Decrease In Electron Transport Rate Research Articles

Use of zinc quantum dot biochar and AMF for alleviation of Cd stress in maize: Regulation of physiological and biochemical attributes

In different heavy metals, cadmium is one of the acute toxins. It also interferes with various physiological processes in plants, such as photosynthesis, respiration, and enzyme activity. To improve the growth of plants in Cd stress, arbuscular mycorrhizae can play an imperative role. On the other hand, the use of quantum dots technology is also gaining the attention of scientists. That's why the current study investigates the effectiveness of using zinc quantum dot biochar (ZQDB) and arbuscular mycorrhizal fungi (AMF) in alleviating cadmium (Cd) stress in maize plants. The results showed that AMF+ZQDB performed significantly best at the highest level of 5Cd (5mgCd/kg soil) for enhancement in plant height (28.20 %), shoot dry weight (48.78 %), chlorophyll a (32.45 %), chlorophyll b (44.03 %) and total chlorophyll (22.02 %) compared to control (NoAMF+NoZQDB). A significant enhancement in photosynthetic rate (12.95 %) and transpiration rate (39.99 %) concentration of carbon dioxide also validated the effectiveness of AMF+ZQDB over control (NoAMF+NoZQDB) at 5Cd. For photochemical quenching decrease was 25.1 %, 2.98 %, 20.7 %, and 20.3 %, respectively, compared to the control group at 5Cd where NoAMF+NoZQDB, AMF, ZQDB, and AMF+ZQDB treatments were applied. AMF+ZQDB treatment of 5Cd showed a 30.22 % decrease in electron transport rate and 34.67 % decrease in non-photochemical quenching compared to control (NoAMF+NoZQDB). In conclusion, AMF + ZQDB is an effective amendment for alleviating Cd stress in maize by regulating biochemical and physiological attributes. Further investigations are recommended at the field level using different crops to validate the effectiveness of AMF + ZQDB as a promising amendment for minimizing Cd toxicity.

Eco‐Physiological Constraints of Deep Soil Desiccation in Semiarid Tree Plantations

AbstractDeep soil water, defined here as the soil water below a certain depth and not recharged by precipitation in one growing season, plays a critical role in maintaining eco‐physiological functioning in thick‐vadose‐zone regions. However, science‐based evidence remains limited on how and the extent to which deep soil desiccation (DSD) affects the eco‐physiological features of apple trees. Here, we improve a process‐based model to disentangle trees' transpiration and photosynthesis responses to precipitation and DSD below 200 cm (DSD200) on the semiarid Loess Plateau. We defined four DSD200 scenarios: 60%–70% of field capacity (FC) as a control, 50%–60%, 40%–50%, and 30%–40% of FC representing mild (MID), moderate (MOD) and severe (SED) desiccation, respectively; and five precipitation scenarios: extremely dry (285.78 mm), dry (392 mm), normal (457.72 mm), wet (524.96 mm), and extremely wet years (630.44 mm). We found that the stomatal conductance, net photosynthetic rate and transpiration under MOD and SED decreased significantly (p < 0.05), independent of precipitation years, indicating clear stomatal limitation induced by DSD200. This phenomenon was greatly enhanced in extremely dry years, with these variables decreasing on average by 29%, 36%, and 37%, respectively. Furthermore, SED resulted in non‐stomatal limitation with a great decrease in electron transport rates (Jmax) and maximum carboxylation rates (VCmax). Jmax and VCmax decreased by 40% and 26%, respectively, on average, in extremely dry years. These findings indicate that the combination of severe meteorological drought and deep soil drought resulted in non‐stomatal limitation for the apple trees. Persistent non‐stomatal limitation may be an important mechanism causing tree mortality in semiarid loess regions.

