reported (e.g. Amin and Jaiswal, 1993), but the mechanism remains unclear. In vitro micrografting has also been used to rejuvenate shoots (Ewald and Kretzschmar, 1996).
wild or unimproved population from which new selections can be derived. The selection population is the somewhat improved population of genotypes which are being tested and which are used in subsequent breeding programmes to create the next generation of potential cultivars. A wide range of genotypes may be kept in this population as long as each has at least one characteristic of possible future interest. The production population consists of the highly selected genotypes, which are used for planting.
As mentioned earlier, there are two basic approaches to the genetic improvement of trees: the seed-based breeding approach typical of forestry, and the clonal approach typical of horticulture. The seed approach typically involves the selection of populations (provenance testing) and/or families (progeny testing) (Zobel and Talbert, 1984; Leakey, 1991). While this approach could be taken for indigenous fruit trees, it is very likely that an examination of the ten situations outlined in the Introduction would indicate that a clonal approach is more appropriate. There are basically two ways to select the best individual trees for cloning from broad and diverse wild populations: (i) selection from a pool of seedlings of virtually unknown quality in a nursery or field trial (although it may be known that the pool originates from a good provenance or progeny); and (ii) selection of proven mature trees in wild or planted populations (Fig. 2.2). In scenario (i), genetic improvement in yield per hectare will undoubtedly require a series of tests, each spanning many years.
Typically, there are four levels of testing (Foster and Bertolucci, 1994):
1. Initial screening with large numbers, preferably tens of thousands, of seedlings or, if seedlings have already been cloned, a few ramets per clone.
2. Candidacy testing with large numbers of cloned genotypes fewer than with initial screening (preferably hundreds or thousands) and two to six ramets per clone.
3. Clonal performance trials with moderate numbers of clones (e.g. fewer than 200) and large numbers (e.g. 0.1 ha plots) of ramets per clone.
4. Compatibility trials with small numbers of clones (e.g. 20–50) with very large plot sizes.
It is important to recognize that there is a trade-off between the accuracy of genetic value estimation and the intensity of selection (i.e. greater accuracy is at the expense of numbers of families, individuals per family, or clones). For cost- effective clonal tree improvement programmes with limited or fixed resources, it has been found that the best strategy is to plant as many clones as possible with relatively few ramets per clone.
To short-circuit the lengthy process of field trials, Ladipo et al. (1991a, b) developed a predictive test for timber tree seedlings in which the initial screening is done on young seedlings in the nursery; it is then possible to jump straight into clonal or compatibility trials with some confidence. To date there is no similar opportunity for fruit trees.
Like the predictive test, scenario (i) is an alternative and much quicker option. In this case, mature trees, which have already expressed their genetic potential at a particular site over many years of growth, are selected and propagated vegetatively and the propagules are planted either in clonal
performance trials or directly into compatibility trials. This raises the question of how the superior mature trees should be identified, especially if it is desirable to select for multiple traits. This can create a problem, as many traits may be weakly or negatively correlated (e.g. fruit size and kernel size in S. birrea;
Leakey, 2005). Consequently, as the number of desirable traits increases, the number of genotypes superior for all traits diminishes rapidly. Thus, the selection intensity (and also the number of trees screened) must be substantially increased, or the expected genetic gain will rapidly decline. For this reason, only the few most economically important traits (e.g. fruit flesh or nut mass, taste) should be concentrated on in the early phases of selection.
Two techniques can be used to assist in the identification of superior mature trees (sometimes called ‘elite’ or ‘plus’ trees) producing indigenous fruits and nuts. The first is to involve indigenous people in the domestication process and to seek their local knowledge about which trees produce the best products. Local people usually have good knowledge about the whereabouts of elite trees, and this knowledge often extends to superiority in a number of different traits, such as size, flavour and seasonality of production. However, access to this knowledge has to be earned by the development of trust between the holder of the knowledge and the potential recipient. Ideally, the recipient should enter into an agreement that the intellectual property rights of the holder will be formally (and legally) recognized if a cultivar is developed from the selected tree. Unfortunately, at present the process of legally recognizing such cultivars is not well developed and requires considerable improvement.
