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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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The now valid generic name Pachyrhizus (pachy = thick, rhizus = root) was originally used by L.C.M. Richard in the illegitimate species name Pachyrhizus angulatus on a herbarium specimen. This name was used by De Candolle spelled in the original way, i.e. with a single ‘r’ when first published in 1825. Sprengel(1827) was the first to introduce the incorrect spelling of ‘Pachyrrhizus’. Later, when conserving the generic name Pachyrhizus against what was considered a barbaric(!) name: ‘Cacara’ (Briquet 1906), the erroneous spelling of ‘Pachyrrhizus’ was retained.

However, according to the present botanical code the spelling used by L.C.M. Richard is the valid one. Further details concerning the origin of the name of the genus and the species are given in Sørensen (1988).

1.2 Nomenclature Genus: Pachyrhizus Rich. ex DC. (1825: 402 nom. cons.); type species: P. angulatus L.C.M. Richard ex DC. (nom. illeg. = P. erosus (L.) Urban)

Generic synonyms:

Cacara Rumph. ex Du Petit-Thouars (1806: 35 nom. rej.); Type species: C. bulbosa Rumph. ex Du Petit-Thouars ( = P. erosus (L.) Urban).

Taeniocarpum Desvaux (1826:421); type species: T. articulatum (Lam.) Desv. ( = P. erosus (L.) Urban).

Robynsia Martens & Galeotti (1843:193); type species R. macrophylla Mart. et Gal.

( = P. erosus (L.) Urban).

1.3 The genus The yam bean, i.e. the Neotropical genus Pachyrhizus Rich. ex DC., is placed taxonomically in the subtribe Diocleinae, tribe Phaseoleae within the legume family (Fabaceae), according to Lackey (1977). The genus currently comprises five species (Sørensen 1988). Three of the five species are cultivated for their edible tubers and the remaining two are only to be found in the wild.

The fact that among the cultivated species only the Mexican yam bean has been spread more or less pantropically cannot be explained by the lack of agriculturally attractive characteristics in the remaining two cultivated species. This situation must be interpreted as either solely due to their specific climatic adaptation and/or possibly combined with the historical progress of the Spanish and Portuguese conquest of Latin America and the general policy of destroying the traditional Andean agricultural systems.

Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) Although the genus had been the subject of a previous taxonomic revision by Clausen (1945), the taxonomy of the genus remained somewhat diffuse, especially for the South American species. This was mainly due to the limitation of available herbarium material caused by World War II. Also, the narrow species concept held by the author of this first revision contributed to the considerable complexity of this work, e.g. made obvious by the great number of infraspecific taxa included. Hence, as the herbarium material available for a new taxonomic revision included material from all relevant European herbaria as well as the material collected during the 40 years elapsed since the work by Clausen (1945), it appeared that the completion of a new revision was justified, see Sorensen (1988).

The genus Pachyrhizus is morphologically delimited by the presence of the following characteristics. Vines or semi-erect herbaceous to somewhat lignified perennial plants. All species have more or less prominently tuberous roots, one or more per plant. Trifoliolate leaves with stipules, pinnately arranged leaflets with caducous stipels. The inflorescence is a complex to simple raceme and the flowers have a tubular calyx and a papilionaceous corolla. The ovary has a basal crenate disc-formed nectarium, the recurving style is ciliated forming a ‘false beard’ of short hairs along the dorsal (= adaxial) side of ovary practically continuing to the base of the stigma along the incurved side of the style and the vertical, subglobose surface of the stigma is placed in the middle or almost terminally. The straight legume is septate between the seeds and the seeds are square, more or less flattened or rounded, kidney-shaped with colours ranging from olive green, deep maroon, lilac, to black or black and white/cream mottled.

Chemosystematic examinations of the phylogeny and interrelationships of Pachyrhizus on the generic as well as on the subtribal level have so far been limited to the studies of canavanine by Lackey (1977) and of isoflavonoid phytoalexins by Ingham (1979). [Lackey (1977) placed Pachyrhizus in the subtribe Diocleinae; Ingham (1979,1990) suggested a close affinity between Pachyrhizus (subtribe Diocleinae) and the Palaeotropical genus Neorautanenia (subtribe Glycininae, according to Ingham (1979) and subtribe Phaseolinae (Ingham 1990)), a relationship that according to Ingham (1979) could justify the transfer of Pachyrhizus to the subtribe Glycininae.

