Brussels Sprout - an overview (2024)

Publisher Summary

Brussels sprouts is a cultivated Brassica in which the marketable parts of the plant are the swollen axillary buds known as “sprouts” or “buttons.” It is generally classified as Brassica oleracea var. gemmifera DC. The origin of this plant remains obscure and has been ascribed to both savoy cabbage and various forms of kale. Brussels sprouts are best adapted to temperate regions of the world and require a long growing season with cool summers and mild winters to achieve high productivity. Brussels sprouts are diploid (2n = 18) and have been little studied cytologically. The chromosomes are very small and no karyotype analyses have been published. The inheritance of most characters of Brussels sprouts has not been studied in detail apart from self-incompatibility and the biometrics of some quantitative characters. This chapter discusses the objectives of breeding of Brussels sprouts. A wide range of pests and diseases afflict Brassica crops, but the relative importance varies from crop to crop. For Brussels sprouts, resistance to foliar fungal diseases is the highest priority. Brussels sprouts are mainly cross-pollinating and have a sporophytic self-incompatibility system. The chapter reviews various methods for breeding Brussels sprouts.

Brussels Sprouts

Brussels sprouts are a form of cultivated Brassica oleracea in which the lateral buds develop as small round vegetative buds. They are important seasonal vegetables in temperate regions. The inflorescence bears a large number of flowers which ripen over a long period of time. The varieties of Brussels sprouts readily cross-fertilize with each other and with other types within the species (cabbage, cauliflower, calabrese, kale, and fodder kale) and wild weedy brassicas. Thus, wide isolation distances are recommended (sometimes over 1km) and good weed management. Modern varieties are mostly F₁ hybrids and so the seed is not true to type. Seed production exploits the strong self-sterility in the species. Commercial crops are mostly established by transplanting seedlings. This is usually done mechanically so high-quality seed is required to produce trays of modular transplants in which there are no blanks (i.e., 100% germination) and all seedlings are the same size.

For seed crops, seeds of the two parent varietal lines are sown in separate nursery beds in spring. Suitable parent plants are selected and transplanted to polytunnels in autumn. In spring the plants flower and blowflies are introduced to ensure pollination. The strong self-incompatibility and wide dispersal of pollen by the flies ensures the seeds are the products of cross-fertilization between the two varieties. This can be tested by growing a sample of seedlings and assessing their uniformity. This method will not work for the parent inbred lines which have to be maintained by hand-pollination at the flower bud stage which overcomes the self-incompatibility.

The seed pods are brittle and may shatter and shed seed before harvest. The decision on timing of harvest is therefore important in determining the quantity and also the quality of seeds. The crop is usually cut and windrowed for up to 2 weeks. The windrows are not turned in order to avoid pod shattering and seed shedding. The seed is then usually dried to reduce seed moisture content to below 10% (if stored in sacks) or 8% (for bulk storage).

Soils and climate

Brussels sprouts can be grown on a wide range of soil types and grows well on moisture retentive soils. Lighter, sandier soil types may require irrigation. Heavy soils should be avoided with later crops where machinery access will be needed in wet conditions. A soil pH level of 7.0 to 7.5 is required, particularly where club root may be a problem. Lime should be applied well before planting/drilling.

Cultivations

A firm soil with a good tilth is required. Compacted soils may require sub-soiling before ploughing and harrowing/discing.

Drilling/planting and timings

Processed (freezing) crops may be direct drilled from March to April at close spacings. Transplanting of modules starts in April, until the first week in June at a target spacing of 60×38cm. Later planting will lead to reduced yield even with late maturing varieties. Early varieties mature in 5–6 months, whereas late varieties mature in 10–11 months.

Rotation

The crop fits well into a cereal rotation on land that is clean and fertile. The earliest crops are cleared by the end of September and so it is possible to follow with winter wheat. Where club root is a problem, particularly on acid soils, brassicas should be grown no more than one year in four or five with a planned liming policy before planting. It is possible to grow brassicas continuously on a high pH soil.

Pests and diseases

Cabbage root fly can be a problem and treatments will be required from April onwards. Aphids, caterpillars, cutworms, pollen beetle, cabbage stem weevil and slugs all attack Brussels sprouts. Damping off and wirestem can be a problem at the seedling stage. Club root, downy mildew, powdery mildew, dark and light leaf spot, ring spot, white blister and canker can all be a problem in the field.

Weed control

Established perennial weeds should be controlled prior to transplanting or drilling. Oilseed rape volunteers and potato groundkeepers can cause a problem in some crops.

