Impact of deficit irrigation on grain quality attributes and milling recovery of aromatic rice

Introduction

The global aromatic rice market exceeds 12 million tonnes annually, driven by consumer preference for the characteristic 2-acetyl-1-pyrroline (2-AP) aroma compound [1,2] . Paradoxically, mild water stress during grain filling can concentrate 2-AP and improve cooking quality, yet excessive stress reduces grain dimensions, increases chalkiness, and lowers milling recovery the proportion of whole kernels after dehusking and polishing [3,4] . Identifying the deficit threshold that enhances quality without unacceptable milling losses is therefore a key challenge for water-scarce aromatic rice production. Deficit irrigation (DI) strategies that supply less than full crop water demand have been widely studied for yield effects, but quality-focused evaluations are rare, particularly under controlled conditions that isolate water deficit from confounding field variables [5,6,7] . This research imposed four precisely controlled irrigation levels in weighing lysimeters to quantify the doseresponse relationship between water deficit and each major grain quality parameter, testing the hypothesis that moderate deficit improves quality while severe deficit degrades it.

Materials and Methods

Study Area

Lysimeter experiments were conducted at the ETH Zurich Research Station Lindau-Eschikon (47°27′N, 8°41′E, 520 m a.s.l.) in temperature-controlled polytunnels simulating tropical lowland conditions (30/24 °C day/night, 75% RH, 12 h photoperiod). Twelve weighing lysimeters (1.2 m diameter, 0.8 m depth) were filled with a Fluvisol from the Rhine floodplain amended with 15% rice hull ash to approximate lowland paddy soil texture.

Irrigation treatments and water monitoring

Lysimeter weight was recorded at 15-minute intervals using precision load cells (±10 g). ETc was computed daily from weight loss, corrected for drainage. Irrigation was applied via drip emitters calibrated to deliver 100, 80, 60, and 40% of the previous day's ETc from panicle initiation to physiological maturity. Pre-treatment irrigation was uniform (100% ETc). Drainage water was collected and analysed for EC and pH.

Materials

Aromatic rice cultivar Basmati-370 was transplanted (25-day seedlings, 4 hills lysimeter⁻¹). Basal fertilisation: 100-50-50 kg NPK ha⁻¹ equivalent. Three replications per irrigation level in a completely randomised design (12 lysimeters total). Treatments were applied for two consecutive seasons (July-November 2021 and 2022).

Methods

At maturity: grain yield, 1000-grain weight, milling recovery (Satake test mill), head rice recovery, amylose (iodine colorimetry), protein (micro-Kjeldahl × 5.95), and aroma score (sensory panel of 8 trained assessors, 1-10 scale). Water productivity (WP) = milled rice yield/total irrigation applied. Data analysed by one-way ANOVA in R 4.2 with Tukey's HSD (p≤0.05).

Results

Fig 1:

Grain quality attributes and milling recovery of Basmati-370 across four deficit irrigation levels (two-season mean) Amylose content increased progressively from 22.4% at 100% ETc to 26.3% at 40% ETc (Figure 1, Table 1). Protein content followed a similar trend (7.2 to 9.4%). Aroma score peaked at 60% ETc (8.6) and declined sharply at 40% ETc (7.1), falling below the full-irrigation score of 7.8. Milling recovery decreased linearly from 68.4% to 54.6% across the deficit gradient. Water use partitioning shifted under deficit: transpiration's share of total water rose from 48.2% to 52.8% between 100% and 60% ETc, while evaporation fell from 28.4% to 24.1% (Figure 2). Water productivity for milled rice peaked at 60% ETc (0.72 kg m⁻³), 50% higher than full irrigation (0.48 kg m⁻³). At 40% ETc, WP declined to 0.58 kg m⁻³ because milling recovery losses outpaced water savings (Table 2). The 60% ETc treatment thus represented the optimal balance: quality improved, water saved 40%, and milling recovery remained above the 60% commercial threshold.

Discussion

The aroma peak at 60% ETc followed by decline at 40% ETc is consistent with the stress-concentration hypothesis: moderate deficit concentrates 2-AP precursors in developing grains, but severe stress disrupts the methylglyoxal biosynthetic pathway responsible for 2-AP synthesis [3,8,9] . The 19.4% protein increase under moderate deficit reflects both a concentration effect (smaller grain volume) and stress-induced upregulation of nitrogen remobilisation from vegetative tissue [10,11] . The linear decline in milling recovery with increasing deficit is driven by grain shrinkage and increased chalkiness, which make kernels more susceptible to breakage during polishing [4,12,13] . The 60% ETc threshold identified here agrees with field estimates from Punjab (55-65% ETc) and Myanmar (60-70% ETc), lending confidence that lysimeter results translate to practice [14,15,16] .

Limitations

Lysimeter conditions cannot fully replicate field-scale water dynamics, particularly lateral seepage and capillary rise from shallow water tables. Two growing seasons provide limited statistical power for detecting small genotype × environment interactions. Only one cultivar was tested; responses may differ across the Basmati and Jasmine aromatic groups.

Conclusion

Deficit irrigation at 60% ETc optimised the quality-water tradeoff for aromatic rice: aroma improved 10.3%, protein rose 19.4%, and milling recovery remained above commercial thresholds while saving 40% of irrigation water. Severe deficit (40% ETc) degraded aroma and reduced milling recovery below acceptable levels. Water productivity peaked at 0.72 kg m⁻³ under moderate deficit. These results provide a quantitative basis for irrigation scheduling in aromatic rice systems facing increasing water scarcity.

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