Impact of super absorbent polymers (hydrogels) on water use parameters of Plum trees under water stress conditions

INTRODUCTION

The Kelsey Plum (Prunus salicina) is one of the deciduous fruit trees which need low chilling requirements (Gao et al., 2001). So, it is cultivated throughout the warmer parts of the world, e.g., in China, America, Europe, and the Kaukasus as well as Egypt. The cultivated area in the world was 2.6 million ha an annual production of 11.8 million tones with an average yield of 4.49 tons/ha (Tareen et al., 2020). China produced 56% of the world production (6,801,187 tonnes) followed by  Romania (842,132 tonnes), Serbia with 430,199 tonnes, and Egypt, with 14,775 tonnes of production per year is ranked at 44 with an average of 13,197 kg/ha (World Plum Production by Country, 2022).

Plum fruit has many advantages including being sweet, juicy, and edible, and it can be eaten fresh or dried or used in making other products or recipes like jams, compotes, mousse, pulp, candied fruit, frozen fruit addition to jelly products (Miloseviae & Miloseviae, 2011). As a result of many nutritional and health properties, eating fruits reduces the risk of chronic diseases and limits the increase in body weight in general (Boeing et al., 2012) and it works prevention and management of osteoporosis (Igwe & Charlton, 2016). Therefore, it is considered one of the most important nutritional means to prevent obesity and problems related to obesity disorders (Siddiqui, 2017). In addition to the above mentioned, plum fruits have many positive effects that improve health, as the fruits contain anti-inflammatory, anti-cancer agents, anti-diabetic, and neuroprotective (Dhalaria et al., 2020). This shows that fruit not only provides better income opportunities for growers but is additionally pivotal to providing more healthy diets for consumers.

In Egypt water resources are limited and more than 95% of all freshwater resources from outside international borders are represented in the Nile River (Amer et al., 2017). In addition to many problems related to it and the possibility of a decrease in the amount of water that flows from it is very large. The scarcity of rainfall and irrigation water is a critical problem in arid and semiarid regions, where water is the determining factor for crop productivity and the cultivated area (Han et al., 2018). In the same context, groundwater is a non-renewable resource, and reliance on it in irrigation operations is costly, especially for the owners of small farms. Soil water conservation techniques are the alternative, to stimulate, collect, store and conserve water for irrigation (Mabhaudhi et al., 2019).

Sandy soils with low productivity as a result of the weak structure, low humus content, and high rate of deep water and nutrient infiltration which affects soil-water plant relationships as well as nutritional status and accordingly plant production (Rajakumar & Jayasree, 2016). Therefore, increasing the total amount of water that soil can hold and available water capacity and thus the amount of water that a plant can absorb from the soil is an important issue to increase the water use efficiency and sustainability. This can be achieved through soil amendment practices (Smith, 2018). Many soil properties addition to moisture conservation the hydrogel can improve such as density, porosity, temperature, water holding capacity, CEC, etc., and biological properties like microbial environment. Agricultural hydrogel polymers are eco-friendly materials since they are naturally degraded over some time, and didn't leave any toxic residue in the soil and plants. Hence use of hydrogel as a soil conditioner will be a productive option for increasing sustainable agricultural productivity in case of soil water stresses (Sri et al., 2019). Agricultural systems that are less able to retain water are prone to reduced crop yield so in arid and semiarid areas soil amendments play a vital role in relieving water stress (Yang et al., 2022).

Hydrogel is a super absorbent polymer (SAP) that can absorb water hundreds of times its dry weight (up to 400-2000 g water g^-1) (Yang et al., 2014 & Guilherme et al., 2015). Depending on its properties there are many uses for hydrogel polymers in the agricultural field to create a suitable growing environment for plants including insoluble water polymers, and soil remediation polymers. The most important hydrogel uses is to absorb water and soluble fertilizer and then release it to plants at the proper time (Abobatta, 2018). Hydrogel is a soil conditioner that can retain water and nutrients in the soil for plants. Polymers are synthetic, water-absorbing monomers of high molecular weight. When soil moisture near the root zone of plants start to dry up hydrogel begins to release water and nutrient to the plants. The soil application of hydrogel led to many avails includes; increasing soil moisture-holding capacity, increase in pore size/number, increase in the stock of nutrients in the soil, and reduction in soil compaction (Nirmala & Guvvali, 2019 & Herawati et al., 2021). It has been found that the use of superabsorbent acrylate polymers (SAPs) in arid regions is a new technology that has been quickly adopted by farmers to reduce soil water loss, and increase crop yield (Li et al., 2014) in many regions (Han et al., 2010, Sharma et al., 2015 & Peng et al., 2016) by maintaining soil moisture content (Chen et al., 2016) thus improves water use efficiency (Liao et al., 2016).

