Plant and Soil 201: 295–305, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands.
295
Water-use efficiency and transpiration efficiency of wheat under rain-fed conditions and supplemental irrigation in a Mediterranean-type environment Heping Zhang, Theib Y. Oweis, Sonia Garabet and Mustafa Pala
International Center for Agricultural Research in the Dry Areas, P.O. Box 5466, Aleppo, Syria∗ Received 24 October 1997. Accepted in revised form 20 March 1998
Key words: nitrogen, soil evaporation, transpiration, water use
Abstract Growth and water use were measured in wheat (Triticum aestivum L.) grown in northern Syria in a typical Mediterranean climate over five seasons 1991/92–1995/96. Water use was partitioned into transpiration (T) and soil evaporation (Es ) using Ritchie’s model, and water-use efficiency (WUE) and transpiration efficiency (TE) were calculated. The aim of the study was to examine the influence of irrigation and nitrogen on water use, WUE and TE. By addition of 100 kg N ha−1 , Es was reduced from 120 mm to 101 mm under rain-fed conditions and from 143 mm to 110 mm under irrigated conditions, and T was increased from 153 mm to 193 mm under rain-fed conditions and from 215 mm to 310 mm under irrigated conditions. Under rain-fed conditions, about 35% of evapotranspiration (ET) may be lost from the soil surface for the fertilized crops and 44% of ET for the unfertilized crops. Transpiration accounted for 65% of ET for the fertilized crops and 56% for the unfertilized crops under rain-fed. As a result of this, WUE was increased by 44% for dry matter and 29% for grain yield under rain-fed conditions, and by 60% for dry matter and 57% for grain yield under irrigated conditions. Transpiration efficiency for the fertilized crops was 43.8 kg ha−1 mm−1 for dry matter and 15 kg ha−1 mm−1 for grain yield, while TE for the unfertilized crops was 33.6 kg ha−1 mm−1 and 12.2 kg ha−1 mm−1 for dry matter and grain yield, respectively. Supplemental irrigation significantly increased post-anthesis water use, transpiration, dry matter and grain yield. Water-use efficiency for grain yield was increased from 9.7 to 11.0 kg ha−1 mm−1 by supplemental irrigation, although WUE for dry matter was not affected by it. Irrigation did not affect transpiration efficiency for grain yield, but decreased transpiration efficiency for dry matter by 16%. This was associated with higher harvest index as a result of good water supply in the post-anthesis period and increased transpiration under irrigated conditions.
Introduction Soil water availability is considered to be the main factor limiting crop production in the Mediterraneantype climate of West Asia and North Africa. Crop production under this climate depends mainly on the amount of water available to the crop and the amount of water transpired by the crops. The amount of water transpired by a crop may be increased either by reducing soil evaporation or by supplemental irrigation. Estimates of soil evaporation range from 20% to 70% of total water used (Cooper, 1983; Cooper et al., ∗ FAX No: +963 21 213490. E-mail:
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1987a; Siddique et al., 1990). Soil evaporation can be reduced by crop structure (Siddique et al., 1990) and agronomic practices that stimulate early ground cover, such as application of fertilizers (Brown et al., 1987; Cooper et al., 1987a; Oweis et al., 1998), early sowing (Oweis et al., 1998) and increased plant density (Van den Boogaard et al., 1996). Water transpired by a crop can also be increased by applying supplemental irrigation, providing the water applied does not simply increase soil evaporation. Transpiration efficiency (TE) has been widely investigated at the leaf and individual plant scales (Condon et al., 1990; Farquhar and Richards, 1984; Van
296 den Boogaard et al., 1997). However, TE has been rarely investigated at the crop level owing to the difficulty in directly measuring transpiration from a field crop or in separating transpiration from evapotranspiration. Soil evaporation under a sparse canopy can be estimated either by using direct measurement with microlysimeters (Boast and Robertson, 1982) or by modelling of the physical process (Cooper et al., 1983; Lascano et al., 1987; Ritchie, 1972). Both approaches enable TE to be estimated from the measurements of soil water content and crop production. In this study, crop growth and water use of wheat were investigated over five seasons at a single site in northern Syria with a typical Mediterranean climate. Four treatments consisting of two nitrogen levels under rain-fed and supplemental irrigation conditions from the agronomic experiments over the five seasons from 1991 to 1996 were used to investigate the effect of N and supplemental irrigation on crop water use, water-use efficiency and transpiration efficiency.
