Plant 378

Zanthoxylum species (Rutaceae)

Sichuan pepper

Zanthoxylum, which comprises over 200 species of evergreen or deciduous trees, shrubs or climbers, is the only genus in the citrus family with a naturally pantropical distribution. The distribution also extends into subtropical and warm temperate regions, whilst fossils show that 48-54 million years ago Zanthoxylum was also found in Europe.

The leaves of Zanthoxylum species, which have leaflets arranged pinnately or in triplets, are prickly with strong, aromatic odours when crushed. Like all members of the citrus family, Zanthoxylum leaves are studded with translucent, oil-filled glands. Prickles on the young stems or sharp-pointed, corky knobs on the trunks and main branches are also distinctive in the genus. The rounded Zanthoxylum fruits (technically called follicles), which are often covered in oil-filled warts, burst open to liberate blue to black, shiny seeds that remain attached to the open fruit by a short, slender thread. Birds are thought to disperse the seeds.

Zanthoxylum fruits have a pungent, pepper-like flavour, with citrus overtones, that creates a numbing sensation in the mouth when eaten. The genus is most familiar as the source of the spice Sichuan pepper, which is produced from numerous species. In China, the spice is produced from the either Zanthoxylum bungeanum or Zanthoxylum armatum. In Japan and Korea, Zanthoxylum piperitum is used, whilst in Indonesia, Zanthoxylum acanthopodium is the species of choice. Differences between the profiles of the fruits’ aromatic compounds mean they have subtly different flavours. However, the numbing effect it produced by a compound called hydroxy-alpha sanshool, which stimulates neurons to produce similar effects to the local anaesthetics used in surgery. The numbing effects of hydroxy-alpha sanshool also produces relief for toothache when the bark and fruit of the eastern North American toothache tree, Zanthoxylum americanum, is chewed. A sip of water after eating Zanthoxylum creates a short-lived effect like a mild electric shock on the tongue.

In British gardens, several hardy Zanthoxylum species are cultivated as curiosities. Zanthoxylum americanum, introduced in the mid-eighteenth century and once commonly cultivated, is now rarely found. Today’s more commonly cultivated species, such as Zanthoxylum armatum and Zanthoxylum bungeanum, were introduced from eastern Asia in the nineteenth century.

Carolus Linnaeus’ generic name, derived from two Greek words meaning yellow (xanthos) and wood (xylon), refers to the colour of the heartwood in some species. Technically, the name should be Xanthoxylum but the rules of plant naming mean Linnaeus’ spelling mistake cannot now be corrected.

Further reading

Appelhans S et al. 2018. Phylogeny and biogeography of the pantropical genus Zanthoxylum and its closest relatives in the proto-Rutaceae group (Rutaceae). Molecular Phylogenetics and Evolution 126: 31-44.

Austin DF and Felger RS 2008. Sichuan peppers and the etymology of Fagara (Rutaceae). Economic Botany 62: 567-573.

Bautista DM et al. 2008. Pungent agents from Szechuan peppers excite sensory neurons by inhibiting two-pore potassium channels. Nature Neuroscience 11: 772-779.

Stephen Harris