Ozone and increased nitrogen supply effects on the yield and nutritive quality of Trifolium subterraneum

The influence of ambient ozone (O₃) concentrations and nitrogen (N) fertilization, singly and in combination, on the growth and nutritive quality of Trifolium subterraneum was assessed. This is an important O₃-sensitive species of great pastoral value in Mediterranean areas. Plant material was enclosed in open-top chambers (OTCs). Three O₃ levels were established: Filtered air with O₃ concentrations below 15 ppb (CFA), non-filtered air with O₃ concentrations in the range of ambient levels (NFA), and non-filtered air supplemented with 40 ppb O₃ over ambient levels (NFA+). Similarly, three N levels were defined: 5, 15 and 30 kg ha⁻¹. The increase in O₃ exposure induced a reduction of the clover aerial green biomass and an increase of senescent biomass. Ozone effects were more adverse in the root system, inducing an impairment of the aerial/subterranean biomass ratio. Compared with the CFA treatment, nutritive quality of aerial biomass was 10 and 20% lower for NFA and NFA+ treatments, respectively, due to increased concentrations of acid detergent fiber, neutral detergent fiber and lignin. The latter effect appears to be related to senescence acceleration. The increment in N supplementation enhanced the increase of ADF concentrations in those plants simultaneously exposed to ambient and above-ambient O₃ concentrations, and reduced the incremental rate of foliar senescence induced by the pollutant.     

Cutleaf coneflower (Rudbeckia laciniata L.) response to ozone and ethylenediurea (EDU)

Cutleaf coneflower (Rudbeckia laciniata L.) seedlings were placed into open-top chambers in May, 2004 and fumigated for 12 wks. Nine chambers were fumigated with either carbon-filtered air (CF), non-filtered air (NF) or twice-ambient (2×) ozone (O₃). Ethylenediurea (EDU) was applied as a foliar spray weekly at 0 (control), 200, 400 or 600 ppm. Foliar injury occurred at ambient (30%) and elevated O₃ (100%). Elevated O₃ resulted in significant decreases in biomass and nutritive quality. Ethylenediurea reduced percent of leaves injured, but decreased root and total biomass. Foliar concentrations of cell-wall constituents were not affected by EDU alone; however, EDU × O₃ interactions were observed for total cell-wall constituents and lignocellulose fraction. Our results demonstrated that O₃ altered the physiology and productivity of cutleaf coneflower, and although reducing visible injury EDU may be phytotoxic at higher concentrations.     

Effect of ambient-level gas-phase peroxides on foliar injury, growth, and net photosynthesis in Japanese radish (Raphanus sativus)

To investigate the effects of ambient-level gas-phase peroxides concurrent with O₃ on foliar injury, photosynthesis, and biomass in herbaceous plants, we exposed Japanese radish (Raphanus sativus) to clean air, 50 ppb O₃, 100 ppb O₃, and 2-3 ppb peroxides + 50 ppb O₃ in outdoor chambers. Compared with exposure to 100 ppb O₃, exposure to 2-3 ppb peroxides + 50 ppb O₃ induced greater damage in foliar injury, net photosynthetic rates and biomass; the pattern of foliar injury and the cause of net photosynthetic rate reduction also differed from those occurring with O₃ exposure alone. These results indicate for the first time that sub-ppb peroxides + 50 ppb O₃ can cause more severe damage to plants than 100 ppb O₃, and that not only O₃, but also peroxides, could be contributing to the herbaceous plant damage and forest decline observed in Japan's air-polluted urban and remote mountains areas.     

