A structural investigation of the ovule in sugar beet, Beta vulgaris: The micropylar nucellus

At the mature stage of ovule development the micropylar part of the crassinucellate nucellus in sugar beet, Beta vulgaris has been investigated by light and electron microscopy. Compared to the main body of the nucellus the columnar cells in the micropylar part appear more meristematic and display a number of structural characteristics indicative of high metabolic activity, e.g. abundant mitochondria, dilated endoplasmic reticulum, numerous dictyosomes and coated vesicles often fusing with irregular and strongly PAS+ cell walls. This tissue undergoes a gradual degeneration after having reached the mature stage and this degeneration seems to be independent of both pollination and fertilization. The cuticle structure of the apical cells in the micropylar nucellus is very similar to cuticles of stigmatic papillae in other plants, which may be related to this tissue being predetermined to be recognized and penetrated by pollen tubes. The ultrastructural features of the micropylar nucellus cells are well in accordance with a secretory activity related to penetration and guidance of pollen tubes. It is discussed whether mitochondrial conversion of ACC to ethylene in these cells might be involved in pre- and postfertilization processes.