What about the detoxification mechanisms underlying ozone sensitivity in Liriodendron tulipifera?

A meta-analysis of elevated O3 effects on herbaceous plants antioxidant oxidase activity

Increases in near-surface ozone (O3) concentrations is a global environmental problem. High-concentration O3 induces stress in plants, which can lead to visible damage to plants, reduced photosynthesis, accelerated aging, inhibited growth, and can even plant death. However, its impact has not been comprehensively evaluated because of the response differences between individual plant species, environmental O3 concentration, and duration of O3 stress in plants. We used a meta-analysis approach based on 31 studies 343 observations) to examine the effects of elevated O3 on malondialdehyde (MDA), superoxide dismutase (SOD), and peroxidase (POD) activities in herbaceous plants. Globally, important as they constitute the majority of the world's food crops. We partitioned the variation in effect size found in the meta-analysis according to the presence of plant species (ornamental herb, rice, and wheat), O3 concentration, and duration of O3 stress in plants. Our results showed that the effects of elevated O3 on plant membrane lipid peroxidation depending on plant species, O3 concentration, and duration of O3 stress in plants. The wheat SOD and POD activity was significantly lower compared to the herbs and rice (P<0.01). The SOD activity of all herbaceous plants increased by 34.6%, 10.5%, and 26.3% for exposure times to elevated O3 environments of 1-12, 13-30, and 31-60 days, respectively. When the exposure time was more than 60 days, SOD activity did not increase but significantly decreased by 12.1%. However, the POD activity of herbaceous plants increased by 30.4%, 57.3%, 21.9% and 5.81%, respectively, when exposure time of herbaceous plants in elevated O3 environment was 1-12, 13-30, 31-60 and more than 60 days. Our meta-analysis revealed that (1) rice is more resistant to elevated O3 than wheat and ornamental herbs likely because of the higher activity of antioxidant components (e.g., POD) in the symplasts, (2) exposure to elevated O3 concentrations for >60 days, may result in antioxidant SOD lose its regulatory ability, and the antioxidant component POD in the symplast is mainly used to resist O3 damage, and (3) the important factors affected the activity of SOD and POD in plants were not consistent: the duration of O3 stress in plants was more important than plant species and O3 concentration for SOD activity. However, for POD activity, plant species was the most important factor.

Integrated metabolomics and transcriptomics analysis reveals γ-aminobutyric acid enhances the ozone tolerance of wheat by accumulation of flavonoids

Wheat is susceptible to atmospheric ozone (O3) pollution, thus the increasing O3 is a serious threat to wheat production. γ-aminobutyric acid (GABA) is found to play key roles in the tolerance of plants to stress. However, few studies elaborated the function of GABA in response of wheat to O3. Here, we incorporated metabolome and transcriptome data to provide a more comprehensive insight on the role of GABA in enhancing the O3-tolerance of wheat. In our study, there were 31, 23, and 32 differentially accumulated flavonoids in the carbon-filtered air with GABA, elevated O3 with or without GABA treatments compared to the carbon-filtered air treatment, respectively. Elevated O3 triggered the accumulation of dihydroflavone, flavonols, and flavanols. Exogenous GABA enhanced dihydroflavone and dihydroflavonol, and also altered the expression of genes encoding some key enzymes in the flavonoid synthesis pathway. Additionally, GABA stimulated proline accumulation and antioxidant enzyme activities under elevated O3, resulting in the less accumulation of H2O2 and malondialdehyde. Consequently, GABA alleviated the grain yield loss from 19.6% to 9.6% induced by elevated O3. Our study provided comprehensive insight into the role of GABA in the alleviating the detrimental effects of elevated O3 on wheat, and a new avenue to mitigate O3 damage to the productivity of crops.

