Effects of dynamic and static deficit and partial root zone drying irrigation strategies on yield, tuber sizes distribution, and water productivity of two field grown potato cultivars

Agricultural Water Management 134 (2014) 126–136

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Effects of dynamic and static deficit and partial root zone drying irrigation strategies on yield, tuber sizes distribution, and water productivity of two field grown potato cultivars Seyed Hamid Ahmadi a,b,∗ , Mohammad Agharezaee b , Ali Akbar Kamgar-Haghighi b , Ali Reza Sepaskhah b a b

Irrigation Department, Faculty of Agriculture, Fasa University, Fasa, Iran Irrigation Department, Faculty of Agriculture, Shiraz University, Shiraz, Iran

a r t i c l e

i n f o

Article history: Received 19 June 2013 Accepted 25 November 2013 Available online 17 December 2013 Keywords: Static and dynamic deficit irrigation Static and dynamic partial root zone drying irrigation Water productivity Potato cultivars

a b s t r a c t New strategies of partial root-zone drying (PRD) and deficit irrigations (DI) were studied on potatoes in a semi-arid area of Iran. A factorial experiment was conducted as a complete randomized design in three replications. The potato cultivar treatments were Agria and Ramos. The whole growth period of both cultivars was divided into three stages based on the BBCH scale. There were five furrow irrigation treatments. The full irrigation treatment (FI) received 100% of potential evapotranspiration (ET); static deficit irrigation (SDI) received 75% of ET during the growth period; dynamic deficit irrigation (DDI) received 90% of ET in the first one-third of growth period, 75% of ET in the second one-third of growth period, and 50% of ET in the last one-third of growth period; static partial root zone drying irrigation (SPRD) received 75% of ET during the growth period; dynamic partial root zone drying irrigation (DPRD) received 90% of ET in the first one-third of growth period, 75% of ET in the second one-third of growth period, and 50% of ET in the last one-third of growth period. Analysis showed that there were significant differences between irrigation strategies as DI did outperform PRD in tuber production. The SDI, DDI, SPRD, and DPRD irrigation treatments decreased the potato tuber yield by 4%, 7%, 56%, and 52% compared to FI, respectively. SPRD and DPRD decreased potato tuber yield by 54% and 48% compared to SDI and DDI, respectively. Results also showed that there were no significant differences between cultivars. Interaction between irrigation strategies and cultivars was not significant. Furthermore, water productivities (WP) were significant among irrigation strategies. Compared to FI, the SDI and DDI increased WP by 28% and 34%, respectively, but SPRD and DPRD decreased WP by 40% and 31%, respectively. In general, the DI strategy (SDI, DDI) is recommended in the study area due to the slight fresh tuber yield reduction (4%, 7%) and considerable increase (28%, 34%) in WP relative to FI. Furthermore, the dynamic irrigation strategies led to higher WP than the static ones. It was also found that Agria outperformed the Ramos because of higher fresh tuber yield under water-saving irrigation strategies. Under non-limiting water conditions, Ramos produced higher fresh tuber yield. Furthermore, it is required to decrease the duration of wet/dry cycle under PRD strategy to guarantee efficient PRD and cope with extra water stress and hot weather in the region. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Potato is a tuber crop that plays an important role in feeding people of the world. Potato production ranks fourth in the world after wheat, maize and rice with the production of 368 million tons from 19.3 million hectares (FAO, 2012). It is well known that potatoes are very sensitive to soil moisture stress (Lynch et al., 1995; Porter et al., 1999; Onder et al., 2005) due to their sparse

∗ Corresponding author at: Irrigation department, Faculty of Agriculture, Fasa University, Fasa, Iran. E-mail address: [email protected] (S.H. Ahmadi). 0378-3774/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2013.11.015

and shallow root system (Opena and Porter, 1999). Therefore, they need frequent irrigations for good growth and tuber yield production (Ahmadi et al., 2010b). However, due to global restriction in fresh water resources, irrigation strategies that have higher water productivity should be adopted in order to save the water resources and improve crop production. Partial root zone drying (PRD) is a modified form of deficit irrigation (DI) that half of the root system is subject to drying soil and the other half is growing in irrigated soil in each irrigation event. The root parts that grow in drying soil produce Abscisic acid (ABA) and that is carried by water flow in xylem to the shoot for regularizing the shoot physiology (Ahmadi et al., 2010a). The PRD irrigation is an improvement of deficit irrigation that its principal

S.H. Ahmadi et al. / Agricultural Water Management 134 (2014) 126–136

impact on plant are reduction in stomatal conductance, reduction on plant growth, and improvement in water use efficiency (Davies et al., 2002). The concept of PRD was first applied by Grimes et al. (1968) in the USA after then the PRD irrigation has been tested for several field crops and fruit trees across the globe such as bean (Sepaskhah et al., 1976; Samadi and Sepaskhah, 1984), sugar beet (Sepaskhah and Kamgar-Haghighi, 1997), grapes (Loveys et al., 2000; Kriedman and Goodwin, 2003), maize (Kang and Zhang, 2004); green bean (Gencoglan et al., 2006); apple (Leib et al., 2006), peach (Gong et al., 2005), potato (Shahnazari et al., 2007; Shayannejad, 2009; Ahmadi et al., 2010b), and tomato (Zegbe et al., 2004; Wang et al., 2013). Among these diverse studies, several researches did some works on potatoes. Ahmadi et al. (2010b) showed that PRD and DI have not significant effects on fresh yield and water productivity of field grown potatoes compared to FI. Liu et al. (2006a) studied the effects of PRD on physiological responses of potato in greenhouse and field conditions, and indicated that in greenhouse conditions, water use efficiency increased in PRD relative to FI, while PRD rduces irrigation water amount by 30% that led to increased water use efficiency by 60%, without no significant reduction in tuber yield. Liu et al. (2006b) studied the effects of FI, PRD (50% of ET), and DI (50% of ET) on water use efficiency (WUE) and yield of potato at tuber initiation stages. They showed that potato tuber yield reduced significantly under DI and PRD relative to FI that was in contrast with their previous study (Liu et al., 2006a) where tuber yield was similar for the FI and PRD and compared with the FI, PRD treatment saved 30% water and increased WUE by 59%. They also indicated that PRD and DI used 37% less water than FI, though water use efficiency was similar for FI and PRD and significantly decreased in DI. However, there are other studies that have indicated improved WUE or WP under PRD and DI compared to FI for potatoes with significant or nonsignificant tuber yield loss compared to FI (Shahnazari et al., 2007; Saeed et al., 2008; Jovanovic et al., 2010; Ahmadi et al., 2010b). In general, most of the previous studies on potatoes have been conducted as a static deficit and partial root zone drying irrigation where a constant level of water stress was imposed throughout the growing season. However, to the best of our knowledge, no field study has yet been undertaken to widely investigate the impact of dynamic deficit irrigation (DDI) and dynamic partial root zone drying irrigation (DPRD) on agronomic components of potatoes. The objectives of this study were to determine water productivity, tuber yield, harvest index, and tuber size distribution of two common potato cultivars subject to different water-saving irrigation managements in a semi-arid area. This was speculated if the dynamic water-saving irrigation strategies could be superior or as agronomically productive as the static water-saving irrigation strategies.

