Effects of exogenous injection of different sugars on leaf photosynthesis, dry matter production and adenosine 5′-diphosphate glucose pyrophosphorylase (AGPase) activity in sweet potato, Ipomoea batatas (Lam.)

Solutions of sucrose, glucose and fructose were artificially injected into the stems of sweet potato plants. The effects of solution injection on both dry matter production and the activity of adenosine 5′-diphosphate glucose pyrophosphorylase (AGPase) in tuberous roots were investigated and compared. The total weight of carbon (T_c) artificially and photosynthetically supplied to a plant during the treatment period of 40 days was 0.987-1.869 times the weight of photosynthetically assimilated carbon alone. At the final sampling time, the dry matter weight of tuberous roots in the plants injected with sugar solutions showed a 2.73-9.13-fold increase over that of the control plant. The root weight linearly increased with T_c. The activity of AGPase was also enhanced by solution injections, with 27-63 % increases compared to the control, but was not significantly related to T_c. The injection of sugar solutions is concluded to have a dual effect on root production in sweet potato. One effect is that the increased sugar concentration in the plant increases AGPase and sink activities, and the other effect is that the increased carbon supply quantitatively promotes starch synthesis and accumulation in roots.