Carbohydrate metabolism and tissue differentiation during potato tuber initiation, growth and dormancy induction

The duration of potato tuber dormancy has economic importance for both ware potatoes and seed tubers. The aim of this study was to shed light on the time at which tuber dormancy is induced. Potato tubers were selected at different stages of tuberisation: initial swelling of the stolon tip and early stages of tuber growth (tuber diameter 3, 7 and 14 mm). At each stage of tuberisation, the diameter of the pith and the cortex was measured, the activity of the enzymes beta-amylase, glucose-6-phosphate dehydrogenase and succinate dehydrogenase was determined, and starch and RNA levels recorded. It was observed that during tuber initiation the pith and perimedullary zone showed the greatest increase in size, whereas the cortical parenchyma increased mainly when the tuber diameter was 7-14 mm. Moreover, during stolon swelling and initial tuber development (3 mm diameter) total RNA accumulation was observed. Starch accumulation varied with the stages of development. Glucose-6-phosphate dehydrogenase and succinate dehydrogenase exhibited their highest activity during stolon swelling whereas beta-amylase activity was highest both during stolon swelling and at the 3 mm diameter stage. From the changes in tuber anatomy/morphology and the assays of enzyme activity, it is clear that dormancy is not induced in all the tissues of the tuber at the time of tuber initiation, but is imposed on the individual tissues as they are formed. Consequently, we may refer to 'tuber dormancy' only when the last bud has completed its differentiation.