Effects of salinity and ozone,individually and in combination,on the growth and ion contents of two chickpea (Cicer arietinum L.) varieties

The aim of this study was to determine the effects of ozone and salinity, singly and in combination, on the growth and ion contents of two chickpea (Cicer arietinum L.) varieties. Chickpea plants were grown in non-saline and saline conditions, with and without a repeated exposure to ozone. Salinity at a concentration of 30 mM NaCl caused a substantial reduction in plant height, number of leaves and the dry weights of the leaves, stems and roots. Biomass allocation to the leaves increased, predominantly at the expense of the roots. Ozone at a concentration of 85 nmol mol(-1) for 6 h per day for 25 days reduced plant height and dry weights but had no effect on leaf number. The results show substantial effects of salinity and ozone on chickpea growth and ion concentrations. When ozonated plants are grown in the presence of salinity, further reductions in growth occur.     

Physiological effects of ozone on cultivars of watermelon (Citrullus lanatus) and muskmelon (Cucumis melo) widely grown in Spain

Two cultivars of watermelon (Citrullus lanatus) and muskmelon (Cucumis melo), which are widely grown in Spain, were exposed to ozone (70 nl litre(-1), 6 h d(-1)) for 21 days. Ozone sensitivity was assessed by recording the extent of visible injury, changes in fast-fluorescence kinetics, the relative-growth rate (R) of root (RR) and shoot (RS), and effects on the number of flowers produced per plant. Leaf gas exchange was measured in order to provide some indication of the factors underlying differential response to ozone. After 9-10 days of fumigation, all the cultivars developed typical visible symptoms of zone injury on the older leaves. However, significant (P < 0.05) changes in fast-fluorescence kinetics were detected prior to the development of visible foliar injury, indicating that detectable effects of ozone on primary photochemical processes supersede the appearance of visible symptoms. In both muskmelon and watermelon, there was a marked reduction in the rate of CO(2) assimilation as a result of exposure to ozone, and this was accompanied by a parallel decrease in stomatal conductance. Mean plant-relative-growth rate (R) was markedly (P < 0.01) suppressed by ozone in the two cultivars of watermelon, but there were no significant effects on R in muskmelon. Ozone reduced root growth relative to the shoot in three out of four cultivars-an effect that may be of considerable ecological significance. Moreover, exposure to ozone reduced flower production in both muskmelon and watermelon, which indicated effects on yield. There was no correlation between a variety of methods used to assess ozone sensitivity and visible injury, and reasons for this are discussed. This observation draws clear attention to the dangers in ranking plants for ozone sensitivity purely on the basis of visible symptoms. It is concluded from this study that ozone-insensitive genotypes should be identified and considered for planting in the major areas of melon production concentrated on the Mediterranean coast of Spain.     

Influence of ozone and ethylenediurea (EDU) on growth and yield of bean (Phaseolus vulgaris L.) in open-top field chambers

Greenhouse and ambient air experiments have shown ethylene diurea (EDU) to be a strong and specific protective suppressant of ozone injury in plants. To examine how EDU affects plant responses to various ozone (O(3)) levels under controlled field conditions, Phaseolus vulgaris L. cv. Lit was treated with 150 ppm EDU every 14 days and exposed in open-top chambers to charcoal-filtered air (CF), nonfiltered air (NF) or two cf treatments with ozone added. The ozone treatments were proportional additions of one (CF1) and two (CF2) times ambient ozone levels. The mean ozone concentrations in the CF, NF, CF1 and CF2 treatments were 0.98, 14.1, 14.98 and 31.56 nl litre(-1). A two-way split plot ANOVA revealed that shoot dry weight was significantly reduced by ozone. EDU treatment was highly significant for leaf dry weight, root dry weight and shoot dry weight, but not for pod dry weight; leading to a higher biomass of EDU-treated plants. Ozone/EDU interactions were significant for root weight only, indicating that EDU reduced growth suppression by ozone. These results show that EDU action on plant biomass could be interpreted as a delay in senescence since EDU-treated plants showed a significant decreased biomass loss even in the CF treatment.     

