3. Results
3
.
1
. Field experiment During the experiment, the atmospheric evapo-
rative demand ET was appreciably higher than
the 30-year mean, with a maximum value of 7.5 mm and maximum air temperature of 30°C
recorded during the month of July. The watering sequence was of 44, 44 and 37 mm as irrigation
at 33, 46 and 53 DAP, respectively, and of 24, 33 and 22 mm as rainfall at 48, 60 and 70 DAP,
respectively. EC
e
values of the two salinity treatments, at any DAP, were significantly different P 5 0.05
from the control, but not from each other, al- though I
2
shows consistently higher values than I
1
Table 2. After the last irrigation 53 DAP, salinity increased with the advance of the season,
reaching the maximum values of 0.6 dS m
− 1
in I ,
3.1 dS m
− 1
in I
1
and 3.6 dS m
− 1
in I
2
at 69 DAP. Despite the high electrical conductivity of the
irrigation water, the resulting EC
e
was damped by the notable buffering capacity of the soil, in con-
junction with the dilution effect of rainfall. The relatively mild salinity regimes of I
1
and I
2
treatments corresponded to a slight reduction in leaf water potentials C
l
, measured along the season data not shown, as compared with the
control I . The values of C
l
in I ranged between
− 0.85 and − 1.3 MPa. C
l
values of I
1
and I
2
were − 0.1 and − 0.3 MPa significantly lower than I
, respectively, on the average. Most likely, the slight reduction in C
l
was sufficient to deter- mine an appreciable reduction in leaf area index
LAI as illustrated in Fig. 1. The maximum LAI differentiation between
treatments was achieved at 52 DAP when leaf area expansion was 10 and 19 significantly
lower than I for I
1
and I
2
, respectively. There- after, this difference remained about constant un-
til the end of anthesis 69 DAP when I reached
the maximum LAI value of 3.4. Contrary to LAI reduction, the variation in C
l
of all treatments from − 0.85 to − 1.67 MPa did not affect stom-
atal conductance for water vapour g
sw
, mol m
− 2
s
− 1
, as shown in Fig. 2, which remained at very high values, ranging between 0.76 and 1.35 mol
m
− 2
s
− 1
. Fig. 2 clearly shows decreasing C
l
values going from I to I
2
. Furthermore, the se- quence of C
l
from higher to lower values within each treatment evolves with the season.
Six Ac
i
curves per treatment were determined to
derive the
non-stomatal photosynthetic
parameters. Two Ac
i
curves per treatment are illustrated in Fig. 3, to show the largest difference
observed within each treatment. The variability in trends between curves is not related to salinity
variation. This observation is confirmed by the analysis performed on carboxylation efficiency
a
, maximum photosynthetic rate A
max
, CO
2
Table 2 Mean values and standard deviations of EC
e
for the three salinity treatments I
, I
1
, and I
2
during the season DAP
EC
e
dS m
− 1
I
1
I
2
I 1.8 9 0.52
b
0.4 9 0.09
a
44 2.6 9 0.62
b
2.4 9 1.0
b
51 2.1 9 0.68
b
0.5 9 0.04
a
0.6 9 0.03
a
3.1 9 0.77
b
69 3.6 9 1.0
b
Significant differences between treatments, at each DAP, were evaluated by t-test P50.05, n = 3 and are indicated by
lower-case letters. The same letters denote a non-significant difference. To obtain salt concentrations mg l
− 1
, multiply the electrical conductivity values dS m
− 1
by 640.
Fig. 1. Seasonal variation of green leaf area index LAI of sunflower during the field experiment. Vertical bars, standard
deviation.
Fig. 2. Stomatal conductance to water vapour g
sw
in relation to leaf water potential C
l
of sunflower, for all salinity treatments during the field experiment. Slope of regression line non-significantly different from zero.
Fig. 3. Ac
i
response curves of sunflower measured in the field experiment. Two curves are reported for each treatment, representing the largest difference observed.
