Impact of a Novel Water-Saving Subsurface Irrigation System on Water Productivity, Photosynthetic Characteristics, Yield, and Fruit Quality of Date Palm under Arid Conditions

Abstract

Water scarcity is a major constraint in arid and semi-arid regions. Crops that require less irrigation water and those, which are considered drought-tolerant such as date palm (Phoenix dactylifera L.), are dominant in these regions. Despite the tolerance of these crops, the development of technologies that ensure efficient use of irrigation water is imperative. Taking these issues into consideration, the study was conducted to investigate the impact of limited irrigation water using a new subsurface irrigation system (SSI) on gas exchange, chlorophyll content, water use efficiency, water productivity, fruit physicochemical characteristics, and yield of date palm (cv. Sheshi). The impact of the SSI system was compared with two surface irrigation systems, namely, surface drip irrigation (SDI) and surface bubbler irrigation (SBI). The field experiment was carried out during 2018 and 2019 at the Date Palm Research Center of Excellence, King Faisal University, Kingdom of Saudi Arabia. The annual crop evapotranspiration (ET_c) was 2544 mm. The applied irrigation water was set at 50%, 75%, and 125% of ET_c for SSI, SDI, and SBI, respectively, which were based on the higher crop water productivity recorded in an initial field study. The total annual volume of water applied for SSI, SDI, and SBI was 22.89, 34.34, and 57.24 m³ palm⁻¹, respectively. The crop water productivity (CWP) at the SSI system was significantly higher, with a value of 1.15 kg m⁻³, compared to the SDI (0.51 kg m⁻³) and SBI systems (0.37 kg m⁻³). The photosynthetic water use efficiency (WUE) was 10.09, 9.96, and 9.56 μmol CO₂ mmol⁻¹ H₂O for SSI, SBI, and SDI, respectively. The maximum chlorophyll content (62.4 SPAD) was observed in SBI, followed by SSI (58.9 SPAD) and SDI (56.9 SPAD). Similarly, net photosynthesis and the transpiration rate were significantly higher in SBI and lowest in SSI. However, the SSI system substantially increased palm yield and enhanced fruit quality. The new SSI system, through its positive impact on the efficiency of irrigation water use and enhancement on fruit yield and fruit quality of date palm, seems quite suitable for the irrigation of palm trees in arid and semi-arid regions.

1. Introduction

Date palm (Phoenix dactylifera L.) is a major crop in most arid and semi-arid regions of the world [1]. These regions are generally characterized by water resource scarcity [2]. Despite water scarcity in such regions, inefficient use of water still prevails and can be commonly noticed on date palm farms, which is blamed for the depletion of precious groundwater sources [3]. In water scarcity regions, the use of irrigation water for agricultural production requires an appropriate transfer of technologies and innovative research [4]. The water productivity should be driving date-water policy, not date palm production [5]. Although highest date palm production is achieved when providing full irrigation water requirements by traditional methods, the same production can be achieved with significantly less water application, up to 50% less, by using modern irrigation systems [5].

Conservation of water and maximization of water use efficiency in arid and semi-arid regions through modern irrigation technologies have become key for sustainable crop production [6]. In most practical situations for date palm and passion fruit irrigation, the required water is calculated based on potential crop evapotranspiration (ET_c) using the Penman–Monteith equation of reference evapotranspiration (ET_o), which requires data of standard prevailed weather [7]. To convert the ET_o to ET_c, a crop factor (K_c) is needed, which varies with the development stage of the crop [8]. The date palm water use can also be predicted from the relation of ET_c = 0.95 LI × ET_o where LI is the light intercepted by the canopy. Therefore, reductions in irrigation water may be achieved by intensive leaf pruning to reduce the light intercepted fraction [9,10]. Kassem [11] used the methods of a soil water-balance approach and Bowen ratio energy balance to calculate the actual annual water use in drip irrigation of 15-year-old palms (cv. Sukariah), i.e., 1640 and 1780 mm, respectively. Al-Omran et al. [12] conducted a study to estimate the water requirements of date palm trees in various regions of the Kingdom of Saudi Arabia. The water requirements, based on the proportion of the cultivated area of date palm (100 palms ha⁻¹) for each year, ranged from 7298.9 to 9495.2 m³ ha⁻¹. Another study was conducted in the western region of Saudi Arabia that recommended 7300 m³ ha⁻¹ of irrigation water for date palm [13].

