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Impact of a Novel Water-Saving Subsurface IrrigationSystem on Water Productivity, PhotosyntheticCharacteristics, Yield, and Fruit Quality of Date Palmunder Arid Conditions
Maged Elsayed Ahmed Mohammed 1,2,* , Mohammed Refdan Alhajhoj 1,Hassan Muzzamil Ali-Dinar 1 and Muhammad Munir 1
1 Date Palm Research Center of Excellence, King Faisal University, Al-Ahsa 31982, Saudi Arabia;[email protected] (M.R.A.); [email protected] (H.M.A.-D.); [email protected] (M.M.)
2 Agricultural Engineering Department, Faculty of Agriculture, Menoufia University,Shebin El Koum 32514, Egypt
* Correspondence: [email protected]
Received: 9 July 2020; Accepted: 17 August 2020; Published: 27 August 2020�����������������
Abstract: Water scarcity is a major constraint in arid and semi-arid regions. Crops that requireless 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. Takingthese issues into consideration, the study was conducted to investigate the impact of limited irrigationwater using a new subsurface irrigation system (SSI) on gas exchange, chlorophyll content, water useefficiency, 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 dripirrigation (SDI) and surface bubbler irrigation (SBI). The field experiment was carried out during2018 and 2019 at the Date Palm Research Center of Excellence, King Faisal University, Kingdomof Saudi Arabia. The annual crop evapotranspiration (ETc) was 2544 mm. The applied irrigationwater was set at 50%, 75%, and 125% of ETc for SSI, SDI, and SBI, respectively, which were basedon the higher crop water productivity recorded in an initial field study. The total annual volume ofwater applied for SSI, SDI, and SBI was 22.89, 34.34, and 57.24 m3 palm−1, respectively. The cropwater 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 useefficiency (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) andSDI (56.9 SPAD). Similarly, net photosynthesis and the transpiration rate were significantly higher inSBI and lowest in SSI. However, the SSI system substantially increased palm yield and enhanced fruitquality. The new SSI system, through its positive impact on the efficiency of irrigation water use andenhancement on fruit yield and fruit quality of date palm, seems quite suitable for the irrigation ofpalm trees in arid and semi-arid regions.
Keywords: date palm; crop water productivity; subsurface irrigation; surface drip irrigation; bubblerirrigation; water use efficiency; photosynthesis; fruit quality
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
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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 technologiesand innovative research [4]. The water productivity should be driving date-water policy, not date palmproduction [5]. Although highest date palm production is achieved when providing full irrigationwater requirements by traditional methods, the same production can be achieved with significantlyless 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 regionsthrough modern irrigation technologies have become key for sustainable crop production [6]. In mostpractical situations for date palm and passion fruit irrigation, the required water is calculatedbased on potential crop evapotranspiration (ETc) using the Penman–Monteith equation of referenceevapotranspiration (ETo), which requires data of standard prevailed weather [7]. To convert the ETo toETc, a crop factor (Kc) is needed, which varies with the development stage of the crop [8]. The datepalm water use can also be predicted from the relation of ETc = 0.95 LI × ETo where LI is the lightintercepted by the canopy. Therefore, reductions in irrigation water may be achieved by intensiveleaf pruning to reduce the light intercepted fraction [9,10]. Kassem [11] used the methods of a soilwater-balance approach and Bowen ratio energy balance to calculate the actual annual water use in dripirrigation 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 theKingdom of Saudi Arabia. The water requirements, based on the proportion of the cultivated area ofdate palm (100 palms ha−1) for each year, ranged from 7298.9 to 9495.2 m3 ha−1. Another study wasconducted in the western region of Saudi Arabia that recommended 7300 m3 ha−1 of irrigation waterfor date palm [13].
