Whole-tree water use efficiency is decreased by ambient ozone and not affected by O3-induced stomatal sluggishness

Ozone-induced oxidative stress alleviation by biogenic silver nanoparticles and ethylenediurea in mung bean (Vigna radiata L.) under high ambient ozone

Ground-level ozone (O3) is the most phytotoxic secondary air pollutant in the atmosphere, severely affecting crop yields worldwide. The role of nanoparticles (NP) in the alleviation of ozone-induced yield losses in crops is not known. Therefore, in the present study, we investigated the effects of biogenicB-AgNPs on the mitigation of ozone-induced phytotoxicity in mung bean and compared its results with ethylenediurea (EDU) for the first time. Two mung bean cultivars (Vigna radiata L., Cv. SML-668 and PDM-139) were foliar sprayed with weekly applications of B-AgNPs (0 = control, 10 and 25 ppm) and EDU (0 = control, 200 and 300 ppm) until maturation phase. Morphological, physiological, enzymatic, and non-enzymatic antioxidant data were collected 30 and 60 days after germination (DAG). The mean O3 and AOT40 values (8 h day-1) during the cultivation period were approximately 52 ppb and 4.4 ppm.h, respectively. More biomass was accumulated at the vegetative phase due to the impact of B-AgNPs and EDU, and more photosynthates were transported to the reproductive phase, increasing yield. We observed that the 10 ppm B-AgNPs treatment had a more noticeable impact on yield parameters and lower Ag accumulation in seeds for both cultivars. Specifically, SML-668 cultivar treated with 10 ppm B-AgNPs (SN1) showed greater increases in seed weight plant-1 (124.97%), hundred seed weight (33.45%), and harvest index (37.53%) in comparison to control. Our findings suggest that B-AgNPs can enhance growth, biomass, yield, and seed quality, and can improve mung bean ozone tolerance. Therefore, B-AgNPs may be a promising protectant for mung bean.

Response of tropical trees to elevated Ozone: a Free Air Ozone Enrichment study

Tropospheric ozone (O₃) has become one of the main urban air pollutants. In the present study, we assessed impact of ambient and future ground-level O₃ on nine commonly growing urban tree species under Free Air Ozone Enrichment (FAOE) condition. During the study period, mean ambient and elevated ozone (EO₃) concentrations were 48.59 and 69.62 ppb, respectively. Under EO₃ treatment, stomatal density (SD) significantly decreased and guard cell length (GCL) increased in Azadirachta indica, Bougainvillea spectabilis, Plumeria rubra, Saraca asoca and Tabernaemontana divaricata, while SD increased and GCL decreased in Ficus benghalensis and Terminalia arjuna. Proline levels increased in all the nine plant species under EO₃ condition. EO₃ significantly reduced photosynthetic rate, stomatal conductance (gs), and transpiration rates (E). Only A. indica and N. indicum showed higher gs and E under EO₃ treatment. Water use efficiency (WUE) significantly increased in F. benghalensis and decreased in A. indica and T. divaricata. Air Pollution Tolerance Index (APTI) significantly increased in Ficus religiosa and S. asoca whereas it decreased in B. spectabilis and A. indica. Of all the plant species B. spectabilis and A. indica were the most sensitive to EO₃ (high g_s and less ascorbic acid content) while S. asoca and F. religiosa were the most tolerant (lowg_s and more ascorbic acid content). The sensitivity of urban tree species to EO₃ is a cause of concern and should be considered for future urban forestry programmes. Our study should guide more such studies to identify tolerant trees for urban air pollution abatement.

Legislative and functional aspects of different metrics used for ozone risk assessment to forests