The adaptation of plant species to water scarcity includes stress sequences detectable at morphological, genetic, and physiological multifunctional responses. The main physiological impact of drought stress is the disruption in the photosynthetic system [14]. Water stress also triggers significant changes in carbon partitioning at the cellular plant levels and noticeable modifications in the composition of membrane proteins and lipids in the photosynthetic apparatus [15]. Plants tolerate low tissue water potential through osmotic changes [16] that alter morphological and/or physiological properties by reducing transpiration or increasing absorption. However, water-stressed plants could induce stomatal adjustment to water potential and carbon dioxide recycling during photosynthesis as an adaptive physiological mechanism [17,18]. In general, many studies have reached similar conclusions regarding responses of plants to water stress. They mainly identified limitation of carbon dioxide supply that affects plants’ metabolic functions and consequently limited the photosynthetic capacity [18,19,20]. Plant stress due to delayed and limited irrigation often leads to economic yield losses. In many date palm producing countries, flood irrigation is still popular despite its low efficiency to conserve water, particularly in sandy soils [21,22,23]. The fruit yield and quality of 17-year-old date palm trees subjected to several irrigation systems revealed substantial differences among these traits. However, adopting a subsurface drip irrigation system showed a noticeable enhancement in yield, up to 163 kg palm⁻¹, and a decrease in water consumption compared to surface drip irrigation [24]. In another study, it was revealed that the subsurface drip irrigation system substantially increased date palm yield, reduced the need for irrigation water, and enhanced WUE compared to surface drip irrigation [23]. It was also reported that date palm yield was 25–60% higher due to an increase in the WUE using a subsurface drip irrigation system [23,25,26]. Similarly, Alikhani-Koupaei et al. [27] obtained higher date palm yield at irrigation intervals of 70% ET_c.

In date palm farming, the crop water productivity (CWP) of a subsurface drip irrigation system was much higher than a bubbler irrigation system although the SDI method has an additional cost but is economical in the long term [28]. Reduction of irrigation water quantity by 49–53% coupled to date palm yield increase of 45–49% was noticed using a low flexibility subsurface drip irrigation system as compared to medium and high flexibility pipes in a subsurface irrigation system [25]. Similarly, deficit irrigation at intervals of 100 mm evaporation resulted in the highest bunch weight, yield, and WUE without any degradation of fruit qualitative properties of date palm (cv. Mazafati) [27]. Similarly, it is also reported that deep drip irrigation leads to noticeable enhancement in fruit quality and marketable yield in addition to increased WUE of 1.55 kg m⁻³ date palm under a deep drip irrigation system and mulched soil compared to a bubbler system and un-mulched soil [29]. The development of new irrigation technologies is imperative to ensure the efficient use of irrigation water in arid regions. The novel SSI system was constructed to efficiently deliver the irrigation water directly to the functional root zone of the palm tree. Hence, it provides a means to save irrigation water by reducing evaporation and infiltration in non-absorbing root zones. The system is characterized by the simplicity of the installation around the palm tree. It only needs a hole with a diameter of 20 cm and a depth of 40 cm and four irrigation units around the date palm tree. Therefore, the study was conducted to investigate the impact of limited irrigation water using this system on various physiological and production palm tree components. The SSI system was compared with two conventional surface irrigation systems (drip irrigation and bubbler irrigation).

2. Materials and Methods

2.1. Experimental Site

This study was conducted in an arid climatic region during 2018 and 2019 at the Date Palm Research Center of Excellence Research and Training Station, King Faisal University, Al-Ahsa, Kingdom of Saudi Arabia (Latitude: 25.2608° N, Longitude: 49.7078° E, Altitude: 155 m above sea level). The soil profile of the experimental site (0–100 cm) was a sandy loam texture consisting of 63.5 ± 2.3% sand, 21 ± 1.9% silt, and 15.5 ± 1.6% clay. The mean volumetric water content (VWC) at field capacity (Fc) was 15.5 ± 1.6% from the surface layer to 100 cm depth at 25 cm intervals. The mean values of the permanent wilting point (PWP), bulk density (BD), pH, and the electrical conductivity (EC) were 5.4 ± 0.12%, 1.6 ± 0.01 kg m⁻³, 7.7 ± 0.08, and 3.17 ± 0.02 ds m⁻¹, respectively, for the same depths (

Table 1. Soil properties in different soil layers of the experimental site. Bulk density (BD); field capacity (Fc); the permanent wilting point (PWP); the electrical conductivity (EC).

Soil Depth	Distribution of Particle Size			BD (g cm−3)	Fc (%)	PWP (%)	pH	EC (dS m−1)
	Sand (%)	Silt (%)	Clay (%)
0–25	67	20	13	1.59	14.8	5.3	7.6	3.15
25–50	61	24	15	1.61	15.3	5.6	7.8	3.18
50–75	62	21	17	1.6	15.1	5.4	7.7	3.16
75–100	64	19	17	1.62	15.5	5.3	7.8	3.21

) [30].

Table 2. Analysis of irrigation water used in the experiment.

Characteristic	Temperature (°C)	pH	Total Dissolved Solids (mg L−1)
Value ± SD	24.3 ± 0.67	7.54 ± 0.15	756 ± 45.6

Values represent means whereas ± values indicate standard deviations (SD).

shows the analysis of the irrigation water used in the experiment [30].

