High Variation in Resource Allocation Strategies among 11 Indian Wheat (Triticum aestivum) Cultivars Growing in High Ozone Environment

Ectomycorrhiza and ethylenediurea reduced the impact of high nitrogen and ozone stresses and increased the growth of Cedrus deodara

Cedrus deodara is the central conifer plant affected by ozone and nitrogen pollutants among forest species worldwide. The growth of C. deodara depends upon the ectomycorrhizal (ECM) association, which is usually disturbed by these factors. This study aims to understand how these factors affect plants at physiological and biochemical levels. Three fungal strain consortiums were inoculated with two-year-old C. deodara seedlings. The stresses of 100 kg N h-1and 100 ppb O3 were applied for six months to study their impact on chlorophyll and antioxidant enzymes (SOD, CAT, and APX). The results showed that C2 (Consortium of Cedrus deodara) positively impacted the growth of selected plant species. The high photosynthesis rate was determined by enhanced chlorophyll content, and C2-treated plants showed high chlorophyll content. Relatively, chlorophyll a and b contents increased significantly in the seedlings treated with Ethylenediurea (EDU) alone and with ozone stress. In addition, a significant difference was observed between EDU and O3-treated plants (14% EDU400-O3 and 23% EDU600-O3) and the control. Overall, antioxidant activities were higher in the treated samples than in the control. The order of SOD activity was C2 (448 U/gFW) and lowest (354.7 U/gFW) in control. APX also showed higher activity in treated plants in C1 ≥ C2 ≥ C3+O3, whereas CAT activity was the highest in C2 treatments. Ozone and nitrogen-stressed plants showed higher activities than EDU-treated plants compared to non-treated ones. Our findings highlight the importance of understanding the signaling effects of numerous precursors. Moreover, an extended investigation of seedlings developing into trees must be conducted to verify the potential of ectomycorrhizal strains associated with C. deodara and comprehend EDU's role as a direct molecular scavenger of reactive toxicants.

Ozone-induced oxidative stress alleviation by biogenic silver nanoparticles and ethylenediurea in mung bean (Vigna radiata L.) under high ambient ozone

Ground-level ozone (O3) is the most phytotoxic secondary air pollutant in the atmosphere, severely affecting crop yields worldwide. The role of nanoparticles (NP) in the alleviation of ozone-induced yield losses in crops is not known. Therefore, in the present study, we investigated the effects of biogenicB-AgNPs on the mitigation of ozone-induced phytotoxicity in mung bean and compared its results with ethylenediurea (EDU) for the first time. Two mung bean cultivars (Vigna radiata L., Cv. SML-668 and PDM-139) were foliar sprayed with weekly applications of B-AgNPs (0 = control, 10 and 25 ppm) and EDU (0 = control, 200 and 300 ppm) until maturation phase. Morphological, physiological, enzymatic, and non-enzymatic antioxidant data were collected 30 and 60 days after germination (DAG). The mean O3 and AOT40 values (8 h day-1) during the cultivation period were approximately 52 ppb and 4.4 ppm.h, respectively. More biomass was accumulated at the vegetative phase due to the impact of B-AgNPs and EDU, and more photosynthates were transported to the reproductive phase, increasing yield. We observed that the 10 ppm B-AgNPs treatment had a more noticeable impact on yield parameters and lower Ag accumulation in seeds for both cultivars. Specifically, SML-668 cultivar treated with 10 ppm B-AgNPs (SN1) showed greater increases in seed weight plant-1 (124.97%), hundred seed weight (33.45%), and harvest index (37.53%) in comparison to control. Our findings suggest that B-AgNPs can enhance growth, biomass, yield, and seed quality, and can improve mung bean ozone tolerance. Therefore, B-AgNPs may be a promising protectant for mung bean.

