Agricultural Water Management 61 (2003) 63–74
Deficit irrigation effects on sweet corn (Zea mays saccharata Sturt) with drip irrigation system in a semi-arid region I. Water-yield relationship Abdullah Oktema,*, Mehmet Simsekb, A. Gulgun Oktemc a
Department of Field Crops, Faculty of Agriculture, Harran University, 63200 Sanliurfa, Turkey b Department of Agricultural Structures and Irrigation, Faculty of Agriculture, Harran University, 63200 Sanliurfa, Turkey c Ministry of Agriculture, Regional Directorate, 63100 Sanliurfa, Turkey Accepted 28 October 2002
Abstract This study was conducted to determine appropriate irrigation frequencies and water-yield relationship for sweet corn irrigated by drip irrigation system in a semi-arid region. The research was carried out at the Agricultural Research Station of Harran University in Sanliurfa, Turkey, in 1998 and 1999. In the study, water was applied to sweet corn as 100, 90, 80 and 70% of evaporation from a Class A Pan corresponding to 2-, 4-, 6-and 8-day irrigation frequencies, respectively. Irrigation water applied to crops ranged between 610 and 876 mm in 1998, while 612–889 mm of water was applied in 1999. The highest values for total water use efficiency (TWUE) were found to be 1.38 and 1.24 kg m3 in the 4day irrigation frequency in 1998 and 1999, respectively. The highest values for irrigation water use efficiency (IWUE) were determined to be 1.66 kg m3 in the 4-day irrigation frequency in 1998 and 1.59 kg m3 in the 6-day irrigation frequency in 1999. Crop response factor (ky) ranged from 0.76 to 1.22 in 1998 and from 0.96 to 1.29 in 1999. Fresh ear yield, based on the irrigation frequencies, was found to be statistically significant (P < 0:01) in both years. The highest fresh ear yields were 13.66 and 13.19 t ha1 for the 2-day irrigation frequency in 1998 and 1999, respectively. The minimum fresh ear yields were found to be 8.55 and 7.29 t ha1 with the 8-day irrigation frequency in 1998 and 1999, respectively. Yield was reduced with deficit irrigation in both years. The results of this research indicate that a 2-day irrigation frequency, with 100% ET water application by a drip system, will be optimal for sweet corn grown in semi-arid regions similar to that in Turkey where the work was conducted. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sweet corn; Deficit irrigation; Drip irrigation; Irrigation frequency; Water-yield relationship * Corresponding author. Tel.: þ90-414-2473680; fax: þ90-414-2474480. E-mail address:
[email protected] (A. Oktem).
0378-3774/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 0 2 ) 0 0 1 6 1 - 0
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1. Introduction Fresh consumption of sweet corn is more beneficial compared to other derivatives of corn due to its soft kernels, thin shells, high concentration of sugar and tastefulness. The production of sweet corn has recently begun in tourist and coastal regions of Turkey. Canned sweet corn or a mixture of sweet corn with other foods are preferred by the consumers. It is used as an appetizer or in salads. It is also consumed as frozen sweet corn and can be kept fresh for a long time. Dough made from dry kernels of sweet corn is used for baby food, chips, dough products, pasta and cakes. In a resent paper (Oktem and Oktem, 1999), reported that sweet corn could be adapted to the Harran Plain due to the suitable climate and soil conditions of the region. Therefore, this will necessitate a study of the water-yield relationship of sweet corn in a semi-arid region, like Harran Plain, especially for water management purposes. Most of the regions in Turkey have an arid or semi-arid climate and the water required for summer crops is obtained