Crops' responses to ozone in Mediterranean environments

The Mediterranean environment, and most of the Italian peninsula, presents some peculiarities in terms of crop response to O₃ since most physiological mechanisms activated upon O₃ exposure, such as stomatal closure, often overlap and interact with those that underlie plant adaptation to drought and hyperosmotic stress, which are typical of these environments. OTC and EDU experiments have demonstrated that O₃ causes strong yield losses when crops are grown without water limitations. However, exposure to water or saline stress significantly reduced O₃ effects on crop yield. In this review, we present the methodological approaches that have been used to study plant-ozone interactions in Italy as well as biochemical, physiological and agronomic responses for representative cropping systems of the Mediterranean climate.     

Growth response of loblolly pine (Pinus taeda L.) seedlings to ozone fumigation

Twenty-two week-old Pinus taeda L. (loblolly pine) seedlings of 30 open-pollinated and five full-sib families, representing a wide range in geographic origin, were grown in charcoal-filtered (CF) air or CF-air supplemented with 160 or 320 nl liter(-1) ozone for 8 h day(-1), 4 days week(-1), for 9 weeks. Visible foliar injury (banded chlorosis, tip burn and premature senescence) was apparent in many families after 3 weeks in 320 nl liter(-1) and 6 weeks in 160 nl liter(-1) ozone. Decreases in relative height and root collar diameter growth rates, total dry weight, root dry weight, shoot dry weight, and root/shoot ratios were evident after 9 weeks of treatment with both 160 and 320 nl liter(-1) ozone. For relative height growth rates, family differences in response to ozone were observed. By the study's end, net photosynthesis rates were 15% less for the 320 nl liter(-1) ozone treatment as compared to the CF-air treatment. Total soluble sugar and total starch content of roots were not changed after 9 weeks of ozone exposure.     

Modelling annual pasture dynamics: Application to stomatal ozone deposition

Modelling ozone (O₃) deposition for impact risk assessment is still poorly developed for herbaceous vegetation, particularly for Mediterranean annual pastures. High inter-annual climatic variability in the Mediterranean area makes it difficult to develop models characterizing gas exchange behaviour and air pollutant absorption suitable for risk assessment. This paper presents a new model to estimate stomatal conductance (g_s) of Trifolium subterraneum, a characteristic species of dehesa pastures. The MEDPAS (MEDiterranean PAStures) model couples 3 modules estimating soil water content (SWC), vegetation growth and g_s. The g_s module is a reparameterized version of the stomatal component of the EMEP DO₃SE O₃ deposition model. The MEDPAS model was applied to two contrasting years representing typical dry and humid springs respectively and with different O₃ exposures. The MEDPAS model reproduced realistically the g_s seasonal and inter-annual variations observed in the field. SWC was identified as the major driver of differences across years. Despite the higher O₃ exposure in the dry year, meteorological conditions favoured 2.1 times higher g_s and 56 day longer growing season in the humid year compared to the dry year. This resulted in higher ozone fluxes absorbed by T. subterraneum in the humid year. High inter-family variability was found in gas exchange rates, therefore limiting the relevance of single species O₃ deposition flux modelling for dehesa pastures. Stomatal conductance dynamics at the canopy level need to be considered for more accurate O₃ flux modelling for present and future climate scenarios in the Mediterranean area.     

Growth response of bigcone Douglas fir (Pseudotsuga macrocarpa) to long-term ozone exposure in southern California

Long-term radial growth of bigcone Douglas fir (Pseudotsuga macrocarpa) was studied throughout its range in the San Bernardino Mountains of southern California, where ambient ozone has been high for approximately the past 40 years. A gradient of both ozone concentration and precipitation exists from west (high) to east (low). Growth rates of bigcone Douglas fir are considerably lower since 1950 throughout the San Bernardino Mountains, with the largest growth reductions in the western part of the range where ozone exposure is highest. Needle retention is also somewhat lower at high ozone sites. Lower annual precipitation since 1950 may have some impact on long-term growth reductions, and short-term growth reductions induced by drought are an important component of long-term growth reductions at sites with high ozone exposure. An ozone-climate stress complex may be responsible for recent reductions in the growth of bigcone Douglas fir.     

