Acceleration of leaf senescence inFagus sylvatica L. by low levels of tropospheric ozone demonstrated by leaf colour,chlorophyll fluorescence and chloroplast ultrastructure

From April 1988 to October 1991 3-year-old seed propagated beech (Fagus sylvatica L.) trees were exposed in open-top chambers to four different levels of air pollution: (1) charcoal filtered air, (2) ambient air, (3) ambient air plus 30 nl 1^-1 ozone during the summer, and (4) ambient air plus 30 nl 1^-1 ozone during the summer and 20 nl 1^-1 SO₂ and NO₂ during the winter. Leaf colour was studied in the autumns of 1989 and 1991 and a close relationship between ozone dose and premature senescence was found. A correlation also exists between the colour groups and chlorophyll fluorescence (F_v/F_m). Ozone fumigation increases the size and speeds up the development of the plastoglobules. This is described using an index based on the volume of plastoglobules as a percentage of chloroplast volume. The index was significantly higher for ozone fumigated plants than for control plants during August to November 1989. According to all three methods it is concluded that low levels of ozone accelerate leaf senescence processes inF. sylvatica. There are indications that leaves of the first and the second flush react differently to the ozone treatment. Irrespective of the ozone treatment a special cell wall structure, probably a local suberization, is confined to the subsidiary cells in leaves of the first flush.     

Sensitivity of Metasequoia glyptostroboides to ozone stress

2-year-old seedlings of Metasequoia glyptostroboides were grown in open top chambers and exposed to four ozone concentrations [O₃] (charcoal-filtered air, CF; 50, 100, and 200 mm³ m⁻³) for 25 d. Measurements of growth, leaf chlorophyll (Chl) content, and gas exchange parameters were made before and/or after O₃ exposure. Leaf length, crown width, Chl a/b, net photosynthetic rate, stomatal conductance, and transpiration rate were significantly reduced at 100 and 200 mm³(O₃) m⁻³. A remarkable decrease in stomatal conductance also occurred at 50 mm³(O₃) m⁻³.     

The influence of ambient and elevated ozone concentrations on photosynthesis in Populus nigra

Light-saturated net leaf photosynthesis (A_sat), CO₂ response curves (A/C_i), current photochemical capacity (F_v/F_m) and pigment contents were measured in leaves of Populus nigra (Clone T107) which had been exposed to ozone stress in open-top chambers for the entire growth period. Surprisingly, not only elevated (ao⁺, i.e. ambient air + 50 mm³ m^?3 ozone) but also ambient (aa) ozone concentrations led to a reduction in A_sat, in comparison with leaves exposed to air containing almost no ozone (cf^?, i.e. charcoal filtered ambient air). The very small change in leaf conductance (g₁) indicated that the decrease in A_sat was not due to stomatal limitation. This finding was supported by the fact that, a decrease in carboxylation efficiency (CE) correlated with a loss in A_sat. In comparison to cf^? leaves, aa leaves showed no change in current photochemical capacity (F_v/F_m) throughout the whole experiment. However, a marked decline in F_v/F_m in ao⁺ leaves was observed at a time when A_sat and CE were already decreased by about 45% and 60% respectively. As the chlorophyll b content of leaves is known to correlate with the amount of LHC and PSII centres, it was used to normalize fluorescence parameters in relation to PSII centres present. The normalized values for F_m and F₀ increased with the dosage of ozone in ao⁺ leaves but not in aa leaves, indicating a change of the pigment content of PSII in the former, but not in the latter. These data led to the conclusion that ozone interacts primarily with components of the Calvin cycle, which results in a decrease in A_sat, with subsequent feedback on the current photochemical capacity of PSII centres.     

