Relation between O₃-inhibition of Photosynthesis and Salicylic Acid in Paddy Rice Grown with Different CO₂ Concentrations

Abstract

This study was conducted to evaluate the role of salicylic acid (SA) on acute ozone (O₃: 0, 0.1, and 0.3 cm³ m⁻³; O⁰, O^0.1 and O^0.3, respectively)-induced photosynthetic inhibition of paddy rice leaves given different atmospheric carbon dioxide concentrations (CO₂: 400 and 800 cm³ m⁻³; C⁴⁰⁰ and C⁸⁰⁰, respectively). Salicylic acid solutions (0, 0.1, and 1 mM; S⁰, S^0.1, and S¹, respectively) were applied as a pretreatment one day before O₃ exposure. Gas exchange, chlorophyll fluorescence, and ascorbic acid were measured immediately before (BE), immediately after (AE-0), and 1 d and 3 d after (AE-1, AE-3) 5-h O₃ exposure. The photosynthesis-related parameters, total ascorbic acid, and redox state of ascorbic acid (RDS) were decreased by O₃ exposure. The O₃-induced reduction of P_N, ascorbic acid content, and its RDS were ameliorated by C⁸⁰⁰. Salicylic acid ameliorated the O₃-inhibition of net photosynthetic rate (P_N) slightly, and O₃-induced depletion of total ascorbic acid and RDS substantially. SA did not increase P_N in non-treated leaves (O⁰ plants). However, O₃ exposure elevated the level of endogenous SA. These results show that SA plays a vital role in the defense response to acute O₃ exposure in paddy rice. Effects of SA on O₃-inhibition of PSII and ascorbic acid content were unaffected by elevated CO₂ (C⁸⁰⁰).