2.2. Description of Irrigation Systems

Currently, different irrigation systems are available in arid regions of the date palm. These systems include furrow irrigation, bubbler irrigation, flood irrigation, surface, and subsurface drip irrigation. In our study, a new subsurface irrigation system that provides irrigation water directly to the absorbing zone of the root system was compared with surface drip irrigation and surface bubbler irrigation systems.

2.2.1. Subsurface Irrigation (SSI)

The SSI unit was constructed to efficiently deliver the irrigation water directly to the functional root zone of the palm tree. The SSI unit (Figure 1), that was designed at the Date Palm Research Center of Excellence, King Faisal University, was used in the experiment. The SSI unit consisted of a water flow regulator, two perforated pipes, and gravel between the outer and inner tubes. The diameter of the inner pipe was 20 mm and the length was 330 mm. The inner pipe was perforated with holes having a diameter of 3 mm arranged in a spiral shape. The outer tube with a diameter of 100 mm and a length of 300 mm was slotted with a tilt angle of 45° with a 2 mm slot width and 40 mm slot length. The pipe was wrapped with a filtering cloth to prevent the movement of fine soil into the tube. The gravitational forces play an important role in water movement in the soil with steady-state water flow. The flow rate of the SSI unit was adjusted to 0.045 m³ h⁻¹ by the head of the water flow regulator at a static pressure of 2 m. The SSI system consisted of a water resource, subsurface irrigation units, electric pump, water tank, delivery pipe, sub-lines, lateral lines, and manifolds. Four subsurface irrigation units were buried around the date palm tree within a circle of diameter 1.40 m (Figure 2).

2.2.2. Surface Drip Irrigation (SDI)

In this system, four low-pressure adjustable drippers (0–0.070 m³ h⁻¹) were used to deliver irrigation water to the same spot around the palm tree. The dripper flow rate was adjusted to 0.045 m³ h⁻¹ by twisting the dripper head at a pressure of 200 kPa, which was regulated by a pressure regulator (Model: DN20, OEM, Zhejiang, China). The dripper head was installed on a plastic pipe around date palm tree within a circle with a diameter of 1.30 m. The dripper ring was connected to the distribution line using a flexible plastic tube with a length of 1 m and diameter of 7 mm.

2.2.3. Surface Bubbler Irrigation (SBI)

In the SBI system, four adjustable bubbler (0–0.120 m³ h⁻¹) were used to deliver irrigation water around the palm. The bubbler flow rate was adjusted to 0.060 m³ h⁻¹ by twisting the bubbler head at a pressure of 100 kPa. The bubbler head was installed on a plastic wedge and was inserted into the ground in a palm basin to prevent runoff when the irrigation water exceeded the soil infiltration. The bubbler was connected to the distribution line using a flexible plastic tube with a length of 1 m and diameter of 7 mm.

Solenoid valves (24 V dc) controlled by an irrigation timer (7 days, 24 h) were used to control the water supply according to the irrigation schedules. The automatic controller (Model: LCD-M, SEA, Zhongjiang, China) with a flow sensor (Model: YF-B8 G1/2, SEA, Guangdong, China) was used to manage the quantitative flow rate of the irrigation water.

2.3. Meteorological Data

The main weather parameters (minimum and maximum air temperature (°C), sunshine duration (h), relative humidity (%), rainfall (mm), wind speed (km h⁻¹), solar radiation (MJ m⁻² day⁻¹)) were inserted into the Penman–Monteith equation to estimate evapotranspiration (ET_o). These parameters were monitored by a weather station installed at the study site. The air–water vapor pressure deficit (kPa) was determined using daily and hourly average relative humidity and temperatures. To adjust the recorded wind speed data at 2 m above the ground surface, the following equation was used [7].

$$u=u_z\frac{4.87}{\ln (67.87 z-5.42)}$$

(1)

where $u$ is the wind speed at 2 m above the ground surface (m s⁻¹), $u_z$ is the measured wind speed (m s⁻¹), and $z$ is the actual height (m).

2.4. Estimation of Evapotranspiration

The reference evapotranspiration (ET_o) is the evaporating power of the atmosphere at a specific time and location of the year. It does not consider the soil factors and crop characteristics [31]. The ET_o was computed from the site weather data using the computer Program (CROPWAT 7) according to the FAO Penman–Monteith method [7]. The method requires the average of solar radiation, air humidity, air temperature, and wind speed [7] as presented in the following equation:

$${\mathrm{ET}}_{\mathrm{o}}=\frac{0.408 \Delta \left(R-G\right)+\gamma [900 u/\left(T+273\right)]\left(e_s-e_a\right) }{\Delta +\gamma \left(1+0.34 u\right)}$$

(2)

where ET_o is the reference evapotranspiration (mm day⁻¹), R is the net radiation at the crop surface (MJ m⁻² day⁻¹), G is the density of soil heat flux (MJ m⁻² day⁻¹), T is the air temperature (°C), u is the wind speed at the height of 2 m (m s⁻¹), e_s is the saturation vapor pressure (kPa), e_a is the actual vapor pressure (kPa), ∆ is the slope vapor pressure curve (kPa °C⁻¹), and γ is the psychrometric constant (kPa °C⁻¹).

