«Andean roots and tubers: Ahipa, arracacha, maca and yacon M. Hermann and J. Heller, editors Promoting the conservation and use of underutilized and ...»
Early selections within lines, derived from the F2 and F3 generations, have only been implemented in the form of visual selection based on plant performance toward pure line development, but not as a chance to reduce the extremely time-consuming period for recombination intervals. To use this in a rapid population improvement is currently under focus for P. ahipa and/or P. tuberosus. A selection, carried out repeatedly, and the recombination of superior F2 or F3 lines, would considerably enhance the probability of breaks in negative linkage groups within and between quantitative traits in connection with the opportunity of selection. Because of the special reproductive biology of Pachyrhizus, selected segregating cross parents may also be reproduced vegetatively and this would easily result in the development of pure lines, as suggested by Jensen (1988). From the view of selection theory this is defined as recurrent selection, targeting on the enhancement of the long-term selection response in the population (Gallais 1984). This is theoretically the most efficient method of population improvement and therefore pre-breeding. In practice the result depends on the control of the negative secondary effects (genetic drift under linkage) in other traits, i.e. a critical point in recurrent selection programmes which has yet to be solved.
The interspecific hybridization experiments – initially established to investigate the compatibility of the three cultivated species – formed the basis of the breeding programmes where P. ahipa is used as a donor for cultivar characteristics. The hybrids of P. erosus x ahipa origin were, as mentioned, selected to combine the tuber quality of P. erosus with the erect, determinate growth habit and earliness of P. ahipa. Similar experiments have been conducted with P. erosus x tuberosus in Mexico and triple hybrid combinations involving P. tuberosus x (P. erosus x ahipa) in Tonga. These are all selected according to the pedigree method (5%) with tuber shape/size, Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) determinate growth habit, pest resistance and seed yield as the selection criteria.
Although the breeding programmes were initially set up to use P. ahipa as a donor parent for cultivar characteristics, amazingly good-performing hybrid lines have been found within these populations with regard to tuber yield and wide adaptability. Nevertheless, negative secondary effects in these hybrid populations have been observed, e.g. reduced seed set and germination rates (pers. observ.). The reduced seed set reached a critical level in the F3 and F4 generations; thus seed yield has had to be included as a selection criterion in later generations. In particular, as these wide crosses resulted in an unbalanced genome, it is probable that an increased recombination within these hybrid populations, by means of increased of cross intensities, would be very suitable. This could be achieved as described above.
10.2 Challenges for the traditional and more advanced production areas When contemplating the breeding challenges of the Andean Yam Bean it is helpful to distinguish between breeding aims for P. ahipa as a vegetable crop, a basic food crop and as a possible industrial crop. However, some breeding aims are common for all usage forms, e.g. reduction of the growth period and reduced pod formation per plant to avoid reproductive pruning. In the following the common aims will be treated under the usage as a vegetable crop.
Fig. 8. Pachyrhizus ahipa, accession AC102. Well-shaped monotuberous root and non-marketable multituberous roots. Grown at the CEBAJ experimental station near Celaya, State of Guanajuato, Mexico. Photo, E. Heredia G.
Promoting the conservation and use of underutilized and neglected crops. 21. 47 All Pachyrhizus species are – in their area of origin and today also in many overseas areas—known as a vegetable/fruit crop. The tubers are eaten raw or to a lesser extent used in salads and juices. As mentioned earlier, this has reached a level of importance where P. erosus today is recognized as the fastest-growing speciality vegetable/fruit on the US market, mainly imported from Mexico (according to Brumback 1990). In general, the breeding challenges for the speciality vegetable like the Andean yam bean ( P. ahipa ) and/or the Mexican yam bean ( P. erosus ) are size and shape of tubers (Fig. 8). On the US market monotuberous plants with a round shape of the single tuber between 0.7 and 1.2 kg is demanded, whereas on the South American market a multituberous plant with more sugar-beet-like shape, between 1 and 2 kg, is accepted (Fig. 8). For trading and selling at the rural markets cultivars with a thicker skin are demanded. Thick skins decrease susceptibility to bruising during transport and prolong shelf-life. The improvement of both traits is named as very important by local producers in Mexico. This is especially the case when the US market is the target.
For the main, large-scale P. erosus production areas in Mexico the major challenge is currently the development of new early cultivars which will allow a continuous production within the country, alternating between the lowland regions (the State of Nayarit) and the areas at higher altitudes (in the States of Guanajuato and Michoacan). The new lines developed as a result of cross-breeding experiments involving P. erosus x ahipa hybrids have been shown to possess a significantly shorter crop duration and may therefore increase the total period of tuber availability.
The introduction of improved yam bean cultivars in the traditional cropping systems practised by smallholders in Mexico and Central America is presently in progress. The same situation applies to India and South East Asia where advanced Mexican cultivars will have to be evaluated extensively under field conditions. Also in a number of West African countries, where both P. erosus and P. tuberosus have recently been introduced, the crop has attained a remarkable success. In these regions the introduction of yam bean cultivars into the traditional intercropping systems is of great interest. Therefore, the evaluation of the crop itself is urgently needed, including a wide range of available genotypes and interspecific hybrids of potential in sustainable intercropping systems at various climatic locations and with a tuber quality similar to the traditional root crops. Hybrids involving P. ahipa may again serve to reduce the cropping period. In spite of the difficulties associated with the statistical treatment of large-scale tests in intercropping designs, such a project should be given a high priority (Federer 1979; Pearce et al. 1988). The first steps in this direction have been conducted within the Yam Bean Project by estimating the nitrogen input of yam beans via the difference method (Castellanos et al. 1997).
