Sweet potato

Use and quality indicators

The high energy value of sweet potato is used in many cases. It is a staple food for many millions of people, and in some countries its consumption can account for 70% of daily caloric intake. In different regions there are large differences in taste and texture preferences among peoples. Most consumers prefer sweet potatoes with high starch content, low sugar content, and dry texture. However, in some production regions, namely the USA, preference is given to varieties with dark skin, high sugar content, and yellow or orange flesh. In other countries, preference is given to varieties with light skin and white starchy flesh.

There are three main types of sweet potato, which after cooking are characterized as:

• firm, dry, and mealy flesh;
• soft, moist, and gelatinous flesh;
• coarse and fibrous flesh.

Most production belongs to the first type, with most varieties having white flesh. In addition to human consumption, a large amount of this type is also used for animal feed and production of industrial products. The second type is mainly intended for human consumption, but in terms of volume it is less significant. The third type is usually used for animal feed and industrial use, for example, for production of starch and alcohol.

In American markets, varieties that have intense orange flesh color and a soft, moist, sweetish texture after cooking are erroneously called "yams." True sweet potato is a completely different plant from the genus Dioscorea (Dioscorea). In the USA, sweet potato is a marketing term distinguishing sweet potatoes with dark red or purple skin and orange flesh from sweet potatoes with light skin and yellow roots, which are less sweet and have a dry firm texture after cooking. The latter are often called "Jersey" varieties.

Besides fresh consumption, sweet potato is processed by canning and used to make chips, noodles, flour, and candies. In Japan and Taiwan, most of the crop goes to animal feed, and part is used for production of starch and alcohol; a significant part of the crop also goes to animal feed in China.

Sweet potato provides a high-calorie contribution to the diet: Average dry matter content is 30%, of which 75-90% is carbohydrates; fat content averages about 0.4%. Sweet potato starch consists of approximately one third amylose and two thirds amylopectin. During cooking, most of the starch is converted to maltose, which is responsible for the sweet taste.

Protein content in most varieties is from 1.5% to 2.5%, which is low or medium compared with other vegetables. Although protein quality is fairly balanced, it is low in sulfur-containing amino acids. Sweet potato is a good source of vitamin C and contains sufficient B-group vitamins. Orange sweet potato is an excellent source of beta-carotene, containing more than yellow, while white has little or none. Raw sweet potato contains a trypsin inhibitor that reduces protein digestibility. However, because sweet potato is eaten after cooking, the inhibitor is destroyed.

Tender leaves, petioles, and shoot tips of sweet potato are also used in Southeast Asia as a vegetable green. They have a high content of provitamins—vitamins A and C—and protein content is much higher than in storage roots, approximately 30% on a dry weight basis, and in nutritional value they are comparable to water spinach (Ipomoea aquatica).

History and origin

Most systematic botanists agree that the place of origin of sweet potato is somewhere in tropical America in regions including Panama, northern parts of South America, and the West Indies. The closest relative Ipomoea trifida is found wild in Mexico. Another related species Ipomoea tiliacea can be found in the West Indies. Ipomoea batatas is a hexaploid, but most of the approximately 400 species of Ipomoea are diploids. The evolutionary stages of sweet potato domestication remain unclear.

Sweet potato was cultivated by the Maya in Central America, the Inca in Peru, and the Maori in New Zealand long before the 16th century. The age of sweet potato remains, possibly of a wild species, found in Peruvian caves is estimated at more than 8,000 years. This plant was introduced to Spain by European explorers around 1600 CE, to western Africa by Portuguese traders, and later to India, the East Indies, China, and Japan.

The early presence of sweet potato in the Philippines and Polynesia suggests that these places are possible centers of origin. However, other data indicate that sweet potato clones were transferred from Central America and/or Mexico to the Philippines and from Peru to islands of the southern Pacific Ocean. Although it is not known exactly, the transfer could have occurred with migrating people and traders or by natural seed dispersal via ocean currents. Seeds have extremely hard seed coats, which could provide protection to survive long periods in the ocean. Differences existing between sweet potato in the West Indies compared with potatoes in the Philippines and Polynesia support the version of dispersal from different places in Central America.