Surface ozone (O₃) is a threat to forests by decreasing photosynthesis and, consequently, influencing the strength of land carbon sink. However, due to the lack of continuous surface O₃ measurements, observational-based assessments of O₃ impacts on forests are largely missing at hemispheric to global scales. Currently, some metrics are used for regulatory purposes by governments or national agencies to protect forests against the negative impacts of ozone: in particular, both Europe and United States (US) makes use of two different exposure-based metrics, i.e. AOT40 and W126, respectively. However, because of some limitations in these metrics, a new standard is under consideration by the European Union (EU) to replace the current exposure metric. We analyse here the different air quality standards set or proposed for use in Europe and in the US to protect forests from O₃ and to evaluate their spatial and temporal consistency while assessing their effectiveness in protecting northern-hemisphere forests. Then, we compare their results with the information obtained from a complex land surface model (ORCHIDEE). We find that present O₃ uptake decreases gross primary production (GPP) in 37.7% of the NH forested area of northern hemisphere with a mean loss of 2.4% year^-1. We show how the proposed US (W126) and the currently used European (AOT40) air quality standards substantially overestimate the extension of potential vulnerable regions, predicting that 46% and 61% of the Northern Hemisphere (NH) forested area are at risk of O₃ pollution. Conversely, the new proposed European standard (POD1) identifies lower extension of vulnerability regions (39.6%).

Flux-Based Ozone Risk Assessment for a Plant Injury Index (PII) in Three European Cool-Temperate Deciduous Tree Species

This study investigated visible foliar ozone (O3) injury in three deciduous tree species with different growth patterns (indeterminate, Alnus glutinosa (L.) Gaertn.; intermediate, Sorbus aucuparia L.; and determinate, Vaccinium myrtillus L.) from May to August 2018. Ozone effects on the timing of injury onset and a plant injury index (PII) were investigated using two O3 indices, i.e., AOT40 (accumulative O3 exposure over 40 ppb during daylight hours) and PODY (phytotoxic O3 dose above a flux threshold of Y nmol m−2 s−1). A new parameterization for PODY estimation was developed for each species. Measurements were carried out in an O3 free-air controlled exposure (FACE) experiment with three levels of O3 treatment (ambient, AA; 1.5 × AA; and 2.0 × AA). Injury onset was found in May at 2.0 × AA in all three species and the timing of the onset was determined by the amount of stomatal O3 uptake. It required 4.0 mmol m−2 POD0 and 5.5 to 9.0 ppm·h AOT40. As a result, A. glutinosa with high stomatal conductance (gs) showed the earliest emergence of O3 visible injury among the three species. After the onset, O3 visible injury expanded to the plant level as confirmed by increased PII values. In A. glutinosa with indeterminate growth pattern, a new leaf formation alleviated the expansion of O3 visible injury at the plant level. V. myrtillus showed a dramatic increase of PII from June to July due to higher sensitivity to O3 in its flowering and fruiting stage. Ozone impacts on PII were better explained by the flux-based index, PODY, as compared with the exposure-based index, AOT40. The critical levels (CLs) corresponding to PII = 5 were 8.1 mmol m−2 POD7 in A. glutinosa, 22 mmol m−2 POD0 in S. aucuparia, and 5.8 mmol m−2 POD1 in V. myrtillus. The results highlight that the CLs for PII are species-specific. Establishing species-specific O3 flux-effect relationships should be key for a quantitative O3 risk assessment.

Ozone effects on plants in natural ecosystems

Tropospheric ozone (O₃ ) is an important stressor in natural ecosystems, with well-documented impacts on soils, biota and ecological processes. The effects of O₃ on individual plants and processes scale up through the ecosystem through effects on carbon, nutrient and hydrologic dynamics. Ozone effects on individual species and their associated microflora and fauna cascade through the ecosystem to the landscape level. Systematic injury surveys demonstrate that foliar injury occurs on sensitive species throughout the globe. However, deleterious impacts on plant carbon, water and nutrient balance can also occur without visible injury. Because sensitivity to O₃ may follow coarse physiognomic plant classes (in general, herbaceous crops are more sensitive than deciduous woody plants, grasses and conifers), the task still remains to use stomatal O₃ uptake to assess class and species' sensitivity. Investigations of the radial growth of mature trees, in combination with data from many controlled studies with seedlings, suggest that ambient O₃ reduces growth of mature trees in some locations. Models based on tree physiology and forest stand dynamics suggest that modest effects of O₃ on growth may accumulate over time, other stresses (prolonged drought, excess nitrogen deposition) may exacerbate the direct effects of O₃ on tree growth, and competitive interactions among species may be altered. Ozone exposure over decades may be altering the species composition of forests currently, and as fossil fuel combustion products generate more O₃ than deteriorates in the atmosphere, into the future as well.

