Domestication potential of Marula (Sclerocarya birrea subsp. caffra) in South Africa and Namibia: 2. Phenotypic variation in nut and kernel traits

As part of a wider study characterizing tree-to-tree variation in fruit traits as a pre-requisite for cultivar development, fruits were collected from each of 63 marula (Sclerocarya birrea) trees in Bushbuckridge, South Africa\ud and from 55 trees from the North Central Region of Namibia. The nuts were removed from the fruit flesh, and the kernels extracted, counted and weighed individually to determine the patterns of dry matter partitioning among the nut components (shell and kernel) of different trees. Mean nut, shell and kernel mass were not significantly different between the two countries. Between sites in South Africa there were highly significant differences in mean nut mass, shell mass, kernel mass and kernel number. In Namibia, there were highly significant differences between geographic areas in mean shell mass, kernel mass and kernel number, but not in nut mass. These differences had considerable impacts on shell:kernel ratios (8.0-15.4). In South Africa, mean kernel mass was significantly greater in fruits from farmers’ fields (0.42 g) than from communal land (0.30 g) or natural woodland (0.32 g). Within all sites, in both South Africa and Namibia, there was highly significant and continuous variation between individual trees in nut mass (South Africa=2.3-7.1 g; Namibia=2.7-6.4 g) and kernel mass (South Africa=0.09-0.55 g; Namibia=0.01-0.92 g). The small and valuable kernels constitute a small part of the nut (Namibia=6.1-11.1%; South Africa=7.6-10.7%). 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, in addition to genetic variation. The nuts and kernels of the Namibian trees were compared with the fruits from one superior tree ('Namibian Wonder': nuts=10.9 g; kernels=1.1 g). Oil content (%) and oil yield (g/fruit) also differed significantly between trees (44.7-72.3% and 8.0-53.0 g/fruit). The percentage frequency distribution of kernel mass was skewed from trees in farmers’ fields in South Africa and in some sites in Namibia, suggesting a level of anthropogenic selection. It is concluded that there is great potential for the development of cultivars for kernel traits, but there is also a need to determine how to increase the proportion of nuts with four kernels, perhaps through improved pollination success

The domestication of indigenous trees as the basis of a strategy for sustainable land use

The sustainability of land use in the tropics has typically been lost when species-rich natural vegetation has been cleared to make way for monocultures of improved staple food crops grown intensively with high inputs of agrochemicals. These intensive fanning systems have indeed helped to feed growing human populations, but at an environment cost that cannot be tolerated into the future. One suggestion for an acceptable alternative that both feeds the people and restores some of the diversity found in natural vegetation, is the development of lower input agroecosystems that combine the cultivation of 'Green Revolution' staple food crops with a number of the indigenous food producing tree species that can restore ecosystem function. This return towards more traditional land use practices can be enhanced by the judicious development of high quality, high yielding cultivars of the trees through the adaptation of standard horticultural practices, and planting of these cultivars in agroforests. In dryland Africa, there are many traditionally important tree species producing marketable nontimber forest products, which have potential to be domesticated in this way. The techniques, methodologies and strategies exist and are being implemented in a small way. A few species are already being domesticated with the participation of local communities. There is the potential to expand these programmes to a scale where they can start to have meaningful impact on land use and create new more biologically diverse, functioning agroecosystems, which also support and enhance the livelihoods of local people. This would be a good example of "Promoting Best Practices for Conservation and Sustainable Use of Biodiversity of Global Significance in Arid and Semi-arid Zones"

Differentiation of the dynamic variables affecting rooting ability in juvenile and mature cuttings of cherry (Prunus avium)

The rooting potential of four types or origins of Prunus avium cuttings from the same mature trees (over 20 years-old) was compared using a mist propagation bed during early Summer (June).The cuttings originated from juvenile sucker shoots of the current and previous year, and mature crown shoots (current year's lateral `long-shoots' and multi-year terminal `short shoots').The morphological differences in inter-node length, stem diameter and leaf area between the four cutting types were highly significant (P = 0.05), leading to large differences in cutting volume and, so it is argued, to assimilate reserves. Juvenile cuttings rooted well (65% and 77% rooting for hardwood and softwood shoots, respectively), while mature cuttings rooted poorly (4% and 7% for mature hardwood and softwood cuttings, respectively). Leaf abscission was significantly more frequent in mature hardwood cuttings (16 - 78%) than in the other cutting types (1.6 - 9%) at the end of the propagation period. Leaf loss resulted from two processes: abscission and leaf rotting. Physiologically (i.e., in carbon assimilation, leaf transpiration and stomatal conductance), the four cutting types were not significantly different early in the post-severance period (day-4); but, by day-22, stomatal conductance was lowest in mature hardwood cuttings that still had leaves. At this time, the most physiologically-active unrooted cuttings were from juvenile hardwood and mature softwood shoots. The extent of physiological and morphological variability between cutting types and their probable impact on processes affecting rooting ability is complex and highly interactive. Consequently, it is not possible to explain the causes of the variation in rooting ability between juvenile and mature cuttings, although this study suggests that the constraints to rooting are likely to reflect physiological differences between the different cutting types. It is concluded that, to resolve the debate about factors that affect the rooting ability of juvenile and mature cuttings (i.e., ontogenetic vs. physiological ageing), there is a need to achieve morphological and physiological comparability in the tissues

The 'ideotype concept' and its application to the selection of cultivars of trees providing agroforestry tree products

The ideotype concept has been developed and modified for a number of different crops, including forest trees, over the last 35 years. In recent years it has been used in the domestication of agroforestry trees producing Agroforestry Tree Products (AFTPs), as an aid to the multiple-trait selection of superior trees for cultivar development. For example in west and central Africa, 'fruit' and 'kernel' ideotypes have been identified in Irvingia gabonensis (Bush Mango), based on quantitative characterization of a number of fruit, nut and kernel traits. Subsequently, recognizing the opportunity to develop different markets for the commercially important kernels, the 'kernel ideotype' has been subdivided into 'oil' and 'food-thickening' ideotypes, with options for further subdivision of the latter into ideotypes with either high viscosity or high drawability characteristics. Similar opportunities for the development of single-purpose cultivars from multi-purpose species have been identified in Sclerocarya birrea (Marula), indicating that with increasing information about the variability of AFTPs it is possible to develop a hierarchy of different ideotypes to meet different market opportunities. In S. birrea and Dacryodes edulis, future options exist to extend ideotypes with recognition of the variability in both nutritional and organoleptic qualities. In Australia, a very broad-based ideotype has been developed for the indigenous shrub Kunzea pomifera (Muntries) established on both morphological and physiological traits. Similar approaches are identified for timber trees (e.g. Triplochiton scleroxylon) and those like Santalum austrocaledonicum producing essential oils. It is concluded that ideotypes are a useful tool for visualizing and conceptualizing how to combine specific rare combinations of visible and invisible traits, aimed at the maximization of harvest index, even when the traits are only weakly related