Longer photoperiod combined with lower intensity: strategies to improve the growth and resource use efficiency of baby-leaves grown in a vertical farm system

Baby-leaf vegetable consumption has gained a prominent role in the food market in recent years, and they represent one of the most widely cultivated crop categories in vertical farms. Considering the novelty of vertical farming, assessments of baby-leaf cultivation with additional strategies to improve growth and resource use efficiency are needed. This study, carried out in the experimental vertical farm of the University of Bologna “AlmaVFarm”, aimed at evaluating the responses of baby-leaf kale (Brassica oleracea L.), in an aeroponic growing system applying different lighting conditions. The environmental parameters of the growing space were set at 850 ppm of CO2, relative humidity at 70/75±10% day/night and air temperature maintained at 24/21°C day/night. The light setting consisted of the same spectrum, a red and blue (RB) mixture with a RB ratio of 3:1 and the same daily light integral which overall provided, on average, 11.9 mol m-2 day-1. The different light treatments consisted of different combinations of light intensities and photoperiods: 24, 20, 16, 14 h combined with 140, 165, 210 and 235 μmol m-2 s-1, respectively. Plants were harvested 21 days after sowing, at the baby-leaf stage. Plants cultivated under the longest photoperiod in combination with the lowest intensity (24 h at 140 μmol m-2 s-1) showed enhanced yield, lighting energy use efficiency and surface use efficiency compared to plants grown with the shortest photoperiod and highest light intensity (14 h at 235 μmol m-2 s-1). Through the insights emerging from this study, further evidence on how to improve resource use efficiency of vertical farming systems is provided.