The adaptation of plant species to water scarcity includes stress sequences detectable atmorphological, genetic, and physiological multifunctional responses. The main physiological impactof drought stress is the disruption in the photosynthetic system [14]. Water stress also triggerssignificant changes in carbon partitioning at the cellular plant levels and noticeable modifications inthe composition of membrane proteins and lipids in the photosynthetic apparatus [15]. Plants toleratelow tissue water potential through osmotic changes [16] that alter morphological and/or physiologicalproperties by reducing transpiration or increasing absorption. However, water-stressed plants couldinduce stomatal adjustment to water potential and carbon dioxide recycling during photosynthesisas an adaptive physiological mechanism [17,18]. In general, many studies have reached similarconclusions regarding responses of plants to water stress. They mainly identified limitation of carbondioxide supply that affects plants’ metabolic functions and consequently limited the photosyntheticcapacity [18–20]. 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 toconserve water, particularly in sandy soils [21–23]. The fruit yield and quality of 17-year-old datepalm 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 increaseddate palm yield, reduced the need for irrigation water, and enhanced WUE compared to surface dripirrigation [23]. It was also reported that date palm yield was 25–60% higher due to an increase inthe 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% ETc.
In date palm farming, the crop water productivity (CWP) of a subsurface drip irrigation systemwas much higher than a bubbler irrigation system although the SDI method has an additional cost butis economical in the long term [28]. Reduction of irrigation water quantity by 49–53% coupled to datepalm yield increase of 45–49% was noticed using a low flexibility subsurface drip irrigation system ascompared to medium and high flexibility pipes in a subsurface irrigation system [25]. Similarly, deficit
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irrigation at intervals of 100 mm evaporation resulted in the highest bunch weight, yield, and WUEwithout any degradation of fruit qualitative properties of date palm (cv. Mazafati) [27]. Similarly, it isalso reported that deep drip irrigation leads to noticeable enhancement in fruit quality and marketableyield in addition to increased WUE of 1.55 kg m−3 date palm under a deep drip irrigation systemand mulched soil compared to a bubbler system and un-mulched soil [29]. The development of newirrigation 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 functionalroot zone of the palm tree. Hence, it provides a means to save irrigation water by reducing evaporationand infiltration in non-absorbing root zones. The system is characterized by the simplicity of theinstallation around the palm tree. It only needs a hole with a diameter of 20 cm and a depth of 40 cmand four irrigation units around the date palm tree. Therefore, the study was conducted to investigatethe impact of limited irrigation water using this system on various physiological and production palmtree 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 PalmResearch Center of Excellence Research and Training Station, King Faisal University, Al-Ahsa, Kingdomof Saudi Arabia (Latitude: 25.2608◦ N, Longitude: 49.7078◦ E, Altitude: 155 m above sea level). The soilprofile 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 thepermanent wilting point (PWP), bulk density (BD), pH, and the electrical conductivity (EC) were5.4 ± 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) [30]. Table 2 shows the analysis of the irrigation water used in the experiment [30].
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).
SoilDepth
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 3.1525–50 61 24 15 1.61 15.3 5.6 7.8 3.1850–75 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
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).
2.2. Description of Irrigation Systems
Currently, different irrigation systems are available in arid regions of the date palm. These systemsinclude 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 theabsorbing zone of the root system was compared with surface drip irrigation and surface bubblerirrigation systems.
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2.2.1. Subsurface Irrigation (SSI)
The SSI unit was constructed to efficiently deliver the irrigation water directly to the functionalroot zone of the palm tree. The SSI unit (Figure 1), that was designed at the Date Palm Research Centerof Excellence, King Faisal University, was used in the experiment. The SSI unit consisted of a waterflow regulator, two perforated pipes, and gravel between the outer and inner tubes. The diameterof the inner pipe was 20 mm and the length was 330 mm. The inner pipe was perforated with holeshaving a diameter of 3 mm arranged in a spiral shape. The outer tube with a diameter of 100 mmand a length of 300 mm was slotted with a tilt angle of 45◦ with a 2 mm slot width and 40 mm slotlength. The pipe was wrapped with a filtering cloth to prevent the movement of fine soil into thetube. The gravitational forces play an important role in water movement in the soil with steady-statewater flow. The flow rate of the SSI unit was adjusted to 0.045 m3 h−1 by the head of the water flowregulator at a static pressure of 2 m. The SSI system consisted of a water resource, subsurface irrigationunits, electric pump, water tank, delivery pipe, sub-lines, lateral lines, and manifolds. Four subsurfaceirrigation units were buried around the date palm tree within a circle of diameter 1.40 m (Figure 2).
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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).
Figure 1. Schematic diagram of the subsurface irrigation unit. Figure 1. Schematic diagram of the subsurface irrigation unit.