Table 3 Mean values and standard deviations of the Ac
i
curve parameters a , A
max
, G, R
l
and corresponding c
s
in the leaves, as obtained from the salinity treatments of sunflower, from 46 to 76 days of growth
R
l
A
max
G a
Treatment c
s
mmol m
− 2
s
− 1
mmol mol
− 1
MPa mmol m
− 2
s
− 1
mol m
− 2
s
− 1
50.5 9 4.41
a
80.4 9 10.69
a
16.0 9 1.71
a
0.33 9 0.04
a
I EC
W
= 0.9 dS m
− 2
− 1.15 9 0.06
A
49.9 9 4.82
a
15.3 9 3.13
a
I
1
EC
W
= 7.8 dS m
− 2
− 1.35 9 0.09
Bb
0.32 9 0.07
a
74.3 9 13.44
a
47.5 9 4.60
a
15.2 9 2.49
a
I
2
EC
W
= 15.6 dS m
− 2
− 1.42 9 0.16
Bb
0.33 9 0.04
a
79.6 9 10.43
a
78.1 9 11.24 49.3 9 4.53
Overall −
1.29 9 0.14 0.33 9 0.05
15.5 9 2.39 Significant differences were evaluated by t-test, with lower-case letters indicating P50.05, and upper-case letters indicating
P50.01 n = 6. The same letters denote non-significant difference.
compensation point G and light respiration R
l
parameters, reported in Table 3, along with leaf osmotic potential c
s
. Leaf osmotic potential measured on the same
leaves of the Ac
i
curves, expresses a relative measure of within-leaf salinity status induced by
I
1
and I
2
treatments, in comparison with the con- trol I
. The mean c
s
values of treatments I
1
and I
2
were − 0.20 and − 0.27 MPa lower than that of I
− 1.15 MPa, respectively. The means dif- ference of the two salinity treatments was highly
significant with respect to I P 5 0.01, but not
between each other. The limited reduction in c
s
for I
1
and I
2
compared with I reflected the mod-
erate salinity difference observed in the soil Table 2, and thus the non-significant differences
in photosynthetic parameters between treatments are not surprising.
3
.
2
. Pot experiment Weather conditions during the pot experiment
were relatively warmer than in the field experi- ment, with ET
and air temperature reaching maximum values of 8 mm and 40°C, respectively,
during the month of July. Having purposely chosen a coarse-textured
soil, the electrical conductivity of drainage water EC
dw
was of the same values as irrigation water EC
w
. Five Ac
i
curves were determined one for each salinity
level to
derive the
photosynthetic parameters Fig. 4. Also, in this case, the ob-
served variability between curves is not related to variation in salinity. Measurements of leaf area
LA and c
s
were obtained from the same leaves of the Ac
i
determinations. The results of a ,
A
max
, G, and R
l
, along with the corresponding c
s
and LA, are reported in Table 4. It is worth noticing that, in this case, the re-
duction of c
s
from − 1.35 to − 2.67 MPa reflected adequately the increased salt concentra-
tion in the irrigation water. Nevertheless, the highest salinity level I
4
, with EC
w
almost 15 times greater than the control I
, corresponded only to a twofold decrease in c
s
. The achieved level of c
s
in the leaves of the pot experiment, however, is high as compared with the field ex-
periment statistically different at P 5 0.05. The leaf area, in fact, reduced progressively by 16
for I
1
, 23 for I
2
and I
3
, and 60 for I
4
treat- ment, as compared with the control, while in the
field experiment, the maximum LA reduction was only 19. Conversely, the mean values of all the
photosynthetic parameters did not significantly differ from those of the field experiment.
Fig. 4. Ac
i
response curves measured in the pot experiment for each salinity treatment.
Table 4 Mean values and standard deviations of the Ac
i
curve parameters a , A
max
, G, R
l
, leaf area LA, and corresponding c
s
in the leaves, from each salinity treatment
Treatment a
c
s
A
max
G R
l
LA mol m
− 2
s
− 1
mmol m
− 2
s
− 1
mmol mol
− 1
MPa mmol m
− 2
s
− 1
cm
2
− 1.35
I EC
w
= 0.9 dS m
− 1
0.28 63.7
48.8 13.1
94.0 0.32
80.2 52.1
− 2.05
15.7 I
1
EC
w
= 3.9 dS m
− 1
79.4 0.31
84.0 46.2
I
2
EC
w
= 7.8 dS m
− 1
13.9 −
2.15 72.6
0.33 73.7
53.3 −
2.23 16.0
I
3
EC
w
= 11.7 dS m
− 1
72.0 0.29
79.0 56.2
15.6 38.1
I
4
EC
w
= 15.6 dS m
− 1
− 2.67
0.31 76.1
51.3 −
2.09 14.9
Mean 71.2
S.D. 0.02
0.48 7.86
3.90 1.29
20.52
3
.
