Photosystem II Electron Transport Research Articles

Hormetic Response of Photosystem II Function Induced by Nontoxic Calcium Hydroxide Nanoparticles

In recent years, inorganic nanoparticles, including calcium hydroxide nanoparticles [Ca Ca(OH)2 NPs], have attracted significant interest for their ability to impact plant photosynthesis and boost agricultural productivity. In this study, the effects of 15 and 30 mg L−1 oleylamine-coated calcium hydroxide nanoparticles [Ca(OH)2@OAm NPs] on photosystem II (PSII) photochemistry were investigated on tomato plants at their growth irradiance (GI) (580 μmol photons m−2 s−1) and at high irradiance (HI) (1000 μmol photons m−2 s−1). Ca(OH)2@OAm NPs synthesized via a microwave-assisted method revealed a crystallite size of 25 nm with 34% w/w of oleylamine coater, a hydrodynamic size of 145 nm, and a ζ-potential of 4 mV. Compared with the control plants (sprayed with distilled water), PSII efficiency in tomato plants sprayed with Ca(OH)2@OAm NPs declined as soon as 90 min after the spray, accompanied by a higher excess excitation energy at PSII. Nevertheless, after 72 h, the effective quantum yield of PSII electron transport (ΦPSII) in tomato plants sprayed with Ca(OH)2@OAm NPs enhanced due to both an increase in the fraction of open PSII reaction centers (qp) and to the enhancement in the excitation capture efficiency (Fv’/Fm’) of these centers. However, the decrease at the same time in non-photochemical quenching (NPQ) resulted in an increased generation of reactive oxygen species (ROS). It can be concluded that Ca(OH)2@OAm NPs, by effectively regulating the non-photochemical quenching (NPQ) mechanism, enhanced the electron transport rate (ETR) and decreased the excess excitation energy in tomato leaves. The delay in the enhancement of PSII photochemistry by the calcium hydroxide NPs was less at the GI than at the HI. The enhancement of PSII function by calcium hydroxide NPs is suggested to be triggered by the NPQ mechanism that intensifies ROS generation, which is considered to be beneficial. Calcium hydroxide nanoparticles, in less than 72 h, activated a ROS regulatory network of light energy partitioning signaling that enhanced PSII function. Therefore, synthesized Ca(OH)2@OAm NPs could potentially be used as photosynthetic biostimulants to enhance crop yields, pending further testing on other plant species.

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.

Mechanistic Approach on Melatonin-Induced Hormesis of Photosystem II Function in the Medicinal Plant Mentha spicata

Melatonin (MT) is considered a new plant hormone having a universal distribution from prokaryotic bacteria to higher plants. It has been characterized as an antistress molecule playing a positive role in the acclimation of plants to stress conditions, but its impact on plants under non-stressed conditions is not well understood. In the current research, we evaluated the impact of MT application (10 and 100 μM) on photosystem II (PSII) function, reactive oxygen species (ROS) generation, and chlorophyll content on mint (Mentha spicata L.) plants in order to elucidate the molecular mechanism of MT action on the photosynthetic electron transport process that under non-stressed conditions is still unclear. Seventy-two hours after the foliar spray of mint plants with 100 μM MT, the improved chlorophyll content imported a higher amount of light energy capture, which caused a 6% increase in the quantum yield of PSII photochemistry (ΦPSII) and electron transport rate (ETR). Nevertheless, the spray with 100 μM MT reduced the efficiency of the oxygen-evolving complex (OEC), causing donor-side photoinhibition, with a simultaneous slight increase in ROS. Even so, the application of 100 μM MT decreased the excess excitation energy at PSII implying superior PSII efficiency. The decreased excitation pressure at PSII, after 100 μM MT foliar spray, suggests that MT induced stomatal closure through ROS production. The response of ΦPSII to MT spray corresponds to a J-shaped hormetic curve, with ΦPSII enhancement by 100 μM MT. It is suggested that the hormetic stimulation of PSII functionality was triggered by the non-photochemical quenching (NPQ) mechanism that stimulated ROS production, which enhanced the photosynthetic function. It is concluded that MT molecules can be used under both stress and non-stressed conditions as photosynthetic biostimulants for enhancing crop yields.

