Cultivated Brassica Species And Forms Brassica oleracea group n the European group

The European (Occidental) Brassica vegetables originate from B. oleracea and probably some closely related Mediterranean species. They can be divided into subordinate groups often at the variety (var.), subvariety or cultivar (cv.) levels. Much of the basis of current understanding of diversity within this group comes from the detailed studies of American horticultural botanist Liberty H. Bailey (1922, 1930, 1940).

Multiple origins and parents

One school of thought suggests that the constituent crops within the B. oleracea group have multiple origins derived from cross-breeding between closely related Brassica species living in geographical proximity to each other. In consequence, the taxonomy of parents and progeny is confused and clouded still further by millennia of horticultural domestication. For example, the progenitors of headed cabbages and kales were postulated by Neutrofal (1927) as B. montana and of kohlrabi as B. rupestris. Later, Schiemann (1932)

Table 1.2. Characteristics of Eruca, Raphanus and Sinapis- genera allied to crop-founding Brassica (after Tsunoda et al., 1984).

Eruca sativa (n = 11, syn. E. vescaria subsp. sativa) commonly know as 'garden rocket', which is a salad vegetable in southern Europe and in India forms an oilseed (taramira) and is used for fodder. E. sativa has recently become a popular salad vegetable in western Europe. It occurs widely distributed across southern Europe, North Africa, western Asia and India. Variation within the cytodeme Eruca is substantial, with ecotypes evolved for several habitats in relation to available water.

Sinapis alba (n = 12) (white mustard) grows wild in Mediterranean areas with abundant moisture and ample soil nutrients, often coexisting with S. arvensis (n = 9) (charlock or wild mustard), although the latter prefers lower soil moisture levels. In dry areas, S. turgida (n = 9) is common. In wet habitats, S. alba reaches 2 m in height with a high mustard oil content; it is grown in Europe as 'white' or 'yellow' mustard. Cultivation is increasing particularly in Canada in combination with 'brown' mustard, B. juncea.

Raphanus (radish). There are about 18 genera in the subtribe Raphininae including Rapistrum, Cakile and Crambe. Possibly all Raphanus spp. with n = 9 form a single cytodeme. The genus is found along Mediterranean coasts, forming the dominant plant on coastal areas of the Sea of Marmara and the Bosphorus Straits. Around field margins, R. raphanistrum and R. rapistrum rugosum tend to coexist. Culinary radish (R. sativum) possibly evolved in southern Asia and provides a wide diversity of root forms and flavours. Fodder radish (R. sativus var. oleiferus) is a source of animal fodder obtained from both roots and foliage. Other varieties radicula, niger and mougri are identified by Banga (1976). Crambe is a source of seed oil and is beginning to be used more extensively in North America.

realized that several of the Mediterranean wild types had formed the origins for locally cultivated land races. Schulz (1936) supported this view, and identified B. cretica as a progenitor of cauliflower and broccoli.

Lizgunova (1959) grouped cultivars into five different species and proposed a multiple origin from wild forms. Helm (1963), in devising a triple origin, combined cauliflower, broccoli and sprouting broccoli in one line, thousand-headed kale and Brussels sprouts in another and all other crop forms in a third. Further analysis was completed by Toxopeus (1974), and Toxopeus et al. (1984) suggested that for simplicity a horticulturally based taxonomy was preferable to attempted botanical versions.

Potential wild species contributing to the B. oleracea group

Populations of wild relatives which cross-fertilize with B. oleracea and form interbreeding groups are found on isolated cliffs and rocky islets, forming distinct units often displaying phenotypic differentiation, leading to several layers of variation superimposed on each other (see Table 1.4).

