Agriculture, Ecosystems and Environment 80 2000 159–168
Elevated CO
2
provides protection from O
3
induced photosynthetic damage and chlorophyll loss in flag leaves of spring wheat
Triticum aestivum L., cv. ‘Minaret’
Alison Donnelly
a,∗
, Mike B. Jones
a
, James I. Burke
b
, Bert Schnieders
b
a
Botany Department, Trinity College, University of Dublin, Dublin 2, Ireland
b
Teagasc, Agricultural and Food Development Authority, Oak Park Research Centre, Carlow, Ireland Received 21 July 1999; received in revised form 11 January 2000; accepted 3 February 2000
Abstract
Spring wheat Triticum aestivum L., cv. ‘Minaret’ was grown in open-top chambers and exposed to two CO
2
concentrations ambient and 680 ppmv and two O
3
concentrations ambient and ambient +50 or +90 ppbv either from anthesis onwards or for the entire growing season. The aim of the experiment was to test whether elevated CO
2
could provide ‘protection’ to the crop against the damaging effects of elevated O
3
on flag leaf chlorophyll content and photosynthetic activity. The decline in flag leaf chlorophyll content following anthesis was unaffected by elevated CO
2
. However, the reduction in chlorophyll content brought about by elevated O
3
was less in elevated compared with ambient CO
2
. The rate of flag leaf photosynthesis was increased in elevated CO
2
and decreased in elevated O
3
. When elevated CO
2
and elevated O
3
were combined the decrease attributed to O
3
was smaller than in elevated O
3
at ambient CO
2
. These results confirm that elevated CO
2
provides some protection to a wheat crop against the damaging effects of O
3
on photosynthetic activity and chlorophyll content. However, the degree of protection varies between growing seasons and also appears to be related to the timing of exposure to elevated
O
3
. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Elevated CO
2
; Elevated O
3
; Chlorophyll; Gas exchange; Triticum aestivum; Ireland
1. Introduction
The tropospheric concentrations of both carbon dioxide CO
2
and ozone O
3
are increasing because of human activity IPCC, 1996. Photosynthesis is
the major physiological process by which plants re- spond directly to changes in the concentration of
∗
Corresponding author. Present address: Division of Environmen- tal Science, School of Biological Sciences, University of Notting-
ham, Sutton Bonington Campus, Nr Loughborough, Leicestershire, LE12 5RD, UK. Tel.: +44-115-951-6349; fax: +44-115-951-6247.
E-mail address: alison.donnellynottingham.ac.uk A. Donnelly
atmospheric gases Long and Drake, 1992; Long et al., 1993. It is generally accepted that any short
term increase in atmospheric concentrations of CO
2
will increase the rate of photosynthesis of C
3
plants as their photosynthetic apparatus is not fully saturated
with CO
2
under present day concentrations Long, 1991; Webber et al., 1994. In addition, stomatal con-
ductance tends to be lower in elevated CO
2
because of partial stomatal closure Morison and Gifford, 1984
and so the water use efficiency WUE of the plant is increased as less water is lost through transpiration
while more CO
2
is fixed in photosynthesis Farquhar and Sharkey, 1982; Sage and Sharkey, 1987.
0167-880900 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 0 0 0 0 1 3 7 - 7
160 A. Donnelly et al. Agriculture, Ecosystems and Environment 80 2000 159–168
Elevated concentrations of O
3
in the atmosphere tends to decrease the photosynthetic rate of C
3
plants Amundson et al., 1987; Lehnherr et al., 1988
either by inducing stomatal closure Heath, 1994 and restricting CO
2
entry to the leaf or by entering the intercellular spaces of the leaf mesophyll and disrupt-
ing cell metabolism Heath, 1980. Cell metabolism is disrupted by the activated oxygen species which
are produced as O
3
instantaneously dismutases on entry into the leaf intercellular spaces. Subsequent
metabolism of O
3
-derived activated oxygen species is dependent on the activities of several antioxidant
enzymes Rao et al., 1995. The effect of elevated O
3
on cell metabolism is frequently associated with increased rates of leaf senescence and the loss of
chlorophyll Grandjean and Fuhrer, 1989; Fangmeier et al., 1994; Finnan et al., 1998; Ojanperä et al.,
1998, thus reducing the duration for photosynthetic activity and resulting in a reduction in growth and
yield Soja and Soja, 1995; Finnan et al., 1996.
The mechanisms which explain the interactive effects of both elevated CO
2
and elevated O
3
on pho- tosynthetic processes in leaves are, as yet, unclear.
One proposal suggests that elevated CO
2
induces partial stomatal closure and so reduces the effective
dose of O
3
reaching the photosynthetic apparatus within the plant Allen, 1990; McKee et al., 1995. A
second proposal, suggested by McKee et al. 1995 and Mulholland et al. 1997, is that an increase in
the amount of in vivo active Rubisco at elevated CO
2
may provide some compensation for damage caused by moderately elevated O
3
. A third proposal from Rao et al. 1995, suggests that increased production
of antioxidants in the leaf mesophyll at elevated lev- els of CO
2
, which increase the rate of destruction of active O
3
as it enters the leaf, may account for the apparent protection from ozone damage.
Experiments were carried out as part of the Envi- ronmental Stress Physiology and Climate Change Ex-
periment on wheat ESPACE-wheat project of the EU Environment and Climate Programme to test whether
elevated CO
2
protects spring wheat flag leaves from a ozone induced chlorophyll loss and b ozone in-
duced damage to the photosynthetic apparatus, and that this is primarily the result of partial stomatal
closure at elevated CO
2
which reduces O
3
flux into the leaf. In this experiment, spring wheat Triticum
asetivum L., cv. ‘Minaret’ was grown in open-top
Table 1 Timetable of events during the 1994 and 1996 growing seasons
Event 1994
1996 Planting
6 April 1 April
Fertilisation NPK 5 April
14 March Fertilisation CAN
3 June 30 April
Insecticide application
a
28 May, 11 July 3 May, 7 June
Fungicide application
b
28 May, 24 June 7 and 18 June
Final harvest 25 August
12 August
a
Insecticide: oxydemeton-methyl 420 mls ha
− 1
.
b
Fungicide: flutriafolchlorothalonil 1.5 l ha
− 1
.
chambers at a site in Carlow, Ireland and exposed to combinations of ambient and elevated CO
2
and O
3
. The chlorophyll content, photosynthetic activity and
stomatal conductance of flag leaves of spring wheat plants exposed to elevated CO
2
and elevated O
3
, singly and in combination, were examined over two
growing seasons.
2. Materials and methods
