Testing a ratio of photosynthesis to O3 uptake as an index for assessing O3-induced foliar visible injury in poplar trees

Visible foliar injury by ozone (ozone visible injury) is known as a biomarker to assess potential phytotoxicity of ozone. We investigated ozone visible injury in an ozone-sensitive poplar (Oxford clone) under a 2-year free-air controlled exposure (FACE) experiment and calculated three ozone indices (i.e., accumulative ozone exposure over 40 ppb during daylight hours (AOT40), phytotoxic ozone dose above a flux threshold of 0 nmol m⁻² s⁻¹ (POD₀), and the cumulative value of the ratio of hourly ozone uptake to net photosynthesis (ΣU/P ₙ) to assess the critical level (CL) at the time of the first symptom onset of ozone visible injury. We tested the hypothesis that ozone injury depends both on the amount of ozone entering a leaf and on the capacity for biochemical detoxification or repair with photosynthesis as a proxy. The CLs at the time of the first symptom onset of ozone visible injury were 19 ppm h for AOT40, 26 mmol m⁻² for POD₀, and 1.2 mol mol⁻¹ for ΣU/P ₙ in Oxford clone at the ozone FACE experiment. Our findings were then verified by 4-year observation-based data in central Italy on Oxford clone and white poplar (Populus alba L.). These observation-based data indicated that we found ozone visible injury in Oxford clone even though AOT40 was relatively low (11.7 ppm h). On the other hand, when values of POD₀ and ΣU/P ₙ exceeded over the CLs, the occurrence of initial symptoms in Oxford clone was shown. White poplar did not show ozone visible injury. ΣU/P ₙ of white poplar at the field sites reached ~1.0 mol mol⁻¹ (less than the CL = 1.2 mol mol⁻¹, which was obtained from O₃ FACE) during May–September, although the values of POD₀ were relatively high in white poplar (44–47 mmol m⁻² during May–September). The result implies that ozone injury may have occurred in poplars when stomatal ozone flux exceeded the critical range of tolerance due to the assimilate shortage for repair and defense against ozone stress.