Functional responses of baobab (Adansonia digitata L.) seedlings to drought conditions: Differences between western and south-eastern Africa

The baobab tree (Adansonia digitata L.) is an important multi-purpose tree species of dryland Africa, commonly found in the driest parts of the Savannah. Although seasonal drought is one of the major constraints for plant growth and survival in these regions, little has been reported about the mechanisms baobab seedlings use to deal with drought. Therefore, the aim of this study was to investigate the functional responses of baobab seedlings under a short-term soil drought stress. As genetic differences between baobab trees from western and south-eastern Africa have been reported, seeds were collected in both Mali and Malawi (western and south-eastern Africa, respectively). Baobab seedlings were found to use a number of mechanisms to cope with drought. First, leaf area was reduced due to leaf shedding, though not all leaves were shed and even some leaves with altered morphology were formed. Second, under drought, relatively more biomass was allocated to the root system. Third, as baobab seedlings had a tight stomatal control, under drought conditions photosynthesis and transpiration were significantly reduced while leaf water potential barely changed. Non-stomatal limitations on photosynthesis became important as drought persevered. Part of the water stored in the taproot was being used for the salvation of part of the old leaves, for the formation of new ones, for the formation of new roots, and for the maintenance of metabolic processes. There was a significant correlation between water content of the taproot and stomatal closure. The former mechanisms, mainly related to water conservation, allow baobab seedlings to keep a high water status during drought events which helps to prevent xylem cavitation and allow them to survive dry periods. Differences between the drought responses of baobab seedlings from different origins were observed: Malawian seedlings were able to retain more leaves and to form more new ones, while Malian seedlings tended to allocate more biomass to their root system. Therefore, baobab seedlings from western Africa showed more drought-avoidance characteristics. These different strategies between Malawi and Mali agree with genetic differences between baobab clades, and suggest that superior planting material in terms of drought tolerance can be selected.