Light plays a crucial role in the development and growth rate of crops by impacting their physiology, morphology as well as the electricity cost for the system's operation which can significantly jeopardize the viability of a commercial-scale system. Different crops have different lighting demands in terms of quality (spectrum), quantity (intensity) and duration (photoperiod). At the same time only a little information is available in the literature concerning the minimum lighting requirements for indoor horticulture. This research aims to define the optimal combination of photoperiod, photosynthetically photon flux density (PPFD) and spectrum for indoor cultivation of Valerianella locusta plants by linking light and energy use efficiency to physiological responses of chlorophyll content and biomass production under different daily light integrals (DLI). Plants were cultivated in two growing towers with three layers of cultivation area under white LED lamps (WHITE) and two identical growing towers with three layers of cultivation area under blue/green/red/far-red lamps (BGRFR). The experimental setup is comprised of three different photoperiods (16, 14, 12 h · d−1) and three different PPFD (150, 250 and 350 μmol/m2s1), resulting in five daily light integrals (8.6, 10.8, 12.6, 14.4, 20.2 mol· m−2 · d−1). All growing towers and layers were equipped with a close-loop Ebb & Flow hydroponic system. Based on data analysis, spectra selection seems to have a significant impact on chlorophyll content, while lower DLIs did not correlate with lower chlorophyll content. The chlorophyll content was found to be similar either the plants received longer daily light hours or/and higher light intensity. Additionally, plants that grew under DLI 10.8 and 12.6 mol/m2d1 resulted in significantly higher fresh and dry biomass while at the same time required significantly lower electricity requirements due to the reduced hours of artificial lighting (14 and 12 h· d−1 respectively). Accordingly, light operation for Valerianella locusta plants under PPFD of 250 μmol/m2s1, 14 h of photoperiod and BGRFR spectra selection could be characterized as optimal for indoor cultivation to optimize the use of resources and ensure high-yield production, considering the specific setup of the cultivation system.
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