exposure of oats, avena sativa l., to filtered and unfiltered air in open-top chambers: effects on...

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ELSEVIER Environmental Pollution 86 (1994) 129-134 © 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0269-7491/94/$07.00 EXPOSURE OF OATS, A VENA SATIVA L., TO FILTERED AND UNFILTERED AIR IN OPEN-TOP CHAMBERS: EFFECTS ON GRAIN YIELD AND QUALITY H. Pleijel, a L. Sk~irby," K. Ojanper~i b* & G. Selld6nb~ ~Swedish Environmental Research Institute ( IVL), Box 47086, S-402 58 GOteborg, Sweden b Department of Plant Physiology, Institute of Botany, University of GOteborg, Carl Skottsbergs gata 22, S-413 19 G6teborg, Sweden (Received 1 March 1993; accepted 28 August 1993) Abstract Field grown oats, Avena sativa L. cv Vital, were exposed to filtered and unfiltered air from anthems until harvest in open-top chambers at a site in south-west Sweden. Ambient plots were used to study the influence of the chamber itself. With the exception of the number of grains per panicle, which was significantly higher in the charcoal-filtered treatment, no significant filtration effects were obtained for any of the plant growth para- meters studied), i.e. grain yield, number of panicles per square metre, lO00-grain weight, straw yield and harvest index). The chamber had a significant negative effect on grain yield, lO00-grain weight and straw yield. None of the yield quality parameters that were measured, such as crude protein content, crude fibre content, fat content, volume weight of the grain and water content of the grain at harvest, were significantly influenced by either air filtration, or the presence of the chamber. The chloro- phyll content of the flag leaves was higher in the char- coal-filtered treatment than in the non-filtered and ambient air treatments towards the end of the experi- ment, indicating that filtering of the air delayed senes- cence. The decline of the shoot area after the onset of plant senescence proceeded faster in both chamber treat- ments. The faster development in the chamber was ex- plained by the faster accumulation of thermal time in the chamber. INTRODUCTION To evaluate the influence of present air pollution on crops in Europe, a series of experiments with open-top chambers was set up by the CEC within the framework of the research programme COST 612 (CEC, 1986). Within that programme, ambient levels of ozone in Europe have been shown to cause yield loss in crops such as spring wheat in Switzerland (Fuhrer et al., 1989) and Sweden (Pleijel et al., 1991), while spring barley did not respond to ambient and slightly elevated ozone con- centrations in a Swedish study (Pleijel et al., 1992). * Present address: Agricultural Research Centre of Finland, SF-31600, Jokioinen, Finland. :~ To whom correspondence should be addressed. 129 To our knowledge, no investigation of the impact of ambient ozone on field-grown oats has so far been published. In a screening test, where several cultivars of different cereals were exposed to rather high ozone con- centrations over a short period of time, oats (as judged by visible injury), appeared to be more sensitive to ozone than wheat and barley (Sechler & Davis, 1964). In addition, photosynthesis of oat leaves was affected negatively by ozone in a controlled laboratory experi- ment (Myhre et al., 1988). However, the relationships between damage at high ozone concentrations in short- term fumigations and the grain yield of an oat crop ex- posed to ambient concentrations of ozone over a long period of time are complex and not yet well under- stood. Several studies have shown that the effects of ozone on cereals may be described, at least partly, as a pre- mature senescence or enhanced development (Grand- jean & Fuhrer, 1989; Ojanper~i et al., 1992) which is expressed, for example as a faster yellowing of leaves and a decrease in chloroplast size in higher ozone con- centrations. The present investigation was undertaken in order to evaluate the impact of ambient ozone concentrations in southern Sweden on grain yield, and the quality of oats. As pointed out in Pleijel et aL (1991), ozone is likely to be the most important gaseous air pollutant in agricul- tural ecosystems in rural south Sweden. This experi- ment, like most other studies of the impact of ozone on crops, used open-top chambers to expose the plants. A problem associated with the use of open-top chambers is that the chambers can affect the plants significantly, and results may therefore not be entirely representative for the ambient situation. Sanders et al. (1991) showed that the presence of the chambers in which thermal time accumulated more quickly than in the ambient air, tended to hasten the development of field beans (Vicia faba) more than in ambient grown plants. MATERIALS AND METHODS The open-top chambers were 1.24 m in diameter and 1.60 m tall including the frustum. The chamber system