Seasonal dynamics of the photosynthetic apparatus parameters of upright and prostrate pine species using the example of Pinus sibirica and P. pumila

The seed progeny photosynthesis intensity and the photosynthetic apparatus state of genus Pinus species of different life forms were studied in seasonal dynamics: P. sibirica is an upright tree, P. pumila is a prostrate species. The seed progeny age was twelve years old. The photosynthesis intensity of both species increased from May to July, was maximum in July and amounted to 9.2 mg CO2 mg-1 fr. m. h-1 in Siberian stone pine and 7.8 mg CO2 mg-1 fr. m. h-1 in Siberian dwarf pine, in August the assimilation activity began to decrease in both species. Studies have shown that in summer P sibirica had photosynthesis values 15-20 % higher than those of P pumila. In the winter months, the chlorophyll content did not differ significantly between the species; at the same time, during the growing season, Siberian stone pine contained more photosynthetic pigments than Siberian dwarf pine. An assessment of the photosynthetic apparatus functional activity showed that the seasonal dynamics of the maximum quantum yield photochemistry (Fv/Fm) of photosystem II (PSII), the real quantum yield of PS II ^psII) in both species had a similar character: Fv/ Fm sharply decreased from November to February, increased from March to June, reached its maximum values and practically did not change in June-Septem-ber and decreased in October. The electron transport rate (ETR) was minimal from November to April, increased sharply in May, and had maximum values in May, June, and July. In August, a significant decrease in ETR was observed. The rate of electron transport significantly differed between species in May-September and was significantly higher in Siberian stone pine. The shown differences between the P sibirica - upright species, compared to P. pumila - prostrate species, indicate species specific physiological parameters.

Differential responses to two heatwave intensities in a Mediterranean citrus orchard are identified by combining measurements of fluorescence and carbonyl sulfide (COS) and CO2 uptake

The impact of extreme climate episodes such as heatwaves on plants physiological functioning and survival may depend on the event intensity, which requires quantification. We unraveled the distinct impacts of intense (HW) and intermediate (INT) heatwave days on carbon uptake, and the underlying changes in the photosynthetic system, in a Mediterranean citrus orchard using leaf active (pulse amplitude modulation; PAM) and canopy level passive (sun-induced; SIF) fluorescence measurements, together with CO2 , water vapor, and carbonyl sulfide (COS) exchange measurements. Compared to normal (N) days, gross CO2 uptake fluxes (gross primary production, GPP) were significantly reduced during HW days, but only slightly decreased during INT days. By contrast, COS uptake flux and SIFA (at 760nm) decreased during both HW and INT days, which was reflected in leaf internal CO2 concentrations and in nonphotochemical quenching, respectively. Intense (HW) heatwave conditions also resulted in a substantial decrease in electron transport rates, measured using leaf-scale fluorescence, and an increase in the fractional energy consumption in photorespiration. Using the combined proxy approach, we demonstrate a differential ecosystem response to different heatwave intensities, which allows the trees to preserve carbon assimilation during INT days but not during HW days.

Higher Atmospheric CO2 Levels Favor C3 Plants Over C4 Plants in Utilizing Ammonium as a Nitrogen Source.

Photosynthesis of wheat and maize declined when grown with NH4+ as a nitrogen (N) source at ambient CO2 concentration compared to those grown with a mixture of NO3– and NH4+, or NO3– as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO2 concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO2 levels. Hydroponic experiments were carried out using a C3 plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C4 plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO3– (NN), sole NH4+ (AN) and a mixture of both NO3– and NH4+ (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO2 (Ca) concentration could be maintained: 280 μmol mol–1 (representing the pre-Industrial Revolution CO2 concentration of the 18th century), 400 μmol mol–1 (present level) and 550 μmol mol–1 (representing the anticipated futuristic concentration in 2050). Under AN, the decrease in net photosynthetic rate (Pn) was attributed to a reduction in the maximum RuBP-regeneration rate, which then caused reductions in the maximum Rubisco-carboxylation rates for both species. Decreases in electron transport rate, reduction of electron flux to the photosynthetic carbon [Je(PCR)] and electron flux for photorespiratory carbon oxidation [Je(PCO)] were also observed under AN for both species. However, the intercellular (Ci) and chloroplast (Cc) CO2 concentration increased with increasing atmospheric CO2 in C3 wheat but not in C4 maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of Pn under AN was relieved in wheat through higher CO2 levels, but that was not the case in maize. In conclusion, elevating atmospheric CO2 concentration increased Ci and Cc in wheat, but not in maize, with enhanced electron fluxes towards photosynthesis, rather than photorespiration, thereby relieving the inhibition of photosynthesis under AN. Our results contributed to a better understanding of NH4+ involvement in N nutrition of crops growing under different levels of CO2.

