The Galileo satellite during its recent passes close to the Earth recorded a planet with an unusual red-absorbing pigment, a poisonous atmosphere, simultaneously rich in oxygen and in methane, with strong, modulated, narrow-band, radio emissions in the MHz frequencies (Sagan et al. 1993). To an observer visiting the solar system, these features; the photo-oxidisable pigment chlorophyll, abundant atmospheric oxygen, the existence of reducing conditions and intelligent life might well appear self-contradictory. While intelligent life is a recent event, the presence of other forms of life based on photosynthesis and survival under both oxygen-rich atmospheres and reducing conditions go back to the earliest times (Table 1). Life on Earth has evolved over nearly 4 G years under atmospheric environments ranging from anoxia, to hypoxia, to hyperoxia (relative to the present day), and not always in that sequence.

Concentrations of ozone have approximately doubled over the past century. In polluted air, photochemical reactions may produce concentrations of ozone up to 100 ppbv or more, even in areas relatively remote from sources of industrial pollution. The large ozone concentrations observed in these episodes, which may persist for several days, are markers for the presence of intense photochemical activity in the atmosphere. Experiments designed to investigate the effects of ozone on plants rarely, if ever, reproduce the chemical conditions which are always associated with ozone episodes.

Exposure of plants to ozone is strongly influenced by topography and meterological conditions. Windy sites which are well coupled to the atmosphere experience only a small diurnal variation in ozone concentration, and peak concentrations during episodes are generally much larger than at sheltered sites.

Stomatal opening effectively controls the amount and rate at which ozone enters leaves. Factors which affect stomatal opening therefore determine the dose of ozone for a given concentration in air. Extrapolation of results from controlled experiments to the field must assess whether growing conditions, including stomatal conductance, are comparable between experiment and field.

The Critical Levels approach has shown that crop plants and natural vegetation are likely to be exposed to potentially damaging ozone concentrations in most industrialised countries.

Although ozone (O3) occurs in both the troposphere and the stratosphere, mechanisms of O3 formation and removal in both regions of the atmosphere are very different. Moreover, the presence of O3 in the two layers has very different implications for the biosphere, especially vegetation. This introductory paper gives an overall appreciation of the two pools of O3 and their relative influences upon plant life. In the troposphere, natural and unburnt hydrocarbons (HCs) not only participate in the formation of atmospheric O3 but are also involved in the formation of free radicals from O3 which go on to injure plant tissues. In the stratosphere, O3 absorbs significant amounts of ultraviolet radiation (285-315 nm, UV-B) from sunlight which would otherwise harm biological systems. Depletion of stratospheric O3 by chlorofluorocarbons (CFCs) has prompted urgent studies to assess the consequences of enhanced UV-B radiation on vegetation. Preliminary assessments of possible changes to UK crops are described in this paper.

Spruce plants (one clone of Picea abies (L.) Karst.) were exposed in environmental chambers to different levels of ozone for 42 days to study the influences of enhanced ozone on root tip chromosomes directly after fumigation had ceased. No visible symptoms resulted from these treatments. The classification of chromosomal aberrations was used as a method to characterise the influences on the root tips caused by ozone. In comparison to the control the fumigated variant showed an increased number of chromosomal abnormalities. Another clone of Picea was used to investigate the ‘post-fumigation long-term effect’ in the genetic material caused by enhanced ozone. These clonal trees appeared to be relatively ozone-sensitive, as pigment reduction and chlorotic spots were observed directly after fumigation had ceased. The spruce trees were also exposed in environmental chambers to different levels of ozone for 24 weeks and then they were transferred to a field near the Institute. Some 21 months after the fumigation had ceased those trees that had been fumigated showed an increased number of chromosomal aberrations compared with the control.

The observed chromosomal abnormalities in all the variants of the experiments consisted of chromosomal ‘stickiness’, chromosome breakage, and fragmentation.