3
. Photosynthetic response and limitations for the whole data set
The assimilation response and the parameters of the limiting components in relation to osmotic
potential are reported in Fig. 5, combining the results of both field and pot experiments. Maxi-
mum values of net assimilation rates Fig. 5a ranging around 40 – 44 mmol m
− 2
s
− 1
were ob- tained at high values of c
s
around − 1.2 MPa reached in the field. Although A tends to decrease
with decreasing c
s
, the regression slope is not significantly different from zero. In fact, minimum
A values about 29 mmol m
− 2
s
− 1
are obtained both at relatively high − 1.6 MPa and low
− 2.6 MPa c
s
values. The regression of A versus c
s
is paralleled by the regression of g
sc
versus c
s
Fig. 5b, because of the strict link between A and g
sc
as illustrated in Fig. 6. This linear relationship implies the con-
stancy of c
i
Fig. 5d as well as of water-use efficiency Fig. 5e when normalized for VPD
Steduto, 1996. The c
i
values remained constant at about 200 mmol mol
– 1
S.D. = 12.5 over the whole c
s
range. The mean value of WUE
N
was constant mostly around 5 mmol mmol
− 1
kPa S.D. = 0.7 over the whole c
s
range and it is close to that found by other authors Rawson and
Constable, 1980; Lauteri et al., 1992. Regarding the role played by stomatal and non-stomatal
components of photosynthesis, there is no varia- tion in g with c
s
Fig. 5b, which reflected the absence of variation in non-stomatal limitation
l
m
to assimilation Fig. 5c. Values of g ranged between 0.12 and 0.20 mol m
− 2
s
− 1
three- to fivefold lower than g
sc
, resulting in l
m
between 72
Fig. 5. Variation of a assimilation rate A, b stomatal g
sc
and non-stomatal g conductances for CO
2
, c stomatal l
s
and non-stomatal l
m
limitations to photosynthesis, d inter- nal CO
2
concentration c
i
and e normalized photosynthetic water use efficiency WUE
N
of sunflower at ambient CO
2
concentration, with variation in leaf osmotic potential c
s
merging both field and pot experiments.
Fig. 6. Relationship between assimilation rate A and stom- atal coductance for CO
2
g
sc
at ambient CO
2
concentration merging both field and pot experiments with
sunflower.
1999, which adjusts its canopy size to water availability, while maintaining adequate cellular
turgour pressure Turner et al., 1978; Gollan et al., 1986, which keeps stomata wide open. This
behaviour seems to be related to the high elastic- ity of sunflower cell walls Jones and Turner,
1980, representing one of the stress avoidance mechanisms Connor and Sadras, 1992; Merrien,
1992 allowing one to cope with environmental stresses. The almost non-existent response of
stomatal conductance to C
l
Fig. 2 resembles what has been reported by other authors for
water stress experiments for example, Rawson, 1979; Connor and Sadras, 1992; Plesnicar et al.,
1995. The observed conductances are typical of sunflower for example, Katerji et al., 1996;
Pankovic et al., 1999, which exhibits some of the highest g
sw
values encountered in the literature for field crops Ko¨rner et al., 1979 due to large
stomatal size and density Merrien, 1992. The reduction in C
l
down to − 1.6 MPa Fig. 2 did not induce any significant reduction in
assimilation rate whose maximum values are typi- cal of sunflower under optimal water and nutri-
tional status Tezara and Lawlor, 1995; Connor and Hall, 1997. In contrast with these results,
Lawlor 1995, in reviewing the effects of water deficit on photosynthesis, found that sunflower
showed a 75 reduction in assimilation at the same C
l
value. However, the findings of this author refer to severe water stress developed dur-
ing a few days. Conversely, some authors for example, Connor et al., 1985; Connor and
Sadras, 1992 found low sensitivity of assimila- tion capacity to C
l
under mild water stress in the field with a gradual increase in water deficit. All
these facts
highlight the
relevance of
the boundary conditions within which stress experi-
ments are carried out. The photosynthetic apparatus was not influ-
enced by salinity, as also clearly shown by the absence of any effect on the Ac
i
curve parame- ters Tables 3 and 4, and Figs. 3 and 4 and by
the remarkably constant g Fig. 5b. While G values are typical of C
3
plants, a and A
max
are consistent with the values found by other authors
for example, Jacob and Lawlor, 1991; Gimenez et al., 1992 and well above those of most C
3
and 87, as opposed to l
s
values between 13 and 27, with both limitations showing slight sloping
regressions with c
s
not significantly different from zero.
4. Discussion
Kategori