Materials and methods Site and crop management The trials were conducted at Tel Hadya, the research station of the International Center for Agricultural Research in the Dry Areas (36◦010 N, 36◦ 560 E) in northern Syria from 1991/92 to 1995/96. The soil at Tel Hadya is classified as a thermic Calcixerollic Xerochrept, which is clay. It has field capacity of about 40% and wilting point at 24% by volume for the top 100 cm soil profile. In the 1991/92 and 1992/93 seasons, a completely randomized design of four nitrogen levels and four irrigation rates was replicated four times. In the other three seasons, a split-split-split-plot design considering three sowing dates, four varieties, four N levels and four irrigation levels was replicated three times. The wheat (Triticum aestivum L.) cultivar was ‘Cham 4’ for the 1991/92–1992/93 seasons and ‘Gomam’ for the 1993–1996 seasons. The crop was planted in 17.5-cm rows at a seed rate of approximately 300 seeds m−2 . The N application was split, half at planting and half top-dressed at early tillering stage. The sowing date was early November in all the seasons except 1992/93 (early December). Phosphorus was applied as a basal dressing each season as triple superphosphate (46% P2 O5 ) at rate of 40–50 kg P ha−1 , depending on the measurement of soil available P level before sowing. Previous studies (Garabet,
1995; Oweis, personal communication) showed that application of 100 kg N ha−1 and 66% of full supplemental irrigation achieved best yields and water-use efficiency. Therefore, we chose the treatments with 100 kg N ha−1 and without fertilization under both rain-fed and 66% of full supplemental irrigation for analysis in this paper. Water was applied when the soil moisture dropped below 50% of the available moisture in the plots receiving full supplemental irrigation. Surface irrigation was used to apply water for the 1991/92–1992/93 seasons and a drip irrigation system for the 1993/94–1995/96 seasons. In all the seasons, a flow meter was used to measure the actual amount of irrigation. The details of date and amounts of water applied in the five seasons are presented in Table 1. Leaf area, dry matter and grain yield Plant growth stages were recorded using the procedures described by Zadocks et al. (1974). Plant sampling started when plants had about three tillers in the 1991/92 and 1992/93 seasons. Plant samples were taken at 15-day intervals until plants approached maturity. Samples were taken from two and three 70-cm rows (0.28 m2 and 0.42 m2 ) for the 1991/92 and 1992/93 seasons, respectively. Plant samples were dried for 48 h in an oven at 65 ◦ C. At final harvest, three 3-m row lengths covering an area of 1.8 m2 were harvested (four replicates per treatment) for estimation of total dry matter and grain yield. In the other three seasons, above-ground dry matter was sampled only at the stages of seedling, stem elongation, booting, anthesis, soft dough and maturity over four areas of 0.2628 m2 and grain yield was harvested from 6.78 m2 . For leaf-area measurements, in the first two seasons, the 10-plant sub-samples were divided into leaves, stems, heads and dead leaves for leaf-area index measurements. Leaf area was measured using a standard moving belt planimeter (Model AAM, Hayashi Denko, Japan). There was no leaf-area measurement from 1993/94 to 1995/96. We plotted the leaf-area index against above-ground dry matter before anthesis using the data from the first two seasons of this experiment and published wheat data from Mediterranean environments (Bonachela et al., 1995; Gregory and Eastham, 1996; Siddique et al., 1989), and found that there was good relationship between the two variables (Figure 1). The regression equation was used to estimate the leaf-area index from the above-ground dry matter before anthesis. The leaf
297 Table 1. Amount of water applied and date of irrigation for irrigated crops and rainfall for the five seasons, 1991/92–1995/96 Season
1991/92 1992/93 1993/94 1994/95 1995/96
1st
Amount (mm) and date of irrigation 2nd 3rd 4th
Total
Rainfall (mm)
86 (11/4/92) 82 (12/4/93) 72 (6/4/94) 33 (9/3/95) 67 (5/5/96)
79 (5/5/92) 76 (26/4/93) 103 (20/4/94) 41 (28/3/95) 33 (15/5/96)
165 203 175 238 100
351 287 358 318 395
45 (12/5/93) 97 (14/4/95)
67 (5/5/95)
except during February and early March when heavy rainfall prevented access to the tubes. Soil evaporation, evapotranspiration and transpiration estimates Ritchie’s (1972) model was used to partition soil evaporation (Es ) and transpiration (T) in this study. According to Ritchie (1972), evaporation from a soil surface beneath a sparse canopy, Es1 (mm day−1 ), after thorough wetting the soil profile, is initially limited by the supply of energy to the soil surface, until a theoretical cumulative amount of water (U) has been evaporated from the soil surface layer: Figure 1. The relationship between leaf-area index and above-ground dry matter before anthesis. Data are from first two seasons in this study, Siddique et al. (1989), Bonachela et al. (1995), and Gregory and Eastham (1996).
