Pollen—stigma Interactions: Development and Cytochemistry of Stigma Papillae and their Secretions in Annona squamosa L. (Annonaceae)

The development and cytochemical features of the stigma and style have been investigated in Sugar apple, Annona squamosa L., using light and electron microscopy. The pistil is a syncarp with an open stylar canal. Papillae of epidermal origin line both the surface of the stigma and the inner face of the stylar canal. The papillae contain organelles characteristic of secretory cells with a highly thickened cellulosic wall. The wall is multi-layered, the zones differing in their microfibrillar stacking and orientation. The stigma is of the ‘wet’ type and the surface exudate is heterogeneous in microscopic appearance and reacts cytochemically for proteins, carbohydrates and lipids. The surface cuticle undergoes dissolution prior to anthesis. A secretion also appears in the thickened middle lamella of the sub-epidermal cell layer which reacts cytochemically for pectinaceous acidic polysaccharides. Esterase activity of papillae is indicative of the receptive areas, and it is also related to the onset of receptivity. Acid phosphatase activity is intense in the sub-epidermal cell layers which probably reflects their secretory activity. Pollination triggers a copious flow of secretion onto the stigma surface which engulfs the pollen grains. It appears that most of the acidic polysaccharides of this secretion come from the middle lamella of the sub-epidermal cell layer. Compatible pollen tubes have no apparent barriers to overcome on their route to the embryo sac and the inherent protogynous dichogamy seems to control the acceptance or rejection of compatible pollen.