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Page 1: Exposure of oats, Avena sativa L., to filtered and unfiltered air in open-top chambers: Effects on grain yield and quality

ELSEVIER

Environmental Pollution 86 (1994) 129-134 © 1994 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0269- 7491/94/$07.00

EXPOSURE OF OATS, A VENA SATIVA L., TO FILTERED AND UNFILTERED AIR IN OPEN-TOP CHAMBERS: EFFECTS

ON GRAIN YIELD AND QUALITY

H. Plei jel , a L. Sk~irby," K . Ojanper~i b* & G. Selld6nb~

~Swedish Environmental Research Institute ( IVL), Box 47086, S-402 58 GOteborg, Sweden b Department of Plant Physiology, Institute of Botany, University of GOteborg,

Carl Skottsbergs gata 22, S-413 19 G6teborg, Sweden

(Received 1 March 1993; accepted 28 August 1993)

Abstract Field grown oats, Avena sativa L. cv Vital, were exposed to filtered and unfiltered air from anthems until harvest in open-top chambers at a site in south-west Sweden. Ambient plots were used to study the influence of the chamber itself. With the exception of the number of grains per panicle, which was significantly higher in the charcoal-filtered treatment, no significant filtration effects were obtained for any of the plant growth para- meters studied), i.e. grain yield, number of panicles per square metre, lO00-grain weight, straw yield and harvest index). The chamber had a significant negative effect on grain yield, lO00-grain weight and straw yield. None of the yield quality parameters that were measured, such as crude protein content, crude fibre content, fat content, volume weight of the grain and water content of the grain at harvest, were significantly influenced by either air filtration, or the presence of the chamber. The chloro- phyll content of the flag leaves was higher in the char- coal-filtered treatment than in the non-filtered and ambient air treatments towards the end of the experi- ment, indicating that filtering of the air delayed senes- cence. The decline of the shoot area after the onset of plant senescence proceeded faster in both chamber treat- ments. The faster development in the chamber was ex- plained by the faster accumulation of thermal time in the chamber.

INTRODUCTION

To evaluate the influence of present air pollution on crops in Europe, a series of experiments with open-top chambers was set up by the CEC within the framework of the research programme COST 612 (CEC, 1986). Within that programme, ambient levels of ozone in Europe have been shown to cause yield loss in crops such as spring wheat in Switzerland (Fuhrer et al., 1989) and Sweden (Pleijel et al., 1991), while spring barley did not respond to ambient and slightly elevated ozone con- centrations in a Swedish study (Pleijel et al., 1992).

* Present address: Agricultural Research Centre of Finland, SF-31600, Jokioinen, Finland. :~ To whom correspondence should be addressed.

129

To our knowledge, no investigation of the impact of ambient ozone on field-grown oats has so far been published. In a screening test, where several cultivars of different cereals were exposed to rather high ozone con- centrations over a short period of time, oats (as judged by visible injury), appeared to be more sensitive to ozone than wheat and barley (Sechler & Davis, 1964). In addition, photosynthesis of oat leaves was affected negatively by ozone in a controlled laboratory experi- ment (Myhre et al., 1988). However, the relationships between damage at high ozone concentrations in short- term fumigations and the grain yield of an oat crop ex- posed to ambient concentrations of ozone over a long period of time are complex and not yet well under- stood.

Several studies have shown that the effects of ozone on cereals may be described, at least partly, as a pre- mature senescence or enhanced development (Grand- jean & Fuhrer, 1989; Ojanper~i et al., 1992) which is expressed, for example as a faster yellowing of leaves and a decrease in chloroplast size in higher ozone con- centrations.

