Pollen tube guidance by the pistil of a solanaceous plant

The guidance of pollen tubes into the stigma, through the style and within the ovary of solanaceous plants is examined. New data based on time-lapse video microscopy of pollen on Nicotiana alata stigmas and in culture, as well as style squashes and pollinations with pollen bearing the marker gene Gus, are presented. The initial guidance of pollen tubes as they emerge From grains on the stigma is probably by a gradient in the concentration of water in the immediate environment of the pollen grain. At later stages of growth on the stigma, the probability that tube tips will encounter an opening is high because of the tendency of tubes to maintain contact with the surface. Styles support pollen tube growth either towards or away from the ovary, suggesting that they have no constitutive directionality. Guidance within solid transmitting tracts is probably by default, i.e. the architecture of the tissue means that pollen tubes can only grow in the intercellular spaces between the parallel files of cells. Pollen tubes within the ovary of N. alata do not appear to be precisely guided to the nearest ovule, and further, quantitative studies of growth in this region are needed. (C) 2000 Annals of Botany Company.