Impact of Salinity, Elevated Temperature, and Their Interaction with the Photosynthetic Efficiency of Halophyte Crop Chenopodium quinoa Willd

Chenopodium quinoa is a genetically diverse crop that can adapt to a wide range of environments, including temperatures and salinities. However, only a few studies have assessed the combined effects of two or more environmental factors on C. quinoa. Here, we investigated the effects of salinity (300 mM NaCl), elevated temperature (35 °C), and their interaction with growth, water–salt balance, the efficiency of photosystem II (PSII), the activity of cyclic electron transport (CET) around photosystem I (PSI), Rubisco and PEPC enzyme content, and the expression of photosynthetic genes. We found that elevated temperature did not decrease the biomass but caused a significant increase in the water and potassium content of C. quinoa leaves. The decrease in PSII efficiency under elevated temperature was accompanied by an increase in the expression of genes encoding the components of PSII (psbA) and linear electron transport (FDI), as well as the main photosynthetic protein Rubisco (rbcL). Moreover, the strongest effect was induced by the combined effect of elevated temperature and salinity, which induced high oxidative stress (a threefold increase in MDA), a threefold decrease in the biomass, a twofold decrease in PSII efficiency, and a two- to eightfold decrease in the expression of the photosynthetic genes psbA, FDI, and rbcL. PSI was more tolerant to all forms of stress; however, the combined effect of elevated temperature and salinity downregulated the expression of PGR5 and FNR1, which may diminish the role of PGR5/PGRL1-dependent CET in favor of the NDH-dependent CET of PSI. The obtained data on the functioning of photosystems and the expression of photosynthetic genes under combined stress (elevated temperature and salinity) can make a significant contribution to understanding the mechanisms of tolerance of C. quinoa to multiple stresses under climate change conditions.

Anthropogenic Dusts Influence Leaf Anatomical and Eco-Physiological Traits of Black Locust (Robinia pseudoacacia L.) Growing on Vesuvius Volcano

In the Mediterranean region, some areas of the Vesuvius National Park (southern Italy) are subjected to a severe anthropogenic impact, especially during spring and summer seasons. The continuous trampling of tourists and buses leads to the formation of “dust-clouds”, exposing plants, especially along the paths, to a great deposition of powder particles on leaves. The aim of this study was to analyze if the dust deposition induces changes in leaf morpho-anatomical and eco-physiological traits of the alien, invasive, species Robinia pseudoacacia L., with particular attention to the photosystem II (PSII) efficiency. We selected plants located near the paths with a high deposition of dust (HD) and plants far away from the paths (low deposition, LD), and tested them over three dates along summer. We analyzed PSII photochemistry, photosynthetic pigments content, and leaf functional (e.g., relative water content and leaf dry matter content) and morpho-anatomical traits (e.g., parenchyma thickness, mesophyll density). HD plants presented a more efficient PSII activity, indicated by the higher quantum yield of PSII electron transport (FPSII) (9%) and electron transport rate (ETR) (38%) in the end of July. Dust deposition also reversibly altered photosynthetic pigments concentration and some lamina traits, adjustable in the short-term (e.g., intercellular spaces and phenolics distribution). We hypothesize that HD leaves were shielded by dusts which would protect their photosynthetic apparatus from the excess of light.

Natural plant inducer 2-Amino-3-Methylhexanoic acid protects physiological activity against high-temperature damage to tea (Camellia sinensis)

Increasing frequency of extreme summer high-temperature (HT) has become one of the most important limiting factors affecting tea production in China. 2-Amino-3-methylhexanoic acid (AMHA) is a unique natural plant inducer against HT stresses. In this study, the field efficacy and physiological mechanism of AMHA for improving tea plant resistance against HT in the summer was studied using the temperature-sensitive tea cultivar “Longjing 43″. It was observed that field-grown tea plants treated with 100 or 1000 nM AMHA maintained erect and normal stems and foliage under HT stress. The optimal concentration of AMHA for counteracting HT in field-grown tea plants was 100 nM. At three days after treatment (DAT), AMHA at 100 nM increased net photosynthetic rate Pn by 199% and stomatal conductance Gs of tea plants by 215% of tea plants. The fast chlorophyll a rise kinetics and JIP-test analysis further confirmed that AMHA improved the overall photosynthetic activity of photosystem II (PSII) mainly through enhancing the fraction of active oxygen evolving complex (OEC) centers, PSII energetic connectivity and electron transport efficiency. PIABS reflecting PSII overall activity in 100 nM AMHA-treated tea plants was increased by 95%, 51%, 39%, and 48% at 3, 5, 7, and 14 DAT, respectively, compared to mock. At the same dose, AMHA also increased osmotic adjustments for the rapid accumulation of amino acids and soluble sugars, which increased by 21% and 145% at 7 DAT, respectively. In addition, AMHA significantly enhanced the activities of peroxidase (POD), catalse (CAT) and superoxide dismutase (SOD), reducing oxidative damage on membrane lipids. Even at 14 DAT, 100 nM AMHA led to 50%, 75% and 19% increase in POD, CAT and SOD activity in tea plants relative to mock, respectively. Clearly, exogenous application of AMHA facilitated HT resistance by alleviating physiological damage in field-grown tea plants due to improved photosynthetic performance, osmotic adjustments and antioxidant enzyme activities.