Effect of ozone stress on crop productivity: A threat to food security

Tropospheric ozone (O3), the most important phytotoxic air pollutant, can deteriorate crop quality and productivity. Notably, satellite and ground-level observations-based multimodel simulations demonstrate that the present and future predicted O3 exposures could threaten food security. Hence, the present study aims at reviewing the phytotoxicity caused by O3 pollution, which threatens the food security. The present review encompasses three major aspects; wherein the past and prevailing O3 concentrations in various regions were compiled at first, followed by discussing the physiological, biochemical and yield responses of economically important crop species, and considering the potential of O3 protectants to alleviate O3-induced phytotoxicity. Finally, the empirical data reported in the literature were quantitatively analysed to show that O3 causes detrimental effect on physiological traits, photosynthetic pigments, growth and yield attributes. The review on prevailing O3 concentrations over various regions, where economically important crop are grown, and their negative impact would support policy makers to implement air pollution regulations and the scientific community to develop countermeasures against O3 phytotoxicity for maintaining food security.

Ethylenediurea protects against ozone phytotoxicity not by adding nitrogen or controlling stomata in a stomata-unresponsive hybrid poplar

Ozone (O₃) pollution is a persistent environmental issue worldwide, which causes widespread damage to vegetation, deteriorating plant health and reducing plant productivity. Ethylenediurea (EDU) is a synthetic chemical that has been widely applied in scientific studies as a protectant against O₃ phytotoxicities. Despite four decades of active research, the exact mechanisms to explain its mode of action remain unclear. Here, we aimed to reveal whether EDU's phytoprotective property is due to its control over stomatal regulation and/or its action as a nitrogen (N) fertilizer, utilizing stomatal-unresponsive plants of a hybrid poplar (Populus koreana × trichocarpa cv. Peace) grown in a free-air O₃-concenctration enrichment (FACE) facility. Plants were treated with water (WAT), EDU (400 mg L^-1), or EDU's constitutive amount of N every nine days, and exposed to ambient (AOZ) or elevated (EOZ) O₃ during a growing season (June-September). EOZ led to extensive foliar injuries (but protected against rust disease), lower photosynthetic rate (A), impaired dynamics of responses of A to changes in light intensity, and smaller total plant leaf area. EDU protected against common phytotoxicities caused by EOZ without inducing stomatal closure, since stomatal conductance (g_s) was generally unresponsive to the experimental treatments. EDU also modulated the dynamic response of A to light fluctuations under O₃ stress. N addition acted as a fertilizer but did not satisfactorily protect plants against O₃ phytotoxicities. The results suggest that EDU protects against O₃ phytotoxicity not by adding N or controlling stomata, which provides a new insight into our understanding of the mode of action of EDU as a protectant against O₃ phytotoxicity.

Substantial yield reduction in sweet potato due to tropospheric ozone, the dose-response function

Impacts of tropospheric ozone on sweet potato (Ipomoea batatas) are poorly understood despite being a staple food grown in locations deemed at risk from ozone pollution. Three varieties of sweet potato were exposed to ozone treatments (peaks of: 30 (Low), 80 (Medium), and 110 (High) ppb) using heated solardomes. Weekly measurements of stomatal conductance (gs) and chlorophyll content (CI) were used to determine physiological responses, along with final yield. gs and CI were reduced with increasing ozone exposure, but effects were partially masked due to elevated leaf senescence and turnover. Yield for the Erato orange and Murasaki varieties was reduced by ∼40% and ∼50% (Medium and High ozone treatments, respectively, vs Low) whereas Beauregard yield was reduced by 58% in both. The DO₃SE (Deposition of Ozone for Stomatal Exchange) model was parameterized for gs in response to light, temperature, vapour pressure deficit and soil water potential. Clear responses of gs to the environmental parameters were found. Yield reductions were correlated with both concentration based AOT40 (accumulated ozone above a threshold of 40 ppb) and flux based POD₆ (accumulated stomatal flux of ozone above a threshold of 6 nmol m^{- 2} s^{- 1}) metrics (R² 0.66 p = 0.01; and R² 0.44 p = 0.05, respectively). A critical level estimate of a POD₆ of 3 (mmol m^-2 Projected Leaf Area^-1) was obtained using the relationship. This study showed that sweet potato yield was reduced by ozone pollution, and that stomatal conductance and chlorophyll content were also affected. Results from this study can improve model predictions of ozone impacts on sweet potato together with associated ozone risk assessments for tropical countries.

