Inspecting the optimal photoperiod and light intensity combination for indoor cultivation of leafy vegetables in a commercial vertical farm

Abstract In plant factories with artificial lighting (PFAL), light-emitting diodes (LEDs) are the sole light source, essential for photosynthesis. Changing light features (e.g. spectrum, photoperiod and intensity) can optimize crop quality and yield, reduce energy consumption and production costs and increase energy use efficiency (EUE). Individually inspecting light features has been extensively explored, whereas comprehensive analyses of varying light features while maintaining a similar daily light integral (DLI, the total daily amount of mol received by plants) have been limited. This study aimed to investigate the influence of different combinations of photoperiod and light intensity on the yield, growth and lighting-energy use efficiency (L-EUE) while maintaining a constant DLI on four leafy vegetable crops, namely crispy lettuce ( Lactuca sativa L.), green kale ( Brassica oleracea L.), pak choi ( Brassica rapa L.) and lettuce var. canasta ( L. sativa L.). The experiment took place in a commercial vertical farm equipped with full white spectrum LED light fixtures. Three different light regimes with a DLI (9.75 ± 0.15 mol m −2 day −1 ) were used for the experiment, obtained by simultaneously varying photoperiod and light intensity, as follows: 14 h day −1 and 190 μmol m −2 s −1 (14/190), 18 h day −1 and 150 μmol m −2 s −1 (18/150), and 24 h day −1 and 115 μmol m −2 s −1 (24/115). Continuous light exposure (24/115) yielded the best morphological results for both lettuce cultivars, whereas pak choi showed higher leaf width at higher light intensity and a shorter photoperiod. L-EUE was not affected by light treatments in crispy lettuce, whereas differences were evident in the other three crops in which the shortest photoperiod with the highest intensity was associated with the lowest L-EUE values. Conversely, differences in light use efficiency (LUE) were observed only in pak choi with the shorter photoperiod (14/190), resulting in higher LUE values than in the continuous day (24/115) treatment. The use of 24/115 and 18/150 is a valid strategy to increase L-EUE for the crops used in this study, but additional research with higher DLI is needed to further investigate the effects of different combinations of light intensity and photoperiod in commercial settings. Significance of the study What is already known on this subject? Light intensity and photoperiod independently affect crop growth and energy-use efficiency in vertical farms. However, their combined effects at constant daily light integral (DLI) need to be deeply investigated, especially in commercial settings. What are the new findings? Reducing light intensity while extending the photoperiod at constant DLI improved lighting-energy use efficiency (L-EUE) by approximately two-fold in canasta lettuce, pak choi and green kale, without compromising yield. Species-specific morphological responses were observed, with lettuce showing increased plant height under continuous light. Additionally, pak choi exhibited higher light use efficiency (LUE) under shorter, more intense exposure. What are the expected impacts on horticulture? These results demonstrate that extending photoperiods at lower light intensities can reduce energy consumption in vertical farms without affecting productivity. The species-specific responses highlight the importance of developing crop-specific light recipes to optimize both yield and energy efficiency in indoor commercial production systems.