Brassica is – as traditionally understood – a genus of ca. 40 Eurasian and Mediterranean species. Its generic limits, however, are quite unclear. Brassica is morphologically closely related with e.g. Eruca, Raphanus and Sinapis and these genera are probably better included in Brassica (Mabberley 2008). Molecular data indeed have demonstrated that Brassica is highly polyphyletic and its current circumscription untenable (Warwick & Black 1993, Al-Shehbaz & al. 2006, Arias & Pires 2012). The genus should either include related genera like Diplotaxis, Eruca, Erucastrum (incl. Hirschfeldia), Raphanistrum, Rapistrum and Sinapis or be further segregated. Pending additional studies, however, it is here maintained in its traditional sense.
Only one species, Brassica nigra (L.) Koch (syn.: Sinapis nigra L.), is originally native in Belgium (perhaps rather archaeophyte) (Lambinon & Verloove 2012). It used to be confined to coastal areas for quite some time but has much expanded lately. Lawalrée (1956) even suggested non-native status in Belgium.
Several species of Brassica are economically very important, mostly as crops (B. napus, B. oleracea and B. rapa). Some are also cultivated as ornamentals in recent times (mostly cultivars of Brassica oleracea).
1 Cauline leaves clasping stem at base. Plants always at least slightly glaucous === 2
Cauline leaves not clasping stem. Plants green or glaucous === 4
2 Sepals erect in flower. Basal leaves entirely glabrous, glaucous. Floral buds much overtopping flowers (inflorescence elongated) === Brassica oleracea
Sepals patent to erecto-patent. Basal leaves densely to sparsely hirsute, glaucous or not. Floral buds overtopped by flowers or slightly overtopping flowers (inflorescence not or scarcely elongated) === 3
3 Petals ca. 6-12 mm, bright yellow. Basal leaves green, hirsute hairy. Inflorescence not elongated, flowers overtopping buds === B. rapa
Petals ca. 11-18 mm, usually paler yellow. Basal leaves glaucous, sparsely hairy. Inflorescence slightly elongated, floral buds slightly overtopping flowers === B. napus
4 Siliqua erect, appressed to stem, 15-20 mm long (native) === B. nigra
Siliqua patent to erecto-patent, 30-70 mm long === 5
5 Basal leaves (rosette) present at flowering time === 6
Basal leaves absent at flowering time === 7
6 Beak of siliqua 10-15(-20) mm long, with 0-2 seeds. Petals pale yellow, 5-6 mm long. Rosette leaves lyrate-pinnatisect, with 6-20 lateral leaflets === B. tournefortii
Beak of siliqua 5-12 mm long, with 0-1 seed. Petals bright yellow, 9-12 mm long. Rosette leaves runcinate-pinnatifid, with 14-20 lateral leaflets === B. barrelieri
7 Lower stem leaves with 0-1 lateral lobes. Beak of siliqua 2,5-6 mm long. Sepals 7-10 mm long === B. carinata
Lower stem leaves with 1-3 lateral lobes. Beak of siliqua 4-10 mm long. Sepals 4,5-7 mm long === B. juncea
Additional alien: Brassica elongata Ehrh. subsp. integrifolia (Boiss.) Breistr. (syn.: subsp. armoracioides (Czern.) Aschers. et Graebn.) (Russia, grain and wool alien).
Al-Shehbaz I. A., Beilstein M.A. & Kellogg E.A. (2006) Systematics and phylogeny of the Brassicaceae (Cruciferae): An overview. Pl. Syst. Evol. 259: 89-120. [available online at: http://www.telomereevolution.org/uploads/8/9/7/6/8976955/fulltext.pdf]
Arias T. & Pires J.C. (2012) A fully resolved chloroplast phylogeny of the brassica crops and wild relatives (Brassicaceae: Brassiceae): Novel clades and potential taxonomic implications. Taxon 61(5): 980-988. [available online at: https://www.researchgate.net/publication/259812488_A_fully_resolved_chlo...
Heywood V.H. (rev. Akeroyd J.R.) (1993) Brassica. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 1 (2nd ed.). Cambridge University Press, Cambridge: 405-409.
Lambinon J. & Verloove F. (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Lawalrée A. (1956) Cruciferae. In: Robyns W. (ed.), Flore Générale de Belgique, vol. 2, fasc. 2. Jardin Botanique de l’Etat, Bruxelles: 160-285.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Schulz O.E. (1919) Brassica. In: Engler A. (ed.), Das Pflanzenreich 70(IV.105). Engelmann, Leipzig: 21-84. [available online at: https://www.biodiversitylibrary.org/item/56527#page/31/mode/1up]
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Sun V.G. (1946) The evaluation of some taxonomic characters of cultivated Brassica with a key to species and varieties – II. The key. Bull. Torrey Bot. Club 73(4): 370-377.
Warwick S.I. & Black L.D. (1991) Molecular systematics of Brassica and allied genera (Subtribe Brassicinae, Brassiceae) -chloroplast genome and cytodeme congruence. Theoretical and Applied Genetics 82(1): 81-92.