1,721,055 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Effect of short-term versus prolonged freezing on freeze-thaw injury and post-thaw recovery in spinach: Importance in laboratory freeze-thaw protocols
Get PDFSpinach (Spinacia oleracea L.) is a cool-season vegetable crop which can be damaged by intermittent spring frost that is needed to better understand freezing tolerance of spinach. This thesis focused on the physiology of spinach's freezing tolerance through duration of freezing, including an ability of recovery, using freeze-thaw injured spinach tissues exposed to various durations of freezing at two sub-lethal temperatures. Various physiological parameters [electrolyte leakage, Fv/Fm ratio (efficiency of PSII), MDA test (an indicator of lipid peroxidation), and histochemical detection of ROS (i.e. superoxide (O2-) and hydrogen peroxide (H2O2)] were quantified. Based on the LT50 of spinach (approximately - 5.5 °C) , -4 and -4.5 °C were selected as two sub-lethal temperatures for prolonged freezing test, but only -4.5 °C for post-thaw recovery test. Enhanced ion-leakage, increased MDA content, accumulated ROS and decreased Fv/Fm ratio were observed in freeze-thaw injured spinach tissues subjected to four duration (0.5, 3.0, 5.5 and 10.5 h) of freezing at both -4 and -4.5 °C; however, all measurements were shown to be worse at -4.5 °C than -4.0 °C during four durations of freezing. For the post-thaw recovery test, spinach tissues frozen for relatively short-term duration of freezing at -4.5 °C was recoverable during post-thaw periods, but was irrecoverable when stressed by a longer duration of freezing at the same temperature. In summary, depending upon duration of freezing, temperatures interpreted as sub-lethal based on LT50 (short-term freezing assays) could be actual lethal. Therefore, "duration of freezing" should be considered as one of critical factors influencing on plant freeze-thaw tolerance when using artificial freeze-thaw protocols.</p
Antioxidants and dehydrin metabolism associated with osmopriming-enhanced stress tolerance of germinating spinach (Spinacia oleracea L. cv. Bloomsdale) seeds
Get PDFOsmopriming is a pre-soaking treatment that improves seed germination performance as well as stress tolerance. In this study, we use spinach (Spinacia oleracea cv. Bloomsdale) as the model to study cellular mechanisms contributing to the improved stress tolerance during post-priming germination. We first determined an optimal osmopriming protocol for `Bloomsdale' spinach, i.e. priming seeds with -0.6 MPa PEG 8000 at 15 yC for 8 d. This protocol also improved germinating seeds' tolerance to temperature (sub- and supra-temperature) and desiccation stress. To explore the biochemistry contributing to the priming-induced stress tolerance, we examined two extensively studied stress-responsive components: antioxidant system and dehydrins. An update of `antioxidant system' was found during osmopriming and early germination stage, as manifested in the repression of antioxidants involved in seed protection during dry storage, and enhancement of those related to seed germinability. Possibly, this system update was resulted from the transition of seeds from dry to imbibing / germinating state. Osmopriming might provide a `head-start' for this transition, and thus resulted in a more robust antioxidant system in primed seeds and increased their germination potential. Consequently, primed seeds exhibited improve tolerance to chilling and desiccation stress. Our study of dehydrin accumulation, on the other hand, suggests an alternative strategy for osmopriming to improve stress tolerance in germinating seeds. Several dehydrins (30, 26, and 19-kD dehydrins, and CAP85) transiently accumulated during osmopriming at both protein and transcript levels. These dehydrins also re-accumulated in primed seeds in response to chilling and desiccation stress, and thus may be associated with the improved stress tolerance. It is possible that osmopriming imposed mild osmotic stress in seeds and induced accumulation of stress responses (dehydrins) to confer cross-tolerance that rendered primed seeds more tolerant to subsequent stress exposures. We assumed that osmopriming might use the above-described two strategies (i.e. increasing seed germination potential and inducing cross-tolerance in seeds) that act together to enhance seed stress tolerance during post-priming germination.</p
Salicylic acid-induced freezing tolerance and the effect of short-term versus prolonged freezing on freeze-thaw injury in spinach (Spinacia oleracea L.): Understanding the cellular mechanism through metabolite profiling
