Partial root-zone drying (PRD), its effects and agricultural significance: a review

Water resources are very important to agricultural crop production due to increasing demand for food, feed, and fiber. There is a growing requirement for more use of our natural resources of land, soil, and water. There is ever-increasing pressure on water resources for our extensive use in agricultural production. There needs to be innovative solutions for more efficient irrigation techniques for better development of agricultural irrigation management. This review paper shows the consequences of partial root-zone drying happening on various plant species, its advantages and disadvantages, and also the hormone production under partial root-zone drying. In this technique of irrigation, a wet-dry cycle irrigates the crop, i.e., irrigation is scheduled at a regular interval with half way root drying. This is a water-saving irrigation strategy used in arid and semi-arid environments for increasing irrigation water use efficiency and water productivity as compared to fully irrigated crop plants in area with limited water resources. Scientists have worked a lot with different morphological, physiological, and yield related parameters of horticultural crops with partial root-zone drying but little work with agronomic crops.


Background
Many environmental factors affect the plant growth, yield, and quality, and the most prominent is drought (Sinclair 2005;Tawfik and El-Mouhamady 2019). Scarcity of water mostly affected growth and yield-related parameters of various crops (Raza et al. 2012a;Tawfik and El-Mouhamady 2019). Production losses due to drought are the most relative comparing with any other abiotic or biotic factors (Bakht et al. 2010;Castillo et al. 2007;Khan et al. 2010). All the physiological and yield-related aspects of a crop were severely affected by drought from the very early stage of seedling to harvesting (Chaves and Oliveira 2004). Drought affects the plants at every stage, i.e., internal functions, processes, physical appearance of plants, and production (Jones et al. 2003;Raza et al. 2012a;Gholipoor et al. 2013;Tawfik and El-Mouhamady 2019).
The world population is increasing at an alarming rate, so there may be deficiency of feed, fiber, and food for humans and animals. With this swift increase in population, there are also chances of increasing drought (Wilhite and Smith 2005). Now, it is necessary to choose the most appropriate methods and strategies to compensate for this drought stress. Selecting the most economic and suitable water irrigation for each specific purpose is an outstanding way to manage the drought problem (Nasrullah et al. 2011). Various methods have been used, e.g., use of different nutrients (either micro or macro) (Raza et al. 2012b), numerous solutes levels (Raza et al. 2012c;Sadak et al. 2020), and agronomic practice, like mulching (Kader et al. 2019;Iqbal et al. 2020), to prevail over insufficient water situations (Schahbazian and Iran-Nejad 2006). Although these techniques can minimize the damaging effects of drought but most of these are expensive, time consuming, or require specific machinery which is only afforded by a limited number of farmers, as most of the farmers in arid and semi-arid regions are subsistent. Moreover, water crisis, energy crisis, and a higher rate of inputs are the major hurdles for adopting these anti-drought strategies by farmers. Henceforth, we need to develop a management technique which not only minimizes water use without a major reduction in crop yield but is both economical and easily approachable by the farmers.
Many investigations have been carried out in order to improve irrigation systems, water use efficiency, and productivity. However, many researches in both less and efficient water consumption are at a standstill (Sleper et al. 2007). Some growers are still using flood irrigation even if the water is plentiful or not. This method provides the crop plants with complete evapo-transpiration needs which lead to highest agricultural growth and development, ultimately providing maximum yield. These days, full irrigation system is viewed as an extravagant utilization of water as it has shown insignificant or zero impact on the production of any crop species (Kang and Zhang 2004). Low water consumption irrigation systems are utilized to enhance water productivity (WP). At this time, deficit watering system (DI) and partial root drying watering system (PRD) are the low water consumption strategies that minimizes the water requirements of crops from maximum utilization to very low water needs. Usage of these types of techniques is mostly related to the better growth with no influences on production and ultimately helpful saving extra water (Ahmadi et al. 2010).
Agriculture is the major consumer of available fresh water (Table 1) and it can use a big portion of water than any other user (Huffaker and Hamilton 2007). The world's demand for food is steadily increasing day by day and at the same time, water resources are diminishing. This conflict needs to be resolved. Decidedly, there is a substantial need to increase irrigated areas and crop yield to account for the increasing world population through 2025 (Lascano and Sojka 2007).