Typical phytopathological defence reactions of tobacco have been studied in response to ozone. After a single standard ozone pulse (5 h, 150 nl 1−1) β-l,3-glucanase was induced up to 40-fold in the sensitive tobacco cv. Bel W3, which developed necrotic lesions following the treatment. The activity was elevated 10-fold in the tolerant Bel B without concomitant injury. A decrease within 24 h occurred only in Bel B. Tyramine hydroxycinnamoyl-CoA transferase activity was induced 6- to 7-fold in both cultivars with a maximum after 5–8 h. Tyramine conjugates as the product of this reaction could not be isolated and were apparently rapidly integrated into the cell wall. Precultivation of Bel B plants in a greenhouse containing up to 70% of the outdoor ozone concentration resulted in a 4-fold induction of β-l,3-glucanase and a further increase after an administered ozone pulse. Pretreatment (30 nl 1−1 ozone, 7 h per day, 23 days) of sensitive Bel W3 caused increased injury and ethylene formation upon a subsequent ozone pulse. β-l,3-glucanase was induced to the same level as in non-pretreated control plants. In a comparison with other major air pollutants (SO2, NO2), only ozone induced the above reactions. However, a combined treatment of ozone and NO2 led to synergistic effects. These data indicate that ambient ozone is able to change the phytopathological disposition of plants. This finding may have practical significance for the usual plant experiments carried out in greenhouses with non-filtered air.

Parsley (Petroselinum crispum L.) is known to respond to pathogen attack by the synthesis of furanocoumarins and to UV-irradiation by the synthesis of flavone glycosides, whereas ozone treatment results in the induction of both pathways. Ozone treatment (200 nl 1−1, 10 h) of parsley seedlings results in an increased mRNA level of early genes within 3 h [pathogenesis related proteins PR1, PR2 and an elicitor-induced protein with unknown function (Eli 16)], of intermediate induced genes within 6 h [phenylalanine ammonia-lyase (PAL), 4-coumaroyl-CoA ligase (4CL), chalcone synthase (CHS)], and of late genes within 12 h [hydroxyproline-rich glycoprotein (HRGP), peroxidase (POD)]. 2D-PAGE of in vitro translated poly(A)+ RNA isolated from ozone-treated parsley seedlings revealed about 20 induced and 10 repressed translation products. A cDNA library from parsley seedlings was differential screened, yielding several induced cDNA clones. One of the ozone-induced cDNA clones could be identified as coding for PR1-1 by hybrid-selected in vitro translation and by DNA sequence analysis.

The responses of photosynthesis to oxygen concentration 2, 21 and 100% by etiolated bean leaves upon illumination were determined. After 0.3, 3, 6, 10, 12 and 24 h of greening the following processes were measured. (1) Functioning of PS II as reflected by changes in Fv/Fm ratios and t1/2 values. (2) Content and composition of pigments. (3) Capacity for carbon dioxide assimilation. The results showed that O2 exerts a complex inhibitory action on examined processes. The magnitudes of inhibition increased with increasing of O2 concentration and time of its action. Under applied conditions the lag phase for development of examined processes (with the exception of carotenoids) was in the range 3-10 h. After 24 h of greening in 2, 21 and 100% of O2 the Fy/Fm ratios attained values 0.55, 0.45, 0.13 respectively as compared with typical 0.80 of normal green leaves of bean. After 24 h of greening the inhibition by 100% O2 as compared with zero inhibition by 2% O2 were, in percentages: Fw/Fm, 78; t1/2, 77; chlorophyll a, 59; chlorophyll b, 69; β-carotene, 78; neoxanthin, 60; zeaxanthin, 43; violaxanthin, 35; CO2 fixation, 95.

This paper presents a chemist's view of the action of ionising radiation on matter with a range of simple examples. Attention is given to the ways in which electron spin resonance spectroscopy (which is described briefly) can be harnessed to give useful information about the initial stages of radiation damage. The effects of radiation are generally indiscriminate and hence damage to water is of special importance in biological systems. Water-derived free radicals will attack biomolecules (the indirect effect) and this mechanism is compared with direct damage events. Also, examples are given of some remarkably discriminate radiation-induced reactions.

