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We determined the effects of tropospheric ozone on the chemical composition of Quercus ilex L. leaves and their decomposition, with a view to assessing the influence of ozone on nutrient cycling and the sustainability of Mediterranean holm oak forests. Forming one of the most widespread thermophilous vegetation communities in the area, Q. ilex is a dominant and widespread evergreen oak in the Mediterranean, where concentrations of tropospheric ozone are particularly high. The dynamics of carbon, nitrogen, lignin and cellulose concentrations were monitored for six months during the decomposition of leaves from plants subjected to controlled ozone exposure in open-top chambers. Ozone-exposed leaves, compared to unexposed leaves, showed no significant differences in C, N, lignin and cellulose concentrations prior to the incubation in mesocosms. However, during decomposition, leaves from plants exposed to ozone lost C significantly more slowly and showed a higher C/N ratio than unexposed leaves. Ozone exposure significantly slowed down the decomposition rate, indicating a negative effect of tropospheric ozone on nutrient cycling, which may reduce long-term sustainability of the holm oak forest.

It’s well known that ozone is the most dangerous atmospheric pollutant for vegetation and reduces yield of several crops (Ferretti et al., 2007). Considering that ozone enters into plant tissues through the stomata, all the factors that reduce stomatal conductance also reduce ozone injuries to crops (Fagnano and Merola, 2007). Among these factors, in Mediterranean area increasing problems of watertable and soil salification are widespread reported. Salt stress reduced ozone sensitivity of some crop (Maggio et al., 2007), thanks to its effect on stomatal conductance and antioxidants production. The aim of this paper was to verify if irrigation with saline water can modify alfalfa responses to ozone pollution. From this experiment, it was confirmed that environmental conditions can strongly modify the response of plants to ozone pollution. Therefore, the thresholds for the protection of vegetation actually based on plant exposure to ozone tropospheric concentration are not suitable for the Mediterranean cropping systems, because these environments are characterized by multiple stresses (i.e. drought, salinity and ozone) that can modify the response of plants to the single stresses. For these reasons a ozone flux approach that takes into account other pedoclimatic stressors, therefore based on the real ozone dose absorbed by plants, will be more suitable for estimating ozone damages to crops in the different agro-environmental conditions of Europe.

Since ozone penetration in vegetation occurs mainly through stomata, all factors which alter plant-atmosphere relations could be able to modify plant response to ozone. Interaction between ozone and water stress in Mediterranean environment was studied on ozone resistant (R) and sensitive (S) biotypes of white clover, which were grown in charcoal filtered and not-filtered Open Top Chambers. Measurements of biomass, leaf area and stomata conductance were made during the growth period. Ozone injuries were estimated by the ratio between biomass yield gained in not-filtered and in filtered OTC; ozone uptake was estimated by the stomata conductance and ozone hourly concentrations. In presence of ozone, biomass yield was higher in R biotype than in S biotype, while no difference between the two biotypes was recorded in filtered air. Water stress partially reduced ozone injuries, as well as water stress effect was partially reduced in S biotype grown in the presence of ozone, while no interaction between ozone and water stress was noticed in R biotype. Yield losses of S was significantly correlated to cumulated ozone uptake, while yield losses of R was not influenced by variations in ozone fluxes. Flux based models could better estimate yield losses due to ozone in Mediterranean environments in which other stresses could be contemporary present; therefore, the new European directives might modify the actual thresholds based on ozone concentration with others based on models of stomata flux of ozone.

Ozone is a secondary pollutant with a mobility and distribution higher than its precursors (VOCs, NOx), which causes injuries to vegetation in rural areas. The experiment was carried out over three years (2000-2002) in three localities of the Campania region characterized by different elevations (plain and hill) and atmospheric pollution (sub-urban and rural). The aim of the present research was to study the pattern of ozone concentration and its injuries to plants and to compare the response of two white clover biotypes with different sensitivities to ozone, which were grown according to the experimental protocol defined by the ICP-Vegetation. In this trial, the pollution threshold (40 ppb) was always overcome. The highest ozone concentrations were recorded during the first year and the sub-urban site was the more polluted. The daily trend of ozone shows reduced nighttime degradation at the hilly sites, which tended to have relatively high concentrations also during the night. This involves the importance to consider also nocturnal stomatal conductance for studying ozone uptake through stomata and damage to vegetation in hill or mountain localities. Ozone injuries were estimated using the yield ratio of the sensitive to the resistant biotype [1-(S/R)*100]. Average yield losses were 20% without differences among the three localities, confirming that ozone pollution can cause injuries to plants also in rural areas located faraway from the emission sources of atmospheric pollutants.

