Review of Amino Acid Foliar Application as a Stress Mitigation Strategy: Unraveling the Protective Mechanisms Against Abiotic Stress

Abstract

The foliar application of amino acids offers a promising strategy to mitigate multiple abiotic stresses—drought, salinity, heat and heavy metals—by enhancing plant physiological, biochemical and molecular responses. This review synthesises recent advances in understanding how external amino acids function as bio-stimulants, focusing on mechanisms such as osmotic regulation, antioxidant defence activation, photosynthetic protection, phytohormone signalling and ion homeostasis. In addition, we explore the environmental dimensions of amino acid foliar fertilisers, including their role in reducing chemical residues, improving soil microbial health, increasing nitrogen use efficiency and aligning with circular bioeconomy principles via resource valorisation. Current limitations—such as variable efficacy across species, low foliar uptake efficiencies and lack of standardised formulations—are critically assessed. Finally, we propose future research directions emphasising advanced delivery technologies, integrative multi-stress trials, life-cycle assessment and policy frameworks. This comprehensive treatment demonstrates not only the agronomic potential but also the sustainability implications of amino acid foliar fertilisers, contributing to the goals of climate-smart agriculture and environmental stewardship.