2.2.2. Surface Drip Irrigation (SDI)
In this system, four low-pressure adjustable drippers (0–0.070 m3 h−1) were used to deliverirrigation water to the same spot around the palm tree. The dripper flow rate was adjusted to0.045 m3 h−1 by twisting the dripper head at a pressure of 200 kPa, which was regulated by a pressureregulator (Model: DN20, OEM, Zhejiang, China). The dripper head was installed on a plastic pipearound date palm tree within a circle with a diameter of 1.30 m. The dripper ring was connected to thedistribution 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 wateraround the palm. The bubbler flow rate was adjusted to 0.060 m3 h−1 by twisting the bubbler headat a pressure of 100 kPa. The bubbler head was installed on a plastic wedge and was inserted intothe 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 anddiameter of 7 mm.
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Figure 2. Setup of the subsurface irrigation system in the root zone of the palm tree.
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: 3/4” DN20). 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: SEA LCD-M,
Figure 2. Setup of the subsurface irrigation system in the root zone of the palm tree.
Solenoid valves (24 V dc) controlled by an irrigation timer (7 days, 24 h) were used to control thewater 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 tomanage 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)) wereinserted into the Penman–Monteith equation to estimate evapotranspiration (ETo). These parameterswere 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 adjustthe recorded wind speed data at 2 m above the ground surface, the following equation was used [7].
u = uz4.87
ln(67.87 z− 5.42)(1)
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where u is the wind speed at 2 m above the ground surface (m s−1), uz is the measured wind speed(m s−1), and z is the actual height (m).
2.4. Estimation of Evapotranspiration
The reference evapotranspiration (ETo) is the evaporating power of the atmosphere at a specifictime and location of the year. It does not consider the soil factors and crop characteristics [31]. The ETo
was computed from the site weather data using the computer Program (CROPWAT 7) according to theFAO 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:
ETo =0.408 ∆ (R−G) + γ[900 u/(T + 273)](es − ea)
∆ + γ (1 + 0.34 u)(2)
where ETo 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 thewind speed at the height of 2 m (m s−1), es is the saturation vapor pressure (kPa), ea is the actual vaporpressure (kPa), ∆ is the slope vapor pressure curve (kPa ◦C−1), and γ is the psychrometric constant(kPa ◦C−1).
The crop evapotranspiration (ETc) was calculated using the following equation:
ETc = Kc × ETo (3)
where ETc is the crop evapotranspiration (mm day−1) Kc is the crop factor, and ETo is the referenceevapotranspiration (mm day−1).
The average values of Kc were 0.85 in the summer and 0.98 in the winter, with an average annualvalue of 0.90. The trend and the average value of Kc 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 usewas 50%, 75%, and 125% of ETc for SSI, SDI, and SBI systems, respectively. These values were basedon higher crop water productivity, as mentioned in the preliminary field study. Ten-year-old date palmtrees (cv. Sheshi) with approximately similar size were selected for the experiment. The three irrigationsystems represented the treatments and were replicated three times based on a Randomized CompleteBlock Design (RCBD). The average height of the palm trunk was 1.5 m, with an average diameter of0.60 m. The experimental orchard had a plant density of 200 palms ha−1 where palm-to-palm androw-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 wereapplied 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 wasconducted to choose the most effective and optimal ETc percentage for SSI, SDI, and SBI irrigationsystems according to the significantly higher crop water productivity (CWP) values. The sameirrigation water amount of 50, 75, 100, and 125% of ETc was applied for all three irrigation systems,SSI, SDI, and SBI (Table 3). Based on the results of the initial trial, we selected the optimum irrigationamount that presented higher CWP values of 50%, 75%, and 125% of ETc for SSI, SDI, and SBI systems,respectively. These parameters were selected to identify irrigation systems that conserve water andproduce reasonable crop yields in arid regions where scarcity of water is a major concern [30].
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Table 3. Crop water productivity (CWP) of date palm (cv. Sheshi) under different irrigation percentagefrom the crop evapotranspiration (ETc) for subsurface irrigation system (SSI), surface drip irrigation(SDI) and surface bubbler irrigation (SBI).
ParameterIrrigationSystems
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
± 0.03SDI 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 meanswhereas ± values indicate standard deviations.
