Number of sweetpotato [Ipomoea batatas] propagules produced, electric energy consumption and storage root yield after transplanting in the field, as affected by planting density in closed type transplant production system

Abstract Sweetpotato (Ipomoea batatas (L.) Lam., cv: Beniazuma) was propagated using single node cuttings as propagules, and grown to plug transplants at a planting density (D) of 225, 473, 840 and 1313 m sup(-2). Growth and development of stock plants and plug transplants, and the amount of electric energy consumed in producing them were investigated. One propagation cycle covered a period of 18 days, and the transplant growing period was 15 days. Photosynthetic photon fluxes were 100, 200 and 300mmol m sup(-2) s sup(-1) during the 0-4, 5-9 and more than 10 day periods after the onset of the experiment, respectively. Air temperature, relative humidity, photoperiod, CO2 concentration were maintained at 30 deg C, 75%, 16 h d sup(-1), 1000 mmol mol sup(-1) respectively during the experimental period. The number of propagules produced (N sub(n), n: number of repetitions of propagation cycle) and electric energy consumption per transplant (E subt(n)) when the cycle was repeated were estimated by using mathematical models with the data derived from the experiment. Additionally, growth and development of the plants after transplanting in the field were investigated. The increase in N sub(n) with increasing number of cycles was maximum at 225 m sup(-2) of D when the planting area in the closed system was unlimited. When the planting area in the closed system was limited to 500 square m, N sub(n) was maximum at 840 m sup(-2) of D after the actual planting area reached 500 square m. The E subt(n) value was lowest (1.15 MJ, 4.3-4.5 Japanese yen) at 840 m sup(-2) of D regardless of the planting area in the closed system. Storage root yield after transplanting in the field decreased with increasing D. For more efficient transplant production by using vegetative propagation in the closed system, it may be necessary to change D depending on the relative priorities in terms of the propagule production rate and electric energy consumption per transplant.