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Yield and water-production functions of two durum
wheat cultivars grown under different irrigation and
nitrogen regimes
Fadi Karam
a,
*, Rabih Kabalan
b
, Joeˆlle Breidi
b
, Youssef Rouphael
c
, Theib Oweis
d
a
Lebanese Agricultural Research Institute, Department of Irrigation and Agro Meteorology, P.O. Box 287, Zahleh, Lebanon
b
Lebanese Agricultural Research Institute, Department of Plant Breeding, P.O. Box 287, Zahleh, Lebanon
c
Department of Crop Production, Faculty of Agricultural and Veterinary Sciences, Lebanese University, Dekwaneh-Al Maten, Lebanon
d
International Center for Agricultural Research in the Dry Areas, P.O. Box 5466, Aleppo, Syria
agricultural water management 96 (2009) 603–615
article info
Article history:
Received 29 July 2008
Accepted 27 September 2008
Published on line 5 November 2008
Keywords:
Irrigation strategy
Nitrogen rate
Supplemental irrigation
Triticum durum L.
Vapor pressure deficit
Water use efficiency
abstract
Wheat (Triticum durumL.) yields in the semi-arid regions arelimited by inadequatewater supply
late in the cropping season. Planningsuitable irrigationstrategy and nitrogenfertilization with
the appropriate crop phenology will produce optimum grain yields. A 3-year experiment was
conducted on deep, fairly drained clay soil, at Tal Amara Research Station in the central Bekaa
Valley of Lebanon to investigate the response of durum wheat to supplemental irrigation (IRR)
and nitrogen rate (NR). Three water supply levels (rainfed and two treatments irrigated at half
and full soil water deficit) were coupled with three N fertilization rates (100, 150 and
200 kg N ha
1
) and two cultivars (Waha and Haurani) under the same cropping practices
(sowing date, seeding rate, row space and seeding depth). Averaged across N treatments
and years, rainfed treatment yielded 3.49 Mg ha
1
and it was 25% and 28% less than half
and full irrigation treatments, respectively,for Waha, while for Haurani the rainfed treatment
yielded 3.21 Mg ha
1
, and it was 18% and 22% less than half and full irrigation,respectively. On
theother hand, Nfertilizationof 150 and 200 kg N ha
1
increasedgrainyieldinWahaby 12%and
16%,respectively,in comparison with Nfertilizationof 100 kg N ha
1
,whilefor cultivarHaurani
the increases were 24% and 38%, respectively. Regardless of cultivar, results showed that
supplemental irrigation significantly increased grain number per square meter and grain
weight withrespect to the rainfed treatment, while nitrogenfertilization was observed to have
significanteffects only on grainnumber per square meter. Moreover, results showed thatgrain
yield for cultivar Haurani was less affected by supplemental irrigation and more affected by
nitrogen fertilization than cultivar Waha in all years. However, cultivar effects were of lower
magnitude compared with those of irrigation and nitrogen. We conclude that optimum yield
was producedfor both cultivars at 50% of soilwater deficit as supplemental irrigation and N rate
of 150 kg N ha
1
. However, Harvest index (HI) and water use efficiency (WUE) in both cultivars
were not significantly affected neither by supplemental irrigation nor by nitrogen rate. Evapo-
transpiration (ET) of rainfed wheat ranged from 300 to 400 mm, while irrigated wheat had
seasonalET ranging from 450 to 650 mm. On the other hand, irrigationtreatments significantly
affected ET after normalizing for vapor pressure deficit (ET/VPD) during the growing season.
Supplemental irrigation at 50% and 100% of soil water deficit had approximately 26 and
52 mm mbar
1
more ET/VPD, respectively, than those grown under rainfed conditions.
#2008 Elsevier B.V. All rights reserved.
*Corresponding author. Tel.: +961 8 90 00 37; fax: +961 8 90 00 77.
E-mail address: fkaram@lari.gov.lb (F. Karam).
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/agwat
0378-3774/$ – see front matter #2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2008.09.018
1. Introduction
Water is considered the most limiting factor for cereal
production in the central plains of the Bekaa Valley of
Lebanon with typically dry Mediterranean climate. Environ-
mental conditions are characterized by adequate amounts of
rainfall during winter (December to February) while few
precipitation events are registered in spring from mid-March
to mid-May. However, the unfavorable distribution of rain
over the growing season and the year-to-year fluctuations
constitute a major constraint to wheat growth and yield.
Under these conditions, wheat plants generally suffer a
midseason drought stress that can reduce grain number per
spike, while grain weight may suffer a terminal stress caused
by high temperatures at the end of the cropping season (Garcı´a
del Moral et al., 2003). As a result, soil water in the root zone
often does not satisfy crop water needs over the whole season,
especially in spring months where the chances of rain to occur
become less, and where most of the crop growth occurs (Loss
and Siddique, 1994). Therefore, irrigation late in the season is
required to match soil water stress and to stabilize yields
(Campbell et al., 1993; Oweis et al., 1999).
Wheat constitutes almost 50% of the area cropped with
cereals in Lebanon and most of the cultivated lands are in the
Bekaa Valley, which by itself accounts for 42% of the total
agricultural land in the country (Karam and Karaa, 2000). Data
reported by the FAO (2005) indicated that despite a slight
increase in cereal yields in Lebanon between 1992 and 2004
(from 2.1 to 2.5 Mg ha
1
), average annual growth rate of cereal
production per capita was found to decrease drastically
between the two periods from 4.6% to 2.5%. Therefore, a
challenge the Lebanese agriculture has to face in the coming
years is to increase cereal yields to higher levels to satisfy the
food requirements of the population and to reduce the gap
between the rapid growth rate and food requirements.
Supplemental irrigation is an alternative to increase and
stabilize yields of crops grown in rainfed areas (Howell et al.,
1975; Zhang et al., 1998; Oweis et al., 1999). It can be defined as
the addition to essentially rainfed crops of small amounts of
water during times when rainfall fails to provide sufficient
water for normal plant growth and optimal yield (Duivenboo-
den et al., 1999; Oweis et al., 1999).
Wheat’s most sensitive growth stages to water stress with
respect tograin yield are stem elongation and booting, followed
by anthesis and grain filling (Blum and Pnuel, 1990; Shpiler and
Blum, 1991; Garcı´a del Moral et al., 2003). Water deficit around
anthesis may lead to a loss in yield by reducing spike and
spikelet number and the fertility of surviving spikelets (Giunta
et al., 1993), while water deficit during grain-filling period
reduces grain weight (Royo et al., 2000). Moreover, Edmeades
et al. (1989) pointed out that the lack of rainfall in spring time
caused a water deficit for rainfed wheat around anthesis that
increases in severity throughout the grain-filling period. Like-
wise, Giuntaet al. (1993) andZhong-hu andRajaram(1994)found
that kernels per spike and spike number per square meter were
the yield components most sensitive to drought, while kernel
weight remains relatively stable due to the high translocation of
assimilates stored during the pre-anthesis period.
Understanding the effect of water stress on yield formation
becomes an essential step for planning a suitable irrigation
strategy for wheat. The amount of water may be scheduled at
booting-anthesis and grain-filling and in dry years irrigation
may be needed as early as at stem elongation stage to ensure
rigorous canopy development (Oweis et al., 1999; Chen et al.,
2003). On the other hand, matching nitrogen (N) supply to
plant water availability is essential for a successful grain yield.
