The influence of ozone exposure dynamics on the growth and yield of kidney bean

A field experiment was conducted in open-top chambers to assess the importance of peak exposure concentration and exposure frequency on the responses of kidney bean plants to O3. There were five treatments in the study: charcoal-filtered air, constant exposure to 0.05 ppm O3 (131 microg m(-3)) daily. fluctuating exposure to 0.08 ppm O3 on three alternate days, cluster exposure to 0.08 ppm O3 on three consecutive days, and peak exposure to 0.12 ppm O3 on two consecutive days. Exposures lasted 4 h and produced an average weekly exposure-period concentration of approximately 0.05 ppm in the O3-addition treatments and 0.025 ppm in the charcoal-filtered treatment. Exposures began on June 23 and terminated on September 8. Plants were harvested weekly and assessed for the number, area, and dry mass of leaves; dry mass of stems; dry mass of roots; the number of pods; and the incidence of foliar O3 injury. Yield was assessed at the end of the study. There were no consistent differences between the plants receiving charcoal-filtered air and those receiving O3 exposure. Significant differences were detected among the treatments for several of the growth variables assessed at the interim harvests, but in the final two harvests these differences had mostly disappeared. There were no significant effects of the O3-addition treatments on yield when compared to the plants receiving charcoal-filtered air. This indicates that there were no cumulative impacts on plants exposed to 0.12 ppm O3 for 4 h on two consecutive days followed by filtered air compared to plants receiving charcoal-filtered air. The seasonal 7-h average concentrations of O3 in the peak and filtered air treatments were approximately 0.040 and 0.025 ppm, respectively.     

Reduction of net photosynthesis in oats after treatment with low concentrations of ozone

Oats (Avena sativa L. cv Titus) were exposed to low concentrations of O3 in an assimilation chamber system. Net photosynthesis (net CO2 uptake), measured before and after O3 fumigation, showed significantly different responses for leaves of different age. The oldest active leaf was the most sensitive to O3. Net photosynthesis was depressed after 2 h with 0.075 ppm (150 microg m(-3)) O3. For leaves exposed to 0.150 ppm (300 microg m(-3)) O3 for 2 h, net photosynthesis was reduced significantly for 4 h, after which recovery occurred, nearly reaching the preexposure level 19 h after the exposure. Dark respiration was initially more than doubled after exposure to 0.130 ppm (260 microg m(-3)) O3. There was no visible injury after any of the experiments. The results indicate that O3 may cause crop losses through effects on photosynthesis even in Scandinavia, where a typical O3 episode lasts 1 to 2 h, and the concentration seldom exceeds 0.150 ppm.     

Indirect effects of air pollutants: Changes in plant/parasite interactions

The impact of air pollutants on plant/parasite-interactions has been investigated. It could be demonstrated that fumigation of Vicia faba L. with 0.15 ppm SO2 (400 microg m(-3)) or 0.2 ppm NO2 (400 microg m(-3)) during 7 days caused changes in plant metabolism which resulted in higher growth rates of the aphid Aphis fabae Scop. feeding on these plants. Fumigation of V. faba with 0.085 ppm O3 during 2 or 3 days, however, caused decreased aphid growth on fumigated plants. That result could be reversed by higher O3 concentrations or through the presence of NOx during O3 fumigation. Ambient air comprising a mixture of pollutant gases had a strong enhancing effect on aphid performance. Thus, the growth of A. fabae on field bean plants was significantly higher in ambient summertime. London air than in charcoal-filtered air. Similarly, the growth of Macrosiphon rosae L. on rose bushes (Rosa sp., cv. Nina Weibull) was improved in ambient summertime Munich air; the increase in growth rate averaged about 20%.     

