Improving the Root Vegetables

IMPROVING THE ROOT VEGETABLES C. F. PooLE, Cytologist, Division of Fruit and Vegetable Crops and Diseases, Bureau of Plant industry LJlD the ancient civilizations arise in the regions where our common cultivated food plants originated and were naturally abundant? Or did man take food plants with him, so that the early centers of civilization only seem to be the centers of origin of the plants? This must remain an interesting subject of speculation, but few students doubt that civilization was dependent on the natural locations of the plants. At any rate, the regions that are now believed to be the natural centers of origin of the root vegetables (40, 4^)/ which were used as food long before recorded history, include practically all of the centers of the oldest civilizations. The present belief is that in the Old World there were six of these, with five of which we are here concerned, and in the New World two major and two minor centers, all of which produced valuable root vegetables: (1) Central and western China—radish, turnip, taro (dasheen). (2) India (except northwestern India)—taro. (3) Middle Asia (Punjab and Kashmir)—turnip, rutabaga, radish, carrot. (4) Near Asia—turnip (secondary center), beet, carrot. (5) The Mediterranean—turnip, rutabaga, beet, parsnip, salsify. (6) Ethiopia—no root vegetables. (7) Mexico—swectpotato. (8) South America (major)—taro, potato. (8a) Chile—potato. (8b) Brazil-Paraguay—cassava. The theory is that the place where a plant exhibits the greatest diversity of subspecies and varieties in its natural state must have been a center of origin of that plant. The value of the root vegetables is due to the fact that they are biennials, storing food^ in their roots during the first season to sup- port the second season's growth. Among the cultivated root vegetables, man has been able to select fairly true breeding varieties, which differ from one another chiefly by the shorter or longer time intervals they require to reach maturity. In some regions an ordinarily biennial crop may be changed into an annual, or vice versa, by planting seed early enough so that the plant will bolt, or produce seed the first season, instead of enlarging its root for winter dormancy. In all the biennial crop plants there are numerous varieties reputed to be non- bolters, meaning that they take longer to mature than varieties not so designated. Genetically, the hereditary factors governing time to ^ Italic uurabers in ])aronthos('s rofcr to Literature Citod, ]). 322. 2 The storod food is most frequently in the form of starch and starchlikn substances, injportant olemonts in the human diet. 300 ROOT VEGETABLES 301 maturity in most of our root crops must be numerous, and the terms annual and biennial are not strictly accurate because of the many gradations. From the beginnings of agriculture almost to the present there has been no conscious effort to improve the root vegetables beyond selecting seed from particular plants whose roots struck the grower^s fancy. Within the last century, however, there has been more or less con- tinuous activity in improving plants by crossing diverse types and selecting new combinations of characters. With the development of Mendelism into the science of genetics it became apparent that cross- breeding was essential for rapid plant improvement, and furthermore that an intelligent hybridization technique required a knowledge of the breeding behavior of chosen parents. In the case of cross- poUinated species, like the root crops, the breeding behavior or the real genetic make-up of the parents can be determined onl}^ by inbreeding to produce lines homozygous or *'pure'' for their own characteristics. This must be combined with the keeping of pedigree records. With all our root vegetables, however, the inbreeding program is sometimes exceedingly difficult, for two reasons to be discussed later. IMPROVEMENT OF ROOT CRUCIFERS THE old method of plant improvement is inadequate for our present needs. The numerous kinds of insects and fungus diseases attacking cruciferous root vegetables (turnip, rutabaga, radish) necessitate the production of new resistant or immune strains by the more recent methods, including inbreeding, crossing, and the use of wild ancestral forms in the search for superior qualities. When established commercial varieties are planted in regions where insects are most numerous or in soils or regions where the fungus diseases are most THE old method of plant improvement is inadequate for our present needs. The numerous kinds of insects and fungus diseases attacking such root vegetables as the turnip^ rutabaga^ and radish necessitate the production of new resistant or immune strains by the more recent methods^ including inbreedings crossings and the use of wild ancestral forms in the search for superior qualities. In all the cruciferous root vegetables the deliberate attempt to breed for resistance to diseases or to produce special kinds of vegetables has been neglected thus far. But if practically nothing has been done in the way of improvement by modern breeding methods^ there has been enough genetic research and working out of adequate techniques to lay the foundations on which to build a practical program. 