DYNAMICS OF POLLEN TUBE GROWTH IN THE WILD RADISH RAPHANUS RAPHANISTRUM (BRASSICACEAE). II. MORPHOLOGY,CYTOCHEMISTRY AND ULTRASTRUCTURE OF TRANSMITTING TISSUES,AND PATH OF POLLEN TUBE GROWTH

The relative importance of prezygotic mechanisms of gametophytic competition and selection are often unclear due to an absence of observations on the gynoecium and pollen tube growth in vivo. We used LM, SEM, and TEM to study the structure of the gynoecium and the path of pollen tube growth in Raphanus raphanistrum, a sporophytically self-incompatible annual. Wild radish has a papillate stigma and a solid style. A septum, which is characteristic of cruciferous gynoecia, is present in the ovary. After germination on the stigma, pollen tubes grow in the secretion of the transmitting tract of the style. The stylar secretion stains positive for acidic polysaccharides and insoluble carbohydrates, and negative for lipids and protein. In the ovary, the transmitting tissue is contained inside the septum. The secretion in the ovary stains positive only for acidic polysaccharides. Pollen tubes travel inside the septum as they enter the ovary and must exit to the surface of this tissue before ovule fertilization can occur. Pollen tube growth on the septum tracks the intercellular junctions of the septum epidermis where the secretion leaks out through a torn cuticle. Tubes must grow across the obturator before reaching the micropyle of an ovule. The temporal pattern with which tubes growing into the ovary exit the septum can contribute to the previously observed nonrandom patterns of fertilization (Hill and Lord, 1986).