Ipomoea batatas (PROSEA)
Ipomoea batatas (L.) Lamk
- Protologue: Tabl. encycl. 1: 465 (1793).
- Family: Convolvulaceae
- Chromosome number: 2n= 90 (hexaploid)
- Convolvulus batatas L. (1753),
- C. edulis Thunb. (1784),
- Batatas edulis (Thunb.) Choisy (1833).
- Sweet potato (En).
- Patate douce (Fr)
- Indonesia: ubi jalar (general), ketela rambat (Javanese), huwi boled (Sundanese)
- Malaysia: ubi keladi, ubi keladek
- Papua New Guinea: kaukau (Pidgin), kaema (Motu)
- Philippines: kamote, kamuti.
- Burma (Myanmar): myonk-ni
- Cambodia: dâmlô:ng chvië
- Laos: man kè:w
- Thailand: man-thet
- Vietnam: khoai lang.
Origin and geographic distribution
It is generally accepted that sweet potato originated from Central America or the northern part of South America. This acceptance is based on archaeological finds, the distribution patterns of wild species, and variation in cultivated clones.
Three lines of dispersion from the centre of origin to other regions have been postulated based on linguistic evidence:
- the "kumara" line: prehistoric transfer from the northern part of South America to eastern Polynesia;
- the "batatas" line: introduction to Africa and Asia through Europe from the first voyage of Columbus;
- the "kamote" line: direct transfer from Mexico to the Philippines via Hawaii and Guam in the 16th Century.
Sweet potato, now an established crop, is widely grown in tropical, subtropical and warm temperate regions.
Storage roots of sweet potato are used mainly for human consumption (70-100%) in most tropical countries. Small portions are used as feed (10-30%); negligible amounts are used for industrial purposes (5-10%). In temperate Asia, 30-35% is produced for industrial purposes, mainly for starch and alcohol.
Human consumption of sweet potato in tropical Asia is in the form of dessert, snacks or supplementary food, while it serves as a staple food on the island of New Guinea and some Oceanian countries. Young shoots are often consumed as a green vegetable.
Production and international trade
The world sweet potato cultivation area peaked at 15 million ha with a total production of over 130 million t in the early 1970s. There has been a significant reduction in the world production area, to 9.4 million ha with 124 million t output in 1994. Asia's production, amounting to 118 million t, accounts for 92% of the world's total production. China is the main producer with 109 million t, about 85% of the world's total production. Other Asian countries with a significant production are: Indonesia (2 million t), Vietnam (2 million t), Japan (1.3 million t) and India (1.2 million t). Sweet potato is usually domestically consumed and not commonly exported.
Nutritional quality and chemical composition of storage roots are genetically determined, but vary widely in response to various environmental and cultural conditions. Freshly harvested storage root consists of 16-40% dry matter of which 75-90% is carbohydrate. The carbohydrates consist mainly of starch (60-80% of dry matter), sugars (4-30% of dry matter) and small amounts of cellulose, hemicellulose and pectins. Sucrose is most commonly found in fresh storage roots. There are also small amounts of glucose and fructose. Maltose increases in cooking due to the activation of ß-amylase. The crude protein ranges from 1.3% to more than 10% on a dry weight basis. The energy value averages 479 kJ/100 g.
The range of ß-carotene (pro-vitamin A) is 0-22 mg/100 g on a fresh weight basis. Orange-fleshed cultivars are rich in ß-carotene. Vitamin C content is high in sweet potato, ranging from 20-50 mg per 100 g on a fresh weight basis.
Sweet potato greens contain on average per 100 g fresh weight: vitamin A 5580 IU, vitamin B20.32 mg, Fe 4 mg and protein 2.7 g.
- A perennial herbaceous plant.
- Root system with fibrous, adventitious roots and enlarged roots, derived from secondary thickening of some adventitious roots, serving as storage organ, variable in shape, size, number, skin colour (white, yellow, brown, red, purple) and flesh colour (white, yellow, orange, purple).
- Stems prostrate or ascending, or occasionally twining, 1-8 m long, much branched from several nodes.
- Leaves arranged spirally with a phyllotaxy of 2/5, simple, lacking stipules; petiole 5-30 cm long, with 2 small nectaries at the base, grooved above; lamina usually ovate, 4-15 cm × 4-12 cm, entire, angular or palmately lobed.
- Flowers axillary, solitary or in cymes; pedicel 3-18 cm long; calyx 5-lobed; corolla funnel-shaped, white or lavender with purple throat; stamens 5, of unequal length, attached near the base of the corolla; ovary surrounded by lobed orange nectary, stigma 2-lobed, white or pale purple.
