Submicron ammonium sulfate particles deposited on leaf surfaces of a leafy vegetable (Komatsuna, Brassica rapa L. var. perviridis) are taken up by leaf and enhance nocturnal leaf conductance

Ammonium sulfate (AS) particles of submicron size are a major component of inorganic aerosol, and are regarded as a long-range transboundary air pollutant in Asia. However, only a few studies have focused on the effects of submicron AS particles on plants. In the present study, we investigated the amount of submicron AS particles deposited on the leaf surface of komatsuna plants (Brassica rapa L. var. perviridis), and their effects on nocturnal leaf (gnight) and cuticular (gcut) conductance. The plants were grown for 23 days after sowing in a naturally lit phytotron at air temperature of 25.0 ± 1.0 °C/18.0 ± 1.0 °C (6:00-18:00/18:00-6:00) and relative air humidity of 70 ± 5%, and were artificially exposed to submicron AS particles at the 21st day after sowing. The exposure to submicron AS particles resulted in average daily atmospheric concentration of SO42− in PM2.5 inside the phytotron of 16.5 μg m−3. After the end of exposure, the plants were kept under light or dark conditions for 48 h in the phytotron. The amount of SO42− deposited on the leaf surface reduced over time, but did not significantly differ between the plants kept under light and dark conditions at 0, 6, 12, 18, 30, and 48 h after the end of exposure. An exposure to submicron AS particles with average daily atmospheric concentration of SO42− in PM2.5 inside the phytotron of 24.7 μg m−3 did not significantly affect the gcut of the plants. At 1.5 and 5 h after the end of exposure, the gnight was significantly higher in the submicron AS particle-exposed plants than in the non-exposed control plants. These results indicate that submicron AS particles deposited on the leaf surface enhance the nocturnal water consumption of komatsuna plants.