area after anthesis was estimated from the leaf area at anthesis. Soil water In the experiments from 1991/92 to 1992/93, two aluminium access-tubes were installed to a depth of 180 cm in each plot of four replicates before the crop was sown. In the experiments from 1993/94 to 1995/96, one single aluminium access-tube was installed to a depth of 180 cm in each plot of one replicate for Gomam. Soil volumetric water content was measured using a neutron moisture probe (MK II, Didcot Instruments, Wallingford, UK) calibrated separately on site. Counts were made for 64 s and 15-cm intervals starting at 23 cm depth. The moisture content at the 0–15 cm layer was measured gravimetrically by sampling undisturbed cores of soil 15 cm long by 5.4 cm diameter from each plot. Measurements were made at approximately 7–14-day intervals,
Es1 =
(1 − L)ρCp rDa 1 Rn (1 − f ) + 1+γ 1+γ for L < 1
Es1 = [1/(1 + γ )]Rn (1 − f )
(1a)
for L ≥ 1 (1b)
where 1 is the slope of the saturation vapor pressure curve at mean air temperature, γ is psychrometric constant, Rn is the net radiation, D is the saturation deficit of the air, f is the fractional light interception, L is the leaf-area index, ra is the aerodynamic resistance at soil surface, ρ is the density of air, and Cp is the specific heat capacity. P When Es1 reaches U, soil evaporation comes into the second stage. Soil evaporation in this stage (Es2) depends on the hydraulic conductivity of the soil and can be estimated as follows: X Es2 = αt1/2 (2) where t is time (days) after the start of Es2 , and α is a soil-specific parameter (mm day−1 ). The value of α for the soil in this study ranged from 0.9 to 3 mm day−1 (Allen, 1990). The value of 2.5 mm day−1 was
298
Figure 2. Seasonal change of mean leaf area index of wheat over the five seasons. The leaf area index was measured in the 1991/92 and 1992/93 seasons, and was estimated in the 1993/94–1995/96 seasons using the relation in Figure 1. The standard errors are shown only for the irrigated crop with N and rain-fed crop without N.
Figure 3. The fractional light interception by crop over the five seasons.
used in the calculations. The value of U of 6 mm for this study was taken for a clay soil from Ritchie (1972). The fractional light interception, f, was calculated from the leaf-area index L, using the relationship
Leaf area index and light interception
f = 1 − exp(−kL)
(3)
where k is the extinction coefficient (0.39). Evapotranspiration (ET) was estimated using a standard water balance equation: ET = 1S + P + I − Dr
(4)
where 1S is the change of soil water storage between the two neutron probe readings, P is precipitation (mm), I is the amount of irrigation (mm) and Dr is the drainage from the bottom of the root zone. The soil water content measurements showed no drainage during the crop-growing season and therefore Dr was taken as zero. Transpiration rate (T) was obtained by T = ET − Es
(5)
Results
Mean seasonal change in leaf-area index (L) and fractional light interception by the crop over the five seasons are presented in Figures 2 and 3. There was little difference in L between the rain-fed and irrigated crops before anthesis under the same N treatment. Leaf-area index was higher for the fertilized crops than for the unfertilized ones during each growing season. Addition of 100 kg N ha−1 doubled the leaf area. Leafarea index was similar for the irrigated and rain-fed crops before anthesis, but post-anthesis green-area duration was longer under irrigation than under rain-fed (data not shown). The proportion of the incident radiation intercepted by crop (f) increased with L, reaching a maximum at about 148 days after sowing (DAS). The maximum values of f ranged from 0.71 to 0.88 for the fertilized crops under irrigation, and from 0.40 to 0.70 for rain-fed crops without fertilization over the five seasons. Dry matter and grain yield
Weather Wind speed, incoming solar radiation, wet and dry bulb air temperatures, rainfall and class A pan evaporation were measured in the weather station located at Tel Hadya research station. Details of major climate factors are presented in Table 2.
