1,720,983 research outputs found
Rhizodeposition of Zea mays L. as affected by heterosis
No full textThe objective of the present work was to investigate in a field experiment the quantity and quality of rhizodeposits of different parental inbred lines (Lo1016 and Lo964, B73 and H99) and hybrids of Zea mays L. Rhizosphere soil was collected after 40 days and several properties were determined: pH, organic carbon, total nitrogen, acid and alkaline phosphatases, organic acids, bioavailable phosphorous, a-amino nitrogen, total phenols, total root-derived rhizodeposits. The results showed that heterosis
induced more qualitative differences within the genotypes as d13C, TOC and TN did not show any significant differences. The two groups of genotypes enhance phosphorous availability adopting two different strategies. In the first group, B736H99 shows the best phenological performance as well as the highest concentration of bioavailable P, result which is correlated with a high organic acid
concentration (in particular succinic and acetic acid), total phenols, a-amino nitrogen and acid phosphatase activity. There is thus a strict relationship between the phenological superiority of this particular hybrid and its ability to modify the chemistry of its rhizosphere whereas the second hybrid (Lo10166Lo964) seems to have developed different strategies as for example changing the root morphology, stimulating microbial biomass or favouring mycorrhizal symbiosis
Ca-polygalacturonate as a useful model to study the interaction of ions at the soil-root interface: aluminium as a case study.
No full textThis chapter presents results of the adsorption of aluminium and its influence on the mobility of nutrients, in particular phosphate at the soil-root interface. Ca-polygalacturonate (Ca-PG) is the major constituent of the root mucilage and thus the soil-root interface. A Ca-PG network, which resembles the natural root mucilages and thus constitutes a good system model of natural systems, was used to study the interaction with aluminium. The first part of the chapter discusses the effect of aluminium adsorption on the structure of a Ca-membrane. The Ca-PG membrane is organised in a fibrillar and porous network acting as a bio-filter for ions and water and playing thereby a fundamental role in plant nutrition. Metal sorption alters this fibrillar structure and consequently its functionality. Structural modifications determined by FT-IR spectroscopy, Thermogravimetric (TG), Differential Thermal Analysis (DTA) and Scanning electron microscopy (SEM) are described. The final part of the chapter examines alterations induced by aluminium of the functionality of the root mucilage
Effect of aluminium and pH on the mobility of phosphate through a soil-root interface model
No full textThe aim of this work was to investigate the effect of aluminium (Al), a toxic metal for plant growth, as
well as pH on the mobility of phosphate across a calcium-polygalacturonate (Ca-PG) network used as a
soil–root interface model. The Ca-PG fibrils, having acidic properties, are able to complex ions selectively:
thus a Ca-PG model could be useful to study the ion uptake by plants. Ca-PG networks were
exposed to Al solutions at different concentrations (25, 100 and 200lM) at pH 3.50, 4.00 and 4.50. These
Ca-PG and Ca–Al-PG networks were subsequently used to measure the phosphate flux at pH 3.50, 4.00
and 4.50. The results showed that the phosphate’s mobility across the soil–root interface is strongly influenced
by pH and aluminium: its mobility is much greater at a low pH. The presence of Al slowed down
the phosphate even more leading to a complete flux impedance in the first 3–5 h at pH 4.00 and 4.50.
This impedance is probably not only due to interactions between phosphate and Al but it is also due to
structural changes: the interaction of Al (hydrolytic and/or polymeric species) at pH 4.00 and 4.50 with
the polygalacturonic chains could lead to a collapse of the porous structure. These results suggest that
the apoplastic-bound Al hinders, especially at pH 4.00 and 4.50, the phosphate uptake by plants
Characterization of Ca- and Al-pectate gels by thermal analysis and FT-IR spectroscopy
No full textThermal analysis (TG-DTA) and FT-IR spectroscopy have been performed on calcium-pectate membranes to investigate their structure and the consequent variation caused by aluminium sorption. Calcium-polygalacturonate (Ca-PG) membranes, system models of the soil-root interface, were exposed to aluminium solutions at different concentrations (25-800 mM). Three different pHs (3.50, 4.00 and 4.50) were chosen to study also the influence of different aluminium species such as Al3+, Al(OH)2+ and Al(OH)2+ on the structure of the Ca-PG membrane.
The DTA profiles and the FT-IR spectra showed how the aluminium sorption induces structural modifications leading to a reorganisation of the chain aggregates and a weakening of the structure. Higher pHs, i.e. 4.00 and 4.50, and thus the presence hydrolytic aluminium species and the related higher calcium content would maintain a more regular structure than at pH 3.50. At pH 3.50, both the effect of Al3+ and a major calcium release induces a greater weakening of the structure causing an earlier collapse of the membrane. This suggests that Al3+ could be more toxic to plants than the hydroxyl-Al species
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
The influence of aluminium availability on phosphate uptake in Phaseolus vulgaris L. and Phaseolus lunatus L.
No full textAluminium toxicity is one of the major limiting factors of crop productivity on acid soils. High levels of available aluminium in soil may induce phosphorus deficiency in plants. This study investigates the influence of Aluminium (Al) on the phosphate (P(i)) uptake of two Phaseolus species, Phaseolus vulgaris L. var. Red Kidney and Phaseolus lunatus L. The two bean species were treated first with solutions of Al at different concentrations (0, 25, 50 and 100microM, pH 4.50) and second with solutions of P(i) (150microM) at pH 4.50. The higher the Al concentration the higher the Al concentration sorbed but P. vulgaris L var. Red Kidney adsorbed significantly more Al than P. lunatus L. Both species released organic acids: P. vulgaris L var. Red Kidney released fumaric acid and P. lunatus L. fumaric and oxalic acids which could have hindered further Al uptake. The two bean species showed a sigmoid P(i) uptake trend but with two different mechanisms. P. vulgaris L var. Red Kidney showed a starting point of 3h whereas P. lunatus L. adsorbed P(i) immediately within the first minutes. In addition, P. vulgaris L var. Red Kidney presented significantly higher P(i) uptake (higher uptake rate 'k' and higher maximum adsorption 'a' of the kinetic uptake model). The Al treatments did not significantly influence P(i) uptake. Results suggest that P. lunatus L. might adopt an external Al detoxification mechanism by the release of oxalic acid. P. vulgaris L var. Red Kidney on the other hand seemed to adopt an internal detoxification mechanism even if the Al sorbed is poorly translocated into the shoots. More detailed studies will be necessary to better define Al tolerance and/or resistance of Phaseolus spp
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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