Main text
Partial root-zone drying technique Partial root drying (PRD) is the changed type of deficit irrigation system (English et al. 1990). In this method of irrigation, during each irrigation time, we will apply irrigation of only half side of the plant root, and in this way one part of root absorbs water and other remain dry for the next irrigation time. For that reason, the partial root drying technique is an imperative irrigation strategy in that one part of the root is put in dry soil and the remaining part of the root is grown in irrigated soil conditions Rashid et al. 2019;Ahmadi et al. 2010). Originally, the PRD concept was primarily used by Grimes et al. (1968) on an experimental cotton trial in the USA by using alternation in furrow irrigation system. This concept was then followed up by Sepaskhah and Sichani (1976), and Samadi and Sepaskhah (1984) on crops (beans) using two trickle irrigation methods known as surface and subsurface. Later on, a lot of studies PRD were carried out in Australia. After those studies, the term PRD was mostly used for grapevine crops Kriedmann and Goodwin 2003). Schematic diagram of FI, DI, and PRD are shown in Fig. 1 (Davies and Hartung 2004).
There are some factors relating to crops such as evaporative demands, growing stage, soil texture, and soil water balances, which affect plants by wetting and drying each side of the roots (Saeed et al. 2008). Farmers have no definitive answer using PRD which will show exactly how much time it should take for each irrigation of either the side of the crop root. Kriedmann and Goodwin (2003) stated that in PRD irrigation, intervals may be changed from irrigated to dry soil when the dry soil root has zero percent extraction of water from the rhizosphere. Furthermore, Liu et al. (2008) stated that irrigation alteration in PRD must be supported on soil moisture threshold content levels which have the ability to produce the greatest level of abscisic acid (ABA) concentrations of xylem. Roots in drying soils during PRD cause the production of plant hormone ABA which is conveyed due to flowing water in xylem vessels through shoots designed for maintaining shoot functionality (Kang and Zhang 2004). Consequently, by using PRD, plant roots are able to judge the drying soil conditions and stimulate ABA concentrations that causes partial closure of stomatal apertures and reduction in leaf expansion while the roots of the wet portion of soil absorb plenty of soil water to sustain an elevated water condition in the plant shoots (Zegbe et al. 2006;Ahmadi et al. 2010;Liu et al. 2006a;Iqbal et al. 2019 a, b). Different research studies on PRD and DI showed that when we use the same amount of water for these above two irrigation strategies, PRD produced more yield against the DI. This resulted in better fruit quality and higher water productivity Rashid et al. 2019;Iqbal et al. 2019a;Shahnazari et al. 2007;Leib et al. 2006;Sepaskhah and Kamgar-Haghighi 1997;Kang and Zhang 2004;Kang et al. 1998;Kriedmann and Goodwin 2003;Kirda et al. 2004;and Liu et al. 2006a). Nevertheless, Wakrim et al. (2005) concluded that there is a similarity between PRD and DI for water use efficiency, but both the strategies have better water use efficiency as compared to full irrigation method (FI). For pot experiments, schematic representation of alternate wettingdrying cycle (PRD) and full irrigation (FI) to plant roots is shown in Fig. 2a, b. Practical implication of PRD is dependent on many factors such as soil conditions, crop species (cultivars), and condition of the surroundings and the method of irrigation. However, alternating furrow irrigation was considered to be the most beneficial over the other irrigation methods (Grimes et al. 1968). Different crops are grown by using alternate furrow irrigation and PRD which has greater water productivity as compared to alternate furrow irrigation (Musick and Dusek 1982;Kang et al. 2000a;Sepaskhah and Kamgar-Haghighi 1997;Kaman et al. 2006;Sepaskhah and Khajehabdollahi 2005;Samadi and Sepaskhah 1984;Kirda et al. 2005;Sepaskhah and Hosseini 2008;Sepaskhah and Parand 2006;Sepaskhah and Ghasemi 2008). Surface and subsurface drip irrigation methods have shown a greater affect when using PRD on different crop species as in hot pepper crop , beans crop (Sepaskhah and Sichani 1976), apple fruit (Leib et al. 2006), cotton crop (Du et al. 2008a), potato crop (Ahmadi et al. 2010;Shahnazari et al. 2007;Liu et al. 2006a;Shahnazari et al. 2008), tomato crop (Kaman et al. 2006;Kirda et al. 2004), and grapevine crop (De la Hera et al. 2007;Du et al. 2008b). Low energy precise application (LEPA) sprinkler method have been successfully used by Schneider and Howell (1999) to apply PRD on maize, winter wheat, and sorghum. For field experiments of PRD, alternate wetting-drying of furrows is carried out in the field with or without drip irrigation system as described in Fig. 3.

