sweet potato

Description

The plant is a herbaceous perennial vine, bearing alternate, triangle-shaped or palmately lobed leaves and medium-sized, sympetalous flowers. The stems usually crawl on the ground and form adventitious roots at the nodes. The leaves are screwed along the stems. The leaf stalk is 13 to 51 centimetres (5 to 20 inches) long. The leaf blades are very variable, 5 to 13 cm (2 to 5 in) long, the shape is heart-, kidney-, to egg-shaped, rounded or triangular, and spear-shaped, the edge can be entire, toothed, or often three- to seven-lobed, cut or divided. Most of the leaf surfaces are bare, rarely hairy, and the tip is rounded to pointed. The leaves are mostly green in color, but the accumulation of anthocyanins, especially along the leaf veins, can make them purple. Depending on the variety, the total length of a stem can be between 0.5 and 4 m (1+1⁄2 and 13 ft). Some cultivars also form shoots up to 16 m (52 ft) in length. However, these do not form underground storage organs.

The hermaphrodite, five-fold, and short-stalked flowers are single or few in stalked, zymous inflorescences that arise from the leaf axils and stand upright. It produces flowers when the day is short. The small sepals are elongated and taper to a point and spiky and (rarely only 7) 10 to 15 millimetres (3⁄8 to 5⁄8 in) long, usually finely haired or ciliate. The inner three are a little longer. The 4 to 7 cm (1+1⁄2 to 2+3⁄4 in) long, overgrown and funnel-shaped, folded crown, with a shorter hem, can be lavender to purple-lavender in color, the throat is usually darker in color, but white crowns can also appear. The enclosed stamens are of unequal length with glandular filaments. The two-chambered ovary is upper constant with a relatively short stylus. Seeds are only produced from cross-pollination.

The flowers open before sunrise and stay open for a few hours. They close again in the morning and begin to wither. The edible, tuberous root is long and tapered, with a smooth skin whose color ranges between beige, yellow, orange, red, brown, and purple. Its flesh ranges from beige through white, red, pink, violet, yellow, orange, and purple. Sweet potato cultivars with white or pale yellow flesh are less sweet and moist than those with red, pink, or orange flesh.

Taxonomy

The sweet potato originated in South America, in what is now Ecuador. The domestication of sweet potato occurred in either Central or South America. In Central America, domesticated sweet potatoes were present at least 5,000 years ago, with the origin of I. batatas possibly between the Yucatán Peninsula of Mexico and the mouth of the Orinoco River in Venezuela. The cultigen was most likely spread by local people to the Caribbean and South America by 2500 BCE.

I. trifida, a diploid, is the closest wild relative of the sweet potato, which originated with an initial cross between a tetraploid and another diploid parent, followed by a second complete genome duplication event. The oldest radiocarbon dating remains of the sweet potato known today were discovered in caves from the Chilca Canyon, in the south-central zone of Peru, and yield an age of 8080 ± 170 BC.

Transgenicity

The genome of cultivated sweet potatoes contains sequences of DNA from Agrobacterium (sensu lato; specifically, one related to Rhizobium rhizogenes), with genes actively expressed by the plants. The T-DNA transgenes were not observed in closely related wild relatives of the sweet potato. Studies indicated that the sweet potato genome evolved over millennia, with eventual domestication of the crop taking advantage of natural genetic modifications. These observations make sweet potatoes the first known example of a naturally transgenic food crop.