Crops developed within Brassica oleracea and allies

The vast array of crop types that have developed within B. oleracea (and also B. rapa) is probably unique within economic botany (Nieuwhof, 1969). This

Table 1.3. Characteristics of natural relatives of crop-founding Brassica. Chromosome Geographical

Name

count

distribution

Habitat

Diplotaxis acris Diplotaxis harra

n = n =

7 13

Saharo-Sindian area

Dry, desert regions

Diplotaxis tenuisiliqua

n =

9

North Africa

Moist areas, sometimes associated with Sinapis alba

Erucastrum cardaminoides

n =

9

Endemic to the Canary Isles

Erucastrum laevigatum

n =

28

Southern Italy, Sicily and North Africa

Probably an autotetraploid of E. virginatum (n = 7)

Erucastrum leucanthum

n =

8

Endemic to North Africa, particularly the seaward side of the Moyen Atlas

Coexists with Hirschfeldia incana

Hirschfeldia incana

n =

7

Dominant component of Mediterranean flora

Large roadside colonies, especially favoured by fine textured soils

Hutera spp.

n =

12

Iberian peninsula

Rocky outcrops and colluvial sites; Hutera and Rhynchosinapis form a cytodeme complex found in the Spanish Sierra Montena regions

Sinapidendron spp.

n =

10

Madeira

Rocky cliffs; perennial habit

After Tsunoda et al. (1984).

After Tsunoda et al. (1984).

has led to acceptance at the subspecies and variety (cultivar)3 levels of descriptions based around the specialized morphology of the edible parts and habits of growth within the crop types (Wellington and Ouartley, 19 72).

Brassica oleracea (the cole or cabbage brassicas)

Brussels sprouts: b. oleracea l. var. gemmifera dc Brussels sprouts may have emerged in the Low Countries in the medieval period and risen to prominence in the 18th century around the city of Brussels. Subsequently, they became established as an important vegetable crop in northeastern Europe, especially the northern Netherlands and parts of the UK. Local open-pollinated land races were developed suited to specific forms of husbandry but usually subdivided into early, midseason and late maturity groups. Often they would be capable of resisting pests and pathogens common within their locality and have morphologies adapted to prevailing climatic conditions. In the 1930s, early maturing types were developed in Japan where many of the original F1 hybrids (e.g. 'Jade Cross') were produced; these and their derivatives formed the basis for cultivars ideally suited to the emerging 'quick-freeze' vegetable processing industry. The entire worldwide crop of Brussels

Table 1.4. Potential progenitors which interacted to found crop-founding Brassica oleracea in Europe.

Brassica cretica: populations occur around the Aegean, including Crete, southern Greece and in southwestern Turkey. The plant has a woody much-branched habit carrying glabrous, fleshy leaves persisting in perennial form for 5-8 years but flowering in the first year under favourable environments. The inflorescence axis elongates between buds prior to opening with light yellow to white flowers. A subspecies B. cretica nivea grows on the mountainous cliffs and gorges of the Peleponnese and Crete, while the subspecies B. cretica cretica populates mountainous cliffs of the same area. Differentiation of B. cretica nivea rests on possession of small white to creamy flowers, strongly reticulated seeds, a tall stem and long-petiolated leaves. Hybrids between both subspecies are found where populations currently overlap or have done so in the recent past.

The Brassica rupestris-incana complex: this group has developed a number of distinct regional variants. The complex is characterized by forming a strong central stem, a large apical inflorescence succeeded by further branching. The top of each partial inflorescence forms a tight grouping of buds which may open before the axis between them elongates. Foliage consists of large petiolate leaves that are poorly structured since they shrivel when past maturity, in contrast to B. cretica which retains mature leaves with a recognizable morphology. Hairs are characteristically found on both seedlings and adults. Distribution includes Sicily, southern and central Italy and parts of the west of the former Yugoslavia. Species which have been separated but which are included within the complex include: B. incana Ten., B. villosa Biv., B. rupestris Rafin., B. tinei Lojac., B. drepanensis (Car.) Dam., B. botteri Vis., B. mollis Vis. and B. cazzae Ginzb. & Teyb.

Brassica macrocarpa Guss: a species restricted to Isole Egadi in the west of Sicily with a habit similar to B. rupestris-incana with which it forms fertile hybrids, and characterized by possessing smooth-surfaced, thick fruits containing seeds produced in two rows within the loculus. There are questions as to whether this species should be placed in the genus Brassica in view of the seed capsule morphology. Conceivably this species is a subordinate of the B. rupestris-incana complex.

Brassica insularis Moris: found in Corsica, Sardinia and Tunisia. The plants are similar in low branching habit to B. cretica but the stiff glabrous leaves with pointed lobes and large white fragrant flowers and seed glucosinolates differ from other Brassica spp. Species rank has been given to the Tunisian form as B. atlantica (Coss.) Schulz., but the morphology and intercrossing indicate clear inclusion in B. insularis without subdivision.

