Irrigating Onions and Potatoes with Chromium and Nickel: Its Effects on Catalase and Peroxidase Activities and the Cross-Contamination of Plants

Tropospheric ozone (O₃) has long been documented to cause an injury to plants, but a plants’ protectant, widely applicable in agronomical practice, does not exist. We evaluated the potential antiozonate efficacy of the antitranspirant di-1-p-menthene (Vapor Gard) compared with ethylenediurea (EDU) on Bel-W3 tobacco plants. Plants were treated either with water, or by EDU (10, 100, and 500 mg dm^?3), or by vapor (1, 5, 10, and 50 ml dm^?3) and were exposed either to O₃-enriched (90 ppb) or O₃-free air, for 12 days and 8 h day^?1. EDU when applied at 10 mg dm^?3 did not protect the plants against O₃, but when applied at 100 and 500 mg dm^?3 offered a significant protection to the plants. Vapor, when applied at 1 ml dm^?3 did not protect the plants against O₃, neither by terms of foliar visible injury nor by terms of aboveground biomass. In addition, when applied at 10 and 50 ml dm^?3 caused phytotoxicity to all the plants, which it was expressed as necrotic spots on the leaves’ surface, misshaping of the leaves, or short plants' height_. It is obvious that vapor does not protect Bel-W3 tobacco plants against O₃. The antiozonate role of di-1-p-menthene is species-specific and probably occurs only under short-term exposures.     

Effects of Antitranspirant Spray and Potassium: Calcium: Magnesium Ratio on Photosynthesis,Nutrient and Water Uptake,Growth, and Yield of Sweet Pepper

Sweet pepper plants (Capsicum annuum L.) were cultivated hydroponically under different nutrient cation ratios at both high potassium (K)/calcium (Ca) (12:2) or magnesium (Mg)/Ca (4:2) ratios, compared with half strength Hoagland's solution (K:Ca:Mg; 3.5:2:0.5). Additionally, antitranspirant (Pinolene) was sprayed every fortnight to the aerial part of the plant at 1% (v/v). The antitranspirant (AT) did not affect dry weight accumulation in the leaves, stems, roots, total plant leaf area, or leaf dry weight percentage. Net carbon dioxide (CO₂) assimilation was not impaired by the AT but the water uptake was reduced significantly independent of the nutrient solution used. The AT did not affect the cation uptake but high Mg significantly reduced Ca concentration in leaves, stems, and fruits, whilst high K had an effect only in old leaves and fruits. The AT reduced fructose and glucose concentration in the leaves but no effect was found in the fruits. Fruit yield was not affected by AT, but it was increased when plants were grown with high Mg/Ca. The percentage of blossom-end rot was reduced with the AT, whilst it was increased with the solutions having high K/Ca or high Mg/Ca. The AT significantly reduced fruit firmness in high Mg/Ca and control solution but no effect was found for fruit color, shape index, total soluble solids, or pericarp thickness.     

Equilibrium, Kinetics, and Thermodynamics of Methylene Blue Adsorption by Pine Tree Leaves

The adsorption capacity of pine tree leaves for removal of methylene blue (MB) from aqueous solution was investigated in a batch system. The effects of the process variables, such as solution pH, contact time, initial dye concentration, amount of adsorbent, agitation speed, salt concentration, and system temperature on the adsorption process were studied. The extent of methylene blue dye adsorption increased with increase in initial dye concentration, contact time, agitation speed, temperature, and solution pH but decreased with increased in amount of adsorbent and salt concentration. Equilibrium data were best described by both Langmuir isotherm and Freundlich adsorption isotherm. The maximum monolayer adsorption capacity of pine tree leaves biomass was 126.58?mg/g at 30?°C. The value of separation factor, R _L , from Langmuir equation and Freundlich constant, n, both give an indication of favorable adsorption. The intrapartical diffusion model, liquid film diffusion model, double exponential model, pseudo-first and second order model were used to describe the kinetic and mechanism of adsorption process. A single stage bath adsorber design for the MB adsorption onto pine tree leaves has been presented based on the Langmuir isotherm model equation. Thermodynamic parameters such as standard Gibbs free energy (??G ⁰), standard enthalpy (??H ⁰), and standard entropy (??S ⁰) were calculated.     