Soil nitrogen transformations under elevated atmospheric CO₂ and O₃ during the soybean growing season

We investigated the influence of elevated CO₂ and O₃ on soil N cycling within the soybean growing season and across soil environments (i.e., rhizosphere and bulk soil) at the Soybean Free Air Concentration Enrichment (SoyFACE) experiment in Illinois, USA. Elevated O₃ decreased soil mineral N likely through a reduction in plant material input and increased denitrification, which was evidenced by the greater abundance of the denitrifier gene nosZ. Elevated CO₂ did not alter the parameters evaluated and both elevated CO₂ and O₃ showed no interactive effects on nitrifier and denitrifier abundance, nor on total and mineral N concentrations. These results indicate that elevated CO₂ may have limited effects on N transformations in soybean agroecosystems. However, elevated O₃ can lead to a decrease in soil N availability in both bulk and rhizosphere soils, and this likely also affects ecosystem productivity by reducing the mineralization rates of plant-derived residues.     

Leaf age affects the responses of foliar injury and gas exchange to tropospheric ozone in Prunus serotina seedlings

We investigated the effect of leaf age on the response of net photosynthesis (A), stomatal conductance (g_wv), foliar injury, and leaf nitrogen concentration (N_L) to tropospheric ozone (O₃) on Prunus serotina seedlings grown in open-plots (AA) and open-top chambers, supplied with either carbon-filtered or non-filtered air. We found significant variation in A, g_wv, foliar injury, and N_L (P < 0.05) among O₃ treatments. Seedlings in AA showed the highest A and g_wv due to relatively low vapor pressure deficit (VPD). Older leaves showed significantly lower A, g_wv, N_L, and higher foliar injury (P < 0.001) than younger leaves. Leaf age affected the response of A, g_wv, and foliar injury to O₃. Both VPD and N_L had a strong influence on leaf gas exchange. Foliar O₃-induced injury appeared when cumulative O₃ uptake reached 8-12 mmol m⁻², depending on soil water availability. The mechanistic assessment of O₃-induced injury is a valuable approach for a biologically relevant O₃ risk assessment for forest trees.     

Exposure to elevated ozone levels differentially affects the antioxidant capacity and the redox homeostasis of two subtropical Phaseolus vulgaris L. varieties

Ozone (O₃) has become one of the most toxic air pollutants to plants worldwide. However, investigations on O₃ impacts on crops health and productivity in South America countries are still scarce. The present study analyzed the differences on the enzymatic and non-enzymatic antioxidant system in foliar tissue of two subtropical Phaseolus vulgaris varieties exposed to high O₃ concentration. Both varieties were negatively impacted by the pollutant, but the responses between each variety were quite distinct. Results revealed that Irai has higher constitutive levels of reactive oxygen species (ROS) and ascorbate (AsA) concentration, but lower total thiol levels and catalase immunocontent. In this variety catalase protein concentration was increased after O₃ exposure, indicating a better cellular capacity to reduce hydrogen peroxide. On the opposite, Fepagro 26-exposed plants increased ROS generation and AsA concentration, but had the levels of total thiol content and catalase protein unchanged. Furthermore, O₃ treatment reduced the levels of chlorophylls a and b, and the relationship analysis between the chlorophyll ratio (a/b) and protein concentration were positively correlated indicating that photosynthetic apparatus is compromised, and thus probably is the biomass acquisition on Fepagro 26. Differently, O₃ treatment of Irai did not affect chlorophylls a and b content, and loss on the protein content was lower. Altogether, these data suggest that early accumulation of ROS on Fepagro 26 are associated with an insufficient leaf antioxidant capacity, which leads to cell structure disruption and impairs the photosynthesis. Irai seems to be more tolerant to O₃ toxic effects than Fepagro 26, and the observed differences on O₃ sensitivity between the two varieties are apparently based on constitutive differences involved in the maintenance of intracellular redox homeostasis.     