Growth-stage dependent crop yield response to ozone exposure

Data from four crop yield-loss field trials were examined to determine if analysis using an imposed phenological weighting function based on seasonal growth stage would provide a more accurate indication of impact of ozone exposure. Alfalfa (Medicago sativa L. cv. Moapa 69), dry bean (Phaseolus vulgaris L. cv. California Dark Red kidney), fresh market and processing tomato (Lycopersicon esculentum Mill. cv. 6718 VF and VF-145-B7879, respectively) were grown at 9-11 ambient field plots within southern California comprising an ambient gradient of ozone. The growing season for each crop was artificially divided into 'quarters' composed of equal numbers of whole days and roughly corresponding to specific growth stages. Ozone exposure was calculated for each of these 'quarters' and regressed against final crop yield using 163 different exposure statistics. Weighting functions were developed using reciprocal residual mean square (1/RMS) or percentage of the best 100 exposure statistics of the 163 tested (TOP100) for each of the quarters. The third quarter of the alfalfa season was clearly most responsive to ozone as measured by both of the weighting functions. Third quarter ozone was also weighted highest by both weighting functions for dry bean. Fresh market and processing tomato were each influenced the greatest by second quartero zone as demonstrated by both weighting functions. The occurrence of ozone during physiologically important events (flowering and initial fruit set in second quarter for tomato; pod development in third quarter for dry bean) appeared to influence the yield of these crops the greatest. Growth-stage-dependent phenological weighting of pollutant exposure may result in more effective predictions of levels of ozone exposure resulting in yield reductions.     

Impact of ozone on Mediterranean forests: a review

Ozone impact on Mediterranean forests remains largely under-investigated, despite strong photochemical activity and harmful effects on crops. As representative of O3 impacts on Mediterranean vegetation, this paper reviews the current knowledge about O3 and forests in Italy. The intermediate position between Africa and European mid-latitudes creates a complex patchwork of climate and vegetation. Available data from air quality monitoring stations and passive samplers suggest O3 levels regularly exceed the critical level (CL) for forests. In contrast, relationships between O3 exposure and effects (crown transparency, radial growth and foliar visible symptoms) often fail. Despite limitations in the study design or underestimation of the CL can also affect this discrepancy, the effects of site factors and plant ecology suggest Mediterranean forest vegetation is adapted to face oxidative stress, including O3. Implications for risk assessment (flux-based CL, level III, non-stomatal deposition) are discussed.     

Atmospheric dynamics and ozone cycles related to nitrogen deposition in the western Mediterranean

Recent results from EU-sponsored projects have shed new light on the workings of the atmosphere in the Mediterranean Basin, where a large airmass becomes trapped and nearly stagnant over the sea during the summer. This work reviews the developments which have led to our present understanding of air pollution dynamics and associated meteorological processes in this region. In summer, the sea breezes combine with upslope winds to create recirculations along the coasts and within the western Mediterranean basin, with residence times in the order of days. Under strong insolation these recirculations become "large natural photo-chemical reactors" where most of the NOx emissions and other precursors are transformed into oxidants, acidic compounds, aerosols and ozone, which can exceed European Union directives for several months of the year. The objective of this paper is to evaluate available experimental evidence and complementary modelling results to help in the interpretation of observed ozone cycles and to anticipate possible scenarios for anthropogenic nitrogen deposition in the region.     