Hydraulic signaling and non-hydraulic signaling using PRD technique
There is an increased production of ABA during the drying phase of roots as in PRD as compared to normal conditions of soil (Iqbal et al. 2019a;Davies and Zhang 1991), and this ABA concentration is moved in upper parts of plants as an anti-stress chemical signal of roots to limit the stomatal conductance and conserve water in plants which would otherwise transpire through stomatal openings (Iqbal et al. 2019b;Stoll et al. 2000;Bauerle et al. 2006;Kang and Zhang 2004;Liu et al. 2006a;Liu et al. 2005b). There was significant leaf expansion rate in maize due to the usage of PRD (Bahrun et al. 2002), wheat crop (Ali et al. 1999), soybean crop (Liu et al. 2005a), potato crop (Liu et al. 2005c), and tomato crop (Topcu et al. 2007). Reduction of leaf growth and development can impede the utilization of more carbon, energy, and a greater portion of plant photosynthates, which is then circulated to the plant root system to enhance more root expansion which may be helpful in the extraction of water from the soil (Taiz and Zeiger 2006). Non-hydraulic (chemical) signaling and hydraulic (water) signaling are two types of signaling which are the consequences of wetting and drying cycling of PRD. When there is a slight water stress occurrence, ABA which is a main non-hydraulic signal, works faster in relation to hydraulic signaling. On the other hand, under harsh conditions of water stress, both hydraulic signaling (HS) and non-hydraulic signaling (NHS) simultaneously regulate the normal physiological functions of plants (Liu et al. 2005b;Liu et al. 2003;Ali et al. 1999). It was found that several crop plants, both HS and NHS, function freely but in some plants they work in conjunction. Equilibrium always exists in HS and NHS using PRD Comstock 2002;Tardieu and Davies 1993).
ABA is a main non-hydraulic signal under the PRD technique. Besides this, other chemical, or non-hydraulic, signals such as concentration of inorganic ions, pH, and other plant hormones (Wilkinson 1999;Stoll et al. 2000). Occurrence of slight water stress reduces the uptake of essential plant nutrients with the booting of pH of xylem sap. High concentration levels of ABA in plant leaves move to the stomata via transpirational stream (Taiz and Zeiger 2006;Dodd 2003;Davies et al. 2002;Iqbal et al. 2019a).The mechanism of ABA dependency reduces leaf elongation due to the higher xylem sap pH (Liu et al. 2003;Iqbal et al. 2019a).
Stomatal conductance and photosynthesis rate using PRD with Co 2 exchange During the wetting and drying cycle of PRD, the wet side absorbs water to sustain the water status of plants and at the same time the dry side causes a partial closing of the stomata which reduces its transpirational losses through producing ABA. However, a small quantity of water is transpired through the stomatal opening and there is absorption of Co 2 which will cause the formation of glucose in the process of photosynthesis. On the other hand, the rate of photosynthesis and stomatal conductance is adversely affected by the changes in stomatal opening and closing. Photosynthetic rate is not affected as much by slight water stress as is leaf expansion (Taiz and Zeiger 2006;Raza et al. 2017). However, stomatal conductance and the photosynthesis rate are severely affected by harsh conditions of water stress on plants.
Intercellular concentration of Co 2 which is the main factor in the formation of organic compounds (photosynthesis) is not affected as much as the rate of transpiration by conductance of stomata at the initial stages of water stresses conditions. Both the photosynthesis rate and stomatal conductance have a very low sensitivity to slight water stress conditions but their water productivity increases with low water availability (Liu et al. 2006a;Liu et al. 2005c;Davies et al. 2002). Hydraulic signaling is related to the water potential of leaves. In very harsh water stress conditions, the mesophyll cells of crop plants lose their water potential in such a way that there is a greater inhibition of their photosynthetic rate (Taiz and Zeiger 2006;Raza et al. 2017).
Partial root-zone drying has many advantages in relation to deficit irrigation. Its main benefit is the irrigated side provides water to the plant and, in this way, the water potential remains at such a level that there is no stress to the crop plants. Secondly, the dry portion causes the production of ABA, which in turn, decreases the conductance of stomata (Iqbal et al. 2019a(Iqbal et al. , 2019bSaeed et al. 2008;Tang et al. 2005;Du et al. 2008b;Costa et al. 2007;Shahnazari et al. 2007) and increases water use efficiency (Davies et al. 2002). Many of the practical studies conducted in different regions show no doubt that stomatal conductance reduces using PRD but the rate of photosynthesis remains the same, as in the case of fully irrigated crop (Raza et al. 2017;Ahmadi 2009;Costa et al. 2007;Ahmadi et al. 2010). Different scientists have conducted many different experiments using diverse crops in PRD such as Du et al. 2008a, Tang et al. 2005, Du et al. 2006, and Iqbal et al. 2019b (cotton crop experiments); Kang et al. 2001 (hot pepper); Kang et al. 2000a, Kang et al. 2000b, and Du et al. 2010 Liu et al. 2006a, and Ahmadi et al. 2010; Campos et al. 2009 andZegbe et al. 2004 (tomato); and Zegbe and Behboudian 2008 conducted experiments on apples and concluded that the photosynthetic rate is not lessened as compared to the crop which is fully irrigated. On the other hand, studies of Liu et al. 2006b (potato) and Kirda et al. 2005 (maize) suggested that by using PRD, there was a significant reduction in the photosynthesis rate. Such disagreements may take place due to the different types of soil found in different regions and that there may be some faults during PRD experimentation (Ahmadi et al. 2010;De la Hera et al. 2007;Costa et al. 2007;Liu et al. 2006a). The two other main factors, selection of variety and agrometeorological conditions, also affect the findings of PRD (Zegbe and Behboudian 2008;Raza et al. 2017).

Rooting system and uptake of water and nutrients in PRD
Growth and development of the rooting system, its circulation, and its movement can vary in different soil profiles depending on the moisture and/or water availability (Wang et al. 2006). Foliage and the shoots of crop plants mostly depend on water and nutrient absorption by the roots in the soil. Root expansion and increased root length density become helpful in the uptake of water in the moist area of PRD through the vigorous growth of roots (Songsri et al. 2008;Benjamin and Nielsen 2006). Previous studies showed that PRD improved the expansion of plant roots and caused the mortification of primary and secondary roots in plants (Kang et al. 2000b), a greater enhancement in root growth and development of crops  and their root biomass (Kang et al. 2000a;Mingo et al. 2004), an improved hydraulic conductivity due to ABA concentrations (Thompson et al. 2007;Taiz and Zeiger 2006;Glinka 1980), and more enhanced absorption of essential plant nutrients (Wang et al. 2009). Dry soil in PRD shows a greater absorption of water due to greater hydraulic conductivity after a low soil water stress condition (Kang and Zhang 2004).
In the wet-dry cycling of PRD, this recompense of hydraulic activity is due to the formation of secondary plant roots and already existing old shoots during wetting phase (Kang and Zhang 2004). Poni et al. (1992) conducted experiments on apple, grape, and peach trees in low water conditions and concluded that hydraulic conductivity of these increased in restricted irrigation. Other studies also proved that uptake of plant nutrients is at its maximum in partial root drying (PRD) than full irrigation (FI) for diverse arable crops Kirda et al. 2005;Li et al. 2007;Wang et al. 2009). The phenomenon behind this is the formation of new roots during PRD resulting in elevated nutrient being absorbed from the soil caused by the supplementary availability of soil water given to the roots (Kang and Zhang 2004).
The irrigated side of PRD used water more effectively as compared to same amount of water in the fully irrigated plant (Rodrigues et al. 2008;Kang et al. 2003;Kang et al. 2000b). This irrigated side can compensate for water requirements of the non-irrigated or dry portion of the root-zone due to hydraulic conductivity of the plant root system in relation to a well-watered crop plant (Liu et al. 2006a). This is because of the enhanced capability of roots in PRD as compared to any other irrigation method. PRD also has an advantage over deficit irrigation (roots have to face much of the time to dry soil conditions) as the plant roots have undergone many physiological and anatomical changes and ultimately there is loss of many of the root cells (North and Nobel 1991).

