Effects of ozone and aerosol pollution on photosynthesis of poplar leaves

由于经济的快速发展, 中国大部分地区正面临着严峻的复合型大气污染, 其中臭氧和气溶胶是两种主要污染物。已有的研究表明臭氧对叶片的氧化性伤害能够抑制光合作用, 而气溶胶可通过增加散射辐射比例或缓解高温抑制促进光合作用。但复合污染下, 臭氧和气溶胶如何共同调控叶片光合作用, 仍缺乏研究。该研究利用北京及周边地区之间的污染梯度, 选择加杨(Populus × canadensis)作为实验对象, 于2012-2013年生长季期间对叶片光合速率进行连续观测, 并同时监测臭氧浓度(AOT40)、气溶胶光学厚度(AOD)、空气温度和冠层内外光合有效辐射(PAR)等环境因子, 以期探讨大气复合污染下臭氧和气溶胶变化对植物叶片光合作用的影响及相关机制。结果表明: (1)臭氧浓度与空气温度、气溶胶浓度之间均呈显著正相关关系, 但气溶胶浓度与空气温度没有显著相关关系; (2)臭氧浓度增加显著抑制了阳生叶片的光合作用, 但气溶胶浓度上升促进了阳生叶片的光合作用; 臭氧浓度升高对阴生叶片光合作用的影响较小, 但气溶胶浓度上升促进了阴生叶片的光合作用; (3)标准化后的结果显示, 臭氧对阳生叶片光合作用的影响最大, 此时气溶胶的促进作用一定程度上补偿了臭氧浓度上升所带来的抑制效应。对于阴生叶片光合作用而言, 气溶胶则是最重要的影响因素。该研究发现复合污染下阴生叶和阳生叶光合响应不同, 这表明冠层结构可能通过影响阴生叶和阳生叶的比例, 从而对植物生长产生不同影响。该研究对理解大气复合污染如何影响光合作用提供了的机理支持, 同时也表明, 为了维持生态系统生产力及功能, 需要同时控制气溶胶和臭氧污染。     

臭氧和气溶胶复合污染对杨树叶片光合作用的影响

由于经济的快速发展, 中国大部分地区正面临着严峻的复合型大气污染, 其中臭氧和气溶胶是两种主要污染物。已有的研究表明臭氧对叶片的氧化性伤害能够抑制光合作用, 而气溶胶可通过增加散射辐射比例或缓解高温抑制促进光合作用。但复合污染下, 臭氧和气溶胶如何共同调控叶片光合作用, 仍缺乏研究。该研究利用北京及周边地区之间的污染梯度, 选择加杨(Populus × canadensis)作为实验对象, 于2012-2013年生长季期间对叶片光合速率进行连续观测, 并同时监测臭氧浓度(AOT40)、气溶胶光学厚度(AOD)、空气温度和冠层内外光合有效辐射(PAR)等环境因子, 以期探讨大气复合污染下臭氧和气溶胶变化对植物叶片光合作用的影响及相关机制。结果表明: (1)臭氧浓度与空气温度、气溶胶浓度之间均呈显著正相关关系, 但气溶胶浓度与空气温度没有显著相关关系; (2)臭氧浓度增加显著抑制了阳生叶片的光合作用, 但气溶胶浓度上升促进了阳生叶片的光合作用; 臭氧浓度升高对阴生叶片光合作用的影响较小, 但气溶胶浓度上升促进了阴生叶片的光合作用; (3)标准化后的结果显示, 臭氧对阳生叶片光合作用的影响最大, 此时气溶胶的促进作用一定程度上补偿了臭氧浓度上升所带来的抑制效应。对于阴生叶片光合作用而言, 气溶胶则是最重要的影响因素。该研究发现复合污染下阴生叶和阳生叶光合响应不同, 这表明冠层结构可能通过影响阴生叶和阳生叶的比例, 从而对植物生长产生不同影响。该研究对理解大气复合污染如何影响光合作用提供了的机理支持, 同时也表明, 为了维持生态系统生产力及功能, 需要同时控制气溶胶和臭氧污染。     

