Water shortage has encouraged the quest for alternative sources of water for food production and agricultural development. Recycled water (RW) is one of the most available water resources with great potential for use in farm irrigation. This experiment was carried out to investigate the use of RW as the irrigation source and its application method, subsurface leaky irrigation (SLI) system or surface irrigation, in an orchard with young olive trees in central Iran. The results revealed that the SLI system was able to enhance tree growth, leaf area, maximum fluorescence (Fv/Fm) and photosynthesis rate by 68%, 26%, 4%, and 42%, respectively. In addition, trees irrigated with the SLI system using RW exhibited increased N and Mg uptakes by 138% and 8%, respectively. Plants irrigated with RW showed a growth improvement (42%), leaf area (26%), and photosynthesis rate (23.4%) compared with those irrigated with clean water. Furthermore, Mg, Na, K, P, and N content increased by 12%, 59%, 30%, 7%, and 92%, respectively, in leaf tissue when RW was applied. The results indicated that RW could be employed as a reliable irrigation source especially when it was delivered with the SLI system.

INTRODUCTION

Water shortage in (semi-)arid regions and its declining availability to a critical level on a global scale dictates the reliance of sustainable and increasing agricultural production on alternative water resources for irrigation (Nirit et al. 2006). Recycled water (RW) is an important alternative source of water for irrigation, which will increase with the growing population and the increasing demand for freshwater (Hassanli et al. 2008). Reusing RW in irrigation has been widely practiced in many countries, especially in (semi-)arid regions. Increasing water reuse as a result of the growing urbanization in Iran, which is located in a dry region, can be considered as the optimal use of RW to overcome the present water crisis. Moreover, urban wastewater can be used not only as a source of irrigation water but also as a source of nutrients for plants, thanks to its large supply of nutrients and organic matter (Meli et al. 2002; Ratton et al. 2005; Singh & Bhati 2005; Aghabarati et al. 2008; Tak et al. 2013). Because water shortage is a limiting factor for increased agricultural production and green-space development, water reuse may be a useful remedy.

When water supplies are restricted, improving irrigation methods to meet water demand becomes important. Malash et al. (2005) reported that drip irrigation has a higher efficiency in irrigating different tomato varieties than the traditional methods such as flooding irrigation. Al-Omran et al. (2010) observed that drip irrigation using saline water would be more useful with more frequent irrigations in order to reduce salt concentration in the root zone. Although drip irrigation is considered to be suitable for RW reuse, its efficiency is mainly limited by emitter clogging (Capra & Scicolone 2004). This limitation justifies the application of other irrigation systems such as subsurface leakage systems.

Olive (Olea europaea L.) is an evergreen tree grown for both oil production for human consumption and for processing as table olives (Fernandez-Escobar et al. 2006) with a high degree of drought tolerance. This species can adapt itself to water scarcity through modification of its leaf area as well as its morphological, anatomical, and physiological characteristics (Bacelar et al. 2006). Olive is one of the most important crops among the horticultural species in Iran. Since almost all the orchards in Iran, including olive ones, are irrigated with fresh water, no information is available about the effects of RW on the growth, production, and quality of olives.

The objectives of this study were: (1) to assess the nutritional value of RW and its usage possibility in olive orchard irrigation; and (2) to evaluate the effects of subsurface leakage irrigation system for RW application and its effect on young olive trees establishment and growth.

MATERIALS AND METHODS

This experiment was conducted in the Department of Horticulture, Isfahan University of Technology, Isfahan (51°W, 31°N; Altitude 1600 m), Iran, during 2010–2012, using an orchard with a sandy-clay soil (pH = 7.3, EC = 2.3 ds/m). The climate of the area is arid with cold winters. Average annual rainfall and average annual maximum temperature are 122.8 mm and 23.4°C, respectively. The experiment was conducted using a split-plot experimental design with two factors and four replicates. The treatments included: irrigation systems – subsurface leaky irrigation (SLI), or surface irrigation (SI); and water quality – RW, or clean water (CW). Young olive trees (Olea europaea L., cv Roghani) (1 year old) were planted at a spacing of 4.0 m by 4.0 m. The trees were irrigated for two consecutive years based on crop evapotranspiration rate (ETc, mm) of about 173.4 L/mo per tree (data not shown). The characteristics of the urban RW used in the experiment are shown in Table 1.

