Yield and water-production functions of two durum wheat ... - ICARDA

agricultural water management 96 (2009) 603–615

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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 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 b

article info

abstract

Article history:

Wheat (Triticum durum L.) yields in the semi-arid regions are limited by inadequate water supply

Received 29 July 2008

late in the cropping season. Planning suitable irrigation strategy and nitrogen fertilization with

Accepted 27 September 2008

the appropriate crop phenology will produce optimum grain yields. A 3-year experiment was

Published on line 5 November 2008

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)

Keywords:

and nitrogen rate (NR). Three water supply levels (rainfed and two treatments irrigated at half

Irrigation strategy

and full soil water deficit) were coupled with three N fertilization rates (100, 150 and

Nitrogen rate

200 kg N ha1) and two cultivars (Waha and Haurani) under the same cropping practices

Supplemental irrigation

(sowing date, seeding rate, row space and seeding depth). Averaged across N treatments

Triticum durum L.

and years, rainfed treatment yielded 3.49 Mg ha1 and it was 25% and 28% less than half

Vapor pressure deficit

and full irrigation treatments, respectively, for Waha, while for Haurani the rainfed treatment

Water use efficiency

yielded 3.21 Mg ha1, and it was 18% and 22% less than half and full irrigation, respectively. On the other hand, N fertilization of 150 and 200 kg N ha1 increased grain yield in Waha by 12% and 16%, respectively, in comparison with N fertilization of 100 kg N ha1, while for cultivar Haurani 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 with respect to the rainfed treatment, while nitrogen fertilization was observed to have significant effects only on grain number per square meter. Moreover, results showed that grain 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 produced for both cultivars at 50% of soil water deficit as supplemental irrigation and N rate of 150 kg N ha1. However, Harvest index (HI) and water use efficiency (WUE) in both cultivars were not significantly affected neither by supplemental irrigation nor by nitrogen rate. Evapotranspiration (ET) of rainfed wheat ranged from 300 to 400 mm, while irrigated wheat had seasonal ET ranging from 450 to 650 mm. On the other hand, irrigation treatments 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 mbar1 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: [email protected] (F. Karam). 0378-3774/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2008.09.018

604 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. Environmental 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 (Garcıá 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 ha1), 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 (Duivenbooden et al., 1999; Oweis et al., 1999). Wheat’s most sensitive growth stages to water stress with respect to grain yield are stem elongation and booting, followed by anthesis and grain filling (Blum and Pnuel, 1990; Shpiler and Blum, 1991; Garcıá 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. Likewise, Giunta et al. (1993) and Zhong-hu and Rajaram (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 irrigation 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 (338510 4400 N lat., 358590 3200 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 (Garcıá 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 m2, 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 m2-experimental field (64 m NS 50 m WE).

605


Table 1 – Some agronomic and management practices carried out during the experiments. Observation a

Sowing date (d.o.y.) Cultivars Seeding depth (cm) Row space (cm) Seeding rate (plants m2) Cultivated area (m2) Effective cultivated area Harvest (d.o.y.) Growing period (days) a

2000–2001

2001–2002

2003–2004

5 November 2000 (310) Waha and Haurani 4–5 20 260–280 3200 (64 m NS 50 m WE) 2400 (48 m NS 50 m WE) 15 June 2001 (166) 222



Day of year (dates are given in parenthesis).

Three water supply treatments (rainfed and two treatments irrigated at 50% and 100% of soil water deficit) were coupled with three nitrogen rates (100, 150 and 200 kg N ha1). Water was applied using a line-source sprinkler at preanthesis 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-elongation, 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 ha1. Therefore, three N rates were applied at 100 (NR1), 150 (NR2) and 200 kg N ha1 (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 ha1. Then, ammonium nitrate (NH4NO3, 34-0-0) was applied in two splits, where an equal amount of 25 kg N ha1 was given to all treatments at stem elongation, and different amounts of 25, 75 and 125 kg N ha1 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 subplots of 20 m2 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 2 – Irrigation dates (day of year) and depth (mm) of wheat treatments. Date of irrigation