The main environmental stresses of Italian croplands are discussed in relation to their interactions with ozone effects on crops. Water deficit and salinization are frequent in Mediterranean environments during spring-summer causing a decrease of soil water potential and water uptake by roots and consequently stomatal closure. These stresses also stimulate secondary metabolism and antioxidant accumulation, which also serves as a stress protection mechanism. High concentrations of tropospheric ozone are common all over Italy during the spring-summer season. Ozone injuries to vegetation are related to its penetration into plant tissues, mostly via stomatal uptake, rather than to tropospheric concentrations per se. In several crops, closure of stomata due to drought/salinization reduces ozone entering into leaf tissues and counteracts possible ozone damages. Furthermore, the stimulation of antioxidant synthesis as a response to environmental stresses can represent a further protection factor from ozone injuries for Mediterranean crops. The co-existence of stress-induced stomatal closure and high ozone levels during spring-summer in Mediterranean environments implies that models that do not take into account physiological responses of crops to drought and salinity stress may overestimate ozone damages when stress responses overlap with seasonal ozone peaks. The shift from concentration-based to flux-based approaches has improved the accuracy of models to assess ozone effects on agricultural crops. It is, however, necessary to further refine the flux concept with respect to the plant abiotic stress defense capacity that can differ among genotypes, climatic conditions, and physiological states.

Tropospheric ozone is the most critical air pollutant for plants. In recent years, common research goals have brought together different groups working on plant/ozone interactions, interested in establishing a European network that could efficiently make progress in this important research area. In this context, the international workshop on “Ozone risk assessment for European vegetation” was held on May 10- 11, 2007 in Capri (Naples, Italy). This was one of the activities promoted by the steering group of the Ozone Risk Assessment Network established in Ispra on February 23-24, 2006. During the Capri workshop, different facets of ozone effects on three major types of European vegetation, crops, grasslands (semi-natural vegetation) and forests, were discussed. The conclusions of the three working-groups, who contributed to a discussion focussed on methodological problems and research needs, are briefly summarized in the following sections.We refer to the contributions reported in this special issue of the Italian Journal of Agronomy for a detailed analysis of specific aspects.

Two clones of white clover (Trifolium repens L.) differing in ozone tolerance were grown in southern Italy during 1997 and 1998 to study the effects of ambient ozone exposure on yield, leaf morphology and water use. Ambient ozone levels were high in both years with values exceeding the threshold for leaf injury reported in the literature. In both years ozone injury was observed on the sensitive clone (NC-S) but not on the resistant one (NC-R), and leaf and stolon dry matter production was significantly lower in NC-S than in NC-R. However, it cannot be excluded that other factors, such as high temperature, interacted with the effect of ozone on biomass production. The clones differed in morphological characteristics. Lower total leaf area in NC-S plants was due to a smaller number of leaves per plant, but the average area per leaf was higher in NC-S. Specific leaf weight and net assimilation rate were higher in the more productive clone (NC-R). Cumulative plant water use was higher in NC-R in each growth period because of the larger leaf area; by contrast, water use per unit leaf area was higher in NC-S, indicating higher leaf conductance to water vapour. The results suggest that ozone significantly reduces the yield of sensitive white clover plants under well-watered conditions, and that the difference in ozone tolerance between clover clones is related to differences in leaf morphology and water use.

Stomatal closure and biosynthesis of antioxidant molecules are two fundamental components of the physiological machinery that lead to stress adaptation during plant's exposure to salinity. Since high stomatal resistance may also contribute in counteracting O 3 damages, we hypothesized that soil salinization may increase O 3 tolerance of crops. An experiment was performed with alfalfa grown in filtered (AOT40 = 0 in both years) and non-filtered (AOT40 = 9.7 in 2005 and 6.9 ppm h in 2006) open-top chambers. Alfalfa yield was reduced by O 3 (−33%) only in plants irrigated with salt-free water, while the increasing levels of soil salinity until 1.06 dS m −1 reduced both stomatal conductance and plant O 3 uptake, thus linearly reducing O 3 effects on yield. Therefore a reliable flux-based model for assessing the effects of O 3 on crop yield should take into account soil salinity.

