Root crops, tuber crops and corm crops are special terms used to refer to specific types of farm crops collectively called root and tuber crops.
The starchy root and tuber crops, also called roots and tubers and tuberous crops, are plants that are grown for their modified, thickened root or stem which generally develop underground. These organs are rich in carbohydrate and are commonly used as staple, livestock feed, or as raw materials for the production of industrial products such as starch and alcohol, or processed into various food products.
Starch is a storage product of photosynthesis and is a direct source of energy for animals. With the aid of glycosidase enzymes, animals hydrolyze the starch to glucose. Starch occurs in plant tissues as starch grains and is composed of two polysaccharides, the amylose and amylopectin (Carey, 1992; Devlin, 1975).
Root crops and tuber crops have very high yield potential although their protein, mineral and vitamin content are generally low compared to cereals. However, taro (gabi) and yams contain up to 6% protein of good quality and the potato tuber provides some minerals and vitamin C (Oregon State University, n.d.). The orange-fleshed sweet potato is also well known as a rich source of beta-carotene, a precursor of vitamin A. These crops are known for their high dry matter production, with a potential of 2.1 tons per hectare from underground storage organs. In contrast, the capacity of cereals is only about 1.5 tons (Kawakami, 1978).
In tropical countries, cassava, sweet potato, taro or gabi, yams and arrowroot are staple food crops. Potato is common in the temperate and subtropical countries (Oregon State University. n.d.).
In the Pacific Islands, there is an ongoing effort to conserve the giant swamp taro, a traditional staple crop. It has been noted that life expectancy in some islands has decreased due to diet-related illnesses attributed to the movement away from traditional staples. The incidence of diabetes has reached up to 44 percent in Tokelau atolls, while it is only about 8 percent in the United States. (Burness Communications, 2010).
The root crops are plants with modified roots that are edible while both the tuber crops and corm crops have modified stems. The modified roots consist of the tuberous and fleshy roots while the tuber and corm, as well as the rhizome and bulb, are modified stems. The description of these underground organs will further shed light on how root crops differ from tuber crops and corm crops.
A tuberous root is a thickened secondary root as in yam bean or sinkamas (Pachirhizus erosus) while the fleshy root is usually an enlarged primary root, as in carrot (Daucus carota), ginseng (Panax spp.) and sugar beet (Beta vulgaris). In these root crops which produce fleshy roots the upper portion on which roots are absent is a hypocotyl, the first internode of the stem. The portion on which secondary roots develop is a root. However, in raddish (Raphanus sativus), the fleshy root consists mainly of the hypocotyl. (Kawakami, 1978).
A tuber is an enlarged tip of an underground stem with leaves reduced to scales or scars subtending the auxillary buds. Tubers are the enlarged storage tips of a rhizome. The “eyes” are actually buds in nodes, arranged in spiral pattern from the base to the apex of the tuber. Aerial tubers, which are common in yams, are called tubercle.
A corm is a short, solid, thickened underground stem found in monocots. Small corms are called cormels. The corms are usually flat in shape having numerous roots at the bottom, and a tuft of leaves at the top. They are distinguished from bulbs with their lack of fleshy leaves but having a covering of dry papery leaves. Corms store food in the stem, unlike the bulbs which store food reserves in the leaves. Examples of plants that form corms are the gladiolus and crocus, in addition to the common starchy corms.
Other modified, underground stems are the rhizome (e.g. ginger, iris) and bulb (e.g. onions, tulips, daffodils, hyacinths, lilies), but they are not produced for starch. A rhizome or rootstock is an underground, horizontally enlarged shoot, more or less fleshy when fresh. Bulbs, like corms, are modified stems found in monocots. Bulbs are erect underground stems having both fleshy and papery leaves with the fleshy leaves serving as food storage organs. The term “bulb” is also used commonly to refer to plants having fleshy underground storage organs including the root crops, but only a few are true bulbs.
1. Arrowroot, arrow root, uraro, araro (Maranta arundinacea)
2. Cassava, tapioca, manioc, kamoteng kahoy, balanghoy, balinghoy (Manihot esculenta)
3. Sweet potato, kamote (Ipomoea batatas)
Being consumed more as a root vegetable or fruit, the yam bean is here excluded from the list of starchy root crops.
1. Asiatic yam, lesser yam, fancy yam, potato yam, tugi, tam-is, apali (Dioscorea esculenta, syn.: D. fasciculata)
2. Jerusalem artichoke, sunchoke, lambchoke, French potato, Canada potato (Helianthus tuberosus)
3. Potato, white potato, Irish potato, patatas (Solanum tuberosum)
4. Yam, ube, ubi, Chinese taro, greater yam, water yam (Dioscorea alata)
1. Elephant’s ear, giant taro, biga, badiang (Alocasia macrorrhiza)
2. Giant swamp taro, swamp taro, galiang (Cyrtosperma chamissonis, syn.: C. merkusii)
3. Taro, gabi, cocoyam, dasheen, lesser yam, lesser asiatic yam (Colocasia esculentum)
4. Tannia, yautia, Chinese taro, new cocoyam, bisol, karlang, palauan, tacudo (Xanthosoma sagittifolium)
(Ben G. Bareja 2010)