Successful cultivation of lettuce in indoor agriculture relies on precise control of environmental factors to optimize crop yields. However, some controlled environment facilities may lose control of humidity due to the net moisture input of transpiration from high-density lettuce overwhelming dehumidification and cooling systems. This study observed the effects of different lighting treatments and air velocities on lettuce at varying levels of humidity as a means of mitigating deleterious effects of elevated humidity. Romaine lettuce (Lactuca sativa, cv. Teton) was cultivated in three growth chambers under three different humidity levels (70%, 80%, and 90%). Each room contained three hydroponic trays setup up for ebb and flow, with one lighting treatment per tray: white light (W), white light supplemented with far red (WFR), and wide amber supplemented with far red (WA). Additionally, each tray contained a low and high velocity zone at 0.5 and 1 m/s respectively, delivered by a polyethylene tube attached to a duct fan. Average velocity ranged from 0.23–3.0 m/s, but no correlation between air velocity and any growth parameter was observed for any treatment. However, tipburn for all treatment combinations was either less severe or eliminated altogether. This can be attributed to the general improvements in plants’ transpiration rates as the air velocity increased. Increasing humidity enhanced growth parameters except for fresh mass, where 70% and 90% RH did not differ significantly but both outperformed 80% RH by 9%. For other metrics—height, head diameter, chlorophyll content, and dry mass—80% RH and 90% RH performed better than 70% RH but did not significantly differ. WA performed the best for all growth parameters, with 7.4% and 14% greater leaf area than W and WFR respectively. Compared to WA, WFR underperformed in most metrics despite also having far-red supplementation, indicating potentially more synergistic interactions of FR with amber wavelengths than blue or red wavelengths. Gas exchange results varied based on the treatments, but it was generally observed that photosynthetic rates were higher at higher air velocities and under WA. The results of this study indicate the effectiveness of air velocity levels up to 3.0 m/s at eliminating tipburn in lettuce up to 90% RH without causing additional stress responses. Further improvements can be achieved with WA supplemented with far-red, improving both morphological development and physiological processes even at higher humidity levels.
