Development and cytochemistry of pollen and tapetum in Mitriostigma axillare (Rubiaceae)

Presence of the protruding oncus is affected by anther dehiscence and acetolysis technique

“…The onci of Mitriostigma axillare Hochst. form at the tetrad stage, protrude through the aperture when the grains separate, contain only minimal cytoplasmic contents and are not shed from the grain (Hansson & El-Ghazaly, 2000). In contrast, the thick onci of Tarenna gracilipes (Hayata) Ohwi form in mature pollen grains just prior to dehiscence .…”

“…The term was proposed to replace 'pollen bud' following Weber and Igersheim's (1994) objection that 'pollen bud' was an inappropriate term. However, pollen buds have usually been described as containing large vacuoles (Igersheim & Weber, 1993;Hansson & El-Ghazaly, 2000;Dessein et al, 2005), whereas depictions of protruding onci show them lacking vacuoles or, at most, possessing very small vacuolar remnants (Tilney & Van Wyk, 1997;Kuang et al, 2008). This distinction was not made by Tilney and Van Wyk (1997).…”

“…Hyde, 1955;Seoane-Camba & Suárez-Cervera, 1986;Rodríguez-García & Fernández, 1988;Ben Nasri-Ayachi & Nabli, 1995;Banks, 2003;Ben Saad-Limam et al, 2005). However, there has been little work on protruding onci, which have been reported to occur in the Rubiaceae (Tilney & Van Wyk, 1997;Hansson & El-Ghazaly, 2000;Dessein et al, 2005;Ma et al, 2005;Kuang et al, 2008).…”

“…Previous studies have investigated the formation and function of the oncus during pollen development and germination (e.g. HeslopHarrison & Heslop-Harrison, 1980;Barnes & Blackmore, 1986;Heslop-Harrison et al, 1986;Hansson & El-Ghazaly, 2000;Ben Saad-Limam et al, 2005), and there has been much discussion on the term and in relationship to its counterpart 'Zwischenkörper' (e.g. Hyde, 1955;Seoane-Camba & Suárez-Cervera, 1986;Rodríguez-García & Fernández, 1988;Ben Nasri-Ayachi & Nabli, 1995;Banks, 2003;Ben Saad-Limam et al, 2005).…”

“…Pollen buds are also usually larger than protruding onci and appear to be shed from the pollen grain before anther dehiscence (Philip & Mathew, 1975;Chennaveeraiah & Shivakumar, 1983;Priyadarshan & Ramachandran, 1984;Igersheim & Weber, 1993;Weber & Igersheim, 1994), a feature that has not been observed for protruding onci. Thus, it seems prudent to follow Hansson and El-Ghazaly (2000) in distinguishing these two terms based on the size of the protrusion, the presence or absence of a well-developed vacuole and whether or not the protrusion separates from the grain before the grain is shed. In this paper, the term 'protruding oncus' will be used for smaller apertural intine protrusions with or without cytoplasmic inclusions, but which lack the large vacuoles that have been reported to occur in pollen buds.…”