The Mediterranean environment, and most of the Italian peninsula, presents some peculiarities in terms of crop response to O 3 since most physiological mechanisms activated upon O 3 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 O 3 causes strong yield losses when crops are grown without water limitations. However, exposure to water or saline stress significantly reduced O 3 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.

In this brief review we analyzed some aspects of tropospheric ozone damages to crop plants. Specifically, we addressed this issue to Mediterranean environments, where plant response to multiple stresses may either exacerbate or counteract deleterious ozone effects. After discussing the adequacy of current models to predict ozone damages to Mediterranean crops, we present a few examples of physiological responses to drought and salinity stress that generally overlap with seasonal ozone peaks in Southern Italy. The co-existence of multiple stresses is then analyzed in terms of stomatal vs. non-stomatal control of ozone damages. Recent results on osmoprotectant feeding experiments, as a non-invasive strategy to uncouple stomatal vs. non stomatal contribution to ozone protection, are also presented. In the final section, we discuss critical needs in ozone research and the great potential of plant model systems to unravel multiple stress responses in agricultural crops.

The main environmental features of Italian cropping systems are described with particular emphasis on their effects on crop responses to ozone pollution. High ozone levels have been recorded all over Italy and daily patterns show, at nighttimes, strong decreases in plain areas, while ozone levels remain high in hilly areas. In the latter sites, therefore, the contribution of nocturnal stomatal conductance (gsto) to ozone uptake should be further studied. It is well known that summer drought and soil salinity reduce the soil water potential, thus causing gsto to decrease. These are likely to be the most important factors reducing crop gas-exchange and yield under environmental conditions occurring in Italy. However, the stressinduced reduction of gsto also restricts ozone uptake and, consequently, its potential damage. In Southern Italy, gasexchange limitations have been also measured in irrigated crops between two successive irrigations. Finally, the effect of water stagnation, which often occurs in clay soils of southern Italy, should be not underestimated. In these soils, in fact, root anoxia will cause stomatal closure and, consequently, will also interfere with ozone uptake and damage.

The main route for ozone entry in plants is through the stomata. Consequently, environmental factors that may expose plants to any stress that will eventually lead to stomatal closure, will also reduce the rate of ozone entry into the plant and will possibly counteract ozone damages. In addition, several abiotic stresses activate the synthesis of antioxidant compounds, which are responsible for neutralizing toxic ozone derivatives. Due to this complex response, it is critical to assess how abiotic stresses and ozone toxicity will interactively affect plant growth and yield, especially in those areas, such as the coastal Mediterranean regions, where these types of stress and ozone exposure typically coexist. In this research we exposed tomato plants to salt stress in presence and absence of ozone to assess how salinity and ozone may interfere in terms of physiological responses and final yield. Plants grown in absence of ozone had a greater total biomass and higher yield compared to those grown in presence of ozone. Nevertheless these differences disappeared upon salinization. The reduced ozone damage in saline environment must be interpreted on relative terms, however, since salinity by itself caused a general inhibition of plant growth and yield. Based on these results, it was concluded that defining environment-specific ozone toxicity thresholds is necessary for developing reliable prediction models and/or assessing environmental safeguard levels.

Yield losses from ozone pollution can be estimated by two methods: one involves the use of sensitive (S) and resistant (R) biotypes of white clover (Trifolium repens L., cv. Regal) exposed in ambient air, the other is based on the use of open-top chambers (OTC) supplied either with charcoal-filtered (CF) or non-filtered (NF) air. In southern Italy the two methods have been compared using the clover biotypes. The aim was (1) to compare the extent of ozone-induced yield reductions estimated by the two methods, (2) to evaluate the effect of the chamber enclosure on the growth of both biotypes, and (3) to compare plant water consumption in the different environments. On the average, the yield reduction was 23% when derived from the S/R yield ratio in ambient air, and 18% obtained by the CF/NF yield ratio of the S-type, without a significant difference between the two values. The slightly lower value for the OTC-based system may be due to the lower ozone levels in NF chambers due to losses in the ventilation system. Thus, both methods yielded equivalent yield reductions of about 20% due to ozone at this Mediterranean site. However, the higher air temperature inside OTCs influenced the plant growth, and this effect was stronger in the case of the R type. Therefore, R/S yield ratios in NF chambers differed from ambient air. Also, plant water consumption was higher in OTCs than in ambient air. The results suggest that the OTC-based method enables yield loss estimates at this Mediterranean site, in spite of chamber effects on plant growth and water use.