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Does partial root-zone drying have advantages over regulated deficit irrigation in pear orchard under desert climates?
Scientia Horticulturae 262 (2020) 109099
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Does partial root-zone drying have advantages over regulated deficit irrigation in pear orchard under desert climates?
T
Yang Wua, Zhi Zhaob, Songzhong Liua, Xingfa Huangb, Wei Wangc,* a
Beijing Academy of Forestry and Pomology Sciences, Beijing Engineering Research Center for Deciduous Fruit Trees, Key Laboratory of Biology and Genetic Improvement of Horticultural Crops (North China), Ministry of Agriculture, Beijing 100093, China b College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China c College of Engineering, China Agricultural University, Beijing 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil water status Fruit yield Vegetative growth Leaf physiological parameters
The impacts of partial root-zone drying (PRD) and regulated deficit irrigation (RDI) with the same irrigation volume on vegetative and fruit growth and leaf physiological parameters of pear trees planted under desert climate were evaluated. The experiment involved RDI and PRD treatment, irrigated with the 50 % replacement of pan evaporation (Ep) during the slow fruit growth stage, and 80 % replacement of Ep during rapid fruit enlargement stage. Control trees were irrigated with the 80 % replacement of Ep during the whole growing season. No significant differences in the soil and leaf water status between PRD and RDI were observed, which resulted in the similar vegetative growth, fruit yield and leaf gas exchange between PRD and RDI trees. In conclusion, the leaf gas exchange, the vegetative and fruit growth of pear trees planted under extreme drought condition were mainly controlled by the irrigation volume rather than the irrigation method. Therefore, application of the PRD technique was not recommended in the extremely arid region compared with the RDI strategy.
1. Introduction Regulated deficit irrigation (RDI) strategy is a technique imposing water stress during the specific growing period to inhibit vigorous vegetative growth (Chalmers et al., 1981; Mitchell et al., 1984; Tejero et al., 2011; Blanco et al., 2019). RDI may improve the fruit yield, quality and irrigation water use efficiency (Mitchell et al., 1986; Romero et al., 2004; Moriana et al., 2013; Rosecrance et al., 2015). Partial root-zone drying (PRD) is an irrigation strategy to alternately wet and dry the two different parts of the root zone (Düring et al., 1997; Wakrim et al., 2005; Marsal et al., 2008; de Lima et al., 2015) and this wetting and drying induces a chemical signal (abscisic acid, ABA) generation (Wakrim et al., 2005; Davies et al., 1994), resulting in stomatal closure, inhibition of vegetative growth (Wakrim et al., 2005; Leib et al., 2006; Romero-Conde et al., 2014), and water use efficiency improvement (Mingo et al., 2003; Leib et al., 2006; Du et al., 2008). Theoretically, the PRD strategy works through manipulating the water stress over space, while the RDI strategy regulates the timing of water deficit to control the vegetative and reproductive growth processes. The effects of PRD and RDI strategy with the same irrigation timing and amount on the vegetative growth of fruit trees has been fully
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concerned in recent years (Loveys, 1999; Stoll et al., 2000; dos Santos et al., 2003, 2007; Çolak and Yazar, 2017). Researches on the grapevine (De la Hera et al., 2007; Çolak and Yazar, 2017) and apple (Leib et al., 2006) show that PRD trees produce higher yield than RDI with the same irrigation amount, while there are no significant differences in vegetative growth of grapevines (Gu et al., 2004; Bravdo et al., 2004; Intrigliolo and Castel, 2009), almond trees (Egea et al., 2009) and papaya trees (de Lima et al., 2015) and in yield or fruit growth of olive trees (Wahbi et al., 2005), almond tress (Egea et al., 2009, 2010) and orange trees (Mossad et al., 2018) between RDI and PRD treatment. Most of the abovementioned researches were conducted in the arid or semi-arid regions with 200−600 mm annual precipitation. The comparative studies of the RDI and PRD effectiveness in extreme drought regions are scarce. The drought under desert climate can effectively separate the dry side from wet side of PRD trees, while the water extraction from deep root zone reduces the synthesis of hormones or chemical signals, such as abscisic acid (ABA), in dried region, reducing the PRD effects, thus drying a larger proportion of active fine roots should be considered (De la Hera et al., 2017). In addition, the level of soil water potential in wetted side of PRD trees is also a key factor to influence the fruit growth
Corresponding author at: China Agricultural University, No.17 Qinghua East Road, Haidian, Beijing, 100083, China. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.scienta.2019.109099 Received 7 September 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 Available online 09 December 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved.
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sample trees.
(Romero and Martinez-Cutillas, 2012). In micro-irrigated orchards, the wetted percentage is usually less than 50 %, which means that more than half of the roots zone will not receive irrigation water. These roots may still maintain its activity and produce abscisic acid (ABA), which consequently induces the control of the leaf stomata under RDI treatment. However, under desert climate, the extremely low availability of soil water in non-irrigated zone may prevent the production of ABA in roots or its transportation to the aboveground portion of trees. Researches on the sunflower planted in substrate show that no sap flow are detected until the volumetric water content is higher than 10.8 % (Dodd et al., 2008). Similar results were reported by Pérez-Pérez and Dodd (2015). Therefore, this study aimed to 1) quantify the effects of PRD and RDI on the vegetative and fruit growth of pear trees and the tree leaf physiological response to the two irrigation strategies; 2) evaluate the advantages of PRD over RDI in an extremely arid region.
2.2. Measurements 2.2.1. Water status and meteorological measurements The volumetric soil water content was measured using a portable capacitance probe (Diviner, 2000, Sentek Pty Ltd). Two 1.5 m long access tubes per replication in RDI and control treatments were installed under the driplines opposite the sampling trees, four access tubes per replication in PRD were installed on both sides of two trees (Fig. 1). The soil water content was measured immediately before each irrigation event. The soil water characteristics, measured with a pressure plate extractor (1500F1, Soil moisture Equipment Corp., USA) were used to convert the volumetric soil water content into the soil water potential. Two fresh leaves from two sampling trees per replication were selected randomly for measuring the leaf water potential. The measurement was conducted before dawn by using a pressure chamber (ZLZ-5, Ningbo Jiangnan Instrument Co., Ltd.). An automatic weather station (Vantage Pro2, Davis Instruments Corp., USA) installed near the experimental orchard was used to record air temperature, net solar radiation, relative humidity (Fig. 2) and rainfall (Table 1). The vapor pressure deficit (VPD) was calculated by using the Goff-Grattch equation (reference).
