Foliar symptoms triggered by ozone stress in irrigated holm oaks from the city of Madrid, Spain

Ante- and post-mortem cellular injury dynamics in hybrid poplar foliage as a function of phytotoxic O3 dose

After reaching phytotoxic levels during the last century, tropospheric ozone (O3) pollution is likely to remain a major concern in the coming decades. Despite similar injury processes, there is astounding interspecific-and sometimes intraspecific-foliar symptom variability, which may be related to spatial and temporal variation in injury dynamics. After characterizing the dynamics of physiological responses and O3 injury in the foliage of hybrid poplar in an earlier study, here we investigated the dynamics of changes in the cell structure occurring in the mesophyll as a function of O3 treatment, time, phytotoxic O3 dose (POD0), leaf developmental stage, and mesophyll layer. While the number of Hypersensitive Response-like (HR-like) lesions increased with higher O3 concentrations and POD0, especially in older leaves, most structural HR-like markers developed after cell death, independent of the experimental factors. The pace of degenerative Accelerated Cell Senescence (ACS) responses depended closely on the O3 concentration and POD0, in interaction with leaf age. Changes in total chlorophyll content, plastoglobuli and chloroplast shape pointed to thylakoid membranes in chloroplasts as being especially sensitive to O3 stress. Hence, our study demonstrates that early HR-like markers can provide reasonably specific, sensitive and reliable quantitative structural estimates of O3 stress for e.g. risk assessment studies, especially if they are associated with degenerative and thylakoid-related injury in chloroplasts from mesophyll.

Elevated ozone decreases the multifunctionality of belowground ecosystems

Elevated tropospheric ozone (O₃ ) affects the allocation of biomass aboveground and belowground and influences terrestrial ecosystem functions. However, how belowground functions respond to elevated O₃ concentrations ([O₃ ]) remains unclear at the global scale. Here, we conducted a detailed synthesis of belowground functioning responses to elevated [O₃ ] by performing a meta-analysis of 2395 paired observations from 222 publications. We found that elevated [O₃ ] significantly reduced the primary productivity of roots by 19.8%, 16.3%, and 26.9% for crops, trees and grasses, respectively. Elevated [O₃ ] strongly decreased the root/shoot ratio by 11.3% for crops and by 4.9% for trees, which indicated that roots were highly sensitive to O₃ . Elevated [O₃ ] impacted carbon and nitrogen cycling in croplands, as evidenced by decreased dissolved organic carbon, microbial biomass carbon, total soil nitrogen, ammonium nitrogen, microbial biomass nitrogen, and nitrification rates in association with increased nitrate nitrogen and denitrification rates. Elevated [O₃ ] significantly decreased fungal phospholipid fatty acids in croplands, which suggested that O₃ altered the microbial community and composition. The responses of belowground functions to elevated [O₃ ] were modified by experimental methods, root environments, and additional global change factors. Therefore, these factors should be considered to avoid the underestimation or overestimation of the impacts of elevated [O₃ ] on belowground functioning. The significant negative relationships between O₃ -treated intensity and the multifunctionality index for croplands, forests, and grasslands implied that elevated [O₃ ] decreases belowground ecosystem multifunctionality.

Dynamics of Foliar Responses to O3 Stress as a Function of Phytotoxic O3 Dose in Hybrid Poplar

With background concentrations having reached phytotoxic levels during the last century, tropospheric ozone (O₃) has become a key climate change agent, counteracting carbon sequestration by forest ecosystems. One of the main knowledge gaps for implementing the recent O₃ flux-based critical levels (CLs) concerns the assessment of effective O₃ dose leading to adverse effects in plants. In this study, we investigate the dynamics of physiological, structural, and morphological responses induced by two levels of O₃ exposure (80 and 100 ppb) in the foliage of hybrid poplar, as a function of phytotoxic O₃ dose (POD₀) and foliar developmental stage. After a latency period driven by foliar ontological development, the gas exchanges and chlorophyll content decreased with higher POD₀ monotonically. Hypersensitive response-like lesions appeared early during exposure and showed sigmoidal-like dynamics, varying according to leaf age. At current POD_{1_SPEC} CL, notwithstanding the aforementioned reactions and initial visible injury to foliage, the treated poplars had still not shown any growth or biomass reduction. Hence, this study demonstrates the development of a complex syndrome of early reactions below the flux-based CL, with response dynamics closely determined by the foliar ontological stage and environmental conditions. General agreement with patterns observed in the field appears indicative of early O₃ impacts on processes relevant, e.g., biodiversity ecosystem services before those of economic significance - i.e., wood production, as targeted by flux-based CL.