Differences in growth, leaf senescence and injury, and stomatal density in birch (Betula pendula Roth.) in relation to ambient levels of ozone in Finland

The differences in growth, leaf senescence, visible ozone injuries and stomatal density between one coastal site (natural ozone) and two inland sites (natural and elevated ozone) in Finland were determined for saplings of Betula pendula clones grown under open-field conditions during two growing seasons. Responses in growth, leaf senescence, visible injuries, and stomatal density were determined in relation to cumulative ozone exposure accumulated over the thresholds of 30, 40 and 50 ppb (10(9)) during the exposure period. In addition, the effects of the different ozone exposures on ultrastructure of chloroplasts were studied. Increasing ozone exposure resulted in reduced shoot dry weight, stimulated (first year) or reduced (second year) height growth, accelerated autumn yellowing of leaves, increased stomatal density, visible symptoms and chloroplast injuries, and increased number and size of plastoglobuli. Newly expanded mature leaves in midsummer were more sensitive to ozone episodes than younger developing leaves in the early growing season. In most parameters, the best correlation was achieved with the exposure index AOT30. Ozone risk for birch is highest in the southern coastal area of Finland, where background ozone concentrations are higher than in inland sites.     

A comparison of measured and modeled ozone uptake into plant leaves

Ozone uptake into plant leaves was measured in gas exchange chambers using a mass balance and a variable conductance approach. The variable conductance approach was found to more reliably measure ozone flux through stomata. Measurements using this approach were contrasted with estimates obtained by measuring stomatal conductance g(sw) and modeling ozone uptake using a diffusion equation, assuming a negligible ozone concentration in the substomatal cavity. Actual measurements of uptake were close, but slightly higher than modeled values, providing some support to the idea that substomatal ozone concentrations are close to zero. However, the difference between measured and modeled uptake values suggests either that (i) variable conductance approach measures more ozone uptake than caused by stomatal uptake alone or (ii) ozone conductance is underestimated.     

The individual and combined effects of ozone and simulated acid rain on growth, gas exchange rate and water-use efficiency of Pinus armandi Franch

The seedlings of Pinus armandi Franch. were exposed to ozone (O(3)) at 300 ppb for 8 h a day, 6 days a week, and simulated acid rain of pH 3.0 or 2.3, 6 times a week, alone or in combination, for 14 weeks from 15 June to 20 September 1993. The control seedlings were exposed to charcoal-filtered air and simulated rain of pH 6.8 during the same period. Significant interactive effects of O(3) and simulated acid rain on whole plant net photosynthetic rate were observed, but not on other determined parameters. The exposure of the seedlings to O(3) caused the reductions in the dry weight growth, root dry weight relative to the whole plant dry weight, net photosynthetic rate, transpiration rate in light, water-use efficiency and root respiration activity, and increases in shoot/root ratio, and leaf dry weight relative to the whole plant dry weight without an appearance of acute visible foliar injury, but did not affect the dark respiration rate and transpiration rate in the darkness. The decreased net photosynthetic rate was considered to be the major cause for the growth reduction of the seedlings exposed to O(3). On the other hand, the exposure of the seedlings to simulated acid rain reduced the net photosynthetic rate per unit chlorophyll a + b content, but did not induce the significant change in other determined parameters.     

