Cell division and cell enlargement during potato tuber formation

Cell division and cell enlargement were studied to reveal the developmental mechanism of potato tuberization using both in vivo and in vitro culture systems. Distribution of cells in S-phase was visualized by immunolabelling of incorporated bromodeoxyuridine (BrdU). Mitosis was detected in DAPI (4,6-di-amidino2-phenylindole) or toluidine blue-stained sections. Timing and frequency of cell division were determined by daily cell counting, and cell enlargement was deduced from measurements of cell diameters. Under in vivo condition, lateral underground buds developed into stolons due to transverse cell divisions and cell elongation in the apical region of the buds. At the onset of tuber formation, the elongation of stolons stopped and cells in pith and cortex enlarged and divided longitudinally, resulting in the swelling of the stolon tip. When tubers had a diameter of 0.8 cm, longitudinal divisions had stopped but randomly oriented division and cell enlargement occurred in the perimedullary region and continued until tubers reached their final diameter. In vitro tubers were formed by axillary buds on single node cuttings cultured under tuber-inducing conditions. They stopped growing at a diameter of 0.8 cm. Pith and cortex were involved in tuberization such as that found during the early stage of in vivo tuberization (< 0.8 cm in diameter). The larger size of in vivo tubers is, however, due to further development of the perimedullary region, which is lacking in in vitro conditions.