Leaf anatomy and photosynthetic acclimation in <Emphasis Type="Italic">Valeriana jatamansi</Emphasis> L. grown under high and low irradiance

Relationship of leaf anatomy with photosynthetic acclimation of Valeriana jatamansi was studied under full irradiance [FI, 1 600 mol(PPFD) m^–2 s^–1] and net-shade [NS, 650 mol(PPFD) m^–2 s^–1]. FI plants had thicker leaves with higher respiration rate (R _D), nitrogen content per unit leaf area, chlorophyll a/b ratio, high leaf mass per leaf area unit (LMA), and surface area of mesophyll cell (S _mes) and chloroplasts (S _c) facing intercellular space than NS plants. The difference between leaf thickness of FI and NS leaves was about 28 % but difference in photon-saturated rate of photosynthesis per unit leaf area (P _Nmax) was 50 %. This indicates that P _Nmax can increase to a larger extent than the leaf thickness with increasing irradiance in V. jatamansi. Anatomical studies showed that the mesophyll cells of FI plants had no open spaces along the mesophyll cell walls (higher S _c), but in NS plants wide open spaces along the mesophyll cell wall (lower S _c) were found. Positive correlation between S _c and P _Nmax explained the higher P _Nmax in FI plants. Increase in mesophyll thickness increased the availability of space along the mesophyll cell wall for chloroplasts (increased S _c) and hence P _Nmax was higher in FI plants. Thus this Himalayan species can acclimate to full sunlight by altering leaf anatomy and therefore may be cultivated in open fields.     

CO2 transfer conductance, leaf structure and carbon isotope composition of Polygonum cuspidatum leaves from low and high altitudes

Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO₂ transfer conductance inside the leaf (g_i) of the highland leaves (0·75 μmol m^?2 s^?1 Pa^?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ¹³C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO₂ partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ¹³C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO₂ partial pressures, would be mainly attributed to their lower g_i.     

Physiological Changes in Phaseolus vulgaris in Response to Long-term Ozone Exposure

The effects of ozone on Phaseolus vulgaris cv. Lit were investigatedusing open-top chambers (OTCs) ventilated with charcoal andPurafil filtered air (CF treatments), ambient air (NF treatments)and ambient air to which low, medium or high concentrationsof ozone were added (NFL, NFM and NFH). Ozone additions of 8,16 and 23 nl l^–1 were made during phase 1 of the experiment(0–44 d after emergence, DAE), and additions of 15, 30and 47 nl l^–1 were made during phase 2 (45–99 DAE).Ozone was added to the chambers between 1100 and 1800 h GMT,for 3 or 4 consecutive days each week. The seasonal 7-h meanozone concentrations were 8, 21, 27, 33 and 38 nl l^–1in the CF, NF, NFL, NFM and NFH treatments, respectively. No visible symptoms of ozone injury or significant physiologicalchanges were detected in P. vulgaris during phase I of the experiment.In phase 2, the photosynthesis (Pn) and stomatal conductance(gs) of NFH-plants were inhibited by 73% and 86%, respectively,during ozone exposure, and recovered to pre-exposure valueson the following day. These observations were made prior tothe appearance, 60 DAE, of bronze lesions on the leaves of NFH-plants.The photosynthetic capacity and gs of NFH-leaves decreased asthe severity of ozone injury increased. Rates of weight lossfrom excised leaves also increased with increasing ozone injury.Microscopic investigations of the bronzed regions revealed extensivecellular breakdown, including tonoplast and chloroplast enveloperupture, and the aggregation of the cytoplasmic contents towardsone end of the cell. Severely damaged leaves abscised from the plants, resultingin premature canopy senescence in the NFM and NFH treatments.This, coupled with the lower photosynthetic capacity of existingleaves led to 25 % lower yield in the NFH than the NF treatment(P < 0.05). Phaseolus vulgaris, green bean, ozone, symptom development, photosynthesis, cell ultrastructure     

Methyl salicylate fumigation increases monoterpene emission rates

We aimed to assess the potential effects of fumigation by methyl salicylate (MeSA) on plant monoterpene production and emissions. We evaluated monoterpene production and emissions both by chromatographic and proton transfer reaction mass spectrometry at the whole plant-and leaf-scales, in MeSa-fumigated (ca. 60 mm³ m⁻³ in air) and control (without MeSa fumigation) holm oak (Quercus ilex L.) plants exposed to temperatures ranging from 25 to 50 °C. The MeSa-fumigated plants showed ca. 3–4-fold greater leaf monoterpene concentrations and emission rates than the control plants between the temperatures of 25 to 45 °C.     

