Wettability of the Plant Growth Regulator 28-HB on Pepper Leaves at Different Developmental Stages

Studying the wettability of plant growth regulators on crop leaf surfaces is essential for enhancing crop yield. In this study, the wetting behavior of the plant growth regulator 28-homo-brassinolide (28-HB), supplemented with different surfactants, was investigated on the adaxial and abaxial surfaces of pepper leaves at the seedling, early flowering, and fruiting stages. The microstructure of the leaf surface was characterized using an ultra-depth field microscope. The surface free energy (SFE) of the leaves was calculated using the Owens-Wendt-Rabel-Kaelble (OWRK) method. Additionally, the surface tension of the 28-HB solutions containing various surfactants, as well as the contact angles on pepper leaves at different growth stages, were measured. The experimental results indicate that the surface free energy (SFE) of pepper leaves significantly decreases with plant maturation. Specifically, the SFE of the adaxial leaf surface declined from 43.4 mJ/m² at the seedling stage to 26.6 mJ/m² at the fruiting stage, while the abaxial surface decreased from 27.5 mJ/m² to 22.5 mJ/m². At all growth stages, the relative polar component (R_P) of the adaxial surface was consistently higher than that of the abaxial surface and showed a gradual decline from 94.70% to 57.34% as development progressed. The contact angle measurement showed that the addition of surfactant decreased the contact angle of 28-HB on the leaf surface and increased the wetting area. Among the tested formulations, the addition of fatty alcohol ethoxylates (AEO-9) significantly reduced the contact angle to below 45°, and resulted in an adhesion tension below 30 mN/m and adhesion work lower than 105 mJ/m². These values indicate superior wetting performance compared to formulations containing sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB). This study integrates the surface free energy characteristics of pepper leaves at different growth stages with the wetting performance of various surfactant systems, providing a quantitative basis for the selection and optimization of surfactants in agricultural spray formulations. The findings offer theoretical support for precise pesticide application strategies, enhancing pesticide adhesion and absorption on leaf surfaces, thereby improving pesticide utilization efficiency throughout the crop growth cycle.