Adding hydrogel amendment to soil increases tree growth and yieldof Khasi mandarin (Pattanaaik et al., 2015a) and Citrus limon (Pattanaaik et al., 2015b) compared to the zero rates (control). Also, Banana growth parameters like the height of pseudostem and circumference, green leaves number, and assimilation area at the bunch shooting stage were significantly increased by applying hydrogel polymers (Kassim et al., 2017). Furthermore, applied polymers under different levels of water stress have an appositive effect on the growth, yield, and water use efficiency of ''Washington Navel'' orange trees (Abo El-Enien & Moursi, 2019). Also, Abdallah (2019) found that the addition of polymers materials to sandy soil led to increasing the amount of available water that the soil can hold and reduced its loss in the depths. This reflected positively on the survival of guava seedlings that grew in sandy soil under water stress conditions compared to those which not added hydrogel polymers.

Thus, the objective of this study was to evaluate the effectiveness of synthetic water retention soil amendments (hydrogels), in separate or combined operations under water deficit irrigation, on yield, and water use parameters of Plum trees.

MATERIALS AND METHODS

1. Experimental plot

This study was carried out during two successive seasons 2019 and 2020, on five years old "Kelsey" plum trees (Prunus salicina L.), budded on Mariana rootstock. The trees were planted in sandy soil in a private orchard located in Salah Al-Abd Village, Bostan Area, West Nubaria, Behaira Governorate, Egypt (Lat. 30.598° and Long. 30.228°). The trees were spaced at 3×3.5 m apart (400 trees/fed.) and irrigated by the drip irrigation system. The fertilization program and other agricultural practices were the same for all trees.

The experimental site is characterized by a semi-arid climate; the weather is hot and dry from May to August. Some climatological data from the experimental site were taken for the   NASA POWER (https://power.larc.nasa.gov/data-access-viewer/) and presented in Table (1).

2. The layout of the experiment

          The experimental trees (105 trees) were selected as very similar in size and shape as possible and arranged in a split-plot design with 5 replicates, in which the irrigation represents the main factor and the water-retaining materials (Hydrogel) represent the subfactor. The selected trees are divided into three groups. Each group is irrigated with a water syringe representing a certain percentage of reference evapotranspiration (100%, as full irrigation), 80 and 60% of the ET₀). The seven  hydrogel treatments (Three rates of  both of AG and BP addition to control) with 5 replicates for each  were distributed  in each of the three irrigation groups.

3. The experimental treatments

3.1. Irrigation practices (main plots)

The irrigation treatments were full irrigation, FI (100% of the ET₀), moderate irrigation treatment, MI (80% of the ET₀), and stress irrigation treatment, SI (60% of the ET₀). The irrigation was controlled via the operating time and using 71 emitters/line (4L/hr). Each row of trees is 25 m in length with two lateral GR lines for each row of the trees with emitters spaced each 0.35 m. The amount of irrigation water was calculated as follows:

The amount of irrigation water = the number of drippers x dripper discharge (4L/hr) x operating time (minutes).

Table (1). Average of some climatic parameters for the experimental site during the growing seasons of Plum trees

3.2. Water retaining materials (subplots) 

The Hydrogel polymers were "Barbary Plant G3", BP (40% Hydro polymer, 6.5% N, 4.8% P, 8.2% K and holding capacity at 300-500%) produced by Lucky Star TG., Egypt, and another one named "Aqua Gool, AG (~ 90% Hydro potassium polymer and hold capacity at 400- 500%) Russian production. The polymers were added once in the last week of January in two trenches around the tree in both seasons. Three levels of each polymer were used as 75, 100, and 125 g/tree in addition to the control treatment (7 treatments).

4. Soil analysis:

         Soil samples (0-30 and 30-60 cm, depth) were collected before the experiment for an analysis of some chemical and physical soil properties. Some physical and chemical properties and moisture content of the experimental site are presented in Tables (2 and 3) according toCarter & ‎Gregorich (2008).‎

Table (2). Some physical and chemical properties of the experimental soil

Table (3). Soil moisture constants for the experimental site

5. Yield:

The producing yield (ton/fed) was expressed by multiplying the weight of fruits/tree (kg) which was attained at the harvest stage by the number of trees/fed.