Get PDFThis dissertation constitutes two main research projects :1) salicylic acid (SA)-induced freezing tolerance, and 2) the effect of short versus prolonged freezing on freeze-thaw injury. Both studies used spinach (Spinacia oleracea L.) as the model. Regarding the SA study, freezing tolerance and metabolome changes were investigated for spinach seedlings sub-fertigated with 0.5 mM SA. Experiments included seedlings sub-fertigated with SA at both the ambient (non-acclimation; NA) and cold acclimation (CA) temperatures, i.e. NASA and CASA, respectively. SA-fed plants exhibited, in general, similar growth performance as non-treated controls. SA application improved freezing tolerance of spinach at both warm- and cold temperatures; CASA-plants were the most freezing tolerant followed by CA-, NASA-, and NA-plants. Metabolite profiling revealed that SA-feeding differentially alters plant metabolism at warm or cold. SA-induced freezing tolerance may be due to improved tolerance to oxidative stress and freeze-desiccation, as indicated by accumulation of antioxidants, compatible solutes, and osmolytes. Additionally, SA-induced FT appears to be mediated cooperatively by NO and H2O2 signaling since removal of H2O2 or NO reduced beneficial effect of SA on freezing tolerance. This study proposed that exogenous SA application could be an important strategy to improve plant freezing tolerance. In the second study, we conducted metabolite profiling to gain metabolic insight into the differential response by spinach leaves to a short (0.5 and 3.0-h freezing) versus prolonged freezing (5.5 and 10.5-h freezing) at a sub-lethal temperature (-4.5 °C) causing a reversible versus irreversible injury, respectively. Some of the key findings from this study are that: 1) SA may induce tolerance/recovery response in leaves exposed to short-term freezing whereas trigger programmed cell death in irreversibly injured leaves following prolonged freezing, 2) GABA accumulation in freeze-thaw stressed tissues serves as a ‘pH-stat’ against cytoplasmic acidification in leaves exposed to short-term freezing, 3) chloroplast functions may be less sensitive to prolonged freezing than mitochondria, 4) increased accumulation of fatty acids and policosanols with increasing freezing duration indicates incremental injury to membrane lipids and epicuticular waxes, respectively, and 5) ascorbic acid and α–tocopherol accumulation during short-term freezing may facilitate recovery by removal of free radicals. This study provided insight into several cellular events/alterations potentially associated with reversible versus irreversible injury following a short-term or prolonged freezing, respectively, and thereby advance our fundamental understanding of plant response to freezing.</p
Freezing tolerance and cold acclimation in guava (Psidium guajava L.)
Get PDFGuava (Psidium guajava L.) is a tropical, woody perennial with nutritious fruit. It exceeds most tropical and subtropical fruit plants in adaptability and productivity. This thesis focused on the physiology of freezing tolerance and cold acclimation in guava. Freezing tolerance tests, growth and leaf relative water content measurements, anthocyanin content analyses, and protein analyses were performed on nonacclimated and cold-acclimated leaves of guava cultivars Lucknow-49 and Ruby y Supreme. The freezing tolerance of nonacclimated leaves were -2.3 yC and 2.7 yC, and enhanced to -4.2 yC and -4.6 yC after cold acclimation for `Lucknow-49' and `Ruby y Supreme', respectively. Growth and leaf relative water content reduced, while anthocyanins accumulated in guava during cold acclimation. Protein analyses which were performed after cold acclimation and drought stress by electrophoresis and immunoblots revealed that three proteins (69, 23.5, and 17.4 kDa) accumulated in response to low temperatures, and three proteins (78, 17.4, and 16 kDa) accumulated in response to drought stress. One common 17.4 kDa dehydrin accumulated in response to cold and drought stresses. In summary, we indicated that guava possesses some freezing tolerance, and also is able to cold acclimated. Diverse mechanisms are used by guava to tolerate freezing stress. Cold acclimation in guava appears to involve complex physiological and biochemical changes, and also overlapping responses with drought stress.</p
Photoprotective strategies in overwintering rhododendrons
Get PDFTwo Rhododendron species (R. catawbiense and R. ponticum) are used to study their different photoprotective strategies during seasonal cold acclimation at morphological, anatomical, physiological, biochemical and molecular levels. In addition, this dissertation also studied whether or not RcPIP2s (R. catawbiense PIP2s) expression patterns were associated with thermonastic leaf curling in rhododendrons.
