Inspecting the optimal photoperiod and lightintensity combination for indoor cultivationof leafy vegetables in a commercialvertical farm

In plant factories with artificial lighting (PFAL), light-emitting diodes (LEDs) are the sole light source, essential forphotosynthesis. Changing light features (e.g. spectrum, photoperiod and intensity) can optimize crop quality and yield, reduceenergy consumption and production costs and increase energy use efficiency (EUE). Individually inspecting light features hasbeen extensively explored, whereas comprehensive analyses of varying light features while maintaining a similar daily lightintegral (DLI, the total daily amount of mol received by plants) have been limited. This study aimed to investigate the influenceof 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 (Brassicaoleracea L.), pak choi (Brassica rapa L.) and lettuce var. canasta (L. sativa L.). The experiment took place in a commercialvertical farm equipped with full white spectrum LED light fixtures. Three different light regimes with a DLI (9.75 ± 0.15 mol m−2day−1) were used for the experiment, obtained by simultaneously varying photoperiod and light intensity, as follows: 14 h day−1and 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 showedhigher 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 wasassociated with the lowest L-EUE values. Conversely, differences in light use efficiency (LUE) were observed only in pak choiwith the shorter photoperiod (14/190), resulting in higher LUE values than in the continuous day (24/115) treatment. The use of24/115 and 18/150 is a valid strategy to increase L-EUE for the crops used in this study, but additional research with higher DLIis needed to further investigate the effects of different combinations of light intensity and photoperiod in commercial settings.