The present investigation was undertaken in order to evaluate the impact of ambient ozone concentrations in southern Sweden on grain yield, and the quality of oats. As pointed out in Pleijel et aL (1991), ozone is likely to be the most important gaseous air pollutant in agricul- tural ecosystems in rural south Sweden. This experi- ment, like most other studies of the impact of ozone on crops, used open-top chambers to expose the plants. A problem associated with the use of open-top chambers is that the chambers can affect the plants significantly, and results may therefore not be entirely representative for the ambient situation. Sanders et al. (1991) showed that the presence of the chambers in which thermal time accumulated more quickly than in the ambient air, tended to hasten the development of field beans (Vicia faba) more than in ambient grown plants.

MATERIALS AND METHODS

The open-top chambers were 1.24 m in diameter and 1.60 m tall including the frustum. The chamber system

Page 2: Exposure of oats, Avena sativa L., to filtered and unfiltered air in open-top chambers: Effects on grain yield and quality

130 H. Pieijel, L. Skdrby, K. Ojanperd, G. Selld~n

Table 1. Timetable of events during the open-top chamber experiment with oats in 1990, Ostad, Sweden

Event Time

PK fertilisation a Autumn 1989 Planting 8 April N fertilisation b 8 May Herbicide c + Mn d 20 May Insecticide e + Mn d 9 June Chambers installed I l June Insecticide e 27 July Removal of the chambers 28 August Harvest of all plots 30 August

a PK 8 : 8 (Hydro-Supra). b Nitrate of lime, 93 kg N ha -1 (Hydro-Supra). c Actril, 2/litre ha ~ (ICI). d Mn SO4, 4 kg ha 1 e Pirimor G, 0.25 kg ha i (ICI).

is described in detail by Fowler et al. (1988). Air was blown, day and night, through a circular perforated an- nulus 0.1 m above the canopy.

Experimental site The experiment was conducted in an oat field at Ostad, 50km north-east of G6teborg, Sweden (N57°54 ', E12°24'). The field was situated about 60 m above sea level and the soil was a loamy sand containing approxi- mately 4% organic matter and with a pH around 6.4. No major air pollution sources are located in the vicinity of the investigation area.

Cultural practices The oats cultivar, Avena sativa, cv Vital, was sown (205 kg ha 1) with 12.5 cm row spacing. The fertiliser application rate was 93 kg N ha 1, 22 kg P ha -l, 22 kg K ha -1. Further details of the cultural practices are given in Table 1. Irrigation of all plots was carried out on 12 June, 15 June and 30 July.

Experimental design Three treatments were used: ambient air plots without chambers (AA), chambers with charcoal-filtered air (CF) and chambers with non-filtered air (NF). There were ten replicates of each treatment. The treatments were distributed in a completely randomised design. Filtration was started as soon as the chambers were in- stalled (Table 1).

Pollutant monitoring Air samples were drawn through 50 m long Teflon (PTFE) tubes (1/4" = 6.35 mm, external diameter) which were continuously ventilated and connected to a time-share sampling system with PTFE solenoid valves. Air was sampled 0.1 m above the canopy in four N F chambers and four CF chambers. Ozone data for June, however, are based only on one NF chamber and two CF chambers. In addition, measurements of ozone con- centrations were made 10 m and 1-1 m above the ground in the ambient air. Ozone concentrations were

monitored using Thermo Environmental 49 UV absorp- tion analysers. The ozone analysers were calibrated on a monthly basis using a portable ozone generator (Monitor Labs 8500). Data were collected with a Campbell Scientific CR 10 data logger.

Climate monitoring Air temperature and relative humidity were monitored with Rotronic YA-100 hygrometer/thermometers at 0-1 m above the canopy in one chamber, and in one ambi- ent plot. The photosynthetically active radiation was measured at one ambient plot with a quantum sensor (Li-1905A). Climate data were stored using a Campbell Scientific CR 10 data logger. Rain was collected in a standardised manual precipitation gauge.