Cultivation areas

Sweet potato is very important in developing countries, which account for about 98% of world production. Sweet potato occupies about 19% of the total land area used for growing root and tuber crops, and accounts for about 22% of world tonnage of these vegetables.

Despite its origin, sweet potato is mainly an Asian crop, where 92% of total world production is produced. China, occupying more than 6 million hectares, produces more than 84% of world volume.

In many countries, sweet potato is proportionally displaced by more prestigious crops such as maize, cereals, legumes, and leafy vegetables. Analysis of production statistics in past years shows that a significant reduction in land area used for production of this crop is compensated by increased yield, which is the result of improved varieties and production technology. Consumption in less developed countries continues to remain high. However, a significant volume for use in livestock feed is likely to remain. On a global scale, sweet potato ranks ninth among the most important food crops. It is mainly a subsistence crop, traditionally consumed locally, and due to its low cost, bulkiness, and perishable appearance when not cured, sweet potato is not the subject of broad trade, except local, and is infrequently found in international trade.

Botanical description

Sweet potato is a perennial dicotyledonous plant with long vines and smooth, flat, heart-shaped or lobed leaves located on upright petioles. The central stem, from which lateral branches extend, is usually prostrate and, depending on internode length, may look more or less bushy. The types of varieties, whether bushy, intermediate, or vining, are determined by internode length, not by the vine; vine branching differs among varieties. The edible parts are enlarged storage roots, shoot tips, and young leaves. Ipomoea aquatica is grown specifically for edible leaves.

Filamentous roots can develop adventitiously from opposite sides of each node on parts of the stem in contact with soil. The edible storage organs, erroneously called tubers, are tuberous roots of thickened secondary roots. Usually about 15% of formed roots thicken and form storage organs, which develop fairly close, within 25 cm of the soil surface. Most storage root growth is usually achieved about 2 months after planting. Diameter continues to increase as long as the foliage remains active. Most of the storage root consists of parenchyma tissues.

Plants usually develop 4-10 storage roots. The mass of most commercial roots ranges from 100 to 400 g. Root enlargement, erroneously called tuberization, is the result of rapid cell division of the central parenchyma, followed by cell enlargement and starch deposition. The color of the root periderm varies by variety: from brown, yellow, orange, copper, or red to purple. Similarly, flesh color can be from white to light yellow, dark orange, red, or purple. Shape varies from elongated to nearly round. Smoothness of root surface varies by variety; some varieties tend to develop surface ridges.

Flowers 3-4 cm in diameter, pale pink with a reddish, lavender, or purple throat in the form of a tube, resemble morning glory flowers. They open early in the morning, close and wilt within a few hours; pollination is carried out by insects. One to four seeds are found in a round capsule. Mature seeds are hard, black, flattened, and often require scarification to facilitate germination.

Sweet potato is generally a short-day plant, and depending on variety, flowering requires a photoperiod of no more than 11 hours. Flowering, although common in tropical regions, is rarely observed in temperate regions. Flowering and seed production are important only for breeding. Most varieties are self-incompatible. To improve flowering characteristics during sexual crossing, sweet potato can be grafted onto rootstocks of other species of the genus Ipomoea (Ipomoea spp.).

Environmental requirements

Being a reliable and productive crop with low costs and a relatively short growing period, sweet potato is an important commodity and is grown in many tropical, subtropical, and warm temperate regions during frost-free periods. This crop is grown in regions up to 40° latitude. It grows best in areas with warm days and nights.

As a rule, high light intensity and long days favor vine growth, predominantly over storage root enlargement. Increased vine growth in response to long days correlates with low yield. The growing period varies from 90 to 150 days in temperate regions. In the tropics, growth occurs continuously, and the crop can be harvested when suitable root size is reached.

Well-drained, loose sandy loam soils with sufficient aeration are preferred. Soil compaction negatively affects root shape and size. Sweet potato has moderate tolerance to low pH and can adapt to a range of 4.5-7.5; optimum is about 6.0-7.5. At pH 5.2 plants are less susceptible to pox and scab. Plants also have moderate salt tolerance.