Meta-analysis of the relative sensitivity of semi-natural vegetation species to ozone

This study identified 83 species from existing publications suitable for inclusion in a database of sensitivity of species to ozone (OZOVEG database). An index, the relative sensitivity to ozone, was calculated for each species based on changes in biomass in order to test for species traits associated with ozone sensitivity. Meta-analysis of the ozone sensitivity data showed a wide inter-specific range in response to ozone. Some relationships in comparison to plant physiological and ecological characteristics were identified. Plants of the therophyte lifeform were particularly sensitive to ozone. Species with higher mature leaf N concentration were more sensitive to ozone than those with lower leaf N concentration. Some relationships between relative sensitivity to ozone and Ellenberg habitat requirements were also identified. In contrast, no relationships between relative sensitivity to ozone and mature leaf P concentration, Grime's CSR strategy, leaf longevity, flowering season, stomatal density and maximum altitude were found. The relative sensitivity of species and relationships with plant characteristics identified in this study could be used to predict sensitivity to ozone of untested species and communities.     

Size-mediated foliar response to ozone in black cherry trees

Local ozone concentration and visible foliar injury were measured over the 1994 growing season on open-grown black cherry (Prunus serotina Ehrh.) trees of varying size (age) within forest stands and adjacent openings at a site in north-central Pennsylvania. Relationships were determined between visible ozone injury and ozone exposure, as well as calculated between injury and ozone uptake expressed as the product of stomatal conductance and ozone concentration. In addition, simultaneous measurements of visible symptoms and leaf gas exchange were also conducted to determine the correlation between visible and physiological injury and ozone exposure. By September, the amount of leaf area affected by visible foliar ozone injury was greatest for seedlings (46%), followed by canopy trees (20%) and saplings (15%). A large amount of variability in foliar ozone symptom expression was observed among trees within a size class. Sum40 and Sum60 (ozone concentration > 40 and > 60 nl liter(-1)) cumulative exposure statistics were the most meaningful indices for interpretation of foliar injury response. Seedlings were apparently more sensitive to ozone injury than larger trees because their higher rates of stomatal conductance resulted in higher rates of ozone uptake. Seedlings also had higher rates of early leaf abscission than larger trees with an average of nearly 30% of the leaves on a shoot abscised by 1 September compared to approximately 5% for larger trees. However, per unit ozone uptake into the leaf, larger trees exhibited larger amounts of foliar injury. The amount of visible foliar injury was negatively correlated (r(2) = 0.82) with net photosynthetic rates, but was not related to stomatal conductance. Net photosynthesis and stomatal conductance thus became uncoupled at high levels of visible foliar injury.     

Leaf conductance response of phaseolus vulgaris to ozone flux density

The effect of ozone flux density on leaf conductance to ozone in Phaseolus vulgaris was examined. The change in conductance was measured within the first two hours of fumigation for mature, fruiting 6-week-old plants of an ozone sensitive cultivar (Seafarer); for young, 14-day-old plants of the same cultivar; and for an ozone resistant cultivar (Gold Crop). Young Seafarer plants showed no change in conductance to ozone over a wide range of ozone flux densities. Gold Crop showed a decrease in conductance of −3.1 % /(mgO₃ m⁻² h⁻¹) whereas mature Seafarer plants exhibited a stronger decrease of −7.7% /(mgO₃ m⁻² h⁻¹). Diffusion porometer measurements taken on fruiting Seafarer plants in the field illustrated that a decrease in leaf diffusive conductance to water is related to visual ozone injury.     

Responses of evergreen and deciduous Quercus species to enhanced ozone levels

Plants of one evergreen oak (Quercus ilex) and three deciduous oaks (Q. faginea, with small leaves; Q. pyrenaica and Q. robur, with large leaves) were exposed both to filtered air and to enhanced ozone levels in Open-Top Chambers. Q. faginea and Q. pyrenaica were studied for the first time. Based on visible injury, gas exchange, chlorophyll content and biomass responses, Q. pyrenaica was the most sensitive species, and Q. ilex was the most tolerant, followed by Q. faginea. Functional leaf traits of the species were related to differences in sensitivity, while accumulated ozone flux via stomata (POD_1.6) partly contributed to the observed differences. For risk assessment of Mediterranean vegetation, the diversity of responses detected in this study should be taken into account, applying appropriate critical levels.     

