Watermelon

Watermelon (Citrullus lanatus) is an annual herbaceous plant that trails along the ground or climbs by tendrils, with stems 1–3 m long covered in soft hairs. The leaves are alternate, deeply pinnately lobed into 3–5 toothed lobes, 5–20 cm long, and hairy on both surfaces. The flowers are unisexual and borne singly in leaf axils on the same plant: male and female flowers are separate, with pale to orange-yellow, veined petals fused at the base; the female flower has a swollen inferior ovary. The fruit is a firm-rinded berry (pepo), spherical to oblong, with a green-striped, dark green or yellow rind, weighing from under 1 kg to more than 30 kg; inside there are no internal divisions, and the juicy flesh may be white, yellow, orange, pink or red depending on the cultivar. The fruit holds 200–800 flat seeds, black when ripe and white when immature. By weight the flesh accounts for about 68%, the rind about 30% and the seeds about 2%. Seedless triploid (3n) cultivars have been produced since 1939.

According to Kew POWO, Citrullus lanatus is native to the E. Sahara, with native countries including Ethiopia, Libya and Sudan–South Sudan; it is an annual of the seasonally dry tropics. The domestication origin of the sweet dessert watermelon is debated: Africa is widely regarded as its home, and the Kordofan melon of Sudan is considered the closest relative of modern cultivated watermelon. Wild watermelon seeds found at Uan Muhuggiag in Libya date to about 3500 BCE; in 2022 a study traced 6,000-year-old seeds from the Libyan desert to the egusi melon (Citrullus mucosospermus) of West Africa. Watermelon was domesticated in Sudan and cultivated in Egypt by about 2000 BCE; dessert watermelons spread across the Mediterranean world in Roman antiquity. POWO records the species as introduced in Vietnam and many other tropical and subtropical countries. In Vietnam watermelon is linked to Tết and is grown mainly in the winter–spring crop (harvested before Tết) and the summer–autumn crop.

Chemically, watermelon is dominated by water (about 91–92% of the flesh) together with bioactive compounds. The carotenoids of red flesh are mainly lycopene (90–95% of total carotenoids, about 4.4–8.0 mg/100 g), which gives the red colour and acts as an antioxidant; yellow flesh contains 30–40% β-carotene, 15–25% α-carotene and 10–20% lutein, while white flesh has almost no carotenoids. Smaller amounts of phytoene, phytofluene, ζ-carotene and β-cryptoxanthin also occur. Watermelon is noted as the richest natural source of L-citrulline — a non-protein amino acid and precursor of L-arginine in the kidney urea cycle, from which nitric oxide is produced to relax blood vessels. The flesh also contains sugars (fructose, glucose and sucrose), fibre/pectin, phenolic compounds, vitamin C, β-carotene (provitamin A), potassium and magnesium. The cucurbitacins (B, C, D, E, I) — triterpenes that cause bitterness and can be toxic at high levels — and unsaturated fatty acids in the seeds have also been isolated. Every part of the fruit (flesh, rind, seeds) contains bioactive compounds, the rind being rich in citrulline.

Watermelon is an important thirst-quenching fruit of the tropics and subtropics: eaten fresh, juiced, blended into smoothies and sorbets; the young rind can be pickled, salted or cooked; the seeds are roasted and pressed for oil; and in dry parts of Africa the fruit is a source of water for people and wildlife. In research, a review in Trends in Food Science & Technology (Montesano et al., 2021) identifies lycopene and citrulline as the two main bioactive compounds of watermelon. A meta-analysis in the British Journal of Nutrition (2021) of 17 randomised controlled trials showed that longer-term L-citrulline supplementation improves flow-mediated dilation (FMD up by 0.9 %-point), while longer-term watermelon consumption lowers pulse wave velocity (PWV down by 0.9 m/s) — a sign of reduced arterial stiffness. In rats fed an atherogenic diet, watermelon powder and L-arginine improved blood lipids and reduced inflammation and oxidative stress by upregulating eNOS (Nutrition Research, Hong et al., 2018). Cucurbitacin E isolated from watermelon has anti-inflammatory activity by inhibiting cyclooxygenase, and cucurbitacins B, C, D, E and I inhibit many cancer cell lines. A 2025 meta-analysis (Frontiers in Nutrition) further supports the benefits of L-citrulline for endothelial function.

As food, ripe watermelon flesh is wholesome and rehydrating; because it contains lycopene and citrulline it has no significant toxicity at normal intakes. Note, however, the bitter cucurbitacins: when watermelon crosses with wild types or is stressed, the fruit can become unusually bitter with high cucurbitacin levels, causing 'cucurbitacin poisoning' (vomiting, diarrhoea, abdominal pain) — so never eat a fruit that tastes strongly bitter. Watermelon is high in sugar, so people with diabetes should watch portion size, and those with kidney disease should mind its potassium content. On conservation, according to Kew POWO the species is regarded as Least Concern in South Africa under IUCN Red List criteria and is now widely cultivated worldwide, so it is not threatened; the species is not listed in the CITES appendices. The main risk is erosion of wild and landrace genetic resources in southern Africa (tsamma and citron types) — populations that are valuable genetic resources but are increasingly shrinking — so the SADC Plant Genetic Resources Centre (SPGRC) and national genebanks have made their conservation a priority.