Variations in physiological and biochemical traits of oak seedlings grown under drought and ozone stress

Drought mitigates the adverse effects of O3 on plant photosynthesis rather than growth: A global meta-analysis considering plant functional types

Tropospheric ozone (O3 ) is a phytotoxic air pollutant adversely affecting plant growth. High O3 exposures are often concurrent with summer drought. The effects of both stresses on plants are complex, and their interactions are not yet well understood. Here, we investigate whether drought can mitigate the negative effects of O3 on plant physiology and growth based on a meta-analysis. We found that drought mitigated the negative effects of O3 on plant photosynthesis, but the modification of the O3 effect on the whole-plant biomass by drought was not significant. This is explained by a compensatory response of water-deficient plants that leads to increased metabolic costs. Relative to water control condition, reduced water treatment decreased the effects of O3 on photosynthetic traits, and leaf and root biomass in deciduous broadleaf species, while all traits in evergreen coniferous species showed no significant response. This suggested that the mitigating effects of drought on the negative impacts of O3 on the deciduous broadleaf species were more extensive than on the evergreen coniferous ones. Therefore, to avoid over- or underestimations when assessing the impact of O3 on vegetation growth, soil moisture should be considered. These results contribute to a better understanding of terrestrial ecosystem responses under global change.

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.

Functional responses of two Mediterranean pine species in an ozone Free-Air Controlled Exposure (FACE) experiment

Effects of the phytotoxic and widespread ozone (O3) pollution may be species specific, but knowledge on Mediterranean conifer responses to long-term realistic exposure is still limited. We examined responses regarding to photosynthesis, needle biochemical stress markers and carbon and nitrogen (N) isotopes of two Mediterranean pine species (Pinus halepensis Mill. and Pinus pinea L.). Seedlings were grown in a Free-Air Controlled Exposure experiment with three levels of O3 (ambient air, AA [38.7 p.p.b. as daily average]; 1.5 × AA and 2.0 × AA) during the growing season (May-October 2019). In P. halepensis, O3 caused a significant decrease in the photosynthetic rate, which was mainly due to a reduction of both stomatal and mesophyll diffusion conductance to CO2. Isotopic analyses indicated a cumulative or memory effect of O3 exposure on this species, as the negative effects were highlighted only in the late growing season in association with a reduced biochemical defense capacity. On the other hand, there was no clear effect of O3 on photosynthesis in P. pinea. However, this species showed enhanced N allocation to leaves to compensate for reduced photosynthetic N- use efficiency. We conclude that functional responses to O3 are different between the two species determining that P. halepensis with thin needles was relatively sensitive to O3, while P. pinea with thicker needles was more resistant due to a potentially low O3 load per unit mass of mesophyll cells, which may affect species-specific resilience in O3-polluted Mediterranean pine forests.

Exogenous Application of Salicylic Acid Improves Physiological and Biochemical Attributes of Morus alba Saplings under Soil Water Deficit

Morus alba L. is a multipurpose and fast-growing tree species. However, its growth and productivity are susceptible to water stress. Therefore, a study was conducted to check the effectiveness of foliar application of salicylic acid (SA) in improving the water stress tolerance of M. alba. A pot experiment was conducted and the morphological, physiological and biochemical attributes of young M. alba saplings were assessed under control (CK, 90% of field capacity (FC)), moderate (MS, 60% of FC) and high soil water deficits (HS, 30% of FC), along with MS and HS + foliar application of SA 0.5 and 1.0 mM (MS + 0.5; HS + 0.5; MS + 1.0, and HS+1.0, respectively). Results demonstrated that the highest decrease in plant growth, leaf, stems and roots’ dry biomass, chlorophyll a, b, carotenoid contents and leaf gas exchange parameters was observed under HS, whereas the lowest decrease was evidenced for HS + 1.0 mM SA. Electrolyte leakage, malondialdehyde contents, hydrogen peroxide and superoxide radicals significantly increased under HS, while the lowest increase was evidenced for HS + 1.0 mM SA. The highest increase in proline content, total soluble sugar, total phenolic content, soluble protein and superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase was also found under HS + 1.0 mM SA. Based on the results, it can be concluded that foliar application of SA can help improve the water deficit tolerance of Morus alba saplings, especially under high soil water deficit.

Are Foliar Nutrition Status and Indicators of Oxidative Stress Associated with Tree Defoliation of Four Mediterranean Forest Species?

Mediterranean forest ecosystems in Croatia are of very high significance because of the ecological functions they provide. This region is highly sensitive to abiotic stresses such as air pollution, high sunlight, and high temperatures alongside dry periods; therefore, it is important to monitor the state of these forest ecosystems and how they respond to these stresses. This study was conducted on trees in situ and focused on the four most important forest species in the Mediterranean region in Croatia: pubescent oak (Quercus pubescens Willd.), holm oak (Quercus ilex L.), Aleppo pine (Pinus halepensis Mill.) and black pine (Pinus nigra J. F. Arnold.). Trees were selected and divided into two groups: trees with defoliation of >25% (defoliated) and trees with defoliation of ≤25% (undefoliated). Leaves and needles were collected from selected trees. Differences in chlorophyll content, hydrogen peroxide content, lipid peroxidation and enzyme activity (superoxide dismutase, catalase, ascorbate peroxidase, non-specific peroxidase), and nutrient content between the defoliated and undefoliated trees of the examined species were determined. The results showed that there were significant differences for all species between the defoliated and undefoliated trees for at least one of the examined parameters. A principal component analysis showed that the enzyme ascorbate peroxidase can be an indicator of oxidative stress caused by ozone. By using oxidative stress indicators, it is possible to determine whether the trees are under stress even before visual damage occurs.

Species-Level Differences in Osmoprotectants and Antioxidants Contribute to Stress Tolerance of Quercus robur L., and Q. cerris L. Seedlings under Water Deficit and High Temperatures

The general aim of this work was to compare the leaf-level responses of different protective components to water deficit and high temperatures in Quercus cerris L. and Quercus robur L. Several biochemical components of the osmotic adjustment and antioxidant system were investigated together with changes in hormones. Q. cerris and Q. robur seedlings responded to water deficit and high temperatures by: (1) activating a different pattern of osmoregulation and antioxidant mechanisms depending on the species and on the nature of the stress; (2) upregulating the synthesis of a newly-explored osmoprotectant, dimethylsulphoniopropionate (DMSP); (3) trading-off between metabolites; and (4) modulating hormone levels. Under water deficit, Q. cerris had a higher antioxidant capacity compared to Q. robur, which showed a lower investment in the antioxidant system. In both species, exposure to high temperatures induced a strong osmoregulation capacity that appeared largely conferred by DMSP in Q. cerris and by glycine betaine in Q. robur. Collectively, the more stress-responsive compounds in each species were those present at a significant basal level in non-stress conditions. Our results were discussed in terms of pre-adaptation and stress-induced metabolic patterns as related to species-specific stress tolerance features.