Evaluating impacts of biogenic silver nanoparticles and ethylenediurea on wheat (Triticum aestivum L.) against ozone-induced damages

Tropospheric ozone (O₃) is a phytotoxic pollutant that leads to a reduction in crop yield. Nanotechnology offers promising solutions to stem such yield losses against abiotic stresses. Silver nanoparticles are major nanomaterials used in consumer products however, their impact on crops under abiotic stress is limited. In this study, we evaluated the anti-ozonant efficacy of biogenic silver nanoparticles (B-AgNPs) and compared them with a model anti-ozonant ethylenediurea (EDU) against ozone phyto-toxicity. Growth, physiology, antioxidant defense, and yield parameters in two wheat cultivars (HD-2967 & DBW-17), treated with B-AgNPs (25 mg/L and 50 mg/L) and EDU (150 mg/L and 300 mg/L), were studied at both vegetative and reproductive stages. During the experimental period, the average ambient ozone concentration and accumulated dose of ozone over a threshold of 40 ppb (AOT40) (8 h day^-1) were found to be 60 ppb and 6 ppm h, respectively, which were sufficient to cause ozone-induced phyto-toxicity in wheat. Growth and yield for B-AgNPs as well as EDU-treated plants were significantly higher in both the tested cultivars over control ones. However, 25 mg/L B-AgNPs treatment showed a more pronounced effect in terms of yield attributes and its lower accumulation in grains for both cultivars. DBW-17 cultivar responded better with B-AgNPs and EDU treatments as compared to HD-2967. Meanwhile, foliar exposure of B-AgNPs (dose; 25 mg/L) significantly enhanced grain weight plant^-1, thousand-grain weight, and harvest index by 54.22 %, 29.46 %, and 14.21 %, respectively in DBW-17, when compared to control. B-AgNPs could enhance ozone tolerance in wheat by increasing biochemical and physiological responses. It is concluded that B-AgNPs at optimum concentrations were as effective as EDU, hence could be a promising ozone protectant for wheat.

Exogenous application of chemicals for protecting plants against ambient ozone pollution: What should come next?

Elevated ground-level ozone (O₃) pollution can adversely affect plants and inhibit plant growth and productivity, threatening food security and ecological health. It is therefore essential to develop measures to protect plants against O₃-induced adverse effects. Here we summarize the current status of phytoprotection against O₃-induced adverse effects and consider recent scientific and engineering advances, to provide a novel perspective for maximizing plant health while reducing environmental/ecological risks in an O₃-polluted world. We suggest that nanoscience and nanotechnology can provide a new dimension in the protection of plants against O₃-induced adverse effects, and recommend that new studies are based upon a green chemistry perspective.

Ethylenediurea (EDU) effects on Japanese larch: an one growing season experiment with simulated regenerating communities and a four growing season application to individual saplings

Japanese larch (Larix kaempferi (Lamb.) Carr.) and its hybrid are economically important coniferous trees widely grown in the Northern Hemisphere. Ground-level ozone (O₃) concentrations have increased since the pre-industrial era, and research projects showed that Japanese larch is susceptible to elevated O₃ exposures. Therefore, methodologies are needed to (1) protect Japanese larch against O₃ damage and (2) conduct biomonitoring of O₃ in Japanese larch forests and, thus, monitor O₃ risks to Japanese larch. For the first time, this study evaluates whether the synthetic chemical ethylenediurea (EDU) can protect Japanese larch against O₃ damage, in two independent experiments. In the first experiment, seedling communities, simulating natural regeneration, were treated with EDU (0, 100, 200, and 400 mg L^-1) and exposed to either ambient or elevated O₃ in a growing season. In the second experiment, individually-grown saplings were treated with EDU (0, 200 and 400 mg L^-1) and exposed to ambient O₃ in two growing seasons and to elevated O₃ in the succeeding two growing seasons. The two experiments revealed that EDU concentrations of 200-400 mg L^-1 could protect Japanese larch seedling communities and individual saplings against O₃-induced inhibition of growth and productivity. However, EDU concentrations ≤ 200 mg L^-1 did offer only partial protection when seedling communities were coping with higher level of O₃-induced stress, and only 400 mg EDU L^-1 fully protected communities under higher stress. Therefore, we conclude that among the concentrations tested the concentration offering maximum protection to Japanese larch plants under high competition and O₃-induced stress is that of 400 mg EDU L^-1. The results of this study can provide a valuable resource of information for applied forestry in an O₃-polluted world.

