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| Last Updated:: 12/07/2024


Ozone in the upper atmosphere acts as a screen that protects us from harmful effects of ultraviolet radiation. While ozone in the lower atmosphere is considered as an important phytotoxic air pollutant. The Ozone in the troposhere is formed when Nitrogen (NO2) is convereted to nitric oxide (NO) by the action of sunlight. The freed oxygen atom reacts with oxygen molecules (O2) to form ozone (O3).
Ecosystem Effects of Ozone Pollution
Ozone affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas. Ozone can especially cause damage during the growing season.
Injury Symptoms
Ozone enters leaves through stomata during normal gas exchange. As a strong oxidant, ozone (or secondary products resulting from oxidation by ozone such as reactive oxygen species) causes several types of symptoms including chlorosis and necrosis. It is almost impossible to tell whether foliar chlorosis or necrosis in the field is caused by ozone or normal senescence. Several additional symptom types are commonly associated with ozone exposure, however. These include flecks (tiny light-tan irregular spots less than 1 mm diameter), stipples (small darkly pigmented areas approximately 2-4 mm diameter), bronzing, and reddening.
Ozone symptoms usually occur between the veins on the upper leaf surface of older and middle-aged leaves, but may also involve both leaf surfaces (bifacial) for some species. The type and severity of injury is dependent on several factors including duration and concentration of ozone exposure, weather conditions and plant genetics. One or all of these symptoms can occur on some species under some conditions, and specific symptoms on one species can differ from symptoms on another. With continuing daily ozone exposure, classical symptoms (stippling, flecking, bronzing, and reddening) are gradually obscured by chlorosis and necrosis.
Yield Loss Caused by Ozone
Field research to measure effects of seasonal exposure to ozone on crop yield has been in progress for more than 40 years. Most of this research utilized open-top field chambers in which growth conditions are similar to outside conditions. The most extensive research on crop loss was performed from 1980 to 1987 at five locations in the USA as part of the National Crop Loss Assessment Network (NCLAN). At each location, numerous chambers were used to expose plants to ozone treatments spanning the range of concentrations that occur in different areas of the world. The NCLAN focused on the most important agronomic crops nationally.
The injury caused by ozone is first characterized by the appearance of minutechlorotic to pale tan or whitish lesions on the upper surface of the affected leaf. The lesions, no more than flecks, initially are less than a millimeter across, but, especially should they coalesce can become much larger. While initially limited to the upper leaf surface, lesions may extend through the leaf when ozone concentrations are higher. The early flecks typically are restricted between the smaller veins but overlap as the injury progresses.
Ozone is not bioaccumulated in plants and can only be detected by sensitive plants. A number of sensitive cultivated and native plant species respond to Ozone episodes rapidly within days by displaying typical Ozone symptoms on their foliage. Plant scientists have taken advantage of this by using sensitive plant species, such as appropriate tobacco and bean cultivars, as biological indicators of relative Ozone pollution in comparisons of the air quality at different geographic locations during a given year and during different years at a given location (Manning and Feder, Krupa et al).
In general, white, fawn, tan grey and brown necrotic streaks on the upper surface of leaves are typical of ozone injury. Kovacs presented typical ozone injury symptoms on certain crop species. The best known and, most classic symptom is exemplified on tobacco Nicotiana tabacum, c.v. Bel-W3, Bel B and Bel C. Numerous small lesions appear mostly on the upper surface of fully expanded leaves although they are often bifacial on the most sensitive varieties. Wilting follows when ozone concentration are higher. Other sensitive species include oats (Avena sativa cv. Clintland) and white flowered petunia (Petunia hybrida, var. Snowstorm). Bean plants show foliar browning and chlorosis, cucumber plants display white stipple, onion plants show white flecks and tip dieback while spinach plant show general shiny or waxy symptom on the upper leaf surface. Which subsequently turn into orange-green color on further fumigation. Tiny, chlorotic or white to tan flecks also best characterize symptoms on cereals and grasses. The initial flecks frequently coalesce between the larger veins to form chlorotic to bleached streaks or oblong lesions. Injury is usually most intense at the apex of a bend, with necrosis most prominent at the leaf tips and margin as with onion tip burn.
Coniferous tree
Chlorotic flecks later becoming pink lesions followed by orange-red tip necrosis and thin and twisted needles observed on current year pine needles in response to ozone. Affected trees are characterized by ‘stunted roots and tops, short, mottled needles and prematures shedding of foliage (Dochinger and seliskar, 1970). Controlled exposures and field observations of ozone effects on western conifer species have confirmed that a distinct visible symptom known as chlorotic mottle typically occurs on needle surfaces. Chlorotic mottle begins as the walls of mesophyll cells below the epidermis degrade, causing the loss of cellular contents and the subsequent degradation of chlorophyll within the cell. Microscopically this condition appears as amorphous staining of cellular contents, plasmolysis of cell contents, and cell death. The degradation of chlorophyll beneath the epidermis appears on the needle surface as amorphous chlorotic blotches with diffuse borders that occur in irregular patterns, giving a yellow “mottled” appearance; hence the terminology “chlorotic mottle”.Chlorotic mottle frequently appears in the one-third of the needle surface nearest the tip on 1-year-old or older needles, and progresses basipetally until the entire needle is affected. This pattern is observed mainly in southern California. In the Sierra Nevada the mottle tends to occur randomly along the entire needle length .