The crop evapotranspiration (ET_c) was calculated using the following equation:

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

(3)

where ET_c is the crop evapotranspiration (mm day⁻¹) K_c is the crop factor, and ET_o is the reference evapotranspiration (mm day⁻¹).

The average values of K_c were 0.85 in the summer and 0.98 in the winter, with an average annual value of 0.90. The trend and the average value of K_c are in good agreement with Al-Amoud et al. [26]; Allen et al. [7]; Dhehibi et al. [28]; FAO [5].

2.5. Experimental Layout

In this study, a new system of subsurface irrigation (SSI) was compared with two irrigation systems, namely: surface drip irrigation (SDI) and surface bubbler irrigation (SBI). The average daily water use was 50%, 75%, and 125% of ET_c for SSI, SDI, and SBI systems, respectively. These values were based on higher crop water productivity, as mentioned in the preliminary field study. Ten-year-old date palm trees (cv. Sheshi) with approximately similar size were selected for the experiment. The three irrigation systems represented the treatments and were replicated three times based on a Randomized Complete Block Design (RCBD). The average height of the palm trunk was 1.5 m, with an average diameter of 0.60 m. The experimental orchard had a plant density of 200 palms ha⁻¹ where palm-to-palm and row-to-row distance was 7 m. A fertilization program that included nitrogen (3 kg tree⁻¹), phosphorus (1.5 kg tree⁻¹), and potassium (3 kg tree⁻¹) was used for each date palm tree. These amounts were applied five times per year in equal doses in the irrigation water.

2.6. Irrigation Water Requirements

Prior to conducting a comprehensive present field study, a preliminary observation trial was conducted to choose the most effective and optimal ET_c percentage for SSI, SDI, and SBI irrigation systems according to the significantly higher crop water productivity (CWP) values. The same irrigation water amount of 50, 75, 100, and 125% of ET_c was applied for all three irrigation systems, SSI, SDI, and SBI (

Table 3. Crop water productivity (CWP) of date palm (cv. Sheshi) under different irrigation percentage from the crop evapotranspiration (ET_c) for subsurface irrigation system (SSI), surface drip irrigation (SDI) and surface bubbler irrigation (SBI).

Parameter	Irrigation Systems	Irrigation Amount
		50% of ETc	75% of ETc	100% of ETc	125% of ETc
CWP (kg m−3)	SSI	1.11 a ± 0.10	0.79 b ± 0.05	0.65 c ± 0.04	0.47 d ± 0.03
	SDI	0.48 b ± 0.03	0.55 a ± 0.03	0.48 b ± 0.02	0.42 c ± 0.02
	SBI	0.34 ab ± 0.03	0.33 ab ± 0.03	0.30 b ± 0.03	0.39 a ± 0.02

Figures with the same letter in a row are non-significant at the 5% level of probability. Values represent means whereas ± values indicate standard deviations.

). Based on the results of the initial trial, we selected the optimum irrigation amount that presented higher CWP values of 50%, 75%, and 125% of ET_c for SSI, SDI, and SBI systems, respectively. These parameters were selected to identify irrigation systems that conserve water and produce reasonable crop yields in arid regions where scarcity of water is a major concern [30].

The amount of irrigation water was expressed per date palm tree, as this would overcome much of the confusion according to FAO recommendations [5]. The irrigation requirement for the irrigation systems was calculated based on ET_c, target soil area, and an adjusted coefficient as below:

$$\mathrm{IWR}=\frac{K_{adj}\times A_s\times {\mathrm{ET}}_{\mathrm{c}}}{1000}$$

(4)

where IWR is the daily irrigation water requirement (m³), Et_c is the crop evapotranspiration (mm day⁻¹), A_s is the target soil area of each date palm tree, and K_adj is the adjusted coefficient (K_adj = 0.5, 0.75, and 1.25 for SSI, SDI, and SBI, respectively).

The target soil area of each date palm tree was equal to 90% of the actual shaded area of the palm tree, which was calculated based on the light intercepted by the canopy [32]. The mean diameter of the shaded area was 5 m, as shown in Figure 2. Irrigation timing was determined by a calendar (every day from May–September, every two days in April and October, and every three days from November–March) using a programmable timer (Model: TM919, HHT, Guangdong, China). The cumulative amount of applied irrigation water throughout the year was monitored by the readings of a digital flow meter (Model: K24-S, SUNNY, Shandong, China).