Ozone sensitivity of the Mediterranean terophyte Trifolium striatum is modulated by soil nitrogen content

The influence of nitrogen (N) fertilization on the sensitivity to ozone (O₃) has been studied using the O₃ sensitive species Trifolium striatum, an annual species frequently found in therophitic grasslands of the dehesas in the central Iberian Peninsula. The experiment was carried out in open-top chambers using three different O₃ exposure treatments: charcoal-filtered air (CFA), non-filtered air (NFA) and non-filtered air plus 40 nl l⁻¹ added over ambient concentrations (NFA+). After 30 days of exposure to the different treatments, plants were placed under ambient conditions until seed ripeness was completed. Three N fertilization treatments were established receiving final doses of 10 (low), 30 (medium) or 60 (high) kg N ha⁻¹ throughout the growing period divided into four proportional doses at every 15 days. Ozone affected phenology and reduced total aerial and flower biomass and seed production in NFA and NFA+ treatments. Carry-over effects of O₃ on the reproductive ability of this species were found resulting in reductions in flower biomass and seed production detected 1 month after O₃ exposure. By contrast, N fertilization increased aerial and flower biomass. Significant O₃×N interactive effects were detected since N fertilization counterbalanced O₃-induced effects only when plants were exposed to moderate O₃ levels (NFA) but not under high O₃ concentrations (NFA+). The observed O₃ effects on reproductive ability of T. striatum might determine changes in species viability and future diversity of the ecosystem.     

The effects of increasing atmospheric ozone on biogenic monoterpene profiles and the formation of secondary aerosols

Monoterpenes are biogenic volatile organic compounds (BVOCs) which play an important role in plant adaptation to stresses, atmospheric chemistry, plant–plant and plant–insect interactions. In this study, we determined whether ozonolysis can influence the monoterpenes in the headspace of cabbage. The monoterpenes were mixed with an air-flow enriched with 100, 200 or 400 ppbv of ozone (O₃) in a Teflon chamber. The changes in the monoterpene and O₃ concentrations, and the formation of secondary organic aerosols (SOA) were determined during ozonolysis. Furthermore, the monoterpene reactions with O₃ and OH were modelled using reaction kinetics equations. The results showed that all of the monoterpenes were unequally affected: α-thujene, sabinene and d-limonene were affected to the greatest extend, whereas the 1,8-cineole concentration did not change. In addition, plant monoterpene emissions reduced the O₃ concentration by 12–24%. The SOA formation was dependent on O₃ concentration. At 100 ppbv of O₃, virtually no new particles were formed but clear SOA formation was observed at the higher ozone concentrations. The modelled results showed rather good agreements for α-pinene and 1,8-cineole, whereas the measured concentrations were clearly lower compared to modelled values for sabinene and limonene. In summary, O₃-quenching by monoterpenes occurs beyond the boundary layer of leaves and results in a decreased O₃ concentration, altered monoterpene profiles and SOA formation.     

Vegetative buffers for fan emissions from poultry farms: 2. ammonia,dust and foliar nitrogen