Effects of ethylenediurea (EDU) on regulatory proteins in two maize (Zea mays L.) varieties under high tropospheric ozone phytotoxicity

Rising tropospheric ozone is a major threat to the crops in the present climate change scenario. To investigate the EDU induced changes in proteins, two varieties of maize, the SHM3031 and the PEHM5, (hereafter S and P respectively) were treated with three EDU applications (0= control, 50 and 200 ppm) (hereafter 0= A, 1 and 2 respectively) (SA, S1, S2, PA, P1, P2 cultivar X treatments). Data on the morpho-physiology, enzymatic activity, and protein expression (for the first time) were collected at the vegetative (V, 45 DAG) and flowering (F, 75 DAG) developmental stages. The tropospheric ozone was around 53 ppb enough to cause phytotoxic effects. Protective effects of EDU were recorded in morpho-physiologically and biochemically. SOD, CAT and APX together with GR performed better under EDU protection in SHM3031 variety than PEHM5. The protein expression patterns in SHM3031 at the vegetative stage (28% proteins were increased, 7% were decreased), and at the flowering stage (17% increased, 8% decreased) were found. In PEHM5, a 14% increase and an 18% decrease (vegetative stage) whereas a 16% increase and a 20% decrease (flowering stage) were recorded in protein expression. Some protein functional categories, for instance, photosynthesis, carbon metabolism, energy metabolism, and defense were influenced by EDU. Rubisco expression was increased in SHM3031 whereas differentially expressed in PEHM5. Germin like protein, APX, SOD, and harpin binding proteins have enhanced defense regulatory mechanisms under EDU treatment during prevailing high tropospheric O₃. The present study showed EDU protective roles in C4 plants as proven in C3.

Plant susceptibility to ozone: A tower of Babel?

In a world with climate change and environmental pollution, modern Biology is concerned with organismic susceptibility. At the same time, policy and decision makers seek information about organismic susceptibility. Therefore, information about organismic susceptibility may have far-reaching implications to the entire biosphere that can extend to several forthcoming generations. Here, we review a sample of approximately 200 published peer-reviewed articles dealing with plant response to ground-level ozone to understand how the information about susceptibility is communicated. A fuzzy and often incorrect terminology was used to describe the responsiveness of plants to ozone. Susceptibility was classified too arbitrarily and this was reflected to the approximately 50 descriptive words that were used to characterize susceptibility. The classification of susceptibility was commonly based on calculated probability (p) value. This practice is inappropriate as p values do not provide any basis for effect or susceptibility magnitude. To bridge the gap between science and policy decision making, classification of susceptibility should be done using alternative approaches, such as effect size estimates in conjunction with multivariate ordination statistics.

