Sporoderm ontogeny in Cabomba aquatica (Cabombaceae)

The study on Cabomba aquatica sporoderm ontogeny was undertaken to reveal the principles of development, their connection with the exine substructure and the resistance of the sporopollenin part of the exine to degradation. Procolumellae appear as the first elements of the exine at early tetrad stage, located in the plasma membrane glycocalyx. These procolumellae correspond to elementary exine units (tufts), by Rowley. Very soon after this, procolumellae are seen consisting of several tufts and turn into columellae. Then the grouping of the columellae occurs: sporopollenin accumulates at the proximal parts of the columellae, uniting several of them. Simultaneously a thin and interrupted tectum is introduced, and the columellae grow in height. The process of grouping of individual columellae persists at the stage of the disintegrating tetrad, so that the diameter of a new-formed complex columellae has increased 3 times. The complex columellae consist of clumps of columellae. The initial striae appear in the tectum at an early free microspore stage. Simultaneously, the foot layer as a very thin layer appears. In the course of further development some striae increase in volume (macro-striae), whereas the groups of the striae between them preserve their initial size (micro-striae). Sporopollenin Acceptor Particles (SAPs) appear on the surface of the striae. The diameter of the complex columellae is 6 times more than of procolumellae, the height of the exine increases 7-9 times. The individual columellae, which are the constructive elements of each complex columella, support adjacent supratectal striae and determine the formation of them. In each depression between the striae a row of radially oriented channels penetrates the tectum. These macro-channels are rather wide - unlike micro-channels which penetrate the columellae and the foot layer in different directions. At the stage of microspore cell vacuolisation, when the intine is introduced, the volume of the microspores increases and the exine is stretched out, the complex columellae are pulled apart and can be fragmented to pieces. The oxidative treatment with potassium permanganate brings about the etching of the superficial sporopollenin layer on the surface of the columellae and reveals a spirally arranged cluster of SAPs, the latter becoming a basis for sporopollenin accumulation in the form of a super-spiral around the complex columellae. These data give evidence of SAPs to be responsible for the growth of the elements of the exine by precisely located accumulation of receptor-dependant sporopollenin. The substructure of the exine - tuft units - becomes also evident as a result of oxidative treatment. These are revealed mainly in the tectum as the basic units of micro- and macro-stria. The binder parts of the tuft units resist degradation, but core zones do not. This gives evidence of the secondary nature of the sporopollenin deposited inside the tufts, whereas the binder zone of them seems to be the location of the primary accumulated, receptor-dependant sporopollenin. (C) 2003 Elsevier B.V. All rights reserved.