The First Post-Kepler Brightness Dips of KIC 8462852

We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in October 2015, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process

{sup 18}O composition of CO{sub 2} and H{sub 2}O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the d18O value of leaf water, depth-resolved soil water, atmospheric water vapor, and Keeling plot d18O intercepts for net soil-surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above-canopy CO2 and d18O gradients, is also presented. To interpret the isotope measurements we applied an integrated land-surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the d18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the delta18O value of above-canopy vapor had a small impact on the predicted delta18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the delta18O value of above-canopy CO2 substantially affected predicted ecosystem discrimination. Leaves dominate the ecosystem 18O-isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O-isofluxes

{sup 18}O composition of CO{sub 2} and H{sub 2}O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the d18O value of leaf water, depth-resolved soil water, atmospheric water vapor, and Keeling plot d18O intercepts for net soil-surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above-canopy CO2 and d18O gradients, is also presented. To interpret the isotope measurements we applied an integrated land-surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the d18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the delta18O val ue of above-canopy vapor had a small impact on the predicted delta18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the delta18O value of above-canopy CO2 substantially a ffected predicted ecosystem discrimination. Leaves dominate the ecosystem 18O-isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O-isofluxes

Estimating mesophyll conductance to CO2: methodology, potential errors and recommendations

The three most commonly used methods for estimating mesophyll conductance (gm) are described. They are based on gas exchange measurements either (i) by themselves; (ii) in combination with chlorophyll fluorescence quenching analysis; or (iii) in combination with discrimination against 13CO2. To obtain reliable estimates of gm, the highest possible accuracy of gas exchange is required, particularly when using small leaf chambers. While there may be problems in achieving a high accuracy with leaf chambers that clamp onto a leaf with gaskets, guidelines are provided for making necessary corrections that increase reliability. All methods also rely on models for the calculation of gm and are sensitive to variation in the values of the model parameters. The sensitivity to these factors and to measurement error is analysed and ways to obtain the most reliable gm values are discussed. Small leaf areas can best be measured using one of the fluorescence methods. When larger leaf areas can be measured in larger chambers, the online isotopic methods are preferred. Using the large CO2 draw-down provided by big chambers, and the isotopic method, is particularly important when measuring leaves with high gm that have a small difference in [CO2] between the substomatal cavity and the site of carboxylation in the chloroplast (Ci-Cc gradient). However, equipment for the fluorescence methods is more easily accessible. Carbon isotope discrimination can also be measured in recently synthesized carbohydrates, which has its advantages under field conditions when large number of samples must be processed. The curve-fitting method that uses gas exchange measurements only is not preferred and should only be used when no alternative is available. Since all methods have their weaknesses, the use of two methods for the estimation of gm, which are as independent as possible, is recommended

The lack of alternative oxidase 1a restricts in vivo respiratory activity and stress-related metabolism for leaf osmoprotection and redox balancing under sudden acute water and salt stress in Arabidopsis thaliana

In plants salt and water stress result in an induction of respiration and accumulation of stress-related metabolites (SRMs) with osmoregulation and osmoprotection functions that benefit photosynthesis. The synthesis of SRMs may depend on an active respiratory metabolism, which can be restricted under stress by the inhibition of the cytochrome oxidase pathway (COP), thus causing an increase in the reduction level of the ubiquinone pool. However, the activity of the alternative oxidase pathway (AOP) is thought to prevent this from occurring while at the same time, dissipates excess of reducing power from the chloroplast and thereby improves photosynthetic performance. The present research is based on the hypothesis that the accumulation of SRMs under osmotic stress will be affected by changes in folial AOP activity. To test this, the oxygen isotope-fractionation technique was used to study the in vivo respiratory activities of COP and AOP in leaves of wild-type Arabidopsis thaliana plants and of aox1a mutants under sudden acute stress conditions induced by mannitol and salt treatments. Levels of leaf primary metabolites and transcripts of respiratory-related proteins were also determined in parallel to photosynthetic analyses. The lack of in vivo AOP response in the aox1a mutants coincided with a lower leaf relative water content and a decreased accumulation of crucial osmoregulators. Additionally, levels of oxidative stress-related metabolites and transcripts encoding alternative respiratory components were increased. Coordinated changes in metabolite levels, respiratory activities and photosynthetic performance highlight the contribution of the AOP in providing flexibility to carbon metabolism for the accumulation of SRMs.This work was financed by the MICINN–project CTM2014-53902-C2-1-P and FONDECYT No. 1191118 from National Agency for Research and Development (ANID). AI-S was supported by a predoctoral fellowship from MICINN (PRE2018-083610). IF-S has received funding the “Ramon y Cajal” contract RYC2019-027244-I/AEI/10.13039/501100011033 and the European Social Fund.Peer reviewe