Brassica montana Pourr. (syn. B. robertiana Gay): found growing in northeastern coastal areas of Spain, southern France and northern Italy, these plants are shrubby perennials with lobed green, glabrous or possibly hairy leaves. An intermediate status between the B. rupestris-incana complex and B. oleracea is suggested by biogeographical and morphological evidence.

Brassica oleracea L. s.str.: found on coasts of northern Spain, western France and southern and southwestern Britain as a stout perennial forming strong vegetative stocks which then flower and branch. The greyish coloration and glabrous nature of leaf surfaces distinguish B. oleracea.

Brassica hilarionis Holmb.: located solely in the Kyrenia Mountains of Cyprus, B. hilarionis has fruit reminiscent of the size and texture of B. macrocarpa and vegetatively a habit and leaf morphology close to B. cretica.

Note: Species such as B. balearica Pers. and B. scopulorum Coss & Dur. have been excluded since their failure to cross with the B. oleracea group make it dubious as to whether they should be included with it. After Tsunoda et al. (1984).

sprouts is now dominated by a very limited F1 germplasm derived by American, Dutch and Japanese breeders originating from the initial crosses. Botanically, the plants are biennial with simple erect stems up to 1 m tall. Axillary buds develop into compact miniature cabbage heads or 'sprouts' that are up to 30 mm in diameter. At the top of the stem is a rosette of leaves; the leaves are generally petiolate and subcircular and the leaf blade is rather small (see Chapter 2 for floral biology).

CAULIFLOWER AND BROCCOLI: b. oleracea L. VAR. botrytis L. (CAULIFLOWER), b. oleracea L. VAR. italica PLENCK (broccoli) A remarkable diversity of cauliflower-and broccoli-like vegetables developed in Europe, probably emanating from original highly localized crops in Italy and possibly evolved from germplasm introduced in Roman times from the eastern Mediterranean. A classification of the colloquial names used to describe these crops was proposed by Gray (1982) and is shown in Table 1.5.

Over the past 400 years, white-headed cauliflowers (derived from the Latin caulis (stem) and floris (flower)) have spread from Italy to central and northern Europe, which became important secondary centres of diversity for the annual and biennial cauliflowers now cultivated worldwide in temperate climates. Cauliflowers adapted to hot humid tropical conditions have evolved in India during the past 200 years from biennial cauliflower mainly of British origins following traders and colonizers.

Crisp (1982) proposed a taxonomic basis for grouping the various types of cauliflower found in cultivation; he admitted that this has limitations but at least it gives order where little previously existed (Table 1.6). Molecular biological techniques offer means to understand the relationships between

Table 1.5. Brassica oleracea varieties botrytis and italica with associated colloquial crop names.

Brassica oleracea L. var. italica Plenck

Purple-sprouting broccoli

Cape broccoli

Purple cauliflower

Calabrese and other green-sprouting forms

(broccoli in North America)

White-sprouting broccoli

Brassica oleracea L. var. botrytis DC

Cauliflower

Heading broccoli

Perennial broccoli

Bouquet broccoli

White-sprouting broccolia

aWhite-sprouting broccolis are thought to have evolved independently in northern Europe. Their close affinity to winter-hardy cauliflower suggests that the late form may be more correctly regarded as a form of B. oleracea var. botrytis. After Gray (1982).

vegetable brassicas. Microsatellites, also known as simple sequence repeats (SSRs), are short tandem repeats of nucleotides of 1-6 bp in length that can be repeated up to 100 times. These offer means for discriminating between varieties and cultivars, and measuring levels of genetic diversity within species such as B. oleracea. By screening large numbers of DNA sequences from B. oleracea, Tongug and Griffiths (2004a) demonstrated that diversity was least in cauliflower, intermediate in broccoli (calabrese) and greatest in cabbage. This illustrates the highly intensive nature of current cauliflower and broccoli commercial breeding programmes frequently centred on the use of a limited genepool, while until very recently at least cabbage breeding has made use of wider crosses. Several other molecular techniques such as the use of RFLPs, randomly amplified polymorphic DNA sequences (RAPDs) and amplified fragment length polymorphisms (AFLPs) are also useful tools for such studies. The relationships between wild Sicilian populations of B. oleracea are discussed by Geraci et al. (2001), while Astarini et al. (2004) have begun to unravel the descent of cauliflower types used in Lembang in Western Java from material introduced from India in the 19 th century and related them to current Australian cultivars.