Assessing the impact of current tropospheric ozone on yield loss and antioxidant defense of six cultivars of rice using ethylenediurea in the lower Gangetic Plains of India

Climate change influences the current tropospheric ozone (O₃) budget due to industrialization and urbanization processes. In recent years, the impact of elevated O₃ on crop development and yield loss has emerged as one of the most important environmental issues, particularly in rural and suburban areas of the lower Indo-Gangetic Plains of India. The impact of the current tropospheric ozone (O₃) on the crop yield, photosynthetic yield, and enzymatic antioxidants of six rice (Oryza sativa L.) cultivars (IR 36, MTU 1010, GB 3, Khitish, IET 4786, and Ganga Kaveri) was investigated with and without the application of ethylenediurea (EDU). The results revealed that O₃ stress significantly affected crop yield, photosynthetic yield, and antioxidant enzymes. The findings showed that O₃ toxicity induces oxidative stress biomarkers, i.e., malondialdehyde (MDA) content, and was manifested by increasing the enzymatic antioxidants, i.e., superoxidase dismutase (SOD) and catalase (CAT) in four rice cultivars (IR 36, GB 3, IET 4786, and Ganga Kaveri). At the same time, the results also illustrated that the rice cultivars MTU 1010 and Khitish are more tolerant to O₃ stress as they had less oxidative damage, greater photosynthetic SPAD value, SOD and CAT activities, and lower MDA activity. The results also elucidated that the application of EDU decreased O₃ toxicity in sensitive cultivars of rice by increasing antioxidant defense systems. The current O₃ level is likely to show an additional increase in the near future, and the use of tolerant genotypes of rice may reduce the negative impacts of O₃ on rice production.

Individual and Interactive Effects of Elevated Ozone and Temperature on Plant Responses

From the preindustrial era to the present day, the tropospheric ozone (O3) concentration has increased dramatically in much of the industrialized world due to anthropogenic activities. O3 is the most harmful air pollutant to plants. Global surface temperatures are expected to increase with rising O3 concentration. Plants are directly affected by temperature and O3. Elevated O3 can impair physiological processes, as well as cause the accumulation of reactive oxygen species (ROS), leading to decreased plant growth. Temperature is another important factor influencing plant development. Here, we summarize how O3 and temperature elevation can affect plant physiological and biochemical characteristics, and discuss results from studies investigating plant responses to these factors. In this review, we focused on the interactions between elevated O3 and temperature on plant responses, because neither factor acts independently. Temperature has great potential to significantly influence stomatal movement and O3 uptake. For this reason, the combined influence of both factors can yield significantly different results than those of a single factor. Plant responses to the combined effects of elevated temperature and O3 are still controversial. We attribute the substantial uncertainty of these combined effects primarily to differences in methodological approaches.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks

Mesophyll conductance (gmCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O3 ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O3 and drought on gmCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O3 (ambient [35.2 nmol mol-1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO2 inside a leaf. The combination of O3 and WD significantly decreased net photosynthetic rate (PN ) regardless of the species. The reduction of photosynthesis was associated with a decrease in gmCO2 and stomatal conductance (gsCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (Vcmax ) and maximum electron transport rate (Jmax ) with decreased diffusive conductance parameters. The reduction of gmCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between gmCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between gsCO2 and ABA under high O3 and WD due to impaired stomatal physiological behaviour, indicating that the reduction of PN was related to gmCO2 rather than gsCO2 . The results suggest that gmCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O3 pollution.

Stress markers and physiochemical responses of the Mediterranean shrub Phillyrea angustifolia under current and future drought and ozone scenarios

Mediterranean plants are particularly threatened by the exacerbation of prolonged periods of summer drought and increasing concentrations of ground-level ozone (O₃). The aims of the present study were to (i) test if selected markers (i.e., reactive oxygen species, ROS; malondialdehyde, MDA; photosynthetic pigments) are able to discriminate the oxidative pressure due to single and combined stress conditions, and (ii) elucidate the physiochemical adjustments adopted by Phillyrea angustifolia (evergreen woody species representative of the maquis, also known as narrow-leaved mock privet) to perceive and counter to drought and/or O₃. Plants were grown from May to October under the combination of two levels of water irrigation [i.e., well-watered (WW) and water-stressed (WS)] and three levels of O₃ [i.e., 1.0, 1.5 and 2.0 times the ambient air concentrations, i.e. AA (current O₃ scenario), 1.5 × AA and 2.0 × AA (future O₃ scenarios), respectively], using a new-generation O₃ Free Air Controlled Exposure (FACE) system. Overall, this species appeared relatively sensitive to drought (e.g., net CO₂ assimilation rate and stomatal conductance significantly decreased, as well as total chlorophyll and carotenoid contents), and tolerant to O₃ (e.g., as confirmed by the absence of visible foliar injury, the unchanged values of total carotenoids, and the detrimental effects on stomatal conductance, total chlorophylls and terpene emission only under elevated O₃ concentrations). The combination of both stressors led to harsher oxidative stress. Only when evaluated together (i.e., combining the information provided by the analysis of each stress marker), ROS, MDA and photosynthetic pigments, were suitable stress markers to discriminate the differential oxidative stress induced by drought and increasing O₃ concentrations applied singly or in combination: (i) all these stress markers were affected under drought per se; (ii) hydrogen peroxide (H₂O₂) and MDA increased under O₃per se, following the gradient of O₃ concentrations (H₂O₂: about 2- and 4-fold higher; MDA: +22 and + 91%; in 1.5 × AA_WW and 2.0 × AA_WW, respectively); (iii) joining together the ROS it was possible to report harsher effects under 2.0 × AA_WS and 1.5 × AA_WS (both anion superoxide and H₂O₂ increased) than under 2.0 × AA_WW (only H₂O₂ increased); and (iv) MDA showed harsher effects under 2.0 × AA_WS than under 1.5 × AA_WS (increased by 49 and 18%, respectively). Plants activated physiological and biochemical adjustments in order to partially avoid (e.g., stomatal closure) and tolerate (e.g., increased terpene emission) the effects of drought when combined with increasing O₃ concentrations, suggesting that the water use strategy (isohydric) and the sclerophyllous habit can further increase the plant tolerance to environmental constraints in the Mediterranean area.

