Photosystem II Yield Research Articles

Effects of Foliar Application of a Lambda-Cyhalothrin Insecticide on Photosynthetic Characteristics of a Fodder Plant Malva moschata

Recently, a large number of pesticides with different chemical structures and modes of action (MOAs) have become regularly used in agriculture. They are used to control the insect populations in various crops. Foliar application of pesticides may negatively affect crop physiology, especially photosynthesis. However, the sensitivity of particular crops, especially their primary and secondary photosynthetic processes, to insecticide application is generally unknown. Our study aimed to evaluate the negative effects of lambda-cyhalothrin (λ-CY) on photosystem II (PSII) in Malva moschata (Musk mallow). We used fast chlorophyll fluorescence transients (i.e., OJIPs) and OJIP-derived parameters, the effective quantum yield of PSII (ΦPSII), induction curves of non-photochemical quenching (NPQ) and spectral reflectance curves and indices. The recommended concentration (0.05 μM) and a 10 times higher concentration (0.5 μM) of λ-CY did not cause any negative effect on photosynthetic parameters. An overdosed foliar application (100 times higher than recommended, i.e., 50 μM) led to changes in OJIP shape; a decrease in performance index (PIABS), maximum photosynthetic yield (FV/FM) and photosynthetic electron transport (ET0/RC); and an increase in protective mechanisms (unregulated quenching, DI0/RC). These changes lasted only tens of minutes after application, after which the parameters returned to pre-application values. An overdosed λ-CY application caused more rapid activation of NPQ, indicating the early response to stress in PSII. The application of 50 μM λ-CY caused an increase in spectral reflectance above 720 nm and changes in the indices that indicated λ-CY-induced stress.

Up-regulation of nitrogen metabolism and chlorophyll biosynthesis by hydrogen sulfide improved photosystem photochemistry and gas exchange in chromium-contaminated bean (Phaseolus vulgaris L.) plants

Hydrogen sulfide (H₂S) is considered as plant growth promoter under heavy metal stress, though its specific effects on photosynthesis are rarely explored. This study investigates the protective effects of exogenous H2S donor sodium hydrosulfide (NaHS) on chlorophyll metabolism and photosystem II (PSII) function in 24-day-old bean plants exposed to 10 μM chromium (Cr) stress. Sodium hydrosulfide (100 μM) reduced Cr accumulation in both roots and leaves, leading to restored plant growth. Concomitantly, H₂S mitigated Cr-induced oxidative damages by decreasing reactive oxygen species levels and further enhancing antioxidant scavenging activities. This resulted in significant reductions in Cr-elevated leaf pheophytin and chlorophyllide levels by 59% and 67%, respectively. Furthermore, NaHS application increased levels of porphyrin and its precursor, 5-aminolevulinic acid (5-ALA), in Cr-stressed bean. The up-regulation in chlorophyll biosynthesis was associated with enhanced activities of glutamine synthetase and glutamate synthase, essential for glutamate (precursor of 5-ALA) production, as well as nitrate and nitrite reductase, leading to increased nitric oxide generation. Under Cr stress, H₂S significantly improved the electron transport rate, effective quantum yield of PSII, and photochemical quenching by 112%, 53%, and 38%, respectively, while reducing non-photochemical quenching by 50%. Furthermore, H₂S promoted net CO₂ assimilation and photosynthesis at saturating light, respectively, while reducing stomatal conductance and transpiration to maintain water balance. Exogenous H₂S restored respiration, as indicated by increased light saturation and compensation points in Cr-treated plants. Overall, these findings indicate that H₂S regulates photosynthesis in Cr-stressed bean by modulating nitrogen and chlorophyll metabolism, thereby optimizing PSII efficiency and gas exchange.

Alleviative Effect of Exogenous Application of Fulvic Acid on Nitrate Stress in Spinach (Spinacia oleracea L.)