Table 1.6. Biogeography and characters of cauliflower (B. oleracea var. botrytis) groups.

Group name

Characteristics

Common types

Italian

Very diverse, includes annuals and

Jezi

biennials and types with peculiar

Naples (= Autumn Giant)

curd conformations and colours

Romanesco

Flora Blanca

North-west

Derived within the last 300 years from

Old English

European

Italian material

Walcheren

biennials

Roscoff

Angers

St Malo

Northern

Developed in northern Europe for at

Le Cerf

European

least 400 years. Origin unknown,

Alpha

annuals

perhaps Italian or possibly eastern

Mechelse

Mediterranean

Erfurt

Danish

Asian

Recombinants of European annuals

Four maturity groups are

and biennials, developed within the

recognized by Swarup and

last 250 years. Adapted to tropical

Chatterjee (1972)

climates

Australian

Recombinants of European annuals

Not yet categorized

and biennials, and perhaps Italian

stocks; developed during the last 200

years

From Crisp (1982).

Cauliflower is a biennial or annual herb, 50-80 cm tall at the mature vegetative stage and 90-150 cm when flowering. The root system is strongly ramified, concentrating in the top 30 cm of soil, with thick laterals penetrating to deeper layers. The stem is unbranched, 20-30 cm long and thickened upwards. The leaves are in a rosette (frame) of 15-25 large oblong more or less erect leaves surrounding the compact terminal flower head (curd). Usually lateral buds do not develop in the leaf axils. The glabrous leaves are almost sessile and coated with a layer of wax; the leaf blade is grey to blue-green in colour with whitish main and lateral veins. Leaves vary in shape from short and wide (40-50 cm X 30-40 cm) with curly edges to long and narrow (70-80 X 20-30 cm) with smooth edges. The curd consists of a dome of proliferated floral meristems that are white to cream or yellow colour growing on numerous short and fleshy peduncles. The curd varies from a rather loose to a very solid structure, with a flattened to a deeply globular shape from 100 to 400 mm in diameter. Young leaves may envelop the curd until a very advanced stage of development is reached. Bolting cauliflower plants often have several flower stalks (see Chapter 2 for floral biology).

Broccoli is an Italian word from the Latin brachium, meaning an arm or branch. In Italy, the term is used for the edible floral shoots on brassica plants, including cabbages and turnips, and was originally applied to sprouting forms, but now includes heading types which develop a large, single, terminal inflorescence. Broccoli with multiple green, purple or white flower heads (sprouting broccoli) became popular in northern Europe in the 18th century. Broccoli with a single main green head (calabrese; the name has been taken from the Calabria region of Italy) was introduced into the USA by Italian immigrants during the early 20th century (Fig. 1.4). It has become a popular 'convenience' vegetable, and has spread back into Europe from the USA and into Japan and other parts of the Pacific Rim over the past 50 years.

The white-heading forms are also colloquially referred to as cauliflower. Broccoli is often used to describe certain forms of cauliflower, notably in the UK where the term heading or winter broccoli is traditionally reserved for biennial types. The term broccoli without qualification is also generally applied in North America to the annual green-sprouting form known in UK and Italy as calabrese. The term sprouting as used in sprouting broccoli refers to the branching habit of this type, the young edible inflorescences often being referred to as sprouts. The term Cape used in conjunction with broccoli or as a noun is traditionally reserved for certain colour heading forms of B. oleracea var. italica. A classification of broccoli is given in Tables 1.7 and 1.8.

Broccoli (the single-headed or calabrese type) differs from cauliflower in the following respects: the leaves are more divided and petiolate; and the main head consists of clusters of fully differentiated green or purple flower buds which are less densely arranged with longer peduncles. Axillary shoots with smaller flower heads usually develop after removal of the dormant terminal shoot. The flower head is fully exposed from an early stage of development.

Brassica Oleracea Sabauda Cropp
Fig. 1.4. Biogeography of the evolution of broccoli and cauliflower in Europe and north Africa (after Gray, 1982).

Table 1.7. Classification of colour-heading and sprouting broccoli (B. oleracea var. italica).