Special issue in honour of Prof. Reto J. Strasser - Chlorophyll fluorescence, leaf gas exchange, and genomic analysis of chromosome segment substitution rice lines exposed to drought stress

This research aims to evaluate the photosynthesis-related parameters in rice chromosome segment substitution lines (CSSL), containing drought-tolerant region from DH212 in a Khao Dawk Mali105 genetic background. Screening at seedling stage indicated that CSSL4 was more tolerant to drought stress than KDML105 with the higher maximal quantum yield of PSII photochemistry. After withholding water, the decline in light-saturated net photosynthetic rate due to drought stress occurred simultaneously with the decrease in electron transport rate and effective quantum yield of PSII photochemistry values, suggesting that stomatal changes affect light-saturated net photosynthetic rate (PNmax) during the initial drought response. KDML105 rice showed the highest level of electron transport rate/PNmax ratio. This suggested that KDML105 has the lowest ability to use reducing power in photosynthesis process under drought stress conditions. Loci containing single nucleotide polymorphisms between CSSL4 and KDML105 were subjected for co-expression network analysis with 0.99 correlation. The co-expression between calmodulin-stimulated calcium-ATPase and C2H2 zinc finger protein was detected. This locus may contribute to the maintenance ability of photosynthesis process under drought stress conditions.

Micronutrient homeostasis and chloroplast iron protein expression is largely maintained in a chloroplast copper transporter mutant.

PAAI is a P-Type ATPase that functions to import copper (Cu) into the chloroplast. Arabidopsis thaliana (L.) Heynh. paa1 mutants have lowered plastocyanin levels, resulting in a decreased photosynthetic electron transport rate. In nature, iron (Fe) and Cu homeostasis are often linked and it can be envisioned that paa1 acclimates its photosynthetic machinery by adjusting expression of its chloroplast Fe-proteome, but outside of Cu homeostasis paa1 has not been studied. Here, we characterise paa1 ultrastructure and accumulation of electron transport chain proteins in a paa1 allelic series. Furthermore, using hydroponic growth conditions, we characterised metal homeostasis in paa1 with an emphasis on the effects of Fe deficiency. Surprisingly, the paa1 mutation does not affect chloroplast ultrastructure or the accumulation of other photosynthetic electron transport chain proteins, despite the strong decrease in electron transport rate. The regulation of Fe-related photosynthetic electron transport proteins in response to Fe status was maintained in paa1, suggesting that regulation of the chloroplast Fe proteins ignores operational signals from photosynthetic output. The characterisation of paa1 has revealed new insight into the regulation of expression of the photosynthetic electron transport chain proteins and chloroplast metal homeostasis and can help to develop new strategies for the detection of shoot Fe deficiency.

The xanthophyll cycle as an early pathogenic target to deregulate guard cells during Sclerotinia sclerotiorum infection

ABSTRACTStomata not only control the important balance between gaseous fluxes and water loss, but also act as a route of invading pathogen entry into the plant. Here, the stomatal opening was observed to be induced by a necrotrophic pathogen Sclerotinia sclerotiorum at the early stages of infection. In contrast to uninfected control, the stomatal pores were still opened in S. sclerotiorum-infected regions after dark adaption. Mutation of violaxanthin de-epoxidase, a key enzyme in the xanthophyll cycle, could partially restore the S. sclerotiorum-induced stomatal opening. Further studies showed that S. sclerotiorum invasion led to a decrease in electron transport rate, but a significant increase in non-photochemical quenching (NPQ). The decay kinetics of NPQ revealed that zeaxanthin epoxidase (ZEP, also known as ABA1) was continuous deactivation in S. sclerotiorum-infected region. HPLC-MS/MS analysis showed a slight increase of jasmonate acid (JA), but a great decrease of abscisic acid (ABA) content in S. sclerotiorum-inoculated tissue. Exogenous application of ABA but not JA could rescue the abnormal stomatal opening. Together, these results suggested that the S. sclerotiorum-induced decrease of ABA biosynthesis reduced stomatal closing via dysfunction of the xanthophyll cycle during early pathogenesis.

Effect of drought stress on the expression of genes linked to antioxidant enzymatic activity in landraces of Zea mays L. and Pennisetum glaucum (L.) R. Br.