In that sense, Tilling et al. (2007) demonstrated that the
response of wheat to nitrogen fertilization is heavily reliant on
rainfall distribution. Moreover, the results of some research
have shown that the first developmental processes that occur
at early growth stages depends mainly on water and nitrogen
availability (Simane et al., 1993).
Efforts to optimize combinations of supplemental irriga-
tion and nitrogen fertilization of wheat were conducted in
many parts of the Mediterranean (Harmsen, 1984; Ryan et al.,
1991; Oweis et al., 1998). The objectives of this study were to
determine the effect of supplemental irrigation and nitrogen
fertilization and their interaction on yield, evapotranspiration
and water use efficiency of two durum wheat cultivars, Waha
and Haurani, widely used by farmers in a typical rainfed
Mediterranean environment, like the central plains of Bekaa
valley of Lebanon.
2. Materials and methods
2.1. Site description
Wheat (Triticum durum L.) seeds were sown under field
conditions at Tal Amara research station in the central Bekaa
Valley (3385104400N lat., 3585903200 E long. and 905 m above sea
level). The experiment was performed during the 2000–2001,
2001–2002 and 2003–2004 growing seasons from November to
June. Tal Amara has a well defined hot and dry season from
May to October and cold extending for the remainder of the
year. Main average rainfall is 592 mm, with a maximum of
145 mm in January. The soil of the study area is characterized
by high clay content and relatively low organic matter content.
Field slope is less than 0.1% and total available water within
the top 90 cm of soil profile is 170 mm.
2.2. Cultural practices and experimental design
Two durum wheat cultivars (Waha and Haurani) were
selected for this experiment. Waha is a high-yielding cultivar
mainly due to its higher spikes per square meter and grains
per spike than other cultivars (Garcı´a del Moral et al., 2003).
Waha has a short straw, straight leaves, short spikes and long
grain, while Haurani has longer straw and spike, making it
sensitive to lodging. Moreover, Haurani is a drought-tolerant
variety.
Waha and Haurani were sown using a mechanical plot drill
planter with 0.20 m row spacing. The seeding rate was
adjusted for a density of 260–280 seeds m
2
, according to
the standard practices in the central Bekaa Valley. In this
experiment, the effects of delayed sowing dates were avoided
by sowing the crops before the end of November (Table 1).
Seeds were planted into 8–10 cm furrows with 4–5 cm soil
cover above the seeds in a 3200 m
2
-experimental field
(64 m NS 50 m WE).
agricultural water management 96 (2009) 603–615604
Three water supply treatments (rainfed and two treat-
ments irrigated at 50% and 100% of soil water deficit) were
coupled with three nitrogen rates (100, 150 and 200 kg N ha
1
).
Water was applied using a line-source sprinkler at pre-
anthesis and by gravity from anthesis onwards when the soil
water content dropped below 50% of the total available water
in the upper 90 cm of the soil depth. In 2001–2002 and 2003–
2004 cropping seasons (normal years) irrigation was scheduled
at booting-flowering and grain-filling stages. In 2000–2001
cropping season (dry year) a drought was recorded early in the
growing season and irrigation was supplied at stem-elonga-
tion, booting-flowering and grain-filling stages. A flow meter
was used to measure the amount of applied irrigation water.
Irrigation was applied at 50% (IRR1) and 100% (IRR2) of soil
water deficit (SWD), while a rainfed treatment (IRR0) was
maintained under no irrigation throughout the growing
season. Irrigation dates and depths are given in Table 2.
In this experiment, nitrogen deficiency was avoided by
applying N fertilization at rates 100 kg N ha
1
. Therefore,
three N rates were applied at 100 (NR1), 150 (NR2) and
200 kg N ha
1
(NR3). All treatments received at sowing
fertilization as NPK (17-17-17) broadcasted mechanically
and incorporated into the upper 10-cm of soil layer at a rate
of 50 kg N ha
1
. Then, ammonium nitrate (NH
4
NO
3
, 34-0-0)
was applied in two splits, where an equal amount of
25 kg N ha
1
was given to all treatments at stem elongation,
and different amounts of 25, 75 and 125 kg N ha
1
were then
given to treatments NR1, NR2 and NR3, respectively, at
booting stage. Dates and amounts of N fertilization are given
in Table 3.
The experimental design was a split plot design. Years were
assigned to blocks and cultivars to main plots and the
combinations (IRR NR) to sub-plots. Three water supply
levels and three nitrogen rates were randomly distributed
within the main plots in three replicates each. In total, 54 sub-
plots of 20 m
2
area each (5 m NS 4 m WE), separated by rows
2 m wide, representing all combinations (IRR NR).
2.3. Crop phenology
Regular observations were made of phenology in terms of days
after sowing (DAS) and sum of temperature-day (8C), assuming
Table 1 – Some agronomic and management practices carried out during the experiments.
Observation 2000–2001 2001–2002 2003–2004
Sowing date (d.o.y.)
a
5 November 2000 (310) 10 November 2001 (314) 22 November 2003 (326)
Cultivars Waha and Haurani Waha and Haurani Waha and Haurani
Seeding depth (cm) 4–5 4–5 4–5
Row space (cm) 20 20 20
Seeding rate (plants m
2
) 260–280 260–280 260–280
Cultivated area (m
2
) 3200 (64 m NS 50 m WE) 3200 (64 m NS 50 m WE) 3200 (64 m NS 50 m WE)
Effective cultivated area 2400 (48 m NS 50 m WE) 2176 (34 m NS 50 m WE) 2000 (40 m NS 50 m WE)
Harvest (d.o.y.) 15 June 2001 (166) 22 June 2002 (173) 27 June 2004 (179)
Growing period (days) 222 224 218
a
Day of year (dates are given in parenthesis).
Table 2 – Irrigation dates (day of year) and depth (mm) of wheat treatments.
Date of irrigation Growth stage Day of year Water depth (mm)
IRR0 IRR1 IRR2
2000–2001
10 March 2001 Booting 69 0.0 40.0 80.0
20 April 2001 Anthesis 110 0.0 40.0 80.0
13 May 2001 Dough stage 133 0.0 25.0 50.0
Total irrigation 0.0 105.0 210.0
Total rain (1 November 2000 onwards) 394.6 394.6 394.6
Total (rain + irrigation) 394.6 499.6 604.6
2001–2002
15 April 2002 Anthesis 105 0.0 45.0 90.0
12 May 2002 Dough stage 132 0.0 45.0 90.0
Total irrigation 0.0 90.0 180.0
Total rain (1 November 2001 onwards) 544.2 544.2 544.2
Total (rain + irrigation) 544.2 634.2 724.2
2003–2004
27 March 2004 Booting 87 0.0 20.0 40.0
22 April 2004 Anthesis 113 0.0 20.0 40.0
12 May 2004 Dough stage 133 0.0 20.0 40.0
Total irrigation 0.0 60.0 120.0
Total rain (1 November 2003 onwards) 654.7 654.7 654.7
Total (rain + irrigation) 654.7 714.7 774.7
agricultural water management 96 (2009) 603–615 605
the base temperature (T
base
) of development and growth for
wheat crop is equal to 6 8C(Rawson and Go
´mez MacPherson,
2000). The length of vegetative period was calculated as days
from sowing to anthesis (growth stage 65 in the Zadoks scale),
where as the grain-filling period was calculated as days from
anthesis to physiological maturity (growth stage 91 in the
Zadoks scale) (Zadoks et al., 1974). The dates of the most
important growth stages of wheat crop were observed when
50% of the plants attained a given stage, i.e., tillering, stem
elongation, booting, anthesis, soft-dough stage, and grain
stiffening (Doorenbos and Kassam, 1980; Zhang and Oweis,
1999; Beuerlein, 2001). Ambient weather data were daily
recorded from the automated weather station of the Institute
(AURIA 12E, DEGREANE, France), 50 m from the experimental
site. Data were used to compute vapor pressure deficit (VPD) at
hourly basis from maximum and minimum air temperatures,
assuming relative humidity was 100% at the daily minimum
air temperature (Allen et al., 1998). Mean daily VPD was then
calculated by averaging hourly VPD and the growing season
mean VPD was calculated by dividing the sum of mean daily
VPD to the length of the growing season, in days (Chen et al.,
2003).