6. Crop Water-Use Parameters

Systematic determination of several water parameters was carried out to provide basic information for the interpretation of experimental results. The following parameters were determined:

6.1. Reference Evapotranspiration (ET₀)

The reference evapotranspiration (ET₀) was calculated with the FAO Penman-Monteith equation (Allen et al., 1998) according to the climatic data collected from NASA POWER. The equation is expressed as:

Where:

ET₀      Reference evapotranspiration, mm day-1 ;

R_n        Net radiation at the crop surface, MJ m-2 day-1;

G          Soil heat flux density, MJ m-2 day-1 (Generally very small and assumed to be zero for daily calculations);

T         Mean daily air temperature at 2.0 m height, C°;

U₂       Wind speed at 2 m height, m s^-1;

e_s          Saturation vapor pressure at 1.5 to 2.5 m height, kPa;

e_a         Actual vapor pressure at 1.5 to 2.5 m height, kPa;

e_s - e_a   Saturation vapor pressure deficit, KPa;

    Slope vapor pressure-temperature curve, kPaC°^-1; and

        Psychrometric constant, kPaC^°-1

6.2. Crop Evapotranspiration (ETc)

The crop evapotranspiration (ETc) is the daily use of water by trees and is calculated from the following equation (Allen et al., 1998):

Where:

Kc  is the crop coefficient ranging from 0.4 (for the initial stage) to 0.95 (for the full development stage)

6.3. Crop Water Requirements

The crop water requirements were calculated according to the Penman-Monteith equation (Allen et al., 1998) using the following equation (Cuenca, 1989):

Where:

ET_drip  is the crop water requirement under a drip  irrigation system

Kr is the reduction factor that reflects the percent of soil covered by crop canopy, Kr can be calculated by the equation described in Karmeli & Keller(1975):

Where:

GC is the ground cover fraction (plant canopy area divided by soil area occupied by one tree, assumed as 0.7).

6.4. Applied Irrigation Water

The amount of applied irrigation water was calculated according to the following equation (Vermeiren & Jopling, 1984):

Where:

AIW is the depth of applied irrigation water (mm),

Ei   is the irrigation efficiency of the drip irrigation system (assumed as 0.90)

LR is the leaching requirement used for salt leaching in the root zone depth (assumed as 0.15).

Irrigation time was calculated before an irrigation event by collecting the actual emitter discharges according to the equation given by Ismail (2002) as follows:

Where:

 t   is the irrigation time (hr),

A  is the wetted area (m²), and

q   is the emitter discharge (m³/hr).

6.5. Water Consumptive Use (CU)

The plant water consumptive use was calculated by the following formula:

Where: CU is the Plum trees' water consumptive use (mm/day).

6.6. Water Use Efficiency (WUE)

The Irrigation Water Use Efficiency (IWUE) was calculated according to Sharma et al. (2015) as follows:

7. Statistical analysis  

All obtained data of the present study were statistically analyzed according to the design used by the Statistix 10 (2019) computer software program and were tested by analysis of variance. least significant difference at a 0.05 level of probability was used to compare the differences among the means of the various treatment combinations as illustrated by Duncan (1955) and Gomez & Gomez (1984).

RESULTS AND DISCUSSION

1. Plum Yield

The results related to the yield show a significant effect of irrigation level and hydrogel application treatments, in addition to the interaction between them, on the tree yield (kg/tree) and finally, the gross yield (ton/fed). The yield of the tree increased after the application of hydrogels as a result of the increase in the weight and number of fruits for each tree, which led to an increase in the gross yield (ton/fed) compared with the control treatment. Statistical analysis showed a significant increase in yield (ton/fed) as a result of adding amendment materials to the soil at the beginning of each season (Table 4). The maximum increase in yield reached (25.65 and 27.84%) when AG was added at 125g/tree and (22.41 and 24.05) when 125 g/tree were added from BP in the two seasons respectively.

Concerning the level of irrigation, the data showed that the yield decreased significantly when the irrigation water was reduced to 80 and then to 60% of ET₀, which caused the greatest decrease in the yield during the two seasons. The positive interaction between the irrigation level and the hydrogel addition treatments led to a reduction in the yield loss as a result of reducing the irrigation level to 80 and then to 60% level.

