Fresh “Pollen Adhesive” Weakens Humidity-Dependent Pollen Adhesion

Abstract

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.

Attachment-based mechanisms underlying capture and release of pollen grains

“…How can we explain this counterintuitive result? In a previous study, we have reported that fresh pollenkitt actually weakens adhesion on high surface energy surfaces under high humidity [14]. We reasoned this result with a water capillary model stating that pollenkitt, the major constituents of which are hydrophobic lipids, leads to an increase of the contact angle of condensed water on the pollen surface, resulting in the reduced water capillary attraction.…”

“…We have recently reported adhesive properties of fresh and aged pollen of Hypochaeris radicata on artificial substrates using atomic force microscopy (AFM) [14]. The study has suggested that capillary bridges formed at the interface between pollen grains and substrates play crucial roles in pollen adhesion, whereas the pollenkitt, traditionally regarded as a kind of pollen adhesive liquid, not necessarily enhances, but rather decreases the pollen adhesion due to weakened water capillarity in some conditions.…”

Influence of Pollen on Solar Photovoltaic Energy: Literature Review and Experimental Testing with Pollen

“…In any case, the resulting pollen adhesion strength is humidity-dependent: while the adhesion force to a hydrophilic surface was found to increase with the relative humidity (RH) up to 70% RH for pollen with and without pollenkitt, the adhesion to a hydrophobic surface was independent of the RH for pollenkitt-free pollen but exhibited a complex dependence on the RH for pollen with pollenkitt [57]. Moreover, the pollenkitt adhesive strength may vary according to its freshness and to the RH: on hydrophilic surfaces (i.e., uncoated glass) a higher RH increased pollen adhesion regardless of the pollen condition (i.e., fresh, aged, without pollenkitt), but on hydrophobic surfaces (i.e., silanized glass) the adherence was significantly lower for fresh pollen than for aged pollen while the ambient humidity had little effect on the pollen adhesion [58].…”

A ballistic pollen dispersal strategy hidden in stylar oscillation

“…Hypochaeris raticata (Asteraceae) was previously adopted as a model species to investigate pollen adhesion 15,16 . In this paper, this species was used, to study the motion of pollen-bearing styles and the resulting pollen dispersal.…”

“…Unlike wind-pollinated pollen, the insectpollinated ones are covered by a thick layer of a viscous oily substance called pollenkitt 26 , which helps to form large dispersal units by bonding pollen grains together 26,27 . In addition to its general function as "pollen adhesive", we recently showed that pollenkitt inhibits pollen adhesion by weakening water capillary attraction 15 . Therefore, we can assume that pollenkitt may play a role in maintaining pollen adhesion in a specific range.…”