2. Materials and methods 2.1. Site and climate conditions The field experiment was carried out in summer 2012 at the experimental fields of Faculty of Agriculture, Shiraz University,   Iran located 16 km north of Shiraz (29◦ , 36 N, 52◦ , 32 E, 1810 m MSL). The climate is warm with an annual average rainfall of about 386 mm. The mean air temperature during the growing period was 23.45 ◦ C. Weather data was collected at a nearby climate station of about 20 m far from the field. Fig. 1 illustrates the seasonal variation of air temperature, relative humidity, air vapor pressure deficit (VPD), and reference evapotranspiration (ETo ) during the growing season. The ETo varied between 3.2 and 10.2 mm day−1 . Mean temperature varied between 12.2 and 29 ◦ C during the growing season. It is seen that in most days the maximum temperature was above 30 ◦ C (Fig. 1A).

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2.2. Soil physical properties The soil texture is silty clay loam. The top soil (0–0.3 m depth) contains 31.2% clay, 48.8% silt, 29.9% sand and 1.44% organic matter. Field capacity (FC), permanent wilting point (PWP) and bulk density of the soil at depths of 0–0.3 m were 0.32 and 0.17 m3 m−3 and 1290 kg m−3 , respectively. These values for the soil depths of 30–60, 60–90, and 90–120 are shown in Table 1.

2.3. Potatoes plantation, experimental design, and irrigation treatments Potato seed tubers (Solanum tuberosum L.) were planted on 19 April 2012, at 75 cm inter row and 25 cm interplant distances. Seed tubers were ridged with 20 cm soil in prepared furrows. The height of the ridge was 30 cm measured from top of the ridge to the bottom of the furrows. The growth stages of the potatoes according to the BBCH scale are shown in Table 2 (Hack et al., 2001). The experimental area was divided into 30 plots of 5 × 4 m2 (length × width). The plots were bordered by a 1 m wide guard area. The experiment was a factorial complete randomized design with three replications and consisted of two cultivar treatments of potatoes as Agria and Ramos, and five furrow irrigation treatments that started at 35 days after planting, DAP, (1) full irrigation (FI) that received 100% of ET, which received 950 mm water; (2) static deficit irrigation (SDI) that received 75% of ET during the whole growth period and received 712 mm water; (3) dynamic deficit irrigation (DDI) that received 90% of ET in the first one-third of growth period (BBCH code 0–39), 75% of ET in the second one-third of growth period (BBCH code 40–69), and 50% of ET in the last onethird of growth period (BBCH code 70–99), and in total received 681 mm water; (4) static partial root zone drying irrigation (SPRD) that received 75% of ET during the whole growth period which accounted for 712 mm water; (5) dynamic partial root zone drying irrigation (DPRD) that received 90% of ET in the first one-third of growth period (BBCH code 0–39), 75% of ET in the second one-third of growth period (BBCH code 40–69), and 50% of ET in the last onethird of growth period (BBCH code 70–99), which in total received 681 mm water. All plots received totally 30 mm pre-irrigation for complete plant establishment during the first two irrigation events (8 and 22 DAP, respectively). The assumed irrigation application efficiency was considered 100% until the irrigation event at 98 DAP, after which it was considered as 70% until the last irrigation at 133 DAP. It is noteworthy that SDI is indeed the same as the traditionally regulated deficit irrigation strategy where the crops receive a fraction of FI during the whole or a prescribed growing season. The philosophy behind the DPRD and SPRD treatments is also basically based on the internationally accepted terminology that has been used so far (Sepaskhah and Ahmadi, 2010). Irrigation interval was 7-day and the amount of water for each irrigation event was determined based on daily evapotranspiration, and the appropriate Kc values as recommended by Allen et al. (1998). The crop evapotranspiration of the previous 7-day was calculated by the following equation: ETc = Kc (ETo ) in which ETc is the crop water requirement between two irrigation events (mm), Kc is the crop coefficient, ETo is the reference evapotranspiration (mm). The ETo was calculated based on the Penman FAO method (Doorenbos and Pruitt, 1977) that has been recommended as the standard method for the study area (Sepaskhah and Fooladmand, 2004). Fig. 1D demonstrates the daily ETo during the growing season.

Air temperature ( )

128

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50

Tmaximum

40

Taverage

A

Tminimum

Table 2 Phonological development in the potato field experiment according to the BBCH scale (Hack et al., 2001).

