Effect of recycled water applied by surface and subsurface irrigation on the growth, photosynthetic indices and nutrient content of young olive trees in central Iran

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 ﬂ uorescence (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. ). 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. ). 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   Net photosynthesis rate (Pn), internal CO 2 , partial pressure (C i ), and stomatal conductance (g s ) 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.
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.

).
After drying, ashing, and digestion in cloridric acid, the mineral content of leaves was determined. Na and K  Table 2 presents the effects of water quality on vegetative measurements and photosynthetic indices of olive trees.

RESULTS
Clearly, no significant differences were observed between the 2 years of study for g s and the number of shoots; however, P n , 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 À2 s À1 ) in plants receiving RW compared to those receiving CW (Table 2).
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 P n , 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). 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).
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).
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).
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. 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 ).    Dass & Chandra ).

DISCUSSION
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. 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.