Salt stress could be a significant factor limiting the growth and development of vegetables. In this study, Fulvic Acid (FA) (0.05%, 0.1%, 0.15%, 0.2%, and 0.25%) was applied under nitrate stress (150 mM), with normal Hoagland nutrient solution as a control to investigate the influence of foliar spray FA on spinach growth, photosynthesis, and oxidative stress under nitrate stress. The results showed that nitrate stress significantly inhibited spinach growth, while ROS (reactive oxygen species) accumulation caused photosystem damage, which reduced photosynthetic capacity. Different concentrations of FA alleviated the damage caused by nitrate stress in spinach to varying degrees in a concentration-dependent manner. The F3 treatment (0.15% FA + 150 mM NO3−) exhibited the most significant mitigating effect. FA application promoted the accumulation of biomass in spinach under nitrate stress and increased chlorophyll content, the net photosynthetic rate, the maximum photochemical quantum yield of PSII (Photosystem II) (Fv/Fm), the quantum efficiency of PSII photochemistry [Y(II)], the electron transport rate, and the overall functional activity index of the electron transport chain between the PSII and PSI systems (PItotal); moreover, FA decreased PSII excitation pressure (1 − qP), quantum yields of regulated energy dissipation of PSII [Y(NPQ)], and the relative variable initial slope of fluorescence. FA application increased superoxide dismutase, peroxidase, and catalase activities and decreased malondialdehyde, H2O2, and O2− levels in spinach under nitrate stress. FA can enhance plant resistance to nitrate by accelerating the utilization of light energy in spinach to mitigate excess light energy and ROS-induced photosystem damage and increase photosynthetic efficiency.

A practical approach for extracting the photosystem II (PSII) contribution to near-infrared solar-induced chlorophyll fluorescence

Recent studies have indicated a good potential for using solar-induced chlorophyll fluorescence (SIF) to estimate photosynthetic CO2 assimilation. SIF can be emitted by both Photosystem I (PSI) and Photosystem II (PSII), but it is the SIF signals from PSII which are related to photosynthetic carbon fixation. However, since top-of-canopy SIF observations (SIFTOC) always contain contributions from both photosystems, to mechanistically estimate gross primary productivity (GPP) from SIF, it is essential to extract PSII SIF from SIFTOC. Based on the differences in the relative contribution of PSII across different wavelengths, we propose a practical approach for extracting PSII contribution to SIFTOC at the near-infrared (NIR) band (fPSII_760) using measurements of SIFTOC in the red and NIR spectral regions. A leaf-scale concurrent instrument was developed to assess the response of fPSII_760 under varying environments. For winter-wheat leaves, as light intensity increased from 0 to 400 μmol m−2 s−1, fPSII_760 rose from 0.6 to 0.8; with further increase in light intensity to 1800 μmol m−2 s−1, fPSII_760 consistently decreased to 0.65. There was a slight decreasing trend in fPSII_760 with rising temperatures, with values dropping from 0.65 at 15 °C to 0.61 at 40 °C. We found that variations in fPSII_760 are due to changes in the fluorescence yield of PSII, with the two having a positively proportional relationship. We also estimated canopy-scale fPSII_760 for a winter-wheat study site: fPSII_760 varied from 0.61 to 0.83, with a mean value of 0.78 during the peak growing season. A comparison with eddy covariance-derived GPP reveals that GPP estimated with dynamic fPSII_760 was more accurate than that calculated using fixed fPSII_760, with R2 increasing from 0.6 to 0.84. This study contributes to a deeper understanding of the link between SIF and photosynthetic CO2 assimilation, paving the way for more effective use of SIF to estimate GPP.

New Data on Phytochemical and Morphophysiological Characteristics of Platycladus orientalis L. Franco and Thuja occidentalis L. Conifer Trees in Polluted Urban Areas of Kazakhstan

The adaptive potential of plants in urban environments, responding to factors like air pollution, electromagnetic radiation, and specific microclimates, remains insufficiently understood. Our study focused on two evergreen Cupressaceae family species, Thuja occidentalis L. and Platycladus orientalis L. Franco, which are commonly found in Kazakhstan’s urban landscapes. Conducted in Almaty, one of Kazakhstan’s most polluted cities, our comparative analysis examined the anatomical features, photosynthetic activity, and secondary metabolite composition of these conifers. Both species exhibited xeromorphic traits, such as submerged stomata, resin passages, and a prominent leaf cuticle. T. occidentalis displayed higher photosynthetic activity values (quantum yield of photosystem II (YII), electron transport rate (ETR), and quantum yield of non-photochemical quenching (Y(NPQ))) than P. orientalis, while P. orientalis exhibited a higher quantum yield of non-regulated energy dissipation in PSII (Y(NO)) values. Chemical analysis revealed 31 components in T. occidentalis and 33 in P. orientalis, with T. occidentalis containing three times more thujone (16.42% and 5.18%, respectively) and a higher monosaccharide content (17.33% and 6.98%, respectively). T. occidentalis also contained 14.53% steroids, whereas P. orientalis showed no steroid presence. The cytotoxic activity of essential oils was determined by the survival of Artemia salina aquatic crustaceans, whereas tested essential oils from both species exhibited acute lethal toxicity to A. salina aquatic crustaceans across all tested concentrations. The connection between physiological traits, adaptation strategies, and cytotoxic effects offers a comprehensive view of the ecological and pharmacological importance of these two observed conifer species, highlighting their diverse roles in urban environments, as well as their potential medical uses.

