Introduction

Over 40% of global food production comes from irrigated land, and the agricultural sector has the highest water consumption, using 70% of our existing freshwater resources (FAO 2017; Mancosu et al. 2015). With an expected 8 billion people needing to be fed by 2025, food security can only be guaranteed if the irrigated land area and crop yield increase by more than 20 and 40%, respectively (Lascano and Sojka 2007). Therefore, the efficiency of water consumption in agriculture needs to be maximized to ensure maximum productivity with minimal water input. Improving agricultural water management is a promising way to achieve this goal (Unver et al. 2017), as it will allow water, energy, and soil resources to be conserved while meeting growing demands for crops, foods, and fibers (Kassam et al. 2007).

Flood and sprinkler irrigation systems are often associated with high water wastage and a low irrigation water use efficiency (IWUE), as well as adverse soil salinity and drainage conditions (Yazar et al. 2002), which has resulted in drip irrigation systems becoming increasingly popular (Şimşek et al. 2004). Drip irrigation supplies a large number of crops with a uniformly distributed and frequent water application using a low-pressure emitter to trickle water onto the soil and is suitable for different soil types and all topographic conditions (Dağdelen et al. 2009; Liu et al. 2017). Emitters are either placed on the surface of the soil (surface drip irrigation, DI), which results in the formation of a shallow pond or buried (subsurface drip irrigation, SDI), which leads to the formation of a saturated bulb surrounded by a wet-bulb of unsaturated soil (Cook et al. 2006). SDI can increase the yield and net profit margin, reduce nitrate loss from deep percolation, minimize soil water evaporation, and increase IWUE and the quality of agricultural products (Douh and Boujelben 2010; Odhiambo and Irmak 2015; Yuan et al. 2003) compared with other irrigation systems, including DI (Camp 1998).

Other water management techniques include deficit irrigation, which allows larger agricultural areas to be irrigated under limited water resources (Chaves et al. 2010; Sezen et al. 2014). This technique strongly affects irrigation scheduling because crops respond to water stress in different ways in their various growth stages. It has been stated that important considerations when applying deficit irrigation are the timing and degree of water stress experienced by the plants (Jovanovic and Stikic 2012; Loveys et al. 2004; Yang et al. 2017), as well as crop evapotranspiration (ETc) and responses to water stress (Capra et al. 2008). Under adequate deficit drip irrigation, crops develop deep roots that allow them to reach the soil water, resulting in significant water savings without crop yield reduction (i.e., IWUE increases) and an increased profit margin for farmers (Chai et al. 2016; Evett and Tolk 2009; Kato et al. 2006; Kirda 2002; Stikic et al. 2010).

Deficit irrigation has been found to promote plant growth and IWUE for a variety of crops in different countries. For example, Geets and Kirk (2009) confirmed that deficit irrigation increases the IWUE of different crops without reducing their yield when water stress is induced during drought-sensitive growth stages, while Ngouajio et al. (2007) showed that tomato (Solanum lycopersicum L.) yields in southern Michigan, USA, could be increased by preventing irrigation in the first few weeks after transplantation, as long as the water content in the soil at the time of transplanting was sufficient. Similarly, Sharma et al. (2014) showed that deficit irrigation under SDI in arid environments of the USA led to a moderate decrease in the yield of melon (Cucumis melo L.) but no reduction in fruit quality. Müller et al. (2016) reported that the optimum soil matric potentials for DI of eggplant (Solanum melongena L.) in Burkina Faso during the early growth and fruit-setting stages were − 15 and − 40 kPa, respectively. Shammout et al. (2018) also found that irrigation at 80% ETc gave the highest yield of bell pepper (Capsicum annum L.) in Jordan and that there was no difference in IWUE between 80 and 100% ETc, while Mattar et al. (2020) indicated that irrigation at 70% ETc increased tomato yield, fruit quality, and IWUE compared with 100% ETc in Saudi Arabia.

Potato (Solanum tuberosum L.) is one of the most economically important crops globally, ranking fourth in the world in terms of its nutritional value (Fabeiro et al. 2001). However, it is sensitive to drought (Obidiegwu et al. 2015), making it particularly difficult to manage (Shock and Feibert 2002). Drip irrigation can significantly improve potato growth under adverse arid conditions (Erdem et al. 2006; Onder et al. 2005; Zin El-Abedin et al. 2017), and several studies have examined the response of potato crops to deficit irrigation to further improve its IWUE (Cantore et al. 2014; Elhani et al. 2019; Jovanovic et al. 2010; Nagaz et al. 2016; Shahnazari et al. 2007). Among these, Shahnazari et al. (2007) showed that there was no significant difference in potato yield under deficit and full irrigation, but IWUE was 1.6 times higher under deficit irrigation (approximately 30% water saving), while Jovanovic et al. (2010) reported that water savings of 38% could be made without significantly affecting tuber yield. In addition, Cantore et al. (2014) indicated that irrigation at 50% ETc generated satisfactory potato yields in Italy, with a tuber quality similar to or better than that obtained under full irrigation, and Nagaz et al. (2016) found that 70% ETc provided the best trade-off between tuber yield and IWUE, while 50% ETc gave the highest IWUE. Finally, Elhani et al. (2019) reported that deficit irrigation improved vegetative growth and tuber quality in a glasshouse and increased IWUE compared with full irrigation.