2000–2001 10 March 2001 20 April 2001 13 May 2001 Total irrigation Total rain (1 November 2000 onwards) Total (rain + irrigation) 2001–2002 15 April 2002 12 May 2002 Total irrigation Total rain (1 November 2001 onwards) Total (rain + irrigation) 2003–2004 27 March 2004 22 April 2004 12 May 2004 Total irrigation Total rain (1 November 2003 onwards) Total (rain + irrigation)

Growth stage

Day of year

Water depth (mm) IRR0

IRR1

IRR2

Booting Anthesis Dough stage

69 110 133

0.0 0.0 0.0 0.0 394.6 394.6

40.0 40.0 25.0 105.0 394.6 499.6

80.0 80.0 50.0 210.0 394.6 604.6

Anthesis Dough stage

105 132

0.0 0.0 0.0 544.2 544.2

45.0 45.0 90.0 544.2 634.2

90.0 90.0 180.0 544.2 724.2

Booting Anthesis Dough stage

87 113 133

0.0 0.0 0.0 0.0 654.7 654.7

20.0 20.0 20.0 60.0 654.7 714.7

40.0 40.0 40.0 120.0 654.7 774.7

606


Table 3 – Dates (days after sowing) and amounts (kg N haS1) of nitrogen fertilization of wheat treatments. Date of fertilization

Growth stage

Days after sowing

Fertilizer source

N application (kg N ha1) NR1

NR2

NR3

2000–2001 15 November 10 March 20 April

Sowing Stem elongation Booting

0 144 167

NPK (17%) NH4NO3 (34%) NH4NO3 (34%)

50 25 25

50 25 75

50 25 125

2001–2002 25 November 20 March 15 April


0 126 170

NPK (17%) NH4NO3 (34%) NH4NO3 (34%)

50 25 25

50 25 75

50 25 125

2003–2004 12 November 27 March 4 May


0 119 157

NPK (17%) NH4NO3 (34%) NH4NO3 (34%)

50 25 25

50 25 75

50 25 125

the base temperature (Tbase) 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 (uv), using the following calibration equation: Y ¼ 0:0079X þ 1:948

(1)

where Y represents uv (in %); X is 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 m3 m3, and the coefficient of determination was 0.91.

TDR readings were used to estimate seasonal evapotranspiration (ET) in the plots using a water balance model as the difference between inputs and outputs within the soil profile, assuming drainage (Dr) and runoff (Ro) in the layer 0–90 cm equal to zero: P þ I Dr Ro ET ðSe Sb Þ ¼ 0

(2)

where P is precipitation, I is irrigation, Dr is drainage, Ro is runoff, Se is the soil water content at the end of a time interval, Sb 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 m2 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). Harvest 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 ha1) 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.

607

14.2 592.0 22.5 0.0 21.8 0.0 19.8 0.0 16.2 17.0 12.7 41.0 9.2 81.0 6.5 103.0 5.9 145.0 7.4 124.0 11.5 58.0 20.0 0.0 1954–2002 Mean air temperature (8C) Rain (mm)

16.5 23.0


17.5 15.8


17.4 35.5

15.3 449.6 23.5 0.0 23.8 0.0 20.7 0.0 17.2 10.2 14.9 5.1 12.9 15.6 7.2 171.3

August July June May April March February January

7.2 58.5 8.4 99.1 11.6 34.8 16.4 50.9 20.2 4.1

Averaged across years, grain yield of cultivar Waha irrigated at 100% of SWD (IRR2) was 4470 kg ha1, showing 50 kg ha1

2000–2001 Mean air temperature (8C) Rain (mm)

3.2. Effects of supplemental irrigation on yield and its components

December

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 other hand, the relatively warmer weather 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 day1 in 2000–2001, 2001–2002 and 2003–2004 cropping 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).

November

Climatic conditions

October

3.1.

September

Results and discussion

Table 4 – Mean daily temperature and total rainfall prevailed during the experiments, compared to the long-run means (1954–2002).

3.