Ozone Amplifies Water Loss from Mature Trees in the Short Term But Decreases It in the Long Term

We measured whole-tree transpiration of mature Fagus sylvatica and Picea abies trees exposed to ambient and twice-ambient O3 regimes (1xO3 and 2xO3 free-air fumigation). After eight years, mean daily total transpiration did not vary with the O3 regime over the 31 days of our study, even though individual daily values increased with increasing daily O3 peaks in both species. Although the environmental parameters were similar at 1xO3 and 2xO3, the main factors affecting daily transpiration were vapour pressure deficit in 2xO3 spruce and O3 peaks in beech. For a mechanistic explanation, we measured O3-induced sluggish stomatal responses to variable light (sunflecks) by means of leaf-level gas exchange measurements only in the species where O3 was a significant factor for transpiration, i.e., beech. Stomata were always slower in closing than in opening. The 2xO3 stomata were slower in opening and mostly in closing than 1xO3 stomata, so that O3 uptake and water loss were amplified before a steady state was reached. Such delay in the stomatal reaction suggests caution when assessing stomatal conductance under O3 pollution, because recording gas exchange at the time photosynthesis reached an equilibrium resulted in a significant overestimation of stomatal conductance when stomata were closing (ab. 90% at 1xO3 and 250% at 2xO3). Sun and shade leaves showed similar sluggish responses, thus suggesting that sluggishness may occur within the entire crown. The fact that total transpiration was similar at 1xO3 and 2xO3, however, suggests that the higher water loss due to stomatal sluggishness was offset by lower steady-state stomatal conductance at 2xO3. In conclusion, O3 exposure amplified short-term water loss from mature beech trees by slowing stomatal dynamics, while decreased long-term water loss because of lower steady-state stomatal conductance. Over the short term of this experiment, the two responses offset each other and no effect on total transpiration was observed.

Effect of Long-Term vs. Short-Term Ambient Ozone Exposure on Radial Stem Growth, Sap Flux and Xylem Morphology of O3-Sensitive Poplar Trees

High ozone (O3) pollution impairs the carbon and water balance of trees, which is of special interest in planted forests. However, the effect of long-term O3 exposure on tree growth and water use, little remains known. In this study, we analysed the relationships of intra-annual stem growth pattern, seasonal sap flow dynamics and xylem morphology to assess the effect of long term O3 exposure of mature O3-sensitive hybrid poplars (‘Oxford’ clone). Rooted cuttings were planted in autumn 2007 and drip irrigated with 2 liters of water as ambient O3 treatment, or 450 ppm ethylenediurea (N-[2-(2-oxo-1-imidazolidinyl)ethyl]-N0-phenylurea, abbreviated as EDU) solution as O3 protection treatment over all growing seasons. During 2013, point dendrometers and heat pulses were installed to monitor radial growth, stem water relations and sap flow. Ambient O3 did not affect growth rates, even if the seasonal culmination point was 20 days earlier on average than that recorded in the O3 protected trees. Under ambient O3, trees showed reduced seasonal sap flow, however, the lower water use was due to a decrease of Huber value (decrease of leaf area for sapwood unit) rather than to a change in xylem morphology or due to a direct effect of sluggish stomatal responses on transpiration. Under high evaporative demand and ambient O3 concentrations, trees showed a high use of internal stem water resources modulated by stomatal sluggishness, thus predisposing them to be more sensitive water deficit during summer. The results of this study help untangle the compensatory mechanisms involved in the acclimation processes of forest species to long-term O3 exposure in a context of global change.

Current ambient and elevated ozone effects on poplar: A global meta-analysis and response relationships

The effects of current and future elevated O₃ concentrations (e[O₃]) were investigated by a meta-analysis for poplar, a widely distributed genus in the Northern Hemisphere with global economic importance. Current [O₃] has significantly reduced CO₂ assimilation rate (P_n) by 33% and total biomass by 4% in comparison with low O₃ level (charcoal-filtered air, CF). Relative to CF, an increase in future [O₃] would further enhance the reduction in total biomass by 24%, plant height by 17% and plant leaf area by 19%. Isoprene emissions could decline by 34% under e[O₃], with feedback implications in reducing the formation of secondary air pollutants including O₃. Reduced stomatal conductance and lower foliar area might increase runoff and freshwater availability in O₃ polluted areas. Higher cumulated O₃ exposure over a threshold of 40 ppb (AOT40) induced larger reductions in P_n, total biomass and isoprene emission. Relationships of light-saturated photosynthesis rates (A_sat), total biomass and chlorophyll content with AOT40 using a global dataset are provided. These relationships are expected to improve O₃ risk assessment and also to support the inclusion of the effect of O₃ in models addressing plantation productivity and carbon sink capacity.