In vivo occurrence of carbonyl residues in Phaseolus vulgaris proteins as a direct consequence of a chronic ozone stress

We aimed to show that a chronic and realistic ozone stress could induce in vivo formation of carbonyl groups in leaf proteins of bean (Phaseolus vulgaris L. cv Bergamo). Plants were grown in three open-top chambers with increasing ozone concentrations: non-filtered air (NF), NF+40 nL·L^–1, NF+80 nL·L^–1 ozone 7 h·d^–1 for 22 d. Carbonyl contents in proteins, evaluation of Rubisco (EC 4.1.1.39) amounts and visible damages were systematically investigated in primary and first trifoliate leaves. Visible ozone injuries clearly reflected the total external ozone dose (expressed as AOT40) that the leaf had suffered. Ozone was effective at inducing aldehydes and ketones formation in bean proteins. This production of carbonyl groups increased with ozone concentration, the most relevant difference being observed on the Rubisco small subunit (Rubisco SSU). Contrary to young first trifoliate leaves, older primary leaves from O₃-enriched atmospheres exhibited a two-fold decrease in Rubisco level. Carbonyl group formation in Rubisco SSU and decrease in Rubisco level were not necessarily linked. Depending on ozone concentration, exposure time and leaf age, these two effects were observed either together or separately for an almost similar external dose of ozone. To conclude, leaf symptoms, loss of Rubisco and oxidized Rubisco SSU could participate in the assessment of the impact of a chronic ozone stress.     

Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95nmol mol^?1, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol^?1) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m^?2 d^?1) to deep shade (0.7mol m^?2 d^?1). Ozone reduced light-saturated rates of net photosynthesis (A_sat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A_sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A_sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.     

Ozone uptake and its effect on photosynthetic parameters of two tobacco cultivars with contrasting ozone sensitivity

Leaf discs of the ozone tolerant tobacco (Nicotiana tabacum L.) cv. Bel B and of the ozone sensitive cv. Bel W3, were exposed to an acute ozone fumigation (300 ppb) for 3 h. We measured ozone uptake by leaves and physiological characteristics before, during and after the treatment, in order to determine if the different O₃ sensitivity was correlated to the leaf uptake. In the tolerant cv. Bel B, O₃ uptake was high during the first 2 h of ozone exposure and then decreased. In the sensitive cv. Bel W3, the rate of O₃ uptake decreased constantly during ozone fumigation. The estimated cumulative uptake over the treatment time was higher (200 ± 30 μmol m^–2) in Bel B than in Bel W3 (130 ± 12 μmol m^–2). Thus, the ozone sensitivity was not correlated with ozone uptake. Stomatal conductance and photosynthesis were significantly inhibited during the fumigation in both cultivars. However, these reductions were strong and irreversible in the cv. Bel W3, while in the cv. Bel B both parameters recovered in the post-fumigation period. Thus, ozone tolerance may be related to a sustained capacity of recovery. There was no linear correlation between ozone uptake and photosynthesis reduction, but a threshold of ozone uptake was found after which photosynthesis was substantially impaired. This threshold may or may not be reached under the same external ozone level, indicating that the AOT40 may not be a sufficiently accurate index for the detection of ozone damage in plants.     

Ozone effects on wheat in relation to CO2: modelling short‐term and long‐term responses of leaf photosynthesis and leaf duration

