Microspore development in Quercus robur (Fagaceae)

Abstract

Tetrad and free microspore stages of Quercus robur L. were studied in thin sections for transmission electron microscopy, and mature pollen grains were observed with scanning electron microscopy before and following permanganate treatment. At a very early tetrad stage, there is no indication of a glycocalyx on the plasma membrane of microspores. Somewhat later, at an early tetrad stage, the first sign of the glycocalyx appears as a thin dark layer. At this stage, the glycocalyx was rather well developed, and developing columellae were apparent as part of the glycocalyx. At the early mid-tetrad stage, the exine has become better defined. Columellae consist of single tuft-units during the tetrad stages. At first, the columellae appeared to be weakly contrasted compared to the surrounding glycocalyx layer. Initially, the outer components of tufts, the binder zone, were low in contrast while the core zone was dark. The reaction of tufts to stains was reversed during the tetrad stages; the binder zone became dark, and the core, low in contrast. Our interpretation of this is that the relative distribution of spiral subunits in the core and binder subunits was changed. By the late tetrad stage, columellae appeared as distinct rodlets separated from one another, and a foot layer had been introduced. By the time of the vacuolate stage of the free microspore period, the form of the ectexine was similar to that of mature pollen. The ectexine consisted of a very thick tectum with supratectal verrucae, columellae and a foot layer. Many columellae at the vacuolate stage consisted of multiple tuft units but some remained singular. The endexine of aperture regions was lamellated. The thick tectum was penetrated by many microchannels. Tapetal cells were bordered by orbicules (Ubisch bodies) linked by a thin lamellation, the pellicle. During the vacuolate stage of free microspores, tapetal cells became invasive, surrounding the microspores in the loculus. The surface of the tectum and the prominent verrucae on it showed no indication of their rod-shaped tuft structure until exposure to acetolysis followed by potassium permanganate. Then, tuft-units were evident as part of verrucae and the rest of the tectum.