Responses of Leaf Processes in a Sensitive Birch (Betula pendulaRoth) Clone to Ozone Combined with Drought

Saplings of an ozone sensitive clone of birch (Betula pendulaRoth,KL-5-M) were well-watered or exposed to mild drought-stresscombined with ambient or elevated (1.5xthe ambient) ozone for11 weeks in open-field conditions in central Finland. Stomatalresponse, visible injury, chlorophyll and nutrient content,and changes in cellular anatomy and plant growth were studied.Drought stress alone, in ambient ozone, reduced stomatal densityand stomatal conductance. Drought stress and ozone effects wereadditive, reducing total leaf number, foliage area and starchformation in mesophyll cells. Drought stress and ozone effectswere additive, increasing the N concentration in the leaves,the thickness of the upper epidermal cell wall, the number ofpectinaceous projections of mesophyll cell walls, and the vacuolartannin-like depositions and phenolic droplets, regarded as signsof activated stress defence mechanisms. The increase in specificfoliage mass, cytoplasmic lipids (younger leaves), and a condensedappearance of the upper epidermal mucilaginous layer were causedby both drought and ozone, but were not additive. The resultsshow that combined drought stress contributed to birch responsesto 1.5xcurrent ambient ozone concentrations, corresponding tocritical-level ozone exposure. The only beneficial effect ofdrought stress was the slight reduction of visible leaf symptomsinduced by ozone in autumnal leaves.Copyright 1998 Annals ofBotany Company Birch,Betula pendula, sensitive clone, ozone, drought, microscopy.     

Effects of 2-month ozone exposure in spinach leaves on photosynthesis,antioxidant systems and lipid peroxidation

The photosynthesis response, antioxidant systems and lipid peroxidation were studied in leaves from spinach plants (Spinacia oleracea L.) in response to ozone fumigation, ambient air and charcoal filtered air treatments. The photosynthetic activity was tested through gas exchange and chlorophyll a fluorescence measurements. Ambient air and ozone fumigation caused a decrease in the photosynthetic rate (25% and 63%, respectively) mainly due to a reduced mesophyll activity, as evidenced by the increased intercellular CO₂ concentration. These data agree with a large reduction in the non-cyclic electron flow (7% and 16%), a lower capacity to reduce the quinone pool and a higher development of non-photochemical quenching upon high O₃ concentration. The results suggest that the oxidative stress produced, together with the stimulation of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (APX, EC 1.11.1.11) activities and the increase in lipid peroxidation (20% and 36%, respectively), generated an alteration of the membrane properties.     

Effects of Ambient and Acute Partial Pressures of Ozone on Leaf Net CO(2) Assimilation of Field-Grown Vitis vinifera L

Mature, field-grown Vitis vinifera L. grapevines grown in open-top chambers were exposed to either charcoal-filtered air or ambient ozone partial pressures throughout the growing season. Individual leaves also were exposed to ozone partial pressures of 0.2, 0.4, or 0.6 micropascals per pascal for 5 hours. No visual ozone damage was found on leaves exposed to any of the treatments. Chronic exposure to ambient O₃ partial pressures reduced net CO₂ assimilation rate (A) between 5 and 13% at various times throughout the season when compared to the filtered treatment. Exposure of leaves to 0.2 micropascals per pascal O₃ for 5 hours had no significant effect on A; however, A was reduced 84% for leaves exposed to 0.6 micropascals per pascal O₃ when compared to the controls after 5 hours. Intercellular CO₂ partial pressure (c_i) was lower for leaves exposed to 0.2 micropascals per pascal O₃ when compared to the controls, while c_i of the leaves treated with 0.6 micropascals per pascal of 0₃ increased during the fumigation. The long-term effects of ambient O₃ and short-term exposure to acute levels of O₃ reduced grape leaf photosynthesis due to a reduction in both stomatal and mesophyll conductances.     

Ultrastructural Responses of Leaf Mesophyll and Trap Wall Cells of Utricularia Vulgaris to Cadmium

Ultrastructure of leaf mesophyll and two-layer trap wall of Utricularia vulgaris L. was studied after 24-h and 48-h treatment with different concentrations of cadmium. The treatment of plants with 0.25 g(Cd²⁺) m⁻³ caused different changes in the chloroplasts of the two organs. The thylakoids swelled in chloroplasts of leaf and the starch content increased in chloroplasts of traps. Higher concentrations of Cd (0.5 and 1.5 g m⁻³) caused osmiophilisation of thylakoids in the chloroplasts of both studied organs. The secretory cells in the trap wall was resistant to Cd. This revised version was published online in June 2006 with corrections to the Cover Date.     