Table 1

Chemical characteristics of the RW and CW used for the irrigation of the young olive trees in this study

Constituent (unit) RW CW 
K (mg l−1136.50 – 
Na (mg l−1118.80 1.20 
P (mg l−11.80 – 
NO3-N (mg l−11.68 – 
SO42– (mg l−1108.96 – 
CaCO3 (mg l−1149.45 – 
pH 8.62 7.40 
EC (dS m−11.00 0.59 
Constituent (unit) RW CW 
K (mg l−1136.50 – 
Na (mg l−1118.80 1.20 
P (mg l−11.80 – 
NO3-N (mg l−11.68 – 
SO42– (mg l−1108.96 – 
CaCO3 (mg l−1149.45 – 
pH 8.62 7.40 
EC (dS m−11.00 0.59 

Water was distributed in the soil through a 10.16 cm diameter drainage pipe in the SLI system. The tubes were wrapped in a soft polyester yarn to prevent clogging with soil. The drainage pipes (Ahwaz Pipe Mills Co., Ahwaz, Iran) were installed at a depth of 30–40 cm below the soil surface, and 40 cm away from each row of trees. Tube length in the SLI irrigation (1 m per tree) was designed based on the tree canopy using a linear arrangement. The vegetative growth was determined at the end of each month by measuring the difference in tree height at the end of each month.

The leaf area was determined using a leaf area meter DT-scan (Delta-Scan Version 2.03, Delta-T Devices Ltd, Burwell, Cambridge, UK). Plant chlorophyll content was measured with a spectrophotometer according to the method developed by Lichtenthaler (1987) and the free proline content was determined according to the method described by Bates et al. (1973).

Net photosynthesis rate (Pn), internal CO2, partial pressure (Ci), and stomatal conductance (gs) were measured in the morning (09:30–11:30) using a portable gas exchange system (Li-6400, LICOR, Lincoln, NE, USA) on the fully expanded leaves situated at the mid-canopy of the plants. The fluorescence measurements (Fv/Fm) were performed using a modulated fluorometer (RS232, Hansatech, Instruments Ltd, UK).

Leaf nutrient content measurements were carried out using the samples collected in July as the best time for diagnosing tree nutritional status (Fernandez-Escobar et al. 2006). After drying, ashing, and digestion in cloridric acid, the mineral content of leaves was determined. Na and K were determined using a flame photometer (PFP7, Jenway, UK). Zn and Mg concentrations were determined by atomic absorption photometry (Perkin Elmer AA3030. Norwalk, CT). The Kjeldahl procedure was used to determine total N (Bremner 1996). P was estimated by colorimetrical determinations using a spectrophotometer (UV-160, A, Shimadzu, Japan). The collected data were subjected to the analysis of variance using the SAS (9.1, SAS institute, 2004) statistical software. Statistical assessments of differences between mean values were performed by the least significant difference (LSD) test at P = 0.05.

RESULTS

Table 2 presents the effects of water quality on vegetative measurements and photosynthetic indices of olive trees. Clearly, no significant differences were observed between the 2 years of study for gs and the number of shoots; however, Pn, height and chlorophyll content showed higher values with RW application. There was no significant difference between the tree height and the leaf area between the RW and the CW in the first year, but they increased by 42% and 29%, respectively, with RW application in the second year. Chlorophyll content in plants receiving RW was higher by 28% and 12% in the first and second year, respectively. Photosynthetic rate was significantly higher (23.4 μmol m−2 s−1) in plants receiving RW compared to those receiving CW (Table 2).

Table 2

Vegetative and photosynthetic indices of young olive trees as influenced by water quality in the 2 years of study

Treatment Height (cm) Leaf area (mm2Chlorophyll ab. (mgg−1Number of shoots Pn (μmol.m−2s−1E (Mmol.m−2s−1gs (μmol.m2s−1Sugar (mg.g−1
2011 CW 12.2a 351.35a 2.1b 13.05a 18.3b 2.95b 0.21a 26.6a 
RW 10.56a 386.8a 2.71a 8.6a 23.6a 4.0a 0.26a 19.8b 
LSD   4.45 56.18 0.5 4.98 4.28 0.94 0.09 4.74 
2012 CW 14.7b 322.35b 1.30b 22.6a 17.57b 4.11a 0.23a 16.45a 
RW 20.9a 418.36a 1.46a 22.8a 23.4a 4.72a 0.28a 17.12a 
LSD   6.01 95.1 0.11 2.8 5.6 1.55 0.11 3.2 
Treatment Height (cm) Leaf area (mm2Chlorophyll ab. (mgg−1Number of shoots Pn (μmol.m−2s−1E (Mmol.m−2s−1gs (μmol.m2s−1Sugar (mg.g−1
2011 CW 12.2a 351.35a 2.1b 13.05a 18.3b 2.95b 0.21a 26.6a 
RW 10.56a 386.8a 2.71a 8.6a 23.6a 4.0a 0.26a 19.8b 
LSD   4.45 56.18 0.5 4.98 4.28 0.94 0.09 4.74 
2012 CW 14.7b 322.35b 1.30b 22.6a 17.57b 4.11a 0.23a 16.45a 
RW 20.9a 418.36a 1.46a 22.8a 23.4a 4.72a 0.28a 17.12a 
LSD   6.01 95.1 0.11 2.8 5.6 1.55 0.11 3.2 

Note: Values with the same letter are not significantly different at 5% probability level according to the LSD test.

