3. Results
3
.
1
. Visible injury No typical visible symptoms of O
3
injury were observed during the study, and leaves showed no
signs of senescence within the measurement period.
3
.
2
. Diurnal gas exchange Fig. 1 shows the effects of O
3
on the rate of CO
2
exchange of the youngest fully expanded leaf following 28 and 42 days fumigation. After 28
days, a decline P B 0.05 in the rate of CO
2
assimilation A was found in the O
3
-treated
Fig. 2. Impact of O
3
on leaf stomatal conductance g
s
of P. major. Values represent the mean of between nine and ten
measurements made on the youngest fully expanded leaf, after 28 a and 42 b days exposure to either CFA open symbols
or O
3
15 nmol mol
− 1
overnight rising to a maximum of 75 nmol mol
− 1
between 12:00 and 16:00 h, shaded symbols. Bars represent the S.E. The thickened parts of the x-axis
indicate the dark period. Significant differences from CFA are indicated: P B 0.05.
Fig. 1. Impact of O
3
on leaf CO
2
assimilation rate A of P. major. Values represent the mean of between nine and ten
measurements made on the youngest fully expanded leaf, after 28 a and 42 b days exposure to either CFA open symbols
or O
3
15 nmol mol
− 1
overnight rising to a maximum of 75 nmol mol
− 1
between 12:00 and 16:00 h, shaded symbols. Bars represent the S.E. The thickened parts of the x-axis
indicate the dark period. Significant differences from CFA are indicated: P B 0.05.
plants. The effect became significant 5 h into the daily period of fumigation and persisted after the
O
3
concentration in the chambers returned to basal night-time levels. In contrast, no significant
effects of the pollutant on A were found after 42 days exposure to O
3
. During the dark period, exposure to O
3
resulted in significantly P B 0.05 enhanced rates of dark respiration R
d
after 28 days, but there were no statistically significant
changes in R
d
after 42 days exposure. Stomatal conductance g
s
, see Fig. 2 was sig- nificantly P B 0.01 reduced by O
3
after 28 and 42 days fumigation − 19 and − 9, respec-
tively. During the dark period, g
s
remained lower in O
3
-exposed plants than in equivalent plants maintained in CFA 28 days: − 45, P B 0.001;
Fig. 3. Impact of O
3
on leaf ethanol soluble and water soluble carbohydrate, starch and total non-structural carbohydrate content of P. major. Values represent the mean of ten measurements made on the youngest fully expanded leaf at 07:00 h a, c and at 21:00
h b, d after 28 a, b and 42 c, d days exposure to either CFA open columns or O
3
B 15 nmol mol
− 1
overnight rising to a maximum of of 75 nmol mol
− 1
between 12:00 and 16:00 h, shaded columns. Bars represent the S.E. Significant differences from CFA are denoted: P B 0.05, P B 0.01, P B 0.001.
42 days: − 35, P B 0.01. However, the response of g
s
to O
3
was not significantly affected by plant age.
3
.
3
. Diurnal leaf carbon budgets Fig. 3 shows the effects of O
3
fumigation on leaf non-structural carbohydrate. The major form
of carbohydrate accumulated over the photope- riod was found to be starch. After 28 days, O
3
- treatment reduced P B 0.05 the ethanol-soluble
carbohydrate fraction in leaves − 28 and caused the enhanced P B 0.05 accumulation of
starch + 21. These effects persisted overnight. Ozone fumigation decreased the level of water-
soluble carbohydrates in the leaf at dawn − 48, P B 0.01. However, no significant difference was
found at 21:00. After 42 days fumigation, dawn levels of starch were reduced in O
3
fumigated leaves − 32, P B 0.01, and at dusk the water-
soluble carbohydrate level was decreased by O
3
− 39, P B 0.001. The partitioning and relative allocation of
newly fixed carbon to export and storage in the light and the relative cost of dark respiration are
shown in Tables 1 and 2, respectively. After 28 days fumigation, net CO
2
fixation was reduced P B 0.05 by 5 over the whole photoperiod in
the O
3
-treatment. There was a significant decline
Table 1 Impact of O
3
on the daily partitioning of photosynthetically fixed carbon to export, storage and respiration of P. major
a
Partitioning of fixed carbon Dark respira-
Storage in Export in light
tion light
28 days 8.3
CFA 61.8
38.1 11.5
52.3 47.8
O
3
42 days 73.9
CFA 26.1
12.1 O
3
66.5 12.2
33.5
a
Values represent the mean of between six and ten measure- ments made on the youngest fully expanded leaf, after 28 and
42 days exposure to either CFA or O
3
15 nmol mol
− 1
overnight rising to a maximum of 75 nmol mol
− 1
between 12:00 and 16:00 h. Data are expressed as a percentage of the
net carbon fixed by leaves over the photoperiod 07:00–21:00 h.