Freshwater resources are scarce not only in arid and semi-arid regions but also in areas that experience heavy rainfall, and agriculture has the greatest potential to solve this global issue. Therefore, there is a need for research that focuses on maintaining the yield and quality of crops and water productivity using pressurized irrigation systems under limited water conditions to ensure the effective use of available freshwater resources in irrigated agriculture. The objectives of this study were to (1) investigate the effects of different water levels on the soil water status under DI and SDI systems; and (2) evaluate the performance of DI and SDI systems with deficit irrigation in terms of vegetative growth, tuber yield, and IWUE in potato.

Materials and methods

Study site

Field experiments were conducted at the Educational Farm of King Saud University, Riyadh (24°44′11.10″N, 46°37′06.61″E; 665 m a.s.l), during the 2017 and 2018 growing seasons (January–May). A weather station was installed near the experimental area to monitor and record meteorological data (Fig. 1). This region has an arid climate, with hot and dry summers. The mean air temperature was 22.6 °C and 22.2 °C, the mean relative humidity was 35.1 and 38.6%, and the total rainfall was 25.7 mm and 72.6 mm in the 2017 and 2018 growing seasons, respectively. Reference evapotranspiration (ETo; mm day−1) was calculated from the recorded meteorological data using the FAO-56 Penman–Monteith equation (Allen et al. 1998) as shown in Eq. (1):

Fig. 1
figure 1

Ten-day average air temperature (T), relative humidity (RH), and reference evapotranspiration (ETo), and the summed rainfall during the 2017 and 2018 growing seasons

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$${\mathrm{ET}}_{\mathrm{o}}=\frac{0.408 \Delta \left({R}_{\mathrm{n}}-G\right)+\gamma \frac{900}{T+273} {u}_{2} \left({e}_{\mathrm{s}}-{e}_{\mathrm{a}}\right)}{\Delta + \gamma \left(1+0.34 {u}_{2}\right)}$$
(1)

where Δ is the slope of the saturation vapor pressure–temperature curve at mean air temperature (kPa °C−1), Rn is the net radiation (MJ m−2 day−1), T is the mean air temperature at 2-m height (°C), u2 is the wind speed at 2-m height (m s−1), γ is the psychrometric constant (kPa °CS−1), G is the soil heat flux (MJ m−2 day−1), and es and ea are the saturation and actual vapor pressures, respectively (kPa). ETo was between 3.1 and 10.3 mm day−1 during the 2017 season and between 2.6 and 9.4 mm day−1 during the 2018 season (Fig. 1).

Before the start of the experiment, soil samples were collected to a depth of 60 cm and the soil physical characteristics were recorded at each 20-cm depth interval (see Table 1). This showed that the soil had a sandy loam texture with a mean bulk density of 1.6 g cm−3 and a soil water content of 138 mm at field capacity (FC) and 78 mm at wilting point (WP), indicating that the available water in the soil profile was 60 mm.

Table 1 Soil physical properties at the experimental site
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Experimental procedure

The experiment was set up using a randomized complete block design with six treatments (two irrigation systems × three irrigation levels) and three replicates per treatment. The two irrigation systems (DI and SDI) were applied at the block level and the three irrigation levels [full irrigation at 100% ETc (I100) and regular deficit irrigation at 70% and 50% ETc (I70 and I50, respectively)] were randomly assigned within these blocks using separate randomization for each block. The experimental area was divided into three fields (one for each replicate) that were separated by 2-m buffer zones (Fig. 2). Each field contained two blocks that were each further divided into three equal plots, leaving a 0.75-m interval between the plots to avoid lateral leakage from the irrigation treatment applied in adjacent plots. The irrigation level plots (I100, I70, and I50) in the DI and SDI blocks were irrigated daily based on the daily ETc (mm day−1), which was calculated as shown in Eq. (2) (Allen et al. 1998):

Fig. 2
figure 2

Schematic diagram of the experimental fields. DI surface drip irrigation, SDI subsurface drip irrigation, I100 100% of crop evapotranspiration (ETc), I70 70% ETc, I50 50% ETc

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$${\mathrm{ET}}_{\mathrm{c}}={\mathrm{ET}}_{\mathrm{o}} \times {K}_{\mathrm{c}}$$
(2)

where Kc is the stage-specific crop coefficient. The lengths of the main growth stages, Kc, and the total amounts of water applied for each treatment during the 2017 and 2018 growing seasons are presented in Table 2. The same amount of water was applied to all treatments until 30 days after planting (DAP), at which time the deficit irrigation treatments (I70 and I50) were applied under both the DI and SDI systems.