It can be observed in Table 3 that, except for the number of shoots, neither the Pn indices nor the growth parameter exhibited any significant differences in the first year between the two treatments. One year after the initiation of the irrigation treatments, the plants irrigated with the SLI system exhibited remarkable increases in their photosynthetic indices and growth rates (Table 3). Statistically significant differences were also observed in the photosynthetic and vegetative indices between plants irrigated with the subsurface irrigation system and those irrigated with the surface system. The results indicated that, compared with the SI system, the SLI system increased Pn, leaf area, height, and number of shoots by 42%, 26%, 68% and 19%, respectively. Sugar content in the SI system, however, was 47% higher compared to that of the SLI system (Table 3).

Table 3

Vegetative and photosynthetic indices of olive trees influenced by irrigation system in 2 years of study

Treatment Height (cm) Leaf area (mm2Chlorophyll ab. (mg.g−1Number of shoots Pn (μmol.m−2s−1E (Mmol.m−2s−1gs (μmol.m2s−1Sugar (mg.g−1
2011 SI 11.75a 379.22a 2.34a 6.7b 23.21a 3.55a 0.218a 23.33a 
SLI 11.03a 359a 2.53a 14.9a 18.77a 3.46a 0.259a 23.15a 
LSD   9.9 50.95 0.92 6.61 6.42 1.96 0.099 10.12 
2012 SI 13.26b 327.5b 1.36b 20.9b 16.9b 3.92b 0.21b 20.03a 
SLI 22.34a 413.2a 1.47a 24.93a 24a 4.92a 0.30a 13.6b 
LSD  2.94 48.19 0.11 2.16 4.9 0.61 0.06 5.85 
Treatment Height (cm) Leaf area (mm2Chlorophyll ab. (mg.g−1Number of shoots Pn (μmol.m−2s−1E (Mmol.m−2s−1gs (μmol.m2s−1Sugar (mg.g−1
2011 SI 11.75a 379.22a 2.34a 6.7b 23.21a 3.55a 0.218a 23.33a 
SLI 11.03a 359a 2.53a 14.9a 18.77a 3.46a 0.259a 23.15a 
LSD   9.9 50.95 0.92 6.61 6.42 1.96 0.099 10.12 
2012 SI 13.26b 327.5b 1.36b 20.9b 16.9b 3.92b 0.21b 20.03a 
SLI 22.34a 413.2a 1.47a 24.93a 24a 4.92a 0.30a 13.6b 
LSD  2.94 48.19 0.11 2.16 4.9 0.61 0.06 5.85 

Note: Values with the same letter are not significantly different at 5% probability level according to the LSD test.

Figure 1 shows the effects of irrigation and water quality treatments on the vegetative growth. Clearly, no significant differences were observed between the two treatments with respect to the tree growth in the first year, but using RW increased the vegetative growth by 26% in the SLI and by 20% in the SI treatments at the end of the second year. In addition, the SLI system using RW led to a better growth rate (54%) than did the SI system (Figure 1).
Figure 1

Growth in young olive trees during the experimental period.

Figure 1

Growth in young olive trees during the experimental period.

Using RW for subsurface irrigation increased Na, Mg, and N contents in the leaf tissue. As shown in Table 4, irrigation with RW, as compared with CW, enhanced N and Mg contents in leaf tissue by 138% and 8%, respectively, in the SLI treatment and by 38% and 12%, respectively, in the SI system (Table 4).

