Onion

The common onion (Allium cepa) is a biennial with large bulbs that are usually single. It is the most important bulb crop and is exceeded in value by only four other vegetable crops grown in the United States, viz., potatoes, sweet potatoes, tomatoes, and cabbages. Like all the other bulb crops it is hardy and, in the North, is planted very early in the spring. In the South it is grown as a winter crop.The bulk of the crop is grown from seed sown in the open, but the plants are also grown from transplanted seedlings that have been started under glass or in a seed bed and by planting sets. ⁹³

Southport White Globe Onion

For root examination the Southport White Globe onion was planted Apr. 10. Especial care was exercised to have the soil fine and loose and with a smooth surface, since the seeds are small and do not germinate quickly. Thorough preparation of the soil is an essential feature in the successful growth of nearly all crops.It is especially important in the production of vegetables. The seed was sown in drill rows in the field where the crop was to mature. The rows were 14 inches apart and the seed was covered 1.5 inch deep. From time to time the soil was shallowly hoed and all weeds were removed from the plots.

Early Development

The onion develops a primary root which, under very favorable conditions, may reach a length of 3 to 4 inches 10 days after the seed is planted. In the meantime the cotyledon comes from the ground in the form of a closed loop. By the time the first foliage leaf emerges from the base of the cotyledon, several new roots make their appearance near the base of the stem.

The first field examination was made June 4. The plants were about a foot tall and had an average of four leaves each. These varied from 4 to 12 inches in length and 2 to 5 millimeters in diameter. The total area of the cylindrical leaves per plant averaged 8.5 square inches. Each plant was furnished with 10 to 12 delicate, shining white, rather poorly branched roots (Fig. 7).The longest usually pursued a rather vertically downward course to a maximum depth of 12 inches. The lateral spread from the base of the bulbous stern did not exceed 4 inches. The roots were rather poorly branched and frequently somewhat curled or pursued a zigzag course.

Fig. 7

Effect of Soil Structure on Root Development

To determine the effect of loose and compact soil on root growth, onions were grown in rectangular containers with a capacity of 2 cubic feet and a cross-sectional area of 1 square foot. A fertile, sandy loam soil of optimum water content was screened and thoroughly mixed and thus well aerated. One container was filled with very little compacting of the soil. It held 173 pounds. Into the second container 232 pounds of the soil were firmly compacted. Surface evaporation was reduced by means of a thin, sand mulch. Onion seeds were planted in each container early in April, and a month later the sides were cut away from the containers and the root systems examined. Owing to clear weather and favorable greenhouse temperatures the plants had grown rapidly and were in the third-leaf stage.

Fig. 8

The effects on root growth are shown in Fig. 8. In the hard soil the plants nearly always possessed six roots but only five were found on those in the loose soil. Under the former conditions the vertically descending roots reached depths not exceeding 5 inches; the others spread laterally 1 to 1.5 inches and then, turning downward, penetrated to a total depth of 2.5 to 3 inches.In the loose soil one root from each plant grew downward to a depth of 12 to 15 inches; the others spread laterally 1 to 2 inches and then turned downward and reached a maximum depth of 7 inches. Thus the root system in the loose soil was not only deeper but more widely spread.The roots in the hard soil, moreover, were much more kinky. In the loose soil they made long, gradual curves; in the compact, hard soil, short abrupt ones. Under the latter soil condition the root ends were often thickened. The roots running laterally from the plant were more horizontal in the hard soil during the first 1 to 2 inches of their course. Thus this portion of the root system was shallower than in loose soil. Branches were shorter throughout.Similar results were obtained with lettuce seedlings (see Chapter Lettuce).

Half-grown Plants

By July 25 the bulbs had reached a diameter of 0.5 to 1.8 inches. From 28 to 33 roots arose from the base of each of the bulbs, i.e., at a depth of 1.5 to 2 inches. Nearly all of these smooth, shining white roots were a millimeter in diameter although a few were only 0.5 millimeters thick. The longest frequently maintained their initial diameter for distances of 5 to 10 inches; others quickly tapered to a thickness of only ½ millimeter.In fact the roots showed considerable variability in this character, sometimes tapering only to enlarge again. The deepest roots penetrated vertically downward or ran obliquely outward for only a few inches and then turned downward. A working level of 20 inches and a maximum depth of 27 inches were attained.