Morpho-Physiological and Biochemical Changes in Syzygium cumini and Populus deltoides: A Case Study on Young Saplings under Water Stress

Drought is one of the most devastating climate factors in terms of its spatial extent and intensity. Therefore, a study was conducted to evaluate the water stress tolerance in young saplings of Syzygium cumini (L.) Skeels and Populus deltoides Marchall that are cultivated in the rain fed areas of Pakistan. Plants were subjected to three levels of moisture regimes: well-watered (WW, 90% of field capacity), mild stress (MS, 60% field capacity), and severe stress (SS, 30% of field capacity). Results showed that dry biomass production (leaf, stem, and root), chlorophyll a, b and carotenoid contents decreased significantly while osmolyte accumulation increased in both species, with the highest increase was evidenced in Populus deltoides saplings. A significant decrease was evidenced in CO2 assimilation rate and stomatal conductance that resulted in a significant increase in intrinsic water use efficiency in both species under MS and SS. In both the species, along with a significant increase in the production of hydrogen peroxide and superoxide radical, the antioxidants enzyme activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase also increased significantly in both species under MS and SS with highest activity evidenced in Syzygium cumini. The results suggest that Syzygium cumini saplings showed better a tolerance mechanism to water stress.

Field Performances of Mediterranean Oaks in Replicate Common Gardens for Future Reforestation under Climate Change in Central and Southern Europe: First Results from a Four-Year Study

Climate change imposes severe stress on European forests, with forest degradation already visible in several parts of Europe. Thus adaptation of forestry applications in Mediterranean areas and central Europe is necessary. Proactive forestry management may include the planting of Mediterranean oak species in oak-bearing Central European regions. Five replicate common gardens of Greek and Italian provenances of Quercus ilex, Q. pubescens and Q. frainetto seedlings (210 each per plantation) were established in Central Italy, NE Greece (two) and Southern Germany (two, including Q. robur) to assess their performance under different climate conditions. Climate and soil data of the plantation sites are given and seedling establishment was monitored for survival and morphological parameters. After 3 years (2019) survival rates were satisfactory in the German and Italian sites, whereas the Greek sites exerted extremely harsh conditions for the seedlings, including extreme frost and drought events. In Germany, seedlings suffered extreme heat and drought periods in 2018 and 2019 but responded well. Provenances were ranked for each country for their performance after plantation. In Greece and Italy, Q. pubescens was the best performing species. In Germany, Q. pubescens and Q. robur performed best. We suggest that Greek or Italian provenances of Q. pubescens may be effectively used for future forestation purposes in Central Europe. For the establishment of Quercus plantations in Northern Greece, irrigation appears to be a crucial factor in seedling establishment.

Salicylic Acid-Induced Morpho-Physiological and Biochemical Changes Triggered Water Deficit Tolerance in Syzygium cumini L. Saplings

Fruit tree culture is at the brink of disaster in arid to semi-arid regions due to low water availability. A pot experiment was carried out to analyze whether foliar application of salicylic acid (SA) can improve water stress tolerance in Syzygiumcumini. Saplings were subjected to control (CK, 90% of field capacity, FC), medium stress (MS, 60% of FC) and high stress (HS, 30% of FC) along with foliar application of 0.5 and 1.0 mM of SA. Results showed that soil water deficit significantly decreased leaf, stem and total dry weight, leaf gas exchange attributes and chlorophyll a, b. However, root dry weight and root/shoot ratio increased under MS and HS, respectively. Contrarily, foliar application of SA significantly improved chlorophyll a, b, leaf gas exchange attributes, and dry weight production under soil water deficit. Concentration of oxidants like hydrogen peroxide and superoxide radicals, along with malondialdehyde and electrolyte leakage increased under soil water deficit; however, decreased in plants sprayed with SA due to the increase in the concentration of antioxidant enzymes like superoxide dismutase, peroxidase, catalase and ascorbate peroxidase. Results suggest that the foliar application of SA can help improve water stress tolerance in Syzygiumcumini saplings; however, validation of the results under field conditions is necessary.

Molecular Research on Stress Responses in Quercus spp.: From Classical Biochemistry to Systems Biology through Omics Analysis

The genus Quercus (oak), family Fagaceae, comprises around 500 species, being one of the most important and dominant woody angiosperms in the Northern Hemisphere. Nowadays, it is threatened by environmental cues, which are either of biotic or abiotic origin. This causes tree decline, dieback, and deforestation, which can worsen in a climate change scenario. In the 21st century, biotechnology should take a pivotal role in facing this problem and proposing sustainable management and conservation strategies for forests. As a non-domesticated, long-lived species, the only plausible approach for tree breeding is exploiting the natural diversity present in this species and the selection of elite, more resilient genotypes, based on molecular markers. In this direction, it is important to investigate the molecular mechanisms of the tolerance or resistance to stresses, and the identification of genes, gene products, and metabolites related to this phenotype. This research is being performed by using classical biochemistry or the most recent omics (genomics, epigenomics, transcriptomics, proteomics, and metabolomics) approaches, which should be integrated with other physiological and morphological techniques in the Systems Biology direction. This review is focused on the current state-of-the-art of such approaches for describing and integrating the latest knowledge on biotic and abiotic stress responses in Quercus spp., with special reference to Quercus ilex, the system on which the authors have been working for the last 15 years. While biotic stress factors mainly include fungi and insects such as Phytophthora cinnamomi, Cerambyx welensii, and Operophtera brumata, abiotic stress factors include salinity, drought, waterlogging, soil pollutants, cold, heat, carbon dioxide, ozone, and ultraviolet radiation. The review is structured following the Central Dogma of Molecular Biology and the omic cascade, from DNA (genomics, epigenomics, and DNA-based markers) to metabolites (metabolomics), through mRNA (transcriptomics) and proteins (proteomics). An integrated view of the different approaches, challenges, and future directions is critically discussed.