Using soil nitrogen amendments in mitigating ozone stress in agricultural crops: a case study of cluster beans

The climate change scenario in the coming years is liable to have serious negative consequences on agricultural productivity. Increasing tropospheric ozone concentration is an important aspect of climate change, which, due to its oxidative nature, is injurious to the plants. Due to the multifarious nature and continuously increasing concentration of tropospheric ozone, it is prerequisite to develop strategies to manage ozone stress in plants. Present study not only evaluates the potential of soil nitrogen amendments in ameliorating ozone stress in plants, but also focuses upon the mechanistic approaches adopted by the different plant cultivars to combat ozone stress. Three doses of nitrogen amendments, recommended (N1), 1.5× recommended (N2) and 2× recommended (N3), were given to two cultivars (S-151 and PUSA-N) of Cymopsis tetragonoloba exposed to ambient ozone stress. Control plants were also maintained in which no nitrogen treatment was given. Nitrogen supplementation reduced the root nodulation frequency and leghaemoglobin content, which subsequently increased the cellular nitrogen metabolism as evident through increase in the activities of nitrate reductase and nitrite reductase in both the test cultivars. The positive effects of nitrogen amendments are clearly evident in the 1D protein profile studies which showed a greater accumulation of larger sub-units of RuBisCO in nitrogen amended plants. The results clearly indicate that N2 treatment effectively enhanced the yield of both the cultivars (84.8% and 76.37%, in S-151 and PUSA-N, respectively); however, the mechanistic approach adopted by the two cultivars was different. Whereas the yield quantity showed higher increments in S-151, the yield quality parameters (carbohydrates and nitrogen contents) responded more positively in PUSA-N.

Secondary metabolites responses of plants exposed to ozone: an update

Tropospheric ozone (O3) is a secondary pollutant that causes oxidative stress in plants due to the generation of excess reactive oxygen species (ROS). Phenylpropanoid metabolism is induced as a usual response to stress in plants, and induction of key enzyme activities and accumulation of secondary metabolites occur, upon O3 exposure to provide resistance or tolerance. The phenylpropanoid, isoprenoid, and alkaloid pathways are the major secondary metabolic pathways from which plant defense metabolites emerge. Chronic exposure to O3 significantly accelerates the direction of carbon flows toward secondary metabolic pathways, resulting in a resource shift in favor of the synthesis of secondary products. Furthermore, since different cellular compartments have different levels of ROS sensitivity and metabolite sets, intracellular compartmentation of secondary antioxidative metabolites may play a role in O3-induced ROS detoxification. Plants' responses to resource partitioning often result in a trade-off between growth and defense under O3 stress. These metabolic adjustments help the plants to cope with the stress as well as for achieving new homeostasis. In this review, we discuss secondary metabolic pathways in response to O3 in plant species including crops, trees, and medicinal plants; and how the presence of this stressor affects their role as ROS scavengers and structural defense. Furthermore, we discussed how O3 affects key physiological traits in plants, foliar chemistry, and volatile emission, which affects plant-plant competition (allelopathy), and plant-insect interactions, along with an emphasis on soil dynamics, which affect the composition of soil communities via changing root exudation, litter decomposition, and other related processes.

Effect of elevated ozone on the antioxidant response, genomic stability, DNA methylation pattern and yield in three species of Abelmoschus having different ploidy levels

The majority of polyploids can withstand many stresses better than their monoploid counterparts; however, there is no proven mechanism that can fully explain the level of tolerance at the biochemical and molecular levels. Here, we make an effort to provide an explanation for this intriguing but perplexing issue using the antioxidant responses, genomic stability, DNA methylation pattern and yield in relation to ploidy level under the elevated level of ozone in Abelmoschus cytotypes. The outcome of this study inferred that the elevated ozone causes an increase in reactive oxygen species leading to enhanced lipid peroxidation, DNA damage and DNA de-methylation in all the Abelmoschus cytotypes. The monoploid cytotype of Abelmoschus, that is Abelmoschus moschatus L., experienced the highest oxidative stress under elevated O₃, resulting in maximum DNA damage and DNA de-methylation leading to the maximum reduction in yield. While the diploid (Abelmoschus esculentus L.) and triploid (Abelmoschus caillei A. Chev.) cytotypes of Abelmoschus with lower oxidative stress result in lesser DNA damage and DNA de-methylation which ultimately leads to lower yield reduction. The result of this experiment explicitly revealed that polyploidy confers better adaptability in the case of Abelmoschus cytotypes under ozone stress. This study can further be used as a base to understand the mechanism behind the ploidy-induced stress tolerance in other plants mediated by gene dosage effect.