mainly through irrigation systems; 1.7 million ha in area will be irrigated when the Southeastern Anatolian Project (GAP) is completed. Currently, traditional irrigation methods which are known to increase the amount of surface run-off are used in Harran Plain. On the other hand, in drip irrigation systems, water and nutrients can be applied directly to the crop at root level, which have positive effects on yield and water savings and thus increase the irrigation performance (Phene and Howell, 1984). For these reasons, drip irrigation systems have seen widespread use in the world in recent years. When water resources or operational costs are limiting factors in yield production, irrigation programs need to be applied to enable maximum production per unit irrigation water (Doorenbos and Kassam, 1979). The relationship between crop yield and water use have been a major focus of agricultural research in arid and semi-arid regions and have been reviewed previously by Hanks (1983), Vaux and Pruitt (1983), and Howell (1990). Also water-yield relationship has been investigated using different methods of limited water applications and programs (Lyle and Bordovsky, 1995; Zhang and Oweis, 1999; Pandey et al., 2000). Stone et al. (2001) developed a model to explain the effects of water stress on crop development. The aim of irrigation is to optimise the yield by minimising the damage caused by water stress during the crop development stages (Stone et al., 2001). Under water stress conditions, leaf area index (LAI) of crops and yield decreases. The reduction in yield and its degree depend upon the timing of water stress on the crops and the period of irrigation events (Jamieson et al., 1995). In previous studies, the average fresh ear yield was reported to be 18.10 t ha1 (Posternak et al., 1995). In addition, fresh ear yield with Sucro variety was found to be between 9.10 and 13.70 t ha1 (Olsen et al., 1990a) and sweet corn yield was between 13.90 and 14.90 t ha1 under a drip irrigation system (Steele et al., 1996). Howell et al. (1998) reported that seasonal water consumption of corn ranged from 465 to 802 mm and water use efficiency (WUE) was between 1.65 and 1.68 kg m3 under wellirrigated conditions. On the other hand, Musick and Dusek (1980) stated that seasonal water consumption of corn was 667–789 mm, WUE was 1.25–1.46 kg m3 and yield varied from 9.52 to 10.85 t ha1.
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Gencoglan (1996) reported that the amount of irrigation water for corn varied from 752 to 823 mm, seasonal water consumption from 999 to 1052 mm, and crop response factor (ky) from 1.08 to 1.61. Also irrigation water use efficiency (IWUE) was 1.0–2.43 and total water use efficiency (TWUE) was 0.22–1.25 kg m3. In another study in Texas, the daily water consumption, seasonal water consumption and the amount of irrigation water for corn were reported to be 12.4, 973 and 644 mm, respectively; the yield was 15.50 t ha1 at insufficient precipitation conditions (Howell et al., 1997). In another study, the maximum yield was found to be 10.15 t ha1 with 5-day irrigation frequency and seasonal water consumption was 1371 mm, whereas the minimum yield was 7.71 t ha1 at 10-day irrigation frequency and the seasonal water consumption was 970 mm. Applied irrigation water was 1303 and 970 mm at 5- and 10-day irrigation frequencies, respectively (Cetin, 1994). It was reported that the maximum yield of 10.82 t ha1 was obtained from an unlimited water application and the yield response factor was 1.02 (Ogretir, 1994). The aim of this study was to determine the appropriate irrigation frequencies and wateryield relationship for sweet corn irrigated by a drip system in a semi-arid region.
