Developmental Aspects of Ultrastructure, Histochemistry and Receptivity of the Stigma of Nicotiana sylvestris

The bilobed papillate stigma of Nicotiana sylvestris Speg. and Comes, is covered at maturity with a copious exudate containing lipid, protein and carbohydrate. The stigma is receptive from the very early stage of development and it also stains positively for esterase activity. The stigma has three distinct zones: an epidermis with papillae; a subepidermal secretory zone; and a parenchymatous ground tissue. The behaviour of the cells of these three zones has been followed from 6 d before anthesis to one day after anthesis and pollination. The cells of the epidermis and the secretory zone stain intensively for lipids, proteins and carbohydrates in the initial stages. The secretory zone develops large intercellular spaces containing heterogenous secretory products which also stain positively for the aforesaid three compounds. At maturity the secretory products are released to the surface through gaps formed in the epidermis by cell separation. The main secretion of the stigma is produced by the cells of the secretory zone. Less secretion is derived from the stigmatic papillae. Some amount of secretion is also released from the stylar transmitting tissue adjoining the stigma. The glandular cells of the stigma contain numerous plastids, mitochondria, ribosomes, ER, cytoplasmic lipid droplets and some dictyosomes. The plastids and the vacuoles in the secretory cells of the stigma have a lot of electron dense (osmiophilic) inclusions respectively in the initial and later stages of development. The former are probably involved in the production of these materials. It is suggested that the proteins are directly secreted by rough ER compartments whereas smooth ER is involved in the synthesis of lipidic materials. The carbohydrate moiety of the exudate is released by the eccrine mode (sugar mono- and dimers) with some addition of polymers by disintegration of the middle lamellae. The means by which the lipidic and osmiophilic material is extruded remains unclear.