«Andean roots and tubers: Ahipa, arracacha, maca and yacon M. Hermann and J. Heller, editors Promoting the conservation and use of underutilized and ...»
Wells placed four species (S. glabratus, S. microcephalus, S. parviceps and S. fruticosus) into a ‘glabrata complex’, a group formed by shrubs or small trees reaching sometimes 10 m or more. Smallanthus jelksii and the related S. pyramidalis also reach tree size. Nevertheless, they appear to be more related to the yacon group than to the glabrata complex.
Promoting the conservation and use of underutilized and neglected crops. 21. 209 4 Species description
4.1 Botanical/morphological description The yacon is a perennial herb, 1.5-3 m tall. The root system is composed of 4-20 fleshy tuberous storage roots that can reach a length of 25 cm by 10 cm in diameter, and an extensive system of thin fibrous roots. Storage roots are mainly-fusiform, but often acquire irregular shapes due to the contact with soil stones or the pressure of neighbouring roots. Roots have an adventitious nature, growing from a developed and ramified stem system formed by short, thick sympodial rhizomes or rootstock (‘corona’, crown) (Fig. 3).
Storage root growth is caused by the proliferation of parenchymatous tissue in the root cortex and particularly in the vascular cylinder. The parenchyma accumulates sugars and, in some cases, pigments typical of certain clone groups.
According to pigments, flesh colour varies considerably: white, cream, white with purple striations, purple, pink and yellow. The tuberous root bark is brown, pink, purplish, cream or ivory white, thin (l-2 mm) and contains resin conduits filled with yellow crystals.
The aerial stems are cylindrical or subangular, hollow at maturity with few branches in most clones or ramified in others, densely pubescent, green to purplish.
Lower leaves are broadly ovate and hastate or subhastate, connate and auriculate at the base; upper leaves are ovate-lanceolate, without lobes and hastate base; upper and lower surfaces are densely pubescent. Lower and upper epidermis have trichomes (0.8-1.5 mm long, 0.05 mm diameter) and glands which contain terpenoid compounds (Fig. 4a, b).
Inflorescences are terminal, composed of 1-5 axes, each one with 3 capitula;
peduncles densely pilose. Phyllaries 5, uniseriate and ovate. Flowers are yellow to bright orange; ray flowers are 2 or 3-toothed, depending on the clone, to 12 mm long x 7 mm broad, pistillate; disc flowers about 7 mm long, staminate. Immature cypselas are purple, and turn dark brown or black at maturity (Fig. 5).
4.2 Reproductive biology Flower production is more reduced in yacon than in other wild Smallanthus species.
Reduced flowering and fruit set are features commonly present in other clonally propagated tuber crops. During yacon evolution, continued vegetative propagation and selection for root yield may have impaired flowering and fruit set.
Flowering is strongly dependent on the environment of the growing area. In some regions, such as northwestern Argentina, flowering happens very late in the growing cycle or not at all. On the contrary, flowering is intense in most clones in northern Bolivia, the growing areas around Cusco, southern Peru and Cajamarca, northern Peru. In the Cajamarca region flowering begins 6-7 months and peaks 8-9 months after planting. But even in the areas where flowering is abundant, seed set is frequently poor or nonexistent and a high proportion of the seeds are non-viable or show low vigour.
Yacon. Smallanthus sonchifolius (Poepp. & Endl.) H. Robinson Fig. 3. Yacon (Smallanthus sonchifolius) morphological aspects (from León 1964). A: flowering branches. B: leaves. C: flowerhead. D-F: tuberous roots. G: transverse section of the tuberous root (x, xylem; c, cortex tissues). H: staminate disk flower. I: pistilate ray flower.
Promoting the conservation and use of underutilized and neglected crops. 21. 211 Poor seed set and low seed vigour can be the result of problems at different levels.
One factor is high pollen sterility. Grau (1993) failed to obtain viable seeds under glasshouse conditions working with a single clone grown commercially in New Zealand. Artificial pollination was tried, but pollen was highly sterile and no filled fruits were produced. Low pollen fertility (0-30% fertility) also was observed in Argentine (Grau and Slanis 1996) and Ecuadorian clones (Grau, unpublished, material supplied by Dr R. Castillo, INIAP). In these cases pollen was stained using the Alexander methodology. Aberrant pollen grains have been observed in other species of Smallanthus (Fisher and Wells 1962; Wells 1971). Abnormal pollen development arises in many cases from irregular meiosis. However, meiosis appears Fig. 4. Scanning microscope image of the upper epidermis showing epidermal trichomes (a) and epidermal glands (b).
