Influence of deficit irrigation scheduling on water use efficiency and oil content of sunflower (Helianthus annuus L.) hybrids

Introduction

Water scarcity is no longer a distant prospect for central European agriculture-summer precipitation on the Hungarian Great Plain has declined by 11% since 1990, and groundwater tables in Hajdú-Bihar county dropped by an average of 1.3 m between 2010 and 2022 [1] . Sunflower, Hungary's most important oilseed crop with roughly 620,000 hectares sown annually, demands 450-550 mm of water across its life cycle, and yield penalties from moisture deficit during flowering and grain fill can exceed 30% [2,3] . Deficit irrigation-deliberately applying less water than the crop's full demand-offers a middle ground between rainfed risk and the high cost of full irrigation [4] . Several studies in Mediterranean sunflower systems showed that cutting supply to 60-75% of ETc reduced yield by only 8-18% while boosting WUE by 40-70% [5] . An additional benefit sometimes observed is an increase in seed oil concentration under mild stress, attributed to a shift in carbon partitioning toward lipid synthesis when vegetative growth is curtailed [6] . However, the interaction between deficit level and hybrid genotype has been poorly characterised under continental-climate conditions [7] . This research aimed to: (a) quantify the yield-WUE trade-off across four irrigation levels and five hybrids, (b) determine the irrigation threshold below which oil content declines, and (c) identify the hybrid best suited to deficit management.

Soil and Climatic Conditions

The trial was established at the Agronomy Research Centre of the University of Debrecen (47°33′ N, 21°36′ E), on the eastern Hungarian Great Plain at 118 m elevation. Soil is a calcareous chernozem, silty clay loam, pH 7.2, organic carbon 2.31%, field capacity 34.8% (v/v). Growing-season rainfall (April-September 2023) totalled 247 mm, which was 18% below the 30-year mean. Mean temperatures were 12.8°C (April), 18.3°C (June), and 22.1°C (July). A 19-day dry spell occurred in late June during the star-bud to early-flowering window [8] .

Materials and Methods

Materials

Five commercial sunflower hybrids registered in the Hungarian National Variety List were used: Tutti (Syngenta), NK Ferti (Syngenta), PR64H42 (Pioneer), ES Generica (Euralis), and LG 5580 (Limagrain). All are mid-maturity (FAO 400-450) higholeic or linoleic types. Seeds were sown at 65,000 plants/ha. A basal fertiliser of 80-60-60 kg N-P₂O₅-K₂O/ha was applied before sowing. Drip irrigation was delivered through T-Tape lines with emitters at 30 cm spacing [9] .

Methods

A split-plot design with four replications assigned irrigation level to the main plot and hybrid to the subplot (80 plots, each 5 m × 4 m). Irrigation treatments were I1 (100% ETc), I2 (75% ETc), I3 (50% ETc), and I4 (25% ETc). ETc was calculated weekly from a nearby automatic weather station using the Penman-Monteith equation with FAO Kc values for sunflower [10] . Sowing took place on 18 April 2023. At physiological maturity (R9), 10 heads per subplot were hand-harvested. Seed yield was adjusted to 9% moisture, oil content measured by NMR (Bruker Minispec mq20), and WUE calculated as seed yield divided by total water input (irrigation + rainfall). Data were analysed by split-plot ANOVA in R (v4.3.1); means separated by Tukey's HSD at p < 0.05.

Seasonal Irrigation Volumes

Total water applied (irrigation + rainfall) was 547, 472, 397, and 322 mm for I1 through I4, respectively. The difference between I1 and I3 amounted to 150 mm saved per hectare, equivalent to 1,500 m³/ha over the season. Seed yield declined progressively with irrigation cutback, but the drop from I1 to I3 (14.7%) was modest relative to the 75.4% gain in WUE (Table 1). Oil content increased under deficit, peaking at I3 across all hybrids. At I4, however, oil content gains plateaued while yield losses steepened sharply. Tutti maintained the highest absolute yield across all irrigation levels, and its yield decline from I1 to I3 was the smallest (14.0%) among the five hybrids (Table 2).

Results

Discussion

The 75% increase in WUE when irrigation was halved confirms earlier Mediterranean findings that sunflower responds well to moderate deficit management [5] . Oil content rose under stress because reduced vegetative sink strength allows more photosynthate to flow into lipid biosynthesis within the achene [6] . The plateau at I4 (25% ETc) indicates that severe stress disrupts seed filling before the oil-enrichment mechanism can operate fully. Tutti's superior performance under deficit likely reflects its deeper root architecture and higher stomatal sensitivity, traits documented in Syngenta's varietal characterisation [9] . The 19-day dry spell during star-bud likely amplified treatment differences, since deficit plots had less stored moisture to buffer the gap [3,11] .

Conclusion

Irrigating sunflower at 50% ETc on the Hungarian Great Plain reduced seed yield by only 14.7% while nearly doubling WUE and raising oil content by 2.9 percentage points. The hybrid Tutti combined the highest oil content (41.3%) with the smallest yield penalty under deficit. Growers facing water restrictions can adopt the 50% ETc schedule without a major quality trade-off; dropping below 25% ETc, however, led to steep yield losses and should be avoided.

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