Impaginato 49 Adv. Hort. Sci., 2020 34(1): 49­60 DOI: 10.13128/ahsc­8401 Improving water productivity and yield of onion crop by combining early planting and straw mulch under different irrigation levels in dry Mediterranean region I. Mubarak Department of Agriculture, Atomic Energy Commission of Syria, P.O. Box 6091, Damascus, Syria. Key words: Allium cepa L., irrigation water use efficiency, onion bulb yield. Abstract: In response to the Sustainable Development Goals (SDGs) adopted by United Nations, combining using straw mulching, the proper crop planting date, and regulated deficit irrigation (RDI) is fundamental to adapt to climate change and to promote sustainable agriculture in the dry Mediterranean region. Two­ year pot experiment under field conditions (2017 and 2018) was conducted in Damascus Countryside, Syria (altitude 600 m), to evaluate the onion crop res­ ponse to early planting, irrigation level, and straw mulching. Treatments com­ posed of three different planting dates with 28­day intervals (two early dates in February and March, and the traditional date in April), three irrigation levels (100, 80, and 60% of crop evapotranspiration, ETc), and two types of soil cover (with and without wheat straw mulch), with three replicates. Findings revealed that the seasonal ETc decreased from about 900 mm under current practice (planting in April without mulch) to only about 550 mm under both straw mulch and earliness in planting. Large bulb yield increases (more than double) were also obtained. Moreover, early planting using straw mulching significantly enhanced the onion crop response to RDI, even at 60% of ETc. Combining early planting in February, straw mulching, and full irrigation represents the best agricultural management. 1. Introduction Onion (Allium cepa L.) is an important crop worldwide. The environ­ mental conditions such as photoperiod and temperature mainly affected its growth, development, and bulb yield. The agronomic practices such as planting date and irrigation water availability have also an effect on onion crop production (Brewster, 2008; Khokhar, 2014; Mubarak and Hamdan, 2018 a). Onion crop thrives best when temperatures are cool during early development period and then warmer and sunny during maturity. Hence, planting date has a profound impact on onion crop growth and develop­ ment. Early planting date tends to have a longer onion growing season before bulb initiation ensuing larger plants. However, large plants are more likely to become sensitive to the cold stimulus resulting in bolting (*) Corresponding author: ascientific4@aec.org.sy Citation: MUBARAK I., 2020 ­ Improving water productivity and yield of onion crop by combining early plan‐ ting and straw mulch under different irrigation levels in dry Mediterranean region. ­ Adv. Hort. Sci., 34(1): 49­60. Copyright: © 2020 Mubarak I. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 25 March 2019 Accepted for publication 31 October 2019 AHS Advances in Horticultural Science http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2020 34(1): 49­60 50 (formation of seed stalk followed by flowering), which represents a highly unfavourable feature for onion bulb production. Large plants are also related with split bulbs. However, late­date­planted onions start forming bulbs before reaching satisfactory plant growth to support the final size of bulbs. This would produce small bulbs, and therefore, decreasing the bulb yield (Brewster, 2008; Rohini and Paramaguru, 2016). Thus, determining the proper onion crop planting date is central to adapt to the regional cli­ mate changes. Onion crop has a shallow rooting system, and therefore, it is considered as a sensitive crop to soil water deficit than other deep­rooted crops. Water productivity (WP, also known as water use efficiency, WUE) is usually used to recognize the environments or management practices by which the yield per unit water can be optimized. In the dry regions where water resources are limited as in the dry Mediterranean region, improving water productivity and crop production represents also a main challenge for agricultural water management. Mulch has been widely adopted because of its agro­pedo­ecological benefits. It constitutes of syn­ thetic (plastic films) or natural (plant residues as wheat or rice straw) materials. Both materials reduce the loss of soil water through evaporation. However, unlike plastic films, straw mulch allows rain and irri­ gation water to penetrate and to reach the soil. This conserves soil water content, and thereby reducing irrigation water requirements, promoting rooting sys­ tem development, and increasing crop growth, devel­ opment and yield (Vavrina and Roka, 2000; Gimenez et al., 2002; Mubarak and Hamdan, 2018 b). From eco­environmental point of view, the use of plastic mulch would not be justified at low crop prices and/or very high plastic films costs, especially it requires to be removed after use annually. The re­ use of plastic films is not practical agronomically and technologically. This could increase the harmful envi­ ronmental impacts from plastic components. For these reasons, straw mulching present an eco­envi­ ronmentally sustainable choice. Unlike plastic films, straw mulch could incorporate into the soil ecosys­ tem, where it is expected to biodegrade. Thus, straw mulching could