Ozone alters carbon allocation in loblolly pine: assessment with carbon-11 labeling

Loblolly pine (Pinus taeda L.) seedlings were exposed to 0.120 micromol mol(-1) (ppm) ozone for 7 h per day, 5 days per week for 12 weeks. No visible damage resulted from this regime. A short-lived radioisotope of carbon ((11)C) was used to characterize changes in plant physiology caused by ozone, the first time this technique has been used for ozone exposure studies. In comparison to plants kept in charcoal-filtered air, pines exposed to ozone exhibited reductions in photosynthesis (16%), speed of phloem transport (11%), phloem photosynthate concentration (40%) and total carbon transport toward roots (45%). Photosynthate not transported to the roots appeared to accumulate in the stems. Primary branches of pines exposed to ozone were some 50-60% heavier than those of control pines. Ozone was thus shown to have a significant short-term impact on phloem transport processes that results in a shift in allocation of photosynthate favoring stems.     

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

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

Effects of ozone on yield, growth, and root starch concentrations of two alfalfa (Medicago sativ A L.) cultivars

Ozone (O3) is considered to be a major air pollutant that affects the yield of several sensitive crop species. Its concentration may reach phytotoxic levels several times during the growing season in Eastern Canada. This study was initiated to evaluate the O3 effects on alfalfa, a major crop species. The objective was to compare the yield and growth parameters of the main alfalfa cultivar used in Québec, Apica, to a cultivar more tolerant to O3, Team. Effects on root starch concentrations were also examined as this parameter is an important indicator of alfalfa perennity. The results obtained have shown that the forage yield of Apica was more reduced by O3 during two growing seasons than the yield of Team. For O3 concentrations of 20 to 40 nl liter(-1), yield reductions were 14-26% for Apica and 0-20% for Team. Whereas Apica could be considered more susceptible to O3 than Team, the latter has shown contrasting responses from year to year. This fact suggests that the mechanisms involved in O3-tolerance could be modulated by environmental conditions. At low O3 levels, Apica has shown reduced root growth in terms of dry matter and length. However, contrary to the current hypothesis that O3 would affect more root than shoot growth, we were unable to show a consistent alteration of the biomass allocation between the two. Ozone seems to reduce globally the growth of the whole plants. The greater O3-tolerance of Team could partly be associated to its capacity to maintain more leaves, to delay their senescence, or to keep a larger leaf:stem ratio under increasing levels of O3. At the end of the two growing seasons, the amount of starch reserves stored below ground was shown to be reduced by the current O3 levels. This reduction was mainly associated with a decrease in root biomass under O3 stress. This result support the hypothesis that O3 may accelerate alfalfa decline under field conditions.     

Responses to ozone of insects feeding on a crop and a weed species

The influence of ozone on insect herbivore growth and population development was investigated. Fumigation of both pea (Pisum sativum L.) and dock (Rumex obtusifolius L.) at a range of O(3) concentrations between 21-206 nl litre(-1) produced changes in mean relative growth rates of the aphids Acyrthosiphon pisum Harris and Aphis rumicis L. of between 24 and -6% relative to controls. However, there was no evidence of a dose-related response to O(3) fumigation and no clear differences in aphid response when fumigated with the plant on prefumigated or previously unfumigated plant material. It is suggested that this may, in part, be due to the presence of NO contamination during O(3) fumigation. However, the MRGR of dock aphids was found to be greater on new compared to old leaves as well as the increase on the new growth and decrease on the old growth of fumigated plants relative to unfumigated controls. The size of egg batches of the chrysomelid beetle Gastrophysa viridula Degeer were found to be larger, survival and productivity of larvae was higher, and the food consumption lower on R. obtusifolius fumigated with 70 nl litre(-1) O(3) compared with unfumigated controls. This meant that these beetle larvae consumed less leaf area per mg of production on fumigated leaves probably because of their better nutritional quality and/or reduced leaf defences. However, the rate of development of larvae was similar on fumigated and control plants.     