2.4. Soil water monitoring
Soil water content in the plots was measured using a Sentry
200-AP TDR (Time Domain Reflectometry, Sentry 200-AP,
1994). The TDR was calibrated to the soil at Tal Amara over a
wide range of soil water content. In all years, access tubes were
installed in the central sub-plot of each treatment to measure
soil water content in 0.15-m increments for the first 0.3 m,
then in 0.30-m increments down to a 1.2 m depth. The TDR
was calibrated in the field, and readings were then converted
to volumetric soil water content (u
v
), using the following
calibration equation:
Y¼0:0079Xþ1:948 (1)
where Yrepresents u
v
(in %); Xis the TDR measurement; 0.0079
and 1.948 are the coefficients of the calibration equation. The
standard error of the regression model estimation was
0.009 m
3
m
3
, and the coefficient of determination was 0.91.
TDR readings were used to estimate seasonal evapotran-
spiration (ET) in the plots using a water balance model as the
difference between inputs and outputs within the soil profile,
assuming drainage (D
r
) and runoff (R
o
) in the layer 0–90 cm
equal to zero:
PþIDrRoET ðSeSbÞ¼0 (2)
where Pis precipitation, Iis irrigation, D
r
is drainage, R
o
is
runoff, S
e
is the soil water content at the end of a time interval,
S
b
is the water content at the beginning of the same time
interval. All terms in Eq. (2) are expressed in mm.
2.5. Yield analyses and water use efficiency
Harvest date was determined at grain moisture of 15% and
ranged from mid to late June. Yield was determined in
sampling areas of 1 m
2
from the central rows of each sub-plot,
where the number of grains, 1000-grain weight, grain yield
and aboveground biomass were measured. The number of
grains per square meter was determined by counting the
grains from all spikes in the harvest area using a seed counter
(Contador, Pfeuffer, Germany). Mean 1000-grain weight was
calculated from the weight of five sets of 1000 grains each
from the sampling area. Moisture content in the grains was
determined using Inframatic 8100 (PerCon, Germany). Har-
vest index (HI), defined as the ratio of grain weight per mature
weight of aboveground parts (Cox and Jolliff, 1986; Moser
et al., 2006) was also calculated. Water use efficiency (WUE) of
grain produced was calculated as the ratio of grain yield at 0%
humidity (in kg ha
1
) after passing it in the oven for 72 h at
105 8C to crop evapotranspiration (in mm) (Caviglia and
Sadras, 2001).
2.6. Statistical analysis
All data were statistically analyzed by ANOVA using the PROC
MIXED procedure of SAS (SAS Institute, 1997). Mean separation
was performed only when the F-test indicated significant
(P<0.05) differences among the treatments, according to the
Fisher’s protected LSD test. The interactions IRR NR were
also reported and significant differences were analyzed at
P<0.05.
Table 3 – Dates (days after sowing) and amounts (kg N ha
S1
) of nitrogen fertilization of wheat treatments.
Date of fertilization Growth stage Days after sowing Fertilizer source N application (kg N ha
1
)
NR1 NR2 NR3
2000–2001
15 November Sowing 0 NPK (17%) 50 50 50
10 March Stem elongation 144 NH
4
NO
3
(34%) 25 25 25
20 April Booting 167 NH
4
NO
3
(34%) 25 75 125
2001–2002
25 November Sowing 0 NPK (17%) 50 50 50
20 March Stem elongation 126 NH
4
NO
3
(34%) 25 25 25
15 April Booting 170 NH
4
NO
3
(34%) 25 75 125
2003–2004
12 November Sowing 0 NPK (17%) 50 50 50
27 March Stem elongation 119 NH
4
NO
3
(34%) 25 25 25
4 May Booting 157 NH
4
NO
3
(34%) 25 75 125
agricultural water management 96 (2009) 603–615606
3. Results and discussion
3.1. Climatic conditions
Annual precipitation totaled 450, 580 and 670 mm in the 2000–
2001, 2001–2002 and 2003–2004 cropping years, compared to
historical average of 592 mm (1954–2002). However, the
rainfall pattern showed monthly variability between the three
growing years (Table 4). In 2000–2001 and 2003–2004 about 95%
of seasonal rain occurred between September and February
and 5% fell between March and May, where a competition for
limiting resources, mainly water, between vegetative and
reproductive organs may occur for wheat (Miralles et al., 2000).
In 2001–2002, 60% of the rain occurred between September and
February, while 40% of the rain fell between March and May,
with more frequent rain during the vigorous growth period.
Moreover, in 2002 rainfall recorded in March was 146.9 mm,
while it was 15.6 mm in March 2001 and 6.5 mm in March 2004,
out of an historical average of 81 mm for this month. The
drought recorded in March of years 2001 and 2004 reoccurred
in April, where rain was 5.1 mm in 2001 and 12.4 mm in 2004
compared to long term average of 41 mm, but in May rain was
below the long average (17 mm) in all three years (Table 4).
Weather conditions that prevailed at Tal Amara were
generally cooler in 2003–2004 than in 2000–2001 and 2001–2002
cropping seasons. When calculated over the whole year,
average air temperature was 1.1 and 0.7 8C warmer in 2000–
2001 and 2001–2002, respectively, and 0.9 8C cooler in 2003–
2004 than the annual historical average (14.2 8C). In 2000–2001
growing year, temperatures from December to June were
higher than the long-term averages (Table 4). Consequently, a
drought was recorded early in the season at stem elongation
stage. In 2001–2002, lower air temperatures and more frequent
rain were observed during the growing season than in 2000–
2001. Moreover, in March and April, with booting-anthesis
stage, average air temperature in all years was 1.5–3.7 8C
warmer than the long run averages. This has led to less
frequent periods during which the soil surface was wet.
On the otherhand, the relatively warmerweather conditions
that prevailed in 2000–2001 and 2001–2002 cropping years also
have increased seasonal and mean daily VPD compared to the
2003–2004 growing year. Indeed, total growing season mean
daily VPD from November 1st to June 30th totaled 403 mbar in
2000–2001, 368 mbar in 2001–2002 and 355 mbar in 2003–2004,
thus giving mean daily vapor pressure deficit of 1.66, 1.52 and
1.48 mbar day
1
in 2000–2001, 2001–2002 and 2003–2004 crop-
ping years,respectively. Moreover,mean daily VPD followed the
same general pattern in all growing years (Fig. 1). However,
there was greater scatter in the 2000–2001 and 2001–2002 data
than for the 2003–2004 data set. Higher mean daily VPD values
were observed late in the season (June–July) where the highest
values of potential evapotranspiration were recorded at Tal
Amara and in the central Bekaa Valley in general (Aboukhaled
and Sarraf, 1970; Karam et al., 2007).