Ozone risk assessment is affected by nutrient availability: Evidence from a simulation experiment under free air controlled exposure (FACE)

Assessing ozone (O₃) risk to vegetation is crucial for informing policy making. Soil nitrogen (N) and phosphorus (P) availability could change stomatal conductance which is the main driver of O₃ uptake into a leaf. In addition, the availability of N and P could influence photosynthesis and growth. We thus postulated that the sensitivity of plants to O₃ may be changed by the levels of N and P in the soil. In this study, a sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha^-1; N80, 80 kg N ha^-1), three P levels (P0, 0 kg P ha^-1; P40, 40 kg P ha^-1; P80, 80 kg P ha^-1) and three levels of O₃ exposure (ambient concentration, AA; 1.5 × AA; 2.0 × AA) for a whole growing season in an O₃ free air controlled exposure (FACE) facility. Flux-based (POD_{0 to 6}) and exposure-based (W126 and AOT40) dose-response relationships were fitted and critical levels (CLs) were estimated for a 5% decrease of total annual biomass. It was found that N and P availability modified the dose-response relationships of biomass responses to O₃. Overall, the N supply decreased the O₃ CLs i.e. increased the sensitivity of poplar to O₃. Phosphorus alleviated the O₃-caused biomass loss and increased the CL. However, such mitigation effects of P were found only in low N and not in high N conditions. In each nutritional treatment, similar performance was found between flux-based and exposure-based indices. However, the flux-based approach was superior, as compared to exposure indices, to explain the biomass reduction when all nutritional treatments were pooled together. The best O₃ metric for risk assessments was POD₄, with 4.6 mmol m^-2 POD₄ as a suitable CL for Oxford poplars grown under various soil N and P conditions.

Tropospheric ozone assessment report: Global ozone metrics for climate change, human health, and crop/ecosystem research

Assessment of spatial and temporal variation in the impacts of ozone on human health, vegetation, and climate requires appropriate metrics. A key component of the Tropospheric Ozone Assessment Report (TOAR) is the consistent calculation of these metrics at thousands of monitoring sites globally. Investigating temporal trends in these metrics required that the same statistical methods be applied across these ozone monitoring sites. The nonparametric Mann-Kendall test (for significant trends) and the Theil-Sen estimator (for estimating the magnitude of trend) were selected to provide robust methods across all sites. This paper provides the scientific underpinnings necessary to better understand the implications of and rationale for selecting a specific TOAR metric for assessing spatial and temporal variation in ozone for a particular impact. The rationale and underlying research evidence that influence the derivation of specific metrics are given. The form of 25 metrics (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts) are described. Finally, this study categorizes health and vegetation exposure metrics based on the extent to which they are determined only by the highest hourly ozone levels, or by a wider range of values. The magnitude of the metrics is influenced by both the distribution of hourly average ozone concentrations at a site location, and the extent to which a particular metric is determined by relatively low, moderate, and high hourly ozone levels. Hence, for the same ozone time series, changes in the distribution of ozone concentrations can result in different changes in the magnitude and direction of trends for different metrics. Thus, dissimilar conclusions about the effect of changes in the drivers of ozone variability (e.g., precursor emissions) on health and vegetation exposure can result from the selection of different metrics.

