Raster-elektronenmikroskopische Studien an Pollen aus Bienenhonig. III. Der Harmomegathie-Mechanismus und seine Auswirkungen auf die Exine-Strukturen am Beispiel unterschiedlicher Pollentypen

Abstract - Scanning electron microscopic studies of pollen from honey. III. The harmomegathy-mechanism and its effect on the exine-structure using different pollen types
Pollen grains found in honey are subjected to changes in volume and shape by hydration of the plasm - the harmomegathy - which enable the pollen grain to harmonize with the surrounding forces. At dehiscence from the anther into the dehydrating atmosphere, pollen grains change their shape so that the apertures close (state of moisture stress = contraction). Conversely, the exine relaxes by hydration when the pollen is on the stigma of the blossom or has fallen into nectar. In addition to volume and shape, the surface pattern of the intercolpium may also change by harmomegathy. Pollen with different sculpturing types were analyzed by SEM in states of contraction and expansion. Contracted pollen grains were transferred into filtered honey for expansion and prepared for SEM after isolation into a wire screen. All tricolpate pollen grains were found to change their prolate-oval into a spheroidal shape by harmomegathy. The analyzed sculpturing patterns (striate, reticulate, foveolate, echinate) show that there is no stretching of the general intercolpium. If such a stretching occurred, the striate sculpturing element should show tectate perforations more visible than in a state of contraction. However, fig 1 shows the opposite : In expanded pollen grains of Acer platanoides, the tectum is closed by supratectate valla. Fig 6 illustrates that the enlargement of the pollen circumference does not require a general stretching, but only a bending of the intercolpium which simultaneously opens the apertures. Due to this harmomegathy mechanism, the external regions of the sculpturing elements draw close as on the pollen of Verbesina persicifolia with its particularly elongated echinate elements (fig 5). Similarly, the expanded pollen of Ligustrum vulgare shows a lumina width decrease of the reticulate pattern (fig 2), and the Leucophyllum frufescens pollen an additional compressive strain of the muri (fig 3). The foveolate sculpturing elements on the Fagopyrum esculentum pollen undergo a compression important enough to induce considerable changes in lumina shape. The knowledge of pollen surface changes described in this study should facilitate the melissopalynological identification of pollen using SEM.