2.7. Gas Exchange Measurements

Gas exchange measurements (net photosynthesis and rate of transpiration) were recorded using a portable photosynthesis system (Model: Li-6400XT LiCor Inc., Lincoln, NE, USA). The Li-6400XT system is an open method to measure gas exchange and enables air from one source to enter both the analysis and reference lines. A leaf with a known area was put in the leaf chamber of Infra-Red Gas Analyzer (IRGA) where the air constantly pass through the leaf chamber to maintain the CO₂ at a fixed level. The system measures the transpiration and photosynthesis on the basis of the differences between the CO₂ and H₂O in the airflow within the leaf cuvette (reference cell) in comparison to the air stream flowing out of it (sample cell). The rate of CO₂ uptake by the leaf in the IRGA leaf chamber is used to calculate the rate of net photosynthesis, and the rate of water loss is used to measure the rate of transpiration [33]. Net photosynthesis (A) and the transpiration rate (T) were evaluated at seven-day intervals between fruit set in early March until July. An airflow of 500 mL min⁻¹ was used, and the readings were performed under ambient temperature, photosynthetically active radiation, and CO₂ concentration of 380 μmol m² s⁻¹. The readings were taken using the middle section of the leaflet (pinnae). The measuring chamber enclosed a circular 2 × 3 cm² leaf area and evaluated the gas fluxes on both sides of the leaf. The leaf chlorophyll content was determined directly at the same time intervals using a portable chlorophyll meter (Model: SPAD 502, Konica–Minolta, Osaka, Japan).

2.8. Photosynthetic Water Use Efficiency

The IRGA data (Li-6400XT LiCor Inc., Lincoln, NE, USA) were used to calculate the photosynthetic water use efficiency (WUE) as below:

$$\mathrm{WUE} = \frac{NP }{Tr} \times 100$$

(5)

where WUE is the photosynthetic water use efficiency (μmol CO₂ mmol⁻¹ H₂O), NP is the net photosynthesis (μmol CO₂), and Tr is the transpiration rate (mmol H₂O).

2.9. Physicochemical Characteristics of Date Fruit

The date fruits were randomly selected from each palm tree at the Tamr fruit maturity stage during the 2018 and 2019 seasons. The collected fruits were used to determine physicochemical parameters. The length and width of fruit were measured using a digital Vernier slide caliper. The fruit weight was measured using a Sartorius electronic balance. Determination of moisture content, pH, and total soluble solids was conducted according to AOAC standard methods of analysis [34]. The fruit moisture content was determined by drying a sample of 25 g under vacuum at 70 °C, then was calculated as the percentage of the weight loss divided by the initial weight of the sample [35]. Total soluble solids and fruit firmness was determined using a laboratory refractometer (Model: RFM 840, Richmond Scientific Ltd. Unit 9, Lancashire, UK) and Koehler penetrometer (Thomas Scientific, Swedesboro, NJ, USA), respectively. Fruit color parameters were measured using a Hunter lab Color Quest −45/0 LAV color difference meter (Hunter Associates Laboratory Inc., Reston, VA, USA) based on the L, a, and b color system. This system is one of the uniform color spaces recommended by the International Commission on Illumination (CIE) in 1976 as a way of closely representing perceived color [36]. The L value is the lightness factor that gives values ranging from zero for black to 100 for white while the values of a and b are chromaticity coordinates. The value of a indicates the degree of greenness–redness (ranging from −60 to zero for green and from 0 to 60 for red), and the b value indicates the blueness–yellowness (ranging from −60 to zero for blue and from 0 to 60 for yellow). Chroma (C) and hue angle (h) were calculated for a random sample of 20 dates according to the following equation:

$$C=\sqrt{a^2+b^2}$$

(6)

$$h=arc tan\frac{b}{a}$$

(7)

where C is Chroma, h is the hue angle (degree), a is the redness, and b is the yellowness.

2.10. Crop Water Productivity

The crop water productivity (CWP) was calculated using the following equation:

$$\mathrm{CWP}=\frac{Y}{Wu}$$

(8)

where CWP is crop water productivity (kg m⁻³), Y is the total marketable date palm yield (kg), and Wu is the annual amount of irrigation water (m³).

2.11. Statistical Analysis

The data of yield, fruit characteristics, chlorophyll content, and gas-exchange were analyzed using Statistical Analysis Software, Release 9.4 (SAS Institute, Cary, NC, USA). Data regarding different irrigation systems were analyzed using IBM SPSS version 23 (SPSS Inc., Chicago, IL, USA). Duncan’s Multiple Range Test (DMRT) was applied to determine the least significant difference between all experimental means at (p < 0.05) probability.

3. Results and Discussion

3.1. Climatic Conditions of the Study Area

The observed mean monthly values of the climatic parameters in the experimental site are shown in Figure 3. The highest mean relative humidity was 55.67% during December and February, while the lowest mean was 27.01% during June and September. The highest mean value of net radiation was 26.1 MJ m⁻² day⁻¹ in May. The data revealed that the highest mean temperature was 36.92 °C during the summer months from June to September, while the lowest mean was 17.18 °C during the winter months from December to February. The mean of the annual cumulative amount of efficient rain 64.2 mm. Scarce rainfall occurs from December to March. The mean value of the wind speed increased from February to September and decreased in the remaining period. The highest mean value of the wind speed was 3.6 km h⁻¹ in February. The mean value of annual sunshine duration, net radiation, and wind speed were 9.1 h and 20.6 MJ m⁻² day, and 1.82 km h⁻¹, respectively.

