1919) 



MCGINTY — DIASTASE ACTIVITY OF SOLANUM TUBEROSUM 247 





The results obtained may also be considered to agree with those 

 found by Keitt ('11) for the sweet-potato. The latter found the 

 total sugars in the sweet-potato tuber to decrease toward matu- 

 rity, and, from the figures given above, the same tendency appears 

 to exist in the Irish potato. In the case of the seed potatoes, the 

 reducing sugars are present in much larger quantity than in the 

 larger sizes of the growing tubers, while the sucrose is lower than 

 the lowest value obtained in the case of the growing potatoes. 



It is somewhat difficult to interpret the sugar relations of the 

 potato tuber, as shown by the figures given, but the following is 

 a possible explanation : Sugars in plants are translocated chiefly 

 in the form of the hexoses, glucose and levulose. When photo- 

 synthesis and growth are proceeding at a rapid rate in the early 

 stages of development and when diastase activity is not so great 

 as later on, it would be expected that the translocated sugars 

 (reducing sugars), would be present in the potato tuber in com- 

 paratively large amounts. Diastase, acting as a synthesizing 

 agent, would gradually convert the glucose into the storage 

 product, starch, but this alone might not suffice to dispose of all 

 the reducing sugars accumulating in the tuber, so the invertase 

 present may also be conceived to act as a synthesizing agent 

 and transform some of it into a temporary storage product, 

 sucrose, which accumulates up to a certain point. Towards 

 maturity, as the translocation of hexoses becomes less rapid and 

 as the glucose which reaches the tuber is more rapidly converted 

 into starch by the more active diastase, less reducing sugar is 

 found present. As the reducing sugar decreases, some of the 

 sucrose is hydrolyzfed to hexose by the invertase, possibly in 

 connection with the general matter of equilibrium relations, and 

 thus a decrease in the sucrose is brought about. Under the 

 influence of the low temperatures which prevail in storage, the 

 starch may be partially hydrolyzed to reducing sugar, some of 

 which may again be built up into sucrose, thus causing an accu- 

 mulation in the tubers, such as has been observed by a number 



of investigators. 



An examination of the values obtained in the starch determina- 

 tions shows that the percentage of this carbohydrate increases 

 gradually as the tuber enlarges in size. The relatively high 



