Citrulline Modulates the Antioxidant Machinery of Two Faba Bean ( Vicia faba ) Cultivars: A Sustainable Biostimulant Strategy for Improving Crop Growth and Productivity

Citrulline (CIT), a natural non-protein amino acid and nitric oxide (NO) precursor, plays a vital role in plant physiological regulation. Its use as an eco-friendly biostimulant aligns with global efforts to reduce dependence on synthetic agrochemicals and strengthen sustainable crop production systems. This study represents the first report investigating the effects of exogenous citrulline (0, 0.5, and 1 mM) as a biostimulant/eustressor in two faba bean ( Vicia faba ) cultivars (Giza 843 and Sakha 1) and the first report to evaluate the variety and dose-dependent responses to foliar CIT application. The morphological, biochemical, and antioxidant responses were assessed. CIT significantly improved several growth and yield attributes in a cultivar-dependent manner, with Giza 843 performing best at 1 mM and Sakha 1 showing optimal shoot performance at 0.5 mM. CIT increased H 2 O 2 levels, flavonoids, and catalase activity, which modulate the response mechanisms of treated plants of two varieties of faba bean. In contrast to Giza 843, Sakha 1 increased proline and the activities of peroxidase and ascorbate peroxidase, which is parallel with decreasing soluble sugars and proteins in response to CIT application. These results showed that Sakha 1 had more effective defense mechanisms than Giza 843. These findings demonstrate that CIT at an optimal dose is a promising, eco-friendly biostimulant. It may be suitable to integrate into sustainable crop management programs to enhance crop resilience and productivity.