Effect of supplemental irrigation on leaf stomatal conductance of field-grown wheat in northern Syria

Abstract

Stomatal conductance (gs) of field-grown wheat during the transition period from rainy to dry seasons in northern Syria was examined in relation to the vapor pressure deficit of the air (VPDa), solar radiation (RS), and the soil available water (AW) under three irrigation regimes. Midday depression of gs was evident in diurnal observations even with sufficient soil water, and an analysis of the data indicated a significant relationship between gs and AW/VPDa. The seasonal estimates of gs showed an apparent decline under high evaporative demand in all moisture regimes. A theoretical explanation of the observed relationship indicated that the effect of supplemental irrigation on stomatal opening declined toward the end of the planting season. These results suggest that reduced irrigation concentrated in the early stage of VPDa escalation would improve the water use efficiency of wheat grown in this area.

Full text

Effect of supplemental irrigation on leaf stomatal
conductance of field-grown wheat in northern Syria
Takahiro Sato
a,
*, Osman S. Abdalla
b,1
, Theib Y. Oweis
c
, Tetsuo Sakuratani
d
a
Germplasm Program, International Center for Agricultural Research in the Dry Areas, P.O. Box 5466, Aleppo, Syria
b
CIMMYT/ICARDA Joint Dryland Wheat Program for the WANA Region, International Maize and Wheat Improvement Center,
P.O. Box 6641, Mexico D.F., Mexico
c
Natural Resource Management Program, International Center for Agricultural Research in the Dry Areas, P.O. Box 5466, Aleppo, Syria
d
Laboratory of Tropical Agriculture, Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwakecho,
Sakyo-ku, Kyoto 606-8502, Japan
1. Introduction
The climate in northern Syria is classified as a warm
continental Mediterranean climate, with a moisture regime
characterized as semiarid to desert (Kassam, 1981). Water
shortage is the major constraint to agricultural production in
this area (Zhang and Oweis, 1999), and the negative impact of
global climate change stresses on rainfed farming systems will
be exacerbated by ubiquitous land degradation and increasing
water scarcity (Araus, 2004). To avoid water shortages,
supplemental irrigation (SI) is widely practiced (Oweis et al.,
2000). More than 80% of the water resources in this area is
allocated to irrigation (Araus, 2004), and data available from
the statistical database of the Food and Agriculture Organiza-
tion of the United Nations (FAO, 2005) indicate that irrigated
agricultural areas in Syria have dramatically increased during
this decade. The principal source of irrigation is groundwater
in the northern part of the country; however, excessive water
use is promoted because water use per se is not charged
according to use (Oweis et al., 1998). The establishment of
effective irrigation practices is therefore essential for agricul-
tural sustainability in this area.
agricultural water management 85 (2006) 105–112
article info
Article history:
Accepted 28 March 2006
Published on line 15 May 2006
Keywords:
Continental mediterranen climate
Supplemental irrigation
Soil available water
Stomatal conductance
Vapor pressure deficit
abstract
Stomatal conductance (g
s
) of field-grown wheat during the transition period from rainy to
dry seasons in northern Syria was examined in relation to the vapor pressure deficit of the
air (VPD
a
), solar radiation (R
S
), and the soil available water (A
W
) under three irrigation
regimes. Midday depression of g
s
was evident in diurnal observations even with sufficient
soil water, and an analysis of the data indicated a significant relationship between g
s
and
A
W
/VPD
a
. The seasonal estimates of g
s
showed an apparent decline under high evaporative
demand in all moisture regimes. A theoretical explanation of the observed relationship
indicated that the effect of supplemental irrigation on stomatal opening declined toward the
end of the planting season. These results suggest that reduced irrigation concentrated in the
early stage of VPD
a
escalation would improve the water use efficiency of wheat grown in this
area.
#2006 Elsevier B.V. All rights reserved.
*Corresponding author. Present address: Laboratory of Tropical Agriculture, Graduate School of Agriculture, Kyoto University, Kitashir-
akawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan. Tel.: +81 75 753 6353; fax: +81 75 753 6352.