Table 2 Dates and crop coefficient (Kc) values for each potato (Solanum tuberosum) growth stage and the total amount of water applied during the study period
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Preliminary field preparations for the potato seedbeds included plowing, grading, and leveling. The soil was ridged with 70-cm spacing and 30-cm height to create furrows, following which a DI and SDI systems were set up that comprised a water tank, a pump unit, a pressure regulator, a fertigation unit, a screen filter, an air vent, manual and solenoid valves, flow meters, and flush valves (Fig. 2). Main and sub-main lines were buried beneath the soil surface, while manifold lines were positioned above the soil surface. The drip lines consisted of built-in emitters that had a discharge of 8 L h−1 at an operating pressure of 100 kPa and were laid manually above (DI) or 10 cm below (SDI) the soil surface at the center of each crop row at 70-cm intervals from adjacent rows and with 50-cm intervals between emitters. The irrigation systems were controlled automatically in this study. Potato tubers of the cultivar Hermes were planted by hand at a density of five plants per m2 on January 27, 2017 and January 24, 2018. A soil ridge was maintained 15 cm above the tubers in the prepared furrows. Compound NPK fertilizer was applied at the same concentration to all treatment plots via the irrigation systems from 7 DAP based on the following scheduled program: N–P2O5–K2O (20:20:20) at rates of 180, 250, and 270 kg ha−1and N–P2O5–K2O (10:10:43) at rates of 150, 250, and 200 kg ha−1 up to 37, 67, and 88 DAP, respectively. Potato tubers were harvested on May 24, 2017 and May 21, 2018.

Soil water measurements

The volumetric soil water content (θv) was measured in 10-cm increments up to a depth of 50 cm using multi-sensor capacitance probes (EnviroSCAN®; Sentek Sensor Technologies, Stepney, Australia). These probes continuously monitored θv in the root zone throughout the growing season after being calibrated at the experimental site following Sentek (2001) and Jabro et al. (2005). A total of 18 probes (six treatments × three replicates) were inserted vertically into the middle of the ridge of each drip line in each experimental field.

Agronomic measurements

In each growing season, three plants were randomly selected from each plot (treatment) at 90 DAP and prepared for analysis of the fresh weight of the vegetative parts (leaf and stem; Mg ha−1), plant height (cm), number of branches (plant−1), and leaf area index (LAI), which was measured using an LI-3100 leaf area meter (Licor Inc., Lincoln, Nebraska, USA). The vegetative samples were then dried at 60 °C for 48 h (Ahmadi et al. 2014) using a forced-air oven to determine the dry weight (Mg ha−1).

The potato harvesting area in each treatment consisted of three 13-m-long rows. After harvesting, the total fresh tuber yield (Mg ha−1), fresh weight (g), mean weight (g), and number of tubers per plant were measured. The tubers were then oven-dried for 24 h at 85 °C (Liu et al. 2006) to measure their dry weights (Mg ha−1). After excluding any deformed and diseased tubers, the remaining tubers were classified according to their diameter, following the method of Shahnazari et al. (2007), whereby < 50 mm was considered small, 50–80 mm was moderate (marketable tubers), and > 80 mm was large. The tubers were then counted and weighed as a percentage of the total weight.

Irrigation water use efficiency (IWUE) and yield response factor

IWUE (kg m−3) was calculated by dividing the total fresh tuber weight (kg ha−1) by the total amount of water applied (irrigation plus rainfall, m3 ha−1) during each growing season (Kirda et al. 2005).