Ozone affects leaf physiology and causes injury to foliage of native tree species from the tropical Atlantic Forest of southern Brazil

In southern Brazil, the recent increase in tropospheric ozone (O₃) concentrations poses an additional threat to the biodiverse but endangered and fragmented remnants of the Atlantic Forest. Given the mostly unknown sensitivity of tropical species to oxidative stress, the principal objective of this study was to determine whether the current O₃ levels in the Metropolitan Region of Campinas (MRC), downwind of São Paulo, affect the native vegetation of forest remnants. Foliar responses to O₃ of three tree species typical of the MRC forests were investigated using indoor chamber exposure experiments under controlled conditions and a field survey. Exposure to 70ppb O₃ reduced assimilation and leaf conductance but increased respiration in Astronium graveolens while gas exchange in Croton floribundus was little affected. Both A. graveolens and Piptadenia gonoacantha developed characteristic O₃-induced injury in the foliage, similar to visible symptoms observed in >30% of trees assessed in the MRC, while C. floribundus remained asymptomatic. The underlying structural symptoms in both O₃-exposed and field samples were indicative of oxidative burst, hypersensitive responses, accelerated cell senescence and, primarily in field samples, interaction with photo-oxidative stress. The markers of O₃ stress were thus mostly similar to those observed in other regions of the world. Further research is needed, to estimate the proportion of sensitive forest species, the O₃ impact on tree growth and stand stability and to detect O₃ hot spots where woody species in the Atlantic Forest are mostly affected.

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.

Increased phytotoxic O3 dose accelerates autumn senescence in an O3-sensitive beech forest even under the present-level O3

Ground-level ozone (O₃) concentrations are expected to increase over the 21^st century, especially in East Asia. However, the impact of O₃ has not been directly assessed at the forest level in this region. We performed O₃ flux-based risk assessments of carbon sequestration capacity in an old cool temperate deciduous forest, consisting of O₃-sensitive Japanese beech (Fagus crenata), and in a warm temperate deciduous and evergreen forest dominated by O₃-tolerant Konara oak (Quercus serrata) based on long-term CO₂ flux observations. On the basis of a practical approach for a continuous estimation of canopy-level stomatal conductance (G_s), higher phytotoxic ozone dose above a threshold of 0 uptake (POD0) with higher G_s was observed in the beech forest than that in the oak forest. Light-saturated gross primary production, as a measure of carbon sequestration capacity of forest ecosystem, declined earlier in the late growth season with increasing POD0, suggesting an earlier autumn senescence, especially in the O₃-sensitive beech forest, but not in the O₃-tolerant oak forest.

Synergism between ozone and light stress: structural responses of polyphenols in a woody Brazilian species

Microscopic studies on isolated ozone (O3) effects or on those in synergy with light stress commonly report the induction of polyphenols that exhibit different aspects within the vacuole of photosynthesizing cells. It has been assumed that these different aspects are randomly spread in the symptomatic (injured) regions of the leaf blade. Interestingly, secretory ducts that constitutively produce polyphenols also exhibit these same variations in their vacuolar aspect, in a spatial sequence related to the destiny of these cells (e.g., programmed cell death (PCD) in lytic secretion processes). Here, we demonstrate that the deposition pattern of polyphenols prior to the establishment of the hypersensitive-like response, a type of PCD caused by O3, follows the same one observed in the epithelial cells of the constitutive lysigenous secretory ducts. Astronium graveolens, an early secondary Brazilian woody species, was selected based on its susceptibility to high light and presence of secretory ducts. The synergism effects were assessed by exposing plants to the high O3 concentrations at an urban site in São Paulo City. Confocal, widefield and light microscopies were used to examine polyphenols' occurrence and aspects. The spatial pattern of polyphenols distribution along the leaflets of plants submitted to the synergism condition, in which a dense vacuolar aspect is the target of a cell destined to death, was also observed in the constitutive secretory cells prior to lysis. This similar structural pattern may be a case of homology of process involving both the constitutive (secretory ducts) and the induced (photosynthesizing cells) defenses.

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

Despite the huge biodiversity characterizing the Mediterranean environment, environmental constraints, such as high sunlight and high temperatures alongside with dry periods, make plant survival hard. In addition, high irradiance leads to increasing ozone (O3 ) concentrations in ambient air. In this era of global warming, it is necessary to understand the mechanisms that allow native species to tolerate these environmental constraints and how such mechanisms interact. Three Mediterranean oak species (Quercus ilex, Quercus pubescens and Quercus cerris) with different features (drought tolerant, evergreen or deciduous species) were selected to assess their biometrical, physiological and biochemical responses under drought and/or O3 stress (80-100 nl l(-1) of O3 for 5 h day(-1) for 77 consecutive days). Leaf visible injury appeared only under drought stress (alone or combined with O3 ) in all three species. Drought × O3 induced strong reductions in leaf dry weight in Q. pubescens and Q. cerris (-70 and -75%, respectively). Alterations in physiological (i.e. decrease in maximum carboxylation rate) and biochemical parameters (i.e. increase in proline content and build-up of malondialdehyde by-products) occurred in all the three species, although drought represented the major determinant. Quercus ilex and Q. pubescens, which co-occur in dry environments, were more tolerant to drought and drought × O3 . Quercus ilex was the species in which oxidative stress occurred only when drought was applied with O3 . High plasticity at a biochemical level (i.e. proline content) and evergreen habitus are likely on the basis of the higher tolerance of Q. ilex.

