Biostimulants Affect Differently Biomass and Antioxidant Status of Onion (Allium cepa) Depending on Production Method

Onions, essential in various salads and cooked dishes, are sought after by producers for high yields, while consumers value their quality, particularly the presence of antioxidant compounds. This study investigates the impact of production methods and biostimulants on the biomass yield and quality of onions. The two-year experiment was conducted in Vojvodina, Serbia, and compared directly seeded (DS) and from-set (FS) onions with four biostimulant treatments: control (C), seaweed extracts (T1), humic and fulvic acids (T2), and Trichoderma sp. (T3). DS onions yielded significantly more biomass, while FS onions had higher dry matter content. DS onions treated with T1 showed a significant increase in phenols (↑ 5.30%), while T2 and T3 led to declines (↓ 8.66% and ↓ 7.55%, respectively). All biostimulants reduced phenol content in FS onions. T1 and T2 significantly increased the flavonoid concentration in DS onions, with no significant changes in FS onions. T1 enhanced antioxidant properties in DS onions and reduced them in FS onions. Additionally, T2 and T3 decreased antioxidant activity in both DS and FS onions, as evidenced by DPPH, FRAP, and ABTS tests. These findings guide onion production, advocating for the fresh consumption of DS onions with higher biomass and industrial processing suitability for FS onions, emphasizing the potential of bio-based products.