The yield response factor (Ky) was calculated from the fresh tuber yield for each irrigation system using the pooled data across the two experimental years (Doorenbos and Kassam 1979), as shown in Eq. (3):

$$\left(1-\frac{{Y}_{\mathrm{a}}}{{Y}_{\mathrm{x}}}\right)=K\mathrm{y}\left(1-\frac{{\mathrm{ET}}_{\mathrm{a}}}{{\mathrm{ET}}_{\mathrm{x}}}\right)$$
(3)

where Ya is the tuber yield (Mg ha−1) obtained from the I70 or I50 treatment, Yx is the tuber yield (Mg ha−1) obtained from the I100 treatment, ETa is the crop water consumption (m3 ha−1) under deficit irrigation (70% or 50% ETc), and ETx is the crop water consumption (m3 ha−1) under full irrigation (100% ETc). Similarly, the biomass response factor (Kss) was calculated from the dry biomass yield (tuber plus vegetative parts), as shown in Eq. (4):

$$\left(1-\frac{{\mathrm{SS}}_{\mathrm{a}}}{{\mathrm{SS}}_{\mathrm{x}}}\right)=K\mathrm{ss}\left(1-\frac{{\mathrm{ET}}_{\mathrm{a}}}{{\mathrm{ET}}_{\mathrm{x}}}\right)$$
(4)

where SSa and SSx are the dry biomass yields (Mg ha−1) under full irrigation and deficit irrigation, respectively. Ky and Kss are indicate the relationship between relative yield and relative crop water consumption (Lovelli et al. 2007; Singh et al. 2010), with values > 1 indicating that the crop is very sensitive to water stress, values < 1 indicating that the crop is more tolerant to water stress, and values of 1 indicating that the relative yield reduction is equal to the relative water use reduction (Steduto et al. 2012).

Data analysis

Differences in the vegetative growth traits, tuber yield, yield components, and IWUE among treatments were statistically analyzed by two-way analysis of variance (ANOVA) (irrigation system × irrigation level) for each experimental year with three replicates per treatment using CoStat version 6.003 (CoHort, USA,1998–2004). In addition, three-way ANOVA (year × irrigation system × irrigation level) was performed to evaluate the statistical significance of annual variations. The significance of differences between means was assessed using the least significant difference (LSD) test at a 5% probability level (p ≤ 0.05). Data are presented as means ± standard errors (SE) of the three replicates. Finally, the relationships among the fresh tuber yield and the number and mean weight of tubers per plant were examined using regression analysis.

Results

Soil water

Differences in soil water storage between the two drip irrigation systems (DI and SDI) and three irrigation levels (I100, I70, and I50) were determined by calculating the average θv of five sampling points at depths of 10, 20, 30, 40, and 50 cm each day throughout the 2017 and 2018 growing seasons for all treated plants (Fig. 3). There were no differences in the θv profile among the different irrigation levels up to 30 DAP in either season because the same amount of irrigation water (corresponding to 100% ETc) was applied to each. However, the average θv values were higher for SDI (18.66 and 18.63% for 2017 and 2018, respectively) than for DI (16.98 and 17.04% for 2017 and 2018, respectively) during this period. From 31 DAP onward, the θv values for the I100 treatment remained above 50% of the available soil water (i.e., the median θv value between FC and WP or 18%) and were close to the FC, averaging 20.14 and 21.39% under the DI and SDI systems, respectively, across both growing seasons. By contrast, the application of deficit irrigation treatments at 31 DAP resulted in the temporal variance of θv under both irrigation systems due to a gradual decrease in soil water content until the end of both seasons. However, while the θv values for the I70 treatment dropped below 50% of the available soil water under the DI system (average: 17.39 and 17.35% for 2017 and 2018, respectively), they remained above 50% of the available soil water under the SDI system (average: 18.86 and 18.84% for 2017 and 2018, respectively), though were still lower than values obtained under the I100 treatment. The θv values for the I50 treatment remained between 50% of the available soil water and the WP and were lower than the values reported for the I100 and I70 treatments, ranging from 12.91 to 16.53% in 2017 and 12.77–17.21% in 2018 under the DI system and from 14.71 to 17.74% in 2017 and 14.69–17.92% in 2018 under the SDI system.

Fig. 3
figure 3

Volumetric soil water content (%) of the average of five points at depths of 10, 20, 30, 40, and 50 cm under different irrigation systems and with different irrigation levels in the 2017 and 2018 growing seasons. DI surface drip irrigation, SDI subsurface drip irrigation, I100 100% of crop evapotranspiration (ETc), I70 70% ETc, I50 50% ETc, FC field capacity, WP wilting point

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Vegetative growth traits

The irrigation system significantly affected the fresh and dry weights of the vegetative parts, with the SDI system resulting in 30.20 and 41.32% higher weights, respectively, in the 2017 season and 32.82 and 34.29% higher weights, respectively, in the 2018 season than the DI system (Table 3). There were also significant differences in these weights between the growing seasons. Irrespective of the irrigation system used, the fresh and dry weights of the vegetative parts were significantly lower with the I70 and I50 treatments than with the I100 treatment in both growing seasons, with the fresh weights decreasing by 33.61 and 34.52%, respectively, and the dry weights decreasing by 28.93 and 35.03%, respectively. There were no significant interaction effects between the irrigation systems and levels on the fresh or dry weights in either season (Table 3).