The relationship between foliar injury, nitrogen metabolism, and growth parameters in ozonated soybeans

A single 12 h ozone exposure peaking at 0.20 ppm proved phytotoxic to greenhouse-grown 'Cutler 71' soybeans at each growth stage tested from V5 to R6. Visible injury occurred within 40 h on the unifoliodate leaves and middle-aged and older trifoliolates while the younger leaves were free from toxicity symptoms. In some instances visible injury was accompanied by a decrease in chlorophyll and an increase in leaf diffusive resistance. Although nitrogen fixation was not significantly altered except at early pod formation (R3), and nitrate reductase activity was significantly reduced only if the ozone exposure occurred at the time of maximal enzyme activity (V5), nitrogen content of the leaves was reduced by ozone treatment. Shoot dry weight was not affected 40 h after ozone treatment, but root dry weight was significantly reduced. Plants grown with supplemental NO(3)(-) were more sensitive to ozone than those dependent on fixed nitrogen. At plant maturity, there was no evidence of an ozone effect on shoot, root, or seed dry weight, NO(3)(-) -grown plants showed a significant increase in growth and yield over N(2)(-) plants; but no ozone effect was observed, despite the increased foliar sensitivity. Multiple ozone exposures at growth stages V3, R1 and R3 exacerbated the effects noted with a single episode and also reduced nitrogenase activity (reflected in specific and total nodule activity) and shoot and root dry weight. At plant maturity, there was again no evidence of a significant effect of multiple ozone treatment on shoot dry weight or seed yield although root weight remained low. The results would tend to support the hypothesis that older leaves of soybean do not make a significant contribution to seed yield. Although they may be injured by ozone during the reproductive phases of growth, seed yield may not be affected if the younger O(3)-tolerant leaves remain functional.     

Growth and disease response of soybeans from early maturity groups to ozone and Fusarium oxysporum

The single and combined effects of ozone (O(3)) and Fusarium oxysporum on growth and disease expression of soybean genotypes differing in foliar sensitivity to O(3) were studied in the greenhouse. O(3) had no effect on root and hypocotyl rot severity of PI 153.283 (O(3)-sensitive, S) or PI 189.907 (O(3)-tolerant, T) maturity group I soybean lines. Plants of both genotypes infected with F. oxysporum and exposed to O(3) had greater reductions in relative growth rate (RGR), net assimilation rate (NAR), and had more stippled leaves per plant than Fusarium-free plants exposed to O(3). O(3) alone had a greater impact on shoot dry weight, RGR, and NAR of PI 153.283 (S) than of PI 189.907 (T). O(3) alone reduced shoot and root dry weights primarily through a depression in NAR and less through reduced leaf area. F. oxysporum alone reduced root dry weight at 35 days; however, infected plants responded with increases in root dry weight from 49 to 63 days. Similarly, F. oxysporum alone lowered early RGR but subsequent RGR decline was less rapid while NAR remained high, particularly during later sampling intervals. Infection by F. oxysporum that causes root and hypocotyl rot increased soybean sensitivity to O(3) by prolonging active vegetative growth.     

Contrasting ozone sensitivity in related evergreen and deciduous shrubs

Plant responses to enhanced ozone levels have been studied in two pairs of evergreen-deciduous species (Pistacia terebinthus vs. P. lentiscus; Viburnum lantana vs. V. tinus) in Open Top Chambers. Ozone induced widespread visible injury, significantly reduced CO₂ assimilation and stomatal conductance (g_s), impaired Rubisco efficiency and regeneration capacity (V_c,max,J_max) and altered fluorescence parameters only in the deciduous species. Differences in stomatal conductance could not explain the observed differences in sensitivity. In control plants, deciduous species showed higher superoxide dismutase (SOD) activity than their evergreen counterparts, suggesting metabolic differences that could make them more prone to redox imbalances. Ozone induced increases in SOD and/or peroxidase activities in all the species, but only evergreens were able to cope with the oxidative stress. The relevancy of these results for the effective ozone flux approach and for the current ozone Critical Levels is also discussed.     

Growth and crown architecture of two aspen genotypes exposed to interacting ozone and carbon dioxide.