Final harvest To facilitate the harvest, each circular plot was marked by a plastic strip with a diameter of 1.10 m and a height of 0.06 m. The strips were installed when the plants were 0.10 m tall. All the above-ground plant material inside each circle was harvested. The panicles were separated from the straw and the number of pani- cles from each plot was counted. The grain from each plot was carefully threshed by hand and then weighed. The water content of the grain was determined by dry- ing three sub-samples (5-10 g fresh weight each) at 70°C to constant weight. The rest of the grain was sent to an authorised agricultural laboratory (AnalyCen, Lidk6ping, Sweden) for the determination of crude protein (Kjeldahl), crude fibre, fat, volume weight and 1000-grain weight. The straw biomass from each plot was dried at about 33°C and 15% relative humidity to constant weight.

Intermediate harvests The chlorophyll content of the flag leaves was deter- mined, according to Arnon (1949), on 20 June, 3 July, 12 July, 30 July and 13 August. At each sampling occa- sion three flag leaves were chosen at random from each plot. Circular leaf tissue samples, with a diameter of 8 mm, were punched out 5 cm from the base of the leaves.

The projected areas of panicles, leaves and straw were measured on 3 July, 16 July, 30 July and 13 August, using an image analyzer (Quantimet 250, Cambridge Instruments, England). Samples were taken in 4 plots per treatment which were chosen at random at the first sampling occasion. At each sampling occa- sion, 3 single shoots were harvested at random from each plot.

Statistical analysis For all harvest parameters, a one-way analysis of vari- ance was performed, least significant differences (LSD) were calculated for the determination of significant differences between individual treatments when the F- test was significant at P = 0.05 (Snedecor & Cochran, 1967). One CF plot was destroyed by rodents. That plot was excluded from the data analysis.

Page 3: Exposure of oats, Avena sativa L., to filtered and unfiltered air in open-top chambers: Effects on grain yield and quality

Effects o f air pollutants on oats 131

Table 2. Ozone concentrations (hi Htre -I) as 24-h and 7-h (11.00-18.00, local time) monthly means for June, July, and August in the ambient at 10 m and 1.1 m above the ground, in the non-filtered air (NF) and charcoal-filtered air (CF), ~)stad

Sweden, 1990

Table 4. Average daily maximum, minimum and mean temper- atures (°C), relative humidity (RH, %) and photosynthetic active radiation (PAR, /Jmol m -z s -i) measured in one NF chamber and at one point in the ambient air for June, July and

August in the oats experiment in 1990, ~)stad, Sweden

24-h 7-h

June July August June July August

AA, 10m 36 30 31 43 36 42 AA, 1.1 m 28 20 20 36 27 33 NF 24 17 18 31 22 28 CF 9 7 7 12 10 13

R E S U L T S

Ozone concentrations 24-h and 7-h (11.00-18.00, local time) monthly means of the ozone concentrations in the different treatments and in the ambient air at 10 m above ground are presented in Table 2. The ozone concentrations in the non-filtered treatment were 2 to 3 times higher than in the charcoal-filtered treatment. The concentrations (7-h means) inside the NF chambers were 3-5 nl litre 1 lower than the corresponding values outside the chambers.

Table 3 shows the number of hours exceeding 50, 60, 70, 80, 90 and 100 nl litre 1 in the different treatments and in the ambient air at 10 m and 1 m above the

Table 3. Number of hours with ozone concentrations above 50, 60, 70, 80, 90, 100 nl litre -I respectively, at two heights above the ground in the ambient air (AA 10 m and AA 1-1 m) in non-filtered (NF) and charcoal-filtered air (CF) between astro- nomical sunrise and sunset for Jane, July and August in the

oats experiment in 1990, ~stad, Sweden

Ozone June July August nl litre l

AA, 10 m 50 69 33 25 60 7 14-5 15 70 0 5 7.5 80 0 0.5 7.5 90 0 0 6

100 0 0 1.5

AA, 1-1 m 50 24.5 6.5 15.5 60 2 1 6.5 70 0 0.5 4 80 0 0 2 90 0 0 1

100 0 0 0

NF 50 9 2.5 12 60 0.5 0 6.5 70 0 0 4.5 80 0 0 1 90 0 0 0

100 0 0 0

CF 50 0 0 0 60 0 0 0 70 0 0 0 80 0 0 0 90 0 0 0

100 0 0 0

Parameter June July August

Number of days with measurements 20 31 17

Max. temperature, chamber 21-2 21.0 24.7 Max. temperature, ambient 19.0 19.2 21.6