3.2. Effects of supplemental irrigation on yield and its
components
Averaged across years, grain yield of cultivar Waha irrigated at
100% of SWD (IRR2) was 4470 kg ha
1
, showing 50 kg ha
1
Table 4 – Mean daily temperature and total rainfall prevailed during the experiments, compared to the long-run means (1954–2002).
September October November December January February March April May June July August Average/tot
2000–2001
Mean air temperature (8C) 20.2 16.4 11.6 8.4 7.2 7.2 12.9 14.9 17.2 20.7 23.8 23.5 15.3
Rain (mm) 4.1 50.9 34.8 99.1 58.5 171.3 15.6 5.1 10.2 0.0 0.0 0.0 449.6
2001–2002
Mean air temperature (8C) 21.3 17.4 11.2 8.2 3.5 9.0 11.0 14.8 16.4 19.9 23.3 22.5 14.9
Rain (mm) 0.0 35.5 51.0 92.0 110.0 63.3 146.9 71.2 9.8 0.0 0.0 0.0 579.7
2003–2004
Mean air temperature (8C) 20.2 17.5 11.1 7.5 6.0 5.9 10.6 14.0 16.5 20.5 23.7 22.5 13.3
Rain (mm) 0.0 15.8 43.8 130.7 257.2 201.4 6.5 12.4 2.7 0.0 0.0 0.0 670.5
1954–2002
Mean air temperature (8C) 20.0 16.5 11.5 7.4 5.9 6.5 9.2 12.7 16.2 19.8 21.8 22.5 14.2
Rain (mm) 0.0 23.0 58.0 124.0 145.0 103.0 81.0 41.0 17.0 0.0 0.0 0.0 592.0
agricultural water management 96 (2009) 603–615 607
more grains than irrigation treatment at 50% of SWD (IRR1)
(Table 5). For cultivar Haurani, similarly to Waha, yield was
slightly higher with irrigation at 100% of SWD than at 50% of
SWD. Mean grain yield of IRR2 treatment was 3900 kg ha
1
and
was 120 kg ha
1
higher than IRR1 treatment, while rainfed
treatment (IRR0) in cultivar Haurani had the lowest yield
(3210 kg ha
1
). However, the magnitude of the differences
between the rainfed and irrigated treatments was observed to
be smaller in Haurani than in Waha. Indeed, while for cultivar
Waha average increases in grain yield in response to irrigation
were 27% and 28% in IRR1 and IRR2, respectively, compared to
IRR0, the increases in the same treatments for cultivar Haurani
were 18% and 22%. This may reflect a higher aptitude of
cultivar Waha to water supply than cultivar Haurani.
Supplemental irrigation was observed to increase grain
number per unit ground area in all years. For cultivar Waha,
the increases in grain number in IRR1 and IRR2 were 20% and
12% (2001), 6% and 10% (2002) and 27% and 19% (2004),
respectively, with respect to IRR0. When compared to the
rainfed treatment, irrigation across years at 100% of SWD
induced less increase (13%) in grain number per unit ground
area than irrigation at 50% of SWD (17%). For cultivar Haurani,
mean increases in grain number due to irrigation were 15%
and 6% (2001), 7% and 21% (2002) in IRR1 and IRR2, respectively.
In 2004, grain number was observed to increase by 9% in IRR1
while no increase was recorded in IRR2 treatment. Averaged
across years and irrigation treatments, supplemental irriga-
tion in cultivar Haurani induced increases in grain number per
unit ground area by 11% and 9% in IRR1 and IRR2, respectively,
in comparison with the rainfed treatment (8130 grains m
2
).
In contrast, the 1000-grain weight seemed to be less
affected than grain number for both cultivars by supplemental
irrigation and nitrogen rate. Indeed, the percentage of increase
of 1000-grain weight in cultivar Waha due to irrigation was
12% (2001) in IRR1 treatment, while no increase was observed
in IRR2 treatment in comparison with the rainfed treatment
(43.97 g). In 2002, the percentages of increase in 1000-grain
weight were 10% and 16% in IRR1 and IRR2, respectively, with
respect to IRR0 (38.98 g), while in 2004 the increases were 3%
and 12%, respectively, in comparison with IRR0 treatment
(43.77 g). Across years, supplemental irrigation at 50% of SWD
increased 1000-grain weight by 8% against an increase of 13%
with supplemental irrigation at 100% of SWD in comparison
with the rainfed treatment (42.24 g). For cultivar Haurani, even
though the weight of 1000-grain was slightly higher than for
cultivar Waha, the magnitude of increase of 1000-grain weight
in response to irrigation was observed smaller than in cultivar
Waha.
Indeed, averages across years and irrigation treatments,
the increases in 1000-grain weight were 7% and 11% in IRR1
and IRR2, respectively, in comparison with IRR0 treatment
(43.57 g). Garcı´a del Moral et al. (2003) demonstrated that
kernel number per spike has a significant contribution to grain
yield, especially under drought conditions, while in cooler
environments the compensatory effects among yield compo-
nents were almost absent, probably because of the relative
availability of water and nitrogen during the critical phases of
plant development.
3.3. Effect of N fertilization on yield and its components
A trend of increasing yields with N rates was observed for both
cultivars with the lowest yield occurring at 100 kg N ha
1
. For
cultivar Waha, even though there were differences among
nitrogen treatments, yield at 200 kg N ha
1
(4380 kg ha
1
) was
slightly higher than yield at 150 kg N ha
1
(4230 kg ha
1
), while
nitrogen treatment of 100 kg N ha
1
had the lowest yield
(3770 kg ha
1
). For cultivar Haurani, nitrogen treatment of
100 kg N ha
1
in Haurani had the lowest yield (3010 kg ha
1
)
while treatments of 150 and 200 kg N ha
1
yielded 3730 and
4150 kg ha
1
, respectively. However, the differences between
the low (NR1) and high (NR3) nitrogen rates were smaller in
Waha than Haurani.
In all three years, grain number per ground area was
influenced by increased N applications. Increasing the rate of
N fertilization from 100 (NR1) to 150 (NR2) and 200 kg N ha
1
(NR3) has stimulated the production of additional grains per
unit ground area by 12% and 8% (2001), 5% and 16% (2002) and
18% and 24% (2004) in NR2 and NR3, respectively, in
comparison with NR1. Averaged across years and N treat-
ments, grain number per unit ground area was observed to
increase in cultivar Waha by 11% and 17% in NR2 and NR3,
Fig. 1 – Daily evolution of vapor pressure deficit (VPD) at noon during the three cropping years.
agricultural water management 96 (2009) 603–615608
respectively, in comparison with NR1 treatment
(8341 grains m
2
). Moreover, N fertilization of 150 and
200 kg N ha
1
has increased significantly (P<0.01) grain
number per unit ground area in cultivar Haurani by 30%
and 44% (2001), 24% and 33% (2002), and 16% and 26% (2004) in
NR2 and NR3, respectively, compared to N fertilization of
100 kg ha
1
. When averaged across years, grain number per
unit ground area was found to increase by 24% and 39%
(P<0.01) in NR2 and NR3, respectively, in comparison with
NR1 (6575 grains m
2
).