This study evaluated the potential of trees planted around commercial poultry farms to trap ammonia (NH₃) and dust or particulate matter (PM). Norway spruce, Spike hybrid poplar, hybrid willow, and Streamco purpleosier willow were planted on five commercial farms from 2003 to 2004. Plant foliage was sampled in front of the exhaust fans and at a control distance away from the fans on one turkey, two laying hen, and two broiler chicken farms between June and July 2006. Samples were analyzed for dry matter (DM), nitrogen (N), and PM content. In addition, NH₃ concentrations were measured downwind of the exhaust fans among the trees and at a control distance using NH₃ passive dosi–tubes. Foliage samples were taken and analyzed separately based on plant species. The two layer farms had both spruce and poplar plantings whereas the two broiler farms had hybrid willow and Streamco willow plantings which allowed sampling and species comparisons with the effect of plant location (control vs. fan). The results showed that NH₃ concentration h^{? 1} was reduced by distance from housing fans (P ≤ 0.0001), especially between 0 m (12.01 ppm), 11.4 m (2.59 ppm), 15 m (2.03 ppm), and 30 m (0.31 ppm). Foliar N of plants near the fans was greater than those sampled away from the fans for poplar (3.87 vs. 2.56%; P ≤ 0.0005) and hybrid willow (3.41 vs. 3.02%; P ≤ 0.05). The trends for foliar N in spruce (1.91 vs. 1.77%; P = 0.26) and Streamco willow (3.85 vs. 3.33; P = 0.07) were not significant. Pooling results of the four plant species indicated greater N concentration from foliage sampled near the fans than of that away from the fans (3.27 vs. 2.67%; P ≤ 0.0001). Foliar DM concentration was not affected by plant location, and when pooled the foliar DM of the four plant species near the fans was 51.3% in comparison with 48.5% at a control distance. There was a significant effect of plant location on foliar N and DM on the two layer farms with greater N and DM adjacent to fans than at a control distance (2.95 vs. 2.15% N and 45.4 vs. 38.2% DM, respectively). There were also significant plant species effects on foliar N and DM with poplar retaining greater N (3.22 vs. 1.88%) and DM (43.7 vs. 39.9%) than spruce. The interaction of location by species (P ≤ 0.005) indicated that poplar was more responsive in terms of foliar N, but less responsive for DM than spruce. The effect of location and species on foliar N and DM were not clear among the two willow species on the broiler farms. Plant location had no effect on plant foliar PM weight, but plant species significantly influenced the ability of the plant foliage to trap PM with spruce and hybrid willow showing greater potential than poplar and Streamco willow for PM_2.5(0.0054, 0.0054, 0.0005, and 0.0016 mg cm^{? 2}; P ≤ 0.05) and total PM (0.0309, 0.0102, 0.0038, and 0.0046 mg cm^{? 2}, respectively; P ≤ 0.001). Spruce trapped more dust compared to the other three species (hybrid willow, poplar, and Streamco willow) for PM₁₀ (0.0248 vs. 0.0036 mg cm^{? 2}; P ≤ 0.0001) and PM_{> 10} (0.0033 vs. 0.0003 mg cm^{? 2}; P = 0.052). This study indicates that poplar, hybrid willow, and Streamco willow are appropriate species to absorb poultry house aerial NH₃–N, whereas spruce and hybrid willow are effective traps for dust and its associated odors.     

Effects of Chronic Doses of Ozone on Field-Grown Loblolly Pine: Seedling Responses in the First Year

A study was designed to examine responses of loblolly pine (Pinus taeda) to chronic exposure to ozone (O₃) in the field. Seedlings of four full-sib families of loblolly pine were planted in a field near Raleigh, NC, and exposed daily (May 27 to October 24, 1985) in open-top chambers to O₃ ranging from 0.5 to 1.96 times the O₃ concentration in non-filtered (NF) air. One-fourth of the plants in each plot were removed during each of two harvests (August and October) to measure effects of O₃ on plant growth. Plants of each family exhibited foliar symptoms characteristic of O3 injury after five months of exposure to any greater-than-ambient O₃ concentration, and one family exhibited symptoms after five months of exposure to NF air. Ozone dose-plant response relationships were quantified by regression for stem height, stem diameter, biomass, and other plant morphological and yield characteristics. All relationships were linear for three families, but one family exhibited no significant growth response relationship of O₃ dose. Dose-response equations suggest a maximum growth suppression of 10 percent for NF air compared to charcoal-filtered air (i.e., 0.5 × NF) in the first season of exposure.     