Table 2 Impact of O
3
on net carbon fixation, carbohydrate export, storage in the light and consumption by dark respiration of P. major
a
Carbon allocation g [CH
2
O] m
− 2
Export in light Storage in light
Net fixation Dark respiration
28 days 7.72
b
4.76
b
12.49
b
1.04
a
CFA 6.19
a
5.66
c
O
3
1.36
b
11.84
a
42 days CFA
12.40
b
9.16
c
3.24
a
1.50
b
8.40
c
4.24
a
12.64
b
1.54
b
O
3
Main effects O
3
Age Ns
ns ns
O
3
× age
a
Values represent the mean of between six and ten measurements made on the youngest fully expanded leaf, after 28 and 42 days exposure to either CFA or O
3
15 nmol mol
− 1
overnight rising to a maximum of 75 nmol mol
− 1
between 12:00 and 16:00 h. Within column, means bearing the same superscript are not significantly different at the 5 level. Significant effects are denoted:
PB0.05. PB0.01.
PB0.001.
Table 3 Impact of O
3
on shoot and root dry weight, relative growth rate R and allometric rootshoot growth K of P. major
a
Shoot
CFA
DW, g Growth age
Shoot Root
CFA
DW, g Root
R
CFA
R K
CFA
K 39
0.094 9 0.01 37
1.43 9 0.01 28 days
83 0.441 9 0.025
0.98 9 0.01 99
91 days 10.7 9 0.4
67 1.55 9 0.16
74 0.35 9 0.01
117 0.87 9 0.03 108
Main effects O
3
ns Age
ns O
3
× age
ns
a
Values represent the mean of ten measurements made after 28 and 91 days exposure to either CFA or O
3
15 nmol mol
− 1
overnight rising to a maximum of 75 nmol mol
− 1
between 12:00 and 16:00 h. Shoot is Shoot
O
3
Shoot
CFA
×100, Root is Root
O
3
Root
CFA
×100, R is R
O
3
R
CFA
×100, K is K
O
3
K
CFA
×100. R and K for 91 days represent the relative growth rate from 28 to 91 days. Significant effects are denoted:
PB0.05. PB0.01.
PB0.001.
P B 0.001 in the amount and relative proportion of the fixed carbon exported during the day and
an increase P B 0.05 in the amount and relative proportion of starch stored in leaves during the
photoperiod. There was also a significant P B 0.05 increase in the utilization of carbohydrate
associated with R
d
. However, after 42 days, O
3
exposure resulted in no significant effects on car- bon allocation.
3
.
4
. Plant growth and resource allocation Table 3 provides a summary of the effects of O
3
on growth and dry-matter partitioning following
28 and 91 days exposure to O
3
. Shoot and root biomass was found to be markedly P B 0.001
reduced by O
3
at both harvests, but effects were found to be less pronounced after 91 days expo-
sure to O
3
in comparison with those after 28 days O
3
× plant age, P B 0.001. Since plant dry
weight represents the cumulative effects of the pollutant over the whole growth period, the rela-
tive growth rate of the plant R was calculated between the two harvests to enable a comparison
of the relative effects of O
3
over the first 28 days and the subsequent 63 days. These data indicated
that R declined substantially P B 0.001 with plant age and that there was a contrasting effect
of O
3
with plant age O
3
× plant age, P B 0.001;
O
3
exposure resulted in a 17 decline in R over the first 28 days of the fumigation, but caused a
17 stimulation in R over the subsequent 63 days. At neither stage of growth was O
3
found to significantly change the balance between root and
shoot growth K.
3
.
5
. Effects of ozone on reproducti6e structures Table 4 shows the effects of O
3
on potential reproductive capacity. Ozone was found to signifi-
cantly reduce the number of flower spikes P B 0.05 and the number of capsules per plant
P B 0.001, resulting in a significant P B 0.001 reduction the number of seeds produced per
plant. There was no significant effect of O
3
on the average number of seeds per capsule or seed dry
weight. Also, the weight of seed produced per plant relative to the dry weight of whole plant
reproductive effort was not significantly affected by exposure to O
3
.
4. Discussion
Kategori