be considered as a slow­acting organ­ ic fertilizer, improving soil fertility and soil physical properties, and consequently, crop yield (Khaledian et al., 2010, 2011). Deficit irrigation (DI) in combination with mulching could be considered as a key water­saving technique that would help in meeting both water scarcity and sustainable crop production (Fereres and Soriano, 2007; Chai et al., 2016). The effects of deficit irrigation under mulching on onion yield have been documented (Vavrina and Roka, 2000; Igbadun et al., 2012; Patel and Rajput, 2013; Tsegaye et al., 2016; Mubarak and Hamdan, 2018 b). Several studies showed that it is better to fractionate the water stress during the cropping season (Regulated deficit irrigation, RDI) rather than applying a water stress during the critical stages of crop growth period (Kadayifci et al., 2005; Patel and Rajput, 2013). For example, deficit irrigation given at 75% of crop evap­ otranspiration (ETc) was recommended for onion crop production (Tsegaye et al., 2016). In the dry Mediterranean area, onion bulb sets as directly planted in the soil is the common method employed to establish onion plantings in the field. Farmers plant onion bulb sets early in the spring and harvest in the summer. The production period between April and August is characterized by no rain­ fall (Ragab and Prudhomme, 2002; Turner, 2004). Moreover, the Mediterranean climate is extremely variable with hot dry summers, and cold wet to dry winters. The Middle East and North Africa are in par­ ticular dry areas, with only 1% of renewable water resources (Joffre and Rambal, 2001; Turner, 2004; Ceccarelli et al., 2007). The increasing climatic change have intensified the vulnerability to drought (Giorgi and Lionello, 2008; Somot et al., 2008; FAO 2011; Polade et al., 2014). An increase by 1.25­2.5°C in temperature is predicted in winter, and the precipita­ tion between October and March will decrease by 10­15 % in the southern Mediterranean countries (Ragab and Prudhomme, 2002). As the onion crop production is already limited by the water availability and local climate, moving towards feasible tools (such as using regulated deficit irrigation under mulching) and agronomic practices (such as determining the proper crop planting date) adapted to the regional climate change is urgently needed for better water saving and cultivation period of the crop (FAO, 2011; Khokhar, 2014; Zinkernagel et al., 2015). In this context, and in response to the ambitious Sustainable Development Goals (SDGs) proposed by United Nations to adapt to climate change and to promote sustainable agriculture, the present study aimed to assess the interactive effects of various planting dates, different irrigation levels, and straw mulching on onion crop production. The outcomes may introduce appropriate agronomic alternatives to meet the ever increasing demand for onions and to Mubarak ‐ Planting date, irrigation level, and mulch for onion crop 51 save irrigation water in the dry Mediterranean area. Moreover, results may contribute to make regulated deficit irrigation with straw mulching familiar for most farmers, and to stimulate them to adopt these techniques in their fields. 2. Materials and Methods Pot experiments were conducted under open field conditions at the Deir Al­Hajar Agricultural Experiment Station, Damascus Countryside, Syria (33°20ʹ N, 36°26ʹ E, altitude 600 m), for different planting dates during February to May in two consec­ utive years 2017 and 2018. The site is located within a dry Mediterranean area, in which the total annual rainfall is about 120 mm, and the annual reference evapotranspiration is about 2000 mm. Some climatic data for the studied site collected during the growing seasons were fairly close to those averaged over the last 16 years (from 2000 to 2016) as can be shown in Table 1. For this reason, testing different planting dates during two years seemed somewhat adequate. The soil is classified as a clay loam (29.5% clay, 42.7% silt, and 27.8% sand). Both volumetric soil water contents at permanent wilting point (PWP) and field capacity (FC) are 0.18 and 0.36 m3 m­3, respec­ tively. Some chemical and physical soil properties are: pH of 8.0; ECe of 0.34 ds m­1; organic matter of 1.00%; available P of 5.7 ppm; NO3 ­ of 28.3 ppm; NH4 + of 12.6 ppm. Pots with dimensions of 25×30 cm and containing 8 kg of soil were used in the experiments. Three bulb sets of onion (Allium cepa L., c.v. Selmouni) were planted in each pot. The pots were set in an open field under natural climatic conditions. Plants were thinned after germination to two bulbs per pot, get­ ting a plant density of about 400000 plants ha­1. Four different planting dates separated with 28 days were tested: PS1 (early February), PS2 (early March), PS3 (early April), and PS4 (early May). Unfortunately, onion bulb sets which were planted in May (PS4) in both studied years did not properly ger­ minate, and therefore, they were ignored. Within a year and at each planting date, the experiment was laid out following a 2×3 factorial experiment arranged in a randomized complete block design (RCB design) with two modes of soil cover and three irrigation levels, replicated three times. The soil cov­ ering comprised of two distinct types. The first one was with mulching using 40 g of wheat straw per pot (about 8 t