Modeling plant response to tropospheric ozone: a critical review

The advantages and disadvantages, benefits and limitations, of a number of published mathematical models representing the effects of ozone on crops and native vegetation are described. Several levels of modeling are addressed: word models, graphic models, mathematical models, and computer simulation implementation. Special attention is given to evaluating: (1) how the interaction between ozone exposure and vegetation effects is quantified, (2) the status of field testing of the model, and (3) the adequacy of information for enabling other investigators to replicate the model for further testing. Original contributions, not previously published, are made in this evaluation in the form of: (1) graphic model flow charts for published models, (2) clarification of mathematical equations for existing models, (3) graphic forms of functional relations comprising portions of models, and (4) graphic displays of model output performance versus observed data. The models that are evaluated cover acute exposure-response models, statistical and mechanistic-process models, including a partial model of ambient exposure versus ozone flux, and uptake. They also cover chronic exposure statistical approaches, including time-series modeling, mechanistic-process models, 'disintegrated' models of forest system simulations, chronic flux density-uptake-response, and models for regional effects assessment in forests and agricultural lands.     

Differential response of buddleia (Buddleia davidii Franch.) to ozone

Five cultivars of buddleia, Buddleia davidii Franch., were exposed to sub-ambient, ambient, and twice-ambient levels of ozone in open-top chambers for 8 weeks (June-August) during 1995: Plants were evaluated for foliar injury, growth index, and inflorescence characteristics during and following exposure. Destructive harvests were conducted at the end of the exposure period to determine dry weights of both above- and below-ground plant components. All cultivars had symptoms of visible injury in the twice-ambient treatment at both three and eight weeks after exposures began. No visible symptoms were observed at ambient ozone concentrations. At three weeks of exposure, 'Pink Delight' had the highest percentage of the leaves injured (PLI), 46.2%, followed by 'Opera' with a PLI of 23.3%. The other three cultivars had similar PLIs of less than 15%. After eight weeks of exposure, visible injury was equally severe on all cultivars with a mean PLI of 50.2% and mean Horsfall-Barratt rating of 5.4, indicating 12 to 25% of the leaf area was injured. No ozone x cultivar interaction was found for any growth variable measured. Across cultivars, growth index was reduced by 6%, total dry weight by 35%, and the number of developing floral buds and inflorescences by 29% for plants in twice-ambient ozone concentrations compared to ambient ozone concentrations. Percent biomass allocated to inflorescences was significantly greater for plants exposed to sub-ambient levels compared to those exposed to ozone at either ambient or twice-ambient concentrations. Results indicate that ozone levels similar to those in large urban areas in the southeastern United States have the potential to reduce growth and flowering of this important landscape plant.     

Exposure to moderate concentrations of tropospheric ozone impairs tree stomatal response to carbon dioxide

With rising concentrations of both atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃), it is important to better understand the interacting effects of these two trace gases on plant physiology affecting land-atmosphere gas exchange. We investigated the effect of growth under elevated CO₂ and O₃, singly and in combination, on the primary short-term stomatal response to CO₂ concentration in paper birch at the Aspen FACE experiment. Leaves from trees grown in elevated CO₂ and/or O₃ exhibited weaker short-term responses of stomatal conductance to both an increase and a decrease in CO₂ concentration from current ambient level. The impairement of the stomatal CO₂ response by O₃ most likely developed progressively over the growing season as assessed by sap flux measurements. Our results suggest that expectations of plant water-savings and reduced stomatal air pollution uptake under rising atmospheric CO₂ may not hold for northern hardwood forests under concurrently rising tropospheric O₃.     