Cultivation

Dispersal history

Before the arrival of Europeans to the Americas, sweet potato was grown in Polynesia, generally spread by vine cuttings rather than by seeds. Sweet potato has been radiocarbon-dated in the Cook Islands to 1210–1400 CE. A common hypothesis is that a vine cutting was brought to central Polynesia by Polynesians who had traveled to South America and back, and spread from there across Polynesia to Easter Island, Hawaii and New Zealand. Genetic similarities have been found between Polynesian peoples and indigenous Americans, including the Zenú, a people inhabiting the Pacific coast of present-day Colombia, indicating that Polynesians may have visited South America and taken sweet potatoes prior to European contact. Dutch linguists and specialists in Amerindian languages Willem Adelaar and Pieter Muysken have suggested that the word for sweet potato is shared by Polynesian languages and languages of South America: Proto-Polynesian *kumala (compare Rapa Nui kumara, Hawaiian ʻuala, Māori kūmara) may be connected with Quechua and Aymara k'umar ~ k'umara. Adelaar and Muysken assert that the similarity in the word for sweet potato is near proof of either incidental or sporadic contact between the Central Andes and Polynesia.

Some researchers, citing divergence time estimates, suggest that sweet potatoes may have been present in Polynesia thousands of years before humans arrived there. However, the present scholarly consensus favours the pre-Columbian contact model.

The sweet potato arrived in Europe with the Columbian exchange. It is recorded, for example, in Elinor Fettiplace's Receipt Book, compiled in England in 1604.

Sweet potatoes were introduced to the Philippines during the Spanish colonial period (1521–1898) via the Manila galleons, along with other New World crops. The plant was introduced to Fujian in about 1594 from Luzon, in response to a major crop failure. Its cultivation was encouraged by the Governor Chin Hsüeh-tseng (Jin Xuezeng).

Sweet potatoes were also introduced to the Ryukyu Kingdom, present-day Okinawa, Japan, in the early 1600s by the Portuguese. Sweet potatoes became a staple in Japan because they were important in preventing famine when rice harvests were poor. Aoki Konyō helped popularize the cultivation of the sweet potato in Japan, and the Tokugawa bakufu sponsored, published, and disseminated a vernacular Japanese translation of his research monograph on sweet potatoes to encourage their growth more broadly. Sweet potatoes were planted in Shōgun Tokugawa Yoshimune's private garden. The crop was introduced to Korea in 1764. Kang P'il-ri and Yi Kwang-ryŏ embarked on a project to grow sweet potatoes in Seoul in 1766, using the knowledge of Japanese cultivators they learned in Tongnae starting in 1764. The project succeeded for a year but ultimately failed in winter 1767 after Kang's unexpected death.

Names

Although the soft, orange sweet potato is often called a "yam" in parts of North America, the sweet potato is distinct from the botanical yam (Dioscorea), which has a cosmopolitan distribution, and belongs to the monocot family Dioscoreaceae. A different crop plant, the oca (Oxalis tuberosa, a species of wood sorrel), is called a "yam" in many parts of the world.

Although the sweet potato is not closely related botanically to the common potato, they have a shared etymology. The first Europeans to taste sweet potatoes were members of Christopher Columbus's expedition in 1492. Later explorers found many cultivars under an assortment of local names, but the name which stayed was the indigenous Taíno name of batata. The Spanish combined this with the Quechua word for potato, papa, to create the word patata for the common potato.

Though the sweet potato is also called batata (בטטה) in Hebrew, this is not a direct loan of the Taíno word. Rather, the Spanish patata was lent into Arabic as baṭāṭā (بطاطا), owing to the lack of a /p/ sound in Arabic, while the sweet potato was called baṭāṭā ḥilwa (بطاطا حلوة); literally sweet potato. The Arabic baṭāṭā was lent into Hebrew as designating the sweet potato only, as Hebrew had its own word for the common potato, תפוח אדמה (tapuakh adama, literally earth apple; compare French pomme de terre).

Some organizations and researchers advocate for the styling of the name as one word—sweetpotato—instead of two, to emphasize the plant's genetic uniqueness from both common potatoes and yams and to avoid confusion of it being classified as a type of common potato. In its current usage in American English, the styling of the name as two words is still preferred.

When soft varieties were first grown commercially there, a need existed to differentiate between the two. Enslaved Africans had already been calling the soft sweet potatoes as yams because they resembled the unrelated yams in Africa. Thus, soft sweet potatoes were referred to as yams to distinguish them from the firm varieties.