Contrasting effects of ozone under different water supplies in two Mediterranean tree species

The effects of ozone (O₃) exposure under different water availabilities were studied in two Mediterranean tree species: Quercus ilex and Ceratonia siliqua. Plants were exposed to different O₃ concentrations in open top chambers (charcoal-filtered air (CF), non-filtered air (NF)) and non-filtered air plus 40 ppb_v of O₃ ((7:00–17:00 solar time) (NF+)) during 2 years, and to different water regimes (IR, sample irrigation, and WS, reduced water dose to 50%) through the last of those 2 years. AOT40 in the NF+ treatment was 59265 ppb_v h (from March 1999 to August 1999) while in the NF treatment, the AOT40 was 6727 ppb_v h for the same period. AOT40 was always 0 in the CF treatment. WS plants presented lower stomatal conductances and net photosynthetic rates, and higher foliar N concentrations than IR plants in both species. The irrigation treatment did not change the response trends to ozone in Q. ilex, the most sensitive species to O₃ ambient concentrations, but it changed those of C. siliqua, the least sensitive species, since its ozone-fumigated WS plants did not decrease their net photosynthetic rates nor their biomass accumulation as it happened to its ozone-fumigated IR plants. These results show interspecific variations in O₃ sensitivity under different water availabilities.     

The efficiency of tobacco Bel-W3 and native species for ozone biomonitoring in subtropical climate, as revealed by histo-cytochemical techniques

We aimed to verify whether hydrogen peroxide (H₂O₂) accumulation and cell death are detected early in three bioindicators of ozone (O₃), Nicotiana tabacum ‘Bel-W3’, Ipomoea nil ‘Scarlet O’Hara’ and Psidium guajava ‘Paluma’, and whether environmental factors also affect those microscopic markers. The three species were exposed to chronic levels of O₃ in a subtropical area and a histo-cytochemical technique that combines 3,3′-diaminobenzidine (DAB) with Evans blue staining was used in the assessments. The three species accumulated H₂O₂, but a positive correlation with O₃ concentration was only observed in N. tabacum. A positive correlation between O₃ and cellular death was also observed in N. tabacum. In I. nil and P. guajava, environmental factors were responsible for symptoms at the microscopic level, especially in P. guajava. We conclude that the most appropriate and least appropriate bioindicator plant for O₃ monitoring in the subtropics are N. tabacum ‘Bel-W3’ and P. guajava ‘Paluma’, respectively.     

Use of the antiozonant ethylenediurea (EDU) in Italy: verification of the effects of ambient ozone on crop plants and trees and investigation of EDU's mode of action

Twenty-four experiments where EDU was used to protect plants from ozone (O₃) in Italy are reviewed. Doses of 150 and 450 ppm EDU at 2-3 week intervals were successfully applied to alleviate O₃-caused visible injury and growth reductions in crop and forest species respectively. EDU was mainly applied as soil drench to crops and by stem injection or infusion into trees. Visible injury was delayed and reduced but not completely. In investigations on mode of action, EDU was quickly (<2 h) uptaken and translocated to the leaf apoplast where it persisted long (>8 days), as it cannot move via phloem. EDU did not enter cells, suggesting it does not directly affect cell metabolism. EDU delayed senescence, did not affect photosynthesis and foliar nitrogen content, and stimulated antioxidant responses to O₃ exposure. Preliminary results suggest developing an effective soil application method for forest trees is warranted.     

Comparison of a diurnal vs steady-state ozone exposure profile on growth and yield of oilseed rape (Brassica napus L.) in open-top chambers in the Yangtze Delta, China

Most available exposure-response relationships for assessing crop loss due to elevated ozone (O₃) have been established using data from chamber and open-top chamber experiments, using a simulated constant O₃ concentration exposure (square wave), which is not consistent with the diurnal variation of O₃ concentration that occurs in nature. We investigated the response of oilseed rape (Brassica napus L.) to O₃ as affected by two exposure regimes: one with a diurnal variation (CF100D) and another with a constant concentration (CF100). Although the two exposure regimes have the same mean O₃ concentration and accumulated O₃ concentration above 40 ppb (AOT40), our results show that O₃ at CF100D reduced biomass and number of pods/plant more than O₃ at CF100. Both O₃ exposures resulted in larger seed weights/100 pods compared to CF. Numbers of seeds/100 pods were reduced by CF100, while numbers of seeds/100 pods in the CF100D chambers were comparable to those in CF. Our results suggest that chamber experiments that use a constant O₃ exposure may underestimate O₃ effects on biomass and yields.     