Average/tot


608


Fig. 1 – Daily evolution of vapor pressure deficit (VPD) at noon during the three cropping years.

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 ha1 and was 120 kg ha1 higher than IRR1 treatment, while rainfed treatment (IRR0) in cultivar Haurani had the lowest yield (3210 kg ha1). 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 irrigation 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 m2). 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). Garcıá 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 components 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 ha1. For cultivar Waha, even though there were differences among nitrogen treatments, yield at 200 kg N ha1 (4380 kg ha1) was slightly higher than yield at 150 kg N ha1 (4230 kg ha1), while nitrogen treatment of 100 kg N ha1 had the lowest yield (3770 kg ha1). For cultivar Haurani, nitrogen treatment of 100 kg N ha1 in Haurani had the lowest yield (3010 kg ha1) while treatments of 150 and 200 kg N ha1 yielded 3730 and 4150 kg ha1, 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 ha1 (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 treatments, grain number per unit ground area was observed to increase in cultivar Waha by 11% and 17% in NR2 and NR3,

609


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

2000–2001

Irrigation level

Nitrogen rate

F-tests

2001–2002

2003–2004

Irrigation level

IRR0 IRR1 IRR2 NR1 NR2 NR3

Irrigation level Nitrogen rate IRR NR Irrigation level

Nitrogen rate

F-tests


Irrigation level Nitrogen rate IRR NR

Nitrogen rate

F-tests

Waha



Irrigation level

Nitrogen rate


GN (m2)

GW (g)

7,406 8,894 8,234 7,684 8,545 8,305

43.97 49.06 49.00 48.03 48.40 45.60

Haurani

GY AB (Mg ha1) (Mg ha1) 3.26 4.37 4.03 3.70 4.16 3.81

9.75 11.45 11.14 9.26 9.94 13.13

HI

GN (m2)

GW (g)

0.34 0.39 0.38 0.40 0.42 0.29

7,258 8,341 7,686 5,958 7,747 9,579

45.97 48.11 50.22 51.12 48.39 44.79

GY AB (Mg ha1) (Mg ha1) 3.30 3.89 3.86 3.05 3.73 4.26

9.69 11.03 11.60 8.96 10.76 12.60

HI 0.34 0.36 0.33 0.34 0.35 0.34

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

***

*

**

*

*

***

*

**

*

*

9,027 9,531 9,881 8,858 9,281 10,299

38.98 42.72 45.22 41.08 42.28 43.55

3.52 4.08 4.48 3.64 3.93 4.50

9.15 9.32 9.74 8.47 9.05 10.69

0.39 0.44 0.46 0.43 0.43 0.42

8,752 9,400 10,598 8,064 9,995 10,691

38.18 43.00 45.42 40.55 42.35 43.70

3.36 4.05 4.83 3.30 4.25 4.70

8.95 9.42 11.46 8.17 10.07 11.59

0.37 0.44 0.42 0.40 0.42 0.40

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

***

*

**

*

*

***

*

**

*

*

8,419 10,665 9,989 8,480 10,044 10,549

43.77 45.18 49.00 46.53 45.73 45.68

3.69 4.80 4.90 3.96 4.61 4.82

9.15 9.98 10.74 9.35 10.32 10.21

0.40 0.48 0.46 0.42 0.45 0.47

6,408 7,008 6,106 5,702 6,632 7,188

46.55 48.22 49.49 47.36 48.49 48.41

2.98 3.39 3.02 2.70 3.21 3.49

8.56 8.35 8.11 8.57 8.51 7.95

0.35 0.42 0.37 0.32 0.38 0.44

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

***

*

**

*

*

***

*

**

*

*

8,284 9,697 9,368 8,341 9,290 9,718

Averages across years 42.24 3.49 9.35 45.65 4.42 10.25 47.74 4.47 10.54 45.22 3.77 9.03 45.47 4.23 9.77 44.94 4.38 11.34

0.38 0.44 0.43 0.42 0.43 0.39

7,472 8,250 8,130 6,575 8125 9,153

Averages across years 43.57 3.21 9.07 46.44 3.78 9.60 48.37 3.90 10.39 46.34 3.01 8.57 46.41 3.73 9.78 45.63 4.15 10.71

