Oxalis tuberosa

Oxalis tuberosa
Yellow and purple Oxalis tuberosa (oca) tubers
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Rosids
Order:	Oxalidales
Family:	Oxalidaceae
Genus:	Oxalis
Species:	O. tuberosa
Binomial name
Oxalis tuberosa Molina

Oxalis tuberosa is an herbaceous perennial plant that overwinters as underground stem tubers. These tubers are known as oca, oka, or New Zealand Yam. The plant was brought into cultivation in the central and southern Andes for its tubers, which are used as a root vegetable. The plant is not known in the wild, but populations of wild Oxalis species that bear smaller tubers are known from four areas of the central Andean region.^[1] Introduced to Europe in 1830 as a competitor to the potato and to New Zealand as early as 1860, it has become popular in that country under the name New Zealand yam and is now a common table vegetable.^[2]

Edibility

The stem tubers of oca form in the ground in the autumn. These are commonly boiled before eating although they can also be eaten raw. The leaves and young shoots can be eaten as a green vegetable.

The flavour is slightly tangy, and texture ranges from crunchy (like a carrot) when undercooked, to starchy or mealy when fully cooked. Though the original Andean varieties are widely variable in colour from purple to yellow, the usual New Zealand variety is a fleshy pink.^[2]

Oca can be boiled, baked or fried. In the Andes it is used in stews and soups, served like potatoes or can be served as a sweet. Oca is eaten raw in Mexico with salt, lemon, and hot pepper.^[2]

Oca are fairly high in oxalates, concentrated in the skin,^[3] but there is significant variation in oxalate concentration among varieties.^[4] The traditional Andean preparation methods are geared towards reducing the oxalate level of the harvested vegetable. This is done by exposure to sunlight which decreases the organic acid content and thereby increases the sweet taste of the oca. The bioavailability of oxalate appears to be similar to spinach.^[5] Recent oca cultivars have a lower oxalate content, and have also been selected for more flexibility in day lengths.^[2]

Cultivation

Pink Oxalis tuberosa (oca) tubers

Oca is one of the important staple crops of the Andean highlands, second only to the potato due to its easy propagation, and tolerance for poor soil, high altitude and harsh climates.^[2]

Nutritional value per 100 g

	fresh	dried
Energy (J)	255	1360
Water (g)	84.1	15.3
Protein (g)	1.0	4.3
Carbohydrates (g)	13.3	75.4
Ash (g)	1.0	3.9
Calcium (mg)	2	52
Phosphorus (mg)	36	171
Iron (mg)	1.6	9.9
Retinol (µg)	1	0
Riboflavin (mg)	0.13	0.08
Niacin (mg)	0.43	0.85
Vitamin C (mg)	38.4	2.4

Distribution

Outside of the Andean region Oxalis tuberosa is cultivated commercially in Mexico and New Zealand.

Propagation

Oca is usually propagated vegetatively by planting whole tubers. Propagation by seed is possible but is rarely used in practice.^[2]

Climate requirements

Oca need a long growing season, and are day length dependent, forming tubers when the day length shortens in autumn. In areas with harsh winter climates, early frosts may cut back the foliage before the tubers have a chance to form. In tropical areas where the days are unchanging in length, oca will not set a crop successfully.^[2]

Soil requirements

The crop grows with very low production inputs and in generall on plots of marginal soil quality and tolerates acidities between about pH 5.3 and 7.8.^[2]

Cropping factors

The cultural practice is similar to potatoes. Planting is done in rows or hills 80–100 cm apart with plants, spaced 40–60 cm apart in the rows.^[6]). Monoculture predominates, but interplanting of several tuber species in one field is common in Andean production. In many instances Andean farmers will allow a mixture containing two or three tuber crops to be planted in one field. Often this mixture consists of several different clones of each species. Such mixed fields may later be sorted into tuber types during harvest or before cooking. Mixed planting with legumes or grain crops is occasionally practiced.^[2]