30 20 10 1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139 145

0 DAP

B

50

30 20 10 0 DAP

3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00

C

1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139 145

VPD (kPa)

Description

2012(dd mm)

00 10 40 69 39 90 99

Planting Emergence Tuber initiation Beginning of flowering About 90% of plants meet between rows Beginning of leaf yellowing Final harvest

19 April 14 May 25 May 28 June 1 July 5 August 12, 13 September

40

1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139 145

Relative humidity (%)

60

Code

DAP

center of each plot for determining and measuring the total tuber fresh weight, total tuber number, shoot dry mass, and tuber dry mass. Stem, leaves and sliced tubers were oven dried at 60 ◦ C for 48 h and 85 ◦ C for 24 h to determine shoot dry mass and tuber dry mass, respectively. Harvested potato tubers were graded into four categories according to their sizes as C1 for those below 40 mm; C2 for those between 40 and 50 mm; C3 for 50–60 mm and C4 for tuber sizes above 60 mm. The tuber size categories C2 and C3 were considered marketable tubers (Shahnazari et al., 2007). Water productivity (WP) of each irrigation treatment was calculated as dividing the total tuber fresh weight (kg) by the total applied water (m3 ) after starting the irrigation treatments at 35 DAP. 2.6. Data analysis

ETo (mm day-1)

12.00

D

10.00 8.00 6.00 4.00 2.00

1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139 145

0.00

Data was subjected to analysis of variance (ANOVA) based on PROC GLM (SAS software 9.0). Duncan’s multiple range tests at p = 0.05 probability level was applied to compare the means of different treatments. Data transformation was done when needed for ANOVA (Grafen and Hails, 2002). 3. Results and discussion

DAP

Fig. 1. Daily variations of (A) mean, minimum, and maximum air temperature, (B) air relative humidity, (C) vapor pressure deficit, and (D) reference evapotranspiration.

2.4. Fertilizers Crop growth was divided into three growth stages according to the BBCH scale (Hack et al., 2001) and the fertilizers were applied at the start of each stage. First fertilizer was applied just before plantation at the rate of 100 kg ha−1 of Di-ammonium Phosphate (18–20–0 NPK). Later 300 kg ha−1 of Urea (46–0–0 NPK) was applied at the second one-third of growth period (BBCH code of 40–69). At the last one-third of growth period (BBCH code of 70–99) Potassium deficiency was observed and then foliar application at the rate of 90 and 72 g l−1 of NPK (20–20–20) was applied at 108 and 115 DAP, respectively. 2.5. Measurement of yield, tuber size and water productivity At the harvest time on 12 and 13 September 2012, (146 and 147 DAP), the crops were harvested from a 1 × 1 m2 area from the

3.1. Fresh and dry tuber yield Fig. 2A and B illustrate the fresh and dry tuber yield of potato cultivars at the final harvest, respectively. The small letters show the ANOVA between the irrigation treatments and potato cultivars. Table 3 summarizes the results of ANOVA for different measured parameters. Table 3 shows that the fresh and dry tuber yields of potatoes were significantly different between irrigation treatments. Although there were no significant differences between the fresh tuber yield of potatoes in FI (34.73 Mg ha−1 ), SDI (33.38 Mg ha−1 ), and DDI (32.41 Mg ha−1 ), there were significant differences between FI and the PRD treatments (SPRD and DPRD of 15.23, 16.88 Mg ha−1 , respectively). The same results are also found for the dry tuber weights as FI and DI (SDI and DDI) had significantly higher tuber dry weights than PRD (SPRD and DPRD) treatments. It is also clear that the DI (SDI and DDI) strategy has outperformed the PRD through producing more than 2 times fresh tuber yield (Table 3). In this study, the PRD strategy (SPRD and DPRD) decreased the potato yield relative to FI, and DI (SDI and DDI) strategy. This result is, however, different from the results of Shahnazari et al. (2007, 2008); Saeed et al. (2008), and Ahmadi

Table 1 Physical properties of the soil used in this study. Depth (cm)

Texture

Clay (%)

Silt (%)

Sand (%)

Bulk density (kg m−3 )

Field capacity (m3 m−3 )

Wilting point (m3 m−3 )

0–30 30–60 60–90 90–120

sil cl sicl sicl

21.2 27.2 33.2 32.7

48.8 48.8 48.8 54.8

29.9 23.9 17.9 12.4

1290 1460 1540 1570

0.32 0.35 0.35 0.36

0.17 0.198 0.212 0.224

sil: silt loam; cl: clay loam; sicl: silty clay loam.

S.H. Ahmadi et al. / Agricultural Water Management 134 (2014) 126–136

ab

Yield FW, (Mg ha-1)

50

a

ab

A

ab

40

129

Agria Ramos

ab

b

30

c

c

c

20

c

10 0 DI75

FI

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50)

Tuber DW, (Mg ha-1)

25

B

a

20 15

ab

b-d

Agria Ramos

ab

a-c ab

10 c-e

ed

5

e

eb

0 DI75

FI

Shoot DW, (Mg ha-1)

5 4

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50)

C

a ab

a-c

3

a-c

a-c

a-c a-c bc

c

2

Agria Ramos

ab

1 0 DI75

Water producvity, (Mg ha-1)

FI

3.5 3 2.5 2 1.5 1 0.5 0

DI( 90 -75- 50) Irrigaon treatment

cd

D

DI75

Agria Ramos

a-c

bc

ef

FI

PRD( 90- 75- 50)

ab

a c-e

PRD 75

DI( 90 -75- 50) Irrigaon treatment

f

PRD 75

d-f

ef

PRD( 90- 75- 50)

Fig. 2. Measured data of (A) tuber fresh weight, (B) tuber dry weight, (C) shoot dry weight, and (D) water productivity. Different letters show significant differences between cultivars across irrigation treatments at 0.05 probability level. Error bars represent ± SD of the mean.

