Abstract

Acidic soil stress can limit sunflower productivity by reducing nutrient availability, impairing root development, and affecting early plant establishment, while foliar nutrient application and seed pretreatment may provide additional support under recommended nitrogen fertilization. This study evaluated the potential of selected nutrient-based products to mitigate acidic environment stress in sunflower through a field experiment and a complementary laboratory germination test. The field experiment was conducted in 2025 at Ivankovo, Croatia, on acidic soil (pH KCl 4.47), under a recommended nitrogen fertilization dose of 85 kg N/ha for sunflower production. Foliar treatments included Barrier, Bioplex, Borealg, and Bor-feed, while the control was left without additional foliar treatment. Seed yield, yield components, seed oil and protein content, and oil and protein yield were determined. In parallel, sunflower seeds were pretreated with the same products and germinated under controlled conditions in aqueous solutions with pH values ranging from 3.5 to 8.5. Germination, seedling morphology, fresh biomass, total phenolic content, antioxidant activity, and free proline content were analysed. Under field conditions, foliar treatments significantly affected plant height, head diameter, seed mass per head, seed oil and protein content, and seed yield. Barrier produced the highest seed yield (5.3 t/ha), oil yield (2.8 t/ha), and protein yield (0.8 t/ha), whereas Bor-feed had the highest seed oil content (54.36%) but the lowest seed yield (2.5 t/ha). In the laboratory experiment, average total germination was 92%, with maximum values of 96–97% depending on treatment and pH. Barrier most strongly promoted root, shoot, and total seedling length, while Borealg increased root, shoot, and total fresh mass. Biochemical responses depended on seed pretreatment and pH, with the highest antioxidant activity recorded in Bioplex at pH 7.5 and higher proline accumulation generally observed in untreated seedlings. The highest antioxidant activity was recorded in Bioplex seed pretreatment at pH 7.5 (5.34 mM Fe(II)/g FW), while the highest free proline content was observed in the control at pH 5.5 (7.87 mM/g FW) and pH 3.5 (7.79 mM/g FW), indicating a stronger stress response in untreated seedlings. The present study indicates that selected foliar products and seed pretreatments may support sunflower productivity and early growth under acidic stress conditions, with Barrier showing the most consistent positive response.

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