ha­1); and the second one was with no mulching. The irrigation levels composed of IL100 (full irrigation, 100% ETc), in which plants received 100% of the crop evapotranspiration; and the root zone was replenished to field capacity. IL80 and IL60 treatments (regulated deficit irrigation) were irrigat­ ed at the same frequency as in IL100 but with water amounts equal to 80 and 60% of the ETc as calculat­ ed in IL100, respectively. In other words, the three watering treatments received at each irrigation event 1.0, 0.8 and 0.6 times the soil water depletion Table 1 ­ Some climatic data for the experimental site during both studied years (2017 and 2018) and the 16 years average (from 2000 to 2016) Variable Year Feb. Mar. Apr. May Jun. Jul. Minimum temperature (°C) 2017 4.0 6.2 9.7 14.4 17.2 20.6 2018 5.7 7.9 10.0 15.6 18.2 19.8 2000­2016 average 4.0 6.8 10.1 14.1 17.6 19.3 Maximum temperature (°C) 2017 14.7 18.7 26.2 31.6 35.7 40.6 2018 18.8 24.3 27.2 31.5 34.6 36.9 2000­2016 average 15.7 20.6 25.3 30.4 35.0 37.4 Mean temperature (°C) 2017 9.1 14.0 19.2 24.9 28.4 31.1 2018 12.7 17.3 19.9 25.7 27.7 28.8 2000­2016 average 10.6 15.0 18.1 23.6 27.7 29.4 Relative air humidity (%) 2017 69.3 74.4 63.1 57.9 56.3 56.0 2018 68.8 65.0 54.8 51.5 59.6 55.6 2000­2016 average 75.0 64.1 60.9 56.5 56.3 60.7 Precipitation (mm) 2017 11.6 42.6 0.0 0.0 0.0 0.0 2018 30.3 1.0 14 9.9 0.0 0.0 2000­2016 average 31.0 31.6 5.9 4.2 0.0 0.0 Adv. Hort. Sci., 2020 34(1): 49­60 52 occurred in the full irrigation treatment (IL100), respectively. Irrigation water was added 3 times per week. Each experiment was started on the planting day with a wet soil at field capacity as measured by pot’s weight. The pots were weighed before and after each irrigation event. The water amount deplet­ ed (mm) between two successive irrigation events (ETc) was regulated by weight and estimated using (Eq. 1) as: ETc = (W1 ‐ W2)/(Pw x S) [1] where ETc = the crop evapotranspiration between two irrigation events (mm); W1 = the weight of the pot (kg) after irrigation (the soil water content in the pot was at the field capacity); W2 = the weight of the pot (kg) just before the next irrigation event; ρw = the water density (g cm­3); and S= the pot soil surface area (m2). The daily crop evapotranspiration (mm day­1) was estimated by dividing the ETc calculated using Eq. (1) by the number of days between two successive irrigations. The seasonal crop evapotran­ spiration was the summation of the daily ETc, which represented the total crop water requirements dur­ ing a growing season. Irrigation water amounts, which were applied to the non mulching treatments, were based on the IL100 treatment under no­ mulching conditions; whereas those applied to the straw mulching treatments were based on the IL100 treatment under mulching conditions. For each planting date, phosphorous and potassi­ um were applied as basal application at planting day; wherease, nitrogen fertilizer was splited into two equal applications, and added during early vegetative stage. Irrigation was stopped when almost 70% of leave­head dropped as signs of maturity. The onions were lifted to field cure. Then the leaves were cut leaving about 2.0 cm tops above the bulb. The length, diameter, and weight of both matured onion bulbs from each pot were measured. The sum of weights of both bulbs represent the bulb yield per pot (Y­pot), and was expressed as g pot­1. Water pro­ ductivity (WP, kg m­3) and irrigation water use effi­ ciency (IWUE, kg m­3) were calculated using equa­ tions [2] and [3] (Mubarak et al., 2018). WP is the relationship between yield and seasonal evapotran­ spiration (ETc). Whereas, IWUE is the relationship between yield and the total amount of irrigation water applied (I, Liter pot­1). WP = Yield/ETc [2] IWUE = Yield/I [3] Within a year, a combined analysis of data over planting seasons was carried out to examine the interaction between planting season and the studied treatments (Gomez and Gomez, 1984). The analysis of variance (ANOVA) was conducted using the DSAA­ STAT add­in version 2011 (Onofri, 2007). Mean com­ parison was made using the LSD test at the 1% level. Trend comparison (regression analysis) was also per­ formed. Data was presented and illustrated accord­ ing to the rules described by Gomez and Gomez (1984). 3. Results As mentioned above, onion bulb sets planted in May did not properly germinate. This could be explained by the fact that onion is a vegetative over­ wintering stage in its life cycle, i.e., it grows best when temperatures are cool during early develop­ ment period (Brewster, 2008). Therefore, it is not recommended to delay planting onion sets after April in the dry Mediterranean area. Bulb shape indicators The shape of onion bulbs was represented herein by two indicators: bulb length (BL) and diameter (BD). The ANOVA revealed that the main effects of all stud­ ied factors (planting date, soil cover system, and irri­ gation levels) highly significantly affected bulb shape indicators in both years (Table 2). Within a year, the data under each factor were pooled over the other factors as can be seen in Table 3. Results indicated that both indicators (BL and BD) found in 2017 were comparable with their homologues in 2018. Early planted onion sets (PS1) produced the tallest onion bulbs (7.9 and 8.5 cm in 2017 and 2018, respectively) than the