Table 3 Summary of the analysis of variance on the effect of experimental factors on Tuber FW (Mg ha−1 ), Tuber DW (Mg ha−1 ), Shoot DW (Mg ha−1 ), water productivity WP (kg m−3 ), and Harvest Index (HI). Different letters in a column of each experimental factors show significant differences at 0.05 probability level. Factor Irrigation treatment FI DI75 (SDI) DI90–75–50 (DDI) PRD75 (SPRD) PRD90–75–50 (DPRD) p-value Cultivar Agria Ramos p-value Irrigation × cultivar p-value a

Tuber FW

Tuber DW

Shoot DW

WP

HI

34.73a ± 8.1a 33.38a ± 7.8 32.41a ± 5.1 15.23b ± 4.2 16.88b ± 3.6 <0.001

8.59a ± 3.2 9.94a ± 2.4 11.89a ± 4.9 2.13b ± 1.6 2.55b ± 2.0 <0.001

2.31a ± 0.47 2.19a ± 1.0 2.32a ± 0.61 2.14a ± 0.69 2.15a ± 0.27 0.96

3.66b ± 0.85 4.69a ± 0.89 4.76a ± 0.75 2.14c ± 0.58 2.48c ± 0.53 <0.001

0.77a ± 0.10 0.81a ± 0.11 0.81a ± 0.10 0.43b ± 0.21 0.47b ± 0.21 <0.001

27.42a ± 10.3 25.65a ± 10.7 0.41

6.01a ± 4.5 7.95a ± 5.4 0.10

2.521a ± 0.62 1.921b ± 0.45 0.006

3.68a ± 1.34 3.41a ± 1.34 0.31

0.61a ± 0.21 0.70a ± 0.24 0.07

0.14

0.39

0.15

0.09

0.02

±SD represent of the means for each treatment.

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et al. (2010b) who showed that there was not significant differences between fresh tuber yields of potato under FI and PRD treatments. A successful partial root zone drying irrigation management depends on crops and cultivars, growing stage, evaporative demands, soil texture, and soil water balance (Saeed et al., 2008; Ahmadi et al., 2010b). There are some reasons that might have let to lower yield production in PRD (SPRD and DPRD) than DI (SDI and DDI). Former studies on applying PRD on potatoes have been mainly conducted in climates with relatively high air humidity such as Denmark (Ahmadi et al., 2010a,b; Shahnazari et al., 2007, 2008), England (Saeed et al., 2008), and Serbia (Jovanovic et al., 2010). Ahmadi et al. (2010a) and Sepaskhah and Ahmadi (2010) reported that PRD is generally a successful irrigation strategy in climates where drought-sensitive crops such as potatoes might not be under severe evaporative demands, which holds true for previous studies. In addition, Ahmadi et al. (2010a) reported that potatoes are drought-sensitive crops and generally close their stomata when are subjected to mild and/or severe water stress. Although in this study the root distribution is not investigated, it is expected that differences in root growth of potatoes under the two water-saving irrigation treatments could be another reason. Though potatoes are shallow-rooted crops, a previous study has shown that DI tended to produce deeper roots than PRD (Ahmadi et al., 2011b). It is noteworthy that the soil texture has significant effect on root distribution under different irrigation treatments (Ahmadi et al., 2011b). Therefore it is required to look for root distribution under different irrigation treatments in a relatively heavy soil as in this study. This further investigation will elucidate if root distribution may affect the PRD effectiveness. Moreover, although the soil moisture content was not monitored during the growing season, it is believed that roots were partially wetted/dried in the DPRD and SPRD due to the geometric specifications and dimensions of the ridges and furrows. In addition, the lesser amount of applied water in the PRD treatments (DPRD, SPRD) than FI, implies that water from the wet side might not considerably move or infiltrate to the dry side (Yactayo et al., 2013; Webber et al., 2006; Du et al., 2006). In agreement with the findings of former researches, Posadas et al. (2008) applied the PRD strategy on the furrow-irrigated potatoes. They measured the soil moisture in the two sides of dry/wet furrows and reported that alternate furrow irrigation caused a distinct and different spatial and temporal pattern of soil moisture in the furrows. However, it is important to note that generally for all plants, the highest amounts of roots are concentrated in the ca. 30–40 cm of top soil layer (Kaman et al., 2011), and the differences of soil moisture between PRD and DI is mainly highlighted in this root zone where roots are very active and accumulated. Under water saving irrigation strategies (PRD and DI), the soil moisture content in the deeper soil layers, however, may not vary considerably because in both irrigation strategies, the root density is generally low and similar (Ahmadi et al., 2011b; Kaman et al., 2011), and probably the dense layer of roots in the top soil layer absorb the water before it would be able to infiltrate to deeper layers (Wang et al., 2009). Kaman et al. (2011) reported that while root densities of five furrow-irrigated maize cultivars subject to FI, DI, and PRD were different in the top 30 cm of soil layer, root densities were exactly the same in deeper soil layers down to 60 cm. Therefore, it is an essential criteria for a successful PRD that the wet/dry cycle should be systematically made in the top soil layer where root are highly active in plant water supply. This pattern, however, is fulfilled if the furrows are well constructed. Analysis revealed that the interaction between irrigation treatments and cultivars was not significant. In addition the fresh tuber yields were not statistically different among cultivars, with an overall crop productivity of 26.5 Mg ha−1 (Table 3). Table 3 shows that

