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
The 1994 growing season had the lowest AOT40’s primarily because of dull weather conditions and as-
sociated low ambient O
3
conditions, and the fact that elevated O
3
exposure did not begin until anthesis 27 June. In 1996 the elevated AOT40 was higher than in
1994 as a result of a combination of climatic condi- tions favouring O
3
production and the long duration of exposure.
Flag leaf chlorophyll content declined from the on- set of senescence and continues to do so throughout
the life of the leaf. The chlorophyll content of the flag leaf was unaffected by elevated CO
2
concentra- tions during the 1996 growing seasons. This result is
consistent with those of Sage et al. 1989, Delgado et al. 1994, Nie et al. 1995 and Mulholland et al.
1997, all of whom found that long term exposure of wheat to elevated concentrations of CO
2
resulted in no significant changes in the chlorophyll content of the
leaves. However, this lack of response is not universal as Greiner de Mothes and Knoppik 1994 reported
an increase in the chlorophyll content whereas Tuba et al. 1994 and Sicher and Bunce 1997 reported a
decrease in the chlorophyll content of winter wheat leaves grown at elevated CO
2
. The light saturated rate of flag leaf photosynthesis
was increased in elevated CO
2
at both ambient and elevated O
3
, irrespective of whether or not fumiga- tion began at anthesis 1994 or was season long
1996. This increase in photosynthetic rate has been reported by many other authors across a wide range
of plants reviewed by Lawlor and Mitchell, 1991; Idso and Idos, 1994. The results of the present study
agree with Mulholland et al. 1997 who reported an increase of up to 100 in the net rate of photosynthe-
sis of all leaves including flag leaf, of spring wheat Triticum aestivum L., cv. ‘Minaret’ in response to
elevated CO
2
680 ppmv. Donnelly et al. 1998 also reported a stimulation in the photosynthetic rate
of juvenile wheat plants Triticum aestivum L., cv. ‘Minaret’ in response to the elevated CO
2
treatment. The increased rates of photosynthesis, in the present
study, were maintained throughout the lifetime of the flag leaf, when fumigation was season long, support-
ing the findings of Arp 1991 that down regulation in photosynthetic activity is uncommon in field grown
wheat with unrestricted rooting volumes. Grain yield was also increased in response to elevated CO
2
in both 1994 and 1996 Donnelly et al., 1999 as a result
of an increase in the individual grain weight which can be attributed to an increase in photosynthetic
rates. Elevated CO
2
also stimulated grain yield in 1996 because of an increase in the number of grains
per ear. However, in 1994, because fumigation with CO
2
began after grain number had been determined, elevated CO
2
did not affect this parameter. The decrease in stomatal conductance observed
under elevated CO
2
is consistent with that observed in wheat and other plants grown under elevated atmo-
spheric CO
2
Cure and Acock, 1986; Morison, 1987; Eamus and Jarvis, 1989. A decrease in stomatal con-
ductance results in a decrease in transpiration rates in plants grown in elevated CO
2
Morison and Gifford, 1984 which in turn results in an increase in instanta-
neous WUE Rogers et al., 1983. Tuba et al. 1994 attributed a greater water use efficiency in wheat
plants, grown from emergence in elevated CO
2
, to both a decrease in transpiration rates at lower stom-
atal conductance and an increase in photosynthetic rates. In the present study, the greater WUE recorded
in elevated CO
2
was also attributable to an increase in the photosynthetic rate and to a decrease in stom-
atal conductance. The increase in WUE in association with elevated atmospheric CO
2
concentrations may be of great importance in terms of growth and yield
for wheat and other plants grown in arid and semi-arid regions Sinoit et al., 1980; Tuba et al., 1993. Under
Irish climatic conditions it is unlikely that a higher WUE would be of any benefit unless summer rainfall
declines markedly under climatic change conditions.