In this study, the increase in yield resulted from an increase in the number of fruits per tree and an increase in the weight of fruits (g). The yield increase may be due to, providing water in the root zone area during periods of shortage which ensures an easier transfer of water and nutrients to the plant, and increasing the rate of carbon dioxide absorption, which improves the process of photosynthesis and increases its outputs. As well as the storage of those products, thus improving plant growth dramatically and increasing biomass accumulation which reflects positively on the yield (El-Hardy et al., 2009 & Jamnika et al., 2013).  It can also be attributed to the soil being wet for a long time and increasing the microbial activity that also reduces the drop of fruit on account of water stress (Pattanaaik et al., 2015b). This explains what was mentioned by Shivakumar et al., (2019) they reported that yield is determined by the overlap of several factors, the most important of which are the environmental factors, which include climate factors and soil factors, in addition to the physical and biological interactions that occur within the plant system which affects the process of photosynthesis, the transmission of its products, and the accumulation of those products inside the plant, especially in economic part.

These results were confirmed by Abd El-Badea et al.,2011, who reported that the application of hydrogel soil amendment (VH) affected yield attributes like total yield positively. In addition,  many studies indicate the same effect of hydrogel applications under reduced irrigation water levels on a lot of crops, for instance, Barakat et al., 2015 found that the yield and yield parameters of bananas were increased when watered soil plants that were treated with 150g hydrogel/plant by 80% of IR. Furthermore, the maximum fruits number per tree of ‘Khasi’ mandarin was recorded by applied100 g of stock absorb hydrogel/tree (Pattanaaik et al., 2015a) and the Citrus limon yield increased by 43% compared with the control when the soil was amended with 100g of stock absorb per tree (Pattanaaik et al., 2015b). Additionally, these findings are consistent with the results of  Safavi et al.,2016 on the pumpkin andbanana. Also, Kassim et al., 2017 found that the plant yield (kg) and the yield (ton/fed) increased significantly by adding 1000 g/mat every year and increased more with 1500 g/mat every year.  In addition, Abdel-Aziz et al., 2020 found a significant increase in ‘Murcott’ mandarin trees yield by the addition of hydrogel to the soil compared with that without hydrogel addition. In addition,  increasing the used rate of hydrogel from 250 up to 750 g/tree significantly increased the yield compared to the control. On the same line, Kumar Nika et al., 2020 registered the positive effect of hydrogel application on yield.

Table (4). Effect of some water retaining materials (hydrogel) at different rate  under different irrigation levels on yield of Kelsey plum trees during the 2019 and 2020 seasons

2. Crop water use parameters

2.1. Reference evapotranspiration

The reference evapotranspiration calculated according to the Penman-Montieth equation is illustrated in Tables (5 and 6). The data indicated that reference evapotranspiration was higher in the first season (1040.69 mm) than in the second season (877.79 mm). The difference between both seasons was due to the changes in climatic conditions.

2.2. Crop evapotranspiration

 The calculated crop evapotranspiration of plum trees reached 774.1, 619.3, and 464.4 mm during the 2019 growing season for 100, 80, and 60% of ET0, respectively. The corresponding values for the 2020 growing season are 662.3, 529.8, and 397.4 mm, respectively. The difference between both seasons was due to the climatic conditions (Tables 5 and 6).

The Kc values, used to calculate the crop evapotranspiration during the evaluated stages, were 0.40 to 0.95, according to (Cotrim et al., 2011). The values of Kc are illustrated in Tables (5 and 6) for the growing season of Plum trees.

The irrigation requirement of Plum trees is still not well investigated. A progressive crop coefficient (Kc) ranging from 0.40 to 0.95 was proposed to calculate crop water requirement (De Azevedo et al., 2003). Also, the crop coefficient is still under study because until now crop coefficient of Plum trees has not been defined and has little investigation.

The present study gave proposed values of Kc as described by the following equation:

Where X represents the days in the year.

Table (5). Monthly reference evapotranspiration and crop evapotranspiration during the 2019 growing season of Plum trees

Table (6). Monthly reference evapotranspiration and crop evapotranspiration during the 2020 growing season of Plum trees

2.3. Applied irrigation water

The applied irrigation water of Plum trees was 3251.13, 2600.90, and 1950.68 m³/fed during the 2019 growing season for 100, 80, and 60% of the ET0, respectively. The corresponding values were 2920.23, 2367.28, and 1813.06 m³/fed, respectively, for the 2020 growing season (Table 7).

Table (7). Applied irrigation water to Plum trees during the 2019 and 2020 growing seasons

2.4. Water Use Efficiency (WUE)

Where water is a limiting factor for production, deficit irrigation can enhance WUE, so that the available water is better allocated. Water use efficiency (WUE) was calculated as the harvested yield (kg) per volume of irrigation water (m³) according to FAO recommendations (Doorenbos & Kassam, 1979). WUE is affected by polymer and the amount of applied water.

