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«Andean roots and tubers: Ahipa, arracacha, maca and yacon M. Hermann and J. Heller, editors Promoting the conservation and use of underutilized and ...»

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Even though the mature seeds have the highest amount of endogenous rotenone of any part of the plant it is the seed which suffers the most serious attack by an insect

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pest, i.e. the bean weevil Bruchideae). The bruchid Caryedes icamae Guerin-Meneville was identified in five seed samples from different localities in Bolivia (in 1994).

Nematodes may be a problem (Fig. 10); during evaluation of a Bolivian accession in Esmeraldas, Ecuador, the nematode Meloidogyne sp. completely destroyed all tubers in the test plants (Bertelsen and Stagegaard, unpubl. data).

In Bolivia the most severe tuber damages observed were rot due to lack of irrigation management, and/or nematodes causing a warty appearance. No other serious damages caused by pests or diseases during the vegetative period were recorded. This may be due to the presence of rotenone in the leaves and stems.

13.5 Harvesting Harvest takes place once the tuberous roots have attained marketable size, i.e.

depending on consumer preference whether small, medium sized or large tubers are preferred. The P. ahipa tubers are generally harvested after 7-9 months in Bolivia (as confirmed in field trials in Portugal), but the species has been found to be the earliest of all genotypes belonging to Pachyrhizus tested in the field trials in Mexico, i.e. marketable tubers were produced after 4 months! The preferred marketable tuber size is from 0.4 kg and up.

Bolivian growers of P. ahipa are reported to harvest the tubers by hand, using a hoe. The vegetative top is left to be incorporated in the soil in some cases, but many will clear the P. ahipa hay from the field.

13.6 Post-harvest handling The chilling sensitivity of the tuberous roots of P. erosus has been demonstrated and low-temperature storage has been found to reduce storage life considerably (Bruton 1983; Barile and Esguerra 1984; Paull and Jung Chen 1988; Cantwell et al. 1992). The optimal storage temperatures are between 12.5 and 17.5°C. Similar storage characteristics may well apply to P. ahipa.

Prolonged storage will serve to change the composition of the starch/sugar ratio.

Paull and Jung Chen (1988) found that after 3 months storage at 12.5°C the sucrose content tripled and only one-sixth of the starch remained. As many consumers prefer a sweeter tuber this may also explain the post-harvest treatment observed for P. ahipa in Bolivia, where some producers leave the tubers in a sunny place for up to 2 weeks prior to marketing (Ørting et al. 1996).

13.7 Yield In Bolivia, according to Ørting et al. (1996), the yield varies from 8 to 30 t/ha regardless of tuber size. Field trials conducted both at the DRATOM experimental Station, Trasos-Montes, Portugal and at the INIFAP/CIFAP-CEBAJ experimental station, Celaya, Guanajuato, Mexico, have during the past 4 years tested the three accessions initially available. With a growth season of 7 months (in Portugal) and 4 months (Mexico), yields of fresh tuberous roots ranging between 29 and 50 t/ha have been recorded with dry matter percentages of 19-25% (Vieira da Silva 1995; Castellanos et al. 1997).

Promoting the conservation and use of underutilized and neglected crops. 21. 59 14 Limitations of the crop The obvious principal constraint in transforming the yam beans from being considered a minor root/tuber crop into one of the major ones can be ascribed to a number of adverse coincidences. If the crop was multiplied in sufficient quantities, i.e. a broad range of genotype-lines, the scientific and developmental interest in the yam beans, which until recently has been negligible, would most certainly increase.

With their numerous apparent advantages, there is little doubt that once the crop receives more attention by scientists engaged in both basic and applied research the knowledge and subsequently the production would increase dramatically.