Min. temperature, chamber 7.1 7-9 8.6 Min. temperature, ambient 6.0 6.9 7.9

Mean temperature, chamber 14.3 14-4 16.1 Mean temperature, ambient 12.9 13.5 14.9

Mean RH, chamber 89 93 90 Mean RH, ambient 89 89 91

PAR 352 342 583

ground between sunrise and sunset. While the maximum concentration of ozone at 10 m exceeded 100 nl litre i, the maximum concentrations at 1 m in the ambient air and in the non-filtered treatment were in the range 90-100 nl litre 1 and 80-90 nl litre -1, respectively. In the charcoal-filtered treatment 50 nl litrff 1 ozone was never exceeded.

Climate The amount of rainfall for 14 May-31 May was 23-2 mm, for June 71.2 mm, for July 84.9 mm and for August 71.3 mm. Monthly average temperatures (maximum, minimum and mean) as well as relative humidities inside and outside a chamber, and the photosynthetically active radiation (PAR) in the field outside the chambers are shown in Table 4. The differ- ence in the mean temperature between the chamber and the ambient air varied between 0.9 and 1-4 °C on a monthly basis. The largest difference between the chamber and the ambient maximum air temperature was 3. I°C in August, and corresponded to the highest monthly average of the photosynthetic active radiation. The monthly averages of the relative humidity were not strongly influenced by the presence of the open-top chamber.

Final harvest Plant growth parameters from the final harvest are pre- sented in Table 5. The number of panicles per unit area and the harvest index did not differ between the three treatments. There was a significant negative influence of the chamber on the grain yield and the straw yield, while the 1000-grain weight was slightly, but signifi- cantly, higher in the chamber treatments than in the ambient air. The only growth parameter for which there was a significant difference between CF and NF was the number of grains per panicle, which was higher in CF and AA compared with NF treatment.

Table 6 shows the results of the determinations of grain quality parameters at the final harvest. No significant (P -- 0.05) treatment effects were obtained in

Page 4: Exposure of oats, Avena sativa L., to filtered and unfiltered air in open-top chambers: Effects on grain yield and quality

132 H. Ple~/el, L. Skdrby, K. Ojanperd, G. Selld6n

&-.. 15o E u

I,IJ a : lOO <

i -

0 o = 5o O ~

a

I = A A

e - N F

i I

190

0 I I i I I I i i I , i i i I i i i i I i i , i I

180 200 210 220 250 500 750 1 0 0 0

JULIAN DAY THERMAL TIME (*C days)

Fig. 1. Changes in area of oat shoots (leaves, stem and panicle) with time. (A) Shoot area plotted against Julian day number. (B) Shoot area plotted against accumulated thermal time (degree days above 0°C). NF, non-filtered air, AA, ambient air.

Table 5. Plant growth parameters for the different treatments in the oats experiment in 1990, Ostad, Sweden

Parameter CF NF AA

Mean SD Mean SD Mean SD

Grain yield, ton ha I 5-38a 0 .30 5-50a 0-33 5 .90b 0.36 Number of panicles 404 34 436 46 430 50

per m 2 Number of grains 35.2a 2.9 32.6b 3.0 37-2a 3.7

per ear 1000-grain weight, § 38.0a 0.8 38.2a 0.8 37.2b 1.0 Straw yield, ton ha 4.46a 0-29 4.62a 0 . 3 9 5 .04b 0.37 Harvest index, % 54.7 1-5 54.4 1.8 54.0 1.5

CF -- charcoal-filtered air; NF -- non-filtered air; AA -- ambient air; SD = standard deviation. Values for each parameter followed by different letters are significantly different at P = 0.05.

any o f the five qual i ty parameters : c rude p ro te in con- tent, c rude fibre content , fat content , vo lume weight and wate r content .