The amount of irrigation water was expressed per date palm tree, as this would overcome muchof the confusion according to FAO recommendations [5]. The irrigation requirement for the irrigationsystems was calculated based on ETc, target soil area, and an adjusted coefficient as below:
IWR =Kadj ×As × ETc
1000(4)
where IWR is the daily irrigation water requirement (m3), Etc is the crop evapotranspiration (mm day−1),As is the target soil area of each date palm tree, and Kadj is the adjusted coefficient (Kadj = 0.5, 0.75, and1.25 for SSI, SDI, and SBI, respectively).
The target soil area of each date palm tree was equal to 90% of the actual shaded area ofthe palm tree, which was calculated based on the light intercepted by the canopy [32]. The meandiameter of the shaded area was 5 m, as shown in Figure 2. Irrigation timing was determined bya calendar (every day from May–September, every two days in April and October, and every threedays 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 readingsof a digital flow meter (Model: K24, SUNNY, Shandong, China).
2.7. Gas Exchange Measurements
Gas exchange measurements (net photosynthesis and rate of transpiration) were recorded usinga portable photosynthesis system (Model: Li-6400XT LiCor Inc., Lincoln, NE, USA). The Li-6400XTsystem is an open method to measure gas exchange and enables air from one source to enter both theanalysis and reference lines. A leaf with a known area was put in the leaf chamber of Infra-Red GasAnalyzer (IRGA) where the air constantly pass through the leaf chamber to maintain the CO2 at afixed level. The system measures the transpiration and photosynthesis on the basis of the differencesbetween the CO2 and H2O in the airflow within the leaf cuvette (reference cell) in comparison to theair stream flowing out of it (sample cell). The rate of CO2 uptake by the leaf in the IRGA leaf chamberis used to calculate the rate of net photosynthesis, and the rate of water loss is used to measure therate of transpiration [33]. Net photosynthesis (A) and the transpiration rate (T) were evaluated atseven-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 theleaflet (pinnae). The measuring chamber enclosed a circular 2 × 3 cm2 leaf area and evaluated the gasfluxes on both sides of the leaf. The leaf chlorophyll content was determined directly at the same timeintervals using a portable chlorophyll meter (Model: SPAD 502, Konica–Minolta, Osaka, Japan).
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2.8. Photosynthetic Water Use Efficiency
The IRGA data (Li-6400XT LiCor Inc., Lincoln, NE, USA) were used to calculate the photosyntheticwater use efficiency (WUE) as below:
WUE =NPTr× 100 (5)
where WUE is the photosynthetic water use efficiency (µmol CO2 mmol−1 H2O), NP is the netphotosynthesis (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 duringthe 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 weightwas measured using a Sartorius electronic balance. Determination of moisture content, pH, and totalsoluble solids was conducted according to AOAC standard methods of analysis [34]. The fruit moisturecontent was determined by drying a sample of 25 g under vacuum at 70 ◦C, then was calculated as thepercentage of the weight loss divided by the initial weight of the sample [35]. Total soluble solids andfruit firmness was determined using a laboratory refractometer (Model: RFM 840, Richmond ScientificLtd. 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 colordifference meter (Hunter Associates Laboratory Inc., Reston, VA, USA) based on the L, a, and b colorsystem. This system is one of the uniform color spaces recommended by the International Commissionon Illumination (CIE) in 1976 as a way of closely representing perceived color [36]. The L value is thelightness factor that gives values ranging from zero for black to 100 for white while the values of aand b are chromaticity coordinates. The value of a indicates the degree of greenness–redness (rangingfrom −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) werecalculated for a random sample of 20 dates according to the following equation:
C =√
a2 + b2 (6)
h = arc tanba
(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:
CWP =Y
Wu(8)
where CWP is crop water productivity (kg m−3), Y is the total marketable date palm yield (kg), and Wuis the annual amount of irrigation water (m3).
2.11. Statistical Analysis
The data of yield, fruit characteristics, chlorophyll content, and gas-exchange were analyzed usingStatistical Analysis Software, Release 9.4 (SAS Institute, Cary, NC, USA). Data regarding differentirrigation systems were analyzed using IBM SPSS version 23 (SPSS Inc., Chicago, IL, USA). Duncan’sMultiple Range Test (DMRT) was applied to determine the least significant difference between allexperimental means at (p < 0.05) probability.
