Effects of exogenous injection of sucrose solution to plant on the carbon distribution to tuberous root and tuberous root production in sweet potato, Ipomoea batatas Lam.

The sink and source balance in a sweet potato plant was artificially changed by exogenously injecting a 6% sucrose solution into the cut end of the stem, and its effect on the photosynthate transport from leaves to roots were studied. The results obtained were as follows : The 13CO2 feeding experiment revealed that the sucrose injection enhanced the distribution ratio of photosynthate in roots. The injection directly before the commencement stage of root tuberization was the most effective in increment of root production, with a significant increase in the activity of adenosine 5'-diphosphate glucose pyrophophorylase (AGPase) . On the other hand, little effect was shown in a plant injected past this stage. It may be considered that the sucrose solution fed at the adequate growth stage quickly moved down to increase the sugar concentration in roots, and this acted as a trigger for increase in the activities of enzymes such as AGPase related to starch synthesis and then enhancement of the sink activity. This may lead to the promotion of photosynthate transport to roots and increase of root yield. Our experimental results suggest that the timely enhancement of source activity is important to activate the sink function of sweet potato roots.