Single and interactive effects of low levels of O3, SO2 and NO2 on the growth and yield of spring rape

During three consecutive seasons (1987-1989), the effects of low-levels of O3, SO2 and NO2 singly and in all possible combinations (NO2 in 1988 and 1989 only) on growth and yield of potted plants of spring rape (Brassica napus L. var. napus, 'callypso') were investigated by means of factorial fumigation experiments in open-top chambers. Plants were exposed from the early vegetative stage of development until seed harvest, to charcoal-filtered air (CF; control) and CF which was supplemented for 8-h per day (8.00-16.00) with O3, for 16-h per day with NO2 (16.00-8.00) and continuously with SO2. Including the controls, the 24-h daily mean concentrations [microg m(-3)] ranged between 6-44 (O3), 9-88 (SO2) and 10-43 (NO2). The corresponding daily mean concentrations during the time of fumigation were 10-121 and 11-60 microg m(-3) for O3 and NO2, respectively. Single effects of O3 on growth and yield parameters were mostly negative and the magnitude of this effect was dependent on the season. O3 reduced plant dry weight by 11.3-18.6% and yield of seeds by 11.4-26.9%. While medium levels of SO2 stimulated the weight of pods up to 33%, higher concentrations (88 microg m(-3)) caused a decline of yield of 12.3%. From the significant interactive effects which were observed, it could be established that SO2 and NO2 alone mostly acted positively, but that their interaction with each other and especially with O3 was antagonistic, as some of the detrimental effects of O3 were mitigated by these pollutants. An important antagonistic effect between SO2 and O3 or NO2 was observed on yield. While 56 microg m(-3) SO2 increased yield by 9.9% compared to the control treatment, it aggravated the yield loss caused by O3 from -16.18% to -21.4%, and it reduced the yield stimulation caused by NO2 from +11.8% to +4.2%. Leaf area was the only parameter which was negatively affected by all pollutants, their joint action being synergistic.     

Avoidance of toxic mixing zones by Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) in the limed River Audna, southern Norway

Chlorophyll a (chl a) fluorescence was used to determine the effects of treatments with gaseous HF or aqueous solutions of NaF on the photosynthetic apparatus of spinach prior to the appearance of visible injury. Placing the petioles in 2 mM NaF for 3 h resulted in the accumulation of 240 ppm F in leaf blades. The second oldest leaves of spinach plants accumulated similar concentrations (270 ppm F) when the plants were exposed to gaseous HF at 5 microg F m(-3) for 6 days. These NaF and HF treatments did not result in visible injury nor did they affect Fo, Fm or Fv/Fm. However, during the slow (>2 s) induction kinetics, fluorescence quenching in fluoride-treated leaves increased during the P to S phase and the M peak was no longer resolved. This was due, in part, to increased photochemical quenching. The results are consistent with a reduced ability to develop or maintain a trans-thylakoid proton gradient in chloroplasts containing elevated concentrations of F.     

The effects of prior exposure to sulphur dioxide and nitrogen dioxide on the water relations of timothy grass (Phleum pratense) under drought conditions

Studies of the growth and water relations of the grass Phleum pratense L. (Timothy) were made after simultaneous exposure to SO(2) and NO(2) at concentrations ranging from 80 + 57.4 microg m(-3) to 240 + 172.2 microg m(-3) (SO(2) + NO(2)). Decreased partitioning to the roots was evident during exposure to the pollutants, but when the plants were returned to clean air restrictions in root growth did not persist. Shoot to root partitioning was, however, complicated by the additional factor of changes in the nutritional status of the soil after additional columns of fresh soil were attached to the original tubes. The rate of use of soil water was nevertheless substantially increased by the pollution treatment and after a period of 23 days in which water was withheld, a clear pollution x water stress interaction was seen. The ability of polluted leaves to conserve water under severe water stress was tested by excising the leaves and measuring their water loss over time. The results from this second experiment showed that conservation of water by the leaves was appreciably affected after exposure to 80 + 57.4 microg m(-3) or 133.3 + 95.6 microg m(-3) SO(2) + NO(2). It seems likely that damage to the cells in the epidermal layer, leading to malfunctioning of stomata, is mainly responsible for the reduced ability to conserve water under conditions of extreme stress.     