2. Materials and methods This study was conducted during 1998 and 1999 at the Field Research Facility of the Faculty of Agriculture at Harran University, Sanliurfa, Turkey. The experimental field is located in Harran Plain (altitude: 465 m; 378080 north and 388460 east) where the climate varies from arid to semi-arid. The weather is hot and dry from May to September where temperatures can reach up to 46 8C. On the other hand, the weather is usually warm during winter months and rainfall is rare. An average of 460 mm of rainfalls each year and the relative humidity averages about 49%. Some of the chemical and physical properties of the experimental field soil are given Table 1 while the chemical compositions of the irrigation water used in the study is given in Table 2. Table 3 provides the climatic data for the city of Sanliurfa. As can be seen from Table 3, in the months of June, July and August for both treatment years, the temperatures were all above 40 8C while the relative humidity was below 50%. Except August 1999, no rainfall was observed in July and September of the treatment years and the rainfall seen in June is fairly low. During the time period for the treatments, the weather conditions were hot, dry and the relative humidity was very low. Table 1 Some of chemical and physical properties of experimental field soil Soil depth (cm)
Field capacity (%)
Wilting point (%)
Volume weight (g cm3)
Total salt (%)
CaCO3 (%)
Organic matter (%)
pH
0–30 30–60 60–90
33.19 32.71 33.84
22.14 21.18 22.55
1.39 1.41 1.37
0.071 0.069 0.073
26.8 29.7 27.9
1.1 0.8 0.8
7.75 7.83 7.36
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Table 2 Chemical composition of irrigation water used in the experiment Source
Deep well
EC25 (106)
309
Cations (me/‘) Na
þ
0.25
K
þ
0.02
Anions (me/‘) 2þ
Ca þ Total Mg2þ
CO3
1.98
–
2.25
2
HCO3 0.90
Cl
SO42
Total
0.60
0.75
2.25
pH
Class
7.0
C2S1
In this study, Merit hybrid sweet corn (Zea mays saccharata Sturt) variety was used as the crop material. The experiment was laid out in a randomized block design with three replications. Each plot consisted of four rows of 5 m in length. The plants were grown 70 cm apart between the rows with 20 cm spacing in each row. The seeds were sown on 21 June 1999 (day of year (DOY: 172)) in the first year and on 26 June 1999 (DOY: 177) in the second year. At sowing, 80 kg ha1 pure N and P (20-20-0 composite) was applied to each plot and this was followed by 160 kg ha1 N as urea when the plant reached to 30–40 cm height. When kernel humidity declined to 70–75% (Olsen et al., 1990b; Stone et al., 2001), ears from two rows in the center of each plot (50 plants) were harvested manually on 11 September 1998 (DOY: 254) and on 18 September 1999 (DOY: 261). The data obtained from the experiments were analysed with analysis-of-variance (ANOVA) and least significant difference (LSD) tests (Cochran and Cox, 1957) using MSTATCTM statistical analysis software package. 2.1. treatments First irrigation water was applied to all treatments using a sprinkler irrigation system during the experiments in 1998 and 1999 to bring the soil water content in 0–90 cm soil depth up to level of field capacity. Irrigation treatments were started using drip irrigation system when the water content of soil decreased to 50% of available soil water.
Table 3 Monthly climate data during the growth period of corn in 1998 and 1999 in Sanliurfaa Months
June July August September October November a
Temperature (8C) Maximum
Minimum
Average
1998
1999
1998
1999
1998
1999
41.2 45.4 43.0 39.6 34.1 27.9
40.0 43.2 43.0 36.6 35.6 25.0
17.8 19.8 22.6 15.1 10.2 8.8
18.8 21.5 20.5 17.0 11.3 1.2
29.4 33.0 33.4 27.0 21.5 16.7
28.8 32.5 31.5 26.2 21.0 13.5
Data collected from Sanliurfa Meteorological Station.