A combined stomatal–photosynthesis model was extended to simulate the effects of ozone exposure on leaf photosynthesis and leaf duration in relation to CO₂. We assume that ozone has a short‐term and a long‐term effect on the Rubisco‐limited rate of photosynthesis, A_c. Elevated CO₂ counteracts ozone damage via stomatal closure. Ozone is detoxified at uptake rates below a threshold value above which A_c decreases linearly with the rate of ozone uptake. Reduction in A_c is transient and depends on leaf age. Leaf duration decreases depending on accumulated ozone uptake. This approach is introduced into the mechanistic crop simulation model AFRCWHEAT2. The derived model, AFRCWHEAT2‐O3, is used to test the capability of these assumptions to explain responses at the plant and crop level. Simulations of short‐term and long‐term responses of leaf photosynthesis, leaf duration and plant and crop growth to ozone exposure in response to CO₂ are analysed and compared with experimental data derived from the literature. The model successfully reproduced published responses of leaf photosynthesis, leaf duration, radiation use efficiency and final biomass of wheat to elevated ozone and CO₂. However, simulations were unsatisfactory for cumulative radiation interception which had some impact on the accuracy of predictions of final biomass. There were responses of leaf‐area index to CO₂ and ozone as a result of effects on tillering which were not accounted for in the present model. We suggest that some model assumptions need to be tested, or analysed further to improve the mechanistic understanding of the combined effects of changes in ozone and CO₂ concentrations on leaf photosynthesis and senescence. We conclude that research is particularly needed to improve the understanding of leaf‐area dynamics in response to ozone exposure and elevated CO₂.     

Evidence of widespread effects of ozone on crops and (semi‐)natural vegetation in Europe (1990–2006) in relation to AOT40‐ and flux‐based risk maps

Records of effects of ambient ozone pollution on vegetation have been compiled for Europe for the years 1990–2006. Sources include scientific papers, conference proceedings, reports to research funders, records of confirmed ozone injury symptoms and an international biomonitoring experiment coordinated by the ICP Vegetation. The latter involved ozone‐sensitive (NC‐S) and ozone‐resistant (NC‐R) biotypes of white clover (Trifolium repens L.) grown according to a common protocol and monitored for ozone injury and biomass differences in 17 European countries, from 1996 to 2006. Effects were separated into visible injury or growth/yield reduction. Of the 644 records of visible injury, 39% were for crops (27 species), 38.1% were for (semi‐) natural vegetation (95 species) and 22.9% were for shrubs (49 species). Owing to inconsistencies in reporting effort from year to year it was not possible to determine geographical or temporal trends in the data. Nevertheless, this study has shown effects in ambient air in 18 European countries from Sweden in the north to Greece in the south. These effects data were superimposed on AOT40 (accumulated ozone concentrations over 40 ppb) and POD3_gen (modelled accumulated stomatal flux over a threshold of 3 nmol m^?2 s^?1) maps generated by the EMEP Eulerian model (50 km × 50 km grid) that were parameterized for a generic crop based on wheat and NC‐S/NC‐R white clover. Many effects were found in areas where the AOT40 (crops) was below the critical level of 3 ppm h. In contrast, the majority of effects were detected in grid squares where POD3_gen (crops) were in the mid‐high range (>12 mmol m^?2). Overall, maps based on POD3_gen provided better fit to the effects data than those based on AOT40, with the POD3_gen model for clover fitting the clover effects data better than that for a generic crop.     

Rate and duration of grain filling in relation to flag leaf senescence and grain yield in spring wheat (Triticum aestivum) exposed to different concentrations of ozone

Effects of different concentrations of ozone on grain filling, flag leaf senescence and final grain yield in field‐grown spring wheat (Triticum aestivum L. cv. Dragon) were studied using open‐top chambers. The hypothesis tested was that an ozone‐induced reduction in grain yield is mainly related to an enhanced senescence and a shortening of the grain‐filling period. The plants were exposed to filtered air (F), non‐filtered air without extra ozone (NF) or non‐filtered air with 3 different levels of ozone added (NF1+, NF2+ and NF3+). The mean daytime (08.00–20.00 h) ozone concentrations during the exposure period (31 days) were 7, 20, 34, 48 and 62 nmol mol?1 in F, NF, NF1+, NF2+ and NF3+, respectively. The corresponding ozone doses, expressed as the accumulated exposure over a concentration threshold of 40 nmol mol?1 (AOT40), were 0, 12, 1 989, 5 881 and 10 375 nmol mol?1 h, respectively, and 884, 2 594, 4 557, 6 188 and 7 900 μmol m?2, respectively, expressed as the calculated cumulative flag leaf ozone flux (CFO30). The flag leaves senesced earlier and the grain‐filling duration was significantly shorter at higher ozone exposure compared to F (?5, ?13 and ?18% in NF1+, NF2+ and NF3+, respectively). The relative grain‐filling rate did not differ between the treatments. The 1000‐grain weights were 10, 28 and 37% lower, and the grain yields were 15, 29 and 46% lower than F in NF1+, NF2+ and NF3+, respectively. Ozone exposure had no significant effect on the number of grains per unit ground area or on straw yield, but significantly reduced the harvest index and increased the grain protein concentration in NF2+ and NF3+ compared to F. The grain yield was negatively correlated with the ozone dose, expressed either as AOT40 or as CFO3 with or without an ozone flux threshold. The 1000‐grain weight was positively correlated with the grain‐filling duration (R2=0.998), which in turn was positively correlated with the leaf area duration (R2=0.989).     