0.35 0.40 0.37 0.35 0.38 0.40

*

Significant at P < 0.05. Significant at P < 0.01. *** Significant at P < 0.001. **

respectively, in comparison with NR1 treatment (8341 grains m2). Moreover, N fertilization of 150 and 200 kg N ha1 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 ha1. 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 m2). 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 ha1 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 ha1 slightly lowered 1000-grain weight by 2–5%, with respect to N fertilization of 100 kg ha1 (48.03 g in 2001 and 46.53 g in 2004). When averaged across years, N fertilization of 150 kg ha1 did not stimulate any increase in 1000-grain weight with respect to N fertilization of 100 kg ha1 (46.34 g), while N fertilization of 200 kg ha1 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 ha1), while for cultivar Haurani the increases were 24% and 38% in NR1 and NR2, respectively, in comparison with NR1 (3010 kg ha1).

610


The relatively small increases in grain yield observed in NR2 and NR3 treatments of cultivar Waha in comparison with N fertilization with 100 kg ha1 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 ha1, in NR1, NR2 and NR3, respectively, exhibiting thus higher difference between the low (100 kg N ha1) and high (200 kg N ha1) 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 ha1 in NR1, 9.78 Mg ha1 in NR2 and 10.71 Mg ha1 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 sufficient 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 ha1) 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 conditions 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. Garcıá del Moral et al. (2003) evaluated grain yield and its components of six ICARDACIMMYT 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 ha1 resulted in grain yield of 3.77 Mg ha1 in Waha against 3.01 Mg ha1 in Haurani, while nitrogen applications at 150 and 200 kg ha1 resulted in grain yield of 4.23 and 4.38 Mg ha1 in Waha against 3.73 and 4.15 Mg ha1 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 m2 in IRR0, IRR1 and IRR2, respectively, against 7472, 8250 and 8130 grain m2 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 m2 in NR1, NR1 and NR3, respectively, against 6575, 8125 and 9153 grain m2 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 day1 (2000–2001), 1.9 mm day1 (2001–2002) and 2.2 mm day1 (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 day1 in 2000–2001 to 2.4 mm day1 in 2001–2002 and to 2.6 mm day1 in 2003–2004, while supplemental irrigation at 100% of soil water deficit increased theses values to 2.6 mm day1 (2000–2001), 2.9 mm day1 (2001–2002) and 3.0 mm day1 (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 evapotranspiration in all three years. Averaged across years, the applications of 150 and 200 kg N ha1 increased ET by 14–18 and 38–46 mm, respectively, in comparison with nitrogen application of 100 kg ha1 (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.

Table 6 – Measured evapotranspiration of wheat treatments according to water balance model (Eq. (2)) during the three cropping years. Growth stage

2001–2002 November December January February March April May June


Irrigation (mm)

Rain + irrigation (mm)

IRR0

IRR1

IRR2

IRR0

IRR1

IRR2

RD (cm) SW (mm)

ET (mm) IRR0

IRR1

IRR2

NR1

NR2

NR3

Establishment Seedling Tillering Stem elongation Booting Anthesis Dough stage Maturity Total

34.8 99.1 58.5 171.3 15.6 5.1 10.2 0.0 394.6

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 35.0 35.0 35.0 0.0 105.0

0.0 0.0 0.0 0.0 70.0 70.0 70.0 0.0 210.0

34.8 99.1 58.5 171.3 15.6 5.1 10.2 0.0 394.6

34.8 99.1 58.5 171.3 50.6 40.1 45.2 0.0 499.6

34.8 99.1 58.5 171.3 85.6 75.1 80.2 0.0 604.6

0.1 0.2 0.3 0.5 0.8 1.0 1.0 1.0

38.3 121.7 100.1 224.5 83.9 69.5 14.6 115.4

3.5 22.6 41.6 53.2 68.3 64.4 4.3 115.4 373.2

3.5 22.6 41.6 53.2 103.3 99.4 39.4 115.4 478.2

3.5 22.6 41.6 53.2 138.3 134.4 74.4 115.4 583.2

3.6 23.0 42.4 54.2 105.3 101.3 40.1 117.7 487.8

3.7 23.7 43.7 55.8 108.4 104.3 41.3 121.1 502.1

3.9 24.8 45.8 58.5 113.6 109.3 43.3 126.9 526.0

Establishment Seedling Tillering Stem elongation Booting Anthesis Dough stage Maturity Total