Average relative humidity (%)
Total precipitation (mm)
1998
1999
1998
1999
46.2 43.8 41.4 53.3 49.5 66.4
43.6 39.7 44.7 46.8 51.2 50.9
0.6 – – – 0.1 22.7
1.6 – 26.0 – 8.4 0.8
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Irrigation water was applied as 100% of evaporation of Class A Pan in the 2-day irrigation frequency (ISC2 treatment), 90% of evaporation in the 4-day irrigation frequency (ISC4 treatment), 80% of evaporation in the 6-day irrigation frequency (ISC6 treatment) and 70% of evaporation in the 8-day irrigation frequency (ISC8 treatment). The amount of required irrigation water was determined by Class A Pan evaporation every day (Kanber, 1984). The total evaporation from Class A Pan was measured with a manual limnimeter which has 0.1 mm accuracy. These measurements were checked with the readings from the water flow meters mounted in every plot. Crop water consumption in the treatments was calculated using Eq. (1) (Garrity et al., 1982; James, 1988): ET ¼ P þ I Rf Dp DS
(1)
where ET is crop water consumption (mm), P rainfall (mm), I irrigation water (mm), R surface runoff (mm), Dp deep percolation (mm), and DS is soil water content variation in crop root depth (mm). In this study, deep percolation (Dp) and surface runoff (Rf) in Eq. (1) were assumed to be negligible because the amount of irrigation water was not increased above the field capacity as a result of drip irrigation and deficit irrigation. It approached field capacity only at the 2-day irrigation frequency. The amount of irrigation water was calculated using Eq. (2): I ¼ AEpan Kcp CAI
(2) 2
where I is the amount of irrigation water (mm), A plot area (m ), Epan cumulative water depth from Class A Pan based on irrigation frequencies (mm), Kcp is crop pan coefficient determined as 100% of total evaporation of Class A Pan in the 2-day irrigation frequency (Kcp1), 90% of total evaporation of Class A Pan in the 4-day irrigation frequency (Kcp2), 80% of total evaporation of Class A Pan in the 6-day irrigation frequency (Kcp3), 70% of total evaporation of Class A Pan in the 8-day irrigation frequency (Kcp4), and CAI is canopy area index which was assumed to be 1. During the experimental period, the variation of soil water content at 0–30, 30–60 and 60–90 cm soil depths in each treatment plot was continuously determined by gravimetric method for calculating the actual evapotranspiration (ETa). Total water use efficiency, defined as the ratio of grain yield per hectare to seasonal water consumption, and irrigation water use efficiency, defined as the ratio of grain yield per hectare to the amount of irrigation water, were calculated using the methodology provided by Tanner and Sinclair (1983). The water use-yield relationship was determined using the Stewart model in which dimensionless parameters in relative yield reduction and relative water consumption are used (Doorenbos and Kassam, 1979): Ya ETa 1 ¼ ky 1 (3) Ym ETm where Ya is actual yield (t ha1), Ym maximum yield (t ha1), Ya/Ym relative yield, 1 (Ya/ Ym) decrease in relative yield, ETa actual crop water consumption (mm), ETm is maximum crop water consumption (mm), ETa/ETm is relative crop water consumption, 1 (ETa/
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ETm) is decrease in relative crop water consumption, ky is yield response factor defined as decrease in yield with respect to per-unit decrease in ET.
3. Result and discussion 3.1. Water use-yield relationship The maximum amount of water applied to the crop was 876 mm in the 2-day irrigation frequency (ISC2 treatment) while the minimum amount was 610 mm in the 8-day irrigation frequency (ISC8 treatment) during the first year of the experiments (Table 4). In the second year, the values were 889 mm for maximum and 612 mm for minimum in the 2- and 8-day irrigation treatments, respectively. The results were similar in both years. The amount of water applied to other treatments ranged between these values. Seasonal water consumption (ETa) ranged from 1008 (2-day irrigation frequency) to 700 mm (8-day irrigation frequency) in 1998 and varied between 1071 (2-day irrigation frequency) and 701 mm (8-day irrigation frequency) in 1999. These result were in