Our colleague Antonio Šiber, in collaboration with Anže Božič from the Jožef Stefan Institute has published an article in prestigious journal, the journal Proceedings of the National Academy of Sciences. The authors investigated how the infolding of the pollen grain depends on its mechanical design. Their elastic model of the grain showed that the elasticity and geometry of the grain must be precisely tuned in order to enable the infolding which reflect directly on the functionality and the evolution of the pollen.

Mechanical design of apertures and the infolding of pollen grain

Anže Božič and Antonio Šiber, Proceedings of the National Academy of Sciences (2020).

DOI: 10.1073/pnas.2011084117

A neat closure of the apertures occurs only in a limited region of elastic parameters which pertain both to the apertures and to the hard parts of the grain wall, which suggests that these parameters may be evolutionary tuned to enable the closure. Investigation of the grains with different number of apertures revealed that the region of elastic parameters which lead to regular closure shrinks as the number of apertures increases. This means that the plant species with larger number of apertures are more sensitive to the modifications of the thickness and elasticity of the grain wall and that small changes can lead to dysfunctionality of the apertures and the desiccation which destroys the pollen. The shape and the area of apertures also influence the infolding and it was shown that long apertures which span from one pole of the grain to the other, like a garlic clove (spherical lune), lead to a more regular closure than is the case in grains with shorter, especially circular apertures.

 This study shows that although the grains with larger number of apertures germinate with larger probability, their mechanics requires a more precise tuning of the geometry and elasticity of the grain in order that it successfully closes when desiccated (the grains with smaller number of apertures close more robustly). The large number of plant species with three apertures (and not more) could thus be a result of a compromise between these two opposing trends.

In spite of a huge diversity of pollen grains, the study attempts to determine some characteristics of their mechanical design which may be common for a large number of species. The results may find application in the design of inhomogeneous elastic shells, which respond to the changes in the surrounding with a programmed deformation. Such shells could be used for encapsulation and delivery of drugs on a microscopic scale. Unlike homogeneous shells which endure the increase of the pressure all until the point of catastrophic collapse, inhomogeneous shells with designed apertures deform continuously and in a predefined fashion, depending on the magnitude of (osmotic) pressure. Soft regions in such shells would effectively guide and localize the deformation which can, in case of homogeneous spherical shells, appear in any surface point.