Coloured-heading types Dark purple heading Copper coloured or purplish-brown Green heading

Sulphur-coloured or yellowish-green heading Sprouting types Green sprouting Purple sprouting

After Giles (1941).

Broccoli plants also carry inflorescences from the lateral branches. Sprouting forms of broccoli bear many, more or less uniform, relatively small flower heads instead of the single large head of the calabrese type.

Chinese kale: b. oleracea L. var. alboglabra (l.h. bailey), musil Chinese kale has formed a cultivated stock since ancient times without apparently obvious wild progenitors, but there are possible similarities to B. cretica subsp. nivea. Following early cultivation in the eastern Mediterranean trade centres, it

Early, intermediate and late maturing cultivars

Table 1.8. Classification of Italian sprouting broccoli (B. oleracea var. italica) by morphological types.

Type Description

Green sprouting of Naples Small shoots produced on long stems, considered to be a counterpart to the white-sprouting broccoli

Early summer broccoli of Naples Shows aggregation of smaller shoots with fewer larger shoots. Stems shorter than the Naples green-sprouting type

Calabrese Further reduction in stem length which gives the plant a heading appearance

After Giles (1944).

could have been taken to China. The lines cultivated in Europe may have lost their identity through uncontrolled hybridization. Recently, much horticultural attention has focused on Chinese kale (B. oleracea var. alboglabra). Chinese kale is now a cultigen native to southern and central China. It is popular and widely cultivated throughout China and Southeast Asia and is used as leaves in salads and other dishes. It is a new vegetable for Japan, Europe and North America. The flower bud, flower stalk and young leaves are consumed. A classification into five groups, which vary in flower colour from white through yellow and in depth of green coloration in the leaves and their shape, was produced by Okuda and Fujime (1996) using cultivars from Japan, Taiwan, China and Thailand as examples.

It is an annual herb, up to 400 mm tall during the vegetative stage and reaching up to 1-2 m at the end of flowering. All the vegetative organs are glabrous and glaucous. The narrow single stem forks at the top. Leaves are alternate, thick, firm and petiolate, and leaf blades are ovate to orbicular-ovate in shape; the margins are irregularly dentate and often undulate and characteristically auriculate at the base or on the petiole. The basal leaves are smaller and sessile without auricles. The inflorescence is a terminal or axillary raceme 300-400 mm long, with pedicels 10-20 mm long. The taproot is strongly branched (see Chapter 2 for floral biology).

other kales: b. oleracea L. var. acephala DC Many groups are distinguished: borecole or curly kale, collard, marrow stem kale, palm tree kale, Portuguese kale and thousand-headed kale. Kales are ancient cole crops, closely related to the wild forms of B. oleracea, and many distinctive types were developed in Europe. There are residual populations of the original progenitors such as the wild kale of Crimea, variously ascribed to B. cretica and B. sylvestris but now identified as a hairy form of B. rupestris-incana. It is suggested that as a consequence of trade around the Mediterranean, this form was transferred to the Crimea and is evidence of early widespread cultivation of B. rupestris-incana types. A similar relic population exists in the wild kale of Lebanon, inhabiting the cliffs near Beirut, which is morphologically similar to B. cretica subsp. nivea. Both are possible evidence for widespread trade by the earliest Mediterranean civilizations which moved the botanical types around, and this allowed interbreeding, resulting in the widening diversity of horticultural crops which were artificially segregated from the botanical populations.

Brussels sprouts, the kales and kohlrabi are part of a similar group of polymorphous, annual or biennial erect herbs growing up to 1.5 m tall, glabrous and often much branched in the upper parts. Kales, in particular, are extremely variable morphologically, most closely resembling their wild cabbage progenitors. The stem is coarse, neither branched nor markedly thickened, and 300-1000 mm tall. At the apex is a rosette of generally oblong, sometimes red coloured leaves; sometimes the leaves are curled. This is caused by disproportionately rapid growth of leaf tissues along the margins. In borecole or curly kale, the leaves are crinkled and more or less finely divided, often green or brownish-purple, and they are used as vegetables. Collards have smooth leaves, usually green; they are most important as forage in western Europe. Marrow stem kale has a succulent stem up to 2 m tall and is used as animal forage. Palm tree kale is up to 2 m tall with a rosette of leaves at the apex; it is mainly used as an ornamental. Portuguese kale has leaves with succulent midribs that are used widely as a vegetable. Thousand-headed kale carries a whorl of young shoots at some distance above the soil; together they are more or less globular in outline. It is mainly used as forage.

