Yacon (Smallanthus sonchifolius syn. Polymnia sonchifolia)
[Much of this information has been gleaned from IPGRI's excellent book on Andean Roots and Tubers: Ahipa, arracha, maca and yacon, edited by M Herrmann and J Heller (Promoting the conservation and use of underutilized and neglected crops 21); the chapter on yacon in this volume has been written by Alfredo Grau and Julio Rea.]
Many of the edible tubers and roots originating from the Andean region of South America were used by the local inhabitants as sources of food energy. However, two (yacon and ahipa, that is Pachyrhizus ahipa) are considered fruits.
Yacon tuberous roots have a sweet flavour and are crunchy to eat, like traditional fruit.
They are eaten raw, usually after being dried in the sun, which increases their sweetness, by partly hydrolysing oligofructans, producing fructose, glucose and sucrose. Drying wrinkles the skin, which is peeled before eating. The roots can also be stewed or can be grated and squeezed through a cloth to produce a drink. Consumption of yacon in some areas is linked to particular cultural or religious festivals.
Yacon roots contain fructose, glucose, sucrose. low polymerization degree (DP) oligosaccharides (DP 3 to 10 fructans) and traces of starch and inulin. The proportions of oligofructans and monosaccharides can vary during growth and after harvest. The roots also contain small amounts of fibre, vitamins and minerals.
Inulin is a high-DP oligofructan with a DP of about 35 and is a main storage compound in many plants of the Compositae family, such as sunflowers and Dahlia sp. In yacon, however, inulin is present only in trace amounts and oligofructans with a DP averaging 4.3 account for 67% of the dry matter at harvest.
However, yacon is more productive as an inulin source than the most likely industrial competitor, topinambur (Helianthus tuberosus, Jerusalem artichoke).
Yacon root carbohydrates can be readily metabolized by ruminants but the palatability of the leaves is believed to be low.
Stems have been reported to contain 11% protein by dry weight and the leaves 17% protein.
Medicinally, yacon has been used for diabetics and those with digestive problems. Dried yacon leaves have been used as a tea with hypoglycemic properties and are commercially sold as such in Brazil.
Hypoglycemic properties have been demonstrated in diabetic rats. However, leaves also contain as yet unidentified toxic products when injected in rats and caution is needed.
Yacon has a relatively low energy value. In modern times, its fibre content and low calorie content may make it an attractive fruit or ornamental vegetable for those individuals who would normally have an excessive intake of carbohydrates and fat and lead a sedentary lifestyle.
Yacon is a perennial herb growing to 3 metres tall. The root system is composed of 4-20 fleshy tuberous storage roots that can reach a length of 25 cm, with a diameter of 10 cm, as well as an extensive system of thin fibrous roots. Productivity in yacon is correlated with the number of roots per plant. The roots accumulate sugars which can be readily monitored by measuring the refractive index of the root juice. The preferred colour for the root flesh in South American markets is yellow but a range of colours is available.
The stems are cylindrical or subangular, hollow at maturity with some branches. Inflorescences are terminal, composed of 1-5 axes, each one with 3 capitula.
Flowering varies greatly with the growing conditions. Yacon is either day-neutral or has a weak short-day response for stem and tuberous root production. It can be artificially induced to flower by grafting onto sunflowers, which belong to the same tribe.
The plant can grow from sea level to 3500 metres above sea level but the optimal temperature range for growth is 18 to 25 C. Low night temperatures appear to be necessary for optimal storage root production. High soil temperatures have a negative impact on productivity.
Yacon requires an optimum of 800mm of rainfall, so irrigation is often needed in many areas.
It does well in rich, moderately deep to deep, light, well-structured and well-drained soils. It tolerates a wide range of soil pH, from acid to weakly alkaline.
Yacon can be propagated vegetatively with 8-12 cm long offsets taken from the underground and above ground rootstock (crown), with a few or no roots attached. The rootstock can be divided into pieces easily. Storage roots with no stem attached are not able to produce shoots.
Aerial stem cuttings can be easily rooted if kept moist.
Rooting occurs best under mist and auxins can improve the response.
Yacon can be propagated in vitro using modified Murashige and Skoog media and tissue culture has been used to select for lines with higher sugar content.
Planting distances in Peru vary from 70-100 cm between rows and 60-80 cm between plants within rows. In Brazil, offsets are planted in furrows 1 m wide and 30-40 cm high at a depth of 15 cm, 90-140 cm apart.
A range of insect pests attack yacon in Peru but natural control agents are usually effective.
Fusarium, Erwinia and Sclerotinia have caused wilting or rotting of the roots or stems.
Being a perennial crop, yacon has been suggested as a crop to control soil erosion on steep sloping areas in the Andes. As well, the leaves can tolerate partial shading, a trait useful for agroforestry.
Roots are mature after 7-12 months, depending on the altitude and can be harvested with similar machinery to that used for potatoes. Roots are brittle when freshly harvested and can suffer postharvest damage in transport, but can be stored for extended periods of time. Yacon can be highly productive, producing 28-100 t fresh weight material per hectare, with 15-30% dry matter.
* Seed production from yacon is often poor and seed frequently non-viable. Reasons for this may include:
* Poor cross pollination, with pistillate ray flowers maturing before staminate disk flowers,
* Pollen sterility, perhaps a result of the tetraploid/octoploid chromosome constitution,
* Poor seed germination, often associated with poorly filled seed,
* Seed dormancy,
* Hard seed coats and
* Possible viral infections influencing seed quality in some regional areas.
Characteristics that would be useful in yacon, if introduced from wild relatives would include:
* thicker root bark,
* looser storage root arrangement (which would reduce the deforming pressures in the soil),
* reduced sweetness (which may indicate a higher oligofructans level) and
* tuberous roots with sprouting capacity for use as propagules.
The original domestication of yacon probably occurred in the region extending from northern Bolivia to central Peru. It was first exhibited in Europe at the beginning of this century and a serious cultivation attempt was made in Italy in the late 1930's. A drastic increase in awareness of yacon occurred in the 1980's and 1990's, following its inclusion in the 'Lost Crops of the Incas' publication (National Research Council, USA, 1989).
Yacon is currently grown as an opportunity crop for family consumption or for local trading in many areas in South America between Ecuador and north-western Argentina. The centre of diversity for yacon appears to be in southern Peru and eastern Bolivia.
Yacon has also been grown recently in the USA and New Zealand (from whence it was introduced into Japan and then into Korea and Brazil). For preliminary assessments of South American tuber crops in New Zealand see Martin et al. (1996, Proceedings First Australian New Crops Conference 2: 155-161).
In situ conservation could be the most appropriate manner in which to preserve the germplasm of yacon. Yacon tends to invade vegetation gaps, such as those that result from the slash-and-burn peasant agriculture of parts of South America. One sanctuary in Peru is ideally suited for this, because peasants have disturbed part of the area and are propagating yacon and several of its wild relatives.
For further information on yacon contact:
Dr Alfredo Grau
Laboratorio de Investigaciones
Ecologicas de las Yungas
Facultad de Ciencias Naturales
Universidad Nacional de Tucuman
Casilla de Correo 34
4107 Yerba Buena, Tucuman
Facsimile: 54 81 254 468
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