agreement with literature data (Musick and Dusek, 1980; Howell et al., 1997, 1998; Gencoglan, 1996; Cetin, 1994). Plants transpire less under high humidity conditions and more when the vapour pressure deficit of the air is high. Since relative humidity is very low in Harran Plain, especially during summertime, the plants transpire more for 1 unit of dry matter accumulation. Moreover, some other factors such as ground cover percentage and low leaf area index in early growing stages accelerate water loss from soil via evaporation. According to variance analysis, fresh ear yield was affected significantly (P < 0:01) by the irrigation treatments in both years (Table 5). The highest fresh ear yield was obtained from the 2-day irrigation frequency as 13.66 t ha1 in 1998 and 13.19 t ha1 in 1999 (Fig. 1). Yield was reduced with deficit irrigation in both years. Darusman et al. (1997) reported the highest yield at the ET coefficient of 100%. Yildirim et al. (1996) studied the effects of different levels of soil moisture on grain yield and stated that the highest yield to be 10.85 t ha–1 under non-waterlimiting conditions while the lowest yield was 3.47 t ha–1 under water-limiting conditions. Table 4 Relationship between the decrease in relative water use and decrease in relative yield and yield response factor for sweet corn irrigated by a drip system Treatments
Ya ETa E T a /Ya/Ym 1 (Ya/Ym) 1 (ETa/ETm) ky (t ha1) (mm) ETm
Irrigation Water water (mm) saving (%)
ISC2-1998 ISC4-1998 ISC6-1998 ISC8-1998 ISC2-1999 ISC4-1999 ISC6-1999 ISC8-1999
13.66 12.86 11.39 8.55 13.19 11.67 11.15 7.29
876 777 693 610 889 792 701 612
1008 930 829 700 1071 944 899 701
1.00 0.92 0.82 0.69 1.00 0.88 0.84 0.65
– 0.94 0.83 0.63 – 0.88 0.85 0.55
0.0000 0.0586 0.1662 0.3741 0.0000 0.1152 0.1547 0.4473
0.0000 0.0774 0.1776 0.3056 0.0000 0.1186 0.1606 0.3455
0.00 0.76 0.94 1.22 0.00 0.97 0.96 1.29
0.00 11.30 20.89 30.36 0.00 10.91 21.14 31.15
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Table 5 Fresh ear yielda irrigated by drip irrigation at different irrigation frequencies and LSD groups Irrigation treatments
ISC2 ISC4 ISC6 ISC8 LSD
Fresh ear yielda (t ha–1) 1998**
1999**
Average**
13.66 ab 12.86 b 11.39 c 8.55 d 0.766
13.19 a 11.67 b 11.15 c 7.29 c 0.464
13.43 a 12.26 b 11.27 c 7.92 d 0.668
a
Fresh ear yield without husk. There is no statistical differences among same letter at 0.05 level according LSD test. ** P < 0:01. b
Howell et al. (1997) reported that yield was 15.50 t ha1 at insufficient precipitation condition in Texas. Maximum yield of 10.82 t ha–1 was reported under unlimited water application (Ogretir, 1994). In addition, yield was found to be 13.90 and 14.90 t ha1 for sweet corn irrigated by a drip system (Steele et al., 1996). In 1998, the rates of water savings were 11.30, 20.89 and 30.36% for the 4-, 6- and 8-day irrigation frequencies, respectively, and 10.91, 21.14 and 31.15% for the treatments in 1999. At the 4-, 6- and 8-day irrigation frequencies, the rates of the decrease in relative yield were 5.86, 16.62 and 37.41% in 1998, and 11.52, 15.47 and 44.73% in 1999, respectively. A reduction in yield was observed with increasing water deficit. Zhang and Davies (1989) reported a reduction in yield as the crops experienced water stress for a long time. The lowest grain yield was observed in the 8-day irrigation frequency treatment in both years (Fig. 1) since low humidity and high air temperatures cause plant stomas to close, resulting in less assimilation due to a decreased CO2 uptake for photosynthesis. Since water stress causes a decrease in leaf area (Jamieson et al., 1995; Stone et al., 2001), a reduction in yield is observed because of low photosynthesis. Pandey et al. (2000) reported that the highest leaf area index for corn was obtained in well-irrigated conditions. A positive correlation between leaf area index and seasonal water consumption was also reported (Kang et al., 1998). Swan et al. (1987) reported a relationship between yield and climate and water holding capacity of soil. Musick and Dusek (1980) cite two reasons for yield reduction at deficit
Fig. 1. Fresh ear yield values at different irrigation frequencies in 1998 and 1999.