A quantitative assessment of hormetic responses of plants to ozone

Evaluations of ozone effects on vegetation across the globe over the last seven decades have mostly incorporated exposure levels that were multi-fold the preindustrial concentrations. As such, global risk assessments and derivation of critical levels for protecting plants and food supplies were based on extrapolation from high to low exposure levels. These were developed in an era when it was thought that stress biology is framed around a linear dose-response. However, it has recently emerged that stress biology commonly displays non-linear, hormetic processes. The current biological understanding highlights that the strategy of extrapolating from high to low exposure levels may lead to biased estimates. Here, we analyzed a diverse sample of published empirical data of approximately 500 stimulatory, hormetic-like dose-responses induced by ozone in plants. The median value of the maximum stimulatory responses induced by elevated ozone was 124%, and commonly <150%, of the background response (control), independently of species and response variable. The maximum stimulatory response to ozone was similar among types of response variables and major plant species. It was also similar among clades, between herbaceous and woody plants, between deciduous and evergreen trees, and between annual and perennial herbaceous plants. There were modest differences in the stimulatory response between genera and between families which may reflect different experimental designs and conditions among studies. The responses varied significantly upon type of exposure system, with open-top chambers (OTCs) underestimating the maximum stimulatory response compared to free-air ozone-concentration enrichment (FACE) systems. These findings suggest that plants show a generalized hormetic stimulation by ozone which is constrained within certain limits of biological plasticity, being highly generalizable, evolutionarily based, and maintained over ecological scales. They further highlight that non-linear responses should be taken into account when assessing the ozone effects on plants.

Ozone Tolerance Found in Aegilops tauschii and Primary Synthetic Hexaploid Wheat

Modern wheat cultivars are increasingly sensitive to ground level ozone, with 7-10% mean yield reductions in the northern hemisphere. In this study, three of the genome donors of bread wheat, Triticum urartu (AA), T. dicoccoides (AABB), and Aegilops tauschii (DD) along with a modern wheat cultivar (T. aestivum 'Skyfall'), a 1970s cultivar (T. aestivum 'Maris Dove'), and a line of primary Synthetic Hexaploid Wheat were grown in 6 L pots of sandy loam soil in solardomes (Bangor, North Wales) and exposed to low (30 ppb), medium (55 ppb), and high (110 ppb) levels of ozone over 3 months. Measurements were made at harvest of shoot biomass and grain yield. Ae. tauschii appeared ozone tolerant with no significant effects of ozone on shoot biomass, seed head biomass, or 1000 grain + husk weight even under high ozone levels. In comparison, T. urartu had a significant reduction in 1000 grain + husk weight, especially under high ozone (-26%). The older cultivar, 'Maris Dove', had a significant reduction in seed head biomass (-9%) and 1000 grain weight (-11%) but was less sensitive than the more recent cultivar 'Skyfall', which had a highly significant reduction in its seed head biomass (-21%) and 1000 grain weight (-27%) under high ozone. Notably, the line of primary Synthetic Hexaploid Wheat was ozone tolerant, with no effect on total seed head biomass (-1%) and only a 5% reduction in 1000 grain weight under high ozone levels. The potential use of synthetic wheat in breeding ozone tolerant wheat is discussed.

Assessment of Ozone Sensitivity in Three Wheat Cultivars Using Ethylenediurea

Three wheat (Triticum aestivum L.) cultivars [HD 2987 (ozone (O₃) sensitive), PBW 502 (intermediately sensitive) and Kharchiya 65 (O₃ tolerant)] with known sensitivity to O₃ were re-evaluated using ethylenediurea (EDU; 400 ppm) to ascertain the use of EDU in determiningO₃ sensitivity under highly O₃-polluted tropical environments. EDU treatment helped in improving the growth, biomass, photosynthetic pigments and the antioxidative defense system of all the wheat cultivars. Under EDU treatment, PBW 502 retained more biomass, while HD 2987 showed better performance and ultimately the greatest increment in yield. Cultivar Kharchiya 65 also showed a positive response to EDU as manifested with an increase in pigment contents, total biomass and enzymatic antioxidants; however, this increment was comparatively lower compared to the other two cultivars. The results indicated that EDU did not have many physiological effects on cultivars but helped in counteracting O₃ primarily by scavenging reactive oxygen species and enhancing the antioxidative defense system where superoxide dismutase emerged as the major responsive biochemical parameter against ambient O₃. The observed results clearly indicated that differential O₃ sensitivity in three wheat cultivars established by the previous study is in accordance with the present study using EDU as a sensitivity tool, which is an easy and efficient technology in comparison to chamber and Free-Air Carbon dioxide Enrichment (FACE) experiments although its mechanistic understanding needs to be further validated.