Optimal for active growth is a uniform water supply, about 25-30 mm each week. Sweet potato tolerates some drought due to deep rooting. About half of the entire root system is within 30 cm of the soil surface; some plants are found at depths up to 2 m. Plants are intolerant of flooding; waterlogging reduces yield. However, irregular heavy rains are tolerated if drainage is good. Moisture deficiency can reduce yield if it occurs during root enlargement.

Sweet potato has the ability to produce reasonably even on infertile soils. Periodic association with mycorrhiza Rhizophagus fasciculatus increases yield on soils with low phosphorus content. This crop is often grown as a second or relay crop to use residual fertilizers from preceding crops. Table 10.1 shows nutrient removal by sweet potato. Petiole tissue analysis can be used to determine the nutrient status of the crop during growth.

If sweet potato is grown in several farming systems, it is important to choose shade-tolerant varieties, since excessive shading reduces yield. Leaf area index is relatively low because growth occurs mainly in a horizontal direction, which limits light penetration under the canopy. However, some shade-tolerant varieties do perform better in light shade than in full sunlight.

Nutrient removal by sweet potato averaged over four varieties (Scott and Bouwkamp, 1974):

• nitrogen with roots — 47 kg/ha, with aboveground part — 52 kg/ha.
• phosphorus with roots — 19 kg/ha, with aboveground part — 8 kg/ha;
• potassium with roots — 179 kg/ha, with aboveground part — 101 kg/ha;
• calcium with roots — 11 kg/ha, with aboveground part — 46 kg/ha;
• magnesium with roots — 9 kg/ha, with aboveground part — 9 kg/ha.

Propagation

Storage roots do not have a natural dormant period and are capable of producing adventitious shoots that develop from the vascular cambium under favorable temperature and moisture. From these sprouts, called shoots or slips, adventitious roots develop. Such seedlings are used for transplanting slips.

In temperate regions, storage roots are used to obtain slips for transplanting. Such use requires a significant number of roots that otherwise could have been consumed. Small stored roots unsuitable for sale are sometimes used for direct field planting.

Due to proximal dominance, sprouts appear earlier and in greater numbers at the "stem end" of the storage root. Dominance is most pronounced in freshly harvested roots, but weakens with aging or transverse division (cutting) of roots. Cut root pieces can be used, since more sprouts form from them than from intact roots. Pieces of storage roots placed at a density of 100/m² usually produce about 800-900 sprouts. Approximately 500-700 kg of roots produce enough sprouts to plant 1 ha.

If growing conditions allow, propagation material can be grown in the open field. However, early production of slips in temperate regions can be achieved on heated nursery beds, which accelerate germination and growth. Soil temperature of 28-30 °C is optimal for producing sprouts, which usually appear after 1-2 weeks. They are ready for pulling and transplanting after 4-6 weeks. Cold temperatures lead to slow emergence and growth. Temperatures above 30 °C lead to thin, wilted sprouts that establish poorly after transplanting.

Uniform intact sweet potatoes or cut pieces are laid in one layer on the nursery bed. They are placed close to each other to maximize the number and uniformity of sprouts produced. After laying, they are covered with a 5-10 cm layer of soil. Beds often have a sandy soil base to provide good drainage, easy placement of root pieces, and easy removal of sprouts. Beds can also be covered with plastic for better temperature control. Low plastic tunnels are often used, which can increase growing temperature and also provide ventilation when needed. Before pulling, sprouts are usually hardened for one or two days by opening and/or not watering for one or two days; the duration of this time depends on temperature and soil type. In situations where transplanting is delayed, pulled plants can be temporarily stored at 16 °C and high humidity; lower or higher temperatures are less favorable. Transplanted sprouts usually root quickly and do best at a soil temperature of 21 °C. At temperatures above 30 °C rooting is suppressed; the minimum critical temperature is 15 °C. Transplanted plants are usually placed in soil to a depth of 5-10 cm.