Agria has produced 7% more fresh tuber yield than Ramos, however, not significant. It is noticeable that Ramos has produced higher fresh tuber yields under FI and DDI, while Agria has produced higher fresh tuber yields in SDI, SPRD and DPRD (Fig. 2). Our result is inconsistent with the results of Shahnazari et al. (2007, 2008), Saeed et al. (2008), Jovanovic et al. (2010), and Ahmadi et al. (2010b) who reported a similar tuber yield of FI and PRD treatments; and is consistent with the results of Liu et al. (2006b) and Brocic et al. (2009) who found that PRD decreased the fresh tuber yield compared to FI. Such inconsistency in the literature could be raised due to the diversity of potato cultivars, climates, and water supply (Bowen, 2003; Ahmadi et al., 2010b). Indeed, there are a plenty of studies that reported very diverse potato yield productions under similar growing conditions (Tourneux et al., 2003a; Tekalign and Hammes, 2005a,b; Geremew et al., 2007). Fig. 2A demonstrates that Agria has outperformed Ramos in fresh tuber yield production under both PRD treatments (SPRD and DPRD), while Ramos produced higher fresh and dry tuber yields under full irrigation. Moreover, the highest and significantly different fresh tuber yield has been obtained for the Agria under SDI (Fig. 2A). Although these findings disagree with the results of Eskandari et al. (2013) who reported that among the three potato cultivars of Agria, Almera and Sinora, the Agria performed well under FI and not DI, it may imply two points under the circumstances of our study. The first point is that in arid and semi-arid areas where water is not sufficient and potatoes are the dominant crops, choosing the suitable potato cultivar such as Agria not only produces the highest amount of fresh tuber yield but also adopting the deficit irrigation practices as seen in this study may save water compared to the full irrigation practices that are indeed luxury use of water (Kang and Zhang, 2004). Our result is in agreement with Fabeiro et al. (2001) who mentioned that deficit irrigation strategies that impose moderate deficit at the beginning of the season (growth and tuber bulking periods) might lead to tuber production results similar to full irrigation. Similarly, Lahlou et al. (2003) realized that potato cultivars that maintain better tuber growth rate under drought conditions during the first three weeks of tuber bulking could maintain better tuber yields. The second point is that potato cultivars show different agronomic and physiological responses to water stresses (Deblonde and Ledent, 2000; Tourneux et al., 2003a,b) and contrary to the general belief that potatoes are drought-sensitive (Ahmadi et al., 2011a), there are potato cultivars that do better under deficit irrigation practices (Fabeiro et al., 2001; Tourneux et al., 2003a). Table 3 shows that DDI and SDI have produced higher tuber dry weights than FI, though the differences are not significant. The tuber dry weights in the PRD treatments (DPRD and SPRD) are substantially and significantly lower than FI and DI treatments (DDI and SDI) regardless of cultivars (Fig. 2B). Analysis has revealed that the interactions between the irrigation strategies and potato cultivars were not significant. Unlike the fresh tuber yields, Table 3 shows that Ramos cultivar has produced higher tuber dry weight (32%), but not significantly, than Agria cultivar. This finding is in agreement with former studies on the effect of water stress on potato cultivars. Jefferies and Mackerron (1993) reported that water stress increased tuber dry matter in potato cultivars but the effect differed between cultivars. Similarly, Nagarajan and Bansal (1991) reported that among the water-stressed potato cultivars, the tolerant potato cultivar got adjusted to the water stress with more dry matter partitioned to tubers compared to other organs of the crop. In fact, Table 3 reveals that Ramos has lower tuber water content (69%) than Agria (78%) and it might imply that the potato cultivars that adjust to water stress has lower tuber water content than the non-tolerant ones. In addition, Tekalign and Hammes (2005a,b) mentioned that differences between potato cultivars regarding the tuber dry mass allocation could be attributed to

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the length of duration of tuber initiation, rate of photosynthesis, efficiency of assimilate partitioning to the tubers, and maturity period, of which the duration of potato growth period and leaf area duration are most important (Geremew et al., 2007). It is noteworthy that the moisture content of tubers was different among irrigation treatments (Table 3). Although PRD (DPRD, SPRD) treatments produced lowest amount of fresh tuber yields, they had the highest tuber moisture content (ca. 85%) compared with the DI (DDI, SDI) with tuber moisture contents of ca. 63–70%. Higher tuber moisture content in the PRD (DPRD, SPRD) may be due to this fact that basically the wet side of root system in PRD strategy receives more water than DI in each irrigation event and roots on the irrigated side of PRD absorb more water to maintain higher crop water balance (Sepaskhah and Ahmadi, 2010; Liu et al., 2006a). Ramos has produced higher tuber dry yields in all of the irrigation strategies except the SPRD where Agria has out-yielded Ramos (Fig. 2B). In addition, it is obvious that Ramos cultivar has significantly produced the highest amount of tuber dry weight under DDI treatment. This finding may imply that under unreliable water resources during summer, the combination of Ramos and DDI irrigation strategy might be the suitable option for producing the highest amount of tuber dry weight. In other words, this cultivar has high efficiency in partitioning the assimilate into tuber dry matter under variable rates of deficit irrigation (DDI). In addition, it may imply that the DDI is potentially a successful irrigation strategy such that the potatoes take the benefits of highest amount of water early in the growing season and during the vegetative growth. Fabeiro et al. (2001) and Sepaskhah and Ahmadi (2010) have also emphasized on the principal role of applied water during the vegetative growth period for the drought-sensitive crops such as maize and potatoes. 3.2. Shoot dry weight Shoot dry weight of potatoes under FI, SDI, DDI, SPRD, and DPRD are 2.31, 2.19, 2.32, 2.14, and 2.15 Mg ha−1 , respectively. Table 3 shows that the shoot dry weight of potatoes were not significantly different between irrigation treatments, though FI and DDI have identically produced the highest amount of shoot dry weight compared to the other irrigation treatments. However, it is observed that shoot dry weight was significantly different between the potato cultivars and Agria has produced ca. 32% more shoot dry weight than Ramos. Comparing the shoot dry weight and tuber dry weight of potato cultivars, it might be concluded that the photosynthetic assimilates in Agria were mainly partitioned to the shoot organ while Ramos has higher efficiency to translocate the assimilates in the tuber organ. Such diversity in assimilate allocation pattern has been reported for other potato cultivars such that drought-sensitive potato cultivars convert the assimilates to shoot, and the drought-tolerant potato cultivars allocate the assimilate to tubers (Nagarajan and Bansal, 1991; Geremew et al., 2007). Higher tuber dry weight under DDI in Ramos (Fig. 2B) implies that under dynamic and variable deficit irrigation the roots might have extended proliferation and distribution in the soil, and therefore increase in root length density at soil profile may enable more water to be exploited (Bingham, 2001; Ahmadi et al., 2011b). It should be noted that the highest and significantly different shoot dry weight has been observed for Agria under SDI (Fig. 3C), which is in agreement with the same observation for fresh tuber yield (Fig. 3A). The interaction between irrigation strategies and potato cultivars was not significant, although the shoot dry weight of Agria was generally higher than Ramos in all irrigation treatments, except in the DPRD that was a bit lower. Jefferies and Mackerron (1993) reported that although the effects of drought treatments on potato tuber dry matter reflect

