Effect of Arbuscular Mycorrhizal Fungus on Drought Tolerance of African Baobab (Adansonia digitata L., Malvaceae)

In semi-arid and arid zones, where the soils are often poor in mineral nutrients and where the dry period lasts for several months, plant growth largely rely upon mycorrhizal symbiosis. Mycorrhizae improve root growth, nutrient uptake, leave growth and are also involved in water uptake and drought tolerance-resistance. A greenhouse experiment was conducted in order to investigate the influence of arbuscular mycorrhizal fungus inoculation (AMF) (Glomus mosseae, G. aggregatum and G. fasciculatum) on drought tolerance in baobab (Adansonia digitata L., Malvaceae). We used seeds collected in different agro-climatic zones in Senegal following a defined rainfall gradient (250-1100 mm). Baobab seedlings (that had grown for one month) were subjected to a progressive drought stress for one month and with weekly records of plant height, leaf number, shoot and root biomass monitored at an interval. Water deficit was imposed by stopping irrigation until the end of the experiment. So, baobab seedlings were subjected to different level of water supply as control, mild water stress, moderate water stress and severe water stress. Seedlings were inoculated with combined inoculums of the three different AMFs, AMF (obtained from the Laboratory of Fungal Biotechnology of the Department of Plant Biology of the Cheikh Anta Diop University /Dakar, Senegal). During drought stress, leaf water potential (LWP), and fluorescence were monitored every two weeks, in order to assess the physiological responses effects of mycorrhizal inoculation. Inoculated plants had greater plant height, leaf number and biomass under well-watered than under drought conditions. Biomass was higher in mycorrhizal (M) than non-mycorrhizal (NM) plants under drought conditions. Mycorrhizal intensity defined as the number of mycorrhizal spores found in seedlings roots were measured by the Trouvelot method. Results show that for all provenances inoculated plants has higher mycorrhizal intensity under well-watered (50-90% of mycorrhizal spores) than under water deficit conditions (less than 50%). Mycorrhizal baobab seedlings from humid areas (Sudano-Sahel and Sudan zone) had higher LWP and lower biomass throughout the experiment in drought conditions. Baobab seedlings responded best to inoculation with G. fasciculatum and G. aggregatum in both dry and wet conditions. These results indicate that baobab trees do derive benefits from AM inoculation. Mycorrhizae significantly improve tolerance to moderate drought stress imposed on young baobab seedlings, especially in those from Sahelian provenance.