A challenging job: Plant pathology in the urban environment

According to Agrios (2005), “Plant pathology is a science that studies plant diseases and attempts to improve the chances for survival of plants when they are faced with unfavorable environmental conditions and parasitic microorganisms that cause disease”. So, it is a discipline that has a practical and noble goal of protecting the food (quantity and quality) available for humans and animals. Nowadays to this crucial task at least another mission must be added: “to ensure the presence of well performing and safe plants (especially trees) in our cities”. This is because plant diseases, by their presence, menace the survival of the plants, shorten their life expectancy and make them dangerous in the urban environment representing a limiting factor for citizen’s security. At world level, far more than 50% of human beings currently live in an urban area and at least 70% will live there in the year 2050. Cities are similar to an organism in that they consume resources from their surroundings and excrete wastes. Urbanization concentrates people, materials and energy into relatively small geographical areas (cities and towns are estimated to be less than a mere 3% of the total land of our planet), whose environmental conditions are often critical. Quality of life in cities relies on a range of components, such as social equity, income and welfare, housing, social relations and education and a healthy environment. The environmental elements for an adequate quality of life include good air quality, low noise levels, clean and sufficient water, fair urban design with sufficient and high-quality public and green spaces, and a good local climate or opportunities to adapt to climate change. Urban trees provide a number of important (but not easily quantified) aesthetic, economic, and psychological benefits (“ecosystem services”) for humans. They increase property values, promote tourism, provide educational opportunities, encourage healthy life styles and outdoor activities, improve the visual appeal of urban areas, mitigate stress and encourage biological diversity. But trees, just as all other plants, may be sick and attacked by biotic and abiotic stress factors, and a diseased tree may represent an intolerable risk factor for human welfare. Plants in the city guarantee multiple benefits and satisfactions, but they are exposed to a variety of site conditions, environmental factors, and physical disturbances which influence their survival. Climate change has the potential to influence almost all components of the urban environment and to raise new, complex challenges for the quality of urban life, health and urban biodiversity. Due to human activity, the temperature in an urban microclimate is higher than that of the surrounding areas

Ecophysiology of Tilia americana under ozone fumigation

The negative effects of the pollutant gas ozone are widely studied in many plant species, but the intimate mechanisms of toxicity have not been completely defined. Generally this contaminant or its free radical by–products impair membrane functions, leading to declines in physiological processes, accelerated foliar senescence and premature leaf abscission. Trees of the genus Tilia do not show any foliar injury induced by ozone under natural conditions. In this study, we investigated the effects of this pollutant on ecophysiological and biochemical parameters of T. Americana saplings exposed to a fumigation (120 ppb for 45 consecutive days, 5 h d–1). At the end of treatment, even if plants did not exhibit any visible foliar injury, several parameters were significantly affected: stomatal conductance for water vapor (–15% compared to control), net photosynthesis (–39%), intercellular CO2 concentration (+30%), as well as chlorophyll fluorescence indexes. After 45 days of fumigation neo–, viola– and anteraxanthin content significantly decreased (–25%, –34% and –63%, respectively, in comparison with controls), but no zeaxanthin induction was detected, suggesting that exposure did not activate the xanthopyll cycle. Under these circumstances, this species should be regarded as “middle tolerant/sensitive”

Ecophysiological and antioxidant traits of Salvia officinalis under ozone stress

Ecophysiological and antioxidant traits were evaluated in sage (Salvia officinalis) plants exposed to 120 ppb of ozone for 90 consecutive days (5 h day−1). At the end of fumigation, plants showed slight leaf yellowing that could be considered the first visual symptom of leaf senescence. Ozone-stressed leaves showed (1) reduced photosynthetic activity (−70 % at the end of exposure), (2) chlorophyll loss (−59 and −56 % of chlorophyll a and b concentrations, starting from 30 days from the beginning of exposure), and (3) cellular water deficit (−12 % of the relative water content at the end of the fumigation). These phenomena are indicative of oxidative stress in the chloroplasts (as confirmed by the strong degradation of β-carotene) despite the photoprotection conferred by xanthophyll cycle [as demonstrated by the significant rise of de-epoxidation index, reaching the maximum value at the end of the treatment (+69 %)], antioxidant compounds [as confirmed by the increase of phenols (in particular caffeic acid and rosmarinic acid)], and water-soluble carbohydrates (especially monosaccharides). By means of combined ecophysiological and biochemical approaches, this study demonstrates that S. officinalis is able to activate an adaptive survival mechanism allowing the plant to complete its life cycle even under oxidative stressful conditions

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

Ozone-elicited secondary metabolites in shoot cultures of Melissa officinalis L.

