Despite the vital role that flavonols play in fertilization and pollen tube growth of a number of species such as petunia and maize, their function is still unclear. Pollen tubes of the flavonol-deficient transformant T17.02 of Petunia hybrida L. are able to germinate and start growing in vitro, but eventually disrupt at the tip approximately 2 h after germination. In order to establish the possible role of flavonols in this process, wild-type and flavonol-deficient pollen tubes were subjected to cytological and ultrastructural analyses and screened for differences. The results showed that before disruption of the flavonol-deficient pollen tubes, the structure of the primary wall at the tip dramatically changed from layered to granular. Secretory vesicles at the tip still fused with the wall but lost their capacity to melt into the wall and to form layers. Instead they remained as dark, electron-dense granular structures surrounded by an electron-translucent matrix. Apparently the matrix is not able to sustain the wall's coherence and as a consequence the tube disrupts. No other remarkable cytological or ultrastructural differences between the transformant and the wild-type pollen tubes could be found before tip disruption. Even a morphometric analysis of abundance and distribution of endoplasmic reticulum, dictyosomes and mitochondria did not reveal any significant difference. However, for the first time, obvious morphological differences were observed in the wall of the flavonol-deficient pollen tubes. We conclude that flavonols act on precursors of the pollen tube wall of petunia and interfere with a cross-linking system in the wall, possibly via extensins.