Sweet corn (Zea mays L.) is a popular crop in many countries. Fresh consumption by boiling or grilling is quickly increasing worldwide. Large husked cops of sweet corn are preferred in the market. Furthermore, vegetative parts (stalks and leaves) are used to feed animals. Its growing popularity increases local and international demand. Due to its high profitability, sweet corn represents one of the most economical crops for the local farmers. However, fficient water use represents an urgent need to meet the sustainability of corn productivity due to the water shortage in the dry Mediterranean area.

Drip irrigation is an efficient method to save water, enhance yield, and yield quality, compared with sprinkle and surface irrigation methods [1, 2]. Unfortunately, its high initial costs represent one of the biggest constraints to the widespread adoption of drip irrigation.

The cost of driplines (laterals) is a large fraction of initial investment costs. According to Bozkurt et al. [3], it represents about 45% of the total costs. In this situation, using one dripline for two crop rows instead of for one row would half the number of driplines per unit-area, and consequently, would decrease the initial costs [3-9]. However, increasing the distance between driplines may need excessive preplant irrigation or using another system for proper seed germination [4, 10, 11]. Moreover, mean crop yield may reduce as dripline spacings increase due to low yields of plant rows located away from driplines [3, 12-14]. This could be worse in soils in which gravity forces control soil water distribution more than capillary forces, as in coarser soils [15], or large-crack soils as in high clay-content soils [10].

Twin-row production systems (paired-rows) have been adopted as a means to increase yields compared to single-row system [16-19]. Twin-rows would be one of the most significant factors in controlling the yield reduction when using wider lateral spacing. Considering this fact, it is necessary to provide supportive data for the adoption of twin-row production system, especially in the dry Eastern Mediterranean region where water is very scarce and valuable.

In this context, this study aimed to compare the effects of both single- and twin-row production systems with two different dripline spacings (75 and 150 cm) on two drip-irrigated corn varieties grown in clay loam soil under the dry Mediterranean climatic conditions. Cop characteristics, yields, and irrigation water use efficiency were evaluated and discussed. Practical alternatives could be introduced based on the obtained results in order to make twin-row production system familiar for most farmers and to stimulate them to adopt it in their fields, to sustain corn productivity and to meet water shortage in the dry Mediterranean region.

A two-year field experiment (2017 and 2018) was carried out at the Agricultural Experiment Station, Damascus, Syria (36°26′ E, 33°20′ N, altitude 600 m). The study area is dominated by a dry Mediterranean climate type with an average precipitation of about 120 mm year^-1 and reference evapotranspiration (ET₀) as calculated by FAO Penman-Monteith equation exceeds 2000 mm year^-1. The meteorological data of the study site, collected during both growing seasons could be summarized overall as being near the 20-year average, as can be seen in Table 1.

The soil texture was clay loam with 29.5% clay, 42.7% silt, and 27.8% sand. Before sowing, topsoil characterstics were: bulk density of 1.35 g cm^-3, pH of 8.0, EC of 0.6 dS m^-1, <1% of organic matter, available P of 22.0 ppm, NH₄⁺ of 14.7 ppm, and NO₃^- of 21.1 ppm.

Field experiment composed of two sweet corn varieties: Silver Queen (var.1) and a local variety “White Kokab” (var.2). Under each variety, two different dripline spacings (75 and 150 cm) with two different crop row production systems with the same plant density (about 67,000 plants ha^-1) were studied. Experiments were arranged in a split-plot design involving two corn varieties (var.1 and var.2) as main-plot treatments and three combinations of crop row and dripline spacings, with three replicates. The main-plot size was 34×12 m², while the size of the sub-plot differed according to the tested dripline spacing. As can be seen in Fig. (1), combinations were composed of:

- S-75: A single-row production system was used with 75-cm crop rows (i.e., 75 cm between crop rows). One dripline was used for each crop row. Therefore, the dripline spacing was 75 cm.

- S-150: A single-row production system was used with 75-cm crop rows (i.e., 75 cm between crop rows). One dripline was used for two crop rows. Therefore, the dripline spacing was 150 cm.

- TS-150: A twin-row production system was used. Corn was planted in paired rows, 37.5 cm apart, on 150 cm centers. Each strip of twin-rows was served by one dripline. Therefore, the dripline spacing was 150 cm.

In both S-75 and S-150, a single-row production system was used, i.e., conventional plant row widths, in which corn was sown with a regular row spacing of 75 cm and spaced 20 cm apart within row. While in TS-150, a twin-row production system was used, so that crop rows were brought closer (37.5 cm) and grouped into strips (ranges or bands) with twin-rows. Thus, the plant density and the number of crop rows in unit-area were maintained equal in the three treatments.