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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 shownin Figure 3. The highest mean relative humidity was 55.67% during December and February, whilethe lowest mean was 27.01% during June and September. The highest mean value of net radiationwas 26.1 MJ m−2 day−1 in May. The data revealed that the highest mean temperature was 36.92 ◦Cduring the summer months from June to September, while the lowest mean was 17.18 ◦C during thewinter months from December to February. The mean of the annual cumulative amount of efficientrain 64.2 mm. Scarce rainfall occurs from December to March. The mean value of the wind speedincreased from February to September and decreased in the remaining period. The highest meanvalue of the wind speed was 3.6 km h−1 in February. The mean value of annual sunshine duration, netradiation, and wind speed were 9.1 h and 20.6 MJ m−2 day, and 1.82 km h−1, respectively.
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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.
.
Figure 3. Mean monthly values of temperature (min. and max temp.), relative humidity, sunshine duration, and solar radiation (Rad) in the experimental area throughout the year (2014–2018).
The mean values of ETo and ETc 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 ETo ranged from 3.52 mm d−1 in February to 12.14 mm d−1 in July, and the ETc rate ranged from 2.99 to 11.77 mm d−1 in the same months. The annual cumulative ETo and ETc were 2755 and 2544 mm, respectively.
Figure 3. Mean monthly values of temperature (min. and max temp.), relative humidity, sunshineduration, and solar radiation (Rad) in the experimental area throughout the year (2014–2018).
The mean values of ETo and ETc at the experimental site are shown in Figure 4. The data revealedthat the daily evaporation rates peaked in the months of June and July. The data were similar to thedata from Al-Amoud et al. [26]; Al-Omran et al. [12]; Kassem [11]. The average daily ETo ranged from3.52 mm d−1 in February to 12.14 mm d−1 in July, and the ETc rate ranged from 2.99 to 11.77 mm d−1 inthe same months. The annual cumulative ETo and ETc were 2755 and 2544 mm, respectively.
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Figure 4. Mean daily evapotranspiration (ETo) and crop evapotranspiration (ETc) rates throughout the year during the study period.
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 ETc. 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].
Figure 4. Mean daily evapotranspiration (ETo) and crop evapotranspiration (ETc) rates throughout theyear during the study period.
3.2. Amount of Applied Irrigation
The amount of applied irrigation in the study site throughout the year was calculated as apercentage of the ETc. FAO recommended that ideally, it is best to express the water requirementper date palm tree rather than per hectare [5]. The results of the study in Figure 5 showed the actualannual water use values of each palm tree as subjected to the irrigation systems of SSI, SDI, andSBI. 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, and451.05 mm day−1 in July, while the lowest values were 46.37, 69.56, and 115.94 mm day−1 in Januaryfor the same irrigation systems (SSI, SDI, and SBI, respectively). The values of annual water use were1271.96, 1907.95, and 3179.92 mm for the irrigation systems of SSI, SDI, and SBI, respectively. Figure 6shows the cumulative water use of date palm tree under the different irrigation systems. The volumeof water applied was calculated using an irrigation area of 18 m2 per palm tree, according to Zaid andArias-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 basisat 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 SDIand SBI systems, the difference in the volume of irrigation water applied was estimated as 11.45 and34.34 m3 palm−1 (0.031 and 0.094 m3 palm−1 day−1); the difference on a hectare basis was estimated tobe 2290 and 6868 m3 ha−1, respectively. The cumulative water use of the date palm tree was within therange reported by Adil et al. [37]; Al-Amoud et al. [26]; FAO [5]; Ismail et al. [13].
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Figure 5. Amount of water used by date palm tree under the different irrigation systems, subsurface irrigation (SSI), surface drip irrigation (SDI) and surface bubbler irrigation (SBI) (2018–2019).
Figure 6. The cumulative amount of irrigation per date palm tree under different irrigation systems (SSI, SDI, and SBI) (2018–2019).
3.3. Chlorophyll and Gas Exchange Measurements
The data in Table 4 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
Figure 5. Amount of water used by date palm tree under the different irrigation systems, subsurfaceirrigation (SSI), surface drip irrigation (SDI) and surface bubbler irrigation (SBI) (2018–2019).
