CRUCIFERS – of cabbages and things

Crucifers possess several common features which make this a more ‘natural’ family of plants than most. With few exceptions, each crucifer species possesses flowers with four sepals, four petals equally spaced to give a cross (the Latin word cruris means a cross), four long stamens and two short ones, and a characteristic seed pod. These features make it easy to determine that a species is a crucifer, but can raise considerable problems in determining which crucifer. The taxonomist has the same problem: estimates of the numbers of crucifer species range from some 2,000 to 4,000, divided into about 200 to 400 genera.

Most crucifers are annual, biennial or perennial herbs, and the majority are restricted to temperate regions of the Northern Hemisphere. Indeed some, such as Cochkaria species, are common in the arctic, whence the name given by sailors, scurvy-grass, as they ate it as a source of vitamin C during long ocean voyages. This, perhaps, epitomizes the way in which crucifers have become useful to man. Most are edible and contain a group of chemicals called glucosinolates, which although not restricted to the crucifers, are another characteristic of the family. When the tissues of crucifers are injured, as they are when eaten, the glucosinolates are broken down by enzymes to a number of volatile chemicals including, notably, the isothiocyanates (mustard oils) and it is principally these which give the crucifers their characteristic pungency. Since ancient times crucifers have therefore been accorded medicinal properties, with or without the justification of scurvy-grass. In addition they have been used as garnishes and condiments. Probably as a consequence of this many have been taken into cultivation, and some have evolved from their wild origins to give our modern crucifer crop plants. These can be roughly divided into four categories: vegetables, animal fodders, oil seed crops and condiments.

Vegetable crucifers

Several vegetable crucifers, such as cabbages and kales (both belonging to the species Brassica okracea) were well-known to the Greeks and Romans several centuries BC. In

Europe, the species was developed further during the Middle Ages to give cauliflower, broccoli (calabrese), kohl rabi and in the early nineteenth century, the Brussels sprout. In Asia, a parallel evolution of B. campestris took place to give a similar range of cabbage and kalelike vegetables as well as the more cosmopolitan turnip. Apart from their flavour, which in general has decreased in pungency as the crops evolved, the frost hardiness of B. okracea and the rapid growth characteristics of B. campestris have contributed to the success of the vegetable brassicas such that at present 20-40 percent of the areas occupied by vegetables in Eurasian countries contain these species.

Other Brassica species are also important as vegetables. A derivative of the hybrid between B. okracea and B. campestris, called B. napus, became established in northern Europe as the swede and swede-rape in the seventeenth century. The radish, Raphanus sativus, like the turnip has been developed in different ways in Asia and Europe. A related species, B. carinata— the Abyssinian mustard—is traditionally an important vegetable in Ethiopia, and another, B. juncea (brown mustard), is grown in Asia for its leaves. Most of these vegetables have also been introduced by immigrants to Australia and America, where they occupy up to 10 percent of the areas devoted to vegetables.

Use as animal fodders

Many vegetables are also grown as animal fodders. The Romans used the turnip as both animal and human food; and nowadays cabbages, kales, turnips, swedes and radishes are used extensively in both roles. However, the fodder crucifers became pre-eminent in northern European agriculture during the seventeenth, eighteenth and nineteenth centuries when many forms were developed to bridge the winter months between the autumn harvest and spring growth with several benefits. Apart from relieving human hunger, more animals could be kept over the winter, the fertility of the soil was improved and rotation farming could be introduced. At their peak perhaps a million hectares (2.5 million acres) of Britain were occupied by these crops. This has since shown a 90 percent decline, in part due to the introduction of agricultural chemicals, but the continuing place of crucifers in agriculture is assured because of their relatively low production costs and high nutritive value.

Oil seed crops

The crucifers contain large quantities of oils in their seeds, and several species have traditionally been cultivated in those parts of the Old World not possessing other oil crops, such as the olive or the sunflower. The Egyptians used the radish. The northern Europeans used various Brassica species, Sinapis alba (white mustard), Eruca and Crambe species (rocket) and Camelina sativa (gold of pleasure). The Asians also used Brassica species, including B. juncea. The oil was used largely for lighting and for culinary purposes, especially in Asia—hence some of the pungency of certain dishes in the Indian subcontinent. More recently, because of the rapid growth and cold resistant properties of the Brassica species they are being increasingly cultivated as the only oil seed crop suitable for northern Europe, the USSR and Canada. The oil is used in the preparation of margarine and in certain industrial processes.

Condiments from crucifers

Various condiments are prepared from crucifers. The most notable is mustard which derived its name from the Roman habit of crushing the seed of crucifers in wine must. The burning sensation of mustard when eaten or inhaled is, again, caused by the breakdown of glucosinolates in the seeds. A similar breakdown gives a characteristic pungency to horseradish sauce (made from the crushed root of Armoracia rusticcma). While the pungency of these condiments is desirable, the same compounds if eaten in excessively large quantities may be toxic. Hence, animals are seldom fed exclusively on rape, kale or any other crucifer material for long periods. Considerable efforts are now being directed towards reducing the quantities of these substances in crucifer seed by breeding new varieties of oil seed rape so that the seed meal, after the oil has been extracted, can be fed to animals.

Other uses ofcrucifers

In addition to their usefulness as major food and oil crops, a few other uses are found for crucifers. Seakale (Crambe maritima) is grown as a salad vegetable. Several types of mustard are used as green manure; they grow rapidly and smother other weeds, and are ploughed in to improve the fertility of the soil. Small industries flourish to supply various green garnishes, notably cress (Lepidium sativum) and watercress (Nasturtium species). The blue dye woad comes from the dried leaves of Isatis tinctoria, which has been cultivated for this purpose since prehistoric times.

Weeds

Some of the characteristics of crucifers which have made them suitable as crop plants such as a rapid growth rate and cold tolerance make them well adapted to be weeds. Thus shepherd’s purse (Capsella bursa-pastoris), charlock (Sinapis arvensis) and wild radish (Raphanus raphani-strum) are common and occasionally troublesome weeds of arable land. The problems with the crucifer weeds lie not only with the fact that they compete with cultivated crops, but that they may harbour many of the pests and diseases which may then spread to the crucifer crops. This is another reflection of the similarities between crucifers; virtually all of them share parasites which are almost restricted to their family, including cabbage root fly, cabbage aphid, clubroot (a fungus disease) and certain viruses.

Reproduction

As might be expected with a group of plants with such a uniform flower morphology, the crucifers tend to be similar in their breeding behaviour. Typically, pollen is transferred between plants by insects, and many crucifcrs possess a biochemical system which ensures that they will not set seed unless pollinated by another plant. Exploitation of this system forms the basis for the commercial production of the F, hybrid varieties of Brussels sprouts and cabbages which have become so important in the last decade. Many crucifers possess features which encourage pollination by insects, such as brightly coloured flowers and powerful scents. Hence, several are cultivated for their appearance, such as the candytuft (Iberis species), and others including Arabis species, alison (Alyssum species and Lobularia species), stock (Mattliiola incana) and wallflower (Clwir-authus cheiri) are grown for both appearance and scent.

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