On the contrary, nitrogen fertilization was observed not to
have significant effects (P<0.05) on 1000-grain weight. Indeed,
supply of 150 and 200 kg N ha
1
did not induce in cultivar
Waha any increase in 1000-grain weight almost in all the three
cropping years, except in 2001–2002 cropping year where the
increases were 3% and 6% in NR2 and NR3, respectively, with
respect to NR1 (41.08 g). In 2001 and 2004, the application of N
fertilization of 150 and 200 kg ha
1
slightly lowered 1000-grain
weight by 2–5%, with respect to N fertilization of 100 kg ha
1
(48.03 g in 2001 and 46.53 g in 2004). When averaged across
years, N fertilization of 150 kg ha
1
did not stimulate any
increase in 1000-grain weight with respect to N fertilization of
100 kg ha
1
(46.34 g), while N fertilization of 200 kg ha
1
was
observed to reduce this parameter by 2% (45.63 g) with respect
to NR1. As a result, cultivar Waha exhibited grain yield
increases in response to N application by 12% and 16%
(P<0.05) in NR2 and NR3, respectively, in comparison with
NR1 (3770 kg ha
1
), while for cultivar Haurani the increases
were 24% and 38% in NR1 and NR2, respectively, in comparison
with NR1 (3010 kg ha
1
).
Table 5 – Main effects of supplemental irrigation (IRR) and nitrogen rate (NR) on grain number (GN), grain weight (GW),
grain yield (GY), aboveground biomass (AB), and Harvest index (HI) of the two wheat cultivars during the three cropping
years.
Growing
year
Treatment Waha Haurani
GN
(m
2
)
GW
(g)
GY
(Mg ha
1
)
AB
(Mg ha
1
)
HI GN
(m
2
)
GW
(g)
GY
(Mg ha
1
)
AB
(Mg ha
1
)
HI
2000–2001 Irrigation level IRR0 7,406 43.97 3.26 9.75 0.34 7,258 45.97 3.30 9.69 0.34
IRR1 8,894 49.06 4.37 11.45 0.39 8,341 48.11 3.89 11.03 0.36
IRR2 8,234 49.00 4.03 11.14 0.38 7,686 50.22 3.86 11.60 0.33
Nitrogen rate NR1 7,684 48.03 3.70 9.26 0.40 5,958 51.12 3.05 8.96 0.34
NR2 8,545 48.40 4.16 9.94 0.42 7,747 48.39 3.73 10.76 0.35
NR3 8,305 45.60 3.81 13.13 0.29 9,579 44.79 4.26 12.60 0.34
F-tests Irrigation level
** ** ** ** ** ** ** ** ** **
Nitrogen rate
** ** ** ** ** ** ** ** ** **
IRR NR
*** * ** * * *** * ** * *
2001–2002 Irrigation level IRR0 9,027 38.98 3.52 9.15 0.39 8,752 38.18 3.36 8.95 0.37
IRR1 9,531 42.72 4.08 9.32 0.44 9,400 43.00 4.05 9.42 0.44
IRR2 9,881 45.22 4.48 9.74 0.46 10,598 45.42 4.83 11.46 0.42
Nitrogen rate NR1 8,858 41.08 3.64 8.47 0.43 8,064 40.55 3.30 8.17 0.40
NR2 9,281 42.28 3.93 9.05 0.43 9,995 42.35 4.25 10.07 0.42
NR3 10,299 43.55 4.50 10.69 0.42 10,691 43.70 4.70 11.59 0.40
F-tests Irrigation level
** ** ** ** ** ** ** ** ** **
Nitrogen rate
** ** ** ** ** ** ** ** ** **
IRR NR
*** * ** * * *** * ** * *
2003–2004 Irrigation level IRR0 8,419 43.77 3.69 9.15 0.40 6,408 46.55 2.98 8.56 0.35
IRR1 10,665 45.18 4.80 9.98 0.48 7,008 48.22 3.39 8.35 0.42
IRR2 9,989 49.00 4.90 10.74 0.46 6,106 49.49 3.02 8.11 0.37
Nitrogen rate NR1 8,480 46.53 3.96 9.35 0.42 5,702 47.36 2.70 8.57 0.32
NR2 10,044 45.73 4.61 10.32 0.45 6,632 48.49 3.21 8.51 0.38
NR3 10,549 45.68 4.82 10.21 0.47 7,188 48.41 3.49 7.95 0.44
F-tests Irrigation level
** ** ** ** ** ** ** ** ** **
Nitrogen rate
** ** ** ** ** ** ** ** ** **
IRR NR
*** * ** * * *** * ** * *
Averages across years Averages across years
Irrigation level IRR0 8,284 42.24 3.49 9.35 0.38 7,472 43.57 3.21 9.07 0.35
IRR1 9,697 45.65 4.42 10.25 0.44 8,250 46.44 3.78 9.60 0.40
IRR2 9,368 47.74 4.47 10.54 0.43 8,130 48.37 3.90 10.39 0.37
Nitrogen rate NR1 8,341 45.22 3.77 9.03 0.42 6,575 46.34 3.01 8.57 0.35
NR2 9,290 45.47 4.23 9.77 0.43 8125 46.41 3.73 9.78 0.38
NR3 9,718 44.94 4.38 11.34 0.39 9,153 45.63 4.15 10.71 0.40
*
Significant at P<0.05.
**
Significant at P<0.01.
***
Significant at P<0.001.
agricultural water management 96 (2009) 603–615 609
The relatively small increases in grain yield observed in
NR2 and NR3 treatments of cultivar Waha in comparison with
N fertilization with 100 kg ha
1
could be attributable to the
‘haying off’ effect, which can occur when N is applied
excessively too, encouraging the crop to produce excessive
biomass and use extra water, reducing water availability
during the grain-filling process (Van Heraarden et al., 1998).
Indeed, across years, the production of aboveground biomass
in response to N fertilization was 9.03, 9.77 and 11.34 Mg ha
1
,
in NR1, NR2 and NR3, respectively, exhibiting thus higher
difference between the low (100 kg N ha
1
) and high
(200 kg N ha
1
) N application than for grain yield. As a
consequence, Harvest index (HI) was observed to decrease
in the high N application (0.39), in comparison with the low
(0.42) and medium (0.43) N applications (Table 5). These values
were very close to those obtained by Przulj and Momcˇilovic
´
(2003) for a series of 20 wheat cultivars in Eastern Europe. For
cultivar Haurani, aboveground biomass at harvest was
8.57 Mg ha
1
in NR1, 9.78 Mg ha
1
in NR2 and 10.71 Mg ha
1
in NR3, giving thus HI values of 0.35 (NR1), 0.38 (NR2) and 0.40
(NR3). Royo et al. (1999) demonstrated that in favorable
seasons during the growth period, plants accumulate suffi-
cient amounts of dry matter for various biological functions,
and a part of the accumulated dry matter is reserved to grain
growth. Moreover, Caviglia and Sadras (2001) showed that
translocation process that takes place in wheat plants
between anthesis and maturity is influenced by the level of
N rate. In this experiment, greater dry matter production at
harvest resulted in greater grain yield. Indeed, in all three
years, the highest grain yield obtained with the high N rate
(200 kg N ha
1
) was accompanied by the highest aboveground
biomass production, with greater values recorded for cultivar
Waha. However, in the least favorable years (2001 and 2002),
grain yields were lower by 10% and 20%, respectively, in the
high N level (NR3) than the low N level (NR1). This could be
explained by the loss of a significant amount of dry matter for
maintaining a large quantity of vegetative mass (Austin et al.,
1977). In growing seasons with unfavorable weather condi-
tions during the vegetative period, plants accumulate dry
matter during the grain-filling period for growth of their
vegetative parts as well for grain development (Bidinger et al.,
1977).
