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The hydraulic architecture of the plants: study of the allometric relations in stem and leaves
Full text linkThe xylem in plants is formed by interconnected dead cells that allow the flow of water from the roots to the leaves. The ascent of sap is mainly passive and it is driven by water evaporation from the mesophyll cell walls in the leaf. The water evaporation generates capillary suction on the menisci at the micro-porous of cell walls, causing negative hydrostatic pressure that propagates down the water column in the xylem. Due to plants grow in height the length of the hydraulic path increases progressively posing the question whether the hydraulic resistance increases accordingly. There is evidence that plants have evolved xylem structures that compensate the possible increase of the hydraulic resistance imposed by path length, namely the tip-to-base conduits widening. Conduits widening has been reported in several species, both angiosperms and conifers, showing that the degree of widening from tip to the base of the stem is very similar among species, or in other words, that plants converge towards a universal xylem structure. Nevertheless, several points on the hydraulic architecture of plants remain to be elucidated.
A largely debated point is whether xylem anatomical traits (e.g. the absolute cell size) change with climatic conditions. Moreover, whether and how the conduits widening in the stem may affect the xylem anatomy of the leaf is still not fully understood. This PhD project aims to widen our understanding of the allometric relations of leaves and stem xylem, considering how the environmental conditions and the height of the plant affect the hydraulic architecture of the water transport system.
A methodological study (Study 1) has been performed on the xylem tissue of stems of Acacia trees grown in different water availability conditions. The main result was that, once the anatomical data were standardized for the tree height, the hydraulic architecture of the xylem did not change in relation to the environmental conditions.
Two studies have been performed on the hydraulic architecture of leaves. The main focus was on the anatomical traits of the xylem conduits in relation to the leaf dimensions and/or the position in the tree crown (height from the base of the stem). The main results were that the xylem traits scaled with the leaf area independently by the position in the crown (Study 2). A fine analysis of the leaf midrib (i.e. major leaf vein) has shown a rigid hydraulic architecture and tissues coordination (Study 3) that was well predicted by the distance from the leaf tip. Both studies showed that the dimensions of the terminal veins were conserved among leaves and within leaf suggesting that the hydraulic architecture of the xylem in the leaf evolved in a way to guarantee an equal distribution of the hydraulic resistances (and thus of the water) among leaves and within the leaf lamina.
Finally, we implemented the anatomical data of both stem and leaf into a hydraulic model to assess the distribution of resistances along the hydraulic path to evaluate how the anatomy of the transport system affects the physiology of the entire tree (Study 4).
This thesis has highlighted that the path length (i.e. the height of the plant and the dimensions of the leaf) is the main factor affecting the hydraulic architecture of the tree. The conduit dimension in both stem and leaf are determined by the distance from the terminal parts, stem apex or leaf tip respectively. Climatic conditions resulted to have marginal (non-significant) effect on the stem anatomical traits. In the leaf, the dimensions of the xylem conduits are statistically invariant with changes in plant size. This rigid hydraulic architecture of the tree, from the stem to the leaf, allows minimizing the effect of the path length on the hydraulic resistance, confining nearly the whole gradient of water potential within the leaves
The total path length hydraulic resistance according to known anatomical patterns: what is the shape of the root-to-leaf tension gradient along the plant longitudinal axis?
Full text linkXylem conduit diameter widens from leaf tip to stem base and how this widening affects the total
hydraulic resistance (RTOT) and the gradient of water potential (Wxyl) has never been thoroughly investigated.
Data of conduit diameter of Acer pseudoplatanus,Fagus sylvatica and Picea abies were used to model the axial variation of RTOT and Wxyl.
The majority of RTOT (from 79 to 98%) was predicted to be confined within the leaf/needle. This means that the xylem conduits of stem and roots, accounting for nearly the total length of the hydraulic path, theoretically provide a nearly negligible contribution to RTOT. Consequently, a steep gradient of water potentials was predicted to develop within the leaf/needle base, whereas lower in the stem water potentials approximate those of rootlets.
Our results would suggest that the strong partitioning of RTOT between leaves/needles coupled with
basal conduit widening is of key importance for both hydraulic safety against drought-induced embolism formation and efficiency, as it minimizes the exposure of stem xylem to high tensions and makes the total plant’s conductance substantially independent of body size
The axial gradient of xylem water potential as theoretically determined by basal conduit widening
No full textAxial anatomy of the leaf midrib provides new insights into the hydraulic architecture and cavitation patterns of Acer pseudoplatanus leaves
Full text linkThe structure of leaf veins is typically described with a hierarchical scheme (e.g. midrib, 1st order, 2nd order), that is used to predict variation in conduit diameter from one order to another overlooking possible variation within the same order. We tested whether xylem conduit diameter changes within the same vein order, with consequences on resistance to embolism. We measured the hydraulic diameter (Dh), and number of vessels (VNo) along the midrib and petioles of Acer pseudoplatanus leaves. We estimated the leaf area supplied (LAsup) at different points of the midrib and how variation in anatomical traits affected embolism resistance. Our results showed that Dh scales with distance from the midrib tip (L) with a power of 0.42, and that VNo scales with LAsup with a power of 0.66. Total conductive area scales isometrically with the LAsup. Embolism events along the midrib occurred first in the basipetal part and afterwards at the leaf tip where vessels are narrower. The distance from the midrib tip well predicts the variations in vessels diameter along the 1st order vein in sycamore maple leaves and this anatomical pattern seems to have an effect on hydraulic safety since wider vessels at the leaf base embolize first
Allometric Trajectories and “Stress”: A Quantitative Approach
Full text linkThe term “stress” is an important but vague term in plant biology. We show situations in which thinking in terms of “stress” is profitably replaced by quantifying distance from functionally optimal scaling relationships between plant parts. These relationships include, for example, the often-cited one between leaf area and sapwood area, which presumably reflects mutual dependence between source and sink tissues and which scales positively within individuals and across species. These relationships seem to be so basic to plant functioning that they are favored by selection across nearly all plant lineages. Within a species or population, individuals that are far from the common scaling patterns are thus expected to perform negatively. For instance, “too little” leaf area (e.g. due to herbivory or disease) per unit of active stem mass would be expected to incur to low carbon income per respiratory cost and thus lead to lower growth. We present a framework that allows quantitative study of phenomena traditionally assigned to “stress,” without need for recourse to this term. Our approach contrasts with traditional approaches for studying “stress,” e.g. revealing that small “stressed” plants likely are in fact well suited to local conditions. We thus offer a quantitative perspective to the study of phenomena often referred to under such terms as “stress,” plasticity, adaptation, and acclimation
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
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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