Ultrastructure and ontogeny of the exine in Tribulus terrestris Linne (Zygophyllaceae)

Abstract

The pollen wall ultrastructure and its ontogeny in Tribulus terrestris L. are described in detail. The distinction between ectexine and endexine have been inconclusive from previous transmission electron and scanning electron microscopy observations, and periodic acid Schiff reaction (PAS) testing. It is observed that, at the beginning of the tetrad stage, the plasmalemma is wavy, with alternating evaginations and invaginations. The bi-layered primexine matrix is concentrated inside invagination, whereas pro-columellae are initiated on evaginations. At the end of the tetrad stage a primordial nexinous lamella (PNL) is formed, it is continuous all around the microspore and, at some levels a white line is observed. On the outer face of the PNL the foot layer develops; while the inner face is the base for progressive formation of the endexine lamellae. At the free microspore stage, all the exine components thicken and the primordial nexinous lamella is no longer discernible. The endexine lamellae, which are lacking in the pore region, form a network; the lumina of this network are progressively reduced and will disappear later. By the free microspore stage, the outer nexine is compact and the inner nexine is distinctly lamellar. © 2009 Collegium Palynologicum Scandinavicum.