Millets are usually considered more drought tolerant than other cereals. Pearl millet [Pennisetum glaucum (L.) R. Br.] could be an alternative to maize (Zea mays L.) for drought hit regions of the world. In this current study, the sensitivity of these two plants was evaluated under a simulated condition of drought stress. Genotypes IP35451 (pearl millet) and LRN1303 (maize) were compared for their tolerance to drought stress. The growth parameters viz., root/shoot ratio and relative water content were reduced drastically in LRN1303 than in IP35451. Photosystem II was also measured and the study revealed a decrease in electron transport rate and photosynthetically active photo flux density in LRN1303 as compared to IP35451 which had an efficient photosynthesis capacity during drought. Malondialdehyde content and concentration of antioxidant enzymes (Ascorbate peroxidase, Catalase, Glutathione reductase and Superoxide dismutase) revealed that the genotype IP35451 (pearl millet) is better adapted to drought as compared to LRN1303 (maize). Moreover, the gene expression study of three identified genes at transcript levels proved their occurrence as indicators for drought tolerance. GBSS11a in leaf and root displayed steady up-regulation under drought stress in IP35451 (pearl millet), and an initial increase with a subsequent down-regulation in LRN1303 (maize). Expression of antioxidant genes revealed that APX1 and CAT1 in leaves and roots were more remarkably sensitive to drought in IP35451 (pearl millet), however, not for LRN1303 (maize) where they might support better detoxifying action against reactive oxygen species effect under drought condition. The results showed different capacities of LRN1303 (maize) and IP35451 (pearl millet) to activate expression of drought related mRNA. The outcome of the study will be helpful to provide new avenues in future research related to attaining drought tolerant landraces and their genomics.

Effect of metformin exposure on growth and photosynthetic performance in the unicellular freshwater chlorophyte, Chlorella vulgaris.

Many pharmaceuticals have negative effects on biota when released into the environment. For example, recent work has shown that the commonly prescribed antidiabetic drug, metformin (N,N-dimethylbiguanide), has endocrine disrupting effects on fish. However, effects of metformin on aquatic primary producers are poorly known. We exposed cultured isolates of a freshwater chlorophyte, Chlorella vulgaris, to a range of metformin concentrations (0–767.9 mg L-1) to test the hypothesis that exposure negatively affects photosynthesis and growth. A cessation of growth, increase in non-photochemical quenching (NPQ, NPQmax), and reduced electron transport rate (ETR) were observed 24 h after exposure to a metformin concentration of 767.8 mg L-1 (4.6 mM). By 48 h, photosynthetic efficiency of photosystem II (Fv/Fm), α, the initial slope of the ETR-irradiance curve, and Ek (minimum irradiance required to saturate photosynthesis) were reduced. At a lower concentration (76.8 mg L-1), negative effects on photosynthesis (increase in NPQ, decrease in ETR) were delayed, occurring between 72 and 96 h. No negative effects on photosynthesis were observed at an exposure concentration of 1.5 mg L-1. It is likely that metformin impairs photosynthesis either through downstream effects from inhibition of complex I of the electron transport chain or via activation of the enzyme, SnRK1 (sucrose non-fermenting-related kinase 1), which acts as a cellular energy regulator in plants and algae and is an ortholog of the mammalian target of metformin, AMPK (5' adenosine monophosphate-activated protein kinase).

Is carbonic anhydrase activity of photosystem II required for its maximum electron transport rate?

It is known, that the multi-subunit complex of photosystem II (PSII) and some of its single proteins exhibit carbonic anhydrase activity. Previously, we have shown that PSII depletion of HCO3−/CO2 as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of α‑carbonic anhydrases, acetazolamide (AZM), was accompanied by a decrease of electron transport rate on the PSII donor side. It was concluded that carbonic anhydrase activity was required for maximum photosynthetic activity of PSII but it was not excluded that AZM may have two independent mechanisms of action on PSII: specific and nonspecific. To investigate directly the specific influence of carbonic anhydrase inhibition on the photosynthetic activity in PSII we used another known inhibitor of α‑carbonic anhydrase, trifluoromethanesulfonamide (TFMSA), which molecular structure and physicochemical properties are quite different from those of AZM. In this work, we show for the first time that TFMSA inhibits PSII carbonic anhydrase activity and decreases rates of both the photo-induced changes of chlorophyll fluorescence yield and the photosynthetic oxygen evolution. The inhibitory effect of TFMSA on PSII photosynthetic activity was revealed only in the medium depleted of HCO3−/CO2. Addition of exogenous HCO3− or PSII electron donors led to disappearance of the TFMSA inhibitory effect on the electron transport in PSII, indicating that TFMSA inhibition site was located on the PSII donor side. These results show the specificity of TFMSA action on carbonic anhydrase and photosynthetic activities of PSII. In this work, we discuss the necessity of carbonic anhydrase activity for the maximum effectiveness of electron transport on the donor side of PSII.

