We recently showed that spinach (Spinacia oleracea L.) transplants produced under a short photoperiod and low air temperature were characterized by a delay of bolting and short flower-stalk length at harvest (Chun et al., 2000a). The present study was conducted to determine whether these changes are caused by the short photoperiod itself or by the lower integrated photosynthetic photon flux (IPPF). Shoot and root dry weights of transplants increased significantly with increasing IPPF, but were not affected by a change in the photoperiod. However, the floral development indices of transplants were significantly greater under a 16-than under a 10- or 13-hours/day photoperiod, but were not affected by a change in IPPF. The percentage of bolted plants 3 days after transplanting (DAT) increased significantly with increasing photoperiod (from 0% at 10 hours/day to more than 85% at 16 hours/day). Flower-stalk length increased with increasing photoperiod (e.g., at 14 DAT, from 15 mm at the shorter photoperiods to 80 mm at 16 hours/day), but was not affected by a change in IPPF. These results show that the delay of bolting that occurs when the photoperiod is reduced during transplant production is due to the delay of floral development and not to retarded vegetative growth as a result of reduced IPPF.