Dry-matter accumulation followed a similar trend in all the treatments over the five seasons and is presented for rain-fed and irrigated treatments and for the fertilized and unfertilized treatments (Figure 4). Dry-matter accumulation was consistently higher for the fertilized crops than for the unfertilized crops. Irrigation did not affect the pre-anthesis accumulation of dry matter, but significantly increased dry-matter accumulation after anthesis. Dry matter and grain yield at final harvest
299 Table 2. Rainfall (P) and vapor pressure deficit (D) at Tel Hadya, northern Syria, 1991/92–1995/96 Month
1991/92 P D (mm) (kPa)
1992/93 P D (mm) (kPa)
1993/94 P D (mm) (kPa)
1994/95 P D (mm) (kPa)
1995/96 P D (mm) (kPa)
Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun.
73 25 74 62 75 16 0.4 23 3.5
0 49 50 58 40 42 0.6 37 0.6
18 42 16 109 137 12 11 13 0
12 109 45 41 18 23 44 27 0.6
9 59 31 74 47 143 27 5 0
1.18 0.51 0.21 0.17 0.18 0.39 0.67 0.85 1.42
1.38 0.69 0.19 0.23 0.32 0.43 0.73 0.85 1.73
1.48 0.43 0.28 0.21 0.32 0.47 0.73 1.18 1.73
1.18 0.42 0.17 0.15 0.31 0.45 0.63 1.43 1.81
1.19 0.42 0.23 0.18 0.31 0.30 0.51 1.20 2.06
Table 3. Two-way analysis of variance of the effect of nitrogen (N) and irrigation (W) on crop water use (ET, Es , T, ETa , ETpa ), dry matter (DM), grain yield (GY), water-use efficiency (WUEdm and WUEgr ) and transpiration efficiency (TEdm and TEgr ). For each independent variable the values indicate the percentage of the total sum of square explained by the model, which could be attributed to that effect Source
d.f.
ET
ETa
ETpa (mm)
Es
T
DM GY (kg ha−1 )
HI
WUEdm
Season N W N×W Residual
4 1 1 1 12
13 10∗ 60∗∗∗ 2 15
40∗ 33∗ 1 0 26
16 1 73∗∗∗ 2 8
35∗ 29∗∗ 11∗ 2 23
22∗ 22∗ 38∗∗ 4 14
24∗∗ 48∗∗∗ 15∗∗ 2 1
12 16∗ 37∗∗ 6 30
24∗∗ 62∗∗∗ 1 0 12
23∗∗ 32∗∗∗ 27∗∗ 5∗ 12
WUEgr TEdm (kg ha−1 mm−1 ) 30∗ 43∗∗∗ 1 4 23
16 50∗∗∗ 17∗∗ 0 17
TEgr
24 35∗∗ 0 0 40
∗ , ∗∗ , ∗∗∗ Statistically significant at P < 0.05, P < 0.01, P < 0.001, respectively.
crop than that for the unfertilized crops (Tables 3 and 4). Irrigation also significantly increased the aboveground dry matter (P < 0.01) and grain yield (P < 0.001). Increase in grain yield in the fertilized crops mainly resulted from the increase in above-ground dry matter because application of N decreased harvest index. Seasonal trend in soil evaporation and transpiration
Figure 4. Changes in mean above-ground dry matter over the five seasons for the fertilized crops under supplemental irrigation, the unfertilized crops and the fertilized crops under rain-fed conditions.
