Sweet potato

Sweet potato is a long-lived herbaceous vine (usually grown as an annual), with slender stems that are smooth or slightly hairy, trailing on the ground or twining to climb, 1–5 m long, much branched and green or purple. The leaves are alternate with long petioles; the blade varies from heart-shaped to palmately divided into 3–7 lobes, with a cordate base and entire or toothed margins, 4–15 cm long. Flowers are borne in the leaf axils, bisexual and bilaterally symmetrical; the fused corolla is funnel-shaped about 4–4.5 cm long, creamy white, pink or purple, often with a darker throat, with 5 stamens and a 2-celled ovary. In the tropics the plant flowers little under short days and usually sets no seed, so it is propagated mainly by stem cuttings. The harvested part is the tuberous root — adventitious roots that swell into elongated tubers with smooth red, purple, brown or white skin and white, yellow, orange or purple flesh depending on the cultivar; tubers may weigh a few hundred grams to several kilograms.

According to Plants of the World Online (Kew POWO), the native range of sweet potato is Mexico to Venezuela and Ecuador — a scrambling tuberous geophyte of the seasonally dry tropics. Archaeological evidence places its domestication in Central America or north-western South America; tubers around 8,000 BCE have been recovered from a cave in Peru. Molecular work by Muñoz-Rodríguez et al. (2018) showed that sweet potato diverged from its closest wild relative Ipomoea trifida at least 800,000 years ago; the species is a hexaploid (2n = 6x = 90) of hybrid (allopolyploid) origin. From the Americas, sweet potato spread to Polynesia (it remains debated whether this predated European contact; recent evidence indicates a presence in Polynesia by around the 1st–2nd millennium CE), while New Guinea became an important secondary centre of diversity (Roullier et al., 2013). Today it is grown throughout the tropics and in subtropical regions with hot summers, with China and several Asian countries among the largest producers; in Vietnam it is widely grown in the lowlands, midlands and on sandy soils.

Chemically, the main component of the root is starch (amylose and amylopectin), which makes up most of its dry weight, together with sugars (sucrose, maltose and glucose released on roasting by amylase, which sweetens the root), fibre and pectin. Root colour reflects two key pigment groups: yellow/orange-fleshed cultivars are rich in beta-carotene and other carotenoids, with beta-carotene often accounting for more than 80% of total carotenoids; purple-fleshed cultivars contain anthocyanins — mainly glycosides of cyanidin, peonidin and malvidin (for example cyanidin-3-sophoroside-5-glucoside and peonidin-3-sophoroside-5-glucoside). Purple roots are also rich in phenolic acids, especially caffeoylquinic acid derivatives (chlorogenic acid, 3,4,5-tri-O-caffeoylquinic acid), at levels several times higher than orange or white cultivars. Roots and leaves also contain the storage protein sporamin, flavonoids, vitamin C, vitamin B6, potassium and manganese. The young leaves (sweet potato greens) are especially rich in polyphenols, caffeoylquinic acids, lutein, beta-carotene, vitamin C, calcium and iron — regarded as nutritionally comparable to spinach — and also contain some oxalate.

Sweet potato is an important staple, vegetable and fodder crop: roots are boiled, roasted, fried, steamed, mashed, made into cakes, desserts, starch, noodles and syrup; in Vietnam “roasted sweet potato” is a familiar cold-season street food. The young shoots and leaves (sweet potato greens) are stir-fried with garlic, boiled or cooked in soup. Pharmacologically, the plant — especially the leaves — is rich in actives: a review by Nguyen et al. (2021, Molecules) systematised the properties of sweet potato leaves; Nagai et al. (2011) showed that sweet potato leaves suppress LDL oxidation in human subjects; a 2023 systematic review confirmed that several compounds (anthocyanins, phenolic acids, caffeoylquinic acid derivatives) contribute to multi-target hypoglycaemic action in type 2 diabetes via α-amylase/α-glucosidase inhibition, increased insulin sensitivity and Glut4/PI3K-AKT regulation; purple sweet potato anthocyanins show antioxidant and anti-inflammatory effects (reducing TNF-α, IL-6, NF-κB) and liver/neuroprotection (reviews 2024–2025). Because of its high beta-carotene, orange-fleshed sweet potato (OFSP) is used in biofortification programmes against vitamin A deficiency in developing countries. The leaves and starch also have industrial uses (fibre, biodegradable plastics) and the plant is grown ornamentally (purple-leaved forms).

As food, sweet potato roots are wholesome, but a few points matter. First, raw roots contain trypsin inhibitors and are hard to digest, so they must be cooked. Second, mouldy, bruised or bitter roots may contain ipomeamarone — a hepatotoxic sesquiterpene produced in response to fungi (for example black rot, Ceratocystis fimbriata); never eat sprouted, mouldy or bitter roots, and cut away damaged parts. Third, the young leaves contain oxalate, so people with kidney stones should eat them in moderation; and orange/yellow roots are rich in carotenoids while purple roots are rich in anthocyanins — eating a variety aids absorption, especially with a little fat. For people with diabetes, boiled sweet potato has a lower glycaemic index than potato, but roasted or well-cooked sweet potato is higher, so sensible portions are advisable. On conservation, sweet potato is Not Evaluated (NE) on the IUCN Red List and is not listed in the CITES appendices; it is a global crop with rich germplasm held at the International Potato Center (CIP, Peru) and many genebanks, and is not threatened. Priorities are conserving local landraces, especially orange-fleshed (vitamin-A-rich) and purple-fleshed (anthocyanin-rich) types.