3.4. Effect of cultivars on grain yield
The effect of irrigation was significant for the two wheat
cultivars, but the effect of nitrogen was not always significant
(Table 5). In all years, irrigation nitrogen interaction was
significant, illustrating varietals differences of yield response
to irrigation. Comparing separate means among cultivars at
each irrigation treatment indicate that cultivar Waha had
higher yield than cultivar Haurani at all water supply levels
and in all years, but for both wheat cultivars the lowest yield
was observed in the rainfed treatment. Garcı´a del Moral et al.
(2003) evaluated grain yield and its components of six ICARDA-
CIMMYT wheat genotypes under Mediterranean conditions
and they found that Waha had the highest yield due to is
higher spikes per square meter and grain number per spike, in
comparison with other cultivars. However, there were no
significant differences in grain yield among nitrogen treat-
ments of the two wheat cultivars in all three years. Across
irrigation treatments, nitrogen application of 100 kg ha
1
resulted in grain yield of 3.77 Mg ha
1
in Waha against
3.01 Mg ha
1
in Haurani, while nitrogen applications at 150
and 200 kg ha
1
resulted in grain yield of 4.23 and 4.38 Mg ha
1
in Waha against 3.73 and 4.15 Mg ha
1
in the same treatments
in cultivar Haurani.
Moreover, data analyses for the 3-year experiment showed
that in spite of significant differences (P<0.05) in grain
number per unit ground area between the two cultivars at all
irrigation and nitrogen treatments, 1000-grain weight was
found to not differ significantly between the two cultivars,
neither in response to supplemental irrigation nor to nitrogen
fertilization. Averaged across years and irrigation treatments,
grain number per unit ground area in cultivar Waha was 8284,
9697 and 9368 grains m
2
in IRR0, IRR1 and IRR2, respectively,
against 7472, 8250 and 8130 grain m
2
in the same treatments
in cultivar Haurani. Across years and N treatments, grain
number per unit ground area in cultivar Waha was 8341, 9290
and 9718 grains m
2
in NR1, NR1 and NR3, respectively,
against 6575, 8125 and 9153 grain m
2
in the same treatments
in cultivar Haurani.
3.5. Evapotranspiration and water use efficiency
Data reported in Table 6 give evapotranspiration of wheat in
all three years according to the water balance (Eq. (2)). Table 6
showed that ET of rainfed wheat ranged from 373 mm in 2000–
2001 to 433 mm in 2001–2002 and to 493 mm in 2003–2004 and
these values were closely related to the amounts of rainfall
registered during the three cropping seasons. In similar
experiments, Zhang and Oweis (1999) pointed out that
evapotranspiration depend on the seasonal rainfall under
rainfed conditions and on the combined amount of water
(irrigation and rainfall) under irrigation conditions. When
averaged over the whole growing season, daily values of
evapotranspiration of rainfed wheat were 1.7 mm day
1
(2000–2001), 1.9 mm day
1
(2001–2002) and 2.2 mm day
1
(2003–2004). Moreover, supplemental irrigation increased
markedly ET of wheat plants and the range of measured ET
values varied from 450 to 650 mm, following to the level of
applied water. Indeed, average daily evapotranspiration of
wheat plants irrigated at 50% of soil water deficit varied from
2.0 mm day
1
in 2000–2001 to 2.4 mm day
1
in 2001–2002 and
to 2.6 mm day
1
in 2003–2004, while supplemental irrigation at
100% of soil water deficit increased theses values to
2.6 mm day
1
(2000–2001), 2.9 mm day
1
(2001–2002) and
3.0 mm day
1
(2003–2004). Moreover, results reported in
Table 6 showed that under both rainfed and irrigation
conditions wheat plants accounted by anthesis for 65–70%
of seasonal evapotranspiration, while the remaining 30–35% is
accumulated during the grain-filling stages.
Irrespective of the rate, N fertilizer increased evapotran-
spiration in all three years. Averaged across years, the
applications of 150 and 200 kg N ha
1
increased ET by 14–18
and 38–46 mm, respectively, in comparison with nitrogen
application of 100 kg ha
1
(Table 6). Similar results observed by
Caviglia and Sadras (2001) showed that N fertilization
increased evapotranspiration of wheat plants in spite of
reducing evaporation from soil.
agricultural water management 96 (2009) 603–615610
Table 6 – Measured evapotranspiration of wheat treatments according to water balance model (Eq. (2)) during the three cropping years.
Growth stage Rain (mm) Irrigation (mm) Rain + irrigation (mm) RD (cm) SW (mm) ET (mm)
IRR0 IRR1 IRR2 IRR0 IRR1 IRR2 IRR0 IRR1 IRR2 NR1 NR2 NR3
2000–2001
November Establishment 34.8 0.0 0.0 0.0 34.8 34.8 34.8 0.1 38.3 3.5 3.5 3.5 3.6 3.7 3.9
December Seedling 99.1 0.0 0.0 0.0 99.1 99.1 99.1 0.2 121.7 22.6 22.6 22.6 23.0 23.7 24.8
January Tillering 58.5 0.0 0.0 0.0 58.5 58.5 58.5 0.3 100.1 41.6 41.6 41.6 42.4 43.7 45.8
February Stem elongation 171.3 0.0 0.0 0.0 171.3 171.3 171.3 0.5 224.5 53.2 53.2 53.2 54.2 55.8 58.5
March Booting 15.6 0.0 35.0 70.0 15.6 50.6 85.6 0.8 83.9 68.3 103.3 138.3 105.3 108.4 113.6
April Anthesis 5.1 0.0 35.0 70.0 5.1 40.1 75.1 1.0 69.5 64.4 99.4 134.4 101.3 104.3 109.3
May Dough stage 10.2 0.0 35.0 70.0 10.2 45.2 80.2 1.0 14.6 4.3 39.4 74.4 40.1 41.3 43.3
June Maturity 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 115.4 115.4 115.4 115.4 117.7 121.1 126.9
Total 394.6 0.0 105.0 210.0 394.6 499.6 604.6 373.2 478.2 583.2 487.8 502.1 526.0
2001–2002
November Establishment 51.0 0.0 0.0 0.0 51.0 51.0 51.0 0.1 51.2 0.2 0.2 0.2 0.2 0.2 0.2
December Seedling 92.0 0.0 0.0 0.0 92.0 92.0 92.0 0.2 117.7 25.7 25.7 25.7 26.2 27.0 28.2
January Tillering 110.0 0.0 0.0 0.0 110.0 110.0 110.0 0.3 151.6 41.6 41.6 41.6 42.4 43.7 45.8
February Stem elongation 63.3 0.0 0.0 0.0 63.3 63.3 63.3 0.5 116.5 53.2 53.2 68.2 59.3 61.1 64.0
March Booting 146.9 0.0 50.0 100.0 146.9 196.9 246.9 0.8 229.2 82.3 132.3 182.3 134.9 138.9 145.5
April Anthesis 71.2 0.0 0.0 0.0 71.2 71.2 71.2 1.0 135.6 64.4 74.4 64.4 69.0 71.1 74.5
May Dough stage 9.8 0.0 50.0 100.0 9.8 59.8 109.8 1.0 45.2 35.4 85.4 135.4 87.1 89.6 93.9
June Maturity 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 130.4 130.4 130.4 130.4 133.0 136.9 143.4
Total 544.2 0.0 100.0 200.0 544.2 644.2 744.2 433.0 543.0 648.0 552.2 568.4 595.5
2003–2004
November Establishment 43.8 0.0 0.0 0.0 43.8 43.8 43.8 0.1 44.0 0.2 0.2 0.2 0.2 0.2 0.2
December Seedling 130.7 0.0 0.0 0.0 130.7 130.7 130.7 0.2 162.6 31.9 31.9 31.9 32.5 33.5 35.1
January Tillering 257.2 0.0 0.0 0.0 257.2 257.2 257.2 0.3 308.1 50.9 50.9 50.9 51.9 53.5 56.0
February Stem elongation 201.4 0.0 0.0 0.0 201.4 201.4 201.4 0.5 254.6 53.2 53.2 53.2 54.2 55.8 58.5
March Booting 6.5 0.0 40.0 80.0 6.5 46.5 86.5 0.8 102.8 96.3 136.3 176.3 139.0 143.1 149.9
April Anthesis 12.4 0.0 0.0 0.0 12.4 12.4 12.4 1.0 90.8 78.4 78.4 78.4 79.9 82.3 86.2
May Dough stage 2.7 0.0 40.0 80.0 2.7 42.7 82.7 1.0 75.3 72.6 112.6 152.6 114.8 118.2 123.8
June Maturity 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 109.4 109.4 109.4 109.4 111.5 114.8 120.3
654.7 0.0 80.0 160.0 654.7 734.7 814.7 492.7 572.7 652.7 584.2 601.3 630.0
agricultural water management 96 (2009) 603–615 611
The growing season mean daily VPD totaled 403 mbar in
2000–2001, 368 mbar in 2001–2002 and 355 mbar in 2003–2004.
