Growth of Lettuce Plants Exposed to Air Anion in a Plant Factory

Air anion is known to be beneficial for plant growth because it may improve photosynthesis and respiration of plants. The objective of this study was to determine the effect of air anion on lettuce growth in a plant factory where environmental conditions for growth and development can be controlled. 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 hours photoperiod) for 18 days were transplanted to NFT systems in a plant factory equipped with LEDs (Red:Blue = 78 : 22, 184 ± 2 µmol∙m^-2∙s^-1, 12 hour photoperiod). Three levels of air anion (low 1×10⁴-5×10⁴, middle 7×10⁵–12×10⁵, and high 15×10⁵–20×10⁵ ION/cc) produced by high voltage air anion generators were applied to lettuce plants for 4 weeks. Lettuce plants exposed to air anion showed vigorous growth 2 and 4 weeks after the treatment. Both middle and high levels of air anion improved growth characteristics such as leaf area and the fresh weight of shoots and roots although there was no significant difference between air anion and control in leaf shape index, number of leaf, and chlorophyll content (SPAD value). As the concentration of air anion increased, leaf area gradually increased at 2 weeks after the onset of anion treatment. Middle level of air anion led to 54% and 45% higher shoot fresh weight than control 2 and 4 weeks after the treatment, respectively. In addition, root fresh weight significantly increased in middle and high levels of air anion at 4 weeks after the onset of anion treatment. In conclusion, this study suggested that the application of air anion in a plant factory has a positive effect on lettuce growth.