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total dry matter production; however, the assimilate allocation which is indicated by the harvest index is affected by both cultivar and irrigation treatment. Statistical analysis showed that harvest index (HI) values defined as tuber dry weight × (tuber dry weight + top dry weight)−1 were not significantly different between FI and DI treatments (DDI, and SDI) while HI values of PRD (SPRD and DPRD) were significantly lower than FI and DI (DDI, and SDI). Furthermore it is observed that Ramos consistently has produced higher HI but not significantly different from Agria, except for the SPRD that Agria has produced higher HI (Fig. 3). It should be highlighted that the only significant difference within each irrigation treatment was seen in the DPRD where Ramos has produced higher HI than Agria. Anyways, the pattern of HI variation among irrigation treatments and potato cultivars is the same as the tuber dry weight (Fig. 2B).

3.3. Tuber sizes distribution Fig. 4 demonstrates the tuber sizes distribution under different irrigation treatments and cultivars. Under the C1 class, Ramos has produced more number of potato tubers than Agria for the FI (26%) and DPRD (18%) treatments. On the other hand, Agria produced more tuber numbers than Ramos under SPRD (28%). However, it is observed that both potato cultivars have identically produced the same number of tubers under the DI strategies (DDI, and SDI). It might be concluded that under water-limiting conditions either of the PRD treatments (DPRD, and SPRD) could be a viable strategy for class C1 tuber production such that Agria and SPRD, and Ramos and SPRD can produce similar tuber numbers compared to FI (Fig. 4). For the C2 class (marketable size), both cultivars have much lower tuber numbers than the C1 class. However, under PRD (DPRD, and SPRD) both cultivars have almost similar tuber numbers but Ramos did produce more tuber numbers in DDI (15%), while Agria did better under SDI with 15% more tuber numbers. It is noteworthy that under FI treatment both cultivars had almost similar number of tubers. Indeed, the number of C2 class tubers does not vary considerably among the irrigation treatments and potato cultivars, which may imply that it is possible to apply the watersaving irrigation strategies to obtain simultaneously almost the same number of C2 class tubers and 28% and 25% saving water in dynamic (DDI, DPRD) and static (SDI, SPRD) irrigation strategies, respectively. Regarding the C3 class (marketable size), it is clear that both PRD treatments (DPRD, and SPRD) under-produced tuber numbers compared to FI and DI (DDI, and SDI). This is in contrast to the former study of Shahnazari et al. (2007) that they reported PRD produced higher numbers of marketable sizes than DI. It is observed that Ramos outperformed Agria by 21%, 36%, and 33% more tuber numbers under FI and dynamic irrigation strategies of DDI and DPRD, respectively. On the other hand, Agria produced more tuber numbers than Ramos under the static irrigation strategy of SDI (8%) and SPRD (59%), respectively. Noticeably, Ramos produced 34% more tuber numbers than Agria under FI which means that under nonlimiting conditions Ramos might be recommended due to higher marketable tuber sizes. Under the C4 class, the cultivars have shown considerable differences across the irrigation treatments. Among all irrigation treatments, Agria outperformed Ramos under SDI by producing 39% more tuber numbers. Furthermore, it is seen that the number of tubers in the PRD treatments (DPRD, and SPRD) is much lower than the DI treatments. However, under SPRD and DPRD, Agria produced 29% and 46% more tuber numbers than Ramos, respectively. In total, it is observed that Agria and Ramos have produced more marketable sizes under SDI and DDI strategies, respectively.

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Harvest index

1.00

ab

a

ab

a

ab

a ab

bc

0.80

Agria Ramos

dc

0.60

d 0.40 0.20 0.00 FI

DI75

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50)

Fig. 3. Harvest index of irrigation strategies within cultivar treatments. Different letters show significant differences between cultivars across irrigation treatments at 0.05 probability level. Error bars indicate ± SD of mean.

Tuber number ha-1

500000 Agria Ramos

C1

400000 300000 200000 100000 0 FI

DI75

Tuber number ha-1

200000

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50)

Agria Ramos

C2

150000 100000 50000 0 FI

DI75

Tuber number ha-1

200000

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50) Agria Ramos

C3

150000 100000 50000 0 FI

DI75

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50)

Tuber number ha-1

200000 Agria Ramos

C4 150000 100000 50000 0 FI

DI75

DI( 90 -75- 50) Irrigaon treatment

PRD 75

PRD( 90- 75- 50)

Fig. 4. Tuber sizes distribution for irrigation strategies and cultivar treatments. Error bars indicate ± SD of the mean.