Effects of Soil Water Deficit on Three Tree Species of the Arid Environment: Variations in Growth, Physiology, and Antioxidant Enzyme Activities

Low water availability predicted under climate change is a major abiotic factor limiting plants growth and productivity. In this study a greenhouse experiment was conducted on three important tree species of arid environment: Conocarpus erectus (CE), Acacia modesta (AM), and Salix tetrasperma (ST). Young saplings were subjected to control (C), medium (MWD) and severe soil water deficit (SWD) treatments and response was evaluated. Results showed that in all the three species leaf, stem and root dry weight production remained similar to C under MWD treatment but decreased significantly under SWD. The highest decrease in total dry weight was noticed in ST and the lowest was evidenced in AM under SWD. Root:shoot ratio increased significantly in both CE and AM under MWD and SWD. Furthermore, chlorophyll content decreased while proline content increased significantly in both MWD and SWD treatments. The production of oxidants (hydrogen peroxide and superoxide anions) and antioxidants (superoxide dismutase, catalase, peroxidase and ascorbate peroxidase) increased significantly under both MWD and SWD treatments and were the highest in AM in both MWD and SWD treatments. Therefore, we may conclude that all the three species can tolerate medium water stress due to increased root production and an effective antioxidant defense mechanism.

Isotopic and Water Relation Responses to Ozone and Water Stress in Seedlings of Three Oak Species with Different Adaptation Strategies

The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O3) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity—WS and WW treatments, respectively) and O3 exposure (1.0, 1.2, and 1.4 times the ambient concentration—AA, 1.2AA, and 1.4AA, respectively) were carried out on Quercus robur L., Quercus ilex L., and Quercus pubescens Willd. seedlings, to study physiological traits (1. isotope signature [δ13C, δ18O and δ15N], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, Asat, and stomatal conductance, gs]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased Asat in Q. robur and Q. pubescens while water stress decreased it in all three oak species. Ozone did not affect δ13C, whereas δ18O was influenced by O3 especially in Q. robur. This may reflect a reduction of gs with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O3 on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by δ13C and by δ18O in all three oak species, while δ15N did not define plant response to stress conditions in any species. The δ13C signal was correlated to leaf water content (LWC) in Q. robur and Q. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O3 exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy.

Effect of water deficit stress on an Indian wheat cultivar (Triticum aestivum L. HD 2967) under ambient and elevated level of ozone

The response of a wheat cultivar (HD 2967) under the combination of elevated ozone (O₃) and water deficit stress (WS) was evaluated in terms of morphological, physiological and yield parameters along with nutrient uptake and their redistribution to different plant parts. An open-top chamber experiment has been conducted under O₃ exposures (ambient (A) and ambient +20 ppb O₃ (E)) along with two different water regimes (well-watered; WW and water deficit with 50% of soil capacity; WS). Most of the growth parameters showed significant reductions due to elevated O₃ under both WW and WS conditions. Stomatal conductance and assimilation rate reduced significantly under the combined stress as compared to their controls (AWW). The maximum decrease in grain yield was observed under the additive effect of both the stresses of water deficit and elevated O₃ (-43.6%), followed by water deficit stress (-19.8%) and elevated O₃ (-17.9%) as compared to the control (AWW). Furthermore, the study displayed that reduced water availability has checked the uptake of nutrients in the roots, shoot and leaves, while, a higher carbon accumulation has been observed with subsequent increases in C: N and C: K ratios in the leaves. Such limitation of nutrients uptake and photosynthates availability weakened the antioxidative defense system of the test cultivar, making it more sensitive against combined stresses. Besides, the study displayed that the defense system has been remarkably suppressed under the presence of interactive stress factors, which allowed us to predict that the distribution of limited carbon pool has inverse relationship between the plant's defense system and growth.

Differential physiological and biochemical responses of Quercus infectoria and Q. libani to drought and charcoal disease

The vast oak-dominated forests of the Zagros Mountains in southwestern Iran currently undergo large-scale dieback driven by a combination of drought and increasing incidence of charcoal disease caused by the fungal pathogens Biscogniauxia mediterranea and Obolarina persica. Here, we explore the interactive effects between drought and charcoal disease agents on the physiology and biochemistry of Quercus infectoria and Quercus libani seedlings. The combination of pathogen attack and water limitation hampered plant development, especially in Q. libani seedlings, negatively affecting growth, biomass production, photosynthetic efficiency, and leaf water potential. An increase in markers of oxidative damage together with the upregulation of the antioxidant defense revealed that drought stress and pathogen infection led to pro-oxidative conditions in both oak species, especially in Q. libani, where larger changes in malondialdehyde and hydrogen peroxide occurred. The upregulation of the antioxidant system was more prominent in Q. infectoria than in Q. libani, resulting in enhanced enzyme activity and accumulation of non-enzymatic antioxidants. Fungal infection stimulated the activity of chitinase, phenylalanine ammonia lyase and β-1,3-glucanase in Q. infectoria leaves and this response became more pronounced under water shortage. Our study highlights that drought stress greatly intensifies the effects of the charcoal disease. Moreover, our findings imply superior stress resistance of Q. infectoria conferred by a highly efficient antioxidant system, strong osmotic adjustment (through proline), and increases in resistance enzymes and secondary metabolites (phenols and flavonoids). Future investigations should focus on adult trees in their natural habitat including interactions with soil factors and other pathogens like nematodes, bacteria and other fungi. Because the present research was conducted on oak seedlings, the findings can be considered by forest nursery managers.

Water use strategy affects avoidance of ozone stress by stomatal closure in Mediterranean trees-A modelling analysis

Both ozone (O₃ ) and drought can limit carbon fixation by forest trees. To cope with drought stress, plants have isohydric or anisohydric water use strategies. Ozone enters plant tissues through stomata. Therefore, stomatal closure can be interpreted as avoidance to O₃ stress. Here, we applied an optimization model of stomata involving water, CO₂ , and O₃ flux to test whether isohydric and anisohydric strategies may affect avoidance of O₃ stress by stomatal closure in four Mediterranean tree species during drought. The data suggest that stomatal closure represents a response to avoid damage to the photosynthetic mechanisms under elevated O₃ depending on plant water use strategy. Under high-O₃ and well-watered conditions, isohydric species limited O₃ fluxes by stomatal closure, whereas anisohydric species activated a tolerance response and did not actively close stomata. Under both O₃ and drought stress, however, anisohydric species enhanced the capacity of avoidance by closing stomata to cope with the severe oxidative stress. In the late growing season, regardless of the water use strategy, the efficiency of O₃ stress avoidance decreased with leaf ageing. As a result, carbon assimilation rate was decreased by O₃ while stomata did not close enough to limit transpirational water losses.