In contrast to the lack of effect of elevated CO
2
on the chlorophyll content of the flag leaves, elevated O
3
exposure clearly lead to decreased chlorophyll con- tent. The chlorophyll content of the leaves declined
with age as anticipated, but the rate of decline was faster in leaves exposed to elevated O
3
. The increased rate of leaf senescence in elevated O
3
at ambient CO
2
can be attributed to the higher O
3
flux into the leaves at ambient CO
2
. The effect of this was to reduce the
166 A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168
leaf chlorophyll content and reduce photosynthate pro- duction and subsequently grain yield Finnan, 1995;
Finnan et al., 1997. Nie et al. 1993 reported a similar result but they distinguished between the leaf tip re-
gion which showed the greatest decline in chlorophyll content and the basal area which showed no difference
in chlorophyll content between leaves exposed to ele- vated O
3
and those exposed to ambient O
3
concentra- tions. They concluded that exposure to O
3
during leaf development had no effect on chloroplast development
or on photosynthetic competence but that O
3
affects the photosynthetic capacity indirectly by inducing the
onset of chlorophyll loss accelerated leaf senescence. The decrease in net photosynthesis of wheat flag
leaves when exposed to elevated concentrations of O
3
is well documented Reich and Amundson, 1985; Lehnherr et al., 1988. In the present experiment, the
decrease in light saturated photosynthesis of spring wheat flag leaves at elevated concentrations of O
3
was coupled with a decrease in chlorophyll content. The decrease in photosynthetic rates at elevated O
3
and ambient CO
2
was apparent towards the end of the growing season when fumigation was season-long.
Donnelly et al. 1998 reported no effect of elevated O
3
on the photosynthetic rate of leaf three of wheat plants, which suggests that it has a greater effect on longer
living leaves when the crop approaches maturity. There have been few reports on the interactive
effects of elevated CO
2
and elevated O
3
on wheat grown to maturity under field conditions. In the 1994
growing season, when fumigation began at anthesis, there was a distinct interaction between elevated CO
2
and elevated O
3
whereby the damage attributed to ele- vated O
3
was greater at ambient than elevated CO
2
. In contrast to this, there was no interaction observed be-
tween CO
2
and O
3
in the 1996 growing season when fumigation was season-long and elevated O
3
caused proportionally similar damage at ambient and elevated
CO
2
. This was despite the O
3
AOT40 being consider- able higher in 1996. In 1994, elevated CO
2
provided partial protection against O
3
damage with respect to rates of light saturated photosynthesis and accelerated
leaf senescence. Rudorff et al. 1996 suggest that CO
2
may compensate the negative effects of elevated O
3
by reducing photorespiration and consequently in- creasing the photosynthetic rate. Nevertheless, in the
present work, as with McKee et al. 1995, elevated CO
2
probably afforded protection against the damag- ing effects of elevated O
3
through reducing stomatal conductance, thus reducing O
3
flux into the leaf. How- ever, it should be noted that Mulholland et al. 1997,
found that the partial stomatal closure observed in response to elevated CO
2
resulted in little difference in the O
3
flux to the leaves at ambient CO
2
and ele- vated CO
2
. In the present experiment, fumigation did not start until anthesis in 1994, so that the exposure
duration was greatly reduced. This, combined with the lower concentration of elevated O
3
used in this particular year, may have allowed the elevated CO
2
to protect against the damaging effects of elevated O
3
. In 1996 the elevated O
3
concentration was increased from ambient +50 ppbv to ambient +90 ppbv. In this
year, elevated CO
2
provided less protection against O
3
damage to photosynthesis. When elevated CO
2
was present at high O
3
, photosynthetic rates were similar to that of the control plants but less than that at
elevated CO
2
in ambient O
3
, indicating the partially protective role of elevated CO
2
. The most consistent parameter affected by elevated
atmospheric O
3
was the chlorophyll content of the flag leaf. Mulholland et al. 1997 also found that the
chlorophyll content of the flag leaves declined faster in elevated O
3
than at elevated CO
2
. The protection afforded by elevated CO
2
against O
3
damage ensured that the leaves remained green for a longer duration,
thus providing an extended period before chlorophyll is lost through enhanced senescence. However, this
protection was not always sustained, as when the concentration of O
3
was increased. In conclusion, this work clearly shows that elevated
CO
2
partially protects spring wheat flag leaves from O
3
induced chlorophyll loss and reductions in photo- synthesis. However, the duration of exposure and the
concentration of O
3
is crucial to the degree of protec- tion afforded by CO
2
. Elevated CO
2
showed the same proportionate effect at ambient and elevated O
3
only when exposure was season-long but on no occasion
did CO
2
fully protect the chlorophyll content or the photosynthetic rates from O
3
damage.
Acknowledgements
This work was funded by the EU Environmental and Climate Programme, Contract No. EV5V-CT93-0301.
A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168 167
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