Effects of Cu2O nanoparticles on the kinetic characteristics of rapid chlorophyll fluorescence induction and related genes in wheat seedlings.

Metal nanoparticles could be accumulated in soils, which threatens the ecological stability of crops. Investigating the effects of cuprous oxide nanoparticles (Cu2O-NPs) on photosystem Ⅱ (PSⅡ) of wheat seedling leaves holds considerable importance in comprehending the implications of Cu2O-NPs on crop photosynthesis. Following the hydroponic method, we investigated the effects of 0, 10, 50, 100, and 200 mg·L-1 Cu2O-NPs on chlorophyll fluorescence induction kinetics and photosynthetic-related genes in wheat seedlings of "Zhoumai 18". The results showed that, with the increases of Cu2O-NPs concentrations, chlorophyll contents in wheat leaves decreased, and the standardization of the OJIP curve showed a clearly K-phase (ΔK＞0). Cu2O-NPs stress increased the parameters of active PSⅡ reaction centers, including the absorption flux per active RC (ABS/RC), the trapping flux per active RC (TRo/RC), the electron transport flux per active RC (ETo/RC), and the dissipation flux per active RC (DIo/RC). Cu2O-NPs stress decreased the parameters of PSⅡ energy distribution ratio including the maximum quantum yield of PSⅡ (φPo), the quantum yield of electron transport from QA (φEo), and the probability that a trapped exciton moved an electron further than QA (Ψo), while increased the quantum ratio for heat dissipation (φDo). Moreover, there was a decrease in photosynthetic quantum yield Y(Ⅱ), photochemical quenching coefficient (qP), net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of leaves with the increases of Cu2O-NPs concentration. Under Cu2O-NPs stress, the expression levels of genes which included PSⅡ genes (PsbD, PsbP, Lhcb1), Rubisco large subunit genes (RbcL), cytochrome b6/f complex genes (PetD, Rieske), and ATP synthase genes (AtpA, AtpB, AtpE, AtpI) were downregulated. These results indicated that Cu2O-NPs stress altered the activity and structure of PSⅡ in wheat seedlings, affected the activity of PSⅡ reaction centers, performance parameters of PSⅡ donor and acceptor sides. PSⅡ related genes were downregulated and exhibited significant concentration effects.

Physiological and agronomical traits effects of titanium dioxide nanoparticles in seedlings of Solanum lycopersicum L

BackgroundTitanium dioxide nanoparticles (TiO2 NPs) have been reported to have contrasting effects on plant physiology, while their effects on sugar, protein, and amino acid metabolism are poorly understood. In this work, we evaluated the effects of TiO2 NPs on physiological and agronomical traits of tomato (Solanum lycopersicum L.) seedlings. Tomato seeds were treated with TiO2 NPs (1000 and 2000 mg L− 1), TiO2 microparticles (µPs, 2000 mg L− 1) as the size control, and ultrapure water as negative control.ResultsThe dry matter of stems (DMs), leaves (DMl) and total dry matter (DMt) decreased as particle concentration increased. This trend was also observed in the maximum quantum yield of light-adapted photosystem II (PSII) (Fv´/Fm´), the effective quantum yield of PSII (ΦPSII), and net photosynthesis (Pn). The concentrations of sugars, total soluble proteins, and total free amino acids were unaffected, but there were differences in the daily dynamics of these compounds among the treatments.ConclusionOur results suggest that treating tomato seeds with TiO2 might affect PSII performance, net photosynthesis and decrease biomass production, associated with a concentration- and size-related effect of TiO2 particles.

Nitrate nitrogen enhances the efficiency of photoprotection in Leymus chinensis under drought stress.

