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

Ozone risk assessment with free-air controlled exposure (FACE) experiments: A critical revisit

Since risk assessments of tropospheric ozone (O3) are crucial for agricultural and forestry sectors, there is a growing body for realistic assessments by a stomatal flux-based approach in Free-Air Controlled Exposure (FACE) facilities. Ozone risks are normally described as relative risks (RRs), which are calculated by assuming the biomass or yield at zero O3 dose as "reference". However, the estimation of the reference biomass or yield is challenging due to a lack of O3-clean-air treatment at the FACEs and the extrapolation without data in a low O3 range increases the bias for estimating the reference values. Here, we reviewed a current methodology for the risk assessment at FACEs and presented a simple and effective way ("modified Paoletti's approach") of defining RRs just using biomass or yield data with a range of expected impacts under the FACE conditions hypothesizing three possible scenarios based on prediction limits using 95% credible intervals (CI) (1. Best fit using the intercept as reference, 2. Optimistic scenario using a lower CI and 3. Worst scenario using an upper CI). As a result, O3-sensitive species show a relatively narrow effect range (optimistic vs. worst scenario) whereas a wide range of response may be possibly taken in resistant species. Showing a possible effect range allows for a comprehensive understanding of the potential risks and its uncertainties related to a species sensitivity to O3. As a supporting approach, we also recommend to use scientifically relevant tools (i.e., ethylenediurea treatments; mechanistic plant models) for strengthening the obtained results for the RRs against O3. Interestingly, the moderately sensitive or resistant species showed non-linear rather than linear dose-response relationships, suggesting a need for the flexible functional form for the risk assessment to properly describe the complex plant response such as hormesis, which depends on their plasticity to O3 stress.

Plant secondary metabolism in a fluctuating world: climate change perspectives

Climate changes have unpredictable effects on ecosystems and agriculture. Plants adapt metabolically to overcome these challenges, with plant secondary metabolites (PSMs) being crucial for plant-environment interactions. Thus, understanding how PSMs respond to climate change is vital for future cultivation and breeding strategies. Here, we review PSM responses to climate changes such as elevated carbon dioxide, ozone, nitrogen deposition, heat and drought, as well as a combinations of different factors. These responses are complex, depending on stress dosage and duration, and metabolite classes. We finally identify mechanisms by which climate change affects PSM production ecologically and molecularly. While these observations provide insights into PSM responses to climate changes and the underlying regulatory mechanisms, considerable further research is required for a comprehensive understanding.

Untangling the role of leaf age specific osmoprotectant and antioxidant responses of two poplar clones under increasing ozone concentrations

Plants possess different degrees of tolerance to abiotic stress, which can mitigate the detrimental effect of environmental inputs affecting carbon balance. Less is known about the functions of osmoprotectants in scavenging of reactive oxygen species (ROS), generated at different sites depending on leaf age. This study aimed to clarify the osmotic adjustments adopted by old and young leaves of Oxford and I-214 poplar clones [differing in ozone (O3) sensitivity] to cope with three levels of O3 [ambient (AA), and two elevated O3 levels]. In both clones, the impact of intermediate O3 concentrations (1.5 × AA) on ROS production appeared to be leaf age-specific, given the accumulation of hydrogen peroxide (H2O2) observed only in old leaves of the Oxford plants and in young leaves of the I-214 ones (2- fold higher than AA and +79%, respectively). The induction of an oxidative burst was associated with membrane injury, indicating an inadequate response of the antioxidative systems [decrease of lutein and β-carotene (-37 and -85% in the old leaves of the Oxford plants), accumulation of proline and tocopherols (+60 and +12% in the young leaves of the I-214 ones)]. Intermediate O3 concentrations reacted with unsaturated lipids of the plasma membrane in old and young leaves of the Oxford plants, leading to an increase of malondialdehyde by-products (more than 2- fold higher than AA), while no effect was recorded for I-214. The impact of the highest O3 concentrations (2.0 × AA) on ROS production did not appear clone-specific, which may react with cell wall components by leading to oxidative pressure. Outcomes demonstrated the ability of young leaves of I-214 plants in contain O3 phytotoxic effects.

Exogenous application of melatonin protects bean and tobacco plants against ozone damage by improving antioxidant enzyme activities, enhancing photosynthetic performance, and preventing membrane damage

Ozone (O3) pollution is harmful to plants and ecosystems. Several chemicals have been evaluated to protect plants against O3 deleterious effects. However, they are not adequately efficient and/or the environmental safety of their application is questioned. Hence, new chemicals that provide sufficient protection while being safer for environmental application are needed. This study investigates the response of two O3-sensitive plant species (Phaseolus vulgaris L. cv. Pinto and Nicotiana tabacum L. cv. Bel-W3) leaf-sprayed with deionized water (W, control), ethylenediurea (EDU, 1 mM) or melatonin at lower (1 mM) or higher (3 mM) concentrations (Mel_L and Mel_H, respectively), and then exposed to a square wave of 200 ppb O3, lasting 1 day (5 h day-1) for bean and 2 days (8 h day-1) for tobacco. In both species, the photosynthetic activity of O3-exposed plants was about halved. O3-induced membrane damage was also confirmed by increased malondialdehyde (MDA) byproducts compared to control (W). In EDU- and Mel-treated bean plants, the photosynthetic performance was not influenced by O3, leading to reduction of the incidence and severity of O3 visible injury. In bean plants, Mel_L mitigated the detrimental effect of O3 by boosting antioxidant enzyme activities or osmoprotectants (e.g. abscisic acid, proline, and glutathione transferase). In Mel_L-sprayed tobacco plants, O3 negatively influenced the photosynthetic activity. Conversely, Mel_H ameliorated the O3-induced oxidative stress by preserving the photosynthetic performance, preventing membrane damage, and reducing the visible injuries extent. Although EDU performed better, melatonin protected plants against O3 phytotoxicity, suggesting its potential application as a bio-safer and eco-friendlier phytoprotectant against O3. It is worth noting that the content of melatonin in EDU-treated plants remained unchanged, indicating that the protectant mode of action of EDU is not Mel-related.

