«Andean roots and tubers: Ahipa, arracacha, maca and yacon M. Hermann and J. Heller, editors Promoting the conservation and use of underutilized and ...»
• Evaluation of pests and diseases and the resistance/tolerance present in P. ahipa.
Further research of the phylogeny on the whole genus as well as on the Andean Yam Bean must be considered as extremely interesting from both the scientific and the applied side. Recent molecular studies of the phylogeny of the different species have indicated that P. ahipa must be considered as a monophyletic group with P. tuberosus as the closest entity.
Nevertheless, interspecific hybridization experiments involving the three cultivated species have demonstrated that the genetic distances between the species allow for the development of fertile hybrids. To substantiate the present perception of the phylogeny of this species, additional field collecting is necessary, targeted on wild as well as cultivated P. ahipa material. The diversity in the appearance of genotypes belonging to P. tuberosus, P. ahipa and P. erosus is remarkable. This has been observed for a considerable number of important agronomic traits. In P. tuberosus both multituberous and monotuberous ‘landraces’ exist as well as types with both low and high dry matter content (Sorensen et al. 1997). However, the approximately 2000 to 3000-year-old mummified tubers found in the South American funeral bundles are superficially similar to P. ahipa tubers (and Jíquima Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) tubers) (Ugent et al. 1986). This yam bean material, the oldest found to date, supports the conclusion that also within P. ahipa a high infraspecific genetic variation had developed and has, perhaps, not yet entirely disappeared. In addition, today virtually nothing is known about the present status of P. ahipa in Peru, even though, with reference to the archaeological remains, this may be where the centre of origin of the (South American) yam bean was located.
Regarding the physiology of the tuber formation, few relationships have yet been examined. Those that have were done in a descriptive manner, based on correlation.
More clarity with regard to the yield physiology of the yam bean is of extreme importance to make this crop more attractive. Like other tuber-forming legumes (Psophocarpus, Sphenostylis, etc.), yam beans exhibit an extremely competitive source/sink relationship between tuber growth and seed production. As was mentioned earlier, this relationship has been known in the yam beans since preColumbian times and led to the traditional, practised technique of reproductive pruning. However, the functioning of the physiological mechanisms of tuber formation in yam beans, as well as the other tuberous legume genera listed above, is as yet poorly understood. For example, it is obvious that, within both Psophocarpus and Sphenostylis, distinction must be made between those genotypes that are cultivated mainly for seed production and those for tuber production. Future research will have to concentrate on the primary effects of tuber initiation in yam beans and tuberous legumes in general by identification of the causal relationship of characters, i.e. characters which are correlated to tuber formation and tuber yield.
Once more is known concerning the functioning of the assimilate distribution into competitive sinks within tuberous legumes, clear breeding targets may be set up for indirect selection (see also Lamaze et al. 1985; Robin et al. 1990; Vaillant et al. 1990, 1991; Zinsou et al. 1987a, 1987b, 1987c, 1988; Zinsou and Vansuyt 1991; Zinsou 1994).
Furthermore, it may become possible to resolve the heritability and identify the genetic factors responsible for tuber formation.
Depending on the availability of rapid, quality screening procedures in future estimations of genetic parameters for quality, in combination with other characteristics of agronomic importance, these most-needed analyses could be conducted (e.g. phenotypic and genotypic correlation). Such information is paramount to facilitate improved dimensions of any breeding programme. To estimate these genetic parameters with sufficient precision, additional information is needed concerning the test/plot size for different yam bean characters: which traits may be evaluated under greenhouse conditions and which characters will need field experiments or even a series of field experiments. Initially such experiments have to be linked with an estimation of genotype x environment variance components of important yam bean characters. Furthermore, the attractiveness of the Andean yam bean for sustainable multiple cropping systems has to be quantified. Considerable positive effects by including P. ahipa in such systems are to be expected: being a legume (N-fixation) and in addition a tuber/root crop (stability under environmental stress) with determinate or semideterminate growth habit (short season/early).
Promoting the conservation and use of underutilized and neglected crops. 21. 65 Therefore, multiple cropping system test/trial and evaluation problems involving yam beans should receive more attention, especially when series of test genotypes are involved.
At this point the authors would like to draw attention to the current research concerning genetic resources which at present is concentrated on the efficiency enhancement of the management and evaluation of genebank material (for an overview, see Brown 1989). From the applied side of breeding, involving underutilized crops - and among tubers/legumes in particular, the yam bean must be regarded as extremely underutilized in view of its agronomic potential - it is essential to increase the breeding time efficiency in order to improve a few of the essential agronomic traits within a population. The importance of such a population improvement by recurrent selection was stressed repeatedly by Gallais (1979,1984, 1990). In recurrent selection schemes with underutilized crops it is vital that a few more or less correlated characters are improved without changing the mean and variability of several attractive characters within a population. In the case of the yam bean such character is obviously the improvement of the dry matter content of the tubers in combination with the assimilate partitioning to the seed/tuber, if the crop is to be used as a staple/basic food rather than a ‘fruit’, but simultaneously without causing negative secondary effects with regard to the nutritional composition and the wide range of adaptability. More research and applied solutions aimed at recurrent multitrait improvement are needed for the initiation of rapid population improvement programmes involving underutilized crops. Such efforts could be defined as pre-breeding research in underutilized crops. Also, in spite of the advantages of genetic molecular markers for distance estimation and characterization within and between accessions, further studies of the implementation of molecular markers in breeding strategies for rapid population improvement must be conducted. In relation to pre-breeding in yam bean, and perhaps most underutilized crops, special attention should be given to comparative examinations of RAPDs and AFLPs, i.e. methods that are easier to manage and less expensive than RFLPs and microsatellites.
Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) 17 Future needs It is of major importance to conserve, both in situ and ex situ, as wide a range of variation as possible in endangered landraces within all three cultivated species.
Also, surveys of wild populations - whether belonging to the cultivated or the exclusively wild species - should be completed. All genotypes yet to be studied cytologically (including molecular analyses) and evaluated for agronomic/breeding potential should be included in research projects as quickly as possible, to ensure the optimum foundation for the publication of a complete manual encompassing the variation available and the identification of wild populations and ‘rare’ landraces most urgently in need of conservation measures.
As indicated above, a number of the present constraints limiting a worldwide breakthrough for the yam beans as an attractive alternative to traditional root/tuber crops may be overcome either by using interspecific hybridization combined with intensive breeding methods or by comprehensive screening and selection within existing landraces.
In conclusion, the yam beans have long been considered minor or even lost crops (National Academy of Sciences 1979; National Research Council 1989; Sorensen et al. 1993) in spite of their obvious potential. The research carried out in the Yam Bean Project has so far served to demonstrate the existence of considerable genetic variation within the genus and genotypes with high yield capacity, adaptability and sustainability. However, in order to establish the yam beans as attractive multiplepurpose crops pantropically (including in many subtropical regions), further research combined with an intensified promotion is needed.
Promoting the conservation and use of underutilized and neglected crops. 21.
Acknowledgements The authors gratefully acknowledge the valuable assistance received from the partners within the Yam Bean Project, the exceptional help provided by Dr Alfredo Grau on the second field collecting trip to Bolivia and northern Argentina, and the many local Bolivian farmers without whom many details on cultivation practices and in situ germplasm conservation would not have been available.
The funding of the Yam Bean Project has been provided by the European Union’s Science and Technology Programmes and the second field trip was funded by the Engineer Svend G. Fiedler and wife’s fund for botanical and archaeological research.
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