Color profile: Disabled Composite Default screen Chapter 5 Cassava Botany and Physiology Alfredo Augusto Cunha Alves Embrapa Cassava and Fruits, Caixa Postal 007, 44.380–000, Cruz das Almas, Bahia, Brazil Introduction cuttings. The seedlings are genetically segre- gated into different types due to their Cassava is a perennial shrub of the family reproduction by cross-pollination. If propagated Euphorbiaceae, cultivated mainly for its starchy by cuttings under favourable conditions, sprout- roots. It is one of the most important food staples ingandadventitiousrootingoccurafter1week. in the tropics, where it is the fourth most impor- tant source of energy. On a worldwide basis it is ranked as the sixth most important source of Morphological and Agronomic calories in the human diet (FAO, 1999). Given Characteristics the crop’s tolerance to poor soil and harsh clima- tic conditions, it is generally cultivated by small Cassava, which is a shrub reaching 1–4 m farmers as a subsistence crop in a diverse range height, is commonly known as tapioca, manioc, of agricultural and food systems. Although mandioca and yuca in different parts of the cassava is a perennial crop, the storage roots can world. Belonging to the dicotyledon family be harvested from 6 to 24 months after planting Euphorbiaceae, the Manihot genus is reported to (MAP), depending on cultivar and the growing have about 100 species, among which the only conditions (El-Sharkawy, 1993). In the humid commercially cultivated one is Manihot esculenta lowland tropics the roots can be harvested after Crantz. There are two distinct plant types: erect, 6–7 months. In regions with prolonged periods with or without branching at the top, or of drought or cold, the farmers usually harvest spreading types. after 18–24 months (Cock, 1984). Moreover, The morphological characteristics of the roots can be left in the ground without cassava are highly variable, which indicate a harvesting for a long period of time, making it high degree of interspecific hybridization. There a very useful crop as a security against famine are many cassava cultivars in several germplasm (Cardoso and Souza, 1999). banks held at both international and national Cassava can be propagated from either stem research institutions. The largest germplasm cuttings or sexual seed, but the former is the bank is located at Centro Internacional de commonest practice. Propagation from true seed Agricultura Tropical (CIAT), Colombia, with occurs under natural conditions and is widely approximately 4700 accessions (Bonierbale used in breeding programmes. Plants from true et al., 1997), followed by EMBRAPA’s collection seed take longer to become established, and they in Cruz das Almas, Bahia, with around 1700 are smaller and less vigorous than plants from accessions (Fukuda et al., 1997), representing ©CAB International 2002. Cassava: Biology, Production and Utilization (eds R.J. Hillocks, J.M. Thresh and A.C. Bellotti) 67 79 Z:\Customer\CABI\A4101 - Hillocks - Cassava\A4212 - Hillocks - Cassava #R.vp Monday, February 04, 2002 11:21:53 AM Color profile: Disabled Composite Default screen 68 A.A.C. Alves the germplasm of the following Brazilian eco- buds under the soil. These roots develop to make systems: lowland and highland semiarid tropics, a fibrous root system. Only a few fibrous roots lowland humid subtropics, lowland subhumid (between three and ten) start to bulk and become tropics, lowland humid tropics, and lowland hot storage roots. Most of the other fibrous roots savannah. The cassava genotypes are usually remain thin and continue to function in water characterized on the basis of morphological and and nutrient absorption. Once a fibrous root agronomic descriptors. Recently, the Interna- becomes a storage root, its ability to absorb water tional Plant Genetic Resources Institute (IPGRI) and nutrients decrease considerably. The storage descriptors (Gulick et al., 1983) were revised, and roots result from secondary growth of the fibrous a new version was elaborated, in which 75 roots; thus the soil is penetrated by thin roots, descriptors were defined, 54 being morpho- and their enlargement begins only after that logical and 21 agronomic (Fukuda et al., 1997). penetration has occurred. Morphological descriptors (for example, lobe Anatomically, the cassava root is not a shape, root pulp colour, stem external colour) tuberous root, but a true root, which cannot have a higher heritability than agronomic (such be used for vegetative propagation. The mature as root length, number of roots per plant and root cassava storage root has three distinct tissues: yield). Among morphological descriptors, the bark (periderm), peel (or cortex) and paren- following were defined as the minimum or basic chyma. The parenchyma, which is the edible descriptors that should be considered for identify- portion of the fresh root, comprises approxi- ing a cultivar: (i) apical leaf colour; (ii) apical leaf mately 85% of total weight, consisting of xylem