Responses to ozone of insects feeding on a crop and a weed species

The influence of ozone on insect herbivore growth and population development was investigated. Fumigation of both pea (Pisum sativum L.) and dock (Rumex obtusifolius L.) at a range of O(3) concentrations between 21-206 nl litre(-1) produced changes in mean relative growth rates of the aphids Acyrthosiphon pisum Harris and Aphis rumicis L. of between 24 and -6% relative to controls. However, there was no evidence of a dose-related response to O(3) fumigation and no clear differences in aphid response when fumigated with the plant on prefumigated or previously unfumigated plant material. It is suggested that this may, in part, be due to the presence of NO contamination during O(3) fumigation. However, the MRGR of dock aphids was found to be greater on new compared to old leaves as well as the increase on the new growth and decrease on the old growth of fumigated plants relative to unfumigated controls. The size of egg batches of the chrysomelid beetle Gastrophysa viridula Degeer were found to be larger, survival and productivity of larvae was higher, and the food consumption lower on R. obtusifolius fumigated with 70 nl litre(-1) O(3) compared with unfumigated controls. This meant that these beetle larvae consumed less leaf area per mg of production on fumigated leaves probably because of their better nutritional quality and/or reduced leaf defences. However, the rate of development of larvae was similar on fumigated and control plants.     

Seedling insensitivity to ozone for three conifer species native to Great Smoky Mountains National Park

Field symptoms typical of ozone injury have been observed on several conifer species in Great Smoky Mountains National Park, and tropospheric ozone levels in the Park can be high, suggesting that ozone may be causing growth impairment of these plants. The objective of this research was to test the ozone sensitivity of selected conifer species under controlled exposure conditions. Seedlings of three species of conifers, Table Mountain pine (Pinus pungens), Virginia pine (Pinus virginiana), and eastern hemlock (Tsuga canadensis), were exposed to various levels of ozone in open-top chambers for one to three seasons in Great Smoky Mountains National Park in Tennessee, USA. A combination of episodic profiles (1988) and modified ambient exposure regimes (1989-92) were used. Episodic profiles simulated an average 7-day period from a monitoring station in the Park. Treatments used in 1988 were: charcoal-filtered (CF), 1.0x ambient, 2.0x ambient, and ambient air-no chamber (AA). In 1989 a 1.5x ambient treatment was added, and in 1990, additional chambers were made available, allowing a 0.5x ambient treatment to be added. Height, diameter, and foliar injury were measured most years. Exposures were 3 years for Table Mountain pine (1988-90), 3 years for hemlock (1989-91), and 1 and 2 years for three different sets of Virginia pine (1990, 1990-91, and 1992). There were no significant (p<0.05) effects of ozone on any biomass fraction for any of the species, except for older needles in Table Mountain and Virginia pine, which decreased with ozone exposure. There were also no changes in biomass allocation patterns among species due to ozone exposure, except for Virginia pine in 1990, which showed an increase in the root:shoot ratio. There was foliar injury (chlorotic mottling) in the higher two treatments (1.0x and 2.0x for Table Mountain and 2.0x for Virginia pine), but high plant-to-plant variability obscured formal statistical significance in many cases. We conclude, at least for growth in the short-term, that seedlings of these three conifer species are insensitive to ambient and elevated levels of ozone, and that current levels of ozone in the Park are probably having minimal impacts on these particular species.     

Emissions of nitric oxide from 79 plant species in response to simulated nitrogen deposition

To assess the potential contribution of nitric oxide (NO) emission from the plants grown under the increasing nitrogen (N) deposition to atmospheric NO budget, the effects of simulated N deposition on NO emission and various leaf traits (e.g., specific leaf area, leaf N concentration, net photosynthetic rate, etc.) were investigated in 79 plant species classified by 13 plant functional groups. Simulated N deposition induced the significant increase of NO emission from most functional groups, especially from conifer, gymnosperm and C(3) herb. Moreover, the change rate of NO emission was significantly correlated with the change rate of various leaf traits. We conclude that the plants grown under atmospheric N deposition, especially in conifer, gymnosperm and C(3) herb, should be taken into account as an important biological source of NO and potentially contribute to atmospheric NO budget.     

