Baobab and tamarind: biology and derived food products. Towards domestication in West Africa
19 mei 2014
UAntwerp - Campus Groenenborger - Lokaal V0.08 - Groenenborgerlaan 171 - 2000 Antwerp
Nina Van den Bilcke
Prof Roeland Samson
PhD defense Nina Van den Bilcke - Faculty of Science
Agroforestry parkland systems in the Sahel region are changing and deteriorating due to agricultural intensification and possibly climatic change. International policy-making and research groups increasingly recognize the importance of local multipurpose tree species in these traditional agricultural systems. Awareness is growing that these trees can play an important role in ecosystem services in the harsh Sahelian climate and in reducing the pressure on forests and protected areas, while at the same time diversifying, increasing and sustaining food security, nutrition and rural incomes, particularly of the most vulnerable groups in society. Most of the locally important trees are considered as undomesticated (wild) or semi-domesticated (selection by farmers), including the priority species baobab (Adansonia digitata L.) and tamarind (Tamarindus indica L.), which were selected as model tree species in this study.
The overall aim of the present research was to gain insight in the biology and food quality of both baobab and tamarind to be able to formulate some suggestions for subsequent selection and domestication. This aim is presented in two parts: drought effects on baobab and tamarind seedlings; and food quality of baobab leaves and tamarind fruits from adult trees.
In the first part, the responses of baobab and tamarind seedlings to short-term and long-term drought, were studied. Therefore, seeds of both species were collected from different provenances in two contrasting climatic zones (Sudano-Sahelian and Sudanian-Guinean) in Mali. Mali was selected as study area, since it covers different climatic zones with a wide range of ecological conditions, possibly co-occurring with some phenotypic/genotypic variation in drought response of baobab and tamarind. Physiological responses to drought may vary from a concomitantly isohydric response to an anisohydric response and various strategies in between. Two physiological mechanisms of plant survival or mortality during drought are hypothesized for species using the isohydric versus anisohydric response: hydraulic failure (irreversibly dessication) and carbon starvation (insufficient carbon supply to maintain metabolism). These mechanisms were also highlighted in this research. Several drought stress experiments were conducted in (semi-) controlled laboratory conditions to investigate: (i) plant-water relations (ecophysiological, morphological and anatomical traits related to the water status); (ii) the impact of stomatal behaviour on plant carbon reserves; and (iii) growth and biomass allocation patterns, and their genetic control. The outcome permitted to draw some conclusions about nursery care, seedling survival rates and drought resistance of different provenances/species, and to identify superior plant material with respect to drought resistance.
In the second part, the food quality of both species was looked into. Therefore, baobab leaves and tamarind fruits from different provenances were collected in the field. A technique that involves the quantitative characterization of several leaf and fruit traits, was used to identify possible superior mature trees. This characterization also determines the extent of the tree-to-tree variation, as well as the frequency distribution, which is typically normal in a wild population but tends to be skewed in populations that have been subjected to some selection by farmers. These data can be used to identify the best combination of traits or ‘ideotype’ to meet a particular target or market opportunity. For baobab leaves, mucilage, calcium and iron content were determined. Mucilage gives food a slimy texture, which is appreciated by local communities. Calcium and iron are essential nutrients that are often lacking in local diets, either due to insufficient intake or to poor absorption from food. For tamarind fruits, sweetness, tartaric acid and sugar content were explored. Tamarind fruit is known to be simultaneously the most acidic fruit with the sweetest taste. Based on sweetness, tamarind types which have particularly sweet or sour tastes could be distinguished. Variation in leaf and fruit traits and influence of the environment (climate and soil) on nutrient content, texture and taste, allowed to make some suggestions for possible ‘plus’ tree selection and domestication.