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Q. Liu, ... S. He, in Encyclopedia of Agriculture and Food Systems, 2014
Botany and Physiology
Sweet potato is a member of the family Convolvulaceae, Genus Ipomoea, section Batatas. The number of chromosomes in the sweet potato plant is 2n=6x=90. This indicates that it is a hexaploid plant with a basic chromosome number x=15. Among the wild species, I. tabascana and I. tiliacea are tetraploids with 2n=4x=60. The other species are diploids with 2n=2x=30. Polyploid species are I. cordatotriloba with 2x and 4x and I. trifida with 2x, 3x, 4x, and 6x (Huaman, 1992). Morphological analysis of the related species indicates that I. trifida is the closest wild relative to sweet potato but I. tabascana is also morphologically very close (Austin, 1977, 1987). Austin (1987) suggests that natural hybridization between I. trifida and I. triloba resulted in generation of the wild ancestors of the present I. batatas (L.) Lam., Jarret et al. (1992) supported a hypothesis that I. trifida (H.B.K.) G. Don and wild Mexican tetraploids are putative ancestors of the cultivated sweet potato. Species in section Batatas have been shown to contain unreduced gametes (Bohac and Jones, 1994), but the derivation of the hexaploid sweet potato remains unknown. Shiotani (1988) considered it an autopolyploid derivative of I. trifida, whereas others favor an allopolyploid origin involving I. trifida and an unidentified tetraploid parent. Tetrasomic segregation patterns in microsatellite primers are consistent with an allopolyploid (Buteler et al., 1999). Zhang et al. (2001) also found tetrasomic patterns of inheritance in sweet potato. The most compelling evidence of an allopolyploid origin is cytological data reported by Magoon et al. (1970). Meiotic patterns showed various pairing configurations among the chromosomes, for example, duplicate, triplicate, and quadrivalents. Their data suggest that I. batatas consists of two related and a third more distant genome (Firon et al., 2009).
The cultivated species, I. batatas (L.) Lam., includes plants that are very variable in their morphology. Sweet potato is a herbaceous and perennial plant. However, it is grown as an annual plant by vegetative propagation using either storage roots or stem cuttings. Its growth habit is predominantly prostrate with a vine system that expands rapidly horizontally on the ground. The types of growth habit of sweet potato are erect, semierect, spreading, and very spreading.
The sweet potato root system consists of fibrous roots, which absorb nutrients and water and anchor the plant, and storage roots that are lateral roots, which store photosynthetic products. A sweet potato stem is cylindrical and its length depends on the growth habit of the cultivar and on the availability of water in the soil. The stem color varies from green to totally pigmented with anthocyanins (red-purple color). The hairiness in the apical shoots, and also in the stems of some cultivars, varies from glabrous (without hairs) to very pubescent. The leaves are simple and are spirally and alternately arranged on the stem in a pattern known as 2/5 phyllotaxis. The edge of the leave lamina can be entire, toothed, or lobed. The shape of the general outline of sweet potato leaves can be round, reniform (kidneyshaped), cordate (heart shaped), triangular, hastate (trilobular and spear shaped with the two divergent basal lobes ), lobed, and almost divided. The leaf color can be green yellowish, green, or can have purple pigmentation in part of or the entire leaf blade. The leaf size and the degree of hairiness vary according to the cultivar and environmental conditions (Huaman, 1992).
Sweet potato cultivars differ in their ability of flowering. Under normal conditions in the field, some cultivars do not flower, some produce very few flowers, and others flower profusely. The inflorescence is generally a cyme in which the peduncle is divided in two axillary peduncles; each one is further divided in two after the flower is produced (biparous cyme). Some cultivars produce white flowers. The flower is bisexual. The androecium consists of five stamens, and the anthers are whitish, yellow, or pink. The gynoecium consists of a pistil with a superior ovary, two carpels, and two locules that contain one or two ovules. The stigma is receptive early in the morning and the pollination is mainly by bees. The fruit is a capsule, more or less spherical with a terminal tip, and can be pubescent or glabrous. The capsule turns brown when mature. Each capsule contains from one to four seeds that are slightly flattened on one side and convex on the other. Seed shape can be irregular, slightly angular, or rounded; the color ranges from brown to black; and the size is approximately 3 mm. The embryo and endosperm are protected by a thick, very hard, and impermeable testa. Seed germination is difficult and requires scarification by mechanical abrasion or chemical treatment. Sweet potato seeds do not have a dormancy period but maintain their viability for many years (Huaman, 1992).
The storage roots are the commercial part of the sweet potato plant, and sometimes are mistakenly named ‘tubers.’ Most cultivars develop storage roots at the nodes of the mother stem cuttings that are underground. However, the very spreading cultivars produce storage roots at some of the nodes that come into contact with the soil. The parts of the storage roots are the proximal end that joins to the stem, through a root stalk, and where many adventitious buds are found from which the sprouts are originated; a central part which is more expanded; and the distal end that is opposite to the root stalk. The adventitious buds that are located in the central and distal part usually sprout later than those located in the proximal end.
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