Enhanced nitrogen deposition exacerbates the negative effect of increasing background ozone in Dactylis glomerata, but not Ranunculus acris

The combined impacts of simulated increased nitrogen (N) deposition (75 kg N ha⁻¹ yr⁻¹) and increasing background ozone (O₃) were studied using two mesotrophic grassland species (Dactylis glomerata and Ranunculus acris) in solardomes, by means of eight O₃ treatments ranging from 15.5 ppb to 92.7 ppb (24 h average mean). A-C_i curves were constructed for each species to gauge effects on photosynthetic efficiency and capacity, and effects on biomass partitioning were determined after 14 weeks. Increasing the background concentration of O₃ reduced the healthy above ground and root biomass of both species, and increased senesced biomass. N fertilisation increased biomass production in D. glomerata, and a significantly greater than additive effect of O₃ and N on root biomass was evident. In contrast, R. acris biomass was not affected by high N. The study shows the combined effects of these pollutants have differential implications for carbon allocation patterns in common grassland species.     

Nopalea cochenillifera, a potential chromium (VI) hyperaccumulator plant

Hexavalant chromium [Cr(VI)] tolerance and accumulation in in vitro grown Nopalea cochenillifera Salm. Dyck. plants was investigated. A micropropagation protocol was establish for a rapid multiplication of N. cochenillifera and [Cr(VI)] tolerance and accumulation was studied in in vitro grown cultures. Cr concentration was estimated by atomic absorption spectroscopy in roots and shoots to confirm plant’s hyperaccumulation capacity. Plants showed tolerance up to 100 μM K₂Cr₂O₇ without any significant changes in root growth after 16 days treatment; whereas, chlorophyll content in plants treated with 1 and 10 μM K₂Cr₂O₇ were not so different than the control plant. The levels of lipid peroxidation and protein oxidation increased significantly (p?<?0.01) with increasing concentration of chromium. Exposures of N. cochenillifera to lower concentrations of K₂Cr₂O₇ (≤10 μM) induced catalase (CAT) and superoxide dismutase (SOD) significantly (p?<?0.001) but higher concentrations of K₂Cr₂O₇ (>100 μM) inhibited the activities of CAT and SOD. Roots accumulated a maximum of 25,263.396?±?1,722.672 mg?Cr?Kg^?1 dry weight (DW); while the highest concentration of Cr in N. cochenillifera shoots was 705.714?±?32.324 mg?Cr?Kg^?1?DW. N. cochenillifera could be a prospective hyperaccumulator plant of Cr(VI) and a promising candidate for phytoremediation purposes.     

Gene expression responses of paper birch (Betula papyrifera) to elevated CO2 and O3 during leaf maturation and senescence

Gene expression responses of paper birch (Betula papyrifera) leaves to elevated concentrations of CO₂ and O₃ were studied with microarray analyses from three time points during the summer of 2004 at Aspen FACE. Microarray data were analyzed with clustering techniques, self-organizing maps, K-means clustering and Sammon's mappings, to detect similar gene expression patterns within sampling times and treatments. Most of the alterations in gene expression were caused by O₃, alone or in combination with CO₂. O₃ induced defensive reactions to oxidative stress and earlier leaf senescence, seen as decreased expression of photosynthesis- and carbon fixation-related genes, and increased expression of senescence-associated genes. The effects of elevated CO₂ reflected surplus of carbon that was directed to synthesis of secondary compounds. The combined CO₂ + O₃ treatment resulted in differential gene expression than with individual gas treatments or in changes similar to O₃ treatment, indicating that CO₂ cannot totally alleviate the harmful effects of O₃.     