Nutrition

For a soil with an index of 2, the fertiliser recommendations for Brussels sprouts would be 270kg/haN, 100kg/ha of P₂O₅ and 150–200kg/ha of K₂O. Nitrogen top-dressing may be required when recommended rates are high. The top-dressing is generally applied within two months of the base-dressing application.

Irrigation

Irrigation may be applied before drilling or after transplanting to aid establishment. Irrigating early and mid-season sprouts may increase yield and quality.

Harvest and storage

In earlier-maturing varieties due to be single-harvested in August and early September the crop is often ‘stopped’ by removing the growing point (or terminal bud) from the plant, or by destroying the bud with a sharp tap using a rubber hammer. This removes apical dominance and stimulates sprout growth producing a 5–10% higher yield. Some varieties tend to be more cylindrical in set and stopping is not needed. Stopping may be done 4–10 weeks before harvest when half of the sprouts are 12mm in diameter.

The tops of the plant, which resembles a small open hearted cabbage can be removed and sold in October and November. The major part of the crop is machine harvested, usually beginning in August and extending until March, according to the variety and season. Occasionally, the earliest maturing varieties are picked over by hand before the whole plant is cut for stripping. Where hand harvested, the crop is usually picked over three or four times during the season. In warm conditions sprouts can become discoloured and deteriorate and need to be cooled rapidly after harvest.

Features of the Edible Parts

In Brussels sprouts, instead of a single head at the top of the stem, miniature heads are born in the axils of leaves distributed all along a tall stem, which is crowned with a cluster of loose leaves. The dwarf varieties of Brussels sprouts have short stems, mostly less than 50cm long. In dwarf cultivars sprouts are of medium size and the distance between sprouts is small. Tall late hardy cultivars are preferred for a long growing season. Early cultivars have light green sprouts. The small solid heads 5–7cm in diameter are tender and delicious.

Michihili, the most common improved variety of Chinese cabbage, is uniform in size and heading. The heads are usually 46cm long and 10cm thick, and cylindrical in shape with a tapered tip. The outer foliage is dark green, the inner leaves are blanched, and the midrib is white and broad. The old Chinese cultivar, Won Bok, has light green leaves. The petsai, which resembles the cos lettuce, produces a much larger, elongated and compact head; the leaves are slightly wrinkled, green, thin, and much veined. The midrib is broad and light in color. Another variety, pakchoi, resembles Swiss chard in its growth habit. The leaves are long, dark green, and oblong or oval, and do not form a solid head.

In the case of Radichetta chicory, the leaves are dark green, narrow, and notched, and the Italian cultivar is a red, nonforcing type.

Endive is a plant with numerous radical leaves, which are smooth, lobed, more or less deeply cut and spreading into a rosette; the stem is hollow, from 50cm to over 1m high, ribbed and branching. Endive grows as a loose head of leaves, which are usually strongly ruffled and serrated. The outermost leaves are green and bitter in taste, but the inner leaves are light green to whitish in color.

Kale is closely related to the wild cabbage, and most of its forms bear a rosette of leaves at the top of the stem. Its tender young leaves are used for salad. It also produces edible inflorescence like cauliflower and broccoli.

Lettuce forms a rosette of large, longish leaves, which are somewhat spoon-shaped and more or less undulated/toothed at the edges. From the center of the rosette springs a nearly cylindrical stem, which narrows very rapidly and branches at about one-third of its height. It is furnished with clasping leaves, which encircle the stem and become narrower as they approach the top. There are six morphological types of lettuce: crisp head, butter head, cos, leaf, stem, and Latin, depending on changes in leaf shape and development of the heading character.

Crisp head-type lettuces have unbranched stems, which generally remain less than 30cm long, owing to the growth being arrested at an early stage. As the growing point continues to form leaf primordia, a rosette of sessile leaves arises. The first leaf unfolds normally, and can reach a length of over 30cm. After some time, other leaves are produced which unfold only partially and form a layer, embracing the laterally formed leaves that do not expand. This continuous segregation and growth of young leaves develops a head. Crisp head cultivars are usually large, heavy, brittle-textured and tightly folded with green outer and white or yellowish inner leaves.

Butter head comprises heads of very soft, buttery, textured crumpled leaves, which have less prominent veins and midribs than in crisp head types. The variety Imperial has relatively soft textured light to medium green leaves with serrated or wavy edges. The leaves of Great Lakes are brittle with ruffled margins, and are bright green to yellow with prominent ribs. Leaves of Empire are light green, deeply serrated and crisp, while the heads are often slightly elongated or conical. Butter head varieties are sometimes referred to as cabbage lettuce. Their broad oily, soft-textured, crumpled leaves form relatively small, loose heads. There are a number of other cultivars, which vary in size, color (including red), and appearance.