Effects of Tropospheric Ozone on Potato Plants Protected by the Antioxidant Diphenylamine (DPA)

To assess the protective effects of diphenylamine (DPA) on yield, DPA- and non-DPA-treated plants of potatoes (Solanum tuberosum cv. Agria) were exposed to ambient air at a rural site in Spain. Concentrations of ambient ozone were high at the site. Untreated plants served as controls. The DPA treatments did not have a significant effect on Chl content. The treatment with DPA (0.2 M) caused a significant increase of fresh weight production and tuber number of the potato plants in both years of study (32 and 47% in 2002 and 9 and 27% in 2003). Changes in the activity of antioxidant enzymes guaiacol peroxidase (GPX) and glutathione reductase (GR) in the leaves were determined in control and DPA-treated plants. The GPX activity in the leaves was significantly lower in the treated plants. GR was also lower, but not significantly, in the treated plants. The highest correlation between enzyme activity and yield was found in the case of GPX. The protective effect of DPA was related inversely to peroxidase activity. This is the first report demonstrating markedly enhanced yield and reduction of peroxides of potentially harmful O₃ by DPA as a mechanism for protecting plants against noxious oxidative stress from the environment.     

Physiological responses of Quercus ilex Leaves to Water Stress and Acute Ozone Exposure Under Controlled Conditions

The combined effect of water stress and ozone (O₃) on stomatal O₃ flux, damage to photosynthesis, and detoxification by biogenic volatile organic compounds (BVOC) in Quercus ilex leaves was studied. A 4-weeks O₃ exposure (250 ppb, 4 h per day) caused a reduction of photosynthesis and stomatal conductance, which was fully recovered 1 week after the end of the treatment, in well-watered and water-stressed plants. Measurements of stomatal O₃ flux revealed a low stomatal flux of the pollutant, which became minimal after stomatal closure caused by water stress. An induction of volatile monoterpenes, important compounds in the O₃ scavenging system in Q. ilex, and a burst of lipoxygenase compounds (LOX), which are released as gaseous by-products of membrane peroxidation, was observed after 2–3 weeks of O₃ fumigation. However, these compounds were also released in control leaves that were exposed to ozone only briefly, to determine stomatal O₃ flux. The low stomatal flux that occurred in water stress conditions helped avoiding permanent damage to Q. ilex leaves, although during the O₃ treatment photosynthesis was severely limited by stomatal closure. In well-watered plants, O₃ fumigation caused a noticeable increase of nocturnal stomatal conductance. If confirmed on adult plants under field conditions, this effect can imply larger flux of O₃ at night and possible detrimental effects of O₃ on leaf functions in plants exposed to high nocturnal O₃ levels.     

Modification of ozone effects on photosynthetic capacity of winter wheat (Triticum aestivum L. CV. perlo) at different levels of water availability

Attached leaves were used for the determination of the photochemical capacity by means of a portable fluorimeter. Repeated fluorescence measurements showed the negative effects of ozone on photochemical capacity and these negative effects increased with increasing ozone doses. But impairments of photochemical capacity were smallest if severe water stress co-occurred with ozone exposures. The upper leaf sides experienced more reduction of photochemical capacity in well-watered plants than the lower leaf sides, possibly by the additional effect of light stress on the upper leaf sides. In diurnal studies, a decline of F _v/F _m was observed at noontime and a recovery at evening in both control and ozone-fumigated leaves at two extreme water capacities (w.c.) (75% and 35% of w.c.). The extent of depression and recovery of F _v/F _m was not significantly varying. The oscillations of F _v/F _m could be due to short-term disturbances in the photosynthetic capacity, due to oxidative stress.     

Alleviation of Alkalinity-Induced Fe Deficiency in Eggplant (Solanum melongena L.) by Foliar Application of Different Fe Sources in Recirculating System

This study was carried out to investigate the effects of foliar sprays of different iron (Fe) sources on eggplant grown in alkaline aquaponic solutions. Four treatments were used, untreated control, foliar application of iron sulfate (FeSO₄), ferric ethylenediaminetetraacetic acid (Fe-EDTA) and ferric ethylenediamine bis(2-hydroxyphenyl)acetic acid (Fe-EDDHA). The results showed that overall growth was significantly increased by foliar Fe application, and the highest values of vegetative growth parameters were recorded in plants treated with FeSO₄. The Fe treatment led to a significant increase of shoot Fe concentration, and the highest Fe was observed in plants sprayed with FeSO₄, compared to Fe-EDTA and Fe-EDDHA. The lowest chlorophyll content was observed in untreated plants. The highest SPAD index, maximal quantum yield of photosystem (PS II) photochemistry (F_v/F_m) and performance index (PI) values of young and old leaves were found with FeSO₄ treatment. It is concluded that application of foliar Fe must be performed in the aquaponic system, to overcome Fe deficiencies in alkaline conditions.     