2. Materials and methods 2.1. Materials and experimental design The experiment was conducted from middle April to early September in 2009 and 2010, in a pear orchard located in Mongolian Autonomous Prefecture of Bayingolin (41°43′N, 86°6′E), China. The experimental site adjacent to Taklimakan desert is under extreme drought conditions, with the average annual evaporation of 1600 mm (USA class A pan), the mean annual precipitation at 53 mm and the average annual temperature of 11 °C. The total precipitation and evaporation during the experimental periods are about 21 mm and 800 mm in 2009, and 24 mm and 773 mm in 2010, respectively. The soil in the experimental orchard is a silt loam comprised of 50.4 % silt, 5.6 % clay and 44.0 % sand, with the bulk density of 1.5 g/cm3. The experimental pear trees (Pyrus bretschneideri Rehd) were 20 years old, grafted on the rootstock of Pyrus betulaefolia and spaced at 6 m by row and 5 m by plant (333 tree·ha−1). The experiment involved 3 treatments: (1) Control, being irrigated with 80 % replacement of the pan evaporation (Ep) through the whole experimental period (reference irrigation); (2) RDI, receiving 50 % of Ep during the slow fruit growth stage (from middle April till early July) and 80 % of Ep during the rapid fruit enlargement stage (from middle July till early September); (3) PRD, the same irrigation timing and amount as RDI (Table 1). Two driplines, located 1 m away from and on each side of the tree row respectively, were used to irrigate the pear trees. The emitters were spaced at 0.5 m with discharge rate of 2.8 L/h. The control and RDI trees were irrigated with both driplines at the same time, while till the early July the PRD trees were irrigated only on one side and alternating the two sides once a week, and during the remaining period the same as RDI treatment (Table 2). All experimental trees were irrigated once a week. The experimental orchard was flood irrigated and shifted to surface drip irrigation from middle April of 2008. The experiment was a complete randomized block and each treatment was replicated 3 times. Each replicate plot consisted of 18 trees in 3 rows, and the 4 central trees in the middle row served as
2.2.2. Tree measurements Five shoots per sampling trees per replication were labeled in the southern part of the canopy. The shoot length was measured with a tape once a week from April through September. Trees were pruned once in July every year and the fresh weight of prunings was scaled immediately. Five fruits per sampling trees per replication were tagged randomly, the fruit diameter was measured once a week, and the pear fruit volume were determined through the method of Wu et al. (2013). All pear fruits of four sampling trees were harvested in early September and the yield of each sampling tree was recorded separately. A photosynthesis system (CB-1102, Yaxinliyi Science and Technology Co., Ltd., China) was used to determine the air CO2 concentration and the leaf physiological indices including the leaf transpiration rate (Tr), photosynthetic rate (Pn), stomatal conductance (gs) and temperature. Two sunlit leaves from two sampling trees per replication were labeled randomly at the same position of the canopies, and the leaf physiological parameters were measured from 10:00 a.m. to 11:30 a.m. one day before the irrigation. Every labeled leaf was measured twice (Cui et al., 2009). The air flow rate was set at 0.5 L/ min, and during the measurement, the CO2 concentration was 360−460 ppm, the temperature in the leaf chamber was 19−33 °C, photosynthetic active radiation (PAR) was 800−1700 μmol m−2·s-1. 2.3. Data analysis The measurements were analyzed with analyses of variance. Tukey tests were used to assess the differences between means and significant differences were based on p < 0.05. The statistical analysis was performed by using the Statistical Analysis Software (SPSS 16.0).
Table 1 The pan evaporation, precipitation and irrigation amount, mm. Year
2009
2010
Growth stage
Slow fruit growth Rapid fruit enlargement Total Slow fruit growth Rapid fruit enlargement Total
Pan evaporation
Precipitation
536.3 264.0 800.3 455.8 317.5 773.3
11.6 9.6 21.2 21.8 2.2 24.0
2
irrigation amount PRD
RDI
Control
272 230 502 229 262 491
270 229 499 230 263 493
430 227 657 360 260 620
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Table 2 The irrigation method of each treatment. Treatment
RDI PRD Control
Irrigation method Slow fruit growth
Rapid fruit enlargement
Both sides of tree row (50 % Ep) Alternating between sides once a week (50 % Ep) Both sides of tree row (80 % Ep)
Both sides of tree row (80 % Ep) Both sides of tree row (80 % Ep) Both sides of tree row (80 % Ep)
3. Results 3.1. Soil and leaf water status The soil water potential of RDI and PRD treatment significantly decreased by the reduction of irrigation water. Under the control, the soil water potential at 30 cm depth maintained about −50 kPa through the whole experimental period, indicating little or no water stress (Fig. 3). The soil water potential of the PRD and RDI treatment were both decreased during the slow fruit growth stage. After the resumption of the reference irrigation during the rapid fruit enlargement stage, the soil water potential of RDI and PRD restored gradually. The similar trends were observed in soil water potential at 60 cm depth (data not shown). Through the whole growth season in both experimental years, the average value of soil water content on both sides of tree rows at 30 cm depth in PRD was similar to that in RDI treatments (Fig. 4), which indicated that there were no significant differences in soil evaporation between PRD and RDI treatments. As shown in Fig. 5, during the slow fruit growth stage, the pre-dawn leaf water potential in RDI and PRD treatment were significantly lower than that in control, which was consistent with the decline trend of the soil water potential. The subsequent reference irrigation in RDI and PRD resulted in the recovering of pre-dawn leaf water potential. No remarkable difference in pre-dawn leaf water potential between RDI and PRD was observed.
Fig. 1. Access tubes location.
3.2. Shoot and fruit growth The vegetative growth of RDI and PRD were significantly reduced by the soil water deficit (Fig. 6). The reduction in shoot extension of RDI and PRD were ranged from 25 % to 28 % in 2009 and 32 % to 30 % in 2010, respectively. The pruning weight of RDI and PRD decreased by 26 % and 28 % in 2009, and 22 % and 25 % in 2010 respectively. No statistical differences in shoot extension and pruning weight between the RDI and PRD treatment were detected in both experimental years. The pear fruit grew slowly before July, after which it entered the rapid enlargement stage. No obvious differences in fruit growth and yield were detected amongst three treatments through the whole growth season, though the soil water potential and the pre-dawn leaf water potential in RDI and PRD was significantly decreased during the water stress period (Table 3). Compared with control, the higher values of total soluble solid and sugar content were observed in PRD and RDI treatment, but the significant difference only existed between PRD and control in 2009. 3.3. Leaf physiological parameters The leaf Pn, Tr and gs of control increased with the air temperature, net solar radiation and VPD from middle April to middle August, and then decreased during remaining growth season (Fig. 7). During the water stress period, the leaf Pn, Tr and gs in PRD and RDI treatments were significantly lower than that of control and these differences between PRD or RDI and control increased with the decreased pre-dawn leaf water potential, while no significant differences in leaf physiological parameters were observed between PRD and RDI treatments. After the reference irrigation resumed, the leaf Pn, Tr and gs in the PRD and RDI treatments recovered to the level of control in 6–7 weeks.
Fig. 2. The net solar radiation, air temperature and VPD in 2009 and 2010. The measurements were conducted during 10:00-11:30 a.m.
3
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Fig. 3. The soil water potential at 30 cm depth in the middle of irrigated zone in 2009 and 2010. The soil water potential were converted from the measured soil water content with soil water characteristics.