Gas-exchange analysis of chloroplastic fructose-1,6-bisphosphatase antisense potatoes at different air humidities and at elevated CO2

Gas-exchange measurements were performed to analyze the leaf conductances and assimilation rates of potato (Solanum tuberosum L. cv. Desireé) plants expressing an antisense construct against chloroplastic fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) in response to increasing photon flux densities, different relative air humidities and elevated CO₂ concentrations. Assimilation rates (A) and transpiration rates (E) were observed during a stepwise increase of photon flux density. These experiments were carried out under atmospheric conditions and in air containing 500 μmol mol⁻¹ CO₂. In both gas atmospheres, two levels of relative air humidity (60–70% and 70–80%) were applied in different sets of measurements. Intercellular CO₂ concentration, leaf conductance, air-to-leaf vapour pressure deficit, and instantaneous water-use efficiency (A/E) were determined. As expected, assimilation rates of the FBPase antisense plants were significantly reduced as compared to the wild type. Saturation of assimilation rates in transgenic plants occurred at a photon flux density of 200 μmol m⁻² s⁻¹, whereas saturation in wild type plants was observed at 600 μmol m⁻² s⁻¹. Elevated ambient CO₂ levels did not effect assimilation rates of transgenic plants. At 70–80% relative humidity and atmospheric CO₂ concentration the FBPase antisense plants had significantly higher leaf conductances than wild-type plants while no difference emerged at 60–70%. These differences in leaf conductance vanished at elevated levels of ambient CO₂. Stomatal response to different relative air humidities was not affected by mesophyll photosynthetic activity. It is suggested that the regulation of stomatal opening upon changes in photon flux density is merely mediated by a signal transmitted from mesophyll cells, whereas the intercellular CO₂ concentration plays a minor role in this kind of stomatal response. The results are discussed with respect to stomatal control by environmental parameters and mesophyll photosynthesis. Received: 24 September 1998 / Accepted: 9 February 1999     

Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Fagus sylvatica)

As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV-B radiation (280–320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5-year-old European beech trees were exposed from 1 July to October 1993 to two levels of UV-B radiation and two levels of ozone, alone and in combination, in open-top chambers equipped with lamps. The simulated UV-B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastern Denmark, resulting in a 23% difference in biologically effective UV-B (UV-B_BE) irradiance. The maximum UV-Bbe given was 8.61 kJ m⁻² day⁻¹. The ozone levels were either the ambient (average 32 nl l⁻¹) or ambient with ozone addition (average resulting concentration 71 nl l⁻¹). Compared to the control treatment (ambient UV-B, ambient O₃) the elevated levels of UV-B and O₃ affected the trees negatively, expressed as declines in net photosynthesis (P_n), stomatal conductance (g_s), chlorophyll fluorescence (F_v/F_m) and acceleration of senescence, measured as yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in P_n and F_v/F_m occurred in the order: control 3 3. Compared to the control, the combination treatment with high levels accelerated the visual senescence processes by ca 27 days, while for high UV-B and O₃ alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O₃ in combination enhanced the negative effects compared with UV-B and O₃ alone. The P_n and F_v/F_m results could be related to this acceleration process. The chamber effect was investigated by comparing the control plots with a plot without open-top chamber. The trees in the chambers showed a higher P_n and F_v/F_m and a 14-day delayed senescence compared to the trees outside, probably caused by higher temperatures, a more protected environment and altered conditions inside the chambers.     

Ozone‐induced reductions in below‐ground biomass: an anatomical approach in potato

Potato plants were grown in open‐top chambers under three ozone concentrations during two complete cropping seasons (93 and 77 d in 2004 and 2005, respectively). The effects of chronic exposure to ozone on leaf anatomy, cell ultrastructure and crop yield were studied. Severe cell damage was found, even at ambient ozone levels, mainly affecting the spongy parenchyma and areas near the stomata. Damage to the cell wall caused loss of cell contact, and loss of turgor pressure due to tonoplast disintegration, contributed to cell collapse. Phloem sieve plates were obstructed by callose accumulation, and damaged mesophyll cells increased their starch stores. Tuber yield fell sharply (24–44%), due to the biggest tubers becoming smaller, which affected commercial yield. These anatomical findings show the mechanisms of ozone effect on assimilate partitioning, and thus crop yield decrease, in potato. Further implications of ozone causing reductions in below‐ground biomass are also discussed.     

