Biofortifying sunflower with Zinc (Zn) aminochelates: a promising strategy for calcareous soil conditions

Zinc (Zn) deficiency in calcareous soils limits crop productivity and restricts nutritional quality in edible plant products. This study assessed the effectiveness of Zn–aminochelates, compared with conventional ZnSO₄, applied through various application methods, in improving physiological traits, growth, yield, and seed Zn biofortification in sunflower (Helianthus annuus L.) under calcareous soil conditions. A field experiment was conducted using a randomized complete block design with three replications. Zn(glycine)₂ [Zn(Gly)₂], Zn(methionine)₂ [Zn(Met)₂], and ZnSO₄ were applied through seed priming (2% solution), fertigation (4 L ha⁻¹ for Zn-aminochelates and 40 kg ha⁻¹ for ZnSO₄), and foliar spray (0.2% w/v, 2 L ha⁻¹ per application) at key growth stages. Control plots received untreated seeds or water. Compared with ZnSO₄, Zn–aminochelates significantly improved seed yield, biomass production, and Zn accumulation. Foliar application of [Zn(Met)₂] was the most effective treatment, increasing seed yield by approximately 85%, oil yield by about 2.5-fold, and seed Zn uptake by nearly twofold, while also enhancing physiological traits, antioxidant enzyme activities, and protein accumulation. The N/S ratio decreased concomitantly with increased seed oil content. The superior performance of methionine-based Zn chelates is likely linked to their role in sulfur metabolism, enhanced Zn mobility, and improved antioxidant regulation. Overall, foliar application of Zn–aminochelates represents an effective and practical agronomic biofortification strategy for improving productivity and nutritional quality of sunflower grown on calcareous soils.