Longan

Longan is an evergreen tree 9–12 m tall (up to 30 m) with a dense rounded crown and thick, corky, grey-brown bark; its long, heavy branches often droop. The leaves are once-pinnate and alternate, with 6–9 pairs of ovate to oblong leaflets 10–20 cm long, blunt-tipped and glossy dark green above with a prominent midrib. The inflorescence is a pale-yellow branched panicle 10–46 cm long at the tips of the branches; the small flowers have 5–6 sepals, brownish-yellow petals, 8 stamens and a two-lobed pistil, and occur in three types (male, functionally female and hermaphroditic) that open in sequence. The fruit is a spherical drupe about 2.5 cm across, borne in drooping clusters; the thin, leathery, tan rind is covered in fine hairs and can be squeezed open like a sunflower seed. The aril is translucent white, juicy and delicately sweet; the large seed is round, hard and lacquered black with a white spot at the base — giving the 'dragon's eye' name. The fruit ripens in late summer to early autumn (July–September in Vietnam), later than lychee; grafted trees bear in 3–5 years.

According to Kew's Plants of the World Online, the native range of Dimocarpus longan is the Indian Subcontinent to southern China and north-west Peninsular Malaysia, in seasonally dry tropical regions. Longan has been cultivated in China since the Han dynasty (206 BCE–220 CE) and spread to Thailand, Vietnam, Taiwan and Indonesia, and from the 18th–19th centuries to Queensland (Australia), Hawaii and Florida. Today China produces about 70% of the world crop and, with Thailand and Vietnam, some 95%. In Vietnam the famous 'nhan long' of Hung Yen is long established, alongside Hai Duong, Bac Giang, Son La and the Mekong Delta.

The longan aril is rich in soluble sugars (sucrose, glucose, fructose) that give its clean sweetness and characteristic aroma; 100 g of fresh fruit supplies about 60 kcal, 83% water, a very high level of vitamin C (~84 mg, nearly 93% of the daily value), riboflavin and the minerals potassium (~266 mg), magnesium and phosphorus. The best-studied actives are polyphenols and flavonoids — gallic acid, ellagic acid, corilagin and (−)-epicatechin (Li et al., 2015) — concentrated in the seed and pericarp. Longan seeds are also rich in polysaccharides and tannins, and water-soluble aril polysaccharides have shown antioxidant, immunomodulatory and anti-glycation effects in reviews published in 2020–2021. Protein and fat contents are very low.

The fruit is eaten fresh or dried as 'guiyuan' in sweet soups, congee, wine and preserves, and is also fermented into longan wine. In traditional Chinese medicine the aril (guiyuan rou) is used to nourish the heart and spleen, calm the spirit and treat insomnia, palpitations and debility, while the seed (guiyuan he) appears in other formulas. As for research, reviews by Li et al. (2015) and in Food Science and Human Wellness (2020) systematise the antioxidant, antimicrobial, immunomodulatory, anticancer and anti-glycation activities of the polyphenols and polysaccharides, and 'An Exhaustive Plant Profile of Dimocarpus longan' (Processes, 2021) compiles its pharmacological properties. Pham & Herrero (Annals of Applied Biology, 2016) described pollination and aril development, underpinning cultivation practice.

On safety, the ripe aril is a safe, nutritious food; the rind and seed are not eaten. Longan is rich in sugar, so people with diabetes and small children should eat it in moderation, and dried longan is even more sugar-dense. Unlike lychee, longan is not associated in the literature with outbreaks of hypoglycaemic encephalopathy. For conservation, the IUCN (Barstow, 2022) lists Dimocarpus longan as Data Deficient: it was formerly listed as Vulnerable and later Near Threatened (1998) because logging depleted wild populations, but recent field data are inadequate for a new assessment; it is not covered by CITES. The tree is widely cultivated, so genetic resources are maintained mainly through cultivation; local cultivars (such as Hung Yen 'nhan long') and infraspecific taxa (var. echinatus, subsp. malesianus, var. obtusus) should be conserved in gene-bank collections.