On a trait basis the major constraint is the 'fruity' tuberous root quality found in the majority of the yam beans if they are to constitute an attractive alternative to the starchy traditional root crops. This is especially of relevance when introducing the crop to areas unfamiliar with tuber/root crops of this type. The solution to the problem has fortunately become within reach with the identification of the Chuin cultivar group in Peru (P. tuberosus complex). This will allow the introduction of a high-yielding tuberous legume with a tuber dry matter content similar to that of the traditional root crops. Once the Chuin has been successfully introduced it will be easier to widen the range of yam bean cultivars in addition to those having the fruity or vegetable-like tuber quality. Therefore if new lines with the fruity quality of the P. ahipa tuber are to be bred, as is taking place at the INIFAP Campo Experimental Bajío, this is accomplished by using P. erosus x ahipa hybrids. At present the limited germplasm available has been included in a number of field trials evaluating yield performance and tuber quality. In general the tuber quality of the P. ahipa is not in accordance with the preferences of the Mexican consumer, i.e. the quality suffers from too high a dry matter content and too much sweetness. This is the tuber quality preferred by the Bolivian and Northern Argentinean consumer. However, the new lines of hybrid origin have the tuber quality of P. erosus in combination with the short cropping period of P. ahipa !

Because of the obvious competitive source/sink relationship between tuber growth and seed production, unpruned plants show a much less attractive tuber formation than those reproductively pruned (see Fig. 9c). Reproductive pruning is considered a major constraint owing to the laboriousness of the process, especially in P. ahipa because of the location and morphology of the inflorescences, i.e. simple, short racemes often placed close to ground level in both determinate and indeterminate genotypes. This is a major reason for the reluctance by Bolivian farmers to continue growing this crop (Ørting 1996b; Ørting et al. 1996) and must also be regarded as a major constraint in advanced production areas. No clear solution to this constraint has been identified so far (Matos and Vieira da Silva 1997).

This problem may be overcome by the introduction of genotypes with reduced flower/pod set, or with a growth habit approaching the habit of P. erosus where the erect inflorescences are produced above the vegetative part of the plant, thus facilitating easy removal. Further field and laboratory studies with different cultivars/landraces targeted on the correlation between tuber yield and other traits Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) in pruned and unpruned plants are needed to obtain more information on the physiological source/sink relationships (Grum et al. 1997). At this point it is of interest that random plants have been observed to initiate tuber growth prior to flowering. However, in yam bean it is necessary to ensure tuber formation in combination with sufficient seed production. In the future, it will be important to breed for new cultivars which will produce high tuber yields without reproductive pruning.

One of the main constraints to the cultivation of yam beans is the rapid decrease in germination when the seeds are stored under humid conditions, which has been observed repeatedly by the Yam Bean Project partners in Ecuador, Costa Rica and Tonga. There is an apparent correlation between seed weight, robustness/thickness of the testa and tolerance to high levels of humidity, i.e. the small and durable seeds of the wild species P. panamensis have been found to retain a high germination percentage for a considerably longer period under adverse storage conditions.

The most serious pest problem is also related to the seeds, i.e. the various species of bean weevils/bruchids, but as the susceptibility to attack by this pest group varies between the species as well as between accessions belonging to the same species it may be feasible to breed for increased resistance (see Section 15).

Among the leaf-damaging insects the genus Diabrotica is doubtless the most serious pest recorded in the humid parts of the Latin American tropics. Again, considerable resistance to attacks has been recorded between the different accessions with an obvious reduced susceptibility recorded in all genotypes possessing various degrees of pubescence due to the presence of strigose hairs.

The bean common mosaic virus (BCMV) is the most serious viral disease in yam beans. Although not fatal, the affected plants will produce yields reduced by 20As this disease maybe seed-transmitted at a low rate, individual infected plants should be removed if seen in plots for seed production.

The splitting or cracking of the tubers prior to harvest may be a serious problem under certain climatic/edaphic conditions (Ramaswamy et al. 1980). If the crop is irrigated, care should be taken not to irrigate for some weeks before harvest (Ing.

Agr. A. Heredia Z., pers. comm.).

Tubers that are physically damaged during harvest are susceptible to attack by common fungi and increased dehydration during storage (Cantwell et al. 1992).

Wounded yam bean tubers suffer greatly from textural changes, decay and internal browning caused by the fungi Rhizopus stolonifer (Ehrenb. ex Link) Lind., Cladosporium sp. and Penicillin sp. when stored at low temperatures and high relative humidity (80%) (Bruton 1983).