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irrigation: high water holding capacity of soil and existence of water stress throughout growing season. Under non-stress conditions, grain yield is directly affected positively by solar radiation (Kiniry et al., 1989; Muchow et al., 1990). The dry matter production of non-stressed plants are usually high compared to stressed plants since water-stressed plants can not utilise solar radiation effectively. Crop roots take up nutrients and water from upper levels of the soil under the condition of low water-stress or non-stress (Rhoads and Bennett, 1990). Frequently watered plants produce a shallow root system whereas occasionally watered plants produce deep root system. The first 25 cm root zone in soil profile supply 40% of water uptake (Kirtok, 1998). Similarly, better utilisation of nutrients and efficient use of soil water by crops might have caused an increase in yield in the 2-day irrigation frequency in the current study. Tassel flowering, ear flowering and milky stage are critical periods for yield in corn. Hot and dry climate and also drought during this period result in yield reduction (Dow et al., 1984; Shaw, 1988; Edmeades et al., 1990; Yildirim et al., 1996; Kirtok, 1998). In this study, plants were exposed to water stress during these periods at the 6- and 8-day irrigation frequency. Linear relationships were observed between the amount of water applied (I) and fresh ear yield (Ya) in both years from the regression analysis. The equation for the relationship was Ya ¼ 0:0188I 2:2953 with R2 ¼ 0:91 for the year 1998 and Ya ¼ 0:0197I 3:897 with R2 ¼ 0:87 for 1999 as seen in Fig. 2. A linear relationship was found between seasonal water consumption (ETa) and grain yield (Ya) in both years: Ya ¼ 0:0167ETa 2:8482 (R2 ¼ 0:98) and Ya ¼ 0:0162ETa 3:8109 (R2 ¼ 0:98) for 1998 and 1999, respectively (Fig. 3). A linear relationship have been reported between crop yield and seasonal water consumption (Stewart et al., 1975; Mogenson et al., 1985; Musick et al., 1994). 3.2. Crop response factor (ky) Crop response factor, ky, indicates a linear relationship between the decrease in relative water consumption and the decrease in relative yield. It shows the sensitivity of yield with
Fig. 2. Irrigation water (I) and fresh ear yield (Ya) relationship. Vertical bars indicate standard errors of the mean.
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Fig. 3. Fresh ear yield (Ya) and seasonal water consumption (ETa) relationship. Vertical bars indicate standard errors of the mean.
respect to the decrease in water consumption. In other words, it explains the decrease in yield caused by the per-unit decrease in water consumption (Stewart et al., 1975; Doorenbos and Kassam, 1979). Crop response factors were determined as 0.76, 0.94 and 1.22 in 1998 and 0.97, 0.96 and 1.29 in 1999 at 4-, 6- and 8-day irrigation frequencies (Table 4). These results were in agreement with the findings reported in the literature. For instance, ky values of 1.25 (Doorenbos and Kassam, 1979), 0.97 (Yildirim et al., 1996), 1.08–1.61 (Gencoglan, 1996), 0.98 (Kanber et al., 1990) and 1.02 (Ogretir, 1994) were reported for total growing season of corn crop. Values of ky were increased with increasing water deficit in both years. Based on research results, a 11.3% water saving in irrigation water and 7.7% less ETa resulted in a decrease of 5.9% in yield at 4-day irrigation frequency in 1998. For the same treatment group in 1999, a 10.9% water saving and 11.9% less ETa resulted in a decrease of 11.5% in yield. Similar findings were observed for other treatments (Table 4). Reduction of water consumption resulted in decreased yield (Braunworth and Mack, 1987; Koksal, 1995; Gencoglan, 1996). 