Mechanism of increasing water productivity in PRD
Most people consider both the terms of water use efficiency (WUE) and water productivity (WP) as the same but there is a difference in both the terminologies, wherein water use efficiency is defined as the crop yield per unit of evapotranspiration; and water productivity is the crop yield per unit of irrigation water applied or the "crop per drop" (Zhang 2003). Dry portion of PRD produces ABA which causes the reduction in stomatal closure or partially opening of stomata and reduction in the formation of foliage in plants; these are the main reasons of controlling the transpirational losses in plants under PRD and enhancing the overall yield ( Table 2) and quality of crops (Davies et al. 2002). Stomatal conductance has a negligible impact on the rate of photosynthesis. It is obvious from the above facts that stomatal conductance has a great influence in controlling the transpiration rate with limited effect on photosynthesis (Morison et al. 2008).
According to Ahmad et al. (2020) 2007), and Fereres and Soriano (2007), water productivity of various crop species mainly increased during PRD application (Table 2). Sadras (2009) concluded in an economic analysis that there is 82% increase in the water productivity using PRD in relation to the fully irrigated plants. On the other hand, some scientists disagree with this in some crop's water productivity. PRD was less efficient than DI in increasing water productivity (Liu et al. 2006b). According to Wakrim et al. (2005) and Kirda et al. (2005), deficit irrigation has more water productivity (Ezzo et al. 2020) in melon as compared to FI. However, fruit quality was greater in PRD as contrasted to the deficit method of irrigation (Shahnazari et al. 2007;Kang and Zhang 2004;Zegbe et al. 2004;Kirda et al. 2004;Leib et al. 2006). De la Hera et al. (2007) and Ahmadi et al. (2010) gave some points to investigate the differences in effectiveness of PRD in relation to DI such as (a) which hormones are involved during the reproductive stages of crops in PRD, (b) rooting system of crops in PRD and water and nutrients uptake system, (c) difference in chemical signaling, and (d) consider the scheduling of PRD as compared to DI and quantity of water applied. Ahmadi (2009) and Dodd (2009) reported significant effects with the application of PRD on different annual crops and tree species with greenhouse and field studies. They reported significant water saving and more economic yield in their studies with PRD. Approximately, 30-50% of irrigation water may be conserved with PRD in different experimental results with no or very low yield reduction. In some research studies, there was even better fruit quality due to partial root drying (PRD) (Du et al. 2008b;Guang-Cheng et al. 2008;Kirda et al. 2004;Leib et al. 2006;Kang and Zhang 2004;Shahnazari et al. 2007;Du et al. 2008a). PRD work is still ongoing from the last decade in reference to different horticultural and agronomical crops (Iqbal et al. 2019a(Iqbal et al. , 2019bAhmad et al. 2020;Rashid et al. 2019), along with the development of some other irrigation techniques and methods (Ahmadi 2009;Morison et al. 2008;Guang-Cheng et al. 2008).

Agricultural advantages of ABA in PRD
In the last few decades, a large number of field studies on PRD are still ongoing and have proved the benefits of chemical signaling of ABA induced in PRD in areas of low water availability. The first who applied the half-way root irrigation techniques were Loveys et al. (2000) where they conducted a field experiment on grapes and concluded its positive effects on the grapevine. There was reduced action foliage growth which avoided tranpirational losses and simultaneously improved the quality of fruit and produced high yield with low water. Some other field experiments of PRD also conducted in Australia on grapevine proved that in addition to the above mentioned benefits of PRD, it can also increase water productivity or water use efficiency of many crops .

Effects of PRD on agronomic, horticultural, and tree species
Agronomic crops Sugarcane and sugar beet Sepaskhah and Kheradnam (1977) conducted a field experiment on sugar beet crops using the alternate furrow irrigation system and concluded that there was only a reduction of 18% in the sugar beet yield with the 10-day interval of alteration of water supplying. The sugar beet consumed 34% less water as compared to the fully irrigated crop plants. So, the sugar beet conserved water using the PRD system, because alternation irrigation method is a PRD technique. De la Hera et al. (2007) concluded that PRD should be scheduled according to the soil conditions (texture and structure), climatic factors, and cultivars grown. As in the case of the sugar beet being a short duration vegetative crop, short intervals of irrigation may be beneficial for yield enhancement and water conservation. Sepaskhah and Kamgar-Haghighi (1997) examined the impacts of each furrow watering system on sugar beet crop for its production and water productivity at various irrigation system interims of (a) 6 days, (b) 10 days, and (c) 14 days. Findings of their research showed that each furrow irrigation system having 10-day irrigation gap consumed very low water quantity for irrigation purpose. In any case, there was a slight reduction in the root biomass. Then again, every alternated furrow irrigation system having 6-day interims lessened 23% irrigation water when contrasted to each furrow watering system having 10-day period. Comparative results were additionally acquired for sugarcane having the inconsistent alternate furrow irrigation, as in PRD, was utilized to decide the impacts of PRD on sugarcane in hot and dry Iranian regions (Shani-Dashtgol et al. 2006).
Outcomes of the study showed, by using the alternated furrow watering system, that there was a 26% reduction of the applied irrigation water as compared to the full watering method producing a 10% yield increase of sugarcane. When there was a comparison of water productivity, PRD increased the WP of about 34% in relation to the fully irrigated system. Equal water productivity was also reported for sugar beet crops by Sepaskhah and Kamgar-Haghighi (1997). In India, a field experiment of sugar cane crop was conducted by Pandias et al. (1992) and reported that 43 to 46% low water utilization in PRD for sugarcane crop which is much higher than suggested by Shani-Dashtgol et al. (2006).

