Lighting Patterns Regulate Flowering and Improve the Energy Use Efficiency of Calendula Cultivated in Plant Factories with Artificial Lighting

Calendula is an edible flower with highly beneficial bioactive compounds for human health. Environmental factors such as light influence flower yield and quality. Calendula is cultivated under controlled environments in plant factories with artificial lighting (PFALs), which enhance its productivity. However, electricity is the main operating cost, with fees based on the time of use in some countries. This study aimed to investigate the effects of lighting patterns on calendula growth and yield. Two varieties of calendula seedlings were cultivated in a PFAL and subjected to six different lighting patterns, i.e., 6 h/6 h, 12 h/12 h, 6 h/2 h, and 18 h/6 h (light/dark) and two continuous lighting patterns with varying light intensities (24 h-200 and 24 h-400). The results indicated that plants cultivated under the 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 conditions showed a more rapid appearance of the first flower bud than those cultivated under the 6 h/6 h and 12 h/12 h conditions. The number of flowers and the fresh and dried weights tended to increase with a longer photoperiod. Interestingly, the total carotenoid content (TCC) of “Citrus Orange” increased under 6 h/6 h and 12 h/12 h conditions compared with the others. For “Orange Gem”, continuous lighting (24 h) increased the total phenolic content (TPC) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity in flowers compared with the 6 h/6 h 12 h/12 h, and 6 h/2 h treatments. The energy use efficiency (EUE) under the 24 h-200 condition was the highest in terms of flower yield and secondary metabolite production. These results suggest that lighting patterns can be used to modulate the growth and flowering of calendula and to maximize EUE.