E-mail address: takahiro@kais.kyoto-u.ac.jp (T. Sato).
1
Present address: Integrated Gene Management Mega-Project, International Center for Agricultural Research in the Dry Areas,
P.O. Box 5466, Aleppo, Syria.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/agwat
0378-3774/$ – see front matter #2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2006.03.015

Wheat in Syria is normally sown in November, headed in
April, and harvested in June. Rainfall decreases during March
and April, and only sporadic rainfall and high evaporative
demand are observed after May. SI applied from March to May
can increase wheat yield (Oweis et al., 1998; Zhang and Oweis,
1999), and reduced SI from one-third to two-thirds of the full
crop requirement can maximize the water use efficiency
(WUE) of wheat (Oweis et al., 1998, 2000). However, the
physiological background of this phenomenon remains
poorly understood.
Van den Boogaard et al. (1997) showed that the wheat yield
could be improved by increasing WUE at the leaf level.
Variation in stomatal conductance (g
s
) rather than in the rate
of photosynthesis caused the differences in WUE (Van den
Boogaard et al., 1996). These studies have indicated the
importance of g
s
in terms of wheat productivity. In addition to
the response of g
s
to different SI applications, stomatal closure
under a high vapor pressure deficit (VPD
a
; e.g., Ishihara et al.,
1972; Turner et al., 1984; Mott and Parkhurst, 1991; Monteith,
1995; Franks et al., 1997; Xue et al., 2004) may play an
important role in optimal irrigation practices, because the SI
application in this area is conducted under high evaporative
demand. Some previous papers (Zhang et al., 1998a; Zhang and
Oweis, 1999) have noted the importance of g
s
and VPD
a
on
WUE in northern Syria, but no studies of these factors have
been conducted directly.
In a previous paper (Sato et al., 2006), we demonstrated
the inability of predawn water potential (c
PL
)asan
irrigation-timing indicator of field-grown wheat in this
climate because the c
PL
under mild water stress did not
respond correctly to the soil water condition. Here, we
sought to examine the effect of SI application on the
stomatal behavior of wheat in this area. Based on the direct
measurement of g
s
, the seasonal changes in g
s
under three
different soil moisture regimes are discussed during the
transition from rainy to dry season.
2. Materials and methods
The experiment was conducted in Tel Hadya, northern Syria
(368010N, 368560E), at the main research station of the
International Center for Agricultural Research in Dry Areas
(ICARDA). The long-term average annual rainfall at this
station is 343.7 mm (ICARDA, 2003), and the soil is classified
as Calcixerollic Xerochrept (Ryan et al., 1997). Five cultivars
(Cham4, Cham6, Bloyka1, Qafza8 and Qimma5) of spring bread
wheat (Triticum aestivum L.) were planted in plots measuring
8m1.6 m at a sowing rate of 13.89 g m
2
on 23 December
2001, and 30 kg ha
1
of CO(NH
2
)
2
was applied twice: before
sowing and during tillering. The plots were arranged in a
randomized complete block design with three replications of
three moisture regimes (rainfed, 50% SI, and 100% SI). Plants
were headed during day of year (DOY) 109–115 and harvested
on DOY 178, 2002.
2.1. Weather data and soil available water
Data on solar radiation (R
S
), air temperature (T
air
), relative
humidity (RH), wind speed, rainfall amount (P), and class-A
pan evaporation were collected at a standard weather station
adjacent to the experimental field. The vapor pressure deficit
of the air in the field (VPD
a
) was calculated using the value of
T
air
, RH, and the equation of Murray (1967). The volumetric soil
water content (u) was measured using a neutron moisture
probe (MK II; Didcot, Wallingford, UK). Access tubes were
installed to a depth of 1.35 m in each plot immediately after
sowing, and counts were made at 0.15-m intervals starting at a
depth of 0.225 m. The uof the top 0.15 m was measured
gravimetrically. Measurements were taken 11 times between
DOY 101 and 168.