Intermediate harvests In F igure I (A) and (B), the shoo t a rea is p lo t t ed agains t Ju l ian day n u m b e r and accumula t ed the rmal t ime, respectively, f rom the ins ta l la t ion o f the chambers unti l the end o f the exper iment . W i t h the except ion o f day 212, when the N F and C F t r ea tmen t s differed significantly f rom the A A t rea tment , there were no significant differences be tween the t rea tments . The difference be tween inside and outs ide the c ha mbe r on

Table 6. Yield quality parameters in the oats experiment in 1990, Ostad, Sweden

Parameter CF NF AA

Mean SD Mean SD Mean SD

Crude protein, % 11-4 0.7 11.6 0.7 11.5 0-7 Crude fibre, % 11.4 0-6 11-3 0-8 11.6 1.2 Fat, % 5.3 0.4 5-4 0-2 5.6 0.3 Volume weight, g litre -1 535 11 533 13 526 24 Water content, % 10.5 0.1 10.8 0.4 11.1 0.4

CF = charcoal-filtered air; NF = non-filtered air; AA = ambient air; SD = standard deviation. No differences between averages. Statisti- cally significant at P = 0.05.

5 I • = AA A = C F

E g .1o +

~ 3 _.1 ..1 >-

"I" 2

O e~

O 1 ,-1 I o

0 5 0 0 1 0 0 0

THERMAL TIME (*C days)

Fig. 2. Changes in chlorophyll a+b content of oat flag leaves with accumulated thermal time (degree days above 0°C). CF,

charcoal-filtered air; NF, non-filtered air, AA, ambient air.

day 212, in a pe r iod o f fast senescence, can pa r t ly be expla ined by the difference in the accumula t ion o f

the rmal time. F igure 2 shows the ch lo rophyl l conten t o f the flag

leaves in A A , C F and N F p lo t ted agains t thermal time. The loss o f ch lo rophyl l in the flag leaves dur ing senes- cence was significantly s lower in the C F t r ea tmen t than in the N F and A A t rea tments . The differences between the N F and A A t rea tments on the last two sampl ing days, significant on day 211, can par t ly be expla ined by the faster accumula t ion o f thermal t ime in the

chamber .

D I S C U S S I O N

In the present invest igat ion, none o f the gra in qua l i ty pa rame te r s was affected, ei ther by ai r f i l t rat ion, or by the presence o f the chamber . However , the number o f grains per ear was significantly h igher in ambien t air and in charcoal - f i l te red air t han in non-f i l tered air. The higher value o f this p a r a m e t e r in the fil tered air was coun te rac ted by a negative, non-s igni f icant effect of f i l t ra t ion on the number o f panicles per uni t area. The

Page 5: Exposure of oats, Avena sativa L., to filtered and unfiltered air in open-top chambers: Effects on grain yield and quality

Effects o f air pol lu tants on oats 133

resulting negative net effect of air filtration on grain yield of oats was small, and not significant, in contrast to the outcome of an earlier wheat study (Pleijel et aL, 1991) where filtration of the air significantly increased the yield in two consecutive years. Furthermore, the sensitivity of oats to ozone manifested as visible damage (Sechler & Davis, 1964) and decrease in net photosynthesis (Myhre et al., 1988) after short-term fumigations with rather high concentrations of ozone was not reflected in significant yield loss under the conditions prevailing during the present study.

There was, however, a significantly higher chloro- phyll content in the flag leaves from the charcoal- filtered treatment towards the end of the experiment compared to that in leaves from the non-filtered treat- ment. This is in accordance with the results from several experiments with wheat. During normal senes- cence cereal leaves become increasingly chlorotic and necrotic. Exposure of wheat to moderately elevated concentrations of ozone has resulted in visible injury of the leaves similar to those occurring during normal senescence (Heagle et al., 1979; Mulchi et al., 1986; Fuhrer et al., 1989; Pleijel et al., 1991), indicating that ozone exposure causes premature senescence. Elevated concentrations of ozone induced chlorosis within a week (Fuhrer et al., 1989; Ojanper~ et al., 1992), while removal of ozone by charcoal filtering retarded senes- cence compared to plants exposed to non-filtered air (Ojanper~. et al., 1992). In wheat, the retarding effect of air filtration on flag leaf senescence was correlated with a positive effect on grain yield (Pleijel et al., 1991). Al- though the higher chlorophyll content in the flag leaves from the filtered treatment indicated that air filtration also retarded flag leaf senescence in oats, no corre- sponding increase in yield was observed.