A California air standard to protect vegetation from ozone

Evidence shows that the current national primary ambient air quality standard, if attained, would still permit substantial injury to vegetation. Thus, in March 1987, the California Air Resources Board (CARB) began consideration of the evidence for the effects of ozone (O3) on vegetation, and of several possible state ambient air quality standards designed to protect vegetation, especially crops, from O3 injury. In its review, the CARB addressed a number of issues relevant to such a standard. One issue considered by the CARB is the relationship of an ambient air quality standard to natural background levels of O3, which would greatly influence the practicality of attainment. Attainment of a standard close to natural background could entail excessive costs. Another issue considered is the occurrence of oxidants other than O3 that can damage vegetation. Throughout much of California, O3 accounts for over 90% of the oxidant air pollutants, and the CARB considered whether, in keeping with current practice, O3 should be used as a surrogate for total oxidant air pollutants. A major new piece of information presented to the CARB was an assessment of the economic effects of several potential standards. This assessment, produced by University of California scientists at Riverside and Davis, calculated the benefits of the potential standards in comparison to current O3 levels and estimated natural O3 background. This assessment was developed using field chamber response data, local crop data, and local O3 concentration data as inputs to the California Agricultural Resources Model, which accounts for both supply and demand effects. Because of California's varied climate, agricultural production occurs on a year-round basis, with overlapping growing seasons for many crops. Over long periods of time, O3 levels may vary markedly because of the influence of various factors, and a 1-h standard may not be an accurate indicator of growing season O3 exposure. A moving three-month averaging time has been proposed as a way to approximate the growing seasons of California's 200 crops. However, a sufficiently stringent 1-h standard would serve as a surrogate for a growing season standard. The CARB reviewed evidence supporting both long-term and short-term standards. Agriculture dominates the economies of some regions within California but is a minor components of other regional economies. Because the San Joaquin Valley is California's most important agricultural area, the CARB reviewed evidence for a regional standard for this area that would be more stringent than standards for other parts of the state.     

Seasonal patterns of non-terpenoid C6-C10 VOC emission from seven Mediterranean woody species

The seasonal pattern of non-terpenoid C6-C10 VOC emission by seven Mediterranean woody species (Bupleurum fruticosum, Cistus albidus, Pinus halepensis, Arbutus unedo, Erica arborea, Quercus coccifera, and Q. ilex) was studied under field conditions. Branch chamber samples were sorbed on carbotrap and analyzed by thermal desorption in combination with GC-MS. These non-terpenoid C6-C10 VOC emissions were large, almost of similar magnitude to those of terpenes. Overall, maximum values were recorded in spring and summer (up to 12 microg g(-1) DM h(-1) in Q. ilex) and minimum values in autumn and winter (up to 5 microg g(-1) DM h(-1) in Q. ilex). These C6-C10 VOC emissions represented 2.82% of the photosynthetic C fixation in summer and 0.22% in winter. Some compounds such as 2-ethoxyethyl acetate were emitted by most species, others such as 3-hexen-1-ol, phenol or decanal were significantly emitted only by few species. The greatest diversity of emitted non-terpenoid C6-C10 VOCs was observed in spring and in Q. ilex. Temperature seemed a strong driver of these seasonal changes but other species-specific and seasonal factors seem involved. These results indicate that C6-C10 non-terpenoid VOCs contribute a rather significant fraction of the total biogenic VOC flux from these Mediterranean species, especially in spring and summer, and therefore should be considered in VOC emission inventories and in model predictions of tropospheric chemistry.     

Within season and carry-over effects following exposure of grassland species mixtures to increasing background ozone

Few studies have investigated effects of increased background ozone in the absence of episodic peaks, despite a predicted increase throughout the northern hemisphere over the coming decades. In this study Leontodon hispidus was grown with Anthoxanthum odoratum or Dactylis glomerata and exposed in the UK to one of eight background ozone concentrations for 20 weeks in solardomes. Seasonal mean ozone concentrations ranged from 21.4 to 102.5 ppb. Ozone-induced senescence of L. hispidus was enhanced when grown with the more open canopy of A. odoratum compared to the denser growing D. glomerata. There was increased cover with increasing ozone exposure for both A. odoratum and D. glomerata, which resulted in an increase in the grass:Leontodon cover ratio in both community types. Carry-over effects of the ozone exposure were observed, including delayed winter die-back of L. hispidus and acceleration in the progression from flowers to seed-heads in the year following ozone exposure.