kohlrabi: b. oleracea l. var. gongylodes l. Kohlrabi first appeared in the Middle Ages in central and southern Europe. The crop has become well established in parts of Asia over the last two centuries and is economically important in China and Vietnam. Kohlrabi are biennials in which secondary thickening of the short stem produces the spherical edible portion, 50-100 mm in diameter and coloured green or purple. The leaves are glaucous with slender petioles arranged in compressed spirals on a swollen stem.

WHITE-HEADED CABBAGE, RED-HEADED CABBAGE AND SAVOY-HEADED CABBAGE White-headed cabbage, B. oleracea L. var. capitata L. f. alba DC; red-headed cabbage, B. oleracea L. var. capitata L. f. rubra (L) Thell; and savoy-headed cabbage, B. oleracea L. var. sabauda L. were defined by Nieuwhof (1969). Heading cabbages are the popular definitive image of vegetable brassicas in Europe; indeed the terms 'cabbage garden' and 'vegetable garden' were synonymous in some literature.

Early civilizations used several forms of 'cabbage' and these were probably refined in domestication in the early Middle Ages in northwestern Europe as important parts of the human diet and medicine and as animal fodder. It is suggested that their progenitors were the wild cabbage (B. oleracea) that is found on the coastal margins of western Europe, especially England and France, and leafy unbranched and thick-stemmed kales that had been disseminated by the Romans. Pliny described methods for the preservation of cabbage, and sauerkraut was of major importance as a source of vitamins in winter and on long sea journeys. In most cabbages, it is chiefly the leaves that are used. Selection pressure in cultivation has encouraged the development of closely overlapping leaves forming tight compact heads, the heart or centre of which is a central undeveloped shoot surrounded by young leaves. Head shape varies from spherical, to flattened to conical. The leaves are either smooth, curled or savoyed (Milan type). Seed propagation of cabbage is relatively straightforward and, in consequence, large numbers of localized regional varieties, or land races, were selected with traits that suited them to particular climatic and husbandry niches such as: Aubervilliers, Brunswick, de Bonneuil, Saint Denis, Strasbourg, Ulm and York. From the 16th century onward, European colonists introduced cabbages worldwide. Scandinavian and German migrants introduced cabbage to North America, especially the mid-Western states such as Wisconsin. In the tropics, cultivation is usually restricted to highland areas and to cooler seasons. White-heading cabbage is especially important in Asia and India. The majority of cultivars are now F2 hybrids coming from a circumscribed group of breeders using similar parental genotypes. Forms derived originally from the Dutch White Langedijk dominated the market for storage cabbage, and more recently fresh white cabbage in supermarkets. Refinement of savoy types through breeding of F1 hybrids has expanded the range of this type now offered to consumers and increased its popularity.

Cabbages are biennial herbs 400-600 mm tall at the mature vegetative stage and 1.5-2.0 m tall when flowering in the second year. Mature plants have a ramified system of thin roots, 90% in the upper 200-300 mm of the soil, but some laterals penetrate down to 1.5-2 m deep. Stems are unbranched, 200-300 mm long, gradually thickening upward. The basal leaves form in a rosette of 7-15 sessile outer leaves each 250-350 X 200-300 mm in size. The upper leaves form in a compact flattened globose to ellipsoidal head, 100-300 mm in diameter, composed of a large number of overlapping fleshy leaves around the single growing point. These leaves are grey to blue-green, glabrous, coated with a layer of wax on the outside of the rosette, and light green to creamy white inside the head, especially with white-headed cabbage. The leaves are red-purple in red-headed cabbage and green to yellow-green and puckered in savoy-headed cabbage. The inflorescence is a 500-1000 mm bractless long raceme on the main stem and on axillary branches of bolted plants. Germination is epigeal and the seedlings have a thin taproot and cordate cotyledons; the first true leaves are ovate with a lobed petiole (see Chapter 2 for floral biology).

