Abstract
Abstract The present study aimed to develop an eco-friendly and efficient approach for the synthesis of silver (AgNPs) and gold nanoparticles (AuNPs) using indole-3-acetic acid (IAA) as the sole reducing and stabilising agent and to evaluate their comparative effects on seed germination, growth and biochemical responses in broccoli (Brassica oleracea var. italica). Although numerous studies report green synthesis of nanoparticles using plant extracts, the direct use of phytohormones such as IAA for synthesis and subsequent evaluation on plant physiological and metabolic traits remains largely unexplored. This research addresses this gap by investigating nanoparticle-auxin interactions and their role in improving early seedling performance. Nanoparticles were synthesised using 1 mM IAA and characterised through UV–visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDX). The analyses confirmed the formation of spherical, crystalline nanoparticles with distinct surface plasmon resonance peaks at 417 nm for AgNPs and 565 nm for AuNPs. Broccoli seeds were treated with different concentrations (10–50 µg/mL) of IAA, IAA-AgNPs, and IAA-AuNPs to assess their effects on germination percentage, morphological parameters, photosynthetic pigments, and secondary metabolites including phenols, tannins, and saponins after seven days of sowing. The results revealed concentration-dependent responses. Remarkably, IAA enhanced photosynthetic pigments, particularly chlorophyll a and chlorophyll b at 30 µg/mL, whereas AgNPs promoted root elongation at 20 µg/mL. AuNPs significantly increased secondary metabolites such as phenols, tannins, and saponins at 20 µg/mL, indicating stimulation of defence-related metabolic pathways. Overall, AgNPs promoted root development, AuNPs enhanced secondary metabolism and IAA supported photosynthesis. The study concludes that IAA-functionalized nanoparticles can differentially modulate growth and metabolism in seedlings and may serve as innovative tools for sustainable agriculture and crop improvement. The novelty of this study lies in the use of IAA as a dual-function reducing and capping agent in nanoparticle biosynthesis. Graphical abstract