Citation/Résumé

Morphological and ecophysiological adaptations of African baobab (Adansonia digitata L.) to drought

The African baobab (Adansonia digitata L.) is one of the most important multi-purpose tree species of the semi-arid regions of Africa. Little has however been reported about the morphological and physiological mechanisms baobab trees use to deal with drought conditions. In this research, some of the drought-coping mechanisms of the species are characterized, which could help in the selection of superior, drought-tolerant plant material.

In a first part of this study geographical variation in baobab leaf, fruit and seed morphology was assessed in western Africa. Environmental factors were shown to affect all of the three aforementioned plant parts. For example, small leaves with a high stomatal density were mainly formed under drought prone conditions. Also within-fruit biomass allocation patterns were related to the surrounding environment, with fruits from drier and hotter regions having a higher seed fraction and lower pulp fraction than fruits from more mesic and cooler sites. In dry areas, more seeds were produced per fruit, while individual seed mass was lower.

As both rainfall and individual seed mass might affect seedling performance and establishment success in the field, an experiment was carried out in which the effects of both factors on seedling performance were assessed. While seedling size scaled positively with seed mass, seedling biomass allocation was related to rainfall of origin, as seedlings from dry study sites tended to invest relatively more biomass in their root system than seedlings from wet study sites.

A greenhouse experiment revealed that baobab seedlings use mainly drought-avoidance mechanisms to cope with soil drought. Due to a tight control over stomatal water losses, gas exchange rates declined rapidly with declining soil moisture, while leaf dropping occurred more gradually. Not all leaves were shed, however, whereas some leaves with altered morphology were formed. As leaf water potential barely changed under drought stress the species can be characterized as isohydric.

Differences in drought responses were observed between baobab seedlings from western and south-eastern Africa, belonging to different genetic clades. In general, baobab seedlings from western Africa seemed to be more water-conservative. Furthermore, baobab seedlings from dry localities tended to be better adapted to drought than seedlings from wet study sites. The observed differences in drought responses point towards genotypic adaptation, and there is thus room for the selection of superior, drought-tolerant plant material.