Ozone has long been known to cause reductions in the yield of crops1 although the precise mechanism of how this occurs is still unclear. Recently, ozone has also been suggested as being involved in the forest decline that has occurred in central Europe2—perhaps in combination with other atmospheric pollutants or with various environmental stresses such as chilling, mineral leaching, insect predation or disease. Consequently, it is desirable that the cellular mechanisms by which ozone is toxic to both crops and conifers, and how they might be modulated by other atmospheric pollutants or environmental stresses, are fully understood. Ethylene is normally produced by all plants and in trace amounts it may interact with other plant growth substances to coordinate a wide variety of developmental processes. However, when plants experience environmental stresses they respond by liberating larger amounts of ethylene—often called stress ethylene3. We have been able to demonstrate, using pea seedlings in an experimental fumigation system4, that the formation of stress ethylene determines the sensivity of plants to atmospheric levels of ozone and to suggest how other air pollutants may enhance ozone-mediated leaf injury.

Stress ethylene formation determines plant sensitivity to ozone

Responses of herbaceous plants to urban air pollution: effects on growth, phenology and leaf surface characteristics.

A definition of stress to plants and a general stress concept are presented including a list of possible major natural and anthropogenic stress factors. It is shown that the white light or laser-induced chlorophyll fluorescence of predarkened leaves is a very suitable tool for early stress detection in plants as well as for regeneration studies. The different chlorophyll fluorescence parameters to be measured are: variable fluorescence (ratio fmax/fo), the vitality index (fluorescence decrease ratio, Rfd-values), the stress adaption index Ap, the fluorescence spectra (ratio F690/F735) and the photochemical Q- and non-photochemical E-quenching (qQ and qE). Some examples of the measurement of these parameters under stress conditions are given.

In Vivo Chlorophyll Fluorescence as a Tool for Stress Detection in Plants

Electron spin resonance (ESR) spectroscopy was used to demonstrate that free radicals are formed in O3-fumigated plant leaves prior to the formation of visible leaf injury. ESR signals with a g-value of 2.0037 to 2.0043, were observed in pea (Pisum sativum L. cv. Feltham first) and bean (Phaseolus vulgaris L. cv. Pinto) plants that had been fumigated for 4 h with 70–300 nl l−1 of ozone after they had been treated with the spin-trap N-t-butyl-α-phenylnitrone (PBN). The size of the ESR signals increased with the concentration of ozone used but the nature of the trapped radicals could not be identified. However, further experiments using an inhibitor of ethylene biosynthesis, arninoethoxyvinyl glycine (AVG), showed that the reaction between ozone and ethylene is the cause for ozone toxicity.

Electron spin resonance evidence for the formation of free radicals in plants exposed to ozone

This book reports on the knowledge of the sensitivities and responses of forests, wetlands and crops to airborne pollutants. Pollutants examined include: acidic depositions, heavy metal particulates, sulphur dioxide, ozone, nitrogen oxides, acid fogs, and mixtures of these. Various types of ecosystem stresses and physiological mechanisms pertinent to acid deposition are also discussed. Related subjects, such as the effects of ethylene on vegetation, the physiology of drought in trees, the ability of soils to generate acidity naturally, the role of Sphagnum moss in natural peatland acidity, the use of lichens as indicators of changing air quality, and the magnitude of natural emissions of reduced sulphur gases from tropical rainforests and temperate deciduous forests, are covered.

https://www.cell.com/trends/ecology-evolution/pdf/0169-5347(88)90110-3.pdf

Effects of atmospheric pollutants on forests, wetlands, and agricultural ecosystems

Effects of long-term exposures to SO2 and NO2 on Poa pratensis and other grasses

The two most widespread air pollutants in Britain are SO2 and NO2 (refs 1, 2). These not only contribute to acidity in rain3, which may seriously affect plant growth, but also cause substantial effects by their direct action as gaseous pollutants. Research into the effects of the gaseous pollutants on grasses has shown that the combination of SO2 and NO2 may be particularly toxic to plant growth4,5. Further research in this laboratory showed that Phleum pratense L. was more sensitive to SO2 when growth was slow and it was suggested that exposures during winter may be particularly phytotoxic6. Here we present the results of experiments in which serious effects on plant growth were detected even during the summer months. Autumn-sown smooth stalked meadow grass (Poa pratensis L.) exposed to SO2 or SO2 plus NO2 showed marked growth reductions during the winter and spring, and although the plants recovered during the summer, flowering was inhibited. Moreover, four broad-leaved tree species showed substantial decreases in growth when exposed to SO2 plus NO2 between March and August.

Growth effects of SO 2 and/or NO 2 on woody plants and grasses during spring and summer

Summary
A series of experiments was conducted in controlled environment chambers to investigate the relationship between the concentration of mixtures of SO2 and NO2, length of exposure and growth of the amenity grass Poa pratensis L. cv. Monopoly. Plants were exposed to concentrations of 40, 70 and 100 nl 1−1 SO2 and NO2, in one to one ratio, for periods of 20, 34 and 38 d and compared with controls grown in partially filtered air. The effects of the pollutants on plant dry weight were related to the product of exposure concentration and duration within the limits of the experimental conditions. A dose-response curve was therefore constructed which illustrated that although small doses may have no effect or may even stimulate growth, slightly larger doses may be severely inhibitory. In order to clarify the effects of small doses a further experiment was performed in which the response was observed at intervals during a 50 d exposure to 40 nl 1−1 SO2 plus 40 nl 1−1 NO2. The results confirmed that low doses could cause small growth stimulations and that the transition from beneficial to inhibitory effects occurs over a narrow range of doses.

A subsequent exploratory experiment provided an example of how the environmental conditions during exposure may affect this relationship, and illustrated that each dose-response curve is only applicable to a given set of conditions.

Relationship between dose of so2 and no2 mixtures and growth of poa pratensis

A large-scale system for fumigating plants with SO2 and NO2

Two case studies are described in which the possibility of injury to vegetation by NO X was investigated. In the first study, the very high levels (up to 3500 ppb) of NO X (mainly NO) in glasshouses have been shown to be inhibitory to plant growth. The effects were not, however, as great as would be expected from such high concentrations. Several reasons why glasshouse crops escape the full impact of the NO X around them are given. It is noted that the NO X in the atmosphere of some glasshouses could theoretically supply all the nitrogen required by the crop. In the second study, effects of NO 2 at concentrations almost two orders of magnitude lower than the NO X in glasshouses are shown to be important when SO 2 is in the atmosphere at the same time. NO 2 and SO 2 normally accompany one another in urban air, and in rural areas of industrialised countries. The effects of exposure to NO 2 + SO 2 are sometimes greater than would be predicted from the action of the two gases separately. SO 2 has been shown to interfere with the induction of the enzyme nitrite reductase, which reduces NO 2 − to NH 3 and may be the main means by which the toxicity of NO 2 − is controlled in plant cells. The results of some new dose-response studies of NO 2 + SO 2 mixtures on seedling grasses are presented.

Mary E. Whitmore

Papers

Effects of long-term exposures to SO2 and NO2 on Poa pratensis and other grasses

Growth effects of SO 2 and/or NO 2 on woody plants and grasses during spring and summer

Relationship between dose of so2 and no2 mixtures and growth of poa pratensis

A large-scale system for fumigating plants with SO2 and NO2

Effects of Nitrogen Oxides on Plants: Two Case Studies