Global climate change exerts a significant impact on the nitrogen supply and photosynthesis ability in land-based plants. The photosynthetic capacity of dominant grassland species is important if we are to understand carbon cycling under climate change. Drought stress is one of the major factors limiting plant photosynthesis, and nitrogen (N) is an essential nutrient involved in the photosynthetic activity of leaves. The regulatory mechanisms responsible for the effects of ammonium (NH4 +) and nitrate (NO3 -) on the drought-induced photoinhibition of photosystem II (PSII) in plants have yet to be fully elucidated. Therefore, there is a significant need to gain a better understanding of the role of electron transport in the photoinhibition of PSII. In the present study, we conducted experiments with normal watering (LD), severe drought (MD), and extreme drought (HD) treatments, along with no nitrogen (N0), ammonium (NH4), nitrate (NO3), and mixed nitrogen (NH4NO3) treatments. We analyzed pigment accumulation, reactive oxygen species (ROS) accumulation, photosynthetic enzyme activity, photosystem activity, electron transport, and O-J-I-P kinetics. Analysis showed that increased nitrate application significantly increased the leaf chlorophyll content per unit area (Chlarea) and nitrogen content per unit area (Narea) (p< 0.05). Under HD treatment, ROS levels were lower in NO3-treated plants than in N0 plants, and there was no significant difference in photosynthetic enzyme activity between plants treated with NO3 and NH4NO3. Under drought stress, the maximum photochemical efficiency of PSII (Fv/Fm), PSII electron transport rate (ETR), and effective quantum yield of PSII (φPSII) were significant higher in NO3-treated plants (p< 0.05). Importantly, the K-band and G-band were higher in NO3-treated plants. These results suggest that drought stress hindered the formation of NADPH and ATP in N0 and NH4-treated L. chinensis plants, thus damaging the donor side of the PSII oxygen-evolving complex (OEC). After applying nitrate, higher photosynthetic enzyme and antioxidant enzyme activity not only protected PSII from photodamage under drought stress but also reduced the rate of damage in PSII during the growth of L. chinensis growth under drought stress.

Long term co-application of biochar and fertilizer could increase soybean yield under continuous cropping: insights from photosynthetic physiology.

Photosynthesis is the key to crop yield. The effect of biochar on photosynthetic physiology and soybean yield under continuous cropping is unclear. We conducted a long-term field experiment to investigate the effects of co-application of biochar and fertilizer (BCAF) on these parameters. Five treatments were established: F2 (fertilizer), B1F1 (3 t hm-2 biochar plus fertilizer), B1F2 (3 t hm-2 biochar plus reduced fertilizer), B2F1 (6 t hm-2 biochar plus fertilizer), and B2F2 (6 t hm-2 biochar plus reduced fertilizer). BCAF increased chlorophyll and leaf area, enhancing soybean photosynthesis. The net photosynthetic rate (Pn ), transpiration rate (Tr ), stomatal conductance (Gs ), water use efficiency (WUE) and intercellular carbon dioxide (CO2 ) concentration (Ci ) were enhanced by BCAF. In addition, BCAF improved soybean photosystem II (PSII) photosynthetic performance, driving force, potential photochemical efficiency (Fv /F0 ), and quantum yield of electron transfer (φE0 ). Furthermore, BCAF enhanced the accumulation of photosynthetic products, such as soluble proteins, soluble sugars and sucrose content, resulting in higher leaf dry weight. Consequently, BCAF increased the soybean yield, with the highest increase of 41.54% in B2F1. The correlation analysis revealed positive relationships between soybean yield and chlorophyll, leaf area, maximal quantum yield of PSII (Fv /Fm ), electron transport flux per cross-section at t = 0 (ET0 /CS0 ), trapped energy flux per cross-section at t = 0 (TR0 /CS0 ), composite blade driving force (DFTotal ), and leaf dry weight. We demonstrated that long-term BCAF enhances soybean photosynthesis under continuous planting, reduces fertilizer use and increases yield. This study reveals a novel way and theory to sustainably increase soybean productivity. © 2023 Society of Chemical Industry.