Experiments examining possible processing procedures of P. ahipa tubers have yet to be conducted.

No negative effects from consuming P. ahipa tubers have been reported.

Promoting the conservation and use of underutilized and neglected crops. 21. 61 15 Prospects The advantageous features shared by all three cultivated species are first and foremost due to the unique combination of the general characteristics present in most

cultivated legumes:

• the producer - good adaptability to a wide climatic and edaphic range with the yield reliability of the root/tuber crops

• the consumer-a well-balanced and nutritious composition of the protein/starch contents with an agreeable taste

• the processor - good post-harvest/ storage characteristics

• the environment - the biological nitrogen fixation (sustainability) and little demand for fuel wood in the preparation of food (all P. ahipa cultivars produce tubers which are consumed/used fresh).

The quality aspects of each of the parts utilized - tuberous root, forage hay and mature seed for the extraction of rotenone and rotenoids - differ among species.

The only results available on seed yield in P. ahipa are based on greenhouse experiments involving 20 different accessions. The number of mature seeds harvested per plant is given in Table 6.

Table 6. Seed yield in P.

ahipa, based on an experiment conducted under greenhouse conditions (Ørting 1996)

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The yield results, but especially the seed yield results, indicate that also within this species sufficient variation may be available to implement a pre-breeding programme aimed at the development of new high-yielding cultivars with acceptable seed production.

A possible future use of the Andean yam bean as an industrial crop (non-food crop) has become a possibility following recent analysis of P. ahipa tubers (see Sections

4.2 and 4.4). The dry matter composition of P. ahipa tubers contains 45-55% starch (practically pure amylopectin), 10-15% sugar (glucose, fructose, saccharose) and 10protein. The protein does not precipitate over a wide range of pH values. The latter characteristic may be attractive to the food industry. However, the main interest in the Andean yam bean tuber as a renewable resource for the industry is due to its high content of virtually pure amylopectin. The pure starch yield of P. ahipa, calculated from landraces, reaches that of high-yielding potato cultivars.

Nevertheless, until now no investigations on pre-industrial processing of P. ahipa starch, sugar and protein have been carried out; this is currently being undertaken.

If industrial processing were to be verified on a laboratory scale, P. ahipa could be considered when breeding new industrial crops.

Thus, mainly two breeding aims must be considered:

• the extension of the production area to include higher latitudes (production further north than Spain and South France would be possible, if the growth period required was to be genetically reduced)

• the avoidance of the laborious reproductive pruning, by selection for those genotypes that have good tuber formation without pruning.

That the necessary genetic variation within the Andean yam bean germplasm exists to allow such breeding efforts has been indicated in the recent P. ahipa greenhouse evaluations (Ørting 1996a).

Promoting the conservation and use of underutilized and neglected crops. 21. 63 16 Research needs From the results of the experiments concluded so far, a number of specific points of interest can be identified.

• Taxonomic and phylogenetic studies using both molecular and numerical systematic methods of analysis in order to clarify the origin of and relationship between the cultivated species

• Field collecting of P. ahipa germplasm (both wild material and rare landraces) in Peru

• Conservation and field evaluation of P. ahipa germplasm (both wild material and rare landraces) originating in northern Argentina, Bolivia and Peru

• Initiation of a breeding programme to improve the existing landraces of P. ahipa in order to ensure continued cultivation of these endangered crops in Argentina and Bolivia

• Breeding of new interspecific hybrids involving: (a) the recently identified P. tuberosus landraces with high dry matter and starch content (the Chuin), (b) high-yielding P. erosus accessions, and (c) early, bushy P. ahipa accessions

• Physiological analysis of the source/sink relationship in P ahipa, with specific reference to the competition between tuberous root growth and legume formation

• Physiological analysis of the biological nitrogen fixation

• Biochemical analysis of the nutritional composition of the most promising interspecific hybrids, i.e. lines which are approaching release as new cultivars

• Biochemical analysis and evaluation of P. ahipa landraces as a potential non-food crop

• Biochemical analysis and evaluation of P. ahipa as sources of rotenone/rotenoids with technical and/or agronomic potential

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