In the current study, the WUE at 80% and 60% of ET₀ was higher compared to 100% of ET₀ as the control treatment as shown in Table (8), which contains the values of the water use efficiency (IWUE and CWUE). The results indicated that the IWUE was affected by irrigation levels, in which the values decreased with increasing the irrigation level. The medium irrigation level (80% of ET₀) reached the highest value for both growing seasons. Irrigation water use efficiency (IWUE) was increased under irrigation deficit by about 13.52 and 12.90% for 80 and 60% of ET₀ in the 2019 growing season, respectively as compared to 100% of ET₀. The corresponding values for the 2020 growing season were 12.52 and 9.68%, respectively. Also, consumptive water use efficiency (CWUE) was more pronounced at 80% of ET₀ than 100% of ET₀ by 13.53 and 14.20% for the 2019 and 2020 growing seasons, respectively. Furthermore, With deficit irrigation, water productivity was 2.174 and 2.485 kg/m3 for the 2019 and 2020 growing seasons, against 1.915 and 2.179 kg/m3 for full irrigation (100% of ET₀), respectively. On the other side using the water retaining materials increase the water use efficiency (IWUE and CWUE) of Plume trees compared with untreated trees. the results in Table (8) showed that the GA125 and BP125 treatments were found to increase IWUE by about 26.63 and 21.57% over the control treatment in the first growing season respectively while it was 27.78   and 23.32 %, respectively in the second growing season. The treatments in terms of water use efficiency came in descending order as follows GA125, BP125, BP100, GA100, BP75 then GA75, and the last treatment was control. As for consumptive water use efficiency (CWUE), the data inducted that, soil addition with GA or BP hydrogels increased CWUE significantly compared to the control treatment in both seasons. The high CWUE values were achieved in the case of GA125 followed by BP125 in both seasons. The increase reached 26.03 and 27.8 over the control treatment for GA125 in the 2019 and 2020 seasons respectively while it was 21.54 and 23.13  over the control in the case of BP125 in the 2019 and 2020 seasons respectively.

WUE is affected by polymer and the amount of applied water.  This trend is true because the increasing values are due to the high productivity of Plum trees per unit of irrigation water (m³) under irrigation deficit treatment compared with full irrigation.  As shown in this study, deficit irrigation especially offers an alternative to conventional irrigation without any detrimental effect on fruit quality. Considering the large increase in WUE, deficit irrigation has shown to be practical for Plum production. In this regard, the present results support the theory that the deficit irrigation shortage in general, is appropriate to make Plum tree production more sustainable (Razouka et al., 2013).  However, Irrigation of Plum trees is necessary to ensure high fruit yields and favorable fruit quality. The replenishment of the water use by ET_C to 100% as it can be calculated by standard methods is the most solid way to ensure the high productivity of the Plum trees.

 In General, this result agreed with those of Razouka et al. (2013) who reported that water use efficiency (WUE) was improved significantly when deficit irrigation was applied to peach; plum, and almond, in the plum tree, the irrigation with 75% or 50% ET₀ increased WUE significantly by the same amplitude by an average of 41%  compared to 100% ET₀. In the opposite of these results, Kassim et al.(2017) found that the highest WUE value was achieved when banana plants were irrigated with 100%  of recommended irrigation water compared with  80% or 75 % of recommended irrigation water. This difference in the results may be due to the different varieties or plant species.

Increasing the productivity of trees as a result of adding polymers per unit of irrigation water (m³) improves water use efficiency. When added both hydrogel granules led to an increase in the irrigation interval considerably for Plum trees this means increasing WUE, especially with the rate of 125g. The productivity of the Plum can be increased by about 23.5 or 26.5 percent by the application of 125g of BP or AG respectively per tree in comparison to the control according to the results of this study. These results agree with those reported by Salvin et al.,2000; Ibrahim 2003; Ibrahim et al., 2012 and Kassim et al., 2017) on the banana.

Improvement of WUE may be attributed to the available water formed in the root zone, but not the amount of applied water (Kassim et al., 2017) and this is due to the BP and AG as soil conditioners able to retain water and plant nutrients. The hydrogel releases water and nutrient to the plants when the surrounding soil near the root zone of plants starts to dry up. This also increases the nutrient use efficiency of soil treated with polymers. Hence, the application of hydrogel in well-drained, gravelly, and sandy soil was found to be effective in increasing the yield of Plum trees. It also increases the WHC of the soil, which provides a conducive atmosphere for better growth of roots in well-drained and sandy soils and ultimately increases Plum yield.

Table (8). Irrigation water use efficiency and consumptive water use efficiency of Plum trees during the 2019 and 2020 growing season