PAB 310: ECONOMIC AND MEDICINAL PLANTS GENERAL INTRODUCTION TO THE TUBER CROPS Introduction Tubers are enlarged structures in some flowering plant families, which act as storage organs for nutrients. The common examples belong to different botanical families, but are often grouped together as all types produce underground food. The word tuber is derived from the Latin word “tuber” which means “lump, bump, or swelling”. They form the second most important group of cultivated food plants after the cereals as global sources of carbohydrates (Anoma and Thamilini, 2016). Tubers are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing season, and as a means of asexual reproduction. The most common examples include cassava (Manihot esculenta), yam (Dioscorea spp.), potato (Solanum tuberosum), and sweet potato (Ipomoea batatas). Less common, but also important examples include cocoyam (Colacasia esculenta), taro ((Xanthosoma sagittifolium), Canna and arrowroot (Maranta arundinacea). In Africa, root and tuber crops are the most important crops consumed for direct human consumption. The most consumed tubers on the African continent, especially sub-Saharan Africa, may be said to be arguably cassava and yams. In fact, the aggregate value of root and tuber crops (yam, cassava, potato and sweet potato) exceeds all other staple crops, and is much higher than the value of cereal crops (producing more than 240 million tons annually on 23 million hectares compared to cereals average annual production of 169 million tons on 108 million hectares of land). According to Nterenya (2015), the following are reasons why tuber crops production must be encouraged for sustainable food production on the African continent: They are versatile staples to address food and nutrition security and produce more food per unit area of land, compared to many other crops, Potato and sweet potato are short cycle crops (3 - 4 months), and thus are well suited to double cropping seasons, particularly the rain-fed systems, Yam and cassava, though longer in their cropping life cycle, are vital in the annual cycle of food availability due to broader agro-ecological adaptation, diverse maturity period and in-ground storage capability, Being a source of cheap, but nutritionally rich staple food that contributes protein, vitamin A, zinc, and iron to meet the dietary demands of the teeming populace, 1 Have high demand in local and international markets, They contribute to a more stable food system and predictable source of income, as they are far less susceptible to large-scale market shocks and price speculations experienced by more widely traded staples. Types of tuber crops Tuber crops may broadly be classified into four groups, based on which organ of the plant is modified for storage of energy reserve and plant nutrients. They are: 1. Tubers: A swollen, fleshy portion of a rhizome involved in water or carbohydrate storage. a) Stem tubers b) Root tubers 2. Rhizomes 3. Corms 4. Bulbs Stem Tubers A stem tuber forms from thickened rhizomes or stolons. The top sides of the tuber produce shoots that grow into typical stems and leaves and the under sides produce roots. They tend to form at the sides of the parent plant and are most often located near the soil surface. The underground stem tuber is normally a short-lived storage and regenerative organ developing from a shoot that branch off a mature plant. The offspring or new tubers are attached to a parent tuber or form at the end of a hypogeonous (initiated below ground) rhizome. In the autumn the plant dies, except for the new offspring stem tubers which have one dominant bud, which in spring regrows a new shoot producing stems and leaves, in summer the tubers decay and new tubers begin to grow. Some plants also form smaller tubers and/or tubercules, which act like seeds, producing small plants that resemble (in morphology and size) seedlings. Some stem tubers are long-lived, such as those of tuberous Begonia, but many plants have tubers that survive only until the plants have fully leafed out, at which point the tuber is reduced to a shriveled-up husk. Stem tubers generally start off as enlargements of the hypocotyl section of a seedling, but also sometimes include the first node or two of the epicotyl and the upper section of the root. The stem tuber has a vertical orientation with one or a few vegetative buds on the top and fibrous roots produced on the bottom from a basal section, typically the stem tuber has an oblong rounded shape. Examples are potato and yam (Diop, 1998). 2 Figure 1: Stem tubers Figure 2: Root tubers Root Tubers A tuberous root or storage root, is a modified lateral root, enlarged to function as a storage organ. The enlarged area of the root-tuber, or storage root, can be produced at the end or middle of a root or involve the entire root. It is thus different in origin, but similar in function and appearance to a stem tuber. Examples of plants with notable tuberous roots include the sweet potato, cassava and Dahlia. 3 Root tubers are perennating organs, thickened roots that store nutrients over periods when the plant cannot actively grow, thus permitting survival from one year to the next. The massive enlargement of secondary roots typically represented by sweet potato have the internal and external cell and tissue structures of a normal root, they produce adventitious roots and stems which again produce adventitious roots. In root-tubers, there are no nodes and internodes or reduced leaves. Root tubers have one end called the proximal end, which is the end that was attached to the old plant; this end has crown tissue that produces buds which grow into new stems and foliage. The other end of the root tuber is called the distal end, and it normally produces unmodified roots. In stem tubers, the order is reversed, with the distal end producing stems. Tuberous roots are biennial in duration; the first year, the plant produces root-tubers, and at the end of the growing season, the plant shoots often die, leaving the newly generated tubers. The next growing season, the root-tubers produce new shoots. As the shoots of the new plant grow, the stored reserves of the root-tuber are consumed in the production of new roots, stems, and reproductive organs; any remaining root tissue dies concurrently to the plant's regeneration of the next generation of root-tubers (Diop, 1998). Rhizome This may be described as a horizontal, swollen stem, more or less underground, bearing scale- like leaves, and roots. The scaly, reduced leaves have resting buds in their axils. It is often called a stolon (or runner), if found above ground, and having an elongated internode. It may be propagated by division, when these resting buds will grow and produce leaves for a new plant e.g. Canna, ginger. Corm This may be described as an underground storage organ formed from the main plant stem. It is usually a mass of short, solid, erect, underground, more or less fleshy stem, covered with thin, dry leaves or leaf bases. Roots form on the underside of the corm. Buds form at the top of the stem, and there may be smaller, secondary buds on the outside of the corm e.g. Crocus, cocoyam. Bulb This is a short, erect, underground, compressed rosette stem (called basal plate), surrounded by thick, fleshy leaves or leaf bases, and roots growing on the underside. In the centre of the bulb is the bud for next year’s flower e.g. onion 4 General characteristics of roots & tubers compared with cereals (FAO, 1983) Cereals and oil seeds Roots and tubers Low moisture content, typically 10% to 15% High moisture content, typically 70% to 80% Small unit size, typically less than 1 gram Large unit size, typically 100 grams to 15 kg Very low respiration rate with very low High respiration rate. Heat production is generation of heat. Heat production is typically typically 0.5 to 10 megajoules/ton/day at 0°C to 0.05 megajoule/ton/day for dry grain 5 to 70 megajoules/ton/day at 20ºC Hard texture Soft texture, easily bruised Stable, natural shelf life is several years Perishable, natural shelf life is a few days to few months Losses usually caused by moulds, insects and Losses usually caused by rotting (bacteria and rodents fungi), senescence, sprouting and bruising Selected References Anoma, C., and Thamilini, J.K. (2016). Roots and Tubers as Functional Foods: A Review on Phytochemical constituents and their potential health benefits. International Journal of Food Science. Hindawi Publishing Corporation. 15pp. http:dx.doi.org/10.1155/2016/3631647 Diop, A. (1998). Storage and Processing of Roots and Tubers in the Tropics. Edited by D. J. B. Caverley. Food and Agriculture Organization of the United Nations. Agro-industries and Post- Harvest Management Service. FAO Publications Division, 00100 Rome, Italy Nterenya, S. (2015). Root and Tuber Crops (Cassava, Yam, Potato and Sweet Potato): Root tubers. Feeding Africa: An action plan for African Agricultural Transformation 5 .