Effects of O₃ and CO₂ on Photosystem II, Nitrate Reductase and Nitrite Reductase in Paddy Rice Leaves

Abstract

To evaluate the interaction between O₃ and CO₂ on photosynthesis and nitrogen metabolism in rice, combined exposures of O₃ [0, 0.1, 0.3 cm³ m⁻³ (abbreviated as O⁰, O^0.1, O^0.3)] and CO₂ [400, 800 cm³ m⁻³ (abbreviated as C⁴⁰⁰, C⁸⁰⁰)] were applied for 5 h using environment-controlled chambers with natural-light. Subsequently, the plants were raised for 3 d under clean air (O⁰+C⁴⁰⁰) conditions. Immediately before (BE), immediately after (AE-0), and 1 and 3 d after (AE-1, AE-3) gas exposure, the chlorophyll fluorescence of photosystem II (PSII) and activities of nitrate (NR) and nitrite (NiR) reductase were determined. Results show that maximum (F_v/F_m) and operating (F_q'/F_m') quantum efficiencies and NiR activity were inhibited by O₃, but they were ameliorated by elevated CO₂, suggesting that the O₃-inhibition of photosynthesis accompanied the reduced electron transport toward the downstream of photosystem and suggesting that the reduction power became deficient. Because NiR locates in the chloroplast and uses reducing power from PSII, it must receive a greater O₃-inhibition than cytoplasmic NR does.