Horseradish
Horseradish is a perennial herb with rosettes of elongated, broadly spear-shaped leaves, a long taproot and small, white flowers, although fruits are rarely set. It has been cultivated and used as a medicinal and food plant for at least two millennia in Eurasia, and introduced into many other parts of the world as Europeans migrated across the globe. In many regions where horseradish has become established, it has a reputation for readily becoming invasive if it is not carefully managed.
Horseradish is a synanthropic plant, whose native distribution is thought to be eastern Europe, where it is known only from cultivation or in areas that have been heavily influenced by humans. Moreover, seed production is very poor, whilst the plant’s propagation is exclusively vegetative. This has led to the suggestion that Armoracia rusticana evolved through domestication of one or more of its wild relatives, Armoracia macrocarpa and Armoracia sisymbrioides.
The plant’s generic name is derived from the ancient Roman name for horseradish, ‘armoracia’, whilst the common name horseradish was apparently introduced by the English herbalist John Gerard in 1597. ‘Horse’ is commonly applied to English names, where the plant bears similarities to an edible plant but is larger, coarser or more strongly scented, e.g., horsemint, horse chestnut and horse bean. Gerard only refers to horseradish as a medicine in Britain, but reported that the pickled root was used as a condiment in northern Europe. In Britain, four decades later, horseradish was being commonly eaten as a distinctively flavoured condiment for meat and fish.
The roots of fresh horseradish have little scent but snap them and a pungent odour is released that irritates mucous membranes in the nose and eyes. The irritant is sulphur-containing mustard oil (allyl isothiocyanate), which is part of horseradish’s chemical defence mechanism to prevent damage by herbivores. Mustard oil is harmful to horseradish so must be rendered harmless for storage inside cells. This is done in the form of glucosinolates, such as sinigrin, where mustard oil molecules are combined with sugar molecules. When cells are damaged an enzyme called myrosinase is released, which breaks the sugar from the glucosinolate to release volatile mustard oil. Very high concentrations of sinigrin are found in other members of the cabbage family valued as food, such as white mustard (Sinapis alba) and wasabi (Eutrema japonicum). Indeed, commercial preparations of the Japanese condiment wasabi are often made of horseradish and green food-colouring.
Further reading
Courter JW and Rhodes AM 1969. Historical notes on horseradish. Economic Botany 23:156-164.
Sampliner D and Miller A 2009. Ethnobotany of horseradish (Armoracia rusticana, Brassicaceae) and its wild relatives (Armoracia spp.): reproductive biology and local uses in their native ranges. Economic Botany 63: 303-313.
Yang J et al. 2020. Brassicaceae transcriptomes reveal convergent evolution of super-accumulation of sinigrin. Communications Biology 3: 779.
Stephen Harris