Air Anions Improve Growth and Mineral Content of Kale in Plant Factories

Air anions affect plant growth by stimulating various biological mechanisms. We investigated the effect of atmospheric anion concentrations on plant growth and mineral concentration in kale (Brassica oleracea var. acephala cv. TBC) plants cultivated in a plant factory. Kale seedlings grown under normal growth conditions for two weeks were transplanted to a nutrient film technique (NFT) system in a plant factory equipped with light-emitting diodes (LEDs) [red:white:blue (RWB) = 8:1:1 and red:green:blue (RGB) = 8:1:1, 181 + - 4.0 micromol.m -2 .s -1 , 12 h photoperiod]. Three concentrations of air anions (low, 2.9 x 10 5 ions.cm -3 ; medium, 5.4 x 10 5 ions.cm -3 ; and high, 7.8 x 10 5 ions.cm -3 ) were applied to the kale seedlings for four weeks using high voltage air anion generators. The medium and high levels of air anions increased shoot fresh weight to approximately 1.5-fold compared to the control after four weeks. Medium and high-level air anion treatments led to significantly higher leaf area than the control. The medium level of air anions improved the photosynthetic rate at four weeks of treatment although there was no significant difference between air anion treatments and the control. In addition, transpiration rate and stomatal conductance were significantly increased in the low and medium levels of air anion treatments, which likely supported biomass accumulation. Air anions also increased mineral uptake. The content of macroelements (P, K, Ca, Mg, and S) was at least 1.5-fold higher for plants exposed to RGB LEDs and 1.3-fold higher under RWB LEDs exposure. Microelements (Fe, Mn, and Zn) were increased at least 1.6- and 1.3-fold in kale shoots treated with air anions under RGB and RWB LEDs, respectively. In conclusion, air anions had a positive effect on kale growth and air anion generators could be used as a new technology for enhancing plant growth in plant factories and greenhouses.