were significantly higher (P < 0.001) for the fertilized
Seasonal variations in daily rate of Es and T for the irrigated and rain-fed crops are shown for five seasons in Figure 5. Although there was variation among seasons, the pattern of Es and T was generally similar. During the first 30 DAS, Es accounted for 92% of ET. During the cool winter, T was on average low (0.1– 0.3 mm day−1 ) due to slow development of green area in winter, and Es was almost approaching potential Es due to frequent rain events. After 80 DAS, as the green area increased rapidly (Figure 2), Es dropped to 0.5 mm day−1, and T rose rapidly. Transpiration
300 Table 4. Dry matter (DM), grain yield (GY) and harvest index (HI) of fertilized and unfertilized wheat under rain-fed and irrigated conditions at Tel Hadya, 1991/92–1995/96 Season
Rain-fed DM GY HI (kg ha−1 )
Irrigated DM GY (kg ha−1 )
HI
13260 10072 13 826 11 974 9167 11660
5221 4629 5350 4907 3275 4676
0.39 0.46 0.39 0.41 0.36 0.40
8665 7008 7536 6626 3019 6751
3636 3177 3090 2670 1174 2749
0.42 0.45 0.41 0.40 0.39 0.41
100 kg N ha−1 1991/92 1992/93 1993/94 1994/95 1995/96 Mean
9093 5422 10803 7745 8983 8409
3233 1646 3537 2823 3062 2860
0.36 0.30 0.33 0.36 0.34 0.34 0 kg N ha−1
1991/92 1992/93 1993/94 1994/95 1995/96 Mean
6843 3947 6929 5119 3105 5189
2832 1554 2750 1887 1114 2027
0.41 0.39 0.40 0.37 0.36 0.39
SE (±)
546
249
0.012
reached its maximum at about 160 DAS (mid-April), when Es was minimal: T was 3–6.4 mm day−1 and Es was 0.3 mm day−1 . The maximum transpiration rate depended on the rainfall and amount of irrigation. In all the seasons, leaf senescence occurred in the post-anthesis period. Transpiration gradually dropped with declining leaf area and Es increased as irrigation or rain events wetted the soil surface during postanthesis. Up to anthesis the patterns of Es and T were generally similar for rain-fed and irrigated conditions (Figure 5). The post-anthesis rate of transpiration under rain-fed conditions was significantly lower than that under irrigation, because of the moisture stress after anthesis. Transpiration rate per unit leaf area, which was estimated by dividing the total transpiration by leaf area for the period from the first irrigation to physiological maturity, was 1.1 ± 0.1 mm m−2 day−1 under supplemental irrigation and only 0.48 ± 0.1 mm m−2 day−1 under rain-fed conditions. Water use and water-use efficiency Evapotranspiration (ET) varied with rainfall under rain-fed conditions, and with the total amount of water applied under supplemental irrigation (Table 5). Under rain-fed conditions, ET was highest in the 1995/96
season with the highest rainfall, and lowest in the 1992/93 season with the least rainfall (Table 2). Crop water use was similar up to anthesis under rain-fed and irrigated conditions in all seasons except 1994/95, in which 74 mm of irrigation before anthesis increased pre-anthesis water use. After anthesis, the irrigated crops used 116 mm more water than rainfed crops. Hence, the ratio of pre-anthesis water use (ETa ) to post-anthesis water use (ETpa ) was lower under irrigation than under rain-fed conditions. The ratio ETa /ETpa was also dependent on the rainfall distribution during the season. Less rainfall after anthesis resulted in higher ratio of ETa /ETpa in the 1992/93– 1994/95 seasons. Application of N increased water use before anthesis by 28–37 mm (P < 0.05) and reduced water use after anthesis by 5–20 mm, resulting in a higher ETa /ETpa ratio. For the fertilized crops, water-use efficiency for dry matter (WUEdm ) ranged from 23 to 33 kg ha−1 mm−1 under rain-fed and irrigated conditions; there was no significant difference in WUEdm between the rain-fed and irrigated conditions. However, water-use efficiency for grain yield (WUEgr ) was slightly increased by irrigation for the fertilized crops, but slightly reduced by irrigation for the unfertilized crops. The fertilized crops consistently had signifi-
301
Figure 5. Seasonal variation of transpiration and soil evaporation for fertilized crops under rain-fed and irrigation conditions for five seasons. 1: 1991/92; : 1992/93; E : 1993/94; : 1994/95 and B : 1995/96. The solid line was estimated using a Gaussian curve for transpiration rate and a four-order polynomial for soil evaporation to represent the average.