The Lack of Alternative Oxidase 1a Restricts in vivo Respiratory Activity and Stress-Related Metabolism for Leaf Osmoprotection and Redox Balancing Under Sudden Acute Water and Salt Stress in Arabidopsis thaliana

In plants salt and water stress result in an induction of respiration and accumulation of stress-related metabolites (SRMs) with osmoregulation and osmoprotection functions that benefit photosynthesis. The synthesis of SRMs may depend on an active respiratory metabolism, which can be restricted under stress by the inhibition of the cytochrome oxidase pathway (COP), thus causing an increase in the reduction level of the ubiquinone pool. However, the activity of the alternative oxidase pathway (AOP) is thought to prevent this from occurring while at the same time, dissipates excess of reducing power from the chloroplast and thereby improves photosynthetic performance. The present research is based on the hypothesis that the accumulation of SRMs under osmotic stress will be affected by changes in folial AOP activity. To test this, the oxygen isotope-fractionation technique was used to study the in vivo respiratory activities of COP and AOP in leaves of wild-type Arabidopsis thaliana plants and of aox1a mutants under sudden acute stress conditions induced by mannitol and salt treatments. Levels of leaf primary metabolites and transcripts of respiratory-related proteins were also determined in parallel to photosynthetic analyses. The lack of in vivo AOP response in the aox1a mutants coincided with a lower leaf relative water content and a decreased accumulation of crucial osmoregulators. Additionally, levels of oxidative stress-related metabolites and transcripts encoding alternative respiratory components were increased. Coordinated changes in metabolite levels, respiratory activities and photosynthetic performance highlight the contribution of the AOP in providing flexibility to carbon metabolism for the accumulation of SRMs.info:eu-repo/semantics/publishedVersio

The in vivo activation of the alternative oxidase pathway mediates the synthesis of osmotic stress-related metabolites in arabidopsis thaliana

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Photoperiod affects the phenotype of mitochondrial complex I mutants

Plant mutants for genes encoding subunits of mitochondrial complex I (CI; NADH:ubiquinone oxidoreductase), the first enzyme of the respiratory chain, display various phenotypes depending on growth conditions. Here, we examined the impact of photoperiod, a major environmental factor controlling plant development, on two Arabidopsis (Arabidopsis thaliana) CI mutants: a new insertion mutant interrupted in both ndufs8.1 and ndufs8.2 genes encoding the NDUFS8 subunit and the previously characterized ndufs4 CI mutant. In the long day (LD) condition, both ndufs8.1 and ndufs8.2 single mutants were indistinguishable from Columbia-0 at phenotypic and biochemical levels, whereas the ndufs8.1 ndufs8.2 double mutant was devoid of detectable holo-CI assembly/activity, showed higher alternative oxidase content/activity, and displayed a growth retardation phenotype similar to that of the ndufs4 mutant. Although growth was more affected in ndufs4 than in ndufs8.1 ndufs8.2 under the short day (SD) condition, both mutants displayed a similar impairment of growth acceleration after transfer to LD compared with the wild type. Untargeted and targeted metabolomics showed that overall metabolism was less responsive to the SD-to-LD transition in mutants than in the wild type. The typical LD acclimation of carbon and nitrogen assimilation as well as redox-related parameters was not observed in ndufs8.1 ndufs8. Similarly, NAD(H) content, which was higher in the SD condition in both mutants than in Columbia-0, did not adjust under LD. We propose that altered redox homeostasis and NAD(H) content/redox state control the phenotype of CI mutants and photoperiod acclimation in Arabidopsi