Leaf lettuce primarily produces a rosette of leaves and has no heading tendency. Like butter head lettuce, it forms a loose head, but the leaves are elongated, resembling those of cos lettuce.

Cos (Romaine) lettuce has elongated heads of long leaves with heavy midribs. The outer leaves are coarse and tough in appearance and dark green, but the inner leaves are fine-textured and light green. The eating quality of cos lettuce is good and it is very popular in southern Europe and the Mediterranean region.

Stem lettuce is sold as Celtuce in the USA; the stems are peeled and used as salad or cooked vegetable.

Latin lettuce leaves are somewhat elongated, but more leathery than Romaine. It is very different in appearance from other types of lettuce, having a thickened, elongated stem, which is peeled and used as salad or cooked vegetable.

Mustard leaves are very strongly flavored and pungent. The inner younger leaves are milder and are best suited for salad. The basal rosette leaves of mustard vary in form, while the stem leaves are mostly entire. Leaves are large, broadening towards the apex, and have smooth or curled margins. Florida Broad Leaf is a vigorous, large, and erect plant with large, thick, broad, oval leaves with toothed margins. Southern Giant (curled leaf) is large and upright with large, wide leaves and curly leaf margins.

Parsley is a rosette of divided leaves on a short stem. The curled-leaf-type is most common and attractive. Its three subtypes – double-curled (moss curled), evergreen, and triple-curled – are distinguished by the degree of leaf curling, coarseness of the leaf, and plant growth habit. Moss curled (double-curled) has a stem about 30cm tall with vigorous, compact and very dark green leaves, which are finely cut, deeply curled, and frost-resistant. Leaves of evergreen type are coarsely cut, while extra triple-curled leaves are finely cut and very closely curled. The triple-curled variety has slightly shorter leaves, which are closely curled. The leaves of the variety Paramount are tall, very uniform, triple-curled, and a very dark green color. The plain leaf parsley (Dark Green Italian) has deeply cut leaves but no curling or fringing. Plain-leaved types include plain (singles) and have flat leaves. The leaves of the Dark Green Italian cultivar are heavy and glossy green.

Spinach, in general, produces rosettes of fleshy leaves, which may be crinkled (Savoy) or smooth in the vegetative phase. The leaves are ovate, rounded or triangular, succulent, and borne on a short stem. In the second stage of growth, the stem elongates, producing a seed stalk with narrow, pointed leaves. The shape, size, and color may differ and be characteristic of a varietal group. Varieties used for processing have large, smooth, dark, or medium green leaves with a semierect or spreading habit.

The watercress plant has three parts: first is an aerial stem portion with an apex, and leaves with no roots projecting above the water; second is a stem portion beneath the flowing water, which has foliage and a system of adventitious roots that arise from the leaf axils and remain free in water; and third is the normal roots. Watercress leaves are pinnately compound with three to 11 leaflets, rounded or oblong, and slightly fleshy with smooth margins.

Handling and Storage

The cultivation of Brussels sprouts is similar to that of cabbage and other Brassica species. Crops may be grown either from direct seeding or from transplanted seedlings raised in open seed beds or, more generally, under protection. Typically, seeds are sown in early spring and harvesting is carried out from fall until the following spring. Continuity of production is achieved primarily through choice of cultivars, but raising transplants under protection enables an advancement of the earlier crops. The sprouts on older cultivars develop from the base of the stem upwards, necessitating selective harvesting by hand. Modern hybrid cultivars, however, mature more uniformly and commercial production is now mainly conducted using mechanical harvesters.

Sprouts are trimmed and marketed loose. Short-term storage in cold stores at 0–1 °C and high humidity can be used to maintain freshness.

Morphology and Taxonomy of the Edible Parts

Dwarf varieties of Brussels sprouts have short stems, <50cm long. ‘Improved Long Island,’ ‘Catskill,’ ‘Early Morn Dwarf Improved,’ and ‘Friither Zwerg Kvik’ are the most important varieties. In dwarf cultivars, sprouts are of medium size and the distance between sprouts is short. ‘Bedfordshire’; ‘Cambridge No. 1, 3, and 5’; ‘Irish Elegance’; and ‘Sherradian’ are some of the important cultivars grown in the United Kingdom and Ireland. Early cultivars such as ‘Breda’ (the Netherlands), ‘Weibulls Rapid’ (Sweden), and ‘Wilhelmsburg’ (Germany) have light green sprouts. Late hardy cultivars include ‘Red Vein,’ ‘Hild's Idea,’ ‘DeRoshy Amager,’ and ‘Polarstjernen.’