#
cantly higher WUEgr than unfertilized crops under both rain-fed and irrigated conditions over five seasons. Addition of 100 kg N ha−1 increased WUEdm and WUEgr by 44% and 29% for the rain-fed crops, and 60% and 57% for the irrigated crops, respectively. Soil evaporation, transpiration and transpiration efficiency The cumulative Es and T from planting to maturity under rain-fed and irrigated conditions at two nitrogen levels are presented in Table 6 for five seasons. Transpiration efficiency for dry matter (TEdm ) and grain yield (TEgr ) were calculated by dividing the dry matter and grain yield (Table 4), respectively, by the total amount of transpiration (T) (Table 6). Application of N significantly (P > 0.05) reduced Es and increased transpiration. Under rain-fed conditions, Es was reduced from 120 mm to 101 mm, and T increased from 153 mm to 193 mm. The ratio Es /ET was 9% lower for the fertilized crops than for the unfertilized crops, and
the ratio Es /T was 27% lower. Under irrigated conditions, Es was reduced from 143 mm to 110 mm and T increased from 357 mm to 420 mm by addition of 100 kg N ha−1 . The variation in Es among seasons was smaller for the fertilized crops than for the unfertilized crops. Irrigation slightly increased Es because it wetted the soil surface two or three times more than rainfed treatments during periods of high vapor pressure deficit (D) (April and May). However, the ratios Es /ET and Es /T were significantly (P < 0.05) lower under irrigated conditions than under rain-fed conditions. Significantly more water was transpired under irrigated conditions than under rain-fed conditions (Tables 3 and 6). On average, TEdm for the fertilized crops was 43.8 kg ha−1 mm−1 , and TEgr 14.9 kg ha−1 mm−1 under rain-fed conditions. Transpiration efficiency for dry matter was significantly higher under rain-fed conditions than under irrigation over all five seasons, indicating that irrigation may decrease TEdm . However, TEgr values were similar under rain-
302 Table 5. Pre-anthesis water use (ETa ), post-anthesis water use (ETpa ), total water use (ET), ratio of pre- to post-anthesis water use (ETa /ETpa ) and water use efficiencies for dry matter (WUEdm ) and grain yield (WUEgr ) of wheat at Tel Hadya, 1991/92–1995/96 Season ETa
ETpa (mm)
ET
Rain-fed ETa /ETpa
WUEdm WUEgr (kg ha−1 mm−1 )
ETa
ETpa (mm)
ET
Irrigated ETa /ETpa
225 170 237 277 223 226
224 207 220 177 141 194
449 377 456 454 364 420
1.0 1.3 1.1 1.6 1.6 1.3
28.8 26.7 30.3 26.3 25.2 27.5
11.3 12.3 11.7 10.8 9.0 11.0
195 171 197 209 180 190
201 198 174 147 115 167
396 369 371 356 295 348
1.0 0.9 1.1 1.4 1.6 1.2
21.9 19.0 20.3 14.4 10.2 17.2
9.2 8.4 8.3 5.3 4.0 7.0
WUEdm WUEgr (kg ha−1 mm−1 )
100 kg N ha−1 1991/92 1992/93 1993/94 1994/95 1995/96 Mean
215 191 255 207 216 217
98 45 74 51 119 77
313 236 329 258 335 294
2.2 4.2 3.4 4.1 1.8 2.8
29.1 23.0 32.9 27.7 26.9 27.9
10.4 7.0 10.8 11.2 9.2 9.7 0 kg N ha−1
1991/92 1992/93 1993/94 1994/95 1995/96 Mean
183 167 220 180 189 188
SE (±)
8.3
103 57 94 59 109 84
286 224 314 258 275 271
1.8 2.9 2.3 3.1 1.8 2.4
23.9 17.6 22.1 21.4 10.4 19.3
9.9 6.9 8.8 7.9 3.8 7.5
13
15
0.26
1.2
0.64
Table 6. Soil evaporation (Es ) and transpiration (T) estimates, ratio of soil evaporation to evapotranspiration (Es /ET) and transpiration efficiencies for dry matter (TEdm ) and grain yield (TEgr ) of wheat at Tel Hadya, 1991/92–1995/96 Season Es
T (mm)
ET
Rain-fed Es /ET (%)
TEdm TEgr (kg ha−1 mm−1 )
Es
1991/92 1992/93 1993/94 1994/95 1995/96 Mean
96 102 96 103 110 101
217 134 232 155 225 193
313 236 329 258 335 294
31 43 29 40 33 35
42.0 40.5 46.5 50.0 40.0 43.8
100 kg N ha−1 15.4 91 12.3 102 15.2 99 18.2 135 13.6 124 14.9 110
1991/92 1992/93 1993/94 1994/95 1995/96 Mean
112 111 113 108 155 120
174 113 201 131 143 153
286 224 314 239 298 272
39 49 36 45 52 44
39.3 34.9 34.5 37.6 21.8 33.6
0 kg ha−1 16.3 13.8 13.7 8.7 7.8 12.1
16
16
2
1.6
SE (±)
6.5
0.97
N 112 139 119 179 164 143
Irrigated Es /ET (%)
T (mm)
ET
TEdm TEgr (kg ha−1 mm−1 )
358 275 357 320 240 310
449 377 456 455 364 420
20 27 22 30 34 27
37.0 36.6 38.7 37.4 38.2 37.6
14.6 16.8 15.0 15.3 13.6 15.0
285 230 251 177 131 215
396 369 370 356 295 358
28 38 32 50 56 40
30.4 30.4 30.0 28.9 23.1 28.6
12.8 13.5 12.3 10.7 9.0 11.7
