Development of hybrids and hybrid seed production of cole crops

Singh BK and Singh B. 2016. Development of hybrids and hybrid seed production of cole crops. In: Principles and Production Techniques of Hybrid Seeds in Vegetables (Singh B, Pandey S, Singh N, Manimurugan C, Devi J and Singh PM Eds). Training Manual No.  67, ICAR-IIVR, Varanasi, UP, pp 112-125.

Cole crops (Brassica oleracea) are a group of highly differentiated plants having 18 numbers of diploid and somatic chromosomes (2n=2x=18), and these are grown all over the world from arctic to tropical climatic conditions. The word ‘cole’ seems to have been derived from the abbreviation of the word ‘caulis’ meaning stem/cabbage/stalk, and it was variously spelt as Kale (English), Kohl (German), Kool (Dutch), Kal (Scandinavian), Kaali (Finnish), Kaol and Kol (Breton), Chou (French), Col (Spanish), Cal (Irish), Cavolo (Italian), and Couve (Portuguese); but usually, the word cole is more recognized in the literature worldwide. Variation within and between subspecies of B. oleracea, and present day cultivated cole crops (kale, broccoli, cauliflower, cabbage, kohlrabi and Brussels sprout) have evolved after a long time of natural/artificial hybridization, mutation, selection and domestication. All these forms are descended from a common kale like ancestor, the wild cabbage (B. oleracea L. var. sylvestris L.) which is still prevalent in the western and southern Europe, and North Africa (Table 1). Taxonomically, the cole crops belong to the order Brassicales (Cruciales), family Brassicaceae (Cruciferae), tribe Brassiceae, subtribe Brassicinae, genus Brassica, section Brassica and species oleracea (Singh 2015). Pachytene chromosome studies have shown that the B. oleracea is a triple tetrasomic for chromosome types B, C and E comprising the genome formula ABBCCDEEF with 6 basic genomes and showing some secondary pairing.

Table 1: Evolution of cultivated B. oleracea crops (Prakash et al. 2011, Singh 2015)

Probable sequence of evolution	Scientific name (B. oleracea var.)	Common name	Ancestor*
1	var. sylvestris L.	Wild cabbage	-
2	var. ramosa DC.	Thousand-head kale, branching bush kale	1
3	var. gemmifera DC.	Brussels sprout	2
4	var. dalechampii		3
5	var. costata DC.	Portuguese tree kale, tronchuda kale	1
6	var. medullosa Thell.	Marrow-stem kale	1
7	Intermediate between 6 & 8		6
8	var. gongylodes L.	Kohlrabi	7
9	var. sabauda L.	Savoy cabbage	5
10	var. capitata L.	White cabbage	9
11	var. capitata L.	Red cabbage	10
12	var. viridis L., var. sabellica L., var. palmifolia DC.	Kale and collards	1
13	var. italica Plencks	Broccoli, Calabrase	12
14	var. botrytis L.	Cauliflower (biennial)	13
15	var. botrytis L.	Cauliflower (annual)	14
16	var. botrytis L.	Cauliflower (Indian) or Tropical cauliflower	15

*Ancestors are represented by numerical letters (1 to 15) of evolution along with corresponding scientific name.

The economic parts used in the present day cultivated cole crops (cauliflower, cabbage, kohlrabi, kale, broccoli and Brussels sprouts) are stem, leaves, flower or modified forms which are named as curd, head, knob or leaf. The curd of cauliflower described as pre-floral fleshy apical meristem in which the lateral buds of shoot meristem are elongated and much branched, and apices of these branches form the structure of curd of which >90% abort prior to flowering. In cabbage, after the rosette stage, new leaves develop with shorter petioles and the leaves begin to cup inward to form head. It has normally smooth leaves, while savoy cabbage has attractive crinkled leaves. Moreover, kohlrabi, grown for its swollen/enlarged stem (knob) just above the soil line which has a short growing season in cool weather because it should be harvested when young and tender. In sprouting broccoli, the economic parts ‘head’ bears on terminal bud and ‘sprouts’ arise from axillary buds, are actually composed of fully functional flower buds of which relatively few abort prior to flowering. The Brussels sprouts, resemble as small cabbage, are grown for sprouts i.e. swollen axillary buds (2.5-5.0 cm in diameter) which arise along the stem of the plant. These axillary buds bear in a spiral arrangement around stem that may reach 65-125 cm in height. Moreover, kale and collards are non-heading cole vegetables grown mostly for tender leaves, used as greens or salad and decorative purposes too. Cole crops are rich in nutrients, including several carotenoids (beta-carotene, lutein, zeaxanthin); vitamins C, E, and K; folate; and minerals (Singh and Devi 2015). In addition, these cruciferous vegetables contain a group of substances known as glucosinolates which are responsible for the pungent aroma and bitter flavours. Many scientific studies have presented the health benefits of these vegetables. For bolting and flowering, most of the cole crops are generally performed on mature vegetative plants either in-situ or ex-situ at temperatures between 5-10 °C for 30-60 days i.e. vernalization other than tropical varieties/genotypes (Table 2). Two main climatic factors should be taken in to consideration while selecting the area for seed production of cole crops. The most important one is the temperature during winter which is critical for vernalization; and other one is the precipitation/rainfall during the season of flowering, seed maturity and harvesting.

Table 2: Low temperature requirements (5-10 °C for 30-60 days) for development of economic parts and flowering in cole crops

Crop	Development of economic/edible parts	Flowering
Broccoli, winter type	Yes	Yes
Broccoli, summer type	No	Yes
Broccoli, tropical type	No	No
Brussel sprouts	No	Yes
Cabbage	No	Yes
Cabbage, tropical type	No	No
Cauliflower, winter type	Yes	Yes
Cauliflower, summer type	No	Yes
Cauliflower, tropical type	No	No
Chinese broccoli	No	No
Collards	No	Yes
Kale	No	Yes
Kale, tropical type	No	No
Kohlrabi	No	Yes

Inflorescence and floral biology

The inflorescence of cole crops is racemose type, except cauliflower having cymose type, being more dwarf and umbrella shape may attain a length of 1-2 m. Flowers are borne on the main stem and its branches whose slender pedicels are only 1.5-2.0 cm long. The flowers are typically cruciferous, having four sepals, four petals, six stamens (two are short) and two carpels along with superior ovary, septum and two rows of campylotropous ovules (Figure 1). The sepals are green and erect. The petals are arranged perpendicular to each other, forming a cross i.e. cruciferous. Generally, the colour of petals may be light yellow, yellow, dark yellow or some time white. The bright yellow petals become 15-25 mm long and about 10 mm wide. The androecium is tetradynamous i.e. there are two short and four long stamens. The two functional nectaries are present which are situated between the bases of ovary and short stamens and the other two inactive nectaries at the bases of pairs of long stamens. Honey bees are the usual pollinating agents, though bumble bees and other syrphid flies may also be responsible for pollination. 

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