Fresh Pollen Adhesive Weakens Humidity-Dependent Pollen Adhesion

This study presents a quantitative investigation of pollen adhesion mediated by pollenkitt using pollen grains in their native state. Here, we attempt to clarify whether the exposure time, pollenkitt losses, and the surrounding humidity (relative humidity, RH) levels influence pollen adhesion. Pollen grains of Hypochaeris radicata (Asteraceae) were tested using atomic force microscopy. Regardless of the pollen condition (fresh, aged, and without pollenkitt), higher RH significantly increased pollen adhesion on hydrophilic surfaces, whereas it had little effect on pollen adhesion on hydrophobic surfaces. On hydrophilic surfaces, adhesion of fresh pollen was less dependent on RH than that of aged pollen or without pollenkitt, resulting in reduced adhesion under high RH. On hydrophobic surfaces, adhesion of fresh pollen was significantly lower than that of aged pollen. We utilized capillary models to explain the counterintuitive results obtained and came to the conclusion that the abundant fresh pollenkitt, which is widely accepted as pollen adhesive, can reduce pollen adhesion in some conditions. This study sheds light on the little-known adhesive properties of pollen and on the pollination mechanics.