Carbon dioxide assimilation in urea‐treated wheat leaves

Application of 10 mM urea to the flag leaf of wheat plants enhanced in vivo urease activity several fold. Photosynthetic rate was also increased considerably. There were significant differences in the leaf internal carbon dioxide (CO₂) concentrations between the urea‐treated and untreated leaves. The finding that carbon (¹⁴C) was detected in the ethanol extract of the leaves fed with ¹⁴C‐urea suggests that CO₂ released from urea is re‐fixed by the leaves.     

The Determination of Ozone in Ambient Air with Free Hanging Filters as Passive Samplers

This paper describes the use of dry free hanging filters, as passive samplers to determine ozone in the ambient air. The filters, with a diameter of 25?mm, were impregnated with 5,5??-disodium indigo disulphonate (IDS), a reagent for ozone. From the amount of reacted indigo compound, found on the filter, and the ozone concentration in the ambient air, a pseudo rate constant k ₁, of the reaction between ozone (O₃) and IDS on the filter, is calculated. The range of measurement is between 9 and 205???g/m³ ambient ozone. The dry filter method is specific for ozone, while the Dutch standard method NEN2789, based on an aqueous solution of IDS, has to be corrected for the presence of NO _x . From wind tunnel and field experiments, k ₁ proved to vary between 0.7 and 1.5?×?10^?6?m³?s^?1 (??g O₃)^?1 at wind velocities between 1 and 3?m/s and at an exposure time of 60?min. Within these conditions, ozone concentrations have been determined with free hanging filters in four busy streets in Yogyakarta, Indonesia and at two background sites using an average value of k ₁ of 1.2?×?10^?6. Subsequently, the traffic NO emission was estimated from the difference of the O₃ concentrations at both sides of a road. For an arbitrary situation, an NO emission of 255???g/s per metre road length was calculated. The filter method is inexpensive and practical, needs no electricity, is easily assembled and can be used to perform measurements in remote areas. It is shown here that this simple measurement technique may support air quality studies, e.g., in developing countries.     

Effects of foliar antitranspirant or calcium nitrate applications on yield and blossom‐end rot occurrence in greenhouse‐grown peppers

In order to reduce blossom‐end rot occurrence in greenhouse‐grown peppers, experiments were conducted to determine whether foliar applications of antitranspirants would decrease transpiration of pepper (Capsicum annuum L. cv. Hungarian Wax or Midal) leaves and subsequently redirect the flow of calcium (Ca) from plant foliage to developing fruit. Foliar calcium nitrate [Ca(NO₃)₂] applications were also carried out as a possible means to increase fruit Ca concentration and reduce blossom‐end rot. Initial studies indicated that two antitranspirant applications of 2.5% ‘Vapor Gard’ (di‐1‐p‐menthene) early in the fruiting phase of ‘Hungarian Wax’ peppers significantly increased early yield and tended to increase total Ca in the fruit. In subsequent experiments, weekly applications of either antitranspirant ‐ 1% ‘Vapor Gard’ or 1% ‘Folicote’ (a wax emulsion concentrate) ‐ throughout the fruiting phase significantly increased fruit Ca of ‘Midal’ peppers during at least part of the reproductive stage, and significantly reduced blossom‐end rot‐affected yield. Weekly Ca(NO₃)₂ applications significantly elevated leaf Ca as well as fruit Ca concentrations, and was the most effective treatment in alleviating blossom‐end rot. However, all weekly Ca(NO₃)₂ and antitranspirant treatments caused significant declines in marketable yield, so that these treatments, although effective in reducing blossom‐end rot, cannot be recommended for commercial use.     