Table 4

Nutritional content of olive trees influenced by water quality and irrigation system

Treatment Proline (μmol g−1Na (mg.g−1Zn (mg.g−1K (mg.g−1P (mg.g−1Mg (mg.g−1N (mg.g−1
SI CW 0.31a 1.07c 0.8b 2.71ab 2.32b 2.1b 1.01b 
RW 0.34a 1.49b 0.14a 3.39a 2.63a 2.37ab 1.43b 
SLI CW 0.21b 1.01c 0.1b 2.49b 2.76a 2.32ab 1.05b 
RW 0.30a 1.84a 0.14a 3.37ab 2.85a 2.51a 2.52a 
LSD  0.085 0.308 0.030 0.807 0.309 0.285 0.503 
Treatment Proline (μmol g−1Na (mg.g−1Zn (mg.g−1K (mg.g−1P (mg.g−1Mg (mg.g−1N (mg.g−1
SI CW 0.31a 1.07c 0.8b 2.71ab 2.32b 2.1b 1.01b 
RW 0.34a 1.49b 0.14a 3.39a 2.63a 2.37ab 1.43b 
SLI CW 0.21b 1.01c 0.1b 2.49b 2.76a 2.32ab 1.05b 
RW 0.30a 1.84a 0.14a 3.37ab 2.85a 2.51a 2.52a 
LSD  0.085 0.308 0.030 0.807 0.309 0.285 0.503 

Note: Values with the same letter are not significantly different at 5% probability level, according to the LSD test.

Leaf content of Zn, K, and Na showed no significant differences between the irrigation treatments; Mg, P, and N, however, increased by 9%, 12%, and 45%, respectively, in the subsurface irrigation system over the 24 months of the experiment. Proline content increased significantly in subsurface leakage irrigation throughout the experiment period. K and Zn concentrations in the leaf tissue remained unaffected by irrigation system treatments (Table 5). A tendency was observed for an increasing Fv/Fm ratio (maximum fluorescence) in plants irrigated with the SLI system compared to the SI system (Table 5).

Table 5

Nutrient content in olive leaf as affected by the irrigation systems

Treatment Fv/Fm Proline (μmol.g−1Na (mg.g−1Zn (mg.g−1K (mg.g−1P (mg.g−1Mg (mg.g−1N (mg.g−1
SI 0.722b 0.32a 1.28a 0.115a 2.93a 2.48b 2.23b 1.22b 
SLI 0.75a 0.25b 1.42a 0.127a 3.05a 2.80a 2.45a 1.78a 
LSD 0.02 0.06 0.678 0.021 0.571 0.219 0.201 0.355 
Treatment Fv/Fm Proline (μmol.g−1Na (mg.g−1Zn (mg.g−1K (mg.g−1P (mg.g−1Mg (mg.g−1N (mg.g−1
SI 0.722b 0.32a 1.28a 0.115a 2.93a 2.48b 2.23b 1.22b 
SLI 0.75a 0.25b 1.42a 0.127a 3.05a 2.80a 2.45a 1.78a 
LSD 0.02 0.06 0.678 0.021 0.571 0.219 0.201 0.355 

Note: Values with the same letter are not significantly different at 5% probability level, according to the LSD test.

RW significantly increased proline, Na, Mg, N, and P concentrations in the leaves of the olive trees compared with CW. Irrigation with RW for 24 months enhanced Mg, Na, K, P, and N concentrations in the leaf tissue by 12%, 59%, 30%, 7%, and 92%, respectively, as compared with CW. Moreover, trees irrigated with RW exhibited an enhanced Fv/Fm intensity by 4% (Table 6). Finally, the proline content of leaf tissue increased (23%) in plants irrigated for 24 months with RW compared to those irrigated with CW.

Table 6

Comparison of the accumulation of nutrient element in olive leaf in water quality treatments

Treatment Fv/Fm Proline (μmol.g−1Na (mg.g−1Zn (mg.g−1K (mg.g−1P (mg.g−1Mg (mg.g−1N (mg.g−1
CW 0.72b 0.26b 1.04b 0.127a 2.6b 2.54b 2.21b 1.03b 
RW 0.75a 0.32a 1.66a 0.115a 3.38a 2.74a 2.47a 1.98a 
LSD 0.03 0.05 0.49 0.015 0.686 0.094 0.196 0.599 
Treatment Fv/Fm Proline (μmol.g−1Na (mg.g−1Zn (mg.g−1K (mg.g−1P (mg.g−1Mg (mg.g−1N (mg.g−1
CW 0.72b 0.26b 1.04b 0.127a 2.6b 2.54b 2.21b 1.03b 
RW 0.75a 0.32a 1.66a 0.115a 3.38a 2.74a 2.47a 1.98a 
LSD 0.03 0.05 0.49 0.015 0.686 0.094 0.196 0.599 

Note: Values with the same letter are not significantly different at 5% probability level, according to LSD test.