other planting dates, while the later planted sets (PS3) produced bulbs significantly shorter by 10­22% than those in PS1 and PS2, in both years. Moreover, onion sets grown under straw mulching produced bulbs considerably taller by about 14% in average than those grown under no­ mulching conditions. With regard to irrigation levels, the higher value of BL was observed under full irriga­ tion (IL100); then, bulb length decreased as the water application rate decreased. The mean value of BL reduced by about 8 and 25% in 2017, and by about 3 and 14% in 2018, when onion sets were planted in March (PS2) and April (PS3), respectively (Table 3). Mubarak ‐ Planting date, irrigation level, and mulch for onion crop 53 mulching conditions. Also, significant differences in BD were observed in relation to irrigation levels. The maximum values of BD of 4.5 cm in 2017 and 4.6 cm in 2018 were found under full irrigation condition (IL100). Then, they significantly decreased as the irri­ gation level decreased. The effect of decreasing water application rate by 40% of ETc (as in IL60 treat­ ment) resulted in a decreased BD by about 48% rela­ tive to those in IL100, in both years (Table 3). Only in 2018, both PS×IL and SC×IL interactions were highly significant (Table 2). To examine the nature of PS×IL interaction, the BD data of planting dates were compared under irrigation levels (data not shown). The decline in BD as planting date delayed was found to be more severe under full irri­ gation compared with water stress conditions. Also, to examine the nature of SC×IL interaction, the BD data of both soil cover systems were compared under irrigation levels (data not shown). The increase in BD as irrigation level increased was found to be more severe under straw mulching compared with no­mulching conditions. These differences could explain the nature of both interactions. On the other hand, the planting date×soil cover system (PS×SC) interaction effect on bulb length was found to be also significant at the 1‰ level only in 2017 (Table 2). To examine the nature of this interac­ tion, the BL data of planting dates were compared under both soil cover systems. The values of BL under straw mulching were 8.23, 7.78, and 7.03 cm, and under no­mulching condition were 7.62, 7.05, and 5.38 cm, for PS1, PS2, and PS3, respectively. The mean values of BL in PS1 and PS2 under no­mulching were smaller by 8­10% than those under straw mulching. While in the traditional planting date in April (PS3), BL under no­mulching was 31% smaller than that with straw mulching. This difference could explain the nature of PS×SC interaction. The largest BD was produced from onion sets planted early in February (PS1) in both year: 4.7 and 4.0 cm in 2017 and 2018, respectively. It then signifi­ cantly reduced as the planting date delayed. The mean values of BD from PS3 were shorter by 35% in 2017 and 23% in 2018 than those from PS1. Furthermore, using straw mulching led to a signifi­ cant increase in BD by 17% compared with no­ Table 2 ­ Analysis of variance of the data of crop responses as affected by planting date, soil cover system, and irrigation level (signifi­ cance of Fisher test) *** = significant at 1‰ level, ** = significant at 1% level, NS = non­significant at 1% level. df = degree of freedom, BL = Bulb length, BD = Bulb diameter, Y­pot = bulb yield per pot, WP = water productivity, IWUE = irrigation water use efficiency. Source of variance df BL BD Y­pot WP IWUE 2017 Planting date (PS) 2 *** *** *** *** *** Rep. within PS 6 Soil cover system (SC) 1 *** *** *** *** *** PS × SC 2 *** NS NS *** *** Irrigation level (IL) 2 *** *** *** *** *** PS × IL 4 NS NS NS NS NS SC × IL 2 NS NS NS NS NS PS × SC × IL 4 NS NS NS NS NS Pooled error 30 Total 53 CV (%) 5.88 6.12 9.14 9.07 9.26 2018 Planting date (PS) 2 *** *** *** *** *** Rep. within PS 6 Soil cover system (SC) 1 *** *** *** *** *** PS × SC 2 NS NS NS *** *** Irrigation level (IL) 2 *** *** *** *** *** PS × IL 4 NS *** NS NS NS SC × IL 2 NS ** NS NS NS PS × SC × IL 4 NS NS NS NS NS Pooled error 30 Total 53 CV (%) 6.90 3.83 5.89 5.49 5.45 54 Adv. Hort. Sci., 2020 34(1): 49­60 Bulb yield per pot (Y‐pot) Analysis of variance shown in Table 2 indicated that the main effects of the three studied factors on Y­pot were significant at the 1‰ level. However, none of the three­factor or two­factor interactions were significant at the 1% level in both studied years. The data under each factor were averaged over all levels of the other factors for mean comparison pur­ poses (Table 3). In both years, earliness in planting date resulted in an increase in the bulb yield. The mean yield of bulbs which were planted in February (PS1), were the highest with 119.7 and 122.3 g pot­1 in 2017 and 2018, respectively. Y­pot significantly decreased with delayed planting. It reduced consid­ erably by 8 and 30% in 2017 and by 6 and 24% in 2018 when onion sets were planted in PS2 and PS3, respectively, compared with sets planted in PS1. In addition, Y­pot was found to be increased sig­ nificantly by 30% in 2017 and 26% in 2018 when straw mulching was used relative to no­mulching conditions (Table 3). On the other hand, decreasing water application rate resulted in a significant decline in the bulb yield. The lowest mean values of Y­pot (67.1 g pot­1 in 2017 and 71.4 g pot­1 in 2018) were observed