3.3. Water use efficiencies TWUE and IWUE were different depending on the treatments and years (Table 6). TWUE were 1.36, 1.38, 1.37 and 1.22 kg m3 in 1998 while the values were 1.23, 1.24, 1.24 and 1.04 kg m3 in 1999 for the 2-, 4-, 6- and 8-day frequencies, respectively. IWUE values were determined to be 1.56, 1.66, 1.64 and 1.40 kg m3 for the 2-, 4-, 6- and 8-day irrigation frequencies, respectively in 1998. The values were 1.48, 1.47, 1.59 and 1.19 kg m3 for the 2-, 4-, 6- and 8-day irrigation frequencies, respectively, in 1999. IWUE values of 1.25–1.46 (Musick and Dusek, 1980) and 1.9 kg m3 (Lyle and Bordovsky, 1995) have been reported. In a different study conducted in the Cukurova region (Cukurova region is located in the southeastern part of Turkey), IWUE values of 1.38– 1.80 kg m3 and TWUE values of 0.87–3.19 kg m3 were found (Koksal, 1995). In addition, IWUE values of 1.02–2.43 kg m3 and TWUE values of 0.22–1.25 kg m3
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Table 6 Total water use efficiency (TWUE) and irrigation water use efficiency (IWUE) values for sweet corn irrigated by a drip system at different frequencies Treatments
TWUE (kg m3)
IWUE (kg m3)
ISC2-1998 ISC4-1998 ISC6-1998 ISC8-1998 ISC2-1999 ISC4-1999 ISC6-1999 ISC8-1999
1.36 1.38 1.37 1.22 1.23 1.24 1.24 1.04
1.56 1.66 1.64 1.40 1.48 1.47 1.59 1.19
were reported in a study conducted in the Cukurova region by another researcher (Gencoglan, 1996). Furthermore, IWUE values of 0.57–0.80 kg m3 were found for corn crop in the Harran Plain (Cetin, 1994). The findings obtained in this study were in good agreement to those values previously reported in the literature for corn crop. 4. Conclusion In this study, the highest fresh ear yield was obtained from the 2-day irrigation frequency as 13.66 and 13.19 t ha1 for both years of investigation while the minimum yield was found to be 8.55 t ha1 in 1998 and was 7.29 t ha1 in 1999 with the 8-day irrigation frequency. Fresh ear yield was reduced as the amount of irrigation water decreased. The rates of reduction in relative yield were 5.86, 16.62 and 37.41% in 1998; 11.52, 15.47 and 44.73% in 1999 at the 4-, 6- and 8-day irrigation frequencies, respectively. The results of this research indicated that a 2-day irrigation frequency, with 100% ET water application by a drip system, would be optimal for sweet corn grown in semi-arid regions similar to the area in Turkey where this work was conducted. References Braunworth, W.S., Mack, H.J., 1987. Effect of deficit irrigation on yield and quality of sweet corn. J. Am. Soc. Hort. Sci. 112 (1), 32–35. Cetin, O., 1994. Harran ovasi kosullarinda ikinci urun misir su gereksinimi. K.H. Aras. Ens. Yay. 90/63, Sanliurfa, Turkey (in Turkish, with English abstract). Cochran, W., Cox, G., 1957. Experimental Design. Wiley, New York. Darusman, A., Khan, H., Stone, L.R., Spurgeon, W.E., Lamm, F.R., 1997. Water flux below the root zone versus irrigation amount in drip-irrigated corn. Agron. J. 89, 375–379. Doorenbos, Kassam, J., A.H., 1979. Yield Response to Water. FAO Irrigation and Drainage. Paper No. 33, Rome. Dow, E.W., Daynard, T.B., Muldoon, J.F., Major, D.J., Thurtell, G.W., 1984. Resistance to drought and density stress in Canadian and European maize hybrids. Can. J. Plant Sci. 64, 575–585. Edmeades, G.O., Bolanos, J., Lafitte, H.R., 1990. Selecting for drought tolerance in maize adapted to the lowland tropics. In: Proceedings of the the 4th Asian Regional Maize Workshop, 23–27 September, Islamabad, Pakistan.
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