Yacon. Smallanthus sonchifolius (Poepp. & Endl.) H. Robinson be normal in yacon (Frías et al. 1997b), in spite of its high ploidy level and likely hybrid origin (see Section 4.3).
Lizárraga et al. (1997) analyzed seed set using paper bags, mesh bags and openpollination. Open-pollination yielded twice as many seeds as mesh bags, which showed slightly better performance than paper bags. These results indicate that pollinators are very important, probably because pistillate ray flowers mature earlier than staminate disk flowers. In northwestern Argentina bumblebees (Bombus sp.) have been observed actively pollinating yacon and S. macroscyphus. Unidentified Hymenoptera have been observed playing the same role in Bolivia.
Other results point to inadequate germination conditions, dormancy or hard coat. Hard coat inhibiting germination is a trait present in S. macroscyphus, a wild species with high pollen fertility and high seed production (Grau and Slanis 1996), and may also be present in yacon. Experiments by Rea (1995a) yielded unfilled and filled cypselas, but he failed to germinate the filled ones. Low germination temperatures (12-15°C) may be partly responsible for this result. Meza (1995) obtained only one seedling out of about 300 seeds sown under glasshouse conditions. Chicata (1996, pers. comm.) obtained better results by selecting the filled cypselas from a sample containing empty and half-filled ones and germinating them at 28°C.
Fig. 5. Typical yacon flowerhead, photographed at Bárcena, Jujuy province, Argentina.
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At present there are still many important gaps in the knowledge of yacon reproductive biology. Most perennial crops are outbreeders and this behaviour is also present in sunflower (Helianthus annuus) and topinambur (Helianthus tuberosus), two crop species in the same tribe as yacon. But there is no experimental report on the yacon mating system. It is also unknown whether yacon seeds are orthodox or recalcitrant. Flowering can be artificially induced in yacon by grafting onto sunflower (Nakanishi and Ishiki 1996), and this technique may represent a useful tool in future reproductive biology studies.
4.3 Chromosome numbers The first report on yacon chromosome number (2n=60) was published by Heiser (1963), working with Ecuadorian material. A year later Leon (1964) reported 2 n=32 using material grown at the La Molina University in Peru. Recent reports by Talledo and Escobar (1996) indicated 2n=60 in Peruvian material. However, more detailed studies (based on 1256 cell counts) by Salgado Moreno (1996) and Ishiki et al. (1997) on 15 clones from Ecuador (1), Peru (8), Bolivia (4) and Argentina (1) showed that all but one had 2n=58. The remaining clone had a somatic number of 87. A somatic value 2n=58 has also been observed by Frías et al. (1997b) in material from northwestern Argentina (Fig. 6).
Talledo and Escobar (1996) suggested that yacon is a tetraploid. Grau and Slanis (1996) speculated with the possibility of yacon being an allotetraploid, with Fig. 6. Typical yacon caryotype with 2n=58 chromosomes (from Frías et al. 1997b).
Yacon. Smallanthus sonchifolius (Poepp. & Endl.) H. Robinson S. macroscyphus as one of the putative parents, a role that could also be played by S. riparius. The studies of Ishiki et al. (1997) are consistent with allopolyploidy, suggesting a yacon caryotype composed by two genomes. They propose an octoploid 6A+2B structure as the dominant in most yacon clones 2n=58, while a dodecaploid 9A+3B structure would explain the 2n=87. Box 1 shows the hypothetical crossings that occurred during the evolution of yacon.
The studies of Salgado Moreno (1996) and Ishiki et al. (1997) are the most detailed and comprehensive so far. Nevertheless more studies are necessary to assess the validity of the reports indicating different chromosome counts. As a clonal crop, yacon could exhibit considerable diversity in chromosome numbers. Another aspect to consider is the presence of B chromosomes, reported in other Smallanthus taxa (Wells 1971; Ishiki et al. 1997), which may be an important factor affecting results.
Further studies are also needed to accept, improve or reject the hypothesis of yacon hybrid origin.