After normalizing ET for vapor pressure deficit (ET/VPD)
during the growing season, supplemental irrigation at 50%
and 100% of soil water deficit had approximately 26 and
52 mm mbar
1
more ET/VPD than rainfed treatment, while N
rates of 150 and 200 kg N ha
1
had 4 and 12 mm mbar
1
more
ET/VPD than N rate of 100 kg N ha
1
.
Averaged acrossyears, water use efficiency (WUE) of cultivar
Waha under rainfed conditions was 8.1 kg ha
1
mm
1
, while
irrigatedtreatments at 50% and 100% of SWD had WUE values of
8.3 and 7.1 kg ha
1
mm
1
, respectively. For cultivar Haurani,
rainfed treatment resulted in WUE of 7.4 kg ha
1
mm
1
, while a
slight decreasein WUE was observed in IRR1 (7.1 kg ha
1
mm
1
)
and IRR2 (6.2 kg ha
1
mm
1
), respectively. Analysis of variance
for the combined data showed that WUE in all years was not
affected either by supplemental irrigation or nitrogen rate
(Table 7). In similar environmental conditions, Oweis et al.
(1999) demonstrated that WUE of rainfed wheat was
6.8 0.31 kg ha
1
mm
1
for grain yields <3Mgha
1
and sea-
sonal rainfall of 330 mm, while for irrigated wheat and grain
yield >3Mgha
1
WUE was 10.8 0.28 kg ha
1
mm
1
. Values
obtained in this experiment were slightly lower than those
obtained by Zhang et al. (1998) and Oweis et al. (1999) mainly
because seasonal rainfall registered at Tal Amara in the central
Bekaa Valley was higher than that registered at the experi-
mental sitein northern Syria. In both cases,however, WUE were
lower than the maximum value of 15 kg ha
1
mm
1
obtained by
Siddique et al. (1990) for wheat in the Mediterranean region.
Averaged across irrigation treatments, WUE at grain basis
over the three seasons increased from 7.0 kg ha
1
mm
1
for
cultivar Waha and 5.6 kg ha
1
mm
1
for cultivar Haurani to 7.6
Table 7 – Main effects of supplemental irrigation (IRR) and nitrogen rate (NR) on evapotranspiration (ET), and water use
efficiency of the two wheat cultivars during the three cropping years.
Cropping
year
Treatment ET
(mm)
VPD
(mbar)
ET/VPD
(mm mbar
1
)
Waha Haurani
GY
(Kg ha
1
)
WUE
(kg ha
1
mm
1
)
GY
(Kg ha
1
)
WUE
(kg ha
1
mm
1
)
2000–2001 Irrigation level IRR0 373.0 403.0 0.93 3258.93 8.74 3301.80 8.85
IRR1 478.0 403.0 1.19 4374.38 9.15 3887.10 8.13
IRR2 583.0 403.0 1.45 4030.36 6.91 3855.38 6.61
Nitrogen rate NR1 487.0 403.0 1.21 3699.25 7.60 3047.93 6.26
NR2 502.0 403.0 1.25 4155.01 8.28 3731.83 7.43
NR3 526.0 403.0 1.31 3809.40 7.24 4264.53 8.11
F-tests
a
Irrigation level
** ns ** ** ns ** ns
Nitrogen rate
* ns * ** ns ** ns
IRR NR
*ns * * ns ** ns
2001–2002 Irrigation level IRR0 433.0 368.0 1.18 3521.56 8.13 3355.34 7.75
IRR1 543.0 368.0 1.48 4077.23 7.51 4054.77 7.47
IRR2 648.0 368.0 1.76 4475.51 6.91 4827.47 7.45
Nitrogen rate NR1 552.0 368.0 1.50 3640.31 6.59 3295.25 5.97
NR2 568.0 368.0 1.54 3933.13 6.92 4246.99 7.48
NR3 595.0 368.0 1.62 4500.87 7.56 4695.34 7.89
F-tests Irrigation level
** ns ** ** ** ** **
Nitrogen rate
** ns ** ** ** ** **
IRR NR
* ns * * * ** *
2003–2004 Irrigation level IRR0 492.0 355.0 1.39 3686.13 7.49 2984.68 6.07
IRR1 572.0 355.0 1.61 4804.37 8.40 3392.43 5.93
IRR2 652.0 355.0 1.84 4895.63 7.51 3021.93 4.63
Nitrogen rate NR1 584.0 355.0 1.65 3956.70 6.78 2698.98 4.62
NR2 601.0 355.0 1.69 4606.56 7.66 3213.60 5.35
NR3 630.0 355.0 1.77 4822.88 7.66 3486.47 5.53
F-tests Irrigation level
** ns ** ** ** ** **
Nitrogen rate
** ns ** ** ** ** **
IRR NR
* ns * * * ** *
Averages across years
Irrigation level IRR0 432.6 375.3 1.16 3488.88 8.12 3213.94 7.43
IRR1 531.0 375.3 1.42 4418.66 8.35 3778.10 7.12
IRR2 627.6 375.3 1.68 4467.17 7.11 3901.59 6.22
Nitrogen rate NR1 541.0 375.3 1.45 3765.42 6.99 3014.05 5.57
NR2 557.0 375.3 1.49 4231.57 7.62 3730.80 6.70
NR3 583.6 375.3 1.57 4377.72 7.49 4148.78 7.11
a
ns, *, **non significant or significant at P<0.05 or P<0.01, respectively.
agricultural water management 96 (2009) 603–615612
and 7.5 kg ha
1
mm
1
for the former, and 6.7 and
7.1 kg ha
1
mm
1
for the latter by applying 50 (IRR2) and
100 kg ha
1
(IRR3) more N than N application of 100 kg ha
1
.