4. Discussion
4.2. Shoot growth
4.1. Soil and leaf water potential
Vegetative growth of pear tree was significantly inhibited by the water stress before the rapid fruit enlargement stage. There no obvious differences in final shoot length and weight of fresh prunings between PRD and RDI treatment. While results of Romero and Martinez-Cutillas (2012) and Romero et al. (2014) show that PRD exhibits less inhibition of the vegetative growth than RDI applied in grapevines and they attribute this effect to the drought-tolerant rootstock, which can produce new roots to uptake deeper soil water resulting from the PRD treatment. In our study, the deeper irrigation in PRD did not benefit the vegetative growth and this may be due to the characteristics of the rootstock or rootstock/scion combination. Moreover, our irrigation frequency is far lower than their 3–5 times per week, because high frequency irrigation can lead to excessive surface evaporation under the extreme drought condition. Our lower irrigation frequency produced a deeper wetting and the water was not readily available to the root in PRD treatment under the poor air permeability condition. However, a stronger control over the vegetative growth by the PRD is observed by Çolak and Yazar (2017) and they ascribe this control to the chemical signals generated in dehydrating roots, which is transported to shoots to inhibit the vegetative growth of grapevine. The
Soil water deficit in PRD and RDI treatments resulted in the significant decrease of leaf water potential. This is consistent with results of Marsal et al. (2002); Intrigliolo and Castel (2009) and Parvizi et al. (2016). Our results show that there were no remarkable differences in soil water content and pre-dawn leaf water potential between PRD and RDI treatment (Figs. 4 and 5). However, Marsal et al. (2008) and Mossad et al. (2018) results show that PRD treatment leads to higher soil water potential and they attribute this to a relatively smaller wetted soil surface causing the decreasing evaporation. The higher irrigation frequent and shorter duration are adopted in the experiment of Mossad et al. (2018), which result in the higher soil water potential (> −200 kPa) in conventional deficit irrigation compared with our experiment. In addition, the potential evaporation was extremely high under our experimental condition and the rapid soil surface drying process caused fracture of soil capillaries close to the soil surface, which reduced the surface soil evaporation. Therefore, this tiny difference in soil evaporation between PRD and RDI affected the soil water content and leaf water potential unremarkably in our experiment.
Fig. 4. The variations of soil water content in the wetted zone (v/v) in 2009 and 2010. Data are mean of water soil content on both sides of tree row at 30 cm depth. 4
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Fig. 5. The variation of pre-dawn leaf water potentials. Each point is the mean of six measurements, and * indicated significant differences between PRD and the control at p < 0.05, △ indicated significant differences between RDI and the control at p < 0.05, no significant differences between RDI and PRD were observed. Vertical bars indicate the standard deviation of the mean.
significant differences were only observed between RDI and control in 2010. The full irrigation after certain period of water stress boosts pear fruit growth due to compensatory mechanism of the osmotic adjustment (Behboudian et al., 1994). In addition, the photesynthate stored in vegetative organs may also transfer to the reproductive organs after a period of water stress (Cui et al., 2009). In our study, the slightly faster fruit growth may be not for these abovementioned reasons, because no significant differences in fruit yield among three treatments were observed. It is indicated that no more photosynthetic products accumulated in the harvested fruit in PRD and RDI treatments than control. The imposed water stress before the rapid fruit growth stage significantly reduced the fruit load (ratio of the yield to final fruit size), which maybe resulted in the faster fruit growth. Caspari et al. (1994) also concluded that the ratio of yield/final fruit weight is decreased by the water stress imposed before rapid fruit enlargement stage of Asian pear trees, though the differences are not significant. The similar results are also obtained by Arseneault and Cline (2016). The similar ratio of the yield to final fruit size were observed in PRD (0.172 in 2009 and 0.159 in 2010) and RDI (0.179 in 2009 and 0.158 in 2010) treatment, which resulted in about the same fruit growth rate during the rapid fruit growth period. The previous researches show that RDI and PRD can improve fruit quality of apple trees (Mpelasoka et al., 2001; Leib et al., 2006), grapevine (Pellegrino et al., 2005; Acevedo-Opazo et al., 2010) and
similar trends of vegetative growth were detected between the PRD and RDI in our extremely arid region. In fact, under RDI treatment the roots near the wetting front of wetted volume also endured alternation of wetting and drying, just like the roots under the emitters of PRD, especially under our lower irrigation frequency and extreme drought conditions. Thus, some ABA was probably generated in the roots of RDI trees. In addition, the xylem ABA in PRD trees decreased with the water stress possibly due to the availability of sap flow, although the leaf ABA concentration in PRD were higher than that in RDI with a certain soil water content, and the constant soil drying resulted in this opposite phenomenon (Yao et al., 2001; Dodd et al., 2008; Romero et al., 2014). Therefore, it indicted that the irrigation amount was also a key factor to influence the tree vegetative growth. 4.3. Yield and fruit quality More photosynthate may translocate from leaves to pear fruit resulting from inhibition of vegetative growth and leaf photosynthesis by imposed water stress (Chalmers et al., 1986). In our study, the similar fruit growth trend during the water stress period was observed in PRD and RDI treatment. The reference irrigation resumed in RDI and PRD treatments two months before harvest, the fruit growth rate of PRD and RDI improved by 5.2 % and 4.3 % in 2009, and 4.8 % and 6.7 % in 2010 respectively during the following growth season, although the
Fig. 6. The shoot extension and weight of fresh prunings. The different lowercase characters indicate significant differences at the p < 0.05). Vertical bars indicate the standard deviation of the mean. 5
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Table 3 The fruit yield, final size, growth before and during rapid fruit enlargement stages, total soluble solid and sugar content. Year
2009
2010
Treatment
PRD RDI Control PRD RDI Control
Final fruit volume (cm3)
Yield (t/ha)
18.8 19.3 19.8 19.9 20.1 21.0
± ± ± ± ± ±
2.0 2.2 2.3 2.2 2.1 2.1
a a a a a a
109 108 105 125 127 122
± ± ± ± ± ±
7.4 6.6 6.5 5.7 5.5 7.1
a a a ab a b
Fruit growth (cm3) Middle Apr. to early Jul.
Middle Jul. to harvest
22.1 21.9 22.4 26.9 26.7 27.8
86.9 ± 6.7 a 86.1 ± 6.2 a 82.6 ± 5.3 a 98.2 ± 4.8 ab 100.0 ± 5.1 a 93.7 ± 7.8 b
± ± ± ± ± ±
3.2 2.9 4.0 4.2 4.1 4.6
a a a a a a
Total soluble solid (%)
Soluble sugar (%)
12.7 12.5 12.0 13.4 13.6 13.3
8.4 7.9 7.5 8.3 8.6 8.1
± ± ± ± ± ±
0.6 0.5 0.5 0.5 0.7 0.7
a ab b a a a
± ± ± ± ± ±
0.6 0.6 0.4 0.5 0.3 0.7
a ab b a a a
Note: Different letters within the same column indicate significant differences at the p < 0.05 level. The numbers are mean ± standard deviation.
Fig. 7. The variation of leaf Pn, Tr and gs measured during 10:00–11:30 a.m. Each point is the mean of twelve measurements, and * indicated significant differences between PRD and the control at p < 0.05, △ indicated significant differences between RDI and the control at p < 0.05, no significant differences between RDI and PRD were observed. Vertical bars indicate the standard deviation of the mean. 6
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Acknowledgements
orange trees (García-Tejero et al., 2010). In our study, the PRD and RDI had comparable effects on pear fruit quality. The total soluble solid and sugar content were increased by the PRD and RDI treatments, but significant difference was observed only between the PRD and control in 2009. This may be due to the varietal differences, the intensity and duration of water stress.