The role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L.

This study investigates the role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2mM DIC and no DIC with different O2 levels (air-equilibrated levels, 3% of air equilibrium and restored air-equilibrated levels). Lowering O2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2mM DIC, air-equilibrated O2; (2) saturating CO2, no O2; and (3) no DIC, air-equilibrated O2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.

Analysis of the Proteome of the Marine Diatom Phaeodactylum tricornutum Exposed to Aluminum Providing Insights into Aluminum Toxicity Mechanisms.

Trace aluminum (Al) concentrations can be toxic to marine phytoplankton, the basis of the marine food web, but the fundamental Al toxicity and detoxification mechanisms at the molecular levels are poorly understood. Using an array of proteomic, transcriptomic, and biochemical techniques, we describe in detail the cellular response of the model marine diatom Phaeodactylum tricornutum to a short-term sublethal Al stress (4 h of exposure to 200 μM total initial Al). A total of 2204 proteins were identified and quantified by isobaric tags for relative and absolute quantification (iTRAQ) in response to the Al stress. Among them, 87 and 78 proteins performing various cell functions were up- and down-regulated after Al treatment, respectively. We found that photosynthesis was a key Al toxicity target. The Al-induced decrease in electron transport rates in thylakoid membranes lead to an increase in reactive oxygen species (ROS) production, which cause increased lipid peroxidation. Several ROS-detoxifying proteins were induced to help decrease Al-induced oxidative stress. In parallel, glycolysis and pentose phosphate pathway were up-regulated in order to produce cell energy (NADPH, ATP) and carbon skeleton for cell growth, partially circumventing the Al-induced toxicity effects on photosynthesis. These cellular responses to Al stress were coordinated by the activation of various signal transduction pathways. The identification of Al-responsive proteins in the model marine phytoplankton P. tricornutum provides new insights on Al stress responses as well as a good start for further exploring Al detoxification mechanisms.

Photosynthesis and photoprotection under drought in the annual desert plant Anastatica hierochuntica

Anastatica hierochuntica is an annual desert plant, which was recently shown to have unusually low nonphotochemical quenching (NPQ) and a high PSII electron transport rate (ETR). In the current study, we examined how these unusual characteristics are related to a lack of CO2 and inhibition of net photosynthetic rate (P N). We compared the photosynthetic and photoprotective response of A. hierochuntica and sunflower (Helianthus annuus), under conditions of photosynthetic inhibition, with either low CO2 or drought. We found that under nonsteady state conditions of low CO2 availability, A. hierochuntica exhibited about half of the NPQ values and almost twice of the ETR values of H. annuus. However, the long-term inhibition of P N under drought caused a similar increase in NPQ and a decrease in ETR in both A. hierochuntica and H. annuus. These results suggest that the unusually low NPQ and high ETR in A. hierochuntica are not directly related to a response to drought conditions.

Scaling nitrogen and carbon interactions: what are the consequences of biological buffering?

Understanding the consequences of elevated CO2 (eCO2; 800 ppm) on terrestrial ecosystems is a central theme in global change biology, but relatively little is known about how altered plant C and N metabolism influences higher levels of biological organization. Here, we investigate the consequences of C and N interactions by genetically modifying the N-assimilation pathway in Arabidopsis and initiating growth chamber and mesocosm competition studies at current CO2 (cCO2; 400 ppm) and eCO2 over multiple generations. Using a suite of ecological, physiological, and molecular genomic tools, we show that a single-gene mutant of a key enzyme (nia2) elicited a highly orchestrated buffering response starting with a fivefold increase in the expression of a gene paralog (nia1) and a 63% increase in the expression of gene network module enriched for N-assimilation genes. The genetic perturbation reduced amino acids, protein, and TCA-cycle intermediate concentrations in the nia2 mutant compared to the wild-type, while eCO2 mainly increased carbohydrate concentrations. The mutant had reduced net photosynthetic rates due to a 27% decrease in carboxylation capacity and an 18% decrease in electron transport rates. The expression of these buffering mechanisms resulted in a penalty that negatively correlated with fitness and population dynamics yet showed only minor alterations in our estimates of population function, including total per unit area biomass, ground cover, and leaf area index. This study provides insight into the consequences of buffering mechanisms that occur post-genetic perturbations in the N pathway and the associated outcomes these buffering systems have on plant populations relative to eCO2.

