This experiment aims to quantify the effects of the water vapor-pressure deficit (VPD) on growth and quality of culinary herbs and lettuce grown inside controlled environments. We postulated that growth will be inhibited at very low and very high VPDs when all other environmental factors are kept constant. We sowed seeds of holy basil (Ocimum tenuiflorum), chamomile (Matricaria chamomilla), lettuce (Lactuca sativa ‘Rex’), mint (Mentha spicata), and lemon balm (Melissa officinalis) in plug trays containing a peat-based soilless substrate. Seedlings grew inside refrigerated chambers at 23°C, with 24-h photoperiod at a photosynthetic photon flux density of 160 µmol∙m–2∙s–1, at five VPD setpoints (kPa): 0.3, 0.7, 1.1, 1.5, and 1.9. We fertigated plants with reverse-osmosis water at a constant fertility of 60 mg N∙L–1. We measured the total leaf area, leaf number, stem length, fresh shoot biomass, and dry shoot biomass, and monitored leaves for tip-burn symptoms. Preliminary results indicate that leaf area and fresh mass of most species were lower at a VPD of 0.3 kPa than at ≥0.7 kPa. In some species (e.g., mint), plants were shortest at the lowest VPD tested, whereas in others (e.g., chamomile), stem length decreased as the VPD increased. These results indicate the importance of managing VPD in controlled environments and especially the potential decreases in growth under low a VPD (i.e., high relative humidity).