Trends and inter-relationships of ground-level ozone metrics and forest health in Lithuania

Lithuania is representative of maritime to continental climate, no water limitation, and moderate ground-level ozone (O₃) pollution. We investigated the trends of meteorological variables and O₃ and how these environmental conditions associate with tree health from 2001 onward. Ozone metrics for forest protection, based on Accumulated O₃ exposure Over a Threshold of X ppb (AOTX) or on Phytotoxic O₃ Dose over a Y threshold (PODY), were modeled at nine ICP-Forests plots over the time period 2001-2014. Tree-response indicators, i.e. crown defoliation and visible foliar O₃ injury, were assessed during annual field surveys carried out at each ICP-Forests plot over the time period 2007-2017. Mann-Kendall and Sen statistical tests were applied to estimate changes over time of meteorological variables, response indicators and O₃ metrics. Finally, the O₃ metrics were correlated (Spearman test) to the response indicators over the common period 2007-2014. Over this time period, trend analyses revealed an increasingly hotter (+0.27 °C decade^-1, on average) and drier climate (rainfall, -48 mm decade^-1). A reduction was found for O₃ annual mean (-0.28 ppb decade^-1, on average) and AOT40 (-2540 ppb·h decade^-1, on average) whereas an increase was found for POD0 (+0.40 mmol m^-2 decade^-1, on average). Visible foliar O₃ injury increased (+0.17% decade^-1), while an improvement of the crown conditions (-5.0% decade^-1) was observed. AOT40 was significantly associated with crown defoliation while PODY and soil water content were correlated with visible foliar O₃ injury. As visible foliar O₃ injury was negligible in all the studied species, the results suggest that moderate O₃ pollution (approximately 30 ppb as annual average) does not induce biologically significant effects on this forest vegetation under the current conditions, however the overall O₃ risk (POD0) is expected to increase in the future under a hotter and drier climate.

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.

When "thirsty" means "less able to activate the signalling wave trigged by a pulse of ozone": A case of study in two Mediterranean deciduous oak species with different drought sensitivity

There is a lack of knowledge about the possibility that plants facing abiotic stressors, such as drought, have an altered perception of a pulse of O₃ and incur in alterations of their signalling network. This poses some concerns as to whether defensive strategy to cope episodic O₃ peaks in healthy plants may fail under stress. In this study, a set of saplings of two Mediterranean deciduous species, Quercus cerris and Q. pubescens, was subjected to water withholding (20% of daily evapotranspiration for 15 days) while another set was kept well-watered. Saplings were then subjected to a pulse of O₃ (200 nl l^-1 for 5 h) or maintained in filtered air. Q. pubescens had a more severe decline of photosynthesis and leaf PDΨ_w (about -65% and 5-fold lower than in well-watered ones) and events of cell death were observed under drought when compared to Q. cerris, which is supportive for a higher sensitivity to drought exhibited by this species. When O₃ was applied after drought, patterns of signalling compounds were altered in both species. Only in Q. pubescens, the typical O₃-induced accumulation of apoplastic reactive oxygen species, which is the first necessary step for the activation of signalling cascade, was completely lost. In Q. cerris the most frequent changes encompassed the weakening of peaks of key signalling molecules (ethylene and salicylic acid), whereas in Q. pubescens both delayed (salicylic and jasmonic acid) or weakened (ethylene and salicylic acid) peaks were observed. This is translated to a higher ability of Q. cerris to maintain a prompt activation of defensive reaction to counteract oxidative damage due to the pollutant. Our results reveal the complexity of the signalling network in plants facing multiple stresses and highlight the need to further investigate possible alteration of defensive mechanism of tree species to predict their behavior.

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.

Different belowground responses to elevated ozone and soil water deficit in three European oak species (Quercus ilex, Q. pubescens and Q. robur)

Effects on roots due to ozone and/or soil water deficit often occur through diminished belowground allocation of carbon. Responses of root biomass, morphology, anatomy and ectomycorrhizal communities were investigated in seedlings of three oak species: Quercus ilex L., Q. pubescens Willd. and Q. robur L., exposed to combined effects of elevated ozone (ambient air and 1.4 × ambient air) and water deficit (100% and 10% irrigation relative to field capacity) for one growing season at a free-air ozone exposure facility. Effects on root biomass were observed as general reduction in coarse root biomass by -26.8% and in fine root biomass by -13.1% due to water deficit. Effect on coarse root biomass was the most prominent in Q. robur (-36.3%). Root morphological changes manifested as changes in proportions of fine root (<2 mm) diameter classes due to ozone and water deficit in Q. pubescens and due to water deficit in Q. robur. In addition, reduced fine root diameter (-8.49%) in Q. robur was observed under water deficit. Changes in root anatomy were observed as increased vessel density (+18.5%) due to ozone in all three species, as reduced vessel tangential diameter (-46.7%) in Q. ilex due to interaction of ozone and water, and as generally increased bark to secondary xylem ratio (+47.0%) due to interaction of ozone and water. Water deficit influenced occurrence of distinct growth ring boundaries in roots of Q. ilex and Q. robur. It shifted the ectomycorrhizal community towards dominance of stress-resistant species, with reduced relative abundance of Tomentella sp. 2 and increased relative abundances of Sphaerosporella brunnea and Thelephora sp. Our results provide evidence that expression of stress effects varies between root traits; therefore the combined analysis of root traits is necessary to obtain a complete picture of belowground responses.

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.

Ozone risk assessment of castor (Ricinus communis L.) cultivars using open top chamber and ethylenediurea (EDU)

Castor bean (Ricinus communis L.) an important non-edible oilseed crop, is a prominent feed stock towards the generation of renewable materials for industrial production which has multiple applications ranging from cosmetics to biofuels industry. India accounts for 76% of the total world production of castor oil seed. However, major concern for developing countries like India where expanding economy led to rapid increases in gases like NOx, CO and VOCs photochemically form ozone. Ozone is strong oxidant that damages agriculture, ecosystems, and materials with considerable reduction in crop yields and crop quality. One way to reduce ozone induced loss is to focus on the adapting crops to ozone exposure by selecting cultivars with demonstrated ozone resistance. An experiment was conducted for ozone risk assessment of castor cultivars to select cultivar with demonstrated resistance against ozone pollution. This study comprise an open top chamber experiment with three treatments viz. (i) control (ambient ozone concentration), (ii) enhanced ozone (average 75 ppb for 4 h daily throughout the growing season), and (iii) EDU application. Results suggested that the ozone pollution substantially affected growth and physiology of castor cultivars. Crop biomass and yield was also negatively influenced by ozone pollution. Developed defence provided strength to withstand against ozone pollution to the experimental crop cultivars. However, developed defence is cultivar specific and positively correlated with the resistance against ozone pollution. Study concluded that the damage to ozone is directly dependent on the antioxidative potential of plant species. However, ozone adaptability is based on the genetic makeup of the cultivar and yield related loss to ozone can be minimizing by selecting ozone tolerant variety as seen in cultivar Nidhi-999.

