CHANGES OF CARBOHYDRATES, METABOLITES AND ENZYME-ACTIVITIES IN POTATO-TUBERS DURING DEVELOPMENT, AND WITHIN A SINGLE TUBER ALONG A STOLON-APEX GRADIENT

The metabolism of C-14-sucrose was investigated in different regions of a growing potato (Solanum tuberosum) tuber attached to the mother plant, by injecting high-specific activity [(UC)-C-14]-sucrose into a fine channel bored in an otherwise intact tuber. The [(UC)-C-14]-sucrose was metabolised more rapidly in the apical region than in the basal (stolon) region. Only small, and statistically non-significant, changes were found in the fate of the metabolised label. The activity of sucrose synthase, other enzymes involved in sucrose and starch metabolism, glycolytic enzymes, and the contents of soluble sugars, UDP-glucose, glycolytic intermediates, and fructose-2,6-bisphosphate were compared along the longitudinal axis of potato tubers. These parameters were also compared in different tubers harvested from a single plant, and in batches of tubers harvested from mother plants of different ages. Sucrose synthase decreased 2-4 fold along the longitudinal axis of a tuber from the apical to the basal region. It also decreased 50-fold between tubers from growing mother plants to senescing mother plants. The individual tubers from growing mother plants had relatively comparable sucrose synthase activities, but a 20-fold variation was found in tubers harvested from a senescent plant. Most other enzymes and metabolites were constant within and between tubers, except the changes of fructose-2,6-bisphosphate which were parallel to sucrose synthase in most tubers, and small trends in ADP-glucose pyrophosphorylase, fructokinase, UDP-glucose, and the pyruvate/phosphoenolpyruvate ratio. The sucrose content declined 2-3 fold from the apical to basal region of individual tubers. However it varied greatly between individual tubers. Glucose showed large and unpredictable changes which did not correlate with sucrose content, activities of invertases or hexokinase, or any other investigated parameters.