Generation of Negative Air Ions by Plants upon Pulsed Electrical Stimulation Applied to Soil

Generation of negative air ions (NAI) by pot plants (aloe Aloe arborescens, haworthia Haworthia rasalata, echinopsis Echinopsis tubiflora, mammillaria Mammillaria prolifera, opuntia Opuntia brunnescens, spider plant Chlorophytum comosum, and jade plant Crassula portulacea) was studied when high-voltage pulses were applied to soil. Plants that generated low amounts of NAI (such as jade plant and echinopsis) elevated NAI level 2–3 times over the mean background level. Plants that produced moderate amounts of NAI (haworthia, opuntia, and mammillaria) increased the NAI level hundredfold, whereas highly active plants (aloe and spider plant) increased the NAI level thousand times. Aloe plants can maintain constant NAI concentration in the ambient air (125 ± 15 × 10³ ions/cm³) for a long time (7–8 h). Negative air ions were predominantly generated by the leaf tips. The capacity of aloe to produce NAI considerably reduced with plant age and was lost, although reversibly, at temperatures below 8.5°C. The rate of NAI generation considerably depended on weather conditions. It was demonstrated that superoxide anion radical is one of NAI species generated by plants.