Effect of Some Herbicides on Primary Photosynthesis in Malva moschata as a Prospective Plant for Agricultural Grass Mixtures

Musk mallow (Malva moschata) is a perennial European plant that grows in pastures and grass mixtures. M. moschata is a medicinal plant with a high content of polysaccharides, flavonoids, and other biologically active compounds. The sensitivity of the species to herbicides is, however, generally unknown. In our study, we investigated the effects of three herbicides with different active compounds: (1) Propaquizafop (PPQ), (2) Clopyralid (CPR), (3) Metamitron+Quinmerac (MMQ) on primary photosynthetic processes of M. moschata plants cultivated in two different temperatures (12 and 18 °C). Non-photochemical quenching (NPQ) of absorbed light energy by chlorophyll molecules and the parameters derived from fast chlorophyll fluorescence transient (OJIP) were evaluated before and then 1, 2, 5, 24, and 48 h after the herbicides application. Among the herbicides, only MMQ negatively affected the functioning of photosystem II (PSII). The effect, however, was reversible, and the plants reached pre-application values after 48 h. No herbicide-induced changes in NPQ were found over the time after the application. The majority of the OJIP-derived chlorophyll fluorescence parameters were found to be temperature-sensitive. The herbicides tested in our study may be ranked among those with only limited effects on primary photosynthetic processes in PSII. While the application of PPQ and CPR did not bring any negative effects on the functioning of PSII, MMQ application induced a decrease in the potential quantum yield of PSII (FV/FM) and activation of protective mechanisms, increased heat dissipation in particular. These changes, however, were fully alleviated 48 h after MMQ application.

Old-growth forest versus generalist lichens: Sensitivity to prolonged desiccation stress and photosynthesis reactivation rate upon rehydration

ABSTRACT Most epiphytic lichens demonstrate high specificity to a habitat type, and sensitive hygrophilous species usually find shelter only in close-to-natural forest complexes. Some of them are considered as old-growth forest and/or long ecological continuity indicators. To evaluate general links between the narrow ecological range and physiological traits, two distinct sets of model lichens, i.e., old-growth forest (Cetrelia cetrarioides (Duby) W.L. Culb. & C.F. Culb., Lobaria pulmonaria (L.) Hoffm., Menegazzia terebrata (Hoffm.) A. Massal.), and generalist (Flavoparmelia caperata (L.) Hale, Hypogymnia physodes (L.) Nyl., Parmelia sulcata Taylor) ones, were examined in terms of sensitivity to long-term desiccation stress (1-, 2-, and 3-month) and photosynthesis activation rate upon rehydration. Desiccation tolerance and response rate to rehydration are specific to a given ecological set of lichens rather than to a particular species. Noticeable delayed and prompt recovery of high photosynthetic activity of photosystem II (PSII) characterize these sets, respectively. At the same time, although a decrease in the potential quantum yield of PSII in lichen thalli with a relative water content (RWC) at the level of 25% was observed, the efficiency remained at a very high level for all species, regardless of habitat preferences. Among the examined lichens, the fluorescence emission parameters for F. caperata were the fastest toward equilibrium upon rehydration, both after a shorter and a longer period of desiccation stress. In contrast to generalist lichens, retrieving of photosynthesis after 3-month desiccation failed in old-growth forest lichens. In the long term, prolonged rainless periods and unfavorable water balance in the environment predicted in the future may have a severely limiting effect on hygrophilous lichens during growing season (also in the sense of species associations) and, at the same time, promote the development of generalists.

Terrestrial Trentepohlia sp. (Ulvophyceae) from alpine and coastal collection sites show strong desiccation tolerance and broad light and temperature adaptation.

For the present study, we collected the Ulvophyceae species Trentepohlia aurea from limestone rock near Berchtesgaden, Germany, and the closely related taxa T. umbrina from Tilia cordata tree bark and T. jolithus from concrete wall both in Rostock, Germany. Freshly sampled material stained with Auramine O, DIOC6, and FM 1-43 showed an intact physiological status. Cell walls were depicted with calcofluor white and Carbotrace. When subjected to three repeated and controlled cycles of desiccation over silica gel (~ 10% relative humidity) followed by rehydration, T. aurea recovered about 50% of the initial photosynthetic yield of photosystem II (YII). In contrast, T. umbrina and T. jolithus recovered to 100% of the initial YII. HPLC and GC analysis of compatible solutes found highest proportions of erythritol in T. umbrina and mannitol/arabitol in T. jolithus. The lowest total compatible solute concentrations were detected in T. aurea, while the C/N ratio was highest in this species, indicative of nitrogen limitation. The prominent orange to red coloration of all Trentepohlia was due to extremely high carotenoid to Chl a ratio (15.9 in T. jolithus, 7.8 in T. aurea, and 6.6. in T. umbrina). Photosynthetic oxygen production was positive up to ~ 1500µmol photons m-2s-1 with the highest Pmax and alpha values in T. aurea. All strains showed a broad temperature tolerance with optima for gross photosynthesis between 20 and 35°C. The presented data suggest that all investigated Trentepohlia species are well adapted to their terrestrial lifestyle on exposed to sunlight on a vertical substrate with little water holding capacity. Nevertheless, the three Trentepohlia species differed concerning their desiccation tolerance and compatible solute concentrations. The lower compatible solute contents in T. aurea explain the incomplete recovery of YII after rehydration.