Stable free radicals in ozone-damaged wheat leaves

Chlorophyll fluorescence measurements were performed on attached leaves of wheat plants (Triticum aestivum L. cv. Nandu) that were exposed to ambient air and to air supplemented with 80 and 120nmol mol^-1 ozone. Decreases in the “current photochemical capacity” were observed that were dependent on both the ozone concentration and duration of exposure. Electron paramagnetic resonance (EPR) spectra on freeze-dried samples from the same batches of plants showed the presence of an unidentified stable free radical, whose spectra had similarities to that of the ubisemiquinone radical. The intensity of this radical signal increased with the duration of ozone exposure in leaves that received an additional 120nmol mol^-1 ozone. In contrast, with exposure to air with 80nmol mol^-1 added ozone, there was little if any change in free radical signal intensity over the 4 week period of the experiment. The increase in intensity of the EPR signal occurred later than the chlorophyll fluorescence changes, which suggests that it is associated with permanent leaf damage.     

Day/night variations in turgor pressure in individual cells of Mesembryanthemum crystallinum L.

Summary Using a pressure probe, turgor pressure was directly determined in leaf-mesophyll cells and the giant epidermal bladder cells of stems, petioles and leaves of the halophilic plant Mesembryanthemum crystallinum. Experimental plants were grown under non-saline conditions. They displayed the photosynthetic characteristics typical of C₃-plants when 10 weeks old and performed weak CAM when 16 weeks old. In 10 week old plants, the turgor pressure (P) of bladder cells of stems was 0.30 MPa; of bladder cells of petioles 0.19 MPa, and of bladder cells of leaves 0.04 MPa. In bladder cells from leaves of 16 week old plants, marked changes in turgor pressure were observed during day/night cycles. Maximum turgor occurred at noon and was paralleled by a decrease in the osmotic pressure of the bladder cell sap. Similar changes in the cell water relations were observed in plants in which traspirational water loss was prevented by high ambient relative humidity. Turgor pressure of mesophyll cells also increased during day-time showing macimum values in the early morning. No such changes in turgor pressure and osmotic pressure were observed in bladder and mesophyll cells of the 10 week old plants not showing the diurnal acid fluctuation typical of CAMAbbreviations CAM crassulacean acid metabolism - V volume of the cells (mm³) - P turgor pressure (MPa) - volumetric elastic modulus (MPa) - ⁱ osmotic pressure of the cell sap (MPa) - T _1/2 half-time of water exchange (s) - Lp hydraulic conductivity of the cell membrane (m·s^-1·MPa^-1) - A surface area of cells (mm²) - P pressure changes (MPa) - V volume changes (mm³) - nocturanal nighttime - diurnal daytime     

Effects of Ozone and Sulfur Dioxide on Phyllosphere Fungi from Three Tree Species

Short-term effects of ozone (O₃) on phyllosphere fungi were studied by examining fungal populations from leaves of giant sequoia (Sequoiadendron giganteum (Lindl.) Buchholz) and California black oak (Quercus kelloggii Newb.). Chronic effects of both O₃ and sulfur dioxide (SO₂) were studied by isolating fungi from leaves of mature Valencia orange (Citrus sinensis L.) trees. In this chronic-exposure experiment, mature orange trees were fumigated in open-top chambers at the University of California, Riverside, for 4 years with filtered air, ambient air plus filtered air (1:1), ambient air, or filtered air plus SO₂ at 9.3 parts per hundred million. Populations of Alternaria alternata (Fr.) Keissler and Cladosporium cladosporioides (Fres.) de Vries, two of the four most common fungi isolated from orange leaves, were significantly reduced by chronic exposure to ambient air. In the short-term experiments, seedlings of giant sequoia or California black oak were fumigated in open-top chambers in Sequoia National Park for 9 to 11 weeks with filtered air, ambient air, or ambient air plus O₃. These short-term fumigations did not significantly affect the numbers of phyllosphere fungi. Exposure of Valencia orange trees to SO₂ at 9.3 parts per hundred million for 4 years reduced the number of phyllosphere fungi isolated by 75% compared with the number from the filtered-air treatment and reduced the Simpson diversity index value from 3.3 to 2.5. A significant chamber effect was evident since leaves of giant sequoia and California black oak located outside of chambers had more phyllosphere fungi than did seedlings within chambers. Results suggest that chronic exposure to ambient ozone or SO₂ in polluted areas can affect phyllosphere fungal communities, while short-term exposures may not significantly disturb phyllosphere fungi.     

