This paper presents a mathematical model which enables the semi-quantification of ozone (O3) detoxification, based upon the direct reaction of the pollutant with ascorbate (ASC) located in the aqueous matrix associated with the cell wall (i.e. the apoplast). The model describes the uptake of ozone into the leaf and its direct reaction with ASC, taking into consideration the regeneration of dehydroascorbic acid in the cytosol, the rate of replenishment of cell wall ASC and the distribution of ASC between sub-cellular compartments based upon the permeability of biomembranes to the neutral species, ascorbic acid and the pH of various sub-cellular compartments. The importance of various physico-chemical characteristics (e.g. stomatal conductance, mesophyll cell wall thickness and tortuosity, chloroplast volume, apoplast pH, ASC:O3 reaction stoichiometry) in mediating the flux of ozone to the plasmalemma is analysed. Model simulations, supported by experimental observations, suggest that the ASC concentration in the leaf apoplast is high enough to scavenge a significant proportion of the O3 taken up into the leaf interior, under environmentally relevant conditions. However, there is considerable variation between taxa in the potential degree of protection afforded by apoplastic ASC, emphasizing the need for an improved understanding of the reaction chemistry of O3 in the cell wall.

Individual tillers of S23 ryegrass, in three paddocks set-stocked with sheep at widely differing grazing pressures, were marked and studied from 11 July to 19 August, 1966, and observations were made on the frequency and severity of grazing. The uprooting of tillers, and the destruction of others, were major sources of loss of records. It was found that an acceptable measure of the frequency of defoliation could be made by recording three times a week. In a subsidiary experiment it was shown that repeated handling did not affect the DM yield, the number of tillers or the length of green leaf (G.L.L.) of single plants. Despite the extreme differences in grazing pressure, there were no significant differences between paddocks in the daily increase in the G.L.L. of tillers which were not grazed during a short period of intensive recording. In all three paddocks the sheep tended to graze the tillers with the greatest G.L.L. at any one time, £ind there was evidence of patchy grazing only at the lowest grazing pressure. An increase in grazing pressure resulted in increases in both the frequency and severity of defoliation. Comparisons were made between the product of the frequency X severity of defoliation of individual tillers, and the stocking rate, grazing pressure and herbage consumption on the three paddocks.

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Two cultivars of spring wheat {Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO2 [ambient (355 pmol mol"^) and elevated (708 |.imol mol"^)] under two O3 regimes [clean air (< 5 nmol mol"' O3) and polluted air (15 nmol mol"' O3 at night rising to a midday maximum of 75 nmol mol""^)l in a phytotron at the University of Newcast!e-upon-Tyne. Between the two-leaf stage and anthesis, measurements of leaf gas-exchange, non-structural carbohydrate content, visible O3 damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO2 and/or O3.Growth at elevated CO2 resulted in a sustained increase in the rate of CO2 assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.-15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO2 assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO2 enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non-structural carbohydrates still accumulated in source leaves. In contrast, long-term exposure to O3 resulted in a decreased CO2 assimilation rate (c. -13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth.In the combined treatment growth of 03-treated plants was enhanced by elevated CO2, but there was little evidence that CO2 enrichment afforded additional protection against O3 damage. The reduction in growth induced by O3 at elevated CO2 was similar to that induced by O3 at ambient CO2 despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O3 flux by 20%, and also CO2-induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO2 enrichment may render plants more susceptible to O3 damage at the cellular level. Possible mechanisms are discussed.

The aim of this work was to examine the correspondence between apoplastic\symplastic antioxidant status and previously reported plant age-related shifts in the ozone (O $ ) resistance of Plantago major L. Seed-grown plants were fumigated in duplicate controlled environment chambers with charcoal\Purafil2-filtered air (CFA) or CFA plus 70 nmol mol −" O $ for 7 h d −" over a 42 d period. Measurements of stomatal conductance and antioxidants were made after 14, 28 and 42 d fumigation, on leaves at an equivalent stage of development (youngest fully expanded leaf, measured c. 9 d after emergence). Ozone exposure resulted in a similar decline in stomatal conductance across plant ages, indicating that increases in O $ resistance with plant age were mediated through changes in the tolerance of leaf tissue rather than enhanced pollutant exclusion. Leaf apoplastic washing fluid was found to contain ' unspecific ' peroxidase, ascorbate peroxidase, superoxide dismutase and ascorbate, but not glutathione and the enzymes required to facilitate the regeneration of ascorbate from its oxidized forms. A weak induction in the activity of certain symplastic antioxidants was found after 14 d O $ fumigation, despite a lack of visible symptoms of injury, but shifts in symplastic antioxidant enzyme activity were not consistent with previously observed increases in resistance to O $ with plant age. By contrast, changes in ' unspecific ' peroxidase activity and in the small pool of ascorbate in the leaf apoplast were found to accompany age-related shifts in O $ resistance. It is concluded that constituents of the leaf apoplast may constitute a potentially important front line defence against O $ .

