Pistil Exudate Production and Pollen Tube Growth in Lilium longiflorum Thunb.

Abstract

Exudate production in the pistil of Lilium longiflorum was studied in relation to pollen tube growth, using scanning electron microscopy (SEM), transmission electron microscopy and light microscopy. In contrast with conventional fixation for SEM, during which the exudate of L. longiflorum largely washes away, the exudate remains present through freezing in case of cryo-SEM. Using the latter method we observed that exudate production on the stigma and in the style started before anthesis. Just underneath the stigma the exudate was first accumulated at the top of each secretory cell, followed by a merging of those accumulations as exudate production proceeded. Exudate is also produced by the placenta. It was however not possible to determine whether any of this fluid originated from the micropyle. Apart from the cell shape and the cuticle present in between the secretory cells, the ultrastructure of the secretory cells covering the placenta was comparable to those of the stylar canal. The transfer wall of the secretory cells of the placenta originated from fusing Golgi vesicles but the endoplasmic reticulum seemed to have an important role as well.After pollination the pollen tubes grew across the stigma and entered the style through one of the slits in the three stigma lobes. The pollen tubes grew straight downward through the style and were covered by exudate. As the pollen tubes approached the ovary their growth was restricted to the areas with secretory cells. In the cavity the pollen tubes formed a bundle and they bent from this bundle in between the ovules towards the micropylar side. There they bent again to stay close to the secretory cells. After bud pollination the pollen tube growth was retarded. Later arriving pollen tubes had a tendency to grow close to the secretory cells of the style, which resulted in a growth between these cells and preceding pollen tubes. If there was still a little exudate produced, it resulted in a lifting up of the pollen tubes, out of the exudate. The relationship between exudate production and pollen tube growth is discussed. Both the speed and the guidance of the pollen tube seemed determined by the properties of the exudate.