In tropical areas, instead of roots, vine tips and stem cuttings are usually used for propagation. The advantage of these materials is that they reduce propagation costs by avoiding the use of consumable storage roots. In addition, shoot tips are less likely to transmit diseases or introduce soil pests such as nematodes, although weevils can be introduced. Cuttings can be taken from existing crops while donor plants continue to grow, or can be obtained from plants during harvest. Cuttings 30-45 cm long with eight or nine nodes are usually used. They are usually planted horizontally or at a slight angle, to a depth of 5-10 cm, with at least three or four nodes covered with soil. Rooting and establishment usually occur quickly. The polarity of sweet potato cuttings, the presence or absence of the shoot tip, do not affect yield. This is explained by the fact that the plant is upright, and new shoots appearing from leaf nodes maintain the polarity of the plant.

Manual transplanting of stem cuttings or rooted sprouts is the most common method of establishment. In the United States, transplanting is mechanized. The intended use of the crop often determines planting density. Wide spacing produces large roots, which are preferable for the food industry and industrial use, while high planting density produces small roots, more suitable for home consumption. Planting density is usually from 20 to 30 thousand plants per hectare.

Plantings are often placed on raised beds or ridges to improve drainage and facilitate soil coverage of developing storage roots. It is important that they are sufficiently covered with soil, since exposure to light prevents initiation and enlargement of storage roots. During partial harvest, roots are recovered with soil for further enlargement of already set roots.

Meristem and tissue culture are used to obtain disease-free propagation material for clean propagation stock. In the USA and other countries, programs have been created for production of certified disease-free material. Although true seeds can be used for propagation, storage root quality and yield may vary. Mainly true seeds are used for breeding purposes. The use of somatic embryos as "synthetic seeds" for propagation, currently being researched, may become possible.

Growth and development

In tropical regions, growth is perennial, and vines continue to grow. Storage roots can be harvested when they reach suitable size; other roots are allowed to develop and enlarge. Lacking clearly defined maturity, yield generally increases with a longer growing period.

In temperate zones, sweet potato is grown as an annual plant; growth stops with cooling or frost. The growth cycle consists of three main phases:

1. active growth of filamentous roots with moderate vine growth;
2. intensive vine growth with formation of a large leaf area and beginning of storage root development;
3. enlargement of storage roots, associated with slowing of leaf and fibrous root growth.

Leaves usually develop quickly, and about 60 days after transplanting, or at the beginning of the middle growth stage, full leaf canopy is usually reached. Leaf area index at this time is about 5. Subsequently, leaf area index gradually decreases. Plants adapt easily to extreme high temperatures, although temperatures much above 30 °C usually suppress growth and yield, especially if high night temperatures persist. The best temperature for growing is 29 °C during the day and 21 °C at night; an average temperature of about 24 °C is optimal.

Storage root enlargement can begin as early as 30-35 days after planting and continues as the main sink until harvest or cessation of growth. High temperatures suppress root growth more than vine growth. Optimal storage root enlargement occurs at a day temperature of 25 °C and night temperature of 20 °C. Soil temperature above 30 °C leads to reduced yield.

Plants are intolerant of frost, and at temperatures below 15 °C active growth may stop. At temperatures below 10 °C frost damage occurs, and plants may die. In addition, as storage root size increases, an anaerobic state may arise due to a large oxygen gradient between internal and external tissues, leading to breakdown of storage tissues. The incidence of storage root rot may increase after heavy rains.

During growth, the ratio of starch and sugar changes. Starch is low in very young roots and remains low during maximum growth, probably because carbohydrates are used to form new tissues. Total sugars also decrease during rapid growth. As roots enlarge, there is a further decrease in sugars, while starch increases.

Harvesting

Most of world sweet potato production is a subsistence crop, harvested as needed, usually by hand digging. Roots can be harvested at early stages without stopping plant growth, by carefully cutting off part of the storage roots with minimal soil disturbance. This is a common practice in Papua New Guinea and some tropical countries. Unharvested mature storage roots lose eating quality due to continuous fiber development.

In large-scale production in many temperate and subtropical areas, harvest usually occurs in one pass, approximately 90-150 days after transplanting. Sometimes the crop is harvested as early as 80 days, but in other cases harvest may be delayed to 150 days. The time of commercial harvest often depends on market needs.