Specific attention is given to radiation-induced damage to proteins, and especially to aqueous DNA and DNA in chromatin and in cells. The importance of DNA strand-breaks is discussed in relation to both the production of mutations and cell death.

In plant tissue, massive doses of ionising radiation (0.5–3 kGy) induce an oxidative burst due to the overproduction of oxygen-centred free radicals. Changes in the protein metabolism of cherry tomato fruits were investigated in response to this peculiar stress. Although DNA damage definitively arrested cell division, the changes observed on a short-term basis were attributed to genetic regulation. Changes in protein metabolism were also maintained long term. Gamma-induced proteins (GIPs) were classified according to their induction kinetics. Group 1 proteins were induced immediately after the treatment and their synthesis was stopped within 24 h. During the same time period, global protein synthesis was restored and a new set of GIPs was induced. The function of these proteins is not yet known; but they may be involved in physiological disorders triggered by irradiation or in repair processes. Short-term typical changes involve the synthesis of ACC synthase – the ethylene pathway regulating enzyme - and most probably of some LMW-HSPs. A non-relevant response to irradiation has also been discovered, namely the long-term accumulation of chitinases. Irradiation induces both specific and non-specific responses which can be analysed by comparison with other types of oxidative stress and some GIPs seem to be specific to the treatment. The ability of irradiation to induce such different responses might be profitably applied for a better understanding of the oxidative mechanisms involved in signal transduction during environmental stress situations.

The photon emission (ultraweak photon emission and delayed luminescence) of duckweeds (Spirodela polyrrhiza) and 1–2 year old tree seedlings (Picea abies, Abies alba and Fagus silvatica) was investigated by means of a single photon counting equipment. The decay kinetics of DL, the temperature dependence of UPE, and the anaerobic phase-induced kinetics of UPE were measured under/after treatment with ozone and UV-radiation. The results suggest that more consideration be given to photon emission as an indicator of stress-induced changes in plants.

Two interdisciplinary projects of the Federal Forest Research Centre have been started in the last ten years as a contribution to the Austrian forest ecosystem research programme. Their main goal was an interdisciplinary investigation of interactions between air pollutants and various ecological parameters. Plant/physiological parameters were used to describe the naturally and anthropogenically induced stress patterns caused by different environmental conditions. Biochemical analyses of needles of Norway spruce (Picea abies) including ascorbic acid concentrations, peroxidase activity, thiols and pigments were taken at two altitudinal profiles.

Little is known about ozone (O3) effects on adult trees in the field, where ecophysiological parameters control pollutant uptake. It was the goal of this study to examine how ambient and above ambient O3 concentrations affect gas exchange of mature Norway spruce (Picea abies (L.) Karst.). Therefore, twigs were enclosed in chambers and exposed to different O3 concentrations for one and two seasons over three years. During winter periods twigs were maintained under ambient conditions. Data from the shade crown of spruce trees at 1000 m a.s.l. are presented. After one and two fumigation periods no clear treatment effects on gas exchange were observed in twigs fumigated with O3 concentrations ranging from zero up to ambient (A) + 60 ppb. However, O3 at 90 ppb reduced photosynthesis and conductance. CO2 response curves indicated that in A + 90 twigs the efficiency of CO2 uptake was diminished. Observed losses in Pn of A + 90 twigs were greater than reductions in conductance indicating that stomatal closure alone did not limit CO2 uptake. We conclude that ambient and slightly above ambient O3 concentrations do not alter gas exchange of mature Norway spruce. Therefore, suppositions on O3 damage on mature spruce trees should be critically questioned.