Some of the main roots ran outward, almost parallel to the soil surface, to distances of 6 to 8 inches before turning downward at an angle of about 45 degrees. These had a maximum lateral spread of about 12 inches on all sides of the plant.Between these horizontal roots and the vertically descending ones, the soil volume thus delimited was filled with numerous roots Which extended outward to various distances and then turned downward, or pursued an outward and downward course throughout their entire extent (Fig. 9).

Fig. 9

As a whole the main roots were poorly branched. In the surface 8 to 10 inches of soil branching occurred at the rate of 2 to 6 laterals per inch of main root, although sometimes as many as 12 were found. Many of the laterals were only 0.5 to 1 inch long, but some reached lengths of 4 to 5 inches. Below 8 inches depth, branches were often fewer and usually shorter, seldom exceeding 1 inch in length. The last few inches of the rapidly growing roots were unbranched.All of the laterals were slender, white, and entirely destitute of branches. Usually they were much kinked and curved and as often extended outward or upward as downward. The absence of roots in the surface 1 or 2 inches of soil is an important character in relation to cultivation. In this respect the onion is quite different from many garden crops.

Mature Plants

By Aug. 21 the plants had reached a height of 19 inches. Each had four to six leaves which varied in diameter from 0.5 to 0.75 inch. Owing to dry weather the plants were not flourishing and the leaf tips were dry. The bulbs averaged about 2 inches in diameter.

From 20 to 25 roots arose from the base of the bulb. A few ran vertically downward but most of them ran outward at various angles, even to near the horizontal, and then gradually turned downward. The volume of soil delimited at the previous excavation (which had an area on the surface of about 4 square feet) had not been increased except in depth. The former working level of 20 inches had been extended to 32 inches. A maximum depth of 39 inches was found (Fig. 10).

Fig. 10

Open Interactive Tool

The uncompleted root growth was shown by the 2 to 5 inches of unbranched or poorly branched, glistening white, turgid root ends. The main roots were very uniform in appearance and about 2 millimeters in diameter throughout their course. They were somewhat kinked and curved, perhaps owing in part to the difficulty in penetrating the rather compact soil.The branching was somewhat uniform throughout and at the rate of three to five, rarely more, branches per inch. These were usually only 0.5 to 1 inch in length although sometimes they reached lengths of 3 to 5 inches. No secondary branches were found. The laterals, as before, were much kinked and curved. Many of them ran in a generally horizontal direction; others extended upward throughout their entire length; still others turned downward or outward and then downward.The surface 1 or 2 inches of soil were entirely free from roots. Compared with most garden crops the onion has a rather meager root system not only in regard to lateral spread and depth but also in degree of branching.

Death of the Older Roots

The decrease in the number of roots from 28 to 33 on July 25 to 20 to 25 on Aug. 21 is of interest for it is one of the few cases found among vegetable crops where the roots die before the plant approaches or reaches maturity. That death of the older roots originating from the center of the bulb actually occurs was further confirmed by greenhouse studies.

Plants were grown from seed in appropriate containers which held 2 cubic feet of soil. When the plants were nearly 3 months old (June 23) and some roots had attained a depth of 22 inches, the sides of the containers were cut away and the roots examined. From one to three roots per plant arising from the center of the base of the bulb were dead. These were surrounded by six to eight living roots. The dead roots, especially one of them, held a more vertically downward course than the live ones.

Similar observations have been made in both water and soil cultures. "It was observed that the roots formed at the time of germination died at about the time of the formation of the bulb, and that new ones then developed and carried the plants through their complete life cycle." Almost all of the original roots of plants which germinated May 12 were dead 2 months later (2.5 months where grown in soil), at which time the new roots were 5 inches long. It is pointed out that the death of the original roots, which come from the older tissue at the base of the bulb, may be due to a number of causes:

(a) to the senility of the stem tissues,
(b) to the senility of the roots proper, and
(c) to the convolutions which are formed in the tissues of the stem as the result of irregular growth between old and new tissues and which bring about, quite often, disconnections in the water- and food-conducting tissues. ¹³⁸

Footnotes

⁹³National LLOYD, J. W. Onion culture. Ill. Agr. Exp. Sta., Circ. 173. 1926. Access Online

¹³⁸ SIDERIS, C. P. Observations on the development of the root system of Allium cepa L. Am. Jour. Bot., 12: 255--258. 1925. Access Online

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