Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Phytolith ecology in an arid environment

Auteur : Katz, Ofir

Université de soutenance : Ben Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2014

Résumé partiel
Many higher land plant taxa accumulate silica in their tissues. In many cases, this silica is deposited in amorphous state as microscopic bodies called phytoliths. Since silica deposition usually occurs inside, outside or between plant cells, many phytoliths tend to preserve cell shapes to a certain degree. Being inorganic, phytoliths are more durable than most other plant structural components and are therefore easily identifiable microscopically. Phytoliths are formed in exceptionally large quantities in grasses and in several other plant families which are closely-related to them. Therefore, most of the current knowledge about phytolith formation processes and patterns and about the roles silica and phytoliths play in plant biology and ecology is based on studies conducted on grasses. Among their many possible roles in plants, which include adaptation to various climatic and edaphic conditions, phytoliths are thought to be a quantitative antiherbivory defence mechanism. Nevertheless, phytoliths occur in smaller quantities in many non-grass taxa, and it is uncertain whether phytolith formation patterns and roles in these taxa are similar to those in grasses. Here, I studied the effects of environmental controls (soil silica availability, water availability and grazing) on phytolith formation patterns and the potential antiherbivory role of phytoliths in a group of Asteraceae species growing in Israel. The Asteraceae was chosen for this study since it is the largest dicotyledonous family and since the Asteraceae species are known to cover a wide range of phytolith concentrations, all below those commonly found in grasses. This family consists in Israel of species of varying life forms (from annual forbs to dwarf shrubs) and spinescence levels. In order to study the effects of environmental controls, three non-spiny and five spiny Asteraceae species were sampled in five sites along a large rainfall gradient in Israel, from the Upper Galilee in the northern Mediterranean district (900 mm mean annual rainfall) to the Negev Highlands in the southern desert district (80 mm mean annual rainfall). In addition, a grass species (Avena sterilis) was sampled for reference. Existing literature on phytolith formation patterns in grasses suggests that phytolith concentrations increase with increasing silica and water availabilities. However, phytolith concentrations in arid and water-limited sites may be higher than in slightly more humid sites. One possible explanation for this is the higher viii transpiration potential in more arid habitats, which is an important component in silica uptake from the soil. Furthermore, if phytoliths have some defensive (antiherbivory) role, they are expected to occur in higher concentrations in more arid habitats in which plants have a more limited ability to compensate for herbivory damage by regrowth, as predicted by the resource availability hypothesis. Phytolith concentration in A. sterilis was found to be positively correlated with water availability and with a minor effect of soil silica availability. Phytolith concentrations in A. sterilis plants growing in the most arid site were slightly (statistically insignificantly) higher than in the nearest more humid site (but still lower than in the more northern and more humid sites), possibly reflecting the predictions of the resource availability hypothesis. The three non-spiny Asteraceae species have a partially similar pattern of higher phytolith concentrations in more humid habitats, but with relatively high phytolith concentrations in the most arid site. The five spiny Asteraceae species have various patterns of phytolith formation, not consistently related to water and silica availabilities

Mots clés : Asteraceae, grass, grazing, herbivory, phytolith, plants, Poaceae, rainfall gradient, silicon

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