under sharp deficit irrigation when only 60% of ETc was applied to irrigate onion plants. Then, Y­pot was highly improved with increasing irrigation level. Y­pot increased by about 110 and 56% when onion plants were irrigated by 100 and 80% of ETc, respectively, compared with those irrigated by only 60% of ETc, in both years. Table 3 ­ Mean comparisons of crop responses as influenced by planting date, soil cover system, and irrigation level for both studied years * Means followed by the same letter within a year and column for each tested factor are not significantly different accord­ ing to LSD at 1% level. BL = Bulb length, BD = Bulb diameter, Y­pot = bulb yield per pot, WP = water productivity, IWUE = irrigation water use efficiency). Tested factor BL (cm) BD (cm) Y­pot (g pot­1) WP (kg m­3) IWUE (kg m­3) 2017 Planting date PS1 (February) 7.9 a 4.7 a 119.74 a 4.60 a 5.71 a PS2 (March) 7.4 b 3.7 b 110.52 b 4.09 b 4.26 b PS3 (April) 6.2 c 3.0 c 83.79 c 2.82 c 2.87 c LSD 0.01 0.4 0.2 8.77 0.32 0.36 Soil cover system with mulch 7.7 a 4.1 a 123.07 a 5.14 a 5.80 a without mulch 6.7 b 3.5 b 86.30 b 2.53 b 2.76 b LSD 0.01 0.3 0.2 7.16 0.26 0.30 Irrigation level IL100 (100% ETc) 7.9 a 4.5 a 141.77 a 4.39 a 4.76 a IL80 (80% ETc) 7.3 b 3.9 b 105.11 b 3.95 b 4.39 b IL60 (60% ETc) 6.3 c 3.1 c 67.11 c 3.18 c 3.70 c LSD 0.01 0.4 0.2 8.77 0.32 0.36 2018 Planting date PS1 (February) 8.5 a 4.0 a 122.32 a 4.96 a 5.44 a PS2 (March) 7.8 b 3.9 a 115.42 b 4.21 b 4.46 b PS3 (April) 7.0 c 3.5 c 92.83 c 3.13 c 3.30 c LSD 0.01 0.5 0.1 5.95 0.21 0. 22 Soil cover system with mulch 8.2 a 4.1 a 126.54 a 5.37 a 5.81 a without mulch 7.3 b 3.5 b 93.83 b 2.83 b 2.99 b LSD 0.01 0.4 0.1 4.86 0.17 0.18 Irrigation level IL100 (100% ETc) 8.2 a 4.6 a 148.65 a 4.52 a 4.86 a IL80 (80% ETc) 8.0 a 3.8 b 110.53 b 4.17 b 4.48 b IL60 (60% ETc) 7.2 c 3.1 c 71.39 c 3.60 c 3.87 c LSD 0.01 0.5 0.1 5.96 0.21 0.22 Mubarak ‐ Planting date, irrigation level, and mulch for onion crop 55 However, for presentation and discussion purpos­ es, all experimental data of Y­pot under all studied treatments were demonstrated in figure 1a for 2017 and 1b for 2018. Trend analysis indicated that the relationships between Y­pot and irrigation level (as a percentage of ETc) for each planting date and under both soil cover systems were linear (equations not presented). As can be seen in figure 1, the rate of increasing yield with increasing irrigation water level was similar regardless of the planting date adopted under both soil cover systems. This confirmed the lack of interaction as found by ANOVA (Table 2). As shown also in figure 1, the maximum value of bulb yield was recorded in the treatment combining between planting in February under straw mulching and 100% of ETc (IL100) conditions. The yield of bulbs produced under such conditions could be attained about 185g pot­1 in both years (Fig. 1). Moreover, irri­ gating onion plants with only 60% of seasonal ETc (IL60) could produce bulb yield comparable with that under 80% of ETc (IL80), provided using straw mulching and planting onion sets as early as possible. Water use parameters As mentioned above, irrigation treatments (IL100, IL80, and IL60) received at each irrigation event 100, 80 and 60% of the amount of soil water depleted under IL100 conditions, respectively. Irrigation water amounts and seasonal crop evapotranspiration (ETc) for the studied treatments are shown in Table 4 for both years. The seasonal ETc were close to the applied water amounts, because water amounts added to the pots were equal to the depleted water amounts, as regulated by weight. As can be seen, both irrigation water amounts and crop water con­ sumption were greatly decreased when straw mulch was applied. For the three planting dates (PS1, PS2, and PS3), an average of 30% of water was saved when straw mulching was used compared with non­ mulching conditions, regardless of the tested irriga­ tion level. Moreover, early planting resulted in a noticeable decrease in both irrigation water amount and crop water use compared with the other planting dates (Table 4). For instance, onion crop planted in February (PS1) required irrigation amount 20­30% lesser than that planted in the traditional planting date (April, PS3). These results highlight the need for changing cultural practices and adopting early plant­ ing in order to conserve water resources. The ANOVA detected that both crop water pro­ ductivity (WP) and irrigation water use efficiency (IWUE) were significantly influenced by the main effects of planting date, soil cover system, and irriga­ tion level at the 1‰ level, within both years. The interaction between planting date and soil cover sys­ tem (PS×SC interaction) was also significant at the 1‰ level (Table 2). It is worth to examine the nature of this interaction. Figure 2 illustrates the data of both WP and IWUE under different planting dates (as averaged over all irrigation levels)under each system of soil covering, for both years. Regression analysis indicated that the relationship between both traits (WP and IWUE) and planting date were linear with significant values of R2 at the 1% level, under both straw mulching and no­mulching conditions. For each trait, both representative straights were not parallel, but did not intersect over the studied period. The slope of straight is about two times greater under