Studies on Effect of Planting Density on the Growth and Yield of Sweet Potato : Ipomoea batatas Lam.

Studies on the effect of planting densities of sweet potato on the above ground in term of dry matter production were executed. The two main crops of sweet potato cultivars Norin No.16 and Shiroyutaka and the two levels of planting density (35 and 70cm X 100-cm spacing) were examined. The differences between the two densities in LA per m^2 were evident in cv. Shiroyutaka. The LA per m^2 of cv. Shiroyutaka under high planting density increased progressively from early to 100 DAP and slightly declined until 150 DAP. It was due to leaf size and leaf length characteristic to cv. Shiroyutaka. Increasing planting density reduced the number of branches per plant in both cultivars. This relationship indicates that branch formation in sweet potato plant is highly plastic, responding to the space which is available during the growing season. Top-DW per plant and per m^2 for both cultivars grown under both densities was decreased after 100 DAP until 150 DAP. On the other hand, tuberous root-DW per plant and per m^2 continuously increased during this period. It was due to the fact that hot day and cool night temperatures during autumn caused leaves to fall and the activity of stems had already been lost at the later growth stages. We considered that starch contents of stems restricted their translocation to tuberous root under ground. No difference was observed in partitioning ratio to the leaves and stems between both cultivars and densities. Sweet potato cv. Shiroyutaka produced higher yields than cv. Norin No.16. Sweet potato cv. Norin No. 16 shows the higher total and marketable yields per plant and per area under low planting density, while cv. Shiroyutaka shows the higher total and marketable yields per ha under high planting density. No differences were observed in total and marketable yields per plant between densities in cv. Shiroyutaka. There was no difference in total and marketable tuber numbers per plant and per m^2 between both densities and cultivars. High planting density increased the average tuber weight (g per tuber) of the total and marketable tubers in cv. Shiroyutaka