Assessment of growth and yield losses in two Zea mays L. cultivars (quality protein maize and nonquality protein maize) under projected levels of ozone

Rapid industrialization and economic developments have increased the tropospheric ozone (O₃) budget since preindustrial times, and presently, it is supposed to be a major threat to crop productivity. Maize (Zea mays L.), a C4 plant is the third most important staple crop at global level with a great deal of economic importance. The present study was conducted to evaluate the performance of two maize cultivars [HQPM1: quality protein maize (QPM)] and [DHM117: nonquality protein maize (NQPM)] to variable O₃ doses. Experimental setup included filtered chambers, nonfiltered chambers (NFC), and two elevated doses of O₃ viz. NFC+15 ppb O₃ (NFC+15) and NFC+30 ppb O₃ (NFC+30). During initial growth period, both QPM and NQPM plants showed hormetic effect that is beneficial due to exposure of low doses of a toxicant (NFC and NFC+15 ppb O₃), but at later stages, growth attributes were negatively affected by O₃. Growth indices showed the variable pattern of photosynthate translocation under O₃ stress. Foliar injury in the form of interveinal chlorosis and reddening of leaves due to increased production of anthocyanin pigments was observed at higher concentrations of O₃. One-dimensional gel electrophoresis of leaves taken from NFC+30 showed reductions of major photosynthetic proteins, and differential response was observed between the two test cultivars. Decline in the number of male flowers at elevated O₃ doses suggested damaging effect of O₃ on reproductive structures which might be a cause of productivity losses. Variable carbon allocation pattern particularly to husk leaves, foliar injury, and damage of photosynthetic proteins led to significant reductions in economic yield at higher O₃ doses. PCA showed that both the cultivars responded more or less similarly to O₃ stress in their respective groupings of growth and yield parameters, but magnitude of their response was variable. It is further supported by difference in the significance of correlations between variables of yield and AOT40. Cultivar response reflects that QPM performed better than NQPM against elevated O₃.     

Effects of chronic elevated ozone exposure on gas exchange responses of adult beech trees (Fagus sylvatica) as related to the within-canopy light gradient

The effects of elevated O₃ on photosynthetic properties in adult beech trees (Fagus sylvatica) were investigated in relation to leaf mass per area as a measure of the gradually changing, within-canopy light availability. Leaves under elevated O₃ showed decreased stomatal conductance at unchanged carboxylation capacity of Rubisco, which was consistent with enhanced δ¹³C of leaf organic matter, regardless of the light environment during growth. In parallel, increased energy demand for O₃ detoxification and repair was suggested under elevated O₃ owing to enhanced dark respiration. Only in shade-grown leaves, light-limited photosynthesis was reduced under elevated O₃, this effect being accompanied by lowered F_v/F_m. These results suggest that chronic O₃ exposure primarily caused stomatal closure to adult beech trees in the field regardless of the within-canopy light gradient. However, light limitation apparently raised the O₃ sensitivity of photosynthesis and accelerated senescence in shade leaves.     

Elevated CO2 response of photosynthesis depends on ozone concentration in aspen

The effect of elevated CO₂ and O₃ on apparent quantum yield (?), maximum photosynthesis (P_max), carboxylation efficiency (V_cmax) and electron transport capacity (J_max) at different canopy locations was studied in two aspen (Populus tremuloides) clones of contrasting O₃ tolerance. Local light climate at every leaf was characterized as fraction of above-canopy photosynthetic photon flux density (%PPFD). Elevated CO₂ alone did not affect ? or P_max, and increased J_max in the O₃-sensitive, but not in the O₃-tolerant clone. Elevated O₃ decreased leaf chlorophyll content and all photosynthetic parameters, particularly in the lower canopy, and the negative impact of O₃ increased through time. Significant interaction effect, whereby the negative impact of elevated O₃ was exaggerated by elevated CO₂ was seen in Chl, N and J_max, and occurred in both O₃-tolerant and O₃-sensitive clones. The clonal differences in the level of CO₂ × O₃ interaction suggest a relationship between photosynthetic acclimation and background O₃ concentration.