J Integr Plant Biol. ›› 1956, Vol. 5 ›› Issue (2): -.

• Research Articles •    

Developmental Morphology of the Tubers of Sweet Potatoes

Lee Shu-hsien and Sheo Chen-hsioh   

Abstract: The present study was carried cut at the Experiment Station of Chekiang Institute of Agriculture in Hangchow. Three varieties, viz., Sun-li No. l00, Nancy Hall, and Red-skin-white-flesh 6o, were used under ordinary cultural practices. Of all the three varieties herein studied, the growth rate of the tops, including vines and leaves, was found to be much higher than that of the tubers during the first two to three months after planting. From July to August, the top growth reached its maximum stage. After that time, however, the top may decrease in weight gradually associated with the cessation of the top growth, whereas the fleshy roots began to increase rapidly up to the harvest time. In case the vines and leaves grew continually thoughout the growing season, tubers would be slender in shape, and smaller in size, and, in the extreme case, tubers may not be formed at all. It is gound that tubers of the higher yield varieties usually begin to form earlier than that of the lower yield varieties. The percentage of dry matter content in the tubers, no matter what varieties they are, is lower in the early stage of growth, and a little higher in the latter stage, and that in the tubers is by far higher than that in the tops. Morphogically, the tuber is a storage organ developed through secondary and tertiary thickening of a lateral root. It should be pointed out that not all the lateral roots of a plant but only those roots with suitable soil and climatic conditions form fleshy roots or tubers. Besides the function of the primary cambium in the usual manner, the extensive activity of the secondary cambiums has much to do with the tuber formation. These secondary cambiums are first found to occur in the parenchyma of the central portion of the stele, surrounding each of the protoxylem points; later, they occur as cylinders surrounding groups of secondary xylem elements; and still later, they appear in the parenchyma of the roots which are not related spatially to any vascular elements. As a result of the different manners of activities of these secondary cambiums, the storage parenchyma produced from them may differentiate in their properties of internal breakdown and susceptibility of frozen injury. A feeding root of sweet potato is anatomically characterized by the limited development of the secondary cambiums and the lignification of the xylem elements; whereas a fleshy root by the extensive activity of the secondary cambiums and the differentiation of abundant thin-walled storage parenchyma. The continuous increase in cell number by the meristematic activity of the secondary cambiums almost throughout the whole period of tuberization is found to be the main cause for the development of tubers. In Nancy Hall, for example, the diameter of the interstitial parenchyma cells increases very little after the middle of July, while the diameter of tubers increases continually up to the end of October. Since the secondary cambiums first occur around each of the protoxylem points, the roots with more number of protoxylem points will enhance the activity of the secondary cambiums to favour the formation of tubers, The number of protoxylem points of the variety Sun-Li No. l00 (average 5.4–5.8) is greater than that of “Nancy Hall” (average 5.0–5.4). And within a single root, the number is more in the base, less in the medium, and still less in the apex part of the root. It should be shown that the number of protoxylem points is one, but not the only one factor effecting the development of the tubers. The activity of the secondary cambiums, the lignification of the xylem elements, and the number of protoxylem points may all be effected by various climatic and soil conditions under which they are grown.

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