Removing foliage before digging facilitates harvesting of storage roots and is believed to thicken (set) the tender skin (periderm). It is important to avoid injuries during harvest, since sweet potato is easily damaged. Sufficient soil moisture and warm temperature during harvest reduce susceptibility to physical damage. After digging, sunscald and drying must be prevented. In some countries, mechanized harvesting is used, and to minimize injuries the crop is often transported in containers on pallets. To avoid increasing rot, sweet potatoes are not washed before curing or storage.

Postharvest handling and storage

Storing roots after harvest minimizes further damage by healing harvest wounds. Curing involves holding storage roots at 27 °C to 30 °C and high relative humidity (85-95%) for 4-7 days to form a cork layer of periderm under damaged areas. Under these conditions, periderm synthesis is optimal. After periderm formation, microbial invasion and water loss are limited. During indoor curing, good air exchange is important to prevent moisture condensation and elevated CO₂ levels. Excessive curing time or high temperatures lead to shrinkage, reduced storage life, and tendency to early sprouting. During curing, dry matter loss occurs and sugars increase.

Field curing provides less management control but is widely used in many countries. Large piles of harvested roots are covered with vines thick enough to serve as insulation and also increase humidity. Root respiration raises the temperature of the pile. Curing is usually completed after about 7-10 days; weight loss during curing can be up to 5%.

Storage

Due to year-round supply in many tropical regions, there are no particular incentives for storing roots. Nevertheless, for short-term storage, ground pits are often used, providing good drainage and insulation. Aboveground clamps are also used for short-term storage. For long-term storage in temperate production regions, after curing the temperature is reduced to 13-16 °C, and relative humidity is maintained at 85-90% to reduce respiration and moisture loss and prevent sprouting. Long-term storage at temperatures below 13 °C leads to chilling injury and increases fibrousness of internal tissues. Temperature should not be below 10 °C, and storages should be ventilated to maintain low CO₂ levels. Weight losses are possible, averaging about 2% per month; good storages are able to minimize such losses. Roots can lose up to 10% fresh weight without visible shriveling. Under optimal conditions, sweet potato can be stored 6 months or more.

Diseases and pests

Sweet potato roots are affected by a number of diseases and pests in the field and after harvest. Although root damage is often more obvious, leaf damage can be serious. Vegetative propagation can lead to transmission of diseases and pests. Prevention consists of using healthy propagation material, hygienic production practices, proper crop rotation, and careful storage of properly cured roots. There is plant resistance to some pests.

Important bacterial diseases are soft rot Dickeya dadantii (syn. Erwinia chrysanthemi), bacterial wilt Pseudomonas solanacearum, and scab Streptomyces ipomoea.

Fungal pathogens affecting sweet potato include the following:

• Alternaria spp. (English Alternaria leaf spot);
• Ceratocystis fimbriata (English Black rot);
• Cercospora spp. (English Cercospora leaf spot);
• Macrophomina phaseolina (English Charcoal rot);
• Plenodomus destruens (English Foot rot);
• Fusarium oxysporum (English Fusarium surface rot);
• Fusarium oxysporum, f. sp. batatas (English Fusarium wilt);
• Fusarium solani (English Fusarium storage root rot);
• Lasiodiplodia theobromae (English Java black rot);
• Sphaceloma batatas (English Leaf and stem scab), currently assigned to species Elsinoe batatas;
• Coleosporium ipomoeae (English Red rust);
• Rhizopus stolonifer (English Rhizopus soft root rot);
• Monilochaetes infuscans (English Scurf);
• Septoria bataticola (English Septoria leaf spot);
• Agroathelia rolfsii (English Southern blight);
• Rhizoctonia solani (English Stem canker);
• Elsinoe batatas (English Stem and Foliage scab);
• Helicobasidium mompa (English Violet root rot);
• Albugo ipomoeae-panduratae (English White rust).

Viruses transmitted by aphids and causing reduced productivity include chlorotic leaf spot virus (English chlorotic leaf spot virus), internal cork virus (English internal cork virus), russet crack virus (English Russet crack virus), sweet potato feathery mottle virus (sweet potato feathery mottle virus, SPFMV), and sweet potato vein mosaic virus (sweet potato vein mosaic virus, SPVMV). Viruses transmitted by whiteflies are sweet potato mild mottle virus (sweet potato mild mottle virus, SPMMV) and sweet potato yellow dwarf virus (sweet potato yellow dwarf virus, SPYDV). A disease caused by mycoplasmas transmitted by leafhoppers is little-leaf disease, also known as witches' broom (English witches-broom).