Table 3 Fresh and dry weights of the vegetative parts of potato (Solanum tuberosum) plants under different irrigation systems and with different irrigation levels in the 2017 and 2018 growing seasons
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There were no significant differences in plant height between treatments in the 2017 season (Table 4). However, in 2018, plants grown under SDI were significantly taller than those grown under DI (p < 0.05), and plants exposed to the I100 treatment were significantly taller than those exposed to the I70 and I50 treatments (p < 0.05), which were not significantly different from each other, irrespective of the irrigation system employed. In 2018, the shortest plants were produced with the I70 treatment under both DI (33.50 cm) and SDI (41.53 cm), while the tallest plants were obtained with the I100 treatment under DI (46.50 cm) and the I50 treatment under SDI (43.20 cm). There was no significant difference in the branch number per plant among the irrigation level treatments when considered alone or in combination with the DI and SDI systems in either season (Table 4). However, plants managed under the SDI system had 40.42 and 28.38% more branches per plant than those managed under the DI system in 2017 and 2018, respectively, irrespective of the irrigation level. By contrast, the irrigation system did not have a significant effect on LAI, but plants treated with I50 and I70 had significantly lower LAI values (average: 3.00 and 3.13, respectively) than those subjected to the I100 treatment (3.53). Under both DI and SDI, the I70 and I50 treatments had a lower LAI than the I100 treatment in both seasons, though these differences were not significant (p > 0.05).

Table 4 Physical characteristics of potato (Solanum tuberosum) plants under different irrigation systems and with different irrigation levels in the 2017 and 2018 growing seasons
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Tuber yield

The fresh tuber yield significantly differed between the irrigation systems and among the irrigation levels in both the 2017 and 2018 growing seasons, while the dry tuber yield only significantly differed among the irrigation levels in both seasons (Table 5). There were also significant differences in the fresh tuber yield between growing seasons, but no such differences for the dry tuber yield. The fresh tuber yield was 7.90 and 5.52% higher under the SDI system than under the DI system in 2017 and 2018, respectively. The fresh and dry tuber yields with I100 were 32.33 and 6.55 Mg ha−1, respectively, in 2017 and 36.53 and 6.78 Mg ha−1, respectively, in 2018. A 30% reduction in the amount of irrigation water (I70) resulted in the fresh and dry tuber yields decreasing by 17.23 and 21.22%, respectively, in 2017 and 16.64 and 34.35%, respectively, in 2018, while a 50% reduction in the amount of irrigation water (I50) decreased these yields by 31.09 and 29.06%, respectively, in 2017 and 29.76 and 47.35%, respectively, in 2018 compared with values obtained with I100. There was no significant interaction effect between the irrigation system and level on the fresh and dry tuber yields in either season.

Table 5 Fresh and dry yields of potato (Solanum tuberosum) tubers under different irrigation systems and with different irrigation levels in the 2017 and 2018 growing seasons
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Components of tuber yield

The fresh tuber weight per plant significantly differed between the irrigation systems and among the irrigation levels in both the 2017 and 2018 growing seasons (Table 6), with plants grown under the SDI system and with the I100 treatment having the highest value. The fresh tuber weight per plant was, on average, (p < 0.05) 10.39 and 35.16% lower for the DI system and 27.65 and 35.55% lower for the SDI system with the I50 treatment than with the I70 and I100 treatments, respectively. The mean tuber weight was, on average, 14.12% lower under the SDI system than under the DI system. The mean tuber weight was significantly lower in plants subjected to the I50 treatment than in those subjected to the I100 and I70 treatments under both irrigation systems (p < 0.001), whereas there was no significant difference between the I100 and I70 treatments. In both growing seasons, the number of tubers per plant was highest with the I100 treatment, with the DI and SDI systems resulting in an average of 11 and 15 tubers per plant, respectively, for I100 compared with 9 and 15 tubers per plant, respectively, for I70 and 10 and 11 tubers per plant, respectively, for I50.

Table 6 Fresh tuber weight per plant, mean tuber weight, and tuber number per plant for potato (Solanum tuberosum) under different irrigation systems and with different irrigation levels in the 2017 and 2018 growing seasons
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There was a strong positive, albeit non-significant, correlation between the fresh tuber yield per hectare and the number of tubers per plant (r = 0.40, p > 0.05; Fig. 4). There was also a strong positive correlation between the fresh tuber yield per hectare and the mean tuber weight (r = 0.63, p < 0.05), indicating that the reduction in fresh tuber yield in treated plants was mainly due to a lower tuber weight, and a negative but non-significant correlation between the mean tuber weight and the number of tubers per plant (r =  − 0.36, p > 0.05).

