Indoor cultivation of baby-leaf kale under constant DLI: Increasing photoperiod with lower PPFD as a strategy to improve crop growth in an aeroponic system

In vertical farming, Light Emitting Diodes (LEDs) allow for specific manipulation of light conditions according to plant needs. Not only do the different spectral regions affect plant growth, but light quantity and duration also influence photosynthetic activity, leaf expansion and dry mass accumulation. Light supplied at a lower photosynthetic photon flux density (PPFD) with an elongated photoperiod was shown to promote growth in various species, compared to providing higher PPFD for fewer hours in a day. Managing the photoperiod and PPFD combinations while maintaining the daily light integral (DLI) constant could be a strategy to boost crop growth. However, additional evidence is needed concerning the optimal combination of light PPFD and photoperiod in under-explored species of growing commercial interest like baby-leaf kale cultivated indoors. This work was carried out in AlmaVFarm, the experimental vertical farm of the University of Bologna. Baby-leaf kale growth in an aeroponic system was tested for a growing cycle of 21 days. In particular, four LED lighting conditions were compared, resulting from various mixtures of light PPFDs and photoperiods, while maintaining constant the DLI at 11.9 mol m-2 d-1 and a standard spectrum with a red and blue (RB) ratio of 3:1. The combinations consisted of increasing photoperiods matched with decreasing light PPFDs: 24 h d-1 at 140 μmol m-2 s-1 (24140), 20 h d-1 at 165 μmol m-2 s-1 (20165), 16 h d-1 at 210 μmol m-2 s-1 (16210) and 14 h d-1 at 235 μmol m-2 s-1 (14235). Plants cultivated under 24140 displayed the highest dry weight (DW) as well as leaf area (LA) and light use efficiency (LUE) compared to other treatments. When the effects of continuous lighting (24140) in comparison with control treatment (16210) were analysed, an increase (+49%) in plant DW was observed. From shoot DW, it emerged that the 24140 treatment significantly increased (+18%) the leaf area ratio (LAR) as a result of enhanced specific leaf area (SLA), rather than leaf weight ratio (LWR), which did not display differences between the two treatments. The study confirmed that in indoor-grown kale, supplying lower PPFD for a prolonged time could enhance plant growth and leaf expansion.