To study the impact of ozone (O3) and O3 plus CO2 on aspen growth, we planted two trembling aspen clones, differing in sensitivity to O3 in the ground in open-top chambers and exposed them to different concentrations of O3 and O3 plus CO, for 98 days. Ozone exposure (58 to 97 microl l(-1)-h. total exposure) decreased growth and modified crown architecture of both aspen clones. Ozone exposure decreased leaf, stem, branch, and root dry weight particularly in the O3 sensitive clone (clone 259). The addition of CO2 (150 microl l(-1) over ambient) to the O3 exposure counteracted the negative impact of O3 only in the O3 tolerant clone (clone 216). Ozone had relatively little effect on allometric ratios such as, shoot/root ratio, leaf weight ratio, or root weight ratio. In both clones, however, O3 decreased the shoot dry weight, shoot length ratio and shoot diameter. This decrease in wood strength caused both current terminals and long shoots to droop and increased the branch angle of termination. These results show that aspen growth is highly sensitive to O3 and that O3 can also significantly affect crown architecture. Aspen plants with drooping terminals and lateral branches would be at a competitive disadvantage in dense stands with limited light.     

Quantification of ozone uptake at the stand level in a Pinus canariensis forest in Tenerife, Canary Islands: an approach based on sap flow measurements

Ozone uptake was studied in a pine forest in Tenerife, Canary Islands, an ecotone with strong seasonal changes in climate. Ambient ozone concentration showed a pronounced seasonal course with high concentrations during the dry and warm period and low concentrations during the wet and cold season. Ozone uptake by contrast showed no clear seasonal trend. This is because canopy conductance significantly decreased with soil water availability and vapour pressure deficit. Mean daily ozone uptake averaged 1.9 nmol m(-2) s(-1) during the wet and cold season, and 1.5 nmol m(-2) s(-1) during the warm and dry period. The corresponding daily mean ambient ozone concentrations were 42 and 51 nl l(-1), respectively. Thus we conclude that in Mediterranean type forest ecosystems the flux based approach is more capable for risk assessment than an external, concentration based approach.     

Ozone fluxes above and within a pine forest canopy in dry and wet conditions

The physiological and physical processes controlling ozone dry deposition to vegetated surfaces are still not fully understood. In particular, the role of the understorey and the possible action of dew on ozone deposition have not received much attention so far. This paper presents the results of an experiment aimed at quantifying ozone dry deposition to a maritime pine forest in the “Les Landes” area in France. Ozone deposition fluxes were measured using the eddy-covariance technique above and within the canopy. We investigate the factors acting on ozone deposition in both dew-wetted and dry conditions. The values obtained for the ozone deposition velocity are well in the range of previously published measurements over coniferous forests. For the present forest, ozone uptake by the understorey is a significant portion of ozone deposition to the whole pine stand. The understorey contributes more to the overall ozone flux than to the other measured scalar fluxes (sensible heat and water vapour). During dry nights the surface conductance for ozone and the friction velocity are strongly correlated, showing that ozone deposition is largely controlled by dynamical processes. During the day, in dry conditions, the canopy stomatal conductance is the major parameter controlling ozone deposition. However, in winter, when the stomatal conductance is low, the influence of dynamical processes persists during day-time. It is also found that surface wetness associated with dew significantly enhances ozone deposition, during the night as well as in the morning.     

Physiological responses of glyphosate-resistant and glyphosate-sensitive soybean to aminomethylphosphonic acid, a metabolite of glyphosate