After harvest, we confirmed that roots had reached a depth
of 1.35 m. However, the vertical distribution of plant roots
normally decreases exponentially (Gerwitz and Page, 1974),
and the contribution of the water taken from the deep soil
layer by the roots to the total plant water uptake may be
recognized only under severe water-stress conditions. Given
the weather conditions during this experiment, we set the
effective soil depth with regard to root water uptake at 0.9 m.
Based on long-term measurements of uat the experimental
site, the minimum and maximum uin each soil layer were
defined in advance; for the 0–90-cm soil layer, the minimum
and maximum uwere 122.0 and 352.2 mm (Oweis, personal
communication, 2002). These values were used to represent
field capacity and permanent wilting point. The soil available
water (A
W
) was calculated by subtracting the above value of
the permanent wilting point from the measured u.
2.2. Supplemental irrigation
A drip irrigation system was used for SI. The A
W
of the top
60 cm soil depth in the 100% SI treatment was conventionally
used to determine the timing and amount of irrigation. In the
100% SI treatment, water was applied to fill the soil profile to
field capacity when the A
W
dropped to 50% of the maximum
A
W
(A
WMax
). Half the amount of water applied to the 100% SI
was applied to the 50% SI treatment, on the same day as the
100% SI. Detailed information on the irrigation practices is
provided by Sato et al. (2006). The amount of water supplied in
the 100% SI treatment was estimated to be 100 mm on 15 April
and 90 mm on 5 May (DOY 105 and 125, respectively).
2.3. Stomatal conductance
Diurnal measurements of stomatal conductance (g
s
)were
made on DOY 118 and 131, corresponding to 13 and 6 days
after SI application, using a steady-state porometer (Li-1600;
Li-Cor, Lincoln, NE, USA). One block in each moisture regime
was selected for sampling to minimize the time difference
between measurements. Two flag leaves in each plot were
sampled approximately every 2 h between 07:00 and 17:00 h.
The obtained values were calculated twice to reflect abaxial
and adaxial leaf surfaces. To examine the relationship
between VPD
a
and g
s
, the vapor pressure deficit of the air
in the sample chamber (VPD
a
-C) was also calculated from RH
and cuvette temperature readings. The sensor head of the
porometer was kept in the shade between measurements to
minimize the difference with open-air conditions. The
measurement time was recorded to take the time lag into
consideration.
agricultural water management 85 (2006) 105–112106

2.4. Seasonal estimation of g
s
and A
W
In addition to the direct measurement of g
s
and A
W
described
above, the simulation of hourly g
s
and A
W
from 2 days after
two SI applications (eliminating the redistribution process in
the 0–90-cm soil layer) was also attempted. Fig. 1 shows a
simulation flow diagram.
According to analyses of A
W
,R
S
, VPD
a
-C, and g
s
, which are
presented in the Results section, an empirical equation to
estimate g
s
was obtained. Hourly g
s
values were estimated by
substituting A
W
,R
S
and VPD
a
into the equation. Evapotran-
spiration (ET) was calculated by the following the model of
Shuttleworth and Wallace (1985). Here, we briefly describe the
model.
ET from the canopy can be expressed as:
ET ¼QE=l¼ðCCPMCþCSPMSÞ=l(1)
where Q
E
is the evaporative heat flux (MJ m
2
), lthe latent heat
of vaporization (MJ kg
1
), and C
C
and C
S
are the model
coefficients given by Shuttleworth and Wallace (1985).PM
C
and PM
S
are the combination equations (Monteith, 1973) for
the canopy and the bare soil under the canopy, respectively,
given by:
PMC¼fðRnGÞþ½rcpVPDaDrbðRns GÞ=ðraþrbÞg=fD
þg½1þrst=ðraþrbÞg (2)
PMS¼fðRnGÞþ½rcpVPDaDrsa ðRnRnsÞ=ðraþrsa Þg=fD
þg½1þrso=ðraþrsa Þg (3)
where R
n
and R
ns
are net radiation above the canopy and on
the soil surface (MJ m
2
), Gis the soil heat flux (MJ m
2
), ris the
mean air density at constant pressure (kg m
3
), c
p
is the
specific heat of the air (MJ kg
1
8C
1
), Dis the slope of the
saturation vapor pressure temperature relationship, and gis
the psychrometric constant (kPa 8C
1
); r
a
,r
b
,r
st
,r
sa
, and r
so
represent canopy aerodynamic resistance (s m
1
), canopy
integrated leaf boundary layer resistance (s m
1
), canopy
integrated stomatal resistance (s m
1
), in-canopy aerody-
namic resistance (s m
1
), and soil surface resistance (s m
1
),
respectively.
