A biological nanofoam: The wall of coniferous bisaccate pollen

Abstract

The outer layer of the pollen grain, the exine, plays a key role in the survival of terrestrial plant life. However, the exine structure in different groups of plants remains enigmatic. Here, modern and fossil coniferous bisaccate pollen were examined to investigate the detailed three-dimensional structure and properties of the pollen wall. X-ray nanotomography and volume electron microscopy are used to provide high-resolution imagery, revealing a solid nanofoam structure. Atomic force microscopy measurements were used to compare the pollen wall with other natural and synthetic foams and to demonstrate that the mechanical properties of the wall in this type of pollen are retained for millions of years in fossil specimens. The microscopic structure of this robust biological material has potential applications in materials sciences and also contributes to our understanding of the evolutionary success of conifers and other plants over geological time.

Fig. 1.. SEM and confocal microscopy images of modern and fossil pollen grains.

SEM images of (A) modern pine pollen in the lateral view and (B) fossil bisaccate pollen in the proximal view. Confocal microscopy measurements (C) of a modern pine pollen grain and (D) of a fossil bisaccate specimen. Eurugulate structures visible on modern pine are shown in (C) in different areas of the cappa at different focusing depths.

Fig. 2.. AFM phase trace measurements on modern pine pollen grains.

Phase trace data (A) over a 3 μm × 3 μm surface and (B) over a 1 μm × 1 μm surface.

Fig. 3.. AFM surface reconstructions and Young’s modulus measurements obtained over 3 μm × 3 μm areas.

(A) 3D surface reconstruction from height trace data from a modern pine pollen specimen. (B) Young’s modulus values obtained using the Oliver-Pharr model on force map measurements from a 6 × 6 grid over the same area. The hatched boxes indicate grid points where the force curves were deemed unreliable due to a deficient contact between the tip and the surface. (C and D) Same type of data for a fossil bisaccate pollen specimen.

Fig. 4.. 3D reconstruction of a modern pine pollen specimen from x-ray nanotomography data.

(A) Lateral view of the full grain (see Fig. 1 for scale). (B) Zoom of the foam structure forming the wall of the pollen sac. (C and D) Slices from the tomographic reconstruction in the cappa region. In (C), five radial slices taken every 20 pixels (800 nm) through the structure of the exine wall illustrate vertical rod-like columella (example indicated by a red arrow), connecting the outer tectum and the inner foot layer (30). In (D), transverse slices taken every 5 pixels (200 nm) reveal detailed eurugulate structures.

Fig. 5.. Density distribution from x-ray nanotomography and XRF data showing Ca and K trace element distributions.

(A) Slice from the reconstruction of a modern pine pollen specimen from nanotomography measurements. The gray map corresponds to mass density in [g cm⁻³]. Following XRF measurements on the same specimen, on the upper right side of the grain oriented as shown in the previous panel, (B) and (C) present the distribution of Ca and K concentrations [ng mm⁻²], respectively, within the pollen grain. There is a notable concentration of Ca contained in the foam structure of the pollen wall (indicated by an orange arrow), whereas elements such as K can be found in much higher quantities in the cytoplasm and cell contents present inside the grain. (D) Composite image showing both Ca (red) and K (blue) trace element distributions.