Amino acid biostimulant increases pine photosynthetic efficiency and growth through optimised mycobiome and nitrogen assimilation

Amino acid biostimulants have emerged as powerful alternatives to conventional inorganic nitrogen fertilisers, yet their potential in forestry species like radiata pine (Pinus radiata) remains largely unexplored. In this study, we reveal physiological mechanisms underlying the enhanced growth of radiata pine seedlings achieved by substituting traditional inorganic fertigation, either partially or fully, with amino-acid-based biostimulants. Amino-acid fertigation notably increased shoot biomass, plant height, and collar diameter. Critically, this approach reshaped the root fungal community, selectively enriching fungi with diverse ecological roles, including several taxa known for auxin production. These microbial shifts correlated directly with elevated auxin concentrations observed in needle tissues, providing a plausible mechanism for the enhanced growth. Additionally, amino-acid fertigation improved nitrogen assimilation, correlating positively with increased chlorophyll content and photosynthetic efficiency. Our findings highlight that transitioning to amino-acid biostimulants from inorganic sources not only enhances plant growth and nitrogen use but also promotes a beneficial root microbiome, thereby offering a sustainable pathway for nursery production of radiata pine.