302 YEARBOOK, 1937 damaging, seed can be produced only by plants that are entirely immune or partially resistant to attack. If no natural immimity or resistance is found in commercial stocks, it can usually be found by the introduction of wild ancestral forms of these cultivated vegetables from their original home sites. In all the cruciferous root vegetables the deliberate attempt to breed for resistance to diseases or to produce special kinds of vegetables has been neglected thus far. But if practically nothing has been done in the way of improvement by modern breeding methods, there has been some genetic research and the working out of adequate technique, as will be brought out later. The ground work has been laid on which to build a practical program. BREEDING OF TURNIP AND RUTABAGA The name turnip is commonly applied to vegetables that, in their present cultivated condition, are botanically classified in three separate species of the genus Brassica, The point should be stressed, however, that cultivated forms of plants represent complexes of characters that make them quite different from their wild prototypes. This has resulted from many generations of selection for characters, usualljr Mendelian récessives, which accumulate in combinations not found in nature. In the United States the name turnip is applied to plants of the species Brassica rapa L.,^ a species wherein all plants examined cytologi- cally have 10 pairs of chromosomes. In Europe the name turnip is frequently applied to the group of plants that we call rutabagas (bagas, or sometimes Sw^edish turnips), known as B. napus var. sativa rapifera Hort, by^ some botanists, and as 5. campestris var. napobrassica DC in Bailey's Standard Cyclopedia of Horticulture. The rutabaga has shown 18 pairs of chromosomes in two cytological examinations by Karpechenko and Frandsen and 19 pairs in one examination by Nagai and Sasaoka (29). It is important to know the number of pairs of chromosomes in a species, or at least in the particular stocks used for parents, because crosses between species having unlike numbers of pairs are less easily made and are less regular in breeding and fertility than crosses between those with the same number. A discussion of chromosome numbers in the genus Brassica is given by Pearson (30). In general, the chief distinguishing characteristics of the turnip are that the roots are mostly disldike or decidedly flattened, though rang- ing from spherical to elongated conical; the leaves are hairy, usually not fleshy, and greatly varied in outline; and the plants reach maturity in from 42 to 80 days. The commonly grown varieties show a wide variation in time to maturity, as the following indicates: ^^^^ White Milan 42 Snow Ball 43 Purple Top Strap Leaved 46 Purple Top White Globe 55-60 Golden Ball 60-65 Cowhorn 70 Yellow Aberdeen '_ 70-80 White Norfolk 76 3 Frandsen and Winge (8), however, in reporting on the cytology and genetics of the progeny from a cross between the turnip and rutabaga, call the turnip parent B. campestris var. sativa rapifera, and Vavilov Ui) calls the turnip B. campestris var. rapifera Metzg. ROOT VEGETABLES 303 The principal turnip varieties reach maturity more quickly than the principal varieties of rutabagas. Since the turnip and the rutabaga will readily cross, however, it is probable that these turnips reaching market size around 65 to 80 days after planting are the results of natural turnip-rutabaga crosses. The chief characteristics of the rutabaga, or Swedish turnip, as it is sometimes known, are a root that is tankard-shaped or elongated, although sometimes globular; a fleshier and larger leaf than the turnip; leaves not hairy; and a longer period of time (from 85 to 90 days) required for reaching the best pulling stage. Among the most commonly available yellow-ñeshed rutabaga varieties on the American market are American Purple Top, Early Neckless, and Bangholm. Two white-fleshed rutabaga varieties are Sweet liussian and White Disease Resistance in the Turnip Although no deliberate attempt has been made to breed disease- resistant varieties of turnips or rutabagas, a new variety of turnip known as The Bruce, that is highly resistant to the slime mold disease called clubroot, has recently been introduced in Great Britain, New Zealand, and Australia. Clubroot attacks many species of the crucifer family, including cabbage, radish, mustard, cauliflower, sweet alys- sum, and many others. It is now present in every country where the common cruciferous vegetables are cultivated. The Bruce is supposed to be a natural hybrid between the turnip and the rutabaga, and it first appeared in Scotland about 1820, some 40 years after the time that clubroot first appeared in Great Britain in 1780.

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