Ozone sensitivity in Triticum durum and T. aestivum with respect to leaf injury, photosynthetic activity and free radical content

Sensitivity to ozone is highly variable in cultivars of different wheat species, leading to differences in leaf injury and yield. Not much is known about the physiological background of these differences. The objective of this study was to compare the effects of ozone on photosynthetic parameters in Triticum aestivum L. (spring wheat cv. Nandu, winter wheat cv. Perlo) and Triticum durum Desf. (cv. Extradur). Plants cultivated in pots were exposed to 80 nmol mol^?1 ozone, or were used as control plants in a greenhouse. Stages of growth and senescence of single leaves were recorded. Light-saturated net photosynthesis, leaf conductance for water vapour, and chlorophyll fluorescence were measured. Stomatal limitation was calculated from CO₂ response curves, and the free radical content of whole leaves measured by EPR spectroscopy. Senescence of single leaves was enhanced by the ozone-treatment in all three cultivars, in the order Nandu > Perlo > Extradur. Development of whole plants was slightly delayed in Perlo and Nandu, but was accelerated significantly in Extradur. The rate of net photosynthesis under light saturation (A_sat) decreased significantly in older, ozone-fumigated leaves of Perlo and Nandu but not of Extradur. Leaf conductance (g₁) showed a similar behaviour, but stomatal limitation (l) was similar between ozone-treated and control plants. Thus, an ozone-induced closure of stomata was not the reason for the observed difference in A_sat. Perlo and Nandu showed a significant, only partly reversible decrease in F_v/F_m in ozone-fumigated leaves, whereas in Extradur the decrease was fully reversible only in older leaves. Whole leaves of Extradur, in contrast to Perlo and Nandu, showed no increase in EPR free radical signals. The higher ozone tolerance of Extradur was thus not caused by decreased ozone uptake via the stomata, but by a better ability of photosynthetically active mesophyll cells to cope with photooxidative stress.     

Effects of sulfur dioxide and ozone on intact leaves and isolated mesophyll cells of groundnut plants (Arachis hypogaea L.)

Difference between effects of sulfur dioxide (SO₂) and ozone (O₃) on groundnut plants (Arachis hypogaea L.) was studied by use of an exposure system of enzymatically-isolated mesophyll cells. SO₂ inhibited photosynthesis of intact groundnut leaves but induced no visible injury on leaves. SO₂ also inhibited photosynthesis of isolated mesophyll cells but did not kill the cells, suggesting that SO₂ inhibits photosynthesis by attacking rather specifically the photosynthetic apparatus in chloroplasts. O₃ inhibited photosynthesis of intact leaves and at the same time induced visible injury corresponding to the extent of photosynthesis inhibition. O₃ also inhibited photosynthesis of isolated mesophyll cells and killed the cells to the extent corresponding to photosynthesis inhibition, suggesting that O₃ inhibits photosynthesis not directly by attacking the photosynthetic apparatus but indirectly by killing cells. Since the response of intact leaves to each pollutant resembled that of isolated mesophyll cells, the difference between responses of intact leaves to both pollutants may considerably reflect that of mesophyll cells.     