The effects of ozone treatment (200 ppb, 3 h) on the accumulation of main secondary metabolites have been investigated in Melissa officinalis (lemon balm) aseptic shoot cultures in order to evaluate the biotechnological application of this gas for improving the yield of secondary metabolites of medicinal plants. During the treatment, we found (i) an activation of enzymes involved in phenolic metabolism [as confirmed by the increase of shikimate dehydrogenase, phenylalanine ammonia-lyase and cinnamyl alcohol dehydrogenase activities (about twofold higher than controls)], (ii) a development of cellular barriers with a higher degree of polymerization of monolignols [as indicated by the increase of lignin (+23% compared to controls)], (iii) an accumulation of phenolic compounds, in particular rosmarinic acid (about fourfold compared to control plants cultivated in filtered air) and (iv) an increase of antioxidant capacity [as documented by the improved 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activity]. The effect of ozone as elicitor of the production of secondary metabolites in lemon balm shoot cultures was dependent on the specific regime, the time of exposure and the concentration of the stressor. After the end of the treatment, we found cell death and hydrogen peroxide (H2O2) deposition concomitant with a prolonged superoxide anion-generation suggesting that a transient oxidative burst had occurre

Physiochemical responses of Ailanthus altissima under the challenge of Verticillium dahliae: elucidating the decline of one of the world’s worst invasive alien plant species

Natural infections of Verticillium spp. (Fungi, Ascomycota) on Ailanthus altissima have suggested to consider the biological control as a promising strategy to counteract this invasive plant, which is otherwise difficult to control by traditional mechanical and chemical treatments. Verticillium wilt is able to lead plants to death, throughout a pathogenic mechanism including vessel occlusions and production of degrading enzymes and phytotoxins. In this study, a 10 weeks open air pot experiment was set to investigate the ecophysiological and biochemical responses of Ailanthus trees artificially inoculated in the trunk with the V. dahliae strain VdGL16, previously isolated in Central Italy from the same host. Inoculated plants showed visible injuries starting from 2 weeks post inoculation (wpi), that progressively developed until a final severe defoliation. The fungal infection rapidly compromised the plant water status, and photosynthesis was impaired due to both stomatal and mesophyll limitations from 4 wpi, with subsequent detrimental effects also on PSII activity. Moreover, the disease altered the translocations of nutrients, as confirmed by cation and carbohydrate contents, probably due to a consumption of simple sugars and starch reserves without replacement of new photosynthesized. An accumulation of osmolytes (abscisic acid and proline) and phenylalanine (a precursor of phenylpropanoids) was also reported at 8 wpi, this being a response mechanism that needs to be further elucidated. However, the activation delay of such defence strategy inevitably did not avoid the premature defoliation of plants and the decline of physiochemical parameters, confirming the key role of Verticillium in Ailanthus decay

Cross‐Talk between Physiological and Metabolic Adjustments Adopted by Quercus cerris to Mitigate the Effects of Severe Drought and Realistic Future Ozone Concentrations

Global climate change represents a moving target for plant acclimation and/or adaptation, especially in the Mediterranean basin. In this study, the interactions of severe drought (20% of the effective daily evapotranspiration) and O3 fumigation (80 ppb, 5 h day−1, for 28 consecutive days) on (i) photosynthetic performance, (ii) cell membrane stability, (iii) hydric relations, (iv) accumulation of compatible solutes, and (v) lipophilic antioxidant compounds were investigated in young Quercus cerris plants. In addition to the typical drought-induced stomatal closure, imposition of water withholding dramatically influenced the profile of stress-associated metabolites, i.e., abscisic acid (ABA), proline, and lipophilic antioxidants. However, plants were not able to delay or prevent the negative effects of water deficit, the greatest impacting factor in this study. This translated into a steep decline of photosynthetic efficiency, leaf hydration, and membrane fluidity and permeability. When water stress was coupled with O3, plants orchestrated cross-talk among ABA, proline, and sugar in fully-expanded mature leaves, partially leading to a premature senescenc

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 dayâ1 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 à O3 (on average â59 vs â70%, respectively); (ii) the steeper decline of photosynthesis found in Q. pubescens under drought (â87 vs â81%) and drought à O3 (â69 vs â59%, 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

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, makes 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, Q. pubescens and Q. 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 d(-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. Q. 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

Trichoderma harzianum T-22 induces systemic esistance in tomato Infected by Cucumber mosaic virus

Understanding the induction of plant defenses against viruses using biocontrol agents is essential for developing new strategies against these pathogens, given the ineffectiveness of chemical treatments. The ability of Trichoderma harzianum, strain T-22 (T22) to control Cucumber mosaic virus (CMV) in Solanum lycopersicum var. cerasiforme plants and the changes in the physiology of tomato treated/infected with T22/CMV were examined. Plant growth-promoting effects, photosynthetic performance, reactive oxygen species scavenging enzymes, and phytohormones were investigated. T22 improved tomato growth in terms of plant height and improved photosynthesis, total chlorophyll content and plant gas exchange. In contrast, CMV induced a negative effect on dry matter accumulation and inhibited the photosynthetic capacity. The analysis of plant hormones demonstrated that treating with T22 before or simultaneously to CMV infection, led to a systemic resistance by jasmonic acid/ethylene and salicylic acid signaling pathways. Conversely, systemic resistance was abscissic acid-dependent when T22 treatment was administered after the CMV infection. In conclusion, the data reported here indicate that the T22-based strategy may be the most effective measure against CMV