Slightly higher values of WUE occurred in N treatments of
cultivar Waha in comparison with cultivar Haurani, and there
was no significant increase in WUE between 100 and
150 kg N ha
1
applications. On average, no significant increase
in WUE occurred in the three growing years for N application
more than 150 kg ha
1
.
3.6. Relationship between grain yield and
evapotranspiration
The relationship between grain yield (GY) and seasonal
evapotranspiration (ET) is presented in Fig. 2. The relation-
ships indicate that for each 10 mm increase in ET, there was a
corresponding grain yield increase of 50 kg for Waha and 35 kg
for Haurani. This reflects a greater response of Waha to
increase in ET than Haurani. Similar linear relationships of
grain yield to seasonal ET were established for rainfed and
irrigated wheat (Zhang and Oweis, 1999). However, the value
of the slope obtained by Zhang and Oweis (1999) was higher
(11.6) than those obtained in this experiment. Although the
threshold for the first gain of grain yield was 373 mm for Waha
and Haurani (estimated from the regression equations), this
value was smaller than the threshold for the first grain yield
increment for winter wheat of 200 mm obtained by Musick
et al. (1994) in the US southern plains and Zhang and Oweis
(1999) in northern Syria.
Moreover, there was a considerable scatter between grain
yield and ET for rainfed data, probably due to the variation in
rainfall distribution within the growing season and tempera-
ture differences between the three growing seasons. Although
the rainfall in the 2000–2001 and 2001–2002 growing years was
less than the long-term average, the crops may have benefited
from the favorable inter-season distribution of rainfall. Crops
may suffer from water stress during a long dry spell lasting
from mid-March to early May in the 2000–2001 and 2001–2002
growing seasons. However, irrigation late in the two seasons
improved soil water status during the grain-filling period,
which apparently improved grain yield. In addition to seasonal
crop water use, vapor-pressure deficit (reflecting temperature
influence) during the grain-filling stage may play an important
role in determining grain yield. In this experiment, more
precipitation was distributed in the spring in 2002 than in 2001
and 2004 (total of 228 mm in 2002 against 31 mm in 2001 and
22 mm in 2004 between March and June), a higher slope of
grain yield vs. ET could be produced in year 2002 than in 2001
and 2004, indicating thus more efficient use of water in this
year in comparison with the other two cropping years.
4. Conclusions
In all the three growing seasons, the N treatments had little
influence on grain yield and irrigation regime being the
predominant limiting factor. The significantly higher yields
obtained from the irrigated plots in 2002 was attributed to the
higher spring rainfall recorded in this year. This indicates that
normal distribution of monthly rainfall, especially during the
spring months, may affect positively grain yield of wheat
rather than total seasonal rainfall.
Analyses of variance for grain yield and its components
revealed that these characters were affected mostly by
supplemental irrigation and nitrogen rate. In fact, the effects
of irrigation regime were observed for grain number, grain
weight, grain yield but not for Harvest index (HI) and water use
efficiency (WUE). Nitrogen rate effects were of lower magni-
tude compared with those of irrigation, though they were
statistically significant for all traits. On the other hand, grain
yield was greater in the cooler than in the warmer year, a
consequence of more grains per square meter, heavier grains,
and a longer plant cycle. Rainfed conditions caused reductions
in grain yield estimated at 25–35% in comparison with
irrigation treatments. Grain number per square meter was
the yield component most sensitive to drought effects and was
Fig. 2 – Relationship between grain yields (GY) and the total evapotranspiration (ET), for Waha and Haurani cultivars wheat
over the three cropping years.
agricultural water management 96 (2009) 603–615 613
reduced by 37% and 34% under rainfed conditions in both
cultivars. Grain weight was the unique yield component that
was moderately insensitive to irrigation regime variations and
appeared to be relatively stable in both cultivars.
Wheat had the greatest yield with supplemental irrigation
at 100% of SWD (IRR2), after which yield decreased slightly
with supplemental irrigation at 50% of SWD (IRR1). The results
obtained in this experiment showed that the degree of yield
decline may depend on weather conditions prevailing during
the cropping year. These different yield responses to irrigation
and nitrogen for the two winter wheat cultivars (Waha and
Haurani) were observed in 2001 and 2004, which was probably
related to little spring rainfall observed in these two years in
comparison with 2002. Irrigation has been observed to be as
important to yield as the total amount of rainfall recorded
during the active growth period. Indeed, Passioura (1983) and
Loss and Siddique (1994) showed that optimal wheat yield was
produced when water supply pattern matches plant growth
and water demand, whereas Rasmussen et al. (1998) found
that wheat yield was generally greater if rain was partially
distributed during the spring months, particularly in May and
June. To avoid possible decrease in wheat yield with small
rainfall amounts in spring months, supplemental irrigation
supply during this period of the crop growth was found to
adjust and stabilize yield under the Mediterranean dry lands of
northern Syria (Oweis et al., 1998, 1999).
However, cultivar Haurani yielded lower than cultivar
Waha under the same weather conditions and cultural
practices, which was especially significant in the production
of number of grains per m
2
. The reason for the lower yields in
Haurani may be attributable to the fact that this cultivar is
considered drought-tolerant cultivar, and therefore its
response to water and nitrogen was not clear due to the
confounding effects of genotype and physiological mechan-
isms of tolerance of this cultivar.
For the same water supply levels and nitrogen rates,
cultivar Waha had slightly higher WUE values than cultivar
Haurani. Irrigation regimes, greatly influenced wheat WUE.
However, wheat cultivars responded differently to irrigation
regime; cultivars Waha appeared to be more sensitive than
Haurani. The optimum WUE was produced with irrigation
treatment at 50% of SWD (IRR1) and nitrogen application of
150 kg ha
1
. WUE data for cultivar Haurani were 1–
2kgha
1
mm
1
less than WUE data for cultivar Waha.
However, there was no difference between the two cultivars
inWUEwhentheyweresuppliedwith200kgha
1
of
nitrogen.
Warmer temperatures recorded in spring months of years
2001 and 2002 may have permitted rigorous growth and more
rapid canopy closure in comparison to year 2004. Higher
temperatures, coupled with increasingly irregular rainfall in
the spring, resulted in more frequent periods during which the
soil surface was dry. Therefore, more water may have been
used through transpiration by plant than through soil surface
evaporation. In other words, the ratio of transpiration to
evaporation may have been greater for years with higher
temperatures than the cooler years. WUE data for wheat in the
two cropping years (2001 and 2002) agreed well, while this
situation does not correspond well in year 2004 because of the
cool weather conditions that dominate this year.
Acknowledgements
The authors wish to thank the technical staff of ICARDA for
providing the TDR and for helping in its calibration at Tal
Amara Research Station. Data elaboration and statistical
analyses at ICARDA headquarters in Aleppo, Syria, were
made possible thanks to WatNitMED Project (INCO-CT-2004-
509107) of the European Commission.
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