Bush bean (Phaseolus vulgaris L) leaf injury,photosynthesis and stomatal functions under elevated ozone levels

Three bush bean cultivars (Lit, Groffy and Stella) were grown under four levels of ozone exposure (ambient air+50 ppb O₃ ambient air+25 ppb O₃, ambient air and charcoal filtered air) in open- top chambers. Number and leaf injury statistics showed significant reduction in the number of healthy leaves as the level of O₃ increased. The area based leaf injury percentages of the cvs. Lit, Groffy and Stella were 69.8, 57.9 and 71.1% at the highest O₃ level, 24.1, 19.6 and 30.3% at the 2nd highest O₃ level, and 4.5, 0.7 and 5.6% at the ambient air, respectively. The plants grown in the filtered air revealed no injury symptoms. The stomatal conductances were found to decrease gradually in each cultivar as the O₃ level increased. At the highest O₃ level, Chlorophyll fluorescence measurements on the 2nd leaf from the top on 24th day of exposure resulted in significantly the highest Fv/Fm values, the lowest f₀ and the highest F_m values whereas the 4th leaf showed the smallest Fm and correspondingly the smallest F_v/F_m values. This is an indication of photosystem II damage after accumulation of a high ozone dose in the 4th leaf. The photosynthetic rate of the 2nd leaf measured on 30th day of exposure was comparatively higher at the highest exposure but the data taken from the same leaf on 40th day of exposure showed significantly lower photosynthetic rate than the plants in ambient air. Both chlorophyll fluorescence and photosynthetic measurements indicated that ozone stressed leaves experience a stimulation of photosynthesis (possibly due to increased assimilate demand) prior to irreversible damage. Bush bean leaves need to accumulate a critical ozone dose(an AOT40 of presumably > 18 ppm-h) for reduction of the photosynthetic capacitys.     

Determination of photosynthetic parameters Vcmax and Jmax for a C3 plant (spring hulless barley) at two altitudes on the Tibetan Plateau

On the Tibetan Plateau, the unique alpine climate factors of low air pressure, low CO₂ partial pressure and low air temperature have significant but non-explicit influences on the photosynthetic capacity of plants. To evaluate these influences, we measured the net photosynthetic rates for spring hulless barley leaves at two altitudes of 3688 m (the low altitude) and 4333 m (the high altitude), respectively. Two photosynthetic parameters—Vc_max, the maximum rate of Rubisco carboxylase activity, and J_max, the maximum rate of photosynthetic electron transport—were determined. The net photosynthetic rate and the photosynthetic parameters Vc_max and J_max were higher for leaves from plants grown at the high altitude than for those at the low altitude. Vc_max and J_max were approximately 24% and 22% greater, respectively, for leaves from plants grown at the high altitude. The CO₂ and air temperature at the high altitude were lower than those at the low altitude. As a consequence, plants exposed to lower CO₂ partial pressure and lower air temperature have a higher photosynthetic capacity on the Tibetan Plateau. The optimal temperatures for Vc_max and J_max were approximately 6.5% and 3.5% higher, respectively, in leaves from plants grown at the high altitude than those grown at the low altitude, and the ratio of J_max to Vc_max was 12.7% lower at the low altitude. Simulation analyses revealed that the photosynthetic capacities of plants decreased after long-term increases in CO₂ partial pressure and temperature associated with global climate change on the Tibetan Plateau.     

Cultivar-Specific Response of Soybean (Glycine max L.) to Ambient and Elevated Concentrations of Ozone Under Open Top Chambers

Two cultivars of soybean (Pusa 9814 and Pusa 9712) were investigated to evaluate the impact of ambient and elevated concentrations of ozone (O₃) in a suburban site of India with and without application of 400 ppm ethylenediurea (EDU) in open top chambers having filtered air (FCs), non-filtered air (NFCs), and non-filtered plus 20 ppb O₃ (NFCs?+?20 ppb). Significant reductions were observed in various growth parameters, biomass accumulation, and yield attributes of soybean cultivars due to ambient O₃ in NFCs and elevated concentration of O₃ in NFCs?+?20 ppb. Reductions in all parameters were of lower magnitude in plants treated with EDU as compared to non-EDU treated plants. Yield (weight of seeds plant^?1) increased by 29.8% and 33% in Pusa 9712 and by 28.2% and 29.0% in Pusa 9814 due to EDU treatment in plants grown at ambient and elevated levels of O₃, respectively. The results clearly showed that (a) EDU can be effectively used to assess phytotoxicity of O₃ by providing protection against its deleterious effects, (b) EDU can be used for biomonitoring of O₃ in areas experiencing its higher concentrations, and (3) EDU is more effective against higher concentrations of O₃.     

