Shortened Light/Dark Cycles Provide Conceptual Basis for Designing Cost-Effective Lighting Regimes for Controlled Environment Horticulture

The effects of shortened light/dark (L/D) cycles (8/4 and 4/2 h) on the growth, productivity, and nutritional value of baby leaf vegetables (borage, fenugreek, pea, and sunflower) and microgreens (arugula, broccoli, mizuna, and radish) were evaluated under controlled environment. Control plants were cultivated under 16/8 h photoperiod. The intermittent lighting influenced the quantitative and qualitative parameters of plant growth and yield (fresh weight, robustness index, dry matter content, nutritional value), yet plant responses were species specific. It is suggested that plant response to shortened L/D cycles depends on their tolerance/sensitivity to circadian asynchrony, which occurs when external cycles do not coincide with the endogenous circadian rhythms. In areas with time-of-use electricity rates, scheduling lighting during off-peak hours can significantly reduce production costs. It was shown that shortened L/D cycles can provide a conceptual basis for designing cost-effective lighting regimes based on electricity tariffs with peak, standard and off-peak time bands. Thus, intermittent irradiation regime with customized periods (7/4/9/4 h) (150 µmol m–2 s–1 PPFD) has been shown to enhance yield and quality of studied crops or not affect them under equivalent energy consumption while based on theoretical scenario analysis cost decreased by 25–45% compared to conventional 16/8 h photoperiod by maximizing the use of off-peak hours.