Comparative Effects of Partial Rootzone Drying and Deficit Irrigation on Water Use Efficiency, Economic Water Productivity, and Productive Traits of Mature Apricot Trees in a Semi-Arid Region

Technical Summary: Comparative Effects of Partial Rootzone Drying and Deficit Irrigation on Mature Apricot Trees

Problem Statement
In semi-arid regions like Iran, particularly the Urmia area, water scarcity is a critical constraint for agricultural production, contributing to the ecological degradation of Lake Urmia. While apricot (Prunus armeniaca L.) is a major crop in the region, its production relies heavily on irrigation. Traditional deficit irrigation (DI) strategies aim to conserve water but often result in significant yield reductions. Partial Rootzone Drying (PRD), a modified deficit irrigation technique, has shown promise in other fruit crops (e.g., grape, pear, pomegranate) to enhance water productivity without severely compromising yield. However, there is limited research on the application of PRD in mature apricot orchards, specifically regarding its physiological impacts, economic viability, and performance under varying levels of water stress compared to conventional DI.

Methodology
The study was conducted over two growing seasons (2015–2016) at the Kahriz Agricultural Research Station in Urmia, Iran. The experiment utilized a randomized complete block design with 30 uniform, 12-year-old apricot trees (cultivar 'Asgar-Abad'). Five irrigation treatments were applied:

1. 100%WW: Full root zone irrigation at 100% of evapotranspiration (ET).
2. 75%DI: Full root zone irrigation at 75% ET.
3. 75%PRD: Alternate wetting of two semicircular root zones at 75% ET.
4. 50%DI: Full root zone irrigation at 50% ET.
5. 50%PRD: Alternate wetting of two semicircular root zones at 50% ET.

In PRD treatments, basins were divided into two equal sections, and irrigation was alternated between them. Key parameters measured included vegetative traits (leaf length, width, area, relative water content), productive traits (yield, fruit weight, size), fruit quality (Titratable Acidity, Total Soluble Solids, pH), and economic indicators.

The study introduced a ranking system termed Economic Deficit Irrigation (EDI), based on four indices: Applied Water (W), Water Use Efficiency (WUE), Gross Margin (GM), and Economic Water Productivity (EWP). GM and EWP calculations incorporated variable costs for water, electricity, and labor (specifically for moving bubblers in PRD treatments), alongside three yield price scenarios.

Key Results

• Vegetative and Physiological Traits: Drought stress significantly reduced leaf relative water content (RWC), leaf area, and dimensions. The 100%WW treatment consistently showed the highest RWC (approx. 60–63%), while 50%PRD showed the lowest (approx. 43–46%). While RWC correlated positively with yield and leaf area, the study found no significant difference in leaf traits between PRD and DI treatments at the same water deficit levels, suggesting PRD did not significantly alter vegetative growth compared to conventional DI.
• Yield and Fruit Quality: Full irrigation (100%WW) produced the highest yields (16.17 and 16.80 Mg ha⁻¹ in 2015 and 2016, respectively) and the highest Gross Margin ($11,890 ha⁻¹). Both DI and PRD strategies reduced yield compared to full irrigation. Notably, under severe stress (50% ET), the PRD method (50%PRD) resulted in significantly lower yields than the DI method (50%DI) in the second year, leading to the lowest Gross Margin ($6,529 ha⁻¹) among all treatments. Regarding quality, water stress increased Total Soluble Solids (TSS) and the TSS/TA ratio but decreased fruit size. Contrary to findings in some other crops, PRD treatments resulted in lower TSS values compared to DI treatments, indicating no improvement in fruit quality under PRD in this specific context.
• Water Use Efficiency (WUE): WUE increased as water application decreased. The 50%PRD treatment achieved the highest WUE (4.03 kg m⁻³ in 2016). Under mild stress (75% ET), PRD (75%PRD) demonstrated higher WUE than DI (75%DI) without a significant yield penalty in the first year. However, under severe stress (50% ET), PRD did not improve WUE over DI and negatively impacted yield.
• Economic Analysis: The EDI ranking system revealed that 50%PRD achieved the highest overall rank (1st), followed by 50%DI and 75%PRD, while 75%DI received the lowest rank. Although 50%PRD achieved the highest Economic Water Productivity (EWP) and the top EDI rank due to its superior water efficiency, it generated the lowest Gross Margin ($6,529 ha⁻¹) compared to the full irrigation treatment ($11,890 ha⁻¹). The analysis indicated that while 50%PRD offered the best economic water productivity, its viability in terms of total profit depends on water and electricity costs; at current market prices, the full irrigation treatment (100%WW) yielded the highest Gross Margin.

Significance and Claims
The paper claims to provide actionable guidance for irrigation managers in semi-arid regions by demonstrating that PRD can improve Water Use Efficiency and Economic Water Productivity, particularly under mild to moderate water stress conditions (75% ET). The study concludes that while PRD is more efficient than DI at the 75% ET level, its application under severe stress (50% ET) may be counterproductive, leading to yield declines without corresponding efficiency gains compared to DI.

The authors introduce the Economic Deficit Irrigation (EDI) index as a novel tool for selecting optimal irrigation strategies by balancing agronomic performance with economic returns. The study modestly concludes that while DI and PRD are recommended to improve WUE in water-scarce areas, farmers should not expect high-quality fruits (in terms of size and sugar content) under these deficit regimes. The research highlights that the effectiveness of PRD is contingent on the severity of water stress and the specific physiological response of mature apricot trees, which may differ from other fruit species due to variations in abscisic acid (ABA) signaling and stomatal regulation.