Effects of cadmium metal on young gametophytes of Gelidium floridanum: metabolic and morphological changes.

By evaluating carotenoid content, photosynthetic pigments and changes in cellular morphology, growth rates, and photosynthetic performance, this study aimed to determine the effect of cadmium (Cd) on the development of young gametophytes of Gelidium floridanum. Plants were exposed to 7.5 and 15 μM of Cd for 7 days. Control plants showed increased formation of new filamentous thallus, increased growth rates, presence of starch grains in the cortical and subcortical cells, protein content distributed regularly throughout the cell periphery, and intense autofluorescence of chloroplasts. On the other hand, plants treated with Cd at concentrations of 7.5 and 15 μM showed few formations of new thallus with totally depigmented regions, resulting in decreased growth rates. Plants exposed to 7.5 μM Cd demonstrated alterations in the cell wall and an increase in starch grains in the cortical and subcortical cells, while plants exposed to 15 μM Cd showed changes in medullary cells with no organized distribution of protein content. The autofluorescence and structure of chloroplasts decreased, forming a thin layer on the periphery of cells. Cadmium also affected plant metabolism, as visualized by a decrease in photosynthetic pigments, in particular, phycoerythrin and phycocyanin contents, and an increase in carotenoids. This result agrees with decreased photosynthetic performance and chronic photoinhibition observed after treatment with Cd, as measured by the decrease in electron transport rate. Based on these results, it was concluded that exposure to Cd affects cell metabolism and results in significant toxicity to young gametophytes of G. floridanum.

Photoprotective mechanism of the non-target organism Arabidopsis thaliana to paraquat exposure

The response of photosystem II (PSII), of the non-target organism Arabidopsis thaliana, to paraquat (Pq) exposure was studied by chlorophyll fluorescence imaging. Effects of 1mM Pq application by spray on A. thaliana leaves were monitored as soon as 20min after application at the deposit areas of the droplets. A decline in the effective quantum yield of photochemical energy conversion in PSII (ΦPSII) was accompanied by an increase in the quantum yield for dissipation by down regulation in PSII (ΦNPQ). The concomitant decrease in the quantum yield of non-regulated energy loss in PSII (ΦNO) pointed out a quick effective photoprotection mechanism to Pq exposure. Even 1h after Pq spray, when the maximum Pq effect was observed, the decrease of electron transport rate (ETR) and the increase in non-photochemical quenching (NPQ) resulted to maintain almost the same redox state of quinone A (QA) as control plants. Thus, maximal photoprotection was achieved since NPQ was regulated in such a way that PSII reaction centers remained open. Arabidopsis plants were protected from Pq exposure, by increasing NPQ that dissipates light energy and decreases the efficiency of photochemical reactions of photosynthesis (down regulation of PSII) via the “water–water cycle”. PSII photochemistry began to recover 4h after Pq exposure, and this was evident from the increase of ΦPSII, the simultaneous decrease of ΦNPQ, and the concomitant decrease of ΦNO. Yet, ETR began to increase, as well as the fraction of open PSII reaction centers.

Akt activation improves oxidative phosphorylation in renal proximal tubular cells following nephrotoxicant injury

