162 A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168
Table 2 Seasonal climate data, CO
2
and O
3
concentrations, exposure duration period together with AOT40 values for the 1994 and 1996 growing seasons
Year Mean daily
Mean daily CO
2
24 h O
3
24h AOT40
a
Start of End of
temperature radiation
seasonal mean seasonal mean
fumigation fumigation
◦
C kW m
− 2
in ppmv in ppbv
period period
1994 13.1
0.14 360 ambient
17 ambient 3356 ambient
27 June 25 August
678 680 ppmv 24 ambient +50
24281
b
elevated 1996
12.6 0.16
362 ambient 27 ambient
33321 ambient 12 April
12 August 675 680 ppmv
53 ambient +90 95930 elevated
20 year average 13.1
0.15
a
Accumulated ozone exposure above a threshold of 40 ppbv.
b
Fumigation started at anthesis.
of variance ANOVA using the statistical package Minitab, version 8.3 extended, for personal comput-
ers. When the multiway ANOVA showed a statis- tically significant effect of one of the parameters a
oneway analysis of variance was carried out to re- veal at what level the effect was significant. In such
cases a two-sided confidence interval for the differ- ences between the treatment means and the control
or for all pairwise differences between level means, was constructed. Data for the intermediate CO
2
con- centration 510 ppmv in 1996 have been omitted
from the results in order to make direct comparisons with the 1994 data as there was no intermediate CO
2
concentration used in that year.
3. Results
3.1. Climate and CO
2
and O
3
concentrations Table 2 shows daily mean values of temperature
and radiation, in addition to the 24 h seasonal mean for CO
2
ppmv and O
3
ppbv, the exposure duration period together with AOT40 Fuhrer, 1994 values,
for both growing seasons. The exposure index AOT40 was calculated for each growing season for both am-
bient and elevated conditions. The 24 h seasonal mean ambient O
3
concentration was lower in 1994 than in 1996. The elevated O
3
concentration was increased from ambient +50 to ambient +90 ppbv in 1996, re-
sulting in higher mean O
3
concentrations. The 1994 growing season had the lower AOT40’s, primarily be-
cause of dull weather conditions and associated low ambient O
3
concentrations, as well as the fact that elevated O
3
exposure did not start until anthesis 27 June. In 1996 a higher AOT40 at ambient O
3
was recorded as a result of a combination of climatic con-
ditions favouring O
3
production and the longer dura- tion of exposure. The higher AOT40 at elevated O
3
was, in addition, the result of the elevated treatment being raised to ambient +90 ppbv. Neither the mean
daily temperature nor the mean daily radiation values differed greatly from the long term average Table 2.
The 24 h seasonal mean CO
2
concentration did not differ from the desired concentrations during either of
the growing seasons. 3.2. Chlorophyll content
Elevated CO
2
did not have a significant effect on the chlorophyll content of the flag leaves grown at am-
bient O
3
in 1996 Fig. 1, Table 3. After anthesis 81 days after sowing, elevated O
3
, ambient +90 ppbv, consistently increased p0.01 the rate of senescence
at ambient CO
2
as indicated by the decrease in flag leaf chlorophyll content Fig. 1. When elevated CO
2
and elevated O
3
were both present the reduction in chlorophyll content attributed to elevated O
3
was con- siderably reduced compared with the effect at ambi-
ent CO
2
Fig. 1 CO
2
× O
3
interaction significant in Table 3. Throughout the senescence period, but par-
ticularly during the middle phase from about 90–110 days after sowing, elevated CO
2
prevented loss of chlorophyll from the flag leaves which was caused by
elevated O
3
.
A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168 163
Fig. 1. The flag leaf chlorophyll content mg cm
− 2
in the 1996 growing season.
d
ambient CO
2
ambient O
3
,
j
680 CO
2
ambient O
3
,
s
ambient CO
2
elevated O
3
,
h
680 CO
2
elevated O
3
. Each point represents 10 measurements and bars are ±1 standard error of the mean. denotes a significant dif-
ference between the control and the ambient CO
2
elevated O
3
treatment at p0.01.
3.3. Flag leaf photosynthesis The light saturated rate of flag leaf photosynthe-
sis was measured under ambient light conditions 1800 mmol m
− 2
s
− 1
in 1994 and under artificial light conditions 2000 mmol m
− 2
s
− 1
in 1996. After anthesis in both growing seasons, flag leaf photosyn-
thetic rates were increased in elevated CO
2
p0.01 in ambient O
3
compared with the control Fig. 2, Table 4 irrespective of the timing of exposure. How-
ever, in 1994, from 106 days after sowing onwards this effect is absent. Elevated CO
2
also increased the photosynthetic rate in elevated O
3
compared with the control in both growing seasons, but this effect
persists for a longer period in 1994. The decline in photosynthetic rates in response to elevated O
3
was apparent, irrespective of whether exposure started at
anthesis 1994 or was season-long 1996 Fig. 2. The effect of O
3
became greater as senescence pro- gressed. In the 1994 growing season, the CO
2
× O
3 Table 3
Effects of elevated CO
2
and elevated O
3
on the chlorophyll content mg cm
− 2
of spring wheat flag leaves in the 1996 growing season
a
ANOVA d.f.