straw mulching than that without mulching. This indi­ cates that the enhancements in both WP and IWUE due to the earliness in planting date, could be dou­ Fig. 1 ­ Response of onion bulb yield (Y­pot) in 2017 (a) and in 2018 (b) to planting date (February, March, and April), soil cover system (with and without mulching), and irri­ gation level (IL100, IL80, and IL60). Error bar represents one standard deviation. Adv. Hort. Sci., 2020 34(1): 49­60 56 bled if straw mulching is used. This could explain the existence of the interaction between planting date and soil cover system. The obtained data of both WP and IWUE under all tested treatments were shown in figure 3a and 3b for WP, and in figure 4a and 4b for IWUE, in 2017 and 2018, respectively. Also, the mean values under each factor were presented in Table 3 for mean compari­ son purposes. Both WP and IWUE as derived from the traditional planting date (PS3, in April) were the lowest: 2.82 and 3.13 kg m­3 for WP, and 2.87 and 3.30 kg m­3 for IWUE, in 2017 and 2018 respectively. Earliness in planting date resulted in a noticeable increase in both efficiencies. For instance, when onion sets were planted early in February (PS1), WP and IWUE were significantly augmented by 63 and 99% in 2017, and by 59 and 65% in 2018, respective­ Table 4 ­ Irrigation water amount (without rainfall) and crop evapotranspiration (ETc) as influenced by planting date (PS1, PS2, and PS3), soil cover system, and irriga­ tion level, for both studied years Parameters Soil cover Irrigation level PS1 (Feb.) PS2 (Mar.) PS3 (Apr.) 2017 Irrigation water amount (mm) Without mulch IL100 705.3 798.8 899.6 IL80 561.0 649.9 734.2 IL60 416.7 502.6 570.8 With mulch IL100 458.4 561.4 636.0 IL80 363.9 461.3 524.9 IL60 269.4 361.2 413.8 Crop evapotranspirationETc Without mulch IL100 800.2 819.1 909.2 IL80 664.5 671.8 745.7 IL60 525.4 528.1 586.2 With mulch IL100 552.5 578.0 641.5 IL80 459.9 479.8 532.5 IL60 367.5 387.3 429.9 2018 Irrigation water amount (mm) Without mulch IL100 706.5 809.6 886.8 IL80 572.3 655.8 718.3 IL60 431.0 493.9 540.9 With mulch IL100 494.6 566.7 620.8 IL80 400.6 459.0 502.8 IL60 301.7 345.7 378.7 Crop evapotranspirationETc Without mulch IL100 760.3 848.1 924.3 IL80 615.8 687.0 748.7 IL60 463.8 517.3 563.8 With mulch IL100 548.4 605.2 658.3 IL80 444.2 490.2 533.2 IL60 334.5 369.2 401.5 Fig. 2 ­ For both years, responses of both (a and c) crop water productivity, WP, and (b and d) irrigation water use effi­ ciency, IWUE, to planting dates under both soil cover systems. Regression equations are fitted and coefficient of determination (R2) is given under each system of soil cover. ** = significant at 1% level. Mubarak ‐ Planting date, irrigation level, and mulch for onion crop 57 ly. Furthermore, both WP and IWUE were found to be enhanced considerably when straw mulching was used: they were two times more than those under no­mulching conditions regardless of the planting dates or irrigation levels chosen. In addition, increas­ ing water application rate resulted in an efficient use of water. The maximum values of both WP and IWUE (4.39 and 4.76 kg m­3 in 2017, and 4.52 and 4.86 kg m­3 in 2018, respectively) were recorded under full irrigation treatment. They then declined dramatically with decreasing irrigation level. Trend analysis revealed that both WP and IWUE were linearly related to the irrigation level (as % of ETc) under both soil cover systems, regardless of the planting date considered, as shown in Figures 3 and 4 (mathematical equations not presented). Such devel­ oped linear functions could be invested for predicting the targeted values of WP and IWUE under similar cli­ matic conditions in the dry Mediterranean area. For instance, the best agricultural management suggested to have maximum values of both traits (WP and IWUE) is to plant onion sets in mulched soil in February under full irrigation conditions. The values of WP and IWUE produced under such conditions could reach in average 6.95 and 8.05 kg m­3 (Figs. 3 and 4). 4. Discussion and Conclusions As the tested onion variety is an oval­ to elongat­ ed­shape onion, the larger the bulb size (both length and diameter), the better the bulb shape for appear­ ance and marketing purposes. The onion bulb size was found to be increased when onion sets were planted early under straw mulching and 100% of ETc conditions. This could be related to the soil water availability. Under water stress, the soil is drier and relatively more compacted. Its mechanical resistance may limit the growth of the bulb and cause it to grow longitudinally. Moreover, straw mulching can reduce Fig. 3 ­ Response of water productivity (WP) in 2017 (a) and in 2018 (b) to planting date (February, March, and April), soil cover system (with and without mulching), and irri­ gation level (IL100, IL80, and IL60). Error bar represents one standard deviation. Fig. 4 ­ Response of irrigation water use efficiency (IWUE) in 2017 (a) and in 2018 (b) to planting date (February, March, and April), soil cover system (with and without mulching), and irrigation level (IL100, IL80, and IL60). Error bar represents one standard deviation. Adv. Hort. Sci., 2020 34(1): 49­60 58 the soil evaporation and conserves soil humidity (Kirda, 2000; Fereres and Soriano, 2007; Igbadun et al., 2012), and consequently, may encourage devel­ oping onion