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

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

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

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

Qualitative and quantitative effects of ozone and/or sulfur dioxide on field-grown potato plants

Solanum tuberosum L. cv Norchip plants were grown in open-top chambers in the summer of 1986. Plants were treated with charcoal-filtered air, nonfiltered air, or nonfiltered air supplemented with 33, 66, or 99% of the ambient ozone (O3) concentrations from 1000 to 2000 h eastern daylight time daily. In addition, plants received charcoal-filtered air plus 0, 0.15 (393 microg m(-3)), 0.34 (891 microg m(-3)), or 0.61 (1598 microg m(-3)) ppm sulfur dioxide (SO2) from 0900 to 1200 h once every 14 d for a total of four treatments. Ozone induced a linear reduction in number and weight of Grade One (> 6.35-cm diameter) potato tubers and in total weight of tubers. Ozone also induced linear reductions in the percentage of dry matter of tubers and linear decreases in glucose and fructose content of Grade One tubers. Sulfur dioxide induced a stimulation and then decline of the number, percentage of dry matter, and sucrose content of Grade One tubers. The SO2 response best fit a quadratic curve. No O3 x SO2 interactions were detected for any of the yield or quality functions measured.     

Sensitivity of three leguminous crops to O3 as influenced by different stages of growth and development

The sensitivity of Cicer arietinum, Vigna mungo and Trigonella foenum-graecum to O(3) has been assessed at different stages of growth and development. Plants of different ages (0, 1, 2, 4 and 6 weeks old) were fumigated with 0 and 120 nl litre(-1) O(3), from 09.30 h to 16.30 h each day for four weeks, in hemispherical chambers located out-of-doors. Seed germination was not affected by O(3) in any of the species, but there were responses (differing between species) on the cotyledons. True leaves were fairly resistant when young but later they became more sensitive. Premature senescence and earlier abscission of leaves (in C. arietinum and T. foenum-graecum) and flowers and abortive fruit drop (in C. arietinum) were also observed. Of the five growth stages examined, 2- and 4-week-old plants seemed to be most sensitive except for Trigonella where sensitivity decreased with increasing age of the plants. The partitioning and distribution of dry matter among different plant parts was also significantly disturbed and root, leaf and stem were adversely affected in a decreasing order. However, the percentage reductions in dry weight per plants for Cicer and Vigna increased with age up to four weeks, then declined abruptly. Growth reductions at the 0- and 6-week-old stages differed only slightly and were very small in magnitude. It may, therefore, be suggested that the plants of these legumes in early stages of exponential growth are more vulnerable to O(3) damage and that the developmental or physiological age is an important factor in O(3) sensitivity.     

Meta-analysis of time-series studies of air pollution and mortality: effects of gases and particles and the influence of cause of death,age, and season

A comprehensive, systematic synthesis was conducted of daily time-series studies of air pollution and mortality from around the world. Estimates of effect sizes were extracted from 109 studies, from single- and multipollutant models, and by cause of death, age, and season. Random effects pooled estimates of excess all-cause mortality (single-pollutant models) associated with a change in pollutant concentration equal to the mean value among a representative group of cities were 2.0% (95% CI 1.5-2.4%) per 31.3 microg/m3 particulate matter (PM) of median diameter < or = 10 microm (PM10); 1.7% (1.2-2.2%) per 1.1 ppm CO; 2.8% (2.1-3.5%) per 24.0 ppb NO2; 1.6% (1.1-2.0%) per 31.2 ppb O3; and 0.9% (0.7-1.2%) per 9.4 ppb SO2 (daily maximum concentration for O3, daily average for others). Effect sizes were generally reduced in multipollutant models, but remained significantly different from zero for PM10 and SO2. Larger effect sizes were observed for respiratory mortality for all pollutants except O3. Heterogeneity among studies was partially accounted for by differences in variability of pollutant concentrations, and results were robust to alternative approaches to selecting estimates from the pool of available candidates. This synthesis leaves little doubt that acute air pollution exposure is a significant contributor to mortality.     