Chlorophyll content mg cm
− 2
1996 days after sowing 66
78 85
92 99
106 113
121 CO
2
1 n.s.
n.s. n.s.
n.s. n.s.
n.s. n.s.
n.s. O
3
1 n.s.
n.s. n.s.
n.s. n.s.
p0.05 p0.01
p0.05 CO
2
× O
3
1 n.s.
n.s. n.s.
n.s. p0.01
p0.001 n.s.
n.s.
a
Results of ANOVA test. Note: significance represents an overall treatment effect. Anthesis was recorded at 81 days after sowing. Fig. 2. The percentage change from control ambient CO
2
, am- bient O
3
of the light saturated rate of flag leaf photosynthesis mmol m
− 2
s
− 1
in the 1994 and 1996 growing seasons. Elevated O
3
concentrations were ambient +50 ppbv in 1994 and ambient +
90 ppbv in 1996. 680 CO
2
ambient O
3
, ambi-
ent CO
2
elevated O
3
, 680 CO
2
elevated O
3
. denotes a significant difference between the control and any mean indi-
vidual treatment at p0.01 and ++ denotes a significant differ- ence between the elevated O
3
treatment and any other treatment at p0.01. Anthesis occurred at 81 days after sowing in both the
growing seasons.
interaction was significant from 98 days after sowing onwards but there was no CO
2
× O
3
interaction in 1996 when fumigation was season-long Table 4.
3.4. Stomatal conductance Stomatal conductance to water vapour was mea-
sured in conjunction with photosynthesis. In both the growing seasons elevated CO
2
decreased p0.01 stomatal conductance in ambient O
3
compared with the control Fig. 3, irrespective of the timing of
fumigation. The decline in stomatal conductance in
164 A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168
Table 4 Effects of elevated CO
2
and elevated O
3
on the photosynthetic rate, stomatal conductance and water use efficiency of spring wheat flag leaves
a
ANOVA d.f. Photosynthetic rate mmol m
− 2
s
− 1
Stomatal conductance mmol m
− 2
s
− 1
Water use efficiency 84
98 106
113 84
98 106
113 84
98 106 113
1994 days after sowing CO
2
1 p0.01 p0.001 n.s.
n.s. p0.01 p0.01
n.s. p0.01
n.s. n.s.
n.s. n.s. O
3
1 n.s.
p0.01 p0.01
p0.001 n.s. p0.01
p0.01 p0.001
n.s. n.s.
n.s. n.s. CO
2
× O
3
1 n.s.
p0.05 p0.001 p0.05
n.s. p0.001 p0.001 p0.05
n.s. n.s.
n.s. n.s. 1996 days after sowing
73 102
73 102
73 102
CO
2
1 n.s.
p0.01 p0.01 n.s.
p0.05 p0.01 O
3
1 n.s.
p0.01 n.s.
p0.01 n.s.
p0.05 CO
2
× O
3
1 n.s.
n.s. n.s.
n.s. n.s.
n.s.
a
Results of ANOVA test. Note: significance represents an overall treatment effect
response to elevated O
3
at ambient CO
2
was greater than the decline attributed to elevated CO
2
at the latter stages of senescence Fig. 3. The CO
2
× O
3
in- teraction Table 4 was apparent in the 1994 growing
season only, and was such that the decrease caused by elevated O
3
was greater in ambient CO
2
than in elevated CO
2
.
Fig. 3. The percentage change from control ambient CO
2
, ambi- ent O
3
of the light saturated rate of flag leaf stomatal conductance mmol m
− 2
s
− 1
in the 1994 and 1996 growing seasons. Elevated O
3
concentrations were ambient +50 ppbv in 1994 and ambient +
90 ppbv in 1996. 680 CO
2
ambient O
3
, ambient
CO
2
elevated O
3
, 680 CO
2
elevated O
3
. denotes a sig- nificant difference between the control and any mean individual
treatment at p0.01. Anthesis occurred at 81 days after sowing in both growing seasons.
3.5. Water use efficiency The instantaneous water use efficiency WUE,
of the flag leaves was increased in the presence of elevated CO
2
at both ambient and elevated O
3
irre- spective of the timing of fumigation Fig. 4, Table
4. Elevated O
3
had no significant effect on WUE
Fig. 4. The percentage change from control ambient CO
2
, ambient O
3
of the light saturated rate of flag leaf water use efficiency in the 1994 and 1996 growing seasons. Elevated O
3
concentrations were ambient +50 ppbv in 1994 and ambient +90 ppbv in 1996.
680 CO
2
ambient O
3
, ambient CO
2
elevated O
3
, 680 CO
2
elevated O
3
. denotes a significant difference between the control and any mean individual treatment at p0.01
and ++ denotes a significant difference between the elevated O
3
treatment and any other treatment at p0.01. Anthesis occurred at 81 days after sowing in both growing seasons. Note differences
in scale.
A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168 165
Table 4 in ambient CO
2
except on one occasion in 1996 Day 102 when WUE was decreased by an
average of 23 p0.05 across the CO
2
treatments Table 4.
4. Discussion