Aminomethylphosphonic acid (AMPA) is formed in glyphosate-treated glyphosate-resistant (GR) and glyphosate-sensitive (GS) soybean [Glycine max (L.) Merr.] plants and is known to cause yellowing in soybean. Although, AMPA is less phytotoxic than glyphosate, its mode of action is different from that of glyphosate and is still unknown. Greenhouse studies were conducted at Stoneville, MS to determine the effects of AMPA on plant growth, chlorophyll content, photosynthesis, nodulation, nitrogenase activity, nitrate reductase activity, and shoot nitrogen content in GR and GS soybeans. AMPA was applied to one- to two-trifoliolate leaf stage soybeans at 0.1 and 1.0 kg ha(-1), representing a scenario of 10% and 100% degradation of glyphosate (1.0 kg ae ha(-1) use rate) to AMPA, respectively. Overall, AMPA effects were more pronounced at 1.0 kg ha(-1) than at 0.1 kg ha(-1) rate. Visual plant injury (18-27%) was observed on young leaves within 3d after treatment (DAT) with AMPA at the higher rate regardless of soybean type. AMPA injury peaked to 46-49% at 14 DAT and decreased to 17-18% by 28 DAT, in both soybean types. AMPA reduced the chlorophyll content by 37%, 48%, 66%, and 23% in GR soybean, and 17%, 48%, 57%, and 22% in GS soybean at 3, 7, 14, and 28 DAT, respectively. AMPA reduced the photosynthesis rate by 65%, 85%, and 77% in GR soybean and 59%, 88%, and 69% in GS soybean at 3, 7, and 14 DAT, respectively, compared to non-treated plants. Similarly, AMPA reduced stomatal conductance to water vapor and transpiration rates at 3, 7, and 14 DAT compared to non-treated plants in both soybean types. Photosynthesis rate, stomatal conductance, and transpiration rate recovered to the levels of non-treated plants by 28 DAT. Plant height and shoot dry weight at 28 DAT; nodulation, nitrogenase activity at 10 DAT, and nitrate reductase activity at 3 and 14 DAT were unaffected by AMPA. AMPA reduced root respiration and shoot nitrogen content at 10 DAT. These results suggest that a foliar application of AMPA could indirectly reduce photosynthesis through decreased chlorophyll content in GR and GS soybean up to 14 DAT, but affected plants can recover to normal growth by 28 DAT.     

Acceleration of 13C-labelled photosynthate partitioning from leaves to panicles in rice plants exposed to chronic ozone at the reproductive stage

To investigate the effects of low (0.05 micromol/mol) and relatively low (0.10 micromol/mol) concentrations of ozone on photoassimilate partitioning, rice plants grown in a water culture were fed with (13)C-labelled carbon dioxide at the reproductive stage in an assimilation chamber with constant concentration of (12)CO(2) and (13)CO(2). Rice plants were exposed to ozone 4 weeks before and 3 weeks after (13)CO(2) feeding. The dry weight of whole plants decreased with increasing ozone concentration, whereas net photosynthetic rate (apparent CO(2) uptake per unit leaf area) was unaffected, compared with the control, at the time of (13)CO(2) feeding. Dry matter distribution into leaf sheaths and culms was reduced more than that into leaf blades by ozone exposure. Although panicle dry weight per plant was reduced by ozone, the percentage of panicle dry weight to the whole plant tended to increase considerably. Exposure to ozone accelerated translocation of (13)C from source leaves to other plant parts. Partitioning of (13)C to panicles and roots was higher under ozone treatment than in the control. Respiratory losses of fixed (13)C from plants tended to decrease under treatment with ozone. The increase in photoassimilate partitioning in panicles can be considered to be an acclimation response of rice plants to complete reproductive stage under the restricted biomass production caused by ozone.     

Nitrogen mediates above-ground effects of ozone but not below-ground effects in a rhizomatous sedge

Ozone and atmospheric nitrogen are co-occurring pollutants with adverse effects on natural grassland vegetation. Plants of the rhizomatous sedge Carex arenaria were exposed to four ozone regimes representing increasing background concentrations (background-peak): 10-30, 35-55, 60-80 and 85-105 ppb ozone at two nitrogen levels: 12 and 100 kg N ha⁻¹ yr⁻¹. Ozone increased the number and proportion of senesced leaves, but not overall leaf number. There was a clear nitrogen × ozone interaction with high nitrogen reducing proportional senescence in each treatment and increasing the ozone dose (AOT40) at which enhanced senescence occurred. Ozone reduced total biomass due to significant effects on root biomass. There were no interactive effects on shoot:root ratio. Rhizome tissue N content was increased by both nitrogen and ozone. Results suggest that nitrogen mediates above-ground impacts of ozone but not impacts on below-ground resource translocation. This may lead to complex interactive effects between the two pollutants on natural vegetation.     

