TRANSFORMATION OF SUGARS IN EXCISED BARLEY SHOOTS

Abstract

Nelson and Auchincloss (10) working with potato tissue showed that sucrose is synthesized from glucose or fructose rather than from starch as has been supposed by some investigators. That such a conversion occurs in plant metabolism has also been shown by Virtanen and Nordlund (14) in red clover and wheat plants, and more recently by Hartt (3) in detached sugar cane leaves. Since sucrose, consisting of a glucose and fructose molecule, can readily be synthesized from either of these monosaccharides, it may be assumed that such synthesis involves a preliminary conversion of each of these hexose sugars into the other. The fructose component of the sucrose molecule exists in the furanose form (unstable ? form with the five-membered ring) and is different from the six-membered fructose (pyranose configuration) from which it can be synthesized. The plant apparently also possesses a mechanism to render this transformation possible. The fact that sucrose is synthesized when plants are artificially supplied with glucose or fructose can serve indirectly in the study of transformation of other hexoses in the plant into these monosaccharides. If, for example, synthesis of sucrose is observed after supplying the plant with mannose, it could be assumed that a preliminary conversion of the latter into glucose and fructose must take place before the synthesis of sucrose occurs. The hexoses that are known to occur naturally in plants are: d-glucose, d-fructose, d-mannose, and dgalactose ; of these the last two have not been detected in the free state and are found in plants only as units in polysaccharides. If, as it is generally assumed, some form of glucose is the first product of photosynthesis, the plant should possess a mechanism whereby this molecule is transformed into mannose or galactose before the synthesis of the corresponding polysaccharide can be possible. In this investigation, using the infiltration method (8), the following compounds were artificially supplied to barley plants : d-glucose, d-fructose, d-mannose, d-galactose, sucrose, lactose, maltose, 1-arabinose, d-xylose, mannitol, sorbitol, gluconic acid, pyruvic acid, and glyceric aldehyde. The increase of sucrose was observed after certain periods of time to determine the ability of the plant to transform these compounds into glucose or fructose and, if possible, to ascertain the mechanism of this transformation.