51.0 92.0 110.0 63.3 146.9 71.2 9.8 0.0 544.2

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 50.0 0.0 50.0 0.0 100.0

0.0 0.0 0.0 0.0 100.0 0.0 100.0 0.0 200.0

51.0 92.0 110.0 63.3 146.9 71.2 9.8 0.0 544.2

51.0 92.0 110.0 63.3 196.9 71.2 59.8 0.0 644.2

51.0 92.0 110.0 63.3 246.9 71.2 109.8 0.0 744.2

0.1 0.2 0.3 0.5 0.8 1.0 1.0 1.0

51.2 117.7 151.6 116.5 229.2 135.6 45.2 130.4

0.2 25.7 41.6 53.2 82.3 64.4 35.4 130.4 433.0

0.2 25.7 41.6 53.2 132.3 74.4 85.4 130.4 543.0

0.2 25.7 41.6 68.2 182.3 64.4 135.4 130.4 648.0

0.2 26.2 42.4 59.3 134.9 69.0 87.1 133.0 552.2

0.2 27.0 43.7 61.1 138.9 71.1 89.6 136.9 568.4

0.2 28.2 45.8 64.0 145.5 74.5 93.9 143.4 595.5

Establishment Seedling Tillering Stem elongation Booting Anthesis Dough stage Maturity

43.8 130.7 257.2 201.4 6.5 12.4 2.7 0.0 654.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 40.0 0.0 40.0 0.0 80.0

0.0 0.0 0.0 0.0 80.0 0.0 80.0 0.0 160.0

43.8 130.7 257.2 201.4 6.5 12.4 2.7 0.0 654.7

43.8 130.7 257.2 201.4 46.5 12.4 42.7 0.0 734.7

43.8 130.7 257.2 201.4 86.5 12.4 82.7 0.0 814.7

0.1 0.2 0.3 0.5 0.8 1.0 1.0 1.0

44.0 162.6 308.1 254.6 102.8 90.8 75.3 109.4

0.2 31.9 50.9 53.2 96.3 78.4 72.6 109.4 492.7

0.2 31.9 50.9 53.2 136.3 78.4 112.6 109.4 572.7

0.2 31.9 50.9 53.2 176.3 78.4 152.6 109.4 652.7

0.2 32.5 51.9 54.2 139.0 79.9 114.8 111.5 584.2

0.2 33.5 53.5 55.8 143.1 82.3 118.2 114.8 601.3

0.2 35.1 56.0 58.5 149.9 86.2 123.8 120.3 630.0



Rain (mm)

611

612


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 ha1 mm1 for grain yields < 3 Mg ha1 and seasonal rainfall of 330 mm, while for irrigated wheat and grain yield >3 Mg ha1 WUE was 10.8 0.28 kg ha1 mm1. 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 experimental site in northern Syria. In both cases, however, WUE were lower than the maximum value of 15 kg ha1 mm1 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 ha1 mm1 for cultivar Waha and 5.6 kg ha1 mm1 for cultivar Haurani to 7.6

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 mbar1 more ET/VPD than rainfed treatment, while N rates of 150 and 200 kg N ha1 had 4 and 12 mm mbar1 more ET/VPD than N rate of 100 kg N ha1. Averaged across years, water use efficiency (WUE) of cultivar Waha under rainfed conditions was 8.1 kg ha1 mm1, while irrigated treatments at 50% and 100% of SWD had WUE values of 8.3 and 7.1 kg ha1 mm1, respectively. For cultivar Haurani, rainfed treatment resulted in WUE of 7.4 kg ha1 mm1, while a slight decrease in WUE was observed in IRR1 (7.1 kg ha1 mm1) and IRR2 (6.2 kg ha1 mm1), respectively. Analysis of variance for the combined data showed that WUE in all years was not

