Effects of ozone on the growth and yield of rice (Oryza sativa L.) under different nitrogen fertilization regimes

To examine whether the sensitivity of growth and yield of rice (Oryza sativa L.) to ozone (O₃) varies under different nitrogen (N) fertilization conditions, rice cultivar 'Koshihikari' was exposed to O₃ under either standard N (SN) fertilization or no N (NN) fertilization. The rice plants were subjected to three gas treatments (charcoal-filtered air (CF) and O₃ at 1.0 (1.0×O₃) and 1.5 (1.5×O₃) times the ambient concentration) in combination with two conditions of N fertilization. At five time points throughout the growth period, plant samples were collected to measure the leaf area and dry mass of each plant organ. At the final harvest, yield, yield components, and harvest index were measured. There was a significant interactive effect of O₃ and N on leaf, stem, root, and whole-plant dry mass at the final harvest. The dry mass of each plant organ and the whole-plant dry mass of rice plants grown in 1.5×O₃ were significantly lower than those in the plants grown in CF and 1.0×O₃ under SN, whereas there were no significant differences in the dry mass among the three gas treatments under NN. Brown rice yield was significantly reduced by the exposure to O₃ under SN, but not under NN. Relative yield loss rate based on the AOT40 (accumulated exposure over a threshold of 40 nmol mol^-1) was pronounced under SN, whereas relative yield was almost unchanged at different AOT40 levels under NN. We concluded that the sensitivity of growth and yield of rice to O₃ is dependent on N levels in the soil; the exposure to ambient levels of O₃ has a negative effect on rice under SN, but not under NN.