An open-top chamber study with filtered and non-filtered air to evaluate the effects of air pollutants on crops

Four non-filtered and four charcoal-filtered open-top chambers were employed to determine the effects of ambient levels of gaseous air pollutants at Braunschweig, FRG, on growth and yield of potted plants of winter and spring barley. During the exposure period (November 1985-August 1986) monthly mean values of gaseous air pollutants (microg m(-3)) ranged between 34 and 127 for SO(2), 34 and 52 for NO(2) and 12 and 33 for O(3) in winter (November-March), and 16 to 26 for SO(2), 20 to 33 for NO(2) and 42 to 53 for O(3) in spring-summer (April-August). Monthly 2% percentile values for these gases reached (microg m (-3)) 561 for SO(2), 140 for NO(2) and 170 for O(3). The filtering efficiencies of the charcoal filters used averaged 60% for SO(2), 50% for NO(2) and 70% for O(3). All plants of winter barley from the unchambered plot were killed by severe frost periods in winter, 1986. Little frost damage occurred on plants grown in the chambers. Air filtration resulted in higher numbers of plants of winter barley per pot, i.e. a higher number of individuals per area, and a higher dry weight of whole plants and ears compared to the non-filtered atmosphere. In the experiments with spring barley, fresh and dry weight of whole plants were lower and dry weight of leaves were higher in the filtered open-top chambers. These effects could not be observed at all harvests which were carried out during the growing season. Grain yield and sulphur content of the leaves of both barley cultivars were not affected by the air filtration. Production of biomass of spring barley grown in ambient air was higher than of that grown in open-top chambers.     

Ozone-induced nitrate formation in needles and leaves of Picea abies, Fagus sylvatica and Quercus robur

Much attention has been paid to ozone as a major cause of novel forest decline in Europe. In combination with acidic mist, O(3) has been observed to increase ion leaching. Besides cations lake Mg(2+), Ca(2+), K(+), NH(4)(+), considerable amounts of nitrate were found to be leached by acidic mist from needles of Norway spruce. Controlled fumigation experiments, with 100, 300, and 600 microg O(3)m(-3) over 22 days continuously, have led to a nitrate accumulation of 94.1 +/- 14.8, 119.4 +/- 28.7 and 198.9 +/- 14.9 microg NO(3)(-1) g(-1) FW, respectively, in leaves of Quercus robur. Similar values were found in leaves of Fagus sylvatica and current and previous year needles of Picea abies. Nitrate levels of controls receiving charcoal filtered air were well below 40 microg NO(3)(-) g (-1) FW. Statistically significant elevated nitrate levels were observed after only 48 h of continuous fumigation with 600 microg O(3)m(-3), in all tree species tested, and after 144 h in the 100 microg O(3)m(-3) treatment. In another experiment, trees of Picea abies were kept in two charcoal (C) and two Purafil plus charcoal (P/C) ventilated chambers, and fumigated with O and 500 microg O(3)m(-3) in cabinets of each filter-type in order to eliminate NO(x) from chamber air. After 29 days of continuous ozone fumigation, NO(3)(-) accumulation in needles amounted to 102.0 +/- 37.7 and 137.4 +/- 40.5 microg g(-1) FW in P/C and C-filtered chambers, respectively. Nitrate contents of controls were below 30 microg NO(3)(-)g(-1) FW at the end of the experiment. No significant differences in NO(3)(-) accumulation between filter treatments were observed. Since NO(x) was reduced by more than 95% in the Purafil/charcoal versus the charcoal treatment, NO(3)(-) accumulation in needles can be attributed predominantly to the influence of ozone and not to direct NO(2) uptake of needles by the possible oxidation of NO to NO(2) in the presence of ozone.     

Effects of ozone impact on the gas exchange and chlorophyll fluorescence of juvenile birch stems (Betula pendula Roth.)

