Orientation and withdrawal of pollination drops in Cupressaceae s. l. (Coniferales)

Abstract

Pollination in the Cupressaceae is studied ex situ, focused on orientation and withdrawal of pollination drops. Orientation of pollination drops is a constant feature in most taxa studied and important for pollen capture. Conspecific pollen causes a withdrawal of pollination drops, varying in time among species from 8 to 24 min, but with little variation within species. Pollination drops of each tested Cupressaceae taxon are also withdrawn when pollinated with foreign, but Cupressaceous pollen. However, they remain unchanged and are not withdrawn immediately when pollinated with pollen of other seed plants. The results clearly indicate that the time for the total withdrawal of pollination drops is strongly influenced by the evolutionary distance of the taxa being involved in the pollination process. Among closely related taxa the withdrawal is much more rapid than in distantly related ones. This points to an effective recognition system regulating the withdrawal of pollination drops, probably controlled by the nucellus. This recognition system can be regarded as an important preadaption for the evolution of a self-incompatibility mechanism. The withdrawal of pollination drops is thus not exclusively a physically induced process as suggested in some earlier studies. Pollination drops of several ovules can fuse to form a large common one, perhaps increasing by this way successful pollen capture.