Changes in volume, surface area, and frequency of nuclear pores on the vegetative nucleus of tobacco pollen in fresh, hydrated, and activated conditions.

The vegetative nucleus (VN) of Nicotiana tabacum L. has been qualitatively and quantitatively studied in fresh, hydrated, and activated pollen. Techniques included the use of optical sectioning by confocal scanning laser microscopy to obtain volume and surface area measurements, and stereoscopic pairs; and freeze-etch electron microscopy to estimate the frequency of nuclear pores per μm(2) in the vegetative nucleus. Several morphological changes were observed to occur in pollen grain nuclei during the early processes of tube growth. In freshly dehisced pollen grain, the vegetative and generative nuclei were side by side, but following hydration and activation of the grain, the elongated generative nucleus became partially surrounded by the vegetative nucleus. It was found that during hydration, the surface area of the vegetative nucleus increased and there was a decrease in the frequency of nuclear pores. The calculated total number of pores remained similar. After activation and pollen-tube growth, the vegetative nucleus retained the same surface area as in the hydrated state but the frequency of nuclear pores decreased; therefore, the calculated total number of pores was significantly lowered. When considered alongside complementary biochemical data, these morphological results indicate that RNA production in the vegetative nucleus decreases following germination.