Field evaluation of a test for phosphorus deficiency in pastures based on dry matter responses induced in detached subterranean clover leaves

Abstract

Four field experiments in widely differing locations in New South Wales were used to evaluate a new approach to the assessment of the phosphorus status of subterranean clover (Trifolium subterraneum L.) based pastures. Leaves sampled from plots which had received different phosphate levels were placed in either water or 6 mM NaH₂PO₄.2H₂O for 7–8 hours. The latter were then transferred to water and both groups placed under fluorescent lights for 7 days (19 hours light, 3000 ft.c, 5 hours dark). There were no differences in dry matter increases between treated and untreated leaves from plots not deficient in phosphorus. Untreated leaves from phosphorus deficient plots showed smaller increases in dry matter than phosphate treated leaves, indicating that it may be possible to distinguish in this way between deficient and non‐deficient pasture plots. Differences between treated and untreated leaves increased with the severity of the phosphorus deficiency of the pasture plots from which the leaves were sampled. This was largely due to a decrease in dry matter produced by the untreated leaves, the increase by phosphate treated leaves being relatively independent of their initial phosphorus status. The correlation coefficient (r) over all four experiments between pasture response and induced leaf dry matter differences, the latter measured as “nutrient stress”;, was ‐0.74.     

Ozone pollution modifies the response of sugarbeet to the herbicide phenmedipham

Experiments were conducted in which sugarbeet plants (Beta vulgaris L. cv. Saxon) with 2 to 3 leaves were exposed to a simulated 2 day ozone episode (100 nl l^?1, 7 h d^?1). Three days later, the plants were sprayed with field rate phenmedipham (1.14 kg a.i. ha^?1) and growth analysis conducted 7 days later indicated an antagonistic interaction was occurring. Physiological and biochemical studies were undertaken to determine the nature of this antagonism. Treatment with phenmedipham increased the ratio of transpiration to photosynthetic rates within 2 days of spraying, whilst exposure to ozone had no effect. When the two treatments were combined, water use efficiency was not significantly different from that when phenmedipham was applied alone. In contrast, trends in the membrane permeability after treatment, indicated that the response of plants exposed to ozone followed by treatment with the herbicide, was intermediate between that of the herbicide (high permeability) and ozone (low permeability). Furthermore, when the two treatments were combined the results of antioxidant enzyme assays indicated greater than expected activities of enzymes which are mainly cytosolic, eg. guaiacol peroxidase, as well as a similar increase in the activity of the mainly chloroplastic superoxide dismutase. Treatment with ozone alone and phenmedipham alone only slightly increased superoxide dismutase. Ozone may therefore induce the activities of these protective enzymes. Thus, when another oxidative stress, such as the photosystem II inhibitor phenmedipham, was applied the plants could then respond more quickly and showed less herbicide visible damage.     

Activated Carbon Adsorption of Fuel Oxygenates MTBE and ETBE from Water

In order to assess as to whether treated textile effluent could be safely used to irrigate some winter vegetables, growth room experiments were conducted. Varying levels of treated and untreated textile effluents were applied to germinating seeds of some winter vegetables and their effect was evaluated on germination and early growth stage using seed germination, growth, and biochemical attributes. From the results, it was obvious that textile effluent reduced seed germination and early growth of all vegetables. However, this effect was more pronounced at the highest concentration of textile effluent. Furthermore, treated textile effluent did not show any inhibitory effect on seed germination of all vegetables. Photosynthetic pigments such as chlorophyll a and b, and protein contents were higher in the leaves of all vegetable plants irrigated with treated textile effluent than those of supplied with untreated textile effluents. It has been observed that heavy metals were lower in concentration in treated textile effluent as compared with untreated textile effluent. However, germination and growth responses of all three vegetables were different to treated or untreated textile effluents. Furthermore, the Raphanus sativus ranked as tolerant followed by Brassica campastris and Brassica napus based on germination and growth responses. In conclusion, in view of shortage of water, textile effluent could safely be used for irrigation to vegetables after proper processing.     

Effects of Two Different Ozone Exposure Regimes on Chlorophyll and Sucrose Content of Leaves and Yield Parameters of Sugar Beet (Beta Vulgaris L.) and Rape (Brassica Napus L.)