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.

Photosynthetic activity and water use efficiency of Salvia verbenaca L. under elevated CO2 and water‐deficit conditions

AbstractInvestigating the combined effects of elevated CO2 concentration and water‐deficit on weed plants is crucial to gaining a thorough understanding of plant performance and modifying agricultural processes under changing climate conditions. This study examined the effect of elevated CO2 concentration and water‐deficit conditions on leaf gas exchange, water use efficiency, carboxylation efficiency and the photosystem II (PSII) activity of two Salvia verbenaca L., varieties. These varieties were grown under two CO2 concentrations (ambient conditions of 400 ppm and elevated conditions of 700 ppm) and two water regimes (well‐watered [100% field capacity] and water‐deficit conditions [60% field capacity]) in laboratory growth chambers. For 12 days, at 2‐day intervals, (i) leaf gas exchange parameters (photosynthesis rate, stomatal conductance, transpiration rate (E) and intercellular CO2 concentration (Ci)), (ii) water use efficiency (WUE), (iii) intrinsic water use efficiency (IWUE), (iv) instantaneous carboxylation efficiency and (v) PSII activity (fluorescence, quantum yield of PSII, photochemical efficiency of PSII, photochemical quenching and photosynthetic electron transport) were measured. Water‐deficit conditions had negative effects on studied parameters of both varieties, whereas elevated CO2 concentration had positive effects on the gas exchange, water use efficiency and PSII activity of both. Salvia verbenaca varieties grown under water‐deficit conditions from Day 0 to Day 5 showed a partial recovery in most of the parameters when the resumption of the well‐watered regime was reinstituted on Day 6. Salvia verbenaca varieties grown under water‐deficit conditions were re‐watered on day 6 and indicated a partial recovery in all the parameters. A comparison of the two varieties showed that var. vernalis recorded higher values of gas exchange, quantum yield of PSII and photochemical efficiency of PSII than var. verbenaca, but the water use efficiency of var. verbenaca was higher than that of var. vernalis. These differences serve to illustrate the complexity of such studies and suggest that a detailed understanding of the nature of weed infestations is essential if optimum management control is to be practiced. Elevated CO2 concentration mitigated the adverse effects of water‐deficit conditions and thereby enhanced the adaptive mechanism of this weed by improving its water use efficiency. It is thus likely that S. verbenaca has the potential to take advantage of climate change by increasing its relative competitiveness with other plants in drought‐prone areas, suggesting that it could significantly expand its invasive range under such conditions.

Photosynthesis and Leaf Traits of Tree Species Influenced by Green Manure Associated with Soil Treatments

ABSTRACT The use of green manure (GM) associated with soil decompaction treatments has shown to be an alternative to improve soil attributes. This study associated the use of GM with tree species nitrogen-fixing and non-nitrogen-fixing (N-fixing and non-N-fixing, respectively), hypothesizing that non-N-fixing species are more beneficial by GM. The experiment was carried out in a disabled clay extraction deposit. Previously, chemical analysis and mechanical resistance to soil penetration was registered in order to characterize the initial soil properties of the experimental site. The soil treatments were scarification (SCA), scarification + harrowing (SCAH), and subsoiling + harrowing (SUBH) with presence/absence of GM (+GM and -GM, respectively) and four native tree species, N-fixing (Dalbergia ecastophyllum (L.) Taub., Inga laurina (Sw.) Willd., and non-N-fixing (Psidium cattleianum Sabine and Schinus terebinthifolius Raddi), were planted. Growth and leaf traits were measured after 6 and 12 months. The response patterns of leaf attributes were well defined in the non-N-fixing species. Over time, GM increases the level of chlorophyll (Chl) in N-fixing species and, consequently, improves the maximum quantum yield for photosystem II (PSII) primary photochemistry (φPo), quantum yield for PSII electron transport (φEo), the number of active reaction centers (RC/CSo), and the performance index (PI/abs). GM increased these same photochemical parameters in non-N-fixing species after 12 months. The samples cultivated in the SCA and SUBH treatments showed a better performance. The species responded in different ways to GM, which improved the leaf traits in non-N-fixing species. GM improved the leaf traits of N-fixing species. The positive effect of GM over time give us a good alternative for fertilization in revegetation areas. Furthermore, our findings shown that the subsoiling promotes the highest soil decompaction, which associated with GM, can be efficiently used in projects of revegetation of degraded areas.