The mean values of ET_o and ET_c at the experimental site are shown in Figure 4. The data revealed that the daily evaporation rates peaked in the months of June and July. The data were similar to the data from Al-Amoud et al. [26]; Al-Omran et al. [12]; Kassem [11]. The average daily ET_o ranged from 3.52 mm d⁻¹ in February to 12.14 mm d⁻¹ in July, and the ET_c rate ranged from 2.99 to 11.77 mm d⁻¹ in the same months. The annual cumulative ET_o and ET_c were 2755 and 2544 mm, respectively.

3.2. Amount of Applied Irrigation

The amount of applied irrigation in the study site throughout the year was calculated as a percentage of the ET_c. FAO recommended that ideally, it is best to express the water requirement per date palm tree rather than per hectare [5]. The results of the study in Figure 5 showed the actual annual water use values of each palm tree as subjected to the irrigation systems of SSI, SDI, and SBI. The data indicate that the maximum value of applied water was during the summer months, especially in July for all irrigation systems. The highest values of the water use were 180.42, 270.63, and 451.05 mm day⁻¹ in July, while the lowest values were 46.37, 69.56, and 115.94 mm day⁻¹ in January for the same irrigation systems (SSI, SDI, and SBI, respectively). The values of annual water use were 1271.96, 1907.95, and 3179.92 mm for the irrigation systems of SSI, SDI, and SBI, respectively. Figure 6 shows the cumulative water use of date palm tree under the different irrigation systems. The volume of water applied was calculated using an irrigation area of 18 m² per palm tree, according to Zaid and Arias-Jimenez [21]. The annual volume of water applied was 22.89, 34.34, and 57.24 m³ palm⁻¹ for SSI, SDI, and SBI, with a daily average of 0.063, 0.094, and 0.157 m³ palm⁻¹, respectively. On a hectare basis at a planting distance of 7 × 7 m (200 palms ha⁻¹), the annual volume of water applied was 4578, 6868, and 11448 m³ ha⁻¹, as related to the SSI, SDI, and SBI systems. Comparing the SSI system to the SDI and SBI systems, the difference in the volume of irrigation water applied was estimated as 11.45 and 34.34 m³ palm⁻¹ (0.031 and 0.094 m³ palm⁻¹ day⁻¹); the difference on a hectare basis was estimated to be 2290 and 6868 m³ ha⁻¹, respectively. The cumulative water use of the date palm tree was within the range reported by Adil et al. [37]; Al-Amoud et al. [26]; FAO [5]; Ismail et al. [13].

3.3. Chlorophyll and Gas Exchange Measurements

The data in

Table 4. Chlorophyll, gas exchange, and water use efficiency of date palm (cv. Sheshi) under different irrigation systems, subsurface irrigation (SSI), surface drip irrigation (SDI), and surface bubbler irrigation (SBI).

Parameters	Irrigation Systems
	SSI	SDI	SBI
Chlorophyll content (SPAD)	58.9 b ± 8.56	56.9 c ± 9.11	62.4 a ± 5.95
Photosynthetic rate (µmol CO2 m−2 s−1)	9.77 c ± 1.63	11.66 b ± 2.01	13.37 a ± 2.45
Transpiration rate (mmol H2O m−2 s−1)	0.98 c ± 0.19	1.21 b ± 0.29	1.44 a ± 0.34
Water Use Efficiency (μmol CO2 mmol−1 H2O)	10.09 a ± 1.70	9.96 a ± 1.82	9.56 b ± 1.65

Figures with the same letter within a row are non-significant at the 5% level of probability. The data presented above indicate the average of each parameter recorded from 7 March to 4 July during 2018 and 2019. Values represent means whereas ± values indicate standard deviations.

indicate significant variations regarding the chlorophyll content, net photosynthesis, stomatal conductance, intercellular CO₂ concentration, transpiration rate, and water use efficiency (WUE) under the different irrigation systems. The maximum chlorophyll content (62.4 SPAD) was measured in SBI followed by the SSI (58.9 SPAD) and SDI (56.9 SPAD) irrigation systems. Generally, water stress has significant adverse effects on the chlorophyll content in some plants where reductions up to 55% were recorded under higher water stress compared to non-stressed studies [38]. Steinberg et al. [39] also reported the harmful effects of water stress on the chlorophyll content in peach tree. Kirnak et al. [38] linked increased electrolyte leakage to a decrease in the chlorophyll content due to leaf senescence, whereas Premachandra et al. [40] reported that the electrolyte leakage was adversely affected by the reduction in water. Our study showed that the chlorophyll content decreased by 8.81% and 5.61% in the SDI and SSI regimes, respectively, when compared to SBI. Usually, the chlorophyll content is negatively affected if the water quantity is reduced. However, in the present study, SSI induced less chlorophyll reduction than SDI despite the fact that SDI resulted in the consumption of relatively higher water quantities. The structure of the SSI regime enables the direct provision of water to the functional absorbing area of the root zone compared to SDI where more water might be evaporated due to the soil surface application. In both water application regimes, reductions in the chlorophyll content may be attributed to irregularities at cell wall membranes [38,41]. Under in vitro drought conditions, date palm (cvs. Shamia and Amri) exhibited an increase in the chlorophyll content [42]. El Rabey et al. [43] reported a non-significant effect of drought-induced by polyethylene glycol 6000 in an in vitro environment on chlorophyll a and b binding proteins of three-month-old date palm (cv. Sagie). The difference in plant age, stress conditions, and stressor types might be the reasons for varied responses between our study and their work.