We are grateful to the research grants from the National Key Technology R&D Program of China (2007BAD38B00) and the China National Natural Science Fund (50879087). References
4.4. Leaf physiological parameters
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Water use efficiency and fruit quality of table grape under alternate partial root-zone drip irrigation. Agric. Water Manage. 95, 659–668. Egea, G., González-Real, M.M., Baille, A., Nortes, P.A., Sánchez-Bel, P., Domingo, R., 2009. The effects of contrasted deficit irrigation strategies on the fruit growth and kernel quality of mature almond trees. Agric. Water Manage. 96, 1605–1614. Egea, G., Nortes, P.A., González-Real, M.M., Baille, A., Domingo, R., 2010. Agronomic response and water productivity of almond trees under contrasted deficit irrigation regimes. Agric. Water Manage. 97, 171–181. Fernández, J.E., Moreno, F., Girón, I.F., Blázquez, O.M., 1997. Stomatal control of water use in olive tree leaves. Plant Soil 190, 179–192. García-Tejero, I., Jiménez-Bocanegra, J.A., Martínez, G., Romero, R., Durán-Zuazo, V.H.,
The leaf Tr was significantly decreased by the RDI and PRD due to the closure of leaf stoma, which was an important mechanism for reducing the leaf water loss, which was consistent with the results of Chaves et al. (2002); Lawlor (2002) and Cifre et al. (2005). In our study, the leaf physiological parameters restored subsequently with the recovery of leaf water potential. It took about 6–7 weeks for RDI and PRD trees to recover these parameters after the resumption of the reference irrigation. It is reported that the leaf An and gs exposed to the low and high soil water stress restored in the following 3 and 50 days after rewatering in pear-jujube trees by Cui et al. (2009) and in orange trees by Pérez-Pérez et al. (2008), respectively. It is indicated that the recovery of leaf physiological parameters depended on the timing and degree of water stress, and the similar results were reported by Fernández et al. (1997) and Ramos and Santos (2010) in olive trees. The PRD grapevines (Parvizi et al., 2016) and pomegranate trees (Romero et al., 2014) have higher leaf Pn due to greater fine root growth and water uptake than RDI. Loveys et al. (2000) and de Souza et al. (2003) reported that the gs of grapevines are significantly reduced due to the ABA from the drying roots in the PRD treatment. In our study, there were no significant differences in leaf Pn, Tr and gs between the RDI and PRD. This may be due to the same soil and plant water status under the two treatments, although PRD and RDI resulted in the different soil water distribution. These results support the theory that the gas exchanges are mainly impacted by the irrigation volume rather than irrigation method (Bravdo et al., 2004; De la Hera et al., 2007; Intrigliolo and Castel, 2009; Mossad et al., 2018).
5. Conclusion In the extremely arid region, there were no significant differences in soil and plant water status between the PRD and RDI pear trees, which resulted in the similar vegetative and fruit growth between two irrigation treatments. This similar soil and tree water status also produced the same variation trend of the leaf gas exchanges between the PRD and RDI treatment. In general, the growth and leaf physiological parameters variation of pear trees planted in the extremely arid region were mainly determined by the irrigation volume rather than the irrigation method. Under desert climate, the RDI technique was more appropriate than PRD due to its low initial investment and labor cost.
CRediT authorship contribution statement Yang Wu: Conceptualization, Methodology, Investigation, Resources, Data curation, Writing - original draft, Visualization, Writing - review & editing. Zhi Zhao: Investigation, Data curation, Writing - review & editing, Visualization. Songzhong Liu: Visualization, Writing - review & editing. Xingfa Huang: Methodology, Writing - review & editing. Wei Wang: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest The authors declare that there are no conflicts of interest. 7
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Romero, P., Botiá, P., Garcia, F., 2004. Effects of regulated deficit irrigation under subsurface drip irrigation conditions on water relations of mature almond trees. Plant Soil 260, 155–168. Romero, P., Martinez-Cutillas, A., 2012. The effects of partial root-zone irrigation and regulated deficit irrigation on the vegetative and reproductive development of fieldgrown Monastrell grapevines. Irrig. Sci. 30 (5), 377–396. Romero, P., Pérez-Pérez, J.G., del Amor, F.M., Martinez-Cutillas, A., Dodd, I.C., Botía, P., 2014. Partial root zone drying exerts different physiological responses on field-grown grapevine (Vitis vinifera cv. Monastrell) in comparison to regulated deficit irrigation. Funct. Plant Biol. 41 (11), 1087–1106. Romero-Conde, A., Kusakabe, A., Melgar, J.C., 2014. Physiological responses of citrus to partial rootzone drying irrigation. Sci. Hortic. 169, 234–238. Rosecrance, R.C., Krueger, W.H., Milliron, L., Bloese, J., Garcia, C., Mori, B., 2015. Moderate regulated deficit irrigation can increase olive oil yields and decrease tree growth in super high density ‘Arbequina’ olive orchards. Sci. Hortic. 190, 75–82. Parvizi, H., Sepaskhah, A.R., Ahmadi, S.H., 2016. Physiological and growth responses of pomegranate tree (Punica granatum (L.) cv. Rabab) under partial root zone drying and deficit irrigation regimes. Agric. Water Manage. 163, 146–158. Pérez-Pérez, J.G., Romero, P., Navarro, J.M., Botía, P., 2008. Response of sweet orange cv ‘Lane Late’ to deficit irrigation in two rootstocks. I: water relations, leaf gas exchange and vegetative growth. Irrig. Sci. 26, 415–425. Pérez-Pérez, J.G., Dodd, I.C., 2015. Sap fluxes from different parts of the rootzone modulate xylem ABA concentration during partial rootzone drying and re-wetting. J. Exp. Bot. 66 (8), 2315–2324. Pellegrino, A., Lebon, E., Simonneau, T., Wery, J., 2005. Towards a simple indicator of water stress in grapevine (Vitis vinifera L.) based on the differential sensitivities of vegetative growth components. Aust. J. Grape Wine Res. 11 (3), 306–315. Stoll, M., Loveys, B., Dry, P., 2000. Hormonal changes induced by partial root-zone drying of irrigated grapevine. J. Exp. Bot. 51, 1627–1634. Tejero, I.G., Zuazo, V.H.D., Bocanegra, J.A.J., Fernández, J.L.M., 2011. Improved wateruse efficiency by deficit-irrigation programmes: implications for saving water in citrus orchards. Sci. Hortic. 128, 274–282. Wahbi, S., Wakrim, R., Aganchich, B., Tahi, H., Serraj, R., 2005. Effects of partial rootzone drying (PRD) on adult olive tree (Olea europaea) in field conditions under arid climate I. Physiological and agronomic responses. Agricult. Ecosyst. Environ. 106, 289–301. Wakrim, R., Wahbi, S., Tabi, H., Aganchich, B., Serraj, R., 2005. Comparative effects of partial root drying (PRD) and regulated deficit irrigation (RDI) on water relations and water use efficiency in common bean (Phaseolus vulgaris L.). Agric. Ecosyst. Environ. 106, 275–287. Wu, Y., Zhao, Z., Wang, W., Ma, Y., Huang, X., 2013. Yield and growth of mature pear trees under water deficit during slow fruit growth stages in sparse planting orchard. Sci. Hortic. 164, 189–195. Yao, C., Moreshet, S., Aloni, B., 2001. Water relations and hydraulic control of stomatal behaviour in bell pepper plant in partial soil drying. Plant Cell Environ. 24, 227–235.