The evaluation of the efficiency of both improved yam bean lines and interspecific hybrid-lines in connection with the development of adapted intercropping systems would likewise have a great impact. This is true not only with reference to the introduction of the crop in more tropical and subtropical regions, but also to retain Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) and improve the attractiveness of P. ahipa and the Jíquima cultivar group ( P. tuberosus complex) with the local farmers, thereby ensuring the conservation of these taxa.
Yam beans are not used or known as a basic food stuff in any parts of the entire distribution area, with one exception: the Chuin cultivar group within the P. tuberosus complex collected along the upper reaches of the Ucayali river. In this region, two P. tuberosus cultivar groups are found – the Ashipa and the Chuin. The last is used similar to the traditional root crops sweet potatoes and manioc. Because of their high dry matter content the tubers are prepared like the roots of the manioc. It seems probable that new cultivars involving the Chuins would have a major impact on Pachyrhizus tuber consumption and perhaps also on crop production systems that include this legume/root crop within intercropping systems. One way could be to include more cultivar characteristics into the Chuin type, such as high yield, determinate or semideterminate growth, shorter growth season and adaptation to a wide range of climatic environments. An alternative, and this is the current main breeding target within the Yam Bean Project, is to introduce the high dry matter character of the Chuin tuber into the high-yielding and widely adapted types of the P. erosus and/or P. ahipa genepool. Here emphasis must be given to designing a breeding strategy that will avoid genetic drift in other traits like protein, starch and sugar contents.
The successful development of the yam bean as a basic food crop will demand a widened breeding focus encompassing important nutritional characteristics such as protein quantity and quality of the tubers. It is most likely that P. ahipa has a composition similar to that of P. erosus, e.g. in P. erosus the amino acid composition is close to the ideal values for human nutrition formulated by FAO/WHO (1989) (see Section 4.3). Here the Mexican yam bean equals the values listed. Evaluations of the P. ahipa and P. tuberosus genepool are still needed to verify whether this also applies for these two species.
Provided the amino acid composition of P. ahipa and P. tuberosus is of the same quality as P. erosus, the main breeding aim for the two species, in terms of nutritional quality, must be the increase of pure protein content. That this is physiologically possible in yam beans can be concluded from the protein content of tubers from Winged Bean (Psophocarpus tetragonolobus (L.) DC.) and Vigna sp. (20-30% on dry matter basis). As suggested by Vavilov’s rule of homologous lines, accessions or single genotypes with a similar high protein content may also be found within the Pachyrhizus genepool. This aspect has been supported in the yam bean genepool by preliminary quality evaluations involving 16 P. ahipa accessions and 2 P. tuberosus accessions (Chuin). A protein content of P. ahipa within the range of 8-18% was recorded, whereas the range for P. tuberosus was 5-8% (pers. observ.).
10.3 Opportunities for modern biotechnology The use of in vitro multiplication techniques constitutes an attractive possibility when contemplating the rapid multiplication of genotypes of limited availability for conservation purposes (new material from field collections) or breeding material Promoting the conservation and use of underutilized and neglected crops. 21. 49 possessing agronomically attractive traits (limited cross-parents). If such genotypes became available instantly in sufficient quantities they could be subjected to broad use almost immediately following their identification/selection.
Several institutions, i.e. in Costa Rica, Denmark, Ecuador and Trinidad, have initiated studies in this field (Forbes and Duncan 1994; Muñoz et al. 1997). The experiments have so far involved regeneration and multiplication from adventitious and axillary shoots (explants) and callus formation with subsequent organogenesis, i.e. the development of protocols for somatic embryogenic systems.
In the area of molecular taxonomy, the relationship between the different species is currently being studied at the School of Biological and Medical Sciences, Plant Sciences Laboratory, University of St Andrews (Estrella E. et al. 1997). The identification of molecular analyses as a new approach appears to be an obvious one as improvement in agricultural production of yam beans is ultimately dependent on the availability of appropriate germplasm for developing drought-tolerant, photothermally neutral, and pest and pathogen-resistant cultivars capable of producing high yields over a wide range of climatic and edaphic conditions. In view of the critical situation of Pachyrhizus germplasm in South America, widely recognized by national and international agencies, a programme of assessment of genetic resources is of the highest priority.
The research at University of St. Andrews involves the estimation of molecular genetic distances and evaluation of cross-progenies, utilizing isozyme variation over 20 enzyme systems and polymerase chain reaction (PCR) to resolve randomly amplified polymorphic DNA sequences (RAPDs) analysis to assess and resolve the level and distribution of genetic diversity within and between species of Pachyrhizus.
The analyses are conducted on a representative sample of the existing germplasm collection of Pachyrhizus to include all species and a wide range of cultivars, landraces and wild material.
The survey of molecular genetic diversity in the germplasm collections of
Pachyrhizus will enable the following to be achieved:
• analysis of phylogenetic relationships within the genus Pachyrhizus with particular emphasis on the origin of the cultivated species P. ahipa, P. erosus and P. tuberosus
• location of natural centres of genetic diversity for collection and conservation
• estimation of genetic diversity levels within and among species
• identification of markers associated with attractive genetic traits, e.g. earliness, protein content.
Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) 11 Major and minor production areas There are no major production areas of P. ahipa. The production is restricted to a few localized villages and individual farmers; P. ahipa is partly cultivated for home consumption, and approximately 50% is marketed. In Bolivia the P. ahipa tubers are principally sold on the markets of the nearest villages which have organized vegetable markets, i.e. only a fraction of the production reaches the—markets in the larger cities.
No international trade in P. ahipa is known to take place.
Promoting the conservation and use of underutilized and neglected crops. 21.