Comparative transcriptomics and bioinformatics analysis of genes related to photosynthesis in Eucalyptus camaldulensis

The timber species Eucalyptus camaldulensis is one of the most important in southern China. Therefore, it is essential to understand the photosynthetic pattern in eucalyptus leaves. In the present study, eighteen photosynthesis-related genes were analyzed using bioinformatics methods. The results indicated that there were ten differentially expressed ribose-5-phosphate isomerase genes (RPI), and six of them were up-regulated in the mature leaves compared to the young leaves, while others were down-regulated. The differential expression of four rubisco methyltransferase genes (RBCMT) were observed. Two of them were up-regulated, while two were down-regulated in mature leaves compared to young leaves. Furthermore, two ribulose-phosphate-3-epimerase genes (RPE) were up-regulated in the mature leaves compared to the young leaves. In contrast, two genes involved in triosephosphate isomerase (TIM) were down-regulated in mature leaves compared with young leaves. The current study provides basic information about the transcriptome of E. camaldulensis and lays a foundation for further research in developing and utilizing important photosynthetic genes.

In the tripartite combination ozone-poplar-Chrysomela populi, the pollutant alters the plant-insect interaction via primary metabolites of foliage

Ozone (O₃)-induced metabolic changes in leaves are relevant and may have several ecological significances. Here, variations in foliar chemistry of two poplar clones (Populus deltoides × maximowiczii, Eridano, and P. × euramericana, I-214) under a chronic O₃ treatment (80 ppb, 5 h d^-1 for 10 consecutive days) were investigated. The aim was to elucidate if leaf age and/or O₃-sensitivity (considering Eridano and I-214 as O₃-sensitive and O₃-resistant, respectively) can affect suitability of poplar foliage for Chrysomela populi L. (Coleoptera Chrysomelidae), in terms of palatability. Comparing controls, only low amino acid (AA) contents were reported in Eridano [about 3- and 4-fold in mature and young leaves (ML and YL, respectively)], and all the investigated primary metabolites [i.e. water soluble carbohydrates (WSC), proteins (Prot) and AA] were higher in YL than in ML of I-214 (+23, +54 and + 20%, respectively). Ozone increased WSC only in YL of Eridano (+24%, i.e. highest values among samples; O₃ effects are always reported comparing O₃-treated plants with the related controls). A concomitant decrease of Prot was observed in both ML and YL of Eridano, while only in YL of I-214 (-41, -45 and -51%, respectively). In addition, O₃ decreased AA in YL of Eridano and in ML of I-214 (-40 and -14%, respectively). Comparing plants maintained under charcoal-filtered air, total ascorbate (Asc) was lower in Eridano in both ML and YL (around -22%), and abscisic acid (ABA) was similar between clones; furthermore, higher levels of Asc were reported in YL than in ML of Eridano (+19%). Ozone increased Asc and ABA (about 2- and 3-fold, respectively) in both ML and YL of Eridano, as well as ABA in YL of I-214 (about 2-fold). Comparing leaves maintained under charcoal-filtered air, the choice feeding test showed that the 2nd instar larvae preferred YL, and the quantity of YL consumed was 9 and 4-fold higher than ML in Eridano and I-214, respectively. Comparing leaves exposed to O₃-treatment, a significant feeding preference for YL disks was also observed, regardless of the clone. The no-choice feeding test showed that larval growth was slightly higher on untreated YL than on untreated ML (+19 and + 10% in Eridano and I-214, respectively). The body mass of larvae fed with O₃-treated YL was also significantly higher than that of larvae fed with untreated YL (3- and 2-fold in Eridano and I-214). This study highlights that realistic O₃ concentrations can significantly impact the host/insect interactions, a phenomenon dependent on leaf age and O₃-sensitivity of the host.

Role of antioxidant pool in management of ozone stress through soil nitrogen amendments in two cultivars of a tropical legume