Muriel-Fernández, J.L., 2010. Positive impact of regulated deficit irrigation on yield and fruit quality in a commercial citrus orchard [Citrus sinensis (L.) Osbeck, cv. salustiano]. Agric. Water Manage. 97, 614–622. Gu, S., Du, G., Zoldoske, D., Hakim, A., Cochran, R., Fugelsang, K., Jorgensen, G., 2004. Effects of irrigation amount on water relations, vegetative growth, yield and fruit composition of Sauvignon blanc grapevines under partial root-zone drying and conventional irrigation in the San Joaquin Valley of California, USA. J. Hortic. Sci. Biotechnol. 79, 26–33. Intrigliolo, D.S., Castel, J.R., 2009. Response of Vitis vinifera cv. “Tempranillo’’ to partial root-zone drying in the field: water relations, growth, yield and fruit and wine quality. Agric. Water Manage. 96, 282–292. Lawlor, D.W., 2002. Limitation to photosynthesis in water-stressed leaves: stomata vs. metabolism and the role of ATP. Ann. Bot. 89, 871–885. Leib, B.G., Caspari, H.W., Redulla, C.A., Andrews, P.K., Jabro, J.J., 2006. Partial rootzone drying and deficit irrigation of ‘Fuji’ apples in a semi-arid climate. Irrig. Sci. 24, 85–99. Loveys, B.R., 1999. Grapevine shoot growth and stomatal conductance are reduced when part of the root system is dried. Vitis 39, 151–156. Loveys, B.R., Dry, P.R., Stoll, M., McCarthy, M.G., 2000. Using plant physiology to improve the water efficiency of horticultural crops. Acta Hortic. 537, 187–197. Marsal, J., Mata, M., Arbonés, A., Rufat, J., Girona, J., 2002. Regulated deficit irrigation and rectification of irrigation scheduling in young pear trees: an evaluation based on vegetative and productive response. Eur. J. Agron. 17, 111–122. Marsal, J., Mata, M., Del Campo, J., Arbones, A., Vallverdú, X., Girona, J., Olivo, N., 2008. Evaluation of partial root-zone drying for potential field use as a deficit irrigation technique in commercial vineyards according to two different pipeline layouts. Irrig. Sci. 26, 347–356. Mpelasoka, B.S., Behboudian, M.H., Mills, T.M., 2001. Effect of deficit irrigation on fruit maturity and quality of ‘Braeburn’ apple. Sci. Hortic. 90, 279–290. Mitchell, P.D., Chalmers, D.J., Jerie, P.H., Burge, G., 1986. The use of initial withholding of irrigation and tree spacing to enhance the effect of regulated deficit irrigation on pear trees. J. Am. Soc. Hortic. Sci. 111 (5), 858–861. Mitchell, P.D., Jerie, P.H., Chalmers, D.J., 1984. The effects of regulated water deficits on pear tree growth, flowering, fruit growth, and yield. J. Am. Soc. Hortic. Sci. 109 (5), 604–606. Mingo, D.M., Bacon, M.A., Davies, W.J., 2003. Non-hydraulic regulation of fruit growth in tomato plants (Lycopersicon esculentum cv Solairo) growing in drying soil. J. Exp. Bot. 54 (385), 1205–1212. Moriana, A., Corell, M., Girón, I.F., Conejero, W., Morales, D., Torrecillas, A., Moreno, F., 2013. Regulated deficit irrigation based on threshold values of trunk diameter fluctuation indicators in table olive trees. Sci. Hortic. 164, 102–111. Mossad, A., Scalisi, A., Lo Bianco, R., 2018. Growth and water relations of field-grown ‘Valencia’ orange trees under long-term partial rootzone drying. Irrig. Sci. 36, 9–24. Ramos, A.F., Santos, F.L., 2010. Yield and olive oil characteristics of a low-density orchard (cv. Cordovil) subjected to different irrigation regimes. Agric. Water Manage. 97, 363–373.
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Does partial root-zone drying have advantages over regulated deficit irrigation in pear orchard under desert climates?
T
Yang Wua, Zhi Zhaob, Songzhong Liua, Xingfa Huangb, Wei Wangc,* a
Beijing Academy of Forestry and Pomology Sciences, Beijing Engineering Research Center for Deciduous Fruit Trees, Key Laboratory of Biology and Genetic Improvement of Horticultural Crops (North China), Ministry of Agriculture, Beijing 100093, China b College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China c College of Engineering, China Agricultural University, Beijing 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil water status Fruit yield Vegetative growth Leaf physiological parameters
The impacts of partial root-zone drying (PRD) and regulated deficit irrigation (RDI) with the same irrigation volume on vegetative and fruit growth and leaf physiological parameters of pear trees planted under desert climate were evaluated. The experiment involved RDI and PRD treatment, irrigated with the 50 % replacement of pan evaporation (Ep) during the slow fruit growth stage, and 80 % replacement of Ep during rapid fruit enlargement stage. Control trees were irrigated with the 80 % replacement of Ep during the whole growing season. No significant differences in the soil and leaf water status between PRD and RDI were observed, which resulted in the similar vegetative growth, fruit yield and leaf gas exchange between PRD and RDI trees. In conclusion, the leaf gas exchange, the vegetative and fruit growth of pear trees planted under extreme drought condition were mainly controlled by the irrigation volume rather than the irrigation method. Therefore, application of the PRD technique was not recommended in the extremely arid region compared with the RDI strategy.