Derivation of ozone flux-yield relationships for lettuce: a key horticultural crop

Ozone flux-response relationships were derived for lettuce, employing a multiplicative approach to model the manner in which stomatal conductance is influenced by key environmental variables, using a dataset collected during field experimentation in Crete and yield-response relationships derived from parallel open-top chamber experiments. Regional agronomic practices were adopted throughout. Computed versus measured data revealed that the derived model explained 51% (P<0.001) of the observed variation in stomatal conductance. Concentration-based indices were compared with flux-based indices. Analyses revealed a significant relationship between accumulated stomatal ozone flux and yield employing flux threshold cut-offs up to 4 nmol m(-2) s(-1). Regressions employing very low or zero flux thresholds resulted in the strongest yield-flux relationships (explaining approximately 80% (P<0.05) of the variation in the dataset).     

Foliar injury, leaf gas exchange and biomass responses of black cherry (Prunus serotina Ehrh.) half-sibling families to ozone exposure

Open pollinated families of black cherry seedlings were studied to determine genotypic differences in foliar ozone injury and leaf gas exchange in 1994 and growth response following three growing seasons. An O(3)-sensitive half-sibling family (R-12) and an O(3)-tolerant half-sibling family (MO-7) planted in natural soil were studied along with generic nursery stock (NS) seedlings. Ozone exposure treatments were provided through open top chambers and consisted of 50, 75, and 97% of ambient ozone, and open plots from May 9 to August 26, 1994. Ambient ozone concentrations reached an hourly peak of 88 ppb with 7-hour averages ranging from 39 to 46 ppb. Seedlings in the 50 and 75% of ambient chambers were never exposed to greater than 80 ppb O(3). Visible foliar ozone injury (stipple) was significantly higher for R-12 seedlings than MO-7 seedlings and increased with increasing ozone exposures. For the chamber treatments averaged over all families, there was no significant difference in stomatal conductance and net photosynthetic rates, but there was a significant decrease in root biomass, and a significant decrease in root/shoot ratio between the 50 and 97% of ambient chambers. Stomatal conductance and net photosynthetic rates were significantly different between families with R-12 seedlings generally greater than MO-7 seedlings. The R-12 seedlings had a 7.5 mmol m(-2) increase in ozone uptake compared to MO-7, and at the same cumulative O(3) exposure R-12 exhibited 40.9% stippled leaf area, whereas MO-7 had 9.2% stippled leaf area. Significant differences were observed in stem volume growth and total final biomass between the open-top chambers and open plots. Although R-12 had the most severe foliar ozone injury, this family had significantly greater stem volume growth and total final biomass than MO-7 and NS seedlings. Root:shoot ratio was not significantly different between MO-7 and R-12 seedlings.     

Ozone induced leaf loss and decreased leaf production of European Holly (Ilex aquifolium L.) over multiple seasons

European Holly (Ilex aquifolium L.) was used to study the impact of one short (28 day) ozone fumigation episode on leaf production, leaf loss and stomatal conductance (g(s)), in order to explore potential longer term effects over 3 growing seasons. Young I. aquifolium plants received an episode of either charcoal-filtered air or charcoal-filtered air with 70 nl l(-1) O(3) added for 7 h d(-1) over a 28 day period from June 15th 1996, then placed into ambient environment, Stoke-on-Trent, U.K. Data were collected per leaf cohort over the next three growing seasons. Ozone exposure significantly increased leaf loss and stomatal conductance and reduced leaf production over all subsequent seasons. Impact of the initial ozone stress was still detected in leaves that had no direct experimental ozone exposure. This study has shown the potential of ozone to introduce long-term phenological perturbations into ecosystems by influencing productivity over a number of seasons.     

