GROWTH-INHIBITION AND RECOVERY IN FREEZE-SUBSTITUTED LILIUM-LONGIFLORUM POLLEN TUBES - STRUCTURAL EFFECTS OF CAFFEINE

In an attempt to correlate structural effects with the known dissipati on of the tip-focused Ca2+ gradient caused by caffeine, we have examin ed the ultrastructure of caffeine-treated lily pollen tubes prepared b y rapid freeze fixation and freeze substitution. We show that treatmen t with caffeine results in a rapid rearrangement of secretory vesicles at the pollen tube tip; the normal cone-shaped array of vesicles is r apidly dispersed. In addition, microfilament bundles appear in the tip region, where they had previously been excluded. Delocalized vesicle fusion continues in the presence of caffeine but tube extension ceases . Removal of caffeine from the growth medium initially causes tip swel ling, delocalized vesicle fusion and presence of microfilaments well i nto the tip before normal structure and growth resume, concurrent with the previously reported return to a normal Ca2+ gradient.