BIOCHEMISTRY OF PLANTS 



may result in sucrose formation by plant tissues. For example, in 

 experiments on barley shoots by infiltration procedures, galactose could 

 be utilized for this purpose, as well as various other carbohydrates or 

 related compounds. In the leaves of the sugar cane, as studied by 

 Hartt, the oxidative system involved was not inhibited by cyanide, 

 although in some experiments on other plant tissues in tliis laboratory 

 the synthesis was cyanide sensitive. While the mechanisms of sucrose 

 synthesis operating in the higher plant are by no means sufficiently 

 elucidated as yet, there is reason for an optimistic view that further 

 developments along the general lines of attack already pursued will 

 eventuafiy lead to a satisfactory biochemical solution of this important 

 problem of plant metabolism and plant nutrition. 



Closely related to the investigations just outlined is the long- 

 standing question of the biochemical nature of the interconversion of 

 starch and sucrose in the plant. As an illustration, the well-known 

 sweetening of potatoes at low^ temperatures may be cited. In this 

 process, starch is converted to sucrose. This conversion is also an 

 aerobic process and is inhibited by cyanide and some other respiratory 

 poisons. A mixture of hexose-6-phosphates has been i.solated from 

 potato juice, and also phosphatases capable of hydrolyzing these com- 

 pounds. A tentative scheme to explain the conversion of starch to 

 sucrose has been advanced on the basis of reactions for which hexose 

 phosphate esters are requisite; and, according to the explanation 

 offered, both glucose-1 -phosphate ester and fructose diphosphate are 

 essential (11). In the earlier experiments in Hawaii on sucrose 

 synthesis by sugar cane leaves, fructose diphosphate was likewise re- 

 garded as an essential substrate. On the other hand, in the bacterial 

 enzyme synthesis referred to above, only glucose-1 -phosphate and 

 fructose could be converted to sucrose. 



The great problem of cellulose synthesis remains without a 

 biochemical explanation. Whether this synthesis can take place only 

 from activities of the organized protoplasm, and whether phosphoro- 

 lytic processes are involved, are in the realm of speculation at the 

 present time. 



The point to be emphasized by the foregoing remarks is that 

 biochemical research, by contributing to the basic knowledge of carbo- 

 hydrate transformations, can influence profoundly the study of plant 

 nutrition and physiology. There is, of course, open for further research 



71 