Age and Adaptive Changes in the Photosynthetic Apparatus of Leaves in Winter Green Herbaceous Plant Ajuga reptans L. in the Natural Conditions of the Taiga Zone

The formation and functioning of the photosynthetic apparatus (PSA) is under the general control of the plant organism and constantly changing environmental conditions. The authors studied age-related and adaptive changes in the pigment complex, spectral properties, and the state of photosystem II (PSII) leaves of partial shoots of a summer-winter-green herbaceous perennial Ajuga reptans L. (bugle) in connection with overwintering. Rosette leaves of a new generation appearing in May–June quickly accumulated photosynthetic pigments and formed PSA with a maximal quantum efficiency of PSII (Fv/Fm) and actual quantum yield of PSII (ФPSII). In autumn (September–October), the content of chlorophylls was 10 mg/g dry weight, the value of Fv/Fm was 0.8 rel. units, and the level of ФPSII at PAR 130 µmol quanta/(m2 s) was approximately 0.7 rel. units. Overwintered leaves contained half as much photosynthetic pigments, accumulated a significant amount of anthocyanins, and were characterized by low photochemical activity and a high level of de-epoxidation of pigments of the violaxanthin cycle. With the resumption of active vegetation of plants (May), a partial restoration of the pigment pool and the repair of PSA of overwintered leaves were noted, as evidenced by an increase in the quantum yield PSII. The completion of the leaf life cycle was accompanied by a decrease in ФPSII up to 0.5 rel. units and a sharp increase in the thermal dissipation of absorbed excitation energy (NPQ) to 0.9 rel. units. Seasonal changes of spectral properties of leaves and photochemical reflectance indices generally corresponded to the dynamics content pigments and the efficiency of the use of light in photosynthesis. The results obtained indicate a significant transformation of the structural and functional organization of PSA in the ontogeny of overwintering leaves. The genetically fixed property of winter green plants to preserve leaves is based on the ability of their PSA to restore functional activity after the shock impact of overwintering, which is facilitated by a complex of adaptive and protective mechanisms.

New foundations for the physical mechanism of variable chlorophyll a fluorescence. Quantum efficiency versus the light-adapted state of photosystem II.

Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons to fix CO2. Although the structure at atomic resolution and the basic photophysical and photochemical functions of PSII are well understood, many important questions remain. The activity of PSII in vitro and in vivo is routinely monitored by recording the induction kinetics of chlorophyll a fluorescence (ChlF). According to the 'mainstream' model, the rise from the minimum level (Fo) to the maximum (Fm) of ChlF of dark-adapted PSII reflects the closure of all functionally active reaction centers, and the Fv/Fm ratio is equated with the maximum photochemical quantum yield of PSII (where Fv=Fm-Fo). However, this model has never been free of controversies. Recent experimental data from a number of studies have confirmed that the first single-turnover saturating flash (STSF), which generates the closed state (PSIIC), produces F1<Fm, and have uncovered rate-limiting steps (Δτ1/2 half-waiting times) in the multi-STSF-induced F1-to-Fm increments that originate from the gradual formation of light-adapted charge-separated states (PSIIL) with significantly increased stability of charges compared to the PSIIC state that is elicited by a single STSF. All the data show that the interpretation of ChlF must be laid on new foundations. Here, we discuss the underlying physical mechanisms and the significance of structural/functional dynamics of PSII as reflected by ChlF and variations in the novel parameter Δτ1/2.

Potassium alleviates over-reduction of the photosynthetic electron transport chain and helps to maintain photosynthetic function under salt-stress.