- Fruit a 5-8 mm long capsule with 1-4 seeds.
- Seed about 3 mm long, black, usually with very hard testa.
Growth and development
Sweet potato is normally grown as an annual. Planted cuttings, which are usually taken from the tip of the vine, start to form adventitious roots from the soil-covered nodes at the base of axillary buds in about 2 days. The adventitious roots form a fibrous root system. New stems also arise from the nodes of the cutting. The stems also form adventitious roots at the nodes when in contact with soil. Storage roots (3-12 per plant) develop in the top 30 cm of the soil by secondary thickening of some of the adventitious roots, both from the original cuttings and from creeping stems.
Approximate duration of sweet potato growth is 3-7 months, depending on cultivar and environment. Growth occurs in three distinct phases:
- an initial phase in which the fibrous roots grow extensively with only moderate growth of the vines;
- an intermediate phase in which the vines grow extensively with a considerable increase in leaf area, and the storage roots are initiated;
- a final phase in which bulking of the storage roots occurs, with little further growth of the vines and fibrous roots, with total leaf area being constant and thereafter declining.
The storage root produces sprouts readily from the vascular cambium region, almost always at the stalk end of freshly harvested storage roots, but sprouts can also arise from the middle and distal parts of aged storage roots.
There are wide differences in flowering habit. Many breeding programmes induce non-flowering plants to flower by grafting onto free-flowering cultivars or onto other Ipomoea species.
Sweet potato is self-incompatible characterized as a sporophytic, multiallelic system. More than 16 cross-incompatibility groups in the crop have been reported. Knowledge of the incompatibility system in a wild diploid makes it possible to interpret the incompatibility; a duplication or possibly triplication of the self-incompatibility explains the cross-incompatibility of sweet potato. Hybridization between genotypes belonging to the same group is mostly hindered. Natural cross-pollination is carried out by hymenopterous insects, particularly bees. The flower opens before dawn and is receptive up to only about 11 a.m.; thus, there is a large chance that any given flower may fail to be pollinated.
The seeds germinate very irregularly due to the hard testa, but germination can be improved by scarification. Scarified seeds germinate in 1-2 days. Germination is epigeal.
Other botanical information
The genus Ipomoea L. section Batatas (Choisy) Griseb. contains 12 species which have chromosome numbers based on a multiple of x = 15. Within the genus, I. batatas (hexaploid) is the only known species with sizable storage roots. There are numerous sweet potato landraces in tropical and subtropical areas, mostly farmers' selections in populations, resulting from natural hybridization and spontaneous mutation. Many improved cultivars have evolved through systematic breeding programmes.
Sweet potato is grown between latitudes 48°N and 40°S. At the equator it is grown at altitudes ranging from sea-level to 3000 m. Its growth is maximum at temperatures above 25°C; when temperatures fall below 12°C or exceed 35°C, growth is retarded. Dry matter production increases with increasing soil temperature from 20-30°C, but declines above 30°C. It is a quantitative short-day plant in terms of flowering response.
Sweet potato is a sun-loving crop; however, it can tolerate a 30-50% reduction of full solar radiation. Light saturation of single leaf photosynthesis occurs at around 800μE/m2per second; light intensity required for saturation in the canopy increases with increasing leaf area index. Optimum leaf area index in the field is 3-4 at solar radiation of 380 gcal/cm2per day. The photosynthetic rate of the canopy in the field is highest between 10 a.m. and 2 p.m.
Sweet potato grows best with a well-distributed annual rainfall of 600-1600 mm during the growing season. Dry weather favours the formation and development of storage roots. Soil moisture at 60-70% of field capacity is favourable for the initial phase, 70-80% for the intermediate phase, and 60% for the final phase. Sweet potato is relatively drought tolerant, mainly because of its capacity for regeneration and root penetration. However, it cannot withstand long periods of drought; the yield is considerably reduced if drought occurs about the time of storage root initiation.
The crop can be grown on a wide range of soil types, but a well-drained, sandy loam with a clayey subsoil is considered ideal. It cannot stand waterlogging and is usually grown on mounds or ridges. Poor aeration or an oxygen concentration of less than 10% in the soil in the initial phase increases the degree of lignification of stele cells and suppresses the primary cambium activity, resulting in young roots developing into fibrous roots. At the final phase, it restrains the secondary cambium activity, favouring vine development at the expense of the storage roots. Flooding shortly before harvest may result in storage roots rotting in the soil or during subsequent storage. The best bulk density of the soil is 1.3-1.5 g/ml. Higher bulk densities tend to reduce storage root formation, resulting in reduced yields or poorly shaped storage roots. The optimum soil pH for sweet potato is 5.6-6.6, but it grows well even in soils with a relatively low pH, e.g. 4.2. It is sensitive to alkaline or saline soils; maximum soil salinity without yield loss (threshold) is about 1.5 dS/m.
