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Abstract To investigate the effect and mechanism of chitosan nanoparticles (CSNPs) on the germination and seedling growth of wheat (Triticum aestivum L.), we conducted systematic research on the impact of different concentrations (1–100 μg/mL) of CSNPs and chitosan (CS). The result of energy-dispersive spectroscopy (EDS) and confocal laser scanning microscopy (CLSM) showed that adsorption of CSNPs on the surface of wheat seeds was higher than that of CS. CSNPs had growth promoting effect at a lower concentration (5 μg/mL) compared with CS (50 μg/mL). In addition, the application of 5 μg/mL CSNPs induced the auxin-related gene expression, accelerated indole-3-acetic acid (IAA) biosynthesis and transport, and reduced IAA oxidase activity resulting in the increase of IAA concentration in wheat shoots and roots. The results suggest that CSNPs have positive effect on seed germination and seedling growth of wheat at a lower concentration than CS due to higher adsorption on the surface of wheat seeds. Highlights • CSNPs could promote growth at a lower concentration about 10 fold than CS. • CSNPs promote wheat growth by activating IAA signaling pathway. • The CSNPs adsorption is stronger than that of CS on the surface of wheat seed by EDS and CLSM. [ABSTRACT FROM AUTHOR]

• Association between quaternary chitosan and alginate were regulated via hydration. • Less hydrated counter anion Cl− bound more strongly to positive groups on polymer. • There was a pH transition point of thermal change of association. • Solution's pH had more effect on the hydration of polymers than counter ions. Studies on interactions between oppositely charged polysaccharides have gathered great interest. We proposed that the association between oppositely charged polymers could be regulated via hydration. A comparison study was carried out by using quaternary chitosan with different counterions(Cl−, Ac−, OH−) and sodium alginate. The results showed that the association between quaternary chitosan with less hydrated counter anion Cl− and sodium alginate was weaker than that between quaternary chitosan with more hydrated counter anion Ac− and sodium alginate. There was a pH transition point of thermal change of association between oppositely charged polymers, as the solution's pH had more effect on the hydration of polymers than counter ions. Further studies showed that a fraction of Cl− was still attracted by polycation in the complex and competed with the interaction of polyanion after complexation. The competitive combination was critical for the property (such as self healing behavior) of the carbohydrate polymer complex. [ABSTRACT FROM AUTHOR]