Sorghum
Sepaskhah and Ghasemi (2008) carried out an experiment on sorghum crop in a hot and dry area of Iran with different intervals of irrigation as (a) 10 days, (b) 15 days, and (c) 20 days. There was a slight reduction in yield of about 28% with increased water productivity of the intervals of 15 days. They concluded that the interval of 10 days is most effective because there is no yield reduction but increased water productivity around 11% as compared to fully irrigated sorghum crop in alternating furrow irrigation system.

Wheat
Sepaskhah and Hosseini (2008) investigated the response of wheat to PRD supplementary irrigation system which is cultivated in rain-fed conditions (less than 250 mm rainfall annually) and they concluded that this supplementary irrigation in rain-fed areas is additionally successful in enhancing the production of wheat crop. They used alternated furrow irrigation system in comparison with the conventional furrow irrigation method in a region having an annual rainfall of about 409 mm. Grain yield obtained in alternate irrigation system was the same as in conventional system of irrigation (just about 15% reduction) and the water usage of this alternate system was approximately 41% less in relation to convention irrigation method. Raza et al. (2017) carried out a pot experiment in nethouse to evaluate the impacts of partial rhizosphere drying (PRD) and control (FI) irrigation on five different wheat genotypes. Findings of that research study showed that higher values of growth, water-related parameters, and physiological attributes were obtained under control treatment except total proline, total sugar content, leaf water, and osmotic potential which were sufficiently higher in PRD applied treatments due to more ABA production. All five wheat varieties showed greater enzyme activities in PRD in relation to control treatment. Iqbal et al. (2019a) concluded that PRD is uncommonly magnificent water system technique to conserve the water needed by plants and boosting the leaf water use efficiency (WUE). Higher development, physiological, and yieldrelated parameters of wheat were seen in full water system applied treatment in correlation of PRD and DI. More ABA and osmotic modification was found in PRD-treated plants than other water systems. Leaf WUE was likewise higher in PRD plants in examination of FI and DI. PRD is the most productive water system technique than DI in water-constrained territories world widely. Ahmad et al. (2020) found that wheat yield properties were more in control/normal water system treatment while grain nitrogen, phosphorus, potassium (NPK) substance and water use efficiency were more in PRD treatment. All ground covers uniquely improved the wheat yield properties and quality substance just as proficiently controlled the weeds when contrasted with open (reveal) ground conditions. Joined utilization of PRD with black plastic spread mulch gave best results than different mulches utilized in the trial. (2005) conducted a PRD (alternating furrow irrigation system) experiment on maize crop, which is a very sensitive crop for moisture. Different intervals of irrigations were kept such as (a) 4-day interval, (b) 7-day intervals, and (c) 10-day intervals. With the use of 7-day interval on maize crop, they concluded that about 28% lessened of grain yield occurred with 31% low consumption of applied irrigation water. On the other hand, use of 4-day interval showed that there was no reduction of grain yield but reduced the applied water of about 6% as compared to 7-day irrigation interval. Sepaskhah and Parand (2006) observed that PRD was less effective in hot and dry areas of the world for grain crops. So, they decided to use PRD in a furrow system but at the susceptible phonological stage of maize (silking and tesseling) crop, there would be added an extra irrigation to avoid the reduction in grain yield. They compared the PRD (alternated furrows irrigation system) with the furrow irrigation system giving an extra irrigation at critical growth stages of maize and concluded that the grain production was almost the same (reduction of 11%) but there was an increased WP of about 30%. Kang et al. (2000aKang et al. ( , 2000b used different alternating irrigation water strategies in a hot and dry region of China in an irrigated maize crop. Two factors were used to study the PRD. One was the different furrow irrigation such as (a) alternate furrow irrigation, (b) fixed furrow irrigation, and (c) conventional furrow irrigation. The second factor was the different levels of irrigation. These irrigation levels are as follows: (i) 45 mm, (ii) 30 mm, and (c) 22.5 mm. Moreover, they reported that alternate furrow irrigation (PRD) resulted in higher increase of grain yield of maize crop with consumption of 50% less irrigated water as compared to fixed and conventional irrigation method. Both the fixed and conventional furrow irrigation systems had reduction in grain yield with limited amount of water in relation to alternate furrow method (PRD). Samadi and Sepaskhah (1984) carried out an experiment on dry beans crop in a dry and hot region to check the effectiveness of PRD using alternate furrow irrigation system. Using alternate furrow irrigation technique, there was reduction of 38% grain production of dry beans with a 22% decrease in the irrigation water. They found that in alternate furrow system, there was the most reduction of yield. So, to compensate for this yield gap, they used supplemental irrigation with the alternate furrow irrigation system at a critical growth stage (podding) of dry beans and concluded that the dry beans yield is statistically significant (reduction of about 9% yield) in relation to furrow irrigation system with 29% less water consumption. Wakrim et al. (2005) conducted a research study of beans in pots to evaluate the effects growth and water relation parameters of partial root drying, deficit irrigation, and fully irrigated crop plants. The water potential of lea was higher in fully irrigated crop than the PRD and DI, but the values of leaf water potential showed no main difference by using PRD and DI. Moreover, the biomass of shoot and pod was statistically higher in FI than both the PRD and DI. These outcomes of the research study are similar with the findings of Samadi and Sepaskhah (1984). Gencoglan et al. (2006) investigated the effects of subsurface drip irrigation of both the conventional and alternate system to study water use efficiency and yield of green beans. Both the subsurface conventional (CSDI) and alternated (ASDI) drip irrigation are the parts of PRD (partial root drying). Production of green beans was the same for both the CSDI and ASDI but there was conservation of 50% water in ASDI in relation to CSDI.