Drought induced changes of leaf-to-root relationships in two tomato genotypes

Water deficit triggers a dynamic and integrated cross-talk between leaves and roots. Tolerant plants have developed several physiological and molecular mechanisms to establish new cell metabolism homeostasis, avoiding and/or escaping from permanent impairments triggered by drought. Two tomato genotypes (a Southern Italy landrace called Ciettaicale and the well-known commercial cultivar Moneymaker) were investigated at vegetative stage to assess leaf and root metabolic strategies under 20 days of water deficit. Physiological and metabolic changes, in terms of ABA, IAA, proline, soluble sugars and phenols contents, occurred in both tomato genotypes under water stress. Overall, our results pointed out the higher plasticity of Ciettaicale to manage plant water status under drought in order to preserve the source-sink relationships. This aim was achieved by maintaining a more efficient leaf photosystem II (PSII) photochemistry, as suggested by chlorophyll fluorescence parameters, associated with a major investment towards root growth and activity to improve water uptake. On the contrary, the higher accumulation of carbon compounds, resulting from reduced PSII photochemistry and enhanced starch reserve mobilization, in leaves and roots of Moneymaker under drought could play a key role in the osmotic adjustment, although causing a feedback disruption of the source-sink relations. This hypothesis was also supported by the different drought-induced redox unbalance, as suggested by H₂O₂ and MDA contents. This could affect both PSII photochemistry and root activity, leading to a major involvement of NPQ and antioxidant system in response to drought in Moneymaker than Ciettaicale.

Effects of ozone (O3) and ethylenediurea (EDU) on the ecological stoichiometry of a willow grown in a free-air exposure system

Ground-level ozone (O₃) concentrations have been elevating in the last century. While there has been a notable progress in understanding O₃ effects on vegetation, O₃ effects on ecological stoichiometry remain unclear, especially early in the oxidative stress. Ethyelenediurea (EDU) is a chemical compound widely applied in research projects as protectant of plants against O₃ injury, however its mode of action remains unclear. To investigate O₃ and EDU effects early in the stress, we sprayed willow (Salix sachalinensis) plants with 0, 200 or 400 mg EDU L^-1, and exposed them to either low ambient O₃ (AOZ) or elevated O₃ (EOZ) levels during the daytime, for about one month, in a free air O₃ controlled exposure (FACE); EDU treatment was repeated every nine days. We collected samples for analyses from basal, top, and shed leaves, before leaves develop visible O₃ symptoms. We found that O₃ altered the ecological stoichiometry, including impacts in nutrient resorption efficiency, early in the stress. The relation between P content and Fe content seemed to have a critical role in maintaining homeostasis in an effort to prevent O₃-induced damage. Photosynthetic pigments and P content appeared to play an important role in EDU mode of action. This study provides novel insights on the stress biology which are of ecological and toxicological importance.

Ameliorating Effects of Leaf Water Extract of Three Aromatic Plant Species on Ozone-Polluted Snap Bean (Phaseolus vulgaris L. 'Jiangjunyoudou')

Ozone (O₃) is one of the major pollutants in near-surface air. In order to protect sensitive plants from O₃ pollution, many kinds of protectants including synthetic ones, were assessed in previous studies. Although they have certain protective effects, some of them are not environment-friendly. In the present study, leaf water extracts of aromatic plants [Plectranthus hadiensis var. tomentosus (PHT), Pelargonium hortorum (PHB), Tagetes patula (TP)] were compared for mitigating the damages caused by O₃ (150 ppb for 3 days, 8 h day^-1) on snap bean (Phaseolus vulgaris 'Jiangjunyoudou'). Our results showed that O₃ fumigation impaired plasma membrane, decreased chlorophyll content, increased contents of malondialdehyde and superoxide anion, inhibited photosynthesis, and caused visible injury. Leaf water extracts of PHT, PHB or TP ameliorated the negative effects of O₃. Among them, extract of PHT showed the greatest potential to alleviate the O₃-caused injury, followed by PHB and TP.

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

Liriodendron tulipifera (known as the tulip tree) is a woody species that has been previously classified as sensitive to ozone (O₃) in terms of visible leaf injuries and photosynthetic primary reactions. The objective of this work is to give a thorough description of the detoxification mechanisms that are at the basis of O₃ sensitivity. Biochemical and molecular markers were used to characterize the response of 1-year-old saplings exposed to O₃ (120 ppb, 5 h day^-1, for 45 consecutive days) under controlled conditions. O₃ effects resulted in a less efficient metabolism of Halliwell-Asada cycle as confirmed by the diminished capacity to convert the oxidized forms of ascorbate and glutathione in the reduced ones (AsA and GSH, respectively). The reduced activity of AsA and GSH regenerating enzymes indicates that de novo AsA biosynthesis occurred. This compound could be a cofactor of several plant-specific enzymes that are involved in the early part of the phenylpropanoid and flavonoid biosynthesis pathway, as confirmed by the significant rise of PAL activity (+75%). The induction of the defence-related secondary metabolites (in particular, rutin and caffeic acid were about threefold higher) and the concomitant increase in transcript levels of PAL and CHS genes (+120 and 30%, respectively) suggest that L. tulipifera utilized this route in order to partially counteract the O₃-induced oxidative damage.

Phenylpropanoids are key players in the antioxidant defense to ozone of European ash, Fraxinus excelsior

Physiological and biochemical responses to ozone (O₃) (150 ppb, 8 h day^-1, 35 consecutive days) of two Italian provenances (Piedmont and Tuscany) of Fraxinus excelsior L. were evaluated, with special attention to the role of phenylpropanoids. Our results indicate (i) the high O₃ sensitivity especially of Piedmont provenance (in terms of visible injury, water status, and photosynthetic apparatus); (ii) although the intra-specific sensitivity to O₃ between provenances differs (mainly due to different stomatal behaviors since only Tuscany plants partially avoided the uptake of the pollutant gas), both provenances showed detoxification and defense mechanisms; (iii) the crucial participation of phenylpropanoids, with a key role played by flavonoids (especially quercitrin): among this class of metabolites, isoquercitrin is the principal player in the lower O₃ sensitivity of Tuscany plants, together with lignins; (iv) although coumarins (typical compounds of Fraxinus) were severely depressed by O₃, isofraxidin was triggered suggesting a key role in reactive oxygen species (ROS) detoxification, as well as trans-chalcone. Furthermore, the different behavior of verbascoside and oleuropein among provenances lead us to speculate on their influence in the tentatively repair or acclimation shown by Piedmont plants at the end of the exposure. Finally, the intra-specific O₃ sensitivity may be also due to de novo peaks triggered by O₃ not yet associated to some chemicals.