Volatile emissions and phenolic compound concentrations along a vertical profile of <Emphasis Type="Italic">Populus nigra</Emphasis> leaves exposed to realistic ozone concentrations

Plants are exposed to increasing levels of tropospheric ozone concentrations. This pollutant penetrates in leaves through stomata and quickly reacts inside leaves, thus making plants valuable ozone sinks, but at the same time triggers oxidation processes which lead to leaf injuries. To counteract these negative effects, plants produce an array of antioxidants which react with ozone and reactive molecules which ozone generates in the leaf tissues. In this study, we measured the effect of an ozone concentration which is likely to be attained in many areas of the world in the near future (80 ppb) on leaves of the vertical profile of the widespread agroforestry species Populus nigra. Changes in (1) physiological parameters (photosynthesis and stomatal conductance), (2) ozone uptake, (3) emission of volatile organic compounds (VOCs, i.e. isoprene, methanol and other oxygenated compounds), (4) concentration of antioxidant surface compounds, and (5) concentration of phenolic compounds were assessed. The aim was to assess whether the defensive pathways leading to isoprenoids and phenolics formation were induced when a moderate and chronic increment of ozone is not able to damage photosynthesis. No visual injuries and minor changes in physiology and ozone uptake were observed. The emission of isoprene and oxygenated six-carbon (C6) volatiles were inhibited by ozone, whereas methanol emission was increased, especially in developing leaves. We interpret these results as suggesting an ontogenetic shift in ozone-treated leaves, leading to a slower development and a faster senescence. Most surface and phenolic compounds showed a declining trend in concentration from the youngest to the fully expanded leaves. Ozone reduced the concentrations of chlorogenic acid derivatives at the leaf surface, whereas in total leaf extracts a metabolic shift towards few phenolics with higher antioxidant capacity was observed.     

农田冬小麦生长和产量对臭氧动态暴露的响应

 评估臭氧(O₃)污染对农田冬小麦生长和产量的影响是污染生态学和生理生态学研究的重要内容之一。该研究运用开顶式气室(OTC),对冬小麦‘ 嘉403’(Triticum aestivum cv. Jia 403)进行了O₃动态暴露的田间原位试验。实验设置过滤空气组(CF)、自然大气组(NF)和两个不同浓度的 O₃动态暴露组(DO100和DO150)。结果表明：1) O₃浓度增加,一方面可以改变灌浆期冬小麦叶片气体交换参数的日变化规律;另一方面引起表观 光量子产额、光饱和点和光补偿点等光响应参数的显著降低。这表明灌浆期叶片光合能力的下降是气孔因素和非气孔因素共同作用的结果。2) O₃暴露可以改变小麦形态特征,如植株变矮、叶片衰老加速、 叶面积变小,并最终导致产量大幅下降。     

Chlorophyll fluorescence assay for ozone injury in intact plants

A chlorophyll fluorescence induction (Kautsky effect) assay predicted ozone-induced injury in bean leaves (Phaseolus vulgaris) at least 20 hours before any visible sign of leaf necrosis. The extent of injury, which could be predicted during exposure to ozone, depended on concentration, exposure time, and leaf development stage. Much more injury occurred in light than in darkness and long exposures to lower ozone concentrations were more injurious than brief exposures to higher ones. The first detectable effect was on the photosynthetic water-splitting enzyme systems, followed by inhibition of electron transport between the photosystems. The fluorescence assay provides a simple, rapid, nondestructive method for observing effects of ozone on plants.     

Image analysis of the leaf vascular network: physiological considerations

The study of leaf vascular systems is important in order to understand the fluid dynamics of water movement in leaves. Recent studies have shown how these systems can be involved in the performance of photosynthesis, which is linked to the density of the vascular network per unit of leaf area. The aim of the present study was to highlight the correlation between a leaf vein density (V_D) and net photosynthetic rate (P_N), which was undertaken using a digital camera, a stereoscopic microscope, and a light source. The proposed hypothesis was tested, for the first time, on the leaves of two cultivars of Vitis vinifera (L.). A significant difference was found between the V_D of mature leaves of the two cultivars. V_D was also significantly correlated with the maximum leaf P_N. These findings support the hypothesis that the vascular system of grape leaves can be correlated with leaf photosynthesis performance.     

