Enhancement of Tuberous Root Production and Adenosine 5'-Diphosphate Pyrophosphorylase (AGPase) Activity in Sweet Potato (Ipomoea batatas Lam.) by Exogenous Injection of Sucrose Solution

Adenosine 5'-diphosphate pyrophosphorylase (AGPase) is a key enzyme governing starch synthesis and is regarded as an important determinant of the sink activity of the sweet potato root. In this study, assuming that the expression of AGPase is under the direct or indirect control of sucrose, we investigated the effect of exogenous injection of sucrose solution into a plant on the activity of AGPase and tuberous root production. Sucrose solutions of 6 and 12 %, and distilled water as the control, were injected into the top of the shoot. The application of sucrose solution was effective in increasing tuberous root production and increasing the ratio of tuberous root weight to total root weight in a plant. AGPase activity in roots was enhanced by about 25 % by injecting sucrose solution. These results suggest that AGPase and sink activities are controlled by sucrose transported from the leaves. To increase sweet potato production effectively, AGPase activity and sink function must be enhanced, and so a genetic and physiological improvement in photosynthetic function or sucrose productivity in the leaves is necessary to increase AGPase activity in sink organs.