Before sowing day, field soil was disked and ploughed to a depth of about 35 cm. A sufficient interdistance (about 2 m) was maintained between experimental plots. Corn was sown on 18^thApril 2017 and 3^rd April 2018. Due to the lack of precipitation during the early stages in both cropping seasons, another lateral move irrigation system, available on the station, was utilized two times to apply 100 mm in order to properly germinate the seeds and to well establish plants. After that, the drip irrigation treatments were started on one month after sowing as planned, using dripline tubes of 16-mm diameter with built-in emitters spaced at 40 cm with a nominal flow rate of 4 litre hr^-1 (i.e., 10 litre hr^-1 m^-1). For each growing season, a quantity of 46.0 kg ha^-1 of P₂O₅ as triple superphosphate was added to the study field in early winter. Moreover, urea (N: 46%) was used as a source of N-fertilizer. A quantity of 150 kg N ha^-1 was applied in two equally split applications: at sowing and about two weeks later. Thus, all crop rows received the same quantities of fertilizers.

The durations of growth stages, i.e., the initial development, mid-season and late-season growth stages were 20, 30, 40, and 10 days long, respectively. Related crop coefficient values (Kc) for sweet corn crop were 0.3, 1.15, and 1.05 for initial, mid-season and late-season growth stages, respectively, as acquired from FAO databases [20]. Daily crop water requirement (daily crop evapotranspiration, ETc), was estimated as the product of multiplying the daily ET₀ by Kc. Weekly crop water requirement, i.e., the weekly sum of daily ETc values was used to adjust the schedule for the following week. Water depth applied in each irrigation event was equal for all treatments. However, treatments differed in terms of irrigation durations, due to the different dripline spacings. Thus, the amount of irrigation water per unit-area was equal among treatments. Each treatment was given about 753 and 700 mm in 2017 and 2018, respectively, as a total irrigation water depth. The drip irrigation durations applied to S-75, S-150, and TS-150 were 49, 98, and 98 hr in 2017, and 45, 90, and 90 hr in 2018, respectively.

Using in-situ-calibrated neutron probe technique (NP), rootzone soil humidity was measured on a weekly basis in plots of “Silver Queen” variety each cropping season. The measurements were conducted about 72 hr after each drip irrigation event. Neutron probe access tubes were inserted into rootzone under plant stems. As can be seen in Fig. (1), one soil profile was probed in each treatment. The total amount of water which is stored in the rootzone soil profile represents the total soil water storage (SWS), as can be estimated by the following equation:

(1)

where z is the soil depth (m), Zm is the root zone depth (=1.2 m), and θ is the soil water content (cm³ cm^-3).

Variations in time of soil water storages for the three irrigation treatments were jointly plotted as can be seen in Fig. (2). The water stress response function as described by Feddes et al. [21] was used in order to determine if the rootzone was exposed to water stress or not. Water uptake is considered maximum when soil pressure head (h) ranged between two values h₂ and h₃. However, it decreases when h ranged between h₃ and h₄. Water uptake becomes zero when h<h₄ (the wilting point pressure head). For sweet corn grown in clay loam soil, soil water content (θ) which corresponded to h₂, h₃, and h₄ were 44.5, 23.2, and 13.6 cm³ cm^-3, respectively, according to Feddes et al. [21]. Soil water storages at these root water uptake parameters were also plotted (Fig. 2).

For fresh marketable products, the harvest was conducted at the milky stage when the water content of seeds was 70-75%. This was after 100 days after sowing for both tested varieties and both growing seasons. A 2m row length, i.e., 10 plants from the center of each plot were selected. Corn cobs from selected plants were hand harvested and husked. Well-filled cobs were considered a marketable product, according to the familiar corn-farming practices in the study area. Weight, length, and diameter of husked cobs were measured. To determine dry matter yield, aboveground vegetative parts of selected plants were also collected and oven-dried at 70°C until constant weight. Cob yield and dry matter were expressed into unit-area yields as t ha^-1. Irrigation water use efficiency (IWUE, kg m^-3) was estimated by dividing yield by the water volume of irrigation (m³). Both values of IWUE for husked cob yield (IWUE_hc) and dry matter (IWUE_dm) were estimated.

Fig. (1). Outline of a replicate according to crop row/dripline spacing configurations.

Fig. (2). Changes over time in soil water storages in 120 cm soil profile for 2017 (a) and 2018 (b) growing seasons. Dashed lines represent soil water storages at h2, h3, and h4 corresponding to the root water uptake parameters for corn crop as suggested by Feddes et al. (1978). S-75, S-150, and TS-150 represent dripline spacings treatments.

The measured traits were submitted to the Analysis of Variance (ANOVA) using the DSAASTAT add-in version 2011 [22]. According to Gomez and Gomez [23], a combined analysis of data over both growing seasons was performed to identify treatment whose mean effect over the years is stable and high. A comparison of treatment means was conducted using the LSD-test (Least Significant Difference Test) at the 5% level of significance.

The local variety “White Kokab” was better than the introduced variety in husked crop yield and Irrigation Water Use Efficiency (IWUE). The new twin-row system 150-cm dripline spacing gave similar cop yield and IWUE as the conventional 75cm dripline spacing, and both were higher in the two traits than the single-row 150cm dripline spacing. With 50% less cost per unit-area on dripline, the twin-row 150cm dripline spacing is more economical as well as environmentally friendly.