Pollen grain apertures in Olea europaea L. (Oleaceae)

Abstract

The apertural zone of Olea europaea L. pollen grain undergoes a differentiation throughout its maturation which is revealed by: (1) The differentiation of the apertural region in the exine which consists of the formation and later transformation of a swelling in the endexine (exinous oncus, EO), and which indicates the place where the endoaperture will appear. (2) The differentiation of the apertural region in the intine which consists in the formation and continuous transformation of a swelling of the intine (intinous oncus, IO), that plays a key role in the main function of the aperture, i.e. the germination of the pollen tube. These onci have a similar form but differ in the development stage (EO appears before IO) and in the wall layer where they are located (EO in the exine and IO in the intine). The confusing terminology which has arisen from the similarity of these structures is discussed. The possible aperture functions are also considered. Finally, in this paper the importance of ontogenetic studies by means of transmission electron microscopy in order to fully understand the complex structure and function of these regions is stressed.