Air Anion Improves Lettuce Growth in a Plant Factory

Air anion is known to be beneficial for plant growth because it may improve photosynthesis and the respiration process of plants. However, there is limited information for the effect of air anion on plant growth. The objective of this study was to determine the effect of air anion on lettuce growth and photosynthesis under a plant factory which controls environmental conditions. Red leaf lettuce (Lactuca sativa L. cv. ‘Jeokchima’) seedlings grown under normal growth conditions (20°C, fluorescent lamp, 150 ± 3 µmol∙m^-2∙s^-1, 12 h photoperiod) for 18 d were transplanted to NFT systems in a plant factory equipped with LEDs (Red:Blue=78 : 22, 184 ± 2 µmol∙m^-2∙s^-1, 12 h photoperiod). Three levels of air anion (low 1×10⁴~5×10⁴, middle 7×10⁵~12×10⁵, and high 15×10⁵~20×10⁵ ION∙cc^-1) were applied to lettuce transplanted using high voltage air anion generators for 4 weeks. Lettuce plants exposed to air anion showed vigorous growth. Both middle and high air anion improved growth characteristics such as leaf area and fresh weight of shoots although there was no significant difference between air anion treatment and control in leaf shape index and SPAD value. Especially, middle level of air anion led to 64% and 44% higher shoot fresh weight and leaf area than control at harvest, respectively. In addition, the photosynthetic rate in middle level of air anion was 30% higher than that in control at 3 weeks of anion treatment, which supported the result of shoot fresh weight. Regarding energy use efficiency, middle level of air anion also showed the highest value. In conclusion, this study suggested that the application of air anion would be a useful technique for enhancing crop yield in plant factories.