Effects of elevated ozone on CO2 uptake and leaf structure in sugar maple under two light environments

The interactive effects of ozone and light on leaf structure, carbon dioxide uptake and short-term carbon allocation of sugar maple ( Acer saccharum Marsh.) seedlings were examined using gas exchange measurements and ¹⁴C-macroautoradiographic techniques. Two-year-old sugar maple seedlings were fumigated from budbreak for 5 months with ambient or 3 × ambient ozone in open-top chambers, receiving either 35% (high light) or 15% (low light) of full sunlight. Ozone accelerated leaf senescence, and reduced net photosynthesis, ¹⁴CO₂ uptake and stomatal conductance, with the effects being most pronounced under low light. The proportion of intercellular space increased in leaves of seedlings grown under elevated ozone and low light, possibly enhancing the susceptibility of mesophyll cells to ozone by increasing the cumulative dose per mesophyll cell. Indeed, damage to spongy mesophyll cells in the elevated ozone × low light treatment was especially frequent. ¹⁴C macroautoradioraphy revealed heterogeneous uptake of ¹⁴CO₂ in well defined areole regions, suggesting patchy stomatal behaviour in all treatments. However, in seedlings grown under elevated ozone and low light, the highest ¹⁴CO₂ uptake occurred along larger veins, while interveinal regions exhibited little or no uptake. Although visible symptoms of ozone injury were not apparent in these seedlings, the cellular damage, reduced photosynthetic rates and reduced whole-leaf chlorophyll levels corroborate the visual scaling of whole-plant senescence, suggesting that the ozone × low light treatment accelerated senescence or senescence-like injury in sugar maple.     

Raman spectroscopy of pigments and oxalates in situ within epilithic lichens: Acarospora from the Antarctic and Mediterranean

The effects of ambient and elevated ozone levels on growth and photosynthesis of beech ( Fagus sylvatica ) were studied by exposing seedlings in open-top chambers for one growing season to three treatments: charcoal-filtered (CF), non-filtered (NF) and non-filtered air with addition of ozone (30 ppb ozone) on clear days for 8–10 h d⁻¹ (NF +). Ambient levels were relatively low and accumulated to an AOT40 (accumulated exposure over a threshold of 40 ppb) of 4055 ppb h (for the period 23 Apr–30 Sept). The NF + chambers received an AOT40 of 8880 ppb h. Throughout the growing season we measured growth and photosynthetic properties. The treatments did not cause strong effects: measurements of gas exchange (light-saturated assimilation rate, CO₂ and light-response curves) and chlorophyll fluorescence showed slight and mostly non-significant reductions of several parameters. No significant differences were found for growth, though in the NF + treatment (AOT40 8880 ppb h) the relative growth rate for diameter increment was at times reduced by 12% compared with the control treatment.     

Differentiation and structural decline in the leaves and bark of birch (Betula pendula) under low ozone concentrations

Summary Leaf and bark structure of a birch clone (Betula pendula Roth) continuously exposed to charcoal-filtered air or charcoal-filtered air plus ozone (0.05, 0.075, 0.1 l 1^-1) was investigated throughout one growing season. Increasing ozone dose influenced leaf differentiation by reducing leaf area and increasing inner leaf air space, density of cells developing into stomata, scales and hairs. When approximately the same ozone dose had been reached, macroscopical and microscopical symptoms appeared irrespective of the ozone concentration used during treatment. Structural decline began in mesophyll cells around stomatal cavities (droplet-like exudates on the cell walls), continued with disintegration of the cytoplasma and ended in cell collapse. Epidermal cells showed shrinkage of the mucilaginous layer (related to water loss). Their collapse marked the final stage of leaf decline. When subsidiary cells collapsed, guard cells passively opened for a transitory period before collapsing and closing. With increasing ozone dose starch remained accumulated along the small leaf veins and in guard cells. IIK-positive grains were formed in the epidermal cells. This contrasted with the senescent leaves, where starch was entirely retranslocated. Injury symptoms in stem and petiole proceeded from the epidermis to the cambium. Reduced tissue area indicated reduced cambial activity. In plants grown in filtered air and transferred into ozone on 20 August, injury symptoms developed faster than in leaves formed in the presence of ozone. Results are discussed with regard to O₃-caused acclimation and injury mechanisms.     

