Sweetpotato Propagule Production Rate and Electric Energy Consumption in a Closed Transplant Production System as Affected by Planting Density

Multiplication ratio (number of average of harvestable propagules per stock plant, M) and electric energy consumption per propagule (Ep) at four levels of planting density (59, 118, 236 and 473 m-2) were investigated for sweetpotato (Ipomoea batatas (L.) Lam.) vegetative propagation in a closed transplant production system. After n multiplication cycles, total number of harvestable propagules (Nn) and electric energy consumption per propagule (Ep, n) in the system were estimated by using mathematical models. Single nodal cuttings were used as propagules and grown to 14 days under photosynthetic photon flux of 140, 200 and 320μ mol·m-2·s-1 during 1 to 5, 6 to 10 and 11 to 14 days after planting, respectively. Other environmental conditions included photoperiod of 16 h·d-1, air temperature of 29°C, and CO2 concentration of 920μmol·mo1-1. At the planting densities of 59, 118, 236 and 473 m-2, M were 5.9, 6.4, 3.8 and 2.3, respectively, and Ep, 1 were 0.41, 0.19, 0.16 and 0.13 kWh (electricity cost : 6.2, 2.9, 2.4 and 2.0 Yen), respectively. Results suggest that production can be optimized and electricity cost can be reduced by manipulating planting densities.