Potassium ions enhance photosynthetic tolerance to salt stress. We hypothesized that potassium ions, by minimizing the trans-thylakoid proton diffusion potential difference, can alleviate over-reduction of the photosynthetic electron transport chain and maintain the functionality of the photosynthetic apparatus. This study investigated the effects of exogenous potassium on the transcription level and activity of proteins related to the photosynthetic electron-transport chain of tobacco seedlings under salt stress. Salt stress retarded the growth of seedlings and caused an outflow of potassium ions from the chloroplast. It also lowered qP (indicator of the oxidation state of QA , the primary quinone electron acceptor in Photosystem II (PSII) and YPSII (average photochemical yield of PSII in the light-adapted state) while increasing YNO+NF (nonregulatory energy dissipation in functional and nonfunctional PSII), accompanied by decreased expression of most light-harvesting, energy-transduction, and electron-transport genes. However, exogenous potassium prevented these effects due to NaCl. Interestingly, lincomycin (an inhibitor of the synthesis of chloroplast-encoded proteins in PSII) significantly diminished the alleviation effect of exogenous potassium on salt stress. We attribute the comprehensive NaCl-induced downregulation of transcription and photosynthetic activities to retrograde signaling induced by reactive oxygen species. There probably exist at least two types of retrograde signaling induced by reactive oxygen species, distinguished by their sensitivity to lincomycin. Exogenous potassium appears to exert its primary effect by ameliorating the trans-thylakoid proton diffusion potential difference via a potassium channel, thereby accelerating ATP synthesis and carbon assimilation, alleviating over-reduction of the photosynthetic electron transport chain, and maintaining the functionality of photosynthetic proteins.

Physiological Variables Influenced by 'Candidatus Liberibacter asiaticus' Infection in Two Citrus Species.

'Candidatus Liberibacter asiaticus' is the bacterium associated with the citrus disease known as huanglongbing (HLB). This study evaluated the influence of 'Ca. L. asiaticus' infection on a number of key plant physiological variables concerning photosynthesis, cell integrity, reactive oxygen species scavengers' activity, and osmoregulation of two different species of citrus-the pomelo Citrus maxima and the mandarin C. reticulata 'Tankan'-relative to their measured 'Ca. L. asiaticus' infection load. Results indicated that all measured physiological variables except soluble sugar were affected by increased 'Ca. L. asiaticus' infection titers, wherein the variety C. maxima proved overall more resistant than C. reticulata. 'Ca. L. asiaticus' infection was linked in both plants to decrease in chlorophyll concentration, cell membrane permeability, and malondialdehyde, as well as increased free proline and starch contents. Chlorophyll fluorescence measurements taken 9 months after grafting the mandarin C. reticulata with 'Ca. L. asiaticus' scions revealed a significant decrease in the photosynthesis variables maximum photochemical quantum yield of photosystem II (PSII), effective photochemical quantum yield of PSII, and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae, whereas nonphotochemical fluorescence quenching increased significantly; C. maxima plants, on the other hand, did not show significant differences until the 12th month from infection exposure. The variables superoxide dismutase, catalase, peroxidase, and soluble protein initially increased and later decreased. In addition, progression of 'Ca. L. asiaticus' replication in both citrus species was accompanied by rapid changes in three reactive oxygen species scavenging enzymes in C. maxima, while the pattern was different in C. reticulata. We hypothesize that the observed interspecific differences in physiological change are related to their relative resistance against 'Ca. L. asiaticus' infection. These results provide a scaffold for better describing the pathogenesis, selecting the most resistant breeds, or even validating pertaining omics research; ultimately, these detailed observations can facilitate the diagnosis of 'Ca. L. asiaticus' infection.

Chlorophyll fluorescence variation in two Mediterranean forest species over a 21-year drought treatment period.

The Mediterranean is among the regions predicted to be most affected by climate change due to rising temperatures and increasing frequency as well as intensity of extreme weather events, such as drought. These shifts in climatic conditions might lead to changes in species community composition by enabling the increase of drought-tolerant species at the expense of less tolerant ones. This hypothesis was tested in the current study using chlorophyll fluorescence data from a 21-year precipitation exclusion experiment in a Mediterranean forest for two co-dominant Quercus ilex and Phillyrea latifolia species with contrasting low and high levels of drought tolerance, respectively. The maximum potential quantum efficiency of photosystem II (PSII) (Fv/Fm), photochemical efficiency of PSII (yield), and non-photochemical quenching (NPQ) varied seasonally. Fv/Fm and NPQ levels were positively correlated with air temperature and Standardized Precipitation-Evapotranspiration Index (SPEI), whereas yield, which was greater under drought treatment was negatively associated with vapor pressure deficit and SPEI. Fv/Fm values showed similar increase in the two species over the 21-year study period regardless of treatment and in parallel to progressive warming. In contrast, yield values were higher in Q. ilex than in P. latifolia, while NPQ values were higher in P. latifolia than in Q. ilex. Notably, high yield values were also observed in the drought treated plots. Throughout the study, plants in the drought treated plots exhibited decreased basal area, leaf biomass, and aerial cover due to high stem mortality. In addition, a continuous increase in temperature was detected in summer and autumn, which might explain the observed increase in Fv/Fm values over the study period. Higher yield and lower NPQ detected in Q. ilex could be attributed to less competition for resources in the drought treated plots and acclimation of Q. ilex plants over the study period. Our results indicate that reduction in stem density could improve forest resilience to climate change-induced drought conditions.