Beans
Cotton DuT et al. (2006) conducted a research study on cotton crop to evaluate the impacts of PRD on cotton production and its physiological related parameters. Three different methods of irrigation were carried out named as (a) fixed PRD furrows irrigation system (FFIS), (b) alternated PRD furrows irrigation system (AFIS), and (c) conventional furrows irrigation system (CFIS). Three irrigation levels were used such as (i) 22.5 mm, (ii) 30 mm, and (iii) 45 mm for every cycling of PRD. The findings of the study revealed that AFIS has the maximum yield of seed cotton and water use efficiency and reduction of water losses occurred in PRD using AFIS. Du et al. (2008a) contrasted PRD and full irrigation system (FI) for the cotton crop by use of drip irrigation system with three different levels of irrigation. These levels were considered as (a) 15 mm, (b) 22.5 mm, and (c) 30 mm and comparable results were achieved by DuT et al. (2006). They demonstrated that comparative seed cotton production was acquired with PRD and FI irrigation systems. Further, 31 to 33% reduced irrigation water in PRD system in relation to FI irrigation system. However, comparative results from an analysis utilizing the alternate furrow watering system (PRD) and conventional furrow watering system (FI) were likewise suggested through Tang et al. (2005). They concluded that using PRD technique reduced watering system by 30%; at the same time, its economic yield was lessened by 8% which was not statistically significant in relation to FI. Iqbal et al. (2019b) concluded that a superior development and a higher photosynthesis rate in cotton were seen under full water system (FI) than under PRD water system; be that as it may, the proline and all other sugar substance, and concentrations of antioxidants were fundamentally higher in cotton plants under PRD water system than under full water system. Then again, mulching affected soil moisture (Kader et al. 2019;Iqbal et al. 2020) and henceforth essentially improved the proficiency of PRD water system. Among mulching applications, M 2 performed the best under PRD water system. In this way, joined utilization of PRD water system and M 2 in the field merits further regard for streamline cotton production with less water in arid regions. Liu et al. (2006a) conducted greenhouse as well as a field experiment to check the effects of PRD on growth and development of potato crop. Two treatments were used in both greenhouse and field experiment one being the PRD and second were the FI (fully irrigated). Used; one being the drip irrigated alternate furrow and one being the fully irrigated. In the field experiment, PRD got maximum water use efficiency as compared to FI. PRD-treated plots lessened the water usage of about 30% with no reduction in production of tuber and gained 60% water use efficiency. Liu et al. (2006b) conducted another experiment to study the effects of different irrigation techniques on tuber initiation stage of potato crop. Three irrigation methods were used: PRD, DI, and FI. Outcomes of the research study showed that PRD and DI significantly reduced potato tuber production in relation to the fully irrigated crop and these findings disagree with their earlier results (Liu et al. 2006a). In addition, both the PRD and DI consumed 37% lower amount of water as compared to FI but the water use efficiency was reduced in DI in relation to both of PRD and FI. The water use efficiency was the same for PRD and FI but PRD showed no additional benefit using the same amount of water in regards to water use efficiency than DI. The main reason behind this is the low water application in PRD which resulted in severe water stress. Some other studies also forced the idea that some unidentified factors should be recognized while studying the PRD and DI that effect the water relations of soil and plant Gencoglan et al. 2006;Costa et al. 2007). Shahnazari et al. (2007) carried out a research study in open field designed for a long time to examine the impacts of FI system and PRD technique, with the use of 70% water of FI on potato production, tuber size, and water profitability of potato. Consequence showed that no critical distinction in leaf area index happened. However, biological and economical yield was somewhat lesser in PRD in relation to FI. The profitable tuber production (size of 40 to 50 mm) was 20% elevated in PRD as compared to FI. At last, PRD conserved 30% of irrigation water and brought about 61% increase in irrigation water profitability combined with keeping up tuber yield and more attractive tuber size. Comparable results on potato were additionally reported by Jovanovic et al. (2010). Ahmadi et al. (2010) found that there is a sound relationship between the PRD effects and characteristics of soil profile. Soil has many influences on the water usage ability of crop plants and water retention capacity which differs with the texture of soil from region to region. By the usage of coarse sand, there was an increase of 11% in water productivity and 36% water productivity was achieved with sandy loam soil as compared to fully irrigated crops in above both type of soils. However, water productivity was lessened by 15% by using loamy sand. There is a need that before the conduction of PRD experiments, we should know the soil features, cultivar characteristics, and climatic factors of the area.