Epigenetic and Physiological Responses to Varying Root-Zone Temperatures in Greenhouse Rocket.

Greenhouse production of baby leaf vegetables grown in hydroponic floating trays has become extremely popular in recent years. Rocket (Eruca sativa Mill.) can grow in temperatures varying between 10 and 20 °C; nevertheless, a root-zone temperature (RZT) range of 18–23 °C is considered optimal for high productivity, photosynthesis, and production of metabolites. Maintaining such temperatures in winter raises production costs and prevents sustainability. In this study, we tested the impact of lower RZT on plants’ status and recorded their responses while providing energy for heating using photovoltaic solar panels. We used three hydroponic tanks for cultivation; a non-heated (control) tank (12 °C) and two heated tanks; a solar panel-powered one (16 °C) and a public grid-powered one (22 °C). Methylation-sensitive amplified polymorphisms (MSAP) analysis of global methylation profiles and chlorophyll fluorescence analysis were employed to assess methylation and physiology levels of rocket leaves. We found that there is demethylation at 16 °C RZT in comparison to 22 °C RZT. Reduction of temperature at 12 °C did not reduce methylation levels further but rather increased them. Furthermore, at 16 °C, the effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII) was significantly higher, with a higher PSII electron transport rate (ETR) and a significantly decreased non-regulated energy loss (ΦΝO), suggesting a better light energy use by rocket plants with higher photosynthetic performance. ΦPSII was significantly negatively correlated with DNA methylation levels. Our results show that at 16 °C RZT, where plants grow efficiently without being affected by the cold, DNA methylation and photosynthesis apparatus systems are altered. These findings corroborate previous results where hydroponic production of rocket at RZT of 16 °C is accompanied by sufficient yield showing that rocket can effectively grow in suboptimal yet sustainable root-zone temperatures.

Responses of Linear and Cyclic Electron Flow to Nitrogen Stress in an N-Sensitive Species Panax notoginseng.

Nitrogen (N) is a primary factor limiting leaf photosynthesis. However, the mechanism of N-stress-driven photoinhibition of the photosystem I (PSI) and photosystem II (PSII) is still unclear in the N-sensitive species such as Panax notoginseng, and thus the role of electron transport in PSII and PSI photoinhibition needs to be further understood. We comparatively analyzed photosystem activity, photosynthetic rate, excitation energy distribution, electron transport, OJIP kinetic curve, P700 dark reduction, and antioxidant enzyme activities in low N (LN), moderate N (MN), and high N (HN) leaves treated with linear electron flow (LEF) inhibitor [3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)] and cyclic electron flow (CEF) inhibitor (methyl viologen, MV). The results showed that the increased application of N fertilizer significantly enhance leaf N contents and specific leaf N (SLN). Net photosynthetic rate (Pn) was lower in HN and LN plants than in MN ones. Maximum photochemistry efficiency of PSII (Fv/Fm), maximum photo-oxidation P700+ (Pm), electron transport rate of PSI (ETRI), electron transport rate of PSII (ETRII), and plastoquinone (PQ) pool size were lower in the LN plants. More importantly, K phase and CEF were higher in the LN plants. Additionally, there was not a significant difference in the activity of antioxidant enzyme between the MV- and H2O-treated plants. The results obtained suggest that the lower LEF leads to the hindrance of the formation of ΔpH and ATP in LN plants, thereby damaging the donor side of the PSII oxygen-evolving complex (OEC). The over-reduction of PSI acceptor side is the main cause of PSI photoinhibition under LN condition. Higher CEF and antioxidant enzyme activity not only protected PSI from photodamage but also slowed down the damage rate of PSII in P. notoginseng grown under LN.