The present experiment was done on two different cultivars of a tropical legume, Cymopsis tetragonoloba L. Taub. (cluster bean) cvv. Pusa-Naubahar (PUSA-N) and Selection-151 (S-151). The experiment was conducted under ambient ozone (O3) conditions with inputs of three different doses of inorganic nitrogen (N1, recommended; N2, 1.5-times recommended and N3, 2-times recommended) as well as control plants. The objective of this study was to evaluate the effectiveness of soil nitrogen amendments in management of ambient ozone stress in the two cultivars of C. tetragonoloba. Our experiment showed that nitrogen amendments can be an efficient measure to manage O3 injury in plants. Stimulation of antioxidant enzyme activities under nitrogen amendments is an important feature of plants that help plants cope with ambient O3 stress. Nitrogen amendments strengthened the antioxidant machinery in a more effective way in the tolerant cultivar PUSA-N, while in the sensitive cultivar S-151, avoidance strategy marked by more reduction in stomatal conductance was more prominent. Enzymes of the Halliwell-Asada pathway, especially ascorbate peroxidase and glutathione reductase, were more responsive and synchronised in PUSA-N than S-151, under similar nitrogen amendment regimes and were responsible for the differential sensitivities of the two cultivars of C. tetragonoloba. The present study shows that 1.5-times recommended dose of soil nitrogen amendments was sufficient in partial mitigation of O3 injury and the higher nitrogen dose (2-times recommended, in our case), did not provide any extra advantage to the plant's metabolism compared with plants treated with the lower nitrogen dose (1.5-times recommended).

Contribution of Bacillus cereus ERBP in ozone detoxification by Zamioculcas zamiifolia plants: Effect of ascorbate peroxidase, catalase and total flavonoid contents for ozone detoxification

Eighteen plant species were screened for ozone (O₃) removal in a continuous system. Zamioculcas zamiifolia had the highest O₃ removal efficiency. To enhance O₃ removal by Z. zamiifolia, adding a compatible endophytic bacteria, Bacillus cereus ERBP into Z. zamiifolia was studied. After operating under an O₃ continuous system (150-250 ppb) at a flow rate of 0.3 L min^-1 for 80 h, inoculated plants (74%) exhibited higher O₃ removal efficiency than non-inoculated ones (53%). In addition, after O₃ exposure (80 h), the population of B. cereus ERBP in inoculated plants was significantly increased in both shoots approximately 35 folds and leaves 13 folds compared to inoculated plants without O₃ exposure. The results also showed that B. cereus ERBP had the ability to protect Z. zamiifolia against O₃ stress conditions. The increase in B. cereus ERBP populations was attributed to the significant increase in ascorbate peroxidase (APX) and catalase (CAT) activity. In addition, increasing B. cereus ERBP populations led to raise total flavonoid contents which is one of antioxidant compounds. Increasing APX, CAT activities, and total flavonoid contents can enhance O₃ detoxification in plant tissues. The mechanism of B. cereus ERBP for enhancing O₃ phytoremediation was proposed in this study. The results suggested that B. cereus ERBP was a potential tool for alleviating O₃ stress on Z. zamiifolia and enhancing O₃ phytoremediation efficiency.

ROS production and its detoxification in early and late sown cultivars of wheat under future O3 concentration

The present field study was planned with an objective to unravel the mechanisms behind the differential responses of early and late sown wheat cultivars with respect to their defense capacity to scavenge ROS induced under elevated O₃ (EO₃). Experiments were performed under ambient and elevated levels of O₃ (ambient + 20 ppb) to plants inside open-top chambers (OTCs). Ozone concentrations, stomatal flux of O₃ and meteorological parameters were measured throughout the experiment. Contents of superoxide radicals (O₂^-) and hydrogen peroxide (H₂O₂) and their localization, lipid peroxidation, antioxidative enzyme activities, ascorbic acid and total phenolic contents were measured at vegetative and reproductive developmental stages. EO₃ exposure induced higher stomatal flux of O₃ in early sown cultivars. Higher contents of O₂^-, H₂O₂ and lipid peroxidation were noticed under EO₃ in all the cultivars but the magnitude of increases was higher in late sown cultivars at the reproductive stage. Activities of glutathione reductase (GR) and ascorbate peroxidase (APX) were higher in late sown cultivars under EO₃. Ascorbic acid and total phenolic contents were significantly higher in early sown than late sown cultivars under EO₃ treatment. The present study concludes that early sown cultivars are more efficient in their defense response due to higher induction of enzymatic and non-enzymatic antioxidants, while the induction of enzymatic antioxidants was more distinct in late sown cultivars. Non-enzymatic linked defense mechanism requires additional metabolic cost than enzymatic defense, making early sown cultivars more susceptible to EO₃. Differential response of early and late sown cultivars with respect to antioxidative defense against O₃ stress suggests that yield responses are governed by the time of sowing and intrinsic defense responses of the cultivars. In future with rising trend of O₃, early sown cultivars are expected to be more vulnerable to oxidative stress compared to late sown cultivars.