Similarly, the average net photosynthesis (13.37 µmol CO₂ m⁻² s⁻¹) and transpiration rate (1.44 mmol H₂O m⁻² s⁻¹) were significantly higher in the SBI treatment. These respective averages were decreased by 12.79% and 15.97% in SDI and 26.93% and 31.94% in SSI regimes. This is mostly because water was applied generously in the SBI system. It was reported in many crops that when plants encountered water stress, a substantial decline in photosynthesis occurred [44]. Those reductions in photosynthesis were attributed to the decrease in CO₂ assimilation per unit leaf area as stomata closed or as photo-oxidation damaged the photosynthetic mechanism [45]. Elshibli et al. [14] studied the photosynthetic response of date palm, and they developed a relationship between the photosynthetic rate and intercellular CO₂ concentration. They indicated that as water stress increased, the photosynthetic rate of the date palms tended to be more dependent on the CO₂ concentration. The present study indicated that the water use efficiency (WUE) based on the net photosynthesis and transpiration rate was higher (10.09 µmol CO₂ mmol⁻¹ H₂O) when the palm tree were irrigated by the SSI system followed by the SDI (9.96 µmol CO₂ mmol⁻¹ H₂O) and SBI (9.56 µmol CO₂ mol H₂O⁻¹) systems, whereas the WUE was significantly higher in both SSI and SDI systems as compared to SBI. The minimum WUE was calculated under the SBI, which was 5.25% lower than that under the SSI. Generally, crop yield increases linearly with increasing water consumption under deficit irrigation management, whereas WUE decreases as the water supply or consumption reaches a certain degree [46]. Lu and Zhuang [47] found that WUE increased with increasing soil moisture under moderate drought conditions. However, it decreased with increasing soil moisture under severe drought conditions. In the present study, WUE was higher in the SSI system, where there was moderate water stress and reduced evaporation. Similarly, Helaly and El-Hosieny [42] reported that WUE increased with an increase in water stress levels in date palm (cvs. Shamia and Amri). These results coincide with the present study, where WUE increased in the SSI system. However, Al-Khateeb et al. [48] observed a negative effect of water stress to a certain degree on WUE in date palm cultivars under in vitro conditions.

3.4. Physicochemical Characteristics of Date Fruit

The data in

Table 5. Physicochemical properties of date palm fruit (cv. Sheshi) at the Tamr stage under different irrigation systems during the 2018 and 2019 seasons.

Physicochemical Characteristics	Irrigation Systems
	SSI	SDI	SBI
Fruit weight (g)	8.27 a ± 0.16	6.42 b ± 0.23	7.85 a ± 0.36
Pulp weight (g)	7.61 a ± 0.17	5.77 b ± 0.21	7.19 a ± 0.39
Fruit length (mm)	35.1 a ± 0.33	31.9 b ± 0.92	33.2 ab ± 1.72
Fruit width (mm)	23.4 a ± 0.86	21.2 b ± 0.16	22.1 ab ± 0.92
Hardness (N mm−2)	2.48 a ± 0.26	2.57 a ± 0.79	2.20 a ± 0.27
Moisture content (%)	12.7 a ± 0.52	12.1 a ± 0.31	13.3 a ± 1.25
Total soluble solids (%)	62.0 a ± 0.24	57.5 b ± 1.49	61.4 a ± 1.85
Fruit pH	5.90 a ± 0.01	5.96 a ± 0.09	5.97 a ± 0.16
L (Lightness	44.0 a ± 0.79	42.8 a ± 1.55	41.5 a ± 0.70
a (Red/green value)	11.6 a ± 0.73	11.4 a ± 0.96	10.3 b ± 0.45
b (Yellow/blue value)	18.7 a ± 0.29	17.6 a ± 1.19	16.9 a ± 0.99
h (Hue angle)	58.1 a ± 1.91	56.6 a ± 183	58.5 a ± 0.92
C (Chroma)	22.2 a ± 0.51	21.1 ab ± 1.34	19.9 b ± 1.05

Figures with the same letter in a row are non-significant at the 5% level of probability. Values represent means whereas ± values indicate standard deviations.