The values of R
n
, and Gwere calculated following the
procedure described by Allen et al. (1998).R
ns
is calculated as a
function of leaf area index (LAI) using Beer’s law:
Rns ¼Rnexp ðkLAIÞ(4)
where kis the extinction coefficient (= 0.39; Zhang et al.,
1998a). The LAI was measured on DOY 101, 111, and 134
(seasonal data on leaf area were provided by Sato et al.
(2006). To obtain the daily value of LAI, linear relationships
with DOY were assumed.
Two canopy surface resistance parameters, r
b
and r
st
, are
related to LAI. These values were calculated as:
rst ¼1=ðgsLAIÞ(5)
rb¼rc=LAI (6)
where r
c
is the mean boundary layer resistance of a single leaf,
obtained using the equation of Choudhury and Monteith
(1988). The eddy diffusion resistance values, r
a
and r
sa
, are
calculated following the process described by Shuttleworth
and Wallace (1985).
In estimating r
so
, a threshold type response with A
W
is
assumed: r
so
= 0 until the A
W
declines to 50% of A
WMax
. Under
the condition that A
W
is less than 50% of A
WMax
,r
so
is assumed
to decrease linearly with A
W
, and reaches 2000 (s m
1
) when
A
W
= 0. This is a typical value of fairly dry soil (Shuttleworth
and Wallace, 1985). These assumptions can be expressed as:
rso ¼0;ð0:5AWMax AWAWMaxÞ(7)
rso ¼17:4AWþ2000;ð0AW0:5AWMaxÞ(8)
Obtained ET values were substituted into the following water
balance equation to determine the changes of A
W
in the
effective rooting zone (DA
W
) during the time step:
DAW¼PET D(9)
Where D(mm) represents drainage into the soil layer below
0.9 m. For simplification, we assumed that Dwas driven only
by gravity. Using the equation of Campbell (1974) for unsatu-
rated hydraulic conductivity, D can be expressed as follows:
D¼K¼KSðu=usÞn¼KSfðAWþ122:0Þ=ð900 usÞgn(10)
where K(mm h
1
) is the unsaturated hydraulic conductivity of
the soil, K
S
is the saturated hydraulic conductivity (6 mm h
1
;
Bruggeman, personal communication, 2002), u
s
(=355.5) is the
volumetric water content at saturation, and n(=12.04) is the
value determined by the moisture retention curve (Campbell,
1974).
Finally, DA
W
was added to the current value of A
W
(A
Wt
)to
calculate the value at the next time step (A
Wt+1
):
AWtþ1¼AWtþDAW(11)
agricultural water management 85 (2006) 105–112 107
Fig. 1 – Simulation flow diagram of the hourly g
s
and A
w
.
The equations and model in the diagram are explained in
the text. The input values surrounded by the thick square
are measured values; other values were calculated.

2.5. Statistical analyses
The data were analyzed using computer software (Microsoft
Excel 2000; Excel Toukei, Syakai Joho Service, Tokyo, Japan). As
no differences were evident among cultivars, all data were
simply categorized into the three moisture regimes. Analyses
were conducted using analysis of variance (ANOVA). If the
ANOVA results were significant (P<0.05), differences among
the means were compared using Tukey’s test. Single and
multiple regression analyses were performed using the least
squares method.
3. Results
Seasonal changesin the daily rainfall amount, pan evaporation,
accumulatedR
S
, and daily averagedVPD
a
are presentedin Fig. 2.
agricultural water management 85 (2006) 105–112108
Fig. 2 – (a) Seasonal changes in daily rainfall amount and
pan evaporation, and (b) daily accumulated solar radiation
(R
S
) and average vapor pressure deficit of the air (VPD
a
)in
the experimental field.