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3.4. Water productivity Compared with FI, the dynamic (DDI, DPRD) and static (SDI, SPRD) irrigation strategies saved 28% and 25% water, respectively. However, water productivities were different such that among the irrigation strategies deficit irrigation treatments (DDI and SDI) had significantly higher WP than the FI and PRD treatments (DPRD and SPRD). The PRD treatments (DPRD and SPRD) had significantly the least WP’s among all irrigation treatments. It should be noted that the interaction between irrigation strategies and cultivars was not significant (Table 3). Both of the PRD (DPRD, SPRD) treatments showed significantly the lowest WP values as they decreased WP by 31% and 41%, respectively, relative to FI. On the other hand, DDI and SDI strategies increased the WP by 34% and 28%, respectively, compared to FI (Table 3). Table 3 shows that Agria has higher WP (1.84 Mg ha−1 m−3 ) than Ramos (1.73 Mg ha−1 m−3 ), though the difference is not significant. Within each irrigation treatment, there was no significant differences between WP’s, but interestingly DDI (2.45 Mg ha−1 m−3 ) and DPRD (1.26 Mg ha−1 m−3 ) had higher WP than SDI (2.34 Mg ha−1 m−3 ) and SPRD (1.08 Mg ha−1 m−3 ), respectively, which implied that the dynamic irrigation strategies were, however, more effective than the static irrigation strategies in improving WP. Such new findings in the context of on-farm agricultural water management shed the light on a wide range of field crops such that DDI and/or DPRD could be tested under different agronomic managements and climates. Consistent with our study, Liu et al. (2006b) reported that DI was more effective than PRD in increasing WP of potatoes, and Wakrim et al. (2005) and Kirda et al. (2005) showed that DI had higher WP than PRD in bean and maize, respectively. In contrast, Shahnazari et al. (2007), Ahmadi et al. (2010b), and Jovanovic et al. (2010) reported that PRD had higher WP than DI for potatoes. A full history of the case studies on application of PRD and DI and their comparison with FI for different growth issues including WP is provided in Sepaskhah and Ahmadi (2010) for various field and horticultural crops. Among all irrigation strategies and cultivars, there was a significant difference between the WP’s of Agria and Ramos under SDI where Agria had significantly higher WP than Ramos (Fig. 2D). This was, however, in agreement with significantly higher fresh tuber yield in Agria (Fig. 2A). Comparing the potato cultivars across the irrigation treatments in Fig. 2D, it is observed that compared to FI, Agria increased WP in SDI and DDI by 56% and 32%, and Ramos increased WP by 2% and 36%, respectively. In contrast, compared to FI Agria and Ramos decreased WP in SPRD and DPRD by 28% and 24%, and 52% and 39%, respectively. Yet, it is observed that DPRD is better than SPRD since the reduction of WP is smaller in DPRD than SPRD. Further analysis across the irrigation strategies revealed that relative to FI treatment, DDI and SDI strategies increased WP by 34% and 28%, respectively, while DPRD and SPRD decreased WP by 31% and 41%, respectively (Table 3). The reduced WP in PRD strategy is mainly due to severe fresh tuber yield loss compared to FI (Table 3). Ahmadi et al. (2010b) reported that under certain soil conditions PRD may increase or decrease WP relative to FI. In their study, they showed that potatoes subject to PRD in loamy sand had decreased WP by 15%, while potatoes grown in sand and sandy loam increased WP by 11% and 28%, respectively. Sadras (2009) did a meta-analysis on WP of diverse horticultural and field crops and realized that in 80% of the experiments the differences of WP’s between PRD and DI varied in the range of ±20%, which is in contrast with our study where SDI and DDI outperformed SPRD and DPRD by 116% and 95%, respectively. He also reported that compared to FI treatment, PRD and DI have increased WP by around 82% and 76%, respectively. Anyway, these reported values are much higher than our findings

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and perhaps these wide gaps could be due to the reasons that he has not included the climate variability between study locations and the growth conditions such as soil texture and soil water balance (Ahmadi et al., 2010b). Therefore, it is likely that the differences between WP values reported in previous studies where DI and PRD have been tested against FI could be due to experimental set up, crop variety, soil textures, climatic demands, soil water balance, and root distribution (De la Hera et al., 2007; Ahmadi et al., 2010b). In the literature the WP of potatoes varied between 12 and 25 kg m−3 in temperate climates (Shahnazari et al., 2007; Ahmadi et al., 2010b; Jovanovic et al., 2010), 10–13 kg m−3 in sub-humid climate (Shae et al., 1999), 9–13 kg m−3 in hot and dry environment (Kang and Zhang, 2004; Erdem et al., 2006; Ferreira and Goncalves, 2007), and 10–16 and 3–5 kg m−3 in a warm tropical environment for three potato cultivars that were grown in winter and summer, respectively (Bowen, 2003). In our study the WP values varied between 2.14 and 4.76 kg m−3 (Table 3) that lie well within the reported ranges of 3–5 kg m−3 for summer-grown potatoes. Several studies in Iran have also indicated that WP of potatoes had a mean value of 3.76 kg m−3 with minimum and maximum values of 1.92–5.28 kg m−3 (Rashidi and Gholami, 2008) that imply low WP might be due to high amount of water lost in potato cultivation in the semi-arid climate. However, Molden et al. (2003) mentioned that WP varies in temporal and spatial scales and is not certainly a characteristic of the crop variety but it is also affected by climate (Bowen, 2003). It is noteworthy that compared to temperate and humid climates a high proportion of applied irrigation water in hot and dry climates is lost through soil evaporation, which results in poor performance of crop and water productivity (Ferreira and Goncalves, 2007; Ahmadi et al., 2010b). Accordingly, low WP in this study is in agreement with the recent findings in the study area where the total soil evaporation from the soil surface constituted about 30% of the total evapotranspiration of wheat and maize (Shahrokhnia and Sepaskhah, 2013). Therefore, the WP in each environment only shows the potential of that climate for achieving a certain level of WP (Ahmadi et al., 2010b). Tanner (1981) and Shae et al. (1999) have reported that due to lower air vapor pressure deficit WP is higher in humid climates than dry climates that support our findings (Fig. 1C). 3.5. Is PRD an effective irrigation strategy to improve crop yield and water productivity in semi-arid area? Results showed that either of the PRD treatments was not as successful as the DI treatments compared to FI. Both tuber yield and WP were considerably low compared to DI and FI. This indeed showed that PRD might not be a successful water-saving irrigation strategy in this semi-arid region based on the current field management. However, it is against the general idea that keeping yield and improving WP are often the advantages of PRD over FI and DI (e.g. Kriedman and Goodwin, 2003). Nevertheless, the lack of PRD effect might be due to some factors that considering them could help for achieving a successful PRD strategy in this region. Most of the experiments that compared PRD with FI have applied different irrigation strategies such as application only half of the water applied in FI (e.g. Zegbe et al., 2004; De la Hera et al., 2007; Sepaskhah and Ahmadi, 2010), a fraction of the water applied in FI as fixed irrigation intervals (Li et al., 2007, 2010; Campos et al., 2009; Yactayo et al., 2013), or a fraction of the water applied in FI for variable irrigation interval (Sepaskhah and KamgarHaghighi, 1997; Sepaskhah and Khajehabdollahi, 2005; Sepaskhah and Hosseini, 2008). However, all of these studies assumed that PRD was responsible for the observed differences among measured traits, but ignored the possible effect of reduced irrigation and imposed water stress (De la Hera et al., 2007). Therefore,