Leaf anatomy of highbush blueberry grownin vitro and during acclimatization toex vitro conditions

Leaves of micropropagated highbush blueberry (Vaccinium corymbosum) cv. ‘Bluetta’ have been observed during the acclimatization phase. In vitro-developed leaf cells were circular and small, the spongy parenchyma was discontinuous and disorganized and formed by 1–2 layers of cells with large intercellular spaces and the palisade to spongy mesophyll thickness ratio was 1:1.5. After rooting ex vitro, the first leaves formed under natural conditions showed substantial changes in the anatomical characteristics. After 6 months, the plants produced leaves similar to those in field-grown plants. The palisade cells were rectangular, the spongy parenchyma was formed by 3–4 layers of cells and the intercellulars were around the stomata. Leaves from field-grown plants lost 24 % of water during 150 min after excision while leaves from in vitro shoots lost about 50 % of water in the same time. Leaves from in vitro shoots showed a higher number of smaller stomata (361 per mm²), with the guard cells forming a circular ring; the stomata frequency in field-grown leaves was 241 per mm² and the guard-cells were elliptical.     

Effects of ozone exposure on growth and photosynthesis of the seedlings of <Emphasis Type="Italic">Liriodendron chinense</Emphasis> (Hemsl.) Sarg,a native tree species of subtropical China

Little is known about the response of trees to elevated ozone (O₃) in the subtropical region of China, where ambient O₃ concentrations are high enough to damage plants. In this study, pigment content, gas exchange and chlorophyll (Chl) a fluorescence in leaves of Liriodendron chinense (Hemsl.) Sarg seedlings, a deciduous broadleaf tree species native in subtropical regions, were investigated at 15, 40, and 58 days after O₃ fumigation (DAF) at a concentration of 150 mm³ m⁻³ (E-O₃). At the end of experiment, seedlings were harvested for biomass measurement. E-O₃ caused visible injuries on the mature leaves e.g. necrotic patches and accelerated early defoliation. Relative to the charcoal-filtered air (CF) treatment, E-O₃ significantly decreased shoot and root biomass, pigment content, light-saturated net photosynthesis (P _Nsat), stomatal conductance (g _s), maximum rate of carboxylation (V_cmax), photochemical quenching coefficient (q_p) and effective quantum yield of PSII photochemistry (Φ_PSII), and also caused a slight reduction in relative increase of basal diameter. Therefore, L. chinense can be assumed to be an O₃-sensitive tree species, which will be threatened by increasing ambient O₃ concentrations in China.     

Assessment of Atmospheric ozone levels in Israel through foliar injury to Bel-W3 tobacco plants

Summary Nicotiana tabacum L. Bel-W3, which is highly sensitive to ozone, was grown in two glass chambers and exposed to the ambient air at the periphery of Tel-Aviv, during winter, spring, summer and autumn 1978. During the exposure time, atmospheric ozone was continuously measured by a chemiluminescent monitor. Throughout the experiments, plants' height was measured and the number of leaves was determined three times weekly. The extent of injury to the tobacco plants was measured by the percentage of injured plants, the percentage of injured leaves and the percentage of leaves' area injured. Necrotic lesions, typical for ozone injury, appeared on the mature leaves of the exposed tobacco plants in three out of four exposures. Appearance of incipient injury and the extent of injury differed among the experiments and depended not only on exposure duration and on ozone concentrations, but also on the exposure conditions. The percentage of injured leaves and the percentage of leaves' area injured, increased with the duration of exposure and with rising cumulative ozone concentrations.This work was partially supported by the Chief Scientist's Office—Israeli Ministry of Health—Contract No. 550