Previously, we showed that protein kinase B (Akt) activation increases intracellular ATP levels and decreases necrosis in renal proximal tubular cells (RPTC) injured by the nephrotoxicant S-(1, 2-dichlorovinyl)-l-cysteine (DCVC) (Shaik ZP, Fifer EK, Nowak G. Am J Physiol Renal Physiol 292: F292-F303, 2007). This study examined the role of Akt in improving mitochondrial function in DCVC-injured RPTC. Our data show a novel observation that phosphorylated (active) Akt is localized in mitochondria of noninjured RPTC, both in mitoplasts and the mitochondrial outer membrane. Mitochondrial levels of active Akt decreased in nephrotoxicant-injured RPTC, and this decrease was associated with mitochondrial dysfunction. DCVC decreased basal, uncoupled, and state 3 respirations; ATP production; activities of complexes I, II, and III; the mitochondrial membrane potential (DeltaPsi(m)); and F(0)F(1)-ATPase activity. Expressing constitutively active Akt in DCVC-injured RPTC increased the levels of phosphorylated Akt in mitochondria, reduced the decreases in basal and uncoupled respirations, increased complex I-coupled state 3 respiration and ATP production, enhanced activities of complex I, complex III, and F(0)F(1)-ATPase, and improved DeltaPsi(m). In contrast, inhibiting Akt activation by expressing dominant negative (inactive) Akt or using 20 microM LY294002 exacerbated decreases in electron transport rate, state 3 respiration, ATP production, DeltaPsi(m), and activities of complex I, complex III, and F(0)F(1)-ATPase. In conclusion, our data show that Akt activation promotes mitochondrial respiration and ATP production in toxicant-injured RPTC by 1) improving integrity of the respiratory chain and maintaining activities of complex I and complex III, 2) reducing decreases in DeltaPsi(m), and 3) restoring F(0)F(1)-ATPase activity.

Temperature response of photosynthesis, excitation energy dissipation and alternative electron sinks to carbon assimilation in Beta vulgaris L.

Photosynthesis, excitation energy dissipation and alternative electron sinks to carbon assimilation at different temperatures (5–35 °C range) were studied in plants of Beta vulgaris L. grown outdoors at springtime. Gas exchange and chlorophyll fluorescence measurements were performed in plants exposed to different temperatures for two hours at saturating irradiance (1000 μmol photons m −2 s −1). The photosynthetic rate decreased significantly either by lowering the temperature below 25 °C or by increasing it above 25 °C. At low temperatures (5, 10 and 15 °C) a significant decrease of electron transport rate (ETR) was found and processes alternative to CO 2 fixation, as sinks of electrons, occurred but they were not affected by the temperature. However, at 5 °C a low and statistically significant photorespiratory rate was observed and the non-photochemical quenching (NPQ) was the highest. At higher temperature (30–35 °C) ETR saturated and hence non-assimilative processes, alternative to CO 2 fixation, sustained the photochemical activity. Among processes consuming O 2 photorespiration increased at 30–35 °C, while processes non-consuming O 2 were non-affected. By increasing the temperature up to 30–35 °C NPQ values did not show appreciable differences compared to 25 °C. The obtained results indicate that in B. vulgaris plants the thermal dissipation represents the main dissipative process of the excess excitation energy at low temperatures. On the contrary, at high temperatures (30–35 °C) the main dissipation of the excess excitation energy is due to alternative pathways to CO 2 assimilation.

Limitations in the use of PAM fluorometry for measuring photosynthetic rates of macroalgae at high irradiances

Pulse amplitude modulated (PAM) fluorometry can be used for measuring photosynthetic electron transport rates (ETR) of marine angiosperms and macroalgae both in the laboratory and in situ. Regarding macroalgae, quantitative values and linear correlations between ETR and rates of photosynthetic O2 evolution have so far been shown only for a few species under low irradiances. As a logical continuation of such work, the aim of the present study was to (a) assess to what degree high irradiances would limit such measurements and (b) evaluate whether PAM fluorometry could be used quantitatively also for other marine macroalgae from different phyla. This was done by comparing ETR with rates of gross O2 evolution (net O2 exhcange corrected for dark respiration) at various irradiances for the green alga Ulva lactuca grown at two irradiances, the brown algae Fucus serratus and Laminaria saccharina and the red algae Palmaria palmata and Porphyra umbilicalis. At low irradiances, there was a clear positive correlation between O2 evolution and fluorescence-based ETR. At high irradiances, however, all algae featured an apparent decrease in ETR while O2 evolution remained relatively constant, and this resulted in markedly increasing O2/ETR ratios. This anomaly could be nicely illustrated in plots of O2/ETR as a function of the effective quantum yield of photosystem II (Y). Such plots showed that the O2/ETR ratio generally started to increase when Y reached a critical low value of c. 0.1. It was further found that the irradiance at which this value was reached varied with species and previous light histories. Thus, it is the Y value, rather than the irradiance per se