Comparing concentration-based (AOT40) and stomatal uptake (PODY) metrics for ozone risk assessment to European forests

Tropospheric ozone (O3) produces harmful effects to forests and crops, leading to a reduction of land carbon assimilation that, consequently, influences the land sink and the crop yield production. To assess the potential negative O3 impacts to vegetation, the European Union uses the Accumulated Ozone over Threshold of 40 ppb (AOT40). This index has been chosen for its simplicity and flexibility in handling different ecosystems as well as for its linear relationships with yield or biomass loss. However, AOT40 does not give any information on the physiological O3 uptake into the leaves since it does not include any environmental constraints to O3 uptake through stomata. Therefore, an index based on stomatal O3 uptake (i.e. PODY), which describes the amount of O3 entering into the leaves, would be more appropriate. Specifically, the PODY metric considers the effects of multiple climatic factors, vegetation characteristics and local and phenological inputs rather than the only atmospheric O3 concentration. For this reason, the use of PODY in the O3 risk assessment for vegetation is becoming recommended. We compare different potential O3 risk assessments based on two methodologies (i.e. AOT40 and stomatal O3 uptake) using a framework of mesoscale models that produces hourly meteorological and O3 data at high spatial resolution (12 km) over Europe for the time period 2000-2005. Results indicate a remarkable spatial and temporal inconsistency between the two indices, suggesting that a new definition of European legislative standard is needed in the near future. Besides, our risk assessment based on AOT40 shows a good consistency compared to both in-situ data and other model-based datasets. Conversely, risk assessment based on stomatal O3 uptake shows different spatial patterns compared to other model-based datasets. This strong inconsistency can be likely related to a different vegetation cover and its associated parameterizations.

An epidemiological assessment of stomatal ozone flux-based critical levels for visible ozone injury in Southern European forests

Southern forests are at the highest ozone (O3) risk in Europe where ground-level O3 is a pressing sanitary problem for ecosystem health. Exposure-based standards for protecting vegetation are not representative of actual field conditions. A biologically-sound stomatal flux-based standard has been proposed, although critical levels for protection still need to be validated. This innovative epidemiological assessment of forest responses to O3 was carried out in 54 plots in Southeastern France and Northwestern Italy in 2012 and 2013. Three O3 indices, namely the accumulated exposure AOT40, and the accumulated stomatal flux with and without an hourly threshold of uptake (POD1 and POD0) were compared. Stomatal O3 fluxes were modeled (DO3SE) and correlated to measured forest-response indicators, i.e. crown defoliation, crown discoloration and visible foliar O3 injury. Soil water content, a key variable affecting the severity of visible foliar O3 injury, was included in DO3SE. Based on flux-effect relationships, we developed species-specific flux-based critical levels (CLef) for forest protection against visible O3 injury. For O3 sensitive conifers, CLef of 19 mmol m(-2) for Pinus cembra (high O3 sensitivity) and 32 mmol m(-2) for Pinus halepensis (moderate O3 sensitivity) were calculated. For broadleaved species, we obtained a CLef of 25 mmol m(-2) for Fagus sylvatica (moderate O3 sensitivity) and of 19 mmol m(-2) for Fraxinus excelsior (high O3 sensitivity). We showed that an assessment based on PODY and on real plant symptoms is more appropriated than the concentration-based method. Indeed, POD0 was better correlated with visible foliar O3 injury than AOT40, whereas AOT40 was better correlated with crown discoloration and defoliation (aspecific indicators). To avoid an underestimation of the real O3 uptake, we recommend the use of POD0 calculated for hours with a non-null global radiation over the 24-h O3 accumulation window.

Concentration- and flux-based ozone dose-response relationships for five poplar clones grown in North China

Concentration- and flux-based O3 dose-response relationships were developed for poplars in China. Stomatal conductance (gs) of five poplar clones was measured to parameterize a Jarvis-type multiplicative gs model. The maximum gs and other model parameters varied between clones. The strongest relationship between stomatal O3 flux and total biomass was obtained when phytotoxic ozone dose (POD) was integrated using an uptake rate threshold of 7 nmol m(-2) s(-1). The R(2) value was similar between flux-based and concentration-based dose-response relationships. Ozone concentrations above 28-36 nmol mol(-1) contributed to reducing the biomass production of poplar. Critical levels of AOT40 (accumulated O3 exposure over 40 nmol mol(-1)) and POD7 in relation to 5% reduction in total biomass for poplar were 12 μmol mol(-1) h and 3.8 mmol m(-2), respectively.

