Africa the Center of Sorgo Origin

The center of origin of domesticated sorgo is on the continent of Africa. Satellite centers were established centuries ago in anterior Asia, India, China, and Malaya, and more recently in southern Europe, the Americas, Australia, and other parts of the world. The recognized wild prototypes of domesticated sorgo (and of all cultivated sorghums) are limited to Africa south of the Sahara, and range far down into the Temperate Zone of South Africa. Sorgo is one of the domesticated plants for which the geographic place of origin is well defined. In common with similar, well-defined centers of origin and distribution of cultivated plants, the situation may be likened to a target in which the greatest number and diversity of varieties is in the bull's-eye, and successively decreasing numbers are found in the concentric rings from the bull's-eye to the outer edge of the target. The situation is not quite so simple, because there are secondary centers, some quite far removed from the primary center, but essentially the situation holds true. The evidence of location of wild prototypes and the relative concentration of diverse varieties show that for sorgo the equatorial part of Africa is the bull's-eye.

It is plain that the plant breeder will not be content with a meager sampling of the forms or varieties available on part of the target, but will try to assemble representative forms from locations on lines bisecting the bull's-eye and extending from the northern to the southern edges.

The first introductions of sorgo into the United States were from the edges of the area of origin and the dispersal areas. About 1850 a variety was obtained from an island in the mouth of the Yangtse River in China, about 31' 30' north of the Equator, at sea level, and 15 varieties from Natal in South Africa, about 30° south of the Equator, at elevations up to 4,000 feet. In mid-continental United States at latitudes 30° to 35° north, this limited sampling of varieties from the fringes of the natural range encountered conditions not incompatible with those of their native homes. The greatest concentration of sorgo in the United States is now precisely in that area.

Later, many races of sorghum were brought to the United States and among them were representatives of the solid-stemmed type with sugary juice. For sugar production none of the latter has proved superior to the original introductions or varieties selected from the original introductions. The significant point is that, in general, the later introductions from points closer to the center of origin in equatorial Africa grew to large size at an astonishing rate but failed to produce flowers and seed. When the imported seed was used up, these desirable forms unfortunately were lost. It is a safe assumption that among them were varieties that varied in quality of juice as well as in size and rate of growth.

We know now that the development of the influoresence in many sorgos is powerfully influenced by photoperiod and that under our conditions the short-day forms of the Tropics must be grown farther south, or planted "off-season," or the daily exposure to light shortened artificially in order to induce blooming. To make use of the short-day forms in breeding, it is now comparatively simple to manipulate the environment so that short-day and long-day forms will bloom simultaneously and produce hybrids.

Quite recently this has been accomplished with sugarcane, a case in which the problem was essentially similar to the sorgo problem. Long-day forms of sugarcane from the North Temperate Zone normally blooming in July–September were crossed with short-day tropical forms normally blooming(north of the Equator) in November–January by artificially advancing the blooming date of the latter. This was accomplished by growing the tropical varieties in flat cars that could be pushed into a large, light-tight photoperiod house on a daily schedule that simulated the tropical short day. Another device used successfully was to ship the pollen across the Equator by air in specially designed refrigerated containers, taking advantage of the reversed seasons. The short-day and long-day forms bloom simultaneously on opposite sides of the Equator.

In some cases, but not all, less expensive methods can be used to induce simultaneous blooming. The simple expedient of planting short-day forms in winter in the greenhouse or at subtropical stations can be used to cross them with intermediate and day-neutral forms. This device has already been used to cross tropical sorgo varieties with certain temperate-zone varieties in southern Florida. The important fact is that by new techniques an obstacle to hybridizing the numerous and important short-day sorgos with other sorgos has been removed and many desirable crosses that formerly seemed impossible now can be accomplished and valuable hybrids can be obtained.

With that background of old objectives and new techniques, a fresh attack was begun in 1941 on the problem of improving sugar and sirup sorgos. Three lines of investigation were started : Intensive exploration for varieties in the Old World and establishing the introduced plants, as a reservoir of breeding material, at stations in the New World where they would complete a normal life cycle with production of viable seed; assembling all sorgo varieties available in the United States at a central breeding station for study and segregation of the best sugar lines; and widespread test plantings of sorgo, including tests in all important sugar-beet and sugarcane districts.