Characterisation of phenotypic variation in marula (Sclerocarya birrea) fruits, nuts and kernels in South Africa and Namibia

Abstract

Twenty four ripe fruits were collected from each of 55 marula (Sclerocarya birrea) trees in Bushbuckridge, South Africa and from 63 trees from the North Central Region of Namibia. The South African trees were in farmers’ fields (20), communal land (29) and natural woodland (14), at three sites: Acornhoek road, Allandale/Green Valley and Andover/Wits Rural Facility. The Namibian trees were all from farmers’ fields in three areas: North East (20), North West (10) and West (25). The fruits were partitioned into skin, flesh/juice, nut, shell and kernel. Each component of each fruit was weighed fresh separately and its individual identity maintained throughout the series of assessments. Detailed analysis of this data provides a good understanding of the extent of the variation found and some of the inter-relationships between the dry matter partitioning among the fruit components of different trees. Namibian fruits were found to be significantly larger than those from South Africa, due to their greater pulp mass, especially the flesh/juice component. In South African fruits, those from farmers’ fields were significantly larger in all components. In Namibia, fruits were larger from the North West, but kernel mass was greater from the West. Within each sample there was highly significant and continuous variation between the mass of individual components, indicating the potential for selection of trees producing larger products. The fruits of the Namibian trees were compared with the fruits from one superior tree (‘Namibian Wonder’) found by a field team while working with farmers. The small and valuable kernels are a small part of the nut. There can be 4 kernels per nut, but even within the fruits of the same tree kernel number can vary between 0-4, suggesting variation in pollination success. Nevertheless there is also clearly a genetic component to the kernel variation. There is considerable variation between trees in the partitioning of dry matter between the different components of the fruits, thus large fruits do not necessarily contain more flesh or larger kernels. Thus to develop cultivars for either fruit juice or kernel oil production it is necessary to do multi-trait selection. To assist in this process, ‘ideotypes’ were identified that would optimize the combination of traits for different products, and the best trees were compared against these ideotypes. None of the sampled trees came very close to the ideotypes, which were based on the data from ‘Namibian Wonder’. However, significant improvements could be achieved by selecting the trees that are the ‘best-fit’ with the ideotypes. These results are discussed within the context of similar studies in other indigenous fruits, and with a view to the potential for developing participatory domestication programmes in South Africa and Namibia. Such participatory programmes aimed at poverty alleviation in the rural communities, the restoration of the natural resource and the maintenance of traditional and social values of marula, would involve the use of commercially prudent selection and improvement strategies.