The rhizosphere, a soil fraction influenced by roots, is an area in which occur important
physico-chemical processes that govern the transfer of nutrients from soil to plant. It is
therefore an ideal entry point for many contaminants including metals. Indeed, given the
similarities between nutrients and contaminants, it is not uncommon that the physiological
mechanisms involved in crop nutrition are diverted from their original function to enhance
the absorption of toxic and harmful elements.
The present study is related to the context of pedosphere contamination increasing and
chose to consider this issue through a focus on rhizosphere and mechanisms that govern
its evolution. This work will be oriented on the relation between uranium – a non-essential
ubiquitous metal – and citrate – a low molecular weight organic acid exudated by specific
species when grown on a phosphorus-deficient environment. Among these particular species
white lupine (Lupinus albus) is considered as a model species which explains why it
was chosen to carry out this work. Furthermore, a natural soil containing huge amounts
of uranium has been collected at the outcrop of a pitchblende vein and used during some
The main aim of this PhD thesis was to determine if the root activity of white lupine
is sufficient to cause the remobilization of uranium into soil solution and so to promote
the transfer of this contaminant to the plant. To answer this problematic, the soil/plant
system has been studied with rhizotests (whole system) and by combining datas obtained
with hydroponics experiments and closed reactors tests (decoupled system).
Thereby, it has been demonstrated that the presence of uranium in a controlled growth
medium (hydropony) stimulates citrate exudation in P-sufficient white lupine. More, this
citrate exudation enhances uranium solubilization in soil solution under specific conditions
in batches. These results have been confirmed by observations on rhizotest even if the
values in this case were more nuanced, probably because of soil complexity. This justifies
the employed approaches and highlights their coherence and complementarity.
The results also allowed to provide additional information concerning uranium toxicity,
its internalization in root cells as well as its transfer from roots to aerial parts.