1. Introduction Regulated deficit irrigation (RDI) strategy is a technique imposing water stress during the specific growing period to inhibit vigorous vegetative growth (Chalmers et al., 1981; Mitchell et al., 1984; Tejero et al., 2011; Blanco et al., 2019). RDI may improve the fruit yield, quality and irrigation water use efficiency (Mitchell et al., 1986; Romero et al., 2004; Moriana et al., 2013; Rosecrance et al., 2015). Partial root-zone drying (PRD) is an irrigation strategy to alternately wet and dry the two different parts of the root zone (Düring et al., 1997; Wakrim et al., 2005; Marsal et al., 2008; de Lima et al., 2015) and this wetting and drying induces a chemical signal (abscisic acid, ABA) generation (Wakrim et al., 2005; Davies et al., 1994), resulting in stomatal closure, inhibition of vegetative growth (Wakrim et al., 2005; Leib et al., 2006; Romero-Conde et al., 2014), and water use efficiency improvement (Mingo et al., 2003; Leib et al., 2006; Du et al., 2008). Theoretically, the PRD strategy works through manipulating the water stress over space, while the RDI strategy regulates the timing of water deficit to control the vegetative and reproductive growth processes. The effects of PRD and RDI strategy with the same irrigation timing and amount on the vegetative growth of fruit trees has been fully
⁎
concerned in recent years (Loveys, 1999; Stoll et al., 2000; dos Santos et al., 2003, 2007; Çolak and Yazar, 2017). Researches on the grapevine (De la Hera et al., 2007; Çolak and Yazar, 2017) and apple (Leib et al., 2006) show that PRD trees produce higher yield than RDI with the same irrigation amount, while there are no significant differences in vegetative growth of grapevines (Gu et al., 2004; Bravdo et al., 2004; Intrigliolo and Castel, 2009), almond trees (Egea et al., 2009) and papaya trees (de Lima et al., 2015) and in yield or fruit growth of olive trees (Wahbi et al., 2005), almond tress (Egea et al., 2009, 2010) and orange trees (Mossad et al., 2018) between RDI and PRD treatment. Most of the abovementioned researches were conducted in the arid or semi-arid regions with 200−600 mm annual precipitation. The comparative studies of the RDI and PRD effectiveness in extreme drought regions are scarce. The drought under desert climate can effectively separate the dry side from wet side of PRD trees, while the water extraction from deep root zone reduces the synthesis of hormones or chemical signals, such as abscisic acid (ABA), in dried region, reducing the PRD effects, thus drying a larger proportion of active fine roots should be considered (De la Hera et al., 2017). In addition, the level of soil water potential in wetted side of PRD trees is also a key factor to influence the fruit growth
Corresponding author at: China Agricultural University, No.17 Qinghua East Road, Haidian, Beijing, 100083, China. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.scienta.2019.109099 Received 7 September 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 Available online 09 December 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved.
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sample trees.
(Romero and Martinez-Cutillas, 2012). In micro-irrigated orchards, the wetted percentage is usually less than 50 %, which means that more than half of the roots zone will not receive irrigation water. These roots may still maintain its activity and produce abscisic acid (ABA), which consequently induces the control of the leaf stomata under RDI treatment. However, under desert climate, the extremely low availability of soil water in non-irrigated zone may prevent the production of ABA in roots or its transportation to the aboveground portion of trees. Researches on the sunflower planted in substrate show that no sap flow are detected until the volumetric water content is higher than 10.8 % (Dodd et al., 2008). Similar results were reported by Pérez-Pérez and Dodd (2015). Therefore, this study aimed to 1) quantify the effects of PRD and RDI on the vegetative and fruit growth of pear trees and the tree leaf physiological response to the two irrigation strategies; 2) evaluate the advantages of PRD over RDI in an extremely arid region.
2.2. Measurements 2.2.1. Water status and meteorological measurements The volumetric soil water content was measured using a portable capacitance probe (Diviner, 2000, Sentek Pty Ltd). Two 1.5 m long access tubes per replication in RDI and control treatments were installed under the driplines opposite the sampling trees, four access tubes per replication in PRD were installed on both sides of two trees (Fig. 1). The soil water content was measured immediately before each irrigation event. The soil water characteristics, measured with a pressure plate extractor (1500F1, Soil moisture Equipment Corp., USA) were used to convert the volumetric soil water content into the soil water potential. Two fresh leaves from two sampling trees per replication were selected randomly for measuring the leaf water potential. The measurement was conducted before dawn by using a pressure chamber (ZLZ-5, Ningbo Jiangnan Instrument Co., Ltd.). An automatic weather station (Vantage Pro2, Davis Instruments Corp., USA) installed near the experimental orchard was used to record air temperature, net solar radiation, relative humidity (Fig. 2) and rainfall (Table 1). The vapor pressure deficit (VPD) was calculated by using the Goff-Grattch equation (reference).
2. Materials and methods 2.1. Materials and experimental design The experiment was conducted from middle April to early September in 2009 and 2010, in a pear orchard located in Mongolian Autonomous Prefecture of Bayingolin (41°43′N, 86°6′E), China. The experimental site adjacent to Taklimakan desert is under extreme drought conditions, with the average annual evaporation of 1600 mm (USA class A pan), the mean annual precipitation at 53 mm and the average annual temperature of 11 °C. The total precipitation and evaporation during the experimental periods are about 21 mm and 800 mm in 2009, and 24 mm and 773 mm in 2010, respectively. The soil in the experimental orchard is a silt loam comprised of 50.4 % silt, 5.6 % clay and 44.0 % sand, with the bulk density of 1.5 g/cm3. The experimental pear trees (Pyrus bretschneideri Rehd) were 20 years old, grafted on the rootstock of Pyrus betulaefolia and spaced at 6 m by row and 5 m by plant (333 tree·ha−1). The experiment involved 3 treatments: (1) Control, being irrigated with 80 % replacement of the pan evaporation (Ep) through the whole experimental period (reference irrigation); (2) RDI, receiving 50 % of Ep during the slow fruit growth stage (from middle April till early July) and 80 % of Ep during the rapid fruit enlargement stage (from middle July till early September); (3) PRD, the same irrigation timing and amount as RDI (Table 1). Two driplines, located 1 m away from and on each side of the tree row respectively, were used to irrigate the pear trees. The emitters were spaced at 0.5 m with discharge rate of 2.8 L/h. The control and RDI trees were irrigated with both driplines at the same time, while till the early July the PRD trees were irrigated only on one side and alternating the two sides once a week, and during the remaining period the same as RDI treatment (Table 2). All experimental trees were irrigated once a week. The experimental orchard was flood irrigated and shifted to surface drip irrigation from middle April of 2008. The experiment was a complete randomized block and each treatment was replicated 3 times. Each replicate plot consisted of 18 trees in 3 rows, and the 4 central trees in the middle row served as
2.2.2. Tree measurements Five shoots per sampling trees per replication were labeled in the southern part of the canopy. The shoot length was measured with a tape once a week from April through September. Trees were pruned once in July every year and the fresh weight of prunings was scaled immediately. Five fruits per sampling trees per replication were tagged randomly, the fruit diameter was measured once a week, and the pear fruit volume were determined through the method of Wu et al. (2013). All pear fruits of four sampling trees were harvested in early September and the yield of each sampling tree was recorded separately. A photosynthesis system (CB-1102, Yaxinliyi Science and Technology Co., Ltd., China) was used to determine the air CO2 concentration and the leaf physiological indices including the leaf transpiration rate (Tr), photosynthetic rate (Pn), stomatal conductance (gs) and temperature. Two sunlit leaves from two sampling trees per replication were labeled randomly at the same position of the canopies, and the leaf physiological parameters were measured from 10:00 a.m. to 11:30 a.m. one day before the irrigation. Every labeled leaf was measured twice (Cui et al., 2009). The air flow rate was set at 0.5 L/ min, and during the measurement, the CO2 concentration was 360−460 ppm, the temperature in the leaf chamber was 19−33 °C, photosynthetic active radiation (PAR) was 800−1700 μmol m−2·s-1. 2.3. Data analysis The measurements were analyzed with analyses of variance. Tukey tests were used to assess the differences between means and significant differences were based on p < 0.05. The statistical analysis was performed by using the Statistical Analysis Software (SPSS 16.0).