303 fed and irrigated conditions when 100 kg N ha−1 was added. Application of 100 kg N ha−1 significantly (P < 0.001) increased TEdm and TEgr under both rainfed and irrigated conditions; TEdm and TEgr were 30% higher for the fertilized crops than for the unfertilized crops. Discussion Soil evaporation estimation Partitioning Es from ET is important for estimating TE. Soil evaporation was estimated using the Ritchie’s (1972) model, giving a value of 101–120 mm, which is about 20 mm lower than the values for wheat and barley reported by Cooper et al. (1983) and for barley by Allen (1990). Cooper et al. (1983) report that Es was 120 mm from a fertilized wheat and 137 mm for an unfertilized wheat, with rainfall of 372 mm, which was considerably higher than the mean rainfall in this study. The mean value of Es in this study is close to the values reported in South Australia (110 mm; French and Schultz, 1984) and New South Wales in Australia (95 mm; Condon et al., 1993). The proportion of Es in the seasonal ET was on average 35% for the fertilized crops and 44% for the unfertilized crops under rain-fed conditions over five seasons. These values are close to those reported for the fertilized and unfertilized wheat and barley under rain-fed conditions in northern Syria (Cooper et al., 1983, 1987a) and for dryland wheat in Australia (Condon et al., 1993; Siddique et al., 1990). We are not able to compare the estimation of Es with the model of Cooper et al. (1983), because the latter requires evaporation from a fallow plot that was not measured in this study. In addition, the fraction light interception was not measured in the 1993/94– 1995/96 seasons and the exclusion of this parameter may also introduce some uncertainty in estimating Es . Yunusa et al. (1993b) report that Ritchie’s model gave the most consistent agreement with the measured Es throughout the seasons compared with other models, because Ritchie’s model discriminates between first and second stages of evaporation. However, Allen (1990) shows that Ritchie’s model did not agree well with the microlysimeter measurements because the model completely ignores water extraction by plant roots. Although the estimation of Es in this study was not tested using microlysimetry, we have confidence in the estimation of Es by Ritchie’s model because of the reasonable agreement with other studies in similar environments.
Water-use efficiency Irrigation increased WUEgr in the fertilized crops, although it did not increase WUEdm . Increase in WUE can result only from changes in TE, the Es /T ratio and harvest index under a given climate condition (Cooper et al., 1987b; Sinclair et al., 1984). The decrease in the Es /T ratio in our study may have increased WUEdm under irrigation. However, the increase in WUEdm from this might have been counteracted by the decrease in TEdm (Table 6), which resulted from both greater leaf area (Figure 2) and higher stomatal conductance as indicated by higher transpiration rate (Figure 5). Transpiration rate per unit leaf area in the irrigated crops was double that in the rain-fed crops from the first irrigation to physiological maturity, indicating that higher stomatal conductance enhanced transpiration under irrigation. Differences in water-use efficiency of wheat at the leaf and field levels were related to variation in stomatal conductance in the same environment as this study (Van den Boogaard et al., 1996, 1997). The increase in WUEgr under irrigation was mainly due to the increase in crop water use after anthesis, which resulted in a higher harvest index. Application of N significantly increased WUEdm and WUEgr both under rain-fed and irrigated conditions. This is in agreement with other experiments in Mediterranean climates (Cooper et al., 1987a). The increase in WUE from application of N is not only due to the decrease of the Es /T ratio, but also to improvement in TE and more rapid crop growth during the season with lower vapor pressure deficit in early spring. Application of N slightly reduced post-anthesis water use due to more pre-anthesis water use under rain-fed conditions. Crop yield and WUE were not affected by this decrease in post-anthesis water use. Transpiration efficiency The values of TE for dry matter and grain yield in the present study compare well with those reported for wheat crops in other Mediterranean environments (Cooper et al., 1983; Siddique et al., 1990). Our study showed that TEdm of the rain-fed crops was on average 16% higher than that for irrigated crops over five seasons, indicating that water stress may increase TEdm , or that irrigation might not increase TEdm . Similar results are reported for wheat grown in eastern Australia (Doyle and Fischer, 1979) and at whole-plant scale in a glasshouse experiment (Condon et al., 1990). Variation in TE at leaf and plant scales for wheat has