Signalling molecules responsive to ozone-induced oxidative stress in Salvia officinalis

Tropospheric ozone (O₃) is the most important gaseous pollutant and induces a mass of negative impacts on vegetation at functional and genic levels. The aim of the present study was to investigate the role of reactive oxygen species and signalling molecules in sage plants exposed to O₃ (200 ppb, 5 h). Ozone exposure induced only a transient oxidative burst, as confirmed by the rapid peak of anion superoxide during the first hours of exposure (+16% compared to controls). The spontaneous reaction of O₃ with membrane fatty acids stimulates peroxidative processes, as demonstrated by the rise of thiobarbituric acid reactive substances concentration starting after 1 h of exposure (+25%). The formation of lipid-based signalling molecules (e.g. jasmonic acid) may be regarded as a sort of O₃-perception. The concomitant accumulation of salicylic acid suggests that sage responds early to O₃ by inducing cellular antioxidants mechanisms in order to minimize O₃-oxidative burst. The transient increase of abscisic acid (+25% at the end of the treatment) twinned with the maximal ethylene emission (about two-fold higher than controls) could be interpreted as a first attempt by plants to regulate the signalling responses induced by O₃. In order to investigate the involvement of transcription factors in managing oxidative protection, BLASTX analysis against the Salvia miltiorrhiza sequence genome was carried out using Arabidopsis thaliana WRKY sequences as queries. Six gene sequences were identified for sage WRKYs and their relative gene expression analyses were characterized. WRKY4, WRKY5, WRKY11 and WRKY46 were up-regulated by O₃ at 2 and 5 h of exposure and they showed similarity with AtWRKY48, AtWRKY22 and AtWRKY53 in A. thaliana. These results suggest that WRKYs could play a pivotal role in the signalling mechanisms during the responses of plants to O₃.

Can nutrient fertilization mitigate the effects of ozone exposure on an ozone-sensitive poplar clone?

We tested the independent and interactive effects of nitrogen (N; 0 and 80 kg ha^-1), phosphorus (P; 0, 40 and 80 kg ha^-1), and ozone (O₃) application/exposure [ambient concentration (AA), 1.5 × AA and 2.0 × AA] for five consecutive months on biochemical traits of the O₃-sensitive Oxford poplar clone. Plants exposed to O₃ showed visible injury and an alteration of membrane integrity, as confirmed by the malondialdehyde by-product accumulation (+3 and +17% under 1.5 × AA and 2.0 × AA conditions, in comparison to AA). This was probably due to O₃-induced oxidative damage, as documented by the production of superoxide anion radical (O₂^-, +27 and +63%, respectively). Ozone per se, independently from the concentrations, induced multiple signals (e.g., alteration of cellular redox state, increase of abscisic acid/indole-3-acetic acid ratio and reduction of proline content) that might be part of premature leaf senescence processes. By contrast, nutrient fertilization (both N and P) reduced reactive oxygen species accumulation (as confirmed by the decreased O₂^- and hydrogen peroxide content), resulting in enhanced membrane stability. This was probably due to the simultaneous involvement of antioxidant compounds (e.g., carotenoids, ascorbate and glutathione) and osmoprotectants (e.g., proline) that regulate the detoxification processes of coping with oxidative stress by reducing the O₃ sensitivity of Oxford clone. These mitigation effects were effective only under AA and 1.5 × AA conditions. Nitrogen and P supply activated a free radical scavenging system that was not able to delay leaf senescence and mitigate the adverse effects of a general peroxidation due to the highest O₃ concentrations.

Cross-talk between physiological and biochemical adjustments by Punica granatum cv. Dente di cavallo mitigates the effects of salinity and ozone stress

