Mechanism of starch-sugar interconversion in Solanum tuberosum

The changes in glucose-1-phosphate, glucose-6-phosphate, fructose-6-phosphate, dihydroxyacetone phosphate, 3-phosphoglycerate, 2-phospho-glycerate, phosphoenol-pyruvate, pyruvate, adenosine mono-, di- and tri-phosphates, NAD and NADH, sugars and respiration of mature potato tubers (variety King Edward) caused by transfer from + 10° to + 2° and back to + 10° were followed throughout 4–8 weeks of storage. The results obtained showed a characteristic two phase pattern. In the case of the transfer from + 10° to + 2° a number of the phosphate esters showed wide individual variations in concentration during the first phase but only slow changes during the second phase when most of the phosphate esters tended to follow a common pattern. In the first phase the sugar concentration remained roughly constant, but in the second a considerable increase in both sucrose and respiration occurred. In the case of potatoes transferred from + 2° to + 10° the two phase character of the results was not so marked. In the case of potatoes transferred from + 10° to + 2° the changes in the phosphate esters in the first phase did not appear to be related to the conversion of starch to sucrose which only occurred to a significant extent in the second phase. Electron micrographs of potato tubers which had been stored at + 2° for 38 days (sugar content 2.4%) showed that the starch grains were still enclosed in a double membrane (amyloplast membrane). Analysis of starch grains prepared by a non-aqueous method from potato tubers stored at + 10° and + 2° indicated that a large part of the K, Na, Cl, citrate and glucose-6-phosphate was inside the amyloplast but that the sugar (storage at + 2°) was outside; sweetening therefore involved the transport of metabolites through the amyloplast membrane. Comparison with other treatments (anaerobiosis, cyanide, ethylene chlorhydrin) which cause sweetening suggested that the regulation of the starch-sugar interconversion was effected at the amyloplast membrane and possibly involved electron transfer. In the case of potatoes which sweetened due to senescence, electron micrographs showed that the amyloplast membranes were disintegrating.