Fig. 4
figure 4

Relationships between the fresh tuber yield per hectare, the number of tubers per plant, and the mean tuber weight of potato (Solanum tuberosum) based on pooled data from the 2017 and 2018 growing seasons

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Plants that were exposed to the I50 treatment had a smaller tuber diameter under both the DI and SDI systems, exhibiting an increased proportion of tubers in the < 50 mm class (39.63 and 34.09%, respectively) and a corresponding decreased proportion in the > 80 mm class (23.73 and 27.58%, respectively) (Fig. 5). The opposite trend was observed with the I100 treatment, though these differences were not significant.

Fig. 5
figure 5

Proportion of tubers in different diameter classes (< 50 mm, 50–80 mm, and > 80 mm) under different irrigation systems and with different irrigation levels in the 2018 growing season. DI surface drip irrigation, SDI subsurface drip irrigation, I100 100% of crop evapotranspiration (ETc), I70 70% ETc, I50 50% ETc. Bars represent means ± SE (n = 3)

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IWUE

IWUE significantly differed between the two growing seasons and was significantly affected by the irrigation system in both seasons (p < 0.01), with plants grown under SDI having 8.38% higher IWUE in 2017 and 5.82% higher IWUE in 2018 than those grown under the DI system (Fig. 6). There were also significant differences in IWUE among the three irrigation levels under each irrigation system in both seasons (p < 0.01), with the exception of DI in 2017 (p = 0.07). Under the DI system, IWUE was significantly higher with the I50 treatment (7.01 kg m−3) than with the I70 and I100 treatments in 2018, with differences of 6.70 and 16.83%, respectively. Under the SDI system, IWUE with the I50 treatment was 6.31 and 19.45% higher in 2017 and 6.25 and 20.45% higher in 2018 compared with the I70 and I100 treatments, respectively.

Fig. 6
figure 6

Irrigation water use efficiency (IWUE) of potato (Solanum tuberosum) plants under different irrigation systems and with different irrigation levels during 2017 and 2018 growing seasons. Bars represent means ± SE (n = 3). Different lower-case letters directly above the bars for each irrigation system and season represent significant differences among irrigation levels, while different lower-case letters in parentheses for each season represent significant differences between the irrigation systems and different upper-case letters in parentheses represent significant differences between the seasons (LSD test, p < 0.05). DI surface drip irrigation, SDI subsurface drip irrigation, I100 100% of crop evapotranspiration (ETc), I70 70% ETc, I50 50% ETc, LSD least significant difference

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Yield and biomass response factors

Ky and Kss were estimated for the DI and SDI systems using the fitted regression approach with pooled data from the two growing seasons to provide an indication of the level of tolerance of the crop to water stress (Fig. 7). Both factors were < 1 under both irrigation systems, with the exception of Kss under the DI system, which was slightly higher than 1. Both Ky and Kss were higher under the DI system than under the SDI system, with values of 0.77 and 1.07, respectively, under DI and 0.68 and 0.94, respectively, under SDI.

Fig. 7
figure 7

Relationship between relative evapotranspiration (1 − ETa/ETx) and both the relative tuber yield (1 − Ya/Yx, solid lines) and relative dry biomass (1 − SSa/SSx, dashed lines) for potato (Solanum tuberosum) under surface drip irrigation (DI) and subsurface drip irrigation (SDI) based on pooled data collected during the 2017 and 2018 growing seasons

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Discussion

The θv value in the root zone of the potato plants was lower with the I70 and I50 treatments under the DI system than with the I100 treatment under the SDI system across both growing seasons, supporting the previous findings of Demir and Sahin (2017) and Mattar et al. (2020) that θv is significantly lower for plants managed with deficit irrigation than for those managed with full irrigation. It has been reported that the high θv provided by the SDI system facilitates water transport to the area around the root (Patel and Rajput 2007; Vyrlas et al. 2014) and results from limited capillary flow to the surface in combination with reduced evaporation losses, which eliminate the effects of surface infiltration properties and saturation status with pond water under the DI system (Ayars et al. 2015; Elmaloglou and Diamantopoulos 2009).