Effects of ozone impact on gas exchange and chlorophyll fluorescence of juvenile birch (Betula pendula) stems and leaves were investigated. Significant differences in the response of leaves and stems to ozone were found. In leaves, O3 exposure led to a significant decline in photosynthetic rates, whereas stems revealed an increased dark respiration and a concomitant increase in corticular photosynthesis. In contrast to birch leaves, corticular photosynthesis appeared to support the carbon balance of stems or even of the whole-tree under O3 stress. The differences in the ozone-response between leaves and stems were found to be related to ozone uptake rates, and thus to inherent differences in leaf and stem O3 conductance.     

Ozone episodes in Southern Lower Saxony (FRG) and their impact on the susceptibility of cereals to fungal pathogens

Spring wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.) plants were exposed to simulated ozone (O(3)) episodes (7 h day(-1) for 7 days) at maximum concentrations of 120, 180 and 240 microg m(-3) O(3), in comparison to a charcoal-filtered air control. Fumigations were conducted in four closed chambers placed in a climate room. Exposures took place prior to inoculation of the plants with six different facultative leaf pathogens. On wheat, significant enhancement of leaf attack by Septoria nodorum Berk. and S. tritici Rob. ex Desm. appeared, particularly on the older leaves and at the highest level of O(3). The same was true for Gerlachia nivalis W. Gams et E. Müll/Fusarium culmorum (W.F.Sm.) Sacc. on wheat and net blotch (Drechslera teres (Sacc.) Shoem.) or G. nivalis leaf spots on barley. Disease development was promoted both on leaves with and without visible injury following exposure to O(3). Sporulation of the two Septoria species increased at 120 and 180 microg m(-3) O(3); however, it was reduced to the level of the control, if 240 microg m(-3) were applied. No significant effects of predisposition were observed with Bipolaris sorokiniana (Sacc.) Shoem. (syn. Helminthosporium sativum Pamm., King et Bakke), the causal agent of spot blotch, neither on wheat nor on barley. Doses and peak concentrations applied in the experiments were in good agreement with measurements of ambient ozone in Southern Lower Saxony, FRG. Six years' ozone data (1984-1989) revealed the annual occurrence of between 3 and 11 ozone episodes with potentially harmful effects on cereals (three or more consecutive 'ozone days' with 8-h means above 80 microg m(-3)). The frequency of ozone episodes followed by weather periods favourable for infections by facultative pathogens was higher in years with low O(3) pollution than in ozone-rich years, and varied between one and five cases per season. The number of ozone days during the main growing season of cereals (1 April until 31 August) varied from 25 in 1984 to 98 in 1989. However, only 7.9% of ozone days during the 6 years examined were concurrent with weather conditions suitable for fungal infections. It is concluded that the majority of leaf infections in the field happens under low-level concentrations of photooxidants.     

Growth stimulation of Triticum aestivum seedlings under Cr-stresses by non-rhizospheric pseudomonad strains

Four chromium-resistant non-rhizospheric strains SPCr-1, SPCr-2, SPCr-3 and SPCr-4 (Pseudomonads), which were isolated from the effluents of an ICI paint factory and could tolerate 2-3 mg ml(-1) chromium in a minimal medium and 40 mg ml(-1) in a rich medium, were used to inoculate seeds of Triticum aestivum. Both inoculated and non-inoculated seeds were germinated and grown under different concentrations of chromium salts (K2CrO4, 0, 100, 250, 500; CrCl3, 0, 250, 500, 1000 microg ml(-1)). Germination and growth parameters were severely affected by chromium-salts. K2CrO4 had more drastic effects than CrCl3 treatments. Seedlings had a hard and brittle texture and showed symptoms of hypertrophy. Brown spots on leaves and stems were visible and the tips of leaves were bifurcated and curled. The root system was also impaired, ranging from a browning of the tip to complete destruction of cortical tissues. Under chromium-stress conditions, inoculated plants had significantly better germination and growth as compared to non-inoculated treatments. Bacterial growth enhancement of seedlings was associated with reduced chromium-uptake, increased auxin content and the formation of stress specific proteins. With bacterial inoculations, symptoms of chromium toxicity were reversed, especially at lower concentrations of chromium salts.