Multiyear CO2 elevation improves PSII efficiency in a japonica rice cultivar

AbstractResponses of crop photosystem activity to the short‐term elevation of carbon dioxide concentration ([CO2]) have been studied extensively. However, long‐term effects of elevated [CO2] (e[CO2]) over multiple cropping generations have received little attention. Using open‐top chambers (OTCs), we set up ambient [CO2] (CK) and two multigeneration e[CO2] treatments during rice (Oryza sativa L.) growing seasons in 2016–2019: a stepwise increase (SI) up to +160 μmol mol−1 in 2019 and a constant increase (CI) of +200 μmol mol−1 above CK over four generations. Beginning in 2017, grains harvested from the previous year in different [CO2] treatments were used as seeds. Seedlings derived from CK OTCs in 2018 were transplanted into both SI and CI OTCs in 2019 to simulate single‐generation abrupt elevation of [CO2]. We measured the diurnal changes in photosystem II (PSII) functionality of leaves in 2019. Single‐generation elevation of [CO2] had no effects on PSII efficiency. However, a stepwise increase of [CO2] over four generations significantly enhanced predawn maximum photochemical efficiency of PSII (Fv/Fm), the efficiency that trapped exciton moves an electron beyond QA− (ψo), and the quantum yield of electron transport (φEo) in PSII at jointing stage. The constant increase of [CO2] over four generations dramatically improved predawn Fv/Fm, ψo, φEo, perform index, and reaction center at grain‐filling stage. Moreover, the multigeneration elevation of [CO2] weakened the depression of Fv/Fm relative to single‐generation elevation at midday. All of these results indicated that e[CO2] had transgenerational effects on PSII functionality.

Heat-induced down-regulation of photosystem II protects photosystem I in honeysuckle (Lonicera japonica).

Honeysuckle (Lonicera japonica Thunb.) is a traditional medicinal plant in China which is often threatened by high temperature at midday during summer. Heat-induced effects on the photosynthetic apparatus in honeysuckle are associated with a depression of the photosystem II (PSII) photochemical efficiency. However, very limited information is available on regulation of photosynthetic electron flow in PSI photoprotection in heat-stressed honeysuckle. Simultaneous analyses of chlorophyll fluorescence and the change in absorbance of P700 showed that energy transformation and electron transfer activity in PSII decreased under heat stress, but the fraction of photo-oxidizable PSI (Pm) remained stable. With treatments at 38 and 42°C, the photochemical electron transport in PSII was suppressed, whereas the cyclic electron flow (CEF) around PSI was induced. In addition, the levels of high energy state quenching (qE) and P700 oxidation increased significantly with increasing temperature. However, a decline of qE in antimycin A (AA)- or 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated leaves after heat treatment was observed, while P700 oxidation decreased only in the presence of AA. The results indicate that heat-induced inhibition of PSII and induction of CEF cooperatively protect PSI from ROS damages through moderate down-regulation of photosynthetic electron flow from PSII to PSI.

Hemolytic Activity in Relation to the Photosynthetic System in Chattonella marina and Chattonella ovata.

Chattonella species, C. marina and C. ovata, are harmful raphidophycean flagellates known to have hemolytic effects on many marine organisms and resulting in massive ecological damage worldwide. However, knowledge of the toxigenic mechanism of these ichthyotoxic flagellates is still limited. Light was reported to be responsible for the hemolytic activity (HA) of Chattonella species. Therefore, the response of photoprotective, photosynthetic accessory pigments, the photosystem II (PSII) electron transport chain, as well as HA were investigated in non-axenic C. marina and C. ovata cultures under variable environmental conditions (light, iron and addition of photosynthetic inhibitors). HA and hydrogen peroxide (H2O2) were quantified using erythrocytes and pHPA assay. Results confirmed that% HA of Chattonella was initiated by light, but was not always elicited during cell division. Exponential growth of C. marina and C. ovata under the light over 100 µmol m−2 s−1 or iron-sufficient conditions elicited high hemolytic activity. Inhibitors of PSII reduced the HA of C. marina, but had no effect on C. ovata. The toxicological response indicated that HA in Chattonella was not associated with the photoprotective system, i.e., xanthophyll cycle and regulation of reactive oxygen species, nor the PSII electron transport chain, but most likely occurred during energy transport through the light-harvesting antenna pigments. A positive, highly significant relationship between HA and chlorophyll (chl) biosynthesis pigments, especially chl c2 and chl a, in both species, indicated that hemolytic toxin may be generated during electron/energy transfer through the chl c2 biosynthesis pathway.

