Pollen Wall and Aperture Development in Helianthus Annuus (Compositae: Heliantheae)

Abstract

Wall development of tricolpate pollen of sunflower was studied by light and by scanning and transmission electron microscopy. The wall and colpi are initiated during the tetrad stage, producing a young, spinulate, two‐layered exine (ektexine and endexine) separated by a “spacer layer.” After release from the tetrads, the individual microspores round up and enlarge. The exine layers increase in thickness and complexity from sporopollenin contributed by the tapetum and microspores. During the mid‐vacuolate microspore stage, the tapetum becomes plasmodial and surrounds the developing microspores. At the vacuolate pollen stage, after the wall and colpi are completely formed, the plasmodial tapetum breaks down and releases its contents into the locule. Some of the contents are presumably utilized by the pollen to make storage reserves while other components, such as lipids and proteins, fill the spaces within the pollen wall exine. Pollen wall ontogeny provides a scheme of terms for mature composite walls in general. The various events associated with microsporogenesis in sunflower are compared with those reported in other pertinent studies.

Ontogeny of pollen wall ofleontodon autumnalis(hypochoeridinae, compositae)

“…It is similar to the spacer layer formed at the same stage in the microspore of ileliarirliirs (Horner & Pearson 1978). The loss of the fibrillar material later results in cavea-like spaces.…”

“…On the other hand, the ontogeny of the tectum in Leoritodori differs from that of other genera in which the tectum is formed over the distal ends of radially oriented probacules inserted into the primexine matrix as in Sriiritpairlia (Ehrlich 1958), Silerie (HeslopHarrison 1963), I'nrliirisoriio (Larson & Lewis 1962), Iporiiueri (Godwin et al 1967) and llelicinthirs (Horner & Pearson 1978). In Leontodori the pretectal period is distinguished from that ofhtenrisin by the deposition of strongly stainable patches appearing to be detached from the plasma membrane.…”

“…2D). A periplasmic zone filled with fibrillar material (spacer layer of Horner & Pearson 1978) separates the pretectal mat from the plasma membrane (Fig. 2 D).…”

Starch distribution in anthers, microspores and pollen grains in Aechmea recurvata (Klotzsch.) L.B.Sm., Dyckia racinae L.B.Sm. and Tillandsia aeranthos (Loisel.) L.B.Sm. (Bromeliaceae)

“…Regarding to the sugars, their importance was demonstrated for maturation of the epidermis, endothecium and tapetum (Clément & Audran 1995;Clément et al 1998;Castro & Clément 2007). In this subject, was also demonstrated that the middle layers also act in the access of sugars to the loculus (Clément & Audran 1995).Besides to supply the metabolic demand, sugars will compose the fibrous thickenings of the endothecium, the primexine and intine, and in some cases, also the exine (Walter & Doyle 1975;Horner & Pearson 1978). When preparing for the developmental events of pollen grains, including the pollination period, sugars can be accumulated as starch, callose, pectins, insoluble polysaccharides, disaccharides or monosaccharides (Pacini 1996;Aouali et al 2001;Clément & Pacini 2001;Pacini et al 2006).…”

“…Besides to supply the metabolic demand, sugars will compose the fibrous thickenings of the endothecium, the primexine and intine, and in some cases, also the exine (Walter & Doyle 1975;Horner & Pearson 1978). When preparing for the developmental events of pollen grains, including the pollination period, sugars can be accumulated as starch, callose, pectins, insoluble polysaccharides, disaccharides or monosaccharides (Pacini 1996;Aouali et al 2001;Clément & Pacini 2001;Pacini et al 2006).…”