Ozone impairs the response of isoprene emission to foliar nitrogen and phosphorus in poplar

Ozone risk assessment with free-air controlled exposure (FACE) experiments: A critical revisit

Since risk assessments of tropospheric ozone (O3) are crucial for agricultural and forestry sectors, there is a growing body for realistic assessments by a stomatal flux-based approach in Free-Air Controlled Exposure (FACE) facilities. Ozone risks are normally described as relative risks (RRs), which are calculated by assuming the biomass or yield at zero O3 dose as "reference". However, the estimation of the reference biomass or yield is challenging due to a lack of O3-clean-air treatment at the FACEs and the extrapolation without data in a low O3 range increases the bias for estimating the reference values. Here, we reviewed a current methodology for the risk assessment at FACEs and presented a simple and effective way ("modified Paoletti's approach") of defining RRs just using biomass or yield data with a range of expected impacts under the FACE conditions hypothesizing three possible scenarios based on prediction limits using 95% credible intervals (CI) (1. Best fit using the intercept as reference, 2. Optimistic scenario using a lower CI and 3. Worst scenario using an upper CI). As a result, O3-sensitive species show a relatively narrow effect range (optimistic vs. worst scenario) whereas a wide range of response may be possibly taken in resistant species. Showing a possible effect range allows for a comprehensive understanding of the potential risks and its uncertainties related to a species sensitivity to O3. As a supporting approach, we also recommend to use scientifically relevant tools (i.e., ethylenediurea treatments; mechanistic plant models) for strengthening the obtained results for the RRs against O3. Interestingly, the moderately sensitive or resistant species showed non-linear rather than linear dose-response relationships, suggesting a need for the flexible functional form for the risk assessment to properly describe the complex plant response such as hormesis, which depends on their plasticity to O3 stress.

A three-year free-air experimental assessment of ozone risk on the perennial Vitis vinifera crop species

Tropospheric ozone (O3) is an oxidative air pollutant that promotes damage to several crops, including grapevine, which is considered moderately resistant to O3 stress. To study the O3 effect on this perennial crop species under realistic environmental conditions, a three-year experiment was performed using an innovative O3-FACE facility located in the Mediterranean climate region, where the target species, Vitis vinifera cv. "Cabernet sauvignon", was exposed to three O3 levels: ambient (AA), 1.5 × ambient (×1.5), and 2 × ambient (×2.0). A stomatal conductance model parameterization was conducted, and O3-exposure (AOT40) and flux-based indices (PODy) were estimated. An assessment of O3-induced visible foliar injury (O3_VFI) was conducted by estimating VFI_Incidence (percentage of symptomatic leaves per branch) and VFI_Severity (average percentage of O3_VFI surface in symptomatic leaves). Biomass parameters were used to assess the cumulative O3 effect and calculate the most appropriate critical levels (CL) for a 5% yield loss and for the induction of 5, 10, and 15% of O3_VFI. We confirmed that the O3 effect on this grapevine variety VFI was cumulative and that POD0 values accumulated over the two or three years preceding the assessment were better related to the response variables than single-year values, with the response increasing with increasing O3 level. The estimated CL for 5% yield loss based on the O3-exposure index was 25 ppm h AOT40 and 21 or 23 ppm h for a 10% of VFI_Incidence or VFI_Severity, respectively. The suggested flux-based index value for 5% yield loss was 5.2 POD3 mmol m-2, and for 10% of VFI_Incidence or VFI_Severity, the values were 7.7 or 8.6 POD3 mmol m-2, respectively. The results presented in this study demonstrate that O3 risk assessment for this grapevine varietyproduces consistent and comparable results when using either yield or O3_VFI as response parameter.

Visible Foliar Injury and Ecophysiological Responses to Ozone and Drought in Oak Seedlings

To verify the responses of visible foliar injury (VFI), we exposed seedlings of three oak species for 4.5 months in an open air facility, using differing ozone (O3) and drought treatments: O3 (three levels from ambient to ×1.4 ambient), and drought (three levels of irrigation from 40% to 100% field capacity). We related the accumulated phytotoxic O3 dose (POD1) and cumulative drought index (CDI) to the O3 and drought VFI and assessed growth increment (height, diameter, leaf number), biomass (of all organs), and physiological parameters: net photosynthesis per plant (Pn), photosynthetic nitrogen (PNUE) and phosphorus use efficiency (PPUE)). The results indicated that an increase in POD1 promoted O3 VFI in Quercus robur and Quercus pubescens, while Quercus ilex was asymptomatic. The POD1-based critical level at the onset of O3 VFI was lower for Q. robur than for Q. pubescens (12.2 vs. 15.6 mmol m−2 POD1). Interestingly, drought reduced O3 VFI in Q. robur but increased it in Q. pubescens. Both O3 and drought were detrimental to the plant biomass. However, Q. robur and Q. pubescens invested more in shoots than in roots, while Q. ilex invested more in roots, which might be related to a hormetic mechanism. Pn, PNUE and PPUE decreased in all species under drought, and only in the sensitive Q. robur (PPUE) and Q. pubescens (PNUE) under O3. This study confirms that POD1 is a good indicator to explain the development of O3 VFI and helps a differential diagnosis of co-occurring drought and O3 VFI in oak forests.