Propagation and planting
In the tropics, sweet potato is propagated vegetatively from vine cuttings, but slips or sprouts obtained as cuttings from storage roots are sometimes used. Vine cuttings are generally taken about 30 cm from the tip, but sometimes from the middle portion as well. In areas where the plant cannot grow all year round, sprouts from storage roots of the previous crop are used as planting material. Direct propagation from storage roots is uneconomic and gives poor yields. Propagation by seed is possible but is used only for breeding purposes.
If there is no critical dry season, sweet potato can be planted at any time. In regions with a critical dry season, planting early in the rainy season is the best. It is usually planted towards the end of the rainy season if this is long and very wet. Land preparation varies from planting on level ground in less intensive systems, to ploughing, harrowing and ridging in more intensive systems. Planting on ridges is recommended. Cultivation on mounds, with several cuttings on each mound, is practised in the tropics, e.g. in the highlands of New Guinea. The mounds vary in shape and may be up to 1.5 m high in extremely wet areas.
Vine cuttings are inserted into the soil horizontally or at an angle, with 3-4 nodes covered by soil. The cuttings are planted manually in most parts of the tropics. The optimum plant density depends on local conditions and practices; however, sweet potato readily compensates to some extent for a low plant density. Number and mean weight of storage roots and the yield per plant decrease with increasing plant density. Normally, vine cuttings are planted 25-30 cm apart in rows with 60-100 cm between rows; the total yield may be expected to be highest at 40 000-50 000 plants per ha.
Weed infestation during the first two months of growth poses a problem in stand development, and requires adequate control to ensure high yield. Thereafter, vigorous growth of the vines results in rapid and effective coverage of the ground and smothers weeds. In the tropics, manual weeding is generally practised, but herbicides are sometimes used in large-scale production.
Sweet potato responds well to fertilizer, particularly if the land has been continuously cropped. However, fertilizer is seldom applied in the tropics. Type and application rate of fertilizer depend on soil type, environment and cultivar. It is estimated that 70 kg N, 20 kg P and 110 kg K are removed from the soil by a sweet potato crop yielding about 15 t/ha of storage roots. Sweet potato plants develop deficiency symptoms when the nutrient levels in the tissues (stems and leaves) fall below 2.5% N, 0.12% P, 0.75% K, 0.16% Mg, 0.2% Ca and 0.08% S. Manure may also be incorporated to improve soil fertility. This is a common practice in smallholdings and traditional agriculture. Sweet potato is used in a wide variety of cropping systems around the world. Rotating sweet potato with other crops such as rice, legumes and maize is desirable to control diseases, pests and weeds in the following crop.
Diseases and pests
In South-East Asia and some Pacific islands, scab caused by Sphaceloma batatas (perfect state: Elsinoe batatas) is the most prevalent disease in sweet potato, followed by Fusarium wilt (Fusarium oxysporum) and witches' broom (caused by mycoplasma-like organisms). Soil rot (Streptomyces ipomoea), black rot (Ceratocystis fimbriata), Java black rot (Diplodia tubericola), scurf (Monilochaetes infuscans), root-knot nematode (Meloidogyne spp.) and some virus diseases also occur in sweet potato, but their distribution and importance vary with the region. The use of disease-free planting material and crop rotation are the most reliable means of controlling these diseases and pests. Some cultivars are resistant to scab, Fusarium wilt, witches' broom, black rot and root-knot nematode.
Among the 300 insect and mite species that feed on sweet potato in the tropics and subtropics, only sweet potato weevil (Cylas formicarius) and vine borer (Omphisa anastomosalis) cause damage and yield loss over wide areas in South-East Asia and Oceania. The sweet-potato weevil is the most destructive insect pest in the tropics and subtropics. No resistant source is available. Integrated pest management for this insect is recommended, consisting of the following measures: crop rotation, eradication of Ipomoea weeds, use of clean planting material, deep planting, regular hilling to fill soil cracks around plants, and the use of sex pheromone which is effective to trap male weevils.