Fig. 3 – Seasonal changes in soil available water (A
w
) under
three irrigation regimes. Arrows indicate the days of SI
application. Differences among treatments on each
measurement date for points without letters are not
significant (ANOVA, P= 0.05). Points with the same letter
are not significantly different (Tukey’s test, P= 0.05).
Fig. 4 – Diurnal changes in solar radiation (R
S
) and the vapor pressure deficit of the air (VPD
a
) on (a) DOY 118 and (b) DOY 131;
stomatal conductance (g
s
) on (c) DOY 118 and (d) DOY 131. Vertical bars indicate standard errors.

Intermittent rainfall was observed until DOY 136, and a dry
spell, during which the daily rainfall was less than 1.0 mm,
followed until DOY 170. The increment of pan evaporation was
evidentfrom between DOY 110 and 120.The change in VPD
a
was
synchronized with pan evaporation; however, the incrementof
R
S
was gradual compared to the VPD
a
. The observed A
W
under
three moisture regimes is presented in Fig. 3.SIapplications
were conductedon DOY 105 and 125. A
W
significantly increased
in the two SI treatments, whereas a continuous decline in A
W
was observedin the rainfed treatment. However, sharp declines
in A
W
were also recognized immediately after SI application in
the two treatments. Although the irrigation amount was
different, no significant difference in A
W
was observed among
the three SI treatments from DOY 146. As presented in our
previous paper (Sato et al., 2006), average yields of each
irrigation regime were 375.58 g m
2
in rainfed, 483.26 g m
2
in
50% SI and 589.61 g m
2
in the 100% SI treatment. Yield differed
significantly among the three irrigation regimes (Sato et al.,
2006).
Fig. 4 illustrates the diurnal changes in R
S
, VPD
a
, and g
s
on
DOY 118 and 131. The A
W
of the sampled plot on these 2 days is
shown in Table 1. Significant differences in A
W
between
rainfed and 100% SI treatments were evident on both days, but
the 50% SI treatment differed from the rainfed treatment only
on DOY 118. The weather on both days was clear and showed a
similar diurnal R
S
pattern. However, an apparent difference in
the VPD
a
change was evident between the 2 days. The peak
values of VPD
a
were 1.93 and 3.19 kPa on DOY 118 and 131,
respectively. Diurnal trends of g
s
were similar in all irrigation
regimes on both days; they showed high values in the early
morning, declined sharply toward midday, and progressively
decreased until sunset. The value of g
s
in the 100% SI
treatment was higher than in the other regimes during
midday. A difference between rainfed and 50% SI treatments
was also observed on DOY 134.
A scatter diagram between VPD
a
-C and g
s
between 09:00
and 16:00 h is plotted in Fig. 5. The data were fairly disperse;
however, significant hyperbolic relationships could be recog-
nized under each irrigation regime, and treatment differences
among these regression curves were observed. The equations
of the regression curves were as follows:
Rainfed :gs¼519:4=VPDa-C þ8:0;ðR2¼0:65Þ(12)
50%SI :gs¼348:9=VPDa-C þ85:1;ðR2¼0:36Þ(13)
100%SI :gs¼328:2=VPDa-C þ177:8;ðR2¼0:42Þ(14)
To identify the effect of A
W
on these relationships, the rela-
tionship between the A
W
value divided by VPD
a
-C and g
s
is
plotted in Fig. 6. A significant linear relationship was observed
between these two variables, but no treatment difference was
evident. The regression line for all points, the intercept of
which was forced to zero, was estimated as:
gs¼5:37 AW=VPDa-C;ðR2¼0:33Þ(15)
Considering the effect of light on stomatal opening and using
A
WMax
to make A
W
as a kind of stress index, the relationship
between (R
S
/VPD
a
-C)(A
W
/A
WMax
) and g
s
is examined empiri-
cally in Fig. 7. A strong linear relationship was recognized
between these variables, and no treatment difference was
evident. The regression line was estimated as:
gs¼1:78 ðRS=VPDa-CÞðAW=AWMax Þ;ðR2¼0:63Þ(16)
As explained in Section 2, the A
W
and g
s
values after two SI
applications were estimated using this empirical relationship.
agricultural water management 85 (2006) 105–112 109
Table 1 – Soil available water (A
w
) of the diurnal g
s
measured plots on DOY 118 and 131
A
wa
(mm)
Moisture regimes DOY 118 DOY 131
Rainfed 89.7 b 60.5 b
50% SI 142.4 a 125.6 ab
100% SI 177.8 a 195.2 a
a
Average of five measurements. Means followed by the same
letter are not significantly different at P= 0.05.