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.

Ozone risk assessment in three oak species as affected by soil water availability

To derive ozone (O₃) dose-response relationships for three European oak species (Quercus ilex, Quercus pubescens, and Quercus robur) under a range of soil water availability, an experiment was carried out with 2-year-old potted seedlings exposed to three levels of water availability in the soil and three levels of O₃ pollution for one growing season in an ozone free-air controlled exposure (FACE) facility. Total biomass losses were estimated relative to a hypothetical clean air at the pre-industrial age, i.e., at 10 ppb as daily average (M24). A stomatal conductance model was parameterized with inputs from the three species for calculating the stomatal O₃ flux. Exposure-based (M24, W126, and AOT40) and flux-based (phytotoxic O₃ dose (POD)_0-3) dose-response relationships were estimated and critical levels (CL) were calculated for a 5% decline of total biomass. Results show that water availability can significantly affect O₃ risk assessment. In fact, dose-response relationships calculated per individual species at each water availability level resulted in very different CLs and best metrics. In a simplified approach where species were aggregated on the basis of their O₃ sensitivity, the best metric was POD_0.5, with a CL of 6.8 mmol m^-2 for the less O₃-sensitive species Q. ilex and Q. pubescens and of 3.5 mmol m^-2 for the more O₃-sensitive species Q. robur. The performance of POD₀, however, was very similar to that of POD_0.5, and thus a CL of 6.9 mmol m^-2 POD₀ and 3.6 mmol m^-2 POD₀ for the less and more O₃-sensitive oak species may be also recommended. These CLs can be applied to oak ecosystems at variable water availability in the soil. We conclude that POD_y is able to reconcile the effects of O₃ and soil water availability on species-specific oak productivity.

Living in a Mediterranean city in 2050: broadleaf or evergreen 'citizens'?

The predicted effects of global change (GC) will be exacerbated in the more densely populated cities of the future, especially in the Mediterranean basin where some environmental cues, such as drought and tropospheric ozone (O₃) pollution, already mine seriously plant survival. Physiological and biochemical responses of a Mediterranean, evergreen, isohydric plant species (Quercus ilex) were compared to those of a sympatric, deciduous, anisohydric species (Q. pubescens) under severe drought (20% of the effective daily evapotranspiration) and/or chronic O₃ exposure (80 ppb for 5 h day^-1 for 28 consecutive days) to test which one was more successful in those highly limiting conditions. Results show that (i) the lower reduction of total leaf biomass of Q. ilex as compared to Q. pubescens when subjected to drought and drought × O₃ (on average -59 vs -70%, respectively); (ii) the steeper decline of photosynthesis found in Q. pubescens under drought (-87 vs -81%) and drought × O₃ (-69 vs -59%, respectively); (iii) the increments of malondialdehyde (MDA) by-products found only in drought-stressed Q. pubescens; (iv) the impact of O₃, found only in Q. pubescens leaves and MDA, can be considered the best probes of the superiority of Q. ilex to counteract the effect of mild-severe drought and O₃ stress. Also, an antagonistic effect was found when drought and O₃ were applied simultaneously, as usually happens during typical Mediterranean summers. Our dataset suggests that on future, the urban greening should be wisely pondered on the ability of trees to cope the most impacting factors of GC, and in particular their simultaneity.

Using foliar spectral properties to assess the effects of drought on plant water potential

Drought frequency is predicted to increase in future environments. Leaf water potential (ΨLW) is commonly used to evaluate plant water status, but traditional measurements can be logistically difficult and require destructive sampling. We used reflectance spectroscopy to characterize variation in ΨLW of Quercus oleoides Schltdl. & Cham. under differential water availability and tested the ability to predict pre-dawn ΨLW (PDΨLW) using spectral data collected hours after pressure chamber measurements on dark-acclimated leaves. ΨLW was measured with a Scholander pressure chamber. Leaf reflectance was collected at one or both of two time points: immediately (ΨLW) and ~5 h after pressure chamber measurements (PDΨLW). Predictive models were constructed using partial least-squares regression. Model performance was evaluated using coefficient of determination (R2), root-mean-square error (RMSE), bias, and the percent RMSE of the data range (%RMSE). ΨLW and PDΨLW were well predicted using spectroscopic models and successfully estimated a wide variation in ΨLW (light- or dark-acclimated leaves) as well as PDΨLW (dark-acclimated leaves only). Mean ΨLWR2, RMSE and bias values were 0.65, 0.51 MPa and 0.09, respectively, with a %RMSE between 8% and 20%, while mean PDΨLWR2, RMSE and bias values were 0.60, 0.44 MPa and 0.01, respectively, with a %RMSE between 9% and 20%. Estimates of PDΨLW produced similar statistical outcomes when analyzing treatment effects on PDΨLW as those found using reference pressure chamber measurements. These findings highlight a promising approach to evaluate plant responses to environmental change by providing rapid measurements that can be used to estimate plant water status as well as demonstrating that spectroscopic measurements can be used as a surrogate for standard, reference measurements in a statistical framework.

Functional indicators of response mechanisms to nitrogen deposition, ozone, and their interaction in two Mediterranean tree species

The effects of nitrogen (N) deposition, tropospheric ozone (O3) and their interaction were investigated in two Mediterranean tree species, Fraxinus ornus L. (deciduous) and Quercus ilex L. (evergreen), having different leaf habits and resource use strategies. An experiment was conducted under controlled condition to analyse how nitrogen deposition affects the ecophysiological and biochemical traits, and to explore how the nitrogen-induced changes influence the response to O3. For both factors we selected realistic exposures (20 kg N ha-1 yr-1 and 80 ppb h for nitrogen and O3, respectively), in order to elucidate the mechanisms implemented by the plants. Nitrogen addition resulted in higher nitrogen concentration at the leaf level in F. ornus, whereas a slight increase was detected in Q. ilex. Nitrogen enhanced the maximum rate of assimilation and ribulose 1,5-bisphosphate regeneration in both species, whereas it influenced the light harvesting complex only in the deciduous F. ornus that was also affected by O3 (reduced assimilation rate and accelerated senescence-related processes). Conversely, Q. ilex developed an avoidance mechanism to cope with O3, confirming a substantial O3 tolerance of this species. Nitrogen seemed to ameliorate the harmful effects of O3 in F. ornus: the hypothesized mechanism of action involved the production of nitrogen oxide as the first antioxidant barrier, followed by enzymatic antioxidant response. In Q. ilex, the interaction was not detected on gas exchange and photosystem functionality; however, in this species, nitrogen might stimulate an alternative antioxidant response such as the emission of volatile organic compounds. Antioxidant enzyme activity was lower in plants treated with both O3 and nitrogen even though reactive oxygen species production did not differ between the treatments.