Wells (1967) published the first review of Smallanthus (Polymnia) chromosome numbers, and the most common value listed is 2n=32 (S. apus, S. oaxacanus, S. maculatus, S. uvedalius). The same value has been obtained for S. riparius (Robinson Box 1. Hypothetical evolution of yacon (by lshiki et al. 1997, modified by A. Grau)
et al. 1981), S. connatus (Wulff 1984) and S. macroscyphus (Rozenblum et al. 1985; Frías et al. 1997a). However, the general picture is still blurred because of the different results reported for the same species by different authors and sometimes the same authors (Table 1). Smallanthus jelksii and S. pyramidalis share the 2n =58 value with yacon. However, they are shrubs or small trees and seem unlikely ancestors.
Table 1. Chromosome numbers of Smallanthus species
5 Origin, evolution and history Several wild Smallanthus species (S. glabratus, S. riparius, S. siegesbeckius, S. macroscyphus and S. connatus) show a clear preference for disturbed habitats, like riverbanks, landslides and roadsides. The growth habit of Smallanthus is well adapted to take advantage of vegetation gaps (Fig. la). The strategy of colonizing areas free of vegetation may be the reason why yacon became associated with humans in the first place. Agriculture in the steep eastern slopes of the Andes, particularly slash-and-burn agriculture practised by the Andean people since prehistoric times, may have provided an ideal niche for yacon relatives. In present times this behaviour can be observed in the Vilcanota river basin, Peru, where S. siegesbeckius is a common invader of abandoned fields and a weed in coffee plantations. The same strategy is used by S. macroscyphus in northwestern Argentina, invading abandoned sugarcane fields and shrubby vegetation patches between cultivated fields. It seems highly possible that a hybrid of two or more Smallanthus species colonizing disturbed habitats gave rise to a species ancestral to yacon.
It is likely that in a very early stage the Andean peasants discovered yacon properties and changed its status from weed to a managed plant, and later to a cultivated plant. The most probable area where these early events took place is the eastern humid slopes of the Andes, in the region extending from northern Bolivia to central Peru, the area with the largest clone diversity, and where native Quechua and Aymara names are used. Diversity of clones is more reduced in Ecuador, where modifications of the Mexican word xicama dominate. Both facts may indicate that the species was introduced there at later stages, perhaps with the Inca conquest of Ecuador, only decades earlier than the Spanish invasion.
Although the mountain forests of central Peru and northern Bolivia are evergreen and supplied with abundant rainfall and mist during most of the year, they are subjected to a relatively dry winter period lasting 2-4 months. This drier and slightly cooler interval may have played a critical evolutionary role, generating the conditions under which large tuberous roots could have an adaptive advantage.
From the humid mountain forests of Peru and Bolivia, yacon may have expanded to the north and south along the humid slopes of the Andes, to the dry inter-Andean valleys and to the Peruvian coast. It is in the coastal archeological sites of Nazca (500-1200 AD), Peru that the oldest phytomorphic representations of yacon have been identified, depicted on textiles and ceramic material (Safford 1917; Yacovleff 1933; O’Neal and Whitaker 1947). Further south, putative remains of tuberous yacon roots have been recovered at a site of the Candelaria culture, which developed between 1 and 1000 AD in the Salta province, south from the present area of cultivation in Argentina (Zardini 1991).
The first written record on yacon (llacum) is by Felipe Guaman Poma de Ayala (1615) in a list of 55 native crops cultivated by the Andeans, including eight crops introduced from Spain. The chronicler priest Bernabé Cobo (1653) produced a more detailed description, pointing out its use as a fruit and its capacity to withstand several days of transport by sea.
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In the 19th century Weddell (1857) called attention to the qualities of yacon roots, named the species Polymnia edulis and collected the herbarium type. According to Perez Arbeláez (1956), yacon was exhibited for the first time in Europe at the Paris exhibition at the beginning of the century. European interest was not very significant though. In Italy there was a serious cultivation attempt in the late 1930s, which faded during World War II (Calvino 1940).
Affected by deep cultural changes, yacon cultivation has declined slowly and steadily throughout the Andes during most of the present century, to the point that the German researcher and Andean crop enthusiast H. Brücher mentioned it in his excellent monograph of useful Neotropical plants (1989), “for the sole reason of completeness.” Fortunately a drastic change in the international awareness of the crop has occurred during the 1980s, particularly after the publication of Lost Crops of the Incas (National Research Council 1989). The growing interest in the crop outside the Andes has stimulated a new wave of attention and research on yacon in the Andean countries.