Table 1 The pan evaporation, precipitation and irrigation amount, mm. Year
2009
2010
Growth stage
Slow fruit growth Rapid fruit enlargement Total Slow fruit growth Rapid fruit enlargement Total
Pan evaporation
Precipitation
536.3 264.0 800.3 455.8 317.5 773.3
11.6 9.6 21.2 21.8 2.2 24.0
2
irrigation amount PRD
RDI
Control
272 230 502 229 262 491
270 229 499 230 263 493
430 227 657 360 260 620
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Table 2 The irrigation method of each treatment. Treatment
RDI PRD Control
Irrigation method Slow fruit growth
Rapid fruit enlargement
Both sides of tree row (50 % Ep) Alternating between sides once a week (50 % Ep) Both sides of tree row (80 % Ep)
Both sides of tree row (80 % Ep) Both sides of tree row (80 % Ep) Both sides of tree row (80 % Ep)
3. Results 3.1. Soil and leaf water status The soil water potential of RDI and PRD treatment significantly decreased by the reduction of irrigation water. Under the control, the soil water potential at 30 cm depth maintained about −50 kPa through the whole experimental period, indicating little or no water stress (Fig. 3). The soil water potential of the PRD and RDI treatment were both decreased during the slow fruit growth stage. After the resumption of the reference irrigation during the rapid fruit enlargement stage, the soil water potential of RDI and PRD restored gradually. The similar trends were observed in soil water potential at 60 cm depth (data not shown). Through the whole growth season in both experimental years, the average value of soil water content on both sides of tree rows at 30 cm depth in PRD was similar to that in RDI treatments (Fig. 4), which indicated that there were no significant differences in soil evaporation between PRD and RDI treatments. As shown in Fig. 5, during the slow fruit growth stage, the pre-dawn leaf water potential in RDI and PRD treatment were significantly lower than that in control, which was consistent with the decline trend of the soil water potential. The subsequent reference irrigation in RDI and PRD resulted in the recovering of pre-dawn leaf water potential. No remarkable difference in pre-dawn leaf water potential between RDI and PRD was observed.
Fig. 1. Access tubes location.
3.2. Shoot and fruit growth The vegetative growth of RDI and PRD were significantly reduced by the soil water deficit (Fig. 6). The reduction in shoot extension of RDI and PRD were ranged from 25 % to 28 % in 2009 and 32 % to 30 % in 2010, respectively. The pruning weight of RDI and PRD decreased by 26 % and 28 % in 2009, and 22 % and 25 % in 2010 respectively. No statistical differences in shoot extension and pruning weight between the RDI and PRD treatment were detected in both experimental years. The pear fruit grew slowly before July, after which it entered the rapid enlargement stage. No obvious differences in fruit growth and yield were detected amongst three treatments through the whole growth season, though the soil water potential and the pre-dawn leaf water potential in RDI and PRD was significantly decreased during the water stress period (Table 3). Compared with control, the higher values of total soluble solid and sugar content were observed in PRD and RDI treatment, but the significant difference only existed between PRD and control in 2009. 3.3. Leaf physiological parameters The leaf Pn, Tr and gs of control increased with the air temperature, net solar radiation and VPD from middle April to middle August, and then decreased during remaining growth season (Fig. 7). During the water stress period, the leaf Pn, Tr and gs in PRD and RDI treatments were significantly lower than that of control and these differences between PRD or RDI and control increased with the decreased pre-dawn leaf water potential, while no significant differences in leaf physiological parameters were observed between PRD and RDI treatments. After the reference irrigation resumed, the leaf Pn, Tr and gs in the PRD and RDI treatments recovered to the level of control in 6–7 weeks.
Fig. 2. The net solar radiation, air temperature and VPD in 2009 and 2010. The measurements were conducted during 10:00-11:30 a.m.
3
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Fig. 3. The soil water potential at 30 cm depth in the middle of irrigated zone in 2009 and 2010. The soil water potential were converted from the measured soil water content with soil water characteristics.
4. Discussion
4.2. Shoot growth
4.1. Soil and leaf water potential
Vegetative growth of pear tree was significantly inhibited by the water stress before the rapid fruit enlargement stage. There no obvious differences in final shoot length and weight of fresh prunings between PRD and RDI treatment. While results of Romero and Martinez-Cutillas (2012) and Romero et al. (2014) show that PRD exhibits less inhibition of the vegetative growth than RDI applied in grapevines and they attribute this effect to the drought-tolerant rootstock, which can produce new roots to uptake deeper soil water resulting from the PRD treatment. In our study, the deeper irrigation in PRD did not benefit the vegetative growth and this may be due to the characteristics of the rootstock or rootstock/scion combination. Moreover, our irrigation frequency is far lower than their 3–5 times per week, because high frequency irrigation can lead to excessive surface evaporation under the extreme drought condition. Our lower irrigation frequency produced a deeper wetting and the water was not readily available to the root in PRD treatment under the poor air permeability condition. However, a stronger control over the vegetative growth by the PRD is observed by Çolak and Yazar (2017) and they ascribe this control to the chemical signals generated in dehydrating roots, which is transported to shoots to inhibit the vegetative growth of grapevine. The
Soil water deficit in PRD and RDI treatments resulted in the significant decrease of leaf water potential. This is consistent with results of Marsal et al. (2002); Intrigliolo and Castel (2009) and Parvizi et al. (2016). Our results show that there were no remarkable differences in soil water content and pre-dawn leaf water potential between PRD and RDI treatment (Figs. 4 and 5). However, Marsal et al. (2008) and Mossad et al. (2018) results show that PRD treatment leads to higher soil water potential and they attribute this to a relatively smaller wetted soil surface causing the decreasing evaporation. The higher irrigation frequent and shorter duration are adopted in the experiment of Mossad et al. (2018), which result in the higher soil water potential (> −200 kPa) in conventional deficit irrigation compared with our experiment. In addition, the potential evaporation was extremely high under our experimental condition and the rapid soil surface drying process caused fracture of soil capillaries close to the soil surface, which reduced the surface soil evaporation. Therefore, this tiny difference in soil evaporation between PRD and RDI affected the soil water content and leaf water potential unremarkably in our experiment.
