Inaug.-Diss.--Erlangen. / Vita. Bibliography: p. [34]

Germination strategies and early seedling development of selected Medicago and Hedysarum species were evaluated to identify plants with high potential for range rehabilitation in central Tunisia. Temperature (5°C to 25°C) and water stress (0 MPa to -0.8 MPa) affected the germination percentage of all Medicago and Hedysarum species. Water stress had a greater effect on percent germination than did temperature, however, the nature of the effect of water potential depended on temperature. Australian medics were more depressed by temperature extremes and low water potential than were Tunisian accessions. Optimum germination of Medicago polvmorpha var. Circle Valley and Medicago trucatula var. Jemalong was at 15 °C and 0 MPa. Germination was highly reduced at higher temperatures and water stress. Tunisian Medicago truncatula germinated better at lower temperatures (5 °C to 15 °C). Hedysarum carnosum germinated more completely at high temperatures. (15°C to 20°C). Medicago laciniata germinated well across a wide range of temperature but germination decreased as water stress increased. Rapid rates of root elongation are beneficial to plants in semiarid environments. High temperature accelerated rates of root elongation and low temperature retarded the rates. The degree of retardation varied with the species and the temperature range. Maximum root elongation occurred at 15°C and minimum root elongation occurred at 5°C. Medicago laciniata had the fastest root elongation rate at 5°C and 15°C . Medicago truncatula was equal to Medicago laciniata at 5°C. Hedysarum carnosum had rapid root elongation at 10°C and 15°C compared to 5°C. A Tunisian accession Medicago polymorpha had the slowest root elongation at all temperatures. A quantitative growth analysis was used to assess the effect of environmental conditions on the species performance over a period of 49 days. Mean relative growth rate (mRGR) varied among species. This variation suggested size hierarchies in relative performance among species. The largest plant, Tunisian Medicago truncatula had the largest mRGR and the smallest plant, The Tunisian Medicago polvmorpha had the lowest mRGR. The derived parameters, leaf area ratio (LAR) and unit leaf ratio (ULR), were not consistent with the size hierarchies obtained by mRGR. The Root to shoot ratio (R/S) varied among the species. The Tunisan Medicago truncatula had high root to shoot ratios at low temperature and its R/S ratio decreased at higher temperature. Slow growing species had high R/S ratio. Medicago laciniata was an exception, it produced low R/S ratio because of its long, and thin root system. / Graduation date: 1991

Les fortes teneurs en éléments traces métalliques (ETMs) présents dans les déblais miniers entraînent généralement une forte réduction de la diversité végétale et microbienne. Selon les caractéristiques physico-chimiques des sols, la nature des métaux, leur concentration et leur spéciation, seules certaines espèces végétales et microbiennes ont développé des mécanismes particuliers et se sont adaptées à la présence d'ETMs. C'est le cas d'Anthyllis vulneraria et d'Hedysarum coronarium, deux Fabacées qui sont présentes sur d'anciennes mines de Zn/ Pb respectivement au sud de la France et au nord de la Tunisie. L'étude de l'absorption du Zn chez quatre sous-espèces d'A. vulneraria a montré que seule la sous-espèce carpatica présente sur l'ancienne mine Zn/Pb des Avinières, peut croître en présence de teneurs élevées en Zn et accumule des quantités élevées de ce métal sans montrer de symptômes de toxicité. Les données obtenues ont révélé qu'A. vulneraria sous espèce carpatica est une plante hypertolérante et accumulatrice de Zn. L'inoculation d'A. vulneraria avec ses bactéries symbiotiques fixatrices d'azote, Mesorhizobium metallidurans STM2683 (souche métallicole) et Mesorhizobium sp. STM2682 (souche non métallicole), a induit une diminution très significative de la teneur en Zn dans la plante. Par contre, seule la souche métallicole a montré un effet protecteur vis-à-vis des sous espèces sensibles d'A. vulneraria lorsque celles-ci sont exposées à une concentration modérée en Zn. La diversité phylogénétique et phénotypique des bactéries symbiotiques fixatrices d'azote associées à H. coronarium isolées du substrat de l'ancienne mine de Zn/Pb de Fej El Hdoum et du sol d'une parcelle agricole à El Krib a mis en évidence (i) l'appartenance de tous les isolats étudiés au genre Rhizobium, (ii) l'existence de différents niveaux de tolérance au Cd et l'absence de tolérance au Zn pour les isolats issus du substrat de Fej El Hdoum (iii) le lien étroit entre la présence de la plante hôte, H. coronarium et de sa bactérie symbiotique fixatrice d'azote sur un site donné. Ainsi, compte tenu des contraintes imposées par la présence des ETMs, l'utilisation de Fabacées en première séquence de végétation pour favoriser l'entrée d'azote et faciliter l'installation d'une couverture végétale stable et pérenne à même de phytostabliser les déblais miniers, rend indispensable la sélection des associations les mieux adaptées aux conditions environnementales locales. / High contents of trace metals present in waste rock mining causes a significant reduction in plant and microbial diversity. According to soil physico-chemical characteristics, metal nature, concentration and speciation, only some plant and strain species have developed specific mechanisms to cope with trace metal toxicity. This is the case of Anthyllis vulneraria and Hedysarum coronarium, two Fabaceae that are present on Zn / Pb ancient mines respectively in southern France and northern Tunisia. The Zn absorption study in four subspecies of A. vulneraria showed that only the carpatica subspecies, present on the Zn / Pb ancient mine of Les Avinières, can grow with high Zn concentrations and accumulate high levels of this metal without showing toxicity symptoms. Thus, A. vulneraria subsp. carpatica should be considered as a Zn-hypertolerant and accumulator plant. Inoculation of A. vulneraria with a nitrogen-fixing symbiotic bacterium, Mesorhizobium metallidurans STM2683 (metallicolous strain) or Mesorhizobium sp. STM2682 (non- metallicolous strain), induced a significant decrease of Zn content in the plant. However, only the metallicolous strain showed a protective effect for sensitive A. vulneraria subspecies when symbiotic plants were exposed to a moderate Zn concentration. Phenotypic and phylogenetic diversity of nitrogen-fixing symbiotic bacteria associated with H. coronarium isolated from the Zn / Pb ancient mine of Fej El Hdoum and an agricultural parcel of El Krib highlighted (i) that all studied isolates belong to the genus of Rhizobium, (ii) the existence of different Cd tolerance levels and the absence of Zn tolerance for isolates from the Fej El Hdoum substrate (iii) the close link between the presence of the host plant, H. coronarium and their symbiotic nitrogen-fixing bacteria on a given site. Thus, given the constraints imposed by heavy metals, the use of Fabaceae in a first sequence of vegetation to promote nitrogen entrance and facilitate the installation of a sustainable plant cover able to phytostabilise waste rock mining, necessitates the selection of the best adapted associations to local environmental conditions.

No description available.