Analysis of the changes of electron transfer and heterogeneity of photosystem II in Deg1-reduced Arabidopsis plants.

Deg1 protease functions in protease and chaperone of PSII complex components, but few works were performed to study the effects of Deg1 on electron transport activities on the donor and acceptor side of PSII and its correlation with the photoprotection of PSII during photoinhibition. Therefore, we performed systematic and comprehensive investigations of electron transfers on the donor and acceptor sides of photosystem II (PSII) in the Deg1-reduced transgenic lines deg1-2 and deg1-4. Both the maximal quantum efficiency of PSII photochemistry (Fv/Fm) and the actual PSII efficiency (ΦPSII) decreased significantly in the transgenic plants. Increases in nonphotochemical quenching (NPQ) and the dissipated energy flux per reaction center (DI0/RC) were also shown in the transgenic plants. Along with the decreased D1, CP47, and CP43 content, these results suggested photoinhibition under growth light conditions in transgenic plants. Decreased Deg1 caused inhibition of electron transfer on the PSII reducing side, leading to a decline in the number of QB-reducing centers and accumulation of QB-nonreducing centers. The Tm of the Q band shifted from 5.7 °C in the wild-type plant to 10.4 °C and 14.2 °C in the deg1-2 and deg1-4 plants, respectively, indicating an increase in the stability of S2QA¯ in transgenic plants. PSIIα in the transgenic plants largely reduced, while PSIIβ and PSIIγ increased with the decline in the Deg1 levels in transgenic plants suggesting PSIIα centers gradually converted into PSIIβ and PSIIγ centers in the transgenic plants. Besides, the connectivity of PSIIα and PSIIβ was downregulated in transgenic plants. Our results reveal that downregulation of Deg1 protein levels induced photoinhibition in transgenic plants, leading to loss of PSII activities on both the donor and acceptor sides in transgenic plants. These results give a new insight into the regulation role of Deg1 in PSII electron transport.

Tocopherol controls D1 amino acid oxidation by oxygen radicals in Photosystem II

Photosystem II (PSII) is an intrinsic membrane protein complex that functions as a light-driven water:plastoquinone oxidoreductase in oxygenic photosynthesis. Electron transport in PSII is associated with formation of reactive oxygen species (ROS) responsible for oxidative modifications of PSII proteins. In this study, oxidative modifications of the D1 and D2 proteins by the superoxide anion (O2•-) and the hydroxyl (HO•) radicals were studied in WT and a tocopherol cyclase (vte1) mutant, which is deficient in the lipid-soluble antioxidant α-tocopherol. In the absence of this antioxidant, high-resolution tandem mass spectrometry was used to identify oxidation of D1:130E to hydroxyglutamic acid by O2•- at the PheoD1 site. Additionally, D1:246Y was modified to either tyrosine hydroperoxide or dihydroxyphenylalanine by O2•- and HO•, respectively, in the vicinity of the nonheme iron. We propose that α-tocopherol is localized near PheoD1 and the nonheme iron, with its chromanol head exposed to the lipid-water interface. This helps to prevent oxidative modification of the amino acid's hydrogen that is bonded to PheoD1 and the nonheme iron (via bicarbonate), and thus protects electron transport in PSII from ROS damage.

Spectral Composition of Light Affects Sensitivity to UV-B and Photoinhibition in Cucumber.