Green plants, within certain limitations, can adapt to a wide variety of unfavourable conditions such as drought, temperature changes, light variations, infectious attacks, air pollution and soil contamination. Depending on the strength of the individual impact(s), fluent or abrupt changes in visible or measurable stress symptoms indicate the deviation from normal metabolic conditions. Most of the visible or measurable symptoms are connected with altered oxygen metabolism principally concerning the transition from mostly heterolytic (two-electron transition) to increased homolytic (one-electron transition) processes. Homolytic reactions within metabolic sequences create, however, free radicals and have to be counteracted by the increase in radical-scavenging processes or compounds, thus warranting reaction sequences under metabolic control. At later states of stress episodes, the above control is gradually lost and more or less chaotic radical processes take over. Finally, cellular decompartmentalisations induce lytic and necrotic processes which are visible as the collapse of darkening cells or tissues. Every episode during this process is governed by a more or less denned balance between pro- and antioxidative capacities, including photosynthetic (strongly under metabolic and oxygen-detoxifying control) and photodynamic (only controlled by scavenger- and/or quencher-availability) reactions. This (theoretical) sequence of events in most cases can only be characterised punctually by strongly defined (analytical) indicator reactions (ESR) and is certainly species- and organ-specific.

Contemporary plant species show a wide range of responses to oxidative attack. Much of this variation may reflect the different environmental selective pressures operating at different geological periods over the course of angiosperm evolution. Evidence is provided to show that the wide range of contemporary responses to oxidative stress may directly reflect the persistence of genes controlling free radical processes under environments of the past.

The activities of a number of enzymes of the ascorbate–glutathione pathway have been shown to rise under conditions of increased oxidative stress. The potential to alter the expression of specific enzymes of this pathway by genetic manipulation has provided the opportunity to attempt to develop transgenic plants with altered levels of oxidative stress defense enzymes which should have improved stress tolerances. We have cloned a cDNA for glutathione reductase from a higher plant (Pisum sativum L.) and have used this to construct chimeric genes for the expression of the pea enzyme in the chloroplast, mitochondrion or cytosol of transgenic tobacco plants. Some of the transformed lines with elevated levels of expression of glutathione reductase accumulate higher concentrations of glutathione and show increased tolerance to paraquat, however, no evidence was found for elevated tolerance to ozone fumigation.

Although chilling of Arabidopsis thaliana cells to 4°C reduced respiration and superoxide generation, the latter still appears significant. Superoxide may contribute to the observed chilling-induced cellular lipid peroxidation and through processes that may be facilitated by the up-regulation of Cu,Zn-superoxide dismutase activity.

Plants of Boea hygroscopica were grown in a controlled environment. Control leaves were detached from plants grown in well-watered conditions and then subjected for 144 h to rapid or slow dehydration. Dehydrated leaves were then rehydrated for 36 h in distilled water. Rapid-dried leaves were not able to revive, whereas slow-dried leaves recovered their normal metabolic status. Results showed a different superoxide radical production in the dehydrated leaves accompanied by an altered lipid composition. Rehydrated leaves recovered the original lipid composition of control leaves. Spin-labelled thylakoid proteins of dehydrated leaves showed lower -SH group levels and increased spin motion.

Hyperoxia (O2 in solution in excess of air-equilibrium values) occurs in certain photosynthesising cells which use inorganic C concentrating mechanisms, and as a result of abiological mechanisms. Geochemical evidence suggests that the atmosphere may have had significantly higher O2 partial pressures in the past (e.g. the Upper Carboniferous) than occurs today. Biochemical effects of high O2 concentrations in solution include inhibition of RUBISCO (competitive with CO2) and nitrogenase, as well as damage caused by higher levels of toxic O species (H2O2, O2 and, especially, 1O2 and OH). The influence of high (twice the extant level) atmospheric O2 on growth of non-N2-fixers is as predicted from the properties of RUBISCO and the occurrence of inorganic C concentrating mechanisms. Acclimation of N2-fixers to twice the extant O2 level involves increased restriction on O2 diffusion to nitrogenase so that growth is not inhibited (unless the high O2 has access to the C3 photosynthesis apparatus). Evidence as to the effect of hyperoxia on quenchers and scavengers of toxic O species is equivocal. Cells exposed to high O2 probably have higher mutation rates as a result of higher levels of toxic O species, although the production and maintenance of ‘stem’ cells may occur in parts of the plants with relatively low O2 levels.