Effects of long-term ambient ozone exposure on biomass and wood traits in poplar treated with ethylenediurea (EDU)

This is the longest continuous experiment where ethylenediurea (EDU) was used to protect plants from ozone (O3). Effects of long-term ambient O3 exposure (23 ppm h AOT40) on biomass of an O3 sensitive poplar clone (Oxford) were examined after six years from in-ground planting. Trees were irrigated with either water or 450 ppm EDU. Above (-51%) and below-ground biomass (-47%) was reduced by O3 although the effect was significant only for stem and coarse roots. Ambient O3 decreased diameter of the lower stem, and increased moisture content along the stem of not-protected plants (+16%). No other change in the physical wood structure was observed. A comparison with a previous assessment in the same experiment suggested that O3 effects on biomass partitioning to above-ground organs depend on the tree ontogenetic stage. The root/shoot ratios did not change, suggesting that previous short-term observations of reduced allocation to tree roots may be overestimated.

Ozone-induced stomatal sluggishness changes carbon and water balance of temperate deciduous forests

Tropospheric ozone concentrations have increased by 60-100% in the Northern Hemisphere since the 19(th) century. The phytotoxic nature of ozone can impair forest productivity. In addition, ozone affects stomatal functions, by both favoring stomatal closure and impairing stomatal control. Ozone-induced stomatal sluggishness, i.e., a delay in stomatal responses to fluctuating stimuli, has the potential to change the carbon and water balance of forests. This effect has to be included in models for ozone risk assessment. Here we examine the effects of ozone-induced stomatal sluggishness on carbon assimilation and transpiration of temperate deciduous forests in the Northern Hemisphere in 2006-2009 by combining a detailed multi-layer land surface model and a global atmospheric chemistry model. An analysis of results by ozone FACE (Free-Air Controlled Exposure) experiments suggested that ozone-induced stomatal sluggishness can be incorporated into modelling based on a simple parameter (gmin, minimum stomatal conductance) which is used in the coupled photosynthesis-stomatal model. Our simulation showed that ozone can decrease water use efficiency, i.e., the ratio of net CO2 assimilation to transpiration, of temperate deciduous forests up to 20% when ozone-induced stomatal sluggishness is considered, and up to only 5% when the stomatal sluggishness is neglected.

Canopy-level stomatal narrowing in adult Fagus sylvatica under O3 stress - means of preventing enhanced O3 uptake under high O3 exposure?

Spatio-temporally consistent O(3) doses are demonstrated in adult Fagus sylvatica from the Kranzberg Forest free-air fumigation experiment, covering cross-canopy and whole-seasonal scopes through sap flow measurement. Given O(3)-driven closure of stomata, we hypothesized enhanced whole-tree level O(3) influx to be prevented under enhanced O(3) exposure. Although foliage transpiration rate was lowered under twice-ambient O(3) around noon by 30% along with canopy conductance, the hypothesis was falsified, as O(3) influx was raised by 25%. Nevertheless, the twice-ambient/ambient ratio of O(3) uptake was smaller by about 20% than that of O(3) exposure, suggesting stomatal limitation of uptake. The O(3) response was traceable from leaves across branches to the canopy, where peak transpiration rates resembled those of shade rather than sun branches. Rainy/overcast-day and nightly O(3) uptake is quantified and discussed. Whole-seasonal canopy-level validation of modelled with sap flow-derived O(3) flux becomes available in assessing O(3) risk for forest trees.

Effects of a three-year exposure to ambient ozone on biomass allocation in poplar using ethylenediurea

We examined the effect of ambient ozone on visible foliar injury, growth and biomass in field-grown poplar cuttings of an Oxford clone sensitive to ozone (Populus maximoviczii Henry × berolinensis Dippel) irrigated with ethylenediurea (EDU) or water for three years. EDU is used as an ozone protectant for plants. Protective effects of EDU on ozone visible injury were found. As a result, poplar trees grown under EDU treatment increased leaves, lateral branches and root density in the third year, although no significant enhancement of stem height and diameter was found. Ambient ozone (AOT40, 24.6 ppm h; diurnal hourly average, 40.3 ppb) may finally reduce carbon gain by reducing the number of branches, and thus sites for leaf formation, in ozone-sensitive poplar trees under not-limiting conditions.