Air Anions Promote the Growth and Mineral Accumulation of Spinach (Spinacia oleracea) Cultivated in Greenhouses

Recently, the positive effects of air anions on the growth of leafy vegetables cultivated in closed plant production systems have been widely reported, but there is limited research on the influence of air anions on plants in greenhouses. Therefore, this study was conducted to determine whether the application of air anions can stimulate various physiological processes, including the growth, photosynthesis, and mineral uptake of spinach grown in different seasons (spring and fall) in greenhouses. Spinach was treated daily with three concentrations of air anions (low [LA], 3.4 × 10 5 ion/cm 3 ; moderate [MA], 5.2 × 10 5 ion/cm 3 ; and high air anion [HA], 8.3 × 10 5 ion/cm 3 ). After 27 days of HA treatment during spring, the fresh weights of treated shoots and roots were at least 26% higher than of control shoots and roots. In fall, the shoot and root fresh and dry weights and the leaf area were significantly higher by at least 50% under all air anion treatments than those under the control treatment. The analyses of the photosynthetic rate, transpiration rate, and stomatal conductance supported the growth results of spinach. Although air anions did not affect the mineral uptake by plants in the spring, the accumulation of all minerals was increased by at least 130% in the fall; Fe ions particularly increased by up to 198% compared with those in the control. The results of this study were consistent with the results of a previous study on a closed production system and demonstrated the possibility of promoting plant growth and increasing production by air anion treatment in a greenhouse. Additional key words: crop productivity, electric field, photosynthesis, seasonal effect, stomatal opening, transpiration