Plants are exposed to a broad range of environmental stresses, such as salinity and ozone (O₃), and survive due to their ability to adjust their metabolism. The aim of this study was to evaluate the physiological and biochemical adjustments adopted by pomegranate (Punica granatum L. cv. Dente di cavallo) under realistic field conditions. One-year-old saplings were exposed to O₃ [two levels denoted as ambient (AO) and elevated (EO) O₃ concentrations] and salinity [S (salt, 50 mM NaCl)] for three consecutive months. No salt (NS) plants received distilled water. Despite the accumulation of Na⁺ and Cl^- in the aboveground biomass, no evidence of visible injury due to salt (e.g. tip yellow-brown lesions) was found. The maintenance of leaf water status (i.e. unchanged values of electrolytic leakage and relative water content), the significant increase of abscisic acid, proline and starch content (+98, +65 and +59% compared to AO_NS) and stomatal closure (-24%) are suggested to act as adaptive mechanisms against salt stress in AO_S plants. By contrast, EO_NS plants were unable to protect cells against the negative impact of O₃, as confirmed by the reduction of the CO₂ assimilation rate (-21%), accumulation of reactive oxygen species (+10 and +225% of superoxide anion and hydrogen peroxide) and malondialdehyde by-product (about 2-fold higher than AO_NS). Plants tried to preserve themselves from further oxidative damage by adopting some biochemical adjustments [i.e. increase in proline content (+41%) and induction of catalase activity (8-fold higher than in AO_NS)]. The interaction of the two stressors induced responses considerably different to those observed when each stressor was applied independently. An analysis of the antioxidant pool revealed that the biochemical adjustments adopted by P. granatum under EO_S conditions (e.g. reduction of total ascorbate; increased activities of superoxide dismutase and catalase) were not sufficient to ameliorate the O₃-induced oxidative stress.

Antioxidative responses of three oak species under ozone and water stress conditions

Plants are frequently exposed to adverse environmental conditions such as drought and ozone (O₃). Under these conditions, plants can survive due to their ability to adjust their metabolism. The aim of the present study was to compare the detoxification mechanisms of three oak species showing different O₃ sensitivity and water use strategy. Two-year-old seedlings of Quercus ilex, Q. pubescens and Q. robur were grown under the combination of three levels of O₃ (1.0, 1.2 and 1.4 times the ambient O₃ concentration) and three levels of water availability (on average 100, 80 and 42% of field capacity i.e. well-watered, moderate drought and severe drought, respectively) in an O₃ Free Air Controlled Exposure facility. Ozone and drought induced the accumulation of reactive oxygen species (ROS) and this phenomenon was species-specific. Sometimes, ROS accumulation was not associated with membrane injury suggesting that several antioxidative defence mechanisms inhibited or alleviated the oxidative damage. Both O₃ and drought increased total carotenoids that were able to prevent the peroxidation action by free radicals in Q. ilex, as confirmed by unchanged malondialdehyde by-product values. The concomitant decrease of total flavonoids may be related to the consumption of these compounds by the cell to inhibit the accumulation of hydrogen peroxide. Unchanged total phenols confirmed that Q. ilex has a superior ability to counteract oxidative conditions. Similar responses were found in Q. pubescens, although the negative impact of both factors was less efficiently faced than in the sympatric Q. ilex. In Q. robur, high O₃ concentrations and severe drought induced a partial rearrangement of the phenylpropanoid pathways. These antioxidative mechanisms were not able to protect the cell structure (as confirmed by ROS accumulation) suggesting that Q. robur showed a lower degree of tolerance than the other two species.

Reflectance spectroscopy: a novel approach to better understand and monitor the impact of air pollution on Mediterranean plants

The Mediterranean basin can be considered a hot spot not only in terms of climate change (CC) but also for air quality. Assessing the impact of CC and air pollution on ecosystem functions is a challenging task, and adequate monitoring techniques are needed. This paper summarizes the present knowledge on the use of reflectance spectroscopy for the evaluation of the effects of air pollution on plants. First, the history of this technique is outlined. Next, we describe the vegetation reflectance spectrum, how it can be scaled from leaf to landscape levels, what information it contains, and how it can be exploited to understand plant and ecosystem functions. Finally, we review the literature concerning this topic, with special attention to Mediterranean air pollutants, showing the increasing interest in this technique. The ability of spectroscopy to detect the influence of air pollution on plant function of all major and minor Mediterranean pollutants has been evaluated, and ozone and its interaction with other gases (carbon dioxide, nitrogen oxides, and sulfur dioxide) have been the most studied. In the recent years, novel air pollutants, such as particulate matter, nitrogen deposition, and heavy metals, have drawn attention. Although various vegetation types have been studied, few of these species are representative of the Mediterranean environment. Thus, major emphasis should be placed on using vegetation spectroscopy for better understanding and monitoring the impact of air pollution on Mediterranean plants in the CC era.