Ultraviolet B (UV-B) (280–315 nm) and ultraviolet A (UV-A) (315–400 nm) radiation comprise small portions of the solar radiation but regulate many aspects of plant development, physiology and metabolism. Until now, how plants respond to UV-B in the presence of different light qualities is poorly understood. This study aimed to assess the effects of a low UV-B dose (0.912 ± 0.074 kJ m–2 day–1, at a 6 h daily UV exposure) in combination with four light treatments (blue, green, red and broadband white at 210 μmol m–2 s–1 Photosynthetically active radiation [PAR]) on morphological and physiological responses of cucumber (Cucumis sativus cv. “Lausanna RZ F1”). We explored the effects of light quality backgrounds on plant morphology, leaf gas exchange, chlorophyll fluorescence, epidermal pigment accumulation, and on acclimation ability to saturating light intensity. Our results showed that supplementary UV-B significantly decreased biomass accumulation in the presence of broad band white, blue and green light, but not under red light. UV-B also reduced the photosynthetic efficiency of CO2 fixation (α) when combined with blue light. These plants, despite showing high accumulation of anthocyanins, were unable to cope with saturating light conditions. No significant effects of UV-B in combination with green light were observed for gas exchange and chlorophyll fluorescence parameters, but supplementary UV-B significantly increased chlorophyll and flavonol contents in the leaf epidermis. Plants grown under red light and UV-B significantly increased maximum photosynthetic rate and dark respiration compared to pure red light. Additionally, red and UV-B treated plants exposed to saturating light intensity showed higher quantum yield of photosystem II (PSII), fraction of open PSII centres and electron transport rate and showed no effect on the apparent maximum quantum efficiency of PSII photochemistry (Fv/Fm) or non-photochemical quenching, in contrast to solely red-light conditions. These findings provide new insights into how plants respond to UV-B radiation in the presence of different light spectra.

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.

Chlorophyll a Fluorescence Transient and 2-Dimensional Electrophoresis Analyses Reveal Response Characteristics of Photosynthesis to Heat Stress in Malus. 'Prairifire'.

Flowering crabapples are a series of precious ornamental woody plants. However, their growth and development are inhibited in the subtropical regions due to the weak photosynthesis under high-temperature environment in the summer. Chlorophyll a fluorescence transient and 2-dimensional electrophoresis (2-DE) analyses were conducted to investigate the response characteristics of photosynthesis under simulated 38 °C heat stress in leaves of Malus. ‘Prairifire’, a spring-red leaf cultivar of flowering crabapple with strong thermal adaptability. In the present study, the net photosynthetic rate (Pn) was significantly decreased during the heat shock process, which showed a similar trend to the stomatal conductance (Gs), indicating a sensitive stomatal behavior to heat stress. Moreover, an efficient reaction center in photosystem II (PSII), and a functionally intact oxygen-evolving complex (OEC) conferred strong photosynthetic adaptability under heat stress. The higher level of transketolase (TK) under 48-h heat shock treatment was considered a protective mechanism of photosynthetic apparatus. However, heat stress inhibited the functions of light harvesting complex II (LHCII), electron transport in PSII, and the levels of key enzymes in the Calvin cycle, which were considered as the reasons causing an increase in the proportion of non-stomatal restrictions.

Eco-Physiological Screening of Different Tomato Genotypes in Response to High Temperatures: A Combined Field-to-Laboratory Approach.

High temperatures represent a limitation for growth and development of many crop species. Several studies have demonstrated that the yield reduction of tomato under high temperatures and drought is mainly due to a photosynthetic decline. In this paper, a set of 15 tomato genotypes were screened for tolerance to elevated temperatures by cultivating plants under plastic walk-in tunnels. To assess the potential tolerance of tomato genotypes to high temperatures, measurements of chlorophyll fluorescence, pigments content and leaf functional traits have been carried out together with the evaluation of the final yields. Based on the greenhouse trials, a group of eight putative heat-sensitive and heat-tolerant tomato genotypes was selected for laboratory experiments aimed at investigating the effects of short-term high temperatures treatments in controlled conditions. The chlorophyll fluorescence induction kinetics were recorded on detached leaves treated for 60 min at 35 °C or at 45 °C. The last treatment significantly affected the photosystem II (PSII) photochemical efficiency (namely maximum PSII quantum efficiency, Fv/Fm, and quantum yield of PSII electron transport, ΦPSII) and the non-photochemical quenching (NPQ) in the majority of genotypes. The short-term heat shock treatments also led to significant differences in the shape of the slow Kautsky kinetics and its significant time points (chlorophyll fluorescence levels minimum O, peak P, semi-steady state S, maximum M, terminal steady state T) compared to the control, demonstrating heat shock-induced changes in PSII functionality. Genotypes potentially tolerant to high temperatures have been identified. Our findings support the idea that chlorophyll fluorescence parameters (i.e., ΦPSII or NPQ) and some leaf functional traits may be used as a tool to detect high temperatures-tolerant tomato cultivars.