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while crops subject to PRD with reduced water compared to FI are exposed to an elevated water stress because of both reduced water and dry root in the non-irrigated side, the crops in DI are just exposed to just water stress due to reduced water. This argument might be valid in our study where the PRD treatments have received a fraction of the applied water in FI throughout the growing season that have affected very adversely on potato yields and WP. Moreover, De la Hera et al. (2007) argued that among other factors the duration of wet/dry cycle in field conditions that is a function of soil type, climate, and cultivar influence the intensity of PRD response. This study area lies in a semi arid region where temperature is high and soil evaporation constitutes nearly 30% of total crop evapotranspiration (Shahrokhnia and Sepaskhah, 2013). Therefore, reduced applied water in the weekly wet/dry cycle of PRD that is accompanied by dry side of PRD and hot weather during the growth period might have imposed extra stress on the potato crops. Former PRD studies in this region have also confirmed this argument. Sepaskhah and Khajehabdollahi (2005) reported that while a 7-day irrigation interval of PRD significantly reduced maize grain yield, the 4-day irrigation interval of PRD alleviated the water stress and produced similar grain yield compared to FI with 7-day irrigation interval. In a subsequent study in this region, Sepaskhah and Parand (2006) reported that applying PRD supplemented with one or two FI during the sensitive growth stages of maize at tasseling or silking produced the same amount of maize grain yield as FI although amount of water was reduced by 30% in PRD. So, it could be concluded that PRD could be a successful irrigation strategy in this hot and semi-arid region provided that the duration of wet/dry cycling in PRD is decreased such that the crops are not exposed to extra and severe water stress. Obviously the irrigation interval and duration depends on soil type, climate and cultivar (De la Hera et al., 2007; Ahmadi et al., 2010b; Yactayo et al., 2013). According to De la Hera et al. (2007) and Yactayo et al. (2013) the duration and correct timing of PRD application, and practical aspects in application of PRD to exploit the chemical signaling mechanism are crucial factors for improved PRD. De la Hera et al. (2007) suggested that yield and water use efficiency of grapevines were more improved by correct timing of PRD application rather than its duration. This might be indeed in agreement with our finding that improved efficiency of DPRD compared to SPRD (Table 3) was because of correct timing of PRD and reduced water stress relative to SPRD. This is also worth noting that under severe water stress the dry side of PRD is disable to transfer the ABA to xylem because of reduced transpiration stream in the dry side and this basically cause malfunction of the PRD principles that in fact relies on increased ABA in leaves through transpiration (Liu et al., 2008). 4. Conclusions This study showed that there were significant differences between the fresh and dry yield of potato tubers under different irrigation strategies. Although there were no significant differences between the fresh yield of potato tubers of FI, SDI and DDI, there were significant differences between FI and PRD irrigation strategies. The same results are also found for the tuber dry weight as FI and DI had significantly higher dry tuber yields than PRD treatments. However, the DI strategy out-yielded the PRD by producing twice of fresh tuber yield. To achieve a strong PRD strategy in this hot and semi-arid region it is recommended to decrease the duration of wet/dry cycling in order to cope with extra and severe water stresses occurs in the dry side. Fresh yields of potato tubers were not statistically different among cultivars, although Agria had produced more tubers yields than Ramos cultivar. However, across the irrigation treatments, Ramos produced higher fresh tuber yields under FI and DDI, while Agria produced more tuber fresh yields in SDI, SPRD and DPRD.

Therefore, Agria is the recommended cultivar under water stressed conditions since it produced higher fresh yield of potato under water-saving irrigations. On the other hand, Ramos is recommended under full irrigation because it produced higher fresh tuber yields. Under PRD and DI strategies, Agria and Ramos produced more marketable sizes under SDI and DDI strategies, respectively. Under full irrigation, Ramos produced more marketable tuber sizes. The shoot dry weight of potatoes was not significantly different between irrigation treatments. FI and DDI produced the highest amount of shoot dry weight. Shoot dry weight was significantly different between the potato cultivars and Agria produced almost 32% more shoot dry weight than Ramos. Analysis showed that harvest indices (HI) differences were not significant between FI, DDI, and SDI while HI values of SPRD and DPRD were significantly lower than FI and DI. Furthermore, Ramos produced higher HI but not significantly different from Agria, except for the SPRD that Agria has produced higher HI. In total, the dynamic and static irrigation strategies saved 28% and 25% water, respectively, compared to FI. However, water productivities were different. Among the irrigation strategies deficit irrigation treatments (DDI and SDI) had significantly higher WP than the FI and PRD treatments (DPRD and SPRD). The PRD treatments had significantly the least WP’s among all irrigation treatments. Agria had higher WP than Ramos, even though the differences were not significant. Among the irrigation treatments, the two deficit irrigation strategies (DDI, SDI) have significantly higher WP than FI and PRD irrigation treatments. However, SPRD and DPRD had significantly the lowest WP values as they decreased WP by 31% and 41%, respectively relative to FI. On the other hand, DDI and SDI strategies increased the WP by 34% and 28%, respectively, relative to FI. Overall, DDI and DPRD had higher WP than SDI and SPRD, respectively, which showed that the dynamic irrigation strategies outperformed the static irrigations strategies. Compared to FI, Agria and Ramos increased WP in SDI and DDI by 56% and 32%, and 2% and 36%, respectively, while Agria and Ramos decreased WP in SPRD and DPRD by 28% and 24%, and 52% and 39% respectively. Based on this study, the SDI and DDI strategies are recommended for the study area because of slight reduction in fresh tuber yield (4%, 7%) and considerable increase in water productivity (28%, 34%), respectively. Furthermore, the dynamic irrigation strategies are more effective than the static ones for improving WP.

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