Horticultural crops Potato
Tomato Kirda et al. (2004) carried out PRD experiments using greenhouse for tomato crops and estimated that PRD lessened about half of the irrigation water requirement by crop with a slight decline in productivity. They demonstrated that in PRD technique, there was a slight reduction in leaf area index and vegetative growth; in this way, end product of photosynthesis relocated to fruit growth and development. Zegbe et al. (2004) directed a comparable study on processing greenhouse tomato utilizing full irrigation system (FI) and half of FI water saved as PRD. They demonstrated that the fruit and economic production was the equivalent for the different treatments; however, water use efficiency for the plants in PRD were 70% higher than that acquired in FI plots. Zegbe et al. (2006) conducted an experiment on different growth stages of tomato to compare the effects of PRD and FI on the production, water use efficiency and quality of tomato crop. Different growth stages of tomato were used as follows: (a) flowering, (b) fruit setting, and (c) maturity. Water was conserved in different growth stages of tomato as in flowering stage (6%), fruit setting (20%), and at maturity/harvesting (25%) than the FI but there was no significant difference for water productivity at different growth stages of tomato. The fresh weight of tomato fruit was reduced at all growth stages, but this fresh weight was compensated by the tomato fruit quality being enhanced in comparison to the FI tomato. These findings emphasize that reproductive crops, such as tomato, have considerations that should be made to achieve better quality and production when applying PRD according to the site specifications and in the scheduling of PRD. Kang et al. (2001) conducted an experiment of PRD in a drip watering system framework for hot pepper crop using pots as alternate trickle watering system (ADI), fixed trickle watering system (FDI), and even drip watering system (EDI). They demonstrated that ADI brought about no decrease in economical production but there was a greater decline in applied water of irrigation about 40% contrasted with EDI. Guang-Cheng et al. (2008) demonstrated in another greenhouse study that PRD essentially decreased yield about 24%, while water efficiency was enhanced in a result of 52% compared to FI; however, the fruit yield and its quality was improved. Considering all the above factors, PRD improved the yield with 17% in relation to DI.

Tree species Pear
Fixed partial root-zone irrigation system (FPRD) was compared with partial root drying technique and entire root-zone watering system (WRI) in a pear plantation in Australia utilizing a surge watering system framework (Kang et al. 2002). The outcomes demonstrated that yield was not lessened while the applied watering system's water was reduced about 52% and 23% and water use proficiency was improved 28% and 12% in FPRD and PRD, separately, in relation to WRI.

Apple
A research study was directed by small-scale sprinkler to examine the impacts of deficit irrigation and partial root drying on apple yield and its fruit quality for a period of 3 years in America (Leib et al. 2006). Using control irrigation system (CI), water content of the soil was reserved beyond 80% of field limit; during the first 2 years experimentation, this field limit was about 50% and in studies last year, it was about 60% of control for DI and PRD. Findings of research showed no statistically significant distinction in apple production and agricultural fruit size among different comparable treatments for the earliest and final time of research study; in any case, in the second year, just DI demonstrated a fundamental decrease in production than that of control irrigation system. After much study, Zegbe and Behboudian (2008) concluded that PRD could not antagonistically influence the economic production and quality of apples and enhanced water productivity by 120%, with saving of 0.14 mega liters of water in 1 ha.

Olive
The main assessment of olive tree species using PRD was made by Wahbi et al. (2005); they demonstrated that with the application of PRD, it may be possible to keep up the yield and end product quality, despite the fact that diminishing portion of the irrigation system water. They demonstrated that PRD slightly affected the yield lessening (15-20%) in relation to the complete irrigation method and it was accomplished with half of decline in the aggregate sum of applied water, which brought about a water use productivity increment about 60 to 70% under PRD contrasted with the FI. Fernandez et al. (2006) carried out a field experiment on olives to evaluate the main difference on physiology of olives by applying the partial root-zone drying and deficit irrigation system. They concluded that there was no significant difference on different physiological parameters of olives by using PRD in comparison of DI. There was neither effect on growth, development nor on yield-related parameters of mature olive trees. They also suggested that the application of PRD is costly and hard to run and had no significant agronomic benefits of PRD in relation to FI.

Grape
The majority of studies of PRD on wooded crop plants were made on grapes that come into view to react well to this sort of low watering system methodology (Fernandez et al. 2006). There are more research studies on grapes and abundant information on the effective use of PRD but limited studies on grapes crop in regards to Iqbal et al increasing water productivity and fruit quality (Kang and Zhang 2004;Sadras 2009). De la Hera et al. (2007) conducted a complete PRD research study of 3 years on grapes in Spain in hot and dry conditions. Parameters of water relations of leaf, development of vegetative growth, and fruit quality and production were taken. Partial root drying and conventional irrigation methods were utilized for irrigation with about 30 percent of crop total evapotranspiration. During the first 2 years, results showed that there were no statistically significant outcomes of PRD about transpirational and assimilation growth rates. Besides this, the effect of PRD on vegetative growth and development, grape production, and quality of fruit was not observed. Vegetative growth and fruit production improved in the final year of research study by using PRD in relation to CI. PRD achieved higher yield production (43% more) and improved (40% more) water use efficiency over CI. Moreover, wetting-drying cycle should be scheduled in order to achieve elevated results of PRD in water deficit areas.