Hybridization between taxa

Crossings occur even between distant taxa of the Brassicaceae giving at least semi-fertile hybrids, and this may be analogous to the means by which genetic mixing between wild forms led to the horticultural types grown commercially today. Meiotic pairing is normal and indicates close identity particularly throughout the 2n = 18 forms. Although pollen fertility and seed set are variable, there is usually enough to provide for the survival of further generations.

These characteristics indicate that where races, varieties or species are cultivated in close proximity, crossings will occur. Self-sterility is found in many of the taxa. It is far from absolute, but sufficiently robust to ensure high proportions of out-breeding. Out-breeding normally results from a high frequency of similar S genes between individuals belonging to the same population (see Chapter 2). It is concluded that present-day cultivars include much introgressed genetic material derived from other cultivated or wild forms. Consequently, it is important to understand and use the historical literature that describes crops derived from B. oleracea alongside that derived from genetic and taxonomic sources in order to interpret the status of modern forms and hybrids.

Comparative taxonomy using ancient and medieval literature and science

Some syntheses of the literature have been attempted specifically for the Brassicaceae, notably that of Toxopeus (19 74). Greek writers, especially Theophrastos (370-285 bc), discussed cole crops. It is evident that 'branching' types were known at that time, and these may have resembled bushy kales that were also found in uncultivated ground. Possibly this indicates the domestication of B. cretica. Comments are found describing bushy kales with curled leaves; thus both forms may have been present and undergoing hybrization before spreading to other parts of Europe. The Romans (Cato, 234-149 bc; Plinius, ad 23-79) knew of both stem kales and heading cabbage which were cultivated together. Since seed would be produced locally, hybrids could form and the best selected for further improvement, thereby developing local cultivars. Highly prized types might then have spread further as items of trade. Zeven (1996) suggested that the 'perpetual kale' (B. oleracea var. ramosa) was the Tritian kale of the Romans (referred to by Pliny in ad 70) which they took throughout their Empire. Some relic populations are still grown in various parts of Europe (Belgium, England, France, Ireland, Portugal, Scotland and The Netherlands) and in Brazil, Ethiopia and Haiti. The crop is known as 'Hungry Gap' in England and 'Cut and Come Again' in Scotland. Plants reach up to 3 m in height, some forms appearing to have lost the capacity to flower, as in those found in the Dutch province of Limburg, resulting from long selection pressure for leafiness and multiple branching habit. The patchwork of dissemination in Europe suggests previously widespread distribution by traders.

The cole and neep crops were grown throughout Europe (Sangers, 1952, 1953). Analysis of archives indicates that cole crops were well recognized in the early 14th century to the extent that the name Coolman or Coelman (cabbage-man) were common surnames, while the term coeltwn (modern Dutch = kooltuin) indicated a cabbage garden (Metzger, 1833). Trade was established between the Low Countries and England for the export of cabbages by the 1390s. Dodenaeus had by 1554 (Zeven and Brandenburg, 1986) classified cole crops as white cabbage, savoy cabbage, red cabbage and curly kale, and had recognized the turnip, which in 1608 he had differentiated into flat-rooted and long-rooted forms.

Useful evidence of the forms of brassicas in cultivation comes from studies of the Dutch and Flemish painters of the 15th and 16th centuries where red and white cabbages and cauliflowers figure prominently. Only turnips (B. rapa) are seen in these paintings, with an apparent absence of swede (B. napus) at this time (Toxopeus, 1974, 1979, 1993). Some evidence is available for the presence of radishes in these paintings, but unfortunately there is also conflict with similarities to turnips; however, it is probable that the French 'icicle' radish can be distinguished. It is likely that all subgroups of Brassica (kohlrabi, cauliflower and sprouting broccoli) were developed by medieval times and spread westwards and northwards. The appearance, origins and colloquial naming of rutabaga in Scandinavia, especially Finland and Sweden in the Middle Ages, is described in detail by Ahokas (2004). Further south and south-east, other Brassica groups were developed for cultivation, but generally with the exception of B. alboglabra.

