CARBOHYDRATE TRANSFORMATIONS IN PLANTS 387 



accumulation is about 5 or 6° C. (41-43° F.). Storage of potatoes at 

 temperatures in this range just above the freezing point will therefore mduce 

 sugar formation in the tubers. 



Apparently it is only in a zone of intermediate temperatures that starch 

 formation at the expense of sugars is favored in potato tubers, as relatively 

 high temperatures (35-45° C.) also appear to favor hydrolysis of starch to 

 sugar. It is possible, however, that this shift in equilibrium towards sugars 

 at'the higher temperatures is a result of the reduction in water content which 

 may be induced by such temperatures (Wolff, 1926). 



Similar effects of temperature on carbohydrate equilibria are shown by 

 other species. In the sweet potato sucrose accumulates rapidly at the expense 

 of starch at temperatures below a critical range of 13-16° C. but above this 

 range most of the stored carbohydrate remains in the form of starch (Hop- 

 kins and Phillips, 1937). In ripening banana fruits hydrolysis of starch 

 occurs rapidly at temperatures between 21 and 26° C. but there is practically 

 no starch hydrolysis at 10° C. It is evident that the critical temperature 

 ranges for hydrolysis and synthesis of starch vary greatly according to species. 

 2 Water Content.— In wilting leaves much of the starch present is 

 digested to sugars (IVIolisch, 1921 ; Ahrns, 1924). A relatively high water 

 content apparently favors "starchiness" in the leaf tissues of many species, 

 while a severe reduction in water content induces a conversion of starch into 

 sugar. Sucrose accumulates as well as simple sugars during wilting. That 

 many species of plants do accumulate sugars rather than starch under drought 

 conditions has already been pointed out in Chap. XVIII. In certain succu- 

 lent plants, such as cacti, desiccation favors the accumulation of polysacchar- 

 ides rather than soluble sugars (Spoehr, 191 9), so evidently no single general 

 principle regarding the inHuence of the water content of the cells upon car- 

 bohydrate equilibria within them can be formulated. 



3. H-ion Concentration.— T\it action of enzymes is very sensitive 10 the 

 H-ion concentration of the medium wherein they operate (Chap. XXVII). 

 Since most of the carbohydrate transformations occurring in plants are prob- 

 ably catalyzed by enzymes, the pH of the medium in which these reactions 

 occur may have an important effect upon the rate of these transformations. 

 Apparently the pH of the medium may influence not only the rate of an 

 enzymatic reaction but may also influence its direction. The reversible 

 carbohydrate transformations occurring in the guard cells of the stomates 

 afford the best known example of this effect. 



4. Concentration of 5//^«r..— Theoretically, a high concentration of 

 sugars in a cell would favor starch synthesis, and vice versa. The daytime 

 accumulation of starch in green leaves which are photosynthesizing rapidly 