Effects of chronic ozone exposure on growth, root respiration and nutrient uptake of rice plants

To clarify the response of growth and root functions to low concentrations of ozone (O(3)), rice plants (Oryza sativa L.) were exposed to O(3) at 0.0 (control), 0.05 and 0.10 ppm for 8 weeks from vegetative to early heading stages. Exposure to 0.05 ppm O(3) tended to slightly stimulate the dry weight of whole plants up to 5 weeks and then slightly decrease the dry weight of whole plants. However, these effects were statistically significant only at 6 weeks. Exposure to 0.10 ppm O(3) reduced the dry weight of whole plants by 50% at 5 and 6 weeks, and thereafter the reduction of the dry weight of whole plants was gradually alleviated. Those changes in dry weight can be accounted for by a decrease or increase in the relative growth rate (RGR). The changes in the RGR caused by 0.05 and 0.10 ppm O(3) could be mainly attributed to the effect of O(3) on the net assimilation rate. Root/shoot ratio was lowered by both 0.05 and 0.10 ppm O(3) throughout the exposure period. The root/shoot ratio which had severely decreased at 0.10 ppm O(3) in the first half period of exposure (1-4 weeks) became close to the control in the latter part of exposure (5-8 weeks). Time-course changes in NH(4)-N root uptake rate were similar to those in the root/shoot ratio especially for 0.10 ppm O(3). On the other hand, root respiration increased from the middle to later periods. Since it is to be supposed that plants grown under stressed conditions change the ratio of plant organ weight to achieve balance between the proportion of shoots to roots in the plant and their activity for maintaining plant growth, these changes in root/shoot ratio and nitrogen uptake rate under long-term exposure can be considered to be an adaptive response to maintain rice growth under O(3) stress.     

Photosynthetic and stomatal conductance responses of Norway spruce and beech to ozone, acid mist and frost--a conceptual model

Two-year-old beech and Norway spruce seedlings were exposed to a combination of ozone and acid mist treatments in open-top chambers in Scotland during the months of July through to September 1988. Replicate pairs of chambers received charcoal-filtered air (control), ozone-enriched air (140 nl ozone litre(-1)) or 140 nl ozone litre(-1) plus a synthetic acid mist (pH 2.5) composed of ammonium nitrate and sulphuric acid. Field measurements of assimilation and stomatal conductance were made during August. In addition, measurements of assimilation and conductance were made during September in the laboratory. Light response curves of assimilation and conductance were determined using a GENSTAT nonrectangular hyperbolic model. During February 1988/9 the Norway spruce were subject to a four day warming period at 12 degrees C and the light response of assimilation determined. The same plants were then subject to a 3-h night-time frost of -10 degrees C. The following day the time-course of the recovery of assimilation was determined. It was found that ozone fumigation did not influence the light response of assimilation of beech trees in the field, although stomatal conductance was reduced in the ozone-fumigated trees. The rate of light-saturated assimilation of Norway spruce was increased by ozone fumigation when measured in the field. Measurements of assimilation of Norway spruce made during the winter showed that prior to rewarming there was no difference in the rate of light-saturated assimilation for control and ozone-fumigated trees. However, the ozone plus acid mist-treated trees exhibited a significantly higher rate. The 4-day period of warming to 12 degrees C increased the rate of light-saturated assimilation in all treatments but only the ozone plus acid mist-treated trees showed a significant increase. Following a 3-h frost to -10 degrees C the control trees exhibited a reduction in the rate of light-saturated assimilation (Amax) to 80% of the pre-frost value. In comparison, following the frost, the ozone-fumigated trees showed an Amax of 74% of the pre-frost value. The ozone plus acid mist-treated trees showed an Amax of 64% of the pre-frost trees. The time taken for Amax to attain 50% of the pre-frost value increased from 30 min (control) to 85 min for ozone-fumigated trees to 190 min (ozone plus acid mist). These results are discussed in relation to the impact of mild, short-term frosts, which are known to occur with greater frequency than extreme, more catastrophic frost events. A simple conceptual framework is proposed to explain the variable results obtained in the literature with respect to the impact of ozone upon tree physiology.