Diversification of Brassica crops is well demonstrated in Portugal where original cole crops were introduced by Celtic tribes several centuries BC and in advance of the Roman conquests. These developed into the Galega kale (B. oleracea var. acephala or B. oleracea var. viridis), Tronchuda cabbage (B. oleracea var. costata (var. tronchuda)) and Algarve cabbage (B. oleracea var. capitata). The Tronchuda types are vigorous growing collard-type plants with a small loose head and large thick leaves, while the Galega types are leafy plants, headless, with large leaves having long petioles and a single indeterminate stem which can attain 2 or 3 m before bolting (Monteiro and Williams, 1989). These crops together with vegetable rape (B. napus var. napus), Nabo (turnip) (B. rapa var. rapa), Nabica (turnip-greens) (B. rapa var. rapa) and Grelos (turnip-tops) (B. rapa var. rapa) form an essential part of the rural diet in Portugal. There are numerous land races of these crops distributed throughout Portugal which have very low within-population uniformity due to the allogamic pollination mechanism associated with the poor isolation used by farmers for seed production. The high levels of variability in shape, size, colour, taste, earliness, and pest and pathogen resistance in these populations constitute an immense reservoir of diversity for breeding purposes.

Comparative morphology gives further information on the origins of brassicas; an important character is the greyish surface texture of the west

European B. oleracea found principally in headed cabbage and Brussels sprouts. The strong dominating central structure of stem kales is found in the B. rupestris-incana from which primary origin could be inferred. B. cretica is probably the origin of the bushy kales since they share common branching, shrubby habit and fleshy leaves. The presence of white flowers may have been derived from B. alboglabra and B. cretica subsp. nivea or combinations between them.

Local stocks (land races)

Various localities still utilize old cultivars and land races, although there is great economic pressure for these to be supplanted by high yielding standardized often hybrid cultivars. Information concerning the older open-pollinated types is fragmentary but of great value in understanding the history of Brassica in cultivation. Around the Aegean, primitive kales closely similar to B. cretica are still cultivated, some with branching inflorescences similar to sprouting broccoli. Even wild types may be utilized in some island villages as salad vegetables. In the former Yugoslavia, wild-type kales grow on field margins, waste areas and building sites, and are still used as animal fodder. Two forms are apparent: a tall single stem type similar to marrow stem kale and a more branching type with high anthocyanin content often with a habit similar to cabbage. Neither of these produces heads, but they are possibly early kale types closely similar to the wild progenitors. In the harsh North Sea habitat of the Shetland Islands, kale has provided winter food for man and fodder for sheep for centuries. Virtually every croft maintains an individual selection which is propagated by seed and grown within a walled 'kale yard'. The plants have high anthocyanin content giving a reddish appearance to the foliage, and grow to 1-1.5 m high.

In general, it is inferred that the west European headed cabbage types derived from B. oleracea on the grounds of morphology with cross-fertilization with Roman kales. The savoy type is possibly a result of further introgression between other coles. The branching bushy kales possibly originate from B. cretica with perhaps 2000 years of hybridization with other forms. Stem kales may well originate from the B. rupestris-incana complex in the Adriatic or more southerly parts of Italy. Hybridization to cabbages and perhaps B. cretica will have added to variation and type differentiation. Origins for the inflorescence of kales, cauliflower and broccoli are still unresolved. Their rapid growth and morphology may have suggested that B. cretica is involved, but the leaf characteristics would also indicate that B. oleracea is a progenitor. The rapid flowering B. alboglabra possibly segregated from B. cretica subsp. nivea. Via cultivation and trade, it then spread from ancient Greece into the eastern Mediterranean and further eastwards.

The relationships between regional groups of Italian land race cauliflower (B. oleracea var. botrytis) and broccoli (B. oleracea var. italica) have recently been unravelled (Massie et al., 1996). Pools of regional genetic diversity exist within cauliflower and broccoli grown throughout Italy following centuries of selection for local conditions and preferences. Different provinces of Italy have been associated with specific variant types, e.g. Romanesco cauliflower in the Lazio Region; Di Jesi, Macerata and Tardivo di Fano varieties in the Marche Region; and Cavolfiore Violetto di Sicilia (Sicilian purple cauliflower) in Sicily, except for the Palermo region where a green cauliflower is typical. Intermediate forms are also reported as between the Macerata and Sicilian Purples (Gray and Crisp, 1985).

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Responses

  • pamphila
    How were cole crops cultivated?
    8 years ago
  • fredrik
    What produces 2030 miniature cabbages botanically axillary that are edible?
    7 years ago

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