Anatomical and physiological responses of Aechmea blanchetiana (Bromeliaceae) induced by silicon and sodium chloride stress during in vitro culture.

Salt stress is one of the most severe abiotic stresses affecting plant growth and development. The application of silicon (Si) is an alternative that can increase the tolerance of plants to various types of biotic and abiotic stresses. The objective was to evaluate salt stress's effect in vitro and Si's mitigation potential on Aechmea blanchetiana plants. For this purpose, plants already established in vitro were transferred to a culture medium with 0 or 14 µM of Si (CaSiO3). After growth for 30 days, a stationary liquid medium containing different concentrations of NaCl (0, 100, 200, or 300 µM) was added to the flasks. Anatomical and physiological analyses were performed after growth for 45 days. The plants cultivated with excess NaCl presented reduced root diameter and effective photochemical quantum yield of photosystem II (PSII) (ΦPSII) and increased non-photochemical dissipation of fluorescence (qN). Plants that grew with the presence of Si also had greater content of photosynthetic pigments and activity of the enzymes of the antioxidant system, as well as higher values of maximum quantum yield of PSII (FV/FM), photochemical dissipation coefficient of fluorescence (qP) and fresh weight bioaccumulation of roots and shoots. The anatomical, physiological and biochemical responses, and growth induced by Si mitigated the effect of salt stress on the A. blanchetiana plants cultivated in vitro, which can be partly explained by the tolerance of this species to grow in sandbank (Restinga) areas.

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.

Insights into melatonin-induced photosynthetic electron transport under low-temperature stress in cucumber.

In this study, the differences in chlorophyll fluorescence transient (OJIP) and modulated 820 nm reflection (MR820) of cucumber leaves were probed to demonstrate an insight into the precise influence of melatonin (MT) on cucumber photosystems under low temperature stress. We pre-treated cucumber seedlings with different levels of MT (0, 25, 50, 100, 200, and 400 μmol · L-1) before imposing low temperature stress (10 °C/6 °C). The results indicated that moderate concentrations of MT had a positive effect on the growth of low temperature-stressed cucumber seedlings. Under low temperature stress conditions, 100 μmol · L-1 (MT 100) improved the performance of the active photosystem II (PSII) reaction centers (PIabs), the oxygen evolving complex activity (OEC centers) and electron transport between PSII and PSI, mainly by decreasing the L-band, K-band, and G-band, but showed differences with different duration of low temperature stress. In addition, these indicators related to quantum yield and energy flux of PSII regulated by MT indicated that MT (MT 100) effectively protected the electron transport and energy distribution in the photosystem. According to the results of WO-I ≥ 1 and MR820 signals, MT also affected PSI activity. MT 100 decreased the minimal value of MR/MRO and the oxidation rate of plastocyanin (PC) and PSI reaction center (P700) (Vox ), while increased △MRslow/MRO and deoxidation rates of PC+ and P700 + (Vred ). The loss of the slow phase of MT 200 and MT 400-treated plants in the MR820 kinetics was due to the complete prevention of electron movement from PSII to re-reduce the PC+ and P700 +. These results suggest that appropriate MT concentration (100 μmol · L-1) can improve the photosynthetic performance of PS II and electron transport from primary quinone electron acceptor (QA) to secondary quinone electron acceptor (QB), promote the balance of energy distribution, strengthen the connectivity of PSI and PSII, improve the electron flow of PSII via QA to PC+ and P700 + from reaching PSI by regulating multiple sites of electron transport chain in photosynthesis, and increase the pool size and reduction rates of PSI in low temperature-stressed cucumber plants, All these modifications by MT 100 treatment promoted the photosynthetic electron transfer smoothly, and further restored the cucumber plant growth under low temperature stress. Therefore, we conclude that spraying MT at an appropriate concentration is beneficial for protecting the photosynthetic electron transport chain, while spraying high concentrations of MT has a negative effect on regulating the low temperature tolerance in cucumber.