The agronomic trait analysis of the potato plants showed that both the fresh and dry weights of the vegetative parts were higher under the SDI system than under the DI system, which agrees with the results of Al-Ghobari and Dewidar (2018) for tomato. The fresh and dry weights of the vegetative parts also decreased with a decreasing irrigation level, which is consistent with the adverse effects of water stress on plant development. Many studies have reported a reduction in the weight of the vegetative parts under deficit irrigation (Badr et al. 2018; Cantore et al. 2014; Eid et al. 2020), indicating a higher fruit absorption strength than the rest of the plant parts (Zegbe et al. 2004). This difference is most likely due to the significant decrease in the soil–root contact area that occurs under soil water depletion (Jensen et al. 1993), which may act as a limiting factor for vegetative growth (Ahmadi et al. 2010a). Karam et al. (2009) and Passioura and Angus (2010) showed that underwater shortage conditions, the most common cause for low vegetative growth in vegetable species is a shortened annual crop cycle and accelerated leaf aging.

Plants that were exposed to the I70 and I50 treatments under DI also had the lowest height while plants treated with DI had fewer branches than SDI, which agrees with the findings of Al-Ghobari and Dewidar (2018) for tomato and Meligy et al. (2020) for potato. However, Elhani et al. (2019) detected no statistically significant difference in either the plant height or the number of branches between different irrigation levels. LAI was not significantly affected by the irrigation system in either season, supporting the results of Çolak et al. (2015) for eggplant. However, there was a significant difference in LAI between the deficit irrigation treatments and the full irrigation treatment in the present study, which is consistent with the results of Liu et al. (2006), Zin El-Abedin et al. (2017), and Romero et al. (2017) for potato, as well as Karam et al. (2011) for eggplant. Shahnazari et al. (2007) and Deshi et al. (2015) reported that LAI values > 3, as observed in this study, are optimal for maximizing the growth rate and light interception in potato.

The SDI system had a significant positive effect on the fresh tuber yield, and similarly Patel and Rajput (2007) and El-Mokh et al. (2014) reported that SDI at 10–15 cm depth can result in a higher tuber yield. An SDI depth of 15–25 cm has commonly been used for crop production in many studies on tomato (Lamm 2016), and it has been shown that SDI outperforms DI in increasing eggplant (Douh and Boujelben 2010) and tomato (Lamm 2016) yields. The ability of the SDI system to improve tuber productivity can be attributed to the reduction in evaporation loss from the soil surface and maintenance of the optimum soil water content in the root zone, resulting in optimal crop production (Ayars et al. 2015; Selim et al. 2009). In terms of the irrigation level, the fresh tuber yield was highest in plants treated with I100 followed by those subjected to the I70 and I50 treatments, respectively, which agrees with the findings of Cantore et al. (2014) and Ahmadi et al. (2014), as well as those of Çolak et al. (2017) for eggplant and Meligy et al. (2020) for potato, who established that crop yield is positively correlated with the amount of irrigation water applied and vegetative growth. Iqbal et al. (1999) also stated that deficit irrigation during the early development stages of potato may cause a sharp drop in the final fresh yield, though treatments in the present study were applied after the tuber initiation stage. By contrast, Patel and Rajput (2007) reported that potato yield was not significantly affected by a 20% decrease in irrigation water, and Onder et al. (2005) reported that while irrigation levels significantly affected the tuber yield of potato, the irrigation system had no significant effect. The dry tuber yield of plants was also higher with the I100 treatment than with the I70 and I50 treatments, supporting the findings of Ahmadi et al. (2014) for potato and Shao et al. (2008) for bell pepper. Similarly, Elhani et al. (2019) showed that the dry tuber yield was significantly reduced under water stress treatments compared with 100% FC. However, Schafleitner (2009), Ahmed et al. (2017), and Petropoulos (2020) found that the dry tuber yield also depends on the crop variety, which is an important factor in the adaptation of potatoes to water-scarce conditions. Camargo et al. (2015) and Ierna and Mauromicale (2018) suggested that increasing levels of irrigation water lead to increases in the dry matter weight in all stages of potato growth. One possible explanation for this may be that potato is a shallow-root crop, so any differences in soil water content in the top 10–30 cm soil layer can have a large effect, while differences in the soil water content in deep soil layers (30–50 cm) may have much less of an effect due to the low density of plant roots (Kaman et al. 2011; Zin El-Abedin et al. 2017). Passioura and Angus (2010) reported that the lower growth and production levels under water-scarce conditions result from a decrease in the transportation of assimilates from the leaves and stems to the fruits in crops that usually mature in increasingly unfavorable water supply conditions.

