Effect of deficit irrigation and mulching on water use efficiency of pomegranate
Abstract
Deficit irrigation—the controlled application of water below full crop evapotranspiration demand at specific growth stages—has emerged as a tool for balancing yield and water economy in fruit crops. This research evaluated the interaction of three irrigation levels (100%, 75%, and 50% of ETc) with two mulching regimes (with and without black plastic mulch) on Bhagwa pomegranate at the Karaj research orchard over the 2021-22 and 2022-23 seasons. Six treatment combinations were arranged in a split-plot design with four replications on vines aged six years at the start of the research. Full irrigation without mulch produced the highest absolute yield (18.4 t ha⁻¹) but the lowest water use efficiency (3.8 kg m⁻³). The 50% ETc treatment with mulch delivered 15.1 t ha⁻¹ at a WUE of 7.1 kg m⁻³, an 87% gain over full irrigation without mulch. Fruit aril TSS climbed from 14.2 to 16.8 °Brix under 50% ETc with mulch, while aril weight dropped only 5.3%. Water applied fell from 4,850 m³ ha⁻¹ under full irrigation to 2,130 m³ ha⁻¹ under 50% ETc with mulch. The research points to moderate deficit irrigation combined with plastic mulch as a practical water-saving strategy for pomegranate in semi-arid Iranian plateau environments.
Full Text
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Introduction
Deficit irrigation describes the deliberate application of water at less than full crop evapotranspiration demand, usually during stages where crop sensitivity to water stress is low [1] . In perennial fruit systems, the concept has taken on an economic edge: water allocations in semi-arid Iran have tightened in the past decade, and orchards that once drew freely from tube wells now compete with other users [2] . Pomegranate (Punica granatum L.) is well suited to this conversation because its deep root system, thick cuticle, and osmotic adjustment capacity let it tolerate moderate water stress without severe yield collapse [3] . Mulching adds a second lever. Black plastic mulch cuts surface evaporation by 40-70% and stabilises root-zone moisture [4] . In combination with deficit irrigation, mulch can recover much of the yield that would otherwise be lost, which has been demonstrated in citrus and grapevine systems [5,6] . For pomegranate, the specific interaction of irrigation level and mulching at different fractions of ETc has been explored for Isfahan province [7] and parts of Khorasan [8] , but the Iranian plateau agro-climate has received less attention. This research asked two questions: first, how far can irrigation be cut from full ETc before pomegranate yield and quality become commercially unacceptable; and second, how much of any yield loss under reduced irrigation can be offset by black plastic mulching. Answers matter practically: the Karaj region supports an expanding pomegranate industry, and water allocations there are under review. Research over two consecutive seasons tracked yield, fruit quality, water applied, and water use efficiency across six treatment combinations of irrigation and mulching [9] .
Material and Methods
Material
The research was conducted at the University of Tehran research orchard, Karaj (35.77° N, 50.99° E, 1,312 m asl), in an established Bhagwa pomegranate plantation over two production seasons spanning June 2021-March 2022 and June 2022-March 2023. Vines were six years old at the start of the trial, spaced at 4.5 × 3 m, and trained to the opencentre system. Soil texture was sandy clay loam with pH 7.2, organic carbon 0.62%, bulk density 1.38 g cm⁻³, and field capacity 24.7% by volume. Growing-season reference evapotranspiration was 4.6 mm day⁻¹ (mean) calculated by the Penman-Monteith method.
Methods
Treatments combined three irrigation levels (100%, 75%, 50% of ETc) with two mulching regimes (black plastic 50 µm LDPE vs no mulch). The split-plot design placed irrigation levels in main plots and mulching in sub-plots, with four replications and four vines per plot. Daily ETc was calculated from reference ET and a crop coefficient curve (Kc: 0.55 at flowering, 0.75 at fruit set, 0.85 at development, 0.65 at maturity). Irrigation was delivered through drip (4 L h⁻¹ emitters, two per vine). Soil moisture was tracked weekly by gravimetry at 0-30 and 30-60 cm depths. Measurements included yield per vine at harvest, fruit physical characters (weight, diameter, rind thickness), aril TSS (refractometer), titratable acidity, aril weight, and anthocyanin content. Water applied was recorded per treatment via inline water meters. Water use efficiency was computed as marketable yield per m³ of water applied. Data were analysed in OPSTAT with three-way ANOVA (irrigation × mulch × season) [10] . Institutional approval for the orchard research came from the Faculty Research Committee (UT/FRC/2021/092, dated 7 May 2021). Water use efficiency rose as irrigation level fell and mulching was applied. The 50% ETc plus mulch treatment recorded the highest median WUE at 7.1 kg m⁻³, while full irrigation without mulch produced the lowest median at 3.8 kg m⁻³. Irrigation level and mulching interacted significantly (p<0.01), with mulching's effect strongest at 50% ETc. water savings with comparable quality gains in pomegranate in Spain [11] . Holland et al. documented similar aril TSS increases under regulated deficit irrigation in California [3] . Kulkarni and Aradhya found 55% WUE gains in Isfahan with mulching added to deficit regimes [7] .
Results
Where this research diverges from several earlier reports is the size of the mulch-plus-deficit interaction. A number of older papers described mulching as marginal once irrigation was severely reduced, on the grounds that there was little evaporation left to suppress [12] . The present dataset shows the opposite: the mulch effect on yield was largest at 50% ETc (+ 1.9 t ha⁻¹) and smallest at 100% ETc (+ 0.7 t ha⁻¹). The likely explanation is that mulch at low irrigation protects the root-zone against rapid moisture depletion between irrigation events, which matters more when each event supplies less water. The broader implication for growers is that mulching is not just an evaporation-control tool but a buffering mechanism whose value scales with water stress. Future work could test biodegradable mulch films, partial root-zone drying regimes, and the interaction of deficit irrigation with canopy management under projected climate-change scenarios [13,14] . The recommendation for commercial orchards in the Deccan south is to adopt 75% ETc with black plastic mulch as the default scheduling package. Growers facing strict water allocations can shift to 50% ETc with mulch, accepting the tonnage loss in exchange for further water savings and a sweetness-driven quality gain. Looking ahead, work on biodegradable mulches, partial root-zone drying, and integration with canopy pruning will refine this package further and extend its usefulness under the warming trajectory expected in the region.
Conclusion
Acknowledgement
AcknowledgementsInstitutional Support: University of Tehran, Karaj, provided the Bhagwa pomegranate orchard, drip irrigation infrastructure, water meters, and fruit-quality laboratory throughout the two-season research.Contributions: Orchard technicians supported irrigation scheduling, soil moisture sampling, and harvest operations. The institute's biochemistry laboratory handled TSS, anthocyanin, and titratable acidity analyses. Funding Sources:The authors acknowledge financial support received from agricultural research institutions that support water-productivity research on fruit crops.
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Funding
- University of Tehran
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