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 andstyle have been investigated in Sugar apple, Annona squamosaL., using light and electron microscopy. The pistil is a syncarpwith an open stylar canal. Papillae of epidermal origin lineboth the surface of the stigma and the inner face of the stylarcanal. The papillae contain organelles characteristic of secretorycells 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 surfaceexudate is heterogeneous in microscopic appearance and reactscytochemically for proteins, carbohydrates and lipids. The surfacecuticle undergoes dissolution prior to anthesis. A secretionalso appears in the thickened middle lamella of the sub-epidermalcell layer which reacts cytochemically for pectinaceous acidicpolysaccharides. Esterase activity of papillae is indicative of the receptiveareas, and it is also related to the onset of receptivity. Acidphosphatase activity is intense in the sub-epidermal cell layerswhich probably reflects their secretory activity. Pollinationtriggers a copious flow of secretion onto the stigma surfacewhich engulfs the pollen grains. It appears that most of theacidic polysaccharides of this secretion come from the middlelamella of the sub-epidermal cell layer. Compatible pollen tubes have no apparent barriers to overcomeon their route to the embryo sac and the inherent protogynousdichogamy seems to control the acceptance or rejection of compatiblepollen. Annona squamosa L., sugar apple, stigma, style, secretions