The second technique for identifying mature elite fruit and nut trees has recently been extended to a study on marula (Leakey, 2005) following its development in Cameroon and Nigeria (Atangana et al., 2002; Leakey et al., 2002, 2005c). This technique involves the quantitative characterization of many traits of fruits and kernels, which are associated with size, flavour, nutritional value, etc. This characterization also determines the extent of the tree-to-tree variation, which is typically three- to sevenfold, as found in marula (Leakey et al., 2005a, b), as well as the frequency distribution, which is typically normal in wild populations but tends to become skewed in populations subjected to some selection. The characterization data can then be used to identify the best combination of traits (the ‘ideotype’) to meet a particular market opportunity (for an example see Fig. 2.3). The development of single-purpose ideotypes (Leakey and Page, 2006) provides a tool for the development of cultivars with different levels of market focus and sophistication (Fig. 2.4). In addition to advancing the selection process for multiple traits, the ideotypes also provide information about opportunities to select for better partitioning of dry matter between desirable and undesirable traits. For example, in marula 90% of the dry matter in nuts is typically found in the shell and only 10% in the valuable kernel, although the range of variation is 3–16%. The shell : kernel ratio is therefore a trait which could usefully be included in the ideotype. However, the inclusion of each additional trait in a multi-trait selection process greatly increases the number of trees that need to be screened, especially if the traits are independent or only weakly related.
Fruit ideotype AR21 AL5
Kernel ideotype AR20 WRF23 Fruit mass
Pulp mass
Skin mass
Flesh/juice mass
Nut mass Shell mass
Kernel mass Oil content
Fig. 2.3. Fruit and kernel ideotypes for marula (Sclerocarya birrea) in South Africa, with the best-fit trees (after Leakey, 2005).
Fruit
Flesh
Pulp Juice Skin
Taste Nutritional
value Processing quality
Beer and wine Distilled
liquor
Nut
Shell Kernel
Waste Other uses
Oil Food
Medicinal products
Cosmetics Edible oils
1 LEVEL
2
3
Fig. 2.4. The development of single-purpose ideotypes as a tool for cultivars with different levels of market focus and sophistication.
It is clear from the above discussion that in the early phases of domestication there is sufficient genetic variation in most tropical tree populations to allow considerable progress in the development of cultivars, but a strategy for clonal agroforestry should not forgo any opportunity for creating new variation.
The selection of clones is not a once-and-for-all event. Domestication is a continuous process, which in wheat, rice, maize, oranges and apples, for example, started thousands of years ago and continues today. Thus, a series of clonal selection trials should be established to seek the best individuals from new accessions of genetically diverse populations or progenies (Fig. 2.2). It is also important to discard old clones as they are superseded, although some of these should be retained in the gene resource population. In the first instance, clones may be selected for yield and quality. With time, this can be extended to include nutritional quality, disease/pest resistance, component products (oils, flavourings, thickening agents, etc.). This continued turnover of the selected clonal population will further ensure the diversity of the commercially planted clones and prevent excessive narrowing of the genetic base. Indeed, it can be argued that in this way clonal plantations of 30–50 superior but unrelated clones can be more diverse than seedlings. This is because seed-lots typically originate from a number of related mother trees and share some genetic material.
Because domestication is a continuous process, commercial plantings have to be made with whatever material is best at a given time, knowing that they will be superseded later. Having a succession of increasingly good planting stock is one of the ways in which the diversity of the genetic base can be maintained, although of course this has to be rigorously enforced as one of the objectives of a breeding and selection programme. For species with existing provenance selection and breeding programmes, clones should be derived from seed collections sampling a wide range of the known variation, as it is not uncommon for a few elite trees to be found with poor provenances.
As the selection process intensifies with time, new traits will be introduced into the programme (e.g. seasonality of production, early fruiting, disease and/or pest resistance and drought tolerance). Capturing variation in the seasonality of production and expanding the harvesting season are likely to be among the best ways of supporting market growth. As the price of end-of- season fruits is likely to be better than the mid-season price, it is also a good way to enhance the benefits of producing households that need sources of income throughout the year. The further the domestication process proceeds, the more important it becomes that the combination of traits being selected is targeted at a particular market (Fig. 2.4). This again is where the ideotype concept can be useful, and ideally advice should be sought from industrial partners who are aware of which characteristics are important in the marketplace or in product processing (Leakey, 1999).
Using the example of Sclerocarya birrea, fruit-producing cultivars could be developed that have large fruit flesh/juice mass (Leakey et al., 2005a) and are nutritious (Thiong’o et al., 2002) as raw fruits, or are good for traditional beer- or wine-making, or meet the needs of the distilling industry. Likewise, other cultivars could be developed for the size and quality of the kernel, with a low
shell : kernel ratio and with either nutritious or medicinal qualities for eating, or with oil yield and quality traits of importance in the cosmetics industry (Leakey, 2005; Leakey et al., 2005b). Similarly, superior phenotypes of U. kirkianawith heavy fruit loads, large fruits and high pulp content have been identified by communities during participatory selection in Malawi, Zambia and Zimbabwe (Akinnifesi et al., 2006). Thus, it is clear that in the domestication of multipurpose tree species, highly productive, single-purpose clones or cultivars are probably the best option. To maximize the market recognition of, and farmer interest in, cultivars, it is a good idea to name them. In Namibia every marula tree already has a name, so it is easy to give a name to a cultivar derived from any particular tree. The name can also be used to recognize the person or community holding the rights to the cultivar.