The Influence of Endogenous Sugar on Potato Tuberization in In Vivo Conditions

Sugar (S) and nitrogen (N) play critical roles in potato tuberization and development, which determines the number, size, and yield of tubers. However, the dynamic models of endogenous sugar and nitrogen during tuberization and development are poorly understood. A pot experiment was performed with the potato cultivar Jizhangshu 12. Tuber and leaf samples were continuously taken to detect the morphological changes in stolon or tuber during tuberization and development. Sugar, starch, nitrogen content, and sucrose metabolism-related enzyme activity were measured. Results showed that the starch, fructose, and sucrose content and S/N ratio in leaves decreased significantly, while significant increases were observed in the activities of alkaline invertase (ALI), sucrose synthetase (SS), and sucrose phosphate synthase (SPS), and the ratios of glucose/sucrose and glucose/fructose from stolon hook initiation to subapical swelling. In contrast, nitrogen content and S/N ratio decreased significantly and starch and glucose content, AI (acid invertase), ALI, SS, and SPS activities, and glucose/sucrose ratio in stolons increased significantly. When the diameter from the stolon subapical swelling to the tuber reached 3.0 cm, starch content and glucose/sucrose and glucose/fructose ratios in leaves and tubers, and contents of fructose and sucrose and the activities of AI and SPS in tubers fluctuated; the S/N ratio in both leaves and tubers increased significantly; the activity of SPS in leaves, and activities of ALI and SS in tubers declined significantly. When the tuber diameter increased from 3.0 to 6.0 cm, the content of starch, sucrose, fructose, and total nitrogen, the activities of AI, ALI, and SPS, and the S/N ratio in leaves increased, while the ratios of glucose/sucrose and glucose/fructose showed an opposite trend; the changes in tubers were slight. We conclude that the content of sugar and total nitrogen and the balance of sugar and nitrogen in leaves and stolons changed significantly at tuberization and gradually thereafter. The significant increases in the ratios of glucose/fructose and glucose/sucrose in leaves could be used as indirect indicators to determine tuberization. The ratio of S/N could replace that of nonstructural carbohydrate/nitrogen to characterize potato tuberization and development.