From Seed to Leaf: Differential Modulation of Wheat Growth and Yield by Lignin-Encapsulated Auxin Depends on Delivery Route

Abstract Nanotechnology has emerged as a promising strategy to improve the efficiency and sustainability of agricultural inputs; however, biological responses to nanoformulations remain strongly influenced by operational factors, such as application method. Plant growth regulators (PGRs) are widely employed to improve crop performance, and indole-3-acetic acid (IAA) is one of the most important hormones regulating plant growth and development. Nanoencapsulation can enhance the stability and delivery efficiency of bioactive compounds, such as PGRs, by protecting them from degradation and enabling controlled release. Among the available nanocarriers, lignin nanoparticles are particularly attractive due to their biodegradability and suitability for controlled-delivery systems. Therefore, we evaluated how different application routes modulate the biological effects of IAA applied either in its free form or encapsulated in lignin nanoparticles during wheat development. Treatments were applied through seed treatment or foliar spraying, and plant responses were assessed across vegetative and reproductive stages. Our results demonstrate that the application method plays a decisive role in determining plant responses. While seed treatment with IAA-loaded lignin nanoparticles resulted in reduced vegetative and reproductive performance, foliar application of the same formulation significantly enhanced reproductive traits, including grain number per spike and other yield-related parameters, without inducing phytotoxic effects. Lignin nanoparticles were successfully visualized within leaf tissues, particularly in mesophyll and bundle sheath cells. These findings demonstrate that the biological behavior of lignin nanoparticles is not solely determined by their intrinsic physicochemical properties or chemical composition but is strongly influenced by the application strategy employed.