Pollenkitt Wetting Mechanism Enables Species-Specific Tunable Pollen Adhesion

Abstract

Plant pollens are microscopic particles exhibiting a remarkable breadth of complex solid surface features. In addition, many pollen grains are coated with a viscous liquid, "pollenkitt", thought to play important roles in pollen dispersion and adhesion. However, there exist no quantitative studies of the effects of solid surface features or pollenkitt on adhesion of pollen grains, and it remains unclear what role these features play in pollen adhesion and transport. We report AFM adhesion measurements of five pollen species with a series of test surfaces in which each pollen has a unique solid surface morphology and pollenkitt volume. The results indicate that the combination of surface morphology (size and shape of echinate or reticulate features) with the pollenkitt volume provides pollens with a remarkably tunable adhesion to surfaces. With pollenkitt removed, pollen grains had relatively low adhesion strengths that were independent of surface chemistry and scalable with the tip radius of the pollen's ornamentation features, according to the Hamaker model. With the pollenkitt intact, adhesion was up to 3-6 times higher than the dry grains and exhibited strong substrate dependence. The adhesion enhancing effect of pollenkitt was driven by the formation of pollenkitt capillary bridges and was surprisingly species-dependent, with echinate insect-pollinated species (dandelion and sunflower) showing significantly stronger adhesion and higher substrate dependence than wind-pollinated species (ragweed, poplar, and olive). The combination of high pollenkitt volume and large convex, spiny surface features in echinate entomophilous varieties appears to enhance the spreading area of the liquid pollenkitt relative to varieties of pollen with less pollenkitt volume and less pronounced surface features. Measurements of pollenkitt surface energy indicate that the adhesive strength of capillary bridges is primarily dependent on nonpolar van der Waals interactions, with some contribution from the Lewis basic component of surface energy.

Diversity of allochtonous substances detected in bee pollen pellets

“…Botanical-palynological identification of pollen grains was settled in the following steps (i) evaluation of general grain morphology; (ii) measurement and calculation of pollen grain uniformity and size by software ImageJ (https://imagej.net/) (iii) detection of specific structures such as surface apertures and (iv) each flower pollen class was finally classified to accommodate "species-specific" features including structure and ornamentation (Almeida-Muradian et al, 2005, Klimko et al, 2000Lin et al, 2013;Punt et al, 2007;Schulz et al, 2000).…”

“…Pollen species also depend on individual levels of adherence to inorganic mineral surfaces. This depends on pollenkit content; the viscous organic substances on the pollen surface which generate strong energy forces between close molecules; such as Van der Wall attraction (Lin et al, 2013). Although the pollen surface is responsible for initial inorganic particle-surface attachment, it is also able to resist attachment during technologically allochtonous material purification and elimination processes with the aid of specially constructed purifiers based on bulk density separation.…”

“…Inset: Energy-dispersive X-ray spectrum showing Si, Al, O, Mg, K, Fe and Ti elements confirming non-homogenous soil aggregate confirmed in our samples. Negative synergic effects of microbial contamination could decrease pollen quality or introduce harmful human health reactions such as allergies (Kačániová et al, 2004;Kačániová et al, 2011) and therefore mycogenic analysis is beneficial in evaluating presence and activity of microscopic fungi, and preclude any of the abundant mycotoxins (Linskens and Jorde, 1997;Kačániová et al, 2011).…”

“…Pollen pellets have long been recognised as beneficial in medical therapy and nutrient supplements (Linskens and Jorde, 1997;Villanueva et al, 2002) and medical research has confirmed bee pollen is valuable in the following human health aspects; (i) allergy desensitisation (ii) tumour presence (iii) prostate problems and (iv) arteriosclerosis. It has also proven beneficial in tissue repair, promoting toxic elimination, rapidly decreasing excessive cholesterol and radical scavenging activity (Estevinho et al, 2012;Nogueira et al, 2012;Linskens and Jorde, 1997).…”

“…These have relatively small size and specific chemical and physical composition with differing positive and negative effects on human health, and they are often relicts of conventional technological procedures that are very difficult to effectively rectify (Linskens and Jorde, 1997).…”