Salt Tolerance of Sea Flax (Linum maritimum L.), a Rare Species with Conservation Interest in Eastern Spain

Seldom found in saltmarshes, Linum maritimum is a halophyte of great conservation interest in the eastern Iberian Peninsula. Although the species has been reported in different plant communities, there is no information on its range of salinity tolerance or mechanisms of response to environmental stress factors. In this study, L. maritimum plants were subjected to increasing salt concentrations in controlled conditions in a greenhouse. After six months of watering with salt solutions, only plants from the control, 50 mM and 100 mM NaCl treatment groups survived, but seeds were produced only in the first two. Significant differences were found between the plants from the various treatment groups in terms of their growth parameters, such as plant height, fresh weight, and the quantity of flowers and fruits. The main mechanism of salt tolerance is probably related to the species' ability to activate K+ uptake and transport to shoots to partly counteract the accumulation of toxic Na+ ions. A biochemical analysis showed significant increases in glycine betaine, flavonoids and total phenolic compounds, highlighting the importance of osmotic regulation and antioxidant compounds in the salt tolerance of Linum maritimum. These findings have implications for the conservation of the species, especially under changing climatic conditions that may lead to increased soil salinity in its Mediterranean distribution area.

Physiological and Biochemical Responses Induced by Plum Pox Virus and Plum Bark Necrosis Steam Pitting Associated Virus in Tuscany Autochthonous Plum cv. Coscia di Monaca

The present study focused on trees of Tuscany autochthonous plum cv. Coscia di Monaca in order to evaluate the presence of viruses and elucidate the physiological and biochemical responses to virus infections under real field conditions. Among the several investigated viruses, plums tested positive only to plum pox virus (PPV) and plum bark necrosis steam pitting associated virus (PBNSPaV), occurring as both singular and co-infections. This is the first report of PBNSPaV in a Tuscany orchard. Furthermore, the present study not only confirmed the detrimental effects of PPV on the carbon dioxide assimilation rate due to both stomatal limitations and mesophyll impairments, but also showed that although PBNSPaV did not induce such photosynthetic impairments when occurring as singular infection, it enhanced this damaging effect when present as a co-infection with PPV, as confirmed by a severe decrease in the chlorophyll content. Infection-specific responses in terms of accessory pigments (i.e., carotenoids and xanthophylls), as well as sugars and organic acids, were also reported, these being likely related to photoprotective mechanisms and osmotic regulations under virus-induced oxidative stress. Overall, the results here presented represent an important step to fill knowledge gaps about the interaction of plant viruses and autochthonous Prunus cultivars.

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.

Secondary metabolites responses of plants exposed to ozone: an update

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

Foliar Application of Wood Distillate Alleviates Ozone-Induced Damage in Lettuce (Lactuca sativa L.)

This study examined whether foliar applications of wood distillate (WD) have a protective effect on photosynthesis and the antioxidant power of lettuce when exposed to an ecologically relevant O₃ concentration. Seedlings of lettuce (Lactuca sativa L.) were fumigated daily with 60 ppb of O₃ for 30 days, five hours per day. Once per week, 50% of the fumigated plants were treated with foliar applications of 0.2% WD, while control plants were treated with water. The results clearly showed the ability of WD to protect lettuce plants from ozone-induced damage. Specifically, WD-treated plants exhibited lower damage to the photosynthetic machinery, assessed through a series of chlorophyll fluorescence parameters, a higher chlorophyll content, higher antioxidant power, as well as antioxidant molecules, i.e., caffeic acid and quercetin, and higher biomass. Counteracting the overproduction of ozone-generated reactive oxygen species (ROS) is speculated to be the main mechanism by which WD protects the plant from ozone-induced damage.

Approaches to investigate crop responses to ozone pollution: from O3 -FACE to satellite-enabled modeling

Ozone (O₃ ) is a damaging air pollutant to crops. As one of the most reactive oxidants known, O₃ rapidly forms other reactive oxygen species (ROS) once it enters leaves through stomata. Those ROS in turn can cause oxidative stress, reduce photosynthesis, accelerate senescence, and decrease crop yield. To improve and adapt our feed, fuel, and food supply to rising O₃ pollution, a number of Free Air Concentration Enrichment (O₃ -FACE) facilities have been developed around the world and have studied key staple crops. In this review, we provide an overview of the FACE facilities and highlight some of the lessons learned from the last two decades of research. We discuss the differences between C₃ and C₄ crop responses to elevated O₃ , the possible trade-off between productivity and protection, genetic variation in O₃ response within and across species, and how we might leverage this observed variation for crop improvement. We also highlight the need to improve understanding of the interaction between rising O₃ pollution and other aspects of climate change, notably drought. Finally, we propose the use of globally modeled O₃ data that are available at increasing spatial and temporal resolutions to expand upon the research conducted at the limited number of global O₃ -FACE facilities.