304 been reported to result from differences in photosynthetic capacity or in stomatal conductance (Condon et al., 1990; Martin et al., 1994; Morgan and Le Cain, 1991). In our study, irrigation certainly reduced postanthesis plant water stress and we assume that the irrigated crops have higher stomatal conductance apart from greater leaf area, resulting in higher values of transpiration (Figure 5) relative to the rain-fed crops. Although the extent to which variation in conductance affects transpiration efficiency is dependent on boundary layer conductance in the field (Condon et al., 1990; Jarvis and McNaughton, 1986), increased stomatal conductance may have enhanced transpiration per unit leaf area by more than 100% in the present study. This enhanced transpiration rate per unit leaf area might have reduced crop TEdm under irrigation, even though the photosynthetic capacity may have been increased. In addition, irrigation was usually applied in April and May when vapor pressure deficit was high (Table 2). This might result in a lower ratio of photosynthesis to transpiration as compared with water used by plants in the season with lower vapor pressure deficit (Sinclair et al., 1984). However, TEgr under supplemental irrigation was similar to that under rain-fed although the irrigated crops had lower TEdm . This was attributed to the adequate water supply during post-anthesis, which not only allows current photosynthesis, but also more importantly, gives plant extra time to translocate reserves to the grains as indicated by a higher harvest index (Table 4). On average, TEdm and TEgr were significantly increased by the addition of N under irrigated and rainfed conditions, although TEgr was slightly higher for the unfertilized crops than for the fertilized crops under rain-fed conditions in the first two seasons. Much of the work on the influence of N on TE reports contradictory results. Cooper (1983) and Pilbeam et al. (1995) report that TE was not influenced by differences in N, while others show that TE was increased by application of N (Cooper et al., 1987a). Photosynthetic capacity at the leaf scale may be increased by higher leaf nitrogen concentration (Evan, 1983; Morgan, 1986), but others have shown that photosynthetic capacity at leaf scale was not increased by the difference in leaf nitrogen concentration (Condon et al., 1992; Gregory et al., 1981). The measurements of N concentration for whole plants at different growing stages showed that the fertilized crops had significantly higher N concentration than the unfertilized crops in the first two seasons of this study (Garabet, pers. comm.). This must have maintained or even in-
creased the photosynthetic capacity of the crops in the early spring, resulting in higher yield as well as higher TE. Increase in TE from the application of N may, probably more importantly, be due to more rapid growth during the period when vapor pressure deficit was low. Brown et al. (1987) report that fertilized wheat consistently grew faster than unfertilized wheat from the beginning of stem extension to about 10 days from maturity, and application of fertilizer resulted in more dry matter at different growing stages. In this study, significantly more water was used by the fertilized crops than by the unfertilized crops before anthesis. As a result, the dry-matter accumulation at anthesis for the fertilized crops was double that of the unfertilized crops. This indicates that higher TE for the fertilized crops was mainly due to water use pre-anthesis when vapor pressure deficit was lower, rather than post-anthesis when vapor pressure deficit was much higher. More rapid crop development and increased growth rate during early spring with lower vapor pressure deficit would increase transpiration efficiency. In conclusion, application of N can significantly increase crop growth during periods with lower vapor pressure deficit and can reduce Es , and hence increase water-use efficiency. The increase in water-use efficiency and TE by application of N is not only due to the decreased ratio of Es to ET, but also to the increase in TE from the more rapid growth during the growing period with lower vapor pressure deficit. Irrigation did not affect water-use efficiency for dry matter. This has been attributed to the offset of an increase in water-use efficiency from the decrease in the ratio of Es /T and a decrease in TE under irrigation. However, irrigation increased water-use efficiency for grain yield due to high harvest index as a result of the good water supply during the reproductive stage. Irrigation may decrease TE due to higher transpiration caused by greater leaf area and higher stomatal conductance under irrigation.
Acknowledgements This work was partially supported through the restricted core program funding (Project No. 94.7860.301.100) from Der the Bundesministerium für Wirtschaftliche Zusammenarbeit (BMZ).
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