Effects of the Antiozonant Ethylenediurea (EDU) on Fraxinus ornus L.: The Role of Drought

Ethylenediurea (EDU) is a synthetic chemical known to protect plants from the phytotoxic effects of tropospheric ozone (O3). Although many studies have proposed the use of EDU for studying the O3 effects under field conditions, its mechanism of action is not fully understood, and it is unclear whether it exerts a specific antiozonant action, or if it may also interact with other oxidative stresses. The aim of this work was to evaluate the effect of EDU on forest species in a Mediterranean environment where, during summer, vegetation is exposed to multiple oxidative stresses, such as O3 and drought. The experiment was conducted on Fraxinus ornus L. (Manna ash) plants growing in six mesocosms, three maintained under full irrigation, while the other three were subjected to drought for 84 days. In each mesocosm, three plants were sprayed every 15 days with 450 ppm EDU. Gas exchange and chlorophyll “a” fluorescence measurements carried out through the experimental period highlighted that EDU did not affect stomatal conductance and had an ameliorative effect on the functionality of drought-stressed plants, thus suggesting that it may act as a generic antioxidant. The implications of these findings for the applicability of EDU in field studies are discussed.

Losing the Warning Signal: Drought Compromises the Cross-Talk of Signaling Molecules in Quercus ilex Exposed to Ozone

Understanding the interactions between drought and acute ozone (O3) stress in terms of signaling molecules and cell death would improve the predictions of plant responses to climate change. The aim was to investigate whether drought stress influences the responses of plants to acute episodes of O3 exposure. In this study, the behavior of 84 Mediterranean evergreen Quercus ilex plants was evaluated in terms of cross-talk responses among signaling molecules. Half of the sample was subjected to drought (20% of the effective daily evapotranspiration, for 15 days) and was later exposed to an acute O3 exposure (200 nL L-1 for 5 h). First, our results indicate that in well-water conditions, O3 induced a signaling pathway specific to O3-sensitive behavior. Second, different trends and consequently different roles of phytohormones and signaling molecules (ethylene, ET; abscisic acid, ABA; salycilic acid, SA and jasmonic acid, JA) were observed in relation to water stress and O3. A spatial and functional correlation between these signaling molecules was observed in modulating O3-induced responses in well-watered plants. In contrast, in drought-stressed plants, these compounds were not involved either in O3-induced signaling mechanisms or in leaf senescence (a response observed in water-stressed plants before the O3-exposure). Third, these differences were ascribable to the fact that in drought conditions, most defense processes induced by O3 were compromised and/or altered. Our results highlight how Q. ilex plants suffering from water deprivation respond differently to an acute O3 episode compared to well-watered plants, and suggest new effect to be considered in plant responses to environmental changes. This poses the serious question as to whether or not multiple high-magnitude O3 events (as predicted) can change these cross-talk responses, thus opening it up possible further investigations.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Combined Effects of Ozone and Drought on the Physiology and Membrane Lipids of Two Cowpea (Vigna unguiculata (L.) Walp) Cultivars

The interactive effects of drought and ozone on the physiology and leaf membrane lipid content, composition and metabolism of cowpea (Vigna unguiculata (L.) Walp.) were investigated in two cultivars (EPACE-1 and IT83-D) grown under controlled conditions. The drought treatment (three-week water deprivation) did not cause leaf injury but restricted growth through stomatal closure. In contrast, the short-term ozone treatment (130 ppb 12 h daily during 14 day) had a limited impact at the whole-plant level but caused leaf injury, hydrogen peroxide accumulation and galactolipid degradation. These effects were stronger in the IT83-D cultivar, which also showed specific ozone responses such as a higher digalactosyl-diacylglycerol (DGDG):monogalactosyldiacylglycerol (MGDG) ratio and the coordinated up-regulation of DGDG synthase (VuDGD2) and ω-3 fatty acid desaturase 8 (VuFAD8) genes, suggesting that membrane remodeling occurred under ozone stress in the sensitive cultivar. When stresses were combined, ozone did not modify the stomatal response to drought and the observed effects on whole-plant physiology were essentially the same as when drought was applied alone. Conversely, the drought-induced stomatal closure appeared to alleviate ozone effects through the reduction of ozone uptake.

The harsh life of an urban tree: the effect of a single pulse of ozone in salt-stressed Quercus ilex saplings

Ozone (O3) and salinity are usually tested as combined factors on plant performance. However, the response to a single episode of O3 in plants already stressed by an excess of NaCl as occurs in the natural environment has never been investigated, but is important given that it is commonly experienced in Mediterranean areas. Three-year-old Quercus ilex L. (holm oak) saplings were exposed to salinity (150 mM NaCl, 15 days), and the effect on photosynthesis, hydric relations and ion partitioning was evaluated (Experiment I). In Experiment II, salt-treated saplings were exposed to 80 nl l-1 of O3 for 5 h, which is a realistic dose in a Mediterranean environment. Gas exchanges, chlorophyll fluorescence and antioxidant systems were characterized to test whether the salt-induced stomatal closure limited O3 uptake and stress or whether the pollutant represents an additional stressor for plants. Salt-dependent stomatal closure depressed the photosynthetic process (-71.6% of light-saturated rate of photosynthesis (A380)) and strongly enhanced the dissipation of energy via the xanthophyll cycle. However, salt-treated plants had higher values of net assimilation rate/stomatal conductance (A/gs) than the controls, which was attributable to a greater mesophyll conductance gm/gs and carboxylation efficiency (higher gm/maximal rate of Rubisco carboxylation (Vcmax)), thus suggesting no damage to chloroplasts. O3 did not exacerbate the effect of salinity on photosynthesis, however a general enhancement of the Halliwell-Asada cycle was necessary to counteract the O3-triggered oxidative stress. Despite the 79.4% gs reduction in salt-stressed plants, which strongly limited the O3 uptake, a single peak in the air pollutant led to an additional burden for the antioxidant system when plants had been previously subjected to salinity.