Ingham (1990) suggests that the genera Pachyrhizus and Calopogonium (both Diocleinae) may bridge the gap between the genera Neorautanenia (Phaseolinae) and Pueraria (Glycininae).] Molecular analyses by Bruneau et al. (1990) studied the significance of a chloroplast DNA inversion as a subtribal character in the Phaseoleae and demonstrated the presence of the inversion in 11 of the 23 genera included in the subtribe Phaseolinae. All six genera (including Pachyrhizus) examined within the subtribe Diocleinae lacked the inversion. The result is thus in agreement with the subtribal classification as suggested by Lackey (1981). A new research project involving the study of isozymes, chloroplast DNA and ribosomal DNA has recently been initiated between the School of Biological and Medical Sciences, Plant Sciences Laboratory, University of St. Andrews, Scotland (Dr R.J. Abbott and Ing. Agr. J.E.





Promoting the conservation and use of underutilized and neglected crops. 21. 19 Estrella E.) and the Botanical Section, Department of Botany, Dendrology and Forest Genetics, Royal Veterinary and Agricultural University, Copenhagen, Denmark (Estrella E. et al. 1997). The results of these studies have revealed considerable agreement between molecular systematic affinities and the numerical taxonomy based on morphological characters. Two main groups/branches have been identified — one containing the species P. tuberosus, P. ahipa and P. panamensis (the species of South American origin) and the second with the Central American and Mexican species of P. erosus and P. ferrugineus. When looking at the species level and below, these examinations indicate that molecular methods of analysis can serve to demonstrate differences, not only between the species, but of even greater interest between different genotypes within a single species — see discussion below on the P. tuberosus complex in Sørensen et al. (1997).

A palynological study of the genus (Sørensen 1989) revealed that the interspecific variation was sufficient to allow unequivocal identification of the single species. The greatest infraspecific variation was, not surprisingly, observed in the pollen grains from the cultivated species.

1.3.1 The species: Pachyrhizus ahipa (Wedd.) Parodi (Fig. 1) Pachyrhizus ahipa (Weddell) Parodi (1935: 137).

Basionym: Dolichos ahipa Weddell (1857: 113).

Type: Mandon, G. 747, Boliviense, prov. á Laracaja, Sorata ad rivum (P. holo).

Heterotypic synonyms: Pachyrhizus ahipa (Wedd.) Parodi var. violacea Parodi (1935:

138).

Type: Parodi, L.R. 12145, ex cult. in Hort. Bot. Fac. Agr. B. Aires (BAA, holo).

Pachyrhizus ahipa (Wedd.) Parodi var. albiflora Parodi (1935: 138).

Type: Parodi, L.R. 12146, ex cult. in Hort. Bot. Fac. Agr. B. Aires (BAA, holo).

Vernacular names: ajipa (ahipa) (Spanish); Andean yam bean (English); Andine Knollenbohne or Andine Jamsbohne (German); l’ahipa or dolique tubereux d’Ande (French).

20 Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) Fig. 1. Pachyrhizus ahipa. Habit, 2/3 of natural size. B. Flower, side view. C. Flower, seen from underneath. E. Calyx, opened. F. Standard. G. Wing. H. Keel. J. Stamens. K. Pistil with basal disc. L. Side view and front view of style and globose stigma. M. Side and top view of seed.

N. Section of abaxial surface of leaf. O. Tuberous root. (All parts from AC102, originally from the Province of Tarija, Bolivia).

Promoting the conservation and use of underutilized and neglected crops. 21.

2 Description of P. ahipa

2.1 Botanical/morphological description of the species Pachyrhizus ahipa is morphologically distinguished by being a herbaceous plant with entire leaflets (a few individual plants possessing dentate leaflets have been recorded), with short racemes (48-92 mm) and the general absence of lateral axes, i.e. simple racemes. The number of flowers per lateral raceme is, if present, as low as 2-6. The wing and keel petals are usually glabrous, but slightly ciliolate specimens have been seen. The wings curl outward following anthesis and this is a feature within the genus seen only in P. ahipa. The pod is 13-17 cm long and 11-16 mm wide, almost circular in cross-section when immature, i.e. only slightly dorsiventrally compressed (Fig. 1). Seeds are black, lilac, maroon or black and white (cream) mottled, never olive-green or red; rounded kidney-shaped, never flattened and square, 9 x 10 mm. The 100-seed weight is (17.3-) 29.2 (-41.2) g.