Advantages and disadvantages of PRD
PRD irrigation has its advantages in regards to water productivity, water use efficiency, fruit quality of most of the crop plants, and nutrient uptake in plants, but it is imperative to evaluate how large of an extent PRD can conserve water in a growing period. The concern for a more economical and efficient use of water has already shown that a positive impact can be made as the majority of PRD treatments use less water (typically 50% less) than control treatments plants. Besides its water saving efficiency, PRD is suggested to also have positive effects on nutrient uptake in plants and best fruit quality with very low, or negligible, losses in crop yield (Dos Santos et al. 2003).

Advantages of PRD Water usage and water use efficiency
Water use efficiency of fully irrigated (FI) treatments is reduced as compared to water use efficiency of PRD Rashid et al. 2019;Ezzo et al. 2020) as suggested in several crop species such as cotton crop, tomato crop, pear tree, grapevine orchard, wheat, and hot pepper crop. In maize crop, PRD irrigation technique reduced water usage by 35% with a biomass decline of about 6-11% in relation to fully irrigated crop plants (Kang and Zhang 2004). In another research study of hot peppers crops using drip irrigation technology, it reported that PRD reduced water consumption in irrigation by 40% and lent the same production as in fully irrigated crop plants . PRD drip technology was used in different areas of the world like China, Yangling and Shaanxi in orchards of peach and apple (Gong et al. 2001), with flood irrigation system in pear orchard in Victoria and Australia (Kang et al. 2002). Conclusions reported water reduction of 52% in peach orchard and 23% in pear orchard correspondingly (Kang and Zhang 2004).

Fruit quality of crops treated PRD
PRD strategy can achieve better quality fruit in many different species such as grapes , cotton crop (Tang et al. 2005;Iqbal et al. 2019b), cantaloupe (Ezzo et al. 2020), wheat Rashid et al. 2019), tomato crop (Kirda et al. 2004), and hot pepper (Dorji et al. 2005) crop in many areas of the world. In grape orchards, the sugar substance of grapes was enhanced with the proper application of the PRD technique Dos Santos et al. 2003). They have concluded that this is mainly an effect of improved management of PRD and vegetative growth with the development of the grapevine species. Furthermore, Dry et al. (2000) established that wine quality was consistently better from PRD vineyards.

Nutrient and water uptake in PRD
An additional advantage from PRD-forced new roots may perhaps be associated to their role in uptake of soil moisture essential plant nutrients/elements. The dryingwetting sequence is a result of PRD forced roots and this can provide the essential elements in soil profile more accessible to the crop plants under PRD (Kang et al. 2001;Dos Santos et al. 2003).

Sugar, proline, and antioxidants (plant defense mechanism)
Antioxidant enzymes like SOD, POD, CAT, and APX act as a defense mechanism to engulf the reactive oxygen species (Sadak et al. 2019;Sadak et al. 2020) which are produced as a result of abiotic stress conditions. Besides this, total sugar and proline contents were also higher under PRD irrigation system than the control irrigation (Abdallah et al. 2019;Iqbal et al. 2019b;Raza et al. 2017). These are powerful weapons for plant defense mechanism against adverse climatic conditions (Abdallah et al. 2019).

Disadvantage of PRD
Production of ABA during PRD results in the partial closure of stomata for the reduction of transpirational losses but at the same time there is reduction in the Co 2 uptake by the plants. Reduced Co 2 causes decrease in biomass production in plants undergoing PRD. In fruit trees, there was no observation recorded of yield reduction but in cereals there is about 10% low yield recorded due to less absorbance of Co 2 (Shahnazari et al. 2007). Proper scheduling is necessary to run PRD technique; otherwise, there may be problem in salinity during the dry phase of PRD if period exceeds than the normal timing of experiment.

Future needs
1. Water requirement of crops vary from climate to climate and taking into account their sensitivity to water (drought sensitive and tolerant crops). So, it is necessary to estimate how much water should be used as PRD, i.e., PRD 100 , PRD 70 , PRD 50 in different crops under different climatic conditions. 2. New roots when exposed to dry periods for a long time may lose their sensibility and contact with soil. It is necessary to know how long these roots can survive under PRD and what effects will be brought on if the wetting and drying cycle is shifted more or less frequently. 3. Some growth stages of crops are critical for water.
So, there is also a need to investigate at which growth stages should PRD be applied or avoided. 4. Crop coefficients under PRD should be evaluated so that they can be applied in irrigation water management. 5. Experiments should be conducted to study the efficiency of PRD under different types of soils. 6. Fertilizer application methods and doses under PRD should be evaluated in order to enhance the efficiency of PRD.

Conclusion
PRD irrigation technique is an imperative strategy of water conservation and, from the last decade, is mostly adopted for horticultural, agronomical, and tree species to increase water productivity in crop plants in water scarce areas. PRD is a more efficient method than the deficit irrigation method and can save agricultural water about 50% without causing reduction in production and improve the fruit quality in comparison of conventional and deficit irrigation technologies of irrigation. Some of the factors which effect the better results of PRD are crop/cultivar grown, environmental factors, and edaphic factors. Some reproductive crops may be very sensitive to limited water as in PRD, so for such crops, one or two extra irrigations should be carried out at critical growth stages. Nowadays, there is a shortage of fresh water for agricultural production of crops. There is no doubt that PRD is a novel irrigation technique but it is recommenced that usage of mulches (Black plastic, wheat straw, and cotton sticks mulch) would be additionally beneficial to PRD (Iqbal et al. 2019b;Ahmad et al. 2020) while still providing a reduction of water losses, an increase in water use efficiency, an improvement in water productivity and better fruit quality.