It has been suggested by Stoy (1965) that the capac- ity for assimilate production after anthesis may act as a yield-limiting factor in wheat. It is well known that ozone affects photosynthesis before visible symptoms are observed. A reduction in net photosynthesis due to ozone has been observed in many plant species, includ- ing wheat and oats (Amundson et al., 1987; Lehnherr et al., 1987; Reich, 1987; Darrall, 1989; Myhre et al., 1988). Furthermore, investigations of the ultrastructure of ozone-exposed wheat flag leaves have shown that chloroplasts are the most sensitive cell organelles, and mitochondria are the least (Ojanper/i et al., 1992). Thus, since the photosynthetic apparatus is one of the prime targets for ozone, one would expect wheat to be sensitive to ozone. However, the yield of a crop plant can be more or less source limited. If oats depend less on current photosynthesis for grain filling than modern wheat cultivars, a minor to moderate ozone-induced re- duction in photosynthesis would not immediately be reflected in a reduction of the oat yield. This could then explain why, while Myhre et al. (1988) found that the rate of net photosynthesis in oats was sensitive to ozone, the present authors observed no effect of ozone on the yield.

Since the final grain yield of a cereal crop is not de-

termined until near the very end of the life of the plant (Evans et al., 1975), one would also expect the stage of crop development to be of importance for the response of the crop to ozone. Results from studies of wheat confirm this assumption (Selld6n & Pleijel, 1993). In the present experiment, a quite strong ozone episode occurred between 2 and 4 August. The ozone concen- trations were above 100 nl litre -1 at the 10 m level in the ambient air (Table 3). However, it was unlikely that this episode could have influenced the grain yield, since the oats had reached the milky ripe stage in the non-fil- tered and ambient air, after which no grain filling takes place. A similar episode, earlier during the growing sea- son, may have affected the yield negatively, however.

In this experiment, as in most other studies of the impact of air pollution on crops, open-top chambers were used. The chamber itself had a negative influence on the grain and straw yields of oats. The main yield component to explain the difference in yield between the ambient and non-filtered treatments seems to be the number of grains per panicle, which was 14% higher in the ambient air than in the non-filtered air, and only to a small extent counteracted by the 3% negative effect on 1000-grain weight.

There was also a hastening effect of the chamber on the development of the plants. This is in accordance with the chamber effect on the pod developmental stage in field beans (Vicia faba) , as described by Sanders et al. (1991). In that experiment, as well as in the experi- ment described in this paper, the difference in develop- mental stage was reduced when the parameters describing the developmental stage (in our case shoot area and chlorophyll content of the flag leaves) were plotted against thermal time instead of Julian day num- ber. However, it is unlikely that the effects of the cham- bers were large enough to invalidate the conclusions about the impact of ozone on oats in southern Sweden. The studies in Sweden in 1987 and 1988 (Pleijel et al., 1991) and in Switzerland, 1986-1988 (Fuhrer et al., 1990) clearly showed that the effect of ozone on the yield of spring wheat was similar from year to year de- spite large differences in weather conditions and associ- ated differences in chamber influence.

We therefore conclude that there were effects of air filtration on the chlorophyll content in flag leaves, number of grains per panicle, and possibly, number of panicles per unit area in the oat crop studied. These effects, however, did not result in any significant changes in yield parameters of agricultural importance.

ACKNOWLEDGEMENTS

Thanks are due to Patrik AlstrOmer for making the in- vestigation area available, to Roy and Karin Olausson for their assistance at the field site and to Gunilla Pihl, Anders /~gren and Pia Almbring for technical assis- tance. The investigation was funded by the National Swedish Environmental Protection Board, the Federa- tion of Swedish Farmers and the Swedish Council for Forestry and Agricultural Research.

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134 H. Pleijel, L. Skgirby, K. Ojanper& G. SelldOn

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