Pest and Diseases

Crops in the Andes are usually infected with viruses, causing chronic yield depression. Adequate techniques to remove viruses have to be applied before the varieties can be used outside the Andean region.^[2] Cultivation is also constrained by the oca weevil (Ariodistus tuberculatus), Andean potato weevil (Premnotrypes spp) and the ulluco weevil (Cylydrorhinus spp), beetles often destroying entire crops. Further notable pests are nematodes.^[7]

Yields

Yields vary with the cultural method. Annals from Andean countries report about 7-10 tons per hectare for Oxalis tuberosa production. But with adequate inputs and virus free propagation material, oca production can range from 35 to 55 tons per hectare.^[2][7]

Limitations

As already mentioned, viruses, day length restrictions and the precence of oxalates can be considered as limiting factors. Scientists work with specific breeding, selection and virus cleaning programs on these purposes.^[2]

Agricultural potential

Potential distribution to other suitable ecogeographical zones of for example Asia and Africa is possible. The cultivation and use of Oxalis tuberosa in New Zealand already indicates a wider utilization and agricultural interest than has been previously recognized.^[8]

Alternative names

• Apilla in Bolivia
• Apiña in Bolivia and Peru
• Hibia in Colombia
• Yam in many other places, such as Polynesia and New Zealand,^[9] where the Dioscorea vegetables known elsewhere as yams are generally very uncommon.

See also

• Mashua
• New World crops
• Yacon
• Ulluco

References

1. ^ Eve Emshwiller, Terra Theim, Alfredo Grau, Victor Nina and Franz Terrazas. (2009). Origins of domestication and polyploidy in oca (Oxalis tuberosa: Oxalidaceae) 3: AFLP data of oca and four wild, tuber-bearing taxa. American Journal of Botany. 96(10):1839-1848. [doi:10.3732/ajb.0800359]
2. ^ ^{a b c d e f g h i j k l} National Research Council. (1989). Lost Crops of the Incas: Little-Known Plants of the Andes with Promise for Worldwide Cultivation. National Academy Press, Washington, D.C.
3. ^ Albihn, P. B. E.; and Savage, G. P. (2001). "The effect of cooking on the location and concentration of oxalate in three cultivars of New Zealand-grown oca (Oxalis tuberosa Mol)". J. of the Science of Food and Agr. 81: 1027-1033.
4. ^ Bradbury, E. Jane and Eve Emshwiller. 2011. The role of organic acids in the domestication of Oxalis tuberosa: a new model for studying domestication resulting in opposing crop phenotypes. Economic Botany. 65(1): 76-84. [doi: 10.1007/s12231-010-9141-0]
5. ^ The Bioavailability of Oxalate from Oca (Oxalis tuberosa)
6. ^ M. Frére, J. Rea, J.Q. Rijks (1975) Ullucus tuberosus. In:Estudio agroclimatolgico de la zona Andina (informe tecnico). Organizacion de las naciones unidas para la alimentacion y la agriculture, Rome
7. ^ ^{a b} Sperling, C.R.; and King, S.R. (1990). "Advances in new crops: Worldwide potential.". J. Janick and J.E. Simon (eds.). Advances in new crops. Timber Press, Portland, OR. p. 428-435.
8. ^ King, S.R.; and Gersoff, S.N. (1986). "Nutritional Evaluation of Three Underexploited Andean Tubers: Oxalis tuberosa (Oxalidaceae), Ullucus tuberosus (Basellaceae), and Tropaeolum tuberosum (Tropaeolaceae) ". Economic Botany, 41 (4). p. 503-511.
9. ^ Albihn, P.B.E.; Savage, G.P. (June 18, 2001). "The effect of cooking on the location and concentration of oxalate in three cultivars of New Zealand-grown oca (Oxalis tuberosa Mol)". Journal of the Science of Food and Agriculture 81: 1027–1033. doi:10.1002/jsfa.890. 

Further reading

• Davidson, Alan. Oxford Companion to Food (1999). "Oca", p. 547 ISBN 0-19-211579-0

External links

• Oca in Lost Crops of the Incas
• NewCROP page on oca
• Genetic Resource Conservation of oca and other Andean tubers at International Potato Center
• Oca, ulluco, and mashua at the International Potato Center
• Oxalis tuberosa at Plants for a Future