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Figure 5. Amount of water used by date palm tree under the different irrigation systems, subsurface irrigation (SSI), surface drip irrigation (SDI) and surface bubbler irrigation (SBI) (2018–2019).
Figure 6. The cumulative amount of irrigation per date palm tree under different irrigation systems (SSI, SDI, and SBI) (2018–2019).
3.3. Chlorophyll and Gas Exchange Measurements
The data in Table 4 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
Figure 6. The cumulative amount of irrigation per date palm tree under different irrigation systems(SSI, SDI, and SBI) (2018–2019).
3.3. Chlorophyll and Gas Exchange Measurements
The data in Table 4 indicate significant variations regarding the chlorophyll content, netphotosynthesis, stomatal conductance, intercellular CO2 concentration, transpiration rate, and wateruse efficiency (WUE) under the different irrigation systems. The maximum chlorophyll content (62.4SPAD) 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 wherereductions 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 peachtree. Kirnak et al. [38] linked increased electrolyte leakage to a decrease in the chlorophyll content dueto leaf senescence, whereas Premachandra et al. [40] reported that the electrolyte leakage was adverselyaffected by the reduction in water. Our study showed that the chlorophyll content decreased by 8.81%
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and 5.61% in the SDI and SSI regimes, respectively, when compared to SBI. Usually, the chlorophyllcontent is negatively affected if the water quantity is reduced. However, in the present study, SSIinduced less chlorophyll reduction than SDI despite the fact that SDI resulted in the consumptionof relatively higher water quantities. The structure of the SSI regime enables the direct provision ofwater to the functional absorbing area of the root zone compared to SDI where more water mightbe evaporated due to the soil surface application. In both water application regimes, reductions inthe chlorophyll content may be attributed to irregularities at cell wall membranes [38,41]. Underin vitro drought conditions, date palm (cvs. Shamia and Amri) exhibited an increase in the chlorophyllcontent [42]. El Rabey et al. [43] reported a non-significant effect of drought-induced by polyethyleneglycol 6000 in an in vitro environment on chlorophyll a and b binding proteins of three-month-old datepalm (cv. Sagie). The difference in plant age, stress conditions, and stressor types might be the reasonsfor varied responses between our study and their work.
Table 4. Chlorophyll, gas exchange, and water use efficiency of date palm (cv. Sheshi) under differentirrigation systems, subsurface irrigation (SSI), surface drip irrigation (SDI), and surface bubblerirrigation (SBI).
ParametersIrrigation 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 aboveindicate the average of each parameter recorded from 7 March to 4 July during 2018 and 2019. Values representmeans whereas ± values indicate standard deviations.
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 averageswere decreased by 12.79% and 15.97% in SDI and 26.93% and 31.94% in SSI regimes. This ismostly because water was applied generously in the SBI system. It was reported in many cropsthat 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 unitleaf 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 relationshipbetween the photosynthetic rate and intercellular CO2 concentration. They indicated that as waterstress 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 netphotosynthesis and transpiration rate was higher (10.09 µmol CO2 mmol−1 H2O) when the palmtree 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 SDIsystems as compared to SBI. The minimum WUE was calculated under the SBI, which was 5.25% lowerthan that under the SSI. Generally, crop yield increases linearly with increasing water consumptionunder deficit irrigation management, whereas WUE decreases as the water supply or consumptionreaches a certain degree [46]. Lu and Zhuang [47] found that WUE increased with increasing soilmoisture under moderate drought conditions. However, it decreased with increasing soil moistureunder severe drought conditions. In the present study, WUE was higher in the SSI system, where therewas moderate water stress and reduced evaporation. Similarly, Helaly and El-Hosieny [42] reported
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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 datepalm cultivars under in vitro conditions.
