Résumé
The general objective of this study was to measure the mycorrhizal dependencies of Cupressus and Lavandula species, to assess the contribution of Lavandula species to the mycorrhizal soil infectivity (MSI) and to determine the main effects of the association between a Lavandula species (L. multifida) and Cupressus arizonica on the soil microbial activities, on the growth and the mycorrhizal status of each plant species in experimental conditions. Three species of Lavandula (L. stoechas L., L. dentata L. and L. multifida L.) and three species of Cupressus (C. atlantica, C. sempervirens and C. arizonica) were tested. Plant species were inoculated with an arbuscular mycorrhizal fungus, Glomus intraradices, to determine their mycorrhizal dependencies in controlled conditions. The results showed that L. multifida and L. dentata were representatives of highly mycorrhizal dependent plant species as well C. arizonica and C. sempervirens. The contribution of L. multifida to the MSI was determined by the most probable number (MPN) method. In six different soils, L. multifida enhanced the mycorrhizal soil infectivity, even more if the soil was P deficient. When C. arizonica and L. multifida were grown together, the growth and mycorrhizal colonization of each species were higher than those recorded when each plant species grew alone. The impact of this dual cultivation on the growth and mycorrhizal colonization of C. arizonica was comparable to those recorded with C. arizonica seedlings previously inoculated with G. intraradices. In contrast, the microbial activities of the soil collected from each treatment were different. These results emphasize the role of “resource islands” and “nurse plants” of Lavender plants in the regeneration processes of tree species such as Cupressus spp. In addition, they confirm the role of AM fungi as a major factor contributing to the growth and co-existence of each of the plant species (L. multifida and C. arizonica).