Fig. 5 – Relationship between vapor pressure deficit of the
air in the cuvette of the porometer (VPD
a
-C) and stomatal
conductance (g
s
) under three moisture regimes between
09:00 and 16:00 h. Lines indicate the regression curve in
each irrigation regime.
Fig. 6 – Relationship between A
w
/VPD
a
-C and g
s
between
09:00 and 16:00 h. The line indicates the regression line for
all points.

The comparisons between the estimated A
W
and observed A
w
on DOY 118, 124, 131, and 138 are presented in Fig. 8. The
estimated A
W
tended to show higher values than the observed
A
W
; however, the regression coefficient was highly significant
(R
2
= 0.91). The seasonal changes in the estimated g
s
at midday
on a sunny day (average R
S
>500 W m
2
) are shown in Fig. 9.
The difference in g
s
seemed to reflect the amount of SI
throughout this simulation period. However, a sharp decline
in g
s
was also estimated from DOY 115 to 120 in all moisture
regimes, and the values were lower after the second SI than
after the first.
4. Discussion
Although a considerable amount of water (190 mm in 100% SI
and 95 mm in 50% SI) was applied in the two irrigation
regimes, the seasonal changes in A
W
(Fig. 3) indicated that the
irrigation effect on soil water conditions did not continue until
the end of the season. As expressed in the pan evaporation
data (Fig. 2), the evaporative demand increased toward the end
of the planting season. Rapid declines of A
W
in the irrigation
treatments may have been caused by higher evapotranspira-
tion mainly due to the rapid increase in VPD
a
. Significant
differences in wheat yield probably resulted from the
differences in soil water conditions during this period.
Treatment differences in g
s
were observed in the diurnal
measurements, but the midday depression of g
s
was also
evident even under sufficient soil water conditions (Fig. 4).
Such midday declines in g
s
were also observed in field-grown
wheat in China with 600 mm annual rainfall (Zhang et al.,
1998b), and even in a flooded paddy (Ishihara et al., 1972). The
relationship with VPD
a
-C (Fig. 5) indicated that the stomata
were strongly influenced by the VPD
a
. Two explanations for
this response are possible. One is the negative feedback
response, i.e., increased water loss through the stomata
caused a water potential decline of the guard cells; the other
is the feedforward response, i.e., the stomata sensed the air
vapor pressure directly (Franks et al., 1997). Xue et al. (2004)
explained the g
s
movement of field-grown wheat in the United
States as a feedforward response. The mechanism of g
s
in
northern Syria could not be explained with our data, but
apparently, VPD
a
affects g
s
in this climate condition, either
directly or indirectly.
agricultural water management 85 (2006) 105–112110
Fig. 7 – . Relationship between (R
S
/VPD
a
-C)(A
W
/A
WMax
) and
g
s
. The line indicates the regression line for all points.
Fig. 8 – Comparison between the observed A
w
and the
average value of estimated A
w
on DOY 118, 124, 131, and
138 in three irrigation treatments.
Fig. 9 – Seasonal changes in estimated g
s
at midday (averaged between 10:00 and 14:00 h), on sunny days (R
S
exceeded
500 W m
S2
), (a) after the first SI, and (b) after the second SI application. Arrows indicate the days of SI application.