In Argentina, Colombia, Venezuela, Puerto Rico, and the Dominican Republic, the sweet potato is called batata. In Brazil, the sweet potato is called batata doce. In Mexico, Bolivia, Peru, Chile, Central America, and the Philippines, the sweet potato is known as camote (alternatively spelled kamote in the Philippines), derived from the Nahuatl word camotli.

Habitat

The plant does not tolerate frost. It grows best at an average temperature of 24 °C (75 °F), with abundant sunshine and warm nights. Annual rainfalls of 750–1,000 mm (30–39 in) are considered most suitable, with a minimum of 500 mm (20 in) in the growing season. The crop is sensitive to drought at the tuber initiation stage 50–60 days after planting, and it is not tolerant to waterlogging, which may cause tuber rots and reduce the growth of storage roots if aeration is poor.

Depending on the cultivar and conditions, tuberous roots mature in 2-9 months. With care, early-maturing cultivars can be grown as an annual summer crop in warm temperate areas, such as the Eastern United States and China. Sweet potatoes rarely flower when the daylight is longer than 11 hours, as is normal outside of the tropics. They are mostly propagated by stem or root cuttings or by adventitious shoots called "slips" that grow out from the tuberous roots during storage. True seeds are used for breeding only.

They grow well in many farming conditions and have few natural enemies; pesticides are rarely needed. Sweet potatoes are grown on a variety of soils, but well-drained, light- and medium-textured soils with a pH range of 4.5–7.0 are more favorable for the plant. They can be grown in poor soils with little fertilizer. However, sweet potatoes are very sensitive to aluminum toxicity and will die about six weeks after planting if lime is not applied at planting in this type of soil. As they are sown by vine cuttings rather than seeds, sweet potatoes are relatively easy to plant. As the rapidly growing vines shade out weeds, little weeding is needed. A commonly used herbicide to rid the soil of any unwelcome plants that may interfere with growth is dimethyl tetrachloroterephthalate, known as trade name Dacthal. In the tropics, the crop can be maintained in the ground and harvested as needed for market or home consumption. In temperate regions, sweet potatoes are most often grown on larger farms and are harvested before first frosts.

Sweet potatoes are cultivated throughout tropical and warm temperate regions wherever water is sufficient to support their growth. Sweet potatoes became common as a food crop in the islands of the Pacific Ocean, South India, Uganda and other African countries.

A cultivar of the sweet potato called the boniato is grown in the Caribbean; its flesh is cream-colored, unlike the more common orange hue seen in other cultivars. The boniato is not as sweet and moist as other sweet potatoes, but its consistency and delicate flavor are different from the common, orange-colored sweet potato.

Production

In 2020, global production of sweet potatoes was 89 million tonnes, led by China with 55% of the world total (table). Secondary producers were Malawi, Tanzania, and Nigeria. It is the fifth-most-important food crop in developing countries. Studies are being done to develop a salt tolerant variety to combat the effects of climate change.

Diseases

Sweet potato suffers from sweet potato chlorotic stunt virus (a Crinivirus). I. batatas suffers from several Phytophthora disease-causing species, including P. carotovorum, P. odoriferum, and P. wasabiae.

Uses

Nutrition

Cooked sweet potato (baked in skin) is 76% water, 21% carbohydrates, and 2% protein,with negligible fat (table). In a 100-gram reference amount, baked sweet potato provides 359 kilojoules (86 kilocalories) of food energy, and rich content (20% or more of the Daily Value, DV) of vitamin A (120% DV), vitamin C (24% DV), manganese (24% DV), and vitamin B6 (20% DV). It is a moderate source (10–19% DV) of some B vitamins and potassium. Between 50 and 90% of the sugar content is sucrose. Maltose content is very low, but baking can increase it from 10 to 20%.

Sweet potato cultivars with dark orange flesh have greater beta-carotene, which is converted to greater vitamin A content once digested, compared to those with light-colored flesh, and their increased cultivation is being encouraged in Africa, where vitamin A deficiency is a serious health problem. Sweet potato leaves are edible and can be prepared like spinach or turnip greens.

