Ccxxx. Synthesis of Sucrose in Plant Tissue

THE transformations which different sugars undergo in plant tissue, independently of the photosynthetic process, are as yet imperfectly understood. There is every reason to assume that all the individual carbohydrates in plant material are capable of conversion into each other, though the experimental evidence at hand is insufficient to warrant definite conclusions in individual cases. This is also true of the most important hexose sugars, glucose and fructose. It is known that starch is readily formed from fructose when leaves that have been previously deprived of starch are kept in fructose solution in the dark. On the other hand, in wilting leaves starch is converted into sucrose. Similarly in the artichoke tuber, inulin-a condensation product of fructose-is in part converted into glucose [Colin, 1919]. It is also known that fructose is converted into glucose in the mammalian body, but evidence is still wanting to show whether a similar transformation takes place also in plants. Although the formation of starch in leaves which are floated on solutions of various carbon compounds in the dark has been extensively studied by numerous workers, very little attention has been paid to other synthetic processes which possibly occur parallel with the synthesis of starch. In view of the very frequent occurrence of sucrose in various plant juices it was therefore of interest to study whether measurable amounts of sucrose were formed in leaves which were previously deprived of starch and then artificially fed with either glucose or fructose solutions in the dark. Such a synthesis would naturally involve a preliminary conversion of each of these hexose sugars into the other. In fact, our experiments showed that sucrose is readily formed both from glucose and from fructose, the rate of synthesis being approximately equal in both cases. In this connection it is interesting to note that Burkard and Neuberg [1934] have recently found in sugar-beet leaves phosphoric esters of both glucose and fructose.