DISCUSSION

In this experiment, the growth improved in young olive trees receiving RW, indicating the positive effect of RW on the plant growth and photosynthetic rate. Generally speaking, RW can be a rich source of nutrients required for plant growth (Toze 2006). Our results are confirmed by those of Aghabarati et al. (2008) who reported that the application of municipal RW led to better growth in olive trees. The enhanced growth may be attributed to the availability of nutrients and the adequacy of the water available in soil for meeting plant water demand. Leaf area was higher in the SLI system with RW application. Large leaf area is of great importance for light interception and Pn in plants, leading to enhanced photosynthetic rate and growth. A decline in leaf area, however, is considered as one of the plant resistance mechanisms under water shortage (Guarnaschelli et al. 2003).

The application of wastewater is recommended for orchard irrigation as it is a rich source of N, P and K that contribute to enhanced leaf area and biomass production (Guo & Sims 2000).

Some researchers have suggested a direct relationship between pigment concentration, which is, in turn, directly related to photosynthetic activity, and environmental factors. Herteman et al. (2011) reported that the application of RW enhances chlorophyll pigment concentration in mangrove leaves. N and Mg are known to be the nutrients that are necessary for the synthesis of chlorophyll (Suntoro 2002). Availability of these nutrients can increase the chlorophyll content on which the photosynthetic rate directly depends (Suharja & Suntoro 2009). Chlorophyll content and photosynthetic rates have been found to have a correlation. There are some reports indicating that an increase in available N increased photosynthetic rates for different mangrove trees (Li et al. 2008; Herteman et al. 2011). Wastewater application reportedly led to enhanced photosynthetic rates, stomatal conductance (gs), and vegetative growth in chickpeas because of the dual effect of wastewater, it is not only a valuable source of water but also a rich source of nutrients (Tak et al. 2013; Ashrafi et al. 2015). Enhanced concentration and uptake of these nutrients by plants through irrigation with RW were the major factors in enhanced chlorophyll content, photosynthetic rate, leaf area, and plant height.

Application of wastewater, compared with CW, significantly increased N, P, K, Mg, and Na concentrations in olive trees. This result is in agreement with the findings of Singh & Bhati (2005) and Aghabarati et al. (2008) who observed higher concentrations of minerals in the leaves of Dalbergia sissoo and olive trees irrigated with wastewater. Increased concentrations of N, P, and K in the RW leads to the accumulation of nutrients in soil and makes them readily available to plants.

Chlorophyll fluorescence (Fv/Fm) ratio is widely used to detect stress conditions and to determine the photosynthetic performance of the plant. The Fv/Fm ratio is a useful guide for the evaluation of maximum quantum efficiency of PSII. It has been widely used to discover stress-induced perturbations in the photosynthetic device (Baker & Eva 2004). Souza et al. (2004) showed that a decrease in maximum quantum yield of PSII (Fv/Fm) was observed at an advanced phase of stress. Our result is in agreement with these findings.

Some of the differences observed in tree performance are directly related to differences in the irrigation efficiency of different irrigation systems. Trees irrigated by surface dripping have been observed to be significantly larger with higher yields than those irrigated by other systems when the same amount of water was applied (David et al. 2003). Subsurface drip systems may further improve upon the efficiencies of irrigation and fertilizer application because water and nutrients are supplied directly to the root zone (Camp 1998). Interaction effect of recycle water and SLI resulted in slower growth in the first year. Probably, this stems from the fact that trees need to develop their root systems and adapted to the SLI system to take advantage of the water and nutrients available in the SLI system and RW.

Photosynthetic indices decline is one of the initial harmful effects of water shortage (Lawlor & Cornic 2002). Lower Pn and Ci under SI compared with the SLI system in the present study could be possibly due to water shortage. Previous studies reported different Pn rate under different irrigation treatments in the rice plant (Zhou et al. 2003; Dass & Chandra 2013).

The SLI system used in the present study was found to have the potential to save irrigation water by creating more favorable conditions in the root zone and to improve water availability by reducing evaporation. These results are in agreement with those reported in Al-Omran et al. (2005), Al-Omran et al. (2010) and Yang et al. (2011).

Sakellariou-Makrantonaki et al. (2007) reported larger leaf area indices in plants irrigated with surface and subsurface drip irrigation methods compared with those irrigated by the other methods.

Soluble sugars accumulation increased in the leaves of young olive trees under the SI system. It is commonly observed that water stress conditions induce an accumulation of soluble sugars in leaves that may act as compatible solutes while they are sources of carbon for plant growth and maintenance (Chaves et al. 2002).

Based on the results of the present study, it may be concluded that trees exhibited enhanced growth rates when irrigated with RW. This is mostly due to the high nutrient concentrations in RW. Also, the subsurface leakage method outperforms the surface method in improving plant growth index and photosynthetic rates in young olive trees in central Iran. Further in-depth study is, therefore, suggested for a deeper understanding of RW used for orchard irrigation.

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