Comparison to other food staples

The table below presents the relative performance of sweet potato (in column)[G] to other staple foods on a dry weight basis to account for their different water contents. While sweet potato provides less edible energy and protein per unit weight than cereals, it has higher nutrient density than cereals.

According to a study by the United Nations Food and Agriculture Organization, sweet potatoes are the most efficient staple food to grow in terms of farmland, yielding roughly 290,000 kilojoules per hectare (28,000 kcal/acre) daily.

Culinary

The starchy, tuberous roots of the sweet potato are by far the most important product of the plant. In some tropical areas, the tubers are a staple food crop and are often cooked before consumption, as this increases nutritive value and digestibility, although the American colonists in the Southeast ate raw sweet potatoes as a staple food.

The vines' tips and young leaves are edible as a green vegetable with a characteristic flavor. Older growths may be used as animal fodder.

Amukeke (sun-dried slices of root) and inginyo (sun-dried crushed root) are a staple food for people in northeastern Uganda. Amukeke is mainly served for breakfast, eaten with peanut sauce. Inginyo is mixed with cassava flour and tamarind to make atapa. People eat atapa with smoked fish cooked in peanut sauce or with dried cowpea leaves cooked in peanut sauce. Emukaru (earth-baked root) is eaten as a snack anytime and is mostly served with tea or with peanut sauce. Similar uses are also found in South Sudan.

The young leaves and vine tips of sweet potato leaves are widely consumed as a vegetable in West African countries (Guinea, Sierra Leone and Liberia, for example), as well as in northeastern Uganda, East Africa. According to FAO leaflet No. 13 – 1990, sweet potato leaves and shoots are a good source of vitamins A, C, and B2 (riboflavin), and according to research done by A. Khachatryan, are an excellent source of lutein.

In Kenya, Rhoda Nungo of the home economics department of the Ministry of Agriculture has written a guide to using sweet potatoes in modern recipes. This includes uses both in the mashed form and as flour from the dried tubers to replace part of the wheat flour and sugar in baked products such as cakes, chapatis, mandazis, bread, buns, and cookies. A nutritious juice drink is made from the orange-fleshed cultivars, and deep-fried snacks are also included.

In Egypt, sweet potato tubers are known as baṭāṭā (بطاطا) and are a common street food in winter, when street vendors with carts fitted with ovens sell them to people passing time by the Nile or the sea. The cultivars used are an orange-fleshed one as well as a white/cream-fleshed one. They are also baked at home as a snack or dessert, drenched with honey.

Molecular gastronomy

Freezing a sweet potato until solid, baking at a low temperature, then increasing to a high temperature brings out the sweetness by caramelizing converted sugars.

Ceramics

Ceramics modeled after sweet potatoes or camotes are often found in the Moche culture.

Dyes

In South America, the juice of red sweet potatoes is combined with lime juice to make a dye for cloth. By varying the proportions of the juices, every shade from pink to black can be obtained. Purple sweet potato color is also used as a natural food coloring.

Aquariums

Cuttings of sweet potato vine, either edible or ornamental cultivars, will rapidly form roots in water and will grow in it, indefinitely, in good lighting with a steady supply of nutrients. For this reason, sweet potato vine is ideal for use in home aquariums, trailing out of the water with its roots submerged, as its rapid growth is fueled by toxic ammonia and nitrates, a waste product of aquatic life, which it removes from the water. This improves the living conditions for fish, which also find refuge in the extensive root systems.

Ornamentals

Ornamental sweet potatoes are popular landscape, container, and bedding plants. Grown as an annual in zones up to USDA hardiness Zone 9, they grow rapidly and spread quickly. Cultivars are available in many colors, such as green, yellow, and purple. Some ornamental varieties, like 'Blackie', flower more than others. These ornamental cultivars are not poisonous, and although the leaves are edible, the tubers do not have a good taste.