaf photosynthesis and redox homeostasis in watermelon were examined under salt stress (300 mM NaCl) along with different doses of melatonin (50, 150, and 500 μM) pretreatment. NaCl stress inhibited photosynthesis and increased accumulation of reactive oxygen species and membrane damage in leaves of watermelon seedlings. However, pretreatment with melatonin on roots alleviated NaCl-induced decrease in photosynthetic rate and oxidative stress in a dose-dependent manner. The protection of photosynthesis by melatonin was closely associated with the inhibition of stomatal closure and improved light energy absorption and electron transport in photosystem II, while the reduction of oxidative stress by melatonin was attributed to the improved redox homeostasis coupled with the enhanced activities of antioxidant enzymes. This study unraveled crucial role of melatonin in salt stress mitigation and thus can be implicated in the management of salinity in watermelon cultivation.