indicate the irrigation water applied through the SSI resulted in significant values regarding fruit weight (8.27 g), fruit length (35.1 mm) and width (23.4 mm), pulp weight (7.61 g), Total soluble solids (62.0%), a fruit color value (11.6), and Chroma (22.2). Generally, the results indicated that the SSI system improved fruit quality parameters. These findings may be due to the efficient use of water within the functional absorbing root zone. Proper utilization of water within the tree system likely enhances and improves plant nutrient uptake [49,50]. A reduction in water quantity uptake normally triggers changes in carbon allocation, which enhances fruit growth and productivity [51]. Similarly, Intrigliolo and Castel [52] suggested that the plants under lower water conditions had an enhanced solute concentration and accumulated more sugars that increased total soluble solids. Sadik et al. [29] recorded higher total soluble solids when date palm was subjected to a deep drip irrigation system, which is in line with the data presented in Table 4. A reduction of water at different fruit development stages such as flowering, fruit setting, and maturation negatively affected the overall productivity and fruit quality of many fruit species [53]. In contrast, both fruit yield and quality were significantly improved under the SSI system that provided a reduced amount of irrigation water to the palm tree. The improvement in both parameters was highly probable due to the efficient use of water by the root system since it was directly provided to the absorbing functional zone. Alikhani-Koupaei et al. [27] reported that physical characters of date palm fruit of cv. Mazafati were improved with a reduction in irrigation water. These results coincide with our present study.

3.5. Date Palm Yield and Crop Water Productivity

Table 6. Yield and Crop water productivity (CWP) of date palm (cv. Sheshi) under different irrigation systems during the 2018 and 2019 seasons.

Parameters	Irrigation Systems
	SSI	SDI	SBI
Yield (kg palm−1)	26.30 a ± 3.03	17.28 b ± 2.75	21.6 ab ± 4.18
CWP (kg m−3)	1.15 a ± 0.13	0.51 b ± 0.08	0.37 b ± 0.07

Figures with the same letter in a row are non-significant at the 5% level of probability. Values represent means whereas ± values indicate standard deviations.

indicates a significant variation regarding the yield and crop water productivity (CWP) of the date palm (cv. Sheshi) under the different irrigation systems. The highest yield (26.30 kg palm⁻¹) and CWP (1.15 kg m⁻³) were recorded under the SSI system, whereas the lowest were recorded under SDI and SBI, respectively. Noticeably, the palms irrigated by the SSI system showed a significant increase of 34.3% in crop yield and 55.6% in CWP compared to the SDI system, though the annual volume of water applied by SSI was 33% lower than that of the SDI system. Likewise, the results showed an increase of 17.9% in crop yield with 67.8% in CWP compared to the SBI system, though the annual volume of water applied by SSI was 60% lower than that of the SBI system. The increase in crop yield and CWP could be due to the high efficiency of the novel SSI system compared to the SDI and SBI ones. It was obvious that although the SSI system dispensed a low amount of water, this amount was used with high efficiency for fruit production. The SSI system does not only minimize the runoff of water but also prevents water loss through soil evaporation. Our results suggested that the increase in yield and yield-related components could be due to the optimal availability of soil water in the SSI system that not only enhanced balanced root growth but also improved soil nutrient uptake [49,50]. Application of just a 65% of the total date palm water requirement also enhanced the yield and resulted in the best CWP [13]. Sadik et al. [29] found that the deep drip irrigation system was the best regarding date palm yield and WUE parameters. The influence of water availability on plant growth is due to the variations in stomatal conductance, carbon uptake, and turgor pressure of plant tissues. Therefore, the restricted application of water affects fruit yield and quality, which vary with vegetative and reproductive growth stages, duration and severity of deficit water, and species diversity [54].

4. Conclusions

Water scarcity is globally a key constraint in arid and semi-arid regions of date palm cultivation. Efforts to design modern irrigation systems that significantly save water and ensure its efficient utilization have been important research areas over the past few years and up to the present time. The novel designed SSI system used in the study is simple, cost-effective, and practical for date palm cultivation in arid regions. It soundly contributes to the reduction of water resource depletion in arid regions while maintaining satisfactory tree growth and production. Our findings demonstrated that the novel SSI system enhanced date palm production and fruit quality by increasing water productivity that significantly reduced the volume of water applied. The estimated amount of water was 4578 m³ ha⁻¹ when the SSI system was used compared to the 11448 m³ ha⁻¹ under the SBI system for 200 palm ha⁻¹. In addition, production costs under the SSI system may be lowered through the reduction of certain cultural practices such as weeding and pest management. Through the experimental period, the SSI units functioned effectively and no constraints were observed. However, sandy soils with high infiltration may need six SSI units around each date palm tree to further improve the water distribution. Based on these inputs, the SSI system could be highly recommended for use in date palm production in arid and semi-arid regions for its high efficiency in water management under these conditions.