Deciduous trees and shrubs
A variety of ozone injury symptoms have been observed in deciduous trees and shrubs. Ash and maple show dense purple or reddish stipple on upper leaf surface. Lime and apple show leaf bronzing while some species such as birch shows leaf bleaching. Leaf curling and tip drying have been observed in lilac. Fruits are often affected and may prematurely drop.The injury pattern initially was observed on the older leaves near the crown of the plant, progressing with time to the younger, more vigorous foliage Visible foliar symptoms caused by ozone are common in watermelon and may be misdiagnosed as nutrient deficiency, rought stress, insect injury, or foliar disease caused by plant pathogens. Consequently, misdiagnosis of ozone injury often leads to misapplication of pesticides that are unlikely to ameliorate the problem. Even when the condition is diagnosed correctly, there is no practical information available on how to ameliorate injury in future years.
Although ozone is generally accepted to be an important phytotoxic air pollutant, little is known of its effects on natural and semi-natural vegetation. Bryophytes, by the very nature of their morphology and physiology are, perhaps, more likely to be susceptible to ozone pollution than higher plants. Mosses like Sphagnum recurvum and Polytrichum commune are sensitive to ozone, when exposed to long-term chronic ozone concentration shows reduction in growth.. A significant negative correlation between Ditrichum pusillum and Ozone concentration were demonstrated under open chamber fumigation by Stanosz et al.
Effects of Ozone AirPollution on Plants
Ambient ozone injury to sensitive and tolerant
Ozone injury in a pumpkin leaf
Ground-level ozone causes more damage to plants than all other air pollutants combined. This Web page describes the ozone pollution situation, shows classical symptoms of ozone injury and shows how ozone affects yield of several major crops.
Tropospheric Ozone Pollution
Ozone is formed in the troposphere when sunlight causes complex photochemical reactions involving oxides of nitrogen (NOx), volatile organic hydrocarbons (VOC) and carbon monoxide that originate chiefly from gasoline engines and burning of other fossil fuels. Woody vegetation is another major source of VOCs. NOx and VOCs can be transported long distances by regional weather patterns before they react to create ozone in the atmosphere, where it can persist for several weeks.
What does ozone exposure do to sensitive plants?
Black cherry
Tulip poplar
  • Reduce photosynthesis, which is the process that plants use to convert sunlight to energy to live and grow.
  • Slow the plant's growth.
  • Increase sensitive plants' risk of
  • Disease tulip poplar
  • Damage from insects
  • Effects of other pollutants
  • Harm from severe weather.
Also, some plants can show visible marks on their leaves when ozone is present under certain conditions.
What happens to the ecosystem?
The effects of ozone on individual plants can then have negative impacts on ecosystems, including:
  • loss of species diversity (less variety of plants, animals, insects, and fish)
  • changes to the specific assortment of plants present in a forest
  • changes to habitat quality
  • Changes to water and nutrient cycles.
Plant Commonly affected by Ozone Foliar Symptoms
  • Bean (Phaseolus sp.)
  • Cucumber(Cucumis sp.)
  • Grape(Vitis sp.)
  • Morning glory (Ipomoea sp.)
  • Onion (Allium sp.)
  • Potato (Solanum sp.)
  • Soyabean (Glycine sp.)
  • Spinach (Spinacia sp.)
  • Tobacco (Nicotiana sp.)
  • Watermelon(Citrullus sp.)Bronzing; chlorosis;
  • bifacial isolated necrosis of small areas
  • White stipple
  • Red to black stipple
  • Chlorosis
  • White flecks; tip dieback
  • Gray fleck; chlorosis; bronzing
  • Red bornzing; chlorosis; purple stipple
  • Gray to white fleck
  • Metallic to white fleck
  • Gray fleck
Deciduous trees
  • Black cherry (Prunus serotina)
  • Green Ash(Fraxinus pennsylvanica var. lanceolata)
  • Quaking aspen(Populus tremuloides)
  • Sycamore (Platanus occidentalis)
  • Tulip Poplar (Liriodendron tulipifera)
  • Red black stipple; reddening and leaf chlorosis; premature defoliation
  • Red purple stipple
  • Black stipple; chlorosis; premature defoliation
  • Chlorosis; early senescence (fall coloring) and defoliation
  • Dark stipple (classic symptoms)
  • Eastern white pine (Pinus strobus)
  • Jeffrey pine(P. Jeffreyi)
  • Ponderosa pine(P. Ponderosa)
  • White fir (Abies concolor)
  • Chlorotic fleck or mottle on older needles; red brown tipburn of current needles.
  • Chlorotic mottle of older needles (banding possible)
  • Chlorotic fleck or mottle on older needles followed by needle dieback from tips (banding possible)
  • Chlorotic mottle on older needles
*Modified From Krupa and Manning,
Higher Plants:-