Early and late adjustments of the photosynthetic traits and stomatal density in Quercus ilex L. grown in an ozone-enriched environment

Quercus ilex L. seedlings were exposed in open-top chambers for one growing season to three levels of ozone (O3 ): charcoal filtered air, non-filtered air supplemented with +30% or +74% ambient air O3 . Key functional parameters related to photosynthetic performance and stomatal density were measured to evaluate the response mechanisms of Q. ilex to chronic O3 exposure, clarifying how ecophysiological traits are modulated during the season in an ozone-enriched environment. Dark respiration showed an early response to O3 exposure, increasing approximately 45% relative to charcoal-filtered air in both O3 enriched treatments. However, at the end of the growing season, maximum rate of assimilation (Amax ) and stomatal conductance (gs ) showed a decline (-13% and -36%, for Amax and gs , respectively) only in plants under higher O3 levels. Photosystem I functionality supported the capacity of Q. ilex to cope with oxidative stress by adjusting the energy flow partitioning inside the photosystems. The response to O3 was also characterised by increased stomatal density in both O3 enriched treatments relative to controls. Our results suggest that in order to improve the reliability of metrics for O3 risk assessment, the seasonal changes in the response of gs and photosynthetic machinery to O3 stress should be considered.

Chronic drought stress reduced but not protected Shantung maple (Acer truncatum Bunge) from adverse effects of ozone (O3) on growth and physiology in the suburb of Beijing, China

A two-year experiment exposing Acer truncatum Bunge seedlings to elevated ozone (O3) concentrations above ambient air (AO) and drought stress (DS) was carried out using open-top chambers (OTCs) in a suburb of Beijing in north China in 2012-2013. The results suggested that AO and DS had both significantly reduced leaf mass area (LMA), stomatal conductance (Gs), light saturated photosynthetic rate (Asat) as well as above and below ground biomass at the end of the experiment. It appeared that while drought stress mitigated the expression of foliar injury, LMA, leaf photosynthetic pigments, height growth and basal diameter, due to limited carbon fixation, the O3 - induced reductions in Asat, Gs and total biomass were enhanced 23.7%. 15.5% and 8.1% respectively. These data suggest that when the whole plant was considered that drought under the conditions of this experiment did not protect the Shantung maple seedlings from the effects of O3.

Ozone phytotoxic potential with regard to fragments of the Atlantic Semi-deciduous Forest downwind of Sao Paulo, Brazil

In the Metropolitan Region of Campinas (MRC), Brazil, high levels of primary pollutants contribute to ozone (O3) formation. However, little is known regarding the O3 effects in the tropics. Objectives in this study were to characterize the present levels of O3 pollution and to evaluate the relevance of current concentration-based indices for assessing the phytotoxic potential of O3. Changes in O3 concentrations and precursors at 5 monitoring stations within towns of MRC were analyzed. The daily O3 profile was typical for urban sites and showed little yearly variation. Given the permanently foliated forest canopy, yearly rather than seasonal O3 indices were thus more appropriate for estimating the effective ozone dose. With yearly SUM00, SUM60 and AOT40 of 156, 16 and 14 ppm h and confirmed by evidence of O3 injury in foliage, oxidative stress in the MRC has reached levels high enough to affect trees from the Atlantic Semi-deciduous Forest.

Ozone-induced responses in Croton floribundus Spreng. (Euphorbiaceae): metabolic cross-talk between volatile organic compounds and calcium oxalate crystal formation

Here, we proposed that volatile organic compounds (VOC), specifically methyl salicylate (MeSA), mediate the formation of calcium oxalate crystals (COC) in the defence against ozone (O3) oxidative damage. We performed experiments using Croton floribundus, a pioneer tree species that is tolerant to O3 and widely distributed in the Brazilian forest. This species constitutively produces COC. We exposed plants to a controlled fumigation experiment and assessed biochemical, physiological, and morphological parameters. O3 induced a significant increase in the concentrations of constitutive oxygenated compounds, MeSA and terpenoids as well as in COC number. Our analysis supported the hypothesis that ozone-induced VOC (mainly MeSA) regulate ROS formation in a way that promotes the opening of calcium channels and the subsequent formation of COC in a fast and stable manner to stop the consequences of the reactive oxygen species in the tissue, indeed immobilising the excess calcium (caused by acute exposition to O3) that can be dangerous to the plant. To test this hypothesis, we performed an independent experiment spraying MeSA over C. floribundus plants and observed an increase in the number of COC, indicating that this compound has a potential to directly induce their formation. Thus, the tolerance of C. floribundus to O3 oxidative stress could be a consequence of a higher capacity for the production of VOC and COC rather than the modulation of antioxidant balance. We also present some insights into constitutive morphological features that may be related to the tolerance that this species exhibits to O3.