SUMMARYPlantago major grows throughout Britain in a range of ozone climates. Because populations have been shown to difFer in ozone resistance, the aim ofthe experiment was to compare the reaction of populations from contrasting ozone climates to different types of ozone exposure. Three populations were grown under controlled conditions in five difFerent ozone treatments (including controls for 10 wk. Development, growth, stomata! conductance and seed production were recorded. Populations were from the south coast of England (Lulhngton), near a mountain summit (Great Dun Fell) and lowland Scotland (Bush)-Ozone treatments were: charcoal and Purafil filtered air (CF); 35 ni I'' for 24 h every day; 70 nl I"' h for 7 h every day; C¥ then three episodes each week oi 70 nJ )"' ior 7 h; and 35 nl 1"^ continuously plus three 7 h episodes each week of 70 nl i"'.The different ozone treatments resulted in different responses in each population. Ozone promoted senescence in the Cireat Dun Fell population hut not in the others; it reduced root growth more in the Lullington population than in the others but those from Lullington and Great Dun Fell maintained seed production to a much greater extent than the Bush population. The reproductive effort (number of seeds g"' of vegetative weight) actually increased in ozone in the Lullington and Great Dun Fell populations. It is suggested that this might be a general stress response rather than being specifically related to ozone. EfFects on stomatal conductance were similar to those previously reported and the converse of effects on seed production.The relative responses of the populations varied according to the ozone treatment. Continuous exposure to 35 nl r^ reduced leaf size only in the Great Dun Fell population, but seed output was reduced in the Bush population. In some cases, giving 3-d episodes of 70 nl l"^ had a greater effect than giving the dose every day but the effects varied with the population. This greater effect was considered to be a result of the time it takes for a plant to develop maximum anti-oxidant defence, which is lost when the ozone decreases after the episode. A plant exposed to episodes might have to re-induce defence with each exposure.Although it is reported frequently that ozone favours allocation of resources to the shoot over the root, it is concluded that this is an over-simpiification of the response. Even within a species there is a compJex suite of responses that varies with the population and with ozone exposure. Describing a population as resistant or sensitive is also an over-simplification.

There is growing evidence that rising atmospheric CO2 concentrations will reduce or prevent reductions in the growth and productivity of C3 crops attributable to ozone (O3) pollution. In this study, the role of pollutant exclusion in mediating this response was investigated through growth chamber-based investigations on leaves 4 and 7 of spring wheat (Triticum aestivum cv. Hanno). In the core experiments, plants were raised at two atmospheric CO2 concentrations (ambient [350 micro l l(-1)] or elevated CO2 [700 micro l l(-1)] under two O3 regimes (charcoal/Purafil-filtered air [<5 nl l(-1) O3] or ozone-enriched air [75 nl l(-1) 7 h d(-1)]). A subsequent experiment used an additional O3 treatment where the goal was to achieve equivalent daily O3 uptake over the life-span of leaves 4 and 7 under ambient and CO2-enriched conditions, through daily adjustment of exposures based on measured shifts in stomatal conductance. Plant growth and net CO2 assimilation were stimulated by CO2-enrichment and reduced by exposure to O3. However, the impacts of O3 decreased with plant age (i.e. leaf 7 was more resistant to O3 injury than leaf 4); a finding consistent with ontogenic shifts in the tolerance of plant tissue and/or acclimation to O3-induced oxidative stress. In the combined treatment, elevated CO2 protected against the adverse effects of O3 and reduced cumulative O3 uptake (calculated from measurements of stomatal conductance) by c. 10% and 35% over the life-span of leaves 4 and 7, respectively. Analysis of the relationship between O3 uptake and the decline in the maximum in vivo rate of Rubisco carboxylation (Vcmax) revealed the protection afforded by CO2-enrichment to be due, to a large extent, to the exclusion of the pollutant from the leaf interior (as a consequence of the decline in stomatal conductance triggered by CO2-enrichment), but there was evidence (especially from flux-response relationships constructed for leaf 4) that CO2-enrichment resulted in additional effects that alleviated the impacts of ozone-induced oxidative stress on photosynthesis.

SUMMARYA mixture of Trifolium repens L. (var. Grasslands Huia) and Loliiim pereniie L. (var. Melle) was established in an area with low ambient ozone concentrations and exposed to episodes of ozone using a simple, unenclosed field fumigation system. Control plots were exposed to ambient air. There were two cutting frequencies to simulate grazing and silage production respectively. This paper reports on the performance of the system and the effects on yield and species composition in the first year of fumigation.The system operated satisfactorily when the wind speed was between 1 and 5 m s"'. When the wind dropped suddenly below 1 m s"' there were short periods with high concentrations but these events were rare and did not last more than a few minutes. Above 5 m s"' there was lateral downwind spillage of ozone and at higher wind speeds it was not possible to maintain the target concentration of 50-70 nl T'. The plots were fumigated only when the weather was suitable for ozone formation and when the wind speed was stable. Because the weather was poor, fumigation was restricted to 22 d between July and September. The mean 4-7 h concentrations were 50-70 nl 1^Ŝ even-hour mean ambient concentrations over the same period were 10-28 nl 1"'. There were significant effects of ozone on yield and herbage composition. The effects varied between harvests, probably because of differences in the timing of cutting relative to the episodes, and to the diflerences in phenology between the species. Most of the loss of yield was due to effects on Trifolium, which was much more sensitive than Lolium. There was a persistent eflect of ozone on the stolon density of Trifolium which lasted into the following year. It is suggested that this arose because the clover replaced damaged leaves very quickly, maintaining the canopy at the expense of the stolons. Persistent effects on stolons might lead to poorer winter survival and lower nitrogen fixation. Gaps in the sward as a result of loss of Trifolium might allow rapid weed invasion. There was no interaction between cutting frequency and ozone, which may have been due to the fact that the poor summer restricted the number of cuts and therefore decreased the difl'erence between the two cutting frequencies.