3.4. Physicochemical Characteristics of Date Fruit
The data in Table 5 indicate the irrigation water applied through the SSI resulted in significantvalues 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 resultsindicated that the SSI system improved fruit quality parameters. These findings may be due to theefficient use of water within the functional absorbing root zone. Proper utilization of water withinthe tree system likely enhances and improves plant nutrient uptake [49,50]. A reduction in waterquantity uptake normally triggers changes in carbon allocation, which enhances fruit growth andproductivity [51]. Similarly, Intrigliolo and Castel [52] suggested that the plants under lower waterconditions had an enhanced solute concentration and accumulated more sugars that increased totalsoluble solids. Sadik et al. [29] recorded higher total soluble solids when date palm was subjectedto a deep drip irrigation system, which is in line with the data presented in Table 4. A reduction ofwater at different fruit development stages such as flowering, fruit setting, and maturation negativelyaffected the overall productivity and fruit quality of many fruit species [53]. In contrast, both fruityield and quality were significantly improved under the SSI system that provided a reduced amountof irrigation water to the palm tree. The improvement in both parameters was highly probable due tothe efficient use of water by the root system since it was directly provided to the absorbing functionalzone. Alikhani-Koupaei et al. [27] reported that physical characters of date palm fruit of cv. Mazafatiwere improved with a reduction in irrigation water. These results coincide with our present study.
Table 5. Physicochemical properties of date palm fruit (cv. Sheshi) at the Tamr stage under differentirrigation systems during the 2018 and 2019 seasons.
PhysicochemicalCharacteristics
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 meanswhereas ± values indicate standard deviations.
3.5. Date Palm Yield and Crop Water Productivity
Table 6 indicates a significant variation regarding the yield and crop water productivity (CWP) ofthe 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 underSDI and SBI, respectively. Noticeably, the palms irrigated by the SSI system showed a significantincrease of 34.3% in crop yield and 55.6% in CWP compared to the SDI system, though the annual
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volume of water applied by SSI was 33% lower than that of the SDI system. Likewise, the resultsshowed an increase of 17.9% in crop yield with 67.8% in CWP compared to the SBI system, thoughthe annual volume of water applied by SSI was 60% lower than that of the SBI system. The increasein crop yield and CWP could be due to the high efficiency of the novel SSI system compared to theSDI 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 minimizethe runoff of water but also prevents water loss through soil evaporation. Our results suggested thatthe increase in yield and yield-related components could be due to the optimal availability of soilwater in the SSI system that not only enhanced balanced root growth but also improved soil nutrientuptake [49,50]. Application of just a 65% of the total date palm water requirement also enhanced theyield and resulted in the best CWP [13]. Sadik et al. [29] found that the deep drip irrigation systemwas the best regarding date palm yield and WUE parameters. The influence of water availability onplant growth is due to the variations in stomatal conductance, carbon uptake, and turgor pressure ofplant tissues. Therefore, the restricted application of water affects fruit yield and quality, which varywith vegetative and reproductive growth stages, duration and severity of deficit water, and speciesdiversity [54].
Table 6. Yield and Crop water productivity (CWP) of date palm (cv. Sheshi) under different irrigationsystems during the 2018 and 2019 seasons.
ParametersIrrigation 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 meanswhereas ± values indicate standard deviations.
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 efficientutilization 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 palmcultivation in arid regions. It soundly contributes to the reduction of water resource depletion in aridregions while maintaining satisfactory tree growth and production. Our findings demonstrated that thenovel SSI system enhanced date palm production and fruit quality by increasing water productivity thatsignificantly 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 certaincultural practices such as weeding and pest management. Through the experimental period, the SSIunits functioned effectively and no constraints were observed. However, sandy soils with highinfiltration 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 productionin arid and semi-arid regions for its high efficiency in water management under these conditions.
Author Contributions: M.E.A.M. and M.R.A. conceptualized the research project while M.E.A.M. designed,constructed, and installed the irrigation systems. M.E.A.M. and M.M. executed the field experiment, collected andanalyzed data, and wrote the first draft of the manuscript. M.E.A.M., M.M., H.M.A.-D., and M.R.A. reviewed andedited the final manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: The authors gratefully acknowledge the financial support from the DPRC-5-2016 project of the DatePalm Research Center of Excellence, King Faisal University, Kingdom of Saudi Arabia.
Acknowledgments: The authors would like to acknowledge Abdelkader A. Sallam for statistical analysis andtechnical assistance in the fieldwork. We are also grateful to Mobark El-Maoid for his assistance during fieldwork.
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Conflicts of Interest: The authors declare that there is no conflict of interest.
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