Sugar beet (Beta vulgaris cv. Loretta) and rape (Brassica napus cv. Licolly) plants were exposed under equaldose conditions in closed fumigation chambers under twodifferent ozone (O₃) pattern: Var₁₃₀ (17:00–09:00 h: 40 μg O₃ m^-3; 09:00–17:00 h: 130μgO₃ m^-3) and Var₂₂₀ (16:00-12:00 h: 40 μgO₃ m^-3; 12:00–16:00 h: 220μg O₃ m^-3).Control plants were exposed under carbon filtered airconditions (ozone free, CF-control). Sugar beet plants wereexposed four weeks each during 6-leaf-stage and lateralgrowth respectively. Rape plants were exposed for 14 daysduring flowering (f) or as young plants (y). In between andafter the end of exposure, plants were kept in carbonfiltered air (CF; < 10 μg O₃ m^-3). With respect to sucrose and chlorophyll content of leaves andseed weight, rape plants were most sensitive duringflowering. Under equal dose conditions, the most pronouncedeffects on chlorophyll and sucrose content of leaves as wellas fresh weight of taproots (CF: 100%; Var₁₃₀: 97%;Var₂₂₀: 83%) and of rape seeds (CF: 100%; y:Var₁₃₀: 122%; Var₂₂₀: 99%; f: Var₁₃₀: 84%;Var₂₂₀: 78%) were detected after exposure under shorttime high ozone peak conditions (Var₂₂₀). Howeverglucose content in taproots (CF: 100%; Var₁₃₀: 43%;Var₂₂₀: 79%) and fatty acid content in rape seeds wasaffected most after exposure to moderate ozone peaks(Var₁₃₀). Var₁₃₀-plants seem to recover better fromozone stress than Var₂₂₀-plants, but glucose content oftaproots and fatty acid content of rape seeds indicate long-lastingeffects especially in Var₁₃₀-plants.Although experiments were conducted in a closed chambersystem it can be concluded, that current ozone concentrationscan induce adverse effects on these crops.     

Effects of ozone on birch (Betula pendula Roth.) clones

Saplings of five birch clones (Betula pendula Roth.) were exposed to ozone doses 1.2–1.7 × higher than the ambient under field conditions during growing seasons 1991–1993. The clones were also fumigated with 40 ppb – 150 ppb ozone in three chamber experiments.The effects of nitrogen supply on ozone responses were studied using field fumigation system and three levels of nitrogen supply (low, medium, high). The ozone-sensitivity was clone-spesific. Even low ozone exposure decreased the biomass production of the most sensitive clone, whereas the most tolerant clone showed unaffected growth rate. In all clones, ozone fumigation accelerated autumn senescence of leaves, increased ultrastructural injuries, especially in chloroplasts, and increased diffusion resistance and stomatal density of leaves. Ozone-induced changes in the tissue and fine structure of leaves were interpreted as enhanced senescence. The plants receiving high nitrogen supply were more resistant to ozone than the plants under lower nitrogen availability. Significant interactions between ozone and nitrogen supply were found in growth and leaf anatomical parameters. The ozone sensitivity and the growth rate of birch clones seem to be related to biochemical and ultrastructural changes of chloroplasts, as reduced growth rate of the most sensitive clone was accompanied by lower contents of Rubisco small subunit and chlorophyll, and lower net photosynthesis. Fast enlarging and young mature leaves without acclimation were more sensitive to ozone fumigation than the leaves emerging under ozone stress.     

Bioindication of ozone in Estonia by using the tobacco variety Bel W3

The presence of tropospheric ozone was monitored by estimating necrotic flecks on the leaves of the ozone-sensitive tobacco variety Bel W3 in Estonia in 1990 and 1991. The indicator and control plants (variety Samsun) were planted in four localities (Käsmu, northern Estonia; Tartu, central Estonia, with two sites and Kooraste, southern Estonia). The percentage of the leaf area covered with necrotic flecks (necrotic index of the leaf) was determined individually for each leaf from mid-June until the end of September. The mean daily increment of the necrotic index for the period between two observations (typically 6–14 days) was calculated for each plant (NII_plant) and for the site (NII_site). The latter parameter indicates the variability of ozone episodes during the vegatation period. Ozone damage to leaves displayed an irregular pattern throughout the vegetation period, the values of NII_site ranging from 0 to 0.3 (Kooraste, 1990) and 0.9 to 2.8 (Käsmu, 1991). The average NII_site for the vegetation period was highest in northern Estonia (0.77 in 1990 and 1.47 in 1991) and lowest in southern Estonia (0.07 in 1990). High ozone damage in Käsmu can be explained by the close location (70–80 km) of the experimental site to major thermal power plants of Estonia and cities (Tallinn and Helsinki). Higher ozone damage in all the observed sites in 1991 as compared to 1990 can be accounted for different conditions for ozone formation.The number of sunny hours was 27% higher in 1991. In July the difference in sunny hours was almost twofold. It is concluded that ozone damage to plants is likely to occur in sunny summers in Estonia.