BEHAVIOR OF SWEET POTATOES IN THE GROUND 



By Heinrich Hasselbring, 



Plant Physiologist, Plant Physiological and Fermentation Investigations, 

 Bureau of Plant Industry, United States Department of Agriculture 



THE PROBLEM 



In the course of former investigations^ on the behavior of sweet 

 potatoes in storage, it was observed that the percentage of starch was 

 always highest and the percentage of sugar lowest in freshly dug pota- 

 toes. This observation was more or less incidental, having been made 

 in the course of experiments whose object was the solution of other prob- 

 lems. It was therefore not based upon a systematic study of the roots 

 throughout the latter part of the growing season. Nevertheless the con- 

 stancy of the condition seemed to justify the conclusion that in the 

 growing sweet potato the reserve materials exist essentially in the form 

 of starch, and that the appearance of sugar in considerable quantities is 

 a phenomenon occuring only in storage or after the destruction of the 

 leaves. 



In order to determine whether these quantitative relations between 

 the starch content and the sugar content of the sweet potato remain 

 constant throughout the latter part of the growing season, and to what 

 extent they are changed by the death of the vines, the carbohydrate 

 metabolism in Big Stem sweet potatoes was followed from the time the 

 roots were large enough to furnish the requisite samples until they were 

 seriously damaged by frost. 



The record thus obtained of the condition of the potatoes during this 

 period may be useful as an aid in determining the time for harvesting 

 the crop; for it is evident that, if marked changes occur in the roots during 

 the latter part of the season, the time of harvest will depend upon the 

 purpose for which they are destined, whether for storage, stock feed, silage, 

 or, as Keitt has suggested, for the manufacture of starch. In the last 

 case it is evident that the crop should be harvested when the starch 

 content is greatest. As a rule, growers are advised to dig sweet potatoes 

 when they are fully matured or after they have thoroughly ripened. 

 While these phrases imply the idea that the roots reach a more or less 

 definite state of ripeness, the characteristics by which this state may be 

 recognized are not precisely defined. On this matter a record of the 



1 Hasselbring, Heinrich, and Hawkins, L. A. physiologicai, changes in sweet potatoes duiuno 

 STORAGE. In Jour. Agr. Research, v. 3, no. 4, p. 331-342. 191S. 



CARBOHYDRATE TRANSFORMATIONS IN SWEET POTATOES. In Jour. Agr. Research, V. s, 



no. 13, p. 543-560. 1915. 



Journal of Agricultural Research, Vol. XII, No. 1 



Washington, D. C. Jan. 7, 1918 



In Key No. G— 131 



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