This species is furthermore unique in that both twining/trailing and semi-erect to short bushy erect growth habits are found, i.e. both determinate and indeterminate genotypes exist. Erect genotypes are 15-40 cm tall, semi-erect 30-60 cm and twining types 60 cm to several metres long.

2.2 Reproductive biology Like all Pachyrhizus species, P. ahipa is a perennial (although cultivated as an annual crop), and may be multiplied vegetatively by cuttings (or in the case of multituberous genotypes, also from tubers). But in contrast to the majority of the tropical root crops all species in the genus are mainly reproduced generatively. Thus all cultivated species, including P. ahipa, are replanted by seed annually.

Flowering: both short- and long-day plants exist in the genus. Even though P. ahipa in its area of origin is grown under a short-day photoperiod, all accessions are short-day insensitive. On the other hand the entire germplasm of P. erosus and P. tuberous must be regarded as short-day sensitive and reproductive development does not normally occur under greenhouse conditions at high latitudes.

Usually sown in August-October (Bolivia), with reproductive pruning (manual removal of flowers) starting 4-7 months after sowing in November-March and with mature pods in April-June. The planting date is mainly determined by the rainy season. During the flowering period the labour-intensive reproductive pruning is conducted once or twice. The flower pruning is only necessary to a certain extent, as the tuber formation is mainly affected by the number of pods produced per plant with fully developed seeds and not by the amount of flowers per plant. Hence, good tuber formation also takes place in profusely flowering plants, but with a high abscission rate. The seeds are easy to harvest owing to the non-dehiscent pods and they generally have high 1000-seed weights.

In the landraces two seed multiplication strategies in this predominantly selfpollinating species were reported (Ørting et al. 1996). (While P. erosus and P. tuberous may be regarded as mainly being self-pollinators, P. ahipa must be regarded as a Ahipa (Pachyrhizus ahipa (Wedd.) Parodi) partial self-pollinator.) The common strategy is to select the most vigorous plant within a field for seed production and reproductively prune the remaining plants in order to increase tuber growth/yield. The second strategy involves leaving the initially produced legume on each plant for seed production, followed by the removal of all subsequently produced flowers. In both practices an indirect selection was said to be implemented when harvesting the seeds by selection on the basis of largest size (Ørting et al. 1996). It is to be expected that these two seed multiplication strategies, practised in the area of origin, have a high impact on the variation within each individual accession. In greenhouse experiments a lower variation was detected within accessions multiplied by the first strategy with respect to a much higher variation recorded in those accessions multiplied by the second multiplication strategy.

This pollination behaviour cannot be considered as a result of the flower morphology only, i.e. with an internally curved stigma in close contact with the anthers. It is also determined by the low pollen fertility and the presence or absence of pollinating insects, i.e. the only natural method of cross-pollination. With respect to pollen fertility and number of flowers produced, highly significant differences between the accessions - and therefore tuber formation - could be detected under greenhouse conditions. In spite of the low pollen fertility, both infra- and interspecific crosses may be carried out successfully. Fertile interspecific hybrids from all combinations (including reciprocal crosses) involving the three cultivated species were obtained with the exception of the cross between P. ahipa (female) x P. tuberous (male) where seeds were produced but were not capable of germinating. The pollen fertility of the hybrids was in general reduced by 10-20% compared with the parental species. Hybridization experiments involving the wild species P. panamensis, conducted in Benin and Tonga, have demonstrated this species to be fully compatible with the three cultivated species (Grum 1994; Prof. D.F. Adjahossou, pers. comm.

and Dr P.E. Nielsen, pers. comm.).

In terms of plant breeding it should be stressed at this point that the combination of vegetative and generative reproduction by selfing is unique to the cultivated species. This combination may be advantageously integrated into a breeding programme. Thus in breeding strategies the species may be treated as a cloneable, annual self-pollinator.



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