Brassica rapa is a highly diverse crop species comprising numerous morphologically distinct cultivated forms selected for different plant organs and uses. Major crop types include root crops (turnip), oilseed crops (turnip rape, toria and yellow sarson), and leafy or inflorescence vegetables, including Chinese cabbage (napa cabbage), bok choy (pak choi), mizuna, tatsoi, komatsuna, choy sum, rapini and other regional vegetables. The species is also cultivated as forage for livestock.
Oilseed forms of B. rapa include both conventional rapeseed and canola-quality varieties. “Canola” is a quality designation rather than a taxonomic designation and refers to oilseed varieties selected for low erucic acid content in the oil and low glucosinolate content in the meal.
Cultivated forms show considerable variation in life history and reproductive biology. Many crop and weedy forms are annual, whereas turnips, some turnip rape cultivars and some Chinese cabbage types are biennial and may require vernalization for flowering. Most B. rapa forms are predominantly self-incompatible and therefore outcrossing, although exceptions occur, notably the self-compatible Asian oilseed form yellow sarson. Pollination occurs primarily by insects, with lesser contributions from wind and physical contact.
Genomic evidence indicates that the extensive morphological diversity of cultivated B. rapa arose through repeated selection and diversification. Turnips were likely among the earliest domesticated forms, while different leafy vegetable forms in Europe and East Asia appear to have been independently selected from geographically distinct turnip lineages. Oilseed forms also have complex origins, with evidence supporting multiple origins.