The harvesting period of sweet potato storage roots is not clearly defined; it varies with cultivar, cultural practices and climate. In South-East Asia, sweet potato is generally harvested 3-4 months after planting. In the Philippines, early-maturing cultivars are harvested 70-80 days after planting, late-maturing ones after 120-150 days. In New Guinea, sweet potato is harvested after 5-6 months in the lowlands, 6-8 months in the highlands and 8-12 months or more in the highest mountainous areas. "Progressive harvesting" (piecemeal harvesting) is common practice in tropical countries where sweet potatoes are grown for home consumption. It is generally recommended to harvest within four months to prevent weevil damage. In the tropics, manual harvesting using simple implements such as a stick, spade or hoe is practised. Mechanical harvesting is done only in large-scale production areas where the terrain is suitable for machinery; a variety of ploughs, either animal- or tractor-drawn, are used.
Average yield of storage roots throughout the world in 1994 was 13 t/ha. The average yield in Asia was 16 t/ha; it varied from 2-22 t/ha. The yield potential of sweet potato is high. However, various abiotic and biotic stresses in the tropics prevent the full expression of this potential.
Handling after harvest
Post-harvest handling procedures differ greatly between temperate and tropical regions. In temperate regions, sweet potatoes are harvested and handled mechanically, often causing damage to the storage roots. In these areas, harvested storage roots are normally cured for 4-7 days at temperatures of 29-35°C with a relative humidity of 85-90% in specially heated storage sheds which must be well ventilated. This treatment promotes the formation of wound cork and the production of phenolics on damaged surfaces, thereby preventing excessive water loss and pathogenic infection. Once the curing procedure is complete, storage roots are stored at 13°C with a relative humidity of 80-90%. Under these conditions, storage roots can be kept for 12 months or longer, depending on the cultivar.
In tropical countries, root storage is difficult because of rotting, weevil damage, and sprouting. Most growers use methods such as progressive harvesting or growing early- and late-maturing cultivars, to avoid the problem of storage. The roots are usually consumed within a few days of harvest. For transport to the market, roots are packed into sacks, boxes or crates soon after harvest. The roots may remain viable for up to a week; however, the quality deteriorates rapidly after a few days. Storage in pits or mounds is often practised but, even then, storage life extends only to 1-2 months.
Continuous vegetative propagation and diversity in the utilization throughout time contributed to the immense amount of genetic variability that exists in sweet potato. In the Asia and Pacific area alone, there exist an estimated 12 000 landraces. The International Potato Center (CIP) in Peru held a total of 6500 accessions in 1994. However, many of them could be duplicates.
CIP is focusing its breeding programme on the improvement of quality and insect resistance within a strategic approach that involves global collaboration with the national agricultural research systems. Well-established research programmes with defined breeding objectives have also been carried out in China, Japan and Taiwan. In the Philippines and Indonesia, breeding activities are also in progress.
Utilization of sweet potato, preferred types, and production constraints vary with region, and breeding goals should reflect the needs of each region. In Asia and Oceania, biotic constraints such as scab and weevils, and abiotic ones such as drought, excess moisture and low soil fertility deserve consideration in genetic improvement programmes. In addition, early maturity, eating quality (flavour, taste and texture), nutritional value, high yield, uniformity and storability are important characteristics which require improvement.
The value of utilizing wild relatives has been demonstrated in several cases, e.g. the Japanese commercial cultivar "Minamiyutaka" has been derived through controlled genetic introgression from the hexaploid Ipomoea trifida (Kunth) G. Don. However, wild germplasm cannot yet be utilized effectively because of the differences in ploidy levels, and the inability of wild species to form storage roots. The efficient utilization of wild relatives requires further information on heterosis effects and specific traits which do not occur in the cultivated sweet potato.
Sweet potato has a great yield potential, high nutritional value, and can survive in a wide range of environments. Great potential exists for using sweet potato for human consumption, animal feed, and industrial processing in Asia and many other tropical regions.
Besides improvement of the important basic characteristics, development of new traits may lead to new applications, e.g. the use of non-sweet sweet potato (lacking ß-amylase) for staple and processing, and the elimination of trypsin inhibitor for better digestibility as feed. In addition, management practices, post-harvest handling, and processing technology should receive due attention.
Conventional breeding methods will remain the mainstay of improving sweet potato. However, the efficiency of the sweet potato breeding programme can be improved through the introduction of appropriate biotechnological tools. In particular, techniques such as restriction fragment length polymorphisms (RFLP) and randomly amplified polymorphic DNA (RAPD) are increasingly being adopted for genome characterization, and to detect markers that are closely linked to genes of strategic interest. Furthermore, transformation with recombinant DNA in conjunction with gene expression technology is being used in the development of virus- and weevil-resistant sweet potatoes.
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H. Takagi, C.G. Kuo & S. Sakamoto