bulbs to grow in both length and diame­ ter directions. Thus, the recommended agricultural management to produce better shape of onion bulbs for consumers, is to plant onion sets in February under straw mulching and full irrigation level condi­ tions. Similar results about the role of straw mulching in enhancing the bulb shape indicator were reported by Mubarak and Hamdan (2018 b). The bulb yield, and both WP and IWUE were also found to be maximized when onion sets were plant­ ed early, under straw mulching and full irrigation. These results could be explained by the fact that the early planting partially covers the late winter time in which onion plants grow well and there would be rel­ atively plenty of water available, compared with the rest of the year from spring to the end of summer. Onion sets planted early also had enough time to benefit from cool period during the vegetative stage, which improved photosynthesis, and therefore pro­ duction, compared with the actual practice followed by farmers (planting in April). This finding is in agree­ ment with similar results obtained by Hamma (2013) and Rohini and Paramaguru (2016). Also, both irriga­ tion water amount and crop water consumption were found to be greatly decreased, saving about of 30% of water when straw mulch was used compared with no­mulch conditions, irrespective of irrigation level. Under current practices of planting in April without mulching, farmers are not in favor of fully irrigating their crops even if the yield is reduced, due to the huge irrigation water needs (about 900 mm). Thus, using straw mulch could regulate such case. Our research results indicated that onion plants planted in February and grown under straw mulch could be fully irrigated with only 550 mm. The favor­ able impact of mulching was preveiously reported (Vavrina and Roka, 2000; Igbadun et al., 2012; Hamma, 2013; Tsegaye et al., 2016; Mubarak and Hamdan, 2018 b). In fact, mulching decreases evapo­ ration from soil surface, remaining more water avail­ able for plants (Kirda, 2000; Fereres and Soriano, 2007; Igbadun et al., 2012). This could also moderate the severity of wetting­drying cycle between irriga­ tions, and therefore, yield could be improved (Vavrina and Roka, 2000; Gimenez et al., 2002; Mubarak and Hamdan, 2018 b). Moreover, Khaledian et al. (2010 and 2011) showed that increasing in crop yield could be also attained under straw mulching due to the enhancements in both soil fertility and soil physical properties. Results indicated that the tested onion variety was very sensitive to regulated deficit irrigation. Many reports cited similar results that onion yield was opti­ mized under full irrigation rather than under regulated deficit irrigation (Bekele and Tilahun, 2007; Kumar et al., 2007; Nagaz et al., 2012; Igbadun et al., 2012). For example, Nagaz et al. (2012) observed that irrigating onion crop with 60% of ETc resulted in considerable reduction in bulb yield, dry matter, and bulbs per hectare, compared with those irrigated by 100% or 80% of ETc. However, an important finding of our experiments is that the onion crop response to regulat­ ed deficit irrigation was found to be significantly enhanced when straw mulching is used. For example, irrigating with only 60% of ETc using straw mulching resulted in WP and IWUE much higher even than those irrigated by 100% of ETc without mulching (Figs. 3 and 4). The irrigation water saving in such treatment (deficit irrigation using straw mulching) could be used to irri­ gate additional cropped area. Patel and Rajput (2013) reported similar outcome that with 40% deficit irriga­ tion throughout the growing period, a water saving obtained could be utilized to irrigate additional ½ ha. To conclude, onion crop was found to be respon­ sive to early planting and straw mulching, so that both onion bulb size and yield were significantly enhanced, compared with those obtained under the traditional agricultural practices (planting in April without mulching). Both crop water productivity and irrigation water use efficiency were also considerably increased; and the seasonal crop water requirements obviously decreased. Our research results suggest that the best agricultural management is to plant onion sets in mulched soil in February under full irri­ gation conditions. Moreover, early planting date and straw mulching improved the response of onion crop to the regulated deficit irrigation. This could be an appropriate agronomic alternative to meet the ever increasing demand for onions and to save irrigation water. Onion bulb responses were predicted to be increased linearly with the increment in water appli­ cation rate and with the earliness of planting date, with an obvious better preference under straw mulching. Both experimental data and the developed equations could be used for predicting onion crop responses under similar agro­pedo­climatic context without carrying out any additional experiments. Moreover, they could be used as a tool for rational management of limited irrigation water. Mubarak ‐ Planting date, irrigation level, and mulch for onion crop 59 Acknowledgements The author would like to thank the Atomic Energy Commission of Syria (AECS) for encouragement and financial support. References BEKELE S., TILAHUN K., 2007 ­ Regulated deficit irrigation scheduling of onion in a semiarid