EVIDENCE FOR THE REPAIR OF OZONE-INDUCED MEMBRANE INJURY

The increase in [¹⁴C]-2-deoxy-D-glucose uptake by leaf discs 24 hr after fumigation was used as a measure of ozone injury to pinto bean leaves (Phaseolus vulgaris). This method showed that the primary leaves were most affected by ozone when plants were 10 to 12 days old, which coincided with the time of maximum leaf necrosis. However, 8-day-old plants, which had no visible leaf injury, still showed a higher uptake rate than controls, indicating that injury occurred at the cellular level. In these younger plants, uptake was shown to return to normal over a 5 day period. Moreover, the rate of this “repair” was retarded by cold or continuous darkness, enhanced by continuous light, and very markedly increased by glucose applied to the leaves. The timing of the glucose application was not critical, nor did H₂O or mannitol have an effect. The results suggest that ozone injury at the cellular level can be repaired by energy-dependent processes so that necrosis of the leaf tissue does not occur. These experiments also show that conditions and treatments after ozone exposure can alter the degree of ozone injury.     

Interactive effects of elevated ozone and springtime frost on growth and physiology of birch (<Emphasis Type="Italic">Betula pendula</Emphasis>) in field conditions

We studied the impact of ozone enrichment and late frost, singly and interactively, on four birch (Betula pendula Roth) families selected from a naturally regenerated birch stand in southeastern Finland. Seedlings were exposed to 1.5× ambient ozone over one and a half growing seasons using free-air ozone enrichment system. Simulated springtime frost was implemented at the beginning of the second study year, 4 weeks after the bud burst. Plants were measured for timing of bud burst, visible ozone injuries, chlorophyll fluorescence, net photosynthesis and concentrations of photosynthetic pigments, as well as for growth and carbon allocation. Frost treatment caused a rapid 60% decline in net photosynthesis. The recovery of net photosynthesis from acute frost treatment was not complete during the subsequent 3 weeks, which led to significant growth reductions, decreased shoot/root ratio and accumulation of excess nitrogen in the leaves. Photosynthetic responses to ozone were very variable and family-specific. Concentrations of photosynthetic pigments were sensitive to both stress factors, while the maximum quantum yield of PSII was unaffected. Ozone exacerbated the effect of frost only on diameter increment. However, ozone and frost affected different seedling characters, e.g., ozone reduced pigments and frost collapsed net photosynthesis, and these effect combined appear to damage birch seedlings more than a single stress situation.     

Effect of tropospheric ozone on morphological characteristics of Nicotiana tabacum L., Phaseolus vulgaris L. and Petunia×Hybrida L. in ambient air conditions

The cumulative ozone effect on morphological parameters (visible leaf injury, plant height and leaf growth, number of bean pods, petunia flowers and stalks) was examined in this study. Well-known ozonesensitive (Bel W3) and ozone-resistant (Bel B) tobacco cultivars as well as bean cv. Nerina and petunia cv. White cascade, both recognized as ozone sensitive, were used in the experiment. Investigations were carried out at two exposure sites varying in tropospheric ozone levels. Ozone negatively affected the leaf growth of both tobacco cultivars and bean. A negative relation was also found for ozone concentration and tobacco plant height. Number of petunia flowers and stalks and bean pods was positively correlated with ozone concentration. This could have been connected with earlier plant maturation due to faster generative development of plants in ozone-stress conditions.