The fresh tuber weight, mean tuber weight, and number of tubers per plant grown were significantly higher under the SDI system with the I100 treatment than with any other treatment combination in both seasons and lowest under the DI system with the I50 treatment, which is consistent with the results of Elhani et al. (2019). Demirel et al. (2014) and Sharma et al. (2014) reported that reduced amounts of irrigation water (50% ETc) decreased the fruit volume in eggplant and melon, respectively. Previous studies have found that water stress induced at different stages of growth reduces the number of tubers per plant (Cantore et al. 2014; Monnem et al. 2015; Nagaz et al. 2016). Similarly, Marouelli and Silva (2007) observed that the number of commercially suitable tomato fruit per plant decreased with soil water tension during growth, and Amer (2011) and Zin El-Abedin et al. (2017) found that fewer fruit were produced when squash (Cucurbita pepo L.) and potato plants, respectively, were exposed to 50% and 75% ETc under a DI system. Ahmadi et al. (2014) reported that the number of potato tubers in the > 80 mm size class was highest with 100% ETc, whereas Erdem et al. (2006) obtained the highest number of tubers per plant with a low irrigation level. This inconsistency may be due to differences in the plant varieties studied, as well as other environmental conditions, such as soil type and air temperature (Rykaczewska 2013; Walworth and Carling 2002). Alva et al. (2012) noted that a decrease in the number, weight, and size of tubers per plant as a result of insufficient irrigation resulted in a lower yield. In the present study, a set amount of water was provided for all irrigation level treatments during the initial growth stage to ensure regular germination and stem elongation. Therefore, the tuber developmental stage was affected by water stress, which reduced tuber weight without increasing the number of tubers (O’Brien et al. 1998), ultimately decreasing the yield of fresh tubers.

Plants that were managed with the SDI system and subjected to the I50 treatment had the highest IWUE. Similarly, Patel and Rajput (2007) and El-Mokh et al. (2014) recommended that a higher IWUE could be achieved by applying the SDI system at a depth of 10–15 cm, and Kashyap and Panda (2003), Liu et al. (2006), Ahmadi et al. (2014), and Ierna and Mauromicale (2018) showed that the IWUE of potato was higher under deficit irrigation than under full irrigation. Increased values of IWUE under deficit irrigation have also been observed for watermelon [Citrullus lanatus (Thunb) Matsum & Nakai] (Leskovar et al. 2004), tomato (Al-Qerem et al. 2012), red pepper (Sezen et al. 2014), bell pepper (Bladia and AbdArahman 2016), and eggplant (Çolak et al. 2015, 2017), while Geets and Kirk (2009) confirmed that deficit irrigation successfully increased the IWUE of different crops without decreasing crop yield in dry regions. The increased IWUE in plants exposed to water stress is induced by physiological processes that control the CO2 and H2O gradients between the leaves of the plant and the air surrounding the leaves. A high CO2 concentration surrounding the leaves is beneficial in water-stressed plants because partial stomatal closure reduces transpiration and enhances photosynthesis, whereas this has little effect on well-watered plants (Bloch et al. 2006; Hatfield and Dold 2019; Lopes et al. 2011). However, severe water stress may lead to complete stomatal closure and reduce IWUE and the potato tuber yield (Zin El-Abedin et al. 2019). By contrast, Ky and Kss were lower in plants grown under the SDI system than in those grown under the DI system, which suggests that potato plants are more sensitive to water stress under the DI system. Unlu et al. (2006) similarly found that different methods of irrigation appeared to affect the Ky values of potato (0.68 for DI and 1.05 for sprinkler irrigation). Since the Ky value is an important indicator of the sensitivity of a crop to drought, it may be influenced by factors besides water deficiency, such as the soil and water management conditions, climatic conditions, length of the growing season, and irrigation method (Ferreira and Gonçalves 2007; Greaves and Wang 2017).

Conclusion

Minimizing water use while maintaining crop production levels has become a key goal of experimental research in agriculture due to the limited availability of water and rising demand for food globally, and this is particularly important in arid environments. Therefore, the present study investigated the effects of full (100% ETc) and deficit (50% and 30% reduction in ETc) irrigation under DI and SDI systems on the soil water, vegetative growth traits, tuber yield and its components, and IWUE of potato during the 2017 and 2018 growing seasons in an arid region of Saudi Arabia. It was found that a 50% reduction in irrigation water under the SDI system improved the distribution of soil water and reduced evaporation, which had a positive effect on IWUE but reduced the tuber yield. Furthermore, the yield response factor to water was less than 1, indicating that potato can tolerate some water deficits, which will allow the yield to be maintained while reducing water loss in arid environments. However, if the purpose is to obtain the highest tuber yield regardless of the IWUE, then full irrigation should be applied with the SDI system. Therefore, the beneficial effects of a given irrigation strategy will depend on both the irrigation method being used and the amount of water being provided to elicit plant responses.