Important nematode pests include the following:

• Pratylenchus spp. (English Lesion);
• Rotylenchulus reniformis (English Reniform);
• Meloidogyne spp. (English Root knot);
• Paratrichodorus spp. (English Stubby root);
• Helicotylenchus spp. (English Spiral);
• Ditylenchus spp. (English Stem and bulb).

There is partial resistance to root-knot nematode, but one should not rely only on resistance, but use it in combination with fumigation and/or crop rotation to limit resurgence of damaging populations.

Physiological disorders affecting sweet potato are souring caused by lack of oxygen due to flooding. Other losses of yield and quality are associated with growth cracks, chilling injury, and sparse stand caused by prolonged warm and dry storage. Hard core is a condition in which storage root tissues inside remain hard after cooking. This occurs as a result of exposure of roots to cold followed by storage at non-cold temperatures. Susceptibility depends on variety and degree of injury, as well as pre-treatment and storage.

Important insect pests include the following:

• Aphis gossypii (English Cotton aphid);
• Spodoptera litura (English Armyworm);
• Diabrotica spp. (English Cucumber beetles);
• Myzus persicae (English Green peach aphid);
• Systena spp. (English Flea beetles);
• Notoxus calcaratus (English Flower beetle);
• Euscepes postfasciatus (English Scarabee);
• Alcidodes dentipes (English Striped sweet potato weevil);
• Chaetocnema confinis (English Sweet potato flea beetle);
• Agrius convolvuli (English Sweet potato horn worm);
• Typophorus nigritus viridicyaneus (English Sweet potato leaf beetle);
• Herse convolvuli (English Sweet potato moth);
• Omphia anastamosalis (English Sweet potato vine borer);
• Cylas formicarius elegantulus, Cylas puncticollis (English Sweet potato weevil);
• Tobacco whitefly (Bemisia tabaci, English Sweet potato whitefly);
• Metriona spp. (English Tortoise beetles);
• Euscepes postfasciatus (English West Indian sweet potato weevil);
• Plectris aliena and Phyllopaga ephilda (English White grub);
• Conoderus spp. (English Wireworms).

Worldwide, sweet potato weevil is the most important insect pest both in the field and in storage.

Sweet potato plants become competitive with weeds after 5-6 weeks, when full canopy is usually established; high plant density also helps reduce early competition with weeds. Effective herbicides exist, although for most of world production the main practice remains manual weeding. Tillage, mechanical or manual, should be shallow so as not to damage the fibrous root system near the surface.

Crop improvement

Because sweet potato is a staple food, several international organizations conduct significant research on improving this crop. The International Potato Center (CIP) in Peru has an improvement program and is a repository of germplasm. Germplasm repositories are also located in Fort Collins, Colorado, and Griffin, Georgia, USA.

Although average world yield is more than 13 t/ha, in many parts of the world, especially Africa, yields are much lower, mainly due to subsistence farming. Researchers have developed varieties with potential for early high yields with limited or no additional fertilizer application. However, the lack of weevil resistance in these varieties limits their spread.

Sweet potato ranks high in affordable nutritional value returned per labor and material costs. This crop is likely to continue to remain an important food, despite its low status as "poor man's food." Progress will depend on improved breeding and crop research. Intraspecific hybridization is difficult due to crossing barriers and differences in ploidy, but there is significant germplasm variability in Ipomoea species (Ipomoea spp.). Therefore, the use of somatic mutations and breeding for improvement may be surpassed by applying new technologies to improve varieties, especially with respect to increased nutritional value. A wide range of crude protein content, reaching 10%, is observed in some germplasm sources, indicating potential for significant quality improvement. Similarly, yield potential has not been fully exploited. Major breeding efforts are aimed at improving taste, weevil resistance, yield and yield stability, and virus resistance. Excellent breeding programs exist in the USA, Japan, Indonesia, and some other countries.

References

1. Rubatzky, Vincent E., Yamaguchi, Mas. (1997). World vegetables: principles, production, and nutritive values. 2nd ed.