The influence of VPD
a
-C on g
s
varied among the treatments
(Fig. 5). However, the significance of the relationship between
A
W
/VPD
a
-C and g
s
(Fig. 6) indicated that this difference could
have resulted from the difference in A
W
. If we assume that leaf
temperature equals air temperature, transpiration rate (E)ofa
single leaf can be expressed as follows:
E¼gsVPDa(17)
When stomatal closure is caused by A
W
, the potential evapo-
transpiration (E
p
) and Ecan be related as follows (Rizza et al.,
2004):
E¼EpAW=pAWMax (18)
where pexpresses the onset point of A
W
for the stomatal
closure. The combination of Eq. (17) and (18) yields:
gs¼ðEp=pAWmaxÞðAW=VPDaÞ(19)
This is a similar form to the relationship observed in Fig. 6.In
the radiation method of Makkink (1957),E
p
is considered to be
proportional to R
S
. If the temperature effect on E
p
is assumed
to be negligible, E
p
can be replaced by aR
S
, such that:
gs¼ða=pÞðRS=VPDaÞðAW=AWmaxÞ(20)
This is the relationship observed in Fig. 7. Eq. (20) indicates
that g
s
increases according to the increment of R
S
when VPD
a
is
constant. However, under field conditions, R
S
and VPD
a
were
positively correlated, as observed in Fig. 4, although the peak
time was different. The value of R
S
/VPD
a
would be expected to
decrease from morning to evening. As diurnal changes in A
W
were relatively smaller than those in R
S
or VPD
a
, the combina-
tion effect of these variables may have determined the diurnal
pattern of g
s
observed in Fig. 4.
Simulated A
W
tended to show slightly higher values than
the observed A
W
in Fig. 8, however, a highly significant
relationship confirmed the utility of estimating A
W
using the
empirical relationship observed in Fig. 7. The difference
observed in Fig. 8 may have been caused by the under-
estimation of drainage into the deep soil layer. As evidenced in
Fig. 2, the seasonal increment of VPD
a
was more rapid than
that of R
S
; the term R
S
/VPD
a
in Eq. (20) was expected to
decrease toward the end of the planting season. This indicates
that more soil water is needed to maintain high stomatal
conductance under high evaporative demand. The decline in
g
s
after the second SI compared to that after the first SI in Fig. 9
reflects this feature of the climate in northern Syria. Although
the difference in irrigation amount in the 100% SI treatment
was only 10 mm between two SI applications, the g
s
on DOY
130 (5 days after the second SI) was less than half of that on
DOY 110 (5 days after the first SI). Thus, the effect of SI on
stomatal opening declines as vapor pressure rises in this
climate.
As indicated by Van den Boogaard et al. (1996), the variation
in g
s
, rather than the photosynthetic rate, caused the
differences in WUE. A decreased contribution of SI to g
s
and
WUE under high evaporative demand may be a reason why the
maximum WUE was achieved in the reduced SI application in
previous studies (Oweis et al., 1998, 2000). In northern China,
Zhang et al. (1998b) demonstrated that a single irrigation at the
end of the internode elongation period achieved higher WUE
than irrigation applied four times. Based on our results, a
reduced SI application concentrated in the early stage of VPD
a
escalation would allow additional improvement of WUE in
northern Syria.
5. Conclusion
Experiments that examined the SI effect on the g
s
of field-
grown wheat have suggested the large impact of VPD
a
on
diurnal g
s
changes. Seasonal g
s
estimations and theoretical
explanations of the observed relationship indicated that the SI
effect on stomatal opening declined toward the end of the
planting season, and irrigation concentrated in the early stage
of VPD
a
escalation may increase WUE in northern Syria.
Findings from this paper may be applicable to years with
normal weather tendencies in this area, although such
irrigation practices were not conducted in this study. The
accumulation of longer-term field data related to these
findings should establish the optimal irrigation practices for
enhancing WUE.
Acknowledgments
We thank Dr. Adriana Bruggeman for providing the precious
information on saturated hydraulic conductivity of the
experimental station. Also would like to thank for Mr.
Mohamed Hamady, Mr. Ali Haj-Dibo, and the other ICARDA
staff members for their support with the labor and for their
hospitality. This research was conducted when T. Sato stayed
at ICARDA as a Japan Overseas Cooperation Volunteer (JOCV),
which was arranged by the Japan International Cooperation
Agency (JICA).
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