region of Ethiopia. ­ Agric. Water Manage., 89: 148­152. BREWSTER J.L., 2008 ­ Onions and other vegetable alliums. Second edition. ­ CAB International, Wallingford, UK, pp. 454. CECCARELLI S., GRANDO S., BAUM M., 2007 ­ Participatory plant breeding in water‐limited environments. ­ Experimental Agriculture, 43: 411­435. CHAI Q., GAN Y., ZHAO C., XU H.L., NIU Y., SIDDIQUE KH. M., 2016 ­ Regulated deficit irrigation for crop produc‐ tion under drought stress. A review. ­ Agronomy for Sustainable Development, 36. FAO, 2011 ­ Climate change, water and food security. ­ FAO, Rome, Italy. FERERES E., SORIANO M.A., 2007 ­ Deficit irrigation for reducing agricultural water use. ­ J. Exp. Bot., 58: 147­ 159. GIMENEZ C., OTTO R.F., CASTILLA N., 2002 ­ Productivity of leaf and root vegetable crops under direct cover. ­ Sci. Hort., 94: 1­11. GIORGI F., LIONELLO P., 2008 ­ Climate change projections for the Mediterranean region. ­ Global and Planetary Change, 63(2­3): 90­104. GOMEZ K.A., GOMEZ A.A., 1984 ­ Statistical procedures for agricultural research. Second edition. ­ Wiley, New York, NY, pp. 680. HAMMA I.L., 2013 ­ Growth and yield of onion as influ‐ enced by planting dates and mulching types in Samaru, Zaria. ­ Intern. J. Adv. Agric. Res., 1: 22­26. IGBADUN H.E., RAMALAN A.A., OIGANJI E., 2012 ­ Effects of regulated irrigation deficit and mulch on yield, water use and crop water productivity of onion in Samaru, Nigeria. ­ Agric. Water Manage., 109: 162­169. JOFFRE R., RAMBAL S., 2001 ­ Mediterranean ecosystems. ­ eLS. John Wiley & Sons Ltd, Chichester, UK. DOI: 10.1038/npg.els.0003196. KADAYIFCI A., TUYLU G.I., UCAR Y., CAKMAK B., 2005 ­ Crop water use of onion (Allium cepa L.) in Turkey. ­ Agric. Water Manage., 72(1): 59­68. KHALEDIAN M.R., MAILHOL J.C., RUELLE P., MUBARAK I., MARAUX F., 2011 ­ Nitrogen balance and irrigation water productivity for corn, sorghum and durum wheat under direct seeding compared with conventional tillage in the Southeastern France. ­ Irrigation Sci., 29(2): 413­422. KHALEDIAN M.R., MAILHOL J.C., RUELLE P., MUBARAK I., PERRET S., 2010 ­ The impacts of direct seeding into mulch on the energy balance of crop production system in the SE of France. ­ Soil Till. Res., 106(2): 218­226. KHOKHAR K.M., 2014 ­ Flowering and seed development in onion. ‐ A Review. ­ Open Access Library Journal, 1: e1049. KIRDA C., 2000 ­ Deficit irrigation scheduling based on plant growth stages showing water stress tolerance. ­ Deficit Irrigation Practices, FAO Water Reports, Rome, Italy, 22. KUMAR S., IMTIYAZ M., KUMAR A., SINGH R., 2007 ­ Response of onion (Allium cepa L.) to different levels of irrigation water. ­ Agric. Water Manage., 89: 161­166. MUBARAK I., HAMDAN T., 2018 a ­ Onion crop response to different irrigation and N‐fertilizer levels in dry Mediterranean region. ­ Adv. Hort. Sci., 32(4): 495­501. MUBARAK I., HAMDAN T., 2018 b ­ Onion crop response to regulated deficit irrigation under mulching in dry Mediterranean region. ­ J. Hort. Res., 26(1): 87­94. MUBARAK I., JANAT M., MAKHLOUF M., 2018 ­ Response of two potato varieties to irrigation methods in the dry Mediterranean area. ‐ Agriculture (Poľnohospodár­ stvo), 64(2): 57­64. NAGAZ K., MASMOUDI M.M., BEN MECHLIA N., 2012 ­ Yield response of drip‐irrigated onion under full and deficit irrigation with saline water in arid regions of Tunisia. ­ ISRN Agronomy, vol. 2012, pp. 8. ONOFRI A., 2007 ­ Routine statistical analyses of field experiments by using an Excel extension. ­ National Conference Italian Biometric Society, “La statistica nelle scienze della vita e dell’ambiente”, Proc. 6th, Pisa, Italy, 20­22: 93­96. PATEL N., RAJPUT T.B.S., 2013 ­ Effect of deficit irrigation on crop growth, yield and quality of onion in subsurface drip irrigation. ­ Inter. J. Plant Prod., 7(3): 417­436. POLADE S.D., PIERCE D.W., CAYAN D.R., GERSHUNOV A., DETTINGER M.D., 2014 ­ The key role of dry days in changing regional climate and precipitation regimes. ­ Scientific Reports, 4: 43­64. RAGAB R., PRUDHOMME C., 2002 ­ Climate change and water resources management in arid and semi arid regions: prospective and challenges for the 21st century. ­ Biosyst. Eng., 81: 3­34. ROHINI N., PARAMAGURU P., 2016 ­ Seasons’ influence on bulb, seed yield and quality of aggregatum onion. ­ Int. J. Farm Sci., 6(1): 174­183. SOMOT S., SEVAULT F., DEQUE M., CREPON M., 2008 ­ 21st century climate change scenario for the Mediterranean using a coupled atmosphere‐ocean regional climate model. ­ Glob. Planet. Change, 63(2­3): 112­126. TSEGAYE B., BIZUAYEHU T., WOLDEMICHAE A., MOHAMMED A., 2016 ­ Yield and yield components of onion (Allium cepa L.) as affected by irrigation schedul‐ ing and nitrogen fertilization at Hawassa Area Districts in Southern Ethiopia. ­ J. Agric. Sci. Food Technol., 2(2): 15­20. TURNER N.C., 2004 ­ Sustainable production of crops and Adv. Hort. Sci., 2020 34(1): 49­60 60 pastures under drought in a Mediterranean environ‐ ment. ­ Ann. Appl. Biol., 144: 139­147. VAVRINA C.S., ROKA F.M., 2000 ­ Comparison of plastic mulch and bareground production and economics for short‐day onions in a semitropical environment. ­ HortTechnology, 10: 326­330. ZINKERNAGEL J., SCHMIDT N., KAHLEN K., 2015 ­ Changing thermal growing season and climatic water balance affect irrigation and cultivation period of vegetables. ­ Procedia Environ. Sci., 29: 51­52.