An improved Chihili (Michihili), the most common variety of Chinese cabbage, is uniform in size with heads around 46cm long, 10cm thick, and cylindrical in shape with a tapered tip. The outer foliage is dark green, inner leaves blanched, and the midrib white and broad. ‘Won bok’ with light green leaves is a very old Chinese cultivar. Pe-tsai resembles cos lettuce but produces a much larger, elongated, and compact head, and the leaves are slightly wrinkled, green, thin, and much veined. The midrib is broad and light in color. Pak choi resembles Swiss chard in its habit of growth. The leaves are long, dark green, and oblong or oval and do not form a solid head.

Endive grows as a loose head of leaves, which are strongly ruffled and serrated. The outer leaves are green and bitter, but the inner leaves are light green to whitish. In the case of Radichetta chicory, the leaves are dark green, narrow, and notched, and the Italian cultivar is red and nonforcing type. Endive is a plant with numerous radical leaves, which are smooth, lobed, and more or less deeply cut, spreading into a rosette, and the stem is hollow (50cm to >1m high), ribbed, and branching.

Kale produces edible inflorescence like cauliflower and broccoli. The most popular varieties of kale in the United States are Dwarf Blue Scotch and Dwarf Green Scotch.

The cultivated lettuce (Lactuca sativa L.) is related to wild lettuce (Lactuca serriola), a common weed in the United States. Heading lettuce was first described as cabbage lettuce in 1543. It forms a rosette of large, longish leaves, which are somewhat spoon-shaped and more or less toothed at the edges. From the center of rosette springs a nearly cylindrical stem, which narrows very rapidly and branches at about one-third of its height. It is furnished with clasping leaves, which encircle the stem and become narrower as they approach the top.

Mustard (Brassica juncea), a member of the cabbage family, is used both as a salad and as a green vegetable. In addition to being grown for its seeds, white mustard may be grown as a salad plant, cover crop, and green manure. Although principally an oilseed crop, it is commonly grown as a leafy vegetable in northern parts of India, China, and tropical and subtropical regions. ‘Ford Hook Fancy,’ ‘Ostrich Plume,’ and ‘Southern Giant Curled’ are the popular varieties of Brassica juncea grown in the United States. The basal rosette leaves vary in form, while the stem leaves are mostly intact. Leaves are large, broadening toward the apex, and have smooth or curled margins. ‘Florida BroadLeaf’ is a vigorous, large, and erect plant, with large, thick, broad, oval leaves with toothed margins. ‘Southern Giant Curled’ (curled leaf) is large and upright, with large leaves and curly margins.

Parsley forms a rosette of divided leaves on a short stem. The most common and attractive is the curled leaf-type containing three subtypes, namely, (i) doubled-curled, (ii) evergreen, and (iii) triple-curled. The degree of leaf curling, coarseness of the leaf, and plant growth habit distinguishes these subtypes. The plain leaf parsley (Dark Green Italian) has deeply cut leaves but no curling or fringing. Double-curled lettuce has a stem 30.5cm tall with compact and very dark green leaves, which are finely cut, deeply curled, and frost-resistant. Evergreen leaves are coarsely cut, while extra triple-curled leaves are finely cut and very closely curled. The triple-curled variety has closely curled shorter leaves. Plain leaf types include plain (singles) and have flat leaves. The leaves of the Dark Green Italian cultivar are heavy and glossy green.

Watercress (Lepidium sativum L.) has three sections, namely, (i) an aerial stem portion with an apex leaves but no roots that is projecting above the water, (ii) a stem portion beneath the flowing water that has foliage and a system of adventitious roots that arise from the leaf axils and remain free in water, and (iii) normal roots. Watercress leaves are pinnately compound with 3–11 leaflets, rounded or oblong, slightly fleshy, and with smooth margins. Cress is a much-branched annual with linear leaves. The moss curled (curled) variety is branching, dark green, and with a mild but distinct flavor.

Varieties

For the fresh market, a smooth, round, dark green button, free from disease, and of a good flavour is required. A large proportion of the crop is size graded for different markets.

