Application of Extended Lighting Cycles to Promote Vegetative Growth of Vegetables Grown in Closed Plant Production Systems

Abstract

In the present study, extended lighting cycle (ELC), a novel lighting pattern was proposed for the production of vegetables in closed plant production systems where artificial lights were adapted as a sole lighting source. The effects of ELC including continuous lighting (CL) on growth of head lettuces, leaf lettuces, Chinese cabbages, pak-chois, hot peppers and tomatoes were examined in the first chapter of this dissertation. When they were grown under various lighting cycle conditions

CL, 16, 24, and 32-h photoperiods combined with an 8-h dark period, and 20, 30 and 40-h photoperiods combined with a 10-h dark period, plant growth and light use efficiency (LUE) increased as the ratio of photoperiod to lighting cycle increased at the earlier growth stage. After that, the greater ratio of photoperiod to lighting cycle, the earlier growth inhibition occurred except head lettuces. In tomatoes growth retardation and chlorosis occurred within 14 days after sowing (DAS) in treatment CL. Those symptoms were found in other ELC treatments and they occurred earlier and more seriously as the ratio of photoperiod to lighting cycle increased. Occurrence time and the severity of those symptoms were varied by plant species, too. Tomatoes were most susceptible to ELC-related symptoms among the six tested vegetable, followed by hot peppers, pak-chois, Chinese cabbages, leaf lettuces, and head lettuces. In head lettuces those symptoms were not found until the end of experiment (28 DAS). In the second chapter, the effect of insertion of dark period in the middle of continuous lighting to avoid growth inhibition caused by continuous lighting in tomatoes was examined. Tomato seedlings were grown under various lighting scenarios

four different insertion timings of dark period (changed from CL to 16/8-h lighting cycle at 6, 8, 10, and 12 DAS) and six different lengths of dark period inserted (8, 16, 24, 32, 40, and 48-h) after 10 days of CL. Growth inhibition caused by CL was avoided by insertion of dark period in the middle of CL. The growth of tomatoes was greatest when the lighting cycle condition was changed from CL to 16/8-h at 6 DAS, and the effectiveness of avoidance decreased as the insertion time of dark period were delayed. The effectiveness of the avoidance did not change, however, as the duration of dark periods inserted was prolonged from 8 up to 48 h. The results of this study imply that ELC or CL with insertion of dark period can be applied for rapid and efficient production of leafy and fruit vegetables and/or their transplants production in closed plant production systems using artificial lights.