To understand the interactive effects of O 3 and CO 2 on rice leaves; gas exchange, chlorophyll (Chl) fluorescence, ascorbic acid and glutathione were examined under acute (5 h), combined exposures of O 3 (0, 0.1, or 0.3 cm 3 m -3 , expressed as O 0 , O 0.1 , or O 0.3 , respectively), and CO 2 (400 or 800 cm 3 m -3 , expressed as C 400 or C 800 , respectively) in natural-light gas-exposure chambers. The net photosynthetic rate (P N ), maximum (F v /F m ) and operating (F q '/F m ') quantum efficiencies of photosystem II (PSII) in young (8 th ) leaves decreased during O 3 exposure. However, these were ameliorated by C 800 and fully recovered within 3 d in clean air (O 0 + C 400 ) except for the O 0.3 + C 400 plants. The maximum PSII efficiency at 1,500 μmol m -2 s -1 PPFD (F v '/F m ') for the O 0.3 + C 400 plants decreased for all measurement times, likely because leaves with severely inhibited P N also had a severely damaged PSII. The P N of the flag (16 th ) leaves at heading decreased under O 3 exposure, but the decline was smaller and the recovery was faster than that of the 8 th leaves. The F q '/F m ' of the flag leaves in the O 0.3 + C 400 and O 0.3 + C 800 plants decreased just after gas exposure, but the F v /F m was not affected. These effects indicate that elevated CO 2 interactively ameliorated the inhibition of photosynthesis induced by O 3 exposure. However, changes in antioxidant levels did not explain the above interaction.

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The role of ascorbic acid on acute O 3 -induced inhibition of photosynthesis in solution-cultured paddy rice was evaluated. As pre-treatment, ascorbic acid (0, 5, and 10 mM) was added to the culture solution for 5 d before 5 h of O 3 exposure (0, .1, and .3 cm 3 m −3 O 3 ) during daytime. O 3 decreased photosynthesis-related parameters, total ascorbic acid content, and the redox state (RDS) of ascorbic acid. Ascorbic acid treatment enhanced the total ascorbic acid contents and its RDS level of rice leaves, but scarcely ameliorated O 3 -induced inhibition of photosynthesis-related parameters. Inhibition of net photosynthetic rate (P N ) by O 3 was slightly ameliorated by exogenous ascorbic acid only at 1 d after O 3 exposure. These results indicate that ascorbic acid is a component of protection from O 3 injury but has a marginal role in the acute inhibition of P N by O 3 in rice leaves.Abbreviations: AA, reduced form of ascorbic acid; CO 2 , carbon dioxide; DHA, dehydroascorbic acid; F q ′/F m ′, operating quantum efficiency; F v /F m , maximum quantum efficiency; g s , stomatal conductance; O 3 , ozone; P N , net photosynthetic rate; PPFD, photosynthetic photon flux density; PSII, photosystem II; RDS, redox state; VPD, vapor pressure deficit this is an open access article distributed under the terms of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Because the transient O 3 injury of leaves is lost with time, the evaluation of O 3 effect on the maximum quantum efficiency of PSII (F v . The effects of O 3 on the xanthophyll cycle-dependent quenching (fast relaxation phase) of q I disappeared when the dark adaptation period was greater than 20 min. However, it was difficult to distinguish the effects of O 3 and other factors (e.g., light) before D 5. The current results demonstrate that the optimum dark adaptation period in rice leaves is 10 min because the effect of O 3 remains maximal, while the effects of other factors on F v /F m disappear during this period. By accurate measurement of F v /F m , the physiology of O 3 effect on PSII in rice leaves is precisely evaluated.