In studies on the chemical basis of frost resistance in plants Siminovitch et al (25,26) observed pronounced seasonal changes in the absolute and relative concentrations of starch and sucrose in the living bark of the black locust tree. Their data suggested that these reserve carbohydrates are interconverted by a reversible process which is dependent on temperature and season. Low temperatures appeared to favor conversion of starch to sucrose while high temperatures favored the reverse process. The in vitro conversion of starch to sucrose observed in maple sap, by Bois and Nadeau (7, 8) was, like the in vivo process, favored by low temperatures. The conversion of sucrose to starch in plants is not as well known as the conversion of starch to sucrose. It is usually assumed that the synthesis of starch is catalyzed by phosphorylase. However, recent demonstrations of the enzymatic synthesis of starch and related polysaccharides from disaccharides without intermediate phosphorylations (3,11,12,18,19,20,21,22,23,24,28) suggest that the phosphorylase reaction may not always be involved in the synthesis of starch in plants. It is the purpose of this paper to discuss some possible mechanisms for starch-sucrose interconversions and to present the results of certain experiments which may have a bearing on the mechanism in the living bark tissue of the black locust tree.The known enzymatic processes which may be involved in starch-sucrose interconversions are summarized in figure 1. The obvious possible route for the conversion of starch to sucrose involves phosphorolytic degradation of starch to glucose-l-phosphate (G-1-P) as catalyzed by phosphorylase (reaction A), followed by combination of G-1-P with fructose to form sucrose as catalyzed by sucrose phosphorylase (reaction B) (10,16,17). The required fructose molecule could be produced from a second molecule of G-1-P by way of glucose-6-phosphate (G-6-P) and fructose-6-phosphate (F-6-P) as intermediates. favors the breakdown rather than the synthesis of sucrose. Yet many plants produce high concentrations of sucrose while the concentrations of fructose and G-1-P remain low. Unless plant cells can remove sucrose from the sphere of influence of the enzyme, this reaction cannot be involved in the synthesis of sucrose.