Fig. 4. The variations of soil water content in the wetted zone (v/v) in 2009 and 2010. Data are mean of water soil content on both sides of tree row at 30 cm depth. 4
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Fig. 5. The variation of pre-dawn leaf water potentials. Each point is the mean of six measurements, and * indicated significant differences between PRD and the control at p < 0.05, △ indicated significant differences between RDI and the control at p < 0.05, no significant differences between RDI and PRD were observed. Vertical bars indicate the standard deviation of the mean.
significant differences were only observed between RDI and control in 2010. The full irrigation after certain period of water stress boosts pear fruit growth due to compensatory mechanism of the osmotic adjustment (Behboudian et al., 1994). In addition, the photesynthate stored in vegetative organs may also transfer to the reproductive organs after a period of water stress (Cui et al., 2009). In our study, the slightly faster fruit growth may be not for these abovementioned reasons, because no significant differences in fruit yield among three treatments were observed. It is indicated that no more photosynthetic products accumulated in the harvested fruit in PRD and RDI treatments than control. The imposed water stress before the rapid fruit growth stage significantly reduced the fruit load (ratio of the yield to final fruit size), which maybe resulted in the faster fruit growth. Caspari et al. (1994) also concluded that the ratio of yield/final fruit weight is decreased by the water stress imposed before rapid fruit enlargement stage of Asian pear trees, though the differences are not significant. The similar results are also obtained by Arseneault and Cline (2016). The similar ratio of the yield to final fruit size were observed in PRD (0.172 in 2009 and 0.159 in 2010) and RDI (0.179 in 2009 and 0.158 in 2010) treatment, which resulted in about the same fruit growth rate during the rapid fruit growth period. The previous researches show that RDI and PRD can improve fruit quality of apple trees (Mpelasoka et al., 2001; Leib et al., 2006), grapevine (Pellegrino et al., 2005; Acevedo-Opazo et al., 2010) and
similar trends of vegetative growth were detected between the PRD and RDI in our extremely arid region. In fact, under RDI treatment the roots near the wetting front of wetted volume also endured alternation of wetting and drying, just like the roots under the emitters of PRD, especially under our lower irrigation frequency and extreme drought conditions. Thus, some ABA was probably generated in the roots of RDI trees. In addition, the xylem ABA in PRD trees decreased with the water stress possibly due to the availability of sap flow, although the leaf ABA concentration in PRD were higher than that in RDI with a certain soil water content, and the constant soil drying resulted in this opposite phenomenon (Yao et al., 2001; Dodd et al., 2008; Romero et al., 2014). Therefore, it indicted that the irrigation amount was also a key factor to influence the tree vegetative growth. 4.3. Yield and fruit quality More photosynthate may translocate from leaves to pear fruit resulting from inhibition of vegetative growth and leaf photosynthesis by imposed water stress (Chalmers et al., 1986). In our study, the similar fruit growth trend during the water stress period was observed in PRD and RDI treatment. The reference irrigation resumed in RDI and PRD treatments two months before harvest, the fruit growth rate of PRD and RDI improved by 5.2 % and 4.3 % in 2009, and 4.8 % and 6.7 % in 2010 respectively during the following growth season, although the
Fig. 6. The shoot extension and weight of fresh prunings. The different lowercase characters indicate significant differences at the p < 0.05). Vertical bars indicate the standard deviation of the mean. 5
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Table 3 The fruit yield, final size, growth before and during rapid fruit enlargement stages, total soluble solid and sugar content. Year
2009
2010
Treatment
PRD RDI Control PRD RDI Control
Final fruit volume (cm3)
Yield (t/ha)
18.8 19.3 19.8 19.9 20.1 21.0
± ± ± ± ± ±
2.0 2.2 2.3 2.2 2.1 2.1
a a a a a a
109 108 105 125 127 122
± ± ± ± ± ±
7.4 6.6 6.5 5.7 5.5 7.1
a a a ab a b
Fruit growth (cm3) Middle Apr. to early Jul.
Middle Jul. to harvest
22.1 21.9 22.4 26.9 26.7 27.8
86.9 ± 6.7 a 86.1 ± 6.2 a 82.6 ± 5.3 a 98.2 ± 4.8 ab 100.0 ± 5.1 a 93.7 ± 7.8 b
± ± ± ± ± ±
3.2 2.9 4.0 4.2 4.1 4.6
a a a a a a
Total soluble solid (%)
Soluble sugar (%)
12.7 12.5 12.0 13.4 13.6 13.3
8.4 7.9 7.5 8.3 8.6 8.1
± ± ± ± ± ±
0.6 0.5 0.5 0.5 0.7 0.7
a ab b a a a
± ± ± ± ± ±
0.6 0.6 0.4 0.5 0.3 0.7
a ab b a a a
Note: Different letters within the same column indicate significant differences at the p < 0.05 level. The numbers are mean ± standard deviation.
Fig. 7. The variation of leaf Pn, Tr and gs measured during 10:00–11:30 a.m. Each point is the mean of twelve measurements, and * indicated significant differences between PRD and the control at p < 0.05, △ indicated significant differences between RDI and the control at p < 0.05, no significant differences between RDI and PRD were observed. Vertical bars indicate the standard deviation of the mean. 6
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Acknowledgements
orange trees (García-Tejero et al., 2010). In our study, the PRD and RDI had comparable effects on pear fruit quality. The total soluble solid and sugar content were increased by the PRD and RDI treatments, but significant difference was observed only between the PRD and control in 2009. This may be due to the varietal differences, the intensity and duration of water stress.
We are grateful to the research grants from the National Key Technology R&D Program of China (2007BAD38B00) and the China National Natural Science Fund (50879087). References
4.4. Leaf physiological parameters
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The leaf Tr was significantly decreased by the RDI and PRD due to the closure of leaf stoma, which was an important mechanism for reducing the leaf water loss, which was consistent with the results of Chaves et al. (2002); Lawlor (2002) and Cifre et al. (2005). In our study, the leaf physiological parameters restored subsequently with the recovery of leaf water potential. It took about 6–7 weeks for RDI and PRD trees to recover these parameters after the resumption of the reference irrigation. It is reported that the leaf An and gs exposed to the low and high soil water stress restored in the following 3 and 50 days after rewatering in pear-jujube trees by Cui et al. (2009) and in orange trees by Pérez-Pérez et al. (2008), respectively. It is indicated that the recovery of leaf physiological parameters depended on the timing and degree of water stress, and the similar results were reported by Fernández et al. (1997) and Ramos and Santos (2010) in olive trees. The PRD grapevines (Parvizi et al., 2016) and pomegranate trees (Romero et al., 2014) have higher leaf Pn due to greater fine root growth and water uptake than RDI. Loveys et al. (2000) and de Souza et al. (2003) reported that the gs of grapevines are significantly reduced due to the ABA from the drying roots in the PRD treatment. In our study, there were no significant differences in leaf Pn, Tr and gs between the RDI and PRD. This may be due to the same soil and plant water status under the two treatments, although PRD and RDI resulted in the different soil water distribution. These results support the theory that the gas exchanges are mainly impacted by the irrigation volume rather than irrigation method (Bravdo et al., 2004; De la Hera et al., 2007; Intrigliolo and Castel, 2009; Mossad et al., 2018).
5. Conclusion In the extremely arid region, there were no significant differences in soil and plant water status between the PRD and RDI pear trees, which resulted in the similar vegetative and fruit growth between two irrigation treatments. This similar soil and tree water status also produced the same variation trend of the leaf gas exchanges between the PRD and RDI treatment. In general, the growth and leaf physiological parameters variation of pear trees planted in the extremely arid region were mainly determined by the irrigation volume rather than the irrigation method. Under desert climate, the RDI technique was more appropriate than PRD due to its low initial investment and labor cost.
CRediT authorship contribution statement Yang Wu: Conceptualization, Methodology, Investigation, Resources, Data curation, Writing - original draft, Visualization, Writing - review & editing. Zhi Zhao: Investigation, Data curation, Writing - review & editing, Visualization. Songzhong Liu: Visualization, Writing - review & editing. Xingfa Huang: Methodology, Writing - review & editing. Wei Wang: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest The authors declare that there are no conflicts of interest. 7
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