Isothiocyanates: Possible Anticancer Agents from Vegetables

Studies with rats have revealed that dietary isothiocyanates can provoke a decrease in activity of enzymes (cytochrome P450) that activate carcinogens, and an increase in enzymes (glutathione S-transferase) used for the detoxification of carcinogens. The task of sorting out how the diet influences the metabolism of carcinogens is complicated by the existence of dozens of different types of cytochrome P450 and several types of GSH S-transferase. For example, human tissues contain the a, μ, p, and θ classes of GSH S-transferase. Another complicating factor is that the transfer of a molecule of GSH to some xenobiotics (such as heterocyclic amines) may either increase or decrease its mutagenic properties. Studies with humans are only beginning to be conducted (Nijhoff et al. 1995; Bogaards et al. 1994). The identification of which glucosinolates and isothiocyanates are actually protective will allow nutritionists to give firmer recommendations on diet. The identification of the genetics of human GSH S-transferases will allow geneticists to pinpoint which human populations are more likely to benefit from cruciferous vegetables.

An enzyme involved in cell cycle control has been identified as the target of a compound supplied by broccoli, cabbage, and Brussels sprouts. One of the glucosinolates of these foods is the source of a chemical called indole-3-carbinol. Indole-3-carbinol enters the cell and influences the activity of a special protein kinase that is used to control the cell cycle. The chemical prevents the synthesis of the enzyme, and thus reduces its activity. The action of this protein kinase (cyclin-dependent protein kinase 6) normally functions to make the cell embark on the G1 phase of the cell cycle, and thus to stimulate progression through the cell cycle. The chronic decrease in activity of this protein kinase serves to prevent cell division, but also prevents spontaneous cancers. The effect of indole-3-carbinol has been shown to occur in cultured cells, as well as in animals (Cover et al. 1998).

11.2.2 Origin and Geographic Distribution

The Brassicaceae family includes broccoli, Brussels sprouts, cauliflower, kale, mustard (greens), and collards. Collectively, these crops are referred to as cole crops or crucifers. Wild cole crops have been found growing along the Mediterranean and in northwestern Europe many years ago; the wild ancestor of cabbage was originally found in Britain and continental Europe. It is, however, believed that because of the wide range of crops developed from the wild B. oleracea, multiple domestications of cabbage may have occurred in the same time period throughout Europe. Cabbages and kale presumably originated in western Europe (i.e., Germany, Italy, and England), and it is believed to have been domesticated over there since the 14th century. Then cabbages spread from Europe into Mesopotamia and Egypt, and later followed trade routes throughout Asia and the Americas. Cabbage seeds traveled to Australia in 1788 with the First Fleet, and were planted the same year on Norfolk Island. It became a favorite vegetable of Australians by the 1830s and was frequently seen at the Sydney Markets.

19.6.4 Crucifers

Crucifer vegetables include broccoli, cabbage, Brussels sprouts, cauliflower, and kale. Currently, crucifer vegetables are among the most popular vegetables in the diet and per capita consumption has risen steadily. The greatest rise in consumption is in broccoli. Crucifers provide the richest sources of the phytochemical glucosinolates in the human diet. They are also rich in vitamin E, tocopherols, vitamin C, fiber, and other phytochemicals. Crucifer vegetables are also rich in total antioxidants, with kale rated as the second highest among 22 vegetables tested (Cao et al., 1996). Brussels sprouts and broccoli were also ranked high in their antioxidant capacity containing significant amounts of vitamins C and E and β-carotene. The variability in the phytochemical constituents was associated with genetic factors, although storage and processing also affect vitamins E and C, tocopherols, and carotenoids (Kidmorse and Hansen, 1999; Kurilich et al., 1999). Significant loss of vitamin C (up to 90%) occured in cold stored, frozen, blanched, and vacuum packaged compared to fresh broccoli, but only a slight decrease in β-carotene was observed (Kidmorse and Hansen, 1999). In a study comparing broccoli florets to intact heads in which the vegetable was stored at 4°C for 21days in air and 100% relative humidity, TAA content was stable and there was no difference between the cut and the intact vegetable (Paradis et al., 1995).

Fonseca et al. (2005) reported beneficial effects of low O₂ and high CO₂ on the quality of shredded Galega kale (Brassica oleraceae var. acephala); they observed an extension of the sensory characteristics of shredded kale as well as a reduction in respiration rate, greater chlorophylls retention, and retardation of AA degradation.

Broccoli florets and whole heads of broccoli had stable vitamin C content and β-carotene during storage and no difference was observed in nutritional quality between the fresh-cut broccoli florets and intact whole heads of broccoli (Paradis et al., 1995). The phytonutrient content of fresh broccoli, both processed and unprocessed was not significantly reduced during storage (Hussein et al., 2000).