pubescence; (iii) central lobe shape; (iv) petiole vessels radially distributed in a matrix of starch- colour; (v) stem cortex colour; (vi) stem external containing cells (Wheatley and Chuzel, 1993). colour; (vii) phyllotaxis length; (viii) root ped- The peel layer, which is comprised of uncule presence; (ix) root external colour; (x) sclerenchyma, cortical parenchyma and phloem, root cortex colour; (xi) root pulp colour; (xii) root constitutes 11–20% of root weight (Barrios epidermis texture; and (xiii) flowering. and Bressani, 1967). The periderm (3% of total Given the large number of cassava genotypes weight) is a thin layer made of a few cells thick cultivated commercially and the large diversity and, as growth progresses, the outermost por- of ecosystems in which cassava is grown, it is tions usually slough off. Root size and shape difficult to make a precise description of the mor- depend on cultivar and environmental condi- phological descriptors as there is a genotype- tions; variability in size within a cultivar is by-environmental conditions interaction. Thus, greater than that found in other root crops in addition to morphological characterization, (Wheatley and Chuzel, 1993). Table 5.1 lists molecular characterization, based mainly on morphological and agronomic characteristics of DNA molecular markers, has been very useful in the root and its variability in cassava. order to evaluate the germplasm genetic diver- sity(Beechingetal.,1993;Fregeneetal.,1994). Stems Roots The mature stem is woody, cylindrical and formed by alternating nodes and internodes. On Roots are the main storage organ in cassava. the nodes of the oldest parts of the stem, there In plants propagated from true seeds a typical are protuberances, which are the scars left by primary tap root system is developed, similar the plant’s first leaves. A plant grown from stem to dicot species. The radicle of the germinating cuttings can produce as many primary stems as seed grows vertically downward and develops there are viable buds on the cutting. In some into a taproot, from which adventitious roots cultivars with strong apical dominance, only originate. Later, the taproot and some adventi- one stem develops. tious roots become storage roots. The cassava plant has sympodial branch- In plants grown from stem cuttings the roots ing. The main stem(s) divide di-, tri- or tetra- are adventitious and they arise from the basal- chotomously, producing secondary branches cut surface of the stake and occasionally from the that produce other successive branchings. These 80 Z:\Customer\CABI\A4101 - Hillocks - Cassava\A4212 - Hillocks - Cassava #R.vp Monday, February 04, 2002 11:21:53 AM Color profile: Disabled Composite Default screen Botany and Physiology 69 Table 5.1. Some morphological and agronomic characteristics of roots and their variability in cassava. Root characteristic Variability Reference Morphological External colour White or cream; yellow; light Fukuda and Guevara (1998) brown; dark brown Cortex colour White or cream; yellow; pink; purple Fukuda and Guevara (1998) Pulp (parenchyma) colour White; cream; yellow; pink Fukuda and Guevara (1998) Epidermis texture Smooth; rugose Fukuda and Guevara (1998) Peduncule Sessile; pedunculate; both Fukuda and Guevara (1998) Constriction None or little; medium; many Fukuda and Guevara (1998) Shape Conical; conical–cylindrical; Fukuda and Guevara (1998) cylindrical; irregular Agronomic No. storage roots/plant 3–14 Dimyati (1994); Wheatley and Chuzel (1993); Ramanujam and Indira (1983); Pinho et al. (1995) Weight of storage roots/plant 0.5–3.4 kg FW Dimyati (1994); Wheatley and Chuzel (1993); Ramanujam and Indira (1983) Weight of one storage root 0.17–2.35 kg Barrios and Bressani (1967); Ramanujam and Indira (1983) Length of storage root 15–100 cm Wheatley and Chuzel (1993); Barrios and Bressani (1967); Pinho et al. (1995) Diameter of storage root 3–15 cm Wheatley and Chuzel (1993); Barrios and Bressani (1967); Pinho et al. (1995) Diameter of fibrous root 0.36–0.67 mm Connor et al. (1981) Depth of fibrous root Up to 260 cm Connor et al. (1981) Amylose in root starch 13–21% FW O’Hair (1990) Protein in whole root 1.76–2.68% FW Barrios and Bressani (1967) Protein in pulp (parenchyma) 1.51–2.67% FW Barrios and Bressani (1967) 1.0–6.0% DW Wheatley and Chuzel (1993) Protein in peel 2.79–6.61% FW Barrios and Bressani (1967) 7.0–14.0% DW Wheatley and Chuzel (1993) DM in whole fresh root 23–43% Barrios and Bressani (1967); Ghosh et al. (1988); Ramanujam and Indira (1983); O’Hair (1989) DM in peel 15–34% Wheatley and Chuzel (1993); Barrios and Bressani (1967); O’Hair (1989) DM in pulp 23–44% Wheatley and Chuzel (1993); Barrios and Bressani (1967); O’Hair (1989) Carbohydrates in whole root 85–91% DW Barrios and Bressani (1967) Carbohydrates in peel 60–83% DW Barrios and Bressani (1967) Carbohydrates in pulp 88–93% DW Barrios and Bressani (1967) (parenchyma) Starch in whole root 20–36% FW Wholey and Booth (1979); O’Hair (1989); Ternes et al.