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 ET/VPD (mbar) (mm mbar1)

Waha

Haurani

GY WUE GY WUE (Kg ha1) (kg ha1 mm1) (Kg ha1) (kg ha1 mm1) 2000–2001

Irrigation level

Nitrogen rate

F-testsa

2001–2002

Irrigation level Nitrogen rate IRR NR Irrigation level

Nitrogen rate

F-tests

2003–2004

Irrigation level



Irrigation level

Nitrogen rate

a



Nitrogen rate

F-tests



373.0 478.0 583.0 487.0 502.0 526.0

403.0 403.0 403.0 403.0 403.0 403.0

0.93 1.19 1.45 1.21 1.25 1.31

3258.93 4374.38 4030.36 3699.25 4155.01 3809.40

8.74 9.15 6.91 7.60 8.28 7.24

3301.80 3887.10 3855.38 3047.93 3731.83 4264.53

8.85 8.13 6.61 6.26 7.43 8.11

**

ns

**

**

ns

**

ns

*

ns

*

**

ns

**

ns

*

ns

*

*

ns

**

ns

433.0 543.0 648.0 552.0 568.0 595.0

368.0 368.0 368.0 368.0 368.0 368.0

1.18 1.48 1.76 1.50 1.54 1.62

3521.56 4077.23 4475.51 3640.31 3933.13 4500.87

8.13 7.51 6.91 6.59 6.92 7.56

3355.34 4054.77 4827.47 3295.25 4246.99 4695.34

7.75 7.47 7.45 5.97 7.48 7.89

**

ns

**

**

**

**

**

ns

**

**

**

**

**

*

ns

*

*

*

**

*

492.0 572.0 652.0 584.0 601.0 630.0

355.0 355.0 355.0 355.0 355.0 355.0

1.39 1.61 1.84 1.65 1.69 1.77

3686.13 4804.37 4895.63 3956.70 4606.56 4822.88

7.49 8.40 7.51 6.78 7.66 7.66

2984.68 3392.43 3021.93 2698.98 3213.60 3486.47

**

6.07 5.93 4.63 4.62 5.35 5.53

**

ns

**

**

**

**

**

ns

**

**

**

**

**

*

ns

*

*

*

**

*

432.6 531.0 627.6 541.0 557.0 583.6

375.3 375.3 375.3 375.3 375.3 375.3

1.16 1.42 1.68 1.45 1.49 1.57

ns, *, **non significant or significant at P < 0.05 or P < 0.01, respectively.

Averages across years 3488.88 8.12 4418.66 8.35 4467.17 7.11 3765.42 6.99 4231.57 7.62 4377.72 7.49

3213.94 3778.10 3901.59 3014.05 3730.80 4148.78

**

7.43 7.12 6.22 5.57 6.70 7.11


613

Fig. 2 – Relationship between grain yields (GY) and the total evapotranspiration (ET), for Waha and Haurani cultivars wheat over the three cropping years.

and 7.5 kg ha1 mm1 for the former, and 6.7 and 7.1 kg ha1 mm1 for the latter by applying 50 (IRR2) and 100 kg ha1 (IRR3) more N than N application of 100 kg ha1. 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 ha1 applications. On average, no significant increase in WUE occurred in the three growing years for N application more than 150 kg ha1.

3.6. Relationship between grain yield and evapotranspiration The relationship between grain yield (GY) and seasonal evapotranspiration (ET) is presented in Fig. 2. The relationships 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 temperature 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 magnitude 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

614


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 m2. 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 mechanisms 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 ha1. WUE data for cultivar Haurani were 1– 2 kg ha1 mm1 less than WUE data for cultivar Waha. However, there was no difference between the two cultivars in WUE when they were supplied with 200 kg ha1 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-2004509107) of the European Commission.

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