Microscopic data revealed that the two Rhododendron species evolved distinct leaf anatomy which could have potential adaptive significance to high light at subfreezing temperatures (chapter 2). Compared with R. catawbiense, R. ponticum has significantly thicker leaf blades but thinner cuticles. There is one layer of upper epidermis and three layers of palisade mesophyll in R. catawbiense, whereas R. ponticum has two distinct layers of upper epidermis and two layers of palisade mesophyll. I suggest that the additional layer of upper epidermis in R. ponticum and thicker cuticle and extra palisade layer in R. catawbiense represent structural adaptations for reducing light injury in leaves and could serve a photoprotective function in winter when leaf photochemistry is generally sluggish. The overall higher opening of stomatal pores per unit leaf area in R. catawbiense indicates that R. catawbiense may be more prone to winter desiccation and that thermonasty may be a particularly beneficial trait in this species by serving as desiccation-avoidance strategy in addition to a photoprotection role.
Comparative study of photoprotection strategies at biochemical and molecular levels during seasonal cold acclimation between the two Rhododendron species suggests that the two species respond differently to winter conditions and have evolved various strategies to reduce and / or tolerate photooxidative stress in winter (chapter 3). In particular, both species have evolved some common strategies including downregulation of photosynthesis capacity, accumulation of early light-induced proteins (ELIPs) and antioxidant systems. R. ponticum accumulates overall higher antioxidant metabolites and enzyme activities, whereas, R. catawbiense has more efficient upregulation of ELIPs and antioxidant systems during seasonal cold acclimation; R. catawbiense also exhibits thermonastic leaf movements. Both species undergo photoinhibition during winter with R. ponticum leaves being relatively less photoinhibited. Such difference may be related to differential sensitivity to excess light in winter and protection efficiencies of ELIPs and antioxidants in the two species, among other adaptations. Although, thermonasty did not seem to provide a clear added advantage to resist photoinhibition in R. catawbiense, its adaptive significance in photoprotection cannot be ruled out. The differences in photoinhibition may also be associated with the distinct leaf anatomies (as shown in chapter 2) in the two species. The additional layer of upper epidermis in R. ponticum, and extra palisade layer and waxy cuticle in R. catawbiense might represent leaf structural adaptations for reducing light injury in winter in these species, and together with ELIPs, antioxidant system and thermonasty constitute photoprotection system in rhododendrons.
To investigate whether or not RcPIP2s expression patterns were associated with leaf curling, the leaves of the thermonastic and nonthermonastic species (R. catawbiense and R. ponticum, respectively) were exposed to a series of temperature regimes in controlled conditions (chapter 4). Progressively cooler temperatures in the subfreezing range employed in this study induced thermonastic leaf curling in R. catawbiense (and uncurling upon warming), whereas R. ponticum leaves showed no curling. Data of gene expression at the transcriptional level using real-time RT-PCR showed similar expression patterns of RcPIP2s across the two species, suggesting that there is no apparent association between RcPIP2s expression and leaf curling / no curling in the two Rhododendron species. However, further investigations involving AQP studies at the protein level as well as at the transcriptional level corresponding to the adaxial and abaxial sides of the leaf with more thermonastic and nonthermonastic Rhododendron species may provide further insights into the role of AQPs in leaf curling in Rhododendron.</p
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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